1. ARTICLE
pubs.acs.org/JPCC
Local Electronic Structure of Lithium-Doped ZnO Films Investigated
by X-ray Absorption Near-Edge Spectroscopy
Shu-Yi Tsai,† Min-Hsiung Hon,† and Yang-Ming Lu*,‡
†
Department of Materials Science and Engineering, National Cheng Kung University, Tainan, Taiwan
‡
Department of Electrical Engineering, National University of Tainan, Tainan, Taiwan
ABSTRACT: Lithium-doped ZnO films were deposited by radio frequency magnetron
sputtering on Corning 1737 glass substrates. The Li content in the films varied from 0 to 10
at. %, as determined by wavelength-dispersive X-ray analysis and inductively coupled plasma
mass spectrometry. The effect of Li content on the microstructure and electrical properties was
studied. The XRD results indicated that all the samples have a ZnO wurtzite structure, and no
secondary phase formed as the Li atoms were incorporated into ZnO thin films. The Hall and
electrical resistance measurements revealed that the resistivity is decreased by Li doping. The
EXAFS measurement showed that the bonding length of both ZnÀO and ZnÀZn was
decreased after converting to p-type conduction due to incorporation of lithium atoms. All
the results confirmed that the Li ions were well incorporated into the ZnO lattices as a result of
substituting Zn sites without changing the wurtzite structure, and no secondary phase appeared in the Li-doped ZnO thin film.
1. INTRODUCTION ZnO:Li thin films in detail using X-ray absorption spectroscopy
Transparent electronics is an advanced technology concerning (XAS), which allows us to understand the primary mechanism of
the realization of invisible electronic devices. Recently, research the p-type behavior of ZnO.
on ZnO thin films has been increasing due to their low cost,
nontoxicity, and high stability in hydrogen plasma. ZnO is one of 2. EXPERIMENTAL DETAILS
the most important semiconductor materials for optoelectronic We have deposited the Li-doped ZnO thin films on a glass
applications based on its wide band gap (3.37 eV) and large substrate (Corning 1737F) at room temperature by radio
exciton binding energy (60 meV). Its considerable applications frequency (rf) sputtering in a mixture of oxygen and argon
in solar cells,1 sensors,2,3 photocatalytics,4 and optoelectronic gases. The target material was zinc metal (99.99% purity). Ar
devices5,6 have also triggered wide research interest. However, (99.995%) and O2 (99.99%) with a ratio of 10:1 were introduced
the fabrication of p-type ZnO, which is an essential step for pÀn as the sputtering gases at a total pressure of 1.33 Pa. The content
junction-based devices, is still a bottleneck because of a self- of Li in the ZnO thin films was adjusted by placing Li2CO3 disks
compensation effect from native defects, such as oxygen vacan- on the target surface. The thickness and diameter of the Li2CO3
cies and zinc interstitials on doping.7À9 p-Type ZnO is achieved disks were controlled to be 0.2 and 1 cm, respectively. The
by the doping of elements from group I (Li, Na, K) and from Li2CO3 disks were made by sintering at high temperature; they
group V (N, P, As) dopants. The theoretical studies demon- will be dissociated into Li2O and CO2 at decomposition.15,16 A
strated, the group I elements might be better p-type dopants than rotating substrate holder was used to obtain uniform composi-
group V elements for introducing shallowness of acceptor tion distributions in the films. After being deposited, the films
levels.10 Lu et al. proposed that Li can be expected to substitute were annealed at 450 °C in Ar ambient for 3 h with heating and
Zn in its site, thus shifting the (002) position to the higher 2θ cooling rates of 3 and 2 °C/min, respectively. The film thickness
values and reducing the c-axis length,11 whereas Wardle et al. was measured using a conventional stylus surface roughness
suggested that lithium doping may be limited by the formation of detector (Alpha-step 200, Tencor, USA). All samples were
complexes, such as LiZnÀLii, LiZnÀH, and LiZnÀAX.12 Never- analyzed in the same thickness of about 200 nm. The film
theless, there remain a lot of open questions and controversial composition was determined by a high resolution hyper probe
opinions. A determination of the dominating mechanism of the (JXA-8500F Fe-EPMA) equipped with a wavelength-dispersive
local electronic structure of lithium-doped ZnO and its valence X-ray spectrometer (WDS) and by an inductively coupled
state is necessary, preferably from experimental results rather plasma mass spectrometer (Hewlett-Packard 4500 ICP-MS).
than a theoretical approach. A way to identify these issues is X-ray The crystalline structure of the films was confirmed by glanc-
absorption spectroscopy (XAS). XAS is a powerful tool to ing incident angle XRD (GIAXRD) using a Cu KR radiation
investigate the local arrangement of atoms in materials, providing
element-specific information about chemistry, site occupancy, Received: January 26, 2011
and the neighboring environment.13,14 In this work, we describe Revised: April 18, 2011
the local environment around Zn and its chemical valence state in Published: April 29, 2011
r 2011 American Chemical Society 10252 dx.doi.org/10.1021/jp200815d | J. Phys. Chem. C 2011, 115, 10252–10255

2. The Journal of Physical Chemistry C ARTICLE
Figure 2. Resistivity (σ), Hall mobility (μ), and carrier concentration
(n) as functions of Li content for ZnO:Li thin films deposited on a glass
substrate.
