Abstract: This Letter presents practical realization of a two-dimensional low loss acoustic cloak for airborne sound obtained by inverse design. The cloak consists of 120 aluminum cylinders of 15 mm diameter surrounding the cloaked object-a cylinder of diameter 22.5 cm. The position of each cylinder in the cloak is optimized using the data from two different techniques: genetic algorithm and simulated annealing. The operation frequency of this cloak is 3061 Hz with the bandwidth of about 100 Hz. Being a multi-step approach to the desired cloaking, the inverse design is also valid, in principle, for non-symmetric cylinders and even for three-dimensional objects. (C) 2011 American Institute of Physics. [doi:10.1063/1.3623761]
Abstract: SiO(2) and TiO(2) thin films with gold nanoparticles (NPs) are of particular interest as photovoltaic materials. A novel method for the preparation of spin-coated SiO(2)-Au and TiO(2)-Au nanocomposites is presented. This fast and inexpensive method, which includes three separate stages, is based on the in situ synthesis of both the metal-oxide matrix and the Au NPs during a baking process at relatively low temperature. It allows the formation of nanocomposite thin films with a higher concentration of Au NPs than other methods. High-resolution transmission electron microscopy studies revealed a homogeneous distribution of NPs over the film volume along with their narrow size distribution. The optical manifestation of localized surface plasmon resonance was studied in more detail for TiO(2)-based Au-doped nanocomposite films deposited on glass (in absorption and transmittance) and silicon (in specular reflectance). Maxwell-Garnett effective-medium theory applied to such metal-doped nanocomposite films describes the peculiarities of the experimental spectra, including modification of the antireflective properties of bare TiO(2) films deposited on silicon by varying the concentration of metal NPs. The antireflective capabilities of the film are increased after a wet etching process.
Abstract: The binding energy E(b) of the acceptor-exciton complex (A(-), X) as a function of the radius (or of the impurity position of the acceptor) and the normalized oscillator strength of (A(-), X) in spherical ZnO quantum dots (QDs) embedded in a SiO(2) matrix are calculated using the effective-mass approximation under the diagonalzation matrix technique, including a three-dimensional confinement of the carrier in the QD and assuming a finite depth. Numerical results show that the binding energy of the acceptor-exciton complexes is particularly robust when the impurity position of the acceptor is in the center of the ZnO QDs. It has been clearly shown from our calculations that these physical parameters are very sensitive to the quantum dot size and to the impurity position. These results could be particularly helpful, since they are closely related to experiments performed on such nanoparticles. This may allow us to improve the stability and efficiency of the semiconductor quantum dot luminescence which is considered critical. (C) 2011 Elsevier Ltd. All rights reserved.
Abstract: We present the results of a comprehensive study carried out on morphological, structural and optical properties of InAs/GaAs quantum dot structures grown by Molecular Beam Epitaxy. InAs quantum dots were deposited at low growth rate and high growth temperature and were capped with InGaAs upper confining layers. Owing to these particular design and growth parameters, quantum dot densities are in the order of 45x10(9) cm(-2) with emission wavelengths ranging from 1.20 to 1.33 mu m at 10 K, features that make these structures interesting for single-photon operation at telecom wavelength. High resolution structural techniques show that In content and composition profiles in the structures depend on the particular epitaxial growth conditions used to reduce the density of quantum dots. Carrier recombination dynamics in quantum dots is studied by analysing the integrated photoluminescence (PL) intensity and recombination times as functions of the temperature. Arrhenius plots of the integrated PL show two activation energies implying two decay processes. The first one is associated to thermal escape of carriers from quantum dot to wetting layer states; the second one is consistent with the thermal population of quantum dot dark states, as suggested by time resolved PL measurements. (C) 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Abstract: We investigate the electronic and optical properties of InAs double quantum dots grown on GaAs (001) and laterally aligned along the [110] crystal direction. The emission spectrum has been investigated as a function of a lateral electric field applied along the quantum dot pair mutual axis. The number of confined electrons can be controlled with the external bias, leading to sharp energy shifts which we use to identify the emission from neutral and charged exciton complexes. Quantum tunneling of these electrons is proposed to explain the reversed ordering of the trion emission lines as compared to that of excitons in our system.
Abstract: We model the time-resolved and time-integrated photoluminescence of a single InAs/GaAs quantum dot (QD) using a random population description. We reproduce the joint power dependence of the single QD exciton complexes (neutral exciton, neutral biexciton and charged trions). We use the model to investigate the selective optical pumping phenomenon, a predominance of the negative trion observed when the optical excitation is resonant to a non-intentional impurity level. Our experiments and simulations determine that the negative charge confined in the QD after exciting resonance to the impurity level escapes in 10 ns.
Abstract: Small (3-5 nm in diameter following HRTEM images) Si nanocrystals were produced in a two-stage process including (1) nanosecond laser ablation of a Si target in an organic liquid (chloroform) that results in formation of big composite polycrystalline particles (about 20-100 nm average diameter) and (2) ultrasonic post-treatment of Si nanoparticles in the presence of HP. The post-treatment is responsible for disintegration of the composite Si particles, release of small individual nanocrystals, and reduction of their size due to HF-induced etching of Si oxide. The downshift and broadening of the similar to S20 cm(-1) Raman phonon band of the small Si nanocrystals with respect to the bulk Si Raman band is consistent with the presence of similar to 4.5 nm Si nanocrystals. The photoluminescence spectra (450-900 nm) and decay kinetics of small Si nanocrystals were detected, and the possible origin of the luminescence is discussed.
Abstract: We report on the growth by molecular beam epitaxy and the study by atomic force microscopy and photoluminescence of low density metamorphic InAs/InGaAs quantum dots. subcritical InAs coverages allow to obtain 10(8) cm(-2) dot density and metamorphic In(x)Ga(1-x)A(s) (x = 0.15, 0.30) confining layers result in emission wavelengths at 1.3 mu m. We discuss optimal growth parameters and demonstrate single quantum dot emission up to 1350 nm at low temperatures, by distinguishing the main exciton complexes in these nanostructures. Reported results indicate that metamorphic quantum dots could be valuable candidates as single photon sources for long wavelength telecom windows. (C) 2011 American Institute of Physics. [doi:10.1063/1.3584132]
Abstract: Assuming finite depth and within the effective mass approximation, the energies of exciton states and of the acceptor-exciton complexes confined in spherical ZnO quantum dots (QDs) embedded in a SiO(2) matrix are calculated using a matrix procedure, including a three-dimensional confinement of carrier in the QDs. This theoretical model has been designed to illustrate the two emission bands in the UV region observed in our experimental Photoluminescence spectrum (PL), with the first emission band observed at 3.04 eV and attributed to the bound ionized acceptor-exciton complexes, and the second one located at 3.5 and assigned to the free exciton. Our calculations have revealed a good agreement between the matrix element calculation method and the experimental results. (C) 2011 Elsevier Ltd. All rights reserved.
Abstract: We have studied the temperature dependence of the photoluminescence of a single layer of InAs/InP(001) self-assembled quantum wires emitting at 1.5 mu m. The non-radiative mechanisms responsible for the quenching of the emission band have been identified. The exciton dynamics has been investigated using time resolved photoluminescence measurements. The results have been explained through the interplay between free excitons and localized states (arising from size fluctuations in the quantum wires). (C) 2011 American Institute of Physics. [doi:10.1063/1.3660260]
Abstract: In this paper, active planar waveguides based on the incorporation of CdSe and CdTe nanocrystal quantum dots in a polymer matrix are demonstrated. In the case of doping the polymer with both types of quantum dots, the nanocomposite film guides both emitted colors, green (550 nm, CdTe) and orange (600 nm, CdSe). The optical pumping laser can be coupled not only with a standard end-fire coupling system, but also directing the beam to the surface of the sample, indicating a good absorption cross-section and waveguide properties. To achieve these results, a study of the nanocomposite optical properties as a function of the nanocrystal concentration is presented and the optimum conditions are found for waveguiding.
Abstract: An nanoparticles are synthesized in situ upon the electron beam exposure of a poly(vinyl alcohol) (PVA) thin film containing Au(III). The e-beam-irradiated areas are insoluble in water (negative-tone resist), and AU-PVA nanocomposite patterns with a variable profile along the structure can be thus generated (3D lithography) in a single step. A local characterization of the generated patterns is performed by high-resolution transmission electron microscopy and UV-vis localized Surface plasmon resonance microspectroscopy. This characterization confirms the presence of crystalline nanoparticles and aggregates.
Abstract: In this work, the formation of silver metal nanoparticles inside a negative-tone resist based on poly(vinyl alcohol) is achieved by electron beam lithography. The chemistry of this sensitive resist allows the production of nanoparticles as well as the polymer crosslinking by the electron radiation. Due to the presence of the silver nanoparticles, the final composite exhibits a plasmonic behavior, which was characterized by measuring the absorbance. The lithographic properties of the resist have been characterized. The technique has also been exported to UV lithography, where silver nanoparticles are obtained inside the polymeric patterns after optical lithography. (C) 2009 Elsevier B.V. All rights reserved.
Notes: 35th International Conference on Micro-and Nano-Engineering, Ghent, BELGIUM, SEP 28-OCT 01, 2009
Abstract: Here we describe a simple, powerful technique based on the laser ablation of a target immersed in a water solution of a metal salt. With this method, nanoparticles of different metals and alloys can be processed very quickly. Both the target and the salt solution can be chosen to produce metal nanoparticles of different sizes, surface-oxidized nanoparticles (silica-silver, for example), or even more complex structures to be defined by the researcher on one or more steps because the technique combines the advantages of both physical and chemical methods. We have applied this technique to the fabrication of inert silica-metal (silver, gold, and silver-gold) nanoparticles with a strong surface plasmon resonance all together in a single step. The advantage of the simultaneous production of silica during laser ablation is the stabilization of the metal nanoparticle colloid but also the possibility to reduce the toxicity of these nanoparticles.
Abstract: We experimentally demonstrate a photonic integrated lens made of holes in a silicon slab operating at lambda(0)=1550 nm. The lens has been designed using a genetic algorithm in conjunction with the two-dimensional multiple scattering theory and fabricated using silicon-on-insulator technology. scanning near field optical microscopy measurements have been performed in order to measure the light intensity distribution on the device surface. The obtained full width at half maximum of the focus is 0.23 lambda(0), which is in good agreement with three-dimensional finite-difference time-domain simulations, and overcomes the diffraction limit in air, where the measurements are made. (C) 2010 American Institute of Physics. [doi:10.1063/1.3479046]
Abstract: The effect of a lateral external electric field F on an exciton ground state in an InAs disc-shaped quantum dot has been studied using a variational method within the effective mass approximation. We consider that the radial dimension of the disc is very large compared to its height. This situation leads to separating the excitonic Hamiltonian into two independent parts: the lateral confinement which corresponds to a two-dimensional harmonic oscillator and an infinite square well in the growth direction. Our calculations show that the complete description of the lateral Stark shift requires both the linear and quadratic terms in F which explains that the exciton possess nonzero lateral dipolar moment and polarizability. The fit of the calculated Stark shift permits us to estimate the lateral permanent dipole moment and the polarizability according to the disc size. Our results are compared to those existing in the literature. In addition the behavior of the optical integral shows that the exciton lifetime is greater than that under zero field which is due to the field-induced polarization.
Abstract: An nanoparticles are synthesized in situ upon the electron beam exposure of a poly(vinyl alcohol) (PVA) thin film containing Au(III). The e-beam-irradiated areas are insoluble in water (negative-tone resist), and AU-PVA nanocomposite patterns with a variable profile along the structure can be thus generated (3D lithography) in a single step. A local characterization of the generated patterns is performed by high-resolution transmission electron microscopy and UV-vis localized Surface plasmon resonance microspectroscopy. This characterization confirms the presence of crystalline nanoparticles and aggregates.
Abstract: In this work we report on a method to synthesize Ag-Au nanoparticles/polymer nanocomposite patterns by UV lithography. The photoresists are based on DNQ-novolac as the polymer matrix, and Ag(I) and Au(III) salts as the nanoparticle precursors. After UV lithography, silver and gold nanoparticles are in situ synthesized inside the polymer patterns during a post bake. The resulting structured nanocomposite shows a characteristic absorbance spectrum related to the plasmon frequency of the synthesized noble metal NPs. This method represents a fast, simple and low-cost approach to the formation of extended polymer patterns with embedded silver or gold NPs. Moreover, it is a mechanism to position nanometric particles with micrometric resolution, which represents a useful tool for nanoscience. Furthermore, even with the polymeric cover, NPs plasmon resonance is affected by the binding of some organic molecules. This concept has been proven with 2-mercaptoethanol molecules, which demonstrates the feasibility of localized surface plasmon resonance chemo/biosensors by using the proposed technology.
Abstract: We report simultaneous oscillation in continuous wave at 1062 and 1337 nm in a Nd(3+):YAl(3)(BO(3))(4) non-linear crystal associated to the infrared laser channels (4)F(3/2) -> 41, 112 and (4)F(3/2) -> (4I)(13/2) of Nd(3+). Generation ofyellow laser light at 592 nm produced by Type I self-sum-frequency-mixing of both fundamental infrared laser waves is observed under non-optimal phase matching conditions. (C) 2008 Elsevier B.V. All rights reserved
Abstract: Here we report on the in situ synthesis of Ag and Au nanoparticles inside several polymer matrixes by solid-state chemical reduction of a metallic salt. Poly(ethyleneimine) (PEI), poly(hydroxyethyl methacrylate) (PHEMA), poly(vinylpyrrolidone) (PVP), novolak, poly(4-vinylphenol) (P4VP), poly(4-vinylphenol)-co-(methyl methacrylate) (P4VP-co-MMA) and poly(styrene-co-allyl alcohol) (PS-co-AA) were able to reduce Ag(I) and Au(III) to the corresponding nanoparticles during the baking process. The nanoparticle diameters of Ag and Au were found to range from 2 to 25 nm. TEM also indicated a uniform distribution of nanoparticles embedded in the thin film. This approach is suitable for controlling the size of the nanoparticles and its homogeneous distribution in the polymer matrix.
Abstract: A bioinspired bottom-up process was developed to produce self-assembled nanocomposites of cellulose synthesized by Acetobacter bacteria and native starch. This process takes advantage of the way some bacteria extrude cellulose nanofibres and of the transport process that occurs during the gelatinization of starch. Potato and corn starch were added into the culture medium and partially gelatinized in order to allow the cellulose nanofibrils to grow in the presence of a starch phase. The bacterial cellulose (BC)-starch gels were hot pressed into sheets that had a BC volume fraction higher than 90%. During this step starch was forced to further penetrate the BC network. The self-assembled BC-starch nanocomposites showed a coherent morphology that was assessed by Atomic Force Microscopy (AFM) and Environmental Scanning Electron Microscopy (ESEM). The nanocomposites structure was studied using X-ray diffraction and ATR-FTIR spectroscopy. The degree of crystallinity of the final nanocomposites was used to estimate the volume fraction of BC. The aim of this paper is to explore a new methodology that could be used to produce nanomaterials by introducing a different phase into a cellulose nanofibre network during its assembly. (C) 2008 Elsevier B.V. All rights reserved.
Abstract: We present a fabrication method to produce site-controlled and regularly spaced InAs/GaAs quantum dots for applications in quantum optical information devices. The high selectivity of our epitaxial regrowth procedure can be used to allocate the quantum dots only in positions predefined by ex-situ local oxidation atomic force nanolithography. The quantum dots obtained following this fabrication process present a high optical quality which we have evaluated by microphotoluminescence and photon correlation experiments.
