P. S. Dutta

Below bandgap optical absorption in tellurium-doped GaSb

Enhancement in below bandgap room temperature infrared transmission has been observed in tellurium (Te)-doped GaSb bulk crystals. The effect of Te concentration on the transmission characteristics of GaSb has been experimentally and theoretically analysed. Undoped GaSb is known to exhibit p-type conductivity with residual hole concentration of the order of (1–2) × 1017 cm−3 at room temperature due to the formation of native defects. For such samples, inter-valence band absorption has been found to be the dominant absorption mechanism. The residual holes could be compensated by n-type dopants such as Te. With increasing Te concentration, free carrier absorption due to electrons and inter-valley transitions in the conduction subband become significant. The dependences of various absorption mechanisms as a function of wavelength have been discussed in this paper.

Below band-gap optical absorption in GaxIn1−xSb alloys

The below band-gap optical-absorption characteristics of GaxIn1−xSb alloy system have been reported. The different dependencies of the hole and electron absorption mechanisms on wavelength result in significant changes of the absorption characteristics with alloy composition. In the undoped Ga-rich alloy compositions that are p type in nature (due to residual holes resulting from native defects), the inter-valence-band absorption has been found to be the dominant absorption mechanism. With decreasing Ga (increasing indium) mole fraction, the hole to electron ratio decreases for undoped samples. For such samples, absorption due to electrons becomes significant. With n-type extrinsic doping, intervalley transitions are seen for certain Ga-rich compositions, which also alter the absorption characteristics. The dependencies of various absorption mechanisms as a function of wavelength have been analyzed and discussed in this paper. Based on the theoretical analysis presented in this paper, one can calculate th...

Donor‐related deep level in bulk GaSb

Deep level transient spectroscopy (DLTS) and thermally stimulated capacitance (TSCAP) studies on bulk grown tellurium‐ selenium‐, and sulphur‐doped gallium antimonide reveal the presence of deep level intrinsic of the dopant species. The trap densities in Te‐ and Se‐doped samples were found to be at least two orders of magnitude lower than the shallow donor concentration. Interestingly, the DLTS spectrum of S:GaSb exhibits DX‐like nature with the trap concentration comparable to that of shallow donor concentration. However, the Te and Se related levels do not exhibit DX‐like nature. The DLTS and TSCAP results are in good agreement with each other.

Auger and radiative recombination coefficients in 0.55-eV InGaAsSb

A radio-frequency (RF) photoreflectance technique, which senses changes in sample conductivity as carriers recombine following excitation by a laser pulse, has been used to measure the recombination parameters in 0.55 eV InGaAsSb lattice matched to GaSb. Doubly-capped lifetime structures with variable active layer thicknesses are used to extract the surface recombination velocity (SRV), while analysis of the samples with different doping concentrations is used to obtain Auger (C) and radiative (B) recombination parameters. Parameter extraction for the samples evaluated gives C = 1 {+-} 0.4 x 10{sup -28} cm{sup 6}/s and B = 3 {+-} 1.5 x 10{sup -11} cm{sup 3}/s for 0.55 eV InGaAsSb lattice matched to GaSb. The Auger and radiative recombination coefficients obtained from high-level injection decay times in low doping concentration samples show very good agreement with values obtained from low-level injection conditions.

A simple single-step diffusion and emitter etching process for high-efficiency gallium-antimonide thermophotovoltaic devices

A single step diffusion followed by precise etching of the diffused layer has been developed to obtain a diffusion profile appropriate for high efficiency GaSb thermophotovoltaic cells. The junction depth was controlled through monitoring of light current-voltage (I-V) curves (photovoltaic response) during the post diffusion emitter etching process. The measured photoresponses (prior to device fabrication) have been correlated with the quantum efficiencies and the open circuit voltages in the fabricated devices. An optimum junction depth for obtaining highest quantum efficiency and open circuit voltage is presented based on diffusion lengths (or monitoring carrier lifetimes), carrier mobility and typical diffused impurity profile in GaSb.

