T. Kärner

Low-temperature excitonic, electron–hole and interstitial-vacancy processes in LiF single crystals

The emission spectra and the excitation spectra of various emissions have been measured in LiF crystals at 9 K using VUV radiation of 10–33 eV. In contrast to the luminescence of self-trapped excitons (3.4 eV), the efficiency of several extrinsic emissions (4.2, 4.6 and 5.8 eV) is very low in the region of an exciton absorption (12.4–14.2 eV). A single exciting photon of 28–33 eV is able to create a primary electron–hole (e–h) pair and a secondary exciton. The tunnel phosphorescence has been detected after the irradiation of LiF by an electron beam or x-rays at 6 K, and several peaks of thermally stimulated luminescence (TSL) at 12–170 K appeared at the heating of the sample. It was confirmed that the TSL at 130–150 K is related to the diffusion of self-trapped holes (VK centres). The TSL peak at ∼160 K is ascribed to the thermal ionization

Trapped-hole centers in MgO single crystals

The properties of the majority trapped-hole centers in MgO, such as g-factors, positions of absorption and luminescence bands, and temperatures of thermal destruction, have been analyzed with the emphasis on the observed regular trends and interrelations between the properties of these centers. Particular emphasis has been placed on the positively charged [Be]+ and [Ca]+ trapped-hole centers, which have a large cross section for recombination with conduction electrons. In these centers, a hole is localized at an oxygen ion near the impurity Be2+ or Ca2+ ion located at a regular cation site. The generation and transformation of defects due to the recombination of either relaxed conduction electrons with OH−-containing hole centers or cold and hot electrons with [Be]+ and [Ca]+ centers have been considered. Using the interrelation of the characteristics of hole centers and taking into account that the recombination emission band revealed at ∼6.8 eV is due to the Ca2+-containing centers that are stable below 50 K, the prospects for the EPR detection of the [Ca]+ center at T < 4.2 K have been discussed.

Multiplication of electron–hole pairs in MgO crystals and ceramics

Abstract The excitation spectra for various impurity emissions have been measured in MgO:Al, MgO:OH, MgO:Cr crystals and MgO:F, MgO:LiF ceramics at 8 K using synchrotron radiation of 5–32 eV. Processes of radiative recombination at the impurity centres of various structure differ significantly for two stages of multiplication of electron–hole (e–h) pairs with threshold photon energies at 20 and 25 eV. The multiplication processes have been analysed taking into account the peculiarities of the electronic structure of MgO.

Electron paramagnetic resonance of the [Be]+ centre in MgO:Be

In Be-doped MgO crystals, a new hole centre has been discovered whose physical properties are similar to those of [Li]0-type centres (a hole localized at a cation impurity). The centre can be observed best by electron paramagnetic resonance (EPR) at liquid He temperature; thermal destruction of the centre through the loss of a hole takes place at about 160 K. The symmetry of the centre is almost tetragonal, with a slight rhombic distortion, and the hyperfine structure of its EPR spectrum suggests the following model for the centre: Be2+–O−. However, a detailed investigation of the temperature dependence of the EPR spectrum did not reveal, contrary to our preliminary announcement, any temperature averaging. At higher temperatures the spectrum is complicated by the wealth of spectral lines induced by the ubiquitous Be impurity ions. In particular, this led to our former misinterpretation. The physical properties of the observed centre are discussed and compared with those of analogous centres in MgO.

Anion interstitials in neutron-irradiated MgO single crystals

Electron paramagnetic resonance (EPR) and high-temperature (300 –775 K) thermoluminescence of neutron-irradiated MgO single crystals were studied. Spin-hamiltonian parameters of the observed anion interstitial centres (O − molecular ion in an oxygen site by a cation vacancy—H centre) were determined. The thermal stability of anion interstitials (H centres) and other EPR-active centres was investigated. It is shown that the thermal decay of H centres is attended by a TL peak due to hole recombination luminescence at 700 K. A possible mechanism for the thermal destruction of anion Frenkel defects in MgO is discussed. c � 2001 Elsevier Science Ltd. All rights reserved.

