4H-SiC Crystals Research Articles

Characterization of partial dislocations for (3, 3, 4), (3, 3, 3, 3), and (3, 3, 2, 2, 4) stacking faults in 4H-SiC crystals

The microstructure of the partial dislocations was characterized with respect to three types of stacking faults found in our previous work on 4H-SiC epitaxial film on p-type substrates. These stacking faults were found to be Shockley-type with five, six, and nine Shockley-type partial dislocations with Burgers vector directions parallel to the basal planes, which are more than previously reported. The correspondence between the number of Shockley partial dislocations, the stacking sequence, and the basal planes in which partial dislocations are introduced was discussed for the number of partial dislocations up to nine with regard to Shockley-type stacking faults with expansion from previous reports.

Experimental and simulation studies of surface segregation-limited nitrogen incorporation at the growth front of physical vapor transport-grown 4H-SiC crystals

Experimental and simulation studies were conducted for surface segregation-limited kinetics of nitrogen incorporation into a 4H-SiC crystal during physical vapor transport (PVT) crystal growth. It was revealed that the nitrogen incorporation is kinetically limited by the step-flow velocity on the growing crystal surface of a 4H-SiC crystal; in this study, the surface step-flow velocity at the growth front was deduced from the local inclination angle of the growth front measured from the (0001¯) plane, assuming a uniform growth rate along the c-axis (crystal growth direction) across the growth front, and the nitrogen concentration across the growth front was measured using Raman scattering microscopy. The step-flow velocity dependence of nitrogen incorporation was theoretically analyzed using a two-site-exchange model, and the simulated dependence using the model was in good agreement with the experimental data. On the basis of these experimental and simulation results, kinematical and energetical aspects of nitrogen incorporation at the growth front of a 4H-SiC crystal during PVT growth are discussed.

Electrical Relaxation and Transport Properties of ZnGeP2 and 4H-SiC Crystals Measured with Terahertz Spectroscopy

Terahertz photoconductivity and charge carrier recombination dynamics at two-photon (ZnGeP2) and three-photon (4H-SiC) excitation were studied. Thermally annealed, high-energy electron-irradiated and Sc-doped ZnGeP2 crystals were tested. The terahertz charge carrier mobilities were extracted from both the differential terahertz transmission at a specified photoexcitation condition and the Drude–Smith fitting of the photoconductivity spectra. The determined terahertz charge carrier mobility values are ~453 cm2/V·s for 4H-SiC and ~37 cm2/V·s for ZnGeP2 crystals. The charge carrier lifetimes and the contributions from various recombination mechanisms were determined at different injection levels using the model, which takes into account the influence of bulk and surface Shockley–Read–Hall (SRH) recombination, interband radiative transitions and interband and trap-assisted Auger recombination. It was found that ZnGeP2 possesses short charge carrier lifetimes (a~0.01 ps−1, b~6 × 10−19 cm3·ps−1 and c~7 × 10−40 cm6·ps−1) compared with 4H-SiC (a~0.001 ps−1, b~3 × 10−18 cm3·ps−1 and c~2 × 10−36 cm6·ps−1), i.e., τ~100 ps and τ~1 ns at the limit of relatively low injection, when the contribution from Auger and interband radiative recombination is small. The thermal annealing of as-grown ZnGeP2 crystals and the electron irradiation reduced the charge carrier lifetime, while their doping with 0.01 mass % of Sc increased the charger carrier lifetime and reduced mobility. It was found that the dark terahertz complex conductivity of the measured crystals is not fitted by the Drude–Smith model with reasonable parameters, while their terahertz photoconductivity can be fitted with acceptable accuracy.

