Si Multicrystals Research Articles

Computational Study of Heat Transfer on Molten Silicon During Directional Solidification for Solar Cell Applications

Computational modeling is an essential tool in modern crystal growth technology and development which is extensively used for promotion of directional solidification of silicon growth processes. The fluctuation of melt flow in the crucible has significant effects on segregation of impurity concentration and the formation of micro - defects in the grown Si multi-crystals. The control of grains as well as the grain boundaries is particularly important to the crystal quality and thus the solar cell efficiency. The numerical study is performed in the framework of the incompressible Navier-Stokes equation in the Boussinesq approximation with convection-conduction equations. The computations are made in two dimensional (2D) axisymmetric model by the finite-element numerical technique. The melt flow properties like velocity field, convective heat flux in molten silicon system are accurately simulated and analyzed at constant Prandtl number for two various Rayleigh numbers Ra= 100 and Ra=1000. In which, we found that Ra=1000 is critical Raleigh number for molten silicon. The main goal is to reduce the grain boundaries, dislocation density and increase the average grain size in whole multi-crystalline silicon ingots through controlling the turbulent melt flow patterns to laminar.

Arrangement of dendrite crystals grown along the bottom of Si ingots using the dendritic casting method by controlling thermal conductivity under crucibles

Dislocations in Si multicrystals strongly affect the efficiency of solar cells, and are usually generated from random grain boundaries during crystal growth. The low density of random grain boundaries and the coherency of random grain boundaries are very important in suppressing dislocations. Controlling arrangement of dendrite crystals grown along the bottom of ingots is effective for decreasing the density of random grain boundaries and for improving the coherency of random grain boundaries. A method of controlling thermal conductivity under crucibles to control the arrangement of dendrite crystals was proposed. Graphite plates with different thermal conductivities were used all over the bottom surface of crucibles. Two types of graphite plates, one with a line-shaped highly cooled part and the other with a ring-shaped one, were used. Using the graphite plates, the distribution of dendrite crystals was well arranged, and dendrite crystals were controlled to be fairly parallel to each other.

Relationship between grain boundary structures in Si multicrystals and generation of dislocations during crystal growth

We attempted to clarify relationship between grain boundary structures in Si multicrystals and generation of dislocations during crystal growth. Systematic variation of grain boundary structures was realized by employing dendritic nucleation at the initial stage of crystal growth. Etch-pit observation revealed that the contact angle of adjacent dendrite crystals to form a grain boundary affects generation of dislocations. Experimentally observed dislocation density was found to be well correlated with shear stress around the grain boundary calculated by finite element analysis.

Control of Microstructures and Crystal Defects in Si Multicrystals Grown by the Dendritic Casting Method

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Investigation on the crystal growth process of spherical Si single crystals by melting

Spherical Si single crystals with a diameter of approximately 1 mm were grown by melting for solar cell applications. The start sources were spherical Si multicrystals fabricated by a dropping method, which had various irregular shapes. Spherical Si multicrystals were melted into droplets and recrystallized on a quartz plate sample holder that was coated with Si 3N 4. It was found that a surface coating of SiO 2 layer on the start sources and oxygen atmosphere during melting and recrystallization were essential to achieve almost perfect spherical shape. Defect-free single crystalline spherical Si could be obtained at recrystallization temperature ranging from 1400 to 1330 °C, corresponding to an undercooling ranging from 14 to 84 °C, with a yield of nearly 100%. At recrystallization temperatures higher than 1380 °C, the recrystallized spherical Si crystals were almost perfect spheres, whereas small protuberances were formed when the recrystallization temperature was lower than 1360 °C. It was also found that that melting at a temperature close to the melting point of Si (at ~1414 °C), a slow cooling rate of ~1 °C/min before recrystallization and relatively fast cooling rate of ~20 °C/min after recrystallization were important for achieving high carrier lifetime. The average carrier lifetime was greatly improved from lower than 2.5 μs of start sources up to ~7.5 μs by melting at optimized conditions. The influences of residual oxygen on the carrier lifetime of recrystallized spherical Si are discussed based on the measurement results with Fourier transform infrared spectrometer.

Microstructures of Si multicrystals and their impact on minority carrier diffusion length

We carried out a systematic investigation of microstructures in a particular zone of Si multicrystals which shows a strong local variation in minority carrier diffusion length (MCDL). It was found that three typical regions with distinct microstructures correspond to the microscopic origins of the local MCDL difference. The region with perfect twin structure (Σ3 boundaries) has a high MCDL, while the regions with either high-angle grain boundaries (Σ9 and Σ27 boundaries) or sub-grain boundaries (high density dislocation) exhibit lower minority carrier diffusion. The relationships between the microstructures and the corresponding MCDLs are briefly discussed. This study has implications for developing improved Si multicrystals with appropriate microstructure for application in solar cells.

