N.S. Savkina

Electrical characteristics of p-3C-SiC/n-6H-SiC heterojunctions grown by sublimation epitaxy on 6H-SiC substrates

Abstract Sublimation epitaxy in a vacuum (SEV) has been used to obtain p-3C–SiC/n-6H–SiC heteroepitaxial structures. Results of a study of epilayers (X-ray diffraction analysis, scanning electron microscopy involving secondary electrons and electron beam induced current modes) and diode structures on their base ( I – V and C – V characteristics, electroluminescence spectra, DLTS) are presented. Band discontinuities are determined and a band diagram of the p-3C–SiC/n-6H–SiC heterostructure is constructed.

EFFECT OF BORON DIFFUSION ON THE HIGH-VOLTAGE BEHAVIOR OF 6H-SIC P+NN+ STRUCTURES

Boron diffusion can be used to compensate the n-type layer of a p(+)nn(+) 6H-silicon carbide structure in order to increase its high-voltage capabilities. Measurements under reverse biases for a current range from 10 to 500 mu A show that this process is very efficient for working temperatures about 300 K. Indeed we obtained a voltage of 670 V for a reverse current of 10 mu A instead of the 120 V calculated for a structure without boron diffusion. Nevertheless, the breakdown voltage decreases rapidly when the temperature increases. Capacitance measurements show that the measured doping level in the n-type layer evolves in the same way as the temperature (it ranges from 10(13) cm(-3) at 300 K to 10(17) cm(-3) at 500 K). A great concentration of boron seems to be responsible for this doping variation with temperature. Admittance spectroscopy reveals the presence of D centers at 0.62 eV above the valence band associated to boron at concentration similar or superior to nitrogen concentration in the n-type layer. The increase of the doping level with the temperature is responsible for this decrease of the breakdown voltage.

Influence of proton irradiation on recombination current in 6H–SiC pn structures

Abstract The effect of proton bombardment on recombination current and the value of the steady state lifetime of nonequilibrium carriers for 6H–SiC pn structures created by sublimation epitaxy was investigated. The irradiation was carried out with 8-MeV protons in the range of doses from 10 14 –10 16 cm −2 . Irradiation with a dose of 3.6×10 14 cm −2 increases the recombination current and decreases the steady-state lifetime for deep-level recombination in the space charge region by up to two orders of magnitude. Irradiation with higher doses (up to 5×10 15 cm −2 ) or annealing at temperatures in the range 300–800 K leaves the recombination current and steady state lifetime practically unchanged.

Investigation of the structure of (p)3C-SiC-(n)6H-SiC heterojunctions

The structure of (p)3C-SiC-(n)6H-SiC epitaxial structures obtained by sublimation epitaxy in vacuum on 6H-SiC substrates was studied by methods of X-ray topography, X-ray diffraction, and transmission electron microscopy. The results showed high structure perfection in the epitaxial layers of both SiC polytypes with a sharp interface between the 3C-SiC and 6H-SiC layers.

Detection of strongly and weakly ionizing radiation by triode structure based on SiC films

Device structures based on thick and thin SiC epitaxialfilms have been studied as detectors of alpha particles and weak ionizationradiation (x ray and UV quanta), respectively. In the first case relatively no transistor effect is observed and the signal is formed similarly to that in a diode structure. The possibility of alpha particle spectrometry in spite of slow carrier transport via diffusion has been demonstrated. In the second case, the signal value of the transistor-like detector on applied voltage is investigated. Different modes are used: single alpha-particle detection and induced-current recording from fluxes of x ray and optical (UV) quanta. A superlinear rise in the resulting signal is observed with increasing voltage. The signal is amplified by a factor of several tens with respect to the value chosen for normalization. A description in terms of the phototriode model gives acceptable values for the main parameters: base width, diffusion length of electrons, and space charge of ionized impurities.

Features of the structure of a porous silicon carbide layer obtained by electrochemical etching of a 6H-SiC substrate

Transverse sections of the (11–20) cuts of a 6H-SiC substrate-porous SiC layer-epitaxial 6H-SiC layer structure were studied using electron microscopy. An intermediate layer is revealed between pores and unetched SiC which consists of a damaged region containing two-dimensional defects and a completely amorphous region. Energy-dispersive X-ray spectra measured within local (∼3 nm) areas in various regions of the transverse sections of the structure studied showed that the intermediate layer is enriched with carbon in comparison to the stoichiometric substrate composition. The excess carbon content is retained in the layer of epitaxial SiC contacting the porous layer.

Structure and properties of silicon carbide grown on porous substrate by vacuum sublimation epitaxy

A SiC layer was grown by vacuum sublimation epitaxy on porous silicon carbide. A porous SiC layer about 10 µm thick was fabricated by electrochemical etching of an off-axis 6H-SiC substrate. The epitaxial layer was ∼ 10 µm thick. Structural and optical properties of the initial substrate and the porous and epitaxial layers were investigated by X-ray, IR-reflection and photoluminescence methods. An epitaxial SiC layer grown on porous SiC exhibits improved characteristics when compared to a SiC layer on a conventional substrate.

Radiation defects in n-4H-SiC irradiated with 8-MeV protons

Capacitance methods and electron spin resonance (ESR) were applied to study deep centers in n-6H-SiC irradiated with 8 MeV protons. Schottky diodes and p-n structures grown by sublimation epitaxy or commercially produced by CREE Inc. (United States) were used. The type of the irradiation-induced centers is independent of the material fabrication technology and the kind of charged particles used. Irradiation results in an increase in the total concentration of donor centers. The possible structure of the centers is suggested on the basis of data on defect annealing and ESR.

Radiation resistance of transistor- and diode-type SiC detectors irradiated with 8-MeV protons

Nuclear-particle detectors based on SiC with a structure composed of an n+-type substrate, a p-type epitaxial layer, and a Schottky barrier are studied. Structures with a ∼10-µm-thick 6H-SiC layer exhibit transistor properties, whereas those with a ∼30-µm-thick 4H-SiC layer exhibit diode properties. It is established that a more than tenfold amplification of the signal is observed in the transistor-type structure. The amplification is retained after irradiation with 8-MeV protons with a dose of at least 5×1013 cm−2; in this case, the resolution is ≤10%. Amplification of the signal was not observed in the structures of diode type. However, there were diode-type detectors with a resolution of ≈3%, which is acceptable for a number of applications, even after irradiation with the highest dose of 2×1014 cm−2.

Characterization of p-n structures grown by sublimation heteroepitaxy of 3C-SiC on 6H-SiC

Abstract A study on forward I – V characteristics of p–n structures grown by sublimation heteroepitaxy of 3C-SiC on 6H-SiC shows that about 90% of all the diodes can be placed in two groups. Current–voltage ( I – V ) characteristics of type I diodes are close to those of high-perfection p–n homostructures based on bulk 3C-SiC, with some indications of tunneling currents. I – V characteristics of type II diodes are close to those of p–n homostructures grown by epitaxial methods on single-crystal 6H-SiC substrates. Diodes of both types emit in the entire visible spectral range. The longer-wavelength emission is predominant in type I diodes, and shorter-wavelength emission in type II diodes. However, the main feature of the injection electroluminescence (IEL) is the qualitative similarity of the IEL spectra for diodes of both types. In particular, the IEL spectra of both diode types contain two bands (with hν max ≈2.3 and 2.9 eV), attributed to free exciton annihilation in 3C-SiC and 6H-SiC, respectively.