GaSb Quantum Dots Research Articles

Engineering carrier lifetimes in type-II In(Ga)Sb/InAs mid-IR emitters

Type-II In(Ga)Sb quantum-confined structures in InAs matrices offer a potential material system for wavelength flexible, high-efficiency, surface-emitting mid-infrared sources. In this work, the authors investigate the carrier dynamics in this material system and demonstrate a number of techniques for engineering carrier lifetimes in such emitters. Samples are grown by molecular beam epitaxy and optically characterized using temperature dependent Fourier transform infrared spectroscopy and mid-infrared time-resolved photoluminescence. The authors investigate both In(Ga)Sb quantum wells and quantum dots, and demonstrate significant improvements in isolated quantum well emitter carrier lifetimes by controlling quantization in the conduction band, or alternatively, by the formation of InGaSb quantum dot structures in InAs matrices. The authors correlate the engineered improvement in carrier lifetime with the emitters temperature performance of our emitters.

Effects of Ga deposition rate and Sb flux on morphology of GaSb quantum dots formed on GaAs

We investigate the effects of Ga deposition rate (RGa) and Sb flux (PSb) on the morphology of GaSb quantum dots (QDs). GaSb QDs are formed on GaAs at 420 °C by Stranski‐Krastanov (SK) mode under various conditions of RGa (0.13‐0.73 monolayer (ML)/s) and PSb (1.0‐4.2 × 10‐7 Torr). An atomic microscope study shows that the density nQD of GaSb QDs is markedly affected by PSb and RGa; no (or few) QDs are formed at high RGa (∼ 0.73 ML/s) and at low PSb (≤1.2 × 10‐7 Torr). As RGa (PSb) increases, nQD first increases and then decreases. nQD reaches its maximum (∼1 × 1011 cm‐2), when the V‐III ratio is about 3.6. We also analyze the experimental data by using a rate equation model and discuss the dependences of nQD on RGa and PSb.

Hole localization energy of 1.18 eV in GaSb quantum dots embedded in GaP

AbstractSelf‐organized GaSb quantum dots are embedded in GaP by molecular beam epitaxy and ‐diodes are fabricated. The structure of the sample is investigated using transmission electron microscopy with atomic resolution. The presence of quantum dots on top of a wetting layer and interdiffusion processes between Sb and P are observed. The localization energy, capture cross‐section, and storage time of holes in the ground state of the quantum dots are determined via deep‐level transient spectroscopy. Their localization energy of eV is found to agree with the theoretical prediction of 1.4 eV once the observed interdiffusion is taken into account. A storage time of holes of days at room temperature is calculated, marking an improvement of 3 orders of magnitude from previous record figures. GaSb/GaP quantum dots are, thus, promising candidates for future non‐volatile DRAMs or fast Flash.Schematic representation of the quantum dots (left) and of the valence band in their vicinity (right). The localization energy is marked on the scheme. The localization of the quantum dots prevents thermal emission of holes, generating a storage time at room temperature of 3.9 days.

Performance improvement of highly mismatched GaSb layers on GaAs by interfacial-treatment-assisted chemical vapor deposition

Strain-relieved GaSb quantum dots on GaAs can be achieved by either periodic interfacial misfit (IMF) or the conventional Stranski–Krastanov (SK) growth modes by changing the growth parameters. In this study, the Sb interfacial treatment was employed to improve the GaSb crystal quality including low defect density, smooth surface morphology, and high hole mobility. This technique yields two-dimensional (2D) islands with a height as low as 1.7 nm and width up to 190 nm in the IMF growth mode. In contrast to the interfacial treatments conventionally employed in the initial strain relaxation of GaSb/GaAs hererostructure, the Sb treatment promotes the formation of strong Ga-Sb bonds on the surface of the grown island, which effectively reduces the interfacial free energy and thus promotes the formation of 2D islands. With the Sb interfacial treatment, a high-relaxation 100-nm GaSb epilayer was grown on the GaAs substrate, the epilayers was strain relaxed and exhibited enhanced electrical properties with a high hole mobility of ~667 cm2 V−1 s−1 and with superior optical properties as evidenced by the photoluminescence B-line peak. The results of this study demonstrate an effective interfacial-treatment growth technique to relax the initial strain for the highly mismatched GaSb layers grown on a GaAs substrate.

