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Thermoluminescence and low-temperature luminescence of beryllium oxide
Radiation Measurements xxx (2015) 1e4
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Radiation Measurements journal homepage: www.elsevier.com/locate/radmeas
Thermoluminescence and low-temperature luminescence of beryllium oxide M.D. Petrenko*, I.N. Ogorodnikov, V. Yu. Ivanov Experimental Physics Department, Ural Federal University, Mira Street, 19, Yekaterinburg 620002, Russia
h i g h l i g h t s We studied pristine and additively-colored BeO crystals and BeO-ceramics. We studied X-rays induced luminescence spectra at 6 and 293 K. We studied spectrally integrated thermoluminescence glow curves at 6-293 K. We determined parameters for dominate TL glow peak located below RT. A number of low-intensity TL peaks were found between 100 and 270K.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 24 October 2015 Received in revised form 21 December 2015 Accepted 24 December 2015 Available online xxx
Beryllium oxide in the forms of either single crystals (pristine, additively-colored) or hot-pressed ceramic samples was studied in the energy range of 1.2e6.2 eV using both the thermoluminescence (TL) and steady-state X-ray induced luminescence (XRL) techniques. The XRL emission spectra were recorded at 6 and 293 K, whereas TL glow curves were studied after X-ray exposure at T0 ¼ 6 K upon linear heating in the temperature range from 6 to 293 K. A search for TL manifestations of shallow trapping centers was carried out using a sensitive channel for TL registration in the range of more than six decades of change in intensity. The participation of shallow trapping centers in the process of recombination luminescence excitation at 6e293 K; branching electronic excitations between different recombination channels; the dominance of the self-trapped exciton and F-center emissions in spectra of the low-temperature recombination luminescence in BeO at 6e293 K were discussed. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Beryllium oxide BeO X-ray luminescence Thermoluminescence Low-temperature luminescence
1. Introduction Since the discovery of intense ultraviolet (UV) emission from beryllium oxide (Albrecht and Mandeville, 1954) and the development of the technology of growing large BeO single crystals (Austerman, 1963), BeO has been the subject of numerous studies. However, the number of research works devoted to luminescence and thermoluminescence (TL) of BeO single crystals at temperatures below 78 K remains relatively low. We are aware of several of them: Cooke et al. (1985, 1984); Petrenko et al. (2014); Shulgin et al. (1988) investigated both the luminescence and TL of BeO:Li, Na single crystals; Pustovarov et al. (2001) have studied relaxation of electronic excitations in BeO crystals using synchrotron radiation spectroscopy. All the other works cited for BeO single crystals, deal
* Corresponding author. E-mail address: [email protected] (M.D. Petrenko).
with temperatures T > 78 K and are devoted to the study of scintillation (Ogorodnikov et al., 1996a; Shulgin et al., 1988) and dosimetric (Ogorodnikov et al., 1991b, 1996b) properties, as well as radiation processing (Belykh et al., 1997; Yakushev et al., 2000, 1999) and doping (Ogorodnikov et al., 1991a) of BeO single crystals. Also, there are several review papers on BeO single crystals (Kruzhalov et al., 1996; Ogorodnikov et al., 1994, 1995a; Ogorodnikov and Kruzhalov, 1997). From the publications cited above, it follows that self-trapped excitons (STE) and F-type lattice defects are dominant luminescence centers in BeO. Pustovarov et al. (2001) have shown the possibility of excitation of the STE during recombination of zone electrons and holes assisted by different lattice defects. Recombination processes in BeO are well studied in the temperature range above 78 K. Elucidation of the role of shallow trapping centers in BeO requires the study of recombination processes at temperatures below 78 K. However, until now only fragmentary data for this
http://dx.doi.org/10.1016/j.radmeas.2015.12.025 1350-4487/© 2015 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Petrenko, M.D., et al., Thermoluminescence and low-temperature luminescence of beryllium oxide, Radiation Measurements (2015), http://dx.doi.org/10.1016/j.radmeas.2015.12.025
2
M.D. Petrenko et al. / Radiation Measurements xxx (2015) 1e4
temperature range are available. The possibility of excitation of STE and F-centers in recombination processes allowed us to use them as luminescence probes for studying manifestations of shallow trapping centers in BeO at temperatures of 6e293 K. The aim of this research work is to study luminescence and thermoluminescence properties of undoped BeO in the forms of both the pristine and additively-colored single crystals in comparison with BeO-ceramics. The research work was carried out in the spectral (1.2e6.2 eV) and temperature (6e293 K) ranges using the methods of both the steady-state X-ray induced luminescence (XRL) and TL techniques. 