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Photoexcitation electron paramagnetic resonance (photo-EPR) was used to determine deep levels related to the carbon vacancy (VC) in 4H-SiC. High-purity free-standing n-type 4H-SiC epilayers with concentration of intrinsic defects (except the photo-insensitive SI1 center) below the detection limit of EPR were irradiated with low-energy (200 keV) electrons to create mainly VC and defects related to the C sublattice. The simultaneous observation of and signals, their relative intensity changes and the absence of other defects in the sample provide a more straight and reliable interpretation of the photo-EPR results. The study suggests that the (+|0) level of VC is located at ~EC–1.77 eV in agreement with previously reported results and its single and double acceptor levels may be at ~ EC–0.8 eV and ~ EC–1.0 eV, respectively.

Defects introduced by electron irradiation at ~80-100 K in 3C-, 4H- and 6H-SiC were studied by electron paramagnetic resonance (EPR). A number of EPR spectra, labelled LE1-10, were detected. Combining EPR and supercell calculations, we will show that the LE1 center in 3C-SiC with C2v symmetry and an electron spin S=3/2 is related to the (VSi-Sii)3+ Frenkel pair between the silicon vacancy and a second neighbour Sii interstitial along the <100> direction. Results on other centers, possibly also related to interstitials, are discussed.

A model is presented for the silicon vacancy in SiC. The previously reported photoluminescence spectra in 4H and 6H SiC attributed to the silicon vacancy are in this model due to internal transitions in the negative charge state of the silicon vacancy. The magnetic resonance signals observed are due to the initial and final states of these transitions.

Semi-insulating (SI) 4H-SiC substrates doped with vanadium (V) in the range 5.5×1015 –1.1×1017 cm–3 were studied by electron paramagnetic resonance. We show that only in heavily V-doped 4H-SiC vanadium is responsible for the SI behavior, whereas in moderate V-doped substrates with the V concentration comparable or slightly higher than that of the shallow N donor or B acceptor, the SI properties are thermally unstable and determined by intrinsic defects. The results show that the commonly observed thermal activation energy Ea~1.1 eV in V-doped 4H-SiC, which was previously assigned to the single acceptor V4+/3+ level, may be related to deep levels of the carbon vacancy. Carrier compensation processes involving deep levels of V and intrinsic defects are discussed and possible thermal activation energies are suggested.

We have studied the small clusters of silicon and carbon interstitials by ab initio supercell calculations in 4H-SiC. We found that silicon interstitials can form stable and electrically active complexes with each other or with a carbon interstitial. Local vibration modes and ionization energies were also calculated in order to help the identification of the defects. We propose that silicon interstitials can emit from these clusters at relatively high temperatures, which may play an important role in the formation of the DI center.

High-purity, semi-insulating 6H-SiC substrates grown by high-temperature chemical vapor deposition were studied by electron paramagnetic resonance (EPR). The carbon vacancy (VC), the carbon vacancy-antisite pair (VCCSi) and the divacancy (VCVSi) were found to be prominent defects. The (+|0) level of VC in 6H-SiC is estimated by photoexcitation EPR (photo-EPR) to be at ~ 1.47 eV above the valence band. The thermal activation energies as determined from the temperature dependence of the resistivity, Ea~0.6-0.7 eV and ~1.0-1.2 eV, were observed for two sets of samples and were suggested to be related to acceptor levels of VC, VCCSi and VCVSi. The annealing behavior of the intrinsic defects and the stability of the SI properties were studied up to 1600°C.

In EPR (electron paramagnetic resonance) identification of point defects, hyperfine (HF) interaction is decisive information not only for chemical identity but also for the local geometry and the electronic state. In some intrinsic defects in SiC, the wave function of the unpaired electron extends quite unevenly among major atoms comprising the defects. In such a case, the determination of the number of equivalent atoms and the chemical identity (Si or C) of those atoms even with weak HF splitting are useful to compare with HF parameters obtained theoretically. For vacancy-related defects of relatively deep levels, the sum of the spin densities on the nearest-neighbor shell is found to be 60-68%.

