Nanocrystalline SiC Research Articles

Planar impacts on nanocrystalline SiC: a comparison of different potentials

Silicon carbide ceramics under shock loading is an important aspect in studying their physical and mechanical properties. Molecular dynamics simulations have been carried out using Tersoff-1989, Tersoff-1994, Tersoff-2005 and Vashishta potentials, respectively. The Hugoniot states including compression stress, shear stress, temperature and shock wave velocity are calculated, as well as shock-induced plasticity and the shock wave fronts. A comprehensive comparison among different potentials, as well as comparison to current available experiments, has been made. Tersoff-1989, Tersoff-1994 and Tersoff-2005 potentials are easily to overestimate the shock stress, shear stress and temperature, as well as shock wave velocity, while Vashishta potential shows excellent agreement with experimental data. The Hugoniot elastic limit is ~ 14.5 GPa and the maximum shear stress is ~ 6 GPa using Vashishta potential which are in good agreement with experiments, while Tersoff-like potentials yield much higher values. Due to differences in radial distribution function among these potentials, Vashishta potential is prone to produce plasticity and structural phase transformation basing on the statistics of the coordination numbers of atoms. Besides, the shock wave fronts show little difference among these potentials under elastic shock compression at low particle velocity. However, when it comes to high shock intensity resulting in plasticity or phase transition, the Tersoff-1989 and Tersoff-1994 produce the widest shock wave fronts, followed by Tersoff-2005, while Vashishta potential has the narrowest wave front. By comprehensive comparisons, the Vashishta potential is demonstrated to be the most suitable one to describe the silicon carbides ceramics under shock loadings. Our work provides useful information to select a suitable potential to study the shock response of silicon carbides ceramics using molecular dynamics simulations.

Nanocrystalline SiC film thermistors for cryogenic applications.

We developed a heat-sensitive material based on nanocrystalline SiC films obtained by direct deposition of carbon and silicon ions onto sapphire substrates. These SiC films can be used for resistance thermometers operating in the 2 K-300 K temperature range. Having high heat sensitivity, they are relatively low sensitive to the magnetic field. The designs of the sensors are presented together with a discussion of their thermometric characteristics and sensitivity to magnetic fields.

Optical properties of a microcavity based on a nanocrystalline SiC film

The optical properties of one-dimensional microcavity based on nanocrystalline SiC (nc-SiC) films containing nanocrystals of cubic and rhombohedric polytypes (R24 and R27, respectively) have been studied. It was revealed that resonance absorption maxima in absorption spectra measured at room temperature varied from 2.17 to 2.79 eV for the nc-SiC films 300–450 thick. To explain the thickness dependence of resonance absorption in the microcavities based on heteropolytypic nc-SiC films, the model of exciton–polariton states in the one-dimensional microcavity based on nanocrystalline SiC film has been proposed.

Spin colour centres in SiC as a material platform for sensing and information processing at ambient conditions

Atomic-scale colour centres in bulk and nanocrystalline SiC are promising systems for quantum photonics compatible with fiber optics, quantum information processing and sensing at ambient conditions. Colour centres which acts as stable single photon sources in SiC can be key elements for quantum photonics and communications. It has been shown that there are at least two families of colour centres in SiC with S = 1 and S = 3/2, which have the property of optical alignment of the spin levels even at room temperature and above. The spin state can be initialized, manipulated and readout by means of optically detected magnetic resonance (ODMR), level anticrossing and cross-relaxation. Recently, we observed the effects of “hole burning” in the ODMR spectra, which made it possible to narrow the ODMR line by approximately an order of magnitude, which substantially increases the possibilities of technological applications of spin centres.

A comparative study of nanocrystalline SiC thin films on multimode optical fiber sensors, synthesized via DBD-NTP and 150 MHz VHF-PECVD

In this study, nanocrystalline SiC films were successfully synthesized on the surface of multimode fiber (MMF) sensors using 150MHz very-high-frequency plasma-enhanced chemical vapor deposition (VHF-PECVD) and dielectric barrier discharge nonthermal plasma (DBD-NTP) methods. The effects of the different utilized methods and various deposition times on the microstructure and chemical composition of the deposited SiC films were investigated For both methods, the SiC film thickness increased with increased deposition time. The dependency of sensitivity on the deposition time and thickness of SiC films was studied by investigating the effects of refractive index and temperature variation on the response of the MMF sensors. The SiC films deposited by 150MHz VHF-PECVD showed more uniformity and higher sensitivity compared to those deposited using DBD-NTP. Using VHF-PECVD with a deposition time of 1min yielded the lowest thickness (54.06nm) and therefore the highest sensitivity (0.0240dB/°C) for the SiC-deposited MMF sensors. Thus, the VHF-PECVD method may be a better option than DBD-NTP for the deposition of thin SiC films to improve the sensitivity of the MMF sensors.

