GaAsSb Layer Research Articles

Basic Concepts and Interfacial Aspects of High-Efficiency III-V Multijunction Solar Cells

Among various types of solar cells, MOVPE-grown triple-junction III-V compound semiconductors are today's most efficient photovoltaic devices with conversion efficiencies exceeding 40%. A next-generation multijunction cell with four or more junctions and optimized band gaps is expected to break the present record efficiency surpassing the 50% mark. High band gap material combinations that are lattice matched to GaAs are already well established, but the required low band gap combinations containing a band gap around 1eV are still to be improved. For this purpose, we have developed a low band gap tandem (two-junction) solar cell lattice matched to InP. For the top and bottom subcells InGaAsP (Eg = 1.03 eV) and InGaAs (Eg = 0.73 eV) were utilized, respectively. A new interband tunnel junction was used to connect the subcells, including thin and highly doped layers of n-type InGaAs and p-type GaAsSb. The delicate MOVPE preparation of critical interfaces was monitored with in-situ reflectance anisotropy spectroscopy (RAS). After a contamination-free transfer, the RAS signals were then benchmarked in ultrahigh vacuum (UHV) with surface science techniques like low energy electron diffraction (LEED) and X-ray photoelectron spectroscopy (XPS). XPS measurements revealed that the sharpest InGaAs/GaAsSb interface was achieved when the GaAsSb layer in the tunnel junction of the solar cell was grown on III-rich (2×4)- or (4×2)-reconstructed InGaAs(100) surfaces. The improved interface preparation had a positive impact on the overall performance of the tandem cell, where slightly higher efficiencies were observed for the cells with the III-rich-prepared tunnel junction interfaces.

InGaAs/GaAsSb-interface studies in a tunnel junction of a low band gap tandem solar cell

A low band gap InP-based double junction (2 J) solar cell was designed, realized and measured. The cell is supposed to work in a 4 junction configuration below an InGaP/GaAs 2 J solar cell. For the lower and higher band gap absorbers InGaAs ( E g = 0.73 eV) and InGaAsP ( E g = 1.03 eV) were utilized, respectively. These two subcells were connected via a tunnel junction including thin, highly doped n-type InGaAs and p-type GaAsSb layers. The growth of GaAsSb layers on different InGaAs surface reconstructions (4 × 3), (2 × 4) and (4 × 2) was investigated with in-situ reflectance anisotropy spectroscopy (RAS). The optical signals were benchmarked in ultrahigh vacuum (UHV) with X-ray photoelectron spectroscopy (XPS) and low energy electron diffraction (LEED). The influence of different preparation procedures on the InGaAs/GaAsSb heterointerface and on the cell performance was investigated. XPS measurements revealed that the sharpest InGaAs/GaAsSb interface was achieved when the GaAsSb layer in the tunnel junction of the solar cell was grown on III-rich, i.e., (2 × 4)- or (4 × 2)-reconstructed InGaAs(100) surfaces. This was also confirmed by solar power conversion efficiency measurements of the solar cells, where slightly lower efficiencies were observed for the cells with the V-rich-prepared tunnel junction interfaces (4 × 3).

Characterization of band gap in GaAsSb/GaAs heterojunction and band alignment in GaAsSb/GaAs multiple quantum wells

Photoreflectance (PR) and photoluminescence (PL) spectra are used to investigate the band alignment of GaAsSb/GaAs multiple quantum wells (MQWs). PR and PL spectra are measured for strained and unstrained GaAs 1− x Sb x /GaAs heterojunctions and coherently strained MQWs grown on a GaAs substrate by molecular beam epitaxy. The band gaps of the unstrained GaAs 1− x Sb x obtained from PL agree well with E g( x) = 1.43 − 1.9 x + 1.2 x 2. For the strained heterojunctions, the strain relaxation factor and the Sb mole fraction determined from PR measurements correspond to the results from X-ray diffraction. In the MQWs, the thickness of the GaAsSb layer is less than its critical thickness so the GaAsSb layer is coherently strained and the band gaps of the GaAs 1− x Sb x layers are estimated under this condition. In this study, the indirect transition from the electron levels in the GaAs layer to the hole levels in the GaAsSb layer is smaller than the band gap of the GaAsSb layer in the MQWs indicating that the band alignment of coherently strained GaAs 1− x Sb x /GaAs MQWs must be type-II.

