Quantitative analysis of the oxygen content in BaTiO3 films deposited by PLD using 16O(α,α)16O resonant elastic scattering

Thin Solid Films 340 (1999) 68±71

Quantitative analysis of the oxygen content in BaTiO3 ®lms deposited by PLD using 16O(a ,a ) 16O resonant elastic scattering Wei Li a, b,*, Fei Lu a, Zhi-Guo Liu b, Yong Zhu c, Feng-Xiang Wang a, Xiang-Dong Liu a, Chun-Yu Tan a, Ke-Ming Wang a, b b

a Department of Physics, Shandong University, Jinan 250100, People's Republic of China National Laboratory of Solid State Microstructures, Nanjing University, Nanjing 210093, People's Republic of China c Institute of Physics, Chinese Academy of Sciences, Beijing 100080, People's Republic of China

Received 3 April 1998; accepted 14 August 1998

Abstract The resonant elastic scattering 16O(a ,a ) 16O at near 3.045 MeV is a very powerful tool for determining the oxygen concentration in oxide ®lms. The resonance yield can be transformed into the stoichiometric ratio of oxygen to Ba±Ti as a function of the probed depth. By means of the resonance yield, we can obtain the oxygen concentration in BaTiO3 ®lms. In this work the method was used to analyze the oxygen concentration of the BaTiO3 ®lms deposited by pulsed laser deposition (PLD) at different oxygen ambience. Our experimental results demonstrate that the oxygen concentration in the ®lms deposited by PLD is homogeneous, and the effect of oxygen pressure in the process of deposition on the oxygen concentration in the ®lms is very signi®cant. It is shown in our experiment that 20 Pa is the lowest value of oxygen pressure for forming optimum BaTiO3 ®lms by the PLD method. q 1999 Elsevier Science S.A. All rights reserved. Keywords: Pulsed laser deposition; Rutherford backscattering spectroscopy; Oxygen concentration; Depth pro®ling

1. Introduction Barium titanate (BaTiO3) as a ferroelectric material possesses a variety of useful properties that have many potential applications in thin-®lm devices. The applications of ferroelectric ®lms require that the properties of deposited ®lms are similar to bulk. Thus, ®lm characteristic such as stoichiometry, crystallinity, density, microstructure, and crystallographic orientations are important. Recently, several depositions methods have been used to deposit BaTiO3 ®lms [1±5]. The ®lm characteristics have been investigated. For instance, Srikant et al. [6] and Kaemmer et al. [7] reported the dielectric and ferroelectric properties of BaTiO3 ®lms. Lu et al. [8], Bihari et al. [9] and Okada et al. [10] reported SHG characteristics of BaTiO3 ®lms. Okada's experimental results show that the non-linear optical coef®cients (d-coef®cients) of BaTiO3 ®lms deposited by pulsed laser deposition (PLD) at different ambient oxygen pressures are quite distinct. So, to determine the oxygen content of BaTiO3 ®lms deposited by PLD is signi®cant. Now, some methods used to probe the composition of

®lms, such as, X-ray photoelectron spectroscopy (XPS), Rutherford backscattering spectrometry (RBS), have lower sensitivity to detect lighter elements. Therefore, it is necessary to use more sensitive methods of measuring oxygen concentration. The resonant elastic scattering (RES) of 16O(a ,a ) 16O at near 3.045 MeV has a large scattering cross section and therefore, provides a powerful tool to determine the absolute quantity of oxygen non-destructively [11±13]. This RES was ®rst described by Cameron [14] in 1953. For a long time this resonance was used only for accelerator calibration. Mezey et al. [15] showed that surface and buried native SiO2 layers can be detected by RES. They also used a glancing incident beam to further increase the technique's sensitivity for detecting oxygen. Up to now, this RES has been widely utilized for analyzing oxide thin ®lms [16], in particular high Tc superconducting materials [17±19]. To our knowledge, quantitative analysis of the oxygen concentration in the BaTiO3 ®lms deposited by PLD have not been done. In this paper, we report: (1) the pro®le of oxygen concentration in the BaTiO3 deposited by PLD; (2) the value of oxygen concentration in the ®lms deposited by PLD at different ambient oxygen pressures.

