Solution Growth Of SiC Research Articles

Analysis of Macrostep Interaction via Carbon Diffusion Field in SiC Solution Growth

In the top-seeded solution growth (TSSG) method for SiC, control of macrostep development is crucial for improving the crystal quality. Dislocation conversion phenomena caused by macrosteps with a certain height on the crystal surface can reduce the dislocation density, while over-developed macrosteps bring macroscopic defects such as solvent inclusions. It is experimentally reported that solution flow direction to the step movement has a substantial impact on the macrostep development: parallel solution flow promotes and anti-parallel solution flow suppresses the increase of macrostep height. Our hypothesis is that this macrostep development is governed by the interaction between the macrosteps not by the instability of the density of the atomical steps. In this study, we constructed a computational fluid dynamic model of the boundary layer around macrosteps on the crystal surface, incorporating the solution flow on the boundary layer and consumption of the carbon solute by the macrostep movement quantitatively. The computational simulation reveals that the macrostep with position shift from the center of the adjacent macrosteps moves to the direction of the nearer macrostep under the parallel flow and moves to the farther macrostep under the antiparallel flow. These macrostep movements result in the bunching and debunching of the macrosteps. The mechanisms of macrostep movements demonstrated in this study will be useful for the precise control of macrostep height aiming to the reduction of the dislocation density during SiC solution growth.

Effect of Solution Components on Solvent Inclusion in SiC Solution Growth

Effect of Solution Components on Solvent Inclusion in SiC Solution Growth

Numerical Modeling of the Cellular Structure Formation Process in SiC Solution Growth for Suppression of Solvent Inclusions

Numerical Modeling of the Cellular Structure Formation Process in SiC Solution Growth for Suppression of Solvent Inclusions

Modeling-Based Design of the Control Pattern for Uniform Macrostep Morphology in Solution Growth of SiC

In the solution growth of the SiC crystal, macrosteps with sufficient height on an off-axis substrate are required to reduce defects and achieve a high-quality grown layer. However, over-developed macrosteps can induce new defects and adversely affect the crystal quality. To better understand and control the behavior of macrosteps corresponding to the control parameters of the growth system, a simulation method that consists of a global two-dimensional computational fluid dynamic (CFD) model, a local three-dimensional CFD model near the growth front, and a kinetics model that describes the movement of macrosteps on the crystal surface is proposed. The simulation method is first applied to investigate the effect of the crystal rotation speed on macrostep morphology. Although the results indicate that a higher crystal rotation speed results in less step bunching, constantly rotating the crystal in one direction is demonstrated to be incapable of yielding a uniform macrostep distribution on the whole surface. Accordingly, a sophisticated control pattern is designed by periodically switching the flow direction underneath the crystal surface, where the proposed simulation method is critical to determine detailed control-parameter values. When the control pattern suggested by the simulation is used, a grown crystal with a uniform macrostep morphology and ideal step height on the whole surface is obtained in the practical experiment.

Enhanced carbon solubility in solvent for SiC rapid solution growth: Thermodynamic evaluation of Cr–Ce–Si–C system

The carbon dissolution in solvent plays a key role in the process of solution growth route for SiC single crystal, which could determine the growth rate and quality of the products. However, the carbon dissolving ability of binary alloy solvent still needs to be improved. Here, we demonstrate the improved carbon dissolution and enlarged carbon supersaturation in Cr–Ce–Si ternary solvent, showing great potential for SiC solution growth. The phase relations of Cr–Ce–Si–C system were determined by using CALPHAD method based on thermodynamic parameters of CeCr2Si2C. It is indicated that the Cr–Ce–Si ternary solvent shows much larger carbon solubility in temperature range from 1700 to 2000 °C compared to Cr–Si binary one. Furthermore, the carbon supersaturation in solvent is also significantly increased in low temperature range after the addition of Ce, leading to a rapid growth rate. Our work not only demonstrates the feasibility of adding Ce in the alloy solvent for rapid growth of SiC crystal, but also provides an example for investigating the C solubility in ternary solvent.

Contribution of Dislocations in SiC Seed Crystals on the Melt-Back Process in SiC Solution Growth.

The melt-back process has a significant effect on the quality of solution-grown SiC crystals. However, the phenomena surrounding the SiC dissolution into the molten alloy during the melt-back process have not been clarified. In this study, the behavior of 4H-SiC dissolution into molten alloy was investigated by using high-temperature in situ observation and subsequent KOH etching, and the effects of different doping conditions and crystal polarity were studied. Local dissolutions with hexagonal pyramid-shape originating from threading screw dislocation (TSD) were observed on the C face of n-type SiC with light nitrogen doping. Our analysis of their behavior revealed that the process was governed by the spiral dissolution. In addition to the dissolution at TSD, local dissolutions at threading-edge dislocations were observed on the Si face of the same crystal. The shape of the local dissolution at the dislocation was significantly affected by the doping conditions and the polarity of the SiC crystal. This local dissolution may occur during the melt-back process, suggesting that it is important to promote the dissolution while maintaining a smooth interface through the selection of the seed crystal and by keeping the degree of interface undersaturation small.

