Oxygen-rich phase segregation in PECVD a-SiO x:H semi-insulators

Abstract

Photosensitive, low H content ( C H = 4–6 at.%) a-SiO x :H, x « 1, semi-insulating films were deposited by means of the admixture of small amounts of oxygen during plasma enhanced chemical vapour deposition (PECVD) to the monosilane process gas. Oxygen incorporation into the films was favoured by a rotating two-pole magnetic field (0–27 mT) and by increasing the power-source frequency from 1.2 to 13.56 MHz. Film characterization was performed measuring UV/VIS/IR transmission, photo- and dark conductivities and determining the gap state distribution by the constant photocurrent method (CPM). IR-analysis included also the previously neglected formation of dihydride configurations. Three SiOSi stretching vibrations at 980, 1030 and 1080 cm −1 assigned to different oxydation states were found. Thus, the reported shift of the SiO stretching vibration frequency with increasing oxygen concentration is caused by a changing distribution of bonding configurations with a different oxidation state. Evidence for the formation of a clustered O-rich phase was found in films with an oxygen content as low as 0.4 at.%. The deposition rate dependence of the defect state density was consistent with a model of surface reactions including a two-step dangling bond termination via an intermediate SiOSi state whose first step of SiOSi formation is rate determining and whose second step is the recombination of two SiOSi configurations releasing O 2 and leaving two SiSi bonds. The distribution of oxygen induced defect states was calculated from measured CPM data.