Vacancies, divacancies and carbon vacancy-carbon antisite pairs are found by electron paramagnetic resonance (EPR) to be dominant defects in high-purity semi-insulating (HPSI) 4HSiC substrates having different thermal activation energies of the resistivity ranging from ~0.8 eV to ~1.6 eV. Based on EPR results and previously reported data, the energy positions of several acceptor states of the vacancies and vacancy-related complexes are estimated. These deep levels are suggested to be associated to different thermal activation energies and responsible for the semiinsulating behaviour in HPSI SiC substrates. Their role in carrier compensation is discussed.

The SI-5 electron-paramagnetic-resonance (EPR) centre is a dominant defect in some high-purity semi-insulating (HPSI) SiC substrates and has recently been shown to originate from the negatively charged carbon vacancy-carbon antisite pair (VC − Si C ). In this work, photoexcitation EPR (photo-EPR) was used for determination of the energy position of deep acceptor levels of VCCSi in 4H-SiC. Our photo-EPR measurements in slightly n-type material show an increase of the EPR signal of VC − Si C for photon energies from ~0.8 eV to ~1.3 eV. Combining the data from EPR, deep level transient spectroscopy and supercell calculations we suggest that the (1–|2–) levels of the different configurations of the defect are located in the range ~0.8-1.1 eV below the conduction band.

We have investigated several aluminum-related complexes in 4H-SiC by ab initio supercell calculations. The binding energies of the defects predict high thermal stability and complex formation between aluminum and carbon interstitials in SiC. We show that the carbon vacancy can be attached to a shallow the aluminum acceptor and form a very stable defect. We also found that aluminum interstitial forms stable and metastable complexes with one or two carbon interstitials. The possible relation of these defects to the recently found aluminum- related DLTS centers is also discussed.

The influence of different cooling rates on deep levels in 4H-SiC after high temperature annealing has been investigated. The samples were heated from room temperature to 2300°C, followed by a 20 minutes anneal at this temperature. Different subsequent cooling sequences down to 1100°C were used. The samples have been investigated using photoluminescence (PL) and IV characteristics. The PL intensities of the silicon vacancy (VSi) and UD-2, were found to increase with a faster cooling rate.

Interaction of boron and aluminum with interstitial carbon is studied using first principles calculations. It is shown that carbon can form very stable complexes with Al and B, forming a family of negative-U bistable defects with deep levels. The influence of this effect on the activation rate of p-type implants is discussed.

Electron paramagnetic resonance (EPR) was used to study 4H- and 6H-SiC doped with P during chemical vapour deposition (CVD) growth. In 6H-SiC, three spectra with C3v symmetry and spin S=1/2, labelled Ph, Pc1 and Pc2, were detected. The g-values and the 31P hyperfine (hf) constants were determined for Ph: g||=2.0046, g^=2.0028, and A||=0.103 mT, A^<0.05 mT; for Pc1: g||=2.0039, g^=2.0025, and A||=0.615 mT, A^=0.43 mT; for Pc2: g||=2.0038(5), g^=2.0025, and A||=0.40 mT, A^=0.22 mT. The hf interaction with nearest 13C neighbours were also observed for the Pc1 and Pc2 centers, confirming that in CVD grown material the shallow P donor occupies the Si site. The Ph, Pc1 and Pc2 centers are assigned to the ground states of the shallow P at the hexagonal (Ph) and quasi-cubic sites (Pc1 and Pc2) in 6H-SiC. In 4H-SiC, an EPR spectrum of C3v symmetry with a larger anisotropy in the g-values (g||=2.0065 and g^=2.0006) was observed. The temperature dependence of the spectrum is similar to that of Ph in the 6H polytype. Its 31P hyperfine constants are determined as A||=0.294 mT and A^=0.21 mT.

Shallow N donors in n-type 4H-SiC were studied by electron paramagnetic resonance (EPR) and electron nuclear double resonance (ENDOR). For the N donor at the cubic site (Nk) in 4H-SiC, the hyperfine (hf) constants of the interaction with the nearest neighbour (NN) 29Si atom along the c axis were determined as A = 41.07 MHz and A^ = 41.31 MHz. For other three NN Si atoms in the basal plane, the hf tensor has C1h symmetry and the principal values Axx = 5.94 MHz, Ayy = 5.06 MHz and Azz = 14.25 MHz. Our EPR and ENDOR observations unambiguously confirm that the N donor occupies the C site in 4H-SiC lattice and also reveal a considerable amount of the spin density of Nk (~23.9%) which was not obtained in previous studies.

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