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Step structure of GaInAsSb grown by OMVPE (Organometallic Vapor Phase Epitaxy)

Description: The microscopic surface morphology of GaInAsSb grown by organometallic vapor phase epitaxy (OMVPE)on GaSb substrates has been studied by atomic force microscopy. The effects of growth temperature, alloy composition, and substrate misorientation on the step structure were investigated. The competition between thermodynamically driven phase separation and kinetically limited surface diffusion is discussed.
Date: May 23, 1999
Creator: Charache, G.W. & Wang, C.A.
Partner: UNT Libraries Government Documents Department

InGaAsSb thermophotovoltaic diode physics evaluation

Description: The hotside operating temperatures for many projected thermophotovoltaic (TPV) conversion system applications are approximately 1,000 C, which sets an upper limit on the TPV diode bandgap of 0.6 eV from efficiency and power density considerations. This bandgap requirement has necessitated the development of new diode material systems, never previously considered for energy generation. To date, InGaAsSb quaternary diodes grown lattice-matched on GaSb substrates have achieved the highest performance. This report relates observed diode performance to electro-optic properties such as minority carrier lifetime, diffusion length and mobility and provides initial links to microstructural properties. This analysis has bounded potential diode performance improvements. For the 0.52 eV InGaAsSb diodes used in this analysis the measured dark current is 2 {times} 10{sup {minus}5} A/cm{sup 2}, versus a potential Auger limit 1 {times} 10{sup {minus}5} A/cm{sup 2}, a radiative limit of 2 {times} 10{sup {minus}6} A/cm{sup 2} (no photon recycling), and an absolute thermodynamic limit of 1.4 {times} 10{sup {minus}7} A/cm{sup 2}. These dark currents are equivalent to open circuit voltage gains of 20 mV (7%), 60 mV (20%) and 140 mV (45%), respectively.
Date: June 1, 1998
Creator: Charache, G.W.; Baldasaro, P.F. & Danielson, L.R.
Partner: UNT Libraries Government Documents Department

Electrical and optical properties of degenerately doped N-type In{sub x}Ga{sub 1{minus}x}As

Description: Degenerately-doped (> 10{sup 19} cm{sup {minus}3}) n-type In{sub x}Ga{sub 1{minus}x}As (x > 0.53) possesses a number of intriguing electrical and optical properties relevant to electro-optic devices and thermophotovoltaic devices in particular. Due to the low electron effective mass of this material and the demonstrated ability to incorporate n-type dopants into the mid-10{sup 19} cm{sup {minus}3} range, both the Moss-Burnstein bandgap shift and plasma reflection characteristics are particularly dramatic. These properties are investigated for In{sub x}Ga{sub 1{minus}x}As as a function of doping concentration, dopant type, and growth conditions. For undoped InGaAs with a nominal bandgap of 0.6 eV, doping this material to 5 {times} 10{sup 19} cm{sup {minus}3} increased the effective optical bandgap to 1.1 eV and has a plasma turn-on wavelength of 5 microns. This filter was coupled to a non-absorbing interference filter, creating a functional tandem filter for thermophotovoltaic applications.
Date: May 1, 1997
Creator: Charache, G.W.; DePoy, D.M. & Egley, J.L.
Partner: UNT Libraries Government Documents Department

Monte Carlo analysis of a monolithic interconnected module with a back surface reflector

Description: Recently, the photon Monte Carlo code, RACER-X, was modified to include wave-length dependent absorption coefficients and indices of refraction. This work was done in an effort to increase the code`s capabilities to be more applicable to a wider range of problems. These new features make RACER-X useful for analyzing devices like monolithic interconnected modules (MIMs) which have etched surface features and incorporates a back surface reflector (BSR) for spectral control. A series of calculations were performed on various MIM structures to determine the impact that surface features and component reflectivities have on spectral utilization. The traditional concern of cavity photonics is replaced with intra-cell photonics in the MIM design. Like the cavity photonic problems previously discussed, small changes in optical properties and/or geometry can lead to large changes in spectral utilization. The calculations show that seemingly innocuous surface features (e.g., trenches and grid lines) can significantly reduce the spectral utilization due to the non-normal incident photon flux. Photons that enter the device through a trench edge are refracted onto a trajectory where they will not escape. This leads to a reduction in the number of reflected below bandgap photons that return to the radiator and reduce the spectral utilization. In addition, trenches expose a lateral conduction layer in this particular series of calculations which increase the absorption of above bandgap photons in inactive material.
Date: October 1, 1998
Creator: Ballinger, C.T.; Charache, G.W. & Murray, C.S.
Partner: UNT Libraries Government Documents Department

