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ABSTRACT: Given an unknown multicomponent alloy, and a set of standard compounds or alloys of known composition, can one improve upon popular standards-based methods for energy dispersive X-ray (EDX) spectrometry to quantify the elemental composition of the unknown specimen? A method is presented here for determining elemental composition of alloys using transmission electron microscopy-based EDX with appropriate standards. The method begins with a discrete set of related reference standards of known composition, applies multivariate statistical analysis to those spectra, and evaluates the compositions with a linear matrix algebra method to relate the spectra to elemental composition. By using associated standards, only limited assumptions about the physical origins of the EDX spectra are needed. Spectral absorption corrections can be performed by providing an estimate of the foil thickness of one or more reference standards. The technique was applied to III-V multicomponent alloy thin films: composition and foil thickness were determined for various III-V alloys. The results were then validated by comparing with X-ray diffraction and photoluminescence analysis, demonstrating accuracy of approximately 1% in atomic fraction.
Microscopy and Microanalysis 01/2013; · 3.01 Impact Factor
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Progress in Photovoltaics Research and Applications 08/2009; 17(8):587 - 593. · 5.79 Impact Factor
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M.W. Wanlass,
S.P. Ahrenkiel,
R.K. Ahrenkiel,
D.S. Albin, J.J. Carapella,
A. Duda,
J.F. Geisz,
S. Kurtz,
T. Moriarty,
R.J. Wehrer,
B. Wernsman
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ABSTRACT: We discuss lattice-mismatched (LMM) approaches utilizing compositionally step-graded layers and buffer layers that yield III-V photovoltaic devices with performance parameters equaling those of similar lattice-matched (LM) devices. Our progress in developing high-performance, LMM, InP-based GaInAs/InAsP materials and devices for thermophotovoltaic (TPV) energy conversion is highlighted. A novel, monolithic, multi-bandgap, tandem device for solar PV (SPV) conversion involving LMM materials is also presented along with promising preliminary performance results.
Photovoltaic Specialists Conference, 2005. Conference Record of the Thirty-first IEEE; 02/2005
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ABSTRACT: Salient advances in the development of thermophotovoltaic (TPV) energy converters based on low‐bandgap, InP‐based, GaInAs/InAsP heterostructures are presented and discussed. InP‐based materials are well‐suited and advantageous for TPV converter applications. Substantial improvements in the quality of lattice‐mismatched (LMM) heterostructures have been realized through an enhanced understanding of the relaxation behavior, and associated microstructure, of InAsP compositionally graded layers and GaInAs/InAsP interfaces. Double‐heterostructure, GaInAs/InAsP test structures with bandgaps as low as 0.5 eV (1.6% lattice mismatch) have been demonstrated with exceptional low‐injection, minority‐carrier lifetimes (several μs) and large estimated diffusion lengths — comparable to those for lattice‐matched materials. The advances in material quality have contributed to a number of notable TPV device achievements. A record in‐cavity efficiency of 23.6% was reported for a 0.6‐eV, GaInAs/InAsP monolithic interconnected module. Additionally, 0.52‐eV GaInAs/InAsP TPV converters were demonstrated with near‐unity internal quantum efficiencies and reverse‐saturation current densities nearly equaling the best reported for lattice‐matched, 0.52‐eV GaInAsSb/GaSb devices. Furthermore, InP‐based, 0.74/0.63‐eV, monolithic, series‐connected, tandem TPV converters are also under development and show promising performance; an in‐cavity efficiency of 11% has been reported for preliminary devices. © 2004 American Institute of Physics
AIP Conference Proceedings. 11/2004; 738(1):427-435.
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ABSTRACT: Low-bandgap, lattice-mismatched GaxIn1−xAs (GaInAs) grown using InAsyP1−y (InAsP) compositional-step grades on InP is a primary choice for lightabsorbing, active layers in high-efficiency thermophotovoltaic
(TPV) devices. The GaInAs/InAsP double heterostructures (DHs) show exceptional minority carrier lifetimes of up to several
microseconds. We have performed a characterization survey of 0.4–0.6-eV GaInAs/InAsP DHs using a variety of techniques, including
transmission electron microscopy (TEM). Dislocations are rarely observed to thread into the GaInAs active layers from the
InAsP buffer layers that terminate the graded regions. Nearly complete strain relaxation occurs in buried regions of the InAsP
grades. The buffer-layer strain prior to deposition of the active layer is virtually independent of the net misfit. Foreknowledge
of this buffer-layer strain is essential to correctly lattice match the buffer to the GaInAs active layer.
