Solidification Process Of Multicrystalline Silicon Research Articles

Dislocation interaction with vicinally faceted groove at grain boundary in multi-crystalline silicon

The morphology of the solid–liquid interface and the GB development during directional solidification process of multi-crystalline silicon (mc-Si) has been studied with an in situ observation system. A small angle grain boundary was observed to have a gradually increasing faceted groove, finally reaching a steady state. A geometric model is used to simulate the growth of groove facets with consideration of different facet growth kinetics. It appears that the kinetics of a face vicinal to a {111} plane is most likely to reproduce the observed behavior. Later on, variations of groove size and grain boundary direction occurred. The geometric simulation results show that such perturbations could be produced by the interaction of single dislocations with one of the vicinal facets.

Analysis of the Effect of a Vertical Magnetic Field on Melt Convection and Oxygen Transport During Directional Solidification of Multi-Crystalline Silicon by Numerical Simulation

Melt convection during the directional solidification process of multi-crystalline silicon plays a critical role in the transport of impurities. The utilization of a static magnetic field is an effective way to control the melt convection pattern. Studying the effect of the Lorentz force induced by the vertical magnetic field (VMF) on the melt convection of silicon in detail is beneficial to optimize the magnetic field parameters in the production process. Based on the numerical simulation method of multi-physics coupling, this paper explores the effects of different VMF intensities on the convection of silicon melt and the transport of oxygen in the melt during the directional solidification of polycrystalline silicon. The results show that in the first 125 minutes of the crystallization stage, the melt convection velocity is affected significantly by the magnetic field intensities. When different convection circulations are present in the silicon melt, the upper circulation easily transports oxygen to the furnace atmosphere, and the subjacent circulation easily lead to the retention and accumulation of oxygen. Enhancing the VMF intensity to a certain extent can reduce the size of the oxygen retention region in the silicon melt, and the time of the first disappearance of the subjacent circulation near the sidewall of the crucible is shortened. Then the average oxygen concentration in the silicon melt can be reduced. However, a larger vertical magnetic field intensity can result in greater average oxygen concentration in the oxygen retention region.

Molecular dynamics simulation of the solidification process of multicrystalline silicon from homogeneous nucleation to grain coarsening

Based on the Stillinger–Weber potential, molecular dynamics simulations of the solidification processes of multicrystalline silicon were carried out.

The effect of radiative heat transfer characteristics on vacuum directional solidification process of multicrystalline silicon in the vertical Bridgman system

In the vertical Bridgman system for multicrystalline silicon vacuum directional solidification process, the radiation heat transfer characteristics have a significant impact on the quality of crystal growth. In this paper, the effect of heat transfer characteristics under different pulling-down rates on the vacuum directional solidification process was investigated. Simplified radiation view factor dependent on pulling-down rate in pilot-scale vertical Bridgman system was derived. Combined with the transient simulation results, we obtained the heat transfer characteristics inside the furnace, and proposed the optimization scheme by using a rate of slowly first and then faster to pull down the crucible, which can not only satisfy thermal conditions of crystal growth, but also shorten the solidification time and effectively reduce the energy consumption. In addition, according to the characteristics of the radiative heat transfer under vacuum condition and the change of view factor, it was found that an improved design by widening the cold zone of the furnace can narrow the temperature gradient in the silicon ingot, and greatly decrease the thermal stresses, which in turn can aid in the design of future industrial equipment and process improvements.

Influence of furnace design on the thermal stress during directional solidification of multicrystalline silicon

Directional solidification is one of the most popular techniques for massive production of multicrystalline silicon (mc-Si). Dislocation is one of the major defects that significantly affect the photovoltaic performance. For the analysis and optimization of stress-induced dislocation, a computational tool has been developed to investigate thermal stress distribution during directional solidification process of multicrystalline silicon. Temperature distribution in the furnace, S/L interface shape and melt flow are simulated. Parametric studies are further conducted to evaluate the effect of furnace design on the interface shape and on the maximum von Mises stress in the growing ingot. To consider the effects of the crucible geometry qualitatively, three-dimensional modeling of the thermal stress is performed with or without the constraint of the crucible. The regions of dislocation multiplication are evaluated by comparing von Mises stress to critical resolved shear stress (CRSS). The results imply that the dislocation in the growing ingot can be reduced by optimizing the design of the directional solidification furnace.

Three-dimensional global analysis of thermal stress and dislocations in a silicon ingot during a unidirectional solidification process with a square crucible

A three-dimensional global model was used to obtain the solution of a thermal field within the entire furnace during a unidirectional solidification process of multicrystalline silicon with a square crucible. Then the thermal stress distribution in the silicon ingot was solved. Based on the solution of thermal stress, relaxation of stress and multiplication of dislocations were performed by using the Haasen–Alexander–Sumino model (HAS model). The influence of crucible constraint on stress levels and dislocations was investigated. It was found that the crucible constraint had significant influence on the thermal stresses and dislocations in the ingot. The results indicated that it is important to reduce the crucible constraint in order to relax thermal stresses and reduce dislocations in a silicon ingot during the solidification process.

Numerical analysis of the formation of Si3N4 and Si2N2O during a directional solidification process in multicrystalline silicon for solar cells

We studied the mechanism of formation of Si3N4 and Si2N2O during the solidification process of multicrystalline silicon by numerical analysis with a phase diagram of the Si(l)–N–O system. Concentrations of oxygen and nitrogen were almost the same as reported values of measurements. Si3N4 was formed on the top of the silicon ingot. Si3N4 was also formed at the middle stage of the solidification process at the center of the ingot. It was clarified from the results that Si2N2O was first formed near the melt–crystal interface, since oxygen concentration in the melt decreases and nitrogen concentration in the melt increases with solidification of the molten silicon. Si3N4 was formed after Si2N2O had been formed.

Effect of crucible rotation on oxygen concentration during unidirectional solidification process of multicrystalline silicon for solar cells

We studied the effects of crucible rotation on distribution of oxygen concentration in a crystal during the unidirectionally solidification process of multicrystalline silicon for solar cells. Oxygen concentration in the melt increased when crucible rotation rate was increased. Oxygen concentration in the silicon crystal was distributed inhomogeneously in the radial direction when crucible rotation rate was increased. This is due to suppression of oxygen transport. Consequently, less oxygen was transported from the crucible wall to the center of the melt. We found that oxygen concentration is small in the whole ingot and homogenized in the radial direction when crucible rotation rate during the solidification process is set to 1 rpm.