Luminescence of high-temperature yttrium-based ... - ScienceDirect

Journal of Luminescence 48 & 49 (1991) 845-848 North-Holland

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Luminescence of high-temperature yttrium-based superconductors V.G. Stankevicha, N.Yu. Svechnikova, K.V. Kaznacheev~’,R.A. Kink”, I.L. Kuusmannb, E.Kh. Fe1dbach~’,G. Zimmerer’~,T. Kloiberc, A.A. Zhokhov’, G.A. Emel’chenko”, M.A. Kalyagine and V.Ya. Kosyeve a J• v• Kurchatov Institute of Atomic Energy, Moscow 123182, USSR b

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Institute of Physics, AS of Eslonian SSR, Tartu 202400, USSR Universität Hamburg, II. Institutfür Experimentaiphysik, 2000 Hamburg 50, FRG Institute of Solid State Physics, Chernogolovka 142432, USSR Institute of Applied Physics, AS of USSR, Gor’ky 603600, USSR

Luminescence spectra of thin films and single crystals of YBa

2Cu3O7_~high-temperature superconductors were measured under optical excitation by synchrotron radiation (E = 10.2 eV) and by electrons (cathodoluminescence) with an energy of 6 keV. It has been shown that in all the spectra the main contribution is made by a blue band with an energy of =2.9 eV, which is, apparently, the luminescence inherent to the crystal. The time characteristics of luminescence were measured. It turned out that 80% of all the luminescence requires a very long time (r> 106 s) to reach its final level. A model of luminescence appearance is proposed, which is based on luminescence of oxygen quasi-molecules formed by the decay of electronic excitations (luminescence of self-localized and defect-localized excitons).

It is already three years since high-temperature superconductivity was discovered [1]. Despite the use of a broad variety of available experimental procedures and theoretical approaches, the mechanism of this phenomenon is not yet clear. Because of this, the investigation of the electronic structure of superconductors, in particular in the vicinity of the Fermi surface arouses particular interest among researchers. At present, there is a large number of papers in which the conventional methods of studying the electronic structure of metals were used to this purpose, like e.g. photoelectron spectroscopy [2], inverted photoelectron spectroscopy [3], the method for measuring characteristic losses of electrons in passing through a sample [4], and optical studies of reflection in infrared and visible regions [5]. However, even the first experiments on luminescence [6] and calculations of the electronic structure [7] of YBa2Cu3O7_~showed that this complicated system should not only manifest the properties of metals but also some specific features of dielectrics with a characteristic band of forbidden energies. This 0022-2313/91/$03.50

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made it possible to use a method of luminescence “probe” for studying the electronic structure and structural variations of superconductors, which is not traditional for physics of superconductors. It should be noted that the information content of such studies increases appreciably and it becomes much easier to interpret the results obtained if the luminescence is intrinsic, i.e. if it complies with the process taking place in the crystalline lattice. Up to now a number of researches into the luminescence of HTSC (mainly, YBa2Cu3O7_~) has been carried out with excitation by an electron beam [6,8-12], or VUV excitation of a synchrotron radiation source [13,14]. Electroluminescence [15] and thermoluminescence [16] have also been investigated. The experiments show that there exists a weak visible luminescence of HTSC but, looking at details, the radiation spectra vary considerably in different articles. Apparently, this is for the biggest part due to the purity and quality ofthe samples used, as well as to the specific nature of excitation. Some authors (e.g. ref. [12]) claim

Elsevier Science Publishers B.V. (North-Holland)

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Luminescence of high-temperature yttrium.based superconductors

that the observed luminescence is, altogether, specified by impurity phases and has nothing to do with the superconducting YBa2Cu-07 cornpound. The present work is a continuation ofthe studies [13,14] and is aimed at additionally investigating the HTSC luminescence of samples being as perfect as possible and prepared by different methods, and at using various types of excitations (synchrotron radiation and electrons) to this purpose. This approach allows us to use the same samples for theexcitation dependence the studying method of and of on luminescence the depth of on its penetration into the sample. The latter circumstance makes it possible to estimate the possible influence of the surface and impurities adsorbed on it on the luminescence, The experiments on the VUV excitation of HTSC samples were performed at the high-resolution “SUPERLUMI” spectrometer [17] on the “DORIS-Il” synchrotron radiation source. The spectrometer is provided with several monochromators allowing the spectral analysis of luminescence to be performed within a broad (501000 nm) spectral range. It should be noted that the use of a focusing toroidal mirror downstream from the exit slit of the exciting monochromator 2) makes crystals. it possible use small-sized x 2 mm single Thetospectrometer also (1 allows one to study the time characteristics of luminescence with a time resolution of 50 ns. The samples were processed under oilless vacuum of °=i09Torr without preheating of the experimental volume. The temperature could vary in the range of 9— 300K. The experiments on the cathodoluminescence of HTSC single crystals were carried out at the spectrometer of the Institute of Physics of the AS of the Estonian SSR [18]. Thin films and single crystals of the YBa 2Cu1O7~composition, being oriented in the [ab] plane, were used as samples. The laser-plasma method was employed to deposit a 4500 A thick film on a substrate made of by 3~strontium laser (A titanate 1.06 ~.rn). means of the YAG:Nd Pellets with stoichiometric composition with that =

