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Thin Solid Films 473 (2005) 218–223

Ultraflat indium tin oxide films prepared by ion beam sputtering

Younggun Hana,b,*, Donghwan Kima, Jun-Sik Chob, Seok-Keun Kohb

aDivision of Materials Science and Engineering, Korea University, 5-1, Anam-Dong, Sungbuk-Ku, Seoul 136-701, South KoreabR and D Center, P and I Corporation, Shinnae Technotown # 405, 485 Sangbong-Dong, Jungrang-Gu, Seoul 131-221, South Korea

Received 11 June 2002; received in revised form 21 May 2004; accepted 21 May 2004

Available online

Abstract

Indium tin oxide (ITO) films with a smooth surface (root-mean-square roughness; Rrms = 0.40 nm) were made using a combination of the

deposition conditions in the ion beam-sputtering method. Sheet resistance was 13.8 V/sq for a 150-nm-thick film grown at 150 jC. Oxygenwas fed into the growth chamber during film growth up to 15 nm, after which, the oxygen was turned off throughout the rest of the

deposition. The surface of the films became smooth with the addition of ambient oxygen but electrical resistance increased. In films grown at

150 jC with no oxygen present, a rough surface (Rrms = 2.1 nm) and low sheet resistance (14.4 V/sq) were observed. A flat surface (Rrms = 0.5

nm) with high sheet resistance (41 V/sq) was obtained in the films grown with ambient oxygen throughout the film growth. Surface

morphology and microstructure of the films were determined by the deposition conditions at the beginning of the growth. Therefore,

fabrication of ITO films with a smooth surface and high electrical conductivity was possible by combining experimental conditions.

D 2004 Elsevier B.V. All rights reserved.

Keywords: Indium tin oxide; Ion beam sputtering; Ambient oxygen

1. Introduction

Indium tin oxide (ITO) is required to have both excellent

optical and electrical properties. In organic light-emitting

devices (OLED), a flat ITO surface is desired for electrolu-

minescence efficiency and display lifetime [1].

The physical properties of ITO films are significantly

dependent on the deposition method [2]. Either dc or rf

magnetron sputtering is used to make high quality ITO films

in large quantities [3,4]. The microstructure of the films

prepared by magnetron sputtering usually shows the domain

type that is a signature of high electrical conductivity [5,6].

However, commercial ITO films deposited by sputtering

have rough surfaces (Rrms = a few 10 s nm at 150–300 nm

film thickness), which can be a source of OLED failure due

to the local electric field variations [7–9]. For this reason,

amorphous ITO films are used for EL devices despite the

low conductivity. A polishing process is commonly used to

smooth the surface. However, the polishing process is

0040-6090/$ - see front matter D 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.tsf.2004.05.125

* Corresponding author. Shinnae Technotown No. 405, 485 Sangbong-

Dong, Jungrang-Gu, Seoul 131-221, South Korea. Tel.: +82-2-3422-4100;

fax: +82-2-3422-4105.

E-mail address: yghan@kist.re.kr (Y. Han).

complicated, time consuming, and low in production yield.

In addition, the process may increase the sheet resistance of

the films and may induce high-resistance contact with the

light-emitting layer. Therefore, it is very important to obtain

a smooth surface in as-deposited films without sacrificing

electrical conductivity. Several methods for the fabrication

of the smooth surface ITO have been studied, such as ion

plating [10] and rf–dc mixed sputtering [11], but these

methods are still not satisfactory. We report the result of our

investigation on growing ITO films with little surface

roughness by employing the ion beam-sputtering method.

In our previous studies [12,13], the relationship between

the ITO film microstructure and the electrical properties was

discussed. High electrical conductivity was observed in

films prepared with Ar ion beam sputter without ambient

oxygen. In this study, we investigated the change in surface

morphology according to the sputtering conditions, espe-

cially in the early part of the film growth.

2. Experimental details

Films were deposited on glass substrates by ion beam

sputtering of the ITO target (90 wt.% In2O3 and 10 wt.%

Fig. 1. Sheet resistance (a) and carrier concentration and hall mobility (b) of

ITO films deposited by Ar ion with and without ambient oxygen.

Fig. 2. SEM images of ITO films deposited at a growth temperature of 150

jC by Ar ion without (a) and with (b) ambient oxygen.

Y. Han et al. / Thin Solid Films 473 (2005) 218–223 219

SnO2, 99.99%; High Purity Chemicals, Japan). Argon gas

was introduced into a cold hollow cathode ion source

having a 10-cm-diameter grid. Base pressure of the chamber

was 1�10� 5 Torr. The deposition was carried out at a

working pressure below 2� 10� 4 Torr. The ion beam

energy was 1.0 keV. Oxygen was used as an ambient gas

for comparison with our previous study [13], in which

oxygen gas was introduced in the ion source. The deposition

rate was 0.26 A/s, which dropped to 0.24 A/s with the

introduction of oxygen into the chamber. Because oxygen

partial pressure even after oxygen gas inlet was kept below

6� 10� 6 Torr, the magnitude of the drop was not signifi-

cant. The pressure measurement was done using a cold

cathode high vacuum gauge (Varian IMG-100) with the

measurement range of 1�10� 3 to 5� 10� 9 Torr, in which

the typical accuracy is F 20%. The deposition rate did not

depend on whether the oxygen was introduced into the ion

gun or into the vacuum chamber as an ambient gas. The

decrease of the deposition rate due to the oxygen is believed

to be caused by the oxidation of the sputtering target

surface. Even when oxygen is introduced into the ion gun,

the calculated ionization efficiency of oxygen of the ion

source is less than 10%. The low ionization rate is probably

the reason for the negligible difference between the two

cases: (i) oxygen as an ambient gas and (ii) oxygen fed into

the ion gun.

