Correlation of superconducting properties and oxygen concentration inY1Ba2Cu3O7−δ films via Raman spectroscopyY. Liu, G. Pangilinan, R. Sooryakumar, Michael J. Sumner, and Thomas R. Lemberger Citation: Journal of Applied Physics 64, 3598 (1988); doi: 10.1063/1.341395 View online: http://dx.doi.org/10.1063/1.341395 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/64/7?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Correlation of structural and superconducting properties of Ba2YCu3O7−δ thin films AIP Conf. Proc. 251, 44 (1992); 10.1063/1.42100 Properties of critical current density in heterostructures of Y1Ba2Cu3O7− y /Y1Ba2(Cu1− x Ni x )3O7− ysuperconducting films AIP Conf. Proc. 251, 108 (1992); 10.1063/1.42070 The properties of Y1Ba2Cu3O7−δ thin films with silver doping prepared by spray pyrolysis J. Appl. Phys. 67, 6946 (1990); 10.1063/1.345038 Highly oriented asdeposited superconducting laser ablated thin films of Y1Ba2Cu3O7−δ on SrTiO3, zirconia, andSi substrates Appl. Phys. Lett. 53, 2330 (1988); 10.1063/1.100515 Structure and superconducting properties of Y1Ba2Cu3O7−δ films prepared by transversely excited atmosphericpressure CO2 pulsed laser evaporation Appl. Phys. Lett. 52, 1008 (1988); 10.1063/1.99228

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Correlation of superconducting properties and oxygen concentration in Y1Ba2Cua07_& fUms via Raman spectroscopy

Yo Uu, G. PangiUnan, R. Sooryakumar, Michael J. Sumner, and Thomas R. Lemberger Department of Physics, The Ohio State University, Columbus, Ohio 43210

(Received 9 May 1988; accepted for pUblication 2 June 1988)

We report Raman scattering and resistance measurements on a YBaZCu30 7 _ 8 film in which tJ is varied by repeated annealing in oxygen or argono As oxygen is removed, the resistance rises, the transition moves to lower temperature and broadens, and "forbidden" Raman modes are observed. Group theory, the Raman activity of another oxygen rich (tJ-O) film of different orientation, and comparison with results on single crystals anow us to assign each Raman phonon. We hence demonstrate that several "forbidden" Raman peaks are associated with depletion of oxygen chain sites. Our results establish that the oxygen concentration D, and hence the superconducting properties, can be varied reversibly by reannealing in oxygen or argon with the Raman spectra providing a useful microscopic probe of the lattice.

I. INTRODUCTION

Properties of the oxide superconductors in the class of y lBa2Cu30 7 __ t5 depend critically on their oxygen content and on the ordering of the vacancies that exist when tJ> 0. 1

While resistance measurements demonstrate clearly that the superconducting properties are degraded by oxygen deple­tion,2--6 they give no microscopic information about the oxy­gen concentration or arrangement of defects, or whether the oxygen depletion is a bulk effect or confined to grain boun­daries or twin planeso Neutron scattering yields this micro­scopic information on bulk samples,7 but is normally unap­plicable to films. Our results demonstrate that Raman scattering is an excellent local probe for studying the micro­scopic effects of oxygen depletion in small samples such as films and single crystals.

In the present study, we repeatedly reanneal an Y 1Ba2Cu30 7 _S film in oxygen and argon at various tem­peratures. Raman scattering clearly shows that the effect of annealing is to change the oxygen concentration on the chain sites in the bulk of the material, in agreement with results on bulk samples. 7 The intensity of the lines that are forbidden for (j = 0 allows us to correlate the quality of the resistive transition with the oxygen concentration. Taken in conjunc­tion with resistivity data, the Raman results confirm that oxygen may reversibly be depleted and reintroduced to the film while completely recovering the structural and super­conducting properties.

U. EXPERIMENT

The Y lBa2Cu30 7 _ Ii film was made following in many ways the procedure of Mankiewich et al. g We codeposited Y from an e-gun, and Cu and BaF2 from resistively heated sources onto a Y -stabilized ZrOz substrate at ambient tem­perature in anion-pumped vacuum system (P< 10-6 Torr). The deposition rate from each source was monitored and controlled by a quartz crystal deposition controller. After deposition the film was slowly introduced into a tube furnace at 900 ·C that had had oxygen flowing in it for several hours. The oxygen was bubbled through distilled water at room

temperature before flowing through the furnace to reduce the annealing time. By control of the deposition rates, we have also grown films on SrTi03 that orient differently com­pared to those on a zr02 substrate. This orientational de­pendence of the films provides us with an added means to confirm our Raman phonon assignments that are central to this work.

