Si-face SiC Research Articles

Defect-Engineered MnO2 as Catalyst for the Chemical Mechanical Polishing of Silicon Carbide Wafer

Chemical mechanical polishing (CMP) of SiC wafer is challenging due to its extreme hardness and inertness. Catalyst assisted CMP is a cost-effective approach to increase material removal rate (MRR) without sacrificing surface quality. Herein, oxygen-deficient α-MnO2 was prepared by mechanochemical synthesis and the effect of catalyst physiochemical structure on the CMP performance of Si-face SiC wafer was systematically studied. The addition of 1% α-MnO2 catalyst increased MRR by 38.8% to 1.11 μm h−1, much higher than commercial γ-MnO2. The synergy of phase structure, oxygen vacancy and surface area & porosity contributed to the high catalytic activity. α-MnO2 is an outstanding oxidation catalyst due to its stable framework, large tunnel size, rich surface area and porosity, which can facilitate the adsorption, activation and transfer of guest species and intermediates and therefore affects the reaction pathway and reaction kinetics. Mechanochemical synthesis generates nano MnO2 particles with rich oxygen vacancies. The presence of more surface oxygen vacancies can improve oxidizing activity of MnO2 catalyst, facilitating the oxidation of C species on wafer surface. The use of defect-engineered α-MnO2 catalyst is promising for overcoming the present bottlenecks of long processing time and high cost of current CMP of SiC wafer.

Structural characteristics of 3C–SiC thin films grown on Si-face and C-face 4H–SiC substrates by high temperature chemical vapor deposition

Hetero-epitaxy of 3C–SiC on Si-face and C-face 4H–SiC is experimentally investigated and corresponding growth model is identified. This study of polytypes of SiC will be useful for band-gap engineering, optical, electrical, and electronic applications of SiC. 3C–SiC was epitaxially deposited on 4H–SiC using a high temperature chemical vapor deposition. The primary factor affecting the epi-quality on Si-face SiC is double positioning boundary (DPB) defect. An adatom migration model elucidates the formation of the V-shaped DPB defect structures. On C-terminated 4H–SiC, super-V-shaped-structure (SVSS) defects and polycrystalline complexes in 3C–SiC are formed. These characteristics of 3C–SiC are measured and analyzed using transmission electron microscopy, X-ray diffraction, atomic force microscopy, and scanning electron microscopy techniques. A model considering adatoms' migration in step-flow direction and anti-step-flow direction explains the SVSS defects. Effects of both step-flow and anti-step-flow growth mode on the atom adsorption near the step edge are discussed.

A novel approach to obtain near damage-free surface/subsurface in machining of single crystal 4H-SiC substrate using pure metal mediated friction

High speed friction induced removal behavior of single crystal silicon carbide (4H-SiC) by pure metals (iron and nickel) were investigated, and the subsurface damage were explored for both C-face and Si-face of SiC substrates. A near damage-free subsurface was obtained for C-face SiC substrate. Crystal defects including cracks, dislocations, stacking faults and lattice distortions were evidenced for Si-face SiC substrate. The highest material removal rate (MRR) was found to be 8.9 μm/min on Si-face using pure nickel. The formation of silicide and/or oxide indicated that the material removal of C-face and Si-face of 4H-SiC were both dominated by the friction-induced chemical interaction. A friction reaction-removal model of single crystal 4H-SiC substrate using pure metal was also proposed in this paper.

Reaction mechanisms of the initial steps for the oxidation of (000[formula omitted]) C and (0001) Si faces of SiC with OH radicals

The reaction mechanisms of OH radicals on the C and Si faces of SiC were analyzed using density functional theory calculations to understand the difference in reactivity. For the first OH radical, it was shown that the O–H bond dissociation occurs on both faces, but at different spin states, i.e., the reaction proceeds at a lower spin multiplicity on the Si face. Then, the generated surface O atom causes dissociation of a C–Si bond to form a C–O–Si structure on the C face, whereas it binds to two surface Si atoms to form a Si–O–Si structure on the Si face. These differences are explained by the atomic charge and spin density of the surface atoms. The second OH radical generates a CO molecule on the C face, and a second Si–O–Si structure is formed on the Si face. These results shed light on the reasons why the grinding technique using a tribocatalytic abrasive, which is considered to be promoted by oxidation of the surface by oxidants such as the OH radical, is inefficient on the Si face of SiC.

