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A study on Ti-doped ZnO transparent conducting thin films fabricated by pulsed laser deposition
Applied Surface Science 305 (2014) 481–486
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
A study on Ti-doped ZnO transparent conducting thin films fabricated by pulsed laser deposition Wenda Zhao, Qianfei Zhou, Xin Zhang, Xiaojing Wu ∗ Department of Materials Science, Fudan University, No. 220 HanDan Road, 200433 Shanghai, PR China
a r t i c l e
i n f o
Article history: Received 21 October 2013 Received in revised form 18 March 2014 Accepted 19 March 2014 Available online 27 March 2014 Keywords: Ti-doped ZnO Transparent conducting oxide Pulsed laser deposition
a b s t r a c t Ti-doped ZnO (TZO) thin films were fabricated on glass substrates by pulsed laser deposition (PLD). The TZO films had wurtzite structure with a preferred orientation along the c-axis. The effects of Ti doping concentration, substrate temperature and oxygen pressure on crystallinity, surface morphology, electrical and optical properties of the films were studied systematically. Under the optimized deposition conditions of Ti content of 1 at% in the target, substrate temperature of 200 ◦ C and oxygen pressure of 2.5 Pa, it was found that a film would display an electrical resistivity as low as 6.34 × 10−3 · cm and also had a 93.3% mean transmittance in the visible region when its thickness was adjusted to 100 nm. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Transparent conducting oxide (TCO) thin film are widely used in microelectronic and optoelectronic devices, such as liquid crystal displays, organic light-emitting diodes and thin film solar cells [1]. High optical transmittance and low electrical resistivity are two of the essential properties required for TCO thin film materials. Indium tin oxide (ITO) is currently the most popular commercially available TCO material and is widely used in many types of optoelectronic devices due to its high transparency (better than 85% in the visible region) and low resistivity (around 2–4 × 10−4 ·cm). However, development of new generation of TCO materials is still a subject of high interest for both the academia and the industry due to the relative scarcity and high cost of indium [2]. Zinc oxide (ZnO) is one of the materials thought to be most promising in replacing ITO [3]. There are several merits to using ZnO as a TCO material. It is highly transparent in the visible region, it has low electrical resistivity with good thermal and chemical stability and it is both environmentally-friendly and low cost. Intrinsic ZnO normally exhibits n-type conductivity due to its off-stoichiometry attributed to native defects, such as oxygen vacancies (Vo ) and zinc interstitials (Zni ), which induce shallow donor states within the forbidden gap [4].
∗ Corresponding author. Tel.: +86 021 65643258; fax: +86 021 65643258. E-mail addresses: [email protected], [email protected] (X. Wu). http://dx.doi.org/10.1016/j.apsusc.2014.03.119 0169-4332/© 2014 Elsevier B.V. All rights reserved.
As a wide bandgap semiconductor material, intrinsic ZnO is highly transparent in the visible region, but relative poor in conductivity. In order to improve its conductivity without sacrificing its optical transmittance, it is necessary to induce a proper amount of carriers into it in an adequate manner. Many studies on doping of Group III elements into the ZnO film, such as B [5], Al [6], Ga [7] and In [8] have already been carried out. In those cases, the Group III elements as the dopants would substitute the bivalent Zn ions inside the film, and then donate an extra free electron [9]. For this substitution to occur, the dopant needs to have a similar radius to that of the host ion. Based on this consideration, the trivalent elements of Ga and Al have been widely studied as a dopant for fabricating high conductive ZnO films. Ga is especially being considered as the most promising candidate because its radius is very similar to that of Zn, and this means that lattice deformation is still small even with a high Ga concentration in the film [10,11]. Kim et al. [12] studied the effects of oxygen pressure and substrate temperature on the structural, electrical and optical properties of GZO thin films deposited on glass substrate by PLD. In their study they have obtained films with high crystalline quality by applying a substrate temperature of 400 ◦ C and an oxygen pressure of 20 mTorr, which displayed low electrical resistivity of 4.6 × 10−4 · cm and high optical transmittance of about 85%. It is known that the radius of a Ti4+ is 0.61 A˚ and the bond length of Ti O is 1.96 A˚ [13], while the radius of a Zn2+ is 0.74 A˚ and the ˚ As a quadrivalent element, a Ti4+ ion bond length of Zn O is 1.97 A. 2+ substituted for a Zn ion could donate two extra free electrons, instead of one in the case of Ga-doping. Consequently, Ti-doping is also considered as a highly efficient way to induce carriers into
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ZnO film. There are many techniques for preparing Ti-doped ZnO films, such as atomic layer deposition (ALD) [2], chemical bath deposition (CBD) [14], RF and DC magnetron sputtering (MS) [15,16], and pulsed laser deposition (PLD) [9]. Among these methods, PLD has several advantages. Firstly, the films deposited by PLD usually have a very similar chemical composition to that of the targets, and therefore controlling the film composition is easier. Moreover, the high kinetic energy of atoms and ionized species produced by laser irradiation allows for the crystallization of films under a lower substrate temperature, which is an important advantage when fabricating crystallized oxide film on a flexible substrate. In 2005, Lee et al. [9] studied the effect of substrate temperature, Ti-doping content and oxygen flow rate on structural, electrical and optical properties of the Ti-doped ZnO (TZO) films deposited by PLD. They found that the TZO films exhibited a strongly preferred orientation along the c-axis, and the film with a thickness of 480 nm and 1 at% Ti content which was fabricated under a substrate temperature of 100 ◦ C and an oxygen flow rate of 10 sccm showed an electrical resistivity of 3.3 × 10−2 · cm and a mean optical transmittance of 92.3%. However, only few reports focused on TZO films deposited by PLD have been published so far. In this study, the effects of substrate temperature, oxygen pressure and Ti-doping content on the electrical properties, optical properties and the structure of the films grown on glass substrates by PLD were studied systematically. The conduction mechanism of the film was also discussed. 