Abstract: We discuss the impact of a 2-D-charged carrier reservoir for high-speed optical amplification and modulated lasing in quantum dot (QD)-based devices by testing the amplification of short trains of high power, femtosecond optical pulses in an In-GaAs QD-in-a-well-based semiconductor optical amplifier (SOA). We adapt a laser-like rate equation model to describe heterodyne pump-and-probe experiments. After an optically induced perturbation, we identify the gain recovery process as a forced steady-state situation which can be consistently described within rate-equation based laser theory. The model is systematically applied to analyze the experimental amplification and the overall SOA dynamics as a function of injected current. We conclude that, under conditions of high optical pump power close to the device saturation regime, the ultrafast SOA dynamics is governed by the overall injection current. The carrier relaxation pathway of a direct capture from the 2-D reservoir to the QD ground state is needed to explain the observed pulse train amplification.
Abstract: We have fabricated an array of closely spaced quantum dashes starting from a planar array of self-assembled semiconductor quantum wires. The array is embedded in a metallic nanogap which we investigate by micro-photoluminescence as a function of a lateral electric field. We demonstrate that the net electric charge and emission energy of individual quantum dashes can be modified externally with the performance limited by the size inhomogeneity of the self-assembling process.
Abstract: We have fabricated an array of closely spaced quantum dashes starting from a planar array of self-assembled semiconductor quantum wires. The array is embedded in a metallic nanogap which we investigate by micro-photoluminescence as a function of a lateral electric field. We demonstrate that the net electric charge and emission energy of individual quantum dashes can be modified externally with the performance limited by the size inhomogeneity of the self-assembling process.
Abstract: We present a fabrication method to produce site-controlled and regularly spaced InAs/GaAs quantum dots for applications in quantum optical information devices. The high selectivity of our epitaxial regrowth procedure can be used to allocate the quantum dots only in positions predefined by ex-situ local oxidation atomic force nanolithography. The quantum dots obtained following this fabrication process present a high optical quality which we have evaluated by microphotoluminescence and photon correlation experiments.
Abstract: Here we report on a new route to synthesize colloidal silver and gold nanoparticles, potentially scalable for massive nanoparticle-production. This method is based on the microwave-assisted heterogeneous reduction of the metal salts with polyvinylalcohol. The reaction is carried out in alcohols, which are non-solvents for polyvinylalcohol. Nanoparticles can be very easily separated by filtration. The reaction kinetics are extremely fast. Size-controlled formation of nanoparticle agglomerates is accomplished with a seed-mediated synthesis of nanoparticles upon MW exposure.
Abstract: We report simultaneous oscillation in continuous wave at 1062 and 1337 nm in a Nd3+:YAl3(BO3)(4) non-linear crystal associated to the infrared laser channels F-4(3/2) -> 41, 112 and F-4(3/2) -> (4I)(13/2) of Nd3+. Generation ofyellow laser light at 592 nm produced by Type I self-sum-frequency-mixing of both fundamental infrared laser waves is observed under non-optimal phase matching conditions. (C) 2008 Elsevier B.V. All rights reserved
Abstract: Here we report on the in situ synthesis of Ag and Au nanoparticles inside several polymer matrixes by solid-state chemical reduction of a metallic salt. Poly(ethyleneimine) (PEI), poly(hydroxyethyl methacrylate) (PHEMA), poly(vinylpyrrolidone) (PVP), novolak, poly(4-vinylphenol) (P4VP), poly(4-vinylphenol)-co-(methyl methacrylate) (P4VP-co-MMA) and poly(styrene-co-allyl alcohol) (PS-co-AA) were able to reduce Ag(I) and Au(III) to the corresponding nanoparticles during the baking process. The nanoparticle diameters of Ag and Au were found to range from 2 to 25 nm. TEM also indicated a uniform distribution of nanoparticles embedded in the thin film. This approach is suitable for controlling the size of the nanoparticles and its homogeneous distribution in the polymer matrix.
Abstract: We discuss the impact of a 2-D-charged carrier reservoir for high-speed optical amplification and modulated lasing in quantum dot (QD)-based devices by testing the amplification of short trains of high power, femtosecond optical pulses in an In-GaAs QD-in-a-well-based semiconductor optical amplifier (SOA). We adapt a laser-like rate equation model to describe heterodyne pump-and-probe experiments. After an optically induced perturbation, we identify the gain recovery process as a forced steady-state situation which can be consistently described within rate-equation based laser theory. The model is systematically applied to analyze the experimental amplification and the overall SOA dynamics as a function of injected current. We conclude that, under conditions of high optical pump power close to the device saturation regime, the ultrafast SOA dynamics is governed by the overall injection current. The carrier relaxation pathway of a direct capture from the 2-D reservoir to the QD ground state is needed to explain the observed pulse train amplification.
Abstract: In this work we demonstrate the use of nanocomposite thin films of poly(vinyl alcohol) with embedded silver NPs for chemosensing purposes. Silver NPs are in situ synthesized inside polyvinyl alcohol during the bake step of the formation of a nanocomposite thin film. The polymer in the nanocomposite provides an appropriate chemical and electromagnetic environment for metal NPs in order to interact with and hence detect the chemical species. A limit of detection below 20 nM is found when detecting 2-mercaptoethanol as the analyte, when measuring spectral changes (peak wavelength, linewidth and intensity) in the Localized Surface Plasmon Resonance. Potential qualitative and semi-quantitative sensors based on such nanocomposites would be easy-to-prepare, easy-to-use and low-cost, which are the basis of a fully disposable sensing platform technology.
Abstract: A bioinspired bottom-up process was developed to produce self-assembled nanocomposites of cellulose synthesized by Acetobacter bacteria and native starch. This process takes advantage of the way some bacteria extrude cellulose nanofibres and of the transport process that occurs during the gelatinization of starch. Potato and corn starch were added into the culture medium and partially gelatinized in order to allow the cellulose nanofibrils to grow in the presence of a starch phase. The bacterial cellulose (BC)-starch gels were hot pressed into sheets that had a BC volume fraction higher than 90%. During this step starch was forced to further penetrate the BC network. The self-assembled BC-starch nanocomposites showed a coherent morphology that was assessed by Atomic Force Microscopy (AFM) and Environmental Scanning Electron Microscopy (ESEM). The nanocomposites structure was studied using X-ray diffraction and ATR-FTIR spectroscopy. The degree of crystallinity of the final nanocomposites was used to estimate the volume fraction of BC. The aim of this paper is to explore a new methodology that could be used to produce nanomaterials by introducing a different phase into a cellulose nanofibre network during its assembly. (C) 2008 Elsevier B.V. All rights reserved.
Abstract: In this work we demonstrate the use of nanocomposite thin films of poly(vinyl alcohol) with embedded silver NPs for chemosensing purposes. Silver NPs are in situ synthesized inside polyvinyl alcohol during the bake step of the formation of a nanocomposite thin film. The polymer in the nanocomposite provides an appropriate chemical and electromagnetic environment for metal NPs in order to interact with and hence detect the chemical species. A limit of detection below 20 nM is found when detecting 2-mercaptoethanol as the analyte, when measuring spectral changes (peak wavelength, linewidth and intensity) in the Localized Surface Plasmon Resonance. Potential qualitative and semi-quantitative sensors based on such nanocomposites would be easy-to-prepare, easy-to-use and low-cost, which are the basis of a fully disposable sensing platform technology.
Abstract: Here we report on a new route to synthesize colloidal silver and gold nanoparticles, potentially scalable for massive nanoparticle-production. This method is based on the microwave-assisted heterogeneous reduction of the metal salts with polyvinylalcohol. The reaction is carried out in alcohols, which are non-solvents for polyvinylalcohol. Nanoparticles can be very easily separated by filtration. The reaction kinetics are extremely fast. Size-controlled formation of nanoparticle agglomerates is accomplished with a seed-mediated synthesis of nanoparticles upon MW exposure.
Abstract: Polymer nanocomposites containing noble metal nanoparticles are promising materials for plasmonic applications. In this paper, we report on a high-resolution negative-tone nanocomposite resist based on poly(vinyl alcohol) where silver nanoparticles and nanopatterns are simultaneously generated by electron-beam lithography. Our results indicate nanostructures with a relatively high concentration of nanoparticles and, consequently, an electromagnetic coupling among the nanoparticles. Therefore, the patternable nanocomposite described in this work may be a suitable material for future plasmonic circuitry.
Abstract: A near-field scanning optical microscope (NSOM) is used to perform structural and optical characterization of the surface layer after Zn diffusion in a channel waveguide fabricated on lithium niobate. A theoretical approach has been developed in order to extract refractive index contrast from NSOM optical transmission measurements (illumination configuration). As a result, different solid phases present on the sample surface can be identified, such as ZnO and ZnNb(2)O(6). They appear like submicrometric crystallites aligned along the domain wall direction, whose origin can be ascribed to some strain relaxation mechanism during the annealing process after Zn diffusion. (C) 2008 American Institute of Physics. [DOI: 10.1063/1.3000464]
Abstract: In this paper we describe the results of temperature dependent photoluminescence intensity and decay time measurements of In(Ga)As/GaAs quantum rings where the depth of barrier is varied from sample to sample. The activation energy found for the reduction of the exciton decay time as a function of the temperature is approximately half the value of the thermionic escape energy of excitons. The temperature dependant behaviour is ascribed to the carriers lost via the excited state to the WL. The time resolved PL study indicates that thermal escape mechanisms is not so affected by reducing the spacer thickness, but it's influenced essentially by the excited state recombination. (c) 2007 Elsevier B.V. All rights reserved.
Abstract: We study the metal-insulator transition in individual self-assembled quantum wires and report optical evidence of metallic liquid condensation at low temperatures. First, we observe that the temperature and power dependence of the single nanowire photoluminescence follow the evolution expected for an electron-hole liquid in one dimension. Second, we find novel spectral features that suggest that in this situation the expanding liquid condensate compresses the exciton gas in real space. Finally, we estimate the critical density and critical temperature of the phase transition diagram at n(c) similar to 1 x 10(5) cm(-1) and T-c similar to 35 K, respectively.
Abstract: Acetobacter sp. growing medium was modified in order to produce bacterial cellulose (BC) nanocomposites using a bottom-up technique that allowed starch to be introduced into the cellulose network. The BC-starch mats were hot-pressed to obtain nanocomposites sheets. Morphological characterisation was carried out using Atomic Force Microscopy and Environmental Scanning Electron Microscopy. The images obtained from microscopy were then processed using image analysis. Network properties, such as mesh size and fibre orientation were characterised. Fracture surfaces of these new nanocomposites were analysed.
Abstract: Photoluminescence and excitation of photoluminescence spectroscopy have been performed for two kinds of single InAs self-assembled quantum dots grown on GaAs. The presence of unintentional impurities (donors and acceptors) offers the possibility to switch from negative to positively charged excitons by selectively exciting impurity related optical transitions. (C) 2007 Elsevier Ltd. All rights reserved.
Abstract: We have investigated the selective optical pumping of charged excitonic species in a sample containing quantum dots of different sizes and low areal density by photoluminescence and excitation of the photoluminescence microspectroscopy. We study the selective optical excitation of negatively charged excitons as an alternative to commonly used electrical methods. We demonstrate that under resonant excitation in impurity related bands, the selective pumping efficiency can be as high as 85% in small quantum dots having one electron shell and emitting at around 930 nm, and around 65% in big quantum dots having four electron shells and emitting at 1160 nm.
Abstract: We demonstrated the effect of reactive ion beam etching (RIBE) process on the PL properties of CdTe/sapphire metal organic vapor phase epitaxy layers. At optimum conditions, the RIBE attack does not make significant morphological changes but it results in an increase of the concentration of acceptor impurities. This was revealed by an increase of the overall photoluminescence (PL) intensity and, simultaneously, a decrease of the PL decay time, more important on the low energy side of PL spectrum due to the recombination of carriers in acceptor pairs. (C) 2008 American Institute of Physics.
Abstract: Continuous wave- and time-resolved micro-photoluminescence spectroscopy has been performed on single InAs self-assembled quantum dots grown on GaAs. The presence of residual impurities (donors and acceptors) in samples with low dot density opens the possibility to switch from trion to neutral exciton states inside quantum dots by selective optical pumping. We propose a microstate model to describe the recombination dynamics of all the excitonic especies (neutral exciton, positive/negative trion and biexciton) under the considered optical pumping conditions when increasing the excitation power. (C) 2007 Elsevier B.V. All rights reserved.
Abstract: Metal nanoparticles can be prepared by a novel technique that consists of the laser ablation of a solid target immersed in a water solution of a metal salt. Silicon was chosen as the most adequate target to synthesize silver and gold nanoparticles from a water solution of either AgNO3 or HAuCl4. The influence of both the silver nitrate concentrations and the irradiation time of the Si target on the optical properties of the Au and Ag nanoparticles have been investigated. The crystalline nature of the metal nanoparticles has been determined by X-ray diffraction (XRD). Average size and particle size distribution have been measured by means of TEM. The absorbance spectra show the characteristic band of the surface resonant plasmon of silver and gold nanoparticles. (C) 2007 Elsevier Ltd. All rights reserved.
Abstract: In the presence of a stationary electric field applied in the growth direction tunneling of electrons out of the quantum dots can take place. This mechanism competes with the quantum confined Stark effect (QCSE) that produces an increase of the exciton lifetime by increasing the electric field, mainly due to a net decrease of the electron-hole wavefunction overlap. The electric field range where QCSE dominates over tunneling will be mainly determined by the size of the nanostructure along the vertical direction (height), as demonstrated in this work. (C) 2007 Elsevier Ltd. All rights reserved.
Abstract: Acetobacter sp. growing medium was modified in order to produce bacterial cellulose (BC) nanocomposites using a bottom-up technique that allowed starch to be introduced into the cellulose network. The BC-starch mats were hot-pressed to obtain nanocomposites sheets. Morphological characterisation was carried out using Atomic Force Microscopy and Environmental Scanning Electron Microscopy. The images obtained from microscopy were then processed using image analysis. Network properties, such as mesh size and fibre orientation were characterised. Fracture surfaces of these new nanocomposites were analysed.
Abstract: Metal nanoparticles can be prepared by a novel technique that consists of the laser ablation of a solid target immersed in a water solution of a metal salt. Silicon was chosen as the most adequate target to synthesize silver and gold nanoparticles from a water solution of either AgNO3 or HAuCl4. The influence of both the silver nitrate concentrations and the irradiation time of the Si target on the optical properties of the Au and Ag nanoparticles have been investigated. The crystalline nature of the metal nanoparticles has been determined by X-ray diffraction (XRD). Average size and particle size distribution have been measured by means of TEM. The absorbance spectra show the characteristic band of the surface resonant plasmon of silver and gold nanoparticles. (C) 2007 Elsevier Ltd. All rights reserved.
Notes: 7th International Conference on Physics of Ligh-Matter Coupling in Nanostructures (PLMCN7), Havana, CUBA, APR 12-17, 2007
Abstract: In this paper we describe the results of temperature dependent photoluminescence intensity and decay time measurements of In(Ga)As/GaAs quantum rings where the depth of barrier is varied from sample to sample. The activation energy found for the reduction of the exciton decay time as a function of the temperature is approximately half the value of the thermionic escape energy of excitons. The temperature dependant behaviour is ascribed to the carriers lost via the excited state to the WL. The time resolved PL study indicates that thermal escape mechanisms is not so affected by reducing the spacer thickness, but it’s influenced essentially by the excited state recombination. (c) 2007 Elsevier B.V. All rights reserved.
Notes: 5th Maghreb/Europe Meeting on Materials and Their Applications for Devices and Physical, Chemical and Biological Sensors (MADICA 2006), Mahdia, TUNISIA, OCT 30-NOV 01, 2006
Abstract: We study the metal-insulator transition in individual self-assembled quantum wires and report optical evidence of metallic liquid condensation at low temperatures. First, we observe that the temperature and power dependence of the single nanowire photoluminescence follow the evolution expected for an electron-hole liquid in one dimension. Second, we find novel spectral features that suggest that in this situation the expanding liquid condensate compresses the exciton gas in real space. Finally, we estimate the critical density and critical temperature of the phase transition diagram at n(c) similar to 1 x 10(5) cm(-1) and T(c) similar to 35 K, respectively.
