SiC Interface Research Articles

Theoretical investigation on SiC(1 1 1)/Al4C3(0 0 0 1) interface using density functional theory calculations

Al4C3 is likely to generate on the interface of SiC reinforced aluminum- and magnesium-matrix composites. In order to clarify the interfacial properties and heterogeneous nucleation of Al4C3 on SiC(1 1 1) substrate, the interfacial adhesion, electronic structure, and bonding nature of SiC(1 1 1)/Al4C3(0 0 0 1) are investigated by using density functional theory (DFT) calculations. Considering different stacking sites (top, center, and hollow sites) and different surface terminations (Si- and C-terminated SiC(1 1 1); Al- and C-terminated Al4C3(0 0 0 1)), totally twelve different interface models are examined in the present work. The calculated interfacial adhesion work suggests top-site stacked Si/C- and C/Al-terminated models are more stable. By comparing different cleaving planes, the interfacial weak point is confirmed on the Al4C3(0 0 0 1) side for the both cases. The valence electron density and partial density of states (PDOS) are also analyzed. The interfacial bonding mainly comes from p orbital hybridizations of C-Si and Al-C atom pairs on shallow energy level (−6 eV to Fermi level), and on the deeper energy level (less than −6 eV), the s electrons will partly contribute to the bonding. The epitaxial stacking style is confirmed on the both interfaces, and epitaxial nucleation and growth of Al4C3 on SiC(1 1 1) substrate could also be identified.

NH3and NO + NH3Annealing of 4H-SiC Trench MOSFETs: Device Performance and Reliability

As the gate oxide (GOX)-SiC interface of SiC MOSFETs is crucial for device performance, it requires special attention. To improve interface quality, device performance, and reliability, commonly, a postoxidation anneal (POA) is applied. Different gas compositions can be used to passivate the SiC interface and bulk oxide. We study the effects of ammonia (NH3) and a new combined NO + NH3 POA on 4H-SiC trench MOSFET test structures and compare them to the common nitric oxide (NO) POA. Our studies are not only limited to device performance represented by channel mobility but also contain reliability studies in the form of bias temperature instability (BTI), sweep hysteresis, Fowler–Nordheim (FN) tunneling and GOX breakdown. We find that both, NH3 and NO + NH3 result in higher mobility than NO annealed test structures. On the other hand, ammonia containing POAs deteriorate device reliability, e.g., leading to larger BTI and higher tunneling currents. As both can be detrimental in certain applications, they must be optimized. We show that NO + NH3 anneals may offer an acceptable compromise between the characteristics of NO and NH3 with regard to tunneling and device performance. However, it shows more positive BTI (PBTI) drift than NH3 and NO. We will show that a full characterization of the GOX is essential to evaluate, optimize, and benchmark new annealing strategies.

Highly electrically and thermally conductive silicon carbide-graphene composites with yttria and scandia additives

Dense silicon carbide/graphene nanoplatelets (GNPs) and silicon carbide/graphene oxide (GO) composites with 1 vol.% equimolar Y2O3–Sc2O3 sintering additives were sintered at 2000 °C in nitrogen atmosphere by rapid hot-pressing technique. The sintered composites were further annealed in gas pressure sintering (GPS) furnace at 1800 °C for 6 h in overpressure of nitrogen (3 MPa). The effects of types and amount of graphene, orientation of graphene sheets, as well as the influence of annealing on microstructure and functional properties of prepared composites were investigated. SiC-graphene composite materials exhibit anisotropic electrical as well as thermal conductivity due to the alignment of graphene platelets as a consequence of applied high uniaxial pressure (50 MPa) during sintering. The electrical conductivity of annealed sample with 10 wt.% of GNPs oriented parallel to the measuring direction increased significantly up to 118 S·cm−1. Similarly, the thermal conductivity of composites was very sensitive to the orientation of GNPs. In direction perpendicular to the GNPs the thermal conductivity decreased with increasing amount of graphene from 180 W·m−1 K−1 to 70 W·m−1 K−1, mainly due to the scattering of phonons on the graphene – SiC interface. In parallel direction to GNPs the thermal conductivity varied from 130 W·m−1 K−1 up to 238 W·m−1 K−1 for composites with 1 wt.% of GO and 5 wt.% of GNPs after annealing. In this case both the microstructure and composition of SiC matrix and the good thermal conductivity of GNPs improved the thermal conductivity of composites.

