APPLIED PHYSICS LETTERS
VOLUME 82, NUMBER 3
20 JANUARY 2003
Ferroelectric properties of Bi3.25La0.75Ti3 O12 thin films grown on the highly oriented LaNiO3 buffered PtÕTiÕSiO2 ÕSi substrates Jiwei Zhai and Haydn Chena) Department of Physics and Materials Science, City University of Hong Kong, Kowloon, Hong Kong, China
共Received 24 September 2002; accepted 29 November 2002兲 Bi3.25La0.75Ti3 O12 共BLT兲 thin films were grown on Pt/Ti/SiO2 /Si and on LaNiO3 共LNO兲 buffered Pt/Ti/SiO2 /Si substrates using sol-gel processing. Scanning electron micrographs showed the BLT films are composed of peg-like or platelet-like grains depending upon annealing temperature and the substrate type. Large platelet grains were found in BLT films deposited on the LNO/Pt/Ti/SiO2 /Si substrates; those thin films showed better polarization–voltage, capacitance–voltage, and current– voltage characteristics. More importantly, they did not show any significant fatigue up to 2⫻1010 switching cycles at a frequency of 1 MHz and electric field 85 kV/cm. © 2003 American Institute of Physics. 关DOI: 10.1063/1.1539928兴
Among many valuable applications of ferroelectric thin films are nonvolatile memories and multifunctional components in microelectronic and optoelectronic devices. Recently, lanthanum-substituted bismuth titanate 共BLT兲 has received much attention because of its improved fatigue behavior over the classical Pb(Zr,Ti)O3 共PZT兲 capacitors. As a fatigue-free material, BLT is of particular interest because it can be crystallized at relatively low processing temperatures below 650 °C, making it more compatible with Sibased IC technology. Fatigue-free BLT films have been grown on Pt/Ti/SiO2 /Si substrates using a number of techniques including pulsed laser deposition,1 metalorganic chemical vapor deposition 共MOCVD兲,2 and sol-gel methods.3– 6 It is well known that the fatigue problem in PZT films can be largely reduced with the use of metal oxide electrodes. However, these electrodes often lead to undesirably large leakage current. The purpose of this study is to use LaNiO3 共LNO兲 as the metal oxide bottom electrode and at the same time to provide a template to grow better quality BLT films with preferred orientation. Our results demonstrate promising properties better than reported data. BLT thin films were prepared by sol-gel process. The targeted nominal composition was Bi3.25La0.75Ti3 O12. The precursor solutions were bismuth nitride, and titanium isopropoxide Ti(OC3 H7 ) 4 . The Bi(NO3 ) 3 •5H2 O and La(NO3 ) 3 •6H2 O were dissolved into glacial acetic acid. Excess 6 mol % Bi precursor was added to compensate Bi evaporation during annealing. The solutions were mixed and the required amount of Ti precursor was slowly added under continuous stirring. Finally, 2-methoxyethanol and ethylene glycol were added to control the viscosity and crack of films. The spin coating technique was employed to deposit the precursor solution on the LNO/Pt/Ti/SiO2 /Si and Pt/Ti/SiO2 /Si substrates at 3000 rpm for 20 s. The corresponding thickness of LNO, Pt, Ti, and SiO2 were 150, 150, 50, and 150 nm, respectively. LNO has a pseudocubic perovskite structure (a⫽0.384 nm) and n-type metallic behavior with the surface resistivity as low as 225 ⍀ cm for 共001兲-preferred thin
films.7 They are prepared by magnetron sputtering technique. Before BLT coating, the LNO-buffered substrates were annealed at 800 °C for 30 min. This process is to promote grain growth and texture in the LNO layer, thereby facilitating growth of good quality BLT films on top of it. XRD scan of annealed LNO-buffered substrate showed highly preferred 共001兲 orientation 关Fig. 1共a兲兴. The LNO layer not only serves as a useful metal oxide bottom electrode, but also forms a template with preferred 共001兲 orientation to enable growth of high quality BLT films. The spun-coated wet BLT films were first dried in air at 300 °C on a hot plate for 5 min, and then moved to a high annealing temperature furnace set between 450 and 700 °C for 20 min to crystallize the amorphous films. The processes were repeated several times to achieve
a兲
FIG. 1. XRD patterns of BLT films deposited on 共a兲 LNO/Pt/Ti/SiO2 /Si, and 共b兲 Pt/Ti/SiO2 /Si substrates.
