Ingot Growth Research Articles

Potential for Recycled Silicon Solar Cells as Feedstock for New Ingot Growth

ABSTRACTThis paper investigates, by modeling, the potential for high‐value recycling of silicon wafers recovered from end‐of‐life PV modules. Technology for PV module recycling is making steady progress, both at recycling companies and R&D institutes, and it is possible that as a result, soon a stream of wafers or wafer fragments recovered from waste modules will become commercially available. Recycling the silicon for manufacturing of new PV modules is an opportunity both for reduction of cost and reduction of environmental footprint of PV. In this paper, we analyze possibilities for recycling of wafer fragments as feedstock for new silicon ingot growth. This could save up to about 0.16 kWh/Wp energy for production of the new PV system. Compared with lower value applications of the recovered silicon, the potential value and energy savings when used as feedstock for ingot growth are considerably higher. This paper presents the possibilities and challenges for recycling wafer fragments from the point of view of dopant type and resistivity control, and mitigation of the impact of recombination activity from possible increased impurity levels or related to boron. Because in current PV production a rapid transition to n‐type wafer doping is taking place, the paper also considers the question what can be done with p‐type doped recycled wafer material. We illustrate how application for perovskite–silicon tandem cells helps to mitigate possible performance loss from metallic impurities or boron. The application in tandem cells is perhaps the only realistic approach to make economically worthwhile use of recycled B‐doped silicon as feedstock for silicon ingot growth.

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.

Exploring popped-out phenomena in large-scale silicon ingot growth

Silicon wafers are essential for the semiconductor industry, providing the foundation for most integrated circuits. As demand for microelectronics grows, larger silicon wafers, have become crucial for increasing chip production and reducing manufacturing costs. However, the crystal separation phase during Czochralski (CZ) ingot growth is particularly challenging for larger ingots, often resulting in defects due to premature detachment (“popped-out” tails). This study investigates the popping-out stage of 18-inch ingots under varying heater power and ingot pull-out speed conditions. Photoluminescence (PL) imaging and a convolutional neural network (CNN) were used to analyze dislocation density in the silicon wafers. Results show that increasing the pull-out speed after detachment can significantly increase dislocation density, while pausing the ingot near the melt surface minimizes dislocations. Additionally, increasing heater power after detachment reduces dislocation density. The optimal condition for minimizing dislocation was found when heater power was doubled, and the ingot was paused near the melt surface for 30 min.

Importance of zero cross-point (Cp = 0) control between the vacancy and interstitial Si atom concentration curves to obtain a perfect Si ingot using Si melt growth

The cross-point (Cp), which appears in the vacancy (CV) and interstitial Si atom (CI) concentration curves, is a simple controlling factor for ingot growth and provides a final goal to obtain a defect-free Si ingot using general Si melt growth. The importance of Cp is demonstrated using the noncontact crucible method (NOC), which has a relatively small temperature gradient due to an ingot grown inside the melt. The control of temperature gradient G at a constant growth rate v is simple for the actual ingot growth; thus, the present simulated results are expressed by G at a constant v. The concentration at the Cp point becomes smaller as G becomes smaller and finally becomes zero (Cp = 0), in which CV and CI are practically zero (CV=CI=0). The estimated G and temperature T at Cp = 0 for v = 0.000144 cm s−1 are 6.3, 12.5, and 18.5 K cm−1 and 1216 K (943 °C), 1354 K (1081 °C), and 1371 K (1098 °C), corresponding to each combination. At 1216 K, G should be precisely controlled within 6.3 ± 3 % K cm−1 in the region substantially close to Cp = 0 to maintain the remaining concentration of point defects substantially lower than 1.0 × 1013 cm−3. G should be changed during growth from larger G to smaller G, such as 6.3 K cm−1, to efficiently grow the ingot within the limited time. The changing point, x of G, is an important key parameter for the timing variation, where x denotes the normalized distance from the growing interface. The initial G is large; thus, the changing point of G approaches the growing interface. The growth condition necessary to obtain a defect-free Si ingot can be demonstrated using the Cp = 0 point, which is fully compatible with the perfect critical point where CV and CI are practically zero. The present results are mainly considered using the CV and CI concentration curves. However, these results largely help understand the approximate trend of G and v for the growth of defect-free ingot and the relation between Cp and the commonly used critical point between CV and CI.

