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Thermoluminescence kinetics of oxygen-related centers in AlN single crystals
Diamond & Related Materials 25 (2012) 59–62
Contents lists available at SciVerse ScienceDirect
Diamond & Related Materials journal homepage: www.elsevier.com/locate/diamond
Thermoluminescence kinetics of oxygen-related centers in AlN single crystals☆☆ I.A. Weinstein ⁎, A.S. Vokhmintsev, D.M. Spiridonov Ural Federal University, REC Nanomaterials and Nanotechnologies, Mira Street, 19, Ekaterinburg, 620002, Russia
a r t i c l e
i n f o
Available online 13 February 2012 Keywords: Aluminum nitride Optical properties Termally stimulated luminescence General order kinetics Oxygen impurity Nitrogen vacancy
a b s t r a c t Excitation and emission spectra of thermoluminescence (TL) in bulk aluminum nitride single crystals irradiated by UV have been studied. TL has been found to be most effectively excited by the 5.04 eV photons. The 3.44 eV band caused by recombination processes with oxygen–vacancy (VAl − ON)-centers dominates in the TL spectrum. Besides, the 2.91 and 2.0 eV emissions have been also observed. The TL mechanisms have been quantitatively analyzed in terms of formal kinetics of general order. On the basis of the obtained values and from their comparison with literature data it has been concluded that the main traps of charge carriers, responsible for the TL peak at 470 К, are formed by the VN vacancy. To interpret the observed regularities, the model of TL has been proposed, which satisfactorily agrees with independent data for thermally and optically stimulated processes in aluminum nitride. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Along with well-established applications in micro- and optoelectronics, AlN single crystals and ceramics are considered to be effective solid-state matrixes for highly sensitive β- , γ- and UV-irradiation detectors [1–4]. Investigations with thermally- (TL), cathodo- (CL), photo- (PL) and optically stimulated luminescence (OSL) have revealed complicated behavior of radiation-induced processes and some limitations for aluminum nitride (ceramics and powders) to be used in personal dosimetry. As for wide application, the difficulties are mainly connected with essential fading and dose nonlinearity [4,5]. In this connection study of the stimulated luminescence processes in AlN is of great interest for analyzing basic properties of the material and improving its functional and applied characteristics. It is known that after exposure to ionizing irradiation the AlN emits in the UV and visible spectral regions [1 and refs. in it]. As a rule this luminescence is connected with recombination processes involving vacancy and oxygen defects (VAl, VN, ON and their complexes) [1,6–10]. At present TL-, CL-, PL-, and OSL-properties of aluminum nitride have been well studied in different modifications (ceramics, films, single crystal, powder, nanostructures) and under different types of corpuscular–photonic irradiation [1,4–6,11–13]. However, kinetics of the processes in bulk nominally pure crystals, particularly under thermal and optical stimulation, has not been analyzed enough in terms of available quantitative approaches. These data could provide additional information about: excitation relaxation in AlN after irradiation, storage mechanisms of dosimetric information, transfer ☆☆ Presented at the Diamond 2011, 22st European Conference on Diamond, DiamondLike Materials, Carbon Nanotubes, and Nitrides, Budapest. ⁎ Corresponding author. E-mail address: [email protected] (I.A. Weinstein). 0925-9635/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2012.02.004
of stored energy between metastable levels of capture and recombination centers of charge carriers. Given the statement above the goal of the present study is to analyze and quantitatively estimate spectral–kinetic parameters of the TL processes, occurring in bulk single crystals of aluminum nitride after UV-irradiation.
2. Samples and experimental technique Single crystals synthesized from fine-dispersed aluminum nitride powder by sublimation–recondensation method at given correlation between temperature and pressure were studied (“Nitride Crystals”, Ltd.). The key features of this growth technique were discussed in Ref. [14]. According to the data of gas discharge mass spectrometry, oxygen impurity in the as-grown crystals does not exceed 10 18 cm − 3 [15]. Finally purposefully unalloyed samples had the form of pellets 15 mm in diameter and ≈0.4 mm in thickness, with epi-ready surface treatment. Spectrally resolved TL was researched with original spectrometer installation. TL emission within 300 to 600 nm was registered with FEU-39A photomultiplier in photon counting regime. To obtain spectral-temperature dependences the AlN samples were subjected to prior UV irradiation at room temperature by DDS-30 (30 W) deuterium lamp for 2 min (emission spectra) and for 5 min (excitation spectra). Then glow curves were registered during linear heating of the sample up to 670 K at the rate of r = 2 К/s. Grating monochromator of the MUM type was used to select the required UV-emission wavelength while measuring TL excitation spectra within 210–300 nm with 5 nm step. To register luminescence the combination of UFS-6 and SZS-22 optical glasses at FEU-39A input was used, thus providing transmission band of maximum 380 nm and halfwidth of 60 nm.
