4H-SiC Crystals Research Articles

Impact of Mechanical Stress and Nitrogen Doping on the Defect Distribution in the Initial Stage of the 4H-SiC PVT Growth Process.

Nitrogen incorporation changes the lattice spacing of SiC and can therefore lead to stress during physical vapor transport (PVT). The impact of the nitrogen-doping concentration during the initial phase of PVT growth of 4H-SiC was investigated using molten potassium hydroxide (KOH) etching, and the doping concentration and stress was detected by Raman spectroscopy. The change in the coefficient of thermal expansion (CTE) caused by the variation of nitrogen doping was implemented into a numerical model to quantitatively determine the stress induced during and after the crystal growth. Furthermore, the influence of mechanical stress related to the seed-mounting method was studied. To achieve this, four 100 mm diameter 4H-SiC crystals were grown with different nitrogen-doping distributions and seed-mounting strategies. It was found that the altered CTE plays a major role in the types and density of defect present in the grown crystal. While the mounting method led to increased stress in the initial seeding phase, the overall stress induced by inhomogeneous nitrogen doping is orders of magnitude higher.

Defect-Induced Luminescence Quenching of 4H-SiC Single Crystal Grown by PVT Method through a Control of Incorporated Impurity Concentration

The structural defect effect of impurities on silicon carbide (SiC) was studied to determine the luminescence properties with temperature-dependent photoluminescence (PL) measurements. Single 4H-SiC crystals were fabricated using three different 3C-SiC starting materials and the physical vapor transport method at a high temperature and 100 Pa in an argon atmosphere. The correlation between the impurity levels and the optical and fluorescent properties was confirmed using Raman spectroscopy, X-ray diffraction, inductively coupled plasma atomic emission spectroscopy (ICP-OES), UV-Vis-NIR spectrophotometry, and PL measurements. The PL intensity was observed in all three single 4H-SiC crystals, with the highest intensities at low temperatures. Two prominent PL emission peaks at 420 and 580 nm were observed at temperatures below 50 K. These emission peaks originated from the impurity concentration due to the incorporation of N, Al, and B in the single 4H-SiC crystals and were supported by ICP-OES. The emission peaks at 420 and 580 nm occurred due to donor–acceptor-pair recombination through the incorporated concentrations of nitrogen, boron, and aluminum in the single 4H-SiC crystals. The results of the present work provide evidence based on the low-temperature PL that the mechanism of PL emission in single 4H-SiC crystals is mainly related to the transitions due to defect concentration.

Comparative study of the MeV ion channeling implantation induced damage in 6H-SiC by the iterative procedure and phenomenological CSIM computer code

Due to its unique material properties, such as extreme hardness and radiation resistance, silicon carbide has been used as an important construction material for environments with extreme conditions, like those present in nuclear reactors. As such, it is constantly exposed to energetic particles (e.g., neutrons) and consequently subjected to gradual crystal lattice degradation. In this article, the 6H-SiC crystal damage has been simulated by the implantation of 4 MeV C3+ ions in the (0001) axial direction of a single 6H-SiC crystal to the ion fluences of 1.359 1015 cm-2, 6.740 1015 cm-2, and 2.02 1016 cm-2. These implanted samples were subsequently analyzed by Rutherford and elastic backscattering spectrometry in the channeling orientation (RBS/C & EBS/C) by the usage of 1 MeV protons. Obtained spectra were analyzed by channeling simulation phenomenological computer code (CSIM) to obtain quantitative crystal damage depth profiles. The difference between the positions of damage profile maxima obtained by CSIM code and one simulated with stopping and range of ions in matter (SRIM), a Monte Carlo based computer code focused on ion implantation simulation in random crystal direction only, is about 10%. Therefore, due to small profile depth shifts, the usage of the iterative procedure for calculating crystal damage depth profiles is proposed. It was shown that profiles obtained by iterative procedure show very good agreement with the ones obtained with CSIM code. Additionally, with the introduction of channeling to random energy loss ratio the energy to depth profile scale conversion, the agreement with CSIM profiles becomes excellent.

