6H-SiC Single Crystals Research Articles

Electron energy-loss spectroscopic evaluation of depth-dependent swelling of He+ ion-irradiated 4H-SiC correlated with defect type

Various defects and amorphous transitions are the primary mechanism behind the accumulation of swelling in silicon carbide (SiC). In this study, selected-area He+ ion irradiation was carried out on single-crystal 4H-SiC using fluences of 1 × 1015, 5 × 1016, and 1 × 1017 cm−2 at room temperature. The defect distribution in the samples with varying irradiation fluences was analyzed using transmission electron microscopy (TEM), while the local swelling of regions under varying damage conditions was estimated using electron energy-loss spectroscopy. The results provide the range of swelling in SiC possessing different primary defect types, such as point defects or tiny clusters, black spot defects, and amorphous SiC. A saturation swelling with a value of 2%–3% in the near-surface region, induced by point defects or tiny clusters (invisible in TEM), was observed at room temperature over the fluence range of 1 × 1015 to 1 × 1017 cm−2. This saturation has already reached at a great low dose of about 0.02 dpa. The swelling of the region containing black spot defects ranges from about 3% to 7%. Helium bubbles increase the volume swelling of SiC, while the He+ ion irradiation may also perform a decreasing effect on the volume swelling below a certain irradiation fluence.

Platinum additive impacts on vapor-liquid-solid growth chemical interface for high-quality SiC single crystal films

Pt additive to Si-based flux in the vapor-liquid-solid (VLS) process for 4H–SiC epitaxial films has been successful in suppressing the step bunching as well as in promoting the step-flow growth mode at growth temperatures much lower than 2000 °C, as evidenced by in situ laser microscope observation of a model solution growth chemical interface. As a result, the SiC film surface became remarkably flat, exhibiting well-regulated step-and-terrace structures with narrow terrace widths and straight step lines; the polytype of 4H–SiC was much more stabilized in the grown SiC films even at a growth temperature of 1250 °C. Furthermore, there is little concern about incorporation of Pt atoms as an impurity into the grown SiC films so that their Schottky junction properties are almost as excellent as those of 4H–SiC single crystals.

Deformation Characteristics in Micromachining of Single Crystal 6H-SiC: Insight into Slip Systems Activation

ABSTRACTSilicon carbide (SiC) is ideally suitable as a sensor material in harsh environments. Despite the brittleness in the macroscopic scale, plasticity in SiC is observed at small component length-scales. Previous nanoindentation based study combining experiment and numerical approaches of single-crystal 6H-SiC has shown that slip activation is rather complex, and that non-basal slip could potentially dominate the plastic deformation behaviour. In this study, we investigated the local deformation response evolution of shear strain directly under and in the vicinity of the indenter tip. The results show the pyramidal slip families contribute significantly to the deformation process.

Model-free determination of the birefringence and dichroism in c-cut crystals from transmission ellipsometry measurements.

In this work, we derive closed-form expressions for determination of the linear birefringence and linear dichroism of uniaxial crystals utilizing transmission ellipsometry measurements at small angles of incidence in $ c $c-cut crystal substrates. The model-free method we use is an algebraic generalization of the method reported in Appl. Opt.44, 3153 (2005).APOPAI0003-693510.1364/AO.44.003153 The optical anisotropy of substrates of sapphire, 4H-SiC, and 6H-SiC single crystals is measured for illustration.

The structural evolution of light-ion implanted 6H-SiC single crystal: Comparison of the effect of helium and hydrogen