Journal of Electronic Materials 02/2004; 33(3):185-193. · 1.47 Impact Factor
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ABSTRACT: Thermophotovoltaic (TPV) tandem converter technology is being explored in an effort to improve both the efficiency and power density of TPV systems. Inverted, tandem structures incorporate epitaxially grown 0.74-eV lattice-matched Ga<sub>0.47</sub>In<sub>0.53</sub>As and 0.55-eV lattice-mismatched Ga<sub>0.28</sub>In<sub>0.72</sub>As diodes. Additionally, a strategy has been developed to allow voltage matching between these two subcells. Performance modeling calculations show that, under typical operating conditions, the 0.74/0.55-eV tandem converter should outperform a 0.5-eV single junction converter by 15% on an efficiency basis and by 15% on a power-density basis. This paper will present details regarding the design, growth, fabrication, and electrical, optical, and structural characterization of voltage-matched tandem TPV devices.
Photovoltaic Specialists Conference, 2002. Conference Record of the Twenty-Ninth IEEE; 06/2002
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ABSTRACT: We measured the recombination lifetime of degenerate n- In <sub>x</sub> Ga <sub>1-x</sub> As for three different compositions that correspond to x=0.53, 0.66, and 0.78 (band gaps of 0.74, 0.60, and 0.50 eV, respectively) over the doping range of 3×10<sup>18</sup>–5×10<sup>19</sup> carriers/cm <sup> 3 </sup>. The Auger recombination rate increases slowly with decreasing band gap, and it matches the behavior predicted for phonon-assisted recombination. © 2001 American Institute of Physics.
Applied Physics Letters 12/2001; · 3.84 Impact Factor
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ABSTRACT: High-quality, thin-film, lattice-matched (LM) InAs <sub>y</sub> P <sub>1-y</sub>/ In <sub>x</sub> Ga <sub>1-x</sub> As double heterostructures (DHs) have been grown lattice mismatched on InP substrates using atmospheric-pressure metalorganic vapor-phase epitaxy. The low-band gap In <sub>x</sub> Ga <sub>1-x</sub> As layers in the DHs have room-temperature band gaps that range from 0.47 to 0.6 eV. Both the optical and electronic properties of these films have been extensively measured. The band-to-band photoluminescence is quite strong and comparable to that found for LM InP/In <sub> 0.53 </sub> Ga <sub> 0.47 </sub> As DHs grown on InP. Recombination lifetime measurements of undoped DH structures show minority-carrier lifetimes in excess of 1 μs in most cases. The earlier properties make the band gap-flexible InAs <sub>y</sub> P <sub>1-y</sub>/ In <sub>x</sub> Ga <sub>1-x</sub> As DH system attractive for applications in high-performance, infrared-sensitive devices. © 2001 American Institute of Physics.
Applied Physics Letters 03/2001; · 3.84 Impact Factor
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ABSTRACT: Recent progress in the development of high-performance, 0.6-eV Ga0.32In0.68As/InAs0.32P0.68 thermophotovoltaic (TPV) converters and monolithically interconnected modules (MIMs) is described. The converter structure design is based on using a lattice-matched InAs0.32P0.68/Ga0.32In0.68As/InAs0.32P0.68 double-heterostructure (DH) device, which is grown lattice-mismatched on an InP substrate, with an intervening compositionally step-graded region of InAsyP1−y. The Ga0.32In0.68As alloy has a room-temperature band gap of ∼0.6 eV and contains a p/n junction. The InAs0.32P0.68 layers have a room-temperature band gap of ∼0.96 eV and serve as passivation/confinement layers for the Ga0.32In0.68As p/n junction. InAsyP1−y step grades have yielded DH converters with superior electronic quality and performance characteristics. Details of the microstructure of the converters are presented. Converters prepared for this work were grown by atmospheric-pressure metalorganic vapor-phase epitaxy (APMOVPE) and were processed using a combination of photolithography, wet-chemical etching, and conventional metal and insulator deposition techniques. Excellent performance characteristics have been demonstrated for the 0.6-eV TPV converters. Additionally, the implementation of MIM technology in these converters has been highly successful. © 1999 American Institute of Physics.
