Optical Materials 29 (2007) 1548–1552
www.elsevier.com/locate/optmat
Conductivity enhancement and semiconductor–metal transition
in Ti-doped ZnO films
J.J. Lu
a
a,*
, Y.M. Lu b, S.I. Tasi c, T.L. Hsiung d, H.P. Wang d, L.Y. Jang
e
Nano-Technology R&D Center, Kun-Shan University, No. 949, Da-Wan Road, Yung-Kang City Tainan, Hsien 71003, Taiwan, ROC
b
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, ROC
d
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 2006
Available 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 sputtering of Ti. In this work, X-ray diffraction (XRD), electrical resistivity, X-ray absorption spectroscopy (XAS), optical transmission spectrum, 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 · 103 X cm, and a metallic conduction behavior was observed in the ZnO: Ti films with
Ti = 1.3%. The enhancement of conductivity and the semiconductor–metal transition are likely attributed to the increase in the free carrier 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 researched
in recent years for the breadth of their technological applications. As a well known wide band gap semiconductor,
ZnO is gaining importance due to possible applications
and desired properties such as low cost and non-toxicity.
In particular, highly c-axis oriented ZnO films can be
applied to acoustic-wave devices due to their large piezoelectric constant. Recently, a number of ZnO films doped
with various metallic ions have been studied extensively
for the manipulation of their optical and electrical properties [1–5]. It is generally agreed that the conductivity of
*
Corresponding author. Tel.: +886 6 2019664; fax: +886 6 2050509.
E-mail address: [email protected] (J.J. Lu).
0925-3467/$ - see front matter 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.optmat.2006.08.002
ZnO film is associated with the free carriers generated from
Zn interstitial atoms and oxygen vacancies [6,7]. However,
the conduction mechanisms of transition-metal doped ZnO
films are still not all clearly understood. By appropriate
doping with Ti, the conductivities were reported to be
improved, and this was attributed by some authors to the
increase of free carrier concentration [5,8]. Furthermore,
a semiconductor–metal transition was reported by Park
et al. [9].
In this study, the ZnO: Ti films were fabricated by simultaneous RF magnetron sputtering of Zn and DC magnetron sputtering of Ti. X-ray diffraction, temperature
dependence of electrical resistivity, X-ray absorption spectroscopy, optical transmission spectrum, and Hall-effect
measurements were carried out in order to investigate the
effects of doping on physical properties as well as study
the conducting mechanism of the Ti-doped ZnO films.
J.J. Lu et al. / Optical Materials 29 (2007) 1548–1552
1549
2. Experimental
3. Results and discussion
Several technologies have been used to prepare the conducting 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 were
deposited onto heated glass substrates (Corning 7059) by
magnetron co-sputtering from both Zn and Ti targets in a
mixture of oxygen and argon gases with a target-to-substrate distance of 6 cm. To avoid strong interference in
plasma during co-sputtering process, RF and DC powers
are chosen for Zn and Ti targets respectively. The targets
used in this study are metal Zn (99.99% purity, 76.2 mm
diameter), and metal Ti (99.999% purity, 76.2 mm diameter). The substrate temperature was kept at 300 C using a
feedback-controlled heater. The variation of the substrate
temperature was maintained within ±5 C during deposition process. A cryo-pump, backed by a rotary pump, was
used to achieve a background pressure of 1 · 106 Torr
before introducing argon gas. The RF power of Zn target
was kept constant at 300 W and DC power of Ti target varied from 50 W to 300 W.
The film thickness was measured using a conventional
stylus surface roughness detector (Alpha-step 200, TENCOR, USA). All the film thicknesses were maintained at
an approximate value of 650 nm. The atomic percentages
of Ti in the ZnO films were determined by the EDS (energy
dispersion spectrum) using a field emission scanning microscope (FE-SEM, JEOL, JSM-6700F).
The crystal structures of the deposited films were examined by X-ray diffraction. The XRD patterns of the deposited films were obtained by an X-ray diffractometer
(Rigaku, RINT 2000) using CuKa-radiation (k =
1.54056 Å). Lattice parameters were determined by
XRAYSCAN [17] by the least-square-fitting method.
The temperature dependence of electrical resistance of
the films was measured by four-point probe method on an
LR-700 AC bridge in a system fully automated for temperature stability and data acquisition. The resistivities, carrier
concentrations, and mobility of the films at room temperature were measured by a Hall-effect measurement system
(LakeShore, Model 7662) using van der Pauw method.
