Czochralski Silicon Research Articles

Real-time prediction of interstitial oxygen concentration in Czochralski silicon using machine learning

We developed a machine learning model to predict interstitial oxygen (Oi) concentration in a Czochralski-grown silicon crystal. A highly accurate prediction can be ensured by selecting the appropriate experimental parameters that represent the change in the furnace conditions. A neural network was trained using the dataset of 450 ingots, and its prediction error for the testing dataset was 4.2 × 1016 atoms cm−3. Finally, a real-time prediction system was developed wherein the crystal growth data are input into the model, and the Oi concentration at the current growth interface is calculated immediately.

Passivation of n- and p-Type Silicon Surfaces With Spray-Coated Sol-Gel Silicon Oxide Thin Film

Thermally grown SiO2 is widely used for silicon surface passivation in solar cells and other applications due to excellent interfacial properties. SiO2 films with thickness in the range of 17 nm deposited by industrially viable spray-coating technique on both n- and p-type silicon are reported. Low values of interface state density of ${1.4}\times {10}^{{10}}$ cm $^{\boldsymbol -{2}}$ eV $^{\boldsymbol -{1}}$ on n-type and ${2.0}\times {10}^{{10}}$ cm $^{\boldsymbol -{2}}$ eV $^{\boldsymbol -{1}}$ on p-type were achieved. Fixed oxide charges in the range of 7.1– ${9.4}\times {10} ^{{11}}$ cm $^{\boldsymbol -{2}}$ and 3.9– ${6.5}\times {10} ^{{11}}$ cm $^{\boldsymbol -{2}}$ on n- and p-type, respectively, are obtained. Excellent passivation results in the effective surface recombination velocity of 0.97 cms $^{\boldsymbol -{1}}$ and 8.07 cms $^{\boldsymbol -{1}}$ at minority carrier concentration of $10^{{15}}$ cm $^{\boldsymbol -{3}}$ on n- and p-type Czochralski silicon, respectively, without the use of capping layer. SiO2 film exhibits dielectric breakdown field strength of 4.3 MVcm $^{\boldsymbol -{1}}$ and 5.6 MVcm $^{\boldsymbol -{1}}$ and leakage current density of ${2.2}\times 10 ^{\boldsymbol -{8}}$ Acm $^{\boldsymbol -{2}}$ and ${1.1}\times {10} ^{\boldsymbol -{8}}$ Acm $^{\boldsymbol -{2}}$ at 1 MVcm $^{\boldsymbol -{1}}$ on n- and p-type silicon, respectively. These values are superior to sol-gel-based SiO2 films reported previously and are comparable to or better than those reported for other methods for the growth or deposition of SiO2 on silicon. Amorphous nature of the film is validated by high-resolution transmission electron microscopy (TEM). The X-ray photoelectron spectroscopy (XPS) analysis shows nearly stoichiometric SiO x film.

Effect of rotating magnetic field on instabilities of thermocapillary flow in a Czochralski silicon melt pool

The effect of the rotating magnetic field on the thermocapillary flow instabilities in a Czochralski silicon melt pool was numerically investigated by means of linear stability analysis. We considered three cases, namely, the non-rotating crystal, the crystal rotating in the same direction as the magnetic field (co-rotating crystal), and the crystal rotating in the opposite direction of the magnetic field (counter-rotating crystal). The linear stability analysis results show that the basic flow becomes unstable against two oscillatory instability modes. With the increase of the magnetic Taylor number (Ta), the overall trend of the critical Marangoni number decreases in the cases of a non-rotating crystal and counter-rotating crystal. However, in the case of a co-rotating crystal, the rotating magnetic field significantly improves the flow stability, and an inversion of the propagation direction of the oscillatory wave is observed with the increase of Ta. Energy analyses reveal that the instabilities are hydrodynamic in nature, which are caused by the combination of the strain-induced instability mechanism and the centrifugal instability mechanism, and the rotating magnetic field only provides little energy for the flow instabilities even at large Ta.

Geometric determination of direction of dislocations using synchrotron X-ray transmission topography.

