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Luminescence of bound excitons in NaI and RbI crystals
Volume 119, number 6
LUMINESCENCE
PHYSICS LETTERS A
29 December 1986
OF BOUND EXCITONS IN NaI AND RbI CRYSTALS
V.G. PLEKHANOV InstituteofPhysics,Academy of Sciencesof the Estonian SSR. Riia 142, 202400 Tartu, USSR
A.U. GRINFELDS Latvian State University,Rainis 19, 226050Riga. USSR Received 31 July 1986; accepted for publication 20 October 1986
X-ray induced luminescence spectra were for the first time investigated at 4.2- 100 K in the vicinity of fundamental absorption of NaI crystals cleaved in liquid helium. Besidea the luminescence of the free exciton and its Lo replicas an additional band due to radiative annihilation of bound excitons appears in NaI crystals, as was earlier observed in RbI crystals. The obtained value of the binding energy of excitons is 50 and 60 meV for NaI and RbI, respectively. The nature of the shallow trap in the crystals of wide-ttap insulators is discussed.
So far there are a lot of investigations about intrinsic luminescence of alkali halide crystals (AI-K) (see, e.g. the review papers [ l-41). This is connected with the fact that luminescence investigations are one of the most delicate and widely used experimental methods for the study of specific peculiarities in the energy spectrum and kinetic characteristics of eleo tronic excitations in a crystal. More attention has been paid to the luminescence of wide-gap insulators after detecting the edge luminescence in Xe [ 51 and alkali metal iodides [ 61 (see also refs. [ 3,7]), which set up a series of new tasks in physics of excitons and exciton-phonon coupling in these systems. An analysis of the shape of LG replicas and temperature dependence of their intensity allows one to determine [ 8- 111 the existence of the quasimomenturn k and kinetic energy of the excitons causing the appearance of extremely sharp emission lines in the region of the fundamental absorption edge of NaI, KI and RbI crystals. An additional evidence of the large value of the kinetic energy of free excitons (FE) is the extremely high velocity of their zone relaxation on Lo phonons that is reflected in the LO structure of the excitation spectrum of FE luminescence [ 8,121. Moreover, as was mentioned in [ 13 1, owing
to the polarization interaction of excitons with phonons, the probability of self-trapping does not depend on the kinetic energy of excitons. The predominance of the FrWich mechanism of FE interaction with Lo phonons in AHC indicates a close analogy of excitonic states in some wide-gap insulators and A2B6 semiconductor crystals (see, e.g. ref. [ 121) . This fact was pointed out already by Knox and Teegarden [ 11. Fertility of the application of models of exciton and exciton-phonon coupling in semiconductors to the physics of excitons in wide-gap insulators is obviously connected with the fact that the effective mass method is applied in these systems. This is indicated also by the parabolic dispersion law of the exciton zone in NaI crystals [ 91. Already in the first investigations of the FE luminescence in alkali iodide crystals, the presence of “excessive” emission in the long wavelength part of 2L0 replica was noted. The intensity of this peculiarity increases with the dose of irradiation [ 81. A more detailed study of the peculiarity with EM= 5.68 eV in the RbI crystal was performed in ref. [ 131. It was shown that this “excessive” emission is also subject to thermal quenching with the activation energy 12f4 meV. 317
Volume 119.number 6
PHYSICS LETTERS A
Fig. 1. Luminescence spectra under X-ray excitation at 4.2 K of NaI crystals cleaved in liquid helium (1) and in the atmosphere of dry air (2). The energy state of a bound exciton is indicated by the arrow BE.
The present communication is the first investigation of intrinsic luminescence in highly hygroscopic NaI crystals cleaved in liquid helium. The experimental device and samples were described already in [ 13,141. Here we only note that an immersion helium cryostat with quartz and beryllium windows was used. The crystals were excited by 45 keV X-rays from an industrial X-ray tube with a tungsten anode. An X-ray induced luminescence (RL) spectrum of NaI crystals cleaved in liquid helium is presented in fig. 1 (curve 1) . Another RL spectrum (fig. 1, curve 2) of NaI crystal cleaved in’an atmosphere of a dry air [8] is presented for comparison. It must be pointed out that the intensity of the zero-phonon line and its 1LO replica has raised by a factor of 2 to 3 for the crystals cleaved in liquid helium*‘. The increase of the intensity is caused obviously by the decrease of non-radiative annihilation of excitons on the crystal surface. Such inference seems to be reasonable for the surface of a sample cleaved in liquid helium (see also refs. [ 14-l 6 1). On the other hand, the cleavage in liquid helium leads to the decrease of luminescence near 5.565 eV by the factor of more than 2. Such changes of luminescence intensity indicate perhaps that this peculiarity is of another nature. As is depicted in fig. 2, an increase of temperature causes a decrease of luminescence intensity around *I Besides, the luminescence spectrum contains ako a broad band of self-trapped excitons (STE) (EM = 4.2 eV) and some other bands (impurity or defect related) (for details see refs. [ l-4, 8- 11I). These bands are out of discussion in this paper.
