Nanomechanical characteristics of BaxSr1−xTiO3 thin films

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Materials Science and Engineering B 131 (2006) 281–284

Short communication

Nanomechanical characteristics of

Ba

x

Sr

1 −x

TiO

3

thin films

Sheng-Rui Jian

a

, Win-Jin Chang

b,∗

, Te-Hua Fang

c

,

Liang-Wen Ji

d

, Yu-Jen Hsiao

e

, Yee-Shin Chang

d

a_{Department of Electrophysics, National Chiao Tung University, Hsinchu 300, Taiwan} b_{Department of Mechanical Engineering, Kun Shan University, Tainan 710, Taiwan} c_{Institute of Mechanical and Electromechanical Engineering, National Formosa University,}

Yunlin 632, Taiwan

d_{Department of Electro-Optics Engineering, National Formosa University,}

Yunlin 632, Taiwan

e_{Department of Materials Science and Engineering, National Cheng Kung University,}

Tainan 701, Taiwan

Received 29 December 2005; received in revised form 7 March 2006; accepted 18 March 2006

Abstract

The nanomechanical properties of barium strontium titanate (BaxSr1−xTiO3, x = 0–1) deposited on a silicon substrate were investigated using the

metalorganic deposition method. The characteristics of the BaxSr1−xTiO3thin film’s crystalline structure and surface roughness were achieved by

means of X-ray diffraction (XRD) and atomic force microscopy (AFM). The results indicated that as the Ba content was decreased and therefore the Sr content was increased a lower leakage current density and a smaller grain size were obtained. Also the mechanical properties such as hardness, Young’s modulus and contact stress–strain were also studied by nanoindentation.

Keywords: BST; XRD; AFM

Perovskite ferroelectric thin films, barium strontium titanate (BaxSr1−xTiO3, BST) have been of great interest for their use in

dynamic random access memories (DRAM), tunable microwave devices, infrared sensors and electro-optical devices[1–5]. BST thin films’ electrical and optical properties, such as, high dielec-tric constant, large electro-optical coefficient and low optical losses are critical for these applications[6–8].

Despite extensive investigations of several different proper-ties of BST thin films, there is still lacking sufficient research systematically correlating the mechanical and structural rela-tionships on a nanometer-scale for designing advanced opto-electronic devices. It is essential that studies continue on the mechanical characterization of these thin films to recognize how the relative parameters, such as Ba content, affect the material structures and the properties for use in advanced applications.

∗_{Corresponding author.}

E-mail address:changwj@mail.ksu.edu.tw(W.-J. Chang).

Modern nanoindentation testing equipment allows penetra-tion depth curves to be measured as a funcpenetra-tion of load where the loads extend down to the range of micro-Newtons, resulting in typical penetration depths in the range of nanometers. This technique is well suited for examining the mechanical properties of thin films[9,10].

In the research presented here, the microstructure and mechanical properties of BaxSr1−xTiO3thin films of different

Ba/Sr composition ratios, produced by use of the metalorganic decomposition (MOD) method, have been studied.

In this study, barium acetate Ba(CH3COO)2, strontium

acetate Sr(CH3COO)2 and Ti-isopropoxide Ti(OC3H7)4 were

used as the starting materials. An optimum ratio of acetic acid and ethylene glycol, Rac/eg= 3:1, was selected as the

sol-vent. Fomamide was selected as an additive to adjust the solu-tion’s viscosity in order to reduce the cracks in the deposited BST thin films. Barium acetate and strontium acetate with a molar ratio of x:1− x (x = 0–1) was completely dissolved into heated glacial acetic acid and then ethylene glycol was

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284 S.-R. Jian et al. / Materials Science and Engineering B 131 (2006) 281–284

Fig. 4. Surface roughness, Ra() and R.M.S. (), as a function of Ba content for BST thin films.

Fig. 5. Load-penetration depth curves for BST thin films at a maximum load of 50␮N.

Fig. 6. Hardness and Young’s modulus of BST thin films measured as a function of Ba content.

Fig. 7. Contact stress–strain relationship of BST thin films.

Ba0.5Sr0.5TiO3film can bear a larger amount of stress and have

a lower surface strain.

Acknowledgement

This work was partially supported by the National Science Council of Taiwan, under Grant Nos. NSC 94-2218-E-150-045 and NSC 94-2212-E-168-004.

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