Surface of amorphous semiconductors and their contacts with metals

Contacts formed by rf sputtering of a chalcogenide glass onto several metals, including Ag, Al, Au, Cu, Mo, Ni, Pd, Pt, Sb, Ta, Te, and nichrome were studied by measuring photoconductivity, contact photovoltage, parallel capacitance Cₚ(ω) and resistance Rₚ(ω), and the I-V characteristic. Aging and annealing effects on the contact properties were found with several metals. In the case of some contact metals alloying produced high- or low-resistance regions depending on preparation conditions. Adsorption of H₂O and other polar molecules was found to shift the surface potential and thus the surface photovoltage. Evidence of the presence of a space-charge region and for compound formation (or alloying) are discussed. The width of the space-charge region was found to be between 300 and 500 A{}. This corresponds to a density of localized gap states between 2 ×{} 10¹⁷ eV^-1 cm^-3 to 8 ×{} 10¹⁷ eV^-1 cm^-3 near the Fermi level. The small size of the field effect in these materials implies a density of interface states of about 10¹⁴ eV^-1 cm^-2. No rectification was observed up to fields of 5 ×{} 10⁴ V/cm. This may be understood in terms of ambipolar conduction in the space-charge region of the nearly intrinsic amorphous semiconductor.