Bulk Silicon Carbide Research Articles

The Creation of True Two-Dimensional Silicon Carbide.

This paper reports the successful synthesis of true two-dimensional silicon carbide using a top-down synthesis approach. Theoretical studies have predicted that 2D SiC has a stable planar structure and is a direct band gap semiconducting material. Experimentally, however, the growth of 2D SiC has challenged scientists for decades because bulk silicon carbide is not a van der Waals layered material. Adjacent atoms of SiC bond together via covalent sp3 hybridization, which is much stronger than van der Waals bonding in layered materials. Additionally, bulk SiC exists in more than 250 polytypes, further complicating the synthesis process, and making the selection of the SiC precursor polytype extremely important. This work demonstrates, for the first time, the successful isolation of 2D SiC from hexagonal SiC via a wet exfoliation method. Unlike many other 2D materials such as silicene that suffer from environmental instability, the created 2D SiC nanosheets are environmentally stable, and show no sign of degradation. 2D SiC also shows interesting Raman behavior, different from that of the bulk SiC. Our results suggest a strong correlation between the thickness of the nanosheets and the intensity of the longitudinal optical (LO) Raman mode. Furthermore, the created 2D SiC shows visible-light emission, indicating its potential applications for light-emitting devices and integrated microelectronics circuits. We anticipate that this work will cause disruptive impact across various technological fields, ranging from optoelectronics and spintronics to electronics and energy applications.

Prominent luminescence of silicon-vacancy defects created in bulk silicon carbide p\u2013n junction diodes

We investigate fluorescent defect centers in 4H silicon carbide p–n junction diodes fabricated via aluminum-ion implantation into an n-type bulk substrate without the use of an epitaxial growth process. At room temperature, electron-irradiated p–n junction diodes exhibit electroluminescence originating from silicon-vacancy defects. For a diode exposed to an electron dose of 1 times 10^{18},{{mathrm{cm}}}^{-2} at 800,{{mathrm{keV}}}, the electroluminescence intensity of these defects is most prominent within a wavelength range of 400–1100,{{mathrm{nm}}}. The commonly observed {{mathrm{D}}}_1 emission was sufficiently suppressed in the electroluminescence spectra of all the fabricated diodes, while it was detected in the photoluminescence measurements. The photoluminescence spectra also displayed emission lines from silicon-vacancy defects.

Fabrication of SiC Sealing Cavity Structure for All-SiC Piezoresistive Pressure Sensor Applications.

High hardness and corrosion resistance of SiC (silicon carbide) bulk materials have always been a difficult problem in the processing of an all-SiC piezoresistive pressure sensor. In this work, we demonstrated a SiC sealing cavity structure utilizing SiC shallow plasma-etched process (≤20 μm) and SiC–SiC room temperature bonding technology. The SiC bonding interface was closely connected, and its average tensile strength could reach 6.71 MPa. In addition, through a rapid thermal annealing (RTA) experiment of 1 min and 10 mins in N2 atmosphere of 1000 °C, it was found that Si, C and O elements at the bonding interface were diffused, while the width of the intermediate interface layer was narrowed, and the tensile strength could remain stable. This SiC sealing cavity structure has important application value in the realization of an all-SiC piezoresistive pressure sensor.

Imaging with Nanometer Resolution Using Optically Active Defects in Silicon Carbide

