Luminescence of F + and F centers in AI 2 O 3. -Y 2 oxide compounds. IOP Conference Series: Materials Science and Engineering.

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1 IOP Conference Series: Materials Science and Engineering Related content Luminescence of + and centers in AI -Y oxide compounds To cite this article: Y Zorenko et al 00 IOP Conf. Ser.: Mater. Sci. Eng View the article online for updates and enhancements. - Luminescence centers in Y Al 5 O :La single crystals Y Zorenko, T Zorenko and T Voznyak - Growth and luminescent properties of Lu SiO 5 and Lu SiO 5 :Ce single crystalline films Yu Zorenko, M Nikl, V Gorbenko et al. - Development of novel UV emitting single crystalline film scintillators Yu Zorenko, V Gorbenko, V Savchyn et al. Recent citations - Drastic Scintillation Yield Enhancement of YAG:Ce with Carbon Doping Oleg Sidletskiy et al - Properties of Ce + -Doped Y Al 5 O Phosphor Nanoparticles ormed by Laser Ablation in Liquid Hiroshi Koizumi et al - Photoluminescence and Thermoluminescence of the Oxygen- Deficient YAG, YAP, and YAM Phosphors Ya. Zhydachevskyy et al This content was downloaded from IP address on 6/0/09 at 5:0

2 th Europhysical Conference on Defects in Insulating Materials (EURODIM 00) IOP Conf. Series: Materials Science and Engineering 5 (00) 0060 doi:0.088/ x/5//0060 Luminescence of + compounds and centers in Al -Y oxide Y Zorenko,, T Zorenko, T Voznyak, A Mandowski, Qi Xia, M Batentschuk and J riеdrich 4 Laboratory of Optoelectronic Materials, Department of Electronics of Ivan ranko National University of Lviv, 7907 Lviv, Ukraine Institute of Physics, Kazimierz Wielki University in Bydgoszcz, Bydgoszcz, Poland Institute of Material Science and Electrical Engineering of University of Erlangen- Nuremberg, 9058 Erlangen, Germany 4 Department Crystal Growth of raunhofer Institute of Integrated Systems and Device Technology (IISB), 9058 Erlangen, Germany yuriyzorenko@gmail.com Abstract We summarize the results of investigation of spectral-kinetic characteristics of the luminescence of + and -centers in Al, YAl (YAP) and Y Al 5 O (YAG) crystals under excitation by synchrotron radiation in the transmittance and fundamental absorption ranges of these oxides. We show that the luminescence of + and centers in the mentioned crystals can be excited via the corresponding intrinsic A A, B and P ( + ) and S P, P () transitions of these centers, as well as via the radiative relaxation of excitons localized around + and centers. In YAG crystal fast (. ns) emission in the 400 nm band, excited at., 5.7 and 6.56 ev, arises from the + -type centers, localized around Y Al antisite defects (ADs). In Al and YAP crystals, the luminescence of excitons localized around + centers (LE( + ) centers) is revealed and the energies of formation of such excitons are determined as well. In YAG crystal the observation of the luminescence of LE( + ) centers is obscured due to presence of the large content (~0. at.%) of Y Al ADs and formation of an excitons localized around Y Al ADs and dimer + -ADs centers.. Introduction Oxygen vacancies play very important role in scintillation and storage processes in phosphors based on wide-gap oxides. Al -Y oxide system presents the series of well-known optical materials, such as Al (sapphire), YAl (YAP) perovskite and Y Al 5 O (YAG) garnet. Apart from the large quantity of works related to studying the luminescence of one- or two- charged oxygen vacancies ( and + centers, respectively) in Al [-5], only several works are dedicated to investigation of the luminescence of these centers in complex oxides, such as YAP [6] and YAG [7-]. Therefore, the regularities of the electronic structure and + centers in Al -Y oxides with different structure type are still unknown by now. Using the synchrotron radiation facilities for excitation of the and + center luminescence in the range of the fundamental absorption edge of the mentioned wide-band oxides gives the possibility to examine the electronic structure of these centers in detail. The energy transfer processes from different oxide hosts to the and + centers can be investigated as well. The aims of our work was comparative investigation of the luminescence of + and centers in single crystals of Al -Y oxide compounds with different structure types, specifically Al, YAP and c 00 Ltd

3 th Europhysical Conference on Defects in Insulating Materials (EURODIM 00) IOP Conf. Series: Materials Science and Engineering 5 (00) 0060 doi:0.088/ x/5//0060 YAG crystals, using the time-resolved luminescence spectroscopy under excitation by SR with an energy of.7-5 ev at the Superlumi experimental station (HASYLAB, DESY) at 0 K and 00 K. In this work, we concern special attention to studying the luminescence of + - and -centers in Al and Al :C crystals. The results of these investigations were compared and summarized in Section 5 together with results of research of the + and center luminescence in YAP and YAG crystals under excitation by SR at approximately the same conditions which have been briefly reported in our previous works [6] and [0], respectively.. Samples and experimental technique The undoped Al crystal was grown in IISB, Erlangen, Germany in vacuum by the Czochralski