Microstructural and conductive properties of BaRuO3 thin films

Davinder Kaura,*, K.V. Raob

aDepartment of Physics, Indian Institute of Technology, Roorkee 247 667, IndiabDepartment of Condensed Matter Physics, Royal Institute of Technology, S-100 44 Stockholm, Sweden

Received 11 September 2002; received in revised form 23 July 2003; accepted 7 August 2003 by A.K. Sood

Abstract

We have grown conducting BaRuO3 films on (100) LaAlO3 substrate using pulsed laser deposition technique over

temperature range varying from 500 to 775 8C. The films are well textured and are c-axis oriented with an in-plane epitaxial

relationship of k010lk100lBaRuO3kk110l LaAlO3. Atomic force microscopy observation shows that they consist of a fine

arranged network of grains and have a mosaic microstructure. Surfaces with smooth terraces have been observed by Scanning

Tunneling Microscopy. The resistivity of the films has been found to be a strong function of substrate temperature during film

deposition. Both metallic and semiconducting behaviour has been observed in these films. Temperature-dependence resistivity

measurement shows that the film has a metallic curve if it is deposited at 700 8C or lower but it transfers to a semiconducting-

metallic twofold curve if the deposition temperature is increased. This unique phenomenon, which is not observed in bulk, may

provide new features useful in the fabrication of novel electronic devices.

q 2003 Elsevier Ltd. All rights reserved.

PACS: 70

Keywords: A. Thin films; A. Ruthenates; B. Laser ablation; C. Microstructure

1. Introduction

The exotic electronic state that leads to superconduc-

tivity at high temperatures in the layered copper oxides has

stimulated research into the physical properties of wide

variety of transition metal oxides. Among most of recent

interest, the simple ruthenates have been the subject of

considerable study, especially since the discovery of

superconductivity near 1 K in the layered compound

Sr2RuO4 [1] without any copper and doping. Further there

have been growing interest in epitaxial growth of these

conducting ruthenium oxide compounds (ARuO3: A ¼

Ba,Sr,Ca) because of their interesting magnetic, transport

properties and potential device applications [2,3]. These

conducting ruthenates have already been used as bottom

electrode for ferroelectric heterostructures [4] and as normal

metal barrier in Superconductor–Normal Metal-Supercon-

ductor Josephson junctions [5]. They are structurally

compatible with ferroelectrics and also can improve the

fatigue resistance of ferroelectric capacitors significantly.

The properties of epitaxial thin films of these perovskite

based oxides especially has been found to be quite different

from the corresponding bulk materials because of the

existence of strain, cation disorder and variation in oxygen

concentration in the films etc. It is therefore important to

understand and control their growth and properties.

The structural chemistry of ARuO3 type ruthenates can

be described in terms of hexagonal and cubic close packing

of AO3 layers. If all AO3 layers are cubic close packed, the

RuO6 octahedra form a cubic like three dimensional array

by sharing only one oxygen to give rise to cubic, tetragonal

and orthorhombic structures. In contrast, if AO3 layers are

entirely hexagonal-close packed, the RuO6 octahedra are

shared by three oxygen to form a hexagonal structure. Due

to two basic packing forms the bulk BaRuO3 has three

different crystal structures. They are nine layered rhombo-

hedral structure (9R) with a ¼ 5:75 �A and c ¼ 21:6 �A [6], the

0038-1098/$ - see front matter q 2003 Elsevier Ltd. All rights reserved.

doi:10.1016/j.ssc.2003.07.005

Solid State Communications 128 (2003) 391–395

www.elsevier.com/locate/ssc

* Corresponding author.

E-mail address: dkaurfph@iitr.ernet.in (D. Kaur).

six layered hexagonal structure (6H) with a ¼ 5:71 �A and

c ¼ 14 �A [7], and the four layered hexagonal structure (4H)

with a ¼ 5:73 �A and c ¼ 9:5 �A [7], depending on the amount

of hexagonal and cubic close packing of the BaO3 layers.

The 9R phase has been reported to be most stable. It consists

of units of three RuO6 octahedra sharing faces in a partial

chain, facilitating direct Ru–Ru d orbital interactions with

in the group, with each of these triple units of octahedra

connected to its neighbors along the hexagonal axis by

perovskitelike corner sharing with the nearly 1808 Ru–O–

Ru bonds favorable for superexchange coupling. The

stacking pattern repeats after nine octahedra. As these low

spin state ðS ¼ 1Þ compounds consists of a narrow itinerant

band composed of Ru t2g and O 2p orbitals, their magnetic

and transport properties depend in an extremely sensitive

way on the degree of band filling and bandwidth. In order to

get a better understanding of electrical properties and their

relation with thin film microstructures, we have prepared

thin films of BaRuO3 and tried to study these interesting

electrical and microstructural properties.

