Abstract: Epitaxially grown, high quality semiconductor InSb nanowires are emerging material systems for the development of high performance nanoelectronics and quantum information processing and communication devices and for the studies of new physical phenomena in solid state systems. Here, we report on measurements of a superconductor-normal conductor-superconductor junction device fabricated from an InSb nanowire with aluminum-based superconducting contacts. The measurements show a proximity-induced supercurrent flowing through the InSb nanowire segment with a critical current tunable by a gate in the current bias configuration and multiple Andreev reflection characteristics in the voltage bias configuration. The temperature dependence and the magnetic field dependence of the critical current and the multiple Andreev reflection characteristics of the junction are also studied. Furthermore, we extract the excess current from the measurements and study its temperature and magnetic field dependences. The successful observation of the superconductivity in the InSb nanowire-based Josephson junction device indicates that InSb nanowires provide an excellent material system for creating and observing novel physical phenomena such as Majorana fermions in solid-state systems.
Abstract: III-V antimonide nanowires are among the most interesting semiconductors for transport physics, nanoelectronics and long-wavelength optoelectronic devices due to their optimal material properties. In order to investigate their complex crystal structure evolution, faceting and composition, we report a combined scanning electron microscopy (SEM), transmission electron microscopy (TEM), and scanning tunneling microscopy (STM) study of gold-nucleated ternary InAs/InAs1-xSbx nanowire heterostructures grown by molecular beam epitaxy. SEM showed the general morphology and faceting, TEM revealed the internal crystal structure and ternary compositions, while STM was successfully applied to characterize the oxide-free nanowire sidewalls, in terms of nanofaceting morphology, atomic structure and surface composition. The complementary use of these techniques allows for correlation of the morphological and structural properties of the nanowires with the amount of Sb incorporated during growth. The addition of even a minute amount of Sb to InAs changes the crystal structure from perfect wurtzite to perfect zinc blende, via intermediate stacking fault and pseudo-periodic twinning regimes. Moreover, the addition of Sb during the axial growth of InAs/InAs1-xSbx heterostructure nanowires causes a significant conformal lateral overgrowth on both segments, leading to the spontaneous formation of a core-shell structure, with an Sb-rich shell.
Abstract: We report and detail a method to achieve growth of vertical self-catalyzed GaAs nanowires directly on Si(111) with a near-perfect vertical yield, using electron-beam-defined arrays of holes in a dielectric layer and molecular beam epitaxy. In our conditions, GaAs nanowires are grown along a vapor-liquid-solid mechanism, using in situ self-forming Ga droplets. The focus of this paper is to understand the role of the substrate preparation and of the pre-growth conditioning. Without changing temperature or the V/III ratio, the yield of vertical nanowires is increased incrementally up to 95%. The possibility to achieve very dense arrays, with center-to-center inter-wire distances less than 100 nm, is demonstrated.
Abstract: The authors present fabrication and electrical measurements of InSb nanowire field-effect transistors (FETs) and quantum dots. The devices are made on a SiO(2)-capped Si substrate from InSb segments of InAs/InSb heterostructured nanowires, which are grown by metalorganic vapor phase epitaxy. For the FETs, both single- and dual-gate devices are fabricated. The Si substrate is employed as the back gate in both the single-and dual-gate devices, while a top metal gate is employed as a second gate in the dual-gate devices. This top gate is made either as a global gate or as a local finger gate by using a thin HfO(2) layer grown by atomic layer deposition as a gate dielectric. The measurements reveal that the fabricated devices show the desired transistor characteristics. The measurements also demonstrate the possibility of realizing ambipolar transistors using InSb nanowires. For InSb nanowire quantum dots, both contact-induced Schottky-barrier-defined devices and top-finger-gate-defined devices are fabricated, and the Si substrate is used as a gate to tune the electron number in the quantum dots. The electrical measurements of these fabricated quantum-dot devices show the Coulomb-blockade effect at 4.2 K. A Fabry-Perot-like interference effect is also observed in a Schottky-barrier-defined quantum device. The authors also discuss in a comparative way, the results of measurements for the InSb nanowire devices made by different fabrication technologies employed in this study.
Abstract: III-V nanowires (NWs) are promising for a wide range of applications, ranging from optics to electronics, energy, and biological sensing. The structural quality of NWs is of paramount importance for the performance of such future NW-based devices. Random structural defects and polytypism occur naturally in semiconductor NWs, but progress both on the theoretical understanding and experimental control have been achieved recently. Here, we review progress towards the realization of perfect wurtzite and zinc-blende phases in III-VNWs, eventually leading to true phase engineering in single NWs.
