Study of plasma enhanced chemical vapor deposition of boron-doped hydrogenated amorphous silicon thin films and the application to p-channel thin film transistor

Abstract

The material and process characteristics of boron doped hydrogenated amorphous silicon (a-Si:H) thin film deposited by plasma enhanced chemical vapor deposition technique (PECVD) have been studied. The goal is to apply the high quality films deposited at low substrate temperature for devices such as thin film transistors (TFTs). The effect of the deposition parameters such as doping gas concentration, substrate temperature, hydrogen dilution, helium dilution, power density, and pressure at 50 kHz rf frequency on the films' characteristics were analyzed. The films' electrical property was characterized by its dark resistivity. The chemical composition and bonding characteristics were discussed. p-channel TFTs were fabricated with these optimized films. Three different levels of dopant concentrations in the channel were used to detect the dopant effect on the TFT properties. Doping resulted in the increase of film deposition rate. The low film deposition rate at the high temperature deposition corresponds to a dense structured film. The increase of gas phase H₂ concentration could increase H₂ etching of the weak bonds in the film, which is consistent with the decrease of the deposition rate. Film's dark conductivity is determined by the atomic B concentration in the film, the substrate temperature, the ion bombardment effect, the surface morphology, and the gas phase and film hydrogen concentration. At high power density and high pressure plasma condition, film with a high deposition rate shows a high conductivity. However, excessive ion bombardment effect, e.g. in powdery plasma region, limits the further increase of the conductivity. Film deposited with He dilution demonstrates a higher conductivity compared to the H₂ dilution counterpart. This might be attributed to a more effective ion bombardment effect of the former. Powder generation in the plasma significantly affects the conductivity of He diluted film compared to the H₂ diluted ones, which might be due to the less H₂ etching effect at the He dilution deposition. The output and transfer characteristics show the normal p-channel TFTs behavior. TFT characteristics, such as mobility, threshold voltage, and on-off current ratio were affected by the doping gas concentration in the channel layer and the deposition process.