Raman spectroscopy has been used to identify defective bonding in neon and silicon ion irradiated single crystals of 6H-SiC. Observable differences exist in the C-C bonding region corresponding to different defect structures for neon and silicon ion implantations. Raman spectra of ion irradiated SiC show less tensile strain than neutron irradiations, explained by a residual compressive stress caused by the swelling constrained by the undamaged substrate. Evidence of oxidation during high temperature ion implantation is observed as C-O and Si-O Raman signals. Annealing irradiated SiC while acquiring Raman spectra shows rapid recovery of Si-C bonding, but not a complete recovery of the unirradiated structure. Annealing irradiated SiC causes surface oxidation where unirradiated SiC does not oxidise. Comparisons are made to the apparent radiation resistance of diamond and silicon which have similar crystal structures, but are monatomic, leading to the suggestion that chemical defects are responsible for increased radiation damage in SiC.
The influence of implantation-induced point defects (PDs) on SiC oxidation is investigated via molecular dynamics simulations. PDs generally increase the oxidation rate of crystalline grains. Particularly, accelerations caused by Si antisites and vacancies are comparable, and followed by Si interstitials, which are higher than those by C antisites and C interstitials. However, in the grain boundary (GB) region, defect contribution to oxidation is more complex, with C antisites decelerating oxidation. The underlying reason is the formation of a C-rich region along the oxygen diffusion pathway t…
The oxidation of SiC and Si provide a unique opportunity for studying oxidation mechanisms because the product is the same, SiO2. Silicon oxidation follows a linear-parabolic law, with molecular oxygen identified as the oxidant. SiC oxidation obeys the same linear-parabolic law but has different rates and activation energies and exhibits much stronger face-dependence. Using results from first-principles calculations, we show that atomic and molecular oxygen are the oxidant for Si- and C-face SiC respectively. Comparing SiC with Si, we elucidate how the interface controls the competition betwee…
When epitaxial graphene layers are formed on SiC(0001), the first carbon layer (known as the "buffer layer"), while relatively easy to synthesize, does not have the desirable electrical properties of graphene. The conductivity is poor due to a disruption of the graphene pi-bands by covalent bonding to the SiC substrate. Here we show that it is possible to restore the graphene pi-bands by inserting a thin oxide layer between the buffer layer and SiC substrate using a low temperature, CMOS-compatible process that does not damage the graphene layer.
Silicon carbide (SiC) is an attractive semiconductor material for applications in power electronic devices. However, fabrication of a high-quality SiC/SiO${}_2$ interface has been a challenge. It is well-known that there is a great difference in oxidation rate between the Si-face and C-face, and that the quality of oxide on the Si-face is greater than that on the C-face. However, the atomistic mechanism of the thermal oxidation of SiC remains to be solved. In this paper, a new Si-C-O interatomic potential was developed to reproduce the kinetics of the thermal oxidation of SiC. Using this newly…
We report first-principles static and dynamic calculations that clarify the microscopic mechanism of carbon annihilation due to phosphorous treatment upon oxidation of silicon carbide (SiC). We identify the most stable form of the phosphorus (P) in the oxide as the four-fold coordinated with the dangling PO unit and find that the unit attracts carbon ejected from the interface, thus operating as a carbon absorber. This finding provides a microscopic reasoning for the first time for the promotion of the oxidation reaction on one hand and the annihilation of the C-related defects at the interfac…
We present a comprehensive structural characterization of ferromagnetic SiC single crystals induced by Ne ion irradiation. The ferromagnetism has been confirmed by electron spin resonance and possible transition metal impurities can be excluded to be the origin of the observed ferromagnetism. Using X-ray diffraction and Rutherford backscattering/channeling spectroscopy, we estimate the damage to the crystallinity of SiC which mutually influences the ferromagnetism in SiC.
