Thickness-dependent growth orientation of F-doped ZnO films formed by atomic layer deposition Kyung-Mun Kang, Yong-June Choi, Geun Young Yeom, and Hyung-Ho Park Citation: Journal of Vacuum Science & Technology A 34, 01A144 (2016); doi: 10.1116/1.4938180 View online: http://dx.doi.org/10.1116/1.4938180 View Table of Contents: http://scitation.aip.org/content/avs/journal/jvsta/34/1?ver=pdfcov Published by the AVS: Science & Technology of Materials, Interfaces, and Processing Articles you may be interested in Nucleation and growth of ZnO on PMMA by low-temperature atomic layer deposition J. Vac. Sci. Technol. A 33, 01A128 (2015); 10.1116/1.4902326 Photoresponse study on transition metal (Co, Ni, Mn) doped ZnO thin films AIP Conf. Proc. 1512, 1050 (2013); 10.1063/1.4791405 Atomic layer deposition of Al-doped ZnO thin films J. Vac. Sci. Technol. A 31, 01A109 (2013); 10.1116/1.4757764 Third harmonic generation in undoped and X doped ZnO films (X: Ce, F, Er, Al, Sn) deposited by spray pyrolysis J. Appl. Phys. 101, 063104 (2007); 10.1063/1.2711143 F-doping effects on electrical and optical properties of ZnO nanocrystalline films Appl. Phys. Lett. 86, 123107 (2005); 10.1063/1.1884256 Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. IP: 115.145.196.174 On: Fri, 09 Dec 2016 06:09:15
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Thickness-dependent growth orientation of F-doped ZnO films formed by atomic layerdepositionKyung-Mun Kang, Yong-June Choi, Geun Young Yeom, and Hyung-Ho Park Citation: Journal of Vacuum Science & Technology A 34, 01A144 (2016); doi: 10.1116/1.4938180 View online: http://dx.doi.org/10.1116/1.4938180 View Table of Contents: http://scitation.aip.org/content/avs/journal/jvsta/34/1?ver=pdfcov Published by the AVS: Science & Technology of Materials, Interfaces, and Processing Articles you may be interested in Nucleation and growth of ZnO on PMMA by low-temperature atomic layer deposition J. Vac. Sci. Technol. A 33, 01A128 (2015); 10.1116/1.4902326 Photoresponse study on transition metal (Co, Ni, Mn) doped ZnO thin films AIP Conf. Proc. 1512, 1050 (2013); 10.1063/1.4791405 Atomic layer deposition of Al-doped ZnO thin films J. Vac. Sci. Technol. A 31, 01A109 (2013); 10.1116/1.4757764 Third harmonic generation in undoped and X doped ZnO films (X: Ce, F, Er, Al, Sn) deposited by spray pyrolysis J. Appl. Phys. 101, 063104 (2007); 10.1063/1.2711143 F-doping effects on electrical and optical properties of ZnO nanocrystalline films Appl. Phys. Lett. 86, 123107 (2005); 10.1063/1.1884256
Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. IP: 115.145.196.174 On: Fri, 09 Dec 2016 06:09:15
Thickness-dependent growth orientation of F-doped ZnO films formedby atomic layer deposition
Kyung-Mun Kang and Yong-June ChoiDepartment of Materials Science and Engineering, Yonsei University, Seoul 120-749, Republic of Korea
Geun Young YeomDepartment of Advanced Materials Science and Engineering, and SKKU Advanced Instituteof Nanotechnology; Sungkyunkwan University, Suwon, Kyunggi-do 440-746, Republic of Korea
Hyung-Ho Parka)
Department of Materials Science and Engineering, Yonsei University, Seoul 120-749, Republic of Korea
(Received 2 September 2015; accepted 8 December 2015; published 22 December 2015)
ZnO thin films were doped with fluorine using atomic layer deposition (ALD) with an in-house F
source at a deposition temperature of 140 �C. Structural and morphological properties of the
resulting F-doped ZnO (ZnO:F) films were investigated by x-ray diffraction analysis, field emis-
sion scanning electron microscopy, and grazing incidence wide-angle x-ray diffraction. During
the initial growth stage of up to 200 ALD cycles, no difference was observed between the pre-
ferred growth orientations of undoped ZnO and ZnO:F films. However, after 300 ALD cycles,
ZnO and ZnO:F films showed (002) and (100) preferred orientation, respectively. This difference
in preferred growth orientation arose from the perturbation-and-passivation effect of F doping,
which involves F anions filling the oxygen-related defect sites in the ZnO lattice. Ultraviolet
photoelectron spectroscopic analyses were carried out to investigate the surface plane depend-
ency of the films’ work functions, which confirmed that the ZnO and ZnO:F films had different
