Quasi-cubic Sites Research Articles

EPR and theoretical studies of positively charged carbon vacancy in 4H-SiC

The carbon vacancy is a dominant defect in $4H\text{\ensuremath{-}}\mathrm{Si}\mathrm{C}$, and the ``EI5'' electron-paramagnetic-resonance (EPR) spectrum originates from positively charged carbon vacancies $(V_{\mathrm{C}}{}^{+})$ at quasicubic sites. The observed state for EI5, however, has been attributed to a motional-averaged state with the ${C}_{3\mathrm{v}}$ symmetry, and its true atomic structure has not been revealed so far. We here report low temperature $(<40\phantom{\rule{0.3em}{0ex}}\mathrm{K})$ EPR measurements on EI5 and show that this center has a ${C}_{1\mathrm{h}}$-symmetric structure due to Jahn-Teller distortion. We also performed ab inito calculations of the hyperfine tensors for EI5, and obtained a good agreement between experiment and theory in not only their principal values but also their principal axis directions. A good agreement was also demonstrated for the EI6 center (hexagonal-site $V_{\mathrm{C}}{}^{+}$) in this paper. The transition from $\mathrm{EI}5({C}_{1\mathrm{h}})$ to $\mathrm{EI}5({C}_{3\mathrm{v}})$ was found to be thermally activated and its activation energy was measured as $0.014\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$.

EPR identification of two types of carbon vacancies in4H−SiC

The EI5 and EI6 centers are typical intrinsic defects in radiation-damaged and semi-insulating $4H\ensuremath{-}\mathrm{SiC}.$ So far, their origins have been assigned to positively charged carbon vacancies ${(V}_{\mathrm{C}}^{+})$ and silicon antisites $({\mathrm{Si}}_{\mathrm{C}}^{+}),$ respectively. However, our complete set of ${}^{29}\mathrm{Si}$ hyperfine (HF) data clearly reveals that both the centers should originate from ${V}_{\mathrm{C}}^{+}$ but their locations are different, i.e., quasicubic sites for EI5 and hexagonal sites for EI6, as recently predicted by the first-principle calculation [M. Bockstedte et al., Phys. Rev. B 67, 193102 (2003)]. The two types of ${V}_{\mathrm{C}}^{+}$ centers showed remarkable differences in their atomic structures as well as in the temperature dependence of HF interactions, which are closely related to the nature of the two sites.

Magnetic resonance investigation on P-doped 6H-SiC

In this paper, we discuss the many interpretations of electron paramagnetic resonance (EPR) results obtained in nuclear transmutation P-doped 6H-SiC. At least six different P-related donor defects have been observed. Among these defects only two can be unambiguously attributed to the shallow P donor on both quasi-cubic sites. The others are still controversial. We present preliminary results of a high-frequency photoluminescence-detected magnetic resonance investigation on a P-doped 6H-SiC in which the doping has been carried out in situ during the PVT growth. Along with Al, B and N, two new defects were observed with lines at g=2.010 and 2.009 for B∥c. They have been observed previously and were associated with P-defects identified with conventional EPR at 9GHz.

EPR studies of the isolated negatively charged silicon vacancies inn-type4H- and6H-SiC: Identification ofC3vsymmetry and silicon sites

The isolated negatively charged silicon vacancy ${(V}_{\mathrm{Si}}^{\ensuremath{-}})$ in the hexagonal lattices of $4H$- and $6H\ensuremath{-}\mathrm{SiC}$ has been studied by electron paramagnetic resonance (EPR). The local structure was suggested to have ${T}_{d}$ symmetry from the isotropic g value within the resolution of the conventional X-band measurements, from the isotropic ${}^{29}\mathrm{Si}$ hyperfine interaction of the next-nearest-neighbor silicon atoms and from the absence of the zero-field splitting with the high spin state of $S=3/2.$ From the ${}^{13}\mathrm{C}$ hyperfine spectrum of the nearest-neighbor carbon atoms, the two kinds of ${V}_{\mathrm{Si}}^{\ensuremath{-}},$ denoted ${V}_{\mathrm{Si}}^{\ensuremath{-}}(\mathrm{I})$ and ${V}_{\mathrm{Si}}^{\ensuremath{-}}(\mathrm{II}),$ respectively have been distinguished. ${V}_{\mathrm{Si}}^{\ensuremath{-}}(\mathrm{I})$ and ${V}_{\mathrm{Si}}^{\ensuremath{-}}(\mathrm{II})$ are assigned to be arising from hexagonal site (h) and quasicubic sites $(k$ in $4H\ensuremath{-}\mathrm{SiC},$ ${k}_{1}$ and ${k}_{2}$ in $6H\ensuremath{-}\mathrm{SiC}),$ respectively. In both ${V}_{\mathrm{Si}}^{\ensuremath{-}}(\mathrm{I})$ and ${V}_{\mathrm{Si}}^{\ensuremath{-}}(\mathrm{II}),$ from the ${}^{13}\mathrm{C}$ hyperfine interactions, the symmetry has been revealed to be ${C}_{3v}$ with the arrangement of the four nearest-neighbor carbon atoms slightly distorted from a regular tetrahedron.

