C-face 4H-SiC Research Articles

Achieving semi-metallic conduction in Al-rich AlGaN: Evidence of Mott transition

The development of high performance wide-bandgap AlGaN channel transistors with high current densities and reduced Ohmic losses necessitates extremely highly doped, high Al content AlGaN epilayers for regrown source/drain contact regions. In this work, we demonstrate the achievement of semi-metallic conductivity in silicon (Si) doped N-polar Al0.6Ga0.4N grown on C-face 4H-SiC substrates by molecular beam epitaxy. Under optimized conditions, the AlGaN epilayer shows smooth surface morphology and a narrow photoluminescence spectral linewidth, without the presence of any secondary peaks. A favorable growth window is identified wherein the free electron concentration reaches as high as ∼1.8 × 1020 cm−3 as obtained from Hall measurements, with a high mobility of 34 cm2/V·s, leading to a room temperature resistivity of only 1 mΩ·cm. Temperature-dependent Hall measurements show that the electron concentration, mobility, and sheet resistance do not depend on temperature, clearly indicating dopant Mott transition to a semi-metallic state, wherein the activation energy (Ea) falls to 0 meV at this high value of Si doping for the AlGaN films. This achievement of semi-metallic conductivity in Si doped N-polar high Al content AlGaN is instrumental for advancing ultrawide bandgap electronic and optoelectronic devices.

SiC-based high electron mobility transistor

This paper describes the fabrication of a SiC-based high electron mobility transistor (HEMT). A single-crystal 3C–SiC layer was grown on a C-face 4H–SiC substrate, and 2D electron gas was induced at the 3C–SiC/4H–SiC heterointerface due to the unique polarization physics. The measured Hall mobility of the 2DEG was 586 cm2/V s at room temperature. Source, gate, and drain electrodes were fabricated on the 3C–SiC surface. The drain current for the fabricated SiC-HEMT was measured to be 47.5 mA/mm, and the transconductance was estimated to be 13.5 mS/mm.

On the origin of epitaxial rhombohedral-B4C growth by CVD on 4H-SiC.

Rhombohedral boron carbide, often referred to as r-B4C, is a potential material for applications in optoelectronic and thermoelectric devices. From fundamental thin film growth and characterization, we investigate the film-substrate interface between the r-B4C films grown on 4H-SiC (0001̄) (C-face) and 4H-SiC (0001) (Si-face) during chemical vapor deposition (CVD) to find the origin for epitaxial growth solely observed on the C-face. We used high-resolution (scanning) transmission electron microscopy and electron energy loss spectroscopy to show that there is no surface roughness or additional carbon-based interlayer formation for either substrate. Based on Raman spectroscopy analysis, we also argue that carbon accumulation on the surface hinders the growth of continued epitaxial r-B4C in CVD.

Heteroepitaxy of N-polar AlN on C-face 4H-SiC: Structural and optical properties

To date, it has remained challenging to achieve N-polar AlN, which is of great importance for high power, high frequency, and high temperature electronics, acoustic resonators and filters, ultraviolet (UV) optoelectronics, and integrated photonics. Here, we performed a detailed study of the molecular beam epitaxy and characterization of N-polar AlN on C-face 4H-SiC substrates. The N-polar AlN films grown under optimized conditions exhibit an atomically smooth surface and strong excitonic emission in the deep UV with luminescence efficiency exceeding 50% at room temperature. Detailed scanning transmission electron microscopy (STEM) studies suggest that most dislocations are terminated/annihilated within ∼200 nm AlN grown directly on the SiC substrate due to the relatively small (1%) lattice mismatch between AlN and SiC. The strain distribution of AlN is further analyzed by STEM and micro-Raman spectroscopy, and its impact on the temperature-dependent deep UV emission is elucidated.

