The impact of processing conditions and post-deposition oxidation on the opto-electrical properties of hydrogenated amorphous and nano-crystalline Germanium films

Low-cost multijunction photovoltaic devices are the next step in the solar energy revolution. Adding a bottom junction with a low bandgap energy material through plasma enhanced chemical vapor deposition (PECVD) processing could potentially provide a low-cost boost in conversion efficiency. A logical candidate for this low bandgap material is germanium. In this work we investigate the growth of PECVD processed hydrogenated amorphous/nano-crystalline germanium (a/nc-Ge:H), by characterizing over 100 samples, processed with a wide range of deposition pressures, powers, temperatures and GeH₄ dilution in hydrogen, using elemental analysis, vibrational analysis and analysis of the opto-electrical properties. We have identified a small processing window in which nc-Ge:H films are processed reproducibly. We also report on the strong correlation between the refractive index of the films and the presence- and extent of post-deposition oxidation. Notably, the oxidation generally increased the photoresponse of the films, as it results in a decrease of room temperature σ_d by 1-3 orders of magnitude. However, oxidation results in an increase of the bandgap energy and therefore impedes the development of a low bandgap material. The lowest E₀₄ we report is about 1.1eV, with an E_Tauc of 0.9eV and an σ_ph/σ_d of 3.4.