Zhiming M. Wang received his B.S. degree in applied physics from Qingdao University, Qingdao, China, in 1992, his M.S. degree in physics from Beijing University in Beijing, China, in 1995, and his Ph.D. in condensed matter physics from the Institute of Semiconductors at the Chinese Academy of Sciences in Beijing, China, in 1998. From 1998 to 2000 he was a Postdoctoral Fellow at the Paul-Drude Institute in Berlin, Germany. In 2000, he joined the Center for Semiconductor Physics in Nanostructures (C-SPIN) at the University of Arkansas in Fayetteville, Arkansas, one of 29 elite Materials Research Science and Engineering Centers in the U.S. His current research interests include low-dimensional semiconductor nanostructures, ferroelectric and ferromagnetic nanostructures, and nanomaterial based biosensors.
Abstract: We have investigated the steady-state and transient optical properties of InGaAs/GaAs quantum chains and found that the photoluminescence (PL) decay time exhibits a strong photon energy dependence. It increases with the decrease of the emission energy. It is also found that the PL decay time increases with the excitation power. When the excitation power is large enough the PL decay time tends to be saturated. All these experimental results show that there is a strong carrier coupling along the chain direction in the quantum dot chain structure. The polarization PL measurements further confirm the carrier transfer process along the chain direction.
Abstract: We present a comparative study for the evolution of utilizing indium gallium (InGa) and aluminum gallium (AlGa) alloys fabricated on GaAs(100) by means of simultaneous and sequential droplet formation. The composite alloys reported using the sequential approach lack the ability to precisely determine the final alloy composition as well as consistency in the density of the droplets. Further, the composition of the InGa alloy is not uniform, as seen by the size distribution using an atomic force microscope (AFM). Although this approach may be acceptable for materials with similar surface kinetics, as in the case of AlGa, it is not acceptable for InGa. This investigation reveals that the simultaneous approach for fabricating composite alloys is the optimum approach for producing InGa alloys with better control on composition for plasmonic applications such as plasmonic waveguides.
Abstract: By utilizing the symmetrical temperature distribution in a tube furnace chamber, a capillary microreactor was designed with the microchannel passing two well-controlled, stable temperatures in steep temperature gradients. The two-temperature microreator, first developed and implemented by this research team, provides an opportunity to separate the nucleation and growth of semiconductor nanocrystals, leading to better control of nucleation and growth kinetics. For the synthesis of CdSe nanocrystals as a model system, we demonstrated the improved size uniformity achieved by the two-temperature approach, confirming the success of the use of high temperature to burst nucleation and low temperature to promote growth.
Abstract: The formation and evolution process of self-assembled InGaAs quantum dot molecules (QDMs) are studied in terms of configuration, volume, and types of QDMs. QDMs are formed around self-assembled GaAs nanoscale island induced by adapting a hybrid growth approach combining droplet homoepitaxy and Stranski-Krastanov mode. In distinction from our previous results [Lee , Appl. Phys. Lett. 89, 202101 (2006)], hexa-QDMs are fabricated without the formation of background QDs, which can be due to a combinational effects of enhanced intermixing of Ga and In atoms, enhanced surface diffusion (high mobility) of adatoms, and higher In desorption rate due to the higher thermal energy provided during the fabrication of QDMs. In addition, a detailed evolution mechanism from bi-QDMs (two QDs per each GaAs island) to hexa-QDMs (six QDs per island) is proposed based on atom diffusion, material transfer, and equilibrium dimension (saturation) of QDs. Under a fixed InAs coverage, depending on postannealing process after liquid Ga droplet formation, highly uniform as well as various types of QDMs can be fabricated and the resulting configurations show a very strong correlation with the size of initial GaAs islands. With relatively smaller GaAs islands, quad-QDMs (four QDs per island) with a squarelike configuration were formed and also, quad-QDMs with a rectangularlike positioning were fabricated with relatively larger size of islands, while hexa-QDMs were formed with middle sized ones. Relatively, broader size distribution of GaAs nanoisland can be a direct result of Ostwald ripening, which can be well controlled by adjusting postgrowth interruption of liquid Ga droplets. (c) 2008 American Institute of Physics.
Abstract: GaAs and AlAs thin capping layers as well as postgrowth rapid thermal annealing (RTA) were applied to InAs quantum dots (QDs) grown by molecular beam epitaxy to study the tunability of optical properties of QDs by photoluminescence (PL) methods. The PL of AlAs-capped QDs shows double-peak structure, as opposed to GaAs-capped QDs, which is due to the formation of two families of QDs in the AlAs-capped sample confirmed by the power dependent PL measurements. The PL peak of the GaAs-capped samples subjected to RTA showed blueshift and narrowing with an increase in RTA temperature. This is the result of thermally enhanced In-Ga intermixing. More complex changes in the PL spectrum of AlAs-capped QDs during the RTA procedure were found and explained by the different In compositions in two branches of QDs. The features observed in the temperature dependences of PL peak energy of GaAs- and AlAs-capped samples were interpreted in terms of thermal escape of carriers from smaller QDs with further redistribution between larger QDs and different InAs content in two families of QDs.
Abstract: We have investigated the self-organized, step bunch assisted formation of In0.19Ga0.81As/GaAs quantum dots in vertical superlattices consisting of one, four, eight, and ten periods. Samples were grown by molecular beam epitaxy on vicinal 2 degrees A and 2 degrees B GaAs(001) substrates. Those with miscut along the [1 (1) over bar0] (2 degrees B) exclusively show step bunches of an aspect ratio larger than 10 but without the formation of quantum dots. This highly linear pattern is improved during subsequent periods as proved by high resolution x-ray diffraction and grazing incidence diffraction. On the other hand, a miscut along the [110] (2 degrees A) initially causes a crosslike pattern of step bunches, which finally becomes a two-dimensional arrangement of individual quantum dots. (C) 2008 American Institute of Physics.
Abstract: We use time-resolved terahertz-pulse spectroscopy to study the ultrafast carrier dynamics in InGaAs/GaAs (311)A quantum wires. Anisotropy in the photoconductive dynamics is observed when aligning the terahertz probe polarization parallel versus perpendicular to the wire direction. The origin of this anisotropy is the carrier capture into localized quantum-wire states from delocalized wetting layer or barrier regions over time scales from 6 to 30 ps. The capture efficiency is found to be strongly temperature dependent, with thermal emission dominating above 125 K, while state-filling effects within the wires influence the capture rate below 125 K. Transient spectroscopy reveals a Drude-like carrier conductivity. (C) 2008 American Institute of Physics.
Abstract: Photoluminescence (PL) of InAs/GaAs surface quantum dots (QDs) is enhanced by implanting a silicon-doped GaAs interlayer beneath surface QDs. It is observed that setting the doping concentration to 2.3 x 10(17) cm(-3) in the doped GaAs interlayer spaced 10 nm from the surface QDs results in optimal QD PL, i.e., highest intensity and narrowest linewidth. This improvement is attributed to the effective enhancement of the photo-excited carrier capture into the surface QDs and a filling of surface states due to free-carrier transfer from the doped GaAs layer to the surface QDs.
Abstract: Using polarized Raman spectroscopy and high resolution X-ray diffraction we have investigated self-organized In0.45Ga0.55As quantum-dot chains in InGaAs/GaAs multilayer structures. It is shown that the formation of InGaAs QDs in InGaAs/GaAs multilayered structures is accompanied by a strong improvement in the uniformity of size and shapes of QDs as well as vertical alignment and lateral ordering. At mean densities, extended chains of QDs (up to 5 mu m) appear along the [1 (1) over bar0] direction; however, increased ordering of QDs along the [110] direction could be observed, too. For the first time, InGaAs dot-chains were investigated using polarized Raman scattering. Observation of optical phonons localized in InGaAs QDs and two-dimensional (2D) layers is demonstrated. An obvious anisotropy in the intensity of Raman modes was observed when the electric field vector of the exciting laser beam is parallel or perpendicular to the wire-like axis [1 (1) over bar0] of dot-chains. This effect may be related to symmetry lowering effects and real anisotropic geometry of the QDs and 2D wetting layers.
Abstract: We report a near-field photoluminescence study of pairs of GaAs quantum dots grown by droplet epitaxy, a growth mode for fabricating quantum dot arrays with controlled geometries. Our study reveals the optical properties of the individual quantum dots within each pair and, by means of a statistical analysis, the correlation between geometrical and optical properties of such arrays. Due to their high optical quality and unique geometry, ordered droplet epitaxy quantum dot arrays may become interesting candidates for coherent exciton and/or spin manipulation.
Abstract: Nanohole formation on an AlAs/GaAs superlattice gives insight to both the "drilling" effect of Ga droplets on AlAs as compared to GaAs and the hole-filling process. The shape and depth of the nanoholes formed on GaAs (100) substrates has been studied by the cross-section transmission electron microscopy. The Ga droplets "drill" through the AlAs layer at a much slower rate than through GaAs due to differences in activation energy. Refill of the nanohole results in elongated GaAs mounds along the [01-1] direction. As a result of capillarity-induced diffusion, GaAs favors growth inside the nanoholes, which provides the possibility to fabricate GaAs and AlAs nanostructures.
Abstract: We demonstrate that by changing the substrate temperature at which Ga droplets form and by varying the InAs deposition, we are able to control the configuration of quantum dots per GaAs mound. The size of the Ga droplets increases with increasing substrate temperature and resulting configurations show a very strong correlation with the size of initial GaAs islands. In distinction from previous reports, we attained two structures: quadmolecules and quantum rod pairs. Quadmolecules are elongated along the [011] crystallographic direction due to strain-driven processes and are directly formed at the edges of the GaAs mounds. On the other hand, quantum rod pairs formed along the [01-1] direction due to higher anisotropic diffusion. (C) 2008 American Institute of Physics.