The microstructure evolution of hydrogen-implanted 6H-SiC at different temperatures and fluences is investigated by using various experimental techniques. In H-implanted samples with relatively low fluence at RT, dense blister cavities are observed after annealing at 1100 °C, while no visible blister cavities appear after annealing at 1100 °C in the sample implanted at RT with high fluence. The absence of blister cavities is due to the loss of elastic energy during the crystalline-to-amorphous transition. With a further increase of implantation temperature to 450 and 900 °C, amorphization did not occur and H-containing microcracks grew laterally below the surface. Thus, blisters appeared on the surface of the samples implanted at 900 °C even without annealing. The results are compared to the microstructural evolution of He-implanted 6H-SiC which was explored in our previous work. The behavior of hydrogen and helium ions in 6H-SiC lattice was rather different. For He implantation, regardless of the fluence and implantation temperature, blisters did not form. The mechanism of migration and coalescence of nanoscale bubbles that are responsible for blistering were studied via density functional theory calculations, which well-supported the presented results. We found that both mechanisms (migration and coalescence) are energetically cheaper in the case of H compared to He.

Mechanisms of enhancing the machining performance in micro abrasive waterjet drilling of hard and brittle materials by vibration assistance

A study of the effect of vibration assistance on the machining performance in the micro slurry jet drilling of hard and brittle materials typified by single crystal 4H-SiC wafers is presented. It shows that this technology provides benefits to the machining process by improving both material removal rate and surface finish, and such improvements are more profound by increasing the vibration amplitude. Ductile-like mode deformation is noticed to be dominant on the surfaces processed with vibration assistance for this highly brittle material. It is found that major material removal is achieved through brittle fractures of the material by the direct impact of the jet. A smoothening process following the jet impact through a crossflow-driven motion of the particles over the brittle-fractured surface at an engaging depth below the critical value for the cleavage fracture of the material improves the processed surface quality. A finite element (FE) analysis of the particle impact reveals that the vibration applied to the target surface adds no energy to the impacting process, but makes the transfer and application of jet energy more effective by promoting a sweeping effect. This effect clears the surface from the accumulation of the after-impact particles, allowing the subsequent particles from the jet to impact directly onto the surface. The FE study also shows that the applied vibration does not change the engaging depth made by the after-impact particles, but raises the crossflow velocity through the sweeping effect. This enables the smoothening process to extend beyond the restriction imposed by a repeating action of dull particles, and results in the brittle-fractured surface to be flattened and smoothened.

Direct observation of discharging phenomena in vibration-assisted micro-electrical discharge machining

It has been previously reported that electrical discharge machining (EDM) performance can be improved by tool electrode vibration. However, the mechanism of vibration-assisted EDM has not been clarified and the optimal vibration condition remains unknown due to the difficulty in direct process observation. In this study, direct observation of the EDM gap by using a high-speed video camera through a transparent workpiece electrode made of single-crystal 4H-SiC was attempted. It was found that tool vibration caused discharge dispersion and discharge frequency increase, and this tendency was more significant at higher amplitudes and higher frequencies. In addition, tool vibration-induced periodic discharge was observed, which prevented concentrated discharge and short-circuiting. Accordingly, the machining speed was improved and surface roughness was reduced. This study succeeded for the first time in observing the discharge behaviors in vibration-assisted EDM, thus revealed the effects of key parameters for process optimization.

Mechanical Behavior Investigation of 4H-SiC Single Crystal at the Micro-Nano Scale.

In this paper, theoretical models of the critical indentation depth and critical force on brittle materials using cleavage strength and contact theory are proposed. A Berkovich indenter is adopted for nanoindentation tests on a 4H-SiC single crystal sample to evaluate its mechanical behaviors. The stages of brittle material deformation (elastic, plastic, and brittle) can be characterized by the load versus indentation depth curves through the nanoindentation test. The curve of the elastic deformation stage follows the Hertz contact theory, and plastic deformation occurs at an indentation depth of up to 10 nm. The mechanism of 4H-SiC single crystal cracking is discussed, and the critical indentation depth and critical force for the plastic–brittle transition are obtained through the occurrence of the pop-in point. This shows that the theoretical results have good coherence with the test results. Both the values of the elastic modulus and hardness decrease as the crack length increases. In order to obtain more accurate mechanical property values in the nanoindentation test for brittle materials such as SiC, GaN, and sapphire, an appropriate load that avoids surface cracks should be adopted.