growth behaviors. VC 2015 American Vacuum Society. [http://dx.doi.org/10.1116/1.4938180]
I. INTRODUCTION
Atomic layer deposition (ALD) is a highly advanced dep-
osition method that allows low-temperature processing and
yields highly uniform films.1–4 ALD thin film growth is
based on self-limiting surface chemistry, and entails a
repeated process of pulsing with precursors and purging,
while keeping the source materials separate throughout the
deposition process. Therefore, ALD is suitable for low-
temperature growth, provides good step coverage (high as-
pect ratio), and good uniformity, and allows film thickness
to be controlled by varying the number of ALD cycles.
Moreover, separation of the precursors prevents gas-phase
reactions, thereby allowing the use of highly reactive precur-
sors and making it possible to provide sufficient time for
each reaction step to reach completion. Thus, ALD enables
the deposition of complex 3D structures onto electronic
devices under relatively low-temperature conditions that
facilitate the use of flexible substrates.5
Transparent conducting oxides (TCOs) are oxide semi-
conductors that are transparent in the visible light region and
that offer control of electrical conductivity.6 TCOs can be
used for transparent electrodes, which are used in next-
generation optoelectronics such as flat-panel displays, photo-
voltaic devices, and organic light-emitting diodes.7–9
Moreover, many TCOs are applicable to flexible substrates,
an increasingly common requirement for optoelectronic
applications. Some methods exist to deposit TCO layers on
plastic substrates as flexible, lightweight, small-volume fea-
tures at temperatures lower than the glass transition
temperatures of the substrates (�150 �C). Among the avail-
able TCO materials, tin-doped indium oxide (indium tin ox-
ide; ITO) is one of those most commonly used for transparent
electrodes in optoelectronic devices because of its excellent
visible-region transparency and high electrical conductivity.10
However, it has long been acknowledged that indium-free
substitute TCOs are needed. Indium is rare and expensive,
requires expensive deposition techniques, is an environmental
pollutant, and is unstable in hydrogen plasma.11 ZnO has
been actively explored as a promising indium-free TCO mate-
rial. Most notably, ZnO is an n-type, direct wide-bandgap
(3.3 eV) semiconductor with a high exciton binding energy
(60 meV).11 However, ZnO has a much lower intrinsic elec-
tron concentration (1018–1019cm�3) than ITO (�1021cm�3).
Thus, in recent years, many investigations have focused on
doping techniques to increase the electron concentration in
ZnO. Such efforts have included doping ZnO with either tri-
valent metal cations (group III elements; Al, Ga, and B)12,13
or halogen anions (group VII elements; F and Cl).14,15 These
studies have yielded degenerately doped semiconductors with
electron concentrations of up to 1021cm�3 while maintaining
ZnO’s high electron mobility. Among the various dopant can-
didates, halogen elements (F and Cl) are the most effective
for oxide semiconductors because oxide semiconductors
intrinsically have O-related defects, such as oxygen vacan-
cies.6 Halogen anions can substitute for oxygen as n-type dop-
ants and also occupy oxygen vacancies, thereby passivating
oxygen defects.16
We have previously studied the mechanism of F doping
in ALD ZnO films and reported on the electrical, structural,
morphological, and optical characteristics of ZnO:F films.17
As the F-doping concentration in a ZnO matrix is increased,a)Electronic-mail: [email protected]
01A144-1 J. Vac. Sci. Technol. A 34(1), Jan/Feb 2016 0734-2101/2016/34(1)/01A144/7/$30.00 VC 2015 American Vacuum Society 01A144-1
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the grain growth orientation of the resulting ZnO thin films
changes twice, with a transition from the c-axis to the a-axis
at 0.7 at. %, and a return to the c-axis above 1.0 at. %. We
attributed these transitions to the perturbation-and-passiva-
tion effect, by which O-related defect sites in ZnO are filled
by F anions. This phenomenon was corroborated by x-ray
diffraction (XRD), surface morphology, and grazing incident
wide-angle x-ray diffraction (GIWAXD) analyses.