Abstract: The structural complexity of GaAs quantum-dot pairs has been revealed by cross-sectional transmission electron microscopy. As a result of high-temperature droplet epitaxy, the AlGaAs substrate beneath the quantum-dot pairs is no longer immobile and its reconstruction is observed to define the crystallization of gallium droplets under an arsenic flux. The GaAs quantum-dot pairs are immersed into the substrate and further confined by the re-distributed AlGaAs materials above the substrate plane; There are two underlying mechanisms responsible for, the final nanostructure configuration, melt-back etching by the gallium droplets and preferential crystallization of gallium around reconstructed sidewalls.
Abstract: Spectroscopic study of the InAs quantum dot (QD) formation in GaAs ringlike nanostructures is carried out. Ga droplet epitaxy is used to form GaAs ringlike nanostructures. Subsequently InAs is deposited to obtain InAs QDs by self-assembly inside the holes of the nanostructures. Regularly spaced bands in the photoluminescence spectra exhibit state filling properties under increased excitation power. However, it is demonstrated that these bands do not represent excited states of a single ensemble of dots, but are separate ensembles with individual ground state energies, which are coupled through the GaAs ring structure on which they form. The most likely cause of these uniformly spaced ensemble energies is monolayer differences in the effective height of the dots. Temperature, excitation power density, and time dependent photoluminescence measurements are used to demonstrate the significance of the interdot coupling. The photoluminescent properties of these novel nanostructures make them candidates for optoelectronic applications. (C) 2008 American Institute of Physics.
Abstract: Ultralow density (similar to 10(6)/cm(2)) of twin InAs quantum dot (QD) hybrid structure was grown by a droplet epitaxy technique. The photoluminescence (PL) from ensemble and individual twin InAs QD structures showed a bimodal behavior and an energy transfer between the well-separated (similar to 190 nm) twin QDs, which was supposedly due to the special wetting ring that built the channel for exciton transfer. This research demonstrates a novel approach to fabricate lateral InAs QD pairs as the candidate for a laterally coupled OD molecule.
Abstract: We have studied the lateral carrier transfer in a specially designed quantum dot chain structure by means of time-resolved photoluminescence (PL) and polarization PL. The PL decay time increases with temperature, following the T-1/2 law for the typical one-dimensional quantum system. The decay time depends strongly on the emission energy: it decreases as the photon energy increases. Moreover, a strong polarization anisotropy is observed. These results are attributed to the efficient lateral transfer of carriers along the chain direction. (c) 2008 American Institute of Physics.
Abstract: We report on the ability to fabricate super low density InGaAs semiconductor ring-shaped nanocrystals on a GaAs (100) surface by molecular beam epitaxy. Specifically, we demonstrate densities down to 2.3 x 10(6) cm(-2) with only self-assembled methods based on droplet epitaxy. This is several orders of magnitude lower than conventional nanostructures. The formation of these ring-shaped nanostructures is driven by a self-assembled indium nanodrilling mechanism and diffusion during crystallization.
Abstract: A comparative study of the steady-state and transient optical properties was made between InGaAs/GaAs quantum do chains (QDCs) and quantum dots (QDs). It was found that the photoluminescence (PL) decay time of QDCs exhibited a strong photon energy dependence, while it was less sensitive in QDs. The PL decay time increased much faster with the excitation power in the QDCs than that in QDs. When the excitation power was large enough, the PL decay time tended to be saturated. In addition, it was also found that the PL rise time was much shorter in QDCs than in QDs. All these experimental results show that there is a strong carrier coupling along the chain direction in the QD chain structure. The polarization PL measurements further confirm the carrier transfer process along the chain direction.
Abstract: Atomic force microscopy and photoluminescence (PL) measurements were carried out to investigate the role of the morphology of GaAs/Al0.3Ga0.7As ringlike nanostructures on their optical properties. A small amount of Ga material (less than three monolayers) was used to fabricate single ringlike and double ringlike (DRL) nanostructures using droplet epitaxy technique. The height of the ringlike nanostructures increased with the increase of the Ga material while the corresponding PL emission energy was found to decrease as the ringlike nanostructure height increased. The PL peak energy showed a blueshift with increasing excitation intensity that can be understood as due to state filling while increasing temperature showed that the peak energy of the larger DRL nanostructures redshifts at a lower rate than the small ones due to larger confinement potential and lower energy emissions. (c) 2007 American Institute of Physics.
Abstract: Electronic states of layered InGaAs/GaAs(001) quantum wire and quantum dot chain structures have been investigated by electroreflectance and surface photovoltage spectroscopy. Band-gap shrinkage and heavy-hole/light-hole state splitting have been observed in the GaAs barrier material. This can be understood by shear strain existing in the GaAs barrier due to strain relaxation and anisotropy within the wires or dot chains. By comparing the experimental results with theoretical calculations, we found that the strain relaxation in the direction perpendicular to the wires or the dot chains has a stronger effect on the heavy-hole-light-hole splitting than on band-gap modification in the InGaAs wires and dot chains. The piezoelectric field induced by the shear strain is also discussed. (c) 2007 American Institute of Physics.
Abstract: The formation of "sidewall nanowires" on shallow patterned mesa strips with a modulation depth of only 35 nm on GaAs (100) was demonstrated using molecular beam epitaxy. While self-assembled GaAs sidewall nanowire formation is observed near mesa strips running along [011], relatively thinner AlAs/GaAs layers are formed on identical mesa strips running along [01-1]. Cross-sectional atomic force microscopy (XAFM) on (011) and (01-1) and AFM on (100) are used to understand the formation of the different morphology of the nanostructures, depending on the direction of the mesas. The data indicates that anisotropic surface diffusion of adatoms, resulting from the characteristic (2 x 4) GaAs (100) surface reconstruction, is responsible for the sidewall nanowire formation and for the different morphology observed along different directions.
Abstract: The surface morphology and optical properties of the (In,Ga)As/GaAs(100) multilayer structures with self-organized quantum dots and quantum wires, which were grown by molecular-beam epitaxy, are investigated. It is found that the ordered arrangement of quantum dots in the heterointerface plane starts to form during the growth of the first periods of the multilayer structure. As the number of periods increases, quantum dots line up in series and form wires along the [011] direction. An increase in the lateral ordering of the structures under consideration correlates with an increase in the optical emission anisotropy governed by relaxation anisotropy of elastic strains and by the shape of nano-objects. A possible mechanism of lateral ordering of quantum dots and wires in multilayer structures, which includes both anisotropy effects of the strain fields and adatom diffusion, as well as the elastic interaction of neighboring quantum dots, is discussed.
Abstract: Structures containing multiple layers of self- assembled InAs quantum dot clusters within a GaAs matrix are investigated on GaAs ( 1 0 0) by atomic force microscopy. Droplet homoepitaxy ( GaAs nanostructures on planar GaAs) is used to create tiny GaAs nano- mound templates elongated along [ 01- 1]. Due to a high density of monolayer steps on the edge of nano- mounds, deposited InAs prefer to form in clusters around the nano- mound templates. By varying the subsequent InAs monolayer coverages and growth temperatures, two distinctive sizes of quantum dots ( QDs) are formed around the nano- mounds for the layer stacking. With a fixed GaAs barrier thickness ( 10 nm) in between the layers, the resulting QDs from the stacked layers show significant improvement in their size uniformity.
Abstract: Self-assembled nanodrill technology based on droplet epitaxy growth was developed to obtain nanoholes on a GaAs(100) surface. In this technology, the gallium droplets act like "electrochemical drills" etching away the GaAs substrate beneath to give rise to nanoholes more than 10 nm deep. The driving force of the nanodrill is attributed to the arsenic desorption underneath the gallium droplet at high growth temperatures and Ga-rich condition. This nanodrill technology provides an easy and flexible method to fabricate nanohole templates on GaAs(100) surface and has great potential for developing quantum dots and quantum dot molecules for quantum computation applications. (c) 2007 American Institute of Physics.
Abstract: Crossed transitions between the wetting layer valence band and the quantum dot (QD) electron states are revealed in the photoluminescence from self-assembled In0.4Ga0.6As/GaAs QDs. The strength of these transitions becomes comparable with the excitonic transitions for below-GaAs barrier excitation and decreases significantly with below wetting layer excitation. The observed peculiar QD photoluminescence dependences on temperature and excitation density are due partly to interdot carrier transfer through the continuum states related to the wetting layer morphology and to phonon-assisted processes. (c) 2007 American Institute of Physics.
Abstract: The density of states for InAs/GaAs quantum dot (QDs) bi-layer arrays placed between two AlGaAs barriers is studied by means of photoluminescence (PL) excitation, resonant PL and time-resolved PL. By varying the excitation energy from above the AlGaAs band gap to values resonant with the QD energies, the energy of states in each layer including the wetting layer, corresponding localized states, defect states and QD states is determined. The creation of asymmetric pairs of quantum dots caused by interlayer coupling is traced starting from the case of weakly correlated systems represented by bi-layer QD arrays with a thick GaAs spacer layer (50 monolayers) to the case of fully correlated systems with a GaAs spacer (30 monolayers). Different mechanisms of carrier relaxation related to the density of states below the barrier and interlayer coupling are explored.
Abstract: The self-organized formation of In0.40Ga0.60As quantum dot chains was investigated using x-ray scattering. Two samples were compared grown on GaAs(100) by molecular beam epitaxy. The first sample with a single layer of In0.40Ga0.60As dots shows weak quantum dot alignment and a corresponding elongated shape along [0 (1) over bar1], while the top layer of a multilayered In0.40Ga0.60As/GaAs sample exhibits extended and highly regular quantum dot chains oriented along [0 (1) over bar1]. Numerical calculations of the three-dimensional strain fields are used to explain the initial stages of chain formation by anisotropic strain relaxation induced by the elongated dot shape.