Effect of InxGa1−xAs Interlayer on Surface Morphology and Optical Properties of GaSb/InGaAs Type-II Quantum Dots Grown on InP (100) Substrates**Supported by the Science and Technology Planning Projects of Guangdong Province under Grant Nos 2014B050505020, 2015B010114007 and 2014B090904045, the Research Fund for the Doctoral Program of Higher Education of China under Grant No 20134407110008, the Guangzhou Science and Technology Project of

The effects of indium composition in InGaAs interlayer on morphology of GaSb/InGaAs quantum dots (QDs) and on optical properties of GaSb/InGaAs QD material system are studied. AFM images show that the change of the indium composition in InGaAs interlayer can alter the GaSb QD morphology. It is found that low indium composition in InGaAs interlayer can promote the formation of QDs, while high indium composition can inhibit the formation of QDs. The photoluminescence (PL) spectra of GaSb/InGaAs QDs at 8K under low excitation power indicate that the third root of the excitation power is linear with the peak position, which provides a direct evidence for their luminescence belonging to type-II material optical transition. The PL spectra at 8K under an excitation power of 90 mW show that the optical properties of GaSb/InGaAs QD material system can be affected by the indium composition in the InGaAs interlayer, and the PL peak position is linear with the indium composition. The optical properties of GaSb/InGaAs QDs can be improved by adjusting the indium composition in the InGaAs interlayer.

Raman and photoluminescence properties of type II GaSb/GaAs quantum dots on (001) Ge substrate

We investigate structural Raman and photoluminescence properties of type II GaSb/GaAs quantum dots (QDs) grown on (001) Ge substrate by molecular beam epitaxy. Array of self-assembled GaSb QDs having an areal density of ∼1.66 × 1010 dots/cm2 is obtained by a growth at relatively low substrate temperature (450 °C) on a GaAs surface segmented into anti-phase domains (APDs). Most of QDs form in one APD area. However, a few QDs can be observed at the APD boundaries. Raman spectroscopy is used to probe the strain in GaAs layer. Slight redshift of both LO and TO GaAs peaks are observed when GaSb QDs are buried into GaAs matrix. Optical properties of capped QDs are characterized by photoluminescence measurement at low temperatures (20 K and 30 K). Emission peaks of GaSb/GaAs QDs are found in the range of 1.0-1.3 eV at both temperatures. Slight redshift is observed when the laser excitation power is increased at 20 K while blueshift of QD peak is observed at 30 K. We attribute this abnormal behavior to the contribution of overlapped GaSb wetting layer peak in the PL emission as well as the feature of type II band structure.

Atomic-column scanning transmission electron microscopy analysis of misfit dislocations in GaSb/GaAs quantum dots

The structural quality of GaSb/GaAs quantum dots (QDs) has been analyzed at atomic scale by aberration-corrected high-angle annular dark-field scanning transmission electron microscopy. In particular, we have studied the misfit dislocations that appear because of the high-lattice mismatch in the heterostructure. Our results have shown the formation of Lomer dislocations not only at the interface between the GaSb QDs and the GaAs substrate, but also at the interface with the GaAs capping layer, which is not a frequent observation. The analysis of these dislocations points to the existence of chains of dislocation loops around the QDs. The dislocation core of the observed defects has been characterized, showing that they are reconstructed Lomer dislocations, which have less distortion at the dislocation core in comparison to unreconstructed ones. Strain measurements using geometric phase analysis show that these dislocations may not fully relax the strain due to the lattice mismatch in the GaSb QDs.

