www.elsevier.com/locate/tsfThin Solid Films 467 (2004) 112–116

Sol–gel derived versus pulsed laser deposited epitaxial La0.67Ca0.33MnO3

films: structure, transport and effects of post-annealing

Rickard Forsa,*, Sergey Khartseva, Alexander Grishina, Annika Pohlb, Gunnar Westinb

aDepartment of Condensed Matter Physics, Royal Institute of Technology, SE-164 40 Stockholm-Kista, SwedenbDepartment of Material Chemistry, Angstrom Laboratory, Uppsala University, SE-751 21 Uppsala, Sweden

Received 2 October 2003; received in revised form 24 February 2004; accepted 11 March 2004

Available online 23 April 2004

Abstract

Epitaxial La0.67Ca0.33MnO3 films have been prepared on LaAlO3 crystals by pulsed laser deposition (PLD) and by a novel all-alkoxide

sol–gel technique. Different out-of-plane lattice parameters are found for the as-prepared films, and scanning electron microscopy shows a

more porous structure for sol–gel films as compared to PLD films. These differences are largely removed by post annealing at 1000 jC.Transport measurements show maximum temperature coefficient of resistivity of 8.2% K�1 at 258 K (PLD) and 6.1% K�1 at 241 K (sol–gel)

and colossal magnetoresistance at 560 kA/m of 35% at 263 K (PLD) and 32% at 246 K (sol–gel).

D 2004 Elsevier B.V. All rights reserved.

Keywords: Sol–gel; Pulsed laser deposition; Post-annealing

1. Introduction

The perovskite manganites with the general formula

La1�x3+Ax

2+MnO3 are very interesting both from a funda-

mental physics standpoint and due to their promise for

potential application in various devices such as uncooled

infrared (IR) bolometers and field effect transistors (FET)

[1–6]. Spin-dependent transport close to the para-to-ferro-

magnetic transition (semiconductor-to-metallic) tempera-

ture, Tc, causes the resistivity to strongly depend on

magnetic field (colossal magnetoresistance) and tempera-

ture. As figures of merit, it is suitable to introduce the

temperature coefficient of resistivity (TCR) and the mag-

netoresistance (MR) to determine the materials performance

as an IR bolometer and as a semiconductor channel material

for FET, respectively. A high TCR and low excess noise in

La0.67Ca0.33MnO3 (LCMO) thin films on Si have recently

been demonstrated as a prototype of an IR bolometer [6],

and the challenge now lies in moving the applications out of

academia and into industry.

To date, most perovskites engineered towards applica-

tions are grown by pulsed laser deposition (PLD) which is

0040-6090/$ - see front matter D 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.tsf.2004.03.022

* Corresponding author. Tel.: +46-8790-4182; fax: +46-8782-7850.

E-mail address: rickardb@kth.se (R. Fors).

not an industrially viable technique since there is almost no

possibility of large area deposition. Sol–gel derived thin

films do not suffer this drawback and represent a cheap and

fast route to industry scale production. High-quality sol–gel

derived, epitaxial LCMO have been demonstrated [7,8], but

a comparison of transport, quality of epitaxy and surface

morphology to PLD-grown films is to the best of our

knowledge lacking. We present results on the characteriza-

tion of both pulsed laser deposited and all-alkoxide sol–gel

derived LCMO on LaAlO3 (LAO) single crystal substrates

with and without post-annealing.

2. Experimental details

The processing technique for sol–gel films is described

in detail in Refs. [9,10] but a self-contained exposition will

be included here for completeness. Sol–gel is a chemical

solution based deposition technique where precursors are

mixed in solution and made to connect to form a sol through

hydrolysis and condensation. The sol is then deposited by

spin coating on the substrate and a gel film is obtained. With

proper heat treatment the gel is then crystallized.

The syntheses of the alkoxide precursors were carried out

in a glove box under argon atmosphere. Lanthanum and

calcium precursor solutions were prepared by dissolving

R. Fors et al. / Thin Solid Films 467 (2004) 112–116 113

metal chips in methoxy-ethanol (moeH), with approximate-

ly 0.5 mg of HgCl2 as catalyst in the lanthanum case.

Resulting solutions contained fine particles, which were

removed by centrifugation. The concentrations were deter-

mined gravimetrically as La2O3 and CaO, respectively,

formed after annealing of hydrolyzed and dried samples at

1050 jC for 12 h.

The [Mn19O12(moe)14(moeH)10]�moeH precursor was

prepared according to the literature, by metathesis of MnCl2and Kmoe in moeH [9]. After removal of KCl and re-

crystallization of the Mn-alkoxide, the alkoxide was dis-

solved in moeH and the concentration checked gravimetri-

cally as Mn3O4 after hydrolysis and annealing at 1050 jCfor 12 h.

