This article is a review of recent progress in our understanding of the optical properties of the important polytypes of SiC: 3C, 4H, 6H, and 15R. We focus on experimental work but also compare results obtained by experiment with theory. The topics of emphasis are: 1. vacuum ultraviolet reflectivity, 2. free excitons, 3. spectroscopy of shallow donors and acceptors, 4. the impurity boron in SiC, 5. the rare earth impurity erbium in SiC, and 6. optical properties of porous SiC.

It has been recognized that Raman scattering spectroscopy is a powerful tool to characterize SiC crystals non-destructively. We review recent significant developments in the use of Raman scattering to study structural and electronic properties of SiC crystals. The areas to be discussed in the first part include polytype identification, evaluation of stacking disorder and ion-implantation damages, and stress evaluation. The Raman scattering by electronic transitions is discussed in the second part of this article. We concentrate on the plasmon LO-phonon coupled modes whose spectral profiles are used to evaluate the carrier concentration and mobility. Anisotropic electronic properties of α-SiC and characteristics of heavily doped crystals are discussed. Semiconductor-to-metal transition and Fano interference effect are also treated.

Recent time resolved optical measurements of carrier and exciton recombination in different SiC polytypes are presented. The recombination of bound excitons, both bound at the nitrogen donor and at the Al acceptor, is extremely fast in 4H and 6H SiC. These recombinations are thus dominated by efficient non-radiative recombinations, assumed to be a phononless Auger process. In high quality epitaxial layers where a free exciton is present, all decay times are determined by the free exciton lifetime. Time resolved luminescence measurements have also been used to measure the minority carrier lifetime mainly in 4H SiC. A significant improvement of the minority carrier lifetime has been obtained in the last two years, with values as high as 2 μs.

The current status and future perspectives of cyclotron resonance (CR) experiments in SiC are given, with a brief review of published results. CR experiments have so far been successful only in 3C, 4H and 6H SiC. Among them, 3C has the best established properties. The location of the conduction band (CB) minima in 3C SiC has been shown by far-infrared CR experiments to lie at the X-point of the Brillouin zone, without the camel's back structure. The constant-energy surfaces near the CB minima are ellipsoids, with the transverse and longitudinal effective mass values as m^*_t = (0.247 ± 0.011) m₀ and m^*_l = (0.677 ± 0.015) m₀. The dominant electron scattering mechanism is shown to be the acoustic-phonon scattering in a very wide temperature range up to room temperature. The electron effective mass tensor in 4H SiC has been fully resolved by the optically detected CR (ODCR) studies at 35 GHz as: m(MΓ) = (0.58 ± 0.01) m₀, m(MK) = (0.31 ± 0.01) m₀, and m(ML) = (0.33 ± 0.01) m₀. The anisotropic electron effective tensor in 6H SiC predicted by theories has, however, not been obtained so far. The limited resolution of the ODCR experiments at 9 GHz only provides the anisotropy of the electron effective masses perpendicular and parallel to the <0001> direction: m^*_t = (0.42 ± 0.02) m₀ and m^*_l = (2.0 ± 0.2) m₀. Future ODCR work at higher frequencies is required to fully resolve the principal values of the mass tensor. Though the CR studies are not able to locate the exact position of the CB minima, they are at least consistent with the theoretical predictions that they lie at the M-point in 4H SiC whereas in 6H SiC they are positioned at the halfway between the M- and L-point in the Brillouin zone. In both 4H and 6H SiC much effort should be made in the future to obtain information on the band non-parabolicity, the exact location of the CB minima, and also on the dominant carrier scattering mechanism. No reliable data of hole effective masses have been obtained for any polytype.