Room temperature deformation of 6H–SiC single crystals investigated by micropillar compression

The room-temperature plastic deformation behavior of 6H–SiC single crystals has been investigated by uniaxial compression of micropillar specimens as a function of crystal orientation and specimen size. Plastic flow is observed even at room temperature by basal and prism slip, latter of which have never been observed in the bulk. The CRSS values for basal and prism slip are as high as above 5 and 6 GPa at the specimen size of 5 μm, respectively, each of which increases with decreasing specimen size, following an inverse power-law relationship with a relatively small power-law exponent of ∼0.10 and ∼0.21, respectively. The CRSS values for basal slip are not virtually affected by the existence of basal dislocations introduced at 1300 °C prior to micropillar compression tests at room temperature. The majority of basal dislocations observed after micropillar compression are perfect (undissociated) screw dislocations, and they are considered to be introduced in the shuffle-set plane during micropillar testing, unlike widely dissociated dislocations introduced in the glide-set plane in the bulk during high-temperature deformation. Prism dislocations are observed also to glide as perfect (undissociated) dislocations and tend to align strongly along their screw orientation. The fracture toughness values are estimated to be 1.37 ± 0.13 and 1.57 ± 0.13 MPa m1/2 by three-point bend tests for chevron-notched single crystalline specimens with a notch plane being parallel to (0001) and {011¯0} planes, respectively.

Nanocutting mechanism of 6H-SiC investigated by scanning electron microscope online observation and stress-assisted and ion implant-assisted approaches

Nanocutting mechanism of single crystal 6H-SiC is investigated through a novel scanning electron microscope setup in this paper. Various undeformed chip thicknesses on (0001) orientation are adopted in the nanocutting experiments. Phase transformation and dislocation activities involved in the 6H-SiC nanocutting process are also characterized and analyzed. Two methods of stress-assisted and ion implant-assisted nanocutting are studied to improve 6H-SiC ductile machining ability. Results show that stress-assisted method can effectively decrease the hydrostatic stress and help to activate dislocation motion and ductile machining; ion implant-induced damages are helpful to improve the ductile machining ability from MD simulation and continuous nanocutting experiments under the online observation platform.

Influence of the growth interface shape on the defect characteristics in the facet region of 4H-SiC single crystals

Two 75mm 4H-SiC single crystals are grown by the physical vapor transport (PVT) technique, using different insulation materials. The insulation material of higher thermal conductivity led to an increased radial temperature gradient. The evolution of the growth front was monitored using the in-situ computed tomography (CT). A slightly bent growth interface and a bigger facet are formed during the growth applying a lower radial temperature gradient while a smaller facet and steeper crystal flanks are formed in the case of the larger radial temperature gradient. Micropipes are deflected laterally by large surface steps on the steep crystal flanks and a reduction of threading edge dislocations by 60% is revealed by KOH defect etching.

Effect of ion implantation on material removal mechanism of 6H-SiC in nano-cutting: A molecular dynamics study

Silicon carbide (SiC) is being increasingly applied in several engineering fields owing to its excellent mechanical and electrical properties. Although much attention has been paid to the material removal mechanism of SiC, producing an efficiently machined SiC surface with a high-quality surface and low-damage subsurface is still a challenging issue. Therefore, there is a need to investigate the material removal mechanism of SiC and explore the appropriate technical parameters for improving the processing performance. In this study, the effect of ion implantation on the material removal mechanism of single-crystal 6H-SiC is investigated numerically using molecular dynamics (MD) simulation. Forty-five silicon atoms were implanted into 6H-SiC individually. The lattice damage and hydrostatic pressure were analysed in detail. Then, nano-cutting simulations were carried out to machine both the 6H-SiC and implanted SiC using a diamond tool. The effect of ion implantation on the ductile deformation, minimum cutting thickness (MCT), and cutting force were determined. The results of this study demonstrate that ion implantation effectively increases MCT and decreases the cutting force, thereby improving the machinability and processing efficiency of 6H-SiC. This study has significant theoretical and practical value in processing technology.