method. The properties of undoped Al crystal was also compared with the properties of Al :C crystal purchased in Landauer Inc, USA []. The undoped YAP and YAG crystals were grown by horizontal direct crystallization (HDC) method in vacuum in VNIISIMS, Alexandrov, Russia. The + and centers in all crystals were formed during the crystallization of these materials in vacuum without any special treatment. Investigations of the time resolved luminescence of Al, YAP and YAG crystals were performed in the framework of II and II projects at the Superlumi experimental station at HASYLAB, DESY under excitation by pulsed (0.6 ns) synchrotron radiation (SR) with an energy of.7-5 ev at 0 and 00 K.. Luminescence of + and -centers in Al and Al :C crystals The luminescence spectra of Al :C and Al crystals under excitation by SR with different energies are shown in ig.. The emission spectrum of undoped Al crystal at 0 K under excitation by SR in the exciton range with an energy of 8.97 ev (ig.a, curve ) presents the superposition of the luminescence of + and centers in the bands peaked at 9 and 46 nm, respectively [-5], and band at 40 nm related to the exciton luminescence of Al host [, 4]. The luminescence of and centers dominates in the emission spectra of undoped Al crystal (ig.a, curves and, respectively) under excitation in the maxima of corresponding excitation bands of these centers at 6.5 and 4.8 ev (ig.b, curves and 5). The emission spectrum of Al :C crystal at 0 K under excitation by SR in the range of the interband transitions (.6 ev) and the exciton range (8.88 ev) (ig.a, curves 4 and 5 respectively) presents the superposition of the luminescence of + and centers in the bands peaked at 4 and 4 nm, respectively, as well as the band peaked at 90 nm, related to the luminescence of excitons localized around oxygen vacancies [4, 5] and weak host excitonic band peaked at 40 nm [, 4]. The detailed comparison of the emission spectra of Al and Al :C crystals at 0 K (ig.a) shows, that (i) the maxima of the emission band of + and centers in Al :C crystals are notably (about 5 nm) shifted in comparison with positions of these band in Al crystals; (ii) the exciton host emission of Al crystals in the 40 nm band is suppressed when intensity of the emission of excitons localized around oxygen vacancies in the 90 nm bands increases. In the RT range, the luminescence of centers in the bands peaked at nm dominates in the emission spectra of Al and Al :C crystals under excitation by SR in the exciton range (8.66 and 8.84 ev, respectively) (ig.b, curve and ). As can see from ig.b, the emission bands of centers in Al and Al :C practically coincide in RT range. At the same time, the luminescence of + centers in the band peaked at 9 nm prevails in the emission spectrum of Al crystal (ig.b, curve ) under excitation at 4.8 ev in the maximum of the corresponding band (ig., curve 5). The luminescence of + centers in Al crystal in the band peaked at 5 nm at 0 K is excited in the characteristic three bands peaked at 4.84, 5. and 5.87 ev (ig.b, curve ), related to the A A, B and P transitions of this center, respectively [4, 5, 6]. The excitation spectrum of the + center luminescence of in Al :C crystal in the 9 nm band shows, on the whole, the same structure as in Al crystal but demonstrates another distribution of the intensity of the 4.84 and 5. ev excitation bands (ig.b curve 4). The observed change of the intensity of these bands in Al :C crystals in

4 th Europhysical Conference on Defects in Insulating Materials (EURODIM 00) IOP Conf. Series: Materials Science and Engineering 5 (00) 0060 doi:0.088/ x/5//0060 comparison with ones for Al crystal can be caused by the influence of the strong excitation bands of centers partly overlapped with the 5.7 ev excitation band of + centers nm 4 nm 46 nm Al 0 K ev ev ev 40 nm ex( + ) 90 nm + 4 nm ex 0 nm + x5 4 5 x0 Wavelength (nm) 44 nm 45 nm ' Al :C 0 K ev ev ev Al ev Al :C 00 K (b) (a) ev; ev; Wavelength (nm) igure. Luminescence spectra of Al and Al :C crystals at 0 K (a) and 00 K (b) under excitation by SR with energies indicated in the legend of the figure in the range of interband transitions (.75 ev), in exciton range ( ev) and maxima of excitation bands of centers (6.5 ev) and + centers ( ev). The excitation spectrum of the -center luminescence in Al crystal consists of the dominating band peaked at 6. ev and the band in the exciton range peaked at ev, corresponding to the energy of creation of excitons bound with -centers (ig.c, curve 5). The main excitation band of the center luminescence in Al :C crystal is widely elongated in the high-energy side (ig.c, curve 6), most probably, due to very large concentration of these centers. The excitation spectrum of the exciton emission in the 40 nm band in Al crystal (ig.a, curve ) shows sharp two-component peak with maxima at 8.8 and ev. We suppose that the last band can be related to excitation of the intrinsic exciton emission of Al host [, 4], most probably, the longwavelength component of self-trapped exciton (STE) emission. The band peaked at 8.8 ev can correspond to the energy of creation of excitons localized around some defects, for instance doubly charged oxygen vacancies ( center).