2. Experimental details The pristine beryllium oxide single crystals of optical quality were grown in the inverse temperature gradient from BeO-solution in a sodium tungstate melt (Maslov et al., 1980). The isomorphous capacity of BeO is fairly low. The Liþ, Zn2þ, Mg2þ, B2þ, Al3þ impurities are the most common substitution ions in beryllium oxide. The background level of their concentration in the undoped crystals was 1e20 ppm. In this research we used only undoped both the pristine and additively-colored synthetic beryllium oxide single crystals. To obtain additively-colored crystals, pristine stoichiometric samples underwent thermal treatment in a metal beryllium vapor in order to produce anion nonstoichiometry (additive coloring). During three hours process the temperature level was 1900 C and the beryllium vapor pressure value was 2.5 kPa (Gorbunov et al., 1987a, b). The concentration of anion vacancies (Fcenters) were determined using an absorption spectroscopy of additively colored crystal in the absorption band at 6.6 eV (FWHM ¼ 0.82 eV), Using Smakula's formula, the concentration value was estimated as 3 1017 cm3. Samples of undoped BeOceramics were investigated in this case for comparison. We used samples of hot pressed (1900 C) ceramics obtained from BeOpowder (Kiiko et al., 2003; Ogorodnikov et al., 1995b). Impurity content in the ceramic BeO-samples does not exceed 100e150 ppm. All studies were performed in the laboratories at Ural Federal University. The steady-state X-ray-induced luminescence (XRL) spectra were recorded in the energy range of 1.2e6.2 eV at T ¼ 6 and 293 K (vacuum better than 104 Pa). The thermoluminescence (TL) glow curves were recorded at spectral-integrated mode (200e650 nm) using a FEU-130 type photomultiplier tube. The BeO samples were irradiated by X-rays (Cu-anode, Ua ¼ 40 kV, Ia ¼ 10 mA) for 40 min in a vacuum at a temperature T0 ¼ 6 K. After exposure, the samples were kept at T0 for a few minutes, and then heated at linear over the temperature range from 6 to 293 K (vacuum better than 104 Pa). All the spectra are normalized in intensity to one unit. 3. Results and discussion Steady-state XRL spectra. Fig. 1 shows steady-state XRL spectra of three BeO samples recorded at temperatures T ¼ 6 and 293 K. Intrinsic XRL-emission band at 4.9 eV dominates the luminescence spectrum in pristine BeO crystals, Fig. 1a. The wide UVluminescence band in beryllium oxide single crystals has previously been described in terms of self-trapped exciton model (Pustovarov et al., 2001). According to this model, the broadband UV emission in BeO is due to radiative annihilation of self-trapped exciton, the hole core of which is a small-radius hole-polaron. Pustovarov et al. (2001) have shown that the excitation of the STE can occur not only during direct creation of a free exciton, but during recombination of zone electrons and holes assisted by different lattice defects. The recombination process is also
Fig. 1. Steady-state XRL spectra of beryllium oxide recorded at T ¼ 6 and 293 K for pristine (a) and additively-colored (b) single crystals and ceramic samples (c).
accompanied by the creation of impurity-bound excitons, which relaxation contributes to the appearance of additional emission bands. This not only explains the appearance of various humps on the dominant XRL-emission band, but also explains an increase in the XRL-intensity when heated from 6 to 293 K, Fig. 1a. Additive coloration resulted in the appearance of new dominant XRLemission band at 3.4 eV, Fig. 1b. Gorbunov et al. (1987b) have been investigated in detail the 3.4 eV-emission band in photoluminescence spectra of BeO single crystals in the temperature range above 78 K, and described it in terms of 3 A1 /1 A1 transitions in F-color centers. From Fig. 1b it follows that the XRL luminescence from F-centers in BeO is efficiently excited in recombination processes at T ¼ 6 K. This is worth noting that during the heating from 6 to 293 K we can see a decrease in the intensity of UV emission band, but at the same time there is an increase in the intensity of the F-band at 3.4 eV. This may indirectly indicate a redistribution of electronic excitations between two excitation channels resulting to the relaxation of the electronic excitation responsible for the UVand F-emission bands. The XRL-emission spectrum of ceramic samples shows a broad band with a maximum of about 4.4 eV. A large spectral width can testify to its complex origin. We can suggest that the observed XRL-spectrum includes apparently the two above mentioned bands at 3.4 and 4.9 eV, Fig. 1c. When heated from 6 to 293 K, the XRL-intensities in the neighborhood of 3.4 and 4.9 eV bands vary in different ways depending on the type of samples, Fig. 1. Low-intensity shoulder at about 3.9 eV in ceramic samples can be explained by the appearance of randomly distributed lattice defects formed during the synthesis of the BeO samples. Briefly summarizing the results derived from the obtained XRLdata, we note the following: X-ray exposure leads to excitation of
Please cite this article in press as: Petrenko, M.D., et al., Thermoluminescence and low-temperature luminescence of beryllium oxide, Radiation Measurements (2015), http://dx.doi.org/10.1016/j.radmeas.2015.12.025
M.D. Petrenko et al. / Radiation Measurements xxx (2015) 1e4
Fig. 2. The TL glow curves of beryllium oxide recorded in the temperature range of 6e110 K using spectral-integrated regime for pristine (1) and additively-colored (2) single crystals, and ceramic sample (3) upon linear heating (5 K/min) after irradiation at T0 ¼ 6 K.
