Czochralski Silicon Research Articles

Oxygen precipitation behavior and its influence on phosphorus gettering in Czochralski silicon

This study investigates the effects of single-step and two-step annealing processes on the formation of oxygen precipitates and their impact on metal impurity gettering in gallium-doped monocrystalline silicon. The research focuses on the competitive relationship between internal gettering by oxygen precipitates and external gettering through phosphorus diffusion. Experimental results show that two-step annealing generates larger and more abundant oxygen precipitates, enhancing the internal gettering effect and reducing the recovery of minority carrier lifetime after phosphorus gettering. Despite this, phosphorus diffusion gettering remains effective in improving minority carrier lifetime, although the degree of improvement depends on the size and quantity of oxygen precipitates. These findings offer valuable insights for optimizing the fabrication process of silicon-based solar cells.

Machine Learning Methods for Structure Loss Classification in Czochralski Silicon Ingots

Machine Learning Methods for Structure Loss Classification in Czochralski Silicon Ingots

Effects of horizontal magnetic field position on oxygen control in 12-inch Czochralski silicon

The application of a horizontal magnetic field is an important method for controlling point defects and impurity distribution in 12-inch Czochralski silicon used in integrated circuits. This work introduces the effects and mechanisms of horizontal magnetic field positioning on oxygen concentration in 12-inch single crystal silicon used for commercial integrated circuits. Effect of melt convection, and temperature distribution on oxygen concentration at the bodying length of 500 mm was presented and analyzed by combining growth experiments and numerical simulations (using CGSim_23_2 from STR) with different horizontal magnetic field positioning. This study found that the mechanism of magnetic field control of oxygen is not directly correlated with the convection suppression effect. It is important to avoid the overlap of strong magnetic regions with high-temperature areas, as this can lead to additional oxygen dissolution.

Investigation of uniformity in fused quartz crucibles for Czochralski silicon ingots

As there is an increased demand for monocrystalline silicon based solar cells, there is an increased interest in stronger and more durable fused quartz crucibles used in the Czochralski process. In this study we focused on the uniformity of hydroxyl impurities in these crucibles, and its potential influence on viscosity of the crucible by combining data from IR-microscopy and a self-made viscosity measurement setup. A half of a fused quartz crucible manufactured from two types of sands was studied, the OH groups content was mapped, and the viscosity at 1500 °C was measured. Local variations in the OH groups content of on average 26.7 ppm difference between the top and the bottom of the crucible were detected. The viscosity measurements further confirmed the crucible’s inhomogeneity as no clear trends or direct correlations to the OH groups content were observed. The correlation between the different sand types used for crucible manufacturing and the OH groups content was also investigated in four samples originating from four different crucibles. Two different trends in the OH groups distribution were found: while two of the studied crucibles showed an increased OH groups content in the boundary between the bubble free and the bubble containing layer, the other two showed an increased and maximum OH groups content in the bubble containing layer, and the link between the sand types and the OH groups content was confirmed. The source of the local inhomogeneities and the two different trends of OH groups distribution can possibly be attributed to the sand quality, the sand particle size, and the difference in the manufacturing process, such as thermal history or distribution of sand.

Experimental Determination of Si Self-Interstitial Emission During Oxide Precipitation in Czochralski Silicon

We used the method of Torigoe and Ono [J. Appl. Phys., 121, 215103 (2017)] to investigate the kinetics of β, the number of self-interstitials emitted per precipitated oxygen atom, during oxide precipitation in Czochralski silicon. For this purpose, we used pp- epitaxial wafers with a buried highly B-doped epitaxial layer which were annealed with and without thermal pre-treatments at 950 °C. From the results we conclude that in the initial phase of oxide precipitation without thermal pre-treatment β is very high before it drops to low values. With a thermal pre-treatment at 800 °C for 2 h, the initial value of β is somewhat lower before the drop also occurs. If a nucleation anneal is carried out before the thermal treatment at 950 °C the β values are low from the beginning. All of these results confirm our previously published theoretical predictions experimentally. This work also shows that the crystal pulling process can affect the initial β value because grown-in oxide precipitate nuclei can reduce their strain by vacancy absorption. Therefore, high vacancy supersaturation during crystal cooling while oxide precipitate nucleate would lead to somewhat lower initial β values.

