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Thin Solid Films 516 (2

Effect of microstructure and surface morphology evolution on opticalproperties of Nd-modified Pb(ZrxTi1− x)O3 thin films

Jarkko Puustinen ⁎, Jyrki Lappalainen, Vilho Lantto

Microelectronics and Materials Physics Laboratories, EMPART Research Group of Infotech Oulu, University of Oulu, Linnanmaa, FIN-90570 Oulu, Finland

Received 13 July 2007; received in revised form 22 February 2008; accepted 29 February 2008Available online 7 March 2008

Abstract

Optical properties of nanocrystalline Nd-modified lead–zirconate–titanate thin films were studied. Pulsed laser deposition was used for thin-filmfabrication at room temperature. Thin films were deposited on single-crystal MgO (100) substrates and post-annealed between temperatures400 °C–1000 °C. The crystal structure and thin film morphology were studied using x-ray diffraction technique and atomic force microscopy,respectively. The optical transmission spectra of the films were measured at UV–vis–IR wavelengths and the results were utilized to obtain therefraction index n, extinction coefficient k, and the value of the band gap Eg. Thin films annealed at different temperatures had a different crystalstructure and morphology. Film thickness and annealing temperature also had an effect on the crystal orientation of the films. Trigonal andtetragonal phases co-existed in analyzed films so that films with thickness of 150 nm had strong tetragonal orientation with increasing post-annealing temperature. Trigonal and tetragonal phase co-existence was present in films with thickness of 300 nm post-annealed at low temperatures.Optical absorption edge shifted to shorter wavelengths with decreasing film thickness and post-annealing temperature which led to increase in bandgap energies. Atomic force microscopy studies showed a clear dependence of films root mean square roughness on post-annealing temperature andcrystal structure.© 2008 Elsevier B.V. All rights reserved.

Keywords: Thin film; PZT; Microstructure; Morphology; Optical properties

1. Introduction

In the rapidly developing field of photonics, there is an in-creasing demand for new devices that are capable of processinglarge quantities of optical information. Perovskite ferroelectricmaterials, such as lead–zirconate–titanate (PZT), La- and Nd-modified lead–zirconate–titanate (PLZT, PNZT), and barium–titanate (BaTiO3), exhibit a combination of properties that makethem attractive for a variety of photonic applications. Propertieslike high optical transmittance, low reflectance, and strongelectro-optic Kerr effect of these ferroelectric materials in theform of thin films can be utilized in various optical applicationsas well.

The film thickness, crystal structure, orientation of the crys-tal, grain size distribution, packing density, and morphology ofthe film surface has a strong effect on the optical properties of

⁎ Corresponding author.E-mail address: jpuust@ee.oulu.fi (J. Puustinen).

0040-6090/$ - see front matter © 2008 Elsevier B.V. All rights reserved.doi:10.1016/j.tsf.2008.02.046

PZT films [1]. Some of the parameters depend on film depositionand heat treatment, to a large extent. For example, the funda-mental issue for manufacturing waveguiding structures is theability to deposit highly transparent films with adequate filmthickness to couple propagating electromagnetic radiation. Hightransparency in addition to strong active properties is achievedby selecting material composition and manufacturing parame-ters carefully for minimizing absorption and scattering.

Miniaturization of active and passive components and dif-ferent semiconductor and photonic devices is needed in today’sintegrated circuits and microelectronics applications. Mostphysical and chemical properties of solid matter change whenthe particle size is decreased to the nanometre scale. Thesechanges can be attributed to quantum size effects, surface andinterface effects, changes in the cell parameter and lattice sym-metry, etc. [2]. As the dimensions become smaller, it is antici-pated that changes in the properties of ferroelectric materials,such as electrical, optical and magnetic properties, should be-come evident. The understanding of finite-size effects in

Fig. 1. X-ray diffraction patterns of thin films with thicknesses of a) 150 nm andb) 300 nm post-annealed at temperatures 700 °C, 800 °C, and 900 °C, res-pectively, deposited at a laser-beam fluence of 1.5 J/cm2.

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ferroelectrics is essential since they effectively define practicallower limit to the miniaturization of devices based on thesematerials [2,3].

