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Investigation of the thermal annealing effect on the defects structure in γ-irradiated CdZnTe crystals by photoluminescence method
Nuclear Instruments and Methods in Physics Research B 290 (2012) 26–29
Contents lists available at SciVerse ScienceDirect
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Investigation of the thermal annealing effect on the defects structure in c-irradiated CdZnTe crystals by photoluminescence method Iu. Nasieka a,b,⇑, L. Rashkovetskyi a, O. Strilchuk a, V. Maslov a, E. Venger a a b
Lashkarev Institute of Semiconductor Physics, NAS of Ukraine, 45 Pr. Nauki, Kyiv, Ukraine Nizhyn Agricultural Institute of National University of Life and Environmental Sciences of Ukraine, 10 Shevchenka Str., Nizhyn, Ukraine
a r t i c l e
i n f o
Article history: Received 26 July 2012 Received in revised form 23 August 2012 Available online 8 September 2012 Keywords: Thermal annealing CdZnTe crystals c-Irradiation Low-temperature photoluminescence
a b s t r a c t The influence of the thermal annealing at 100 and 200 °C on the low-temperature photoluminescence properties of different doses c-irradiated Cd0.95Zn0.05Te crystals is investigated. It was obtained such thermal annealing induces increasing of the amplitude of donor bound exciton lines and A-centers line which were reduced by preliminary c-irradiation. Such effect can be explained by the reconstruction of the initial donor defects structure. In this case the reconstruction process is caused by the transmission of the additional thermal energy which leads to the substantial movement of the radiation induced vacancies and interstitials ions with their further annihilation. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction As it generally known [1,2] CdZnTe crystals have the ability to detect and perform energy-dispersive spectroscopy of high energy radiation such as X-rays, c-rays and other types of ionizing radiation. Except this Cd0.95Zn0.05Te is a promising substrate material for the growing of high quality CdHgTe infrared detectors. Therefore the noted materials have a wide application in the field of the manufacturing of uncooled semiconductor radiation and infrared detectors. Such types of detectors have the ability to work at room temperature and can be characterized by high sensitivity and selectivity. Another advantage of pointed detectors is very small functional sizes which favorably distinguish them from, for example, gas ionizing cameras [3]. But despite the recent the wide range of the questions, particularly concerning radiation and thermal stimulated defect creations in CdZnTe crystals and influence of noted defects on their optical properties are remain unexplored. In this paper we present an analysis of the changes in the low-temperature photoluminescence properties of different doses c-irradiated CdZnTe crystals under thermal annealing. It is worth noting the following facts there. In the work [6] we reported the study of the effect of c-irradiation (with the doses in the range 10–100 kGy) on the low-temperature photoluminescence of Cd0.95Zn0.05Te crystals. We pointed that the following two phenomena induced by c-irradiation were observed. The first was the essential decrease of the intensities of the initial (as⇑ Corresponding author at: Lashkarev Institute of Semiconductor Physics, NAS of Ukraine, 45 Pr. Nauki, Kyiv, Ukraine. E-mail address: [email protected] (Iu. Nasieka). 0168-583X/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.nimb.2012.09.002
grown) luminescence lines – the deep level defects line (general peak centered at 1.409 eV), donor–acceptor pairs (peak centered at 1.547 eV) and shallow acceptor (peak centered at 1.556 eV) lines as well as intensity of the lines related to excitons bound to shallow neutral acceptors and donors (peaks centered at 1.592 and 1.599 eV, respectively). Such changes in the photoluminescence spectra we have explained by the decrease in the concentration of the corresponding luminescence centers due to their interaction with radiation-induced defects. The second phenomena was an appearance of new luminescence lines which were probably caused by radiation-induced cadmium vacancies (VCd) bound to other defects (donor–acceptor pairs) and isolated cadmium vacancies as well as excitons bound to the indicated Cd-vacancies. The intensities of the radiation-induced lines changed non-monotonically with the increase of the c-irradiation dose (see Fig. 1): at low doses (650 kGy) it increased due to the increase in the concentration of radiation-induced Cd-vacancies and then considerably decreased at high doses (>50 kGy) due to generation of a large number of effective centers of radiationless recombination of excess charge carriers [6–10]. The recent work is the continuation of the above study.