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To resolve the spectra composition of TL the samples were irradiated with unfiltered UV light from the deuterium lamp before each measurement. Then TL intensity was analyzed within 300–600 nm with 10 nm step by MUM monochromator at photomultiplier input. 3. Results Fig. 1 demonstrates the spectral and temperature dependences of glow curves intensity for AlN single crystal after UV irradiation. The degree of trap filling with irradiation energy can be estimated by Fig. 1a. It is seen that the traps are most effectively occupied near Eirr = 5 eV resulting in intensive TL with its maximum at T = 480 K. Fig. 1b demonstrates the spectral composition of thermally stimulated emission for UV irradiated AlN single crystals. Maximum response is observed at Em = 3.44 eV and Tm = 470 К. A less intensive peak is seen within the same temperature range at E ≤ 2.48 eV. It is to be noted that the results obtained agree well with the independent data for AlN powders, where emission and excitation energies are shifted toward the low-energy range [1]. Namely, thermoluminescence is most effectively excited under irradiation with Eirr = 4.59 eV (270 nm) and the TL spectrum consists of two bands at Em = 3.10 eV (400 nm) and Em = 2.58 eV (480 nm). 4. Discussion To quantitatively estimate luminescence parameters spectral and temperature cross sections of the three-dimensional dependences were analyzed. Fig. 2 shows normalized TL excitation and emission spectra for the crystals under study at 470 К. The obtained curves were approximated by elementary Gaussian components. Calculated values of parameters are listed in Table 1. It is seen that the TL excitation band can be with high accuracy described by Gaussian at 5.04 eV (R 2 = 0.997, solid blue line in Fig. 2). It should be noted the excitation maximum of room temperature OSL is observed near 5 eV in AlN ceramics also [2]. So it could conclude about common origin of OSL and TL active traps. In turn the TL emission spectrum can be described as superposition of three Gaussian components (dash green lines in Fig. 2). In this case the G1 band with Em = 3.44 eV dominates and can be ascribed to recombination processes with oxygen–vacancy defects of type (VAl − ON) [6] or donor–acceptor pairs of (VAl − ON) and isolated ON [8]. It is known that spectral position of the band under discussion in the AlN ceramics essentially depends on oxygen concentration [7]. It was shown [7] for low content of impurity the ON and VAl defects formed the correlated (VAl − ON)-center, that was stabilized due to Coulomb interaction, and emission was observed at Em b 3.44 eV (360 nm). With increasing oxygen concentration above 5∙1020 cm− 3 new types of extended defects were formed including inversion domain boundaries resulting in emission shift toward long-wave range up to E = 3.26 eV (380 nm) [7].
Fig. 2. Spectral dependencies of TL response at T = 474 К. Symbols — experiment; dashed lines — Gauss resolution components; solid lines — resulting approximation.
Thus, in our case the presence of the G1 component in the TL spectrum is caused by the oxygen-related (VAl − ON)-centers. Earlier the 2.0-eV luminescence (G3 component) was observed in the TL spectra of the AlN based ceramics [1]. This emission is assigned to uncontrolled impurities of Mn or Cr, which substitute aluminum in the cation sublattice [16,17]. According to other data, the 2.0-eV band can be related to the centers based on nitrogen vacancy VN and excess Al [18]. Parameters of the G2 component agree with the 2.58-eV luminescence (480 nm), which dominates in the AlN samples with developed surface and is also ascribed to oxygen–vacancy complexes of the second type (VAl − 2ON) [1 and refs. in it, 19]. In Ref. [6] the 2.96-eV luminescence in crystalline AlN:O powders was assigned to recombination of donor–acceptor pairs, which levels are formed by a neutral oxygen center ON0 (atom of oxygen is in the position of nitrogen with captured electron) and aluminum vacancy, i.e. (VAl − ON)-complexes respectively. In our case the observed 2.91-eV luminescence in the TL spectrum can be also attributed to recombination processes with oxygen-related centers. To describe and quantitatively analyze the obtained TL curves we used the formalism of general order kinetics [20]: 0 1b 1−b ″ ⋅ðb−1Þ T −EA E s A IðTÞ ¼ s″ ⋅n0 ⋅e ⋅ ∫ e kθ ⋅dθA ; ⋅@1 þ r kT T
ð1Þ
0
where n0 is the initial electron concentration in traps, m − 3; k is Boltzman constant, eV К − 1; s″ is effective frequency factor, s − 1; EA is activation energy, eV; b is kinetics order; T is current temperature of the sample, К; T0 is initial temperature, К. All of the TL curves obtained are characterized by wide structureless peaks, which parameters are given in Table 2. As an example Fig. 3 shows the TL curves measured within the 3.44 eV band (cross
Fig. 1. 3D plots for TL excitation (a) and emission (b) spectra in AlN single crystals.
I.A. Weinstein et al. / Diamond & Related Materials 25 (2012) 59–62
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Table 1 Spectral parameters of thermoluminescence.
