SURFACE AND INTERFACE ANALYSISSurf. Interface Anal. 29, 325–329 (2000)

Oriented growth of sol–gel-modified PbTiO3thin films on Si-based substrates†

A. Gonzalez, R. Poyato, R. Jimenez, J. Mendiola, L. Pardo and M. L. Calzada*Inst. Ciencia de Materiales de Madrid (CSIC), Cantoblanco, 28049 Madrid, Spain

A sol–gel Pb0.76Ca0.24TiO3 solution was deposited by spin-coating onto Pt/Ti/SiO2/Si(100), Ti/Pt/Ti/SiO2/Si(100) and Pt/TiO2/SiO2/Si(100) substrates. As-prepared substrates and those subjected to a thermaltreatment in air at 650 °C for 1800 s were used. The wet films were dried at 350°C for 60 s and crystallized inair by rapid thermal processing (RTP) at 650°C for 50 s, with a heating rate of ~30°C s−1. This procedure wasrepeated four times to obtain crystalline films with a thickness of ~250–300 nm. Perovskite films with differenttextures were obtained after crystallization. This texture was a consequence of the multilayer structure of thesubstrate. The formation of Pb0.76Ca0.24TiO3 films with a preferred orientation in the 〈111〉 direction of theperovskite was observed when Ti was present at the top surface of the Pt. Texture of the films was studied byx-ray diffraction (XRD) and the structure of the substrate surfaces was studied by grazing incidence x-raydiffraction (GIXRD). Spontaneous pyroelectric coefficients were measured in the films. Copyright 2000John Wiley & Sons, Ltd.

KEYWORDS: thin film; sol–gel; lead titanate; growth

INTRODUCTION

Calcium-modified lead titanate perovskite with a compo-sition of Pb0.76Ca0.24TiO3 has proved, as bulk ceramic, topresent appropriate pyroelectric and piezoelectric proper-ties for its use in infrared sensors and electromechanicaltransducers.1 The sensitivity of pyroelectric devices isincreased when this perovskite is prepared as a thin filmon silicon substrates. This implies the preparation of aheterostructure in which the bottom electrode and otherinterface layers have to be placed between the ferroelec-tric film and the substrate, with the aim of collecting theresponse of the material. Such a response depends on theremaining polarization in a direction perpendicular to thefilm plane. Therefore, the preparation of films with a pre-ferred orientation in the polar direction of the perovskiteis desired in these materials, because it allows eliminationof the polarization step under an electric field for the fab-rication of a device. Also, the device is more stable if thisspontaneous polarization cannot be switched easily.2

It has been shown that nucleation mechanisms of filmson substrates depend on the preparation process and onthe surface characteristics of the substrates.3 Epitaxialfilms have been obtained mainly by physical depositionmethods.4 Preparation of films with a high degree of ori-entation is also possible through chemical solution depo-sition methods such as sol–gel if appropriate preparationconditions are selected.5 These conditions have to pro-mote heterogeneous growth of the crystalline film at the

* Correspondence to: M. L. Calzada, Inst. Ciencia de Materiales deMadrid (CSIC), Cantoblanco 28049 Madrid, Spain.E-mail: lcalzada@icmm.csic.es

† Paper presented at ECASIA 99, 4–8 October 1999, Seville, Spain.Contract/grant sponsor: BRITE EURAM; Contract/grant number:

BRPR-CT98-0777.Contract/grant sponsor: CICYT; Contract/grant number: MAT98-

1068.

film/substrate interface, therefore, the substrate surface hasan important role in the growth of oriented films, and thematching between lattices of the top surface of the sub-strate and of the crystalline film facilitates the orientedgrowth of films.3

In this paper, the effect of the nature of the surfaceof the platinized silicon substrates on the orientation ofsol–gel spin-on Pb0.76Ca0.24TiO3 thin films is studied. Thetexture developed in the crystalline films was followedby x-ray diffraction (XRD) and the nature of the substratesurface was studied by grazing incidence x-ray diffraction(GIXRD). These techniques have been used previously byother authors for the study of the texture of lead zirconatetitanate thin films.6

