Deposition And Characteristics Of Hydrogenated Silicon Films By Hot-filament Chemical Vapor Deposition

The silicon films solar cells have been concerned in the field of PV at present. Compared with crystal silicon solar cells and polysilicon solar cells, the silicon-based thin film solar cells have less silicon materials consumption and much lower cost, especially, the application of low-cost substrates makes it more competitive in cost-control. At the same time, hydrogenated silicon film attracts extensively attention due to its application on many kinds of microelectronic devices, such as infrared imaging system, and thin film transistor. Hot-wire chemical vapor deposited (HWCVD), or catalytic-CVD, is a low temperature Si film preparation technology which is widely used to deposit a-Si:H and poly-Si films, due to many advantages compared with other methods, including being much simpler and easier to adopt for large-area deposition at a lower cost, low deposition temperature and high deposition rate. This thesis is focused on hydrogenated silicon film prepared by HWCVD. The main research work and the results as listed as follows:1. Hydrogenated nanocrystalline silicon films were deposited onto the glass substrates at different H2/SiH4 ratio by hot-filament chemical vapor deposition. The results demonstrated that the crystalline fraction increased with the increasing of the dilution ratio (H2/SiH4).In addition, we also studied the relationship between the crystallinity of films and the distance between filament and substrate. The crystallinity of films increased with the decrease of the distance between filament and substrate. We initially consider that such changes of crystalline structure may be ascribed to the increasing of the surface temperature of thin films caused by hot-filament radiant heating. The surface diffusion coefficient (length) on the hydrogen-covered surface is enhanced by elevating the substrate temperature. As a consequence, film precursors adsorbed on the surface can find energetically favorable (stable) sites, leading to an enhancement of the crystalline fraction.2. A probability distribution model of H radical was proposed. As mentioned above, the surface diffusion coefficient (length) on the hydrogen-covered surface is enhanced by elevating the substrate temperature. Thus, hydrogenated silicon thin films were prepared by HFCVD at filament temperature (Tfil) of 1600℃and 1650℃, respectively. A stair-like substrate holder was used to supply different distance between the filament and the substrate (Ds-f) at the same experimental condition. A structural transformation from amorphous phase to crystalline phase and the increase of the crystallinity of films was observed. The temperature field is simulated to obtain the surface temperature. The result indicates that the crystallization does not result from the increasing of the surface temperature. A probability distribution model of H radical was proposed to elucidate this phenomenon.3. Effects of substrate temperature on properties of nanocrystalline silicon films. nc-Si:H films were prepared by HFCVD at different substrate temperatures (Ts) from 140℃to 400℃. The effects of the substrate temperatures on structural, electrical and optical properties of nanocrystalline silicon films were discussed. The Change of crystalline volume fraction, microstructure factor and deposition rate have a turning point at 250℃. A structural transformation from columnar structure to coagulated structure is observed above 250℃. The optical band gap and dark conductivity do not have a turning point at 250℃. The dark conductivity increases with Ts while Eopt decreases with the increase of Ts.4. Effects of substrate on properties of nanocrystalline silicon films. hydrogenated amorphous and nanocrystalline silicon thin films were prepared by HFCVD on c-Si (100) and glass substrates. The volume fraction of silicon films on c-Si substrate is higher than that on the glass substrate. All the Si films exhibit a columnar structure. The columnar growth becomes more pronounced with the increase of Pw. At relatively low Pw (10 Pa,20 Pa), the growth rate of the Si films on c-Si substrate is lower than that on the glass substrate by～20 nm/min. However, there is no significant difference in growth rate at Pw = 40 Pa. The scratch test results show that the adhesion increases with Pw for samples prepared on c-Si substrates while there is no significant difference in adhesion among these on glass substrates.