The influences of technological parameters on the ultraviolet photocatalytic reaction rate and photocatalysis-assisted polishing effect for SiC

Abstract At first, aiming at ultraviolet (UV) photocatalysis-assisted polishing of single-crystal 6H-SiC (silicon carbide), the particle size and zeta potential of colloidal silica abrasives on photocatalytic reaction conditions were detected. By doing so, it was guaranteed that colloidal silica abrasives retained their stability with the assistance of photocatalysis. Afterward, the effects of various factors (including light intensity, H2O2 concentration, TiO2 concentration, and pH) on the reaction rate of UV photocatalysis were explored and an orthogonal experiment was conducted on the four factors. The test results showed that H2O2 concentration and light intensity delivered the most significant and the least influences on the rate of photocatalytic reaction, respectively. As the rate of photocatalytic reaction increased, the material removal rate (MRR) increased while the surface roughness Ra generally decreased. In terms of the influence on MRR, the four factors were, in descending order: H2O2 concentration, pH, light intensity, and TiO2 concentration; as for the influence on surface roughness Ra, the four factors were, in descending order: pH, H2O2 concentration, TiO2 concentration, and light intensity. The difference in rank of the four factors arose because MRR was mainly affected by the rate of photocatalytic reaction while surface quality (roughness) was synchronously influenced by the reaction rate and the stability of abrasives. At a light intensity at 1000 mW/cm2, H2O2 concentration of 4.5 vol%, TiO2 concentration of 3 g/L, and a pH of 11, the optimal polished surface can be attained, with a surface roughness Ra of 0.423 nm.

Ohmic contact formation mechanism of Ge-doped 6H-SiC

Abstract We present a comprehensive investigation of mechanism of ohmic contact characteristic in Ge-doped 6H-SiC single crystals. Raman mapping and Rutherford backscattering spectrometry reveal high crystalline quality of Ge-doped 6H-SiC and the Ge as a substitutional impurity. Deep-level transient spectroscopy (DLTS) spectrum was measured. Its Arrhenius fitting shows two shallow energy levels. They are below the conductive band bottom 0.298 eV and 0.323 eV respectively. The first-principles calculations indicate that the no new levels are in ( Ge ) Si configuration, but ( Ge ) C can induce deep levels in 6H-SiC. Thus, the shallow energy levels are caused by native defects which arise in a process of crystal growth. The two characteristics shallow energy levels, the small trap cross sections and positions near to conductive band bottom, can emit electron under effect of an applied field. In addition, the Ge Si bonds are relatively easily to break for generating electrons to jump into conductive band bottom though the shallow energy levels.

Amorphization and dislocation evolution mechanisms of single crystalline 6H-SiC

The amorphization and dislocation evolution mechanisms of a single crystal 6H-SiC were systematically investigated by using nano-indentation, high-resolution transmitted electron microscope (HRTEM), molecular dynamics (MD) simulations and the generalized stacking fault (GSF) energy surface analysis. Two major plastic deformation mechanisms of 6H-SiC under nano-indentation were revealed by HRTEM, i.e., (1) an amorphization region near the residual indentation mark, and (2) dislocations below the amorphization region in both the basal and prismatic planes. MD results showed that the amorphization process corresponds to the first “pop-in” event of the indentation load-displacement curve, while the dislocation nucleation and propagation are related to the consequent “pop-in” events. The amorphization is confirmed to achieve via an initial transformation from wurtzite structure to an intermediate structure, and then a further amorphization process.

