The characteristics of transparent conducting Al-doped zinc oxidethin films deposited on polymer substrates

J. Y. Kao • C. Y. Hsu • G. C. Chen •

D. C. Wen

Received: 19 September 2011 / Accepted: 12 December 2011 / Published online: 4 January 2012

� Springer Science+Business Media, LLC 2011

Abstract Al-doped zinc oxide (AZO) transparent, con-

ductive thin films were deposited on inexpensive polyeth-

ylene terephthalate substrates, using radio frequency (rf)

magnetron sputtering, with an AZO ceramic target (the

Al2O3 content is approximately 2 wt%). This paper pre-

sents an effective method for the optimization of the

parameters for the deposition process for AZO thin films

with multiple performance characteristics, using the

Taguchi method, combined with grey relational analysis.

Using the Taguchi quality design concept, an L9 orthogonal

array was chosen for the experiments. The effects of var-

ious process parameters (rf power, substrate-to-target dis-

tance, substrate temperature and deposition time) on the

electrical, structural, morphological and optical properties

of AZO films were investigated. In the confirmation runs,

using grey relational analysis, the electrical resistivity of

the AZO films was found to have decreased from

5.0 9 10-3 to 1.6 9 10-3 X-cm and the optical transmit-

tance was found to have increased from 74.39 to 79.40%.

The results demonstrate that the Taguchi method combined

with grey relational analysis is an economical way to

obtain the multiple performance characteristics of AZO

films with the fewest experimental data. Additionally, by

applying an Al buffer layer, of thickness 10 nm, the results

show that the electrical resistivity was 3.1 9 10-4 X-cm

and the average optical transmittance, in the visible part of

the spectrum, was approximately 79.12%.

1 Introduction

Transparent conducting oxides (TCO) on a glass substrate

are used in various devices, such as liquid crystal displays,

energy efficient windows and transparent electrodes,

because of their excellent electrical and optical properties

[1, 2]. Tin-doped indium oxide (ITO) is the most popular

commercial TCO material, because of its excellent elec-

trical and optical performance, but indium is toxic, rare and

expensive, so the development of alternative TCO mate-

rials is essential [3]. The advantages of zinc oxide (ZnO)

are its low material cost, environmental friendliness, wide

energy band gap (*3.3 eV), high crystallinity and stability

in hydrogen plasma processes, as compared to ITO films

[4]. In addition, ZnO can be doped with a wide variety of

ions, to meet the demands of several fields of application.

Non-doped ZnO usually has a high resistivity, due to a

low carrier concentration. In order to increase the electrical

conductivity and transparency and to stabilize the film at

high temperatures, ZnO is commonly doped with Group III

elements (B, Al, Ga, or In) [5]. Of these impurity-doped

ZnO films, Al-doped ZnO (AZO) films have a wider band

gap, good optical characteristics and lower resistivity,

compared with ITO [6]. Several deposition techniques have

been used to grow AZO films. Sputtering is considered to

be a suitable technique for the preparation of AZO films,

because it is inexpensive and offers good uniformity of

deposition, over large areas [7].

Traditional experimental methods would be too com-

plicated and difficult to use. These methods require a large

number of experiments, when the number of process

parameters increases [8]. The Taguchi method offers an

efficient way to increase experimental efficiency [9]. The

mixed orthogonal table in the Taguchi quality design

derives important deposition factors. A statistical analysis

J. Y. Kao � C. Y. Hsu � G. C. Chen

Department of Mechanical Engineering, Lunghwa University

of Science and Technology, Taoyuan, Taiwan, ROC

D. C. Wen (&)

Department of Mechanical Engineering, China University

of Science and Technology, Taipei, Taiwan, ROC

e-mail: dcwen@cc.cust.edu.tw

123

J Mater Sci: Mater Electron (2012) 23:1352–1360

DOI 10.1007/s10854-011-0598-0

of signal-to-noise (S/N) ratio is followed by an analysis of

variance (ANOVA) [10]. Grey relational analysis can be

used to optimize the complicated inter-relationships

between multiple performance characteristics [11]. Grey

relational analysis provides a measurement method for the

analysis of the relationship between sequences, using less

data and multiple factors, which is more helpful for sta-

tistical regression analysis.

