We have measured the excitation-dependent radiative efficiency in a set of lattice-matched In x Ga 1Ϫx As/InAs y P 1Ϫy double heterostructures incrementally lattice mismatched to InP substrates. We find that the overall rate of defect-related recombination shows little change from the lattice-matched case. However, the excitation-dependent transition between defect-related and radiative recombination changes dramatically with mismatch. While a simple defect recombination model assuming defect

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... vels concentrated near the middle of the band gap fits well for the lattice-matched material, the model does not fit the shape of the efficiency curve for the mismatched structures. We show that the addition of band edge exponential tails to the defect-related density of states gives a much better theoretical fit. Thermophotovoltaic ͑TPV͒ energy conversion systems that utilize broadband, blackbody-like radiators require highperformance, low-band gap ͑0.4 -0.7 eV͒ photovoltaic converters. 1 Our approach to meeting this requirement involves a converter design based on a lattice-matched ͑LM͒ InAs y P 1Ϫy /In x Ga 1Ϫx As/InAs y P 1Ϫy double-heterostructure device ͑LM when yϭ2.14xϪ1.14͒, which is grown latticemismatched ͑LMM͒ on an InP substrate with an intervening compositionally step-graded region of InAs y P 1Ϫy . The lowband gap In x Ga 1Ϫx As alloy serves as the light absorber and the LM InAs y P 1Ϫy cladding layers passivate the interfaces and confine carriers in the In x Ga 1Ϫx As material. Deleterious effects of the LMM ͑e.g., dislocation formation and morphological defects͒ are alleviated by including an appropriate number of Ϫ0.2% mismatch InAs y P 1Ϫy steps between the substrate and the double heterostructure. 2 The lowest band gap ͑ϳ0.5 eV at room temperature͒ In x Ga 1Ϫx As alloys under investigation experience severe ͑up to Ϫ1.7%͒ LMM, which usually leads to large increases in the rate of defect-related recombination. For TPV applications, such an increase in carrier recombination is problematic because it reduces the conversion efficiency of the cells. Our step-graded design, combined with the passivation afforded by LM InAs y P 1Ϫy cladding layers, has resulted in dramatic improvements in converter performance. In this letter, we explore the physical basis for these improvements by considering the effect of LMM on defect-related recombination in the In x Ga 1Ϫx As layer. We report excitation-dependent radiative efficiency measurements on several different epistructures that range from the LM condition (xϭ0.53) where no grading is employed to step-graded structures accommodating significant LMM ͑xϭ0.60, 0.72, and 0.78͒. We measure the radiative efficiency as a function of excitation power at 77 K to study the changeover between defect-related ͑nonradiative͒ and radiative recombination in the structures ͑see Fig. 1͒ . The band gap energy in the active layer (E g ) is determined from 77 K photoluminescence spectra. Since Auger recombination is slow at low temperatures and defect recombination saturates at high carrier densities, we scale the peak internal quantum efficiency to 100% at 77 K. The low-temperature, high-excitation result provides a a͒ Electronic mail: tigfroerer@davidson.edu FIG. 1. Internal radiative quantum efficiency ͑integrated photoluminescence intensity divided by the excitation power͒ vs the rate of electron-hole pair generation and recombination in steady state. Structures are identified by the InGaAs band gap energy at 77 K. All curves are scaled to 100% peak efficiency and incrementally shifted by 20% for clarity. The solid lines are included to emphasize the difference in the shape of the efficiency curve for the nominally lattice-matched (E g ϭ0.80 eV) and mismatched structures.