crystalline phases was seen, suggesting good crystallinity with a
high preferential c-axis orientation and formation of LiZn in the
films. With the Li-doped content increasing, the full width at half-
maximum (fwhm) became weak and broad, and the diffraction
angle shifted toward the high angle direction, as shown in
Figure 1b. It is known generally that dopants can be substituted
or inserted, depending on the doping ions' size. Yamamoto24 and
Onodera22 reported that most doping ions substituted for Zn ion
sites in the doping case due to a decrease in the Madelung energy.
If Liþ ions interstitial to Zn2þ ions, the lattice parameter of the
Figure 1. (a) XRD diffraction patterns of undoped ZnO and ZnO:Li ZnO crystal increases and the (0 0 2) peak should shift to low
thin films with different Li contents. (b) Positions of the (002) peak and
full width at half-maxima (fwhm) of ZnO:Li thin films.
angle. In addition, Li at a substitutional site creates an energy
level at 0.09 eV. However, Li at an interstitial site creates an
(λ = 0.15406 nm). The Zn K-edge (9659 eV) XAS spectra were energy level at 1.58 eV, and it is more stable, according to Park
recorded on a wiggler C (BL-17C) beamline at the National et al. According to their result, the XRD peak shifts toward high
Synchrotron Radiation Research Center (NSRRC) of Taiwan. angle, which implies that the highly incorporated Liþ ions exist in
The XAS data analyses were performed using standard methods the substitutional sites, not in the interstitial sites.
and WinXAS software. The fittings of the EXAFS were per- The electrical resistivity values of ZnO:Li films with different
formed using least-squared fittings from outputs from FEFF8.0 Li dopant contents can be seen in Figure 2. The Hall coefficient
software. General EXAFS data analysis has been described in and hot probe measurements method were employed to identify
the literature.17À19 The parameters calculated from the fittings the type of conduction in these films. The p-type conductivity
were the interatomic distances, coordination numbers, and the behavior could be achieved only in the Li content from 1 to 5 at. %.
DebyeÀWaller factors. The resistivity and carrier concentrations As is well-known, in Li-doped ZnO specimens, Li doping mainly
of the ZnO:Li thin film at room temperature were measured by a occurs as follows25
Hall-effect measurement system (Lake Shore, model 7662) using ZnO 0
the van der Pauw method. Li2 O s LiZn þ Li• þ Oo
f i
where LiZn represents lithium on the zinc lattice site, Lii lithium
3. RESULTS AND DISCUSSION in an interstitial position, and Oo oxygen on the lattice site of
Figure 1 shows the XRD pattern of ZnO thin films with itself. Significantly, LiZn is theoretically predicted to have a
different Li-doping contents on glass substrates prepared by an rf shallow acceptor level.10 For the 1 at. % Li-doped ZnO films,
magnetron sputtering method. The compositions of the doped weak p-type conduction was found to have high resistivity
ZnO films were determined by both WDS and ICP-MS. The and low carrier concentration due to the fact that holes may
O/Zn atomic ratios were obtained from WDS, and the Li/Zn be compensated for by n-type native defects. For the 3 at. %
atomic ratios were measured by ICP-MS. The Li content in the Li-doped ZnO film, more Li atoms substituted for Zn, which
films increased with an increasing number of Li2CO3 disks acted as an effective acceptor, thus achieving optimized p-type
mounted on the Zn target surface. The maximum Li content conduction. By doping a I group impurity into the IIÀVI
obtained in this study was approximately 10 at. %. A similar semiconductor of ZnO, the impurity became the acceptor, and
content has been reported by Wang.20À22The solubility of Li in the electrons decreased, thus transforming the film from an
single-crystal ZnO is very high, with up to around 30% of the Zn n-type to a p-type conductive behavior. In a certain amount of
sites being occupied by Li has been reported by Onodera et al.23 doping, the electronic holes increased with doping Li concentra-
Only one peak corresponding to a (002) plane was observed for tions. The optimized doping amount obtained in this study is at
all the samples, and no diffraction peaks reflected from other 3 at. % Li-doped ZnO thin films with 0.11 Ω 3 cm in electrical
10253 dx.doi.org/10.1021/jp200815d |J. Phys. Chem. C 2011, 115, 10252–10255

3. The Journal of Physical Chemistry C ARTICLE
resistivity, 0.22 cm2/V 3 s in Hall mobility, and 3.13 Â 1018 cmÀ3 structural aspects, X-ray spectroscopy (XAS) provides comple-
in concentration. The conversion of the conducting type from mentary details on the electronic environments of the metals and
p-type to n-type at a higher doping level (5 at. %), which may be on the short-range structure. The XAS, including X-ray absorp-
attributed to the formation of the defects (Lii or LiZnÀLii) acting tion near-edge structure (XANES) and extended X-ray absorp-
