Time resolved luminescence spectroscopy of wide bandgap insulators

Journal of Electron Spectroscopy and Related Phenomena 79 (1996) 99-102

Time resolved luminescence spectroscopy of wide bandgap insulators J.Becker a, A.N.Belsky b, D.Bouttet °, C.Dujardin f, A.V.Gektin d, A.Hopkirk °, S.N.Ivanov b, I.A.Kamenskikh b, N.Y.Kirikova f, V.Klimenko f, V.N.Kolobanov b, V.N.Makhov r, P.Martin 8 V.V.Mikhailin b, I.H.Munro ', C.Mythen ', P.A.Orekhanov b, C.Pedrini ° A.Schroeder a D.A.Shaw" N.Shiran a, I.N.Shpinkov b, A.N.Vasirev b, G.Zimmerer ~ a b ° d

II. Institut fur Experimentalphysik, Universitat Hamburg, Germany Physics Faculty, Moscow State University, Moscow, Russia LPCML, Universite Lyon-I, Villeurbanne, France Institute for Single Crystals, Kharkov, Ukraine DRAL, Daresbury Laboratory, Warrington, UK f Lebedev Physical Institute, Moscow, Russia s LURE, Orsay, France This is a review of the recent activity of a collaboration on the basis of an INTAS grant 'The luminescence process - a study of the excitation and radiation dynamics of materials using synchrotron radiation'. For the following crystals (CeF3, LaF3:Ce, PWO, LSO:Ce, CsI, RbI, KI, CsF, CsC1, BaF2, CsPbCI3, etc.) in the UV, VUV, XUV, and X-ray regions the sequential steps of the energy conversion of a high-energy photon into a luminescence quantum have been studied: (i) absorption; (ii) multiplication of electronic excitations; (iii) migration in the process of thermalisation; (iv) emission center excitation.

I. INTRODUCTION An understanding of the principles which follow the absorption of relatively high energy radiation (photons) by matter is of the greatest importance both in the physical and in the life sciences. The measurement of the UV/VUV luminescence generated by photoabsorption offers probably the most important single parameter for study. A detailed study of luminescence properties can lead to an understanding of the mechanisms which lead to radiation damage effects in organic materials, may help identify the most suitable materials for use as high efficiency X-ray scintillators and could assist with the choice of new improvement materials for X-ray imaging plates or for direct readout, two' dimensional X-ray position detectors. The mechanisms for energy transformation within solids and liquids following an X-ray absorption event are nevertheless exceedingly complex, involving the processes of energy conversion by means of Auger cascades of core holes, inelastic electron-electron scattering, thermalization, energy transfer to emission centers, 0368-2048/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved PII S0368-2048 (96) 02812-5

surface states, defect creation. We perform a progressive study of luminescence properties for different excitation energies, starting with the visible/UV region for a characterization of the luminescent centers, through VUV and XUV to the X-ray region. This allow us to generate many different processes for energy relaxation and thus to separately identify them and to assess their relative importance. 2.

DESCRIPTION

OF

COLLABORATING

GROUPS The merit of this research collaboration is based on drawing together the varied, but essential skills for the project which are located separately within the different institutions to the common scientific advantage of all participants, and which will give the CIS participants access to major technical and scientific facilities which are not normally available to them. The production of materials is the primary responsibility of Prol~ Gektin and Prof. Pedrini, who produce single crystals, powder samples and amorphous layers of a variety of oxides, garnets,

100

alkali halides and organic crystals respectively. Careful characterization of samples using conventional techniques in the visible/UV/near VUV is undertaken by Prof. Mikhailin, Prof. Pedrini and Dr. Makhov. The primary experimental programs is undertaken at Orsay (Dr. Martin), at 'Hamburg (Prof. Zimmerer) and at Daresbury (Prof. Munro) using the synchrotron radiation facilities at LURE, HASYLAB and the SRS. The group in Orsay has developed several experimental stations for luminescent spectroscopy at SuperACO and DCI storage rings, which cover the excitation range from 4 eV to 180 eV and from 7 to 22 KeV with time resolution about 100 psec. The group in Hamburg has developed a unique SR experimental station for luminescent spectroscopy CSuperhimi") for excitation in the spectral range 6 eV to 30 eV and in the time region from -psec to millisec. At Daresbury laboratory (Prof. Munro) luminescence characterization is extended using the SRS from the VUV through the SXR into the X-ray regimes. 3. RESEARCH AIMS