First Principles Study of the Stability and Diffusion Mechanism of a Carbon Vacancy in the Vicinity of a SiO2/4H–SiC Interface

The carbon vacancy in bulk 4H‐SiC and in the vicinity of an SiO2/(0001)‐4H‐SiC interface using density‐functional theory is studied. It is found that the migration is hindered in the immediate vicinity of the interface, with the energy barrier for diffusion being ≈15% greater than the same defect in bulk 4H‐SiC. Herein, it is shown that the increased barrier is a consequence of the stabilization of the vacancy in the immediate interface due to a combination of strengthened reconstructions and interfacial relaxation, coupled with the destabilization of the transition‐state structure.

Impact of interface traps/defects and self‐heating on the degradation of performance of a 4H‐SiC VDMOSFET

The reliability of silicon carbide metal oxide semiconductor field-effect transistors remains a challenge in power applications and relates to the SiO2–SiC interface. The presence of unwanted interface traps/defects degrades the device performance. The impact of acceptor traps/defects on the performance of a 4H-SiC vertical Diffused Metal Oxide Semiconductor Field Effect Transistor (DMOSFET) with a breakdown voltage of 1700 V is investigated. - and - characteristics were simulated, using a drift-diffusion model coupled to Fourier heat equations, and are in a good agreement with experimental results. The presence of interface traps/defects were shown to produce degradation of threshold voltage, but the impact diminishes as temperature increases. A threshold voltage shift of 3.5 V occurs for a trap concentration of 2 × 1013 cm–2/eV at room temperature. The transfer characteristics obtained from electro-thermal modelling show a larger degradation than those at a constant temperature. This degradation increases with the drain bias increase. The threshold voltage from the electro-thermal simulations is 5 V compared to 4 V observed in constant 423 K temperature simulations at. Finally, the interface traps/defects increases breakdown voltage exhibiting a strong dependency on the trap density and their energy decay characteristics.

Atomic Coordination Analysis of Nitrogen Introduced in SiO<sub>2</sub>/ SiC Interface and SiO<sub>2</sub> Layer by XAFS Measurement

Although the nitridation by N2O or NO oxidation with annealing has been used in order to reduce interface trap density, the atomic location or coordination environment of nitrogen atom have been still unclear. In this study, we have investigated atomic coordination environment of nitrogen atom in both SiO2/SiC interface and SiO2 layer after nitridation by N2O annealing using XAFS measurements and ab initio multiple scattering calculations. Nitrogen in SiO2 layer was suggested to have the same atomic coordination environments as general SiON thin film. On the other hand, in SiO2/SiC region, nitrogen atoms were isolated and occupied the C-site of SiC structure, and it was confirmed that nitrogen atoms were surrounded by silicon atoms and strong Si-N bonds were formed in the interface region that was not eliminated by HF etching.

G‐C3N4‐SiC‐Pt for Enhanced Photocatalytic H2 Production from Water under Visible Light Irradiation

Graphite carbon nitride (g‐C3N4) and SiC have drawn increasing attention for application in visible light photocatalytic hydrogen evolution by water splitting due to their unique band structure and high physicochemical stability. Herein, a g‐C3N4‐SiC heterojunction with loaded noble metal is constructed. The g‐C3N4‐SiC‐Pt composite photocatalysts are successfully prepared by the combination method of bio‐reduction, sol deposition, and calcination. The layers of g‐C3N4 are thinned, and both SiC and Pt nanoparticles are simultaneously tightly bound to g‐C3N4 by calcination during the preparation of g‐C3N4‐SiC‐Pt. The heterojunction formed at the interface of SiC and g‐C3N4 enhances the separation efficiency of the photogenerated electron–hole pairs. These composite photocatalysts achieve a high hydrogen evolution rate of 595.3 μmol h−1 g−1 with 1 wt% of deposited Pt, which is 3.7‐ and 2.07‐fold higher than those of g‐C3N4‐bulk and g‐C3N4‐SiC under visible light irradiation with a quantum efficiency of 2.76% at 420 nm, respectively.

