Hexagonal 6H-SiC Research Articles

Growth and Study of Thick 3C-SiC Epitaxial Layers Produced by Epitaxy on 6H-SiC Substrates

3C-SiC epitaxial layers with a thickness of up to 100 μm and area of ~0.3-0.5 cm2 have been grown by sublimation epitaxy on hexagonal (6H-SiC) substrates at a maximum growth rate of about 200 μm per hour. The epilayers obtained are of n-type (Nd-Na ~ 1017 -1018 cm-3). According to X-ray data, the epitaxial layers are composed of the 3C-SiC polytype, without inclusions of other polytypes. The donor-acceptor (Al-N) recombination band with hνmax ~ 2.12 eV predominates in the photoluminescence (PL) spectrum. A detailed analysis of a PL spectrum measured at 6 K is presented. A conclusion is made that the epitaxial layers can be used as substrates for electronic devices based on 3C-SiC.

Thin, crystalline MgO on hexagonal 6H-SiC(0001) by molecular beam epitaxy for functional oxide integration

MgO thin films are proposed as a template for the effective integration of three and four element oxides on wide band gap SiC for next generation multifunctional devices. Oriented, crystalline MgO(111) of 20–380Å is grown on 6H-SiC(0001) by molecular beam epitaxy at a substrate temperature of 140°C using a magnesium effusion cell and a remote oxygen plasma source with ion deflection plates located at the end of the plasma discharge tube and approximately 7in. from the sample surface. Films are conformal to the steps of the cleaned SiC surface with a rms roughness of 0.45±0.05nm. Magnesium adsorption controls the growth rate in an excess oxygen environment with Mg:O flux ratios of 1:99–1:20, where the oxygen flux is the equivalent molecular oxygen. The oxygen plasma, which was determined to be free of ions when the ion deflection plates are energized, does impact nucleation and initial stages of the MgO film formation, and there may be evidence of etching mechanisms involved in the thicker film growth. Chemical and structural thermal stability of 20Å MgO(111)‖6H-SiC(0001) was demonstrated up to 740°C in vacuum for 90min through reflection high-energy electron diffraction and x-ray photoelectron spectroscopy analyses. X-ray diffraction was used to further test the thermal stability of 380Å films in vacuum and in an oxygen environment up to 790°C. As a proof of concept for MgO(111) as an interface for aligned functional oxide growth, barium titanate (111) was deposited on 100Å MgO(111)‖6H-SiC(0001) by rf magnetron sputtering.

Influence of substrate temperature on growth of nanocrystalline silicon carbide by reactive magnetron sputtering

Hydrogenated nanocrystalline silicon carbide were grown at various deposition temperatures Td from 200to600°C by means of reactive magnetron sputtering in a plasma of 80% H2 and 20% Ar mixture. A detailed investigation of the structural, compositional, phase nature, and morphology was carried out by complementary sophisticated techniques, such as Fourier transform infrared spectroscopy, x-ray diffraction (XRD), Rutherford backscattering, nuclear reaction, and elastic recoil detection analysis techniques, in addition to conventional and high-resolution transmission electron microscopy (HRTEM) observations. A crystallization onset with a fraction of 35% was observed for Td=300°C, which improved to 80% for Td=600°C, reflected by an increasing density of the SiC nanocrystals which kept an average size of about 5nm. The observed fiber textures present ⟨102⟩ and ⟨11ℓ⟩ texture components, with ℓ larger than 2, while SiC nanocrystals elongated along the [111] direction are also evidenced. These latter are supported by the careful analyses of the HRTEM images which show evidence of faulted growing cubic SiC, as the origin of the very close hexagonal 6H–SiC structure taken into account in the XRD refinement. These various features were found quite consistent with the optical properties of the layers, and, in particular, the evolutions of both optical gap and static refractive index.

First-principles studies of the electronic and optical properties of 6H–SiC

We study the electronic and optical properties of hexagonal 6H–SiC crystal, using ab initio full potential augmented plane wave method. The density of states (DOS) and band structure are presented based on local density function theory. From the electronic structure calculation, the imaginary part of the dielectric function has been obtained directly using the joint DOS and the optical matrix elements. With band gap correction, the real part of dielectric function can be derived from the imaginary part by the Kramers–Kronig relationship. The reflectivity for normal incidence is also calculated. The resulting spectrum is in good agreement with available experimental data in a wide energy range.

