Quasi-cubic Sites Research Articles

Fine structure in electronic transitions attributed to nitrogen donor in silicon carbide

Nitrogen in group-IV semiconductors has become a well-established element of qubits capable of room-temperature operation. In silicon carbide, nitrogen can occupy different nonequivalent lattice sites, giving rise to different shallow donor states. We report a triplet fine structure in electronic transitions of nitrogen donors on the quasi-cubic carbon site in 4H silicon carbide with activation enthalpies of around 100 meV. The intensities of triplet components have a prominent dependence on the voltage bias. The activation enthalpies of the transitions exhibit the Poole–Frenkel effect, while no bias dependence is observed for the magnitude of splitting. A tentative explanation of the fine structure involves local symmetry changes due to stacking faults.

Thermally activated double-carrier transport in epitaxial graphene on vanadium-compensated 6H-SiC as revealed by Hall effect measurements

In this report we demonstrate the results of charge carriers transport studies in graphene using a Hall effect sensor fabricated on quasi-free-standing monolayer graphene grown on a semi-insulating on-axis vanadium-compensated 6H-SiC(0001) substrate in an epitaxial Chemical Vapor Deposition process. The sensor is passivated with aluminum oxide through atomic layer deposition and offers current-mode sensitivity of 140 V/AT with thermal stability of − 0.02%/K within the range between 80 and 573 K. The electrical properties of the graphene layer are determined as a function of temperature ranging from 300 to 770 K. High-temperature characteristics of passivated and not passivated graphene are compared and their profiles explained through a double-carrier transport involving the spontaneous-polarization-induced holes in the graphene layer and the thermally activated electrons from a shallow donor level of nitrogen in the quasi-cubic (k1) site and a deep acceptor level of vanadium in the hexagonal (h) site both present in the bulk of the vanadium-compensated SiC substrate. Finally, we conclude that this mechanism is directly responsible for the limitation of the thermal stability of the sensor's current-mode sensitivity.

Evidence for near-infrared photoluminescence of nitrogen vacancy centers in4H-SiC

We present evidence of near-infrared photoluminescence (PL) signature of nitrogen vacancy centers (NCVSi)− in silicon carbide (SiC). This center exhibits an S=1 ground state spin similar to the NV− center in diamond. We have performed photoluminescence excitation measurements at cryogenic temperature and demonstrated efficient photoexcitation of distinct photoluminescence from (NCVSi)− in 4H-SiC. Furthermore, by correlating the energies of measured zero phonon lines (ZPLs) with theoretical values derived from hybrid density functional theory each of the ZPLs has been associated to the respective occupation of hexagonal (h) and quasicubic (k) lattice sites in close analogy to neutral divacancy centers (VCVSi)0 in the same material. Finally, with the appropriate choice of excitation energy we demonstrated the selective excitation of (NCVSi)− PL with no contamination by (VCVSi)0 PL, thereby opening the way towards the optical detection of (NCVSi)− electron spin resonance.

Temperature dependent behavior of localized and delocalized electrons in nitrogen-doped 6H SiC crystals as studied by electron spin resonance

We have studied the temperature behavior of the electron spin resonance (ESR) spectra of nitrogen (N) donors in n-type 6H SiC crystals grown by Lely and sublimation sandwich methods (SSM) with donor concentration of 1017 cm−3 at T = 60–150 K. A broad signal in the ESR spectrum was observed at T ≥ 80 K with Lorentzian lineshape and g|| = 2.0043(3), g⊥ = 2.0030(3), which was previously assigned in the literature to the N donors in the 1s(E) excited state. Based on the analysis of the ESR lineshape, linewidth and g-tensor we attribute this signal to the conduction electrons (CE). The emergence of the CE ESR signal at T > 80 K was explained by the ionization of electrons from the 1s(A1) ground and 1s(E) excited states of N donors to the conduction band while the observed reduction of the hyperfine (hf) splitting for the Nk1,k2 donors with the temperature increase is attributed to the motional narrowing effect of the hf splitting. The temperature dependence of CE ESR linewidth is described by an exponential law (Orbach process) with the activation energy corresponding to the energy separation between 1s(A1) and 1s(E) energy levels for N residing at quasi-cubic sites (Nk1,k2). The theoretical analysis of the temperature dependence of microwave conductivity measured by the contact-free method shows that due to the different position of the Fermi level in two samples the ionization of free electrons occurs from the energy levels of Nk1,k2 donors in Lely grown samples and from the energy level of Nh residing at hexagonal position in 6H SiC grown by SSM.