Defect reduction in SiC epilayers by different substrate cleaning methods

Low defect epitaxial layers are of highest importance for the fabrication of high power SiC devices using large chip area. To minimize the impact of defects and particles on the substrates, the influence of different automated cleaning procedures on defects of epitaxial layers subsequently grown on 150 mm 4H–SiC substrates was investigated. Therefore, 12 μm thick n-type epitaxial layers were grown on commercially available 150 mm 4H–SiC Si-face substrates with lowest micropipe density using a warm-wall chemical vapor deposition reactor. The defects of all samples were characterized utilizing surface microscopy as well as ultra-violet photoluminescence techniques. With these methods, a qualitative, as well as quantitative, characterization of surface defects and crystallographic defects from the as-received substrate to the finished epitaxy was carried out and is discussed. Yield prediction maps for high voltage metal-oxide-semiconductor field-effect-transistors were generated and the reduction of critical defects, which results in a predicted die yield increase, as well as an outlook on future investigations, is discussed.

Correlation between Contact Angle and Surface Roughness of Silicon Carbide Wafers

Two-inch diameter 6H-SiC wafers were sliced from a SiC ingot and the wafers were ground and polished using different diamond slurries (1 μm and 0.1 μm in particles size) to investigate their dependence on wetting on surface roughness (Ra) and polarity using precisely dispensed de-ionized (DI) water drops. The Ra of the Si-face (0001) SiC wafer, after grinding and polishing, was 5.6 and 1.6 nm, respectively, as measured by atomic force microscopy (AFM). For C-face (000–1) SiC wafers, the Ra was 7.2 nm after grinding and 3.3 nm after polishing. The average contact angle measurement of the SiC wafers after final polishing showed clear differences between surface polarity; the contact angle for the Si-face (0001) was ~7o greater than that for the C-face (000–1). The difference in contact angles between the Si-face (0001) and the C-face (000–1) tends to increase as the reduction of surface roughness approaches the final stage of polishing. The uniformity of Raman peak intensity in the folded transverse optical phonon band at ~780 cm−1 in scanned areas correlated well with the surface roughness measured by AFM. The contact angle measurement can be used as a convenient surface polarity and surface roughness testing technique for SiC wafers.

Growth Mechanism of InN Nucleation Layers on Epitaxial Graphene Using Metal Organic Vapor Phase Epitaxy and Radio-Frequency Molecular Beam Epitaxy

This paper presents a growth mechanism and a comparative study of the InN nucleation layers grown on epitaxial graphene (EG) by metal organic vapor phase epitaxy (MOVPE) and radio-frequency molecular beam epitaxy (RF-MBE). Before growing InN nucleation layers, the EG surface was formed on 4° off 6H-SiC substrates (both Si- and C-faces) by thermal annealing in Ar ambient. The MOVPE grown InN nucleation layers show three-dimensional (3D) growth from the very beginning due to lack of nucleation centers and for the long migration length on the EG surface. The MOVPE InN on the EG formed on Si-face SiC is nucleated with the step edge, while it is nucleated randomly over the entire EG surface on C-face SiC. In contrast, the RF-MBE InN nucleation layers grown on EG formed on Si-face SiC show a two-dimensional growth mode up to 3 ML, and then a further increase of nucleation layer changes the growth mode to 3D mode. The formation of adsorption sites on the graphene surface by introduction of defects plays a signif...

Controllable growth of P-type graphene on the boron ion-implanted Si-face of SiC (0001)

Realizing the reliable modulation of the carrier type and density in graphene is a challenging task, especially for epitaxial graphene (EG) grown in situ on SiC via thermal decomposition. In this process, doping is seldom adopted for substrate pretreatment, and the residual carriers in EG are determined by the properties of the SiC substrate and the random defects in EG. Here, we demonstrate a simple and effective technique for hole doping in EG on the Si-face of SiC (0001) implanted with boron ions. P-type EG with a carrier concentration of approximately 1–2 × 1012 cm−2 and a room-temperature mobility of 100–300 cm2/V s was achieved at the boron ion dose of approximately 1 × 1015 cm−2 at a centroid depth of approximately 140 nm from the SiC surface. These transport properties are comparable with the data reported for unintentionally doped P-type EG on the Si-face of SiC [Tedesco et al., Appl. Phys. Lett. 95, 122102 (2009)], for which the Hall device size was 40 times smaller compared to our device. The formation of P-type EG on the Si-face of SiC (0001) in this study was attributed to the synergetic effects of boron doping in EG and the transfer of electrons from EG into SiC. The detailed mechanisms are studied and discussed in this paper. The method demonstrated herein can be universally applied to downshift the Fermi level of EG on SiC, regardless of the crystallization orientation or polytype. The developed method is also compatible with modern semiconductor procedures.

