The roles of deposition pressure and rf power in opto-electronic properties of a-SiO:H films

Abstract

Hydrogenated amorphous silicon oxide (a-SiO:H) films have been prepared in a conventional single-chamber radio frequency (13.56 MHz) plasma enhanced chemical vapour deposition system with a carbon dioxide , silane and hydrogen gas mixture. Variation in optical gap from 1.96 to 2.12 eV has been obtained by controlling the deposition pressure and radio frequency (rf) power. The photoconductivities of films in each set are similar to each other for similar optical gaps. High rf power does not have any remarkable deteriorative effect on material but the rate of deposition of film and optical gap are enhanced. At rf power density the and are and 2.11 eV respectively whereas under similar other conditions but with rf power density the corresponding and are and 1.96 eV respectively. By lowering the deposition pressure the optical gap of films has also been enhanced. At 1.5 Torr pressure, the optical gap is 1.96 eV and at 0.13 Torr it becomes 2.12 eV. Here variation of the dilution of hydrogen is adopted in order to look into the role of hydrogen atoms in plasma and characteristics of the deposited films. A model in which incorporation of oxygen is described through plasma kinetics is proposed in this regard. With a higher optical gap the photoconductivity reduces but this reduction is mostly due to there being a lower optical absorption coefficient , partly due to there being a lower value of the product of the mobility , lifetime and quantum efficiency and also partly due to the mid-gap defect density . For the power variation set of samples, with optical gap up to about 1.99 eV, and the photoconductivity remain around and respectively and, with this optical gap, the dark conductivity is . When the material is degraded by soaking in light, a modification of mid-gap states and also of the valence band tail state takes place and falls, together with the photoconductivity. For material of 1.95 eV optical gap the initial and are reduced to and respectively after 3 h of soaking in light.