X-ray absorption spectroscopy was carried out at the
NSRRC (National Synchrotron Radiation Research Center, Hsinchu, Taiwan) on X-ray wiggler beamline BL17C
by using a Si(1 1 1) double-crystal monochromator. The
Zn K-edge XANES (X-ray absorption near-edge structure)
and EXAFS (extended X-ray absorption fine structure)
were obtained by the fluorescence mode in conventional
ionization chambers. The estimated energy resolution was
1.5 eV for the near-edge structure.
The optical transmission spectra were obtained by using
a UV spectrophotometer (MFS-630, Multi-Purpose Optical Characteristics Measurement System) with a continuous wave He–Cd laser in a wavelength range of 300–
800 nm.
Fig. 1 displays the XRD patterns of the pure and Tidoped ZnO films. The XRD patterns of the films are consistent with the hexagonal lattice structure, and a strong
(0 0 2) preferential orientation is observed. Neither TiO4
nor Zn2TiO4 phase is detected in the scanning range. It
implies that the Ti atoms may substitute the zinc sites substitutionally or incorporate interstitially in the lattice.
From Fig. 1, it can be found that the locations of the diffraction peaks shift towards lower angles, and the peaks
become broader as the powers of Ti are increased. These
experimental evidences indicate that the Ti atoms are more
likely to substitute the zinc sites substitutionally. According
to previous reports [8,18], the crystallite of ZnO: Ti films
will be distorted by Ti atoms substituting into the zinc sites,
and the films suffer a compressive stress in the direction
parallel to the surface. This effect could result in increasing
the interplanar spacing (d), hence lead to the observed
decrease in the diffraction angle. Besides, compared with
the pure ZnO film, the peaks become broader indicating
that the crystallinity of the Ti-doped films was weakened
as the Ti power was increased. The degradation of crystallinity may be interpreted as follows. While higher DC powers are applied, more Ti atoms can incorporate into the
lattice of ZnO, whereas the residual stress increases as well.
The increasing residual stress is likely to distort the wellestablished crystal structure of the ZnO lattice.
The atomic percentages of Ti in the ZnO films, which
were semi-quantitatively determined by EDS, are listed in
Table 1. The Ti content of the sputtered films increases
Fig. 1. X-ray diffraction patterns of ZnO: Ti films with different Ti
contents.
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J.J. Lu et al. / Optical Materials 29 (2007) 1548–1552
Table 1
Measured Ti composition, carrier concentration, resistivity, and optical
energy gap (Eg) of ZnO: Ti films at different Ti target powers
Ti power
(W)
Ti content
(%)
Carrier concentration
(cm3)
Resistivity
(X cm)
Eg
(eV)
0
50
100
200
300
0
1.3
2.0
3.1
4.2
1.54 · 1019
3.16 · 1020
8.47 · 1019
1.46 · 1018
7.31 · 1017
1.12 · 101
3.82 · 103
2.74 · 102
9.83 · 101
2.24 · 102
3.35
3.32
3.37
3.39
3.46
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 of
ZnO:Ti films. The most intense absorption structure, which
is produced by Zn 1s ! 4p transition, is the so-called white
line. The white line intensity, varying with the transition
probability, is related to the structure geometry of the Zn
atom, and the energy shifts recorded in the white line are
primarily affected by the bond character, charge distribution, and valence state [19]. According to Fig. 2, all of
the XANES spectra are similar (expect for 4.2% Ti), indicating that overall bond character and geometrical structure of the Zn atom remain basically unchanged. Besides,
the Zn valence state of all samples is +2, with no evidence
of other valence states detectable from the XANES spectra.
The k3-weighted Fourier transformed EXAFS spectra of
the films, which are obtained by multiplying the weighting
scheme (k3) after background subtraction and normalization [20], are shown in the Fig. 3. The EXAFS spectra
are usually significantly influenced by the environments
of the zinc atom, and can be used to verify the local structure 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.
Fig. 3. The k3-weighted Fourier transformed EXAFS spectra of the pure
and Ti-doped ZnO films.
ing the information on the oxygen nearest-neighbor of the
Zn atoms, is equal to 1.942 Å. The second peak, which is
situated at 2.573 Å, corresponds to the Zn–Zn distance.
These distances are in good agreement with those of
reported ZnO films in the würtzite form [21]. In addition,
the second peak almost smears out at Ti = 4.2%, indicating
a poor crystallinity at that Ti concentration. These results
are consistent with our XRD measurements.
