Multi-crystalline Silicon Growth Research Articles

Enhancement of the quality of mc-Si ingot grown by vacuum directional solidification furnace with growth rate increase reducing the cost of the wafer for PV application: Carbon and oxygen analysis

In this paper, we propose a numerical model to investigate the dissolution of oxygen atoms from the porous silica crucible, SiO gas formation, back diffusion of CO gas and segregation of carbon and oxygen during the growth of multi-crystalline silicon (mc-Si) by vacuum directional solidification (VDS) at different growth rates (3, 6 and 9 mm/h) by silt valve opening rate. The dissolution of oxygen decreases during the rapid growth of ingot because the reaction between the molten silicon and the porous silica crucible is constrained. This limitation on oxygen dissolution from the porous silica crucible further diminishes chemical reaction inside the VDS furnace. Consequently, the segregation pattern of the carbon and oxygen is affected at higher growth rate. During the VDS mc-Si growth, a growth rate of 9mm/h yields better quality of the VDS grown mc-Si ingots compared to other growth rates, this rate reduces SiC precipitation and SiO2 cluster formation due to lower concentration of carbon and oxygen. The obtained carbon concentration minimizes the wire saw defects. Based on these numerical results (9 mm/h), we have implemented experimental work. Prepared samples are analyzed using FTIR spectra and minority carrier lifetime measurements. The effect of oxygen concentration in the samples is analyzed in the minority carrier lifetime measurements. The oxygen and carbon concentrations are calculated by FTIR spectra and their results are compared with the numerical simulation.

Growth of multicrystalline silicon assisted by electrophoretic deposited quartz

Cast multicrystalline silicon (mc-Si) requires high quality at low cost, while fully-melted seed-assisted growth has a reasonable utilization rate but urges better initial grain control. An electrophoretic deposited quartz nucleation layer is introduced to obtain a uniform and oriented nucleation layer. Carrier recombination characterization, including photoluminescence and carrier lifetime mapping, shows that the density of recombination-active structural defects is reduced due to better grain distribution. The average conversion efficiency values of silicon particle, sprayed quartz, and deposited quartz assisted wafers are 20.52%, 20.48%, and 20.68%, respectively Moreover, deposited quartz assisted mc-Si can obtain oriented grains, which offers a potential to apply alkaline texturing on mc-Si wafers.

Influence of growth rate on minority carrier lifetime of multicrystalline silicon ingot

Multicrystalline silicon was grown by directional solidification method, and different growth rates was obtained by optimizing the growth process. The influence of the growth rate on the minority carrier lifetime of the multicrystalline silicon ingot was studied. The results show that the infrared image of the middle and lower part of the silicon block with an average growth rate of 1.0cm/h has a lighter shade. The area with minority carrier lifetime of the silicon block is less than 4 us accounts for about 50%, and the average minority carrier lifetime is 5.28 us. Reduce the growth rate to 0.94cm/h, the infrared image of the crystal is pure and the crystal grains are coarse, the average minority carrier lifetime of the silicon block is increase to 5.65 us. When the growth rate is further reduced to 0.87cm/h, the crystal growth deviates from the vertical direction. The area of silicon block less than 4 us accounts for about 40%, and the average minority carrier lifetime is reduced to 5.39 us. The phase field method is used to calculate the growth of multicrystalline silicon. The results show that the greater the flow rate, the faster the growth of crystal nucleus and the easier the formation of competitive growth, which is in good agreement with the experiment.

The effect of sintering temperature of Si3N4 photovoltaic solar crucible coating on the adhesion strength

Multicrystalline silicon is widely used in solar cell production. Silicon nitride is commonly applied as a coating material for the silica crucibles utilized for the growth of multicrystalline silicon by directional solidification. In this work, we will study the effect of the sintering temperature of the Si3N4 coating applied to the photovoltaic solar crucible on the adhesion strength of the Si3N4 coating in order to optimize its final sintering temperature. In this case, the sintering treatment temperature was from 750 to 1200 °C; the effect of the temperature on the morphology of the coating was studied too. For this study, a PosiTest A type automatic peel adhesion tester was used for measuring the adhesion strength and “JEOL JSM-7610F plus” SEM imaging for studying the coating microstructure.

