Argon gas pressure dependence of the properties of transparentconducting ZnO:Al ®lms deposited on glass substrates

Yasuhiro Igasaki*, Hirokazu KanmaResearch Institute of Electronics, Shizuoka University, Johoku 3-5-1, Hmamatsu 432-8011, Japan

Received 12 July 1999; accepted 8 November 1999

Abstract

Aluminium doped zinc oxide (ZnO:Al) ®lms were deposited on amorphous substrates heated up to 2008C with a radio

frequency (rf) power of 100 W by rf magnetron sputtering from a ZnO target mixed with Al2O3 of 2 wt.%. Argon gas pressure

during deposition was in the range 0.08±2.7 Pa. As argon gas pressure was increased, the deposition rate and the grain size

were decreased and the surface roughness was increased. Furthermore, the carrier concentration and the Hall mobility were

decreased and thus the electrical resistivity was increased. However, the optical transmittance of about 90% was maintained

over the argon pressure range. The resistivity of the ®lm deposited at argon gas pressure of 0.13 Pa was about

2:5� 10ÿ4 O cm, a value comparable to that for indium tin oxide ®lm presently used as a transparent electrode.

# 2001 Elsevier Science B.V. All rights reserved.

Keywords: Transparent conducting oxide ®lm; Transparent electrode; Zinc oxide ®lm; Aluminum-doped zinc oxide ®lm; Sputtering; rf

magnetron sputtering

1. Introduction

The increasing use of transparent electrodes for

solid state display devices, solar cells, and transmit-

tance-variable windows has promoted the develop-

ment of inexpensive materials such as zinc oxide

(ZnO), and Al-doped zinc oxide (ZnO:Al) ®lms have

been actively investigated as a transparent conducting

material [1±4]. The electrical properties of ZnO:Al

®lms are so heavily affected by adsorption of oxygen

on the surfaces of the crystallites [5] that values for

their electrical resistivity differ with morphological

structure of each ®lm. Therefore, in order to know the

attainable value for the electrical resistivity of ZnO:Al

®lm, we attempted to measure the electrical resistivity

of a single-crystal ®lm which can be considered to

have no effect of the morphological structure and

obtained the value comparable to that for ITO ®lms

[6,7].

From the viewpoint of practical use, however, the

®lms should be deposited on an amorphous substrate

such as glass substrates. Krikorian and Sneed have

established that the three most important parameters

which determine the ®lm properties in sputtering for a

given material and substrate are substrate temperature,

deposition rate, and background pressure [8]. In this

paper, we report the argon gas pressure dependence of

electrical properties, structural properties and optical

properties of the ZnO:Al ®lms deposited on glass

substrates.

2. Experimental

ZnO:Al ®lms were prepared by rf magnetron sput-

tering. The apparatus used was a NEVA type FP-45 rf

Applied Surface Science 169±170 (2001) 508±511

* Corresponding author. Tel/Fax: �81-53-478-1308.

0169-4332/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved.

PII: S 0 1 6 9 - 4 3 3 2 ( 0 0 ) 0 0 7 4 8 - 0

sputtering system modi®ed for magnetron sputtering,

made up of a water-cooled stainless steel jar of inside

radius 300 mm connected with an oil diffusion pump

through a liquid nitrogen trap. Magnets stored in an

aluminum receptacle were installed on a water-cooled

cathode. A sintered disc of ZnO (purity, 99.99%)

mixed with 2 wt.% Al2O3 (purity, 99.99%), 125 mm

in diameter, was used as a target. A substrate heating

device was mounted on a water-cooled anode. The

distance between target and substrate was about 50 or

70 mm. A stainless steel mesh with 70% transmittance

maintained at earth potential was sometimes laid

between target and substrate. The residual gas and

the sputtering gas were monitored with a B-A type

ionization gauge and a capacitance manometer,

respectively. Quartz glass was used as a substrate.

The substrates were ultrasonically cleaned in a weak

alkaline cleaning solution provided by Furuuchi Che-

mical Laboratory, in acetone and ®nally in methyl

alcohol.

