- Email: [email protected]
Preparation, microstructure and photoelectrical properties of Tantalum-doped zinc oxide transparent conducting films
Journal of Alloys and Compounds 608 (2014) 85–89
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Preparation, microstructure and photoelectrical properties of Tantalum-doped zinc oxide transparent conducting films Yunlang Cheng, Ling Cao, Gang He, Guang Yao, Xueping Song, Zhaoqi Sun ⇑ School of Physics & Materials Science, Anhui University, Hefei 230601, PR China
a r t i c l e
i n f o
Article history: Received 20 January 2014 Received in revised form 4 March 2014 Accepted 5 March 2014 Available online 20 March 2014 Keywords: Transparent conductive films Tantalum-doped zinc oxides RF magnetron sputtering Electrical and optical properties
a b s t r a c t Transparent and conducting Tantalum-doped zinc oxides thin films have been prepared for the first time by RF magnetron sputtering method on glass and silicon substrates at room temperature. The doped contents are 0 wt.%, 2 wt.%, 5 wt.%, 7 wt.% and 10 wt.%. The test result shows that Ta element in the film is in the state of Ta5+. The ratio of O/Zn is 84.73% for ZnO:Ta film doped with 5 wt.% Ta2O5, which indicates the films is in a state of oxygen deficiency. As the content of Ta increases, the crystallite size has been estimated to be in the range of 9.4–13.5 nm. AFM studies indicate the maximum average particle size 94.46 nm and the minimum surface roughness 4.480 nm can be obtained for the ZnO:Ta films with the Ta2O5 content of 5 wt.%. These structural changes are accompanied by significant variations of electrical property and optical property. The resistivity of this film first decreases and then increases with the increase of the Ta2O5 content. The optical property analysis shows that the average transmittance in the visible range is above 85% for all the films. The optical band gap value of the film initially increases and then shows a decrease with the increase of the Ta2O5 content. The ZnO:Ta films containing 5 wt.% Ta2O5 presents the maximum optical band gap 3.38 eV. These findings shows that the highest figure of merit obtained is 2.20 104 X1 for the as-grown ZnO:Ta films doped with 5 wt.% Ta2O5. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction The technology of photovoltaic conversion and display has been developed to a new level in the past few decades, giving a push to the field of transparent conducting oxide. Transparent conducting oxide films have been widely applied as transparent electrodes for light-emitting diodes and solar cells [1,2]. The materials of TCO are the oxidate of Zn, Sn, In, and Cd. As an important wide band gap (3.37 eV) semiconductor with a large exciton binding energy (60 meV), ZnO has triggered great interest due to its great performance and potential applications in electro-optical, ferroelectric, pyroelectric and piezoelectric [3,4]. Compared with other materials of TCO, ZnO based transparent conducting film has the advantages of lower cost, non-poisonous, environmental friendly, relatively lower deposition temperature and the ability of keeping steady under the hydrogen plasma. Usually, undoped ZnO has higher electrical resistivity, while doping Al, Ga, Ta, etc., can reduce the resistivity. Ta has good chemical stability, relatively higher refractive index and lower absorption rate in the visible spectral region and high dielectric constant [5]. The doped Ta5+ can give more electrons ⇑ Corresponding author. Tel./fax: +86 551 63861767 (O). E-mail address: [email protected] (Z. Sun). http://dx.doi.org/10.1016/j.jallcom.2014.03.031 0925-8388/Ó 2014 Elsevier B.V. All rights reserved.
than Zn2+ that compared with ITO it will decrease the impurity level. In this study, the series of different mass fractions of ZnO:Ta films were prepared by RF magnetron sputtering. The chemical composition, structure, surface, topography, electrical and optical property were investigated using X-ray photoelectron spectrometer (XPS), X-ray diffractometer (XRD), Atomic Force Microscopy (AFM), ultraviolet–visible spectroscope (UV–Vis) and four-point probe instrument.
2. Experimental The substrates of glass and silicon slices were firstly cleaned by ultrasonic in acetone for 10 min, in deionized water for 10 min, then in ethyl alcohol for 10 min. After drying, they were placed into the ultra high vacuum magnetron sputtering apparatus (JGP560I). The target materials were ceramics of different mass fraction of Ta2O5 (0 wt.%, 2 wt.%, 5 wt.%, 7 wt.% and 10 wt.%) and ZnO(99%) sinter forging. Sputter for 30 min. The sputtering conditions are in Table 1. The microstructures of ZnO:Ta were tested by XRD (MAC, M18XHF) employing CuKa radiation. The thickness of the films was characterized by surface profiler (XP-1). The surface morphology of ZnO:Ta films was tested by AFM (AJ-III). XPS (Thermo, ESCALAB250) was employed to analyze the surface composition of the samples. The absorption spectra of samples were recorded by an UV–Vis spectrophotometer (Shimadzu, UV-2550) within the wavelength range of 200–900 nm. And the electrical resistivity of the films was characterized by four-point probe instrument (SZ-82).
