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Luminescence of defect centers in yttrium–aluminum garnet crystals
PERGAMON
Solid State Communications 120 (2001) 491±494
www.elsevier.com/locate/ssc
Luminescence of defect centers in yttrium±aluminum garnet crystals M.Kh. Ashurov*, A.F. Rakov, R.A. Erzin Scienti®c Industrial Association ªPhononº, 50 Sobir Yusupov Str., 700054 Tashkent, Uzbekistan Received 17 August 2001; accepted 1 October 2001 by C.N.R. Rao
Abstract Spectral±kinetic properties of the 2.38 eV emission in neutron-irradiated and as-received yttrium aluminum garnet (Y3Al5O12) crystals are investigated. This luminescence band as well as its lifetime and the temperature dependence are analyzed. It is suggested that the 2.38 eV luminescence band is due to the strongly allowed transition of the F 1-center in garnet crystals. Possible reasons that may explain the observed temperature dependence of this emission intensity are discussed. q 2001 Elsevier Science Ltd. All rights reserved. PACS: 78.40.Ha; 78.55.2m; 78.60.2b Keywords: E. Luminescence; C. Intrinsic defect; C. YAG; A. Garnets; B. Neutron irradiation
1. Introduction
2. Experimental
Yttrium±aluminum garnet crystals (YAG) are interested as laser host and as scintillate material. The optical properties of this crystal have been investigated for quite some time by Bernhardt [1], who gave his attention to studies of absorption and luminescent behaviors of transient metals in this solid host. Several essential works by Robbins et al. [2,3] were dedicated to the investigation of luminescence and energy transfer in YAG crystals activated by rareearth elements. Radiative relaxation of elemental electron excitations was studied in Refs. [4±6]. Relatively little is known about the origin of intrinsic defects in YAG. The theoretical [7] and the experimental [8,9] attempts to identify the absorption band of F-type centers were undertaken. Bunch [8] and Chakrabarti [9] suggested the 6.1 eV absorption band in neutron-irradiated YAG crystals to be due to F-centers. The present work follows earlier investigations [10,11] and refers to studies of luminescence due to intrinsic defects in YAG crystals.
The samples were high-purity YAG crystals grown by Stockbarger method. The neutron-irradiated crystals described in this work were exposed to ¯uences of 10 16 ±10 19 fast-neutrons/cm 2 at the temperature of 330 K. Photoluminescence (PL) and excitation spectra were taken using the MPF-2A `Hitachi' luminescent spectrometer. g-luminescence (GL) was excited from 60Co-source, dispersed by the ZMR-3 prism monochromator and detected by the FEU-39 model photomultiplier. Absorption measurements were taken using the EPS-3T `Hitachi' double-beam recording spectrometer. An electron beam with a current density of 100 A/cm 2 about 200 eV and 10 ns duration generated by the GIN-600 accelerator was used for pulse excitation. A decay curve at a speci®c wavelength was recorded by passing the luminescence through the MDR-3 monochromator to the FEU-97 model photomultiplier and recording the signal in the I2-9A model oscilloscope. 3. Results and discussion
* Corresponding author. Fax: 1998-71-2755501. E-mail address: [email protected] (M.Kh. Ashurov).
Fig. 1 shows the GL spectra of YAG crystals. The initial GL is predominantly intrinsic emission in a shortwavelength region. The band appearing at about 2 eV arises from an incidental Eu 31 impurity in the component oxide.
0038-1098/01/$ - see front matter q 2001 Elsevier Science Ltd. All rights reserved. PII: S 0038-109 8(01)00434-3
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M.Kh. Ashurov et al. / Solid State Communications 120 (2001) 491±494
Fig. 3. Time-resolved spectra of YAG sample obtained in (1) 20 and (2) 300 ns after electron pulse excitation. Curve (3) is the difference between (1) and (2).
Fig. 1. The GL spectra of neutron-irradiated YAG samples at 300 K. The fast neutron ¯uence labeled by the number at the lower righthand side of the each spectrum.
After neutron-irradiation, a luminescence band appears near 2.38 eV. The number at the right-hand side indicates neutron ¯uence. All spectra are normalized to the same height and displaced vertical to one another. The same
Fig. 2. The room temperature optical absorption, emission and excitation from YAG sample irradiated to 5 £ 10 17 fast neutrons/cm 2.