Many films have been made with this procedure and characterized with resistivity, x-ray, and SEM measure­ments. The film on Zr02 with the highest Tc and sharpest transition, onset at Tel = 93 K and zero resistance at T cO

= 85 K, was annealed at 900 °C for 20 minutes and then cooled at a rate of 2°C/min to room temperature. For this film, oxygen was removed from (and reintroduced to) the lattice by heating to 400-450·C in an argon (oxygen) atmo­sphere for about 20 min. The oxygen-rich film grown on SrTi03 had an onset temperature Tel of 92 K and a very sharp transition to zero resistance at T cO = 91 Ko Figure 1 shows x-ray diffraction from the two types of superconduct­ing films, confirming their different orientations and absence of any impurity phases. The diffraction from a powder of YBa2Cu30 7 is also shown for comparison.

We measured the Raman scattering spectrum for the best film on Zr02 discussed above which, by repeated an­nealing, was cycled reversibly through the orthorhombic ->

tetragonal -> orthorhombic phases. X-ray diffraction-Figo l-confirms that this film is oriented with the c axis lying paranel to the substrate surface over most of the film. This is supported by SEM results (Fig. 1) that reveal in addition that the surface has a smooth structure on the scale of mi­crons. The Raman spectrum was also recorded from the film on SrTi03 for which the c axis is predominantly oriented normal to the substrate surfaceo SEM from this film shows that the surface is also largely smooth but supports a few random thin rod-shaped features, that may be misoriented growth with the c axis along the surface.

Fi.ve different oxygen concentrations in the film on Zr02

were studied, with each film labeled chronologically from a to e. Based on the results presented here, films a and e had the highest oxygen concentrations, near {j = 0; band d had

3598 J. Appl. Phys. 64 (7), i October 1988 0021-8979/88/193598-04$02.40 © 1988 American Institute of Physics 3598

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!? film g Of) Z(O:o

~ ~ § ~ ." 8~ <i<:) g

g~ .,;n ~ -;:1iJ

10 20 30 40 50 60 10

28

FIG. 1. X-ray diffraction from YBa2Cu30 7 _ 8 films a and f (cunres 3 and 2.)

revealing their different orientations. The diffraction from II sintered pow­der (curve 1) of the material is provided for comparison. Also shown are SEM pictures 3 and 2. of the films a and f, that have, respectively, parallel and normal orientation of the c axis relative to the film surface.

lower oxygen concentrations but were still orthorhombic and superconducting; and film c had the lowest oxygen con­centration and was tetragonal and not superconducting. Film f, grown on SrTi03• is oxygen rich with 8-0.

Figure 2 illustrates the resistivity of the films. The onset Tel and width A.Tc (10%-90% width) of the transition are indicated and confirm the degraded superconducting prop-

l I

o 100 200 300

T (K) FIG. 2. Temperature dependence of the resistance of several YBa2Cu30 7 _ &

films for various oxygen deficiency. The curves are labeled chronologically according to oxygen removal (a, b,c) and reintroduction (d,e). Cur".: f is f .. om the film grown on SrTi03• Tel and ATc are the onset and width of the superconducting transition. Specific atoms on the unit cel! are labeled in the inset.

3599 J. Appl. Phys., Vol. 64, No.7, 1 October 1988

erties as oxygen is removed. The weak fall in resistance for the tetragonal film, curve c, probably is related to small quantities of the nontransformed orthorhombic phase.

III. RAMAN RESUI.. TS

Before getting into the details of our Raman spectra, we note the group theory assignment of modes. The orthorhom­bic structure (D ih) of Y I Ba2Cu30 7' shown in the inset to Fig. 2, supports 15 Raman (5A g • SB2g , SBlg ) and 21 IR (7B 1u • 7Bzu ' 7B3u ) active modes. In the semiconducting te­tragonal (D 111) phase at 8 = 1, in which the chain oxygens are missing, three IR vibrations disappear while the number of Raman phonons is unchanged. In the transition to the tetragonal phase the E21! and B3g modes become degenerate Eg phonons while the two Ag Raman modes related to the planar oxygen atoms transform to an A Ig and a E Ig vibration in the semiconductor. The systematic change in symmetry of the crystal structure with 0 is accompanied by changes to the frequency and intensity of the vibrations and are of primary importance to this work.