TCO films deposited on polymer substrates have many

merits, compared to those deposited on a glass substrate;

these include their lighter weight, smaller volume, better

impact resistance, cheapness and ease of transport and

flexibility [12]. To realize flexible devices such as flexible

displays and electronic paper, TCO on polymer substrates

are requisite [13]. In this study, AZO thin films were

deposited on flexible polyethylene terephthalate (PET)

substrates, using conventional rf magnetron sputtering.

Grey relational analysis [14] was used to investigate mul-

tiple performance characteristics in the Taguchi design, in

order to optimize the deposition process with a limited

number of experiments. The effects of deposition param-

eters on the electrical, structural, morphological and optical

properties of AZO films were determined. An L9 (34, with

four columns and nine rows) orthogonal array was

employed. Four influential deposition parameters were

selected: rf power, substrate-to-target distance, substrate

temperature and deposition time, each of which was

assigned high, medium and low levels, as shown in

Table 1. Additionally, the effects of an Al buffer layer on

the structure and properties of AZO films are also reported.

2 Experimental

Thin AZO films and Al buffer were deposited on inex-

pensive PET substrates, using a magnetron sputtering

system. The rf (13.56 MHz) and dc power were applied to

a ceramic oxide AZO target (purity; 99.99%) and metal Al

target (purity; 99.95%), respectively. The diameter and

thickness of the AZO and Al targets were 50.8 mm and

6 mm, respectively. The amount of Al2O3 doping in the

ZnO powder ranged from 2 to 5% [15, 16]. A commer-

cially available, hot-pressed and sintered disc of ZnO was

mixed with 2 wt% Al2O3 (Elecmat, USA). The sputtering

was carried out with a deposition pressure of 20 mTorr, in

a pure argon atmosphere. The substrate was rotated at

20 rpm. Table 2 lists the sputtering conditions for the Al

buffers; the direct current (dc) power was 150 W and the

pulse frequency and pulse time were 30 kHz and 3 ls,

respectively. The Al buffer thicknesses were measured to

be 3 and 10 nm. Before deposition, AZO and Al targets

were pre-sputtered with Ar plasma, for 5 min, in order to

remove any contamination. All of the PET substrates were

ultrasonically cleaned in acetone, rinsed in de-ionized

water and blow-dried with nitrogen. The magnetron sput-

tering system was microprocessor controlled.

The structural properties and crystallite size were

determined using X-ray diffraction (Rigaku-2000 X-ray

Generator) using Cu Ka radiation (40 kV, 30 mA and

k = 0.1541 nm), with a grazing incidence angle of 1o. The

scanning rate was 5�/min. The surface morphologies were

analyzed using field emission scanning electron micros-

copy (SEM, JEOL, JSM-6500F). The sheet resistance was

measured by the four-point probe method (Mitsubishi

Table 1 Deposition process

parameters and their levelsSubstrate Polyethylene terephthalate (PET)

Target 98 wt% ZnO, 2 wt% Al2O3 (99.99% purity)

Gas Argon (99.995%)

Base pressure 5.0 9 10-6 Torr

Substrate rotate vertical axis 20 rpm

Deposition pressure 20 mTorr

Symbol Factor Level 1 Level 2 Level 3

A rf power (W) 80 100 120

B Substrate-to-target distance (mm) 70 85 100

C Substrate temperature (oC) 40 80 120

D Deposition time (min) 30 40 50

Table 2 Experimental conditions for Al buffer

Target Al; 99.95%, purity

Gas Argon (99.995%)

Base pressure 5.0 9 10-6 Torr

Substrate rotate vertical axis 20 rpm

Deposition pressure 20 mTorr

Substrate-to-target distance 85 mm

dc power 150 W

Pulse frequency 30 kHz

Pulse time 3 ls

Al buffer thickness 3 and 10 nm

J Mater Sci: Mater Electron (2012) 23:1352–1360 1353

123

chemical MCP-T600). Film thickness was measured using

a surface profilometer (a-step, AMBIOS XP-1). The optical

transmittance measurement was performed with a UV–VIS

spectrophotometer, for wavelengths ranging from 300 to

800 nm.

3 Results and discussion

Figure 1 shows the X-ray diffraction patterns of AZO films

grown on PET substrates (samples No. 1 to No. 9 of the L9

orthogonal array). The experimental results show that there

is no significant change in orientation for films deposited

under different deposition parameters. It is clear that, for all

of the AZO films, only the (0 0 2) diffraction peak, located

at 2h * 34.2� was observed, demonstrating that the films

have a hexagonal ZnO wurtzite structure and a preferred

orientation along the c-axis, perpendicular to the substrate

surfaces. For orthogonal arrays No. 8 (A3B2C1D3), the

diffraction peaks were more intense and the full widths at

half maximum (FWHM) were relatively narrow. This is

caused by the increase in crystallite size and the

improvement in the crystallinity of the films. Table 3

shows the experimental results for electrical resistivity and

the corresponding S/N ratios for AZO grown on PET.

