Very Wide-Gap and Device-Quality a-Si:H from Highly H₂ Diluted SiH₄ Plasma Decomposed by High RF Power

Abstract

The H₂ dilution technique at a high deposition rate (R_D) was investigated by depositing hydrogenated amorphous silicon (a-Si:H) under a high if power density of 750 mW/cm², which is 20 times as large as that of conventional conditions. It was found that the H₂ dilution ratio γ (= [H₂ gas flow rate] / [SiH₄ gas flow rate]) tendency of the film properties, such as the H content (C_H), optical gap (E_opt), SiH₂/SiH and photoconductivity (σ_ph) of a-Si:H is different for the high rf power (750 mW/cm²) and the medium rf power (75 mW/cm²) conditions. Under medium rf power, the C_H, E_opt and SiH₂/SiH decrease as γ increases. Under the high if power, on the contrary, the C_H and E_opt, monotonously increase while maintaining a low SiH₂/SiH and a high σ_ph of 10⁻⁶ S/cm as γ increases. These results suggest that increasing the rf power enhances the H incorporation reactions due to H₂ dilution. It is thought that a high rf power causes the depletion of SiH₄ and hence the extinction of H radicals, expressed by SiH₄ + H* → SiH₃* + H₂, is suppressed. A high H radical density enhances the incorporation of H into _a-Si:H, resulting in very wide-gap a-Si:H with a high C_H, Consequently, very wide-gap a-Si:H with device-quality (E_opt of 1.82 eV with an (αhv)^1/3 plot, corresponding to > 2.1 eV with Tauc’s plot, and σ_ph of 10⁻⁶ S/cm) can be obtained at a high R_D of 12 Å/s without carbon alloying.