Silicon Ingots Research Articles

Design of an irradiation rig using screen method for silicon transmutation doping at the Dalat research reactor

The neutron transmutation doping of silicon (NTD-Si) at research reactors has beensuccessfully implemented in many countries to produce high-quality semiconductors. In the late 1980s, NTD-Si has been tested at the Dalat Nuclear Research Reactor (DNRR) but the results have been limited. Therefore, the design and testing of an irradiation rig for NTD-Si at the DNRR are necessary to have a better understanding in order to apply the NTD-Si in a new research reactor of the Research Centre for Nuclear Science and Technology (RCNEST), which has planned to be built in Viet Nam. This paper presents the design and testing of a new irradiation rig using screen method for testing NTD-Si at the DNRR. The important parameters in the rig such as neutron spectrum and thermal neutron flux distribution were determined by both calculation using MCNP5 computer code and experiment. The aluminum ingots, which have similar neutronic characteristics with silicon ingots, were irradiated in the rig to verify the appropriate design. The uniformity of thermal neutron flux in the rig is less than 5% in axial and 2% in radial directions, respectively. However, the thermal/fast flux ratio of the irradiation rig is 4.38/1 would affect target resistivity of testing Silicon ingots after irradiation.

Quality Control of High Carbon Steel for Steel Wires.

High-carbon steel wires used for bridge cables, tire reinforcement materials and cutting materials of silicon ingot for photovoltaic industry require an extremely fine diameter and high strength. Poor control of centerline segregation, inclusion and microstructure of high-carbon steel is detrimental to drawability and subsequent fatigue performance. Prof. Weiqing Chen’s group at the University of Science and Technology Beijing (USTB) has been investigating the quality control of high-carbon steel through a low-cost one-stage hot rolling process since 2000. This paper reviews the main research from this group. The laboratory-scale and industrial results are presented. Intensive secondary cooling, final electromagnetic stirring (F-EMS), final permanent magnetic stirring (F-PMS), and soft reduction are investigated and applied to control centerline segregation, and the application scope is also discussed. A combination of redesign of submerged entry nozzle (SEN) and refining slag, utilization of Al-free refractory and the addition of low-melting-point compounds is studied and applied effectively to control inclusions. Measurements and mechanisms to control network cementite, martensite, banded structure and undesired texture are investigated and discussed. Integration of the above research has achieved industrial application in more than 10 steelworks and was further extended to application in spring steel, welding wire steel and some other wire rods.

Evaluation of improvement strategies of grain structure properties in high performance multi-crystalline silicon ingots

High performance multi-crystalline silicon (HPM-Si) for the use in photovoltaics is characterized by a very fine grain structure and a high content of random grain boundaries, finally resulting in a low dislocation density and consequently in a high material quality. Typically, the grain size increases and the fraction of random grain boundaries decreases over ingot height due to annihilation mechanisms, especially in the first 150 mm. One approach for further material improvement is to further increase the initial random grain boundary fraction and to maintain it as high as possible over the complete ingot height.In this work, several theoretical approaches to achieve these points were evaluated by experiments in G1 scale. Firstly, the influence of the silicon seeding material on the initial grain structure was investigated regarding the effect of extremely fine Si particles in the µm to nm range and the bulk density of the particle layer. Secondly, the effect of the initial geometrical grain boundary arrangement in the seed layer was evaluated. For that purpose, special seed alignments similar to the Quasimono approach were tested. Finally, the process parameter growth rate was varied in a wide range to investigate its influence on the evolution of the grain boundaries during growth.The results show that the optimum for the initial random grain boundary fraction is already reached by the existing methods/commonly used seed materials. Concerning the decrease of the random grain boundary fraction over ingot height, some technical aspects were identified which are able to keep the amount of random grain boundaries at a high level. However, the practical realization within an industrial setup might be difficult.

