- Email: [email protected]
Self-trapped exciton luminescence in crystalline α-quartz under two-photon laser excitation
Solid State Communications 127 (2003) 655–659 www.elsevier.com/locate/ssc
Self-trapped exciton luminescence in crystalline a-quartz under two-photon laser excitation Anatoly Trukhina,*, Margarita Kinkb, Yuri Maksimovb, Rein Kinkb a
Institute of Solid State Physics, University of Latvia, Kengarga St., 8, LV-1063 Riga, Latvia b Institute of Physics, University of Tartu, Estonia Received 9 December 2002; accepted 25 June 2003 by J.H. Davies
Abstract The luminescence of pure crystalline a-quartz is studied under pulsed ArF laser excitation. The luminescence parameters obtained correspond well with those of self-trapped excitons (STEs) in a-quartz, indicating that the excitation process is twophoton. The efficiency of two-photon excitation is of the same order of magnitude as the one-photon excitation of sodium salicylate. The STE luminescence decay kinetics and their temperature dependence under photoexcitation were recorded with higher accuracy than previously. Changes in the decay kinetics with temperature are explained by the splitting of the STE triplet state in zero magnetic field and are analyzed with the assumption of the Orbach process of spin– lattice relaxation. No trace of other luminescence was detected in the pure sample. q 2003 Elsevier Ltd. All rights reserved. PACS: 61.82. 2 d; 61.66. 2 f; 71.35.Aa; 78.47. þ p; 78.60. 2 b; 78.60.Hk Keywords: A. a-Quartz; E. Luminescence; E. Self-trapped exciton; B. Laser excitation
1. Introduction The luminescence of pure crystalline quartz excited by ionizing radiation has been attributed to the self-trapped exciton (STE) in works of Trukhin and Plaudis [1] and Griscom [2] although these employed somewhat different models. The energetic yield of the STE luminescence under such conditions of excitation is very high being approximately 20% of the absorbed energy [3]. The problem of exciting this luminescence with photons from the beginning of the intrinsic absorption already arises in Ref. [1] because the yield of photoluminescence (PL) is very small [1,3]. This makes it difficult to be certain of the appropriate parameters to use in measuring STE luminescence under photoexcitation. The possible cause of low yield may be related to the non-ideal surface structure of the polished surface. Indeed, the first measurement of the STE lumines* Corresponding author. Tel.: þ371-7260-686; fax: þ 371-7132778. E-mail address: [email protected] (A. Trukhin).
cence excitation spectrum using synchrotron radiation [4,5] reveals that a small peak previously identified at 8.6 eV [1] corresponds to a sharp, intense peak at 8.7 eV when measured with good resolution. The onset of this peak represents the threshold for intrinsic absorption with a fairly small absorption coefficient, where the penetration of the exciting light is sufficiently high. The cut off of this 8.7 eV peak may be related to the growth of the absorption coefficient and, as the near surface damaged layer of sample was excited, then losses of absorbed energy may take place. Such an interpretation was made in Ref. [6] where the measured low yield of STE PL measurement was explained as being due to the non-ideal material near the surface. Optical excitation by two photons allows luminescence in the bulk of the studied specimen to be studied, unaffected by the conditions of the surface. Also, the use of a pulsed laser allows the more precise measurement of the decay kinetics of STE photoluminescence of a-quartz, which has previously been well measured only with the use of ionizing radiation [1– 3,6 – 8]. The two-photon excitation of the STE luminescence was performed in Ref. [9] for temperature
0038-1098/03/$ - see front matter q 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0038-1098(03)00526-X
656
A. Trukhin et al. / Solid State Communications 127 (2003) 655–659
higher that 30 K, without any detailed measurement of decay kinetics. It was noted that, visually the luminescence under conditions of the experiment [9] is very bright. The temperature dependence of the STE luminescence obtained in Ref. [9] was different from known behavior, i.e. the two stages of thermal quenching discussed in Ref. [6] were not detected. The measurements of STE luminescence under pulsed ArF laser excitation (6.4 eV) were thus performed for crystalline quartz for temperature range from 10 K to complete thermal quenching at 210 K. The attribution of the luminescence to STE under these excitation conditions is strongly supported by the detection of the decay time constant; the temperature dependence including the decay behavior at 10 K (the slow and fast components of the decay correspond to the STE triplet state affected by zero-field splitting (ZFS)), as well as the detection of specific twostages of thermal quenching of the luminescence. All these parameters have been reported previously [3,8,10] for the case of one-photon, X-ray and cathodoexcitation. The efficiency of the two-photon STE excitation (luminescence photons out per two photons absorbed) was estimated to be of the same order of magnitude as the one-photon excited luminescence of sodium salicylate (ArF excitation) in the same geometry of detection. According to Ref. [11], sodium salicylate has a luminescence yield of about 0.5 over a wide spectral range at least from 4 to 40 eV. Because of light scattering by the powdered sodium salicylate and other uncertainties (such as the two-photon absorbed energy), the comparison has only been made within an order of magnitude. The high efficiency of STE luminescence in aquartz under two-photon excitation supports the idea that the damaged surface of a polished sample may be responsible for the low yield of one-photon excited luminescence. The relatively high efficiency of the twophoton excitation allows determination of the measured parameters with an improved signal-to-noise ratio than in previous one-photon measurement, approaching the quality of cathodoexcitation, [8].
