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Formation of color centers at X-ray irradiation of ZnSe single crystals
Radiation Measurements 131 (2020) 106232
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Radiation Measurements journal homepage: http://www.elsevier.com/locate/radmeas
Formation of color centers at X-ray irradiation of ZnSe single crystals V. Ya Degoda a, G.P. Podust a, *, N. Yu Pavlova b, M. Alizadeh a a b
Taras Shevchenko National University of Kyiv, Physics Department, 03680, Kyiv, Ukraine National Pedagogical Dragomanov University, Kyiv, Ukraine
A R T I C L E I N F O
A B S T R A C T
Keywords: ZnSe Monocrystal Broadband semiconductors X-ray luminescence Photoluminescence Color centers
In the present work we study the dose dependences of X-ray luminescence and X-ray conductivity at different temperatures and different angles of incidence of the exciting radiation and the registration of luminescence. Low-energy X-ray quanta (with energies up to 20 keV) do not create new structural defects in ZnSe crystals, but can only recharge existing ones. It was established from the dose dependences of the luminescence intensity of the 630-nm and 970-nm bands that X-rays do not create the color centers in ZnSe crystals, even those that are temperature unstable, which absorb the luminescent radiation. This fact is confirmed by the absence of con duction current change at additional optical intensive irradiation of ZnSe samples by optical quanta with wavelengths that coincide with luminescent quanta.
1. Introduction Zinc selenide (ZnSe) belongs to the АІІВVI wide-gap semiconductors (Gavrilenko et al., 1987; Morozova et al., 1992; Atroshchenko et al., 1998; Ryzhikov et al., 2001a; Starzhinkiy et al., 2008; Dafnei et al., 2010; Degoda et al., 2010) and has been studied for a long time. The use of especially undoped ZnSe as a semiconductor detector became possible only after development of technologies for growing sufficiently high-quality monocrystals with low concentrations of uncontrolled im purities and high specific resistance of the material at ~1010–1014 Ohm∙cm. It should be noted that a relatively high value of the effective atomic number Zef ¼ 32 and a bandgap width Eg ¼ 2.7 eV (at 300 K) make ZnSe a promising material for creating X-ray detectors that do not require cooling (Sofiienko et al., 2012; Brodyn et al., 2014). Such semiconductor detectors of irradiation work well in the direct current registration mode, but only in some samples can the current pulses be registered. This is due to the presence of a significant concentration of traps in ZnSe crystals, which reduce the amplitude of the current pulse and increase its duration (Degoda et al., 2013). The ZnSe crystal-based scintillators are not hygroscopic, their radi ation stability is relatively good, their light output is 1.1–1.5 times higher, and afterglow level after 10 ms – by 2–3 orders of magnitude lower with respect to oxides and CsI (TI), which are the most widely used in low-energy (E < 100 keV) detectors (Ryzhikov et al., 2001b, 2007, 2013; Lee et al., 2008). The emission spectrum of ZnSe-based scintillators (maximum at ~600 nm) is very convenient for
photodiode recording. There is a large amount of data on the radiation resistance of ZnSebased scintillators regarding the creation of structural defects. High radiation resistance is observed for ZnSe crystals under the influence of ionizing radiation when compared to most other scintillators: gammaquanta (~1.3 MeV, up to 500 Mrad), protons (~18 MeV, up to 1016 protons/cm2), electrons (from 0.54 to 2.26 MeV, up to 50 Mrad) and neutrons (source channel-heat reactor, up to 1018 neutrons/cm2) (Starzhinskiy et al., 2005). As a rule, radiation resistance is commonly understood as the ability to resist the creation of new structural defects in a material that change the optical transmission of crystals after irradiation by ionizing radia tion. This approach does not take into account temperature-instable radiation defects that can absorb luminescence during irradiation. Ionizing radiation may not create new crystalline lattice (vacancies (ions)) defects but may recharge the existing ones. Such recharged centers are commonly called the color centers which also significantly affect the detector’s operation. Therefore, the purpose of this work is to verify if X-ray radiation creates the color centers that absorb lumines cent quanta in high-resistance ZnSe single crystals. 2. Experimental methodology The ZnSe crystals were grown from the prepurified charge and were especially not doped during crystal growth. As a result, the samples were obtained with a minimum concentration of point defects and maximum
* Corresponding author. E-mail address: [email protected] (G.P. Podust). https://doi.org/10.1016/j.radmeas.2019.106232 Received 2 August 2019; Received in revised form 13 December 2019; Accepted 17 December 2019 Available online 18 December 2019 1350-4487/© 2019 Elsevier Ltd. All rights reserved.
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Radiation Measurements 131 (2020) 106232
specific resistance (ρ � 1012–1014 Ohm∙cm). To research various crys talline boules, samples of 18 mm � 9 mm � 2 mm were cut out and polished. Studies of photoconductivity (PC), X-ray conductivity (XRC), pho toluminescence (PL), X-ray luminescence (XRL), their dose and tem perature dependences, phosphorescence (Ph), conduction current relaxation (CR), thermally stimulated conductivity (TSC), thermally stimulated luminescence (TSL) were carried out. The two three-layered metal contacts were sprayed by a resistive method on the single crystals to study the conductivity. The chemical composition of each layer was selected purposefully to obtain highresistance contacts with proper adhesion. Earlier, it was experimen tally verified that the application of contacts and heating to 450 K did not change the luminescence characteristics of the samples. It was also found that the luminescence and conductivity of the samples were not changed for 4 years, when they were irradiated with X quanta, cooled to 8 K and heated to 450 K (Degoda et al., 2018a,b). The contacts were the rectangular-shaped strips of L ¼ 5 mm length, 1 mm width and the distance between them was d ¼ 5 mm. The copper conductors were soldered to electrical contacts to measure conductivity. The stabilized voltage of 0–1000 V was applied to one of the electrodes, while the other was earthed through nanoamperemeter. The stabilized voltage unit and the nanoamperemeter were fabricated and calibrated according to standard circuits. For all values of conductivity current, the input impedance of the nanoamperemeter was several orders of magnitude smaller than the electrical resistance of the ZnSe sample. Since the value of the X-ray current was 104 times greater than the magnitude of the dark conductivity, this allowed us to ignore leakage currents and surface currents. The ZnSe sample was placed in the copper vacuum cryostat with beryllium and quartz windows in the direction perpendicular to the sample surface. The V-I curves were recorded at temperatures of 8 K, 85 K, 295 K and 420 K. The luminescence and conductivity studies were carried out in vacuum (<1 Pa). The sample was cooled with liquid ni trogen or liquid helium. The temperature of the sample was monitored using a semiconductor sensor and chromel-copel thermocouple to an accuracy of 0.3 K. X-ray excitation (X-excitation) of ZnSe samples was carried out by integral radiation of X-ray tube BXV-7 (Re, 20 kV, 25 mA, anode of the tube was at a distance of 130 mm from the sample, the maximum in tensity of X-ray irradiation was IX ¼ 0.635 mW/cm2) through the beryllium window of cryostat. The radiation intensity of the X-ray tube was calculated by the well-known formula IX tube ¼ 0; 9⋅ 10 9 Z2 U2 i. The peculiarity of such X-excitation (hvX < 20 keV) is the absence of generation of new radiation defects (threshold field for the generation of vacancies and interstitial ions), it is possible only to recharge the existing defects in the crystal. For photo-excitation (UV-excitation) seven ultraviolet light-emitting diodes (seven LEDs) of UF-301 type with maximum radiation at 395 nm were used, i.e. the energy of UV-quanta was greater than the width of the ZnSe bandgap. All seven LEDs were placed on the same installation site Ø ¼ 20 mm and had a common power supply unit, which allowed changing the value of stabilized current through the LEDs in the range of 30–180 mA. The radiation of each LED from a distance of 55 mm, through the quartz window of the cryostat was directed to the sample under study. Previously it was experimentally established that the form of the LED’s radiation spec trum did not depend on the magnitude of the current flowing through it. The value of the sample illumination at different supply currents of the seven LEDs (0.032–0.156 mW/cm2) was visualized. The value of the illumination of sample at various supply currents was measured by a meter of average power and laser radiation energy. The accumulated lightsum was made by red LEDs (FTN33042, 620 nm, 15 mA) and IR LEDs (FTN33046, 942 nm, 15 mA). The sample luminescence was recorded through two independent channels: integrally through an optical filter (OC-13 with a trans mittance at 580 nm) and spectrally through a high-speed
Fig. 1. XRL spectra of the ZnSe sample at 85 K.
