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Tailoring the refractive index of aluminum doped zinc oxide thin films by co-doping with titanium
Applied Surface Science 263 (2012) 210–214
Contents lists available at SciVerse ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Tailoring the refractive index of aluminum doped zinc oxide thin films by co-doping with titanium Tiefeng Wei a , Pinjun Lan a , Ye Yang a , Xianpeng Zhang a , Ruiqin Tan b , Yong Li a , Weijie Song a,∗ a b
Ningbo Institute of Material Technology and Engineering, Chinese Academy of Sciences, No. 519 Zhuangshi Road, Zhenhai District, Ningbo 315201, PR China Faculty of Information Science and Engineering, Ningbo University, No. 818 Fenghua Road, Jiangbei District, Ningbo 315211, PR China
a r t i c l e
i n f o
Article history: Received 12 July 2012 Received in revised form 5 September 2012 Accepted 6 September 2012 Available online 14 September 2012 Keywords: Al–Ti co-doped ZnO thin films Refractive index Resistivity
a b s t r a c t The refractive index of transparent conductive oxides has a direct effect on the transmission of lights into thin film solar cells. Here we report the study of improving the refractive index of aluminum doped zinc oxide through titanium co-doping. The Al–Ti co-doped zinc oxide (ATZO) thin films with different Ti doping concentration were deposited on glass substrates by radio frequency magnetron sputtering with ATZO targets in an argon atmosphere. The structural, optical and electrical properties of the thin films were investigated using X-ray diffraction, ultraviolet–visible-near-infrared spectroscopy and hall measurements, respectively. The results showed that the as-deposited thin films were all textured along c-axis and perpendicular to the surface of substrate. The average transmittance in the visible region were more than 80% for all the ATZO thin films. The minimum resistivity of the obtained ATZO (1 wt% TiO2 doping) thin films were 2.6 × 10−3 cm and 1.4 × 10−3 cm before and after annealing in vacuum, respectively. The refractive index of the thin films (at 0 = 550 nm) increased from 1.91 to 2.05 as the TiO2 content increased from 0 wt% to 3 wt%. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Aluminum-doped zinc oxide (AZO) thin films have attracted much attention as transparent conductive oxide (TCO) electrodes in Si-based thin film solar cells because of their low cost and excellent optical and electrical properties [1–3]. Several methods, such as hydrogen plasma treatment [4], surface texturing [5–7], and buffer layers deposition [8,9] on AZO thin films, have been developed to improve its optoelectronic properties for applications in solar cells. Although many efforts have been made, the refractive index (RI) of the AZO layer should be further optimized according to the Fresnel’s law in order to reduce the light reflections at the glass/AZO/Si interfaces [10]. The RI of the AZO layer is calculated to be 2.29 at 0 = 550 nm in the case of glass (n ∼ 1.5)/AZO/Si (n ∼ 3.5) to minimize the interfacial reflection. However, the mostreported RIs of AZO thin films were varied between 1.8 and 1.9 at 0 = 550 nm [11,12]. Optimizing the RI of AZO thin films is required for improving the light transmission into the Si layer. Several studies have been reported on tailoring the RI of the zinc oxide thin films. Xue et al. showed the effect of annealing temperature and the Al doping concentration on the RI of ZnO thin films [13,14]. Caglar et al. described the variation of the RI of ZnO thin
∗ Corresponding author. Tel.: +86 574 87913375. E-mail address: [email protected] (W. Song). 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2012.09.029
films with different doping elements [15]. Kang et al. reported the influence of the substrate temperature on the RI of ZnO films [16]. Most of the reported results revealed only a limited variation of the RI, which could not meet the requirement. The studies to tailor the RI of the AZO thin films in a relatively larger range are still necessary. Ye et al. studied the variation of RI of the nanocrystalline compound ZnO–TiO2 thin films with different Zn/Ti. The RI of the ZnO–TiO2 thin films changed from 1.9 to 2.4 with the variation of TiO2 contents from 0% to 100% [17]. This indicated that modulation of the RI of AZO thin films by co-doping with Ti might be practically applicable. In this work, the (ATZO) thin films were fabricated by radio frequency (RF) magnetron sputtering, and the influence of the TiO2 content on the structural, electrical and optical properties of the ATZO thin films was investigated.