Author to whom correspondence should be addressed; electronic mail:
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Appl. Phys. Lett., Vol. 82, No. 3, 20 January 2003
FIG. 2. Surface morphology of BLT thin films deposited on Pt/Ti/SiO2 /Si substrates annealed at 共a兲 600 °C, 共b兲 700 °C and deposited on LNO/Pt/Ti/SiO2 /Si substrates annealed at 共c兲 700 °C, and 共d兲 crosssectional SEM micrograph of sample 共c兲.
desired film thickness. For the electrical measurements top gold electrodes of 200⫻200 m2 were made by dc sputtering. Electrical properties and fatigue tests were carried out using an Agilent 4284A LCR meter and a Radiant Technology RT66 ferroelectric tester. Phase identification and degree of crystallinity of the films were studied by powder x-ray diffraction, whereas SEM was used to determine the film thickness and the surface morphology. XRD patterns for BLT films undergone annealing at various temperatures are shown in Fig. 1 for 共a兲 LNO/Pt/Ti/SiO2 /Si and 共b兲 Pt/Ti/SiO2 /Si substrate, respectively. It is observed that all BLT films show amorphous structure after low temperature 共e.g., at 450 °C) annealing; they become polycrystalline above 550 °C. Amorphous phase continues to exist for films grown on Pt/Ti/SiO2 /Si substrate at as high as 650 °C annealing, whereas for films grown on LNO-buffered substrates a complete crystallization seems to have occurred above 550 °C. The degree of c-axis preferred orientation can be determined by comparing a series of (00l) Bragg peaks to the 共117兲 peak; the latter is the most dominant peak in BLT with random orientation. At 550 °C annealing, the XRD scans taken from both types of substrates show similar intensity pattern to that of Bi4 Ti3 O12 powder with no obvious preferred orientation. In the case of BLT films on Pt/Ti/SiO2 /Si, the intensity of 共006兲 peak notably increases after annealing at 700 °C. For the same condition the BLT films grown on LNO/Pt/Ti/SiO2 /Si substrate show an even stronger 共006兲 peak, indicating an improved c-axis preferred orientation. The surface morphology of BLT films on Pt/Ti/SiO2 /Si substrates is illustrated in Fig. 2 after annealing at 共a兲 600 °C, 共b兲 700 °C, and 共c兲 700 °C depostied on LNObuffered substrate. There is an obvious grain growth with annealing temperature. Larger platelet like grains are observed for the BLT films deposited on the LNO/Pt/Ti/SiO2 /Si substrates after a 700 °C anneal, as shown in Figs. 2共c兲 and 2共d兲. The large platelet grains have been reported,4 but were found only after annealing at higher temperatures 共e.g., 900 °C). The platelet BLT microstructure found at lower annealing temperature, e.g., 700 °C in our study, is attributed to the use of the LNO-buffered Pt/Ti/SiO2 /Si substrates. Hysteresis loops of 700 °C annealed BLT thin films are
J. Zhai and H. Chen
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FIG. 3. P – E hysteresis loops of films deposited on 共a兲 LNO/Pt/Ti/SiO2 /Si, and 共b兲 Pt/Ti/SiO2 /Si substrates annealed at 700 °C, measurements were made at 1 kHz.