Investigations of nitrogen donors in 6H-, 4H- and 3C-SiC using conventional electron paramagnetic resonance (EPR), electron nuclear double resonance (ENDOR) and optical detection of EPR and ENDOR as well as optical absorption and emission spectroscopy are reviewed and critically discussed. An attempt is presented to interpret the experimentally found large differences in hyperfine interactions of the ¹⁴N nuclei on the various inequivalent sites in the different polytypes of SiC in terms of valley-orbit splittings and "central-cell corrections" in the framework of the effective mass theory (EMT). P-doping by neutron transmutation in 6H-SiC resulted in various P-related EPR spectra previously associated with shallow P donors and P-vacancy complexes. In analogy to the new interpretation of the N donor spectra in various polytypes, it is proposed that all P-related spectra found hitherto in 6H-SiC are due to isolated P donors in ground and excited EMT states. A detailed discussion is presented of the electronic structure of B acceptors, as determined by EPR and in particular by ENDOR investigations: The B atom itself has only very little unpaired hole density, while the hole resides mainly on a neighbouring relaxed C atom B acceptors have a rather "deep" character and pronounced dynamical properties. A discussion of the present understanding of the so-called deep B centre (D centre) is also given. In contrast to B, the Al acceptor behaves as expected from the effective mass theory. It shows, however, two optical absorption bands identified by optical detection of EPR which are related to an ionization transition to the valence band and another transition, probably to a V impurity.

A review is given on the results of magnetic resonance studies of transition metal impurities in SiC polytypes. The data are presented for the elements titanium (Ti), vanadium (V), chromium (Cr), molybdenum (Mo), manganese (Mn), scandium (Sc) and copper (Cu). Most of these transition metals were found to occur in multiple charge states, underlining their role as deep level defects in SiC. A compilation of relevant ESR parameters for transition metal defects in various SiC polytypes is presented in the Appendices.

Irradiation of fast particles like 1 MeV electrons and 2 MeV protons was made for single crystalline cubic silicon carbide (3C-SiC) grown epitaxially on Si by chemical vapor deposition in order to introduce point defects in the material. Intrinsic point defects in 3C-SiC have been characterized by electron spin resonance (ESR), positron annihilation spectroscopy (PAS), Hall and photoluminescence (PL) techniques. The structure and annealing behavior of intrinsic defects, e.g. monovacancies at silicon and carbon sublattice sites, are described based on the results obtained by ESR and PAS. The contributions of such point defects to electrical and optical properties of 3C-SiC are discussed using the Hall and PL results, with a brief review of published work.

Electrical data obtained from deep level transient spectroscopy investigations on deep defect centers in the 3C, 4H, and 6H SiC polytypes are reviewed. Emphasis is put on intrinsic defect centers observed in as-grown material and subsequent to ion implantation or electron irradiation as well as on defect centers caused by doping with or implantation of transition metals (vanadium, titanium, chromium, and scandium).

By analyzing the temperature dependence of the capacitance and AC conductance of a zero-biased Schottky diode one obtains the activation energy of the primary shallow dopant in a semiconductor. This technique has become known as Thermal Admittance Spectroscopy. Thermal admittance spectroscopy has been used to characterize nitrogen donors in 4H-, 6H-, and 15R-SiC. p-type dopants (Al, B) have also been characterized in 6H-SiC. In addition, an activation energy attributable to hopping conduction has been obtained in n-type specimens of all three polytypes. The change in the admittance of a Schottky diode caused by illumination, measured as a function of wavelength, is called Optical Admittance Spectroscopy. Optical admittance spectroscopy has been used to study deep centers in silicon carbide where traditional electrical detection is made difficult by the requirement for very high measurement temperatures. Using this technique, the energies of the midgap donor-like levels attributed to vanadium atoms substitutionally occupying the inequivalent lattice sites have been determined. The bandgaps of 6H- and 4H-SiC polytypes have been measured, and the phonon spectra associated with the indirect transitions from the valence band to the conduction band have been determined.

Thermoluminescence (TL) and thermally stimulated conductivity (TSC) in the temperature range 10 to 300 K is a phenomenon to be observed in 6H/4H-SiC boule crystals. The effect is explained by the classical two-center model consisting of one species of trap and one species of recombination center. One is able to make an excellent fit of the calculated thermoluminescence obtained from the model to the experimental data (glow curves). The analysis of the glow peaks leads to the ionization energies of the involved traps. By comparing the obtained ionization energies of the traps with the known ionization energies of the doping species, the TL-active traps are identified with nitrogen for n-type crystals and aluminum or boron for p-type crystals. Analyzing the charge transfer during the TL process shows that one of the two involved centers acts as doping the other as compensation in thermal equilibrium. Information about the involved recombination center is obtained from excitation spectroscopy or from the recombination spectrum. The cut-off energy of the excitation spectrum determines the energetic position of the recombination centers in the band gap. TL turns out to be a method to distinguish between different species of compensation.