We report on the low-energy electron irradiated 4H-SiC material studied by means of deep-level transient spectroscopy (DLTS) and Laplace-DLTS. Electron irradiation has introduced the following deep level defects: EH1 and EH3 previously assigned to carbon interstitial-related defects, and M-center, a metastable defect also recently assigned to carbon interstitial defects. We propose that EH1 and EH3 are identical to M1 and M3 and assign them to C_i^(= ) (h) and C_i^(0 ) (h), respectively. Moreover, we provide direct evidence that Laplace-DLTS can be used as an excellent tool to distinguish othe…
The doping dependence of dry thermal oxidation rates in n-type 4H-SiC was investigated. The oxidation was performed in the temperature range 1000C to 1200C for samples with nitrogen doping in the range of 6.5e15/cm3 to 9.3e18/cm3, showing a clear doping dependence. Samples with higher doping concentrations displayed higher oxidation rates. The results were interpreted using a modified Deal-Grove model. Linear and parabolic rate constants and activation energies were extracted. Increasing nitrogen led to an increase in linear rate constant pre-exponential factor from 10-6m/s to 10-2m/s and the …
We have performed electronic state calculations to clarify the initial stage of the oxidation of the Si- and C-faces in 4H-SiC based on the density-functional theory. We investigate how each Si and C atomic site is oxidized on C- and Si-face, and explore most probable reaction pathways, corresponding energy barriers, and possible defects generated during the oxidation. We have found that carbon annihilation process is different between on Si- and on C-face, and this difference causes different defects in interface; In C-face case, (1), carbon atoms are dissociated directly from the substrate a…
We demonstrate on the basis of first-principles calculations that the formation of carbonate-like moiety in SiO$_2$ could be the intrinsic origin of negative fixed charge in SiC thermal oxidation. We find that two possible origins for the negative fixed charges are O-lone-pair state and a negatively charged CO$_3$ ion in SiO$_2$. Such CO$_3$ ion is able to be formed as a result of the existence of residual C atoms in SiO$_2$, which are expected to be emitted from the interface between SiC and SiO$_2$, and the incorporation of H atoms during wet oxidation.
We report first-principles calculations that reveal the atomic forms, stability, and energy levels of carbon-related defects in SiC (0001)/SiO$_{\rm 2}$ systems. We clarify the stable position (SiC side, SiO$_{\rm 2}$ side, or just at the SiC/SiO$_{\rm 2}$ interface) of defects depending on the oxidation environment. Under an O-rich condition, the di-carbon antisite ((C$_{\rm 2}$)$_{\rm Si}$) in the SiC side is stable and critical for $n$-channel MOSFETs, whereas the di-carbon defect (Si-C-C-Si) at the interface becomes critical under an O-poor condition. Our results suggest that the oxidation…
The effects of the oxidation atmosphere and crystal faces on the interface-trap density was examined by using constant-capacitance deep-level transient spectroscopy to clarify the origin of them. By comparing the DLTS spectra of the low-mobility interfaces oxidized in a N$_2$O atmosphere with those of the high-mobility interfaces on C-face oxidized in a wet atmosphere, it was found that a high density of traps are commonly observed around the energy of 0.16 eV from the edge of the conduction band ($C1$ traps) in low-mobility interfaces irrespective of crystal faces. It was also found that the …
Based on the parameters from published ab-inito theoretical and experimental studies, and combining Molecular Dynamics (MD) and kinetic Monte Carlo (KMC) simulations, a framework of multi-scale modeling is developed to investigate the long-term evolution of displacement damage induced by heavy-ion irradiation in cubic silicon carbide. The isochronal annealing after heavy ion irradiation is simulated, and the annealing behaviors of total interstitials are found consistent with previous experiments. Two annealing stages below 600K and one stage above 900K are identified. The mechanisms for those…