Abstract: Polymer nanocomposites containing noble metal nanoparticles are promising materials for plasmonic applications. In this paper, we report on a high-resolution negative-tone nanocomposite resist based on poly(vinyl alcohol) where silver nanoparticles and nanopatterns are simultaneously generated by electron-beam lithography. Our results indicate nanostructures with a relatively high concentration of nanoparticles and, consequently, an electromagnetic coupling among the nanoparticles. Therefore, the patternable nanocomposite described in this work may be a suitable material for future plasmonic circuitry.
Abstract: Photoluminescence and excitation of photoluminescence spectroscopy have been performed for two kinds of single InAs self-assembled quantum dots grown on GaAs. The presence of unintentional impurities (donors and acceptors) offers the possibility to switch from negative to positively charged excitons by selectively exciting impurity related optical transitions. (C) 2007 Elsevier Ltd. All rights reserved.
Notes: 7th International Conference on Physics of Ligh-Matter Coupling in Nanostructures (PLMCN7), Havana, CUBA, APR 12-17, 2007
Abstract: In the presence of a stationary electric field applied in the growth direction tunneling of electrons out of the quantum dots can take place. This mechanism competes with the quantum confined Stark effect (QCSE) that produces an increase of the exciton lifetime by increasing the electric field, mainly due to a net decrease of the electron-hole wavefunction overlap. The electric field range where QCSE dominates over tunneling will be mainly determined by the size of the nanostructure along the vertical direction (height), as demonstrated in this work. (C) 2007 Elsevier Ltd. All rights reserved.
Notes: 7th International Conference on Physics of Ligh-Matter Coupling in Nanostructures (PLMCN7), Havana, CUBA, APR 12-17, 2007
Abstract: We demonstrated the effect of reactive ion beam etching (RIBE) process on the PL properties of CdTe/sapphire metal organic vapor phase epitaxy layers. At optimum conditions, the RIBE attack does not make significant morphological changes but it results in an increase of the concentration of acceptor impurities. This was revealed by an increase of the overall photoluminescence (PL) intensity and, simultaneously, a decrease of the PL decay time, more important on the low energy side of PL spectrum due to the recombination of carriers in acceptor pairs. (C) 2008 American Institute of Physics.
Abstract: Continuous wave- and time-resolved micro-photoluminescence spectroscopy has been performed on single InAs self-assembled quantum dots grown on GaAs. The presence of residual impurities (donors and acceptors) in samples with low dot density opens the possibility to switch from trion to neutral exciton states inside quantum dots by selective optical pumping. We propose a microstate model to describe the recombination dynamics of all the excitonic especies (neutral exciton, positive/negative trion and biexciton) under the considered optical pumping conditions when increasing the excitation power. (C) 2007 Elsevier B.V. All rights reserved.
Notes: 13th International Conference on Modulated Semiconductor Structures (MSS13)/17th International Conference on Electronic Properties of 2-Dimensional Systems, Genova, ITALY, JUL 15-20, 2007
Abstract: We have investigated the selective optical pumping of charged excitonic species in a sample containing quantum dots of different sizes and low areal density by photoluminescence and excitation of the photoluminescence microspectroscopy. We study the selective optical excitation of negatively charged excitons as an alternative to commonly used electrical methods. We demonstrate that under resonant excitation in impurity related bands, the selective pumping efficiency can be as high as 85% in small quantum dots having one electron shell and emitting at around 930 nm, and around 65% in big quantum dots having four electron shells and emitting at 1160 nm.
Abstract: A near-field scanning optical microscope (NSOM) is used to perform structural and optical characterization of the surface layer after Zn diffusion in a channel waveguide fabricated on lithium niobate. A theoretical approach has been developed in order to extract refractive index contrast from NSOM optical transmission measurements (illumination configuration). As a result, different solid phases present on the sample surface can be identified, such as ZnO and ZnNb2O6. They appear like submicrometric crystallites aligned along the domain wall direction, whose origin can be ascribed to some strain relaxation mechanism during the annealing process after Zn diffusion. (C) 2008 American Institute of Physics. [DOI: 10.1063/1.3000464]
Abstract: In this work we explore the first stages of quantum wires (QWRs) formation studying the evolution of the growth front for InAs coverage below the critical thickness, theta(c), determined by reflection high-energy electron diffraction. Our results show that at a certain InAs coverage theta(InAs)<theta(c), the elastic relaxation process starts by spontaneous formation of isolated QWRs that evolves towards QWRs covering the whole surface with increasing InAs thickness. These results allow for a better understanding of the self-assembling process of QWRs in the InAs/InP system and for the first time enable the study of the novel properties of single self-assembled QWR. (c) 2006 Elsevier B.V. All rights reserved.
Notes: 14th International Conference on Molecular Beam Epitaxy (MBE XIV), Waseda Univ, Tokyo, JAPAN, SEP 03-08, 2006
Abstract: Scanning near-field optical microscopy is capable of measuring the topography and optical signals at the same time. This fact makes this technique a valuable tool in the study of materials at nanometric scale and, in particular, of ferroelectric materials, as it permits the study of their domains structure without the need of chemical etching and, therefore, not damaging the surface (as will be demonstrated later). We have measured the scanning near-field optical microscopy transmission, as well as the topography, of an RbTiOPO4 single crystalline slab, which exhibits two different of macroscopic ferroelectric domains. A chemical selective etching has been performed to distinguish between them, obtaining areas with a noticeable roughness (C- domain) in comparison with the original flat aspect of the other ones (C+ domain). The effects of the selective chemical etching have been quantified in topographic images obtained by means of our fibre tip probe, and have been compared to topographic images obtained by Atomic Force Microscopy, with a better resolution. The near-field optical transmission images recorded have been obtained under different excitation wavelengths. These images are modulated by the light scattering due to the grains at the rough surface, which depends on the excitation wavelength used. In addition, they show a significant optical contrast due to the variations of the dielectric constant on the proximity of the ferroelectric domain wall.
Abstract: We present a study of the structural and optical properties of InAs quantum dots formed in a low density template of nanoholes fabricated by droplet epitaxy on GaAs (001). The growth conditions used here promote the formation of isolated quantum dots only inside the templated nanoholes. Due to the good optical quality and low density of these nanostructures, their ensemble and individual emission properties could be investigated and related to the particular growth method employed and the quantum dot morphology. (C) American Institute of Physics.
Abstract: We report generation of continuous-wave yellow laser radiation in a Nd3+ -doped periodically poled lithium niobate bulk crystal. Simultaneous oscillation in pi polarization of the two infrared laser channels F-4(3/2) -> I-4(11/2) and F-4(3/2) -> I-4(13/2) Of Nd3+ is achieved at 1084.4 and 1373.6 nm by end pumping with a Ti: Sapphire laser. Intracavity quasiphase-matched self-sumfrequency-mixing of both infrared laser waves in the poled crystal leads to the generation of yellow laser light at 606 nm. The yellow laser power obtained is 15.6 mW from an absorbed pump power of 314 mW.
Abstract: In this work we explore the first stages of quantum wire ( QWR) formation studying the evolution of the growth front for InAs coverages below the critical thickness, theta(c), determined by reflection high energy electron diffraction (RHEED). Our results obtained by in situ measurement of the accumulated stress evolution during InAs growth on InP(001) show that the relaxation process starts at a certain InAs coverage theta(R) < theta(c). At this theta(R), the spontaneous formation of isolated quantum wires takes place. For theta > theta(R) this ensemble of isolated nanostructures progressively evolves towards QWRs that cover the whole surface for theta = theta(c). These results allow for a better understanding of the self-assembling process of QWRs and enable the study of the individual properties of InAs/InP self-assembled single quantum wires.
Abstract: We have carried out continuous wave and time resolved photoluminescence experiments in self-assembled In(Ga)As quantum dots and quantum rings embedded in field effect structure devices. In both kinds of nanostructures, we find a noticeable increase of the exciton radiative lifetime with the external voltage bias that must be attributed to the field-induced polarizability of the confined electron hole pair. The interplay between the exciton radiative recombination and the electronic carrier tunneling in the presence of a stationary electric field is therefore investigated and compared with a numerical calculation based on the effective mass approximation.
Abstract: Gain recovery dynamics are studied in electrically pumped quantum dot (QD) based semiconductor optical amplifiers (SOAs) after amplification of double femtosecond laser pulses using ultrafast pump-probe spectroscopy with heterodyne detection. The authors observe a distinct change in gain recovery in the ground state when a significant excited state population is achieved. A complete gain recovery is found when two 150 fs pulses with 5 ps time delay pass through the SOA in resonance to the ground state under high injection currents of 80-100 mA. The obtained results open the way for ultrafast (> 200 GHz) operation in p-doped QD based SOAs at 1.3 mu m telecommunications wavelengths. (c) 2007 American Institute of Physics.
Abstract: This paper investigates the electronic structure of self-assembled InAs quantum wires (QWrs), grown under different conditions by molecular beam epitaxy on InP, by means of photoluminescence measurements under pressure. In samples with regularly distributed QWrs, room pressure photoluminescence spectra consist of a broad band centred at about 0.85 eV, which can be easily de-convoluted in a few Gaussian peaks. In samples with isolated QWrs, photo luminescence spectra exhibit up to four clearly resolved bands. Applying hydrostatic pressure, the whole emission band monotonously shifts towards higher photon energies with pressure coefficients ranging from 72 to 98 meV/GPa. In contrast to InAs quantum dots on GaAs, quantum wires photoluminescence is observed up to 10 GPa, indicating that InAs QWrs are metastable well above pressure at which bulk InAs undergoes a phase transition to the rock-salt phase (7 GPa). (C) 2007 WILEY-VCH Vertag GmbH & Co. KGaA, Weinheim.
Notes: 12th International Conference on High Pressure Semiconductor Physics (HPSP-12), Barcelona, SPAIN, JUL 31-AUG 03, 2006
Abstract: We study the gain dynamics in QD-based SOAs after excitation with fs-pulse trains of up to THz repetition rates. A complete ground-state gain recovery is found for 200GHz repetition rates and injection currents around 90mA. (C)2007 Optical Society of America
Abstract: We have carried out continuous wave and time resolved photoluminescence experiments in self-assembled In(Ga)As quantum dots and quantum rings embedded in field effect structure devices. In both kinds of nanostructures, we find a noticeable increase of the exciton radiative lifetime with the external voltage bias that must be attributed to the field-induced polarizability of the confined electron hole pair. The interplay between the exciton radiative recombination and the electronic carrier tunneling in the presence of a stationary electric field is therefore investigated and compared with a numerical calculation based on the effective mass approximation.
Abstract: In this work we explore the first stages of quantum wire ( QWR) formation studying the evolution of the growth front for InAs coverages below the critical thickness, theta(c), determined by reflection high energy electron diffraction (RHEED). Our results obtained by in situ measurement of the accumulated stress evolution during InAs growth on InP(001) show that the relaxation process starts at a certain InAs coverage theta(R) < theta(c). At this theta(R), the spontaneous formation of isolated quantum wires takes place. For theta > theta(R) this ensemble of isolated nanostructures progressively evolves towards QWRs that cover the whole surface for theta = theta(c). These results allow for a better understanding of the self-assembling process of QWRs and enable the study of the individual properties of InAs/InP self-assembled single quantum wires.
Abstract: Scanning near-field optical microscopy is capable of measuring the topography and optical signals at the same time. This fact makes this technique a valuable tool in the study of materials at nanometric scale and, in particular, of ferroelectric materials, as it permits the study of their domains structure without the need of chemical etching and, therefore, not damaging the surface (as will be demonstrated later). We have measured the scanning near-field optical microscopy transmission, as well as the topography, of an RbTiOPO4 single crystalline slab, which exhibits two different of macroscopic ferroelectric domains. A chemical selective etching has been performed to distinguish between them, obtaining areas with a noticeable roughness (C- domain) in comparison with the original flat aspect of the other ones (C+ domain). The effects of the selective chemical etching have been quantified in topographic images obtained by means of our fibre tip probe, and have been compared to topographic images obtained by Atomic Force Microscopy, with a better resolution. The near-field optical transmission images recorded have been obtained under different excitation wavelengths. These images are modulated by the light scattering due to the grains at the rough surface, which depends on the excitation wavelength used. In addition, they show a significant optical contrast due to the variations of the dielectric constant on the proximity of the ferroelectric domain wall.
Abstract: In this work we explore the first stages of quantum wires (QWRs) formation studying the evolution of the growth front for InAs coverage below the critical thickness, theta(c), determined by reflection high-energy electron diffraction. Our results show that at a certain InAs coverage theta(InAs)<theta(c), the elastic relaxation process starts by spontaneous formation of isolated QWRs that evolves towards QWRs covering the whole surface with increasing InAs thickness. These results allow for a better understanding of the self-assembling process of QWRs in the InAs/InP system and for the first time enable the study of the novel properties of single self-assembled QWR. (c) 2006 Elsevier B.V. All rights reserved.
Abstract: We present a study of the structural and optical properties of InAs quantum dots formed in a low density template of nanoholes fabricated by droplet epitaxy on GaAs (001). The growth conditions used here promote the formation of isolated quantum dots only inside the templated nanoholes. Due to the good optical quality and low density of these nanostructures, their ensemble and individual emission properties could be investigated and related to the particular growth method employed and the quantum dot morphology. (C) American Institute of Physics.
Abstract: We report generation of continuous-wave yellow laser radiation in a Nd3+ -doped periodically poled lithium niobate bulk crystal. Simultaneous oscillation in pi polarization of the two infrared laser channels F-4(3/2) -> I-4(11/2) and F-4(3/2) -> I-4(13/2) Of Nd3+ is achieved at 1084.4 and 1373.6 nm by end pumping with a Ti: Sapphire laser. Intracavity quasiphase-matched self-sumfrequency-mixing of both infrared laser waves in the poled crystal leads to the generation of yellow laser light at 606 nm. The yellow laser power obtained is 15.6 mW from an absorbed pump power of 314 mW.
Abstract: This paper investigates the electronic structure of self-assembled InAs quantum wires (QWrs), grown under different conditions by molecular beam epitaxy on InP, by means of photoluminescence measurements under pressure. In samples with regularly distributed QWrs, room pressure photoluminescence spectra consist of a broad band centred at about 0.85 eV, which can be easily de-convoluted in a few Gaussian peaks. In samples with isolated QWrs, photo luminescence spectra exhibit up to four clearly resolved bands. Applying hydrostatic pressure, the whole emission band monotonously shifts towards higher photon energies with pressure coefficients ranging from 72 to 98 meV/GPa. In contrast to InAs quantum dots on GaAs, quantum wires photoluminescence is observed up to 10 GPa, indicating that InAs QWrs are metastable well above pressure at which bulk InAs undergoes a phase transition to the rock-salt phase (7 GPa). (C) 2007 WILEY-VCH Vertag GmbH & Co. KGaA, Weinheim.
Abstract: Gain recovery dynamics are studied in electrically pumped quantum dot (QD) based semiconductor optical amplifiers (SOAs) after amplification of double femtosecond laser pulses using ultrafast pump-probe spectroscopy with heterodyne detection. The authors observe a distinct change in gain recovery in the ground state when a significant excited state population is achieved. A complete gain recovery is found when two 150 fs pulses with 5 ps time delay pass through the SOA in resonance to the ground state under high injection currents of 80-100 mA. The obtained results open the way for ultrafast (> 200 GHz) operation in p-doped QD based SOAs at 1.3 mu m telecommunications wavelengths. (c) 2007 American Institute of Physics.
