- Email: [email protected]
Electronic excitations and luminescence in MgO:Ge single crystals
Nuclear Instruments and Methods in Physics Research B 166±167 (2000) 232±237
www.elsevier.nl/locate/nimb
Electronic excitations and luminescence in MgO:Ge single crystals T. K arner
a,* ,
S. Dolgov a, M. Kirm b, P. Liblik a, A. Lushchik S. Nakonechnyi a,c b
a,c
, A. Maaroos a,
a Institute of Physics, University of Tartu, Riia 142, 51014 Tartu, Estonia Institute of Experimental Physics, University of Hamburg, Hamburg, Germany c Institute of Material Sciences, University of Tartu, Tartu, Estonia
Abstract MgO single crystals, doped with Ge2 Ômercury-likeÕ ions, were grown. The emission of Ge2 centres (3.2 eV) can be eciently excited at 4s2 ® 4s4p electron transitions in Ge2 ions (4.8±6.4 eV) as well as at the formation of electron± hole (e±h) pairs by 8±36 eV photons. The absorption of one photon of 25 or 30 eV leads to the creation of two or three e±h pairs, respectively. The thermal quenching of the emission begins at 500 K and follows the Mott law with an activation energy E 0.52 eV. Taking advantage of the relatively high thermal stability of Ge2 luminescence, high temperature thermostimulated luminescence (up to 775 K) of MgO:Ge crystals has been investigated. Ó 2000 Elsevier Science B.V. All rights reserved. PACS: 72.20.Jv; 78.60.Hk; 78.60.Kn Keywords: Electronic excitation; Luminescence; MgO
1. Introduction The investigation of electron±hole (e±h) and interstitial±vacancy (i±v) processes in oxide crystals is signi®cantly favoured in the presence of stable ecient luminescent centres. However, a relatively small radius of a Mg2 ion makes it dicult to introduce most well-known luminescent ions into MgO. A relatively large physical radius of a Ca2 ion (0.114 nm) easily enables to
*
Corresponding author. Tel.: +372-7-428-946; fax: +372-7383-033. E-mail address: tiit@®.tartu.ee (T. KaÈrner).
substitute cations in a CaO crystal for various luminescent impurity ions, e.g., for Sn2 , Pb2 , Bi3 s2 -ions. On the other hand, only a few ions can substitute a small-radius Mg2 ion (0.086 nm) in a MgO crystal. It is very dicult to reach a high concentration of Bi3 , Pb2 and even of Sn2 impurity ions in MgO. At the same time, a number of alkali halide and phosphate crystals were doped with Ga and Ge2 s2 -ions [1]. Therefore, we made an attempt to dope a MgO crystal with Ge2 ions with a small physical radius (0.087 nm). We have succeeded in growing bulk doped MgO:Ge single crystals with intensive blue (maximum at 3 eV) luminescence under X-ray and electron excitation.
0168-583X/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 9 9 ) 0 0 7 5 8 - 2
T. K arner et al. / Nucl. Instr. and Meth. in Phys. Res. B 166±167 (2000) 232±237
2. Experimental The MgO:Ge single crystals used in this study were grown at the Institute of Physics, University of Tartu. Two methods were used: the doping of existing MgO single crystals and a variation of the arc fusion technique. In the doping of the MgO single crystals with germanium the method of thermal diusion was used. The starting objects were the MgO single crystals and high grade purity (99.97%) GeO2 powder. The thermal diusion of Ge into MgO was carried out in a microwave stove within a closed graphite crucible. The MgO crystals were heated in GeO2 vapors at 3040±3050 K for 1 h and then cooled down to the room temperature in 1.5 h. The activated MgO:Ge crystals were annealed in a quartz container at 933 K for 1.5 h and then slowly (in 6 h) cooled down to the room temperature. The second method has been described in detail in [2]. In this case the starting material was a mixture of high-purity (99.9%) MgO and GeO2 powders. The concentration of GeO2 in the starting mixture varied from 0.017 to 0.5 mol%. The mixture was carefully stirred, dried and compressed under a pressure of 9 ´ 107 Pa. The MgO:Ge crystals were grown in a carbon arc furnace with two spectrographic-grade graphite electrodes. The stainless-steel furnace reactor was water-cooled and the mixed and pressed MgO± GeO2 powder was melted during 1 h, using an arc current up to 300 A and voltage 70 V. In the course of cooling, crystals up to 15 15 10 mm3 were formed. The crystals were relatively fragile and hard to cleave. An experimental setup for the X-ray luminescence studies in the range of 77±650 K consisted of a tungsten anode X-ray tube operating at 55 kV and 15 mA, and a liquid nitrogen cryostat. A SPM2 monochromator and a FEU-39A photomultiplier were used to record X-ray luminescence spectra during the irradiation of the crystals. The cathodoluminescence spectra were recorded using a 6 keV electron beam (current density 1.0 lA/cm2 to 10 mA/cm2 ), a helium cryostat, and a vacuum double monochromator in the Johnson± Onaka mounting (gratings 1200 L/mm, R 0.5 m, For more details, see spectral resolution 1.5 A). [3].