Excitation Emission
G1 G2 G3
Peak position Em, ± 0.1 eV
Halfwidth ωE, ±0.05 eV
Determination coefficient R2
Defect
5.04 3.44 2.91 ≈ 2.0
0.67 0.64 0.45 0.61
0.997
(VAl–ON) [8] (VAl–ON) [6,7]; (VAl–ON) + ON [8] (VAl–ON) [6]; (VAl–2ON)+ON [19] VN + Ali [18]; Cr, Mn [16,17]
section, Fig. 1b) and under the 4.96 eV excitation (cross section, Fig. 1a). The curves in Fig. 3 are normalized for convenience of comparison. It is seen that the peaks are of the same width, but their maxima are shifted by ≈15 K. Insignificant difference in peak position may result from different time of the UV-exposure of the sample during registration of the TL emission and excitation spectra. As a rule change of Tm depending on irradiation dose indicates the presence of competing processes with kinetics order b > 1 in the mechanisms of the observed TL [21]. Form factor, calculated via the ratio of the high-temperature shoulder of peak halfwidth to total halfwidth, lies within 0.43 b μg b 0.52 (see Table 2), that is also typical for the kinetics exponents b > 1 [20,21]. The use of Eq. (1) shows that the TL curves for the data in Fig. 1a and b are with high accuracy (R2 = 0.997) described by one and the same TL component. The averaged parameters of the calculated curves are given in Table 2. Note, that from the analysis of the peaks, all the geometric and kinetic parameters, except Tm, have similar values within the error. The obtained value of b = 1.35 agrees well with the observed shape of the experimental curves and thus suggests the high rate of repeated capture of released charge carriers by active traps. The value of the activation energy (Table 2) satisfactorily agrees with the independent estimates of EA = 0.4–0.5 [10] and 0.75 eV [12], where the trap under study is assigned to a nitrogen vacancy VN. It is to be noted that in Ref. [12] estimate may be slightly overstated, since the calculations are made assuming first order processes b = 1 with frequency factor s = 2 × 10 7 s − 1. In our case b = 1.35, and effective frequency factor is s″ = s(n0/N) b − 1 [20], where N is total concentration of active traps in the crystal, m − 3. Using the fractional glow technique it has been found that TL processes to 400 K in AlN:O powder with oxygen content > 2% are caused by electron traps with continuous distribution of EA = 0.2, 0.26, 032 and 0.38 eV [22]. As in our calculations, the extremely low values in the range of 1 ÷ 10 4 s − 1 were obtained for frequency factor. According to kinetic Eq. (1) it is the ratio between EA and s (in case of general order EA and s″) that dictates temperature position and halfwidth of TL peak. For example in anion-defective Al2O3 crystals, where Tm = 480 ÷ 500 К and ωT = 40 ÷ 50 K, kinetic parameters of thermally stimulated processes have values EA = 0.5 eV, s″ = 2 × 107 s − 1 , b = 1.56 [21,23]. Consequently, trap activation with low effective frequency factor can cause the large halfwidth of experimental peak in aluminum nitride. All the above confirms the reliability of the estimates for the AlN single crystals.
0.998
Summarizing the results of calculation and literature analysis one may conclude that the observed processes of thermally stimulated recombination involve defects of the oxygen–vacancy origin — (VAl − ON) and VN. In this case a nitrogen vacancy appears to be the center of electron capture and plays the role of a donor during TL, being in agreement with the conclusions of Refs. [12,24–26]. In turn the (VAl − ON)-based recombination centers are characterized by a complicated structure of ground and excited states. Aluminum vacancy provides the ground levels in the vicinity of the valence band top, while oxygen impurity forms excitation levels with the upper one near the bottom of the conduction band [6]. According to calculations by the plane-wave pseudopotential 3− method [26], the doubly negative centre VAl + ON+ → (VAl − ON) 2 − turns out to be the dominant form of oxygen–vacancy complex. In addition, upon extra excitation of crystal (in our case UV-irradiation) one may get the (−/2−) and (0/−) ionization levels [26]. As for the isolated defects in anion sublattice, positively charged ON+ and VN+ are energetically most favorable, i.e.: oxygen atom is in nitrogen site and nitrogen vacancy has two trapped electrons, respectively [25]. So the charge compensation of a cation vacancy requires three singly positive donors [26]. With account for the aforesaid we can write an equation for photo-thermal recharging of centers under study during UV- irradiation and further thermal stimulation: − − þ ðV Al −ON Þ2− þ V þ N þ hν 5:04eV →ðV Al −ON Þ þ e þ V N →
→ðV Al −ON Þ− þ V 0N →heating→ 2− →ðV Al −ON Þ− þ e− þ V þ þ Vþ N →ðV Al −ON Þ N þ hν 3:44eV :
ð2Þ
Fig. 4 demonstrates the corresponding band diagram for the TL processes observed. According to this diagram and Eq. (2), irradiation of the samples with the 5.04-eV photons results in free electrons within the conduction band due to ionization of the oxygen-related centers — (VAl − ON) 2 − → (VAl − ON) −. Then charge carriers are captured by an isolated nitrogen vacancies — VN+ → VN0 , the depth of which according to our estimates is EA = 0.49 eV. Upon further heating the VN0 trap is emptied and (VAl − ON) −-complex returns to its initial state (VAl − ON) 2 −, emitting thus the 3.44-eV and 2.91-eV
Table 2 Kinetic parameters of thermoluminescence. Parameter
Value
Peak position Tm, ±5 K
467 (emission) 482 (excitation) 100 0.48 0.49 (0.8 ÷ 8) × 104 1.35 0.997
Halfwidth ωT, ± 2 K Form factor μg, ± 0.01 Activation energy EA, ±0.03 eV Effective frequency factor s″, s− 1 Kinetic order b, ±0.10 Determination coefficient R2
Fig. 3. TL glow curves. Symbols — experiment; solid lines — approximation with Eq. (1).