Precursor solutions were mixed in stoichiometric ratio

and the total metal concentration adjusted to 0.6 mol�dm�3.

Gel films were deposited on the LAO substrates by spin

coating at 3500 rpm for 30 s and finally converted to oxide

by heating in air at 2 jC�min�1 to 800 jC.The soluble Mn precursor [Mn19O12(moe)14(moeH)10]�

moeH enables pure alkoxide sol–gel routes to manganese

containing materials. One advantage of alkoxides compared

to, e.g., acetates or citric acid complexes is that alkoxides

yield more homogeneous and pure gels with less organic

residues, leading to well-controlled low temperature con-

version of the gel to oxide. The precursor synthesis, espe-

cially of the La and Ca precursors, is also much simpler and

straightforward than, e.g., in the propionic acid route, which

involves distillation and drying/re-dissolution steps, and

also requires filtering of the mixed precursor solution before

it can be used [11].

The simplicity of the alkoxide system also makes it easy

to prepare films or powders of different compositions. After

determining the concentrations of the different alkoxide

solutions, they are mixed stoichiometrically to the desired

composition(s). The mixed solution is then ready to be used

immediately; there is no need for sol preparation, addition of

a gelling agent or heating of reaction mixture, as in many

other sol–gel systems [7]. The high reactivity of the

alkoxides to the moisture in the air is enough to form very

pure gel films when spin-coated.

Pulsed laser deposition is a technique in which a high

intensity laser is used to vaporize a surface layer of a target.

A plume consisting of the vaporized material is formed

which will re-condensate on an appropriately placed sub-

strate where film growth occurs. One of the largest advan-

tages of the PLD technique is the stoichiometric transfer of

material from target to substrate. Since pulsed laser deposi-

tion is a more well known and accepted thin film deposition

technique than sol–gel it will not be described in more

detail here.

Processing of PLD films was carried out according to the

outline below: A 248 nm KrF excimer Lambda Physik-

Compex-102 laser was focused on a ceramic La0.67Ca0.33MnO3 target through the optical window of a vacuum

chamber. Powders used in making the target were MnCO3

(99.9%), CaCO3 (99.5%) and La2O3 (99.9%) where the

number within parenthesis represents the purity of the

powders. The radiation energy density was 3–4 J/cm2, the

pulse repetition rate 20 Hz and the substrate-to-target

distance 72 mm. With these parameters the deposition rate

was 0.21 A per pulse. Deposition onto the LAO was carried

out in an oxygen pressure of 250 mTorr, with a substrate

temperature of 750 jC and was followed by annealing at

500 Torr oxygen for 15 min at 730 jC. Finally, the film was

slowly cooled down to room temperature. Some samples

were post-annealed at 800 jC for 6 h and at 1000 jC for 2 h.

These optimized growth conditions were chosen to maxi-

mize Tc which served as an initial figure of merit for the

samples.

Surface morphology and cross-section characteristics of

the films were investigated by means of FEG-SEM (Leo

1550-ISIS-EDS). Thicknesses of samples as derived from

the cross-sections were 200 and 50 nm for PLD and sol–

gel, respectively.

X-ray diffraction measurements were carried out with a

three-circle powder X-ray diffractometer (Siemens D5000)

with CuKa radiation (k=1.54056 A).

Transport measurements were performed in an electro-

magnet, capable of fields up to 560 kA/m, using a standard

four-probe technique with direct current density in the

range 0.2–40 A/cm2, well in the Ohmic regime. By switch-

ing the current + �, effects of thermoelectric voltages were

eliminated.

3. Results and discussion

3.1. Structure

SEM in Fig. 1 shows layer-by-layer growth in PLD and

more porous, columnar growth in sol–gel. As-grown PLD

exhibits a geometric, layered surface which stands in stark

contrast to the porous, percolation-like structure of the sol–

gel film. The porosity of the sol–gel film can be explained

by the removal of carbonate groups from the deposited gel

at temperatures (and time scales) where re-crystallization is

not expected to occur to any greater extent [10]. Post-

annealing dramatically increases surface smoothness and

there are no visible grain boundaries in the sol–gel film.

X-ray diffraction measurements were performed to de-

termine quality of epitaxy and phase purity. Fig. 2 shows the

(002) peak positions in the h–2h scans of different films. As

only (00l) reflections are present in the h–2h scan of the

films (only the results for post-annealed sol–gel film are

shown in Fig. 3) the film is c-axis-oriented.