Prior to pre-sputtering, the jar and the substrates

were heated, respectively, at several ten degrees and at

about 4008C for degassing the equipment. After the jar

was evacuated to a pressure below 3� 10ÿ4 Pa, pre-

sputtering of 10 min was carried out at an argon gas

pressure between about 0.08 and 2.7 Pa with an rf

power of 150 W. A constant ¯ow of argon gas was

maintained at a value between 1.5 and 5.0 cm3/min

with a STEC type SEC-400 Mark 3 mass ¯ow con-

troller. After the pre-sputtering, the jar was re-evac-

uated to a pressure below 3� 10ÿ5 Pa.

ZnO:Al ®lms were deposited on the substrates

heated at 100 or 2008C with an rf power of 100 W

at same argon pressure as that during pre-sputtering.

However, the substrate temperature after deposition

was in a value between 150 and 2108C depending

on the deposition conditions. Deposition time was

10±90 min. A ®nal thickness and the deposition rate

were 250±900 nm and 4±21 nm/min, respectively.

Film thickness was determined with a surface rough-

ness detector DEKTAK.

The crystal structure was studied by X-ray diffrac-

tion using CuKa line with a RIGAKU type RAD-IIA.

A JEOL type JSM-T330A scanning microscope

equipped with a wavelength dispersive X-ray spectro-

scope (WDS) JXA-840AP was used for the observa-

tion of surface roughness and for the estimation of

aluminum content. The electrical resistivity r and the

Hall coef®cient RH were measured using the van der

Pauw method. An electric current I ranging from 0.1

to 1.0 mA at 0.1 mA intervals was applied through

electrodes and each potential difference V between the

electrodes was measured. The resistivity was calcu-

lated from the V versus I curve using the least-squares

method. The Hall voltage was measured under an

electric and a magnetic ®eld and then reversed ®elds;

the current and the magnetic ®eld used were, respec-

tively, 10 mA and 3100 G. The carrier concentration N

and the Hall mobility m were calculated from the

electrical resistivity and the Hall coef®cient using

the following relations:

N � 1

eRH

(1)

m � 1

Ner(2)

The optical transmittance and the absorption coef-

®cient for ZnO:Al ®lms deposited on quartz substrates

were measured by a double beam spectrophotometer

(Hitachi type-340) at room temperature with unpolar-

ized light in the spectral range 300±2600 nm.

3. Results and discussion

In the application of ZnO:Al ®lms to a transparent

conductor, especially in solid state display devices, the

electrical resistivity and the optical transmittance of

the ®lms must be most important factors. Therefore,

the emphasis of studies of transparent conducting

®lms has been concentrated mainly on how to prepare

transparent and low resistivity ®lms.

Fig. 1 presents the electrical resistivity r and the

optical transmittance T as a function of argon gas

pressure. T was the average transmittance in the

wavelength range 400±850 nm. Each of the marks

in Fig. 1, means the data obtained from ®lms prepared

under the following conditions;

1. (&): The ®lms were deposited on the substrates

heated at about 1008C for 60 min, substrate±target

distance was 50 mm and a stainless steel mesh

was laid at 3 cm under the target, ®lm thickness

was in the range 300±900 nm.

2. (&): The ®lms were deposited on the

substrates heated at about 1008C for 30±60 min,

Y. Igasaki, H. Kanma / Applied Surface Science 169±170 (2001) 508±511 509

substrate±target distance was 70 mm and a

stainless steel mesh was laid at 3 cm under the

target, ®lm thickness was in the range 270±

360 nm.

3. (*): The ®lms were deposited on the substrates

heated at about 1008C for 15±40 min, substrate±

target distance was 50 mm, ®lm thickness was in

the range 260±390 nm.

4. (*): The ®lms were deposited on the substrates

heated at about 2008C for 15±40 min, substrate±

target distance was 50 mm, ®lm thickness was in

the range 240±370 nm.

As shown in Fig. 1, the average transmittance T of

the ®lm is about 90% independent of argon gas

pressure during deposition, enough for a transparent

conducting ®lm. On the other hand, the resistivity r of

the ®lm increases with increase in argon gas pressure.

The resistivity r is proportional to the reciprocal of

the product of the carrier concentration N and the Hall

mobility m. Therefore, the change in resistivity with

argon gas pressure shown in Fig. 1, is ascribed to the

change in N and/or m which are characteristic para-

meters re¯ecting the ®lm structure and/or the impurity

contents. In order to explain the dependence of the

electrical resistivity on argon gas pressure, we inves-

tigated the change in structural characteristics such as

crystal structure and surface roughness, and the

change in the aluminum content.