Abstract: We present a study of the primary optical transitions and recombination dynamics in InGaAs self-assembled quantum nanostructures with different shape. Starting from the same quantum dot seeding layer, and depending on the overgrowth conditions, these new nanostructures can be tailored in shape and are characterized by heights lower than 2 nm and base lengths around 100 nm. The geometrical shape strongly influences the electronic and optical properties of these nanostructuctures. We measure for them ground state optical transitions in the range 1.25-1.35 eV and varying energy splitting between their excited states. The temperature dependence of the exciton recombination dynamics is reported focusing on the intermediate temperature regime (before thermal escape begins to be important). In this range, an important increase of the effective photoluminescence decay time is observed and attributed to the state filling and exciton thermalization between excited and ground states. A rate equation model is also developed reproducing quite well the observed exciton dynamics.
Abstract: Exciton recombination dynamics in vertical stacks of InGaAs quantum rings have been studied by means of continuous wave and time resolved photoluminescence under low excitation density conditions. We have paid special attention to the effect of the carrier coupling on the exciton radiative lifetime: weak (14 nm spacer sample), intermediate (4.5 nm spacer sample), where the size filtering effects (towards small rings) compensate partially that arising from carrier coupling (towards lower energies), and strong electron and hole coupling (1.5 nm spacer sample) between layers. Experimental decay times in the latter two cases have been compared to the times simulated with a multi-quantum well based model, which accounts for the observed change of carrier coupling regime. The most important effect is observed when the hole wave function overlap along the growth direction becomes important (1.5 nm spacer sample). This situation makes important the lateral tunneling of excitons between rings, given their large lateral size, which is characterized by times around 5 ns at the emission peak energy (rings with the most probable size of the distribution).
Abstract: Isolated InAs/InP self-assembled quantum wires have been grown using in situ accumulated stress measurements to adjust the optimal InAs thickness. Atomic force microscopy imaging shows highly asymmetric nanostructures with average length exceeding more than ten times their width. High resolution optical investigation of as-grown samples reveals strong photoluminescence from individual quantum wires at 1.5 mu m. Additional sharp features are related to monolayer fluctuations of the two-dimensional InAs layer present during the early stages of the quantum wire self-assembling process. (c) 2006 American Institute of Physics.
Abstract: We present a study of the primary optical transitions and recombination dynamics in InGaAs self-assembled quantum nanostructures with different shape. Starting from the same quantum dot seeding layer, and depending on the overgrowth conditions, these new nanostructures can be tailored in shape and are characterized by heights lower than 2 nm and base lengths around 100 nm. The geometrical shape strongly influences the electronic and optical properties of these nanostructuctures. We measure for them ground state optical transitions in the range 1.25-1.35 eV and varying energy splitting between their excited states. The temperature dependence of the exciton recombination dynamics is reported focusing on the intermediate temperature regime (before thermal escape begins to be important). In this range, an important increase of the effective photoluminescence decay time is observed and attributed to the state filling and exciton thermalization between excited and ground states. A rate equation model is also developed reproducing quite well the observed exciton dynamics.
Abstract: In this work, an imaging tool to investigate optical inhomogeneities with site and chemical sensitivities has been integrated in a hard x-ray microprobe. Freestanding GaN and epitaxially grown GaN:Mn on alpha-Al2O3 are used to exploit the unprecedented scanning x-ray excited luminescence technique. Optical images of the radiative recombination channels are reported for several impurities and defect centers in sapphire and GaN compounds. Within the experimental accuracy, a visible nonuniformity characterizes the Mn centers in good correlation with former x-ray fluorescence map. Expanding the microprobe versatility, x-ray absorption spectroscopy in both photon collection modes (x-ray excited luminescence and x-ray fluorescence) is finally presented from a freestanding GaN layer.
Abstract: Isolated InAs/InP self-assembled quantum wires have been grown using in situ accumulated stress measurements to adjust the optimal InAs thickness. Atomic force microscopy imaging shows highly asymmetric nanostructures with average length exceeding more than ten times their width. High resolution optical investigation of as-grown samples reveals strong photoluminescence from individual quantum wires at 1.5 mu m. Additional sharp features are related to monolayer fluctuations of the two-dimensional InAs layer present during the early stages of the quantum wire self-assembling process. (c) 2006 American Institute of Physics.
Abstract: In the present work we study the influence of stacking self-assembled InAs quantum wires (QWRs) on the emission wavelength and the excitonic recombination dynamics. The reduction in the InP spacer layer thickness, d(InP), produces both a size filtering effect towards large wire ensembles and an increase in the vertical coupling for electrons and holes along the stack direction. The different vertical coupling for electrons and holes induces a different behaviour in the exciton recombination dynamics, depending on the InP spacer layer thickness: weak electron coupling and negligible hole coupling for d(InP) > 10 nm, intermediate electron coupling and weak hole coupling for 5 nm <= d(InP) <= 10 nm and strong electron coupling and moderate hole coupling for d(InP) < 5 nm. Such exciton dynamics have been established by comparing the experimental time decay results with a multi-quantum well model accounting for the vertical carrier coupling.
Abstract: We have developed a Scanning Near-Field Optic Microscope (SNOM), which is based on a commercial tuning-fork sensor, capable of measuring the topography and the optical signal, at the same time. We have measured the SNOM transmission, as well as the topography, of an RbTiOPO4 single crystal, which exhibits two kinds of macroscopic ferroelectric domains. A chemical selective attack has been performed to distinguish them. As a consequence, we obtained zones with a noticeable roughness (C-) in comparison with the flat aspect of the other ones (C+). The SNOM topography images have been compared to Atomic Force Microscopy ones, which has a better resolution. The changes observed in the transmission measurements are due to different effects: i) variations of the dielectric constant at the interface walls between domains; ii) light scattering, wavelength dependent, at the grains in the C- face. Finally, we have determined the angles formed at the domain-walls between inverted domains, which are measured to be higher than 90 and close to 135, as could be expected, because of the combination between [110], [-110], [10 0] and [010] directions of the orthorhombic lattice.
Abstract: We present a systematic study of closely In(Ga)As/InAs quantum rings (QRs) grown by molecular beam epitaxy (MBE). Photoluminescence (PL) experiments show a strong filtering effect in the ring being stacked and simultaneous linewidth narrowing for the appropriate layer thickness (thinner thickness). If the spacer thickness is further reduced, a strong coupling between the nanostructures is produced and the signal shifts to low energy. (c) 2005 Elsevier B.V. All rights reserved.
Abstract: A commercial Atomic Force Microscope (AFM) and a semi-home made Scanning Near-Field Optical Microscope (SNOM) have been used to characterize electrically, topographically and optically the domain walls among natural ferroelectric domains in a KNbO3 crystal. The AFM measurements have been performed with a metallic coated tip in order to detect electrostatic forces between the polarization field at the ferroelectric surface and the tip. An external electric field has also been applied between the sample surface and the tip to tune this electrostatic interaction over the atomic forces. In optical transmission images, acquired under near field conditions, we observe a clear contrast of the signal at the domain walls between 180 spontaneous polarization domains; while the images of the surface topography, obtained simultaneously, show a reasonably flat surface of the crystal. The scanning probe microscopy techniques used in this work are valuable tools for the investigation of ferroelectric materials and, in particular, to characterize the domain walls, without needing a either especial preparation or damage of the sample surface.
Abstract: We present results concerning the carrier transfer between In( Ga) As quantum rings in a stacked multilayer structure, which is characterised by a bimodal size distribution. This transfer of carriers explains the observed temperature behaviour of diode lasers based on that kind of stacked layer structures. The inter-ring carrier transfer can be possible by phonon assisted tunnelling from the ground state of the small-ring family towards the big-ring family of the bimodal size distribution. This process is thermally activated in the range 40 - 80 K.
Abstract: In this work, an imaging tool to investigate optical inhomogeneities with site and chemical sensitivities has been integrated in a hard x-ray microprobe. Freestanding GaN and epitaxially grown GaN:Mn on alpha-Al2O3 are used to exploit the unprecedented scanning x-ray excited luminescence technique. Optical images of the radiative recombination channels are reported for several impurities and defect centers in sapphire and GaN compounds. Within the experimental accuracy, a visible nonuniformity characterizes the Mn centers in good correlation with former x-ray fluorescence map. Expanding the microprobe versatility, x-ray absorption spectroscopy in both photon collection modes (x-ray excited luminescence and x-ray fluorescence) is finally presented from a freestanding GaN layer.
Abstract: Exciton recombination dynamics in vertical stacks of InGaAs quantum rings have been studied by means of continuous wave and time resolved photoluminescence under low excitation density conditions. We have paid special attention to the effect of the carrier coupling on the exciton radiative lifetime: weak (14 nm spacer sample), intermediate (4.5 nm spacer sample), where the size filtering effects (towards small rings) compensate partially that arising from carrier coupling (towards lower energies), and strong electron and hole coupling (1.5 nm spacer sample) between layers. Experimental decay times in the latter two cases have been compared to the times simulated with a multi-quantum well based model, which accounts for the observed change of carrier coupling regime. The most important effect is observed when the hole wave function overlap along the growth direction becomes important (1.5 nm spacer sample). This situation makes important the lateral tunneling of excitons between rings, given their large lateral size, which is characterized by times around 5 ns at the emission peak energy (rings with the most probable size of the distribution).
Abstract: In the present work we study the influence of stacking self-assembled InAs quantum wires (QWRs) on the emission wavelength and the excitonic recombination dynamics. The reduction in the InP spacer layer thickness, d(InP), produces both a size filtering effect towards large wire ensembles and an increase in the vertical coupling for electrons and holes along the stack direction. The different vertical coupling for electrons and holes induces a different behaviour in the exciton recombination dynamics, depending on the InP spacer layer thickness: weak electron coupling and negligible hole coupling for d(InP) > 10 nm, intermediate electron coupling and weak hole coupling for 5 nm <= d(InP) <= 10 nm and strong electron coupling and moderate hole coupling for d(InP) < 5 nm. Such exciton dynamics have been established by comparing the experimental time decay results with a multi-quantum well model accounting for the vertical carrier coupling.
Abstract: We present results concerning the carrier transfer between In( Ga) As quantum rings in a stacked multilayer structure, which is characterised by a bimodal size distribution. This transfer of carriers explains the observed temperature behaviour of diode lasers based on that kind of stacked layer structures. The inter-ring carrier transfer can be possible by phonon assisted tunnelling from the ground state of the small-ring family towards the big-ring family of the bimodal size distribution. This process is thermally activated in the range 40 - 80 K.
Abstract: We have developed a Scanning Near-Field Optic Microscope (SNOM), which is based on a commercial tuning-fork sensor, capable of measuring the topography and the optical signal, at the same time. We have measured the SNOM transmission, as well as the topography, of an RbTiOPO4 single crystal, which exhibits two kinds of macroscopic ferroelectric domains. A chemical selective attack has been performed to distinguish them. As a consequence, we obtained zones with a noticeable roughness (C-) in comparison with the flat aspect of the other ones (C+). The SNOM topography images have been compared to Atomic Force Microscopy ones, which has a better resolution. The changes observed in the transmission measurements are due to different effects: i) variations of the dielectric constant at the interface walls between domains; ii) light scattering, wavelength dependent, at the grains in the C- face. Finally, we have determined the angles formed at the domain-walls between inverted domains, which are measured to be higher than 90 and close to 135, as could be expected, because of the combination between [110], [-110], [10 0] and [010] directions of the orthorhombic lattice.
Abstract: We present a systematic study of closely In(Ga)As/InAs quantum rings (QRs) grown by molecular beam epitaxy (MBE). Photoluminescence (PL) experiments show a strong filtering effect in the ring being stacked and simultaneous linewidth narrowing for the appropriate layer thickness (thinner thickness). If the spacer thickness is further reduced, a strong coupling between the nanostructures is produced and the signal shifts to low energy. (c) 2005 Elsevier B.V. All rights reserved.
Notes: 4th Maghreb/Europe Meeting on Material and Their Applications for Devices and Physical, Chemical and Biological Sensors (MADICA 2004), Tunis, TUNISIA, NOV 29-DEC 01, 2004
Abstract: A commercial Atomic Force Microscope (AFM) and a semi-home made Scanning Near-Field Optical Microscope (SNOM) have been used to characterize electrically, topographically and optically the domain walls among natural ferroelectric domains in a KNbO3 crystal. The AFM measurements have been performed with a metallic coated tip in order to detect electrostatic forces between the polarization field at the ferroelectric surface and the tip. An external electric field has also been applied between the sample surface and the tip to tune this electrostatic interaction over the atomic forces. In optical transmission images, acquired under near field conditions, we observe a clear contrast of the signal at the domain walls between 180 spontaneous polarization domains; while the images of the surface topography, obtained simultaneously, show a reasonably flat surface of the crystal. The scanning probe microscopy techniques used in this work are valuable tools for the investigation of ferroelectric materials and, in particular, to characterize the domain walls, without needing a either especial preparation or damage of the sample surface.
Abstract: Photoluminescence and excitation of the photoluminescence spectroscopy has been performed in single InGaAs self-assembled quantum rings embedded in a field effect structure device. To determine their electronic structure, bias-dependent optical transitions have been analyzed both, for individual quantum rings, and for the averaged ensemble. Our results are compared with a theoretical model, and also with results reported by other authors studying similar nanostructures.
Abstract: For a certain heteroepitaxial system, the optical properties of self-assembled nanostructures basically depend on their size. In this work, we have studied different ways to modify the height of InAs/InP quantum wires (QWrs) in order to change the photoluminescence emission wavelength. One procedure consists of changing the QWr size by varying the amount of InAs deposited. The other two methods explored rely on the control of As/P exchange process, in one case during growth of InAs on InP for QWr formation and in the other case during growth of InP on InAs for QWr capping. The combination of the three approaches provides a fine tuning of QWr emission wavelength between 1.2 and 1.9 mu m at room temperature. (c) 2005 American Institute of Physics.
Abstract: In this work we investigate the exciton recombination dynamics in InAs/InP semiconductor self-assembled quantum wires, by means of continuous wave and time resolved photoluminescence. The continuous wave photoluminescence results seem to indicate that the temperature quenching of the emission band seems to be more probably due to unipolar thermal escape of electrons towards the InP barrier. On the other hand, the analysis of time resolved photoluminescence reveals that the temperature dependence of the radiative and nonradiative recombination times is mainly determined by the dynamics of excitons localized by disorder (high energy tail of the PL band) and strongly localized (low energy tail of the PL band) in local size fluctuations of the quantum wires.
Abstract: For a certain heteroepitaxial system, the optical properties of self-assembled nanostructures basically depend on their size. In this work, we have studied different ways to modify the height of InAs/InP quantum wires (QWrs) in order to change the photoluminescence emission wavelength. One procedure consists of changing the QWr size by varying the amount of InAs deposited. The other two methods explored rely on the control of As/P exchange process, in one case during growth of InAs on InP for QWr formation and in the other case during growth of InP on InAs for QWr capping. The combination of the three approaches provides a fine tuning of QWr emission wavelength between 1.2 and 1.9 mu m at room temperature. (c) 2005 American Institute of Physics.
Abstract: In this work we investigate the exciton recombination dynamics in InAs/InP semiconductor self-assembled quantum wires, by means of continuous wave and time resolved photoluminescence. The continuous wave photoluminescence results seem to indicate that the temperature quenching of the emission band seems to be more probably due to unipolar thermal escape of electrons towards the InP barrier. On the other hand, the analysis of time resolved photoluminescence reveals that the temperature dependence of the radiative and nonradiative recombination times is mainly determined by the dynamics of excitons localized by disorder (high energy tail of the PL band) and strongly localized (low energy tail of the PL band) in local size fluctuations of the quantum wires.