5 th Europhysical Conference on Defects in Insulating Materials (EURODIM 00) IOP Conf. Series: Materials Science and Engineering 5 (00) 0060 doi:0.088/ x/5// (a) Al () Al :C () - em =40 nm (ex) - em =90 nm (ex( + )) 0 K ev 8.8 ev ev ev 5.7 ev Al () 4 4 Al :C (4), 4 - em=0 nm ( + ) 5.9 ev 0 K (b) ev 9.0 ev 8.58 ev 8.8 ev ev Al (5) Al :C (6) 5, 6-40 nm () 0 K ev Energy (ev) igure. Excitation spectra of exciton (, ), + (, 4) and (5, 6) centers luminescence in Al (,, 5) and Al :C (, 4, 6) crystals at 0 K. The registration wavelength is indicated in the legend of the figure. The excitation spectrum of the exciton luminescence in the Al :C crystal in the 90 nm band at 0 K (ig.a, curve ) significantly differs from that in Al crystal for the 40 nm band (ig.a, curve ) and consists of other two intensive bands in the exciton range peaked at 8.58 and 8.8 ev. The structure of this spectrum, which is closer to that of the excitation spectrum of the + center luminescence in Al :C 4

6 th Europhysical Conference on Defects in Insulating Materials (EURODIM 00) IOP Conf. Series: Materials Science and Engineering 5 (00) 0060 doi:0.088/ x/5//0060 crystal in the same range (ig.b, curve ), supports the conclusion that the emission in the 90 nm band is the luminescence of excitons localized around + centers. This conclusion is also strongly supported by the decay kinetics of the exciton luminescence of Al and Al :C crystals at 0 K (ig.). As can see from ig., the decay kinetics of the exciton luminescence in the 90 nm band in Al :C crystal (ig., curve 4) substantially differs from the decay kinetics of the intrinsic exciton luminescence of Al crystal in the 40 nm band (ig., curve ) and is very close to the decay kinetic of the + center luminescence in both crystals (ig., curves and ). Namely, in Al :C crystal, the decay time of the main component of the exciton emission in the 80 nm band (.6 ns) is very close to the decay times of the + center luminescence (.0 ns). Somewhat lower decay time of the + emission in Al :C crystal in comparison with undoped Al sample (. ns) can be caused by concentration quenching of the luminescence due to very large concentration of + centers in this crystal. The decay kinetics of the exciton luminescence of Al crystal in the 40 nm band (ig., curve ) shows typical form for the intrinsic exciton emission in oxides from the triplet relaxed excited state [4, 5]. The decay curve can be approximated by the fast component with a decay time of 8. ns and slow component with a decay time in the several hundred µs range which can not be well evaluated due to the narrow (~ 00 ns) time interval for observation of the luminescence between two pulses of SR. or the same reason we can not even estimate the decay time of the center luminescence in Al and Al :C crystals in the ms range. 0, 0,0 τ()=. ns τ()=.0 ns τ()=8. ns τ(4)=.6 ns 4, - Al, 4 - Al :C 0 K - E ex =4.76 ev; λ em =0 nm; - E ex =4.89 ev; λ em =5 nm - E ex = 40 nm; λ em =40 nm 4 - E ex = 8.54 ev; λ em =80 nm Time (ns) igure. Decay kinetics of + center luminescence at 5-0 nm (, ) and exciton luminescence at 40 nm () and 80 nm (4) in Al (, ) and Al :C (, 4) crystals at 0 K 4. Luminescence of + and -centers in YAP and YAG crystals The results of investigation of the + and luminescence in Al and Al :C crystals were compared with the results of research of the luminescence of these centers in YAP and YAG crystals under excitation by SR with approximately the same conditions (igs.4-6). The luminescence of + centers in YAP and YAG crystals is observed in the bands peaked at 57 and 400 nm and 4 and 464 nm under above band-gap excitation, excitation in the exciton rage or intra-center excitation of these centers by quanta with energies of 5.5 and.69 еv (ig.4, curves a and b, respectively). The luminescence of centers in YAP and YAG crystals under analogical conditions of excitation is observed in the main bands peaked at 4 and 464 nm with the shoulders peaked at 570 and 550 nm (ig.4, curves a and b, respectively). The luminescence of + centers in YAP and YAG crystals is excited in the characteristic three bands peaked at 5.54, 6.0, 6.5 ev (YAP) and., 5.7, 6.56 ev (YAG) in the transmittance range of these 5

