Q. Wahab

Ionization rates and critical fields in 4H silicon carbide

Epitaxial p-n diodes in 4H SiC are fabricated showing a good uniformity of avalanche multiplication and breakdown. Peripheral breakdown is overcome using the positive angle beveling technique. Photomultiplication measurements were performed to determine electron and hole ionization rates. For the electric field parallel to the c-axis impact ionization is strongly dominated by holes. A hole to electron ionization coefficient ratio of up to 50 is observed. It is attributed to the discontinuity of the conduction band of 4H SiC for the direction along the c axis. Theoretical values of critical fields and breakdown voltages in 4H SiC are calculated using the ionization rates obtained.

High-carbon concentrations at the silicon dioxide–silicon carbide interface identified by electron energy loss spectroscopy

High carbon concentrations at distinct regions at thermally-grown SiO2/6H-SiC(0001) interfaces have been detected by electron energy loss spectroscopy (EELS). The thickness of these C-rich regions is estimated to be 10-15 Angstrom. The oxides were grown on n-type 6H-SiC at 1100 degrees C in a wet O-2 ambient for 4 h immediately after cleaning the substrates with the complete RCA process. In contrast, C-rich regions were not detected from EELS analyses of thermally grown SiO2/Si interfaces nor of chemical vapor deposition deposited SiO2/SiC interfaces. Silicon-rich layers within the SiC substrate adjacent to the thermally grown SiO2/SiC interface were also evident. The interface state density D-it in metal-oxide-SiC diodes (with thermally grown SiO2) was approximately 9x10(11) cm(-2) eV(-1) at E- E-v=2.0 eV, which compares well with reported values for SiC metal-oxide-semiconductor (MOS) diodes that have not received a postoxidation anneal. The C-rich regions and the change in SiC stoichiometry may be associated with the higher than desirable D-its and the low channel mobilities in SiC-based MOS field effect transistors

Influence of epitaxial growth and substrate-induced defects on the breakdown of 4H-SiC Schottky diodes

Morphological defects and elementary screw dislocations in 4H-SiC were studied by high voltage Ni Schottky diodes. Micropipes were found to severely limit the performance of 4H-SiC power devices, w ...

A 3 kV Schottky barrier diode in 4H-SiC

High-voltage Schottky barrier diodes with low reverse leakage current were processed on hot-wall chemical vapor deposition grown 4H-SiC films. A metal overlap onto the oxide layer was employed to reduce electric field crowding at the contact periphery. By utilizing a 42–47 μm thick, high-quality epitaxial layers with doping in the range of 7×1014–2×1015 cm−3, a record blocking voltage of above 3 kV was achieved. The large diodes with 1.0 mm diameter showed breakdown at 2.1 kV. The reverse leakage current density at 1.0 kV was measured to be 7.0×10−7 A cm−2. Specific on-resistance of the diode with breakdown voltage at 3 kV was 34 mΩ cm2.

Growth of 6H and 4H—SiC by sublimation epitaxy

The epitaxial sublimation growth process of SiC has been investigated. Layers with specular surfaces and growth rates up to 2 mm/h have been obtained. No step bunching is observed by optical microscopy even on very thick layers which indicates a stable step growth mechanism. Under certain growth conditions the morphology degrades. The morphological stability is investigated and discussed in relation to the growth kinetics. Impurities in the epitaxial layers are investigated by secondary ion mass spectroscopy and low-temperature photoluminescence. The carrier concentration is measured by capacitance–voltage measurements. The structural quality of the grown material is improved compared to the substrate as shown by X-ray diffraction measurements.

Inhomogeneities in Ni/4H-SiC Schottky barriers: Localized Fermi-level pinning by defect states

We investigated arrays of Ni, Pt, or Ti Schottky diodes on n-type 4H-SiC epitaxial layers using current-voltage (I-V) measurements, electron beam induced current (EBIC), polarized light microscopy, x-ray topography, and depth-resolved cathodoluminescence spectroscopy. A significant percentage of diodes (∼7%–30% depending on epitaxial growth method and diode size) displayed “nonideal” or inhomogeneous barrier height characteristics. We used a thermionic emission model based on two parallel diodes to determine the barrier heights and ideality factors of high- and low-barrier regions within individual nonideal diodes. Whereas high-barrier barrier heights increased with metal work function, low-barrier barrier heights remained constant at ∼0.60, 0.85, and 1.05eV. The sources of these nonidealities were investigated with a variety of spectroscopic and imaging techniques to determine the nature and energy levels of the defects. EBIC indicated that clusters of defects occurred in all inhomogeneous diodes. Cathod...