N-polar AlN nucleation layers grown by hot-wall MOCVD on SiC: Effects of substrate orientation on the polarity, surface morphology and crystal quality

Hot-wall metalorganic vapor phase epitaxy enables a superior quality of group-III nitride epitaxial layers and high electron mobility transistor structures, but has not yet been explored for N-polar growth. In this work, we aim at achieving N-polar AlN nucleation layers (NLs) with optimized properties for subsequent growth of GaN device heterostructures. The effects of substrate orientation on the polarity, surface morphology and crystalline quality of AlN NLs on on-axis C-face SiC (0001̄), C-face SiC (0001̄) off-cut towards the [112̄0] by 4°, and Si-face SiC (0001) are investigated. The results are discussed in view of growth mode evolution with growth temperature and substrate orientation. It is demonstrated that N-polar AlN NLs with step-flow growth mode and 0002 rocking curve widths below 20 arcsec can be achieved on off-axis C-face SiC substrates.

Enhanced Raman spectra of hydrogen-intercalated quasi-free-standing monolayer graphene on 4H-SiC(0001)

Abstract Epitaxial Chemical Vapor Deposition graphene on silicon carbide is a promising material for application in electronics. Its most developed form, i.e. quasi-free-standing monolayer that comes through hydrogen atom intercalation of carbon buffer layer grown on the Si-face of SiC, constitutes an excellent platform for sensor technology. Raman spectroscopy allows for a non-destructive analysis of graphene properties. The G and 2D bands widths and positions bring information on the number of graphene layers as well as charge carrier type and doping level. Their intensity is strong. However, the analysis of Si-H bonds remains a challenge. In this work we present how analytical methods can improve the visibility of hydrogen-related Raman bands and help monitor hydrogen presence in quasi-free-standing graphene.

Interaction of epitaxial graphene with heavy metals: towards novel sensing platform

Development of next-generation sensors based on graphene materials, especially epitaxial graphene (EG) as the most promising representative, with desirable cross-reactivity to heavy metals (HMs) is of great technological significance in the virtue of enormous impact on environmental sensorics. Nevertheless, the mechanisms by which EG responds to toxic HMs exposure and then produces the output signal are still obscure. In the present study, the nature of interaction of toxic HMs, e.g. Cd, Hg and Pb in neutral charge state and EG on Si-face SiC in the absence and in the presence of pure water solution has been investigated using density functional theory with the inclusion of dispersion correction and cluster model of EG. The gas-phase calculations showed that adsorbed electron-donating Cd and Hg adatoms on EG are most stable when bonded to hollow sites, while Pb species prefer to sit above bridge sites. By using non-covalent interaction analysis, charge decomposition analysis, overlap population density of states analysis and topological analysis, it was found that the interaction between Cd or Hg and EG is non-bonding in nature and is mainly governed by van der Waals forces, while Pb adsorption is followed by the formation of anti-bonding orbitals in vacuum conditions and bonding orbitals in water. The role of solvent in the adsorption behavior of HMs is studied and discussed. The present theoretical analysis is in good agreement with recent experimental results towards discriminative electrochemical analysis of the toxic HMs in aqueous solutions at critically low concentrations.

Electronic and structural properties of H-intercalated graphene-SiC (0001) interface

In this study, we investigate the changes in electronic and structural properties of a graphene layer deposited on a () R30° SiC (0001) surface before and after hydrogen intercalation using first-principles energy band calculations. Before hydrogen adsorption, the bond distance between the first graphene layer (FGL) and an Si atom on SiC surface is calculated to be 0.2285 nm, which is close to the value reported in the literature. After hydrogen adsorption, the layer distance between SiC surface and FGL tends to increase. The adsorption energy of hydrogen on SiC surface as estimated from the total energy change before and after hydrogen adsorption is relatively small. Thus, we suggest that hydrogen easily bonds to Si dangling bonds on SiC surface. When Si dangling bonds on SiC surface are passivated by hydrogen, the graphene layer on SiC functions as a quasi-free-standing graphene layer, whether the graphene exfoliates or not. Therefore, our results indicate the effectiveness of the hydrogen intercalation technique to create quasi-free-standing epitaxial graphene on Si-face SiC (0001) substrate.

Novel Gate Insulator Process by Nitrogen Annealing for Si-Face SiC MOSFET with High-Mobility and High-Reliability

The gate insulator process for SiC-MOSFET was examined and high-quality interface was realized by employing the pre-annealing process before high-temperature N2 annealing. The pre-annealing evidently activated the interface to introduce nitrogen, and then field-effect mobility exceeded 50 cm2/Vs. The fabricated sample also demonstrated superior bias temperature instability (BTI) and excellent breakdown electric field of 11.7 MV/cm.

Atomically manipulated proton transfer energizes water oxidation on silicon carbide photoanodes

Preferential exposure of Si-face of SiC will mechanistically shift the rate limiting step of water oxidation from sluggish proton-coupled electron transfer on C-face to a more energy-favorable electron transfer.