86
Y. Cheng et al. / Journal of Alloys and Compounds 608 (2014) 85–89
Table 1 Experimental conditions for RF sputtering ZnO:Ta films. Parameter
Data
Background vacuum degree (Pa) Flow of Ar (SCCM) Sputtering pressure (Pa) Voltage (V) Sputtering current (A) Target-substrate distance (mm) Substrate temperature (K) Sputtering time (min)
9.0 104 30 0.7 450 0.18 60 293 30
3. Results and discussion 3.1. Composition analysis Fig. 1 is the XPS survey spectrum of ZnO:Ta film doped with 5 wt.% Ta2O5. It shows that surface layer of the films contains C, O, Zn and Ta. Fig. 2 is the binding energy of Zn 2p3/2 on the face of the film. From the figure, the peak locates on 1021.85 eV, just according with the bulks of ZnO (1021.75 eV), which shows that the form of Zn in the film is Zn2+, not elemental Zn [6,7]. Fig. 3 is the Photoelectron spectroscopy of O 1s. The map can be exploded to three peaks of 530.31 eV (OI), 530.97 eV (OII) and 532.17 eV (OIII). OI stands for the O2 in the hexagonal wurtzite, while OII correspond to the O2 of anoxic area in the ZnO. OIII stands for the absorbed Oxygenium, which exist in –CO3, H2O, –OH and O2. To calculate the proportion of O/Zn, the OI and OII are effective. Chen [8] discovered that in the Al-doped ZnO films the O 1s also has three different chemical states. In Fig. 4 photoelectron spectroscopy of Ta 4f, the characteristic peaks is on 26.24 eV and 28.04 eV, which stand for Ta 4f7/2 and Ta 4f5/2. Obviously, the valence state of Ta is more approaching to the Ta5+ in Ta2O5 (26.3 eV) than the zero valence Ta (21.7 eV) [9,10]. So the form of Ta in the films is Ta5+. The percentage of each component in the film can be calculated by the formula [11]:
x% ¼
Ax Sx PN Ai i¼1 Si
The result is in Table 2. The O/Zn is 84.73%, less than the ideal stoichiometric ratio of ZnO, explaining that the film is oxygenpoor. Then, the proportion of Ta/(Zn + Ta) is 3.29%, higher than
Fig. 2. Photoelectron spectroscopy of Zn 2p3/2 for ZnO:Ta films doped with 5 wt.% Ta2O5.
Fig. 3. Photoelectron spectroscopy of O 1s for ZnO:Ta films doped with 5 wt.% Ta2O5.
the calculation percentage 1.9% of 5 wt.% Ta2O5 target. The reason is the different saturated vapor pressure of ZnO and Ta2O5, which the higher one of ZnO leads to the desorption of Zn on the surface of the film. 3.2. Microstructure analysis
Fig. 1. XPS survey spectrum of ZnO:Ta film doped with 5 wt.% Ta2O5.
Fig. 5 shows the XRD patterns of the ZnO:Ta film for different Ta2O5 content. The diffraction peaks are consistent with standard spectrum diagram of ZnO. For the undoped ZnO film, the highest diffraction peak is on the 2h = 34.18, which represents the crystal face of (0 0 2). Besides, there are relatively lower peaks of (1 0 0), (1 0 1) and (1 0 2). While doped, all of these films only have peaks of (0 0 2) and (1 0 3) on the diffraction pattern. It illustrates that there is no new phase producing in the film, which another way of saying is the doping of Ta did not change the structure of hexagonal wurtzite. The Ta just replaced the position of Zn in the crystal lattice. The crystal quality of the films can be evaluated by the diffraction half peak width (FWHM) and crystal particle size. Fig. 6 is FWHM and crystallite size of ZnO:Ta film with different Ta2O5 content. The figure shows that as the proportion of the impurities increased from 0 wt.% to 5 wt.%, the intensity of (0 0 2) diffraction
Y. Cheng et al. / Journal of Alloys and Compounds 608 (2014) 85–89
87
content increased further, the crystal particle sizes decreased. It means the higher percentage of doping Ta destroyed the crystalline, because the atoms of Ta tend to concentrate on the crystal boundary, which leads to the disorder of lattice and the decrease of film property [13–15]. So the right amount of Ta doped can improve the structural performance of ZnO films. 3.3. Surface topography analysis
Fig. 4. Photoelectron spectroscopy of Ta 4f for ZnO:Ta films doped with 5 wt.% Ta2O5.