A new model for the crystal field and the quadrupolar phase transitions of UPd3

The double-hexagonal close-packed localized moment 5f system UPd3exhibits four phase transitions below 8 K. We present newmeasurements of the magnetic susceptibility of UPd3single crystals. The phase transitions are attributed to a sequenceof antiferroquadrupolar ordered structures of the U 5f electrons onthe quasi-cubic sites.From a comprehensive analysis of the neutronscattering, x-ray scattering, and bulk property measurements of thissystem, we have deduced a new crystal field model for UPd3.This model, which has a doublet ground state on the quasi-cubic sites with a firstexcited singlet some 4 meV above it, provides a qualitative understanding of thesuccession of quadrupolar phase transitions and order parameters of UPd3.Moreover, the new model also explains the excitationsobserved, by inelastic neutron scattering, in UPd3at 2 K.

Silicon and carbon vacancies in neutron-irradiated SiC: A high-field electron paramagnetic resonance study

Electron-paramagnetic-resonance (EPR) and electron-spin-echo (ESE) studies have been performed that show that isolated ${V}_{\mathrm{Si}}^{\ensuremath{-}},$ ${V}_{\mathrm{Si}}^{0},$ and ${V}_{C}$ vacancies are the dominant intrinsic paramagnetic defects in SiC treated by room-temperature neutron irradiation with doses up to ${10}^{19}{\mathrm{cm}}^{\ensuremath{-}2}.$ This conclusion is supported by the observation of high concentrations of all these defects in 4H- and 6H-SiC that are almost proportional to the irradiation dose. The 95-GHz EPR spectra at 1.2 K prove that the ground state of ${V}_{\mathrm{Si}}^{0}$ corresponds to $S=1$ and that the zero-field splitting parameter D is positive. A possible energy-level scheme and optical pumping process which induces the spin polarization of the ground triplet state of the ${V}_{\mathrm{Si}}^{0}$ vacancy in SiC is presented. In the EPR spectra of ${V}_{\mathrm{Si}}^{\ensuremath{-}}$ in 4H-SiC an anisotropic splitting of the EPR lines is observed. This splitting is assumed to arise from small differences in the g tensor of the quasicubic (k) and hexagonal (h) sites. Anisotropic EPR spectra with $S=\frac{1}{2}$ that are related to the carbon vacancy have also been observed in the n-irradiated SiC crystals. The hyperfine (hf) interaction with the first shell of Si atoms is almost identical to that observed in electron-irradiated SiC crystals. The observed additional 6.8-G hf splitting with 12 carbon atoms in the second shell is considered as a confirmation for the isolated carbon vacancy model.