The electron spin resonance study of heavily nitrogen doped 6H SiC crystals

The magnetic and electronic properties of heavily doped n-type 6H SiC samples with a nitrogen concentration of 1019 and 4 × 1019 cm−3 were studied with electron spin resonance (ESR) at 5–150 K. The observed ESR line with a Dysonian lineshape was attributed to the conduction electrons (CE). The CE ESR (CESR) line was fitted by Lorentzian (insulating phase) (T < 40 K) and by Dysonian lineshape (metallic phase) above 40 K, demonstrating that Mott insulator-metal (IM) transition takes place at ∼40 K, accompanied by significant change in the microwave conductivity. The temperature dependence of CESR linewidth follows the linear Korringa law below 40 K, caused by the coupling of the localized electrons (LE) and CE, and is described by the exponential law above 40 K related to the direct relaxation of the LE magnetic moments via excited levels driven by the exchange interaction of LE with CE. The g-factor of the CESR line (g‖ = 2.0047(3), g⊥ = 2.0034(3)) is governed by the coupling of the LE of nitrogen donors at hexagonal and quasi-cubic sites with the CE. The sharp drop in CESR line intensity (25–30 K) was explained by the formation of antiferromagnetic ordering in the spin system close to the IM transition. The second broad ESR line overlapped with CESR signal (5–25 K) was attributed to the exchange line caused by the hopping motion of electrons between occupied and non-occupied positions of the nitrogen donors. Two mechanisms of conduction, hopping and band conduction, were distinguished in the range of T = 10–25 K and T > 50 K, respectively.

Pulsed EPR and ENDOR Study of SiC Nanopowders

The great potential of the micro(mp) nano-particle (np) silicon carbide (SiC) for future applications in spintronics stimulates detailed studies of the effects of impurities and defects in their electronic properties. In particular, mpand np-SiC doped with nitrogen (N) atoms is an important issue to be investigated, since N is a common contaminant in SiC bulk, acting as n-type dopant. Although N has been found to be present in the nanomaterials, there is no experimental evidence indicating for the formation of the N shallow donor state in np-SiC. The np-SiC samples with R 50 nm which was identified by pulsed ENDOR as the N in quasi-cubic (Nk2) site. As the size of the np-SiC decrease, the signal from VC in 6H SiC phase appeared in the FS ESE spectrum, which gives rise to the compensation of the N donors and as a result the FS ESE spectrum intensity of N decrease significantly. Only two signals due to the VC located in 3C and 6H SiC phase were observed in the FS ESE spectrum of the np-SiC samples with the R < 50 nm. The fact that the ENDOR signal centered at the hydrogen (H) nuclear Larmor frequency (n) was observed in the np-SiC with R < 100 nm indicates that H atoms are localized in the vicinity of the VC and can form the H-related defect like (VC+H). As a conclusion, two phases (and -SiC crystalline phases) are presented in np-SiC. The work was supported by GA CR 13-06697P and SAFMAT project CZ.2.16/3.1.00/22132.

Identification of the Negative Carbon Vacancy at Quasi-Cubic Site in 4H-SiC by EPR and Theoretical Calculations

In freestanding n-type 4H-SiC epilayers irradiated with low-energy (250 keV) electrons at room temperature, the electron paramagnetic resonance (EPR) spectrum of the negative carbon vacancy at the hexagonal site, VC- (h), and a new signal were observed. From the similarity in defect formation and the spin-Hamiltonian parameters of the two defects, the new center is suggested to be the negative C vacancy at the quasi-cubic site, VC- (k). The identification is further supported by hyperfine calculations.

Superhyperfine Interactions of the Nitrogen Donors in 4H SiC Studied by Pulsed ENDOR and TRIPLE ENDOR Spectroscopy

The nitrogen donors residing at quasi-cubic lattice site (Nk) in 4H SiC were investigated by field sweep electron spin echo (FS ESE), pulsed electron nuclear double resonance (ENDOR) and pulsed General TRIPLE ENDOR spectroscopy. The 29Si and 13C superhyperfine lines observed in the FS ESE and ENDOR spectra of Nk in n-type 6H SiC were assigned by pulsed General TRIPLE resonance spectroscopy to the specific carbon (C) and silicon (Si) atoms located in the nearest environment of Nk in 4H SiC. The superhyperfine interaction constants and their relative signs for Nk with 29Si and 13C nuclei located in the nearest-neighbor shells are found from the General TRIPLE ENDOR spectra to be positive for C atoms and negative for Si atoms.

