Mechanical and transparent conductive properties of ZnO and Ga-doped ZnO filmssputtered using electron-cyclotron-resonance plasma on polyethylene naphtalatesubstratesHousei Akazawa

Citation: Journal of Vacuum Science & Technology A 32, 021503 (2014); doi: 10.1116/1.4831979 View online: http://dx.doi.org/10.1116/1.4831979 View Table of Contents: http://scitation.aip.org/content/avs/journal/jvsta/32/2?ver=pdfcov Published by the AVS: Science & Technology of Materials, Interfaces, and Processing Articles you may be interested in Optical and electrical properties of transparent conducting B-doped ZnO thin films prepared by variousdeposition methodsa) J. Vac. Sci. Technol. A 29, 041504 (2011); 10.1116/1.3591348 Modification of transparent conductive ZnO and Ga-doped ZnO films by irradiation with electron cyclotronresonance argon plasma J. Vac. Sci. Technol. A 29, 031304 (2011); 10.1116/1.3571603 Combinatorial characterization of transparent conductive properties of Ga-doped ZnO films cosputtered fromelectron cyclotron resonance and rf magnetron plasma sources J. Vac. Sci. Technol. A 28, 314 (2010); 10.1116/1.3328053 Effect of fluorine addition on transparent and conducting Al doped ZnO films J. Appl. Phys. 100, 063701 (2006); 10.1063/1.2347715 Transparent and conductive Ga-doped ZnO films grown by low pressure metal organic chemical vapordeposition J. Vac. Sci. Technol. A 15, 1063 (1997); 10.1116/1.580430

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Mechanical and transparent conductive properties of ZnO and Ga-dopedZnO films sputtered using electron-cyclotron-resonance plasmaon polyethylene naphtalate substrates

Housei Akazawaa)

NTT Microsystem Integration Laboratories 3-1 Morinosato Wakamiya, Atsugi, Kanagawa 243-0198, Japan

(Received 5 June 2013; accepted 4 November 2013; published 18 November 2013)

Transparent conductive ZnO and Ga-doped ZnO (GZO) films were deposited on polyethylene

naphtalate (PEN) sheet substrates using electron cyclotron resonance plasma sputtering. Both ZnO

and GZO films were highly adhesive to the PEN substrates without inserting an intermediate layer

in the interface. When compared at the same thickness, the transparent conductive properties of

GZO films on PEN substrates were only slightly inferior to those on glass substrates. However, the

carrier concentration of ZnO films on PEN substrates was 1.5 times that of those on glass

substrates, whereas their Hall mobility was only 60% at a thickness of 300 nm. The depth profile of

elements measured by secondary ion mass spectroscopy revealed the diffusion of hydrocarbons out

of the PEN substrate into the ZnO film. Hence, doped carbons may act as donors to enhance carrier

concentration, and the intermixing of elements at the interface may deteriorate the crystallinity,

resulting in the lower Hall mobility. When the ZnO films were thicker than 400 nm, cracks became

prevalent because of the lattice mismatch strain between the film and the substrate, whereas GZO

films were free of cracks. The authors investigated how rolling the films around a cylindrical pipe

surface affected their conductive properties. Degraded conductivity occurred at a threshold pipe

radius of 10 mm when tensile stress was applied to the film, but it occurred at a pipe radius of 5 mm

when compressive stress was applied. These values are guidelines for bending actual devices

fabricated on PEN substrates. VC 2014 American Vacuum Society.

[http://dx.doi.org/10.1116/1.4831979]