The aim of the project is to study the role of the various processes which exist for the relaxation of electronic excitations created in inorganic and organic crystals by high-energy photons through measurements of luminescence quantum yield and luminescence kinetics in the nanosecond and subnanosecond range, as well as the measurement of defect creation processes. The band gap energies of ionic and rare gas crystals fall into the VUV, so for the study of exciton and electron-hole pair creation, localization and decay with the formation of defects and photostimulated desorption, this region is of an essential importance. For higher photon energies of the order of several band gaps (but still in the U V ) the energy of a photon becomes sufficient for the creation of several electron-hole pairs of excitons, excited centers or any combinations of these electronic excitations situated in very close proximity [1-3]. The interaction between these electronic excitations in the regions where their local density is high can lead to fresh defect creation, to the formation of complex luminescence centers responsible for novel types of luminescence

or to the quenching of luminescence of "conventional" types. This quenching becomes manifest not only through the reduction of the expected quantum yield but also via the modification (which can be pronounced) of the decay kinetics. These "many-body" effects are not well studied, though they can play a crucial role for higher excitation energies. The investigation of the contribution of short-lived and stationary defects created by absorbed radiation and the opportunity to control it by the crystal growth conditions or introduction of impurities is also performed. In the XUV region, in addition to these effects, other processes such as the localization of two excitations on the same ion following the Auger relaxation of a core hole come into effect. Understanding these effects through the construction of a model for their description are verified by the measurements in the X-ray region. 4. MAIN RESULTS The main results of the present work are presented in references listed in Table 1. For the following crystals (CeF3, LaF3:Ce, PWO, LSO:Ce, CsI, RbI, KI, CsF, CsCI, BaF2, CsPbCI3, etc.) in the UV, VUV, XUV, and X-ray regions the sequential steps of the energy conversion of a highenergy photon into a luminescence quantum have been studied: (i) absorption; (ii) multiplication of electronic excitations; (iii) migration in the process of thermalisation; (iv) emission center excitation. In addition to fundamental processes conunon for all the systems some features specific for the system can be named: • the role of localized subbands in the density of states (rare-earth compounds) and of the corelevels (crossluminescent crystals) in the manifestation of the multiplication threshold; • the creation of spatially correlated excitations in the process of the core hole relaxation, resulting in the formation of complex emission centers (iodine compounds); • the bimolecular processes as a probe for the local density of excitations (CsPbCI3); • efficient mechanisms of the energy transfer to the emission centers (LSO).

101

Table 1 Various objects treated by different methods Objects

Excitation spectra

VUV Crossluminescent crystals [16-22] CeF3 and LaF3:Ce

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The emphasis of the collaboration is placed on sample production and characterization. During next years, the emphasis will shift to model creation based on the assembly of experimental data leading to the selection of materials with optimum properties for various applications (e.g. photo stimulated luminescence, direct readout 2D X-ray area detectors, fast scintillators).

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CONCLUSIONS AND PERSPECTIVES

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Photo- Photo- SimulaTime-resolved electron desorp- tion of luminescence spectro- tion procesVUV XUV X-rays scopy ses I16-221 [6,1617,8,18] 221 [12,16, [6,16] [7,81 21,23] [9,13] [4,6] [4,111 [141 [6] [14] [141

ACKNOWLEDGMENTS The authors gratefully acknowledge the support of INTAS 93-2554 grant. The group from Moscow State University also acknowledge the support of RFFI 94-0205695 and ISF JBT100 grants. REFERENCES

10

Figure 2. Experimental and calculated CL excitation spectra for CsBr crystal (LNT) and CsBr absorption. Lc is the cation hole diffusion length [17]

[1] V.V.Mikhailin, NIM, A261, 107 (1987). [2] A.N.Belsky, I.A.Kamenskikh, V.V.Mikhailin, I.N.Shpinkov, and A.N.Vasil'ev, Physica Scripta, voi. 41,530 (1990).

