4H-SiC Surface Research Articles

Structural study of single Shockley stacking faults terminated near substrate/epilayer interface in 4H-SiC

Structural analysis is carried out of a single Shockley stacking fault (1SSF) that terminates near the substrate/epilayer interface and originally expanded from a basal plane dislocation (BPD) segment near the epilayer surface of 4H-SiC. The characteristic zigzag structure is found for the partial dislocations (PDs), with microscopic connecting angles of almost 120°. It has been suggested that the microscopic construction of PDs might be limited by the Peierls valley. The termination line near the substrate/epilayer interface was found to have 30° Si-core and 90° Si-core PDs. This combination is the same as that found near the surface of the epilayer in commonly observed 1SSFs. Penetrating BPDs of this kind were also found experimentally for the first time. For the currently proposed charts for the 1SSF expansions, photoluminescence imaging during UV illumination is one of the nondestructive analysis methods that can provide the structural information and expected expansion shapes of the 1SSFs.

Simulation and experimental studies of the dissolution corrosion of 4H-SiC in liquid Pb/Bi

The dissolution corrosion of liquid Pb/Bi on 4H-SiC (0001) surfaces are studied based on first-principles calculations and experimental studies. The calculation models of 4H-SiC (0001) surfaces are proposed according to the experimental results, and the rationality of the models is verified by our repeated experiments. Clean surface stability is mainly determined by the strength of surface states due to the dangling bonds. Pb/Bi adatoms accelerate the dissolution of C atoms more severely than that of Si atoms and liquid Bi is more corrosive than liquid Pb, which are consistent with our experimental results. Energetics, microstructural and electronic properties are analyzed to reveal the corrosion mechanism of liquid Pb/Bi. Moreover, we provide a theoretical scheme to improve the corrosion resistance of 4H-SiC by adjusting the charges of vacancies and changing the relative position of Fermi level.

Femtosecond laser nanostructuring on a 4H-SiC surface by tailoring the induced self-assembled nanogratings.

Ultrafast laser micromachining of crystalline silicon carbide (SiC) has great perspectives in aerospace industry and integrated circuit technique. In this report, we present a study of femtosecond laser nanostructuring on the surface of an n-type 4H-SiC single crystal. Except for uniform nanogratings, new types of large-area periodic structures including nanoparticle array and nanoparticle-nanograting hybrid structures were induced on the surface of 4H-SiC by scanning irradiation. The effects of pulse energy, scan speed, and the polarization direction on the morphology and periodicity of nanogratings were systematically explored. The proper parameter window for nanograting formation in pulse energy-scan speed landscape is depicted. Both the uniformity and the periodicity of the induced nanogratings are polarization dependent. A planar light attenuator for linear polarized light was demonstrated by aligning the nanogratings. The transition between different large-area periodic structures is achieved by simultaneous control of pulse energy and scan interval using a cross scan strategy. These results are expected to open up an avenue to create and manipulate periodic nanostructures on SiC crystals for photonic applications.

The structural and electronic properties of Carbon-related point defects on 4H-SiC (0001) surface

Carbon-related point defects on the 4H-SiC surface are essential for understanding the origin of defects at the SiO2/SiC interface and improving the quality of epitaxial materials. In this work, a first principle calculation was carried out to study the structural and electronic properties of carbon antisite (CSi), vacancy (VC) and interstitial defects (Ci1, Ci2, Ci3 and Ci4) on the 4H-SiC (0001) surface. The optimized structures showed that interstitial defects except Ci2 caused the surface reconstruction of 4H-SiC. The variation in formation energies with chemical potentials of the carbon for all defects indicated that the C-rich condition was beneficial to the formation of CSi, Ci1, Ci3 and Ci4, whereas the Si-rich condition was more favorable to VC. We also observed that these defects except Ci4 generated the corresponding defect energy levels in the bandgap of 4H-SiC by calculating the density of states and local charge densities. Furthermore, the effects of defect coverage and lateral lattice strain on structural and electronic properties of these defects were provided.

