Fabrication and Characterization of Wafer-Bonded GaInAsSb Epitaxy for Monolithically Interconnected Thermophotovoltaic D
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Fabrication and Characterization of Wafer-Bonded GaInAsSb Epitaxy for Monolithically Interconnected Thermophotovoltaic Devices C.A. Wang, D.A. Shiau, P.G. Murphy, P.W. O’Brien, R.K. Huang, M.K. Connors, A.C. Anderson, D.M. Depoy1, G. Nichols1, and M.N. Palmisiano2 Lincoln Laboratory, Massachusetts Institute of Technology, Lexington, MA 02420-9108 1 Lockheed Martin Corporation, Schenectady, NY 12301 2 Bechtel Incorporated, West Mifflin, PA 15122 ABSTRACT The fabrication, characterization, and performance of wafer-bonded (WB) GaInAsSb thermophotovoltaic (TPV) devices for monolithically series-interconnected cells are reported. TPV epilayers were bonded to GaAs handle wafers with SiOx/Ti/Au. This dielectric/metal layer is multi-functional in that it is used as the adhesive to bond the epilayers; it provides electrical isolation; and it is an internal back surface reflector. Considerations were made to minimize residual stress and provide high reflectivity. Excellent structural and optical properties of WB TPV structures are observed. The external quantum efficiency of WB and unbonded TPV cells is comparable. TPV cells were monolithically series interconnected, and a 10-junction device exhibited linear voltage building with an open-circuit voltage of 1.8 V. INTRODUCTION InP- and GaSb-based III-V semiconductors with energy gap in the range 0.5 to 0.6 eV are being developed for thermophotovoltaic (TPV) energy conversion systems that operate with thermal sources heated to ~1000 °C [1]. Recently, there has been interest in being able to monolithically series interconnect TPV devices to build voltage and simplify fabrication, and lattice-mismatched 0.6 ev-InGaAs/InP devices were reported for such devices fabricated on semi-insulating (SI) InP substrates [2-4]. The parallel approach for fabrication of GaSb-based devices, however, is not possible, since SI GaSb substrates are not available. Therefore, electrical isolation must be achieved by alternative means. One possible approach reported incorporation of a lattice-matched AlGaAsSb cell-isolation diode with 0.6-eV GaInAsSb/GaSb TPV cells, and an open-circuit voltage Voc of 0.42 V for a 15-cell device was measured [5]. In this paper, electrical isolation is achieved by wafer bonding 0.54-eV GaInAsSb-based TPV epilayers to a SI GaAs handle wafer with a SiOx/Au layer. The resulting TPV device structure consists of an electrically insulating, broad-band, high-reflectivity mirror sandwiched between the GaSb-based device layers and the GaAs wafer [6]. Figure 1 schematically illustrates the device. An earlier report of GaAs photovoltaic cells on a Pd-coated Si wafer suggested that performance enhancements might be expected due to light trapping and photon recycling [7]. These performance advantages for TPV devices are also anticipated. In addition, the internal reflector, which is designed to provide electrical isolation, allows monolithic series interconnection of TPV cells and can aid in spectral control of below-bandgap photons. Considerations were made to minimize resid
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