Abstract: Molecular beam epitaxy growth of multilayer In (x) Ga1-x As/GaAs(001) structures with low indium content (x = 0.20-0.35) was studied by X-ray diffraction and photoluminescence in order to understand the initial stage of strain-driven island formation. The structural properties of these superlattices were investigated using reciprocal space maps, which were obtained around the symmetric 004 and asymmetric 113 and 224 Bragg diffraction, and omega/2 theta scans with a high-resolution diffractometer in the triple axis configuration. Using the information obtained from the reciprocal space maps, the 004 omega/2 theta scans were simulated by dynamical diffraction theory and the in-plane strain in the dot lattice was determined. We determined the degree of vertical correlation for the dot position ("stacking") and lateral composition modulation period (LCM) (lateral ordering of the dots). It is shown that initial stage formation of nanoislands is accompanied by LCM only for [110] direction in the plane with a period of about 50 to 60 nm, which is responsible for the formation of a quantum wire like structure. The role of In (x) Ga1-x As thickness and lateral composition modulation in the formation of quantum dots in strained In (x) Ga1-x As/GaAs structures is discussed.
Abstract: The evolution of InAs quantum dot (QD) formation is studied on GaAs ring-like nanostructures fabricated by droplet homo-epitaxy. This growth mode, exclusively performed by a hybrid approach of droplet homo-epitaxy and Stransky-Krastanor (S-K) based QD self-assembly, enables one to form new QD morphologies that may find use in optoelectronic applications. Increased deposition of InAs on the GaAs ring first produced a QD in the hole followed by QDs around the GaAs ring and on the GaAs (100) surface. This behavior indicates that the QDs prefer to nucleate at locations of high monolayer (ML) step density.
Abstract: We present a study on the formation of In droplets on GaAs(100) substrates as functions of substrate temperature and monolayer (ML) deposition by using molecular beam epitaxy (MBE) and atomic force microscopy (AFM). We specifically reveal the change in critical thickness of In deposition to form In droplets at different substrate temperatures. At a relatively high substrate temperature, the critical thickness of In droplets becomes relatively thinner as the amount of As atoms on the surface decreases. The control of the size and density of In droplets is also systematically discussed. This study provides an aid in understanding the formation of In droplets and thus can find applications in the formation of quantum structures and/or nanostructures based on droplet epitaxy.
Abstract: InAs/GaAs heterostructures have been simultaneously grown by molecular beam epitaxy on GaAs (100), GaAs (100) with a 2 degrees misorientation angle towards [01-1], and GaAs (n11) B (n = 9, 7, 5) substrates. While the substrate misorientation angle increased from 0 degrees to 15.8 degrees, a clear evolution from quantum dots to quantum well was evident by the surface morphology, the photoluminescence, and the time-resolved photoluminescence, respectively. This evolution revealed an increased critical thickness and a delayed formation of InAs quantum dots as the surface orientation departed from GaAs (100), which was explained by the thermal-equilibrium model due to the less efficient of strain relaxation on misoriented substrate surfaces.
Abstract: We have investigated lateral self-assembling in In0.4Ga0.6As/GaAs quantum dot (QD) multilayers, which were grown by molecular beam epitaxy on GaAs (100) and (n11)B substrates with n = 9,8,7,5,4,3. The lateral self-assembling and the QD size distribution have been studied by atomic force microscopy depending on substrate orientation and the number of periods within the multilayers. The observed two-dimensional ordering can be described by a centered rectangular surface unit cell. Derived autocorrelation functions exhibit the most pronounced lateral QD assembling along the elastically soft directions [(1) over bar n0]. This can be attributed to elastic interaction, the particular elastic anisotropy of the high index substrates, and the minimization of the strain energy. (C) 2007 American Institute of Physics.
Abstract: We report on the ability to grow InAs quantum dots into patterns of any shape. We specifically demonstrate the spatial localization of InAs quantum dots on mesa and trench patterns varying from line, square and triangle patterns on GaAs (100) substrates by molecular beam epitaxy. Based on the underlying science, this growth approach enables the localization of InAs QDs on GaAs (100) by controlling the sidewall facets and InAs monolayer coverage.
Abstract: We have studied the structural properties of ordered InGaAs/GaAs(00 I) quantum dot chains multi layer by high-resolution X-ray diffraction. Two systems of lateral satellites, one of which being inclined with respect to the sample surface normal, i.e. the growth direction [001], were observed. The measured inclination of 30.0 degrees +/- 2.5 degrees does not affect the diffraction profile from planar superlattice (SL), i.e. SL peaks are not inclined with respect to the GaAs substrate peak. We identify the splitting of coherent SL satellites for all orders as well as for two perpendicular directions. This splitting most likely indicates that two discrete periods exist in SL structure. (c) 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
Abstract: Monodisperse zinc blende CdSe nanocrystals (NCs) ranged from 2.5 to 4.3 nm have been reproducibly synthesized directly open to air in a microfluidic reactor with PTFE capillaries acting as the reaction channels. The foregoing features led to superior operational stability, allowing the assessment of kinetic data such as temperature, dwelling time and ligand concentration. Red shifts of the absorption spectra were observed with increasing residence time and temperature. 'Crossover time' of size distribution was first reported in a microfluidic reactor preceded by encouraged Ostwald ripening at high reaction temperature. Oleic acid concentration influenced the formation of CdSe NCs, and high OA to Cd ratio resulted in large NCs size and decreased CdSe concentration. By systematic optimization of the parameters, CdSe NCs with FWHM for the photoluminescence peak in the range from 35 to 42 nm were thus made possible.
Abstract: GaAs spacer thicknesses are varied to tune the coupling between InGaAs surface quantum dots (QDs) and multilayers of buried QDs. Temperature and excitation intensity dependence of the photoluminescence together with time resolved photoluminescence reveal that coupling between layers of QDs and consequently the optical properties of both the surface and the buried QDs significantly depend on the GaAs spacer. This work provides an experimental method to tune and control the optical performance of surface QDs. (C) 2007 Optical Society of America
Abstract: The optical properties of uncapped InAs/GaAs quantum dots sitting on multiple layers of buried quantum dots are investigated through a series of samples with a different number of buried quantum dot layers. Two photoluminescence peaks are observed, one from surface quantum dots and one from buried quantum dots. Close spatial and optical correlations between surface and buried quantum dots are demonstrated. Due to the carrier transfer from buried to surface quantum dots, the photoluminescence emission from surface quantum dots is significantly improved. As a result, the surface quantum dot-buried quantum dot sample shows potential for biological sensing. (c) 2006 American Institute of Physics.
Abstract: Surface pit formation is observed to occur for conventional thermal oxide desorption, regardless of the GaAs surface index. The resulting surface roughening can be substantially reduced by depositing an appropriate amount of Ga in the absence of As flux. The amount of Ga required to optimize the quality of the oxide-free GaAs surface is found to be different for different indexed substrates and is dependent on the oxide thicknesses. Due to the flexibility of the GaAs surface reconstruction, the Ga-triggered oxide desorption is observed to be much more robust than was previously believed. (c) 2006 American Institute of Physics.
Abstract: Selective formation of InAs quantum dots on the sidewalls of mesa strips along both [01-1] and [011] directions of a GaAs(100) surface is demonstrated. This result is in sharp contrast to observations on traditionally deep-patterned substrates, where quantum dots are formed on top mesas and at bottom trenches. This distinction is explained kinetically and energetically. These results may encourage application of organized arrays of quantum dots. (c) 2006 American Institute of Physics.
Abstract: A growth technique combining droplet epitaxy and molecular beam epitaxy (MBE) is developed to obtain a low density of InAs quantum dots (QDs) on GaAs nanoholes. This growth technique is simple, flexible, and does not require additional substrate processing. It makes possible separate control of the QD density via droplet epitaxy and the QD quality via MBE growth. In this letter the authors report the use of this technique to produce InAs QDs with a low density of 2.7x10(8) cm(-2) as well as good photoluminescence properties. The resulting samples are suitable for single QD device fabrication and applications. (c) 2006 American Institute of Physics.
Abstract: We systematically investigated the correlation between morphological and optical properties of InGaAs self-assembled quantum dots (QDs) grown by solid-source molecular beam epitaxy on GaAs (n11) B (n = 9, 8, 7, 5, 3, 2) substrates. Remarkably, all InGaAs QDs on GaAs( n11) B under investigation show optical properties superior to those for ones on GaAs( 100) as regards the photoluminescence (PL) linewidth and intensity. The morphology for growth of InGaAs QDs on GaAs ( n11) B, where n = 9, 8, 7, 5, is observed to have a rounded shape with a higher degree of lateral ordering than that on GaAs( 100). The optical property and the lateral ordering are best for QDs grown on a ( 511) B substrate surface, giving a strong correlation between lateral ordering and PL optical quality. Our results demonstrate the potential for high quality InGaAs QDs on GaAs( n11) B for optoelectronic applications.
Abstract: The authors report on x-ray diffuse scattering at nominally strain-free GaAs(001) quantum dot molecules (QDMs). Al0.3Ga0.7As deposited by molecular beam epitaxy on GaAs(001) acts as barrier layer between the GaAs(001) substrate and subsequently grown QDMs; the adjusted thickness of 50 nm preserves the in-plane lattice parameter. Pairs of lenselike quantum dots are created with preferential orientation along [1 (1) over bar0] placed on shallow hills. Grazing incidence diffraction along with kinematical scattering simulations indicate completely strain-free QDs which prove a strongly suppressed intermixing between QDMs and the underlying AlGaAs barrier layer.
