Investigation of structural disorder using electron temperature in VHF-PECVD on hydrogenated amorphous silicon films for thin film solar cell applications

Electrode distances and gas flow ratios are important parameters 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, we investigated the relationship between the electrode distances and gas flow ratios on the properties of i-type a-Si:H films. The electrical, chemical and structural properties are improved with decreasing electrode distances (20-40 mm) at a hydrogen ratio [R (H₂/SiH₄) = 4], due to the low electron temperature and heating effect. A low electron temperature generates silane-related-reactive species (SiH₃) and decreases structural disorder resulting in high quality i-type a-Si:H films. The electrical, chemical and structural properties of the a-Si:H films are confirmed using Al coplanar electrodes, FTIR, Raman spectroscopy, and spectroscopy ellipsometry (SE). When a solar cell is fabricated using the a-Si:H film, J_sc of 13.2-14.8 mA/cm², photoconductivity of 1.5×10^-5-8.6×10^-5 S/cm, Si-H₂ content of 0-1.24 at.%, and hydrogen content of about 10 at.% are obtained. These results together with a model of the plasma chemistry indicate that H atoms and SiH₃ radicals play an important role in the deposition process.