I. INTRODUCTION

The family of ZnO materials provides transparent con-

ductive oxide (TCO) electrodes feasible for use in a wide

range of optoelectronic devices. A very recent trend has

been the fabrication of ZnO-based transparent devices on

flexible organic substrates, making a product that is portable,

lightweight, and bendable.1 Polymer substrates employed

for this purpose include polyimide,2 polycarbonate,3,4 poly-

ethylene terephtalate (PET),5–11 polyethylene naphtalate

(PEN),11–14 and polyester.15 The main concern in such appli-

cations is the durability of the conductive and mechanical

properties of the TCO films. It is believed that crystalline

films cannot withstand bending because the crystallites are

destroyed by mechanical stress. Amorphous conductors such

as indium gallium zinc oxide (IGZO) have advantages over

polycrystalline TCO films because of their low process tem-

peratures as well as their flexibility to absorb stress when

bent. While IGZO has been utilized as a channel layer in

thin film transistors,16 its resistivity is rather high for use as

transparent electrodes. Although the most severe treatment

the films will experience is being folded like a newspaper,

enduring such stress is overly demanding. The first barrier

that should be overcome is the bending of TCO films along a

gentle curvature. However, the mechanical properties of

ZnO films on flexible substrates have only recently been

investigated using bending tests.6,11,15

In this paper, we investigate undoped ZnO and Ga-doped

ZnO (GZO) films on PEN substrates in terms of their

mechanical and transparent conductive properties. These

properties are compared to those of ZnO and GZO films on

glass substrates. Since GZO films are suitable for transparent

electrodes because of their low resistivities,17–25 the durabil-

ity of GZO films against mechanical stress is important in a

practical sense. We use electron cyclotron resonance (ECR)

plasma sputtering to deposit these films. It has been well rec-

ognized that when the ZnO family of films are deposited on

polymer substrates, ensuring high adhesive strength is an

essential requirement. In this respect, ECR sputtering is suit-

able since continuous exposure to the plasma stream pro-

motes intermixing of the elemental atoms across the

film/substrate interface. Up to now, however, it has not been

clear whether the transparent conductive properties of ZnO

and GZO films on PEN substrates are similar to those on

glass substrates. We present the exact difference between

them and provide information whether these films deposited

on PEN substrates are useful as transparent electrodes. In

addition, we identify the extent of applied stress at which

conductive properties are maintained to clarify the deforma-

tion limits of flexible substrates.

II. EXPERIMENTAL PROCEDURE

The configuration of our ECR sputtering apparatus com-

bined with a conventional RF magnetron-sputtering gun has

been described elsewhere.26,27 The base pressure in the dep-

osition chamber was 5� 10�5 Pa. The ECR plasma was gen-

erated around the ECR point when microwaves were

transmitted through double silica windows under the applica-

tion of an external magnetic field. The microwave powera)Electronic mail: akazawa.housei@lab.ntt.co.jp

021503-1 J. Vac. Sci. Technol. A 32(2), Mar/Apr 2014 0734-2101/2014/32(2)/021503/7/$30.00 VC 2014 American Vacuum Society 021503-1

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was 500 W and the argon gas pressure was 0.032 Pa.

Applying RF voltage (500 W) to the cylindrical ZnO target

caused sputtering of the Zn and O atoms, which were depos-

ited on a substrate placed in a downstream position. Undoped

ZnO films were deposited by ECR sputtering only from the

ZnO target without introducing any oxygen source gases.26

The GZO films were deposited by simultaneous ECR sputter-

ing from the ZnO target and RF magnetron sputtering from a

planar Ga2O3 target.27 The GZO films analyzed in this paper

had a fixed Ga2O3 content of 5 wt. %. When the composition

at the ZnO target surface reached a steady state after presput-

tering, the deposition rate and the properties of ZnO and

GZO films were highly reproducible. The deposition rate on

a glass substrate was 4.9 nm min�1 for ZnO films and 4.2 nm

min�1 for GZO films. The deposition rate of ZnO films on

the PEN substrate was 1.5 times higher (7.4 nm min�1) while

that of GZO films on PEN was the same as on glass sub-

strates. The PEN substrate (TeonexTM) is a biaxially oriented

polycrystalline sheet (200 lm-thick) supplied by the Teijin

DuPont Co. The glass transition temperature of the PEN sub-

strates is 155 �C, which is slightly higher than the 110 �C of

PET. Although the substrate was not annealed with a heater

from its rear side, illuminating the substrate with ECR plasma

raised the surface temperature to 70 �C, which was well

below the glass transition temperature.

The crystal structures of deposited films were character-

ized by x-ray diffraction (XRD) (Rigaku, RINT1500) using

a Cu Ka line under the Bragg–Brentano configuration.