OMVPE growth of GaInAsSb in the 2 to 2.4 {micro}m range

Description: Ga{sub 1{minus}x}In{sub x}As{sub y}Sb{sub 1{minus}y} epilayers were grown lattice matched to GaSb substrates by organometallic vapor phase epitaxy using all organometallic precursors, which include triethylgallium, trimethylindium, tertiarybutylarsine, and trimethylantimony. Layers were grown over a temperature range between 525 and 575 C, a V/III ratio range between 0.9 and 1.7, x < 0.2 and y < 0.2, and on (100) GaSb substrates with 2{degree} toward (100) or 6{degree} toward (111)B. The overall material quality of these alloys depends on growth temperature, In content, V/III ratio, and substrate misorientation. A mirror-like surface morphology and room temperature photoluminescence (PL) could be obtained for GaInAsSb layers with peak emission in the wavelength range between 2 and 2.4 {micro}m. Based on epilayer surface morphology and low temperature PL spectra, the crystal quality improves for growth temperature decreasing from 575 to 525 C, and with decreasing In content. In general, GaInAsSb layers grown on (100) GaSb substrates with a 6{degree} toward (111)B misorientation exhibited smoother surfaces and narrower full width at half-maximum values of 4 K PL spectra than layers grown on the more standard substrate 9100 2{degree} toward (110). Nominally undoped GaInAsSb layers grown at 550 C are p-type with 300 K hole concentration of {approximately} 5 {times} 10{sup 15} cm{sup {minus}3} and hole mobility of {approximately} 430 to 560 cm{sup 2}/V-s. The n- and p-type doping of GaInAsSb with diethyltellurium and dimethylzinc, respectively, are also reported.
Date: December 1, 1997
Creator: Charache, G.W. & Wang, C.A.
Partner: UNT Libraries Government Documents Department

Bulk single crystal ternary substrates for a thermophotovoltaic energy conversion system

Description: A thermophotovoltaic energy conversion device and a method for making the device are disclosed. The device includes a substrate formed from a bulk single crystal material having a bandgap (E{sub g}) of 0.4 eV < E{sub g} < 0.7 eV and an emitter fabricated on the substrate formed from one of a p-type and an n-type material. Another thermophotovoltaic energy conversion device includes a host substrate formed from a bulk single crystal material and lattice-matched ternary or quaternary III-V semiconductor active layers.
Date: December 31, 1996
Creator: Charache, G.W.; Baldasaro, P.F. & Nichols, G.J.
Partner: UNT Libraries Government Documents Department

A thermophotovoltaic energy conversion device

Description: A thermophotovoltaic device and a method for making the thermophotovoltaic device are disclosed. The device includes an n-type semiconductor material substrate having top and bottom surfaces, a tunnel junction formed on the top surface of the substrate, a region of active layers formed on top of the tunnel junction and a back surface reflector (BSR). The tunnel junction includes a layer of heavily doped n-type semiconductor material that is formed on the top surface of the substrate and a layer of heavily doped p-type semiconductor material formed on the n-type layer. An optional pseudomorphic layer can be formed between the n-type and p-type layers. A region of active layers is formed on top of the tunnel junction. This region includes a base layer of p-type semiconductor material and an emitter layer of n-type semiconductor material. An optional front surface window layer can be formed on top of the emitter layer. An optional interference filter can be formed on top of the emitter layer or the front surface window layer when it is used.
Date: December 31, 1996
Creator: Charache, G.W.; Baldasaro, P.F. & Egley, J.L.
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

Recent progress in GaInAsSb thermophotovoltaics grown by organometallic vapor phase epitaxy

