SiC Bulk Crystal Growth Research Articles

Advances in fast 4H–SiC crystal growth and defect reduction by high-temperature gas-source method

This paper reviews recent progress in the development of high-speed 4H–SiC bulk crystal growth technology using the high-temperature gas-source method (high-temperature chemical vapor deposition). Vertical-type reactors capable of growing SiC bulk crystals of 50–75 and 100–150 mm in diameter have been developed, as have reactor configurations and growth conditions to achieve a high growth rate and material quality. Using the reactors, high growth rates up to or exceeding 3 mm/h have been achieved by employing an H2–SiH4–C3H8 gas system with high partial pressures of source gases and high growth temperature of 2500–2550 °C. It is also confirmed that the dislocation densities decline significantly in the direction of growth under proper growth conditions. Key factors for enhancing growth rate and improving crystal quality in the growth process are discussed.

A niching genetic algorithm applied to optimize a SiC-bulk crystal growth system

A niching genetic algorithm (NGA) was presented to optimize a SiC-bulk crystal growth system by PVT. The NGA based on clearing mechanism and its combination method with heat transfer model for SiC crystal growth were described in details. Then three inverse problems for optimization of growth system were carried out by NGA. Firstly, the radius of blind hole was optimized to decrease the radial temperature gradient along the substrate while the center temperature on the surface of substrate is fixed at 2500K. Secondly, insulation materials with anisotropic thermal conductivities were selected to obtain much higher growth rate as 600, 800 and 1000µm/h. Finally, the density of coils was also rearranged to minimize the temperature variation in the SiC powder. All the results were analyzed and discussed.

(Invited) Sublimation Growth of 3C-SiC: Status and Prospects

Cubic silicon carbide (3C-SiC) has been considered suitable for a number of device applications like MOSFETs, MEMS, solar cells, water splitting devices, and etc. However, the growth technologies for 3C-SiC are lagging behind its hexagonal counterparts (6H- and 4H-SiC). A primary reason is the lack of high quality 3C-SiC seeds which could be explored for the growth of bulk crystals or homoepitaxial layers. Although, some thermodynamic aspects on 3C-SiC stability should be accounted as well. During the initial developments of hexagonal SiC crystal growth techniques the hexagonal SiC seeds were grown by Acheson or Lely processes. Unfortunately, the 3C-SiC is rarely obtained using these processes. Therefore, it has to be heteroepitaxially grown on silicon or SiC substrates. In both cases there are still challenges in controlling the formation of structural defects at the 3C-SiC/substrate interface. A reproducible growth of 3C-SiC with a crystal size and crystalline quality similar to the hexagonal equivalents has not been demonstrated. Commonly, growth studies of 3C-SiC on SiC substrates by sublimation techniques were done using nominally on-axis hexagonal SiC surfaces. However, this approach has not succeeded in growth of crystals with reasonable structural quality. The main reason is the mismatch of symmetries between the SiC (0001) and 3C-SiC (111) which induces rotational twinning and formation of structural defects – double positioning boundaries (DPBs). Such defects could be avoided only if one 3C-SiC domain or several with the same orientation nucleate on the surface. In contrast to on-axis substrates, the off-orientated substrates have been a typical choice for the growth of homoepitaxial hexagonal SiC layers or bulk crystals. The off-oriented surfaces contain high density of steps which ensures reproduction of the substrate polytype. This has been considered by most of the researchers as an obstacle towards stable 3C-SiC growth. However, under certain conditions, off-oriented SiC substrates can be used to grow 3C-SiC with well controlled initial nucleation. In this paper we will present our recent results on the growth of thick (~1 mm) 3C-SiC layers on off-oriented SiC substrates using sublimation epitaxy. The design of graphite crucible was optimized using numerical simulations to obtain high polytype stability and reproducible growth of 3C-SiC at temperatures below 2000oC in vacuum (10-5 mbar). Based on our knowledge of the initial 3C-SiC nucleation on nominally on-axis and low off-axis surfaces we developed and optimized 3C-SiC growth approach using 4H-SiC (0001) surfaces with the off-orientation of 4 degrees. We will also present results on the growth of 3C-SiC on 6H- and 15R-SiC. The 3C-SiC layer grown using this approach pass through three interconnected growth stages which include formation of a large terrace/facet with an on-axis area at the edge of the sample, preferential nucleation of 3C-SiC domains on the large terrace, and their lateral enlargement along the step-flow direction. In order to obtain a single domain layers we have developed a two-step growth process and applied geometrical control using spacers, separating the source and the substrate, with different geometries. The HRXRD and LTPL analysis of 3C-SiC layers indicate high crystalline quality. The average value of the full width at half maximum (FWHM) of ω rocking curves is about 40 arcseconds. The LTPL spectra demonstrate well resolved near-band-edge features and lines associated with multiple bound-exciton complexes with up to four electron−hole pairs. The layers are usually unintentionally doped with nitrogen and exhibit resistivity of about 10-50 Ωcm. Furthermore, such layers were used as substrates for sublimation growth of homopeitaxial layers with intentional doping of B and V. The dopants were introduced by co-doping from the source material. Single domain 3C-SiC layers were used as seeds to explore the growth of bulk (thickness 4-5 mm) material by sublimation. The absence of DPBs allows us to analyze the development of a bulk crystal and the effect of other defects like stacking faults, secondary inclusions and threading defects on the crystalline quality of the bulk material. We will present our recent results and discuss further prospects for the growth of bulk 3C-SiC crystals.