7 th Europhysical Conference on Defects in Insulating Materials (EURODIM 00) IOP Conf. Series: Materials Science and Engineering 5 (00) 0060 doi:0.088/ x/5//0060 oxides (ig.5, curves a and b, respectively). The luminescence of centers in the same range in YAP and YAG crystals is excited in the main bands peaked at 5.9 and 4.9 ev, with the shoulders at 5.6 and 4.7 ev (ig.5, curves a and b, respectively). All of the mentioned excitation bands correspond to the A A, B and P and S P, P optical transitions of + and centers, respectively [6](ig.7). The significant difference in the positions of these bands is reflected the differences in the wide of band gap, crystal field strength and symmetry of oxygen sites in YAP and YAG host.,0 0,8 0,6 0,4 0, 57 nm + 4 nm YAP (a) 570 nm Іntensity (arb. units),0 0,8 0,6 0,4 0, 400 nm 464 nm YAG (b) 550 nm + 0, Wavelength (nm) 0, Wavelength (nm) igure 4. Luminescence spectra (normalized) of + and -centers in YAP (a) and YAG (b) crystals under excitation by SR with energies of 5.5 еv (a), 4.95 еv (a),.69 еv (b) and 4.89 ev (b) at 0 K,0 0,8 0,6 0,4 0, ev 5.6 ev 6.0 ev ev YAP (a) 7.8 ev 7.54 ev 7.9 ev,0 0,8 0,6 0,4 0,. ev 4.7 ev 4.9 ev YAG ev ev 6.95 ev (b) 0, Energy (ev) 0, Еnergy (еv) igure 5. Excitation spectra (normalized) of + () and -centers () luminescence at 0 K in YAP (a) and YAG (b) crystals monitored at 5 nm (a), 40 nm (a), 400 nm (b) and 470 nm (b), respectively. Due to strong overlapping of the excitation spectra of the + and center luminescence in YAG crystals the positions of excitation bands of these centers are specified separately in ig.6b The luminescence of + centers in YAP and YAG crystals is also excited in the exciton range in the bands peaked at 7.8, 7.9 ev and 6.56, 7.0 ev which correspond to the energies of creation excitons localized around + centers and self-trapped excitons (STE) (ig.5, curves a and b, respectively). The luminescence of centers is also excited in the exciton range in the bands peaked at 7.54 and 6.95 ev, corresponding to creation of excitons localized around centers (ig.5, curves a and b, respectively). At the same time, in YAP and especially in YAG crystals the excitations of the + and center 6

8 th Europhysical Conference on Defects in Insulating Materials (EURODIM 00) IOP Conf. Series: Materials Science and Engineering 5 (00) 0060 doi:0.088/ x/5//0060 luminescence in the exciton range can be obscured due to presence of the large content (~0.-0. at. %) of Y Al antisite defects (ADs) (Y cations in the positions of Al cations) as the main type defects in these complex oxides [5, 6, 7]. Therefore, it is not excluded that in such crystals the formation of + -AD dimer centers can also occur [7, ]. Recenly, for YAP and YAG crystals the formation of excitons localized around ADs (LE(AD) centers) and excitons bound with Y Al ADs has been found [5, 8]. Thus, in YAP and YAG crystals with large concentration of + and centers, the formation of excitons, localized around +-AD or -AD dimer centers can be also observed. Due to allowed character of singlet-singlet radiative transition, the decay kinetics of the + center luminescence in YAP and YAG crystals is very fast (ig.6, curves a and b, respectively). The decay time of the main components of the + center luminescence in YAP and YAG is equal to.6 and. ns, respectively. Due to forbidden triplet-singlet character of radiative transitions, the luminescence of centers is very slow and decays in the ms time range (ig., curves a and b), respectively. 