Effect of rf power on the structure and related gap states in hydrogenated amorphous silicon

Abstract

The intricate relationship between the structure and processing parameters in a-Si:H is not well understood. The effect of variation in rf power on the structural, optical and electronic properties of glow discharge deposited hydrogenated amorphous silicon (a-Si:H) has been studied using deep level transient spectroscopy (DLTS), photoluminescence (PL), spectroscopic ellipsometry and Raman spectroscopy. The DLTS results show a predominant increase in the gap state density around 0.5 eV below the conduction band, which is also supported by the decrease in the intensity of the PL peak around 0.8 eV with rf power. Ellipsometry results show an overall decrease in the value of ε₂, while the energy corresponding to the ε₂ peak position remains constant. Significant modifications of spectral features are observed in the Raman spectra of film deposited at higher rf powers. These results suggest that the a-Si:H films grown at higher rf power are less dense and have larger vacancy or void concentration. It is proposed that such structural defects lead to the observed energy levels in the gap and associated physical properties of the a-Si:H films.