M. Springis

Polarization of luminescence of colour centres in YAG crystals

The polarization diagrams of the luminescence at 400 nm of colour centres in YAG crystals have been measured under excitation in absorption bands at 370 and 235 nm. The results suggest that the centre is most likely to be a one-electron centre involving an oxygen vacancy and a perturbing agent in the adjacent Al(Oh) site.

The F-type centres in YAG crystals

Abstract The comparative study of optical properties of thermochemically reduced undoped YAG (Y3Al5O12) crystals is reported. A particular type of the centres often observed in YAG crystals synthesized as well as treated under the reducing atmosphere is related to an anion vacancy with one or more trapped electrons (F-type centres in YAG crystals). The changes of photoluminescence intensity and absorption bands intensity of the observed centres under X-irradiation support the F- and F+-centre models. It is shown that the F-centre absorption involves at least two bands, at 195 nm and 240 nm, where the broad F-centre luminescence band at 460 nm can be excited. It is suggested the existence of the F−-centre could be connected with the absorption bands at 360 nm, 480 nm, and 830 nm.

Colour centres in LiBaF3 crystals

Abstract The origin of the absorption bands in LiBaF 3 created by X-ray irradiation at RT has been investigated. It is found that three absorption bands at 270, 320 and 430 nm represent different electron transitions within a radiation defect effectively created in LiBaF 3 single crystals. Following the Mollwo–Ivey relation we discuss investigations of optical dichroism, magnetic optical dichroism, as well as the assumptions regarding the accumulation kinetics of these absorption bands, the F centre being the main radiation defect created by X-rays in undoped LiBaF 3 crystals at RT.

F-type centres in LiBaF3 crystals

Abstract The origin of the absorption bands in LiBaF3 created by X-ray irradiation at RT has been investigated. It is found that three absorption bands at 270, 320 and 430 nm represent different electron transitions within the same radiation defect effectively created in LiBaF3. The Mollwo-Ivey relation as well as investigations of optical dichroism, magnetic circular dichroism and the assumptions regarding the accumulation kinetics of these absorption bands lead us to suggest that the F-type centre being the main radiation defect created by X-rays in LiBaF3 crystals at RT.

Photo- and thermostimulated processes in α-Al2O3

Abstract We reported on the recombination processes determined by the release of electrons from defects connected with the dosimetric 430 K thermostimulated luminescence (TSL) peak as well as with the 260 K TSL peak. These TSL peaks appear in thermochemically reduced α-Al 2 O 3 crystals containing hydrogen and emission of these TSL peaks corresponds to luminescence of the F-center. The X-ray exposure or UV excitation in the absorption band of F-centers at 6.0 eV of reduced α-Al 2 O 3 crystals doped with acceptor impurities results in the appearance of a broad anisotropic complex absorption band in the spectral region 2.5–3.5 eV and in the appearance of a predominant TSL peak at 430 K. Above 430 K the above-mentioned broad absorption band disappears. Optical bleaching of the 2.5–3.5 eV band is accompanied by the disappearance of the 430 K TSL peak and results in F-center emission. The X-ray or UV excitation of reduced α-Al 2 O 3 crystals with donor-type impurities results in the appearance of an anisotropic absorption band at 4.2 eV and the appearance of a dominant TSL peak at 260 K. Above 260 K the 4.2 eV absorption disappears and photostimulated luminescence (PSL) of the F-center recombination luminescence in the 4.2 eV region is no longer observed. Optical bleaching of the 4.2 eV absorption band is accompanied by the disappearance of the 260 K TSL peak. The successful use of reduced α-Al 2 O 3 in dosimetry needs the optimization of the concentration of all components (acceptors, hydrogen, intrinsic defects) involved in the thermo- and photostimulated processes.

Nature of the blue luminescence bands in PbWO4

Abstract The photoluminescence spectrum of PbWO 4 is composed of blue and green bands, previously attributed to the regular WO 4 group and to the defect-related WO 3 group, respectively. Untill now only green emission was observed in the thermostimulated luminescence (TSL) above 77 K. Investigation of the TSL spectra starting from 20 K indicates that the blue band, being definitely present at least in the 28 K TSL peak, is also due to recombination emission at defect sites.

Self-trapped holes and recombination luminescence in LiBaF3 crystals

Abstract We investigated electron paramagnetic resonance (EPR) angular dependencies, recombination afterglow and thermostimulated luminescence of undoped LiBaF3 crystals, X-irradiated at low temperatures. EPR parameters of the F2− molecule oriented along the [1 1 0] direction have been obtained. Based on the value of the g-shift Δg=g⊥−gII=0.02, characteristic for the VK-centres in similar perovskites, we propose that we indeed observed the VK-centres, not the H-centres. X-irradiation below 170 K results in creation of a long-time temperature-independent afterglow due to the tunnelling recombination between close electron and hole centres. The F-type electron centres and the VK as well as probably O2− centres are proposed to be the tunnelling recombination partners, responsible for the 4.1 and 3.15 eV luminescence bands, respectively.

Defect assisted intrinsic luminescence in Al2O3 crystals

Abstract The origin of the luminescence bands at 7.5 eV anv 3.8 eV appearing additionaly to the luminescence of F- and F+- centres in pure Al2O3 are investigated. The time - resolved luminescence spectra, absorption and luminescence excitation spectra as well as trap spectroscopy data depending on deviation from the stochiometry of crystals are discussed in terms of self - trapping of excitons in two configurations. The role of defects due to annihilation of excitons is considered.

Thermal relaxation of colour centres in LiBaF3 crystals

Abstract Processes in LiBaF3 crystals caused by the thermal decay of F-type centres created by X-irradiation at room temperature have been examined. It is shown that the thermal decay of F-type centres results in the formation of two kinds of electron centres peaking at 630 nm and 740 nm differing in thermal stability. Weak TSL intensity, accompanying the decay of F-centres, also observed as well as the low value of the process activation energy suggest that due to the presence of moving anion vacancies a random walk of the F-centres occur. We propose that in course of the random walk of the F-centres both the aggregate F-centres are created and the annihilation with some complementary radiation defects take place.

Luminescence and electron transport properties of GaN and AlN layers

Abstract The transport properties of free charge carriers, photo- and cathodo-luminescence (CV) in GaN and AlN films obtained by MOCVD technique on sapphire and Si substrates, are investigated. The concentration of free charge carriers in GaN is of order 10 17 – 10 19 cm −3 whereas AlN thin films are insulating. The Hall mobility of electrons are 80– 140 cm 2 / V s ). In undoped GaN films the spectral composition of CL is close to photoluminescence (PL) when excited in the region of band–band transitions. The decay time constant of the 3.44 eV UV emission attributed to the bound exciton is considerably less than 1 ns , whereas the 3.26 eV violet (VI) band shows a slow hyperbolical decay over about 1 μs . The known yellow band appears at 2.25 eV due to transitions via localised states. In AlN the spectral composition of the broad CL band is close to that from bulk materials attributed to charge transfer transitions in deep oxygen-related donor–acceptor centres.