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A luminescence and absorption spectroscopy study of KH2PO4 crystals doped with Tl+ ions
Optical Materials 34 (2012) 1522–1528
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A luminescence and absorption spectroscopy study of KH2PO4 crystals doped with Tl+ ions I.N. Ogorodnikov a,⇑, V.A. Pustovarov a, V.M. Puzikov b, V.I. Salo b, A.P. Voronov b a b
Ural Federal University, 19, Mira Street, 620002 Ekaterinburg, Russia STC Institute for Single Crystals NAS of Ukraine, 60, Lenina Avenue, 61001 Kharkov, Ukraine
a r t i c l e
i n f o
Article history: Received 27 August 2011 Received in revised form 28 February 2012 Accepted 15 March 2012 Available online 10 April 2012 Keywords: Luminescence spectroscopy Luminescent materials Scintillation crystals Potassium dihydrogen phosphate Thallium ion
a b s t r a c t We report experimental study on luminescence and optical properties of single crystals KH2PO4 (KDP) doped with Tl+ ions (KDP:Tl) carried out at 10–480 K. The 4.5 eV photoluminescence (PL) of KDP:Tl originates from radiative electronic transitions 3P1 ? 1S0 in the Tl+ ions upon excitation by UV-photons, X-rays, and electron beam. This luminescence can be induced by direct photoexcitation, or through the recombination process with participation of the lattice defects located in the vicinity of Tl+ ion. These excitation mechanisms lead to different temperature behavior of the luminescence intensity. The PL excitation spectra of KDP crystals containing a small amount of the Tl+ ions (0.001–0.008 wt.%) comprises peaks at 5.7–5.8, 6.9, and 7.3–7.4 eV, corresponding subsequently to the A-, B- and C-optical transitions in Tl+. The PL time response has single-exponential behavior with an average lifetime of s = 280 ± 8 ns. Under exposure to ionizing radiation (X-rays, or electron beam) the intensity of the Tl+ luminescence increases depending on the exposure time. The origin of this phenomenon was associated with creation of defects in the hydrogen sublattice of KDP. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Potassium dihydrogen phosphate KH2PO4 (KDP) in the crystalline form is a well known optical material, which was successfully used in many areas of applications, including nonlinear and integrated optics, laser technology. This material is successfully utilized in many kinds of optical devices such as non-linear optical elements, electro-optical polarizers, solid state detectors and transformers of radiation, operating in a broad spectral range from the middle infrared to the vacuum ultraviolet (VUV) spectral regions [1]. It is important that dihydrogen phosphate crystals (in particular, KDP) are a very unique inorganic material with a high content of hydrogen ions (protons) in the crystal lattice. In this regard, KDP holds great promise for use in radiation detectors of neutrons. Many research works last 30 years were devoted to studying the luminescent properties of KDP in the visible and ultraviolet (UV) spectral ranges, see e.g. [2]. These results show that KDP at temperatures below 20 K exhibits an intense intrinsic luminescence, which however is subject to thermal quenching when heated. The main reason is related to the thermal creation of defects in the hydrogen sublattice of KDP, which are the centers of an efficient non-radiative recombination for free electrons and holes [3,4]. ⇑ Corresponding author. E-mail address: [email protected] (I.N. Ogorodnikov). 0925-3467/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.optmat.2012.03.018
In this connection, many research works have been focused on a study of the doped KDP crystals. However, the concentration of intrinsic defects in the hydrogen sublattice is far exceed possible concentration of any impurity in KDP. This makes extremely inefficient any transfer of energy from the matrix to the impurity at temperatures above 20 K. The thallium impurity in KDP seems to be the only dopant, which able to accept the energy transferred after non-radiative recombination of electrons and holes at the defects in the hydrogen sublattice of KDP [5]. The thallium impurity ion is known as an activator that forms s2 centers in cubic alkali halide crystals [6–8]. Owing to the large optical transparency band (Eg 8.8 eV), KDP is a unique noncubic host system, where the s2 centers of Tl+ ion can exhibit several UV-absorption bands, which allow study of various interactions between the impurity ion and the host lattice [7,9]. Until today, Tl+ center in KDP is the only center type s2 in the non-cubic hostlattice, where are possible all the s2 ? sp transitions corresponding to A, B, and C absorption bands in alkali halide crystals [5]. In previous papers we started a detailed study of undoped KDP crystals by the means of the low-temperature luminescence and optical VUV-spectroscopy with a time-resolution [3,4,10]. Detection of practically important scintillation properties of crystals KDP:Tl [11,12] stimulated further detailed studies of this material. The present work continues theses studies and focuses on spectroscopy of the KDP crystals doped with Tl+ ions. The main goal of this paper is to study the luminescent and optical properties
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of ferroelectric crystals KH2PO4:Tl over the broad temperature range (10–450 K) using the method of luminescence UV and VUV spectroscopy with a time-resolution. 2. Experimental details Single crystals of KDP containing 0.001, 0.008, and 0.1 wt.% of Tl+ ions were grown at the STC Institute for Single Crystals NAS of Ukraine (Kharkov, Ukraine) [11–13]. The samples measuring 7 7 2 mm3 were cut from the perfect part of the grown crystal and their surfaces were polished to the high optical quality. The present study was carried out mainly by the means of the low-temperature luminescence VUV spectroscopy. Photoluminescence (PL) spectra in the energy range of Em from 1.2 to 6.2 eV, PL excitation (PLE) spectra in the broad energy range of hm from 3.7 to 15 eV (0.32 nm resolution) were measured at 10 and 293 K for these crystals at the SUPERLUMI experimental station of HASYLAB [14] using synchrotron radiation. Samples were mounted in a sample holder attached to a He-flow cryostat with vacuum not less than 7 1010 Torr. At the storage ring DORIS the full width at half maximum (FWHM) of SR pulses was 130 ps with the repetition period of 192 ns. Such pulses excitation enables the recording of spectra within a time-window correlated with the arrival of SR pulses. In the present experiments we recorded time-resolved spectra within two independent time-windows (TWs) set for detection of luminescence signal within 0.5–2.3 ns (TW1) and 14–58 ns (TW2) relative to the beginning of the SR pulse. Timeintegrated (TI) spectra were recorded within the full time range available between two sequential excitation pulses, viz. 192 ns. The 0.3 m ARC Spectra Pro-300i monochromator and R6358P (Hamamatsu) photomultiplier were used as a registration system. The PLE spectra were corrected to an equal number of photons incident on the sample using sodium salicylate. The luminescence characteristics in the temperature range from 90 to 480 K were measured under excitation with either X-rays (BSW2:Cu X-ray tube with Ua = 40 kV and Ia = 10 mA), or electron beam (Ee = 180 keV, I = 800 A/cm2, t = 3 ns), or UV light (400 W deuterium discharge lamp with a continuous UV emission spectrum). Spectra of the X-rays induced luminescence (XRL), spectra of the pulsed cathodoluminescence (PCL), and decay kinetics of PCL were recorded by the means of the MDR-23 monochromator, the FEU-106 photomultiplier, operating in a photon counting mode, and the digital TDS-1030 oscilloscope. All measurements were performed in vacuum (residual gas pressure 104 Pa). The sample chamber was a quick response vacuum cryostat with quartz windows, which made it possible to control the sample temperature in the range of 80–500 K.