recombination processes involving zone charge carriers, smallradius hole-polarons, and various lattice defects. All these processes lead to the excitation of different bands of recombination luminescence. Although the photoluminescence of STE and F-centers in BeO are known already for a long time, experimental data on the recombination processes involving shallow trapping centers at temperatures below 78 K are still rather limited. We used the luminescence of STE and F-centers as luminescent probes for studying recombination processes involving shallow trapping centers. Analysis of the spectra and the temperature dependences of XRL-emission allowed us to speak about the participation of shallow trapping centers in the process of recombination luminescence excitation at 6e293 K, branching electronic excitations between different recombination channels, and the dominance of the luminescence of the STE and F-centers in recombination luminescence spectra of BeO in the temperature range of 6e293 K. TL glow curves. Fig. 2 shows TL glow curves recorded in the temperature range of 6e110 K using spectral-integrated regime for beryllium oxide samples after irradiation at 6 K. All three investigated samples showed the dominant low-temperature TL glow peak at about 85 K. Table 1 shows thermoluminescence parameters determined for the dominant TL glow peak at 85 K for three BeO samples. The peakeshape parameters u (FWHM) and d (high-temperature half width at half maximum) allow us to estimate an asymmetry parameter mg ¼ d=u. Chen (1969a) derives values for mg based upon the order of kinetics, the result being that mg ¼ 0.42 for first order kinetics. This is quite consistent with our results: we found mg z 0.42 for all three samples discussed. The activation energy as given by Chen (1969a, b) for the first order kinetics is
. 2 u 2kB Tm ; E ¼ 2:52kB Tm
3
where kB is the Boltzmann constant. For the first two samples (pristine and additively colored single crystals) there is not much difference in the values of the parameters. However, the luminescence intensity recorded for BeO-ceramics, exceeds the luminescence intensity of single crystals by several orders of magnitude. Moreover, the full width at half-maximum of TL-peak is halved (approximately from 13 to 6 K), and the activation energy is increased (from 0.10 to 0.24 eV), Table 1. This, however, is not surprising. We interpreted this phenomenon in terms of the fluctuation restructuring of the neighboring environment of trapping centers, which is the relaxation of various dipoles originating from either lattice defects or impurity related centers. Ogorodnikov et al. (1995b) has previously observed a similar phenomenon in the BeO ceramic samples during thermoluminescence studies using fractional glow technique in the temperature range above room temperature. The increase in the intensity and narrowing of the TL glow peak in the BeO-ceramic was interpreted in terms of the fluctuation restructuring of the neighboring environment of trapping centers. Ogorodnikov et al. (1995b) have shown that such a restructuring in BeO-ceramics might lead to an apparent increase in the observed activation energy, an it is due to the relaxation of various dipoles originating from either lattice defects or impurity related defects. Bearing in the mind this hypothesis, we conducted a search for thermoluminescent manifestations of shallow trapping centers, which could be due to such defects. To do this, we used a sensitive channel for the registration of thermoluminescence, which allowed us to record the thermoluminescence in the range of more than six decades of change in intensity. Fig. 3 shows TL glow curves recorded in spectral-integrated regime for beryllium oxide samples after irradiation at 6 K. All TL glow curves are shown in log-scale in intensity covering six orders of magnitude. From Fig. 3 it follows that TL glow curves of BeO single crystals do not contain manifestation of shallow trapping centers that might occur in the temperature range from 6 to 293 K. But it is not such a situation with the ceramic samples. The BeOceramic samples exhibit a number of shallow trapping centers being responsible for TL glow peaks at 150, 169, 187, 210 and 244 K, Fig. 3. These TL-peaks are lower in intensity by 2e5 orders of magnitude than the intensity of the dominant TL peak. Revealing a series of shallow trapping centers in the ceramic samples, manifested in the form of TL glow peaks at 120e270 K, can be considered as an indirect confirmation of the possibility of a fluctuation restructuring in BeO.
(1)
Table 1 Thermoluminescence parameters for dominant TL glow peak at 85 K: temperature position of TL glow peak (Tm), FWHM (u), and activation energy (E). Parameter
Tm , K u, K E, eV
Beryllium oxide sample Pristine
Additively colored
Ceramics
85.2 12.8 0.108
86.7 13.8 0.103
85.5 6.1 0.240
Fig. 3. The TL glow curves of beryllium oxide recorded in spectral-integrated regime for pristine (1) and additively-colored (2) single crystals and ceramic samples (3) upon linear heating of 5 K/min after irradiation at T0 ¼ 6 K.
