Investigation of radiation damage and hardness of H- and He-implanted SiC

In this study, an investigation was conducted in order to determine the effects of high-dose H and He ion implantation on the structure of 3C-SiC and 6H-SiC as well as the effects on material hardness in order to understand the role of H and He produced in SiC by neutron-induced transmutations as described by Heinisch et al. [J. Nucl. Mater. 2004, 327, 175–181.]. H and He ions were implanted into polycrystalline 3C-SiC on a Si substrate and single-crystal bulk 6H-SiC, respectively, at an ion energy of 100 keV, and the total dose that was used for both species was 10¹⁷ ions/cm² in the temperature range of 473–573 K. The specimens were annealed at 1000°C for 20 min in an inert Ar atmosphere. The damaged region in the He-implanted 6H-SiC had a high density of small bubbles, but no cracks were observed. Severe cracking was observed along the damaged region in the H-implanted 3C-SiC specimens as well as a high density of hydrogen platelets. Neither specimen displayed any amorphization. Nanoindentation hardness measurements showed a marked increase in the hardness of the annealed He-implanted 6H-SiC, which was ascribed to the creation of point defects inhibiting interplanar slip. There was also a large decrease in hardness corresponding to the depth of the ion damage.     

Formation of MgO nanorods in the reaction zone of a Mg–CuO powder mixture by in-situ reaction

We report the in-situ formation of MgO nanorods during sintering of a Mg–20?wt%?CuO powder mixture at 450°C. After sintering, we identified three regions with distinct microstructures in the reaction zone between the Mg grain and the newly formed Cu grain. Region I contained MgO nanorods and Cu nanoparticles, region II was composed of MgO nanorods, while larger nanometre-sized MgO crystals were found in region III. The MgO nanorods were single crystals with a diameter of about 20?nm and a length of about 100?nm. The growth of these nanorods was controlled by the vapour–solid mechanism. The progressive change in morphology of the MgO phase had induced a hardness gradient across the reaction zone. As a result, the interfacial bonding between the major phases in the sintered product was enhanced.     

氢氟酸刻蚀对Ni/6H-SiC接触性质的作用

采用浓度为10%的氢氟酸（HF）刻蚀6H-SiC单晶片,研究了HF刻蚀时间对Ni/6H-SiC接触性质的影响．经24?h刻蚀的SiC基片在溅射Ni层后,其接触表现良好线性的电流-电压（I-V）曲线．低于这个腐蚀时间的接触具有明显的势垒,但在大于1000℃快速退火后,也得到了良好线性的I-V曲线．X射线衍射（XRD）和俄歇能谱（AES）深度元素分析表明Ni₂Si和C是快速退火后的主要产物．XRD和低能反射电子能量损失谱表明表层的C 关键词： 欧姆接触 SiC 富碳层 互扩散     

The effect of SiC nanoparticle addition on the flux pinning properties of MgB2

The nanoparticle–MgB₂ composite superconducting sample, (SiC)_4wt.%(MgB₂)_96wt.% ((SiC)₄–MgB₂), was prepared, and the effect of nanoparticle additions on the magnetic flux pinning was investigated. The measurement and comparison of isothermal magnetization M(H), for pure-MgB₂ powder and sintered pellets of (SiC)₄–MgB₂ and pure-MgB₂ were carried out at temperatures between 5 and 50 K in fields up to 8.5 T. The magnetic irreversibility ΔM(H) curves of the (SiC)₄–MgB₂ follow almost identical lines of both pure-MgB₂ powder and sintered bulk MgB₂ in the region above a specific magnetic field (called a merged field), which gradually decreases as the temperature increases. The (SiC)₄–MgB₂ composite superconductor has exhibited the flux pinning effect which comes from both the grain boundaries and the point defect weak pinning centers in the region below the merged field line. This is different from the case of pure-MgB₂ powder and sintered bulk MgB₂ which showed mostly the grain boundaries pinning.     

