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Al-doped ZnO thin films deposited by reactive frequency magnetron sputtering: H2-induced property changes
Thin Solid Films 515 (2007) 3057 – 3060 www.elsevier.com/locate/tsf
Al-doped ZnO thin films deposited by reactive frequency magnetron sputtering: H2-induced property changes Weifeng Liu a , Guotong Du a,b,⁎, Yanfeng Sun b , Yibin Xu a , Tianpeng Yang b , Xinsheng Wang a , Yuchun Chang b , Fabin Qiu b a
b
State Key Laboratory for Materials modification by laser, ion, electron beams, Department of Physics, Dalian University of Technology, Dalian Liaoning 116024, China State Key Laboratory on Integrated Optoelectronics, College of Electronic Science and Engineering, Jinlin University, Changchun Jilin 130023, China Received 24 April 2005; received in revised form 29 December 2005; accepted 24 August 2006 Available online 28 September 2006
Abstract Al-doped ZnO (AZO) transparent conductive thin films have been prepared by radio-frequency magnetron sputtering with a ceramic target (98 wt.% ZnO, 2 wt.% Al2O3) in different Ar + H2 ambient at a substrate temperature of 200 °C. To investigate the influence of H2-flow on the properties of AZO films, H2-flow was changed during the growth process with a fixed Ar-flow of 60 sccm. The results indicate that H2-flow has a considerable influence on the transparent conductive properties of AZO films. The low resistivity in the order of 10− 4 Ω cm and the high average transmittance more than 92% in the visible range were obtained for the samples prepared in the optimal H2-flow range from 0.4 sccm to 1.0 sccm. In addition, the influence of H2-flow on the structure and composition of AZO films have also been studied. © 2006 Elsevier B.V. All rights reserved. Keywords: Al-doped zinc oxide; Zin oxide; Hydrogen; Sputtering; Transparent conductive oxides
1. Introduction ZnO is a wide-band-gap (3.3 eV) semiconductor that has been used extensively in many fields such as solar cells, surface acoustic wave devices, piezoelectric transducers, gas sensor, phosphors, and transparent conducting electrodes of optoelectronic devices. In serving as the transparent electrode of optoelectronic devices, ZnO film has several advantages including high transmittance in the visible range, abundance in natural resource, and stability under the exposure to hydrogen plasma. Impurity doped ZnO, such as H2, Al or Ga-doped ZnO, has recently gained much attention as an alternative material for Sndoped In2O3 thin films. As for sputtered Al-doped zinc oxide AZO films, there have been numerous reports concerning the influence of Al content, substrate temperature, sputtering ⁎ Corresponding author. State Key Laboratory for Materials modification by laser, ion, electron beams, Department of Physics, Dalian University of Technology, Dalian Liaoning 116024, China. Tel.: +86 411 84707865; fax: +86 411 84709304. E-mail address: [email protected] (W. Liu). 0040-6090/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2006.08.021
power, sputtering pressure (oxygen/argon pressure ratio), target to substrate distance, and post-deposition annealing (in air or argon) on AZO film quality, and many parameters in magnetron sputtering process may influence the electronic and optical properties of the deposited AZO films obviously [1–5]. Presently, researchers have done some work on ZnO and AZO films prepared in H2 ambient, ZnO films prepared in H2 ambient have been studied by Van de Walle [6] and in other reports [7–9]. Hydrogen incorporated into ZnO during the deposition is potentially an excellent candidate for further optimizing the properties of ZnO films. And AZO films prepared in Ar + H2 gas ambient have been developed by Rajesh Das and Swati Ray; they mainly studied the optical transmission and infrared reflectance of these films [10]. The results suggest that hydrogen incorporated into AZO during deposition also optimized the performances of AZO film. In this paper, the effects of different Ar + H2 ambient on the structural, electrical and optical properties of AZO films deposited on glass substrates were investigated. This work aims at further optimizing the conductivity and transmittance of AZO film suitable for transparent electrodes and revealing the reason
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for different stage variations of conductivity of the films prepared in different Ar + H2 ambient.
