Excitation-energy dependence of optically induced ESR in a-Si:H

Comparative studies of red (633 nm) and infrared (IR) (1064 nm) light-induced electron spin resonance (LESR) have been performed on various samples of hydrogenated amorphous silicon. Red-light excitation always yields the well-known LESR spectra for that material which can be ascribed to a superposition of two resonances due to band-tail state carriers of both types in a 1:1 ratio. For the a-Si:H films examined in this study, which were all deposited on quartz substrates, IR excitation results in different LESR spectra with considerably less spin density in the broad line attributed to holes in the valence-band tail than in the narrow line attributed to electrons in the conduction-band tail. This asymmetry is not observed in a powdered sample, which points to interface states at the quartz surface of the films as the mechanism responsible for the excess spin density in the narrow line. The observed dependence on film thickness confirms this interpretation. Surprisingly, the number of these interface states is as large as 5×${10}^{17}$ ${\mathrm{cm}}^{\mathrm{-}3}$ for a region which is assumed to be 1000 Å thick near the a-Si:H/quartz interface. An increased density of dangling bonds in the interface region due to different growth conditions for the first few hundred angstroms of the film growth, which are doubly occupied because of band-bending effects, is one possibility to explain the results.