Taguchi approach for characterization of three-body abrasive wear of carbon-epoxy composite with and without SiC filler

Fiber–matrix interfacial bonding plays a critical role in controlling performance properties of polymer composites. Carbon fibers have major constraints of chemical inertness with the matrix and need the surface treatment to improve the adhesion with the matrix. In this work, parametric appraisal of three-body abrasive wear behavior was presented for silane treated carbon fabric reinforced epoxy (C-E) composites with and without silane treated silicon carbide (SiC) as filler. The fiber content was fixed at 60?wt.%, while the weight fraction of SiC was varied (5 and 10?wt.%) to obtain three different compositions. Three-body abrasive wear tests were conducted using design of experiments approach based on Taguchi’s orthogonal arrays. The findings of experiments indicate that the wear loss is greatly influenced by load and grain size of abrasive. An optimal parameter combination was determined, which leads to maximization of abrasion resistance. Inclusion of SiC filler reasonably increased the abrasion resistance of C-E composite. Analysis of variance results showed that the load significantly influenced the abrasion of SiC filled C-E composites. Efforts were also made to correlate the abrasive wear performance of SiC filled C-E composites using artificial neural network (ANN). The wear behavior of composite by ANN prediction closely matched the experimental results and finally, optimal wear settings for minimum wear were identified.     

Exciton related photoluminescence stimulation in SiC nanocrystals

The paper presents the results of porous SiC characterization using scanning electron microscopy (SEM), X-ray diffraction (XRD) and photoluminescence spectroscopy techniques. XRD study shows the investigated porous 6H-SiC layers contain inclusions of 4H-SiC and 15R-SiC polytypes as well as the amorphous graphite phase. Photoluminescence study of PSiC layers with different thicknesses and SiC NC sizes reveals the intensity stimulation for exciton and defect-related PL bands. The intensity stimulation of defect-related PL bands is due to the increase, at the etching process, of the concentration of surface defects, apparently to deal with carbon. The intensity enhancement for exciton-related PL bands is attributed to the exciton recombination rate increasing due to the realization of exciton confinement and exciton–polariton effects in big size SiC NCs of different polytypes (6H-PSiC with inclusions of 15R- and 4H-PSiC).     

Characterization of nanostructured ceramics prepared by both high energy ball milling and fast firing sintering processes

High-energy ball milling technique was successfully applied to calcinated lead zirconate titanate (PZT 60/40) powders. After 20?h of ball milling, large PZT particles were completely broken down, reducing its initial size in three orders of magnitude. Experimental results show a huge sinterability enhancement of the PZT powders by using this technique, achieving its maximum sintering rate at ～800°C. Relatively low densities (～91%) were achieved in stoichiometric samples, while in 3% lead excess samples sintered at 950°C for 30, 45, 60, 90 and 120?min using a fast firing process and a post-annealing treatment at 800°C for 4 h, densities of ～97% of the theoretical were achieved. PZT nanostructured ceramics prepared under optimized processing conditions (60?h of powder milling, 950°C of sintering temperature, 60?min of sintering time and a post-annealing process at 800°C during 4?h) show high dielectric constant (ε′) values (900) and low dielectric loss (tan?δ) at room temperature and a ferroelectric-paraelectric transition temperature at 375°C.     

Single crystal growth of SiC and electronic devices

Single crystal growth of silicon carbide (Sic) and application to electronic devices are reviewed. In the crystal growth, bulk and homoepitaxial growth are picked up, and crystal quality and electrical properties are described. For electronic devices, various device processes are argued. Power devices based on Sic are stressed in this review.

Bulk single crystals of SiC can be grown by a sublimation method, and large-area 6H-SiC and 4H-SiC single crystals are obtained. The occurrence of SiC polytypes is affected by the growth condition, and can be controlled successfully by optimizing these conditions. 6H-SiC is grown on 6H-SiC (0001) Si-faces, and 4H-SiC on 6H-SiC (0001) C-faces. The crystallinity of bulk crystals is investigated by reflection high-energy electron diffraction (RHEED) and X-ray analysis, and characterization is carried out in detail by optical and electrical measurement.

Successful homoepitaxial vapor phase growth of SiC can be realized using off-axis (0001) substrates prepared by a sublimation method called “step-controlled epitaxy”. Since the crystallinity of epilayers is improved during the step-controlled epitaxy, this growth technique is a key for getting high-quality crystal surfaces. Impurity doping is controlled during homoepitaxial growth by employing impurity gases, such as N₂, trimethylaluminum (TMA), and B₂H₆. A wide-range of carrier concentrations of 5 × 10¹³～3 × 10¹⁸ cm^?3 for n-type and 5 × 10¹⁶～3 × 10²⁰ cm^?3 for p-type are realized. The impurity-incorporation mechanism in the step-controlled epitaxy is discussed based on the C/Si ratio dependence of impurity doping.

