C-face 4H-SiC Research Articles

Multi-scale investigation of interface properties, stacking order and decoupling of few layer graphene on C-face 4H-SiC

In this work, we report a multi-scale investigation using several nano-, micro and macro-scale techniques of few layer graphene (FLG) sample consisting of large monolayer (ML) and bilayer (BL) areas grown on C-face 4H-SiC (000-1) by high-temperature sublimation. Single 1 × 1 diffraction patterns are observed by micro-low-energy electron diffraction for ML, BL and trilayer graphene with no indication of out-of-plane rotational disorder. A SiOx layer is identified between graphene and SiC by X-ray photoelectron emission spectroscopy and reflectance measurements. The chemical composition of the interface layer changes towards SiO2 and its thickness increases with aging in normal ambient conditions. The formation mechanism of the interface layer is discussed. It is shown by torsion resonance conductive atomic force microscopy that the interface layer causes the formation of non-ideal Schottky contact between ML graphene and SiC. This is attributed to the presence of a large density of interface states. Mid-infrared optical Hall effect measurements revealed Landau-level transitions in FLG that have a square-root dependence on magnetic field, which evidences a stack of decoupled graphene sheets. Contrary to previous works on decoupled C-face graphene, our BL and FLG are composed of ordered decoupled graphene layers without out-of-plane rotation.

Extremely uniform epitaxial growth of graphene from sputtered SiC films on SiC substrates

ABSTRACTA novel method to fabricate uniform epitaxial graphene on C-face SiC substrates was investigated. Graphene was grown on the C-face 6H-SiC substrates with a sputtered SiC film by annealing temperatures ranging from 1400 to 1900 °C under an Ar ambient. The fractional area of the graphene having the layer number of two was about 95% in a 75×75 μm square by a Raman mapping and a low energy electron microscopy. Graphene on the C-face SiC fabricated by this method is quite uniform compared to that made by a conventional method without the sputtered SiC films and is thus suitable for high frequency analog devices.

Super-V-shaped structure on 3C-SiC grown on the C-face of 4H-SiC

We investigate a unique super-V-shaped structure (SVSS) on the C-face of 3C/4H-SiC heteroepitaxial films. Atomic force microscopy (AFM) and optical images reveal that two low protruding straight lines of an SVSS join at a common higher protruding point. Transmission electron microscopy images determine that the protruding point is induced by polycrystalline nucleation, and straight lines are found to be 3C crystals without any specific defects. High-resolution AFM reveals that the directions of the SVSSs are strongly related to step-flow. It is believed that the straight lines are caused by redundant adatoms gathering and crystallizing when they meet and deviate around the protruding point defect on their original migration path. The effects of step-flow and anti-step-flow in the formation of SVSSs are discussed.

Relationship between C-Face Defects and Threshold-Voltage Instability in C-Face 4H-SiC MOSFETs Studied by Electrically Detected Magnetic Resonance

We present EDMR (electrically detected magnetic resonance) observations on “C-face defects” in C-face 4H-SiC MOSFETs. We found that negative threshold-voltage shifts of C-face MOSFETs are increased in association with EDMR signals of C-face defects as well as with the dissociation of hydrogen atoms induced by gamma-ray irradiation.

Microscopic Difference between Dry and Wet Oxidations of C-Face 4H-SiC MOSFFETs Studied by Electrically Detected Magnetic Resonance

We studied interface defects of C-face 4H-SiC metal-oxide-semiconductor field-effect transistors (MOSFETs) by means of electrically-detected-magnetic-resonance (EDMR) spectroscopy. EDMR measurements were carried out on opposite types of C-face MOSFETs, which were prepared by dry oxidation and wet oxidation, and we found EDMR signals of interface defects from both the MOSFETs. Judging from their spectroscopic features, the interface signals of the two MOSFETs are assigned to be the same type, and we call them “C-face defects.” The density of C-face defects was found to be larger in the dry-oxide MOSFETs than in the wet-oxide MOSFETs. It is also revealed that part of C-face defects in wet-oxide MOSFETs are coupled with hydrogen atoms.

