Crystallization Of Hydrogenated Amorphous Silicon Thin Film Processing

Hydrogenated amorphous silicon (a-Si:H) thin film has attracted considerable attention and been a subject of extensive studies worldwide on account of its important applications such as thin film solar cells, thin film transistors, radiation detectors, and liquid crystal displays based on its good electrical and optical properties. However, light-induced degradation, also named Staeble-Wronski effect, has been a major bottleneck since it was observed in practical device applications. Due to this technical difficulty, several approaches aiming at eliminating it have been proposed. It has been reported that partially nanocrystallization of the thin films improves stability and properties of a-Si:H thin films.In this dissertation, the argon dilution to silane in PECVD and argon plasma anneal methods are used to crystallize a-Si:H thin films. The influence of argon dilution ratio, RF power density and argon plasma anneal time on the structure and properties of partially nc-Si:H thin films are studied with the help of many characterization methods. The main results in the dissertation are shown as follows:(1) With the increase of argon dilution ratio, the structure of thin films changes intensely. The crystallization happens when dilution ratio reaches to a certain point. The crystalline volume fraction and size of crystal grains increase with the increase of dilution ratio. The quality of thin film surface is also improved when the dilution ratio is increased, but the intensity of the optical absorption coefficient gradually declines. The optical gap decreases from 3.2eV to 2.1eV when dilution increases from 3 to 24. The photo taken by TEM proves the existence of crystal grains and the partially crystalline or crystalloid status of the film.(2) The influence of RF power density on the crystallization of a-Si:H thin films are more complicated. The influence of RF power on crystal grain size, the Si-H bonding pattern and crystalline fraction is obvious and nonlinear.(3) The research and analysis of Ar+ and Ar*, two high-energy particles in argon plasma, explain the process of dilution argon dilution ratio and RF power density on the different performance in crystallization. Relative to Ar+, Ar* is more moderate and favorable on grain growth and film quality. Increasing argon dilution ratio will effectively increase the content of Ar*. When the RF power density is increased to a certain degree, the Ar+ content in argon plasma will proliferate.(4) With the increase of argon plasma anneal time, the increase of crystalline volum fraction and the size of crystal grains are observed and studied through Raman spectra and TEM. But the influence of anneal time on the electrical properties of thin films is negative. With the increase of anneal time, the dark conductivity of films decreases while the active energy increases.