2. Experimental part Our investigations were performed for p-Cd0.95Zn0.05Te crystals with the room-temperature resistivity q 60 Ohm cm and q ? 1 at T = 5 K (the conduction of the crystals at 5 K is determined by excess electrons and holes) which grown with the help of the Bridgman technique in V.E. Lashkarev Institute of semiconductor
Iu. Nasieka et al. / Nuclear Instruments and Methods in Physics Research B 290 (2012) 26–29
27
Fig. 2. Photoluminescence spectra of Cd0.95Zn0.5Te crystals observed at 5 K: (1) initial sample and (2) sample irradiated with the dose of 10 kGy.
Fig. 1. Dose dependencies of low-temperature (T = 5 R) intensities of the radiationinduced photoluminescence lines: (1) D⁄V0cd (donor–acceptor pair), (2) eV0cd (band to acceptor transitions), (3) V0cd X (acceptor bound exciton) in c-irradiated Cd1xZnxTe ([ = 0.05) crystals.
physics of NAS of Ukraine [4,6]. The Zn content (5%) was verified using X-ray diffractometry technique on X’Pert PRO MRD set. From the ingots with the diameter of 40 mm the samples of the size 5 5 2 mm were cut. With the aim to reduce the broken surface layer (150–200 lm) the samples were mechanically polished with the different fraction diamond pastes then etched in the bromine– methanol [5]. After pointed previous preparation the samples were irradiated with the different doses of c-quanta in the dose range of 10–100 kGy at room temperature. Corresponding c-quanta flux was Nc = 1.69 1015–1.69 1016 quanta/cm2. 60Co source with the photon energy of about 1.2 MeV was used. After irradiation low-temperature photoluminescence measurements were done. It was the first part of the general experiment. Thereafter c-irradiated samples were annealed at the temperature of 100 and 200 °C in the quartz ampoules with argon atmosphere. The ampoules with the crystals were placed to the furnaces which had fixed at 100 and 200 °C. The noted temperatures were kept during the half an hour. The low-temperature photoluminescence measurements also were done after that. It was the second part of the general experiment. The temperature of all photoluminescence experiments was 5 K. For the keeping at such temperature the samples were loaded in A-240 optical helium bath-flow cryostat with automatic thermoregulated system. For the photoluminescence excitation He–Ne laser with the kex = 632.8 nm was used. Photoluminescence signals were registered using the photomultiplier with the antimony–cesium emitter. With the aim to reduce the own noises the photomultiplier was cooled with liquid nitrogen.
One can see, low-temperature photoluminescence spectrum of initial crystal consists of the following emission lines [6,11,12]: 1. Shallow donor bound exciton emission lines D10X and D20X with the peaks centered at 1.605 and 1.598 eV respectively. 2. Shallow acceptor bound exciton line A0X with the peak position at 1.592 eV. Also the first longitudinal optical (LO)-phonon replica of the pointed line (A0X–LO) was observed. The LO-phonon energy is equal to 21.5 meV. 3. Impurity related line I with the peak centered at 1.548 eV which is the composition of the recombination in donor–acceptor pair and band to acceptor transition emission lines. 4. Deep level defects line D (generally known as line of the A-centers – complexes which contain Cd-vacancy and donor atom) with their numerous LO-phonon replicas (LO-phonon energy is the same as for the A0X line). Peak position of the zero-phonon line is 1.45 eV. The radiation-induced changes in the low-temperature photoluminescence of the studied crystals are the same as in previous case in [6]. This fact indicates the similar impurity-defects structure of initial samples in previous [6] and recent works. Fig. 3 shows the influence of the thermal annealing at 200 °C on the low-temperature photoluminescence properties of initial crystals. One can see such annealing of the unirradiated sample does not affect substantially on their low-temperature photoluminescence. Only weak decrease of the intensity of I emission line was observed
3. Results and discussion Thermal annealing at the temperature of 100 and 200 °C of the
c-irradiated crystals Cd0.95Zn0.05Te leads to the substantial changes in their photoluminescence properties which will be discussed further. The first general feature of the thermal effect is the similar and synchronous decrease of the amplitudes of all emission lines observed in the spectra for all crystals irradiated at different doses. Such effect can be explained by increasing in the concentration of radiationless luminescence centers. Therefore with the aim to minimize the influence of such centers on the luminescence lines amplitudes we have normalized all photoluminescence spectra. Fig. 2 presents photoluminescence spectra of initial sample and sample which had been c-irradiated with the dose of 10 kGy.
