MEASUREMENT OF URBACH EDGE AND MIDGAP STATES IN AMORPHOUS-SILICON P-I-N DEVICES

Urbach energy of valence band tails and mid-gap defect densities are i mportant parameters for determining the performance of amorphous silic on (a-Si:H) devices. In this paper, we examine a technique which allow s one to determine these parameters in device structures, as opposed t o in films. The technique consists of measuring sub-gap quantum effici ency as a function of reverse bias and photon wavelength. We show that there is a distinction between the response of tail states and mid-ga p states to the application of reverse bias, and that by analyzing the se differences, one can locate the energies of the mid-gap states in t he device. The technique gives an accurate measurement of Urbach edge of tail states, even on textured substrates, but only estimates mid-ga p states within a factor of 2.