Table 2 The atomic percentage of constitutive components for ZnO:Ta films doped with 5 wt.% Ta2O5. Element
O
Zn
Ta
Percentage %
12.71
15.00
0.51
Fig. 7 shows the AFM images of ZnO:Ta film with different Ta2O5 content. The concentration of Ta brings a marked impact to the surface and crystal shape and size of the film. The surfaces of undoped ZnO films are short cylindrical. Moderate amount of doping films have more homogeneous particle size and no obvious empty and defects, proving that the films have better crystal quality. But when the Ta content increased to 10 wt.%, the granules are refined in different size, and the surface is rough with more empty. The reason may be the over content of Ta which leads segregation and influences the surface of the film [16]. Fig. 8 shows the average surface grain size and roughness of ZnO:Ta film with different Ta2O5 content. When the doping content grew from 0 wt.% to 5 wt.%, the surface roughness (RMS) decreased from 10.4 nm to 4.5 nm. Then as the content kept growing to 10 wt.%, the surface roughness grew to 7.0 nm. Undoped ZnO has relatively bigger surface grain size, and the doped films’ average surface grain sizes increase firstly and then decrease. The 5 wt.% doped film has the largest average particle size of 94.5 nm, met up with the result of XRD. 3.4. Electrical property analysis Fig. 9 gives the resistivity of ZnO:Ta film with different Ta2O5 content. For the record, the undoped ZnO film has high resistance that beyond the maximum span of the equipment. Firstly, when the content of Ta rose up, the resistivity of the film rushed down significantly. And the 5 wt.% film had the lowest resistivity of 7.81 102 X cm. But when the percentage increased to 10 wt.%, the resistivity grew to 1.25 101 X cm. For the doped films, the production of charge carrier contains two ways. One is due to oxygen vacancy. From the XPS result, the film was oxygen-poor. It was easy to form the oxygen vacancy, producing two electrons. On the other hand, the doping Ta5+ gives surplus electrons. Then the structure of crystals became homogeneous and had better crystal quality, which leads to the lower block to the charge carrier. So the resistivity got low fast as the Ta increased first. But, as the doping ions have attraction to oxygen
Fig. 5. XRD patterns of ZnO:Ta film with different Ta2O5 content.
peak grew higher, while the half peak width decreased from 0.73° to 0.61°, proving the increasing quality of crystal. On the other hand, the intensity of (0 0 2) diffraction peak decreased when the proportion of the impurities increased further to 10 wt.%. The half peak width grew to 0.72°, indicated that the crystal quality become worse. The average crystal particle size can be calculated through Scherrer formula [12]:
D¼
Kk b cos h
The results are in Fig. 6, which the crystal particle sizes are among 10.5–13.7 nm. When the Ta content of the target is 5 wt.%, the film has best structural performance. But as the doping
Fig. 6. FWHM and crystallite size of ZnO:Ta film with different Ta2O5 content.
88
Y. Cheng et al. / Journal of Alloys and Compounds 608 (2014) 85–89
Fig. 7. AFM images of ZnO:Ta film with different Ta2O5 content: (a) 0 wt.%, (b) 2 wt.%, (c) 5 wt.%, (d) 7 wt.%, (e) 10 wt.%.
Fig. 8. Average surface grain size and roughness of ZnO:Ta film with different Ta2O5 content.
Fig. 9. Resistivity of ZnO:Ta film with different Ta2O5 content.
ions, they can fetter the oxygen around them, which leads to the reduction of charge carrier. So when the doping content further increased, more and more Ta5+ gathered to form neutral defects. They can not produce charge carriers, instead, becoming the traps of electrons, causing the rising trend of resistivity [13–18]. 3.5. Optical property analysis Fig. 10 is the visible transmission and reflectance spectrals of ZnO:Ta film with different Ta2O5 content, with the influence of glass substrate. The film thickness is 400 nm. From the result, all of the samples perform a high transmission in visible area, and have a strong absorbent in ultraviolet region, corresponding to the absorbent of charge carriers. Compared with the undoped film, the transmission of the samples in visible area increased, for the reduction of surface roughness [19]. The figure also shows the reflectance of 5 wt.% doped film, which the result is among 10%– 20%. The absorptivity can be calculated by transmission and reflectance through the formula [20]:
1 d
a ¼ ln
1R T
For the direct-gap semiconductor, the absorptivity has relation with optical band gap Eg as this: [21]
Fig. 10. The visible transmission and reflectance spectrals of ZnO:Ta film with different Ta2O5 content: (a) 0 wt.%, (b) 2 wt.%, (c) 5 wt.%, (d) 7 wt.%, (e) 10 wt.%.
2
ðahmÞ ¼ Aðhm Eg Þ So, as Figs. 11 and 12 show, when the dope content grew from 0 wt.% to 5 wt.%, the optical band gap increased from 3.27 eV to
Y. Cheng et al. / Journal of Alloys and Compounds 608 (2014) 85–89
89
all filled and the Fermi level got higher to conduction band. The film became degenerate semiconductor. So the valence electron must be excited higher above the Fermi level in order to become free charge carrier, which means the band gap of the film became wider. In the doped film, the raising concentration of charge carrier leads to blue shift absorption edge. The similar result can be seen in the work by Yang [24]. To evaluate the transparent conducting film, Fig. 13 shows the merit of ZnO:Ta film with different Ta2O5 content. When the content is 5 wt.%, the merit get the highest peak of 2.20 104 X1. 4. Conclusions
Fig. 11. The relationship graph of (ahm)2 and photon energy for ZnO:Ta film with different Ta2O5 content.