Table 1 Atomic percentage of AZO films measured by XPS Samples (H2-flow)
O (at.%)
Zn (at.%)
Al (at.%)
2. Experiments
0.0 sccm 0.7 sccm 2.2 sccm
59.0 52.8 50.1
44.0 44.9 47.6
1.9 2.2 2.2
A ceramic disc of ZnO (99.99% in purity) mixed with 2 wt.% Al2O3 (99.99% in purity) was used as the target (diameter 52 mm). AZO transparent conductive thin films were deposited by reactive frequency magnetron sputtering on microscope slide glass substrate (made by a Chinese company, Feizhou-Bosu). The substrate-to-target distance, substrate temperature, working power and depositional time was 70 mm, 200 °C, 200 W and 35 min, respectively. During the AZO deposition, H2 and Ar mixed gas was fed into the reaction chamber, the working pressure was maintained at about 4.5 Pa with 60 sccm Ar-flow, the flow rate of H2 was varied from 0.0 sccm to 2.2 sccm. To investigate the effects H2 ambient on the properties of AZO films, eight samples were deposited with different H2-flow which were 0.0 sccm, 0.2 sccm, 0.3 sccm, 0.4 sccm, 0.7 sccm, 1.0 sccm, 1.6 sccm and 2.2 sccm, and other conditions were remained unchanged.
The thickness of thin films was measured using a surface profilometer. The carrier resistivities, concentration and Hall mobility were studied by means of room temperature Bio-Rad HL5500 system by the van de Pauw method. The transmittance of the films was tested by an UV-2450 spectrophotometer in the wavelength range from 300 nm to 850 nm. The crystal structure was determined by X-ray diffraction (XRD, 2θ = 25°–75°) with a Cu Kα radiation (λ = 0.154056 nm). The surface atomic composition of O, Zn and Al were measured by an ESCALAB Mark II X-ray photoemission spectrometer (XPS) with a Mg Kα radiation source (hν = 1253.6 eV). The binding energy was calibrated by the peak energy of C1s. In order to avoid the influence of surface absorption in the atmosphere, Ar ion etching (4000 V, 50 μA) was performed for about 30 min with an etching rate of 0.5 nm/min. Survey spectra, covering a wide range of energies, have been taken from 0 eV to 1200 eV in binding energy in a constant pass energy mode of 187.85 eV. 3. Results and discussion 3.1. Electrical properties The conduction characteristics of AZO are primarily dominated by electrons generated from Al3+ ions on substitutional sites of Zn2+ ions and Al interstitial atoms. The resistivity ρ is proportional to the reciprocal of the product of carrier concentration n and mobility μ as the following equation [11]: q¼
Fig. 1. The electrical properties of AZO films: (a) resistivity; (b) carrier concentration and Hall mobility.
1 nel
ð1Þ
n of AZO film is related to the O-vacancies, H- and Al-doping concentration, μ of AZO film is related to grain boundaries and impurity scattering. The electrical properties of AZO films as a function of H2flow are shown in Fig. 1. As shown in Fig. 1(a), the resistivity of AZO film prepared without H2 is 2.3 × 10− 3 Ω cm, and the resistivities of all AZO films prepared with H2 are markedly lower than that deposited without H2. It is also found that the resistivity in the order of 10− 4 Ω cm (samples prepared with H2flow of 0.4, 0.7 and 1.0 sccm) was obtained in the H2-flow range from 0.4 sccm to 1.0 sccm. But the samples prepared with H2-flow of 1.6 and 2.2 sccm show bad resistivities. In order to study the change of electrical properties, Fig. 1(b) is divided into three zones. When comparing zones I, II and III, it was found that with the H2-flow increased from 0.0 sccm to 0.3 sccm as shown in zone I, the carrier concentration increased and the Hall mobility decreased rapidly. The increase of carrier concentration is attributed to the formation of oxygen vacancy
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average transmittance in the visible range is found to be above 90% for most films, and the best transmittance is 96.3%. However the sample prepared with 2.2 sccm H2-flow shows bad transparency, which is due to the increase of intergranular zinc atoms caused by hydrogen. It is found that the transmittance edge shifts to a lower wavelength with H2-flow increasing. The phenomenon might be due to the Burstein–Moss effect [12–14], pointing out that the Fermi level inside the conduction band moves upward with increasing donor concentration due to the filling of conduction band by the increase of electron carriers. 3.3. Structure
Fig. 2. Optical transmittance spectra of the AZO films prepared with different H2-flow.