Deep-Level Defects in Electron Irradiated 6H-SiC

AbstractAn effect of electron irradiation on the concentrations of deep-level centers in C-rich and Si-rich 6H-SiC wafers is investigated. In the former material, the main deep-level centers with activation energies of Ec-0.50, Ec-0.64 and Ec-0.67 eV are found to be related to dicarbon interstitials and CiNC complexes located in hexagonal and quasi-cubic lattice sites, respectively. In the latter material, the dicarbon interstitials are dominant after the irradiation with 1.5-MeV electrons. At the energy of bombarding electrons equal to 0.3 and 0.7 MeV, the activation energies of the dominant deep-level centers are Ec-0.38 and Ec-0.52 eV, respectively. The first center is related to carbon vacancies and the second to silicon interstitials.

EPR, ESE and pulsed ENDOR study of the nitrogen donor pairs on quasi-cubic lattice sites in 6H SiC

Abstract D-band EPR, X-band FS ESE and pulsed ENDOR studies of the additional nitrogen (N) related centers observed in highly compensated n-type 6H-SiC wafers are presented. The D-band EPR and X-band FS ESE spectrum consists of the 14N hyperfine (hf) triplet lines of the N donors incorporated at the two quasi-cubic (Nc1, Nc2) sites with the intensity ratio of INc1/INc2=0.7 and three additional triplet lines. X-band pulsed ENDOR spectra have shown intense 14N ENDOR signals of Nc1, Nc2 centers and new 14N lines due to the N related centers N1, N2, N3 with the isotropic hf splitting: 21.04, 26.43, 29.77 MHz, respectively. It was found that the g-tensors of the N2 and N3 triplet lines coincide with those of N on quasi-cubic Nc2 site. The g-tensor of the third N1 triplet lines corresponds to the average value of the Nc1 and Nc2 spectrum: g ( N 1 ) ≈ 1 2 [ g ( N c 1 ) + g ( N c 2 ) ] . It was suggested that N1 center with Aiso=21.04 MHz is due to the spin coupling between N on two quasi-cubic Nc1 and Nc2 sites while two others are tentatively attributed to the N pairs formed between N atoms on one quasi-cubic Nc2 site.

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.

X-ray resonant scattering determination of the antiferroquadrupolar ordering inUPd3 at low temperatures

We have used x-ray resonant scattering techniques to make a comprehensive study of the quadrupolarorder in UPd3. New measurements reveal that the order parameter describing the in-phase stacking along thec-axis of the quadrupole moments of the uranium ions on the quasi-cubic sites is primarilyQxy in the two lowest temperature phases. These results are discussedwithin the context of a three-level crystal field model for the uranium5f2 electrons, the allowed quadrupolar operators, and the constraints deducedfrom the analysis of heat capacity measurements. We have now deduced, andsummarize, the order parameters for each of the four antiferroquadrupolar phases ofUPd3.

High-resolution photoinduced transient spectroscopy of defect centers in vanadium-doped semi-insulating SiC

High-resolution photoinduced transient spectroscopy (HRPITS) has been applied to studying deep-level defects controlling the charge compensation in semi-insulating (SI), vanadium-doped, bulk 6H– and 4H–SiC. The photocurrent relaxation waveforms were digitally recorded in the temperature range of 300–750 K and a new approach to extract the parameters of defect centers from the temperature-induced changes in the waveforms’ time constants has been implemented. It is based on a two-dimensional analysis using the numerical inversion of the Laplace transform. As a result, the images of spectral fringes depicting the temperature dependences of the emission rate of charge carriers for defect centers are created. For 6H–SiC:V, 11 deep defect centers with activation energies ranging from 660 to 1405 meV were resolved. For 4H–SiC:V, 13 deep traps with activation energies ranging from 560 to 1530 meV were detected. In the both polytypes, the predominant traps were found to be related to vanadium donors located at quasi-cubic and hexagonal lattice sites.