233
The luminescence and excitation spectra of the MgO:Ge single crystals were measured by means of synchrotron radiation at the SUPERLUMI station of the HASYLAB, DESY, Hamburg. A detailed description of the experimental setup is given elsewhere [4]. A 0.5 m ultraviolet-visible (UV-VIS) monochromator equipped with a fast R2059 (HAMAMATSU) photomultiplier operating in the time-correlated single photon counting mode was used in the present study. The primary and the UV-VIS monochromator were set to the band pass of 0.3 and 12.8 nm, respectively. For the high temperature (295±573 K) thermoluminescence measurements we used a Brown Teledyne Engineering SYSTEM 310 TLD Reader.
3. Results and discussion In Fig. 1 the room temperature cathodoluminescence spectra of MgO:Ge single crystals of various dopant concentrations are presented. The emission band has almost a perfect Gaussian shape with the maximum at 3.18 eV and FWHM 0.94 eV. The intensity of the 3 eV luminescence band depends on the Ge concentration and is the biggest for the intermediate GeO2 con-
Fig. 1. Cathodoluminescence spectra (1 ± 0.5 mol% GeO2 , 2 ± 0.05 mol% GeO2 , 3 ± 0.017 mol% GeO2 in the initial MgO± GeO2 powder) of MgO:Ge single crystal at 295 K. The excitation spectrum of the time-integrated 3 eV emission (4) for the MgO:Ge single crystal at 7.6 K.
234
T. K arner et al. / Nucl. Instr. and Meth. in Phys. Res. B 166±167 (2000) 232±237
centration (0.05 mol% of GeO2 in the starting MgO±GeO2 mixture). Several intrinsic and extrinsic emission bands are known in MgO, contributing to the luminescence in this spectral region. The emission of F centres has maximum at 3.16 eV [5], whereas the emission bands of deformation-induced centers ± vacancy complexes ± [6], and Sn2 centers [7], are both situated at 2.9 eV. In order to avoid a misinterpretation we measured the excitation spectrum of the observed luminescence. Fig. 1 presents also the excitation spectrum for the luminescence (time-integrated) of Ge2 centers in MgO:Ge at 8 K, measured with synchrotron radiation. The recorded spectrum is typical for a `mercury-like' ion and diers from the excitation spectra of the emissions mentioned above. The most intensive excitation band at 5.8±6.4 eV can be ascribed to 1 S0 ® 1 P1 totally allowed electron transitions in a free Ge2 ion. The probability of intercombination 1 S0 ® 3 P1 transitions in a Ge2 ion is lower. Considering the characteristics of Ge2 centers in other systems, 1 S0 ® 3 P1 , 3 P2 intracenter transitions in MgO:Ge are expected in the region of 4.6±5.6 eV. The maximum of an absorption band of F centers lies at 5.05 eV (FWHM 0.6 eV at 80 K). The emission band with the maximum at 3.18 eV (FWHM 0.6 eV) can be excited in the region of this absorption band. According to our data, the intensity of F centre emission undergoes thermal quenching (decreases by 10 times) at the heating of a proton-irradiated MgO crystal from 70 to 295 K. In contrast to F centre emission, the luminescence of Ge2 centers in X-irradiated MgO:Ge does not undergo thermal quenching up to 500 K (see below). Fig. 2 displays the thermal run of the Ge2 luminescence. Both cathodoluminescence (from 10 to 270 K) and X-ray luminescence (from 77 to 650 K) data are shown. As can be seen from Fig. 2, the Ge2 luminescence is thermally very stable: the intensity of the luminescence starts to decline at 500 K and its intensity at 600 K is about one half of that at room temperature. In the same ®gure, a result of ®tting of the X-ray luminescence data to the Mott formula is also depicted. According to our measurements, during thermal quenching the shape of the luminescence band remains un-
Fig. 2. Thermal stability of Ge2 emission. 1 ± cathodoluminescence, 2 ± X-ray luminescence, 3 ± Mott function, ®tted to X-ray luminescence data.
