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Materials Science and Engineering A 452453 (2007) 715720

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, TaiwancDepartment 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 23 m 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 magnetronsputtering system. The crystallographic and surface properties of the films were characterized by X-ray diffraction (XRD) and atomic force

microscopy (AFM). Results showed that Youngs modulus and the hardness of the films increased as the sputtering power was increased. The

hardness and Youngs 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 [14], due to their

high transparency, piezoelectric properties, wide band-gap and

electro-optical characteristics [58].

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 ofinternal stresses has given rise to serious concerns about the

mechanical properties of sputteredZnOfilms. It hasbeen 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.

Nanoindentationtechniqueshave beendeveloped for probing

mechanical properties, such as the hardness and Youngs 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 researchershave usedthenanoinden-

tation technique to study the indentation-produced deformation

and dislocation mechanismsof bulksingle-crystalZnO [1619],

but the influence of the indentation load, the indentation-loading

time and thecreep behavior duringnanoindentation still warrant

further research and discussion.

In this article, the nanoindentation-induced behavior of poly-

crystalline ZnOthinfilmsdepositedat various sputteringpowers

was investigated. The microstructural properties of ZnO filmswere 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.

doi:10.1016/j.msea.2006.11.008
mailto:[email protected]://dx.doi.org/10.1016/j.msea.2006.11.008http://dx.doi.org/10.1016/j.msea.2006.11.008mailto:[email protected]


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716 T.-H. Fang et al. / Materials Science and Engineering A 452453 (2007) 715720

using a 99.99% pure, 2-diameter Zn target, and was depositedat sputtering powers of 150, 175, 200 and 225 W. The films

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

2 degrees. The morphological properties of the ZnO films

surface were measured by atomic force microscopy (Shimadzu

SPM-9500J2, Japan). Typical scans were taken over an area of

1m 1m 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.

Loadunloading experiments were performed to understand

the effects of different loads. A loading time of 10s, a hold time

of 1 s and an unloading time of 10s were used. The loads rangedfrom1000to3000 N.Forthe loading rate tests,the indentation-

loading and unloading times ranged from 10 to 50s 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 Youngs modulus as a function of

the displacement of the indenter were measured from the

loadingunloading of the indenter. A loadingunloading curveis shown in Fig. 1. In the AFM micrograph of Fig. 1a, trian-

Fig. 1. The indentation loadingunloading 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 Pmax divided by

the actual projected contact area Ac and written as:

H=

Pmax

Ac(1)

In depth-sensing nanoindentation, the composite modulus E* is

calculated by [22]:

E = S2Ac

(2)

where S is the measured stiffness and is a shape constant of

1.034 for the Berkovich tip. Youngs modulus Em is defined by:

Em = (1 2m)

1

E 1

2i

Ei

1(3)

where is Poissons ratio, E the Youngs modulus, and thesubscripts i and m refer to indenter and test material, respec-

tively. Indenter properties used in this studys calculations were

Ei = 1141 GPa and i =0.07 [22] and m is assumed to be 0.3.

In Fig. 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 theenergydissipated into thefilmdueto 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 differentsputtering powers areshown in Fig.2 [20].

In Fig. 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.

In Fig. 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 200W. In addition, the best crystalline struc-

ture appeared at 225 W and did not have different crystalliteorientations of the same phase. The use of sputtering powers

largerthan225W ledto a higherdeposition rate causing thesput-

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 theScherrer formula [23]. The average grain size (D) on the ZnO

films ranged from 34 to 42 nm as shown in Table 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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T.-H. Fang et al. / Materials Science and Engineering A 452453 (2007) 715720 717

Fig. 2. XRD spectra of ZnO thin films for various sputtering powers of: (a) 150,

(b) 175, (c) 200 and (d) 225W.

decrease as the grain size got larger. Since the grains are gener-

ally cone-like in shape, the structure can be characterized by the

surface area of the grains.

An AFM image of a 1m 1m surface region of a ZnOfilm deposited at 225 W is shown in Fig. 3. Cone-like grains

covering the ZnO surface can be seen. The average surface

roughness (Ra) and root-mean-square surface roughness (RMS)

are listed in Table 1. The average surface roughness of the ZnO

films at the sputtering powers of 150, 175, 200 and 225 W were

12.7, 12.0, 12.4 and 12.3nm, respectively. There are no spe-cial statistical differences between these surface roughness for

films formed at different sputtering powers. The smoothest sur-

face was created at the highest sputtering power of 225 W. High

frequency surface acoustic wave devices function much better

when the surface of the film is smoother [24]. This could be very

useful in further SAW application development.

4.2. Investigation of loadingunloading behaviors

Fig. 4(ac) shows the (Ph) curves and creep behavior (pen-

etration depth versus indentation load) of ZnO films deposited

at 225 W and indented at various loads, loading times and hold

times. In this current study, pop-ins were not observed. How-

ever, pop-ins have been observed in previous studies [1518].

Table 1

Grain size (D) and surface roughness (Ra, RMS) of ZnO films for different

sputtering powers

Sputtering powers (W)

150 175 200 225

D 34 5 37 6 40 6 42 7Ra 12.7 1.3 12.0 1.0 12.4 1.0 12.3 0.8RMS 16.4 1.6 15.7 1.1 15.9 1.1 14.3 0.5

Unit: nm.

Fig. 3. Three-dimensional AFM image of a ZnO film for a sputtering power of

225W. The surface region: 1m 1m, Zmax =97nm.

This most likely can be attributed to the preparation methods of

the different films used.

