Surface & Coatings Technology 201 (2007) 5318 – 5322
www.elsevier.com/locate/surfcoat
Metal-doped ZnO thin films: Synthesis and characterizations
S.H. Jeong, B.N. Park, S.-B. Lee, J.-H. Boo ⁎
Department of Chemistry and Institute of Basic Science, Sungkyunkwan University, Suwon 440-746, Republic of Korea
Available online 23 August 2006
Abstract
The metal doped ZnO (MZO, M = Al, Ag) films were prepared by RF magnetron sputtering on glass substrates with extraordinary designed the
MZO targets. For the doping sources contained in each MZO target, we used Al(OH)3, AgNO3 powders by mixing the powders into pure ZnO
powder with various rate (0–10 wt.%), respectively. We investigated on the optical and electrical properties of the as-sputtered MZO films as
dependences of the dopant contents in targets. All the MZO films had shown a preferred orientation in the [001] direction. As the quantity and
variety of metal dopants were changed, the crystallinity and the transmittance as well as optical band gap were changed. The electrical resistivity
was also changed with changing metal doping amounts and a kind of dopant. To investigate these phenomena in details, the O K-edge
configurations of the MZO films were studied.
© 2006 Elsevier B.V. All rights reserved.
PACS: 81.05.Dz
Keywords: Metal doped ZnO; Rf magnetron sputtering; Transparent conducting oxide; NEXAFS
1. Introduction
Zinc oxide have been investigated to several applications
such as transparent conducting oxide (TCO), photodetectors,
and light emitting diodes because it had wide band gap (3.37 eV)
[1–3]. Generally, undoped ZnO thin films typically exhibit ntype conduction. It was caused by a deviation from stoichiometry due to native defects like oxygen vacancies or zinc
interstitials [4]. Many different methods such as rf/dc sputtering
[5], sol-gel method [6], metal organic chemical vapor deposition
[7], and pursed laser deposition [8] have been used for the
preparation of ZnO thin films. The structural, physical and
electrical properties of ZnO films were governed by dopants,
deposition parameters [9] and post treatment [10]. In our
previous works, we researched effect of sputtering parameters
such as target-to-substrate distance (Dts) and dopants [11–13]. In
addition, the resistivity of ZnO films was changed by extrinsic
impurities. Therefore, ZnO films have been doped to enhance
their properties with elements of Li, Al, Ga, In, and Ag, etc.
⁎ Corresponding author. Tel.: +82 31 290 5972; fax: +82 31 290 7075.
E-mail address: [email protected] (J.-H. Boo).
0257-8972/$ - see front matter © 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.surfcoat.2006.07.185
However, there are not many reports on the systematic study of
dopant (Ag, Al) effect on film properties. In this paper, we
investigated the effect of dopant on the structural, optical and
electrical properties of Ag and Al doped ZnO (SZO and AZO)
thin films.
2. Experimental
Two different targets were prepared with different weight
percent (wt.%) of dopant (AgNO3, Al(OH)3) in the targets (0–
10 wt.% MZO). ZnO films were deposited on the Si(001) and
glass substrates at room temperature (RT), rf power of 150 W
and target-to-substrate distance (Dts) of 45 mm. The thickness
of MZO films were made of 150–200 nm on glass substrates for
optical measurements. The crystal structure, microstructure, and
the thickness were observed using X-ray diffraction (XRD) and
scanning electron microscope (SEM), respectively. X-ray
photoelectron spectroscopy (XPS) and energy dispersive Xray spectroscopy (EDX) were also utilized to analyze the chemical ratio of MZO films. The optical transmittance measurements were performed with an UV/visible spectrophotometer.
The oxygen K-edge features of grown MZO films on Si(001)
substrates were also investigated using near edge X-ray absorption fine structure (NEXAFS) spectroscopy.
S.H. Jeong et al. / Surface & Coatings Technology 201 (2007) 5318–5322
5319
Fig. 1. Typical XRD patterns of MZO films deposited on glass substrates at room temperature (a) and high resolution XRD patterns as a function of dopant wt.% : Al
(OH)3 (b) and AgNO3 (c) in the target.
Fig. 2. Optical transmittance of MZO films prepared at R.T.: (a) AZO, and (b)SZO. Fig. 2(c) and (d) show the optical band gap of the MZO films : (c) Al-doped ZnO
films and (d) Ag-doped ZnO films.