GaAs-based triangular barrier photodiodes with embedded type-II GaSb quantum dots

We fabricate GaAs-based triangular barrier photodiodes (TBPs), in which type-II GaSb quantum dots (QDs) are embedded in the vertex part of their triangular barriers. Their current–voltage characteristics and photo-responses are studied at low temperatures to show that GaSb QDs enhance the number and lifetime of photo-generated holes that are trapped by QDs in the barrier, resulting in the increase in the electron current around positively charged QDs. An extremely high responsivity of 109 A/W is achieved.

Contribution of a single quantum dots layer in intermediate band solar cells: A capacitance analysis

Theoretical model and numerical analysis of charge accumulation within a single GaSb quantum dots layer embedded in GaAs-based Schottky diode is performed. We hereby give an analytical calculation of the capacitance–voltage (C–V) characteristic of GaAs-based Schottky barrier structure incorporating GaSb self-assembled quantum dots layer. The Schottky barrier is derived in different bias voltage region based on solving analytically Poisson׳s equation, including the effects of the dots size dispersion and the Fermi statistics of the holes in the quantum dots. The numerical simulation of capacitance–voltage curves exhibits a plateau that is caused by the high carrier concentration and the saturation of the quantum dots levels upon the applied voltage. These results are in good agreement with experiments done by Hwang et al.

Investigation of GaSb/GaAs Quantum Dots Formation on Ge (001) Substrate and Effect of Anti-Phase Domains

ABSTRACTThe effects of GaAs anti-phase domains (APDs) on the growth of GaSb quantum dots (QDs) are investigated by molecular beam epitaxial growth of GaAs on Ge (001) substrate. Ge is a group-IV element and GaAs is a polar III-V compound semiconductor. Due to polar/non polar interface, GaAs APDs are formed. Initial formation of APD relates to a non-uniform growth of high index GaAs surfaces. However, due to high sticking coefficient of Sb atoms at low substrate growth temperature, GaSb QDs can be formed on the whole surface of the sample without any effects from APD boundary. The buffer layer growth temperature is one of the key roles to control the APDs formation. Therefore we tried to adjust the optimum conditions such as buffer layer thickness and growth temperature to get nearly flat sample surface with large APDs for high QDs density (∼ 8×109 dots/cm2). Low-temperature photoluminescence is conducted and GaSb QDs peak is observed at the energy range of 1.0 eV-1.3 eV.

Delta doping and positioning effects of type II GaSb quantum dots in GaAs solar cell

GaSb quantum dot (QD) solar cell structures were grown by molecular beam epitaxy on GaAs substrates. We investigate the reduction in open-circuit voltage and study the influence of the location of QD layers and their delta doping within the solar cell. Devices with 5 layers of delta-doped QDs placed in the intrinsic, n- and p-regions of a GaAs solar cell are experimentally investigated, and the deduced values of Jsc, Voc, fill factor, efficiency (η) are compared. A trade-off is needed to minimize the Voc degradation while maximizing the short circuit current density (Jsc) enhancement due to sub-bandgap absorption. The voltage recovery is attributed to the removal of the QDs from the high-field region which reduces SRH recombination. The devices with p- or n-doped QDs placed in the flat band potential (p- or n-region) show a recovery in Jsc and Voc compared to devices with delta-doped QDs placed in the depletion region. However, there is less photocurrent arising from the absorption of sub-band gap photons. Furthermore, the long wavelength photoresponse of the n-doped QDs placed in the n-region shows a slight improvement compared to the control cell. The approach of placing QDs in the n-region of the solar cell instead of the depletion region is a possible route towards increasing the conversion efficiency of QD solar cells.

GaSb/GaAs quantum dots and rings grown under periodical growth mode by using molecular beam epitaxy

GaSb/GaAs quantum dots (QDs) and quantum rings (QRs) are investigated. By using periodical growth interrupts, precise coverage control can be achieved for GaSb QD growth by using a single Ga source. With direct As irradiation to the substrate surface during the post soaking time, the soaking time can be effectively reduced while full ring morphologies and room-temperature QR luminescence can still be obtained by using this method.