31.8
29 December 1986
Fig. 2. Temperature dependence of the luminescence intensity of the maximum of 2L0 replica (1) and peculiarity at 5.565 eV (2) for NaI crystals cleaved in liquid helium.
5.565 eV. The temperature dependence of luminescence intensity can be described by a barrier conception, as in the case .of FEs2. Still, a comparison of the temperature dependence of 2L0 replica (see, e.g. ref. [8]) and of the peculiarity at 5.565 eV shows that the latter is quenched more rapidly. Taking into account the spectral location of this peculiarity, one obtains that the depth of the trap responsible for the luminescence at 5.565 eV is M%50 meV, where E,,= Isis the energy of E,,,,-E the maximum of the FE emission zero-phonon line. For RbI crystals the trap depth is about 60 meV (the results of intrinsic luminescence from ref. [ 13 ]) . Now,let us briefly discuss the nature of the trap to which free excitons are bound. First of all one must remember the results of Williams and Kabler [ 181, where transient absorption bands were obtained under 500 keV electron beam excitation at. 10 K for KI and RbI crystals (EM= 5.74 and 5.6 1 eV, respectively). For NaI crystals the peculiarity at 5.57 eV in the transient absorption spectrum was detected. The same decay time of this absorption and triplet luminescence of STE leads the authors of ref. [ 18 ] to a conclusion that this absorption is caused by the excitation of a FE next to STE. Further it was shown that if optical binding energy of STE exceeds that of FE by 2 to 4 times, then the binding energy of an exciton bound to STE in NaI is 50 meV. This value coincides ** The possible importance of including quantum corrections in the determination of the barrier [ 171 will be demonstrated by further investigations.
Volume 119, number 6
PHYSICS LETTERS A
with the one obtained from RL spectra (see fig. 1). As for RbI crystals, the binding energy of a trapped excitont3, obtained from RL spectra, is by a factor of 2 less than the one obtained by Williams and Kabler. Note, that in the region of the detected transient absorption of RbI (EM= 5.6 1 eV) , an emission is also detected [ 81. The nature of the luminescence at 5.68 eV in RbI crystals is not clear at the moment and further investigations are necessary to elucidate it. At the end note that the described peculiarities of emission spectra can be detected not only under X-ray excitation, but also when luminescence is excited by longer wavelength photons. In the case of NaI crystals the peculiarity at 5.565 eV was detected in the photoluminescence spectrum [ 111, and prolonged longwavelength tail of 2L0 replica was observed in ref. [ 131. Q Note that the conclusion about the existence of the bondiig states of an exciton causing luminescence in the fundamental absorption region, was obviously made in ref. [ 191 already, when the authors analysed the reasons of discrepancies in the results of photoluminescence [ 191 and cathodoluminescence [3,6].
References [ 1] R.S. Knox and K.J. Teegarden, in: Physics of color centers, ed. W. Beall Fowler (Academic Press, New York, 1968) p. [2] MN. Rabler, in: Point defects in solids, Vol. 1, eds.J.H. Crawford and L.M. Slitkin (Plenum, New York, 1975) p. 327.