2. Experimental The ATZO thin films were deposited on glass substrates using radio-frequency (RF) planar magnetron sputtering (JS-8000, ULVAC, Japan). Before deposition, the glass substrates were ultrasonically cleaned in acetone, rinsed in deionized water and dried in nitrogen. The sputtering targets were high density (>98%) ATZO ceramics co-doped with 2 wt% Al2 O3 and different TiO2 contents (0, 1, 2 and 3 wt%) which were fabricated in our laboratory. The sputtering process was performed in pure Ar at a pressure of 0.6 Pa and the RF power was fixed at 100 W. The distance between the
T. Wei et al. / Applied Surface Science 263 (2012) 210–214
Fig. 1. X-ray diffraction patterns (–2) of the films prepared by ATZO targets with different TiO2 contents.
target and the substrate was 140 mm. The substrate temperature was kept at 350 ◦ C. The samples were rotated at a constant speed of 10 rpm during sputtering. The RI and film thickness of the AZTO thin films were determined using a spectroscopic ellipsometry (M2000-DI, JA Woollam, USA). Crystallographic and phase structure of the ATZO thin films were determined using a Bruker D8 Advance X-ray diffractometer ˚ The optical transmittance of with Cu K␣ radiation ( = 1.5406 A). the ATZO thin films on glass substrates were measured using an ultraviolet-visible-near-infrared (UV-Vis-NIR) spectrophotometer (Lambda 950, Perkin Elmer, USA). The surface morphology of the AZO thin films was characterized using an atomic force microscope (AFM, Vecoo Dimension V, USA). Hall measurements (HL5500PC, Nanometrics, USA) were performed to determine the electrical resistivity, carrier concentration, and mobility. 3. Results and discussion Fig. 1 shows the X-ray diffraction patterns of the as-obtained ATZO thin films. All the diffraction patterns exhibited strong (0 0 2) diffraction peaks of the hexagonal ZnO structure (34.45◦ , JCPDS card No. 079-0205), which revealed the obvious c-axis orientation of the ATZO thin films. This indicated that the preferred orientation of the films was not changed after Ti doping. In addition, no secondary phase TiO2 or Al2 O3 diffraction peak was observed, which implied that the doping atoms have replaced zinc atoms sites or incorporated interstitially in the lattice. Similar results on the structures of Ti-doped ZnO thin films prepared by magnetron sputtering were reported previously [18,19]. Fig. 2 shows the dependence of diffraction angle (2) and fullwidth at half-maximum (FWMH) on the TiO2 content. It was observed that the positions of the (0 0 2) peaks (curve a) shifted toward lower diffraction angles and the FWHM of (0 0 2) peaks (curve b) increased from 0.221◦ to 0.585◦ as TiO2 content increased from 0 to 3 wt%. The broadening of the (0 0 2) peak indicated that the crystallite of ATZO thin films was distorted by more Ti atoms substituting into the Zn sites, and the films suffered a compressive stress in the direction parallel to the surface [20]. This resulted in the inter-planar spacing (d) increasing, which led to the shift of (0 0 2) diffraction peaks to lower angles. The average grain size (D) of the ATZO thin films was estimated using the Scherrer’s equation: ˚ B is the D = 0.9/Bcos, where is the X-ray wavelength (1.5406 A), FWMH, and is the Bragg diffraction angle. The grain sizes along caxis were estimated to be 37.2 ± 0.3, 37.0 ± 0.3, 15.7 ± 0.3, 14.0 ± 0.3
211
Fig. 2. The (0 0 2) peak position and the FWMH of the ATZO thin films with different TiO2 contents.