Quantification of nitrogen in heavily doped silicon carbide by soft X-ray emission spectroscopy

Nitrogen (N) is a crucial component in silicon carbide (SiC) crystals to control conductivity. It acts as an electron carrier that offers n-type conduction, and it is also employed for growing low-resistivity p-type SiC by co-doping with aluminum (Al). In this study, we quantified the N content in 4H-SiC using soft X-ray emission spectroscopy (SXES) combined with a scanning electron microscope (SEM). SXES analyses of 4H-SiC crystals with different N concentrations successfully quantified N concentration of 1.5 × 1019 cm−3 or higher, which fall within the range for heavily N-doped SiC crystals. Further, N concentration mapping was performed, and Al concentration in the same region was evaluated by a wavelength dispersive spectrometer (WDS) attached to the SEM. High concentrations of Al and N were identified and both dopants tended to increase with crystal growth, suggesting that their incorporation correlated with the Al-N interactions. Therefore, SXES and its combination with WDS are promising non-destructive technologies that can be employed to study the growth mechanism of SiC.

Development of High Quality 8 Inch 4H-SiC Substrates

8 inch 4H-silicon carbide (SiC) development faces challenges first from obtaining high-quality 8 inch SiC seed substrate, then reducing grown-in crystal residual stress and defects in the following crystal growth process. Here we report the diameter expansion process from 6 inch 4H-SiC seed substrate to 8 inch 4H-SiC crystal. Based on simulation and experimental results, it is deduced that an optimized radial temperature gradient (RTG) zone in the range of 0.10-0.12 °C/mm is essential for high-quality and efficient SiC crystal diameter expansion. According to the RTG calculation, diameter expansion process is designed and 8 inch 4H-SiC crystal as well as seed substrate is achieved. With the obtained seed substrate, high-quality 8 inch 4H-SiC crystal is developed and the following polished 4H-SiC substrate quality is characterized.

Effective Penetration Depth Investigation for Frank Type Dislocation (Deflected TSDs/TMDs) on Grazing Incidence Synchrotron X-Ray Topographs of 4H-SiC Wafers

In 4H-SiC crystals, Frank type dislocations are created through the deflection of threading screw/mixed dislocations onto the basal plane. Grazing-incidence X-ray topographs are often used to evaluate the density of such dislocations and a knowledge of the effective penetration depth is therefore essential. In this study, a systematic analysis is performed to investigate the effective penetration depth, which is the depth from which contrast from the dislocation is still discernible. This is achieved by comparison between observed topographic images and detailed ray tracing simulations. Simulations shows no significant contrast difference between a deflected TSD and a deflected TMD with the same line direction since the large c component is the dominant contributor to the effective misorientation, whereas the effect of a component is rather negligible. Therefore, this effective penetration depth study uses ray tracing simulation images of deflected TSDs with photoelectric absorption applied to compare with all topographically observed Frank type dislocations. Analysis first reveals that the effective penetration depth varies with the line direction of a Frank type dislocation, and the effective penetration depth is significantly deeper compared to that of a BPD. Further, the effective penetration depth on ray tracing simulations with absorption applied matches well with experimentally measured depth. The study also evaluated the effectiveness of a simplified model based on an approximate expression for the effective misorientation of a dislocation modulated by photoelectric absorption. This was also found to yield satisfactory results and can be used as a universal method to determine the effective penetration depth for Frank type dislocations with c component of Burgers vector.

Atomic scale localization of Kohn–Sham wavefunction at SiO2/4H–SiC interface under electric field, deviating from envelope function by effective mass approximation

To clarify the cause of the low channel conductivity at the SiO2/4H–SiC interface, the wavefunction at the SiC conduction band minimum was calculated using density functional theory under an applied electric field. We found that the wavefunction for a 4H–SiC (0001) slab tends to be localized at the cubic site closest to the interface. Importantly, because the conduction electrons are distributed closer to the interface (<5 Å) than expected from the effective mass approximation (EMA), they are more frequently scattered by interface defects. This is expected to be the reason why the channel conductivity for the (0001) face is particularly low compared with that for other faces, such as (112¯0). The breakdown of the EMA for the (0001) interface is related to the long structural periodicity along the [0001] direction in 4H–SiC crystals.

