Andrey O. Konstantinov

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.

PHOTOLUMINESCENCE STUDIES OF POROUS SILICON CARBIDE

A detailed investigation of the dependence of the photoluminescence from porous silicon carbide on preparation conditions and starting material is presented. Porous silicon carbide prepared from different polytypes shows almost identical emission spectra, demonstrating a clear impedance of the band‐gap energy of a particular SiC polytype. Emission bands with peak energies of 2.43, 2.22, 2.07, and 1.93 eV were resolved with the use of selective excitation by tuning the excitation wavelength. The origin of luminescence is suggested to relate to defect states produced at the etched surface.

Electrical properties and formation mechanism of porous silicon carbide

Variation of the preparation conditions of porous silicon carbide is shown to have a strong effect on the structural and electrical properties of the material obtained. A correlation has been observed between the fiber size and resistivity of porous SiC, a decrease of fiber size results in a semi‐insulating material due to Fermi‐level pinning to surface states. A model is proposed for the mechanism of fiber size self‐regulation responsible for the porous material formation. The model relates the blocking of the fiber dissolution process to the increase of resistivity in a thin fiber due to Fermi‐level pinning. We suggest that the Fermi‐level pinning model is also applicable to the formation mechanism of porous silicon.

Progress towards SiC products

Since the late 1980s, there has been a significant effort in the development of technology based on the semiconductor SiC, aimed at utilising the unique physical and electrical properties of this material to achieve high performance devices. Many factors governing the progress in development are beginning to converge with the result that the first commercial devices are appearing in the market place. The following article reviews some of the factors that are determining the progress in different application areas, including power electronics, high frequency devices and sensors. Breakthroughs in the technology in these areas will be believed to accelerate the adoption of SiC as a mature semiconductor material.

Temperature dependence of the photoelectric conversion quantum efficiency of 4H–SiC Schottky UV photodetectors

Ultraviolet Schottky photodetectors based on n-4H–SiC (Nd − Na = 4 × 1015 cm−3) epitaxial layers of high purity have been fabricated. Their spectral sensitivity range is 3.2–5.3 eV peaking at 4.9 eV (quantum efficiency is about ~0.3 electron/photon), which is close to the bactericidal ultraviolet radiation spectrum. The temperature dependence of the quantum efficiency of 4H–SiC Schottky structure has been investigated to determine the temperature stability and the mechanism of the photoelectric conversion process. At low temperatures (78–175 K) the quantum efficiency increases with increasing temperature for all photon energy values and then tends to saturate. We suppose that some imperfections in the space-charge region act as traps that capture both photoelectrons and photoholes. After some time the trapped electron–hole pairs recombine due to the tunnelling effect. At high temperatures (more than 300 K), the second enhancement region of the quantum efficiency is observed in the photon energy range of 3.2–4.5 eV. It is connected with a phonon contribution to indirect optical transitions between the valence band and the M-point of the conduction band. When the photon energy is close to a direct optical transition threshold this enhancement region disappears. This threshold is estimated to be 4.9 eV. At photon energies more than 5 eV a drastic fall of the quantum efficiency has been observed throughout the temperature interval. We propose that in this case the photoelectrons and photoholes are bound to form hot excitons in the space-charge region due to the Brillouin zone singularity, and do not contribute to the following photoelectroconversion process.

Temperature dependence of avalanche breakdown for epitaxial diodes in 4H silicon carbide

The temperature dependence of avalanche breakdown is investigated for uniform and microplasma-related breakdown in epitaxial 4H SiC p-n junctions. P-n mesa diodes fabricated with positive angle beveling and oxide passivation can withstand temperatures of up to 300–400 °C in the breakdown regime. Uniform avalanche breakdown in 4H silicon carbide appears to have a positive temperature coefficient, in contrast to the 6H polytype, where the temperature coefficient is negative. The influence of deep levels on avalanche breakdown in epitaxial diodes is of minor importance for uniform breakdown, but appears to be significant for breakdown through microplasmas. A negative temperature coefficient for the avalanche breakdown voltage can be observed even for 4H SiC if the breakdown is dominated by microplasmas.

Effect of vapor composition on polytype homogeneity of epitaxial silicon carbide

The silicon to carbon precursor ratio is demonstrated as strongly affecting the spontaneous nucleation of cubic SiC upon the growth of epitaxial layers of 4H and 6H silicon carbide using the chemical vapor deposition (CVD) technique. High C/Si ratios appear to promote the nucleation of cubic SiC. A model of CVD process chemistry that relates the effect to a decrease of SiC surface mobility with an increase of the C/Si ratio is proposed. The resulting increase of supersaturation at the surface terraces promotes the spontaneous nucleation of cubic SiC.

Homoepitaxy of 6H and 4H SiC on nonplanar substrates

Growth by vapor phase epitaxy around stripe mesas and in trenches formed by reactive ion etch on 6H and 4H SiC substrates has been investigated. The mesas were aligned with the low index 〈1120〉 and 〈1100〉 directions, as well as with the high index 〈1,1+∛,2+∛¯,0〉 directions, in order to reveal and study the growth habit. It was found that a low C:Si ratio gave a smooth growth and small differences in growth rate between lattice planes. A larger C:Si ratio gave more faceted growth, both limited by surface kinetics and surface diffusion, and the growth rate was 10% lower in the [1100] direction and 10% higher in the [1120] direction, than on the substrate. Growth on mesas oriented parallel to the substrate off-orientation shows clear step-flow growth, while growth on mesas oriented perpendicular to the off-orientation reveals the singular (0001) plane, where islands are observed, which might indicate Stranski–Krastanov growth.

High-dose Al-implanted 4H-SiC p+-n-n+ junctions

p+-n-n+ junctions were fabricated by ion implantation with Al of low-doped epitaxial n layers of 4H-SiC grown by chemical vapor deposition on commercial 4H-SiC wafers both with and without reduction of micropipe densities. It was shown that, using high levels of Al ion doping (5×1016 cm−2) in combination with rapid thermal anneal, single-crystal p+-4H-SiC layers can be obtained. These layers do not form barriers at the contact metal–semiconductor interface and do not introduce additional resistance into structures with p+-n junctions. This significantly reduces the forward voltage drop across the structure in a wide range of current densities up to 104 A cm−2.

Material defects in 4H-silicon carbide diodes

Crystallographic defects revealed by synchrotron white beam x-ray topography, electron beam induced current, optical microscopy, and electroluminescence are correlated with the electrical characteristics of medium-voltage epitaxial 4H-silicon carbide diodes. Diodes that include macroscopic crystallographic defects show a significantly reduced reverse breakdown voltage with typical microplasma current fluctuations under reverse bias. Microplasma current paths are revealed by increased electroluminescence both under forward and reverse bias of the diodes and coincide with the locations of screw dislocations in the epitaxial layers of the diodes. The role of crystallographic imperfections on the formation of stacking faults responsible for the degradation of bipolar silicon carbide components is discussed.