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Photoluminescence and thermoluminescence in SBN:Cr crystals
Journal of Physics and Chemistry of Solids 61 (2000) 1959±1971
www.elsevier.nl/locate/jpcs
Photoluminescence and thermoluminescence in SBN:Cr crystals M. Gao, S. Kapphan*, R. Pankrath FB Physik, UniversitaÈt OsnabruÈck, 49069 OsnabruÈck, Germany Received 2 March 1999; accepted 20 September 1999
Abstract Cr doping in Strontium Barium Niobate (SBN) crystals is found to enhance the photorefractive response. Absorption bands around 650 nm and below 600 nm can be attributed to Cr in SBN crystals. In addition to XPS experimental results, the appearance of broad R line absorptions at low temperature con®rms that most Cr ions are in the Cr 31 charge state in the SBN crystals. Luminescence bands around 765 nm were observed with different peak positions in nominally pure and in Crdoped SBN crystals. The positions of the maxima in the excitation spectra in SBN:Cr, coinciding with the absorption bands, indicate that this emission band comes from the excitation of Cr 31 ions. Two thermoluminescence (TL) peaks are found in SBN:Cr and nominally pure SBN crystals: one at 88 K and the other around 220 K. The identical spectral distribution of the two TL bands points to the same recombination process following the liberation of two light-induced electron trapping centers: Nb 41 polarons and VIS-centers created at low temperatures. A model of the thermal recovery process involved is proposed. The thermal activation energy for the hopping motion of Nb 41 polarons is estimated to be 0:18 ^ 0:02 eV: For the VIS-centers, the thermal activation energy estimated from their decay dynamics to be 0:36 ^ 0:05 eV (Ming Gao, PhD thesis, University of OsnabruÈck, Germany, 1998), agrees with the activation energy value 0:30 ^ 0:05 eV determined from the TL process. q 2000 Elsevier Science Ltd. All rights reserved. Keywords: A. Oxides; D. Defects; D. Luminescence; D. Optical properties
1. Introduction Strontium barium niobate SrxBa12xNb2O6 (SBN, x 0:61 for the congruent compound, where melt and crystal have the same composition) has been considered one of the most promising materials for a number of non-linear optical applications, like volume holographic storage [1±3] and phase conjugation mirrors [4,5], because of its many advantages over other photorefractive crystals such as BaTiO3 and LiNbO3 [6,7]. SBN crystals exhibit an exceptionally large transverse electro-optic coef®cient r33 420 £ 10212 m=V; making possible a sizable modulation of the refractive index by the electro-optic effect under non-uniform illumination. Furthermore, the photorefractive properties of SBN crystals can be enhanced by suitable dopants. Ce doping is found to increase the photorefractive sensitivity by two orders of magnitude in comparison with pure SBN crystals [7,8]. SBN with Cr and Rh doping shows enhanced photo* Corresponding author. Fax: 149-541-9692670. E-mail address: [email protected] (S. Kapphan).
refractive sensitivity in the red spectral region due to the additional absorption bands of dopants in the red [9±11]. Compared with SBN:Ce, a faster response speed by one order of magnitude was also observed in SBN:Cr [9]. Also, the ferroelectric properties can be altered by doping and composition. Pure congruent SBN crystals possess a ferroelectric phase 4mm at room temperature and the phase transition temperature from paraelectric 4/mmm to ferroelectric 4mm is at about 808C. The composition can vary in a wide range 0:25 # x # 0:75 and strongly in¯uence the phase transition temperature Tc, ranging from 558C for x 0:25 to 2008C for x 0:75 [12]. Additionally, Ce doping also decreases Tc with increasing Ce doping [13]. Therefore, properties related to the phase transition can be controlled by composition or dopants. SBN has one unique fourfold axis and 908 domains do not exist, instead only 1808 domains are permitted, so it is relatively easy to pole the crystals above the phase transition temperature and keep them in a monodomain state at room temperature or below. SBN crystals belong to the tungsten bronze family of crystals with the general chemical formula: (A1)2(A2)4C4(B1)2(B2)8O30 [14]. One unit cell consists of
0022-3697/00/$ - see front matter q 2000 Elsevier Science Ltd. All rights reserved. PII: S 0022-369 7(00)00184-0
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M. Gao et al. / Journal of Physics and Chemistry of Solids 61 (2000) 1959±1971
Wavelength ( nm ) 1000
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-1
Absorption Coefficient ( cm )
30 R lines of Cr
25
T=2K
0.4
3+
0.3
E⊥ C
0.2
T=300K
0.1
E//C
20
0.0 1.60
1.65
1.70
1.75
15 SBN:Cr 5000 ppm
pure SBN
SBN:Ce 0.025 wt.%
10 E⊥ C 5
R-lines
E//C E//C
E⊥ C
E//C
E⊥ C
0 1.5
2.0
2.5
3.0
3.5
Photon Energy ( eV ) Fig. 1. Polarized absorption spectra of congruent pure SBN, SBN:Cr (5000 ppm in the melt) and SBN:Ce (0.025 wt% in the melt) at RT. The inset shows the R lines absorption spectra of SBN:Cr at 2 K.
ten BO6 octahedra linked by their corners to form three types of tunnels along the ~c axis of crystals, which are the A1, A2 and C tunnels corresponding to 15-, 12- and 9-fold oxygen coordinated lattice sites. Both B1 and B2 are 6-fold sites located inside the BO6 oxygen octahedra units. In the case of SBN, only ®ve of the six available A1 and A2 sites are occupied by Sr 21 and Ba 21 cations on an average. B1 and B2 sites are completely ®lled with Nb51 cations while C sites are empty. The partially ®lled structure of SBN provides large possibilities to accommodate various dopants from transition metal ions to rare earth ions. The instrumental neutron activation analysis (INAA) of SBN shows Ce 31 ions are occupying Sr/Ba sites and Cr 31 ions are sitting on Nb sites [15,16]. Even though most investigations of SBN crystals have focused on the improvement of crystal quality and the characterization of photorefractive properties of materials, some efforts to discover the microscopic mechanism of photorefractive effect and related light-induced charge transport process have also been made in parallel in order to optimize the applications of photorefractive devices using SBN crystals. The 3 1 majority charge state of dopants (Ce 31, Cr 31concentration . 90%) was determined in accordance from FIR absorption [17], ESR [18], XPS [19,20] and photoconductivity [21]. Photoluminescence (PL) [18]and the INAA [15,16] of SBN and SBN:Ce crystals point to the existence of trace impurities like Fe 31, Cr 31, Er 31, Ho 31, Ta 51 and so on, in the ppm level even in nominally pure crystals. The majority of the photo-excited charge carriers have been determined by laser beam coupling experiments [22] and Hall effect measurements [23] to be electrons. Two light-
induced absorption centers are observed: the VIS-centers in the visible around 600 nm [24] and the other, observable only below 130 K or in reduced SBN crystals in the NIR (around 0.72 eV), which is assigned to Nb 41 polarons [25,26]. The creation of Nb 41 and VIS-centers under illumination is the ®rst step in the charge transfer from bright to dark areas and the build-up of space charge ®elds which modify the refractive index and constitute the basis of the photorefractive effect under non-uniform spatial illumination. The investigation of the build-up and decay of the light induced absorption is therefore of direct relevance for the understanding of the mechanisms of the photorefractive effect. Since Cr doping enhances the photorefractive response of SBN crystals especially in the red spectral region, it is of interest to investigate its optical properties and its role in the light-induced charge transfer process. In this paper we will report about the absorption and PL of SBN:Cr in comparison with the results from SBN:Ce and pure SBN. The thermoluminescence (TL) technique is employed to study the charge trapping centers created under illumination at low temperature. The related light-induced charge transfer process and the thermal recovery process of the trapping centers at elevated temperatures are discussed and a simpli®ed model is proposed.
2. Experimental techniques SBN:Cr crystals with congruent melting composition
M. Gao et al. / Journal of Physics and Chemistry of Solids 61 (2000) 1959±1971
1961
50
E ⊥C
-1
α at 650 nm ( cm )
40
T=300 K 30
20
10
0 0
5000
10000
15000
20000
Cr doping in the melt ( ppm ) Fig. 2. The absorption coef®cient a at 650 nm in SBN:Cr crystals at RT as a function of Cr concentration in the melt. The solid line is the result of a least square ®t from 50 to 7500 ppm Cr doping.
x 0:61 were grown with the Czochralski method by R. Pankrath in the crystal growth laboratory of the Physics Department at the University of OsnabruÈck. Cr2O3 with various doping levels: 50, 100, 150, 250, 500, 750, 1000, 1500, 2000, 3000, 5000, 7500, 10 000, 15 000, 20 000 ppm were added to the melt during the crystal growth. Small temperature gradients along with a pulling rate of 0.8 mm per hour ensured that optical-quality crystals with natural facets and free of striations could be obtained. In order to study the possible in¯uence of poling, some crystals were poled under an electric ®eld of about 10 kV/cm at T 708C: A Fourier spectrometer (Bruker IFS 120HR) was employed to measure the absorption spectra of crystals from the UV to the FIR spectral region using different combinations of light sources, beam splitters and detectors. PL and excitation spectra were measured by a photon counting system with a SPEX1402 double grating monochromator and a cooled RCA31034 GaAs photomultiplier. A high pressure Xenon lamp (ILC, Cermax, 300 W) was used as the excitation source and a SPEX Minimate for the wavelength selection together with variable bandpass ®lters. A Helium bath cryostat (Leybold) was employed in the absorption measurement and the crystals were immersed in the super¯uid helium (2 K). A closed cycle cryostat (Leybold) was used for the PL and TL measurements. A programmable temperature controller could be set to different heating rates for the TL measurement. For the luminescence lifetime measurement, a Xenon lamp, a chopper system and a multichannel scaler (EG&G 914P) were used.