photomultiplier (PM) FEU-71 with a silica glass window. An oscilloscope Textronic TDS 380 was used to record decay curves with following transmittance to PC. The PM was used in current regime with a 50 ohm resistance, providing impedance matching with 50 ohm cable with connecting to the oscilloscope. The intensity of luminescence was such that the slits of the monochromator needed to be narrow in order to avoid PM overload. Measured decay curves were fitted to a sum of exponents using the leastsquares-fit method. The same method was used for other mathematical treatment of experimental results.
3. Results In Fig. 1 the photoluminescence spectrum and decay time constant of the fast component at 10 K are presented. (Note that a slow component is also present, but cannot be conveniently displayed using the scale of Fig. 1.) It is seen that the usual PL band of STE with decay time constant of the fast component about 0.35 ms which is characteristic of this temperature appears under two-photon excitation with the ArF laser giving good agreement with known data for single photon excitation [1– 8,10]. The decay time constant is of the same value within the broad PL band. The decay kinetics curves and lifetimes for different temperatures are shown in Figs. 2 and 3, respectively. Two components of decay are observed, converting to a single component with increase in temperature. At the same time, the intensity of the PL does not significantly change with temperature. The temperature dependence of the PL intensities, determined as the integrals under the decay curves, show little variation when the decay curves themselves show strong changes. The corresponding temperature dependence of the time constants for the range of strong changes in decay is shown in Fig. 3. The points correspond to time constants determined by an
2. Experimental The sample of crystalline quartz used for the two-photon measurements exhibited no luminescence in the one-photon regime. The ArF laser (model PSX-100, made by Neweks, Estonia) has a pulse energy of about 5 mJ with a duration of 5 ns and a repetition rate up to 100 Hz. The light beam was concentrated with a lens, focusing after the sample so as to avoid destruction of the cryostat window and the sample. The power in the sample was ,1 GW/cm2. The power of sample’s destruction is , 3 GW/ cm2 in conditions of our experiment. A liquid helium cryostat was used to maintain the temperature in the range 10 –290 K. Luminescence was detected using a grating monochromator MDR-23 with
Fig. 1. PL spectrum (line—continuous measurements, points—time resolved for fast component) and decay time constant (fast component) dependence on the wavelength within the PL band of pure crystalline quartz STE luminescence under ArF laser excitation.
A. Trukhin et al. / Solid State Communications 127 (2003) 655–659
657
Fig. 2. Decay kinetics at different temperatures of pure crystalline quartz STE luminescence under ArF laser excitation.
Fig. 4. Pure crystalline quartz STE luminescence decay kinetics at temperature quenching range excited by ArF laser.
approximation of the decay curve by two exponents. The lines correspond to fitting with formulae taken from Ref. [8], which describe the dependence of decay time constant temperature for the triplet radiative level of STE affected by ZFS (see Section 4). The decay curves could be divided relatively well into two exponents, fast and slow, whose time constants decrease with increase in temperature in the range of liquid helium temperatures. Both components convert to one exponential component with a time constant about 1 ms for temperatures higher than 30 K. For temperatures above 100 K the PL intensity undergoes two-stage thermal quenching, as has been previously reported [1,3]. That disagrees with previous two-photon excitation of a-quartz’s luminescence in Ref. [9], where only one stage was determined. The causes of that could not be yet understood. We believe that in Ref. [9] STE luminescence was detected, so two-stages should be observed. The decay curve at 118 K becomes faster than for lower and higher temperatures, Fig. 4. This is reflected as a drop in the time constant in Fig. 5, where the PL intensity and time-
constant temperature dependences are presented. In the decay curve, a fast component appears in the range of 145 K and this is reflected in both Fig. 4 and in Fig. 5. This is shown by the disappearance of one STE component, whereas another component remains with almost the same time constant as for T below 100 K. The second stage of the PL thermal quenching, with corresponding decrease in time constant, takes place up to 200 K. Previously [10], the different sign of STE luminescence polarization was determined for both stages under X-ray irradiation and the two stages with different polarization’s signs were explained as two types of differently oriented STEs with respect to crystal’s axes.