Quantitative analysis of subgrain boundaries in Si multicrystals and their impact on electrical properties and solar cell performance

We implement spatially resolved x-ray rocking curve for quantitative analysis of subgrain boundary (sub-GB) density in Si multicrystals with controlled microstructures by “dendritic casting method.” The oriented crystal grains made it possible to analyze a large area over different crystal grains by using the same optical configuration. Sub-GBs were found to be spatially localized in a particular region and spread in the growth direction of the ingot. By combining sub-GB density with a separate measurement of minority carrier diffusion length, carrier recombination velocity at sub-GBs was revealed to be at the same order as random GBs. Furthermore, by analyzing electroluminescence images of a solar cell with different detection wavelengths, sub-GBs were shown to behave as shunts with stronger activity than random GBs.

Impact of Defect Density in Si Bulk Multicrystals on Gettering Effect of Impurities

We investigated the impact of defect density in Si multicrystals on the efficiency of gettering of impurities. Samples with low defect density were prepared by controlling crystal growth and compared with those grown by a conventional cast method with high defect density. As a result, increment in minority carrier diffusion length after gettering can be enlarged by decreasing defect density. Therefore, control of microstructures in Si multicrystals by controlling crystal growth is concluded to be beneficial not only for improvement of macroscopic properties of as-grown samples but for realization of high-performance solar cells.

Modification of local structures in multicrystals revealed by spatially resolved x-ray rocking curve analysis

We show that spatially resolved x-ray rocking curve analysis can be a useful technique to investigate local structures in bulk multicrystals and their modification during crystal growth when combined with appropriate samples. For this purpose, a model crystal growth experiment was carried out using a Si seed crystal with artificially controlled ⟨110⟩ tilted grain boundaries. The growth orientation was chosen as the scan axis for rocking curve measurements in different crystal grains. Thanks to the superior angle resolution, the gradual structural changes can be monitored by the changes in the rocking curve profile such as the peak shift and the peak splitting. The amount as well as the sign of the peak shift was found to be strongly dependent on the initial grain boundary structure. Furthermore, the technique was applied to investigate local structures of Si multicrystals with controlled grain orientation grown without any seeds.

In situ observations of crystal growth of spherical Si single crystals

Spherical Si single crystals for solar cell substrates have been grown successfully with a yield of almost 100%. Spherical Si multicrystals with diameters of approximately 400 μm in a teardrop shape were initially fabricated by a dropping method. The as-dropped spherical Si multicrystals were melted into droplets on a silica plate in an oxygen atmosphere, and the Si droplets were then recrystallized to form single crystals by supercooling within a specific temperature range. It was found that the recrystallization process occurred unidirectionally in each Si droplet, and recrystallized spherical Si single crystals free of defects (dislocations and oxidation-induced stacking faults, OSF) were obtained at a supercooling in the range of 12–42 °C. For supercooling ranging from 42 to 87 °C, spherical Si single crystals could still be obtained; however, many defects such as dislocations and OSF were generated. When the supercooling was larger than 87 °C, the crystal growth showed a dendritic growth mode and only multicrystals were obtained.

Si multicrystals grown by the Czochralski method with multi-seeds

Si multicrystals with several large grains have been grown by the Czochralski (CZ) method with multiple seeds using semiconductor-grade high-purity Si raw material in order to investigate the influence of defects such as dislocations and sub-grain-boundaries on the minority carrier lifetime. Two or three single-crystal Si seeds with [1 0 0], [1 1 0] and [1 1 0] orientations were used for the growth. It was found that the minority carrier lifetime in the Si multicrystals decreased considerably in the region of grains near the semicoherent interfaces with others, e.g., interfaces between [2 3 2] and [1 1 1] grains, [1 0 0] and [1 1 1] grains, and [1 1 0] and [1 1 1] grains. Numerous dislocations with densities as high as 10 6/cm 2 in these grains moved to align and generate sub-grain boundaries by the shear stress caused by the semicoherence near the interfaces, which was determined to be responsible for the decrease in the minority carrier lifetime.

Influence of crystalline defects in Czochralski-grown Si multicrystal on minority carrier lifetime

Si multicrystals have been grown by the Czochralski (CZ) method using commercially available semiconductor grade Si raw material (11N) and high purity silica crucibles for investigating the influence of crystalline defects such as grain boundaries including sub-grain-boundaries, and high-density dislocations on the minority carrier lifetime. The minority carrier lifetime in the CZ-Si multicrystal was compared with that in a CZ-Si single crystal, both with and without intentional Fe doping. It was found that the minority carrier lifetime in regions with high density of crystalline defects in the CZ-Si multicrystal decreased drastically to 1 μs from 8 μs, which was the value in the CZ-Si single crystal. Fe doping with a concentration of 3×10 13 atoms/cm 3 decreased the minority carrier lifetime to 2 μs in the CZ-Si single crystal. We therefore conclude that both crystalline defects and Fe doping play a very important role in the deterioration of the minority carrier lifetime, and the influence of the crystalline defects is more crucial by considering that the occurrence of crystalline defects in a multicrystal is an intrinsic problem whereas Fe contamination is an extrinsic one.