Diodes of nanocrystalline SiC on n−/n+-type epitaxial crystalline 6H-SiC

The diodes of nanocrystalline SiC on epitaxial crystalline (n−/n+)6H-SiC wafers were investigated, where the (n+)6H-SiC layer was treated as cathode. For the first unit, a heavily boron doped SiC film as anode was directly deposited by plasma enhanced chemical vapor deposition method on the wafer. As to the second one, an intrinsic SiC film was fabricated to insert between the wafer and the SiC anode. The third one included the SiC anode, an intrinsic SiC layer and a lightly phosphorus doped SiC film besides the wafer. Nanocrystallization in the yielded films was illustrated by means of X-ray diffraction, transmission electronic microscope and Raman spectrum respectively. Current vs. voltage traces of the obtained devices were checked to show as rectifying behaviors of semiconductor diodes, the conduction mechanisms were studied. Reverse recovery current waveforms were detected to analyze the recovery performance. The nanocrystalline SiC films in base region of the fabricated diodes are demonstrated as local regions for lifetime control of minority carriers to improve the reverse recovery properties.

Shock-induced spall in single and nanocrystalline SiC

Shock-induced spall in SiC is investigated via high strain-rate loading molecular dynamics simulations. The dynamic response under different shock intensities is characterized along the low-index 3C-SiC crystallographic directions, [001], [110], and [111], and in a nanocrystalline sample with 5 nm average grain size. The simulation results show that all single crystal samples generate elastic, plastic, and structural phase transformation waves for increasing particle velocities in good agreement with previous investigations. However, crystal anisotropy effects affect the exact shock response and the corresponding wave structures. The Hugoniot elastic limit is significantly higher along the [111] and [110] directions while the patterns of plastic deformation, based on deformation twinning, contrast along the three crystallographic directions. The spall behavior, both for single and nanocrystalline samples vary from classical to micro-spall. The predicted spall strength is at maximum along the [111] direction, at 34 GPa, followed by the [110], and [001] directions, at 32 and 30 GPa, respectively. Nanocrystalline SiC displays a spall strength over 66.7% lower than single crystals. Spall strengths from direct and indirect methods agree well for both classical and micro-spall regimes after applying an elastic-plastic correction and considering the change in sound velocity, in particular for the case where the structural phase transformation occurs.

Strong effect of atmosphere on the microstructure and microwave absorption properties of porous SiC ceramics

Excellent microwave absorption properties of porous SiC ceramics were successfully synthesized using SiC/camphene slurries with various polycarbosilane (PCS) contents related to the SiC powder. The compositions of the nanowires (NWs) growth in the pore channels of porous SiC ceramics strongly depended on the pyrolysis atmosphere, with N2-generating Si3N4 NWs and Ar SiC NWs. With the increase of PCS content, the minimum reflection coefficient (RC) of porous SiC ceramics decreased from −7.6dB to −67.4dB in Ar and from −10.9dB to −24.7dB in N2, respectively. The effective absorption bandwidth (EAB) of porous SiC ceramics could be up to 8.1GHz in Ar and 4.5GHz in N2. The enhanced microwave absorption properties of porous SiC ceramics could be attributed to the formation of SiC nano-crystalline, nanosized carbon and the NWs, which would increase the amount of boundaries and defects, leading to the electronic dipole polarization and interfacial scattering.