Fig. 3. Pure crystalline quartz STE luminescence decay kinetics temperature dependences under ArF laser excitation. Points— experimental, lines—fitted with expression (3) and (4). Calculation parameters: ta , 10 ms; tb ¼ 0:33 ms; Ea ¼ 0:0099 eV; Eb ¼ 0:0098 eV; na ¼ 8 £ 105 s21 ; nb ¼ 4 £ 105 s21 :
4. Discussion Under two-photon ArF laser excitation of crystalline aquartz we have obtained luminescence which, considering all the measure, corresponds to the known luminescence of the self-trapped exciton in a-quartz [1 – 8]. Two-stage
Fig. 5. Temperature dependences of PL intensity and decay time constants in the range of STE thermal quenching of the pure crystalline quartz excited by ArF laser.
658
A. Trukhin et al. / Solid State Communications 127 (2003) 655–659
thermal quenching was observed with a broad band at 2.7 eV (Fig. 1). A qualitative estimate of the quantum yield is of the same order of magnitude as sodium salicylate, i.e. closer to 0.5 than to the value 0.05 characterizing previous measurements [3,12] of one-photon-excited STE photoluminescence. However, this latter low yield could be attributed to the quality of the near-surface, whereas twophoton excited luminescence in the bilk of the specimen reflects the real situation with regard to STE creation. In addition there is no contradiction with the high energetic yield of STE luminescence under X-ray and cathodoexcitation. The peculiarities in the decay kinetics in the range of low temperatures (Figs. 2 and 3) (up to 10 K) were observed in the two-photon experiment to be similar to previous cases of one-photon excitation [3] and cathodoexcitation [8]. These peculiarities correspond to ZFS of the triplet state of the STE. The fast component may correspond to transition to the ground singlet state from m ¼ 1 or from m ¼ 21; whereas the slow component may correspond to that from m ¼ 0: This behavior of the kinetics corresponds to the measured ODMR of the STE in a-quartz [12] where the splitting parameters D and E of ZFS are determined. The D and E have unusually high values (D ¼ 22 GHz and E ¼ 1:5 GHz). The sequence on the model of STE from these data could be better understood by comparison with the analogous decay kinetics data obtained in Refs. [8,11] for STE in GeO2 crystals with the a-quartz structure. An analytical model describing the peculiarities of the luminescence decay kinetics was performed in Ref. [8] for the case of quartz and GeO2 under cathodoexcitation. This model is based on a previous investigation of STE in alkali halides (Ref. [8] and references cited therein). At low temperatures, when the spin– lattice relaxation is frozen, the exchange between the ZFS sublevel of the triplet state is also frozen. The decay time constant of about 1/3 that at high temperatures shows that one of the sublevels m ¼ 1 or m ¼ 21 is capable of radiation transitions, [13]. However, both D and E are determined for quartz, the sublevel of distance 2E could be accounted for as degenerated for the studied range of temperatures. In this case, the sublevel occupation could be expressed as the corresponding kinetic equations [8]: dna 1 ¼2 þ va na þ vb nb ; dt ta
ð1Þ
dnb 1 ¼2 þ vb nb þ va na : dt tb
ð2Þ
where ni ði ¼ a; bÞ and ti correspond to the population and the radiative time constant of each sublevel, and vi is the non-radiative transition rate from the level i to the other. For these kinetic equations it is possible to obtain two exponential components with two decay time constants tS and tF ; with temperature dependence given by expressions
[8]: 1 1 ¼ 2 tS
( "
2
1 1 ¼ 2 tF
( "
þ
1 1 þ þ va þ vb ta tb
!
1 1 2 þ va 2 vb ta tb 1 1 þ þ va þ vb ta tb
#1=2 )
!2 þ4va vb
ð3Þ
;
!