2. Experimental A Nd:YAG pulsed laser (wavelength: 355 nm, and pulse duration: 10 ns) was employed for depositing TZO films on soda-lime glass substrates. The laser was operated at a repetition rate of 10 Hz with an energy density of 2 J/cm2 . The distance from target-tosubstrate was about 45 mm. Pure ZnO (99.99%) and TiO2 (99.99%) powders were mixed for preparing the targets with Ti concentrations (TiO2 /TiO2 + ZnO) of 0.5, 0.75, 1, 2 and 5 at%, respectively. The mixed powder of ZnO and TiO2 was formed by 10 min of stirring and then 4 h of sintering at 800 ◦ C. The sintered targets were typically 12 mm in diameter and 2.5 mm in thickness. The soda-lime glass substrates were cut into 25 mm × 20 mm pieces and cleaned inside an ultrasonic bath 30 min at a time using each of acetone, ethanol and distilled water and then dried with nitrogen gas. The deposition chamber was first evacuated to 1.0 × 10−4 Pa and the substrate was heated from room temperature to the target temperature. During deposition, oxygen gas was flowed into the chamber to keep the working pressure in the range of 1–4 Pa. After deposition, the substrate was naturally cooled down to room temperature in vacuum. Crystallinity and crystal orientation of the films were characterized by X-ray powder diffractometer of BRUKER D8 ADVANCE. The composition and content of the films were identified by X-ray Photoelectron Spectroscopy (XPS) which was carried out on a RBD upgraded PHI-5000C ESCA system (Perkin Elmer) with Mg K␣ radiation (h = 1253.6 eV). Electrical resistivity, Hall mobility and carrier concentration were determined using a HMS 5000 Hall effect measurement system. Optical transmission spectra of the films in the visible region were measured with a SHIMADZU UV-2450 UV–vis spectrophotometer. The film thickness determined by a Kosaka Laboratory ET3000 profilometer was controlled at about 100 nm. 3. Results and discussions 3.1. The effect of Ti content Fig. 1(a) shows X-ray diffraction (XRD) patterns of TZO targets prepared with different Ti-doping of 0, 1, 2 and 5 at%. Several peaks
Fig. 1. X-ray diffraction patterns of TZO targets (a) and films (b) with different Ti content.
from Zn2 TiO4 (PCPDF card No. 73-0578) phase can be observed in these patterns, indicating that a chemical reaction occurred during sintering process in the targets. It is clear from this figure that the intensities of the peaks are proportional to the concentration of Ti in the targets. Unmarked peaks match well with the hexagonal wurtzite phase of ZnO. Fig. 1(b) shows XRD patterns of TZO films that were deposited at 200 ◦ C under working pressure of 2.5 Pa by using targets that have Ti-doping concentrations of 0, 0.5, 0.75, 1, 2 and 5 at%, respectively. For all films, only a (0 0 2) peak of ZnO at 2 ≈ 34.4◦ was observed, indicating that the TZO films were polycrystalline with wurtzite structure and they had a preferred orientation along the c-axis. No peaks related with TiO2 or Zn2 TiO4 phases were found in these patterns. If any such phases did exist in the films, the ratio of the phase must have been too low or the crystallinity of the phase must have been too poor to be detected by XRD. The Ti content in the films were measured by XPS as shown in Fig. 2. It was found that when we took the film with Ti-doping of 5% as a standard, the Ti concentrations in the films were almost equal to that of the targets. This implies that Ti concentration of TZO films can be controlled simply by using targets with the desired Ti concentration. Fig. 3 shows the varying electrical properties of films between the different Ti-doping concentrations. It is well-known that the relationships between electrical resistivity (), carrier concentration (n) and Hall mobility () could be expressed as [17]: = 1/ne
(1)
The symbol e here represents the charge of an electron. It was found from Fig. 3 that the resistivity of the TZO films initially decreased from 1.92 · cm to 6.34 × 10−3 · cm with the increase of Ti-doping content from 0 to 1 at%. Thus, change can be mainly attributed to the dramatic increase of carrier concentration in the film, which mainly came from the substitution of Ti4+ into Zn2+ sites, or the presence of Ti4+ ions at some interstitial positions. However, the further increase of Ti-doping content did not decrease its resistivity furthermore. When the Ti-doing content have reached and excessed 2 at%, its Hall mobility was kept almost unchanged, but it showed a clearly drop in its carrier concentration. Taken as a
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Fig. 4. Optical mean transmittance for the TZO films with different Ti content.
Fig. 2. The relationship between the Ti content in the films and Ti content in the targets.
whole, the resistivity of the film exhibited an increasing tendency. Similar phenomenon has previously been reported by Kim et al. [18], and according to their explanation, this happened due to the excess Ti atoms forming some non-conducting Ti O clusters in the film, which act as carrier traps rather than electron donors. Clearly, excess Ti-doping is not an efficient way to enhance carrier concentration as this also forms impurity phases and induces much more crystal defects. Fig. 4 plots the transmittance of the films against their Ti concentration. All of them exhibited a transmittance higher than 85%, while the film with 1 at% Ti-doping reached the peak value of 93.3%. After that peak, the mean transmittance decreased as the Ti content increased furthermore. This could also be attributed to the appearance of the second phases.