Fig. 1. The optical absorption spectra of KDP:Tl for the Tl+ concentrations of 0.001 – (1), 0.008 – (2), and 0.1 wt.% – (3) measured at 293 K.
we have measured all the spectra (TW1, TW2 and TI), but in each case they were identical in the profile. All the presented PL and PLE spectra were normalized in intensity to unity at the maximum. Fig. 2 shows the PL emission spectra of KDP:Tl under excitation with the 5.75 eV photons (the A-band excitation) measured at 10 and 293 K. A Stokes-shifted luminescence band was observed in the energy interval of 3.50–5.0 eV. The luminescence intensity of this band depends on the concentration of thallium ions in the same way as the absorption spectra: the higher the concentration, the greater the intensity. However, the band shape does not depend on the concentration of thallium ions in the entire investigated concentration range. In this regard, Fig. 2 shows only the luminescence spectra for KDP:Tl (0.001 wt.%). At 10 K the PL emission band has almost a Gaussian shape with a maximum at 4.50 eV and FWHM of 0.32 eV. The low-temperature band-shape does not depend on the excitation energy value. On heating to room temperature the PL emission intensity decreases by a factor of about eight. In contrast to the low-temperature case, at room temperature the band-shape and its energy position depend on excitation energy, Figs. 2 and 3. Depending on the excitation energy, the position of the PL band is red- or blue-shifted from its low-temperature position, and bandwidth is broadened to FWHM of 0.55–0.58 eV, Table 1. Decomposition shows a low intensive sub-band at 3.9–4.25 eV, which position and intensity depend on both the temperature and excitation energy. Its contribution to
3. Experimental results and discussion 3.1. UV and VUV spectroscopy of Tl+ ion in KDP Fig. 1 shows the optical absorption spectra of KDP:Tl measured at room temperature. The different curves represent the absorption spectra of KDP:Tl crystals with different concentrations of the Tl+ ions. The single absorption band at 5.76 eV (FWHM = 0.27 eV) dominates in the optical absorption spectra over the energy region from 1.2 to 6.2 eV. The band intensity depends strongly on the Tl+ impurity concentration. The label ‘A’ depicts the absorption band corresponding to the A-band of Tl+ centers measured in alkali halide crystals for the impurity positions of the Oh symmetry. Bearing this in mind, the 5.76 eV absorption band in KDP:Tl can be assigned to a direct photoexcitation of the Tl+ impurity ion. Discussing the luminescence of KDP:Tl, we present in figures only the time-integrated PL and PLE spectra. Although in each case,
Fig. 2. The photoluminescence spectra of KDP:Tl (0.001 wt.%) at 10 – (1) and 293 K – (2). The spectra were normalized in intensities to unity at the maximum. Dashed lines show the principal Gaussians, solid lines are the results of approximation, open and black circles depict the experimental data.
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Fig. 3. The PL emission spectra of KDP:Tl (0.001 wt.%) at 293 K. The spectra were normalized in intensity to unity at the maximum.
Table 1 Parameters of the PL emission bands of KDP:Tl (0.001 wt.%). hm (eV)
Em (eV)
DE (eV)
S (%)
10
5.75
4.25 4.50
0.26 0.32
10.2 89.8
10
6.90
4.25 4.50 5.00
0.20 0.32 0.34
13.0 85.5 1.5
293
5.75
3.90 4.45
0.57 0.55
7.5 92.5
293
6.90
4.05 4.55
0.55 0.58
3.3 96.7
T (K)
Note. hm is excitation energy; Em is energy position of the band maximum; DE is FWHM; S is the band contribution into the total light yield of the observed PL emission spectrum.
the total light yield ranges from 3.3% to 10.2%. In one case, a very minor band at 5.0 eV has been added to the best fit, Table 1. We focus our discussion at the properties of the dominant PL emission band. Fig. 4 shows the PLE spectra for the 4.50 eV emission band of KDP:Tl measured for the Tl+ concentrations of 0.001, 0.008, and 0.1 wt.% at 10 K. The PLE spectra were measured over the energy range from 4.7 to 15 eV, however we found no excitation bands in the energy range of the band-to-band transitions. The sufficient intensities were detected only between 5.5 and 8 eV. This means that the energy transfer from the host lattice to the impurity Tl+ ions has a very insignificant efficiency, as it was found earlier by us for various lattice defects in crystal KDP [10]. In this connection, only relevant part of each PLE spectrum is shown in Fig. 4. The set of the PLE bands matches well the known optical absorption bands of KDP:Tl single crystals containing a small amount of the Tl+ ions and exhibiting five characteristic polarized absorption bands Az, Axy, Bxy, Cxy and Cz in the ultraviolet region [5,9]. Fig. 4 shows also a possible interpretation of the observed peaks in the PLE spectra. The PLE spectrum of KDP:Tl (0.001 wt.%) comprises three dominant peaks (Fig. 4a), which match well the energy positions of the Axy, Bxy, and Cxy absorption bands reported in [5]. With an increase in concentration to 0.008 wt.%, the PL intensity increases respectively, but the structure of the PLE spectrum remains unchanged, except for increasing the relative intensity of the Axy peak, Fig. 4b. Further increase in concentration to 0.1 wt.% does not only increase the luminescence intensity, but it changes the structure of the PLE spectrum. New
Fig. 4. The PL excitation spectra recorded monitoring emission at 4.5 eV in KDP:Tl at 10 K for the Tl+ concentrations of 0.001 – (a), 0.008 – (b), and 0.1 wt.% – (c). The spectra were normalized in intensity to unity at the maximum.
peaks appear in the PLE spectrum. Three of them are comparable to the Az, Bz, and Cz absorption bands (Fig. 4c), however the 6.0 eV PLE peak has no correspondence with the known absorption peaks reported in [5]. A high-energy slope of the supposed Cz peak overlaps with the low-energy tail of the KDP host absorption, so its shape can be significantly distorted. When heated to 293 K, the PLE-spectra change in the profile, they broaden and become less resolved, Fig. 5. A new broad excitation band appears at room temperature in the energy range above 7.5 eV. This PLE band is located in the energy range of the low energy tail of the KDP host absorption, but its relative intensity depends strongly on the thallium concentration, Fig. 5. This can indirectly point to the creation of impurity-bound excitons in KDP:Tl. In this connection, the 7.5–9 eV band in the PLE spectra
Fig. 5. The PL excitation spectra of KDP:Tl at 293 K recorded monitoring emission at 4.5 eV for the Tl+ concentrations of 0.001 – (1), 0.008 – (2), and 0.1 wt.% – (3). The spectra were normalized in intensities to unity at the maximum. Curve (4) shows the energy position of the low energy tail of the KDP host absorption in accordance with the data of Baldini et al. [15] and Ogorodnikov et al. [16].
