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

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Abstract
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 °C. 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 °C 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.