1 1 2 þ va 2 vb ta tb
ð4Þ
#1=2 )
!2 þ4va vb
;
As in Ref. [8], the best fitting procedure was obtained by introducing the Orbach process. The Orbach process dominates the relaxation pathway in the case when a real low-lying excited spin manifold exists about the ground state manifold. The spin system is promoted into a virtual excited spin state by absorption of a phonon of energy Ea above the ground-state manifold, and returns to the lowlying excited spin state by emitting a phonon of energy Eb : For low temperatures, the Orbach process relaxation rate varies exponentially with inverse temperature: ni expð2Ei =kB TÞ; where kB is the Boltzmann constant. The ni is the temperature-independent rate of non-radiative transition and Ei is the energy separation between each sublevel and the additional state. The fitting of experimentally obtained dependences tS and tF is shown in Fig. 3 with parameters given in the caption analogous to Ref. [8]. It is important to underline that the fitting procedure is sufficiently good for a wide range of ni and Ei : The larger the Ei ; is, the larger is ni ; as a consequence, some independent source of those parameters is necessary. The obtained difference in energy is on the level of accuracy of the Ei determination. Therefore, the finding of ZFS parameters from luminescence kinetics data is not the best way. The crystal-field, spin – orbit interaction and spin – spin interaction could determine ZFS. As the efficiency of the spin– orbit interaction increases rapidly with atomic number, the effect of the group-IV element spin – orbit interaction should be much larger in GeO2, than in SiO2, [8]. For a-quartz the ZFS of the STE is attributed to a strong spin– spin interaction [14]. Similar values in GeO2 and SiO2 of the energy of thermal activation of spin – lattice relaxation, leading to an exchange of population between three levels of the triplet state for STE in GeO2 and in SiO2 (and therefore independent of the atomic numbers of Si and Ge) are indicated by the similar values of ZFS in both materials and by the significance of spin – spin interactions in determination of ZFS parameters [8]. On the other hand, whether triplet– singlet transitions are allowed is dependant primarily on spin– orbit interactions [15,16]. The completely forbidden triplet– singlet transitions become partly allowed only by the mixture with the triplet state of some
A. Trukhin et al. / Solid State Communications 127 (2003) 655–659
singlet state by spin– orbit interaction. The influence of spin– spin interaction is not significant for singlet – triplet mixing [15,16]. Since the ZFS does not change much from Ge to Si oxide with structure of a-quartz, it must be the oxygen contribution to spin– orbit coupling that is most important. This conclusion is modified from that in Ref. [8]. This together with the existence of the two kinds of STE, differing in luminescence polarization with respect to crystal orientation, and thermal activation energy of STE luminescence quenching, are the basis for the STE model, proposed in Refs. [3,6], where the exciton self-trapping begins with the appearance of an electron in an anti-bonding state leading to weakening of the Si– O bond. Under such conditions, a non-bridging oxygen relaxes to the direction of a bonding oxygen so that the hole of STE is shared with this bonding oxygen. This provides a fixing of STE with O– O bond creation of that NBO of STE with bonding oxygen on the other side of the c or x; y channels. These two cases explain the existence of the two kinds of STE with different energies of thermal quenching and different polarization of luminescence with respect to crystal orientation. The bond strength of the quasi-molecule O – O determines the energy of thermal quenching.
Acknowledgements This work was supported by the grants 01.0822 of the scientific society of Latvia and by Estonian SF grant 4508.
659
References [1] A.N. Trukhin, A.E. Plaudis, Fizika Tverdogo Tela (Sov. Sol St. Phys.) 21 (1979) 1109. [2] D.L. Griscom, 32nd Freq. Control Symp. Electr. Indust Assn. W-DC (1979) 98. [3] A.N. Trukhin, Fizika Tverdogo Tela (Sov. Sol St. Phys.) 33 (1991) 1631. [4] Yu.M. Alexandrov, V.M. Vishnyakov, V.N. Makhov, K.K. Sidorin, A.N. Trukhin, M.N. Yakimenko, Nucl. Instrum. Methods Phys. Res. A282 (1989) 580. [5] C. Itoh, K. Tanimura, M. Itoh, Phys. Rev. B 39 (1989) 11183. [6] A.N. Trukhin, J. Nucl. Instrum. Method Phys. Res. B91 (1994) 334. [7] C. ltoh, K. Tanimura, N. Itoh, J. Phys. C 21 (1988) 1869. [8] C. Itoh, K. Tanimura, A.N. Trukhin, Nucl. Instrum. Methods Phys. Res. B 116 (1996) 72. [9] W. Joosen, S. Guizard, P. Martin, G. Petite, P. Agostini, A. Dos Santos, G. Grillon, D. Hulin, A. Migus, P. Antonetti, Appl. Phys. Lett. 61 (1992) 2260. [10] A.N. Trukhin, Phys. Status Solidi B 142 (1987) K83. [11] J.A.R. Samson, Vacuum Ultraviolet Spectroscopy, Wiley, New York, 1967. [12] A.N. Trukhin, Mater. Sci. Forum 239–241 (1996) 531. [13] A.N. Trukhins, Solid State Commun. 85 (1993) 723. [14] W. Hayes, M.J. Kane, O. Salminen, R.L. Wood, S.P. Doherty, J. Phys. C 17 (1984) 2943. [15] M.A. El-Sayed, J. Chem. Phys. 54 (1971) 680. [16] S.P. McGlynn, T. Azumi, Molecular Spectroscopy of the Triplet State, Prentice Hall, Englewood Cliffs, NJ, 1969.
