Probing Wave Functions of Electrically Active Shallow Level Defects by Means of High-Frequency Pulsed ENDOR in Wide Bandgap Materials: SiC, AlN, ZnO, and AgCl

In the high-frequency ENDOR experiments, the hyperfine (HF) interaction between the unpaired electron of the shallow donor or shallow acceptor and the nuclear spins of the Coulombic center and the surrounding atoms is determined, which is then translated into the spin density of the electronic wave function at the various atomic positions. The results of studying the spatial distribution of wave functions for shallow donors in ZnO, AgCl, AlN, and SiC crystals, ZnO-based nanostructures, and shallow boron acceptors in SiC will be presented. The change of the electronic wave function of a shallow donor in ZnO quantum dots (QDs) when entering the regime of quantum confinement by using the nuclear as probes has been observed. The model, based on the effective mass approximation (EMA), that describes a 1s-like wave function with the Bohr radius of ~ 1.5 nm for distant shells was tested. The EMA does not yield an appropriate description of the electronic wave function when the radius of the QD is reduced below the Bohr radius. The direct reconstruction of the wave function of the intrinsic shallow electronic center (SEC) and self-trapped excitons in AgCl was presented. The SEC was suggested to be an electron that is shallowly trapped by two adjacent silver ions on a single cationic site (split-interstitial position), so-called “latent image” in silver halides. The shallowly trapped electron of the STE is shown to behave like a hydrogen 1s electron, centered on the Ag+ lattice position, with a Bohr radius r0 = 1.51 nm that is in agreement with Bohr radius of SEC (r0 = 1.66 nm). For SEC in AgBr, r0 = 2.48 nm. It was demonstrated that dynamic nuclear polarization of nuclear spins due to hyperfine interactions with ligand nuclei can be achieved in ZnO (and based QDs) and AgCl by saturating the high-frequency EPR transition of a shallow donor at low temperatures corresponding to a high Boltzmann factor. Several types of shallow donors were indicated in AlN crystals: (i) affected by the DX-relaxation and (ii) with normal behavior. The strong HF interaction for light-induced SD in AlN support the assignment to the impurity in anionic sublattice (e.g. oxygen in N position). At the same time, a shallow donor with normal behavior can belong to Si or C in the Al position. The electronic structure of shallow donors and shallow acceptors in silicon carbide was investigated by the ENDOR method. The spin density of the N donor corresponding to the observed ENDOR lines was established to be p like in character and located mainly on the Si atoms for the k site in 4H-SiC, whereas for the three sites in 6H-SiC the spin density is s-like in character and located mainly on the C atoms. An explanation for the difference in the electronic wave function of the N donor in 4H-SiC and 6H-SiC can be found in the large difference in the band structure of the two polytypes and in the position of the minima in the Brillouin Zone. The electronic density for shallow B acceptor substituting Si in the k position is distributed in an ellipsoidal shape with the main symmetry axis making an angle of 70° with the c axis, i.e., along the direction of the B–C with main spin density

Hyperfine and nuclear quadrupole splitting of the NV- ground state in 4H -SiC

Optically addressable spin-triplet defects in silicon carbide, such as divacancies and negatively charged nitrogen vacancy (NV-) allow to develop modern quantum technologies operating in the near-infrared range based on the well-developed semiconductor material. Here, by means of both high-frequency (94 GHz) pulsed electron paramagnetic resonance (EPR) and electron-nuclear double Rresonance (ENDOR) techniques the ground state properties of the negatively charged NV- defect in 4H-SiC were studied. We experimentally determined the ordering of the ground state spin sublevels and established the sign of the zero-field splitting to be positive as predicted by theory. Analysis of nuclear magnetic resonance transitions in ENDOR spectra allowed to determine the sign, symmetry, and absolute values of the hyperfine interaction of the NV- defect electron spin with N14 nuclear spin as A∥=-1.142MHz and A⊥=-1.184MHz. The absolute value of the nuclear quadrupole interaction constant reflecting an interaction between the N14 nuclear electric quadrupole moment with the electric field gradient was determined to be |Cq|=2.44MHz. This large value is compatible with a threefold coordinated N14 nucleus with uniaxial symmetry and proves conclusively the existence of a nearestneighbor NCVSi pair in the material. For this NV- defect, an ensemble (Hahn-echo) coherence time of T2=49μs was measured, a value which is in the range previously reported for silicon vacancy spin ensembles and slightly longer than T2=40μs measured here on the divacancy spin ensemble

Relaxation processes and high-field coherent spin manipulation in color center ensembles in 6 H-SiC