GaSb and Ga1−xInxSb Thermophotovoltaic Cells using Diffused Junction Technology in Bulk Substrates

This paper presents results of experimental and theoretical research on antimonide‐ based thermophotovoltaic (TPV) materials and cells. The topics discussed include: growth of large diameter ternary GaInSb bulk crystals, substrate preparation, diffused junction processes, cell fabrication and characterization, and, cell modeling. Ternary GaInSb boules up to 2 inches in diameter have been grown using the vertical Bridgman technique with a novel self solute feeding technique. A single step diffusion process followed by precise etching of the diffused layer has been developed to obtain a diffusion profile appropriate for high efficiency, p‐n junction GaSb and GaInSb thermophotovoltaic cells. The optimum junction depth to obtain the highest quantum efficiency and open circuit voltage has been identified based on diffusion lengths (or minority carrier lifetimes), carrier mobility and experimental diffused impurity profiles. Theoretical assessment of the performance of ternary (GaInSb) and binary (GaSb) cells f...

Bulk crystal growth of antimonide based III-V compounds for TPV applications

In this paper, the bulk growth of crack-free GaInSb and single phase GaInAsSb alloys are presented. A new class of III-V quasi-binary semiconductor alloys [AIIIBV]1−x[CIIIDV]x has been synthesized and bulk crystals grown from the melt for the first time. The present investigation is focused on the quasi-binary alloy (GaSb)1−x(InAs)x (0<x<0.05) due to its importance for thermophotovoltaic applications. The structural properties of this melt-grown quasi-binary alloy are found to be significantly different from the conventional quaternary compound Ga1−xInxAsySb1−y with composition x=y. Synthesis and growth procedures are discussed. For the growth of ternary alloys, it was demonstrated that forced convection or mixing in the melt during directional solidification of InxGa1−xSb (9<x<0.1) significantly reduces cracks in the crystals.

Performance Limits of Low Bandgap Thermophotovoltaic Antimonide-Based Cells for Low Temperature Radiators

This paper assesses the performance of antimonide-based thermophotovoltaic cells fabricated by different technologies. In particular, the paper compares the performance of lattice matched quaternary (GaInAsSb) cells epitaxially grown on GaSb substrates to the performance of ternary (GaInSb) and binary (GaSb) cells fabricated by Zn diffusion on bulk substrates. The focus of the paper is to delineate the key performance advantages of the highest performance-to-date of the quaternary cells to the performance of the alternative ternary and binary antimonide-based diffusion technology. The performance characteristics of the cells considered are obtained from PC-1D simulations using appropriate material parameters.

Development of Low Cost III–V Ternary and Quaternary Bulk Substrates and Epilayers for High Efficiency Thermophotovoltaic Applications

This paper presents two of our recent developments in the area of III–V ternary and quaternary materials growth technology for high efficiency thermophotovoltaic (TPV) cells. It is well recognized that growth of TPV cell materials from melts or solutions presents the most economical option. We have investigated bulk substrate growth from melt as well as epitaxial growth from liquid phase. In the bulk crystal growth area, we have been concentrating on the growth of 50 mm or larger diameter device quality ternary III–V substrates. In the epitaxial growth arena, we have developed a new thermochemistry for growing ternary and quaternary compounds on binary substrates from quaternary solutions. Some of the recent results pertaining to the bulk growth of GaInSb and epitaxial growth of GaInAs, InAsSb and GaInAsSb on GaAs substrates will be discussed here.

Morphological Evolution and Properties of LPE Grown GaSb, AlGaSb and AlGaAsSb

The nucleation morphologies of LPE grown GaSb, AlGaSb and AlGaAsSb layers on GaSb substrates are presented. The morphology of the GaSb layers grown from Sb-rich melts showed facets on highly terraced surface, whereas those grown from Ga-rich melts exhibited fine terraces without facets. An optimum temperature in the range of 500 – 550°C was found to be suitable for the growth of mirror smooth layers from Ga-melts. The surface morphology of the Al x Ga 1- x Sb layers degrades drastically with increase in Al content beyond x = 0.5. The surface morphology of AlGaAsSb epilayers has been found to depend strongly on the pre-growth melt dissolution sequence.