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Optical Materials 29 (2007) 1548–1552

Conductivity enhancement and semiconductor–metal transitionin Ti-doped ZnO films

J.J. Lu a,*, Y.M. Lu b, S.I. Tasi c, T.L. Hsiung d, H.P. Wang d, L.Y. Jang e

a Nano-Technology R&D Center, Kun-Shan University, No. 949, Da-Wan Road, Yung-Kang City Tainan, Hsien 71003, Taiwan, ROCb Graduate Institute of Electro-Optical and Materials Science, National Formosa University, Huwei, Yunlin, Taiwan, ROC

c Department of Materials Science and Engineering, National Cheng-Kung University, Tainan, Taiwan, ROCd Department of Environmental Engineering, National Cheng-Kung University, Tainan, Taiwan, ROC

e National Synchrotron Radiation Research Center, Hsinchu, Taiwan, ROC

Received 30 June 2006; received in revised form 2 August 2006; accepted 9 August 2006Available online 18 September 2006

Abstract

Ti-doped ZnO films were deposited onto Corning 7059 glass substrates by simultaneous RF sputtering of Zn and DC magnetron sput-tering of Ti. In this work, X-ray diffraction (XRD), electrical resistivity, X-ray absorption spectroscopy (XAS), optical transmission spec-trum, and Hall-effect measurements were utilized in order to study the properties of the Ti-doped ZnO films. The resistivities of the ZnO:Ti films were reduced to a value of 3.82 · 10�3 X cm, and a metallic conduction behavior was observed in the ZnO: Ti films withTi = 1.3%. The enhancement of conductivity and the semiconductor–metal transition are likely attributed to the increase in the free car-rier concentration, along with the band-gap shrinkage effects caused by Ti doping.� 2006 Elsevier B.V. All rights reserved.

PACS: 78.20.Ci; 78.66.Hf

Keywords: ZnO thin films; Semiconductor–metal transition; Optical properties

1. Introduction

Transparent conducting oxides, such as SnO2 (NESA),In2O3 (ITO) and ZnO, have been extensively researchedin recent years for the breadth of their technological appli-cations. As a well known wide band gap semiconductor,ZnO is gaining importance due to possible applicationsand desired properties such as low cost and non-toxicity.In particular, highly c-axis oriented ZnO films can beapplied to acoustic-wave devices due to their large piezo-electric constant. Recently, a number of ZnO films dopedwith various metallic ions have been studied extensivelyfor the manipulation of their optical and electrical proper-ties [1–5]. It is generally agreed that the conductivity of

0925-3467/$ - see front matter � 2006 Elsevier B.V. All rights reserved.

doi:10.1016/j.optmat.2006.08.002

* Corresponding author. Tel.: +886 6 2019664; fax: +886 6 2050509.E-mail address: jjlu@mail.ksu.edu.tw (J.J. Lu).

ZnO film is associated with the free carriers generated fromZn interstitial atoms and oxygen vacancies [6,7]. However,the conduction mechanisms of transition-metal doped ZnOfilms are still not all clearly understood. By appropriatedoping with Ti, the conductivities were reported to beimproved, and this was attributed by some authors to theincrease of free carrier concentration [5,8]. Furthermore,a semiconductor–metal transition was reported by Parket al. [9].

In this study, the ZnO: Ti films were fabricated by simul-taneous RF magnetron sputtering of Zn and DC magne-tron sputtering of Ti. X-ray diffraction, temperaturedependence of electrical resistivity, X-ray absorption spec-troscopy, optical transmission spectrum, and Hall-effectmeasurements were carried out in order to investigate theeffects of doping on physical properties as well as studythe conducting mechanism of the Ti-doped ZnO films.

Fig. 1. X-ray diffraction patterns of ZnO: Ti films with different Ticontents.