Nanostructured and bulk silicon carbide ($\mathrm{Si}\mathrm{C}$) materials are relevant for electronics, nano- and micromechanical systems, and biosensing applications. $\mathrm{Si}\mathrm{C}$ has recently emerged as an alternative platform for nanophotonics and quantum applications due to its intra-band-gap point defects, emitting from the visible to the near-infrared, which are ideal for photoluminescent probes. Here, we use a single-molecule localization microscope to study the photoluminescence (PL) properties of $\mathrm{Si}\mathrm{C}$ point defects in bulk, quantum dots, and nanoparticles of different sizes in the 3C and $4H\text{\ensuremath{-}}\mathrm{Si}\mathrm{C}$ polytypes using intra-band-gap excitation. We study the PL dynamics by using different excitation wavelengths, and we use the point-defect PL intermittency to achieve superresolved images, with a resolution of 20 nm and a minimum distance between emitters of 40 nm. We observe that, while 561 nm is an ideal excitation wavelength to obtain a sufficient blinking behavior, 638 nm is mostly quenching the PL. We further incubate $4H\text{\ensuremath{-}}\mathrm{Si}\mathrm{C}$ nanoparticles with MCF10A cells in vitro and observe superresolved images of the nanoparticles in cells by combining 561 and 638 nm excitation. This approach is very promising for the application of $\mathrm{Si}\mathrm{C}$ fluorescent nanocrystals and their quantum dots, hosting a combination of intra-band-gap color centers and surface defects, for quantum nanophotonics, magnetic sensing, and biomedical imaging, paving the way for single-particle tracking combined with spin sensing within a cellular environment using near-infrared emission.

Spatial density neural network force fields with first-principles level accuracy and application to thermal transport

Constrained by the fixed mathematical form of most empirical potentials used in classical molecular dynamics (MD) simulations, many properties of materials cannot be captured within experimental accuracy. On the other hand, accurate electronic structure calculations based on quantum theory, most notably density functional theory (DFT), are limited to several hundred atoms within a picosecond, which makes the method inadequate for modeling systems beyond the nanoscale. A combination of speed from classical MD and fidelity from DFT can be achieved through machine learning methods. Herewith, we developed an approach named spatial density neural network force fields (SDNNFFs) by training neural networks to ``learn'' and predict DFT-level forces. Our model focuses on the usage of a three-dimensional mesh of density functions, which together act as a mapping of the atomic environment and provides a physical representation of the forces acting on the central atom. Several notable advantages arise from the SDNNFF, including (1) the avoidance of the chain rule on the total energy and other variables by direct calculation of the forces from the neural network, (2) the ever large $N\ifmmode\times\else\texttimes\fi{}t$ scaling of the training data, where $N$ is the number of atoms in a supercell and $t$ is the number of evaluated structures by first-principles, and (3) the significant reduction in parameters and human effort needed to successfully train a force- and/or property-converged neural network force field. Overall, we focus on modeling DFT-level forces with minimal computational cost and parametrization for rapid prediction of phonon-based properties and future molecular dynamics of large-scale systems. To demonstrate the SDNNFF, we trained several models on diamond structures, including bulk silicon (Si), diamond, silicon carbide (SiC), and boron arsenide (BAs), and predicted their phonon dispersions and lattice thermal conductivities using the direct solution to the phonon Boltzmann transport equation. For phonon properties, we utilized a fitting method for obtaining the second- and third-order force constants, which outperforms the highly force-sensitive finite displacement method when employing neural network force fields. In comparison to DFT lattice thermal conductivity, we obtained high precision results from our SDNNFF within 0.7% for Si, 6.2% for diamond, 2.76% for SiC, and 7.46% for BAs, with further agreement with experiments. The phonon dispersions from the SDNNFF also matched those from direct DFT and experiments. The developed approach for accurately predicting phonon transport properties of crystalline materials would largely benefit the design of advanced materials with improved performance, such as complex thermoelectric devices and low thermal resistance interfaces for nanoelectronics. Future applications of our SDNNFF model could be extended toward including atomic energy into the algorithm and simulating large-scale heterogeneous systems for quasielectronic representations for various properties.

