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

Description: 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 [A{sub III}B{sub V}]{sub 12{minus}x}[C{sub III}D{sub V}]{sub x} semiconductor alloys 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){sub 1{minus}x}(InAs){sub 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 Ga{sub 1{minus}x}In{sub x}As{sub y}Sb{sub 1{minus}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 In{sub x}Ga{sub 1{minus}x}Sb (0 < x < 0.1) significantly reduces cracks in the crystals.
Date: October 1, 1998
Creator: Dutta, P.S.; Ostrogorsky, A.G. & Gutmann, R.J.
Partner: UNT Libraries Government Documents Department

RF/microwave non-destructive measurements of electrical properties of semiconductor wafers for thermophotovoltaic applications

Description: A radio-frequency/microwave measurement system has been designed for non-contacting determination of sheet resistance and excess carrier lifetime of low-bandgap materials and junctions, specifically GaSb-based alloys for thermophotovoltaic (TPV) applications. The design incorporates RF circuitry in the 100--500 MHz frequency range and utilizes a Q-switched YAG laser at 1.32 microns to photo-generate electron-hole pairs and conductivity modulate the material and/or junction under test. Supplementary measurements with a GaAs pulsed diode laser at 904 nm provides a faster transient response with near-surface photogeneration. Initial measurements on GaSb substrates, Zn-diffused materials and epitaxially grown layers are presented and discussed.
Date: May 1, 1997
Creator: Saroop, S.; Borrego, J.M. & Gutmann, R.J.
Partner: UNT Libraries Government Documents Department

Bulk growth of GaSb and Ga{sub 1{minus}x}In{sub x}Sb

Description: GaSb and InGaSb have been demonstrated to be suitable choices for high efficiency thermophotovoltaic (TPV) cells. Synthesis and growth of bulk GaSb single crystals and GaInSb polycrystals have been carried out by the vertical Bridgman technique, with a baffle immersed in the melt and by complete encapsulation of the melt by low melting temperature alkali halides or oxides. The critical roles of the baffle and the encapsulation are discussed. Efforts in obtaining device grade GaSb with superior structural and electrical properties and compositionally homogeneous GaInSb are described, emphasizing the key steps in the growth cycle developed to obtain good crystalline quality.
Date: May 1, 1997
Creator: Dutta, P.S.; Ostrogorsky, A.G. & Gutmann, R.J.
Partner: UNT Libraries Government Documents Department

Growth of Uniform Ga{sub 1-x}In{sub x}Sb Bulk Crystals by Self-Solute Feeding Technique

Description: Compositionally homogeneous, crack-free bulk crystals of Ga{sub 1-x}In{sub x}Sb with x as high as 0.4 has been grown for the first time using a self-solute feeding method. A balance between the growth rate and the spacing between the solute and the growth interface has been found to be crucial in maintaining uniform alloy composition.
Date: August 29, 2003
Creator: Dutta, P.S.; Rajagopalan, G.; Gutmann, R.J. & Nichols, G.
Partner: UNT Libraries Government Documents Department

Interface Reactions and Electrical Characteristics of Au/GaSb Contacts

Description: The reaction of Au with GaSb occurs at a relatively low temperature (100 C). Upon annealing, a AuSb{sub 2} compound and several Au-Ga phases are produced. Phase transitions occur toward higher Ga concentration with increasing annealing temperatures. Furthermore, the depth of the contact also increases with increased annealing temperature. They found that the AuSb{sub 2} compound forms on the GaSb surface, with the compound crystal partially ordered with respect to the substrate. The transition of Schottky- to ohmic-contact behavior in Au/n-type GaSb occurs simultaneously with the formation of the AuGa compound at about a 250 C annealing temperature. This ohmic contact forms without the segregation of dopants at the metallic compound/GaSb interface. Therefore it is postulated that transition from Schottky- to ohmic-contact behavior is obtained through a series of tunneling transitions of electrons through defects in the depletion region in the Au/n-type GaSb contacts. Contact resistivities of 6-7 x 10{sup -6} {Omega}-cm{sup 2} were obtained with the annealing temperature between 300 and 350 C for 30 seconds. In Au/p-type GaSb contacts, the resistivity was independent of the annealing temperature. This suggested that the carrier transport in p-type contact dominated by thermionic emission.
Date: July 7, 2000
Creator: Ehsani, H.; Gutmann, R.J. & Charache, G.W.
Partner: UNT Libraries Government Documents Department

