Thermochromism and photoluminescence in SrTiO3

Physica B 154 (1989) 209-213 North-Holland, Amsterdam

THERMOCHROMISM AND PHOTOLUMINESCENCE IN SrTiO, N.N. LEBEDEVA,

A.G. ARUSHANOV,

Azerbaijan State University, Baku-370073,

A.Kh. ZEYNALLY

USSR

Received 1 October 1987 Revised manuscript received 15 June 1988

The present paper deals with the study of electrical, optical and luminescent properties of the reduced SrTiO, single crystals. It is shown that the annealing in vacuum at T = 11CWC results in defects responsible for the increase in electroconductivity, the change in optical absorption coefficient, and in impurity photoluminescence (PL) quenching caused by Cr3’ ions. Temperature conditions of annealing in reducing atmosphere and oxidation medium allowing to vary the concentration of defects have been determined experimentally. Transmission spectra at h = 200 to 1100 nm, the dark- and photoconductivity (PC) at T = 77 to 300 K, the excitation spectra of PL and PC, as well as the kinetics of PL and PC have been measured in crystals with different degrees of reduction. On the basis of the obtained results, the model of defects responsible for the change in properties of the crystals studied is also discussed in the present paper.

1. Introduction It is well known that when reducing the “pure” and doped SrTiO, single crystals by transition metals, considerable changes are observed in their optical and electrical properties, e.g. coloration, the increase of IR absorption and conductivity etc. [l-4]. The conditions of the reduction process used by different authors are approximately the same: the crystals studied are exposed in vacuum or in H, for a certain time at temperatures from 950 to 1250” and then cooled down sharply. The annealing of the samples in air at high temperatures reduces their original properties: they become transparent in the visible light up_ to 5 pm and have a low conductivity of lo-l3 R-’ cm-‘. There is no single opinion concerning the nature of defects appearing in the reduced SrTiO, crystals. Experimentally (by EPR method) only one type of intrinsic structural defect - i.e. when the Ti3+ ion substitutes the Sr*’ ion, the socalled “noncentral” Ti ion - was observed [5]. Another type of structural defects, discussed elsewhere, are oxygen vacancies forming the shallow donors, polaron states [6] and color cen-

ters (the type of F-centers) [4]. The oxygen vacancies have not been identified experimentally. However, this version has been supported in ref. [6] by theoretical calculations of electron states of oxygen vacancies. Depending on the ion charge, the oxygen vacancies form the local states at 0.1, 0.27, 0.41, 1.07 and 1.27 eV below the c-band. The absorption in the IR region (A > 0.6 Frn) correlates with the increase of charge carrier concentration and is accounted for by free carrier absorption [3]. The oxygen vacancies are assumed to form in the forbidden band of SrTiO, a narrow impurity band of acceptor type at 0.8 eV above the v-band with the absorption at 2.4eV [7]. Fe4+ and Fe3+ ions along with the oxygen vacancy are supposed to be responsible for another two absorption bands (2.9 and 2.1 eV). The “pure” SrTiO, crystals contain the Fe ion as an uncontrollable impurity. As in other transition metals, according to EPR studies [8], the Fe ion appears in the SrTiO, lattice substitution the Ti4+ ion in the center of the TiO, oxygen octahedron. Consequently, it is obvious that all the supposed types of defects due to reduction process

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lead to the distortion of TiO, octahedra. As known, the TiO, octahedra form the c- and v-bands in SrTiO, [9, lo]. Therefore, they should be predominant in processes such as the charge carrier transfer, optical absorption and luminescence [ll]. On this basis, the purpose of the present paper is to study the effect of the structural deficiency due to reduction process in SrTiO, on the electrical, optical and luminescent properties.

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Fig. 1. Spectral dependence of the absorption coefficient of the basic (curve 1) and reduced (curve 2: sample A-l, curve 3: sample A-3) crystals.

2. Experimental Three samples cut off the same boule, A-l (8 X 7 X 1 mm3), A-2 (6.4 X 4 X 3.4 mm’) and A3 (10 X 7 X 2.4 mm3) were chosen as the basic (nonreduced) materials. The permissible impurity concentration was as follows: for Fe 5 x 10m3mol%, for Cr, Co, Ni, Al, Mg lo-* mol%, and for Mn, Pb less than 10d4 mol%. The plates cut were oriented along the [llO] axis. At room temperature the resistivity of samples studied was p-6x 101lflcm. The crystals were reduced in vacuum (5 x 10m5mm Hg) when heated continuously from room temperature up to 1100°C. At 1100°C the exposure time for different samples was also different: the A-l sample was reduced for 10 min, A-2 for 20 min, and A-3 for 2 h. All the samples were cooled down to room temperature at a rate of 40 deg/min. In the basic and reduced crystals the electroconductivity, the optical absorption spectra with A = 0.3-1.1 km and the photoluminescence excited by light from the fundamental absorption region were measured.

dependence of the perature The A-3 sample with a higher degree tion changed its resistivity by an order on cooling down to 90 K the resistivity constant.