When performing transmission polychromatic beam topography, the extensions to the line segments of the diffraction images of a straight dislocation are shown to intersect at a single point on the X-ray film. The location of this point, together with the diffraction pattern recorded on the film by synchrotron radiation, gives the crystallographic direction [hkl] of the dislocation unambiguously. The results of two synchrotron topography experiments are presented. Very long dislocations found in the center of a large 450 mm-diameter Czochralski silicon crystal align with the growth direction [001]. In the other silicon sample, the dislocations are of mixed type and along the [011] direction.

AC-coupled n-in-p pixel detectors on MCz silicon with atomic layer deposition (ALD) grown thin film

We report initial characterization of our novel sensor process solutions with AC-coupled n+/p−/p+ pixel detectors made on 150 mm diameter p-type Magnetic Czochralski silicon (MCz-Si) wafers. The pixels were segmented in a 52 × 80 dual column array and designed to be AC capacitive coupled. The resistive coupling between pixels, allowing quality assurance probing prior the flip chip bonding, was realized with thin film metal-nitride resistors fabricated by sputtering deposition. This approach allows us to omit punch-through resistor structures, which reduces the overall process complexity. Moreover, our previous studies have emphasized that applying ALD Aluminum Oxide (Al2O3) field insulator and passivation layer results in negative net oxide charge and thus additional p-spray or p-stop surface current termination structures are not necessary. Our focused application is a radiation-hard ALD AC-coupled pixel detector to be used in future particle physics experiments, such as the High-Luminosity Large Hadron Collider (HL-LHC), as well as photon counting applications. The pixel detectors were tested at Helsinki Institute of Physics (HIP) Detector laboratory and Ruđer Bošković Institute (RBI). We show measurement data of pixel detectors and other test structures. For the TiN resistors surrounding pixels, the resistance values were measured to be about 15kΩ. Data of electrical properties, full depletion voltage and leakage current are shown as well. Our Transient Current Technique (TCT) measurements indicated clear pixel segmentation with excellent homogeneity. For further study, AC-coupled sensors were hybridized to PSI46dig read out chips (ROC) by flip-chip interconnection technique and tested with a radioactive source.

Numerical method for thermal donors formation simulation during silicon Czochralski growth

Various upgrades of the Czochralski (Cz) growth process are currently being investigated in order to increase throughput and reduce production cost of high efficiency silicon-based solar cells. However, as-grown thermal donors (TD) in Cz silicon can significantly reduce the conversion efficiency of such solar cells. An accurate simulation tool is therefore required to investigate and optimize TD formation during crystal growth. A numerical method combining thermo-hydraulic simulations and a kinetic TD formation model was improved by the implementation of a more appropriate TD formation model, identified through a benchmark of the different models available in the literature. Three different Cz growth processes were investigated both numerically and experimentally. Numerical results are in remarkable agreement with TD concentrations measured along the three ingots by the OxyMap technique developed at CEA. The simulations were then used to detect when TD were formed during Cz processes. The reliability of the method was also assessed through sensitivity analyses, highlighting the critical importance of the input interstitial oxygen concentration. These results show that accurate estimates of axial TD concentration profiles can be obtained and so for very different Cz processes, supporting the robustness of the developed method and its relevance to process optimization and furnace design to reduce TD concentrations.

Influence of Tabula Rasa on Process- and Light-Induced Degradation of Solar Cells Fabricated From Czochralski Silicon

