4H-SiC Polytype Research Articles

Thermochromic Properties of 3C-, 6H- and 4H-SiC Polytypes up to 500°C

The thermochromic properties (color change with temperature) of n type doped SiC wafers of different polytypes (3C, 4H and 6H) have been investigated up to 500°C under air. It was found that 3C-SiC color passes from bright yellow at room temperature to deep orangeat 500°C leading to a color contrast (ΔE) as high as 64. The hexagonal polytypes undergo also a color change upon heating but far less pronounced, with ΔE values <20. All these semiconductors undergo band gap shrinkage upon heating which effect largely participated to the observed color change. This effect is very sensitive for 3C polytypesince its bandgap is already in the visible energy range at room temperature. The thermochromicity of 3C-SiC was found to be reversible thanks to its thermal stability and its resistance towards oxidation.

Formation and stability of point defect color centers in 6H silicon carbide

Point defect color centers acting as single-photon emitters are promising for quantum technology applications and have been extensively studied, e.g., in the 4H polytype of silicon carbide (SiC). However, the physics of such color centers in other SiC polytypes is much less explored. Herein, we study the formation and thermal stability of such color centers in 6H-SiC using photoluminescence spectroscopy. The emissions from typical single-photon emitters, such as silicon vacancies, divacancies, and carbon antisite-vacancy pairs in 6H-SiC, were monitored as a function of the proton irradiation fluence and post-irradiation annealing, and compared to that in 4H-SiC. Overall, at the background of similarities between the emission behavior in 4H- and 6H-SiC polytypes, we observed prominent differences; e.g., for the thermal stability of the carbon antisite-vacancy pair, which exhibited maximized emissions upon 300 and 900 °C anneals in 4H- and 6H-SiC, respectively. Moreover, we observed a range of defect emission signatures not previously reported for 6H-SiC in the literature and discussed their potential origin in the context of the thermal stability. For example, among the PL-lines in 6H-SiC, we detected periodically repeatable emission signatures, resembling the so-called L-lines recently reported in 4H-SiC, even though their exact origin has not yet been settled in the literature. Thus, we use color centers comparison in different polytypes to better understand the nature of the single-photon emitters in SiC.

Zero-Field ODMR and relaxation of Si-vacancy centers in 6H-SiC

Silicon vacancies in silicon carbide (SiC) have been proposed as interesting candidates for quantum technology applications such as quantum sensing and quantum repeaters. SiC exists in many polytypes with different plane stacking sequences, and in each polytype, the vacancies can occupy a variety of different lattice sites. In this work, we focus on the three important charged silicon vacancies in the 6H-SiC polytype. We record the photoluminescence and continuous-wave optically detected magnetic resonance (ODMR) spectra at different radio-frequency power levels and different temperatures. We individually select the zero-phonon lines of the different silicon vacancies at low temperatures and record the corresponding ODMR spectra. ODMR allows us to correlate optical and magnetic resonance spectra and thereby separate signals from V 1 and V 3. The results also explain the observed sign change of the ODMR signal as a function of temperature.

Enhancing densification rate and the unusual 4H-SiC polytype stabilization in ultrafast high-temperature sintering of α-SiC

Novel Powder medium-based Ultrafast High-temperature Sintering (P-UHS) technique brings unexplored opportunities for the ultrarapid consolidation of large-sized and difficult-to-sinter ceramics with a complex shape. Herein, the P-UHS was demonstrated to be very effective means for the densification of large-sized α-SiC bulks (∼16 cm3) with boron and carbon additions under heating rates exceeding 1500 °C/min. Bulk ceramics with ∼97% density and homogenous microstructure were obtained within only 6 min. The ultrafast heating led to two orders of magnitude increase in the densification rate contributing to an energy saving of 90% if compared to conventional sintering process. In addition, the preferential 4H-SiC polytype stabilization was observed only when ultrafast heating rate was employed. The non-equilibrium transient chemistry and the ultrarapid shrinkage were considered as the major drivers for this unusual phenomenon.

NV-centers in SiC: A solution for quantum computing technology?

