Electronic transport in amorphous and microcrystalline phosphorus-doped silicon films prepared by thermal LPCVD

Abstract

Abstract Phosphorus-doped silicon films have been prepared by thermal low-pressure vapour deposition method using a wide range of coating conditions. Detailed results on dark electrical conductivity, photoconductivity and thermoelectric power are presented. The films can be roughly classified, according to their properties, into four categories, namely, low-phosphorus-content amorphous films (aLP), high-P-content amorphous films (aHP), low-P-content microcrystalline films (μcLP) and high-P-content microcrystalline films (μcHP). By analysing the experimental data by suitable two-conduction-path models, the following conclusions can be drawn. μcHP films are highly degenerate semiconductors with the Fermi energy E F lying inside the conduction band. μcLP films are slightly degenerate with a phosphorus impurity band situated at about 0·05 to 0·07 eV below E C. The as-deposited aHP films are high-quality n-doped films. The best doped sample so far achieved has E F lying 0·2 eV below E C. For as-deposited aLP films, E F is pinned at mid-gap by dangling bonds. Detailed analysis showed that the experimental results are consistent with a Hubbard model for the dangling bonds with a positive correlation energy of approximately 0·4 eV. The upper Hubbard band when occupied corresponds to negatively charged dangling bonds and is situated at 0·55 eV below E C. Post-hydrogenation removes the dangling bonds and the aLP films then become effectively doped, similar to the aHP films.