Temperature dependence of the normalized electrical
resistances of the films between 77 and 300 K are shown
in Fig. 4. Those of pure and Ti-doped ZnO films with Ti
content of more than 2.0% exhibit basically a semiconductor-type conduction behavior as expected. However, the
resistivity of Ti-doped ZnO films at 2.0% Ti remains almost
constant throughout the entire temperature range. Furthermore, at Ti = 1.3% the resistance of the Ti-doped ZnO film
decreases as the temperature decreases, i.e., the material
exhibits a metallic type conduction behavior. From
Fig. 4, it is apparent that a semiconductor–metal transition
occurs in the Ti-doped ZnO films.
The resistivities and carrier concentrations of the films,
obtained from Hall-effect measurements at room temperature, are also listed in Table 1. One can see that the carrier
concentrations increase abruptly and the resistivity of the
ZnO:Ti films reaches a minimum value of 3.82 ·
103 X cm in the film with 1.3% Ti. However, the resistivity
increases as further increases the Ti contents. The decrease
in conductivity could be attributed to the increase in
induced stress field, which will reflect free carriers and
reduce the conductivity. These results imply that only
appropriate amount of Ti could induce more free carriers
and prevent from acting as scattering centers. The increase
J.J. Lu et al. / Optical Materials 29 (2007) 1548–1552
1551
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 Tidoped ZnO films.
in carrier concentrations may originate from the following
two mechanisms: (1) Substituting Ti4+ for Zn2+ in the ZnO
structure will result in two more free electrons that contribute to the electrical conductivity. (2) The point defects
(oxygen vacancies) increase due to a lower atomic O/Zn
ratio with Ti doping [8].
Another mechanism affecting the conductive behavior is
the band structure. In order to obtain the optical energy
gap (Eg) of ZnO:Ti films, we exploited the optical transmittance measurements. Using the relation for a parabolic
band, the optical band gap Eg of the films at zero temperature can be determined from the absorption coefficient a
by the relation [22] (see Fig. 5):
films: the Burstein–Moss (BM) band-filling effect [24] and
band gap shrinkage phenomenon [23]. The former has
the effect of widening the band gap due to the conducting
band edge being filled by excessive carriers donated by
the doped impurity. Contrarily, the band gap shrinkage
effect, stemming from the change in the nature and strength
of interaction potentials between donors and the host lattice, will result in band tailing of both valence and conducting bands, and lead to a merging of valence and conducting
bands. A red-shift in Eg and a metallic type conducting
behavior indicate that the latter effect prevails at
Ti = 1.3%. Due to large difference in electron configuration
(Ti: [Ar]3d24s2, Zn: [Ar]3d104s2), Ti atoms substitute the
zinc sites in ZnO lattice will result in extra weakly bound
electrons. The overlapping of the wave functions of these
weakly bound electrons will lead to an impurity band in
the band gap. The impurity band becomes broader as the
density of conducting electrons (free carrier concentration)
is increased. Once the impurity band becomes broad
enough to reach the edge of the conducting band, the effective ionization energy of these shallow donors will vanish,
and result in a metallic type conducting behavior [25].
a ¼ Aðhm Eg Þ1=2 :
However, at higher temperatures the actual band structure must be taken into account. If a Lorentzian type with
a broadening d is chosen, then the absorption coefficient (a)
will be modified as
2
a ¼ Afðhm Eg Þ þ ½ðhm Eg Þ þ d2 1=2 1=2
g
:
From the above relation, the optical energy gap Eg can
then be determined [23]. The obtained Eg energies are listed
in Table 1. Compared to Eg = 3.35 eV for pure ZnO film,
almost all the samples exhibit a blue-shift of Eg. In contrast, a red-shift (Eg = 3.32 eV) was observed in the film
with Ti = 1.3%. This result indicates that only proper content of Ti could result in a red-shift of the band-gap energy.
Our observation is also consistent with the report of Park
et al. [9].
It is generally agreed that two competing mechanisms
are dominant in affecting the optical energy gap in ZnO
4. Conclusions
In this work, Ti-doped ZnO films were deposited by
simultaneous RF sputtering of Zn and DC magnetron
sputtering of Ti. These films were examined by XRD, temperature dependence of electrical resistance, Hall-effect,
and optical transmittance measurements as well as Zn
K-edge XAS. The XRD and XAS measurements revealed
that all of the films have hexagonal würtzite type structure
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J.J. Lu et al. / Optical Materials 29 (2007) 1548–1552
with a strong (0 0 2) preferential orientation. When higher
powers of Ti target were applied, the crystallinity of the
films became poorer. The distortion of the lattice is likely
to stem from the increase of residual stress due to more
Ti atoms incorporated into the zinc sites.