2.38 eV emission band can be detected in PL. As shown in Fig. 2, the 3.38 eV band is only an excitation band in the spectral range 2.5±6.2 eV. An absorption band at 3.38 eV cannot be clearly seen in the absorption spectrum of neutron-irradiated sample but more intense absorption bands may cover it. The intensity of the 2.38 eV emission band both in GL and PL grows with the fast-neutron ¯uence, as an evidence of this band belonging to the neutron induced center. It must be noted that no emission at 2.38 eV was detected from the neutron-irradiated sample in the thermostimulated luminescence, which means that there is no recombination of carriers of the opposite signs. It was found during an isochronal annealing that the 2.38 eV PL band disappears when the temperature reaches 700 K, but this disappearance can be fully restored by a short exposure to g-radiation. This cycle can be repeated many times. The annealing to temperatures above 1300 K is necessary to eliminate the restoration of the 2.38 eV band in both GL and in PL. It is clear that in the ®rst case, the emission center destroys by releasing a charge while in the second, the loss of the center occurs because of interstitial-vacancy recombination during the process of annealing. As it was mentioned in Ref. [12], the same 2.38 eV emission band can be created in garnet crystals by the thermo-chemical treatment in reducing conditions designed to introduce a stoichiometric excess of the metallic constituent and hence, anion vacancies. Therefore, it seems reasonable to suggest that the emission band at 2.38 eV is associated with the defect in the anion (oxygen) sublattice of YAG crystal i.e. the oxygen vacancy. Two types centers relating to an anion vacancy are now known in oxide crystals [13,14]: F 1and F- centers (one or two electrons, respectively trapped at an anion vacancy). A very similar cathodoluminescence band can be also
M.Kh. Ashurov et al. / Solid State Communications 120 (2001) 491±494
493
Fig. 4. The temperature dependence of the 2.38 eV emission band intensity of YAG crystal exposed to 5 £ 10 17 fast neutrons/cm 2. WAÐPL, XOÐGL, XWÐtemperature increase regime, OAÐtemperature decrease regime. In the inset, the semilogarithmic plots of PL (W) and GL (X) 2.3 eV emission intensity decay are given.
excited very weakly in the as-received YAG sample. The fact that this emission was excited in such a sample does not rule out the possibility of it being due to anion defect center because crystals of oxides are often partially reduced during the growth. In order to de®ne the origin of 2.38 eV emission more precisely, the radiative lifetime was measured under the electron beam pulse excitation. The YAG single crystal grown in a suf®ciently deep vacuum was selected for these studies, deliberately. Such a strongly reducing condition results an anion-de®cient stoichiometric imbalance and a much higher concentration of intrinsic anion defects. In addition, unlike the neutron-irradiated sample, no other undesirable complicating defect centers, i.e. interstitial oxygen clusters are present. Time-resolved spectra for the nominally undoped YAG crystal grown in vacuum are shown in Fig. 3. The earliest spectrum in Fig. 3 resembles the steady state luminescence spectrum of the neutronirradiated sample mentioned above. An extremely shortlived luminescence band is seen at 2.38 eV with a decay time of about 4 £ 10 29 s. The decay time measured indicates that transition is strongly allowed by nature. The magnitude obtained is very close to that one of radiative lifetime of the 3.8 eV luminescence band in Al2O3 associated with 1B ! 1A electric-dipole-allowed transition of the F 1center [13]. By analogy with this oxide, it is therefore tempting to tentatively assign the 2.38 eV-luminescence band to the luminescence from the F 1- center in the YAG crystal. To clarify to what degree the behavior of the defect luminescent band depends on temperature, the PL and GL intensity at 2.38 eV has been investigated in the neutron-