Electrical properties of SiC grown by step-controlled epitaxy are determined precisely. A high electron mobility of 720 cm²/Vs is obtained in an undoped 4H-SiC epilayer with an electron concentration of 2.5 × 10^l6 cm^?3 at 300 K. This electron mobility is about two times higher than that of 6H-Sic (～380 cm²/Vs). High breakdown fields of 1～5 × 10⁶ V/cm are obtained for both 6H- and 4H-SiC, one order of magnitude higher than those for Si. A high saturation electron drift velocity of 1.6 × 10⁷ cm/s is obtained in 4H-Sic, which may make possible high performance of high-frequency 4H-SiC power devices. Impurity levels and deep levels are investigated by Hall effect, admittance spectroscopy, and DLTS measurement. Metal/4H-SiC Schottky barrier heights are characterized and a strong dependence on metal work function without strong “pinning” is elucidated.

Device processes are described for ion implantation. Interface properties of SiO₂/SiC are characterized in detail using metal-oxide-semicond.     

Fracture toughness evaluation of WC–10 wt% Co hardmetal sintered under high pressure and high temperature

This work focused on fracture toughness studies of WC–10?wt% Co hardmetal fabricated through the high pressure/high-temperature technique. A powder mixture of WC–10?wt% Co was sintered at 1500–1900°C under a pressure of 7.7?GPa for 2 and 3?min. Vickers hardness test at two different loads of 15 and 30?kgf was done and fracture toughness of the sintered bodies was measured using the indentation method to obtain the effect of sintering parameters. Structural analyses were also performed via X-ray diffraction to investigate structure-related properties. Full density was achieved for high sintering temperature along with abnormal grain growth that reduced hardness. High hardness was observed ranging from 1200 to 1670?HV and fracture toughness increased with increasing sintering temperature up to the highest value of 17.85?MPa/m^1/2.     

Electromagnetic shielding of multiwalled,bamboo-like carbon nanotube/methyl vinyl silicone composite prepared by liquid blending

Multiwalled, bamboo-like carbon nanotube (BCNT)/methyl vinyl silicone (MVQ) composites with different concentrations of BCNT were fabricated by liquid blending method with an aim to investigate the behavior of such composites as effective electromagnetic interference shielding material in the frequency range of 1–6?GHz. The morphology and structure of BCNT were characterized and elucidated. Scanning electron microscopy examination showed that the BCNTs homogeneously dispersed in MVQ. The electrical conductivity (σ) and shielding effectiveness (SE) of the composite were measured and discussed. The results showed that the BCNTs/MVQ composites had a relatively low percolation threshold at 0.92?wt. % BCNT, and the σ showed a decreasing linear relation with temperature, i.e., the σ slightly decreased with increasing temperature. The BCNTs/MVQ composites with SE of 33–38?dB were obtained at 7?wt. % BCNT loading. Shielding mechanism was studied by resolving the total incident energy into absorbed, reflected, and transmitted contribution, and the result showed that the dominated shielding mechanism was reflection loss.     

Microstructure and conductivity of alumina-fiber-doped YSZ membranes

Alumina-fiber-doped YSZ membranes were fabricated by tape casting and pressureless sintering. With a milling time of 4 h and a slurry viscosity of ≈ 600 mPa·s (at a shear rate of 350 s⁻¹), the fibers could be dispersed homogeneously in the slurry with an average aspect ratio of ≈ 20. With 1.5 vol% Al₂O₃ fibers, the relative density of the alumina-fiber-doped YSZ membranes reached a maximum of 93%, and then decreased gradually with higher fiber addition. For the 6 vol% alumina-fiber-doped YSZ membranes, the density decreased by 7.1%, accounting to 85.1% of the theoretical value. However, the density of the 6 vol% Al₂O₃/YSZ composites (fabricated by tape lamination) was 93.4%, which has benefited from the isostatic pressing of green compacts at a pressure of 300 MPa. Obvious fiber pull-out was observed in the fracture section of the alumina-fiber-doped green tapes while not in the cross-section of the sintered membranes. With the addition of alumina fibers, the ionic conductivity of the sintered membranes decreased much more than the ROM (Rule of Mixtures) theoretical calculation, which may be attributed to impurities in fibers and pores accompanied with fiber addition.     

