Study of the electronic structure of amorphous and crystallized low pressure chemically vapor deposited silicon films near the absorption threshold

Abstract

Silicon films were chemically vapor deposited on quartz substrates from SiH4 decomposition in a conventional reactor at 230 mTorr and at a temperature of 550 °C. Samples were subsequently doped with phosphorus at a concentration of 1018 cm−3 and oxidized, in order to increase the grain size and to enhance crystallization. The optical properties of these films were studied with optical transmission measurements below their absorption threshold taken after every processing step, which were analyzed with the aid of a two-band model proposed by R. Forouhi and I. Bloomer [Phys. Rev. B 34, 7018 (1986)]. It was found that the energetic distance, separation, and magnitude of the Forouhi–Bloomer (FB) bands change with the various post-deposition treatments. For the as-deposited (amorphous) films, the FB model provides a picture of the distribution of the density of states (DOS) in very good agreement with that of vacuum evaporated films measured in the past with photoelectron spectroscopy. X-ray diffraction measurements taken on crystallized samples have shown that grains were oriented with the (111) and (220) crystallographic axes normal to the substrate. It was shown that the orientation of grains affects significantly the DOS and hence the optical properties of films, which are in this case influenced by electronic transitions near the L and K points of the Brillouin zone of Si [corresponding to the (111) and (220) crystallographic directions].