Fig. 5. X-ray diffraction patterns deposited with different substrate temperature.
Fig. 5 shows XRD patterns of 1 at% Ti-doped films deposited at different substrate temperatures under a working pressure of 2.5 Pa. Again, all the films exhibited the wurtzite structure with a preferred orientation along the c-axis. The diffraction peak at (0 0 2) became stronger with the increase of substrate temperature,
indicating that the proportion of TZO crystalline grew with the increase of substrate temperature. Fig. 6 shows the full width at half maximum (FWHM) of (0 0 2) diffraction peaks as a function of substrate temperature. FWHM decreased with increase in substrate temperature in the beginning, from room temperature up to 200 ◦ C, and then increased only gradually with further increase of substrate temperature up to 400 ◦ C. Usually, there are two main factors that determine the FWHM for a
Fig. 3. Electrical properties of TZO films deposited at 200 ◦ C under working pressure of 2.5 Pa as a function of Ti content.
Fig. 6. FWHM of (0 0 2) diffraction peaks as a function of substrate temperature.
3.2. The effect of substrate temperature
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Fig. 7. Electrical properties of TZO films as a function of substrate temperature. Fig. 8. Optical mean transmittance of TZO films as a function of substrate temperature.
specific diffractometer: average grain size in the film and perfection of the crystal in the film. The larger the grain size is, the narrower the FWHM is, and the more defects inside the grains, the broader the FWHM becomes. It is known that the average grain size in the film must increase with the rise of substrate temperature, which should result in a decrease of FWHM. Hence the first section of the graph is rather easy to understand: when the substrate temperature rose from room temperature to 200 ◦ C, the kinetic energy of ions on the film surface increased, and the ions could migrate more easily, which was benefit for crystal growth [19]. Consequently, the grain size increased, which lead to the decrease of FWHM. However, it was found that FHWM increased in stead of decreasing for the films prepared at 300 and 400 ◦ C. Referring to the film preparation process, it could be recognized that a lot of defects must have appeared inside the films prepared at 300 or 400 ◦ C. As aforementioned, the films were cooled naturally in vacuum. In this setting, if the temperature was high enough, many oxygen atoms could escape from the film during the cooling process, resulting in formations of a number of oxygen vacancies in the film. In practice, the average grain size indeed increased for those films, and the number of internal structural defects inside the grains also increased simultaneously. Such structural defects are thought to have played a key role to broaden FWHM. Fig. 7 plots the changes in electrical properties of the films against rising substrate temperature. Compared with the film prepared at room temperature, the film prepared at 100 ◦ C did not show any remarkable changes in carrier concentration, but owing to the improved crystallinity of the film, its Hall mobility was enhanced dramatically. As a result, the resistivity of the film showed a significant decline from 4.60–1.04 × 10−2 ·cm. When the substrate temperature rose to 200 ◦ C, the carrier concentration clearly increased. This can be attributed to the substitution of Ti4+ into Zn2+ sites as an electron donor [20]. The simultaneous slight decrease of Hall mobility can be considered as a result of the increased number of crystal defects in the film. In this case, the resistivity of the film dropped to 6.34 × 10−3 ·cm. Similar tendency continued, i.e., the carrier concentration increased and the Hall mobility decreased with rise in substrate temperature. For the films prepared at higher temperatures, much more oxygen atoms could be escaping from the films to form vacancies during the cooling process, which was confirmed by XRD analysis. Such vacancies could enhance the number of carriers, but at same time they become new scattering centers. Correspondingly, their carrier concentration increased, but their Hall mobility decreased further. As a net result their resistivity increased slightly. Therefore, high substrate temperature could indeed enhance carrier concentration effectively through creating a number of oxygen
vacancies, but it was not an efficient way to fabricate a low resistivity TZO film. Fig. 8 shows the mean transmittance as a function of the substrate temperature in the visible region. The mean transmittance reached a maximum value of 93.3% for the film prepared at 200 ◦ C, and then decreased gradually with the rise of substrate temperature. Clearly, the increase of carrier concentration for the films prepared at higher temperature must be a major factor in the reduction of mean transmittance. 3.3. The effect of oxygen pressure Fig. 9 shows XRD patterns for the 1 at% Ti-doped films deposited with different oxygen pressure at 200 ◦ C. All the films had the wurtzite structure with a preferred orientation along the c-axis. Fig. 10 shows a plot of FWHM of (0 0 2) diffraction peaks as a function of oxygen pressure. It can be seen that FWHM shows a “V” shaped change with the increase of oxygen pressure. It is well-known that the influences of oxygen pressure manifest in two aspects during growth of the film. First of all, it functions as a source of oxygen supplier. By adjusting oxygen pressure, we can control the number of oxygen vacancies in the film, which in turn plays a key role in controlling the carrier concentration as well as the number of structural defects. Secondly, higher oxygen pressure means a lower vacuum, and the sputtered species would be
Fig. 9. XRD patterns for the films deposited with different oxygen pressure.
