Raman spectra of ultrathin YBaCuO7 filmsP. Zhang, T. Haage, H.U. Habermeier, T. Ruf, and M. Cardona Citation: Journal of Applied Physics 80, 2935 (1996); doi: 10.1063/1.363149 View online: http://dx.doi.org/10.1063/1.363149 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/80/5?ver=pdfcov Published by the AIP Publishing

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Raman spectra of ultrathin YBaCuO 72d filmsP. Zhang,a) T. Haage, H.-U. Habermeier, T. Ruf, and M. CardonaMax-Planck-Institut fu¨r Festkorperforschung, Heisenbergstrasse 1, 70569 Stuttgart, Germany

~Received 27 March 1996; accepted for publication 16 May 1996!

Thin films of YBa2Cu3O72d ~YBCO! on LaSrAlO4 ~001! substrates with various thicknesses havebeen investigated by Raman spectroscopy. The Raman spectra of the films have been obtained byproperly subtracting the spectrum of the substrate from the recorded spectra. This data anaysisenables us to determine the Raman spectra of YBCO films with thicknesses down to 12 nm~about10 unit cells!. The evolution of the peak positions and the linewidths of theB1g mode with thicknessallow us to conclude that the residual strains in these films are negligibly small within the measuredthickness range. The influence of the birefringence on measurement of the degree of epitaxy byRaman has been studied. With the reduction of film thickness the degree of epitaxy is slightlyincreased in these films. ©1996 American Institute of Physics.@S0021-8979~96!06716-3#

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I. INTRODUCTION

The basic physical properties of high-temperature supconductor~HTS! thin films are sensitively related to thestructure, the oxygen deficiency and distribution, the lattstrains, and their morphology.1,2 Recently, it was shown thathe early stages of growth play an important role in detmining the structure and the physical properties of thefilms.3,4 Consequently, reliable methods to characterizeultrathin films are of great interest. On the other hand, apcations of HTS films, such as junction and microwave dvices, or thermoelectric detectors,5–7 also require fast andconvenient nondestructive characterization methods.

Raman spectroscopy is one of the most commonly unondestructive tools for characterizing HTS materials.8 How-ever, it has been difficult to obtain reliable spectra from vethin films due to the small scattering cross section of Hfilms and the interference of the signals from the substra

In this article we report on Raman scattering froYBa2Cu3O72d ~YBCO! thin films grown on LaSrAlO4 ~001!with a thickness ranging from 12 to 240 nm. It will be showthat the as-recorded Raman spectra of the samples stuhere are a mixture of the spectra resulting from the YBCfilm and the substrate material. Therefore, the true Ramspectrum which exclusively results from the film can onlyobtained by properly subtracting the substrate spectrum fthe as-recorded one. The Raman spectra have been recat two scattering geometries in order to identify the Rammodes and to determine thea(b)-plane degree of epitaxy othe films. It was found that for YBCO films grown onLaSrAlO4 ~001! the peak position and the linewidth of thB1g mode show no apparent variation with decreasing fithickness down to 10 unit cells. This result suggests thatfilm strain which might be caused by the lattice mismatbetween the substrate and the film is negligibly small. Wshall discuss the thickness related Raman spectra in contion with the growth morphology of the films.

II. EXPERIMENTSThe YBCO films were grown on crystalline LaSrAlO4

~001! substrates in an ultrahigh vacuum chamber by pul

a!Electronic mail: zhang@servix.mpi-stuttgart.mpg.de

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laser ablation under optimized deposition parameters~in 0.2mbar oxygen pressure at a substrate temperature of 760 °!.9

The oxygen pressure was increased immediately afterdeposition and the films were cooled down to 450 °C arate of 20 °C/min. The films were kept for 20 min at thtemperature and then cooled to room temperature. The nonal film thickness was derived from the deposition rawhich was calibrated to be 0.2 nm per pulse.