The enhancement of conductivity and the semiconductor–metal transition were verified by the temperature
dependence of the normalized resistance and Hall-effect
measurements. The improvement of conductance can be
attributed to the increase in carrier concentration due to
more free electrons and more oxygen vacancies while
substituting Ti4+ for Zn2+. A red-shift in Eg observed in
the film with Ti = 1.3% implies a prevailing of the bandgap shrinkage effect at that Ti content. From the results,
we conclude that only proper concentration of Ti could
greatly enhance the conductivity and prevent from distorting the lattice appreciably. The enhancement of conductivity along with the merging of the donor and conducting
band are likely to result in the observed semiconductor–
metal transition in the ZnO:Ti films.
Acknowledgement
This work was supported by National Science Council
of the Republic of China under contract No. NSC-952112-M-168-001.
References
[1] S.S. Lin, J.L. Huang, P. Šajgalik, Surf. Coat. Technol. 190 (2005) 39.
[2] S. Kohiki, M. Nishitani, T. Wanda, J. Appl. Phys. 75 (1994) 2069.
[3] K. Tominaga, N. Umezu, I. Mori, T. Ushiro, T. Moriga, I.
Nakabayashi, J. Vac. Sci. Technol. A 16 (1998) 1213.
[4] Z. Jin, K. Hasegawa, T. Fukumura, Y.Z. Yoo, T. Hasegawa, H.
Koinuma, M. Kawasaki, Physica E 10 (2001) 256.
[5] Y.M. Lu, C.M. Chang, S.I. Tsai, T.S. Wey, Thin Solid Films 447–448
(2004) 56.
[6] Y. Igasaki, H. Saito, J. Appl. Phys. 69 (1991) 2190.
[7] F.R. Blom, F.C.M. Van de Pol, G. Bauhuis, Th.J.A. Popma, Thin
Solid Films 204 (1991) 365.
[8] S.S. Lin, J.L. Huang, P. Šajgalik, Surf. Coat. Technol. 191 (2005) 286.
[9] Y.R. Park, K.J. Kim, Solid State Commun. 123 (2002) 147.
[10] A.P. Roth, D.F. Williams, J. Appl. Phys. 52 (1981) 6685.
[11] O.F. Khan, P. O’Brien, Thin Solid Films 173 (1989) 95.
[12] P. Petrou, R. Singh, D.E. Brodie, Appl. Phys. Lett. 35 (1979) 930.
[13] J.H. Morgan, D.E. Brodie, Can. J. Phys. 60 (1982) 1387.
[14] Furusaki, J. Takahashi, K. Kodaira, J. Ceram. Soc. Jpn. 102 (2)
(1994) 200.
[15] P. Fons, A. Yamada, K. Iwata, K. Matsubara, S. Niki, K. Nakahara,
H. Takasu, Nucl. Inst. Methods Phys. Res. B 199 (2003) 190.
[16] B.E. Sernelius, K.F. Berggren, Z.C. Jin, I. Hamberg, C.G. Granqvist,
Phys. Rev. B 37 (1988) 10244.
[17] J.S. Hwang, C. Tien, Chin. J. Phys. (Taipei) 34 (1996) 41.
[18] R. Lohmann, E. Österschulze, K. Thoma, H. Gärtner, W. Herr, B.
Matthes, E. Broszeit, K.H. Kloos, Mater. Sci. Eng. A 139 (1991) 259.
[19] G. Liang, Ph.D. dissertation, Rutgers, The State University of New
Jersey, New Brunswick, 1990.
[20] B.K. Teo, EXAFS: Basic Principles and Data Analysis, Springer,
Berlin, 1986.
[21] R.W.G. Wyckoff, Crystal Structure, vol. 1, Interscience, New York,
1960, p. 19.
[22] M. Jiles, Introduction to Electronic Properties of Materials, Chapmam and Hall, 1994 (chapter 9).
[23] A.P. Roth, J.B. Webb, D.F. Williams, Phys. Rev. B 25 (1982) 7836.
[24] E. Burstein, Phys. Rev. 93 (1954) 632;
T.S. Moss, Proc. Phys. Soc. London, Ser. B 67 (1954) 775.
[25] N.F. Mott, Metal–Insulator Transitions, second ed., Taylor &
Francis, London, 1990 (chapter 4 and 5).