Materials Science and Engineering B 144 (2007) 3842

Thickness dependence of the conductivitygl

r VaLaborark

Abstract

Thin films gle crdeposited fil electand by the V e temof different s incof all films i versu(Tmax). This asessamples sho e lim 2007 Else

Keywords: L condu(PLD); Thin fi

1. Introdu

Mixed ionic and electronic conducting materials can be usedin solid oxide fuel cells and in ceramic membranes for gasseparation including oxygen separation and partial oxidation ofmethane to syn? gas in a membrane reactor. One group of promi-sing materistability inthe lanthannic and ionby Patrakee

While thstudied, lesthin films.electrical preported foet al. [4] hafilms of Srthe dependconductivitfilm thickn

CorresponE-mail ad

obj0.6 r0.4F 3

dependence on the film thickness. MgO was chosen as substrateto avoid reaction between the substrate and the film and to avoidany contribution from the substrate conductivity.

0921-5107/$doi:10.1016/jals with high oxygen conductivity and high chemicalreducing atmosphere for use as oxygen membranes isum strontium ferrites (La1xSrxFeO3). Both electro-ic conductivity of La1xSrxFeO3 has been reportedv and co-workers [13].e bulk transport properties of La1xSrxFeO3, are wells is known about the properties of La1xSrxFeO3It has been found that the microstructure and the

roperties of thin films are different from the propertiesr the bulk material with the same composition. Pardove studied the electrical conductivity of epitaxial thin4Fe6O13 deposited on single crystal NdGaO3 andency of the conductivity on the film thickness. They of the films was found to increase with decreasingess.

ding author. Tel.: +45 46775837; fax: +45 46775858.dress: majid.mosleh@risoe.dk (M. Mosleh).

2. Experimental

(La0.6Sr0.4)0.99FeO3 (LSF64) powders were prepared usingthe glycine nitrate process (GNP). The synthesised powderwas calcined at 900 C for 24 h. The resultant powders wereball-milled in ethanol for 1 week and subsequently dried. Thetargets for the PLD were prepared by pressing the powder ini-tially uniaxially followed by isostatic pressing. The pressedtargets were then sintered in air at 1200 C for 20 h. The X-ray diffraction (XRD) analysis shows that after sintering, thetarget becomes well crystalline with a structure consist only ofLa0.6Sr0.4FeO3.

The samples were irradiated with 20 ns laser pulsed from anKrF excimer laser at 248 nm with a fluence of 4 J/cm2. During thedeposition the substrate temperature was kept around 750 C andan oxygen background pressure of 10 Pa. The ablated materialwas collected on the substrate located at a distance of 88 mmfrom the target.

To study the variation of conductivity with film thickness,the deposition is carried out with variable deposition time under

see front matter 2007 Elsevier B.V. All rights reserved..mseb.2007.07.089deposited on MgO sinMajid Mosleh , Nini Pryds, Pete

Department of Fuel Cells and Solid State Chemistry, Ris NationalDK-4000 Roskilde, Denm

of La0.6Sr0.4FeO3 of different thicknesses have been deposited on sinms are characterized by XRD before and after annealing, by scanningan der Pauw (VDP) technique for determination of the conductivity. Ththickness has been investigated. The electrical conductivity of the films less than the value of the bulk material. The apparent conductivitycharacteristic temperature (Tmax) decreases as the film thickness increw the same activation energy of the conductivity in the low temperaturvier B.V. All rights reserved.

anthanum strontium ferrite (LSF); Perovskite; Oxygen membrane; Electricallm

ction TheLa Sof thin films (La,Sr)FeO3e crystalng Hendriksenatory, Technical University of Denmark,

ystal MgO substrate by pulsed laser deposition (PLD). Theron microscopy (SEM) for morphological characterizationperature dependence of the conductivity in air for samplesreases with increasing film thickness but the conductivitys temperature shows a maximum at a certain temperatureand reaches the value for bulk for thicker films. All of theit.

ctivity; Van der Pauw (VDP) technique; Pulsed laser deposition

ective of this work is to study the conductivity ofeO deposited on a MgO single crystal and its

M. Mosleh et al. / Materials Science and Engineering B 144 (2007) 3842 39

the same deposition conditions. The thicknesses of the filmswere determined by comparing energy dispersive spectrome-try (EDS) dwith Montedeterminedgeometry wped with aused wasof the depand cross-1540XB Cscope (FESand a gas i

3. Results

Fig. 1 shwith the dXRD pattebut for thedening the35.

