Conductivity enhancement and semiconductor metal transition in Ti-doped ZnO films

Transcription

1 Optical Materials 29 (2007) Conductivity enhancement and semiconductor metal transition in 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, 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 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. Ó 2006 Elsevier B.V. All rights reserved. PACS: Ci; Hf Keywords: ZnO thin films; Semiconductor metal transition; Optical properties 1. Introduction Transparent conducting oxides, such as SnO 2 (NESA), In 2 O 3 (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.: ; fax: address: jjlu@mail.ksu.edu.tw (J.J. Lu). 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 /$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi: /j.optmat

2 J.J. Lu et al. / Optical Materials 29 (2007) Experimental 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 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, TEN- COR, 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 = 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 nm. 3. Results and discussion 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 (002) preferential orientation is observed. Neither TiO 4 nor Zn 2 TiO 4 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.

3 1550 J.J. Lu et al. / Optical Materials 29 (2007) Table 1 Measured Ti composition, carrier concentration, resistivity, and optical energy gap (E g ) of ZnO: Ti films at different Ti target powers Ti power (W) Ti content (%) Carrier concentration (cm 3 ) Resistivity (X cm) E g (ev) Fig. 2. Zn K-edge XANES spectra of the pure and Ti-doped ZnO films. Fig. 3. The k 3 -weighted Fourier transformed EXAFS spectra of the pure and Ti-doped ZnO films. 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 k 3 -weighted Fourier transformed EXAFS spectra of the films, which are obtained by multiplying the weighting scheme (k 3 ) 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, containing the information on the oxygen nearest-neighbor of the Zn atoms, is equal to Å. The second peak, which is situated at 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 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

4 J.J. Lu et al. / Optical Materials 29 (2007) 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 Ti 4+ for Zn 2+ 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 (E g ) of ZnO:Ti films, we exploited the optical transmittance measurements. Using the relation for a parabolic band, the optical band gap E g of the films at zero temperature can be determined from the absorption coefficient a by the relation [22] (see Fig. 5): a ¼ Aðhm E g Þ 1=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 a ¼ Afðhm E g Þþ½ðhm E g Þ 2 þ d 2 Š 1=2 g 1=2 : From the above relation, the optical energy gap E g can then be determined [23]. The obtained E g energies are listed in Table 1. Compared to E g = 3.35 ev for pure ZnO film, almost all the samples exhibit a blue-shift of E g. In contrast, a red-shift (E g = 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 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 E g 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]3d 2 4s 2, Zn: [Ar]3d 10 4s 2 ), 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]. 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

5 1552 J.J. Lu et al. / Optical Materials 29 (2007) 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 Ti 4+ for Zn 2+. A red-shift in E g 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 M 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) [3] K. Tominaga, N. Umezu, I. Mori, T. Ushiro, T. Moriga, I. Nakabayashi, J. Vac. Sci. Technol. A 16 (1998) [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 (2004) 56. [6] Y. Igasaki, H. Saito, J. Appl. Phys. 69 (1991) [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) [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) [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) [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, [20] B.K. Teo, EXAFS: Basic Principles and Data Analysis, Springer, Berlin, [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) [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).