Description: Studies on the materials development of Ga{sub 1{minus}x}In{sub x}As{sub y}Sb{sub 1{minus}y} alloys for thermophotovoltaic (TPV) devices are reviewed. Ga{sub 1{minus}x}In{sub x}As{sub y}Sb{sub 1{minus}y} epilayers were grown lattice matched to GaSb substrates by organometallic vapor phase epitaxy (OMVPE) using all organometallic precursors including triethylgallium, trimethylindium, tertiarybutylarsine, and trimethylantimony with diethyltellurium and dimethylzinc as the n- and p-type dopants, respectively. The overall material quality of these alloys depends on growth temperature, In content, V/III ratio, substrate misorientation, and to a lesser extent, growth rate. A mirror-like surface morphology and room temperature photoluminescence (PL) are obtained for GaInAsSb layers with peak emission in the wavelength range between 2 and 2.5 {micro}m. The crystal quality improves for growth temperature decreasing from 575 to 525 C, and with decreasing In content, as based on epilayer surface morphology and low temperature PL spectra. A trend of smaller full width at half-maximum for low temperature PL spectra is observed as the growth rate is increased from 1.5 to 2.5 and 5 {micro}m/h. In general, GaInAsSb layers grown on (100) GaSb substrates with a 6{degree} toward (111)B misorientation exhibited overall better material quality than layers grown on the more standard substrate (100)2{degree} toward (110). Consistent growth of high performance lattice-matched GaInAsSb TPV devices is also demonstrated.
Date: June 1, 1998
Creator: Wang, C.A.; Choi, H.K.; Oakley, D.C. & Charache, G.W.
Partner: UNT Libraries Government Documents Department

Extending the cutoff wavelength of lattice-matched GaInAsSb/GaSb thermophotovoltaics devices

Description: This paper reports the growth, materials characterization, and device performance of lattice-matched GaInAsSb/GaSb thermophotovoltaic (TPV) devices with cutoff wavelength as long as 2.5 {micro}m. GaInAsSb epilayers were grown lattice matched to GaSb substrates by organometallic vapor phase epitaxy (OMVPE) using all organometallic precursors including triethylgallium, trimethylindium, tertiarybutylarsine, and trimethylantimony with diethyltellurium and dimethylzinc as the n- and p-type dopants, respectively. The growth temperature was 525 C. Although these alloys are metastable, a mirror-like surface morphology and room temperature photoluminescence (PL) are obtained for alloys with PL peak emission at room temperature as long as 2.5 {micro}m. In general, however, a trend of decreasing material quality is observed as the wavelength increases. Both the surface roughness and PL full width at half-maximum increase with wavelength. In spite of the dependence of material quality on PL peak emission wavelength, the internal quantum efficiency of TPV devices with cutoff wavelengths of 2.3 to 2.5 {micro}m is as high as 86%.
Date: October 1, 1998
Creator: Wang, C.A.; Choi, H.K.; Oakley, D.C. & Charache, G.W.
Partner: UNT Libraries Government Documents Department

OMVPE growth and characterization of GaInAsSb for thermophotovoltaics

Description: Studies on the materials development of Ga{sub 1{minus}x}In{sub x}As{sub y}Sb{sub 1{minus}y} alloys for thermophotovoltaic (TPV) devices are reviewed. Ga{sub 1{minus}x}In{sub x}As{sub y}Sb{sub 1{minus}y} epilayers were grown lattice matched to GaSb substrates by organometallic vapor phase epitaxy (OMVPE) using all organometallic precursors including triethylgallium, trimethylindium, tertiarybutylarsine, and trimethylantimony with diethyltellurium and dimethylzinc as the n- and p-type dopants, respectively. The overall material quality of these alloys depends on growth temperature, In content, V/III ratio, substrate misorientation, and to a lesser extent, growth rate. A mirror-like surface morphology and room temperature photoluminescence (PL) are obtained for GaInAsSb layers with peak emission in the wavelength range between 2 and 2.5 {micro}m. The crystal quality improves for growth temperature decreasing from 575 to 525 C, and with decreasing In content, as based on epilayer surface morphology and low temperature PL spectra. A trend of smaller full width at half-maximum for low temperature PL spectra is observed as the growth rate is increased from 1.5 to 2.5 and 5 {micro}m/h. In general, GaInAsSb layers grown on (100) GaSb substrates with a 6{degree} toward (111)B misorientation exhibited overall better material quality than layers grown on the more standard substrate (100) 2{degree} toward (110). Consistent growth of high performance lattice-matched GaInAsSb TPV devices is also demonstrated.
Date: June 1, 1998
Creator: Wang, C. A. & Charache, G. W.
Partner: UNT Libraries Government Documents Department