Visible and Infrared Photoluminescence in Hexagonal Silicon Carbide by Direct Femtosecond Laser Writing

Optically active color centers in silicon carbide have attracted considerable attention in the past few years as candidates for quantum technologies such as single-photon sources, nanomagnetic resonance imaging, and spintronic devices. Control over defect position and their placement at the desired location within a chip, necessary to integrate them within optical and electronic devices, is still a challenge. Recently, laser writing emerged as a new tool to generate vacancies in crystals as a starting point for the formation of color centers. In this work, a laser writing method has been used to produce color centers in 4H and 6H bulk silicon carbide by using a femtosecond laser. An array of color centers was fabricated by different pulse laser energies in sites of square grids at varying depths (from the surface to 10 µm below the surface). We optically characterized the fabricated color centers using confocal imaging, photoluminescence, and Raman spectroscopy. We show that the technique can produce specifically vacancy color centers with a relevant emission in the visible (peak around 700 nm) and near-infrared (peak at 900 nm) with the latter identified as the silicon-vacancy. This method can be adapted to engineer color centers in silicon carbide at different depths in the material, for the above-mentioned applications, in addition to the fabrication of light-emitting diodes.

Role of Microstructure and Interfaces in Governing the Mechanical Properties of Nanocomposites Manufactured in the Solid State

Considerable efforts have been made to manufacture composite materials with low density and to enhance the specific strength of metals and alloys by adding boron carbide particles. One of the major factors in enhancing the mechanical properties of these composites is the interfacial characteristics of the metal/ceramic interface, particularly the strength of the bonding between the metal/ceramic interface. Here we investigate the microstructure and interfacial characteristics of the boron carbide-nickel (B4C-Ni) and silicon carbide-boron carbide (SiC-B4C) composites processed in the solid state at relatively high pressure, using transmission electron microscopy. The sintered boron carbide and nickel powders showed improved hardness and modulus, which is attributed to the formation of nickel boride (Ni4B3) at the nickel-boron carbide interface. Using fine-probe energy dispersive spectroscopy and high-angle annular dark field imaging, we observed a Si-rich intermixing layer for the silicon carbide-boron carbide interface, which could enhance the strength of the interface. The density functional theory calculations show that the decohesion energy of silicon carbide-boron carbide (without intermixing) is smallest at the interface as compared with the bulk silicon carbide and boron carbide, and that the structure would fail along the interface.

Effect of cobalt addition to Si–Cr solvent in top-seeded solution growth

Top-seeded solution growth (TSSG) has received considerable attention as a promising growth method for obtaining high-quality silicon carbide (SiC) single crystals. However, low carbon solubility in the TSSG method prevents rapid growth of bulk SiC single crystals. In this study, we propose cobalt (Co), for the first-ever use in this method, with the aim of improving the SiC growth rate and crystal quality realized through the TSSG method with a Si0.6Cr0.4 solvent. The surface morphology of crystals grown with the Si0.6Cr0.4 solvent was characterized by significant step bunching. However, the growth surface generated by a Si0.56Cr0.4Co0.04 solvent was characterized by a smooth surface with step-flow growth, and the crystal grown with this solvent contained almost no Co. Furthermore, the growth rate in the Si0.56Cr0.4Co0.04 solvent was three times higher than that in the Si0.6Cr0.4 solvent. Based on the aforementioned results, Si0.56Cr0.4Co0.04 represents a promising solvent that can be used to enhance the growth rate and improve the quality of SiC single crystals.

High-Q suspended optical resonators in 3C silicon carbide obtained by thermal annealing.

We fabricate suspended single-mode optical waveguides and ring resonators in 3C silicon carbide (SiC) that operate at telecommunication wavelength, and leverage post-fabrication thermal annealing to minimize optical propagation losses. Annealed optical resonators yield quality factors of over 41,000, which corresponds to a propagation loss of 7 dB/cm, and is a significant improvement over the 24 dB/cm in the case of the non-annealed chip. This improvement is attributed to the enhancement of SiC crystallinity and a significant reduction of waveguide surface roughness, from 2.4 nm to below 1.7 nm. The latter is attributed to surface layer oxide growth during the annealing step. We confirm that the thermo-optic coefficient, an important parameter governing high-power and temperature-dependent performance of SiC, does not vary with annealing and is comparable to that of bulk SiC. Our annealing-based approach, which is especially suitable for suspended structures, offers a straightforward way to realize high-performance 3C-SiC integrated circuits.