2. Experimental

The BaRuO3 thin films were in situ grown by pulsed

laser deposition using 30 mm diameter target. The target

was prepared by mixing appropriate molar ratios of BaCO3

and hydrated RuO2 powders, grinding and heating the mixed

powder in air at 800 8C for 12 h. The powder was further

reground and pressed into pellets under a pressure of about

80 MPa. Finally, it was sintered at 1400 8C for 2 h. The laser

used was Nd: YAG with wavelength of 355 nm in tripled

mode. The pulse repetition rate was 10 Hz with laser fluence

of about 3 J/cm2. Before ablation the deposition chamber

was evacuated to a base pressure of 1025 Torr, then the pure

oxygen was introduced and maintained at pressure of

200 mTorr. Single crystal LaAlO3 (100) wafers were used

as substrates and were cleaned sequentially in acetone,

methanol and deionized water prior to deposition. The target

substrate distance was kept at 55 mm. The deposition

temperature was varied from 400 to 775 8C. The thickness

of the films was in the range of 100–150 nm.

A Siemens D 5000 four circle diffractometer with Cu Ka

radiation was used to see the orientation and crystallinity of

the films. The surface morphology of the films was

investigated using AFM and STM. The resistivity of the

films were measured using standard four probe technique

over a temperature range from 300 K down to 10 K.

3. Results and discussion

Fig. 1(a) shows the XRD u–2u scan of BaRuO3 film

deposited at 775 8C. It reflects prominent ð0010Þ reflection

of BaRuO3 film along with two intense peaks of LaAlO3

substrate. Rocking curve measurements on the ð0010Þ

reflection show that the full width at half maximum

(FWHM) reduces from 2.9 to 1.98 when substrate

temperature increases from 720 to 775 8C (Table 1). This

indicates an improvement of film epitaxial quality. Fig. 1(b)

shows the X-ray f scans of BaRuO3 ð0111Þ and LaAlO3

(113) reflections for the sample deposited at 775 8C. The

film has also been found highly oriented in the ab plane.

From the peak positions, it is easy to find that the in-

plan epitaxial relationship of BaRuO3 film on LaAlO3

(100) substrate is k001lBaRuO3kk100lLaAlO3 and

k010lk100lBaRuO3kk110lLaAlO3. This suggests that unit

cell axis of BaRuO3 thin film is parallel to the diagonals of

the unit cell of LaAlO3 substrates. The interfacial relation-

ship of this diagonal-type epitaxy gives rise to a lattice

mismatch of 6–7%, which is larger than that of SrRuO3 or

CaRuO3 on LaAlO3[9].

Deposition temperature has an important influence on

resistivity of BaRuO3 films. Temperature dependence of

film resistivity (r–T curves in Fig. 2) shows that the films

Fig. 1. (a) X-ray u–2u scan for a BaRuO3 epitaxial film on (100)

LaAlO3. The film was deposited at 775 8C. The inset is the rocking

curve measurement showing the full width at half maximum of

BaRuO3-ð1110Þ and LuAlO3-(200) reflections. (b) f-Scans on the

BaRuO3-ð0111Þ and LaAlO3-(113) reflections.

D. Kaur, K.V. Rao / Solid State Communications 128 (2003) 391–395392

deposited at lower temperatures i.e. at 500, 600 and 700 8C

are metallic. While the films deposited at higher temperature

i.e. above 700 8C shows better texture quality and undergoes

transition from semiconducting to metallic in their r–T

curves, i.e. they are semiconductor-like at low temperature

region and become metallic at higher temperature region.

The transition temperature of these films increase with

increasing the deposition temperature. It is also interesting

to note that the film resistivity rð250 KÞ for sample

deposited at 700 8C is lowest i.e. 135 mV cm. This value

is comparable to that of single crystal BaRuO3

(,100 mV cm) [10,11] and is much lower than

810 mV cm value of sputtered films [8]. Either increasing

or decreasing deposition temperature results in increasing

film resistivity, regardless of film quality. As these low spin

state ðS ¼ 1Þ ARuO3 compounds consists of a narrow

itinerant band composed of Ru t2g and O 2p orbitals, their

magnetic and transport properties depend in an extremely

sensitive way on the degree of band filling and bandwidth.

Moreover in these perovskite oxides a small orthorhombic

or rhombohedral distortion can change the Ru–O–Ru bond

angle which changes the resistivity considerably as seen in

case of NdNiO3 [12]. The increase in value of the resistivity

at higher deposition temperature in present case of BaRuO3

thin films could possibly be due to the oxygen deficiency.

In order to understand the cause of the observed

conductive properties, we investigated the structure of the

BaRuO3 films more accurately using XRD. It was found that

the films deposited at low temperature are amorphous, and it

began to crystallize at deposition temperature of ,400 8C.