Abstract: The results of investigation of superlattice with quantum wires by high-resolution x-ray scattering are presented. The influence of lattice distortions on rocking curves (RC) are analysed within the framework of the dynamical theory of diffraction. It allows to explain the azimuth dependence of experimental rocking curves. Anisotropic changes in the shape of lattice unit cell are revealed and measured. An influence of the smooth boundaries between heterolayers is analysed. Comparative analysis of different gradient functions on the heteroboundary influence on the rocking curves is carried out. Using RC modelling, the parameters of heterojunctions in specimens at issue are determined.
Abstract: The results of investigation of In0.3Ga0.7As/GaAs superlattice by high-resolution X-ray scattering are presented. The influence of lattice distortion on diffraction curves (DC) were analyzed with dynamical diffraction theory. It allowed to explain azimuth dependence of experimental diffraction curves. Anisotropic changes in the shape of InGaAs lattice unit cell were shown and measured. The influence of smooth borders between hetero-layers were analyzed. Comparative analysis of different gradient functions on the hetero-border influence on diffraction curves was done. Parameters of heterojunction in investigated samples were determined with the help of DC modelling.
Abstract: The evolution between lattice-matched GaAs/Al0.3Ga0.7As single and double ring-like nanostructures is studied, with an emphasis on the construction and destruction of the observed outer ring. Using droplet epitaxy, this was achieved by directly controlling the Ga surface diffusion on GaAs( 100). Double ring-like nanostructures were observed at relatively low temperatures under a fixed As-4 flux (beam equivalent pressure (BEP) of 6.4 mu Torr) and at a fixed temperature under a high As-4 flux. The construction of the outer ring can be controlled through surface diffusion by varying the substrate temperature or the As-4 flux. Single ring-like nanostructures were realized both at relatively high temperatures under a fixed As-4 flux, and at low temperatures under a relatively low As-4
Abstract: Using high-resolution x-ray diffraction (HRXRD), Raman scattering, photoluminescence, and atomic-force microcopy, we investigated InxGa1-xAs/GaAs quantum dot (QD) stacks grown by molecular-beam epitaxy with nominal In contents of 0.30 and 0.35. The analysis of x-ray-diffraction rocking curves using symmetrical (004), asymmetrical (113), and quasiforbidden (002) reflections within the framework of dynamical theory allowed us to determine the average values of strains parallel and perpendicular to the growth direction. We also measured nonuniform In profiles in the InxGa1-xAs layers along the growth direction. This observation confirms the important role of surface segregation of In atoms and interdiffusion of Ga atoms from GaAs layers in the formation of InxGa1-xAs QDs. Both HRXRD and Raman scattering in InxGa1-xAs/GaAs-stacked QD structures demonstrate that the InGaAs inserts in these structures can be modeled effectively as sublayers with two different compositions: sufficiently thick InxGa1-xAs sublayer with the In concentration lower than the nominal one, which includes the thin layer of InGaAs islands with the In concentration much higher than the nominal one. (c) 2006 American Institute of Physics.
Abstract: Anisotropic surface diffusion and strain are used to explain the formation of three-dimensional (In,Ga)As quantum dot lattices. The diffusion characteristics of the surface, coupled with the elastic anisotropy of the matrix, provides an excellent opportunity to influence the dot positions. In particular, quantum dots that are laterally organized into long chains or chessboard two-dimensional arrays vertically organized with strict vertical ordering or vertical ordering that is inclined to the sample surface normal are accurately predicted and observed.
Abstract: Self-assembled quantum dots (QDs) in double-layer InAs/GaAs structures are studied by resonant photoluminescence and photoluminescence excitation spectroscopy. A weakly correlated (50%) double-layer system with an array of vertically coupled QDs (asymmetric quantum-dot molecules) was formed in a structure consisting of the 1.8-monolayer-thick first and the 2.4-monolayer-thick second InAs layers separated by 50 monolayers of GaAs. The nature of discrete quantum states in this system was studied and resonances corresponding to vertically coupled QDs were clearly observed for the first time.
Abstract: Self-organized InGaAs quantum dot chains grown by molecular beam epitaxy were investigated using vertical stacking techniques on pre-patterned GaAs( 100) substrates. The results demonstrate the formation of quantum dot (QD) chains only on desired spatial regions. In addition to QD chains, the results show that almost any shape of lines of QDs are possible depending on the faceted pre-patterned substrate. The experimental results suggest that this approach has the potential to be used to fabricate single QD chains or necklaces or almost any pattern.
Abstract: Long chains of quantum dots formed in InGaAs/GaAs(100) multiple layers have been systematically investigated by scanning electron, transmission electron, and atomic force microscopies. In addition to the usual two-dimensional wetting layer involved in the Stranski-Krastanov growth, we have directly observed a one-dimensional postwetting layer along the [01-1] direction that strings together the quantum dots in each chain. In sharp contrast with the two-dimensional wetting layer, which exists before the quantum-dot chains form, this one-dimensional postwetting layer develops during the GaAs capping of the existing dot chains. This one-dimensional layer forms through the anisotropic surface diffusion of In atoms that accompanies the change in strain profile during capping and therefore produces the steady-state material distribution that includes a one-dimensional postwetting layer as a result.(c) 2006 American Institute of Physics.
Abstract: The authors report on the use of GaAs islands, formed by the droplet epitaxy growth technique, as a template for the growth of clusters of InAs quantum dots. Surface morphology measurements show that the shape and dimensions of the GaAs islands and consequently the formation of InAs quantum dots depend strongly on the annealing temperature and annealing time. This can be explained by the diffusion of gallium atom during the annealing process and the selective formation of InAs quantum dots on the misoriented GaAs island sidewalls. (c) 2006 American Institute of Physics.
Abstract: Several distinctive self-assembled InGaAs quantum dot molecules (QDMs) are studied. The QDMs self-assemble around nanoscale-sized GaAs moundlike templates fabricated by droplet homoepitaxy. Depending on the specific InAs monolayer coverage, the number of QDs per GaAs mound ranges from two to six (bi-QDMs to hexa-QDMs). The Ga contribution from the mounds is analyzed in determining the morphologies of the QDMs, with respect to the InAs coverages ranging between 0.8 and 2.4 ML. Optical characterization shows that the resulting nanostructures are high-quality nanocrystals. (c) 2006 American Institute of Physics.
Abstract: The spontaneously formation of epitaxial GaAs quantum-dot pairs was demonstrated on an AlGaAs surface using Ga droplets as a Ga nanosource. The dot pair formation was attributed to the anisotropy of surface diffusion during high-temperature droplet epitaxy.
Abstract: Coupling between the surface and the buried quantum dots (QDs) is studied by photoluminescence (PL) measurement for a heterostructure with InAs surface QDs sitting above four layers of buried QDs. Temperature and excitation intensity dependences of the PL together with the PL temporal decay behavior reveal fast carrier transfer from the buried QDs to the surface QDs. The PL signals from the surface and buried QDs are shown to be closely correlated with each other and very surface sensitive. These features support the promise this heterostructure holds for sensor devices. (c) 2006 American Institute of Physics.
Abstract: Using ( 100) GaAs substrates as a reference, we present a study of the formation of Ga droplets on ( 311) A and ( 511) A GaAs substrates in which the effect of both the substrate temperature and the amount of Ga supplied on the droplet density and height for the three different surfaces have been investigated. Droplets on ( 100) substrates show a round shape; however, they appear as elongated balls with tails along the [233] direction of the ( 311) A substrate and the [255] direction of the ( 511) A substrate. It has been found that the Ga droplets on ( 511) A surfaces have lower densities and higher heights than those on ( 100) substrates. In contrast, Ga droplets on ( 311) A surfaces have lower heights and much higher densities compared to those for both ( 100) and ( 511) A. We observed that the decrease in the droplet density with increasing growth temperature for both ( 311) A and ( 511) A is more than twice that for the ( 100) GaAs surface due to the larger drop in the nucleation rate. Based on these observations, we offer a physical explanation based on the thermodynamics and the anisotropy of the high-index surfaces.
Abstract: Significant surface pitting and a degraded surface roughness are almost always observed on GaAs (100) surface after conventional thermal oxide desorption. Here we report on the use of a Ga-triggered low temperature oxide desorption method that can be used to preserve the atomic monolayer (ML) steps. By providing an external supply of atomic Ga at a relatively low substrate temperature of similar to 450 degrees C without an As-4 overpressure, this technique resulted in an atomically smooth GaAs ML steps with a root mean square roughness of 0.25 nm, nearly identical to as-grown GaAs surface (0.2 nm). The demonstrated results show the potential for applications in optoelectronics such as regrowth and patterned substrate growth. (c) 2006 American Institute of Physics.
Abstract: Peculiar mechanism of carrier transfer, excitonic trapping, from quantum well (QW) states to quantum dot (QD) states is clearly observed for the intentionally designed strained InAs:In0.3Ga0.7As/GaAs QD:QW structure. This transfer occurs very efficient at low excitation densities and low temperatures and explains the excitation density and nonmonotonic temperature dependences of the QW photoluminescence. (c) 2006 American Institute of Physics.
Abstract: New morphologies of InAs quantum dot ( QD) ensembles forming on self-assembled GaAs nano-holed island templates are demonstrated. Droplet homoepitaxy ( GaAs/GaAs) is used to generate holed nanoscale-sized mounds that appear to elongate along [ 0 1 (1) over bar]. Depending on the InAs monolayer ( ML) coverages, subsequent InAs deposition forms different sizes and shapes of QD ensembles. While we initially observe the formation of the QDs at the hole sites when less InAs is deposited, QDs form around the edges of the mounds with greater InAs deposition. By varying the InAs depositions and growth temperatures, we demonstrate an ability to control the size and density of QDs. The observed decrease in the necessary critical thickness required for the InAs 2D - 3D transition may be due to the higher density of monolayer steps on the sidewalls of the holes and on the edges of the mounds. This hybrid growth approach overcomes some limitations of typical QD growth on planar GaAs surfaces and may find applications in optoelectronics.