Two sets of samples were prepared. In the first set (Set

1), the entire film was deposited either with or without

ambient oxygen. In the second set (Set 2), combinations of

Fig. 3. XRD patterns of ITO films prepared (a) without ambient oxygen, (b)

with oxygen, (c) with oxygen only up to 15 nm thick, and (d) without

oxygen up to 15 nm thick. The growth temperature was 150 jC.

Y. Han et al. / Thin Solid Films 473 (2005) 218–223220

the two experimental conditions were used, i.e., sputtering

with ambient oxygen up to 15 nm in growth and off for the

rest of the deposition. A reverse sequence was used for the

other sample for comparison. The total thickness of the

films was about 150 nm. More details of the experimental

conditions are given elsewhere [13].

The film was evaluated by X-ray diffraction (XRD;

Diffractometer D5000, Siemens, Germany) using Cu Ka

radiation. The X-ray tube voltage and the current were 30

kV and 40 mA, respectively. Film thickness and surface

morphology were measured by scanning electron microsco-

py (SEM; Hitachi, S-4200FE). Surface morphology was

observed by contact mode atomic force microscopy (AFM;

PSIA, Korea, XE-100 SPM system). The root-mean-square

roughness (Rrms) and the peak-to-valley roughness (Rp–v) of

the surface were estimated over the area of 5� 5 Am2.

Electrical properties of the films were examined by a four-

point probe and Hall effect measurement.

3. Results and discussion

Fig. 1 shows sheet resistance (Rs), carrier concentration,

and mobility of the films that belong to the first set (Set 1);

Fig. 4. AFM images of ITO films deposited at a growth temperature of 150 jC b

roughness of the films with increasing of growth temperature (c).

that is, the entire film was deposited either with or without

ambient oxygen. As shown in Fig. 1a, the lower Rs is

obtained in the films prepared without ambient oxygen

mainly because the carrier concentration was higher (see

Fig. 1b), probably due to the higher density of oxygen

vacancies in the films [12]. The films show higher mobility

values despite the higher carrier concentration. It is well

accepted that the ITO films having the ‘‘domain structure’’

generally show higher electron mobility due to lower grain

boundary scattering [14].

It is well known that the domain structure is obtained

in ITO films deposited in an oxygen-deficient environ-

ment in sputtering methods. A single domain consists of

many grains having the same crystallographic orientation

perpendicular to the substrate [14]. Fig. 2 shows SEM

images of the films prepared by ion beam sputtering with

and without the ambient oxygen during the entire film

deposition (Set 1). As shown in Fig. 2a, the domain

structure is observed in the films by Ar ion sputtering

without ambient oxygen, whereas a ‘‘grain structure’’ is

obtained in the deposition with ambient oxygen (see Fig.

2b). In our previous reports [12,13], the ITO films

prepared with oxygen ion also show the grain structure.

Fig. 3 shows the XRD patterns of two sets of ITO films

y Ar ion without (a) and with (b) ambient oxygen, and changes of surface

Y. Han et al. / Thin Solid Films 473 (2005) 218–223 221

grown at 150 jC. The film grown in an oxygen environ-

ment has a strong preferred orientation in [111], and the

film without ambient oxygen is randomly oriented. The

preferential growth in the direction of [111] is a signature

of the grain structure in ITO films, and the grain structure

is observed in ITO films with higher oxygen concentra-

tion [12]. From our studies including the previous reports

[12,13], whether oxygen was introduced in the ion gun or

Fig. 5. Surface morphologies of ITO films deposited (a) with oxygen only up t

temperature of 150 jC, and (c) changes of surface roughness of the films with in

in the vacuum chamber as an ambient gas did not make a

difference in properties of ITO films as long as the partial

pressure of oxygen was kept the same. It seems that the

concentration of oxygen in the growing film was not

affected whether oxygen was ionized or not. Moreover,

the small increment of oxygen partial pressure seemed to

be enough to induce the structural change in our energetic

ion beam-sputtering system. Our result is well agreed with

o 15 nm thick and (b) without oxygen up to 15 nm thick at the substrate

creasing of growth temperature.

Fig. 6. Sheet resistance (a) and carrier concentration and hall mobility (b) of

ITO films deposited with a combination of deposition conditions, i.e., with

oxygen only up to 15 nm thick and without oxygen for the rest of the

growth. Result of the reverse combination is also shown.

Y. Han et al. / Thin Solid Films 473 (2005) 218–223222

that by Choi et al. [15] who grew ITO films by using dc

magnetron sputtering.