In-plane self-arrangement of high-density InAs quantum dots on GaAsSb∕GaAs(001) by molecular beam epitaxy

InAs quantum dots (QDs) were grown on GaAsSb∕GaAs(001) buffer layers by molecular beam epitaxy using a Stranski-Krastanov mode. A high QD density of about 1×1011 cm−2 was obtained for an Sb flux ratio of more than 0.14. In the case of high-density QD growth on the GaAsSb layers, many nanoholes were observed on the surface and located beside the QDs. In addition, one-dimensional QD chains were formed near nanogrooves and step edges. As the InAs growth proceeded, an in-plane arrangement of the InAs QDs was partially formed along ⟨010⟩ directions. It is considered that the in-plane arrangement of the QDs originates from the QD chains and the nanogrooves, aligned along ⟨010⟩ step edges. From transmission electron microscopy and photoluminescence measurements, it was found that the high-density InAs QDs on the GaAsSb buffer layer reveal high crystal quality.

Atomic arrangement in a CuPt-B-type ordered GaAsSb layer grown by molecular beam epitaxy observed by cross sectional scanning tunneling microscope

Using an ultra-high-vacuum cross-sectional scanning tunneling microscope (XSTM), we observed the atomic arrangement of a double period (CuPt)-B-type ordered GaAsSb layer grown on (0 0 1) InP by molecular beam epitaxy (MBE). XSTM images of the ordered GaAsSb region revealed that the arrangement of the CuPt-B-type ordered GaAsSb layer was partially and non-uniformly formed in the epitaxial layer. The image also revealed that the ordered region was composed of one GaAs-like planes and one GaSb-like planes alternatively aligned in the [ 1 ¯ 1 1 ] and [ 1 1 ¯ 1 ] directions.

In Ga As ∕ Ga As /alkanethiolate radial superlattices

A radial InGaAs∕GaAs∕1-hexadecanethiol superlattice is fabricated by the roll-up of a strained InGaAs∕GaAs bilayer passivated with a molecular self-assembled monolayer. The technique allows the formation of multiperiod inorganic/organic hybrid heterostructures. The authors investigate the radial superlattices in a detailed transmission electron microscopy study. The structure consists of 11 tightly bonded semiconductor/organic layers with thicknesses and chemical compositions accurately controlled by epitaxial growth and self-assembly. Their chemical analysis reveals that neither any detectable oxygen contamination nor amorphization is present at the superlattice interfaces.

Growth of an InGaAs/GaAsSb tunnel junction for an InP-based low band gap tandem solar cell

The hetero-interfaces of an InGaAs/GaAsSb tunnel junction, embedded into two InP barrier layers, as used in a tandem solar cell, were studied. With regard to the sharpness of these interfaces, different surface preparations and growth procedures were tested. For that, several surface reconstructions of In 0.53 Ga 0.47 As grown by metal organic chemical vapor deposition (MOCVD) were investigated in-situ with reflectance difference spectroscopy (RDS) and analyzed in ultrahigh vacuum (UHV) with low energy electron diffraction (LEED) and X-ray photoemission spectroscopy (XPS). Depending on the annealing temperature, three different surface reconstructions of MOCVD prepared InGaAs were found: As-rich ( 4 × 3 ) , ( 2 × 4 ) and III-rich ( 4 × 2 ) / c ( 8 × 2 ) . Next, thin GaAs 0.51 Sb 0.49 layers were grown on both As-rich and III-rich InGaAs surfaces. LEED patterns showed the expected c ( 4 × 4 ) reconstruction of the As-terminated GaAsSb surface for both preparations, also of the same quality. A significant difference for the Sb to As ratios of thin GaAsSb layers were measured by XPS, indicating that the unusual growth of GaAsSb on a III-rich InGaAs surface results in a sharper InGaAs/GaAsSb hetero-interface. Moreover, we searched for the best preparation procedure of the InP barrier layer on GaAsSb. RDS and a stable growth of the top cell suggested a favorable growth procedure by ramping the growth temperature from 500 to 600 ∘ C during the growth of the InP layer. The challenge here was to prepare GaAsSb at the optimum temperature of 500 ∘ C , to switch then to the optimum growth temperature of the subsequent layers of 600 ∘ C and to produce a well-defined and sharp interface.

Photoluminescence beyond 1.5 μm from InAs quantum dots

Photoluminescence measurements were carried out to investigate the origin of long wavelength emissions (∼1.6μm at room temperature) observed from wafers with InAs quantum dots capped with GaAsSb layers. For wafers with high Sb content (22% and 26%) photoluminescence peak energies were found to be linearly proportional to third root of optical excitation power, a characteristic of emission due to a type-II band alignment. This work therefore presents unambiguous evidence that the long wavelength emission of the wafers comes from type-II band alignment between the InAs quantum dots and the GaAsSb capping layers.

