Experimental demonstration of a silicon carbide IMPATT oscillator

Silicon carbide (SiC) is an excellent material for high-power and high-frequency applications because of its high critical field, high electron saturation velocity, and high thermal conductivity. In this letter, we report the first experimental demonstration of microwave oscillation in 4H-SiC impact-ionization-avalanche-transit-time (IMPATT) diodes. The prototype devices are single-drift diodes with a high-low doping profile. DC characteristics exhibit hard, sustainable avalanche breakdown, as required for IMPATT operation. Microwave testing is performed in a reduced-height waveguide cavity. Oscillations are observed at 7.75 GHz at a power level of 1 mW     

Nitridation of silicon-dioxide films grown on 6H silicon carbide

This letter addresses the question of why it is possible to grow high-quality oxide films on N-type but not on P-type SiC. It provides results which indicate that the oxide/SiC interface would be inferior to the oxide/Si interface for both N-type and P-type SiC, if it were not for the beneficial effects of nitrogen incorporation. The letter presents, for the first time, results on nitridation of thermally grown oxides in NO and N₂O. The results demonstrate that the oxides grown on P-type can be improved by NO annealing, but not by N₂O annealing     

Experimental characterization of electron-hole generation in silicon carbide

Thermal generation in wide bandgap semiconductors can be observed by monitoring the capacitance recovery transients of npn (or pnp) storage capacitors in which the middle layer is floating. In this article, we report a study of thermal generation in 4H and 6H silicon carbide (SiC). Three generation mechanisms are identified: bulk generation in the depletion regions of the pn junctions, surface generation at the periphery of the capacitors, and defect generation associated with imperfections in the material. All three generation mechanisms are thermally activated. Bulk generation and surface generation have activation energies of approximately half bandgap, while defect generation exhibits field-induced barrier lowering resulting in an apparent activation energy less than half bandgap. Because the generation rate is extremely low, most measurements are conducted at elevated temperatures (250-350°C). However, we also describe a long-term measurement at room temperature in which the 1/e recovery time appears to be in excess of 100 years.     

Four-point probe characterization of 4H silicon carbide

We report on four-point probe measurements on SiC wafers as such measurements give erratic data. Current-voltage measurements on n-type SiC wafers doped to 3 × 10¹⁸ cm⁻³ are non-linear and single probe I-V measurements are symmetrical for positive and negative voltages. For comparison, similar measurements of p-type Si doped to 5 × 10¹⁴ cm⁻³ gave linear I-V, well-defined sheet resistance and the single probe I-V curves were asymmetrical indicating typical Schottky diode behavior. We believe that the reason for the non-linearity in four-point probe measurements on SiC is the high contact resistance. Calculations predict the contact resistance of SiC to be approximately 10¹² Ω which is of the order of the input resistance of the voltmeter in our four-point probe measurements. There was almost no change in two-probe I-V curves when the spacing between the probes was changed from 1 mm to 2 cm, further supporting the idea that the I-V characteristics are dominated by the contact resistance.     

Normally-off trench JFET technology in 4H silicon carbide

A double gate normally-off silicon carbide (SiC) trench junction field effect transistors (JFET) design is considered. Innovative migration enhanced embedded epitaxial (ME³) growth process was developed to replace the implantation process and realize high device performance. Strong anisotropic behavior in electrical characteristics of the pn junction fabricated on (1 1 −2 0) and (1 −1 0 0) trench a-planes was observed, although quality of the pn diodes was found to be independent of trench plane orientations. Fabricated normally-off trench 4H-SiC JFET demonstrates the potential for lower specific on-resistance (R_onS) in the range of 5-10 mΩ cm² (1200 V class). A relative high T^−2.6 dependence of R_onS is observed. A breakdown voltage of 400 V in the avalanche mode was confirmed at zero gate bias conditions for cell design without edge termination. It was demonstrated that the normally-off JFETs are suitable for high temperature applications. Average temperature coefficient of threshold voltage (V_th) was calculated as −1.8 mV/°C, which is close to the MOS based Si power devices.     

Study of avalanche breakdown and impact ionization in 4H silicon carbide

Epitaxial p-n diodes in 4H SiC are fabricated with uniform avalanche multiplication and breakdown. Photomultiplication measurements were performed to determine electron and hole ionization rates. Theoretical values of critical fields and breakdown voltages in 4H SiC are calculated using the ionization rates obtained. We discuss ionization mechanisms in 4H SiC and make a comparison between silicon carbide and gallium nitride.     

