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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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