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                                                                                                                                        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
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
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
The Journal of Physical Chemistry C                                                                                                                      ARTICLE

length of RLiÀO in ZnO is 1.661 Å, and the RZnÀLi bond length                     (8) Saw, K. G.; Ibrahim, K.; Lim, Y. T.; Chai, M. K. Thin Solid Films
is 2.703 Å,28 which is much different than the bond length of                   2007, 515, 2879.
RZnÀO, excluding the possibility that the interstitial mechanism                  (9) Zhang, S. B.; Wei, S. H.; Zunger, A. Phys. Rev. B 2001,
was executed. For the 3 at. % Li-doped ZnO films, only the                      63, 075205.
coordination number and interatomic distance of the second                        (10) Park, C. H.; Zhang, S. B.; Wei, S. H. Phys. Rev. B 2002,
                                                                               66, 073202.
shell decreased, whereas that of the first shell was similar to the                (11) Lu, J. G.; Zhang, Y. Z.; Ye, Z. Z.; Zeng, Y. J.; He, H. P.; Zhu,
undoped ZnO thin films. The second shell interatomic distance                   L. P.; Huang, J. Y.; Wang, L.; Yuan, J.; Zhao, B. H.; Li, X. H. Appl. Phys.
RZnÀZn of 3 at. % Li-doped ZnO decreased from 3.270 to 3.211                   Lett. 2006, 89, 112113.
Å, which implied a decreased lattice parameter for the Li-doped                   (12) Wardle, M. G.; Goss, J. P.; Briddon, P. R. Phys. Rev. B 2005,
ZnO films. These results indicate that the substitution of Li                   71, 155205.
atoms for parts of Zn atoms in the ZnO lattice leads to a decrease                (13) Liu, X. J.; Song, C.; Zeng, F.; Pan, F. J. Phys.: Condens. Matter
in the nearest-neighbor bond length between the Zn and Zn                      2007, 19, 296208.
atoms of Li-doped ZnO films. For the second coordination shell,                    (14) Norton, D. P.; Pearton, S. J.; Hebard, A. F.; Theodoropoulou,
it can be seen that the DebyeÀWaller factor (σ2) is larger for the             N.; Boatner, L. A.; Wilson, R. G. Appl. Phys. Lett. 2003, 82, 239.
3 at. % Li-doped ZnO as compared with the undoped one. This                       (15) Ktalkherman, M. G.; Emelkin, V. A.; Pozdnyakov, B. A. Theor.
                                                                               Found. Chem. Eng. 2009, 43, 88.
result is also consistent with the XRD measurement because local                  (16) Timoshevskii, A. N.; Ktalkherman, M. G.; Emel’kin, V. A.;
structure distortions may occur as a consequence of a lattice                  Pozdnyakov, B. A.; Zamyatin, A. P. High Temp. 2008, 46, 414.
mismatch induced at a higher doping amount.                                       (17) Koningsberger, D. C.; Prins, R. X-ray Absorption: Principles,
                                                                               Applications, Techniques of EXAFS, SEXAFS, and XANES; Wiley:
                                                                               New York, 1988.
4. CONCLUSIONS                                                                    (18) Dimitrov, D. A.; Ankudinov, A. L.; Bishop, A. R.; Conradson,
    The XRD results indicated that all of the ZnO:Li films had                  S. D. Phys. Rev. B 1998, 58, 14227.
(002) preferred orientations with hexagonal wurtzite structures.                  (19) Ankudinov, A. L.; Ravel, B.; Rehr, J. J.; Conradson, S. D. Phys.
The electrical measurements by hot probe and the Hall coeffi-                    Rev. B 1998, 58, 7565.
                                                                                  (20) Wang, D. Y.; Zhou, J.; Liu, G. Z. J. Alloys Compd. 2009, 481, 802.
cient showed that the lithium-doped ZnO thin films had a p-type
                                                                                  (21) Mohamed, G. A.; Mohamed, E.; Abu El-Fadl, A. Physica B 2001,
conductive behavior. The optimized doping amount obtained in                   308, 949.
this study is at 3 at. % Li-doped ZnO thin films with 0.11 Ω 3 cm in               (22) Onodera, A.; Yoshio, K.; Satoh, H.; Yamashita, H.; Sakagami,
electrical resistivity, 0.22 cm2/V 3 s in Hall mobility, and 3.13 Â            N. Jpn. J. Appl. Phys., Part 1 1998, 37, 5315.
1018 cmÀ3 in carrier concentration. The XANES and EXAFS                           (23) Onodera, A.; Tamaki, N.; Kawamura, Y.; Sawada, T.; Yamashita,
analyses indicate that the Li substituted for Zn2þ without                     H. Jpn. J. Appl. Phys., Part 1 1996, 35, 5160.
changing the crystalline structure of ZnO. From the EXAFS                         (24) Yamamoto, T.; Katayama-Yoshida, H. J. Cryst. Growth 2000,
results, it indicates that decreases in the second shell of the                214, 552.
ZnÀZn coordination number are caused by incorporation of                          (25) Bonasewicz, P.; Hirschwald, W.; Neumann, G. J. Electrochem.
lithium in the substitutional sites rather than in the interstitial            Soc. 1986, 133, 2270.
                                                                                  (26) Wang, B.; Tang, L. D.; Qi, J. A.; Du, H. L.; Zhang, Z. B. J. Alloys
sites in p-type ZnO sputtering film.
                                                                               Compd. 2010, 503, 436.
                                                                                  (27) Kelly, R. A.; Andrews, J. C.; DeWitt, J. G. Microchem. J. 2002,
’ 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.

’ REFERENCES
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Chen, Y. F. Superlattices Microstruct. 2010, 47, 160.
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2011, 22, 035601.
   (3) Zheng, K. B.; Gu, L. L.; Sun, D. L.; Mo, X. L.; Chen, G. R. Mater.
Sci. Eng., B 2010, 166, 104.
   (4) Guo, M. Y.; Fung, M. K.; Fang, F.; Chen, X. Y.; Ng, A. M. C.;
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                                                                            10255                dx.doi.org/10.1021/jp200815d |J. Phys. Chem. C 2011, 115, 10252–10255

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Zn ofilmlidoped

  • 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 length of RLiÀO in ZnO is 1.661 Å, and the RZnÀLi bond length (8) Saw, K. G.; Ibrahim, K.; Lim, Y. T.; Chai, M. K. Thin Solid Films is 2.703 Å,28 which is much different than the bond length of 2007, 515, 2879. RZnÀO, excluding the possibility that the interstitial mechanism (9) Zhang, S. B.; Wei, S. H.; Zunger, A. Phys. Rev. B 2001, was executed. For the 3 at. % Li-doped ZnO films, only the 63, 075205. coordination number and interatomic distance of the second (10) Park, C. H.; Zhang, S. B.; Wei, S. H. Phys. Rev. B 2002, 66, 073202. shell decreased, whereas that of the first shell was similar to the (11) Lu, J. G.; Zhang, Y. Z.; Ye, Z. Z.; Zeng, Y. J.; He, H. P.; Zhu, undoped ZnO thin films. The second shell interatomic distance L. P.; Huang, J. Y.; Wang, L.; Yuan, J.; Zhao, B. H.; Li, X. H. Appl. Phys. RZnÀZn of 3 at. % Li-doped ZnO decreased from 3.270 to 3.211 Lett. 2006, 89, 112113. 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