Optical transmittance spectra in the UV to infrared range

were measured with a spectrophotometer (Shimadzu, UV-

3100), where transmittance of a film with a substrate was

measured with reference to the bare substrate. Electrical

conduction parameters were determined by Hall measure-

ments following the van der Pauw method. The hardness

of the ZnO and GZO films on PEN and glass substrates

was evaluated with a pencil scratch test according to the

JIS K5600-5-4 standard. Here, nine Uni Mitsubishi pencils

from H to 9 H hardness were prepared so that they

scratched the film surface at a fixed angle of 45�. Five

trials were conducted for each hardness level, and the max-

imum hardness number that the pencil did not make

grooves on the film surface was judged as the hardness of

the sample.

The standard way to test the fracture nature of thin films

on flexible substrates is by bending.6,11,28 However, in our

mechanical tests, the strains were applied by rolling. As

depicted in Fig. 1(a), ZnO and GZO films deposited on PEN

substrates were rolled around a cylindrical pipe, maintained

in this position for 1 min, and then unfolded to recover a flat

surface. Resistivities before and after rolling were measured

with a four-point probe. We prepared two specimens with

the same thickness and resistivity for the two test menus in

Fig. 1(b), where the PEN sheet was rolled with the film on

the inside or the outside. The test began using a pipe with a

radius of 20 mm, and the tested sample was transferred to

another test using a pipe with the next smallest radius. Such

measurements were continued until reaching a pipe with a

minimum radius of 2 mm.

III. RESULTS

A. Structural and transparent conductive properties

Figures 2(a) and 2(b) compare the dependence of carrier

concentration (n), Hall mobility (l), and resistivity (q) upon

the thickness of undoped ZnO films on glass and PEN sub-

strates, respectively. The evolution in the electrical transport

parameters of ZnO films on glass substrates is typical

with increasing thickness, wherein both the carrier concentra-

tion and the Hall mobility increase as the film becomes

thicker.29–32 Although the carrier concentration saturates at

200 nm, the Hall mobility further continues to increase until

finally saturating at 300 nm. According to these changes, the

resistivity steeply decreases with increasing thickness and

reaches a steady state at 300 nm. On the other hand, the elec-

trical conduction parameter dependence of ZnO films on PEN

substrates upon the thickness is quite different. For instance,

at a thickness of 300 nm, the carrier concentration of the ZnO

film on a PEN substrate (1.7� 1020 cm�3) is 1.5 times that

of the ZnO film on a glass substrate (1.1� 1020 cm�3).

However, the Hall mobility of the ZnO film on a PEN

substrate (10 cm2 V�1 s�1) is 60% that of the ZnO film on a

glass substrate (18 cm2 V�1 s�1). The difference is particu-

larly significant for very thin films. The Hall mobility of a

27 nm-thick ZnO film on a PEN substrate is only

1.3 cm2 V�1 s�1, while that of a 24 nm-thick ZnO film on a

glass substrate is 8.5 cm2 V�1 s�1. The Hall mobility of the

ZnO film on a PEN substrate saturates at 150 nm, stays nearly

constant up to 440 nm, and suddenly decreases as the film

thickens further. We confirmed that this unusual dependence

resulted from the formation of cracks in the ZnO films.

Inspecting the film surface with optical microscopy revealed

that miniature cracks emerged at thicknesses around 200 nm,

grew to small cracks at 300 nm, and became prevalent at

thicknesses above 440 nm. The developing cracks obviously

depressed the Hall mobility. These cracks form when

the stress due to the lattice mismatch between the

oxygen-deficient ZnO and the PEN exceeds some critical

level. When the films become thicker, the stress builds up and

cracks widen. Once the cracks are created, both the carrier

concentration and the Hall mobility significantly decrease,

which means that cracks affect the carrier generation as well.

We evaluated the distribution of constituent atoms by

means of secondary ion mass spectroscopy (SIMS) to under-

stand the origin of the increased carrier concentration in

FIG. 1. (Color online) (a) Procedure for the mechanical test and (b) the con-

figuration for rolling.