Figs. 2 and 3 show the carrier concentration and the

aluminum content, and the Hall mobility and FWHM

Fig. 1. Optical transmittance T and electrical resistivity r as a

function of argon gas pressure during deposition. The meaning of

marks is presented in text.

Fig. 2. Carrier concentration N and aluminum content Al/

(Al� Zn) as a function of argon gas pressure during deposition.

Marks correspond to those in Fig. 1.

Fig. 3. The Hall mobility m and the half-width of X-ray diffraction

peak FWHM as a function of argon gas pressure during deposition.

Marks correspond to those in Fig. 1.

510 Y. Igasaki, H. Kanma / Applied Surface Science 169±170 (2001) 508±511

of X-ray diffraction from (0001) plane, respectively,

as a function of argon gas pressure.

As shown in Fig. 2, the carrier concentration N

decreases with increasing argon gas pressure in the

range 0.27±2.7 Pa, and the aluminum content is

almost constant in the argon pressure range 0.08±

1.0 Pa and then increases slightly above about

1.0 Pa. In the aluminum doped zinc oxide, an alumi-

num atom is generally considered to be substituted

for zinc atom and to act as donor. Therefore, the

decrement of N shown in ®gure is not ascribed to

the change in the aluminum content namely donor

concentration.

As shown in Fig. 3, the Hall mobility m is decreased

and FWHM is increased with increasing argon gas

pressure in the range 0.27±2.7 Pa. The change in

FWHM generally re¯ects the change in the grain size

of crystallites, that is, the increase in FWHM corre-

sponds to the decrease in grain size. Therefore, one of

the causes of decrement of m with increasing argon gas

pressure can be ascribed to the decrease in grain size,

namely the increase in the grain-boundary i.e. the

scattering centre for carriers. On the other hand, from

SEM observation, we found that the surface roughness

was increased with increase in argon pressure. The

increase in surface roughness of ®lm decreases the

effective thickness, not measured thickness by DEK-

TAK, of conducting path in ®lm for the carrier and

then increases the apparent electrical resistivity.

Furthermore, the increase in surface roughness

increases the effective surface area of ®lms.

In the ZnO ®lm, an oxygen adsorbed on the surface

of crystallites traps an electron and decreases the

carrier concentration and also decreases the Hall

mobility by increasing the potential height at the

surface of crystallites [9]. On the other hand, as

mentioned above, the decrease in the crystallite size

and the increase in the surface roughness increase the

effective surface area of ®lm and then cause the

increase in the number of adsorption site for oxygen.

Therefore, we can conclude that the increase of resis-

tivity of ZnO:Al ®lm with increasing argon gas pres-

sure can be ascribed to the decrease in the carrier

concentration and the Hall mobility caused by the

decrease in grain size and the increase in surface

roughness.

4. Conclusions

ZnO:Al ®lms were deposited on quartz substrates

heated to 100 or 2008C with an rf power of 100 W

under an argon gas pressure between 0.08 and 2.7 Pa

by rf magnetron sputtering from a ZnO target with

2 wt.% Al2O3, and their structural, optical and elec-

trical properties such as electrical resistivity r, carrier

concentration N and the Hall mobility m were studied

as a function of argon gas pressure from the viewpoint

of their application to the transparent electrodes. As a

result, it was found that the electrical properties were

closely related to the grain size and the surface rough-

ness of ®lm, and that as argon pressure during deposi-

tion was increased, the grain size was decreased and

the surface roughness was increased and thus the

electrical resistivity was increased. The minimum

resistivity of about 2:5� 10ÿ4 O cm obtained with

a ®lm grown on a quartz substrate heated at 1008Cwith an rf power of 100 W at argon pressure of 0.13 Pa

is comparable to one for ITO ®lm. Therefore, we can

claim that ZnO:Al ®lms deposited on glass substrates

at lower pressure can be used as transparent conduct-

ing ®lm.

Acknowledgements

One of the authors (Y.I.) wishes to express his

appreciation to the Takahashi Foundation for their

®nancial support.

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Y. Igasaki, H. Kanma / Applied Surface Science 169±170 (2001) 508±511 511