Attosecond-fast internal photoemission

The photoelectric effect has a sister process relevant in optoelectronics called internal photoemission. Here an electron is photoemitted from a metal into a semiconductor. While the photoelectric effect takes place within less than 100 attoseconds, the attosecond time scale has so far not been measured for internal photoemission. Based on the new method CHArge transfer time MEasurement via Laser pulse duration-dependent saturation fluEnce determinatiON, CHAMELEON, we show that the atomically thin semi-metal graphene coupled to bulk silicon carbide, forming a Schottky junction, allows charge transfer times as fast as (300 $\pm$ 200) attoseconds. These results are supported by a simple quantum mechanical model simulation. With the obtained cut-off bandwidth of 3.3 PHz for the charge transfer rate, this semimetal-semiconductor interface represents the first functional solid-state interface offering the speed and design space required for future light-wave signal processing.

SiC based charged particle strip sensor spectrometer with neutron detection capability

Silicon carbide (SiC) devices have gained much attention owing to their superior characteristics that make them high-temperature and radiation-hard. The advantage of the SiC arises from its unique combination of electronic and physical properties such as a wide band-gap, high breakdown electric field strength, high saturated electron velocity, and high thermal conductivity. The wide band-gap results in a low intrinsic charge carrier concentration and a radiation hardness. The low intrinsic charge carrier concentration leads to low device leakages at high temperature. The high breakdown strength allows SiC devices to operate at much higher voltages. The aim of this publication is to present current status of a charged particle spectrometer based on a SiC strip detector. The sensor is made of a 4H-SiC (α-SiC) hexagonal crystalline structure material which manifests good spectroscopic characteristics for charged particle detection similar to a standard silicon diode (20 keV FWHM with 5,4857 MeV 241Am alpha particle). To obtain sensors for the charged particle detection out of the SiC bulk material we created Schottky contacts on the top and the Ohmic contact on the bottom. Preparation of the contacts will be discussed alongside the electric characterization of the sensor material. Results of the charged particle and the gamma detection and detection of thermal neutron detection (after a neutron converter deposition) will be presented. There will be also a discussion regarding fast neutron detection. The SiC sensor material was attached to a VATA GP8 based 128 strip readout to form the handheld spectrometer which will be demonstrated.

Impact of Varying Parameters on the Temperature Gradients in 100 mm Silicon Carbide Bulk Growth in a Computer Simulation Validated by Experimental Results

AbstractThe investigation of the interplay of experimental crystal growth runs and computer simulations of the physical vapor transport (PVT) growth process of silicon carbide (SiC) strongly supports the development of the growth technology toward larger crystalline diameters. To apply computer‐aided designs of the growth process, a material database is developed that enables quantitative calculations of the temperature distribution inside the growth setup. The model utilizing COMSOL Multiphysics is validated by experimental means using five 100 mm SiC crystal growth runs, two measurement runs in a 100 mm PVT setup, and one SiC crystal growth run in a 75 mm PVT setup, varying in power, pressure, coil position, and reactor geometry. Material data are varied and the impact on the thermal gradients is investigated to trace out critical and noncritical parameters for the accuracy of the simulation. The influence of ambient pressure on the isolation's performance is studied experimentally and this correlation is implemented into the model. Beyond the state of the art, this work presents a modeling approach and handling of material properties for a true scale up of the current 100 mm and 150 mm SiC sublimation growth technologies to 200 mm.

Near-field radiative heat transfer between rough surfaces modeled using effective media with gradient distribution of dielectric function