Simple and quick enhancement of SiC bulk crystal growth using a newly developed crucible material

Newly developed highly reliable low-cost TaC-coated graphite materials prepared by a wet ceramic process were applied to SiC sublimation growth. We demonstrated an increased long-duration growth rate and a resultant increase in crystal size by a factor of ∼1.2 (experimental value) after 24 h of growth [and ∼1.5 (extrapolated value) after the optimum duration of 53.1 h] by simply and quickly replacing graphite crucibles with TaC-coated graphite crucibles. Growth with the TaC-coated graphite crucibles reduced source gas leakage and increased the material yield for single crystals because the TaC layers were gas-tight and had a low emissivity.

ChemInform Abstract: Growth of SiC Bulk Crystals for Application in Power Electronic Devices — Process Design, 2D and 3D X‐Ray in situ Visualization and Advanced Doping

AbstractReview: 59 refs.

Growth of SiC bulk crystals for application in power electronic devices – process design, 2D and 3D X‐ray in situ visualization and advanced doping page 2–9 by Peter Wellmann et al.

In-situ 3D computed tomography (CT) view into the hot growth cell during bulk crystallization of SiC at 2200°C. Colors indicate the evolution of SiC powder consumption (bottom) and SiC crystal growth (top).

Growth of SiC bulk crystals for application in power electronic devices – process design, 2D and 3D X‐ray in situ visualization and advanced doping

Silicon carbide single crystals have become widely used as substrates for power electronic devices like diodes and electronic switches. Today, 4 inch and 6 inch wafer diameters are commercially available which are processed from vapor grown crystals. The state of the art physical vapor transport method may be called mature. Nevertheless, low defect density and uniform doping are still topics which can be further improved by current research and development of more sophisticated processes and process control. The aim of the paper is to review the physical vapor transport growth method as applied today. Special emphasis will be put on currently less advanced in situ growth monitoring tools based on 2D and 3D X‐ray imaging that could be a tool for production monitoring. These techniques allow a precise determination of the crystal and source material evolution. Another topic will be the processing of highly conductive p‐type 4 H‐SiC which is of particular interest for power electronic switches.

Bulk Growth of SiC

ABSTRACTThe paper reviews the basics of SiC bulk growth by the physical vapor transport (PVT) method and discuss current and possible future concepts to improve crystalline quality. In-situ process visualization using x-rays, numerical modeling and advanced doping techniques will be briefly presented which support growth process optimization. The “pure” PVT technique will be compared with related developments like the so called Modified-PVT, Continuous-Feeding-PVT, High-Temperature-CVD and Halide-CVD concepts. Special emphasis will be put on dislocation generation and annihilation and concepts to reduce dislocation density during SiC bulk crystal growth. The dislocation study is based on a statistical approach. Rather than following the evolu-tion of a single defect, statistic data which reflect a more global dislocation density evolution are interpreted. In this context a new approach will be presented which relates thermally induced strain during growth and dislocation patterning in networks.