Synthesis of SiC nanofibers with superior electromagnetic wave absorption performance by electrospinning

A method combined electrospinning and preceramic polymer pyrolysis, annealing has been developed to fabricate SiC nanofibers. In this work, the effects of polycarbosilane (PCS) mass ratio (8.7 wt% ∼ 11.8 wt%) in precursor solution on microstructure and electromagnetic (EM) absorption properties of SiC nanofibers are systematically studied. The phase composition of SiC nanofibers are mainly inclusive of abundant SiC nanocrystallines, a small quantity of graphitic-like carbon coated on fibers surface and randomly distributed amorphous SiOxCy phase. When PCS mass ratio is 10 wt%, the SiC nanofibers present a minimum reflection loss (RL) value of −57.8 dB at 14.6 GHz with a absorber coating thickness of 1.9 mm and the effective absorption bandwidth (RL < −10 dB, 90% EM wave absorbed) covers from 6 to 18 GHz. As PCS mass ratio increasing, the EM absorption performance of SiC nanofibers gradually becomes poor. Multi-reflection among SiC nanofibers gradually weaken the incident EM wave. Moreover the interfacial polarization originated from heterogeneous interfaces which exist among SiC nanocrystallines, graphite carbon, amorphous SiOxCy dissipates EM wave energy contributed to high EM performance of SiC nanofibers. This work demonstrates a new kind of nano SiC EM absorption materials that has potential to be applicated in harsh environments.

Origin of Photovoltage Enhancement via Interfacial Modification with Silver Nanoparticles Embedded in an a-SiC:H p-Type Layer in a-Si:H Solar Cells.

We used silver nanoparticles (Ag-NPs) embedded in the p-type semiconductor layer of hydrogenated amorphous silicon (a-Si:H) solar cells in the Schottky barrier contact design to modify the interface between aluminum-doped ZnO (ZnO:Al, AZO) and p-type hydrogenated amorphous silicon carbide (p-a-SiC:H) without plasmonic absorption. The high work function of the Ag-NPs provided a good channel for the transport of photogenerated holes. A p-type nanocrystalline SiC:H layer was used to compensate for the real surface defects and voids on the surface of Ag-NPs to reduce recombination at the AZO/p-type layer interface, which then enhanced the photovoltage of single-junction a-Si:H solar cells to values as high as 1.01 V. The Ag-NPs were around 10 nm in diameter and thermally stable in the p-type a-SiC:H film at the solar-cell process temperature. We will also show that a wide range of photovoltages between 1.01 and 2.89 V could be obtained with single-, double-, and triple-junction solar cells based on the single-junction a-Si:H solar cells with tunable high photovoltage. These solar cells are suitable photocathodes for solar water-splitting applications.

99% Efficient Three-Phase Buck-Type SiC MOSFET PFC Rectifier Minimizing Life Cycle Cost in DC Data Centers

Due to the increasing power consumption of data centers, efficient dc power distribution systems have become an important topic in research and industry over the last years and according standards have been adopted. Furthermore the power consumed by telecommunication equipment and data centers is an economic factor for the equipment operator, which implies that all parts of the distribution system should be designed to minimize the life cycle cost, i.e. the sum of first cost and the cost of the power conversion losses. This paper demonstrates how semiconductor technology, chip area, magnetic component volumes and switching frequency can be selected based on life cycle cost, using analytical and numerical optimizations. A three-phase buck-type PFC rectifier with integrated active filter for 380V dc distribution systems is used as an example system, which shows that a peak efficiency of 99% is technically and economically feasible with state-of-the-art SiC MOSFETs and nanocrystalline or ferrite cores. Measurements taken on an 8 kW, 4 kWdm-3 hardware prototype demonstrate the validity and feasibility of the design.

(p+)Nanocrystalline/(n−)crystalline/(n+)nanocrystalline Si fast recovery diode with (p+)nanocrystalline SiC inserted in cathode junction

Surge suppressed by (p+)nanocrystalline SiC islands buried in cathode junction of operating (p+)nanocrystalline/(n−)crystalline/(n+)nanocrystalline Si fast recovery diode was studied in this paper. The proposed diode was prepared by plasma enhanced chemical vapor deposition (PECVD) technique. Firstly, heavily boron doped Si film was prepared with PECVD on one end of the lightly doped n−-type cubic Si (c-Si) substrate to form an anode. Hereafter, heavily boron doped SiC layer was deposited in the lithography mask etched pits in another side of c-Si, heavily phosphorus doped Si film was subsequently grown on the end inserted SiC to form a cathode. For a comparison, a traditional p+-n−-n+ type device without SiC was fabricated by the PECVD growing heavily boron and heavily phosphorus doped films on two sides of the c-Si respectively. Nanocrystallization was demonstrated by X-ray diffraction and high resolution transmission electron microscopy in the fabricated films. Capacitance vs. voltage relation, current vs. voltage feature and reverse recovery waveform were measured to reveal the performance of the studied components. The important roles of (p+)nanocrystalline SiC in the static and dynamic conduction processes in operating device were probed, which indicates that the reverse recovery behaviors were improved.