EXPERIMENTAL METHODS

Preparation and characterization of substrates

The substrates used in this work were Pt/Ti/SiO2/Si(100),Ti/Pt/Ti/SiO2/Si(100) and Pt/TiO2/SiO2/Si(100). Herein-after, these substrates will be called I, II and III, respec-tively. The Si(100) was a commercial wafer with a topspontaneous-oxidized SiO2 layer. The Pt, Ti and TiO2layers were deposited onto the silicon by r.f. magnetronsputtering. The thickness of the Pt electrode was¾100 nmand it had a highly preferredh111i orientation. The thick-ness of the Ti and TiO2 layers placed between the Pt andthe Si was¾50 nm. The Ti layer deposited onto the plat-inum had a thickness<10 nm. Equivalent substrates toI, II and III were thermally treated at 650°C for 1800 s,with a heating rate of 10°C min�1. These substrates willbe called T-I, T-II and T-III.

The surface of the substrates was examined by GIXRDwith an incidence angle of D 2°. A Siemens D500diffractometer equipped with 0.4° divergence soller slits

Copyright 2000 John Wiley & Sons, Ltd. Received 22 November 1999Revised 18 February 2000; Accepted 18 February 2000

326 A. GONZALEZ ET AL.

and a plane LiF monochromator was used for this study.Copper K radiation with an acceleration voltage of 40 kVand a current of 25 mA were used.

Preparation and characterization of thin films

A calcium-modified lead titanate precursor solution wassynthesized by a sol–gel method described elsewhere.7

The concentration of this solution was 0.3M (moles ofPb0.76Ca0.24TiO3 perovskite formed after crystallization,per litre of solution). This solution was spin-coated ontothe silicon substrates using a velocity of 2000 rpm for45 s. Wet films were dried on a hot plate at 350°C for60 s. These films were crystallized in air by rapid thermalprocessing (RTP) at 650°C for 50 s, using a heatingrate of¾30°C s�1. Deposition, drying and crystallizationwere repeated four times. Each of these crystalline layershad a thickness of¾60–70 nm and a total of fourlayers were prepared on each substrate to obtain a finalfilm thickness of¾250–300 nm. The thickness of thefilms was measured by profilometry. The composition ofthe crystalline films was Pb0.76Ca0.24TiO3 as analysed byRutherford backscattering spectroscopy.8 The films willbe denoted hereinafter as the name of the substrates onwhich they were deposited (I, II, III, T-I, T-II or T-III).

Crystal phase and orientation of the films were anal-ysed by XRD using the above-mentioned diffractometerwith the Bragg-Brentano geometry. The 111 peaks ofthe Pb0.76Ca0.24TiO3 film and of the Pt electrode appearedoverlapped in the patterns recorded, with strong intensitydifferences between them and with a higher preferred ori-entation of the Pt than of the film along directionh111i.To avoid this,� and 2� were misaligned by an angle of¾5° during the recording of the patterns. In this way, itwas possible to separate the two peaks and to obtain asemi-quantitative value of the orientation degree of thefilms along the different directions of the perovskite. Theorientation of the films has been estimated from the inten-sities of theI001/I100, I101/I110 andI111 perovskite doubletsand peaks with Miller indexes 001/100, 101/110 and 111,respectively. Relative intensities have been calculated byconsidering the intensity of the highest peak recorded foreach sample as a value of 100%. These intensities havebeen compared with the JCPDS-ICDD 39–1336 file thatcorresponds to a bulk ceramic with the same nominal com-position as that of the films of this work.9

Spontaneous pyroelectric currents of the samples weremeasured with a Keithley 6512 electrometer by applyinga triangular thermal wave ofš1.5 °C and 5ð10�3 Hz. Theeffective heating rate used was 1.8 °C min�1. Pyroelectriccoefficients, , were calculated from these measurements.

RESULTS

Figure 1 shows GIXRD patterns of substrates I, II, III,T-I, T-II and T-III in the 2� interval where the Pt(III)peak is observed. These substrates show similar patternsafter their thermal treatment. A broadening in the Pt peakscan be observed in the patterns of the substrates I, II, T-Iand T-II.