021503-2 Housei Akazawa: ECR sputtered ZnO and GZO TCO films on PEN substrate 021503-2

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ZnO films on PEN substrates compared to that of the films

on glass substrates. The SIMS depth profiles of Zn, O, C,

and Si atoms are compared in Fig. 3 for PEN and glass sub-

strates. The deposition time for both is 1 h. In agreement

with a previous report,33 the profiles of glass substrates have

a sharp cutoff in the distribution of Zn and Si at the interface,

and the incorporation of Si atoms into the ZnO film is negli-

gible. In contrast, a substantial amount of carbon diffuses

into the ZnO film from the PEN substrate and reaches the

ZnO film surface. The concentration of carbon in the ZnO

film on the PEN substrate is more than two orders of magni-

tude higher than the impurity level in the ZnO film on the

glass substrate. Furthermore, the deposition rate of ZnO

films on the PEN substrate is enhanced compared to that of

films on the glass substrate. This can be seen from the fact

that the ZnO film on the PEN is 1.5 times thicker than that

on the glass substrate, although they have a common deposi-

tion time of 1 h (Fig. 3).

Figures 4(a) and 4(b) plot the electrical conduction pa-

rameters of GZO films, respectively, deposited on glass and

PEN substrates. If compared at the same thickness, the car-

rier concentration is comparable in the glass and PEN sub-

strates. However, the Hall mobility of GZO films on a glass

substrate is slightly higher than that on a PEN substrate, sug-

gesting inferior crystallinity on the PEN substrate. The simi-

lar dependence behavior upon thickness is due to the GZO

being fully oxidized with oxygen atoms supplied from the

Ga2O3 target. Reduced ZnO may have a high affinity with

carbon, which can become trapped at oxygen vacancies.

Figure 5 depicts the XRD patterns of ZnO and GZO films

on a PEN substrate. The broad diffraction signal from the

polycrystalline PEN substrate is centered at 2h¼ 27� and

56�. The ZnO(002) at 2h¼ 34� is the primary diffraction

peak for both ZnO and GZO. The crystalline grain size can

be evaluated using Scherrer’s formula and was found to be

16 nm for the ZnO film and 15 nm for the GZO film. Their

shoulder structure at 2h¼ 36� is due to the ZnO(101) signal

contribution. The diffraction peaks from ZnO crystals are ba-

sically the same as those on a glass substrate except for the

lower peak intensities.

Figure 6 plots the evolution of ZnO(002) peak intensities

with increasing thickness. It is obvious that both ZnO and

GZO films on a PEN substrate have inferior crystallinity

compared to those on a glass substrate. Smaller ZnO crystal-

lites are seen on the PEN substrate than those on a glass sub-

strate, partially due to the intermixing of Zn and O atoms

with hydrocarbons supplied by the PEN substrate, which

hinders the nucleation of the ZnO crystalline lattice at an

early growth stage. We found that x-scan rocking curves

tuned to the ZnO(002) peak of ZnO and GZO films on PEN

have a maximum intensity at x angles lower than those cor-

responding to the Bragg reflection angle. Such observations

mean that the crystallites are too small to produce a typical

Bragg-reflection-type rocking curve.

The optical transmittance spectra of ZnO films deposited

on glass and PEN substrates are shown in Figs. 7(a) and

FIG. 2. (Color online) Carrier concentration (n), Hall mobility (l), and resis-

tivity (q) of undoped ZnO films on (a) glass and (b) PEN substrates as a

function of thickness.

FIG. 3. (Color online) SIMS depth profile of elements for ZnO films on

(a) glass and (b) PEN substrates.

021503-3 Housei Akazawa: ECR sputtered ZnO and GZO TCO films on PEN substrate 021503-3

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7(b), respectively, for various film thicknesses; and those of

GZO films on glass and PEN substrates are shown in Figs.

8(a) and 8(b), respectively. These transmittance spectra cor-

respond to the films only, because the contribution from the

substrates has already been subtracted. The higher concen-

trations of carriers in the GZO films markedly reduce the

FIG. 4. (Color online) Carrier concentration (n), Hall mobility (l), and resis-

tivity (q) of GZO films on (a) glass and (b) PEN substrates as a function of

thickness.

FIG. 5. XRD patterns of 660-nm-thick ZnO and 340-nm-thick GZO films on

PEN substrates.

FIG. 6. ZnO(002) XRD peak intensity of (a) ZnO and (b) GZO films on glass

(solid circles) and PEN (open circles) substrates.

FIG. 7. (Color online) Optical transmittance spectra of ZnO films on (a) glass

and (b) PEN substrates with various film thicknesses.