changed. The activation energy of the quenching process E equals 0.52 eV. The reason of the decline of the luminescence intensity at 50±100 K remains unclear. Partly it may be due to the gradual thermal decay of the broad complex luminescence band, centered at 5.4 eV and overlapping the excitation band of Ge2 luminescence. However, the main part of this luminescence band, connected with the recombination of electrons with the holes, localized at the impurity ions and cation vacancies [7], is stable at least to 180 K. To investigate the carrier mobility at low temperature, we measured the edge luminescence of MgO:Ge crystals at LHeT (Fig. 3). As it is evident from Fig. 3, both the luminescence of free (small peak at 7.71 eV) and weakly bound excitons of large radius (the structure at 7.6±7.7 eV) are detectable, indicating the high mobility of excitons and perfect structure of the crystal. However, in comparison with the edge luminescence of the undoped MgO crystals [8], the observed luminescence peaks are shifted about 40 meV to higher energies. This shift is far too big to fall into the limits of measurement accuracy and is tentatively assigned to the in¯uence of Ge doping (for the measured crystal the concentration of GeO2 in the starting powder was 0.07 m%). By using synchrotron radiation and the Ge2 emission, the multiplication of electronic excitations (MEE) in MgO:Ge was investigated. Fig. 4 presents the excitation spectrum for 3 eV lumi-
T. K arner et al. / Nucl. Instr. and Meth. in Phys. Res. B 166±167 (2000) 232±237
Fig. 3. The cathodoluminescence spectrum of MgO:Ge (0.017 mol% GeO2 ) at T 8 K near the absorption edge of MgO. The inset shows the ®ne structure for emission of free exciton (FE) and bound excitons (BE) of large radius.
Fig. 4. The excitation spectrum of the time-integrated 3 eV emission and a re¯ection spectrum for the MgO:Ge single crystal at 7.6 K.
nescence (time-integrated), along with a re¯ection spectrum of MgO, measured in a spectral region of 8±36 eV at 8 K. In the inset the excitonic structure near the absorption edge of MgO is shown. The photoexcitation of MgO:Ge in the region of interband transitions causes the formation of e±h pairs, only small part of which recombine near Ge2 centers causing the appearance of 3 eV emission. For high absorption coecients (105 cmÿ1 ) e±h pairs are formed only in a thin nearsurface layer increasing the probability of nonradiative e±h recombination at the surface. The role of near-surface e±h recombination signi®cantly decreases in the photon energy region of 21±36 eV
235
(absorption coecients are less than 104 cmÿ1 ). A quantitative estimation of near-surface losses has been carried out for a MgO:Al crystal [9]. It was shown recently that the eciency of impurity emissions (5.0, 5.4, 5.9 eV), connected with the recombination of electrons and VOH , VAl or other hole centers, increases in two stages if the energy of the exciting photons exceeds the threshold value Et 21:5 eV [10]. So, the intrinsic MEE process in MgO occurs in two stages. The absorption of one photon of 21±26 eV (the ®rst stage) leads to the creation of two e±h pairs, while one photon of 25±30 eV (the second stage) causes the formation of three e±h pairs. Both these stages of MEE are observed in the excitation spectrum for 3 eV timeintegrated emission in MgO:Ge (see Fig. 4). Taking advantage of the relatively high thermal stability of Ge2 luminescence, we investigated the high temperature thermostimulated luminescence (HTTSL) (up to 775 K) of MgO:Ge crystals. In this temperature region one can expect a manifestation of intrinsic ionic processes, above all a thermally induced diusion of anion interstitials. According to [11], the thermal annealing of H centres (Oÿ 2 molecular ions, formed due to the trapping of an interstitial oxygen atom by a cation vacancy, single or associated with a heterovalent impurity ion) occurs at 550±680 K. According to [12], the diusion of oxygen interstitials is characterized by an activation energy 1.45 eV. We made an attempt to ®nd the tracks of these processes in the high temperature thermoluminescence of MgO:Ge. In Fig. 5 the thermoluminescence of a MgO:Ge crystal, X-irradiated at room temperature but unirradiated with heavy particles, is shown. The thermoluminescence curve was measured using the oscillating temperature regime proposed by Gobrecht [13]. This enabled to calculate the activation energies and frequency factors of the relaxation processes (we followed the procedure proposed by Tale [14]). The steps on the activation energy curve mark the peaks and shoulders on the thermoluminescence curve (Fig. 5). The main TL peak at 400 K is characterized by a distribution of activation energies, connected apparently with the thermal decay of various V-centers in this temperature region. The other peaks at 450, 520 and 625 K have wide distribution of frequency
236
T. K arner et al. / Nucl. Instr. and Meth. in Phys. Res. B 166±167 (2000) 232±237
Fig. 5. High temperature thermoluminescence glow curve of MgO:Ge (1); the activation energy (2) and frequency factor (3) of the relaxation processes involved, measured by use of an oscillating temperature regime.
