Physical Vapor Transport Growth Research Articles

Bulk Growth of Low Resistivity n-Type 4H-SiC Using Co-Doping

The growth of n-type 4H-SiC crystal was performed by physical vapor transport (PVT) growth method by using nitrogen and aluminum (N-Al) co-doping. Resistivity of N-Al co-doped 4H-SiC was as low as 5.8 mΩcm. The dislocation densities of N-Al co-doped substrates were evaluated by synchrotron radiation X-ray topography (SXRT). In addition, epitaxial growth was performed on the N-Al co-doped substrates by chemical vapor deposition (CVD). No double Shockley type stacking fault was observed in the epitaxial layer.

Investigation of the Surface Morphology and Stacking Fault Nucleation on the (000-1)C Facet of Heavily Nitrogen-Doped 4H-SiC Boules

The stacking fault formation during physical vapor transport growth of heavily nitrogen-doped (mid-1019 cm−3) 4H-SiC crystals was investigated. Low-voltage scanning electron microscopy (LVSEM) observations detected the stacking fault formation on the (000-1) facet of heavily nitrogen-doped 4H-SiC crystals. Stacking faults showed characteristic morphologies, and atomic force microscopy (AFM) studies revealed that these morphologies of stacking faults stemmed from the interaction between surface steps and stacking faults. Based on these results, the stacking fault formation mechanism in heavily nitrogen-doped 4H-SiC crystals is discussed.

Freestanding single crystal AlN layers grown using the SiC substrate evaporation method

The physical vapor transport growth of aluminium nitride (AlN) layers was performed in one process with the evaporation of silicon carbide (SiC) substrates. In this paper, we show that the rate of SiC evaporation is slow in a narrow process window for AlN growth but increases in the presence of AlN vapor. The properties of the substrates influence the evaporation process; therefore an investigation is done to understand this phenomenon. We first describe the evaporation behavior of a SiC substrate depending on its size, thickness, structure and polarity. Then we demonstrate the surface morphology and structural quality of the AlN layers grown on C-polar SiC. In the case of a step-flow growth, our method allows us to obtain thin freestanding layers with moderate dislocation densities and rocking curve widths comparable to those of bulk crystals.

Difference of double Shockley-type stacking faults expansion in highly nitrogen-doped and nitrogen-boron co-doped n-type 4H-SiC crystals

The growth of n-type 4H-SiC crystals has been performed by physical vapor transport (PVT) growth method, with nitrogen and boron (N-B) co-doping. It was revealed that, in the growth of 4H-SiC with N-B co-doping, the generation of double Shockley-type stacking faults (DSSFs) was suppressed, as in the case of the nitrogen and aluminum co-doping. The resistivity of the N-B co-doped 4H-SiC was as low as 5.5mΩcm. In addition, we investigated the expansion velocities of DSSFs, which were intentionally caused by indention of crystal surfaces with a diamond indenter, while the crystals were annealed for 2h at 1000°C. The DSSF expansion velocities of the N-B co-doped crystals were found to be lower than the velocities of nitrogen doped crystals. The difference in expansion velocities is discussed in terms of the quantum well action model.

Prismatic Slip in PVT-Grown 4H-SiC Crystals

Basal plane slip is the most frequently observed deformation mechanism in 4H-type silicon carbon (4H-SiC) single crystals grown by the physical vapor transport (PVT) method. However, it was recently reported that dislocations in such crystals can also glide in prismatic slip systems. In this study, we observed nonuniform distributions of three sets of prismatic dislocations in a commercial 4H-SiC substrate wafer. The nonuniformity is a result of the distribution of resolved shear stress on each prismatic slip system caused by radial thermal gradients in the growing crystal boule. A radial thermal model has been developed to estimate the thermal stress across the entire area of the crystal boule during PVT growth. The model results show excellent agreement with the observations, confirming that radial thermal gradients play a key role in activating prismatic slip in 4H-SiC during bulk growth.