In summary, Ta doped ZnO transparent conducting films are produced by magnetron sputtering apparatus. The chemical composition, structure, surface, topography, electrical and optical property were systematically investigated. The results are as follows: Ta doping did not change the crystal structure of ZnO, instead, the doping ions took possession of Zn and become solid solution. The minimum resistivity of the ZnO:Ta films is 7.81 102 X cm. The average transmittance in visible area is over 85%. The maximum optical band gap is 3.38 eV. These prove that the doped Tantalum can help improve the structure and property of ZnO film. The merit of 5 wt.% is the highest, for 2.20 104 X1. So the best doping content is 5 wt.%. Acknowledgments This work is supported by the National Natural Science Foundation of China (No. 51272001), National Key Basic Research Programs (2013CB632705), and the National Science Research Foundation for Scholars Return from Overseas, Ministry of Education, China. References
Fig. 12. Optical band gap of ZnO:Ta film with different Ta2O5 content.
Fig. 13. Figure of merit of ZnO:Ta film with different Ta2O5 content.
3.38 eV. When the percentage get to 10 wt.%, the optical band gap fell to 3.34 eV. This transformation law might be explained by Burstein–Moss Move [22,23]. According to this theory, as the Ta doped, the concentration of the charge carrier increased. This made that the energy states in the bottom of the conduction band were
[1] Z. Pan, P. Zhang, X. Tian, G. Cheng, Y. Xie, H. Zhang, X. Zeng, C. Xiao, G. Hu, Z. Wei, J. Alloys Comp. 576 (2013) 31–37. [2] R. Yousefi, B. Kamaluddin, J. Alloys Comp. 479 (2009) L11–L14. [3] K.W. Liu, M. Sakurai, M. Aono, Sens. Actuators, B 157 (1) (2011) 98–102. [4] K. Zhong, J. Xu, J. Su, Y.L. Chen, Appl. Surf. Sci. 257 (8) (2011) 3495–3498. [5] I. Drouelle, C. Servant, J. Alloys Comp. 551 (2013) 293–299. [6] H. Xue, Y. Chen, X.L. Xu, G.H. Zhang, H. Zhang, S.Y. Ma, Physica E 41 (5) (2009) 788–791. [7] Y. Zhang, G. Du, X. Wang, W. Li, X. Yang, Y. Ma, B. Zhao, H. Yang, D. Liu, S. Yang, J. Cryst. Growth 252 (1–3) (2003) 180–183. [8] M. Chen, X. Wang, Y.H. Yu, Z.L. Pei, X.D. Bai, C. Sun, R.F. Huang, L.S. Wen, Appl. Surf. Sci. 158 (1–2) (2000) 134–140. [9] S.V.J. Chandra, S. Uthanna, G.M. Rao, Appl. Surf. Sci. 254 (7) (2008) 1953–1960. [10] E. Atanassova, D. Spassov, Appl. Surf. Sci. 135 (1–4) (1998) 71–82. [11] E. Holmelund, J. Schou, S. Tougaard, N.B. Larsen, Appl. Surf. Sci. 197–198 (1) (2002) 467–471. [12] R. Sharma, K. Sehrawat, A. Wakahara, R.M. Mehra, Appl. Surf. Sci. 255 (11) (2009) 5781–5788. [13] B.H. Choi, H.B. Im, J.S. Song, J. Am. Ceram. Soc. 73 (5) (1990) 1347–1350. [14] M. Lv, X. Xiu, Z. Pang, Y. Dai, L. Ye, C. Cheng, S. Han, Thin Solid Films 516 (8) (2008) 2017–2021. [15] Z.B. Fang, Y.S. Tan, H.X. Gong, C.M. Zhen, Z.W. He, Y.Y. Wang, Mater. Lett. 59 (21) (2005) 2611–2614. [16] D.S. Liu, F.C. Tsai, C.T. Lee, C.W. Sheu, Jpn. J. Appl. Phys. 47 (4) (2008) 3056– 3062. [17] G. Frank, H. Kostlin, Appl. Phys. A: Mater. 27 (1) (1982) 197–206. [18] C. Jeong, H.S. Kim, D.R. Chang, K. Kamisako, Jpn. J. Appl. Phys. 47 (7) (2008) 5656–5658. [19] J.L. Chung, J.C. Chen, C.J. Tseng, J. Phys. Chem. Solids 69 (2–3) (2008) 535–539. [20] B.G. Choi, I.H. Kim, D.H. Kim, K.S. Lee, T.S. Lee, B. Cheong, Y.J. Baik, W.M. Kim, J. Eur. Ceram. Soc. 25 (12) (2005) 2161–2165. [21] C. Fournier, O. Bamiduro, H. Mustafa, R. Mundle, R.B. Konda, F. Williams, A.K. Pradhan, Semicond. Sci. Technol. 23 (8) (2008). 85019–1–85019–5. [22] E. Burstein, Phys. Rev. 93 (3) (1954) 632–633. [23] T.S. Moss, Proc. Phys. Soc. Sect. B 67 (10) (1954) 775–782. [24] J.H. Yang, X.T. Zhao, X.N. Shan, H.G. Fan, L.L. Yang, Y.J. Zhang, X.Y. Li, J. Alloys Comp. 556 (2013) 1–5.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Preparation, microstructure and photoelectrical properties of Tantalum-doped zinc oxide transparent conducting films Yunlang Cheng, Ling Cao, Gang He, Guang Yao, Xueping Song, Zhaoqi Sun ⇑ School of Physics & Materials Science, Anhui University, Hefei 230601, PR China
a r t i c l e
i n f o
Article history: Received 20 January 2014 Received in revised form 4 March 2014 Accepted 5 March 2014 Available online 20 March 2014 Keywords: Transparent conductive films Tantalum-doped zinc oxides RF magnetron sputtering Electrical and optical properties