monochromator MDR-2 (Sp ¼ 8 nm) and was registered by photo multiplier tube: PMT-106 in the visible region, or PMT-83 (in cooling mode) in the IR-region. Simultaneously the current was registered through the sample of monocrystalline ZnSe. Upon excitation, the phosphorescence and conduction CR were measured (5 � 10 min). After that, the sample was heated and the TSL and TSC curves were con structed. The heating rate for TSL and TSC studies was ~0.30 K/s. All luminescence spectra were corrected for spectral sensitivity of the recording system. 3. Luminescence spectra The luminescence spectra of ZnSe crystals have been widely studied (Ryzhikov et al., 2001c, 2013; Focsha et al., 2002; Lee et al., 2008). In different samples, band-edge luminescence (exciton recombination at small centers), donor–acceptor pair luminescence, as well as different luminescence bands in visible and IR regions are observed. In specially undoped high-resistance ZnSe crystals at temperatures of 8–450 K, as a rule, two wide bands are observed with maxima near 630 and 970 nm (Fig. 1). These two luminescence bands are recombination bands since they are observed in the phosphorescence and thermostimulated lumi nescence spectra (Degoda et al., 2010; Ryzhikov et al., 2007) after their X-ray and UV irradiation. The photoluminescence spectra of these ZnSe samples, upon exci tation by UV quanta with energy greater than the bandgap width, practically coincide with the XRL spectra at temperatures of 8–420 K (Ryzhikov et al., 2001b, 2001c; Starzhinskiy et al., 2005). There are slight differences (up to several nm) in the fixed spectral position of the maximum 630 nm band at room temperature. Lux-luminescent and lux-ampere characteristics obtained at different temperatures (Ryzhikov et al., 2001c; Starzhinskiy et al., 2005) under UV excitation are not fundamentally different from those obtained under X-excitation. 4. Dose dependencies of PL, XRL, PC, XRC We understand dose dependences as the dependence of lumines cence intensity and conductivity current on the duration of X-radiation. The luminescence (for different luminescence bands) and conductivity dependencies are recorded simultaneously, and they do not remain constant in the process of X-excitation. In general, changes in lumines cence intensity and conduction current at constant excitation can be caused by several factors. The first is the change in the concentration of recharged lumines cence centers and recharged traps in the process of excitation, which always occurs due to the accumulation of the light-sum. Such changes are quite predictable if the kinetics of the light-sum accumulation is known. As a rule, they lead to a slow asymptotic increase in lumines cence intensity and conduction current. The second factor is the creation of color centers in the excitation process, which absorb the luminescence radiation in the sample. In this case, the luminescence will experience a monotonous decrease in intensity after initial growth, and the 2
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Fig. 2. Dose dependences of the PL intensities of the 630-nm (1) and 970-nm (2) bands and the PC (3) current at U0 ¼ 3.2 V of the ZnSe sample at 8 K.
Fig. 3. Dose dependences of the XRL intensities of the 630-nm (1) and 970-nm (2) bands and the XRC (3) current at U0 ¼ 15 V of the ZnSe sample at 85 K.