Generation of Compositionally Graded Ga{sub 1-x}In{sub x}Sb Seed by Solute Diffusion

Description: Compositionally graded single crystalline &lt;100&gt; seed of Ga{sub 1-x}In{sub x}Sb has been grown in a single experiment using a solute diffusion method. The present technique is simple and less time consuming compared to the conventional boot-strapping approach previously used for generating ternary seeds. Starting from an InSb &lt;100&gt; single crystalline seed, a seed of Ga{sub 0.6}In{sub 0.4}Sb has been grown. The effect of temperature gradient on the crystalline quality of seeds grown using this method has been discussed.
Date: August 29, 2002
Creator: Dutta, P.S.; Rajagopalan, G.; Gutmann, R.J. & Nichols, G.
Partner: UNT Libraries Government Documents Department

Silicon as the P-type dopant in GaSb and Ga{sub 0.8}In{sub 0.2}Sb grown by metalorganic vapor phase epitaxy

Description: P-type GaSb and Ga{sub 0.8}In{sub 0.2}Sb layers have been grown on GaSb and GaAs substrates by metalorganic vapor phase epitaxy (MOVPE) using silane as the doping precursor. Hall measurements show that the concentration and mobility of holes in GaSb and Ga{sub 0.8}In{sub 0.2}Sb are higher when the layers are grown on GaSb substrates than when grown on GaAs substrates. Secondary ion mass spectroscopy (SIMS) results show that the incorporation of Si is higher when GaSb substrates are used. The compensation of Si acceptors is negligible in GaSb, but is as high as 25% in Ga{sub 0.8}In{sub 0.2}Sb.
Date: June 1, 1998
Creator: Ehsani, H.; Bhat, I.; Hitchcock, C. Gutmann, R.J.; Charache, G. & Freeman, M.
Partner: UNT Libraries Government Documents Department

Tellurium doping of Ga{sub 0.8}In{sub 0.2}Sb layers grown by metalorganic vapor phase epitaxy

Description: N-type Ga{sub 0.8}In{sub 0.2}Sb epitaxial layers have been grown on GaSb and GaAs substrates by metalorganic vapor phase epitaxy (MOVPE) using diethyltelluride (DETe) as the dopant source. The incorporation efficiency of Te in Ga{sub 0.8}In{sub 0.2}Sb and the electron mobility were found to be higher with GaSb substrates compared to using GaAs substrates. The electron concentration increased from 5 {times} 10{sup 16} cm{sup {minus}3} to 1.5 {times} 10{sup 18} cm{sup {minus}3} as the Te concentration was increased from 1 {times} 10{sup 17} cm{sup {minus}3} to 5 {times} 10{sup 18} cm{sup {minus}3}. As the Te concentration was increased further, the electron concentration decreased, with only about 1% of the Te electrically active at a Te concentration of 2 {times} 10{sup 20} cm{sup {minus}3}.
Date: June 1, 1998
Creator: Ehsani, H.; Bhat, I.; Hitchcock, C.; Gutmann, R.J.; Charache, G. & Freeman, M.
Partner: UNT Libraries Government Documents Department

TPV energy conversion: A review of material and cell related issues

Description: This paper presents an overview of thermophotovoltaic (TPV) energy conversion using low band gap semiconductor photovoltaic cells. Physics of PN junctions related to TPV cells is described and the factors that affect overall cell efficiencies are outlined. Current status of bulk and epitaxial growth of TPV materials and cell fabrication issues are also described.
Date: August 1, 1996
Creator: Bhat, I.B.; Borrego, J.M.; Gutmann, R.J. & Ostrogorsky, A.G.
Partner: UNT Libraries Government Documents Department

P-type and N-type doping in GaSb and Ga{sub 0.8}In{sub 0.2}Sb layers grown by metalorganic vapor phase epitaxy