Lattice parameters c(00l), calculated from single (00l)

reflections, are fitted against the Nelson–Riley function

cos2h(1/sinh+1/h) and the lattice parameter c is found by

extrapolating at cosh!0. Structural properties as measured

by XRD are presented in Table 1. It is apparent that the

difference in out-of-plane lattice parameter of as-grown

Fig. 1. SEM images of as-grown and post-annealed PLD films, (a) and (b),

respectively, and as-grown and post-annealed sol–gel films, (c) and (d),

respectively.

Fig. 3. XRD of post-annealed sol–gel LCMO on LAO showing h–2h scan

(main), rocking curves (left inset), and phi scans (right inset) of off-normal

(103)-planes at the oblique geometry: hsample=57.755j, 2hdetector=78.510jand hsample=58.480j, 2hdetector=80.030j for film and substrate, respectively.

R. Fors et al. / Thin Solid Films 467 (2004) 112–116114

films, 3.869(9) and 3.849(7) A for PLD and sol–gel,

respectively, vanishes upon post-annealing for which the

values 3.854(7) and 3.856(6) A are obtained. A gain (deple-

tion) of oxygen ions results in a contraction (expansion) of

the lattice indicating that as-grown sol–gel films are oxygen-

rich and that as-grown PLD films are oxygen-depleted [12].

From the FWHM of the LCMO-(00l) peaks it is possible

to derive the crystallite size by using Scherrer’s formula

L=Kk/((D2h)cosh), where K is a parameter close to 1.

Assuming that the instrumental broadening is equal to the

smallest recorded FWHM of LAO-(002) peaks and subtract-

Fig. 2. Portion of XRD h–2h scan around the (002) reflections.

ing this width from measured FWHM of the LCMO-(002)

peaks, the crystallite sizes in Table 1 are obtained. Absolute

values of L are very uncertain but useful information can be

determined by the ratio Lpost-annealed/Las-grown which indicates

a 22% and 27% size increase upon post-annealing for sol–gel

and PLD, respectively. The change in crystallite size is thus

not quite as dramatic as suggested by the smooth SEM

surface images but the microstructure of PLD and sol–gel

definitely converges with post-annealing. Also, it is impor-

tant to note that sol–gel film thickness, f50 nm, will be a

limiting factor in the measured crystallite size.

The degree of c-axis orientation, or texture, of the films

can be obtained from rocking curves (N-scans) and results

are presented in Table 1. Substrates used for PLD and sol–

Table 1

Summary of structural properties

Sol–gel PLD

As grown Post-annealed As-grown Post-annealed

From h–2hLattice parameter

c (A)

3.849(7) 3.856(6) 3.869(9) 3.854(7)

LCMO-(002)

FWHM (j)0.274 0.237 0.276 0.232

LAO-(002)

FWHM (j)0.076 0.068 0.070 0.067

Crystallite size

L (nm)

50 61 49 62

From x-scan

LCMO-(002)

FWHM ()

0.292 0.208 0.870 0.640

LAO-(002)

FWHM ()

0.285 0.230 0.295 0.230

Fig. 4. Transport measurements on (a) PLD and (b) sol–gel films. Filled

symbols correspond to as-grown films and open symbols correspond to

post-annealed films. Magnetic measurements were performed in an applied

field of 560 kA/m.

R. Fors et al. / Thin Solid Films 467 (2004) 112–116 115

gel have comparable texture both before and after post-

annealing as derived from the FWHMs of LAO-(002)

reflections. This allows us to directly compare the FWHM

of post-annealed LCMO-(002) reflections from sol–gel,

0.208j, and PLD, 0.640j. Sol–gel films thus exhibit a

much larger, by a factor 3, degree of c-axis orientation than

PLD films.

For both as-grown and post-annealed sol –gel the

FWHM of LCMO-(002) and LAO-(002) reflections are

approximately equal, indicating that the texture of LCMO

is as good as can be expected for the substrate used (see

Table 1). As mentioned above, texture of PLD films is not as

good. Post-annealing results in f30% decrease in FWHM

of the LCMO-(002) reflections for both sol–gel and PLD.

In the right inset of Fig. 3, a B-scan of the oblique (103)-

planes of post-annealed sol–gel for LCMO is shown.

Perfect coincidence of film and substrate peaks and fourfold

symmetry was found for all samples. High degree of c-axis

orientation, as found from h–2h and N-scans, together withstrong in-plane texture, as found from B-scan, indicates

epitaxial quality of post-annealed sol–gel films.