Abstract: We present temperature dependent electrical measurements on n-type InAs/InSb nanowire heterostructure field-effect transistors. The barrier height of the heterostructure junction is determined to be 220 meV, indicating a broken bandgap alignment. A clear asymmetry is observed when applying a bias to either the InAs or the InSb side of the junction. Impact ionization and band-to-band tunneling is more pronounced when the large voltage drop occurs in the narrow bandgap InSb segment. For small negative gate-voltages, the InSb segment can be tuned toward p-type conduction, which induces a strong band-to-band tunneling across the heterostructucture junction. (c) 2011 American Institute of Physics. [doi: 10.1063/1.3633742]
Abstract: Crystal structure and defects have been shown to have a strong impact on III-V nanowire properties. Recently, it was demonstrated that the issue of random stacking and polytypism in semiconductor nanowires can often be controlled using accessible growth parameters (such as temperature, diameter, and V/III ratio). In addition, it has been shown that crystal phase can be tuned selectively between cubic zinc blende and hexagonal wurtzite within individual nanowires of III-V materials such as InAs. In order for such results to be generally applied to different growth setups, it is necessary to fully explore and understand the trends governing crystal phase dependencies on all accessible growth parameters, including how they relate to each other. In this study, the authors have systematically investigated the influence of temperature, diameter, V/III ratio, and total mass flow on the crystal structure of InAs nanowires grown by metal-organic vapor phase epitaxy over a broad parameter range. The authors observed that each of these accessible parameters can affect the resulting crystal structure, and that the trends for each parameter are affected by the magnitude of the others. The authors also noted that most of the parameter dependencies are nonlinear and, in fact, exhibit threshold values at which structure changes discontinuously. By optimizing each of the growth parameters, it is shown that pure ZB or pure WZ phase can be achieved for several different sets of growth conditions. The roles of nucleation kinetics, thermodynamics, and precursor chemistry are also discussed to compare the results to current nanowire growth models. The results in this work should facilitate comparison of data and transfer of knowledge between different growth systems and techniques, which, in turn, should lead to greater understanding of polytypism in nanowires and greater control and freedom in nanowire crystal phase engineering. (C) 2011 American Vacuum Society. [DOI: 10.1116/1.3593457]
Abstract: We report a systematic study of the relationship between crystal quality and electrical properties of InAs nanowires grown by MOVPE and MBE, with crystal structure varying from wurtzite to zinc blende. We find that mixtures of these phases can exhibit up to 2 orders of magnitude higher resistivity than single-phase nanowires, with a temperature-activated transport mechanism. However, it is also found that defects in the form of stacking faults and twin planes do not significantly affect the resistivity. These findings are important for nanowire-based devices, where uncontrolled formation of particular polytype mixtures may lead to unacceptable device variability.
Abstract: Crystal phase control in single III-V semiconductor nanowires has emerged recently as an important challenge and possible complement to conventional bandgap engineering in single material systems. Here we investigate a supply interruption method for precise crystal phase control in single nanowires. The nanowires are grown by metalorganic vapor phase epitaxy using gold particles as seeds and are analyzed by transmission electron microscopy. It is observed that wurtzite segments with controlled length and position can be inserted on demand into a pure InAs zincblende nanowire. The interface between wurtzite and zincblende segments can be made atomically sharp and the segments can be made only a few bilayers in thickness. The growth mechanisms, applicability and limitations of the technique are presented and discussed.
Abstract: The thermal conductivity of wurtzite and zinc blende indium arsenide nanowires was measured using a microfabricated device, with the crystal structure of each sample controlled during growth and determined by transmission electron microscopy. Nanowires of both phases showed a reduction of the thermal conductivity by a factor of 2 or more compared to values reported for zinc blende indium arsenide bulk crystals within the measured temperature range. Theoretical models were developed to analyze the measurement results and determine the effect of phase on phonon transport. Branch-specific phonon dispersion data within the discretized first Brillouin zone were calculated from first principles and used in numerical models of volumetric heat capacity and thermal conductivity. The combined results of the experimental and theoretical studies suggest that wurtzite indium arsenide possesses similar volumetric heat capacity, weighted average group velocity, weighted average phonon-phonon scattering mean free path, and anharmonic scattering-limited thermal conductivity as the zinc blende phase. Hence, we attribute the differing thermal conductivity values observed in the indium arsenide nanowires of different phases to differences in the surface scattering mean free paths between the nanowire samples.
Abstract: The atomic distances in hexagonal polytypes of III-V compound semiconductors differ from the values expected from simply a change of the stacking sequence of (111) lattice planes. While these changes were difficult to quantify so far, we accurately determine the lattice parameters of zinc blende, wurtzite, and 4H polytypes for InAs and InSb nanowires, using X-ray diffraction and transmission electron microscopy. The results are compared to density functional theory calculations. Experiment and theory show that the occurrence of hexagonal bilayers tend to strech the distances of atomic layers parallel to the c-axis and to reduce the in-plane distances compared to those in zinc blende. The change of the lattice parameters scales linearly with the hexagonality of the polytype, defined as the fraction of bilayers with hexagonal character within one unit cell.