conduction current will not vary when the maximum value is reached. The color centers that can be generated by X-rays will be either new structural defects in the lattice or the recharged existing centers. It is possible to determine the X-ray defective ability by studying the asymptotic dependence of Ilum (t → ∞) (Degoda et al., 2007): for the first case, Ilum (t → ∞) � 0; and for the second case, when there is no for mation of new structural defects, Ilum (t → ∞) 6¼ 0. In ZnSe single crystals the luminescence intensity does not approach zero. It should be noted that changes in luminescence intensity and con duction current may not be observed experimentally in the process of excitation (the changes remain within the measurement error) if the concentration of traps or generated color centers in the excitation area of the sample is significantly lower than the concentration of generated free electron-hole pairs. This is the characteristics and mechanism of ultraviolet (UV) excitation, where, due to the huge absorption coeffi cient, the excitation area thickness does not exceed a few microns. At UV excitation of ZnSe crystals at temperatures of 8–420 K, a rapid (1–3 s) transition to a stationary value of the luminescence intensity of both bands (630 and 970 nm) and photoconductivity current is observed (Degoda et al., 2019). That is, there is practically no influence of traps and recharged luminescence centers on the kinetics of luminescence and conductivity. Also, at UV excitation and low temperatures, there is a small value of the accumulated light-sum compared to X-excitation. This is due to the absorption of UV radiation and the mechanism of free charge carrier generation and, as a result, free charge carriers will be generated only in the near-surface layer of the sample. It is easy to es timate the total number of generated e-h pairs in the near-surface area per unit of sample surface per unit of time. At the intensity of UV exci tation IUV ¼ 0.156 mW/cm2 per unit of time will generated e-h pairs GPL ¼ 3 � 1018 s 1cm 2 in the near-surface area. The number of centers where free charge carriers (traps and recombination centers) can be located at a thickness of 1 μm is less than the number of free carriers generated in this region in 1 s. For example, Fig. 2 shows the initial sections of dose dependences of PL and PC (these dependencies were registered for 2 h) at 8 K of one of the ZnSe samples. Since the dose dependences of PL and PC at all temperatures go to saturation for ~10 s, it can be assumed that the traps and luminescence centers are filled during this time, which is facilitated by a high concentration of e–h pairs generated by UV excitation in the surface layer up to 1 μm. A completely different situation is observed at X-excitation, which is caused by another mechanism of generation of electronic excitations at absorption of a single X-ray quantum (hvX) (Gumenyuk et al., 1986; Degoda et al., 2018a,b) and a much smaller coefficient of X-ray radiation absorption (κX � 250 cm 1 for quanta with an energy of 13 keV,
corresponding to the maximum braking radiation of the X-ray tube at 20 kV). It means that the depth of the excitation area is ~40 μm. It is possible to estimate the total number of generated e-h pairs in the excitation area per unit of sample surface per unit of time at IX ¼ 0.635 mW/cm2. The concentration of generated e-h pairs in the area of X-excitation will be GXRL ¼ 1.2 � 1017 cm 3. The total number of e-h pairs generated by both UV- and X-excitation are approximately the same, and the concentration of free carriers will be ~40 times greater for UV excitation. This confirms the fact that the stationary intensities of PL and XRL will be approximately the same. The concentrations of lumi nescence centers (responsible for the 630- and 970-nm bands) are large enough (>1018 cm 3) to ensure the entire flow of e-h pairs’ recombi nation (Gurvich, 1976). The dose dependences of the XRL intensity and the XRC current at high temperatures (295 and 420 K) change very little in the process of excitation in comparison with the dose dependences at low tempera tures (8 and 85 K). This indicates the effect of deep traps on dose de pendencies. Typical initial areas of dose dependences are shown in Fig. 3 for the ZnSe sample (dependences recorded for 2 h). As for the other samples, the main changes in the intensity of the XRL and XRC occur during the first 20 min. Then, slow changes in XRL intensity are observed, which vary for different luminescence bands. One more
Fig. 4. TSC (1), TSL-630 (2), TSL-970 (3) curves of ZnSe sample after X-exci tation (up to 1 h) at 85 K, U0 ¼ 15 V. 3
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Radiation Measurements 131 (2020) 106232
Fig. 5. Scheme of three registration geometries to determine the dose de pendences of XRL intensity from the registration angle.
peculiarity should also be noted, at low temperatures, the XRC current starts to increase several seconds later than the XRL intensity (Degoda et al., 2019). For all ZnSe samples, there are opposite patterns of the slow change of intensity of the 630- and 970-nm bands at X-excitation. After a sharp increase in the intensity of both bands with applied X-irradiation (due to generation of scintillation pulses): the 630-nm band continues to grow with a constant time ~ 20–35 s, and then slowly decreases. At the same time, the 970-nm band slightly decreases during this time and then slowly increases.
Fig. 7. Normalized dose dependences of the intensity of the 630 -nm (1) and 970-nm (2) bands averaged by the three geometries of the XRL registration at 85 K for the ZnSe sample.
5. Thermally stimulated luminescence and conductivity of ZnSe In oxides, as a rule, the color centers generated by x-excitation are filled deep traps, and at the absorption of luminescent quanta, there is an optical delocalization of charge carriers from traps. Therefore, it is necessary to check whether there are traps in highresistance ZnSe samples using the TSL and TSC methods (Chen and Mckeever, 1997). Fig. 4 shows TSC and TSL curves during registering in the luminescence bands 630- and 970-nm. An increase in TSC current at T > 320 K is due to an increase in the dark conductivity of the ZnSe sample. The magnitude of this current is independent of the X-excitation dose and is the same for different excitation geometries. The trap energy spectra of ZnSe crystals obeys the formula of the harmonic oscillator (Degoda et al., 2016). Comparing phosphorescence, current relaxation, TSL, and TSC ob tained after X-ray excitation at different excitation and registration ge ometries, it turns out that all these curves are similar. The CR curves and phosphorescence are not only similar in nature but also almost coincide in value for different registration geometries. The TSC and TSL curves for the two luminescence bands have a similar character, the same set, and the ratio of peak intensities.
6. Dose dependencies of XRL at different angles of X-excitation To determine whether X-excitation in ZnSe crystals produces color centers that absorb luminescent radiation, it is easier to use the method used for yttriumaluminumgarnet crystals (Degoda et al., 2007). The relative XRL intensity in the case of absorption of luminescence by the color centers is a function of the registration angle. Depending on the geometry of the X-excitation and the XRL registration, the luminescent radiation passes a different distance in the excited area of the sample (Fig. 5). In the presence of absorption of luminescent radiation by the color centers, the luminescence intensity will differ for different di rections of XRL registration. By comparing the dose dependences of the XRL intensity for different registration angles relative to the normal to the sample surface, it is possible to clearly establish the presence of luminescence absorption in the sample. Three geometries were used (angles α were calculated relative to the normal to the sample surface): (a) αX ¼ 0, α630 ¼ π/4, α970 ¼ (b) αX ¼