J.J. Lu et al. / Optical Materials 29 (2007) 1548–1552 1549

2. Experimental

Several technologies have been used to prepare the con-ducting ZnO films, including chemical vapor deposition[10,11], reactive evaporation [12,13], sol–gel [14], MBE(molecular beam epitaxy) [15], DC and RF co-sputtering[5,8,16], etc. In this study, the Ti doped ZnO films weredeposited onto heated glass substrates (Corning 7059) bymagnetron co-sputtering from both Zn and Ti targets in amixture of oxygen and argon gases with a target-to-sub-strate distance of 6 cm. To avoid strong interference inplasma during co-sputtering process, RF and DC powersare chosen for Zn and Ti targets respectively. The targetsused in this study are metal Zn (99.99% purity, 76.2 mmdiameter), and metal Ti (99.999% purity, 76.2 mm diame-ter). The substrate temperature was kept at 300 �C using afeedback-controlled heater. The variation of the substratetemperature was maintained within ±5 �C during deposi-tion process. A cryo-pump, backed by a rotary pump, wasused to achieve a background pressure of 1 · 10�6 Torrbefore introducing argon gas. The RF power of Zn targetwas kept constant at 300 W and DC power of Ti target var-ied from 50 W to 300 W.

The film thickness was measured using a conventionalstylus surface roughness detector (Alpha-step 200, TEN-COR, USA). All the film thicknesses were maintained atan approximate value of 650 nm. The atomic percentagesof Ti in the ZnO films were determined by the EDS (energydispersion spectrum) using a field emission scanning micro-scope (FE-SEM, JEOL, JSM-6700F).

The crystal structures of the deposited films were exam-ined by X-ray diffraction. The XRD patterns of the depos-ited films were obtained by an X-ray diffractometer(Rigaku, RINT 2000) using CuKa-radiation (k =1.54056 A). Lattice parameters were determined byXRAYSCAN [17] by the least-square-fitting method.

The temperature dependence of electrical resistance ofthe films was measured by four-point probe method on anLR-700 AC bridge in a system fully automated for temper-ature stability and data acquisition. The resistivities, carrierconcentrations, and mobility of the films at room tempera-ture were measured by a Hall-effect measurement system(LakeShore, Model 7662) using van der Pauw method.

X-ray absorption spectroscopy was carried out at theNSRRC (National Synchrotron Radiation Research Cen-ter, Hsinchu, Taiwan) on X-ray wiggler beamline BL17Cby using a Si(1 11) double-crystal monochromator. TheZn K-edge XANES (X-ray absorption near-edge structure)and EXAFS (extended X-ray absorption fine structure)were obtained by the fluorescence mode in conventionalionization chambers. The estimated energy resolution was1.5 eV for the near-edge structure.

The optical transmission spectra were obtained by usinga UV spectrophotometer (MFS-630, Multi-Purpose Opti-cal Characteristics Measurement System) with a continu-ous wave He–Cd laser in a wavelength range of 300–800 nm.

3. Results and discussion

Fig. 1 displays the XRD patterns of the pure and Ti-doped ZnO films. The XRD patterns of the films are con-sistent with the hexagonal lattice structure, and a strong(002) preferential orientation is observed. Neither TiO4

nor Zn2TiO4 phase is detected in the scanning range. Itimplies that the Ti atoms may substitute the zinc sites sub-stitutionally or incorporate interstitially in the lattice.From Fig. 1, it can be found that the locations of the dif-fraction peaks shift towards lower angles, and the peaksbecome broader as the powers of Ti are increased. Theseexperimental evidences indicate that the Ti atoms are morelikely to substitute the zinc sites substitutionally. Accordingto previous reports [8,18], the crystallite of ZnO: Ti filmswill be distorted by Ti atoms substituting into the zinc sites,and the films suffer a compressive stress in the directionparallel to the surface. This effect could result in increasingthe interplanar spacing (d), hence lead to the observeddecrease in the diffraction angle. Besides, compared withthe pure ZnO film, the peaks become broader indicatingthat the crystallinity of the Ti-doped films was weakenedas the Ti power was increased. The degradation of crystal-linity may be interpreted as follows. While higher DC pow-ers are applied, more Ti atoms can incorporate into thelattice of ZnO, whereas the residual stress increases as well.The increasing residual stress is likely to distort the well-established crystal structure of the ZnO lattice.