W. Zhao et al. / Applied Surface Science 305 (2014) 481–486
Fig. 10. FWHM of (0 0 2) diffraction peaks as a function of oxygen pressure.
able to impact with oxygen molecules more easily, which leads to a shorter mean free path and a lower average speed for the species. This phenomenon has a significant influence on crystal perfection. For the films prepared under low oxygen pressure, the mean free path of the sputtered species was long and the average speed of the sputtered species was high in the path from target to substrate. Such high energy sputtered species bombarded the surface of the films, and deteriorated the crystallinity [21]. Consequently, the films prepared at 1.0, 1.5 and 2.0 Pa had more crystal defects, and subsequently showed a broader FWHM. On the other hand, for the films prepared at high oxygen pressure, such as 3.0 and 4.0 Pa, the mean free path of the sputtered species became shorter, and the average speed of the sputtered species became lower, resulting in a decrease of the particle’s mobility on the surface of the film. The particles with a lower kinetic energy could not migrate into more suitable lattice sites, adjust their own bond direction and length to obtain optimum bonding to the adjacent ones [22]. In those cases, their grain size was small, the crystallinity of the film was poor, though oxygen vacancies might decline under a high oxygen pressure. For these films, the FWHM values were larger, too. Thus, it is concluded that an optimum oxygen pressure should be between 2 and 3 Pa. Fig. 11 shows the variation of electrical properties as a function of oxygen pressure. With the increase of oxygen pressure, the electrical resistivity of TZO thin films first decreased slightly and then increased very sharply. The decrease of resistivity could be attributed to the improvement of crystallinity, i.e., the reduction of the ionized impurities and oxygen vacancies, which resulted in an increase of Hall mobility [23]. When oxygen pressure increased further, the number of oxygen vacancies decreased remarkably, which resulted in the decrease of carrier concentration. Furthermore, the
Fig. 11. Electrical properties of TZO films as a function of oxygen pressure.
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Fig. 12. Optical mean transmittance of TZO films as a function of oxygen pressure.
degradation of crystallinity with the increase of oxygen pressure resulted in a dramatic reduction of Hall mobility. Accordingly, the reductions of Hall mobility as well as carrier concentration led to a sharp increase of electrical resistivity. For the film prepared at 4.0 Pa, the resistivity was too high to be detected by HMS 5000, and therefore could not be represented in this figure. Fig. 12 shows the mean transmittance of TZO films as a function of oxygen pressure in the visible region. The mean transmittance of TZO films increased from 88.1% to 93.3% with the increase of oxygen pressure from 1.0 to 2.5 Pa. It subsequently decreased to 85.1% at oxygen pressure of 3.0 and 4.0 Pa. The increase of mean transmittance was related to the improvement of the film crystallinity. Vice versa, the decrease of optical mean transmittance was mainly due to the poorer crystallinity of the film, which was caused by decreasing kinetic energy of particles onto the substrate surface [24].
4. Summary Ti concentration in the target, substrate temperature and oxygen pressure all have a great influence on the properties of the films. Ti-doping could induce much more carrier concentration, which was beneficial for enhancing the electroconductivity of the film. But excessive Ti-doping would form Ti O impurity phase in the film, which might function as carrier traps, which would decrease the electroconductivity of the film. The presence of such impurity phase might obstruct the transmission of light in the visible region, too. Increase of substrate temperature could enhance the grain size of ZnO in the film and produce much more oxygen vacancies in ZnO grains. The former was beneficial for improving Hall mobility, while the latter was beneficial for increasing carrier concentration. However, oxygen vacancies might also function as scattering center to enhance resistivity of the film, and if too much carriers are present they could severely impact the transmission of light. The films prepared at lower oxygen pressure could offer higher carrier concentration. When the oxygen pressure was too high, the carrier concentration became too low, and resulting in a poor conductivity for the films. Furthermore, the films prepared at higher oxygen pressures contained much more crystal defects, which reduced the transmittance in the visible region. After striking a balance between these factors, the optimum conditions for preparing Ti-doped ZnO films by PLD were determined as Ti-doping content of 1 at%, substrate temperature of 200 ◦ C and oxygen pressure of 2.5 Pa. Under these conditions, the film had an electrical resistivity of 6.34 × 10−3 ·cm, being about one fifth of
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that reported by Lee et al. [9], and a mean transmittance of 93.3% in the visible region. References [1] H. Ma, J.S. Cho, C.H. Park, Surf. Coat. Technol. 153 (2002) 131–137. [2] Z.Y. Ye, H.L. Lu, Y. Geng, Y.Z. Gu, Z.Y. Xie, Y. Zhang, Q.Q. Sun, S.J. Ding, D.W. Zhang, Nanoscale Res. Lett. 8 (2013) 108. [3] T. Minami, Thin Solid Films 516 (2008) 5822–5828. [4] N.J. Begum, K. Ravichandran, J. Phys. Chem. Solids 74 (2013) 841–848. [5] G. Kim, J. Bang, Y. Kim, S.K. Rout, S.I. Woo, Appl. Phys. A 97 (2009) 821–828. [6] W. Yang, Z. Liu, D.L. Peng, F. Zhang, H. Huang, Y. Xie, Z. Wu, Appl. Surf. Sci. 255 (2009) 5669–5673. [7] F. Wu, L. Fang, Y.J. Pan, K. Zhou, H.B. Ruan, G.B. Liu, C.Y. Kong, Thin Solid Films 520 (2011) 703–707. [8] M.S. Wang, Y.P. Zhang, H.Y. Yu, Q.H. Li, S.H. Hahn, E.J. Kim, J. Alloys Compd. 561 (2013) 211–213. [9] H.W. Lee, B.G. Choi, K.B. Shim, Y.J. Oh, J. Ceramic Process. Res. 6 (2005) 52–56. [10] S.J. Henley, M.N.R. Ashfold, D. Cherns, Surf. Coat. Technol. 177–178 (2004) 271–276. [11] W.S. Liu, Y.H. Liu, W.K. Chen, K.P. Hsueh, J. Alloys Compd. 564 (2013) 105–113.