Seed Size: Influence of Seed Size on the Quality of High‐Performance Multicrystalline Silicon Ingots Grown by Fused Quartz Particle Seeding (Crystal Research and Technology 3/2019)

Crystal Research and TechnologyVolume 54, Issue 3 1970014 Cover PictureFree Access Seed Size: Influence of Seed Size on the Quality of High-Performance Multicrystalline Silicon Ingots Grown by Fused Quartz Particle Seeding (Crystal Research and Technology 3/2019) First published: 12 March 2019 https://doi.org/10.1002/crat.201970014AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat Graphical Abstract Growing Silicon ingots by fused quartz particle seeding is critically dependent on seed size. In article number 1800098, Xinming Huang and co-workers report on the investigation of four different seed sizes. Experimental results show that ingots obtained in the presence of 50-70 mesh seeds have the best quality. Volume54, Issue3March 20191970014 RelatedInformation

Reduced metal contamination from crucible and coating using a silicon nitride based diffusion barrier for the growth of cast quasi-single crystalline silicon ingots

During the crystallization of directionally solidified silicon such as multicrystalline or cast quasi-single crystalline silicon, the diffusion of metal impurities from crucible and coating into the solid silicon ingot at high temperatures creates a severe contamination issue in the edge region of the ingot, the so called red zone. We present an effective diffusion barrier which greatly reduces this detrimental metal diffusion. Silicon wafers are coated with a layer system of silicon oxide and silicon nitride by chemical vapor deposition and are placed on the bottom of the crucible. Using this technique, the metal contamination in the red zone can be reduced by about one order of magnitude. This leads to better solar cell performance of wafers fabricated from this ingot area. Also, less ingot material has to be discarded leading to higher wafer yield per ingot.

Evolution of the light sensitive defects in high performance multicrystalline silicon wafers

Sequential degradation measurements have been performed on passivated high performance multicrystalline silicon wafers, first at room temperature under low intensity illumination followed by a higher intensity illumination at an elevated temperature. The presence of two main degradation mechanisms, affecting the lifetime under different conditions has been demonstrated, namely, the well-studied light induced degradation caused by boron-oxygen-complexes and the less understood light and elevated temperature induced degradation. Light and elevated temperature induced degradation is the main lifetime limiting the recombination path when fully activated, but the contribution from boron-oxygen complexes is not negligible. This separation of the two degradation mechanisms might, therefore, be necessary for proper evaluation of the dominant recombination mechanism. Experiments also show regeneration of the minority carrier lifetimes caused by deactivation of both the lifetime limiting defects at comparable time scales, and under similar illumination and temperature conditions. Wafers from different heights in a high performance multicrystalline silicon ingot have been evaluated to better understand the underlying causes for the different degradation mechanisms. Effects of the iron-boron-splitting on the carrier lifetime are only visible in ungettered wafers.

Influence of Seed Size on the Quality of High‐Performance Multicrystalline Silicon Ingots Grown by Fused Quartz Particle Seeding

AbstractThe performance of high‐performance multicrystalline silicon ingots is studied as a function of fused quartz seed particle size. Four kinds of Si ingots are grown using fused quartz particles having four distinct particle sizes, seed Nos. 1–4, corresponding to mesh sizes of 10–30, 30–50, 50–70, and 70–100, respectively. Each seed type is employed to have the same particle density area of ≈220 particles per cm2. The experimental results show that the Si ingot grown using 50–70 mesh seeds (seed No. 3) has the best uniformity of initial nucleation and subsequent grains, resulting in the lowest density of dislocation clusters. The Si ingots are cut into bricks and Si wafers are sliced from different positions of each brick and further studied. As a result, the average conversion efficiency is the highest for the ingot grown using No. 3 seeds (50–70 mesh) at 18.34%, ≈0.22% (absolute value) higher than that observed when employing 10–30 mesh seeds (seed No. 1, 18.12%).

Numerical investigation of Directional Solidification process for improving multi-crystalline silicon ingot quality for photovoltaic applications

Numerical simulation is used as a comprehensive tool for analysis and optimization of the multi-crystalline silicon (mc-Si) growth by the Directional Solidification (DS) method. We have used a transient global heat transfer model to optimize the temperature profile of the DS process to produce high-quality multi-crystalline silicon ingots for solar cell application. The melt-crystal interface shape, impurities and von Mises stress were studied by the numerical simulation with different temperature profiles. A slightly convex interface shape, lower thermal stress and lower impurities in the mc-Si were obtained due to the lower temperature gradient by the optimized temperature profile. Simulation results show that the optimized temperature profile is particularly suitable for high quality mc-Si ingot.