3. Experimental results 3.1. Absorption Fig. 1 shows the typical polarized absorption spectra of pure SBN, SBN:Cr and SBN:Ce crystals. The UV absorption band edge of pure SBN crystals at room temperature lies at about 370 nm with a slight dichroism for extraordinary and ordinary light. In the visible region pure SBN is transparent and shows no absorption. Ce doping shifts the onset of the absorption edge to longer wavelengths and an absorption shoulder around 500 nm appears. This absorption shoulder is attributed to the 4f ! 5d transition of Ce 31 ions, which is an allowed electric dipole transition. The spin±orbit and crystal ®eld splitting of 4f 1 level of Ce 31 ions is not resolved in the visible, but this splitting indeed exists and shows in characteristic absorption bands in the FIR (around 2100 cm 21) [17]. SBN:Cr also shows broad absorption bands around 650 nm (1.9 eV) as well as strong absorption below 600 nm, very similar to the absorption bands of Cr 31 ions in other oxide crystals [27]. The appearance of R line absorptions con®rms the existence of Cr 31, which is consistent with XPS measurement results of SBN:Cr, yielding more than 90% of Chromium in the 3 1 charge state. However, R line absorption in SBN:Cr is structurally broadened even at 2 K (full width at half maximumFWHM: 300 cm 21 or 16 nm) as shown in the inset of Fig. 1. For comparison, the R lines in other oxides like ruby and Ê ) [27]. The bandemerald are very sharp (FWHM: about 1 A width (FWHM) of the absorption band around 650 nm is about 0.3 eV for all Cr-doped SBN crystals. Fig. 2 shows
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Wavelength ( nm ) 880
860
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820
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740
20000
-1
Photon Count Rate ( s )
pure, λex =380 nm 15000
50 ppm Cr, λex =488 nm 500 ppm Cr, λex =488 nm 1000 ppm Cr, λex =488 nm
10000
2000 ppm Cr, λex =488 nm 10000 ppm Cr, λex =488 nm
5000
0 1.40
1.45
1.50
1.55
1.60
1.65
1.70
Photon Energy ( eV ) Fig. 3. Emission in pure SBN and SBN:Cr with different Cr concentration in the melt at 10 K.
the absorption coef®cient a at 650 nm as a function of Cr concentration in the melt. The roughly linear relationship between a and the concentration CCr in the melt indicates that the effective distribution coef®cient Keff CCr2 O3 in crystal =CCr2 O3 in melt changes very little from 50 to 7500 ppm Cr2O3 doping level. The quantitative determination of the Cr concentration in crystals is in progress [28]. The poling of crystals does not show any measurable in¯uence on the absorption measurements. 3.2. Photoluminescence A strong luminescence band in the NIR is observed in all Cr-doped SBN crystals as shown in Fig. 3. In the earlier literature [18,29], pure SBN crystals are reported to exhibit a similar emission band in the same spectral range (around 760 nm). Our measurements do con®rm the existence of such an emission band around 765 nm in nominally pure SBN crystals. More interestingly, Cr doping increases the intensity of this band very much. For example, a 500 ppm Cr-doped SBN crystal shows about two orders of magnitude stronger emission than pure SBN. The peak position of the emission band shifts to longer wavelength with increasing Cr concentration in the melt, from 765 nm for pure SBN to 780 nm for 10 000 ppm Cr-doped SBN. No R line luminescence could be observed in SBN:Cr crystals. SBN:Ce crystals also show the same emission band but with at least a factor of ®ve lower intensity under the same excitation conditions compared with pure SBN. The poling of the crystals did not show any in¯uence on the PL. The excitation spectra are displayed in Fig. 4. The emission intensity was corrected for the variation of the light
source with exciting energy (number of impinging photons) as well as for detector ef®ciency and for throughput of the grating spectrometers used at the exciting side and at the emission detection side. Above 500 nm, the excitation spectra resemble the absorption spectra, indicating Cr 31 ions actually participate in the excitation processes. For the pure SBN the excitation around the band edge is the most effective for the emission. In essence, the excitation near the band edge can result in an emission in all Cr-doped SBN crystals as well. However, for higher Cr-doped crystals the excitation in the red induces a much stronger intensity in the emission compared with the excitation in the UV band edge, where the penetration depth is limited due to the high absorption value. The temperature dependence of the emission band shape is shown in Fig. 5(a) and the temperature dependence of the integrated intensity normalized at 10 K is shown in Fig. 5(b). The decrease of the integrated intensity with increasing temperature can be interpreted as usual due to competitive non-radiative processes increasing with the phonon population with increasing temperature. The time dependent decay of this emission band after shutting-off the excitation is found to be almost monoexponential with a lifetime of 2.9 ms at 19 K and getting shorter at higher temperatures, as shown in Fig. 6. Zeinally et al. measured the lifetime of the emission to be 1.4 ms at 80 K in nominally pure SBN crystals, also with a decreasing trend of lifetime with increasing temperature [29]. 3.3. Thermoluminescence TL measurements are performed in the following way: SBN:Cr or pure SBN crystals were ®rst cooled from room
M. Gao et al. / Journal of Physics and Chemistry of Solids 61 (2000) 1959±1971
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W avelength ( nm ) 800700 600
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T=10K
pure SBN
Photon Count Rate ( a.u. )
λem =760 nm
50 ppm Cr λem =766 nm
500 ppm Cr λem =766 nm
2000 ppm Cr λem =775 nm
10000 ppm Cr λem =780 nm
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
Photon Energy ( eV ) Fig. 4. The excitation spectra at 10 K for pure SBN and for SBN:Cr crystals with different Cr concentration in the melt.
temperature to 10 K and then illuminated 10 min at 10 K with light of one speci®c wavelength from a Xenon lamp using an interference ®lter; then after shutting-off the lamp and waiting another 10 min, the crystals were heated at a constant heating rate (5.2 K/min) from 10 to 320 K. The TL measurement results are demonstrated in Fig. 7. Two quite resolved TL peaks emerge in SBN:Cr and nominally pure
SBN crystals: one at 88 K and the other around 220 K, indicating two different charge trapping centers. The illumination with UV light results in a stronger intensity for both TL peaks compared with 488 and 647 nm wavelength light. The ratio among the intensities of three illumination wavelengths (380, 488 and 647 nm) was 1.0:2.1:1.8. The ®rst TL peak has a ®xed peak position at 88 K regardless of the
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M. Gao et al. / Journal of Physics and Chemistry of Solids 61 (2000) 1959±1971
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Integrated Intensity ( a.u. )
-1
Photon Count Rate ( s )
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30K
15000
λex=488 nm
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(a) 5000
150K 180K 210K
0
1.40
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1.45
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λex=488 nm
0.8
0.6
0.4
(b) 0.2
0.0
0
50
100
150
200
250
Temperature ( K )
Photon Energy ( eV )
Fig. 5. Temperature dependence of emission in SBN:Cr (500 ppm Cr in the melt). (a) Emission band at different temperatures. (b) Integrated emission intensity (from 1.4 to 1.7 eV) as a function of temperature, normalized at 10 K. lexc 488 nm:
illumination wavelength. However, the position of the second TL peak is depending on the illumination wavelength as shown in Fig. 7. The peak position and bandwidth (FWHM) in temperature of the two TL peaks with different illumination wavelengths are listed in Table 1. The possible reason for the shift of the peak position of the second TL band will be discussed in the later section. The spectral distribution of the two TL peaks are shown in Fig. 8. For comparison, the PL emission of this crystal is
also given with normalized amplitude (solid line and dashed line in Fig. 8). The similarity between PL band and the TL spectral distribution indicates that they originate from the same emission process. The time dependence of the two TL peaks at ®xed temperatures are demonstrated in Fig. 9. The crystal was illuminated at 10 K for 10 min with UV light (380 nm) and then heated to 88 K. The temperature of the crystal was kept at 88 K for 30 min and the spectrometer recorded the decay process of the ®rst TL peak at 88 K. The
2.90 2.85
τ ( ms )
2.80 2.75 2.70 2.65 2.60 2.55 2.50 0
30
60
90
120
150
180
Temperature ( K ) Fig. 6. Lifetime of the emission in SBN:Cr (750 ppm Cr in the melt) at different temperatures. The dashed line is a guideline for the eyes.
M. Gao et al. / Journal of Physics and Chemistry of Solids 61 (2000) 1959±1971
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Crystal was illuminated
5000
-1
Photon Count Rate ( s )
10 mins at 10K with 380 nm
4000
488 nm 647 nm
3000 214K
2000 88K
229K
1000
0 238K
0
50
100
150
200
250
300
350
Temperature ( K ) Fig. 7. Thermoluminescence in SBN:Cr (750 ppm Cr in the melt) from 10 to 320 K. The intensity ratio of the illumination light 380, 488 and 647 nm is 1.0:2.1:1.8. Heating rate: 5.2 K/min. Detection wavelength: 766 nm.
decay process of the second TL peak at 220 K could be obtained in the same way. The second TL peak shows a more monoexponential decay behavior, indicating a simple ®rst-order kinetics for this peak. The deviation from the monoexponential decay process for the ®rst TL peak points to a more complicated thermal recovery mechanism, as discussed also for the light-induced absorption kinetics of the Nb 41 polarons [40].
4. Discussion 4.1. Absorption and luminescence of Cr 31 ions The preliminary INAA result [15,16] has shown that the concentration of Nb in SBN:Cr decreases with increasing Cr concentration and the concentration of Sr/Ba does not vary with the different Cr doping, which strongly indicates that Cr 31 ions prefer to substitute for Nb 51 ions. In the structure of SBN, Nb ions occupy B1 and B2 sites, which lie inside the distorted oxygen octahedra along the ~c axis of the crystal with slightly different Nb±O distances and different next
nearest neighbor ions. The substitution of Cr 31 for Nb 51 ions will result in the level splitting of Cr 31 in octahedral symmetry. The 3d electron con®guration of Cr 31 is 3d 3. The ground energy level of free Cr 31 ions is 4F and the excited energy levels are 4P, 2G, 2F and so on. In octahedral crystal ®elds, the energy levels of Cr 31 ions will split into ground level 4A2g and excited levels 2Eg, 2T1g, 2T2g, 4T2g, 4T1g, 2A1g and so on [30]. The excited levels could exchange their position depending on the strength of the crystal ®elds. The typical absorption spectra of Cr 31 in octahedral symmetry consist of several broad spin-allowed absorption bands due to transitions like 4A2g ! 4T2g, 4T1g, and a series of sharp lines due to the spin-forbidden transitions: 4A2g ! 2E1g, 2T1g and so on [27]. The peak positions of the broad bands strongly depend on crystal ®elds and vary from material to material. However, the positions of the sharp lines are not sensitive to the crystal ®eld because the energy levels 2E1g and 2T1g do not depend much on the crystal ®eld strength. Surprisingly there is only one broad absorption band at 650 nm resolved in SBN:Cr, as shown in Fig. 1 even though extensive efforts to look for other resolved broad absorption bands by using very thin crystals or low Cr-doped crystals
Table 1 The temperatures at the maximum of the TL bands (peak) and the width of the TL bands in temperature (FWHM) under illumination with different wavelengths: 380, 488 and 647 nm at 10 K Illumination (nm)
380 488 647
TL1
TL2
Peak (K)
FWHM (K)
Peak (K)
FWHM (K)
88 88 88
34 27 25
214 229 238
61 43 40
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Wavelength ( nm ) 800 795 790 785 780 775 770 765 760 755
88 K 220 K
Illuminated at 10K
-1
TL Count Rate ( s )
4000
8 mins with 380 nm 3000
UV light
2000
1000
1.56
1.58
1.60
1.62
1.64
Photon Energy ( eV ) Fig. 8. The spectral distribution of the two thermoluminescence peaks in SBN:Cr (750 ppm in the melt). The photoluminescence emission band of the crystal at 10 K with the excitation of 380 nm UV light is normalized to the TL maximum at 88 K (solid line) or to the TL maximum at 220 K (dashed line).
