Silane FBR-CVD Method For Granular Polysilicon Production: Studies On Silicon Particle Growth Numerically And Cluster Properties Modification Experimentally

Developing the solar photovoltaic industry is an effective way to solve the resources and environmental problems in China as well as the whole world. The production of polysilicon, as one of the most abundant resources on the earth with high efficiency, represents the advanced level of solar energy industry. The mainstream technology is on the basis of Siemens method, which carries out chemical vapor deposition(CVD) of silicon source gas in a bell-jar reactor(BJR). However, the disadvantages of BJR such as batch production, small scale, high energy consumption and cost have appeared to be a major limitation for the development of solar energy industry. As an alternative, fluidized bed reactor(FBR) can overcome the shortcomings mentioned above, and the technique of silane pyrolysis in a FBR is considered to be one of the most promising manufacturing processes for polysilicon production in the solar photovoltaic industry.Due to the complexities of multiphase flow and reaction mechanism in the system, the main projective of present research is to provide a comprehensive insight into the particle growth in a three-dimensional CVD-FBR by using computational fluid dynamics(CFD) coupled with population balance model(PBM). The multiple features of gas-solid flow hydrodynamics, gaseous reactions, particle growth kinetics and their couplings are considered through Eulerian-Granular model, species transport model and QMOM-based PBM. The influences of growth mechanism, aggregation effect,operating conditions and silane pyrolysis kinetics on silicon growth are especially studied. The simulated results of flow parameters and growth rates are verified by the empirical values and experimental data. This work also investigates the possibility of sintering treatment on modifying the properties of silicon clusters prepared by silane homogenous pyrolysis in a quartz tube reactor, with particular focus on sintering condition of temperature, duration and pressure. The achievements of preset work help to understand the occurrings of flow behavior, silane decomposition and silicon growth in the FBR, and provide valuable reference for industrial production.The main contents and results are summarized as follows.First of all, the fundamental gas-solid flow behavior in the lab-scale FBR is simulated through Eulerian-Granular model. The effect of fluidization ratio on flow regime is investigated, and the simulated bed expansion height and heat transfer coefficient are compared with the empirical values and existing theories. The results show that the present Eulerian-Granular model is feasible to describe the variation of flow regime, and a uniform bed temperature is obtained as the heating wall applied.Secondly, the influece of Eulerian-Granular model parameters on the flow behavior of large-scale FBR is studied and a suitable simulation condition is obtained by comparing the minimum fluidization velocity(Umf) and bed pressure drop with the empirical values. The results suggest that the EMMS-based drag model considering meso-scale flow structure can provide a more accurate prediction on solid distribution than the widely-used Gidaspow drag model, and the influence of collision parameters can be ignored due to the large reactor scale.Thirdly, a narrow particle size distribution(PSD) is introduced in the above larege scale FBR through PBM. It is found that the solid flow pattern of the polydisperse system is similar to the uniform one, but still with discernable separation phenomenon due to particle size difference. The small particles cause dilute top region and dense wall region. When aggregation effect is considered, the small particles are gathered towards the middle of the reactor, and the bed separation phenomenon becomes obvious.Fourthly, the CFD-PBM coupled model is established to describe the particle growth during FBR-CVD process on the basis of Tejero-Ezpeleta’s experiment. The influences of aggregation, SiH4/SiH2 surface deposition and cluster scavenging are discussed. The obtained results show that SiH4 surface deposition and cluster scavenging are the two main contributions for silicon growth under atmospheric pressure and 923 K with silane mole fraction of 0.1. The contribution of SiH2 surface deposition can be neglected due to the limited quantity, which is calculated on the basis of Ho’s silane pyrolysis mechanism in CHEMKIN format. It is also found that Si2H6 and Si3H8 are the main silicon hydrides in the gas phase.Fifthly, the influence of silane homogeneous and heterogeneous pyrolysis kinetic models on particle growth is also focused on the basis of Hsu’s experiments through the established coupled model, in which cluster scavenging is related to the concentration of Si-cluster in the gas phase. The results show that the obtained growth rates agree well with the experimental data when the combined kinetics of Hogness homogenous and Furusawa heterogeneous models applied, but the prediction is overestimated with high silane concentration. The mass transfer of silane surface deposition and cluster scavenging are also investigated, which indicates that silane surface deposition occurs in the dense bottom region with high mass transfer rate, and cluster scavenging almost takes place in the whole solid region but with relative low rate.Finally, the effect of sintering treatment on crystallinity, hydrogen bond structure, morphology and PSD of clusters prepared by silane homogeneous pyrolysis is investigated. The XRD, SEM, DR-FT-IR and ZPA analyzer are used to characterize the properties. The results indicate that high sintering temperature and vacuum condition are advantage to the aggregation of polycrystalline silicon grains and the release of hydrogen in the clusters. As the sintering temperature close to melting point, the powders fuse into big mass, but still with high crystallinity.