Spin S = 1 centers in diamond and recently in silicon carbide, have been identified as interesting solid-state qubits for various quantum technologies. The largely-studied case of the nitrogen vacancy center (NV) in diamond is considered as a suitable qubit for most applications, but it is also known to have important drawbacks. More recently it has been shown that divacancies (VSiVC)° and NV (VSiNC)- centers in SiC can overcome many of these drawbacks such as compatibility with microelectronics technology, nanostructuring and n- and p-type doping. In particular, the 4H-SiC polytype is a widely used microelectronic semiconductor for power devices for which these issues are resolved and large-scale substrates (300mmm) are commercially available. The less studied 3C polytype, which can host the same centers (VV, NV), has an additional advantage, as it can be epitaxied on Si, which allows integration with Si technology. The spectral range in which optical manipulation and detection of the spin states are performed, is shifted from the visible, 632 nm for NV centers in diamond, to the near infrared 1200–1300 nm (telecom wavelength) for divacancies and NV centers in SiC. However, there are other crucial parameters for reliable information processing such as the spin-coherence times, deterministic placement on a chip and controlled defect concentrations. In this review, we revisit and compare some of the basic properties of NV centers in diamond and divacancies and NV centers in 4H and 3C-SiC.

Effect of cerium impurity on the stable growth of the 4H-SiC polytype by the physical vapour transport method

Effect of cerium impurity in the SiC source material on the 4H-SiC growth was investigated. 4H-SiC crystals were grown on 6H-SiC crystal seeds by physical vapor transport and using the open seed backside method. Cerium is the impurity used for promotion of the 4H polytype nucleation. The optimal amount of CeO2 for the growth of 4H-SiC with a good crystalline quality and without undesired 15R- and 6H-SiC inclusions was identified. The open seed backside method combined with the presence of cerium impurity in the growth chamber were more effective for the crystal growth process due to a lower degradation of the backside surface of the grown crystal.

Inverted fine structure of a 6H-SiC qubit enabling robust spin-photon interface

Controllable solid-state spin qubits are currently becoming useful building blocks for applied quantum technologies. Here, we demonstrate that in a specific type of silicon-vacancy in the 6H-SiC polytype the excited-state fine structure is inverted, compared to 4H-SiC. From the angular polarization dependencies of the emission, we reconstruct the spatial symmetry and determine the optical selection rules depending on the local deformation and spin–orbit interaction. We show that this system is well suited for the implementation of robust spin–photon entanglement schemes. Furthermore, the inverted fine structure leads to unexpected behavior of the spin readout contrast. It vanishes and recovers with lattice cooling due to two competing optical spin pumping mechanisms. Our experimental and theoretical approaches provide a deep insight into the optical and spin properties of atomic-scale qubits in SiC required for quantum communication and distributed quantum information processing.

Systematic Characterization of Plasma-Etched Trenches on 4H-SiC Wafers.

Silicon carbide power semiconductors overcome some limitations of silicon chips, and therefore, SiC is an attractive candidate for next-generation power electronics. In addition, the number of possible vertical devices that can be obtained on a given surface using the trench technique is significantly larger than that attainable using a planar setup. Moreover, a SiC trench power metal oxide semiconductor field-effect transistor (power MOSFET) structure removes the junction field-effect transistor (JFET) region (that would decrease the current flow width) and improves the channel density, thus reducing the specific electrical resistance. Consequently, in the present study, we report on the chemical bonding state of elements and structural characterization of trenches, obtained using SF6-based plasma etching, on the 4H-SiC polytype substrate. An interferometric algorithm that finds the endpoint to stop etching governed the trench depth. Scanning electron microscopy, transmission electron microscopy, atomic force microscopy, and X-ray photoelectron spectroscopy analyses stated the high quality and uniformity of the trenches. These materials are particularly promising for the fabrication of the SiC MOSFET to be implemented in the manufacturing of power devices.

A Simulation Approach to Study the Effect of SiC Polytypism Factor on Sensitivity of Piezoresistive MEMS Pressure Sensor

SiC is a well known wide band gap semiconductor explored for realizing the piezoresistive micro-electro-mechanical systems (MEMS) pressure sensors for harsh environments. In this work a thin SiC diaphragm based piezoresistive pressure sensor was designed by locating the resistors of different SiC polytypes such as 3C, 4H, and 6H-SiC, on highly stressed zone of the diaphragm and analyzed. The sensor design parameters were extensively studied by executing the finite element method (FEM) and piezoresistive simulation using device simulation software. The sensor characteristics were measured for different SiC polytypes and compared for different design parameter variations to obtain the optimum sensor performance under the influence of the pressure upto 8 MPa. Influence of temperature for heavily doped SiC polytypes is also studied. The simulation results shows that the pressure sensor with 3C-SiC polytype offers higher sensitivity 7.3 μV/V/KPa as compared to 4H-SiC and 6H-SiC polytypes.