(nm) with the TiO2 content increased from 0 wt% to 3 wt%. This suggested that the lattice of ATZO thin films became more distorted and a higher density of defects was generated with increasing the TiO2 content. Fig. 3 shows the AFM images of ATZO thin films with different TiO2 contents. The root-mean-square (RMS) roughness of the ATZO thin films decreased from 9.9 nm to 5.7 nm with increasing the TiO2 content. This indicated that the heavier doping resulted in not only lattice distortion but also surface roughness decrease of the ATZO thin films. In addition, it was clear that the particle size of the ATZO thin films decreased with increasing the TiO2 content, which was in consistent with the calculated results of grain size from Fig. 2. Fig. 4 shows the electrical resisitivity,carrier concentration and mobility of the ATZO thin films with different TiO2 contents before and after annealing. The resistivity of as deposited ATZO thin films increased from 8.8 × 10−4 cm to 6.5 × 10−3 cm when TiO2 content increased from 0 wt% to 3 wt%. The increase of the resistivity could be explained that Al–Ti impurity co-doped into ZnO films acted as effective donor resulting from the substitution of Al3+ and Ti4+ for Zn2+ or the incorporation of Al and Ti ions into interstitial positions, shown in the following equations: Al−→AlZn + OX O+
1 O2 + e 2
(1)
Ti−→TiZn + OX O+
1 O2 + 2e 2
(2)
ZnO
ZnO
Substitution of Ti4+ for Zn2+ could contribute one more free electron to the conduction of thin films than the substitution Al3+ for ˚ was much smaller than that of Zn2+ . But the radius of Ti4+ (0.42 A) ˚ a small dopant concentration will exceed the solid Zn2+ (0.75 A), solubility. As reported by Chen et al. [19,21], the formation of TiOx took place at the grain boundaries due to co-doping of more Ti into the AZO films. It could result in the decrease of carrier concentration and mobility, as observed in our experiment. The optical transmittance of the ATZO thin films are shown in Fig. 5. The thickness of the as-deposited films were about 250 nm fitted by spectroscopic ellipsometry. It was observed that all the AZTO thin films had an average transmittance of more than 80% in the visible range. When the TiO2 content increased, the transmittance at around 400 nm decreased abruptly and the absorption edge revealed a red shift. It was known that ZnO was a direct transition type semiconductor [22] and its optical energy gap could be estimated by plotting ˛2 (˛ is absorption coefficient) versus h (h is the photon energy) according to the following equation: ˛2 = A(h − Eg )
(3)
212
T. Wei et al. / Applied Surface Science 263 (2012) 210–214
Fig. 4. The electrical resistivity (), carrier concentration (N) and mobility () of the ATZO thin films before and after annealing in vaccum at 400 ◦ C for an hour.
Fig. 5. The transmittance spectra of the ATZO thin films with different TiO2 contents.
Fig. 3. AFM images of the ATZO thin films with different TiO2 contents.
Fig. 6. The RI of the ATZO thin films with different TiO2 contents.
T. Wei et al. / Applied Surface Science 263 (2012) 210–214
213
Fig. 7. The RI of the ATZO thin films before and after annealing in vacuum at 400 ◦ C for an hour.
The intercept on the energy axis gave the value of Eg by extrapolating the straight-line part of the plot to the photon energy axis. It was observed that the Eg of the ATZO thin films decreased from 3.52 eV to 3.38 eV with the increase of TiO2 contents from 0 wt% to 3 wt%. The shrinkage of the optical energy gap could be due to the decrease of carrier concentration according to the equation as follows [23]:
Eg =
2 2m∗cv
(3 2 N)
2/3
(4)
where the Eg is the shift of the optical energy gap, mcv is the effective mass of the electron, h is Planck’s constant, and N is the carrier concentration. Fig. 6 shows the RI of ATZO thin films as a function of the wavelength. Cauchy model was used to extract the RI (n) and extinction coefficient (k) of ATZO thin films. The following formulas were used to define the n and k. The fitting procedure to determine the optical constants of samples was described in Refs. [24,25]. n() = A +
B C + 4 2
k() = ˛ exp 12 400
(5)
1
−
1
(6)
The six fitting parameters in the model are A, B, C, the extinction coefficient amplitude ˛, the exponent factor ˇ and the band edge
. Three phase model was used to describe the material system, i.e., air/ATZO film/1 mm glass substrate. It was observed that the RI of the ATZO thin films increased with increasing TiO2 content, and the highest RI was 2.05 at 0 = 550 nm with the TiO2 at 3 wt%. The increase of RI was mainly attributed to two reasons: one was the decrease of carrier concentration with increasing the TiO2 content in the ATZO thin films [26], which could be inferred from Fig. 4(b);
the other was the forming of relatively higher RI of TiOx . To further clarify the relationship of RI and carrier concentration, the ATZO thin films were annealed in vacuum (4 × 10−4 Pa) at 400 ◦ C for 1 h. The changes of carrier concentration and RI of the ATZO thin films were then estimated. Fig. 4(b) and Fig. 7 revealed the variation of carrier concentration and RI of the ATZO thin films after annealing in vacuum. The carrier concentration of the ATZO films increased, while the RI showed a declined tendency after annealing in vacuum. The increase of carrier concentration was due to the generate of the oxygen vacancies, which can also result in the decrease in the RI of the films. Based on the above results, the RI of the AZO thin films could be tailored by tuning the content of co-doped Ti, which was important for applications in designing the integrated optoelectronic devices such as a-Si thin film solar cells. 4. Conclusions In this work, ATZO thin films were deposited on glass substrates by RF magnetron sputtering in pure Ar ambient. The ATZO thin films revealed hexagonal würtzite structure with (0 0 2) preferential orientation. The RI and the resistivity increased with the increase of the doping amount of Ti, and the highest RI of the film was 2.05 at 0 = 550 nm with the 3 wt% doping TiO2 . The RI of the ATZO thin films was improved by co-doping with Ti, which was due to the decrease of conductivity and the increase of the TiO2 content. Acknowledgements This work was financially supported by the NSFC (20975107), the “Hundred Talents Program,” from the Chinese Academy of Sciences, the Zhejiang Natural Science Foundation (Y4110463), and the Ningbo Innovative Research Team Program.