Self-trapped exciton luminescence in crystalline a-quartz under two-photon laser excitation Anatoly Trukhina,*, Margarita Kinkb, Yuri Maksimovb, Rein Kinkb a
Institute of Solid State Physics, University of Latvia, Kengarga St., 8, LV-1063 Riga, Latvia b Institute of Physics, University of Tartu, Estonia Received 9 December 2002; accepted 25 June 2003 by J.H. Davies
Abstract The luminescence of pure crystalline a-quartz is studied under pulsed ArF laser excitation. The luminescence parameters obtained correspond well with those of self-trapped excitons (STEs) in a-quartz, indicating that the excitation process is twophoton. The efficiency of two-photon excitation is of the same order of magnitude as the one-photon excitation of sodium salicylate. The STE luminescence decay kinetics and their temperature dependence under photoexcitation were recorded with higher accuracy than previously. Changes in the decay kinetics with temperature are explained by the splitting of the STE triplet state in zero magnetic field and are analyzed with the assumption of the Orbach process of spin– lattice relaxation. No trace of other luminescence was detected in the pure sample. q 2003 Elsevier Ltd. All rights reserved. PACS: 61.82. 2 d; 61.66. 2 f; 71.35.Aa; 78.47. þ p; 78.60. 2 b; 78.60.Hk Keywords: A. a-Quartz; E. Luminescence; E. Self-trapped exciton; B. Laser excitation
1. Introduction The luminescence of pure crystalline quartz excited by ionizing radiation has been attributed to the self-trapped exciton (STE) in works of Trukhin and Plaudis [1] and Griscom [2] although these employed somewhat different models. The energetic yield of the STE luminescence under such conditions of excitation is very high being approximately 20% of the absorbed energy [3]. The problem of exciting this luminescence with photons from the beginning of the intrinsic absorption already arises in Ref. [1] because the yield of photoluminescence (PL) is very small [1,3]. This makes it difficult to be certain of the appropriate parameters to use in measuring STE luminescence under photoexcitation. The possible cause of low yield may be related to the non-ideal surface structure of the polished surface. Indeed, the first measurement of the STE lumines* Corresponding author. Tel.: þ371-7260-686; fax: þ 371-7132778. E-mail address: [email protected] (A. Trukhin).
cence excitation spectrum using synchrotron radiation [4,5] reveals that a small peak previously identified at 8.6 eV [1] corresponds to a sharp, intense peak at 8.7 eV when measured with good resolution. The onset of this peak represents the threshold for intrinsic absorption with a fairly small absorption coefficient, where the penetration of the exciting light is sufficiently high. The cut off of this 8.7 eV peak may be related to the growth of the absorption coefficient and, as the near surface damaged layer of sample was excited, then losses of absorbed energy may take place. Such an interpretation was made in Ref. [6] where the measured low yield of STE PL measurement was explained as being due to the non-ideal material near the surface. Optical excitation by two photons allows luminescence in the bulk of the studied specimen to be studied, unaffected by the conditions of the surface. Also, the use of a pulsed laser allows the more precise measurement of the decay kinetics of STE photoluminescence of a-quartz, which has previously been well measured only with the use of ionizing radiation [1– 3,6 – 8]. The two-photon excitation of the STE luminescence was performed in Ref. [9] for temperature
0038-1098/03/$ - see front matter q 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0038-1098(03)00526-X
656
A. Trukhin et al. / Solid State Communications 127 (2003) 655–659
higher that 30 K, without any detailed measurement of decay kinetics. It was noted that, visually the luminescence under conditions of the experiment [9] is very bright. The temperature dependence of the STE luminescence obtained in Ref. [9] was different from known behavior, i.e. the two stages of thermal quenching discussed in Ref. [6] were not detected. The measurements of STE luminescence under pulsed ArF laser excitation (6.4 eV) were thus performed for crystalline quartz for temperature range from 10 K to complete thermal quenching at 210 K. The attribution of the luminescence to STE under these excitation conditions is strongly supported by the detection of the decay time constant; the temperature dependence including the decay behavior at 10 K (the slow and fast components of the decay correspond to the STE triplet state affected by zero-field splitting (ZFS)), as well as the detection of specific twostages of thermal quenching of the luminescence. All these parameters have been reported previously [3,8,10] for the case of one-photon, X-ray and cathodoexcitation. The efficiency of the two-photon STE excitation (luminescence photons out per two photons absorbed) was estimated to be of the same order of magnitude as the one-photon excited luminescence of sodium salicylate (ArF excitation) in the same geometry of detection. According to Ref. [11], sodium salicylate has a luminescence yield of about 0.5 over a wide spectral range at least from 4 to 40 eV. Because of light scattering by the powdered sodium salicylate and other uncertainties (such as the two-photon absorbed energy), the comparison has only been made within an order of magnitude. The high efficiency of STE luminescence in aquartz under two-photon excitation supports the idea that the damaged surface of a polished sample may be responsible for the low yield of one-photon excited luminescence. The relatively high efficiency of the twophoton excitation allows determination of the measured parameters with an improved signal-to-noise ratio than in previous one-photon measurement, approaching the quality of cathodoexcitation, [8].