Abstract: Photoluminescence and excitation of the photoluminescence spectroscopy has been performed in single InGaAs self-assembled quantum rings embedded in a field effect structure device. To determine their electronic structure, bias-dependent optical transitions have been analyzed both, for individual quantum rings, and for the averaged ensemble. Our results are compared with a theoretical model, and also with results reported by other authors studying similar nanostructures.
Abstract: The size and emission wavelength of self-assembled InAs/InP(001) quantum wires (QWrs) is affected by the P/As exchange process. In this work, we demonstrate by in situ stress measurements that P/As exchange at the InAs/InP interface depends on the surface reconstruction of the InAs starting surface and its immediate evolution when the arsenic cell is closed. Accordingly, the amount of InP grown on InAs by P/As exchange increases with substrate temperature in a steplike way. These results allow us to engineer the size of the QWr for emission at 1.3 and 1.55 mum at room temperature by selecting the range of substrate temperatures in which the InP cap layer is grown. (C) 2004 American Institute of Physics.
Abstract: Excitonic transitions of single InAs self-assembled quantum dots were directly measured at 4.2 K in an optical transmission experiment. We use the Stark effect in order to tune the exciton energy of a single quantum dot into resonance with a narrow-band laser. With this method, sharp resonances in the transmission spectra are observed. The oscillator strengths as well as the homogeneous line widths of the single-dot optical transitions are obtained. A clear saturation in the absorption is observed at modest laser powers. (C) 2003 Published by Elsevier B.V.
Abstract: Excitonic transitions of single InAs self-assembled quantum dots were directly measured at 4.2 K in an optical transmission experiment. We use the Stark effect in order to tune the exciton energy of a single quantum dot into resonance with a narrow-band laser. With this method, sharp resonances in the transmission spectra are observed. The oscillator strengths as well as the homogeneous line widths of the single-dot optical transitions are obtained. A clear saturation in the absorption is observed at modest laser powers. (C) 2003 Published by Elsevier B.V.
Notes: 11th International Conference on Modulated Semiconductor Structures (MSS11), Nara, JAPAN, JUL 14-18, 2003
Abstract: In this work we have studied the dependence of the optical properties of self-assembled InAs quantum wires (QWr) grown on InP(001) on the growth temperature of the InP cap layer, as a mean for controlling the InAs QWr size. Our main result is that we can tune the emission wavelength of InAs QWr either at 1.3 mum or 1.55 mum at room temperature. We suggest that the role of growth temperature is to modify the As/P exchange at the InAs QWr/InP cap layer interface and consequently the amount of InAs involved in the nanostructure. In this way, due to the enhancement of the As/P exchange, the higher the growth temperature of the cap layer, the smaller in height the InAs quantum wires. Accordingly, the emission wavelength is blue shifted with InP cap layer growth temperature as the electron and hole ground state moves towards higher energies. Optical studies related to the dynamics of carrier recombination and light emission quenching with temperature are also included.
Abstract: The size and emission wavelength of self-assembled InAs/InP(001) quantum wires (QWrs) is affected by the P/As exchange process. In this work, we demonstrate by in situ stress measurements that P/As exchange at the InAs/InP interface depends on the surface reconstruction of the InAs starting surface and its immediate evolution when the arsenic cell is closed. Accordingly, the amount of InP grown on InAs by P/As exchange increases with substrate temperature in a steplike way. These results allow us to engineer the size of the QWr for emission at 1.3 and 1.55 mum at room temperature by selecting the range of substrate temperatures in which the InP cap layer is grown. (C) 2004 American Institute of Physics.
Abstract: In this work we have studied the dependence of the optical properties of self-assembled InAs quantum wires (QWr) grown on InP(001) on the growth temperature of the InP cap layer, as a mean for controlling the InAs QWr size. Our main result is that we can tune the emission wavelength of InAs QWr either at 1.3 mum or 1.55 mum at room temperature. We suggest that the role of growth temperature is to modify the As/P exchange at the InAs QWr/InP cap layer interface and consequently the amount of InAs involved in the nanostructure. In this way, due to the enhancement of the As/P exchange, the higher the growth temperature of the cap layer, the smaller in height the InAs quantum wires. Accordingly, the emission wavelength is blue shifted with InP cap layer growth temperature as the electron and hole ground state moves towards higher energies. Optical studies related to the dynamics of carrier recombination and light emission quenching with temperature are also included.
Abstract: We have performed photoluminescence experiments in samples containing self-assembled quantum dots with different spacer layer thicknesses.. A strong filtering effect produced by the GaAs spacer layer on the dots size being stacked is observed for spacers thinner than 10 nm. This effect produces a blue shift of the emission band from stacked dots and A simultaneous line width narrowing. At the same time, given the existence of a broad dot size distribution in the first layer; bigger dots can evolve towards InAs cylinder-like structures, whose emission occurs at appreciably lower energies as compared to the emission band associated to dot stacks (with some GaAs separation). (C) 2002 Elsevier Science B.V. All rights reserved.
Abstract: The temperature dependence of the effective mobility edge in semiconductor quantum wells containing disorder is reported for the first time. It is deduced by reproducing the experimental continuous wave and time resolved luminescence spectra by means of a two-class exciton kinetic model, which considers the co-existence of free and localized excitons and introduces a mobility edge defined by a Fermi function. The mobility edge varies faster than the PL peak energy when the temperature increases, passing from the high energy side of the PL band at low temperatures (recombination of localized excitons dominates) to the low energy side above 125 K (recombination of quasi-free excitons dominates). (C) 2002 Elsevier Science B.V. All rights reserved.
Notes: International Conference on Superlattices Nano-Structures and Nano-Devices (ICSNN-02), TOULOUSE, FRANCE, JUL 22-26, 2002
Abstract: The temperature dependence of the effective mobility edge in semiconductor quantum wells containing disorder is reported for the first time. It is deduced by reproducing the experimental continuous wave and time resolved luminescence spectra by means of a two-class exciton kinetic model, which considers the co-existence of free and localized excitons and introduces a mobility edge defined by a Fermi function. The mobility edge varies faster than the PL peak energy when the temperature increases, passing from the high energy side of the PL band at low temperatures (recombination of localized excitons dominates) to the low energy side above 125 K (recombination of quasi-free excitons dominates). (C) 2002 Elsevier Science B.V. All rights reserved.
Abstract: Multilayer structure containing vertically stacked InAs/InP self-assembled quantum wires (QWR) have been successfully grown by molecular beam epitaxy. We have found that spacer layer thickness fully determines the structural and optical properties of the multilayer structure, being observed a strong improvement of the homogeneity and uniformity of the QWR stacks in the case of an appropriate layer spacing. (C) 2002 Elsevier Science B.V. All rights reserved.
Notes: International Conference on Superlattices Nano-Structures and Nano-Devices (ICSNN-02), TOULOUSE, FRANCE, JUL 22-26, 2002
Abstract: Multilayer structure containing vertically stacked InAs/InP self-assembled quantum wires (QWR) have been successfully grown by molecular beam epitaxy. We have found that spacer layer thickness fully determines the structural and optical properties of the multilayer structure, being observed a strong improvement of the homogeneity and uniformity of the QWR stacks in the case of an appropriate layer spacing. (C) 2002 Elsevier Science B.V. All rights reserved.
Abstract: We have performed photoluminescence experiments in samples containing self-assembled quantum dots with different spacer layer thicknesses.. A strong filtering effect produced by the GaAs spacer layer on the dots size being stacked is observed for spacers thinner than 10 nm. This effect produces a blue shift of the emission band from stacked dots and A simultaneous line width narrowing. At the same time, given the existence of a broad dot size distribution in the first layer; bigger dots can evolve towards InAs cylinder-like structures, whose emission occurs at appreciably lower energies as compared to the emission band associated to dot stacks (with some GaAs separation). (C) 2002 Elsevier Science B.V. All rights reserved.
Notes: International Conference on Superlattices Nano-Structures and Nano-Devices (ICSNN-02), TOULOUSE, FRANCE, JUL 22-26, 2002
Abstract: Multilayer structure containing vertically stacked InAs/InP self-assembled quantum wires have been successfully grown by molecular-beam epitaxy. The influence of the InP spacer layer thickness on the structural and optical properties of the wire superlattice has been studied by means of transmission electron microscopy and photoluminescence. The coherent propagation of the strain field in the sample with a 5-nm-thick spacer determines by a size filtering effect a good homogeneity and uniformity of the wire stacks, and hence a good optical quality. The exciton recombination dynamics in the wire superlattice cannot be related to thermal escape of carriers out to the barriers, as occurs in single layer samples.
Abstract: The effects of external electric field on the photoluminescence (PL) of self assembled InAs / GaAs quantum dots (QDs) have been studied at different temperatures with different energies and power densities of excitation. Photocurrent measurements on the same sample were carried out for additional information on the transitions. The test structure consists of a single layer of InAs QDs with the InAs wetting layer buried in intrinsic GaAs, sandwiched between an n(++)GaAS at the back and a semitransparent Cr Schottky contact on the top GaAs capping layer. The power of the excitation source for PL measurements was varied in the range of 200-4000 W/cm(2), approximately. The results of comparatively low electric fields ate to be reported.
Abstract: Temperature dependent photoluminescence studies have been carried out on several samples containing self-assembled GaxIn1-xAs/InP quantum wires. A red-shift of the emission wavelength is observed when increasing the Ga content in the ahoy, but the overall optical quality decreases. In the case of x = 0.15, the photoluminescence is not sensible to temperature and the emission band is conserved until room temperature is reached, which could be explained if the nanostructures are considered to be almost amorphous.
Notes: 7th International Conference on Optics and Excitons in Confined Systems (OECS7), MONTPELLIER, FRANCE, SEP 03-07, 2001
Abstract: GaAs/InxGa1-xP quantum wells, with x = 0.541 and 0.427, have been investigated by continuous wave and time resolved photoluminescence. Spatial localization of excitons due to alloy compositional defects seem to be responsible of the observed phenomenology: non-exponential decay behavior at long times and slow decrease of the effective recombination time by increasing temperature. A two-class exciton kinetic model, based on a mobility edge varying with the temperature explain the experimental results. (C) 2002 Elsevier Science B.V. All rights reserved.
Notes: 20th European Conference on Surface Science, KRAKOW, POLAND, SEP 04-07, 2001
Abstract: In this work, a study of the relaxation mechanisms in InAs self-assembled quantum dots have been performed by means of photoluminescence (PL), resonant PL (RPL) and PL excitation. The observed. phenomenology is different for samples with different InAs coverage. At low InAs coverages, two resonant Raman scattering lines are observed when the excitation energy is chosen inside the emission hand. These lines are related to the TO and LO GaAs phonons. Multi-phonon relaxation of carriers (GaAs phonons) is clearly observed in RPL and excitation PL in the lowest InAs coverage sample. Neither such mechanism nor resonant Raman scattering is present in samples with larger InAs coverages, for which the average quantum dot size and areal density remain essentially constant. (C) 2002 Elsevier Science B.V. All rights reserved.
Notes: 20th European Conference on Surface Science, KRAKOW, POLAND, SEP 04-07, 2001
Abstract: Multilayer structure containing vertically stacked InAs/InP self-assembled quantum wires have been successfully grown by molecular-beam epitaxy. The influence of the InP spacer layer thickness on the structural and optical properties of the wire superlattice has been studied by means of transmission electron microscopy and photoluminescence. The coherent propagation of the strain field in the sample with a 5-nm-thick spacer determines by a size filtering effect a good homogeneity and uniformity of the wire stacks, and hence a good optical quality. The exciton recombination dynamics in the wire superlattice cannot be related to thermal escape of carriers out to the barriers, as occurs in single layer samples.
Abstract: Resonant photoluminescence experiments have been performed on self-assembled quantum dots. The emission bands measured in the investigated samples can be deconvoluted in several Gaussian components, which could be related to different size families of dots. The experimental results reveal the importance of GaAs phonons for the carrier relaxation of excess energy, specially for the sample with low dot density. With increasing temperature, electrons from smaller dots can be activated to reach the wetting layer electron states, from which they can be trapped at larger dots (below 100 K) or escape to the GaAs barriers (above 100 K).
Notes: 7th International Conference on Optics and Excitons in Confined Systems (OECS7), MONTPELLIER, FRANCE, SEP 03-07, 2001
Abstract: Photoluminescence measurements as a function of temperature (30 - 300K) have been carried out on self assembled GaXIn1-XAs / InP (x=0, 0.05, 0.15) quantum wires. The nanostructures emerge when the thickness of the deposited, strained GaXIn1-XAs layer exceeds the critical thickness for 2D growth on InP substrates. Atomic Force Microscopy (AFM) images show how the regular structure and perfection of the wires are lost as the amount of Ga in the alloy increases, which is also reflected in the corresponding PL spectra. The energy and envelope of the PL spectra change with the Gallium content of the alloy. This leads to the fact that the emission energy can be tuned in the range of 1.48 -1.72 mum, an interesting range for optoelectronics, by changing the Gallium content of the alloy. A new striking feature is observed in the PL spectra. The temperature dependence of the PL spectra is strongly dependent on the Gallium content of the alloy. When x=0.15 the PL spectra becomes almost temperature invariant. This inference is highly relevant for temperature stable optical sources. The paper reports and discusses these interesting observations.
Abstract: Temperature dependent photoluminescence studies have been carried out on several samples containing self-assembled GaxIn1-xAs/InP quantum wires. A red-shift of the emission wavelength is observed when increasing the Ga content in the ahoy, but the overall optical quality decreases. In the case of x = 0.15, the photoluminescence is not sensible to temperature and the emission band is conserved until room temperature is reached, which could be explained if the nanostructures are considered to be almost amorphous.
Abstract: GaAs/InxGa1-xP quantum wells, with x = 0.541 and 0.427, have been investigated by continuous wave and time resolved photoluminescence. Spatial localization of excitons due to alloy compositional defects seem to be responsible of the observed phenomenology: non-exponential decay behavior at long times and slow decrease of the effective recombination time by increasing temperature. A two-class exciton kinetic model, based on a mobility edge varying with the temperature explain the experimental results. (C) 2002 Elsevier Science B.V. All rights reserved.
Abstract: The effects of an external electric field on the photoluminescence of small self-assembled InAs/GaAs quantum dots (3-4 nm high) have been studied at different temperatures, laser power, and laser wavelength excitation. The shallow nature of the only confined electron level is revealed by the strong photocurrent signal measured at low temperatures. In this QD system, recombination from that electron level with ground and first excited hole states can be observed in high power excited PL and low power excited PL under direct applied bias, when the hole population is high enough under these conditions.
Notes: 7th International Conference on Optics and Excitons in Confined Systems (OECS7), MONTPELLIER, FRANCE, SEP 03-07, 2001
Abstract: The effects of an external electric field on the photoluminescence of small self-assembled InAs/GaAs quantum dots (3-4 nm high) have been studied at different temperatures, laser power, and laser wavelength excitation. The shallow nature of the only confined electron level is revealed by the strong photocurrent signal measured at low temperatures. In this QD system, recombination from that electron level with ground and first excited hole states can be observed in high power excited PL and low power excited PL under direct applied bias, when the hole population is high enough under these conditions.
Abstract: Resonant photoluminescence experiments have been performed on self-assembled quantum dots. The emission bands measured in the investigated samples can be deconvoluted in several Gaussian components, which could be related to different size families of dots. The experimental results reveal the importance of GaAs phonons for the carrier relaxation of excess energy, specially for the sample with low dot density. With increasing temperature, electrons from smaller dots can be activated to reach the wetting layer electron states, from which they can be trapped at larger dots (below 100 K) or escape to the GaAs barriers (above 100 K).
