Irradiated 4H-SiC Research Articles

Intrinsic defects in high purity semi-insulating 6H SiC

AbstractElectron paramagnetic resonance (EPR) and photo EPR measurements were performed on undoped semi-insulating (SI) 6H SiC material grown by the physical vapor transport (PVT) method. EPR lines from a carbon-related surface defect, two photosensitive high temperature stable intrinsic defects with S=1/2 and shallow nitrogen and boron were observed. The EPR spectrum of the intrinsic defect labeled XX was observed in the dark and consists of three single lines XX1, XX2, XX3 due to three inequivalent sites in the 6H SiC. The second defect labeled PP appeared in EPR spectrum during photo-excitation of the SI 6H SiC and consisted of a single EPR line. The photo EPR data placed the energy level of the defects in the region EV + 1.24 ÷ 1.29 eV. The EPR parameters and symmetry behavior of XX agree well with those of the carbon vacancy-related Ky center observed in p-type electron irradiated 6H SiC and we tentatively identify the XX defect with VC+. The electronic processes occurring in undoped SI 6H SiC under photo-excitation are also described.

Displacement damage effects on the forward bias characteristics of SiC Schottky barrier power diodes

Commercial SiC Schottky barrier power diodes have been subjected to 203 MeV proton irradiation and the effects of the resultant displacement damage on the I-V characteristics have been observed. The diodes show excellent resistance to radiation damage. Changes in forward and reverse bias I-V characteristics are reported for irradiated 4H SiC commercial Schottky barrier diodes at fluences up to 2.5/spl times/10/sup 14/ p/cm/sup 2/. Small changes are seen in the reverse bias I-V characteristics with the reverse leakage actually decreasing with increasing irradiation fluence. In forward bias, the series resistance is observed to increase as the fluence increases. The changes in series resistance are interpreted as being due to changes in the effective dopant density due to carrier removal by the defects produced.

Effect of heavy proton and neutron irradiations on epitaxial 4H-SiC Schottky diodes

In this work we report electrical characterizations on heavily irradiated epitaxial 4H-SiC Schottky diodes. Even after an irradiation at a fluence of 1.4 × 10 16 p / cm 2 and 7 × 10 15 n / cm 2 , we found the diodes still able to detect α and β particles with a charge collection efficiency (CCE) ranging from 25 to 30% after proton irradiation and about 18% after neutron irradiation, at the highest reverse bias applied. This corresponds to a charge collection distance (CCD) of 7 μ m after the proton irradiation and 5 μ m after the neutron irradiation. As the irradiation level approaches the range ∼ 10 15 / cm 2 , the material behaves as intrinsic due to a very high compensation effect.

Optical Centres with Local Vibrational Modes Created by High Temperature Annealing of Electron Irradiated 4H and 6H Silicon Carbide

New results are presented concerning several optical centres having local vibrational modes in electron irradiated and annealed 4H and 6H SiC. Some of these centres are common to both polytypes, others have only been found in 6H SiC. They appear, typically, after annealing in the range 1000°C - 1300°C. Additional results have been obtained about mode splitting from 13C isotope enriched 6H SiC.

Electron Irradiation Induced Vacancy Defects Detected by Positron Annihilation in 6H-SiC

This paper presents positron lifetime results which give information on the nature of vacancy defects induced by electron irradiation in bulk nitrogen doped (nD-nA= 2.3x1017 cm-3) Cree 6H-SiC. The electron irradiations have been performed at different energies and with different fluences from 51017 e-cm-2 to 31018 e-cm-2. Positron lifetime have been measured with a 22NaCl source as a function of temperature between 15 and 300 K. The lifetime spectra were analyzed as sums of two exponential lifetime components i weighted by the intensities Ii, convoluted with the resolution function. From the temperature dependence of the lifetime spectra we can infer that several vacancy defects exist in the electron irradiated n-type 6H-SiC. The nature of detected vacancy defects depends on the electron energy.

Defect Evolution in Ion Irradiated 6H-SiC Epitaxial Layers

Deep-Level Transient Spectroscopy and room temperature photoluminescence were used to characterise a 6H-SiC epitaxial layer irradiated with 10 MeV C+ and to follow the defect annealing in the temperature range 300-1400 °C. The intensity of luminescence peak at 423 nm, related to band to band transitions, decreases after irradiation and it is slowly recovered after annealing in the temperature range 1000-1400 °C. The DLTS spectra of low temperature annealed samples show the presence of several overlapping traps, which anneal and evolve at high temperatures. After 1200 °C a main level at Ec-0.43 eV (E1/E2) is detected. The comparison between luminescence and DLTS results indicates that the defect associated with the E1/E2 level is mainly responsible for the luminescence quenching after irradiation.