The development of defect populations after proton irradiation of n-type 4H-SiC and subsequent annealing experiments is studied by means of deep level transient (DLTS) and photoluminescence (PL) spectroscopy. A comprehensive model is suggested describing the evolution and interconversion of irradiation-induced point defects during annealing below 1000°C. The model proposes the EH4 and EH5 traps frequently found by DLTS to originate from the (+/0) charge transition level belonging to different configurations of the carbon antisite-carbon vacancy (CAV) complex. Furthermore, we show that the tran…
Silicon Carbide (SiC) ceramic matrix composite (CMC) cladding is currently being pursued as one of the leading candidates for accident-tolerant fuels. To enable an improved understanding of SiC-SiC composite performance, the development of non-destructive evaluation techniques to assess critical defects is needed. Three-dimensional (3D) X-ray imaging, also referred to as X-ray computed tomography (CT), is a non-destructive, data-rich characterization technique that can provide surface and subsurface spatial information. This paper discusses the design and implementation of a fully automatic wo…
We present the first findings of the vacancy-related centers identified by the electron spin resonance (ESR) and electrically-detected (ED) ESR method in the non-irradiated 6H-SiC nanostructure. This planar 6H-SiC nanostructure represents the ultra-narrow p-type quantum well confined by the δ-barriers heavily doped with boron on the surface of the n-type 6H-SiC (0001) wafer. The EDESR method by measuring the only magnetoresistance of the 6H SiC nanostructure under the high frequency generation from the δ-barriers appears to allow the identification of the silicon vacancy centers as well as the…
The effect of oxygen adsorption on the local structure and electronic properties of monolayer graphene grown on SiC(0001) has been studied by means of Low Energy Electron Microscopy (LEEM), microprobe Low Energy Electron Diffraction (\muLEED) and microprobe Angle Resolved Photoemission (\muARPES). We show that the buffer layer of epitaxial graphene on SiC(0001) is partially decoupled after oxidation. The monitoring of the oxidation process demonstrates that the oxygen saturates the Si dangling bonds, breaks some Si-C bonds at the interface and intercalates the graphene layer. Accurate control …
Distribution of black spot defects and small clusters in 1 MeV krypton irradiated 3C-SiC has been investigated using advanced scanning transmission electron microscopy (STEM) and TEM. We find that two thirds of clusters smaller than 1 nm identified in STEM are invisible in TEM images. For clusters that are larger than 1 nm, STEM and TEM results match very well. A cluster dynamics model has been developed for SiC to reveal processes that contribute to evolution of defect clusters and validated against the (S)TEM results. Simulations showed that a model based on established properties of point d…
Oxidation of SiC produces SiO2 while CO is released. A `reoxidation' step at lower temperatures is, however, necessary to produce high-quality SiO2. This step is believed to cleanse the oxide of residual C without further oxidation of the SiC substrate. We report first-principles calculations that describe the nucleation and growth of O-deficient C clusters in SiO2 under oxidation conditions, fed by the production of CO at the advancing interface, and their gradual dissolution by the supply of O under reoxidation conditions. We predict that both CO and CO2 are released during both steps.
We report static and dynamic first-principles calculations that provide atomistic pictures of the initial stage of the oxidation processes occurring at the ($11\bar20$) surface of 4H-SiC. Our results unveil reaction pathways and their associated free-energy barriers for the adsorption of oxygen and the desorption of carbon atoms. We find that oxygen adsorption shows structural multi-stability and that the surface-bridge sites are the most stable and crucial sites for subsequent oxidation. We find that an approaching O$_2$ molecule is adsorbed, then dissociated and finally migrates toward these…
We present a detailed investigation of the magnetic properties in SiC single crystals bombarded with neon ions. Through careful measuring of the magnetization of virgin and irradiated SiC, we decompose the magnetization of SiC into paramagnetic, superparamagnetic, and ferromagnetic contributions. The ferromagnetic contribution persists well above room temperature and exhibits a pronounced magnetic anisotropy. We qualitatively explain the magnetic properties as a result of the intrinsic clustering tendency of defects.
