The influence of excess Bi2O3 on the characteristics of Bi4Ti3O12 thin film Chien-Chen Diao1, Cheng-Fu Yang*2, Kauo-Chun Huang3, and Chien-Min Cheng3 Dept Electronic Engineering, K.Y.I.T., Kaohsiung, Taiwan, R.O.C. Dept Chemical and Material Engineering, N.U.K., Kaohsiung, Taiwan, R.O.C. 3 Dept Electronic Engineering, S.T.U.T., Tainan, Taiwan, R.O.C. 1

2

∗

Corresponding author. Email: [email protected]

ABSTRACT: Bi4Ti3O12 thin film was deposited on ITO glass substrate by R.F. magnetron sputtering method at room temperature for 10~60 minutes using the target of Bi4Ti3O12+ 4wt% Bi2O3 ceramics. The excess Bi2O3 was used to compensate the vaporization of Bi2O3 in the deposition process. A rapid thermal processed with different temperature (450oC-575oC) in oxygen was used to re-crystal the deposition Bi4Ti3O12 thin film. The crystallization and dielectric characteristics of Bi4Ti3O12 thin film were developed with the aid of X-ray diffraction patterns, SEM, and impendence analyzer. KEYWORDS: Bi4Ti3O12, layer structured, ferroelectric 1. Introduction In recent years, Bi4Ti3O12 had been attracting much attention, because of its excellent ferroelectric, piezoelectric, and electro-optic properties. And Bi4Ti3O12 ceramics had been extensively applied in electric industry, as capacitor, memory and sensor [1-4]. Especially the advantage of low leakage current, high dielectric constant and high remnant polarization make it become one of the promising candidates for nonvolatile digital memory. The layer perovskite structured Bismuth compound ferroelectrics has the general formula: An-1Bi2BnO3n+3 , where A is usually a divalent ion, such as Sr , Ba , or Pb, and B is Ti4+ , N5+, or Ta5+, when A=Bi, B=Ti, and n=3, it make Bi4Ti3O12. Bi4Ti3O12 ceramic had a plate-like crystal growth mechanism because of its orthorhombic structure [5], and it had the lattice constants a=5.45Å, b=5.41 Å, c=32.82Å, and the Curie temperature 675oC. The structure could be written as a formula of (Bi2O2)2+(Bi2TiO10)2-, which was constructed by alternative stacking of a triple layer of TiO6 octahedra and a monolayer of (Bi2O2)2+ along c-axis[6], as shown in Fig1. A number of methods had been successfully developed to prepare Bi4Ti3O12 thin film, such as metalorganic chemical vapor deposition (MOCVD) [7], sol-gel processing [8], pulsed laser deposition (PLD) [9], and R.F. sputtering [10]. Bi2O3 was a unstable species similar to PbO and it was easy to evaporate during deposition processing in elevated temperature [11], and mostly excess Bi source was added in the process of manufacturing Bi4Ti3O12 thin film [11,12]. In this study, we combined the R.F. sputtering and rapid thermal annealing (RTA) process, Bi4Ti3O12 + 4wt% Bi2O3 thin film, which was sintered at 950oC and used as source target, was deposited on substrate without heating, then annealed by RTA process. Random oriented Bi4Ti3O12 thin film was successfully fabricated on ITO glass substrate, and the effect of Bi4Ti3O12 thin film with various annealing temperature and time was investigated. 2. Experiment HOYA NA-40 glass coated with Indium-Tin Oxide (ITO) of 0.2µm thickness is chosen as the substrate in this study. Sheet resistance and transmittance of the ITO films was 15Ω/□ and larger than 80%, respectively. ITO glass substrates were ultrasonically cleaned in acetone, 2-Propanal and D.I. water for 20 minutes, respectively, dried by pure N2 and baked in oven at 110oC for 30mins before film deposition. Bi4Ti3O12 + 4wt% Bi2O3 thin films were deposited on

ITO glass substrate by R.F. magnetron sputtering system, and part area of the ITO glass was covered by glass to form bottom electrode. The deposition conditions are shown in Table1. Table1. Typical growth condition of Bi4Ti3O12 + 4wt% Bi2O3 thin film Target

Bi4Ti3O12 + 4 wt% Bi2O3 (2 inches)