The atomic percentages of Ti in the ZnO films, whichwere semi-quantitatively determined by EDS, are listed inTable 1. The Ti content of the sputtered films increases

Table 1Measured Ti composition, carrier concentration, resistivity, and opticalenergy gap (Eg) of ZnO: Ti films at different Ti target powers

Ti power(W)

Ti content(%)

Carrier concentration(cm�3)

Resistivity(X cm)

Eg

(eV)

0 0 1.54 · 1019 1.12 · 10�1 3.3550 1.3 3.16 · 1020 3.82 · 10�3 3.32

100 2.0 8.47 · 1019 2.74 · 10�2 3.37200 3.1 1.46 · 1018 9.83 · 10�1 3.39300 4.2 7.31 · 1017 2.24 · 102 3.46

Fig. 3. The k3-weighted Fourier transformed EXAFS spectra of the pureand Ti-doped ZnO films.

1550 J.J. Lu et al. / Optical Materials 29 (2007) 1548–1552

as the DC power of the Ti target increases from 1.3%(50 W) to 4.2% (300 W).

Fig. 2 shows the Zn K-edge XANES spectrum ofZnO:Ti films. The most intense absorption structure, whichis produced by Zn 1s! 4p transition, is the so-called whiteline. The white line intensity, varying with the transitionprobability, is related to the structure geometry of the Znatom, and the energy shifts recorded in the white line areprimarily affected by the bond character, charge distribu-tion, and valence state [19]. According to Fig. 2, all ofthe XANES spectra are similar (expect for 4.2% Ti), indi-cating that overall bond character and geometrical struc-ture of the Zn atom remain basically unchanged. Besides,the Zn valence state of all samples is +2, with no evidenceof other valence states detectable from the XANES spectra.

The k3-weighted Fourier transformed EXAFS spectra ofthe films, which are obtained by multiplying the weightingscheme (k3) after background subtraction and normaliza-tion [20], are shown in the Fig. 3. The EXAFS spectraare usually significantly influenced by the environmentsof the zinc atom, and can be used to verify the local struc-ture of the ZnO films. The first peak in the spectra, contain-

Fig. 2. Zn K-edge XANES spectra of the pure and Ti-doped ZnO films.

ing the information on the oxygen nearest-neighbor of theZn atoms, is equal to 1.942 A. The second peak, which issituated at 2.573 A, corresponds to the Zn–Zn distance.These distances are in good agreement with those ofreported ZnO films in the wurtzite form [21]. In addition,the second peak almost smears out at Ti = 4.2%, indicatinga poor crystallinity at that Ti concentration. These resultsare consistent with our XRD measurements.

Temperature dependence of the normalized electricalresistances of the films between 77 and 300 K are shownin Fig. 4. Those of pure and Ti-doped ZnO films with Ticontent of more than 2.0% exhibit basically a semiconduc-tor-type conduction behavior as expected. However, theresistivity of Ti-doped ZnO films at 2.0% Ti remains almostconstant throughout the entire temperature range. Further-more, at Ti = 1.3% the resistance of the Ti-doped ZnO filmdecreases as the temperature decreases, i.e., the materialexhibits a metallic type conduction behavior. FromFig. 4, it is apparent that a semiconductor–metal transitionoccurs in the Ti-doped ZnO films.

The resistivities and carrier concentrations of the films,obtained from Hall-effect measurements at room tempera-ture, are also listed in Table 1. One can see that the carrierconcentrations increase abruptly and the resistivity of theZnO:Ti films reaches a minimum value of 3.82 ·10�3 X cm in the film with 1.3% Ti. However, the resistivityincreases as further increases the Ti contents. The decreasein conductivity could be attributed to the increase ininduced stress field, which will reflect free carriers andreduce the conductivity. These results imply that onlyappropriate amount of Ti could induce more free carriersand prevent from acting as scattering centers. The increase

Fig. 4. Temperature dependence of the normalized electrical resistances(q(T)/q(300 K)) of the ZnO: Ti films with different Ti contents.

Fig. 5. The absorption coefficient (a) vs. photon energy of pure and Ti-doped ZnO films.