In situ ultrahigh-vacuum scanning tunneling microscop~STM! has been used to investigate the surface morpholof the YBCO films with various thicknesses. The lattice prameters of the epitaxial films have been measuredgrazing-incidence x-ray diffraction~GIXRD!. The informa-tion about the growth mechanism as revealed by STM aGIXRD will be compared with the results obtained by Rman scattering. Raman experiments were performed inusing a DILOR micro-Raman spectrometer equipped withliquid-nitrogen-cooled CCD detector. The excitation sourwas an argon ion-laser operating at 514.5 nm with a powof less than 5 mW. Two scattering geometries, with thecident light polarization along the@110# direction of theYBCO films and the analyzer either parallel or perpendicuto @110#, defined asX8X8 andX8Y8 according to Ref. 10,were used to obtain the Raman spectra and also to evalthe degree ofa(b)-plane epitaxy of these films. Spectra othe single crystalline LaSrAlO4 substrate were also measurefor the same scattering geometries. These spectra werefor the subtraction required to obtain true spectra of thHTS films. The subtraction procedure is described below

III. RESULTS AND DISCUSSION

In Fig. 1~a! the as-recorded Raman spectra from tYBCO/LaSrAlO4 system for the scattering geometryX8Y8,and in Fig. 1~b! the corresponding spectra after subtractiof the substrate signal are shown. Figures 2~a! and 2~b! showthe corresponding spectra obtained for theX8X8 scatteringgeometry. The spectra of LaSrAlO4 are shown in the bottomcurves of Figs. 1~a! and 2~a! for the two corresponding scattering geometries.

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A. Subtraction of the substrate features

It is clear from Figs. 1, and 2 that there are features frthe substrate visible even in the as-recorded spectra fYBCO films with a thickness of 240 nm. A sharp peak300.2 cm21, resulting from the substrate, appears at the lfrequency tail of YBCO’sB1g mode inX8Y8 scattering ge-

FIG. 1. Raman spectra of YBCO thin films with thicknesses of~a! 240, ~b!120, ~c! 60, ~d! 36, ~e! 24, and ~f! 12 nm obtained atX8Y8 scatteringgeometry;~a! shows the as-recorded spectra, and~b! the ones after subtrac-tion of the substrate features. The bottom curve of~a! is the spectrum ofLaSrAlO4 substrate obtained for the same scattering geometry.

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ometry, and even more features from the substrate are visiin X8X8 geometry. The thinner the films, the more pronounced are these peaks. With decreasing film thicknessRaman spectra are dominated by such substrate contributisince more light penetrates through the film and reachessubstrate. Hence, the Raman signal from the substratecomes stronger than that from the YBCO film. One therefo

FIG. 2. The Raman spectra of YBCO thin films with thicknesses of~a! 240,~b! 120, ~c! 60, ~d! 36, ~e! 24, and~f! 12 nm obtained atX8X8 scatteringgeometry;~a! shows the as-recorded spectra, and~b! the ones after subtrac-tion of the substrate features. The bottom curve of~a! is the spectrum ofLaSrAlO4 substrate obtained for the same scattering geometry.

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has to make a subtraction in order to eliminate the substfeatures, and to obtain the true Raman spectra of the film

The key point to perform a reliable subtraction espcially for very thin films, is to find strong reference peaksthe spectra of the substrate, which do not overlap withpeaks of the YBCO film itself. We have chosen thLaSrAlO4 peaks at 300.2 and 439.7 cm21 as the referencesfor the two scattering geometries~X8Y8 andX8X8!, respec-tively: They are the strongest peaks for these scatteringometries. The 300.2 cm21 peak does not overlap with anmodes from the YBCO film. The 439.7 cm21 mode doesoverlap with the 430 cm21 mode of the YBCO film; how-ever, the intensity of the latter mode is very weak for thscattering geometry. Hence, satisfactory subtractions aretained.

All the recorded Raman spectra are first smoothed ousing the software provided byDILOR to reduce the statisticaerror in the recorded photon counts. Then the intensity ofreference peak in the sample spectrum is measured. Wemalize the whole spectrum of the substrate using this intsity, and then subtract this renormalized spectrum from tof the sample. The spectra obtained in this manner are shin Figs. 1~b! and 2~b!.