a b s t r a c t Transparent and conducting Tantalum-doped zinc oxides thin films have been prepared for the first time by RF magnetron sputtering method on glass and silicon substrates at room temperature. The doped contents are 0 wt.%, 2 wt.%, 5 wt.%, 7 wt.% and 10 wt.%. The test result shows that Ta element in the film is in the state of Ta5+. The ratio of O/Zn is 84.73% for ZnO:Ta film doped with 5 wt.% Ta2O5, which indicates the films is in a state of oxygen deficiency. As the content of Ta increases, the crystallite size has been estimated to be in the range of 9.4–13.5 nm. AFM studies indicate the maximum average particle size 94.46 nm and the minimum surface roughness 4.480 nm can be obtained for the ZnO:Ta films with the Ta2O5 content of 5 wt.%. These structural changes are accompanied by significant variations of electrical property and optical property. The resistivity of this film first decreases and then increases with the increase of the Ta2O5 content. The optical property analysis shows that the average transmittance in the visible range is above 85% for all the films. The optical band gap value of the film initially increases and then shows a decrease with the increase of the Ta2O5 content. The ZnO:Ta films containing 5 wt.% Ta2O5 presents the maximum optical band gap 3.38 eV. These findings shows that the highest figure of merit obtained is 2.20 104 X1 for the as-grown ZnO:Ta films doped with 5 wt.% Ta2O5. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction The technology of photovoltaic conversion and display has been developed to a new level in the past few decades, giving a push to the field of transparent conducting oxide. Transparent conducting oxide films have been widely applied as transparent electrodes for light-emitting diodes and solar cells [1,2]. The materials of TCO are the oxidate of Zn, Sn, In, and Cd. As an important wide band gap (3.37 eV) semiconductor with a large exciton binding energy (60 meV), ZnO has triggered great interest due to its great performance and potential applications in electro-optical, ferroelectric, pyroelectric and piezoelectric [3,4]. Compared with other materials of TCO, ZnO based transparent conducting film has the advantages of lower cost, non-poisonous, environmental friendly, relatively lower deposition temperature and the ability of keeping steady under the hydrogen plasma. Usually, undoped ZnO has higher electrical resistivity, while doping Al, Ga, Ta, etc., can reduce the resistivity. Ta has good chemical stability, relatively higher refractive index and lower absorption rate in the visible spectral region and high dielectric constant [5]. The doped Ta5+ can give more electrons ⇑ Corresponding author. Tel./fax: +86 551 63861767 (O). E-mail address: [email protected] (Z. Sun). http://dx.doi.org/10.1016/j.jallcom.2014.03.031 0925-8388/Ó 2014 Elsevier B.V. All rights reserved.
than Zn2+ that compared with ITO it will decrease the impurity level. In this study, the series of different mass fractions of ZnO:Ta films were prepared by RF magnetron sputtering. The chemical composition, structure, surface, topography, electrical and optical property were investigated using X-ray photoelectron spectrometer (XPS), X-ray diffractometer (XRD), Atomic Force Microscopy (AFM), ultraviolet–visible spectroscope (UV–Vis) and four-point probe instrument.
2. Experimental The substrates of glass and silicon slices were firstly cleaned by ultrasonic in acetone for 10 min, in deionized water for 10 min, then in ethyl alcohol for 10 min. After drying, they were placed into the ultra high vacuum magnetron sputtering apparatus (JGP560I). The target materials were ceramics of different mass fraction of Ta2O5 (0 wt.%, 2 wt.%, 5 wt.%, 7 wt.% and 10 wt.%) and ZnO(99%) sinter forging. Sputter for 30 min. The sputtering conditions are in Table 1. The microstructures of ZnO:Ta were tested by XRD (MAC, M18XHF) employing CuKa radiation. The thickness of the films was characterized by surface profiler (XP-1). The surface morphology of ZnO:Ta films was tested by AFM (AJ-III). XPS (Thermo, ESCALAB250) was employed to analyze the surface composition of the samples. The absorption spectra of samples were recorded by an UV–Vis spectrophotometer (Shimadzu, UV-2550) within the wavelength range of 200–900 nm. And the electrical resistivity of the films was characterized by four-point probe instrument (SZ-82).