We investigate nanoscale local anodic oxidation (LAO) on hydrogen-intercalated graphene grown by controlled sublimation of silicon carbide (SiC). Scanning probe microscopy (SPM) was used as a lithographic and characterization tool in order to investigate the local properties of the nanofabricated structures. The anomalous thickness observed after the graphene oxidation process is linked to the impact of LAO on the substrate. Micro-Raman spectroscopy was employed to demonstrate the presence of two oxidation regimes depending on the applied bias. We show that partial and total etching of monolay…
We found that carbon-associated byproducts formed at the dry-oxidized SiO2/SiC(0001) interface could be decomposed and be taken out to the SiO2 side by high-purity Ar annealing. We evaluated the concentration of the ejected carbon atoms in the SiO2 by secondary ion mass spectrometry, and discovered that it clearly depends on the condition of oxide formation (dry-oxidation, nitridation treatment, and phosphorus treatment). This work provides an indirect but unambiguous evidence for the carbon-byproducts existing at the SiO2/SiC interfaces, and also indicates that the phosphorus treatment remove…
Photoluminescence of Femtosecond Laser-irradiated Silicon Carbide
RPTU University Kaiserslautern-Landau（凯泽斯劳滕-兰道莱布尼茨技术大学）；Universität des Saarlandes（萨尔兰州立大学）；Luxembourg Institute of Science and Technology (LIST)（卢森堡科学与技术研究所）；University of Luxembourg（卢森堡大学）；Institut für Oberflächen- und Schichttechnik GmbH（表面与薄膜技术研究所有限公司）
We investigate the effect of SiC stacking on the 4H-SiC/SiO$_2$ interface, both in the presence and absence of O defects, which appear during thermal oxidation, via first principles calculations. It is known that 4H-SiC(0001) has two different surface types, depending on which of the two lattice sites, $h$ or $k$, is at the surface [K. Arima \textit{et al}., Appl. Phys. Lett. \textbf{90}, 202106 (2007)]. We find interlayer states along the conduction band edge of SiC, whose location changes depending on the interface type, and thus too the effect of defects. When $h$ sites are directly at the …
Graphene films prepared by heating the SiC(000-1) surface (the C-face of the {0001} surfaces) in a Si-rich environment are studied using low-energy electron diffraction (LEED) and low-energy electron microscopy (LEEM). Upon graphitization, an interface with rt(43) x rt(43)-R7.6 degree symmetry is observed by in situ LEED. After oxidation, the interface displays rt(3) x rt(3)-R30 degree symmetry. Electron reflectivity measurements indicate that these interface structures arise from a graphene-like "buffer layer" that forms between the graphene and the SiC, similar to that observed on Si-face Si…
Materials with good carrier mobilities are desired for device applications, but in real devices the mobilities are usually limited by the presence of interfaces and contacts. Mobility degradation at semiconductor-dielectric interfaces is generally attributed to defects at the interface or inside the dielectric, as is the case in Si/SiO2 structures, where processing does not introduce detrimental defects in the semiconductor. In the case of SiC/SiO2 structures, a decade of research focused on reducing or passivating interface and oxide defects, but the low mobilities have persisted. By invoking…
Silicon carbide silicon carbide (SiC SiC) composites are often used in oxidizing environments at high temperatures. Measurements of the thermal conductance of the oxide layer provide a way to better understand the oxidation process with high spatial resolution. We use time domain thermoreflectance (TDTR) to map the thermal conductance of the oxide layer and the thermal conductivity of the SiC SiC composite with a spatial resolution of 3 μm. Heterodyne detection using a 50 kHz modulated probe beam and a 10 MHz modulated pump suppresses the coherent pick-up and enables faster data acquisition th…
Using positive muons as local probes implanted at low energy enables gathering information about the material of interest with nanometer depth resolution (low energy muon spin rotation spectroscopy (LE-$μ$SR). In this work, we leverage the capabilities of LE-$μ$SR to perform an investigation of the SiO$_\text{2}$-SiC interface. Thermally oxidized samples are investigated before and after annealing in nitric oxide (NO) and argon (Ar) ambience. Thermal oxidation is found to result in structural changes both in the SiC crystal close to the interface and at the interface itself. Annealing in NO en…
