XPS study of overlayer formation on a-Si(H) due to high dose ion implantation

Doping of a-Si(H) during sample preparation has been the subject of numerous studies. The ion implantation process can be almost free from contamination because of the high purity of beam used. However, our results show that for high-dose implantation severe contamination takes place. Films of a-Si(H) with a thickness of about 103 nm on Silicon(111) substrates were obtained by the silane glow discharge decomposition (Ar + 4% SiH4). The hydrogen content in the film used was estimated to be 12 at%. The surface of as-prepared a-Si(H) samples is characterized by a thin oxide film and some carbon contamination caused mostly by an air exposure handling. Arsenic and chlorine ions were implanted by means of a commercial implanter. The surface was examined by XPS. The experimental details are given. The results are discussed. After the ion implantation an additional contamination by carbon and oxygen atoms was observed. It is known from the SIMS and AES measurements that carbon atoms can be adsorbed from residual gas molecules even in a vacuum cryopumped target chamber. So, formation of a contaminated overlayer is probably due to the lack of UHV conditions in standard implanters. (U.K.)