86
Y. Cheng et al. / Journal of Alloys and Compounds 608 (2014) 85–89
Table 1 Experimental conditions for RF sputtering ZnO:Ta films. Parameter
Data
Background vacuum degree (Pa) Flow of Ar (SCCM) Sputtering pressure (Pa) Voltage (V) Sputtering current (A) Target-substrate distance (mm) Substrate temperature (K) Sputtering time (min)
9.0 104 30 0.7 450 0.18 60 293 30
3. Results and discussion 3.1. Composition analysis Fig. 1 is the XPS survey spectrum of ZnO:Ta film doped with 5 wt.% Ta2O5. It shows that surface layer of the films contains C, O, Zn and Ta. Fig. 2 is the binding energy of Zn 2p3/2 on the face of the film. From the figure, the peak locates on 1021.85 eV, just according with the bulks of ZnO (1021.75 eV), which shows that the form of Zn in the film is Zn2+, not elemental Zn [6,7]. Fig. 3 is the Photoelectron spectroscopy of O 1s. The map can be exploded to three peaks of 530.31 eV (OI), 530.97 eV (OII) and 532.17 eV (OIII). OI stands for the O2 in the hexagonal wurtzite, while OII correspond to the O2 of anoxic area in the ZnO. OIII stands for the absorbed Oxygenium, which exist in –CO3, H2O, –OH and O2. To calculate the proportion of O/Zn, the OI and OII are effective. Chen [8] discovered that in the Al-doped ZnO films the O 1s also has three different chemical states. In Fig. 4 photoelectron spectroscopy of Ta 4f, the characteristic peaks is on 26.24 eV and 28.04 eV, which stand for Ta 4f7/2 and Ta 4f5/2. Obviously, the valence state of Ta is more approaching to the Ta5+ in Ta2O5 (26.3 eV) than the zero valence Ta (21.7 eV) [9,10]. So the form of Ta in the films is Ta5+. The percentage of each component in the film can be calculated by the formula [11]:
x% ¼
Ax Sx PN Ai i¼1 Si
The result is in Table 2. The O/Zn is 84.73%, less than the ideal stoichiometric ratio of ZnO, explaining that the film is oxygenpoor. Then, the proportion of Ta/(Zn + Ta) is 3.29%, higher than
Fig. 2. Photoelectron spectroscopy of Zn 2p3/2 for ZnO:Ta films doped with 5 wt.% Ta2O5.
Fig. 3. Photoelectron spectroscopy of O 1s for ZnO:Ta films doped with 5 wt.% Ta2O5.
the calculation percentage 1.9% of 5 wt.% Ta2O5 target. The reason is the different saturated vapor pressure of ZnO and Ta2O5, which the higher one of ZnO leads to the desorption of Zn on the surface of the film. 3.2. Microstructure analysis
Fig. 1. XPS survey spectrum of ZnO:Ta film doped with 5 wt.% Ta2O5.
Fig. 5 shows the XRD patterns of the ZnO:Ta film for different Ta2O5 content. The diffraction peaks are consistent with standard spectrum diagram of ZnO. For the undoped ZnO film, the highest diffraction peak is on the 2h = 34.18, which represents the crystal face of (0 0 2). Besides, there are relatively lower peaks of (1 0 0), (1 0 1) and (1 0 2). While doped, all of these films only have peaks of (0 0 2) and (1 0 3) on the diffraction pattern. It illustrates that there is no new phase producing in the film, which another way of saying is the doping of Ta did not change the structure of hexagonal wurtzite. The Ta just replaced the position of Zn in the crystal lattice. The crystal quality of the films can be evaluated by the diffraction half peak width (FWHM) and crystal particle size. Fig. 6 is FWHM and crystallite size of ZnO:Ta film with different Ta2O5 content. The figure shows that as the proportion of the impurities increased from 0 wt.% to 5 wt.%, the intensity of (0 0 2) diffraction
Y. Cheng et al. / Journal of Alloys and Compounds 608 (2014) 85–89
87
content increased further, the crystal particle sizes decreased. It means the higher percentage of doping Ta destroyed the crystalline, because the atoms of Ta tend to concentrate on the crystal boundary, which leads to the disorder of lattice and the decrease of film property [13–15]. So the right amount of Ta doped can improve the structural performance of ZnO films. 3.3. Surface topography analysis
Fig. 4. Photoelectron spectroscopy of Ta 4f for ZnO:Ta films doped with 5 wt.% Ta2O5.