Recent progress in InGaAsSb/GaSb TPV devices

Description: AstroPower is developing InGaAsSb thermophotovoltaic (TPV) devices. This photovoltaic cell is a two-layer epitaxial InGaAsSb structure formed by liquid-phase epitaxy on a GaSb substrate. The (direct) bandgap of the In{sub 1{minus}x}Ga{sub x}As{sub 1{minus}y}Sb{sub y} alloy is 0.50 to 0.55 eV, depending on its exact alloy composition (x,y); and is closely lattice-matched to the GaSb substrate. The use of the quaternary alloy, as opposed to a ternary alloy--such as, for example InGaAs/InP--permits low bandgap devices optimized for 1,000 to 1,500 C thermal sources with, at the same time, near-exact lattice matching to the GaSb substrate. Lattice matching is important since even a small degree of lattice mismatch degrades device performance and reliability and increases processing complexity. Internal quantum efficiencies as high as 95% have been measured at a wavelength of 2 microns. At 1 micron wavelengths, internal quantum efficiencies of 55% have been observed. The open-circuit voltage at currents of 0.3 A/cm{sup 2} is 0.220 volts and 0.280 V for current densities of 2 A/cm{sup 2}. Fill factors of 56% have been measured at 60 mA/cm{sup 2}. However, as current density increases there is some decrease in fill factor. The results to date show that the GaSb-based quaternary compounds provide a viable and high performance energy conversion solution for thermophotovoltaic systems operating with 1,000 to 1,500 C source temperatures.
Date: May 1, 1996
Creator: Shellenbarger, Z.A.; Mauk, M.G.; DiNetta, L.C. & Charache, G.W.
Partner: UNT Libraries Government Documents Department

Substrate misorientation effects on epitaxial GaInAsSb

Description: The effect of substrate misorientation on the growth of GaInAsSb was studied for epilayers grown lattice-matched to GaSb substrates by low-pressure organometallic vapor phase epitaxy. The substrates were (100) misoriented 2 or 6{degree} toward (110), (111)A, or (111)B. The surface is mirror-like and featureless for layers grown with a 6{degree} toward (111)B misorientation, while, a slight texture was observed for layers grown on all other misorientations. The optical quality of layers, as determined by the full width at half-maximum of photoluminescence spectra measured at 4K, is significantly better for layers grown on substrates with a 6{degree} toward (111)B misorientation. The incorporation of Zn as a p-type dopant in GaInAsSb is about 1.5 times more efficient on substrates with 6{degree} toward (111)B misorientation compared to 2{degree} toward (110) misorientation. The external quantum efficiency of thermophotovoltaic devices is not, however, significantly affected by substrate misorientation.
Date: December 1, 1997
Creator: Wang, C.A.; Choi, H.K.; Oakley, D.C. & Charache, G.W.
Partner: UNT Libraries Government Documents Department

Optical constants of Ga{sub 1{minus}x}In{sub x}As{sub y}Sb{sub 1{minus}y} lattice-matched to GaSb(001): Experiment and modeling