High-temperature thermoluminescence manifestations of anion interstitials in neutron-irradiated pure and doped single crystals of MgO

Abstract The thermal stability of anion interstitials (H centers) was studied in neutron irradiated single crystals of MgO. The ESR spectra of H centers were investigated, the annealing of their ESR absorption and the optical absorption of F+ centers was measured. Both defects were found to disappear in the 650—750 K temperature region. Thermal decay of H centers was investigated using fractional thermoactivation spectroscopy and a common high-temperature (300—775 K) thermoluminescence, combined with the method of controlled X-colouration. A possible mechanism for the thermal destruction of complex anion Frenkel defects in MgO, including the hopping diffusion of neutral oxygen interstitials to F centres, is proposed.

Continuous-wave room-temperature laser oscillation of Cr(3+):MgO.

Continuous-wave laser oscillation of Cr(3+):MgO at room temperature was realized under argon-ion laser pumping at 476 and 514 nm. The free-running laser wavelength was 840 nm, and with different mirror sets laser oscillation at 824, 830, 870, and 878 nm was also realized. With different crystals a maximum output power of 48 mW and a lowest threshold with respect to the absorbed pump power of 80 mW were realized. The slope efficiencies with respect to the absorbed pump power were as high as 2.3%. No thermal effects were observed for absorbed pump powers as great as 2.7 W.

Defect structure relaxation process in the Si–SiO2 system

Abstract The Si–SiO 2 system with a SiO 2 film prepared by high-temperature oxidation can be in a nonequilibrium state and a defect structure relaxation process is possible in this system. The paper presents the results of the electron spin resonance (ESR) and MOS capacitance technique studies of the defect structure relaxation process in the Si–SiO 2 system. It has been found that after sample preparation, the intensity of the ESR signal, connected with vacancy-type defects, increases; and the intensity of the ESR signal connected with nonsaturated SiSi · bonds at the Si–SiO 2 interface ( P b centres) decreases with different time constants, indicative of different mechanisms of the defect structure relaxation process. At a longer storage time, nonmonotonous changes in the ESR signal intensity, the charge in the SiO 2 films and the density of surface states have been observed. The nonmonotonous change in the ESR signal and the charge in SiO 2 can be explained on the basis of the charge carrier (electrons and holes) transfer from Si to SiO 2 on the excited and ground state of the defects, respectively, and on the basis of their subsequent recombination. The defect structure relaxation process can be accelerated and the reliability of the manufactured devices improved by an appropriate choice of the oxidation conditions (oxide thickness, cooling rate).

VOH?Be?a new and unusual member in the family of V centres

In Be-doped MgO crystals a new V centre has been discovered whose physical properties differ significantly from those of other V centres. The centre can be observed by means of EPR at room temperature; the shift of its g-factor from the free electron value is about 1.6 times smaller than that of a VOH centre, the optical absorption band is shifted by ~1 eV to higher energies, and the thermal destruction of the centre through the loss of a hole takes place at about 400 K. The axial symmetry of the centre and the hyperfine structure of its EPR spectrum suggest the following model of the centre: Be2+?O??vc?OH?. The specific physical properties of the VOH?Be centre are explained by an interaction of the effective electric dipole, created by the off-centre position of the Be2+ ion substituting for the Mg2+, with the nearby localized hole.

Paramagnetic Centres in Be-Doped MgO Single Crystals

Using a variation of arc-fusion technique, Be-doped MgO single crystals were grown, in which about 0.01% of the cation sites are occupied by Be2☎. This gives rise to a variety of Be-containing paramagnetic centres, easily detectable by EPR. The models of the centres are proposed and the values of their spin-Hamiltonian parameters are determined and discussed. Two of them—VOH-Be and H-Be centres—stem from well-known paramagnetic centres such as VOH, and interstitial H atom. In addition, because of the non-central position of the ion, an isolated Be2☎ can trap a hole forming a Be2☎O− centre. The symmetry of the Be2☎O− centre at T<30K is rhombic, at a higher temperature a motional averaging of the spectrum takes place. It is shown that unusually for V centres physical properties of the VOH-Be centre (a relatively small g-factor anisotropy and high thermal stability, optical absorption energy and spin-lattice relaxation time) arc caused by the non-central position of the Be2☎ ion.