Abstract: In this work, a study of the relaxation mechanisms in InAs self-assembled quantum dots have been performed by means of photoluminescence (PL), resonant PL (RPL) and PL excitation. The observed. phenomenology is different for samples with different InAs coverage. At low InAs coverages, two resonant Raman scattering lines are observed when the excitation energy is chosen inside the emission hand. These lines are related to the TO and LO GaAs phonons. Multi-phonon relaxation of carriers (GaAs phonons) is clearly observed in RPL and excitation PL in the lowest InAs coverage sample. Neither such mechanism nor resonant Raman scattering is present in samples with larger InAs coverages, for which the average quantum dot size and areal density remain essentially constant. (C) 2002 Elsevier Science B.V. All rights reserved.
Abstract: InAs self-assembled quantum wire structures have been grown on InP substrates and studied by means of photoluminescence and polarized-light absorption measurements. According to our calculations, the observed optical transitions in each sample are consistent with wires of different heights, namely from 6 to 13 monolayers. The nonradiative mechanism limiting the emission intensity at room temperature is related to thermal escape of carriers out of the wires. (C) 2001 American Institute of Physics.
Abstract: We have investigated the electronic structure of the zig-zag ladder (chain) compound SrCuO2 combining polarized optical absorption, reflection, photoreflectance, and pseudo-dielectric-function measurements with the model calculations. These measurements yield an energy gap of 1.42 eV (1.77 eV) at 300 K along (perpendicular to) the Cu-O chains. We have found that the lowest-energy gap, the correlation gap, is temperature independent. The electronic structure of this oxide is calculated using both the local-spin-density approximation with gradient correction method and the tight-binding theory for the correlated electrons. The calculated density of electronic states for noncorrelated and correlated electrons shows quasi-one-dimensional character. The correlation gap values of 1.42 eV (indirect transition) and 1.88 eV (direct transition) have been calculated with the electron hopping parameters t = 0.30 eV (along a chain), t(yz) = 0.12 eV (between chains), and the Anderson-Hubbard repulsion on copper sites U = 2.0 eV. We concluded that SrCuO2 belongs to the correlated-gap insulators.
Abstract: We have investigated the electronic structure of the zig-zag ladder (chain) compound SrCuO2 combining polarized optical absorption, reflection, photoreflectance, and pseudo-dielectric-function measurements with the model calculations. These measurements yield an energy gap of 1.42 eV (1.77 eV) at 300 K along (perpendicular to) the Cu-O chains. We have found that the lowest-energy gap, the correlation gap, is temperature independent. The electronic structure of this oxide is calculated using both the local-spin-density approximation with gradient correction method and the tight-binding theory for the correlated electrons. The calculated density of electronic states for noncorrelated and correlated electrons shows quasi-one-dimensional character. The correlation gap values of 1.42 eV (indirect transition) and 1.88 eV (direct transition) have been calculated with the electron hopping parameters t = 0.30 eV (along a chain), t(yz) = 0.12 eV (between chains), and the Anderson-Hubbard repulsion on copper sites U = 2.0 eV. We concluded that SrCuO2 belongs to the correlated-gap insulators.
Abstract: InAs self-assembled quantum wire structures have been grown on InP substrates and studied by means of photoluminescence and polarized-light absorption measurements. According to our calculations, the observed optical transitions in each sample are consistent with wires of different heights, namely from 6 to 13 monolayers. The nonradiative mechanism limiting the emission intensity at room temperature is related to thermal escape of carriers out of the wires. (C) 2001 American Institute of Physics.
Abstract: We have studied the influence of InP buffer-layer morphology in the formation of InAs nanostructures grown on InP(001) substrates by solid-source molecular-beam epitaxy. Our results demonstrate that when InP buffer layers are grown by atomic-layer molecular-beam epitaxy, InAs quantum dot-like structures are formed, whereas InP buffer layers grown by MBE produce quantum-wire-like structures. The optical properties of these corrugated structures make them potential candidates for their use in light-emitting devices at 1.55 mu m. (C) 2000 American Institute of Physics. [S0003-6951(00)00309-0].
Abstract: We have studied the influence of InP buffer-layer morphology in the formation of InAs nanostructures grown on InP(001) substrates by solid-source molecular-beam epitaxy. Our results demonstrate that when InP buffer layers are grown by atomic-layer molecular-beam epitaxy, InAs quantum dot-like structures are formed, whereas InP buffer layers grown by MBE produce quantum-wire-like structures. The optical properties of these corrugated structures make them potential candidates for their use in light-emitting devices at 1.55 mu m. (C) 2000 American Institute of Physics. [S0003-6951(00)00309-0].
Abstract: InxGa1-xP/GaAs (x = 0.541 and 0.427) heterostructures, grown by Atomic Layer Molecular Beam Epitaxy (ALMBE) on low temperature substrates, have been characterised by pressure-dependent and time-resolved photoluminescence experiments. The excitonic optical transitions and recombination dynamics are both influenced by the particular band alignments of these systems. The valence band offset has been found to have approximately the same absolute value (Delta E-VB approximate to 380 meV), independent of the In content of the alloy in the barrier, whereas the conduction band offset varies appreciably depending on the alloy band gap. The huge valence band offset implies a strong asymmetry in the confinement of carriers, affecting the exciton recombination dynamics in the quantum wells.
Abstract: InxGa1-xP/GaAs (x = 0.541 and 0.427) heterostructures, grown by Atomic Layer Molecular Beam Epitaxy (ALMBE) on low temperature substrates, have been characterised by pressure-dependent and time-resolved photoluminescence experiments. The excitonic optical transitions and recombination dynamics are both influenced by the particular band alignments of these systems. The valence band offset has been found to have approximately the same absolute value (Delta E-VB approximate to 380 meV), independent of the In content of the alloy in the barrier, whereas the conduction band offset varies appreciably depending on the alloy band gap. The huge valence band offset implies a strong asymmetry in the confinement of carriers, affecting the exciton recombination dynamics in the quantum wells.
Notes: 6th International Conference on Optics of Excitons in Confined Systems (OECS-6), ASCONA, SWITZERLAND, AUG 30-SEP 02, 1999
Abstract: The transmission properties of different metallic photonic lattices (square and rectangular! have been experimentally studied. A numerical algorithm based on time domain finite differences has been used for simulating these photonic structures. The introduction of defects in the two-dimensional metallic lattice modifies its transmission spectrum. If metal rods are eliminated from (or added to) the lattice, extremely narrow peaks are observed at some particular frequencies below (or above) the band pass edge. (C) 1999 American Institute of Physics. [S0021-8979(99)00415-6].
Abstract: The transmission properties of different metallic photonic lattices (square and rectangular! have been experimentally studied. A numerical algorithm based on time domain finite differences has been used for simulating these photonic structures. The introduction of defects in the two-dimensional metallic lattice modifies its transmission spectrum. If metal rods are eliminated from (or added to) the lattice, extremely narrow peaks are observed at some particular frequencies below (or above) the band pass edge. (C) 1999 American Institute of Physics. [S0021-8979(99)00415-6].
Abstract: In this work high structural and optical quality InxGa1-xP/GaAs quantum wells in a wide range of thicknesses have been successfully grown on GaAs substrates by low temperature atomic layer molecular beam epitaxy. We demonstrate that compositional fluctuations in the barrier alloy are responsible for the inhomogeneous broadening and spatial localization effects observed in the excitonic recombination, the influence of quantum well width fluctuations being negligible in comparison. An important change of the optical transition energies in these quantum wells is observed when tuning a 10% In-Ga ratio in the alloy around the lattice match composition (x = 0.48). This change is related to the barrier band gap variation and the intrinsic characteristics of the InGaP/GaAs heterostructure: different exciton binding energy from tensile to compressive strain in the barrier, and a possible dependence of the conduction band offset on the In composition. (C) 1998 American Institute of Physics. [S0021-8979(98)04023-7].
Abstract: We report a comprehensive study of carrier-recombination dynamics in shallow AlxGa1-xAs/GaAs quantum wells. At low crystal temperature (2 K), the excitonic radiative recombination time is shown to be strongly enhanced in shallow quantum wells with x>0.01, consistently with a model that takes into account the thermal equilibrium between the three-dimensional exciton gas of the barrier and the two-dimensional exciton gas, which are closer in energy as I decreases. Furthermore, we demonstrate the existence of a thermally activated escape mechanism due to the low effective barrier height in these structures. The nonradiative recombination is shown to dominate the carrier dynamics for temperatures as low as 10 K for x approximate to 0.01. Our experimental observations are analyzed using three different variational exciton calculations. In particular, we study the crossover from the two-dimensional to the three-dimensional behavior of the exciton, which occurs far x as low as 0.01 and affects mainly the oscillator strength, whereas the transition energies in shallow quantum wells can be calculated, to a large extent, using the same approximations as for conventional quantum wells. The peculiar behavior of the oscillator strength at the crossover to the weak confinement regime is obtained by expansion in a large basis.
Abstract: We present results on the optical characterization of InxGa1-xP layers grown by atomic layer molecular beam epitaxy on GaAs (001) substrates at a growth temperature of 420 degrees C. Our results show that the optical characteristics of these layers, which do not show ordering effects, are strongly dependent on surface stoichiometry during growth. In this way, we can obtain either highly homogeneous alloys with a predictable band-gap energy or layers with optical properties indicative of spatial localization effects, like an anomalous behavior of photoluminescence peak energy with temperature and a large shift between the emission energy and absorption edge. (C) 1998 American Institute of Physics.
Abstract: We present results on the optical characterization of InxGa1-xP layers grown by atomic layer molecular beam epitaxy on GaAs (001) substrates at a growth temperature of 420 degrees C. Our results show that the optical characteristics of these layers, which do not show ordering effects, are strongly dependent on surface stoichiometry during growth. In this way, we can obtain either highly homogeneous alloys with a predictable band-gap energy or layers with optical properties indicative of spatial localization effects, like an anomalous behavior of photoluminescence peak energy with temperature and a large shift between the emission energy and absorption edge. (C) 1998 American Institute of Physics.
Abstract: In this work high structural and optical quality InxGa1-xP/GaAs quantum wells in a wide range of thicknesses have been successfully grown on GaAs substrates by low temperature atomic layer molecular beam epitaxy. We demonstrate that compositional fluctuations in the barrier alloy are responsible for the inhomogeneous broadening and spatial localization effects observed in the excitonic recombination, the influence of quantum well width fluctuations being negligible in comparison. An important change of the optical transition energies in these quantum wells is observed when tuning a 10% In-Ga ratio in the alloy around the lattice match composition (x = 0.48). This change is related to the barrier band gap variation and the intrinsic characteristics of the InGaP/GaAs heterostructure: different exciton binding energy from tensile to compressive strain in the barrier, and a possible dependence of the conduction band offset on the In composition. (C) 1998 American Institute of Physics. [S0021-8979(98)04023-7].
Abstract: We report a comprehensive study of carrier-recombination dynamics in shallow AlxGa1-xAs/GaAs quantum wells. At low crystal temperature (2 K), the excitonic radiative recombination time is shown to be strongly enhanced in shallow quantum wells with x>0.01, consistently with a model that takes into account the thermal equilibrium between the three-dimensional exciton gas of the barrier and the two-dimensional exciton gas, which are closer in energy as I decreases. Furthermore, we demonstrate the existence of a thermally activated escape mechanism due to the low effective barrier height in these structures. The nonradiative recombination is shown to dominate the carrier dynamics for temperatures as low as 10 K for x approximate to 0.01. Our experimental observations are analyzed using three different variational exciton calculations. In particular, we study the crossover from the two-dimensional to the three-dimensional behavior of the exciton, which occurs far x as low as 0.01 and affects mainly the oscillator strength, whereas the transition energies in shallow quantum wells can be calculated, to a large extent, using the same approximations as for conventional quantum wells. The peculiar behavior of the oscillator strength at the crossover to the weak confinement regime is obtained by expansion in a large basis.
Abstract: In this work we study the exciton recombination of InAs/InP self-organized quantum dots by means of photoluminescence (PL) as a function of temperature and excitation density. Well defined islands, spatially separated in most cases, and with different size distribution, make localized exciton recombination the dominant contribution to the PL spectrum. From our experimental results, we propose the co-existence of two types of islands, one with small height whose contribution to the PL spectra is important in samples with low InAs coverage (below two monolayers), and the properly 3D islands, whose dimensions and sheet concentration increase with the InAs coverage. Good quality structures are obtained for an InAs coverage of about two to three monolayers, showing only one PL band centered around 1.11 eV with a reasonable linewidth below 40 meV.
Notes: 5th International Meeting on Optics of Excitons in Confined Systems (OECS 5), GOTTINGEN, GERMANY, AUG 10-14, 1997
Abstract: We show that by measuring the separation between the Stokes and anti-Stokes peaks excited by two different laser lines we obtain a very precise determination of absolute phonon energies. The method is useful for measuring small changes of these energies with strain, temperature, laser power, etc. It doubles the changes and avoids the necessity of using the reference lines in the Raman spectra. The method can be applied for the determination of phonon deformation potentials, for the characterization of strained heteroepitaxial layers, and for micro-Raman analysis of strain in silicon integrated circuits. We give examples of phonon shifts in Si, Ge, GaAs, InAs, and Cap as a function of applied biaxial strain, laser power, and temperature. (C) 1997 American Institute of Physics.
Abstract: It is shown that the phonon deformation potentials in semiconductors can be determined by Raman scattering on hydrostatically and biaxially deformed samples. The complete data includes biaxial deformation in the (100), (111), and (110) planes. Biaxial strain is applied to the sample using the recently developed membrane method. The phonon displacement under biaxial strain can be directly obtained from Raman measurements on a single membrane provided we determine the strain from the splitting of the band gap using e.g. the photoreflectance technique. Alternatively, the ratios of different phonon shifts can supply the necessary information. The method is illustrated with experimental results for GaP.
Notes: XXVI International School on Physics of Semiconducting Compounds, JASZOWIEC, POLAND, JUN 06-13, 1997
Abstract: We report on time-resolved optical experiments performed resonantly onto the fundamental excitonic transition on several (Zn,Cd)Se/ZnSe quantum wells, for temperatures from 10 K up to room temperature. We show that the thermal quenching of the photoluminescence intensity is due to extrinsic non-radiative channels, whose activation energy is found to be 23 meV.
Notes: 5th International Meeting on Optics of Excitons in Confined Systems (OECS 5), GOTTINGEN, GERMANY, AUG 10-14, 1997
Abstract: We show that the insertion of extremely narrow AlAs layers in double-barrier GaAs/AlAs/AlxGa1-xAs quantum wells results in a variety of electronic configurations, thus providing a powerful tool for tailoring the electronic transitions in GaAs heterostructures. In particular, the transition from type-I to type-II recombination is shown to occur in correspondence with variations by a single monolayer in the thickness of the AlAs and/or GaAs layers. Drastic changes in the recombination lifetimes are correspondingly observed; at the same time, the photoluminescence efficiency is found to be almost independent of the type-I-type-Il character of the transition.
Abstract: We report on time-resolved optical experiments performed resonantly onto the fundamental excitonic transition on several (Zn,Cd)Se/ZnSe quantum wells, for temperatures from 10 K up to room temperature. We show that the thermal quenching of the photoluminescence intensity is due to extrinsic non-radiative channels, whose activation energy is found to be 23 meV.
Abstract: We show that by measuring the separation between the Stokes and anti-Stokes peaks excited by two different laser lines we obtain a very precise determination of absolute phonon energies. The method is useful for measuring small changes of these energies with strain, temperature, laser power, etc. It doubles the changes and avoids the necessity of using the reference lines in the Raman spectra. The method can be applied for the determination of phonon deformation potentials, for the characterization of strained heteroepitaxial layers, and for micro-Raman analysis of strain in silicon integrated circuits. We give examples of phonon shifts in Si, Ge, GaAs, InAs, and Cap as a function of applied biaxial strain, laser power, and temperature. (C) 1997 American Institute of Physics.
