Photoinduced absorption spectra ina-Ge: H anda-Si: H

Measurements of steady-state photoinduced absorption in $a$-Ge: H and $a$-Si: H were extended to cover the energy range from 0.25 to 1.9 eV. The subgap photoinduced-absorption bands in both materials are interpreted in terms of four kinds of optical transitions of photogenerated carriers from traps in the gap into the bands; two transitions produce absorption and two bleaching. This model explains the approximately symmetric form of the photoinduced-absorption band in $a$-Ge: H previously ascribed to polaron absorption.