AIP Conference Proceedings. 03/1999; 460(1):132-141.
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ABSTRACT: Thermophotovoltaic (TPV) systems have recently rekindled a high level of interest for a number of applications. In order to meet the requirement of low-temperature (∼1000 °C) TPV systems, 0.6-eV Ga0.32In0.68As/InAs0.32P0.68 TPV monolithically interconnected modules (MIMs) have been developed at the National Renewable energy Laboratory (NREL) [1]. The successful fabrication of Ga0.32In0.68As/InAs0.32P0.68 MIMs depends on developing and optimizing of several key processes. Some results regarding the chemical vapor deposition (CVD)-SiO2 insulating layer, selective chemical etch via sidewall profiles, double-layer antireflection coatings, and metallization via interconnects have previously been given elsewhere [2]. In this paper, we report on the study of contacts and back-surface reflectors. In the first part of this paper, Ti/Pd/Ag and Cr/Pd/Ag contact to n-InAs0.32P0.68 and p-Ga0.32In0.68As are investigated. The transfer length method (TLM) was used for measuring of specific contact resistance Rc. The dependence of Rc on different doping levels and different pre-treatment of the two semiconductors will be reported. Also, the adhesion and the thermal stability of Ti/Pd/Ag and Cr/Pd/Ag contacts to n-InAs0.32P0.68 and p-Ga0.32In0.68As will be presented. In the second part of this paper, we discuss an optimum back-surface reflector (BSR) that has been developed for 0.6-eV Ga0.32In0.68As/InAs0.32P0.68 TPV MIM devices. The optimum BSR consists of three layers: ∼1300 Å MgF2 (or ∼1300 Å CVD SiO2) dielectric layer, ∼25 Å Ti adhesion layer, and ∼1500 Å Au reflection layer. This optimum BSR has high reflectance, good adhesion, and excellent thermal stability. © 1999 American Institute of Physics.
AIP Conference Proceedings. 03/1999; 460(1):517-524.
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ABSTRACT: Recent progress in the development of high-performance, 0.6-eV Ga{sub 0.32}In{sub 0.68}As/InAs{sub 0.32}P{sub 0.68} thermophotovoltaic (TPV) converters and monolithically interconnected modules (MIMs) is described. The converter structure design is based on using a lattice-matched InAs{sub 0.32}P{sub 0.68}/Ga{sub 0.32}In{sub 0.68}As/InAs{sub 0.32}P{sub 0.68} double-heterostructure (DH) device, which is grown lattice-mismatched on an InP substrate, with an intervening compositionally step-graded region of InAs{sub y}P{sub 1{minus}y}. The Ga{sub 0.32}In{sub 0.68}As alloy has a room-temperature band gap of {approximately}0.6 eV and contains a p/n junction. The InAs{sub 0.32}P{sub 0.68} layers have a room-temperature band gap of {approximately}0.96 eV and serve as passivation/confinement layers for the Ga{sub 0.32}In{sub 0.68}As p/n junction. InAs{sub y}P{sub 1{minus}y} step grades have yielded DH converters with superior electronic quality and performance characteristics. Details of the microstructure of the converters are presented. Converters prepared for this work were grown by atmospheric-pressure metalorganic vapor-phase epitaxy (APMOVPE) and were processed using a combination of photolithography, wet-chemical etching, and conventional metal and insulator deposition techniques. Excellent performance characteristics have been demonstrated for the 0.6-eV TPV converters. Additionally, the implementation of MIM technology in these converters has been highly successful. {copyright} {ital 1999 American Institute of Physics.}
AIP Conference Proceedings 02/1999; 460(1).
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ABSTRACT: The optical bowing behavior of polycrystalline thin film Culn1-yGaySe2 alloys is dependent upon the Cu stoichiometry. The variation in optical band gap, Eg, for alloys in which Cu is near stoichiometric (25 at.%) is parabolic and follows the relationship: Eg(y) = 1.011 + 0.421 y + 0.244 y2 (eV), where y is the alloy parameter, [at.% Ga] / [at.%Ga + at.%In]. Contrary to this, films with Cu-poor stoichiometries (∼19 at.% Cu) exhibit little alloy bowing: Eg(y) = 1.01 + 0.733 y - 0.046 y2 (eV). The increase in Eg with Cu deficiency appears to be the result of both a structural effect associated with tetragonal lattice shrinkage, ΔV = Vstoichiometric - VCu-poor (resulting from the presence of Cu vacancies) and a chemical effect, possibly associated with a counterbalance between p-d repulsion and anion displacement effects.
