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Thin Solid Films 516 (2

Microstructure control of (Pb,Sr)TiO3 films on Pt/Ti/SiO2/Sisubstrates by a TiO2 buffer layer

Lili Chen, Mingrong Shen ⁎, Liang Fang, Yu Xu

Department of Physics and Jiangsu Key Laboratory of Thin Films, Suzhou University, Suzhou 215006, People's Republic of China

Received 20 August 2006; received in revised form 14 May 2007; accepted 29 May 2007Available online 6 June 2007

Abstract

A thin TiO2 buffer layer was used to control the microstructure and electrical properties of the polycrystalline (Pb,Sr)TiO3 (PST) filmsproduced by a Sol–Gel method on Pt(111)/Ti/SiO2/Si(100) substrates. The PST films included (Pb0.6Sr0.4)TiO3 (PST40) and (Pb0.4Sr0.6)TiO3

(PST60). It was found that a crystallized TiO2 buffer layer with a thickness of nearly 5 nm was critical for improving the crystallinity and surfacemorphology of both the thinner (about 40 nm) and thicker (about 330 nm) PST films, which exhibited a (l00) preferred orientation and muchsmoother surface comparing with those without the buffer layer. The electrical properties of the PST films having TiO2 buffer layer were alsoimproved. For 330-nm-thick PST40 films, the dielectric constant and its tunability by dc voltage were increased from 482 and 26.8% at 10 kHz to590 and 51.2%, while the loss and leakage current density were reduced from 0.04 and 4.26×10−4 A/cm2 at 100 kV/cm to 0.034 and7.63×10−6 A/cm2, respectively. Similar results were also found in the PST60 films.© 2007 Elsevier B.V. All rights reserved.

Keywords: Sol–Gel; Buffer layer; Electrical properties

1. Introduction

Due to the excellent dielectric, piezoelectric and opticalproperties, ferroelectric films have attracted much attention inrecent years. They have been applied tomany electrical and opticaldevices, such as electro-optic modulators [1] and ferroelectricrandom access memory devices with excellent potential for use inboth dynamic and permanent data storage systems [2,3]. Effort hasbeen devoted to the control of the morphology and crystallinity ofthe films due to the great significance of the microstructure fromthe point of view of applications [4,5].

Ferroelectric (Pb,Sr)TiO3 (PST) films have recently beenconsidered to be important candidates for dynamic randomaccess memory and tunable microwave devices [6–8]. It wasreported that highly (100) epitaxial PST films had a dielectricconstant up to 3100 and tunability near 48% under a bias field of40 kV/cm [7]. The films were deposited on the single-crystalsubstrates such as LaAlO3 and MgO providing good lattice

⁎ Corresponding author. Tel.: +86 512 6511 2251; fax: +86 512 6511 2597.E-mail address: mrshen@suda.edu.cn (M. Shen).

0040-6090/$ - see front matter © 2007 Elsevier B.V. All rights reserved.doi:10.1016/j.tsf.2007.05.061

match [7,8]. However, these substrates are very expensive andcomponents mounting techniques to fabricate a hybid micro-wave-integrated circuit using these substrates are not so simple.The use of Si substrates has great advantages of low-cost,reduced power consumption and high volume production [9].Unfortunately, most previous works related to PST films growndirectly on Si substrates have no microstructure control andsuffer from low dielectric constant and its tunability by dcvoltage, due to the polycrystalline in nature and the formation ofa dead layer with low dielectric constant between the film andsubstrate [10–13]. So, it is necessary to introduce a buffer layerbetween the PST films and Pt/Ti/SiO2/Si substrates, in an effortto modulate the morphology and crystallinity of the films andthus improve their electrical properties.