Chemical vapor deposition growth of [formula omitted] on Si- and C- face off-axis 4H–SiC at high temperature

Realization of β‐Ga2O3 on a high-thermal-conductivity substrate is crucial to overcome the poor thermal conductivity of β‐Ga2O3 which is one of the major roadblocks against its era in power electronics. We investigated low-pressure chemical vapor deposition (LPCVD) growth on both Si and C- face (0001) 4H–SiC at a high growth temperature of 925 °C using Ga metal and O2 gas source. Growths on C-face 4H–SiC resulted polycrystalline β‐Ga2O3 for the entire O2-flow range, whereas under optimized O2-flow conditions, step flow growth of (−201) β‐Ga2O3 was achieved on Si-face 4H–SiC with 3.7 nm rms surface roughness and rocking curve FWHM of 0.54° at a growth rate reaching 2.48 μm/h 4° off-axis nature of the substrate promoted the growth of some of the in-plane rotational domains while suppressing others. Raman measurements further verified the pure β‐Ga2O3 nature of the grown films. Using a customized LPCVD with solid source Ga, a cost-effective, safe and industrially scalable method was shown to pave the way to enable β‐Ga2O3 heteroepitaxy on 4H–SiC for high-power electronics.

Tunable bandgap and Si-doping in N-polar AlGaN on C-face 4H-SiC via molecular beam epitaxy

N-polar AlGaN is an emerging wide-bandgap semiconductor for next-generation high electron mobility transistors and ultraviolet light emitting diodes and lasers. Here, we demonstrate the growth and characterization of high-quality N-polar AlGaN films on C-face 4H-silicon carbide (SiC) substrates by molecular beam epitaxy. On optimization of the growth conditions, N-polar AlGaN films exhibit a crack free, atomically smooth surface (rms roughness ∼ 0.9 nm), and high crystal quality with low density of defects and dislocations. The N-polar crystallographic orientation of the epitaxially grown AlGaN film is unambiguously confirmed by wet chemical etching. We demonstrate precise compositional tunability of the N-polar AlGaN films over a wide range of Al content and a high internal quantum efficiency ∼74% for the 65% Al content AlGaN film at room temperature. Furthermore, controllable silicon (Si) doping in high Al content (65%) N-polar AlGaN films has been demonstrated with the highest mobility value ∼65 cm2/V-s observed corresponding to an electron concentration of 1.1 × 1017 cm−3, whereas a relatively high mobility value of 18 cm2/V-s is sustained for an electron concentration of 3.2 × 1019 cm−3, with an exceptionally low resistivity value of 0.009 Ω·cm. The polarity-controlled epitaxy of AlGaN on SiC presents a viable approach for achieving high-quality N-polar III-nitride semiconductors that can be harnessed for a wide range of emerging electronic and optoelectronic device applications.

Angle-Dependent Raman Scattering Studies on Anisotropic Properties of Crystalline Hexagonal 4H-SiC.

Raman scattering spectroscopy (RSS) has the merits of non-destructiveness, fast analysis, and identification of SiC polytype materials. By way of angle-dependent Raman scattering (ADRS), the isotropic characteristics are confirmed for c-face 4H-SiC, while the anisotropic properties of a-face 4H-SiC are revealed and studied in detail via combined experiments and theoretical calculation. The variation functional relationship of the angle between the incident laser polarization direction and the parallel (perpendicular) polarization direction was well established. The selection rules of wurtzite 4H-SiC are deduced, and the intensity variations of the A1, E2, and E1 Raman phonon modes dependent on the incident angle are calculated, and well-matched with experimental data. Raman tensor elements of various modes are determined.

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.

High-mobility 2D electron gas in carbon-face 3C-SiC/4H-SiC heterostructure with single-domain 3C-SiC layer

We epitaxially grew a single-domain 3C layer on a step-controlled C-face 4H-SiC substrate to create a 3C/4H-SiC heterostructure. The effectiveness of using such a substrate to grow a thin highly crystalline single-domain 3C layer was demonstrated. The heterostructure exhibited an electron Hall mobility of 7224 cm2/V s at 32 K, which is more than one order of magnitude higher than the best value reported for this structure. From a comparison with the value of 134 m2/V s for a structure with a multi-domain 3C layer, we attribute the high mobility to single-domain formation. The nearly constant sheet carrier density of ∼1.5 × 1013 cm−2 in the temperature range from 34 to 573 K and the high mobility suggest that conduction occurred in a two-dimensional electron gas. Thus, the high potential of C-face 3C/4H heterostructures for high electron mobility transistor applications is displayed.

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.