Fig. 3. Photoluminescence spectra of Cd0.95Zn0.5Te crystals observed at 5 K: (1) initial sample and (2) sample annealed at 200 °C during half an hour in the argon atmosphere.
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Iu. Nasieka et al. / Nuclear Instruments and Methods in Physics Research B 290 (2012) 26–29
Fig. 4. Photoluminescence spectra of c-irradiated (50 kGy) Cd0.95Zn0.5Te crystals observed at 5 K: (1) only c-irradiated sample, (2) c-irradiated sample annealed at 100 °C during half an hour in the argon atmosphere and (3) c-irradiated sample annealed at 200 °C during half an hour in the argon atmosphere.
which is due to decreasing in the concentration of the corresponding luminescence centers. However, thermal annealing at 100 and 200 °C leads to more substantial changes in the spectra of c-irradiated crystals. Fig. 4 demonstrates the effect on pointed thermal annealing on the low-temperature photoluminescence of the Cd0.95Zn0.05Te crystals irradiated with the dose of 50 kGy. As we can see thermal annealing of c-irradiated samples leads to the increase in the intensities of donors bound exciton lines and deep level defects line which can be explained by thermal-induced reconstruction of the as-grown shallow donor centers D1 and D2. Also, blue-shift in the spectra of the thermal-annealed and c-irradiated samples was observed. Such effect denotes about certain reconstruction of the as-grown crystalline properties of the investigated c-irradiated crystals. However, decrease of the integral photoluminescence intensity in the spectra of c- and thermal-treated samples also takes place. We believe that pointed effect is due to increase in the concentration of the radiationless recombination centers because of the possible deterioration of the surface crystalline quality. The observed decrease in the amplitudes of donors bound exciton lines and return of the photoluminescence emission lines peak position to the initial one indicates compensation effect of the annealing of the radiation-induced defects. The general physical model of the crystalline lattice process which can explain all regularities observed in the spectra of c- and thermal-treated Cd0.95Zn0.05Te crystals can be the following. The prevailing result of the interaction of c-quanta flux with Cd0.95Zn0.05Te material is the creation of the radiation-induced defects – Frenkel’s pairs (in the simplest case). In this case the main effect of c-irradiation is creation of Cd-vacancies (shallow acceptors in CdTe and CdZnTe [6,10]) and related interstitial atoms, i.e. we suppose that radiation induced increase in the intensities of the luminescence lines attributed to the shallow acceptors determined by the increase in the concentration of Cd-vacancies. We have not performed a direct experiment proves the radiation-induced acceptors, however the high probability of the assumption pointed above is confirmed by the following facts. Firstly, the concentration of radiation-induced Cdvacancies in irradiated crystals is somewhat higher than the concentration of tellurium (Te)-vacancies [2]. The latter induce the luminescence line with the energy peak position at 1.1 eV in Cd0.9Zn0.1Te at 4.2 K [1,2]. There no such peak in our spectra and luminescence line significantly differing from that observed in irradiated and annealed crystals. The Cd-vacancy concentration
also exceeds the concentration of zinc (Zn)-vacancies which is due to rather low content of zinc in the investigated Cd0.95Zn0.5Te crystals. Secondly, it is worth expecting that radiation-induced interstitial atoms of Cd0.95Zn0.5Te lattice (isolated ones and bound to impurities or other defects) do not make a considerable contribution to the observed c-induced luminescence as follows from the known positions of some levels they create [1,2,8,9]. They considerably differ from the corresponding ones for the observed c-induced luminescence centers. At the increasing of the c-quanta dose, radiation induced Cd-vacancies associate in the larger units (conglomerates). In this case the nearest neighbor atoms of the crystalline lattice are displaced from pointed vacancy type defects, but the next neighbor atoms displaced to the radiation-induced defects [13]. Such redistribution in the lattice of the neighbor to the vacancies atoms (relaxation) leads to the decreasing in the energy of vacancy type defects. The consequence of such lattice relaxation is local increase of the lattice constant. Due to translation symmetry of the crystalline lattice there are many equivalent energy states for radiation-induced defects confined by the energy barriers. Under the thermal annealing an intensive movement of single vacancies and their larger conglomerates begins. During such movement space-confined defects can close to each other or to the localized radiation-induced or as-grown defects with further defect–defect interaction [14]. This interaction leads to disappearance of the simple radiation-induced defects as well as complex defects and defects conglomerates which include radiation-induced defects. Such interaction acts through the field of the mechanical strains and caused by the change of the defect complex volume and decrease in the interatomic repulsion forces in the less densely packed crystalline regions. Such regions are radiation-induced vacancies, conglomerates of the vacancies (dislocations) as well as impurity atoms and ions [13–15].