Near-field radiative heat transfer (NFRHT) between rough surfaces, due to its widespread presence in engineering practice of near-field energy utilization, requires indepth studies, especially from the perspective of physical mechanism. In this paper, an effective multilayer model is built to approach the NFRHT between random rough surfaces of silicon carbide (SiC). Using the effective medium theory (EMT), the effective dielectric function of each layer is obtained, which forms a gradient distribution of dielectric function along the depth of the medium. The influence of the effective dielectric function on surface phonon polaritons (SPhPs) is analyzed, showing that the effective layers with small filling fraction of SiC feature lower SPhP resonance frequencies than SiC bulk. The coupling of SPhPs from the gradient distribution of dielectric function produces new surface modes that dominates the NFRHT. Investigation on the effect of root mean square height (RMS height, σ) reveals that the peaks of local density of states (LDOS) and spectral heat flux are red-shifted as σ increases, while the spectral heat flux below the peak frequency gets larger. This can be attributed to the coupling of SPhPs inside the rough layer. We also found the total net heat flux between rough surfaces separated by an average distance exceeds that between smooth plates and increases with increasing σ, which offer a new way to enhance NFRHT. This work provides a reference for the simulation and understanding of the NFRHT between rough surfaces.

Cobalt Intercalation of Graphene on Silicon Carbide

In this paper, we studied cobalt intercalation of single-layer graphene grown on the 4H-SiC(0001) polytype. The experiments were carried out in situ under ultrahigh vacuum conditions by high energy resolution photoelectron spectroscopy using synchrotron radiation and low energy electron diffraction. The nominal thicknesses of the deposited cobalt layers varied in the range of 0.2–5 nm, while the sample temperature was varied from room temperature to 800°C. Unlike Fe films, the annealing of Co films deposited on graphene at room temperature is shown to not intercalate graphene by cobalt. The formation of the graphene–cobalt–SiC intercalation system was detected upon deposition of Co atoms on samples heated to temperatures of above ~400°C. Cobalt films with a thickness up to 2 nm under graphene are formed using this method, and they are shown to be magnetized along the surface at thicknesses of greater than 1.3 nm. Graphene intercalation by cobalt was found to be accompanied by the chemical interaction of Co atoms with silicon carbide leading to the synthesis of cobalt silicides. At temperatures of above 500°C, the growth of cobalt films under graphene is limited by the diffusion of Co atoms into the bulk of silicon carbide.

Recent development of wide bandgap semiconductor SiC substrates and device

As a key representative material for the third-generation wide bandgap semiconductors, silicon carbide (SiC) is a promising wide band gap semiconductor material and can be used for the fabrication of high-power and high-frequency electronics, due to its superior physical properties，such as high thermal conductivity, wide band gap and high critical breakdown field. In recent years, bulk growth of SiC single crystals and the fabrication of devices have made significant progress. The paper introduces the growth techniques for SiC bulk and presents the relationship between the on-state resistance and voltage or laser energy. It also analyses the failure of devices.

Phase field modeling of heterogeneous microcrystalline ceramics

Diffuse interface models and simulations capture deformation and failure of polycrystalline ceramics with multiple phases. Two heterogeneous ceramic solids are investigated. The first consists of a boron carbide matrix phase embedded with titanium diboride grains. Boron carbide may undergo cleavage fracture, twinning, and amorphization under sufficient mechanical loading. Titanium diboride demonstrates cleavage fracture and limited plastic slip. The second ceramic composite consists of diamond crystals with a smaller fraction of silicon carbide grains, where the latter may encapsulate the former in a micro- or nano-crystalline matrix and/or may be interspersed as larger micro-crystals among the diamond grains. Diamond may undergo cleavage fracture while the cubic phase of silicon carbide may fracture and twin. A general constitutive framework suitable for representing behaviors of all phases of each material system is reported. This phase field framework is implemented in three-dimensional (3D) finite element (FE) simulations of polycrystalline aggregates under compressive loading. Numerical results demonstrate effects of grain and phase morphology and activation or suppression of slip or twinning mechanisms on the overall strength and ductility of each material system. Efforts to inhibit localization and cleavage in boron carbide crystals, including addition of titanium diboride, promote increases in strength of the first composite, though residual stresses may be necessary to realize toughness improvements observed experimentally. In the second composite, layers of softer silicon carbide nanocrystals along grain boundaries (GBs) improve overall strength and ductility relative to addition of larger bulk silicon carbide grains.