Cone-shaped hexagonal 6H–SiC nanorods

In this Letter, we report the synthesis of cone-shaped 6H–SiC nanorods by an arc-discharge process. The axis of nanorods is preferentially along the 〈1 0 1 ̄ 0〉 direction. The diameters of the thin tips are about 10–30 nm , and the lengths range from tens of nanometers up to 1 μm . Branched nanostructures are also formed and the branches form 60° or 120° angles with the main nanorods. The formation of gradually enlarged Fe–Al–Si–C droplets and their high-enthalpy have led to the growth of cone-shaped hexagonal structure SiC nanorods by a VLS mechanism.

K-resolved inverse photoemission of four different 6H-SiC (0001) surfaces

We have investigated the unoccupied electronic states of the Si-terminated (0001) surface of hexagonal 6H-SiC. The main problem with these surfaces is the reliable preparation of well defined surface reconstructions. We give reproducible methods to prepare the $(1\ifmmode\times\else\texttimes\fi{}1),$ $(\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3})R30\ifmmode^\circ\else\textdegree\fi{},$ $(3\ifmmode\times\else\texttimes\fi{}3),$ and the $(6\sqrt{3}\ifmmode\times\else\texttimes\fi{}6\sqrt{3})R30\ifmmode^\circ\else\textdegree\fi{}$ surface by controlled heating of the SiC sample in a Si flux. These surface reconstructions show a characteristic LEED pattern and a characteristic Si/C peak ratio in Auger electron spectroscopy. We present k-resolved inverse photoemission spectra for the $(1\ifmmode\times\else\texttimes\fi{}1),$ $(\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3})R30\ifmmode^\circ\else\textdegree\fi{},$ and $(3\ifmmode\times\else\texttimes\fi{}3)$ surface. We compare the measured dispersion relations with ab initio local density approximation surface band structure calculations of the $(1\ifmmode\times\else\texttimes\fi{}1)$- and the $(\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3})R30\ifmmode^\circ\else\textdegree\fi{}$-reconstructed 6H-SiC(0001) surface and with a Mott-Hubbard model of the electronic ground state of the $(\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3})R30\ifmmode^\circ\else\textdegree\fi{}$ and $(3\ifmmode\times\else\texttimes\fi{}3)$ reconstruction. The comparison between experiment and theory supports the Hubbard model: The experiment determines a value of $U=2.0\mathrm{eV}$ for the Mott-Hubbard Coulomb interaction parameter for the $(\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3})R30\ifmmode^\circ\else\textdegree\fi{}$ reconstruction and $U=1.25\mathrm{eV}$ for the $(3\ifmmode\times\else\texttimes\fi{}3)$-reconstructed surface, respectively.

The [formula omitted] reconstruction on hexagonal 6H–SiC([formula omitted]) surface with and without a Si flux

The (3×3)R30° reconstruction on hexagonal 6H–SiC(0001) surface and its properties were studied by using reflection high-energy electron diffraction (RHEED) and RHEED multislice dynamical calculations. A Si coverage of 1/3 monolayer occupying the threefold-symmetric T4 or H3 sites develops upon a lower Si flux and/or at a higher annealing temperature and/or upon a longer time annealing. However, Si trimers centered on the T4 positions with 1 monolayer coverage may be another appropriate candidate for the (3×3)R30° reconstruction obtained under a very Si-rich condition and/or at a lower annealing temperature and/or upon a shorter time annealing.

The Coulomb effects in the dynamics of polar lattices

Zone-center phonon frequencies of polar lattices are calculated for uniaxial crystals from the symmetry arguments. Long-range Coulomb forces and crystal anisotropy are explicitly taken into account. Free-carrier contributions into a dielectric constant are included. The angular dispersion of optical-phonon modes is compared with data for hexagonal 6H-SiC politype.

Electronic and chemical passivation of hexagonal 6H–SiC surfaces by hydrogen termination

Hydrogenation of 6H–SiC (0001) and (0001̄) is achieved by high-temperature hydrogen treatment. Both surfaces show a low-energy electron diffraction pattern representative of unreconstructed surfaces of extremely high crystallographic order. On SiC(0001), hydrogenation is confirmed by the observation of sharp Si–H stretching modes. The absence of surface band bending for n- and p-type samples is indicative of electronically passivated surfaces with densities of charged surface states in the gap of below 7×1010 cm−2 for p-type and 1.7×1012 cm−2 for n- type samples, respectively. Even after two days in air, the surfaces show no sign of surface oxide in x-ray photoelectron spectroscopy.