4. Conclusions In the present work the changes in the low-temperature photoluminescence properties of c-irradiated Cd0.95Zn0.05Te crystals under thermal annealing at 100 and 200 °C was analyzed. It was obtained such annealing leads to the reconstruction of the initial donor concentrations which were reduced by preliminary c-irradiation. In this case the reconstruction processes are caused by thermal annealing which leads to the substantial movement of the radiation-induced defects. As a result of a movement, space-confined defects can close to each other and annihilate. Except this, thermal treatment induces blue-shift in the spectra of c-irradiated samples which we associate with the post-radiation thermal-induced lattice constant recovery due to compensation between vacancy type defects and interstitial ions. References [1] K. Hjelt, M. Juvonen, T. Tuomi, S. Nenonen, E. Eissler, M. Bavdaz, Phys. Status Solidi A 162 (1997) 747. [2] T.E. Schlesinger, J.E. Toney, H. Yoon, E.Y. Lee, B.A. Branett, R.B. James, Mater. Sci. Eng. 32 (2001) 103. [3] V. Komar, V. Puzikov, Group Single Crystals: Growth, Properties, Application, Institute for Single Crystals, Kharkov, 2002, 214 pp. [4] P.P. Moskvin, L.V. Rashkovetskii, F.F. Sizov, Russ. J. Phys. Chem. 84 (2010) 1324. [5] H. Chen, J. Tong, Z. Hu, D.T. Shi, G.H. Wu, K.-T. Chen, M.A. George, W.E. Collins, A. Burger, J. Appl. Phys. 80 (1996) 3509. [6] K.D. Glinchuk, N.M. Litovchenko, Y.M. Naseka, A.V. Prohorovich, L.V. Rashkovetskyi, O.M. Strilchuk, F.F. Sizov, O.O. Voitsihovska, B.O. Danilchenko, Ukr. J. Phys. 55 (2010) 776. [7] T. Taguchi, J. Shirafuji, Y. Inuishi, Nucl. Instr. Meth. 150 (1978) 43. [8] A. Castaldini, A. Cavallini, B. Fraboni, J. Appl. Phys. 83 (1998) 2121. [9] A. Cavallini, B. Fraboni, W. Dusi, M. Zanarini, P. Siffert, Appl. Phys. Lett. 77 (2000) 3212. [10] S.G. Krylyuk, D.V. Korbutyak, Y.V. Kryuchenko, I.M. Kupchak, N.D. Vakhnyak, J. Alloys Compd. 371 (2003) 142.
Iu. Nasieka et al. / Nuclear Instruments and Methods in Physics Research B 290 (2012) 26–29 [11] A. Medvid, A. Mychko, O. Strilchuk, N. Litovchenko, Y. Naseka, P. Onufrijevs, A. Pludonis, Phys. Stat. Sol. 1 (6) (2009) 209. [12] A. Medvid, A. Mychko, O. Strilchuk, N. Litovchenko, Y. Naseka, P. Onufrijevs, A. Pludonis, Nucl. Instr. Meth. A 607 (2009) 110. [13] M.A. Elango, Elementary Inelastic Radiation-Induced Processes, Nauka, Moscow, 1988. 192 pp.