π/4;
π/4, α630 ¼ 0, α970 ¼ π/2;
Fig. 6. The normalized dose dependences of the XRL for the 630- (a) and 970-nm (b) bands of the ZnSe sample with three recording geometries: 1 – α ¼ 0, 2 – α ¼ -π/4 and 3 – α ¼ π/4. 4
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(c) αX ¼ π/4, α630 ¼
Radiation Measurements 131 (2020) 106232 Chen, R., Mckeever, S.W.S., 1997. Theory of Thermoluminescence and Related Phenomena. World Scientific, Singapore. https://doi.org/10.1142/2781. Dafnei, I., Fasoli, M., Ferroni, F., et al., 2010. Low temperature scintillation in ZnSe crystals. IEEE Trans. Nucl. Sci. 57, 1470–1474. https://doi.org/10.1109/ TNS.2009.2035914. Degoda, V.Y., Sofiienko, A.O., 2007. Spatial generation of electronic excitations at the absorption of X-rays. Ukrainian J. Phys. 52, 256–264. Degoda, V.Y., Sofiienko, A.O., 2010. Specific features of the luminescence and conductivity of zinc selenide on exposure to X-ray and optical excitation. Semiconductors 44, 1–7. https://doi.org/10.1134/S1063782610050040. Degoda, V.Ya, Sofiienko, A.O., 2013. Effect of traps on current impulse from X-ray induced conductivity in wide-gap semiconductors. Phys. B Condens. Matter 426, 24–30. Degoda, V., Gumenjuk, A., Pavlova, N., Sofiienko, A., Sulima, S., 2016. Oscillatory regularity of charge carrier trap energy spectra in ZnSe single crystals. Acta Phys. Pol. 129, 304–309. https://doi.org/10.12693/APhysPolA.129.304. Degoda, V.Ya, Alizadeh, M., Kovalenko, N.O., Pavlova, N.Yu, 2018. V-I characteristics of X-ray conductivity and UV photoconductivity of ZnSe crystals. J. Appl. Phys. 123, 075702. https://aip.scitation.org/doi/abs/10.1063/1.5012597. Degoda, V.Ya, Alizadeh, M., Martynyuk, N.V., Pavlova, N.Yu, 2018. Dose dependences of the conductivity and luminescence in ZnSe single crystals. Acta Phys. Pol., A 133, 4. https://doi.org/10.12693/APhysPolA.133.984. Degoda, V.Ya, Alizadeh, M., Kogut, YaP., Pavlova, N.Yu, Sulima, S.V., 2019. The influence of UV excitation intensity on photoconductivity and luminescence in ZnSe crystals. J. Lumin. 205, 540–547. https://doi.org/10.1016/j.jlumin.2018.09.051. Focsh, A.A., Gashin, P.A., Ryzhikov, V.D., Starzhinskiy, N.G., Ga�lchinetskii, L.P., Silin, V. I., 2002. Properties of semiconductor scintillators and combined detectors of ionizing radiation based on ZnSe(Te,O)/pZnTe–nCdSe structures. Opt. Mater. 19, 213–217. Gavrilenko, V.I., Grehov, A.M., Korbutyak, D.V., Litovchenko, V.G., 1987. Optical Properties of Semiconductors. Naukova dumka, Kiev [in Russian]. Gumenyuk, A.F., Degoda, V.Ya, Kuchakova, T.A., 1986. X-ray luminescence and color centers in YAG:Nd3þ crystals. Ukrainian J. Phys. 31, 831–836 [in Russian]. Gurvich, A.M., 1976. X-Ray Luminophors and X-Ray Screens. Atomizdat, Moscow [in Russian]. Lee, Woo Gyo, Kim, Yong Kyun, Kim, Jong Kyung, Starzhinskiy, Nicolai, Ryzhikov, Vladimir, Grinyov, Boris, 2008. Growth and properties of new ZnSe(Al,O, Te) semiconductor scintillator. Radiat. Meas. 43, 502–505. https://doi.org/10.1016/ j.radmeas.2008.02.008. Morozova, N.K., Kuznetsov, V.A., Ryzhikov, V.D., 1992. Zinc Selenide: Receiving and Optical Properties. Science, Moscow [in Russian]. Ryzhikov, V.D., et al., 2001. Properties of semiconductor scintillators ZnSe(Te,O) and integrated scintielectronic radiation detectors based thereon. IEEE Trans. Nucl. Sci. 48, 356–359. https://doi.org/10.1109/23.940080. Ryzhikov, V., Starzhinskiy, N., Gal’chinetskii, L., et al., 2001. Behavior of new ZnSe(Te, O) semiconductor scintillators under high doses of ionizing radiation. IEEE Trans. Nucl. Sci. 48, 1561–1564. Ryzhikov, V.D., Starzhinskiy, N.G., Gal’chinetskii, L.P., et al., 2001. The role of oxygen in formation of radiative recombination centers in ZnSe1-xTex crystals. Int. J. Inorg. Mater. 3, 1227–1229. https://doi.org/10.1016/S1466-6049(01)00138-6. Ryzhikov, V., Grynyov, B., Opolonin, A., Naydenov, S., Lisetska, O., Galkin, S., Voronkin, E., 2007. Scintillation materials and detectors on their base for nondestructive two energy testing. Radiat. Meas. 42, 915–920. https://doi.org/10.1016/ j.radmeas.2007.02.025. Ryzhikov, V., Grinyov, B., Galkin, S., Starzhinskiy, N., Rybalka, I., 2013. Growing technology and luminescent characteristics of ZnSe doped crystals. J. Cryst. Growth 364, 111–117. Sofiienko, A.O., Degoda, V.Y., 2012. X-ray induced conductivity of ZnSe sensors at high temperatures. Radiat. Meas. 47, 27–29. Starzhinkiy, N., Grinyov, B., Zenya, I., Ryzhikov, V., Gal’chinetskii, L., Silin, V., 2008. New trends in the development of AIIBVI-based scintillators. IEEE Trans. Nucl. Sci. 55, 1542–1546. https://doi.org/10.1109/TNS.2008.921929. Starzhinskiy, N.G., Ryzhikov, V.D., Gal’chinetskii, L.P., Nagornaya, L.L., Silin, V.I., 2005. Radiation-induced processes in A2B6 compounds under proton and gammairradiation. VANT 3, 43–46.
π/2, α970 ¼ 0.
For each type of geometry, dose dependences of the XRL intensity of the 630 and 970-nm bands at 85 K were measured for 80 min (Fig. 6). The dose dependences for each band at different geometries are similar. If the corresponding bands are normalized to the maximum intensity, then they are practically identical. Fig. 7 shows the normalized dose dependences of the intensities of 630- and 970-nm bands, averaged over the three geometries. Thus, we confirm the fact that at X-xcitation in ZnSe crystals there is no absorption of luminescence quanta. Optical delocalization of charge carriers from traps should increase the concentration of free carriers. Therefore, the value of conductivity current at additional illumination by optical quanta with wavelengths of 620 or 942 nm at X-excitation and relaxation of conduction current after excitation was checked. For this purpose, we used red and infrared IR LEDs with maximum radiation at 620 and 942 nm, respectively. It has been established that the illumination of the ZnSe sample by optical quanta of 605–645 nm and 920–980 nm, both during X-excitation and after a long X-ray excitationX-excitation, does not change the XRC сurrent or XRC relaxation current with an accuracy up to 1%. Note that the total number of quanta from both LEDs that passed through the ZnSe sample was at least 10 times higher than the number of luminescent quanta. Thus, it is possible to assert that in ZnSe monocrystals there is no optical delocalization of carriers from traps at illumination by quanta which coincide with quanta of luminescent radiation. 7. Conclusions By using various geometries of X-excitation and registration of luminescent radiation we established that X-excitation in ZnSe crystals does not create color centers which absorb luminescent radiation. A slow decrease in the luminescence intensity of the 630-nm band and the same slow increase in the intensity of the 970-nm band at long X-exci tation, is probably due to slow changes in the concentrations of various recharged recombination centers. The absence of generation of color centers confirms the absence of conduction current change at additional optical intensive irradiation of ZnSe samples by optical quanta with wavelengths that coincide with luminescent quanta. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. References Atroshchenko, L.V., Burachas, S.F., Galchinetski, L.P., Grinev, B.V., Ryzhikov, V.D., Starzhinskii, N.G., 1998. Scintillation Crystals and Ionization Radiation Detectors on Their Base. Naukova dumka, Kiev [in Ukrainian]. Brodyn, M.S., Degoda, V.Ya, Kozhushko, B.V., Sofiienko, A.O., Vesna, V.T., 2014. Monocrystalline ZnSe as an ionising radiation detector operated over a wide temperature range. Radiat. Meas. 65, 36–44.