Simulation analysis on furnace pressure for reducing the impurities concentrations distribution during the growth of mc-Si ingot by DS process: Solar cell applications

A numerical investigation is carried out for the directional solidification (DS) process with various furnace pressures such as 300 mbar, 400 mbar, 500 mbar, and 600 mbar. During the multi-crystalline silicon (mc-Si) ingot growth process, the pressure inside the furnace affects the species' transport. The oxygen and carbon concentrations for various furnace pressures have been analyzed, and the results were compared. In the gas phase, a furnace pressure of less than 500 mbar enhances the strong back diffusion of the species into the silicon melt. The furnace pressure is higher than 500 mbar, which enhances the convection transport of the species in the lab-scale furnace. The optimized furnace pressure (500 mbar) enhances the quality of multi-crystalline silicon ingot. The oxygen concentration and carbon concentration values are reduced when the furnace pressure is around 500 mbar, which reduces the light-induced degradation (LID) effects and SiC precipitation.

Simulation of Solidification Structure in the Vacuum Arc Remelting Process of Titanium Alloy TC4 Based on 3D CAFE Method

Vacuum arc remelting is the main production method of titanium alloy ingots at present. In order to obtain good quality ingots, it is of great significance to study the formation of the solidification structure of ingots via vacuum arc remelting. In order to select and optimize the nucleation parameters for the solidification microstructure simulation of an ingot, a 3D CAFE model for microstructure evolution during vacuum arc remelting was established, taking into account heat transfer, flow, and solute diffusion. The Gaussian distribution continuous nucleation model and extended KGT model were used to describe the grain nucleation and dendrite tip growth rates, respectively. The multi-point mass source and moving boundary method were used to simulate the ingot growth. The results show that there are three typical crystal regions in the solidification structure of vacuum arc remelting titanium alloy ingots, namely the surface fine crystal region, columnar crystal region, and central equiaxed crystal region. The proportion of the columnar crystal region in the solidification structure of an ingot increases gradually with the increase in the undercooling of the maximum bulk nucleation. With an increase in the maximum bulk nucleation density, the equiaxed grain zone gradually increases, and the grain size gradually decreases. The proportion of the columnar crystal region in the solidification structure of an ingot increases gradually with an increase in the undercooling of the maximum bulk nucleation. The maximum volume nucleation variance has no obvious effect on the change in the solidification structure. When the maximum volume nucleation undercooling is 5.5 K, the maximum volume nucleation standard deviation is 4 K, and the maximum volume nucleation density is 5 × 108. The solidification structure simulation results are in good agreement with the experimental results.

Numerical Investigation of Argon Gas Flow Patterns and Their Effects on Mc-Si Ingot Growth Process: Solar Cell Applications

Numerical Investigation of Argon Gas Flow Patterns and Their Effects on Mc-Si Ingot Growth Process: Solar Cell Applications

Dislocations in Crystalline Silicon Solar Cells

Dislocation is a common extended defect in crystalline silicon solar cells, which affects the recombination characteristics of solar cells by forming deep‐level defect states in the silicon bandgap, thereby reducing the lifetime of minority carrier. Hence, reducing the impact of defects on device performance is an effective strategy to optimize the performance of photovoltaic devices. This article reviews the observation and engineering of dislocation in Si solar cell. The structure and deformation of Si can be directly observed by chemical etching combined with electron microscopy. Also, more information about dislocation is obtained indirectly by monitoring the electrical and optical properties of Si. The classification, density, distribution of dislocations, and their interactions with other defects in Si can affect the lifetime of minority carriers and thereby reduce the performance of Si solar cells. In order to achieve higher cell efficiency, crystals with less or even no dislocation should be obtained. In addition to the specification of controlling the relevant parameters during the growth of silicon ingots to obtain the minimum dislocation density, it is necessary to study the behavior of dislocation in Si wafers under the combined action of external stress, temperature, and other defects.

High-Rate Epitaxial Growth of Silicon Using Electron Beam Evaporation at High Temperatures

This paper describes the high-rate (~1.5 μm/min) growth of Si films on Si supporting substrates with (100) crystallographic orientation at 600 °C, 800 °C, and 1000 °C in a vacuum environment of ~1 × 10−5 mbar using electron beam (e-beam) evaporation. The microstructure, crystallinity, and conductivity of such films were investigated. It was established that fully crystalline (Raman spectroscopy, EBSD) and stress-free epi-Si layers with a thickness of approximately 50 µm can be fabricated at 1000 °C, while at 600 °C and 800 °C, some poly-Si inclusions were observed using Raman spectroscopy. Hall effect measurements showed that epi-Si layers deposited at 1000 °C had resistivity, carrier concentration, and mobility comparable to those obtained for c-Si wafers fabricated through ingot growth and wafering using the same solar grade Si feedstock used for the e-beam depositions. The dislocation densities were determined to be ∼2 × 107 cm−2 and ∼5 × 106 cm−2 at 800 and 1000 °C, respectively, using Secco etch. The results highlight the potential of e-beam evaporation as a promising and cost-effective alternative to conventional CVD for the growth of epi-Si layers and, potentially, epi-Si wafers. Some of the remaining technical challenges of this deposition technology are briefly indicated and discussed.