Description: P-type and n-type GaSb and GA{sub 0.8}In{sub 0.2}Sb layers have been grown on GaSb and GaAs substrates by metalorganic vapor phase epitaxy (MOVPE) using silane and diethyltellurium (DETe) as the dopant precursors, respectively. Hall measurements show that the concentration and mobility of holes and electrons in GaSb and GA{sub 0.8}In{sub 0.2}Sb are higher when the layers are grown on GaSb substrates than when grown on GaAs substrates. Secondary ion mass spectrometry (SIMS) results show that the incorporation of Si and Te is higher when GaSb substrates are used. The electron concentration increased from 5 {times} 10{sup 16} cm{sup {minus}3} to 1.5 {times} 10{sup 18} cm{sup {minus}3} as the Te concentration was increased from 1 {times} 10{sup 17} cm{sup {minus}3} to 5 {times} 10{sup 18} cm{sup {minus}3}. As the Te concentration was increased further, the electron concentration decreased, with only about 1% of the Te electrically active at a Te concentration of 2 {times} 10{sup 20} cm{sup {minus}3}.
Date: June 1, 1998
Creator: Ehsani, H.; Bhat, I.; Hitchcock, C.; Gutmann, R.J.; Charache, G. & Freeman, M.
Partner: UNT Libraries Government Documents Department

Modeling of InGaSb thermophotovoltaic cells and materials

Description: A closed form computer program has been developed for the simulation and optimization of In{sub x}Ga{sub 1{minus}x}Sb thermophotovoltaic cells operating at room temperature. The program includes material parameter models of the energy bandgap, optical absorption constant, electron and hole mobility, intrinsic carrier concentration and index of refraction for any composition of GaInSb alloys.
Date: May 1, 1997
Creator: Zierak, M.; Borrego, J.M.; Bhat, I.; Gutmann, R.J. & Charache, G.
Partner: UNT Libraries Government Documents Department

Thermophotovoltaic Cells on Zinc Diffused Polycrystalline GaSb

Description: For the first time, it has been demonstrated that thermophotovoltaic cells made of polycrystalline GaSb with small grain sizes (down to 100 x 100 {micro}m) have similar characteristics to the best Zinc diffused single crystal GaSb cells with identified device parameters. The grain boundaries in polycrystalline GaSb do not degrade TPV cell parameters, indicating that such material can be used for high-efficiency thermophotovoltaic cells.
Date: May 1, 2000
Creator: Sulima, O.V.; Bett, A.W.; Dutta, P.S.; Ehsani, H. & Gutmann, R.J.
Partner: UNT Libraries Government Documents Department

A Simple Single Step diffusion and Emitter Etching Process for High Efficiency Gallium Antimonide Thermophotovoltaic Devices

Description: 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.
Date: August 29, 2003
Creator: Rajagopalan, G.; Reddy, N.S.; Ehsani, E.; Bhat, I.B.; Dutta, P.S.; Gutmann, R.J. et al.
Partner: UNT Libraries Government Documents Department

Ternary and quaternary antimonide devices for thermophotovoltaic applications

Description: Thermophotovoltaic (TPV) devices have been fabricated using epitaxial ternary and quaternary layers grown on GaSb substrates. GaInSb ternary devices were grown by metalorganic vapor phase epitaxy (MOVPE) with buffer layers to accommodate the lattice mismatch, and GaInAsSb lattice-matched quaternaries were grown by MOVPE and by liquid phase epitaxy (LPE). Improved devices are obtained when optical absorption occurs in the p-layer due to the longer minority carrier diffusion length. Thick emitter p/n devices are limited by surface recombination, with highest quantum efficiency and lowest dark current being achieved with epitaxially grown surface passivation layers on lattice-matched MOVPE quaternaries. Thin emitter/thick base n/p devices are very promising, but require improved shallow high-quality n-type ohmic contacts.
Date: June 1, 1998
Creator: Hitchcock, C.W.; Gutmann, R.J.; Ehsani, H.; Bhat, I.B.; Wang, C.A.; Freeman, M.J. et al.
Partner: UNT Libraries Government Documents Department

RF photoreflectance characterization of binary and quasi-binary substrates and antimonide-based TPV devices

