eV For 6H-SiC Research Articles

A Study of Deep Defect Levels in Semi-Insulating SiC Using Optical Admittance Spectroscopy

Optical admittance spectroscopy (OAS), supported by electron paramagnetic resonance (EPR) measurements, is used to identify the controversial vanadium acceptor levels in vanadium-doped semi-insulating (SI) 4H-SiC and 6H-SiC. The V3+/4+ levels for the cubic site are likely located at E c − 0.67 ± 0.02 eV and E c − 0.70 ± 0.02 eV in 6H-SiC and E c − 0.75 ± 0.02 eV in 4H-SiC. A peak at 0.87 ± 0.02 eV in the 6H-SiC is tentatively assigned to the same transition at the hexagonal site and the associated transition in 4H-SiC is thought to occur near 0.94 eV. All assignments are supported by the observation of V3+ in the EPR spectrum.

Vanadium donor and acceptor levels in semi-insulating 4H- and 6H-SiC

The electronic levels of vanadium in semi-insulating 4H- and 6H-SiC have been reinvestigated using temperature dependent Hall effect and resistivity measurements at temperatures up to 1000K in conjunction with electron paramagnetic resonance (EPR) and optical absorption measurements which were used to identify the charge state of vanadium in the material. Two distinct thermal activation energies were found for each polytype. The shallower of the two levels correlated with the presence of both V3+ and V4+ in the EPR and absorption experiments, demonstrating that this level is the vanadium acceptor level while the deeper level is the donor level for which the V4+ charge state was observed. The results for the V4+∕5+ donor level, EC−1.57±0.09eV for 4H-SiC and EC−1.54±0.06eV for 6H-SiC, are in agreement with the generally accepted values. However, the results for the V3+∕4+ acceptor level, EC−0.85±0.03eV in 6H-SiC and EC−1.11±0.08eV in 4H-SiC, are significantly higher than previously assumed. Variations in crystal quality and purity may explain the differences in the previously reported values for the donor and acceptor levels.

Electrical Measurement of the Vanadium Acceptor Level in 4H- and 6H-SiC

AbstractTemperature dependent Hall effect, Fourier transform infrared absorption, and electron paramagnetic resonance (EPR) studies have been performed on both 6H and 4H vanadium doped semi-insulating SiC samples grown by the physical vapor transport technique. Nitrogen and boron concentrations have been measured in some samples by secondary ion mass spectrometry (SIMS). Unlike undoped s.i. SiC, where several different thermal ionization energies have been observed, the ionization energies for all of the vanadium doped s.i. samples studied here were found to cluster around only two values for the two polytypes, EC – 0.85 eV and EC – 1.54 eV for 6H and EC – 1.11 eV and EC – 1.57 eV for 4H. SIMS measurements indicate that the nitrogen concentration exceeds the boron concentration in samples with the shallower of two values while the opposite is true for the deeper level samples. EPR detected both V3+ and V4+ in shallower level samples while only V4+ was detected in the deeper level samples. These results indicate that the vanadium acceptor level, V3+/4+, is located at EC – 0.85 eV in 6H-SiC and EC – 1.11 eV in 4H-SiC. However, some EPR results do show a small, unexpected asymmetry in the angular dependence of the V4+ signal, most noticeably in the 4H samples. This suggests that at least some of the vanadium related levels might be complexed with another defect or be under higher local strain than expected.

Deep-level luminescence at 1.0 eV in 6H SiC

ABSTRACTResults from photoluminescence (PL) and Zeeman effect measurements of a PL center, labeled UD-1, in 6H SiC are presented. The spectrum consists of three no phonon-lines (NPLs) at 0.9952, 1.0015 and 1.0020 eV. The luminescence starts decreasing in intensity above 40 K and is completely quenched at 80 K. The observed Zeeman splitting reveals a spin one half of the ground state of the two highest energy lines. No splitting of the 0.9952 eV line is detected. The g║-value for the 1.0015 eV and 1.0020 e║ lines are g║ = 1:4 and g║ = 1:7, respectively. For both lines, g⊥ = 0. The C3v symmetry indicates that the UD-1 center is either a substitutional defect or a complex with its constituents lying along the c-axis of the lattice.

