Emerging SiC Applications beyond Power Electronic Devices.

In recent years, several new applications of SiC (both 4H and 3C polytypes) have been proposed in different papers. In this review, several of these emerging applications have been reported to show the development status, the main problems to be solved and the outlooks for these new devices. The use of SiC for high temperature applications in space, high temperature CMOS, high radiation hard detectors, new optical devices, high frequency MEMS, new devices with integrated 2D materials and biosensors have been extensively reviewed in this paper. The development of these new applications, at least for the 4H-SiC ones, has been favored by the strong improvement in SiC technology and in the material quality and price, due to the increasing market for power devices. However, at the same time, these new applications need the development of new processes and the improvement of material properties (high temperature packages, channel mobility and threshold voltage instability improvement, thick epitaxial layers, low defects, long carrier lifetime, low epitaxial doping). Instead, in the case of 3C-SiC applications, several new projects have developed material processes to obtain more performing MEMS, photonics and biomedical devices. Despite the good performance of these devices and the potential market, the further development of the material and of the specific processes and the lack of several SiC foundries for these applications are limiting further development in these fields.

High-voltage SiC power devices for improved energy efficiency.

Silicon carbide (SiC) power devices significantly outperform the well-established silicon (Si) devices in terms of high breakdown voltage, low power loss, and fast switching. This review briefly introduces the major features of SiC power devices and then presents research works on breakdown phenomena in SiC pn junctions and related discussion which takes into account the energy band structure. Next, recent progress in SiC metal-oxide-semiconductor field effect transistors, which are the most important unipolar devices, is described with an emphasis on the improvement of channel mobility at the SiO2/SiC interface. The development of SiC bipolar devices such as pin diodes and insulated gate bipolar transistors, which are promising for ultrahigh-voltage (>10 kV) applications, are introduced and the effect of carrier lifetime enhancement is demonstrated. The current status of mass production and how SiC power devices can contribute to energy saving are also described.

Characterization of Thermal Oxides on 4H-SiC Epitaxial Substrates Using Fourier-Transform Infrared Spectroscopy.

Fourier transform infrared (FT-IR) spectra were measured for thermal oxides with different electrical properties grown on 4H-SiC substrates. The peak frequency of the transverse optical (TO) phonon mode was blue-shifted by 5 cm-1 as the oxide-layer thickness decreased to 3 nm. The blue shift of the TO mode indicates interfacial compressive stress in the oxide. Comparison of data for the oxide on a SiC substrate with that for similar oxides on a Si substrate implies that the peak shift of the TO mode at the SiO2/SiC interface is larger than that of SiO2/Si, which suggests that the interfacial stress for the oxide on the SiC substrate is larger than that on the Si substrate. For the SiO2/SiC interfacial region (<3 nm oxide thickness), despite the fact that the blue shift of the TO modes becomes larger while approaching the oxide/SiC interface, the peak frequency of the TO modes red-shifts at the oxide/SiC interface. The peak-frequency shift of the TO mode for the sample without post-oxidation annealing was larger than that for the samples post-annealed in a nitric oxide atmosphere. The channel mobilities are correlated with the degree of shift of the TO mode when the oxide thickness is <3 nm. It appears that the compressive stress at the SiO2/SiC interface generates silicon suboxide components and weakens the Si-O bonds. As the result, the TO mode was red-shifted and the oxygen deficiency increased to relax the compressive stress in the oxide with <3 nm thickness. Fourier transform infrared spectroscopy measurements provide unique and useful information about stress and inhomogeneity at the oxide/SiC interface.

Stress Characterization of 4H-SiC Metal-Oxide-Semiconductor Field-Effect Transistor (MOSFET) using Raman Spectroscopy and the Finite Element Method.

We measured the depolarized and polarized Raman spectra of a 4H-SiC metal-oxide-semiconductor field-effect transistor (MOSFET) and found that compressive stress of approximately 20 MPa occurs under the source and gate electrodes and tensile stress of approximately 10 MPa occurs between the source and gate electrodes. The experimental result was in close agreement with the result obtained by calculation using the finite element method (FEM). A combination of Raman spectroscopy and FEM provides much data on the stresses in 4H-SiC MOSFET.

Characterization of Thermal Oxides on 4H Silicon Carbide (4H-SiC) Epitaxial Substrate Using Fourier Transform Infrared Spectroscopy.