Abstract: One of the difficulties in understanding energy transfer in bilayer quantum dot structures is the complex role of carrier tunneling. This limitation is due to the fact that, for most studies to date, QDs in each layer have only one confined energy level making it difficult to study resonant tunneling effects. In this work, we have investigated the low growth-rate technique to produce dislocation-free very large QDs in the second layer that are characterized by several confined energy levels. The high quality surface morphology and optical behavior of these structures were demonstrated by AFM and PL measurements. (c) 2006 WILEYNCH Verlag GmbH & Co. KGaA, Weinheim.
Abstract: Strain relaxation mechanisms for the growth of m-layers of In0.18Ga0.82As/GaAs on a GaAs( 100) substrate, tilted 2 degrees towards the [ 0 - 11] direction, have been studied by molecular beam epitaxy ( MBE) and atomic force microscopy (AFM). While dislocations alone provide a strain relaxation mechanism for nominal GaAs( 100), additional strain relaxation mechanisms were observed for a vicinal GaAs( 100) substrate. For m <= 8, step bunching provided a mechanism for strain relaxation. For m >= 10, in addition to the step bunching, bunched corners along two [ 051] and [ 0 - 1 - 5] directions provided the mechanism for strain relaxation. These surface patterns provide potential to act as a template for the growth of more uniform and organized nanostructures. For m = 16, the formation of dislocations provided an additional route for strain relaxation.
Abstract: We report direct spectroscopic evidence for the formation of one-dimensional ( 1D) wetting wires ( WWs) during the Stransky-Krastanov growth of InGaAs/GaAs quantum dot ( QD) chains. The wire-like nature of these 1D WWs was demonstrated by their 1D density of states and absorption anisotropies from the photoluminescence excitation measurements. Two groups of QDs were found sitting on top of these 1D WWs and the traditional two-dimensional wetting layers, respectively, with size-dependent emission polarization anisotropies of ca. 6-25% because of their elongated shapes.
Abstract: Time-resolved photoluminescence measurements are carried out for InAs quantum dots (QDs) coupled through a GaAs barrier to an In0.3Ga0.7As quantum well (QW). It is found that decay time of photoluminescence response from the QW is significantly shortened when compared to a reference sample only containing a QW with similar parameters while the decay time of photoluminescence response for the QD layer is significantly lengthened compared to reference sample only containing a QD layer. A rate equation model is developed to describe the observed behavior via carrier capture from QW to QD states in the QD:QW structure. (c) 2006 American Institute of Physics.
Abstract: Time-resolved photoluminescence (PL) measurements are carried out for the InGaAs quantum dots (QDs) grown on (001) and (311) oriented GaAs substrates. The detection energies scan the spectral region from the energy of the QD excitonic transition up to the wetting layer absorption edge. A convex-shaped decay of the PL signal in this region gives evidence of carrier relaxation through the continuum states below the wetting layer similar to a diffusion process. Strong dependence of the decay time on the excitation density observed for the QD ground-state PL is consistent with a presented theoretical model. (c) 2006 American Institute of Physics.
Abstract: We report on the formation of (In,Ga)As self-assembled quantum structures grown on different orientations of GaAs along one side of the stereographic triangle between (100) and (111)A. The samples were characterized by atomic force microscopy. A systematic transition from zero-dimensional (In,Ga)As quantum dots to one-dimensional quantum wires was observed as the substrate was varied along the side of the triangle between (100) and (111)A. An explanation for the role of the substrate in determining the size of the nanostructure is proposed. (c) 2004 Elsevier B.V. All rights reserved.
Abstract: We report selective chemical etching as a promising procedure to study the buried quantum dots in multiple InGaAs/GaAs layers. The dot layer-by-dot layer etching is demonstrated using a mixed solution of NH4OH:H2O2:H2O. Regular plan-view atomic force microscopy reveals that all of the exposed InGaAs layers have a chain-like lateral ordering despite the potential of significant In-Ga intermixing during capping. The vertical self-correlation of quantum dots in the chains is observed. (C) 2005 American Institute of Physics.
Abstract: In this paper, the growth dynamics and the surface morphology of the GaAs(331)B surface have been studied by both in situ reflection high-energy election diffraction (RHEED) and in situ scanning tunneling microscopy (STM). For the first time, a RHEED oscillation is reported on high index GaAs(331)B faceted surface with (110) and (111)B facets. The RHEED oscillation was observed only along the [116] direction. Absence of any RHEED oscillations along [116], [110], and [110] indicates a possible growth model in which the GaAs(331)B surface is moving frontward through fractional growth of its (111)B facets. These results help us to better understand the nature of RHEED oscillation on high index GaAs. (c) 2005 Elsevier B.V. All rights reserved.
Abstract: Nucleation and growth of (In,Ga)As-GaAs(100) islands with low In content by molecular-beam epitaxy is investigated by scanning tunneling microscopy. The islands tend to nucleate at upper convex edges of surface steps due to elastic strain relaxation. They are elongated along [01-1] with a flat top (100) facet. The growth of the islands, mainly through uphill transport of the (In,Ga)As material, is characterized by shrinking of the top (100) facet but the ratio of island elongation keeps constant. (C) 2005 American Institute of Physics.
Abstract: The transition from two-dimensional (2D) pseudomorphic growth to the three-dimensional (3D) (nanoisland) growth in InxGa1 - xAs/GaAs multilayer structures grown by molecular-beam epitaxy was investigated by atomic force microscopy, photoluminescence, and Raman scattering. The nominal In content x in InxGa1 - xAs was varied from 0.20 to 0.50. The thicknesses of the deposited InxGa1 - xAs and GaAs layers were 14 and 70 monolayers, respectively. It is shown that, at these thicknesses, the 2D-3D transition occurs at x greater than or equal to 0.27. It is ascertained that the formation of quantum dots (nanoislands) does not follow the classical Stranski-Krastanov mechanism but is significantly modified by the processes of vertical segregation of In atoms and interdiffusion of Ga atoms. As a result, the InxGa1 - xAs layer can be modeled by a 2D layer with a low In content (x < 0.20), which undergoes a transition into a thin layer containing nanoislands enriched with In (x > 0.60). For multilayer InxGa1 - xAs structures, lateral alignment of quantum dots into chains oriented along the [110] direction can be implemented and the homogeneity of the sizes of quantum dots can be improved. (C) 2005 Pleiades Publishing, Inc.
Abstract: In0.2Ga0.8As molecular-beam heteroepitaxial growth on GaAs(331)B was investigated by scanning tunneling microscopy. While GaAs(331)B homoepitaxial growth always leads to a faceted ridgelike surface, In0.2Ga0.8As growth over this surface can lead to either wirelike corrugations or a flat surface depending on growth parameters. The transition between the phases of wirelike corrugations and a flat surface is reversible, indicating that both phases are thermodynamic favored at different temperatures. Based on this observation, we also demonstrate a novel approach for the fabrication of (In,Ga)As quantum wires in the GaAs matrix with low In contents. The carriers are confined to one-dimensional quantum wires in the In0.2Ga0.8As layer bounded by a lower corrugated In0.2Ga0.8As-on-GaAs interface and an upper flat GaAs-on-In0.2Ga0.8As interface.
Abstract: We demonstrate that interface optical phonons can efficiently pump electrons from the quasi-X states to the quasi-T states in short-period type-II GaAs-AlAs superlattices. As a result, peculiar behaviors on these superlattices have been observed. First, photoluminescence intensity for the quasi-direct transition drastically increases as the temperature or pump power increases. Second, the dependence of the integrated photoluminescence intensity on the pump power exhibits a square power law.
Abstract: Electrons in short-period type-II GaAs/AlAs superlattices jump from the quasi-X states to the quasi-Gamma states by absorbing interface longitudinal optical phonons. Such a transition is characterized by several unique features. First of all, the photoluminescence intensity for the quasi-direct transition drastically increases as the temperature or the pump power increases. Secondly, the dependence of the integrated photoluminescence intensity on the pump power exhibits a square power law.
Abstract: Transient photoluminescence from a series of asymmetric InAs quantum-dot bilayers with a GaAs barrier layer thickness varying from 30 to 60 monolayers between the quantum-dot planes is investigated. The interdot carrier transfer process is analyzed. In the framework of a three-level system, interdot carrier transfer times between 200 and 2500 ps are derived and compared with similar data from the literature. Within the semiclassical Wentzel-Kramers-Brillouin approximation, the observed "transfer time-barrier thickness-relation" supports nonresonant tunneling as the microscopic carrier transfer mechanism. (C) 2005 American Institute of Physics.
Abstract: Strain-induced laterally ordered In0.4Ga0.6As on (311)A GaAs template quantum wires have been fabricated and identified with cross-section transmission electron microscopy technique to be of average length similar to 1 mu m, and on average width and height of 23 and 2 nm, respectively, under InGaAs coverage of six monolayers. The photoluminescence spectrum of a sample demonstrates unusually strong optical nonlinearity even at moderate excitation densities. The excitonic peak energy blueshifts by similar to 25 meV without essential contribution of the quantum wire excited states at elevating excitation density. Strong decrease of the polarization anisotropy and increase of the energy of excitonic photoluminescence are attributed to a combined action of the phase-space filling effects and the screening of the internal piezoelectric field by free carriers.(c) 2005 American Institute of Physics.
