Vicinal 6H-SiC Research Articles

Morphological Transition of Vicinal 4H-SiC Surface Observed during Repeated Annealing

Morphological Transition of Vicinal 4H-SiC Surface Observed during Repeated Annealing

Stepped morphology on vicinal 3C- and 4H-SiC (0001) faces: A kinetic Monte Carlo study

Stepped morphologies on vicinal 3C- and 4H-SiC (0001) surfaces with the miscut toward [11¯00] or [112¯0] directions have been studied with a three-dimensional kinetic Monte Carlo model. In the model, a three-dimensional lattice mesh was established based on the crystal lattice of 3C-and 4H-SiC to fix the positions of atoms and interatomic bonding. Periodic boundary conditions were applied in the lateral direction while helicoidal boundary conditions were used in the direction of crystal growth. Events, such as adatoms attachment, detachment and interlayer transport at the step edges, and adatoms adsorption and diffusion on the terraces were considered in the model. Effects of Ehrlich–Schwoebel barriers at downward step edges and incorporation barriers at upwards step edges were also considered. Moreover, the atoms of silicon and carbon were treated as the minimal diffusing species independently to achieve more elaborate information for the behavior of atoms in the crystal surface. The simulation results showed that multiple-height steps were formed on the vicinal 4H-SiC (0001) surfaces, whereas single bilayer-height stepped morphologies were observed on the vicinal 3C-SiC (0001) surfaces. Furthermore, zigzag shaped edges were observed for both of 3C- and 4H-SiC (0001) surfaces with the miscut toward [112¯0] direction. At last, the formation mechanism of the stepped morphology was also analyzed.

Wide (0001) terrace formation due to step bunching on a vicinal 4H-SiC (0001) epitaxial layer surface

The wide (0001) terrace formation due to step bunching on a vicinal 4H-SiC (0001) epitaxial layer surface was investigated using low-energy electron channeling contrast (LE-ECC) imaging and atomic force microscopy. LE-ECC imaging revealed that step bunching resulted in the formation of wide atomically-flat (∼200 nm) (0001) terraces on the surface, and the terraces tended to form in pairs. Terraces in a pair had almost the same width and often showed the same LE-ECC; moreover, the contrast of the two terraces, either bright or dark, appeared to be determined by the orthogonal misorientation of substrates. On the basis of these results, the formation mechanism of the paired terraces with the same LE-ECC on a vicinal 4H-SiC (0001) surface is discussed herein.

SEM and ECC Imaging Study of Step-Bunched Structure on 4H-SiC Epitaxial Layers

Step bunching on a vicinal 4H-SiC (0001) epitaxial layer surface was investigated using low-voltage electron scanning microscopy (LVSEM) and electron channeling contrast (ECC) imaging. LVSEM observations revealed that the step bunching resulted in the formation of atomically flat wide (~250 nm) terraces on the surface, and the terraces tended to form in pairs. The two terraces in paired terraces often showed the same electron channeling contrast as each other, and the contrast of the two terraces, either bright or dark, appeared to be determined by the orthogonal misorientation of substrates. On the basis of these results, the formation mechanism of the step-bunched structure on a vicinal 4H-SiC (0001) surface is discussed.

Investigation of Low Off-Angled 4H-SiC Epitaxial Wafers for Power Device Applications

We investigated the potential of low off angled 4H-SiC epitaxial wafers for use in power device applications. To this end, we successfully grew epitaxial layers on 2 degree off-angled and vicinal 4H-SiC Si-face substrate with good surface morphology and low defects density. Schottky diodes, metal oxide semiconductor (MOS) capacitors, and trench metal oxide semiconductor field effective transistors (MOSFETs) were fabricated on these epitaxial layers, and we investigated their characteristics. The results were comparable, with respect to the Schottky diodes and trench MOSFETs and the reliability of MOS capacitors, to those for similar devices fabricated on 4 degree off-angled 4H-SiC Si-face epitaxial wafers. These results suggest that low off-angled 4H-SiC Si-face epitaxial wafers may be effectively utilized in power device applications.

Origin of the Current Transport Anisotropy in Epitaxial Graphene Grown on Vicinal 4H-SiC (0001) Surfaces

In this paper, the electronic transport in epitaxial graphene (EG) grown on the Si face of 8° off-axis 4H-SiC has been investigated, using both electrical characterization of macroscopic devices and conductive atomic force microscopy (CAFM). In particular, current measurements on linear transmission line model (TLM) structures with different orientations showed a current transport anisotropy related to steps orientation, with the resistance of EG in the direction orthogonal to the steps ~2× higher than in the parallel direction. Two dimensional morphology and current maps in EG over the stepped SiC surface were obtained by CAFM and revealed a local resistance increase of EG over the (11-2n) facets with respect to the (0001) basal planes. This effect allows to account for the observed macroscopic current transport anisotropy and can be explained in terms of a different interface nature between EG and SiC on the two faces, leading to a locally different substrate induced doping of EG.