Abstract: InAs/HfO(2) nanowire capacitors using capacitance-voltage (CV) measurements are investigated in the range of 10 kHz to 10 MHz. The capacitors are based on vertical nanowire arrays that are coated with an 8 nm-thick HfO(2) layer by atomic layer deposition. CV characteristics are measured at temperatures in the range between -140 and 40 degrees C and the CV characteristics for nanowires with different Sn and Se n-type doping levels are compared. The comparison of the data at various doping levels points towards large number of traps for highly doped samples, caused by the preferential dopant precursor incorporation at the nanowire surface. We also evaluate the frequency dispersion of the accumulation capacitance and determine values below 2% with weak temperature dependence, indicating the existence of border traps in these nanowire capacitors. (C) 2010 Elsevier B.V. All rights reserved.
Abstract: We demonstrate that the crystal structure of InAs nanowires call be controlled with nanowire diameter and growth temperature. At small diameters, the nanowires exhibit a wurtzite Structure. As the diameter is increased, there is a crossover to the zinc blende structure. The crossover is less sharp at lower growth temperature and L, the crossover diameter decreases as the growth temperature is increased. We explain these results with classical nucleation theory. The strong diameter dependence is accounted for by including the Gibbs-Thomson effect in the chemical potential.
Abstract: In this work we investigate the variation of the crystal structure of gold-seeded III-V nanowires with growth parameters, in order to gain a cohesive understanding of these effects. We investigate six III-V materials: GaAs, InAs, GaP, InP, GaSb and InSb, over a variation of growth conditions. All six of these materials exhibit a cubic zinc blende structure in bulk, but twin planes and stacking faults, as well as a hexagonal wurtzite structure, are commonly observed in nanowires. Parameters which may affect the crystal structure include growth temperature and pressure, precursor molar fraction and V/III ratio, nanowire diameter and surface density, and impurity atoms. We will focus on temperature, precursor molar fraction and V/III ratio. Our observations are compared to previous reports in the literature of the III-V nanowire crystal structure, and interpreted in terms of existing models. We propose that changes in the crystal structure with growth parameters are directly related to changes in the stable side facets.
Abstract: The large, level-dependent g factors in an InSb nanowire quantum dot allow for the occurrence of a variety of level crossings in the dot. While we observe the standard conductance enhancement in the Coulomb blockade region for aligned levels with different spins due to the Kondo effect, a vanishing of the conductance is found at the alignment of levels with equal spins. This conductance suppression appears as a canyon cutting through the web of direct tunneling lines and an enclosed Coulomb blockade region. In the center of the Coulomb blockade region, we observe the predicted correlation-induced resonance. Our findings are supported by numerical and analytical calculations.
Abstract: We present temperature dependent electrical measurements on InSb and InAs nanowire field-effect transistors (FETs). The FETs are fabricated from InAs/InSb heterostructure nanowires, where one complete transistor is defined within each of the two segments. Both the InSb and the InAs FETs are n-type with good current saturation and low voltage operation. The off-current for the InSb FET shows a strong temperature dependence, which we attribute to a barrier lowering due to an increased band-to-band tunneling in the drain part of the channel.
Abstract: Growth of GaAs/GaAsSb heterostructure nanowires on silicon without the need for gold seed particles is presented. A high vertical yield of GaAs nanowires is first obtained, and then GaAs(x)Sb(1-x) segments are successfully grown axially in these nanowires. GaAsSb can also be integrated as a shell around the GaAs core. Finally, two GaAsSb segments are grown inside a GaAs nanowire and passivated using an Al(x)Ga(1-x)As shell. It is found that no stacking faults or twin planes occur in the GaAsSb segments.
Abstract: The electrical and structural properties of < 111 > B-oriented InAs nanowires grown using metal-organic precursors have been studied. On the basis of electrical measurements it was found that the trends in carbon incorporation are similar to those observed in the layer growth, where an increased As/In precursor ratio and growth temperature result in a decrease in carbon-related impurities. Our results also show that the effect of non-intentional carbon doping is weaker in InAs nanowires compared to bulk, which may be explained by lower carbon incorporation in the nanowire core. We determine that differences in crystal quality, here quantified as the stacking fault density, are not the primary cause for variations in resistivity of the material studied. The effects of some n-dopant precursors (S, Se, Si, Sn) on InAs nanowire morphology, crystal structure and resistivity were also investigated. All precursors result in n-doped nanowires, but high precursor flows of Si and Sn also lead to enhanced radial overgrowth. Use of the Se precursor increases the stacking fault density in wurtzite nanowires, ultimately at high flows leading to a zinc blende crystal structure with strong overgrowth and very low resistivity.