Atomic models of (√3×√3)R30° reconstruction on hexagonal 6H–SiC(0001) surface

By using reflection high-energy electron diffraction (RHEED) and RHEED multislice dynamical calculations, the atomic structures of the (∛×∛)R30° reconstruction on 6H–SiC(0001) surface were solved. Both the simple adatom structure with a Si coverage of one-third monolayer occupying the threefold-symmetric T4 or H3 sites and a bit complex structure with Si trimers centered on the T4 positions with 1 monolayer coverage are all compatible with our results.

Ion beam synthesis of semiconductor nanoparticles for Si based optoelectronic devices

Intense white (to the eye) luminescence has been obtained by multiple implantation of Si + and C + ions into thermal SiO 2 and a post-implantation annealing process. This white emission is a consequence of the convolution of three luminescence peaks centred at about 1.45 eV (infrared with a long tail in the red), 2.1 eV (yellow) and 2.8 eV (blue). These emissions have been correlated to the synthesis of nanocrystals of Si and SiC, and the existence of C-rich precipitates. Cross section TEM shows a buried layer with dark contrast, which correlates with the maximum of the C implanted profile, and likely with a high density of C-rich amorphous domains. Besides, two kinds of nanocrystalline precipitates are found, which have been identified as Si and hexagonal 6H–SiC by electron diffraction experiments. To our knowledge, these data provide the first experimental evidence on the ion beam synthesis of nanocrystalline 6H–SiC embedded in SiO 2. Correlation with previous data gives support to the assignment of the infrared, yellow and blue peaks with the Si, C-rich and SiC precipitate phases and/or its interfaces with SiO 2.

The phonon dispersion in 6H-SiC investigated by inelastic neutron scattering

Using coherent inelastic neutron scattering we have investigated the phonon dispersion relation of bulk hexagonal 6H-SiC. The complete set of phonon branches has been determined along the Γ−A and 14 out of 36 branches along the Γ−M direction. The experimental data are compared to ab initio calculations for cubic 3C-SiC transposed to 6H-SiC. Good agreement between theory and experiment is found for the Γ−A direction. The discrepancies encountered for the Γ−M direction are interpreted as evidence for a dependence of the dynamical response in the SiC system on the stacking sequence.

Optical Properties and Temperature Dependence of the Interband Transitions of 3C- and 6H-SiC in the Energy Region 5 to 10 eV

A synchrotron radiation study of the optical properties of cubic (3C) and hexagonal (6H) SiC in the energy region 5 to 10 eV and over the temperature range 90 to 550 K is presented. Analysis of the dielectric function spectra reveals the location and temperature dependence of the occurring interband transitions. The results on 3C-SiC are compared in terms of transition energies with theoretical calculations in the literature and favour the assignment of the first direct gap to the X-point. The relation between the gaps of the polytypes is discussed. The results on the temperature dependence of the first direct gaps of 3C-SiC at the symmetry points Γ, X and L are compared to those of Si and diamond. Finally, the reflectivity spectra of both polytypes indicate very good sample quality and are compared in terms of absolute values and features to the existing experimental and theoretical data in the literature.

Structure of 6H–SiC(0001) Surfaces from AB-Initio Calculations

We report the results of ab-initio calculations of structural properties of hexagonal 6H–SiC(0001) surfaces. The calculations have been carried out self-consistently within local density approximation employing supercell geometries, smooth norm-conserving pseudopotentials in separable form and Gaussian orbital basis sets. We have investigated several structural models for adatom-induced [Formula: see text] reconstructions with adsorbed Si or C adatoms or trimers residing in threefold-symmetric T 4 or H 3 positions above Si- or C-terminated substrate surfaces, respectively. In the case of the Si-terminated substrate surface our results favor Si adatoms in T 4 sites as optimal configuration in very good agreement with experimental data. For the C-terminated substrate surface our results indicate that none of the investigated [Formula: see text] adatom or trimer configurations is the optimal surface structure.

A Model for Double Positioning Twin Formation in Cubic SiC on Noncubic SiC Substrates

ABSTRACTAs an attempt to explain polytype formation and crystal growth of SiC we propose an electrostatic model in which electrostatic Coulomb force is considered as an interatomic force. The dependence of surface potential on the stacking sequence of atoms in substrates was investigated, and the growth simulation was tried considering the energetic balance under various interaction lengths.The model predicts the double positioning twin formation and the difference in area between twins in cubic 3C-SiC growth on hexagonal 6H-SiC and rhombohedral 15R-SiC substrates. The prediction was experimentally tested by simultaneous chemical vapor deposition growth on basal planes of 6H-SiC(0001) and 15R-SiC(0001). Polytype and interface structure of both specimens were investigated by reflection high energy electron diffraction (RHEED) and high resolution transmission electron microscopy (HRTEM).