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of KDP:Tl can be tentatively compared with the D-band in the PLE spectra of alkali halide crystals doped with thallium, see e.g. [7]. The absence of this band in the PLE spectra of KDP:Tl at 10 K may be due to the existence of a competing process of self-trapping of excitons at low temperatures, as it was shown previously for undoped KDP crystals [4,10]. The 4.5 eV luminescence can be efficiently excited upon excitations onto the A, B and C absorption bands associated with the impurity Tl+ ions in KDP (Fig. 3). The PL decay kinetics of KDP:Tl (0.001 wt.%) upon selective photoexcitation at the A-, B-, and C-bands at room temperature obeys a single-exponential behavior, Fig. 6. However, the decay time exceeds the time interval between two sequential excitation pulses, viz. 192 ns. In this connection, the observed lifetime value showed little change for varying the experimental conditions. The average value of the lifetime was estimated as 280 ± 8 ns. Fig. 6 shows an example of the PL decay kinetics recorded monitoring emission at 4.5 eV upon excitation at 5.7 eV. The luminescence time-response at 10 K in our measurements appears as a pedestal, indicating a time-constant of the microsecond and millisecond timeranges, Fig. 6. Temperature dependence of the PL intensity was recorded in the range of 90–480 K monitoring emission at 4.5 eV upon selective photoexcitation at 5.6 eV from the laboratory light source, Fig. 7. Temperature dependence of the PL intensity shows two maxima, the first one is located in the vicinity of the ferroelectric phase transition of KDP at 123 K. The composite band origin may cause the peculiar behavior of intensity of the 4.5 eV emission band near the ferroelectric phase transition temperature. The second maximum is located at 325 K, its possible origin will be discussed in Section 3.2. The final temperature quenching of PL intensity occurs at 475 K, however the complicate temperature behavior of the luminescence intensity does not obeys the Mott law, Fig. 7. More likely, there are superposition of several thermally stimulated processes responsible for this thermal quenching of the PL intensity. Fig. 8 summarizes all the discussed data and shows the schematic configuration coordinate diagram of energy levels for the Tl+ ion in KDP lattice. For the simplicity, only discussed levels are shown. The EA label depicts an activation energy for an intracenter temperature quenching of the 4.5 eV luminescence, which occurs at 475 K. From Fig. 8 it follows that the 4.50 eV luminescence band should be ascribed to a radiative transition from the 3P1 excited state to the 1S0 ground state of the Tl+ ions (A-transition) since there is a strong response on the A-absorption band of Tl+ ions. The expected high-energy transitions either 3P2 ? 1S0 (B-transition), or 3P3 ? 1S0 (C-transition) are not inherent to the Tl+ ion in
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Fig. 7. Temperature dependencies of steady-state intensities of PL upon excitations at hm = 5.6 eV – (1) and XRL – (2) recorded monitoring emission at 4.5 eV; and spectrally integrated TL glow curve recorded at heating rate of 0.3 K/s – (3) for KDP:Tl (0.001 wt.%). The intensities were normalized to unity at the maximum, the vertical dashed line corresponds to Tc = 123 K.
Fig. 8. Schematic configuration coordinate diagram of energy levels for Tl+ ion in KDP lattice. For the simplicity, only discussed levels are shown.
the KDP host lattice. In fact, the high-energy transitions would be expected in the PL emission spectrum at higher energies from 4.5 to 7.0 eV. One can envision that the B- and C-transitions occur through the fast nonradiative relaxations from 3P2- and 3P3-highenergy excited states onto the 1P1 lowest excited state of the Tl+ ion. The proper 3P1 ? 1S0 radiative transition occurs at the later stage. 3.2. Luminescence upon exposure to ionizing radiation
Fig. 6. The PL decay kinetics of KDP:Tl (0.001 wt.%) at 10 – (1) and 293 K – (2) recorded monitoring emission at 4.5 eV upon excitation at 5.7 eV. The curves were normalized in intensities to unity at the maximum.
The luminescence spectra were measured over the broad energy range from 1.5 to 8 eV upon excitation with either X-rays, or electron beam. The luminescence intensity upon excitation with ionizing radiation is much more higher than the PL emission intensity upon band-to-band excitation with VUV-photons in the energy range above 9 eV. Both the XRL and PCL spectra are similar. In this connection, Fig. 9 shows only the XRL spectrum of KDP:Tl (0.001 wt.%) recorded at 293 K. The spectrum comprises the dominant emission band at 4.51 eV resulted from the 3P1 ? 1S0 radiative transition in the Tl+ impurity ion. Fig. 9 shows also the PL
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Fig. 9. The PL emission spectra recorded at 293 K upon photoexcitation at 5.6 eV for KDP:Tl (0.001 wt.%) before irradiation – (1), and after the 10 min exposure to X-rays at this temperature – (2). Curve (3) shows the steady-state XRL spectrum recorded during this exposure. The XRL spectrum is normalized in intensity to unity at the maximum.
emission spectra of the same sample of KDP:Tl (0.001 wt.%) recorded at 293 K before starting the exposure to X-rays and after terminating this exposure. Although the XRL and PL spectra are similar at first glance, but they differ in many details. In fact, the bandwidth of the 4.51 eV band changes from 0.51 eV for PL to 0.61 eV for XRL, and the PL emission intensity increases almost twice after irradiation, Fig. 9. The possible reason for these changes is the creation of lattice defects during irradiation. A persistent elevated level of the PL intensity after X-ray irradiation of the crystals also may be explained by accumulation of the lattice defects during the exposure. Fig. 7 shows the temperature dependence of the steady-state XRL intensity and the spectrally integrated thermoluminescence (TL) glow curve measured at a heating rate of 0.3 K/s after irradiation with X-rays for 15 min at 90 K. The observed TL glow peaks at 130, 175 and 240 K correspond to the most significant changes in the temperature behavior of the luminescence intensity, Fig. 7. At the same time, these TL glow peaks do not depend on the concentration of thallium ions and hence should be associated with other lattice defects. Approximate estimates of the activation energies using the peak shape method [17] are 0.55 and 0.40 eV for TL peaks at 175 and 240 K, respectively. From Fig. 7 it follows that the electron–hole recombinations at the lattice defects affect significantly the excitation efficiency for the 4.5 eV luminescence. In this case, the most high-temperature peak at 325 K in the temperature dependencies of both the XRL and PL intensities can be compared with the thermally stimulated delocalization from the main recombination center in KDP. In our opinion, this recombination center can be tentatively assigned to the thallium impurity ion associated with a lattice defect. The most likely defects in the crystal lattice of KDP are hydrogen vacancy (L-defect) and an interstitial hydrogen ion (D-defect). It is known [19], that such defects can be created with exposure to a fairly low-energy ionizing radiation (X-rays, electron beam). On the other hand, such defects are created to compensate for a lattice distortion and the excess charge arising from the introduction of impurities into the crystal during the growth process. In any case these defects are very efficient recombination centers in KDP, which are responsible for broad emission bands at 2.6 and 3.5–3.6 eV [3,16,18]. Low but nonzero emission intensity at energies below 4 eV in XRL spectrum of KDP:Tl may be a manifestation of these defects in recombination processes, Fig. 9. There is no doubt that the lattice defects in KDP can serve as recombination centers, which transfer the obtained energy to the
Tl+ impurity ions at the next stage. Let us discuss the possible channels for such energy transfer. First, the recombination of free electrons from the conduction band (CB) with self-trapped holes (Bradicals) leading to a typical intrinsic luminescence of undoped KDP at 5.2 eV [3]. This broad emission band has FWHM of 0.8 eV and overlaps the A-absorption band of the Tl+ ion. Second, the L-defect (hydrogen vacancy) can trap a hole and become the A-radical. Recombination of free electrons from CB with trapped holes at the A-radicals leads to the luminescence at 2.6 and 3.5–3.6 eV. These emission bands do not overlap with the absorption bands of thallium ion. However, if the A-radical is located in the vicinity of a Tl+ ion, it is possible creation of a dipole coupling between a hole of the A-radical and an electron of the Tl+ ion. When free electron recombine with the trapped hole at the A-radical, the released energy can be transferred to the thallium ion. In order to confirm these assumptions, we carried out measurements of temporal behavior of the luminescence of KDP crystals under exposure to ionizing radiation. A time response of the pulsed cathodoluminescence of KDP:Tl was measured in the microsecond lifetime range at various temperatures from 100 K to 450 K. The PCL decay kinetics of the 4.5 eV emission band obeys a singleexponential law, and the pedestal is less than 1%, Fig. 10. The PCL lifetime is subject to shortening on heating. Fig. 10 shows a temperature dependence of the PCL lifetime recorded monitoring emission at 4.5 eV for KDP:Tl (0.001 wt.%). The PCL lifetime exhibits a drastic change in the vicinity of the ferroelectric transition of KDP at Tc = 123 K. These result indicates clearly that the relaxed excited electronic states of the Tl+ ions in KDP are strongly influenced by the ferroelectric local field due to surrounding ligand ions. For the numerical analysis of experimental data on s(T), we used the Mott law in the form
sðTÞ ¼
s0 1 þ x0 exp ðE=kb T Þ
;
ð1Þ
where s0 is the low temperature lifetime value; E is the activation energy of thermal quenching; x0 is the dimensionless preexponential factor. Temperature dependence of the lifetime in the coordinates of Y = ln(s0/s(T) 1), X = 1/T should straighten with slope of the line equal to E/kb. From Fig. 10 it follows that there are two thermally stimulated processes obeying the Mott law. The first process occupies the low-temperature part of the s(T) curve down from Tc. The second process dominates at temperatures above Tc, Fig. 10.