Moreover, the growth mode of the ALD ZnO:F films was
confirmed by GIWAXD analysis.
Numerous reports have been published regarding the
preparation and characterization of ZnO thin films with a
strong c-axis orientation preference.18,19 These reports have
focused mainly on the optimization of deposition conditions.
However, no systematic study has been performed to investi-
gate the mechanism of orientation preference or microstruc-
tural evolution in relation to thickness in ALD-grown ZnO
thin films. A thin film’s growth orientation is an important
determinant of its film properties due to the anisotropic
behavior it represents. The ability to manipulate crystal
growth orientation is an essential requirement in modern
materials science.20 Achieving control over the crystalliza-
tion orientation of thin film would enable the realization and
optimization of many potential applications including solar
cells, transparent transistors, and gas sensors.21–23 In the
present study, we investigated how the growth orientation in
ZnO:F thin films depends on thickness of films deposited by
ALD at the low temperature of �140 �C. Structural and mor-
phological characteristics of ZnO:F films were measured as
a function of the number of ALD cycles used to fabricate
them.
II. EXPERIMENT
Undoped ZnO and ZnO:F thin films were deposited on Si
and LCD glass (Fusion 1737) substrates using ALD con-
ducted at the extremely low deposition temperature of
140 �C and a working pressure of �1 Torr. ALD was per-
formed using a traveling-wave-type Lucida D100 system
(NCD Technology, Inc., Korea). The base pressure of this
ALD system was �50 mTorr. Diethylzinc (DEZ, Hansol
Chemical Co., Ltd., Korea) and deionized (DI) water were
used as the Zn and O precursors, respectively. The F source
was a H2O/HF mixture, which was made in-house by adding
0.5 ml of aqueous hydrogen fluoride (HF of 48%–51%
diluted in water) to 50 ml DI water, as described in our previ-
ous work.17 DEZ was kept at 10 �C using a chiller and was
delivered into the reaction chamber with a carrier gas of
high-purity N2 (99.999%) at a flow rate of 20 sccm. Each
cycle of ALD growth used to fabricate undoped ZnO (or
ZnO:F) thin films was conducted according to the following
program: 0.1 s DEZ pulse, 10 s N2 purge, 0.1 s pulse of H2O
(or H2O/HF mixture), and 10 s N2 purge. Film samples of
various thicknesses were fabricated by conducting various
numbers of ALD cycles: 20, 50, 100, 200, 300, 600, and
1200; hereafter, the corresponding films are denoted as
ZnO(20), ZnO:F(20), etc.
Film thicknesses were verified using ellipsometry. Crystal
structures and surface morphologies of undoped ZnO and
ZnO:F films were analyzed using XRD (D/MAX-2000,
Rigaku) with a CuKa radiation source (k¼ 1.5418 A) and
field emission scanning electron microscopy (FE-SEM, S-
4800, Hitachi). Surface topographies were analyzed using
atomic force microscopy (AFM, Bruker Nanoscope V).
GIWAXD experiments were carried out using the 9A beam
line at the Pohang Accelerator Laboratory (PLS). Ultraviolet
photoelectron spectroscopy (UPS) analyses were carried out
using the 4D beam line of the PLS, with a He I radiation
source (21.22 eV) and a bias voltage of �5 V. Au foil was
used as a reference material to calibrate kinetic energy shifts.