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Radiation Measurements journal homepage: http://www.elsevier.com/locate/radmeas
Formation of color centers at X-ray irradiation of ZnSe single crystals V. Ya Degoda a, G.P. Podust a, *, N. Yu Pavlova b, M. Alizadeh a a b
Taras Shevchenko National University of Kyiv, Physics Department, 03680, Kyiv, Ukraine National Pedagogical Dragomanov University, Kyiv, Ukraine
A R T I C L E I N F O
A B S T R A C T
Keywords: ZnSe Monocrystal Broadband semiconductors X-ray luminescence Photoluminescence Color centers
In the present work we study the dose dependences of X-ray luminescence and X-ray conductivity at different temperatures and different angles of incidence of the exciting radiation and the registration of luminescence. Low-energy X-ray quanta (with energies up to 20 keV) do not create new structural defects in ZnSe crystals, but can only recharge existing ones. It was established from the dose dependences of the luminescence intensity of the 630-nm and 970-nm bands that X-rays do not create the color centers in ZnSe crystals, even those that are temperature unstable, which absorb the luminescent radiation. This fact is confirmed by the absence of con duction current change at additional optical intensive irradiation of ZnSe samples by optical quanta with wavelengths that coincide with luminescent quanta.
1. Introduction Zinc selenide (ZnSe) belongs to the АІІВVI wide-gap semiconductors (Gavrilenko et al., 1987; Morozova et al., 1992; Atroshchenko et al., 1998; Ryzhikov et al., 2001a; Starzhinkiy et al., 2008; Dafnei et al., 2010; Degoda et al., 2010) and has been studied for a long time. The use of especially undoped ZnSe as a semiconductor detector became possible only after development of technologies for growing sufficiently high-quality monocrystals with low concentrations of uncontrolled im purities and high specific resistance of the material at ~1010–1014 Ohm∙cm. It should be noted that a relatively high value of the effective atomic number Zef ¼ 32 and a bandgap width Eg ¼ 2.7 eV (at 300 K) make ZnSe a promising material for creating X-ray detectors that do not require cooling (Sofiienko et al., 2012; Brodyn et al., 2014). Such semiconductor detectors of irradiation work well in the direct current registration mode, but only in some samples can the current pulses be registered. This is due to the presence of a significant concentration of traps in ZnSe crystals, which reduce the amplitude of the current pulse and increase its duration (Degoda et al., 2013). The ZnSe crystal-based scintillators are not hygroscopic, their radi ation stability is relatively good, their light output is 1.1–1.5 times higher, and afterglow level after 10 ms – by 2–3 orders of magnitude lower with respect to oxides and CsI (TI), which are the most widely used in low-energy (E < 100 keV) detectors (Ryzhikov et al., 2001b, 2007, 2013; Lee et al., 2008). The emission spectrum of ZnSe-based scintillators (maximum at ~600 nm) is very convenient for
photodiode recording. There is a large amount of data on the radiation resistance of ZnSebased scintillators regarding the creation of structural defects. High radiation resistance is observed for ZnSe crystals under the influence of ionizing radiation when compared to most other scintillators: gammaquanta (~1.3 MeV, up to 500 Mrad), protons (~18 MeV, up to 1016 protons/cm2), electrons (from 0.54 to 2.26 MeV, up to 50 Mrad) and neutrons (source channel-heat reactor, up to 1018 neutrons/cm2) (Starzhinskiy et al., 2005). As a rule, radiation resistance is commonly understood as the ability to resist the creation of new structural defects in a material that change the optical transmission of crystals after irradiation by ionizing radia tion. This approach does not take into account temperature-instable radiation defects that can absorb luminescence during irradiation. Ionizing radiation may not create new crystalline lattice (vacancies (ions)) defects but may recharge the existing ones. Such recharged centers are commonly called the color centers which also significantly affect the detector’s operation. Therefore, the purpose of this work is to verify if X-ray radiation creates the color centers that absorb lumines cent quanta in high-resistance ZnSe single crystals. 2. Experimental methodology The ZnSe crystals were grown from the prepurified charge and were especially not doped during crystal growth. As a result, the samples were obtained with a minimum concentration of point defects and maximum
* Corresponding author. E-mail address: [email protected] (G.P. Podust). https://doi.org/10.1016/j.radmeas.2019.106232 Received 2 August 2019; Received in revised form 13 December 2019; Accepted 17 December 2019 Available online 18 December 2019 1350-4487/© 2019 Elsevier Ltd. All rights reserved.
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specific resistance (ρ � 1012–1014 Ohm∙cm). To research various crys talline boules, samples of 18 mm � 9 mm � 2 mm were cut out and polished. Studies of photoconductivity (PC), X-ray conductivity (XRC), pho toluminescence (PL), X-ray luminescence (XRL), their dose and tem perature dependences, phosphorescence (Ph), conduction current relaxation (CR), thermally stimulated conductivity (TSC), thermally stimulated luminescence (TSL) were carried out. The two three-layered metal contacts were sprayed by a resistive method on the single crystals to study the conductivity. The chemical composition of each layer was selected purposefully to obtain highresistance contacts with proper adhesion. Earlier, it was experimen tally verified that the application of contacts and heating to 450 K did not change the luminescence characteristics of the samples. It was also found that the luminescence and conductivity of the samples were not changed for 4 years, when they were irradiated with X quanta, cooled to 8 K and heated to 450 K (Degoda et al., 2018a,b). The contacts were the rectangular-shaped strips of L ¼ 5 mm length, 1 mm width and the distance between them was d ¼ 5 mm. The copper conductors were soldered to electrical contacts to measure conductivity. The stabilized voltage of 0–1000 V was applied to one of the electrodes, while the other was earthed through nanoamperemeter. The stabilized voltage unit and the nanoamperemeter were fabricated and calibrated according to standard circuits. For all values of conductivity current, the input impedance of the nanoamperemeter was several orders of magnitude smaller than the electrical resistance of the ZnSe sample. Since the value of the X-ray current was 104 times greater than the magnitude of the dark conductivity, this allowed us to ignore leakage currents and surface currents. The ZnSe sample was placed in the copper vacuum cryostat with beryllium and quartz windows in the direction perpendicular to the sample surface. The V-I curves were recorded at temperatures of 8 K, 85 K, 295 K and 420 K. The luminescence and conductivity studies were carried out in vacuum (<1 Pa). The sample was cooled with liquid ni trogen or liquid helium. The temperature of the sample was monitored using a semiconductor sensor and chromel-copel thermocouple to an accuracy of 0.3 K. X-ray excitation (X-excitation) of ZnSe samples was carried out by integral radiation of X-ray tube BXV-7 (Re, 20 kV, 25 mA, anode of the tube was at a distance of 130 mm from the sample, the maximum in tensity of X-ray irradiation was IX ¼ 0.635 mW/cm2) through the beryllium window of cryostat. The radiation intensity of the X-ray tube was calculated by the well-known formula IX tube ¼ 0; 9⋅ 10 9 Z2 U2 i. The peculiarity of such X-excitation (hvX < 20 keV) is the absence of generation of new radiation defects (threshold field for the generation of vacancies and interstitial ions), it is possible only to recharge the existing defects in the crystal. For photo-excitation (UV-excitation) seven ultraviolet light-emitting diodes (seven LEDs) of UF-301 type with maximum radiation at 395 nm were used, i.e. the energy of UV-quanta was greater than the width of the ZnSe bandgap. All seven LEDs were placed on the same installation site Ø ¼ 20 mm and had a common power supply unit, which allowed changing the value of stabilized current through the LEDs in the range of 30–180 mA. The radiation of each LED from a distance of 55 mm, through the quartz window of the cryostat was directed to the sample under study. Previously it was experimentally established that the form of the LED’s radiation spec trum did not depend on the magnitude of the current flowing through it. The value of the sample illumination at different supply currents of the seven LEDs (0.032–0.156 mW/cm2) was visualized. The value of the illumination of sample at various supply currents was measured by a meter of average power and laser radiation energy. The accumulated lightsum was made by red LEDs (FTN33042, 620 nm, 15 mA) and IR LEDs (FTN33046, 942 nm, 15 mA). The sample luminescence was recorded through two independent channels: integrally through an optical filter (OC-13 with a trans mittance at 580 nm) and spectrally through a high-speed
Fig. 1. XRL spectra of the ZnSe sample at 85 K.