Defect behavior during growth of heavily phosphorus-doped Czochralski silicon crystals. I. Experimental study

Defect behavior during growth of heavily phosphorus-doped Czochralski silicon crystals. I. Experimental study

Defect behavior during growth of heavily phosphorus doped Czochralski silicon crystals (II): Theoretical study

Defect behavior during growth of heavily phosphorus doped Czochralski silicon crystals (II): Theoretical study

Surface Molecular Engineering for Fully Textured Perovskite/Silicon Tandem Solar Cells.

Developing large-scale monolithic perovskite/silicon tandem devices based on industrial Czochralski silicon wafers will likely have to adopt double-side textured architecture, given their optical benefits and low manufacturing costs. However, the surface engineering strategies that are widely used in solution-processed perovskites to regulate the interface properties are not directly applicable to micrometric textures. Here, we devise a surface passivation strategy by dynamic spray coating (DSC) fluorinated thiophenethylammonium ligands, combining the advantages of providing conformal coverage and suppressing phase conversion on textured surfaces. From the viewpoint of molecular engineering, theoretical calculation and experimental results demonstrate that introducing trifluoromethyl group provide more effective surface passivation through strong interaction and energy alignment by forming a dipole layer. Consequently, the DSC treatment of this bifunctional molecule enables the tandem cells based on industrial silicon wafers to achieve a certified stabilized power conversion efficiency of 30.89 %. In addition, encapsulated devices display excellent operational stability by retaining over 97 % of their initial performance after 600 h continuous illumination.

Influence of Illumination Spectrum on Dissociation Kinetics of Iron–Boron Pairs in Silicon

Photodissociation kinetics of iron–boron (FeB) pairs in boron‐doped Czochralski silicon is studied experimentally using different light sources. It is shown that the FeB dissociation rate depends not only on integrated light intensity and overall carrier generation rate, but also on spectral composition of illumination. The value of the material constant of dissociation K varies and has been determined to be within s. The investigation reveals an increase in the dissociation rate with increase in photon energy. The results indicate that recombination‐enhanced defect reaction is the primary factor in the second stage of pair dissociation.

Why is gallium-doped silicon (sometimes) stable? Kinetics of light and elevated temperature induced degradation

Light and elevated temperature induced degradation (LeTID) can induce high power losses in photovoltaic modules built from various types of silicon wafers. After the industry's rapid transition from Boron- to Gallium-doping, it is still unclear how the new dopant atom affects the degradation process and why it entails an apparently higher resistance to LeTID. We treat identically processed Gallium-doped Czochralski silicon wafers at 16 different conditions by screening temperature and minority charge carrier density (in the following “injection”). The illumination source is regulated to keep the injection constant at each condition. This method allows for an improved quantitative analysis of the LeTID kinetics based on effective lifetime measurements. Our results show that Gallium-doping shifts the equilibrium between the formation of LeTID defects and their temporary recovery (TR) to the latter, leading to a reduced degradation extent at temperatures up to 80 °C. The efficacy of this TR-induced LeTID suppression depends delicately on both temperature and injection which explains why Gallium-doped silicon appears to be LeTID-immune at high illumination intensities. By accounting for the influence of TR, we extract activation energies and injection exponents that relate to the dominant defect transitions separately, revealing a large discrepancy to effective values reported in literature. The increased accuracy of our kinetic parameters enhances the reliability of existing efficiency models. Finally, we find that a degradation as expected under field conditions can be probed at a 100-fold accelerated rate in the lab. By shining light on LeTID kinetics in Gallium-doped silicon, we explain the dopant atoms' influence on the degradation behavior, establish a basis for precise yield modelling and formulate guidelines for accelerated test protocols.