TiO2 buffer layer has been used to control the microstructureof PbTiO3 and Pb(Zr,Ti)O3 (PZT) films [14–16]. In P. Muralt'swork [14,15], a crystalline and a few nanometer thick TiO2 filmwas proved to be a very efficient seeding layer for the nucleationof (111) oriented PZT films prepared by a sputtering depositionmethod. In this study, a thin TiO2 buffer layer was introducedbetween the PST films and Pt/Ti/SiO2/Si substrates using a Sol–

Fig. 1. AFM micrographs of (a) Pt/Ti/SiO2/Si substrate, (b) thin TiO2 bufferlayer on the Pt/Ti/SiO2/Si substrate, 40 nm thick PST40 films (c) without and(d) with TiO2 buffer layer, and 330 nm thick PST40 films (e) without and(f) with TiO2 buffer layer.

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Gel method. Special attention was paid to the influence of thebuffer layer on the microstructure of the initial PST layer, inorder to clarify the role of the buffer layer.

2. Experimental procedures

(Pb0.6Sr0.4)TiO3 (PST40) and (Pb0.4Sr0.6)TiO3 (PST60)precursor solutions were prepared by a Sol–Gel method fromlead acetate trihydrate [Pb(CH3COO)33H2O], strontium acetate1/2 hydrate [Sr(CH3COO)1/2H2O], and titanium isopropoxide[Ti(OCH(CH3)2)4] as starting materials. Acetic acid [CH3-

COOH] and 2-methoxyethanol [CH3OCH2CH2OH] with a ratioof 1:1 in volume were used as solvents. 10 mol% excess amountof lead acetate trihydrate was used to compensate the Pb lossoccurring during the annealing process. 2.64×10−3 mol Pb(CH3COO)33H2O and 1.6×10−3 mol Sr(CH3COO)1/2H2O forPST40, 1.76 × 10−3 mol Pb(CH3COO)33H2O and2.4×10−3 mol Sr(CH3COO)1/2H2O for PST60 were dissolvedin 10 mL acetic acid in air, respectively, which were stirred at110–120 °C for more than 1 h to remove water. 4×10−3 moltitanium isopropoxide and 10 mL 2-methoxyethanol were thenadded to the mixed precursor solutions at room temperature.Finally, 0.8 mL acetylacetone was added as a stabilizing agent.The clear and transparent sols with a molar concentration of0.2 mol/L were obtained after stirring the mixtures for severalhours. As another precursor solution for the TiO2 buffer layer,4×10−4 mol titanium iso-propoxide was dissolved in a mixtureof acetic acid and 2-methoxyethanol (the volume ratio of thetwo solvents was 1:1), and the molar concentration wascontrolled to be as small as 0.02 mol/L, in order to get a thin(near 5 nm) TiO2 buffer layer on Pt/Ti/SiO2/Si substrates.

TiO2 buffer layers and PST films were deposited onto the Pt(111)/Ti/SiO2/Si(100) substrates by spin-coating the alkoxidesolutions. Each layer was deposited using two steps of1000 rpm for 3 s and subsequent 3000 rpm for 30 s. Thedeposited layer was dried at 150 °C for 1 min, pre-annealed at400 °C for 10 min in air, and then heated to 650 °C by rapidthermal annealing in an oxygen flow. The ramp rate was 100 °C/s and the heating period lasted 5 min. This coating process wasreferred to as one deposition. The buffer layer and the thinnerPST films were fabricated using 1 deposition, respectively,while the thicker PST films were fabricated using 8 depositionsteps. The thicknesses of the thicker and the thinner PST filmswere about 330 nm and 40 nm, respectively, as determined froma surface profilometer and compared against values taken fromthe scanning electron microscopy cross-section images.

