Ion diffusion-controlled thermally stimulated processes in X-ray irradiated halide crystals

The ionic and ion diffusion-controlled thermally stimulated relaxation (TSR) processes in CaF 2 , BaF 2 , LiBaF 3 and KBr crystals were investigated above 290 v K by means of the ionic conductivity, ionic thermally stimulated depolarisation current (TSDC) and thermal bleaching techniques. Under a DC field the halide crystals store large ionic space charge. We were able to detect in CaF 2 , BaF 2 , LiBaF 3 and KBr in the extrinsic ionic conductivity region a series of the ionic defect (the interstitial anion and/or anion vacancies - in fluorides; the cation vacancies - in KBr) release stages: 3-6 wide and overlapping ionic TSDC peaks. The correlated data of the ionic TSDC and the F band thermal evidence that above 290 v K the TSR processes are initiated and controlled by the ionic defect thermal detrapping, migration and interaction with the localised electronic and ionic charges and colour centres. The ion diffusion-controlled TSR processes take place in the above halide crystals.     

Ion diffusion-controlled thermally stimulated processes in x-ray irradiated halide crystals

The ionic and ion diffusion-controlled thermally stimulated relaxation (TSR) processes in CaF₂, BaF₂, LiBaF₃ and KBr crystals were investigated above 290 K by means of the ionic conductivity, ionic thermally stimulated depolarisation current (TSDC) and thermal bleaching techniques. Under a DC field the halide crystals store large ionic space charge. We were able to detect in CaF₂, BaF₂, LiBaF₃ and KBr in the extrinsic ionic conductivity region a series of the ionic defect (the interstitial anion and/or anion vacancies - in fluorides; the cation vacancies - in KBr) release stages: 3-6 wide and overlapping ionic TSDC peaks. The correlated data of the ionic TSDC and the F band thermal show that above 290 K the TSR processes are initiated and controlled by the ionic defect thermal detrapping, migration and interaction with the localised electronic and ionic charges and colour centres. The ion diffusion-controlled TSR processes take place in the above halide crystals.     

Thermoluminescence kinetics of lithium borate crystals

The paper reports the results of experimental study and modeling of thermally stimulated recombination processes in undoped crystals of lithium borates Li₂B₄O₇ and LiB₃O₅. For both crystals in the unified experimental conditions, we have measured the glow curves of thermally stimulated luminescence, the spectra and the temperature dependencies of intensity of the steady-state luminescence over the temperature range of 90–500 K under X-rays excitation. In the framework of the unified model, we carried out the calculations of the recombination processes and interpretation of the results with the involvement of all the available experimental data on undoped lithium borate crystals.     

Luminescence VUV spectroscopy of cerium-and europium-doped lithium borate crystals

The paper reports on a study of the luminescence of lithium borate crystals (Li₆Gd(BO₃)₃ doped by Eu³⁺ and Ce³⁺ ions, Li_5.7Mg_0.15Gd(BO₃)₃: Eu, and Li₆Eu(BO₃)₃) initiated by selective excitation by synchrotron radiation at excitation energies of 3.7–27 eV at 10 and 290 K. Efficient energy transfer between the rare-earth ions Gd³⁺ → Ce³⁺ and Gd³⁺ → Eu³⁺ was found to proceed by the resonance mechanism, as well as by electron-hole recombination. Fast decay kinetics of luminescence of the Ce³⁺ activator centers was studied under intracenter photoexcitation and excitation in the interband transition region. The mechanisms involved in luminescence excitation and radiative relaxation of electronic states of rare-earth ions are analyzed, and the energy transfer processes operating in these crystals are discussed.     

