Optical and photoconductive properties of discharge-produced amorphous silicon

Optical and photoconductive properties of discharge-produced amorphous silicon (a-Si) of the type used in efficient thin-film solar cells have been studied as a function of a wide range of deposition conditions. The optical absorption, optical band gap, photoconductivity, hydrogen content, and the characteristics of the Si-H vibrational mode in a-Si were determined. Both substrate temperature in the range ∼200–400 °C and the type of discharge used are found to be important factors in determining the measured optical and photoconductive properties of a-Si. For films produced at substrate temperatures near 200 °C, dihydride bonding occurs, and the optical band gap is about 1.7 eV. As the substrate temerature increases, monohydride bonding is favored, the optical band gap decreases, the optical absorption increases, and the photoconductive properties improve. These properties are, in part, associated with the presence of bonded hydrogen. For substrate temperatures between 300 and 400 °C, the photoconductive properties are not strongly affected by changes in the substrate temperature, but are strongly influenced by the deposition technique. The Si-H bond is shown to be stable to thermal treatments at or below the deposition temperature. This is critical for the stability of a-Si properties in the fabrication of solar cells and other devices at elevated temperatures.