The formwas confirmclose to thfiles for La0MgO(1 0 0here) for a(0 2 4) orie

The subter a = 0.42structure wand latticea pseudo-cspacing ofthe film anthus 7.6%MgO(1 0 0

For filmthe (0 2 4) ware much lothe predompositions oplan of theand at 32.5respectivel

The X-rshift and lilated for thin Fig. 2. Tof the filmafter it is aved that thwith an incvalue at a dAs the filmincreases.

omparison between the XRD patterns of the target and the synthesised.4FeO3.

he inter-planar spacing of different films as a function of film thickness.etermined in a scanning electron microscope (SEM)Carlo simulations [5]. The structure of the films wasby X-ray diffraction (XRD) using BraggBrentanoith a STOE & CIE / powder diffractometer equip-n energy-dispersive Kevex detector. The radiationCu K. The morphology and the microstructureosited films were investigated by SEM, imagingsectioning of the films was performed on a ZeissrossBeam field emission scanning electron micro-EM) equipped with a focused ion beam (FIB) columnnjection system (GIS).

and discussions

ows the XRD patterns of the target material togethereposited layer after different deposition times. Therns were originally recorded in the range 2 = 2090comparison of the peak position and the line broa-XRD data are shown with a 2 scan from 20 to

ation of a perovskite structure at all deposition timeed by these measurements. The peak positions are

ose of the target and the corresponding ICDD-card.6Sr0.4FeO3. The LSF64 films on the single crystal

) show the reflections at 22.8 and 46.6 (not shownll films, which can be identified with the (0 1 2) andntation, respectively.strate, MgO, has a cubic structure with lattice parame-16 nm while bulk La0.6Sr0.4FeO3 has a perovskiteith a rhombohedral structure (space group: R-3c)

parameters a = 0.55273 nm and c = 1.3421 nm. It hasubic lattice parameter 0.3897 nm, equal to the d-the (0 1 2) planes. The lattice mismatch between

d the substrate defined as e = (asub abulk)/asub, isfor LSF64(0 1 2) on MgO(1 0 0). The LSF64 film on) substrate is thus under tensile strain.s thicker than 130 nm other peaks than the (0 1 2) and

ere observed. However, the intensity of these peakswer then for the (0 1 2) so the (0 1 2) orientation is stillinant one. The vertical lines in Fig. 1 indicate the peakf the target material. It should be noted that the (1 1 0)

LSF64 is in fact composed of two peaks at 32.2, which belong to the (1 1 0) and the (1 0 4) plans,

y. However, these peaks could not be distinguished.ay diffraction profiles of the films show both a peakne broadening. The inter-planar spacing was calcu-e (0 1 2) planes from the X-ray results and is plottedhe inter-planar spacing increases with the increasethickness up to a film thickness of 130 nm where

lmost constant. From the XRD data, it is also obser-e full width at half maximum (FWHM) decreasesrease in film thickness and approaches a minimumeposition time of 20 min (film thickness 130 nm).thickness increases further, the width of the peaks

Fig. 1. CLa0.6Sr0

Fig. 2. T

40 M. Mosleh et al. / Materials Science and Engineering B 144 (2007) 3842

Fig. 3. Time dependence of conductivity of film of LSF64 in air for differentthickness at 950 C.

The conductivity of the films was measured with help of theVan der PaThe measuroom tempat 950 C uthen decreawere perfo

Fig. 3 dfilm over tiof the filmsis attributedfilms thinnewe found ato 4.1 S/cmfirst increasteady stateconductivitthat as thesteady statHowever, tthe equilibr[1].