5320
S.H. Jeong et al. / Surface & Coatings Technology 201 (2007) 5318–5322
3. Results and discussion
3.1. Structural characterization
Typical XRD patterns of MZO films in the Fig. 1(a) were
observed only (002) peaks at 2θ ≈ 34°. High resolution XRD
patterns as a function of dopant wt.% : Al(OH)3 (b) and AgNO3
(c) in the target were shown in Fig. 1. Fig. 1(b) showed highresolution XRD patterns of AZO (002) plane. From this figure,
positions of (002) peaks were shifted to high 2θ values with
increasing the variation of the Al content to 4 wt.%. It meant
that the lattice parameter of ZnO had been decreasing while Al
dopants were doping until 4 wt.%. Above 4 wt.% doping, 2θ
values of (002) peak were decreased with increasing Al dopants.
In Fig. 1(c), positions of (002) peaks were shifted to low 2θ
values with increasing amounts of Ag content. It was indicated that the lattice parameter of MZO films was changed in the
c-axis with adding metal dopant (Al, Ag). Moreover, intensities of (002) peaks were broadened with increasing metal dopants. We could conclude that metal ions (Ag2+: 122 pm or
Al3+: 53 pm) were substituted into the Zn2+ (72 pm) in the
MZO crystal.
Fig. 4. (a) Variations of the resistivities and dopant contents in both the MZO
films and the targets; (a) AZO, and (b) SZO.
3.2. Optical properties
We measured that the average transmittance in the visible
range was over 80% for all MZO films (in the Fig. 2(a), (b)). The
optical band gap (Eg) of the SZO film could be obtained by
plotting α2 vs. hν (α is the absorption coefficient and hν is the
photon energy) and extrapolating the straight-line portion of this
plot to the photon energy axis (Fig. 2(c), (d)) [14]. As aluminum
contents increased, the absorption edge shifted to a shorter
wavelength region (Fig. 2(a)). With increasing silver contents, the
absorption edge slightly shifted to a longer wavelength region
(Fig. 2(b)). Fig. 2(c) and (d) showed the variation of optical band
gap as a function of dopant contents, respectively. Optical band
gap were widened with increasing Al contents until 4 wt.% (in the
Fig. 2(c)). The reasons were that the densities of electrons were
decreased while Al ions were substituted into Zn2+ sites in the
films. However, the energy band gap narrowed because the excess
Al atoms were segregated into the grain boundaries over the 4 wt.
% dopant. These segregated Al atoms did not act as dopant. The
Fig. 2(b) was shown the variation of optical band gap as a function
of Ag contents. It showed that a band gap narrowed with increasing Ag contents. It meant that the Ag+ was substituted into
the Zn+.
3.3. Chemical ratio and electrical resistivity
Fig. 3. EDS data of pure ZnO film (a), 10 wt.% AZO film (b), 10 wt.% SZO film
(c) grown on glass substrates at RT.
The EDS spectra were shown in Fig. 3. Positions of peak,
which were O–Kα, Zn–Lα, Zn–Kα, and Zn–Kβ, were appeared at
S.H. Jeong et al. / Surface & Coatings Technology 201 (2007) 5318–5322
5321
0.518, 1.109, 8.635, and 9.577 keV, respectively. Additional peaks
at 1.496 and 2.984 keV were attributed to Al–Kα and Ag–Lα,
respectively. It estimated the contents of dopant in the films by the
EDS results. Fig. 4(a) showed that Al percentages were 0%,
1.24%, 2.78%, 3.04%, 3.64%, and 4.71% when Al dopants
increased from 0 to 10 wt.% by 2 wt.%, respectively. Fig. 4(b)
showed that Ag percentages were 0%, 3.04%, 7.07%, 9.65%,
13.21%, and 13.47% when Ag dopants increased from 0 to 10 wt.
% by 2 wt.%, respectively. It seemed that doping strongly influenced on the electrical properties of MZO films because metal
dopants acted donors. On the whole electrical resistivities were
related to oxygen vacancies, Al dopant and Zn concentrations at
the interstitial sites, grain boundary [15]. With increasing the Al
content from 0 to 4 wt.%, the resistivity in Fig. 4(a) decreases. At
the same Dts, electrical resistivity was from 5.0 × 10− 1 Ω cm for
pure ZnO films to a minimum value of 9.8 × 10− 2 Ω cm for AZO
films. However, the AZO films prepared with above 6 wt.%
resistivities of which were increased to 12 Ω cm because both hall
mobility and carrier concentration were decreased [14]. With
increasing the Ag contents from 0 to 10 wt.% by 2 wt.%, the
resistivity in Fig. 4(b) was decreased from 5.0 × 10− 1 Ω cm to
1.4 × 10− 2 Ω cm. The result of decreased resistivity produced
silver's metallic characteristic because Ag dopant had aggregated.