(Invited) Schematic Description of the Internal Stress Distribution Responsible for Defect Generation in Larger-Diameter PVT-Grown 4H-SiC Single Crystals

Accurate control of the growth conditions, specifically in view of the temperature field, is more important as the diameter of SiC single crystals becomes larger. The temperature field during the physical vapor transport (PVT) growth inevitably induces the residual internal stresses depending on its actual temperature distribution profile, and the strength of the residual stress will become more significant for larger diameter SiC crystals, probably affecting on the crystal quality such as the dislocation density. Such views has, therefore, been well-accepted so-far as an essential key issue for realizing SiC crystals with lower levels of the dislocation density as well as the appropriately-controlled mechanical properties suitable for device productions such as BOW, and a number of literatures regarding numerical calculations of the stress distribution inside the crystals have hence been performed worldwide [1]. For PVT-growths of 4H-SiC with higher crystallinity applicable to power device applications, it is important to establish first the stabilization of 4H-SiC single polytype growth, and then realize the minimization of residual internal stress levels inside the crystal. However, these guiding principles usually lead to results in contradiction; for example, an appropriate convex shape of the grown crystal surface is necessary for single 4H-SiC polytype crystal growths, but in principle such growth conditions give rise to unwanted generation of the internal stress inside the crystal [2], and therefore a compromise has to be required for careful optimization of designing the growth conditions. In our last presentation at ECS conference in 2012, we have demonstrated that the PVT process of SiC can be described successfully in terms of the phase transition between equilibrium phases in Si-C binary system [3]. In this presentation, we first discuss our model, in particular, in the viewpoint of microscopic elementary reaction processes which are ignored in the original representation scheme despite of its crucial importance for stabilization of 4H-SiC crystal growth. Upon the macroscopic growth stability described above, intense descriptions for possible emergence of thermoelastic inertia forces inside the SiC crystal will be developed by introducing a schematic representation of stress distributions in terms of the thermodynamic formulations [4]. The discussions are concentrated mainly on the shear stress component, σrz, and based upon the results obtained from the representation, the effect of the growth parameters on the SiC crystal quality will be discussed in order to obtain larger-diameter 4H-SiC single crystals with properties of importance for applications, optimized in views of both crystallinity and mechanical characteristics.

(Invited) Study on the Role of Thermal Stress on Prismatic Slip of Dislocations in 4H-SiC Crystals Grown by PVT Method

Basal plane slip is most frequently observed deformation mechanism in 4H type silicon carbon (4H-SiC) single crystals grown by physical vapor transport (PVT) method. It has recently been reported, however, dislocations in such crystals can also glide in the prismatic slip systems. In our study, we have observed non-uniform distributions of three sets of prismatic dislocations in a commercial 4H-SiC substrate wafer. The non-uniformity is likely a result of the distribution of resolved shear stress on each prismatic slip system caused by the radial thermal gradients in the growing crystal boule. A radial thermal model during PVT growth has been developed to estimate the thermal stress across the entire area of the crystal boule. The model makes an excellent agreement with the actual observation, confirming that radial thermal gradients play a key role in activating prismatic slip in 4H-SiC during bulk growth.

Wafer-Scale Graphene on 4-Inch SiC

Wafer-scale graphene on SiC with uniform structural features was grown on semi-insulating 4 inch on-axis 4H-SiC (0001) face. Growth was carried out in a conventional physical vapor transport (PVT) growth system. Atmospheric pressure graphitization and a “face-down” orientation were account for the high uniformity of graphene. Atomic force microscopy, electrostatic force microscopy and Raman spectroscopy were used to confirm the uniformity of surface morphology and layer number. Electrical properties were also characterized by Hall measurements on 15×15mm2 samples sawed from the wafer. An average Hall mobility of about 2000cm2/Vs was obtained.

Total pressure‐controlled PVT SiC growth for polytype stability during using 2D nucleation theory

A total pressure‐controlled physical vapor transport growth method that stabilizes SiC polytype is proposed. The supersaturation of carbon during SiC growth changed as a function of the growth time due to changes in the temperature difference between the surfaces of the source and the grown crystal. Supersaturation also varied as a function of the pressure inside the furnace. Therefore, modification of the pressure as a function of growth time allowed for constant supersaturation during growth. The supersaturation was calculated based on classical thermodynamic nucleation theory using data for heat and species of Si2C and SiC2 transfer in a furnace obtained from a global model. Based on this analysis, a method for polytype‐stabilized SiC growth was proposed that involves decreasing the pressure as a function of growth time. The 4H‐SiC prepared using this pressure‐controlled method was more stable than that of 4H‐SiC formed using the conventional constant‐pressure method.