The Correlation between the Reduction of Interface State Density at the SiO<sub>2</sub>/SiC Interface and the NO Post-Oxide-Annealing Conditions

We fabricated SiO2/4H-SiC (0001) MOS capacitors with oxidation temperature at 1350°C, followed by post-oxide-annealing (POA) in NO simply by the control of POA temperatures and times. A correlation between the reduction of interface state density and the increasing of N concentration at the interface has been indicated by C-ψs measurement and secondary ion mass spectrometry (SIMS). The SiO2/4H-SiC interface density decreased when POA temperature was elevated, and the sample annealed at 1300°C for 30min showed the lowest interface state density about 1.5×1012 cm-2eV-1 at Ec-E=0.3 eV when the N concentration is 11.5×1020 cm-3. Meanwhile, the SiO2 /4H-SiC interface annealed at 1200°C for 120min showed the highest N concentration at the 4H-SiC/SiO2 interface is 12.5×1020 cm-3, whereas the interface state density is 2.5×1012 cm-2eV-1 at Ec-E=0.3 eV higher than 1300°C for 30min. The results suggested that higher temperature POA might be much more efficiency in decreased the 4H-SiC MOS interface density with increasing the N area concentration.

First-principle calculations on the structure of 6H-SiC/Al interface

In this study, two most common 6H-SiC/Al interfaces, Al(100)/SiC(0001) and Al(111)/SiC(0001), are studied by first principles. The results show that for both interfaces, C-terminated interfaces show stronger interfacial binding ability than Si-terminated. In comparison, Al(111)/SiC(0001) interface has higher bonding strength. However, the tensile strength of the composites can not be completely determined by the initial interfacial separation work. During the stretching deformation, the atoms of Al are reconstructed and the distribution of metal electrons of Al can also change, which makes the fracture positions of the composites change regularly. The results show that Al(111)/SiC(0001) has better tensile strength. But for Al(100)/SiC(0001) interfacae, the brittle fracture is less prone to occur.

Effects of proton radiation on field limiting ring edge terminations in 4H–SiC junction barrier Schottky diodes

In this study, the effects of high-energy proton radiation on the effectiveness of edge terminations using field limiting rings (FLRs) in 4H–SiC junction barrier Schottky (JBS) diodes were examined in detail. The devices were irradiated using 5-MeV protons at fluences ranging from 5× 1012 cm−2 to 5× 1014 cm−2. Further, the reverse breakdown performances of the investigated devices were measured both before and after irradiation. Proton irradiation initially decreased the breakdown voltage (BV); subsequently, the BV was increased as the proton fluence increased. At a fluence of 5× 1013 cm−2, the BV was reduced by approximately 18%, whereas it was reduced by approximately 5% at a higher proton fluence of 5× 1014 cm−2. The related degradation mechanism that was associated with this phenomenon was also investigated using the numerical simulations of the current-voltage ( I - V ) and capacitance-voltage ( C - V ) characteristics of the device. The main contribution to the radiation-induced changes in BV originates from the variations of charge distribution at the SiO2/4H–SiC interface and the reduction of the net carrier density in the drift region. Both the aforementioned variations affect the spread of the electric field in the FLR edge termination regions.

Macroscopic simulations of the SiC thermal oxidation process based on the Si and C emission model

The interface defects formed at the oxide–SiC interface impact not only on channel mobility of MOSFET but also on many scenes such as generation or extinction of Si/C atom vacancies and formation of single-photon sources. Therefore, it is very important to know the oxidation mechanism of SiC more in detail. We have previously proposed a unified SiC oxidation model based on the Si and C emission phenomenon. In addition, it has been revealed that this model accurately reproduces the oxide growth rate in the entire oxide thickness range for various oxidation conditions and substrate surface orientations. In this report, based on the experimental verifications for the four oxidation stages deduced from this unified oxidation model, the validity of this model was reviewed. Furthermore, the interface-emitted Si and C concentrations were simulated for various oxidation temperatures and substrate surface orientations. Based on the simulation results together with experimental results of interface state density, the optimum oxide growth condition has been discussed.