Design and numerical analysis of a novel argon gas tube to reduce impurities in large size casting crystalline silicon furnace

A novel design of argon gas tube within the silicon casting system was developed employing pipe flow and side jet analysis, which facilitates horizontal ejection of argon gas along the silicon melt surface. The aim was to enhance control over gas flow velocity for removing impurities during large size silicon ingot (1400 kg) growth, with special attention on the edge region of ingot. It was found that the diameter of side-facing outlet holes, when oriented along the corner direction, is about 2 times of that along the side center direction, to achieve an extended jet distance reaching corner region. The gas flow and transport of SiO above the silicon melt surface were studied for the proposed gas tube configuration based on 3D numerical simulations. The simulation results reveal the correlation between argon flow rate and resultant gas velocity at the edge of melt surface, which is consistent with the theoretical analysis, particularly for moderate to high argon flow rates. The numerical simulations demonstrated that the proposed gas tube, equipped with eight optimized outlet holes, outperformed the conventional tube in extracting SiO from both central and corner regions above the silicon melt surface.

Twenty years crystal growth of solar silicon: My serendipity journey

The past two decades have been a transformative era for solar silicon crystal growth, especially in the competition between multi-crystalline silicon (Multi-Si) and mono-crystalline silicon (Mono-Si). As a critical sector of the solar photovoltaic (PV) industry, the demand for this crucial material has surged exponentially, expanding over a thousand-fold. This remarkable increase has led to an accumulative deployment of silicon solar panels, which now approach a striking terawatt (TW), capturing over 95 % of the global PV market share. Furthermore, the rapid advancements in crystal growth technology during this period have set an unprecedented historical benchmark. This paper reviewed our early effort in the hot-zone design of Czochralski (CZ) solar silicon and then discussed its recent advancements. In between, the small-grain high-performance multi-crystalline silicon (HPM-Si), which we proposed, and the mono-like silicon (ML-Si) were briefly introduced. Because of the much better ingot quality and growth yield, HPM-Si became the mainstream until 2018. Its market share peaked at more than 70 % in 2017 but slid down quickly and was replaced by CZ silicon due to the emergence of diamond wire sawing (DWS); the CZ silicon was easier to cut, and its kerf loss was much less. Since then, the CZ ingot size has also started to increase quickly. The wafer size increased from 156 mm × 156 mm (M0) to 217 mm × 217 mm (M12+) in a few recent years; the increase in wafer area was almost doubled. Nowadays, the market share of CZ mono-Si is over 80 %.

Schmidt number's impact during the growth of mc-silicon ingot through directional solidification furnace for photovoltaic applications: A computational investigation

The use of multi-crystalline silicon for photovoltaic solar cell applications is highly advantageous due to its low manufacturing costs. In this study, numerical simulations were conducted to investigate mass transport phenomena during the directional solidification of molten silicon for different Schmidt values at the critical Prandtl number of 0.01. The simulations were performed using the diffusion model and convection-conduction equations as the framework for the incompressible Navier-Stokes equation. The finite-volume method was used to carry out computations in a two-dimensional (2D) axisymmetric model. The diffusion flux of impurities (oxygen, carbon, and nitrogen) and dopants (phosphorus and boron) during the mc-silicon growth process was examined for Schmidt numbers Sc=0.5, Sc=5, and Sc=10. It was found that the Schmidt number Sc=0.5 is suitable for producing higher-grade mc-silicon ingots by regulating the production of impurities and dopants. The simulation results obtained in this study may provide a better understanding of the mc-silicon growth process and can be used to improve the production of high-quality mc-silicon ingots for photovoltaic applications.