resistivity. of reducof 9 while remained

3.2. Optical absorption The basic crystals were transparent in the visible light. The fundamental absorption in the UV was observed at A G 390 nm (fig. 1, curve 1). The optical absorption was found to change due to reduction process (fig. 1): the impurity absorption band appears at 2.9 eV and the absorption in IR region slightly increases at A > 0.6 km (fig. 1, curve 2). Furthermore, with increasing degree of reduction (sample A-3) the band at 2.4 eV is also resolved (fig. 1, curve 3) and the absorption in IR region increases considerably. 3.3. Photoluminescence

(PL)

In the basic crystals two PL bands were observed (fig. 2): the visible band (curve 1) and the

3. Results

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as The resistivity of the reduced crystals was as follows: for the A-l sample p = 7 x 10’ Q cm; for A-2 p=1.4x105Rcm and for A-3 p=2x lo* s1 cm. The A-l sample with a low degree of reduction changed its resistivity by an order of 4, preserving a semiconductive character of tem-

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Fig. 2. Radiation spectrum (a) and excitation spectrum (b) of the visible (curves 1,3) and IR bands (curves 2,4) of the PL.

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Fig. 3. Temperature dependence of the intensity of the visible band of PL in the basic (curve 1) and the reduced (curve 2: A-l, curve 3: A-2) samples. Temperature dependence of the intensity of IR band of the PL in the basic crystals (curve 4).

IR band (curve 2). The visible band with the maximum at 1.97 eV was broad (N 0.5 eV) and non-structural. It was excited by light from the fundamental absorption region of 3.35 eV (fig. 2, curve 3) and could be observed in a small temperature range (fig. 3, curve 1). On increasing the temperature from 77 to 110 K, the PL intensity decreased by two orders. This PL decay after the pulse excitation was independent of brad and is characterized by the exponential decay with the time constant r = 0.7 ps. During the thermal quenching process T remained constant. The IR band of the PL was observed as a narrow peak with the maximum at 1.56eV The excitation spectrum maximum of the IR band was at 3.26eV (fig. 2, curve 4). It should be noted that the PL studied was observed at temperatures from 77 to 200 K with the maximum at 110K (fig. 3, curve 4). It was found that the PL decay was of the exponential character. However, the relaxation time constant, T, considerably depends on temperature (fig. 4, curve 1). Based on the available

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concepts of such PL mechanism [12] as the radiative transition in the Cr3+ ion excited during the light absorption by the lattice, one can assume that the relaxation time of the PL can be considerably affected by processes preceeding the radiation of the Cr ion and connected with the presence of photoelectrons in the conduction band and on the trapping levels. With thermal emptying of the trapping levels the relaxation time decreases. To confirm what is said above the thermoluminescent (TL) measurements were carried out. The temperature dependence of TL intensity at the IR maximum is shown in fig. 4, curve 2. As seen in fig. 4, one of the TL peaks is observed in that temperature region where r decreases sharply. These are the results of our PL investigations in the basic crystals. During the reduction process both PL bands were found to change. For the IR band the total quenching at a low degree of reduction is typical. Therefore, this PL band is already absent in the A-l sample. To study the dependence of its parameters on the reduction degree, one can either change the reduction degree by choosing different annealing conditions in vacuum, or anneal the defects occurring during the reduction. We choose this second way. Unlike the IR band; the intensity of the visible PL band in reduced crystals grows. However, there is no quantitative correlation between the IR quenching and the growth of the visible band. In the A-l sample at 77 K the visible PL intensity is 2 to 2.5 times more than that in the basic crystals. With the increase of degree of reduction (samples A-2 and A-3) the PL intensity at 77 K does not change any further; however the temperature range where it exists broadens substantially towards higher temperatures (fig. 3). The radiation and excitation spectra of this PL as well as the relaxation kinetics both in the reduced crystals and in the basic crystals are all the same.

4. Reduction of crystal properties when annealing in air Fig. 4. Temperature dependence of the relaxation time of the IR band of PL (curve 1) and of the thermoluminescence intensity at A = 792 nm (curve 2).

It is known that the defects formed due to reduction process and affecting the properties of

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Fig. 5. The change of Aa (curve l), AI (curve 2) and Ap (curve 3) versus the annealing temperature (T,,,,,,).

SrTiO, crystal can be annealed at high temperatures in oxygen atmosphere or simply in air. We have annealed the A-l sample. Isochronous annealing mode (for 30 min) at different annealing temperatures, Tanneal= 50 to 300°C was used. The resistivity (p), absorption coefficient at 2.9eV (a) and IR intensity of PL (I) were measured in the annealed crystals. Assuming Ap, Aa, AZ to be the total (100%) changes of these values due to reduction process, we observed how these changes disappeared with the annealing temperature (fig. 5). As seen in fig. 5, crystal properties are not affected by annealing at 50 to 150°C while at Tanneal= 150°C one can observe a decrease of A,cyand AZ (curves 1,2). The curves 1 and 2 essentially coincide: it means that the structural defects responsible for the coloring at 2.9 eV and the quenching of IR band of the PL are annealed in the same temperature region from 150 to 280°C. In contrast, the defects responsible for the change of electrical resistance are annealed at Tanneal= 250 to 320°C (curve 3).