021503-4 Housei Akazawa: ECR sputtered ZnO and GZO TCO films on PEN substrate 021503-4

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optical transmittance in the infrared range. Besides interfer-

ence fringes, many small absorption features assigned to

hydrocarbons are superimposed between the wavelengths of

800 and 1300 nm for both ZnO and GZO. These wavelengths

coincide with that of absorption by CH2 units in polyethylene

chains as well as benzene. Moreover, ZnO films on PEN

reveal a characteristic absorption at 2750–2800 nm. When

comparing the same thickness, ZnO on a PEN substrate is

more photoabsorptive than that on a glass substrate between

400 and 700 nm wavelengths, as well as within the

near-infrared range between 1500 and 2800 nm. In contrast,

there is not as marked an absorption level difference between

the optical transmittance spectra of GZO films on PEN and

glass substrates except for the superposition of hydrocarbon

signals.

B. Mechanical properties

Apart from the formation of cracks, ZnO and GZO films

were tightly bonded to the PEN substrate, and there was no

evidence of delamination. We examined the durability of the

electric conducting properties of crackfree ZnO and GZO

films after they were rolled around pipes. When the PEN

sheet is rolled with the film surface in contact with the pipe,

the film suffers from compressive stress. When the PEN

sheet is rolled with the film surface outside, the film suffers

from tensile stress. Figures 9(a) and 9(b) plot the results of a

mechanical test on ZnO specimens with the thicknesses of

220 and 300 nm, respectively. When compressive stress is

exerted on the ZnO films, the resistivity is actually main-

tained down to a radius of 5 mm, after which the resistivity

increases steeply. When exerting tensile stress, the onset of

degradation occurs at 10 mm. Cairns et al.34 also observed

sharp increases in the resistivity of tin-doped indium oxide

films on PET substrates when the threshold stress was

exceeded. After the film was rolled around the pipe with a ra-

dius of 5 mm, cracks were apparent. Figures 10(a) and 10(b)

plot the results from a rolling test on 130 and 170 nm-thick

GZO samples, respectively. Similar to the ZnO samples, the

resistivity is constant for pipe radii down to 5 mm for com-

pressive stress, whereas degradation occurs for pipe radii less

than 10 mm when tensile stress is exerted. The increased

damage to both ZnO and GZO films during tensile stress

compared with compressive stress agrees with the results

obtained by Ni et al.6 Atomic-level crystal structures probed

by XRD showed no difference before and after folding. We

thus confirm that the formation of cracks in ZnO and GZO

films is responsible for the deteriorated conductivity.

The hardness indices of ZnO and GZO films on PEN sub-

strates evaluated with the pencil scratch test are plotted in

Fig. 11 as a function of thickness. The hardness indices

increase with increasing thickness, and the value for suffi-

ciently thick films converges at 6H for ZnO. However, ZnO

and GZO films on glass substrates showed no effects from

FIG. 8. (Color online) Optical transmittance spectra of GZO films on (a)

glass and (b) PEN substrates with various film thicknesses.

FIG. 9. Results from the rolling test for (a) 220-nm-thick and (b) 300-nm-

thick ZnO films.

021503-5 Housei Akazawa: ECR sputtered ZnO and GZO TCO films on PEN substrate 021503-5

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the scratch test, even with a 9H pencil. Considering that the

hardness becomes high with increasing thickness, the hard-

ness mainly reflects the properties of the substrates rather

than those of the films. We thus conclude that while the ad-

hesion of ZnO and GZO films to the soft PEN substrates is

satisfactory, they are easily scratched.

IV. DISCUSSION

Lau and Fonash reported that the resistivities of undoped

ZnO films that were deposited on soda lime glass and

Corning glass were similar.35 Matsuda et al., however,

observed considerably different crystallinities for ZnO films

on substrates made of quartz, glass, SiOx, SiOxNy, and

SiNx.36 Revisiting Fig. 6, we can see that the data also show

that the crystallinity of ZnO and GZO films in our system

reflects the substrate type, which can be interpreted as the

manner of initial nucleation governing crystallinity. The

delay in nucleation at the beginning of growth is not easily

resolved even when the film becomes thicker. Considering

that the scattering of electrons at grain boundaries signifi-

cantly contributes to electron mobility,37,38 securing good

crystallinity as early as possible is crucial for obtaining high

conductivity. Viewed more roughly, however, the transpar-

ent conductive properties of GZO films on PEN and glass

substrates do not greatly differ. This is because interfacial

reactions are not significant for fully oxidized GZO films.