Step-controlled homoepitaxial growth of 4H–SiC on vicinal substrates

A comprehensive study on the step-controlled homoepitaxial growth on the (0001)Si-face of vicinal 4H–SiC substrates was performed in order to establish epitaxial growth on 2° towards 〈112¯0〉 off-cut substrates and 4° towards 〈11¯00〉 off-cut substrates. A standard epitaxial growth process was developed by optimizing the growth temperature T, Si/H ratio and C/Si ratio for growth on 4° towards 〈112¯0〉 off-cut substrates. Thereby, step-controlled epitaxial growth was achieved within a broad operating window. The surface roughness of such epilayers varies typically between rms=0.5nm and rms=2.5nm and step-controlled growth is conserved even at a growth rate of 27μm/h. Then, the standard growth process was applied to substrates with different off-cut angles α of 2°, 4° and 8° as well as with different off-cut directions 〈112¯0〉 and 〈11¯00〉. The step-controlled growth was achieved also within a wide range of Si/H ratio and C/Si ratio for growth on 8° and 4° off-cut substrates, but the process window narrows strongly for growth on 2° off-cut substrates. The epilayers' surface roughness increases with decreasing off-cut angle of the substrate. Epilayers grown on 4° towards 〈11¯00〉 off-cut substrates were significantly smoother than epilayers grown on 4° towards 〈112¯0〉 off-cut substrates. No influence of the substrates' off-cut angle and direction on the growth rate was found. The experimental results of this comprehensive study are discussed globally in consideration of other relevant publications.

Reduction of Threading Screw Dislocation Utilizing Defect Conversion during Solution Growth of 4H-SiC

Reduction of threading screw dislocation without polytype transformation from 4H-SiC was performed by the combination of step-flow growth and spiral growth. On a vicinal 4H-SiC seed crystal, threading screw dislocations are converted to Frank-type stacking faults by step-flow during solution growth. As the growth proceeds, the defects are excluded to the crystal. Thus utilizing the conversion, high quality SiC crystal growth without threading screw dislocations is expected to achieve. However, at the same time, polytype transformation is caused by the occurrence of 2D nucleation. By using the special shape of seed crystal, we successfully grew high quality 4H-SiC crystal without threading screw dislocation and polytype transformation.

Optical and structural characterization of epitaxial graphene on vicinal 6H-SiC(0001)–Si by spectroscopic ellipsometry, Auger spectroscopy, and STM

The authors report results of spectroscopic ellipsometry (SE) measurements in the near-IR, visible, and near-UV spectral ranges using a Woollam dual rotating-compensator ellipsometer, analyzing data in terms of both epitaxial graphene and interface contributions. The SiC samples were cleaned by standard methods of CMP and HF etching prior to mounting in UHV and growing epitaxial graphene by thermal annealing at ∼1400 °C. Most samples were vicinally cut 3.5° off (0001) toward [11−20]. STM measurements show that the initial regular step edges were replaced by somewhat irregular edges after graphene growth. From growth-temperature and Auger data the authors estimate that the graphene is ∼3–4 ML thick. The authors find significant differences among the spectral features of the interface “buffer” layer and those of graphene. Specifically, the hyperbolic-exciton peak reported previously at ∼4.5 eV in graphene shifts to a similarly shaped peak at ∼4 eV in the interface buffer layer. The authors attribute this shift to a significant component of sp3 bonded carbon in the buffer, which occurs in addition to the sp2 bonded carbon that is present in the graphene layer. SE data in the terahertz range obtained by Hoffman et al. [Thin Solid Films 519, 2593 (2011)] show that the mobility values of graphene grown on the carbon face of SiC vary with proximity to the substrate. This leads to the question as to whether an interface layer at the Si face has properties (i.e., dielectric function/complex refractive index) that are different from and/or affect those of the graphene layers.

A Raman imaging study of growth process of few-layer epitaxial graphene on vicinal 6H–SiC

Few-layer epitaxial graphenes grown on vicinal 6H–SiC (0001) were characterized by confocal Raman imaging. In the beginning of the growth, the surface of SiC substrate was covered with monolayer graphene. Next, few-layer graphenes started to grow toward directions perpendicular to [11–20] of the SiC substrate. The shift in the G-peak was not straightforward with the increase in number of graphene layers. This result can be interpreted that the in-plane compressive stress from the substrate depends on the domain size of graphene. The 2D-peak frequency shifted to higher frequency side due to strong compressive strain from the substrate with increasing of the growth times.