Changes in crystalline defects in ZnO were monitored using
photoluminescence (PL) spectroscopy at room temperature
with a 325-nm laser excitation source.
III. RESULTS AND DISCUSSION
The thicknesses of ZnO and ZnO:F thin films formed on
glass substrates by ALD increased linearly with the number
of growth cycles (Fig. 1). Under our experimental condi-
tions, the relationship of t¼ 0.2 c was observed, where t is
the thickness of the film in nanometers and c is the number
of growth cycles. This observation confirmed that fluorine
anion doping in ZnO did not affect the formation or growth
rates of ZnO films.
A. Crystal structure
Crystal structures of undoped ZnO and ZnO:F films of
various thicknesses grown on glass substrates were analyzed
by XRD. The diffraction peaks observed for all the films
were consistent with the standard diffraction pattern for a
hexagonal wurtzite ZnO structure (Fig. 2). The crystal
growth of ALD-grown ZnO films is influenced primarily by
growth temperature.24 However, in the present experiments,
changes observed in the preferred orientation of ZnO and
ZnO:F thin films between the (002) and (100) should be
attributed to F-doping and variations in film thicknesses.
During the initial growth stage of up to 200 ALD cycles, nei-
ther ZnO nor ZnO:F thin films showed a preferred
FIG. 1. (Color online) Thicknesses of undoped and F-doped ZnO thin films
deposited on glass substrates vs number of ALD growth cycles applied to
grow the films.
01A144-2 Kang et al.: Thickness-dependent growth orientation of F-doped ZnO films 01A144-2
J. Vac. Sci. Technol. A, Vol. 34, No. 1, Jan/Feb 2016
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orientation, and both displayed nearly the same growth
behavior. However, after 300 ALD cycles, (002) preferred
orientation was observed in the undoped films, as is com-
monly observed in other ZnO films.17 In contrast, the ZnO:F
thin films showed (100) preferred orientation after 300 ALD
cycles. No change in peak position resulting from O vacancy
filling or O site substitution by F dopants was observed. This
is because the F� ion (�1.31 A) has an ionic radius similar
to the O2� ion (�1.34 A).17
Analyses of XRD patterns alone did not fully elucidate
the growth behavior of ZnO:F films. Thus, we also con-
ducted a GIWAXD study to better understand the growth
mechanism. GIWAXD analysis allows growth orientation to
be characterized for the entire range of ZnO:F film thick-
nesses covered in the present work. Figure 3 shows 2D
GIWAXD patterns for undoped and F-doped ZnO films of
various thicknesses grown on Si substrates at 140 �C. To
determine the 2D intensity distribution in each image, the
horizontal (or out-of-plane) component of the scattering vec-
tor, qy, was plotted along the x-axis, and the vertical (or in-
plane) component of the scattering vector, qz, was plotted
along the y-axis.
The GIWAXD results for the growth component (q) per-
pendicular to the substrate verify the growth mode suggested
by the XRD results [Fig. 2(a)] for the undoped ZnO films
[Figs. 3(1a)–3(1g)]. As already observed in XRD analyses,
the growth mode of ZnO:F films showed nearly identical
behavior to undoped ZnO films for thinner films fabricated
using 20–100 ALD cycles [Figs. 3(1a)–3(1d) and
3(2a)–3(2d)]. However, as the number of ALD cycles
increased to 200, growth of the (100) plane was enhanced,
and the intensity of the (100) plane was similar to that of the
(002) plane [Fig. 3(2d)]. With a further increase in the num-
ber of ALD cycles from 300 to 1200, the preferred orienta-
tion changed to (100) [Figs. 3(2e)–3(2g)]. This change in
preferred orientation with increasing film thickness revealed
that the (100) preferred orientation resulted from the growth
of films perpendicular to the substrate.