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[14] L.S. Smirnov, Physical Processes in Irradiated Semiconductors, Nauka, Novosibirsk, 1977. 254 pp. [15] A.M. Kurakin, S.A. Vitusevich, S.V. Danylyuk, H. Hardtdegen, N. Klein, Z. Bougrioua, B.A. Danilchenko, R.V. Konakova, A.E. Belyaev, J. Appl. Phys. 103 (2008) 083707.
Contents lists available at SciVerse ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Investigation of the thermal annealing effect on the defects structure in c-irradiated CdZnTe crystals by photoluminescence method Iu. Nasieka a,b,⇑, L. Rashkovetskyi a, O. Strilchuk a, V. Maslov a, E. Venger a a b
Lashkarev Institute of Semiconductor Physics, NAS of Ukraine, 45 Pr. Nauki, Kyiv, Ukraine Nizhyn Agricultural Institute of National University of Life and Environmental Sciences of Ukraine, 10 Shevchenka Str., Nizhyn, Ukraine
a r t i c l e
i n f o
Article history: Received 26 July 2012 Received in revised form 23 August 2012 Available online 8 September 2012 Keywords: Thermal annealing CdZnTe crystals c-Irradiation Low-temperature photoluminescence
a b s t r a c t The influence of the thermal annealing at 100 and 200 °C on the low-temperature photoluminescence properties of different doses c-irradiated Cd0.95Zn0.05Te crystals is investigated. It was obtained such thermal annealing induces increasing of the amplitude of donor bound exciton lines and A-centers line which were reduced by preliminary c-irradiation. Such effect can be explained by the reconstruction of the initial donor defects structure. In this case the reconstruction process is caused by the transmission of the additional thermal energy which leads to the substantial movement of the radiation induced vacancies and interstitials ions with their further annihilation. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction As it generally known [1,2] CdZnTe crystals have the ability to detect and perform energy-dispersive spectroscopy of high energy radiation such as X-rays, c-rays and other types of ionizing radiation. Except this Cd0.95Zn0.05Te is a promising substrate material for the growing of high quality CdHgTe infrared detectors. Therefore the noted materials have a wide application in the field of the manufacturing of uncooled semiconductor radiation and infrared detectors. Such types of detectors have the ability to work at room temperature and can be characterized by high sensitivity and selectivity. Another advantage of pointed detectors is very small functional sizes which favorably distinguish them from, for example, gas ionizing cameras [3]. But despite the recent the wide range of the questions, particularly concerning radiation and thermal stimulated defect creations in CdZnTe crystals and influence of noted defects on their optical properties are remain unexplored. In this paper we present an analysis of the changes in the low-temperature photoluminescence properties of different doses c-irradiated CdZnTe crystals under thermal annealing. It is worth noting the following facts there. In the work [6] we reported the study of the effect of c-irradiation (with the doses in the range 10–100 kGy) on the low-temperature photoluminescence of Cd0.95Zn0.05Te crystals. We pointed that the following two phenomena induced by c-irradiation were observed. The first was the essential decrease of the intensities of the initial (as⇑ Corresponding author at: Lashkarev Institute of Semiconductor Physics, NAS of Ukraine, 45 Pr. Nauki, Kyiv, Ukraine. E-mail address: [email protected] (Iu. Nasieka). 0168-583X/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.nimb.2012.09.002
grown) luminescence lines – the deep level defects line (general peak centered at 1.409 eV), donor–acceptor pairs (peak centered at 1.547 eV) and shallow acceptor (peak centered at 1.556 eV) lines as well as intensity of the lines related to excitons bound to shallow neutral acceptors and donors (peaks centered at 1.592 and 1.599 eV, respectively). Such changes in the photoluminescence spectra we have explained by the decrease in the concentration of the corresponding luminescence centers due to their interaction with radiation-induced defects. The second phenomena was an appearance of new luminescence lines which were probably caused by radiation-induced cadmium vacancies (VCd) bound to other defects (donor–acceptor pairs) and isolated cadmium vacancies as well as excitons bound to the indicated Cd-vacancies. The intensities of the radiation-induced lines changed non-monotonically with the increase of the c-irradiation dose (see Fig. 1): at low doses (650 kGy) it increased due to the increase in the concentration of radiation-induced Cd-vacancies and then considerably decreased at high doses (>50 kGy) due to generation of a large number of effective centers of radiationless recombination of excess charge carriers [6–10]. The recent work is the continuation of the above study.