Physics of molecular deformation mechanism in 6H-SiC

Even though there have been several studies in literature of 6H SiC, a proper physics based understanding of the molecular deformation mechanisms of the material under different loading conditions is still lacking. Experimentally, the brittle nature of the material leads to difficulties associated with in-situ determination of molecular deformation mechanisms of the material under an applied load; whereas, the complex material structure along with the bonding environment prevents proper computational identification of different types of inelasticity mechanisms within the material. Molecular dynamics study (on successful verification of the interatomic potential with experimental results) of pristine single crystals of 6H SiC have been used to probe the physics of molecular deformation mechanisms of the material along with its inherent orientational anisotropy. The study elucidates the experimentally observed mechanisms of defect nucleation and evolution through a detailed analysis of radial distribution functions, x-ray diffraction as well as phonon vibrational studies of the single crystal. Studies have been presented at room temperature, initial high temperature and different types of confinement effects of the material (including hydrostatic and different biaxial loading cases). The confinement resulted in an increase in stress and stiffness whereas increase in initial temperature resulted in a decrease compared to uniaxial stress loading conditions at room temperature.

Study on the Surface Structure of N-Doped 4H-SiC Homoepitaxial Layer Dependence on the Growth Temperature and C/Si Ratio Deposited by CVD

The quality of the N-doped 4H-SiC homoepitaxial layers grown via hot-wall horizontal chemical vapor deposition (CVD) was evaluated at various C/Si ratios (1.0–1.2) and growth temperatures (1570–1630 °C). The microstructure and morphology of the epilayers were studied through a comparative analysis of the AFM patterns under different growth conditions. X-ray photoelectron spectroscopy and Raman spectroscopy revealed the quality of the 4H-SiC epilayers and the amount of N-doping. It was found that an increase in the C/Si ratio enabled obtaining a quite smooth epitaxial layer surface. Moreover, only the 4H-SiC crystal type was distinguished in the epilayers. In addition, the epitaxial quality was gradually improved, and the amount of defect-related C-C bonds significantly dropped from 38.7% to 17.4% as the N doping content decreased from 35.3% to 28.0%. An increase in the growth temperature made the epitaxial layer surface smoother (the corresponding RMS value was ~0.186 nm). According to the Raman spectroscopy data, the 4H-SiC forbidden mode E1(TO) in the epilayers was curbed at a higher C/Si ratio and growth temperature, obtaining a significant enhancement in epitaxial quality. At the same time, more N dopants were inserted into the epilayers with increasing temperature, which was opposite to increasing the C/Si ratio. This work definitively shows that the increase in the C/Si ratio and growth temperature can directly enhance the quality of the 4H-SiC epilayers and pave the way for their large-scale fabrication in high-power semiconductor devices.

Study on the damage evolution of 6H-SiC under different phosphorus ion implantation conditions and annealing temperatures

In this work, we investigate the radiation damage of 6H-SiC crystals by phosphorus (P) ion implantation. The 6H-SiC samples were implanted at different energy with the same fluence of 5.0 × 1014 ions/cm2 at room temperature, one of which was subsequently annealed. Raman and absorption spectra were obtained to probe the structure and optical properties of 6H-SiC crystals after P ion implantation. In addition, Rutherford backscattering/channeling spectroscopy and transmission electron microscopy were used to explore further the damage behavior at different implantation energy and damage evolution with post-annealing treatment. It was found that the damage of the 200 keV implanted sample partially recovered after annealing at 600 °C for 60 min and disappeared at an annealing temperature of 800 °C. The damage evolution behavior of phosphorus ions implantation into 6H-SiC crystals with different implantation and annealing conditions is presented in our work.

Suppression of stacking-fault expansion in 4H-SiC PiN diodes using proton implantation to solve bipolar degradation

4H-SiC has been commercialized as a material for power semiconductor devices. However, the long-term reliability of 4H-SiC devices is a barrier to their widespread application, and the most important reliability issue in 4H-SiC devices is bipolar degradation. This degradation is caused by the expansion of single Shockley stacking-faults (1SSFs) from basal plane dislocations in the 4H-SiC crystal. Here, we present a method for suppressing the 1SSF expansion by proton implantation on a 4H-SiC epitaxial wafer. PiN diodes fabricated on a proton-implanted wafer show current–voltage characteristics similar to those of PiN diodes without proton implantation. In contrast, the expansion of 1SSFs is effectively suppressed in PiN diodes with proton implantation. Therefore, proton implantation into 4H-SiC epitaxial wafers is an effective method for suppressing bipolar degradation in 4H-SiC power-semiconductor devices while maintaining device performance. This result contributes to the development of highly reliable 4H-SiC devices.