Thermopower evidence for Wigner crystallization in the insulating phase of two-dimensional GaAs bilayer hole systems

We report on low-temperature thermopower measurements of interacting GaAs bilayer hole systems in the limit of no interlayer tunneling. These systems exhibit a reentrant insulating phase near the many-body quantum Hall state (QHS) at total filling factor ν = 1 , when both layers have the same density. The diffusion thermopower is expected to diverge as T - 1 in the presence of an energy gap (Wigner crystal) or to vanish in the case of a disordered induced mobility gap. Our results show that, as the temperature is decreased, the diffusion thermopower exhibits a T - 1 dependence in the insulating phase around ν = 1 . This behavior clearly indicates the opening of an energy gap at low temperature, in agreement with the formation of a pinned Wigner solid. Finally, we report on the T-dependence of the thermopower at ν = 1 .

Bilayer counterflow transport at filling factor 1 in the strong interacting regime

We review magneto-transport properties of interacting GaAs bilayer hole systems, with very small inter-layer tunneling, in a geometry where equal currents are passed in opposite directions in the two, independently contacted layers (counterflow). In the quantum Hall state at total bilayer filling ν = 1 both the longitudinal and Hall counterflow resistances tend to vanish in the limit of zero temperature, suggesting the existence of a superfluid transport mode in the counterflow geometry. As the density of the two layers is reduced, making the bilayer more interacting, the counterflow Hall resistivity ( ρ xy ) decreases at a given temperature while the counterflow longitudinal resistivity ( ρ xx ) , which is much larger than ρ xy , hardly depends on density. Our data suggest that the counterflow dissipation present at any finite temperature is a result of mobile vortices in the superfluid created by the ubiquitous disorder in this system.

Probing residual strain in InGaAs∕GaAs micro-origami tubes by micro-Raman spectroscopy

We have experimentally investigated nanostructures consisting of free-standing microtubes with diameters in the micrometer range fabricated by rolling-up InGaAs∕GaAs bilayers grown by molecular-beam epitaxy on a GaAs substrate. The formation of the microtubes is powered by the built-in strain in the InGaAs layer and they develop after releasing the bilayer structure from the substrate by selective etching. Through micro-Raman spectroscopy we were able to detect the residual strain of the microtube, which results in a frequency shift of phonon modes measured on the tube as compared with reference unstrained material. We developed a simple elastic model to describe the measured phonon frequency shifts, from which we estimate the strain status of the microtube. Results demonstrate the power of Raman spectroscopy as a diagnostic tool for engineering of strain-driven self-positioning microelectromechanical systems.

Microstructure of ordered nanodomains induced by Bi surfactant in OMVPE-grown GaAsSb

Bismuth surfactant is shown to have dramatic effects on the microstructure of GaAsSb alloys grown on nominally [0 0 1] InP substrates by organometallic vapor phase epitaxy. The addition of Bi at a ratio of 1% Bi/Ga in the gas flow results in the formation of CuAu {1 0 0} and chalcopyrite {2 1 0} ordered and disordered structures. Domains are approximately 10–20 nm in diameter for both undoped and heavily carbon-doped GaAsSb layers, and they account for ∼40% (CuAu) and ∼3% (chalcopyrite) of the total surface area. The addition of this small amount of Bi does not affect growth rate, Sb concentration, or surface morphology. Ordered domains with c-axes along the [0 0 1] growth direction were not observed. Photoluminescence and optical absorption measurements did not detect band gap changes in GaAsSb samples with and without {1 0 0} and {2 1 0} ordering, respectively.

Tailoring of high-temperature photoluminescence in InAs∕GaAs bilayer quantum dot structures

Temperature-dependent photoluminescence is investigated in bilayer InAs∕GaAs quantum dot structures with constant InAs deposition θ1 in the seed layer, but variable deposition θ2 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 θ2 and the GaAs spacer thickness.

Interaction and disorder in bilayer counterflow transport at filling-factor one

We study high-mobility, interacting GaAs bilayer hole systems exhibiting counterflow superfluid transport at total filling-factor $\ensuremath{\nu}=1$. As the density of the two layers is reduced, making the bilayer more interacting, the counterflow Hall resistivity $({\ensuremath{\rho}}_{xy})$ decreases at a given temperature, while the counterflow longitudinal resistivity $({\ensuremath{\rho}}_{xx})$, which is much larger than ${\ensuremath{\rho}}_{xy}$, hardly depends on density. On the other hand, a small imbalance in the layer densities can result in significant changes in ${\ensuremath{\rho}}_{xx}$ at $\ensuremath{\nu}=1$, while ${\ensuremath{\rho}}_{xy}$ remains vanishingly small. Our data suggest that the finite ${\ensuremath{\rho}}_{xx}$ at $\ensuremath{\nu}=1$ is a result of mobile vortices in the superfluid created by the ubiquitous disorder in this system.

