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;H-2/SiH4: 4) and short electrode distance conditions at which a lot of SiH3 radicals are generated. When the substrate temperate is above 200 degrees C, hydrogen effusion decreases Si-H 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. (C) 2014 Elsevier Ltd. All rights reserved.