The microstructure and electromagnetic wave absorption properties of near-stoichiometric SiC fibre

The microstructure and electromagnetic (EM) properties of near-stoichiometric SiC fibres (with C/Si ratio of 1.125) were analyzed and evaluated in detail. The SiC fibres consisted of β-SiC nanocrystallines and free carbon, and exhibited a uniquely specific skin-core structure with thin carbon layer of 5nm on their surfaces. The relative complex permittivity increased with the increasing fibre volume fraction from 13vol% to 27.5vol%. The imaginary part of permittivity increased from 1.36 to 2.13 at 10GHz, due to more SiC nanocrystallines and interfaces generating. The EM wave absorption properties were enhanced by the increasing fibre volume fraction and the effective absorption bandwidth was approximately 2.6GHz when the fibre volume fraction was 27.5 vol%.

Electron transport in nanocrystalline SiC films obtained by direct ion deposition

Electrical conductivity of nanocrystalline SiC films obtained by direct ion deposition was investigated within the temperature interval from 2 to 770 K. It were investigated the samples of films with 3С-SiC polytype structure and the heteropolytype films formed by layers of different polytypes SiC (3C-SiC/21R-SiC, 21R-SiC/27R-SiC, 3C-SiC/15R-SiC). The films had n-type conductivity that ensured a small excess of silicon ions. The thermally activated character of electron transport in the 3С-SiC polytype films was established. In the heteropolytype films the temperature dependence of the electrical resistance was described by the relation R(T) = R0 × exp[–kT/E0]. It was shown that the charge transport mechanism in the heteropolytype samples is electron tunneling through potential barriers formed by the conduction band offset in the contact region of the heterojunction. Tunnel charge transport occurs due to the presence of discrete energy states in the forbidden band caused the dimensional quantization.

Formation of silicon nanocrystals in silicon carbide using flash lamp annealing

During the formation of Si nanocrystals (Si NC) in SixC1−x layers via solid-phase crystallization, the unintended formation of nanocrystalline SiC reduces the minority carrier lifetime and therefore the performance of SixC1−x as an absorber layer in solar cells. A significant reduction in the annealing time may suppress the crystallization of the SiC matrix while maintaining the formation of Si NC. In this study, we investigated the crystallization of stoichiometric SiC and Si-rich SiC using conventional rapid thermal annealing (RTA) and nonequilibrium millisecond range flash lamp annealing (FLA). The investigated SixC1−x films were prepared by plasma-enhanced chemical vapor deposition and annealed at temperatures from 700 °C to 1100 °C for RTA and at flash energies between 34 J/cm2 and 62 J/cm2 for FLA. Grazing incidence X-ray diffraction and Fourier transformed infrared spectroscopy were conducted to investigate hydrogen effusion, Si and SiC NC growth, and SiC crystallinity. Both the Si content and the choice of the annealing process affect the crystallization behavior. It is shown that under certain conditions, FLA can be successfully utilized for the formation of Si NC in a SiC matrix, which closely resembles Si NC in a SiC matrix achieved by RTA. The samples must have excess Si, and the flash energy should not exceed 40 J/cm2 and 47 J/cm2 for Si0.63C0.37 and Si0.77C0.23 samples, respectively. Under these conditions, FLA succeeds in producing Si NC of a given size in less crystalline SiC than RTA does. This result is discussed in terms of nucleation and crystal growth using classical crystallization theory. For FLA and RTA samples, an opposite relationship between NC size and Si content was observed and attributed either to the dependence of H effusion on Si content or to the optical absorption properties of the materials, which also depend on the Si content.

Enhancement of second harmonic generation in nanocrystalline SiC films based natural microcavities

We explore second harmonic generation (SHG) features of nanocrystalline SiC films based natural microcavities (MC) with resonance modes in the vicinity of the fundamental radiation at 1064 nm wavelength for nanocrystalline films of 24R, 27R polytypes characterized by high radiation doubling frequency coefficients. The natural walls of the SiC NC serve a role of cavities. To learn the nonlinear resonance features of the MC near the fundamental wavelength 1064 nm three series of the nc-SiC films indicated as 7495, 7517 and 7522 with thicknesses varying within 250–600 nm were fabricated. The role of the coherent and non-coherent contributions with taking into an account of cavity resonance modes is discussed. The giant increase of the SHG was achieved which may be used for further fabrication of the frequency transducers and nonlinear optical triggers.