Numerical modeling and experimental verification of modified-PVT crystal growth of SiC

This work focuses on numerical modeling of temperature field and mass transport in a so-called modified physical vapor transport (M-PVT) growth configuration, which is a further development of the state-of-the-art SiC bulk crystal growth technique. Modeling has been carried out by adapting the commercial computer code CFD-ACE+ (CFD Research Corporation) to the special application. In particular, linked to experimental results in the own lab, we will first show that modeling may be applied to numerically reproduce the impact of an additional gas inlet on the resulting effective mass transport inside the growth setup. In a second step, we will use modeling to understand the physical and chemical processes during crystal growth. We will show how additional gases may be used to tailor the gas phase composition, i.e. C/Si ratio, in front of the SiC growth interface.

Vapor growth of SiC bulk crystals and its challenge of doping

The paper reviews the so-called Modified-PVT (M-PVT) technique which combines the state of the art PVT technique for SiC crystal growth with physical and chemical vapor deposition (PVD and CVD) for fine tuning of growth parameters and improved doping. Using this technique, currently the highest aluminum doping levels and lowest resistivity values in p-type bulk SiC were achieved that for the first time meet device fabrication needs. The paper will address fundamentals of the Modified-PVT technique including a comparison of experimental results with numerical modeling of the gas flow. As additional gas feeding helium, helium–aluminum vapor for p-type doping, phosphine for n-type doping and propane for fine tuning of the C/Si gas phase composition will be discussed. So far, the M-PVT concept, i.e. mixture of conventional PVT and fine tuning by PVD/CVD, enables the most flexible doping of SiC single crystals.

Growth of Silicon Carbide Bulk Crystals with a Modified Physical Vapor Transport Technique

AbstractIn this paper, the development of the modified physical vapor transport (M‐PVT) method for SiC bulk crystal growth is reviewed. For this technique, elements from CVD are combined with the conventional PVT (physical vapor transport) method to achieve a better process and particularly dopant control. To this end, an additional gas flow is introduced directly into the growth chamber of a PVT growth system. For stable crystal growth it is important that the additional gas flow is of the same order of magnitude as the SiC‐species flow by sublimation. Doping experiments resulted in phosphorous concentrations up to 1.5 × 1018 cm–3 and aluminum concentrations up to 1.3 × 1020 cm–3 using a PH3‐flow and a solid aluminum source in an external reservoir as dopant source, respectively. Also, the growth of a crystal with alternating p‐ and n‐type regions for the investigation of the charge carrier‐dependant dislocation evolution in SiC is presented to illustrate the outstanding flexibility that is offered by the M‐PVT method.

Investigation of Dislocation Behavior during Bulk Crystal Growth of SiC

AbstractIt is well-known that SiC crystal deficiencies are delaying the realization of outstandingly superior SiC power electronics. Efforts to date have centered on eradicating micropipes, and 4H-SiC substrates with extremely low micropipe densities have been achieved. Nevertheless, SiC substrates and epilayers still contain several types of dislocations in densities on the order of thousands per square centimetres, which are nearly 100-fold micropipe densities. While not nearly as detrimental to SiC device performance as micropipes, it has recently been demonstrated that dislocations existing in SiC crystals degrade several characteristics of SiC devices, e.g., the forward bias characteristics of SiC pin diodes and the gate oxide reliability of SiC MOSFETs.This paper reports several dislocation processes occurring during the growth of hexagonal SiC bulk crystals. Particularly, we focus on the dislocation formation and propagation processes in SiC crystals. We have investigated dislocation processes in 4H-SiC bulk crystals grown by the physical vapor transport (PVT) growth method, using defect selective etching and transmission electron microscopy (TEM).It was found that foreign polytype inclusions introduced a high density of dislocations at the polytype boundary. In the polytype-transformed areas of the crystal, very few medium size hexagonal etch pits due to threading screw dislocations were observed, indicating that the polytype transformation ceased the propagation of threading screw dislocations. The oval-shaped etch pit arrays observed on the etched vicinal (0001)Si surface, indicative of the dislocation multiplication in the basal plane, showed characteristic distribution around micropipes and low angle grain boundaries. Based on the results, we will argue the dislocation behavior in PVT grown SiC crystals, suggesting that dislocation interaction and conversion are relevant processes to understanding the behavior.

Faceted Growth of SiC Bulk Crystals

In this paper, we suggest a model of facet formation during bulk SiC growth by Physical Vapor Transport (PVT). The model considers the step-flow growth, with the step density dependent on the local orientation of the crystallization front with respect to the close-packed crystal planes. The growth kinetics employs the Burton-Cabrera-Frank approach extended to binary compounds and a multi-component vapor. Being implemented into a 2D simulator, the model is applied to analysis of faceting in free-spreading bulk SiC growth. The computations predict the crystal shapes very similar to those observed experimentally. The faceting influence on the overall crystal growth rate is discussed.