Physical-Vapor-Transport growth of 4H silicon carbide single crystals by a tiling method

Increasing the diameter and reducing the dislocation density of 4H silicon carbide (4H-SiC) single crystals are the development tendency of 4H-SiC single-crystal substrate to reduce the cost of 4H-SiC based devices. In this work, we use the growth of a 100 mm 4H-SiC single crystal as an example to demonstrate that the tiling approach is attractive to enlarge the diameter of physical-vapor-transport (PVT)-grown 4H-SiC single crystals by a single growth period. By titling four pieces of 4H-SiC seed crystals, a 100 mm 4H-SiC boule without crystal deterioration is obtained by the PVT growth method. The shape of the growth interface is convex and complete after the PVT growth, indicating the tiling gaps between neighboring seed-crystal pieces are healed after the PVT growth of 25-mm thick 4H-SiC. Wafers are obtained after sequential slicing, lapping, and chemical mechanical polishing (CMP). The high-resolution X-ray diffraction (HRXRD) rocking curve measurements of the wafer at the latest growth stage indicate that the 4H-SiC crystal is basically healed, with mesoscopic boundaries being created at the central area of the wafer. Raman spectra indicate that the healing polymorph across the tiling gaps is 4H-SiC. We find that threading edge dislocations (TEDs) dominate the dislocation types in the 4H-SiC substrate wafers at the initial, middle, and latest growth stages. As the PVT growth proceeds, the density of dislocations in 4H-SiC wafers significantly decreases, as a result of the crystalline healing. Our work opens a pathway to enlarge the size of 4H-SiC single crystals in a single PVT growth period, which paves the way for the high-efficiency growth of large-size 4H-SiC single-crystals.

Effective Penetration Depths Investigation on Synchrotron X-Ray Topographic Images of Basal Plane Dislocations in 4H-SiC Crystals through Ray-Tracing Simulation

The demand for power electronic devices fabricated from silicon carbide (SiC) has increased rapidly in recent years for a diverse range of existing and emerging applications. The development of device technology highly depends on the quality of substrate and epitaxial material and the number of deleterious defects within, as those are critical factors that impact the device performance and long-term reliability [1, 2]. Therefore, understanding the nature of defects is of great importance for preventing the degradation or premature breakdown of devices. Synchrotron X-ray topography (XRT) is a dominant analytical method for the imaging and characterization of crystallographic defects. For 4° off-axis (0001) 4H-SiC wafers, the grazing-incidence geometry topographic technique is commonly employed to focus on the defect investigation near surface region. In this geometry, a small incidence angles (~1-2°) is used for allowing X-ray to only penetrate the sample with a shallow depth below the surface, therefore, it is essential to establish a procedure to define the effective penetration depth for detailed analysis of dislocation structures and density calculations. In earlier studies, Dudley in 1989 concluded that the observed penetration depths are limited by photoelectric absorption rather than extinction [3]. Ishiji in 2017 estimated the penetration depth based on the photoelectric absorption experienced by the diffracted X-rays [4]. However, only basal plane dislocations (BPDs) with screw segments parallel to the off-cut [11-20]-direction were evaluated in that study, which is insufficient for evaluating other types of BPDs. Therefore, a universal method to determine the effective penetration depth for all dislocation types would be of more practical importance.In this study, a procedure to define this effective penetration depth is demonstrated by investigating physical vapor transport (PVT) grown 4° off-axis 4H-SiC crystals through preforming systematic analysis of topographic and ray-tracing simulated contrast of BPDs with different Burgers vector and line direction combinations. The topographic images are recorded through synchrotron monochromatic beam in grazing incidence geometry. The simulation model is based on the principle of orientation contrast mechanism [5, 6] and developed by considering both the effects of surface relaxation [7] and X-ray absorption [8, 9] for evaluating the contributed factors of dislocation contrast formation. Analysis of the results indicates the observable dislocation contrast depends on the effective misorientation associated with the dislocation modulated by the photoelectric absorption effect. The dislocations with larger effective misorientation are associated with longer projected lengths and corresponding deeper effective penetration depths. A more simplified factor based on an approximate expression for the misorientation is also introduced in this study as an alternative to the full ray-tracing simulation approach. The effective penetration depth determination method presented in this study enables three-dimensional dislocation configuration analysis and accurate density calculations.Reference: Das, H., et al., Materials Science Forum, 2019. 963: p. 284-287.Kimoto, T., et al. 2017 IEEE International Reliability Physics Symposium (IRPS). 2017.Dudley, M., J. Wu, and G.D. Yao, Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, 1989. 40-41: p. 388-392.Ishiji, K., et al., Japanese Journal of Applied Physics, 2017. 56(10): p. 106601.Dudley, M., X.R. Huang, and W. Huang, Journal of Physics D: Applied Physics, 1999. 32(10A): p. A139-A144.Huang, X.R., et al., Journal of Applied Crystallography, 1999. 32(3): p. 516-524.Peng, H., et al., Journal of Applied Crystallography, 2021. 54(2): p. 439-443.Fujie, F., et al., Acta Materialia, 2021. 208: p. 116746.Ailihumaer, T., et al., Materials Science and Engineering: B, 2021. 271: p. 115281. Figure 1

