Metamorphic antimonides on GaAs for thermophotovoltaic devices

Abstract

The growth of metamorphic III-V semiconductors on substrates of dissimilar lattice parameter is of interest for the engineering of optoelectronic devices for which the cost of the substrate would otherwise limit the applicability of the material. Such is the case of the antimonides, which are very appropriate for small band gap photovoltaics or thermophotovoltaics. To further increase the efficiency of tandem solar cells (typically based on Ge or GaAs substrates) it is necessary to integrate materials with band gap < 1.4 eV. Metamorphic InGaAs has been successfully integrated in GaAs based tandems, but requires the growth of thick step graded buffers of InGaP.i Here we present a study on the nucleation and subsequent growth of AlSb on GaAs (001), and compare our results with the more extensively studied case of GaSb on GaAs,ii, iii and the case of AlSb grown on Si.iv We have observed the nucleation of almost completely relaxed AlSb islands after 0.11 ML of AlSb. The lattice parameter of the metamorphic material is slightly smaller than that of bulk AlSb. Measuring the deformation of the substrate during growth, we have observed that residual stress accumulates due to the incomplete plastic relaxation at the interface. Transmission Electron Microscopy (TEM) images show almost all of the plastic relaxation accommodated by an array of 90° misfit dislocations at the interface (IMF). A very small fraction of the plastic relaxation is accommodated by stacking faults (in the case of AlSb) or threading dislocations (in the case of GaSb). We have observed InAs layers grown on GaSb to be quite effective filters of threading dislocations, but stacking faults in AlSb propagate undisturbed though thin InAs and GaSb layers.