Luminescence and defects creation at the relaxation of various electronic excitations in wide-gap materials

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Institute of Physics University of Tartu (Dorpat) Estonia Workshop

1 Luminescence and defects creation at the relaxation of various electronic excitations in wide-gap materials Aleksandr Lushchik Institute of Physics University of Tartu (Dorpat) Estonia Workshop Scintillating Screen Applications in Beam Diagnostics, GSI, Darmstadt, February 15, 2011

2 Outline Intracenter and recombination impurity luminescence. Thermal quenching Emission of free and self-trapped excitons in metal halides and oxides Core-valence luminescence (crossluminescence) Types of hot luminescence. Electron and hole intraband luminescence Multiplication of electronic excitations. Enhanced local density of EEs Creation spectra of Frenkel defects by synchrotron radiation. A role of hot e-h recombination in the materials with E g < E FD Competition between decay channels of EEs 2

3 Intrinsic and impurity excitations in a wide-gap crystal Absorption spectrum of a typical ionic crystal (KF) Absorption constant (cm -1 ) Electronic Excitations K + E g F - Tl + excitons, e 0 Vibronic Excitations H Photon energy (ev) 3

4 E 2 Free ion Free ion I A Impurity luminescence E 1 Ion Ion in a crystal a 2 I Emission 1 I 3 4 (2) (1) E, hν a b A Absorption A large Stokes shift mercury-like ions 1s 2 2s 2 2p 6 ns 2 np 6 nd 10 (n +1)s 2 : Ga +, In +, Tl + Ge 2+, Sn 2+, Pb 2+ intermediate Stokes shift I c A small Stokes shift e.g., Eu 2+ in CaF 2 4 E, hν

5 Thermal stability of impurity luminescence 5

6 Thermal quenching of impurity luminescence d=d 0 exp( ε /kt) f ηo η( T ) = η( 0 ) = f + d 1+ d τ exp( ε / kt d 0 even at 5 K 0 f = τ ) For KCl:Tl ε 1.3 ev Usually, d s -1 ϖ eff. n = ± 1 and even in case of 50 phonons relaxation time < s 6

7 Potential curves for a ground and an excited state of different impurity centres Small Stokes shift excited Large Stokes shift Optical quenching No luminescence even at T=0 K ground weak luminescence 7

8 Introduction of RE 3+ into binary and complex metal oxides with high resistance against radiation Rare-earth ions with 1 to 14 4f-electrons: Ce 3+ Pr 3+ Nd 3+ Pm 3+ Sm 3+ Eu 3+ Gd 3+ Tb 3+ Dy 3+ Ho 3+ Er 3+ Tm 3+ Yb 3+ Lu 3+ Line emission spectra, f f transitions, from IR to UV 8

9 Recombination luminescence (RL) E d a e b f c - band E h E e σ r recombination cross-section Two types of hole traps: with an effective charge or neutral ones with respect to the crystal lattice σ r Coulomb >> σ r neutral v - band e-h RL h-e RL Quenching of impurity recombination luminescence: intracentre nonradiative transitions + external thermal quenching 9

10 Tunnel luminescence (TL) E d a e b f gc c - band E e Deep electron and hole traps are needed to observe TL above RT. In AHCs, E e for an F centre equals 2-3 ev E h v - band Self-trapping of holes takes place in some WGMs e-h RL h-e RL tunnel RL 10

11 Free and self-trapped excitons (FE and STE) in AHC activation barrier q FE τ s STE 11

12 Free exciton Self-trapped exciton F-H pair Luminescence Intensity Number of Frenkel Defects Temperature (K) 12

13 Free and self-trapped excitons (FE and STE) in metal oxides Exciton-phonon interaction is responsible for the coexistence of FE and STE. AHCs: NaI NaBr KI KBr Free (FE) and self-trapped exciton (STE) emission. Luminescence Intensity 8 Κ BeO δ-al 2 O 3 25 STE STE e o MgO FE α-al 2 O 3 e o e o BeO FE 8 Κ Photon Energy (ev) MgO FE BeO FE STE α-al 2 O 3 STE All the above-mentioned exciton emissions are lowtemperature ones. Emissions of bound excitons!! 13