Temperature-sensitive hybridization of propagating and localized surface phonon polaritons in polar 4H-SiC nano-resonators

Polar 4H-SiC nano-resonators can host localized surface phonon polaritons (SPhPs) with low optical loss and fully exploit light–matter interactions for promising nanophotonic applications such as surface-enhanced spectroscopies and thermal imaging. To expand the mid-infrared and infrared application spaces, a sound knowledge of temperature effect on infrared dielectric functions and SPhPs of 4H-SiC is required, yet it remains largely unexplored. Herein, we focus on exploiting the temperature influence on dielectric functions, hybridization of propagating and localized SPhPs, and tailed spectral radiation properties of 4H-SiC nanopillar arrays through spectroscopic ellipsometry (SE) measurements as well as multiscale simulations. The 4H-SiC crystal is grown using the physical vapor transport method, and SE experiments measure infrared dielectric functions at temperatures between 300 and 800 K. Finite-element electromagnetic simulations confirm the emerged Monopole and transverse dipoles (TD1 and TD2) resonance modes in 4H-SiC nano-resonators, which agrees with the literature experiment. At high temperatures with strong lattice vibration, the amplitudes of resonant optical absorption peaks gradually decrease and the linewidths broaden, accompanied by the weakened electric resonances. First-principles calculations show that the anharmonic phonon scattering strengthens and less optical phonons are coupled to incident photons as temperature increases. Moreover, the propagating and localized SPhPs’ hybridization and spectral radiation properties of 4H-SiC nanopillar arrays can be largely tailed by modifying the morphology and incident angle of light. This work provides physical insights into the temperature-induced spectral tuning of 4H-SiC nano-resonators and helps exploit their applications in the high-temperature working conditions.

Threading dislocation increase in the initial stage of growth of nitrogen and boron co-doped 4H-SiC by physical vapor transport

In this study, n-type N-B co-doped 4H-SiC crystals were grown by the physical vapor transport method. We investigated the relationship between the boron concentration in the grown crystal and the mixing ratio of boron carbide (B4C) in the SiC source powder. In addition, we analyzed the relationship between the boron concentration and dislocation increase in the initial stage of growth. We found that an increase of threading dislocation occurred when the boron concentration was higher than 1 × 1019 cm−3 in the initial stage of growth. This threading dislocation increase can be suppressed by performing a pre-sublimation process of the mixed SiC and B4C powder.