photomultiplier (PM) FEU-71 with a silica glass window. An oscilloscope Textronic TDS 380 was used to record decay curves with following transmittance to PC. The PM was used in current regime with a 50 ohm resistance, providing impedance matching with 50 ohm cable with connecting to the oscilloscope. The intensity of luminescence was such that the slits of the monochromator needed to be narrow in order to avoid PM overload. Measured decay curves were fitted to a sum of exponents using the leastsquares-fit method. The same method was used for other mathematical treatment of experimental results.
3. Results In Fig. 1 the photoluminescence spectrum and decay time constant of the fast component at 10 K are presented. (Note that a slow component is also present, but cannot be conveniently displayed using the scale of Fig. 1.) It is seen that the usual PL band of STE with decay time constant of the fast component about 0.35 ms which is characteristic of this temperature appears under two-photon excitation with the ArF laser giving good agreement with known data for single photon excitation [1– 8,10]. The decay time constant is of the same value within the broad PL band. The decay kinetics curves and lifetimes for different temperatures are shown in Figs. 2 and 3, respectively. Two components of decay are observed, converting to a single component with increase in temperature. At the same time, the intensity of the PL does not significantly change with temperature. The temperature dependence of the PL intensities, determined as the integrals under the decay curves, show little variation when the decay curves themselves show strong changes. The corresponding temperature dependence of the time constants for the range of strong changes in decay is shown in Fig. 3. The points correspond to time constants determined by an
2. Experimental The sample of crystalline quartz used for the two-photon measurements exhibited no luminescence in the one-photon regime. The ArF laser (model PSX-100, made by Neweks, Estonia) has a pulse energy of about 5 mJ with a duration of 5 ns and a repetition rate up to 100 Hz. The light beam was concentrated with a lens, focusing after the sample so as to avoid destruction of the cryostat window and the sample. The power in the sample was ,1 GW/cm2. The power of sample’s destruction is , 3 GW/ cm2 in conditions of our experiment. A liquid helium cryostat was used to maintain the temperature in the range 10 –290 K. Luminescence was detected using a grating monochromator MDR-23 with
Fig. 1. PL spectrum (line—continuous measurements, points—time resolved for fast component) and decay time constant (fast component) dependence on the wavelength within the PL band of pure crystalline quartz STE luminescence under ArF laser excitation.
A. Trukhin et al. / Solid State Communications 127 (2003) 655–659
657
Fig. 2. Decay kinetics at different temperatures of pure crystalline quartz STE luminescence under ArF laser excitation.
Fig. 4. Pure crystalline quartz STE luminescence decay kinetics at temperature quenching range excited by ArF laser.
approximation of the decay curve by two exponents. The lines correspond to fitting with formulae taken from Ref. [8], which describe the dependence of decay time constant temperature for the triplet radiative level of STE affected by ZFS (see Section 4). The decay curves could be divided relatively well into two exponents, fast and slow, whose time constants decrease with increase in temperature in the range of liquid helium temperatures. Both components convert to one exponential component with a time constant about 1 ms for temperatures higher than 30 K. For temperatures above 100 K the PL intensity undergoes two-stage thermal quenching, as has been previously reported [1,3]. That disagrees with previous two-photon excitation of a-quartz’s luminescence in Ref. [9], where only one stage was determined. The causes of that could not be yet understood. We believe that in Ref. [9] STE luminescence was detected, so two-stages should be observed. The decay curve at 118 K becomes faster than for lower and higher temperatures, Fig. 4. This is reflected as a drop in the time constant in Fig. 5, where the PL intensity and time-
constant temperature dependences are presented. In the decay curve, a fast component appears in the range of 145 K and this is reflected in both Fig. 4 and in Fig. 5. This is shown by the disappearance of one STE component, whereas another component remains with almost the same time constant as for T below 100 K. The second stage of the PL thermal quenching, with corresponding decrease in time constant, takes place up to 200 K. Previously [10], the different sign of STE luminescence polarization was determined for both stages under X-ray irradiation and the two stages with different polarization’s signs were explained as two types of differently oriented STEs with respect to crystal’s axes.