monochromator MDR-2 (Sp ¼ 8 nm) and was registered by photo multiplier tube: PMT-106 in the visible region, or PMT-83 (in cooling mode) in the IR-region. Simultaneously the current was registered through the sample of monocrystalline ZnSe. Upon excitation, the phosphorescence and conduction CR were measured (5 � 10 min). After that, the sample was heated and the TSL and TSC curves were con structed. The heating rate for TSL and TSC studies was ~0.30 K/s. All luminescence spectra were corrected for spectral sensitivity of the recording system. 3. Luminescence spectra The luminescence spectra of ZnSe crystals have been widely studied (Ryzhikov et al., 2001c, 2013; Focsha et al., 2002; Lee et al., 2008). In different samples, band-edge luminescence (exciton recombination at small centers), donor–acceptor pair luminescence, as well as different luminescence bands in visible and IR regions are observed. In specially undoped high-resistance ZnSe crystals at temperatures of 8–450 K, as a rule, two wide bands are observed with maxima near 630 and 970 nm (Fig. 1). These two luminescence bands are recombination bands since they are observed in the phosphorescence and thermostimulated lumi nescence spectra (Degoda et al., 2010; Ryzhikov et al., 2007) after their X-ray and UV irradiation. The photoluminescence spectra of these ZnSe samples, upon exci tation by UV quanta with energy greater than the bandgap width, practically coincide with the XRL spectra at temperatures of 8–420 K (Ryzhikov et al., 2001b, 2001c; Starzhinskiy et al., 2005). There are slight differences (up to several nm) in the fixed spectral position of the maximum 630 nm band at room temperature. Lux-luminescent and lux-ampere characteristics obtained at different temperatures (Ryzhikov et al., 2001c; Starzhinskiy et al., 2005) under UV excitation are not fundamentally different from those obtained under X-excitation. 4. Dose dependencies of PL, XRL, PC, XRC We understand dose dependences as the dependence of lumines cence intensity and conductivity current on the duration of X-radiation. The luminescence (for different luminescence bands) and conductivity dependencies are recorded simultaneously, and they do not remain constant in the process of X-excitation. In general, changes in lumines cence intensity and conduction current at constant excitation can be caused by several factors. The first is the change in the concentration of recharged lumines cence centers and recharged traps in the process of excitation, which always occurs due to the accumulation of the light-sum. Such changes are quite predictable if the kinetics of the light-sum accumulation is known. As a rule, they lead to a slow asymptotic increase in lumines cence intensity and conduction current. The second factor is the creation of color centers in the excitation process, which absorb the luminescence radiation in the sample. In this case, the luminescence will experience a monotonous decrease in intensity after initial growth, and the 2
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Fig. 2. Dose dependences of the PL intensities of the 630-nm (1) and 970-nm (2) bands and the PC (3) current at U0 ¼ 3.2 V of the ZnSe sample at 8 K.
Fig. 3. Dose dependences of the XRL intensities of the 630-nm (1) and 970-nm (2) bands and the XRC (3) current at U0 ¼ 15 V of the ZnSe sample at 85 K.
conduction current will not vary when the maximum value is reached. The color centers that can be generated by X-rays will be either new structural defects in the lattice or the recharged existing centers. It is possible to determine the X-ray defective ability by studying the asymptotic dependence of Ilum (t → ∞) (Degoda et al., 2007): for the first case, Ilum (t → ∞) � 0; and for the second case, when there is no for mation of new structural defects, Ilum (t → ∞) 6¼ 0. In ZnSe single crystals the luminescence intensity does not approach zero. It should be noted that changes in luminescence intensity and con duction current may not be observed experimentally in the process of excitation (the changes remain within the measurement error) if the concentration of traps or generated color centers in the excitation area of the sample is significantly lower than the concentration of generated free electron-hole pairs. This is the characteristics and mechanism of ultraviolet (UV) excitation, where, due to the huge absorption coeffi cient, the excitation area thickness does not exceed a few microns. At UV excitation of ZnSe crystals at temperatures of 8–420 K, a rapid (1–3 s) transition to a stationary value of the luminescence intensity of both bands (630 and 970 nm) and photoconductivity current is observed (Degoda et al., 2019). That is, there is practically no influence of traps and recharged luminescence centers on the kinetics of luminescence and conductivity. Also, at UV excitation and low temperatures, there is a small value of the accumulated light-sum compared to X-excitation. This is due to the absorption of UV radiation and the mechanism of free charge carrier generation and, as a result, free charge carriers will be generated only in the near-surface layer of the sample. It is easy to es timate the total number of generated e-h pairs in the near-surface area per unit of sample surface per unit of time. At the intensity of UV exci tation IUV ¼ 0.156 mW/cm2 per unit of time will generated e-h pairs GPL ¼ 3 � 1018 s 1cm 2 in the near-surface area. The number of centers where free charge carriers (traps and recombination centers) can be located at a thickness of 1 μm is less than the number of free carriers generated in this region in 1 s. For example, Fig. 2 shows the initial sections of dose dependences of PL and PC (these dependencies were registered for 2 h) at 8 K of one of the ZnSe samples. Since the dose dependences of PL and PC at all temperatures go to saturation for ~10 s, it can be assumed that the traps and luminescence centers are filled during this time, which is facilitated by a high concentration of e–h pairs generated by UV excitation in the surface layer up to 1 μm. A completely different situation is observed at X-excitation, which is caused by another mechanism of generation of electronic excitations at absorption of a single X-ray quantum (hvX) (Gumenyuk et al., 1986; Degoda et al., 2018a,b) and a much smaller coefficient of X-ray radiation absorption (κX � 250 cm 1 for quanta with an energy of 13 keV,