2. Experimental part Our investigations were performed for p-Cd0.95Zn0.05Te crystals with the room-temperature resistivity q 60 Ohm cm and q ? 1 at T = 5 K (the conduction of the crystals at 5 K is determined by excess electrons and holes) which grown with the help of the Bridgman technique in V.E. Lashkarev Institute of semiconductor
Iu. Nasieka et al. / Nuclear Instruments and Methods in Physics Research B 290 (2012) 26–29
27
Fig. 2. Photoluminescence spectra of Cd0.95Zn0.5Te crystals observed at 5 K: (1) initial sample and (2) sample irradiated with the dose of 10 kGy.
Fig. 1. Dose dependencies of low-temperature (T = 5 R) intensities of the radiationinduced photoluminescence lines: (1) D⁄V0cd (donor–acceptor pair), (2) eV0cd (band to acceptor transitions), (3) V0cd X (acceptor bound exciton) in c-irradiated Cd1xZnxTe ([ = 0.05) crystals.
physics of NAS of Ukraine [4,6]. The Zn content (5%) was verified using X-ray diffractometry technique on X’Pert PRO MRD set. From the ingots with the diameter of 40 mm the samples of the size 5 5 2 mm were cut. With the aim to reduce the broken surface layer (150–200 lm) the samples were mechanically polished with the different fraction diamond pastes then etched in the bromine– methanol [5]. After pointed previous preparation the samples were irradiated with the different doses of c-quanta in the dose range of 10–100 kGy at room temperature. Corresponding c-quanta flux was Nc = 1.69 1015–1.69 1016 quanta/cm2. 60Co source with the photon energy of about 1.2 MeV was used. After irradiation low-temperature photoluminescence measurements were done. It was the first part of the general experiment. Thereafter c-irradiated samples were annealed at the temperature of 100 and 200 °C in the quartz ampoules with argon atmosphere. The ampoules with the crystals were placed to the furnaces which had fixed at 100 and 200 °C. The noted temperatures were kept during the half an hour. The low-temperature photoluminescence measurements also were done after that. It was the second part of the general experiment. The temperature of all photoluminescence experiments was 5 K. For the keeping at such temperature the samples were loaded in A-240 optical helium bath-flow cryostat with automatic thermoregulated system. For the photoluminescence excitation He–Ne laser with the kex = 632.8 nm was used. Photoluminescence signals were registered using the photomultiplier with the antimony–cesium emitter. With the aim to reduce the own noises the photomultiplier was cooled with liquid nitrogen.
One can see, low-temperature photoluminescence spectrum of initial crystal consists of the following emission lines [6,11,12]: 1. Shallow donor bound exciton emission lines D10X and D20X with the peaks centered at 1.605 and 1.598 eV respectively. 2. Shallow acceptor bound exciton line A0X with the peak position at 1.592 eV. Also the first longitudinal optical (LO)-phonon replica of the pointed line (A0X–LO) was observed. The LO-phonon energy is equal to 21.5 meV. 3. Impurity related line I with the peak centered at 1.548 eV which is the composition of the recombination in donor–acceptor pair and band to acceptor transition emission lines. 4. Deep level defects line D (generally known as line of the A-centers – complexes which contain Cd-vacancy and donor atom) with their numerous LO-phonon replicas (LO-phonon energy is the same as for the A0X line). Peak position of the zero-phonon line is 1.45 eV. The radiation-induced changes in the low-temperature photoluminescence of the studied crystals are the same as in previous case in [6]. This fact indicates the similar impurity-defects structure of initial samples in previous [6] and recent works. Fig. 3 shows the influence of the thermal annealing at 200 °C on the low-temperature photoluminescence properties of initial crystals. One can see such annealing of the unirradiated sample does not affect substantially on their low-temperature photoluminescence. Only weak decrease of the intensity of I emission line was observed
3. Results and discussion Thermal annealing at the temperature of 100 and 200 °C of the
c-irradiated crystals Cd0.95Zn0.05Te leads to the substantial changes in their photoluminescence properties which will be discussed further. The first general feature of the thermal effect is the similar and synchronous decrease of the amplitudes of all emission lines observed in the spectra for all crystals irradiated at different doses. Such effect can be explained by increasing in the concentration of radiationless luminescence centers. Therefore with the aim to minimize the influence of such centers on the luminescence lines amplitudes we have normalized all photoluminescence spectra. Fig. 2 presents photoluminescence spectra of initial sample and sample which had been c-irradiated with the dose of 10 kGy.