29 December 1986
[ 31 C. Lush&k, in: Excitons, eds. E.I. Rashba and M.D. Sturge (North-Holland, Amsterdam, 1982) p. 502. [4] N. Itoh, Adv. Phys. 31 (1982) 491. [5] J.M. Debever, A. Bonnot, A.M. Bonnot, F. Coletti and J. Hanus, Solid State. Commun. 14 (1974) 989; F. Coletti and A.M. Bonnot, Chem. Phys. Lett. 55 (1978) 92. [6] I.L. Kuusmann, P.H. Liblik and C.B. Lushchik, Pis’ma Zh. Eksp. Tear. Fix. 21 (1975) 161; Sov. Phys. JETP Lett. 21 (1975) 72. [ 71 I.Y. Fugol, Adv. Phys. 27 (1978) 1. [ 81 V.G. Plekhanov, A.A. O’Konnel-Bronin, Pis’ma Zh. Eksp. Teor. Fix 27 (1978) 30 [Sov. Phys. JETP Lett. 27 (1978) 271; Phys. Stat. Sol. (b) 95 (1979) 75. [ 91 V.G. Plekhanov, Proc. Int. Conf. Lasers 1980, McLean, VA, USA ( STS Press, McLean, 198 1) p. 94; Enlarged Abstracts of All-Union Conf. on Physics of Insulators, Baku (1982) p. 83. [lo] A. Nouailhat, G. Guillot, E. Mercier and T. van Khiem, J. Lumin. 18/19(1979) 305. [ 111 H. Nishimura and T. Yamano, J. Phys. Sot. Japan 5 1(1982) 2947. [ 121 A.A. Klochikin, S.A. Permogorov and A.N. Rex&sky, Zh. Eksp.Teor. Fix. 71 (1976) 2230 [SovPhys. JETP46 (1976) 11761. [ 131 V.G. Plekhanov, V.V. Shepelev and A.U. Grinfelds, Phys. Stat. Sol. (b) 119 (1983) 493. [ 141 V.G. Plekhanov, A.V. Emel’yanenko and A.U. Grinfelds, Phys. Lett. A 101 (1984) 291. [ 151 R. Planel, A. Bonnot and C. Benoit a la Guilaume, Phys. Stat. Sol. (b) 58 (1973) 251. [ 161 L. Schultheis and I. Balslev, Phys. Rev. B 28 (1983) 2292. [ 171 H. Sumi, J. Phys. !loc. Japan 53 (1984) 3498; 53 (1984) 3512. [ 181 R.T. Williams and M.N. Kabler, Solid State Commun. 10 ( 1972) 49; Phys. Rev. B 9 (1974) 1897. [ 191 H. Nishimura, C. Ohigashi, Y. Tanaka and M. Tomura, J. Phys. Sot. Japan 43 ( 1977) 157.
319
LUMINESCENCE
PHYSICS LETTERS A
29 December 1986
OF BOUND EXCITONS IN NaI AND RbI CRYSTALS
V.G. PLEKHANOV InstituteofPhysics,Academy of Sciencesof the Estonian SSR. Riia 142, 202400 Tartu, USSR
A.U. GRINFELDS Latvian State University,Rainis 19, 226050Riga. USSR Received 31 July 1986; accepted for publication 20 October 1986
X-ray induced luminescence spectra were for the first time investigated at 4.2- 100 K in the vicinity of fundamental absorption of NaI crystals cleaved in liquid helium. Besidea the luminescence of the free exciton and its Lo replicas an additional band due to radiative annihilation of bound excitons appears in NaI crystals, as was earlier observed in RbI crystals. The obtained value of the binding energy of excitons is 50 and 60 meV for NaI and RbI, respectively. The nature of the shallow trap in the crystals of wide-ttap insulators is discussed.