Micropipes are hollow tubular defects penetrating SiC single crystals. Different mechanisms of formation and stabilization as well as energetic aspects are discussed.

N⁺ implantation into p-type α-SiC (6H-SiC, 4H-SiC) epilayers at room and elevated temperatures, mainly obtained by the authors' group, has been reviewed. Since recrystallization of SiC is difficult, the implantation-induced damage should be minimal during implantation to achieve higher electrical activation. The effects of hot implantation are pronounced in high-dose (>10¹⁵ cm ^{- 2}) implantation. The lowest sheet resistance of 542 Ω/□ was obtained by implantation at 500 to 800 °C with a 4 x 10¹⁵cm ^{- 2} dose. The properties of pn junctions formed by N⁺ implantation into p-type epilayers are characterized in detail. The forward current is clearly divided into two components of diffusion and recombination currents. The diodes exhibited high breakdown voltages of 615 to 820 V, which are almost ideal values expected from device structure. The reverse leakage current can significantly be reduced by employing hot implantation at 800 °C.

Experimental studies on aluminum (Al) and boron (B) implantation in 4H/6H SiC are reported; the implantation is conducted at room temperature or elevated temperatures (500 to 700 °C). Both Al and B act as "shallow" acceptors in SiC. The ionization energy of these acceptors, the hole mobility and the compensation in the implanted layers are obtained from Hall effect investigations. The degree of electrical activity of implanted Al/B atoms is determined as a function of the annealing temperature. Energetically deep centers introduced by the Al⁺/B⁺ implantation are investigated. The redistribution of implanted Al/B atoms subsequent to anneals and extended lattice defects are monitored. The generation of the B-related D-center is studied by coimplantation of Si/B and C/B, respectively.

The orientation dependence of the thermal oxidation rates in 6H-SiC has been investigated. The wet oxidation was performed in steam and in the temperature range of 1000 to 1200 °C. The linear rate sharply changes at an off-angle around 30° from the {0001}-face. This large anisotropy in oxidation rate will cause a difficulty in controlling the bird's beak length in LOCOS. The activation energy of the linear rate gradually increases from C-face to Si-face. The situation is quite different from silicon in which the orientation dependence of the activation energy is explained by the steric hindrance.

Silicon carbide (SiC) is the only compound semiconductor whose native oxide is SiO₂. This places SiC in a unique position to compete with silicon in applications involving high power, high voltages, or high temperatures. SiC MOS technology has made substantial progress in recent years. This article aims to summarize the present status of this field, including MOS analysis techniques, oxidation procedures, experimental results, reliability considerations, alternative insulators, and remaining questions. In addition, we hope to convince the reader of the following: 1. Great care must be exercized in interpreting MOS data on wide bandgap semiconductors. This is due to the extremely long response times for interface states deep in the bandgap. 2. Recent results do not support the argument that interface quality on p-type SiC is inferior to that of n-type.

The energy distribution of electron states at SiC/SiO₂ interfaces produced by oxidation of various (3C, 4H, 6H) SiC polytypes is studied by electrical analysis techniques and internal photoemission spectroscopy. A similar distribution of interface traps over the SiC bandgap is observed for different polytypes indicating a common nature of interfacial defects. Carbon clusters at the SiC/SiO₂ interface and near-interfacial defects in the SiO₂ are proposed to be responsible for the dominant portion of interface traps, while contributions caused by dopant-related defects and dangling bonds at the SiC surface are not observed.

The properties of electron transport in silicon carbide inversion layers are reviewed. Emphasis is put on the thermally-activated conductivity which prevails at low and room temperatures. The impact of interface disorder on the performance of SiC MOSFETs is discussed in detail. These transport properties are related to the conclusions that can be drawn from the electrical characterization of the SiC/SiO₂ interface. High temperature operation is discussed more briefly.