Low Resistance Ohmic Contact Formation on 4H-SiC C-Face with NbNi Silicidation Using Nanosecond Laser Annealing

Non-equilibrium laser silicidations for low-resistance ohmic contact to 4H-SiC C-face with carbon-interstitial type metal, Nb was demonstrated. In a conventional nickel silicide (NiSi) electrode on SiC, a carbon agglomeration at the silicide/SiC interface occurs, and constant resistance between NiSi and SiC substrate becomes larger. For suppressing the carbon agglomeration, in this research, nanoseconds non-equilibrium laser annealing was introduced, and also carbon-interstitial type metal Nb to form metal carbides was introduced. Ni/Nb/SiC multilayer contact and NbNi mixed contact were formed on C-face side of 4H-SiC wafers. Electrical contact properties were investigated after 45 nanoseconds pulse laser annealing in N2 ambient. As the result, at the NbNi mixed contact, specific contact resistance of 2.4×10-4 Ωcm2 was realized.

Reduction in Background Carrier Concentration for 4H-SiC C-face Epitaxial Growth

ABSTRACT Reduction in background carrier concentration has been investigated for 4H-SiC C-face epitaxial growth in order to be applied for ultra-high voltage power devices. Optimizing epitaxial growth parameters made it possible to achieve 7.6x1013 cm-3 as the background carrier concentration within a whole area of specular 3-inch wafers. In addition to the background carrier concentration reduction, epitaxial film thickness variation, surface defect density and the carrier lifetime have been confirmed to fulfill the requirements for the devices.

Effects of substrate on the domains and electrical properties of epitaxial graphene formed on on-axis C-face 4H-SiC

The effect of substrate surface preparation on the quality of epitaxial graphene grown on on-axis C-face 4H-SiC has been investigated. Structural epitaxial graphene has been achieved using a graphite enclosure. The homogeneity and domain size of epitaxial graphene on different pretreated substrates were studied using Raman spectroscopy, field emission scanning electron microscopy and hall measurements. The etching of C-face SiC substrate using C3H8/H2 is shown to have a pronounced effect on the domain size and hall mobility of multilayer epitaxial graphene. From Raman measurements, it is found that the pretreatment of substrate has a great influence on the domain size of epitaxial graphene. From hall results, it is found that the effect of substrate pretreatment on carrier mobility follows the same tendency reflected in the results of Raman spectra. It is concluded that the pretreatment of substrate surface, mainly the etching for generating high density, regular and periodic step structure with flatter terrace, would allow the growth of graphene with significantly large area and high mobility, even on large on-axis C-face 4H-SiC wafer.

Nanoselective area growth of GaN by metalorganic vapor phase epitaxy on 4H-SiC using epitaxial graphene as a mask

We report the growth of high-quality triangular GaN nanomesas, 30-nm thick, on the C-face of 4H-SiC using nanoselective area growth (NSAG) with patterned epitaxial graphene grown on SiC as an embedded mask. NSAG alleviates the problems of defects in heteroepitaxy, and the high mobility graphene film could readily provide the back low-dissipative electrode in GaN-based optoelectronic devices. A 5–8 graphene-layer film is first grown on the C-face of 4H-SiC by confinement-controlled sublimation of silicon carbide. Graphene is then patterned and arrays of 75-nm-wide openings are etched in graphene revealing the SiC substrate. A 30-nm-thick GaN is subsequently grown by metal organic vapor phase epitaxy. GaN nanomesas grow epitaxially with perfect selectivity on SiC, in the openings patterned through graphene. The up-or-down orientation of the mesas on SiC, their triangular faceting, and cross-sectional scanning transmission electron microscopy show that they are biphasic. The core is a zinc blende monocrystal surrounded with single-crystal wurtzite. The GaN crystalline nanomesas have no threading dislocations or V-pits. This NSAG process potentially leads to integration of high-quality III-nitrides on the wafer scalable epitaxial graphene/silicon carbide platform.