The appearance of hydrocarbon signals assigned in the

transmittance spectra indicates diffusion of hydrocarbons out

of the PEN substrates into the ZnO films. Intermixing of the

elements through the film/substrate interface may be pro-

moted by continuous irradiation of the film and the substrate

with ECR plasma. Since the carbon concentration in the

SIMS profile is constant within the ZnO film, its incorpora-

tion may be enhanced by segregation of the hydrocarbons at

the surface of the growing ZnO film. Such hydrocarbon dop-

ing will result in higher carrier concentrations in ZnO films

deposited on PEN substrates compared to those on glass sub-

strates. Clues necessary to explain these results are found in

the work done by Honjo et al., where Mg(OH)2 films became

conductive after doping with carbon atoms.39 The segregated

carbon layer changes the sticking probability of Zn and/or

oxygen atoms that arrive at the growing film surface. The

enhanced deposition rate of ZnO films on the PEN substrate

can be explained in this respect.

The deposition temperature yielding the best performance

of GZO films on glass substrates has been found to be

between 200 and 300 �C.20,21,24,27,40 This means that room

temperature does not represent the optimum deposition con-

dition. Nevertheless, our near-room-temperature ECR-

sputtered ZnO and GZO films performed well. This is due to

continuously irradiating the film surface with argon plasma

during deposition,41 where the impact of the argon ions pro-

motes improvement in crystallinity through bond rearrange-

ment as well as generating carriers. Considering the

limitations of process temperature on organic substrates,

low-temperature ECR sputtering has proven to be a suitable

technique for obtaining highly conductive and adherent

GZO films on PEN substrates.

V. CONCLUSION

ZnO and GZO films sputtered on PEN sheets using ECR

plasma are sufficiently adhesive without inserting any inter-

mediate layers at the interface. The conductive properties of

GZO films on PEN substrates are slightly inferior to those

observed for films prepared on a glass substrate. In contrast,

the carrier concentration of ZnO films on PEN is higher than

that of films deposited on glass substrates, while the Hall

FIG. 10. Results from the rolling test for (a) 130-nm-thick and (b) 170-nm-

thick GZO films.

FIG. 11. Hardness index of ZnO (open squares) and GZO (open circles) films

on PEN substrate as a function of thickness.

021503-6 Housei Akazawa: ECR sputtered ZnO and GZO TCO films on PEN substrate 021503-6

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mobility is lower. Hydrocarbons in the PEN substrate exten-

sively diffused out into the ZnO films due to abundant oxy-

gen vacancies, and doped carbons may act as additional

donors. The ZnO films thicker than 200 nm suffered from

cracking and a deteriorated Hall mobility. Hence, we con-

clude that GZO films deposited via ECR plasma on PEN

substrates are feasible for use as transparent electrodes simi-

lar to those deposited on glass substrates, whereas ZnO films

deposited on PEN substrates exhibit an inferior performance

compared to ZnO films on glass substrates. Our pencil

scratch test revealed that ZnO and GZO films on PEN sub-

strates are more easily scratched than those on glass sub-

strates, reflecting the softness of the substrate. The changes

in the conductivity that occur after rolling ZnO and GZO

film samples around pipes were investigated. The conductiv-

ity deteriorated when a minimum threshold pipe radius of

10 mm was exceeded and tensile stress was exerted on the

film, but it deteriorated at a minimum threshold pipe radius

of 5 mm when compressive stress was exerted. The different

critical radii of curvature reflect the increased damage to

both ZnO and GZO films when applying tensile stress com-

pared to compressive stress.

ACKNOWLEDGMENT

The authors wish to thank Teijin DuPont Co. for kindly

providing us with the PEN sheet samples.

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021503-7 Housei Akazawa: ECR sputtered ZnO and GZO TCO films on PEN substrate 021503-7

JVST A - Vacuum, Surfaces, and Films

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