A sessile drop approach for studying 4H-SiC/liquid silicon high-temperature interface reconstructions

Accurate description, understanding and control of the high temperature interface behavior between the SiC single crystal and liquid phase is critical for further development of SiC solution growth. The different parameters which create morphological instabilities, such as step bunching, micro-faceting and solvent trapping, remain unclear because they are very difficult to address in high temperature experiments. We combined experiments and numerical simulation to design a specific sessile drop approach where the liquid is removed before cooling down. The advantage of this method that it activates or suppresses the solute (carbon) transport by an AC magnetic field and thus separates the physical–chemical and hydrodynamic contributions. The method was demonstrated through observation of the morphological evolution of a 4°off 4H-SiC (0001) surface in contact with pure liquid silicon at 1600 °C according to the time factor. Several parasitic effects were also analyzed and suppressed in order to obtain a well-controlled uniform interface.

Direct visualization of kinetic reversibility of crystallization and dissolution behavior at solution growth interface of SiC in Si-Cr solvent

The present study is a challenge to visualizing the solution growth behavior of SiC in a high-temperature solvent of Si-Cr. In situ observation of SiC-solution growth interfaces with a confocal scanning laser microscope was successful, revealing the effects of Cr addition on the solution growth behavior. The most outstanding finding in this study is that the Cr addition significantly enhances the kinetic reversibility with high reaction rates of both elemental crystallization and dissolution at the SiC solution interface, besides the well-known increase of the net crystal growth rate that has been explained primarily from the viewpoint of thermodynamics, i.e., by the strong temperature dependence of carbon solubility in Si-based solvents.

Numerical investigation of solute evaporation in crystal growth from solution: A case study of SiC growth by TSSG method

Evaporation of the volatile solute from the liquid phase is a common problem for the crystal grown from solution, especially for the growth under high temperature. In the present study, a numerical model was constructed to quantitatively investigate the evaporation process in crystal growth. This model was applied in top-seeded solution growth (TSSG) of SiC crystal to simulate the transport of aluminum (Al), which is important for crystal surface morphology but easy to vaporize. The transport path of Al in the growth system was determined by analyzing the possible reactions on different boundaries. Accordingly, an improved structure was proposed to suppress the evaporation loss of Al during long-term growth, and was compared with the original case both numerically and experimentally. The simulation results showed that the improved structure could effectively decrease the Al loss by over 70%, and meanwhile had almost no influence on the thermal and flow environment in the solution. For the experimental results, the improved case presented much lower spontaneous nucleation possibilities and higher step height on the crystal surface, which matched well with the features of high Al addition in literatures. Therefore, the improved structure proposed in the present study was proven to be effective to enhance the composition stability of solution during long-term SiC solution growth, and this numerical method could be applied in the growth of other crystals facing the similar problem.

Solvent design aiming at solution property induced surface stability: A case study using SiC solution growth

For solution growth of silicon carbide, it is significant to understanding the evolutionary mechanism of step bunching. This study infers that solute’s incorporation into steps and transport together determines step bunching progress. The occurrence of step bunching is due to the depletion of solute in the region with high step density, caused by a high step kinetic coefficient. On the other hand, by promoting the transport of the solute in the solution, the step speed becomes uniform, thereby the step bunching can be prevented. Furthermore, we proposed a non-dimensional Damköhler number for crystal growth in step-flow mode. It correlates incorporation rates with bulk diffusion rates and can build a phase map of growth rates and step bunching stability. Several solvents are located in the phase map, demonstrating the possible usage of the phase map as a pointer for solvent designing.

Density, surface tension, and viscosity of liquid Si–Cr alloys and influence on temperature and fluid flow during solution growth of SiC

Solution growth using a Si–Cr-based solvent is a promising process to produce high quality 4H–SiC single crystal. Computational fluid dynamics simulation is an effective method to estimate and control the temperature and fluid flow in the process, although simulations are presently performed using the physical properties of liquid silicon due to the lack of data for properties of the alloy solvent. In the present work, the density, surface tension, and viscosity of liquid Si–18, 40, and 50 mol% Cr alloys were measured by electrostatic levitation. Computational fluid dynamics simulation was carried out using the measured melt properties and compared with those of liquid silicon. Simulation of the Si–40 mol%Cr solvent predicted a larger fluid velocity and smaller temperature distribution in solution than those of the pure silicon solvent.

Adaptive process control for crystal growth using machine learning for high-speed prediction: application to SiC solution growth

A time-dependent recipe designed by an adaptive control method can consistently maintain the optimal growth conditions despite the unsteady growth environment.

