V. A. Khramtsov

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 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.

Oriented silver chloride microcrystals and nanocrystals embedded in a crystalline KCl matrix, as studied by means of electron paramagnetic resonance and optically detected magnetic resonance

Self-trapped holes (STH), self-trapped excitons (STE) and shallow electron centres in small AgCl crystals embedded in a KCl crystalline matrix have been observed by means of optically detected magnetic resonance (ODMR). The existence of the impurity clusters in heavily doped KCl:AgCl single crystals, ranging from single and paired Ag ions to AgCl nanometre- and micrometre-size crystals (nanocrystals and microcrystals) retaining the orientation of the matrix, was confirmed. ODMR spectra were used as a fingerprint of the embedded AgCl microcrystals and a signature of their crystallinity. For AgCl nanocrystals the anisotropy of the g-factor both for isolated STH and for STH forming STE was found to be substantially reduced compared with those of bulk AgCl crystals and AgCl microcrystals embedded in KCl. This implies a considerable suppression of the Jahn-Teller effect in nanoparticles. A rather general mechanism of the suppression of the Jahn-Teller effect in nanocrystals is developed, taking into account the additional deformation field appearing because of the strong vibronic interaction at the interface. It allows evaluation of the critical size of the embedded AgCl nanocrystals, at which the suppression of the Jahn-Teller effect has its onset; the value obtained is ≈10 nm, in agreement with experiment.

Electron-hole recombination confinement in self-organized AgBr nanocrystals in a crystalline KBr matrix

The phenomenon of spatial confinement of the electron-hole recombination in exchange-coupled donor-acceptor pairs was observed by optically detected magnetic resonance in AgBr nanocrystals formed as a result of the self-organized growth in an ionic KBr crystal matrix. The effect is manifested by the maximum distance between recombining donors and acceptors being restricted to the nanocrystal size and by a change in the g value of shallow electron donor centers. Based on an analysis of the exchange interactions, the distribution of distances in the donor-acceptor pairs is determined and the dimensions of nanocrystals are estimated.

Suppression of the local Jahn-Teller effect in nanostructures: Self-trapped holes and excitons in AgCl nanocrystals

A strong decrease in the g-factor anisotropy was revealed by optically detected magnetic resonance for self-trapped Jahn-Teller holes (both isolated and forming self-trapped excitons) in AgCl nanocrystals embedded into the KCl crystal lattice. This is evidence for considerable suppression of the Jahn-Teller effect in nanoobjects. The suggested mechanism of suppression of the Jahn-Teller effect in nanocrystals is associated with an additional deformation field arising in nanocrystals owing to a strong vibronic interaction at the interface.

Erbium in silicon carbide crystals: EPR and high-temperature luminescence

Erbium ions have been incorporated for the first time in bulk 6H-SiC crystals during growth, and they were unambiguously identified from the 167Er EPR hyperfine structure. High-temperature luminescence of erbium ions at a wavelength of 1.54 µm has been detected. The observed luminescence exhibits an increase in intensity with increasing temperature. The observation of Er luminescence in 6H-SiC offers a promising potential for development of semiconductor light-emitting devices at a wavelength within the fiber-optics transparency window.

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.

Electron paramagnetic resonance of Dy3+ ions in lead thiogallate single crystals

The results of electron paramagnetic resonance (EPR) studies of Dy3+ ions in lead thiogallate PbGa2S4 single crystals have been presented. It has been shown that the ground state of these ions corresponds to the lowest Stark sublevel Γ6 of the term 6H15/2. The spectra are well described by the axially symmetric spin Hamiltonian with the effective spin S = 1/2 with the factors g‖ = 15.06 and g⊥ = 2.47. The Dy3+ ions substitute Pb2+ ions in the crystal lattice of PbGa2S4. The observed hyperfine structure has allowed to unambiguously interpret the EPR spectra. The hyperfine interaction constants of two odd isotopes of dysprosium in lead thiogallate single crystals have been found to be A‖ = 675 × 10−4 cm−1 and A⊥ = 111 × 10−4 cm−1 for 163Dy and A‖ = 472 × 10−4 cm−1 and A⊥ = 77 × 10−4 cm−1 for 161Dy.

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.

Recombination processes in systems based on doped ionic crystals with impurity-related nanostructures

Recombination processes leading to the tunnelling afterglow and photostimulated luminescence in systems based on host ionic crystals with impurity-related nanostructures-promising phosphors for x-ray storage-were studied by means of luminescence, EPR and optically detected magnetic resonance. It was found that in the x-ray irradiated CsBr:Eu and CsBr:Pb crystals the energy released in the spin-dependent tunnelling recombination of electron-hole pairs and self-trapped excitons in the host crystal is directionally transferred to the impurity-related low-dimensional structures. To identify the origin of recombining electron and hole centres, their EPR spectra were detected by monitoring the tunnelling afterglow and the photostimulated luminescence including the emission bands of the low-dimensional structures.