Detection of Microcracks in Cz‐Si Wafer Manufacturing by Photoluminescence Imaging

Photoluminescence imaging (PLI) is a widely accepted, fast, and contactless method for detecting crystal defects in crystalline silicon solar cells and solar‐grade silicon wafers. However, it is less known by semiconductor wafer manufacturers despite the similarities between photovoltaic (PV) and semiconductor wafers. This study focuses on the detection of microcracks by PLI during high‐quality Czochralski silicon (Cz‐Si) wafer manufacturing. The results show that in case of low resistivity (<25 mΩ cm) wafers, microcracks can be detected at any stage of the processing—even after diamond‐wire slicing. When resistivity increases, visibility of microcracks reduces in process steps that produce uneven surfaces. Nevertheless, they can still be detected after slurry‐wire slicing, lapping, alkaline etching, and polishing. According to the results, unlike resistivity, other material parameters such as dopant species, crystal orientation, and wafer thickness have no similar impact on visibility of microcracks in PLI. Furthermore, all wafers produce a decent photoluminescence (PL) signal without a need for separate sample preparation. Based on these results, general recommendations for the in‐line detection of microcracks for Cz‐Si wafer manufactures are provided. While this study focuses on microcracks, the results and discussion include broader perspectives on the defect characterization in Cz‐Si wafer manufacturing via PLI.

Physical mechanism underlying the enhancement effect of carbon in heavily phosphorus-doped Czochralski silicon substrate on phosphorus out-diffusion within n/n+ epitaxial wafer

The effects of carbon in heavily phosphorus-doped Czochralski (HP-Cz) silicon (Si) substrates, with the concentrations across an order of magnitude from 1016 to 1017 cm−3, on the out-diffusion of phosphorus impurities within n/n+ epitaxial Si (Epi-Si) wafers have been investigated. It is found that the increase in the carbon concentration ([C]) from 1.0 × 1016 to 1.0 × 1017 cm−3 leads to the enhanced phosphorus out-diffusion within the n/n+ Epi-Si wafer when subjected to anneal at 1100 °C in an N2 or an O2 ambient or to anneal at 1150 °C in an N2 ambient, but hardly affects the phosphorus out-diffusion within the n/n+ Epi-Si wafer when subjected to anneal at 1150 °C in an O2 ambient. Based on the density functional theory calculations, it is derived that the increase in the [C] from 1.0 × 1016 to 1.0 × 1017 cm−3 in the HP-Cz Si substrate results in a significantly increased thermal equilibrium concentration of self-interstitial silicon (SiI) atoms at 1100 or 1150 °C, which, in turn, leads to the increased concentration of the SiI atoms that out-diffuse from the substrate to the epitaxial layer because the SiI atoms diffuse extremely fast in Si. Such a carbon-induced increase in the concentration of SiI atoms is believed to be responsible for the aforementioned enhanced phosphorus out-diffusion, which is predominantly dictated by the interstitialcy mechanism. In the case of anneal in the O2 ambient at sufficiently high temperatures such as 1150 °C, a large number of excessive SiI atoms injected into the Epi-Si wafer substantially mask the carbon enhancement effect on the phosphorus out-diffusion. Of technological significance, it is deduced that the [C] should be not higher than 1.0 × 1016 cm−3 in the HP-Cz Si wafers as the substrates of n/n+ Epi-Si wafers used for power devices.