Porous AlN films grown on C-face SiC by hydride vapor phase epitaxy

We report the growth of porous AlN films on C-face SiC substrates by hydride vapor phase epitaxy (HVPE). The influences of growth condition on surface morphology, residual strain and crystalline quality of AlN films have been investigated. With the increase of the V/III ratio, the growth mode of AlN grown on C-face 6H-SiC substrates changes from step-flow to pit-hole morphology. Atomic force microscopy (AFM), scanning electron microscopy (SEM) and Raman analysis show that cracks appear due to tensile stress in the films with the lowest V/III ratio and the highest V/III ratio with a thickness of about 3 μm. In contrast, under the medium V/III ratio growth condition, the porous film can be obtained. Even when the thickness of the porous AlN film is further increased to 8 μm, the film remains porous and crack-free, and the crystal quality is improved.

Optimization of thermal oxidation for 4H-SiC and fabricate/characterize MOS capacitor

On the 4H-SiC substrate, C-face and Si-face oxide layers have been grown by thermal oxidation process and sputtering. The thermal oxidation temperature dependence of 4H-silicon carbide (SiC) is systematically investigated using capacitance-voltage (C–V) measurements. The oxidation quality and thickness vary according to the temperature and time duration of the thermal oxidation. The layers’ thicknesses are determined by atomic force microscopy (AFM), and the temperature range is between 800°C and 1110 °C. The primary reason to fabricate the Metal-Oxide-Silicon (MOS) capacitor is to know the thermal oxidation process and a working principle. In this paper, we optimize a thermal oxidation process and fabricate the MOS structure. Then we determine the various parameters such as flat band voltage (Vf b), Inversion threshold voltage (Vt), Surface depletion capacitance (Cdep), Oxide capacitance (Cox), the total capacitance of the device (Co), doping concentration (Nd), Depletion width (Xd), Maximum depletion width (Xdt) and Interface trap density (Dit). Finally, we analyze and discuss the MOS capacitance.

Revelation of the dislocations in the C-face of 4H-SiC substrates using a microwave plasma etching treatment

A novel microwave plasma etching technique was used to reveal various types of dislocation on the C-face of 4H-SiC.

Investigation of Factors Influencing the Occurrence of 3C-Inclusions for the Thick Growth of on-Axis C-Face 4H-SiC Epitaxial Layers.

In this study, we grew homoepitaxial layers on 3-inch on-axis carbon-face 4H-silicon carbide substrates and attempted to suppress the generation of 3C-inclusions. It was found that the 3C-inclusion density decreased with increasing time spent on reaching an objective flow rate for the precursors. It is suggested that 3C-SiC nucleation occurred on large terraces of the on-axis substrates, which existed before the substrates were covered with spiral hillocks. This nucleation was suppressed owing to the decrease in the degree of supersaturation at the initial growth stage. Moreover, we found that the 3C-inclusions were also generated owing to contamination in the form of graphite products. Furthermore, we succeeded in growing a thick on-axis 4H-SiC homoepitaxial layer on a 3-inch substrate and demonstrating its free-standing epitaxial layer with a thickness of 182 μm and a 3C-inclusion density of 2.0 cm−2.

Etching Rate Profile of C-Face 4H-SiC Wafer Depending on Total Gas Flow Rate of Chlorine Trifluoride and Nitrogen

A 50-mm diameter silicon carbide wafer thinning technique by means of a chemical reaction using a chlorine trifluoride (ClF3) gas was studied accounting for the gas distributor design and the total gas flow rate. The entire etching depth profile could become uniform with the increasing total gas flow rate at the fixed chlorine trifluoride gas concentration. A relationship between the pinhole arrangement of the gas distributor and the local etching rate profile was clarified by comparing the quick calculation and the measurement.

The Effect of Surface Polarity on the CMP Behavior of 6H-SiC Substrates

The chemical mechanical planarization (CMP) performance of the Si-face and C-face 6H-SiC substrates were compared using potassium permanganate (KMnO4) as an oxidizer and the alumina (Al2O3) nanoparticles as the abrasive particles over a pH range from 2 to 10. The results indicate that there was a significant difference in the CMP performance between Si-face and C-face 6H-SiC substrates, indicating that the CMP process was sensitive to the surface polarity of the 6H-SiC substrates. A higher material removal rate (MRR) was obtained during the CMP of C-face 6H-SiC substrates, as compared to that of Si-face 6H-SiC substrates. The maximum MRR of the C-face 6H-SiC substrates was reached to 6412 nm/h with an average surface roughness Ra of 0.54 nm when using the slurries containing 2 wt% alumina nanoparticles, 0.05 M KMnO4 at pH 2. While the Si-face 6H-SiC substrates was 1554 nm/h with an average surface roughness Ra of 0.53 nm. Furthermore, the polishing mechanism was also discussed based on the MRR, atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS) analysis of the dipped and polished 6H-SiC surfaces.