Abstract: Achieving phase purity and control in III-V nanowires is a necessity for future nanowire-based device applications. Many works have focused on cleaning specific crystal phases of defects such as twin planes and stacking Faults, using parameters such as diameter, temperature, and impurity incorporation. Here we demonstrate an improved method for crystal phase control, where crystal structure variations in single InAs nanowires are designed with alternating wurtzite (WZ) and zinc blende (ZB) segments of precisely controlled length and perfect interfaces. We also demonstrate the inclusion of single twin planes and stacking faults with atomic precision in their placement, designed ZB quantum dots separated by thin segments of WZ, acting as tunnel barriers for electrons, and structural superlattices (polytypic and twin plane). Finally, we present electrical data to demonstrate the applicability of these designed structures to investigation of Fundamental properties. From electrical measurements we observe clear signatures of controlled structural quantum dots in nanowires. This method will be directly applicable to a wide range of nanowire systems.
Abstract: We report growth by molecular beam epitaxy and structural characterization of gallium-nucleated GaAs nanowires on silicon. The influences of growth temperature and V/III ratio are investigated and compared in the case of oxide-covered and oxide-free substrates. We demonstrate a precise positioning process for Ga-nucleated GaAs nanowires using a hole array in a dielectric layer thermally grown on silicon. Crystal quality is analyzed by high resolution transmission electron microscopy. Crystal structure evolves from pure zinc blende to pure wurtzite along a single nanowire, with a transition region.
Abstract: Sn and Se doped InAs nanowires are characterized using a capacitance-voltage technique where the threshold voltages of nanowire capacitors with different diameter are determined and analyzed using an improved radial metal-insulator-semiconductor field-effect transistor model. This allows for a separation of doping in the core of the nanowire from the surface charge at the side facets of the nanowire. The data show that the doping level in the InAs nanowire can be controlled on the level between 2 X 10(18) to 1 X 10(19) cm(-3), while the surface charge density exceeds 5 X 10(12) cm(-2) and is shown to increase with higher dopant precursor molar fraction. (C) 2010 American Institute of Physics. [doi:10.1063/1.3475356]
Abstract: We present a growth study and structural characterization of InP-InSb nanowire heterostructures. In contrast to planar epitaxy, this heterostructure can be realized in nanowires without the formation of dislocations, despite an extreme lattice-mismatch (10.4%). We obtain high crystal quality in the InSb nanowires, confirmed by a narrow 111 reflection peak measured by XRD. Additionally, the diameter dependence of the nanowire growth rate was investigated. An original competition between surface growth and nanowire growth is found, which can be controlled by varying the nanowire surface coverage. Finally, HRTEM and X-EDS investigations reveal that the InSb nanowire is always defect-free zinc-blende, and that the InP-InSb heterointerface is free from misfit dislocations, although single twin planes are common.
Abstract: A wavelet generator producing 100 ps short pulses at 60 GHz is presented. The wavelet generator consists of a gated tunnel diode (GTD) integrated in parallel with an inductor. This forms a negative differential conductance (NDC) oscillator with the ability to switch the NDC property on and off, which makes it possible to generate short pulses. In the experiments described, the wavelet generator drives a 50 Omega load and delivers 206 mV(pp) when generating 97 ps short pulses at 60 GHz. It Is demonstrated that it is possible to generate pulses of different length and phase. An explanation of the almost instantaneously startup and decay lapse of the oscillator, including generation of signals with opposite phase, is presented. This novel circuit may find use in ultra-wideband impulse radio communication.
Abstract: We investigated the brain-tissue response to nanowire implantations in the rat striatum after 1, 6, and 12 weeks using immunohistochemistry. The nanowires could be visualized in the scar by confocal microscopy (through the scattered laser light). For the nanowire-implanted animals, there is a significant astrocyte response at week 1 compared to controls. The nanowires are phagocytized by ED1 positive microglia, and some of them are degraded and/or transported away from the brain.
Abstract: We demonstrate the growth of InSb-based nanowire heterostructures by metalorganic vapour phase epitaxy and use it to integrate InSb on extremely lattice-mismatched III-V nanowire templates made of InAs, InP, and GaAs. Influence of temperature, V/III ratio, and diameter are investigated in order to investigate the growth rate and morphology. The range of growth temperatures used for InSb nanowire growth is very similar to that used for planar growth due to the nature of the precursor decomposition. This makes optimization of growth parameters very important, and more difficult than for most other nanowire III-V materials. Analysis of the InSb nanowire epitaxial quality when grown on InAs, InP, and GaAs, along with InSb segment and particle compositions are reported. This successful direct integration of InSb nanowires, on nanowire templates with unprecedented strain levels show great promise for fabrication of vertical InSb devices.
