A structural study of InGaAs/InGaAs strain-balanced MQW for TPV applications

摘要

Multi-quantum well photovoltaic cells offer a number of advantages over conventional "single-gap" cells for thermophotovoltaic applications, first of all because they can reach a higher open circuit voltage under the same radiation source and with the same absorption edge. Material quality issues and the constraints imposed by the commercial available substrates indicate that In_xGa_(1-x)As/In_yGa_(1-y)As/InP strain-balanced heterostructures are suitable to obtain good quality multi-quantum wells with an absorption edge just below 2.0 μm. Structural stability in the presence of a high density of elastic energy such as in the case of a strain-balanced multi-layer is a very important issue to be addressed by optimising key parameters like composition, thickness of wells and barriers and number of periods. In this paper we present and discuss the mechanisms of plastic relaxation of these structures with a particular attention to the impact of the extended defects generated by the local breakdown of the crystal lattice to the electrical properties of the devices. Then, after the presentation of the optimum structure with an absorption edge at 1.96 μm, we discuss the issue of a further extension of the absorption edge through the use of a so-called virtual substrate, that is a buffer structure between the substrate and the device designed to relax to a given extent with a minimum number of dislocations propagating towards the active region. On the basis of a recipe based on the experimental results on InGaAs single and multi-layers grown on GaAs, we have designed a series of step-graded buffer structures providing good virtual substrates with a lattice parameter larger than GaAs. Strain-balanced multi-quantum wells have been grown on In_xGa_(1-x)As virtual substrates with 0.14 < x < 0.35 with a residual density of threading dislocations of about 10~5 cm~(-2). Work is in progress to remove the residual morphological undulation (cross hatch) induced by the misfit dislocations confined in the buffer structure and to extend this approach to InP.