and a small quantity of hydrogen atoms situates in the Zn–O bond center [6]. And the Hall mobility decrease implies that scattering centers are contained. With H2-flow increased from 0.4 sccm to 1.0 sccm as shown in zone II, the carrier concentration and Hall mobility did not change. The irregular changes of the carrier concentration and the Hall mobility may be because the hydrogen atoms situated in the Zn–O bond center [6] reach H-doping saturated stage. At the same time, the films exhibited smaller full-width at half-maximum (FWHM) shown in Fig. 3(b), the crystallinity of the films are improved and the grain size become larger when the hydrogen atoms situated in the Zn–O bond center is at saturation stage, scattering centers of impurity defects and grain boundaries decrease, so the Hall mobility increased. However, as shown in zone III with H2-flow increased from 1.6 sccm to 2.2 sccm, the carrier concentration and Hall mobility increased and decreased, respectively. This implies that hydrogen atoms exceed Hdoping saturated stage, and many intergranular zinc atoms acting as donors of electrons are present in the films, so the carrier concentration increases. At the same time, intergranular zinc atoms form many scattering centers, the decrease of the Hall mobility being due to these scattering centers. The XPS survey spectra were used to detect the element components in the AZO thin films and the concentration of O, Zn and Al are shown in Table 1. With H2-flow increasing, the atomic concentration of Al presents only slightly changes, and the concentration of O decreases and the concentration of Zn increases. The changes of the concentration of O and Zn are mainly due to the introduction of H. These results indicate that H plays an important role in varying the electrical properties of AZO film with different electrical mechanism.
XRD results are shown in Fig. 3. All films exhibit the (0002) peak due to a self-texturing mechanism as discussed by Jiang [15]. The (0002) plane in ZnO crystals had the lowest surface energy so that continuous films tend to change into (0002) orientation films to minimize the surface energy [16]. The (0002) peak of the AZO film prepared without H2 is situated at 2θ = 34.5°, which implies that Al3+ ions substitute Zn2+ ions, and the lattice parameter of ZnO crystal decreases due to the smaller radius of Al3+ ions compared to Zn2+ ions [17]. K. C.
3.2. Optical properties The optical transmittance of the AZO thin films was measured by spectrophotometry for the wavelength ranging from 350 to 850 nm. The thickness of the AZO thin films prepared in 0.0, 0.2, 0.4, 1.0 and 2.2 sccm H2-flow ambient was 460, 450, 400, 400 and 280 nm, respectively. As shown in Fig. 2, the
Fig. 3. (a,b) X-ray diffraction patterns of AZO films of films prepared with different H2-flow.
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Park [18] reported that the length of c-axis is expected to be shorter if the Al atoms are substituted into the Zn site in the crystal because the ionic radii of Zn2+ and Al3+ were 72 pm and 53 pm, respectively. However, our results for the samples grown under Ar + H2 ambient were inconsistent with Park's conclusion. This is because hydrogen atoms could be situated in the Zn–O bond center when H2-flow is increased from 0.4 sccm to 1.0 sccm, which meant the lattice parameter of ZnO films, would increase, so the diffraction angle became smaller [6]. As H2-flow continuously increased beyond 1.0 sccm, the intensity of (0002) peak decreased markedly. It implies that when H dopant concentration exceeded the saturation stage, the increased number of grain boundary defects caused by the excess H-atoms lead to the deterioration of the crystal quality of AZO film. It is known that the grain size can be characterized by the full-width at half-maximum (FWHM) of XRD peaks generally reflects the change in the grain size of crystallites, a decrease in FWHM corresponding to an increase in grain size [19]. The FWHM values of samples prepared with H2-flow of 0.0, 0.4 sccm and 1.0 sccm were 0.361, 0.219 and 0.209, respectively; the FWHM values of samples prepared with H2flow of 0.4 sccm and 1.0 sccm show a tendency to decrease markedly. Therefore, it can be concluded that the H2-flow should be controlled in a small range of 0.4 sccm to 1.0 sccm to grow the good crystallinity AZO thin films. 