Formation of Ohmic contacts on laser irradiated n-type 6H-SiC without thermal annealing

In this work, KrF excimer laser irradiation of n-type SiC is used to form Ohmic contacts at the interfaces between the irradiated SiC and various types of metals with different work functions without subsequent thermal annealing. Ohmic contacts are formed between laser-treated 6H-SiC and Ti at a laser fluence of 0.7 J/cm². Moreover, in the fluence range of 0.7–1.3 J/cm², Ohmic characteristics are also observed between irradiated 6H-SiC and Au, which is a representative inert metal. The laser-induced heavy doping effect reduces the thickness of the Schottky barrier between the metal and SiC, and the formation of graphene sheets on the irradiated SiC surface reduces the barrier height, resulting in the direct formation of Ohmic contacts. Our findings thus demonstrate the potential of this laser treatment method to achieve Ohmic contacts between n-type SiC and a broad range of metal electrodes without requiring high-temperature annealing.     

Defect Characterization of 6H-SiC Studied by Slow Positron Beam

利用单能慢正电子束流,对原生的和经过电子辐照的6H-SiC内的缺陷形成及其退火行为进行研究．发现在n型6H-SiC中,经过退火后缺陷浓度降低．这主要是因为在退火过程中缺陷和间隙子的相互作用所引起．n型6H-Si经过1400 oC、30 min真空退火后,在SiC表面形成一个大约20 nm的Si层,这是在高温退火过程中Si原子向表面逸出的有力证明．在高温退火中,在样品的近表面区域有一个明显的表面效应,既在这些区域的S参数整体较大,这种现象与高温退火中Si不断向表面逸出有关．经过10 MeV的电子辐照,在n型6H-SiC中,正电子有效扩散长度从86.2 nm减少至39.1 nm,说明在样品中由于电子辐照产生大量缺陷．但是对p型6H-SiC,经过10 MeV电子辐照后有效扩散长度变化不大,这与其中缺陷的正电性有关．同时还对n型6H-SiC进行了1.8 MeV电子辐照后的300 oC退火实验,发现退火后缺陷浓度不减反增,这主要是因为在退火过程中,一些双空位缺陷和Si间隙子互相作用从而产生了VC缺陷的缘故．     

Formation of the intermediate semiconductor larger for the Ohmic contact to silicon carbide using Germanium implantation

By formation of an intermediate semiconductor layer (ISL) with a narrow band gap at the metallic contact/SiC interface, this paper realises a new method to fabricate the low-resistance Ohmic contacts for SiC. An array of transfer length method (TLM) test patterns is formed on N-wells created by P⁺ ion implantation into Si-faced p-type 4H-SiC epilayer. The ISL of nickel-metal Ohmic contacts to n-type 4H-SiC could be formed by using Germanium ion implantation into SiC. The specific contact resistance ρ_c as low as 4.23× 10^-5~Ωega \cdotcm² is achieved after annealing in N₂ at 800~°C for 3~min, which is much lower than that (>900~°C) in the typical SiC metallisation process. The sheet resistance R_sh of the implanted layers is 1.5~kΩega /\Box. The technique for converting photoresist into nanocrystalline graphite is used to protect the SiC surface in the annealing after Ge⁺ ion implantations.     

Recrystallization of hexagonal silicon carbide after gold ion irradiation and thermal annealing

Silicon carbide (SiC) single crystals with the 6H polytype structure were irradiated with 4.0-MeV Au ions at room temperature (RT) for increasing fluences ranging from 1?×?10¹² to 2?×?10¹⁵ cm^?2, corresponding to irradiation doses from ~0.03 to 5.3 displacements per atom (dpa). The damage build-up was studied by micro-Raman spectroscopy that shows a progressive amorphization by the decrease and broadening of 6H-SiC lattice phonon peaks and the related growth of bands assigned to Si–Si and C–C homonuclear bonds. A saturation of the lattice damage fraction deduced from Raman spectra is found for ~0.8?dpa (i.e. ion fluence of 3?×?10¹⁴ cm^?2). This process is accompanied by an increase and saturation of the out-of-plane expansion (also for ~0.8?dpa), deduced from the step height at the sample surface, as measured by phase-shift interferometry. Isochronal thermal annealing experiments were then performed on partially amorphous (from 30 to 90%) and fully amorphous samples for temperatures from 200 °C up to 1500 °C under vacuum. Damage recovery and densification take place at the same annealing stage with an onset temperature of ~200 °C. Almost complete 6H polytype regrowth is found for partially amorphous samples (for doses lower than 0.8 dpa) at 1000 °C, whereas a residual damage and swelling remain for larger doses. In the latter case, these unrelaxed internal stresses give rise to an exfoliation process for higher annealing temperatures.     

