| As low-temperature preparation techniques of silicon films, hot wire chemical vapor deposition (HWCVD) and plasma enhance chemical vapor deposition (PECVD) get more and more attention. Compared to PECVD, HWCVD technique has the advantages of high deposition rate, no ion bombardment damage during the deposition process, high utilization degree of gas and weak S-W effect. HWCVD technology has been achieved amorphous, microcrystalline (especially nanocrystalline) and polysilicon-based thin-film solar cell, as well as single-junction, tandem, crystal-amorphous heterojunction thin film solar cellsTherefore, microcrystalline silicon (especially the "edge states"-nanocrystalline silicon) film, as well as large-grain polycrystalline silicon thin film are considered a strong contender for next-generation device manufacturing. In HWCVD system, people used to change the hydrogen dilution ratio to achieve the preparation of nanocrystalline silicon thin film. Several methods have been developed to obtain large-grain polycrystalline silicon thin film, such us solid-phase crystallization, laser crystallization and so on. Deposition parameter directly impact the decomposited radical by hot wire, the following chemical vapor reactions, and the radical growth, so the film crystallinity and property. We focus on the issue that how the deposition parameters influence film crystallinity and get the following conclusions:1. When the substrate temperature Ts is less than250℃, the crystalline fraction of samples increases with Ts increasing. R*values increase too, which indicate the H atoms have effectively passivated grain boundaries of films. Effective H passivation, SiH3diffusion and the enhanced vibration of lattice atoms are main causes of improved crystallinity. When Ts is higher than250℃, the crystalline fraction of films decreases when Ts increases. Through SEM testing, we found the columnar grains of films at low Ts have been changed into a gel state. Through combination of the reducing R*value, the fracture surfaces of films and relevant literatures, we believe that the enhanced H atom desorption leads to this result.2. Under the deposition condition of P=23Pa and Tf=1600℃or1650℃, the crystalline fraction of samples decreases with larger ds.f (distance from hot wire to substrate) and the crystalline fraction of Tf=1650℃samples is higher than Tf=1600℃’s. Through thermal radiation law and the Matlab software simulation analysis, we found the distribution of temperature caused by the radiation of hot wire just like an elliptical sphere. Compared to substrate temperature, the transformation of radicals on the surface of film has a greater impact on the film crystallinity during deposition process.3. In the situation of middle deposition pressure, crystallinity of samples at all deposition parameters shows a continuous increasing phenomence when P increases. We conclude that SiH3and H radicals play important roles in the crystallization processes of films. The two radicals’ concentration increaseing with pressure is the root cause of the enhanced film crystallinity. By analyzing films prepared at different H2dilution ratio, we know that the higher the concentration of H atoms is, the larger the film crystalline is. So we propose the H atom model. The probability distribution of H atoms in the space likes an elliptical sphere. H atom density on each surface ellipsoid is equal, and becomes smaller when it’s close to the edge of the elliptical sphere. The experimental phenomenon of the samples prepared at different ds-f has been well explained by this model. We believe that transformation of H atom elliptical sphere at different filament temperature is the major cause of the film crystallinity changing in a large scale. |
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