Czochralski-grown Silicon Crystals Research Articles

Effect of crucible rotation on the temperature and oxygen distributions in Czochralski grown silicon for photovoltaic applications

Oxygen concentration is sensitive to changes in the rotation rate for Czochralski grown silicon crystals.

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.

Formation of Oxygen-Containing Centers in Irradiated Silicon Crystals during Annealing in the Temperature Range of 450–700°С

The processes associated with transformations of oxygen-related radiation-induced defects in Czochralski-grown silicon crystals irradiated with fast electrons or neutrons and subjected to heat-treatment in the temperature range of 450–700°C have been studied by means of IR absorption spectroscopy. It is found that, upon disappearance of the VO3 and VO4 defects, new vacancy-oxygen-related complexes, which give rise to a number of vibrational absorption bands in the wavenumber range of 980–1115 cm–1, are formed. It is argued that these complexes are radiation-induced VOm centers (m ≥ 5), which serve as nucleation centers of enhanced oxygen precipitation in silicon.

Формирование кислородосодержащих центров в облученных кристаллах кремния при отжиге в интервале температур 450 – 700 °С

The processes associated with transformations of oxygen-related radiation-induced defects in Czochralski-grown silicon crystals irradiated with fast electrons or neutrons and subjected to heat-treatments in the temperature range 450 – 700 °C have been studied by means of IR absorption spectroscopy. It is found that upon disappearance of the VO3 and VO4 defects the new vacancy-oxygen-related complexes, which give rise to a number of vibrational absorption bands in the 980 – 1115 cm–1 vawenumber range, are formed. It is argued that these complexes are VOm (m ≥ 5) defects which serve as radiation-induced nucleation centers of enhanced oxygen precipitation.

Analysis of the geometry of the growth ridges and correlation to the thermal gradient during growth of silicon crystals by the Czochralski-method

A contactless, non-destructive approach to measure the geometrical parameters of the growth ridge, based on surface topography, is presented and established. It allows a systematic, large scale analysis of growth ridges of single crystals of almost any type. Here, it is applied to Czochralski-grown silicon crystals. Based on the measurement results, Voronkov’s theory of the shape of the growth ridge is verified. This theory is used to calculate the temperature gradient at the growth ridge from the geometrical parameters. The presented method gives an easy, direct experimental access to the thermal conditions, both qualitative and quantitative, at the solid-liquid interface during the growth process.

Detection of interstitial oxygen contents in Czochralski grown silicon crystals using internal calibration in laser-induced breakdown spectroscopy (LIBS)

We use an internal calibration approach in laser-induced breakdown spectroscopy (LIBS) for quantitative detection of dead load interstitial oxygen contents (Oi) in industrial-grade silicon (Si) crystal ingots. Si crystal samples were grown via Czochralski technique and supplied by SunEdison Semiconductor Ltd. with known Oi contents measured via gas fusion analysis (GFA) and Fourier transform infrared (FTIR) spectroscopy. The LIBS analyses reported here use and compare a direct approach based on the known oxygen atomic emission line at 777. 19 nm and an indirect approach based on an internal calibration technique using an emission line at 781 nm associated to Si I. Unlike the first direct approach, the latter exhibited much higher sensitivity, reliability and less error. In this approach, an internal calibration uses systematic variations in the 781 nm emission line in conjunction with observed changes in plasma excitation temperatures as a quantitative measure of changes in plasma conditions and laser-matter interactions due to varying Oi contents in the analyte matrix. Using this technique, we establish the detection limit of LIBS in measuring Oi in Si crystal ingots down to 8 ± 1 ppma level. The approach assists to overcome the limitations of common industrial techniques such as FTIR that cannot provide accurate quantitative measurements for heavily doped Si crystals and GFA that is significantly cumbersome to be an online technique. Our results establish LIBS at the forefront of alternative industrial analytical tools heretofore not considered for rapid, potential on-line monitoring of dead loads in commercial grade Si wafers during their growth processes.

Do thermal donors reduce the lifetimes of Czochralski-grown silicon crystals?

High-performance electronics require long carrier lifetimes within their silicon crystals. This paper reports the effects of thermal donors on the lifetimes of carriers in as-grown n-type silicon crystals grown by the Czochralski method. We grew silicon crystals with two different concentrations of thermal donors using the following two cooling processes: one was cooled with a 4-h halt after detaching the crystal from the melt, and the other was cooled continuously. The crystal grown with the cooling halt contained higher concentrations of thermal donors of the order of 1 × 1013 cm−3, while the crystal without the halt had no thermal donors. The measured bulk lifetimes were in the range of 15–18 ms. We concluded that thermal donors in Czochralski-grown silicon crystals do not act to reduce their lifetimes.