3D cellular automaton modelling of silicon crystallization including grains in twin relationship

Production of silicon for solar cells in photovoltaic systems is mainly based on directional casting processes. Twin nucleation is favoured during silicon growth due to the low-level twin energy of formation. As a consequence, in all solidification process, a large amount of grain boundaries (GB) are in twin relationship. A 3D cellular automaton (CA) model has been recently developed for the growth of multi-crystalline silicon including facet formation and nucleation of new grains in twin relationship. Activation of facets is based on an undercooling parameter assigned to each grain and for each of the <111> crystal directions. The model also considers nucleation and growth of grains on <111> facets corresponding to Σ3 twin relationships between twin grains. This model is first applied to comparison with experimental observations. It is found that impingement of growing grains that nucleated in Σ3 twin relationships meet during growth and form Σ3, Σ9 and Σ27 GB, in good agreement with experimental observations. Finally, the model is applied at a larger scale to generate grain structures representative of industrial practice. While quantitative experimental data is missing for comparison at such scale, the model is promising and its implementation in heat and mass transfer models should be considered for assistance to production of silicon for solar cells dedicated to photovoltaic systems.

The Artificial Mixed Fused Quartz Particles and Silicon Particles-Assisted High-Performance Multicrystalline Silicon

Mixed seeds of fused quartz particles and silicon particles were laid at the bottom of the crucible to assist the growth of multicrystalline silicon crystals. The full melting process was used, and then we found that the grown crystals had higher quality. The effect of mixed seeds on the growth of multicrystalline silicon was studied. The results showed that fine and uniform initial grains could be obtained by mixed seeds assisting the growth of crystals. Increasing the number of grain boundaries can better release thermal stress and inhibit the proliferation and diffusion of dislocations. The defect density of multicrystalline silicon decreased and the minority carrier lifetime increased, thus improving the conversion efficiency of multicrystalline silicon cells.

Random angle grain boundary formation and evolution dynamics during Si directional solidification

The growth of multicrystalline silicon and the formation of a random angle grain boundary, as well as the dislocation generation and expansion is observed dynamically in situ, by Synchrotron X-ray imaging techniques. The focus is kept on a random angle grain boundary since its behavior is particularly important to better understand the HP mc-Si (High Performance Multi-crystalline Silicon) photovoltaic properties. Due to the process conditions and to the grain competition that occurs during the solidification, a facetted {111}/facetted {111} groove is formed by this random angle grain boundary at the solid/liquid interface. It is shown how the shape of the solid/liquid interface allows the change of the preferential {111} growth facet and affects the grain boundary propagation direction. In one of the groove configurations, the two adjacent {111} facets do not have the same growth velocity and as a consequence the corresponding grain boundary does not follow the bisector of the angle between the two facets. Indeed, the direction of the grain boundary is determined by the growth velocities of the facets which control the grain competition. Moreover, under these experimental conditions a clear relationship is observed between the existence of random angle grain boundaries and the local generation of dislocations as well as their expansion. By comparison, dislocation emission is not observed at the level of Σ3 {111} grain boundaries.

An investigation of the wetting behavior of heat-treated silicon nitride particles with liquid silicon

Silicon nitride is commonly used as a coating material for the silica crucibles utilized for the growth of multicrystalline silicon by directional solidification. The coating material might introduce impurities into the silicon melt during the growth. To investigate the wetting and incorporation behavior of the silicon nitride particles, silicon nitride particles were put intentionally into silicon rods, which were then grown by the float zone technique. To ensure that the particles stay within the melt and are not pushed out of the molten zone, the particles must show good wetting. Superheating the silicon nitride powders up to 105 K over the melting point of silicon was a successful approach to ensure wetting and thus to investigate the incorporation behavior of the silicon nitride in photovoltaic silicon. The heat treated silicon nitride particles stayed within the melt zone during the float zone experiments and the distribution behavior of the particles could be studied. Incidentally, the results imply that to suppress incorporation of silicon nitride particles (as coating material) into molten silicon during directional solidification, superheating the melt must be avoided.