Abstract: Surface morphology of microsize defects on the surface of various high-index GaAs substrates was investigated using an atomic force microscope (AFM). The surfaces investigated were the top layer of 1- and 17-period In(0.45)GaAS(0.55)/GaAs structures with quantum dots or buffer layer. These structures were characterized by the formation of oval defects on (10 0) surfaces, and microsize defects possessing the shape of multifaceted pits and hillocks on (N11)A/B (N = 7, 5,4, 3) surfaces. The microsize defects were found to chaotically distribute on the surface and, as a rule, gathering in groups with some number of defects. Their density did not depend on the substrate orientation while the shape and orientation of the microsize defects were found to depend on the crystallographic orientation of the substrate. This dependence was determined to be the result of anisotropy of surface diffusion and surface elastic properties. The anisotropy of elastic properties of high-index surfaces was found to be the dominating factor in determining the microsize defect shape. We also report direct evidence of the fact that the effect of quantum dot lateral ordering observed on high-index (N I I)B surfaces is determined by the anisotropy of surface elastic properties as well as elastic interaction between adjacent quantum dots. (c) 2005 Elsevier B.V. All rights reserved.
Abstract: Laterally ordered multilayered arrays of InGaAs quantum dots are investigated by photoluminescence as a function of high index GaAs substrates. Different laser wavelengths are used to investigate the photoluminescence from quantum dots layer-by-layer. High optical quality is demonstrated for laterally ordered quantum dot arrays. GaAs(511)B is identified as the optimum high index substrate for growth of InGaAs/GaAs multilayered quantum dots, demonstrating strong photoluminescence with a narrow full width at half maximum linewidth of 23 meV in spite of the potential for misfit dislocations. (c) 2005 American Institute of Physics.
Abstract: We have characterized self-assembled InAs quantum dots grown on the top of a single InGaAs/GaAs quantum well (QD's:QW) by measuring photoluminescence (PL) spectra as an effective technique. We have found that the linewidths of the PL peaks for the QD's:QW are narrower than that for the InAs quantum dots grown on a GaAs layer by an amount as large as 35 meV. In addition, the wavelength-integrated PL intensity for the QD's:QW is about twice higher. Within the lateral plane, the energies for the optical transitions stay more or less the same for the QD's:QW whereas f6i die InAs quantum dots they shift quite a lot from one location to the next. Furthermore, the QD's:QW exhibit much stronger band-filling effect than the InAs quantum dots. In order to explain our experimental results, we have introduced the concept of a strong coupling between the InAs quantum dots and the InGaAs/GaAs quantum well strained by the quantum dots.
Abstract: We report on the use of reflection high-energy electron diffraction (RHEED) and scanning tunneling microscopy (STM) study that indicates that the GaAs (711)A is right at the transition between vicinal GaAs (100) and vicinal GaAs (511)A surfaces and that a variation of the As overpressure switches the surface morphology between the two vicinal surfaces. The steps on the vicinal (100) surface have a width of 1.5 nm creating a staircase surface with excellent possibilities for. growth of quantum wells. As-rich conditions can be described by vicinal (511)A surfaces with a width of 3.5 nm. This surface could find applications as a template for quantum wire growth. The observation suggests that the transition between these two morphologies is understandable based on the increase in surface energy of a vicinal (100) surface as the step separation approaches the dimer reconstructed separation. (c) 2005 Elsevier B.V. All rights reserved.
Abstract: Temperature-dependent photoluminescence is investigated in bilayer InAs/GaAs quantum dot structures with constant InAs deposition theta(1) in the seed layer, but variable deposition theta(2) in both the second layer and the GaAs spacer layer. It is shown that interlayer coupling, leading to the formation of asymmetric quantum dot pairs, strengthens the high-temperature photoluminescence and strongly influences carrier relaxation channels. We report that radiative recombination and carrier capture efficiency by the quantum dots in the second layer can be tailored using the deposition theta(2) and the GaAs spacer thickness. (c) 2005 American Institute of Physics.
Abstract: We report on Ga nano-droplets on GaAs(100) that are not stable under arsenic flux. Spontaneous evolution in shape leads to many interesting GaAs nanostructures. GaAs nanocrystals shaped like lighted candles and square-holed round coins are observed under different growth conditions. The underlying physics of the formation of these interesting nano-structures can be understood in terms of GaAs growth under a uniform arsenic flux and a non-uniform Ga supply from the Ga nano-droplets. These novel shaped GaAs nanostructures, in an AlGaAs matrix, offer promising applications in optoelectronics. (c) 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
Abstract: X-ray diffuse scattering from a 17-fold InGaAs/GaAs quantum dot (QD) multilayer grown by an advanced molecular beam epitaxy process was investigated. The QDs in each layer form extended chains that are oriented along the [110] planar direction. A prominent three-dimensional satellite structure in the x-ray diffuse scattering is found, which is caused by a strong lateral chain-chain correlation and vertical correlation of the lateral chain positions in the multilayer. In contrast, the dot-dot correlation within the chains are comparatively small. Numerical x-ray scattering simulations have been carried out. A comparison with corresponding experimental intensity patterns provides information on the QD size and QD spatial distribution.
Abstract: Carrier transfer in InAs/GaAs asymmetric quantum dot pairs has been studied by means of continuous-wave and time-resolved photoluminescence in a bilayer InAs/GaAs quantum dots system. The dependence of the tunneling time on the thickness of the separation layer is determined and the tunneling time is found to span the range from 250 to 2500 ps. A microscopic model of carrier transfer, including nonresonant electron tunneling from a direct into a cross exciton state, with subsequent generation of two direct excitons in adjacent quantum dot layers, is proposed.
Abstract: The spontaneous formation of long chains of quantum dots during the growth of InGaAs/GaAs multiple layers has been reported recently. The effects of In content and spacer on the evolution of dotchains are investigated in the present work. By reducing the In content in the InGaAs layer, the quantum dots in chains are more connected and finally arrays of quantum wires would form. By changing the GaAs spacer layer thickness, the vertical and also lateral spacing between dotchains can be continually tuned. The capability to insert a thick layer of AlGaAs as part of the spacer layer enables us to fabricate InGaAs quantum-dot chains without vertical electronic coupling. The achieved control of self-assembly of organized InGaAs quantum dots may be advantageous for novel optoelectronic applications. (c) 2005 American Vacuum Society.
Abstract: Utilizing the naturally curved surface contours provided by oval defects on a GaAs(100) surface, we demonstrate that alignment of quantum-dot chains formed during the growth of (In,Ga)As multilayers is unyielding to a modest deviation of surface orientation from (100) of about 0.7degrees along [01-1] and 8degrees along [011]. This finding suggests that the strain-driven kinetic anisotropy responsible for the formation of the quantum dot chains dominates over selective island formation at steps due to surface misorientation. The robustness of the quantum dot chain adds to its potential for its future application. (C) 2004 American Institute of Physics.
Abstract: Nanostructure evolution during the growth of multilayers of In0.5Ga0.5As/GaAs (100) by molecular-beam epitaxy is investigated to control the formation of lines of quantum dots called quantum-dot chains. It is found that the dot chains can be substantially increased in length by the introduction of growth interruptions during the initial stages of growth of the GaAs spacer layer. Quantum-dot chains that are longer than 5 mum are obtained by adjusting the In0.5Ga0.5As coverage and growth interruptions. The growth procedure is also used to create a template to form InAs dots into chains with a predictable dot density. The resulting dot chains offer the possibility to engineer carrier interaction among dots for novel physical phenomena and potential devices. (C) 2004 American Institute of Physics.
Abstract: Using (331)B GaAs templates with nanoscale fluctuations, we have fabricated InGaAs/GaAs quantum wires (QWRs) with a density of similar to2.0x10(6) cm(-1) and the degree of polarization as high as similar to28%. In the samples with weak lateral confinement, we observed thermal delocalization of carriers from the one-dimensional QWR states to the two-dimensional quantum-well states with increasing temperature, which is almost absent in QWR samples with strong lateral confinement. (C) 2004 American Institute of Physics.
Abstract: Optical absorption spectra of intersubband transitions in In0.3Ga0.7As/GaAs multiple quantum dots were investigated using the optical absorption as a function of the number of In0.3Ga0.7As monolayers deposited using the molecular-beam epitaxy Stranski-Krastanow technique. The peak position energy reached 13.7 mum for a sample containing 50 monolayers of In0.3Ga0.7As. The lack of the observation of intersubband transitions in small quantum dots, where the number of the deposited monolayer is less than 15 monolayers, is an indication of the absence of quantum confinement. On the other hand, the presence of high dislocations density in larger quantum dots, where the deposited number of monolayers exceeds 50, could be the reason of why the intersubband transitions are degraded. (C) 2004 American Institute of Physics.
Abstract: Generally (In,Ga)As strained growth on GaAs surfaces results in zero-dimensional quantum dots. The formation of one-dimensional quantum wires is demonstrated during (In,Ga)As molecular-beam-epitaxial growth on GaAs(311)A at high temperature. The wires are running along the [-233] direction. Atomically resolved scanning tunneling microscopy images reveal that the wires are triangular-shaped in cross section and the two side bonding facets are {11,5,2}. These results are discussed in terms of a mechanism of strain-driven facet formation. (C) 2004 American Institute of Physics.
Abstract: We report results of time-resolved terahertz (THz) pulse spectroscopy experiments on laterally ordered chains of self-assembled InGaAs quantum dots photoexcited with 400 nm, 100 fs laser pulses. A large anisotropy in the transient photoconductive response is observed depending on the polarization of the THz probe pulse with respect to the orientation of the dot chains. Fast (3.5-5 ps) and efficient carrier capture into the dots and one-dimensional wetting layers underneath the dot chains is observed below 90 K. At higher temperatures, thermionic emission into the two-dimensional wetting layers and barriers becomes significant and the anisotropy in the photoconductive signal is reduced. (C) 2004 American Institute of Physics.
