In this work we describe a novel system for photovoltaic applications which combines InGaAs based strain-balanced multiple quantum wells (MQWs) with a “virtual substrate”, designed to extend the absorption edge of the photovoltaic devices to about 1 eV. The virtual substrate is designed by properly choosing a sequence of InGaAs layers having different In content, in order to obtain the desired lattice parameter at the topmost layer and to confine at the deepest interfaces the misfit dislocations, well away from the QW active region.
A series of InGaAs p-i-n junctions, containing a strain balanced MQW in the intrinsic region, were deposited by metallorganic chemical vapor deposition on different virtual substrates. In all the samples the virtual substrates were proved to be successful to grow zero net strain MQW and to confine defects at the buffer/substrate interface. Transmission electron microscopy observation shows that no defects propagate from the strain accommodating layers to the active region. The total density of threading dislocations reaching the surface was found to be less than 1*E5/cm2.
The confined misfit dislocation network, however, results in marked cross-hatched morphology that was found to affect the lateral strain distribution in the whole structure. By optimizing the growth condition of the structures, the influence of the surface roughness induced by CH pattern is partially suppressed.