Structural and optical properties of a-Si1−xCx:H films synthesized by dc magnetron sputtering technique

Hydrogenated amorphous SiC films (a-Si_1−xC_x:H) were prepared by dc magnetron sputtering technique on p-type Si(1 0 0) and corning 9075 substrates at low temperature, by using 32 sprigs of silicon carbide (6H-SiC). The deposited a-Si_1−xC_x:H film was realized under a mixture of argon and hydrogen gases. The a-Si_1−xC_x:H films have been investigated by scanning electronic microscopy equipped with an EDS system (SEM-EDS), X-ray diffraction (XRD), secondary ions mass spectrometry (SIMS), Fourier transform infrared spectroscopy (FTIR), UV-vis-IR spectrophotometry, and photoluminescence (PL). XRD results showed that the deposited film was amorphous with a structure as a-Si_0.80C_0.20:H corresponding to 20 at.% carbon. The photoluminescence response of the samples was observed in the visible range at room temperature with two peaks centred at 463 nm (2.68 eV) and 542 nm (2.29 eV). In addition, the dependence of photoluminescence behaviour on film thickness for a certain carbon composition in hydrogenated amorphous SiC films (a-Si_1−xC_x:H) has been investigated.     

Enhanced single photon emission in silicon carbide with Bull's eye cavities

The authors demonstrate a Bull's eye cavity design that is composed of circular Bragg gratings and micropillar optical cavity in 4H silicon carbide (4H-SiC) for single photon emission. Numerical calculations are used to investigate and optimize the emission rate and directionality of emission. Thanks to the optical mode resonances and Bragg reflections, the radiative decay rates of a dipole embedded in the cavity center is enhanced by 12.8 times as compared to that from a bulk 4H-SiC. In particular, a convergent angular distribution of the emission in far field is simultaneously achieved, which remarkably boost the collection efficiency. The findings of this work provide an alternative architecture to manipulate light—matter interactions for achieving high-efficient SiC single photon sources towards applications in quantum information technologies.     

Self-ordering of nanofacets on vicinal SiC surfaces

Vicinal 4H and 6H-SiC(0001) surfaces have been investigated using atomic force microscopy and cross-sectional high-resolution transmission electron microscopy. We observed the characteristic self-ordering of nanofacets on any surface, regardless of polytypes and vicinal angles, after gas etching at high temperature. Two facet planes are typically revealed: (0001) and high index (112;n) that are induced by equilibrium surface phase separation. A (112;n) plane may have a free energy minimum due to attractive step-step interactions. The differing ordering distances in 4H and 6H polytypes imply the existence of SiC polytypic dependence on nanofaceting. Thus, it should be possible to control SiC surface nanostructures by selecting a polytype, a vicinal angle, and an etching temperature.     

Structural and optical properties of thin films porous amorphous silicon carbide formed by Ag-assisted photochemical etching

In this work, we present the formation of porous layers on hydrogenated amorphous SiC (a-SiC: H) by Ag-assisted photochemical etching using HF/K₂S₂O₈ solution under UV illumination at 254 nm wavelength. The amorphous films a-SiC: H were elaborated by d.c. magnetron sputtering using a hot pressed polycrystalline 6H-SiC target. Because of the high resistivity of the SiC layer, around 1.6 MΩ cm and in order to facilitate the chemical etching, a thin metallic film of high purity silver (Ag) has been deposited under vacuum onto the thin a-SiC: H layer. The etched surface was characterized by scanning electron microscopy, secondary ion mass spectroscopy, infrared spectroscopy and photoluminescence. The results show that the morphology of etched a-SiC: H surface evolves with etching time. For an etching time of 20 min the surface presents a hemispherical crater, indicating that the porous SiC layer is perforated. Photoluminescence characterization of etched a-SiC: H samples for 20 min shows a high and an intense blue PL, whereas it has been shown that the PL decreases for higher etching time. Finally, a dissolution mechanism of the silicon carbide in 1HF/1K₂S₂O₈ solution has been proposed.