have been undertaken. However, the absorption below 600 nm is increasing with increasing Cr doping level. The broad band at 650 nm is similar to the 4A2g ! 4T2g absorption band of Cr 31 in emerald, LiNbO3:Cr and LiTaO3:Cr oxide crystals from the shape and polarization dependence [27,31], and is therefore assumed to come from the same transition of Cr 31 ions in octahedral symmetry of SBN crystals. The second broad absorption band due to the transition 4 A2g ! 4T1g cannot be resolved as a separate band. The structural disorder of SBN crystals alone cannot account for such a behavior since this band of Cr 31 ions is quite resolved even in oxide glasses with much more disorderly structures [32]. As long as Cr 31 lies inside the oxygen octahedra, the splitting of Cr 31 levels can be well explained by crystal ®eld theory. However, the absence of the resolved 4 A2g ! 4T1g band could result from the following reasons. One possibility is that the level 4T1g lies already inside or higher than the bottom of the conduction band of SBN crystals, and the transition 4A2g ! 4T1g of Cr 31 ions results in a charge transfer transition 4A2g(Cr 31) ! 4d(Nb 51). As a result, the onset of the absorption edge is shifted to the longer wavelength with increasing Cr concentration,
which is consistent with our experimental observation. The second possibility lies at the substitution of Cr 31 for different sites. If some Cr 31 (,1%) replaces not only Nb 51 sites (B1 and B2) but also Sr 21/Ba 21 (A1 and A2) or even C sites, the situation will become very complicated. The level splitting of Cr 31 in 15-, 12- and 9-fold co-ordinations has not been extensively studied so far and it could be rather different from 6-fold (octahedral) and 4-fold (tetrahedral) co-ordinations. The energy levels of Cr 31 ions in these 15, 12- and 9-fold co-ordinations could mix with the levels of Cr 31 like 4T1g in 6-fold co-ordination (octahedral). As a result, the absorption band due to the transition 4 A2g ! 4T1g of Cr 31 ions in octahedra could be overlapping with other transitions of Cr 31 ions in higher-fold co-ordinations and therefore become unresolvable. At present no independent experimental evidence for different Cr 31-sites has been found and the second possibility mentioned above, is therefore considerate to be not very likely. The oscillator strength f for the transition 4A2g ! 4T2g can be estimated from the absorption spectra. The integrated absorption for this transition in SBN:Cr (Cr ion concentration:1:7 £ 1021 cm23 for 5000 ppm Cr doping,
M. Gao et al. / Journal of Physics and Chemistry of Solids 61 (2000) 1959±1971
1967
9
TL Count Rate ( Log scale )
8
T=88K
7
6
T=220K
5
4
3 0
3
6
9
12
15
Time ( mins ) Fig. 9. Time dependence of the two TL peaks at two temperatures in SBN:Cr (750 ppm in the melt). The circles indicate the experimental data for ldet 770 nm: The dashed lines are the ®tting results with monoexponential functions and the solid line is the ®tting result with Eqs. (7) and (8).
where the concentration of Cr in crystal is regarded as same as in the melt) can be calculated to be Z aE ' dE 3:5 cm21 eV Z
aE k dE 1:8 cm21 eV
Based upon the Smakula equation [33], the oscillator strengths can be derived as f 4 A2g ! 4 T2g E 7:5 £ 1026 ' f 4 A2g ! 4 T2g A 3:8 £ 1026 k The integrated intensity ratio between R lines and 4T2g bands is about 5 £ 1023 ; indicating a two or three orders of magnitude smaller oscillator strength for the transition 2 A2g ! 2E1g. The R lines absorption bands in SBN:Cr are extraordinarily broad even at 2 K, re¯ecting the structural disorder of SBN crystals. The structural disorder of SBN crystals is also re¯ected by a broad Raman linewidth for NbO6 in SBN crystals [18]. Different occupation sites of Cr 31 in SBN crystals can also lead to several unresolved lines. Similar to the situation in LiNbO3:Cr [30,31], the
zero-phonon level of the 4T2g bands in SBN:Cr lies close to the 2E1g level. If an electron is excited from the ground state 4A2g to the excited state 2E1g, the electron in this excited state could relax to the zero-phonon level of the 4 T2g band through a non-radiation process and then the electron will transit from this state to the ground state or to the other excited metastable states. This may explain the absence of the R line luminescence in the SBN:Cr crystal. The origin of the emission band in the NIR in nominally pure SBN crystals is not clear. Zeinally et al. [29] assigned the emission around 760 nm to the de-excitation from 4d orbital of Nb to 2p orbital of O in NbO6, but Giles et al. [18] regarded this emission as the transition of unwanted Cr 31 in SBN crystals in trace amounts of Cr in the ppm level. Our luminescence measurement in SBN:Cr crystals suggests that the excitation of this emission band indeed originates from Cr 31 ions in SBN. The direct corresponding relationship between excitation spectra and absorption spectra in SBN:Cr, especially from 500 to 720 nm indicates that this emission band could come from the de-excitation process of Cr 31 ions: 4T2g ! 4A2g. A similar luminescence in the same spectral range is observed in LiNbO3:Cr and LiTaO3:Cr [31], which is due to the transition of Cr 31: 4T2g ! 4A2g. The luminescence measurements in
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CB Ex
Eh
Nb4+ Polaron Emission
VIS-Centers
Cr4+ Cr3+
VB Fig. 10. Model of the thermal recovery processes in SBN:Cr crystals. Ex is the average thermal activation energy for the VIS-centers and Eh is the energy needed for the hopping motion of the Nb 41 polarons.
other niobates [34±37] show that the intrinsic emission band of NbO6 usually lies around 500 nm. Actually, upon band edge excitation, a very weak additional emission around 480 nm was observed in pure SBN crystals [38], which might be the emission of NbO6 in SBN crystals. The INAA data show that Cr with trace amount in the ppm level exists even in nominally pure SBN and SBN:Ce crystals and that the Cr amount decreases with Ce doping [15,16]. Such a small amount of Cr is suf®cient for an observable luminescence. However, the lifetime of the emission of Cr 31 ions: 4T2g ! 4A2g is usually in the range of ms [31]. The quite long lifetime (in ms range) of the emission in SBN:Cr crystals leads one to consider other emission mechanisms. Since the excitation originates from Cr 31 ions and the appearance of light-induced Nb 41 polarons at low temperature is a well-known fact [25,26], one can also assume that the direct recombination of the electrons from the Nb 41 polarons with Cr 41 ions results in the emission band around 765 nm. However, the lifetime of Nb 41 polarons after switching-off illumination is in the range of 100 s at T , 60 K [28], indicating that the direct recombination of the electrons from Nb 41 polarons with Cr 41 ions contributes only to a small fraction of the total emission intensity. The peak position shift of the emission bands with increasing Cr concentration could result from the increasing re-absorption of the emission by Cr 31 centers in SBN in this spectral region (around 760 nm). Although the absorption due to the transition of Ce 31 ions: 4f ! 5d is strong in SBN:Ce crystals, no luminescence is observed for the de-excitation: 5d ! 4f of Ce 31 ions. Blasse et al. [39] discussed the quenching of the luminescence of Ce 31 ions in detail and proposed that Ce 31 ions may be expected to luminesce only in host materials which have their optical absorption edge in the far ultraviolet and the quenching by electron transfer could rapidly eliminate the luminescence of Ce 31 ions if the optical absorption edge
of host materials shifts to the low energy side. The absorption band of Ce 31 ions in SBN:Ce is very close to the UV absorption edge of the pure crystal, suggesting that the ®rst excited state 5d 1 of Ce 31 ions lies nearby the bottom of the conduction band. In this case an electron transfer transition from the excited state level of the Ce 31 ions to the conduction band (4d orbital of Nb 51) becomes possible, quenching the luminescence of Ce 31 ions. 4.2. Thermal recovery process The TL results show that two charge trapping centers exist in SBN crystals under illumination, which is in good agreement with the light-induced absorption results. Since one trapping center is known as the electronic Nb 41 polaron, the same spectral distribution of two TL peaks indicates that the other charge trapping center is also an electron trapping center. The light-induced Nb 41 polaron shows a broad absorption band around 0.72 eV [25,26]. Actually most light-induced Nb 41 polarons are found to decay even at 2 K after the illumination is switched off, indicating that most Nb 41 polarons are formed nearby Cr 41 recombination centers and the decay of polarons is due to the recombination of the electrons from the Nb 41 polarons with Cr 41 by tunneling processes or phonon-assisted hopping processes [40]. However, there are always several percent of Nb 41 polarons remaining very stable at low temperatures (i.e. 2 K) for a long time after switching off the illumination, indicating that this part of polarons are created far away from their recombination centers. At temperatures above 70 K, the hopping motion of polarons tends to dominate the thermal recombination process and therefore the recombination of Cr 41 ions with electrons from Nb 41 polarons being liberated far away from Cr 41 centers by a polaron hopping process at increasing temperatures results in the ®rst TL peak at 88 K. Here we are dealing with a unique
M. Gao et al. / Journal of Physics and Chemistry of Solids 61 (2000) 1959±1971
1969
9
Experiment
8
curve fitting ( total 7
glow peak method )
ln(A/I)
6 5 4
TL2 ( 229K )
3 2 4.4
4.8
5.2
5.6
6.0
-1
1000/T ( K ) Fig. 11. Analysis of the second TL peak with the total glow peak method (see text). Previous low temperature illumination was performed with l 488 nm: The slope of the straight line yields a value of the average thermal activation energy: Ex 0:30 ^ 0:05 eV:
trapping center, the polaron, and the thermal recovery is due to the hopping motion of the polaron instead of the normal band transport. The second electron trapping center (VIScenter) has a broad absorption band in the VIS (around 2.1 eV or 600 nm) [24,26]. The physical nature of this center is not clear so far. At room temperature this light-induced absorption center is weak and dif®cult to observe, but it becomes more pronounced with decreasing temperature. The decay process of this light-induced absorption center after the illumination is switched off is thermally activated with an activation energy estimated to be Ex 0:36 ^ 0:05 eV [41]. The decay rate is practically zero at T , 100 K; but is increasing with rising temperature. The thermal recovery process of this center results in the second TL peak around 220 K. The identical spectral distribution of the two TL processes and the agreement with the PL emission show that the two TL peaks come from the same emission process, i.e. the direct recombination of electrons from the Nb 41 polarons with the Cr 41 recombination centers. Fig. 10 brie¯y depicts a model which could explain the thermal recovery processes involved for the TL emission in previously illuminated SBN:Cr crystals at low temperature. Two channels of the thermal release of the electrons from the VIS-centers are shown in this picture: one with the electrons transported in the conduction band and the other with the thermal dissociation of the VIS-center as the ®rst step and then the hopping motion of Nb 41 polarons and subsequent recombination of electrons and Cr 41 centers. The latter model part could suggest to consider the VIS-centers as bipolarons. Indeed there are many similarities in the optical properties and thermal stability with polarons and bipolarons in LiNbO3 crystals [42]. However, more experimental
and theoretical considerations are needed to ascertain these assumptions for the SBN systems. The intensities of the two TL peaks strongly depend on illumination conditions, respectively the photon energy. An electron absorbing a photon of higher energy (in case of illumination with UV light like 380 nm) is transferred to higher conduction band states and has more possibilities to migrate a longer distance in the conduction band before it is ®nally trapped. We should keep in mind that only those Nb 41 polarons far away from Cr 41 recombination centers contribute to the ®rst TL peak and the electrons with higher energy have more chances to form such polarons. For an electron absorbing a photon of lower energy (in case of illumination with red light like 647 nm), the possibility to form a polaron far away from Cr 41 ions will be small. Therefore, the number of Nb 41 polarons effective for TL, created under illumination of UV light (380 nm) is much larger than that under illumination with visible light (488 and 647 nm) as shown in Fig. 7. A similar situation exists for the second TL peak. Since trapping centers around the original Cr 31 ions are limited, electrons with higher energy have more chances to walk away and be trapped in other positions, forming the VIS-centers responsible for the total intensity increase of the second TL peak. One interesting feature for the second TL peak is that the peak position shifts to lower temperature with increasing TL intensity and the bandwidth FWHM is also broadened in parallel. The in¯uence of the heating rate can be excluded because we use the same heating rate for all three curves in Fig. 7. One very probable situation is that the thermal depth (or the thermal activation energy) for the VIS trapping centers is not a single level, but rather, there is an energy distribution of
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M. Gao et al. / Journal of Physics and Chemistry of Solids 61 (2000) 1959±1971
the thermal activation energy which varies with the concentration of the VIS-centers. Energy distributions of thermal depths for trapping centers have been observed in amorphous materials and polymeric compounds [43]. Such a picture usually exists in the situation where there are random differences among the con®gurations between nearest neighbors like in bond angles and positions of nearest neighbors, with the result that the thermal depth tends to be smeared out. The structural disorder of SBN crystals may account for the energy distribution of thermal depth for the VIS-centers involved in the second TL peak. According to Urbach's formula [43], E Tm =500 eV; where E is the activation energy and Tm is the temperature in K at the maximum of a TL band, the thermal activation energy for the VIS-centers is estimated to be about 0.44 eV. A more accurate method called total glow peak method [43,44] can be employed to determine the average thermal activation energy for the VIS trapping centers. In this method for the ®rst-order kinetics process one plots ln(A/ I) versus the reciprocal of temperature 1/T, where I is the TL intensity at one ®xed temperature T, and A is the area of TL curve from that temperature to the end of the peak. The plot should yield a straight line with a slope of E/k. Additionally one should take into account the temperature dependence of the emission for the recombination process since thermal quenching increases with rising temperature as shown in Fig. 5(b). Fig. 11 shows the analysis using the total glow peak method with consideration of the temperature dependence of the integrated emission intensity. The average thermal activation energy Ex 0:30 ^ 0:05 eV from this analysis is smaller, as expected, than the estimated value from the Urbach's law. This value of Ex can be compared with light-induced absorption measurements which yield an average thermal activation energy with the value Ex 0:36 ^ 0:05 eV for the VIS trapping centers [41]. To analyze the thermal recovery process for the ®rst TL peak, we follow the treatment described in Ref. [45]. For a luminescence recombination process Nb41 polaron 1 Cr41 ! Nb51 1 Cr31 1 hn; the rate equation is ! dnp dnCr41 r0 2x 1 1 p np nCr41 dt dt pDp t
1
Here np and nCr41 are the concentrations of Nb 41 polarons and Cr 41 ions, respectively; r0 denotes the critical interaction distance for this recombination process and x 4pr0 Dp with Dp being the diffusion coef®cients for Nb 41 polarons. Cr 41 ions are considered localized here and only Nb 41 polarons are mobile. Under the approximation pDp t q r02 ; dnp 24pr0 Dp np nCr41 dt
2
Let us consider the situation where all the electrons trapped in polarons come from the photoionization of Cr 31, and we
then have nCr41 np 1 N0
3 41
Here N0 is the concentration of Cr centers before illumination, together with Cr 41 centers whose electrons are excited by illumination and trapped by VIS-centers. By using the random walk model the diffusion ef®cient Dp can be written as E Dp 16 a2 n0 exp 2 h 4 kT where a is the random hopping distance, n 0 the average hopping frequency, Eh the activation energy for hopping motion, k Boltzmann's constant and T is the absolute temperature. We combine Eqs. (2)±(4) and obtain dnp 2hnp np 1 N0 dt with
h
2 3
E pr0 a2 n0 exp 2 h kT
5
6
being a constant for one ®xed temperature. The solution of Eq. (5) is given by np t "
N0 11 np 0
#
N0
7
exp 2hN0 t 2 1
The decay of the TL intensity ITL at one ®xed temperature is proportional to 2dnp(t)/dt ITL / 2
dnp t hnp np 1 N0 dt
8
Eqs. (7) and (8) are used to ®t the time dependence of the ®rst TL peak at 88 K and indeed a better agreement with the experimental result is obtained (solid line) than using a monoexponential function (dashed line) as shown in Fig. 9. If np p N0 ; Eq. (5) becomes a ®rst-order kinetic equation with a monoexponential solution. If np q N0 ; a secondorder kinetics is reached. The agreement of the relationship between lnI , t for the ®rst TL peak (as shown in Fig. 9) and the small deviation from a straight line indicates that the kinetics governing the thermal recovery process by polaron hopping motion is close to a ®rst-order kinetic process. According to the Urbach's formula, the activation energy for the polaron hopping motion can be inferred from the TL data: Eh 0:18 ^ 0:02 eV; which agrees with the value estimated from the optical transition 0:72=4 0:18 eV [40]. 5. Conclusion Broad absorption bands due to the Cr-doping have been observed around 650 nm and below 600 nm in SBN:Cr, and these absorption bands extend and enhance the spectral response of SBN crystals to the red. The R line absorption
M. Gao et al. / Journal of Physics and Chemistry of Solids 61 (2000) 1959±1971 31
of Cr ions in SBN:Cr is extremely broad (FWHM: 300 cm 21) even at 2 K, re¯ecting the structural disorder of SBN crystals. Under UV or visible light excitation, an emission band appears in the NIR (around 765 nm) with slightly different peak positions in pure and different Cr-doped SBN crystals. No R line luminescence was observed in SBN:Cr crystals. Although the absorption of Ce 31 ions in SBN is strong, a luminescence due to the transition of Ce 31 ions: 5d ! 4f has not been found. Two TL peaks are observed in the temperature range from 10 to 320 K. The ®rst TL peak (at 88 K) comes from the thermal hopping motion of Nb 41 polarons created far away from Cr 41 centers and the recombination of their electrons with Cr 41 ions. The thermal activation energy for the hopping motion of Nb 41 polarons is estimated to be: Eh 0:18 ^ 0:02 eV: The second TL peak (around 220 K) originates from the thermal liberation of electrons from another electron trapping centers (VIScenters) and their recombination with Cr 41 centers. The average thermal activation energy of the VIS-centers is estimated to be: Ex 0:30 ^ 0:05 eV; agreeing well with the value estimated from the light-induced VIS absorption results [41]. Acknowledgements This work is supported by DFG (SFB225, C7). We thank Ms Eden and Mr Aulich for stimulating discussions and assistance. Ming Gao thanks the VW Foundation for a doctoral fellowship. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]
J. Thaxter, M. Kestigian, Appl. Opt. 13 (1974) 913. S. Red®eld, L. Hesselink, Opt. Lett. 13 (1988) 880. M. Horowitz, A. Bekker, B. Fisher, Opt. Lett. 18 (1993) 1964. G. Salamo, M. Miller, W. Clark III, G. Wood, E. Sharp, Opt. Commun. 59 (1986) 417. G. Wood, W. Clark III, M. Miller, E. Sharp, G. Salamo, R. Neurgaonkar, IEEE J. Quantum Elect. QE-23 (1987) 2126. R. Neurgaonkar, W. Cory, J. Oliver, M. Ewbank, W. Hall, Opt. Engng 26 (1987) 392. R. Neurgaonkar, W. Cory, J. Opt. Soc. Am. B 3 (1986) 274. K. Megumi, H. Kozuka, M. Kobayashi, Y. Furahata, Appl. Phys. Lett. 30 (1977) 631. K. Sayano, A. Yariv, R.R. Neurgaonkar, Appl. Phys. Lett. 55 (1989) 328. R. Vazquez, M. Ewbank, R. Neurgaonkar, Opt. Commun. 80 (1991) 253. R. Vazquez, R. Neurgaonkar, M. Ewbank, J. Opt. Soc. Am. B 9 (1992) 1416. R. Neurgaonkar, W. Hall, J. Oliver, W. Ho, W. Cory, Ferroelectrics 87 (1988) 167. N. Wittler, G. Greten, S. Kapphan, R. Pankrath, J. Seglins, Phys. Status Solidi (b) 189 (1995) K37.
