States in the gap of doped and undoped a-Si:H studied by the photomodulation spectroscopy

The steady state photomodulation spectrum, its temperature and excitation intensity dependences have been studied in phosphorous doped and undoped a-Si:H. The spectra are analyzed in terms of photocarriers trapped in band-tail states and dangling bonds (DB) defects in undoped samples and impurities and charged DB states introduced by doping in P-doped samples. In undoped a-Si:H we found that the two DB energy levels D± are symmetrically located about midgap with an effective correlation energy Ue f f of 0.6±0.2eV. In P-doped a-Si:H D" is pushed toward midgap by about 0.25 eV and Ueff=0.4±0.2eV.