irradiated YAG crystal over the temperature interval from 77 to 600 K (Fig. 4). The number of salient features could be noted. The intensity of the 2.38 eV PL band was approximately independent of the temperature over the range 80± 350 K and was not affected by the prolonged exposure of the crystal to the incident light. On the contrary, the GL intensity at the 2.38 eV decreases below 200 K. There is considerable similarity in the general shape between these curves in the temperature range 350±600 K. The curves of the thermal decay of the GL and PL, plotted as the logarithm, from the luminescence intensity inversely with the temperature is the straight line (Fig. 4, inset). Thus, the intensity of GL and PL at the 2.38 eV, which has an inverse exponential dependence with the temperature and the activation energy for the thermal quenching, estimated from the slope of the straight line is about 0.56 eV. With a subsequent temperature decrease, the GL intensity is fully restored while the 2.38 eV PL band is not observed. In a simple three-level model, an electron in the excited state of the F 1-center can either decay radiatively to the ground state or escape into the conduction band. The process governing the 2.38 luminescence decay seems to be a thermal release of the electron from the F 1-center excited state to the conduction band. In the case of PL, the term `photo-thermal release' would be more correct to be used; in the simplest scenario, we envision the incident light at 3.38 eV as a placing of the F 1-center in a state which ionizes by thermal electron release to the conduction band. If the excited F 1-center ionizes by the electron release, as we have proposed, then in the case of GL, the F 1-luminescence results from the recombination of the free electron with
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M.Kh. Ashurov et al. / Solid State Communications 120 (2001) 491±494
the bare oxygen vacancy 2 1 V22 0 1 e ! F 1 hn
2:38 eV
1
from the ground state of the F 1-center occurs by means of the non-radiative recombination.
1
If Eq. (1) correctly describes the F -luminescence, the leaking of electrons must occur to support the steady recombination process mentioned above. This suggests that the hole of the bottom of the valence band could be localized near the oxygen vacancy creating the channel of the non-radiative recombination. In other words, an electron moves from the F 1-center to the neighboring oxygen ions followed by the non-radiative annihilation with the trapped hole. Such a representation is quite suitable to explain the low-temperature decay of the 2.38 eV emission in the GL, taking into account that holes can be (self) trapped in YAG lattice regular sites at temperatures below 180 K [4]. 4. Final conclusions The experimental observations and discussion suggest that the 2.38 eV emission band in the neutron-irradiated and as-received YAG crystal is possibly due to the strongly allowed transition of the F 1-center. The 2.38 eV PL and PL quenching in the temperature range of 350±600 K is due to the thermal release of electrons from the excited state of the F 1-center into a conduction band. From this work, it can be observed that, during GL, holes from the valence band are localized near oxygen vacancies so that ¯ow of electrons
References [1] H.J. Bernhardt, Phys. Status Solidi B 87 (1979) 213. [2] D.J. Robbins, B. Cockayne, J.L. Glasper, B. Lent, J. Electrochem. Soc. 126 (1979) 1213. [3] D.J. Robbins, B. Cockayne, J.L. Glasper, B. Lent, J. Electrochem. Soc. 126 (1979) 1221. [4] Sh.A. Vakhidov, E.M. Ibragimova, G.I. Ikramov, I. Nuritdinov, A.F. Rakov, Phys. Status Solidi B 106 (1980) 31. [5] A.V. Pujats, L.M. Kuzmin, Z.A. Rachko, J.L. Jansons, Solid state Commun. 54 (1985) 61. [6] M.Kh. Ashurov, A.F. Rakov, Phys. Status Solidi A 139 (1993) K61. [7] Yu.B. Rozenfeld, S.R. Rotman, Phys. Status Solidi A 139 (1993) 249. [8] J.M. Bunch, Phys. Rev. B 16 (1977) 724. [9] K. Chakrabarti, J. Phys. Chem. Solidi 49 (1988) 1009. [10] A.F. Rakov, Phys. Status Solidi A 76 (1983) K57. [11] Sh.A. Vakhidov, A.F. Rakov, Phys. Status Solidi A 80 (1983) K175. [12] V.A. Adrichuk, L.G. Volzhenskaya, Yu.M. Zakharko, Yu.V. Zorenko, Zh. Prikl. Spektrosk. 47 (1987) 3. [13] B.D. Evans, M. Stapelbroek, Phys. Rev. B18 (1978) 7089. [14] G.H. Rosenblatt, M.W. Rowe, G.P. Williams Jr., R.T. Williams, Phys. Rev. B39 (1989) 10309.