Investigation of pinholes in Czochralski silicon ingots in relation to structure loss

In the production of monocrystalline silicon by the Czochralski process, structure loss frequently reduces yield and increases production time. Structure loss means the transition from monocrystalline to multicrystalline structure. It is typically preceded by generation of dislocations and slip. Previous work concluded that a pinhole (i.e. a cavity from gas bubble in the melt) was the main cause of structure loss in an n-type Czochralski ingot. In the present study, seven industrial n-type Czochralski silicon ingots were investigated to assess whether pinholes frequently contribute to cause structure loss. Pinholes were found within 5 cm above the position where dislocations were generated in four out of the seven ingots. Slices were cut with the pinholes at one surface and etched with Sopori etchant. The pinholes did not appear to have affected the arrangements of etch pits in these samples and investigation of pinholes did not reveal the cause of structure loss. Furthermore, there were not significantly higher pinhole frequency before structure loss compared to ingots without structure loss among 4041 industrial ingots. Modeling of thermal stresses showed stress concentration around a pinhole, and it gives new insight into the level of stress the ingot can endure without generating dislocations and slip.

Quasi-steady-state photoconductance bulk lifetime measurements on silicon ingots with deeper photogeneration

Quasi-steady-state photoconductance measurements on silicon ingots and blocks with different photo-generation profiles are simulated in this work. The results show that deeper generation profiles, achieved by using a long-pass optical filter with a longer cut-off wavelength, can reduce the impact of the high surface recombination velocity of the ingot surface. This results in higher measured effective lifetimes and reduces the reliance on transfer functions to convert the measured lifetimes into bulk lifetimes. However, there exists a trade-off between generating carriers further from the surface to reduce surface recombination and ensuring that the generated carriers are within the sensitivity range of the photoconductance sensing coil. The simulations are compared with experimental results measured on a monocrystalline silicon block using both quasi-steady-state and transient photoconductance decay, as the transient method is relatively less prone to the impact of surface recombination, and provides a lower bound on the bulk lifetime. The results confirm an increased accuracy in bulk lifetimes extracted from quasi-steady-state measurements when measured using deeper photo-generation profiles.

The dependence of the crystallization front shape on the heat exchange regime in the Bridgman-Stockbarger method

The growth of a silicon ingot in the Bridgman-Stockbarger crystal-melt-crucible system, similar to that used in real technology is studied numerically in the axisymmetric formulation by the finite element method. The simulation was carried out taking into account the heat of phase transition under conditions of unsteady thermal conductivity and natural convection at the initial heating of the melt up to 40 K and two rates of crucible lowering. The temperature gradient along the lower part of the side wall of the crucible is linear and equal to 35 K/cm or 70 K/cm. A comparison between the crystallization processes in the unsteady heat conduction regime and the natural convection regime is carried out. The dependence of the shape of the crystallization front on the heat transfer modes is investigated. It has been shown that in all heat conduction modes being considered, the crystallization front has a convex shape during the entire crystallization process. Moreover, the shape of the crystallization front in the examined range of parameters weakly depends on the crucible lowering rate, but is drastically influenced by the temperature gradient on the crucible walls. Under conditions of hermogravitational convection the shape of the crystallization front strongly depends on the crucible lowering rate and temperature gradient on the side walls of the crucible. At some parameters, a secondary convective vortex can be generated over the crystal during the crystallization process. Depending on the combinations of temperature gradient on the side walls and the lowering rate of the crucible, the shape of the crystallization front can be either concave or convex.

Using Life Cycle Assessment to Determine the Environmental Impacts Caused by Solar Photovoltaic Systems

The paper presents research that investigated the Life Cycle Assessment of multi-crystalline photovoltaic (PV) panels, by considering environmental impacts of the entire life cycle for any solar PVsystems. The overall manufacturing process of a solar PV panel ranging from silica extraction, crystalline silicon ingot growth, wavering to module fabrication and packing of the solar PV panel. The results from this research showed that the module assembly and cell processing of the manufacturing process contributed towards the main environmental impacts of the life cycle of solar PV systems.

