Recombination processes ina-Si:H: Spin-dependent photoconductivity

We report on photoconductivity measurements and on the resonant photoconductivity change induced by ESR in undoped $a$-Si: H in the temperature range 100-300 K. We investigate samples with various defect densities (5×${10}^{15}$-${10}^{18}$ ${\mathrm{cm}}^{-3\mathrm{}}$) which are achieved by electron bombardment and subsequent annealing. Samples with a high-defect density ($N_s>{10}^{17}$ ${\mathrm{cm}}^{-3\mathrm{}}$) exhibit a resonant quenching effect which is attributed to spin-dependent tunneling of localized band-tail electrons to singly occupied defect states (dangling bonds). In low-defect material an additional quenching resonance is observed which is identified with spin-dependent diffusion (thermalization) of localized band-tail holes to doubly occupied dangling bonds. The results confirm our recombination model in which trapped electrons and holes recombine via defect states by tunneling and diffusion rather than by capture of free carriers.