Solid State Communications 120 (2001) 491±494
www.elsevier.com/locate/ssc
Luminescence of defect centers in yttrium±aluminum garnet crystals M.Kh. Ashurov*, A.F. Rakov, R.A. Erzin Scienti®c Industrial Association ªPhononº, 50 Sobir Yusupov Str., 700054 Tashkent, Uzbekistan Received 17 August 2001; accepted 1 October 2001 by C.N.R. Rao
Abstract Spectral±kinetic properties of the 2.38 eV emission in neutron-irradiated and as-received yttrium aluminum garnet (Y3Al5O12) crystals are investigated. This luminescence band as well as its lifetime and the temperature dependence are analyzed. It is suggested that the 2.38 eV luminescence band is due to the strongly allowed transition of the F 1-center in garnet crystals. Possible reasons that may explain the observed temperature dependence of this emission intensity are discussed. q 2001 Elsevier Science Ltd. All rights reserved. PACS: 78.40.Ha; 78.55.2m; 78.60.2b Keywords: E. Luminescence; C. Intrinsic defect; C. YAG; A. Garnets; B. Neutron irradiation
1. Introduction
2. Experimental
Yttrium±aluminum garnet crystals (YAG) are interested as laser host and as scintillate material. The optical properties of this crystal have been investigated for quite some time by Bernhardt [1], who gave his attention to studies of absorption and luminescent behaviors of transient metals in this solid host. Several essential works by Robbins et al. [2,3] were dedicated to the investigation of luminescence and energy transfer in YAG crystals activated by rareearth elements. Radiative relaxation of elemental electron excitations was studied in Refs. [4±6]. Relatively little is known about the origin of intrinsic defects in YAG. The theoretical [7] and the experimental [8,9] attempts to identify the absorption band of F-type centers were undertaken. Bunch [8] and Chakrabarti [9] suggested the 6.1 eV absorption band in neutron-irradiated YAG crystals to be due to F-centers. The present work follows earlier investigations [10,11] and refers to studies of luminescence due to intrinsic defects in YAG crystals.
The samples were high-purity YAG crystals grown by Stockbarger method. The neutron-irradiated crystals described in this work were exposed to ¯uences of 10 16 ±10 19 fast-neutrons/cm 2 at the temperature of 330 K. Photoluminescence (PL) and excitation spectra were taken using the MPF-2A `Hitachi' luminescent spectrometer. g-luminescence (GL) was excited from 60Co-source, dispersed by the ZMR-3 prism monochromator and detected by the FEU-39 model photomultiplier. Absorption measurements were taken using the EPS-3T `Hitachi' double-beam recording spectrometer. An electron beam with a current density of 100 A/cm 2 about 200 eV and 10 ns duration generated by the GIN-600 accelerator was used for pulse excitation. A decay curve at a speci®c wavelength was recorded by passing the luminescence through the MDR-3 monochromator to the FEU-97 model photomultiplier and recording the signal in the I2-9A model oscilloscope. 3. Results and discussion
* Corresponding author. Fax: 1998-71-2755501. E-mail address: [email protected] (M.Kh. Ashurov).
Fig. 1 shows the GL spectra of YAG crystals. The initial GL is predominantly intrinsic emission in a shortwavelength region. The band appearing at about 2 eV arises from an incidental Eu 31 impurity in the component oxide.
0038-1098/01/$ - see front matter q 2001 Elsevier Science Ltd. All rights reserved. PII: S 0038-109 8(01)00434-3
492
M.Kh. Ashurov et al. / Solid State Communications 120 (2001) 491±494
Fig. 3. Time-resolved spectra of YAG sample obtained in (1) 20 and (2) 300 ns after electron pulse excitation. Curve (3) is the difference between (1) and (2).
Fig. 1. The GL spectra of neutron-irradiated YAG samples at 300 K. The fast neutron ¯uence labeled by the number at the lower righthand side of the each spectrum.
After neutron-irradiation, a luminescence band appears near 2.38 eV. The number at the right-hand side indicates neutron ¯uence. All spectra are normalized to the same height and displaced vertical to one another. The same
Fig. 2. The room temperature optical absorption, emission and excitation from YAG sample irradiated to 5 £ 10 17 fast neutrons/cm 2.