Abstract: We report on magnetotransport measurements on InSb nanowire quantum dots. The measurements show that the quantum levels of the InSb quantum dots have giant g factors, with absolute values up to similar to 70, the largest value ever reported for semiconductor quantum dots. We also observe that the values of these g factors are quantum level dependent and can differ strongly between different quantum levels. The presence of giant g factors indicates that considerable contributions from the orbital motion of electrons are preserved in the measured InSb nanowire quantum dots, while the level-to-level fluctuations arise from spin-orbit interaction. We have deduced a value of Delta(so) = 280 mu eV for the strength of spin-orbit interaction from an avoided level crossing between the ground state and first excited state of an InSb nanowire quantum dot with a fixed number of electrons.
Abstract: We investigate the Au-assisted growth of InAs nanowires on two different kinds of heterostructured substrates: GaAs/AlGaAs structures capped by a 50 nm thick InAs layer grown by molecular beam epitaxy and a 2 mu m thick InAs buffer layer on Si(111) obtained by vapor phase epitaxy. Morphological and structural properties of substrates and nanowires are analyzed by atomic force and transmission electron microscopy. Our results indicate a promising direction for the integration of III-V nanostructures on Si-based electronics as well as for the development of novel micromechanical structures incorporating nanowires as their active elements.
Abstract: We report on the critical thickness for InAs quantum dot (QD) formation on (311)B InP substrates. Firstly, critical thicknesses for InAs QD formation on InP surfaces have been measured by reflection high-energy electron diffraction. Large change of the critical thickness has been observed as a function of substrate temperature. We assume that is related to large As/P exchange on InP surface which leads to the formation of extra InAs on surface. Then, change of critical thickness during QD stacking has been investigated. When capping layers were grown continuously a large decrease of the critical thickness was observed as a function of the number of QD layers. In contrast, when capping layers were grown in two steps (double cap procedure) a nearly constant critical thickness was measured. We propose an explanation based on stress-driven mass transport and As/P exchange on InP surface to interpret such results. (C) 2009 Elsevier B.V. All rights reserved.
Abstract: Semiconductor nanowires show promise for use in nanoelectronics, fundamental electron transport studies, quantum optics and biological sensing. Such applications require a high degree of nanowire growth control, right down to the atomic level. However, many binary semiconductor nanowires exhibit a high density of randomly distributed twin defects and stacking faults, which results in an uncontrolled, or polytypic, crystal structure. Here, we demonstrate full control of the crystal structure of InAs nanowires by varying nanowire diameter and growth temperature. By selectively tuning the crystal structure, we fabricate highly reproducible polytypic and twin-plane superlattices within single nanowires. In addition to reducing defect densities, this level of control could lead to bandgap engineering and novel electronic behaviour.
Abstract: We report Au-assisted growth of GaAs/GaSb nanowire heterostructures on GaAs(1 1 1)B-substrates by metal-organic vapor phase epitaxy. The growth is studied at various precursor molar fractions and temperatures, in order to optimize the growth conditions for the GaSb nanowire segment. In contrast to most other III-V nanowire systems, the GaSb nanowire growth is Group V-limited under most conditions. We found that depending on the TMSb molar fraction, the seed particle is either supersaturated AuGa or AuGa2 during GaSb growth. The high Ga content in the particle gives a characteristic diameter increase between the GaAs and GaSb segment. From TEM and XEDS measurements we conclude that the GaSb nanowire growth occurs along either the AuGa-GaSb or AuGa2-GaSb pseudo-binaries of the Au-Ga-Sb ternary phase diagram. Finally, the GaSb nanowires exhibit untapered radial growth on the {1 (1) over bar 0} side facets. (C) 2008 Elsevier B.V. All rights reserved.
Abstract: We demonstrate GaSb overgrowth over tungsten patterns and that selective area epitaxy is achievable in the W/GaSb system. By controlling the facet growth at low temperatures, it is possible to embed a metal grating in a thin layer.
Abstract: In this paper, we report on the development of a vertical wrap-gated field-effect transistor based on epitaxially grown InAs nanowires. We discuss some of the important steps involved in the growth and processing, such as nanowire position control in situ doping, high-kappa dielectric deposition, spacer layer formation: and metal wrap-gate fabrication. In particular, we compare a few alternative methods for deposition of materials onto vertical structures and discuss their potential advantages and limitations. Finally, we also present a comparison of transistor performance for nanowires grown using two different epitaxial techniques.
Abstract: We report the growth and characterization of GaSb nanowires grown by MOVPE. The structural properties of the nanowires are investigated by the means of transmission electron microscopy, X-ray diffraction and single nanowire photoluminescence. The measurements confirm a high material quality in the GaSb nanowires. Also, a back-gated nanowire transistor structure is used to extract values for the polarity and resistivity of the GaSb. Finally, a simple kinetic model is presented to explain the non-linear time dependence of the GaSb nanowire growth. (C) 2008 Elsevier B.V. All rights reserved.
Abstract: We report on long-wavelength photoluminescence (PL) emission at room temperature from self-organized InAs surface quantum dots grown by gas-source molecular beam epitaxy on a GaInAsP/InP (113)B substrate. The influence of arsenic pressure conditions during growth on the PL emission of surface quantum dots is detailed as well as oxide/contamination layer formation after growth. Experimental results are in good agreement with six-band k.p theory in the envelope function approximation. (C) 2008 American Institute of Physics.
