Defect density and hydrogen bonding in hydrogenated amorphous silicon as functions of substrate temperature and deposition rate

We present the results of a detailed investigation of hydrogen bonding and optical absorption in the 0-6-2 eV range for four series of hydrogenated amorphous silicon films (a-Si:H) deposited under four different plasma conditions and for substrate temperatures ranging from 100 to 300°C. We have found that, for each plasma condition, the subgap absorption shows a sharp decrease at a well defined temperature, which allows us to define, for each series of samples, an equilibrium temperature T_ED for defects above which the subgap absorption remains constant. A similar trend is observed for the structural properties of the films (hydrogen content, hydrogen bonding, optical gap and refractive index). However, for each series, the equilibrium temperature T_ED for the defects is about 50°C lower than the equilibrium temperature T_EH for the hydrogen-related properties. Our results clearly show that the defect structure and the hydrogen-related structure of these a-Si: H samples do not follow the same substrate temperature dependence. Moreover, the substrate temperature and growth rate dependences of the defect density cannot be explained in the context of existing models for a-Si: H defect density in which only hydrogen-mediated thermal equilibrium processes are taken into account. We suggest that the plasma parameters (radical species contributing to deposition, ion bombardment, photon irradiation, etc.) play a crucial part in the determination of the growth processes and the optoelectronic properties of the film.