Abstract: We show that the insertion of extremely narrow AlAs layers in double-barrier GaAs/AlAs/AlxGa1-xAs quantum wells results in a variety of electronic configurations, thus providing a powerful tool for tailoring the electronic transitions in GaAs heterostructures. In particular, the transition from type-I to type-II recombination is shown to occur in correspondence with variations by a single monolayer in the thickness of the AlAs and/or GaAs layers. Drastic changes in the recombination lifetimes are correspondingly observed; at the same time, the photoluminescence efficiency is found to be almost independent of the type-I-type-Il character of the transition.
Abstract: It is shown that the phonon deformation potentials in semiconductors can be determined by Raman scattering on hydrostatically and biaxially deformed samples. The complete data includes biaxial deformation in the (100), (111), and (110) planes. Biaxial strain is applied to the sample using the recently developed membrane method. The phonon displacement under biaxial strain can be directly obtained from Raman measurements on a single membrane provided we determine the strain from the splitting of the band gap using e.g. the photoreflectance technique. Alternatively, the ratios of different phonon shifts can supply the necessary information. The method is illustrated with experimental results for GaP.
Abstract: We report on optical experiments performed on a ZnSe/Zn0.73Mn0.27Se strained quantum well. The measurements were done by means of resonant time-integrated and time-resolved photoluminescence spectroscopies at temperatures from 1.7 to 100 K. We show that recombination arising from the first excited excitonic state of the heterostructure is present even at low temperature. The quite unusual relaxation channel from this state, namely, a heavy-hole bound interface exciton essentially located in the (Zn,Mn)Se layer, to the fundamental one, a light-hole bound exciton located in the ZnSe layer, is quantitatively studied. In particular, when an external magnetic field is applied, we observe directly the consequences of the giant Zeeman effect occurring in the (Zn,Mn)Se layer on both types of excitons.
Abstract: In this work we study the exciton recombination of InAs/InP self-organized quantum dots by means of photoluminescence (PL) as a function of temperature and excitation density. Well defined islands, spatially separated in most cases, and with different size distribution, make localized exciton recombination the dominant contribution to the PL spectrum. From our experimental results, we propose the co-existence of two types of islands, one with small height whose contribution to the PL spectra is important in samples with low InAs coverage (below two monolayers), and the properly 3D islands, whose dimensions and sheet concentration increase with the InAs coverage. Good quality structures are obtained for an InAs coverage of about two to three monolayers, showing only one PL band centered around 1.11 eV with a reasonable linewidth below 40 meV.
Abstract: We report on optical experiments performed on a ZnSe/Zn0.73Mn0.27Se strained quantum well. The measurements were done by means of resonant time-integrated and time-resolved photoluminescence spectroscopies at temperatures from 1.7 to 100 K. We show that recombination arising from the first excited excitonic state of the heterostructure is present even at low temperature. The quite unusual relaxation channel from this state, namely, a heavy-hole bound interface exciton essentially located in the (Zn,Mn)Se layer, to the fundamental one, a light-hole bound exciton located in the ZnSe layer, is quantitatively studied. In particular, when an external magnetic field is applied, we observe directly the consequences of the giant Zeeman effect occurring in the (Zn,Mn)Se layer on both types of excitons.
Abstract: High structural and optical quality InxGa1-xP/GaAs quantum wells, with x from 0.51 to 0.45, have been successfully grown by atomic layer molecular beam epitaxy. In that compositional range, an important blue shift of the quantum well luminescence Lines is observed, which is explained by an increase of the conduction band gap offset from compressive to tensile strain conditions. The luminescence intensity decreases with temperature above 20-30 K, which is attributed to impurities located at the interfaces and inside the quantum wells. The influence of the In content on the oscillator strength of the optical transitions is also evaluated. (C) 1996 American Institute of Physics.
Abstract: High structural and optical quality InxGa1-xP/GaAs quantum wells, with x from 0.51 to 0.45, have been successfully grown by atomic layer molecular beam epitaxy. In that compositional range, an important blue shift of the quantum well luminescence Lines is observed, which is explained by an increase of the conduction band gap offset from compressive to tensile strain conditions. The luminescence intensity decreases with temperature above 20-30 K, which is attributed to impurities located at the interfaces and inside the quantum wells. The influence of the In content on the oscillator strength of the optical transitions is also evaluated. (C) 1996 American Institute of Physics.
Abstract: Resonant photoluminescence and excitation spectroscopy and time-resolved photoluminescence are performed on a 50 Angstrom thick ZnSe/(Zn, Mn)Se strained quantum well. From these experiments, the existence of a relaxation from a type-II heavy-hole bound interface exciton to a type-I light-hole exciton is demonstrated. The time transfer from the heavy-hole exciton to the light-hole exciton is found to be of the same order of magnitude as the recombination time of the heavy-hole exciton. This transfer is enhanced when temperature increases.
Notes: IV International Conference on Optics of Excitons in Confined Systems, CORTONA, ITALY, AUG 28-31, 1995
Abstract: We report on time-integrated and time-resolved optical experiments performed on a 26 Angstrom thick Zn-0.85 Cd-0.15 Se/ZnSe quantum well, for temperatures in the 10-200 K range. Excitation spectroscoy allows an estimation of the relative valence band offset, which is found to be 10%. From the temperature variation of the decay time of the photoluminescence performed reasonantly on the e1h1 excitonic transition, we deduce that the main non-radiative mechanism is the heavy-hole thermal escape out of the well.
Notes: IV International Conference on Optics of Excitons in Confined Systems, CORTONA, ITALY, AUG 28-31, 1995
Abstract: Optical properties of GaAs/AlxGa1-xAs shallow quantum wells have been studied in the 2-200 K range, both by c.w. and time-resolved experiments. The results agree with a variational calculation of the excitonic transitions. The LO-phonon-assisted carrier relaxation and a temperature-activated non-radiative channel are evidenced.
Notes: IV International Conference on Optics of Excitons in Confined Systems, CORTONA, ITALY, AUG 28-31, 1995
Abstract: Resonant photoluminescence and excitation spectroscopy and time-resolved photoluminescence are performed on a 50 Angstrom thick ZnSe/(Zn, Mn)Se strained quantum well. From these experiments, the existence of a relaxation from a type-II heavy-hole bound interface exciton to a type-I light-hole exciton is demonstrated. The time transfer from the heavy-hole exciton to the light-hole exciton is found to be of the same order of magnitude as the recombination time of the heavy-hole exciton. This transfer is enhanced when temperature increases.
Abstract: We report on time-integrated and time-resolved optical experiments performed on a 26 Angstrom thick Zn-0.85 Cd-0.15 Se/ZnSe quantum well, for temperatures in the 10-200 K range. Excitation spectroscoy allows an estimation of the relative valence band offset, which is found to be 10%. From the temperature variation of the decay time of the photoluminescence performed reasonantly on the e1h1 excitonic transition, we deduce that the main non-radiative mechanism is the heavy-hole thermal escape out of the well.
Abstract: Optical properties of GaAs/AlxGa1-xAs shallow quantum wells have been studied in the 2-200 K range, both by c.w. and time-resolved experiments. The results agree with a variational calculation of the excitonic transitions. The LO-phonon-assisted carrier relaxation and a temperature-activated non-radiative channel are evidenced.
Abstract: The dependence of the hole spin relaxation on the electron density is studied in a n-modulation doped 75 angstrom GaAs/AlGaAs quantum well by means of cw and time-resolved photoluminescence techniques. A slow hole spin relaxation time has been measured (approximately 1 ns) and the polarization has been found to be strongly dependent on the in-plane wavevector of the photocreated holes. Calculations are presented which support the experimental findings.
Notes: 10th International Conference on Electronic Properties of Two-Dimensional Systems (EP2DS-10), NEWPORT, RI, MAY 31-JUN 04, 1993
Abstract: We have performed c.w. and time-resolved photoluminescence measurements on a 30/30 Cd0.82Mn0.18Te/CdTe superlattice containing two enlarged wells with different widths. Excitation above the alloy band gap allows to study the Bloch transport along the growth direction of the superlattice. Both ambipolar transport and hopping of carriers are observed in the 4-100 K temperature range. At low temperatures (<20 K), mobilities of the order of 4 x 10(4) cm2/Vs and 3 x 10(2) cm2/Vs are estimated for the two processes, respectively.
Notes: 6th International Conference on Modulated Semiconductor Structures, GARMISCH PARTENKIR, GERMANY, AUG 23-27, 1993
Abstract: The origin of the red shift of the excitonic luminescence with respect to the absorption peak (Stokes shift) in good quality quantum wells, usually ascribed to exciton trapping, is explained in terms of a thermal population of the inhomogeneous distribution of excitonic states. Within this new picture the Stokes shift turns out to be quadratic on the absorption linewidth and inversely proportional to the excitonic temperature. The predictions are found to be in good agreement with careful measurements in a set of GaAs/AlGaAs single quantum well structures.
Abstract: The dependence of the hole spin relaxation on the electron density is studied in a n-modulation doped 75 angstrom GaAs/AlGaAs quantum well by means of cw and time-resolved photoluminescence techniques. A slow hole spin relaxation time has been measured (approximately 1 ns) and the polarization has been found to be strongly dependent on the in-plane wavevector of the photocreated holes. Calculations are presented which support the experimental findings.
Abstract: We have performed c.w. and time-resolved photoluminescence measurements on a 30/30 Cd0.82Mn0.18Te/CdTe superlattice containing two enlarged wells with different widths. Excitation above the alloy band gap allows to study the Bloch transport along the growth direction of the superlattice. Both ambipolar transport and hopping of carriers are observed in the 4-100 K temperature range. At low temperatures (<20 K), mobilities of the order of 4 x 10(4) cm2/Vs and 3 x 10(2) cm2/Vs are estimated for the two processes, respectively.
Abstract: The origin of the red shift of the excitonic luminescence with respect to the absorption peak (Stokes shift) in good quality quantum wells, usually ascribed to exciton trapping, is explained in terms of a thermal population of the inhomogeneous distribution of excitonic states. Within this new picture the Stokes shift turns out to be quadratic on the absorption linewidth and inversely proportional to the excitonic temperature. The predictions are found to be in good agreement with careful measurements in a set of GaAs/AlGaAs single quantum well structures.
Abstract: We have performed cw and time-resolved photoluminescence measurements on a 30/30 Cd0.82Mn0.18Te/CdTe superlattice containing two enlarged wells with different widths, namely, 60 and 200 angstrom. The distance between the two enlarged wells is 2400 angstrom and the wider one is located 0.96 mum away from the sample surface. Excitation above the alloy band gap allows us to study the Bloch transport along the growth direction of the superlattice. Both ambipolar transport and hopping of carriers are observed. Mobilities of the order of 4 X 10(4) cm2/V s and 3 X 10(2) cm2/V s are estimated for the two processes, respectively, at low temperatures (< 20 K). Transfer of excitation between the two wells is also observed when exciting resonantly the thinner enlarged well, i.e., below the superlattice band gap. This transfer is assigned to carrier hopping on localized states of the superlattice.
Abstract: In this work we demonstrate the existence of above-barrier quasibound states in Al0.3Ga0.7As/AlAs/GaAs double-barrier quantum-well structures, by using both continuous-wave and time-resolved photoluminescence techniques in addition to reflectivity measurements. A series of clearly resolved resonances is found in the optical spectra. Their energy positions agree fairly well with the energies found for the quasibound states when analyzing the density of states of the heterostructure continuous spectrum. As shown by the analysis of the envelope wave function these resonances are localized in the Al0.3Ga0.7As barrier regions and are found to induce both an increase of the interband absorption and a reduction of the carrier capture efficiency into the wells.
Abstract: We have performed cw and time-resolved photoluminescence measurements on a 30/30 Cd0.82Mn0.18Te/CdTe superlattice containing two enlarged wells with different widths, namely, 60 and 200 angstrom. The distance between the two enlarged wells is 2400 angstrom and the wider one is located 0.96 mum away from the sample surface. Excitation above the alloy band gap allows us to study the Bloch transport along the growth direction of the superlattice. Both ambipolar transport and hopping of carriers are observed. Mobilities of the order of 4 X 10(4) cm2/V s and 3 X 10(2) cm2/V s are estimated for the two processes, respectively, at low temperatures (< 20 K). Transfer of excitation between the two wells is also observed when exciting resonantly the thinner enlarged well, i.e., below the superlattice band gap. This transfer is assigned to carrier hopping on localized states of the superlattice.
Abstract: Photoluminescence (PL), PL excitation and time resolved PL experiments have been Performed On Al0.42Ga0.58As/AlAs/GaAS symmetric double barrier quantum wells (DBQW) with only one or two AlAs monolayers constituting the intermediate barriers. In agreement with the envelope function predictions we show that such DBQWs undergo a type I - type II transition when the GaAs thickness is reduced below 7 and 5 monolayers for 2 and 1 AlAs molecular planes respectively In type II configuration the PL decay time is found to be strongly dependent on the energy difference between AlAs X(Z) - and GaAs GAMMA - electron confined states and the coupling parameter of the GAMMA and X(Z) valleys can be deduced (4.2 meV on average). Similar experiments on DBQWs grown on 2-degrees off (001) vicinal surfaces show that the exciton lifetime remains close to that of nominal samples. Considering also former results obtained in type II GaAs/AlAs superlattices, it is concluded that the GAMMA-X potential in quantum structures is intrinsically linked to the AlAs/GaAs interface.
Abstract: In this work we demonstrate the existence of above-barrier quasibound states in Al0.3Ga0.7As/AlAs/GaAs double-barrier quantum-well structures, by using both continuous-wave and time-resolved photoluminescence techniques in addition to reflectivity measurements. A series of clearly resolved resonances is found in the optical spectra. Their energy positions agree fairly well with the energies found for the quasibound states when analyzing the density of states of the heterostructure continuous spectrum. As shown by the analysis of the envelope wave function these resonances are localized in the Al0.3Ga0.7As barrier regions and are found to induce both an increase of the interband absorption and a reduction of the carrier capture efficiency into the wells.
Abstract: We report on continuous-wave and time-resolved measurements of the photoluminescence of good-quality single quantum wells at low temperatures (4-30 K). Luminescence arising from both free and localized excitons is observed and the influence of excitation localization on the photoluminescence decay time is investigated. Resonant pumping at the light-hole exciton transition is found to greatly increase the generation of free heavy-hole excitons. In this case, the free-exciton lifetime increases linearly with temperature (10-30 K) and the dependence of the slope on the well thickness is found to be in good quantitative agreement with the theoretical model of Andreani et al. [Solid State Commun. 77, 641 (1991)].
Abstract: The nature of transport through superlattice minibands is addressed in this paper. All-optical techniques have been used to study the vertical transport, monitoring the luminescence from an enlarged well grown within the superlattice. Photoluminescence, photoluminescence excitation spectroscopy, and time-resolved luminescence-decay measurements have been performed as a function of temperature and excitation intensity. A consistent picture emerges from the experimental data, showing the key role played by the superlattice miniband widths and interface disorder on the vertical transport at low temperatures (4-40 K).
Abstract: A photoluminescence-excitation (PLE) study of the exciton binding energy in GaAs/AlxGa1-xAs quantum-well (QW) structures is reported. A line-shape fit of the PLE spectra, based on a correct evaluation of the absorption probability in QW’s, is proposed as a Powerful tool in order to deduce accurately the exciton binding energies even in samples where the 2s peak is unresolved. The experimental results obtained are in good agreement with recent accurate theories. In particular, we find a strong dependence of the heavy-hole exciton binding energy E1 on the aluminum concentration of the AlxGa1-xAs barrier, in agreement with the predicted importance of the dielectric mismatch and conduction-band nonparabolicity in enhancing E1. Finally, evidence of exciton binding energies larger than the two-dimensional limit in GaAs is found even in relatively thick QW’s (50 angstrom) with AlAs barriers.