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References [1] S. Ghosh, A. Sarkar, S. Chaudhuri, A.K. Pal, Thin Solid Films 205 (1991) 64. [2] K.H. Kim, K.C. Park, D.Y. Ma, J. Appl. Phys. 81 (1997) 7764. [3] V. Sittinger, F. Ruske, W. Werner, B. Szyszka, B. Rech, J. Hüpkes, G. Schöpe, H. Stiebig, Thin Solid Films 496 (2006) 16–25. [4] F.H. Wang, H.P. Chang, C.C. Tseng, C.C. Huang, H.W. Liu, Curr. Appl. Phys. 11 (2011) 12–16. [5] W.T. Yen, Y.C. Lin, J.H. Ke, Appl. Surf. Sci. 257 (2010) 960–968. [6] O. Kluth, B. Rech, L. Houben, S. Wieder, G. Schöpe, C. Beneking, H. Wanger, A. Löffl, H.W. Schock, Thin Solid Films 351 (1999) 247–253. [7] W.L. Lua, K.C. Huanga, C.H. Yehb, C.I. Hungc, M.P. Hounga, Mater. Chem. Phys. 127 (2011) 358–363. [8] C.Y. Hsu, T.F. Ko, Y.M. Huang, J. Eur. Ceram. Soc. 28 (2008) 3065–3070. [9] J.H. Shi, S.M. Huang, J.B. Chu, H.B. Zhu, Z.A. Wang, X.D. Li, D.W. Zhang, Z. Sun, W.J. Cheng, F.Q. Huang, X.J. Yin, J. Mater. Sci.: Mater. Electron. 21 (2010) 1005–1013. [10] C.J. Brinker, M.S. Harrington, Sol. Energy Mater. 5 (1981) 159–172. [11] T. Schuler, M.A. Aegerter, Thin Solid Films 35 (1999) 1125–1131. [12] M. Caglar, S. Ilican, Y. Caglar, F. Yakuphanoglu, J. Mater. Sci.: Mater. Electron. 19 (2008) 704–708.
[13] S.W. Xue, X.T. Zu, W.L. Zhou, H.X. Deng, X. Xiang, L. Zhang, H. Deng, J. Alloys Compd. 448 (2008) 21–26. [14] S.W. Xue, X.T. Zua, W.G. Zheng, H.X. Deng, X. Xiang, Physica B 3813 (2006) 209–213. [15] Y. Caglar, S. Ilican, M. Caglar, F. Yakuphanoglu, Spectrochim. Acta Part A 67 (2007) 1113–1119. [16] S.J. Kang, Y.H. Joung, Appl. Surf. Sci. 253 (2007) 7330–7335. [17] C. Ye, S.S. Pan, X.M. Teng, H.T. Fan, G.H. Li, Appl. Phys. A 90 (2008) 375–378. ˇ [18] S.S. Lin, J.L. Huang, P. Sajgalik, Surf. Coat. Technol. 191 (2005) 286–292. [19] J.L. Chung, J.C. Chen, C.J. Tseng, Appl. Surf. Sci. 255 (2008) 2494–2499. [20] J.J. Lu, Y.M. Lu, S.I. Tasi, T.L. Hsiung, H.P. Wang, L.Y. Jang, Opt. Mater. 29 (2007) 1548–1552. [21] J.L. Chung, J.C. Chen, C.J. Tseng, Appl. Surf. Sci. 254 (2008) 2615–2620. [22] S.T. Tan, B.J. Chen, W.W. Sun, W.J. Fan, H.S. Kwok, X.H. Zhang, S.J. Chua, J. Appl. Phys. 98 (2005), 013505(1–5). [23] C. Jeong, H.S. Kim, D.R. Chang, K. Kamisako, Jpn. J. Appl. Phys. 47 (2008) 5656–5658. [24] Y.C. Liu, S.K. Tung, J.H. Hsieh, J. Cryst. Growth 105 (2006) 287. [25] Y.C. Liu, J.H. Hsieh, S.K. Tung, Thin Solid Films 32 (2006) 510. [26] H. Kim, A. Pique, J.S. Horwitz, et al., Thin Solid Films 798 (2000) 377–378.
