Yuehu Wang

Raman analysis of epitaxial graphene on 6H-SiC (0001̄) substrates under low pressure environment

This article investigates the formation mechanism of epitaxial graphene on 6H-SiC (000) substrates under low pressure of 2 mbar environment. It is shown that the growth temperature dramatically affects the formation and quality of epitaxial graphene. The higher growing temperature is of great benefit to the quality of epitaxial graphene and also can reduce the impact of the substrate for graphene. By analyzing Raman data, we conclude that epitaxial graphene grown at 1600 °C has a turbostratic graphite structure. The test from scanning electron microscopy (SEM) indicates that the epitaxial graphene has a size of 10 μm. This research will provide a feasible route for fabricating larger size of epitaxial graphene on SiC substrate.

Improved empirical DC I–V model for 4H-SiC MESFETs

A novel empirical large signal direct current (DC) I-V model is presented considering the high saturation voltage, high pinch-off voltage, and wide operational range of drain voltage for 4H-SiC MESFETs. A comparison of the presented model with Statz, Materka, Curtice-Cubic, and recently reported 4H-SiC MESFET large signal I-V models is made through the Levenberg-Marquardt method for fitting in nonlinear regression. The results show that the new model has the advantages of high accuracy, easily making initial value and robustness over other models. The more accurate results are obtained by the improved channel modulation and saturation voltage coefficient when the device is operated in the sub-threshold and near pinch-off region. In addition the new model can be implemented to CAD tools directly, using for design of 4H-SiC MESFET based RF&MW circuit, particularly MMIC (microwave monolithic integrate circuit).

High Performance of 5.7kV 4H-SiC JBSs with Optimized Non-Uniform Field Limiting Rings Termination

In this paper, a 5.7kV 4H-SiC Junction Barrier Schottky diode(JBS) with non-uniform field limiting rings termination is simulated and fabricated successfully based on a epitaxial thickness of 49μm and the doping concentration about 1.04×1015cm-3 respectively. The reverse breakdown voltage could reach to 5.7kV at least at reverse current of 200μA. And the on-state voltage is 3V at the forward current of 2A, corresponding to an on-resistance of 32mΩ•cm2. The corresponding figure-of- merit of VB2/ RSP-ON for our fabricated device is 1.026 GW/cm2, which is closing to the optimal levels among several reported SiC JBS.

Low Pressure Homoepitaxial Growth of 4H-SiC on 4°off-Axis Substrates

Step-bunching and triangular defects are significant problems in achieving higher growth rate 4H-SiC epilayers in a horizontal hot wall CVD reactor using a standard non-chlorinated chemistry of silane-propane-hydrogen on 4°off-axis substrates. In this work, the impact of growth pressure on generation of step-bunching and triangular defects and the correlations between the surface roughness and the formation of defects were investigated. It has been found that the impact of growth pressure on concentration of the triangle defects and surface roughness is obviously different. An overall reduction of defects was observed with decreasing growth pressure while the surface roughness increased. The increased adatom surface mobility in low pressure range and minimization of surface free energy are the main reasons for the phenomenon above. High Resolution X-Ray Diffraction (HRXRD) indicated that the structural quality of 4H-SiC epilayers performed at low pressure was higher than that obtained at high pressure.

Raman Spectroscopy and XPS Analysis of Epitaxial Graphene Grown on 4H-SiC (0001) Substrate under an Argon Pressure of 900 mbar Environment

This paper investigated a feasible process of growing epitaxial graphene on 4H hexagonal poly-type of silicon carbide Si-faced polar surface (0001) under an argon pressure of 900 mbar conditions. Using Raman Spectroscopy, Scanning Electron Microscopy and X-ray Photoelectron Spectroscopy, epitaxial graphene grown at temperature 1600°C is confirmed to take shape weakly on 4H-SiC (0001) with an average domain size of several tens of nanometers, and this can be seen as the characteristic of initial formation of epitaxial graphene on substrate.

4H-SiC Junction Barrier Schottky diode with embedded P-layer

4H-SiC Junction Barrier Schottky diode with embedded P-layer (JBS-EPL) is reported in this paper. JBS-EPL has attractive advantages, such as remarkably increasing the breakdown voltage and decreasing the power loss and die area comparing with conventional Junction Barrier Schottky diode. The estimates for power loss and die area are proposed that can be used for evaluating the different structure design of power devices.

Semi-insulating SiC formed by Vanadium ion implantation

With Vanadium ion implantation semi-insulating 4H-SiC layer has been investigated. For n-type and p-type 4H-SiC, resistivities have been reached 7.6times106middotcm and 1.6times1010middotcm respectively after 1650degC annealing. Perfect surface morphology has been observed using a simple Carbon coating film protection. The Vanadium energy levels in forbidden band of n-type 4H-SiC were confirmed as 0.8 eV and 1.1 eV by different measurements.

The effect of different mechanism on the characteristics of SiC Schottky barrier diode

A number of experiments demonstrate that the Schottky barrier height for Metal-SiC is different from the prediction of theory. Based on non-uniform barrier height assumption for metal-SiC contact, a numerical simulation with 2D simulator MEDICI is performed in this paper. The simulation results show that present model matches the experimental data very well. Patch defects make the Schottky barrier height decreased. This may give a reasonable explanation for the nonideal behaviors observed from many experiments. The effect of interface state density on Schottky barrier height is also discussed.

Effect of Low Pressure on Surface Roughness and Morphological Defects of 4H-SiC Epitaxial Layers

In this work, 4H-SiC epilayers are performed on 4° off-axis substrates under low pressure condition by horizontal hot wall chemical vapor deposition (HWCVD) with a standard chemistry of silane-propane-hydrogen, which focuses on the effects of growth pressure on morphology, basal plane dislocations (BPDs) and crystalline quality. It is found that morphological defects reduce with the decreasing of growth pressure, since the surface diffusion length of absorbed adatoms increases under low growth pressure, which suppresses the nucleation of adatoms on terraces and the formation of morphological defects. However, as the surface diffusion length increases under low growth pressure, the difference of growth velocity at steps is enhanced, which leads to the extension of the steps’ width and the formation of step-bunching. Besides variation of surface diffusion length, the phenomenon described above can be correlated with different dominate modes for the minimization of surface energy at varied growth pressure. Because of the contrary influence of increased C/Si ratio and enhanced step-flow growth on the propagation of BPDs, the dislocation densities of BPDs and threading edge dislocations (TEDs) in epilayers grown at varied pressures remain basically unchanged. The crystalline quality is almost independent of growth pressure based on high resolution X-ray diffraction (HRXRD) measurements.

Fabrications and characterizations of high performance 1.2 kV, 3.3 kV, and 5.0 kV class 4H–SiC power SBDs*

In this paper, 1.2 kV, 3.3 kV, and 5.0 kV class 4H–SiC power Schottky barrier diodes (SBDs) are fabricated with three N-type drift layer thickness values of 10 μm, 30 μm, and 50 μm, respectively. The avalanche breakdown capabilities, static and transient characteristics of the fabricated devices are measured in detail and compared with the theoretical predictions. It is found that the experimental results match well with the theoretical calculation results and are very close to the 4H–SiC theoretical limit line. The best achieved breakdown voltages (BVs) of the diodes on the 10 μm, 30 μm, and 50 μm epilayers are 1400 V, 3320 V, and 5200 V, respectively. Differential specific-on resistances (R on−sp) are 2.1 mΩ cm2, 7.34 mΩcm2, and 30.3 mΩcm2, respectively.