2.38 eV emission band can be detected in PL. As shown in Fig. 2, the 3.38 eV band is only an excitation band in the spectral range 2.5±6.2 eV. An absorption band at 3.38 eV cannot be clearly seen in the absorption spectrum of neutron-irradiated sample but more intense absorption bands may cover it. The intensity of the 2.38 eV emission band both in GL and PL grows with the fast-neutron ¯uence, as an evidence of this band belonging to the neutron induced center. It must be noted that no emission at 2.38 eV was detected from the neutron-irradiated sample in the thermostimulated luminescence, which means that there is no recombination of carriers of the opposite signs. It was found during an isochronal annealing that the 2.38 eV PL band disappears when the temperature reaches 700 K, but this disappearance can be fully restored by a short exposure to g-radiation. This cycle can be repeated many times. The annealing to temperatures above 1300 K is necessary to eliminate the restoration of the 2.38 eV band in both GL and in PL. It is clear that in the ®rst case, the emission center destroys by releasing a charge while in the second, the loss of the center occurs because of interstitial-vacancy recombination during the process of annealing. As it was mentioned in Ref. [12], the same 2.38 eV emission band can be created in garnet crystals by the thermo-chemical treatment in reducing conditions designed to introduce a stoichiometric excess of the metallic constituent and hence, anion vacancies. Therefore, it seems reasonable to suggest that the emission band at 2.38 eV is associated with the defect in the anion (oxygen) sublattice of YAG crystal i.e. the oxygen vacancy. Two types centers relating to an anion vacancy are now known in oxide crystals [13,14]: F 1and F- centers (one or two electrons, respectively trapped at an anion vacancy). A very similar cathodoluminescence band can be also
M.Kh. Ashurov et al. / Solid State Communications 120 (2001) 491±494
493
Fig. 4. The temperature dependence of the 2.38 eV emission band intensity of YAG crystal exposed to 5 £ 10 17 fast neutrons/cm 2. WAÐPL, XOÐGL, XWÐtemperature increase regime, OAÐtemperature decrease regime. In the inset, the semilogarithmic plots of PL (W) and GL (X) 2.3 eV emission intensity decay are given.
excited very weakly in the as-received YAG sample. The fact that this emission was excited in such a sample does not rule out the possibility of it being due to anion defect center because crystals of oxides are often partially reduced during the growth. In order to de®ne the origin of 2.38 eV emission more precisely, the radiative lifetime was measured under the electron beam pulse excitation. The YAG single crystal grown in a suf®ciently deep vacuum was selected for these studies, deliberately. Such a strongly reducing condition results an anion-de®cient stoichiometric imbalance and a much higher concentration of intrinsic anion defects. In addition, unlike the neutron-irradiated sample, no other undesirable complicating defect centers, i.e. interstitial oxygen clusters are present. Time-resolved spectra for the nominally undoped YAG crystal grown in vacuum are shown in Fig. 3. The earliest spectrum in Fig. 3 resembles the steady state luminescence spectrum of the neutronirradiated sample mentioned above. An extremely shortlived luminescence band is seen at 2.38 eV with a decay time of about 4 £ 10 29 s. The decay time measured indicates that transition is strongly allowed by nature. The magnitude obtained is very close to that one of radiative lifetime of the 3.8 eV luminescence band in Al2O3 associated with 1B ! 1A electric-dipole-allowed transition of the F 1center [13]. By analogy with this oxide, it is therefore tempting to tentatively assign the 2.38 eV-luminescence band to the luminescence from the F 1- center in the YAG crystal. To clarify to what degree the behavior of the defect luminescent band depends on temperature, the PL and GL intensity at 2.38 eV has been investigated in the neutron-