Figure 3 shows the Raman spectra of the five different oxygen concentrations in the film on ZrOz (a-e), and ofthe film on SrTi03 ( f), which has a different crystal orienta­tion. Films a and e have the best resistive transitions for growth on zrOz and very similar Raman spect.ra, showing the reversibility of the oxygen variation. Important features to note in Fig. 3 are the five allowed modes evident in aU spectra of films a-e, the "forbidden" modes (identified by

Ag Ag

100 200 300 400 500 600 700 800

Raman shift (em-I) FIG. 3. Raman spectra at 5 K from films a--e and f. The arrows on spectra b and d identify "forbidden" Raman phonons. Tel and T to identify the onset and zerooresistance temperatures. The spectra were measured with 150 m W of 5308-A radiation.

Liu eta!. 3599

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arrows on spectra band d) seen only when oxygen is partial­ly depleted, and the systematic differences between spectra with different oxygen concentrations. We further note that the Raman spectrum from d is close to the spectrum report­ed on a single-crystal sample with b~0.5,9 thus confirming the high quality of the films. In film f we observe the domi­nance of one peak, a result, as discussed below, consistent with its specific orientation.

Let us examine first how the allowed modes are affected by oxygen depletion, Le., spectra a-e of Fig. 3. Of these only five Ag modes are expected for 8 = 0 since depolarized scat­tering from single crystals9--ll have confirmed that the .B2g

and B3g vibrations are absent. The allowed phonons are ap­parent in all spectra from oxygen rich samples at roughly 114, 150, 336, 440, and 504 cm-- 1

• The peak around 504 cm -I softens to 418 em -J in the tetragonal phase and broad­ens somewhat. The phonon at 336 cm -1 increases by a few wave numbers in the oxygen-depleted orthorhombic films b and d. There is no evidence for an increase in intensity of this mode in the orthorhombic phase, in contrast to some reports from sintered pellets. 12 In the tetragonal phase, fUm c, this peak has stiffened to 342 em -1, and is clearly narrower and more intense than in the other spectra of Fig. 3. The mode at 150 em - I softens to 141 cm - I in the tetragonal phase. In f, a film characterized by an oxygen concentration close to a and e, the peak at 336 cm --1 is strong while the 504-cm- 1 mode is suppressed. The other allowed modes of film fat 114, 150, and 440 cm -,1 are as in spectra a and e.

When some oxygen is removed, but the lattice is still orthorhombic and superconducting (films band d), the weak feature seen in the other films at about 570 em - I be­comes a strong band containing several sharp modes. Addi­tional modes appear at 232, 265, and 290 em - [. As discussed below, many of these features are associated with modes in­volving the chain oxygens. These modes are forbidden for 8 = 0, observed when some oxygen is removed, and subse­quently disappear when aU of the chain oxygens are elimin­ated and the lattice is tetragonal.

IV. DISCUSSION

We now discuss our assignments of the modes to show that the changes in the spectra with oxygen stoichiometry are consistent with the depletion of oxygen from the chain sites. We note that many Raman reports on these oxides have appeared9

-17 with disagreements between some assign­

ments. Raman studies specifically probing the effects of oxy­gen stoichiometry in bulk pellets have also been report­ed.12·18.19 While assignments are best determined by polarized light scattering from single crystals, the lack of high-quality, untwirmed crystals of the orthorhombic phase have hindered progress in this respect. Indirect criteria such as behavior under atom substitutions, while valuable, must in general be considered only as supportive and not the pri­mary reason for an assignment. Hence, we mainly rely upon results from single crystals9-11 to aid us in our assignments, although even in this situation the possibility of twinning, which causes the a and b axes to be misoriented, can be a problem.

3600 J. AppJ. Phys., Vol. 64, No.7, 1 October 1986

Several groups have assigned the peak at 336 cm - 1 (440 cm - 1 ) for the highest oxygen concentrations (films a and e) to modes involving the out-of-phase (in-phase) vibrations of the planar 0(2), [0(3)] atoms along the c axis. Recent9

results have shown that the component azz of the diagonal Raman tensor of the 336-cm - 1 vibration is virtually zero so that for an oriented film this phonon should be absent from the spectra for light polarized along the c axis. The observa­tion of this mode in our film from backscattering normal to the surface confirms the nonuniform in-plane orientation of the c axis as also suggested by the SEM result in Fig. 1. This mode moves to 342 cm-! and becomes BIg symmetry where a zz = 0 in the tetragonal phase. The increased intensity of this B 19 mode implies that the other diagonal components of its Raman tensor are weakly enhanced in comparison to the orthorhombic structure. This assignment of the 336-cm --I

mode is confirmed in f, where the backscattered photon wave vectors are along the c axis. Hence, the spectrum would be sensitive to the finite a xx , a yy Raman tensor components Jeading to the strong 3 36-cm - 1 peak observed in f.

