Optical Properties of Amorphous Semiconductors

The optical absorption coefficient alpha of doped and undoped hydrogenated amorphous silicon (a-Si:H) thin films are studied for photon energies ranging from 2.4 eV to 0.8 eV using the Photo-Pyroelectric Spectroscopy (PPES) technique. In this technique the temperature rise induced in the a-Si:H film due to weakly absorbed light is detected via a pyroelectric polymer in thermal contact with the thin film sample. PPES presently has a sensitivity of alphad > 10 ^{-3}, where d is the sample thickness. PPES also provides separate determinations of the spectral dependence of the non-radiative efficiency eta_ {rm nr}(hnu), which provides a measure of the heat generated due to the absorption of photons. The observed non-constant spectrum of the non-radiative efficiency eta_ {rm nr}(hnu) can be understood through a model in which the recombination and thermalization of the photo-generated electron-hole pairs provide separate contributions to the PPES signal due to their different time constants.