Preparation, Properties And Growth Mechanism Of Polycrystalline Silicon Thin Films By Aluminum Induced Crystallization

Polycrystalline silicon (poly-Si) thin film is considered as one of highly efficient and low depleted photovoltaic materials. Metal induced crystallization (AIC) is a new technique of producing high quality silicon polycrystalline film at a relatively low temperature with large grains. But the poly-Si thin films by AIC often contain Si islands on the surface. It is believed that the Si islands have a negative effect on epitaxy and will limit further improvement of the absorbent layer quality. In order to prepare large-grained high quality poly-Si with amooth surface, the material preparing technique and growth mechanism of AIC poly-Si is investigated in this thesis and obtaine the following main achievements.1. Double layered poly-Si thin film with large grain was prepared from Al/Al₂O₃/a-Si stack. The poly-Si thin film is with strong (111) preferential and of very high crystalline quality. Initial thickness of Si and Al layers, alumina film at the Si/Al interface and annealing condition are the important factors that affect the AIC process.2. Single dendritic large grain (about 60?m) was obtained by removing the upper layer of the poly-Si double film with discontinuous lower layer. The fractal dimension characterized by sand box method of the dendritic crystalline grains is 1.86. The dendritic grain has high crystal quality close to single c-Si wafer. High crystalline quality poly-Si thin films were obtained by peeling off the upper layer of the double poly-Si films with continuous lower layer.3. Mechanism models are built to discuss the effect of alumina at the Si/Al interface during the AIC process and the formation of double layers. When there is no alumina at the Si/Al interface,Si diffuses into Al layer with rapid speed and more Si nukes form in Al layer and the grain size formed at the end is small. When there is alumina at the Si/Al interface, Si diffuses into Al layer with slow speed and less Si nukes form in Al layer and the grain size formed at the end is large. And because of the blocking of alumina at the Si/Al interface all the process, when Al atom diffuses into a-Si layer, it induce the a-Si to crystallize laterally, then double poly-Si layers form.The formation of the continuous upper layer is related to the slow crystallization rate during AIC process. The direct effect of the slow rate is that the Al diffuses into the upper a-Si layer slowly giving enough time for the upper a-Si to crystallize continuously by the Al lateral induction. Because the lower layer is covered by continuous poly-Si film, it has few surface defects. After the upper layer is removed, high quality poly-Si thin film is obtained. On the other hand, when the initial thickness ratio of Si/Al thickness is large, the lower poly-Si thin film is continuous and after the upper layer is removed, the continuous high quality poly-Si thin film is obtained. While when the initial thickness ratio of Si/Al thickness is small, the lower poly-Si thin film is discontinuous and after the upper layer is removed, the single large high quality poly-Si grain is obtained.4. Large-grained poly-Si thin film with smooth surface was prepared at 400?-500?from a-Si/SiO₂/Al. The poly-Si thin film is with strong (111) preferential and of very high crystalline quality. The annealing temperature lower than 500?make it possible to use common galss as substrate. The poly-Si thin film has double layers too and the upper layer is of higher quality. In the experiment, wheatear morphology not ever reported for the poly-Si thin film prepared by AIC was found except for the common dendritic morphology. Initial thickness ratio of Si and Al layers, silica film at the Si/Al interface and annealing temperature are inportant factors that affect the AIC process. When initial thickness ratio of Si and Al layers increase from 5:4 to 5:1, the crystallization of amorphous silicon becomes more and more easy and the poly-Si crystalline quality becomes higher and higher. While when the ratio continues to increase from 5:1 to 25:3 the poly-Si crystalline quality becomes worse and worse and the ration of 5:1 is optical. The silica film at the Si/Al interface make the diffuse speed of Si into Al layer slower, but it is helpful to form union large poly-Si grains. The optical natural oxidation time of a-Si layer is 47 h. The research on effect of annealing temperature showed that the lower is annealing temperature, the larger is the grain size formed but the slower is the crystallization rate. High quality poly-Si was grown by HWCVD on high quality poly-Si thin film with smooth surface from substrate/a-Si/SiO₂/Al stack.5. Poly-Si thin film with grain size larger than 100?m has been prepared by AIC with a shorter process time at lower temperature. The condition whether there is new nucleation when the annealing temperature increases is first discussed based on the relationship among grain radium, depletion region thickness and distance between adjacent grains. It is also the first time to propose a method to shorter the diffusion and nucleation stage time. The situation whether it forms smaller grains when the annealing temperature decreases is discussed. It depends on whether the concentration of Si in Al formed at higher temperature is higher than the critical nucleation concentration of lower temperature. If it is higher, smaller grains form and if not, large grains form.