of the film serve as targets. In the course of deposition of erosion products the substrate temperature

increased to 650°C,the residual oxygen pressure in the chamber was 10 Torr, the growth rate was 3 A/s. The oxygen concentration in the films was established depending on the oxygen pressure in the chamber in the process of cooling down the deposited structure. According to the estimates taken in compliance with ref.[19], thexvalue amounted to 0.05 (TR=~= 90 K) in the film under investigation. The transition width ~T~°=0.6 K. The current density of 2 critical at liquid nitrogen ternthe film a= 106 A/cm perature. Single crystals measuring 3x3x0.1 mm3 were grown by using the method of solution in a melt, employing the BaCuO 2—CuO eutectics [20] as a flux. The crystals were grown in a platinum crucible under atmospheric pressure in air. After forming, the crystals displayed a small deficiency of oxygen (x=0.15), which was eliminated by annealing in the oxygen atmosphere at T 450°Cduring 20 h. It has been established by the combination scattering spectra that in an annealed crystal x 0. Measurements of the magnetic susceptibility x( T) revealed the presence of a superconducting transition at T~ 92 K with width ~ T~= I K, the Meissner effect up to analysis 87%. According toamounted the microprobe data, the chemical composition conformed to the stoichiometry of the compound 1: 2 : 3 for cations. Figure 1 presents the luminescence spectra of the single crystal (1) and of the film (2) in the =

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superconducting state at 60 K, obtained under VUV excitation by synchrotron radiation with an

We think that the blue luminescence (°=2.9eV) is inherent to the YBa2Cu3O7 crystal and is not

energy of 10.2 eV. These spectra were taken with open slits of the analyzing monochromator having a 30 nm resolution, and they were not corrected for the detection system sensitivity, As seen from the figure, the band at =2.9 eV makes the main contribution to both the spectra. In the case of the film, a small (°° 20%) contribution made by additional bands at energies of =4 and =2.4 eV is observed. A similar shape of the luminescence curve was observed by us earlier [13,14] for the YBa2Cu3O7_~films with a thickness of 1500 A. Single crystal cathodoluminescence spectra measured at three different temperatures and corrected for the detection system sensitivity are presented in fig. 2. In the same way as in the previous spectra, here the main contribution to the luminescence is made by the same blue band at an energy of =2.9 eV. However, at high temperature (T = 190 K), after its partial temperature quenching, an additional band is clearly seen at 2.4 eV. The intensity of this band increases sharply when irradiating the sample by large doses of electrons. Apparently, it is assigned to radiation-induced defects in the crystal, which originate under exposure to sufficiently large doses of radiation. This is also in agreement with the results of the photoexcitation (fig. 1), according to which the weak band of =2.4 eV is observed in a film known to be more defective, and is, practically, absent from the single crystal spectrum.

connected to luminescence of random impurities in the crystal or on its surface, the surface oxygen included. This statement is based on the following experimental data. (1) The blue luminescence is excited in all the high-quality samples single crystals and thin crystalline films. (2) The aa~2.9eV band is dominant at various methods of excitation and different depth values of the excitation penetration into the crystal. The estimates show that under optical excitation the light penetrates into the crystal to a depth of hundreds of A, while electrons with an energy of 6 keV, to that of thousands of A. (3) The luminescence yield under VUV excitation is sufficiently high (about 1%) in spite of the large absorption of the blue light ( 15% of the light passes through a film of 1500 A thick [15]). (4) The intensity of the blue band of =2.9 eV correlates with the superconductivity state [13], which is a volumetric effect. The synchrotron radiation source has been used to study the YBa2Cu3O7 single crystal luminescence kinetics. It turned out that the luminescence displays a weak (=20%) fast component of nanosecond duration, while the main part of luminescence (=80%) decays with r~5 ~ts. The latter value is an upper estimate of the luminescence slow component decay time, since in the single-bunch mode of the DORIS-Il storage unit the interval between the exciting pulses amounts to °‘=lp~sand the detection of long time periods of luminescence turns out to be difficult without special choppers (fig. 3). The time characteristics of the luminescence