MRS Proceedings. 12/1990; 228.
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ABSTRACT: The optical and microstructural properties of polycrystalline CuIn
<sub>1-y</sub>Ga<sub>y</sub>Se<sub>2</sub> (CIGS) thin film deposited by
coevaporation are reported within the boundaries of an orthogonal
experimental design investigating the effects of Cu flux, Ga/(Ga+In)
composition. Se rate, substrate temperature, T <sub>s</sub> and
substrate type. The optical bandgaps for near-stoichiometric
CuIn<sub>1-y</sub>Ga<sub>y</sub>Se<sub>2</sub> are smaller and exhibit
bowing behavior which follows the relationship
E <sub>g</sub>=1.011+0.664 y +0.249 y ( y
-1). In comparison, Cu-poor films exhibit a linear variation with
zero bowing given by E <sub>g</sub>=1.0032+0.71369 y .
The increase in E <sub>g </sub>with decreasing Cu may result in
part from lattice shrinkage as measured by X-ray diffraction (XRD).
Optical absorption below the band edge appears to be dependent upon both
Cu and Ga content. Absorption coefficients of α⩾10<sup>3</sup>
cm<sup>-1</sup> within this region are indicative of Cu-rich films.
Absorption ⩽10<sup>3</sup> cm<sup>-1</sup> may be dictated more by
surface morphology and possible phase separation in films containing
⩾50% Ga. The magnitude of α varies from ≅2×10<sup>4
</sup> near the band edge tip to 10<sup>5</sup> cm<sup>-1</sup> at 1 ev
above the edge for near-stoichiometric films, with the absorption in
Cu-poor films being slightly less
Photovoltaic Specialists Conference, 1990., Conference Record of the Twenty First IEEE; 06/1990
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M W Wanlass,
S P Ahrenkiel,
D S Albin, J J Carapella,
A Duda,
K Emery,
J F Geisz,
K Jones,
Sarah Kurtz,
T Moriarty,
M J Romero
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ABSTRACT: We present a new approach for ultra-high-performance tandem solar cells that involves inverted epitaxial growth and ultra-thin device processing. The additional degree of freedom afforded by the inverted design allows the monolithic integration of high-, and medium-bandgap, lattice-matched (LM) subcell materials with lower-bandgap, lattice-mismatched (LMM) materials in a tandem structure through the use of transparent compositionally graded layers. The current work concerns an inverted, series-connected, triple-bandgap, GaInP (LM, 1.87 eV)/GaAs (LM, 1.42 eV)/GaInAs (LMM, ~1 eV) device structure grown on a GaAs substrate. Ultra-thin tandem devices are fabricated by mounting the epiwafers to pre-metallized Si wafer handles and selectively removing the parent GaAs substrate. The resulting handle-mounted, ultra-thin tandem cells have a number of important advantages, including improved performance and potential reclamation/reuse of the parent substrate for epitaxial growth. Additionally, realistic performance modeling calculations suggest that terrestrial concentrator efficiencies in the range of 40-45% are possible with this new tandem cell approach. Laboratory-scale (0.243 cm 2), prototype GaInP/GaAs/GaInAs tandem cells with terrestrial concentrator efficiencies as high as 37.9% have already been demonstrated at low concentration ratios (10.1 suns).
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ABSTRACT: We optimize InAsyP1−y buffer layers and compositional grades for lattice-mismatched heteroepitaxy of GaxIn1−xAs/InAsyP1−y double-heterostructures on InP. The strains of the active and buffer layers depend on the bulk misfit difference between these layers. The misfit difference is adjusted to eliminate strain in the active layer, thus avoiding misfit dislocations and surface topography that would otherwise form to relieve strain. The optimized structure uses an “overshoot” with respect to the conventional design in the misfit and As composition of the InAsyP1−y buffer. Nearly optimized heterostructures typically show excellent structural quality and extended minority-carrier lifetimes.
Solar Energy Materials and Solar Cells.