Dosimetric parameters and radiation tolerance of epitaxial diodes for diagnostic radiology and computed tomography beams

A custom-made EPI diode-based dosimetry system is thoroughly characterized for diagnostic radiology and computed tomography beams. The diode has a thin n-type epitaxial layer (50 μm) grown on a thick (300 μm) Czochralski silicon substrate. It operates as an online radiation dosimeter in the short-circuit current mode. In this case, the key dosimetric quantity is the dose rate, correlated with the output current from the diode exposed to radiation. The corresponding collected charge (the integral of the current signal) is proportional to the dose. Irradiations are performed with the Pantak-Seifert 160HS Isovolt X-ray generator previously standardized by Radcal RC6-RD and RC3-CT ionization chambers. The data gathered with all radiation quality beams confirm the linearity with dose and dose rate despite a slight energy dependence. Independently from the beam energy, the dosimetric parameters of repeatability (<0.3%), long-term stability (0.4%/year), angular response (<3%, ± 5°), dose rate dependence (<3%), and signal-to-noise ratio (≥4900) fully adhere to the IEC 61674 recommendations. Compliance with the accumulated dose stability requirement (1.0%/40 Gy) is almost achieved with the pristine diode and effectively accomplished through radiation conditioning with 60Co gamma rays. Under the latter condition, the lifespan of the diode can easily reach 15 kGy, assuring the high reusability of this diode for diagnostic radiology and computed tomography dosimetry before requiring recalibrations.

Influence of Thermal Conductivity and Emissivity of Heat Shield Surface Material on the Thermal Field of Czochralski Silicon Crystal Growth

Influence of Thermal Conductivity and Emissivity of Heat Shield Surface Material on the Thermal Field of Czochralski Silicon Crystal Growth

Effect of horizontal magnetic field position on oxygen distribution in CZ silicon crystal growth

The horizontal magnetic field-assisted Czochralski(CZ) method is gaining significant popularity in the semiconductor industry for producing 200 mm semiconductor-grade silicon wafers. In this study, the impact of the horizontal magnetic field position on the oxygen content and radial uniformity of semiconductor-grade silicon crystals produced by the CZ method was investigated. Three-dimensional simulations were conducted using finite element software to calculate the melt convection and oxygen content before and after adjustment of the magnetic field position, respectively. The simulations results predicted a decrease in the oxygen content of the crystals following the adjustment of the magnetic field position from 0 to 75 mm below the free surface, and this was experimentally verified. The velocity distribution of the free melt surface exhibits better uniformity when lowering the magnetic field position. The influence of the magnetic field position on the temperature distribution and Marangoni flow in the melt was investigated, and the result of oxygen concentration difference identified the adjustment of the magnetic field position promoted the greater Marangoni flow below the free melt surface, which promoted the volatilization of SiO gas and thus reduced the oxygen concentration in the silicon ingot. Therefore, adjusting the position of the magnetic field is an effective way to increase the Marangoni flow and to obtain semiconductor-grade silicon crystal with lower oxygen content and better oxygen radial homogeneity.

The effects on heat and oxygen transport of different heat shield and sidewall insulation designs during continuous Czochralski silicon crystal growth

Numerical work has been done to optimize the hot zone for the continuous Czochralski growth of 8-inch diameter silicon crystals using a double-crucible system. Thicker insulation layers on a high-heel-shaped heat shield and the sidewall insulation led to a reduction in the heater power of more than 40%, a decrease in the oxygen content, and a significant flattening of the growth interface. This design also allowed a stronger argon gas flow near the outer free melt surface and the quartz crucible wall, which prevented the generation of an argon gas vortex in this area. Stronger flow of argon gas would assist in moving more SiO gas and dust out of the chamber. However, the reduction in the outer melt temperature compared to the original design could slow down the melting of the silicon granules.

