Electron and hole-spin densities in undoped illuminated a-Si:H

Light-induced electron spin resonance in undoped hydrogenated amorphous silicon (a-Si:H) was excited with red light (1.7 eV) or/and infra-red light (1.16 eV), respectively. For both kinds of excitation, the spin density of the narrow electron line is smaller than that of the broad hole line, the ratio being smaller than 0.44 for 1.7 eV light but larger than this value for 1.16 eV excitation. Assuming that 0.44 is the fraction of singly-occupied states in the conduction-band tail, it is deduced from the data that there is a deficit of electrons for 1.7 eV light but a deficit of holes for 1.16 eV excitation. These differences may be due to the existence of both positively and negatively-charged Si-dangling bonds (D⁺, D^-) in the dark. However, it is also conceivable that these states are only induced by the illumination.