Structural characterization of ZnO nanopillars grown by atmospheric-pressure metalorganic chemical vapor deposition on vicinal 4H-SiC and SiO2/Si substrates

The structural characteristics of ZnO nanocrystals epitaxially grown on p-type (0001) 4H-SiC substrates were studied by transmission electron microscopy (TEM). The nanocrystallites were grown by atmospheric-pressure metalorganic chemical vapor deposition. The ZnO nanocrystals were formed at terraces introduced by vicinal 4H-SiC substrates toward the [112¯0] direction. They had the shape of hexagonal nanopillars, with their edges parallel to the 〈112¯0〉 directions and a top c-plane facet, reflecting the crystal symmetry of ZnO. The free surface between the hexagonal nanopillars was covered by a very thin and highly defected epitaxial ZnO film, which strongly suggests the Stranski-Krastanov mode of growth. The ZnO/SiC interface was systematically studied by plane view TEM and cross sectional high resolution TEM. The residual strain in the thin continuous film as well as in the nanopillars was estimated from Moiré patterns and by geometrical phase analysis. ZnO was also deposited on the SiO2/Si substrate for comparison. The films were polycrystalline exhibiting strong preferred orientation, with the c-axes of the grains almost perpendicular to the substrate resulting in the formation of nanopillars. The differences of nanopillar formation in the two substrates, 4H-SiC and SiO2 is also discussed.

Few-layer epitaxial graphene grown on vicinal 6H–SiC studied by deep ultraviolet Raman spectroscopy

Few layer epitaxial graphenes (1.8–3.0 layers) grown on vicinal 6H–SiC (0001) were characterized by deep ultraviolet Raman spectroscopy. Shallow penetration depth of the probe laser enabled us to observe G-peak of graphene without subtraction of the SiC substrate signal from observed spectra. The G-peak was greatly shifted to higher frequency compared to that of graphite due to in-plane compressive stress deriving from the substrate. The frequency shift decreased with the number of graphene layers because of stress relaxation from layer to layer. Our experiment suggests that the stress is completely relaxed within five to six graphene layers.

Deep UV Raman Spectroscopy of Epitaxial Graphenes on Vicinal 6H-SiC Substrates

We report microscopic Raman scattering studies of epitaxial graphene grown on SiC substrates using a deep-ultraviolet (UV) laser excitation at 266 nm to elucidate the interaction between the graphene layer and the substrate. The samples were grown on the Si-face of vicinal 6H-SiC (0001) substrates by sublimation of Si from SiC. The G band of the epitaxial graphene layer was clearly observed without any data manipulation. Increasing the number of graphene layers, the peak frequency of the G-band decreases linearly, while the peak width and the intensity increase. The G-band frequency of the graphene layers on SiC is higher than those of exfoliated graphene, which has been ascribed to compression from the substrate.

Growth mechanism for epitaxial graphene on vicinal6H-SiC(0001)surfaces: A scanning tunneling microscopy study

The inability to grow large well-ordered ultra high vacuum (UHV) graphene with a specific number of layers on SiC(0001) is well known. The growth involves several competing processes (Si desorption, carbon diffusion, island nucleation, etc.) and because of the high temperatures, it has not been possible to identify the growth mechanism. Using scanning tunneling microscopy and a vicinal 6H-SiC(0001) sample, we determine that the Si desorption from steps is the main controlling process. Adjacent steps retract with different speeds and the released carbon produces large areas of bilayer graphene with characteristic ``fingers'' emanating from steps. If faster heating rates are used, the different Si desorption rates are avoided and single-layer graphene films extending over many microns are produced.

Stable surface termination on vicinal 6H–SiC(0 0 0 1) surfaces

Ordered nanofacet structures on vicinal 6H–SiC(0 0 0 1) surfaces, consisting of pairs of a (0 0 0 1) basal plane and a ( 1 1 2 ¯ n ) facet, are investigated in terms of stable surface stacking of the (0 0 0 1) basal planes. The surface termination of S3 (or S3*), i.e., ABC (or A*C*B*), was suggested by a structural model based on quantized step-bunching, which typically gives a one-unit-cell bunched step configuration at the ( 1 1 2 ¯ n ) facet. Here, we evaluate the surface termination at basal planes covered with a layer of silicon oxynitride by means of quantitative low-energy electron diffraction (LEED) analysis combined with scanning tunneling microscopy (STM), and show the validity of the structural model proposed.