A new design and processing concepts have been applied to develop practical SiC trench MOSFETs for high power applications. The designed trench MOSFET has a MOS structure consisting of epitaxially grown n-type SiC trench sidewall layers. The current flows via an accumulation mode through the channel defined in the epitaxially grown SiC sidewall layer. The channel is depleted by the built-in fields of the p-type SiC base layer and the p-poly-Si gate, that control the channel conditions. The simulation based investigations revealed that the formed channel can withstand up to the avalanche breakdown condition. The structure of the n-type SiC trench sidewall epi-layer has been optimized to realize the blocking voltage of more than 1000 V for SiC MOSFETs with low on-state resistance. Moreover, our designed structure can address most of the open issues related to the MOS interface, viz., high surface state density, low channel mobility and high electric field at the trench base of the MOS structure. We have fabricated the first 2 mm square large size 6H-SiC trench MOSFET chip, in which 2380 hexagonal structural microcells of 23 μm pitch were integrated. The fabricated 6H-SiC trench MOSFET on (0001-) C-face wafers feature the on-state resistance as low as 23.84 mΩ cm² with blocking voltage of more than 450 V.

Schottky contacts of metal/3C-, 6H-, and 4H-SiC systems are investigated in this review. Most Schottky contacts having large barrier heights show good characteristics with low ideality factors. The barrier height depends on the metal work function without strong Fermi-level pinning for all polytypes, and linear relationships with slopes of about 0.2 to 0.7 are observed between the barrier height and the metal work function. Based on the analysis of metal/SiC systems, the fabrication of high-voltage rectifiers has been reported, and high voltages from 400 to 1100 V have been achieved using Pt/, Ti/, and Au/6H-SiC structures. In addition, high-temperature operation at 400 °C is performed for an Au/6H-SiC structure while supporting a high reverse bias (460 V). Using Ti/4H-SiC structures, high-voltage (≈1000 V) and low-power loss characteristics are realized, which is superior to Ti/6H-SiC Schottky rectifiers. To improve the reverse bias characteristics, an edge termination technique is employed for Ti/4H-SiC Schottky rectifiers, and the devices show excellent characteristics with a higher blocking voltage up to 1750 V compared with unterminated devices.

A variety of RF and microwave electronic devices can be fabricated from SiC. These SiC-based devices have many properties that make them near ideal for high temperature, high frequency, high power, and radiation hard applications. Progress in SiC bulk and epitaxial layer growth has been rapid in recent years and corresponding progress has been achieved in device fabrication and contact technology. Both 6H- and 4H-SiC substrates are commercially available, and high quality epitaxial layers can be grown. Prototype SiC electronic devices with very good DC and RF performance have been demonstrated and devices such as diodes are commercially available, while RF and high frequency transistors are rapidly approaching the commercialization state. In particular, SiC transistors such as MESFETs (MEtal Semiconductor Field-Effect Transistors) and SITs (Static Induction Transistors) with excellent DC and RF performance have been demonstrated and these devices are being developed for microwave power amplifier and oscillator applications. One of the most promising devices for microwave power applications is the MESFET. In this work the performance of microwave SiC MESFETs is investigated with a simulator that contains a physically based device model. The simulator permits the operation of the device to be examined and optimum device structures to be determined. This, in turn, permits performance capability and limitations to be investigated. The results of the simulations are compared to experimental measurements where possible, and excellent agreement between the simulated and measured data are obtained.

The impact of SiC on high power devices and their applications is analysed using simulations in a very wide range of design voltages. First, a detailed presentation of the anisotropic form of the basic equations and of the physical models for 4H-SiC used in the simulations is given. Following that the application ranges of unipolar and bipolar devices in the domains of voltage and frequency are predicted in the case of IGBTs versus MOSFETs and PiN versus Schottky rectifiers based on comparisons of the on-state voltage and of the total losses. The application limit of the MOSFETs compared to IGBTs and of the Schottky rectifiers compared to PiN rectifiers is predicted to be about 4.5 and 2.5 kV, respectively, in the case of the 4H-SiC polytype. The impact of technological limitations of SiC is illustrated by the case of low channel mobility. The merits of SiC as compared to Si are illustrated by the case of a SiC rectifier operating together with a Si IGBT. Dramatically reduced turn-on losses are demonstrated. The superiority of SiC from the point of view of dynamic avalanche is predicted and illustrated. Finally, some novel SiC switch structures are introduced in response to the reliability problems encountered in ordinary trench MOSFETs.