The difference in epitaxy of PLD and sol–gel films can

be related to the nature of the different processing techni-

ques. PLD is a process where the film is deposited layer-by-

layer resulting in dense films, without voids, but due to the

high energy particles impinging on the surface many defects

are introduced [13]. Sol–gel on the other hand is deposited

as a gel and converted to a film by heat treatment during

which decomposing carbonates will give a porous and

sometimes also cracked film [10]. It is also possible that

nucleation of crystallites is preferred in the sol–gel heat

treatment process whereby a thin film will have a more

porous structure. This could result in a sol–gel film with

good epitaxial quality, despite its porosity, since free stand-

ing crystallites are free to grow epitaxially, with less strain

induced from substrate mismatch and substrate defects. PLD

films on the other hand will have more extrinsic defects due

to the nature of the PLD technique and more accumulated

strain and defects induced from the substrate resulting in

worse epitaxial quality.

3.2. Transport

Results of transport measurements are summarized in

Table 2 and shown in Fig. 4. The temperature coefficient of

Table 2

Summary of transport properties

Sol–gel PLD

As-grown Post-annealed As-grown Post-annealed

Tqpeak (K) 269 258 268 274

TCRpeak (% K�1) 4.4 6.1 4.6 8.2

TTCRpeak

(K) 239 241 249 258

MRpeak

(% at 560 kA/m)

26 32 27 35

TMRpeak

(K) 249 246 249 249

resistivity is defined as TCRudlnq/dT and the magnetore-

sistance ratio as MRu(q0�q560 kA/m)/q0. Post-annealing

greatly enhances the transport properties of both films but

PLD films perform better overall in spite of sol–gel

showing better epitaxy.

Increasing peak TCR and MR with annealing is expected

since it depends mainly on crystallite size (which is shown

to increase by SEM and XRD) [12]. The decrease in

resistivity and the peak shift to higher temperature in PLD

films is mainly due to the incorporation of more oxygen

[12,14,15], which is also evident from the decrease in out-

of-plane lattice parameter as shown above. Increasing crys-

tallite size also acts to decrease the contribution of grain

boundary resistance.

Previous thermogravimetric, XRD and IR spectroscopy

studies on all-alkoxide derived sol–gel films have indicated

the presence of excess oxygen which is lost during post-

annealing [10]. An increasing out-of-plane lattice parameter

with annealing as described above is consistent with this

view. This can explain the absence of significant peak shift

in transport measurements of sol–gel films.

Due to the porosity and surface roughness of sol–gel

films and the definition of film thickness used, the calcu-

lated resistivity of sol–gel films is consistently higher than

Fig. 5. Cross-sectional SEM of as-grown sol–gel film.

R. Fors et al. / Thin Solid Films 467 (2004) 112–116116

for PLD films. The thickness t of sol–gel and PLD films

was measured on as-grown films from the substrate to the

top of the films, disregarding any surface roughness. Cross-

sectional SEM of an as-grown sol–gel film as shown in Fig.

5 reveal free standing crystallites on top a thinner denser

film so that the effective thickness teffective is less than the

measured thickness t. The resistivity of all samples was

calculated from sheet resistance according to q=Rsheett and

replacing t with teffectivect/2 for as-grown sol–gel films, as

obtained from inspection of Fig. 5, would result in compa-

rable resistivity for as-grown PLD and sol–gel films. In

post-annealed sol–gel films the effects of porosity are

reduced but the densification will again lead to a film

thickness smaller than that measured on as-grown films.

As a result, the absolute magnitudes of the resistivities of as-

grown and post-annealed films cannot be directly compared

and the expected increase in resistivity associated with

oxygen loss cannot be verified. This analysis shows the

inherent problem of calculating the resistivity of porous

sol–gel films. Thickness measurements should be per-

formed after the last annealing step and be complemented

with surface roughness measurements allowing an effective

thickness to be determined.

4. Conclusions

In summary we have found that all-alkoxide sol–gel

derived La0.67Ca0.33Mno3/LaAlO3 film structures can com-

pete with PLD in terms of structure and transport, making

sol–gel a prime candidate for future industrial manganite

thin film production.

While sol–gel film exhibits better epitaxial quality, PLD

films have the higher peak TCR and MR. These differences

are mainly due to different growth mechanisms which

results in sol–gel films which contain excess oxygen, have

columnar growth and very porous structure while PLD films

are oxygen-depleted, have layer-by-layer growth, and a

more dense structure. Deficiencies in the microstructure

and oxygen content can be largely eliminated by post-

annealing, which results in sol–gel film of remarkable

epitaxial quality and with desirable transport properties.

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