Pre-infiltration and brazing behaviors of Cf/C composites with high temperature Ti[sbnd]Si eutectic alloy

A novel method is developed to join carbon fiber reinforced carbon composites (Cf/C composites) via a two-step approach using Ti14Si86 metal foil as the joining material. The infiltration behavior, joint microstructure, interface reaction, fracture characteristics and joining mechanism are investigated. The preparation of the almost compact deposit layer and the infiltration carbide can immediately heal the pores of Cf/C composites and form the gradient composite structure, which can effectively minimize the impact of the coefficient of thermal expansion (CTE) mismatch between the outer reaction layer and the carbon matrix and avoid the excessive filler infiltration in the brazing step. The brazing process reveals that the growing SiC interface and TiSi eutectic structure in the seam also behave as a graded CTE composite structure, which is beneficial for strengthening the joint. Compared with direct brazing, this method can obtain more reliable Cf/C composites joints due to strong interfacial bonding and the gradient structure. As a result, a favorable shear strength of 26 MPa is achieved within 10 min at a brazing temperature of 1400 °C.

First-principles calculations on wetting interface between Ag-Cu-Ti filler metal and SiC ceramic: Ag (1 1 1)/SiC (1 1 1) interface and Ag (1 1 1)/TiC (1 1 1) interface

Interfacial properties of Ag (1 1 1)/SiC (1 1 1) interface and Ag (1 1 1)/TiC (1 1 1) interface were researched by first-principles calculations to investigate the reason that Ag-Cu-Ti filler metal shows the superior reactive wettability on SiC ceramic. The calculated results show that, 7 atom-layers Ag (1 1 1) surface, 11 atom-layer SiC (1 1 1) surface and 9 atom-layer TiC (1 1 1) surface can represent the Ag bulk, SiC bulk and TiC bulk, effectively. Ag (1 1 1)/SiC (1 1 1) interface and Ag (1 1 1)/TiC (1 1 1) interface with C-terminated structure and TL stacking sequence show the highest interfacial stability. Chemical bonds at Ag (1 1 1)/SiC (1 1 1) interface and Ag (1 1 1)/TiC (1 1 1) interface are mainly formed by the interaction between 1st layer C atoms in SiC (or TiC) slab and 1st layer Ag atoms in Ag slab. Interfacial energy of Ag (1 1 1)/SiC (1 1 1) interface is much larger than that of Ag (1 1 1)/TiC (1 1 1) interface, which indicates that the formed TiC reaction layer between Ag-Cu-Ti filler metal and SiC ceramic indeed plays the positive role on improving the wettability of Ag-Cu-Ti filler metal on SiC ceramic.

(Invited) Characterization of UV Excitation Accelerated Material Changes on As-Grown SiC Epitaxial Layers and Their Impact on Defect Detection

