Czochralski Growth Process Research Articles

Investigation of Doping Processes to Achieve Highly Doped Czochralski Germanium Ingots

Highly doped germanium (HD-Ge) is a promising material for mid-infrared detectors, bio-sensors, and other devices. Bulk crystals with a doping concentration higher than 1018 cm−3 would be desirable for such device fabrication technologies. Hence, an effective method needs to be developed to dope germanium (Ge) ingots in the Czochralski (Cz) growth process. In this study, a total of 5 ingots were grown by the Cz technique: two undoped Ge ingots as a reference and three doped ingots with 1018, 1019 , and 1020 atoms/cm3 respectively. To obtain a uniform p-type doping concentration along the crystal, co-doping of boron-gallium (B-Ga) via the Ge feed material was also attempted. Both B and Ga are p-type dopants, but with a large difference in their segregation behavior (contrary segregation profile) in Ge, and hence it is expected that the incorporation of dopants in the crystal would be uniform along the crystal length. The distribution of the dopants followed the Scheil-predicted profile. The etch pit density maps of the grown crystals showed an average dislocation density in the order of 105 cm−2. No increase in the overall etch pit count was observed with increasing dopant concentration in the crystal. The grown highly doped Ge crystals have a good structural quality as confirmed by x-ray diffraction rocking curve measurements.

Modification of calcium orthovanadate single crystal due to cobalt doping

Activation of calcium orthovanadate crystals by cobalt ions was carried out by introducing cobalt oxide into the melt during Czochralski growth process or during high-temperature annealing of nominally pure Ca3(VO4)2 single-crystal using Co3O4 powder as a solid-phase diffusant. The maximum concentration of the Co ions in the matrix is achieved in the case of the diffusion doping process. For the concentration series of Ca3(VO4)2:nCo crystals, the local structure was investigated and the crystal chemical composition was determined. Polarized absorption spectra of doped crystals are obtained. The influence of the atmosphere and the annealing temperature on the redistribution of the intensities of the characteristic absorption bands in the visible and near-IR spectral regions is shown. Luminescence parameters of the doped crystals were studied.

Particularities of the thermal and oxygen concentration instabilities in a Czochralski process for solar silicon growth

A previous presented numerical model was used to study the time behavior particularities of temperature and oxygen concentration in a Czochralski (Cz) growth process of 200 mm diameter silicon single crystals for photovoltaic applications. First, some important model issues have been addressed: the dependence of the numerical results on the refinement level of the grid and on the model for oxygen equilibrium concentration at the melt-crucible interface. Then a parametrical study have been performed to study how the pulling rate and the rotation rate of the crucible influence the fluctuations of temperature and oxygen concentration near the crystallization interface. The numerical simulations have revealed that the temperature oscillations are closer to a normal distribution than the oxygen concentration, which is closer to a skewed left distribution.Both the increase of crucible rotation rate and pull speed has an effect of “pushing to the left”: the histogram of temperature fluctuations tends from a skewed right distribution to a normal distribution with the increase of the pull speed and crucible rotation values while the oxygen concentration is more skewed to the left. As a general conclusion, the numerical simulations have shown that the oxygen concentration is more sensitive to the crucible rotation than the temperature field, while the pull speed has a stronger effect on temperature oscillations than on the oxygen concentration oscillations.

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.

Numerical simulation and global heat transfer computations of thermoelastic stress in Cz silicon crystal

AbstractGlobal heat transfer computations were performed for the investigation of thermal stress in a Czochralski silicon crystal. The temperature distribution, thermal stress properties including maximum shear stress and von Mises stress distributions at two different axial crystal positions have been investigated with the help of heat transfer simulations. By analyzing the obtained results, the thermal stress maxima during the Czochralski growth process can be controlled by applying optimal crystal rotation of 8 rpm and counter crucible rotations of 5 and 15 rpm for a crystal position of 100 mm and 300 mm. This shows that by applying optimal crystal and counter crucible rotations throughout the growth process, the thermal stress maxima of a growing crystal can be reduced.