Growth and analysis of polycrystalline silicon ingots using recycled silicon from waste solar module

The proliferated growth of the Photovoltaic industry (PV) will eventually lead to unprecedented volumes of silicon-based solar waste. Failing to manage high volumes of waste, can lead to a huge amount of silicon metal loss and is also highly conducive to posing an environmental hazard. Therefore, efficient recovery and re-utilization after proper purification of the PV-waste-based silicon are highly obligatory for the sustainable development of the PV industry. Herein, we report the growth of small-size silicon ingots (φ = 0.5 inch, 1.0 inch) produced from recovered silicon from the waste crystalline silicon (c-Si) solar module through the Spark Plasma Sintering (SPS) technique. The silicon feedstock was prepared, after the extraction of silicon cells from the used panel and chemically etching contacts, ARC (anti-reflection coating), from the cells in order to recover the silicon wafer. The silicon wafer pieces were pulverized to produce very fine powder and subsequently processed at a different set of temperature-pressure for the study of density variation. The highest density (2.33 gm/cm3) equal to the theoretical density of silicon was obtained for sample grown at temperature 1200 °C and pressure 60 MPa. The purity of the obtained ingots was found to be greater than 3 N and therefore, can find use in different industries.

Analysis of the formation of interweaved strips in cast quasi-single crystal silicon ingots

Unlike the impurity behavior of conventional polycrystalline silicon ingots, we have found that impurities are generated at the solid–liquid interface during the growth of quasi-single crystal silicon ingots. Due to the absence of large-angle grain boundaries in quasi-single crystal silicon ingots, impurities become enriched at the solid–liquid interface of the silicon ingot and exhibit a specific directional tilt during crystal growth. The accumulated impurities areas often exhibit a dark interweaved strip morphology detected by an infrared detector of quasi-single crystal silicon ingots. In this study, the dark interweaved strip is identified as an “interweaved shadow”. Additionally, the use of chemical corrosion technology and microscope observations revealed that interweaved shadows were present within the silicon ingot and could be extracted as white filament impurities. Subsequently, the interweaved shadows and dislocations of various morphologies in the silicon brick were analyzed. The interweaved shadows contained a mixture of carbon, oxygen, and nitrogen impurities. The degree of dislocation depended on the size of the filament impurities in the interweaved shadow. Specifically, the concentrations of carbon and nitrogen exceeded the solid solubility and the filament impurities with a diameter greater than 2.1 μm resulted in more severe dislocation defects.

Analyses of Distributions of Different Impurities for Growth of Polysilicon Ingots in Directional Solidification System

Analyses of Distributions of Different Impurities for Growth of Polysilicon Ingots in Directional Solidification System

Distributed data filtering and modeling for fog and networked manufacturing

Fog Manufacturing applies both Fog and Cloud Computing collaboratively in Smart Manufacturing to create an interconnected network through sensing, actuation, and computation nodes. Fog Manufacturing has become a promising research component to be integrated into the existing Smart Manufacturing paradigm and provides reliable and responsive computation services. However, Fog nodes' relatively limited communication bandwidth and computation capabilities call for reduced data communication load and computation time latency for modeling. There has long been a lack of an integrated framework to automatically reduce manufacturing data and perform computationally efficient modeling/machine learning. This research direction is increasingly important as both the computational demands and Fog/networked Manufacturing become prevalent. This paper proposes an integrated and distributed framework for data reduction and modeling of multiple systems in a Smart Manufacturing network considering the system similarities. A simulation study and a Fog Manufacturing testbed for ingot growth manufacturing validated that the proposed framework significantly reduces the sample size used for improved computational runtime metrics while outperforming various other data reduction methods in modeling performance.

Copper in compensated p- and n-type Czochralski silicon: Diffusivity, influence on the majority charge carrier density and mobility

Copper contamination risks are present throughout the whole crystalline silicon solar cell process flow, namely during ingot growth processes using recycled feedstock, wafer sawing and metallisation. Copper is a fast-diffusing impurity which can modify the carrier transport and recombination properties. The aim of this study is to present new insights into the properties of copper-contaminated highly doped and compensated silicon, with a focus on the copper diffusivity and the influence of this impurity on the majority carrier mobility and density. For this, compensated Czochralski silicon ingots were grown from electronic grade feedstock doped with boron and phosphorus, and the wafers intentionally contaminated with copper. Its concentrations were measured by Glow Discharge Mass Spectrometry, while the electrical properties were extracted from Hall Effect measurements. The dopant concentrations were used to compute the effective diffusivity of interstitial copper and important changes in the copper diffusivity are expected in the vicinity of the p-type/n-type transition. For the n-type compensated wafers, a strong migration of copper atoms towards the surface and a reduced copper bulk content were highlighted. Furthermore, the presence of precipitates was experimentally revealed. Both the equilibrium electron density and electron mobility were not influenced by the contamination. In p-type compensated material, copper atoms did not accumulate at the surface, probably due to both the limited copper precipitation in p-type silicon and the lower copper diffusivity. In addition, the equilibrium hole density was not influenced. However, the hole carrier mobility was affected, likely associated to the higher bulk copper content. • Highly doped compensated silicon was intentionally contaminated with copper. • Electrical properties were obtained from Hall Effect measurements. • Large differences in copper diffusivity close to the p-to n-type transition. • Strong migration of copper towards the surface in n-type compensated wafers. • Equilibrium electron density and mobility are not influenced by the contamination.