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observed TL response with its maximum near 470–480 К was formed due to emptying of electron trap with activation energy EA = 0.49 eV. Kinetics order of the given processes is b = 1.35 ± 0.10. On the basis of the data obtained the band model of the TL process is proposed. The mechanism is based on ionization of the (VAl − ON) 2 −-centers under photon irradiation. The ionization results in capture of electrons by levels formed by a nitrogen vacancy. Further heating leads to emptying of VN0 → VN+ traps and radiative return of the oxygen-related complexes to the initial state — (VAl −ON) − → (VAl − ON) 2 −. This model agrees with independent data of other authors as far as thermally stimulated processes in AlN-based materials are concerned. In our opinion, further comprehensive studies of optically stimulated luminescence and photoconductivity in aluminum nitride bulk single crystals are to be made. It will allow deeper understanding of interaction of complexes of intrinsic and impurity defects under UV irradiation, as well as elucidation of the origin of a number of traps, which accumulate energy under irradiation and then uncontrollably lose it in the form of persistent afterglow. Acknowledgment
Fig. 4. Band model of TL in AlN single crystal after UV-irradiation.
photons. These emissions are observed in photoluminescence of the AlN-based ceramics and powder during excitation near the 5.0and 4.43 eV bands, respectively [1,6]. We did not observe the 4.43eV transition in the TL excitation spectra (Figs. 1a and 2) because the corresponding excited level is located deeply in the forbidden gap, so ionization of the oxygen-related complex and thus filling of the VN+-trap does not occur. It is to be noted that our diagram (Fig. 4) does not show radiative transitions at 2.0 eV, which can be connected with recombination at other centers , for example, in complexes of nitrogen vacancy and interstitial aluminum [16]. Besides, the crystal contains other traps of different origin, which also are filled under irradiation and activated at room and higher temperatures [12,22,24]. A part of these donors is assigned to isolated oxygen impurities ON in various charged states [9,10]. Presumably, processes of direct donor–acceptor recombination between ON0 and (VAl − ON) − centers are responsible for intensive afterglow observed in different structural modifications of AlN just after UV-irradiation [1,4]. This afterglow is one of the obstacles for the AlN crystals and ceramics to be used as TL detectors of ionizing radiation because it contributes to essential fading and loss of stored dosimetric data [5]. Note, that considered band scheme quite well agrees with independent models of TL, PL, and photoconductivity mechanisms reported elsewhere [1,12,27]. 5. Conclusion Thus, the present study investigates thermoluminescence processes in the AlN bulk single crystals, grown by sublimation sandwich technique. Oxygen impurities and complexes based on these impurities involving cation and anion vacancies form main luminescent centers in such purposefully unalloyed crystals. Spectral and temperature dependences of emission intensity after irradiation by UV photons have been obtained. The 3.44-eV band with halfwidth of 0.64 eV was found to be dominated in the TL emission. Besides, there are also the 2.91- and 2.0-eV bands. It should be emphasized that the most efficient energy storage and further thermally stimulated luminescence was observed under irradiation with the 5.04 eV photons. Formal kinetics of general order was used to analyze the TL curves within the temperature range of RT—673 К. It is shown that the
This work was fulfilled in terms of a State contract under the Federal Program “Scientific and Scientific-Pedagogical Staff of Innovational Russia” 2009–2013. Vokhmintsev A.S. thanks UrFU Development Program “Provedenie nauchnyh issledovanii molodymi uchyonymi, kandidatami nauk” for financial support. References [1] L. Trinkler, B. Berzina, in: C. Sikalidis (Ed.), Advances in Ceramics — Characterization, Raw Materials, Processing, Properties, Degradation and Healing, InTech, 2011, pp. 59–82. [2] L. Trinkler, B. Berzina, A. Auzina, M. Benabdesselam, P. Iacconi, Nucl. Instrum. Methods Phys. Res. A 580 (2002) 354. [3] S. Tanaka, Q. Hu, V. Grishmanov, T. Yoneoka, Nucl. Instrum. Methods Phys. Res. B 141 (1998) 547. [4] A.S. Vokhmintsev, I.A. Weinstein, D.M. Spiridonov, D.A. Beketov, A.R. Beketov, Tech. Phys. Lett. 38 (2012). [5] L. Trinkler, L. Bøtter-Jensen, P. Christensen, B. Berzina, Radiat. Prot. Dosim. 92 (2000) 299. [6] S. Pacesova, L. Jastrabik, Czech. J. Phys. B 29 (1979) 913. [7] J.H. Harris, R.A. Youngman, R.G. Teller, J. Mater. Res. 5 (1990) 1763. [8] S. Schweizer, U. Rogulis, J.-M. Spaeth, L. Trinkler, B. Berzina, Phys. Status Solidi B 219 (2000) 171. [9] S.M. Evans, N.C. Giles, L.E. Halliburton, G.A. Slack, S.B. Schujman, L.J. Schowalter, Appl. Phys. Lett. 88 (2006) 062112-1–062112-3. [10] V.A. Soltamov, I.V. Ilyin, A.A. Soltamova, E.N. Mokhov, P.G. Baranov, J.Appl. Phys. 107 (2010) 113515-1–113515-3. [11] Yongbing Tang, Hongtao Cong, Feng Li, Hui-Ming Cheng, Diamond Relat. Mater. 