1971
[14] P. Jamieson, S. Abrahams, J. Bernstein, J. Chem. Phys. 48 (1968) 5048. [15] Th. Woike, G. Weckwerth, H. Palme, R. Pankrath, Solid State Commun. 102 (1997) 743. [16] Th. Woike, INAA of SBN:Cr, private communication. [17] G. Greten, S. Hunsche, U. KnuÈpffer, R. Pankrath, U. Siefker, N. Wittler, S. Kapphan, Ferroelectrics 185 (1996) 289. [18] N. Giles, J. Wolford, G. Edwards, R. Uhrin, J. Appl. Phys. 77 (1995) 976. [19] R. Niemann, K. Buse, R. Pankrath, M. Neumann, Solid State Commun. 98 (1996) 209. [20] M. Neumann, XPS experiment of SBN:Cr shows more than 90% of Cr are in Cr 31 state, private communication. [21] M. Simon, K. Buse, R. Pankrath, E. KraÈtzig, J. Appl. Phys. 80 (1996) 251. [22] M. Ewbank, R. Neurgaonkar, W. Cory, J. Feinberg, J. Appl. Phys. 62 (1987) 374. [23] A. Gerwens, K. Buse, E. KraÈtzig, J. Opt. Soc. Am. B 15 (1998) 2143. [24] K. Buse, U. van Stevendaal, R. Pankrath, E. KraÈtzig, J. Opt. Am. B 13 (1996) 1461. [25] G. Greten, S. Kapphan, R. Pankrath, Rad. Eff. Def. Solids 135 (1995) 195. [26] M. Gao, S. Porcher, R. Pankrath, S. Kapphan, J. Korean Phys. Soc. 32 (1998) S542. [27] D.L. Wood, J. Ferguson, K. Knox, J.F. Dillon Jr., J. Chem. Phys. 39 (1963) 890. [28] Th. Woike, R. Pankrath, private communication. [29] A. Zeinally, N. Lebedeva, A. Mordukhayev, M. Osman, Ferroelectrics 45 (1982) 83. [30] G. Imbusch, Inorganic luminescence, in: M. Lumb (Ed.), Luminescence Spectroscopy, Academic Press, London, 1978. [31] A. Glass, J. Chem. Phys. 50 (1969) 1501. [32] L. Andrews, A. Lempicki, B. McCollum, J. Chem. Phys. 74 (1981) 5526. [33] F. Agullo-Lopez, C. Catlow, P. Townsend, Point Defects in Materials, Academic Press, London, 1988. [34] G. Blasse, Phys. Status Solidi (a) 20 (1973) K99. [35] D. Crol, G. Blasse, R. Powell, J. Chem. Phys. 70 (1980) 4681. [36] G. Blasse, A. Bril, Z. Phys. Chem. Neue Folge Bd. 57 (1968) 187. [37] T. Volk, M. WoÈhlecke, N. Rubinina, F. Jermann, C. Fisher, R. BoÈwer, Appl. Phys. A 60 (1995) 217. [38] G. Greten, PhD thesis, University of OsnabruÈck, Germany, 1996. [39] G. Blasse, W. Schipper, J.J. Hamelink, Inorg. Chim. Acta 189 (1991) 77. [40] M. Gao, R. Pankrath, S. Kapphan, V. Vikhnin, Appl. Phys. B 68 (1999) 849. [41] Ming Gao, PhD thesis, University of OsnabruÈck, Germany, 1998. [42] O. Schirmer, O. Thiemann, M. WoÈhlecke, J. Phys. Chem. Solids 52 (1991) 185. [43] S. Mckeever, Thermoluminescence of Solids, Cambridge University Press, London, 1988 (p. 105). [44] G. Taylor, E. Lilley, J. Phys. D: Appl. Phys. 11 (1978) 567. [45] F. LoÂpez, M. Aguilar, F. AgulloÂ-LoÂpez, Phys. Rev. B 23 (1981) 3041.
www.elsevier.nl/locate/jpcs
Photoluminescence and thermoluminescence in SBN:Cr crystals M. Gao, S. Kapphan*, R. Pankrath FB Physik, UniversitaÈt OsnabruÈck, 49069 OsnabruÈck, Germany Received 2 March 1999; accepted 20 September 1999
Abstract Cr doping in Strontium Barium Niobate (SBN) crystals is found to enhance the photorefractive response. Absorption bands around 650 nm and below 600 nm can be attributed to Cr in SBN crystals. In addition to XPS experimental results, the appearance of broad R line absorptions at low temperature con®rms that most Cr ions are in the Cr 31 charge state in the SBN crystals. Luminescence bands around 765 nm were observed with different peak positions in nominally pure and in Crdoped SBN crystals. The positions of the maxima in the excitation spectra in SBN:Cr, coinciding with the absorption bands, indicate that this emission band comes from the excitation of Cr 31 ions. Two thermoluminescence (TL) peaks are found in SBN:Cr and nominally pure SBN crystals: one at 88 K and the other around 220 K. The identical spectral distribution of the two TL bands points to the same recombination process following the liberation of two light-induced electron trapping centers: Nb 41 polarons and VIS-centers created at low temperatures. A model of the thermal recovery process involved is proposed. The thermal activation energy for the hopping motion of Nb 41 polarons is estimated to be 0:18 ^ 0:02 eV: For the VIS-centers, the thermal activation energy estimated from their decay dynamics to be 0:36 ^ 0:05 eV (Ming Gao, PhD thesis, University of OsnabruÈck, Germany, 1998), agrees with the activation energy value 0:30 ^ 0:05 eV determined from the TL process. q 2000 Elsevier Science Ltd. All rights reserved. Keywords: A. Oxides; D. Defects; D. Luminescence; D. Optical properties
1. Introduction Strontium barium niobate SrxBa12xNb2O6 (SBN, x 0:61 for the congruent compound, where melt and crystal have the same composition) has been considered one of the most promising materials for a number of non-linear optical applications, like volume holographic storage [1±3] and phase conjugation mirrors [4,5], because of its many advantages over other photorefractive crystals such as BaTiO3 and LiNbO3 [6,7]. SBN crystals exhibit an exceptionally large transverse electro-optic coef®cient r33 420 £ 10212 m=V; making possible a sizable modulation of the refractive index by the electro-optic effect under non-uniform illumination. Furthermore, the photorefractive properties of SBN crystals can be enhanced by suitable dopants. Ce doping is found to increase the photorefractive sensitivity by two orders of magnitude in comparison with pure SBN crystals [7,8]. SBN with Cr and Rh doping shows enhanced photo* Corresponding author. Fax: 149-541-9692670. E-mail address: [email protected] (S. Kapphan).
refractive sensitivity in the red spectral region due to the additional absorption bands of dopants in the red [9±11]. Compared with SBN:Ce, a faster response speed by one order of magnitude was also observed in SBN:Cr [9]. Also, the ferroelectric properties can be altered by doping and composition. Pure congruent SBN crystals possess a ferroelectric phase 4mm at room temperature and the phase transition temperature from paraelectric 4/mmm to ferroelectric 4mm is at about 808C. The composition can vary in a wide range 0:25 # x # 0:75 and strongly in¯uence the phase transition temperature Tc, ranging from 558C for x 0:25 to 2008C for x 0:75 [12]. Additionally, Ce doping also decreases Tc with increasing Ce doping [13]. Therefore, properties related to the phase transition can be controlled by composition or dopants. SBN has one unique fourfold axis and 908 domains do not exist, instead only 1808 domains are permitted, so it is relatively easy to pole the crystals above the phase transition temperature and keep them in a monodomain state at room temperature or below. SBN crystals belong to the tungsten bronze family of crystals with the general chemical formula: (A1)2(A2)4C4(B1)2(B2)8O30 [14]. One unit cell consists of
0022-3697/00/$ - see front matter q 2000 Elsevier Science Ltd. All rights reserved. PII: S 0022-369 7(00)00184-0
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M. Gao et al. / Journal of Physics and Chemistry of Solids 61 (2000) 1959±1971
Wavelength ( nm ) 1000
800
600
400
-1
Absorption Coefficient ( cm )
30 R lines of Cr
25
T=2K
0.4
3+
0.3
E⊥ C
0.2
T=300K
0.1
E//C
20
0.0 1.60
1.65
1.70
1.75
15 SBN:Cr 5000 ppm
pure SBN
SBN:Ce 0.025 wt.%
10 E⊥ C 5
R-lines
E//C E//C
E⊥ C
E//C
E⊥ C
0 1.5
2.0
2.5
3.0
3.5
Photon Energy ( eV ) Fig. 1. Polarized absorption spectra of congruent pure SBN, SBN:Cr (5000 ppm in the melt) and SBN:Ce (0.025 wt% in the melt) at RT. The inset shows the R lines absorption spectra of SBN:Cr at 2 K.
ten BO6 octahedra linked by their corners to form three types of tunnels along the ~c axis of crystals, which are the A1, A2 and C tunnels corresponding to 15-, 12- and 9-fold oxygen coordinated lattice sites. Both B1 and B2 are 6-fold sites located inside the BO6 oxygen octahedra units. In the case of SBN, only ®ve of the six available A1 and A2 sites are occupied by Sr 21 and Ba 21 cations on an average. B1 and B2 sites are completely ®lled with Nb51 cations while C sites are empty. The partially ®lled structure of SBN provides large possibilities to accommodate various dopants from transition metal ions to rare earth ions. The instrumental neutron activation analysis (INAA) of SBN shows Ce 31 ions are occupying Sr/Ba sites and Cr 31 ions are sitting on Nb sites [15,16]. Even though most investigations of SBN crystals have focused on the improvement of crystal quality and the characterization of photorefractive properties of materials, some efforts to discover the microscopic mechanism of photorefractive effect and related light-induced charge transport process have also been made in parallel in order to optimize the applications of photorefractive devices using SBN crystals. The 3 1 majority charge state of dopants (Ce 31, Cr 31concentration . 90%) was determined in accordance from FIR absorption [17], ESR [18], XPS [19,20] and photoconductivity [21]. Photoluminescence (PL) [18]and the INAA [15,16] of SBN and SBN:Ce crystals point to the existence of trace impurities like Fe 31, Cr 31, Er 31, Ho 31, Ta 51 and so on, in the ppm level even in nominally pure crystals. The majority of the photo-excited charge carriers have been determined by laser beam coupling experiments [22] and Hall effect measurements [23] to be electrons. Two light-
induced absorption centers are observed: the VIS-centers in the visible around 600 nm [24] and the other, observable only below 130 K or in reduced SBN crystals in the NIR (around 0.72 eV), which is assigned to Nb 41 polarons [25,26]. The creation of Nb 41 and VIS-centers under illumination is the ®rst step in the charge transfer from bright to dark areas and the build-up of space charge ®elds which modify the refractive index and constitute the basis of the photorefractive effect under non-uniform spatial illumination. The investigation of the build-up and decay of the light induced absorption is therefore of direct relevance for the understanding of the mechanisms of the photorefractive effect. Since Cr doping enhances the photorefractive response of SBN crystals especially in the red spectral region, it is of interest to investigate its optical properties and its role in the light-induced charge transfer process. In this paper we will report about the absorption and PL of SBN:Cr in comparison with the results from SBN:Ce and pure SBN. The thermoluminescence (TL) technique is employed to study the charge trapping centers created under illumination at low temperature. The related light-induced charge transfer process and the thermal recovery process of the trapping centers at elevated temperatures are discussed and a simpli®ed model is proposed.