Evidence for a Deep Two Charge State Defect in High Energy Electron Irradiated 4H-SiC

Evidence for a Deep Two Charge State Defect in High Energy Electron Irradiated 4H-SiC

Ab initiostudy of the annealing of vacancies and interstitials in cubic SiC: Vacancy-interstitial recombination and aggregation of carbon interstitials

The annealing kinetics of mobile intrinsic defects in cubic SiC is investigated by an ab initio method based on density-functional theory. The interstitial-vacancy recombination, the diffusion of vacancies, and interstitials to defect sinks (e.g., surfaces or dislocations) as well as the formation of interstitial clusters are considered. The calculated migration and reaction barriers suggest a hierarchical ordering of competing annealing mechanisms. The higher mobility of carbon and silicon interstitials as compared to the vacancies drives the annealing mechanisms at lower temperatures including the vacancy-interstitial recombination and the formation of interstitial carbon clusters. These clusters act as a source of carbon interstials at elevated temperatures. In p-type material the transformation of the silicon vacancy into the more stable vacancy-antisite complex constitutes an annealing mechanism which is activated before the vacancy migration. Recent annealing studies of vacancy-related centers in irradiated 3C-SiC and 4H-SiC and semi-insulating 4H-SiC are interpreted in terms of the proposed hierarchy of annealing mechanisms.

Electrically active defects in irradiated 4H-SiC

4H-SiC epilayers were irradiated with either protons or electrons and electrically active defects were studied by means of deep level transient spectroscopy. Motion of defects has been found to occur at temperature as low as 350–400 K. Indeed, the application of an electric field has been found to enhance modifications in defect concentrations that can also occur during long time annealing at elevated temperature. Two levels have been revealed and labeled B and M. Two other levels, referred to as S1 and S2 and located at 0.40 and 0.71 eV below the conduction band edge have been studied in detail (capture cross sections, profiling, formation energy, activation energy during annealing). The S1 and S2 levels have been found to exhibit a one to one relation and are proposed to be two charge states of the same acceptor center, labeled the S center.

Identification of Dumb-Bell Shaped Interstitials in Electron Irradiated 6H SiC by Photoluminescence Spectroscopy

Identification of Dumb-Bell Shaped Interstitials in Electron Irradiated 6H SiC by Photoluminescence Spectroscopy

Performance of gamma irradiated P-channel 6H SiC MOSFETs: high total dose

We present the first observation of total dose effects of gamma-ray irradiation on enhancement mode 6H-SiC p-channel MOSFETs. The electrical characterization of these transistors was performed before and after irradiation up to a total dose of 10/sup 8/ rad (SiO/sub 2/) by measuring the drain-source current (I/sub ds/) as a function of gate voltage (V/sub g/) and drain-source voltage (V/sub ds/). These transistors were compared to 6H-SiC n-channel MOSFETs. The p-channel devices remain fully functional up to 10/sup 6/ rad (SiO/sub 2/), while the n-channel devices are fully functional through 10/sup 8/ rad (SiO/sub 2/). We found that the generation of radiation-induced interface states in the n-channel transistors is substantially lower than that of the p-channel devices.

The Neutral Silicon Vacancy in SiC: Ligand Hyperfine Interaction

The isolated silicon vacancy in its neutral charge state has unambiguously been confirmed in electron irradiated 4H and 6H SiC. This was achieved by the observation of the ligand hyperfine lines arising from interaction with C-13 atoms in the nearest-neighbor (NN) shell and With Si-29 atoms in the next-nearest-neighbor (NNN) shell in optically detected magnetic resonance (ODMR) experiments. The complete hyperfine tensors for all inequivalent lattice sites have been deduced and are compared to the known hyperfine parameters for the negatively charged silicon vacancy in the two polytypes.

Defect Accumulation and Recovery in Ion-Implanted 6H-SiC

AbstractSingle crystal wafers of <0001>-oriented 6H-SiC were irradiated at different temperatures using a variety of ion species. The disorder on both the Si and C sublattices has been studied in situ using a combination of ion beam analyses in multiaxial channeling geometry. The fraction of the irradiation-induced defects surviving simultaneous recovery processes decreases with decreasing ion mass and with increasing irradiation temperature. Some of the Si and C defects are well aligned with the <0001> axis and the rate of C disordering is higher than that of Si disordering. Three recovery stages in Au2+-irradiated 6H-SiC have been identified.