The impact of diffusion gettering on solar cell efficiency and light induced degradation

In this paper, the influence of an annealing process at 500 °C for 30 min. after emitter diffusion of multi-crystalline solar cells was investigated. Neighboring sister wafers from a silicon ingot were processed using light doped emitter (LDE) and low temperature annealing (LTA). Emitter profiles measured by Electrochemical Capacitance Voltage (ECV) showed that there is no obvious difference between LDE and LTA due to the lower annealing temperature. The lifetime and iron concentration measurement indicated that the gettering of LTA diffusion is much better than LDE. Therefore, the solar cell efficiency of LTA was up to 19.01%, which is 0.28%abs higher than LDE due to the higher open circuit voltage (Voc) and short-circuit current (Isc). The Internal Quantum Efficiency (IQE) measurement with better long wavelength response showed the wafer quality was much improved after LTA diffusion. To further investigate diffusion gettering performance, light-induced-degradation (LID) was analyzed both for LDE and LTA diffusion. The LID of LDE diffusion with 1.15%rel. is obviously higher than LTA diffusion of 0.46%rel., attributing to the higher iron concentration of LDE diffusion emitter to result in higher iron-boron pair dissociation.

Reduction of chromium ore by recycled silicon cutting sludge waste with carbon addition

A basic study on the feasibility of producing ferrochrome (silicon) alloys using Si sludge waste collected from the silicon ingot cutting process was carried out, and the effects of the addition of carbon components, reaction time, and reaction temperature on the silicothermic reduction of chromium ore by Si sludge were studied. The cordierite (Mg2Al4Si5O18) phase was generated in the slag, and the Fe–Cr(–Si)–C alloy was formed by the silicothermic reduction. Moreover, the addition of carbon powder lowered the reduction initiating temperature, and the reduction ratio based on the oxygen content was evaluated at around 68–88% at 1573 K, which increased with an increase in carbon. However, it was difficult to find a significant difference in the reduction behavior in response to increasing the holding time. The reduced ferrochrome (Fe–Cr) metal alloy droplets coalesced more intensively with an increase in reduction temperature, and for manufacturing the Fe–Cr alloy, it is estimated that a temperature of 1773 K or higher is required for good separation of the slag and the metal. Furthermore, the metallization ratio was defined, and higher values are evaluated for Fe than for Cr.

The influence of travelling magnetic field on phosphorus distribution in n-type multi-crystalline silicon

The influence of different melt streams on the distribution of phosphorus in multi-crystalline silicon ingot was studied. Phosphorus-doped multi-crystalline silicon (mc-Si) ingots were directionally solidified using travelling magnetic fields (TMF) to alter axial phosphorus profiles. Resistivity distributions in the crystallized n-type ingots were measured along the ingot length. Different Lorentz forces were applied in order to enhance melt stirring and with that to transport phosphorus more rapid towards the melt surface. A new rectangular setup was developed, which enables simultaneous directional solidification of 4 G0-sized mc-Si ingots (80x80x60 mm3) under the influence of TMF. 900 g ingots with different initial level of phosphorus doping were grown, and dopant concentrations in ingots were related to stirring intensities. The phosphorus evaporation rate significantly affects axial dopant profile of mc-Si material, thus this approach can be used as a powerful tool to control and tailor resistivity distribution along phosphorus-doped mc-Si ingots.

The influence of Marangoni effect on the growth quality of multi-crystalline silicon during the vacuum directional solidification process

A multi-field coupling model of heat-flow-thermal stress was established to investigate the influence of Marangoni effect on the growth quality of multi-crystalline silicon (mc-Si) during the vacuum directional solidification (VDS) process. The simulation results showed that the Marangoni effect has a significant effect on the distribution of temperature and thermal stress as well during the VDS process due to the increased velocity of melts. The enhanced flow incurred more homogeneous temperature distribution of silicon melts and reduced radial temperature gradient, which then leads to the flatter solid/liquid (s/l) interface. However, as temperature gradient of the crystal increased, thermal stress of the silicon ingot was strengthened as a result. The growth quality of crystal was more desirable when adopting a pulling-down rate of 10 µm/s, in which case the (111) surface was advantageous throughout the entire crystal growth process by XRD detection. Finally, the reliability of the numerical simulation result was verified by the experiment.