2. Experimental techniques SBN:Cr crystals with congruent melting composition
M. Gao et al. / Journal of Physics and Chemistry of Solids 61 (2000) 1959±1971
1961
50
E ⊥C
-1
α at 650 nm ( cm )
40
T=300 K 30
20
10
0 0
5000
10000
15000
20000
Cr doping in the melt ( ppm ) Fig. 2. The absorption coef®cient a at 650 nm in SBN:Cr crystals at RT as a function of Cr concentration in the melt. The solid line is the result of a least square ®t from 50 to 7500 ppm Cr doping.
x 0:61 were grown with the Czochralski method by R. Pankrath in the crystal growth laboratory of the Physics Department at the University of OsnabruÈck. Cr2O3 with various doping levels: 50, 100, 150, 250, 500, 750, 1000, 1500, 2000, 3000, 5000, 7500, 10 000, 15 000, 20 000 ppm were added to the melt during the crystal growth. Small temperature gradients along with a pulling rate of 0.8 mm per hour ensured that optical-quality crystals with natural facets and free of striations could be obtained. In order to study the possible in¯uence of poling, some crystals were poled under an electric ®eld of about 10 kV/cm at T 708C: A Fourier spectrometer (Bruker IFS 120HR) was employed to measure the absorption spectra of crystals from the UV to the FIR spectral region using different combinations of light sources, beam splitters and detectors. PL and excitation spectra were measured by a photon counting system with a SPEX1402 double grating monochromator and a cooled RCA31034 GaAs photomultiplier. A high pressure Xenon lamp (ILC, Cermax, 300 W) was used as the excitation source and a SPEX Minimate for the wavelength selection together with variable bandpass ®lters. A Helium bath cryostat (Leybold) was employed in the absorption measurement and the crystals were immersed in the super¯uid helium (2 K). A closed cycle cryostat (Leybold) was used for the PL and TL measurements. A programmable temperature controller could be set to different heating rates for the TL measurement. For the luminescence lifetime measurement, a Xenon lamp, a chopper system and a multichannel scaler (EG&G 914P) were used.
3. Experimental results 3.1. Absorption Fig. 1 shows the typical polarized absorption spectra of pure SBN, SBN:Cr and SBN:Ce crystals. The UV absorption band edge of pure SBN crystals at room temperature lies at about 370 nm with a slight dichroism for extraordinary and ordinary light. In the visible region pure SBN is transparent and shows no absorption. Ce doping shifts the onset of the absorption edge to longer wavelengths and an absorption shoulder around 500 nm appears. This absorption shoulder is attributed to the 4f ! 5d transition of Ce 31 ions, which is an allowed electric dipole transition. The spin±orbit and crystal ®eld splitting of 4f 1 level of Ce 31 ions is not resolved in the visible, but this splitting indeed exists and shows in characteristic absorption bands in the FIR (around 2100 cm 21) [17]. SBN:Cr also shows broad absorption bands around 650 nm (1.9 eV) as well as strong absorption below 600 nm, very similar to the absorption bands of Cr 31 ions in other oxide crystals [27]. The appearance of R line absorptions con®rms the existence of Cr 31, which is consistent with XPS measurement results of SBN:Cr, yielding more than 90% of Chromium in the 3 1 charge state. However, R line absorption in SBN:Cr is structurally broadened even at 2 K (full width at half maximumFWHM: 300 cm 21 or 16 nm) as shown in the inset of Fig. 1. For comparison, the R lines in other oxides like ruby and Ê ) [27]. The bandemerald are very sharp (FWHM: about 1 A width (FWHM) of the absorption band around 650 nm is about 0.3 eV for all Cr-doped SBN crystals. Fig. 2 shows
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M. Gao et al. / Journal of Physics and Chemistry of Solids 61 (2000) 1959±1971
Wavelength ( nm ) 880
860
840
820
800
780
760
740
20000
-1
Photon Count Rate ( s )
pure, λex =380 nm 15000
50 ppm Cr, λex =488 nm 500 ppm Cr, λex =488 nm 1000 ppm Cr, λex =488 nm
10000
2000 ppm Cr, λex =488 nm 10000 ppm Cr, λex =488 nm
5000
0 1.40
1.45
1.50
1.55
1.60
1.65
1.70
Photon Energy ( eV ) Fig. 3. Emission in pure SBN and SBN:Cr with different Cr concentration in the melt at 10 K.
the absorption coef®cient a at 650 nm as a function of Cr concentration in the melt. The roughly linear relationship between a and the concentration CCr in the melt indicates that the effective distribution coef®cient Keff CCr2 O3 in crystal =CCr2 O3 in melt changes very little from 50 to 7500 ppm Cr2O3 doping level. The quantitative determination of the Cr concentration in crystals is in progress [28]. The poling of crystals does not show any measurable in¯uence on the absorption measurements. 3.2. Photoluminescence A strong luminescence band in the NIR is observed in all Cr-doped SBN crystals as shown in Fig. 3. In the earlier literature [18,29], pure SBN crystals are reported to exhibit a similar emission band in the same spectral range (around 760 nm). Our measurements do con®rm the existence of such an emission band around 765 nm in nominally pure SBN crystals. More interestingly, Cr doping increases the intensity of this band very much. For example, a 500 ppm Cr-doped SBN crystal shows about two orders of magnitude stronger emission than pure SBN. The peak position of the emission band shifts to longer wavelength with increasing Cr concentration in the melt, from 765 nm for pure SBN to 780 nm for 10 000 ppm Cr-doped SBN. No R line luminescence could be observed in SBN:Cr crystals. SBN:Ce crystals also show the same emission band but with at least a factor of ®ve lower intensity under the same excitation conditions compared with pure SBN. The poling of the crystals did not show any in¯uence on the PL. The excitation spectra are displayed in Fig. 4. The emission intensity was corrected for the variation of the light
source with exciting energy (number of impinging photons) as well as for detector ef®ciency and for throughput of the grating spectrometers used at the exciting side and at the emission detection side. Above 500 nm, the excitation spectra resemble the absorption spectra, indicating Cr 31 ions actually participate in the excitation processes. For the pure SBN the excitation around the band edge is the most effective for the emission. In essence, the excitation near the band edge can result in an emission in all Cr-doped SBN crystals as well. However, for higher Cr-doped crystals the excitation in the red induces a much stronger intensity in the emission compared with the excitation in the UV band edge, where the penetration depth is limited due to the high absorption value. The temperature dependence of the emission band shape is shown in Fig. 5(a) and the temperature dependence of the integrated intensity normalized at 10 K is shown in Fig. 5(b). The decrease of the integrated intensity with increasing temperature can be interpreted as usual due to competitive non-radiative processes increasing with the phonon population with increasing temperature. The time dependent decay of this emission band after shutting-off the excitation is found to be almost monoexponential with a lifetime of 2.9 ms at 19 K and getting shorter at higher temperatures, as shown in Fig. 6. Zeinally et al. measured the lifetime of the emission to be 1.4 ms at 80 K in nominally pure SBN crystals, also with a decreasing trend of lifetime with increasing temperature [29]. 3.3. Thermoluminescence TL measurements are performed in the following way: SBN:Cr or pure SBN crystals were ®rst cooled from room
M. Gao et al. / Journal of Physics and Chemistry of Solids 61 (2000) 1959±1971
1963
W avelength ( nm ) 800700 600
500
400
300
T=10K
pure SBN
Photon Count Rate ( a.u. )
λem =760 nm
50 ppm Cr λem =766 nm
500 ppm Cr λem =766 nm
2000 ppm Cr λem =775 nm
10000 ppm Cr λem =780 nm
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
Photon Energy ( eV ) Fig. 4. The excitation spectra at 10 K for pure SBN and for SBN:Cr crystals with different Cr concentration in the melt.
temperature to 10 K and then illuminated 10 min at 10 K with light of one speci®c wavelength from a Xenon lamp using an interference ®lter; then after shutting-off the lamp and waiting another 10 min, the crystals were heated at a constant heating rate (5.2 K/min) from 10 to 320 K. The TL measurement results are demonstrated in Fig. 7. Two quite resolved TL peaks emerge in SBN:Cr and nominally pure
SBN crystals: one at 88 K and the other around 220 K, indicating two different charge trapping centers. The illumination with UV light results in a stronger intensity for both TL peaks compared with 488 and 647 nm wavelength light. The ratio among the intensities of three illumination wavelengths (380, 488 and 647 nm) was 1.0:2.1:1.8. The ®rst TL peak has a ®xed peak position at 88 K regardless of the
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M. Gao et al. / Journal of Physics and Chemistry of Solids 61 (2000) 1959±1971
W avelength ( nm ) 840
820
800
780
760
740
1.0
10K
20000
Integrated Intensity ( a.u. )
-1
Photon Count Rate ( s )
880 860
30K
15000
λex=488 nm
60K
90K
10000 120K
(a) 5000
150K 180K 210K
0
1.40
240K
1.45
1.50
1.55
1.60
1.65
1.70
λex=488 nm
0.8
0.6
0.4
(b) 0.2
0.0
0
50
100
150
200
250
Temperature ( K )
Photon Energy ( eV )
Fig. 5. Temperature dependence of emission in SBN:Cr (500 ppm Cr in the melt). (a) Emission band at different temperatures. (b) Integrated emission intensity (from 1.4 to 1.7 eV) as a function of temperature, normalized at 10 K. lexc 488 nm:
illumination wavelength. However, the position of the second TL peak is depending on the illumination wavelength as shown in Fig. 7. The peak position and bandwidth (FWHM) in temperature of the two TL peaks with different illumination wavelengths are listed in Table 1. The possible reason for the shift of the peak position of the second TL band will be discussed in the later section. The spectral distribution of the two TL peaks are shown in Fig. 8. For comparison, the PL emission of this crystal is
also given with normalized amplitude (solid line and dashed line in Fig. 8). The similarity between PL band and the TL spectral distribution indicates that they originate from the same emission process. The time dependence of the two TL peaks at ®xed temperatures are demonstrated in Fig. 9. The crystal was illuminated at 10 K for 10 min with UV light (380 nm) and then heated to 88 K. The temperature of the crystal was kept at 88 K for 30 min and the spectrometer recorded the decay process of the ®rst TL peak at 88 K. The
2.90 2.85
τ ( ms )
2.80 2.75 2.70 2.65 2.60 2.55 2.50 0
30
60
90
120
150
180
Temperature ( K ) Fig. 6. Lifetime of the emission in SBN:Cr (750 ppm Cr in the melt) at different temperatures. The dashed line is a guideline for the eyes.
M. Gao et al. / Journal of Physics and Chemistry of Solids 61 (2000) 1959±1971
1965
Crystal was illuminated
5000
-1
Photon Count Rate ( s )
10 mins at 10K with 380 nm
4000
488 nm 647 nm
3000 214K
2000 88K
229K
1000
0 238K
0
50
100
150
200
250
300
350
Temperature ( K ) Fig. 7. Thermoluminescence in SBN:Cr (750 ppm Cr in the melt) from 10 to 320 K. The intensity ratio of the illumination light 380, 488 and 647 nm is 1.0:2.1:1.8. Heating rate: 5.2 K/min. Detection wavelength: 766 nm.
decay process of the second TL peak at 220 K could be obtained in the same way. The second TL peak shows a more monoexponential decay behavior, indicating a simple ®rst-order kinetics for this peak. The deviation from the monoexponential decay process for the ®rst TL peak points to a more complicated thermal recovery mechanism, as discussed also for the light-induced absorption kinetics of the Nb 41 polarons [40].