Design of High-quality SiC Solution Growth Condition Assisted by Machine Learning

Design of High-quality SiC Solution Growth Condition Assisted by Machine Learning

The effect of Al addition to a Cr solvent without molten Si on the surface morphology in a solution growth of SiC

This work assessed the effects of adding Al to a molten Cr solvent on the surface morphology of the SiC crystals resulting from solution growth without molten Si. Al was added to a pure Cr solvent at compositions of 0, 4 or 20 mol%, and the heights of macrosteps for the solvents with any composition of Al were at most one fifth of those obtained without Al. The frequency of two-dimensional nucleation was examined, based on calculating the surface energy of each solvent from viscosity values. The viscosity of the pure Cr solvent was found to decrease with increasing Al addition, such that the surface energy was also lowered. This phenomenon in turn lowered the two-dimensional nucleation frequency, such that surface roughening is considered to be reduced. The addition of Al to the Cr solvent is evidently beneficial in terms of growing smoother crystals.

Effect of Melt-Back Process on the Quality of Grown Crystal in SiC Solution Growth

We investigated the effect of a melt-back process on the quality of the grown crystal in 4H-SiC solution growth. In our experiments, the crystal was grown by top-seeded solution growth (TSSG) method with and without melt-back, following which the quality of the obtained crystals was compared. When solution growth was carried out without melt-back, solvent inclusions and a different polytype were observed. When molten KOH etching was conducted, the dislocation density in the crystals at the early stage of growth became much higher than that in a seed crystal. Solvent inclusions, a different polytype, and an increase in dislocations were suppressed when solution growth was performed with melt-back. It was confirmed that melt-back is necessary to prevent the deterioration of crystal quality at the early stage of solution growth.

Effect of the Growth Conditions on the Crystal Quality in Solution Growth of SiC Using Cr Solvent without Molten Si

We have succeeded in solution growth of SiC from Cr solvent without Si using ceramic SiC as the SiC source. The effect of the growth conditions, such as the liquid height in the crucible, on the crystal quality in solution growth of SiC from Cr solvent was investigated. For a liquid height in the crucible of up to 10 mm, the growth rate increases with increasing liquid height and the SiC crystals are a single polytype, while the growth rate decreases and the crystals are polycrystalline for a liquid height above 10 mm. In the former case, the balance between dissolution and transportation of the solute are comparable. The latter case is expected to be transportation limited because transportation of free C and Si atoms is inhibited by excrescent crystals in solution and the increase in the distance for solute transportation. In addition, a higher growth temperature leads to growth of only 4H-SiC.

Solution Growth of SiC from the Crucible Bottom with Dipping under Unsaturation State of Carbon in Solvent

SiC crystals are grown using a Si-Cr-based solvent by a top-seeded solution growth (TSSG) method by changing the dipping time after when the growth temperature is reached. Step-flow-like curve morphologies were observed for a dipping time after 15 min, while polycrystallization occurred at the periphery for that after 120 min, which corresponded to the dipping under unsaturated and supersaturated carbon in the solvent, respectively. Furthermore, the solution growth of SiC with dipping under unsaturated carbon was easily realized by the growth from the crucible bottom, step-flow-like growth was achieved. Using this technique, dominant polytypes of SiC in various growth conditions after stable seed dipping under the unsaturation in the solvent can be demonstrated.

Experimental Determination of Carbon Solubility in Si<sub>0.56</sub>Cr<sub>0.4</sub>M<sub>0.04</sub> (M = Transition Metal) Solvents for Solution Growth of SiC

Si0.56Cr0.4M0.04 (M = Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Rh, and Pd) solvents were investigated to identify new multi-component materials in which carbon is highly soluble, because solubility is a key parameter in solution growth of SiC. The solubility of carbon in Si0.56Cr0.4Co0.04 was 8.37 at%, the highest value among the tested multi-component materials. This is about 2.5 times the solubility of carbon in Si0.6Cr0.4. These results show that addition of a small amount of a transition metal enhanced the solubility of carbon in Si0.6Cr0.4. This technique for determining carbon solubility is effective for investigating crystal growth using solvents with several components, for which complex thermodynamic calculations are necessary.

Development of Solvent Inclusion Free 4H-SiC Off-Axis Wafer Grown by the Top-Seeded Solution Growth Technique

This study reports our newly developed technology for SiC solution growth. In particular, we succeed in completely suppressing solvent inclusions, which have been a serious technological problem peculiar to the solution growth method. Then, we fabricate two-inch-diameter 4° off-axis SiC wafers without solvent inclusions. Moreover, we performed their crystal defects evaluation. It was found that our wafers were low resistance n-type 4H-SiC and contain almost no basal plane dislocation. As a result, the superior quality of our solution-grown crystal was confirmed.

In Situ Interface Observation of 3C-SiC Nucleation on Basal Planes of 4H-SiC During Solution Growth of SiC from Molten Fe-Si Alloy

Solution growth is a promising process to obtain high-quality 4H-SiC crystal for application in high-voltage-power devices. Polytype transition from 4H-SiC into 6H-, 15R-, or 3C-SiC is one of the major issues to be solved to achieve rapid growth of large-diameter 4H-SiC crystals. In this study, the mechanism for the polytype transition to 3C-SiC during the solution growth process was determined from in situ interface observation of SiC growth on basal planes of 4H-SiC from Fe-36 mol.%Si alloy at 1680–1885 K. The nucleation behavior of 3C-SiC on 4H-SiC was discussed in terms of classical nucleation theory.