Figure 2 shows the GIXRD patterns of films I, II, III,T-I, T-II and T-III. The formation of a single perovskite

Figure 1. The GIXRD patterns of the top surfaces of thesubstrates: (a) I, II and III; (b) T-I, T-II and T-III.

phase is observed in all the samples. The non-perovskitepeaks of the figure correspond to a substrate phase, PtSi,formed during thermal annealing. The GIXRD analysisat different incident angles has shown that this phaseis placed at the substrate surface and it appears in thepatterns mainly when the substrate is not totally coveredby the ferroelectric film.

Note the small ratio of the intensity of the perovskitepeaks in relation to the background for the patterns of thefilm T-II. This film is thus suspected of having a preferredorientation. To get an approximation of the texture of thefilms, XRD analysis with conventional Bragg-Brentano

Surf. Interface Anal. 29, 325–329 (2000) Copyright 2000 John Wiley & Sons, Ltd.

SOL–GEL-MODIFIED PbTiO3 FILMS ON Si 327

geometry was used, as shown by other authors as a usefultechnique to determine film orientations.6,10 The XRDpattern obtained for film II is shown in Fig. 3. In thisfigure a shoulder is detected on the left of the Pt(III)peak. This shoulder corresponds to the 111 peak of theperovskite. It indicates the growth of this film with apreferredh111i orientation, because the intensity of thispeak is higher than that of the others of the perovskite.Similar results were obtained for film T-II, but not forthe films I, III, T-I and T-III, where no shoulders in thePt(111) peak were detected. X-ray patterns of the filmswere also carried out by misalignment of the� and 2�angles. In this way, the 111 peaks of the Pt and of theperovskite could be separated. The peak profiles shownin Fig. 4 were obtained together with band splitting andpeak fitting to a pseudo-Voigt function. Data of the relativeintensities of the perovskite peaks in Fig. 4 are givenin Table 1. This table also shows the film thickness andthe measured spontaneous pyroelectric coefficients. Theseresults indicate that films II and T-II have a preferredorientation along theh111i direction of the perovskite,whereas the other films have a preferred orientation alongthe h100i direction.

DISCUSSION

Other authors11,12 have shown that lead titanate zirconate(PZT) thin films grow on platinized silicon substrates with

Figure 2. The GIXRD patterns of the films: (a) I, II and III: (b) T-I,T-II and T-III. The asterisk corresponds to a PtSi phase from theJCPDS-ICDD 7-251 file.

Figure 2. (continued).

Figure 3. The XRD pattern of film II.

a preferredh111i orientationwheneverTi is presentonthesubstratesurface.They indicatedthat thecauseof thisorientationwas the formation of a Pt3Ti compoundwitha cubic structure.The GIXRD patternof the substrates(Fig. 1) seemsto show that the surfacesof substratesIIIand T-III only havepure Pt. However,the GIXRD pat-ternsof the othersubstratesshowa broadeningof the Ptpeaks,probablyindicatingthatotherspeciesarepresentat

Copyright 2000JohnWiley & Sons,Ltd. Surf. InterfaceAnal. 29, 325–329 (2000)

328 A. GONZALEZ ET AL.

Table 1. Thickness, preferred orientation and spontaneouspyroelectric coefficients of the Pb0.76Ca0.24TiO3 thinfilms

Thickness ð 10�9

(nm)a I001 I100 I101 I110 I111 .C cm�2 K�1/

I 300 100 7 6 31 C4.7II 300 6 34 5 5 100 C6.5III 300 8 100 23 16 28 C4.8T-I 240 20 100 17 26 40 C14.0T-II 260 39 16 17 100 C7.3T-III 240 100 54 37 60 C13.5Theoretical 14 21 100 45 35valuesb

a Average estimated error of 20%.b Values from the JCPDS-ICDD 39-1336 file.

the top surface. This broadening could be associated withthe overlapping of peaks of the Pt3Ti cubic compound.In this structure, the Ti–Ti distance in the (111) planeis 5.55 A. For the Pb0.76Ca0.24TiO3 perovskite, the Ti–Tidistance in the (111) plane is 5.52A (Fig. 5).13 Thus,the matching between both structures is good enough forgrowing perovskite on this surface with ah111i preferredorientation. The films of this work were prepared by mul-tiple deposition, drying and crystallization steps of thinlayers of perovskite. This preparation method facilitates

Figure 4. The XRD patterns of the films: (a) I, II and III; (b) T-I, T-IIand T-III. Peaks shown in the figures have been obtained fromthe experimental peaks after band splitting and peak fitting to apseudo-Voigt function.