1 (6) (2007) 537. [12] V.A. Soltamov, I.V. Ilyin, A.A. Soltamova, D.O. Tolmachev, E.N. Mokhov, P.G. Baranov, Phys. Solid State 53 (2011) 1186. [13] M. Bickermann, B.M. Epelbaum, O. Filip, P. Heimann, M. Feneberg, S. Nagata, A. Winnacker, Phys. Status Solidi C 7 (2010) 1743. [14] E.N. Mokhov, O.V. Avdeev, I.S. Barash, T.Y. Chemekova, A.D. Roenkov, A.S. Segal, A.A. Wolfson, Yu.N. Makarov, M.G. Ramm, H. Helava, J. Cryst. Growth 281 (2005) 93. [15] J.-M. Maki, I. Makkonen, F. Tuomisto, A. Karjalainen, S. Suihkonen, J. Raisanen, T.Yu. Chemekova, Yu.N. Makarov, Phys. Rev. B 84 (2011) 081204-1. [16] M. Benabdesselam, P. Iacconi, D. Lapraz, P. Grosseau, B. Guilhot, J. Phys. Chem. 99 (1995) 10319. [17] T. Miyajima, Y. Kudo, T. Uruga, K. Hara, Phys. Status Solidi C 3 (2006) 1742. [18] A. Sarua, S. Rajasingam, M. Kuball, N. Garro, O. Sancho, A. Cros, A. Cantarero, D. Olguin, B. Liu, D. Zhuang, J.H. Edgar, Mater. Res. Soc. Symp. Proc. 798 (2004) 297. [19] Quanli Hu, Tetsuji Noda, Hisashi Tanigawa, Toshiaki Yoneoka, Satoru Tanaka, Nucl. Instrum. Methods Phys. Res. B 19 (1) (2002) 536. [20] R. Chen, S.W.S. McKeever, World Scientific, Singapore, 1997. [21] I.A. Weinstein, A.S. Vokhmintsev, V.S. Kortov, Radiat. Meas. 43 (2008) 259. [22] I.A. Weinstein, V.S. Kortov, A.S. Vohmintsev, J. Lumin. 122–123 (2007) 342. [23] V.A. Soltamov, I.V. Ilyin, A.A. Soltamova, D.O. Tolmachev, E.N. Mokhov, P.G. Baranov, Diamond Relat. Mater. 20 (2011) 1085. [24] I. Tale, J. Rosa, Phys. Status Solidi A 86 (1984) 319. [25] T. Mattila, R.M. Nieminen, Phys. Rev. B 54 (1996) 16776. [26] T. Mattila, R.M. Nieminen, Phys. Rev. B 55 (1997) 9571. [27] H.M. Huang, R.S. Chen, H.Y. Chen, T.W. Liu, C.C. Kuo, C.P. Chen, H.C. Hsu, L.C. Chen, K.H. Chen, Y.J. Yang, Appl. Phys. Lett. 96 (2010) 062104-1–062104-3.
Contents lists available at SciVerse ScienceDirect
Diamond & Related Materials journal homepage: www.elsevier.com/locate/diamond
Thermoluminescence kinetics of oxygen-related centers in AlN single crystals☆☆ I.A. Weinstein ⁎, A.S. Vokhmintsev, D.M. Spiridonov Ural Federal University, REC Nanomaterials and Nanotechnologies, Mira Street, 19, Ekaterinburg, 620002, Russia
a r t i c l e
i n f o
Available online 13 February 2012 Keywords: Aluminum nitride Optical properties Termally stimulated luminescence General order kinetics Oxygen impurity Nitrogen vacancy
a b s t r a c t Excitation and emission spectra of thermoluminescence (TL) in bulk aluminum nitride single crystals irradiated by UV have been studied. TL has been found to be most effectively excited by the 5.04 eV photons. The 3.44 eV band caused by recombination processes with oxygen–vacancy (VAl − ON)-centers dominates in the TL spectrum. Besides, the 2.91 and 2.0 eV emissions have been also observed. The TL mechanisms have been quantitatively analyzed in terms of formal kinetics of general order. On the basis of the obtained values and from their comparison with literature data it has been concluded that the main traps of charge carriers, responsible for the TL peak at 470 К, are formed by the VN vacancy. To interpret the observed regularities, the model of TL has been proposed, which satisfactorily agrees with independent data for thermally and optically stimulated processes in aluminum nitride. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Along with well-established applications in micro- and optoelectronics, AlN single crystals and ceramics are considered to be effective solid-state matrixes for highly sensitive β- , γ- and UV-irradiation detectors [1–4]. Investigations with thermally- (TL), cathodo- (CL), photo- (PL) and optically stimulated luminescence (OSL) have revealed complicated behavior of radiation-induced processes and some limitations for aluminum nitride (ceramics and powders) to be used in personal dosimetry. As for wide application, the difficulties are mainly connected with essential fading and dose nonlinearity [4,5]. In this connection study of the stimulated luminescence processes in AlN is of great interest for analyzing basic properties of the material and improving its functional and applied characteristics. It is known that after exposure to ionizing irradiation the AlN emits in the UV and visible spectral regions [1 and refs. in it]. As a rule this luminescence is connected with recombination processes involving vacancy and oxygen defects (VAl, VN, ON and their complexes) [1,6–10]. At present TL-, CL-, PL-, and OSL-properties of aluminum nitride have been well studied in different modifications (ceramics, films, single crystal, powder, nanostructures) and under different types of corpuscular–photonic irradiation [1,4–6,11–13]. However, kinetics of the processes in bulk nominally pure crystals, particularly under thermal and optical stimulation, has not been analyzed enough in terms of available quantitative approaches. These data could provide additional information about: excitation relaxation in AlN after irradiation, storage mechanisms of dosimetric information, transfer ☆☆ Presented at the Diamond 2011, 22st European Conference on Diamond, DiamondLike Materials, Carbon Nanotubes, and Nitrides, Budapest. ⁎ Corresponding author. E-mail address: [email protected] (I.A. Weinstein). 0925-9635/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2012.02.004
of stored energy between metastable levels of capture and recombination centers of charge carriers. Given the statement above the goal of the present study is to analyze and quantitatively estimate spectral–kinetic parameters of the TL processes, occurring in bulk single crystals of aluminum nitride after UV-irradiation.
