The structural, morphological, and defect properties of mixed anion, InAs y P 1Ϫy and mixed cation, In x Al 1Ϫx As metamorphic step-graded buffers grown on InP substrates are investigated and compared. Two types of buffers were grown to span the identical range of lattice constants and lattice mismatch ͑ϳ1.1-1.2%͒ on ͑100͒ InP substrates by solid source molecular beam epitaxy. Symmetric relaxation of ϳ90% in the two orthogonal ͗110͘ directions with minimal lattice tilt was observed for the terminal InAs 0.4 P 0.6 and In 0.7 Al 0.3 As overlayers of each graded buffer type, indicating nearly equal numbers of ␣ and  dislocations were formed during the relaxation process and that the relaxation is near equilibrium and hence insensitive to asymmetric dislocation kinetics. Atomic force microscopy reveals extremely ordered crosshatch morphology and very low root mean square ͑rms͒ roughness of ϳ2.2 nm for the InAsP relaxed buffers compared to the InAlAs relaxed buffers ͑ϳ7.3 nm͒ at the same degree of lattice mismatch with respect to the InP substrates. Moreover, phase decomposition is observed for the InAlAs buffers, whereas InAsP buffers displayed ideal, step-graded buffer characteristics. The impact of the structural differences between the two buffer types on metamorphic devices was demonstrated by comparing identical 0.6 eV band gap lattice-mismatched In 0.69 Ga 0.31 As thermophotovoltaic ͑TPV͒ devices that were grown on these buffers. Clearly superior device performance was achieved on InAs y P 1Ϫy buffers, which is attributed primarily to the impact of layer roughness on the carrier recombination rates near the front window/emitter interface of the TPV devices.
Monolit hic Interconnected Modules (MIM) are under dev e lopment for thermophotovoltaic (TPV) energy conversion apphcations. MIM de vices are typifi ed by series-interconnected photovoltaic cells on a common , semi-insul ati ng su bstrate and generally include rear-surface infrared (lR) reflectors. The MIM arc hitecture is being implemented in InGaAsSb materials without semi-insulating substrates through the development of alternative isolation methodologies. Moti vations for developing the MIM structure include: reduced resistive losses , higher output power density than for systems utilizing fro nt surface spectral control, improved thermal coupling and ultimately higher system efficiency. Numerous design and material changes have been investigated since the introduction of the MIM concept in 1994. These developments as well as the current design strategies are addressed.
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