Fig. 10. Temperature dependence of the PCL lifetime recorded monitoring emission at 4.5 eV for KDP:Tl (0.001 wt.%). Open circles relate to the experimental data, the solid and dashed straight lines are the results of approximation for s0 = 5.5 ls, the vertical arrow corresponds to Tc = 123 K. In the inset: examples of the PCL decay kinetics measured at 100 – (1), 130 – (2), 170 – (3), and 293 K – (4).
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Fig. 11. The exposure time-dependence of the 4.5 eV luminescence band intensity of KDP:Tl (0.001 wt.%) at room temperature under excitation with X-rays. Open circles relate to the experimental data, and dashed line is the result of approximation.
The best fitting parameters for s0 = 5.3 ls were E1 = 0.216 eV, x01 = 1.4 108 and E2 = 0.044 eV, x02 = 71 for the first and second process, respectively. They match satisfactory the parameters of two basic physical processes known for undoped KDP. In fact, Diéguez et al. [2] revealed that thermally stimulated delocalization of self-trapped hole (B-radical) in KDP occurs at 73 K with an activation energy of 42 meV and frequency factor of 19 s1. At the same time, thermally stimulated delocalization of trapped holes from Aradicals occurs at Tc = 123 K with an activation energy of 220 meV and frequency factor of 8 106 s1 [2]. We can suggest that the first low temperature process (E1 = 216 meV) in the temperature dependence of the PCL lifetime s(T) (Fig. 10) should be associated with delocalization of holes from A-radicals. The second process (E2 = 44 m eV) should be assigned to delocalization of holes from B-radicals. It can be assumed that the hole recombination process involving A- and B-radicals in KDP:Tl leads to excitation of the Tl+ impurity ions. The steady-state luminescence intensity I0 in KDP:Tl upon exposure to ionizing radiation (X-rays, an electron beam) at 293 K depends strongly on the exposure time t. Fig. 11 shows this dependence for an X-ray irradiation. The luminescence intensity at 4.5 eV increases by the factor higher than 25, obeying an exponential law
I0 ðtÞ / 1 expðt=sÞ;
ð2Þ
with the time-constant s = 55 s. Our interpretation of this phenomenon is based on the fact that the ionizing radiation creates lattice defects in the hydrogen sublattice of KDP crystal: interstitial hydrogen ions (D-defects) and appropriate hydrogen vacancies (L-defects) [3,18,19]. The hydrogen vacancies under excitation create trapped hole centers, which have an elevated cross-section for nonradiative recombination for delocalized electrons. The realizing energy can be transferred through the dipole–dipole interaction to the neighboring Tl+ ions. This excitation of the Tl+ ions results in radiative A-transitions, increasing the luminescence yield at the 4.5 eV emission band.
4. Conclusion The main essential results on the luminescence and absorption spectroscopy study of KH2PO4 crystals doped with Tl+ ions are as follows:
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1. The 4.5 eV luminescence of KDP:Tl occurs under action of UVphotons, X-rays, and electron beam. This luminescence originates from radiative electronic transitions 3P1 ? 1S0 in the Tl+ ions. Owing to cascade nonradiative transitions from higher excited levels onto the lowest excited state of the Tl+ ion only the 4.5 eV emission band can be observed. The schematic configuration coordinate diagram of energy levels for Tl+ ion in KDP lattice was proposed on the basis of the obtained experimental data. 2. The PLE spectra of KDP single crystals containing a small amount of the Tl+ ions (0.001–0.008 wt.%) comprises peaks at 5.7–5.8, 6.9, and 7.3–7.4 eV, corresponding subsequently to the A-, B- and C-optical transitions. Identification of these bands was based on estimation of their positions in suggestion of linear compression of energy of the excited states in a crystal field in respect of the free Tl+ ion levels. 3. The PL decay kinetics of ion Tl+ in KDP is almost independent on the A-, B-, or C-excitation bands. The time response has singleexponential behavior with an average PL lifetime of s = 280 ± 8 ns. A radiative lifetime of the center in the excited state at T = 293 K in KDP:Tl is comparable with that for the Tl+ ion in alkali halide crystals. 4. Temperature dependence of the 4.5 eV luminescence yield under excitation with X-rays does not correlate with temperature dependence of the PL yield. It is connected with different mechanisms of excitation of the impurity center: a direct photoexcitation of an impurity ion and excitation of the impurity center through the recombination process under action of ionizing radiation. 5. Temperature dependence of a PCL lifetime measured in the temperature range from 90 to 480 K, is characterized by two processes, each of them has a specific activation energy of 200 and 44 eV. These processes were associated with delocalization of holes from A- and B-radicals, respectively. It was assumed that the recombination process involving A- and B-radicals in KDP:Tl leads to excitation of the Tl+ impurity ions. 6. Under exposure to ionizing radiation (X-rays, or electron beam) the intensity of the Tl+ luminescence increases depending on the exposure time. The origin of this phenomenon was associated with creation of defects in the hydrogen sublattice of KDP.
Acknowledgement Authors are grateful to N.S. Mezenina for her help in conducting this study. The work was supported by HASYLAB (Project No. I20110050). References [1] V.G. Dmitriev, G.G. Gurzadyan, D.N. Nikogosyan, Handbook of Nonlinear Optical Crystals, Springer, Berlin, New York, 2000. [2] E. Diéguez, J.M. Cabrera, F. Agulló López, J. Chem. Phys. 81 (1984) 3369–3374. [3] I.N. Ogorodnikov, V.A. Pustovarov, M. Kirm, V.S. Cheremnykh, Opt. Spectrosc. 95 (2003) 385–389. [4] I.N. Ogorodnikov, M. Kirm, V.A. Pustovarov, Radiat. Measurem. 42 (2007) 746– 750. [5] I. Fujita, Phys. Rev. B 49 (1994) 6462–6469. [6] F.D. Klement, Ch.B. Lushchik, J. Appl. Spectrosc. 8 (1972) 329–333. [7] S. Zazubovich, A. Voloshinovskii, G. Stryganyuk, Phys. Stat. Sol. B 233 (2002) 238–249. [8] A. Voloshinovskii, S. Zazubovich, G. Stryganyuk, I. Pashuk, J. Luminesc. 111 (2005) 9–15. [9] K. Ichimura, T. Fuyuki, T. Kawai, S. Hashimoto, T. Hirai, N. Ohne, Phys. Stat. Sol. C 3 (2006) 3607–3610. [10] I.N. Ogorodnikov, M. Kirm, V.A. Pustovarov, V.S. Cheremnykh, Radiat. Measurem. 38 (2004) 331–334. [11] A.P. Voronov, V.I. Salo, V.M. Puzikov, V.F. Tkachenko, Yu.T. Vydai, Crystallogr. Rep. 51 (2006) 696–701. [12] A.P. Voronov, Yu.T. Vyday, V.I. Salo, V.M. Puzikov, S.I. Bondarenko, Radiat. Measurem. 42 (2007) 553–556.