factors. The activation energy varies in the range of 1.0 to 2.0 eV and the frequency factor ± of 1012 to 1016 sÿ1 , respectively. However, these values can be in¯uenced by the possible contribution of tunnel recombination luminescence that was not taken into account in the data evaluation. The drop of both the activation energy and frequency factor at T > 700 K may be connected with the in¯uence of the emission of the sample holder, developing at these temperatures. However, an analogous decrease of the frequency factor and activation energy is expected in the case of thermally induced hopping diusion of anion intersitials. Our measurements of the optical absorption of neutron-irradiated MgO crystals indicated that the anion vacancies (F -centers), created by neutron irradiation, decayed at the ®rst heating up to 750 K. It makes it promising to search for the thermoluminescent manifestations of the ionic processes in MgO. However, it is dicult to separate the thermoluminescent peaks, originating from ionic processes, from the thermoluminescence peaks, created in the electronic processes. These investigations on the neutron-irradiated MgO crystals are being continued.
4. Conclusions MgO single crystals, doped with Ge2 ions, have been synthesized. The crystals emit intensive
blue luminescence (room temperature cathodoluminescence maximum at 3.18 eV and FWHM 0.94 eV) in the wide temperature range of LHeT to 650 K. The measurement of the excitation spectrum of Ge2 luminescence in the range of photon energies 4.5±35 eV at 8 K revealed the process of the direct excitation of the 4s2 -type Ge2 ions (4.7±6.7 eV) and the process of the multiplication of electronic excitations when the absorption of one photon of 25 or 30 eV leads to the creation of two or three electron±hole pairs, respectively. The high thermal stability of this luminescence makes it especially suitable for the high temperature thermoluminescence measurements. The HTTSL measurements of the MgO:Ge crystals with view to discriminating between electron±hole and interstitial±vacancy mechanisms of light sum storage in MgO are in progress. Acknowledgements We would like to thank Prof. Ch.B. Lushchik for helpful discussions. This work has been supported by Grant 3867 of the Estonian Science Foundation. M.K acknowledges the ®nancial support from the STINT Foundation (Sweden). References [1] N.E. Lushchik, Trudy Inst. Fiz. Acad. Nauk Est. SSR 7 (1958) 134. [2] A. Maaroos, Trudy Inst. Fiz. Acad. Nauk Est. SSR 53 (1982) 49. [3] I.L. Kuusmann, P.H. Liblik, R.A. Mugur, V.M. Tiit, E.H. Feldbach, R.V. Chatskina, J.J. Edula, Trudy Inst. Fiz. Acad. Nauk Est. SSR 51 (1980) 57. [4] G. Zimmerer, Nucl. Instr. and Meth. A 308 (1991) 178. [5] L.A. Kappers, R.L. Kroes, E.B. Hensley, Phys. Rev. B 1 (1970) 4151. [6] Y. Chen, M.M. Abraham, T.J. Turner, C.M. Nelson, Philos. Mag. 32 (1975) 99. [7] K.A. Kalder, T.N. K arner, C.B. Lushchik, A.F. Malysheva, R.V. Milenina, Izv. Akad. Nauk SSSR, Ser. Fiz. 40 (1976) 2313. [8] I.L. Kuusmann, E.H. Feldbach, Sov. Solid State Phys. 23 (1981) 461. [9] Y. Aleksandrov, A. Vasil'ev, V. Kolobanov, C. Lushchik A. Maaroos et al., Trudy Inst. Fiz. Acad. Nauk Est. SSR 53 (1982) 31.
T. K arner et al. / Nucl. Instr. and Meth. in Phys. Res. B 166±167 (2000) 232±237 [10] M. Kirm, E. Feldbach, T. K arner, A. Lushchik, C. Lushchik et al., Nucl. Instr. and Meth B 141 (1998) 431. [11] L.E. Halliburton, L.A. Kappers, Sol. State Commun. 26 (1978) 111.