Study on electrical activity of grain boundaries in silicon through systematic control of structural parameters and characterization using a pretrained machine learning model

We report on the effects of grain boundary (GB) structures on the carrier recombination velocity at GB (vGB) in multicrystalline Si (mc-Si). The fabricated artificial GBs and an originally developed machine learning model allowed an investigation of the effect of three macroscopic parameters, misorientation angle α for Σ values, asymmetric angle β, and deviation angle θ from the ingot growth direction. Totally, 13 GBs were formed by directional solidification using multi-seeds with controlled crystal orientations. vGB was evaluated directly from photoluminescence intensity profiles across GBs using a pre-trained machine learning model, which allowed a quantitative and continuous evaluation along GBs. The evaluation results indicated that the impact of θ on vGB would be relatively large among the three macroscopic parameters. In addition, the results for the Σ5 and Σ13 GBs suggested that the minimum vGB would be related to the GB energy. These results were discussed in terms of the complexity of the local reconstruction of GB structures. The deviation would make a more complex reconstructed GB structure with local distortion, resulting in an increase in the electrical activity of GBs. The obtained knowledge will contribute to improving various polycrystalline materials through a comprehensive understanding of the relationship between GB structures and their properties.

Recent Progress on Single-Crystal Growth and Epitaxial Growth of 4H Silicon Carbide

The review article describes the recent progress on SiC single-crystal and epitaxial growth technology. SiC is a third-generation semiconducting material with wide bandgap and high electrical breakdown field. Thanks to its excellent properties, it becomes an advantageous material in the field of high-temperature and high-power electronic device applications. Power devices fabricated of SiC are able to be operated at higher power density and higher switching frequency. This review focus on the growth, doping control and defect control of SiC single-crystal ingot and epilayer. The process of PVT, CVD, defect control, doping control and some recent applications of SiC are described. Various types of defects are described, including Micropipes, Dislocations, Stacking Faults etc. The wafering and polishing technology are also described.

Two-dimensional distribution of vacancy concentration at cross points for the ingot length and the temperature gradients to largely reduce point defects and the growth of a dislocation-free NOC ingot for evaluation of its quality

The noncontact crucible (NOC) method has a large and deep low-temperature region in Si melt to grow a uniform large Si single ingot. The cross point with the same concentration of vacancy (CV) and interstitial Si atom (CI) is very important to reduce the point defects in NOC ingot. The relation between the temperature gradient, G, the position and concentration of cross point was estimated using equations derived from Voronkov’ explanation. For NOC growth, two stages for temperature gradient are required as G1 at the growing interface and G2 at the melt surface. The linear T profile is useful for the two-stage growth. Concentration curve of vacancy and interstitial Si atom was calculated on the two stages as a function of the distance from the growing interface. A cross point appears above the melt surface even for the two-stage calculation with a long ingot inside the melt. By the two dimensional-distribution of CV at cross point for G2/G1 and d1, CV has a tendency to become higher as G2/G1 becomes larger. On the whole, the higher CV area appears in the larger G2/G1 and shorter d1 region, and the lower CV area appears in the smaller G2/G1 and longer d1 region. The latter condition is suitable for the NOC growth. Such trend of CV at cross points should be utilized to reduce the remained point defects in ingot. Lower CV and CI at the cross point can reduce the remained point defects in a NOC ingot using smaller G2/G1, smaller G1 and longer d1 near the critical point. To confirm the quality of NOC ingot grown using the present method, a dislocation-free Si single ingot was grown utilizing silica crucibles by the NOC growth method. The maximum diameter and total length of the grown ingot are 17.5 and 15.5 cm, respectively. A Si ingot was experimentally grown by the NOC method. The bottom of the NOC ingot had a convex shape in the growth direction which is very advantageous to NOC growth.It was confirmed by habit lines and etch-pit density of dislocation whether the ingot was dislocation-free or not. A dislocation-free Si single ingot was realized by the NOC method for the first time.