Ion implantation is widely used as a surrogate for neutron irradiation in the investigation of radiation damage on the properties of materials. Due to the small depth of damage, micromechanical methods must be used to extract material properties. In this work, nanoindentation has been applied to ion irradiated silicon carbide to extract radiation-induced hardening. Residual stress is evaluated using HR-EBSD, AFM swelling measurements, and a novel microcantilever relaxation technique coupled with finite element modelling. Large compressive residual stresses of several GPa are found in the irrad…
The recombination of Frenkel pairs resulting from low energy recoils in 3C-SiC has been investigated using first principles and Nudged Elastic Band calculations. Several recombination mechanisms have been obtained, involving direct interstitial migration, atoms exchange, or concerted displacements, with activation energies ranging from 0.65 eV to 1.84 eV. These results are in agreement with experimental activation energies. We have determined the lifetime of the $V_{Si}+Si_{TC}$ Frenkel pair, by computing phonons and the Arrhenius prefactor. The vibrational contributions to the free energy bar…
We report on the low-energy electron and fast neutron irradiated 4H-SiC studied by deep-level transient spectroscopy (DLTS) and Laplace DLTS. Irradiations introduced two defects, Ec-0.4eV and Ec-0.7eV. They were previously assigned to carbon interstitial (Ci) labeled as EH1/3 and silicon-vacancy (VSi) labeled as S1/2, for the low-energy electron and fast neutron irradiation, respectively. This work demonstrates how Laplace DLTS can be used as a useful tool for distinguishing the EH1 and S1 defects. We show that EH1 consists of a single emission line arising from the Ci(h), while S1 has two emi…
Chemical oxidation of multilayer graphene grown on silicon carbide yields films exhibiting reproducible characteristics, lateral uniformity, smoothness over large areas, and manageable chemical complexity, thereby opening opportunities to accelerate both fundamental understanding and technological applications of this form of graphene oxide films. Here, we investigate the vertical inter-layer structure of these ultra-thin oxide films. X-ray diffraction, atomic force microscopy, and IR experiments show that the multilayer films exhibit excellent inter-layer registry, little amount (<10%) of int…
The detection of the 11.3-micron emission feature characteristic of the Si--C stretch in carbon-rich evolved stars reveals that silicon carbide (SiC) dust grains are condensed in the outflows of carbon stars. SiC dust could be a significant constituent of interstellar dust since it is generally believed that carbon stars inject a considerable amount of dust into the interstellar medium (ISM). The presence of SiC dust in the ISM is also supported by the identification of presolar SiC grains of stellar origin in primitive meteorites. However, the 11.3-micron absorption feature of SiC has never b…
The annealing kinetics of mobile intrinsic defects is investigated by an ab initio method based on density functional theory. The interstitial-vacancy recombination, the diffusion of vacancies and interstitials to defect sinks (e.g. surfaces or dislocations) as well as the formation of interstitial-clusters are considered. The calculated migration and reaction barriers suggest a hierarchical ordering of competing annealing mechanisms. The higher mobility of carbon and silicon interstitials as compared to the vacancies drives the annealing mechanims at lower temperatures including the vacancy-i…
Superconductor based quantum computing has the major drawback of working temperatures which require liquid helium for cooling. A promising approach to overcome this obstacle for quantum technologies is based on deep level defects in semiconductors, with the nitrogen vacancy (NV) center in diamond being the most prominent example. Unfortunately, diamond in sufficient quality is scarce, which motivated efforts to find similar defects in silicon carbide (SiC). So far, many reports focus on investigations of point defects in irradiated 3C-SiC and as grown material. However, the investigated defect…
We propose a table-top method to obtain bilayer quasi-free-standing epitaxial-graphene (QFSEG) on SiC(0001). By applying a microwave annealing in air to a monolayer epitaxial graphene (EG) grown on SiC(0001), the buffer layer is decoupled from the SiC substrate and becomes the second EG layer as confirmed by the low energy electron diffraction, high-resolution transmission electron microscopy, Raman scattering spectroscopy, X-ray photoelectron spectroscopy, and angle-resolved photoelectron spectroscopy. The most likely mechanism of the decoupling is given by the oxidation of the SiC surface, w…