237
[12] T. Brudevoll, E.A. Kotomin, N.E. Christensen, Phys. Rev. B 53 (1996) 53. [13] H. Gobrecht, D. Hofmann, J. Phys. Chem. Solids 27 (1966) 1759. [14] I.A. Tale, Phys. Stat. Sol. A 66 (1981) 65.
www.elsevier.nl/locate/nimb
Electronic excitations and luminescence in MgO:Ge single crystals T. K arner
a,* ,
S. Dolgov a, M. Kirm b, P. Liblik a, A. Lushchik S. Nakonechnyi a,c b
a,c
, A. Maaroos a,
a Institute of Physics, University of Tartu, Riia 142, 51014 Tartu, Estonia Institute of Experimental Physics, University of Hamburg, Hamburg, Germany c Institute of Material Sciences, University of Tartu, Tartu, Estonia
Abstract MgO single crystals, doped with Ge2 Ômercury-likeÕ ions, were grown. The emission of Ge2 centres (3.2 eV) can be eciently excited at 4s2 ® 4s4p electron transitions in Ge2 ions (4.8±6.4 eV) as well as at the formation of electron± hole (e±h) pairs by 8±36 eV photons. The absorption of one photon of 25 or 30 eV leads to the creation of two or three e±h pairs, respectively. The thermal quenching of the emission begins at 500 K and follows the Mott law with an activation energy E 0.52 eV. Taking advantage of the relatively high thermal stability of Ge2 luminescence, high temperature thermostimulated luminescence (up to 775 K) of MgO:Ge crystals has been investigated. Ó 2000 Elsevier Science B.V. All rights reserved. PACS: 72.20.Jv; 78.60.Hk; 78.60.Kn Keywords: Electronic excitation; Luminescence; MgO
1. Introduction The investigation of electron±hole (e±h) and interstitial±vacancy (i±v) processes in oxide crystals is signi®cantly favoured in the presence of stable ecient luminescent centres. However, a relatively small radius of a Mg2 ion makes it dicult to introduce most well-known luminescent ions into MgO. A relatively large physical radius of a Ca2 ion (0.114 nm) easily enables to
*
Corresponding author. Tel.: +372-7-428-946; fax: +372-7383-033. E-mail address: tiit@®.tartu.ee (T. KaÈrner).
substitute cations in a CaO crystal for various luminescent impurity ions, e.g., for Sn2 , Pb2 , Bi3 s2 -ions. On the other hand, only a few ions can substitute a small-radius Mg2 ion (0.086 nm) in a MgO crystal. It is very dicult to reach a high concentration of Bi3 , Pb2 and even of Sn2 impurity ions in MgO. At the same time, a number of alkali halide and phosphate crystals were doped with Ga and Ge2 s2 -ions [1]. Therefore, we made an attempt to dope a MgO crystal with Ge2 ions with a small physical radius (0.087 nm). We have succeeded in growing bulk doped MgO:Ge single crystals with intensive blue (maximum at 3 eV) luminescence under X-ray and electron excitation.
0168-583X/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 9 9 ) 0 0 7 5 8 - 2
T. K arner et al. / Nucl. Instr. and Meth. in Phys. Res. B 166±167 (2000) 232±237
2. Experimental The MgO:Ge single crystals used in this study were grown at the Institute of Physics, University of Tartu. Two methods were used: the doping of existing MgO single crystals and a variation of the arc fusion technique. In the doping of the MgO single crystals with germanium the method of thermal diusion was used. The starting objects were the MgO single crystals and high grade purity (99.97%) GeO2 powder. The thermal diusion of Ge into MgO was carried out in a microwave stove within a closed graphite crucible. The MgO crystals were heated in GeO2 vapors at 3040±3050 K for 1 h and then cooled down to the room temperature in 1.5 h. The activated MgO:Ge crystals were annealed in a quartz container at 933 K for 1.5 h and then slowly (in 6 h) cooled down to the room temperature. The second method has been described in detail in [2]. In this case the starting material was a mixture of high-purity (99.9%) MgO and GeO2 powders. The concentration of GeO2 in the starting mixture varied from 0.017 to 0.5 mol%. The mixture was carefully stirred, dried and compressed under a pressure of 9 ´ 107 Pa. The MgO:Ge crystals were grown in a carbon arc furnace with two spectrographic-grade graphite electrodes. The stainless-steel furnace reactor was water-cooled and the mixed and pressed MgO± GeO2 powder was melted during 1 h, using an arc current up to 300 A and voltage 70 V. In the course of cooling, crystals up to 15 15 10 mm3 were formed. The crystals were relatively fragile and hard to cleave. An experimental setup for the X-ray luminescence studies in the range of 77±650 K consisted of a tungsten anode X-ray tube operating at 55 kV and 15 mA, and a liquid nitrogen cryostat. A SPM2 monochromator and a FEU-39A photomultiplier were used to record X-ray luminescence spectra during the irradiation of the crystals. The cathodoluminescence spectra were recorded using a 6 keV electron beam (current density 1.0 lA/cm2 to 10 mA/cm2 ), a helium cryostat, and a vacuum double monochromator in the Johnson± Onaka mounting (gratings 1200 L/mm, R 0.5 m, For more details, see spectral resolution 1.5 A). [3].