Growth of Shockley type stacking faults upon forward degradation in 4H-SiC p-i-n diodes

The growth of Shockley type stacking faults in p-i-n diodes fabricated on the C-face of 4H-SiC during forward current operation was investigated using Berg-Barrett X-ray topography and photoluminescence imaging. After forward current experiment, Shockley type stacking faults were generated from very short portions of basal plane dislocations lower than the conversion points to threading edge dislocations in the epitaxial layer. The growth behavior of Shockley type stacking faults was discussed. Growth of stacking faults in the substrates was not observed.

Characterization of Grapho-Silicidation on n+ 4H-SiC C-Face for Back Side Ohmic Contacts of Power Devices

In this research, a novel grapho-silicidation is introduced to form a low resistance ohmic contact for n+ 4H-SiC power semiconductor devices. In this method, amorphous line and space patterns with sizes of 100 nm and 200 nm were formed in SiC substrate with ion implantation, and Ni silicide was formed on the SiC substrate. By the grapho-silicidation, carbon agglomeration was controlled, and low contact resistance of 1.9 × 10−3 Ωcm2 was realized.

Nitrogen Incorporation during Seeded Sublimation Growth of 4H-SiC and 6H-SiC

We address the problem of nitrogen incorporation during bulk crystal growth of 4H-SiC and 6H-SiC by seeded sublimation method. The partial pressure of nitrogen and temperature dependence were considered in bulk SiC crystals. Free carrier concentration and incorporated nitrogen were determined using Raman spectroscopy and Secondary Ion Mass Spectrometry, respectively. The incorporated nitrogen at the (000-1) C-face of 4H-SiC and 6H-SiC is found to be independent of the polytype of the crystal. Higher desorption rate at Si-face compared to C-face is found, using a Langmuir equation, which is attributed to the difference in bond density between the two polar faces. The increased nitrogen desorption when growth temperature increases is believed to be the most contributing factor, based on the temperature dependent trends.

Characterization of Comet-Shaped Defects on C-Face 4H-SiC Epitaxial Wafers

The crystallographic structure of comet-shaped defects observed on C-face 4H-SiC epitaxial film was investigated. The comet-shaped defects consist of head and tail part. The tail part shows symmetrical shape with respect to the (1-100) plane in cross section and narrowing along the step-flow direction. The tail part consists of four 3C domains with characteristic twin boundaries of Σ3 and Σ27. The head part consists of 3C poly-crystalline grains formed during epitaxial film growth and its formation is triggered by 3C-SiC particle contamination.

Direct growth of freestanding GaN on C-face SiC by HVPE.

In this work, high quality GaN crystal was successfully grown on C-face 6H-SiC by HVPE using a two steps growth process. Due to the small interaction stress between the GaN and the SiC substrate, the GaN was self-separated from the SiC substrate even with a small thickness of about 100 μm. Moreover, the SiC substrate was excellent without damage after the whole process so that it can be repeatedly used in the GaN growth. Hot phosphoric acid etching (at 240 °C for 30 min) was employed to identify the polarity of the GaN layer. According to the etching results, the obtained layer was Ga-polar GaN. High-resolution X-ray diffraction (HRXRD) and electron backscatter diffraction (EBSD) were done to characterize the quality of the freestanding GaN. The Raman measurements showed that the freestanding GaN film grown on the C-face 6H-SiC was stress-free. The optical properties of the freestanding GaN layer were determined by photoluminescence (PL) spectra.

Scalable control of graphene growth on 4H-SiC C-face using decomposing silicon nitride masks

Selective epitaxial graphene growth is achieved in pre-selected areas on the 4H-SiC C-face with a SiN masking method. The mask decomposes during the growth process leaving a clean, resist free, high temperature annealed graphene surface, in a one-step process. Depending on the off-stoichiometry composition of a Si3 + xN4 mask evaporated on SiC prior to graphitization, the number of layers on the C-face increases (Si-rich) or decreases (N-rich). Graphene grown in masked areas shows excellent quality as observed by Raman spectroscopy, atomic force microscopy and transport data.