Substrate

0.7mm thickness, 15Ω/□ ITO glass

Substrate temperature

room temperature

Atmosphere

pure Ar

Base pressure

5x10-6 Pa

Work gas pressure

1.5Pa

Deposition time

10~60 min

Subsequently, these resultant films were annealed by various temperature and period with heating rate of 900oC /min and an oxygen atmosphere in a rapid thermal annealing furnace. For electrical measured, Al was evaporated on the surface of the thin films, forming the diameter of 0.05mm, 100nm-thick top dot electrodes. The crystal structure of the films was identified by X-ray diffraction (XRD) with Cu-kα wavelength. Surfaces and cross-section microstructures of the thin films were observed using field emission scanning electronic microscopy (FE-SEM), the grain size and thickness of BTO thin film were also estimated from the SEM results. The composition of thin film was examined by energy dispersive spectrometer (EDS). Electrical characterization was performed to determine the dielectric and ferroelectric properties. HP 4194 frequency impedance analyzer was used to measure capacitance- voltage (C-V) and dielectric loss with DC-bias from –5V to +5V at 1k Hz, and the relative dielectric constant εr was calculated from the capacitance at zero DC-bias. 3. Results and discussion Fig.2 shows the micrographs of deposited Bi4Ti3O12+ 4wt% Bi2O3 thin film as a function of deposited time and annealing at 550oC for 10 min. For short deposited time, 10 min and 20 min, the annealing surface reveals a non-uniform surface. This may be caused by that short deposited time, the thickness of deposited Bi4Ti3O12+ 4wt% Bi2O3 material was not enough for shrinkage and crystallization. For longer deposited time, 40 min and 60 min, the annealed Bi4Ti3O12+ 4wt% Bi2O3 thin film shows a uniform grain growth. This result suggests that the deposited time is important factor will influence the characteristics of deposited Bi4Ti3O12+ 4wt% Bi2O3 thin film and at least 40 min deposited time is needed. Fig.3 shows the cross section observation of the deposited Bi4Ti3O12+ 4wt% Bi2O3 thin film, the deposited time is 60 min, and annealed at 550oC for 10 min. As Fig.3 shows, the thickness of the deposited Bi4Ti3O12+ 4wt% Bi2O3 thin film is about 0.8µm, and the annealed film shows a densifed structure. Fig.4 shows the X-ray diffraction patterns of deposited Bi4Ti3O12+ 4wt% Bi2O3 thin film, as a function of annealing temperature and annealing time =10 min. As Fig.4 shows, the crystal intensities of Bi4Ti3O12 phase and ITO phase increase with the increase of annealing temperature. This result suggest that higher annealing temperature is, the more densified Bi4Ti3O12+ 4wt% Bi2O3 thin film will be and the thickness of Bi4Ti3O12 thin film will be decreased. For that the crystallization of Bi4Ti3O12 phase will be better, and the crystal intensity of ITO phase will also increase. The variations of the relative dielectric constant and dielectric loss of deposited Bi4Ti3O12+ 4wt% Bi2O3 thin film by different annealing temperatures are shown in Fig5, the annealing time is 10 min. The dielectric constant increase with the increase of annealing temperature of 550oC, then slightly decreases in the 575oC-annealed film. The dielectric

loss increases with the increase of annealing temperature and ranged 6.4~20 x 10-3. Even the dielectric loss increases with the increase of annealing temperature, but the fabricated film is enough for real use. 4. Conclusions Bismuth titanate Bi4Ti3O12+ 4wt% Bi2O3 thin film was deposited on ITO glass substrate by the combination of R.F. magnetron sputtering and rapid anneal process. In this study, the deposited Bi4Ti3O12+ 4wt% Bi2O3 thin film has the dielectric characteristics of dielectric constant 100~275 and dielectric loss 6.4~20 x 10-3. For the optimum deposited and annealing condition are: deposited time =60min, annealing temperature =550oC, and annealing time 10~15 min. References [1] M.J.Forbess, S.Seraji, Y.Wu, C.P.Nguyen and G.Z.Cao, Appl.Phys.Lett. 76 (2000) 2934. [2] X.Y.Zhang, Z.Z.Huang, H.L.W.Chan, K.W.Kwok and C.L.Choy, J.Europ.Ceram.Soc. 19 (1999) 985. [3] Y.Noguchi, M.Miyayama, AND T.Kudo, Appl.Phys.Lett. 77 (2000) 3639. [4]Y.TOrii, K.Tato, A.T.Suzuki, H.J.Hwang and S.K.Dey, J.Mater. Sci.Lett. 17 (1998) 827. [5] Masaki Yamaguchi, Takao Nagamoto, thin solid films 348 (1999) 294. [6] Wei F.Yao , Hong Wang , Xiao H.Xu, Materials Letters 57 (2003) 1899. [7] J.Si.S.B.Desu, J.Appl.Phys.73(11) (1993) 7910. [8] L.B.Kong,J.Ma, Thin Solid Films 379 (2000) 89. [9] Minoru Noda, Toshiyuki Nakaiso, Kentaro Takarabe, Journal of Crystal Groth 237-239 (2002) 478. [10] Masaki Yamaguchi, Takao Nagamoto, Osamu Omoto, thin solid films 300 (1997) 299. [11] T.J.Bukowski, T.P.Alexander, D.R.Uhlmann, 07803-3355-1, 1996 IEEE p589-592. [12] G.D.Hu, J.B.Xu, I.H.Wlison,W.Y.Cheung, 0-7803-4959-8/98, 1998 IEEE, p163-166.

Fig1 The schematic crystal structure of Bi4Ti3O12.

(a)

(b)

(c)

(d)

Fig. 2 The micrographs of Bi4Ti3O12+ 4wt% Bi2O3 thin film as a function of deposited time and annealing at 550oC for 10 min. (a) 10min, (b) 20min, (c) 40 min, and (d) 60min.

(ITO)

550oC

Intensity

(1,1,7)

Fig.3 The cross section of the deposited Bi4Ti3O12+ 4wt% Bi2O3 thin film.

525oC

500oC

450oC

20

30

40

50

60

2θ value Fig.4 The X-ray diffraction patterns of deposited Bi4Ti3O12+ 4wt% Bi2O3 thin film, as a function of annealing temperature and annealing time =10 min.

22

250 16

150

10

50

Dielectric loss (x 10-3)

Dielectric constant

x dielectric constant + dielectric loss

4

450

500

550

600

Annealing temperature(oC)

Fig.5 The dielectric constant and dielectric loss of deposited Bi4Ti3O12+ 4wt% Bi2O3 thin film. The deposited time = 60min, and the annealing time =10 min.