Contents lists available at SciVerse ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Tailoring the refractive index of aluminum doped zinc oxide thin films by co-doping with titanium Tiefeng Wei a , Pinjun Lan a , Ye Yang a , Xianpeng Zhang a , Ruiqin Tan b , Yong Li a , Weijie Song a,∗ a b
Ningbo Institute of Material Technology and Engineering, Chinese Academy of Sciences, No. 519 Zhuangshi Road, Zhenhai District, Ningbo 315201, PR China Faculty of Information Science and Engineering, Ningbo University, No. 818 Fenghua Road, Jiangbei District, Ningbo 315211, PR China
a r t i c l e
i n f o
Article history: Received 12 July 2012 Received in revised form 5 September 2012 Accepted 6 September 2012 Available online 14 September 2012 Keywords: Al–Ti co-doped ZnO thin films Refractive index Resistivity
a b s t r a c t The refractive index of transparent conductive oxides has a direct effect on the transmission of lights into thin film solar cells. Here we report the study of improving the refractive index of aluminum doped zinc oxide through titanium co-doping. The Al–Ti co-doped zinc oxide (ATZO) thin films with different Ti doping concentration were deposited on glass substrates by radio frequency magnetron sputtering with ATZO targets in an argon atmosphere. The structural, optical and electrical properties of the thin films were investigated using X-ray diffraction, ultraviolet–visible-near-infrared spectroscopy and hall measurements, respectively. The results showed that the as-deposited thin films were all textured along c-axis and perpendicular to the surface of substrate. The average transmittance in the visible region were more than 80% for all the ATZO thin films. The minimum resistivity of the obtained ATZO (1 wt% TiO2 doping) thin films were 2.6 × 10−3 cm and 1.4 × 10−3 cm before and after annealing in vacuum, respectively. The refractive index of the thin films (at 0 = 550 nm) increased from 1.91 to 2.05 as the TiO2 content increased from 0 wt% to 3 wt%. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Aluminum-doped zinc oxide (AZO) thin films have attracted much attention as transparent conductive oxide (TCO) electrodes in Si-based thin film solar cells because of their low cost and excellent optical and electrical properties [1–3]. Several methods, such as hydrogen plasma treatment [4], surface texturing [5–7], and buffer layers deposition [8,9] on AZO thin films, have been developed to improve its optoelectronic properties for applications in solar cells. Although many efforts have been made, the refractive index (RI) of the AZO layer should be further optimized according to the Fresnel’s law in order to reduce the light reflections at the glass/AZO/Si interfaces [10]. The RI of the AZO layer is calculated to be 2.29 at 0 = 550 nm in the case of glass (n ∼ 1.5)/AZO/Si (n ∼ 3.5) to minimize the interfacial reflection. However, the mostreported RIs of AZO thin films were varied between 1.8 and 1.9 at 0 = 550 nm [11,12]. Optimizing the RI of AZO thin films is required for improving the light transmission into the Si layer. Several studies have been reported on tailoring the RI of the zinc oxide thin films. Xue et al. showed the effect of annealing temperature and the Al doping concentration on the RI of ZnO thin films [13,14]. Caglar et al. described the variation of the RI of ZnO thin
∗ Corresponding author. Tel.: +86 574 87913375. E-mail address: [email protected] (W. Song). 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2012.09.029
films with different doping elements [15]. Kang et al. reported the influence of the substrate temperature on the RI of ZnO films [16]. Most of the reported results revealed only a limited variation of the RI, which could not meet the requirement. The studies to tailor the RI of the AZO thin films in a relatively larger range are still necessary. Ye et al. studied the variation of RI of the nanocrystalline compound ZnO–TiO2 thin films with different Zn/Ti. The RI of the ZnO–TiO2 thin films changed from 1.9 to 2.4 with the variation of TiO2 contents from 0% to 100% [17]. This indicated that modulation of the RI of AZO thin films by co-doping with Ti might be practically applicable. In this work, the (ATZO) thin films were fabricated by radio frequency (RF) magnetron sputtering, and the influence of the TiO2 content on the structural, electrical and optical properties of the ATZO thin films was investigated.