Silicon Carbide continues to gain acceptance and enjoy widespread adoption in mainstream high voltage applications. Even as both substrate and epitaxial material quality keeps improving, new failure mechanisms can get introduced due to various factors like diameter expansion, process and volume scale up. This forces the industry to evaluate and implement a variety of characterization methods to identify and eliminate any potential root causes for failure modes. Currently every substrate and epitaxial wafer is scanned for defects even in mass production due to the variable presence of various types of killer visual defects and pure crystal defects. Due to the bandgap of SiC, a convenient technique to non-destructively reveal crystal defects in the material is to use excitation light in the Ultraviolet (UV) range. Recently all volume mass production characterization systems have adopted this UV excitation along with detecting crystal defects using either near-ultraviolet (NUV) or infra-red (IR) PL bands [1,2]. Both the widely used KLA CS920 and the Lasertec SICA 88 platforms use UV light excitation using either a laser or a lamp. In our previous reports [3-6] while detecting faint PL signals, we have observed that exposing the wafers to this UV excitation results in material changes and increase in background PL that can be detected via the various PL acquisition channels. This raises concerns on the exact mechanism of the SiC material change. This constantly changing background also causes issues with measurement repeatability and calibration of the tools. In this work, we offer a detailed study of the effects of both UV laser and UV lamp exposure on as-grown SiC epitaxial wafers and explain the changes in the material. Additionally we replicate exactly the changes using various independent processing steps, while also offering various ways to both prevent and remove this effect. Performing repetitive scans on as-grown epitaxial layers by using either an UV laser or UV lamp results in an observable increase in the background PL. This effect is highly repeatable in samples and results in a 25%-35% change in the signal to noise ratio both in the NUV and IR wavelength ranges. This can either adversely affect or enhance the defect signatures depending on which wavelength range is being captured. This causes repeatability issues for the detecting recipes and algorithms due to the signal variation. A few surface treatments done to the affected surfaces have been successful in restoring the surface PL signals to their original values. From this and from performing EDX on pre- and post HF etch samples the formation of a very thin layer of UV-exposure accelerated oxide was observed. This accelerated oxide occurred only in the present of the UV excitation. The cause for the formation of this oxide is proposed to be the highly reactive surface state of the as-grown epitaxial wafers. Passivating the as-grown surface before repetitive scanning completely eliminates the change in background PL. This effect is further replicated on epitaxial wafers by growing thermal oxides of different thicknesses and also depositing PECVD oxide. The effect of the thermal oxides very closely mimics the observed effect, while the deposited oxide does not cause a big change in the PL signals. This shows the oxide – SiC interface is an important factor for this effect, and not just the presence of the oxide. The other effect of UV excitation on the expansion of basal plane dislocations (BPD) into stacking faults (SF) [2] was also studied. SF nucleation was only observed after about 150 scans and even then only in a small fraction of the BPDs. Characterization of the affected surface after repetitive exposures will be presented. High intensity constant exposure low temperature PL spectrum results which are very different in nature to the relative low intensity exposures will also be presented. Both surface treatments to completely recover from the effect and passivation schemes to prevent this effect from UV excitation exposures will be shown. Further characterization of the effect of both thermal and deposited oxides on PL of epitaxial layers will be shown.

A GaN–SiC hybrid material for high-frequency and power electronics

We demonstrate that 3.5% in-plane lattice mismatch between GaN (0001) epitaxial layers and SiC (0001) substrates can be accommodated without triggering extended defects over large areas using a grain-boundary-free AlN nucleation layer (NL). Defect formation in the initial epitaxial growth phase is thus significantly alleviated, confirmed by various characterization techniques. As a result, a high-quality 0.2-μm thin GaN layer can be grown on the AlN NL and directly serve as a channel layer in power devices, like high electron mobility transistors (HEMTs). The channel electrons exhibit a state-of-the-art mobility of >2000 cm2/V-s, in the AlGaN/GaN heterostructures without a conventional thick C- or Fe-doped buffer layer. The highly scaled transistor processed on the heterostructure with a nearly perfect GaN–SiC interface shows excellent DC and microwave performances. A peak RF power density of 5.8 W/mm was obtained at VDSQ = 40 V and a fundamental frequency of 30 GHz. Moreover, an unpassivated 0.2-μm GaN/AlN/SiC stack shows lateral and vertical breakdowns at 1.5 kV. Perfecting the GaN–SiC interface enables a GaN–SiC hybrid material that combines the high-electron-velocity thin GaN with the high-breakdown bulk SiC, which promises further advances in a wide spectrum of high-frequency and power electronics.

Investigation of the structural and electrical properties of air-annealed ruthenium thin films on 6H–SiC at different temperatures

Ruthenium (Ru) Schottky contacts and thin films on n-type 6H–SiC were fabricated and characterised by physical and electrical methods. The characterisation was done after annealing the samples in air at various temperatures. Rutherford backscattering spectroscopy (RBS) analysis of the thin films indicated the oxidation of Ru after annealing at a temperature of 400 °C, and interdiffusion of Ru and Si at the Ru–6H–SiC interface at 500 °C. XRD analysis of the thin films indicated the formation of RuO2 and RuSi in Ru–6H–SiC after annealing at a temperature of 600 °C. The formation of the oxide was also corroborated by Raman spectroscopy. The ideality factor of the Schottky barrier diodes (SBD) was seen to generally decrease with annealing temperature. The series resistance increased astronomically after annealing at 700 °C, which was an indication that the SBD had broken down. The failure mechanism of the SBD is attributed to deep inter-diffusions of Ru and Si at the Ru–6H–SiC interface as evidenced by the RBS of the thin films.

