Directional Solidification Of Multicrystalline Silicon Research Articles

A modified cooling process in directional solidification of multicrystalline silicon

Directional solidification is the most popular technique for massive production of multicrystalline silicon in the solar industry. Conventional process aims to prevent the breakage of silicon ingots. To lower dislocation density in ingots, a significant modification to conventional cooling processes in productions of mc-Si ingots is proposed, and a three-dimensional numerical simulation with a simplified model of dislocation multiplication has been carried out to analyze its effect on dislocation multiplication. The results show that with the modified cooling process, the final dislocation density could be reduced to about one fifth of that formed in normal cooling process.

Investigation of grain boundary grooves at the solid–liquid interface during directional solidification of multi-crystalline silicon: in situ characterization by X-ray imaging

X-ray radiography imaging has been used to investigate the formation of the grain structure during directional solidification of multi-crystalline silicon (mc-Si). A unique experimental apparatus has been developed and allows following the evolution of the solid/liquid interface and describing its features accurately. In the present paper, grain boundary groove evolution mechanisms during solidification have been studied. Their shape and evolution revealed grain competition phenomena and were drastically modified by the occurrence of new grains or twins. Moreover, the grain boundary groove repeatedly deepens and shrunks during the experiments, with different behaviours depending on the groove type. We studied two groove types in detail: facetted on both sides, or rough on one side and facetted on the other. In the case of grain boundary grooves facetted on both sides, two mechanisms were observed. In the first situation, both facets that form the grain boundary groove grow at the same rate. The grain boundary followed the bisector of the angle of this groove during the progression of the interface during solidification. In the second situation, a small grain or a twin nucleated inside the groove. This new grain entered into competition with previously existing grains and modified the evolution of the grain boundary groove in a manner depending on the crystallographic orientation of the new grain. In the case of a rough/facetted grain boundary groove, the grain boundary followed the facet.

Non-isothermal model experiments and numerical simulations for directional solidification of multicrystalline silicon in a traveling magnetic field

A new experimental setup containing a GaInSn melt with a square horizontal cross section of 10×10cm2 and a variable melt height up to 10cm has been developed. The melt is positioned in the center of a coil system generating a traveling magnetic field (TMF). Using a cooling system at the bottom and a heating system at the top of the melt, a vertical temperature difference up to approximately 50K can be applied to the melt, imitating the thermal conditions during the directional solidification of multicrystalline silicon. Direct measurements of the time-dependent velocity and the temperature profiles were performed using ultrasonic Doppler velocimetry and thermocouples, respectively. Complementary three-dimensional (3D) numerical simulations of the model experiments were used to validate the numerical tools and to gain a deeper insight into the characteristics of TMF flows in square melts. The classical toroidal flow structure known from isothermal cylindrical melts is shown to obtain a large horizontal central vortex at a small height of the square melt, whereas a distinct 3D asymmetry appears at a large height. A vertical temperature gradient tends to suppress the vertical melt motion and leads to new complex horizontal flow structures.

Temperature Field Analysis of Directional Solidification of Multi-Crystalline Silicon

A cost effective directional solidification (DS) technique is one of the main methods to produce multi-crystalline silicon (mc-Si) ingots for solar cells. A detailed understanding of the DS process is very important to control the formation and distribution of impurities, precipitates, thermal stress and dislocation defects in an ingot. All these factors have direct effects on the solar cells efficiency. The quality of crystal grown by DS is largely determined by the temperature field. In order to optimize the technique parameters and obtain high quality silicon ingots, the temperature fields with different heat transfer coefficients at different positions have been calculated during the silicon DS process. The influence of the heat transfer coefficients at the ingot top(ht), the ingot bottom (hb), and between ingot and crucible (hs) on the DS process of mc-Si have been analyzed. The calculation results may provide important theoretical basis for optimizing technological recipe in the productive practice.

Dynamic simulation of impurity transport and chemical reactions in a Bridgman furnace for directional solidification of multi-crystalline silicon

Numerical results from a dynamic simulation of impurity transport and reactions in a Bridgman furnace for directional solidification of multi-crystalline silicon are presented and compared to experimental results. The simulation includes the calculation of the thermal field, melt and gas flow velocity field, transport and chemical reactions of oxygen and carbon impurities for the entire process based on heating, melting and solidification phases. Carbon and oxygen distribution in the ingot is analyzed experimentally by means of FT-IR spectroscopy and LECO combustion method, the CO development by means of an μ-GC gas analyzer.The simulated impurity distribution in the ingot and the CO development above the free melt surface are in good agreement with the experimental results. Furthermore the results indicate that the carbon solubility limit is already reached at the stage of melting and SiC precipitates are likely to form at the early stage of growth.

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.

Real Time Observation of the Directional Solidification of Multicrystalline Silicon: X-ray Imaging Characterization

Directional solidification of multicrystalline silicon for photovoltaic applications is investigated by means of synchrotron X-ray imaging techniques at the European Synchrotron Radiation Facility. Our experimental device combines two complementary modes: X-ray radiography imaging giving information on the dynamical evolution of the solid/liquid interface and X-ray topography giving information on the grain structure and phenomena occurring during the solidification process such as strains and twinning. In this paper, we report on the experimental details of the developed device and on some preliminary results obtained by using both imaging techniques.

