Analysis of the composition, strain-relaxation, layer-tilt, and the crystalline quality of InyGa1-yAs/ InP1-xAsx thermophotovoltaic (TPV) diodes grown by metal-organic vapor phase epitaxy (MOVPE) is demonstrated using triple-axis X-ray reciprocal space mapping techniques. [n(0.53)Ga(0.47)s (E-gap = 0.74 eV) n/p junction diodes are grown lattice matched (LM) to InP substrates and lattice-mismatched (LMM) In0.67Ga0.33As (E-gap = 0.6 eV) TP%/ diodes are grown on three-step InP1-xAsx (0 <x<0.32) buffer layers on InP substrates. X-ray reciprocal space maps about the symmetric (400) and asymmetric (5 3 3) reciprocal lattice points (RELPs) determine the in-plane and out-of-plane lattice parameters and strain of the InyGa1-yAs TPV active layer and underlying InP1-xAsx. buffers. Triple-axis X-ray rocking Curves about the LMM In0.67Ga0.33As RELPshow an order of magnitude increase of its full-width at half-maximum (FWHM) compared to that from the LM In0.53Ga0.47As (250 vs. 30 arcsec). Despite the significant RELP broadening, the photovoltaic figure of merits show that the electronic quality of the LMM In0.67Ga0.33As approaches that of the LM diode material. This indicates that misfit-related crystalline imperfections are not dominating the photovoltaic response of the optimized LMM In0.67Ga0.33As material compared with the intrinsic recombination processes and/or recombination through native point defects, which would be present in both LMM and LM diode material. However, additional RELP broadening in non-optimized LMM In0.67Ga0.33As n/p junction diodes does correspond to significant degradation of TPV diode open-circuit voltage and minority carrier lifetime demonstrating that there is correlation between X-ray FWHM and the electronic performance of the LMM TPV diodes. (C) 2008 Elsevier B.V. All rights reserved.

The results of study of minority carrier lifetime in p-type and n-type InGaAs double heterostructures (DH) and thermophotovoltaic (TPV) cells with InPAs step-graded buffer layers grown on InP substrates are summarized. The active layer carrier concentration was varied in the range from 1015 to 1018 cm-3. The carrier lifetime constants were determined from photoluminescence (PL) transient and frequency responses under low-injection conditions. The rapid decrease of electron lifetime with decrease of excitation was observed in p-type InGaAs DHs and was attributed to capture of electrons on positively-charged deep-donor recombination centers. It was found that Te-doping of the InPAs buffer layers improves the low-injection electron lifetime. Temperature dependences of radiative efficiency and minority carrier lifetime were studied in sets of DHs with different doping level in order to separate the radiative and non-radiative recombination processes

Thermophotovoltaic (TPV) diodes fabricated from InGaAsSb alloys lattice-matched to GaSb substrates are grown by Metal Organic Vapor Phase Epitaxy (MOVPE). 0.53eV InGaAsSb TPV diodes utilizing front-surface spectral control filters have been tested in a vacuum cavity and a TPV thermal-to-electric conversion efficiency ({eta}{sub TPV}) and a power density (PD) of {eta}{sub TPV} = 19% and PD=0.58 W/cm{sup 2} were measured for T{sub radiator} = 950 C and T{sub diode} = 27 C. Recombination coefficients deduced from minority carrier measurements and the theory reviewed in this article predict a practical limit to the maximum achievable conversion efficiency and power density for 0.53eV InGaAsSb TPV. The limits for the above operating temperatures are projected to be {eta}{sub TPV} = 26% and PD = 0.75 W/cm{sup 2}. These limits are extended to {eta}{sub TPV} = 30% and PD = 0.85W/cm{sup 2} if the diode active region is bounded by a reflective back surface to enable photon recycling and a two-pass optical path length. The internal quantum efficiency of the InGaAsSb TPV diode is close to the theoretically predicted limits, with the exception of short wavelength absorption in GaSb contact layers. Experiments show that the open circuit voltage of the 0.53eV InGaAsSb TPV diodes is not strongly dependent on the device architectures studied in this work where both N/P and P/N double heterostructure diodes have been grown with various acceptor and donor doping levels, having GaSb and AlGaAsSb confinement, and also partial back surface reflectors. Lattice matched InGaAsSb TPV diodes were fabricated with bandgaps ranging from 0.6 to 0.5eV without significant degradation of the open circuit voltage factor, quantum efficiency, or fill factor as the composition approached the miscibility gap. The key diode performance parameter which is limiting efficiency and power density below the theoretical limits in InGaAsSb TPV devices is the open circuit voltage. The open circuit voltages of state-of-the-art 0.53eV InGaAsSb TPV diode are {approx}10% lower than the predicted semi-empirical limit to open circuit voltage for a device having absorbing substrate; the voltages are {approx}17% below that for an Auger-limited device having back surface reflector and two-pass optical design.