233
The luminescence and excitation spectra of the MgO:Ge single crystals were measured by means of synchrotron radiation at the SUPERLUMI station of the HASYLAB, DESY, Hamburg. A detailed description of the experimental setup is given elsewhere [4]. A 0.5 m ultraviolet-visible (UV-VIS) monochromator equipped with a fast R2059 (HAMAMATSU) photomultiplier operating in the time-correlated single photon counting mode was used in the present study. The primary and the UV-VIS monochromator were set to the band pass of 0.3 and 12.8 nm, respectively. For the high temperature (295±573 K) thermoluminescence measurements we used a Brown Teledyne Engineering SYSTEM 310 TLD Reader.
3. Results and discussion In Fig. 1 the room temperature cathodoluminescence spectra of MgO:Ge single crystals of various dopant concentrations are presented. The emission band has almost a perfect Gaussian shape with the maximum at 3.18 eV and FWHM 0.94 eV. The intensity of the 3 eV luminescence band depends on the Ge concentration and is the biggest for the intermediate GeO2 con-
Fig. 1. Cathodoluminescence spectra (1 ± 0.5 mol% GeO2 , 2 ± 0.05 mol% GeO2 , 3 ± 0.017 mol% GeO2 in the initial MgO± GeO2 powder) of MgO:Ge single crystal at 295 K. The excitation spectrum of the time-integrated 3 eV emission (4) for the MgO:Ge single crystal at 7.6 K.
234
T. K arner et al. / Nucl. Instr. and Meth. in Phys. Res. B 166±167 (2000) 232±237
centration (0.05 mol% of GeO2 in the starting MgO±GeO2 mixture). Several intrinsic and extrinsic emission bands are known in MgO, contributing to the luminescence in this spectral region. The emission of F centres has maximum at 3.16 eV [5], whereas the emission bands of deformation-induced centers ± vacancy complexes ± [6], and Sn2 centers [7], are both situated at 2.9 eV. In order to avoid a misinterpretation we measured the excitation spectrum of the observed luminescence. Fig. 1 presents also the excitation spectrum for the luminescence (time-integrated) of Ge2 centers in MgO:Ge at 8 K, measured with synchrotron radiation. The recorded spectrum is typical for a `mercury-like' ion and diers from the excitation spectra of the emissions mentioned above. The most intensive excitation band at 5.8±6.4 eV can be ascribed to 1 S0 ® 1 P1 totally allowed electron transitions in a free Ge2 ion. The probability of intercombination 1 S0 ® 3 P1 transitions in a Ge2 ion is lower. Considering the characteristics of Ge2 centers in other systems, 1 S0 ® 3 P1 , 3 P2 intracenter transitions in MgO:Ge are expected in the region of 4.6±5.6 eV. The maximum of an absorption band of F centers lies at 5.05 eV (FWHM 0.6 eV at 80 K). The emission band with the maximum at 3.18 eV (FWHM 0.6 eV) can be excited in the region of this absorption band. According to our data, the intensity of F centre emission undergoes thermal quenching (decreases by 10 times) at the heating of a proton-irradiated MgO crystal from 70 to 295 K. In contrast to F centre emission, the luminescence of Ge2 centers in X-irradiated MgO:Ge does not undergo thermal quenching up to 500 K (see below). Fig. 2 displays the thermal run of the Ge2 luminescence. Both cathodoluminescence (from 10 to 270 K) and X-ray luminescence (from 77 to 650 K) data are shown. As can be seen from Fig. 2, the Ge2 luminescence is thermally very stable: the intensity of the luminescence starts to decline at 500 K and its intensity at 600 K is about one half of that at room temperature. In the same ®gure, a result of ®tting of the X-ray luminescence data to the Mott formula is also depicted. According to our measurements, during thermal quenching the shape of the luminescence band remains un-
Fig. 2. Thermal stability of Ge2 emission. 1 ± cathodoluminescence, 2 ± X-ray luminescence, 3 ± Mott function, ®tted to X-ray luminescence data.