[12] J.H. Kim, K.J. Lee, J.H. Roh, S.W. Song, J.H. Park, I.H. Yer, B.M. Moon, J. Korean Phys. Soc. 60 (2012) 2025–2028. [13] V.V. Atuchin, V.G. Kesler, N.V. Pervukhina, Z. Zhang, J. Electron Spectrosc. Relat. Phenom. 152 (2006) 18–24. [14] C.H. Hsu, W.S. Chen, C.H. Lai, S.F. Yan, Adv. Mater. Res. 194–196 (2011) 2254–2258. [15] J. Liu, S.Y. Ma, X.L. Huang, L.G. Ma, F.M. Li, F.C. Yang, Q. Zhao, X.L. Zhang, Superlattices Microstruct. 52 (2012) 765–773. [16] H.F. Liu, C.X. Lei, Vacuum 86 (2011) 483–486. [17] C.C. Huang, F.H. Wang, C.C. Wu, H.H. Huang, C.F. Yang, Nanoscale Res. Lett. 8 (2013) 206. [18] H. Kim, A. Piqué, J.S. Horwitz, H. Murata, Z.H. Kafafi, C.M. Gilmore, D.B. Chrisey, Thin Solid Films 377/378 (2000) 798–802. [19] T. Yamada, A. Miyake, S. Kishimoto, H. Makino, N. Yamamoto, T. Yamamoto, Surf. Coat. Technol. 202 (2007) 973–976. [20] H. Kim, C.M. Gilmore, A. Pique, J.S. Horwitz, H. Mattoussi, H. Murata, Z.H. Kafafi, D.B. Chrisey, J. Appl. Phys. 86 (1999) 6451–6461. [21] Y. Shigesato, D.C. Paine, Thin Solid Films 238 (1994) 44–50. [22] D.H. Zhang, T.L. Yang, J. Ma, Q.P. Wang, R.W. Gao, H.L. Ma, Appl. Surf. Sci. 158 (2000) 43–48. [23] E. Fortunato, V. Assuncao, A. Goncalves, A. Marques, H. Aguas, L. Pereira, I. Ferreira, P. Vilarnho, R. Martins, Thin Solid Films 451/452 (2004) 443–447. [24] H.F. Liu, H.F. Zhang, C.X. Lei, C.K. Yuan, J. Semicond 30 (2009) 023001.
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
A study on Ti-doped ZnO transparent conducting thin films fabricated by pulsed laser deposition Wenda Zhao, Qianfei Zhou, Xin Zhang, Xiaojing Wu ∗ Department of Materials Science, Fudan University, No. 220 HanDan Road, 200433 Shanghai, PR China
a r t i c l e
i n f o
Article history: Received 21 October 2013 Received in revised form 18 March 2014 Accepted 19 March 2014 Available online 27 March 2014 Keywords: Ti-doped ZnO Transparent conducting oxide Pulsed laser deposition
a b s t r a c t Ti-doped ZnO (TZO) thin films were fabricated on glass substrates by pulsed laser deposition (PLD). The TZO films had wurtzite structure with a preferred orientation along the c-axis. The effects of Ti doping concentration, substrate temperature and oxygen pressure on crystallinity, surface morphology, electrical and optical properties of the films were studied systematically. Under the optimized deposition conditions of Ti content of 1 at% in the target, substrate temperature of 200 ◦ C and oxygen pressure of 2.5 Pa, it was found that a film would display an electrical resistivity as low as 6.34 × 10−3 · cm and also had a 93.3% mean transmittance in the visible region when its thickness was adjusted to 100 nm. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Transparent conducting oxide (TCO) thin film are widely used in microelectronic and optoelectronic devices, such as liquid crystal displays, organic light-emitting diodes and thin film solar cells [1]. High optical transmittance and low electrical resistivity are two of the essential properties required for TCO thin film materials. Indium tin oxide (ITO) is currently the most popular commercially available TCO material and is widely used in many types of optoelectronic devices due to its high transparency (better than 85% in the visible region) and low resistivity (around 2–4 × 10−4 ·cm). However, development of new generation of TCO materials is still a subject of high interest for both the academia and the industry due to the relative scarcity and high cost of indium [2]. Zinc oxide (ZnO) is one of the materials thought to be most promising in replacing ITO [3]. There are several merits to using ZnO as a TCO material. It is highly transparent in the visible region, it has low electrical resistivity with good thermal and chemical stability and it is both environmentally-friendly and low cost. Intrinsic ZnO normally exhibits n-type conductivity due to its off-stoichiometry attributed to native defects, such as oxygen vacancies (Vo ) and zinc interstitials (Zni ), which induce shallow donor states within the forbidden gap [4].
∗ Corresponding author. Tel.: +86 021 65643258; fax: +86 021 65643258. E-mail addresses: [email protected], [email protected] (X. Wu). http://dx.doi.org/10.1016/j.apsusc.2014.03.119 0169-4332/© 2014 Elsevier B.V. All rights reserved.