Separation and purification of silicon from cutting kerf-loss slurry waste by electromagnetic and slag treatment technology

The recovery of silicon from cutting kerf-loss slurry waste for the polycrystalline silicon production has been becoming a meaningful and urgent issue. In this paper, a united method of combining electromagnetic separation and slag treatment technology was used to separate and purify silicon from silicon kerf. The low melting compound of Na3AlF6 is added to the synthetic binary CaO-SiO2 slag, which can improve the separation effect of silicon from slag phase. The mechanism of physical separation and chemical purification of silicon from cutting kerf-loss slurry waste was investigated. The experimental results show that the electromagnetic stirring has an obvious effect for silicon separation in spite of no slag addition. The addition of CaO-SiO2 or CaO-SiO2-Na3AlF6 slag can further improve the separation rate of silicon. The purity of separated silicon is about 99.47% after the synthetic CaO-SiO2-Na3AlF6 slag treatment and reaches 99.98% followed by a vacuum directional solidification treatment. The recovered silicon from cutting kerf-loss slurry waste by this method can meet the purity requirement to the raw materials for silicon ingot casting.

Calculated and experimental studies at critical facility in view of development of a technology for neutron transmutation doping of a large size silicon specimen in WWR-K reactor

The Institute of Nuclear Physics using WWR-K research reactor were carried out studies to develop the technology of neutron-transmutation doping of silicon ingot with a diameter of 150 mm and a height of 200 mm. As a result, was obtained silicon ingot with an uneven electrical resistivity at the level of ∼6%. In 2017, together with the Chiyoda Technol Corporation, (Japan) were started the activities to develop a technology of neutron-transmutation doping of large sized silicon with a diameter of more than 150 mm and a height of up to 500 mm for the WWR-K reactor. The main goal is to achieve an irregularity electrical resistivity in terms of the volume of the silicon ingot at the level of 3-4 %. At the first stage, were performed the works on the formation of the thermal neutron field in irradiation device. The object of investigation is a silicon ingot with a diameter of 150 mm and a height of 500 mm. For reduction of the height irregularity are used neutron absorbers arranged along the height of the ingot. The development of irradiation device with a neutron absorber and the corresponding neutron measurements were performed at the critical facility. The silicon ingot is modeled by a block of the aluminium. At the second stage it is planned to perform the works on the WWR-K reactor to test the developed technology with the further irradiation of a silicon ingot.

Distribution of Phosphorus in n-Type Multicrystalline Silicon Produced by Directional Solidification

n-type multicrystalline silicon ingot with a doping element of phosphorus was produced by directional solidification under industrial conditions. The evaporation and distribution of phosphorus were studied by content analysis in different positions. The results show that it is mainly controlled by melting and solidification processes. During the melting process, the content of phosphorus reduced from 16.6 and 6.7 ppmw to 11.1 and 6.0 ppmw under different melting times, respectively. The evaporation of phosphorus is mainly affected by the temperature. During the solidification process, the segregation of phosphorus was promoted by evaporation. A formula similar to the Scheil equation was established to calculate the longitudinal distribution of phosphorus in silicon. The results show that its longitudinal distribution is influenced by the evaporation coefficient, the solidification rate, and the temperature gradient. The effective segregation coefficient of phosphorus in ingots a and b was 0.31 and 0.33, respectively, which could be obtained from the fitting result. During the whole process, the phosphorus removal efficiency in ingots a and b was 60.8% and 50.5%, respectively. Its horizontal distribution is related to the morphology of the solid–liquid interface.

Crystallization of multicrystalline silicon from reusable silicon nitride crucibles: Material properties and solar cell efficiency

In this paper experimental results on High Performance multicrystalline silicon ingots (HPmc-Si) grown from reusable silicon nitride crucibles are presented. Five ingots are grown from the same crucible and the material is analyzed for carbon and oxygen, and metallic impurities, by means of Fourier transform infrared (FTIR) spectroscopy and Neutron Activation Analysis (NAA), respectively. Microwave Photoconductance Decay (μW-PCD) and Quasi-Steady-State Photoconductance (QSSPC) is applied on samples to reveal the resulting charge carrier lifetime distribution. Full-area aluminum back surface field (BSF) solar cells are processed from 156 mm × 156 mm wafers and the solar cell efficiency is analyzed. Material properties and solar cell efficiencies are comparable to reference material grown from high purity electronic grade (EG) quartz crucibles. Obtained results are discussed with respect to results published recently.