Deep-Level Defects in Nitrogen-Doped 6H-SiC Grown by PVT Method

ABSTRACTAn effect of the nitrogen concentration on the concentrations of deep-level defects in bulk 6H-SiC single crystals is investigated. Six electron traps labeled as T1A, T1B, T2, T3, T4 and T5 with activation energies of 0.34, 0.40, 0.64, 0.67, 0.69, and 1.53 eV, respectively, were revealed. The traps T1A (0.34 eV) and T1B (0.40 eV), observed in the samples with the nitrogen concentration ranging from ∼2×1017 to 5×1017 cm−3, are attributed to complexes formed by carbon vacancies located at various lattice sites and carbon antisites. The concentrations of traps T2 (0.64 eV) and T3 (0.67 eV) have been found to rise from ∼5×1015 to ∼1×1017 cm−3 with increasing the nitrogen concentration from ∼2×1017 to ∼2.0×1018 cm−3. These traps are assigned to complexes involving silicon vacancies occupying hexagonal and quasi-cubic sites, respectively, and nitrogen atoms. The trap T4 (0.69 eV) concentration also substantially rises with increasing the nitrogen concentration and it is likely to be related to complexes formed by carbon antisites and nitrogen atoms. The midgap trap T5 (1.53 eV) is presumably associated with vanadium contamination. The presented results show that doping with nitrogen involves a significant change in the defect structure of 6H-SiC single crystals.

High-resolution Photoinduced Transient Spectroscopy of Defect Centers in Undoped Semi-Insulating 6H-SiC

ABSTRACTHigh-resolution photoinduced transient spectroscopy (HRPITS) has been applied to studying defect centers controlling the charge compensation in semi-insulating (SI), vanadium-free, bulk 6H- SiC. The photocurrent relaxation waveforms were digitally recorded in the temperature range of 50 − 750 K and a new approach to extract the parameters of defect centers from the temperature-induced changes in the time constants of the waveforms has been implemented. It is based on a two-dimensional analysis using the numerical inversion of the Laplace transform. As a result, the images of spectral fringes depicting the temperature dependences of the emission rate of charge carriers for defect centers are created. Using the new procedure for the analysis of the photocurrent relaxation waveforms and the new way of the visualization of the thermal emission rate dependences, a number of shallow and deep defect levels ranging from 80 to 1900 meV have been detected. The obtained results indicate that defect structure of undoped SI bulk 6H-SiC is very complex and the material properties are affected by various point defects occupying the hexagonal and quasi-cubic lattice sites.

Spin-flop transition in samarium metal investigated by capacitance dilatometry in a steady magnetic field of45T

The longitudinal and transversal forced magnetostrictions of a single crystal of samarium metal have been investigated in a static magnetic field up to $45\phantom{\rule{0.3em}{0ex}}\mathrm{T}$. The data show pronounced features of a spin-flop transition initiating at an applied field of about $30\phantom{\rule{0.3em}{0ex}}\mathrm{T}$. The measured forced magnetostriction is about 1 order of magnitude smaller than the spontaneous magnetostriction. Based on a model calculation of the magnetic phase diagram these features have been interpreted within the exchange striction model. The moments of the quasicubic Sm sites enter a spin-flop phase at about $30\phantom{\rule{0.3em}{0ex}}\mathrm{T}$ and ferromagnetic saturation is predicted at $60\phantom{\rule{0.3em}{0ex}}\mathrm{T}$. Similarly the model predicts a spin-flop phase for the hexagonal Sm sites at $140\phantom{\rule{0.3em}{0ex}}\mathrm{T}$ and ferromagnetic saturation at $390\phantom{\rule{0.3em}{0ex}}\mathrm{T}$.