Modeling anisotropic shape evolution during Czochralski growth of oxide single crystals

We present a technique for modeling three-dimensional (3D) anisotropic crystal shape evolution in meniscus defined growth systems. In particular we apply this model to a simplified low gradient Czochralski growth process used to grow bismuth–germanium–oxide–like crystals. Our Lattice Boltzmann Model (LBM) based approach involves a two-dimensional unstructured mesh (embedded within the 3D LBM grid) which accurately tracks the crystal/melt and melt/gas interfaces while accounting for anisotropic interface attachment kinetics as well as capillarity, involving the enforcement of either isotropic or anisotropic growth angles (GA) at the crystal/melt/gas triple phase line (GA-TPL). The robustness of the algorithm is demonstrated by a series of tests involving mesh refinement, comparisons with results from an older study and an evaluation of the impact on accuracy of a time-saving artificial pull-rate speedup scheme described within the manuscript. Physically significant results (which do not involve diameter control) include a demonstration of the impact of changes in kinetic parameters on resultant ingot shapes, calculations showing how (for conditions considered here) the diameter of the growing crystal decreases with increasing isotropic growth angle values, and growth simulations demonstrating how anisotropic growth angles can impact the shape of the evolving ingot. The dependence of crystal diameter on isotropic growth angle values is reproduced in a local sense in the case of anisotropic growth angles. A specific point on the GA-TPL will tend to extend inwards or outwards (respectively exhibiting a decreased or increased local radial position value) when the growth angle value is locally (and respectively) increased or decreased. As a result, considering crystals whose non-circular cross-sectional shapes are due to kinetic anisotropy as a baseline, growth angle anisotropy acts to either decrease or increase shape anisotropy when the extended (rough) parts of the GA-TPL respectively support large or small local growth angle values as compared to reduced-radius (faceted) parts of the GA-TPL. It should finally be mentioned that our results, concerning the sensitivity of the (local or average) radial position value of the GA-TPL to the growth angle, are consistent with other computational results exhibited in the literature (concerning solidifying semiconductor drops and floating zone growth of oxides).

Growth rate effect on colony formation in directional solidification of Al2O3/YAG/ZrO2

AbstractMechanical properties of Al2O3/Y3Al5O12/ZrO2 ternary eutectic ceramics are strongly affected by structural defects as pores or colonies. Experimental investigation of the microstructure of this ternary composite indicates that the colonies are generally observed when the solidification occurs at high rates. In this work, the influence of the growth rate on the solid‐liquid interface shape and formation of colonies in directional solidification of Al2O3/Y3Al5O12/ZrO2 by Bridgman, Edge‐defined Film‐fed Growth (EFG), and Czochralski (Cz) methods is numerically and experimentally investigated. Numerical modeling of the Bridgman growth process shows large curvatures of the solid‐liquid interface when the pulling rate is increased up to 80 mm/h. The ingots solidified at rates between 5 and 80 mm/h exhibit colony type microstructure. The analysis of EFG growth of ceramic ribbons reveals less curved solid‐liquid interfaces in this system. Numerical modeling shows significant increase in the interface curvature with increasing pulling rate. The microstructure of ribbons grown at pulling rates between 6 and 12 mm/h exhibits colonies only for the ingots solidified at higher rate. Simulations carried out for Czochralski growth process show that the solidification front is almost plane in this system. These results are in agreement with experimental observations showing good structural quality of Cz grown crystals with a flat solid‐liquid interface. Finally it is concluded that formation of colonies in directional solidification of this ternary eutectic composite is linked to large curvatures of the growth interface.

Numerical simulation of the oxygen distribution in silicon melt for different argon gas flow rates during Czochralski silicon crystal growth process

The effects of argon gas flow rate on the oxygen concentration in Czochralski (CZ) grown silicon crystal were examined. To analyze the influence of the argon gas flow rate in CZ growth process, a 200 mm length silicon single crystal was grown. Different argon gas flow rates are considered. The melt flow pattern, temperature and oxygen concentration distributions in the melt and crystal-melt interface are calculated. The results show that the transport of oxygen impurity is quite dependent on the flow motion in the melt. As the argon gas flow rate increases, there is no fundamental change in flow motion of the melt and the oxygen concentration decreases to a minimum value. When the argon gas flow rate increases further, the flow pattern under the melt-crystal interface starting changes and the oxygen concentration has increased after. Therefore, there is an optimum value for the argon gas flow rate for obtaining the lowest oxygen concentration in the melt.