Muon spin rotation with low-energy muons (LEμSR) is a powerful nuclear method where electrical and magnetic properties of surface-near regions and thin films can be studied on a length scale of $\approx$\SI{200}{\nano\meter}. In this work, we show the potential of utilizing low-energy muons for a depth-resolved characterization of oxide-semiconductor interfaces, i.e. for silicon (Si) and silicon carbide (4H-SiC). Silicon dioxide (SiO$_2$) grown by plasma-enhanced chemical vapor deposition (PECVD) and by thermal oxidation of the SiO$_2$-semiconductor interface are compared with respect to inter…
The formation of graphene on the SiC(000-1) surface (the C-face of the {0001} surfaces) has been studied, utilizing both disilane and neon environments. In both cases, the interface between the graphene and the SiC is found to be different than for graphene formation in vacuum. A complex low-energy electron diffraction pattern with rt(43) x rt(43)-R\pm7.6° symmetry is found to form at the interface. An interface layer consisting essentially of graphene is observed, and it is argued that the manner in which this layer covalently bonds to the underlying SiC produces the rt(43) x rt(43)-R\pm7.6° …
This work reports on the morphological and electrical properties of Ni-based back-side Ohmic contacts formed by laser annealing process for SiC power diodes. Nickel films, 100 nm thick, have been sputtered on the back-side of heavily doped 110 um 4H-SiC thinned substrates after mechanical grinding. Then, to achieve Ohmic behavior, the metal films have been irradiated with an UV excimer laser with a wavelength of 310 nm, an energy density of 4.7 J/cm2 and pulse duration of 160 ns. The morphological and structural properties of the samples were analyzed by means of different techniques. Nanoscal…
The effects of ion beam induced atomic mixing and subsequent thermal treatment in Si/C multilayer structures are investigated by use of the technique of grazing incidence X-ray diffraction (GIXRD) and Raman spectroscopy. The [Si (3.0 nm) / C (2.5 nm)]x10 /Si multilayer films were prepared by electron beam evaporation under ultra high vacuum (UHV) environment. The layer thicknesses were measured using in-situ quartz crystal oscillator. These multilayer films were subjected to 40 keV Ar+ ion irradiation with fluences 5E-16 (low fluence) and 1E-17 ions / cm2 (high fluence).The as-prepared and irr…
Electrical properties of point defects in 4$H$-SiC have been studied extensively, but those related to carbon interstitials (C$_{i}$) have remained surprisingly elusive until now. Indeed, when introduced via ion irradiation or implantation, signatures related to C$_{i}$ observed by deep level transient spectroscopy (DLTS) tend to overlap with those of other primary defects, making the direct identification of C$_{i}$-related levels difficult. Recent literature has suggested to assign the so-called M center, often found in as-irradiated 4H-SiC, to charge state transitions of the C$_{i}$ defect …
Resonantly-enhanced Raman response in graphene-capped bismuthene on SiC
Universität Würzburg（维尔茨堡大学）；Würzburg-Dresden Cluster of Excellence ctd.qmat, Universität Würzburg（维尔茨堡大学 würtzburg-dresden 卓越集群 ctd.qmat）；Sapienza University of Rome（罗马第一大学）；RWTH Aachen University（亚琛工业大学）；Forschungszentrum Jülich（于利希研究中心）；Institut für Theoretische Physik und Astrophysik, Universität Würzburg（维尔茨堡大学理论与天体物理研究所）；TUD Dresden University of Technology（德累斯顿工业大学）
We determined the interface state density ($D_{\rm it}$) distributions in the vicinity of the conduction band edge in silicon carbide (SiC) metal-oxide-semiconductor (MOS) structures by reproducing the experimental current-voltage characteristics of MOS field effect transistors (MOSFETs) with numerical calculations. In the calculation, potential distributions and energy sub-bands in the inversion layer were calculated by solving Poisson and Schrödinger equations, respectively. We demonstrate that gate characteristics of the MOSFETs are well described by considering that the interface states ar…
The effect of argon ion bombardment on the chemical properties of crystalline Ta$_2$O$_5$ films grown on Si(100) substrates by radio frequency magnetron sputtering was investigated by X-ray photoelectron spectroscopy. All samples were irradiated for several time intervals [(0.5, 3, 6, 9) min] and the Ta $4f$ and O $1s$ core levels were measured each time. Upon analysis at the surface of the films, we observe the Ta $4f$ spectrum characteristic of Ta$_2$O$_5$. Irradiated samples exhibit the formation of Ta suboxides with oxidation states Ta$^{1+}$, Ta$^{2+}$, Ta$^{3+}$, Ta$^{4+}$, and Ta$^{5+}$…