4. Discussion 4.1. Absorption and luminescence of Cr 31 ions The preliminary INAA result [15,16] has shown that the concentration of Nb in SBN:Cr decreases with increasing Cr concentration and the concentration of Sr/Ba does not vary with the different Cr doping, which strongly indicates that Cr 31 ions prefer to substitute for Nb 51 ions. In the structure of SBN, Nb ions occupy B1 and B2 sites, which lie inside the distorted oxygen octahedra along the ~c axis of the crystal with slightly different Nb±O distances and different next
nearest neighbor ions. The substitution of Cr 31 for Nb 51 ions will result in the level splitting of Cr 31 in octahedral symmetry. The 3d electron con®guration of Cr 31 is 3d 3. The ground energy level of free Cr 31 ions is 4F and the excited energy levels are 4P, 2G, 2F and so on. In octahedral crystal ®elds, the energy levels of Cr 31 ions will split into ground level 4A2g and excited levels 2Eg, 2T1g, 2T2g, 4T2g, 4T1g, 2A1g and so on [30]. The excited levels could exchange their position depending on the strength of the crystal ®elds. The typical absorption spectra of Cr 31 in octahedral symmetry consist of several broad spin-allowed absorption bands due to transitions like 4A2g ! 4T2g, 4T1g, and a series of sharp lines due to the spin-forbidden transitions: 4A2g ! 2E1g, 2T1g and so on [27]. The peak positions of the broad bands strongly depend on crystal ®elds and vary from material to material. However, the positions of the sharp lines are not sensitive to the crystal ®eld because the energy levels 2E1g and 2T1g do not depend much on the crystal ®eld strength. Surprisingly there is only one broad absorption band at 650 nm resolved in SBN:Cr, as shown in Fig. 1 even though extensive efforts to look for other resolved broad absorption bands by using very thin crystals or low Cr-doped crystals
Table 1 The temperatures at the maximum of the TL bands (peak) and the width of the TL bands in temperature (FWHM) under illumination with different wavelengths: 380, 488 and 647 nm at 10 K Illumination (nm)
380 488 647
TL1
TL2
Peak (K)
FWHM (K)
Peak (K)
FWHM (K)
88 88 88
34 27 25
214 229 238
61 43 40
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M. Gao et al. / Journal of Physics and Chemistry of Solids 61 (2000) 1959±1971
Wavelength ( nm ) 800 795 790 785 780 775 770 765 760 755
88 K 220 K
Illuminated at 10K
-1
TL Count Rate ( s )
4000
8 mins with 380 nm 3000
UV light
2000
1000
1.56
1.58
1.60
1.62
1.64
Photon Energy ( eV ) Fig. 8. The spectral distribution of the two thermoluminescence peaks in SBN:Cr (750 ppm in the melt). The photoluminescence emission band of the crystal at 10 K with the excitation of 380 nm UV light is normalized to the TL maximum at 88 K (solid line) or to the TL maximum at 220 K (dashed line).
have been undertaken. However, the absorption below 600 nm is increasing with increasing Cr doping level. The broad band at 650 nm is similar to the 4A2g ! 4T2g absorption band of Cr 31 in emerald, LiNbO3:Cr and LiTaO3:Cr oxide crystals from the shape and polarization dependence [27,31], and is therefore assumed to come from the same transition of Cr 31 ions in octahedral symmetry of SBN crystals. The second broad absorption band due to the transition 4 A2g ! 4T1g cannot be resolved as a separate band. The structural disorder of SBN crystals alone cannot account for such a behavior since this band of Cr 31 ions is quite resolved even in oxide glasses with much more disorderly structures [32]. As long as Cr 31 lies inside the oxygen octahedra, the splitting of Cr 31 levels can be well explained by crystal ®eld theory. However, the absence of the resolved 4 A2g ! 4T1g band could result from the following reasons. One possibility is that the level 4T1g lies already inside or higher than the bottom of the conduction band of SBN crystals, and the transition 4A2g ! 4T1g of Cr 31 ions results in a charge transfer transition 4A2g(Cr 31) ! 4d(Nb 51). As a result, the onset of the absorption edge is shifted to the longer wavelength with increasing Cr concentration,
which is consistent with our experimental observation. The second possibility lies at the substitution of Cr 31 for different sites. If some Cr 31 (,1%) replaces not only Nb 51 sites (B1 and B2) but also Sr 21/Ba 21 (A1 and A2) or even C sites, the situation will become very complicated. The level splitting of Cr 31 in 15-, 12- and 9-fold co-ordinations has not been extensively studied so far and it could be rather different from 6-fold (octahedral) and 4-fold (tetrahedral) co-ordinations. The energy levels of Cr 31 ions in these 15, 12- and 9-fold co-ordinations could mix with the levels of Cr 31 like 4T1g in 6-fold co-ordination (octahedral). As a result, the absorption band due to the transition 4 A2g ! 4T1g of Cr 31 ions in octahedra could be overlapping with other transitions of Cr 31 ions in higher-fold co-ordinations and therefore become unresolvable. At present no independent experimental evidence for different Cr 31-sites has been found and the second possibility mentioned above, is therefore considerate to be not very likely. The oscillator strength f for the transition 4A2g ! 4T2g can be estimated from the absorption spectra. The integrated absorption for this transition in SBN:Cr (Cr ion concentration:1:7 £ 1021 cm23 for 5000 ppm Cr doping,
M. Gao et al. / Journal of Physics and Chemistry of Solids 61 (2000) 1959±1971
1967
9
TL Count Rate ( Log scale )
8
T=88K
7
6
T=220K
5
4
3 0
3
6
9
12
15
Time ( mins ) Fig. 9. Time dependence of the two TL peaks at two temperatures in SBN:Cr (750 ppm in the melt). The circles indicate the experimental data for ldet 770 nm: The dashed lines are the ®tting results with monoexponential functions and the solid line is the ®tting result with Eqs. (7) and (8).
where the concentration of Cr in crystal is regarded as same as in the melt) can be calculated to be Z aE ' dE 3:5 cm21 eV Z
aE k dE 1:8 cm21 eV
Based upon the Smakula equation [33], the oscillator strengths can be derived as f 4 A2g ! 4 T2g E 7:5 £ 1026 ' f 4 A2g ! 4 T2g A 3:8 £ 1026 k The integrated intensity ratio between R lines and 4T2g bands is about 5 £ 1023 ; indicating a two or three orders of magnitude smaller oscillator strength for the transition 2 A2g ! 2E1g. The R lines absorption bands in SBN:Cr are extraordinarily broad even at 2 K, re¯ecting the structural disorder of SBN crystals. The structural disorder of SBN crystals is also re¯ected by a broad Raman linewidth for NbO6 in SBN crystals [18]. Different occupation sites of Cr 31 in SBN crystals can also lead to several unresolved lines. Similar to the situation in LiNbO3:Cr [30,31], the
zero-phonon level of the 4T2g bands in SBN:Cr lies close to the 2E1g level. If an electron is excited from the ground state 4A2g to the excited state 2E1g, the electron in this excited state could relax to the zero-phonon level of the 4 T2g band through a non-radiation process and then the electron will transit from this state to the ground state or to the other excited metastable states. This may explain the absence of the R line luminescence in the SBN:Cr crystal. The origin of the emission band in the NIR in nominally pure SBN crystals is not clear. Zeinally et al. [29] assigned the emission around 760 nm to the de-excitation from 4d orbital of Nb to 2p orbital of O in NbO6, but Giles et al. [18] regarded this emission as the transition of unwanted Cr 31 in SBN crystals in trace amounts of Cr in the ppm level. Our luminescence measurement in SBN:Cr crystals suggests that the excitation of this emission band indeed originates from Cr 31 ions in SBN. The direct corresponding relationship between excitation spectra and absorption spectra in SBN:Cr, especially from 500 to 720 nm indicates that this emission band could come from the de-excitation process of Cr 31 ions: 4T2g ! 4A2g. A similar luminescence in the same spectral range is observed in LiNbO3:Cr and LiTaO3:Cr [31], which is due to the transition of Cr 31: 4T2g ! 4A2g. The luminescence measurements in
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M. Gao et al. / Journal of Physics and Chemistry of Solids 61 (2000) 1959±1971
CB Ex
Eh
Nb4+ Polaron Emission
VIS-Centers
Cr4+ Cr3+
VB Fig. 10. Model of the thermal recovery processes in SBN:Cr crystals. Ex is the average thermal activation energy for the VIS-centers and Eh is the energy needed for the hopping motion of the Nb 41 polarons.