Extremely Thermal Stable Ni/W/TaSi<sub>2</sub>/Pt Simultaneous Ohmic Contacts to n-Type and p-Type 4H-SiC

Studying the ohmic contacts formed on ion-implanted SiC layers is fundamental to understand and to predict the behavior of practical devices. Ohmic contacts to n- (1×1019 cm−3) and p-type (1×1020 cm−3) ion-implanted 4H-SiC using Ni/W/TaSi2/Pt were investigated. No degradation of the specific contact resistance nor a minute change of the surface morphologies was observed after 300 h of 500 oC thermal treatment in air. From auger electron spectroscopy (AES) depth profiles, it was found that the oxidation of the protective platinum silicide overlayer significantly slowed down further migration of oxygen to the SiC interface. In addition, Pt and W played the role of mutual blocking, which guarantees the stability of the contact. This research suggests that the contacts are very promising for applications in harsh environments, where the simultaneously completed both on n-and p-type stability ohmic contacts is crucial.

Electrical leakage phenomenon in heteroepitaxial cubic silicon carbide on silicon

Heteroepitaxial 3C-SiC films on silicon substrates are of technological interest as enablers to integrate the excellent electrical, electronic, mechanical, thermal, and epitaxial properties of bulk silicon carbide into well-established silicon technologies. One critical bottleneck of this integration is the establishment of a stable and reliable electronic junction at the heteroepitaxial interface of the n-type SiC with the silicon substrate. We have thus investigated in detail the electrical and transport properties of heteroepitaxial cubic silicon carbide films grown via different methods on low-doped and high-resistivity silicon substrates by using van der Pauw Hall and transfer length measurements as test vehicles. We have found that Si and C intermixing upon or after growth, particularly by the diffusion of carbon into the silicon matrix, creates extensive interstitial carbon traps and hampers the formation of a stable rectifying or insulating junction at the SiC/Si interface. Although a reliable p-n junction may not be realistic in the SiC/Si system, we can achieve, from a point of view of the electrical isolation of in-plane SiC structures, leakage suppression through the substrate by using a high-resistivity silicon substrate coupled with deep recess etching in between the SiC structures.

Platinum-catalyzed hydrolysis etching of SiC in water: A density functional theory study

A comprehensive study of the physicochemical interactions and the reaction mechanism of SiC etching with water by Pt catalysts can reveal key details about the surface treatment and catalytic phenomena at interfaces. Therefore, density functional theory simulations were performed to study the kinetics of Pt-assisted water dissociation and breaking of a Si–C bond compared to the HF-assisted mechanism. These calculations carefully considered the elastic and chemical interaction energies at the Pt–SiC interface, activation barriers of Si–C bond dissociation, and the catalytic role of Pt. It was found that the Pt-catalyzed etching of SiC in water is initiated via hydrolysis reactions that break the topmost Si–C bonds. The activation barrier strongly depends on the elastic and chemical interactions. However, chemical interactions are a dominant factor and mainly contribute to the lowering of the activation barrier, resulting in an increased rate of reaction.

Free-Standing Binder-Free Multi-Wall Carbon Nanotube/Silicon Anodes for Lithium-Ion Batteries

Silicon is of interest owing to its high theoretical capacity (4200 mAh/g). However, its enormous volume expansion of silicon during cycling is one of the main reasons for the rapid capacity fading and pulverisation of the anode commonly observed. Here, freestanding multi-wall carbon nanotube (MWCNT)/silicon anodes were prepared using so-called MWCNT buckypapers (BPs) as current collectors instead of the traditional metallic foils. To prepare the anodes, silicon was deposited on these MWCNT BPs to form silicon nanoparticles (SiNPs) and subsequent annealing resulted in the formation of a connecting SiC interface preventing the SiNPs from detaching. An amorphous carbon coating was applied in order to enhance electrical conductivity and overall integrity of these hierarchical anode materials. The morphology and chemical composition of the Si-BP anodes were characterised by means of SEM, TEM, EDX, STEM, XRD, RAMAN, etc. Cycling tests of these binder and additive-free anodes revealed an areal capacity of 0.55 mAh/cm2 after 300 cycles with 99.7% of Columbic efficiency and showed that the presence of the SiC interface and a-C coating played a crucial role in improving their rate capability and stability.