Fig. 3. Pure crystalline quartz STE luminescence decay kinetics temperature dependences under ArF laser excitation. Points— experimental, lines—fitted with expression (3) and (4). Calculation parameters: ta , 10 ms; tb ¼ 0:33 ms; Ea ¼ 0:0099 eV; Eb ¼ 0:0098 eV; na ¼ 8 £ 105 s21 ; nb ¼ 4 £ 105 s21 :
4. Discussion Under two-photon ArF laser excitation of crystalline aquartz we have obtained luminescence which, considering all the measure, corresponds to the known luminescence of the self-trapped exciton in a-quartz [1 – 8]. Two-stage
Fig. 5. Temperature dependences of PL intensity and decay time constants in the range of STE thermal quenching of the pure crystalline quartz excited by ArF laser.
658
A. Trukhin et al. / Solid State Communications 127 (2003) 655–659
thermal quenching was observed with a broad band at 2.7 eV (Fig. 1). A qualitative estimate of the quantum yield is of the same order of magnitude as sodium salicylate, i.e. closer to 0.5 than to the value 0.05 characterizing previous measurements [3,12] of one-photon-excited STE photoluminescence. However, this latter low yield could be attributed to the quality of the near-surface, whereas twophoton excited luminescence in the bilk of the specimen reflects the real situation with regard to STE creation. In addition there is no contradiction with the high energetic yield of STE luminescence under X-ray and cathodoexcitation. The peculiarities in the decay kinetics in the range of low temperatures (Figs. 2 and 3) (up to 10 K) were observed in the two-photon experiment to be similar to previous cases of one-photon excitation [3] and cathodoexcitation [8]. These peculiarities correspond to ZFS of the triplet state of the STE. The fast component may correspond to transition to the ground singlet state from m ¼ 1 or from m ¼ 21; whereas the slow component may correspond to that from m ¼ 0: This behavior of the kinetics corresponds to the measured ODMR of the STE in a-quartz [12] where the splitting parameters D and E of ZFS are determined. The D and E have unusually high values (D ¼ 22 GHz and E ¼ 1:5 GHz). The sequence on the model of STE from these data could be better understood by comparison with the analogous decay kinetics data obtained in Refs. [8,11] for STE in GeO2 crystals with the a-quartz structure. An analytical model describing the peculiarities of the luminescence decay kinetics was performed in Ref. [8] for the case of quartz and GeO2 under cathodoexcitation. This model is based on a previous investigation of STE in alkali halides (Ref. [8] and references cited therein). At low temperatures, when the spin– lattice relaxation is frozen, the exchange between the ZFS sublevel of the triplet state is also frozen. The decay time constant of about 1/3 that at high temperatures shows that one of the sublevels m ¼ 1 or m ¼ 21 is capable of radiation transitions, [13]. However, both D and E are determined for quartz, the sublevel of distance 2E could be accounted for as degenerated for the studied range of temperatures. In this case, the sublevel occupation could be expressed as the corresponding kinetic equations [8]: dna 1 ¼2 þ va na þ vb nb ; dt ta
ð1Þ
dnb 1 ¼2 þ vb nb þ va na : dt tb
ð2Þ
where ni ði ¼ a; bÞ and ti correspond to the population and the radiative time constant of each sublevel, and vi is the non-radiative transition rate from the level i to the other. For these kinetic equations it is possible to obtain two exponential components with two decay time constants tS and tF ; with temperature dependence given by expressions
[8]: 1 1 ¼ 2 tS
( "
2
1 1 ¼ 2 tF
( "
þ
1 1 þ þ va þ vb ta tb
!
1 1 2 þ va 2 vb ta tb 1 1 þ þ va þ vb ta tb
#1=2 )
!2 þ4va vb
ð3Þ
;
!