Table 4 lists ANOVA results for electrical resistivity. The

contribution ratio for each parameter can be seen in this

table. It can be seen that the substrate-to-target distance has

a dominant effect on electrical resistivity, with a contri-

bution ratio of almost 83%. Figure 2 shows the S/N

response graph for electrical resistivity, which demon-

strates that a substrate-to-target distance of 85 mm pro-

duces lower electrical resistivity. Figure 3 shows the SEM

micrographs of the AZO thin films, obtained using the

experimental conditions for Nos. 1, 2 and 3. It is apparent

that the grain sizes become larger, for a substrate-to-target

distance less than the optimal distance of 85 mm. These

Fig. 1 XRD patterns of AZO films grown on PET substrates, for the

L9 orthogonal array from No. 1 to No. 9 (b: full width at half

maximum, FWHM)

Table 3 Experimental results for electrical resistivity and S/N ratio,

for AZO grown on PET substrates

Experiment

no.

Control factors Resistivity (910-3 X-cm) S/N (dB)

A B C D R1 R2

1 1 1 1 1 54.4 58.6 -35.05

2 1 2 2 2 5.1 7.5 -16.14

3 1 3 3 3 53.1 59.7 -35.04

4 2 1 2 3 76.2 78.2 -37.75

5 2 2 3 1 5.1 7.3 -15.98

6 2 3 1 2 63.0 67.4 -36.29

7 3 1 3 2 10.3 14.3 -21.91

8 3 2 1 3 3.8 6.2 -14.22

9 3 3 2 1 60.9 66.9 -36.12

Table 4 ANOVA results for electrical resistivity of AZO grown on

PET substrates

Factor Degree of

freedom

Sum of

square

Variance Contribution

(P %)

A 2 58.40 29.20 6.95

B 2 692.78 346.39 82.52

C 2 52.33 26.17 6.23

D 2 36.07 18.04 4.30

Total 8 839.58 419.80 100

S/N

rat

io o

f re

sist

ivit

y (d

B)

12010080

-15

-20

-25

-30

-35

1008570 1208040 504030

A B C D

Fig. 2 S/N graph for AZO electrical resistivity. A rf power (W),

B substrate-to-target distance (mm), C substrate temperature (oC),

D deposition time (min)

1354 J Mater Sci: Mater Electron (2012) 23:1352–1360

123

results are consistent with the XRD observation. This is

similar to the results achieved by Assuncao et al. [17]. The

highest values for the Hall mobility are obtained for the

optimal substrate-to-target distance. This behavior can be

ascribed to the increase in the crystallite sizes, which yields

a lower value for resistivity. Atomic force micrographs of

the surface morphology of the films deposited at experi-

mental conditions for Nos. 1, 2 and 3 are shown in Fig. 4. It

can be seen that the film is crack-free and smooth. Grains

are tightly packed, and the roughness diminution is related

to grain size reduction. Considering the importance of high

transmission in the visible range, the effect of the deposi-

tion parameters on the optical properties of the AZO films

was investigated. Table 5 shows the experimental results

for the optical transmittance in the visible region and the

corresponding S/N ratios of the AZO films grown on PET.

Fig. 3 SEM micrographs of AZO thin films, obtained using the

experimental conditions for Nos. 1, 2 and 3, a experimental condition

No. 1, substrate-to-target distance of 70 mm, with grain size of

20.27 nm, b experimental condition No. 2, substrate-to-target

distance of 85 mm, with grain size of 24.47 nm and c experimental

condition No. 3, substrate-to-target distance of 100 mm, with grain

size of 19.75 nm

Fig. 4 AFM images of the films corresponding to Fig. 3 a experi-

mental condition No. 1, Ra = 4.59 nm; b experimental condition No.

2, Ra = 4.63 nm; c experimental condition No. 3, Ra = 4.32 nm

J Mater Sci: Mater Electron (2012) 23:1352–1360 1355

123

For comparison purposes, the optical transmittance of the

PET substrate was approximately 81.57%, also presented.