0, 0,0 τ=.6 ns YAP - E ex =7.0 ev; E em =.695 ev; - E ex = 5.0 ev; E em =.9 ev; + Time (ns) (a) 0, τ=. ns YAG - E ex = 4.89 ev; E em =.69 ev; - E ex =.7 ev; E em =.095 ev; + 0, Time (ns) igure 6. Decay kinetics of + () and center () luminescence in YAP (a) and YAG (b) crystals at 0 K monitored at 5 nm (a), 40 nm (a), 400 nm (b) and 470 nm (b), respectively, under excitation by SR with different energies in characteristic points of excitation spectra in ig Systematization of characteristics of the luminescence of + and -centers in Al -Y oxide compounds The spectral-kinetic characteristics of the luminescence of + and centers in Al -Y oxides with different structural types (Al, perovskite (YAP) and garnet (YAG)) are summarized in Table. The luminescence of + and centers in YAG, YAP and Al crystals can be excited as a result of the corresponding intrinsic A A, B and P and S P, P optical transitions in these centers [6] (ig.7). The presence of the excitation bands of the + and center luminescence in the exciton absorption range (ig. and 6) indicates also the possibility of exciting the luminescence of these centers via creation of excitons localized around such centers. Table. Parameters of luminescence of + and -centers in Al, YAP and YAG at 0 K (b) + LE( + ) E g Crystal em, ev ex, ev τ, ns em, ev ex, ev τ, ns em, ev ex, ev еv 4.84; 5.; 5.9; 6.0; Al ~ ms ; ; 6.0; 6.5; 5.9; 5.6; YAP ; ms ; ; 7.8 YAG.095.; 5.7; 6.56; (weak); 4.9; ~6.6; 6.95 ~µs-ms

9 th Europhysical Conference on Defects in Insulating Materials (EURODIM 00) IOP Conf. Series: Materials Science and Engineering 5 (00) 0060 doi:0.088/ x/5//0060 Conduction band Conduction band center + center B 5.6 ev 5.9 ev.9ev τ ~ ms 5.54 ev 6.0 ev 6.5 ev.47ev τ ~.6 ns Valence band Valence band igure 9. Scheme of optical transitions of and + centers in YAP crystals at 0 K The shift in the positions of the luminescence bands of + and centers in Al (.76 and.98 ev), YAP (.47 and.9 ev) and YAG (.095 and.67 ev) is quite consistent with decreasing the band gaps of these oxides (9., 8. and 7.95 ev, respectively) (see Table). The values of the Stokes shift between the main excitation and luminescence bands of centers in Al (. ev), YAP (.6 ev) and YAG (. ev) reflect also the changes of electron phonon interaction in these crystals. or all the oxides studied, the excitation bands of centers are located in the. 5.5 ev range, which, on the whole, is also consistent with the calculations of Bunch [9] for the absorption energy of + centers in oxides on the basis of the modified Molvo Ivy equation (see [0]). The positions of the main excitation bands of the + center luminescence at 5.56, 4.84 and.4 ev for YAP, Al and YAG crystals also reflect an increase of the symmetry and crystal field strength of the respective oxygen sites in series of the mentioned oxide hosts. Specifically, the last conclusion is supported by increasing the difference between the positions of the lowest- and highest excitation bands of + centers in Al, YAP and YAG crystals, which are equal to.08,.07 and.04 ev, respectively. Specifically, these values for YAG crystal are enormously large and this phenomenon needs additional explanation. or our opinion, an important peculiarity of + centers formation in complex oxides, to which YAP and YAG crystals belong, is the possibility of aggregation of + centers with other type of intrinsic defects. The influence of antisite defects (AD) of Y Al type on the formation of + centers in YAG crystal was considered in [7]. It is important to note, that the concentration of YAl AD in YAG is very high and can reach up to 0.0 at. % [6]. Therefore, it is highly likely that ADs really affect the formation of + centers in garnet crystals. The local symmetry of an oxygen site near an ADs suggests the formation of a tetrahedron consisting of two Y D + cations and two other cations, Al Oh and Al Td, one of