Photoluminescence and Zeeman effect in chromium-doped 4H and 6H SiC

Photoluminescence (PL) and Zeeman effect measurements in near-infrared luminescence bands in Cr-doped 4H and 6H SiC are presented. The PL spectrum consists of two no-phonon lines (NPLs) at 1.1583 and 1.1898 eV in 4H SiC and three NPLs at 1.1556, 1.1797, and 1.1886 eV in 6H SiC. The observed Zeeman splittings and temperature dependence studies reveal the spin triplet of the ground state and the orbital doublet structure of the excited state of the Cr-related center. All the triplets have almost isotropic g values close to 2 with trigonal symmetry and small zero-field splitting values D. In contrast, the effective g values of the excited state of the center are very anisotropic with g∥ in the range of 0.22–0.64 and g⊥=0 for different NPLs in both polytypes. Based on the Zeeman results, the PL is attributed to the internal transition 1E(D)→3A2(F) within the d shell of a substitutional, neutral chromium (Cr4+) in the 3d2 electronic configuration.

Electronic structure of silicon carbide polytypes studied by soft x-ray spectroscopy

The electronic structure of SiC polytypes is investigated with soft x-ray absorption (SXA) and emission (SXE) spectroscopy studying the local C p and Si $(s+d)$ partial density of states (LPDOS) of n-doped cubic 3C-SiC and hexagonal 4H- and 6H-SiC. The shape and the energetic position of the occupied LPDOS of the valence band measured by nonresonantly excited SXE spectroscopy is nearly identical for the three polytypes, reflecting the local identity of the crystalline structures. The variation of the band gap from $2.2$ eV for 3C-SiC to $3.0$ eV for 6H-SiC, and $3.3$ eV for 4H-SiC is caused by changes of the conduction band alone as reflected by the unoccupied LPDOS measured by SXA. Additionally, by resonantly excited SXE information on the band dispersion and the local symmetry character of the valence states is obtained.

P-type 4H and 6H-SiC high-voltage Schottky barrier diodes

High-voltage Schottky barrier diodes have been successfully fabricated for the first time on p-type 4H- and 6H-SiC using Ti as the barrier metal. Good rectification was confirmed at temperatures as high as 250/spl deg/C. The barrier heights were estimated to be 1.8-2.0 eV for 6H-SiC and 1.1-1.5 eV for 4H-SiC at room temperature using both I-V and C-V measurements. The specific on resistance (R/sub on,sp/) for 4H- and 6H-SiC were found to be 25 m/spl Omega/ cm/sup -2/ and 70 m/spl Omega/ cm/sup -2/ at room temperature. A monotonic decrease in resistance occurs with increasing temperature for both polytypes due to increased ionization of dopants. An analytical model is presented to explain the decrease of R/sub on,sp/ with temperature for both 4H and 6H-SiC which fits the experimental data. Critical electric field strength for breakdown was extracted for the first time in both p-type 4H and 6H-SiC using the breakdown voltage and was found to be 2.9/spl times/10/sup 6/ V/cm and 3.3/spl times/10/sup 6/ V/cm, respectively. The breakdown voltage remained fairly constant with temperature for 4H-SiC while it was found to decrease with temperature for 6H-SiC.

Study of Different Edge Terminations Used for 6H-SiC Power Diodes

Silicon Carbide is a wide bandgap semiconductor (3 eV for 6H-SiC at 300 K) suitable for high voltage, high temperature, high frequency and power devices. Its drift velocity is high (2 x 10 7 cm s -1 ), its thermal conductivity is similar to that of copper (5 Wm -1° C- 1 ) and its critical electric field is almost 10 times higher than that of silicon. In the field of power devices, many publications refer to PN structures (4.5 kV), Schottky diodes (1.3 kV) as well as MOSFET and JFET. PN junction diodes have been designed and characterized with two different peripheral protections to achieve high breakdown voltage. The first protection is the MESA structure. In this approach, the curvature region of the main junction where the electric field is higher than in the bulk, is etched off. The efficiency of a P + NN + MESA structure can be increased by decreasing the doping level of the N-type layer. For this purpose compensation by boron atoms is used for P + NN + MESA structures. The second peripheral protection is a planar structure in which equipotential lines are spread around the lateral P-type low-doping implanted zones for P+N junction or N-type Schottky diodes. The experimental breakdown voltage of these diodes lies between 600 and 1500 V.