Model experiments and numerical simulations for directional solidification of multicrystalline silicon in a traveling magnetic field

Low-temperature model experiments and 3D, time-dependent flow simulations with relevance to the melt motion during directional solidification of multicrystalline silicon under a traveling magnetic field are presented. The influence of the inductor current, the relative inductor–melt position, and the melt height on the flow pattern and velocity is studied in a square shaped GaInSn melt. Numerical simulations show a good agreement with measurements of the flow velocity by the ultrasonic Doppler velocimetry method. The toroidal flow structure already known from cylindrical melts is observed for a large parameter range. However, at small melt heights, the 3D melt geometry leads to a new flow pattern with a central horizontal vortex. The results obtained from the model experiments are transferred to silicon solidification processes using the proposed scaling laws.

About the formation and avoidance of C and N related precipitates during directional solidification of multi-crystalline silicon from contaminated feedstock

Multi-crystalline silicon ingots with a diameter of 6 cm and a height of 4–5 cm were directionally solidified in a laboratory scale crystal growth facility within a Si 3N 4-coated fused silica crucible with a growth rate of 1 cm/h for two different conditions of convective transport in the melt. The feedstock quality was varied from “pure” (electronic grade) to highly carbon-contaminated and highly nitrogen-contaminated. It will be demonstrated that under certain convective process conditions even for a highly contaminated feedstock (C and N) a mc silicon ingot can be grown which has an axially and radially homogenous carbon and nitrogen concentration in the range 3–4×10 17 C-atoms/cm 3 and 2–4×10 15 N-atoms/cm 3, respectively, and is free of SiC- and Si 3N 4-precipitates in the bulk.

Generation mechanism of dislocations during directional solidification of multicrystalline silicon using artificially designed seed

We investigated the generation mechanism of dislocations by comparing dislocation occurrence in multicrystalline silicon with calculated results of the shear stress on the slip plane by finite element analysis. To mimic the multicrystalline Si and to observe structural modification around grain boundaries a model crystal growth set-up was applied using artificially designed seed. We found that the dislocations occur at grain boundary and propagate as crystal growth proceeds. The generation of dislocations was not spatially uniform but often localized in one of the grains. The calculated stress distribution, which depends on crystallographic orientation, implies that the shear stress on the slip plane around the grain boundary is likely to cause occurrence of dislocations.

Directional Solidification of Multicrystalline Silicon Using the Accelerated Crucible Rotation Technique

Employing the accelerated crucible rotation technique, we grew multicrystalline silicon by the directional solidification process. The distribution of carbon concentration determined by Fourier transform infrared spectroscopy demonstrated that application of accelerated crucible rotation homogenized the carbon concentration in the grown ingot. Attempts were made to explain the effect of crucible rotation on homogenization of carbon concentration in terms of segregation phenomena. Moreover, growth striations induced by the crucible rotation were observed in the axial direction of the ingot.

Carbon concentration and particle precipitation during directional solidification of multicrystalline silicon for solar cells

The content and uniformity of carbon and silicon carbide (SiC) precipitates have an important impact on the efficiency of solar cells made of multicrystalline silicon. We established a dynamic model of SiC particle precipitation in molten silicon based on the Si–C phase diagram. Coupling with a transient global model of heat transfer, computations were carried out to clarify the characteristics of carbon segregation and particle formation in a directional solidification process for producing multicrystalline silicon for solar cells. The effects of impurity level in silicon feedstock and solidification process conditions on the distributions of substitutional carbon and SiC precipitates in solidified silicon ingots were investigated. It was shown that the content of SiC particles precipitated in solidified ingots increases markedly in magnitude as well as in space with increase in carbon concentration in silicon feedstock when it exceeds 1.26×1017 atoms/cm3. The distribution of SiC precipitates can be controlled by optimizing the process conditions. SiC precipitates are clustered at the center-upper region in an ingot solidified in a fast-cooling process but at the periphery-upper region for a slow-cooling process.

Silicon nitride coating and crucible—effects of using upgraded materials in the casting of multicrystalline silicon ingots

AbstractA method for removal of metallic contaminants from commercial Si3N4‐powders has been developed. The method is based on acid leaching and shows promising results. Analyses show that the content of both intermetallic compunds as Fe, Ti and Al as well as O are significantly reduced. Clean, synthetic fused silica crucibles have been used along with normal sintered ones based on natural quartz. The crucibles were coated with normal, commercial silicon nitride powder and with purified Si3N4 in a cleanest possible environment. The crucibles were then used as vessels for directional solidification of multicrystalline silicon in a pilot scale furnace. The average lifetime of minority charge carriers in the cast silicon was determined by quasi steady state photoconductance (QSSPC) from the bottom to the top of the ingots. These varied in a systematic way, so that the materials cast in the pure environment had significantly higher values than the materials cast with conventional coating‐ and crucible materials. The maximum values for the lifetimes in the individual ingots varied from 7 to 135 µs. Copyright © 2007 John Wiley & Sons, Ltd.