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4. Conclusions We performed a study of the recombination processes in undoped BeO in the forms of the pristine and additively-colored single crystals, or ceramic samples. The study was carried out in the energy range of 1.2e6.2 eV, using both the steady-state XRL and TL techniques in the temperature range of 6e293 K. The main essential conclusions are as follows. Our experimental results showed that the X-ray exposure leads to an efficient excitation of recombination luminescence in the entire investigated temperature range from 6 to 293 K. The dominating bands of the lowtemperature recombination luminescence observed in our study, are consistent with previous discussions of the photoluminescence bands made for BeO in terms of the models of self-trapped excitons (4.9 eV), and F-type color centers (3.4 eV). Both the 3.4 and 4.9 eV emission bands were used in our research as luminescent probes for studying recombination processes involving shallow trapping centers. The experimental data indicate the presence of shallow trapping centers and their participation in the recombination processes resulting in excitation of the probe luminescence bands at T ¼ 6e293 K. Unfortunately, the experimental methods used do not allow us to identify the detected shallow trapping centers in BeO. Unfortunately, the experimental methods used do not allow us to identify the detected shallow trapping centers in BeO. The most notable manifestation of shallow trapping center we associate with TL glow peak at 85 K, which appeared in all BeO-samples studied. Its parameters are virtually identical for all single crystals, however the ceramic samples differ essentially in amplitude, FWHM and activation energy of the peak. We presumably interpreted this phenomenon in terms of the fluctuation restructuring of neighboring environment of trapping centers. It is worth noting that Ogorodnikov et al. (1995b) has previously observed a similar phenomenon in BeO-ceramics using fractional glow technique in the temperature range above room temperature. Acknowledgments The authors are grateful to A.V. Kruzhalov and I.N. Antsigin for collaboration. All the examined BeO crystals were grown and kindly put at our disposal by V.A. Maslov. This work was partially supported by the Center of Excellence “Radiation and Nuclear Technologies” (Competitiveness Enhancement Program of Ural Federal University, Russia). References Albrecht, H.O., Mandeville, C.E., 1954. The luminescence of beryllium oxide. Phys. Rev. 94 (3), 776e777. Austerman, S.B., 1963. Growth of beryllia single crystals. J. Am. Ceram. Soc. 46 (6), 6e10. Belykh, T.A., Ogorodnikov, I.N., Porotnikov, A.V., Bautin, K.V., Neshov, F.G., Kruzhalov, A.V., 1997. Variation of the properties of single crystals of BeO and
LiB3O5 irradiated with helium ions. Phys. Chem. Mater. Treat. 31 (6), 555e560. Chen, R., 1969a. Glow curves with general order kinetics. J. Electrochem. Soc. 116 (9), 1254e1257. Chen, R., 1969b. On the calculation of activation energies and frequency factors from glow curves. J. Appl. Phys. 40 (20), 570e585. Cooke, D.W., Jahan, M.S., Alexander Jr., C., Shulgin, B.V., 1985. A model for low temperature luminescence in lithium- and sodium doped beryllium oxide single crystals. J. Appl. Phys. 58 (6), 2302e2307. Cooke, D.W., Lianos, D., Gavathas, E., Alexander Jr., C., Shulgin, B.V., Jahan, M.S., 1984. Thermally induced optical scintillations from single crystalls of unirradiated BeO: Li and BeO: Na. Ferroelectrics 59 (3e4), 187e195. Gorbunov, S.V., Ivanov, V.Y., Kruzhalov, A.V., et al., 1987a. Luminescence excitation of colour centers in beryllium oxide. Nucl. Instrum. Methods Phys. Res. A 261, 148e149. Gorbunov, S.V., Kruzhalov, A.V., Springis, M.J., 1987b. Optical properties of the Fcentre in beryllium oxide. Phys. Status Solidi (b) 141 (1), 293e301. Kiiko, V.S., Sofronov, A.A., Makurin, Yu N., Ivanovskii, A.L., 2003. Temperature dependence of luminescence in beryllium ceramics. Glass Ceram. 60 (3e4), 120e122. Kruzhalov, A.V., Ogorodnikov, I.N., Kudyakov, S.V., 1996. Radiative relaxation of lowenergy electron excitations and point defects in beryllium oxide. Russ. Phys. J. 39 (11), 1067e1081. Maslov, V.A., Rylov, G.M., Mazurenko, V.G., Kruzhalov, A.V., Shulgin, B.V., 1980. Conditions of growth, structure, spectral and luminescent properties of crystals BeO. In: Proc. 6th Internat. Confer. On Crystals Growth (Moscow, 1980). Inst. of Crystallogr., Moscow, pp. 268e269. Ogorodnikov, I.N., Ivanov, V.Y., Kruzhalov, A.V., 1991a. Spectral luminescence properties of zinc-activated beryllium oxide crystals. J. Appl. Specrosc. 54 (4), 376e381. Ogorodnikov, I.N., Ivanov, V.Y., Kruzhalov, A.V., 1994. Point defects and radiative decay of low-energy electronic excitations in BeO. Phys. Solid State 36 (11), 1748e1753. Ogorodnikov, I.N., Ivanov, V.Y., Kruzhalov, A.V., 1995a. Short-wavelength luminescence and thermostimulated processes in single crystals of BeO. Radiat. Meas. 24 (4), 417e421. Ogorodnikov, I.N., Kirpa, V.I., Kruzhalov, A.V., 1991b. Characteristics of the relaxation processes in the dosimetric region of BeO thermoluminescent dosimeters. Sov. Phys. Tech. Phys. 36 (7), 759e763. Ogorodnikov, I.N., Kirpa, V.I., Kruzhalov, A.V., 1995b. Spontaneous emission from BeO and the fluctuational rearrangement of its structure. 