J.J. Lu et al. / Optical Materials 29 (2007) 1548–1552 1551

in carrier concentrations may originate from the followingtwo mechanisms: (1) Substituting Ti4+ for Zn2+ in the ZnOstructure will result in two more free electrons that contrib-ute to the electrical conductivity. (2) The point defects(oxygen vacancies) increase due to a lower atomic O/Znratio with Ti doping [8].

Another mechanism affecting the conductive behavior isthe band structure. In order to obtain the optical energygap (Eg) of ZnO:Ti films, we exploited the optical transmit-tance measurements. Using the relation for a parabolicband, the optical band gap Eg of the films at zero temper-ature can be determined from the absorption coefficient aby the relation [22] (see Fig. 5):

a ¼ Aðhm� EgÞ1=2:

However, at higher temperatures the actual band struc-ture must be taken into account. If a Lorentzian type witha broadening d is chosen, then the absorption coefficient (a)will be modified as

a ¼ Afðhm� EgÞ þ ½ðhm� EgÞ2 þ d2�1=2g1=2:

From the above relation, the optical energy gap Eg canthen be determined [23]. The obtained Eg energies are listedin Table 1. Compared to Eg = 3.35 eV for pure ZnO film,almost all the samples exhibit a blue-shift of Eg. In con-trast, a red-shift (Eg = 3.32 eV) was observed in the filmwith Ti = 1.3%. This result indicates that only proper con-tent of Ti could result in a red-shift of the band-gap energy.Our observation is also consistent with the report of Parket al. [9].

It is generally agreed that two competing mechanismsare dominant in affecting the optical energy gap in ZnO

films: the Burstein–Moss (BM) band-filling effect [24] andband gap shrinkage phenomenon [23]. The former hasthe effect of widening the band gap due to the conductingband edge being filled by excessive carriers donated bythe doped impurity. Contrarily, the band gap shrinkageeffect, stemming from the change in the nature and strengthof interaction potentials between donors and the host lat-tice, will result in band tailing of both valence and conduct-ing bands, and lead to a merging of valence and conductingbands. A red-shift in Eg and a metallic type conductingbehavior indicate that the latter effect prevails atTi = 1.3%. Due to large difference in electron configuration(Ti: [Ar]3d24s2, Zn: [Ar]3d104s2), Ti atoms substitute thezinc sites in ZnO lattice will result in extra weakly boundelectrons. The overlapping of the wave functions of theseweakly bound electrons will lead to an impurity band inthe band gap. The impurity band becomes broader as thedensity of conducting electrons (free carrier concentration)is increased. Once the impurity band becomes broadenough to reach the edge of the conducting band, the effec-tive ionization energy of these shallow donors will vanish,and result in a metallic type conducting behavior [25].

4. Conclusions

In this work, Ti-doped ZnO films were deposited bysimultaneous RF sputtering of Zn and DC magnetronsputtering of Ti. These films were examined by XRD, tem-perature dependence of electrical resistance, Hall-effect,and optical transmittance measurements as well as ZnK-edge XAS. The XRD and XAS measurements revealedthat all of the films have hexagonal wurtzite type structure

1552 J.J. Lu et al. / Optical Materials 29 (2007) 1548–1552

with a strong (002) preferential orientation. When higherpowers of Ti target were applied, the crystallinity of thefilms became poorer. The distortion of the lattice is likelyto stem from the increase of residual stress due to moreTi atoms incorporated into the zinc sites.

The enhancement of conductivity and the semiconduc-tor–metal transition were verified by the temperaturedependence of the normalized resistance and Hall-effectmeasurements. The improvement of conductance can beattributed to the increase in carrier concentration due tomore free electrons and more oxygen vacancies whilesubstituting Ti4+ for Zn2+. A red-shift in Eg observed inthe film with Ti = 1.3% implies a prevailing of the band-gap shrinkage effect at that Ti content. From the results,we conclude that only proper concentration of Ti couldgreatly enhance the conductivity and prevent from distort-ing the lattice appreciably. The enhancement of conductiv-ity along with the merging of the donor and conductingband are likely to result in the observed semiconductor–metal transition in the ZnO:Ti films.

Acknowledgement

This work was supported by National Science Councilof the Republic of China under contract No. NSC-95-2112-M-168-001.

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