Defect Formation of Nickel-Incorporated Large-Diameter Czochralski-Grown Silicon and Their Effect on Gate Oxide Reliability

Nickel, the related defect distribution of as-grown nickel-incorporated p-type Czochralski-grown silicon crystals, and its effect on the gate oxide reliability were investigated with respect to the boron doping concentration. The segregation of nickel at the surface was more likely in the doping range of 5.5–7.4 × 1014 cm−3 than in the doping range of 1.1–1.7 × 1015 cm−3, which was due to the active formation of Ni–B pairs in the crystal with a higher boron doping concentration. The formation of surface defects was affected by the atomic nickel distribution; that is, the surface defect density was relatively high in the crystal with a lower boron doping concentration. The gate-oxide reliability was aggravated by nickel-related defects, but this effect was noticeably suppressed in the crystal with a higher boron doping concentration. From the breakdown yield and measured laser scattering tomographic defect density, we found the “transferred portion” of bulk nickel defects that causes a degradation in the gate-oxide reliability.

Comparative Spatially Resolved Characterization of a Czochralski-Grown Silicon Crystal by Different Laser-Based Imaging Techniques

The axial distribution of electrical and optical properties of a 4 inch Czochralski-grown silicon single crystal were analyzed by different methods that can be applied in the scanning mode. These methods were tested with respect to the suitability to reveal growth striations. The residual stress was visualized by SIRIS (Scanning Infrared Stress Inspection System) and SIREX (Scanning Infrared Stress Explorer), the electrical resistivity by LPS (Lateral Photovoltage Scanning) and SRP (Spreading Resistance Profiling), and the lifetime of the minority charge carriers by MDP (Microwave Detected Photoconductivity) mapping. The concentration of interstitial oxygen (Oi) across the growth striations was determined by FTIR (Fourier Transform Infrared) spectroscopy. We demonstrate for the first time on the micrometer scale that the Oi scan is very well-correlated with the profile of Δσ (difference of the in-plane principal stress components). The stress field is tensile oriented in growth direction, i. e. perpendicularly to the growth striations. The stress-concentration coefficient has been estimated to be in the order of 10-13 Pa cm-3 what does agree well with previous XRD results.

Three-dimensional evaluation of gettering ability for oxygen atoms at small-angle tilt boundaries in Czochralski-grown silicon crystals

Three-dimensional distribution of oxygen atoms at small-angle tilt boundaries (SATBs) in Czochralski-grown p-type silicon ingots was investigated by atom probe tomography combined with transmission electron microscopy. Oxygen gettering along edge dislocations composing SATBs, post crystal growth, was observed. The gettering ability of SATBs would depend both on the dislocation strain and on the dislocation density. Oxygen atoms would agglomerate in the atomic sites under the tensile hydrostatic stress larger than about 2.0 GPa induced by the dislocations. It was suggested that the density of the atomic sites, depending on the tilt angle of SATBs, determined the gettering ability of SATBs.

Radial oxygen precipitation of a 12” CZ silicon crystal studied in-situ with high energy X-ray diffraction

The radial oxygen nucleation and precipitation in Czochralski-grown silicon crystals is observed in real time via the local evolution of the X-ray Bragg intensity. Using a high X-ray energy and a divergent beam setup, it becomes possible to examine the impact of the local as-grown point defect configuration on the formation of bulk micro defects in a crystal of macroscopic dimensions. As a salient feature the transition from vacancy- to interstitial-dominated regions in a 12” crystal can be identified, and its influence on the precipitation kinetics is quantified through effective oxygen diffusion constant. It appears enhanced by up to a factor of six in vacancy-rich regions as compared to crystal volumes which are identified as point-defect free where it agrees well with the literature value.

Efficient computation of population distribution of microdefects at any location in growing Czochralski silicon single crystals

A computationally efficient model to quantify the distribution of microdefect population at any given location in Czochralski grown silicon crystals is proposed and numerically solved. For this purpose, all microdefects are approximated as spherical clusters and the concentration fields of intrinsic point defects and other intermediate species are evolved using a lumped model which eliminates the requirement to store formation and path histories of the clusters. The formation of all clusters is modeled by the classical nucleation theory, while they grow by diffusion-limited kinetics. The model is validated by comparing its predictions against that of a rigorous model, where actual cluster population distribution is captured on the basis of formation and path histories of the clusters.

Grown-in microdefects and photovoltaic characteristics of heavily Ge co-doped Czochralski-grown p-type silicon crystals

The minority carrier lifetime (MCL)/flow pattern defect of B and Ge co-doped Czochralski silicon (CZ-Si) was increased/decreased with the initial Ge concentration up to 3 × 10 20 cm −3 and decreased/increased at higher Ge concentrations. Secco etch pit defects were increased in heavily (⩾3 × 10 20 cm −3 ) Ge co-doped CZ-Si (grown at low growth rate), which resulted in a low MCL. The influence of Ge on microdefect variation was relatively lower than the crystal growth rate in heavily Ge co-doped CZ-Si.