Numerical Investigation of Three-Dimensional Heat Transfer and Natural Convection in the Sapphire Melt for Czochralski Growth Process

Numerical Investigation of Three-Dimensional Heat Transfer and Natural Convection in the Sapphire Melt for Czochralski Growth Process

Temperature stable operation of YCOB crystal for giant-pulse green microlaser.

In this work the performance of two yttrium calcium oxyborate (YCOB) crystals made by Czochralski and Bridgman growth process was measured. By using high peak power, passively Q-switched Nd3+:YAG/Cr4+:YAG microlaser, high conversion second harmonic generation efficiency were obtained. Laser pulses at 532 nm with 1.14 mJ energy and 223 ps duration were obtained with a 15-mm long YCOB crystal that was grown by Bridgman method. The conversion efficiency was 70.2%, comparable with the conversion efficiency of 72.8% that was achieved with 10-mm long lithium triborate (LBO) nonlinear crystal. Also, for the first time, experimental data on temperature tuning in type I YCOB crystal was measured with linear slope in 200°C temperature range equal to -0.057%/°C and -0.064%/°C for the Czochralski and Bridgman grown crystals, respectively. Such YCOB nonlinear crystal can become a serious option for developing laser sources with high-peak power at high repetition rate that can operate in harsh environment.

Effect of crucible and crystal rotations on the convexity and the thermal stress in large size sapphire crystals during Czochralski growth

In this study, the effect of the temperature and flow fields generated by the rotation of the crucible and the crystal on the convexity of a c-axis, large-diameter sapphire crystal during the Czochralski growth process is investigated numerically. The thermal stress distributions in different sizes of crystal are also considered. The computational results show that the convexity and the thermal stress of the crystal are strongly dependent on the crucible and crystal rotation rates. The counter rotation between the crucible and the crystal results in a flatter crystal-melt interface, compared to the case of no crucible rotation or crystal rotation. Maximum thermal stress occurs at the highest curvature of the crystal-melt interface which appears near the center of the growing crystal, and the value is directly proportional to the crystal's size. Moreover, there is a significant decrease in the von Mises stress for the crystal-melt interface with lower convexity due to a reduction in the temperature gradient in the radial direction along the interface. As the crystal length gets larger, the maximum von Mises stress rapidly reduces.

Numerical simulation of the oxygen concentration distribution in silicon melt for different crystal lengths during Czochralski growth with a transverse magnetic field

A three-dimensional simulation model is used to study the oxygen concentration distribution in silicon crystal during the Czochralski growth process under a transverse uniform magnetic field. The flow, temperature, and oxygen concentration distributions inside the furnace are calculated for different crystal lengths. There is significant variation in the flow structure in the melt with the growth length. The results show that in the initial stages, there is a decrease in the oxygen concentration at the crystal-melt interface as the length of the growing crystal increases. As the crystal lengthens further, a minimum value is reached after which the oxygen concentration increases continuously. This trend is consistent with that shown in the experimental results. The variation of the oxygen concentration with the growth length is strongly related to the depth of the melt in the crucible and the flow structure inside the melt. Better uniformity of the axial oxygen concentration can be achieved by proper adjustment of the crucible rotation rate during the growth process.