corresponding to the maximum braking radiation of the X-ray tube at 20 kV). It means that the depth of the excitation area is ~40 μm. It is possible to estimate the total number of generated e-h pairs in the excitation area per unit of sample surface per unit of time at IX ¼ 0.635 mW/cm2. The concentration of generated e-h pairs in the area of X-excitation will be GXRL ¼ 1.2 � 1017 cm 3. The total number of e-h pairs generated by both UV- and X-excitation are approximately the same, and the concentration of free carriers will be ~40 times greater for UV excitation. This confirms the fact that the stationary intensities of PL and XRL will be approximately the same. The concentrations of lumi nescence centers (responsible for the 630- and 970-nm bands) are large enough (>1018 cm 3) to ensure the entire flow of e-h pairs’ recombi nation (Gurvich, 1976). The dose dependences of the XRL intensity and the XRC current at high temperatures (295 and 420 K) change very little in the process of excitation in comparison with the dose dependences at low tempera tures (8 and 85 K). This indicates the effect of deep traps on dose de pendencies. Typical initial areas of dose dependences are shown in Fig. 3 for the ZnSe sample (dependences recorded for 2 h). As for the other samples, the main changes in the intensity of the XRL and XRC occur during the first 20 min. Then, slow changes in XRL intensity are observed, which vary for different luminescence bands. One more
Fig. 4. TSC (1), TSL-630 (2), TSL-970 (3) curves of ZnSe sample after X-exci tation (up to 1 h) at 85 K, U0 ¼ 15 V. 3
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Fig. 5. Scheme of three registration geometries to determine the dose de pendences of XRL intensity from the registration angle.
peculiarity should also be noted, at low temperatures, the XRC current starts to increase several seconds later than the XRL intensity (Degoda et al., 2019). For all ZnSe samples, there are opposite patterns of the slow change of intensity of the 630- and 970-nm bands at X-excitation. After a sharp increase in the intensity of both bands with applied X-irradiation (due to generation of scintillation pulses): the 630-nm band continues to grow with a constant time ~ 20–35 s, and then slowly decreases. At the same time, the 970-nm band slightly decreases during this time and then slowly increases.
Fig. 7. Normalized dose dependences of the intensity of the 630 -nm (1) and 970-nm (2) bands averaged by the three geometries of the XRL registration at 85 K for the ZnSe sample.
5. Thermally stimulated luminescence and conductivity of ZnSe In oxides, as a rule, the color centers generated by x-excitation are filled deep traps, and at the absorption of luminescent quanta, there is an optical delocalization of charge carriers from traps. Therefore, it is necessary to check whether there are traps in highresistance ZnSe samples using the TSL and TSC methods (Chen and Mckeever, 1997). Fig. 4 shows TSC and TSL curves during registering in the luminescence bands 630- and 970-nm. An increase in TSC current at T > 320 K is due to an increase in the dark conductivity of the ZnSe sample. The magnitude of this current is independent of the X-excitation dose and is the same for different excitation geometries. The trap energy spectra of ZnSe crystals obeys the formula of the harmonic oscillator (Degoda et al., 2016). Comparing phosphorescence, current relaxation, TSL, and TSC ob tained after X-ray excitation at different excitation and registration ge ometries, it turns out that all these curves are similar. The CR curves and phosphorescence are not only similar in nature but also almost coincide in value for different registration geometries. The TSC and TSL curves for the two luminescence bands have a similar character, the same set, and the ratio of peak intensities.
6. Dose dependencies of XRL at different angles of X-excitation To determine whether X-excitation in ZnSe crystals produces color centers that absorb luminescent radiation, it is easier to use the method used for yttriumaluminumgarnet crystals (Degoda et al., 2007). The relative XRL intensity in the case of absorption of luminescence by the color centers is a function of the registration angle. Depending on the geometry of the X-excitation and the XRL registration, the luminescent radiation passes a different distance in the excited area of the sample (Fig. 5). In the presence of absorption of luminescent radiation by the color centers, the luminescence intensity will differ for different di rections of XRL registration. By comparing the dose dependences of the XRL intensity for different registration angles relative to the normal to the sample surface, it is possible to clearly establish the presence of luminescence absorption in the sample. Three geometries were used (angles α were calculated relative to the normal to the sample surface): (a) αX ¼ 0, α630 ¼ π/4, α970 ¼ (b) αX ¼
π/4;
π/4, α630 ¼ 0, α970 ¼ π/2;
Fig. 6. The normalized dose dependences of the XRL for the 630- (a) and 970-nm (b) bands of the ZnSe sample with three recording geometries: 1 – α ¼ 0, 2 – α ¼ -π/4 and 3 – α ¼ π/4. 4
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(c) αX ¼ π/4, α630 ¼
Radiation Measurements 131 (2020) 106232 Chen, R., Mckeever, S.W.S., 1997. Theory of Thermoluminescence and Related Phenomena. World Scientific, Singapore. https://doi.org/10.1142/2781. Dafnei, I., Fasoli, M., Ferroni, F., et al., 2010. Low temperature scintillation in ZnSe crystals. IEEE Trans. Nucl. Sci. 57, 1470–1474. https://doi.org/10.1109/ TNS.2009.2035914. Degoda, V.Y., Sofiienko, A.O., 2007. Spatial generation of electronic excitations at the absorption of X-rays. Ukrainian J. Phys. 52, 256–264. Degoda, V.Y., Sofiienko, A.O., 2010. Specific features of the luminescence and conductivity of zinc selenide on exposure to X-ray and optical excitation. Semiconductors 44, 1–7. https://doi.org/10.1134/S1063782610050040. Degoda, V.Ya, Sofiienko, A.O., 2013. Effect of traps on current impulse from X-ray induced conductivity in wide-gap semiconductors. Phys. B Condens. Matter 426, 24–30. Degoda, V., Gumenjuk, A., Pavlova, N., Sofiienko, A., Sulima, S., 2016. Oscillatory regularity of charge carrier trap energy spectra in ZnSe single crystals. Acta Phys. Pol. 129, 304–309. https://doi.org/10.12693/APhysPolA.129.304. Degoda, V.Ya, Alizadeh, M., Kovalenko, N.O., Pavlova, N.Yu, 2018. V-I characteristics of X-ray conductivity and UV photoconductivity of ZnSe crystals. J. Appl. Phys. 123, 075702. https://aip.scitation.org/doi/abs/10.1063/1.5012597. Degoda, V.Ya, Alizadeh, M., Martynyuk, N.V., Pavlova, N.Yu, 2018. Dose dependences of the conductivity and luminescence in ZnSe single crystals. Acta Phys. Pol., A 133, 4. https://doi.org/10.12693/APhysPolA.133.984. Degoda, V.Ya, Alizadeh, M., Kogut, YaP., Pavlova, N.Yu, Sulima, S.V., 2019. The influence of UV excitation intensity on photoconductivity and luminescence in ZnSe crystals. J. Lumin. 