Spin Centres in SiC for Quantum Technologies

Atomic-scale colour centres in bulk and nanocrystalline SiC are promising for quantum information processing, photonics and sensing at ambient conditions. Their spin state can be initialized, manipulated and readout by means of optically detected magnetic resonance. It has been shown that there are at least two families of colour centres in SiC with S = 1 and S = 3/2, which have the property of optical alignment of the spin levels and allows a spin manipulation. The ground state and the excited state were demonstrated to have spin S = 3/2 and a population inversion in the ground state can be generated using optical pumping, leading to stimulated microwave emission even at room temperature. By controlling the neutron irradiation fluence, the colour centres concentration can be varied over several orders of magnitude down to a single defect level. Several, separately addressable spin centres have been identified in the same crystal for each polytype, which can be used either for magnetic field or temperature sensing. Some of these spin centres are characterised by nearly temperature independent zero-field splitting, making these centres very attractive for vector magnetometry. Contrarily, the zero-field splitting of the centres in the excited state exhibits a giant thermal shift, which can be used for thermometry applications. Finally coherent manipulation of spin states has been performed at room temperature and even at temperatures higher by hundreds of degrees. SiC is taking on a new role as a flexible and practical platform for harnessing the new quantum technologies.

Simulation and experimental study of CVD process for low temperature nanocrystalline silicon carbide coating

There is a huge requirement for development of a coating technique in nuclear industry, which is environmentally safe, economical and applicable to large scale components. In this view, simulation of gas-phase behavior in specially designed CVD reactor was carried out using computational tool, COMSOL. There were two important zones in CVD reactor first one is precursor vaporization zone and second one is coating zone. Optimized parameters for coating were derived from the simulation of gas phase dynamics in both zone of CVD reactor. The overall effect of fluid velocity, heat flow and concentration profile showed that Re=54 is the optimum reaction condition for uniform coating in CVD system. In CVD coating experiments a synthesized halogen free, non-toxic and non-corrosive silicon carbide precursor was used. Uniform coating of SiC was obtained on zircaloy substrate at 900°C using as synthesized organosilicon precursor. The X-ray diffraction and scanning electron microscopy analysis show that dense nano crystalline SiC film was deposited on zircaloy substrate.

In situ synthesis and electrical properties of porous SiOC ceramics decorated with SiC nanowires

Porous SiOC ceramics decorated with SiC nanowires were synthetized in situ from wood powders impregnated with silicone resins followed by pyrolysis in argon atmosphere. The content of wood powders is a crucial parameter affecting the microstructure and electrical properties of the composites. The multiphase SiOC ceramics contain a glassy matrix, nanosized β-SiC, together with cristobalite and graphite-like carbon. With increasing the content of wood powders, SiC nanowires display significant changes in morphology and growth mechanism. The composite with a lower content of wood powders exhibits higher specific surface area and pore volume. The electrical resistivities of the composites are in the range of 11.3–1.2×10−2Ωcm. An increase in electrical conductivity is attributed to an improvement of the ordering of graphitic-like domains, and to a possible contribution of SiC nanocrystallines and nanowires when the content of wood powders increases.

Investigations of Ar ion irradiation effects on nanocrystalline SiC thin films

The effects of 800keV Ar ion irradiation on thin nanocrystalline SiC films grown on (100) Si substrates using the pulsed laser deposition (PLD) technique were investigated. On such PLD grown films, which were very dense, flat and smooth, X-ray reflectivity, glancing incidence X-ray diffraction and nanoindentation investigations were easily performed to evaluate changes induced by irradiation on the density, surface roughness, crystalline structure, and mechanical properties. Results indicated that the SiC films retained their crystalline nature, the cubic phase partially transforming into the hexagonal phase, which had a slightly higher lattice parameter then the as-deposited films. Simulations of X-ray reflectivity curves indicated a 3% decrease of the films density after irradiation. Nanoindentation results showed a significant decrease of the hardness and Young's modulus values with respect to those measured on as-deposited films. Raman and X-ray photoelectron spectroscopy investigations found an increase of the CC bonds and a corresponding decrease of the SiC bonds in the irradiated area, which could explain the degradation of mechanical properties.