Surface effects in hydrogenated amorphous silicon studied by photothermal-deflection experiments

The optical-absorption spectra of glow-discharge undoped a-Si:H films deposited on silica and Corning 7059 glass substrates have been determined between 0.6 and 2 eV, using both the photothermal-deflection-spectroscopy (PDS) and constant-photocurrent (CPM) methods. All the samples, with thicknesses ranging from 0.5 to 10 m, had identical values of the optical gap and of the dark-conductivity activation energy (with the minor exception of the thinnest film for the latter). The results of conventional PDS experiments at low modulation frequency on films with different thicknesses confirm previously reported conclusions regarding the existence of substantial surface or interface absorption in device-quality undoped a-Si:H films. In contrast, the CPM-derived absorption spectra show very little, if any, surface or interface effect. They coincide within experimental uncertainties with the PDS-derived spectra obtained for the thickest films, down to about 1.1 eV. At lower energies, the CPM-derived absorption coefficient values decrease rapidly with decreasing energy, while the PDS-derived ones remain roughly constant for both types of substrates. In order to discriminate between bulk, surface, and interface absorption in this low-energy range, we used PDS experiments at high modulation frequency, in which we compared the values of the phase lag of the PDS signal with respect to the exciting beam, measured on thick (710-m) films for front and rear illumination, the probe-beam sample geometry remaining identical in both cases. We derive conclusive evidence that, for our films deposited either on silica or on glass substrates, the absorption measured by PDS at energies lower than 1 eV comes essentially from the free-surface region. © 1991 The American Physical Society.