Modeling of facet formation in SiC bulk crystal growth

Control of the crystallization front profile is of great importance for various aspects of bulk SiC crystal growth by physical vapor transport. The structural defect density, doping uniformity, and polytype stability are largely dependent on the profile evolution. In this paper, we consider the binary crystal growth from the multicomponent vapor and suggest a model of facet formation. The model is based on the microscopic consideration of the crystal growth by step-flow mechanism, with the step density dependent on the local front orientation with respect to the close-packed crystal planes. The growth kinetics employs the Burton–Cabrera–Frank approach extended to binary crystals and a multicomponent vapor. The model was implemented into the “Virtual Reactor” code and used to simulate the faceting during growth of the free-spreading bulk SiC crystals. The computations are compared with observations, providing a reasonable agreement between the theory and experiment. The developed approach can be readily extended to other materials grown from the vapor.

Growth of Faceted Free-Spreading SiC Bulk Crystals by Sublimation

Growth of Faceted Free-Spreading SiC Bulk Crystals by Sublimation

Substrate and epitaxial issues for SiC power devices

Realization of the full potential of semiconductor SiC for electronic and optical applications is critically dependent on the production of large diameter SiC single-crystals of high crystalline quality and controlled impurity content as well as high quality uniform epitaxial layers. In this paper, current substrate and epitaxial issues are presented in the light of recent results. Progress in monocrystalline SiC bulk crystal growth is characterized by the attainment of substrate diameters up to 100-mm; residual impurities in the 10/sup 15/ cm/sup -3/ range; and micropipe densities as low as 1.1 cm/sup -2/ over an entire 50-mm diameter 4H-SiC wafer. Epitaxial growth progress is highlighted by growth rates as high as 50 /spl mu/m/h, residual impurities of 10/sup 14/ cm/sup -3/, and doping uniformities of less than 4.7% in commercial multiwafer reactors. Current materials challenges are the reduction of micropipes, dislocations and dislocation related defects.

Dislocation Constraint by Etch-Back Process of Seed Crystal in SiC Bulk Crystal Growth

Dislocation Constraint by Etch-Back Process of Seed Crystal in SiC Bulk Crystal Growth

Incorporation of Boron and the Role of Nitrogen as a Compensation Source in SiC Bulk Crystal Growth

Incorporation of Boron and the Role of Nitrogen as a Compensation Source in SiC Bulk Crystal Growth

Global modeling of the SiC sublimation growth process: prediction of thermoelastic stress and control of growth conditions

The current status of the mathematical model for heat and mass transfer during SiC bulk crystal growth from the vapor phase in inductively heated reactors is reviewed. Results on the simulation of thermoelastic stresses during the growth process are presented. Stresses have been analyzed to exceed considerably the critical resolved shear stress σ CRS =1 MPa which is generally assumed to be the indicator for the onset of dislocation formation in SiC. It is shown that the conditions for stress formation at fixed positions in the crystal vary considerably during growth and that geometric modifications can contribute significantly to a reduction of the stress level. The possible impact of semitransparency of SiC on additional stress generation is discussed. As effective tool for process control and optimization an inverse modeling procedure is introduced.

Virtual reactor as a new tool for modeling and optimization of SiC bulk crystal growth

We propose a new approach to optimization of SiC bulk crystal growth based on modeling. The idea is to employ a special software tool “virtual reactor” (VR) operated by the user of the code as an actual crystal growth system. The software tool includes the models necessary to simulate global heat transfer in the whole growth system and inside the crucible including radiative transport through the semi-transparent SiC crystal. It is known that accurate material properties are crucial for thermal modeling of SiC growth. A database with material properties of SiC crystal and powder, graphites and insulation is included into the VR-software. An advanced model of species transport during sublimation growth of SiC crystals is developed. The model includes convective and diffusive species transport, surface kinetics based on the Hertz–Knudsen equations and chemical models for all solid surfaces (SiC crystal, SiC source, graphite wall). A model to predict type of parasitic deposit and the corresponding deposition rate is combined with the mass transport model available in the VR-software. In this paper, we show the results of simulation of a large-size SiC bulk crystal growth using the VR-software tool with the focus on poly-SiC deposit formation on the graphite crucible lid around the crystal.