Some problems in determination of gap-state density in amorphous silicon

A brief review is provided of the measurement techniques of gap-state density in amorphous silicon (a-Si), giving particular attention to the applicability of these techniques to a-Si which has rather high densities of states in the gap. It is pointed out that a-Si produced by a chemical-vapor deposition (CVD) technique is pure and exhibits properties relating only to positional disorder. A description is given of two approaches for measuring gap-state density in CVD a-Si. One approach represents basically an improvement of the field-effect method, while the other is the high-frequency capacitance-voltage method. Results of measurements of the gap-state density in CVD a-Si are also presented.