Fig. 3. Photoluminescence spectra of Cd0.95Zn0.5Te crystals observed at 5 K: (1) initial sample and (2) sample annealed at 200 °C during half an hour in the argon atmosphere.
28
Iu. Nasieka et al. / Nuclear Instruments and Methods in Physics Research B 290 (2012) 26–29
Fig. 4. Photoluminescence spectra of c-irradiated (50 kGy) Cd0.95Zn0.5Te crystals observed at 5 K: (1) only c-irradiated sample, (2) c-irradiated sample annealed at 100 °C during half an hour in the argon atmosphere and (3) c-irradiated sample annealed at 200 °C during half an hour in the argon atmosphere.
which is due to decreasing in the concentration of the corresponding luminescence centers. However, thermal annealing at 100 and 200 °C leads to more substantial changes in the spectra of c-irradiated crystals. Fig. 4 demonstrates the effect on pointed thermal annealing on the low-temperature photoluminescence of the Cd0.95Zn0.05Te crystals irradiated with the dose of 50 kGy. As we can see thermal annealing of c-irradiated samples leads to the increase in the intensities of donors bound exciton lines and deep level defects line which can be explained by thermal-induced reconstruction of the as-grown shallow donor centers D1 and D2. Also, blue-shift in the spectra of the thermal-annealed and c-irradiated samples was observed. Such effect denotes about certain reconstruction of the as-grown crystalline properties of the investigated c-irradiated crystals. However, decrease of the integral photoluminescence intensity in the spectra of c- and thermal-treated samples also takes place. We believe that pointed effect is due to increase in the concentration of the radiationless recombination centers because of the possible deterioration of the surface crystalline quality. The observed decrease in the amplitudes of donors bound exciton lines and return of the photoluminescence emission lines peak position to the initial one indicates compensation effect of the annealing of the radiation-induced defects. The general physical model of the crystalline lattice process which can explain all regularities observed in the spectra of c- and thermal-treated Cd0.95Zn0.05Te crystals can be the following. The prevailing result of the interaction of c-quanta flux with Cd0.95Zn0.05Te material is the creation of the radiation-induced defects – Frenkel’s pairs (in the simplest case). In this case the main effect of c-irradiation is creation of Cd-vacancies (shallow acceptors in CdTe and CdZnTe [6,10]) and related interstitial atoms, i.e. we suppose that radiation induced increase in the intensities of the luminescence lines attributed to the shallow acceptors determined by the increase in the concentration of Cd-vacancies. We have not performed a direct experiment proves the radiation-induced acceptors, however the high probability of the assumption pointed above is confirmed by the following facts. Firstly, the concentration of radiation-induced Cdvacancies in irradiated crystals is somewhat higher than the concentration of tellurium (Te)-vacancies [2]. The latter induce the luminescence line with the energy peak position at 1.1 eV in Cd0.9Zn0.1Te at 4.2 K [1,2]. There no such peak in our spectra and luminescence line significantly differing from that observed in irradiated and annealed crystals. The Cd-vacancy concentration