Structural and Electrical Characterization of the Initial Stage of Physical Vapor Transport Growth of 4H-SiC Crystals

Defect formation during the initial stage of physical vapor transport (PVT) growth of 4H-SiC crystals in the [000-1] and [11-20] directions was investigated by x-ray diffraction, defect-selective etching, and micro Raman scattering imaging. X-ray diffraction studies showed that the growths in the [000-1] and [11-20] directions exhibited markedly different behaviors with respect to the defect formation during the initial stage of growth. While a characteristic lattice plane bending was observed for the PVT growth along [000-1], a tilted domain structure was revealed near the grown crystal/seed interface for the growth in the [11-20] direction. Micro Raman scattering imaging revealed that nitrogen enrichment occurred near the grown crystal/seed interface and was associated with compressive stress parallel to the interface. Based on the results, the defect formation mechanisms during the initial stage of PVT growth of 4H-SiC are discussed.

Polarity Inversion of SiC(0001) during the Al Doped PVT Growth

We found that the polarity of the 4H-SiC is reversed from Si-face to C-face by high Al doping during the physical vapor transport (PVT) growth. KOH etching and deep ultraviolet (DUV) Raman spectroscopy were used to confirm the polarity of the grown crystals. The results show the polarity inversion is occurred in the samples grown on Si-face SiC with using Al doped SiC source material.

Physical Vapor Transport Growth of 4H-SiC on {000-1} Vicinal Surfaces

Bulk 4H-SiC crystals were grown on 4° off-axis seeds by the physical vapor transport technique. Two completely different surface morphologies of as-grown crystals were observed by laser scanning confocal microscopy. The formation mechanisms of the different surface morphologies are proposed and discussed. We found that the facet formation and migration on the 4° off-axis seeds largely depended on the profile evolution of the temperature field in the growth cell over a long-term growth run. At the interface between the two growth regimes, some grown-in defects, including micropipes, dislocations and polytype inclusions, were frequently observed by polarizing optical microscopy and chemical etching. The stress induced by step coalescence could be responsible for the formation of these grown-in defects.

Quantitative Study of the Role of Supersaturation during Sublimation Growth on the Yield of 50 mm 3C-SiC

We have investigated the growth of 3C-SiC using sublimation growth in the temperature range from 1800°C to 1950°C. The supersaturation was determined using numerical modeling of the temperature field and gas phase composition by applying quasi-equilibrium thermodynamic conditions. Analysis of the 3C-SiC yield was carried out by optical microscopy, optical absorption, Raman spectroscopy and x-ray analysis. Quantitative data on supersaturation are compared with most stable 3C-SiC nucleation and growth condition. Finally the application to large area growth in a physical vapor transport growth reactor is briefly addressed.

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.

Magnetic, optical and electrical characterization of SiC doped with scandium during the PVT growth

Scandium is introduced into bulk SiC during the physical vapor transport (PVT) growth. SiC crystals grown with different Sc contents (from 0.5wt% up to 2.5wt%, added to the SiC source material) are studied. Magnetic properties of SiC doped with scandium during the PVT growth are reported for the first time. The presence of antiferromagnetic interactions between magnetic moments of Sc ions is concluded from the temperature dependence of magnetic susceptibility. Detailed PL spectra of 4H-/6H-SiC:B and 4H-/6H-SiC:Sc crystals are presented. A new energy level of 35–37meV is found on SiC:Sc samples and its possible assignment to a complex defect, consisting of nitrogen donor and scandium acceptor, is proposed.