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Figure 3. DecaT curves for CsBr CL at different temperatures [3] A.N.Vasil'ev, V.V.Mikhailin, I.V.Ovchinnikova, Bull. of the Acad. of Sci. of the USSR, Phys. Set., v. 49, p. 164, 1985. [4] A.N.Belsky, V.V.Mikhailin, A.N.Vasil'ev, I.Dailnei, P.Lecoq, C.Pedrini, P.Cbevallier, P.Dhez, P.Martin, Preprint CERN CMS TN95073, 1995; Chem. Phys. Lett., 1995, to be published. [5] J. Becker, M. Runne, A. Schroeder, G.Zimmerer, V. Kolobanov, V. Mikhailin, N. Klassen, S. Shmurak, HASYLAB Annual Report 1994, to be published. [6] N. Belsky, P. Chevallier, P. Dhez, P. Martin, C. Pedrini, A. N. Vasil'ev, NIM, A, to be published in 1995. [7] Vasil'ev A. N., NIM B, to be published in 1995. [8] Vasil'ev A. N., Abstracts of SCINT-95, Delft, 1995. [9] Kamenskikh I. A., Mikhailin V. V., Petrovykh D. Y., Vasil'ev A. N., Munro I. H., Mythen C., Shaw D. A., Becker J., Schroeder A., Zimmerer G., Makhov V. N., ibid. [10]Belsky A. N., Gektin A. V., Klimov S. N., Krupa J. C., Martin P., Mayolet A., Mikhailin V. V., Pedrini C., Vasil'ev A. N., ibid. [ll]Belsky A. N., Gektin A. V., Korzik M. V., Lecoq P., Martin P., Mikhailin V. V., Nikl M., Pedrini C., Schekoldin V. N., Vasil'ev A. N., ibid. [12]Belsky A. N., Bouttet D., Kamenskikh I. A., Makhov V. N., Mikhailin V. V., Pedrini C., Petrovykh D. Y., Terekhin M. A., Vasil'ev A. N., Abstracts of the 1 lth International Conference on Vacuum Ultraviolet Radiation Physics VUV-XI, Tokio, 1995, Tu79. [13]Becker J., Kamenskikh I. A., Mikhailin V. V., Mythen C., Munro I. H., Petrovykh D. Y., Schroeder A., Shaw D. A., Vasil'ev A. N., Zimmerer G, ibid., Tu80. [14]Belsky A. N., Comtet G., Dujardin G., Gektin A. V., Hellner L., Kamenskikh I. A., Martin P.,

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Figure 4. PbWO4 reflectivity (1), 400 nm excitation spectrum (2) measured at room temperature in 4150 eV photon energy region, total PbWO4 absorption (3) and partial Pb absorption (4) calculated from atomic data [4] Mikhailin V. V., Pedrini C., Vasil'ev A. N., ibid., M47. [15]Belsky A. N., Kamenskikh I. A., Martin P., Mikhailin V. V., Munro I. H., Pedrini C., Shaw D. A., Vasil'ev A. N., ibid., W69. [16]I.A.Kamenskikh, M.A.MacDonald, V.N.Makhov, V.V.Mikhailin, I.H.Munro and M.A.Terekhin, Nucl.Instr.& Meth. A348 (1994) 542-545. [17]N.Yu.Kirikova, V.E.Klimenko, V.A.Kozlov, V.N.Makhov, N.M.Khaidukov and T.V.Uvarova, Nucl.Instr.& Meth. A359 (1995) 351. [18]N.Yu.Kirikova and V.N.Makhov, Nucl.Instr.& Meth. A359 (1995) 354. [19]N.Yu.Kirikova, N.M.Khaidukov, V.E.Klimenko, V.A.Kozlov, V.N.Makhov and T.V.Uvarova, Record of the Int.Workshop "PHYSCI-94" Physical Processes in Fast Scintillators, St.Petersburg, September 30 October 3, 1994, p.172. [20]V.Makhov, J.Becker, L.Frankenstein, I.Kuusmann, M.Runne, A.Schroder and G.Zimmerer, Radiation Effects and Defects in Solids, 133-134 (1995), to be published [21]E.G.Devitsin, N.M.Khaidukov, N.Yu.Kirikova, V.E.Klimenko, V.A.Kozlov, V.N.Makhov and T.V.Uvarova, ibid., to be published [22] V.N.Makhov, I.A.Kamenskikh, M.A.Terekhin, I.H.Munro, C.Mythen and D.A.Shaw, Preprint DL-P-95-001 Daresbury Laboratory, June 1995. [23]M.A.Terekhin, I.A.Kamenskikh, V.N.Makhov, V.A.Kozlov, I.H.Munro, D.A.Shaw, C.M.Gregory and M.A.Hayes, Preprint DL-P-95-003 Daresbury Laboratory, June 1995.