B. Surface morphology
The surface morphologies of the ZnO and ZnO:F films
were monitored using FE-SEM (Fig. 4). The undoped and
F-doped ZnO films formed at the same deposition temperature
(140 �C) exhibited surface structures consisting of grains of
different sizes and morphologies. Grain size increased slightly
with the increasing film thickness due to grain growth,
because ALD was carried out slowly, with 1200 ALD cycles
requiring almost 10 h of processing time. Root mean square
roughness of the samples was measured using AFM. The
AFM results are provided in the supplementary material.25
Normally, columnar and wedgelike morphologies are attrib-
uted to c- and a-axis growth directions, respectively.26 XRD
FIG. 2. (Color online) Thin-film XRD patterns of (a) undoped and (b) F-doped ZnO films grown on glass substrates for various numbers of ALD growth
cycles.
01A144-3 Kang et al.: Thickness-dependent growth orientation of F-doped ZnO films 01A144-3
JVST A - Vacuum, Surfaces, and Films
Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. IP: 115.145.196.174 On: Fri, 09 Dec 2016 06:09:15
[Fig. 2(a)] and SEM [Figs. 4(1a)–4(1g)] observations of
undoped ZnO thin films showed that the c-axis was the pre-
ferred growth direction. In the case of ZnO:F thin films, c-
axis-oriented grains decreased and a-axis-oriented grains
increased with the increasing film thickness [Figs.
4(2a)–4(2g)]. When 600 ZnO:F ALD cycles were applied
[Fig. 4(2f)], large wedge-shaped grains were clearly observed,
suggesting that the (100) planes grew more than the (002)
planes, which was consistent with the XRD results [Fig. 2(b)].
This phenomenon may be a result of the F doping mechanism.
It has been demonstrated that F� anions can passivate surface
dangling bonds.16 To confirm the passivating effect of F dop-
ing, room-temperature PL spectra were obtained, showing
that deep-level defects, such as O vacancies, were effectively
FIG. 3. (Color online) Two dimensional GIWAXD patterns of [(1a)–(1g)] undoped and [(2a)–(2g)] F-doped ZnO films grown at 140 �C on Si substrates, using
various numbers of ZnO or ZnO:F ALD cycles: (1a) ZnO(20), (1b) ZnO(50), (1c) ZnO(100), (1d) ZnO(200), (1e) ZnO(300), (1f) ZnO(600), (1g) ZnO(1200),
(2a) ZnO:F(20), (2b) ZnO:F(50), (2c) ZnO:F(100), (2d) ZnO:F(200), (2e) ZnO:F(300), (2f) ZnO:F(600), and (2g) ZnO:F(1200). Observed diffraction peaks
are labeled with their corresponding Miller indices (hkl).
01A144-4 Kang et al.: Thickness-dependent growth orientation of F-doped ZnO films 01A144-4
J. Vac. Sci. Technol. A, Vol. 34, No. 1, Jan/Feb 2016
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removed through F doping (see the supplementary material25).
In fact, it has been reported that in nanocrystalline ZnO thin
films of ZnO, surface dangling bonds can act as trap sites for
free electron carriers, thereby decreasing mobility and conse-
quently increasing resistivity.27 Therefore, the passivation
effect of saturating these surface dangling bonds by F anions
at grain boundaries may decrease the resistivity of ZnO:F
films, as observed in our previous study.17
Fujimura et al.27 suggested that the surface energy density
of the (002) plane is the lowest in ZnO crystals. Normally,
grain growth occurs on the lowest-surface-energy plane of
the crystal, causing the preferred orientation to develop into
a single crystallographic orientation corresponding to this
plane. However, in the present work, as ZnO:F film thick-
ness increased, the intensities of the (002) diffraction peaks
decreased, and those of the (100) peaks increased.
According to the evolutionary selection rule,28 it is expected
that the direction of fastest grain growth will survive and
will ultimately govern the preferred orientation of the film.
In the case of ZnO:F films, F� anions substituted O2� anion
FIG. 4. Top-view FE-SEM images of [(1a)–(1g)] undoped and [(2a)–(2g)] F-doped ZnO films grown at 140 �C on glass substrates, using various numbers of
(2b) ZnO:F(50), (2c) ZnO:F(100), (2d) ZnO:F(200), (2e) ZnO:F(300), (2f) ZnO:F(600), and (2g) ZnO:F(1200). All scale bars represent 100 nm.