Description: The optical constants {epsilon}(E)[={epsilon}{sub 1}(E)+i{epsilon}{sub 2}(E)] of two epitaxial layers of GaInAsSb/GaSb have been measured at 300 K using spectral ellipsometry in the range of 0.35--5.3 eV. The {epsilon}(E) spectra displayed distinct structures associated with critical points (CPs) at E{sub 0} (direct gap), spin-orbit split E{sub 0}+{Delta}{sub 0} component, spin-orbit split (E{sub 1}, E{sub 1}+{Delta}{sub 1}) and (E{sub 0}{prime}, E{sub 0}{prime}+{Delta}{sub 0}{prime}) doublets, as well as E{sub 2}. The experimental data over the entire measured spectral range (after oxide removal) has been fit using the Holder model dielectric function [Phys.Rev.B 56, 4037 (1997)] based on the electronic energy-band structure near these CPs plus excitonic and band-to-band Coulomb enhancement effects at E{sub 0.}, E{sub 0}+{Delta}{sub 0} and the E{sub 1}, E{sub 1}+{Delta}{sub 1} doublet. In addition to evaluating the energies of these various band-to-band CPs, information about the binding energy (R{sub 1}) of the two-dimensional exciton related to the E{sub 1}, E{sub 1}+{Delta}{sub 1} CPs was obtained. The value of R{sub 1} was in good agreement with effective mass/k{sup {rightharpoonup}}{center_dot}p{sup {rightharpoonup}} theory. The ability to evaluate R{sub 1} has important ramifications for recent first-principles band structure calculations which include exciton effects at E{sub 0}, E{sub 1}, and E{sub 2} [M.Rohlfing and S.G.Louie, Phys.Rev.Lett. 81, 2312 (1998) and S. Albrecht et al., Phys.Rev.Lett. 80, 4510 (1998)]. The experimental absorption coefficients in the region of E{sub 0} were in good agreement with values obtained from a linear interpolation of the end point materials. The experimental results were compared to a recent evaluation and fitting (Holden model) of the optical constants of GaSb.
Date: June 1999
Creator: Charache, G. W.; Muñoz, M.; Wei, K.; Pollak, F. H.; Freeouf, J. L. & Wang, C. A.
Partner: UNT Libraries Government Documents Department

Spectral ellipsometry of GaSb and GaInAsSb: Experiment and modeling

Description: The optical constants {epsilon}(E)[={epsilon}{sub 1}(E)+i{epsilon}{sub 2}(E)] of single-crystal GaSb at 300K have been measured using spectral ellipsometry in the range of 0.3-5.3 eV. The {epsilon}(E) spectra displayed distinct structures associated with critical points (CPs) at E{sub 0} (direct gap), spin-orbit split E{sub 0}+{Delta}{sub 0} component, spin-orbit split (E{sub 1}, E{sub 1}+{Delta}{sub 1}) and (E{sub 0}{prime}, E{sub 0}{prime}+{Delta}{sub 0}{prime}) doublets, as well as E{sub 2}. The experimental data over the entire measured spectral range (after oxide removal) has been fit using the Holden model dielectric function based on the electronic energy-band structure near these CPs plus excitonic and band-to-band Coulomb enhancement effects at E{sub 0}, E{sub 0}+{Delta}{sub 0} and the E{sub 1}, E{sub 1}+{Delta}{sub 1} doublet. In addition to evaluating the energies of these various band-to-band CPs, information about the binding energy (R{sub 1}) of the two-dimensional exciton related to the E{sub 1}, E{sub 1}+{Delta}{sub 1} CPs was obtained. The value of R{sub 1} was in good agreement with effective mass/k{sup {rightharpoonup}}{center_dot}p{sup {rightharpoonup}} theory. The ability to evaluate R{sub 1} has important ramifications for recent first-principles band structure calculations which include exciton effects at E{sub 0}, E{sub 1}, and E{sub 2}. The experimental results were compared to other evaluations of the optical constants of GaSb.
Date: June 30, 1999
Creator: Charache, G.W.; Mu {tilde n}oz, M.; Wei, K.; Pollak, F.H. & Freeouf, J.L.
Partner: UNT Libraries Government Documents Department

Lapped substrate for enhanced backsurface reflectivity in a thermophotovoltaic energy conversion system

Description: A method is described for fabricating a thermophotovoltaic energy conversion cell including a thin semiconductor wafer substrate having a thickness ({beta}) calculated to decrease the free carrier absorption on a heavily doped substrate; wherein the top surface of the semiconductor wafer substrate is provided with a thermophotovoltaic device, a metallized grid and optionally an antireflective (AR) overcoating; and, the bottom surface (10 ft) of the semiconductor wafer substrate is provided with a highly reflecting coating which may comprise a metal coating or a combined dielectric/metal coating.
Date: December 31, 1996
Creator: Baldasaro, P.F.; Brown, E.J.; Charache, G.W. & DePoy, D.M.
Partner: UNT Libraries Government Documents Department

