Optical properties and transport in microcrystalline silicon prepared at temperatures below 400 °C

Microcrystalline silicon (grain size ∼100 Å) films have been prepared at temperatures below 400 °C through an H2-plasma assisted transport process. The films contain an average of ∼2 at. % of hydrogen which is predominantly situated in the grain boundary regions in the form of Si-H2. The optical absorption coefficient of these films exceeds that of crystalline Si by up to two orders of magnitude below 3.0 eV. The films can be doped by adding PH3 to the hydrogen plasma. The highest room-temperature conductivity so achieved was ∼1 (Ω−1 cm−1). From an analysis of the activation energies vs dopant concentration we deduce that the high-temperature transport (T≳330 °K) is barrier limited. The upper limit for the density of trapping states in the grain boundaries is 1×1013 cm−2.