The Initial Oxidation of the 4h-Sic (0001) Surface with C-Related Point Defects：Insight by First-Principles Calculations

The Initial Oxidation of the 4h-Sic (0001) Surface with C-Related Point Defects：Insight by First-Principles Calculations

A sessile drop approach for studying 4H-SiC/liquid silicon high-temperature interface reconstructions

Accurate description, understanding and control of the high temperature interface behavior between the SiC single crystal and liquid phase is critical for further development of SiC solution growth. The different parameters which create morphological instabilities, such as step bunching, micro-faceting and solvent trapping, remain unclear because they are very difficult to address in high temperature experiments. We combined experiments and numerical simulation to design a specific sessile drop approach where the liquid is removed before cooling down. The advantage of this method that it activates or suppresses the solute (carbon) transport by an AC magnetic field and thus separates the physical–chemical and hydrodynamic contributions. The method was demonstrated through observation of the morphological evolution of a 4°off 4H-SiC (0001) surface in contact with pure liquid silicon at 1600 °C according to the time factor. Several parasitic effects were also analyzed and suppressed in order to obtain a well-controlled uniform interface.

Ultraviolet antireflective porous nanoscale periodic hole array of 4H-SiC by Photon-Enhanced Metal-assisted chemical etching

Benefitting from the outstanding stability and suitable bandgap energy, silicon carbide (SiC) shows promising applications especially for ultraviolet light detection in harsh environments. Traditionally, 4H-SiC surface antireflection textures which boost light harvesting have been realized by plasma dry etching due to its chemical inertness, nevertheless causing surface damage which is detrimental to device performance. This paper presents 4H-SiC porous nanoscale periodic hole array with outstanding ultraviolet antireflection capability by highly efficient plasma-free photon-enhanced metal-assisted chemical etching. Its formation process is carefully monitored with etching mechanism explained by carrier generation and mass transport. Effect of pattern dimension on etching is also investigated, which is closely related with catalyst coverage. The 4H-SiC porous nanoscale periodic hole array by photon-enhanced metal-assisted chemical etching sheds light on novel applications in ultraviolet light harvesting and detection.

Thermodynamics and kinetics of H adsorption and intercalation for graphene on 6H-SiC(0001) from first-principles calculations

Previous experimental observations for H intercalation under graphene on SiC surfaces motivate the clarification of configuration stabilities and kinetic processes related to intercalation. From first-principles density-functional-theory calculations, we analyze H adsorption and intercalation for graphene on a 6H-SiC(0001) surface, where the system includes two single-atom-thick graphene layers: the top-layer graphene (TLG) and the underling buffer-layer graphene (BLG) above the terminal Si layer. Our chemical potential analysis shows that in the low-H coverage regime (described by a single H atom within a sufficiently large supercell), intercalation into the gallery between TLG and BLG or into the gallery underneath BLG is more favorable thermodynamically than adsorption on top of TLG. However, intercalation into the gallery between TLG and BLG is most favorable. We obtain energy barriers of about 1.3 and 2.3 eV for a H atom diffusing on and under TLG, respectively. From an additional analysis of the energy landscape in the vicinity of a step on the TLG, we assess how readily one guest H atom on the TLG terrace can directly penetrate the TLG into the gallery between TLG and BLG versus crossing a TLG step to access the gallery. We also perform density functional theory calculations for higher H coverages revealing a shift in favorability to intercalation of H underneath BLG and characterizing the variation with H coverage in interlayer spacings.

Silicon carbide of 4H-SiC type Schottky diode current-voltage characteristics in small-sized type metal-polymeric package SOT-89

The forward and reverse current–voltage characteristics of Ti/Al/4H-SiC Schottky diode type DDSH411A91 in modern small-sized (SOT-89) type metal-polymeric package have been obtained. In forward direction (current up to 2 A) on the basis of analysis it is shown that Schottky diode corresponds to the "ideal" diode with ideality factor n=1.12 and effective Schottky barrier height φB =1.2 eV. It is shown that reverse current-voltage characteristics (breakdown voltage 1200 V) can be well approximated by mechanism of field dependence of barrier height lowering by the presence of the intermediate layer in the form of oxide on the 4H-SiC surface.