Stacking Fault Formation During the Seeding Process of Physical Vapor Transport Growth of 4h-Sic Crystals

Stacking Fault Formation During the Seeding Process of Physical Vapor Transport Growth of 4h-Sic Crystals

Growth of p-type 4H-SiC single crystals by physical vapor transport using p-type SiC powder

In this study, a novel Al doping method was used to grow p-type SiC. p-Type 4H-SiC crystals were grown by a physical vapor transport (PVT) method using p-type SiC powder.

Undulated Step Structure on the (0001¯) Facet of Physical Vapor Transport-Grown 4H-SiC Crystals.

The step structure on the (000)C facet of 4H-SiC boules grown by the physical vapor transport growth method with different nitrogen doping concentrations was examined in various scales, using different types of microscopy, such as differential interference contrast optical microscopy (DICM) and atomic force microscopy (AFM). DICM observations unveiled characteristic macroscopic surface features of the facet dependent on the nitrogen doping concentration. AFM observations revealed the existence of step trains of half unit-cell height (0.5 nm) on the facet and found that their separation was undulated with a characteristic wavelength dependent on the nitrogen doping concentration; the higher the nitrogen concentration, the longer was the undulation wavelength of step separation. Based on these results, we discussed the origin and formation mechanism of the separation-undulated step structure observed on the (000)C facet of nitrogen-doped 4H-SiC boules.

The quantitative 6H-SiC crystal damage depth profiling

The hexagonal silicon carbide (6H-SiC) is one of materials used in nuclear applications, and as such is exposed to crystal damage inducing by variety of energetic particles like neutrons. In this article the 6H-SiC crystal lattice damage was introduced by the 4 MeV C3+ and 4 MeV Si3+ channelling ion implantation at the room temperature. The implantation of C and Si ions (so called self-ions) to the set of different fluences, achieves a 6H-SiC crystal lattice damage more similar to what the exposure to neutrons would produce. The 6H-SiC crystal damage has been investigated by the Elastic Backscattering spectra taken in the channeling orientation (EBS/C). EBS/C spectra of the implanted 6H-SiC samples were taken with 1.725 MeV and 1.860 MeV protons. By fitting the EBS/C spectra, the quantitative 6H-SiC crystal damage depth profiles were obtained. Further, the cross section of crystal's implanted region has been scanned with the micro-Raman (μR) technique for a comparison. In this way, the qualitative analysis of a non-crystalline phase as a function of the crystal depth was independently determined. Additionally, a scanning electron microscopy (SEM) image was taken of the implanted crystal cross sections. The comparison of the crystal damage profiles obtained by fitting EBS/C spectra with the corresponding ones obtained with the μR and SEM techniques shows very good consistency between them.

Analysis of Dislocation Contrast in Synchrotron Grazing-incidence X-ray Topographs and Ray-tracing Simulation in Off-axis 4H-SiC Crystals

The contrast of dislocations lying on the basal plane in off-axis 4H-SiC crystals, such as basal plane dislocations, deflected threading screw and mixed dislocations are simulated using a more sophisticated ray-tracing simulation method, which is based on the orientation contrast mechanism. The effects of surface relaxation combined with X-ray absorption are both considered in this simulation method in order to further evaluate the factors contributing to the dislocation contrast formation. Comparison of the simulated results with the actual grazing-incidence topographic images of the dislocations reveals that the dislocation contrast is dominated by surface relaxation effect, which mainly occurs for diffractions near the crystal surface. Depth analysis shows that the diffracted X-ray beam from regions below the dislocation contribute additional contrast to the image formation. With consideration of X-ray absorption, the simulated contrast of the dislocation gradually weakens as the diffraction position gets deeper into the crystal. Detailed discussion on each type of dislocation on the basal plane including inclined dislocation contrast is presented in this paper. The full BPD loop contrast is also simulated and thus the Burgers vectors of all types of BPDs can be determined by comparing with the simulated results.

Micropipes in SiC Single Crystal Observed by Molten KOH Etching.