The depture in air ithe coolingFig. 3).

Fig. 4. Tempeair for differe

Table 1Maximum temperature and conductivity of films as a function of depositiontime (or film thickness) together with activation energy (Eac) from conductivitymeasurements

Deposition time(min)

Thickness(nm)

Tmax(C)

max(S/cm)

950 (S/cm) Eac (eV)

5 33 646 5.55 4.14 10 66 631 42.3 29.3 0.30820 130 583 92.5 56.3 0.30650 330 579 97.4 59.9 0.309100 650 580 98.3 60.4 0.309Bulk 574 99 0.3

hick

. 4 sed bytotallmsgend in several perovskite ceramics [6,7].clearly seen that the conductivity increases rapidly as an of film thickness up to the 130 nm thick film until itches a constant value.linear dependence of log (T) versus 1/T in the low tem-

re range allows us to determine activation energy (Eac)h sample. This behaviour is consistent with the Arrhe-lation [6]. The activation energy is found to be constant,o 0.307 0.02 eV. The results from Fig. 4 are summarizedle 1. As seen from Table 1, the peak temperature and thetivity change significantly with changing the film thick-uw (VDP) technique in air at different temperatures.rements were first performed during heating fromerature to 950 C. The temperature was then keptntil a steady state is reached. The temperature wassed down to room temperature and the measurementsrmed again on the heat-treated samples.emonstrates the variation of the conductivity of eachme at 950 C. At this temperature the oxygen content

equilibrates fast, so the change in the conductivityto the change in the microstructure of the films. Forr than 66 nm, for example, a film of thickness 33 nm,strong reduction of the conductivity from 49 S/cm

. For the film with thickness 66 nm the conductivityses from 33 S/cm to 47 S/cm, then decreases to the

value of 29 S/cm. For films thicker than 66 nm, they increases until steady state is reached. One can seefilm thickness increases from 33 nm to 650 nm, thee conductivity increases from 4.1 S/cm to 60 S/cm.he conductivity for the thickest film is still far fromium bulk value, which at this temperature is 99 S/cm

endence of the total conductivity () on the tempera-s presented in Fig. 4 for all samples as determined inruns after the long time equilibration at 950 C (cf.

Fig. 5. T

Figfollowtion ofin all fiof oxyreporte

It isfunctioapproa

Theperatufor eacnius reequal tin Tabconducrature dependence of total conductivity of thin film of LSF64 innt thickness. Fig. 6. XRDness dependence of conductivity of LSF64 films in air at 950 C.

hows a thermally activated behaviour below 650 Ca maximum and a decreasing to 950 C. This reduc-

conductivity with increasing temperature is observedin the high temperature region and is due to the lossfrom the perovskite. A similar behaviour has beenof 20 min deposition before and after heat treatment at 950 C.

M. Mosleh et al. / Materials Science and Engineering B 144 (2007) 3842 41

Fig. 7. Scann the c

ness especitemperaturconductivit130 nm, Tmaround 580

Fig. 5 dfunction offilms thickthe value fockness effesize withou

In orderthe film crsamples weAn XRD pprepared iorientationnew peakssubstrate. Ato the MgOposition tochange in o

The effewas investitures of thmicrographfor the thinnuous unif200500 nmthese two duniform laytop was cothe thickneof the condand the tria

d drnt meat tly pof dnduologent. Wentsreme

film,orphthetheins cing electron micrograph of the synthesised LSF64 before (left) and after (right)

ally for films less than 130 nm in thickness. The peake (denoted as Tmax), which corresponds to maximumy decreases as the film thickness increases. Aboveax decreases very slowly and reaches a minimumC which is close to the value found for bulk samples.emonstrates the variation of the conductivity as athe film thickness at 950 C. One can see that for

er than 130 nm, the conductivity slowly approachesr the bulk material. A possible explanation of the thi-ct on the conductivity is the change in the crystallitet any effect on crystal orientation.to investigate the effect of the heat treatment on

ystallinity, XRD measurements of the heat-treatedre taken right after the conductivity measurements.