irradiated YAG crystal over the temperature interval from 77 to 600 K (Fig. 4). The number of salient features could be noted. The intensity of the 2.38 eV PL band was approximately independent of the temperature over the range 80± 350 K and was not affected by the prolonged exposure of the crystal to the incident light. On the contrary, the GL intensity at the 2.38 eV decreases below 200 K. There is considerable similarity in the general shape between these curves in the temperature range 350±600 K. The curves of the thermal decay of the GL and PL, plotted as the logarithm, from the luminescence intensity inversely with the temperature is the straight line (Fig. 4, inset). Thus, the intensity of GL and PL at the 2.38 eV, which has an inverse exponential dependence with the temperature and the activation energy for the thermal quenching, estimated from the slope of the straight line is about 0.56 eV. With a subsequent temperature decrease, the GL intensity is fully restored while the 2.38 eV PL band is not observed. In a simple three-level model, an electron in the excited state of the F 1-center can either decay radiatively to the ground state or escape into the conduction band. The process governing the 2.38 luminescence decay seems to be a thermal release of the electron from the F 1-center excited state to the conduction band. In the case of PL, the term `photo-thermal release' would be more correct to be used; in the simplest scenario, we envision the incident light at 3.38 eV as a placing of the F 1-center in a state which ionizes by thermal electron release to the conduction band. If the excited F 1-center ionizes by the electron release, as we have proposed, then in the case of GL, the F 1-luminescence results from the recombination of the free electron with
494
M.Kh. Ashurov et al. / Solid State Communications 120 (2001) 491±494
the bare oxygen vacancy 2 1 V22 0 1 e ! F 1 hn
2:38 eV
1
from the ground state of the F 1-center occurs by means of the non-radiative recombination.
1
If Eq. (1) correctly describes the F -luminescence, the leaking of electrons must occur to support the steady recombination process mentioned above. This suggests that the hole of the bottom of the valence band could be localized near the oxygen vacancy creating the channel of the non-radiative recombination. In other words, an electron moves from the F 1-center to the neighboring oxygen ions followed by the non-radiative annihilation with the trapped hole. Such a representation is quite suitable to explain the low-temperature decay of the 2.38 eV emission in the GL, taking into account that holes can be (self) trapped in YAG lattice regular sites at temperatures below 180 K [4]. 4. Final conclusions The experimental observations and discussion suggest that the 2.38 eV emission band in the neutron-irradiated and as-received YAG crystal is possibly due to the strongly allowed transition of the F 1-center. The 2.38 eV PL and PL quenching in the temperature range of 350±600 K is due to the thermal release of electrons from the excited state of the F 1-center into a conduction band. From this work, it can be observed that, during GL, holes from the valence band are localized near oxygen vacancies so that ¯ow of electrons
References [1] H.J. Bernhardt, Phys. Status Solidi B 87 (1979) 213. [2] D.J. Robbins, B. Cockayne, J.L. Glasper, B. Lent, J. Electrochem. Soc. 126 (1979) 1213. [3] D.J. Robbins, B. Cockayne, J.L. Glasper, B. Lent, J. Electrochem. Soc. 126 (1979) 1221. [4] Sh.A. Vakhidov, E.M. Ibragimova, G.I. Ikramov, I. Nuritdinov, A.F. Rakov, Phys. Status Solidi B 106 (1980) 31. [5] A.V. Pujats, L.M. Kuzmin, Z.A. Rachko, J.L. Jansons, Solid state Commun. 54 (1985) 61. [6] M.Kh. Ashurov, A.F. Rakov, Phys. Status Solidi A 139 (1993) K61. [7] Yu.B. Rozenfeld, S.R. Rotman, Phys. Status Solidi A 139 (1993) 249. [8] J.M. Bunch, Phys. Rev. B 16 (1977) 724. [9] K. Chakrabarti, J. Phys. Chem. Solidi 49 (1988) 1009. [10] A.F. Rakov, Phys. Status Solidi A 76 (1983) K57. [11] Sh.A. Vakhidov, A.F. Rakov, Phys. Status Solidi A 80 (1983) K175. [12] V.A. Adrichuk, L.G. Volzhenskaya, Yu.M. Zakharko, Yu.V. Zorenko, Zh. Prikl. Spektrosk. 47 (1987) 3. [13] B.D. Evans, M. Stapelbroek, Phys. Rev. B18 (1978) 7089. [14] G.H. Rosenblatt, M.W. Rowe, G.P. Williams Jr., R.T. Williams, Phys. Rev. B39 (1989) 10309.