As a wide bandgap semiconductor material, intrinsic ZnO is highly transparent in the visible region, but relative poor in conductivity. In order to improve its conductivity without sacrificing its optical transmittance, it is necessary to induce a proper amount of carriers into it in an adequate manner. Many studies on doping of Group III elements into the ZnO film, such as B [5], Al [6], Ga [7] and In [8] have already been carried out. In those cases, the Group III elements as the dopants would substitute the bivalent Zn ions inside the film, and then donate an extra free electron [9]. For this substitution to occur, the dopant needs to have a similar radius to that of the host ion. Based on this consideration, the trivalent elements of Ga and Al have been widely studied as a dopant for fabricating high conductive ZnO films. Ga is especially being considered as the most promising candidate because its radius is very similar to that of Zn, and this means that lattice deformation is still small even with a high Ga concentration in the film [10,11]. Kim et al. [12] studied the effects of oxygen pressure and substrate temperature on the structural, electrical and optical properties of GZO thin films deposited on glass substrate by PLD. In their study they have obtained films with high crystalline quality by applying a substrate temperature of 400 ◦ C and an oxygen pressure of 20 mTorr, which displayed low electrical resistivity of 4.6 × 10−4 · cm and high optical transmittance of about 85%. It is known that the radius of a Ti4+ is 0.61 A˚ and the bond length of Ti O is 1.96 A˚ [13], while the radius of a Zn2+ is 0.74 A˚ and the ˚ As a quadrivalent element, a Ti4+ ion bond length of Zn O is 1.97 A. 2+ substituted for a Zn ion could donate two extra free electrons, instead of one in the case of Ga-doping. Consequently, Ti-doping is also considered as a highly efficient way to induce carriers into
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ZnO film. There are many techniques for preparing Ti-doped ZnO films, such as atomic layer deposition (ALD) [2], chemical bath deposition (CBD) [14], RF and DC magnetron sputtering (MS) [15,16], and pulsed laser deposition (PLD) [9]. Among these methods, PLD has several advantages. Firstly, the films deposited by PLD usually have a very similar chemical composition to that of the targets, and therefore controlling the film composition is easier. Moreover, the high kinetic energy of atoms and ionized species produced by laser irradiation allows for the crystallization of films under a lower substrate temperature, which is an important advantage when fabricating crystallized oxide film on a flexible substrate. In 2005, Lee et al. [9] studied the effect of substrate temperature, Ti-doping content and oxygen flow rate on structural, electrical and optical properties of the Ti-doped ZnO (TZO) films deposited by PLD. They found that the TZO films exhibited a strongly preferred orientation along the c-axis, and the film with a thickness of 480 nm and 1 at% Ti content which was fabricated under a substrate temperature of 100 ◦ C and an oxygen flow rate of 10 sccm showed an electrical resistivity of 3.3 × 10−2 · cm and a mean optical transmittance of 92.3%. However, only few reports focused on TZO films deposited by PLD have been published so far. In this study, the effects of substrate temperature, oxygen pressure and Ti-doping content on the electrical properties, optical properties and the structure of the films grown on glass substrates by PLD were studied systematically. The conduction mechanism of the film was also discussed. 2. Experimental A Nd:YAG pulsed laser (wavelength: 355 nm, and pulse duration: 10 ns) was employed for depositing TZO films on soda-lime glass substrates. The laser was operated at a repetition rate of 10 Hz with an energy density of 2 J/cm2 . The distance from target-tosubstrate was about 45 mm. Pure ZnO (99.99%) and TiO2 (99.99%) powders were mixed for preparing the targets with Ti concentrations (TiO2 /TiO2 + ZnO) of 0.5, 0.75, 1, 2 and 5 at%, respectively. The mixed powder of ZnO and TiO2 was formed by 10 min of stirring and then 4 h of sintering at 800 ◦ C. The sintered targets were typically 12 mm in diameter and 2.5 mm in thickness. The soda-lime glass substrates were cut into 25 mm × 20 mm pieces and cleaned inside an ultrasonic bath 30 min at a time using each of acetone, ethanol and distilled water and then dried with nitrogen gas. The deposition chamber was first evacuated to 1.0 × 10−4 Pa and the substrate was heated from room temperature to the target temperature. During deposition, oxygen gas was flowed into the chamber to keep the working pressure in the range of 1–4 Pa. After deposition, the substrate was naturally cooled down to room temperature in vacuum. Crystallinity and crystal orientation of the films were characterized by X-ray powder diffractometer of BRUKER D8 ADVANCE. The composition and content of the films were identified by X-ray Photoelectron Spectroscopy (XPS) which was carried out on a RBD upgraded PHI-5000C ESCA system (Perkin Elmer) with Mg K␣ radiation (h = 1253.6 eV). Electrical resistivity, Hall mobility and carrier concentration were determined using a HMS 5000 Hall effect measurement system. Optical transmission spectra of the films in the visible region were measured with a SHIMADZU UV-2450 UV–vis spectrophotometer. The film thickness determined by a Kosaka Laboratory ET3000 profilometer was controlled at about 100 nm. 3. Results and discussions 3.1. The effect of Ti content Fig. 1(a) shows X-ray diffraction (XRD) patterns of TZO targets prepared with different Ti-doping of 0, 1, 2 and 5 at%. Several peaks
Fig. 1. X-ray diffraction patterns of TZO targets (a) and films (b) with different Ti content.
