Optical absorption edge of hydrogenated amorphous silicon studied by photoacoustic spectroscopy

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Abstract
Optical absorption spectra of undoped hydrogenated amorphous silicon (a-Si: H) were investigated as a function of the ambient temperature, the deposition temperature and the post-annealing temperature using photoacoustic spectroscopy. On the basis of the temperature dependence of the photoacoustic signal, the optical gap and the characteristic energy of the exponential tail have been discussed in relation to the thermal and structural disorder. From the dependence of the optical absorption spectra on the deposition as well as post-annealing temperature, the optical gap of a-Si: H has been found to be mainly affected by the bonded hydrogen content rather than structural disorder. A weak absorption band overlapping the absorption tail has been well correlated with a spin density of electron spin resonance (ESR) signals (g = 2.0055), namely, a density of singly occupied dangling bonds. It has been demonstrated that the structural disorder and a number of defect states are not so much affected by post-annealing temperature but mainly by deposition temperature. From the temperature dependence of the photoacoustic signal, the absolute luminescence quantum efficiency of the undoped a-Si: H film with an ESR spin density of 5 × 1015 cm−3 has been estimated to be 25% at around 50 K.