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Nonequilibrium Occupancy of Dangling Bond Defects in Undoped Amorphous Silicon Studied by Subgap-Light-Induced Electron Spin Resonance

A detailed investigation of the nonequilibrium occupancy of band tail states and dangling bond (DB) states in undoped amorphous silicon has been performed using the subgap-light-induced electron spin resonance (subgap LESR) technique. Measurements performed over a wide range of excitation intensities and temperatures reveal a systematic change of the LESR line-shape. Lowering the excitation intensity transforms the superposition of band-tail electron and hole absorption lines, corresponding to the creation of these carriers, into a reversed single DB line, corresponding to the annihilation of neutral DB states. The LESR behavior is reasonably well interpreted in terms of the charge neutrality requirement for band-tail carriers and charged DBs under illumination. A careful inspection of the experimental results on the basis of numerical calculation as well as analytical consideration proves that neutral DBs predominantly occur in thermal equilibrium, and that the ratio of charged to neutral defect density is not more than 10%.