Table 6 lists ANOVA results for optical transmittance,

which show that the rf power has the dominant effect on

AZO optical transmittance, with a contribution ratio of

almost 49.80%. Figure 5 shows the S/N response graph for

the optical transmittance, which indicates that the optical

transmittance of the AZO films on PET gradually decrea-

ses, as rf power is increased. Similar behavior was

observed by Kuo et al. [18]. It is possible that the greater

thickness of the films deposited using higher rf powers

causes increased scattering, reflection and optical absorp-

tion in the films, which causes a greater degree of surface

roughness and greater amounts of amorphous content.

Deposition time also influences the AZO deposition rate,

with a contribution ratio of about 26.33%.

In order to optimize the deposition parameters with

reference to electrical resistivity and optical transmittance,

an analysis of multiple performance characteristics was

initiated. Grey relational analysis can be used to effectively

determine the complicated interrelationships between

multiple performance characteristics. The grey relational

coefficient is [14]

rðx0ðkÞ; xiðkÞÞ

¼min

imin

kx0ðkÞ � xiðkÞj j þ n max

imax

kx0ðkÞ � xiðkÞj j

x0ðkÞ � xiðkÞj j þ n maxi

maxk

x0ðkÞ � xiðkÞj j

ð1Þ

where xiðkÞ is the normalized value of the kth performance

characteristic, in the ith experiment, n is a distinguishing

coefficient, n 2 ½0; 1�. The value of n can be adjusted,

according to actual system requirements. The coating

parameters all have equal weighting, in this paper, so n is

0.5.

The grey relational grade is a weighting-sum of the grey

relational coefficient. It is defined as follows [14]:

rðx0; xiÞ ¼1

n

Xn

k¼1

rðr0ðkÞ; xiðkÞÞ ð2Þ

Using Eqs. (1) and (2), the complicated multiple

performance characteristics can be converted into a

single grey relational grade. The grey relational grade for

each experiment, using the L9 orthogonal array, is shown in

Table 7. Experiment No. 2 exhibits the highest grey

Table 5 Experimental results for optical transmittance, in the visible

region, and the S/N ratio of AZO grown on PET substrates

Experiment

no.

Control factors Transmittance (%) S/N (dB)

A B C D T1 T2

1 1 1 1 1 79.08 79.50 37.98

2 1 2 2 2 78.10 78.50 37.88

3 1 3 3 3 77.51 78.17 37.82

4 2 1 2 3 75.51 76.15 37.60

5 2 2 3 1 77.71 78.01 37.83

6 2 3 1 2 79.02 79.26 37.97

7 3 1 3 2 74.10 74.16 37.40

8 3 2 1 3 74.17 74.61 37.43

9 3 3 2 1 77.56 77.99 37.82

For comparison purposes, the optical transmittance of the PET sub-

strate, in the visible region, was approximately 81.57%

Table 6 ANOVA results for optical transmittance of AZO grown on

PET substrates

Factor Degree of

freedom

Sum of

square

Variance Contribution

(P %)

A 2 0.1903 0.0951 49.80

B 2 0.0716 0.0358 18.74

C 2 0.0196 0.0098 5.13

D 2 0.1006 0.0503 26.33

Total 8 0.3821 0.1910 100

S/N

rat

io o

f tr

ansm

itta

nce

(dB

)

12010080

37.9

37.8

37.7

37.6

37.51008570 1208040 504030

A B C D

Fig. 5 S/N graph for AZO transmittance. A rf power (W), B sub-

strate-to-target distance (mm), C substrate temperature (oC), D depo-

sition time (min)

Table 7 Grey relational grade and its order in the optimization

process

Experiment

no.

Grey relational

grade

Order

1 0.7060 3

2 0.8439 1

3 0.5264 7

4 0.3803 9

5 0.8056 2

6 0.6599 5

7 0.5826 6

8 0.6725 4

9 0.5051 8

1356 J Mater Sci: Mater Electron (2012) 23:1352–1360

123

relational grade, indicating that the optimal process

parameter set of (A1B2C2D2) has the best multiple

performance characteristics of the nine experiments. The

mean value of the grey relational grade, for each deposition

process parameter level, is shown in Fig. 6. It can be seen

that the predicted optimal process parameter set is

(A1B2C1D2), based on the grey relational analysis.

