Czochralski Growth Process Research Articles

Dislocation reduction in GaAs crystal grown from the Czochralski process

The dislocation density in the gallium arsenide (GaAs) crystal is generated by excessive thermal stresses during the Czochralski (CZ) growth process. A constitutive equation which couples the dislocation density with the plastic deformation is employed to simulate the dislocation density in the crystal. The temperature distribution in the crystal during the growth process is obtained by solving the quasi-steady-state (QSS) heat-transfer equation. The thermal stresses induced by the temperature distribution are calculated using the finite-element method. The crystal is assumed to be an axisymmetrical ingot. The resolved shear stress (RSS) in each slip system is obtained by stress transformation. The RSS in each slip system is no longer axisymmetric. The dislocation motion and multiplication in each slip system are simulated using the constitutive equation. The total dislocation density in the crystal is obtained by summing the dislocation densities in all of the slip systems. Since the thermal stresses are sensitive to the temperature gradients and the dislocations move faster at a higher temperature, the dislocation densities are generated most near to the solid-melt interface. The dislocation density is also found to be affected by the growth orientation, growth speed, ambient temperature and the radius of the crystal. The dislocation density in GaAs crystals grown with the different growth orientation, growth speed, and crystal radius at various ambient temperatures has been calculated so that the influence of these growth parameters on the dislocation density can be understood. Consequently, the growth parameters can be controlled to reduce the dislocation density generated in the crystal during the CZ growth process.

引上げCu-希薄Al合金結晶の転位形成

引上げCu-希薄Al合金結晶の転位形成

The numerical modelling of dislocation generation in semiconductor crystals during Czochralski growth

The dislocation densities in semiconductor compound crystals have been found to be affected by the growth orientation, crystal radius and the ambient temperature employed during the Czochralski (CZ) growth process. The objective of this paper is to investigate the influence of these growth parameters on the dislocation density and spell out ways to control the growth parameters in order to reduce the dislocation density. The finite-element technique is utilized to solve the governing equations of the quasi-steady-state (QSS) heat transfer and the thermoelasticity to obtain the temperature distribution and the thermal stresses in a growing crystal. The resolved shear stresses (RSSs) in each slip system are obtained through the transformation of thermal stresses. The RSSs are then employed to calculate the plastic deformation and dislocation density in each slip system of the crystal using the constitutive equation. The total dislocation densities in the crystal are obtained by summing the dislocation densities in all the slip systems. The results indicate that ambient temperature and growth direction can be effectively used to reduce the dislocation density in the CZ process.

The fracture behavior of single-crystal Y3Al5O12

Abstract The fracture behavior of single-crystal yttrium aluminum garnet in ambient air has been studied as a function of temperature for two different crystallographic orientations. A Chevron notch specimen geometry was used so that stable crack propagation could be achieved. It was found that the room temperature fracture toughness, K Ic = 1.8 MPa m 1 2 , increased to K Ic = 4.0 MPa m 1 2 at 1600°C. At 1750°C, samples did not fracture even when the load was brought to an equivalent stress intensity factor of 11.3 MPA m 1 2 . The fracture toughness was found not to depend significantly on crystallographic orientation. The increase in toughness with temperature was attributed to the activation of slip systems, which were evaluated by trace analysis. It was found that the degree of plasticity is extremely sensitive to the strain rate. Finally, periodic striations were observed on the fracture surfaces which are believed to be related to periodic chemical or structural fluctuations which arise during the Czochralski growth process.

Optical studies of Czochralski and hydrothermal bismuth silicate

Bismuth silicate in the sillenite structure, (Bi 12SiO 20), is a photorefractive material of interest for applications in two-wave and four-wave mixing, optical correlation and holographic storage devices. It has higher sensitivity and speed than other visible light photorefractive materials. Currently available material is obtained by the Czochralski growth process or the directional gradient freeze technique. These methods procedure material of adequate quality for most purposes but problems with crystal uniformity and reproductibility limit the material for some applications. The hydrothermal growth technique will potentially yield highly reproducible material of large size and excellent uniformity. Doping studies on both Czochralski and hydrothermal crystal indicate that the photorefractive properties may be tailored to individual applications.