Hybrid type-I InAs/GaAs and type-II GaSb/GaAs quantum dot structure with enhanced photoluminescence

We investigate the photoluminescence (PL) properties of a hybrid type-I InAs/GaAs and type-II GaSb/GaAs quantum dot (QD) structure grown in a GaAs matrix by molecular beam epitaxy. This hybrid QD structure exhibits more intense PL with a broader spectral range, compared with control samples that contain only InAs or GaSb QDs. This enhanced PL performance is attributed to additional electron and hole injection from the type-I InAs QDs into the adjacent type-II GaSb QDs. We confirm this mechanism using time-resolved and power-dependent PL. These hybrid QD structures show potential for high efficiency QD solar cell applications.

Molecular beam epitaxial growth of GaSb quantum dots on (0 0 1) GaAs substrate with InGaAs insertion layer

We report on the molecular beam epitaxial growth of self-assembled GaSb quantum dots (QDs) on (001) GaAs substrates with an insertion layer. The insertion layer, which is a 4-monolayers (MLs) InxGa1−xAs (x=0.00, 0.07, 0.15, 0.20 and 0.25), is grown prior to the QD growth. With this InGaAs insertion layer, the obtained QD density decreases substantially, while the QD height and diameter increase as compared with typical GaSb QDs grown on conventional (001) GaAs surface under the same growth condition. The GaSb QDs on GaAs have the dome shape with elliptical base and the elongation direction of the base is along the [110] direction. When the InGaAs insertion layer is introduced, the distinct elongation disappears and the QD sidewall shows facet-related surfaces with (001) plateau on top.

Growth and optical properties of GaSb/GaAs type-II quantum dots with and without wetting layer

We investigated the effect of a wetting layer (WL) on the optical properties of GaSb type-II quantum dots (QDs) in GaAs. GaSb QDs were grown by droplet epitaxy, where Ga droplets are first formed on GaAs and then exposed to Sb flux, followed by an annealing step. By adjusting the annealing temperature, we fabricated GaSb QDs with and without a well-defined WL-like structure. Photoluminescence (PL) measurements showed that the high-temperature-annealed samples exhibit a strong PL of the WL, whereas the WL luminescence is quite weak for the samples annealed at low temperatures. As the measurement temperature T increases, the PL energy decreases for the GaSb QDs without the WL. In contrast, the PL energy of the GaSb QDs with the WL has little dependence on T. These PL energy shifts are explained by considering the effects of the hole population and energy-dependent absorption.

Height stabilization of GaSb/GaAs quantum dots by Al-rich capping

GaSb quantum dots (QDs) in a GaAs matrix are investigated with cross-sectional scanning tunneling microscopy (X-STM) and photoluminescence (PL). We observe that Al-rich capping materials prevent destabilization of the nanostructures during the capping stage of the molecular beam epitaxy (MBE) growth process and thus preserves the QD height. However, the strain induced by the absence of destabilization causes many structural defects to appear around the preserved QDs. These defects originate from misfit dislocations near the GaSb/GaAs interface and extend into the capping layer as stacking faults. The lack of a red shift in the QD PL suggests that the preserved dots do not contribute to the emission spectra. We suggest that a better control over the emission wavelength and an increase of the PL intensity is attainable by growing smaller QDs with an Al-rich overgrowth.