Abstract: We demonstrate the use of high index GaAs (41 1)A substrates as templates for the growth of (In,Ga)As quantum wires and quantum dots by molecular beam epitaxy. Scanning tunneling microscopy is used to characterize the facets of these nanostructures. For a deposition of six monolayers of (In,Ga)As on GaAs (4 1 1)A, quantum wire structures were observed to form along the [-122] direction with side facets indexed to 1115 2}. By increasing the (In,Ga)As deposition to twelve monolayers, three-dimensional islands were observed to form above the wires bounded by similar 1115 2} facets as well as steeper {1 10) and (1 11)A facets and a convex curved region composed of (100) facets. (C) 2004 Elsevier B.V. All rights reserved.
Abstract: A formation process for long chains of quantum dots during the molecular-beam epitaxial growth of (In,Ga)As/GaAs(100) multilayers is presented. The morphology evolution monitored by atomic force microscopy for a series of (In,Ga)As layers demonstrates that the highly anisotropic lateral alignment of dots is gradually developed as the result of the strain field interaction mediated by the GaAs spacer coupled with the anisotropic surface kinetics that occurs during capping the dots. The dot-chain structure, providing unique properties of its own, is demonstrated to serve as a template for the spatially controlled growth of strained quantum dots in general. (C) 2004 American Institute of Physics.
Abstract: The behavior of InAs deposition on GaAs(111)B substrates and the corresponding routes toward strain relaxation have been investigated. InAs growth was for depositions ranging from 2 monolayers to 30 monolayers. Over this deposition range, different routes for strain relaxation caused by the lattice mismatch were observed. The strain relaxed through ragged step edge formation and Ga-In intermixing for low InAs deposition and through the formation of step bunching and dislocations for thicker depositions.
Abstract: Self-organized surface ordering of (In,Ga)As quantum dots in a GaAs matrix was investigated using stacked multiple quantum dot layers prepared by molecular-beam epitaxy. While one-dimensional chain-like ordering is formed on singular and slightly misorientated GaAs(100) surfaces, we report on two-dimensional square-like ordering that appears on GaAs(n11)B, where n is 7, 5, 4, and 3. Using a technique to control surface diffusion, the different ordering patterns are found to result from the competition between anisotropic surface diffusion and anisotropic elastic matrix, a similar mechanism suggested before by Solomon [Appl. Phys. Lett. 84, 2073 (2004)]. (C) 2004 American Institute of Physics.
Abstract: The surface morphology of the GaAs(331) surface was investigated by in situ reflection high-energy electron diffraction and scanning tunneling microscopy. It was found, that GaAs(331) A and B surfaces are both faceted on a nanometer scale, containing (110) and (111) facets which are atomically resolved in real space. The resulting highly anisotropic ridge-like surfaces can prove useful in the fabrication of quantum wire structures. (C) 2003 American Institute of Physics.
Abstract: The piezoelectric (PZ) effect is demonstrated for the elongated three-dimensional (In,Ga)As islands grown on a GaAs (100) substrate. The photoluminescence (PL) spectrum is studied as a function of excitation intensity. With increasing excitation intensity, a blue shift and a linewidth reduction of the PL peak from the (In,Ga)As islands are observed. The observed phenomena are attributed to the screening of the internal strain-induced PZ field in the (In,Ga)As islands.
Abstract: The shape of InAs three-dimensional islands grown on GaAs(311)A substrates by molecular-beam epitaxy was investigated by in situ scanning tunneling microscopy. The island is found to be laterally surrounded by (111)A and {110} facets together with a convex curved region close to the (100) facet. The top ridge of the islands is atomically resolved to be the most recently discovered high-index surface {11,5,2}. This observation points to the importance of the study of nanostructure growth on high-index surfaces and their characterization. (C) 2003 American Institute of Physics.
Abstract: Scanning tunneling microscopy is used to capture the initial stage of the transition from c(4x4) to (2x4) reconstruction on a GaAs(100) surface. A model for the transition is proposed in which surface atoms escape from the c(4x4) reconstructed area to form (2x4) reconstructed islands and pits. The proposed explanation is consistent with c(4x4) models having species intermixing in the first or second layer.
Abstract: Molecular-beam-epitaxy growth of strained (In,Ga)As on GaAs vicinal (100) surfaces is investigated by scanning tunneling microscopy. Surface roughing as the consequence of step bunching driven by strain is explored. By tuning the In content over the range from 0.05 to 0.2, the step bunching is observed to exhibit considerable uniformity and order. These results experimentally demonstrate that strain-driven step bunching is a viable approach to provide templates for nanostructure growth. (C) 2003 American Institute of Physics.
Abstract: Photoluminescence (PL) of self-organized quantum dots (QDs) in bilayer InAs/GaAs structures is studied with a fixed seed layer and spacer, but variable second-layer coverage. Careful line shape analysis reveals modulation in the high-energy tail of the seed-layer PL spectrum. The oscillation-like behavior is reproducible with variations in both the temperature and optical excitation energy. These oscillations are attributed to carrier relaxation through inelastic phonon scattering from the wetting layer to the QD excited states. (C) 2003 American Institute of Physics.
Abstract: We report on the first fabrication of (In,Ga)As/GaAs quantum dots with both vertical and lateral ordering forming a three-dimensional array. An investigation of the photoluminescence spectra from the ordered array of quantum dots, as a function of both temperature and optical excitation intensity, reveals both a lateral and vertical transfer of excitation. (C) 2003 American Institute of Physics.
Abstract: Photoluminescence (PL) properties of self-organized quantum dots (QDs) in a vertically aligned double-layer InAs/GaAs QD structure are studied as a function of temperature from 10 to 290 K. The QDs in a sample with a 1.8 ML InAs seed layer and a second 2.4 ML InAs layer are found to self-organize in pairs of unequal sized QDs with clearly discernible ground-states transition energy. The unusual temperature behavior of the PL for such asymmetrical QD pairs provides clear evidence for carrier transfer from smaller to larger QDs by means of a nonresonant multiphonon-assisted tunneling process in the case of interlayer transfer and through carrier thermal emission and recapture within one layer. (C) 2002 American Institute of Physics.
Abstract: GaAs(311) surfaces grown by molecular beam epitaxy are investigated by in situ ultrahigh-vacuum scanning tunnelling microscopy. The observation of an atomically flat Ga(2x1)-reconstructed GaAs(311) surface and its transformation to a 8x1-reconstructed GaAs(311) surface leads to an improved understanding of the processes involved in the step formation. The high density of steps observed on the 8x1-reconstructed GaAs(311) surface along the [(2) over bar 33] direction originates from the change of surface atomic density required to accommodate the surface transition from the Ga(2x1) surface to the 8x1 surface. This understanding is further supported by the observation of independent step formation.
Abstract: Morphologies of GaAs(311) surfaces grown by molecular beam epitaxy were investigated by in situ reflection high-energy electron diffraction and scanning tunnelling microscope. In addition to the (8x1) reconstruction, two surface phases, GaAs(311)A-(4x1) and GaAs(311)B-(2x1) were observed. Both of these surfaces are characterized by wider, atomically smooth terraces with much lower structural anisotropy, when compared to the (8x1) reconstructed GaAs(311) surfaces. The observed surfaces have potential as templates for the growth of organized quantum dots, wires, and wells. (C) 2002 American Institute of Physics.
Abstract: Ferromagnetic MnAs layers with abrupt interfaces were grown by MBE on (001) and (311)A oriented GaAs substrates. At low growth rates and As-rich conditions, the films are ((1) over bar 1 0 0) oriented on both templates. For MnAs/GaAs(0 0 1) layers it is demonstrated that realizing a well-ordered reconstructed growth surface and the layer-by-layer growth mode on a thin MnAs buffer, films of high structural quality and very promising magnetic properties are obtained. The quality of thick films is limited by processes accompanying the beta --> alpha MnAs phase transition during sample cooling. The magnetic domains, their increase in size with increasing external magnetic field and their relation to topographical defects was studied by magnetic force microscopy combined with atomic force microscopy. (C) 2001 Elsevier Science B.V. All rights reserved.
Abstract: Reflection high-energy electron diffraction (RHEED) and scanning tunneling microscopy (STM) are used to study the surface and growth of GaAs (311)B. The RHEED pattern reveals a lateral periodicity of 3.2 nm along the [01 (1) over bar] direction, which is confirmed in real space by STM images. Pronounced RHEED intensity oscillations during the homoepitaxial growth on GaAs(311)B were observed in a wide substrate temperature range. (C) 2001 American Institute of Physics.
Abstract: The optical properties of InAs quantum dots in n-i-p-i GaAs superlattices are investigated by photoluminescence (PL) characterization. We have observed an anomalously large blueshift of the PL peak and increase of the PL linewidth with increasing excitation intensity, much smaller PL intensity decrease, and faster PL peak redshift with increasing temperature as compared to conventional InAs quantum dots embedded in intrinsic GaAs barriers. The observed phenomena can all be attributed to the filling effects of the spatially separated photogenerated carriers. (C) 2000 American Institute of Physics. [S0003-6951(00)03515-4].
Abstract: By using scanning tunneling microscopy and reflection high-energy electron diffraction it is demonstrated that self-organized ordered Si structures develop during submonolayer Si deposition on vicinal GaAs(001) surfaces, provided the preferred adsorption sites in the trenches of the (2 x 4) reconstruction are filled with Ga. The evolution of different reconstructions with increasing Si coverages is accompanied by step bunching and de-bunching processes. This unexpected behavior is explained by the interaction between reconstructions and steps from a thermodynamic equilibrium view. For particular coverages the complex process leads to a separation of different surface phases and Si coverages on terraces and in step regions.
