Measurement of surface photovoltage in high-rate deposited a-Si:H films and comparison with photothermal deflection spectroscopy and conductivity data

Abstract

A series of extensively characterized films of hydrogenated amorphous silicon (a-Si:H) has been used to study influences on and the reliability of the measurement of minority carrier diffusion length by the surface photovoltage method (SPV). The films were prepared by high-rate VHF glow-discharge deposition at plasma excitation frequencies f between 38 to 145 MHz. Defect density N D and Urbach tail parameter E o, measured by photothermal deflection spectroscopy (PDS), and photoluminescence (PL) yield at 4.2 K do not change as a function of f. However, photoconductivity, open-circuit voltage of Schottky barrier structures and diffusion length decrease with f. The increasing dark conductivity activation energy E a shows that the films become more intrinsic with increasing f. Therefore basic assumptions of the conventional SPV theory, like the existence of a field-free region, may be violated. We discuss whether drift and diffusion of holes and electrons contribute to the SPV signal. We also show how the power law governing the relation between photocurrent and light intensity can change the SPV result.