14 Core-valence or crossluminescence (CL) τ 1 ns = 10-9 s energetic yield (a) We have both cross- and recombination luminescence E gc <2E ga (c) Auger processes, no CL at E gc >2E ga (d) We have CL at 2E ga < E gc <2(E ga + E v ) and no CL at E gc >2(E ga + E v ) E ga Egc Auger CL processes (a) (b) (c) (d) E gc < 2E ga E gc = 2E ga E gc > 2E ga E gc > 2(E ga + E v ) CL RL c - band E v 14

15 Core-valence or crossluminescence (CL) CsBr CsCl CsF Intensity, a.u. KMgF 3 KF BaF 2 RbF Wavelength, nm 15

16 Volume density ( ev/cm 3 ) BaF 2 CL 5.5 ev Quenching of crossluminescence under high-dense irradiation more than e-h per 1cm 3 (>10 19 ev) (a) (b) Exatation by γ-quanta Exatation by α-particlel (high density) Peak intensity η (CL/IBL) IBL 2.3 ev 295 K 10 Yield (CL/IBL) Current density (A/cm 2 ) 2 STEL STEL 1 1' c - band 0 e c CL-crossluminescence, IBL-(hole) intraband luminescence 1 CL STEL- self-trapped exciton luminescence M. Kirm, A. Lushchik, Ch. Lushchik, A.I. Nepomnyashikh, F. Savikhin, Radiat. Meas (2001) no CL! anion v-band cation v-band 16

17 Hot luminescence excited state Hot luminescence of complex molecules in water solution (S.Vavilov s group) Energy excitation thermalized emission ground state Configuration coordinate hot luminescence of impuriry centre 17

18 Hot luminescence excited state Hot luminescence of complex molecules in water solution (S.Vavilov s group) Energy excitation thermalized emission ground state Configuration coordinate hot luminescence of impuriry centre hot STE luminescence 18

19 E c non-radiative relaxation c-band radiative relaxation e-ibl Intraband luminescence (IBL) E v E g E v v-band E c > E g > E v Spectrum of hole-ibl Photon energy h-ibl Intensity Intensity 5 0 LiF 80 K 300 K MgO 730 K Photon Energy (ev) 50 0 Intensity Spectra of fast (τ < 2 ns) intraband (IBL) luminescence under irradiation by single nanosecond 300-keV electron pulses of the Kovalchuk-Mesyats-type generator A. Lushchik, Ch. Lushchik, M. Kirm, V. Nagirnyi, F. Savikhin, E. Vasil'chenko, Nucl. Instr. and Meth. B (2006) 19

20 K 2 SO c band Intensity CaSO 4 SrSO K 300 K2 Energy (ev) E g h-ibl DOS BaSO v band Photon Energy (ev) Schematic energy diagram of K 2 SO 4 F. Savikhin, M. Kerikmäe, E. Feldbach, A. Lushchik, D. Onishchik, D. Rakhimov, I. Tokbergenov, "Phys. stat. sol. (c), (2005) 20

(2) Y Ba T c T c YBa 2 Cu 3 O 6.9 HTSC YBa 2 Cu 3 O 6.6 YBa 2 Cu 3 O 6.2 no HTSC Ch.

21 Usage of fast IBL (excitation by single electron pulses, 300 kev, 2ns) for the investigation of high-temperature superconductivity in ceramics Cu (1) O Cu (2) Y Ba T c T c YBa 2 Cu 3 O 6.9 HTSC YBa 2 Cu 3 O 6.6 YBa 2 Cu 3 O 6.2 no HTSC Ch. Lushchik, F. Savikhin, E. Feldbach, I. Meriloo, Sov. J. Low. Temp. Phys (1991) 21

22 Multiplication of electronic excitations (MEE) (a) (b) (c) (d) c-band Energy E g 1 2 2' 1 3 3' 1 4 E id * 4',5' 1 E e E id E v v-band 5 Three mechanisms of MEE in dielectrics: (a) electron-hole, (b) excitonic and (c, d) solid-state analogue of the Franck-Hertz effect in gases. QY >1 22