Quality evaluation of diamond wire-sawn gallium-doped silicon wafers

Most of the world's solar cells in photovoltaic industry are currently fabricated using crystalline silicon. Czochralski-grown silicon crystals are more expensive than multicrystalline silicon crystals. The future of solar-grade Czochralski-grown silicon crystals crucially depends on whether it is usable for the mass-production of high-efficiency solar cells or not. It is generally believed that the main obstacle for making solar-grade Czochralski-grown silicon crystals a perfect high-efficiency solar cell material is presently light-induced degradation problem. In this work, the substitution of boron with gallium in p-type silicon single crystal is studied as an alternative to reduce the extent of lifetime degradation. The diamond-wire sawing technology is employed to slice the silicon ingot. In this paper, the quality of the diamond wire-sawn gallium-doped silicon wafers is studied from the chemical, electrical and structural points of view. It is found that the characteristic of gallium-doped silicon wafers including texturing behavior and surface metallic impurities are same as that of conventional boron-doped Czochralski crystals.

Magnetomechanical effect in silicon (Cz-Si) surface layers

The mechanical properties of near-surface layers of Czochralski-grown silicon crystals Cz-n-Si(111) have been found to undergo changes in response to an external constant magnetic field (B ∼ 0.1 T). A magnetically induced variation in the microhardness, Young’s modulus, and coefficient of plasticity of silicon crystals correlates with the change in the lattice parameter and internal stresses of the sample. The growth of an oxide film under exposure to a magnetic field plays the principal role in the magnetomechanical effect due to a decrease in the concentration of oxygen complexes in the near-surface layers of the sample. In microstructured silicon, where the surface is considerably more developed, the magnetic field induces more profound changes in the internal stresses as compared to single crystals.

Analysis of W-shape melt/crystal interface formation in Czochralski silicon crystal growth

The shape of the melt/crystal interface in Czochralski grown silicon crystal is a result of the interaction of several phenomena. In particular, melt convection plays a key role as it affects the heat and mass transfer inside the melt. Depending on the growth conditions and heat transfer in the melt, the interface shape can evolve from a purely concave or convex shape to a W-shape. The aim of this study is to explore the heat transfer conditions and melt flow patterns leading to the formation of W-shape interfaces. The 2D simulations are performed for 6in. diameter silicon in order to identify conditions leading to W-shape formation. The numerical results show that the formation and development of W-shape interfaces is associated with growth conditions favoring the appearance of a vortex underneath the crystal. This vortex significantly influences the thermal field within the crystallization zone and locally alters the interface deflection. This study suggests that the formation of the W-shape interfaces influences significantly the grown-in defects and crystal quality.

Interaction of dopant atoms with stacking faults in silicon crystals

Variation in stacking fault energy with annealing at 1173 K were identified in Czochralski-grown silicon crystals heavily doped with n- or p-type dopant atoms. In n-type crystals, the energy decreased with increasing annealing time. The higher the concentration of dopant atoms, the larger the degree of the decrease. On the other hand, the energy was unchanged during annealing in p-type and nondoped crystals. These results imply that n-type dopant atoms segregate nearby a stacking fault, via their thermal migration, under an electronic interaction leading to a reduction in the stacking fault energy.

Interaction of A-centers with isovalent impurities in silicon

An A-center is an oxygen interstitial atom near a lattice vacancy and is one of the most common impurity-defect pairs in Czochralski-grown silicon crystals. In the present study, density functional theory calculations have been used to predict the binding energies of A-centers that are at nearest neighbor (NN) and next NN sites to isovalent impurities (carbon, germanium, and tin) in silicon. Interestingly, we predict that the A-center is more bound in isovalent-doped and, in particular, tin-doped silicon. We calculate that most of the binding energy of these A-centers originates from the interaction between the isovalent atoms and the vacancies.

Comparative Study of Electrical and Optical Properties of Plastically Deformed Silicon

The Czochralski-grown silicon crystals plastically deformed at 680±C to the residual strain of 5% and subsequently annealed in the temperature range of 650{850±C were studied by the IR absorption and DLTS techniques. The formation and disappearance kinetics were investigated for several weak IR absorption lines around the 1000 cm¡1 wave number, which were previously related to the deformation-induced defects of non-dislocation nature in the dislocation trails. In parallel, the spectrum and concentrations of the deep-level centers in the upper half of the gap were measured by DLTS. It is found that the deep-level centers and op- tically active defects exhibit a common behavior during the post-deformation heat treatments. However, a direct correspondence between the IR and DLTS features cannot be established be- cause of the higher concentration of the deep-level centers as compared to the optically active defects.

Interaction of phosphorus with dislocations in heavily phosphorus doped silicon

Effects of annealing at 1173 K, that is comparable to the typical temperatures for the fabrication of Si-based devices, on the dissociated dislocations in Czochralski-grown silicon crystals heavily doped with phosphorus atoms were determined. Dislocation segments with edge component are constricted. They climbed out of the slip plane toward the compression side, forming complete dislocation segments. The dissociation width of the rest segments is increased. These results suggest that phosphorus atoms segregate nearby dislocations and the high doping level at the dislocations lowers the formation energy of negatively charged vacancies.