2. Ceram. Tech. Phys. 40 (12), 1249e1253. Ogorodnikov, I.N., Kruzhalov, A.V., 1997. Defect properties of beryllium oxide. Mater. Sci. Forum 239e241, 51e56. Ogorodnikov, I.N., Kruzhalov, A.V., Ivanov, V.Y., 1996a. Mechanisms of fast UVscintillations in oxide crystals with self-trapped excitons. In: Proc. of Int. Confer. “Inorganic Scintillators and their Applications, SCINT95” (Delft, The Netherlands, Aug.28eSept.1, 1995). Delft University Press, Delft, The Netherlands, pp. 216e219. Ogorodnikov, I.N., Kruzhalov, A.V., Kuznetsov, A.Y., 1996b. Thermoluminescence, point defects and recombination processes in BeO and LiB3O5 single crystals. Radiat. Prot. Dosim. 65 (1e4), 109e112. Petrenko, M.D., Ogorodnikov, I.N., Ivanov, V.Y., Sarychev, M.N., Milman, I.I., 2014. Thermoluminescence of nonstoichiometric beryllium oxide crystals. Izv. Vuzov Phys. (Russ. Phys. J.) 57 (12/3), 66e69. Pustovarov, V.A., Ivanov, V.Y., Kirm, M., Kruzhalov, A.V., Korotaev, A.V., Zimmerer, G., 2001. Relaxation of electronic excitations in beryllium oxide: a time-resolved vacuum-UV spectroscopy study. Phys. Solid State 43 (7), 1233e1240. Shulgin, B.V., Kruzhalov, A.V., Ogorodnikov, I.N., Viktorov, L.V., Maslov, V.A., Alexander, C., Cooke, D.W., 1988. Scintillation properties of BeO single crystals. J. Appl. Specrosc. 49 (2), 848e852. Yakushev, M.V., Ogorodnikov, I.N., Varaksin, A.N., Kruzhalov, A.V., Nagirnyi, V.P., Denks, V.P., Zhivoderov, A.A., 2000. A stady of beryllium oxide crystals implanted by zinc. Poverkhnost 5, 83e86. Yakushev, M.V., Varaksin, A.N., Ogorodnikov, I.N., Kruzhalov, A.V., 1999. A Rutherford backscattering spectrometry study of Zn implanted BeO single crystals. Radiat. Eff. Defect Solid 150 (1e4), 157e160.
Please cite this article in press as: Petrenko, M.D., et al., Thermoluminescence and low-temperature luminescence of beryllium oxide, Radiation Measurements (2015), http://dx.doi.org/10.1016/j.radmeas.2015.12.025
Contents lists available at ScienceDirect
Radiation Measurements journal homepage: www.elsevier.com/locate/radmeas
Thermoluminescence and low-temperature luminescence of beryllium oxide M.D. Petrenko*, I.N. Ogorodnikov, V. Yu. Ivanov Experimental Physics Department, Ural Federal University, Mira Street, 19, Yekaterinburg 620002, Russia
h i g h l i g h t s We studied pristine and additively-colored BeO crystals and BeO-ceramics. We studied X-rays induced luminescence spectra at 6 and 293 K. We studied spectrally integrated thermoluminescence glow curves at 6-293 K. We determined parameters for dominate TL glow peak located below RT. A number of low-intensity TL peaks were found between 100 and 270K.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 24 October 2015 Received in revised form 21 December 2015 Accepted 24 December 2015 Available online xxx
Beryllium oxide in the forms of either single crystals (pristine, additively-colored) or hot-pressed ceramic samples was studied in the energy range of 1.2e6.2 eV using both the thermoluminescence (TL) and steady-state X-ray induced luminescence (XRL) techniques. The XRL emission spectra were recorded at 6 and 293 K, whereas TL glow curves were studied after X-ray exposure at T0 ¼ 6 K upon linear heating in the temperature range from 6 to 293 K. A search for TL manifestations of shallow trapping centers was carried out using a sensitive channel for TL registration in the range of more than six decades of change in intensity. The participation of shallow trapping centers in the process of recombination luminescence excitation at 6e293 K; branching electronic excitations between different recombination channels; the dominance of the self-trapped exciton and F-center emissions in spectra of the low-temperature recombination luminescence in BeO at 6e293 K were discussed. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Beryllium oxide BeO X-ray luminescence Thermoluminescence Low-temperature luminescence
1. Introduction Since the discovery of intense ultraviolet (UV) emission from beryllium oxide (Albrecht and Mandeville, 1954) and the development of the technology of growing large BeO single crystals (Austerman, 1963), BeO has been the subject of numerous studies. However, the number of research works devoted to luminescence and thermoluminescence (TL) of BeO single crystals at temperatures below 78 K remains relatively low. We are aware of several of them: Cooke et al. (1985, 1984); Petrenko et al. (2014); Shulgin et al. (1988) investigated both the luminescence and TL of BeO:Li, Na single crystals; Pustovarov et al. (2001) have studied relaxation of electronic excitations in BeO crystals using synchrotron radiation spectroscopy. All the other works cited for BeO single crystals, deal
* Corresponding author. E-mail address: [email protected] (M.D. Petrenko).