Барьер Шоттки на контакте магнитного 3d-металла с полупроводником

With the aim of the Friedel transition metal d-band model and semiconductor local defect state model the analytical expressions for metal – semiconductor charge transfer and Schottky barrier height are obtained. It is shown that the account of metal magnetization leads to the increase of the charge transfer and corresponding contribution to the Schottky barrier height. Numerical estimates are made for the contacts of Co and Ni with 6H- and 4H-SiC polytypes.

Theoretical study of the band structure of 2H-SiC and 4H-SiC of silicon carbide polytypes

We have studied the electronic band properties of 2H-SiC and 4H-SiC silicon carbide polytypes. The structures of the electronic bands and density of state (DOS) using ab initio Density Functional Theory (DFT) were calculated for the first Brillouin zone both in the generalized gradient approximation and taking into account quasiparticle effects according to the GW scheme. The calculated bandgaps obtained using the GW approximation Eg2H-SiC = 3.17 eV and Eg4H-SiC = 3.26 eV agree well with experimental values. The shape and values of total DOS are within agreement with calculations performed by other authors. The calculated total energy values for 2H-SiC and 4H-SiC were close, but they satisfy the condition E2H > E4H, which implies that the 4H-SiC structure is more stable than 2H-SiC. Our calculations of the band structure and DOS of 2H-SiC and 4H-SiC by the DFT method showed that the application of the GW approximation is an optimum approach to the study of the electronic structure of 2H-SiC and 4H-SiC polytypes.

Adsorption of Group-I and -VII Atoms on Silicon-Carbide Polytypes

In the context of Haldane–Anderson’s model, the transferred charge and the energy of adsorption of alkali metal and halogen atoms on the C and Si faces of the 3C-, 6H-, and 4H-SiC polytypes are estimated. The contributions of the band and local states to the formation of the charge of adatoms and the metallic and ionic components of the adsorption energy are clarified.

Amorphous and photoluminescent crystalline silicon carbide nanoparticles synthesized by laser ablation in liquids

Amorphous and crystalline SiC nanoparticles were synthesized via laser ablation of SiC micro-sized powder in water and ethanol. X-ray and HRTM analysis showed the amorphous nature of SiC nanoparticles with an average particle size of 44 nm in water and crystalline 6H-SiC polytype nanoparticles with an average size of 18 nm in ethanol. The direct and indirect bandgap of SiC nanoparticles were determined from UV–visible absorption spectra. PL emission characteristics were studied and crystalline SiC nanoparticles exhibit tunable PL emission covering UV to the visible region.

Experimental characterization of spin- 32 silicon vacancy centers in 6H -SiC

Silicon carbide (SiC) hosts many interesting defects that can potentially serve as qubits for a range of advanced quantum technologies. Some of them have very interesting properties, making them potentially useful, e.g. as interfaces between stationary and flying qubits. Here we present a detailed overview of the relevant properties of the spins in silicon vacancies of the 6H-SiC polytype. This includes the temperature-dependent photoluminescence, optically detected magnetic resonance, and the relaxation times of the longitudinal and transverse components of the spins, during free precession as well as under the influence of different refocusing schemes.

Intercalation Synthesis of Cobalt Silicides under Graphene Grown on Silicon Carbide

The process of formation of cobalt silicides near the graphene-silicon carbide interface by intercalation of single-layer graphene grown on the 4H- and 6H-SiC(0001) polytypes with cobalt and silicon is studied. The experiments were carried out in situ in ultrahigh vacuum. The analysis of the samples is performed by high-energy-resolution photoelectron spectroscopy using synchrotron radiation, low-energy electron diffraction, and also Raman spectroscopy, atomic-force and kelvin-probe microscopies. The thicknesses of the deposited cobalt and silicon layers is varied to 2 nm, and the sample temperature, from room temperature to 1000°C. Co and Si atoms deposited on heated samples is found to penetrate under graphene and are localized between the buffer layer and the substrate, which leads to a transformation of the buffer layer into additional graphene layer. It is shown that the result of intercalation of the system with cobalt and silicon is the formation under two-layer graphene of a Co–Si solid solution and silicide CoSi coated by the surface Co3Si phase. It is shown that the thickness and the composition of the formed silicide films can be changed by varying the amount of the intercalated material and the order of their depositions.

