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

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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: hhpark@yonsei.ac.kr

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

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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.

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[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

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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

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). All scale bars represent 100 nm.

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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.

1D. C. Look, Mater. Sci. Eng. B-Solid 80, 383 (2001).2S. M. George, Chem. Rev. 110, 111 (2010).3M. Leskela and M. Ritala, Thin Solid Films 409, 138 (2002).4E. Guziewicz et al., J. Appl. Phys. 103, 033515 (2008).5P. Poodt, D. C. Cameron, E. Dickey, S. M. George, V. Kuznetsov, G. N.

Parsons, F. Roozeboom, G. Sundaram, and A. Vermeer, J. Vac. Sci.

Technol. A 30, 010802 (2012).6T. Minami, Semicond. Sci. Technol. 20, S35 (2005).7T. Minami, H. Nanto, and S. Takata, Jpn. J. Appl. Phys. 23, L280 (1984).8V. V. Simakov, O. V. Yakusheva, A. I. Grebennikov, and V. V. Kisin,

Tech. Phys. Lett. 31, 339 (2005).9P. F. Carcia, R. S. McLean, M. H. Reilly, and G. Nunes, Jr., Appl. Phys.

Lett. 82, 1117 (2003).10R. B. H. Tahar, T. Ban, Y. Ohya, and Y. Takahashi, J. Appl. Phys. 83,

2631 (1998).11T. Minami, Thin Solid Films 516, 5822 (2008).12K. I. Bolotin, K. J. Sikes, Z. Jiang, M. Klima, G. Fudenberg, J. Hone, P.

Kim, and H. L. Stormer, Solid State Commun. 146, 351 (2008).13H. Serier, A. Demourgues, and M. Gaudon, Inorg. Chem. 49, 6853 (2010).14J. Hu and R. G. Gordon, Sol. Cells 30, 437 (1991).15J. Rousset, E. Saucedo, and D. Lincot, Chem. Mater. 21, 534 (2009).16B. Liu, M. Gu, X. Liu, S. Huang, and C. Ni, Appl. Phys. Lett. 97, 122101

(2010).17Y.-J. Choi and H.-H. Park, J. Mater. Chem. C 2, 98 (2014).18J. H. Choi, H. Tabata, and T. Kawai, J. Cryst. Growth 226, 493 (2001).19J. B. Lee, S. H. Kwak, and H. J. Kim, Thin Solid Films 423, 262 (2003).20Q. Zhang, S.-J. Liu, and S.-H. Yu, J. Mater. Chem. 19, 191 (2009).

FIG. 5. (Color online) UPS spectra of ZnO and ZnO:F films: (a) low-kinetic-

energy cut-off region and (b) valence band region.

01A144-6 Kang et al.: Thickness-dependent growth orientation of F-doped ZnO films 01A144-6

J. Vac. Sci. Technol. A, Vol. 34, No. 1, Jan/Feb 2016

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

21K. Keis, E. Magnusson, H. Lindstrom, S.-E. Lindquist, and A. Hagfeldt,

Sol. Energy Mater. Sol. Cells 73, 51 (2002).22E. Fortunato, P. Barquinha, A. Pimentel, A. Goncalves, A. Marques, L.

Pereira, and R. Martins, Thin Solid Films 487, 205 (2005).23J. Xu, Q. Pan, Y. Shun, and Z. Tian, Sens. Actuator B-Chem. 66, 277

(2000).24L. Rivas, S. Sanchez-Cortes, J. V. Garc�ıa-Ramos, and G. Morcillo,

Langmuir 17, 574 (2001).25See supplementary material at http://dx.doi.org/10.1116/1.4938180 for

additional details concerning the brief discussion and analysis of addi-

tional results from PL and AFM.26S.-H. Ko Park and Y. E. Lee, J. Mater. Sci. 39, 2195 (2004).

27N. Fujimura, T. Nishihara, S. Goto, J. Xu, and T. Ito, J. Cryst. Growth

130, 269 (1993).28J. G. E. Gardeniers, Z. M. Rittersma, and G. J. Burger, J. Appl. Phys. 83,

7844 (1998).29J. Cui, J. Phys. Chem. C 112, 10385 (2008).30D.-J. Yun, K. Hong, S. H. Kim, W.-M. Yun, J.-Y. Jang, W.-S. Kwon, C.

E. Park, and S.-W. Rhee, ACS Appl. Mater. Interfaces 3, 43 (2011).31P. J. Møller, S. A. Komolov, and E. F. Lazneva, J. Phys.-Condens. Mater.

11, 9581 (1999).32H. Moormann, D. Kohl, and G. Heiland, Surf. Sci. 80, 261 (1979).33A. Klein, C. K€orber, A. Wachau, F. S€auberlich, Y. Gassenbauer, R. Schafranek,

S. P. Harvey, and T. O. Mason, Thin Solid Films 518, 1197 (2009).

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