2. Experimental details

A pulsed XeCl-excimer laser (Lambda Physik COMPex 201)with a wavelength of 308 nm and Pb0.97Nd0.02(Zr0.55Ti0.45)O3

target with a density of 7.49×103 kg/m3 were used to depositamorphous PNZT thin films with a thicknesses of 150 nm and300 nm onMgO (100) substrates. The deposition was carried outat room temperature at a background pressure of 6–7 mPa. Thesubstrate and the target were placed in parallel at a distance of35 mm. The repetition rate of laser pulses was 5 Hz and the laserfluence at the target surface was 1.5 J/cm2. After deposition, thinfilms were post-annealed in ambient air at temperatures from400 °C to 1000 °C for 30 min under the inverted zirconia cru-cible with some PNZT powder. The heating and cooling rate of5 °C/min was used in every temperature profile. Atomic zirco-nium content x ¼ Zr

ZrþTið Þ, of PNZT films stays around x≈0.51 ina morphotropic phase boundary when these processing parame-ters are used [4].

After annealing, the crystal structures of the thin films werestudied by x-ray diffraction (XRD) measurement using CuKα

radiation with a wavelength λ=1.54 Å. The x-ray diffractionintensities were recorded with a constant speed of 1o/minbetween 2θ angles 10o–80o. The instrumental broadening wasobtained using a large-grain polycrystalline silicon sample as astandard specimen. Atomic force microscopy (AFM) (VeecoDimension 3100) was used to study the surface morphology ofpost-annealed thin films in contact mode. Based on amplitudedistribution functions (ADF), representing the statistical dis-tribution of the film surface height data, bearing area curves(BAC) and the values of root mean square roughness Rq werecalculated in order to evaluate the morphology of the thin films.

Optical transmission spectra in the wavelength range from180 nm to 3000 nm were measured using a spectrophotometer(Varian Cary 500). Refractive index n and extinction coefficientk were calculated from transmission data fitted with a multipleLorentz-oscillator model using SCI Film Spectrum software.

3. Results

3.1. XRD

XRD patterns from PNZT films with thicknesses of 150 nmand 300 nm post-annealed at temperatures of 700 °C, 800 °C,and 900 °C, respectively, are shown in Fig. 1. MgO (200)substrate reflection was used to correct the peak positions. Fromthe patterns it is clearly seen that film with a thickness of 150 nmpost-annealed at a temperature of 900 °C had tetragonal structurewith strong c-axis orientation preferred while, at lower post-annealing temperatures, a-axis oriented crystals were preferred.Both trigonal and tetragonal phases co-existed in films with athickness of 300 nm with both a- and c-axis orientated crystalspresent. From Fig. 1(b) it is clearly seen that reflection of the(110) crystal plane started to split, intensity of reflections from

crystal planes (002/200) increased, and intensity ratio I(100)/I(110) increased as the post-annealing temperature increasedfrom 700 °C to 800 °C indicating a change in the main phasefrom trigonal to tetragonal. As the post-annealing temperatureincreased tetragonal orientation increased also for films withthickness of 300 nm.

For the tetragonal crystal structure, the values of lattice cons-tant a=4.08237 Å, 4.0548 Å, and 4.1123 Å, and c=4.1186 Å,4.11224 Å, and 4.1583 Å were calculated using a tetragonallattice symmetry model for films with thickness of 150 nm post-annealed at temperatures 700 °C, 800 °C, and 900 °C, leading totetragonal distortion values c/a=1.00887, 1.01667, and1.01119, respectively. Lattice constants for 300 nm film post-annealed at 700 °C were a(=b=c)=4.0303 Å using a trigonallattice symmetry model. Lattice constants for 300 nm films post-annealed at the temperatures of 800 °C and 900 °C werea=4.071 Å and 4.0698 Å and c=4.10163 Å and 4.10677 Å,respectively, using a tetragonal lattice symmetry model and thecorresponding c/a-ratios were 1.00752 and 1.00908. Valuesquantifying the degree of the c- or a-axis grain orientation of thefilms were defined by comparing the sum of intensities of (001)and (002) (c-axis orientation, p(c)) and (100) and (200) (a-axis