Development of 4H-SiC Schottky np diode with high blocking voltage and ultralow on-resistance

A Schottky np diode (SNPD) was fabricated on a 4H-SiC C-face epitaxial wafer, and its I-V characteristics were investigated. The diode showed a high blocking voltage of 300 V and ultralow on-resistance of 0.18 m Ω cm2 at a forward bias of 2.4 V. This value is almost the same as the resistance of the 4H-SiC bulk substrate, indicating that the resistance of the drift layer is almost zero and does not contribute to the observed on-resistance. As the temperature was increased, the forward I-V curve moved in parallel as the built-in bias shifted to a lower voltage due to the reduction in the barrier height of the np junction. This means that the resistance of SNPD above built-in bias is the same regardless of temperature. These results suggest that the diode is different from the conventional Schottky diodes and pn diodes. This diode has great potential for devices with both ultralow on-resistance and high blocking voltage.

Origin of layer decoupling in ordered multilayer graphene grown by high-temperature sublimation on C-face 4H-SiC

We study the origin of layer decoupling in ordered multilayer graphene grown by high temperature sublimation on C-face 4H-SiC. The mid-infrared optical Hall effect technique is used to determine the magnetic field dependence of the inter-Landau level transition energies and their optical polarization selection rules, which unambiguously show that the multilayer graphene consists of electronically decoupled layers. Transmission electron microscopy reveals no out-of-plane rotational disorder between layers in the stack, which is in contrast to what is typically observed for C-face graphene grown by low temperature sublimation. It is found that the multilayer graphene maintains AB-stacking order with increased interlayer spacing by 2.4%–8.4% as compared to highly oriented pyrolytic graphite. Electron energy loss spectroscopy mapping reveals Si atoms trapped in between layers, which are proposed to be the cause for the observed increased interlayer spacing leading to layer decoupling. Based on our results, we propose a defect-driven growth evolution mechanism for multilayer graphene on C-face SiC via high temperature sublimation.

Growth of high-quality 4H-SiC epitaxial layers on 4° off-axis C-face 4H-SiC substrates

• 4H-SiC epilayers are prepared on C-face substrates. • Surface morphologies show a strong dependence on the growth conditions. • A specular surface morphology was obtained. 4H-Silicon carbide (SiC) epitaxial layers on the C-face SiC substrates are potentially useful for fabricating high-performance power SiC MOSFET device applications. In this research, 4H-SiC epilayers are prepared on 4° off-angle C-face 4H-SiC substrates through a low pressure chemical vapor deposition (CVD). Surface morphologies of the epilayers show a strong dependence on C/Si ratio, growth temperature and etching time. A specular surface morphology was obtained at the temperature of about 1550 °C, with the etching time of 15 min and C/Si ratio of 1.2.

Electrically detected-magnetic-resonance identifications of defects at 4H-SiC(0001¯)/SiO2 interfaces with wet oxidation

We present electrically detected-magnetic-resonance (EDMR) identification of major and minor interface defects at wet-oxidized 4H-SiC(0001¯)/SiO2 interfaces for C-face 4H-SiC metal-oxide-semiconductor field-effect transistors. The major interface defects are identified as c-axial types of carbon-antisite-carbon-vacancy (CSiVC) defects. Their positive (+1) charge state generates a spin-1/2 EDMR center named “C-face defects” and behaves as an interfacial hole trap. This center is responsible for the effective hydrogen passivation of the C face. We also identify a minor type of interface defect at this interface called “P8 centers,” which appear as spin-1 centers. Judging from their similarity to the P7 centers (divacancies, VSiVC) in SiC, they were assigned to be a sort of basal-type interfacial VSiVC defect. Since both the CSiVC and VSiVC defects are known as promising single photon sources (SPSs) in SiC, the wet oxidation of the C face will have good potential for developing SPSs embedded at SiC surfaces.