205, 540–547. https://doi.org/10.1016/j.jlumin.2018.09.051. Focsh, A.A., Gashin, P.A., Ryzhikov, V.D., Starzhinskiy, N.G., Ga�lchinetskii, L.P., Silin, V. I., 2002. Properties of semiconductor scintillators and combined detectors of ionizing radiation based on ZnSe(Te,O)/pZnTe–nCdSe structures. Opt. Mater. 19, 213–217. Gavrilenko, V.I., Grehov, A.M., Korbutyak, D.V., Litovchenko, V.G., 1987. Optical Properties of Semiconductors. Naukova dumka, Kiev [in Russian]. Gumenyuk, A.F., Degoda, V.Ya, Kuchakova, T.A., 1986. X-ray luminescence and color centers in YAG:Nd3þ crystals. Ukrainian J. Phys. 31, 831–836 [in Russian]. Gurvich, A.M., 1976. X-Ray Luminophors and X-Ray Screens. Atomizdat, Moscow [in Russian]. Lee, Woo Gyo, Kim, Yong Kyun, Kim, Jong Kyung, Starzhinskiy, Nicolai, Ryzhikov, Vladimir, Grinyov, Boris, 2008. Growth and properties of new ZnSe(Al,O, Te) semiconductor scintillator. Radiat. Meas. 43, 502–505. https://doi.org/10.1016/ j.radmeas.2008.02.008. Morozova, N.K., Kuznetsov, V.A., Ryzhikov, V.D., 1992. Zinc Selenide: Receiving and Optical Properties. Science, Moscow [in Russian]. Ryzhikov, V.D., et al., 2001. Properties of semiconductor scintillators ZnSe(Te,O) and integrated scintielectronic radiation detectors based thereon. IEEE Trans. Nucl. Sci. 48, 356–359. https://doi.org/10.1109/23.940080. Ryzhikov, V., Starzhinskiy, N., Gal’chinetskii, L., et al., 2001. Behavior of new ZnSe(Te, O) semiconductor scintillators under high doses of ionizing radiation. IEEE Trans. Nucl. Sci. 48, 1561–1564. Ryzhikov, V.D., Starzhinskiy, N.G., Gal’chinetskii, L.P., et al., 2001. The role of oxygen in formation of radiative recombination centers in ZnSe1-xTex crystals. Int. J. Inorg. Mater. 3, 1227–1229. https://doi.org/10.1016/S1466-6049(01)00138-6. Ryzhikov, V., Grynyov, B., Opolonin, A., Naydenov, S., Lisetska, O., Galkin, S., Voronkin, E., 2007. Scintillation materials and detectors on their base for nondestructive two energy testing. Radiat. Meas. 42, 915–920. https://doi.org/10.1016/ j.radmeas.2007.02.025. Ryzhikov, V., Grinyov, B., Galkin, S., Starzhinskiy, N., Rybalka, I., 2013. Growing technology and luminescent characteristics of ZnSe doped crystals. J. Cryst. Growth 364, 111–117. Sofiienko, A.O., Degoda, V.Y., 2012. X-ray induced conductivity of ZnSe sensors at high temperatures. Radiat. Meas. 47, 27–29. Starzhinkiy, N., Grinyov, B., Zenya, I., Ryzhikov, V., Gal’chinetskii, L., Silin, V., 2008. New trends in the development of AIIBVI-based scintillators. IEEE Trans. Nucl. Sci. 55, 1542–1546. https://doi.org/10.1109/TNS.2008.921929. Starzhinskiy, N.G., Ryzhikov, V.D., Gal’chinetskii, L.P., Nagornaya, L.L., Silin, V.I., 2005. Radiation-induced processes in A2B6 compounds under proton and gammairradiation. VANT 3, 43–46.
π/2, α970 ¼ 0.
For each type of geometry, dose dependences of the XRL intensity of the 630 and 970-nm bands at 85 K were measured for 80 min (Fig. 6). The dose dependences for each band at different geometries are similar. If the corresponding bands are normalized to the maximum intensity, then they are practically identical. Fig. 7 shows the normalized dose dependences of the intensities of 630- and 970-nm bands, averaged over the three geometries. Thus, we confirm the fact that at X-xcitation in ZnSe crystals there is no absorption of luminescence quanta. Optical delocalization of charge carriers from traps should increase the concentration of free carriers. Therefore, the value of conductivity current at additional illumination by optical quanta with wavelengths of 620 or 942 nm at X-excitation and relaxation of conduction current after excitation was checked. For this purpose, we used red and infrared IR LEDs with maximum radiation at 620 and 942 nm, respectively. It has been established that the illumination of the ZnSe sample by optical quanta of 605–645 nm and 920–980 nm, both during X-excitation and after a long X-ray excitationX-excitation, does not change the XRC сurrent or XRC relaxation current with an accuracy up to 1%. Note that the total number of quanta from both LEDs that passed through the ZnSe sample was at least 10 times higher than the number of luminescent quanta. Thus, it is possible to assert that in ZnSe monocrystals there is no optical delocalization of carriers from traps at illumination by quanta which coincide with quanta of luminescent radiation. 7. Conclusions By using various geometries of X-excitation and registration of luminescent radiation we established that X-excitation in ZnSe crystals does not create color centers which absorb luminescent radiation. A slow decrease in the luminescence intensity of the 630-nm band and the same slow increase in the intensity of the 970-nm band at long X-exci tation, is probably due to slow changes in the concentrations of various recharged recombination centers. The absence of generation of color centers confirms the absence of conduction current change at additional optical intensive irradiation of ZnSe samples by optical quanta with wavelengths that coincide with luminescent quanta. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. References Atroshchenko, L.V., Burachas, S.F., Galchinetski, L.P., Grinev, B.V., Ryzhikov, V.D., Starzhinskii, N.G., 1998. Scintillation Crystals and Ionization Radiation Detectors on Their Base. Naukova dumka, Kiev [in Ukrainian]. Brodyn, M.S., Degoda, V.Ya, Kozhushko, B.V., Sofiienko, A.O., Vesna, V.T., 2014. Monocrystalline ZnSe as an ionising radiation detector operated over a wide temperature range. Radiat. Meas. 65, 36–44.
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