Solid State Communications 150 (2010) 1637–1640

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Solid State Communications

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The structural and electrical properties of oriented SrTiO3 films prepared bymetal organic deposition methodXiaofei Wang a,b, Xiaomei Lu a,∗, Huifeng Bo a, Yaoyang Liu a, Yanchi Shen a, Xiaobo Wu a, Wei Cai a,Yi Kan a, Chao Zhang a, Yunfei Liu a, Fengzhen Huang a, Jinsong Zhu aa National Laboratory of Solid State Microstructures, Physics Department, Nanjing University, Nanjing, 210093, Chinab School of Physics and Engineering, Henan University of Science and Technology, Luoyang, 471003, China

a r t i c l e i n f o

Article history:Received 23 March 2010Received in revised form10 June 2010Accepted 19 June 2010by T. KimuraAvailable online 26 June 2010

Keywords:A. Oriented SrTiO3 filmB. Metal organic depositionD. Electrical property

a b s t r a c t

Strontium titanate films with high a-axis orientation [a(100) = 94.1%] and random orientation weredeposited on (111) Pt/Ti/SiO2/Si substrates by a concentration controlling of the precursor solutionduring the metal organic deposition process. Topography of samples was investigated by atomic forcemicroscopy after annealing at 800 °C. X-ray diffraction found that the degree of a-axis orientationincreased with increasing annealing temperature. The leakage current and the dielectric property werestrongly dependent on the film orientation, and the possible causes of orientation dependence werediscussed.

© 2010 Elsevier Ltd. All rights reserved.

1. Introduction

Quantumparaelectric strontium titanate (STO) is one of the bestinvestigated perovskite oxides in the field of electronic industry,due to their potential application in tunable microwave devices,including tunable oscillators and phase shifters [1–3]. At the sametime, these studies also promote the development of preparativetechnique of thin films. Most of the current studies are focusedon the high-quality STO thin film with preferred orientation,because the oriented films can present a better property andavoid some limitations and non-uniformity problems comparedwith random-oriented polycrystalline films [4]. The epitaxial (001)SrTiO3 films on (001) Si substrateswere grown viamolecular beamepitaxy (MBE) by Warusawithana et al. [5] and also the (100)-epitaxial STO films were deposited on (110) NdGaO3 substratesvia pulsed laser deposition (PLD) by Setter et al. [6]. Besides, therewere many reports on the other oriented STO films on DyScO3,SrTiO3/Si, Nb-doped SrTiO3, LaAlO3 substrates and so on [7–12].Unfortunately, all of the above oriented films were grownepitaxially by some special equipments on single-crystallinesubstrates [5,6], and oriented film has not yet been grown simplyon conventional substrates such as (111) Pt/Ti/SiO2/Si substrates.Recently, Lu et al. [13,14] showed that (100)-predominant and

∗ Corresponding author. Tel.: +86 25 83594730; fax: +86 25 83595535.E-mail addresses: xiaomeil@nju.edu.cn (X. Lu), jszhu@nju.edu.cn (J. Zhu).

0038-1098/$ – see front matter© 2010 Elsevier Ltd. All rights reserved.doi:10.1016/j.ssc.2010.06.035

c-axis-oriented Bi3.15Nd0.85Ti3O12 (BNT) films were grown bycontrolling the heating rates on (111) Pt/Ti/SiO2/Si substratesthrough a sol–gel process. Subsequently, the BNT, Bi3.15Pr0.85Ti3O12(BPT) and Pb(Zr, Ti)O3 films with the preferred orientations werealso deposited on Pt substrates, which strongly relied on the layerthickness or the films thickness [15–18]. In this letter, we providean optimum route for the preparation of (100)-preferential-oriented STO films on (111) Pt/Ti/SiO2/Si substrates through asimple metal organic deposition (MOD) method. In addition, thedifferent electric properties are also discussed for the sampleswithdifferent orientations. At 30 kHz, the dielectric constant of the(100)-oriented film is about 300, which is twice as large as thatof random-oriented film.