Description: Both starting substrates and complete TPV device structures have been characterized using a radio-frequency (RF) photoreflectance technique, in which a Nd-YAG pulsed laser is used to excite excess carriers, and the short-pulse response and photoconductivity decay are monitored with an inductively-coupled non-contacting RF probe. The initial exponential transient decay, indicative of bulk recombination and surface recombination mechanisms as demonstrated previously for doubly-capped sample structures, is approximately 30--40 ns for GaSb substrates, with the decay constant increasing with increasing optical excitation (similar to Shockley-Read-Hall (SRH) high injection behavior). In the InGaAsSb quasi-binary substrates two distinct decays are observed, an initial decay transient of 15--20 ns which is independent of optical intensity and a subsequent decay of 30--60 ns which decreases with increasing optical intensity. This latter dependence on optical intensity was observed with doubly-capped epitaxial layers and is indicative of radioactive recombination. Similar measurements on quaternary device structures indicate that both the pulse amplitude and initial decay are reduced significantly without a front-surface capping layer that reduces surface recombination velocity. With reduction of the front surface recombination velocity, initial decays of 20--25 ns were obtained under open-circuit conditions. These results indicate that the RF photoreflectance technique can be useful in characterizing and qualifying starting substrates and can be used to qualify epitaxial structures as well, particularly when doubly-capped standards are available for initial understanding of recombination processes in the material systems being investigated.
Date: October 1, 1998
Creator: Saroop, S.; Borrego, J.M.; Gutmann, R.J.; Dutta, P.S.; Ostrogorsky, A.G. & Charache, G.W.
Partner: UNT Libraries Government Documents Department

Antimonide based devices for thermophotovoltaic applications

Description: Thermophotovoltaic (TPV) devices have been fabricated using epitaxial ternary and quaternary layers grown on GaSb substrates. GaInSb ternary devices were grown by metalorganic vapor phase epitaxy (MOVPE) with buffer layers to accommodate the lattice mismatch, and GaInAsSb lattice-matched quaternaries were grown by MOVPE. Improved devices are obtained when optical absorption occurs in the p-layer due to the longer minority carrier diffusion length. Thick emitter p/n devices are limited by surface recombination, with highest quantum efficiency and lowest dark current being achieved with epitaxially grown surface passivation layers on lattice-matched MOVPE quaternaries. Thin emitter/thick base, n/p devices are very promising, but require improved shallow high-quality n-type ohmic contacts. Diffused junction devices using quasi-binary substrates offer the possibility of good performance and low manufacturing cost.
Date: December 1, 1998
Creator: Hitchcock, C.W.; Gutmann, R.J.; Borrego, J.M.; Bhat, I.B. & Charache, G.W.
Partner: UNT Libraries Government Documents Department

Recombination Parameters for Antimonide-Based Semiconductors using RF Photoreflection Techniques

Description: RF photoreflection measurements and PC-1D simulations have been used to evaluate bulk and surface recombination parameters in antimonide-based materials. PC-1D is used to simulate the photoconductivity response of antimonide-based substrates and doubly-capped epitaxial layers and also to determine how to extract the recombination parameters using experimental results. Excellent agreement has been obtained with a first-order model and test structure simulation when Shockley-Reed-Hall (SRH) recombination is the bulk recombination process. When radiative, Auger and surface recombination are included, the simulation results show good agreement with the model. RF photoreflection measurements and simulations using PC-1D are compatible with a radiative recombination coefficient (B) of approximately 5 x 10{sup -11} cm{sup 3}/s, Auger coefficient (C) {approx} 1.0 x 10{sup -28} cm{sup 6}/s and surface recombination velocity (SRV) {approx} 600 cm/s for 0.50-0.55 eV doubly-capped InGaAsSb material with GaSb capping layers using the experimentally determined active layer doping of 2 x 10{sup 17} cm{sup -3}. Photon recycling, neglected in the analysis and simulations presented, will affect the extracted recombination parameters to some extent.
Date: October 10, 2002
Creator: Kumar, R.J.; Borrego, J.M.; Dutta, P.S.; Gutmann, R.J.; Wang, C.A.; Martinelli, R.U. et al.
Partner: UNT Libraries Government Documents Department

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

Description: 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.
Date: August 29, 2000
Creator: Borrego, J.M.; Wang, C.A.; Dutta, P.S.; rajagopalan, G.; Bhat, I.B.; Gutmann, R.J. et al.
Partner: UNT Libraries Government Documents Department