The ground state of silicon vacancies in 6H–SiC and 15R–SiC

We investigated irradiated 6H–silicon carbide (SiC) and 15R–SiC with the magnetic circular dichroism of the absorption (MCDA) and MCDA-detected electron paramagnetic resonance (EPR). In neutron- and electron-irradiated 6H–SiC, we observed two MCDA transitions at photon energies of 1.435 and 1.369 eV. Photoluminescence (PL) lines at these photon energies (the so-called V1 and V3 lines) are presently assigned to the neutral silicon vacancy at the two quasicubic lattice sites in 6H–SiC. In electron-irradiated 15R–SiC, which has three inequivalent quasicubic lattice sites, we observed three MCDA lines at 1442, 1438 and 1373 meV, respectively. At the photon energy expected for the hexagonal site in 6H–SiC (the so-called V2 PL line), no MCDA signal was observed. From the temperature and field dependence of the MCDA, a spin S= 1 2 of the quasicubic sites was determined. EPR spectra detected via these MCDA lines consist of single EPR lines at g=2.005(2). We conclude that the ground state of V Si giving rise to these optical transitions is paramagnetic with S= 1 2 , which is predicted theoretically for the ground state of the triply negative charge state. The previous assignment of the optical transitions to the neutral charge state must, therefore, be corrected.

Neutral and negatively charged silicon vacancies in neutron irradiated SiC: a high-field electron paramagnetic resonance study

High-field pulsed and continuous-wave electron paramagnetic resonance (EPR) techniques at 95 GHz have been used to investigate radiation defects in neutron-irradiated SiC. In the temperature range between 1.2 and 300 K three types of EPR spectra were observed in 4H-and 6H-SiC crystals that are attributed to the neutral silicon vacancy, the negatively charged silicon vacancy, and the carbon vacancy. In the EPR spectra of V - Si in 4H- and 6H-SiC an anisotropic splitting of the EPR lines is observed. This splitting is assumed to arise from small differences in the g-tensor of the quasi-cubic (k) and hexagonal (h) sites. The g-factor for the k site g (k) is found to be isotropic with g(k) = 2.0032 and the g-factor of the h-site is found to be slightly anisotropic with g λ (h) is = g(k) - 0.00004 and g⊥(h) = g(k) - 0.00002. The 95 GHz EPR spectra at 1.4 K show that the ground state of V u Si , is a triplet state. Additional 9.5 GHz EPR experiments reveal signals that are attributed to the carbon vacancy on the basis of the observed hyperfine splitting. The results demonstrate that V - Si , V 0 Si and V c are dominant defects after neutron irradiation of SiC to doses up to 10 19 cm -2 .

Positively charged carbon vacancy in 6H–SiC: EPR study

The low-temperature X-band EPR study of Ky1 and Ky2 centers assigned to positively charged carbon vacancy (V C +) in two quasicubic sites of 6H–SiC crystal is presented. The C S symmetry, spin S=1/2 and close coincidence of the g-tensor components have been revealed. The principal values of g-tensor are determined as g z =2.0048, g x =2.0022 and g y =2.0037 for Ky2 defect, 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 with g z =2.0058, g x =2.0025 and g y =2.0023. For Ky2 center, the principal values and corresponding directional cosines of the hyperfine (HF) interaction tensor A are determined for four Si atoms in the nearest neighborhood of the defect. Ky3 center is tentatively proposed as V C + defect in quasihexagonal site of 6H–SiC. The results of ab initio DFT calculations using cubic C 18Si 16H 36 and hexagonal C 18Si 20H 40 clusters corroborate the main features of proposed V C + defect models including the point symmetry and values of HF parameters.

Proton-implantation-induced defects inn-type6H- and4H−SiC:An electron paramagnetic resonance study

The microscopic structure and introduction rate of point defects in n-type 6H- and 4H-SiC generated by room-temperature proton implantation have been studied by the electron paramagnetic resonance technique. In order to selectively study the effects of defect introduction in the trace region, 12-MeV implantation in 300-\ensuremath{\mu}m-thick samples was employed, for which the protons completely cross the sample. In both polytypes we observe three dominant paramagnetic defects attributed to the Si monovacancy in the negative charge state and the neutral Si monovacancy in the hexagonal and quasicubic lattice sites, respectively. The concentration of all three defects increases linearly with proton dose. Their total introduction rate is \ensuremath{\sim}19 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$, which amounts only to 4% of the concentration expected from SRIM simulations. No carbon-vacancy-related defect is observed. Thermal annealing at 1100 \ifmmode^\circ\else\textdegree\fi{}C is sufficient to anneal out the ${V}_{\mathrm{Si}}$ defects and to restore n-type conductivity. The observation of the neutral Si vacancy at hexagonal and quasicubic sites under thermal equilibrium conditions at 4 K does not support their previous assignment to an excited state.