as donors. They may act as a compensative and scattering centers tion fine structure (EXAFS), is a nonintrusive technique
that reduce the hole concentration and result in further deteriora- intended to investigate the molecular environment around a
ting of hole mobility and depress the p-type behavior of ZnO. target element in various matrices of different states. For
Wardle et al. suggested that excess lithium may occupy interstitial example, XANES can be used to determine the oxidation state
sites as well and lead to the formation of electrically inactive of an absorbing element by measuring the energy shift of the
LiZnÀLii pairs.12 absorption edge. With higher oxidation states, the absorption
ZnO:Li thin films have previously been partly characterized by edge shifts to higher energy by a few electronvolts. Furthermore,
X-ray powder diffraction and transmission electron microscopy.26 the shape of the XANES profile often reflects the geometry of the
Whereas X-ray diffraction yields information on long-range first coordination sphere of many transition elements with
unfilled d orbitals and can be used to qualitatively assess the
coordination environment of the absorbing atom. Figure 3
illustrates the normalized Zn K-edge spectra of undoped and 3
at. % Li-doped ZnO films. The result shows a sharp increase in
absorption edge energy of 9664 eV, caused by excitation of Zn 1s
electrons.27 The XANES in Figure 3 for both samples are
virtually identical, indicating that the Zn is predominantly
present in a formal 2þ oxidation state in tetrahedral coordina-
tion. As the amount of doped Li increased, the edge energy
corresponding to the Zn2þ oxidation state has a small structural
distortion. The enlarged near-edge spectra are shown in the inset
of Figure 3. Because the intensity is approximately proportional
to the density of the unoccupied Zn 3d-derived states, the results
indicate that increases in the absorption intensity will decrease
the number of 3d electrons in Zn.
For the purpose of studying in more detail the local structure
of the ZnO host lattice upon Li incorporation, we performed
Figure 3. Normalized Zn K-edge XANES spectra of undoped ZnO and extended X-ray absorption fine structure (EXAFS) measure-
ZnO:Li samples. The inset shows enlargements of the peaks associated ments at the Zn K edges. The Zn K-edge EXAFS spectrum was
with the 1s-to-3d transitions. quantitatively simulated using the FEFF 8.0 program.19 Both the
experimental results and the fitting curve are displayed in R-space
and are provided in Figure 4. In the simulation, Liþ is assumed to
substitute for the Zn2þ site in the ZnO lattice. The first shell of
the radial distribution function indicates the position of the
ZnÀO bonding distance, and the second shell peak denotes a
combination of ZnÀZn bonding distances. From the results, the
fitting curve was shown to be in good agreement with the
experimental results, which provided evidence that Li occupied
Zn sites in the ZnO lattice without forming impurity phases. In
the case of the Li-doped ZnO, the intensity of the second peak
decreased, revealing degradation in the crystal structure. This
result was also consistent with the XRD measurement.
To obtain quantitative structural information, the best-fit
values for the Zn K edge are listed in Table 1. From the results,
it can be seen that the undoped ZnO thin films exists at the same
local structure as the wurtzite ZnO, in which Zn atoms are
surrounded by four O atoms in the first-coordination shell. The
Figure 4. Fourier transform magnitude of Zn K-edge EXAFS of first shell ZnÀO coordination number NZnÀO was 4.018 Å, and
undoped ZnO and 3 at. % Li-doped ZnO films. the bond length RZnÀO was 1.971 Å. As we know, the bond
Table 1. Structural Parameters of ZnO:Li from EXAFS Analyses, where R is the Interatomic Distance, N is the Coordination
Number, and σ2 is the DebyeÀWaller Factor
sample interaction type interatomic distance (R) coordination number (N) DebyeÀWaller (σ2)
undoped ZnO thin film ZnÀO 1.971 4.018 0.002
ZnÀZn 3.270 12.09 0.001
3 at. % Li-doped ZnO films ZnÀO 1.969 4.013 0.004
ZnÀZn 3.211 11.89 0.006
10254 dx.doi.org/10.1021/jp200815d |J. Phys. Chem. C 2011, 115, 10252–10255

4. The Journal of Physical Chemistry C ARTICLE
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’ AUTHOR INFORMATION 71, 231.
(28) Fu, Z. W.; Zhang, L. N.; Qin, Q. Z.; Zhang, Y. H.; Zeng, X. K.;
Corresponding Author
Cheng, H.; Huang, R. B.; Zheng, L. S. J. Phys. Chem. A 2000, 104, 2980.
*Telephone: þ886-6-2606123, ext. 7771. Fax: þ886-6-2602305.
E-mail: ymlumit@yahoo.com.tw and ymlu@mail.nutn.edu.tw.
’ ACKNOWLEDGMENT
The authors are grateful to the National Science Council in
Taiwan for financially supporting this research under 99-2221-
E-024-003 and 98-2221-E-006-075-MY3.
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