23 3 2E g 17,5 ev Excitonic mechanism of multiplication (secondary excitons) E 0 eh th E th NaCl:Ag 8 K I Ag Intensity Ratio (I Ag /I σ ) 2 1 I Ag /I σ Intensity Reflection Photon Energy (ev) secondary excitons see peak in intensity ratio spectrum at ev Ag + emission e 0 are more efficient than e-h σ -STE emission due to e + h E. Feldbach, M. Kirm, A. Lushchik, Ch. Lushchik, I. Martinson, J. Phys: Condens. Matter (2000) F.-J. Himpsel and W.Steinmann, Phys. Rev. B 17 (1978)

24 Intensity E (ev) 10 CB 0 FB -10 VB 1-20 VB 2 a b c 3p 3d 3s 2p 6 O 2s 2 O α-al 2 O 3 :Sc 3+ Auger process Sc 3+ emission 5.6 ev Photon Energy (ev) One exciting photon of ~32 ev(start from 2s 2 oxygen shell) is able to form up to 3 e-h pairs A. Lushchik, Ch. Lushchik, P. Liblik, A. Maaroos, V.N. Makhov, F. Savikhin, E. Vasil'chenko, J. Lumin (2009) 24

25 IBL CL Energetic yield ~10-5 ~10-3 Lifetime ~0.1 ns ~ 1 ns Emission Intensity NaCl 300 K 80 K F Photon Energy (ev) 3 2 Reabsorption of IBL by short-lived Frenkel defects 25

26 Dielectric materials with high radiation resistance (E g < E FD ) BeO SiO 2 ZrO 2 MgO Al 2 O 3 MgAl 2 O 4 Lu 3 Al 5 O 12 CaO Sc 2 O 3 CaAl 2 O 4 Y 3 Al 5 O 12 SrO Y 2 O 3 SrAl 2 O 4 Y 3 SiO 5 NaCl (T = 4-80 K) NaBr NaI MgF 2 E (ev) MgO c-band v-band E g E FD Dielectric materials with low radiation resistance (E g > E FD ) LiH LiF LiD NaF NaCl (T > 200 K) KF KCl KBr KI CaF 2 RbCl RbBr RbI SrF 2 CsCl CsBr CsI BaF 2 26

27 Creation spectra of Frenkel defects by VUV radiation In NaCl, the first creation spectrum of F centers was measured already in 1964 by Cheslav Lushchik and collaborators at room temperature (i.e. when E FD < E g ). e 0 and e-h mechanisms of Frenkel defect creation stable F centers and complementary defects (halogen interstitials) are efficiently formed at the recombination of separated electrons and holes. Ch.B. Lushchik, G.G. Liidya, M.A. Elango, "Electron-hole mechanism of color center creation in ionic crystals," Sov. Phys. Solid State, 6, pp , KCl and KBr efficient creation of stable radiation Frenkel defects at both 10 and 300K NaCl highly resistant against radiation at T 80 K 27

28 Low-temperature creation of long-lived F-H pairs in NaCl (E FD > E g ) Reflectivity NaCl e-h e-h T = 9.1 K T = 12 K e 0 E g Photon energy (ev) Number of F centers N(F)~ light sum of (3-4 ev)- flash stimulated in the maximum of F-absorption band (2.75 ev). Synchrotron radiation - FINEST beamline with undulator, MAX-III, Lund Synchrotron radiation (7-40 ev) Lumisescence, ~ N(F) F-stimulation 80 K F Photon energy (ev) F... H + hν F F *... H hν Lum + α. I R v a e i 0 a v a i - a 28

29 Three decay channels of electronic excitations (anion excitons or e-h pairs) in a typical wide-gap crystal c-band E g Energy luminescence heat release Frenkel defects hν L phonons, nhω e s0 v a e (F) + i a0 (H) v-band 29

30 Competition between radiative and non-radiative (heat, defect creation) channels of STE decay Dependence of STE luminescence quenching on high hydrostatic pressure in NaCl 8.6 kbar A.Laisaar, V.Scherbakov, A.Kuznetsov, High Peressure Research 3, (1990) 30

31 Concluding Remarks Radiation-hardened materials Fast and temperature-stable electron or hole intraband luminescence IBL under extremely high-dense irradiation? An obstacle a pre-breakdown emission due to electron avalanches in the bulk of WGMs Materials with additional gaps or at least pseudo-gaps (at certain wave vectors) inside energy bands should be preferred. 31

32 Acknowledgements To all present and former colleagues from the Laboratory of Ionic Crystals et al. 32

33 THANK YOU 33

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