Vanadium-related Center in 4H Silicon Carbide

The V4+ (3d(1)) center in 4H SiC is investigated using photoluminescence (PL) and photoluminescence excitation (PLE). The energy position of the ground state of the defect is determined to be 2.1 +/- 0.1 eV below the conduction band for both the hexagonal and the quasi-cubic site. A broad peak in the PLE spectrum is tentatively ascribed to the excited A(1) state, previously believed to be located in the conduction band.

Effective mass donors in silicon carbide — a study with electron nuclear double resonance

With electron nuclear double resonance the superhyperfine interactions of numerous 29Si and 13C lattice neighbours of 14N donors in 3C and 4H–SiC have been measured. It was attempted to explain them using the effective mass theory (EMT). The interactions of N donors on cubic sites in 3C–SiC can be well explained with EMT assuming N to be on C sites, but not on Si sites. N in 3C–SiC represents probably the only true EMT donor, since for shallow donors in Si EMT is not sufficient as shown previously. The interactions of N on the hexagonal sites in 4H–SiC are still reasonably well explained, while the EMT approach fails to explain N on the quasi-cubic sites. Comparison between 3C–SiC and 4H–SiC shows that the “central cell corrections” due to the chemical nature of N introduced into EMT previously for silicon is very small. The major contribution comes from inter-valley interactions.

EPR and ENDOR investigations of B acceptors in 3C-, 4H- and 6H-silicon carbide

The shallow boron acceptors in 3C-, 4H- and 6H-SiC were investigated with electron paramagnetic resonance (EPR) at high frequency (142 GHz) providing a precise knowledge of the electronic g tensors. The hyperfine (hf) interactions of the boron acceptor on the hexagonal site and the quasi-cubic site in 4H-SiC were determined precisely with electron nuclear double resonance (ENDOR). The hf interactions and superhyperfine (shf) interactions with surrounding and neighbours were interpreted within a semiempirical analysis. The microscopic model suggested from the EPR and ENDOR results and from the semiempirical analysis is as follows: the shallow boron acceptors have the same electronic structure in 6H-, 4H- and 3C-SiC and have to be viewed as B-induced C acceptors. The hole is located in the connection line between and the adjacent C. Depending on the microwave frequency used in the measurements the hole at the quasi-cubic site defects experiences a thermally activated motion about the hexagonal crystal axis at high temperatures.

Electron paramagnetic resonance of nitrogen pairs and triads in 6H-SiC: Analysis and identification

A study of electron paramagnetic resonance (EPR) of nitrogen-doped 6H-SiC is reported here. By use of both first- and second-harmonic EPR detection, the predicted spectral fingerprints for nitrogen pairs and triads were found at low temperatures. The pair spectrum was present in the second harmonic, while the spectral feature for both nitrogen pairs and triads was evidenced in the first harmonic. At least one nitrogen donor of the identified pairs and triads is located at one of the two available quasicubic carbon sites, and the other(s) could be at any one of the two quasicubic sites and the hexagonal carbon site.

Electronic structure of the shallow boron acceptor in 6H-SiC:mA pulsed EPR/ENDOR study at 95 GHz

A high-frequency pulsed electron paramagnetic resonance electron-nuclear double-resonance (ENDOR) study on a $^{13}$ C-enriched 6H-SiC single crystal is reported. This type of spectroscopy, owing to its superior spectral resolution, allows us to obtain detailed information about the electronic structure of the shallow boron acceptor centers in 6H-SiC. It is concluded that around 40% of the spin density is localized in the ${\mathrm{p}}_{\mathrm{z}}$ orbital of a carbon which is nearest to boron. The ${\mathrm{p}}_{\mathrm{z}}$ orbital is directed along the C-B connection line and is parallel to the c axis for the hexagonal site and 70\ifmmode^\circ\else\textdegree\fi{} away from the c axis for the two quasicubic sites. We conclude that the C-B bond is a dangling bond, that boron is neutral, and that there is no direct spin density on boron. There is a relaxation of the carbon atom, carrying the spin density, and the boron atom away from each other. From a $^{29}$ Si and $^{13}$ C ENDOR study it is further concluded that around 60% of the spin density is distributed in the crystal with a Bohr radius of 2.2 \AA{}.

