Influence of substrate bias on the optical properties of amorphous hydrogenated silicon films prepared by sputtering

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Abstract
The influence of a negative d.c. substrate bias VB on the properties of sputtered a-Si: H films has been investigated. The analysis of the deposited species was carried out using He+ Rutherford backscattering (Ar, Si and O concentration) and boron-induced nuclear reactions for hydrogen profiling. In particular, the optical absorption properties of films in the infrared range, the fundamental absorption near the optical gap and the specular reflectivity in the ultraviolet were investigated. The essential role of ionic bombardment is emphasized. Films prepared under high bias (-140 V) exhibit no post-oxidation, in contrast to those prepared under a less negative bias. From Rutherford backscattering measurements it is shown that the film density increases with | VB|. This, and the evolution with time of film oxidation and infrared absorption bands at 2000 and 2100 cm-1, indicate two types of hydrogen bonding in dilute sites and cluster sites, respectively. The optical gap is found to decrease in a similar way to the oxygen content when |VB| increases. The incorporation of oxygen leads to a more disordered structure, accompanied by a higher density of states in the gap. All the results show that a hitherto neglected parameter, namely the bombardment of the film by Ar+ ions, appears to be of major importance in sputtered films. Good control of the substrate bombardment allows the quality of the films to be improved and can drastically reduce ageing effects.