* Corresponding author. 0040-6090/99/$ - see front matter q 1999 Elsevier Science S.A. All rights reserved. PII: S00 40-6090(98)0134 2-X

W. Li et al. / Thin Solid Films 340 (1999) 68±71

2. Principle of measurement 2.1. The ratio of the oxygen to metal element The well known 16O(a ,a ) 16O 3.045 MeV resonance is mostly used to detect the oxygen distribution. The resonant scattering yield can be used to quantify the oxygen content. When an oxide ®lm target is bombarded with alpha particles at the resonance energy, Er, resonant scattering occurs at the surface. If the alpha particles have higher incident energies, the resonant scattering occurs from deeper layers in the oxide ®lm. The resonant yield is obtained by subtracting the non-resonance contribution from the total area under the oxygen peak. This resonant yield provides information on the oxygen±metal element stoichiometry. The total yield Ai at the incident energy Ei is given by   dE ox …1† Ai / Ni n0 …x†= dx Er where Ni is the number of alpha particles incident on the target, n0(x) is the average atomic density of oxygen in the ®lm   dE ox dx Er is the rate of energy loss of the alpha particles in the oxide at the resonant energy Er. If we express the normalized resonant yield (Ai/Ni) by AN and note that   dE ox ˆ nox …x†1Er ox …2† dx Er where n0(x) is the molecular density of the oxide MpOq at the depth x and 1ox Er is the stopping cross section of the alpha particles in the oxide at the energy Er. We have AN / q=1Er ox

…3†

According to Bragg's rule [20], 1Er ox can be expressed in terms of the constituents of each atomic stopping cross section. In our experiment, the SiO2 ®lm was treated as a reference sample for calibration, and the BaTiO3 ®lms were treated as the analyzed samples. For SiO2 ®lm, we have O 2 1SiO ˆ ps 1Si Er 1 qs 1Er Er

…4†

2 AsN / qs =1SiO Er

…5†

where qs =ps ˆ 2. For BaTiO3 ®lms, if we assumed that the atomic ratio of Ba and Ti in the ®lms is an ideal value, Ba=Ti ˆ 1 : 1, similarly we have   Ti O 3 1BaTiO ˆ p 1Ba …6† E r 1 1E r 1 q1E r Er 3 AN / q=1BaTiO Er

…7†

69

Hence, the absolute stoichiometric ratio, r, of oxygen to the elements Ba±Ti in the ®lms is given by   Ti AN 1Ba Er 1 1Er ÿ
AsN ! r ˆ q=p ˆ …8† AN Si ps O 1E r 1 1E r 1 2 s qs AN 2.2. Depth scaling The 16O(a ,a ) 16O resonant energy is known and ®xed at 3.045 MeV. When the incident energy of alpha particles is higher than 3.045 MeV, the alpha particles will enter certain depth in the target, this place corresponds to the position of oxygen resonant peak. When the alpha particles penetrate the oxide layer, their backscattering yield is the Rutherfordtype until they are suf®ciently slowed down to take the place of resonant scattering. Let us denote the incident energy as Ei, the resonant energy as Er, the depth of resonant scattering event as Xi. u 1 and u 2 are the angle between the sample normal and the direction of the incident beam and of the scattered particle, respectively. Then, with the mean energy approximation, we have Ei 2 Er ˆ ‰1 ŠBaTiO3 £N £ dXi ˆ ‰1 ŠBaTiO3 £N £ Xi ‰1 ŠBaTiO3 ˆ



ÿ
ÿ
K 1BaTiO3 E in 1 1BaTiO3 E out cosu1 cosu2

…9†

 …10†

where Ein ˆ 1=2…E 1 Ei †, Eout ˆ 1=2…E1 1 KE†; K is the kinematics factor of oxygen atom and N is the atomic density in oxide ®lms. Using the energy loss ratio method [21], E can be given by ÿ
…11† E ˆ E1 1 aEi =…K 1 a† h ÿ
ÿ
i cosu1 a ˆ 1BaTiO3 KEi =1BaTiO3 Ei cosu2

…12†

Hence, the depth Xi of resonant scattering is expressed by ÿ
…13† Xi ˆ Ei 2 Er =‰1 ŠBaTiO3 £N where the depth Xi is represented in units of mg/cm 2. 3. Experimental procedure Five sets of samples were studied. First, a homogeneous SiO2 ®lm with a thickness of 600 nm grown on a silicon wafer was irradiated as a reference sample. The others are the BaTiO3 ®lms deposited on a polished silicon wafer (100) by PLD at different oxygen ambient. The values of oxygen pressure are 6, 15, 20 and 30 Pa, respectively. All samples were irradiated at normal incidence with alpha particles in the energy range of 3.04±3.13 MeV obtained from the tandem accelerator (General lonex 1.7 MeV). The beam energy was pre-calibrated by

