External rf substrate biasing as a tool to improve the material properties of hydrogenated amorphous silicon at high deposition rates by means of the expanding thermal plasma

An external rf substrate bias (ERFSB) has been employed during the deposition of hydrogenated amorphous silicon (a-Si:H) by means of the expanding thermal plasma (ETP) deposition technique. The effects of the additional created ion bombardment on the a-Si:H material properties, such as the microstructure and defects, have been studied. The incorporation of nano-sized voids is reduced under ERFSB whereas the incorporation of vacancies is enhanced. The mass density increases with increasing rf power (P_rf). For moderate ERFSB (P_rf>12 W) the sub gap absorption of the a-Si:H material is reduced drastically. Possible plasma and ion bombardment induced surface processes responsible for these observations are discussed. Finally, it is demonstrated that the performance of the pin cells with a-Si:H films deposited with ERFSB improves.