GaSb quantum dots on GaAs with high localization energy of 710 meV and an emission wavelength of 1.3 µm

We report on the epitaxial growth of GaSb quantum dots (QDs) that show photoluminescence at a wavelength of around 1.3µm and a high hole localization energy of up to 710meV with a capture cross-section of 1×10−13cm2. The QDs were grown in Stranski–Krastanov (SK) epitaxial mode with molecular beam epitaxy. The characteristics of these QDs are a high dot density of up to 2.6×1010cm−2 as well as narrow dot size and dot density distributions. To achieve the desired values for emission wavelength, hole localization energy, and dot density, the influence of the growth parameters must be controlled precisely. The influence of the V/III (i.e. Sb/Ga) partial pressure or flux ratio, growth temperature, nominal coverage, and growth interruption after quantum dot deposition are investigated. The QD samples are analyzed with atomic force microscopy (AFM), photoluminescence (PL), and deep-level transient spectroscopy (DLTS). Theoretical simulations are performed with the nextnano++ software.

Impact of spatial separation of type‐II GaSb quantum dots from the depletion region on the conversion efficiency limit of GaAs solar cells

AbstractThe purpose of this work is to look for a practical structure for application of quantum dots (QDs) in solar cells. We focus on a stack of strain‐compensated GaSb/GaAs type‐II QDs. We propose a novel structure with GaSb/GaAs type‐II QD absorber embedded in the p‐doped region of ideal solar cell but spatially separated from the depletion region. We developed the model and used the detailed balance principle along with Poisson and continuity equations for calculating of the energy band bending and photovoltaic characteristics of the proposed solar cell. Our model takes into account both single‐photon and two‐photon absorption as well as non‐radiative processes in QDs and predicts that concentration from 1‐sun to 500‐sun increases the efficiency from 30% to 50%. We showed that accumulation of charge in the QD absorber is the clue to understanding of potentially superior performance of the proposed solar cell. An attractive feature of the proposed solar cell is that QDs do not reduce the open‐circuit voltage but facilitate generation of the additional photocurrent to the extent that photovoltaic characteristics reduce to that of ideal solar cell while the efficiency meets the Luque‐Marti limit. It should be noted that although non‐radiative processes like relaxation in QDs and recombination through QDs degrade photovoltaic characteristics of the proposed solar cell, its conversion efficiency is still predicted to be above the Shockley‐Queisser limit by 5% to 10%. This study is an important step toward producing practical solar cells that benefit from additional photocurrent generated by sub‐band gap photons. Copyright © 2014 John Wiley & Sons, Ltd.

Growth of GaSb and AlSb quantum dots on high-index GaAs substrates

We investigate the growth of GaSb and AlSb quantum dots (QDs) on high-index GaAs substrates at various substrate temperatures (Ts). The GaSb QD density on GaAs(311)A is about 1.5–2 times larger than that on GaAs(100) substrates. Small and dense AlSb QDs are formed by AlSb deposition on GaAs(311)A substrates at relatively high temperatures (Ts ≥ 430 °C) with QD densities as high as 29 × 1010 cm−2. In contrast, no QDs are observed for low-temperature AlSb deposition (Ts ∼ 410 °C). In contrast to InAs QDs, GaSb and AlSb QDs are easily formed even on GaAs(111)A surfaces. We use a rate equation to analyze the experimental data and discuss QD density and size and their Ts dependences.

Molecular beam epitaxial growth of GaSb/GaAs quantum dots on Ge substrates

We perform structural and optical investigations of GaSb/GaAs quantum dots (QDs) grown on Ge (001) substrates by molecular beam epitaxy. Anti-phase domains (APDs) of GaAs are distributed on Ge substrate after the growth of GaAs due to the growth nature of III–V compound on group IV semiconductors having polar and non-polar behaviors. The APDs affect the QD growth as demonstrated by the growth of conventional InAs QDs on this surface. For GaSb QDs, the GaSb layer is grown on GaAs APD surface and compared with the GaSb layer on conventional (001) GaAs surface. Self-assembled QDs are formed on both surfaces but structural analysis reveals evidence of shape and size differences, which is attributed to the influence of the initial surface. Photoluminescence of GaSb/GaAs QDs grown on both Ge and GaAs substrates is studied. Emission from GaSb/GaAs QDs on Ge substrate can be detected till near room temperature (270K).