Hyrogenated Amorphous Silicon with High Growth Rate, Gas Utilization and Homogeneous Deposition by Amplitude Modulated Vhf-Pecvd for Solar Cell Application

Abstract

AbstractHydrogenated amorphous silicon samples have been deposited by very high frequency plasma enhanced chemical vapour deposition (VHF PECVD), using a square-wave amplitudemodulated radio-frequency excitation. It is observed that the gas-utilization efficiency improves by a considerable amount when amplitude modulation is combined with a reduction in the gas flows. Using a conventional continuous wave (cw) 50 MHz plasma with SiH4 and H2 gas flows of 30 sccm each at a pressure of 0.2 mbar, the gas-utilization efficiency is about 8%. It increases up to 50%, by modulating the amplitude of the radio-frequency excitation signal and reducing both gas flows to 10 sccm, keeping the pressure constant. In this case, the deposition rate amounted to 0.55 nm/s; which is twice as large as compared to the deposition rate of a cw deposition. Device-quality opto-electronic properties are obtained under these conditions. The refractive index at 2 eV is about 4.25 and the microstructure parameter has a value around 0.02. The materials exhibited a low defect density (CPM) which is in the order of 3-8x1015 per cubic centimeter and photo-to-dark-conductivity ratio of 4-6x106. N-i-p solar cells of size 0.16 cm2 deposited on 10cmx10cm stainless steel (SS) substrate in the configuration SS/n-a-Si:H/i-a- Si:H/buffer/p-μc-Si/ITO/Ag grid (without back reflector) using amorphous silicon i-layer made by amplitude-modulated VHF plasma CVD showed an efficiency of 6.5%. This is a similar efficiency to the cell with standard device-quality cw a-Si:H in the same n-i-p structure, but at a high growth of 0.55 nm/s and gas utilization of ∼50%.