Germanium-Tin Diode for Thermophotovoltaic Energy Collection

The bandgap of germaniumtin alloys extends from 0.8 e V into the metallic as the Sn composition increases, which overlaps the strongly absorbing range of the dominant 111- V systems used for thermophotovoltaic (TPV) power collection and for narrow bandgap diodes in multijunction PV cells. Si and Ge wafers are more physically resilient than III - V wafers and material abundance is higher than indium-containing cells. Si wafers are also available in larger formats, with a broader array of processing technologies. In fully relaxed GeSn films with greater than 6% Sn, the bandgap transitions to direct; in compressively strained films, the transition is higher. For high-stress and cost-limited applications, Group-IV devices have the potential to be a useful substitute for existing options. We designed a Gel-xSnx device and simulated its performance in Silvaco Atlas using material properties obtained from preliminary growths. We deposited the Gel-xSnx TPV device by molecular beam epitaxy, targeting 3 % Sn to achieve a similar bandgap to GaSb. Boron and antimony were used as p and n dopants. Material quality was characterized by XRD, spectroscopic ellipsometry, photoluminescence, and microscopy techniques. Component material electrical characterization was performed via Hall effect.