Structural properties and dielectric function of graphene grown by high-temperature sublimation on 4H-SiC(000-1)

Understanding and controlling growth of graphene on the carbon face (C-face) of SiC presents a significant challenge. In this work, we study the structural, vibrational, and dielectric function properties of graphene grown on the C-face of 4H-SiC by high-temperature sublimation in an argon atmosphere. The effect of growth temperature on the graphene number of layers and crystallite size is investigated and discussed in relation to graphene coverage and thickness homogeneity. An amorphous carbon layer at the interface between SiC and the graphene is identified, and its evolution with growth temperature is established. Atomic force microscopy, micro-Raman scattering spectroscopy, spectroscopic ellipsometry, and high-resolution cross-sectional transmission electron microscopy are combined to determine and correlate thickness, stacking order, dielectric function, and interface properties of graphene. The role of surface defects and growth temperature on the graphene growth mechanism and stacking is discussed, and a conclusion about the critical factors to achieve decoupled graphene layers is drawn.

Characterization of comet-shaped defects on C-face 4H-SiC epitaxial wafers by electron microscopy

The crystallographic structures of comet-shaped defects observed on the C-face 4H-SiC epitaxial film were investigated using electron microscopy. The comet-shaped defects consist of head and tail parts. The tail part is symmetric with respect to the (11¯00) plane in the cross-sectional image and narrows along the [1¯1¯20] direction, i.e., along the step-flow direction of epitaxial film growth on the C-face. The tail part consists of four 3C domains with characteristic twin boundaries of Σ3 and Σ27. The head part is formed by 3C and defective hexagonal-SiC polycrystalline grains during epitaxial film growth.

Homoepitaxial growth and investigation of stacking faults of 4H-SiC C-face epitaxial layers with a 1° off-angle

We grew epitaxial layers on 4H-SiC C-face substrates with a 1° off-angle, and discussed important factors related to stacking fault (SF) density reduction by investigating the causes of SFs. Three types of SFs were generated, namely 3C inclusions, 8H-SFs and -SFs. The 3C inclusions were caused by 3C-SiC particles, which were present on the substrates before epitaxial growth, or which had fallen onto the substrates during epitaxial growth from the inside walls of a chemical vapor deposition reactor. The 3C-inclusion density decreased when the in-situ H2 etching depth exceeded 0.4 µm because the 3C-SiC particles, which were present on substrates before epitaxial growth, were removed. For 8H-SFs and -SFs, high dislocation density areas on the substrates rather than the dislocations themselves cause these SFs. To reduce the density of these SFs, it is important to suppress generation of the high dislocation density areas on the substrates.

Hydrogen assisted growth of high quality epitaxial graphene on the C-face of 4H-SiC

We demonstrate hydrogen assisted growth of high quality epitaxial graphene on the C-face of 4H-SiC. Compared with the conventional thermal decomposition technique, the size of the growth domain by this method is substantially increased and the thickness variation is reduced. Based on the morphology of epitaxial graphene, the role of hydrogen is revealed. It is found that hydrogen acts as a carbon etchant. It suppresses the defect formation and nucleation of graphene. It also improves the kinetics of carbon atoms via hydrocarbon species. These effects lead to increase of the domain size and the structure quality. The consequent capping effect results in smooth surface morphology and suppression of multilayer growth. Our method provides a viable route to fine tune the growth kinetics of epitaxial graphene on SiC.

Growth and surface analysis of SiO2 on 4H-SiC for MOS devices

The SiO2 layers have been grown onto C-face and Si-face 4H-SiC substrates by two different techniques such as wet thermal oxidize process and sputtering. The deposition recipes of these techniques are carefully optimized by trails and error method. The growth effects of SiO2 on the C-face and Si-face 4H-SiC substrates are thoroughly investigated by AFM analysis. The growth mechanism of different species involved in the growth process of SiO2 by wet thermal oxide is now proposed by adopting two body classical projectile scattering. This mechanism drives to determine growth of secondary phases such as α-CH nano-islands in the grown SiO2 layer. The effect of HF etchings on the SiO2 layers grown by both techniques and on both the C-face and Si-face substrates are legitimately studied. The thicknesses of the layers determined by AFM and ellipsometry techniques are widely promulgated. The MOS capacitors are made on the Si-face 4H-SiC wafers by wet oxidation and sputtering processes, which are studied by capacitance versus voltage (CV) technique. From CV measurements, the density of trap states with variation of trap level for MOS devices is estimated.