As mentioned above, ITO films with a domain structure

are more favorable because of enhanced electrical proper-

ties. However, for OLED application, besides high electrical

conductivity, the surface smoothness of ITO films is also

critical. Fig. 4 shows AFM 3D images and roughness (root-

mean-square; Rrms and peak-to-valley; Rp–v) changes of the

films deposited by Ar ion with and without ambient oxygen.

In films deposited without ambient oxygen (Fig. 4a), a

rough surface (Rrms = 2.1 nm and Rp–v = 16.3 nm) was

obtained compared to the other case (Fig. 4b; Rrms = 0.5

nm and Rp–v = 4.0 nm). Related to the structural studies, it is

thought that the difference in surface flatness is closely

related to the microstructure of the ITO films. The ITO films

with a domain structure exhibit rough surfaces, while those

with a grain structure have flat surfaces. Surface roughness

of the domain structure seems to be induced from the

thickness variation between the domains [16]. From the

SEM images in Fig. 2, the size of domains is between 300

Table 1

Summary of electrical and structural properties of ITO films

O2 Used Preferred orientation Microstructure root-mean-sq

100 jC

No Polycrystalline Domain 2.2

Yes [111] Grain 0.4

Only at the beginning [111] Grain 0.3

Not at the beginning Polycrystalline Domain 1.0

and 500 nm. Therefore, the AFM image in Fig. 4a contains

at least 100 domains for the scan area of 5� 5 Am2. As for

the films with a grain structure, a relatively smooth surface

was obtained, which is probably due to the preferential

growth into [111] over the entire surface. Deposition of ITO

films in ambient oxygen is favorable for obtaining a flat

surface. (Fig. 4c)

In order to obtain films with a high electrical conductiv-

ity and a smooth surface, we used a combination of the

experimental conditions as described in the Experimental

details. Fig. 5 shows SEM and AFM images of ITO films

belonging to Set 2, i.e., sputtering with ambient oxygen up

to 15 nm in thickness and then without oxygen for the rest

of the deposition (Fig. 5a) and vice versa (Fig. 5b). A grain

structure is observed in Fig. 5a, whereas a domain structure

is shown in Fig. 5b. Apparently, the first 15-nm-thick layer

acted as a template determining the final structure of the

entire film. The effect of the deposition condition of the

template layer on the entire film structure was discussed

minutely in our previous report [13]. The growth conditions

for the template layer also determined the crystallographic

orientation of the entire films as evidenced in XRD patterns

shown in Fig. 3. For example, the film that is deposited with

ambient oxygen only during the early part of the growth

showed the same XRD pattern as that of the film grown with

oxygen throughout the entire deposition (Fig. 3b and c).

Both films have a strong preferred orientation in [111],

whereas the other cases (Fig. 3(a) and (d)) showed a random

orientation. As a result of the strong preferred orientation,

the film with a combination of the growth conditions

(oxygen flow only in the early part of the growth) showed

a grain structure and a flat surface, as given in Fig. 5c

(Rrms = 0.4 nm and Rp–v = 3.7 nm).

Finally, the electrical properties of the films in Set 2

were investigated by changing the growth temperature from

100 to 200 jC (Fig. 6). As shown in Fig. 6a, the sheet

resistance of the films deposited with ambient oxygen only

during the early part of the growth is lower than in the

opposite case (oxygen off for the template layer and on for

the growth of the bulk layer). It is evident that the

microstructure of the film is determined by the deposition

condition of the template layer, whereas the electrical

properties are determined by the deposition of the bulk

layer. The relationship between the electrical properties and

the microstructure was explained in detail in our previous

two reports [12,13].

uare roughness (nm; F 0.2 nm) Sheet resistance (V/sq)

150 jC 200 jC 100 jC 150 jC 200 jC

2.1 2.1 18.3 14.4 14.0

0.5 0.4 304.0 41.0 29.3

0.4 1.6 16.1 13.8 12.5

1.1 1.2 446.0 52.6 24.5

Y. Han et al. / Thin Solid Films 473 (2005) 218–223 223

The surface roughness and the sheet resistance of ITO

films for the four deposition conditions are summarized in

Table 1. As shown in Table 1, considering both surface

roughness and sheet resistance, the film grown with a

combination of the deposition conditions (oxygen on in

the early growth phase and off later) at 150 jC showed the

best results, i.e., Rrms = 0.4 nm and Rs = 13.8 V/sq.

4. Conclusion

Ultraflat ITO films (RrmsV 1 nm) having excellent

electrical conductivity were fabricated without the post-

polishing process. Using a combination of deposition

conditions, i.e., ambient oxygen in the growth phase up

to 15 nm and without oxygen during the rest of the

deposition, we obtained an ultraflat ITO film (Rrms = 0.4

nm) with a low sheet resistance (13.8 V/sq) for 150-nm-

thick films. The microstructure of the films was deter-

mined by the growth conditions in the early phase of the

deposition, whereas the electrical properties were influ-

enced by the growth condition of the bulk layer. The ITO

films are expected to improve the durability of organic

EL devices.

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