So far there are a lot of investigations about intrinsic luminescence of alkali halide crystals (AI-K) (see, e.g. the review papers [ l-41). This is connected with the fact that luminescence investigations are one of the most delicate and widely used experimental methods for the study of specific peculiarities in the energy spectrum and kinetic characteristics of eleo tronic excitations in a crystal. More attention has been paid to the luminescence of wide-gap insulators after detecting the edge luminescence in Xe [ 51 and alkali metal iodides [ 61 (see also refs. [ 3,7]), which set up a series of new tasks in physics of excitons and exciton-phonon coupling in these systems. An analysis of the shape of LG replicas and temperature dependence of their intensity allows one to determine [ 8- 111 the existence of the quasimomenturn k and kinetic energy of the excitons causing the appearance of extremely sharp emission lines in the region of the fundamental absorption edge of NaI, KI and RbI crystals. An additional evidence of the large value of the kinetic energy of free excitons (FE) is the extremely high velocity of their zone relaxation on Lo phonons that is reflected in the LO structure of the excitation spectrum of FE luminescence [ 8,121. Moreover, as was mentioned in [ 13 1, owing
to the polarization interaction of excitons with phonons, the probability of self-trapping does not depend on the kinetic energy of excitons. The predominance of the FrWich mechanism of FE interaction with Lo phonons in AHC indicates a close analogy of excitonic states in some wide-gap insulators and A2B6 semiconductor crystals (see, e.g. ref. [ 121) . This fact was pointed out already by Knox and Teegarden [ 11. Fertility of the application of models of exciton and exciton-phonon coupling in semiconductors to the physics of excitons in wide-gap insulators is obviously connected with the fact that the effective mass method is applied in these systems. This is indicated also by the parabolic dispersion law of the exciton zone in NaI crystals [ 91. Already in the first investigations of the FE luminescence in alkali iodide crystals, the presence of “excessive” emission in the long wavelength part of 2L0 replica was noted. The intensity of this peculiarity increases with the dose of irradiation [ 81. A more detailed study of the peculiarity with EM= 5.68 eV in the RbI crystal was performed in ref. [ 131. It was shown that this “excessive” emission is also subject to thermal quenching with the activation energy 12f4 meV. 317
Volume 119.number 6
PHYSICS LETTERS A
Fig. 1. Luminescence spectra under X-ray excitation at 4.2 K of NaI crystals cleaved in liquid helium (1) and in the atmosphere of dry air (2). The energy state of a bound exciton is indicated by the arrow BE.
The present communication is the first investigation of intrinsic luminescence in highly hygroscopic NaI crystals cleaved in liquid helium. The experimental device and samples were described already in [ 13,141. Here we only note that an immersion helium cryostat with quartz and beryllium windows was used. The crystals were excited by 45 keV X-rays from an industrial X-ray tube with a tungsten anode. An X-ray induced luminescence (RL) spectrum of NaI crystals cleaved in liquid helium is presented in fig. 1 (curve 1) . Another RL spectrum (fig. 1, curve 2) of NaI crystal cleaved in’an atmosphere of a dry air [8] is presented for comparison. It must be pointed out that the intensity of the zero-phonon line and its 1LO replica has raised by a factor of 2 to 3 for the crystals cleaved in liquid helium*‘. The increase of the intensity is caused obviously by the decrease of non-radiative annihilation of excitons on the crystal surface. Such inference seems to be reasonable for the surface of a sample cleaved in liquid helium (see also refs. [ 14-l 6 1). On the other hand, the cleavage in liquid helium leads to the decrease of luminescence near 5.565 eV by the factor of more than 2. Such changes of luminescence intensity indicate perhaps that this peculiarity is of another nature. As is depicted in fig. 2, an increase of temperature causes a decrease of luminescence intensity around *I Besides, the luminescence spectrum contains ako a broad band of self-trapped excitons (STE) (EM = 4.2 eV) and some other bands (impurity or defect related) (for details see refs. [ l-4, 8- 11I). These bands are out of discussion in this paper.
31.8
29 December 1986
Fig. 2. Temperature dependence of the luminescence intensity of the maximum of 2L0 replica (1) and peculiarity at 5.565 eV (2) for NaI crystals cleaved in liquid helium.
5.565 eV. The temperature dependence of luminescence intensity can be described by a barrier conception, as in the case .of FEs2. Still, a comparison of the temperature dependence of 2L0 replica (see, e.g. ref. [8]) and of the peculiarity at 5.565 eV shows that the latter is quenched more rapidly. Taking into account the spectral location of this peculiarity, one obtains that the depth of the trap responsible for the luminescence at 5.565 eV is M%50 meV, where E,,= Isis the energy of E,,,,-E the maximum of the FE emission zero-phonon line. For RbI crystals the trap depth is about 60 meV (the results of intrinsic luminescence from ref. [ 13 ]) . Now,let us briefly discuss the nature of the trap to which free excitons are bound. First of all one must remember the results of Williams and Kabler [ 181, where transient absorption bands were obtained under 500 keV electron beam excitation at. 10 K for KI and RbI crystals (EM= 5.74 and 5.6 1 eV, respectively). For NaI crystals the peculiarity at 5.57 eV in the transient absorption spectrum was detected. The same decay time of this absorption and triplet luminescence of STE leads the authors of ref. [ 18 ] to a conclusion that this absorption is caused by the excitation of a FE next to STE. Further it was shown that if optical binding energy of STE exceeds that of FE by 2 to 4 times, then the binding energy of an exciton bound to STE in NaI is 50 meV. This value coincides ** The possible importance of including quantum corrections in the determination of the barrier [ 171 will be demonstrated by further investigations.