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orientation, p(a)) reflections to the sum of all peak intensities ofthe XRD patterns. For films post-annealed at 900 °C, values of p(c)=0.56598 and p(a)=0.11026 for 150 nm film were definedshowing that tetragonal c-axis orientation dominated. For300 nm film the value of p(a)=0.42638 was calculated usingthe tetragonal lattice symmetry model. In addition to perovskitephases of PNZT, XRD patterns of films post-annealed attemperatures below 700 °C contained clear peaks originatingfrom the pyrochlore phase showing that the films pass throughan intermediate phase before transforming into the ferroelectricperovskite phase. This has been observed in many differentceramic thin films [5–7]. As the post-annealing temperatureincreased, the perovskite phase continued to grow at the expenseof the pyrochlore phase. Complete crystallization into theperovskite phase occurred at the post-annealing temperature of700 °C.

3.2. Surface analysis

AFM micrographs of thin films post-annealed at tempera-tures of 600 °C, 700 °C, 800 °C, and 900 °C with thicknessesof 150 nm and 300 nm are presented in Fig. 2. Scanning area inthe micrographs is 10 µm×10 µm. Secondary grain growth isclearly seen in thin film with a thickness of 150 nm post-annealed at 900 °C. After comparing the AFM micrograph andXRD pattern of 150 nm film post-annealed at 900 °C, it waspossible to define trigonal and tetragonal oriented grains atthe surface of the film. Rosette-like structure in the AFMmicrographs can be defined as tetragonal oriented grains andthe background as trigonal oriented grains. Diffractionintensity of the (001) and (002) crystal planes increased asthe area of rosette-like structure defining tetragonal orientedgrains increased.

Values of the surface roughness of the films are shown in Fig.3. Surface roughness tended to increase due to different reasons.

Fig. 2. AFM micrographs of films with thicknesses of a) 150 nm and b) 300

As thickness of the films increased Rq increased. Rq values forfilms with thicknesses of 150 nm and 300 nm post-annealed at700 °C were Rq=7.75 nm and 28.2 nm, respectively. As thepost-annealing temperature increased, Rq tended to increase dueto surface evolution. Grain and phase boundary stresses in add-ition to macroscopic compressive stress increases localizedcurvature in films due to mass transfers causing higher filmroughness. Especially co-existence of tetragonal and trigonalphases in separate entities in the films led to an increase in Rq

values [8]. Values of Rq increased from 4.68 nm to 49.9 nm andfrom 12 nm to 47.1 nm for films with thicknesses of 150 nm and300 nm, respectively, as the post-annealing temperature in-creased. At high post-annealing temperatures, surface rough-ness was high due to agglomeration of the films.

In Fig. 4, there are BAC graphs for thin films with a thicknessof 300 nm post-annealed at different temperatures. BAC des-cribes the cumulative coverage of the film surface area at acertain value of a film’s surface height, or amplitude distribution.The BAC can be calculated from the ADF which represent thedistribution histogram of the heights zi, by the equation

BAC ¼ 1Atot

Pn

i¼1A zið Þ, where Atot is the total area of the surface

andA(zi) is the sum of area of heights from z1 to zi [9]. BAC stayedsteep until post-annealing temperature increased to 700 °C. At thistemperature, BAC becomes gentler when phase co-existenceis strongly present. From Fig. 4, it can be seen that films with athickness of 300 nm post-annealed at 800 °C and 900 °C hadsteeper BAC and narrower ADF, and thus better surfacemorphologies than the film with same thickness post-annealedat 700 °C. As the temperature increased, Rq of the film decreasedfrom the value of Rq=28.2 nm to Rq=21.5 nm and BAC becamesteeper. This has also been observed with films deposited withlaser-beam fluence of 2.0 J/cm2 [10]. As the post-annealingtemperature increased from 700 °C to 800 °C orientation of thefilm tends to move towards tetragonal orientation. Improvement

nm post-annealed at temperature of 600 °C, 700 °C, 800 °C, and 900 °C.

Fig. 5. Measured and fitted transmission spectra of 150 nm thick film post-annealed at temperature of 700 °C together with calculated refractive index nand extinction coefficient k.