Spectral ellipsometry of GaSb: Experiment and modelling

Description: The optical constants {epsilon}(E)[{equals}{epsilon}{sub 1}(E) + i{epsilon}{sub 2}(E)] of single crystal GaSb at 300K have been measured using spectral ellipsometry in the range of 0.3--5.3 eV. The {epsilon}(E) spectra displayed distinct structures associated with critical points (CPs) at E{sub 0}(direct gap), spin-orbit split E{sub 0} + {Delta}{sub 0} component, spin-orbit split (E{sub 1}), E{sub 1} + {Delta}{sub 1} and (E{sub 0}{prime}), E{sub 0}{prime} + {Delta}{sub 0}{prime} doublets, as well as E{sub 2}. The experimental data over the entire measured spectral range (after oxide removal) has been fit using the Holden model dielectric function [Phys.Rev.B 56, 4037 (1997)] based on the electronic energy-band structure near these CPs plus excitonic and band-to-band Coulomb enhancement effects at E{sub 0}, E{sub 0} + {Delta}{sub 0}and the E{sub 1}, E{sub 1} + {Delta}{sub 1} doublet. In addition to evaluating the energies of these various band-to-band CPs, information about the binding energy (R{sub 1}) of the two-dimensional exciton related to the E{sub 1}, E{sub 1} + {Delta}{sub 1} CPS was obtained. The value of R{sub 1} was in good agreement with effective mass/{rvec k} {center_dot} {rvec p} theory. The ability to evaluate R{sub 1} has important ramifications for recent first-principles band structure calculations which include exciton effects at E{sub 0}, E{sub 1}, and E{sub 2}.
Date: May 1, 1999
Creator: Charache, G.W.; Mu {tilde n}oz, M.; Wei, K.; Pollak, F.H. & Freeouf, J.L.
Partner: UNT Libraries Government Documents Department

Theoretical prediction of the plasma frequency and Moss-Burstein shifts for degenerately doped In{sub x}Ga{sub 1{minus}x}As

Description: Theoretical predictions for the plasma frequency and Moss-Burstein shift (optical band gap) of degenerately doped (n > 10{sup 19} cm{sup {minus}3}) In{sub x}Ga{sub 1{minus}x} As are presented. This system is of interest because it possesses desirable optical properties for thermophotovoltaic (TPV) applications. The studies presented are based on electronic band structures calculated using the Full Potential Linearized Augmented Plane Wave (FLAPW) method which includes non-local screened exchange (sX-LDA) and spin-orbit effects. The plasma frequency and Moss-Burstein shift are calculated vs. doping assuming a rigid band approximation (i.e. conduction band filling of the undoped bands). The doping dependence of the effective mass (band non-parabolicity) plays an important role at the high dopings considered here. This effect leads to a maximum in the plasma frequency vs. doping (2--3 {times} 10{sup 14}/s) and a significant departure from the constant effective mass prediction for the optical band gap vs. doping. These calculations are in good agreement with measurements.
Date: October 1, 1998
Creator: Raynolds, J. E.; Charache, G. W.; Geller, C. B.; Holden, T. & Pollak, F. H.
Partner: UNT Libraries Government Documents Department

Thermodynamic analysis of Thermophotovoltaic Efficiency and Power Density Tradeoffs

Description: This report presents an assessment of the efficiency and power density limitations of thermophotovoltaic (TPV) energy conversion systems for both ideal (radiative-limited) and practical (defect-limited) systems. Thermodynamics is integrated into the unique process physics of TPV conversion, and used to define the intrinsic tradeoff between power density and efficiency. The results of the analysis reveal that the selection of diode bandgap sets a limit on achievable efficiency well below the traditional Carnot level. In addition it is shown that filter performance dominates diode performance in any practical TPV system and determines the optimum bandgap for a given radiator temperature. It is demonstrated that for a given radiator temperature, lower bandgap diodes enable both higher efficiency and power density when spectral control limitations are included. The goal of this work is to provide a better understanding of the basic system limitations that will enable successful long-term development of TPV energy conversion technology.
Date: February 22, 2000
Creator: Baldasara, P.F.; Reynolds, J.E.; Charache, G.W.; DePoy, D.M.; Ballinger, C.T.; Donovan, T. et al.
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

Microstructural evaluation of Sb-adjusted Al{sub 0.5}Ga{sub 0.5}As{sub 1{minus}y}Sb{sub y} buffer layer systems for IR applications