2. Experimental The ATZO thin films were deposited on glass substrates using radio-frequency (RF) planar magnetron sputtering (JS-8000, ULVAC, Japan). Before deposition, the glass substrates were ultrasonically cleaned in acetone, rinsed in deionized water and dried in nitrogen. The sputtering targets were high density (>98%) ATZO ceramics co-doped with 2 wt% Al2 O3 and different TiO2 contents (0, 1, 2 and 3 wt%) which were fabricated in our laboratory. The sputtering process was performed in pure Ar at a pressure of 0.6 Pa and the RF power was fixed at 100 W. The distance between the
T. Wei et al. / Applied Surface Science 263 (2012) 210–214
Fig. 1. X-ray diffraction patterns (–2) of the films prepared by ATZO targets with different TiO2 contents.
target and the substrate was 140 mm. The substrate temperature was kept at 350 ◦ C. The samples were rotated at a constant speed of 10 rpm during sputtering. The RI and film thickness of the AZTO thin films were determined using a spectroscopic ellipsometry (M2000-DI, JA Woollam, USA). Crystallographic and phase structure of the ATZO thin films were determined using a Bruker D8 Advance X-ray diffractometer ˚ The optical transmittance of with Cu K␣ radiation ( = 1.5406 A). the ATZO thin films on glass substrates were measured using an ultraviolet-visible-near-infrared (UV-Vis-NIR) spectrophotometer (Lambda 950, Perkin Elmer, USA). The surface morphology of the AZO thin films was characterized using an atomic force microscope (AFM, Vecoo Dimension V, USA). Hall measurements (HL5500PC, Nanometrics, USA) were performed to determine the electrical resistivity, carrier concentration, and mobility. 3. Results and discussion Fig. 1 shows the X-ray diffraction patterns of the as-obtained ATZO thin films. All the diffraction patterns exhibited strong (0 0 2) diffraction peaks of the hexagonal ZnO structure (34.45◦ , JCPDS card No. 079-0205), which revealed the obvious c-axis orientation of the ATZO thin films. This indicated that the preferred orientation of the films was not changed after Ti doping. In addition, no secondary phase TiO2 or Al2 O3 diffraction peak was observed, which implied that the doping atoms have replaced zinc atoms sites or incorporated interstitially in the lattice. Similar results on the structures of Ti-doped ZnO thin films prepared by magnetron sputtering were reported previously [18,19]. Fig. 2 shows the dependence of diffraction angle (2) and fullwidth at half-maximum (FWMH) on the TiO2 content. It was observed that the positions of the (0 0 2) peaks (curve a) shifted toward lower diffraction angles and the FWHM of (0 0 2) peaks (curve b) increased from 0.221◦ to 0.585◦ as TiO2 content increased from 0 to 3 wt%. The broadening of the (0 0 2) peak indicated that the crystallite of ATZO thin films was distorted by more Ti atoms substituting into the Zn sites, and the films suffered a compressive stress in the direction parallel to the surface [20]. This resulted in the inter-planar spacing (d) increasing, which led to the shift of (0 0 2) diffraction peaks to lower angles. The average grain size (D) of the ATZO thin films was estimated using the Scherrer’s equation: ˚ B is the D = 0.9/Bcos, where is the X-ray wavelength (1.5406 A), FWMH, and is the Bragg diffraction angle. The grain sizes along caxis were estimated to be 37.2 ± 0.3, 37.0 ± 0.3, 15.7 ± 0.3, 14.0 ± 0.3
211
Fig. 2. The (0 0 2) peak position and the FWMH of the ATZO thin films with different TiO2 contents.
