High band-gap hydrogenated amorphous silicon-selenium alloys

Hydrogenated amorphous silicon-selenium alloy thin films were prepared by the decomposition of SiH4 and H2Se gas mixtures in a radio-frequency plasma glow discharge at a substrate temperature of 250 °C. The alloy composition was varied by changing the gas volume ratio Rv = {[H2Se]/[SiH4]}. Infrared and Raman spectroscopies were used to probe the bonding structure of the material. In addition to the hydrogen induced bands normally observed in a-Si:H, a new selenium induced band at 390 cm−1, assigned to the stretching mode of the Si—Se bond, was observed. Analysis of the vibrational SiH stretching region reveals the presence of a significant level of (Si)xSe3−xSiH and (Si)ySe2−ySiH2 configurations. Optical and electrical measurements show that increasing the selenium content results in an increase in the optical (Tauc) gap and a decrease in the dark conductivity (σD) and photoconductivity (σph). However, the photosensitivity (σph/σD) remains high for the entire composition range. The Urbach energy and defect density were obtained from subgap absorption measurements. Upon increasing the Se content, a broadening of the band tails and an increase in the defect density were observed. The photoluminescence intensity and width at half-maximum data for the alloys are consistent with the results drawn from the optical absorption measurements.