Optical band gap of hydrogenated amorphous-boron thin films: The effect of thermal treatment

Hydrogenated amorphous-boron thin films produced by glow-discharge decomposition of a gas mixture of 3.1 vol. % diborane and 96.9 vol. % hydrogen have been heat treated at 360 and 400 0C. The resulting absorption coefficients and optical band gaps have been measured as a function of heat-treatment time and temperature. Both the absorption coefficients and the optical band gaps of these films can be drastically changed by such treatments. Thermal treatment at 400 0C for 200 h results in a systematic decrease in the optical band gap from 2.19 to 0.90 eV. From quantitative hydrogen analyses it is shown that hydrogen evolution during heating plays a strong role in reducing the apparent optical band gap. These results indicate that it might be possible to optimize theoretical amorphous-boron thin films through the tailoring of the optical band gap by such thermal treatment