Table 2 The atomic percentage of constitutive components for ZnO:Ta films doped with 5 wt.% Ta2O5. Element
O
Zn
Ta
Percentage %
12.71
15.00
0.51
Fig. 7 shows the AFM images of ZnO:Ta film with different Ta2O5 content. The concentration of Ta brings a marked impact to the surface and crystal shape and size of the film. The surfaces of undoped ZnO films are short cylindrical. Moderate amount of doping films have more homogeneous particle size and no obvious empty and defects, proving that the films have better crystal quality. But when the Ta content increased to 10 wt.%, the granules are refined in different size, and the surface is rough with more empty. The reason may be the over content of Ta which leads segregation and influences the surface of the film [16]. Fig. 8 shows the average surface grain size and roughness of ZnO:Ta film with different Ta2O5 content. When the doping content grew from 0 wt.% to 5 wt.%, the surface roughness (RMS) decreased from 10.4 nm to 4.5 nm. Then as the content kept growing to 10 wt.%, the surface roughness grew to 7.0 nm. Undoped ZnO has relatively bigger surface grain size, and the doped films’ average surface grain sizes increase firstly and then decrease. The 5 wt.% doped film has the largest average particle size of 94.5 nm, met up with the result of XRD. 3.4. Electrical property analysis Fig. 9 gives the resistivity of ZnO:Ta film with different Ta2O5 content. For the record, the undoped ZnO film has high resistance that beyond the maximum span of the equipment. Firstly, when the content of Ta rose up, the resistivity of the film rushed down significantly. And the 5 wt.% film had the lowest resistivity of 7.81 102 X cm. But when the percentage increased to 10 wt.%, the resistivity grew to 1.25 101 X cm. For the doped films, the production of charge carrier contains two ways. One is due to oxygen vacancy. From the XPS result, the film was oxygen-poor. It was easy to form the oxygen vacancy, producing two electrons. On the other hand, the doping Ta5+ gives surplus electrons. Then the structure of crystals became homogeneous and had better crystal quality, which leads to the lower block to the charge carrier. So the resistivity got low fast as the Ta increased first. But, as the doping ions have attraction to oxygen
Fig. 5. XRD patterns of ZnO:Ta film with different Ta2O5 content.
peak grew higher, while the half peak width decreased from 0.73° to 0.61°, proving the increasing quality of crystal. On the other hand, the intensity of (0 0 2) diffraction peak decreased when the proportion of the impurities increased further to 10 wt.%. The half peak width grew to 0.72°, indicated that the crystal quality become worse. The average crystal particle size can be calculated through Scherrer formula [12]:
D¼
Kk b cos h
The results are in Fig. 6, which the crystal particle sizes are among 10.5–13.7 nm. When the Ta content of the target is 5 wt.%, the film has best structural performance. But as the doping
Fig. 6. FWHM and crystallite size of ZnO:Ta film with different Ta2O5 content.
88
Y. Cheng et al. / Journal of Alloys and Compounds 608 (2014) 85–89
Fig. 7. AFM images of ZnO:Ta film with different Ta2O5 content: (a) 0 wt.%, (b) 2 wt.%, (c) 5 wt.%, (d) 7 wt.%, (e) 10 wt.%.
Fig. 8. Average surface grain size and roughness of ZnO:Ta film with different Ta2O5 content.
Fig. 9. Resistivity of ZnO:Ta film with different Ta2O5 content.
ions, they can fetter the oxygen around them, which leads to the reduction of charge carrier. So when the doping content further increased, more and more Ta5+ gathered to form neutral defects. They can not produce charge carriers, instead, becoming the traps of electrons, causing the rising trend of resistivity [13–18]. 3.5. Optical property analysis Fig. 10 is the visible transmission and reflectance spectrals of ZnO:Ta film with different Ta2O5 content, with the influence of glass substrate. The film thickness is 400 nm. From the result, all of the samples perform a high transmission in visible area, and have a strong absorbent in ultraviolet region, corresponding to the absorbent of charge carriers. Compared with the undoped film, the transmission of the samples in visible area increased, for the reduction of surface roughness [19]. The figure also shows the reflectance of 5 wt.% doped film, which the result is among 10%– 20%. The absorptivity can be calculated by transmission and reflectance through the formula [20]:
1 d
a ¼ ln
1R T
For the direct-gap semiconductor, the absorptivity has relation with optical band gap Eg as this: [21]
Fig. 10. The visible transmission and reflectance spectrals of ZnO:Ta film with different Ta2O5 content: (a) 0 wt.%, (b) 2 wt.%, (c) 5 wt.%, (d) 7 wt.%, (e) 10 wt.%.
2
ðahmÞ ¼ Aðhm Eg Þ So, as Figs. 11 and 12 show, when the dope content grew from 0 wt.% to 5 wt.%, the optical band gap increased from 3.27 eV to
Y. Cheng et al. / Journal of Alloys and Compounds 608 (2014) 85–89
89
all filled and the Fermi level got higher to conduction band. The film became degenerate semiconductor. So the valence electron must be excited higher above the Fermi level in order to become free charge carrier, which means the band gap of the film became wider. In the doped film, the raising concentration of charge carrier leads to blue shift absorption edge. The similar result can be seen in the work by Yang [24]. To evaluate the transparent conducting film, Fig. 13 shows the merit of ZnO:Ta film with different Ta2O5 content. When the content is 5 wt.%, the merit get the highest peak of 2.20 104 X1. 4. Conclusions
Fig. 11. The relationship graph of (ahm)2 and photon energy for ZnO:Ta film with different Ta2O5 content.