5. Discussion

Prior to reduction, the SrTiO, samples studied exhibited properties similar to those described previously: they were transparent, high-Ohmic and showed two PL bands: the visible and IR band. According to the well-known work of G. Blasse on luminescence in oxygen-octahedral crystals of ABO, type [ll], the visible PL band in SrTiO, is considered as the fundamental transition of localized type in TiO, oxygen octahedron at the lattice excitation of the crystal. The regularities of this PL band are correctly described by the configuration coordinate model,

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according to which the energy of TiO, octahedron depends on the the relative positions of ions in a given octahedron. The reduction process results in an increase of the visible PL intensity and of the quenching temperature. Both types of structural defects (oxygen vacancies and non-central Ti3’ ions) may lead to such octahedral distortions, which will explain the observed changes in the visible PL in reduced crystals. The IR band of PL was observed as a narrow peak with the maximum at A = 792 nm (1.56 eV) at the lattice excitation. However, the excitation band maximum shifts towards the wavelength region relative to the absorption edge (A = 380 nm). According to [12], the PL observed is identified as a radiative transition (2E, + 4A,,) in the Cr3+ ion which is present in the crystal lattice as the uncontrollable impurity substituting the Ti4+ ion. This PL is attributed to the charge transfer process: the Cr3+ ion traps the photohole (fig. 6, transition 1) and transforms into the Cr4+ ion; the subsequent electron trapping (transition 2) leads to the formation of (Cr ‘)* ion in the excited state; the transition to the ground state is accompanied by IR radiation, Cr4’ + e+ (Cr3+)* + Cr3+ + hv (792.5 nm). Although at present the mechanism of PR luminescence in SrTiO, is regarded to be strictly determined, a number of problems connected with the excitation spectrum and the nature of PL thermal quenching at temperatures below 110 K still remain to be solved. To explain both phenomena, the presence of a certain sensitive center near the valence band associatively connected with Cr ions is supposed in [12]. At the photoexcitation . t

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of the above center the electron transfers to the conduction band, while the hole passes to the valence band and is immediately captured by Cr3+ ion, leading subsequently to luminescence. Such a mechanism of excitation is more effective as compared to that of band-band excitation. This fact accounts for the shift of excitation band of IR luminescence relative to the absorption edge. At T < 100 K the transfer of the hole to the valence band is difficult and thus the efficiency of the luminescence drops. We consider the possible mechanisms of changing PL in the reduced crystals. The changes in the visible PL band may be due to the following reasons: (1) The defects, occurring during the reduction, change the configuration of the TiO, radiation center in such a way that the efficiency and the quenching temperature of this PL increase. (2) In the reduced crystals the energy transfer from TiO, to Cr3+ ions decreases, thus causing the intrinsic radiation from TiO, to grow. Because there is no exact correlation between the increasing intensity near the visible PL band and the IR band quenching, this second suggestion cannot be adopted as the only true one. The PL quenching of the IR band during the reduction process can occur (1) either due to the change in valency and/or in the crystallographic position of Cr3+, (2) or when other centers appear, which can trap the holes from the v-band more actively than Cr3+. Such IR radiation quenching centers can be color centers which occur during the reduction and are responsible for the absorption band at 2.9 eV. The validity of our first suggestion should be confirmed experimen-

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tally (using the EPR method). The validity of the second suggestion is indirectly confirmed by our experiment: the color centers and the PL quenching centers are annealed simultaneously within the same temperature range of 150 to 280°C. According to [7] these color centers are the Fe4+ ions formed during the reduction from the Fe3+ ions and giving rise to local states at 0.3 eV above the v-band (fig. 6). It will be reasonable to suggest that these centers are at the same time the IR radiation quenching centers which retrap the photocarriers needed to excite Cr3+. The defects causing the decrease in the p value are usually associated with oxygen vacancies. Our experiment on annealing reveals the difference between these defects and those leading to the light absorption at 2.9eV and IR radiation quenching.

References [l] H.W. Gandy, Phys. Rev. 113 (1959) 795. [2] P.H. Frederikse, Phys. Rev. 134 (1964) A442. [3] F.J. Mot-in, Phys. Rev. B 8 (1973) 5847. (41 W.S. Baer, Phys. Rev. 144 (1966) 734. [5] P.P. Van Engelen, Phys. Lett. A 25 (1967) 733. [6] M.O. Selme and P.J. Pecheur, J. Phys. C 16 (1983) 2559. [7] R.L. Wild, Phys. Rev. B 13 (1973) 3828. [8] B.W. Faughnan, Phys. Rev. B 4 (1971) 3623. [9] C.K. Jorgensen, Absorption Spectra and Chemical Boneling (Pergamon Press, New York, 1962). [lo] M. Postemak, A.E. Freeman and D.E. Ellis, Phys. Rev. B 19 (1979) 6555. [ll] G. Blasse, Mat. Res. Bull. 18 (1983) 525. [12] T. Feng, Phys. Rev. 25 (1982) 629.