Microstructural evolution of helium-irradiated 6H–SiC subjected to different irradiation conditions and annealing temperatures: A multiple characterization study

The microstructural phenomena occurring in 6H–SiC subjected to different irradiation conditions and annealing temperatures were investigated to assess the suitability of 6H–SiC as a structural material for nuclear applications. To this aim, a single crystal of 6H–SiC was subjected to He+ irradiation at 300 keV with different fluences and at temperatures ranging from 25 to 750 °C. Rutherford backscattering/channeling (RBS/C), X-ray diffraction (XRD) and transmission electron microscopy (TEM) analyses were combined to shed light on the microstructural changes induced by irradiation and subsequent annealing (750 to 1500 °C). At room temperature, amorphization starts to occur at a fluence of 2.5 × 1016 cm−2 (0.66 dpa). On the contrary, amorphization was prevented at high irradiation temperatures and fluences. Furthermore, a thin and highly strained region located around the maximum He concentration (Rp) formed. This region results from the accumulation of interstitial atoms which are driven toward the highly damaged region under the actions of a strain gradient and high temperature. Regardless of the fluence and irradiation temperature, the material stores elastic energy, which leads to the trapping of He in dissimilar defect geometries. For irradiation temperatures below 750 °C, helium was accumulated in bubbles which coarsened after annealing. On the other hand, for an irradiation temperature of 750 °C, helium was trapped in platelets (even for medium fluence), which evolved into a homogeneous dense array of cavities during annealing. DFT calculations show that the bubbles are under high pressure and contribute to developing the overall tensile strain in the single crystal 6H–SiC.

On the study of microstructure of annealed C+and H2+ Co_implanted 6H-SiC

The microstructure of annealed single crystal 6H-SiC implanted with C+ and H2+ ions were characterized by glancing incidence X-ray diffraction (GIXRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). After annealing, cracks were observed to form in the surface of C+ ions implanted sample. However, blisters as circular clusters were observed in the surface of the H2+ ions implanted and annealed sample. Compared with the samples implanted with only single type of ions, both cracks and blisters with different morphologies were found to appear in the surface of C+ and H2+ ions co-implanted sample. The morphology and distribution of blisters and exfoliations in the C+ and H2+ co-implanted samples were more irregular and inhomogeneous than those in the H2+ ions implanted sample, which can be attributed to the randomly distributed columnar crystals formed during the annealing process. The grain boundaries of these columnar crystals could be used as migration channels for hydrogen, which influence the size evolution of blisters and exfoliations in the sample surface during annealing process.

Influence of Morphological Changes in a Source Material on the Growth Interface of 4h-Sic Single Crystals.

In this study, the change of mass distribution in a source material is tracked using an in situ computer tomography (CT) setup during the bulk growth of 4H- silicon carbide (SiC) via physical vapor depostion (PVT). The changing properties of the source material due to recrystallization and densification are evaluated. Laser flash measurement showed that the thermal properties of different regions of the source material change significantly before and after the growth run. The Si-depleted area at the bottom of the crucible is thermally insulating, while the residual SiC source showed increased thermal conductivity compared to the initially charged powder. Ex situ CT measurements revealed a needle-like structure with elongated pores causing anisotropic behavior for the heat conductivity. Models to assess the thermal conductivity are applied in order to calculate the changes in the temperature field in the crucible and the changes in growth kinetics are discussed.

Polytype Control by Pretreatment of SiC Source Powder for 4H-SiC Single Crystal Growth

4H-SiC single crystal was successfully grown with source powder modified by the pretreatment process in order to improve polytype stability of SiC crystal. To increase C/Si ratio in SiC source powder, SiC source powder was mixed with liquid carbon source and then pre-heated at 1200°C. SiC single crystal grown with modified source powder exhibited complete 4H polytype and the crystal quality of SiC crystal grown by modified source power was definitely better than conventional source powder

Modified Hot-Zone Design for Large Diameter 4H-SiC Single Crystal Growth

6-inch 4H-SiC single crystal was grown with modified hot-zone design for large diameter crystal. The simulation data confirmed reduced temperature gradient between center and edge region of growing front, and actual growth experiment exhibited that SiC crystal with good quality was obtained with modified hot-zone design without any quality degradation in edge region of bulk crystal. Based on the mapping measurement of FWHM (Full width at half maximum) value in X-ray rocking curve, the crystal quality of SiC crystals from middle and top region of grown ingot was observed to be almost identical. Furthermore, various properties of SiC crystal grown with modified hot-zone design have been systematically investigated.