Deposition of high quality amorphous silicon, germanium and silicon-germanium thin films by a hollow cathode reactive sputtering system

Abstract

High quality hydrogenated amorphous silicon (a-Si:H), germanium (a-Ge:H) and silicon–germanium (a-SiGe:H) thin films have been deposited by means of a d.c. hollow cathode system with magnetic field confinement. High purity single-crystal silicon and germanium nozzles were reactively sputtered in a high-density hollow cathode discharge of argon and hydrogen. This process avoids the use of the toxic and pyrophoric gases, germane and silane. The amorphous silicon thin films had light to dark conductivity ratios >10 6 with light conductivity in the 10 −5 S/cm range. The best a-Si:H films have a Tauc band gap near 1.8 eV with an atomic hydrogen concentration of approximately 14%. The growth rate was in the 2–3 μm/h range. For the a-Ge:H films the FTIR results indicate that these films have hydrogen bonding as a single atom, as did the hydrogenated silicon films. The Tauc bandgap was approximately 1.0 eV for all the germanium films. A slight photoresponse was noted for these films, which were deposited at a rate of from 2 to 6 μm/h. For the a-SiGe:H films, two hollow cathodes of single crystal Si and Ge are reactively sputtered simultaneously. A description of the complete system will be presented. The optical and electronic properties of the initial films are promising. The photoresponse is dependent upon the bandgap, i.e. the germanium content, as expected. A light to dark ratio of 2600 has been achieved for a film with a bandgap of 1.53 eV. The FTIR data indicates that SiH bonds dominate over Ge:H bonds by the absence of peaks at 570 and 1880 cm −1.