Measuring the dielectric and optical response of millimeter-scale amorphous and hexagonal boron nitride films grown on epitaxial graphene

Monolayer epitaxial graphene (EG), grown on the Si face of SiC, is an advantageous material for a variety of electronic and optical applications. EG forms as a single crystal over millimeter-scale areas and consequently, the large scale single crystal can be utilized as a template for growth of other materials. In this work, we present the use of EG as a template to form millimeter-scale amorphous and hexagonal boron nitride (a-BN and h-BN) films. The a-BN is formed with pulsed laser deposition and the h-BN is grown with triethylboron (TEB) and NH3 precursors, making it the first metal organic chemical vapor deposition (MOCVD) process of this growth type performed on epitaxial graphene. A variety of optical and non-optical characterization methods are used to determine the optical absorption and dielectric functions of the EG, a-BN, and h-BN within the energy range of 1 eV–8.5 eV. Furthermore, we report the first ellipsometric observation of high-energy resonant excitons in EG from the 4H polytype of SiC and an analysis on the interactions within the EG and h-BN heterostructure.

Probing the dielectric response of the interfacial buffer layer in epitaxial graphene via optical spectroscopy.

Monolayer epitaxial graphene (EG) is a suitable candidate for a variety of electronic applications. One advantage of EG growth on the Si face of SiC is that it develops as a single crystal, as does the layer below, referred to as the interfacial buffer layer (IBL), whose properties include an electronic band gap. Though much research has been conducted to learn about the electrical properties of the IBL, not nearly as much work has been reported on the optical properties of the IBL. In this work, we combine measurements from Mueller matrix ellipsometry, differential reflectance contrast, atomic force microscopy, and Raman spectroscopy, as well as calculations from Kramers-Kronig analyses and density functional theory (DFT), to determine the dielectric function of the IBL within the energy range of 1 eV to 8.5 eV.

Transport properties of graphene films grown by thermodestruction of SiC (0001) surface in argon medium

We present the results of investigations of the transport properties of graphene films obtained by thermodestruction of a 4H-SiC (0001) surface in argon. The charge-carrier concentration in the graphene layer was within 7 × 1011–1 × 1012 cm–2, and the maximum mobility of electrons approached 6000 cm2/(V · s). The achieved parameters of mobility are close to theoretical values calculated for graphene films with intrinsic conductivity on the Si face of SiC at Т = 300 К in the absence of intercalated hydrogen.

Evaluation of silicon- and carbon-face SiO2/SiC MOS interface quality based on scanning nonlinear dielectric microscopy

A strong positive correlation was found between the trap density (Dit) at the SiO2/SiC interface and signal variation in a scanning nonlinear dielectric microscopy (SNDM) image. Si-face and C-face SiC wafers with a 45-nm-thick oxide layer were examined by the conventional high-low method and SNDM, which is a type of scanning probe microscopy. The Dit value measured by the high-low method and the standard deviation of normalized SNDM images exhibited a strong positive correlation, which means that the standard deviation of the normalized SNDM image can be used as a universal measure of the SiO2/SiC interface quality. Using this measure, a quick evaluation of Dit using SNDM is possible.

Epitaxial graphene homogeneity and quantum Hall effect in millimeter-scale devices

Quantized magnetotransport is observed in 5.6 × 5.6 mm2 epitaxial graphene devices, grown using highly constrained sublimation on the Si-face of SiC(0001) at high temperature (1900 °C). The precise quantized Hall resistance of Rxy=h2e2 is maintained up to record level of critical current Ixx = 0.72 mA at T = 3.1 K and 9 T in a device where Raman microscopy reveals low and homogeneous strain. Adsorption-induced molecular doping in a second device reduced the carrier concentration close to the Dirac point (n ≈ 1010 cm−2), where mobility of 18760 cm2/V is measured over an area of 10 mm2. Atomic force, confocal optical, and Raman microscopies are used to characterize the large-scale devices, and reveal improved SiC terrace topography and the structure of the graphene layer. Our results show that the structural uniformity of epitaxial graphene produced by face-to-graphite processing contributes to millimeter-scale transport homogeneity, and will prove useful for scientific and commercial applications.

Effective approach for accurately calculating individual energy of polar heterojunction interfaces

We propose a direct approach for calculating individual energy of polar semiconductor interfaces using density functional theory calculations. This approach is applied to polar interfaces between group-III nitrides (AlN and GaN) and SiC and clarifies the interplay of chemical bonding and charge neutrality at the interface, which is crucial for the stability and polarity of group-III nitrides on SiC substrates. The ideal interface is stabilized among various atomic arrangements over the wide range of the chemical potential on Si-face SiC, whereas those with intermixing are favorable on C-face SiC. The stabilization of the ideal interfaces resulting in Ga-polar GaN and Al-polar AlN films on Si-face SiC is consistent with experiments, suggesting that our approach is versatile to evaluate various polar heterojunction interfaces as well as group-III nitrides on semiconductor substrates.