Adsorption of Barium Atoms on Silicon Carbide

The adsorption of barium atoms on C and Si faces of 3C-, 6H-, and 4H-SiC polytypes is examined using the Haldane–Anderson model. The charge transfer from a barium adatom to the substrate is analyzed with band and local states of the adsorption system taken into account. The ionic and metallic contributions to the adsorption energy are estimated. The surface molecule model is used to verify some of the results.

Carbothermal Synthesis, Properties, and Structure of Ultrafine SiC Fibers

This paper briefly reports the results of the carbothermal synthesis of SiC from natural graphite and quartz in a reactor with a self-organizing protective CO–SiO atmosphere and in argon at a temperature between 1600 and 1650°C. The chemical and phase compositions of the synthesized SiC have been determined using chemical analysis, X-ray diffraction, and Raman spectroscopy. We have found conditions for obtaining the 6H-SiC or 3C-SiC polytype with a total impurity concentration within 0.3 ppm (with no allowance for the SiO2 film). The structural properties and particle size of the SiC powder obtained in a reactor with a CO–SiO atmosphere have been studied using scanning electron microscopy, laser diffraction, ellipsometry, and mathematical description of physical adsorption. The SiC powder has been shown to consist of platelike agglomerates in which the average particle size (40 to 60 μm) and shape have been inherited from the graphite used in the synthesis process and which have a specific surface area of 10.4 m2/g. The agglomerates have a fibrous structure, with fiber diameters from 100 to 300 nm (the thickness of the forming SiO2 film is 53 ± 2 nm).

Адсорбция атомов бария на карбиде кремния

Adsorption of barium atoms on the C- and Si-faces of the 3C- 6H- and 4H-SiC polytypes is considered within the scope of the Haldane-Anderson model. Charge transfer from Ba atoms to the substrate is analyzed with the account of the band and local states of the adsorbed system. The ionic and metallic con-tributions to the adsorption energy are estimated. To improve some obtained results surface molecule model is used.

Comparison of graphene films grown on 6H-SiC and 4H-SiC substrates

Atomic force microscopy, Kelvin-probe microscopy and Raman spectroscopy have been used to examine graphene films grown by thermal decomposition of the Si face of semi-insulating substrates of 6H-SiC and 4H-SiC polytypes in the atmosphere of argon. It was demonstrated that the quality of graphene grown on substrates of various polytypes at identical technological growth regimes is about the same. A conclusion was made that the differences in crystal structure between 6H-SiC and 4H-SiC does not lead to significant dissimilarities in the mechanism of sublimation of silicon carbide components from the surface of a crystal and in that of graphene crystallization.

Fabrication of Gas-Sensor Chips Based on Silicon–Carbon Films Obtained by Electrochemical Deposition

In this study, we designed two types of gas-sensor chips with silicon–carbon film, doped with CuO, as the sensitive layer. The first type of gas-sensor chip consists of an Al2O3 substrate with a conductive chromium sublayer of ~10 nm thickness and 200 Ω/□ surface resistance, deposited by magnetron sputtering. The second type was fabricated via the electrochemical deposition of a silicon–carbon film onto a dielectric substrate with copper electrodes formed by photoelectrochemical etching. The gas sensors are sensitive to the presence of CO and CH4 impurities in the air at operating temperatures above 150 °C, and demonstrated p- (type-1) and n-type (type-2) conductivity. The type-1 gas sensor showed fast response and recovery time but low sensitivity, while the type-2 sensor was characterized by high sensitivity but longer response and recovery time. The silicon–carbon films were characterized by the presence of the hexagonal 6H SiC polytype with the impurities of the rhombohedral 15 R SiC phase. XRD analysis revealed the presence of a CuO phase.