2. Experiments

The STO filmswere fabricated by theMOD technique. Strontiumacetate and Tetrabutyl titanate were used as precursors for Sr andTi, respectively. The films were spin-coated on (111) Pt/Ti/SiO2/Sisubstrates with 3500 rpm for 25 s using precursor solution withtwo different concentrations. And then the films were pyrolyzedat 350 °C for 5min to evaporate solvents and organic addenda. Theabove processes were repeated for several times until a desiredfilm thickness of 200 nm was achieved. The coated films werecrystallized via one-time post-annealing in an oxygen ambience at800 °C for 30 min. Moreover, to calculate the orientation degreeof STO films, a random-oriented STO powder sample was also

1638 X. Wang et al. / Solid State Communications 150 (2010) 1637–1640

a b

c

Fig. 1. X-ray diffraction patterns of (a) the highly a-axis-oriented SrTiO3 films, (b) the random-oriented films annealed at different temperatures from 550 to 800 °C on(111) Pt/Ti/SiO2/Si substrates, and (c) the highly a-axis-oriented film, the random-oriented film annealed at 800 °C and the STO powder sample. The inset of Fig. 1(c) givesthe annealing temperature dependence of the orientation degree for a-axis-oriented STO films.

prepared for comparing by sintering the dried gel derived from thecoating solution at 1100 °C for 3 h in air. Pt dot electrodes weredeposited by sputtering through a shadowmask for measuring theelectrical properties of the films. X-ray diffraction (XRD, D/Max-RB) with Cu–Kα radiation and atomic force microscope (AFM,Nanoscope IV) were used for analyzing the microstructure ofthe films. Leakage current of the films was measured by theKeithley 6517A Electrometer/High resistance meter. The dielectriccharacteristics were evaluated using HP4194A impedance/phaseanalyzer.

3. Results and discussions

Fig. 1(a) shows the XRD patterns of the highly a-axis-orientedSTO films at different annealing temperatures from 550 to800 °C, which are fabricated using precursor solution of lowerconcentration. It can be seen that the STO films crystallizegradually with increasing annealing temperature. When theannealing temperature reaches 650 °C, (110) and (200) peaksobviously appear.With increasing temperature from650 to 800 °C,(200) peak intensity increases rapidly and is higher than thegrowth rate of (110) peak. Furthermore, the (200) reflectionpeak becomes the strongest among all the peaks and (100) peakalso appears at 800 °C. Except for (100), (200) and weak (110)peaks, there are no other obvious STO diffraction peaks. Basedon these phenomena, a preferential a-axis-oriented STO filmis obtained. Fig. 1(b) shows the XRD patterns of the random-oriented STO films at different annealing temperatures from 550to 800 °C, which are fabricated using precursor solution of higherconcentration. Similar to a-axis-oriented film, with increasingannealing temperature, the diffraction peaks intensity of STO films

increases gradually. The crystalline phase formation appears at600 °C that is lower than that of the a-axis-oriented STO film. Thediffraction peaks of (110), (200) and (211) of STO films coexistfrom 600 to 800 °C, which show a random orientation. Fig. 1(c)shows the XRD patterns of the two films with a-axis orientationand random orientation annealed at 800 °C as well as that ofthe STO powders for comparison. All of the XRD peaks of STOpowders correspond well to the standard powder diffraction data.To determine the degree of preferred orientation, the volumefraction (100) of a-axis-oriented grains in a STO film is definedas [13]

a(100) =∑

(In00/I∗n00)/(Ihkl/I∗

hkl), (1)

where Ihkl is the measured intensity of the (hkl) peak for thefilms, I∗hkl is the intensity for powders, and n is the number ofreflections. According to Eq. (1), the orientation degree of a-axis-oriented STO films is a(100) = 66.9%, 69.6%, 77.8% and 94.1% atannealing temperatures of 650 °C, 700 °C, 750 °C and 800 °C,respectively. The degree of preferred orientation increases withincreasing annealing temperature as shown in the inset of Fig. 1(c).Based on the above results, it is concluded that the better a-axis-oriented STO film can be obtained using the precursor solutionwith a lower concentration at 800 °C annealing temperature.The surfacemorphology of STO filmswas detected using atomic

force microscope (AFM). Fig. 2(a) and (b) show the highly a-axis-oriented and the random-oriented film surface annealed at 800 °C,respectively. As shown in Fig. 2, both the two kinds of STO films arecrystallized well and dense. The a-axis-oriented film is composedof more or less flat-shaped big grains, which is similar to theoriented BNT films [13,16], and the average grain size is about160 nm in diameter. In contrast, the random-oriented STO film has

X. Wang et al. / Solid State Communications 150 (2010) 1637–1640 1639

a b

Fig. 2. AFM topography images of (a) the highly a-axis-oriented and (b) the random-oriented STO films annealed at 800 °C.

smaller average grain size down to about 120 nm andmany grainsare quasi-circular and rarely flat-shaped. The easier growth of a-axis-oriented grains in SrTiO3 film at 800 °C can be attributed to theminimization of surface and grain boundary energies as describedby Yan et al. [19]. As a matter of fact, the lower concentrationof the precursor solution results in lower condensation rate andthinner thickness of each spin-coating layer [19,20]. Therefore,local grain-on-grain or homoepitaxy growth would easily occurat the interface between the new and the previous coating layer,resulting in highly preferred orientation.For learning about the electrical properties of a-axis-oriented

