| 其他摘要 | Microcrystalline silicon films were deposited using Ar diluted SiH4 gaseous mixture by plasma-enhanced chemical vapor deposition (PECVD) with different technological conditions and different substrates. With the aid of SEM, TEM, AFM, XRD, Raman and FTIR, the crystallinity, morphologies, and phase structures of the films were analyzed. In the present study, growth mechanism and properties of the μ-Si films were studied to provide improved understanding of growth process of μ-Si. It is expected that this work could provide some experimental and theoretical support for the preparation of high-quality Si with argon as discharge gas. At the same time, this work would also be significant to improve the preparation technology of polycrystalline silicon and to decrease the energy consumption.
By comparing with H2+SiH4, it is indicated that the film’s deposition rate and growth state are more superior with Ar+SiH4. Influence of argon flow rate, microwave power, and substrate temperature on the film’s deposition rate, crystallinity, morphology, and concentration of H were investigated. Growth mechanism and model were studied with the investigation results. The results indicated that the film crystallinity increased with the increase of argon flow rate and the optimized flow ratio of SiH4 to Ar was obtained as F (SiH4): F (Ar) = 10:70 for the highest deposition rate and least H concentration in the films. The optimized microwave power is 600W, with which the optimum of the deposition rate and film roughness is obtained. At the same time, in this case, the film crystallinity is close to 65% with less H concentration. When the substrate temperature increased, the deposition rate increased and the concentration of hydrogen decreased monotonously, but the crystallinity of the films exhibited sophisticated trends. The crystalline volume fraction increases firstly, and then decreases. A minimum value is obtained at 400℃, which is followed by an increase. It is proposed that the crystallinity of the films is determined by a competing balance of the self-diffusion activity of Si atoms and the deposition rate. The effect of the substrate on the initial nucleation rate was significant, so that the influence of the substrate on the deposition rate became less obvious with the increases of silane flow rate, microwave power, and substrate temperature. On the other hand, substrate would also influence the preferred orientation and roughness of the films remarkably. The vertical growth of the film is more rapid when the substrate is in its preferred orientation with a rougher surface, in contrast to a quicker transversal growth for the amorphous substrate. It is confirmed that the combination of diffusion and etching is indispensable to describe the deposition of μ-Si. The growth model was considered as follows: Thermal fluctuation atoms absorbed on the growth surface plays an important role in the beginning of the film growth. Nucleation would be initiated after the combination and merging of these atoms. The dangling bond could be saturated by hydrogen atoms existing in the plasmas. Then mobility and diffusion of the SiH2-radical on the surface of the films would remove these H atoms and produce the Si-Si bond. It is suggested that Ar+ etching on the film surface would participate in the growth process which would remove the weak Si-Si bond and the crystalline silicon film would be formed.
The effects of the film microstructure and micro-morphology on the optical and electrical properties were studied and the influence mechanism was discussed subsequently. The film crystallinity and H concentration was proved to play important roles for the film optical properties, but the electrical properties are mainly determined by the film crystallinity and grain size. With the decrease of the hydrogen concentration, the film optical properties would be improved. It is attributed to that the absorption coefficient of the films at visible region would be enhanced. Since the band gap of crystalline silicon is 1.1eV in contrast to a 1.7eV of amorphous silicon, the band gap would be cut down when the film structure changed from amorphous to microcrystalline phase domination, accompanying the decrease of absorption coefficient of the films at visible region. Consequently, favorable optical properties of the films could be obtained by aid of optimized hydrogen concentration and microstructure. As for μ-Si film, there are two different materials, the a-Si in the interface regions and the nc-Si composing the grains with an unequally distributed size. So it is advised that μ-Si film is a network of n-Si/a-Si:H hetero-junction structure. Heteroquantum dots model was applied to explain the distinct electrical quantities of μ-Si film. The higher barrier potential existed at the interface for the bigger grains due to less quantum effect, while the lower barrier potential for the smaller grains. On the other hand, the relationship between the crystallinity and the electrical conductivity is more complicated. The dark electrical conductivity would be improved when the film crystallinity is enhanced, but lower than 65%. In contrast to that the electrical conductivity would decrease when the film crystallinity is higher than 65%. So that, relatively small grain size and appropriate crystallinity would be benefical to improve the electrical properties of μ-Si films. |
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