attern of a heat-treated sample is compared to the as

changediffereAfter hand onchainrent comorphtreatmsurem

measu

of thefilm m

Forduringthe gran Fig. 6. The heat-treated samples still had the (0 1 2). It should be noted that we have not observed any, which can indicate any reaction between film and

ll the observed peaks belong either to the LSF64 orsubstrate. However, there is a slight shift in peak

wards the lower angles, which probably reflects axygen stoichiometry in the sample.ct of heat treatment on the morphology of the filmsgated by SEM (see Fig. 7). Fig. 7 shows SEM pic-e samples before and after the heat treatment (thes to the left are the one before heat treatment). Asfilms, it was observed that the films contain a conti-

orm layer and triangular shape particulates of aboutin diameter on the top of the layer. The existence of

istinct components in the produced coatingsthiner of LSF64 and triangular shape particulates on the

nfirmed by the EDS measurement technique duringss determination [5]. In that case, the measurementuctivity reflects the average conductivity of the filmngular shape particulates on the top. The thin film

thick filmsments theand flawles

4. Conclu

Thin filnesses havsingle crysto investigaThe total aMgO singlthickness. Itivity willthe sampleconductivittivity reflecduring heaonductivity measurements for thinnest film and thickest film.

astically after the heat treatment and has a totallyicrostructure (see the top right micrograph in Fig. 7).reatment it seems as if the film ceases to be continuousartially covers the substrate plane with a connectedeposited materials. The observed changes in appa-ctivity with time (Fig. 3) are therefore caused by theical/microstructural changes occurring during heat

hen the thin films break up and form holes, the mea-are no longer valid, due to tortuosity. In that case thents will underestimate the microscopic conductivityi.e. the measured conductivity over time reflects theology rather than a change in material conductivity.thick films samples SEM images (Fig. 7) show thatconductivity measurements at 950 C the shape ofhanges and the size of the grains increases. For the

expressing the results of the conductance measure-

conductivity is warranted, as the film is continuouss.

sions

ms of La0.6Sr0.4FeO3 (LSF64) of different thick-e been prepared by pulsed laser deposition on MgOtal substrates. The Van der Pauw technique was usedte the effect of film thickness on total conductivity.pparent conductivity of the thin layer of LSF64 one crystal was found to increase with increasing filmt seems that as the film becomes thicker the conduc-asymptotically reach a value close to bulk. All ofs are found to have the same activation energy of they. The observed thickness dependence of the conduc-ts different tendencies to re-crystallize and re-shape

t treatment.

42 M. Mosleh et al. / Materials Science and Engineering B 144 (2007) 3842

Acknowledgments

The conductivity cell was designed by Dr. Nikolaos Bonanos.We are grateful to him and to Dr. Jrgen Bilde-Srensen and Dr.Jesper Knudsen for SEM pictures.

References

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[3] J.A. Bahteeva, I.A. Leonidov, PatrakeevF M.V., E.B. Mitberg, Kozhevni-kovF V.L., K.R. Poepplmeier, J. Solid State Electrochem. 8 (2004) 578584.

[4] J.A. Pardo, J. Santios, C. Solis, G. Garcia, A. Figueras, M.D. Rossell, SolidState Ionics 177 (2006) 423428.

[5] N. Pryds, B. Toftmann, J.B. Srensen, J. Schou, S. Linderoth, Appl. Surf.Sci. 252 (2006) 48824885.

[6] J.W. Stevensen, T.R. Armstrong, R.D. Carneim, L.R. Pedersen, W.J. Weber,J. Electrochem. Soc. 143 (1996) 27222729.

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Thickness dependence of the conductivity of thin films (La,Sr)FeO3 deposited on MgO single crystalIntroductionExperimentalResults and discussionsConclusionsAcknowledgmentsReferences