The crystal phase and crystallinity of the PST films wereidentified by an X-ray diffractometer (XRD) with Ni filteredCuKα. The acceleration voltage and current were 40 kV and50 mA, respectively. The chemical composition of the TiO2

buffer layer was investigated using an X-ray photoelectronspectroscopy (XPS) which was carried out with a VG-ScientificESCA-LAB using an Al Kα (1486.6 eV) X-ray sourceoperating at 240 W. The surface morphology of the films wasobserved using an NT-MDT Solver P47-PRO scanning probemicroscope operating in the contact atomic force microscope(AFM) mode. Prior to electrical measurements, Pt was sputtered

on the surface of the films using a mask to produce topelectrodes with an area of 5×10−4 cm2. Dielectric propertieswere measured at room temperature using a HP4294 impedanceanalyzer. The current–voltage (I–V) data were acquired using aKeithley 6517A electrometer as a voltage source and picoam-pere meter. Hysteresis loops of the films were recorded using aRadiant Precision Ferroelectric Analyzer at room temperature.

3. Results and discussion

Fig. 1a and b show the AFM micrographs of the Pt/Ti/SiO2/Sisubstrate and the TiO2 buffer layer on it, respectively. ComparingFig. 1b with Fig. 1a, we can see that the TiO2 layer had appearedon the Pt surface with small grains. Fig. 2 is the XPS spectrum ofthe TiO2 layer deposited on Pt/Ti/SiO2/Si substrate. Photoelectronpeaks for Ti 2p, O 1s and C 1s (at 284.6 eV) were recorded. For Ti2p peak, two pronounced features were observed at bindingenergies of 458.5 and 464.2 eV, evoked by the Ti 2p3/2 andTi 2p1/2states respectively. The measured binding energy of the Ti 2p3/2peak and the splitting of the doublet (5.7 eV) indicated a Ti4+ state[17–19]. These results confirmed that the coating had an almoststoichiometric composition of the TiO2. To clarify the micro-structure of the buffer layer, we also performed the XRD

Fig. 3. XRD patterns of the 20 nm thick TiO2 layer.

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measurement on the TiO2 buffer layer. However, no diffractionpeak was found, even if high X-ray current of 200 mA instead ofthe typical one of 50 mAwas used. However, we may not get theconclusion that the TiO2 buffer layer was amorphous in nature,since it may be too thin to get XRD peaks. To check for this, wedeposited a thicker TiO2 layer (nearly 20 nm) prepared by fourdeposition cycles instead of one. Fig. 3 shows its XRD pattern.The testing condition was 40 kV and 200 mA. It was found thatthis thicker TiO2 buffer layer was crystallized as the rutile (110)and anatase (101) TiO2 peaks appeared in the XRD pattern. Wealso verified that these two peaks became more evident as thethickness of the TiO2 layer increased. Thus, we may concludedthat the TiO2 buffer layer was crystallized in this study. This wasreasonable since the layer was annealed under a high temperatureof 650 °C.

Fig. 1c and d present the AFM micrographs of the 40 nmthick PST40 thin films coated onto the Pt/Ti/SiO2/Si substrateswithout and with the TiO2 buffer layer. It was seen that thecrystal grains in Fig. 1d were more evident than that in Fig. 1c,indicating the better development of the PST on the TiO2 bufferlayer. Additionally, the PST film with the buffer layer wassmoother. The root mean square (RMS) surface roughness ofthe PST film reduced from 18.8 nm (without the buffer layer) to11.4 nm (with the buffer layer).

To further understand the effect of the buffer layer on thecrystallization of the PST films, XRD patterns of the initial40 nm thick PST40 and PST60 films without and with the TiO2

buffer layer were recorded in Fig. 4. It can be seen that the PSTfilms deposited directly on the Pt/Ti/SiO2/Si (100) substratesshowed very weak (100), (110) and (200) peaks, indicating apoor crystallization. When inserting a TiO2 buffer layer, the(100) and (200) diffraction peaks became stronger to a largeextent. These distinct peaks meant that the TiO2 buffer layerimproved the crystallinity of the PST layers obviously, whichwas in good accordance with the observation in the AFMpictures that the initial PST grains on the buffer layer were moreevident.