In summary, Ta doped ZnO transparent conducting films are produced by magnetron sputtering apparatus. The chemical composition, structure, surface, topography, electrical and optical property were systematically investigated. The results are as follows: Ta doping did not change the crystal structure of ZnO, instead, the doping ions took possession of Zn and become solid solution. The minimum resistivity of the ZnO:Ta films is 7.81 102 X cm. The average transmittance in visible area is over 85%. The maximum optical band gap is 3.38 eV. These prove that the doped Tantalum can help improve the structure and property of ZnO film. The merit of 5 wt.% is the highest, for 2.20 104 X1. So the best doping content is 5 wt.%. Acknowledgments This work is supported by the National Natural Science Foundation of China (No. 51272001), National Key Basic Research Programs (2013CB632705), and the National Science Research Foundation for Scholars Return from Overseas, Ministry of Education, China. References
Fig. 12. Optical band gap of ZnO:Ta film with different Ta2O5 content.
Fig. 13. Figure of merit of ZnO:Ta film with different Ta2O5 content.
3.38 eV. When the percentage get to 10 wt.%, the optical band gap fell to 3.34 eV. This transformation law might be explained by Burstein–Moss Move [22,23]. According to this theory, as the Ta doped, the concentration of the charge carrier increased. This made that the energy states in the bottom of the conduction band were
[1] Z. Pan, P. Zhang, X. Tian, G. Cheng, Y. Xie, H. Zhang, X. Zeng, C. Xiao, G. Hu, Z. Wei, J. Alloys Comp. 576 (2013) 31–37. [2] R. Yousefi, B. Kamaluddin, J. Alloys Comp. 479 (2009) L11–L14. [3] K.W. Liu, M. Sakurai, M. Aono, Sens. Actuators, B 157 (1) (2011) 98–102. [4] K. Zhong, J. Xu, J. Su, Y.L. Chen, Appl. Surf. Sci. 257 (8) (2011) 3495–3498. [5] I. Drouelle, C. Servant, J. Alloys Comp. 551 (2013) 293–299. [6] H. Xue, Y. Chen, X.L. Xu, G.H. Zhang, H. Zhang, S.Y. Ma, Physica E 41 (5) (2009) 788–791. [7] Y. Zhang, G. Du, X. Wang, W. Li, X. Yang, Y. Ma, B. Zhao, H. Yang, D. Liu, S. Yang, J. Cryst. Growth 252 (1–3) (2003) 180–183. [8] M. Chen, X. Wang, Y.H. Yu, Z.L. Pei, X.D. Bai, C. Sun, R.F. Huang, L.S. Wen, Appl. Surf. Sci. 158 (1–2) (2000) 134–140. [9] S.V.J. Chandra, S. Uthanna, G.M. Rao, Appl. Surf. Sci. 254 (7) (2008) 1953–1960. [10] E. Atanassova, D. Spassov, Appl. Surf. Sci. 135 (1–4) (1998) 71–82. [11] E. Holmelund, J. Schou, S. Tougaard, N.B. Larsen, Appl. Surf. Sci. 197–198 (1) (2002) 467–471. [12] R. Sharma, K. Sehrawat, A. Wakahara, R.M. Mehra, Appl. Surf. Sci. 255 (11) (2009) 5781–5788. [13] B.H. Choi, H.B. Im, J.S. Song, J. Am. Ceram. Soc. 73 (5) (1990) 1347–1350. [14] M. Lv, X. Xiu, Z. Pang, Y. Dai, L. Ye, C. Cheng, S. Han, Thin Solid Films 516 (8) (2008) 2017–2021. [15] Z.B. Fang, Y.S. Tan, H.X. Gong, C.M. Zhen, Z.W. He, Y.Y. Wang, Mater. Lett. 59 (21) (2005) 2611–2614. [16] D.S. Liu, F.C. Tsai, C.T. Lee, C.W. Sheu, Jpn. J. Appl. Phys. 47 (4) (2008) 3056– 3062. [17] G. Frank, H. Kostlin, Appl. Phys. A: Mater. 27 (1) (1982) 197–206. [18] C. Jeong, H.S. Kim, D.R. Chang, K. Kamisako, Jpn. J. Appl. Phys. 47 (7) (2008) 5656–5658. [19] J.L. Chung, J.C. Chen, C.J. Tseng, J. Phys. Chem. Solids 69 (2–3) (2008) 535–539. [20] B.G. Choi, I.H. Kim, D.H. Kim, K.S. Lee, T.S. Lee, B. Cheong, Y.J. Baik, W.M. Kim, J. Eur. Ceram. Soc. 25 (12) (2005) 2161–2165. [21] C. Fournier, O. Bamiduro, H. Mustafa, R. Mundle, R.B. Konda, F. Williams, A.K. Pradhan, Semicond. Sci. Technol. 23 (8) (2008). 85019–1–85019–5. [22] E. Burstein, Phys. Rev. 93 (3) (1954) 632–633. [23] T.S. Moss, Proc. Phys. Soc. Sect. B 67 (10) (1954) 775–782. [24] J.H. Yang, X.T. Zhao, X.N. Shan, H.G. Fan, L.L. Yang, Y.J. Zhang, X.Y. Li, J. Alloys Comp. 556 (2013) 1–5.