We measured the Fourier transform infrared (FT-IR) spectra of thermal oxides with various thicknesses, grown thermally on 4H silicon carbide (4H-SiC) substrates. For the thin (8 nm thick) thermal oxide, the transverse optical (TO) phonon peak frequency in the thermal oxide on the 4H-SiC substrate was observed at ~1080 cm-1 and was higher than that recorded in thermal oxides on a Si substrate (1074 cm-1). This shows that the thin thermal oxide was under compressive stress, calculated to be approximately 0.4 GPa, at the interface between the thermal oxide and 4H-SiC substrate. The shift of the TO phonon for s-polarized light was found to be larger than that for p-polarized light. In contrast, for the thick (85 and 130 nm thick) thermal oxides, the TO phonon peak frequency tended to shift toward lower frequencies with increasing oxide-layer thickness. By comparing the FT-IR and cathodoluminescence (CL) measurements, we conclude that the TO phonon redshift with increasing oxide-layer thickness can mainly be attributed to a corresponding increase in inhomogeneity in the thick thermal oxides.

Characterization of inhomogeneity in silicon dioxide films on 4H-silicon carbide epitaxial substrate using a combination of Fourier transform infrared and cathodoluminescence spectroscopy.

We measured the Fourier transform infrared (FT-IR) and cathodoluminescence (CL) spectra of silicon dioxide (SiO2) films grown on 4H-silicon carbide (4H-SiC) substrates and confirmed that the phonon observed at around 1150-1250 cm(-1) originates from the upper branch of the surface phonon polaritons (SPPs) in the SiO2 films and that its frequency is sensitive to the oxide thickness. The relative intensity of the upper branch of SPPs normalized by that of the transverse optical phonon (TO) tended to increase with decreasing channel mobility (CM). A comparison of the FT-IR and CL measurements shows that the relative intensity is correlated with an inhomogeneity in the SiO2-SiC interface and the CM of SiC devices. A combination of FT-IR spectroscopy and CL spectroscopy provides us with a large amount of data on the inhomogeneity, defect, and oxide thickness of SiO2 films on 4H-SiC substrates.

Abnormal behavior of longitudinal optical phonon in silicon dioxide films on 4H-SiC bulk epitaxial substrate using Fourier transform infrared (FT-IR) spectroscopy.

We report the abnormal behavior of longitudinal optical (LO) phonon in a silicon dioxide (SiO2) film on a 4H-SiC bulk epitaxial substrate using an attenuated total reflection (ATR) technique. The peak frequency of the LO phonon in the ATR spectrum was observed at around 1165 cm(-1) and red-shifted by approximately 92 cm(-1) relative to that at the grazing incidence (40°), whereas the peak frequency of the transverse optical (TO) phonon in the ATR spectrum agreed well with that at the grazing incidence. Furthermore, the peak frequency of the TO phonon hardly depends on change in the incident angle and thickness, suggesting that the microstructure of the sample is homogeneous within a thickness of 100 nm. On the other hand, we found that the microstructure of the sample was inhomogeneous within a thickness less than 5 nm. Fourier transform infrared (FT-IR) spectroscopy provides us with a large amount of data on microstructures in the SiO2 films on a 4H-SiC substrate.

Characterization of silicon dioxide films on a 4H-SiC Si(0001) face by fourier transform infrared (FT-IR) spectroscopy and cathodoluminescence spectroscopy.

We used Fourier transform infrared (FT-IR) spectroscopy to characterize silicon dioxide (SiO(2)) films on a 4H-SiC(0001) Si face. We found that the peak frequency of the transverse optical (TO) phonon in SiO(2) films grown on a 4H-SiC substrate agrees well with that in SiO(2) films grown on a Si substrate, whereas the peak frequency of the longitudinal optical (LO) phonon in SiO(2) films on a 4H-SiC substrate is red-shifted by approximately 50 cm(-1) relative to that in SiO(2) films on a Si substrate. We concluded that this red-shift of the LO phonon is mainly caused by a change in inhomogeneity due to a decrease in density in the SiO(2) films. Furthermore, cathodoluminescence (CL) spectroscopy results indicated that the channel mobility of the SiC metal-oxide-semiconductor field-effect transistor (MOSFET) decreases roughly in proportion to the increase in the intensity of the CL peak at 460 and 490 nm, which is attributed to the increase in the number of oxygen vacancy centers (OVCs). FT-IR and CL spectroscopies provide us with a large amount of data on OVCs in the SiO(2) films on a 4H-SiC substrate.