Numerical Study on Effect of Side Insulation Movement in the Multi-crystalline Silicon Growth Process

Numerical simulation is the best tool to understand and optimize the directional solidification (DS) process to grow good quality multi crystalline silicon (mc-Si) ingot for PV application. We have numerically investigated the effect of side insulation lifting distance. Here the mc-Si ingot was grown for four different heights of side insulation movement, such as 50, 100, 150 and 160 mm height from the bottom insulation. During the solidification process, the side insulation is lifted at constant velocity, 0.1 mm/min. which was fixed at four heights for four different processes till the end of solidification. From the investigations lower maximum thermal stress, lower maximum shear stress, lower dislocation rate and convex isothermal lines were obtained at the side insulation lifting distance of 150 mm.

Impurity Distribution in Multicrystalline Silicon Growth

This paper presents experimental data on the growth of multicrystalline silicon with tailored electrical properties using starting silicon of purity better than 99.999 at %. Our calculations demonstrate that the effective distribution coefficients of Fe-group impurities decrease with increasing impurity concentration in the starting charge.

Possible Twinning Operations during Directional Solidification of Multicrystalline Silicon

Twinning is an important phenomenon during crystal growth of multicrystalline silicon. During twinning, the nucleated new grain is in twin relation with the parent grain, and the twin relation math...

Numerical Simulation of Multi-Crystalline Silicon Crystal Growth Using a Macro–Micro Coupled Method during the Directional Solidification Process

In this work, the crystal growth of multi-crystalline silicon (mc-Si) during the directional solidification process was studied using the cellular automaton method. The boundary heat transfer coefficient was adjusted to get a suitable temperature field and a high-quality mc-Si ingot. Under the conditions of top adiabatic and bottom constant heat flux, the shape of the crystal-melt interface changes from concave to convex with the decrease of the heat transfer coefficient on the side boundaries. In addition, the nuclei form at the bottom boundary while columnar crystals develop into silicon melt with amzigzag-faceted interface. The higher-energy silicon grains were merged into lower energy ones. In the end, the number of silicon grains decreases with the increase of crystal length.

Investigation of different cases of dislocation generation during industrial Cz silicon pulling

AbstractThis study aims at better understanding the generation of dislocations causing a structure loss issue during the pulling of Czochralski silicon ingots. Several industrial‐scale n‐type ingots containing this issue were characterized. The generation and propagation of dislocations were observed in each ingot along with an investigation of the perturbation at the origin of the structure loss. The results show that for the two ingots characterized in this study, particles entering in contact with the ingot edge during crystal growth are responsible for the generation of dislocations.Moreover, the propagation of dislocations was compared for two different structure loss origins: particles in contact with the ingot edge and the crystallization around a gas bubble (pinhole) in the middle of the ingot. In each case, a fast transition between the original dislocation‐free growth and a growth with dislocations was observed at the perturbation position. Differences in the propagation of slip dislocations backward in the crystal are discussed as well as differences in the transition to a multicrystalline silicon growth regime. (© 2016 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Effect of grain boundary grooves at the crystal/melt interface on impurity accumulation during the unidirectional growth of multicrystalline silicon

Impurity accumulation at the grain boundaries in multicrystalline Si (mc-Si) was investigated by in situ observation of the crystal/melt interface, analysis of the grain boundary characteristics, and measurement of impurity concentrations. The impurity concentration was higher at grain boundaries that formed a groove at the crystal/melt interface than that at regions which did not form a groove at the crystal/melt interface. We conclude that groove formation at the crystal/melt interface is the cause of impurity accumulation at the grain boundary.

Computational Modelling on Heat Transfer Study of Molten Silicon During Multi-crystalline Silicon Growth Process for PV Applications

Multi-crystalline silicon is an important material with advantages of low-production cost and high conversion efficiency for photovoltaic solar cells. Directional solidifi cation has become the main technique for producing mc-Si ingots for solar cell applications. The study is performed in the framework of the incompressible Navier-Stokes equation in the Boussinesq approximation with convection-conduction equations. The computations are carried out in a two dimensional (2D) axisymmetric model by the finite- element method. The influence of the Reynolds numbers, total heat flux and velocity streamline pattern on the silicon melt was simulated and analyzed for various Rayleigh numbers between 10 to 10 6 with the help of a numerical technique. The following key findings are presented in this paper. The velocity field value is increased above 0.02(m/s), heat flux value is increased to 10 4(W/m 2), when the Rayleigh number is increased above 1000. Reynolds numbers are also studied in five parallel horizontal cross-sections of the melt silicon region for various Prandtl numbers at a critical Rayleigh number of 1000. Reynolds numbers are varied between 100 and 10 5 for the Rayleigh numbers between 10 to 10 6. Meanwhile, the melt has high fluctuation when the Prandtl number is increased above 0.01. The flow is converted from laminar to turbulence at a critical Rayleigh number 1000 and Prandtl number 0.01. These results provide important information for controlling the melt fluctuations during the solidification process which are used to increase the average grain size in growing silicon multicrystals and reduce the dislocation density.