The peak at 504 cm -I, the strongest feature in films a and e, results from vibrations along c involving the bridging oxygen O( 4).9-11 Due to modifications of the c-axis lattice parameter on oxygen depletion, the mode softens to 478 cm-- 1 in film c. We attribute the width to a distribution of oxygen concentrations 8 and the proximity of 0 (4) to the chain oxygen O( 1) from which oxygen is preferentially de­pleted. The latter could allow phonons associated with O( 4), and having wave vectors away from the center of the Brillouin zone, to yield Raman scattering that would be for­bidden in a perfect lattice. The absence of the 504-cm -I peak in f supports this assignment as the polarizability compo­nents a xx , a yy for this vibration are very small compared to a zz ." We hence note that the 504-cm- 1 Raman mode pro­vides a means to determine the orientation of the c axis rela­tive to the film surface. The peak is very weak for a c-axis orientation normal to the plane ( f), and it is a strong fea­ture when the c axis lies paranel to the film surface (a and e).

The features at 114 and 150cm,-1 areAg optic modes of the Ba and Cu (2) atoms moving along the c axis.9 Their softening in the semiconducting phase is consistent with the expansion of the unit cell along c. To assign the weak broad feature at 580 cm -I, we note that lattice dynamical caIcula­tions9 have placed infrared active B lu phonons at 502 and 555 em -1. The observed enhancement of this structure with decreased oxygen concentration (films band d) is consistent with its being assigned to these IR phonons associated with the chain and bridge oxygens rendered Raman active by the loss of translational symmetry. Indeed, peaks at 580 em-I have been observed in IR data from the ceramic materia1. 16

The structures at 232 and 630 cm- I, identified by ar­

rows in band d of Fig. 2, are assigned to the chain 0 ( 1 ) and CuO) atoms moving out-of-phase along the a and b axes, respectively. The highest frequency optic phonon has been estimated to be an IR active vibration at 576 cm --I (except for a B2g mode calculated at 586 em -1 whose Raman polar­izability is negligible9

). This mode supports the oscillations ofthe chain Cu ( 1) and 0 ( 1 ) atoms that we identify with the forbidden scattering at 630 em - I. Note that in the tetragonal

Liu et al. 3600

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structure, these forbidden peaks are absent/weak. The above assignments are, hence, consistent with oxygen being prefer­entially depleted from the chains, with the Cu-O planes sandwiching the Y being least susceptible to disruption.2o,21

Identification of the remaining weak oxygen-defect-in­duced structures at 264 and 290 cm -! is more difficult with several likely candidates available. Based on the lattice dy­namic calculations,9 these would include B3u and B2u in­plane vibrations and the B1u 0(2), 0(3) vibrations along the c axis. More data are required for any definitive conclu­sions.

The Raman results, hence, provide several specific fea­tures that characterize the quality of the films. In the oxygen rich /5 - 0 phase, these are the 336- and 504-cm- 1 peaks. The strong mode at 342 cm - I and vibrations at 456 em --I are specific to the tetragonal semiconducting phase. For inter­mediate oxygen stoichiometries in the orthorhombic phase, the intensity of the 232-cm -I forbidden Raman peak pro­vides a measure of the oxygen deficiency and, hence, of Tc'

V. CONCLUSIONS

In summary, we have reported resistance and Raman measurements on a single, high-quality Y I Ba2Cu307 _ fj film in which 8 was varied reversibly through repeated reanneal­iog in oxygen or argon. As oxygen was removed, the re~ds­tance of the film increased, the transition moved to lower temperatures and broadened, and finally disappeared alto­gether. By assigning the peaks in the corresponding Raman spectra to specific phonons, based on data on single crystals and IR data, we demonstrated that the oxygen was removed primarily from the Cu-O chains. Moreover, the oxygen con­tent can be monitored by the intensity of the "forbidden" Raman scattering from the Cu-O chain IR-active mode, which is made Raman active from the 0 defects in the Cu~O chains. We have supported our assignments of the allowed phonons by also investigating a different film with a c-axis orientation normal to the film surface.

ACKNOWLEDGMENTS

We thank Professor M. Cardona for useful discussions and sending us a preprint of Ref. 9. This work was supported in part by the Office of Graduate Studies and Research at the

3601 J. Appl. Phys., Vol. 64, NOe 7,1 October 1988

Ohio State University and by the National Science Founda­tion through Grants Nos. DMR 87-03980 and DMR 85-15370.

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