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  • ARTICLE pubs.acs.org/JPCCLocal Electronic Structure of Lithium-Doped ZnO Films Investigatedby X-ray Absorption Near-Edge SpectroscopyShu-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 ofthe 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 DETAILSthe most important semiconductor materials for optoelectronic We have deposited the Li-doped ZnO thin films on a glassapplications based on its wide band gap (3.37 eV) and large substrate (Corning 1737F) at room temperature by radioexciton binding energy (60 meV). Its considerable applications frequency (rf) sputtering in a mixture of oxygen and argonin solar cells,1 sensors,2,3 photocatalytics,4 and optoelectronic gases. The target material was zinc metal (99.99% purity). Ardevices5,6 have also triggered wide research interest. However, (99.995%) and O2 (99.99%) with a ratio of 10:1 were introducedthe 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 contentjunction-based devices, is still a bottleneck because of a self- of Li in the ZnO thin films was adjusted by placing Li2CO3 diskscompensation effect from native defects, such as oxygen vacan- on the target surface. The thickness and diameter of the Li2CO3cies and zinc interstitials on doping.7À9 p-Type ZnO is achieved disks were controlled to be 0.2 and 1 cm, respectively. Theby the doping of elements from group I (Li, Na, K) and from Li2CO3 disks were made by sintering at high temperature; theygroup V (N, P, As) dopants. The theoretical studies demon- will be dissociated into Li2O and CO2 at decomposition.15,16 Astrated, 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 filmslevels.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 andZn in its site, thus shifting the (002) position to the higher 2θ cooling rates of 3 and 2 °C/min, respectively. The film thicknessvalues and reducing the c-axis length,11 whereas Wardle et al. was measured using a conventional stylus surface roughnesssuggested that lithium doping may be limited by the formation of detector (Alpha-step 200, Tencor, USA). All samples werecomplexes, such as LiZnÀLii, LiZnÀH, and LiZnÀAX.12 Never- analyzed in the same thickness of about 200 nm. The filmtheless, there remain a lot of open questions and controversial composition was determined by a high resolution hyper probeopinions. A determination of the dominating mechanism of the (JXA-8500F Fe-EPMA) equipped with a wavelength-dispersivelocal electronic structure of lithium-doped ZnO and its valence X-ray spectrometer (WDS) and by an inductively coupledstate 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 radiationinvestigate the local arrangement of atoms in materials, providingelement-specific information about chemistry, site occupancy, Received: January 26, 2011and the neighboring environment.13,14 In this work, we describe Revised: April 18, 2011the 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 theFigure 1. (a) XRD diffraction patterns of undoped ZnO and ZnO:Li ZnO crystal increases and the (0 0 2) peak should shift to lowthin films with different Li contents. (b) Positions of the (002) peak andfull 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 Parkrecorded on a wiggler C (BL-17C) beamline at the National et al. According to their result, the XRD peak shifts toward highSynchrotron Radiation Research Center (NSRRC) of Taiwan. angle, which implies that the highly incorporated Liþ ions exist inThe 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 differentformed using least-squared fittings from outputs from FEFF8.0 Li dopant contents can be seen in Figure 2. The Hall coefficientsoftware. General EXAFS data analysis has been described in and hot probe measurements method were employed to identifythe literature.17À19 The parameters calculated from the fittings the type of conduction in these films. The p-type conductivitywere 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 mainlyof the ZnO:Li thin film at room temperature were measured by a occurs as follows25Hall-effect measurement system (Lake Shore, model 7662) using ZnO 0the van der Pauw method. Li2 O s LiZn þ Li• þ Oo f i where LiZn represents lithium on the zinc lattice site, Lii lithium3. 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 adifferent 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 resistivityZnO films were determined by both WDS and ICP-MS. The and low carrier concentration due to the fact that holes mayO/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, whichfilms increased with an increasing number of Li2CO3 disks acted as an effective acceptor, thus achieving optimized p-typemounted on the Zn target surface. The maximum Li content conduction. By doping a I group impurity into the IIÀVIobtained in this study was approximately 10 at. %. A similar semiconductor of ZnO, the impurity became the acceptor, andcontent has been reported by Wang.20À22The solubility of Li in the electrons decreased, thus transforming the film from ansingle-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 ofsites 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 atall 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 ARTICLEresistivity, 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 andp-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 techniquethat reduce the hole concentration and result in further deteriora- intended to investigate the molecular environment around ating of hole mobility and depress the p-type behavior of ZnO. target element in various matrices of different states. ForWardle et al. suggested that excess lithium may occupy interstitial example, XANES can be used to determine the oxidation statesites as well and lead to the formation of electrically inactive of an absorbing element by measuring the energy shift of theLiZnÀLii 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 theWhereas 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 performedFigure 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 waswith 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. TheFigure 4. Fourier transform magnitude of Zn K-edge EXAFS of first shell ZnÀO coordination number NZnÀO was 4.018 Å, andundoped ZnO and 3 at. % Li-doped ZnO films. the bond length RZnÀO was 1.971 Å. As we know, the bondTable 1. Structural Parameters of ZnO:Li from EXAFS Analyses, where R is the Interatomic Distance, N is the CoordinationNumber, 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 ARTICLElength 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 Filmsis 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. Matterin 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. Alloyssites 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 inTaiwan for financially supporting this research under 99-2221-E-024-003 and 98-2221-E-006-075-MY3.’ REFERENCES (1) Chao, H. Y.; Cheng, J. H.; Lu, J. Y.; Chang, Y. H.; Cheng, C. L.;Chen, Y. F. Superlattices Microstruct. 2010, 47, 160. (2) Lim, M. A.; Lee, Y. W.; Han, S. W.; Park, I. Nanotechnology2011, 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. 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