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W. Li et al. / Thin Solid Films 340 (1999) 68±71

Fig. 1. Energy spectrum of the alpha particles scattered at 1658 from the BaTiO3/Si target. Incident energy equals 3.078 MeV. 27

Al(p,g ) 28Si resonance. The alpha particles scattered at 1658 were measured with a surface barrier detector. The spectra were collected by a multichannel analyzer. For each sample, a series of spectra were collected at different incidence energy. 4. Results and discussion A typical energy spectrum of alpha particles scattered at 1658 from the BaTiO3 ®lm deposited by PLD at 20 Pa oxygen pressure is shown in Fig. 1. From this spectrum, the total backscattering yields and its full width at half maximum (FWHM) of Ba and Ti can be obtained, therefore, the atomic ratio of Ba and Ti in the ®lm and the thickness of the ®lm can be calculated [22]. For other samples, using the Rutherford backscattering spectrometries, then, we can obtain the thickness and the atomic ratio of Ba and Ti. The results are listed in Table 1. It shows that the atomic ratio of Ba and Ti in the ®lms is consistent with that of BaTiO3. Fig. 2 is a typical energy spectrum of the alpha particles scattered at 1658 from the SiO2 target. The oxygen resonant peak rides over the background of non-resonant scattering from oxygen. This background comes from elastic scattering by the silicon nuclei in the unoxidized part of the target. The relation between the normalized resonant yields of SiO2 and incident energy of alpha particles is shown in Fig. 3. The depth pro®le of oxygen concentration for the BaTiO3 deposited by PLD at 20 Pa oxygen pressure is shown in Fig.

Fig. 2. Energy spectrum of the alpha particles scattered at 1658 from the SiO2/Si target. Incident energy equals 3.078 MeV.

4. Its shape is ladder-shaped. The reason for the rise at the left side is that the stopping powers of BaTiO3 and SiO2 are different. However, the decline at the right side is formed by alpha particles penetrating through the BaTiO3 ®lm. Where essence is concerned, the rise of the left side and the decline of the right side are caused by our experimental method, and does not represent the actual situation of oxygen concentration in the ®lm. The top side of ladder-shape describes the real situation of oxygen concentration in the ®lm, and corresponds to the depth from 15 to 80 mg/cm 2. The average value of the stoichiometric ratio of O and Ba±Ti is 2.993. From Fig. 4 we have: (1) the oxygen concentration in the ®lm is homogeneous; (2) the stoichiometric ratio of O and Ba±Ti in the ®lm deposited at 20 Pa oxygen pressure is consistent with that of BaTiO3. Fig. 5 describes the effect of oxygen pressure during deposition on oxygen concentration in the BaTiO3 ®lms. The results indicate that when oxygen pressure is lower than 20 Pa, the oxygen atoms in deposited ®lms are de®cient. When oxygen pressure is higher than 20 Pa, the oxygen concentration in deposited ®lms basically is a constant, and equal to the normal value of BaTiO3. In

Table 1 Preparation condition, thickness and elemental composition of BaTiO3 ®lms Sample T (8C) Oxygen pressure (Pa) Thickness (nm) Ba/Ti/O S1 S2 S3 S4

740 740 720 740

6 15 20 30

372 345 215 262

0.999:1:2.675 1.001:1:2.908 1.035:1:2.993 1.034:1:3.001

Fig. 3. Normalized resonant yield of oxygen in SiO2 as a function of incident energy of alpha particles, the vertical scale is in arbitrary units.

W. Li et al. / Thin Solid Films 340 (1999) 68±71

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Our results indicate: (1) the oxygen concentration in the BaTiO3 ®lms deposited by PLD is homogeneous; (2) the effect of oxygen pressure in the deposition process on the oxygen concentration in the ®lms is very marked; (3) when the oxygen pressure is lower than 20 Pa, there is oxygen de®ciency in the deposited ®lms. However, if the oxygen pressure is higher than 20 Pa, the stoichiometric ratio of O and Ba±Ti in the ®lms roughly keeps constant, and is consistent with the normal value of BaTiO3. Acknowledgements

Fig. 5. Stoichiometric ratio (O/Ba±Ti) as a function of oxygen pressure for the BaTiO3 ®lms deposited by PLD at different oxygen ambient.

other words, if oxygen pressure is higher than 20 Pa, the effect of oxygen pressure on oxygen concentration in the ®lms can be neglected. The experimental error in the stoichiometric ratios arises from the errors in determination of the area under the oxygen resonance peak, it can be estimated from the quantitative difference between the upper limit or lower limit and the average value of normalized yields of SiO2 (Fig. 3). This error was found to be 5%. The depth resolution chie¯y comes from the width of the resonance which is 10 keV. The depth resolution corresponding to the resonant width is 6 mg/cm 2 [16]. 5. Conclusion RES of 16O(a ,a ) 16O at near 3.045 MeV is a very powerful tool for determining the absolute quantity of oxygen in oxide ®lms. It has high sensitivity with a depth resolution of 8 mg/ cm 2, and is a non-destructive measurement method. Using this method, we have analyzed the oxygen concentration in BaTiO3 ®lms deposited by PLD at different oxygen ambient.