Abstract: We report the successful growth of high quality InAs films directly on Si( 111) by Metal Organic Vapor Phase Epitaxy. A nearly mirror-like and uniform InAs film is obtained at 580 C for a thickness of 2 mu m. We measured a high value of the electron mobility of 5100 cm(2)/Vs at room temperature. The growth is performed using a standard two-step procedure. The influence of the nucleation layer, group V flow rate, and layer thickness on the electrical and morphological properties of the InAs film have been investigated. We present results of our studies by Atomic Force Microscopy, Scanning Electron Microscopy, electrical Hall/van der Pauw and structural X-Ray Diffraction characterization.
Abstract: We studied the dynamic response of lnAs/lnP quanturn dot transitions with a resonant pump-probe experiment. A 72-stacked InAs/InP quantum dot layer sample is grown on (311)B substrate. The variation of carrier radiative lifetimes with increasing excitation powers is measured and is attributed to the exciton and biexciton lifetime. The implications of such a difference on differential transmission are discussed, and fmally exciton and biexciton lifetimes are measured to be about 1720 ps and 530 ps respectively.
Abstract: We propose a new type of long-wavelength vertical cavity surface emitting laser (VCSEL) which consists of quantum wires (QWires) layers of InAs/InGaAsP grown on InP(001) and dielectrics Bragg mirrors, in order to control the in plane polarization of output power. QWires and quantum wells growth are performed by molecular beam epitaxy. QWires present a strong photoluminescence dependence to the polarization in contrast to the quantum wells, a polarization rate of 33% is measured. The optically pumped VCSEL is fabricated by metallic bonding, which allows the deposition of two dielectrics Bragg mirrors. The VCSEL with an active region based on InGaAs/InGaAsP quantum wells exhibits a lasing emission at 1.578 mu m at room temperature under continuous wave operation. The VCSEL with an active region based on quantum wires shows a luminescence at 1.53 mu m strongly polarized along the direction [101] which is promising for the stabilization of in plane polarization of VCSEL emission. (c) 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
Abstract: Two samples of 12 and 72 stacks of InAs/InP quantum dots were grown by molecular beam epitaxy to study the photoluminescence and the dynamical response by a pump-probe experimental set-up respectively. The carrier's lifetimes and differential transmission are performed in a function of the excitation density. A comparison of the differential transmission ratio for one quantum dot layer (InAs/InP) and one quantum well (InGaAs/InP) is also investigated. (c) 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
Abstract: Theoretical and experimental studies of the electronic properties of InAs(Sb) quantum dots (QDs) grown on InP substrate are presented. Unstrained bulk InAsSb present direct gap between 0.1 eV to 0.35 eV suitable for mid infrared emitters (2-5 mu m). However, strain and quantum confinement effects may limit the extension of the emission spectrum of these nanostructures towards the longer wavelengths. Various combinations of barrier materials are considered in the simulations. Photoluminescence (PL) spectroscopy experiments on molecular beam epitaxy (MBE) grown samples show promising results. We obtained PL peak beyond 2 mu m at room temperature (RT) with InAS/InGaAsP/InP(100) QDs High arsine flow rate during the growth of the QDs makes possible this long emission wavelength. Two ways are investigated to incorporate antimony in InAs/InGaAs/InP(100) nanostructures: growth interrupt of InAs QDs under Sb flux and direct growth of InAs(Sb). A red-shift of the PL peak from 1.85 mu m to 2 mu m is observed in the first case whereas emission wavelengths as long as 2.35 mu m are observed in second case. (c) 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
Abstract: We report the observation of enhanced charge-carrier redistribution in laterally organized and coupled InAs/InP quantum dots (QDs). We show that a periodic organization appears in the QD plane for a high in-plane QD density (QDD). This organization enhances the lateral coupling between the dots, which is evidenced by photoluminescence and magnetophotoluminescence experiments. Electronic inter-QD lateral coupling results in an improved charge-carrier distribution at low temperature, as shown by electroluminescence on high QDD QD lasers. We conclude that the inter-QD tunneling occurs via the tunneling of excited states through the wetting layer, and discuss the prospects of using coupled QDs for improving the quantum efficiency and dynamical properties of QD lasers.
Abstract: The formation of InAs quantum dots by Stransky-Krastanow method on (311)B InP substrates has been studied. On Al0.48In0.52As alloy lattice matched on InP, large changes of the quantum dot structural characteristics have been observed as a function of the amount of InAs deposited and of the arsenic pressure during the InAs quantum dot formation. Small quantum dots (minimum diameter = 20 nm) in very high density (1.3 x 10(11) quantum dots per cm(2)) have been achieved in optimized growth conditions. These results are interpreted from the strong strain field interaction through the substrate at high density and from the InAs surface energy evolutions with the Arsenic pressure. The effect on quantum dot characteristics of the arsenic pressure during the growth of Al0.48In0.52M buffer layers has also been investigated. Despite the importance of this parameter on the Al0.48In0.52As clustering, weak changes have been observed. (c) 2006 Elsevier B.V. All rights reserved.