Description: The authors report on a transmission electron microscopy (TEM) study of Sb-adjusted quaternary Al{sub 0.5}Ga{sub 0.5}As{sub 1{minus}y}Sb{sub y} buffer-layers grown on <001> GaAs substrates. A series of structures were grown by MBE at 470 C that utilize a multilayer grading scheme in which the Sb content of Al{sub 0.5}Ga{sub 0.5}As{sub 1{minus}y}Sb{sub y} buffer-layers grown on <001> GaAs substrates. A series of structures were grown by MBe at 470 C that utilize a multilayer grading scheme in which the Sb content of Al{sub 0.5}Ga{sub 0.5}As{sub 1{minus}y}Sb{sub y} is successively increased in a series of 125 nm thick layers. Post growth analysis using conventional bright field and weak beam dark field imaging of these buffer layers in cross-section reveals that the interface misfit dislocations are primarily of the 60{degree} type and are distributed through out the interfaces of the buffer layer. When optimized, the authors have shown, using plan view and cross-sectional TEM, that this approach can reduce the threading defect density to below the detectability limit of TEM (< 10{sup 5}/cm{sup 2}) and preserve growth surface planarity. The Sb-graded approach was used to fabricate two 2.2 {micro}m power converter structures fabricated using InGaAs grown on Sb-based buffer layers on GaAs substrates. A microstructural and electrical characterization was performed on these device structures and the results are contrasted with a sample in which InP was selected as the substrate. Microstructure, defect density and device performance in these not-yet-optimized Sb-based buffer layers compares favorably to equivalent devices fabricated using InP substrates.
Date: June 1, 1998
Creator: Chen, E.; Paine, D.C.; Uppal, P.; Ahearn, J.S.; Nichols, K. & Charache, G.W.
Partner: UNT Libraries Government Documents Department

Lattice-matched epitaxial GaInAsSb/GaSb thermophotovoltaic devices

Description: The materials development of Ga{sub 1{minus}x}In{sub x}As{sub y}Sb{sub 1{minus}y} alloys for lattice-matched thermophotovoltaic (TPV) devices is reported. Epilayers with cutoff wavelength 2--2.4 {micro}m at room temperature and lattice-matched to GaSb substrates were grown by both low-pressure organometallic vapor phase epitaxy and molecular beam epitaxy. These layers exhibit high optical and structural quality. For demonstrating lattice-matched thermophotovoltaic devices, p- and n-type doping studies were performed. Several TPV device structures were investigated, with variations in the base/emitter thicknesses and the incorporation of a high bandgap GaSb or AlGaAsSb window layer. Significant improvement in the external quantum efficiency is observed for devices with an AlGaAsSb window layer compared to those without one.
Date: May 1, 1997
Creator: Wang, C.A.; Choi, H.K.; Turner, G.W.; Spears, D.L.; Manfra, M.J. & Charache, G.W.
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

Optical properties of thin semiconductor device structures with reflective back-surface layers

Description: Ultrathin semiconductor device structures incorporating reflective internal or back surface layers have been investigated recently as a means of improving photon recuperation, eliminating losses associated with free carrier absorption in conductive substrates and increasing the above bandgap optical thickness of thermophotovoltaic device structures. However, optical losses in the form of resonance absorptions in these ultrathin devices have been observed. This behavior in cells incorporating epitaxially grown FeAl layers and in devices that lack a substrate but have a back-surface reflector (BSR) at the rear of the active layers has been studied experimentally and modeled effectively. For thermophotovoltaic devices, these resonances represent a significant loss mechanism since the wavelengths at which they occur are defined by the active TPV cell thickness of {approximately} 2--5 microns and are in a spectral range of significant energy content for thermal radiators. This study demonstrates that ultrathin semiconductor structures that are clad by such highly reflective layers or by films with largely different indices of refraction display resonance absorptions that can only be overcome through the implementation of some external spectral control strategy. Effective broadband, below-bandgap spectral control using a back-surface reflector is only achievable using a large separation between the TPV active layers and the back-surface reflector.
Date: November 1, 1998
Creator: Clevenger, M.B.; Murray, C.S.; Ringel, S.A.; Sachs, R.N.; Qin, L.; Charache, G.W. et al.
Partner: UNT Libraries Government Documents Department