Investigation on dislocation and deflection morphology of PVT-grown on-axis 4H-SiC crystals

The morphologies of dislocation etch pits and dislocation deflections of on-axis 4H-SiC substrate etched by molten KOH were observed with the help of a microscope. Based on experimental observation and etch mechanism, a method for the identification of threading edge dislocations, threading screw dislocations (TSDs) and threading mixed dislocations was proposed. The details about the inner micro-structure of threading edge dislocations and TSDs were observed by laser scanning confocal microscope and scanning electron microscopy. The morphologies and the cross-sectional views of the basal plane dislocation formed by threading edge dislocation were observed and two models were formed to explain it.

Seed surface orientation dependence of the defect formation at the initial stage of physical vapor transport growth of 4H-SiC crystals

The influence of the seed crystal surface orientation on the defect formation at the initial stage of physical vapor transport (PVT) growth of 4H-SiC crystals was investigated using X-ray topography and Raman scattering microscopy. To examine the detailed influence of the seed surface orientation, a seed crystal having a groove on the growth surface was used for PVT growth; the seed crystal with a groove provides a continuously varying seed surface orientation from (0 0 0 1¯) (on-axis c-face growth) to (1 1 2¯ 0) (a-face growth), and using the seed crystal, the effect of the seed surface orientation can be examined in a single growth experiment. X-ray topography observations revealed that a number of threading screw dislocations (TSDs) were formed at the bottom corners of the groove, while their formation was largely suppressed at the bottom of the groove, indicating that an off-oriented (0 0 0 1¯) seed crystal surface tends to generate TSDs. Characteristic dopant striations were also observed in the crystal portions grown on the off-oriented seed crystal surface, and the generated TSDs were found to propagate along the boundaries of these dopant striations. Based on these results, we discuss the TSD formation mechanism on an off-oriented (0 0 0 1¯) seed crystal surface at the initial stage of PVT growth of 4H-SiC crystals and suggest an important role of macrostep formation on the surface.

Enhanced nitrogen incorporation in the 〈112̄0〉 directions on the (0001̄) facet of 4H-SiC crystals

Enhanced nitrogen doping in the 〈110〉 directions on the (000) facet of 4H-SiC crystals grown by the physical vapor transport (PVT) growth method was investigated using Raman scattering microscopy and atomic force microscopy (AFM). The enhanced nitrogen doping showed either single or double peak structure of nitrogen incorporation in the azimuthal direction around 〈110〉. AFM observations revealed that these two characteristic doping structures stemmed from different propagation morphologies of spiral steps on the (000) facet. It was also unveiled that the change in the step-flow direction was always associated with the change of the terrace-width ratio on the facet, and that the interplay between these two parameters determined the nitrogen incorporation on the (000) facet. On the basis of these experimental results, the mechanism of the azimuthal dependence of nitrogen incorporation on the (000) facet during PVT growth of 4H-SiC crystals is discussed.

Fabrication and polarization modulation of waveguides in 4H-SiC crystals by femtosecond laser direct writing

Fabrication and polarization modulation of waveguides in 4H-SiC crystals by femtosecond laser direct writing

Investigation of propagation and coalescence of threading screw and mixed dislocations in 4H-SiC crystals grown by the high-temperature gas source method

Propagation of threading screw dislocations (TSDs) and mixed dislocations (TMDs) in 4H-SiC crystals grown by the high-temperature gas source method were investigated. Cross-sectional samples cut in the growth direction were prepared and X-ray transmission topography with a diffraction vector g = 0004¯ was performed to reveal the dislocation lines. The topography image revealed that some pairs of TSDs/TMDs extending in the growth direction approached each other and merged in the middle of the grown crystal. Direct observations by synchrotron X-ray section topography confirmed that a pair of TSD and TMD with opposing c-component Burgers vectors had merged and the c-components of the dislocations were annihilated during crystal growth. Some pairs of TSDs/TMDs, however, showed no coalescence, although they could be annihilated topologically. The mechanism behind the success and failure of TSD/TMD coalescence was also discussed.