Abstract: We have performed time-resolved resonant pump-probe experiment to study the dynamic response of InAs/InP quantum dot transitions. A 72-stacked InAs/InP quantum dot layer sample is grown on (311)B substrate. Photoluminescence at high excitation power reveals ground and excited transitions. Carrier radiative lifetimes and differential transmission are determined under strong excitation powers. The variation of measured carrier radiative lifetimes with increasing excitation powers is attributed to the exciton and biexciton lifetimes difference. The implications of such a difference on differential transmission are discussed, and finally exciton and biexciton lifetimes are measured to be about 1720 and 530 ps, respectively. (c) 2006 American Institute of Physics.
Abstract: Quantum dot (QD) lasers exhibit many interesting and useful properties such as low threshold current, temperature insensitivity or chirpless behavior. In order to reach the standards of long-haul optical transmissions, 1.55 mu m InAs QD lasers on InP substrate have been developed. Based on time resolved photoluminescence (PL) measurements, carrier dynamics behavior is at first investigated. Electroluminescence (EL) results are then shown at room temperature exhibiting a laser emission centered at 1.61 mu m associated to a threshold current density as low as 820 A/cm(2) for a six InAs QD stacked layers. Finally, a rate equation model based on the reservoir theory is used to model both time-resolved photoluminescence (TRPL) and electroluminescence results. It is shown that carrier dynamic calculations are in a good agreement with measurements since the saturation effect occurring at high injected power is clearly predicted.
Abstract: This paper presents the temperature dependent characteristics of a semiconductor laser with a single quantum dot (QD) layer on InP(311)B substrate. The laser emits in the optical telecommunication wavelength band near 1.55 mu m and shows a threshold current density of 6 A/cm(2) at low temperature, attesting excellent material quality. Laser spectra obtained under electrical injection for different temperatures show a drastic influence on their shape. A large spectral broadening is observed at low temperature, while it dramatically narrows close to room temperature. This is due to homogeneous broadening of each QD with increasing temperature, favouring a collective emission of the QDs and homogeneous gain. (c) 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinbeim.
Abstract: We present new results on the simulation of two-dimensional (2D) quantum dot(s) (QDs) InAs/InP superlattices, emitting at 1.55 mu m, the optical telecommunication wavelength. QDs and wetting layer (WL) electronic energy states are close in such a system. Using the Fourier-transformed Schrodinger equation developed on a mixed basis, we describe the wetting layer-assisted inter-QDs lateral (WLaiQD) coupling by studying the influence of WL states on QDs states and vice versa. The results show that WL and QDs have to be considered as a unique system, in strong coupling conditions. The increase of QDs density on the WL leads to enhanced splitting and miniband effects on QDs states. It induces a fragmentation of WL density of states interpreted as a 0D-like confinement of WL states. A comparison is made with a real high QDs density sample. It is expected to have an impact on carrier-capture phenomena in optoelectronic devices using high-density QDs in the active region.
Abstract: We present a modified growth procedure used to control the emission wavelength of InAs quantum dots in a GaInAsP matrix grown on InP(311)B substrates by gas source molecular beam epitaxy. In this procedure, the quantum dots are grown on a GaInAsP buffer layer and partially capped with the same GaInAsp alloy. Then As/P exchanges during a growth interruption are used in order to reduce quantum dots height and thus to control the emission wavelength. Two GaInAsP quaternary alloys composition have been studied in this work. Both could be used as optical waveguide in 1.55 μ m laser diodes. We present and compare photoluminescence results on test structures with these two quaternary alloys. Finally, the new growth procedure is successfully applied and we demonstrate a quantum dot laser emission at 1.58-1.61 μ m at 300 K. © 2005 Elsevier B.V. All rights reserved.
Abstract: We have used magnetophotoluminescence to study the impact of different capping layer material combinations (InP, GaInAsP quaternary alloy, or both InP and quaternary alloy) on lateral confinement in InAs/InP quantum dots (QDs) grown on (311)B orientated substrates. Exciton effective masses, Bohr radii, and binding energies are measured for these samples. Conclusions regarding the strength of the lateral confinement in the different samples are supported by photoluminescence at high excitation power. Contrary to theoretical predictions, InAs QDs in quaternary alloy are found to have better confinement properties than InAs/InP QDs. This is attributed to a lack of lateral intermixing with the quaternary alloy, which is present when InP is used to (partially) cap the dots. The implications of the results for reducing the temperature sensitivity of QD lasers are discussed. (c) 2005 American Institute of Physics.