which (Al Oh ) is replaced by the Y + cations [7]. In this case, the formation of an oxygen vacancy becomes energetically more favourable than in the case of an unperturbed oxygen site [7, ]. Therefore, the fast (. ns) emission of YAG crystals in the 400 nm band, excited at., 5.7 and 6.56 ev, arises from the + centers, which consist of an oxygen vacancy with one trapped electron located around of Y Al AD. In opposite to the luminescence of the + centers in Al and YAP crystals, the luminescence of + centers in YAG and other Al-garnet crystals are characterized by a weak electron-phonon interaction []. Apparently, the influence of Y Al ADs on the formation of + centers can be less probable in YAP crystal, where the concentration of ADs is presumably much lower than in YAG crystals [6]. In this case the luminescent characteristics of + centers are close to properties of these centers in Al crystal. Conclusions The spectral-kinetic characteristics of the luminescence of + and centers in Al, YAl perovskite (YAP) and Y Al 5 O garnet (YAG) crystals under excitation by synchrotron radiation in the transmitance 8

10 th Europhysical Conference on Defects in Insulating Materials (EURODIM 00) IOP Conf. Series: Materials Science and Engineering 5 (00) 0060 doi:0.088/ x/5//0060 and fundamental absorption ranges of these oxides are investigated and summarized. We confirmed in this work that the luminescence of + and centers in YAG, YAP and Al crystals can be excited as a result of the corresponding intrinsic A A, B and P and S P, P optical transitions of these centers, as well as via the radiative relaxation of excitons localized around + and centers. In Al and YAP crystals the luminescence of excitons localized around + centers (LE( + ) centers) is also revealed and the energies of formation of such excitons (Table) are determined. In YAG crystal observation of the luminescence of LE( + ) centers can be obscured due to presence of the large content (~0. at. %) of Y Al antisite defects (ADs) and formation of an excitons localized around Y Al ADs. Specifically, in YAG crystal the fast (. ns) emission in the 400 nm band, excited at., 5.7 and 6.56 ev, can arise from the + -type centers, perturbed by the nearest Y Al ADs. References [] Akselrod M S, Kortov V S, Kravetsky D J and Gotlib V I 990 Radiat. Prot. Dosimetry 5. [] Akselrod M S, Kortov V S and Gorelova E A 99 Radiat. Prot. Dosimetry [] Yang X, Li H, Cheng Y, Tang Q, Su L and Xu J, 008 Journal of Crystal Growth, [4] Surdo A I, Kortov V S and Pustovarov V A. 00 Radiat. Measur [5] Surdo A I and Kortov V S 004 Radiation Measurements [6] Zorenko Yu, Voloshinovskii A and Konstankevych I 004 Optics and Spectroscopy [7] Springis M. Pujats A and Valbis J 99 J. Phys.: Condens. Matter [8] Pujats A and Springis M, 00 Radiat. Eff. Def. Solids [9] Ashurov M Kh, Rakov A and Erzin R A 00 Solid State Commun [0] Zorenko Y, Zorenko T and Voznyak T 00 J. Phys. Conf. series, to be published [] Babin V, Laguta V, Maaroos A, Makhov A, Nikl M and Zazubovich S 00 Phys. Stat. Sol. RRL, to be published [] [] Makhov V N, Lushchik A, Lushchik Ch B, Kirm M, Vasil chenko E, Vielhauer S, Harutunyan V V and Aleksanyan E 008 Nuclear Instruments and Methods in Physics Research Section B [4] Kirm M, Zimmerer G, eldbach E, Lushchik A, Lushchik Ch and Savikhin, 999 Phys. Rev. B [5] Zorenko Yu., Voloshinovskii A, Savchyn V, Vozniak T, Nikl M, Nejezchleb K, Mikhailin V, Kolobanov V and Spassky D 007 Phys. Stat. Sol. (b) [6] Ashurov M, Voronko Yu, Osiko V and Solol A 977 Phys. Status Solidi (a) 4, 0. [7] Stanek C R, Levy M R, McClellan K J and Grimes R W, J. 006 Appl. Phys. 99. [8] Zorenko Yu, Gorbenko V, Voloshinovskii A, Vistovskii V, Nikl M, Mihokova E and Nejezchleb K 008 IEEE Transaction on Nuclear Science [9] Bunch J M, 977 Phys. Rev. B 6, 74. [0] owler W B 968 Physics of Colour Centers (Academic, New York) 9