Observation of a Non-stoichiometric Layer at the Silicon Dioxide – Silicon Carbide Interface: Effect of Oxidation Temperature and Post-Oxidation Processing Conditions

Thermal oxides were grown on n-type 6H-SiC(0001) at 1100 °C for 2 hrs in a wet oxygen ambient after the substrates were cleaned using the complete RCA cleaning process. Metal-oxide-semiconductor (MOS) diodes were then fabricated and subsequently cleaned under different annealing conditions, including re-oxidation-, NO-, and forming gas (10% H 2 + 90% N 2 )-annealing at 950 °C for one hour. Measurements of the interface state densities (D it ) at room temperature showed that post oxidation annealing (POA) reduced the D it values to a varying degree depending on the specific annealing condition. Annealing in NO and forming gas resulted in the largest reduction in D it values. Oxides were grown at 950, 1100, and 1250 °C for varying amounts of time without receiving POA. A non-stoichiometric (Si x C, x>1) transition layer adjacent to the SiO 2 /SiC interface has been observed by electron energy loss spectroscopy (EELS). The thickness of this layer was found to increase with oxidation temperature and was not observed at all for a thin oxide grown at 950 °C. The D it values (close to the conduction band) for diodes with oxides grown at 950°C without POA were lower than the D it values for the samples oxidized at 1100 °C with any of the POA treatments. While the thickness of the transition layer was found to be dependent on temperature, our results indicate that it is independent of oxide thickness. This transition layer may be associated with the high D it values and low channel mobilities for SiC MOSFETs.

Growth of epitaxial 3C-Sic films on (111) silicon substrates at 850 "C by reactive magnetron sputtering

Reactive magnetron sputtering in a mixed Ar/CH4 discharge has been used to deposit 3C‐SiC films on (111)‐oriented Si substrates. The carbon content as well as the crystalline structure was found to depend on both the CH4 and Ar pressures. At a total pressure of 3 mTorr and a CH4 partial pressure of 0.6 mTorr, epitaxial stoichiometric films were obtained at growth temperatures as low as 850 °C. The epitaxial nature of the films was established by x‐ray diffraction using a combination of reciprocal space maps, texture scans, and 360° φ scans. Based on these analyses it could also be concluded that double‐positioning domains rotated 60° to one another as well as other defects, giving rise to a mosaic broadening in the reciprocal space maps, were present in the films. Furthermore, based on plasma probe measurements and determination of the electron energy distribution functions in the near‐substrate vicinity, the low growth temperature of 850 °C is suggested to be a consequence of an effective decomposition o...

Defect-driven inhomogeneities in Ni∕4H–SiC Schottky barriers

Nanoscale depth-resolved cathodoluminescence spectroscopy (DRCLS) of Ni diode arrays on 4H-SiC epitaxial wafers reveals a striking correspondence between deep level defects and electrical transport measurements on a diode-by-diode basis. Current-voltage measurements display both ideal and nonideal diode characteristics due to multiple barriers within individual contacts. Near-interface DRCLS demonstrates the presence of three discrete midgap defect levels with 2.2, 2.45, and 2.65eV emission energies whose concentrations vary on a submicron scale among and within individual diodes, correlating with barrier inhomogeneity. These results also suggest that SiC native defect levels can account for the maximum range of n-type barrier heights.

Growth of high‐quality 3C‐SiC epitaxial films on off‐axis Si(001) substrates at 850 °C by reactive magnetron sputtering

Single crystal 3C‐SiC films of thickness ∼10 μm were grown by reactive magnetron sputtering on off‐axis Si(001) at 850 °C. The film quality was comparable to the best chemical vapor deposited 3C‐SiC as characterized by x‐ray diffraction, transmission electron microscopy, and photo‐ luminescence (PL). The lattice mismatch and difference in thermal contraction between SiC and Si resulted in complete arrays of misfit dislocations at the film/substrate interface and a residual in‐plane strain of (−6.9±1)×10−4, respectively. The full width at half‐maximum (FWHM) of the 3C‐SiC(004) peak was 59 arcsec in ω‐2θ direction. The structural defects in the SiC film were planar faults on {111} planes and no voids or plastic deformation in the Si substrate were observed. PL spectra of SiC films showed characteristic appearances with N bound‐exciton (N‐BE) lines with a FWHM value of 3.4 meV. The deposition conditions leading to low defect‐density 3C‐SiC films are discussed in terms of low‐energy ion‐surface interactions.