D.V. Savchenko

Nitrogen Donor Aggregation in 4H-SiC: g-Tensor Calculations

The microscopic origin of the Nx EPR-lines observed in heavily nitrogen doped 4H-SiC and 6H-SiC is discussed with the help of EPR parameters calculated from first principles. Based on the symmetry of the g-tensors we propose a model with distant NC donor pairs on inequivalent lattice sites which are coupled to S=1 centers but with nearly vanishing zero-field splittings, giving rise to an essentially S=1/2 like spectrum. The proposed aggregation in neutral donor pairs can contribute to the saturation of the free concentration observed in heavily nitrogen doped SiC.

EPR, ESE and pulsed ENDOR study of nitrogen related centers in 4H-SiC wafers grown by different technologies

D-band electron paramagnetic resonance (EPR) measurements as well as X and Q-band field-swept Electron Spin Echo (ESE) and pulsed Electron Nuclear Double Resonance (ENDOR) studies were performed on a series of n-type 4H-SiC wafers grown by different techniques including sublimation sandwich method (SSM), physical vapor transport (PVT) and modified Lely method. Depending on the C/Si ratio and the growth temperature the n-type 4H-SiC wafers revealed, besides a triplet due to nitrogen residing on the cubic site (Nc), two nitrogen (N) related EPR spectra with g||=2.0055, g⊥=2.0010 and g||=2.0063, g⊥=2.0005 with different intensities. In the samples with low C/Si ratio the EPR spectrum with g|| =2.0055, g⊥=2.0010 consists of a triplet with low intensity which is tentatively explained as a N-related complex, while in the samples with high C/Si ratio the triplet is transformed into one structureless line of high intensity, which is explained as being due to an exchange interaction between N donors. In the samples grown at low temperature with enhanced carbon concentration the EPR line with g||=2.0063, g⊥=2.0005 and a small hyperfine (hf) interaction dominates the EPR spectrum. It is attributed to N on the hexagonal lattice site. The interpretation of the EPR data is supported by activation energies and donor concentrations obtained from Hall effect measurements for three donor levels in this series of 4H-SiC samples.

Photo-EPR and Hall Measurements on Undoped High Purity Semi-Insulating 4H-SiC Substrates

EPR measurements at 37 GHz and 77K have been made on undoped High Purity SemiInsulating 4H-SiC in the dark and after illumination. Two intrinsic defects with donor-like behavior have been detected by EPR. Temperature dependent Hall effect measurements confirm that the conduction is n-type in these samples. It is suggested that that the deep donor-like defects with S =1/2 are the VSi 3− and probably VC 1− defects and that they might be responsible for the compensation of shallow nitrogen and boron in SI4H SiC. Introduction Undoped high purity semi-insulating (HPSI) physical vapor transport (PVT) grown 4H SiC material shows a variety of activation energies ranging in our measurements from 0.95 to 1.4 eV. But the deep level or levels responsible for compensation in this material is not known at present. Recent EPR measurements at several laboratories have detected intrinsic defects in undoped semiinsulating (s.i.) SiC. Photo-EPR experiments in these laboratories suggest that these defects have energy levels in the bottom half of the bandgap [1-3]. Here we report the results of a photo-EPR study of HPSI s.i. 4H-SiC samples along with high temperature Hall effect (HTHE) measurements. Experimental Details A series of undoped PVT HPSI 4H-SiC samples were investigated by EPR and HTHE measurements. EPR measurements were performed on Q-band EPR spectrometer. The photo-EPR experiments were performed using a 250 W high-pressure mercury vapor lamp and a 100 W halogen lamp combined with interference filters.

Spin-Coupling in Heavily Nitrogen-Doped 4H-SiC

EPR and ESE in nitrogen doped 4H- and 6H-SiC show besides the well known triplet lines of 14N on quasi-cubic (Nc,k) and hexagonal (Nc,h) sites additional lines (Nx) of comparatively low intensity providing half the hf splitting of Nc,k. Frequently re-interpreted as spin-forbid¬den lines, arising from Nc,k pairs and triads or resulting from hopping conductivity, only re¬cent¬ly the theoretical calculation of the corresponding g-tensors lead to a tentative model of distant NC donor pairs on inequivalent lattice sites which are coupled to S = 1 assuming a fine-struc¬ture splitting too small to be observed in the EPR and ESE experiments. In this work, we pre¬sent ESE nutation measurements confirming S = 1 for the Nx center. Analysing the nutation frequencies in comparison with that of the Nc,k (S = 1/2) spectrum as well as the line width of ESE and EPR spectra we obtain a rough estimate between 5uf0d7104 cm-1 and 50uf0d7104 cm-1 for the fine-structure splitting demonstrating efficient spin-coupling between nitrogen donors in 4H-SiC.

Trapping Recombination Process and Persistent Photoconductivity in Semi-Insulating 4H SiC

The decay kinetics of a persistent photoconductivity (PPC) in undoped semi-insulating 4H SiC and intercenter charge transfer were studied with EPR, photo-EPR and optical admittance spectroscopy (OAS). A thermally activated charge transfer process that occurs in the dark has been observed. The PPC effect was observed directly in changes in the quality factor of the EPR cavity before and after illumination and by the decay of the OAS signal for deep levels, and indirectly by the excitation and decay of the nitrogen and boron EPR lines that were not observed in the dark before illumination. The decay kinetics of the PPC and photo-induced carrier capture by nitrogen and boron levels were found to follow a stretched exponential form. The PPC in the temperature range from 77 to 300K was found to be produced by a thermally induced charge transfer process involving deep trap levels.

Possible Role of Hydrogen within the So-Called X Center in Semi-Insulating 4H-SiC

Multi-frequency electron paramagnetic resonance (EPR) measurements between 9 and 140 GHz as well as Hall measurements were performed on a series of nominally undoped high-purity semi-insulating (HPSI) 4H-SiC. The investigations in a temperature range from 4 K to 300 K were focused on the photosensitive intrinsic X-defect residing at two inequivalent lattice sites (X h and X k ). Photo-EPR and Hall effect measurements indicate donor-like energy levels at 1.26 ± 0.06 eV and 1.36 ± 0.06 eV below the conduction band for X h and X k , respectively. The EPR spectra recorded at 77 K and at 9 GHz consist of superimposed EPR lines, which were characterized by axially symmetric g-tensors and resolved hyperfine (hf) lines. The hf interactions determined are identical to those published earlier in two independent works for the carbon vacancy in a single positive charge state. However, it is a surprising fact that for one hf line the intensity ratio is different in all works. With the help of additional theoretical investigations, these variations are tentatively assigned to the contamination of a sample-dependent fraction of carbon vacancies with hydrogen.

Luminescence and EPR Characterization of Vanadium Doped Semi-Insulating 4H SiC

In this paper, we report on a photoluminescence (PL) and EPR study of several semiinsulating (SI) 4H SiC samples showing the different compensation regimes due to the presence of V4+ and V3+of different concentration. The samples which contain only V3+ indicates the compensation regime NV≅ND-NA>0 with the Fermi level located in the upper half of the band gap. The presence of V4+ along with V3+ in the other two set of samples indicates the SI behavior of the samples with compensation regime NV>NA-ND>0. Considering that the samples revealed EPR spectrum of vanadium V3+, position of the Fermi level should be also in the upper half of the band gap and mixed conductivity could be expected. UD-3 PL spectrum was observed in vanadium doped SI 4H SiC presented in the samples in V3+/V4+ and V4+/V5+ charge states with compensation regime NV>NA-ND>0. The data obtained prove that the PL and EPR are suitable techniques in determination SI yield in SiC crystal.