Study on the mechanism of second phase formation in high-purity fused silica materials for semiconductor application

Gas bubble is a common defect in high-purity fused silica materials, but the formation of the second phase particles related to gas bubble and its mechanism remain unclear till now. In this work, we have presented a method for characterization of colored gas bubbles in high-purity fused silica crucibles for Czochralski silicon by the means of TEM and EDX. Based on it, the origin of the coloring of the gas bubbles is demonstrated to be induced by the gathering of Fe impurity and then, the formation of high-density second phase—ferric oxide (Fe2O3) crystals around the gas bubbles. Moreover, our results provide insight into the formation mechanism of second phase crystals which can be induced by stress field variations around gas bubbles during the cooling process that probably promoted by volume shrinkage effect and increasing solubility of gas from gas bubbles into fused silica substrate. The understanding of the second phase particle formation mechanism in high-purity fused silica materials is valuable for its further controlling.

Adjustment of oxygen transport phenomena for Czochralski silicon crystal growth

During silicon crystal growth, oxygen, a well-known major impurity, affects the final silicon wafer's mechanical and electrical properties. This study focused on regulation of discharge of different concentrations of oxygen from the quartz crucible into the silicon melt while considering the crucible angular speed and the friction at the melt–crucible interface. The three-dimensional transient governing equations for heat transfer, fluid flow, and impurity transportation in the Czochralski (CZ) puller were solved numerically. The oxygen solvation equation representing the crucible to silicon melt was modified to evaluate the accuracy of oxygen concentration calculations during the CZ process. Experimental measurements using the Fourier-transform infrared (FTIR) technique were used to confirm the simulation results. The results demonstrate that the crucible angular speed affects the oxygen concentration near the crucible wall and therefore in the silicon ingot. The proposed modifications for evaluating oxygen concentration offer a more comprehensive understanding of the oxygen dynamics during the CZ crystal growth.

Surface Examination of Structure Loss in N-Type Czochralski Silicon Ingots

In principle, growing a dislocation-free Czochralski silicon ingot is possible if the growth process is kept stable and below the critical resolved shear stress value. However, in practice, a considerable proportion of the Si ingots are remelted due to the generation of dislocations or the so-called structure loss. The assessment of the failed ingots is a crucial step toward higher yield. However, the characterization of Si ingots is challenging due to their high brittleness and the high concentration of dislocations related to slip. In this work, we develop a non-destructive method to investigate the ingots that have experienced structure loss and reveal the root causes of this failure. Many characteristic features have been found on the surface of Czochralski silicon ingots. Based on these features, the ingots are classified into seven major groups that could be related to the main causes of the structure loss. Furthermore, the temperature gradient of several ingots is revealed by careful measurements of the growth ridges’ widths of these ingots. The results show that most of the failed ingots experience low-temperature gradients before the dislocation generation which agrees with the previous results. Three ingots have a clear particle hit on the surface, which caused an immediate transition to a multi-crystalline silicon structure. Particles are found on atomically smooth and rough interfaces, growth ridges, and surfaces in between. The surface examination method is a promising, fast, low-cost, and non-destructive technique that can be used to identify the most critical factors of structure loss in industrial ingots.

A Soft Measurement Method for the Tail Diameter in the Growing Process of Czochralski Silicon Single Crystals

In the Czochralski silicon single crystal growth process, the tail diameter is a key parameter that cannot be directly measured. In this paper, we propose a real-time soft measurement method that combines a deep belief network (DBN) and a support vector regression (SVR) network based on system identification to accurately predict the crystal diameter. The main steps of the proposed method are as follows: First, we address the delay problem of the effects of the temperature and crystal pulling speed on the tail diameter growth by using a back propagation (BP) neural network based on the mean impact value (MIV) method to determine the optimal delay time. Second, we construct a prediction model of the tail diameter by using the DBN network with the temperature and crystal pulling speed as input variables in the crystal growth process. Third, we improve the DBN network by using the SVR network to enhance its linear regression capability. We also employ the ant colony optimization (ACO) algorithm to obtain the optimal parameters of the SVR network. Finally, we compare the performance of the DBN-ACO-SVR network based on system identification with the DBN and SVR networks, and the results show that our method can effectively deal with the delay problem and achieve the accurate prediction of the tail diameter in the Czochralski silicon single crystal growth process.