STO films, the room-temperature leakage currents and dielectricproperties were measured for both the a-axis-oriented and therandom-oriented STO films annealed at 800 °C. Fig. 3 plots theJ–E characteristics at room temperature for these two kinds ofSTO films. The leakage current density (J) increases gradually withincreasing the applied electric field (E) and is about the order ofmagnitude of 10−5 A/cm2 when E increases to about 50 kV/cmaccording to the results of Rozier et al. [21]. With the furtherincrease of E, J of the a-axis-oriented STO film is larger than thatof random-oriented STO film. We ascribe this to the decrease inthe volume fraction of grain boundaries in the a-axis-oriented STOfilm, since the grain boundaries can reduce the leakage currents[21,22]. In addition, it is noticed that the break down region (Eg)with a sudden increase of J appears at about 100 kV/cm and200 kV/cm for the a-axis-oriented and random-oriented STO film,respectively. The existence of Eg suggests that the conductionmechanism of the STO films falls into the category of grainboundary limited conduction (GBLC) [23], which indicates thatalmost all the small grains with similar barrier height are suddenly‘‘turned on’’ at Eg . The higher Eg of the random-oriented STO filmdenotes the greater barrier at the grain boundary.Fig. 4 shows the dielectric constant (ε) and the loss (tan δ)

as a function of the measuring frequency for the a-axis-orientedand the random-oriented STO films in the range from 1 kHz to1 MHz at room temperature. (1) For ε, it can be observed that thedielectric constant (292 at 100 kHz) of a-axis-oriented STO filmis larger than that (142 at 100 kHz) of the random-oriented film,which shows that the orientation of the film effectively improvesthe dielectric properties. Similar results were reported in BNTfilms [13] that the dielectric constant of the a-axis-oriented filmwas larger than that of the random-oriented and c-axis-orientedfilms because the polarization vector in BNTwas close to the a axis.Lu [24] and Keane [25] et al. also reported the effect of the differentorientations of films on dielectric properties. Both the permittivityand the dielectric-loss tangent of the (111)-oriented SrTiO3 filmwere greater than those of the (110)/(111)-textured film accordingwith the appearance of a ferroelectric phase transition for (111)-oriented film due to biaxial tensile stress. The effect of a small

Fig. 3. J–E characteristics at room temperature for the highly a-axis-oriented andthe random-oriented STO films annealed at 800 °C.

biaxial stress on dielectric properties may exist as well in our case.The a-axis-oriented STO film shows a larger dielectric constantbecause the stress may counteract with each other due to differentgrain orientations in random-oriented STO film. In addition to thestress effect, the larger dielectric constant may also relate to thelarger average grain size in a-axis-oriented STO film (as shownin Fig. 2) [26]. (2) For tan δ, in the high frequency regime, thedielectric loss (tan δ = 0.036 at 100 kHz) of the a-axis-orientedfilm is smaller than that of the random-oriented film (tan δ =0.048 at 100 kHz), which indicates better interfacial state andlattice integrality in the a-axis-oriented film. In the low frequencyregime, tan δ of the a-axis-oriented STO film is largerwhich reflectscommonly higher dc leakage current in the sample [27], and theobserved increase of tan δ is consistent with the results of theleakage currents (Fig. 3). According to above discussion, it is clearthat ε and tan δ of the STO films have close relationship with itsorientation in the range of measuring frequency. The preferentialorientation may improve the properties of the film and thusachieve a better practical application.

4. Conclusions

In conclusion, we deposited successfully the a-axis-orientedSTO films on (111) Pt/Ti/SiO2/Si substrates through a simpleMOD process with a lower cost. The films are annealed throughone-time-annealing process. For oriented STO film, the precursorsolution concentration is a key factor and the degree of preferredorientation increases with increasing the annealing temperature.The substrate and its orientation show less influence on theorientation of our samples, though they were important for

1640 X. Wang et al. / Solid State Communications 150 (2010) 1637–1640

Fig. 4. Frequency dependence of dielectric constant ε and loss tan δ at roomtemperature for the highly a-axis-oriented and the random-oriented STO filmsannealed at 800 °C.

the fabrication of epitaxial films [11,28,29]. Compared with therandom-oriented STO films, the highly a-axis-oriented STO filmspresent a better dielectric property, which is favorable for actualapplications in microwave devices.

Acknowledgements

This work was supported by the National Science Foundation(Grant Nos. 50972056 and 50832002), the 973 Project of MOST(Grant Nos. 2006CB921804 and 2009CB929501) and NCET-06-0443.

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