(nm) with the TiO2 content increased from 0 wt% to 3 wt%. This suggested that the lattice of ATZO thin films became more distorted and a higher density of defects was generated with increasing the TiO2 content. Fig. 3 shows the AFM images of ATZO thin films with different TiO2 contents. The root-mean-square (RMS) roughness of the ATZO thin films decreased from 9.9 nm to 5.7 nm with increasing the TiO2 content. This indicated that the heavier doping resulted in not only lattice distortion but also surface roughness decrease of the ATZO thin films. In addition, it was clear that the particle size of the ATZO thin films decreased with increasing the TiO2 content, which was in consistent with the calculated results of grain size from Fig. 2. Fig. 4 shows the electrical resisitivity,carrier concentration and mobility of the ATZO thin films with different TiO2 contents before and after annealing. The resistivity of as deposited ATZO thin films increased from 8.8 × 10−4 cm to 6.5 × 10−3 cm when TiO2 content increased from 0 wt% to 3 wt%. The increase of the resistivity could be explained that Al–Ti impurity co-doped into ZnO films acted as effective donor resulting from the substitution of Al3+ and Ti4+ for Zn2+ or the incorporation of Al and Ti ions into interstitial positions, shown in the following equations: Al−→AlZn + OX O+
1 O2 + e 2
(1)
Ti−→TiZn + OX O+
1 O2 + 2e 2
(2)
ZnO
ZnO
Substitution of Ti4+ for Zn2+ could contribute one more free electron to the conduction of thin films than the substitution Al3+ for ˚ was much smaller than that of Zn2+ . But the radius of Ti4+ (0.42 A) ˚ a small dopant concentration will exceed the solid Zn2+ (0.75 A), solubility. As reported by Chen et al. [19,21], the formation of TiOx took place at the grain boundaries due to co-doping of more Ti into the AZO films. It could result in the decrease of carrier concentration and mobility, as observed in our experiment. The optical transmittance of the ATZO thin films are shown in Fig. 5. The thickness of the as-deposited films were about 250 nm fitted by spectroscopic ellipsometry. It was observed that all the AZTO thin films had an average transmittance of more than 80% in the visible range. When the TiO2 content increased, the transmittance at around 400 nm decreased abruptly and the absorption edge revealed a red shift. It was known that ZnO was a direct transition type semiconductor [22] and its optical energy gap could be estimated by plotting ˛2 (˛ is absorption coefficient) versus h (h is the photon energy) according to the following equation: ˛2 = A(h − Eg )
(3)
212
T. Wei et al. / Applied Surface Science 263 (2012) 210–214
Fig. 4. The electrical resistivity (), carrier concentration (N) and mobility () of the ATZO thin films before and after annealing in vaccum at 400 ◦ C for an hour.
Fig. 5. The transmittance spectra of the ATZO thin films with different TiO2 contents.
Fig. 3. AFM images of the ATZO thin films with different TiO2 contents.
Fig. 6. The RI of the ATZO thin films with different TiO2 contents.
T. Wei et al. / Applied Surface Science 263 (2012) 210–214
213
Fig. 7. The RI of the ATZO thin films before and after annealing in vacuum at 400 ◦ C for an hour.
The intercept on the energy axis gave the value of Eg by extrapolating the straight-line part of the plot to the photon energy axis. It was observed that the Eg of the ATZO thin films decreased from 3.52 eV to 3.38 eV with the increase of TiO2 contents from 0 wt% to 3 wt%. The shrinkage of the optical energy gap could be due to the decrease of carrier concentration according to the equation as follows [23]:
Eg =
2 2m∗cv
(3 2 N)
2/3
(4)
where the Eg is the shift of the optical energy gap, mcv is the effective mass of the electron, h is Planck’s constant, and N is the carrier concentration. Fig. 6 shows the RI of ATZO thin films as a function of the wavelength. Cauchy model was used to extract the RI (n) and extinction coefficient (k) of ATZO thin films. The following formulas were used to define the n and k. The fitting procedure to determine the optical constants of samples was described in Refs. [24,25]. n() = A +
B C + 4 2
k() = ˛ exp 12 400
(5)
1
−
1
(6)
The six fitting parameters in the model are A, B, C, the extinction coefficient amplitude ˛, the exponent factor ˇ and the band edge
. Three phase model was used to describe the material system, i.e., air/ATZO film/1 mm glass substrate. It was observed that the RI of the ATZO thin films increased with increasing TiO2 content, and the highest RI was 2.05 at 0 = 550 nm with the TiO2 at 3 wt%. The increase of RI was mainly attributed to two reasons: one was the decrease of carrier concentration with increasing the TiO2 content in the ATZO thin films [26], which could be inferred from Fig. 4(b);
the other was the forming of relatively higher RI of TiOx . To further clarify the relationship of RI and carrier concentration, the ATZO thin films were annealed in vacuum (4 × 10−4 Pa) at 400 ◦ C for 1 h. The changes of carrier concentration and RI of the ATZO thin films were then estimated. Fig. 4(b) and Fig. 7 revealed the variation of carrier concentration and RI of the ATZO thin films after annealing in vacuum. The carrier concentration of the ATZO films increased, while the RI showed a declined tendency after annealing in vacuum. The increase of carrier concentration was due to the generate of the oxygen vacancies, which can also result in the decrease in the RI of the films. Based on the above results, the RI of the AZO thin films could be tailored by tuning the content of co-doped Ti, which was important for applications in designing the integrated optoelectronic devices such as a-Si thin film solar cells. 4. Conclusions In this work, ATZO thin films were deposited on glass substrates by RF magnetron sputtering in pure Ar ambient. The ATZO thin films revealed hexagonal würtzite structure with (0 0 2) preferential orientation. The RI and the resistivity increased with the increase of the doping amount of Ti, and the highest RI of the film was 2.05 at 0 = 550 nm with the 3 wt% doping TiO2 . The RI of the ATZO thin films was improved by co-doping with Ti, which was due to the decrease of conductivity and the increase of the TiO2 content. Acknowledgements This work was financially supported by the NSFC (20975107), the “Hundred Talents Program,” from the Chinese Academy of Sciences, the Zhejiang Natural Science Foundation (Y4110463), and the Ningbo Innovative Research Team Program.