Radiative Heat Transfer and Thermocapillary Effects on the Structure of the Flow during Czochralski Growth of Oxide Crystals

A numerical study was carried out to describe the flow field structure of an oxide melt under 1) the effect of internal radiation through the melt (and the crystal), and 2) the impact of surface tension-driven forces during Czochralski growth process. Throughout the present Finite Volume Method calculations, the melt is a Boussinnesq fluid of Prandtl number 4.69 and the flow is assumed to be in a steady, axisymmetric state. Particular attention is paid to an undulating structure of buoyancy-driven flow that appears in optically thick oxide melts and persists over against forced convection flow caused by the externally imposed rotation of the crystal. In a such wavy pattern of the flow, particularly for a relatively higher Rayleigh number , a small secondary vortex appears nearby the crucible bottom. The structure of the vortex which has been observed experimentally is studied in some details. The present model analysis discloses that, though both of the mechanisms 1) and 2) end up in smearing out the undulating structure of the flow, the effect of thermocapillary forces on the flow pattern is distinguishably different. It is shown that for a given dynamic Bond number, the behavior of the melt is largely modified. The transition corresponds to a jump discontinuity in the magnitude of the flow stream function.

Applying the chemical bonding theory of single crystal growth to a Gd3Ga5O12Czochralski growth system: both thermodynamic and kinetic controls of the mesoscale process during single crystal growth

The chemical bonding theory of single crystal growth was applied to a Gd3Ga5O12 (GGG) Czochralski growth system. On the basis of anisotropic chemical bonding characteristics, the mesoscale process controlled by both thermodynamics and kinetics has been studied in GGG single crystal growth. Starting from the unit-scale in the growth system, the mesoscale structures undergo evolution from GdO8, GaO6, and GaO4 to Gd3Ga5O12 clusters and to a GGG single crystal via chemical bonding between Gd, Ga, and O. On the basis of the chemical bonding theory of single crystals, the GGG single crystal thermodynamically prefers to exhibit a hexagonal configuration along the [111] pulling direction. Kinetically, isotropic mass transfer in the Czochralski growth process leads to formation of a GGG single crystal with a circular configuration, as viewed down along the [111] direction. In such a case, the GGG single crystal maintains the lowest energy. In order to satisfy both thermodynamic and kinetic controls, the thermodynamically preferred {−110} microfacets are exposed on the microscale and the single crystal adopts a circular shape on the macroscale. The present work deepens our understanding of the mesoscale process in the GGG Czochralski growth system.

Ky 및 CZ 방법을 사용한 사파이어 단결정 성장공정의 수치해석

Sapphire crystals are used in a substrate of the LED devices. Both Kyropoulos (Ky) and Czochralski (CZ) growth process are widely applied techniques for growing high quality sapphire single crystal. A successful growth of the sapphire crystals requires the control of heat and mass transport phenomena. In this study, the growth processes of the sapphire crystal using the resistivity-heated Ky method and the inductively-heated CZ method have been analyzed numerically using finite element method. Based on the simulation results, the melt-crystal interface of the crystal changed from the concave to the flat shape as the Ky process progressed. In case of the CZ method, the high temperature positions moved from the crucible surface to inside the melt and the interface changed to the flat shape when the RPM was increased. Also the interface shape of the grown crystal has been influenced by the formed shoulder shape at the initial stage.

Global simulation of the Czochralski silicon crystal growth in ANSYS FLUENT

Silicon crystals for high efficiency solar cells are produced mainly by the Czochralski (CZ) crystal growth method. Computer simulations of the CZ process established themselves as a basic tool for optimization of the growth process which allows to reduce production costs keeping high quality of the crystalline material. The author shows the application of the general Computational Fluid Dynamics (CFD) code ANSYS FLUENT to solution of the static two-dimensional (2D) axisymmetric global model of the small industrial furnace for growing of silicon crystals with a diameter of 100mm. The presented numerical model is self-sufficient and incorporates the most important physical phenomena of the CZ growth process including latent heat generation during crystallization, crystal–melt interface deflection, turbulent heat and mass transport, oxygen transport, etc. The demonstrated approach allows to find the heater power for the specified pulling rate of the crystal but the obtained power values are smaller than those found in the literature for the studied furnace. However, the described approach is successfully verified with the respect to the heater power by its application for the numerical simulations of the real CZ pullers by “Bosch Solar Energy AG”.

