Germanium-on-silicon virtual substrate for lateral multijunction photovoltaics

Abstract

Lateral multijunction photovoltaics based on III-V direct band gap semiconductors enable efficient energy conversion. However, lattice matching between cell and substrate requires the use of expensive Ge or III-V substrates, which limits widespread application of III-V solar cells. Cost reduction can be achieved by using Ge-on-Si virtual substrate where a thin layer of Ge is grown on relatively inexpensive Si substrates, thanks to the greater material abundance and larger wafer diameters of Si. However, the lattice mismatch between Si and Ge can bring about threading dislocations that can significantly impair the efficiency of solar cells. This thesis presents patterned epitaxial growth of pure Ge on Si wafer through ultra-high vacuum chemical vapor deposition that achieves low threading dislocation density. This unlocks the potential for growing lattice-matched III-V photovoltaics of high quality on top of the virtual substrate. In addition, this thesis seeks to understand the mechanisms behind trapping of dislocations. The dislocation studies in this thesis not only shed light on dislocation motion in the Ge-on-Si epitaxy, but can be applied to other lattice mismatched materials systems as well. Lastly, the potential of lateral multijunction photovoltaics is demonstrated through simulation approaches.