Quasi-donor-acceptor pair transitions in GaAsSb and AlGaAsSb on InP

We identify quasi-donor-acceptor pair transitions in the photoluminescence spectra of GaAsSb and AlGaAsSb layers, lattice matched to InP, and grown by molecular-beam epitaxy. These alloys show compositional inhomogeneity due to phase separation resulting from miscibility gaps. The presence of Al in the quaternary alloy increases the fluctuation of the electrostatic potential in the epitaxial layer, increasing the variation of recombination energy as a function of intensity excitation in the range of low temperatures.

Thermopower of Interacting GaAs Bilayer Hole Systems in the Reentrant Insulating Phase Nearν=1

We report thermopower measurements of interacting GaAs bilayer hole systems. When the carrier densities in the two layers are equal, these systems exhibit a reentrant insulating phase near the quantum Hall state at total filling factor nu=1. Our data show that, as the temperature is decreased, the thermopower diverges in the insulating phase. This behavior indicates the opening of an energy gap at low temperature, consistent with the formation of a pinned Wigner solid. We extract an energy gap and a Wigner solid melting phase diagram.

Molecular beam epitaxy growth and photoluminescence study of room temperature 1.31 μm (In yGa 1−yAs/GaAs 1−x Sb x)/GaAs bilayer quantum wells

Molecular beam epitaxy (MBE) growth of (In yGa 1− y As/GaAs 1− x Sb x )/GaAs bilayer quantum well (BQW) structures has been investigated. It is evidenced by photoluminescence (PL) that a strong blue shift of the PL peak energy of 47 meV with increasing PL excitation power from 0.63 to 20 mW was observed, indicating type II band alignment of the BQW. The emission wavelength at room temperature from (In yGa 1− y As/GaAs 1− x Sb x )/GaAs BQW is longer (above 1.2 μm) than that from InGaAs/GaAs and GaAsSb/GaAs SQW structures (1.1 μm range), while the emission efficiency from the BQW structures is comparable to that of the SQW. Through optimizing growth conditions, we have obtained room temperature 1.31 μm wavelength emission from the (In yGa 1− y As/GaAs 1− x Sb x )/GaAs BQW. Our results have proved experimentally that the GaAs-based bilayer (In yGa 1− y As/GaAs 1− x Sb x )/GaAs quantum well is a useful structure for the fabrication of near-infrared wavelength optoelectronic devices.

Growth of strained GaAsSb layers on GaAs (0 0 1) by MOVPE

We investigated the growth of GaAsSb layers and quantum wells (QW) on GaAs (0 0 1) by MOVPE. Sb concentrations up to 12% were achieved at low arsine partial pressure and low growth temperature. Varying the tri-methyl antimony pressure primarily changed the growth rate but not the Sb incorporation. The in situ reflectance anisotropy spectrum during growth resembles that of a GaSb surface. Sb has a strong tendency towards segregation and is only incorporated after reaching a certain critical surface coverage. These findings can be explained by a very mobile and highly Sb enriched surface layer which forms by surface melting due to strain.

Optical Characterization of Heavily Sn-Doped GaAs1-xSbx Epilayers Grown by Molecular Beam Epitaxy on (001) GaAs Substrates

We have optically investigated ternary GaAs1-xSbx (x<0.58) epilayers and Sn-doped GaAs1-xSbx (x=0.10–0.14) epilayers grown by molecular beam epitaxy on GaAs (001) substrates. Sn-doped GaAsSb layers were grown as a function of Sn Knudsen-cell temperature, and then characterized by low-temperature photoluminescence (PL) measurements and Hall effect measurements. The Sn-doped GaAsSb films grown at a K-cell temperature of 670°C changed from exhibiting p-type conduction to exhibiting n-type conduction, and showed a maximum PL intensity and a maximum electron mobility of 1900 cm2/V s. The PL intensities obtained for Sn-doped GaAsSb films showed a relatively good correlation with the variations in Hall mobility.

Photoluminescence study of (GaAs1－xSbx/InyGa1－yAs)/GaAs bilayer quantum well grown by molecular beam epitaxy

Photoluminescence study of (GaAs1-xSbx/InyG a1-yAs)/GaAs bilayer quantum wells (BQWs) grown by molecular beam epitaxy (MBE) were carried out. Temperature and excitation power dependent photoluminescence (PL) s tudy indicated that the band alignment of the BQWs is type - Ⅱ. The origin of the double-peak luminescence was discussed. Under optimized growth conditions, t he PL emission wavelength from the BQWs has been extend up to 131 μm with a s ingle peak at room temperature.