In Fig. 4(a), it was found that as the indentation loads

increased, thepenetration depths, theplastic deformation andthe

area Wp all increased. The deepest plastic depth was produced

under the larger indentation loads. It was also discovered from

theoverlapping loading curves of all three indentation loads that

all the ZnO films surface properties were similar.

The effect of the indentation-loading time on a nanoscale

level is shown in Fig. 4(b). The indentation-loading time was

definedas the time from thebeginningof the indentation loading

to when the maximum indentation load was reached. It can be

seen that theslope oftheloadingcurvedecreased, thepenetration

depthsandthearea Wp both increasedas the indentation-loading

time was increased, i.e. as the loading rate was decreased. These

results could be explained by the dislocation and slip behaviors

causing larger plastic deformation depths to take place at the

longer indentation-loading times.

Thecreepbehaviorat threedifferenthold timescanbe seen in

Fig. 4(c). It was found that the loading curves were quite similar,

but the unloading curves started from slightly greater values of

hmax as the hold times were increased. In this study, the maxi-

mumindentation depth was less than 200nm, i.e. approximately

1/10th of the films thicknessand the mechanical properties were

not influencedby thepresence of thesilicon substrate. At shorterhold times, a slightly steeper unloading curve occurred, which

would reduce the calculated elastic surface deformation and the

area Wp increased as the hold time was increased. This implies

that the longer hold times allowed for more energy to be dissi-

pated and therefore caused the creep behavior to extend further

out, which in turn created the larger indentation deformations

[25].

4.3. Youngs modulus and hardness

Fig.5(a) shows theYoungs modulus andthe hardnessofZnO

thin films measured under different indentation loads. The ZnO


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Fig. 4. Loaddisplacement curves of ZnO films sputtered at 225 W under various indentation: (a) loads, (b) loading times and (c) hold times.

films Youngs modulus rangedfrom 68to 125GPa andthe hard-

ness rangedfrom 4 to 6 GPa. The hardness andYoungs modulus

slightly decreasedas the indentation loads were increased. Simi-larnanoindentationtests forZnOmaterials havebeen performed

by previous studies, such as Lucca et al. [18] and Klopfstein et

al. [19] studied mechanical responses for the chemomechani-

cally polished single-crystal ZnO and obtained that the values

of elastic modulus andhardnessranged from 104to 134GPa and

2 to 7 GPa, respectively. Thegreater values forYoungs modulus

and hardness were achieved at the higher sputtering powers due

to the ZnO films having had better crystalline structures. The

ZnO film deposited at the sputtering power of 225 W appeared

to have had the best mechanical properties in this study. The

previous study showed that the wear volume decreased as the

sputtering power was increased [20]. Thus, the combined data

from this study and that found in ref. [20] show that the films

with higher Eand Hhave greater wear resistance.

The ZnO films Youngs modulus ranged from 61 to 123 GPaandthehardness rangedfrom 4 to 5 GPa at different indentation-

loading times as shown in Fig. 5(b). As the indentation-loading

time was decreased both the Youngs modulus and the hardness

of the films increased. This was due to the longer indentation-

loading times, and leading to dislocation slip to the materials

surface causing a larger degree of plastic deformation.

Youngs modulus and hardness of the thin films under the

different hold times of 30, 60 and 120 s are shown in Fig. 5(c).

Youngs modulus and hardness ranged from 70 to 90 GPa, and

from 4 to 6 GPa, respectively. As seen in Fig. 5(c), the indenta-

tion creep tests showed that the hardness and Youngs modulus

slightly decreased as the hold time was increased. These results


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T.-H. Fang et al. / Materials Science and Engineering A 452453 (2007) 715720 719

Fig. 5. Youngs modulus and hardness for ZnO films subjected to various indentation: (a) loads, (b) loading times and (c) hold times.

might be due to the surface diffusion and reorganization pro-

cesses that occurred at the longer holding times and caused

the larger indentation depths. In addition, the variation of the

hardness with the sputtering powers for different hold times in

Fig. 5(c) is not obvious when the powers range from 175 to225 W. However, it appears a relatively large variation when the

sputtering power of 150 W is used. This because the ZnO film

deposited at sputtering powers of 150 W has a larger roughness

as listed in Table 1, and that induced a higher uncertainty and

error of measurement. In order to reduce the measurement error,

a longer hold time in the experiment may be used, such as 120s.

5. Conclusion

The mechanical properties of ZnO films deposited at vari-

ous sputtering powers were investigated by nanoindentation. At

higher sputtering powers, the crystalline structures of the ZnO

thin films are well ordered with a high (0 0 2)-orientation. The

films exhibited higher quality crystalline structures as the sput-

tering power was increased. At sputtering powers that ranged

from 150 to 225 W during depth-sensing indentation recording,

the Youngs modulus of the films ranged from 61 to 125 GPa andthe hardness ranged from 4 to 6 GPa. In addition, the nanoinden-

tation characteristics affected by the indentation loads, loading

rates and creep behaviors are presented. In summary, the results

confirmed that various sputtering powers played an important

role in the nanomechanical characteristics of the ZnO film and

that the films deposited at 225 W appeared to have the best

mechanical properties.

Acknowledgements

The author would like to thank Mr. Sheng-Rui Jian, Chun-

Chin Chang andShi-ChengLiao for their technicalsupport.This


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work was partially supported by the National Science Council

of Taiwan, under Grant Nos. NSC94-2212-E150-045, NSC94-

2212-E150-046 and NSC95-2221-E-168-008.

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ZnO MagSputt Taiwan MSEA 2007