Effective control of oxygen impurity in multicrystalline silicon is required for the production of a high-quality crystal. The basic principle and some techniques for reducing oxygen impurity in multicrystalline silicon during the unidirectional solidification process are described in this paper. The oxygen impurity in multicrystalline silicon mainly originates from the silica crucible. To effectively reduce the oxygen impurity, it is essential to reduce the oxygen generation and enhance oxygen evaporation. For reduction of oxygen generation, it is necessary to prevent or weaken any chemical reaction with the crucible, and for the enhancement of oxygen evaporation, it is necessary to control convection direction of the melt and strengthen gas flow above the melt. Global numerical simulation, which includes heat transfer in global furnace, argon gas convection inside furnace, and impurity transport in both melt and gas regions, has been implemented to validate the above methods.
Multicrystalline silicon has now become the main material in the photovoltaic market because of its low production cost and because of the high conversion efficiency of solar cells made from this material. The unidirectional solidification method is a cost-effective technique for large-scale production of multicrystalline silicon material. Similar to the Czochralski method for crystal growth, the unidirectional solidification method is also related to transport of impurities [
Effective control of oxygen concentrations in a crystal is required for the production of a high-quality crystal. Some papers about carbon and oxygen impurities [
Incorporation of oxygen impurity into multicrystalline silicon occurs during the global unidirectional solidification process. To effectively illuminate the mechanism of oxygen incorporation, the global solidification process is divided into several substeps: melting process, solidification process, and cooling process. During these processes, there are some differences in the main mechanism of oxygen incorporation into silicon.
During the melting process, there are two reactions occurring. The first is rapid reaction of the melt silicon with the silica crucible:
The silicon and oxygen atoms are transported onto the melt surface and react with each other to produce SiO gas:
The resultant SiO gas evaporates from the melt surface. Reaction (
During the melting process, there is another chemical reaction that can contribute to oxygen incorporation. The graphite susceptor of the crucible can react with the crucible [
Experimental results [
During the solidification process, besides reactions (
During the cooling process, the increase of oxygen impurity in solid phase is mainly due to diffusion from the crucible wall into the crystal. This contribution is very small due to small diffusion coefficient of oxygen in crystal, which is close to
The above analysis of oxygen incorporation indicates that oxygen impurity in multicrystalline silicon mainly originates from the silica crucible. To effectively reduce oxygen impurity, it is essential to prevent or reduce any reaction with the crucible, such as reaction between the crucible and silicon and reaction between the crucible and its graphite susceptor. Meanwhile, it is also important to enhance the evaporation of SiO gas by controlling the melt and gas convection. Therefore, the basic principle for oxygen reduction is to reduce oxygen generation and to increase oxygen evaporation.
Global numerical modeling, which includes heat transfer in global furnace, argon gas convection inside furnace, and impurity transport in both melt and gas regions, has been implemented to validate the above methods.
Our simulation implementation involves three steps: first, the temperature distribution of furnace components, due to heat transfer and heat radiation, is computed without gas flow; second, the flow field and temperature field of the cooling argon gas are computed using the temperature boundary conditions from the first step; and third, carbon and oxygen impurities in the gas and melt are computed using the flow field and temperature field from the second step.
The modeling of heat transfer in the furnace involves convective heat transfer of the melt in the crucible, conductive heat transfer in all solid components, and radiation heat transfer in all enclosures of the furnace. The melt flow in the crucible is assumed to be an incompressible laminar flow. The radiative heat exchange in all radiative enclosures is modeled on the basis of the assumption of diffuse-gray surface radiation.
The flow of argon gas through the furnace is considered to be a compressible and axisymmetric flow. The compressible flow solver can accurately simulate the buoyancy-driven flow due to the large density variation in the furnace. Although the flow velocity in this furnace is low, yet the density variation is significant for this buoyancy-driven flow, which is similar to the combustion problem.
The concentration of oxygen impurity is calculated by coupling the calculation of carbon impurity inside the global furnace. The SiO and CO concentrations inside gas and the C and O atom concentrations inside melt are calculated by a set of fully coupled program [
Since an important source of oxygen impurity in the crystal is the reaction between the crucible and silicon, an effective method to weaken that reaction is to use Si3N4 liner along the inner wall of the crucible. This liner can prevent direct contact between the crucible and silicon and thus reduce the intensity of chemical reaction between them. One simulation has been implemented to test the effect of the liner on oxygen impurity in a crystal [
Oxygen concentration distributions inside a silicon crystal for different crucible conditions: (a) without Si3N4 liner, (b) with Si3N4 liner.
Oxygen impurity measures at different solidified fraction with Si3N4 liner have been implemented in our group [
Another important source of oxygen impurity is the reaction between the crucible and its graphite susceptor. Numerical simulations have been implemented to test the effect of that reaction on oxygen impurity [
SiO gas distribution inside the gas space after considering the reaction between the silica crucible and the susceptor.
Figure
Oxygen concentrations in silicon melt with different carbon activities at the SiO2 surface.
The increase of oxygen impurity in melt is due to the rapid generation of SiO and CO in gas phase:
Therefore, an effective method for reducing oxygen impurities in the crystal is to prevent reaction between the silica crucible and the graphite susceptor by setting a free space between them or by depositing a layer of SiC film on the surface of the susceptor.
Since reaction (
Inside the melt, the oxygen concentration at the melt surface is always minimal due to evaporation. Thus, the flux along the surface is determined by the concentration just beneath the surface, denoted by
Oxygen concentration distributions inside silicon melt for different convection directions: (a) first from the crucible wall to the bottom and then to the surface, (b) first from the crucible wall to the surface and then to the bottom.
Inside the gas space, the oxygen concentration at the gas/melt interface is always maximal. Thus, the oxygen flux along the interface is determined by the concentration just above the interface, denoted by
Oxygen concentration distributions inside silicon melt: (a) without a crucible cover, (b) with a crucible cover.
To indicate the validation of our calculation, some measurements and numerical simulations have been done for the effect of strengthening the gas flow on impurities [
The oxygen impurity inside multicrystalline silicon mainly originates from the silica crucible. To effectively reduce the oxygen impurity, it is essential to reduce the oxygen generation and enhance oxygen evaporation. For the reduction of oxygen generation, it is necessary to weaken the chemical reaction between the crucible and silicon by using a layer of Si3N4 liner and to prevent the reaction between the crucible and the graphite susceptor by setting a free space between them or by depositing a layer of SiC film on the surface of the susceptor. For the enhancement of oxygen evaporation, it is necessary to increase the oxygen concentration value beneath the melt surface as much as possible by adjusting the convection direction and to reduce the oxygen concentration just above the melt-gas interface as much as possible by strengthening gas flow. The feasibility of the above methods has been validated by the global numerical simulation.
This work was partly supported by the New Energy and Industrial Technology Development Organization (NEDO) under the Ministry of Economy, Trade and Industry (METI).