other niobates [34±37] show that the intrinsic emission band of NbO6 usually lies around 500 nm. Actually, upon band edge excitation, a very weak additional emission around 480 nm was observed in pure SBN crystals [38], which might be the emission of NbO6 in SBN crystals. The INAA data show that Cr with trace amount in the ppm level exists even in nominally pure SBN and SBN:Ce crystals and that the Cr amount decreases with Ce doping [15,16]. Such a small amount of Cr is suf®cient for an observable luminescence. However, the lifetime of the emission of Cr 31 ions: 4T2g ! 4A2g is usually in the range of ms [31]. The quite long lifetime (in ms range) of the emission in SBN:Cr crystals leads one to consider other emission mechanisms. Since the excitation originates from Cr 31 ions and the appearance of light-induced Nb 41 polarons at low temperature is a well-known fact [25,26], one can also assume that the direct recombination of the electrons from the Nb 41 polarons with Cr 41 ions results in the emission band around 765 nm. However, the lifetime of Nb 41 polarons after switching-off illumination is in the range of 100 s at T , 60 K [28], indicating that the direct recombination of the electrons from Nb 41 polarons with Cr 41 ions contributes only to a small fraction of the total emission intensity. The peak position shift of the emission bands with increasing Cr concentration could result from the increasing re-absorption of the emission by Cr 31 centers in SBN in this spectral region (around 760 nm). Although the absorption due to the transition of Ce 31 ions: 4f ! 5d is strong in SBN:Ce crystals, no luminescence is observed for the de-excitation: 5d ! 4f of Ce 31 ions. Blasse et al. [39] discussed the quenching of the luminescence of Ce 31 ions in detail and proposed that Ce 31 ions may be expected to luminesce only in host materials which have their optical absorption edge in the far ultraviolet and the quenching by electron transfer could rapidly eliminate the luminescence of Ce 31 ions if the optical absorption edge
of host materials shifts to the low energy side. The absorption band of Ce 31 ions in SBN:Ce is very close to the UV absorption edge of the pure crystal, suggesting that the ®rst excited state 5d 1 of Ce 31 ions lies nearby the bottom of the conduction band. In this case an electron transfer transition from the excited state level of the Ce 31 ions to the conduction band (4d orbital of Nb 51) becomes possible, quenching the luminescence of Ce 31 ions. 4.2. Thermal recovery process The TL results show that two charge trapping centers exist in SBN crystals under illumination, which is in good agreement with the light-induced absorption results. Since one trapping center is known as the electronic Nb 41 polaron, the same spectral distribution of two TL peaks indicates that the other charge trapping center is also an electron trapping center. The light-induced Nb 41 polaron shows a broad absorption band around 0.72 eV [25,26]. Actually most light-induced Nb 41 polarons are found to decay even at 2 K after the illumination is switched off, indicating that most Nb 41 polarons are formed nearby Cr 41 recombination centers and the decay of polarons is due to the recombination of the electrons from the Nb 41 polarons with Cr 41 by tunneling processes or phonon-assisted hopping processes [40]. However, there are always several percent of Nb 41 polarons remaining very stable at low temperatures (i.e. 2 K) for a long time after switching off the illumination, indicating that this part of polarons are created far away from their recombination centers. At temperatures above 70 K, the hopping motion of polarons tends to dominate the thermal recombination process and therefore the recombination of Cr 41 ions with electrons from Nb 41 polarons being liberated far away from Cr 41 centers by a polaron hopping process at increasing temperatures results in the ®rst TL peak at 88 K. Here we are dealing with a unique
M. Gao et al. / Journal of Physics and Chemistry of Solids 61 (2000) 1959±1971
1969
9
Experiment
8
curve fitting ( total 7
glow peak method )
ln(A/I)
6 5 4
TL2 ( 229K )
3 2 4.4
4.8
5.2
5.6
6.0
-1
1000/T ( K ) Fig. 11. Analysis of the second TL peak with the total glow peak method (see text). Previous low temperature illumination was performed with l 488 nm: The slope of the straight line yields a value of the average thermal activation energy: Ex 0:30 ^ 0:05 eV:
trapping center, the polaron, and the thermal recovery is due to the hopping motion of the polaron instead of the normal band transport. The second electron trapping center (VIScenter) has a broad absorption band in the VIS (around 2.1 eV or 600 nm) [24,26]. The physical nature of this center is not clear so far. At room temperature this light-induced absorption center is weak and dif®cult to observe, but it becomes more pronounced with decreasing temperature. The decay process of this light-induced absorption center after the illumination is switched off is thermally activated with an activation energy estimated to be Ex 0:36 ^ 0:05 eV [41]. The decay rate is practically zero at T , 100 K; but is increasing with rising temperature. The thermal recovery process of this center results in the second TL peak around 220 K. The identical spectral distribution of the two TL processes and the agreement with the PL emission show that the two TL peaks come from the same emission process, i.e. the direct recombination of electrons from the Nb 41 polarons with the Cr 41 recombination centers. Fig. 10 brie¯y depicts a model which could explain the thermal recovery processes involved for the TL emission in previously illuminated SBN:Cr crystals at low temperature. Two channels of the thermal release of the electrons from the VIS-centers are shown in this picture: one with the electrons transported in the conduction band and the other with the thermal dissociation of the VIS-center as the ®rst step and then the hopping motion of Nb 41 polarons and subsequent recombination of electrons and Cr 41 centers. The latter model part could suggest to consider the VIS-centers as bipolarons. Indeed there are many similarities in the optical properties and thermal stability with polarons and bipolarons in LiNbO3 crystals [42]. However, more experimental
and theoretical considerations are needed to ascertain these assumptions for the SBN systems. The intensities of the two TL peaks strongly depend on illumination conditions, respectively the photon energy. An electron absorbing a photon of higher energy (in case of illumination with UV light like 380 nm) is transferred to higher conduction band states and has more possibilities to migrate a longer distance in the conduction band before it is ®nally trapped. We should keep in mind that only those Nb 41 polarons far away from Cr 41 recombination centers contribute to the ®rst TL peak and the electrons with higher energy have more chances to form such polarons. For an electron absorbing a photon of lower energy (in case of illumination with red light like 647 nm), the possibility to form a polaron far away from Cr 41 ions will be small. Therefore, the number of Nb 41 polarons effective for TL, created under illumination of UV light (380 nm) is much larger than that under illumination with visible light (488 and 647 nm) as shown in Fig. 7. A similar situation exists for the second TL peak. Since trapping centers around the original Cr 31 ions are limited, electrons with higher energy have more chances to walk away and be trapped in other positions, forming the VIS-centers responsible for the total intensity increase of the second TL peak. One interesting feature for the second TL peak is that the peak position shifts to lower temperature with increasing TL intensity and the bandwidth FWHM is also broadened in parallel. The in¯uence of the heating rate can be excluded because we use the same heating rate for all three curves in Fig. 7. One very probable situation is that the thermal depth (or the thermal activation energy) for the VIS trapping centers is not a single level, but rather, there is an energy distribution of
1970
M. Gao et al. / Journal of Physics and Chemistry of Solids 61 (2000) 1959±1971
the thermal activation energy which varies with the concentration of the VIS-centers. Energy distributions of thermal depths for trapping centers have been observed in amorphous materials and polymeric compounds [43]. Such a picture usually exists in the situation where there are random differences among the con®gurations between nearest neighbors like in bond angles and positions of nearest neighbors, with the result that the thermal depth tends to be smeared out. The structural disorder of SBN crystals may account for the energy distribution of thermal depth for the VIS-centers involved in the second TL peak. According to Urbach's formula [43], E Tm =500 eV; where E is the activation energy and Tm is the temperature in K at the maximum of a TL band, the thermal activation energy for the VIS-centers is estimated to be about 0.44 eV. A more accurate method called total glow peak method [43,44] can be employed to determine the average thermal activation energy for the VIS trapping centers. In this method for the ®rst-order kinetics process one plots ln(A/ I) versus the reciprocal of temperature 1/T, where I is the TL intensity at one ®xed temperature T, and A is the area of TL curve from that temperature to the end of the peak. The plot should yield a straight line with a slope of E/k. Additionally one should take into account the temperature dependence of the emission for the recombination process since thermal quenching increases with rising temperature as shown in Fig. 5(b). Fig. 11 shows the analysis using the total glow peak method with consideration of the temperature dependence of the integrated emission intensity. The average thermal activation energy Ex 0:30 ^ 0:05 eV from this analysis is smaller, as expected, than the estimated value from the Urbach's law. This value of Ex can be compared with light-induced absorption measurements which yield an average thermal activation energy with the value Ex 0:36 ^ 0:05 eV for the VIS trapping centers [41]. To analyze the thermal recovery process for the ®rst TL peak, we follow the treatment described in Ref. [45]. For a luminescence recombination process Nb41 polaron 1 Cr41 ! Nb51 1 Cr31 1 hn; the rate equation is ! dnp dnCr41 r0 2x 1 1 p np nCr41 dt dt pDp t
1
Here np and nCr41 are the concentrations of Nb 41 polarons and Cr 41 ions, respectively; r0 denotes the critical interaction distance for this recombination process and x 4pr0 Dp with Dp being the diffusion coef®cients for Nb 41 polarons. Cr 41 ions are considered localized here and only Nb 41 polarons are mobile. Under the approximation pDp t q r02 ; dnp 24pr0 Dp np nCr41 dt
2
Let us consider the situation where all the electrons trapped in polarons come from the photoionization of Cr 31, and we
then have nCr41 np 1 N0
3 41
Here N0 is the concentration of Cr centers before illumination, together with Cr 41 centers whose electrons are excited by illumination and trapped by VIS-centers. By using the random walk model the diffusion ef®cient Dp can be written as E Dp 16 a2 n0 exp 2 h 4 kT where a is the random hopping distance, n 0 the average hopping frequency, Eh the activation energy for hopping motion, k Boltzmann's constant and T is the absolute temperature. We combine Eqs. (2)±(4) and obtain dnp 2hnp np 1 N0 dt with
h
2 3
E pr0 a2 n0 exp 2 h kT
5
6
being a constant for one ®xed temperature. The solution of Eq. (5) is given by np t "
N0 11 np 0
#
N0
7
exp 2hN0 t 2 1
The decay of the TL intensity ITL at one ®xed temperature is proportional to 2dnp(t)/dt ITL / 2
dnp t hnp np 1 N0 dt
8
Eqs. (7) and (8) are used to ®t the time dependence of the ®rst TL peak at 88 K and indeed a better agreement with the experimental result is obtained (solid line) than using a monoexponential function (dashed line) as shown in Fig. 9. If np p N0 ; Eq. (5) becomes a ®rst-order kinetic equation with a monoexponential solution. If np q N0 ; a secondorder kinetics is reached. The agreement of the relationship between lnI , t for the ®rst TL peak (as shown in Fig. 9) and the small deviation from a straight line indicates that the kinetics governing the thermal recovery process by polaron hopping motion is close to a ®rst-order kinetic process. According to the Urbach's formula, the activation energy for the polaron hopping motion can be inferred from the TL data: Eh 0:18 ^ 0:02 eV; which agrees with the value estimated from the optical transition 0:72=4 0:18 eV [40]. 5. Conclusion Broad absorption bands due to the Cr-doping have been observed around 650 nm and below 600 nm in SBN:Cr, and these absorption bands extend and enhance the spectral response of SBN crystals to the red. The R line absorption
M. Gao et al. / Journal of Physics and Chemistry of Solids 61 (2000) 1959±1971 31
of Cr ions in SBN:Cr is extremely broad (FWHM: 300 cm 21) even at 2 K, re¯ecting the structural disorder of SBN crystals. Under UV or visible light excitation, an emission band appears in the NIR (around 765 nm) with slightly different peak positions in pure and different Cr-doped SBN crystals. No R line luminescence was observed in SBN:Cr crystals. Although the absorption of Ce 31 ions in SBN is strong, a luminescence due to the transition of Ce 31 ions: 5d ! 4f has not been found. Two TL peaks are observed in the temperature range from 10 to 320 K. The ®rst TL peak (at 88 K) comes from the thermal hopping motion of Nb 41 polarons created far away from Cr 41 centers and the recombination of their electrons with Cr 41 ions. The thermal activation energy for the hopping motion of Nb 41 polarons is estimated to be: Eh 0:18 ^ 0:02 eV: The second TL peak (around 220 K) originates from the thermal liberation of electrons from another electron trapping centers (VIScenters) and their recombination with Cr 41 centers. The average thermal activation energy of the VIS-centers is estimated to be: Ex 0:30 ^ 0:05 eV; agreeing well with the value estimated from the light-induced VIS absorption results [41]. Acknowledgements This work is supported by DFG (SFB225, C7). We thank Ms Eden and Mr Aulich for stimulating discussions and assistance. Ming Gao thanks the VW Foundation for a doctoral fellowship. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]
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