Recombination Parameters in InGaAsSb Epitaxial Layers for Thermophotovoltaic Applications
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Recombination Parameters in InGaAsSb Epitaxial Layers for Thermophotovoltaic Applications R. J. Kumara, R. J. Gutmanna, J. M. Borregoa, P. S. Duttaa, C. A. Wangb, R. U. Martinellic and G. Nicholsd a Center for Integrated Electronics, Rensselaer Polytechnic Institute, Troy, NY 12180 b Lincoln Laboratory, Massachusetts Institute of Technology, Lexington, MA 02420 c Sarnoff Corporation, Princeton, NJ 08543 d Lockheed Martin, Schenectady, NY 12301 ABSTRACT Radio-frequency (RF) photoreflectance measurements and one-dimensional device simulations have been used to evaluate bulk recombination parameters and surface recombination velocity (SRV) in doubly-capped 0.55-eV p-InGaAsSb epitaxial layers, doped at 2 x 1017 cm-3, for thermophotovoltaic (TPV) applications. Bulk lifetimes of 90 to 100 ns and SRVs of 680 cm/s to 3200 cm/s (depending on the capping layer) are obtained, with higher doping and higher bandgap capping layers most effective in reducing SRV. RF photoreflectance measurements and one-dimensional device simulations are compatible with a radiative recombination coefficient (B) of 3 x 10-11 cm3/s and Auger coefficient (C) of 1 x 10-28 cm6/s. INTRODUCTION Since thermophotovoltaic (TPV) devices generate electricity directly from a relatively low temperature heat source, low bandgap (Eg < 0.75 eV) semiconductors are required to simultaneously maximize both efficiency and power density [1]. A source of heat is coupled with a radiator to emit infrared radiation. An array of interconnected semiconductor diodes converts above-bandgap photons to electricity. To improve efficiency, the TPV system rechannels belowbandgap photons back to the radiator using a spectral control filter [2]. The bandgap of the TPV diode is placed near the peak of the incident greybody spectrum from the radiator to simultaneously maximize both efficiency and power density. While higher power density can be achieved by lowering the bandgap and capturing a larger fraction of the spectrum, the diode dark current increases which adversely affects both the open circuit voltage and fill factor. For a radiator temperature of ~ 1000 oC, the best compromise between power density and efficiency is obtained by using a bandgap in the range of 0.50 to 0.60 eV. The material systems investigated for TPV energy conversion have included GaSb, InGaAs, InGaSb and InGaAsSb. The highest quality antimonide-based devices have been observed with InGaAsSb epitaxial layers grown lattice matched to GaSb substrates [3-5]. Figure 1(a) shows the layers of a typical InGaAsSb TPV cell grown by organometallic vapor phase epitaxy (OMVPE), with the InGaAsSb alloy composition corresponding to the desired bandgap (~ 0.55 eV). The structure uses a thick p-type emitter layer compared to the ntype base layer because of the large minority carrier diffusion length in the p-type quaternary alloy compared to the n-type alloy and difficulty in making ohmic contacts to thin n-type layers. The AlGaAsSb or GaSb window layer is used to reduce surface recombination which is detrimental to
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