2. Samples and experimental technique Single crystals synthesized from fine-dispersed aluminum nitride powder by sublimation–recondensation method at given correlation between temperature and pressure were studied (“Nitride Crystals”, Ltd.). The key features of this growth technique were discussed in Ref. [14]. According to the data of gas discharge mass spectrometry, oxygen impurity in the as-grown crystals does not exceed 10 18 cm − 3 [15]. Finally purposefully unalloyed samples had the form of pellets 15 mm in diameter and ≈0.4 mm in thickness, with epi-ready surface treatment. Spectrally resolved TL was researched with original spectrometer installation. TL emission within 300 to 600 nm was registered with FEU-39A photomultiplier in photon counting regime. To obtain spectral-temperature dependences the AlN samples were subjected to prior UV irradiation at room temperature by DDS-30 (30 W) deuterium lamp for 2 min (emission spectra) and for 5 min (excitation spectra). Then glow curves were registered during linear heating of the sample up to 670 K at the rate of r = 2 К/s. Grating monochromator of the MUM type was used to select the required UV-emission wavelength while measuring TL excitation spectra within 210–300 nm with 5 nm step. To register luminescence the combination of UFS-6 and SZS-22 optical glasses at FEU-39A input was used, thus providing transmission band of maximum 380 nm and halfwidth of 60 nm.
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I.A. Weinstein et al. / Diamond & Related Materials 25 (2012) 59–62
To resolve the spectra composition of TL the samples were irradiated with unfiltered UV light from the deuterium lamp before each measurement. Then TL intensity was analyzed within 300–600 nm with 10 nm step by MUM monochromator at photomultiplier input. 3. Results Fig. 1 demonstrates the spectral and temperature dependences of glow curves intensity for AlN single crystal after UV irradiation. The degree of trap filling with irradiation energy can be estimated by Fig. 1a. It is seen that the traps are most effectively occupied near Eirr = 5 eV resulting in intensive TL with its maximum at T = 480 K. Fig. 1b demonstrates the spectral composition of thermally stimulated emission for UV irradiated AlN single crystals. Maximum response is observed at Em = 3.44 eV and Tm = 470 К. A less intensive peak is seen within the same temperature range at E ≤ 2.48 eV. It is to be noted that the results obtained agree well with the independent data for AlN powders, where emission and excitation energies are shifted toward the low-energy range [1]. Namely, thermoluminescence is most effectively excited under irradiation with Eirr = 4.59 eV (270 nm) and the TL spectrum consists of two bands at Em = 3.10 eV (400 nm) and Em = 2.58 eV (480 nm). 4. Discussion To quantitatively estimate luminescence parameters spectral and temperature cross sections of the three-dimensional dependences were analyzed. Fig. 2 shows normalized TL excitation and emission spectra for the crystals under study at 470 К. The obtained curves were approximated by elementary Gaussian components. Calculated values of parameters are listed in Table 1. It is seen that the TL excitation band can be with high accuracy described by Gaussian at 5.04 eV (R 2 = 0.997, solid blue line in Fig. 2). It should be noted the excitation maximum of room temperature OSL is observed near 5 eV in AlN ceramics also [2]. So it could conclude about common origin of OSL and TL active traps. In turn the TL emission spectrum can be described as superposition of three Gaussian components (dash green lines in Fig. 2). In this case the G1 band with Em = 3.44 eV dominates and can be ascribed to recombination processes with oxygen–vacancy defects of type (VAl − ON) [6] or donor–acceptor pairs of (VAl − ON) and isolated ON [8]. It is known that spectral position of the band under discussion in the AlN ceramics essentially depends on oxygen concentration [7]. It was shown [7] for low content of impurity the ON and VAl defects formed the correlated (VAl − ON)-center, that was stabilized due to Coulomb interaction, and emission was observed at Em b 3.44 eV (360 nm). With increasing oxygen concentration above 5∙1020 cm− 3 new types of extended defects were formed including inversion domain boundaries resulting in emission shift toward long-wave range up to E = 3.26 eV (380 nm) [7].
Fig. 2. Spectral dependencies of TL response at T = 474 К. Symbols — experiment; dashed lines — Gauss resolution components; solid lines — resulting approximation.
