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 $C_p\left(\omega \right)$ and resistance $R_p\left(\omega \right)$, 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 ${\mathrm{H}}_2$O 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 AÅ. This corresponds to a density of localized gap states between 2 × ${10}^{17}$ ${\mathrm{eV}}^{-1\mathrm{}}$ ${\mathrm{cm}}^{-3\mathrm{}}$ to 8 × ${10}^{17}$ ${\mathrm{eV}}^{-1\mathrm{}}$ ${\mathrm{cm}}^{-3\mathrm{}}$ near the Fermi level. The small size of the field effect in these materials implies a density of interface states of about ${10}^{14}$ ${\mathrm{eV}}^{-1\mathrm{}}$ ${\mathrm{cm}}^{-2\mathrm{}}$. No rectification was observed up to fields of 5 × ${10}^4$ V/cm. This may be understood in terms of ambipolar conduction in the space-charge region of the nearly intrinsic amorphous semiconductor.