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[13] V.I. Salo, V.F. Tkachenko, A.P. Voronov, V.M. Puzikov, V.A. Tsurikov, Funct. Mater. 12 (2005) 658–662. [14] G. Zimmerer, Radiat. Measurem. 42 (2007) 859–864. [15] G. Baldini, M. Cottini, E. Grilli, Solid State Commun. 11 (1972) 1257–1260. [16] I.N. Ogorodnikov, V.A. Pustovarov, B.V. Shulgin, V.T. Kuanyshev, M.K. Satybaldieva, Opt. Spectrosc. 91 (2001) 224–231.
[17] R. Chen, J. Electrochem. Soc. 116 (1969) 1254–1257. [18] I.N. Ogorodnikov, V.Yu. Yakovlev, B.V. Shulgin, M.K. Satybaldieva, Phys. Solid State 44 (2002) 880–887. [19] V.T. Kuanyshev, T.A. Belykh, I.N. Ogorodnikov, B.V. Shulgin, M.K. Satybaldieva, M.M. Kidibaev, Radiat. Measurem. 33 (2001) 503–507.
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A luminescence and absorption spectroscopy study of KH2PO4 crystals doped with Tl+ ions I.N. Ogorodnikov a,⇑, V.A. Pustovarov a, V.M. Puzikov b, V.I. Salo b, A.P. Voronov b a b
Ural Federal University, 19, Mira Street, 620002 Ekaterinburg, Russia STC Institute for Single Crystals NAS of Ukraine, 60, Lenina Avenue, 61001 Kharkov, Ukraine
a r t i c l e
i n f o
Article history: Received 27 August 2011 Received in revised form 28 February 2012 Accepted 15 March 2012 Available online 10 April 2012 Keywords: Luminescence spectroscopy Luminescent materials Scintillation crystals Potassium dihydrogen phosphate Thallium ion
a b s t r a c t We report experimental study on luminescence and optical properties of single crystals KH2PO4 (KDP) doped with Tl+ ions (KDP:Tl) carried out at 10–480 K. The 4.5 eV photoluminescence (PL) of KDP:Tl originates from radiative electronic transitions 3P1 ? 1S0 in the Tl+ ions upon excitation by UV-photons, X-rays, and electron beam. This luminescence can be induced by direct photoexcitation, or through the recombination process with participation of the lattice defects located in the vicinity of Tl+ ion. These excitation mechanisms lead to different temperature behavior of the luminescence intensity. The PL excitation spectra of KDP crystals containing a small amount of the Tl+ ions (0.001–0.008 wt.%) comprises peaks at 5.7–5.8, 6.9, and 7.3–7.4 eV, corresponding subsequently to the A-, B- and C-optical transitions in Tl+. The PL time response has single-exponential behavior with an average lifetime of s = 280 ± 8 ns. Under exposure to ionizing radiation (X-rays, or electron beam) the intensity of the Tl+ luminescence increases depending on the exposure time. The origin of this phenomenon was associated with creation of defects in the hydrogen sublattice of KDP. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Potassium dihydrogen phosphate KH2PO4 (KDP) in the crystalline form is a well known optical material, which was successfully used in many areas of applications, including nonlinear and integrated optics, laser technology. This material is successfully utilized in many kinds of optical devices such as non-linear optical elements, electro-optical polarizers, solid state detectors and transformers of radiation, operating in a broad spectral range from the middle infrared to the vacuum ultraviolet (VUV) spectral regions [1]. It is important that dihydrogen phosphate crystals (in particular, KDP) are a very unique inorganic material with a high content of hydrogen ions (protons) in the crystal lattice. In this regard, KDP holds great promise for use in radiation detectors of neutrons. Many research works last 30 years were devoted to studying the luminescent properties of KDP in the visible and ultraviolet (UV) spectral ranges, see e.g. [2]. These results show that KDP at temperatures below 20 K exhibits an intense intrinsic luminescence, which however is subject to thermal quenching when heated. The main reason is related to the thermal creation of defects in the hydrogen sublattice of KDP, which are the centers of an efficient non-radiative recombination for free electrons and holes [3,4]. ⇑ Corresponding author. E-mail address: [email protected] (I.N. Ogorodnikov). 0925-3467/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.optmat.2012.03.018
In this connection, many research works have been focused on a study of the doped KDP crystals. However, the concentration of intrinsic defects in the hydrogen sublattice is far exceed possible concentration of any impurity in KDP. This makes extremely inefficient any transfer of energy from the matrix to the impurity at temperatures above 20 K. The thallium impurity in KDP seems to be the only dopant, which able to accept the energy transferred after non-radiative recombination of electrons and holes at the defects in the hydrogen sublattice of KDP [5]. The thallium impurity ion is known as an activator that forms s2 centers in cubic alkali halide crystals [6–8]. Owing to the large optical transparency band (Eg 8.8 eV), KDP is a unique noncubic host system, where the s2 centers of Tl+ ion can exhibit several UV-absorption bands, which allow study of various interactions between the impurity ion and the host lattice [7,9]. Until today, Tl+ center in KDP is the only center type s2 in the non-cubic hostlattice, where are possible all the s2 ? sp transitions corresponding to A, B, and C absorption bands in alkali halide crystals [5]. In previous papers we started a detailed study of undoped KDP crystals by the means of the low-temperature luminescence and optical VUV-spectroscopy with a time-resolution [3,4,10]. Detection of practically important scintillation properties of crystals KDP:Tl [11,12] stimulated further detailed studies of this material. The present work continues theses studies and focuses on spectroscopy of the KDP crystals doped with Tl+ ions. The main goal of this paper is to study the luminescent and optical properties
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of ferroelectric crystals KH2PO4:Tl over the broad temperature range (10–450 K) using the method of luminescence UV and VUV spectroscopy with a time-resolution. 2. Experimental details Single crystals of KDP containing 0.001, 0.008, and 0.1 wt.% of Tl+ ions were grown at the STC Institute for Single Crystals NAS of Ukraine (Kharkov, Ukraine) [11–13]. The samples measuring 7 7 2 mm3 were cut from the perfect part of the grown crystal and their surfaces were polished to the high optical quality. The present study was carried out mainly by the means of the low-temperature luminescence VUV spectroscopy. Photoluminescence (PL) spectra in the energy range of Em from 1.2 to 6.2 eV, PL excitation (PLE) spectra in the broad energy range of hm from 3.7 to 15 eV (0.32 nm resolution) were measured at 10 and 293 K for these crystals at the SUPERLUMI experimental station of HASYLAB [14] using synchrotron radiation. Samples were mounted in a sample holder attached to a He-flow cryostat with vacuum not less than 7 1010 Torr. At the storage ring DORIS the full width at half maximum (FWHM) of SR pulses was 130 ps with the repetition period of 192 ns. Such pulses excitation enables the recording of spectra within a time-window correlated with the arrival of SR pulses. In the present experiments we recorded time-resolved spectra within two independent time-windows (TWs) set for detection of luminescence signal within 0.5–2.3 ns (TW1) and 14–58 ns (TW2) relative to the beginning of the SR pulse. Timeintegrated (TI) spectra were recorded within the full time range available between two sequential excitation pulses, viz. 192 ns. The 0.3 m ARC Spectra Pro-300i monochromator and R6358P (Hamamatsu) photomultiplier were used as a registration system. The PLE spectra were corrected to an equal number of photons incident on the sample using sodium salicylate. The luminescence characteristics in the temperature range from 90 to 480 K were measured under excitation with either X-rays (BSW2:Cu X-ray tube with Ua = 40 kV and Ia = 10 mA), or electron beam (Ee = 180 keV, I = 800 A/cm2, t = 3 ns), or UV light (400 W deuterium discharge lamp with a continuous UV emission spectrum). Spectra of the X-rays induced luminescence (XRL), spectra of the pulsed cathodoluminescence (PCL), and decay kinetics of PCL were recorded by the means of the MDR-23 monochromator, the FEU-106 photomultiplier, operating in a photon counting mode, and the digital TDS-1030 oscilloscope. All measurements were performed in vacuum (residual gas pressure 104 Pa). The sample chamber was a quick response vacuum cryostat with quartz windows, which made it possible to control the sample temperature in the range of 80–500 K.