1 1 2 þ va 2 vb ta tb
ð4Þ
#1=2 )
!2 þ4va vb
;
As in Ref. [8], the best fitting procedure was obtained by introducing the Orbach process. The Orbach process dominates the relaxation pathway in the case when a real low-lying excited spin manifold exists about the ground state manifold. The spin system is promoted into a virtual excited spin state by absorption of a phonon of energy Ea above the ground-state manifold, and returns to the lowlying excited spin state by emitting a phonon of energy Eb : For low temperatures, the Orbach process relaxation rate varies exponentially with inverse temperature: ni expð2Ei =kB TÞ; where kB is the Boltzmann constant. The ni is the temperature-independent rate of non-radiative transition and Ei is the energy separation between each sublevel and the additional state. The fitting of experimentally obtained dependences tS and tF is shown in Fig. 3 with parameters given in the caption analogous to Ref. [8]. It is important to underline that the fitting procedure is sufficiently good for a wide range of ni and Ei : The larger the Ei ; is, the larger is ni ; as a consequence, some independent source of those parameters is necessary. The obtained difference in energy is on the level of accuracy of the Ei determination. Therefore, the finding of ZFS parameters from luminescence kinetics data is not the best way. The crystal-field, spin – orbit interaction and spin – spin interaction could determine ZFS. As the efficiency of the spin– orbit interaction increases rapidly with atomic number, the effect of the group-IV element spin – orbit interaction should be much larger in GeO2, than in SiO2, [8]. For a-quartz the ZFS of the STE is attributed to a strong spin– spin interaction [14]. Similar values in GeO2 and SiO2 of the energy of thermal activation of spin – lattice relaxation, leading to an exchange of population between three levels of the triplet state for STE in GeO2 and in SiO2 (and therefore independent of the atomic numbers of Si and Ge) are indicated by the similar values of ZFS in both materials and by the significance of spin – spin interactions in determination of ZFS parameters [8]. On the other hand, whether triplet– singlet transitions are allowed is dependant primarily on spin– orbit interactions [15,16]. The completely forbidden triplet– singlet transitions become partly allowed only by the mixture with the triplet state of some
A. Trukhin et al. / Solid State Communications 127 (2003) 655–659
singlet state by spin– orbit interaction. The influence of spin– spin interaction is not significant for singlet – triplet mixing [15,16]. Since the ZFS does not change much from Ge to Si oxide with structure of a-quartz, it must be the oxygen contribution to spin– orbit coupling that is most important. This conclusion is modified from that in Ref. [8]. This together with the existence of the two kinds of STE, differing in luminescence polarization with respect to crystal orientation, and thermal activation energy of STE luminescence quenching, are the basis for the STE model, proposed in Refs. [3,6], where the exciton self-trapping begins with the appearance of an electron in an anti-bonding state leading to weakening of the Si– O bond. Under such conditions, a non-bridging oxygen relaxes to the direction of a bonding oxygen so that the hole of STE is shared with this bonding oxygen. This provides a fixing of STE with O– O bond creation of that NBO of STE with bonding oxygen on the other side of the c or x; y channels. These two cases explain the existence of the two kinds of STE with different energies of thermal quenching and different polarization of luminescence with respect to crystal orientation. The bond strength of the quasi-molecule O – O determines the energy of thermal quenching.
Acknowledgements This work was supported by the grants 01.0822 of the scientific society of Latvia and by Estonian SF grant 4508.
659
References [1] A.N. Trukhin, A.E. Plaudis, Fizika Tverdogo Tela (Sov. Sol St. Phys.) 21 (1979) 1109. [2] D.L. Griscom, 32nd Freq. Control Symp. Electr. Indust Assn. W-DC (1979) 98. [3] A.N. Trukhin, Fizika Tverdogo Tela (Sov. Sol St. Phys.) 33 (1991) 1631. [4] Yu.M. Alexandrov, V.M. Vishnyakov, V.N. Makhov, K.K. Sidorin, A.N. Trukhin, M.N. Yakimenko, Nucl. Instrum. Methods Phys. Res. A282 (1989) 580. [5] C. Itoh, K. Tanimura, M. Itoh, Phys. Rev. B 39 (1989) 11183. [6] A.N. Trukhin, J. Nucl. Instrum. Method Phys. Res. B91 (1994) 334. [7] C. ltoh, K. Tanimura, N. Itoh, J. Phys. C 21 (1988) 1869. [8] C. Itoh, K. Tanimura, A.N. Trukhin, Nucl. Instrum. Methods Phys. Res. B 116 (1996) 72. [9] W. Joosen, S. Guizard, P. Martin, G. Petite, P. Agostini, A. Dos Santos, G. Grillon, D. Hulin, A. Migus, P. Antonetti, Appl. Phys. Lett. 61 (1992) 2260. [10] A.N. Trukhin, Phys. Status Solidi B 142 (1987) K83. [11] J.A.R. Samson, Vacuum Ultraviolet Spectroscopy, Wiley, New York, 1967. [12] A.N. Trukhin, Mater. Sci. Forum 239–241 (1996) 531. [13] A.N. Trukhins, Solid State Commun. 85 (1993) 723. [14] W. Hayes, M.J. Kane, O. Salminen, R.L. Wood, S.P. Doherty, J. Phys. C 17 (1984) 2943. [15] M.A. El-Sayed, J. Chem. Phys. 54 (1971) 680. [16] S.P. McGlynn, T. Azumi, Molecular Spectroscopy of the Triplet State, Prentice Hall, Englewood Cliffs, NJ, 1969.