Figure 4. (continued).

Figure 5. Configurations for the (111) plane of Pt3Ti and ofPb0.76Ca0.24TiO3 (PTCa) perovskite.

heterogeneousnucleationin thefilm on its surfacein con-tact with the substrate.6 Becausein eachcrystallizationstep only a thin new layer (¾70 nm) crystallizeswitha strong tendencyto replicatethe underlayingstructure,growth of the perovskitestructurewith a preferredori-entation is promoted.This justifies the preferredh111iorientationbeingobtainedonly for film II, whereasfilmsI andIII grow in the h100i orientation,which is knowntobe the easiestcrystalgrowth direction.Orientationalongtheh111i directionof theperovskiteprovidesacomponentalongthe[001] polardirectionof theperovskite.This pro-ducesa spontaneouspolarizationandit is associatedwiththelargerspontaneouspyroelectriccoefficient, , obtainedfor film II comparedto films I andIII (Table1).

Surf. InterfaceAnal. 29, 325–329 (2000) Copyright 2000JohnWiley & Sons,Ltd.

SOL GEL-MODIFIED PbTiO3 FILMS ON Si 329

The thermal treatment of the substrates promotes solid-state reaction between the Pt and the underlying Ti orTiO2 layers11 in substrates T-I and T-III, which shouldinduce the formation of the Pt3Ti phase and thus theh111iorientation of the Pb0.76Ca0.24TiO3 film. The XRD resultsobtained for these films are shown in Fig. 4(b). A largepreferredh111i orientation is not observed in these filmsbut again instead ah100i preferred orientation. However,by comparing the x-ray patterns of Figs 4(a) and 4(b)it can be observed that films T-I and T-III have highercontributions of theh001i, h101i and/orh111i preferentialorientations perpendicular to the film surface than films Iand III. These orientations contribute to larger values of for films T-I and T-III than for films I and III. On theother hand, the thermal treatment of the T-II substrate pro-duces oxidation of the top Ti layer, probably hindering theformation of a thicker Pt3Ti layer in this substrate than insubstrate II. Consequently,h111i orientation and the spon-taneous of film T-II are quite similar to those of film II.More experimental data (pole figures, transmission elec-tron microscopy, Rutherford backscattering spectroscopy,etc.) have to be carried out on these films to establish thecomposition of the surface Pt layer and the texture of thissurface and the film.

Owing to the centro-symmetry of the XRD measure-ments, the preferred orientation obtained from these exper-iments only gives us information about the direction of thepolarization vector. The sign of this vector is given by the

spontaneous pyroelectric coefficient, (Table 1). The pos-itive value of obtained in all the films means that thesamples behave as a material poled with the positive signat the top film surfaces. This sign of spontaneous polar-ization is probably linked to the driving force during thefilm growth and to the development of spatial charge atthe surface of the Pt bottom electrode during film crystal-lization. This polarisation was difficult to switch,14 whichis relevant for the use of these materials integrated intopyro- and piezoelectric devices.

CONCLUSIONS

Calcium-modified lead titanate thin films have been grownby sol–gel on platinized silicon substrates with ah111ipreferred orientation. Such a texture occurs when thereis some Ti on the surface of the platinum electrode,otherwise ah100i preferred orientation takes place. Thisorientation provides a net spontaneous polarization in thepolar direction of the perovskite film. Consequently, thesefilms have an appreciable spontaneous pyroelectricity.

Acknowledgements

This work has been supported by the European BRITE EURAM ProjectBRPR-CT98-0777 and the Spanish CICYT Project MAT98-1068.

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Copyright 2000 John Wiley & Sons, Ltd. Surf. Interface Anal. 29, 325–329 (2000)