Kinetics of thermally stimulated luminescence from alkaline earth borates

The kinetics of thermally stimulated emission (TSL) from XB₂O₄ (X= Ca, Sr, Ba) has been investigated employing both isothermal decay and peak shape analysis methods. For BaB₂O₄ the emission was observed to lie in the visible region and the kinetics of TSL emission could be determined without any ambiguity. However, the emission spectra of CaB₂O₄ and SrB₂O₄ were found to consist of two broad emissions: one in the UV and the other in the blue green regions. Consequent to the presence of two emission bands, the determination of TSL kinetics required collection of TSL data for the two components of emission separately. Otherwise, an unambiguous determination of the TSL kinetics just could not be possible. The kinetics of TSL emission for the three borates is found to be of second order.     

Radiation-induced processes and defects in alkali and alkaline-earth borate crystals

The paper presents the results of a study of the radiation-induced processes and defects in nonlinear optical crystals Li₂B₄O₇ (LTB), LiB₃O₅ (LBO), CsLiB₆O₁₀, KB₅O₈·4H₂O, β-BaB₂O₄. It was revealed that a pulsed electron beam irradiation at 290 K forms the radiation-induced pairs of the ‘vacancy—interstitial atom’ defects in the cation sublattice of these crystals. This gives rise to a creation of metastable electronic (interstitial atom) and hole (small-radius polaron near the cation vacancy) centers in high concentrations. Optical hole-transitions from the local level of the trapped hole centers to the valence band states are responsible for the transient optical absorptions (TOA) of borates in the visible and UV spectral ranges. A sublattice of the weakly bound mobile lithium cations in LTB and LBO favors a spatial separation of the radiation-induced pair defects ‘hole polaron near Li-vacancy—mobile interstitial Li⁰ atom’. Their decay rated by the electron–hole nonradiative tunnel recombination determines a peculiar feature of the TOA decay kinetics in LTB and LBO.     

The pre-exponential factor of the escape probability in thermally stimulated processes

The pre-exponential factor of the transition probability of a trapped charge carrier into an excited state is evaluated from two different approaches. The semi-classical treatment based on a thermodynamical concept involves the calculation of the vibrational entropy change from the partition sum. The quantum-mechanical treatment deals with non-radiative transitions due to electron-phonon coupling. The factor is found to be explicitely dependent on both the temperature and the lattice vibration frequencies represented by a single Einstein oscillator. In addition, the results from different concepts show similar behaviour. As a consequence the effect of lattice vibrations on the position as well as on the shape of glow peaks in thermally stimulated measurements are investigated on a simple phenomenological base. The analysis of some experimental results can be performed in accordance with the theoretical predictions.     

Femtosecond time-resolved absorption processes in lithium niobate crystals

Femtosecond pump pulses are strongly attenuated in lithium niobate owing to two-photon absorption; the relevant nonlinear coefficient beta(p) ranges from approximately 3.5 cm/GW for lambda(p) = 388 nm to approximately 0.1 cm/GW for 514 nm. In collinear pump-probe experiments the probe transmission at the double pump wavelength 2lambda(p) = 776 nm is controlled by two different processes: A direct absorption process involving pump and probe photons (beta (r) = 0.9 cm/GW) leads to a pronounced short-duration transmission dip, whereas the probe absorption by pump-excited charge carriers results in a long-duration plateau. Coherent pump-probe interactions are of no importance. Hot-carrier relaxation occurs on the time scale of < or approximately equal to 0.1 ps.     

Effects of exposure temperature on thermally stimulated currents in InSe single crystals

The thermally stimulated currents (TSC) in InSe single crystals are investigated as a function of initial temperature T₀ at which the samples examined are excited. In particular, experiments show that the peak at low temperature increases considerably in height and shifts markedly to lower temperatures as T₀ is reduced. A kinetic model is proposed for the interpretation of the results.     