Monocrystalline Si solar cells are fabricated from Czochralski (Cz) Si, which contains 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">17</sup> -10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">18</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> oxygen atoms. Cz Si undergoes degradation during high-temperature thermal processing steps, such as dopant diffusion to form the p-n junction. This degradation in the bulk minority carrier lifetime can be related to the formation of oxygen precipitates. We found that a high-temperature annealing process known as tabula rasa (TR) not only mitigates process-induced degradation via oxygen precipitate nuclei dissolution, but also modifies subsequent light-induced degradation. We report on the bulk carrier lifetime of n- and p-type Cz Si after TR, which homogenizes the interstitial oxygen in the bulk Si to its monoatomic form in either an N2 or O2 environment. A control sample, which was not subjected to a TR processing step, experienced severe process-induced degradation during a boron emitter thermal budget as oxygen precipitates were formed in the Si bulk. These precipitates could be imaged using photoluminescence. Additionally, samples that underwent a TR processing step prior to the boron emitter thermal budget show efficient gettering of metallic impurities compared to the control sample, which showed a decline in the implied open-circuit voltage after the gettering step. Furthermore, modification of the interstitial oxygen bonding by TR had a strong effect on the light-induced degradation kinetics. Instead of a typically observed monotonic decrease, minority carrier lifetime increases first, followed by a nonmonotonic decrease over a ~20 h period. We conclude that by modifying the interstitial oxygen bonding via TR pretreatment, p-type Cz Si wafers become substantially resistant to harsh solar cell processes and strongly modified light-induced degradation, which would open alternative ways to mitigate this degradation mechanism.

Optimization of the melt/crystal interface shape and oxygen concentration during the Czochralski silicon crystal growth process using an artificial neural network and a genetic algorithm

The melt/crystal interface shape and oxygen concentration during the Czochralski silicon crystal growth process significantly influence the crystal quality. In this paper, an optimization system with the combination of an artificial neural network and a genetic algorithm is proposed to optimize the growth parameters during the growth process. Flattening the melt/crystal interface and reducing the oxygen concentration along the interface were chosen as the optimization targets. Two important growth parameters, the crystal rotation rate and crucible rotation rate, were chosen as optimization variables. First, a global heat and mass transfer model was developed to simulate the crystal growth process and then tested with experimental data. The verified heat and mass transfer model was then used to train an artificial neural network with the aim of rapidly assessing the complex nonlinear dependence of the interface shape and oxygen concentration on the growth parameters. The trained neural network combined with a genetic algorithm was then used to obtain the optimal growth parameters. Both deflection of the melt/crystal interface and the oxygen concentration along the interface decreased after optimization. Finally, the optimal growth parameters were checked in the heat and mass transfer model to evaluate the performance of the optimization system. The proposed method will also be useful for optimization of other crystal growth processes.

Large‐Area Boron‐Doped 1.6 Ω cm p‐Type Czochralski Silicon Heterojunction Solar Cells with a Stable Open‐Circuit Voltage of 736 mV and Efficiency of 22.0%

Herein, large‐area defect‐engineered p‐type silicon heterojunction (SHJ) solar cells using standard 1.6 Ω cm commercial‐grade boron‐doped Czochralski (Cz) silicon wafers are fabricated. It is demonstrated that despite achieving an open‐circuit voltage of 735 mV with an efficiency of 21.6% for gettered samples, without appropriate treatment, the cells are heavily susceptible to boron–oxygen‐related light‐induced degradation (LID), with the effective lifetime at maximum power point decreasing to 13 μs. This degradation results in a loss of efficiency of more than 3.1%abs (14.3%rel) after 48 h of light soaking. However, the addition of an advanced hydrogenation postcell fabrication process increases the efficiency by 0.2%abs to 21.8%, and dramatically reduces susceptibility of LID, decreasing the extent of degradation to 0.2%abs (0.9%rel). A peak stable independently measured efficiency of 22.0% with an open‐circuit voltage (VOC) of 736 mV is achieved with the addition of a dedicated high‐temperature prefabrication hydrogenation. These results indicate that p‐type Cz wafers can be used to fabricate stable, next‐generation high‐efficiency solar cells using silicon heterojunctions or other passivated contact architectures requiring VOCS well above 700 mV.