from Zn2 TiO4 (PCPDF card No. 73-0578) phase can be observed in these patterns, indicating that a chemical reaction occurred during sintering process in the targets. It is clear from this figure that the intensities of the peaks are proportional to the concentration of Ti in the targets. Unmarked peaks match well with the hexagonal wurtzite phase of ZnO. Fig. 1(b) shows XRD patterns of TZO films that were deposited at 200 ◦ C under working pressure of 2.5 Pa by using targets that have Ti-doping concentrations of 0, 0.5, 0.75, 1, 2 and 5 at%, respectively. For all films, only a (0 0 2) peak of ZnO at 2 ≈ 34.4◦ was observed, indicating that the TZO films were polycrystalline with wurtzite structure and they had a preferred orientation along the c-axis. No peaks related with TiO2 or Zn2 TiO4 phases were found in these patterns. If any such phases did exist in the films, the ratio of the phase must have been too low or the crystallinity of the phase must have been too poor to be detected by XRD. The Ti content in the films were measured by XPS as shown in Fig. 2. It was found that when we took the film with Ti-doping of 5% as a standard, the Ti concentrations in the films were almost equal to that of the targets. This implies that Ti concentration of TZO films can be controlled simply by using targets with the desired Ti concentration. Fig. 3 shows the varying electrical properties of films between the different Ti-doping concentrations. It is well-known that the relationships between electrical resistivity (), carrier concentration (n) and Hall mobility () could be expressed as [17]: = 1/ne
(1)
The symbol e here represents the charge of an electron. It was found from Fig. 3 that the resistivity of the TZO films initially decreased from 1.92 · cm to 6.34 × 10−3 · cm with the increase of Ti-doping content from 0 to 1 at%. Thus, change can be mainly attributed to the dramatic increase of carrier concentration in the film, which mainly came from the substitution of Ti4+ into Zn2+ sites, or the presence of Ti4+ ions at some interstitial positions. However, the further increase of Ti-doping content did not decrease its resistivity furthermore. When the Ti-doing content have reached and excessed 2 at%, its Hall mobility was kept almost unchanged, but it showed a clearly drop in its carrier concentration. Taken as a
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Fig. 4. Optical mean transmittance for the TZO films with different Ti content.
Fig. 2. The relationship between the Ti content in the films and Ti content in the targets.
whole, the resistivity of the film exhibited an increasing tendency. Similar phenomenon has previously been reported by Kim et al. [18], and according to their explanation, this happened due to the excess Ti atoms forming some non-conducting Ti O clusters in the film, which act as carrier traps rather than electron donors. Clearly, excess Ti-doping is not an efficient way to enhance carrier concentration as this also forms impurity phases and induces much more crystal defects. Fig. 4 plots the transmittance of the films against their Ti concentration. All of them exhibited a transmittance higher than 85%, while the film with 1 at% Ti-doping reached the peak value of 93.3%. After that peak, the mean transmittance decreased as the Ti content increased furthermore. This could also be attributed to the appearance of the second phases.
Fig. 5. X-ray diffraction patterns deposited with different substrate temperature.
Fig. 5 shows XRD patterns of 1 at% Ti-doped films deposited at different substrate temperatures under a working pressure of 2.5 Pa. Again, all the films exhibited the wurtzite structure with a preferred orientation along the c-axis. The diffraction peak at (0 0 2) became stronger with the increase of substrate temperature,
indicating that the proportion of TZO crystalline grew with the increase of substrate temperature. Fig. 6 shows the full width at half maximum (FWHM) of (0 0 2) diffraction peaks as a function of substrate temperature. FWHM decreased with increase in substrate temperature in the beginning, from room temperature up to 200 ◦ C, and then increased only gradually with further increase of substrate temperature up to 400 ◦ C. Usually, there are two main factors that determine the FWHM for a
Fig. 3. Electrical properties of TZO films deposited at 200 ◦ C under working pressure of 2.5 Pa as a function of Ti content.
Fig. 6. FWHM of (0 0 2) diffraction peaks as a function of substrate temperature.
3.2. The effect of substrate temperature
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Fig. 7. Electrical properties of TZO films as a function of substrate temperature. Fig. 8. Optical mean transmittance of TZO films as a function of substrate temperature.
specific diffractometer: average grain size in the film and perfection of the crystal in the film. The larger the grain size is, the narrower the FWHM is, and the more defects inside the grains, the broader the FWHM becomes. It is known that the average grain size in the film must increase with the rise of substrate temperature, which should result in a decrease of FWHM. Hence the first section of the graph is rather easy to understand: when the substrate temperature rose from room temperature to 200 ◦ C, the kinetic energy of ions on the film surface increased, and the ions could migrate more easily, which was benefit for crystal growth [19]. Consequently, the grain size increased, which lead to the decrease of FWHM. However, it was found that FHWM increased in stead of decreasing for the films prepared at 300 and 400 ◦ C. Referring to the film preparation process, it could be recognized that a lot of defects must have appeared inside the films prepared at 300 or 400 ◦ C. As aforementioned, the films were cooled naturally in vacuum. In this setting, if the temperature was high enough, many oxygen atoms could escape from the film during the cooling process, resulting in formations of a number of oxygen vacancies in the film. In practice, the average grain size indeed increased for those films, and the number of internal structural defects inside the grains also increased simultaneously. Such structural defects are thought to have played a key role to broaden FWHM. Fig. 7 plots the changes in electrical properties of the films against rising substrate temperature. Compared with the film prepared at room temperature, the film prepared at 100 ◦ C did not show any remarkable changes in carrier concentration, but owing to the improved crystallinity of the film, its Hall mobility was enhanced dramatically. As a result, the resistivity of the film showed a significant decline from 4.60–1.04 × 10−2 ·cm. When the substrate temperature rose to 200 ◦ C, the carrier concentration clearly increased. This can be attributed to the substitution of Ti4+ into Zn2+ sites as an electron donor [20]. The simultaneous slight decrease of Hall mobility can be considered as a result of the increased number of crystal defects in the film. In this case, the resistivity of the film dropped to 6.34 × 10−3 ·cm. Similar tendency continued, i.e., the carrier concentration increased and the Hall mobility decreased with rise in substrate temperature. For the films prepared at higher temperatures, much more oxygen atoms could be escaping from the films to form vacancies during the cooling process, which was confirmed by XRD analysis. Such vacancies could enhance the number of carriers, but at same time they become new scattering centers. Correspondingly, their carrier concentration increased, but their Hall mobility decreased further. As a net result their resistivity increased slightly. Therefore, high substrate temperature could indeed enhance carrier concentration effectively through creating a number of oxygen
vacancies, but it was not an efficient way to fabricate a low resistivity TZO film. Fig. 8 shows the mean transmittance as a function of the substrate temperature in the visible region. The mean transmittance reached a maximum value of 93.3% for the film prepared at 200 ◦ C, and then decreased gradually with the rise of substrate temperature. Clearly, the increase of carrier concentration for the films prepared at higher temperature must be a major factor in the reduction of mean transmittance. 3.3. The effect of oxygen pressure Fig. 9 shows XRD patterns for the 1 at% Ti-doped films deposited with different oxygen pressure at 200 ◦ C. All the films had the wurtzite structure with a preferred orientation along the c-axis. Fig. 10 shows a plot of FWHM of (0 0 2) diffraction peaks as a function of oxygen pressure. It can be seen that FWHM shows a “V” shaped change with the increase of oxygen pressure. It is well-known that the influences of oxygen pressure manifest in two aspects during growth of the film. First of all, it functions as a source of oxygen supplier. By adjusting oxygen pressure, we can control the number of oxygen vacancies in the film, which in turn plays a key role in controlling the carrier concentration as well as the number of structural defects. Secondly, higher oxygen pressure means a lower vacuum, and the sputtered species would be
Fig. 9. XRD patterns for the films deposited with different oxygen pressure.
