Effects of pressure and inter-electrode distance on deposition of nanocrystalline silicon under high pressure conditions

Abstract

Pressure (p) and inter-electrode distance (d) are important parameters in the process of depositing hydrogenated nanocrystalline silicon (nc-Si:H) by very high frequency plasma enhanced chemical vapour deposition (VHF PECVD). High quality nc-Si:H materials are normally deposited at high pressure (1 mbar <p <7 mbar). However, systematic research on the combined effects of p and d is rare. In order to optimize nc-Si:H for solar cells, such effects are investigated for a silane-hydrogen discharge at high pressure conditions. All nc-Si:H layers were deposited at fixed hydrogen dilution ratio (H2/SiH4), power and frequency. With optical emission spectroscopy, direct images taken by a photo camera and by 1D SiH4/H2 plasma simulation, three different series were analyzed to study the combined effects of p and d at high pressure. The effects on the crystalline ratio and the porosity of the deposited silicon layers were also investigated. When the p ·d product is constant, the plasma sheath becomes relatively thinner when d increases. When p or d increases independently, the electron density decreases. All the above modifications can increase the deposition rate, but by different mechanisms. When nc-Si:H is deposited at a p ·d product of 30 mbar·mm, compact material with high crystalline ratio is obtained