Top-Seeded Solution Growth Process Research Articles

Optimal Control of SiC Crystal Growth in the RF-TSSG System Using Reinforcement Learning

We have developed a reinforcement learning (RL) model to control the melt flow in the radio frequency (RF) top-seeded solution growth (TSSG) process for growing more uniform SiC crystals with a higher growth rate. In the study, the electromagnetic field (EM) strength is controlled by the RL model to weaken the influence of Marangoni convection. The RL model is trained through a two-dimensional (2D) numerical simulation of the TSSG process. As a result, the growth rate under the control of the RL model is improved significantly. The optimized RF-coil parameters based on the control strategy for the 2D melt flow are used in a three-dimensional (3D) numerical simulation for model validation, which predicts a higher and more uniform growth rate. It is shown that the present RL model can significantly reduce the development cost and offers a useful means of finding the optimal RF-coil parameters.

Numerical Study of Three-Dimensional Melt Flows during the TSSG Process of SiC Crystal for the Influence of Input Parameters of RF-Coils and an External Rotating Magnetic Field

Three-dimensional numerical simulations were conducted for the Top-Seeded Solution Growth (TSSG) process of silicon carbide (SiC) crystals. We investigated the influence of coils frequency and peak current, and an applied rotating magnetic field (RMF) on the melt flow developing in this system. Numerical simulation results show that the Marangoni flow in the melt becomes stronger at higher coils frequencies due to the decreasing coils-induced electromagnetic field strength. Results also show that the use of external RMF may improve supersaturation uniformity along the seed if it is properly adjusted with respect to the coils-induced electromagnetic field strength. Furthermore, it is predicted that the application of RMF and seed rotation in the same direction may enhance supersaturation below the seed.

Bayesian optimization for a high- and uniform-crystal growth rate in the top-seeded solution growth process of silicon carbide under applied magnetic field and seed rotation

Abstract The Top-Seeded Solution Growth (TSSG) method is a promising technique for the production of high-quality SiC single crystal. To achieve a high- and uniform-growth rate in the TSSG process of SiC, the fluid flows developing in the growth solution (melt), due to the applied and induced electromagnetic fields, buoyancy, seed rotation, and free surface tension gradient, need to be controlled. Previous numerical analysis has shown that such complex flows in the TSSG melt can be controlled by the applications of a static magnetic field and seed rotation. However, the requirement of significant computational resources prevented us from carrying out the needed optimization for the process parameters involved. In order to resolve the computational demand issue, in this study, we utilized the Bayesian optimization algorithm for an efficient optimization of the associated control parameters of the TSSG process of SiC. It was shown that the Bayesian algorithm determines the optimal state at about roughly 1/4 of the computational cost of a conventional optimization, and accurately predicts the growth-rate evaluation function around the optimal state. The optimal state obtained by the present optimization process predicts a high- and uniform-growth rate in the TSSG system of SiC considered in this work.

Uncovering ferroelectric polarization in tetragonal (Bi1/2K1/2)TiO3\u2013(Bi1/2Na1/2)TiO3 single crystals

We report the robust ferroelectric properties of (1 − x)(Bi1/2Na1/2)TiO3–x(Bi1/2K1/2)TiO3 (x = 33%) single crystals grown by a top-seeded solution growth process under a high oxygen-pressure (0.9 MPa) atmosphere. The sample exhibit a large remanent polarization of 48 μC/cm2 and a sizeable piezoelectric strain constant of 460 pm/V. Neutron powder diffraction structural analysis combined with first-principles calculations reveals that the large ferroelectric polarization comparable to PbTiO3 stems from the hybridization between Bi-6p and O-2p orbitals at a moderately negative chemical pressure.

Global simulation of the induction heating TSSG process of SiC for the effects of Marangoni convection, free surface deformation and seed rotation

A global numerical simulation was performed for the induction heating Top-Seeded Solution Growth (TSSG) process of SiC. Analysis included the furnace and growth melt. The effects of interfacial force due to free surface tension gradient, the RF coil-induced electromagnetic body force, buoyancy, melt free surface deformation, and seed rotation were examined. The simulation results showed that the contributions of free surface tension gradient and the electromagnetic body force to the melt flow are significant. Marangoni convection affects the growth process adversely by making the melt flow downward in the region under the seed crystal. This downward flow reduces carbon flux into the seed and consequently lowers growth rate. The effects of free surface deformation and seed rotation, although positive, are not so significant compared with those of free surface tension gradient and the electromagnetic body force. Due to the small size of the melt the contribution of buoyancy is also small.

Analysis of Macrostep Formation during Top Seeded Solution Growth of 4H-SiC

We used numerical and analytical modeling to investigate fluid flow behaviors close to the growing 4H-SiC crystal surface in the top seeded solution growth process. First, we calculated the azimuthal and radial components of the fluid flow in front of the rotation disc. Second, we developed an analytical model describing the interaction between the step flow (of the vicinal crystal surface) and fluid flow components, considering the crystallography of 4H-SiC and introducing a phase parameter. The correlation of both models allows us to describe qualitatively the conditions for which macrosteps form and destabilize. This phenomenological description is in good agreement with the corresponding experimental observations that are also presented in this paper.

The role of an SiC interlayer at a graphite–silicon liquid interface in the solution growth of SiC crystals

SiC crystal growth using the top seeded solution growth (TSSG) method involves the precipitation of solid SiC from carbon that is dissolved in a silicon melt. The growth rate of SiC is strongly influenced by the solubility of C in liquid Si, which is quite low. In this study, the dissolution of C from graphite to the Si melt was explored by observing the formation of an SiC interlayer at a graphite – Si liquid interface. The SiC interlayer was observed to become thickened during the several hours needed to reach a certain thickness at 1500°C. Assuming that the SiC interlayer is a direct C source, a pre-formed SiC layer was coated on the graphite crucible to evaluate its effect on the concentration of C in the Si melt. As a result, the concentration of C in the Si melt increased within a short time, especially at low temperatures. By applying the SiC coated crucible to the TSSG process for SiC crystal growth, we confirmed that the development of a pre-formed SiC layer enhanced the growth rate of SiC crystals, especially at the initial stage of crystal growth at low temperatures.

Modeling of the Growth Rate during Top Seeded Solution Growth of SiC Using Pure Silicon as a Solvent

To assist the development of high quality single crystalline SiC ingot using the top seeded solution growth process, we have implemented a numerical model with the aim of giving quantitative outcomes in addition to qualitative information. The major role of the convection patterns on the carbon flux is demonstrated. We also evidence that the carbon solubility in liquid silicon is the actual limiting parameter of the SiC solution growth process. A good agreement between computed and experimental growth rates is obtained as a function of temperature, making simulation an adapted predictive tool for the further development of the process.