Oscillatory convection in low aspect ratio Czochralski melts

Modeling of the crucible in bulk crystal growth simulations as a right circular cylinder may be adequate for high aspect ratio melts but this may be unrealistic when the melt height is low. Low melt height is a unique feature of a solid feed continuous Czochralski growth process for silicon single crystals currently under investigation. At low melt heights, the crucible bottom curvature has a dampening effect on the buoyancy-induced oscillations, a source of inhomogeneities in the grown crystal. The numerical results demonstrate how the mode of convection changes from vertical wall-dominated recirculating flows to Benard convection as the aspect ratio is lowered. This phenomenon is strongly dependent on the boundary condition at the free surface of the melt, which has been generally considered to be either adiabatic or radiatively cooled. A comparison of the flow oscillations in crucibles with and without curved bottoms at aspect ratios in the range of 0.25 to 0.50, and at realistic Grashof numbers (10 7 < Gr < 10 8) illustrate that changing the shape of the crucible may be an effective means of suppressing oscillations and controlling the melt flow.

Numerical and experimental study of a solid pellet feed continuous Czochralski growth process for silicon single crystals

A polysilicon pellets (≅1 mm diameter) feed continuous Czochralski (CCZ) growth process for silicon single crystals is proposed and investigated. Experiments in an industrial puller (14–18 inch diameter crucible) successfully demonstrate the feasibility of this process. The advantages of the proposed scheme are: a steady state growth process, a low aspect ratio melt, uniformity of heat addition and a growth apparatus with single crucible and no baffle(s). The addition of dopant with the solid charge will allow a better control of oxygen concentration leading to crystals of uniform properties and better quality. This paper presents theoretical results on melting of fully and partially immersed silicon spheres and numerical solutions on temperature and flow fields in low aspect ration melts with and without the addition of solid pellets. The theoretical and experimental results obtained thus far show a great promise for the proposed scheme.

Hydrothermal growth of bismuth silicate (BSO)

Bismuth silicate (Bi 12 Si 20 ) is a promising material for use in optical signal processing. It is a photorefractive material with good response at argon laser wavelengths (488 nm) and is well suited to holography and four wave mixing applications. It exhibits a high response speed and good sensitivity, although the gain is far less than that of the ferroelectric materials (barium titanate, etc.). Most of the currently available material is obtained by the Czochralski growth process or the directional gradient freeze process. Nominally undoped crystals from these processes yield material of acceptable research quality but crystal uniformity and reproducibility have been a problem. Improved growth techniques are needed for advanced applications. Hydrothermal growth of this material dramatically changes the “intrinsic” optical properties. There are indications that lower temperature aqueous growth prevents the formation of defects created in the melt processes. This yields a baseline material ideally suited for the study of these defects. In addition, by modifying the optical properties with dopants, material tailored to specific wavelengths and applications may possibly be produced. Growth procedures and preliminary optical characterization results are reported.

Computational simulation of melt flow in magnetic Czochralski growth process

Melt flow phenomena of magnetic Czochralski processes are simulated numerically. A model is established for the system that can grow a 5 inch diameter single crystal from the melt in a 14 inch diameter cylindrical crucible. Effective conditions to suppress convection adequately are studied by varying rotation rates of the crucible and the crystal for given magnetic fields. When the axial magnetic force is used, the method of co-rotation of the crystal and the crucible is found more effective to suppress meridional circulation than the conventional counter-rotation operation.

The effect of an external magnetic field on natural convection during the crystal growth process from a melt.

The effect of an external magnetic field on the natural convention of a melt, which is electrically conductive, has been studied in relation to the process of crystal growth. It is known that the convective flow of a melt seriously affects the quality of the crystal, and that the application of a magnetic field could be one of the most promising procedures. Both the Bridgman and the Czochralski growth processes have been modeled under the assumption that the external magnetic field is imposed vertically. The result of a numerical analysis has revealed that there is a minimum intensity of the magnetic field that is necessary to suppress the convective motion of the liquid, and that above this threshold intensity the flow velocity decreases as inversely proportional to the magnetic field squared.

The Role of Fluid Flow Phenomena in the Czochralski Growth of Oxides.

ABSTRACTIn the Czochralski growth of single crystals from large melts, fluid flow phenomena have a major effect on interface shape, growth striations, defect density and the length of crystals which can be grown from a melt of given volume and thermal geometry. Because of the technical difficulties encountered in making direct measurements in molten oxides, simulation experiments have been extensively utilized to gain insight into melt behavior.Both temperature profile and flow geometry results from simulation experiments are discussed. This data is supported by direct melt observations and results from the characterization of grown crystals. When reviewed together, this information offers new insights into the complex behavior of Czochralski growth processes, including the role of thermal gradients, crystal rotation, and surface tension driven (Marangoni) convection.