Effect of electron irradiation on carrier removal rate in silicon and silicon carbide with 4H modification

Comparative study of the effect of successive (up to fluences of 3 × 10¹⁶ cm^?2) irradiation with 900 keV electrons of samples made of FZ-Si and 4H-SiC (CVD) has been performed for the first time. Measurements on initial and irradiated samples were made using the van der Pauw method for silicon and the capacitance-voltage technique at a frequency of 1 kHz for silicon carbide. In addition, the spectrum of the defect levels introduced was monitored by the DLTS method for SiC. The carrier removal and defect introduction rates were determined for the two materials. It was found that the rates of defect introduction into FZ-Si and 4 H-SiC (CVD) are close to eachy other (～0.1 cm^?1), which is largely due to the almost identical threshold energies of defect generation.     

Experimental performance of the buried-channel acoustic charge transport device

High-speed electron transport is accomplished in a new type of buried-channel charge transfer device which is based on the nonlinear acoustoelectric interaction between large amplitude surface acoustic waves (SAW) and electrons in GaAs. The performance of the acoustic charge transport (ACT) device is enhanced by design improvements which provide precise channel definition and charge injection structures. A transfer efficiency in excess of 0.996 has been measured in an experimental device operating at the SAW clock frequency of 358 MHz.     

Anisotropy of the solid-state epitaxy of silicon carbide in silicon

A new method for the solid-state synthesis of epitaxial layers is developed, in which a substrate participates in the chemical reaction and the reaction product grows not on the substrate surface, as in traditional epitaxial methods, but inside the substrate. This method offers new opportunities for elastic-energy relaxation due to a mechanism operating only in anisotropic media, specifically, the attraction of point defects formed during the chemical reaction. The attracting point centers of dilatation form relatively stable objects, dilatation dipoles, which significantly reduce the total elastic energy. It is shown that, in crystals with cubic symmetry, the most favorable arrangement of dipoles is the 〈111〉 direction. The theory is tested by growing silicon carbide (SiC) films on Si (111) substrates by chemical reaction with carbon monoxide CO. High-quality single-crystal SiC-4H films with thicknesses of up to 100 nm are grown on Si (111). Ellipsometric analysis showed that the optical constants of the SiC-4H films are significantly anisotropic. This is caused not only by the lattice hexagonality but also by a small amount (about 2–6%) of carbon atoms remaining in the film due to dilatation dipoles. It is shown that the optical constants of the carbon impurity correspond to strongly anisotropic highly oriented pyrolytic graphite.     

Photoluminescence of anodized silicon carbide

The luminescence of single-crystalline 6H-SiC plates after electrochemical etching has been investigated. The photoluminescence spectrum was found to change strongly after etching; the decay times of separate bands were determined. Just as in the case of silicon, the change in the photoluminescence could be due to the formation of nanostructures. Fiz. Tekh. Poluprovodn. 31, 315–317 (February 1997)     

Characteristics of inversion-channel and buried-channel MOS devicesin 6H-SiC

Inversion-channel and buried-channel gate-controlled diodes and MOSFET's are investigated in the wide bandgap semiconductor 6H-SiC. These devices are fabricated using thermal oxidation and ion implantation. The gate-controlled diodes allow room temperature measurement of surface states, which is difficult with MOS capacitors due to the 3 eV bandgap of 6H-SiC. An effective electron mobility of 20 cm²/Vs is measured for the inversion-channel devices and a bulk electron mobility of 180 cm²/Vs is found in the channel of the buried-channel MOSFET. The buried-channel transistor is the first ion-implanted channel device in SIC and the first buried-channel MOSFET in the 6H-SiC polytype     

Growth of 4H silicon carbide crystals on a (11 \bar 2 2) seed

The features of the defect structure of 4H silicon carbide ingots grown by the modified Lely method on (11 $ \bar 2 $ 2)-oriented seeds are studied. It is shown that this seed plane can be used to improve the structural quality of silicon carbide ingots. The defect structure of grown ingots features a complete lack of micropores and a decreased (by an order of magnitude) dislocation density in comparison with the seed. At the same time, growth on the (11 $ \bar 2 $ 2) seed results in stacking fault accumulation. The type of stacking faults corresponds to formula (5, 2) in the Zhdanov notation (internal Frank stacking fault).     