changed. The activation energy of the quenching process E equals 0.52 eV. The reason of the decline of the luminescence intensity at 50±100 K remains unclear. Partly it may be due to the gradual thermal decay of the broad complex luminescence band, centered at 5.4 eV and overlapping the excitation band of Ge2 luminescence. However, the main part of this luminescence band, connected with the recombination of electrons with the holes, localized at the impurity ions and cation vacancies [7], is stable at least to 180 K. To investigate the carrier mobility at low temperature, we measured the edge luminescence of MgO:Ge crystals at LHeT (Fig. 3). As it is evident from Fig. 3, both the luminescence of free (small peak at 7.71 eV) and weakly bound excitons of large radius (the structure at 7.6±7.7 eV) are detectable, indicating the high mobility of excitons and perfect structure of the crystal. However, in comparison with the edge luminescence of the undoped MgO crystals [8], the observed luminescence peaks are shifted about 40 meV to higher energies. This shift is far too big to fall into the limits of measurement accuracy and is tentatively assigned to the in¯uence of Ge doping (for the measured crystal the concentration of GeO2 in the starting powder was 0.07 m%). By using synchrotron radiation and the Ge2 emission, the multiplication of electronic excitations (MEE) in MgO:Ge was investigated. Fig. 4 presents the excitation spectrum for 3 eV lumi-
T. K arner et al. / Nucl. Instr. and Meth. in Phys. Res. B 166±167 (2000) 232±237
Fig. 3. The cathodoluminescence spectrum of MgO:Ge (0.017 mol% GeO2 ) at T 8 K near the absorption edge of MgO. The inset shows the ®ne structure for emission of free exciton (FE) and bound excitons (BE) of large radius.
Fig. 4. The excitation spectrum of the time-integrated 3 eV emission and a re¯ection spectrum for the MgO:Ge single crystal at 7.6 K.
nescence (time-integrated), along with a re¯ection spectrum of MgO, measured in a spectral region of 8±36 eV at 8 K. In the inset the excitonic structure near the absorption edge of MgO is shown. The photoexcitation of MgO:Ge in the region of interband transitions causes the formation of e±h pairs, only small part of which recombine near Ge2 centers causing the appearance of 3 eV emission. For high absorption coecients (105 cmÿ1 ) e±h pairs are formed only in a thin nearsurface layer increasing the probability of nonradiative e±h recombination at the surface. The role of near-surface e±h recombination signi®cantly decreases in the photon energy region of 21±36 eV
235
(absorption coecients are less than 104 cmÿ1 ). A quantitative estimation of near-surface losses has been carried out for a MgO:Al crystal [9]. It was shown recently that the eciency of impurity emissions (5.0, 5.4, 5.9 eV), connected with the recombination of electrons and VOH , VAl or other hole centers, increases in two stages if the energy of the exciting photons exceeds the threshold value Et 21:5 eV [10]. So, the intrinsic MEE process in MgO occurs in two stages. The absorption of one photon of 21±26 eV (the ®rst stage) leads to the creation of two e±h pairs, while one photon of 25±30 eV (the second stage) causes the formation of three e±h pairs. Both these stages of MEE are observed in the excitation spectrum for 3 eV timeintegrated emission in MgO:Ge (see Fig. 4). Taking advantage of the relatively high thermal stability of Ge2 luminescence, we investigated the high temperature thermostimulated luminescence (HTTSL) (up to 775 K) of MgO:Ge crystals. In this temperature region one can expect a manifestation of intrinsic ionic processes, above all a thermally induced diusion of anion interstitials. According to [11], the thermal annealing of H centres (Oÿ 2 molecular ions, formed due to the trapping of an interstitial oxygen atom by a cation vacancy, single or associated with a heterovalent impurity ion) occurs at 550±680 K. According to [12], the diusion of oxygen interstitials is characterized by an activation energy 1.45 eV. We made an attempt to ®nd the tracks of these processes in the high temperature thermoluminescence of MgO:Ge. In Fig. 5 the thermoluminescence of a MgO:Ge crystal, X-irradiated at room temperature but unirradiated with heavy particles, is shown. The thermoluminescence curve was measured using the oscillating temperature regime proposed by Gobrecht [13]. This enabled to calculate the activation energies and frequency factors of the relaxation processes (we followed the procedure proposed by Tale [14]). The steps on the activation energy curve mark the peaks and shoulders on the thermoluminescence curve (Fig. 5). The main TL peak at 400 K is characterized by a distribution of activation energies, connected apparently with the thermal decay of various V-centers in this temperature region. The other peaks at 450, 520 and 625 K have wide distribution of frequency
236
T. K arner et al. / Nucl. Instr. and Meth. in Phys. Res. B 166±167 (2000) 232±237
Fig. 5. High temperature thermoluminescence glow curve of MgO:Ge (1); the activation energy (2) and frequency factor (3) of the relaxation processes involved, measured by use of an oscillating temperature regime.