Micropipe, a “killer” defect in SiC crystals, severely hampers the outstanding performance of SiC-based devices. In this paper, the etching behavior of micropipes in 4H-SiC and 6H-SiC wafers was studied using the molten KOH etching method. The spectra of 4H-SiC and 6H-SiC crystals containing micropipes were examined using Raman scattering. A new Raman peak accompanying micropipes located near −784 cm−1 was observed, which may have been induced by polymorphic transformation during the etching process in the area of micropipe etch pits. This feature may provide a new way to distinguish micropipes from other defects. In addition, the preferable etching conditions for distinguishing micropipes from threading screw dislocations (TSDs) was determined using laser confocal microscopy, scanning electron microscopy (SEM) and optical microscopy. Meanwhile, the micropipe etching pits were classified into two types based on their morphology and formation mechanism.

Fully Optical Detection of Hyperfine Electron–Nuclear Interactions in Spin Centers in 6H-SiC Crystals with a Modified 13C Isotope Content

Fully Optical Detection of Hyperfine Electron–Nuclear Interactions in Spin Centers in 6H-SiC Crystals with a Modified 13C Isotope Content

Effects of boron doping on the fabrication of dense 6H-SiC ceramics by high-temperature physical vapor transport

In this paper, boron-doped dense 6H-SiC ceramics was fabricated by the high-temperature physical vapor transport (HTPVT) method. The effect of B doping on the crystal structure stability of 6H-SiC was investigated based on density functional theory (DFT). The results show that B doping can be realized even under thermodynamical equilibrium conditions. Nevertheless, it is found that the B doping effects on the (0001) of Si-plane and (000-1) of C-plane are significantly different. The doping experiments demonstrated that B can observably change the crystal growth morphology, leading to the formation of elongated 6H-SiC crystals.

Analysis of Dislocation Contrast in Synchrotron Grazing-incidence X-ray Topographs and Ray-tracing Simulation in Off-axis 4H-SiC Crystals

A sophisticated simulation model is developed based on the principle of ray-tracing to simulate the grazing-incidence synchrotron X-ray topographic contrast of dislocations lying on the basal plane including basal plane dislocations and deflected threading screw and mixed dislocations in off-axis 4H-SiC crystals. The model incorporates effects of surface relaxation as well as the photoelectric absorption to predict dislocation contrast. Compared to conventional ray-tracing images, surface relaxation effects dominate dislocation contrast for diffraction near the crystal surface. The simulated dislocation contrast gradually weakens with increasing depth of the diffracted beam position within the crystal due to photoelectric absorption. The distinctive features of the net simulated dislocation images obtained by aggregating through the effective penetration depth correlate well with contrast features observed on the experimental topographic images. Depth analysis reveals that in some cases the diffracted X-rays from regions below the dislocation can contribute additional contrast features previously not considered.

Dislocation contrast on X-ray topographs under weak diffraction conditions

The contrast of dislocations in 4H-SiC crystals shows distinctive features on grazing-incidence X-ray topographs for diffraction at different positions on the operative rocking curve. Ray-tracing simulations have previously been successfully applied to describe the dislocation contrast at the peak of a rocking curve.The present work shows that the dislocation images observed under weak diffraction conditions can also be simulated using the ray-tracing method. These simulations indicate that the contrast of the dislocations is dominated by orientation contrast. Analysis of the effective misorientation reveals that the dislocation contrast in weak-beam topography is more sensitive to the local lattice distortion, consequently enabling information to be obtained on the dislocation sense which cannot be obtained from the peak.

Massive reduction of threading screw dislocations in 4H-SiC crystals grown by a hybrid method combined with solution growth and physical vapor transport growth on higher off-angle substrates

Propagation of threading screw dislocations (TSDs) during hybrid growth, combined with TSD conversion by solution growth and bulk growth by physical vapor transport, has been investigated for 4° off and 15° off C-face seeds. The converted defects on basal planes (BP-defects) are able to be eliminated by continuing to guide them on basal planes with a 15° off seed, while BP-defects revert to TSDs with a 4° off seed. The difference in the BP-defects propagation is discussed based on the elastic energy balance between TSDs and BP-defects in the unit growth thickness for higher off-angle seeds.