Growth of multicrystalline silicon in a cone-shaped crucible

In this paper, a novel, vertical Bridgman-type technique for growing multicrystalline silicon (mc-Si) ingots in an induction furnace is described. In contrast to conventional growth, a modified setup with a cone-shaped crucible and susceptor is used for the first time. The temperature field and melt flow in the modified setup are calculated numerically and compared with the situation in a cylindrical standard setup. A cone-shaped mc-Si ingot is presented and analyzed with focus on the microstructure (inclusions, dislocations, grains) and the minority carrier lifetime, which are compared with the properties of a cylindrical ingot grown under similar conditions. Results of numerical simulations and growth experiments are discussed with respect to the influence of the cone-shaped setup on the temperature and flow fields in the melt, as well as on the microstructure and the minority carrier lifetime in the crystal. They indicate the potential of the novel technology to produce mc-Si ingots with a globular grain structure, low dislocation density, and high carrier lifetime.

Effect of crucible rotation rate on the growth and macrostructure of multicrystalline silicon

This paper examines the influence of uniform rotation of the crucible-melt-crystal system in a flat-bottom Bridgman geometry on the columnar silicon structure formed by a flat solidification front. We analyze the key features of heat exchange via natural convection using numerical simulation of experimental conditions of the growth of multicrystalline silicon from high-purity metallurgical-grade silicon.

The effect of silica nucleation layers on grain control of multi-crystalline silicon in directional solidification

The grain control in multi-crystalline silicon growth by directional solidification is crucial to the ingot quality. In order to study the nucleation and grain growth behavior, we carried out ingot growth using porous silica coatings, with different Si/SiO2 ratios, and the traditional nitride coating for comparison, at the bottom of a small crucible. It was observed that with more silica in the coating, smaller and uniform grains, as well as columnar growth, could be obtained. As a result, more non-Σ grain boundaries were found as well. On the contrary, as silicon became a continuous phase in the coating, the grain refinement was less effective and their grain development in the porous coating was examined. The possible grain nucleation and growth mechanisms in the porous coatings were further proposed.

Characterization of high-efficiency multi-crystalline silicon in industrial production

A new technique for the directional solidification growth of multi-crystalline silicon (mc-Si) ingot was developed by GCL-POLY Energy Holdings Ltd. This technique is called as S2 and has been used recently for industrial production. The average conversion efficiency of the solar cells fabricated by S2 mc-Si wafers is increased by 0.62% compared with the traditional mc-Si solar cells using conventional solar cell processing. In order to understand the origin of the high cell performance, ensure the process reproducibility and further improve the technique, this paper analyzes the grain structures of the S2 mc-Si wafers by light microscopy and scanning electron microscopy supported with electron back scatter diffraction. Our analysis indicates that the increased performance of the S2 mc-Si solar cells is contributed to low dislocation density, uniform and highly oriented grains with high percentage of electrically inactive grain boundary (∑3 grain boundary).

Effect of flow pattern on the segregation of impurities in vertical Bridgman growth of multi-crystalline silicon

Numerical experiments are used to study thermally driven flow during directional solidification of multi-crystalline (mc) silicon. A solid–liquid (s–l) interface shape was imposed by varying the absolute value and the direction of the radial temperature gradient in the melt. The flow pattern and the final impurity distribution were calculated for a solidification process with a planar, concave and convex s–l interface shape. When solidification with a planar s–l interface occurs the flow intensity is low and the impurity segregation is diffusion controlled and nearly uniform in radial direction. With concave and convex s–l interfaces the flow intensity becomes one order of magnitude higher. If a concave interface occurs impurities are transported towards the center of the ingot and radial segregation increases. The flow direction reverses when the interface becomes convex and the impurities are carried more effectively away from the interface to the bulk of the melt. It is for this case that the most uniform radial distribution was obtained.