3.4. Oxygen K-edge features of MZO films
Chen et al. [16] reported a comparison of oxygen K-edge
NEXAFS spectra of several 3d transition metal oxides. Here, the
four O K-edge features could be assigned to the one-electron
transition from the O 1s orbital to the 2t2g, 3eg, 3a1g, and 4t1u
orbitals, respectively. The ten d-orbital electrons in ZnO completely
occupied both the 2t2g, and 3eg orbitals. The lowest unoccupied
molecular orbital (LUMO) for ZnO was the 3a1g orbital, which was
most likely the origin for the broad O K-edge feature at 538 eV.
Fig. 5 showed the O K-edge features of MZO films by the amounts
of the Al and Ag dopant. In pure ZnO, the O K-edge features
showed broad peak at 540 eV and shoulder peak at 543 eV. The
whole changes were not observed greatly with increasing Ag
dopant. In detail, the peak at 543 eV increased gradually because of
the density increment of conduction band by metal doping. The
shoulder peaks at 543 eV had intensified that Al dopants were more
influential than Ag dopants. The shoulder peaks intensified to
create metal–oxygen bond by the metal dopant. This result was in
good agreement with electrical resistivity data shown in Fig. 4.
NEXAFS data has measured after Ar ion sputtering for 10 min to
remove adsorbed species on the surface of the MZO film.
4. Conclusions
MZO films with various metal contents (Ag and Al of 0–
10 wt.%) were prepared by rf magnetron sputtering on Si(001)
and glass substrates with especially designed ZnO targets. The
structural, optical and electrical properties of MZO films
depended on dopant contents in target. The deposited MZO
films have a preferred crystalline orientation of [001] direction.
As amounts of the Al dopant in the target were increased, the
angles of (002) peaks were shifted to higher angle, while peaks
Fig. 5. NEXAFS spectra measured from the MZO films with different dopant
contents at R.T : (a) AZO, and (b) SZO.
of [001] direction were shifted to lower with increasing Ag
dopant in the target, suggested different doping mechanism.
These results indicated that the lattice parameters of MZO film
were changed in the c-axis with doping metal dopant. The
optical band gap and electrical resistivities were decreased with
increasing metal dopants.
Acknowledgments
Support of this research by the KOSEF(Project No. R012003-000-10019-0) is gratefully acknowledged. This work was
also supported by the BK21 project of the Ministry of Education
Korea and by the Center for Advanced Plasma Surface
Technology as well as CNNC at the Sungkyunkwan University.
The NEXAFS experiments at PLS (Pohang Accelerator
Laboratory) were supported in part by MOST (the Korean
Ministry Of Science and Technology) and POSTECH (Pohang
University of Science and Technology).
References
[1] S.H. Jeong, S. Kho, D. Jung, S.B. Lee, J.-H. Boo, Surf. Coat. Technol. 174
(2003) 187.
[2] S. Nakamura, T. Mukai, M. Snoh, Appl. Phys. Lett. 64 (1994) 1687.
[3] D.M. Bagnall, Y.F. Chen, Z. Zhu, T. Yao, S. Koyama, M.Y. Shen, T. Goto,
Appl. Phys. Lett. 70 (1997) 2230.
[4] S. Major, K.L. Chopra, Sol. Energy Mater. 17 (1998) 319.
5322
[5]
[6]
[7]
[8]
S.H. Jeong et al. / Surface & Coatings Technology 201 (2007) 5318–5322
K. Ellmer, R. Wendt, Surf. Coat. Technol. 93 (1997) 21.
T. Schuler, M.A. Aegerter, Thin Solid Films 351 (1999) 125.
J.L. Deschanvres, B. Bochu, J.C. Joubert, J. Phys. 4 (3) (1993) 485.
H. Kim, A. Pique, J.S. Horwitz, h. Murata, Z.H. Kafafi, C.M. Gilmore, D.B.
Chrisey, Thin Solid Films 377 (2000) 798.
[9] F.-J. Haug, Z.S. Geller, H. Zogg, A.N. Tiwari, C. Vignali, J. Vac. Sci.
Technol. A 19 (2001) 171.
[10] P. Nunes, E. Fortunato, R. Martins, Thin Solid Films 383 (2001) 277.
[11] S.H. Jeong, J.-H. Boo, Thin Solid Films 435 (2003) 78.
[12] S.H. Jeong, J.-H. Boo, Thin Solid Films 447–448 (2004) 105.
[13] S.H. Jeong, B.N. Park, S.B. Lee, J.-H. Boo, Surf. Coat. Technol. 193
(2005) 340.
[14] K.C. Park, D.Y. Ma, K.H. Kim, Thin Solid Films 305 (1997) 201.
[15] Y. Igasaki, H. Saito, J. Appl. Phys. 70 (1991) 3613.
[16] J.G. Chen, B. Fruhberger, M.L. Colaianni, J. Vac. Sci. Technol. A 14
(1996) 1668.