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Fig. 10. FWHM of (0 0 2) diffraction peaks as a function of oxygen pressure.
able to impact with oxygen molecules more easily, which leads to a shorter mean free path and a lower average speed for the species. This phenomenon has a significant influence on crystal perfection. For the films prepared under low oxygen pressure, the mean free path of the sputtered species was long and the average speed of the sputtered species was high in the path from target to substrate. Such high energy sputtered species bombarded the surface of the films, and deteriorated the crystallinity [21]. Consequently, the films prepared at 1.0, 1.5 and 2.0 Pa had more crystal defects, and subsequently showed a broader FWHM. On the other hand, for the films prepared at high oxygen pressure, such as 3.0 and 4.0 Pa, the mean free path of the sputtered species became shorter, and the average speed of the sputtered species became lower, resulting in a decrease of the particle’s mobility on the surface of the film. The particles with a lower kinetic energy could not migrate into more suitable lattice sites, adjust their own bond direction and length to obtain optimum bonding to the adjacent ones [22]. In those cases, their grain size was small, the crystallinity of the film was poor, though oxygen vacancies might decline under a high oxygen pressure. For these films, the FWHM values were larger, too. Thus, it is concluded that an optimum oxygen pressure should be between 2 and 3 Pa. Fig. 11 shows the variation of electrical properties as a function of oxygen pressure. With the increase of oxygen pressure, the electrical resistivity of TZO thin films first decreased slightly and then increased very sharply. The decrease of resistivity could be attributed to the improvement of crystallinity, i.e., the reduction of the ionized impurities and oxygen vacancies, which resulted in an increase of Hall mobility [23]. When oxygen pressure increased further, the number of oxygen vacancies decreased remarkably, which resulted in the decrease of carrier concentration. Furthermore, the
Fig. 11. Electrical properties of TZO films as a function of oxygen pressure.
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Fig. 12. Optical mean transmittance of TZO films as a function of oxygen pressure.
degradation of crystallinity with the increase of oxygen pressure resulted in a dramatic reduction of Hall mobility. Accordingly, the reductions of Hall mobility as well as carrier concentration led to a sharp increase of electrical resistivity. For the film prepared at 4.0 Pa, the resistivity was too high to be detected by HMS 5000, and therefore could not be represented in this figure. Fig. 12 shows the mean transmittance of TZO films as a function of oxygen pressure in the visible region. The mean transmittance of TZO films increased from 88.1% to 93.3% with the increase of oxygen pressure from 1.0 to 2.5 Pa. It subsequently decreased to 85.1% at oxygen pressure of 3.0 and 4.0 Pa. The increase of mean transmittance was related to the improvement of the film crystallinity. Vice versa, the decrease of optical mean transmittance was mainly due to the poorer crystallinity of the film, which was caused by decreasing kinetic energy of particles onto the substrate surface [24].
4. Summary Ti concentration in the target, substrate temperature and oxygen pressure all have a great influence on the properties of the films. Ti-doping could induce much more carrier concentration, which was beneficial for enhancing the electroconductivity of the film. But excessive Ti-doping would form Ti O impurity phase in the film, which might function as carrier traps, which would decrease the electroconductivity of the film. The presence of such impurity phase might obstruct the transmission of light in the visible region, too. Increase of substrate temperature could enhance the grain size of ZnO in the film and produce much more oxygen vacancies in ZnO grains. The former was beneficial for improving Hall mobility, while the latter was beneficial for increasing carrier concentration. However, oxygen vacancies might also function as scattering center to enhance resistivity of the film, and if too much carriers are present they could severely impact the transmission of light. The films prepared at lower oxygen pressure could offer higher carrier concentration. When the oxygen pressure was too high, the carrier concentration became too low, and resulting in a poor conductivity for the films. Furthermore, the films prepared at higher oxygen pressures contained much more crystal defects, which reduced the transmittance in the visible region. After striking a balance between these factors, the optimum conditions for preparing Ti-doped ZnO films by PLD were determined as Ti-doping content of 1 at%, substrate temperature of 200 ◦ C and oxygen pressure of 2.5 Pa. Under these conditions, the film had an electrical resistivity of 6.34 × 10−3 ·cm, being about one fifth of
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