Control of micro void fraction and optical band gap in intrinsic amorphous silicon thin films (VHF-PECVD) for thin film solar cell application

Plasma parameters are important factors for fabricating intrinsic (i-type) layers of hydrogenated amorphous silicon (a-Si:H) films using a very high frequency plasma enhanced chemical vapor deposition (VHF-PECVD) system. In this work, the effects of hydrogen ratio (R), power density, pressure, electrode instances (E_d) and substrate temperature (T_s) on the growth and the properties of intrinsic amorphous silicon (i-type a-Si:H) thin films are investigated. The structural defect (micro void fraction: R∗) and optical band gap (E_g) can be controlled by changing the plasma conditions. High quality i-type a-Si:H is obtained with low power, pressure, moderate hydrogen ratio (R&9552;H₂/SiH₄: 4) and short electrode distance conditions at which a lot of SiH₃ radicals are generated. When the substrate temperate is above 200 °C, hydrogen effusion decreases SiH bonding and optical band gap of 1.74 eV is obtained. The properties of i-type a-Si:H depending on plasma conditions show hydrogen content (C(H)) = 6-11%, R∗= 0-0.2 and E_g = 1.74-1.86 eV. The properties of region of low structural defect and good passivation represent C(H) = about 10 at.%, R∗= 0 and E_g = 1.77 eV.