Shallow and deep levels in n-type 4H-SiC

The nitrogen levels in 4H-SiC have been determined using thermal admittance spectroscopy. The values of Ec−0.053 eV for nitrogen at the hexagonal site and Ec−0.10 eV for nitrogen at the quasicubic site agree with those reported using other techniques. The deep levels in 4H-SiC were studied using optical admittance spectroscopy. The optical admittance spectrum showed, besides the conductance peak corresponding to band to band transitions, four other conductance peaks. These peaks correspond to photoexcitation of carriers from the defect levels to the conduction band. It is inferred from a comparison with 6H-SiC that the conductance peak b4 is due to excitation of electrons from the vanadium donor at Ec−1.73 eV. The photoconductance build up transients of the Ec−1.73 eV level are described fully by one exponential term. This suggests that only one center contributed to the observed conductance. The decay kinetics of persistent photoconductance due to the Ec−1.73 eV level follow the stretched exponential form. The potential barrier against recapture of photoexcited carriers was determined to be 18 meV for the vanadium donor level in 4H-SiC.

The Microscopic Structure Of Shallow Donors In Silicon Carbide

AbstractThe electronic structure of nitrogen donors in 6H-, 4H- and 3C-SiC is investigated by measuring the nitrogen hyperfine (hf) interactions with electron nuclear double resonance (ENDOR) and the temperature dependence of the hf split electron paramagnetic resonance (EPR) spectra. Superhyperfine (shf) interactions with many shells of 13C and 29Si were measured in 6H-SiC. The hf and shf interactions are discussed in the framework of effective mass theory. The temperature dependence is explained with the thermal occupation of the lowest valley-orbit split A1 and E states. It is proposed that the EPR spectra of P donors observed previously in neutron transmuted 6H-SiC at low temperature (<10K) and high temperature (>60K) are all due to substitutional P donors on the two quasi-cubic and hexagonal Si sites, whereby at low temperature the E state is occupied and at high temperature the A1 state. The low temperature spectra are thus thought not to be due to P-vacancy pair defects as proposed previously.

Shallow levels in n-type 6H-silicon carbide as determined by admittance spectroscopy

Admittance spectroscopy has been used to study shallow levels in n-type 6H-SiC single crystals. A total of eight unintentionally doped n-type samples obtained from three different sources were used in this study. Two of the samples were grown by the Lely method, while the others were grown by physical vapor transport. Two electron traps at EC−0.04 eV and EC−0.03 eV were detected in the more heavily n-type (ND−NA=1018 cm−3) samples. These defects may be due to contaminants other than nitrogen. A defect level at EC−0.08 eV as detected in a sample with ND−NA=8.9×1017 cm−3. This level is associated with nitrogen at the hexagonal site (h). An electron trap at EC−0.11 eV was detected and is associated with nitrogen at the quasicubic sites (k1k2). This level was observed only in the lightly n-type samples (ND−NA=4.7 ×1015–6.4×1017 cm−3).

Magnetic circular dichroism of a vanadium impurity in 6H-silicon carbide

The magnetic circular dichroism of the absorption (MCDA) of VSi4+ in 6H-silicon carbide shows a strong temperature dependence. Within a crystal field approach the MCDA and g-factors of the hexagonal site defect could be explained, whereas the situation is more complicated for both quasi-cubic site defects owing to a dynamical Jahn-Teller effect.

ENDOR investigation of the microscopic structure of the boron acceptor in 6H-SiC

The boron acceptor in 6H-SiC was investigated using electron paramagnetic resonance (EPR) and electron nuclear double resonance (ENDOR). The hyperfine interactions with 11B could be determined precisely for the two quasi-cubic and the hexagonal sites. The microscopic model suggested from the EPR and ENDOR results is as follows: boron occupies a silicon site and is a negatively charged ligand to an adjacent carbon atom on which most of the unpaired spin density is located. At low temperatures the symmetry of the two quasi-cubic site defects is monoclinic, while the hexagonal site defect has C3V symmetry about the hexagonal axis. At about 50 K the two quasi-cubic site defects experience a thermally activated motion of the hole at the adjacent carbon about the hexagonal crystal axis and the apparent defect symmetry also becomes C3V. The boron acceptor should be regarded as a boron-induced carbon acceptor.