Fig. 1. The optical absorption spectra of KDP:Tl for the Tl+ concentrations of 0.001 – (1), 0.008 – (2), and 0.1 wt.% – (3) measured at 293 K.
we have measured all the spectra (TW1, TW2 and TI), but in each case they were identical in the profile. All the presented PL and PLE spectra were normalized in intensity to unity at the maximum. Fig. 2 shows the PL emission spectra of KDP:Tl under excitation with the 5.75 eV photons (the A-band excitation) measured at 10 and 293 K. A Stokes-shifted luminescence band was observed in the energy interval of 3.50–5.0 eV. The luminescence intensity of this band depends on the concentration of thallium ions in the same way as the absorption spectra: the higher the concentration, the greater the intensity. However, the band shape does not depend on the concentration of thallium ions in the entire investigated concentration range. In this regard, Fig. 2 shows only the luminescence spectra for KDP:Tl (0.001 wt.%). At 10 K the PL emission band has almost a Gaussian shape with a maximum at 4.50 eV and FWHM of 0.32 eV. The low-temperature band-shape does not depend on the excitation energy value. On heating to room temperature the PL emission intensity decreases by a factor of about eight. In contrast to the low-temperature case, at room temperature the band-shape and its energy position depend on excitation energy, Figs. 2 and 3. Depending on the excitation energy, the position of the PL band is red- or blue-shifted from its low-temperature position, and bandwidth is broadened to FWHM of 0.55–0.58 eV, Table 1. Decomposition shows a low intensive sub-band at 3.9–4.25 eV, which position and intensity depend on both the temperature and excitation energy. Its contribution to
3. Experimental results and discussion 3.1. UV and VUV spectroscopy of Tl+ ion in KDP Fig. 1 shows the optical absorption spectra of KDP:Tl measured at room temperature. The different curves represent the absorption spectra of KDP:Tl crystals with different concentrations of the Tl+ ions. The single absorption band at 5.76 eV (FWHM = 0.27 eV) dominates in the optical absorption spectra over the energy region from 1.2 to 6.2 eV. The band intensity depends strongly on the Tl+ impurity concentration. The label ‘A’ depicts the absorption band corresponding to the A-band of Tl+ centers measured in alkali halide crystals for the impurity positions of the Oh symmetry. Bearing this in mind, the 5.76 eV absorption band in KDP:Tl can be assigned to a direct photoexcitation of the Tl+ impurity ion. Discussing the luminescence of KDP:Tl, we present in figures only the time-integrated PL and PLE spectra. Although in each case,
Fig. 2. The photoluminescence spectra of KDP:Tl (0.001 wt.%) at 10 – (1) and 293 K – (2). The spectra were normalized in intensities to unity at the maximum. Dashed lines show the principal Gaussians, solid lines are the results of approximation, open and black circles depict the experimental data.
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Fig. 3. The PL emission spectra of KDP:Tl (0.001 wt.%) at 293 K. The spectra were normalized in intensity to unity at the maximum.
Table 1 Parameters of the PL emission bands of KDP:Tl (0.001 wt.%). hm (eV)
Em (eV)
DE (eV)
S (%)
10
5.75
4.25 4.50
0.26 0.32
10.2 89.8
10
6.90
4.25 4.50 5.00
0.20 0.32 0.34
13.0 85.5 1.5
293
5.75
3.90 4.45
0.57 0.55
7.5 92.5
293
6.90
4.05 4.55
0.55 0.58
3.3 96.7
T (K)
Note. hm is excitation energy; Em is energy position of the band maximum; DE is FWHM; S is the band contribution into the total light yield of the observed PL emission spectrum.
the total light yield ranges from 3.3% to 10.2%. In one case, a very minor band at 5.0 eV has been added to the best fit, Table 1. We focus our discussion at the properties of the dominant PL emission band. Fig. 4 shows the PLE spectra for the 4.50 eV emission band of KDP:Tl measured for the Tl+ concentrations of 0.001, 0.008, and 0.1 wt.% at 10 K. The PLE spectra were measured over the energy range from 4.7 to 15 eV, however we found no excitation bands in the energy range of the band-to-band transitions. The sufficient intensities were detected only between 5.5 and 8 eV. This means that the energy transfer from the host lattice to the impurity Tl+ ions has a very insignificant efficiency, as it was found earlier by us for various lattice defects in crystal KDP [10]. In this connection, only relevant part of each PLE spectrum is shown in Fig. 4. The set of the PLE bands matches well the known optical absorption bands of KDP:Tl single crystals containing a small amount of the Tl+ ions and exhibiting five characteristic polarized absorption bands Az, Axy, Bxy, Cxy and Cz in the ultraviolet region [5,9]. Fig. 4 shows also a possible interpretation of the observed peaks in the PLE spectra. The PLE spectrum of KDP:Tl (0.001 wt.%) comprises three dominant peaks (Fig. 4a), which match well the energy positions of the Axy, Bxy, and Cxy absorption bands reported in [5]. With an increase in concentration to 0.008 wt.%, the PL intensity increases respectively, but the structure of the PLE spectrum remains unchanged, except for increasing the relative intensity of the Axy peak, Fig. 4b. Further increase in concentration to 0.1 wt.% does not only increase the luminescence intensity, but it changes the structure of the PLE spectrum. New
Fig. 4. The PL excitation spectra recorded monitoring emission at 4.5 eV in KDP:Tl at 10 K for the Tl+ concentrations of 0.001 – (a), 0.008 – (b), and 0.1 wt.% – (c). The spectra were normalized in intensity to unity at the maximum.
peaks appear in the PLE spectrum. Three of them are comparable to the Az, Bz, and Cz absorption bands (Fig. 4c), however the 6.0 eV PLE peak has no correspondence with the known absorption peaks reported in [5]. A high-energy slope of the supposed Cz peak overlaps with the low-energy tail of the KDP host absorption, so its shape can be significantly distorted. When heated to 293 K, the PLE-spectra change in the profile, they broaden and become less resolved, Fig. 5. A new broad excitation band appears at room temperature in the energy range above 7.5 eV. This PLE band is located in the energy range of the low energy tail of the KDP host absorption, but its relative intensity depends strongly on the thallium concentration, Fig. 5. This can indirectly point to the creation of impurity-bound excitons in KDP:Tl. In this connection, the 7.5–9 eV band in the PLE spectra
Fig. 5. The PL excitation spectra of KDP:Tl at 293 K recorded monitoring emission at 4.5 eV for the Tl+ concentrations of 0.001 – (1), 0.008 – (2), and 0.1 wt.% – (3). The spectra were normalized in intensities to unity at the maximum. Curve (4) shows the energy position of the low energy tail of the KDP host absorption in accordance with the data of Baldini et al. [15] and Ogorodnikov et al. [16].
