Plasma enhanced chemical vapor deposition of silicon thin films: characterization of film growth at different frequencies and gas compositions utilizing plasma diagnostics

Zhu, Lala
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University of Delaware
Hydrogenated amorphous Si (a-Si:H) and nano-crystalline silicon (nc-Si:H) thin films with unique properties have provoked wide research interest and technology applications for thin film silicon solar cells, and active layer in thin film transistors for liquid crystal display. The technologies investigated for both a-Si:H and nc-Si:H thin film preparation have included Sputtering , Hot Wire Chemical Vapor Deposition (HWCVD), Photochemical-CVD and Plasma Enhanced CVD (PECVD). Of these, PECVD is the most recognized and utilized technology for high quality, low temperature and large area thin film deposition. The effect of PECVD silicon thin film growth condition on the film properties, device performance and the plasma characterization need to be deeply understood. This dissertation analyzes the growth rate and material properties of thin film silicon at different plasma excitation frequencies and gas compositions by PECVD based on in-situ Plasma Diagnostics by Optical Emission Spectroscopy (OES) and Langmuir Probe. A relatively unique aspect of this research is evaluating the effect of adding a small amount of higher order silane gas to catalyze decomposition of the dominant silane species to enhance the growth rate. It has been found that the addition of 1.7% Si2 H6 flow into SiH4 /H2 mixture increased the a-Si:H growth rate by 60%. The optimization of a-Si:H deposition utilizing the SiH 4 / Si2 H6 /H2 mixture resulted in films grown at high rate and power with low microstructure factor which correlates with better stability of a-Si:H. The process window for transition from a-Si:H to nc-Si:H deposition was increased at higher H2 /SiH4 ratio and large grain size was achieved at either high pressure for RF 13.56 MHz or low pressure for Very High Frequency (VHF) 40.56MHz discharge. Si films grown at high H2 /SiH4 ratio or RF power, corresponding to a higher H alpha/SiH* intensity ratio, have lower microstructure factor (Rmf ) from Fourier Transform Infrared Spectroscopy for a-Si:H and higher fraction crystallinity (Xc ) from Raman spectroscopy for nc-Si:H films, respectively where the growth rate is proportional to the electron density of the plasma analyzed by Langmuir Probe. The plasma Electron Energy Distribution calculated through Druyvesteyn Method indicates an increase of high energy electron percentage at higher RF power, resulting in larger H 2 dissociation level for higher H alpha/SiH* detected by OES. The high energy electron density and plasma potential is decreased at higher process pressure, resulting in less ion-surface bombardment and larger grain size formation of nc-Si:H film. Comparing RF with VHF plasma, the VHF plasma had one order of magnitude higher electron density leading to better H 2 /SiH 4 utilization. An a-Si:H solar cell with initial efficiency of 7.4% was fabricated with intrinsic layer deposited from SiH4 /Si2 H 6 /H2 discharge at a high growth rate of >5Å/s and the stabilized performance after light soaking was similar with the baseline cells grown at 1.8Å/s. nc-Si:H solar cell with efficiency of 4.5% was obtained with a growth rate of 4Å/s by SiH4 /Si2 H6 /H2 discharge.