Abstract: A systematic study of the electron transport and shallow impurity distribution in indium selenide above room temperature or after an annealing process is reported by means of far-infrared-absorption and Hall-effect measurements. Evidences are found for the existence of a large concentration of deep levels (10(12)-10(13) cm-2), related to impurities adsorbed to stacking faults in this material. Above room temperature impurities can migrate from those defect zones and then become shallow in the bulk. The subsequent large increase of 3D electrons can change the dimensionality of the electron transport, which in most cases was 2D. The temperature dependence of the resistivity parallel to the c axis can be explained by the observed increase of the 3D electron concentration, whose motion across the layers is limited by stacking-fault-related potential barriers. The observed macroscopic resistivity is thus determined by tunneling through those barriers.
Abstract: In this paper we report an overview of some points that we believe mostly relevant to the exciton dynamics in quantum well structures. We will concentrate our attention to the intrinsic radiative lifetime of the free excitons and the effects that localization has on the recombination kinetics. It is shown that a good agreement is eventually found between the theoretical predictions and the experimental data, even if some key aspects for a quantitative comprehension of the recombination mechanisms, namely the effects of phonon and/or interface defect scattering, still lack a complete understanding.
Notes: 3rd International Conference on Optics of Excitons in Confined Systems, UNIV MONTPELLIER II, MONTPELLIER, FRANCE, AUG 30-SEP 02, 1993
Abstract: A systematic study of the electron transport and shallow impurity distribution in indium selenide above room temperature or after an annealing process is reported by means of far-infrared-absorption and Hall-effect measurements. Evidences are found for the existence of a large concentration of deep levels (10(12)-10(13) cm-2), related to impurities adsorbed to stacking faults in this material. Above room temperature impurities can migrate from those defect zones and then become shallow in the bulk. The subsequent large increase of 3D electrons can change the dimensionality of the electron transport, which in most cases was 2D. The temperature dependence of the resistivity parallel to the c axis can be explained by the observed increase of the 3D electron concentration, whose motion across the layers is limited by stacking-fault-related potential barriers. The observed macroscopic resistivity is thus determined by tunneling through those barriers.
Abstract: Photoluminescence (PL), PL excitation and time resolved PL experiments have been Performed On Al0.42Ga0.58As/AlAs/GaAS symmetric double barrier quantum wells (DBQW) with only one or two AlAs monolayers constituting the intermediate barriers. In agreement with the envelope function predictions we show that such DBQWs undergo a type I - type II transition when the GaAs thickness is reduced below 7 and 5 monolayers for 2 and 1 AlAs molecular planes respectively In type II configuration the PL decay time is found to be strongly dependent on the energy difference between AlAs X(Z) - and GaAs GAMMA - electron confined states and the coupling parameter of the GAMMA and X(Z) valleys can be deduced (4.2 meV on average). Similar experiments on DBQWs grown on 2-degrees off (001) vicinal surfaces show that the exciton lifetime remains close to that of nominal samples. Considering also former results obtained in type II GaAs/AlAs superlattices, it is concluded that the GAMMA-X potential in quantum structures is intrinsically linked to the AlAs/GaAs interface.
Notes: 3rd International Conference on Optics of Excitons in Confined Systems, UNIV MONTPELLIER II, MONTPELLIER, FRANCE, AUG 30-SEP 02, 1993
Abstract: In this paper we report an overview of some points that we believe mostly relevant to the exciton dynamics in quantum well structures. We will concentrate our attention to the intrinsic radiative lifetime of the free excitons and the effects that localization has on the recombination kinetics. It is shown that a good agreement is eventually found between the theoretical predictions and the experimental data, even if some key aspects for a quantitative comprehension of the recombination mechanisms, namely the effects of phonon and/or interface defect scattering, still lack a complete understanding.
Abstract: The nature of transport through superlattice minibands is addressed in this paper. All-optical techniques have been used to study the vertical transport, monitoring the luminescence from an enlarged well grown within the superlattice. Photoluminescence, photoluminescence excitation spectroscopy, and time-resolved luminescence-decay measurements have been performed as a function of temperature and excitation intensity. A consistent picture emerges from the experimental data, showing the key role played by the superlattice miniband widths and interface disorder on the vertical transport at low temperatures (4-40 K).
Abstract: A photoluminescence-excitation (PLE) study of the exciton binding energy in GaAs/AlxGa1-xAs quantum-well (QW) structures is reported. A line-shape fit of the PLE spectra, based on a correct evaluation of the absorption probability in QW's, is proposed as a Powerful tool in order to deduce accurately the exciton binding energies even in samples where the 2s peak is unresolved. The experimental results obtained are in good agreement with recent accurate theories. In particular, we find a strong dependence of the heavy-hole exciton binding energy E1 on the aluminum concentration of the AlxGa1-xAs barrier, in agreement with the predicted importance of the dielectric mismatch and conduction-band nonparabolicity in enhancing E1. Finally, evidence of exciton binding energies larger than the two-dimensional limit in GaAs is found even in relatively thick QW's (50 angstrom) with AlAs barriers.
Abstract: We report on continuous-wave and time-resolved measurements of the photoluminescence of good-quality single quantum wells at low temperatures (4-30 K). Luminescence arising from both free and localized excitons is observed and the influence of excitation localization on the photoluminescence decay time is investigated. Resonant pumping at the light-hole exciton transition is found to greatly increase the generation of free heavy-hole excitons. In this case, the free-exciton lifetime increases linearly with temperature (10-30 K) and the dependence of the slope on the well thickness is found to be in good quantitative agreement with the theoretical model of Andreani et al. [Solid State Commun. 77, 641 (1991)].
Abstract: Nonconventional GaAs/Al1-xGaxAs quantum-well structures, with one or two monolayers of AlAs inserted between the GaAs layers and the Al1-xGaxAs barriers (double-barrier quantum wells), have been investigated to observe the quasi-three-dimensional behavior of the excitons in thin quantum wells. These samples provide higher confinement energies than single quantum wells of the same thickness, in such a way that relatively thick wells produce shallow carrier subbands. An increase of the photoluminescence lifetime of excitons has been found when narrowing the well thickness. The analysis of intensity and shape of the photoluminescence spectra made us confident in interpreting this increase as an effect of the exciton delocalization, and a qualitative agreement with a simple model for the exciton radiative lifetime is obtained.
Abstract: Shallow impurities in n-type indium selenide (InSe) have been investigated by means of Fourier-transform spectroscopy in the far-infrared region. Three electric dipole transitions have been identified: 1s-2p+/-, 1s-2p0, and 1s-3p+/-, corresponding to electrons bound to native donors and tin-, silicon-, and chlorine-related donors, whose ionization energies (17.6, 18.1, 18.8, and 19 meV, respectively) have been determined through the Guerlach-Pollmann model. That model was also used to calculate the oscillator strengths of those dipole transitions, and then to estimate the shallow-donor concentrations in each sample. Native donors tum out to be the most hydrogenic ones, and the energies of their related transitions are used to determine a more accurate value of the low-frequency dielectric-constant product. An absorption line is observed in the low-energy side of the spectra (80 cm-1) for samples with a donor concentration higher than 10(15) cm-3, which is attributed to impurity pairing. A mechanism is proposed to explain the large value of the full width at half maximum associated with the 1s-2p+/- absorption line (almost-equal-to 6.6 cm-1 in the purest samples): Longitudinal-acoustic phonons polarized parallel to the c axis create dielectric-constant waves that modulate the dipole transition energies of shallow donors. In highly doped samples, compensating acceptors give rise to internal electric fields that largely broaden the absorption lines.
Abstract: We present a study of the dependence of the hole-spin relaxation on the electron density in an n-modulation-doped 75-angstrom GaAs/AlxGa1-xAs quantum well by using cw and time-resolved photo-luminescence techniques, at low temperature. The electron concentration has been continuously varied from 10(11) to 10(12) cm-2. A slow hole-spin relaxation time has been found (almost-equal-to 1 ns). A polarization decrease has also been observed when the in-plane wave vector of the photocreated holes increases. Calculations are presented which qualitatively support the latter experimental findings.
Abstract: Low temperature mobility of 2-D electrons in indium selenide is calculated, taking into account neutral and ionized impurity scattering. Two-dimensional electric subbands are originated due to quantum size effects, at both sides of thin epsilon-polytype layers, separated by two stacking faults from the bulk gamma-InSe. Ionized impurities are in the epsilon-layer and then, spatially separated from 2-D electrons. Neutral impurities are adsorbed to stacking faults in the epsilon-gamma interface. A relaxation time for dipole-like neutral impurity scattering is deduced. Calculated mobilities are compared to previous experimental results and the areal concentration of the neutral impurities is so estimated. The increase of the Hall mobility above 10 K is attributed to the presence of a low concentration of higher mobility 3-D electrons that are excited from shallow donors in the gamma-regions.
Abstract: Low temperature mobility of 2-D electrons in indium selenide is calculated, taking into account neutral and ionized impurity scattering. Two-dimensional electric subbands are originated due to quantum size effects, at both sides of thin epsilon-polytype layers, separated by two stacking faults from the bulk gamma-InSe. Ionized impurities are in the epsilon-layer and then, spatially separated from 2-D electrons. Neutral impurities are adsorbed to stacking faults in the epsilon-gamma interface. A relaxation time for dipole-like neutral impurity scattering is deduced. Calculated mobilities are compared to previous experimental results and the areal concentration of the neutral impurities is so estimated. The increase of the Hall mobility above 10 K is attributed to the presence of a low concentration of higher mobility 3-D electrons that are excited from shallow donors in the gamma-regions.
Abstract: We present a study of the dependence of the hole-spin relaxation on the electron density in an n-modulation-doped 75-angstrom GaAs/AlxGa1-xAs quantum well by using cw and time-resolved photo-luminescence techniques, at low temperature. The electron concentration has been continuously varied from 10(11) to 10(12) cm-2. A slow hole-spin relaxation time has been found (almost-equal-to 1 ns). A polarization decrease has also been observed when the in-plane wave vector of the photocreated holes increases. Calculations are presented which qualitatively support the latter experimental findings.
Abstract: Nonradiative recombination processes in thin GaAs/AlxGa1-xAs quantum-well (QW) and double-barrier quantum-well structures have been investigated by means of continuous-wave and time-resolved photoluminescence (PL) measurements. We find that, due to the temperature dependence of the radiative time constant, the Arrhenius plot of the PL intensity cannot be used to extracting the activation energy of the nonradiative channels. In fact it is the temperature dependence of the PL decay time T(L) which directly gives information on the thermal activation of the loss mechanism. The activation energies obtained from the Arrhenius plot of T(L) demonstrate that, at least in the case of thin wells, the main nonradiative mechanism is the thermal escape of the less-confined species of carriers.
Abstract: Shallow impurities in n-type indium selenide (InSe) have been investigated by means of Fourier-transform spectroscopy in the far-infrared region. Three electric dipole transitions have been identified: 1s-2p+/-, 1s-2p0, and 1s-3p+/-, corresponding to electrons bound to native donors and tin-, silicon-, and chlorine-related donors, whose ionization energies (17.6, 18.1, 18.8, and 19 meV, respectively) have been determined through the Guerlach-Pollmann model. That model was also used to calculate the oscillator strengths of those dipole transitions, and then to estimate the shallow-donor concentrations in each sample. Native donors tum out to be the most hydrogenic ones, and the energies of their related transitions are used to determine a more accurate value of the low-frequency dielectric-constant product. An absorption line is observed in the low-energy side of the spectra (80 cm-1) for samples with a donor concentration higher than 10(15) cm-3, which is attributed to impurity pairing. A mechanism is proposed to explain the large value of the full width at half maximum associated with the 1s-2p+/- absorption line (almost-equal-to 6.6 cm-1 in the purest samples): Longitudinal-acoustic phonons polarized parallel to the c axis create dielectric-constant waves that modulate the dipole transition energies of shallow donors. In highly doped samples, compensating acceptors give rise to internal electric fields that largely broaden the absorption lines.
Abstract: Nonconventional GaAs/Al1-xGaxAs quantum-well structures, with one or two monolayers of AlAs inserted between the GaAs layers and the Al1-xGaxAs barriers (double-barrier quantum wells), have been investigated to observe the quasi-three-dimensional behavior of the excitons in thin quantum wells. These samples provide higher confinement energies than single quantum wells of the same thickness, in such a way that relatively thick wells produce shallow carrier subbands. An increase of the photoluminescence lifetime of excitons has been found when narrowing the well thickness. The analysis of intensity and shape of the photoluminescence spectra made us confident in interpreting this increase as an effect of the exciton delocalization, and a qualitative agreement with a simple model for the exciton radiative lifetime is obtained.
Abstract: Nonradiative recombination processes in thin GaAs/AlxGa1-xAs quantum-well (QW) and double-barrier quantum-well structures have been investigated by means of continuous-wave and time-resolved photoluminescence (PL) measurements. We find that, due to the temperature dependence of the radiative time constant, the Arrhenius plot of the PL intensity cannot be used to extracting the activation energy of the nonradiative channels. In fact it is the temperature dependence of the PL decay time T(L) which directly gives information on the thermal activation of the loss mechanism. The activation energies obtained from the Arrhenius plot of T(L) demonstrate that, at least in the case of thin wells, the main nonradiative mechanism is the thermal escape of the less-confined species of carriers.
Abstract: Electron-scattering mechanisms in n-type indium selenide doped with different amounts of tin are studied by means of the Hall effect (30-300 K) and photo-Hall effect (300 K). The electron mobility at room temperature is found to increase with the free-electron concentration in samples with low tin content. The same behavior is observed when the electron concentration increases due to thermal annealing or photogeneration. That is explained through the presence of two kinds of free electrons contributing to the charge transport along the layers: high-mobility three-dimensional (3D) electrons in the conduction band, and low-mobility two-dimensional electrons in the electric subbands. These 2D subbands are proposed to exist in InSe due to size-quantization effects in thin layers located between two stacking faults. In these regions electron states become higher than conduction-band states. Electrons are transferred outside these regions and are confined in 2D subbands by the resulting electric field. In regard to the electron-scattering mechanisms, it is shown that LO polar phonons play an important role in the scattering of 3D electrons, whose mobility has been calculated by an iteration method. From a comparison with experimental results, we show that the coupling constant for the 3D electron-phonon deformation-potential interaction has been overestimated in previous calculations. The relaxation time for the scattering of 2D electrons by homopolar phonons is determined by using a variational wave function, and the calculated 2D mobility decreases when the localization of the 2D subbands along the c axis increases.
Abstract: Electron-scattering mechanisms in n-type indium selenide doped with different amounts of tin are studied by means of the Hall effect (30-300 K) and photo-Hall effect (300 K). The electron mobility at room temperature is found to increase with the free-electron concentration in samples with low tin content. The same behavior is observed when the electron concentration increases due to thermal annealing or photogeneration. That is explained through the presence of two kinds of free electrons contributing to the charge transport along the layers: high-mobility three-dimensional (3D) electrons in the conduction band, and low-mobility two-dimensional electrons in the electric subbands. These 2D subbands are proposed to exist in InSe due to size-quantization effects in thin layers located between two stacking faults. In these regions electron states become higher than conduction-band states. Electrons are transferred outside these regions and are confined in 2D subbands by the resulting electric field. In regard to the electron-scattering mechanisms, it is shown that LO polar phonons play an important role in the scattering of 3D electrons, whose mobility has been calculated by an iteration method. From a comparison with experimental results, we show that the coupling constant for the 3D electron-phonon deformation-potential interaction has been overestimated in previous calculations. The relaxation time for the scattering of 2D electrons by homopolar phonons is determined by using a variational wave function, and the calculated 2D mobility decreases when the localization of the 2D subbands along the c axis increases.