High-efficiency wafer-scale finishing of 4H-SiC (0001) surface using chemical-free electrochemical mechanical method with a solid polymer electrolyte

Silicon carbide (SiC) has been extensively studied for applications in next-generation high-power electronic devices. High-quality and low-cost electronic devices require a surface finishing process that can produce smooth and defect-free 4H-SiC (0001) surfaces. However, SiC surfaces are difficult to remove because of their extremely high mechanical strength and chemical stability. Herein, we present a high-efficiency, wafer-scale, chemical-free finishing process for a 4H-SiC (0001) surface using electrochemical mechanical polishing (ECMP) with a solid polymer electrolyte (SPE). The ECMP method comprises electrochemical oxidation at the SiC/SPE interface and subsequent oxide removal by CeO2 particles. Electrolysis with a SiC (anode)/SPE/cathode electrochemical system demonstrated that the use of SPE as an alternative to liquid electrolytes allows the efficient electrochemical oxidation of highly inert SiC (0001) surfaces without requiring any chemicals. The ECMP process achieved a high material removal rate for SiC (0001) in the range of 1.8–9.2 μm/h, although strongly dependent on a 50 to 450 mA electrolytic current. Moreover, an electrolytic current of 250 mA produced a smooth and defect-free surface with a sub-nanometer-scale roughness (0.68 nm Sa) and excellent uniformity over an entire 2-inch SiC (0001) surface. Atomic force microscopy observations showed that ECMP under a low electrolytic current (30 mA) combined with subsequent CeO2 polishing without electrolysis can help improve microscale surface morphologies. The proposed ECMP, which produces a smooth and uniform surface without requiring chemicals, is a highly efficient and environmentally friendly finishing process for the fabrication of SiC wafers.

Positronium emission from GaN(0001) and AlN(0001) surfaces

Positronium emission from wurtzite GaN(0001) and AlN(0001) surfaces was observed by positronium time-of-flight spectroscopy. The positronium energy spectra contained two positronium components distinguished by their energies. Through detailed analyses based on Monte Carlo simulations, these two components were attributed to positronium formed from valence and conduction electrons. The obtained results augment the previous arguments regarding the contribution of conduction electrons to positronium emission from 4H SiC(0001) and Si(111) surfaces.

Infrared erbium photoluminescence enhancement in silicon carbide nano-pillars

Color centers that emit light at telecommunication wavelengths are promising candidates for future quantum technologies. A pressing challenge for the broad use of these color centers is the typically low collection efficiency from bulk samples. Here, we demonstrate enhancements of the emission collection efficiency for Er3+ incorporated into 4H-SiC surface nano-pillars fabricated using a scalable top-down approach. Optimal Er ion implantation and annealing strategies are investigated in detail. The substitutional fraction of Er atoms in the SiC lattice is closely correlated with the peak photoluminescence intensity. This intensity is further enhanced via spatial wave-guiding once the surface is patterned with nano-pillars. These results have broad applicability for use with other color centers in SiC and also demonstrate a step toward a scalable protocol for fabricating photonic quantum devices with enhanced emission characteristics.

Platinum silicide formation on selected semiconductors surfaces via thermal annealing and intercalation

In this work, platinum silicide formation on selected semiconductors surfaces: Si(100), Si(111), 4H-SiC(0001) and graphene–4H-SiC(0001), via thermal annealing and intercalation process are investigated. Growth morphology and thermal stability of formed silicides are studied using X-ray Photoelectron Spectroscopy (XPS) and Low Energy Electron Diffraction (LEED). Results of chemical composition and observed reconstructions corresponding to each substrate are discussed and compared. Despite graphene surface, formation of platinum silicide takes place already at room temperature. However to obtain characteristic Pt-Si structures additional processes are required. System annealing at various temperatures allows to observe the effects of platinum atoms diffusion and intercalation into the deeper substrate regions, which causes formation of different silicide structures and chemical composition. Presented results allow to determine the Si(100) and graphene–4H-SiC(0001) surfaces as the most proper for formation of stabile and well-ordered platinum silicide.

