Electronic structure of Ge1-x-ySixSny ternary alloys for multijunction solar cells

Abstract

Ternary group-IV alloys have a wide potential for applications in infrared devices and optoelectronics. In connection with photovoltaic applications, they are among the most promising materials for inclusion in the next generation of high-efficiency multijunction solar cells, because they can be lattice matched to substrates as GaAs and Ge, offering the possibility of a range of band gaps complementary to III-V semiconductors. Apart from the full decoupling of lattice and band structures in Ge1-x-ySixSny alloys, experimentally confirmed, they allow preparation in a controllable and large range of compositions, thus enabling to tune their band gap. Recently, optical experiments on ternary alloy-based films, photodetectors measured the direct absorption edges and probed the compositional dependence of the direct gap. The nature of the fundamental gap of Ge1-x-ySixSny alloys is still unknown, as neither experimental data on the indirect edges nor electronic structure calculations are available, as yet. Here, we report a first calculation of the electronic structure of Ge1-x-ySixSny ternary alloys, employing a combined tight-binding and virtual crystal approximation method, which proved to be useful to describe group-IV semiconductor binary alloys. Our results confirm predictions and experimental indications that a 1 eV band gap is indeed attainable with these ternary alloys, as required for the fourth layer plan to be added to present-day record-efficiency triple-junction solar cells, to further increase their efficiency, for example, for satellite applications. When lattice matched to Ge, we find that Ge1-x-ySixSny ternary alloys have an indirect gap with a compositional dependence reflecting the presence of two competing minima in the conduction band.