Microcrystalline Germanium Carbide: A New, Almost Direct Gap, Thin Film Material for Photovoltaic Energy Conversion

Abstract

ABSTRACTWe report on the growth and electrical and optical properties of micro- and poly- crystalline Ge1−x Cxfilms. This is a new material which does not exist in nature. The films were grown using ECR plasma techniques under conditions of low pressure and high power from a mixture of germane, methane and hydrogen. The bandgap of the material could be varied from that of c-Ge (∼0.7 eV) to almost 1.15 eV by changing the methane/germane ratio. X ray diffraction and Raman measurements indicated good crystallinity in the film. The optical absorption spectrum of the film resembles that of c-Ge, with a near direct bandgap, implying that as C is added, both the central and the L valley in the conduction band move up simultaneously, while maintaining a small (∼0.15 eV) separation between the two. The Hall mobility was measured to be 70 cm2/V-sec. The films were doped n- type using phosphine, and p- type using diborane. P-n junctions with good rectification characteristics were fabricated. Because of its excellent optical absorption, this material may be suitable for photovoltaic applications, particularly for tandem junction cells. We have also observed that grain size can be controlled using H etching, and have observed distinct changes in optical properties related to quantum size effects in c-Ge films.