Coherent spin manipulations of spin-32 color center ensembles in 6H-SiC crystal have been studied in high magnetic fields using methods of pulsed electron paramagnetic resonance, Rabi oscillations, and pulsed electron-electron double resonance under optical alignment conditions of the spin level populations. Rabi oscillation experiments show room temperature coherent control of these spin-32 color center ensembles in strong magnetic fields. A sharp decrease of the spin-lattice relaxation time T1, ∼40 times, was observed in 6H-SiC at magnetic field of ∼3.5 T with increasing temperature from 100 to 300 K, while the spin-spin relaxation time T2 is only shortened by ∼1.3 times. With an increase in the magnetic field, the times T1 and T2 were shown to decrease. The relaxation time T1 in the case of magnetic field directed along the axis of the spin-32 center is ∼2 times longer than T1 in magnetic field perpendicular to this axis. Relaxation times of the spin center in crystal grown with a reduced concentration of an isotope Si29 are significantly longer than crystal, with the natural content of isotopes. With a decrease in the Si29 content in our experiments by a factor of ∼5, the effective nuclear spin bath in SiC is reduced by a factor of ∼2. In a zero magnetic field resonance, transitions are allowed as magnetic dipole transitions with frequency ω0 which correspond to the zero-field splitting. In zero magnetic field and in fixed magnetic fields, the Rabi frequency was shown, using so-called "Feynman-Vernon-Hellwarth transformation,"to be ωR=|γ|B1. In pulsed electron-electron double resonance experiments, a change in the intensity of the electron spin echo signal corresponding to one of the spin-allowed fine structure transitions is recorded depending on the sweep of the second frequency. The experiments show the possibility to coherently detect the optical spin alignment between MS=±32 via optically pumped silent MS=±12 sublevels of the spin-32 color centers, including a detection of Rabi oscillations

Electron nuclear interactions in spin-3/2 color centers in silicon carbide: A high-field pulse EPR and ENDOR study

High-frequency pulsed electron paramagnetic resonance (EPR) and electron nuclear double resonance (ENDOR) were used to determine electron nuclear interactions on remote ligand shells of silicon and carbon in spin-3/2 color centers with an optically induced high-temperature spin alignment in hexagonal -, -, and rhombic -silicon carbide (SiC) polytypes. The EPR and ENDOR experimental data relate unambiguously to spin-3/2 centers in which the optically induced alignment of the spin-level populations occurs. The identification is based on resolved ligand hyperfine interactions with carbon and silicon nearest, next-nearest, and the more distant neighbors and on the determination of the unpaired electron spin densities. The hyperfine interactions with and nuclei are unambiguously separated due to the selective population of the fine-structure levels with certain values of . The signs of these interactions and, as a result, the signs of oscillating spin density at and nuclei, are determined. On the basis of the EPR and optically induced ENDOR measurements, signs of the fine-structure splitting for all the centers were demonstrated, which made it possible to establish the character of optically induced spin alignment, including the inverse populations of the spin levels for these centers. The values of hyperfine interaction with and nuclei, including those remote from the localization of the spin-3/2 center, are tabulated, which can be used by a number of algorithms in quantum information processing as long-term memory

Creation of negatively charged boron vacancies in hexagonal boron nitride crystal by electron irradiation and mechanism of inhomogeneous broadening of boron vacancy-related spin resonance lines

Optically addressable high-spin states (S ≥ 1) of defects in semiconductors are the basis for the development of solid-state quantum technologies. Recently, one such defect has been found in hexagonal boron nitride (hBN) and identified as a negatively charged boron vacancy (V−B ). To explore and utilize the properties of this defect, one needs to design a robust way for its creation in an hBN crystal. We investigate the possibility of creating V−B centers in an hBN single crystal by means of irradiation with a high-energy (E = 2 MeV) electron flux. Optical excitation of the irradiated sample induces fluorescence in the near-infrared range together with the electron spin resonance (ESR) spectrum of the triplet centers with a zero-field splitting value of D = 3.6 GHz, manifesting an optically induced population inversion of the ground state spin sublevels. These observations are the signatures of the V−B centers and demonstrate that electron irradiation can be reliably used to create these centers in hBN. Exploration of the V−B spin resonance line shape allowed us to establish the source of the line broadening, which occurs due to the slight deviation in orientation of the two-dimensional B-N atomic plains being exactly parallel relative to each other. The results of the analysis of the broadening mechanism can be used for the crystalline quality control of the 2D materials, using the V−B spin embedded in the hBN as a probe