Surface relaxation and photoelectric absorption effects on synchrotron X-ray topographic images of dislocations lying on the basal plane in off-axis 4H-SiC crystals

A more sophisticated simulation model is developed based on the principle of ray-tracing to simulate the grazing-incidence synchrotron X-ray topographic contrast of dislocations lying on the basal plane including basal plane dislocations and deflected threading screw and mixed dislocations in off-axis 4H-SiC crystals. The model incorporates effects of surface relaxation as well as the photoelectric absorption to predict dislocation contrast. Compared to conventional ray-tracing images, surface relaxation effects dominate dislocation contrast for diffraction near the crystal surface. The simulated dislocation contrast gradually weakens with increasing depth of the diffracted beam position within the crystal due to photoelectric absorption. The distinctive features of the net simulated dislocation images obtained by aggregating through the effective penetration depth correlate well with contrast features observed on the experimental topographic images. Depth analysis reveals that in some cases the diffracted X-rays from regions below the dislocation can contribute additional contrast features previously not considered.

Reduction of threading screw dislocations in 4H-SiC crystals by a hybrid method with solution growth and physical vapor transport growth

The propagation of defects during the hybrid growth, which combines solution growth and physical vapor transport (PVT) growth, for bulk 4H-polytype silicon carbide was examined. During solution growth, more than 80% of the threading screw dislocations (TSDs) were converted to defects on basal planes by formation of an 80-μm-thick TSD conversion layer. The layer thickness was limited by the balance between the conversion ratio and the surface roughness. During the subsequent PVT growth, most of the converted defects reverted to TSDs when the TSD conversion layer was planarized prior to PVT growth. In contrast, the converted defects were eliminated by continuous propagation on basal planes when the PVT growth was carried out on the as-grown surface of the TSD conversion layer. Microscopic imaging revealed that continuous formation of macrosteps in the PVT layer is essential for the elimination of the converted defects.

Relaxation processes and high-field coherent spin manipulation in color center ensembles in 6 H -SiC

Coherent spin manipulations of spin-$\frac{3}{2}$ color center ensembles in 6H-SiC crystal have been studied in high magnetic fields using methods of pulsed electron paramagnetic resonance, Rabi oscillations, and pulsed electron-electron double resonance under optical alignment conditions of the spin level populations. Rabi oscillation experiments show room temperature coherent control of these spin-$\frac{3}{2}$ color center ensembles in strong magnetic fields. A sharp decrease of the spin-lattice relaxation time ${T}_{1}$, \ensuremath{\sim}40 times, was observed in 6H-SiC at magnetic field of \ensuremath{\sim}3.5 T with increasing temperature from 100 to 300 K, while the spin-spin relaxation time ${T}_{2}$ is only shortened by \ensuremath{\sim}1.3 times. With an increase in the magnetic field, the times ${T}_{1}$ and ${T}_{2}$ were shown to decrease. The relaxation time ${T}_{1}$ in the case of magnetic field directed along the axis of the spin-$\frac{3}{2}$ center is \ensuremath{\sim}2 times longer than ${T}_{1}$ in magnetic field perpendicular to this axis. Relaxation times of the spin center in crystal grown with a reduced concentration of an isotope $^{29}\mathrm{Si}$ are significantly longer than crystal, with the natural content of isotopes. With a decrease in the $^{29}\mathrm{Si}$ content in our experiments by a factor of \ensuremath{\sim}5, the effective nuclear spin bath in SiC is reduced by a factor of \ensuremath{\sim}2. In a zero magnetic field resonance, transitions are allowed as magnetic dipole transitions with frequency ${\ensuremath{\omega}}_{0}$ which correspond to the zero-field splitting. In zero magnetic field and in fixed magnetic fields, the Rabi frequency was shown, using so-called ``Feynman-Vernon-Hellwarth transformation,'' to be ${\ensuremath{\omega}}_{R}=|\ensuremath{\gamma}|{B}_{1}$. In pulsed electron-electron double resonance experiments, a change in the intensity of the electron spin echo signal corresponding to one of the spin-allowed fine structure transitions is recorded depending on the sweep of the second frequency. The experiments show the possibility to coherently detect the optical spin alignment between ${M}_{S}=\ifmmode\pm\else\textpm\fi{}\frac{3}{2}$ via optically pumped silent ${M}_{S}=\ifmmode\pm\else\textpm\fi{}\frac{1}{2}$ sublevels of the spin-$\frac{3}{2}$ color centers, including a detection of Rabi oscillations.