The advantages of SiC for high power, microwave devices are discussed. The design considerations, fabrication, and experimental results are described for SiC MESFETs and SITs. The highest reported f_max for a 0.5μm MESFET using semi-insulating 4H-SiC is 42 GHz. These devices also showed a small signal gain of 5.1 dB at 20 GHz. Other 4H-SiC MESFETs have shown a power density of 3.3 W/mm at 850 MHz. The largest SiC power transistor reported is a 450 W SIT measured at 600 MHz. The power output density of this SIT is 2.5 times higher than that of comparable silicon devices. SITs have been designed to operate as high as 3.0 GHz, with a 3 cm periphery part delivering 38 W of output power.

The research and development activities carried out to demonstrate the status of MOS planar technology for the manufacture of high temperature SiC ICs will be described. These activities resulted in the design, fabrication and demonstration of the World's first SiC analog IC - a monolithic MOSFET operational amplifier. Research tasks required for the development of a planar SiC MOSFET IC technology included characterization of the SiC/SiO₂ interface using thermally grown oxides; high temperature (350 °C) reliability studies of thermally grown oxides; ion implantation studies of donor (N) and acceptor (B) dopants to form junction diodes; epitaxial layer characterization; N channel inversion and depletion mode MOSFETs; device isolation methods and finally integrated circuit design, fabrication and testing of the World's first monolithic SiC operational amplifier IC. These studies defined a SiC n-channel depletion mode MOSFET IC technology and outlined tasks required to improve all types of SiC devices. For instance, high temperature circuit drift instabilities at 350 °C were discovered and characterized. This type of instability needs to be understood and resolved because it affects the high temperature reliability of other types of SiC devices. Improvements in SiC wafer surface quality and the use of deposited oxides instead of thermally grown SiO₂ gate dielectrics will probably be required for enhanced reliability. The slow reverse recovery time exhibited by n⁺-p diodes formed by N ion implantation is a problem that needs to be resolved for all types of planar bipolar devices. The reproducibility of acceptor implants needs to be improved before CMOS ICs and many types of power device structures will be manufacturable.

Silicon carbide has been used to fabricate a variety of short wavelength optoelectronic devices including blue LEDs, green LEDs and UV photodiodes. As a light emitter, 6H-SiC junctions can be tailored to emit light across the visible spectrum. The most widely commercialized device is the blue LED. Over the past years, the quantum efficiency of the Cree Research blue LED has increased significantly. The devices emit light with a peak wavelength of 470 nm with a spectral halfwidth of ≈70 nm. The optical power output is typically between 25 and 35 μW at a forward current of 20 mA and 3.2 V. This represents an external quantum efficiency of ≈0.05 to 0.07%. Green LEDs have been demonstrated which emit with a peak wavelength of 530 nm. As opposed to the epitaxial junction used in the blue LED, the green devices use ion implanted junctions. The typical output power is similar to that of the blue LED. However, with respect to photometric units, the die luminous intensity is a factor of two higher than the blue LED, 1.2 mcd (millicandela) for a radiant flux output of 33 μW. In addition to short wavelength light emission, the energy bandgap of ≈3.0 eV allows for inherently low dark currents and high quantum efficiencies for ultraviolet photodiode detectors made in 6H-SiC, even at high temperatures. These devices typically exhibit a quantum efficiency of 80 to 100% and peak response of ≈250 to 280 nm. These characteristics are maintained to at least 350 °C. The dark current density at - 1.0 V and 473 K is ≈10 ^{- 11} A/cm². This corresponds to an extrapolated room temperature current density of ≈2 x 10 ^{- 17} A/cm² at - 1.0 V.

High temperature gas sensors based on catalytic metal-insulator-silicon carbide (MISiC) devices are developed both as capacitors and Schottky diodes. A maximum operation temperature of 1000 °C is obtained for capacitors based on 4H-SiC, and all sensors work routinely for several weeks at 600 °C. Reducing gases like hydrocarbons and hydrogen lower the flat band voltage of the capacitor and the barrier height of the diode. The time constants for the gas response are in the order of milliseconds and because of this good performance the sensors are tested for combustion engine control. For temperatures around 600 °C total combustion occurs on the sensor surface and the signal is high for fuel in excess and low for air in excess. At temperatures around 400°C the response is more linear. The high temperature operation causes interdiffusion of the metal and insulator layers in these devices, and this interdiffusion has been studied. At sufficiently high temperatures the inversion capacitance shows different levels for hydrogen free and hydrogen containing ambients, which is suggested to be due to a reversible hydrogen annealing effect at the insulator-silicon carbide interface.