with temperatures T > 78 K and are devoted to the study of scintillation (Ogorodnikov et al., 1996a; Shulgin et al., 1988) and dosimetric (Ogorodnikov et al., 1991b, 1996b) properties, as well as radiation processing (Belykh et al., 1997; Yakushev et al., 2000, 1999) and doping (Ogorodnikov et al., 1991a) of BeO single crystals. Also, there are several review papers on BeO single crystals (Kruzhalov et al., 1996; Ogorodnikov et al., 1994, 1995a; Ogorodnikov and Kruzhalov, 1997). From the publications cited above, it follows that self-trapped excitons (STE) and F-type lattice defects are dominant luminescence centers in BeO. Pustovarov et al. (2001) have shown the possibility of excitation of the STE during recombination of zone electrons and holes assisted by different lattice defects. Recombination processes in BeO are well studied in the temperature range above 78 K. Elucidation of the role of shallow trapping centers in BeO requires the study of recombination processes at temperatures below 78 K. However, until now only fragmentary data for this
http://dx.doi.org/10.1016/j.radmeas.2015.12.025 1350-4487/© 2015 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Petrenko, M.D., et al., Thermoluminescence and low-temperature luminescence of beryllium oxide, Radiation Measurements (2015), http://dx.doi.org/10.1016/j.radmeas.2015.12.025
2
M.D. Petrenko et al. / Radiation Measurements xxx (2015) 1e4
temperature range are available. The possibility of excitation of STE and F-centers in recombination processes allowed us to use them as luminescence probes for studying manifestations of shallow trapping centers in BeO at temperatures of 6e293 K. The aim of this research work is to study luminescence and thermoluminescence properties of undoped BeO in the forms of both the pristine and additively-colored single crystals in comparison with BeO-ceramics. The research work was carried out in the spectral (1.2e6.2 eV) and temperature (6e293 K) ranges using the methods of both the steady-state X-ray induced luminescence (XRL) and TL techniques. 2. Experimental details The pristine beryllium oxide single crystals of optical quality were grown in the inverse temperature gradient from BeO-solution in a sodium tungstate melt (Maslov et al., 1980). The isomorphous capacity of BeO is fairly low. The Liþ, Zn2þ, Mg2þ, B2þ, Al3þ impurities are the most common substitution ions in beryllium oxide. The background level of their concentration in the undoped crystals was 1e20 ppm. In this research we used only undoped both the pristine and additively-colored synthetic beryllium oxide single crystals. To obtain additively-colored crystals, pristine stoichiometric samples underwent thermal treatment in a metal beryllium vapor in order to produce anion nonstoichiometry (additive coloring). During three hours process the temperature level was 1900 C and the beryllium vapor pressure value was 2.5 kPa (Gorbunov et al., 1987a, b). The concentration of anion vacancies (Fcenters) were determined using an absorption spectroscopy of additively colored crystal in the absorption band at 6.6 eV (FWHM ¼ 0.82 eV), Using Smakula's formula, the concentration value was estimated as 3 1017 cm3. Samples of undoped BeOceramics were investigated in this case for comparison. We used samples of hot pressed (1900 C) ceramics obtained from BeOpowder (Kiiko et al., 2003; Ogorodnikov et al., 1995b). Impurity content in the ceramic BeO-samples does not exceed 100e150 ppm. All studies were performed in the laboratories at Ural Federal University. The steady-state X-ray-induced luminescence (XRL) spectra were recorded in the energy range of 1.2e6.2 eV at T ¼ 6 and 293 K (vacuum better than 104 Pa). The thermoluminescence (TL) glow curves were recorded at spectral-integrated mode (200e650 nm) using a FEU-130 type photomultiplier tube. The BeO samples were irradiated by X-rays (Cu-anode, Ua ¼ 40 kV, Ia ¼ 10 mA) for 40 min in a vacuum at a temperature T0 ¼ 6 K. After exposure, the samples were kept at T0 for a few minutes, and then heated at linear over the temperature range from 6 to 293 K (vacuum better than 104 Pa). All the spectra are normalized in intensity to one unit. 3. Results and discussion Steady-state XRL spectra. Fig. 1 shows steady-state XRL spectra of three BeO samples recorded at temperatures T ¼ 6 and 293 K. Intrinsic XRL-emission band at 4.9 eV dominates the luminescence spectrum in pristine BeO crystals, Fig. 1a. The wide UVluminescence band in beryllium oxide single crystals has previously been described in terms of self-trapped exciton model (Pustovarov et al., 2001). According to this model, the broadband UV emission in BeO is due to radiative annihilation of self-trapped exciton, the hole core of which is a small-radius hole-polaron. Pustovarov et al. (2001) have shown that the excitation of the STE can occur not only during direct creation of a free exciton, but during recombination of zone electrons and holes assisted by different lattice defects. The recombination process is also
Fig. 1. Steady-state XRL spectra of beryllium oxide recorded at T ¼ 6 and 293 K for pristine (a) and additively-colored (b) single crystals and ceramic samples (c).