also exceeds the concentration of zinc (Zn)-vacancies which is due to rather low content of zinc in the investigated Cd0.95Zn0.5Te crystals. Secondly, it is worth expecting that radiation-induced interstitial atoms of Cd0.95Zn0.5Te lattice (isolated ones and bound to impurities or other defects) do not make a considerable contribution to the observed c-induced luminescence as follows from the known positions of some levels they create [1,2,8,9]. They considerably differ from the corresponding ones for the observed c-induced luminescence centers. At the increasing of the c-quanta dose, radiation induced Cd-vacancies associate in the larger units (conglomerates). In this case the nearest neighbor atoms of the crystalline lattice are displaced from pointed vacancy type defects, but the next neighbor atoms displaced to the radiation-induced defects [13]. Such redistribution in the lattice of the neighbor to the vacancies atoms (relaxation) leads to the decreasing in the energy of vacancy type defects. The consequence of such lattice relaxation is local increase of the lattice constant. Due to translation symmetry of the crystalline lattice there are many equivalent energy states for radiation-induced defects confined by the energy barriers. Under the thermal annealing an intensive movement of single vacancies and their larger conglomerates begins. During such movement space-confined defects can close to each other or to the localized radiation-induced or as-grown defects with further defect–defect interaction [14]. This interaction leads to disappearance of the simple radiation-induced defects as well as complex defects and defects conglomerates which include radiation-induced defects. Such interaction acts through the field of the mechanical strains and caused by the change of the defect complex volume and decrease in the interatomic repulsion forces in the less densely packed crystalline regions. Such regions are radiation-induced vacancies, conglomerates of the vacancies (dislocations) as well as impurity atoms and ions [13–15].
4. Conclusions In the present work the changes in the low-temperature photoluminescence properties of c-irradiated Cd0.95Zn0.05Te crystals under thermal annealing at 100 and 200 °C was analyzed. It was obtained such annealing leads to the reconstruction of the initial donor concentrations which were reduced by preliminary c-irradiation. In this case the reconstruction processes are caused by thermal annealing which leads to the substantial movement of the radiation-induced defects. As a result of a movement, space-confined defects can close to each other and annihilate. Except this, thermal treatment induces blue-shift in the spectra of c-irradiated samples which we associate with the post-radiation thermal-induced lattice constant recovery due to compensation between vacancy type defects and interstitial ions. References [1] K. Hjelt, M. Juvonen, T. Tuomi, S. Nenonen, E. Eissler, M. Bavdaz, Phys. Status Solidi A 162 (1997) 747. [2] T.E. Schlesinger, J.E. Toney, H. Yoon, E.Y. Lee, B.A. Branett, R.B. James, Mater. Sci. Eng. 32 (2001) 103. [3] V. Komar, V. Puzikov, Group Single Crystals: Growth, Properties, Application, Institute for Single Crystals, Kharkov, 2002, 214 pp. [4] P.P. Moskvin, L.V. Rashkovetskii, F.F. Sizov, Russ. J. Phys. Chem. 84 (2010) 1324. [5] H. Chen, J. Tong, Z. Hu, D.T. Shi, G.H. Wu, K.-T. Chen, M.A. George, W.E. Collins, A. Burger, J. Appl. Phys. 80 (1996) 3509. [6] K.D. Glinchuk, N.M. Litovchenko, Y.M. Naseka, A.V. Prohorovich, L.V. Rashkovetskyi, O.M. Strilchuk, F.F. Sizov, O.O. Voitsihovska, B.O. Danilchenko, Ukr. J. Phys. 55 (2010) 776. [7] T. Taguchi, J. Shirafuji, Y. Inuishi, Nucl. Instr. Meth. 150 (1978) 43. [8] A. Castaldini, A. Cavallini, B. Fraboni, J. Appl. Phys. 83 (1998) 2121. [9] A. Cavallini, B. Fraboni, W. Dusi, M. Zanarini, P. Siffert, Appl. Phys. Lett. 77 (2000) 3212. [10] S.G. Krylyuk, D.V. Korbutyak, Y.V. Kryuchenko, I.M. Kupchak, N.D. Vakhnyak, J. Alloys Compd. 371 (2003) 142.
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