Recognition and Imaging of Point Defect Diffusion, Recombination, and Reaction During Growth of Czochralski-Silicon Crystals

The behavior of point defects was visualized in lightly and heavily boron (B)-doped Czochralski-silicon (CZ-Si) crystals by employing a special growth technique, namely, rapidly cooling a growing crystal after it is detached from the Si melt. In the case of crystal growth with a high pulling rate, an anomalous oxygen precipitation (AOP) region dominated by vacancies appeared, whereas in the case of crystal growth with a low pulling rate, a dislocation loop region dominated by self-interstitials appeared. In the crystals cooled rapidly after halting growth for several hours, self-interstitials flowed into the AOP region and dislocation loop regions formed and expanded, while the AOP region shrunk due to diffusion of excess vacancies to the crystal surface and void regions. These transient changes in the point defect distribution were reproduced using a point defect simulator. Defect regions related to self-interstitials could not be confirmed in the heavily B-doped crystals with resistivities of 10 mΩcm or less, where the void- and oxidation-induced stacking fault (OSF)-ring regions disappeared completely at the center of the crystal. These results show that the behaviors of point defects in heavily doped CZ-Si crystals with various impurities are important research subjects in relation to future advanced power applications.

Investigation of nitrogen enriched silicon for particle detectors

This article explores the viability of nitrogen enriched silicon for particle physics application. For that purpose silicon diodes and strip sensors were produced using high resistivity float zone silicon, diffusion oxygenated float zone silicon, nitrogen enriched float zone silicon and magnetic Czochralski silicon. The article features comparative studies using secondary ion mass spectrometry, electrical characterization, edge transient current technique, source and thermally stimulated current spectroscopy measurements on sensors that were irradiated up to a fluence of 1015 neq/cm2. Irradiations were performed with 23 MeV protons at the facilities in Karlsruhe (KIT), with 24 GeV/c protons at CERN (PS-IRRAD) and neutrons at the research reactor in Ljubljana. Secondary ion mass spectrometry measurements give evidence for nitrogen loss after processing, which makes gaining from nitrogen enrichment difficult.

Effects of vacancy defects on the mechanical properties in neutron irradiated Czochralski silicon

We have investigated the mechanical properties of neutron irradiated Czochralski (NICZ) silicon using nanoindentations combined with micro-Raman spectroscopy. It is found that NICZ silicon shows higher hardness (∼13% higher) than non-irradiated silicon, with a slightly lower Young’s modulus. When the samples were subjected to isochronal anneals in the temperature range of 250 °C–650 °C, the hardness of NICZ silicon gradually decreases as the temperature increases and it is finally comparable to that of the non-irradiated silicon. The vacancies and vacancy–oxygen defects induced by neutron irradiation in NICZ silicon annihilate or transform into more complex defects during the annealing processes. It suggests that the vacancy defects play a role in the evolution of hardness, which promote phase transition from the Si-I phase to the stiffer Si-II phase in NICZ silicon during indentation. In addition, the irradiation induced vacancy defects could lead to the lower Young’s modulus.

Solid state diffusion of metallic impurities from crucible and coating materials into crystalline silicon ingots for PV application

A setup for annealing experiments was developed which allows the detailed investigation of metal impurity solid state diffusion into silicon crystals. In order to simulate the impurity diffusion into multicrystalline or mono-cast silicon ingots during directional solidification processes, small cuboidal Czochralski silicon ingots with extremely low metal contamination, placed on various substrates, were annealed for several hours at temperatures just below the melting point of silicon. As substrates, different crucible materials, partly including SiO2 diffusion barrier layers as well as different Si3N4-coatings with widely varying content of metals (Fe, Cr, Ti, Cu, Co and Ni) were used. After ingot annealing, vertical samples were prepared out of the ingot center and the red zone dimensions were determined by lifetime measurements. Additionally, highly sensitive neutron activation elemental analysis was carried out on the silicon samples. In combination with the concentrations of transition elements of various crucible materials and silicon nitride powders the solid state diffusion process of these impurity species into and through the silicon was investigated. The results indicated, that the red zone area mainly contains the fast and intermediate diffusing elements Fe, Co, Cr and Cu. By variation of certain metal element concentration (Fe, Cr, Co, Ni, Cu and Ti) in the substrates it turned out that Fe and Co have the major impact on red zone formation and charge carrier lifetime reduction. Whereas for increasing Fe concentrations in the substrate material (>1017 at/cm3) no further increase of red zone can be observed due to the formation of precipitates in the lower areas of the red zone, fast diffusing Co atoms are able to reduce also the charge carrier lifetime in the center of the silicon samples.