accompanied by the creation of impurity-bound excitons, which relaxation contributes to the appearance of additional emission bands. This not only explains the appearance of various humps on the dominant XRL-emission band, but also explains an increase in the XRL-intensity when heated from 6 to 293 K, Fig. 1a. Additive coloration resulted in the appearance of new dominant XRLemission band at 3.4 eV, Fig. 1b. Gorbunov et al. (1987b) have been investigated in detail the 3.4 eV-emission band in photoluminescence spectra of BeO single crystals in the temperature range above 78 K, and described it in terms of 3 A1 /1 A1 transitions in F-color centers. From Fig. 1b it follows that the XRL luminescence from F-centers in BeO is efficiently excited in recombination processes at T ¼ 6 K. This is worth noting that during the heating from 6 to 293 K we can see a decrease in the intensity of UV emission band, but at the same time there is an increase in the intensity of the F-band at 3.4 eV. This may indirectly indicate a redistribution of electronic excitations between two excitation channels resulting to the relaxation of the electronic excitation responsible for the UVand F-emission bands. The XRL-emission spectrum of ceramic samples shows a broad band with a maximum of about 4.4 eV. A large spectral width can testify to its complex origin. We can suggest that the observed XRL-spectrum includes apparently the two above mentioned bands at 3.4 and 4.9 eV, Fig. 1c. When heated from 6 to 293 K, the XRL-intensities in the neighborhood of 3.4 and 4.9 eV bands vary in different ways depending on the type of samples, Fig. 1. Low-intensity shoulder at about 3.9 eV in ceramic samples can be explained by the appearance of randomly distributed lattice defects formed during the synthesis of the BeO samples. Briefly summarizing the results derived from the obtained XRLdata, we note the following: X-ray exposure leads to excitation of
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Fig. 2. The TL glow curves of beryllium oxide recorded in the temperature range of 6e110 K using spectral-integrated regime for pristine (1) and additively-colored (2) single crystals, and ceramic sample (3) upon linear heating (5 K/min) after irradiation at T0 ¼ 6 K.
recombination processes involving zone charge carriers, smallradius hole-polarons, and various lattice defects. All these processes lead to the excitation of different bands of recombination luminescence. Although the photoluminescence of STE and F-centers in BeO are known already for a long time, experimental data on the recombination processes involving shallow trapping centers at temperatures below 78 K are still rather limited. We used the luminescence of STE and F-centers as luminescent probes for studying recombination processes involving shallow trapping centers. Analysis of the spectra and the temperature dependences of XRL-emission allowed us to speak about the participation of shallow trapping centers in the process of recombination luminescence excitation at 6e293 K, branching electronic excitations between different recombination channels, and the dominance of the luminescence of the STE and F-centers in recombination luminescence spectra of BeO in the temperature range of 6e293 K. TL glow curves. Fig. 2 shows TL glow curves recorded in the temperature range of 6e110 K using spectral-integrated regime for beryllium oxide samples after irradiation at 6 K. All three investigated samples showed the dominant low-temperature TL glow peak at about 85 K. Table 1 shows thermoluminescence parameters determined for the dominant TL glow peak at 85 K for three BeO samples. The peakeshape parameters u (FWHM) and d (high-temperature half width at half maximum) allow us to estimate an asymmetry parameter mg ¼ d=u. Chen (1969a) derives values for mg based upon the order of kinetics, the result being that mg ¼ 0.42 for first order kinetics. This is quite consistent with our results: we found mg z 0.42 for all three samples discussed. The activation energy as given by Chen (1969a, b) for the first order kinetics is
. 2 u 2kB Tm ; E ¼ 2:52kB Tm
3
where kB is the Boltzmann constant. For the first two samples (pristine and additively colored single crystals) there is not much difference in the values of the parameters. However, the luminescence intensity recorded for BeO-ceramics, exceeds the luminescence intensity of single crystals by several orders of magnitude. Moreover, the full width at half-maximum of TL-peak is halved (approximately from 13 to 6 K), and the activation energy is increased (from 0.10 to 0.24 eV), Table 1. This, however, is not surprising. We interpreted this phenomenon in terms of the fluctuation restructuring of the neighboring environment of trapping centers, which is the relaxation of various dipoles originating from either lattice defects or impurity related centers. Ogorodnikov et al. (1995b) has previously observed a similar phenomenon in the BeO ceramic samples during thermoluminescence studies using fractional glow technique in the temperature range above room temperature. The increase in the intensity and narrowing of the TL glow peak in the BeO-ceramic was interpreted in terms of the fluctuation