Moreover, the films are randomly oriented at Ts below

700 8C and becomes well c-axis oriented if Ts is more than

700 8C. The u–2u scans of ð0010Þ reflection of films display

a clear difference in the film peak positions, as shown in Fig.

3. Firstly, we found that the film deposited at 500 or 600 8C

has a very broad peak, corresponding to a wide range of

c-lattice constant varying from 20.9 to 21.9 A. However, the

peaks are much narrower for the c-axis oriented films

deposited at 700 8C or higher deposition temperature and

they shift to small angle values when the deposition

temperature increases (corresponding to larger c-lattice

constants as shown in the inset of Fig. 3 and Table 1). Thus,

the volume of BaRuO3 unit cell increases with increase in

deposition temperature. In-plane X-ray diffraction of all

these films show almost the same a-lattice constant, which is

of ,5.73 A.

To get better insight to the conducting properties and

volume expansion we measured the composition of the films

deposited at various substrate temperature using energy

dispersive X-ray analysis technique (EDAX). The films

which show metallic behaviour in r–T curve are found to be

stoichiometric, however, the films which show semicon-

ducting behaviour were found to be slightly Ba rich and Ru

deficient with a composition close to Ba1.2Ru0.8Ox with in

Table 1

Various parameters of BaRuO3 thin films

Substrate tempratutre Ts; (8C) Lattice constant (A) FWHM (degrees) rRT ð250 KÞ (mV) rð20 KÞ (mV)

a c

700 5.73 21.38 4.0 139 119

720 5.73 21.41 3.2 154 140

750 5.73 21.45 2.2 203 196

775 5.73 21.46 1.7 466 494

Fig. 2. Temperature-dependence film resistivity curves (r–T

curves) for BaRuO3 films deposited at substrate temperature of

(a) 600 8C, (b) 700 8C, (c) 750 8C, and (d) 775 8C.

Fig. 3. X-ray u–2u scans of ð0010Þ reflection of BaRuO3 films

which display a clear difference in the film peak positions. The inset

shows the corresponding c-lattice length.

D. Kaur, K.V. Rao / Solid State Communications 128 (2003) 391–395 393

experimental error. This leads to the fact that interesting

conductive properties of these BaRuO3 thin films could be

due to the change in stoichiometry of the films with change

in substrate temperature. This change in stoichiometry of the

semiconducting films could result either in information of

secondary phases or in cation disorder with the substitution

of the Ru cation by the Ba cation in the lattice. As no

impurity peaks has been observed in the XRD pattern of

these semiconducting films, therefore we believe that there

is a partial cation substitution with Ba substituting for Ru,

which is responsible for the change in r–T curve from

metallic to metallic–semiconducting. The substitution of

larger Ba cations for smaller Ru will result in the observed

enlargement of the unit cell as also seen in case of CaRuO3

[9].

Besides the difference of unit cell volume, we also

believe that defects mainly the grain boundaries scattering

in the films may be another cause, as it has been seen in

epitaxial SrRuO3 films [2]. We then studied the film surface

morphology using atomic force microscopy and Scanning

Tunneling microscopy (STM). As shown in Fig. 4, the film

deposited at 600 8C looks like partially crystallized, and has

more amount of grain boundary phases (or amorphous

phases) which can result in stronger electrical scattering and

hence a low conductivity. Where as those epitaxial samples

(i.e. Ts ¼ 700 8C or higher) are well crystallized and consist

of fine arranged network of grains. The film surfaces are

smooth with rms value of surface roughness as determined

from STM is about 1.6 nm over the area of 1 mm £ 1 mm.

Increase of the grain size at the film surface with increasing

deposition temperature has been observed. Therefore, the

films deposited at Ts ¼ 700 8C with large grain size shows

lower resistivity than the 600 8C deposited sample, though it

has a larger unit cell volume.

4. Conclusion

In summary, we have grown highly conducting BaRuO3

films on LaAlO3 substrates using PLD and have studied

their structural and electrical transport properties. The films

are well textured and are c-axis oriented with an in-plane

epitaxial relationship of k010lk100lBaRuO3kk110l LaAlO3.

The electrical conductivity of the films undergoes a metallic

to semiconducting–metallic transition, depending on the

deposition process. We believe that the partial cation

substitution with Ba substituting for Ru, in these films is

responsible for the change in r–T curve from metallic to

metallic-semiconducting and unit cell enlargement. Mean-

while, the value of resistivity is also dependent on the grain

boundary scattering in the film. Such interesting conductive

properties of BaRuO3, which has not been seen in the bulk,

may provide new features useful in making novel electronic

devices.

Fig. 4. Surface morphology of the films deposited at (a) 600 8C, (b)

720 8C, and (c) 775 8C. The pictures were measured by atomic force

microscopy and the scan area is 2.3 mm £ 2.8 mm.

D. Kaur, K.V. Rao / Solid State Communications 128 (2003) 391–395394

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