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References [1] S. Ghosh, A. Sarkar, S. Chaudhuri, A.K. Pal, Thin Solid Films 205 (1991) 64. [2] K.H. Kim, K.C. Park, D.Y. Ma, J. Appl. Phys. 81 (1997) 7764. [3] V. Sittinger, F. Ruske, W. Werner, B. Szyszka, B. Rech, J. Hüpkes, G. Schöpe, H. Stiebig, Thin Solid Films 496 (2006) 16–25. [4] F.H. Wang, H.P. Chang, C.C. Tseng, C.C. Huang, H.W. Liu, Curr. Appl. Phys. 11 (2011) 12–16. [5] W.T. Yen, Y.C. Lin, J.H. Ke, Appl. Surf. Sci. 257 (2010) 960–968. [6] O. Kluth, B. Rech, L. Houben, S. Wieder, G. Schöpe, C. Beneking, H. Wanger, A. Löffl, H.W. Schock, Thin Solid Films 351 (1999) 247–253. [7] W.L. Lua, K.C. Huanga, C.H. Yehb, C.I. Hungc, M.P. Hounga, Mater. Chem. Phys. 127 (2011) 358–363. [8] C.Y. Hsu, T.F. Ko, Y.M. Huang, J. Eur. Ceram. Soc. 28 (2008) 3065–3070. [9] J.H. Shi, S.M. Huang, J.B. Chu, H.B. Zhu, Z.A. Wang, X.D. Li, D.W. Zhang, Z. Sun, W.J. Cheng, F.Q. Huang, X.J. Yin, J. Mater. Sci.: Mater. Electron. 21 (2010) 1005–1013. [10] C.J. Brinker, M.S. Harrington, Sol. Energy Mater. 5 (1981) 159–172. [11] T. Schuler, M.A. Aegerter, Thin Solid Films 35 (1999) 1125–1131. [12] M. Caglar, S. Ilican, Y. Caglar, F. Yakuphanoglu, J. Mater. Sci.: Mater. Electron. 19 (2008) 704–708.
[13] S.W. Xue, X.T. Zu, W.L. Zhou, H.X. Deng, X. Xiang, L. Zhang, H. Deng, J. Alloys Compd. 448 (2008) 21–26. [14] S.W. Xue, X.T. Zua, W.G. Zheng, H.X. Deng, X. Xiang, Physica B 3813 (2006) 209–213. [15] Y. Caglar, S. Ilican, M. Caglar, F. Yakuphanoglu, Spectrochim. Acta Part A 67 (2007) 1113–1119. [16] S.J. Kang, Y.H. Joung, Appl. Surf. Sci. 253 (2007) 7330–7335. [17] C. Ye, S.S. Pan, X.M. Teng, H.T. Fan, G.H. Li, Appl. Phys. A 90 (2008) 375–378. ˇ [18] S.S. Lin, J.L. Huang, P. Sajgalik, Surf. Coat. Technol. 191 (2005) 286–292. [19] J.L. Chung, J.C. Chen, C.J. Tseng, Appl. Surf. Sci. 255 (2008) 2494–2499. [20] J.J. Lu, Y.M. Lu, S.I. Tasi, T.L. Hsiung, H.P. Wang, L.Y. Jang, Opt. Mater. 29 (2007) 1548–1552. [21] J.L. Chung, J.C. Chen, C.J. Tseng, Appl. Surf. Sci. 254 (2008) 2615–2620. [22] S.T. Tan, B.J. Chen, W.W. Sun, W.J. Fan, H.S. Kwok, X.H. Zhang, S.J. Chua, J. Appl. Phys. 98 (2005), 013505(1–5). [23] C. Jeong, H.S. Kim, D.R. Chang, K. Kamisako, Jpn. J. Appl. Phys. 47 (2008) 5656–5658. [24] Y.C. Liu, S.K. Tung, J.H. Hsieh, J. Cryst. Growth 105 (2006) 287. [25] Y.C. Liu, J.H. Hsieh, S.K. Tung, Thin Solid Films 32 (2006) 510. [26] H. Kim, A. Pique, J.S. Horwitz, et al., Thin Solid Films 798 (2000) 377–378.