Low surface recombination velocity is critical to the performance of minority carrier devices. Minority carrier lifetime in double heterostructures (DHs) of 0.53-eV p-GaInAsSb confined with 1.0-eV p-AlGaAsSb, and grown lattice-matched to GaSb, was measured by time-resolved photoluminescence. The structures were designed to be dominated by the heterointerface while minimizing the contribution of photon recycling to minority carrier lifetime. Surface recombination velocity as low as 30cm∕s for DHs was achieved. This value is over an order of magnitude lower than that reported in previous studies.

Thermophotovoltaic (TPV) diodes fabricated from 0.52eV lattice-matched InGaAsSb alloys are grown by Metal Organic Vapor Phase Epitaxy (MOVPE) on GaSb substrates. 4cm(2) multi chip diode modules with front-surface spectral filters were tested in a vacuum cavity and attained measured efficiency and power density of 19% and 0.58 W/cm(2) respectively at operating at temperatures of T-radiator = 950 degreesC and T-diode = 27 degreesC. Device modeling and minority carrier lifetime measurements of double heterostructure lifetime specimens indicate that diode conversion efficiency is limited predominantly by interface recombination and photon energy loss to the GaSb substrate and back ohmic contact. Recent improvements to the diode include lattice-matched p-type AlGaAsSb passivating layers with interface recombination velocities less than 100 cm/s and new processing techniques enabling thinned substrates and back surface reflectors. Modeling predictions of these improvements to the diode architecture indicate that conversion efficiencies from 27-30% and similar to0.85 W/cm(2) could be attained under the above operating temperatures.

GaInAsSb/AlGaAsSb/InAsSb/GaSb epitaxial layers were bonded to semi-insulating (SI) GaAs handle wafers with SiOx/Ti/Au as the adhesion layer for monolithic interconnection of thermophotovolatic (TPV) devices. Epitaxial transfer was completed by removal of the GaSb substrate, GaSb buffer, and InAsSb etch-stop layer by selective chemical etching. The SiOx/Ti/Au provides not only electrical isolation, but also high reflectivity and is used as an internal backsurface reflector (BSR). Characterization of wafer-bonded (WB) epitaxy by high-resolution x-ray diffraction (HRXRD) and time-decay photoluminescence (PL) indicates minimal residual stress and enhancement in optical quality. The 0.54-eV GaInAsSb cells were fabricated and monolithically interconnected in series. A ten-junction device exhibited linear voltage building with an opencircuit voltage of 1.8 V.

This work summarizes recent data on minority carrier lifetime in n- and p-type double heterostructures (DHs) of 0.5-0.6 eV GaInAsSb confined with GaSb and AlGaAsSb cap layers. Recombination times were measured by time-resolved photoluminescence (TRPL) and by optical frequency response (OFR) to sinusoidal excitation. It was shown that one of the mechanisms responsible for interface recombination in GaSb/GaInAsSb/GaSb DHs is thermionic emission of carriers over the heterobarrier. Considerable improvement of carrier confinement was obtained with 1 eV AlGaAsSb cap layers. Optimization of the epitaxial growth resulted in a recombination velocity at GaInAsSb/AlGaAsSb interface as low as 30 cm/s.

The effects of growth interruption on interfacial quality of GaInAsSb/AlGaAsSb heterostructures grown by organometallic vapor-phase epitaxy are reported. In situ reflectance monitoring and ex situ characterization by high-resolution X-ray diffraction, 4K photoluminescence (PL), and time-resolved PL indicate that GaInAsSb is extremely sensitive to growth interruption time as well as the ambient atmosphere during interruption. By optimizing the growth switching sequence, surface recombination velocity as low as 30cm/s was achieved for GaInAsSb/AlGaAsSb double heterostructures.

Auger recombination in p-type GaSb, InAs, and their alloys is enhanced due to the proximity of the band gap energy and the energy separation to the spin split-off valence band. This can affect the device performance even at moderate doping concentration. We report electron lifetime measurements in a p-type 0.54 eV GaInAsSb alloy, commonly used in a variety of infrared devices. We have studied a series of double-capped heterostructures with varied thicknesses and doping levels, grown by organometallic vapor phase epitaxy on GaSb substrates. The Auger coefficient value of 2.3×10−28 cm6/s is determined by analyzing the photoluminescence decay constants with a systematic separation of different recombination mechanisms.

Minority carrier lifetimes in 0.55 eV band-gap GaInAsSb epitaxial layers that are double capped with GaSb or AlGaAsSb layers were determined using time-resolved photoluminescence. It was found that accumulation of electrons at the p-doped GaInAsSb/GaSb type-II interface contributes significantly to the interfacial recombination velocity S, which was measured to be 3100 cm/s. The use of heavily p-doped GaSb cap layers was proposed to eliminate the potential well of electrons and barrier for holes at the interface. Increasing the GaSb cap doping level from 1×1016 to 2×1018 cm−3 resulted in a 2.7 times reduction of S down to 1140 cm/s. The smallest value of S was determined to be 720 cm/s, which was obtained for structures with AlGaAsSb cap layers that have no valence band offset.