Strain relaxation in GaN grown on vicinal 4H-SiC(0001) substrates

The strain of GaN layers grown by metal organic chemical vapor deposition on three vicinal 4H-SiC substrates (0°, 3.4°, and 8° off cut from [0001] towards [11−20] axis) is investigated by x-ray diffraction, Raman scattering, and cathodoluminescence. The strain relaxation mechanisms are analyzed for each miscut angle. At a microscopic scale, the GaN layer grown on on-axis substrate has a slight and homogeneous tensile in-plane stress due to a uniform distribution of threading dislocations over the whole surface. The GaN layers grown on miscut substrates presented cracks, separating areas which have a stronger tensile in-plane stress but a more elastic strain. The plastic relaxation mechanisms involved in these layers are attributed to the step flow growth on misoriented surfaces (dislocations and stacking faults) and to the macroscopical plastic release of additional thermoelastic stress upon cooling down (cracks).

Superior material properties of AlN on vicinal 4H-SiC

The crystal structure and optical properties of thick (>100nm) AlN layers grown by hot-wall metalorganic chemical vapor deposition are characterized by infrared spectroscopic ellipsometry, cathodoluminescence, and transmission electron microscopy. The choice of substrates among the available SiC wafer polytype modifications (4H∕6H) and misorientations (on-/off-axis cut) is found to determine the AlN defect interaction, stress homogeneity, and luminescence. The growth of thick AlN layers benefits by performing the epitaxy on off-axis substrates because, due to stacking faults, the propagation of threading defects in AlN layers is stopped in a narrow interface region.

반응성 RF 마그네트론 스퍼터링 법을 이용한 AIN/SiC 구조의 제작 및 특성

Al/AlN/n-type 6H-SiC (0001) MIS structures were prepared by AlN layers on vicinal 6H-SiC(0001) substrates with reactive RF magnetron sputtering method. The AlN films were annealed at <TEX>$900^{\circ}C$</TEX>, <TEX>$N_2$</TEX> atmosphere lot 1 minutes showed the best result. With XRD analysis, AlN(0002) peak was clearly found. The typical dielectric constant value of the AlN film in the MIS capacitors was obtained as 8.4 from photo C-V. Also, the gate leakage current density of the MlS capacitor was <TEX>$10^{-10}\;A/cm^2$</TEX> order within the electric field of 1.8 MV/cm. Finally, the amount of interface trap densities, <TEX>$D_{it}$</TEX>, was evaluated as <TEX>$5.3\times10^{10}\;eV^{-1}cm^{-2}$</TEX> at (Ec-0.85) eV.

Intersecting basal plane and prismatic stacking fault structures and their formation mechanisms in GaN

A systematic study of intersecting planar boundary structures observed in a GaN epifilm grown on a vicinal 6H-SiC substrate (offcut towards [12¯10]) with an AlN buffer is presented. These structures are shown to comprise stacking faults that fold back and forth from the basal plane [I1 basal plane stacking faults (BSFs)] to the prismatic plane [prismatic stacking faults (PSFs)]. The PSFs, with fault vector 12⟨101¯1⟩ nucleate at steps on the substrate surface as a consequence of the different stacking sequences exposed on either side of the step. Once nucleated, PSFs intersecting the vertical step risers in the AlN buffer and eventually in the GaN film are replicated during the predominantly step-flow growth and propagate into the growing crystal. As a consequence of the different growth rates experienced on either side of the intersection of a PSF with a vertical step riser, the PSF may be redirected onto an equivalent {112¯0} plane, leaving an I1 BSF between the bottom of the redirected section of PSF and the top of that portion of the original PSF which was below the terrace. This leads to the formation of folded PSF/BSF fault structures which exhibit various configurations. Such folded stacking fault configurations form walls which enclose domains of different stacking sequence. Stair-rod dislocations (SRDs) are observed to form at the intersections of these various faults and the Burgers vectors of some of these are discussed. In some cases, reconstruction of these SRDs occurs through the formation of partner dislocations which serve to minimize the energy associated with the lattice disconnections located at the cores of the SRDs.

Step control of vicinal 6H–SiC(0001) surface by H2 etching

Anisotropic step bunching on vicinal 6H–SiC(0001) surface induced by H2 etching was investigated. Step structures were observed using atomic force microscopy and high-resolution transmission electron microscopy at off angle &amp;lt;2°. The etched surfaces exhibited three types of step bunching: (1) straight six-bilayer steps, (2) straight six-bilayer steps and nanofacets, and (3) zigzag three-bilayer steps. The step height and density can be controlled by a tilt angle and direction of a vicinal surface. Mechanisms of the step bunching processes are discussed at points of an anisotropy of lateral-etching rates and surface-free energies.