Abstract: After capping InAs islands with a thin enough GaAs layer, growth interruption has been introduced. Ejected energy of self-organized InAs/GaAs quantum dots has been successfully tuned in a controlled manner by changing the thickness of GaAs capping layer and the time of growth interruption and InAs layer thickness. The photoluminescence (PL) spectra showing the shift of the peak position reveals the tuning of the electronic states of the QD system. Enhanced uniformity of Quantum dots is observed judging from the decrease of full width at half maximum of FL. Injection InAs/GaAs quantum dot lasers have been fabricated and performed on various frequencies. (C) 2000 Published by Elsevier Science B.V. All rights reserved.
Abstract: Reflection high-energy electron diffraction and reflectance difference spectroscopy are used to monitor in real-time the Si incorporation as well as nucleation and growth of MnAs on different well-defined GaAs(001) templates. It is demonstrated that Si incorporation into the trenches of the (2 x 4)beta 2 structure occurs randomly whereas for Si deposition on a (2 x 4)alpha template a high degree of ordering is observed. This is confirmed by real-space imaging using scanning tunneling microscopy. Despite the dissimilar NiAs structure ferromagnetic MnAs can be grown in high structural quality on GaAs(001)-d(4 x 4), as evidenced by real-time measurements as well as high-resolution transmission electron microscopy. The exposed ((1) over bar 1.0)MnAs surface develops stoichiometry dependent reconstructions. These findings are promising in view of the integration of ferromagnetic material within the framework of the well developed GaAs technology. (C) 2000 Elsevier Science S.A. All rights reserved.
Abstract: The evolution of the morphology of the vicinal GaAs (001) surface induced by Si deposition was studied by scanning tunneling microscopy. The observed step bunching is accompanied by a spatial separation of surface phases with different Si coverages on terraces and in step regions. The spacing and height of the bunches depend on the substrate temperature. A model is proposed to account for these effects by considering a kinetic pathway of the surface to an accessible lowest energy. state. (C) 2000 Elsevier Science B.V. All rights reserved.
Abstract: Both the photoluminescence peaks corresponding to the vertical transitions and the nonvertical transitions in an n-i-p-i GaAs superlattice are clearly observed. The redshifts of the two peaks with increasing temperature are: discussed in terms of the temperature-dependent carrier separation effect.
Abstract: Scanning tunneling microscopy studies of the restructuring of GaAs (001) sui-faces induced by Si deposition have been performed. With increasing Si coverages, different reconstructions are developing. Their interaction with the surface step structures results in a distinct separation into different surface phases with different Si coverages, revealing unique Si distribution patterns. This phenomenon is explained by considering kinetically accessible thermodynamically determined structures. (C) 2000 American Vacuum Society. [S0734-211X(00)00304-8].
Abstract: Growth interruption was introduced after the deposition of GaAs cap layer, which is thinner than the height of quantum dots. Uniformity of quantum dots has been enhanced because the full-width of half-maximum of photoluminescence decrease from 80 to 27 meV in these samples as the interruption time is increased. Meanwhile, we have observed that the peak position of photoluminescence is a function of interruption time, which can be used to modulate energy level of quantum dots. All of the phenomenon mentioned above can be attributed to the diffusion of In atoms from the tops of InAs islands to the top of GaAs cap layer caused by the difference between the surface energies of InAs and GaAs. (C) 1999 Elsevier Science B.V. All rights reserved.
Abstract: In situ ultra high vacuum scanning probe microscopy (SPM) and low-temperature :photoluminescence (PL) studies have been performed on Si-doped self-organized InAs/GaAs quantum dots samples to investigate the Si doping effects. Remarkably, when Si is doped in the sample, according to the SPM images, more small dots are formed when compared with images from undoped samples. On the PL spectra, high-energy band tail which correspond to the small dots appear, with increasing doping concentration, the integral intensity of the high-energy band tail account for the whole peak increase too. We relate this phenomenon to a model that takes the Si atom as the nucleation center for QDs formation. (C) 1999 Elsevier Science B.V. All rights reserved.
Abstract: Growth interruption was introduced after the deposition of GaAs cap layer, which is thinner than the mean height of Quantum dots. Uniformity of quantum dots has been enhanced because the full width of half maximum of photoluminescence decrease from 80meV to 27meV in these samples as the interruption time increasing from 0 to 120 second. Meanwhile, we have observed that the peak position of photoluminescence is a function of interruption time. This effect can be used to control the energy level of quantum dots. The phenomena mentioned above can be attributed to the diffusion of In atoms from the top of InAs islands to the top of GaAs cap layer caused by the difference of surface energies between InAs and GaAs.
Abstract: We investigate the annealing behavior of InAs layers with different thicknesses in a GaAs matrix. The diffusion enhancement by strain, which is well established in strained quantum wells, occurs in InAs/GaAs quantum dots (QDs). A shift of the QD luminescence peak toward higher energies results from this enhanced diffusion. In the case of structures where a significant portion of the strain is relaxed by dislocations, the interdiffusion becomes negligible, and there is a propensity to generate additional dislocations. This results in a decrease of the QD luminescence intensity, and the QD peak energy is weakly affected.
Abstract: The effect of growth interruption on the InAs deposition and its subsequent growth as self-assembled island structures, in particular the material transport process of the InAs layers has been investigated by photoluminescence and transmission electron microscopy measurements. InAs material in structures with only coherent islands transfers from the wetting layer to the formed islands and the growth interruption causes a red shift of PL peak energy. On the other hand, the PL peak shifts to higher energy in structures containing simultaneously coherent and noncoherent islands with dislocations. In this case, the noncoherent islands capture InAs material from the surrounding wetting layer as well as coherent islands, which casues a reduction in the size of these islands. The variations in the PL intensity and line width are also discussed. (C) 1998 Elsevier Science B.V. All rights reserved.
Abstract: We investigate the annealing behavior of Photoluminescence (PL) from self-assembled InAs quantum dots (QDs) with different thicknesses GaAs cap layers. The diffusion introduced by annealing treatment results in a blue-shift of the QD PL peak, and a decrease in the integrated intensity. The strain present in QDs enhances the diffusion, and the QDs with the cap layers of different thicknesses will experience a strain of different strength. This can lend to a, better understanding of the larger blue-shift of the PL peak of the deeper buried QDs, and the different variance of the full width at half maximum of the luminescence from QDs with the cap layers of different thicknesses.
Abstract: The influence of interdot electronic coupling on photoluminescence (PL) spectra of self-assembled InAs/GaAs quantum dots (QDs) has been systematically investigated combining with the measurement of transmission electron microscopy. The experimentally observed fast red-shift of PL energy and an anomalous reduction of the linewidth with increasing temperature indicate that the QD ensemble can be regarded as a coupled system. The study of multilayer vertically coupled QD structures shows that a red-shift of PL peak energy and a reduction of PL linewidth are expected as the number of QD layers is increased. On the other hand, two layer QDs with different sizes have been grown according to the mechanism of a vertically correlated arrangement. However, only one PL peak related to the large QD ensemble has been observed due to the strong coupling in InAs pairs. A new possible mechanism to reduce the PL linewidth of QD ensemble is also discussed.
Abstract: Introducing the growth interruption between the InAs deposition and subsequent GaAs growth in self-assembled quantum dot (QD) structures, the material transport process in the InAs layers has been investigated by photoluminescence and transmission electron microscopy measurement. InAs material in structures without misfit dislocations transfers from the wetting layer to QDs corresponding to the red-shift of PL peak energy due to interruption. On the other hand, the PL peak shifts to higher energy in the structures with dislocations. In this case, the misfit dislocations would capture the InAs material from the surrounding wetting layer and coherent islands leading to the reduction of the size of these QDs. The variations in the PL intensity and Linewidth are also discussed.
Abstract: Deep Level Transient Spectroscopy (DLTS) has been applied to investigate the electronic properties of self-organized InAs quantum dots. The energies of electronic ground states of 2.5ML and 1.7ML InAs quantum dots (QDs) with respect to the conduction band of bulk GaAs are about 0.21 eV and 0.09 eV, respectively. We have found that QDs capture electrons by lattice relaxation through a multi-phonon emission process. The samples are QDs embedded in superlattices with or without a 500 Angstrom GaAs spacing layer between every ten periods of a couple of GaAs and InAs layers. The result shows that the density of dislocations in the samples with spacer layers is much lower than in the samples without the spacer layers.
Abstract: The growth of InGaAs on GaAs with sufficient lattice mismatch typically proceeds in the Stranski–Krastanov (SK) mode. In the SK growth mode, the first few monolayers (MLs) of InGaAs form a pseudomorphic twodimensional (2D) layer, traditionally called the wetting layer (WL). After a critical thickness, the development of three-dimensional (3D) InGaAs islands, which partially relieve the built-up strain, is more energetically favorable than continuous layers. Such 3D islands standing on the 2D WL, usually referred to as self-assembled quantum dots (QDs), have been commonly used for low-dimensional semiconductor research in the last decade. Generally, the InGaAs QDs are randomly distributed on the 2D WL due to the stochastic nature of the self-assembly process. The resulting selfassembled InGaAs QDs have emerged as an important class of materials with potential for modern optoelectronic devices, such as QD-based lasers and detectors. In principle, however, more control over uniformity and spatial organization of the InGaAs QD arrays is desirable for many applications. For example, organized 3D arrays are important for addressing QDs and for a collective behavior uniquely different from the individual InGaAs QDs.For the vertical ordering along the growth direction, the SK growth mode has produced excellent results through stacking of multiple InGaAs layers and the corresponding strain interaction through GaAs spacer layers.