Formation of thermal donor enhanced by oxygen precipitation in silicon crystal

The formation of thermal donors in silicon is investigated using Czochralski silicon crystals grown with different grown-in defect regions, such as voids and nuclei of oxidation-induced stacking faults. It was found that the formation rate of thermal donors during annealing at 450 °C increases with an increase in the density of oxide precipitates in the regions containing different grown-in defects. The thermodynamic model for the formation of thermal donors shows that the electrically inactive oxygen trimers as nuclei of thermal donors in an as-grown crystal increase with an increase in the density of oxide precipitates, suggesting that the formation of the nuclei is enhanced due to silicon self-interstitials emitted by oxygen precipitations during the crystal growth.

Seed‐Assisted Growth of Cast‐Mono Silicon for Photovoltaic Application: Challenges and Strategies

Silicon has dominated the photovoltaic material market for a few decades. In contrast to perfectly crystallized Czochralski (CZ) silicon, casting multi‐crystalline silicon attracts much more attention due to its low cost and high throughput, which meets the increasing demands of the rapidly developing photovoltaic industry. However, the unavoidable appearance of crystallographic defects seriously limits the performance improvement of solar cells. In this Review, a general overview of the recent efforts in high‐quality casting silicon crystal techniques is provided, including the dendritic cast method, high‐performance multi‐crystalline silicon, and cast‐mono silicon. Then, special attention is focused on the seed‐assisted growth of cast‐mono silicon, which shares some of the most favorable features of high solar cell efficiency offered by well‐established CZ silicon and cost‐effectiveness of cast multi‐crystalline silicon. Nevertheless, this innovative growth technique facing a few challenges once impeded its mass scale production. The main issues concerning nucleation and growth control, extended defects as well as impurity contamination are addressed, which pave the way for this high‐potential technology in applications for high‐efficiency and low‐cost solar cells.

Ab initio study of the effect of hydrogen passivation on boron-oxygen-carbon related defect complexes in silicon

We present hybrid density functional theory (DFT) study of defect complexes formed by oxygen, boron and carbon, in ion-implanted and electron-irradiated Czrolaski (CZ) silicon containing boron. The effect of hydrogen passivation on the defect complexes is also studied. The defects considered are the interstitial boron-interstital oxygen (BiOi) complex as well as interstitial boron-substitutional carbon (BiCs) complex, which are found in ion-implanted Czochralski silicon. Other defect complexes which are found in electron-irradiated CZ silicon and have been studied are the interstitial boron (Bi), substitutional boron-interstitial oxygen (BsOi), substitutional boron-interstitial boron (BsBi), interstitial boron-interstitial oxygen dimer (BiO2i), and substitutional boron-interstitial oxygen dimer (BsO2i). We found the defects to be stable in neutral charge state with the stability of the Bi, BiOi, BsBi, BiO2i and BsO2i increasing after hydrogen passivation. However, the stability of BsOi and BiCs defect complexes decreases after the hydrogen passivation. For the BsBi defect complex in particular, a deep donor level appeared after hydrogen passivation. Furthermore, we found a shift in the charge state transition levels in to the valence band for the donor levels, and a shift in to the conduction band for the acceptor levels for all the defects after hydrogen passivation. We compare our results with available theoretical and experimental studies.

Development of advanced hydrogenation processes for silicon solar cells via an improved understanding of the behaviour of hydrogen in silicon

AbstractThe understanding and development of advanced hydrogenation processes for silicon solar cells are presented. Hydrogen passivation is incorporated into virtually all silicon solar cells, yet the properties of hydrogen in silicon are still poorly understood. This is largely due to the complex behaviour of hydrogen in silicon and its ability to exist in many different forms in the lattice. For commercial solar cells, hydrogen is introduced into the device through the deposition of hydrogen‐containing dielectric layers and the subsequent metallisation firing process. This process can readily passivate structural defects such as grain boundaries but is ineffective at passivating numerous defects in silicon solar cells such as the boron‐oxygen complex, responsible for light‐induced degradation in p‐type Czochralski silicon. This difficulty is due to the need to first form the boron‐oxygen defect and also due to atomic hydrogen naturally occupying low‐mobility and low‐reactivity charge states. However, these challenges can be overcome using advanced hydrogenation processes incorporating excess carrier generation from illumination or current injection that increase the concentration of the highly mobile and reactive neutral charge state. As a result, after fast firing, additional low‐temperature advanced hydrogenation processes incorporating illumination can be implemented to enable the passivation of difficult defects like the boron‐oxygen complex. With the implementation of such processes for industrial silicon solar cells, efficiency improvements of 1.1% absolute can be obtained.