factors. The activation energy varies in the range of 1.0 to 2.0 eV and the frequency factor ± of 1012 to 1016 sÿ1 , respectively. However, these values can be in¯uenced by the possible contribution of tunnel recombination luminescence that was not taken into account in the data evaluation. The drop of both the activation energy and frequency factor at T > 700 K may be connected with the in¯uence of the emission of the sample holder, developing at these temperatures. However, an analogous decrease of the frequency factor and activation energy is expected in the case of thermally induced hopping diusion of anion intersitials. Our measurements of the optical absorption of neutron-irradiated MgO crystals indicated that the anion vacancies (F -centers), created by neutron irradiation, decayed at the ®rst heating up to 750 K. It makes it promising to search for the thermoluminescent manifestations of the ionic processes in MgO. However, it is dicult to separate the thermoluminescent peaks, originating from ionic processes, from the thermoluminescence peaks, created in the electronic processes. These investigations on the neutron-irradiated MgO crystals are being continued.
4. Conclusions MgO single crystals, doped with Ge2 ions, have been synthesized. The crystals emit intensive
blue luminescence (room temperature cathodoluminescence maximum at 3.18 eV and FWHM 0.94 eV) in the wide temperature range of LHeT to 650 K. The measurement of the excitation spectrum of Ge2 luminescence in the range of photon energies 4.5±35 eV at 8 K revealed the process of the direct excitation of the 4s2 -type Ge2 ions (4.7±6.7 eV) and the process of the multiplication of electronic excitations when the absorption of one photon of 25 or 30 eV leads to the creation of two or three electron±hole pairs, respectively. The high thermal stability of this luminescence makes it especially suitable for the high temperature thermoluminescence measurements. The HTTSL measurements of the MgO:Ge crystals with view to discriminating between electron±hole and interstitial±vacancy mechanisms of light sum storage in MgO are in progress. Acknowledgements We would like to thank Prof. Ch.B. Lushchik for helpful discussions. This work has been supported by Grant 3867 of the Estonian Science Foundation. M.K acknowledges the ®nancial support from the STINT Foundation (Sweden). References [1] N.E. Lushchik, Trudy Inst. Fiz. Acad. Nauk Est. SSR 7 (1958) 134. [2] A. Maaroos, Trudy Inst. Fiz. Acad. Nauk Est. SSR 53 (1982) 49. [3] I.L. Kuusmann, P.H. Liblik, R.A. Mugur, V.M. Tiit, E.H. Feldbach, R.V. Chatskina, J.J. Edula, Trudy Inst. Fiz. Acad. Nauk Est. SSR 51 (1980) 57. [4] G. Zimmerer, Nucl. Instr. and Meth. A 308 (1991) 178. [5] L.A. Kappers, R.L. Kroes, E.B. Hensley, Phys. Rev. B 1 (1970) 4151. [6] Y. Chen, M.M. Abraham, T.J. Turner, C.M. Nelson, Philos. Mag. 32 (1975) 99. [7] K.A. Kalder, T.N. K arner, C.B. Lushchik, A.F. Malysheva, R.V. Milenina, Izv. Akad. Nauk SSSR, Ser. Fiz. 40 (1976) 2313. [8] I.L. Kuusmann, E.H. Feldbach, Sov. Solid State Phys. 23 (1981) 461. [9] Y. Aleksandrov, A. Vasil'ev, V. Kolobanov, C. Lushchik A. Maaroos et al., Trudy Inst. Fiz. Acad. Nauk Est. SSR 53 (1982) 31.
T. K arner et al. / Nucl. Instr. and Meth. in Phys. Res. B 166±167 (2000) 232±237 [10] M. Kirm, E. Feldbach, T. K arner, A. Lushchik, C. Lushchik et al., Nucl. Instr. and Meth B 141 (1998) 431. [11] L.E. Halliburton, L.A. Kappers, Sol. State Commun. 26 (1978) 111.
237
[12] T. Brudevoll, E.A. Kotomin, N.E. Christensen, Phys. Rev. B 53 (1996) 53. [13] H. Gobrecht, D. Hofmann, J. Phys. Chem. Solids 27 (1966) 1759. [14] I.A. Tale, Phys. Stat. Sol. A 66 (1981) 65.