Characterization of Dislocations in 6H-SiC Wafer Through X-Ray Topography and Ray-Tracing Simulations

Silicon carbide (SiC) is an important semiconductor material for a variety of electronic and optoelectronic applications owing to the unique combination of its superior electronic and physical properties. In order to continuously improve the crystal quality and improve device performance, obtaining a clear understanding of the defect types and their distribution and potential influence on device operation is of great importance. In this study, 6H-SiC crystals grown by physical vapor transport (PVT) have been characterized by synchrotron monochromatic beam x-ray topography (SMBXT). By recording six different $$11{\bar{2}},12$$ grazing incidence reflections and analyzing the contrast patterns of threading screw dislocations (TSDs), threading edge dislocations (TEDs), threading mixed dislocations (TMDs), and basal plane dislocations (BPDs) observed in conjunction with ray-tracing simulation, the Burgers vectors of these dislocations have been determined. This successfully demonstrated a direct Burgers vector determination approach for each type of dislocation. Understanding these dislocation types and their distributions in 6H-SiC wafers can provide crucial feedback for pursuing crystal quality enhancement during growth process. High-resolution x-ray diffraction (HRXRD) has been performed on the wafer to carry out the rocking curve analysis of areas with different degrees of lattice distortion.

Synchrotron X-ray Topography Studies of Dislocation Behavior During Early Stages of PVT Growth of 4H-SiC Crystals

With the increasing attention of 4H-silicon carbide (4H-SiC) crystals in the applications of high-power electronics, it has become necessary to further improve the development of the 4H-SiC crystal growth process, especially the initial stage of physical vapor transport (PVT) growth, which is a critical step to obtain high quality SiC crystals with polytype stability and low dislocation density. This paper describes a study on dislocation behavior of large diameter 4H-SiC crystals at the early stages of PVT growth. Synchrotron x-ray topography is applied to 6-inch PVT-grown crystals with a thickness of several hundred microns on 4H-SiC seeds. Grazing-incidence topographs in $$g = 11\overline{2}8$$ and $$g = 1\overline{1}09$$ recorded from both the seed crystal and newly grown layer show the presence of screw-type basal plane dislocations (BPDs) with $$b = 1/3\left[ {11\overline{2}0} \right]$$ at the inner region of the wafers, which is further confirmed by comparing with ray tracing simulated images of these dislocations. Their origins are likely from deflection of threading edge dislocations (TEDs) onto the basal plane by the overgrowth of macro-steps. Pairs of threading screw/mixed dislocations (TSDs/TMDs) are found to be newly generated at the initial growth stage and some are deflected onto the basal plane. A high density of newly generated TEDs is observed in the early-grown crystals, which are either nucleated in pairs at the seed/crystal interface or converted from BPDs in the seed crystal. Furthermore, dislocations with unique shapes are observed and found to be associated with deflection of TMDs and TEDs, which become the major source for BPD generation during the early PVT growth stage. Possible models to explain their formation mechanism are developed.

Extended Hückel Semi-Empirical Approach as an Efficient Method for Structural Defects Analysis in 4H-SiC.

This paper presents an efficient method to calculate the influence of structural defects on the energy levels and energy band-gap for the 4H-SiC semiconductor. The semi-empirical extended Hückel method was applied to both ideal 4H-SiC crystal and different structures with defects like vacancies, stacking faults, and threading edge dislocations. The Synopsys QuatumATK package was used to perform the simulations. The results are in good agreement with standard density functional theory (DFT) methods and the computing time is much lower. This means that a structure with ca. 1000 atoms could be easily modeled on typical computing servers within a few hours of computing time, enabling fast and accurate simulation of non-ideal atomic structures.