Figures 1 and 2 demonstrate that clean Raman spectthe YBCO films are generated after the subtraction ofsubstrate spectra. Reliable Raman spectra can thus betained for films with thicknesses down to 10 unit cells. Athe Raman active modes can be identified from the two stering geometries. TheA1g mode at about 117 cm

21 ~Ba–Bamotion! overlaps a plasma line at 117 cm21, which is notcompletely suppressed by the bandpass filter for the illunating light beam. There are also vibrations of air molecunearby, since the measurements were conducted in air.B1g mode, which has been investigated extensively, is strenough to determine its position and linewidth accurately

B. Possible Raman shifts due to interface stresses

From Figs. 1 and 2, the thickness dependencies ofposition and linewidth of theB1g mode have been obtainedNo apparent variations, both, in the peak position andlinewidth of this mode were found. The impact of the iplane mismatch ofc-axis oriented YBCO films with the in-dividual substrate material on the growth mode is wknown from the STM and GIXRD studies. In general,small lattice mismatch causes a stressed overlayer growpseudomorphically on the substrate until a critical thickneis reached. Beyond this thickness, stress relaxation octhrough the generation of dislocations or via islanding.room temperature, LaSrAlO4 ~001! has a smaller in-planelattice constant~a5b53.754 Å! than YBCO ~a53.819 Å,b53.886 Å!. At the early stages of growth we thus expestressed YBCO overlayers on LaSrAlO4. However, STM im-ages of the films under investigations show island growthno morphological changes, which would be related torelaxation of overlayer stress.3 The lattice vibrational fre-quencies of a stressed YBCO film should be different frothose of the unstressed or stress relaxed film. The frequeshifts of, say, the stressed epitaxial semiconductor filmssubstrate or in superlattices have been measured by Ra

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spectroscopy.12 The shifts are small, but usually larger thathe detection limit of Raman experiments. Assuming pseudmorphic growth, one can estimate the strain developed inc direction due to the compressive strain ina(b) plane ofYBCO films on LaSrAlO4. These strains should have direcimpact on the frequency shift of theB1g mode. According toHooke’s law

s i j5Ci jkl ekl ,

with the conditionszz50 at the epitaxial film surface, intro-ducing the lattice parameters,3 and the elastic constants11 ofYBCO and LaSrAlO4 in Hooke’s law, then the lattice com-pression of exx520.018 and eyy520.035, respectively,alonga andb direction should result in an expansion in thc direction of ezz50.039. There is, however, no precisevaluation of how theB1g mode should react to these strainbut one can make an estimation using the Gru¨neisen param-eter g~52d ln v/d ln V!. For YBCO crystals with oxygencontent of 6.95,g ~B1g!51.7,13 the change of volumeDV/Vin this case is 1.5%, thus the pseudomorphic growth mlead to a Raman shift of about 9 cm21 for the B1g mode.Experimentally, no apparent shift within the experimenterror~61.0 cm21! was observed in our case. This means theither the strains are negligibly small or theB1g mode is notsensitive to the strains. X-ray grazing incidence diffractiowas used to measure the lattice parameters of the 10 andunit-cell-thick films. It indicates that there is no pseudomophic growth and the YBCO layer seems like a free-standifilm.14 This result is consistent with the Raman observatiothat the films are grown with negligibly small strain. It isvery likely that the stresses are released at a very early stof the growth, say, in thickness of less than 12 nm by eithislanding, oxygen deficiency, or mismatch dislocations.