Laser doping of silicon carbide substrates

A direct-write laser conversion technique was used to produce n-type and p-type doped tracks on SiC substrates. Polycrystalline and single-crystal SiC substrates were investigated. The tracks irradiated in an inert gas exhibit a higher electrical resistance than those generated in dopant-containing atmospheres. The effects of various processing parameters, such as the laser-matter interaction time, laser-beam characteristics, number of exposures to the laser beam, and ambient gas in the laser processing chamber, are examined. The laser conversion technique can be used for selective-area doping of silicon carbide substrates to build semiconductor devices for high-temperature applications.     

Electro-chemical mechanical polishing of silicon carbide

In an effort to improve the silicon carbide (SiC) substrate surface, a new electro-chemical mechanical polishing (ECMP) technique was developed. This work focused on the Si-terminated 4H-SiC (0001) substrates cut 8° off-axis toward 〈1120〉. Hydrogen peroxide (H₂O₂) and potassium nitrate (KNO₃) were used as the electrolytes while using colloidal silica slurry as the polishing medium for removal of the oxide. The current density during the polishing was varied from 10 μA/cm² to over 20 mA/cm². Even though a high polishing rate can be achieved using high current density, the oxidation rate and the oxide removal rate need to be properly balanced to get a smooth surface after polishing. A two-step ECMP process was developed, which allows us to separately control the anodic oxidation and removal of formed oxide. The optimum surface can be achieved by properly controlling the anodic oxidation current as well as the polishing rate. At higher current flow (>20 mA/cm²), the final surface was rough, whereas a smoother surface was obtained when the current density was in the vicinity of 1 mA/cm². The surface morphology of the as-received wafer, fine diamond slurry (0.1 μm) polished wafer, and EMCP polished wafer were studied by high-resolution atomic force microscopy (AFM).     

Carcinogenic properties of silicon carbide whiskers

In experiments with intrapleural injections of silicon carbide whiskers (20 mg X 3, with one month interval) to random-inbred rats their carcinogenic activity close to that of chrysotile B UICC has been established. The pleural mesotheliomas were induced in 47.7 and 34.1% of rats, respectively.     

Surface chemistry of porous silicon carbide

The anodization reaction of SiC using HF solution makes a porous silicon carbide (PSC) layer develop. The luminescence behavior of PSC, however, is somewhat different from that of porous Si in that the so-called blue shift is not observed. Though the quantum confinement effect is said to be responsible for light emission in porous SiC, the surface state of PSC plays an important role. The effects of thermal annealing under various atmospheres on the luminescence properties were studied. Some spectroscopic analyses were adopted to elucidate the surface chemistry of PSC. The surface of PSC, which seems to be an origin of the luminescence, had C-H termination but Si-H or Si-O bonds were not detected. X-ray photoelectron spectroscopy analysis also showed that the Si-O bond that usually exists on the surface of bulk SiC was depressed and a strong peak assigned to -CH- appeared. The oxidation treatment reconstructed the Si-O bonds on the PSC surface, and this surface depressed the luminescence. Two other thermal treatments also depressed the PL spectra from the higher energy region, which is due to alternation from C-H to C-C on the surface.     

Carrier drift mobility in porous silicon carbide

The time-of-flight technique was used to determine the drift mobilities of electrons and holes in porous silicon carbide produced by surface anodization of n-type 4H-SiC wafers. The electron and hole mobilities at 300 K in an electric field of 10⁴ V/cm were μ_e=6×10^?3 and μ_h=3×10^?3 cm² V^?1 s^?1, respectively. The low values of the mobilities are accounted for by carrier capture in localized states.     

Progress in silicon carbide semiconductor electronics technology

Silicon carbide’s demonstrated ability to function under extreme high-temperature, high-power, and/or high-radiation conditions is expected to enable significant enhancements to a far-ranging variety of applications and systems. However, improvements in crystal growth and device fabrication processes are needed before SiC-based devices and circuits can be scaled-up and incorporated into electronic systems. This paper surveys the present status of SiC-based semiconductor electronics and identifies areas where technological maturation is needed. The prospects for resolving these obstacles are discussed. Recent achievements include the monolithic realization of SiC integrated circuit operational amplifiers and digital logic circuits, as well as significant improvements to epitaxial and bulk crystal growth processes that impact the viability of this rapidly emerging technology.     

Comparison of the radiation hardness of silicon and silicon carbide

The radiation hardness of silicon carbide and silicon are compared. It is shown that one of the main characteristics of the radiation hardness of a semiconductor, the carrier removal rate V _d , strongly depends on its measurement conditions in the case of wide-gap semiconductors. A conclusion is made that comparison of the values of V _d , obtained at room temperature for SiC and Si, is not fully adequate from the physical standpoint.