Solidification of Silicon for Solar Cells

Silicon is the dominating material in solar cells. Monocrystalline and multicrystalline cells have approximately equal market shares and are produced from wafers, cut from single crystals produced by Czochralski (CZ) pulling or from polycrystalline ingots made by directional solidification, respectively. The present paper reviews how demands for lower cost, better yield, higher efficiency and use of less pure silicon in solar cells are addressed by advanced solidification processing. In monocrystalline solar silicon, careful growth control results in less point defects, and better efficiency. Continuous- or semi-continuous CZ growth processes are being developed for better productivity and lower cost. In multicrystalline solar silicon, extended defects such as dislocations and grain boundaries decrease efficiency, particularly in combination with new, less expensive, but more contaminated silicon feedstock. This problem is addressed by control of nucleation and growth of ingots with larger grains, preferred grain orientation and lower dislocation density.

사파이어 단결정의 초크랄스키 성장공정에 대한 유한요소분석

최근 사파이어 결정은 LED 응용부품에 사용되고 있고, CZ 성장공정은 고 품질의 사파이어 단결정을 성장시키기 위한 중요한 기술중의 하나이다. 고 품질의 단결정을 성장하기 위해서는 CZ 성장로 내부의 열 및 질량 전달현상의 제어가 필요하다. 본 연구에서는 유도 가열된 CZ 성장로에 대한 사파이어 결정의 성장공정을 FEM을 사용하여 수치적으로 분석하였다. 본 연구의 결과, 성장로의 rpm이 증가함에 따라 고온부는 도가니 표면에서 융액의 내부로 이동하고, 고-액 계면은 평편한 형태로 변화되는 것으로 분석되었다. 또한 성장된 결정의 고-액 계면은 초기에 형성된 결정의 shoulder 형상에 의해서도 영향을 받는 것으로 나타났다. Recently sapphire crystals are used in LED applications. The Czochralski (CZ) growth process is one of the most important techniques for growing high quality sapphire single crystal. A successful growth of perfect single crystals requires the control of heat and mass transport phenomena in the CZ growth furnace. In this study, the growth processes of the sapphire crystal in an inductively heated CZ furnace have been analyzed numerically using finite element method. The results shown that the high temperature positions moved from the crucible surface to inside the melt and the crystal-melt interface changed to the flat shape when the rpm was increased. Also the crystal-melt interface shape has been influenced by the shoulder shape of the grown crystal during the initial stage.

Study of the Spontaneous Nucleation of 3C-SiC Single Crystals Using CF-PVT Technique

The control of the nucleation step is a critical issue for a future development of 3C-SiC bulk growth. The possibility to get very high quality 3C-SiC single crystal through self-nucleation on graphite was already demonstrated but the large number of nuclei limits the growth of only one crystal. In this study, we have investigated different configurations that help improving the nucleation step. For that, the “necking” stage, well established in Bridgman or Czochralski growth processes, has been successfully applied to the growth of 3C-SiC with the CF-PVT technique. This has allowed getting only one 3C-SiC crystal. The enlarged parts, after having passed the neck, are of high structural quality.

On the characterisation of grown-in defects in Czochralski-grown Si and Ge

High yield processing of advanced integrated devices poses stringent demands on substrate and active device layer quality. Wafers have to be free of electrically active defects and should therefore be free of so called large pit defects and Crystal Originated Particles (COP’s) which can be formed during Czochralski (Cz) crystal growth. These COP’s are surface pits formed by large vacancy clusters and are observed by surface inspection tools based on light scattering as “particles”. They are formed by vacancy clustering during crystal growth. In Cz Si these defects can also be observed inside the bulk of the material by using infra red light scattering tomography and transmission electron microscopy. Recently similar defects were observed on polished Cz Ge wafers using optical and scanning electron microscopy and the same surface inspection tools as used for silicon wafers. In the present paper the characterisation of grown-in voids in Si and Ge using these various techniques is discussed. The observed void size-density distributions are compared with results of the simulation of vacancy incorporation and clustering during the Czochralski growth process.