I. V. Il’in

EPR identification of the triplet ground state and photoinduced population inversion for a Si-C divacancy in silicon carbide

It is shown that intrinsic defects responsible for the semi-insulating properties of SiC represent Si-C divacancies in a neutral state (VSi-VC)0, which have the triplet ground state. The energy level scheme and the mechanism of creating the photoinduced population inversion of the triplet sublevels of the divacancy ground state are determined. It is concluded that there is a singlet excited state through which spin polarization is accomplished, and this fact opens the possibility of detecting magnetic resonance on single divacancies.

Electron spin resonance detection and identification of nitrogen centers in nanodiamonds

Individual nitrogen centers N0 and nitrogen pairs N2+ have been detected and identified in natural diamond nanocrystals by means of the high-frequency electron spin resonance method. The N0 nitrogen centers have been observed in synthetic diamond nanocrystallites with a size of less than 10 nm produced by high-temperature high-pressure sintering of detonation nanodiamonds. Thus, the possibility of the stable state of impurity nitrogen atoms in diamond nanoparticles has been demonstrated.

Properties of Erbium Luminescence in Bulk Crystals of Silicon Carbide

The infrared luminescence of Er3+ ions has been studied in bulk crystals of silicon carbide 6H-SiC doped with erbium in the process of their growth. The erbium centers of different symmetry in the crystals are revealed by the EPR technique. A number of intense luminescence bands of erbium ions are observed at a wavelength of about 1.54 µm. The luminescence can be excited by the light with quantum energies above and below the band gap of SiC. It is found that the luminescence exhibits unusual temperature behavior: as the temperature increases, the luminescence intensity abruptly rises starting with 77 K, passes through a maximum at ∼240 K, and, in the vicinity of ∼400 K, decreases down to the values observed at 77 K. The activation energies for the flare-up and quenching of the Er3+ luminescence are estimated at EA ≈130 and ≈350 meV, respectively. The mechanisms of the flare-up and quenching of the Er3+ luminescence in SiC are discussed.

Electron Paramagnetic Resonance Detection of the Giant Concentration of Nitrogen Vacancy Defects in Sintered Detonation Nanodiamonds

A giant concentration of nitrogen vacancy defects has been revealed by the electron paramagnetic resonance (EPR) method in a detonation nanodiamond sintered at high pressure and temperature. A high coherence of the electron spins at room temperature has been observed and the angular dependences of the EPR spectra indicate the complete orientation of the diamond system.

Electron paramagnetic resonance of scandium in silicon carbide

The EPR spectra of scandium acceptors and Sc2+(3d) ions are observed in 6H-SiC crystals containing a scandium impurity. The EPR spectra of scandium acceptors are characterized by comparatively small hyperfine interaction constants, whose values are consistent with the constants for other group III elements in SiC: boron, aluminum, and gallium acceptors. The EPR spectra of scandium acceptors undergo major changes in the temperature interval 20–30 K. In the low-temperature phase the EPR spectra are characterized by orthorhombic symmetry, whereas the high-temperature phase has higher axial symmetry. The EPR spectra observed at temperatures above 35 K and ascribed by the authors to Sc2+(3d) ions, or to the A2− state of scandium, have significantly larger hyperfine structure constants and narrower lines in comparison with the EPR spectra of scandium acceptors. The parameters of these EPR spectra are close to those of Sc2+(3d) in ionic crystals and ZnS, whereas the parameters of the EPR spectra of scandium acceptors correspond more closely to the parameters of holes localized at group III atoms, in particular, at scandium atoms in GeO2. It is concluded that in all centers the scandium atoms occupy silicon sites.

