Materials Science and Engineering A 452–453 (2007) 715–720
Nanoindentation characterization of ZnO thin films
Te-Hua Fang
a, Win-Jin Chang
b,∗, Chao-Ming Lin
caInstitute of Mechanical and Electromechanical Engineering, National Formosa University, Yunlin 632, Taiwan bDepartment of Mechanical Engineering, Kun-Shan University, Tainan 710, Taiwan
cDepartment of Mechanical Engineering, WuFeng Institute of Technology, Chiayi 621, Taiwan Received 14 August 2006; received in revised form 24 October 2006; accepted 3 November 2006
Abstract
The effects of the indentation load, indentation-loading time and the creep behavior of 2–3m thick ZnO films deposited on a Si(1 0 0) substrate were investigated by nanoindentation. The ZnO thin films were deposited under different sputtering powers by a radio frequency magnetron sputtering system. The crystallographic and surface properties of the films were characterized by X-ray diffraction (XRD) and atomic force microscopy (AFM). Results showed that Young’s modulus and the hardness of the films increased as the sputtering power was increased. The hardness and Young’s modulus slightly decreased as the indentation rate and creep time were increased. The best ZnO film mechanical properties were found at a sputtering power of 225 W.
© 2006 Elsevier B.V. All rights reserved.
Keywords: Thin films; X-ray diffraction; Mechanical properties
1. Introduction
Recently, ZnO thin films have increasingly been used for various technological applications in sensor, surface acous-tic wave (SAW) and piezoelectric devices[1–4], due to their high transparency, piezoelectric properties, wide band-gap and electro-optical characteristics[5–8].
Many different deposition techniques, such as spray pyrol-ysis[9], pulsed laser deposition[10], sputtering method[11], metal organic chemical vapor deposition (MOCVD)[12]and molecular beam epitaxy (MBE) [13]have been developed to prepare ZnO thin films.
Each method has its relative advantages for certain applica-tions. Among them, the sputtering method is one of the most commonly used techniques due to several advantages, such as low substrate temperature, high deposition rate, uniform surface and excellent crystalline orientation. However, the build-up of internal stresses has given rise to serious concerns about the mechanical properties of sputtered ZnO films. It has been a chal-lenge to understand what effects the load, the loading rate and the creep behavior have on the mechanical properties of these films. The condition that will subject the ZnO thin films to
tri-∗Corresponding author. Fax: +886 2050883.
E-mail address:[email protected](W.-J. Chang).
bological interaction is caused by rubbing damage during the piezoelectric component manufacture.
Nanoindentation techniques have been developed for probing mechanical properties, such as the hardness and Young’s modu-lus of thin films[14]. Mayo et al. studied the effect of grain size on the hardness strain-rate sensitivity of nanocrystalline bulk ZnO and showed that lower sintering temperatures, which pro-vide finer grain sizes, tended to promote strain rate sensitivity [15]. Recently, a number of researchers have used the nanoinden-tation technique to study the indennanoinden-tation-produced deformation and dislocation mechanisms of bulk single-crystal ZnO[16–19], but the influence of the indentation load, the indentation-loading time and the creep behavior during nanoindentation still warrant further research and discussion.
In this article, the nanoindentation-induced behavior of poly-crystalline ZnO thin films deposited at various sputtering powers was investigated. The microstructural properties of ZnO films were investigated by X-ray diffraction (XRD) and atomic force microscopy (AFM). The influence on a nanometer-scale that the indentation loads, the indentation-loading time, the creep behavior and the sputtering power had on the deposited films were also investigated.
2. Experimental details
In this study, ZnO thin films were produced on a Si(1 0 0) substrate by a radio frequency magnetron sputtering system, 0921-5093/$ – see front matter © 2006 Elsevier B.V. All rights reserved.
716 T.-H. Fang et al. / Materials Science and Engineering A 452–453 (2007) 715–720
using a 99.99% pure, 2-diameter Zn target, and was deposited at sputtering powers of 150, 175, 200 and 225 W. The film’s thickness was measured using a stylus profiler (Tencor Alpha Step 200, USA). The thicknesses of the films were between 2 and 3m. The detailed growth conditions of the ZnO films have been reported in a previous study[20].
