Studies of the frequency-dependent admittances of Schottky barriers formed on sputtered hydrogenated amorphous silicon

Abstract

Measurements of the frequency-dependent capacitance and conductance of Schottky barriers formed on sputtered hydrogenated amorphous silicon made over a temperature range 300–450 K are described. The data are interpreted using a trap-release model, which fits the zero d.c.-bias data well at all temperatures. For measurements made at finite bias other physical effects have to be considered. At negative biases, generation/recombination processes via deep states are found to be important. These are incorporated qualitatively into the model and, as a result, information is obtained about the relative decay rates for holes and for electrons. Considerable evidence is found for a higher density of deep states close to the barrier than in the bulk of the film; in far-forward bias these extra states can be directly probed by the capacitance measurement. Our best films have a density of states in the region of the Fermi level of around 10¹⁶ eV⁻¹ cm⁻³, with little energy dependence, in agreement with other recent observations using similar techniques.