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-degrees-C. The alloy composition was varied by changing the gas volume ratio R(upsilon) = {[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 (sigma-D) and photoconductivity (sigma-ph). However, the photosensitivity (sigma-ph/sigma-D) 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.