Fig. 3. The Rq values of PNZT thin films with thicknesses of 150 nm and 300nm post-annealed at various temperatures.

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in the surface morphology was also confirmed by optical trans-mission measurements. BAC for films with a thickness of 150 nmalso became gradually gentler as the post-annealing temperatureincreased.

3.3. Optical measurements

Measured and fitted optical transmission spectrum of 150 nmfilm post-annealed at 700 °C and calculated values of n and k asa function of UV–vis–IR wavelengths are presented in Fig. 5.Lorentz multiple oscillator model was fitted to the data with rootmean square error of 0.461. At the UV–vis region, refractiveindex showed a typical dispersion curve and saturated to thevalue of 2.35 at IR wavelengths. In Fig. 6, there are transmissionspectra at UV–vis–IR wavelengths of PNZT films with thick-ness of 300 nm post-annealed at temperatures from 400 °C to1000 °C. Well oscillating optical transmission indicates that thefilm has a flat surface and uniform thickness. As the opticallength of height differences due to surface roughness of the film,are in comparison to the fraction of wavelength λ, interference

Fig. 4. Bearing area curves for thin films with thickness of 300 nm post-annealedat temperatures from 400 °C to 1000 °C.

maxima and minima are modulated due to diffuse reflection,which is clearly seen also in Fig. 6. Bah et al. [11] and Swanepoel[12] have proposed an approximate analytic method for thedetermination of optical parameters for an inhomogeneous filmwith nonparallel surfaces from transmittance spectra using twoseparate layers in their model.

In many previous studies it has been shown that, as thethickness of the film decreases optical absorption edge shiftstowards shorter wavelengths. Shifting of the optical absorptionedge to shorter wavelengths indicates an increase in the value ofthe band gap energies. An increase in the band gap energiespossibly indicates the thickness-dependent energy band gap thatwas also found in other PNZT films deposited by PLD [13,14],and also in the case of sol–gel derived SrTiO3 thin films [15].Values of the energy band gap Eg were calculated using Tauc-plot analysis. Tauc-plots were derived from the calculated valuesof extinction coefficients using the expression α=4πk/λ, where

Fig. 6. Transmission spectra measured at UV–vis–IR wavelengths of PNZT thinfilms with thickness of 300 nm post-annealed at different temperatures.Evolution of thin films surface morphology is clearly seen from transmissionspectra.

Fig. 7. Values of energy band gap of thin films with thicknesses of 150 nm and300 nm post-annealed at temperatures from 400 °C to 1000 °C.

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α is absorption coefficient, and assuming direct band-to-bandtransition according to the formula (αhv)2 =C(hv−Eg), where hvis the incident photon energy, and C is a constant, in order todetermine the value of the Eg. Values of the energy band gap ofthe analyzed thin films are presented in Fig. 7. Eg values in-creased from 4.16 eV to 4.28 eV as the thickness of the filmdecreased from 300 nm to 150 nm. The blue shift effect with anincrease of ΔEg≈0.12 eV in energy band gap value withdecreasing thickness was similar to the effect in PNZT andSrTiO3 thin films found previously [14,15]. Shifting of theoptical absorption edge to the shorter wavelengths was also seenin the films post-annealed at 400 °C. According to XRD pat-terns, thin films at a post-annealing temperature of 400 °C con-sists of mixture of amorphous and pyrochlore phases. The samekind of blue shift was also observed in nanocrystalline ZrO2 thinfilms, which is most likely a pure size effect [16].

4. Discussion

Optical properties of the films are strongly dependent on thestructure of the thin film. For optical applications, the surfacequality of the films is very important in order to avoid Rayleighscattering and losses, which can be significant even for relativelysmooth surfaces, because the propagating waves stronglyinteract with the surfaces of the waveguides [17]. Post-annealingheat treatment of asdeposited thin films can be used to controlthe grain growth, orientation, and the surface morphology of thefilms. At room-temperature deposition films are initially amor-phous. During post-annealing heat treatment films form a poly-crystalline film structure. Grain growth starts at different placesin the thin films at the same time and is dependent on mini-mization of energies originating from the grain and phase boun-daries and interface and surface energies, and stresses. Theexistence of stress is mainly caused by a thermal mismatch orlattice mismatch between substrate and film.