On the relation between thermally activated and magnetically stimulated processes during dislocation movement in InSb crystals in a magnetic field

It is shown that, in contrast to conventional mobility of dislocations in InSb crystals, which is characterized by a thermally activated temperature dependence of velocity (v ∝ exp(-U/kT), where U is the activation energy), relaxation displacements of dislocation in the same crystals in a magnetic field in the absence of external load are described by a more complex temperature dependence. The V(T) dependence in the temperature range 120–250°C studied here exhibits a tendency to linearization in the ln v vs. 1/T coordinates only in its low-temperature part and rapidly attains saturation upon an increase in temperature. The observed decrease in the thermal sensitivity of relaxation mobility of dislocations in the magnetic field upon heating is interpreted in the framework of the model describing the detachment of a dislocation from a point defect as a sequence of two processes: (i) magnetically stimulated lowering of the barrier, U → U′ (over time τ_sp of the spin evolution in the system) and (ii) expectation of thermal fluctuation (over a time τ_th ∝ exp(U′/kT)). Thus, at low temperatures, we have , and the total time before detachment amounts to τ_th + τ_sp ≈ τ_th. On the contrary, at high temperatures, we have , and τ_th + τ_sp ≈ τ_sp (i.e., the motion becomes athermal). It is shown that this model correctly describes the results of measurements and makes it possible to separate the effects. In particular, it is found that the barrier height decreases from the activation energy U = 0.8 eV under a load of 10 MPa to U′ = 0.25 eV in a magnetic field of B = 0.8 T.     

Anomalous luminescence of impurity-bound excitons in lithium borate crystals doped with cerium ions

The spectra and relaxation kinetics of the anomalous (?? < 10 ns) luminescence of Li₆GdB₃O₉:Ce³⁺ crystals have been experimentally detected. The time-resolved vacuum ultraviolet spectroscopy study has shown that optical transitions at 6.2 eV, caused by the transfer of an electron from the 4f ¹ ground state of Ce³⁺ to autoionizing states near the conduction band bottom of a crystal, lead to the formation of an impurity-bound exciton with the hole component localized on the 4f state of Ce³⁺ and the electron localized on states of the conduction band bottom. It has been found that the decay of such an exciton in Li₆GdB₃O₉:Ce³⁺ occurs through radiative recombination, leading to fast luminescence at 4.25 eV. The energy threshold for the formation of the impurity-bound exciton has been determined. The distribution functions of elementary relaxations over the reaction rate constants H(k), which determine the relaxation kinetics and luminescence quenching processes, have been calculated.     

Thermoluminescence and thermally stimulated currents in PbMoO4 crystals

Thermally stimulated depolarization (TSD) currents have been studied in the range from 4 K to room temperature by utilizing direct contacts as well as blocking contacts. Charge carrier injection and subsequent trapping seem to be responsible for internal electric fields. A glow peak near 186 K arises from a dipolar relaxation. The results of additional optical excitation during polarization provide some more information on charge transport involved in relaxation processes. The correlation to the results of photoconductivity and annealing procedure leads to the assumption of oxygen vacancies as the dominant defect centres. The study of thermoluminescence (TL) after X-irradiation gives evidence on defect states in question. In addition, from the results quasi-free charge carriers are assumed to be thermally generated above 100 K.     

Low-temperature luminescence and thermally stimulated luminescence of BeO: Mg single crystals

Luminescence and thermally stimulated luminescence (TL) of BeO: Mg crystals are studied at T = 6–380 K. The TL glow curves and the spectra of luminescence (1.2–6.5 eV), luminescence excitation, and reflection (3.7–20 eV) are obtained. It is found that the introduction of an isovalent magnesium impurity into BeO leads to the appearance of three new broad luminescence bands at 6.2–6.3, 4.3–4.4, and 1.9–2.6 eV. The first two are attributed to the radiative annihilation of a relaxed near-impurity (Mg) exciton, the excited state of which is formed as a result of energy transfer by free excitons. The impurity VUV and UV bands are compared with those for the intrinsic luminescence of BeO caused by the radiative annihilation of self-trapped excitons (STE) of two kinds: the band at 6.2–6.3 eV of BeO: Mg is compared with the band at 6.7 eV (STE₁) of BeO, and the band at 4.3–4.4 eV is compared with the band at 4.9 eV (STE²) of BeO. In the visible region, the luminescence spectrum is due to a superposition of intracenter transitions in an impurity complex including a magnesium ion. The manifestation of X-ray-induced luminescence bands at T = 6 K in BeO: Mg indicates their excitation during band-to-band transitions and in recombination processes. The energy characteristics of the impurity states in BeO: Mg are determined; the effect of the isovalent impurity on the fluctuation rearrangement of the BeO: Mg structure in the thermal transformation region of STE₁ → STE₂ is revealed.     

