Ekaterina N. Kalabukhova

On the microscopic structures of shallow donors in 6H SiC: studies with EPR and ENDOR

Abstract Nitrogen and phosphorus in 6H-SiC were investigated with electron paramagnetic resonance (EPR) at 10 GHz and at 140 GHz and with electron nuclear double resonance (ENDOR). The phosphorus defects were prepared by neutron transmutation of 30 Si in 6H-SiC. We observed two sets of EPR spectra due to two different phosphorus-related defects. Both EPR spectra exhibit a strong temperature dependence. It is proposed, that one of these EPR spectra is due to the isolated shallow phosphorus donor on Si sites and the other due to a P-vacancy-pair defect. The electronic structures of the shallow donors P and N are discussed using effective mass theory. It is shown that the shallow P donor on the hexagonal site has not yet been observed.

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.

Electrical and Multifrequency EPR Study of Nitrogen and Carbon Antisite-Related Native Defect in n-Type As-Grown 4H-SiC

Two donor paramagnetic states originated from nitrogen and a native de fect have been resolved and studied in n-type 4H SiC bulk material at 9.6 and 140 GHz in the temperature range of 4.2 K to 77 K. Additional small intensity lines symmetrically posi ti ned about the nitrogen and native defect EPR lines were attributed to the four nearest C atoms surrounding nitrogen and the native defect using the line intensities and the natural abundance of m agnetic isotopes Si (4.7%) and C (1.1%). A remarkable result is that the value of the C isotropic shf interaction constant for nitrogen on the quasi-cubic site was found to be an order of magnit ude larger than those determined from Si and C ENDOR spectra. This fact enabled us to conclude that nitrogen in 4 H SiC occupies the silicon site in the lattice and not the carbon site as was previously ac cepted. On the basis of the measured EPR data and the C shf interaction constant, the native defect has been determined to be the carbon antisite defect − Si C i.e. a carbon atom in the hexagonal silicon site with one captured electron. Two donor levels have been determined from Hall measurements and the values of the valleyorbit splitting of nitrogen on the quasi-cubic site and the native defe ct on the hexagonal site have been determined from the analysis of the temperature dependencies of the EPR s pectra.

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.

Magnetic Resonance and Optical Study of Carbonized Silica Obtained by Pyrolysis of Surface Compounds

The carbonized silica (SiO2:C) nanopowders were prepared by chemical modification of fumed silica (aerosil) by phenyltrimethoxysilane followed by thermal annealing at temperature in range of 500-800 °C in nitrogen flow. Their magnetic properties were investigated by electron paramagnetic resonance (EPR) in the temperature range from 4.2 K to 292 K. The initial and annealed SiO2:C samples revealed carbon (C) related defects. The carbon related radicals (CRR) in annealed SiO2:C nanopowders with g-factors 2.0042, 2.0039 were attributed to the oxygen (O)-centered CRR and C-centered CRR with a nearby O heteroatom, respectively. The EPR data were compared with infrared (IR) and photoluminescence (PL) data. It was found that the position of the PL band depends on the type of CRR formed after sample annealing. The PL with maximum intensity at 440 nm was found for the sample annealed at 500°C in which O-centered CRR was observed while in the sample annealed at 600°C in which C-centered CRR with a nearby O heteroatom was observed and graphite-like amorphous C clusters were appeared the peak of the PL band was shifted to the 510-520 nm.

Electron paramagnetic resonance of the scandium acceptor in 6H silicon carbide

Abstract In a Sc-doped 6HSiC epitaxial layer on an n-type 6HSiC substrate three new electron paramagnetic resonance (EPR) spectra were observed. They are explained as being due to the isolated Sc acceptor on the three inequivalent sites in 6HSiC having S = 1 2 . The analysis of the EPR spectra shows that Sc resides on carbon sites. The results are discussed on the basis of selfconsistent calculations for ScSi in silicon.

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 5104 cm-1 and 50104 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.

Electrically-detected electron paramagnetic resonance of point centers in 6H-SiC nanostructures

The results of investigation of electrically-detected electron paramagnetic resonance (EDEPR) and classical electron paramagnetic resonance (EPR) (X-band) for the identification of shallow and deep boron centers, NVSi defects, and isolated silicon vacancies (VSi), which are formed directly during the preparation of planar nanostructures under conditions of silicon-vacancy injection at the SiO2/n-6H-SiC interface without any subsequent irradiation, are presented. The prepared sandwich nanostructures are an ultra-narrow p-type quantum well, confined by δ barriers heavily doped with boron on an n-6H-SiC surface, which are self-ordered during pyrolytic-oxide deposition and subsequent short-time boron diffusion. The EDEPR data of point centers in sandwich nanostructures, prepared within the framework of Hall geometry, are recorded by measuring the field dependences of the magnetoresistance without an external cavity, microwave source and detector, due to the presence of microcavities embedded in the quantum-well plane and microwave generation under conditions of a stabilized source-drain current from δ barriers containing dipole boron centers. The obtained EDEPR spectra of the shallow and deep boron centers are analyzed using the data of EPR studies in 6H-SiC bulk crystals [10]. The EDEPR spectrum of the isolated silicon vacancy reveals both the negatively charged state VSi− (S = 3/2) and the neutral state in hexagonal (VSi(h)) and quasicubic (VSi(k1, k2)) states (S = 1). In turn, NVSi defects are detected not only by the EDEPR method at 77 K, but also through the use of a Bruker ELEXSYS E580 EPP spectrometer at 9.7 GHz, in a temperature range of 5–40 K. The EDEPR and EPR spectra recorded on the same sandwich nanostructure are virtually identical and correspond to the center in the triplet state with spin S = 1. The EPR spectrum, which is a lowintensity line doublet with a splitting value equal to ΔB = 237.6 mT, is observed in the background of the EPR spectrum from donors of nitrogen, the concentration of which in the n-6H-SiC initial sample was 5 × 1018 cm−3, whereas nitrogen donor centers are not revealed in the EDEPR spectrum because of total occupation by silicon vacancies inside the 6H-SiC sandwich nanostructure.