From the XRD patterns in Fig. 1 and AFM micrographs inFig. 2, it is clearly seen that grain growth and orientation of thefilms is strongly activated by the post-annealing temperature.

Crystal orientation of the films with thickness of 150 nm wastetragonal with c-axis orientation preferred as the post-annealingtemperature increased. Both tetragonal and trigonal phases co-existed in 300 nm films at low post-annealing temperatures. Thiscorresponds quite well with previous studies of PNZT thin films[4]. High Rq values of the films may be due to the developmentof the ferroelectric phase during heating via the pyrochlore phasewith a much larger unit cell, due to two phase co-existence in thefilms, and increased localized curvature originating from grainand phase boundary stresses. The effect of the co-existence oftetragonal and trigonal phases in the film on Rq value was clearlyobserved in the films with a thickness of 300 nm. As the post-annealing temperature increased from 700 °C to 800 °C, Rq

values decreased. At that temperature range, the main phase inthe films changed from trigonal to tetragonal. Rq values tend tobe smaller for films with high orientation. As the post-annealingtemperature increased to 1000 °C films tended to be very rough.This may be due to different degrees of agglomeration of thefilms.

Surface morphology of the films has a great effect on theoptical properties of the thin films. Well oscillating opticaltransmission indicated that the films had a flat surface, uniformthickness, and high orientation. As the surface of the filmbecame rougher or trigonal and tetragonal phases co-existed inthe films, interference maxima and minima were modulated. Asthe Rq decreased the BAC became steeper and the fall in trans-mittance near the band gap energy got sharper in films withthickness of 300 nm as the post-annealing temperature increasedfrom 700 °C to 800 °C. Cause for degradation of interferenceof the film may also be due to inhomogeneity of the film andcontamination [18,19].

Shifting of the optical absorption edge to a shorter wave-length may be due to quantum confinement size effect like Baoet al. [15] and Teng et al. [20] have proposed, or by reduction ofthe unit cell volume and reduction of interatomic separationcaused by strong residual compressive stress [14]. Blue shifteffect in thin films post-annealed at temperatures of 400 °C and500 °Cmay be due to size effect and nonuniformity of the crystalstructure of the film as proposed by Kosacki et al. [16] or due toan increase in extended localization in the conduction andvalence bands as the fraction of the amorphous phase increasesas Tan et al. have proposed in the case of ZnO thin films [21]. Asthe fraction of the amorphous phase increases the absorption ofphotons is mainly contributed by amorphous PNZT and ab-sorption edge blue shifts. From the XRD patterns of the filmspost-annealed at 400 °C it was clearly seen that amorphous andpyrochlore phases co-existed in the films, and at a post-an-nealing temperature of 500 °C there was a mixture of amor-phous, pyrochlore and perovskite phases.

5. Conclusion

PLD with XeCl-eximer was used for PNZT thin films, withthicknesses of 150 nm and 300 nm fabrication on MgO (100)substrates. After deposition thin films were post-annealed attemperatures from 400 °C to 1000 °C. The crystal structure wasstudied using XRD technique and morphology was studied

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using AFM. The optical transmission spectra of the thin filmswere measured at UV–vis–IR wavelengths. Refraction index nand extinction coefficient k were calculated using multiple Lo-rentz-oscillator model and the value of the energy band gap wasdetermined using Tauc-plot analysis. Thin films with differentthicknesses and post-annealed at different temperatures had adifferent crystal structure and morphology. Films with a thick-ness of 150 nm had a strong tetragonal orientation with an in-creasing post-annealing temperature with a preference for c-axisorientation. Tetragonal and trigonal phases were present in thefilms with a thickness of 300 nm at low post-annealing tempe-ratures. Post-annealing temperature had a clear dependence onroughness of the thin films. Films that had high orientation hadlower Rq values. Roughness increased as tetragonal and trigonalphases co-existed in analyzed films. Surface morphology of thefilms had a clear effect on the optical transmission spectra of thefilms. Optical absorption edge shifted to shorter wavelengths asthe thickness of the films decreased from 300 nm to 150 nmwhich led to an increase in the band gap energies by the amountof ΔE≈0.12 eV.

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