X-ray diffraction (Rigaku D/MAX-RA, Japan) was used to analyze the crystallographic structure of the ZnO films. Crys-tallographic orientation was determined by XRD rotation over θ–2θ degrees. The morphological properties of the ZnO film’s surface were measured by atomic force microscopy (Shimadzu SPM-9500J2, Japan). Typical scans were taken over an area of 1m × 1 m at a constant scan speed of 2 m/s.
The mechanical properties of ZnO thin films were charac-terized by nanoindentation (Hysitron Triboscope, USA) using a Berkovich diamond indenter with a radius of 50 nm[21]. All indentation tests were performed at room temperature.
Load–unloading experiments were performed to understand the effects of different loads. A loading time of 10 s, a hold time of 1 s and an unloading time of 10 s were used. The loads ranged from 1000 to 3000N. For the loading rate tests, the indentation-loading and unindentation-loading times ranged from 10 to 50 s at a constant load of 1000N, the hold time was constant at 1 s. Hold time creep behavior experiments were performed using hold times of 30, 60 and 120 s at the peak load and kept at a constant load of 1000N using loading and unloading times of 10 s.
3. Analysis
The hardness and Young’s modulus as a function of the displacement of the indenter were measured from the loading–unloading of the indenter. A loading–unloading curve is shown inFig. 1. In the AFM micrograph ofFig. 1a,
trian-Fig. 1. The indentation loading–unloading curve and the associated ZnO thin film AFM indentation image.
gular indent can be clearly seen, with plastic behavior pile-up around the indentation. The hardness of a material is defined as its resistance to plastic deformation. Thus, hardness H is determined from maximum indentation load Pmaxdivided by the actual projected contact area Acand written as:
H =PmaxAc (1)
In depth-sensing nanoindentation, the composite modulus E*is calculated by[22]:
E∗= πS
2β√Ac (2)
where S is the measured stiffness andβ is a shape constant of 1.034 for the Berkovich tip. Young’s modulus Emis defined by: Em= (1 − ν2 m) 1 E∗ − 1− νi2 Ei −1 (3) where ν is Poisson’s ratio, E the Young’s modulus, and the subscripts i and m refer to indenter and test material, respec-tively. Indenter properties used in this study’s calculations were Ei= 1141 GPa andνi= 0.07[22]andνmis assumed to be 0.3.
InFig. 1, the area under the unloading curve represents the elastic energy deformation and is represented by the area des-ignated as We. The area between the loading and unloading curve represents the energy dissipated into the film due to plastic deformation and is represented by the area designated as Wp.
4. Results and discussion
4.1. Structural and surface characterizations
The XRD patterns for the ZnO films deposited on Si(1 0 0) substrates at different sputtering powers are shown inFig. 2 [20]. InFig. 2, the intensities of the (0 0 2)-orientation can be seen to have been enhanced as the sputtering power was increased, indicating that the crystalline film is more uniformly oriented at higher sputtering powers.
InFig. 2(a), the film deposited at 150 W had the poorest crys-tallinity. As the sputtering powers increased, the (0 0 2)-peaks of ZnO films became sharper. Base on the XRD data, the ZnO films started to gain better crystallization when the sputtering power was above 200 W. In addition, the best crystalline struc-ture appeared at 225 W and did not have different crystallite orientations of the same phase. The use of sputtering powers larger than 225 W led to a higher deposition rate causing the sput-tered atoms to pile-up on the surface of the film and to not have enough time to diffuse. The film also exhibited poor substrate adhesive properties at sputtering powers greater than 225 W.
The average grain size (D) of the ZnO films was calculated from the full width at half maximum (FWHM) of the XRD (0 0 2)-peak at around a diffraction angle of 34.4◦, using the Scherrer formula[23]. The average grain size (D) on the ZnO films ranged from 34 to 42 nm as shown inTable 1. It was found that the size of the grains increased slightly as the sputtering power was increased and the surface roughness appeared to
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work was partially supported by the National Science Council of Taiwan, under Grant Nos. 2212-E150-045, NSC94-2212-E150-046 and NSC95-2221-E-168-008.
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