Investigation of gas bubble growth in fused silica crucibles for silicon Czochralski crystal growth

Abstract Gas bubbles in crucibles for Czochralski (Cz) silicon growth are both necessary and detrimental: In the outer, bubble-containing (BC) layer of the crucible, they are required for mechanical stability, while in the inner, bubble-free (BF) layer, bubbles can cause the release of particles from the crucible into the melt which may disrupt the single-crystalline growth. In this work, a vacuum bake-out test (VBT) procedure was set up for unused crucible parts and a microscopic characterization routine was developed to systematically investigate bubble formation and growth. Longer process time, higher temperature, and lower atmospheric pressure lead to an increased bubble growth in both, the BC and BF layer. During the VBT, no new bubbles form in the BF layer, while existing bubbles grow. The comparison to experimental data from crucibles used in an industrial Cz process indicates that VBTs can simulate this process. This allows the prediction of the gas-bubble formation in Cz crucibles using a cost-effective and less time-consuming analyzation method.

Improved Surface Passivation by Wet Texturing, Ozone‐Based Cleaning, and Plasma‐Enhanced Chemical Vapor Deposition Processes for High‐Efficiency Silicon Heterojunction Solar Cells

Silicon heterojunction (SHJ) solar cells rely on excellent surface passivation of the crystalline wafer. This article reports on the development of wet chemical processes varying the texturing and optimizations of the final clean processes for Czochralski–silicon wafers used in SHJ solar cells. Three different additives are used to modify both the pyramid size and the texture homogeneity on the wafers. As an alternative to standard Radio Corporation of America (RCA) sequence, ozonized deionized (DI) water‐based procedures are used to clean the silicon surfaces, reducing process time and the amount of chemicals to obtain the same cleaning quality. In addition, two different amorphous silicon (a‐Si:H) passivation stacks are discussed in this article demonstrating an improvement in the open‐circuit voltage, Voc, by 15 mV. Combining these wet chemistry improvements with optimized process conditions for the passivating a‐Si:H layers deposited by plasma‐enhanced chemical vapor deposition (PECVD), allows to obtain high‐efficiency devices both in small‐ and full‐area solar cells. On 6 in. full‐area solar cells (215.3 cm2 aperture area), open‐circuit voltages, fill factors, and efficiency values of 738 mV, 81.4%, and 23.2% are obtained, respectively; for 4 cm2 cell size, the best obtained values in equivalent devices are 741 mV, 80.6%, and 23.2%.

Transient global modeling for the pulling process of Czochralski silicon crystal growth. II. Investigation on segregation of oxygen and carbon

Abstract We conducted the transient global simulations for the crystal pulling process of Czochralski silicon (CZ-Si) growth with the cusp-shaped magnetic field (CMF). The generation, transport, and segregation of oxygen (O) were considered for the crystal growing process. Further, the incorporation, accumulation, and segregation of carbon (C) were also predicted based on our previous studies on CZ-Si growth. The distributions of O and C at the growth interface were dynamically predicted for the diameter pulling stages, and their segregations were plotted as a function of solidified fraction and crystal length. The O level and uniformities at the growth interface exhibit a strong correlation with the depth of melt in the crucible and the flow structures inside the melt. The comparisons of different CMFs indicated that adjustments of the zero-Gauss plane (ZGP) have the potential to optimize the O segregation into the pulling crystal from the level, axial, and radial uniformities. Further, the C concentration increased with the increase in the crystal length because of the continuous contamination and the lower segregation coefficient. Hence, the developed dynamic global model also has applications in the segregation prediction of other dopants and impurities in the CZ-Si growing process.