Thus, in our case the presence of the G1 component in the TL spectrum is caused by the oxygen-related (VAl − ON)-centers. Earlier the 2.0-eV luminescence (G3 component) was observed in the TL spectra of the AlN based ceramics [1]. This emission is assigned to uncontrolled impurities of Mn or Cr, which substitute aluminum in the cation sublattice [16,17]. According to other data, the 2.0-eV band can be related to the centers based on nitrogen vacancy VN and excess Al [18]. Parameters of the G2 component agree with the 2.58-eV luminescence (480 nm), which dominates in the AlN samples with developed surface and is also ascribed to oxygen–vacancy complexes of the second type (VAl − 2ON) [1 and refs. in it, 19]. In Ref. [6] the 2.96-eV luminescence in crystalline AlN:O powders was assigned to recombination of donor–acceptor pairs, which levels are formed by a neutral oxygen center ON0 (atom of oxygen is in the position of nitrogen with captured electron) and aluminum vacancy, i.e. (VAl − ON)-complexes respectively. In our case the observed 2.91-eV luminescence in the TL spectrum can be also attributed to recombination processes with oxygen-related centers. To describe and quantitatively analyze the obtained TL curves we used the formalism of general order kinetics [20]: 0 1b 1−b ″ ⋅ðb−1Þ T −EA E s A IðTÞ ¼ s″ ⋅n0 ⋅e ⋅ ∫ e kθ ⋅dθA ; ⋅@1 þ r kT T
ð1Þ
0
where n0 is the initial electron concentration in traps, m − 3; k is Boltzman constant, eV К − 1; s″ is effective frequency factor, s − 1; EA is activation energy, eV; b is kinetics order; T is current temperature of the sample, К; T0 is initial temperature, К. All of the TL curves obtained are characterized by wide structureless peaks, which parameters are given in Table 2. As an example Fig. 3 shows the TL curves measured within the 3.44 eV band (cross
Fig. 1. 3D plots for TL excitation (a) and emission (b) spectra in AlN single crystals.
I.A. Weinstein et al. / Diamond & Related Materials 25 (2012) 59–62
61
Table 1 Spectral parameters of thermoluminescence.
Excitation Emission
G1 G2 G3
Peak position Em, ± 0.1 eV
Halfwidth ωE, ±0.05 eV
Determination coefficient R2
Defect
5.04 3.44 2.91 ≈ 2.0
0.67 0.64 0.45 0.61
0.997
(VAl–ON) [8] (VAl–ON) [6,7]; (VAl–ON) + ON [8] (VAl–ON) [6]; (VAl–2ON)+ON [19] VN + Ali [18]; Cr, Mn [16,17]
section, Fig. 1b) and under the 4.96 eV excitation (cross section, Fig. 1a). The curves in Fig. 3 are normalized for convenience of comparison. It is seen that the peaks are of the same width, but their maxima are shifted by ≈15 K. Insignificant difference in peak position may result from different time of the UV-exposure of the sample during registration of the TL emission and excitation spectra. As a rule change of Tm depending on irradiation dose indicates the presence of competing processes with kinetics order b > 1 in the mechanisms of the observed TL [21]. Form factor, calculated via the ratio of the high-temperature shoulder of peak halfwidth to total halfwidth, lies within 0.43 b μg b 0.52 (see Table 2), that is also typical for the kinetics exponents b > 1 [20,21]. The use of Eq. (1) shows that the TL curves for the data in Fig. 1a and b are with high accuracy (R2 = 0.997) described by one and the same TL component. The averaged parameters of the calculated curves are given in Table 2. Note, that from the analysis of the peaks, all the geometric and kinetic parameters, except Tm, have similar values within the error. The obtained value of b = 1.35 agrees well with the observed shape of the experimental curves and thus suggests the high rate of repeated capture of released charge carriers by active traps. The value of the activation energy (Table 2) satisfactorily agrees with the independent estimates of EA = 0.4–0.5 [10] and 0.75 eV [12], where the trap under study is assigned to a nitrogen vacancy VN. It is to be noted that in Ref. [12] estimate may be slightly overstated, since the calculations are made assuming first order processes b = 1 with frequency factor s = 2 × 10 7 s − 1. In our case b = 1.35, and effective frequency factor is s″ = s(n0/N) b − 1 [20], where N is total concentration of active traps in the crystal, m − 3. Using the fractional glow technique it has been found that TL processes to 400 K in AlN:O powder with oxygen content > 2% are caused by electron traps with continuous distribution of EA = 0.2, 0.26, 032 and 0.38 eV [22]. As in our calculations, the extremely low values in the range of 1 ÷ 10 4 s − 1 were obtained for frequency factor. According to kinetic Eq. (1) it is the ratio between EA and s (in case of general order EA and s″) that dictates temperature position and halfwidth of TL peak. For example in anion-defective Al2O3 crystals, where Tm = 480 ÷ 500 К and ωT = 40 ÷ 50 K, kinetic parameters of thermally stimulated processes have values EA = 0.5 eV, s″ = 2 × 107 s − 1 , b = 1.56 [21,23]. Consequently, trap activation with low effective frequency factor can cause the large halfwidth of experimental peak in aluminum nitride. All the above confirms the reliability of the estimates for the AlN single crystals.