Once the optimal deposition process parameter set was

obtained, the confirmation tests were run, to verify the

improved characteristics. The results of the confirmation

experiment were compared with the outcome of the

orthogonal array and the grey theory prediction of the design

operating parameters. Table 8 shows the multiple perfor-

mance characteristics for the orthogonal array and the grey

relational optimal predicted deposition process parameters,

for AZO films. When the grey theory prediction (A1 B2 C1

D2) is compared with the orthogonal array process param-

eters (A1 B2 C2 D2), it can be seen that the AZO electrical

resistivity decreases from 5.0 9 10-3 to 1.6 9 10-3 X-cm

and optical transmittance increases from 74.39 to 79.40%

(including the PET substrate), respectively.

Figure 7 also shows the XRD diffraction patterns for the

AZO films. Using the parameters of the grey theory pre-

diction, it can be seen that the (0 0 2) diffraction peaks

become sharper and their FWHM become narrower

(FWHM is reduced from 0.340� to 0.322�, for AZO films).

This demonstrates that the crystalline structure of the films

is improved and the crystallite sizes increase. The larger

crystallite size results in lower density for grain boundaries,

so these grain boundaries behave as traps for free carriers

and barriers for carrier transport. This leads to an increase

of conductivity, due to the increase in both carrier con-

centration and Hall mobility [19].

Zhang et al. [20] showed that the incorporation of a

buffer layer, between the film and the substrate, improved

the crystalline quality and structure of the film. Bang et al.

[21] found that the surface morphology and structural and

optical properties of the films depended on the thickness of

the buffer layer. For this study, the Al thin film was grown,

at room temperature, on PET substrates, as a buffer layer.

The dc power was 150 W, the pulse frequency and pulse

time was 30 kHz and 3 ls. The thicknesses of the Al

buffers were 3 and 10 nm (experimental conditions are

listed in Table 2) and the AZO films were deposited using

the parameter set predicted by the grey theory (A1B2C1D2).

Experimental results show that the electrical resistivity of

the AZO films was 1.6 9 10-3, 7.0 9 10-4 and 3.1 9

10-4 X-cm, for Al buffer thicknesses of 0, 3 and 10 nm,

respectively. Figure 8 shows the XRD patterns of the AZO

films, for Al buffer thicknesses of 3 and 10 nm. For buffer

thicknesses of 10 nm, the diffraction peaks of the AZO

films became sharper and more intense, and correspond

with the SEM micrographs for AZO films grown on Al/

PET shown in Fig. 9. As mentioned above, the AZO grain

size increased with increased buffer thickness, and the

electrical resistivity was reduced.

S/N

rat

io o

f gr

ey r

elat

iona

l gra

de (

dB)

12010080

-2

-3

-4

-5

-61008570 1208040 504030

A B C D

Fig. 6 Grey relational grade graph

Table 8 Results of confirmation experiment, for multiple perfor-

mance characteristics, for the orthogonal array and optimal predicted

deposition parameters

Orthogonal

array

Grey theory

prediction

design

Improvement

rate (%)

Level A1B2C2D2 A1B2C1D2

Resistivity (10-3 X-cm) 5.0 1.6 68.0

Transmittance (%) 74.39 79.40 6.7

20 25 30 35 40 45 50 55 60

Inte

nsit

y (a

.u.)

(b)

AZO (002)

2θ (degree)

b:0.322

b:0.340

(a)

Fig. 7 X-ray diffraction spectrum for AZO films grown on PET

substrates, a orthogonal array parameters (A1B2C2D2) and b grey

theory prediction (A1B2C1D2) (b: full width at half maximum,

FWHM)

J Mater Sci: Mater Electron (2012) 23:1352–1360 1357

123

In the deposition process for AZO/Al/PET, the buffer

layer between the film and the substrate causes a decrease in

resistance, improving the crystalline quality of the film [22].

It is also possible that, since the Al has a high diffusion

coefficient and it can migrate quickly into the AZO film,

during the deposition process [23], the existence of the Al

buffer increases the carrier concentrations and reduces the

resistance of the film. For high transmittance, the thickness

of the metal layer (Au, Ag and Al) is not allowed to exceed

a certain threshold. For thicker Ag layers (10 nm), reduced

transmission is due to absorption in the aggregated Ag film

[23]. As the thickness of the metal layer increases, the

transmittance decreases and reflection increases, as the film

becomes a mirror [24]. Considering the effect of a metallic

buffer layer, Kim et al. [25] reported that an Au buffer, of

10 nm thick, had an optical transmission of 71%, at

550 nm. In Fig. 10 shows our experimental results, for

optical transmittance. The PET substrate was approxi-

mately 81.57% (left side) and the AZO films, grown on Al

10 nm/PET, was approximately 79.12% (right side). The

visibility of the lettering through the AZO (grown on Al

10 nm/PET) film demonstrates high transmittance. The

experiments were repeated three times and the experimental

results were highly reproducible. Figure 11 shows the TEM

images of the AZO crystalline particles and the selected-

area electron diffraction (SAED) pattern. The SAED pattern

is composed of a series of rings, which confirm that of the

AZO film was typical wurtzite single crystalline, with the

(002) plane parallel to the substrate surface [13].