Fig. 4. Stoichiometric ratio (O/Ba±Ti) as a function of depth for the BaTiO3 ®lm, which was deposited by PLD under 20 Pa oxygen pressure.

We would like to thank Ji-Tian Liu, Jian-Hua Zhang, JuXin Lu, Xiao-Ting Yu, Xiao-Yuan Chen, Zhuang-Chun Wu and Jiang Yin for their help. This work is supported by the foundation of the national key laboratory of solid state microstructures, Nanjing University. References [1] F. Fujimoto, K. Kobaayashi, K. Kubota, Thin Solid Films 169 (1989) 249. [2] K. Iijima, T. Terashima, K. Yamamoto, K. Hirata, Y. Bando, Appl. Phys. Lett. 56 (1990) 527. [3] T. Chiba, K. Itoh, O. Matsumoto, Thin Solid Films 300 (1997) 6. [4] G.M. Davis, M.C. Gower, Appl. Phys. Lett. 55 (1989) 112. [5] L.A. Wills, B.W. Wessels, D.S. Richeson, T.J. Marks, Appl. Phys. Lett. 60 (1) (1992) 6. [6] V. Srikant, E.J. Tarsa, D.R. Clarke, J.S. Speck, J. Appl. Phys. 77 (1995) 1517. [7] K. Kaemmer, H. Huelz, B. Holzapfel, W. Haessler, L. Schultz, J. Phys. D: Appl. Phys. 30 (1997) 522. [8] H.A. Lu, L.A. Wills, B.W. Wessels, et al., Appl. Phys. Lett. 62 (1993) 1314. [9] B. Bihari, J. Kumar, G.T. Stauf, P.C. Van Buskirk, C.S. Hwang, J. Appl. Phys. 76 (1994) 1169. [10] T. Okada, Y. Nakata, H. Kaibara, M. Maeda, Jpn. J. Appl. Phys. L 34 (1995) 1536. [11] G. Battistig, E.F. Kennedy, P. Revesz, J. Gyulai, G. Kadar, J. Gyimesi, G. Drozdy, G. Vizkelethy, Nucl. Instrum. Methods B 15 (1986) 372. [12] B. Blanpain, P. Revesz, L.R. Doolitle, K.H. Purser, J.W. Mayer, Nucl. Instrum Methods B 34 (1988) 459. [13] V.I. Soroka, M.V. Artsimovich, I.Yu. Lobach, et al., Nucl. Instrum Methods B 83 (1993) 311. [14] J.R. Cameron, Phys. Rev. 90 (1953) 839. [15] G. Mezey, J. Gyulai, T. Nagy, E. Kotai, A. Manuba, in: O. Meyer, G. Linker, F. Kappeler (Eds.), Ion Beam Surface Layer Analysis, Plenum Press, New York, 1976, p. 303. [16] B.K. Patnaik, C.V. Barros Leite, G.B. Baptista, E.A. Schweikert, D.L. Cocke, L. Quinones N. Magnussen, Nucl. Instrum Methods B 35 (1988) 159. [17] O. Meyer, F. Weschenfelder, X.X. Xi, G.C. Xiong, G. Linker, J. Geerk, Nucl. Instrum Methods B 35 (1988) 292. [18] M. Watamori, F. Shoji, K. Oura, Jpn. J. Appl. Phys. 33 (1994) 6039. [19] C.C. Chin T. Morishita, Physica C 243 (1995) 373. [20] W.H. Bragg, R. Kleeman, Philos. Mag. 10 (1905) S318. [21] R.F. Lever, in: O. Meyer, G. Linker, F. Kappeler (Eds.), Ion Beam Surface Layer Analysis, Vol. 1, Plenum Press, New York, 1976, p. 111. [22] W.K. Chu, J.W. Mayer M.A. Nicolet, Backscattering Spectrometry, Academic Press, New York, 1978, p. 91.