Thermostimulated recombination processes in LiBaF3 crystals

The creation of radiation defects in LiBaF₃ crystals at 10 K and the processes of their thermostimulated recombination are investigated. The methods of optical absorption, thermal bleaching of color centers, thermostimulated luminescence and fractional glow technique are used. The radiation defects anneal in a multi-stage process accompanied with thermo-luminescence at 20, 46, 105, 130, 170, 210 and 270 K. Differences in the optical absorption spectra measured before and after the TSL peaks are obtained and recombination parameters are determined. The TSL peak at 20 K arises from the delocalization of H-centers. The presence of two TSL peaks related to V_K-centers at 105 and 130 K indicates that 60° and 90° migration hops occur.     

Quantum mechanical treatment of the escape probability from traps in thermally stimulated processes

In thermally stimulated processes the calculation of the transition probability of a trapped charge carrier into an excited state is based on an atomistic model involving multiphonon processes. The displacement of lattice particles from equilibrium positions gives rise to electron-lattice interaction. As a result the temperature dependence is found to be of non-Arrhenius type except in some limiting cases. The difference of the probabilities for non-radiative transitions, as calculated by the quantum mechanical and by semi-classical methods, is discussed. Thermoluminescence proves to be an example for a non-radiative transition controlled phenomenon and the calculated glow curve differs appreciably from one model to the other. As a result the glow curves calculated in the quantum mechanical concept exhibit shapes which are broadened and shifted to lower temperatures, as compared to that calculated in the semi-classical concept. In turn, the activation energy, as determined by the initial rise method or the method of heating rate variation, is found to be much smaller in the quantum mechanical case.     

Optical and magnetic resonance spectroscopy of stimulated recombination processes in defect studies

Optical and magnetic resonance spectroscopy is widely used in the investigation of radiation-induced processes in wide-gap solids. This paper discusses the present understanding of applications of the experimental methods of optical and thermoactivation spectroscopy in the research of new materials, for applications in radiation dosimetry and digital imaging and in the basic research into the fundamental physics and chemistry of radiation. The advantages of the simultaneous use of optical and magnetic resonance techniques for the investigation of stimulated processes are considered.     

ODMR study of recombination processes in ionic crystals and silicon carbide

The paper presents the results of optically detected magnetic resonance (ODMR) study of tunneling and photostimulated recombination processes in irradiated ionic crystals, observation of resonance effects using optically pumped F-centres, and investigation of spin-dependent recombination processes in silicon carbide doped with boron, aluminium, gallium and scandium.     

Nanoscale peculiarities of the thermally stimulated surface autosegregation of covalent crystals: Gallium arsenide

Nanoscale changes in the morphology and elemental composition of cleavage surfaces of gallium arsenide crystals, which arise due to thermally stimulated surface autosegregation, are investigated in detail and systematized. It is shown that, depending on the annealing conditions (temperature, duration, and evacuation), gallium arsenide dissociates, forming submicron nonstoichiometric layers on the surface or local phases of arsenic, gallium, and gallium oxide. The mechanism and nature of autosegregation are determined by the competing processes of arsenic sublimation and its surface diffusion with an activation energy of ～31 kJ/mol. The migration of arsenic atoms is described with the help of a crystallochemical model. The nanomorphology of the surface phases includes arsenic and gallium nanoparticles with sizes of 10-200 nm, their agglomerates, and gallium oxide nano- and microcrystallites combined in plate- and chainlike configurations.