01A144-5 Kang et al.: Thickness-dependent growth orientation of F-doped ZnO films 01A144-5
JVST A - Vacuum, Surfaces, and Films
Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. IP: 115.145.196.174 On: Fri, 09 Dec 2016 06:09:15
sites and filled O vacant sites during the growth of (100)
plane, and preferential filling continued with the growth of
(002) plane along the longitudinal direction of the hexagonal
wurtzite structure as the ZnO:F film thickness increased.
Reducing the number of O-related defect sites in ZnO:F
films may enhance these films’ growth rates.29 This implies
that a-axis growth occurs at the highest rate during crystal
growth. Therefore, the (100) preferred orientation is domi-
nant during the overall growth of ZnO:F thin films, whereas
the (002) preferred orientation due to c-axis growth occurred
only at the initial growth stage, because it initially minimizes
the surface energy of the film.
C. Work function
UPS spectra were collected for ZnO and ZnO:F thin films
grown for 200 and 1200 ALD cycles. Low-kinetic-energy
cutoff regions and valence-band regions of these UPS spec-
tra are shown in Figs. 5(a) and 5(b), respectively. The inelas-
tic cutoff of the samples was clearly distinguished by
applying a �5.0 V sample bias [Fig. 5(a)]. The Fermi energy
(EF) of the films was 21.40 eV, corresponding to the onset of
energy intensity that can be seen in Fig. 5(b). The work func-
tions of the films were determined using Eq. (1)30
U ¼ h� þ Ecutoff � EF: (1)
The work functions of the ZnO(200), ZnO:F(200),
ZnO(1200), and ZnO:F(1200) films were calculated to be
4.60, 4.61, 4.61, and 4.70 eV, respectively. The ZnO(200),
ZnO:F(200), and ZnO(1200) films had nearly the same work
function values due to their shared (002) preferred orienta-
tion. A material’s work function is known to be strongly
related to surface plane orientation; for example, in the case
of ZnO, the work function values of 4.50 and 4.64 eV have
been reported for the nonpolar (002) and (100) planes.31,32
Note that a greater work function was reported for the (100)
plane than the (002) plane. This trend was also observed in
the present work, even in the case of the (100) preferred ori-
entation containing more (101) facets from the wedgelike
surface morphological shape. As more ALD cycles were
applied, textured film surfaces were formed, and finally, the
proportion of (100) planes relative to (002) planes was able
to increase as the (100) planes became exposed. This change
in surface polarity of the films allowed the ionization poten-
tial of the films to increase as more ZnO:F ALD cycles were
applied, thereby increasing the work function of the resulting
TCO films.33
IV. CONCLUSIONS
ALD was used to fabricate undoped ZnO films and
ZnO:F films. Films of various thicknesses were fabricated by
varying the number of ALD cycles applied. As more ZnO:F
ALD cycles were applied, the preferred orientation changed
from (002) to (100). This change in behavior indicated that
a-axis was the preferred growth direction of ZnO:F film,
whereas c-axis was the most probable growth direction dur-
ing the initial stage of growth to minimize the surface energy
of the film. This phenomenon could be explained by the sub-
stitution of O sites with F anions, a theory well supported by
XRD and GIWAXD results. Moreover, the difference in
growth behavior between ZnO and ZnO:F films grown for
300 ALD cycles was also confirmed by a UPS study in
which the relationship between the films’ surface planes and
work functions was investigated.
ACKNOWLEDGMENTS
This work was supported by the Industrial Strategic
Technology Development Program (10041926, Development
of high density plasma technologies for thin film deposition of
nanoscale semiconductor and flexible display processing),
funded by the Ministry of Knowledge Economy of Korea.
This work was supported by the National Research Foundation
of Korea(NRF) grant funded by the Korea government(MSIP)
(No. 2015R1A2A1A15054541). Experiments at the PLS were
supported in part by MSIP and POSTECH.
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J. Vac. Sci. Technol. A, Vol. 34, No. 1, Jan/Feb 2016
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