Understanding the quadrupolar structures of [formula omitted

UPd 3 exhibits four phase transitions below T 0 = 7.8 K , attributed to a succession of antiferroquadrupolar (AFQ) orderings of the 5 f 2 uranium ions localised on the quasi-cubic sites of the dhcp crystal structure. From earlier polarised neutron diffraction measurements in a magnetic field, we proposed that the order parameter of the phase below T 0 is Q x 2 - y 2 and a model for the order parameters of the four phases was subsequently developed. This model has now been tested experimentally with measurements of the azimuthal dependence of the intensities of the quadrupolar reflections in the different phases, by means of X-ray resonant scattering (XRS) studies at ESRF. The results indicate that the order parameter, in zero field, of the phase below T 0 is Q zx . Our model provides an explanation for these apparently contradictory results. New measurements of the heat capacity of UPd 3 at low temperatures have revealed the entropy changes at each of the four transitions. We find that the entropy changes ( Δ S ) at T 0 and T + 1 = 6.9 K are minimal, whereas Δ S is large at T - 1 = 6.7 K . From this information together with the new XRS results, we have extended our model to provide an explanation of the AFQ structures of UPd 3 .

Pulsed EPR studies of Phosphorus shallow donors in diamond and SiC

Phosphorus shallow donors having the symmetry lower than T d are studied by pulsed EPR. In diamond:P and 3 C –SiC:P, the symmetry is lowered to D 2 d and the density of the donor wave function on the phosphorus atom exhibits a predominant p-character. In 4 H –SiC:P with the site symmetry of C 3 v , the A 1 ground state of the phosphorus donors substituting at the quasi-cubic site of silicon shows an axial character of the distribution of the donor wave function in the vicinity of the phosphorus atom.

A Study of V3+/4+ Levels in Semi-insulating 6H-SiC using Optical Admittance and Electron Paramagnetic Resonance Spectroscopies

AbstractThe purpose of this study is to identify the vanadium acceptor levels in semi-insulating (SI) 6H-SiC using optical admittance spectroscopy (OAS) and electron paramagnetic resonance (EPR) spectroscopy. OAS conductance peaks near at 0.67 ± 0.02 eV and 0.70 ± 0.02 eV are identified as V3+/4+ levels at the quasi-cubic sites. An OAS peak at 0.87 eV is assigned to the same transition at the hexagonal site. EPR measurements before illumination revealed the characteristic spectrum of V3+. The presence of the V3+ signal supports the identification of the OAS peaks as transitions from the V3+/4+ level to the conduction band. Photo-induced EPR measurements reveal a change in the intensity of V3+ and V4+ at 0.8 ± 0.1 eV, where the amplitude of the V3+ charge state decreases and that of V4+ increases by approximately equal amounts. Although the individual sites are not resolved in the photo-induced EPR data, the 0.8 eV feature strongly supports the assignment of the three OAS peaks as acceptor levels.

Positively charged carbon vacancy in three inequivalent lattice sites of6H−SiC: Combined EPR and density functional theory study

The $Ky1$, $Ky2$, and $Ky3$ centers are the dominant defects produced in the electron-irradiated $p$-type $6H\text{\ensuremath{-}}\mathrm{Si}\mathrm{C}$ crystals. The electron paramagnetic resonance study of these defects has been performed in the temperature range of $4.2--300\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ at $X$, $K$, and $Q$ bands. The centers are characterized by the fourfold silicon coordination established on a basis of the observed hyperfine structure. At low temperatures both $Ky1$ and $Ky2$ defects reveal the ${C}_{S}$ symmetry that only slightly deviates from the ${D}_{2d}$ one. At high temperatures, the thermally activated reorientation from one Jahn-Teller distortion to the others causes the averaging of the $Ky1$ and $Ky2$ spectra in such a manner that their spin-Hamiltonians correspond to the axial symmetry. The $Ky3$ center has axial symmetry in all the temperature range under investigation. Its hyperfine parameters for the first-shell silicon atoms are substantially different from those determined for the $Ky1$ and $Ky2$ centers. Based on the density functional theory, the calculations of the electronic structure of a number of fourfold silicon coordinated defects have been carried out for the unambiguous identification of the observed defects through the comparison of experimentally determined and calculated hyperfine parameters. The present study proves an assignment of the $Ky1$, $Ky2$, and $Ky3$ centers to the positively charged carbon vacancy located in two quasicubic and hexagonal sites of the $6H\text{\ensuremath{-}}\mathrm{Si}\mathrm{C}$ lattice, respectively. The features of the ${V}_{\mathrm{C}}^{+}$ defect related to the multivalley character of its potential energy surface are also discussed. It is shown that this defect can be localized in the minima of different symmetry depending on the occupied lattice site, and these minima are experimentally distinguishable by the values of hyperfine parameters.