Fig. 1e and f are the AFM micrographs of the 330 nm thickPST40 films deposited on Pt/Ti/SiO2/Si substrate without andwith TiO2 buffer layer. Both the films showed good uniformity,

Fig. 2. XPS spectrum of the 5 nm TiO2 layer deposited on Pt/Ti/SiO2/Sisubstrate.

smooth and dense microstructure. The average grain size ofboth the films was almost the same as about 60 nm. Comparedwith Fig. 1c and d, it can be noted that the surfaces of the330 nm thick PST films were more even than the initial 40 nmones. We proposed that this was owing to the different growingmechanisms for the first PST layer on Pt (according to an islandmode) and the subsequent layer on PST (according to a layer-island mode) [20]. In addition, it can also be found from Fig. 1eand f that the surface roughness of the PST film with TiO2

buffer layer was better than the one without the buffer layer. TheRMS surface roughness was 8.8 nm for the PST40 film withoutthe buffer layer, and reduced to 5.8 nm for the one with thebuffer layer. Similar result can be observed in PST60 films. Thisindicated that the nucleation of the initial layer had largeinfluence on the surface roughness of the PST film. Fig. 5 is theXRD patterns of the corresponding samples. It was observedthat the microstructure of the 330 nm thick PST40 and PST60films with the TiO2 buffer layer was different from thosewithout the buffer layer. When a thin TiO2 buffer layer wasintroduced between the PST films and the Pt/Ti/SiO2/Sisubstrates, the (l00) diffraction peak became stronger, whilethe (110) peak was relatively weaker, indicating a (l00)preferred orientation appeared in the PST films. Thus, we canconclude that the TiO2 buffer layer influenced the growth mode

Fig. 4. XRD patterns of (a) 40 nm thick PST40 films and (b) 40 nm thick PST60films deposited on Pt/Ti/SiO2/Si substrates without and with TiO2 buffer layer.

Fig. 5. XRD patterns of 330 nm thick (a) PST40 films and (b) PST60 filmsdeposited on Pt/Ti/SiO2/Si substrates without and with TiO2 buffer layer.

Fig. 7. Dielectric constant and loss changing with the applied electric field forthe capacitors having the 330 nm thick PST40 films (a) without and (b) withTiO2 buffer layer, and 330 nm thick PST60 films (c) without and (d) with TiO2

buffer layer.

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of the PST films. Combined with Fig. 4, it was clear that theeffect of the TiO2 buffer layer emerged when the first PST layerwas deposited, and the crystallinity of the 330 nm thick PSTfilms was strongly dependent on that of the initial 40 nm PSTlayers. We note that P. Muralt et al. [14,15] reported that a thinsputtered TiO2 buffer layer leads to (111)-oriented PZT thinfilms. However, in present study the PST films having the TiO2

buffer layer exhibited a (100) preferred orientation. Thedifference may be due to the following two factors: 1) thegrowth modes of the perovskite films in the Sol–Gel and

Fig. 6. Variation of the dielectric constant and loss as a function of frequency forthe capacitors having 330 nm thick (a) PST40 films and (b) PST60 filmsdeposited on Pt/Ti/SiO2/Si substrates without and with TiO2 buffer layer.

sputtering fabrication process may be different; and 2) thesurface state including the microstructure and grain size of theTiO2 buffer layers may also differ from each other. The detailedmechanism for the (100) preferred orientation should be ourfurther study.

For 330 nm thick PST films, we found that the TiO2 bufferlayer improved their electrical properties obviously. Fig. 6shows the variation of dielectric constant and loss as a functionof frequency for PST40 and PST60 films with and without theTiO2 buffer layer. The dielectric constant and loss of the Pt/PST40/Pt and Pt/PST60/Pt capacitors without the TiO2 bufferlayer were 482, 0.040 and 900, 0.067 at 10 kHz, respectively.After inserting the thin TiO2 buffer layer, the room-temperaturedielectric constant of the films increased significantly, while thedielectric loss decreased. At 10 kHz, the dielectric constant andloss factor of the Pt/PST40/TiO2/Pt and Pt/PST60/TiO2/Ptcapacitors were 590, 0.034 and 1126, 0.044, respectively. Notewe took the thickness of the TiO2 layer into account whencalculating the dielectric constant. It was proposed that the (l00)texture of the TiO2 buffered PST films leads to an enhancement

Fig. 8. I–V characteristics for the capacitors having the 330 nm thick PST40films (a) without and (b) with TiO2 buffer layer, and PST60 films (c) without and(d) with TiO2 buffer layer.