Volume 119, number 6
PHYSICS LETTERS A
with the one obtained from RL spectra (see fig. 1). As for RbI crystals, the binding energy of a trapped excitont3, obtained from RL spectra, is by a factor of 2 less than the one obtained by Williams and Kabler. Note, that in the region of the detected transient absorption of RbI (EM= 5.6 1 eV) , an emission is also detected [ 81. The nature of the luminescence at 5.68 eV in RbI crystals is not clear at the moment and further investigations are necessary to elucidate it. At the end note that the described peculiarities of emission spectra can be detected not only under X-ray excitation, but also when luminescence is excited by longer wavelength photons. In the case of NaI crystals the peculiarity at 5.565 eV was detected in the photoluminescence spectrum [ 111, and prolonged longwavelength tail of 2L0 replica was observed in ref. [ 131. Q Note that the conclusion about the existence of the bondiig states of an exciton causing luminescence in the fundamental absorption region, was obviously made in ref. [ 191 already, when the authors analysed the reasons of discrepancies in the results of photoluminescence [ 191 and cathodoluminescence [3,6].
References [ 1] R.S. Knox and K.J. Teegarden, in: Physics of color centers, ed. W. Beall Fowler (Academic Press, New York, 1968) p. [2] MN. Rabler, in: Point defects in solids, Vol. 1, eds.J.H. Crawford and L.M. Slitkin (Plenum, New York, 1975) p. 327.
29 December 1986
[ 31 C. Lush&k, in: Excitons, eds. E.I. Rashba and M.D. Sturge (North-Holland, Amsterdam, 1982) p. 502. [4] N. Itoh, Adv. Phys. 31 (1982) 491. [5] J.M. Debever, A. Bonnot, A.M. Bonnot, F. Coletti and J. Hanus, Solid State. Commun. 14 (1974) 989; F. Coletti and A.M. Bonnot, Chem. Phys. Lett. 55 (1978) 92. [6] I.L. Kuusmann, P.H. Liblik and C.B. Lushchik, Pis’ma Zh. Eksp. Tear. Fix. 21 (1975) 161; Sov. Phys. JETP Lett. 21 (1975) 72. [ 71 I.Y. Fugol, Adv. Phys. 27 (1978) 1. [ 81 V.G. Plekhanov, A.A. O’Konnel-Bronin, Pis’ma Zh. Eksp. Teor. Fix 27 (1978) 30 [Sov. Phys. JETP Lett. 27 (1978) 271; Phys. Stat. Sol. (b) 95 (1979) 75. [ 91 V.G. Plekhanov, Proc. Int. Conf. Lasers 1980, McLean, VA, USA ( STS Press, McLean, 198 1) p. 94; Enlarged Abstracts of All-Union Conf. on Physics of Insulators, Baku (1982) p. 83. [lo] A. Nouailhat, G. Guillot, E. Mercier and T. van Khiem, J. Lumin. 18/19(1979) 305. [ 111 H. Nishimura and T. Yamano, J. Phys. Sot. Japan 5 1(1982) 2947. [ 121 A.A. Klochikin, S.A. Permogorov and A.N. Rex&sky, Zh. Eksp.Teor. Fix. 71 (1976) 2230 [SovPhys. JETP46 (1976) 11761. [ 131 V.G. Plekhanov, V.V. Shepelev and A.U. Grinfelds, Phys. Stat. Sol. (b) 119 (1983) 493. [ 141 V.G. Plekhanov, A.V. Emel’yanenko and A.U. Grinfelds, Phys. Lett. A 101 (1984) 291. [ 151 R. Planel, A. Bonnot and C. Benoit a la Guilaume, Phys. Stat. Sol. (b) 58 (1973) 251. [ 161 L. Schultheis and I. Balslev, Phys. Rev. B 28 (1983) 2292. [ 171 H. Sumi, J. Phys. !loc. Japan 53 (1984) 3498; 53 (1984) 3512. [ 181 R.T. Williams and M.N. Kabler, Solid State Commun. 10 ( 1972) 49; Phys. Rev. B 9 (1974) 1897. [ 191 H. Nishimura, C. Ohigashi, Y. Tanaka and M. Tomura, J. Phys. Sot. Japan 43 ( 1977) 157.
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