restructuring of the neighboring environment of trapping centers. Ogorodnikov et al. (1995b) have shown that such a restructuring in BeO-ceramics might lead to an apparent increase in the observed activation energy, an it is due to the relaxation of various dipoles originating from either lattice defects or impurity related defects. Bearing in the mind this hypothesis, we conducted a search for thermoluminescent manifestations of shallow trapping centers, which could be due to such defects. To do this, we used a sensitive channel for the registration of thermoluminescence, which allowed us to record the thermoluminescence in the range of more than six decades of change in intensity. Fig. 3 shows TL glow curves recorded in spectral-integrated regime for beryllium oxide samples after irradiation at 6 K. All TL glow curves are shown in log-scale in intensity covering six orders of magnitude. From Fig. 3 it follows that TL glow curves of BeO single crystals do not contain manifestation of shallow trapping centers that might occur in the temperature range from 6 to 293 K. But it is not such a situation with the ceramic samples. The BeOceramic samples exhibit a number of shallow trapping centers being responsible for TL glow peaks at 150, 169, 187, 210 and 244 K, Fig. 3. These TL-peaks are lower in intensity by 2e5 orders of magnitude than the intensity of the dominant TL peak. Revealing a series of shallow trapping centers in the ceramic samples, manifested in the form of TL glow peaks at 120e270 K, can be considered as an indirect confirmation of the possibility of a fluctuation restructuring in BeO.
(1)
Table 1 Thermoluminescence parameters for dominant TL glow peak at 85 K: temperature position of TL glow peak (Tm), FWHM (u), and activation energy (E). Parameter
Tm , K u, K E, eV
Beryllium oxide sample Pristine
Additively colored
Ceramics
85.2 12.8 0.108
86.7 13.8 0.103
85.5 6.1 0.240
Fig. 3. The TL glow curves of beryllium oxide recorded in spectral-integrated regime for pristine (1) and additively-colored (2) single crystals and ceramic samples (3) upon linear heating of 5 K/min after irradiation at T0 ¼ 6 K.
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4. Conclusions We performed a study of the recombination processes in undoped BeO in the forms of the pristine and additively-colored single crystals, or ceramic samples. The study was carried out in the energy range of 1.2e6.2 eV, using both the steady-state XRL and TL techniques in the temperature range of 6e293 K. The main essential conclusions are as follows. Our experimental results showed that the X-ray exposure leads to an efficient excitation of recombination luminescence in the entire investigated temperature range from 6 to 293 K. The dominating bands of the lowtemperature recombination luminescence observed in our study, are consistent with previous discussions of the photoluminescence bands made for BeO in terms of the models of self-trapped excitons (4.9 eV), and F-type color centers (3.4 eV). Both the 3.4 and 4.9 eV emission bands were used in our research as luminescent probes for studying recombination processes involving shallow trapping centers. The experimental data indicate the presence of shallow trapping centers and their participation in the recombination processes resulting in excitation of the probe luminescence bands at T ¼ 6e293 K. Unfortunately, the experimental methods used do not allow us to identify the detected shallow trapping centers in BeO. Unfortunately, the experimental methods used do not allow us to identify the detected shallow trapping centers in BeO. The most notable manifestation of shallow trapping center we associate with TL glow peak at 85 K, which appeared in all BeO-samples studied. Its parameters are virtually identical for all single crystals, however the ceramic samples differ essentially in amplitude, FWHM and activation energy of the peak. We presumably interpreted this phenomenon in terms of the fluctuation restructuring of neighboring environment of trapping centers. It is worth noting that Ogorodnikov et al. (1995b) has previously observed a similar phenomenon in BeO-ceramics using fractional glow technique in the temperature range above room temperature. Acknowledgments The authors are grateful to A.V. Kruzhalov and I.N. Antsigin for collaboration. All the examined BeO crystals were grown and kindly put at our disposal by V.A. Maslov. This work was partially supported by the Center of Excellence “Radiation and Nuclear Technologies” (Competitiveness Enhancement Program of Ural Federal University, Russia). References Albrecht, H.O., Mandeville, C.E., 1954. The luminescence of beryllium oxide. Phys. Rev. 94 (3), 776e777. Austerman, S.B., 1963. Growth of beryllia single crystals. J. Am. Ceram. Soc. 46 (6), 6e10. Belykh, T.A., Ogorodnikov, I.N., Porotnikov, A.V., Bautin, K.V., Neshov, F.G., Kruzhalov, A.V., 1997. Variation of the properties of single crystals of BeO and
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Please cite this article in press as: Petrenko, M.D., et al., Thermoluminescence and low-temperature luminescence of beryllium oxide, Radiation Measurements (2015), http://dx.doi.org/10.1016/j.radmeas.2015.12.025