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in the in-plane oriented polar axis and thus results in theimprovement of the dielectric properties, similar to the case ofTiO2 buffered BST films [21].

Fig. 7 presents the variations of the dielectric constant andloss as a function of bias voltage for PST40 and PST60 filmswith and without the TiO2 buffer layer. Capacitors with PST40film showed a typical ferroelectric butterfly shape, while thePST60 capacitors showed the absence of splitting in thecapacitance curve, characterizing the paraelectric nature.The tunability was defined as (ε0−εv) /ε0, where ε0 and εvwere the dielectric constant values measured at 10 kHz underzero and 180 kV/cm electric field, respectively. From Fig. 7, itcan be calculated that the tunabilities of the PST40 and PST60film with TiO2 buffer layer were 51.2% and 60.7%, respec-tively. These values were all much higher than those of the filmwithout the TiO2 buffer layer, which was 26.8% and 45.8%,respectively. The enhanced tunability may also be related to(l00) preferred orientation of PST40 and PST60 films due to thepresence of the TiO2 buffer layer.

Fig. 8 is the I–V characteristics for PST40 and PST60 filmswith and without TiO2 buffer layer. The leakage current of boththe PST40 and PST60 filmswith TiO2 buffer layer dropped nearlytwo orders of magnitude, comparing with those of the filmswithout the buffer layer in the testing voltage range between −160and 160 kV/cm, showing much more resistive property. Whenmeasured at 100 kV/cm, the leakage current densities of PST40and PST60 films without the TiO2 buffer layer were4.26×10−4 A/cm2 and 2.96×10−4 A/cm2, which reduced to7.63×10−6 A/cm2 and 1.95×10−6 A/cm2, respectively, as thebuffer layer was introduced. This suggests that the buffer layer iseffective for obtaining insulating PST40 and PST60 films.

The polarization–electric field (P–E) hysteresis loops mea-sured for PST40 and PST60 films with and without the TiO2

buffer layer are displayed in Fig. 9. It can be observed that thehysteresis loops of both the PST60 films with and without TiO2

buffer layer were very slim, due to their paraelectric nature. Butthe PST40 capacitors showedwell hysteresis loops irrespective ofthe TiO2 buffer layer. The 2Pr value (12.9 μC/cm

2) of the PST40

Fig. 9. P–E hysteresis loops for the capacitors having the 330 nm thick PST40films (a) without and (b) with TiO2 buffer layer, and PST60 films (c) without and(d) with TiO2 buffer layer.

film with the buffer layer was increased in comparison with that(8.8 μC/cm2) of the film without the buffer layer, while thecoercive value was not changed. Since the leakage current forthe PST40/TiO2 film was strongly reduced in comparison to thepure PST 40 film, the broadening of the hysteresis loop seemsnot due to the leakage effects. Instead, we proposed again thatthis is due to the improved microstructure in the PST40/TiO2

film [22].

4. Conclusions

In this work, a thin crystallized TiO2 buffer layer wasintroduced between the PST films and Pt/Ti/SiO2/Si substrates,and the microstructure was found to be modified significantlyfor both the thinner (about 40 nm) and thicker (about 330 nm)PST films. In addition, their electrical properties were improvedto a large extent, including the increase of the dielectric constantand its tunability by dc voltage, as well as the 2Pr value of thePST40 films, and the decrease of the dielectric loss and leakagecurrent density. This enhancement of electrical properties wasascribed to the (l00) preferred orientation and smoother surfacein the PST films with the TiO2 buffer layer.

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