Abstract: InAs quantum-dot (QD) laser structures are grown on (113)B-oriented InP substrate by gas-source molecular-beam epitaxy. Following an optimized growth procedure, a high density of 1.1x10(11) cm(-2) of uniformly sized QDs is achieved. Broad-area lasers containing three stacked QD layers have been realized and tested. Laser emission on the ground-state transition (lambda=1.59 mu m) is obtained at room temperature (RT), at a threshold current density as low as 190 A/cm(2). Ground-state modal gain and transparency current density is measured to be 7 cm(-1) and 23 A/cm(2) per dot layer. Ground-state laser emission is also demonstrated from low temperature (100 K, J(th)=33 A/cm(2)) to high temperature (350 K), exhibiting an insensitive threshold in the [100, 170] K range, and a 55 K characteristic temperature at RT. (c) 2005 American Institute of Physics.
Abstract: InAs/InGaAsP/InP(I 13)B quantum-dots are studied as active mediums for laser structures emitting near 1.55 mu m under optical and electrical injection. In order to precisely tune the emission wavelength of QDs, the double cap growth procedure is used. Laser emission on the ground states is obtained under optical pumping at room temperature. On equivalent structures doped for electrical injection, laser emission is also observed at low temperatures up to 200 K. The difference between the optical and electrical pumping is ascribed to low carrier injection efficiency due to the presence of a 3 nm InP hole blocking barrier at each quantum dot layer which is inherent to the double cap growth procedure. Room temperature laser emission has been reached when the InP first cap layer is substituted by a quaternary GaInAsP (1.18 mu m gap) layer in the double cap growth procedure. The threshold current density of the new structure with QD capped only by quaternary is as low as 840 A cm(-2) at room temperature.
Abstract: We present the control of the emission wavelength of InAs quantum dots in a GaInAsP matrix grown on InP(311)B substrates by g-as source molecular beam epitaxy. By growing the capping layer in two steps, the control of the dot height and thus of the emission wavelength is achieved. The dot height is tuned using As/P exchange during a growth interrupt. We have studied the changes induced by the nature of the overpressure (As, P, As and P) during the growth interrupt. Photoluminescence spectra at room temperature show the effects of the different growth parameters on the peak energy and width. (C) 2004 Elsevier B.V. All rights reserved.
Abstract: The As flux effect on InAs quantum dots formed by gas source molecular beam epitaxy on lnP substrates, oriented following the (311)B crystallographic direction has been studied. Atomic force microscopy images show that the quantum dot (QD) density dramatically increases and quantum dot sizes decrease, when decreasing the As pressure. Moreover, the size dispersion is narrowed. Photoluminescence measurements on the high QD density samples is shifted to higher energy, toward the telecommunication important 1.55 mu m emission.
Abstract: The formation of InAs self-assembled islands on the InP(3 1 1)B surface in Stranski-Krastanow growth mode is investigated. First, we study the island nucleation on (1 0 0) and (3 1 1)B InP surfaces by means of low-temperature photoluminescence. We show that the InAs deposition at a substrate temperature of 480degreesC leads to formation of islands during the deposition on (3 1 1)B InP surface. On the other hand, morphological transformation occurs only during an annealing stage on (1 0 0) surfaces. Such a difference is related to the lower surface energy cost of island formation on InP(3 1 1)B. Then, we study the island characteristics as a function of substrate temperature and growth rate. Smaller islands are achieved at lower growth temperature. It should be related to a fast nucleation step, followed by a diffusion controlled island growth stage. Finally, we investigate the island stability by performing annealing after growth. The island height increases and density decreases as a function of the annealing time. This implies that Oswald ripening takes place and thus island arrays on (3 1 1)B surface do not correspond to a free energy minimum. We conclude that the island formation on InP(3 1 1)B is mainly governed by kinetic effects. (C) 2003 Elsevier B.V. All rights reserved.
Abstract: Room- and low-temperature magnetization measurements have been performed on magnetic tunnel junctions before patterning to evidence the presence of an indirect ferromagnetic exchange interaction between the hard and the soft ferromagnetic electrodes through the ZnS barrier. The magnetic junctions are composed of an artificial ferrimagnet CoFe/Ru/CoFe sandwich as a hard layer separated by a ZnS barrier from a soft CoFe/Fe soft layer as follows: Fe6 nmCu3 nm(CoFe)(1.8 nm)Ru-0.8 nm(CoFe)(3 nm)ZnSx nmCoFe1 nmFe4 nmCu1 nmRu3 nm. At room temperature, a large shift of about -25 Oe is observed in a magnetization minor loop that indicates the presence of a ferromagnetic interaction. The decrease of the amplitude of this shift at low temperature provides a signature of an indirect exchange coupling mediated by spin-polarized quantum tunneling. (C) 2003 American Institute of Physics.
