Recombination at defects in amorphous silicon (a-Si:H)

Optical excitation with energy E_x = 1.16 eV was used to study the defect-related photoluminesce in a-Si:H. Light-induced electron spin resonance shows that both electrons and holes are generated in the band tails by defect absorption. The carriers appear to be created in a two-step process and cluster around the defects. The photoluminescence spectra display both the familiar intrinsic band near 1.3 eV and a defect-related band near 0.8 eV, the latter one being strongly enhanced compared with excitation with E_x above the optical gap (E_x > 1.8 eV). It is proposed that the defect bands in undoped, phosphorus- and boron-doped films originate from tunneling transitions of majority carriers from band-tail states into the respective predominating dangling-bond states (D⁰, D^- or D⁺, in the dark). The influence of temperature and of the defect density on both emission bands is studied, the defects being generated by doping, electron bombardment and light exposure. The general behavior suggests that the tunneling transitition into the defect states in general is non-radiative. However, depending on the specific local configuration, this transition can occur radiatively as well.