0.998
Summarizing the results of calculation and literature analysis one may conclude that the observed processes of thermally stimulated recombination involve defects of the oxygen–vacancy origin — (VAl − ON) and VN. In this case a nitrogen vacancy appears to be the center of electron capture and plays the role of a donor during TL, being in agreement with the conclusions of Refs. [12,24–26]. In turn the (VAl − ON)-based recombination centers are characterized by a complicated structure of ground and excited states. Aluminum vacancy provides the ground levels in the vicinity of the valence band top, while oxygen impurity forms excitation levels with the upper one near the bottom of the conduction band [6]. According to calculations by the plane-wave pseudopotential 3− method [26], the doubly negative centre VAl + ON+ → (VAl − ON) 2 − turns out to be the dominant form of oxygen–vacancy complex. In addition, upon extra excitation of crystal (in our case UV-irradiation) one may get the (−/2−) and (0/−) ionization levels [26]. As for the isolated defects in anion sublattice, positively charged ON+ and VN+ are energetically most favorable, i.e.: oxygen atom is in nitrogen site and nitrogen vacancy has two trapped electrons, respectively [25]. So the charge compensation of a cation vacancy requires three singly positive donors [26]. With account for the aforesaid we can write an equation for photo-thermal recharging of centers under study during UV- irradiation and further thermal stimulation: − − þ ðV Al −ON Þ2− þ V þ N þ hν 5:04eV →ðV Al −ON Þ þ e þ V N →
→ðV Al −ON Þ− þ V 0N →heating→ 2− →ðV Al −ON Þ− þ e− þ V þ þ Vþ N →ðV Al −ON Þ N þ hν 3:44eV :
ð2Þ
Fig. 4 demonstrates the corresponding band diagram for the TL processes observed. According to this diagram and Eq. (2), irradiation of the samples with the 5.04-eV photons results in free electrons within the conduction band due to ionization of the oxygen-related centers — (VAl − ON) 2 − → (VAl − ON) −. Then charge carriers are captured by an isolated nitrogen vacancies — VN+ → VN0 , the depth of which according to our estimates is EA = 0.49 eV. Upon further heating the VN0 trap is emptied and (VAl − ON) −-complex returns to its initial state (VAl − ON) 2 −, emitting thus the 3.44-eV and 2.91-eV
Table 2 Kinetic parameters of thermoluminescence. Parameter
Value
Peak position Tm, ±5 K
467 (emission) 482 (excitation) 100 0.48 0.49 (0.8 ÷ 8) × 104 1.35 0.997
Halfwidth ωT, ± 2 K Form factor μg, ± 0.01 Activation energy EA, ±0.03 eV Effective frequency factor s″, s− 1 Kinetic order b, ±0.10 Determination coefficient R2
Fig. 3. TL glow curves. Symbols — experiment; solid lines — approximation with Eq. (1).
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I.A. Weinstein et al. / Diamond & Related Materials 25 (2012) 59–62
observed TL response with its maximum near 470–480 К was formed due to emptying of electron trap with activation energy EA = 0.49 eV. Kinetics order of the given processes is b = 1.35 ± 0.10. On the basis of the data obtained the band model of the TL process is proposed. The mechanism is based on ionization of the (VAl − ON) 2 −-centers under photon irradiation. The ionization results in capture of electrons by levels formed by a nitrogen vacancy. Further heating leads to emptying of VN0 → VN+ traps and radiative return of the oxygen-related complexes to the initial state — (VAl −ON) − → (VAl − ON) 2 −. This model agrees with independent data of other authors as far as thermally stimulated processes in AlN-based materials are concerned. In our opinion, further comprehensive studies of optically stimulated luminescence and photoconductivity in aluminum nitride bulk single crystals are to be made. It will allow deeper understanding of interaction of complexes of intrinsic and impurity defects under UV irradiation, as well as elucidation of the origin of a number of traps, which accumulate energy under irradiation and then uncontrollably lose it in the form of persistent afterglow. Acknowledgment
Fig. 4. Band model of TL in AlN single crystal after UV-irradiation.
photons. These emissions are observed in photoluminescence of the AlN-based ceramics and powder during excitation near the 5.0and 4.43 eV bands, respectively [1,6]. We did not observe the 4.43eV transition in the TL excitation spectra (Figs. 1a and 2) because the corresponding excited level is located deeply in the forbidden gap, so ionization of the oxygen-related complex and thus filling of the VN+-trap does not occur. It is to be noted that our diagram (Fig. 4) does not show radiative transitions at 2.0 eV, which can be connected with recombination at other centers , for example, in complexes of nitrogen vacancy and interstitial aluminum [16]. Besides, the crystal contains other traps of different origin, which also are filled under irradiation and activated at room and higher temperatures [12,22,24]. A part of these donors is assigned to isolated oxygen impurities ON in various charged states [9,10]. Presumably, processes of direct donor–acceptor recombination between ON0 and (VAl − ON) − centers are responsible for intensive afterglow observed in different structural modifications of AlN just after UV-irradiation [1,4]. This afterglow is one of the obstacles for the AlN crystals and ceramics to be used as TL detectors of ionizing radiation because it contributes to essential fading and loss of stored dosimetric data [5]. Note, that considered band scheme quite well agrees with independent models of TL, PL, and photoconductivity mechanisms reported elsewhere [1,12,27]. 5. Conclusion Thus, the present study investigates thermoluminescence processes in the AlN bulk single crystals, grown by sublimation sandwich technique. Oxygen impurities and complexes based on these impurities involving cation and anion vacancies form main luminescent centers in such purposefully unalloyed crystals. Spectral and temperature dependences of emission intensity after irradiation by UV photons have been obtained. The 3.44-eV band with halfwidth of 0.64 eV was found to be dominated in the TL emission. Besides, there are also the 2.91- and 2.0-eV bands. It should be emphasized that the most efficient energy storage and further thermally stimulated luminescence was observed under irradiation with the 5.04 eV photons. Formal kinetics of general order was used to analyze the TL curves within the temperature range of RT—673 К. It is shown that the
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