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of KDP:Tl can be tentatively compared with the D-band in the PLE spectra of alkali halide crystals doped with thallium, see e.g. [7]. The absence of this band in the PLE spectra of KDP:Tl at 10 K may be due to the existence of a competing process of self-trapping of excitons at low temperatures, as it was shown previously for undoped KDP crystals [4,10]. The 4.5 eV luminescence can be efficiently excited upon excitations onto the A, B and C absorption bands associated with the impurity Tl+ ions in KDP (Fig. 3). The PL decay kinetics of KDP:Tl (0.001 wt.%) upon selective photoexcitation at the A-, B-, and C-bands at room temperature obeys a single-exponential behavior, Fig. 6. However, the decay time exceeds the time interval between two sequential excitation pulses, viz. 192 ns. In this connection, the observed lifetime value showed little change for varying the experimental conditions. The average value of the lifetime was estimated as 280 ± 8 ns. Fig. 6 shows an example of the PL decay kinetics recorded monitoring emission at 4.5 eV upon excitation at 5.7 eV. The luminescence time-response at 10 K in our measurements appears as a pedestal, indicating a time-constant of the microsecond and millisecond timeranges, Fig. 6. Temperature dependence of the PL intensity was recorded in the range of 90–480 K monitoring emission at 4.5 eV upon selective photoexcitation at 5.6 eV from the laboratory light source, Fig. 7. Temperature dependence of the PL intensity shows two maxima, the first one is located in the vicinity of the ferroelectric phase transition of KDP at 123 K. The composite band origin may cause the peculiar behavior of intensity of the 4.5 eV emission band near the ferroelectric phase transition temperature. The second maximum is located at 325 K, its possible origin will be discussed in Section 3.2. The final temperature quenching of PL intensity occurs at 475 K, however the complicate temperature behavior of the luminescence intensity does not obeys the Mott law, Fig. 7. More likely, there are superposition of several thermally stimulated processes responsible for this thermal quenching of the PL intensity. Fig. 8 summarizes all the discussed data and shows the schematic configuration coordinate diagram of energy levels for the Tl+ ion in KDP lattice. For the simplicity, only discussed levels are shown. The EA label depicts an activation energy for an intracenter temperature quenching of the 4.5 eV luminescence, which occurs at 475 K. From Fig. 8 it follows that the 4.50 eV luminescence band should be ascribed to a radiative transition from the 3P1 excited state to the 1S0 ground state of the Tl+ ions (A-transition) since there is a strong response on the A-absorption band of Tl+ ions. The expected high-energy transitions either 3P2 ? 1S0 (B-transition), or 3P3 ? 1S0 (C-transition) are not inherent to the Tl+ ion in
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Fig. 7. Temperature dependencies of steady-state intensities of PL upon excitations at hm = 5.6 eV – (1) and XRL – (2) recorded monitoring emission at 4.5 eV; and spectrally integrated TL glow curve recorded at heating rate of 0.3 K/s – (3) for KDP:Tl (0.001 wt.%). The intensities were normalized to unity at the maximum, the vertical dashed line corresponds to Tc = 123 K.
Fig. 8. Schematic configuration coordinate diagram of energy levels for Tl+ ion in KDP lattice. For the simplicity, only discussed levels are shown.
the KDP host lattice. In fact, the high-energy transitions would be expected in the PL emission spectrum at higher energies from 4.5 to 7.0 eV. One can envision that the B- and C-transitions occur through the fast nonradiative relaxations from 3P2- and 3P3-highenergy excited states onto the 1P1 lowest excited state of the Tl+ ion. The proper 3P1 ? 1S0 radiative transition occurs at the later stage. 3.2. Luminescence upon exposure to ionizing radiation
Fig. 6. The PL decay kinetics of KDP:Tl (0.001 wt.%) at 10 – (1) and 293 K – (2) recorded monitoring emission at 4.5 eV upon excitation at 5.7 eV. The curves were normalized in intensities to unity at the maximum.
The luminescence spectra were measured over the broad energy range from 1.5 to 8 eV upon excitation with either X-rays, or electron beam. The luminescence intensity upon excitation with ionizing radiation is much more higher than the PL emission intensity upon band-to-band excitation with VUV-photons in the energy range above 9 eV. Both the XRL and PCL spectra are similar. In this connection, Fig. 9 shows only the XRL spectrum of KDP:Tl (0.001 wt.%) recorded at 293 K. The spectrum comprises the dominant emission band at 4.51 eV resulted from the 3P1 ? 1S0 radiative transition in the Tl+ impurity ion. Fig. 9 shows also the PL
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Fig. 9. The PL emission spectra recorded at 293 K upon photoexcitation at 5.6 eV for KDP:Tl (0.001 wt.%) before irradiation – (1), and after the 10 min exposure to X-rays at this temperature – (2). Curve (3) shows the steady-state XRL spectrum recorded during this exposure. The XRL spectrum is normalized in intensity to unity at the maximum.