Paramagnetic defects in silicon carbide crystals irradiated with gamma-ray quanta

The results of the first observations of paramagnetic defects in SiC crystals irradiated with gamma-ray quanta are reported. Three types of defects, designated as γ1, γ2, and γ3, were detected in irradiated 4H-SiC:Al and 6H-SiC:Al crystals using electron spin resonance (ESR) measurements. All these centers have almost the same parameters of the spin-related Hamiltonian with S=1/2 and feature an appreciable anisotropy of the g-factors. The γ1 centers are almost coaxial with the local z-axis oriented approximately along one of the directions of the Si-C bond that does not coincide with the c-axis. The γ2 and γ3 centers have a lower symmetry, although the orientation along the above bonds is clearly pronounced. The values of the largest g-factor (g z) decreases in the sequence from γ1 to γ3. The γ1 signal can be detected at temperatures of 3.5–15 K; the γ2 and γ3 signals are detectable at temperatures of 10–35 and 18–50 K, respectively. The hyperfine interaction of an unpaired electron in the γ1 center with a nucleus of the 29Si isotope is detected for certain orientations of the crystal. The γ1, γ2, and γ3 centers cease to exist at a temperature of 160°C; it is concluded that the ESR signals of these centers are related to defects in the C sublattice. It is assumed that the γ1, γ2, and γ3 centers have a common origin and are related to the low-temperature (γ1) and high-temperature (γ2 and γ3) modifications of the same center. The models of a defect in the form of either a carbon vacancy or a complex incorporating an Al impurity atom and a C atom that occupies the silicon site or interstice are discussed.

Characterization of deep centers in bulk n-type 4H–SiC

Nitrogen-related shallow centers and defect- and impurity-related deep centers in nitrogen doped bulk n-type 4H–SiC were characterized by high temperature Hall effect and deep level transient spectroscopy (DLTS) measurements. Two nitrogen centers at E c−0.048 and −0.098 eV, with a concentration close to each other, and a poorly resolved deep donor level, with activation energy of 0.58 eV, were found by fitting the Hall-effect data. A dominant DLTS deep center is observed at E c−0.61–0.63 eV due to electric field effect, with a concentration of ∼1.2×10 15 cm −3. The center is believed to be the Z 1 center (also called Z 1/ Z 2(4H)), which is often observed in as-grown and implanted or irradiated 4H–SiC epilayers and is speculated to be a vacancy-type defect. Correlation between the Hall effect deep donor and Z 1 suggests that Z 1 is donor-like.

Vacancy-related defects in ion-beam and electron irradiated 6H–SiC

A brief review is given on EPR study of irradiation-induced defects in SiC. The results of low-temperature study of Ky1 and Ky2 centers reveal for both of them the C S symmetry, spin S= 1 2 and close coincidence of the g-tensor components. For Ky2 defect the principal values of g-tensor have been determined as g z =2.0048, g x =2.0022 and g y =2.0037, where z and x directions reside in the (1 1 2 ̄ 0) plane and the z-axis makes up an angle 65° with the c-axis. The same residence of z- and x-axis and an angle 59° are found for Ky1 center, g z =2.0058, g x =2.0025 and g y =2.0023. A comparison of experimental and calculated hyperfine (HF) parameters is presented which suggests that Ky2 and Ky1 defects can be assigned to the positively charged carbon vacancy in 6H–SiC. The EPR study of defects created along the Al + ion track in n-type 6H–SiC shows that lineshape, linewidth and integral intensity of the EPR signal reflect the state of damaged layer generated by ion implantation. A variation of defect density with annealing is reported and defect origin is discussed.

Effects of Illumination on Positron Lifetime of Electron Irradiated n-type 6H-SiC

Effects of Illumination on Positron Lifetime of Electron Irradiated n-type 6H-SiC

Accumulation and recovery of disorder on silicon and carbon sublattices in ion-irradiated 6H–SiC

Irradiation experiments have been performed at 100, 170 and 300 K for 6H–SiC single crystals using Au 2+ and He + ions over a range of fluences. The evolution of disorder on both the Si and C sublattices has been simultaneously investigated using 0.94 MeV D + Rutherford backscattering spectrometry (RBS) in combination with 12C(d,p) nuclear reaction analysis (NRA) in a 〈0 0 0 1〉 axial channeling geometry. At low doses, a higher rate of C disordering is observed, which is consistent with molecular dynamics simulations that suggest a smaller threshold displacement energy on the C sublattice. At higher doses for He + irradiation, the C disordering appears to increase less rapidly than the Si disordering. Three distinct recovery stages are observed on both the Si and C sublattices in the Au 2+-irradiated 6H–SiC. However, complete recovery of irradiation-induced disorder does not occur during isochronal annealing at temperatures up to 970 K.

X-Ray Diffraction Line Profile Analysis of Neutron Irradiated 6H-SiC

X-Ray Diffraction Line Profile Analysis of Neutron Irradiated 6H-SiC

Optical Characterization of Lattice Damage and Recovery in Ion-Implanted and Pulsed Excimer Laser Irradiated 4H-SiC

Optical Characterization of Lattice Damage and Recovery in Ion-Implanted and Pulsed Excimer Laser Irradiated 4H-SiC