High-temperature CO2 treatment for improving electrical characteristics of 4H-SiC(0001) metal-oxide-semiconductor devices

High-temperature CO2 treatments for 4H-SiC(0001) surfaces and SiO2/SiC structures were investigated. A stoichiometric SiO2 insulating layer was found to be grown on SiC by thermal oxidation in atmospheric CO2 ambient with an activation energy of 7.5 eV. Post-oxidation annealing (POA) in CO2 at a temperature of 1200 °C or more was effective in reducing interface fixed charges, oxide traps, and defect precursors, which are intrinsically involved in SiO2/SiC system grown with O2 oxidant. Furthermore, modification of the SiO2/SiC interface promoted by oxidizing SiC with CO2 at 1400 °C or more reduced the amount of interface states, thus the SiC metal-oxide-semiconductor field-effect transistors (MOSFETs) treated with CO2 at 1400 °C exhibited improved channel mobility with a stable threshold voltage.

The atomic arrangement-dependent electrical transport behavior of excimer laser induced graphene ridges on 4H-SiC

In this paper, the sub-micron graphene ridge structures commonly observed in laser irradiation on the 4H-SiC surface are analyzed to explore the formation mechanism and the electrical transfer properties. The cross-section morphologies of irradiated SiC indicate that the atomic arrangement of graphene sheets in flat and ridge areas is different significantly. The atom distributed direction of laser induced graphene layer is more prefer to be morphology-dependent rather than the SiC atomic distribution-dependent. The difference of carbon atoms arrangement is considered to be the reason of the various electrical transport properties on the graphene ridge and the flat graphene sheets. The electron transfer behavior along the gap between graphene layers is found to be stronger than that from layer to layer. The formation of such graphene sheets is attributed to the high energy photons irradiation-induced decomposition of SiC and the reconstruction of carbon atoms on SiC surface. The study on the formation of laser induced graphene ridge with different atomic distributions and the corresponding mechanism of electrical transport property is significant for the growth analysis of graphene on silicon carbide during thermal destruction.

Novel photoelectrochemically combined mechanical polishing technology for scratch-free 4H-SiC surface by using CeO2-TiO2 composite photocatalysts and PS/CeO2 core/shell abrasives

4H-SiC is a promising next-generation semiconductor. To obtain a scratch-free SiC surface with less/no residual oxide layer at an ideal material removal rate (MRR), a novel photoelectrochemically combined mechanical polishing (PECMP) technique was proposed. Polystyrene (PS)/CeO2 core/shell abrasives and CeO2-TiO2 composite photocatalysts immobilized on a titanium mesh were synthesized and characterized. The results of hydroxyl radical trapping tests present that the strongest photocatalytic activity is achieved by CeO2-TiO2 when UV-light and anodic bias are applied simultaneously (the optimal conditions). SiC-PECMP comparison tests were conducted in water containing 6.5 wt% PS/CeO2 on a home-made polisher, and after polishing, surface morphologies, surface roughness values (Ra) and MRRs were compared. The best polishing results (Ra: 0.738 nm, MRR: 1.109 μm/h) are realized under the optimal conditions. This was further demonstrated by another verification test, and a scratch-free surface (Ra: 0.113 nm) with less residual oxide layer was obtained. The scratch-free surface is attributed to the elastic core/shell abrasives and to the mild photocatalytic reaction. The ideal MRR is ascribed to the enhanced photocatalytic activity of the composite photocatalysts. Therefore, the feasibility of the proposed method is demonstrated, and this technique may be utilized to manufacture other semiconductors.

Hydrogen etching of the SiC(0001) surface at moderate temperature

Hydrogen etching of a 4H-SiC(0001) surface at a moderate temperature of 1200 °C with molecular hydrogen gas was investigated to obtain enough flat and clean surface for large-scale high-quality epitaxial graphene synthesis. We found after a prolonged hydrogen etching that micro scratches, large depressions, and contaminations produced on the wafer in the manufacturing process disappeared and that a periodic array of atomic steps appeared, maintaining initial flat surface morphology. One hour of etching with a flow of 1.0 l/min was the optimum condition to obtain a flat and clean SiC surface in the present study. Using such surfaces, we were able to synthesize the so-called zero layer graphene by thermal annealing in ultrahigh vacuum.