High temperature ferromagnetism in mechanically milled 6H-SiC nanoparticles

Silicon carbide (SiC) nanoparticles with different sizes were successfully obtained by high energy ball milling. Room temperature magnetic measurements showed that the saturation magnetization of milled-SiC nanoparticles increases monotonically from 0.0006 to 0.0032 emu/g with increasing milling time, while the bulk SiC powders present diamagnetism. Besides, the high temperature hysteresis curves indicated that the Curie temperature of the milled-SiC nanoparticles was much over 800 K. Meanwhile, the results of X-ray diffraction, Raman spectrometer and scanning electron microscope suggested that the surface defects were introduced into the milled-SiC nanoparticles. Further studies via electron spin resonance and photoluminescence proved that the surface defects are responsible for the origin of ferromagnetism in milled-SiC nanoparticles.

Интеркалирование графена на карбиде кремния кобальтом

AbstractIn this paper, we studied cobalt intercalation of single-layer graphene grown on the 4 H -SiC(0001) polytype. The experiments were carried out in situ under ultrahigh vacuum conditions by high energy resolution photoelectron spectroscopy using synchrotron radiation and low energy electron diffraction. The nominal thicknesses of the deposited cobalt layers varied in the range of 0.2–5 nm, while the sample temperature was varied from room temperature to 800°C. Unlike Fe films, the annealing of Co films deposited on graphene at room temperature is shown to not intercalate graphene by cobalt. The formation of the graphene–cobalt–SiC intercalation system was detected upon deposition of Co atoms on samples heated to temperatures of above ~400°C. Cobalt films with a thickness up to 2 nm under graphene are formed using this method, and they are shown to be magnetized along the surface at thicknesses of greater than 1.3 nm. Graphene intercalation by cobalt was found to be accompanied by the chemical interaction of Co atoms with silicon carbide leading to the synthesis of cobalt silicides. At temperatures of above 500°C, the growth of cobalt films under graphene is limited by the diffusion of Co atoms into the bulk of silicon carbide.

High-Q-factor nanobeam photonic crystal cavities in bulk silicon carbide

Silicon carbide (SiC) is a promising optical material for stable and broadband nanophotonics. To date, thin crystalline SiC layers for nanophotonic platforms have been created by ion implantation or growth on other materials, which may cause optical absorption in the SiC layer. We fabricated SiC nanobeam photonic crystal cavities directly from a crystalline (4H) SiC bulk wafer using oblique plasma etching to avoid material-based optical absorptions. The measured quality (Q) factor of the nanobeam photonic crystal cavity reaches 4 × 104, which is the highest recorded Q factor in crystalline SiC cavities. Furthermore, we investigated theoretical Q factors by taking into account structural imperfections unique to this fabrication process and compared them with the experimental results.

Electronic exchange-correlation, many-body effect issues on first-principles calculations of bulk SiC polytypes

The first-principles Projector-Augmented Wave method (PAW) is used to investigate the electronic, phonon band structure and dielectric properties of four bulk silicon carbide (SiC) polytypes. We employ PAW pseudopotential density functional theory with Perdew, Burke and Ernzerhof (PBE) and hybrid HSE06 approximations of the exchange-correlation functional. Many-body effects are incorporated using the GW approximation of the self-interaction to study SiC properties. GW method in its single-shot variant, which is based on the many-body perturbation theory (MBPT), is used to calculate the quasi-particle (QP) energies of the band structure and the dielectric properties for different polytypes. The electronic band structure determination within GW method uses the Wannier procedure where a basis set of maximally localized Wannier function (MLWF) is constructed to interpolate the QP energies of few regular mesh k-points to the high-symmetry lines in Brillouin zone. As a consequence of QP correction to the Kohn–Sham energies, bandgap is increased by upto 3 eV in case of 4H–SiC, as compared to PBE bandgap. GW results are comparable to those of hybrid functionals and are in good agreement with the experimental results. The optical properties are then studied within PBE, HSE06 and include many-body effects. In addition, the phonon band structure has been investigated within HSE06 and compared to previous PBE results. We found good agreement with the previous theoretical results and the experimental available data.