C. Evaluation of degree of a(b )-plane epitaxy and theinfluence of birefringence

Additionally, we have estimated the degree ofa(b)-plane epitaxy of the films grown at different thicknesses bRaman scattering. According to Ref. 10, the degree ofa(b)orientation of YBCO films can be determined by the ratio othe Raman intensities of theB1g mode measured in theX8X8andX8Y8 scattering geometries. The birefringence of YBCcrystals, which leads to the rotation of the polarizationincident and scattered light propagating not along the optiaxes of the crystal, should also be taken into consideratiThe influence of the piezobirefringence on the scatteringtensities of the two scattering geometries in opaque samphas been evaluated by Chandrasekharet al.15 Along the sameline we obtain the Raman intensity ratioRb due to the bire-fringence of the crystal

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where l is the laser wavelength,a and nxx(nyy) are theabsorption coefficient and the refractive index ina(b) orien-tation, respectively. Kircheret al.16 have measured the anisotropic optical constants of YBCO at different wavelength

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Introducing the constantsa51.33105 cm21, nxx51.8,nyy51.6 for 514.5 nm excitation we obtainRb50.02. Forsmall values ofRb the measured ratioRm5Rb1Re, can beexpressed as the sum of contribution due to birefringen~Rb) and misorientation of the film from the ideal epitax~Re!. For thick films, based on the directly measured rat~Rm50.12! one obtains a degree of epitaxy of 82%. Takinthe birefringence into account, the real degree of epitawould be increased to 85%. For ultrathin films, the preciparameters, such asa andnxx(nyy) are not available. How-ever, the ratio of the Raman intensities due to misorientatshould be smaller than the measured one, hence the degreepitaxy should be higher than that evaluated without takithe birefringence into consideration. For ultrathin films, thmeasured ratioRm is 0.06, which corresponds to a degreeepitaxy of 90%. The real degree of epitaxy should be evhigher. This means that for our samples, the thinner tfilms, the higher the degree of epitaxy. The nearly perfea(b)-plane growth found at the very beginning, is slightlreduced with increasing thickness even under optimizgrowth conditions. We have also noticed that the relatiintensity of the 500 cm21 peak shows a thickness dependence, which means that more non-c-axis growth takes placewith increasing film thickness. In the above conditions, thbirefringence does not have a dramatic influence onevaluation of the degree of epitaxy of the films. However, wshould point out that at some other wavelengths, say 64nm, the ratioRb can be as large as 0.07, which has to btaken into consideration if a precise evaluation of the degof epitaxy is required. The influence of birefringence on thmeasurement of the degree of epitaxy is complicated fortrathin films. The involved parameters are both thickness awavelength dependent. Further studies are required, whwill be published elsewhere.17

D. Abnormal intensity variation of Cu-Cu modes withreduction of film thickness

Some unusual variation of the peak intensity with thfilm thickness for the 147 cm21 mode is evident, as shown inFigs. 1 and 2. Considering that there are some other fawhich may influence the intensities, positions, and tlinewidths of the Raman peaks, we cannot reach a definconclusion at present from these results. More experimeare going on and detailed analyses on the thickness depdence of more Raman modes of YBCO will be publisheelsewhere.18

IV. CONCLUSIONS

In summary, we have recorded Raman spectra fromtrathin YBCO films with thicknesses down to 10 unit cells

2938 J. Appl. Phys., Vol. 80, No. 5, 1 September 1996rticle is copyrighted as indicated in the abstract. Reuse of AIP content is su

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Reliable Raman spectra of YBCO films were obtainedproperly subtracting the features of the substrate fromrecorded spectra. The success of the technique stronglypends on the existence of intense Raman peaks of thestrate which do not coincide with features from the film toinvestigated. The thickness dependent Raman spectratained in YBCO on LaSrAlO4 demonstrate that there is negligibly small stress in the films with a thickness larger tha10 unit cells. The influence of the birefringence on measument of the degree of epitaxy by Raman has been studTaking the influence into account, the measured degreeepitaxy should be even higher. It was found that the hpercentage ofa(b)-plane growth is slightly reduced with theincrease of the thickness of the films even under optimizgrowth conditions.

ACKNOWLEDGMENTS

The authors would like to thank T. Sekinger for manhelpful discussions and technical assistance. Partial supof this work by the Fonds der Chemischen Industrie is grafully acknowledged.

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