Dy adsorption on and intercalation under graphene on 6 H -SiC(0001) surface from first-principles calculations

Previous experimental observations motivate clarification of configuration stabilities and kinetic processes for intercalation of guest atoms into a layered van der Waals material such as a graphene-SiC system. From our first-principles density functional theory (DFT) calculations, we analyze Dy adsorption and intercalation for graphene on a 6H-SiC(0001) surface, where the system includes two single-atom-thick graphene layers: the top-layer graphene (TLG) and the underling buffer-layer graphene (BLG) above the terminal Si layer. Our chemical potential analysis shows that intercalation of a single Dy atom into the gallery between TLG and BLG is more favorable than adsorption on TLG but that intercalation into the gallery underneath BLG is highly unfavorable. We obtain diffusion barriers of \ensuremath{\sim}0.45 and 0.54 eV for a Dy atom diffusing on and under TLG, respectively. We find that the direct penetration of a Dy atom from the graphene top into the gallery under TLG is almost inhibited below a temperature of \ensuremath{\sim}1400 K due to a large global barrier of at least \ensuremath{\sim}3.5 eV. Instead, we find that a single Dy atom on TLG can easily intercalate by crossing a TLG step (e.g., a zigzag step presaturated by a Dy chain or a reconstructed zigzag step zz57). We also perform DFT calculations for different Dy coverages to demonstrate how the favorability of Dy intercalation, as well as the corresponding interlayer spacings, depend on the coverage. Consequently, we can provide general insight and guidance for extensively studied systems involving intercalation of foreign atoms into graphene on a SiC substrate.

Eigenmodes and resonance vibrations of graphene nanomembranes

Natural and resonant oscillations of suspended circular graphene and hexagonal boron nitride (h-BN) membranes (single-layer sheets lying on a flat substrate having a circular hole of radius $R$) have been simulated using full-atomic models. Substrates formed by flat surfaces of graphite and h-BN crystal, hexagonal ice, silicon carbide 6H-SiC and nickel surface (111) have been used. The presence of the substrate leads to the forming of a gap at the bottom of the frequency spectrum of transversal vibrations of the sheet. The frequencies of natural oscillations of the membrane (oscillations localized on the suspended section of the sheet) always lie in this gap, and the frequencies of oscillations decrease by increasing radius of the membrane as $(R+R_i)^{-2}$ with nonezero effective increase of radius $R_i>0$. The modeling of the sheet dynamics has shown that small periodic transversal displacements of the substrate lead to resonant vibrations of the membranes at frequencies close to eigenfrequencies of nodeless vibrations of membranes with a circular symmetry. The energy distribution of resonant vibrations of the membrane has a circular symmetry and several nodal circles, whose number $i$ coincides with the number of the resonant frequency. The frequencies of the resonances decrease by increasing the radius of the membrane as $(R+R_i)^{\alpha_i}$ with exponent $\alpha_i<2$. The lower rate of resonance frequency decrease is caused by the anharmonicity of membrane vibrations.

Combination of Plasma Electrolytic Processing and Mechanical Polishing for Single-Crystal 4H-SiC.

Single-crystal 4H-SiC is a typical third-generation semiconductor power-device material because of its excellent electronic and thermal properties. A novel polishing technique that combines plasma electrolytic processing and mechanical polishing (PEP-MP) was proposed in order to polish single-crystal 4H-SiC surfaces effectively. In the PEP-MP process, the single-crystal 4H-SiC surface is modified into a soft oxide layer, which is mainly made of SiO2 and a small amount of silicon oxycarbide by plasma electrolytic processing. Then, the modified oxide layer is easily removed by soft abrasives such as CeO2, whose hardness is much lower than that of single-crystal 4H-SiC. Finally a scratch-free and damage-free surface can be obtained. The hardness of the single-crystal 4H-SiC surface is greatly decreased from 2891.03 to 72.61 HV after plasma electrolytic processing. By scanning electron microscopy (SEM) and X-ray Photoelectron Spectroscopy (XPS) observation, the plasma electrolytic processing behaviors of single-crystal 4H-SiC are investigated. The scanning white light interferometer (SWLI) images of 4H-SiC surface processed by PEP-MP for 30 s shows that an ultra-smooth surface is obtained and the surface roughness decreased from Sz 607 nm, Ra 64.5 nm to Sz 60.1 nm, Ra 8.1 nm and the material removal rate (MRR) of PEP-MP is about 21.8 μm/h.