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Fig. 2. Luminescence spectra of the same single crystal at excitation by electron beam versus temperature.

slow component were additionally measured under laser excitation of the thin YBa2Cu1O7~ film by light quanta having an energy of 4.05 eV. As a result, it has been established experimentally that this luminescence has a component with a still longer decay time of 100 p.s. As known, such long times of luminescence decaygiveevidenceofalargedegreeofprohibition of the corresponding radiative transition. Taking into account this circumstance, as well as the spectral position of the band (=2.9 eV) and its weakly

848

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Luminescence of high-temperature yttrium-based superconductors

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2Cu,O7_,~single crystal at excitation by synchrotron radiation with energy of 10.2 eV. The inset presents the instrument function (2) and time behaviour of the fast component (3). The sample temperature T = 60K.

Fig. 4. Reflectivity (T = 300 K) and excitation of luminescence 2.9 eV) spectra of thin YBa2Cu3O7_~film (T~= 90 K) under excitation of synchrotron radiation.

(T = 80 K, E

[5] I. Bozovic et al., Phys. Rev. Lett. 59 (1987) 2219.

pronounced vibrational structure, we earlier proposed [14] the model of the luminescence centre, according to which the quasimolecule of oxygen glows in the crystalline lattice (the self-localized or defect-localized exciton). The corresponding transition in the free molecule is forbidden. In the superconductor crystalline lattice, the composition of such a quasimolecule includes not only oxygen atoms but also other atoms of the unit cell. However, since there are five different positions of oxygen atoms in the unit cell of YBa2Cu5O7, it is not yet possible to determine precisely where the quasimolecule is produced. The reflectivity spectrum of thin YBa2Cu3O7_~ film (fig. 4), obtained recently, correlates with the excitation of the luminescence spectrum, measured at =2.9 eV, and the spectra have the antipode character. These spectra also confirm the internal character of luminescence of the 2.9 eV band. ~

[6] Ch.B. Lushchik et al., Pis’ma v ZhETF 46 (1987) 122. [7] WY. Ching et al., Phys. Rev. Lett. 59 (1987) 1333. [8] V.N. Andreev, B.P. Zakharchenya, SE. Nikitin et al., Pis’ma v ZhETF 46 (1987) 391. [9] yE. Eremenko, 1.Ya. Fugol’, V.N. Samovarov and V.M. Zhuravlev, Pis’ma v ZhETF 47 (1988) 529.

[10] B.J. Luff, P.D. Townsend and J. Osborne, J. Phys. D 21 (1988) 663.

[11]

M N. Popova, A.V. Puyats,

M.E. Springis

and E.P.

Khlybov, Pis’ma v ZhETF 48 (1988) 616.

[12] Ch.B. Lushchik, IL. Kuusmann and E.Kh. Feldbach, Works of the IF. AS of the Estonian SSR 63(1989)137. [13]

Stankevich, NYu. Svechnikov, K.V. Kaznacheev, R.A. Kink, Kh.E. Niedrays, V.N. Golubev, V.Ya. Kosyev, Yu N. Simirsky and M.B. Tsetlin, Pis’ma v ZhETF 47 V.G.

(1988) 321. [14] V.G. Stankevich, NYu. Svechnikov, Ky. Kaznacheev, R.A. Kink, Kh.E. Niedrays, K.A. Kalder, V.N. Golubev and V.Ya. Kosyev, NucI. Instr. and Meth. A 282 (1989)

684. [15] S.H. Pawar, H T. Lokhande, CD. Lokhande, R.N. Patil, B. Jayavam, S.K. Agavwal, A. Gupta and A.V. Navlikav, Solid State Commun. 67 (1988) 47. [16] M. Roth, A. Halperin and S. Katz, Solid State Commun.

67(1988)105.

References

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P. Gurtler, E. Roick, G. Zimmerer and M. Pouey, Interner

Bericht, DESY F41, SR-82-l4. [18] E.Kh. Feldbach, Ch.B. Lushchik and IL. Kuusmann, [1] G. Bednords and K.A. Muller, Z. Phys. B 64 (1989) 189. [2] A. Samsavar et al., Phys. Rev. B 37 (1988) 5164. [3] Al. Viescas et al., Phys. Rev. B 37 (1988) 3738. [4] C. Tarrio and SE. Schnatterly, Phys. Rev. B 38(1988)921.

Pis’ma v ZhETF 39 (1984) 54. [19] K. Kishio et a!., J. AppI. Phys. 26 (1987) Ll228. [20] V.A. Tatarchenko, GA. Emelchenko and Abrosimov, J. Modern Physics B 3 (1989) 583.

NV.