Polytype-controlled single-crystal growth of silicon carbide using 3C→6H solid-state phase transformation

The phase transformation from 3C-SiC to 6H-SiC at high temperature has been applied to crystal growth of SiC. Single-crystal growth of controlled polytype has been carried out on chemical-vapor-deposition-grown 3C-SiC(001) films by using a sublimation method. The polytype of grown layers was controlled as either cubic 3C-SiC or hexagonal 6H-SiC. The growth temperature was varied in the range of 1800–2400 °C. A growth rate up to several hundred micrometers per hour was achieved. The polytypes, crystallographic planes, and crystallinity of the grown layers were examined using photoluminescence, Raman spectroscopy, x-ray diffraction, reflection high-energy electron diffraction, and molten KOH (potassium hydroxide) etching. The polytype of the grown layers was changed from 3C-SiC to 6H-SiC with an increase in substrate temperature, and 6H-SiC(011̄4) planes were grown on 3C-SiC(001) at high temperatures. The crystallographic relationship between 6H-SiC(0114) and 3C-SiC(001) is described. The growth mechanism of 6H-SiC(011̄4) on 3C-SiC(001) is discussed based on experimental results on the thermal stability of 3C-SiC.

Single crystal growth of hexagonal SiC on cubic SiC by intentional polytype control

Crystal growth of SiC on 3C-SiC(100) substrates has been carried out by using a sublimation method, utilizing the phase transformation from cubic 3C-SiC to hexagonal 6H-SiC at high temperatures. Polytype control in SiC growth on CVD-grown 3C-SiC(100) substrates was successfully achieved for the first time. The growth rate was several hundred microns per hour. The polytypes and crystallographic planes of the grown layers were examined using photoluminescence, Raman spectroscopy, X-ray diffraction and RHEED analyses. The polytypes of the grown layers change from cubic 3C modification to hexagonal 6H modification with increase in temperature. 6H-SiC with (01 1 4) planes can be grown on 3C-SiC(100). Ingots of 6H-SiC up to 15 mm in diameter and 6 mm in length were grown in 6 h. The grown mechanism of 6H-SiC on 3C-SiC substrates based on the experimental results is discussed.

Muonium states in silicon carbide

Implanted muons in samples of silicon carbide have been observed to form paramagnetic muonium centers (μ+ e−). Muonium precession signals in low applied magnetic fields have been observed at 22 K in a granular sample of cubic β-SiC, however it was not possible to determine the hyperfine frequency. In a single crystal sample of hexagonal 6H-SiC, three apparently isotropic muonium states were observed at 20 K and two at 300 K, all with hyperfine frequencies intermediate between those of the isotropic muonium centers in diamond and silicon. No evidence was seen of an anisotropic muonium state analogous to the Mu* state in diamond and silicon.

Thermal expansion of the hexagonal (4H) polytype of SiC

Thermal expansion of the hexagonal (4H) polytype of SiC was measured from 20 to 1000 °C by the x-ray diffraction technique. The principal axial coefficients of thermal expansion are expressed by the second-order polynomials: α11=3.21×10−6+3.56×10−9T−1.62×10−12T2, and α33=3.09×10−6+2.63×10−9T−1.08×10−12T2 (°C−1). The α11 is larger than the α33 over the entire temperature range, yielding a thermal expansion anisotropy ‘‘A’’ of 0.04 at room temperature which increases to 0.11 at 1000 °C. The thermal expansion of the (4H) structure is compared with previously published results for the cubic (3C) and the hexagonal (6H) SiC polytypes.

Reduction of SiC surface oxides by a Ga molecular beam: LEED and electron spectroscopy studies

SiC surface composition and structure appear to be sensitive to the method of surface preparation used in ultra-high vacuum. New diffraction and electron spectroscopy results have been obtained after removal of native oxide by specimen heating in a Ga molecular beam. This process, unlike oxide removal by heating alone, apparently does not deplete the surface of Si. For cubic β-SiC, the results suggest a mosaic gross surface structure with Si oxide residing in grain boundaries. Results are also presented for the hexagonal 6H-SiC polytype.