emission spectra of the same sample of KDP:Tl (0.001 wt.%) recorded at 293 K before starting the exposure to X-rays and after terminating this exposure. Although the XRL and PL spectra are similar at first glance, but they differ in many details. In fact, the bandwidth of the 4.51 eV band changes from 0.51 eV for PL to 0.61 eV for XRL, and the PL emission intensity increases almost twice after irradiation, Fig. 9. The possible reason for these changes is the creation of lattice defects during irradiation. A persistent elevated level of the PL intensity after X-ray irradiation of the crystals also may be explained by accumulation of the lattice defects during the exposure. Fig. 7 shows the temperature dependence of the steady-state XRL intensity and the spectrally integrated thermoluminescence (TL) glow curve measured at a heating rate of 0.3 K/s after irradiation with X-rays for 15 min at 90 K. The observed TL glow peaks at 130, 175 and 240 K correspond to the most significant changes in the temperature behavior of the luminescence intensity, Fig. 7. At the same time, these TL glow peaks do not depend on the concentration of thallium ions and hence should be associated with other lattice defects. Approximate estimates of the activation energies using the peak shape method [17] are 0.55 and 0.40 eV for TL peaks at 175 and 240 K, respectively. From Fig. 7 it follows that the electron–hole recombinations at the lattice defects affect significantly the excitation efficiency for the 4.5 eV luminescence. In this case, the most high-temperature peak at 325 K in the temperature dependencies of both the XRL and PL intensities can be compared with the thermally stimulated delocalization from the main recombination center in KDP. In our opinion, this recombination center can be tentatively assigned to the thallium impurity ion associated with a lattice defect. The most likely defects in the crystal lattice of KDP are hydrogen vacancy (L-defect) and an interstitial hydrogen ion (D-defect). It is known [19], that such defects can be created with exposure to a fairly low-energy ionizing radiation (X-rays, electron beam). On the other hand, such defects are created to compensate for a lattice distortion and the excess charge arising from the introduction of impurities into the crystal during the growth process. In any case these defects are very efficient recombination centers in KDP, which are responsible for broad emission bands at 2.6 and 3.5–3.6 eV [3,16,18]. Low but nonzero emission intensity at energies below 4 eV in XRL spectrum of KDP:Tl may be a manifestation of these defects in recombination processes, Fig. 9. There is no doubt that the lattice defects in KDP can serve as recombination centers, which transfer the obtained energy to the
Tl+ impurity ions at the next stage. Let us discuss the possible channels for such energy transfer. First, the recombination of free electrons from the conduction band (CB) with self-trapped holes (Bradicals) leading to a typical intrinsic luminescence of undoped KDP at 5.2 eV [3]. This broad emission band has FWHM of 0.8 eV and overlaps the A-absorption band of the Tl+ ion. Second, the L-defect (hydrogen vacancy) can trap a hole and become the A-radical. Recombination of free electrons from CB with trapped holes at the A-radicals leads to the luminescence at 2.6 and 3.5–3.6 eV. These emission bands do not overlap with the absorption bands of thallium ion. However, if the A-radical is located in the vicinity of a Tl+ ion, it is possible creation of a dipole coupling between a hole of the A-radical and an electron of the Tl+ ion. When free electron recombine with the trapped hole at the A-radical, the released energy can be transferred to the thallium ion. In order to confirm these assumptions, we carried out measurements of temporal behavior of the luminescence of KDP crystals under exposure to ionizing radiation. A time response of the pulsed cathodoluminescence of KDP:Tl was measured in the microsecond lifetime range at various temperatures from 100 K to 450 K. The PCL decay kinetics of the 4.5 eV emission band obeys a singleexponential law, and the pedestal is less than 1%, Fig. 10. The PCL lifetime is subject to shortening on heating. Fig. 10 shows a temperature dependence of the PCL lifetime recorded monitoring emission at 4.5 eV for KDP:Tl (0.001 wt.%). The PCL lifetime exhibits a drastic change in the vicinity of the ferroelectric transition of KDP at Tc = 123 K. These result indicates clearly that the relaxed excited electronic states of the Tl+ ions in KDP are strongly influenced by the ferroelectric local field due to surrounding ligand ions. For the numerical analysis of experimental data on s(T), we used the Mott law in the form
sðTÞ ¼
s0 1 þ x0 exp ðE=kb T Þ
;
ð1Þ
where s0 is the low temperature lifetime value; E is the activation energy of thermal quenching; x0 is the dimensionless preexponential factor. Temperature dependence of the lifetime in the coordinates of Y = ln(s0/s(T) 1), X = 1/T should straighten with slope of the line equal to E/kb. From Fig. 10 it follows that there are two thermally stimulated processes obeying the Mott law. The first process occupies the low-temperature part of the s(T) curve down from Tc. The second process dominates at temperatures above Tc, Fig. 10.
Fig. 10. Temperature dependence of the PCL lifetime recorded monitoring emission at 4.5 eV for KDP:Tl (0.001 wt.%). Open circles relate to the experimental data, the solid and dashed straight lines are the results of approximation for s0 = 5.5 ls, the vertical arrow corresponds to Tc = 123 K. In the inset: examples of the PCL decay kinetics measured at 100 – (1), 130 – (2), 170 – (3), and 293 K – (4).
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Fig. 11. The exposure time-dependence of the 4.5 eV luminescence band intensity of KDP:Tl (0.001 wt.%) at room temperature under excitation with X-rays. Open circles relate to the experimental data, and dashed line is the result of approximation.
The best fitting parameters for s0 = 5.3 ls were E1 = 0.216 eV, x01 = 1.4 108 and E2 = 0.044 eV, x02 = 71 for the first and second process, respectively. They match satisfactory the parameters of two basic physical processes known for undoped KDP. In fact, Diéguez et al. [2] revealed that thermally stimulated delocalization of self-trapped hole (B-radical) in KDP occurs at 73 K with an activation energy of 42 meV and frequency factor of 19 s1. At the same time, thermally stimulated delocalization of trapped holes from Aradicals occurs at Tc = 123 K with an activation energy of 220 meV and frequency factor of 8 106 s1 [2]. We can suggest that the first low temperature process (E1 = 216 meV) in the temperature dependence of the PCL lifetime s(T) (Fig. 10) should be associated with delocalization of holes from A-radicals. The second process (E2 = 44 m eV) should be assigned to delocalization of holes from B-radicals. It can be assumed that the hole recombination process involving A- and B-radicals in KDP:Tl leads to excitation of the Tl+ impurity ions. The steady-state luminescence intensity I0 in KDP:Tl upon exposure to ionizing radiation (X-rays, an electron beam) at 293 K depends strongly on the exposure time t. Fig. 11 shows this dependence for an X-ray irradiation. The luminescence intensity at 4.5 eV increases by the factor higher than 25, obeying an exponential law
I0 ðtÞ / 1 expðt=sÞ;
ð2Þ
with the time-constant s = 55 s. Our interpretation of this phenomenon is based on the fact that the ionizing radiation creates lattice defects in the hydrogen sublattice of KDP crystal: interstitial hydrogen ions (D-defects) and appropriate hydrogen vacancies (L-defects) [3,18,19]. The hydrogen vacancies under excitation create trapped hole centers, which have an elevated cross-section for nonradiative recombination for delocalized electrons. The realizing energy can be transferred through the dipole–dipole interaction to the neighboring Tl+ ions. This excitation of the Tl+ ions results in radiative A-transitions, increasing the luminescence yield at the 4.5 eV emission band.
4. Conclusion The main essential results on the luminescence and absorption spectroscopy study of KH2PO4 crystals doped with Tl+ ions are as follows:
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1. The 4.5 eV luminescence of KDP:Tl occurs under action of UVphotons, X-rays, and electron beam. This luminescence originates from radiative electronic transitions 3P1 ? 1S0 in the Tl+ ions. Owing to cascade nonradiative transitions from higher excited levels onto the lowest excited state of the Tl+ ion only the 4.5 eV emission band can be observed. The schematic configuration coordinate diagram of energy levels for Tl+ ion in KDP lattice was proposed on the basis of the obtained experimental data. 2. The PLE spectra of KDP single crystals containing a small amount of the Tl+ ions (0.001–0.008 wt.%) comprises peaks at 5.7–5.8, 6.9, and 7.3–7.4 eV, corresponding subsequently to the A-, B- and C-optical transitions. Identification of these bands was based on estimation of their positions in suggestion of linear compression of energy of the excited states in a crystal field in respect of the free Tl+ ion levels. 3. The PL decay kinetics of ion Tl+ in KDP is almost independent on the A-, B-, or C-excitation bands. The time response has singleexponential behavior with an average PL lifetime of s = 280 ± 8 ns. A radiative lifetime of the center in the excited state at T = 293 K in KDP:Tl is comparable with that for the Tl+ ion in alkali halide crystals. 4. Temperature dependence of the 4.5 eV luminescence yield under excitation with X-rays does not correlate with temperature dependence of the PL yield. It is connected with different mechanisms of excitation of the impurity center: a direct photoexcitation of an impurity ion and excitation of the impurity center through the recombination process under action of ionizing radiation. 5. Temperature dependence of a PCL lifetime measured in the temperature range from 90 to 480 K, is characterized by two processes, each of them has a specific activation energy of 200 and 44 eV. These processes were associated with delocalization of holes from A- and B-radicals, respectively. It was assumed that the recombination process involving A- and B-radicals in KDP:Tl leads to excitation of the Tl+ impurity ions. 6. Under exposure to ionizing radiation (X-rays, or electron beam) the intensity of the Tl+ luminescence increases depending on the exposure time. The origin of this phenomenon was associated with creation of defects in the hydrogen sublattice of KDP.
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