The AZO films deposited on PET substrates showed

very good adherence. There was no crack or peel off

observed after deposition. Figure 12a shows a schematic

diagram for the pull-off adhesion test between PET sub-

strate and AZO thin film [26, 27]. A steel bar, of diameter

5 mm, was used for the test. Adhesive epoxy (3 M Scotch-

Weld, Epoxy adhesives DP-460) was applied between the

steel bar and the AZO thin film. Figure 12b shows the

photograph of AZO films (zone 1), which was the original

peel-test zone, and zone 2, a circular region which shows

the fractured surface, after mechanical testing of the AZO

film. The pulling test was done at room temperature and

atmospheric pressure, using a motor crosshead speed of

1 mm/min. Tests were repeated three times, for each

sample. Table 9 shows the adhesion strength of AZO films,

as measured by a pull test, with and without Al buffer

layers. This demonstrates that the Al buffer layer increases

the adhesive strength of the AZO thin films.

20 25 30 35 40 45 50 55 60

Inte

nsit

y (a

.u.)

(b)

AZO (002)

2θ (degree)

b:0.320

b:0.317

(a)Al (111)Al (200)

Fig. 8 X-ray diffraction spectrum for AZO films grown on Al/PET: a10 nm Al buffer layer, b 3 nm Al buffer layer (b: full width at half

maximum)

(a) 10 nm Al buffer layer

(b) 3 nm Al buffer layer

Fig. 9 SEM micrographs for AZO films grown on Al/PET corre-

sponding to Fig. 8. a 10 nm Al buffer layer, b 3 nm Al buffer layer

1358 J Mater Sci: Mater Electron (2012) 23:1352–1360

123

4 Conclusions

Conductive and transparent AZO films were grown on

flexible PET substrates, with and without an Al buffer

layer, using magnetron sputtering. The effects of changing

selective deposition parameters (rf power, substrate-to-

target distance, substrate temperature and deposition time)

on the electrical, structural, morphological and optical

properties of AZO films were investigated. In the confir-

mation runs, when using the results from grey relational

analysis (A1 B2 C1 D2), electrical resistivity was improved

by 68.0%, visible range transmittance was improved by

6.7% and the (0 0 2) diffraction peaks became sharper, with

a narrower FWHM, showing that the crystalline structure

of the AZO films is improved as the crystallite sizes

increases. The electrical resistivity of AZO film with Al

buffers of 0, 3 and 10 nm thickness was 1.6 9 10-3,

7.0 9 10-4 and 3.1 9 10-4 X-cm, respectively. The opti-

cal transmittance decreased slightly, from 81.57% (for PET

substrate) to 79.12% (for AZO films grown on Al 10 nm/

PET). For AZO films deposited on PET substrates, with

and without an Al buffer layer, the peel-off stresses were

studied. The experimental results show that the Al buffer

layer increases the adhesive strength of the AZO thin films.

Fig. 10 The optical transmittance for the PET substrate was

approximately 81.57% (left side) and that for AZO film grown on

Al 10 nm/PET was approximately 79.12% (right side)

Fig. 11 TEM images of AZO thin film and SAED pattern of sample

Fig. 12 a Scheme of the pull-off test device, b photograph of AZO

films (zone 1) and fracture surface (zone 2), after mechanical test

(950)

Table 9 Adhesive strength of AZO films, measured by pull test, with

and without Al buffer layers

Maximum

loading (kgf)

Maximum loading

displacement (mm)

Peel off

stress

(MPa)

AZO/Al 3 nm/PET 12.4 0.102 21.35

AZO/Al 10 nm/PET 13.6 0.122 22.01

AZO no buffer layer 10.1 0.096 12.61

J Mater Sci: Mater Electron (2012) 23:1352–1360 1359

123

This technique greatly simplifies the procedure for opti-

mization of multiple performance characteristics, as veri-

fied by experiment.

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