Studies of the Origins of Half-Loop Arrays and Interfacial Dislocations Observed in Homoepitaxial Layers of 4H-SiC

Synchrotron x-ray topography and KOH etching studies have been carried out on n-type 4H-SiC offcut substrates before and after homoepitaxial growth to study defect replication and strain relaxation processes and identify the nucleation sources of both interfacial dislocations (IDs) and half-loop arrays (HLAs), which are known to have a deleterious effect on device performance. Two cases are reported. In one, they nucleate from short segments of edge-oriented basal plane dislocations (BPDs) in the substrate which are drawn into the epilayer. In the other, they form from segments of half-loops of BPD that are attached to the substrate surface prior to growth which glide into the epilayer. The significance of these findings is: (1) It is demonstrated that it is not necessary for a BPD to intersect the substrate surface in order for it to be replicated into the homoepitaxial layer and take part in nucleation of IDs and HLAs; (2) The conversion of the surface intersections of a substrate BPD half-loop into threading edge dislocations (TEDs) does not prevent it from also becoming involved in nucleation of IDs and HLAs. This means that, while BPD to TED conversion can eliminate most of the BPD transfer into the epilayer, further mitigation may only be possible by continued efforts to reduce the BPD density in substrates by control of temperature-gradient- induced stresses during their physical vapor transport (PVT) growth.

Vapor-transport growth of high optical quality WSe2 monolayers

Monolayer transition metal dichalcogenides are atomically thin direct-gap semiconductors that show a variety of novel electronic and optical properties with an optically accessible valley degree of freedom. While they are ideal materials for developing optical-driven valleytronics, the restrictions of exfoliated samples have limited exploration of their potential. Here, we present a physical vapor transport growth method for triangular WSe2 sheets of up to 30 μm in edge length on insulating SiO2 substrates. Characterization using atomic force microscopy and optical microscopy reveals that they are uniform, monolayer crystals. Low temperature photoluminescence shows well resolved and electrically tunable excitonic features similar to those in exfoliated samples, with substantial valley polarization and valley coherence. The monolayers grown using this method are therefore of high enough optical quality for routine use in the investigation of optoelectronics and valleytronics.

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.

Defect formation during the initial stage of physical vapor transport growth of 4H–SiC in the [formula omitted] direction

Defect formation during the initial stage of physical vapor transport (PVT) growth of 4H–SiC in the [112¯0] direction has been investigated by x-ray reciprocal space mapping (RSM), defect-selective etching, and low-voltage scanning electron microscopy. RSM studies showed that 4H–SiC crystals grown in the [112¯0] direction showed a significant degradation of crystalline quality during the initial stage of PVT growth, compared with crystals grown in the [0001¯] direction; the growth in the [112¯0] direction resulted in a misoriented domain structure near the grown crystal/seed interface. At the interface, high densities of basal plane dislocations, extending parallel to the interface, and threading edge dislocations along the c-axis were observed. The former caused a tilt domain boundary around an axis parallel to [11¯00]. Based on the results, the paper discusses the defect formation mechanism during the initial stage of PVT growth in the [112¯0] direction and points to the difference in nitrogen concentration between the seed and the grown crystal as a cause of the observed domain and dislocation structure at the grown crystal/seed interface.

Role of SiC substrate polarity on the growth and properties of bulk AlN single crystals

Two symmetrically nonequivalent silicon carbide (SiC) substrate orientations, (0001) Si-terminated and \((000\overline{1} )\) C-terminated, were used in the physical vapour transport growth of bulk aluminium nitride (AlN) single crystals. The crystals grown on Si-faces always exhibit an Al-polar growth surface. AlN growth on \((000\overline{1} )\) C-terminated surfaces of the SiC substrates was performed to obtain N-polar growth surfaces. An abrupt interface was observed between the AlN crystal and the C-face substrate which is in contrast to the growth on Si-faces where hexagonally shaped SiC hillocks are formed. The growth on C-faces is usually dominated by multi-site nucleation. Applying similar supersaturation conditions that led to step-flow growth on Si-faces to the C-faces resulted in a spiral growth mode, even on highly off-oriented substrates. The obtained broad X-ray diffraction rocking curves of such samples (full-width at half-maximum ≈380 arcsec) indicate the presence of more misfit dislocations and significant misfit stress. In addition, polarity inversion is observed in C-face grown crystals. Though the structural properties of the crystals grown on C-face are inferior to that of the crystals grown on Si-face, the incorporation of unintentional Si impurity was found to be lower (<2 wt%).