shown in Fig. 3. It can be seen that BLT films on the LNO/Pt/Ti/SiO2 /Si substrates have clearly saturated P – E loops with larger remanent polarization, P r , and lower coercive field, E c 共Fig. 4兲. This behavior can be ascribed to the larger and platelet-like grains of the BLT films. The polarization switching in films with finer grains is usually more difficult. The domain walls in larger and platelet grains are easier to be switched under external field, while smaller domains in peg-like shape seem more stable. Similar phenomena have been found in SBT and PT thin films.8,9 It has been reported10 that, in Bi4 Ti3 O12 material, the spontaneous polarization vector lies the a–c plane 共using orthorhombic indexing兲. It is reasonable to expect that the ferroelectric BLT show similar polarization orientation. Therefore, for BLT films with preferred c-axis orientation, such as the ones grown on LNO/Pt/Ti/SiO2 /Si substrates films, a lower E c than those grown on plain Pt/Ti/SiO2 /Si substrates is consistent with the expectation. The current–voltage 共or current–field兲 characteristics have also been studied because the leakage current is one of the most important properties for applications. Figure 5 shows J – V curves of the thin films. The leakage current density of BLT on LNO/Pt/Ti/SiO2 /Si is only about 1
FIG. 4. P r and E c of 700 °C annealed BLT thin films as function of applied field. Downloaded 14 May 2003 to 144.214.25.22. Redistribution subject to AIP license or copyright, see http://ojps.aip.org/aplo/aplcr.jsp
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Appl. Phys. Lett., Vol. 82, No. 3, 20 January 2003
J. Zhai and H. Chen
FIG. 5. J – E characteristics of BLT on 共a兲 LNO/Pt/Ti/SiO2 /Si, and 共b兲 Pt/Ti/SiO2 /Si substrates annealed at 700 °C.
⫻10⫺7 A/cm2 at 200 kV/cm, which is comparable to the films prepared on Pt/Ti/SiO2 /Si. Moreover, no dielectric break down was detected up to 550 kV/cm. The dielectric constant shows dielectric dispersion, typical of thin films, and low losses at frequencies less than 1 MHz. The dielectric constant, , of the films deposited on LNO/Pt/Ti/SiO2 /Si and Pt/Ti/SiO2 /Si, respectively, were 128 and 84 at 10 kHz, with a dielectric loss of 0.024 and 0.019. The fatigue characteristics of the BLT capacitor are displayed in Fig. 6. The cycling frequency and cycling field 共square waveform兲 were kept at 1 MHz and 85 kV/cm, respectively. P * is the switched polarization between two opposite polarity pulses and P ∧ is the nonswitched polarization between the same two polarity pulses. The P * – P ∧ or – P * – ( – P ∧ ) denote the switchable polarization, which is an important variable for nonvolatile memory application. The BLT thin films deposited on LNO/Pt/Ti/SiO2 /Si show only 7% change in the switching polarization up to 2⫻1010 switching cycles, but for BLT thin films deposited on Pt/Ti/SiO2 /Si substrates there is a 18% change. In order to examine the effects of fatigue frequency and switching voltage, the polarization fatigue test of the BLT thin films deposited on both types of substrates were carried out under a cycling field of 250 kV/cm and bipolar pulses of frequencies 1, 10, 100, and 1000 kHz. It was found that the fatigue behaviors are independent of fatigue frequency and switching field. In conclusion, Bi3.25La0.75Ti3 O12 thin films deposited on highly oriented LaNiO3 -buffered Pt/Ti/SiO2 /Si substrates by
FIG. 6. Switchable polarization P * – P ∧ and – P * – ( – P ∧ ) of the BLT thin films on 共a兲 LNO/Pt/Ti/SiO2 /Si, and 共b兲 Pt/Ti/SiO2 /Si substrates, as a function of switching cycles.
sol-gel process exhibited very good ferroelectric and fatigue properties. The platelet-like grains obtained on the LNO/Pt/Ti/SiO2 /Si substrates were instrumental to the desirable properties. The BLT thin films have good polarization– voltage, capacitance–voltage, and current–voltage characteristics. More importantly, the BLT thin films did not show any significant fatigue up to 2⫻1010 switching cycles at a frequency of 1 MHz and electric field 85 kV/cm. This research was supported by a grant from the City University of Hong Kong under Project No. of 9380015. 1
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