Probing of the shallow donor and acceptor wave functions in silicon carbide and silicon through an EPR study of crystals with a modified isotopic composition

The spatial distributions of the unpaired-electron wave functions of shallow N donors in SiC crystals and of shallow P and As donors in silicon crystals were determined by studying crystals with a modified content of the 29Si and 13C isotopes having a nonzero nuclear magnetic moment. As follows from the present EPR and available ENDOR data, the distribution of donor electrons in SiC depends substantially on the polytype and position in the lattice; indeed, in 4H-SiC, the unpaired electrons occupy primarily the Si s and p orbitals, whereas in 6H-SiC these electrons reside primarily in the s orbitals of C. The electron distributions for the N donor in the hexagonal position, which has a shallow level close to that obtained for this material in the effective-mass approximation, and for the donor occupying the quasi-cubic position differ substantially. The EPR spectrum of N in quasi-cubic positions was observed to have a hyperfine structure originating from a comparatively strong coupling with the first two coordination shells of Si and C, which were unambiguously identified. The effective-mass approximation breaks down close to the N donor occupying the quasi-cubic position, and the donor structure and the donor electron distribution become less symmetric. In silicon, reduction of the 29Si content brought about a substantial narrowing of the EPR line of the shallow P and As donors and an increase in the EPR signal intensity, as well as a noticeable increase in the spin-lattice relaxation time T1. This offers the possibility of selectively studying these spectra by optically exciting a region of the crystal in order to shorten T1 and thereby precluding EPR signal saturation only in the illuminated part of the material. This method may be used to advantage in developing materials for quantum computers based on donors in silicon and SiC.

Electron paramagnetic resonance of deep boron acceptors in 4H-SiC and 3C-SiC crystals

EPR spectra of deep boron in 4H-SiC and 3C-SiC crystals have been observed and studied. Two sites in 4H-SiC produced deep-boron EPR signals, quasi-cubic k and hexagonal h. In both cases the deep-boron center symmetry is close to axial along the c crystal axis, and the g factor anisotropy is about an order of magnitude larger than that for shallow boron centers. In the 3C-SiC crystal, the deep-boron symmetry is also close to axial along one of the four 〈111〉 directions. The model proposed for the deep boron center with acceptor properties is BSi-vC, where BSi is the boron substituting for silicon, and vC is the carbon vacancy, with the BSi-vC direction coinciding in 4HSiC with the hexagonal axis of the crystal for both k and h positions. In the cubic 3C-SiC crystal, there are four equivalent deep boron centers, which represent BSi-vC pairs with the bond directed along one of the four 〈111〉 crystal directions.

Transition and rare-earth elements in the SiC and GaN wide-gap semiconductors: Recent EPR studies

EPR studies of transition-element ions in SiC and GaN and of erbium in 6H-SiC are reported. Data are presented on Sc2+ ions and scandium acceptors, and chromium and molybdenum ions in various charge states in SiC. A study was made of nickel and manganese in nominally pure GaN grown by the sandwich sublimation method. The first EPR investigation of Er in 6H-SiC is reported. Erbium was identified from the hfs of the EPR spectra. Various possible models of erbium centers in silicon carbides are discussed. Strong room-temperature erbium-ion luminescence was observed.

Neutron transmutation doping of silicon 30Si monoisotope with phosphorus

Phosphorus-doped silicon 30Si monoisotope samples with a highly homogeneous impurity distribution at a concentration of 5 × 1016 cm−3 were obtained for the first time by means of neutron transmutation doping.

Electron paramagnetic resonance in neutron-transmutation-doped semiconductors with a changed isotopic composition

Potential applications of electron paramagnetic resonance (EPR) for investigating and controlling the process of neutron transmutation doping (NTD) of semiconducting germanium, silicon, and silicon carbide are discussed. It is shown that EPR enables one to control the process of annealing of radiation-induced defects in semiconductors subject to neutron irradiation and to detect the shallow donors restored in the process of annealing of donor-compensating defects by observing EPR signals from these donors. EPR can be used to separately detect isolated donors and clusters of two, three, and more exchange-bound donor atoms and thereby determine the degree of nonuniformity of the impurity distribution over the crystal. Neutron transmutation doping is demonstrated to produce a fairly uniform arsenic-donor distribution in a germanium crystal. It is argued that semiconductors enriched in the selected isotopes should be used for NTD. The results of an investigation of phosphorus donors in silicon carbide are presented.