4. Conclusion AZO thin films with low resistivity and high transmittance have been obtained by the magnetron sputtering using a ZnO target mixed with 2 wt.% Al2O3 in Ar + H2 ambient. The results showed that H2 has a strong influence on the properties of AZO films prepared at relatively low growth temperature. The electrical, optical and structural properties of the AZO films prepared with different H2-flow present variations related to the different H-doping state. The optimal H2-flow range for preparing low resistivity and high transmittance AZO films was 0.4 sccm to 1.0 sccm. Such method for improving the electrical properties of AZO films prepared at low growth
temperature can be especially useful for some low-melting point photoelectric devices and substrates. Acknowledgements This work was financially supported by the National Natural Science Foundation of China under Grant No. 60307002. References [1] O. Vigil, F. Cruz, G. Santana, L. Vaillant, A.M. Acevedo, G.C. Puente, Appl. Surf. Sci. 161 (2000) 27. [2] T. Minami, H. Sonohara, T. Kakumu, S. Takata, Jpn. J. Appl. Phys. 34 (1995) L971. [3] T. Minami, T. Yamamoto, T. Miyata, Thin Solid Films 366 (2000) 63. [4] Y.M. Lu, W.S. Hwang, W.Y. Liu, J.S. Yang, Mater. Chem. Phys. 72 (2001) 269. [5] Y. Igasaki, H. Kanma, Appl. Surf. Sci. 169–170 (2001) 508. [6] C.G. Van de Walle, Phys. Rev. Lett. 85 (2000) 1012. [7] D.M. Hofmann, A. Hofstaetter, F. Leiter, H. Zhou, F. Henecker, B.K. Meyer, S.B. Orlinskii, J. Schmidt, P.G. Baranov, Phys. Rev. Lett. 88 (2002) 045504. [8] B. Theys, V. Sallet, F. Jomard, A. Lusson, J.F. Rommeluere, Z. Teukam, J. Appl. Phys. 91 (2002) 3922. [9] N. Ohashi, T. Ishigaki, N. Okada, T. Sekiguchi, I. Sakaguchi, H. Haneda, Appl. Phys. Lett. 80 (2002) 2869. [10] R. Das, S. Ray, J. Phys. D: Appl. Phys. 36 (2003) 152. [11] Y. Igasaki, H. Saito, Thin Solid Films 199 (1991) 223. [12] M.K. Hudait, P. Modak, S.B. Krupanidhi, Mater. Sci. Eng., B, Solid-State Mater. Adv. Technol. 60 (1999) 1. [13] N.Y. Lee, K.J. Lee, C. Lee, J.E. Kim, H.Y. Park, D.H. Kwak, H.C. Lee, H. Lim, J. Appl. Phys. 78 (1995) 3367. [14] L.Y. ih Chen, W.H. Chen, J.J. Wang, F.C.N. Hong, Appl. Phys. Lett. 85 (2004) 2628. [15] X. Jiang, C.L. Jia, B. Szyszka, Appl. Phys. Lett. 80 (2002) 3090. [16] R.J. Hong, K. Helming, X. Jiang, B. Szyszka, Appl. Surf. Sci. 226 (2004) 378. [17] B.Y. Oh, M.C. Jeong, W. Lee, J.M. Myoung, J. Cryst. Growth 274 (2005) 453. [18] K.C. Park, D.Y. Ma, K.H. Kim, Thin Solid Films 305 (1997) 201. [19] B.D. Cullity, Elements of X-ray Diffraction, Second Ed., Addison-Wesley, Reading MA, 1978, p. 162.
Al-doped ZnO thin films deposited by reactive frequency magnetron sputtering: H2-induced property changes Weifeng Liu a , Guotong Du a,b,⁎, Yanfeng Sun b , Yibin Xu a , Tianpeng Yang b , Xinsheng Wang a , Yuchun Chang b , Fabin Qiu b a
b
State Key Laboratory for Materials modification by laser, ion, electron beams, Department of Physics, Dalian University of Technology, Dalian Liaoning 116024, China State Key Laboratory on Integrated Optoelectronics, College of Electronic Science and Engineering, Jinlin University, Changchun Jilin 130023, China Received 24 April 2005; received in revised form 29 December 2005; accepted 24 August 2006 Available online 28 September 2006
Abstract Al-doped ZnO (AZO) transparent conductive thin films have been prepared by radio-frequency magnetron sputtering with a ceramic target (98 wt.% ZnO, 2 wt.% Al2O3) in different Ar + H2 ambient at a substrate temperature of 200 °C. To investigate the influence of H2-flow on the properties of AZO films, H2-flow was changed during the growth process with a fixed Ar-flow of 60 sccm. The results indicate that H2-flow has a considerable influence on the transparent conductive properties of AZO films. The low resistivity in the order of 10− 4 Ω cm and the high average transmittance more than 92% in the visible range were obtained for the samples prepared in the optimal H2-flow range from 0.4 sccm to 1.0 sccm. In addition, the influence of H2-flow on the structure and composition of AZO films have also been studied. © 2006 Elsevier B.V. All rights reserved. Keywords: Al-doped zinc oxide; Zin oxide; Hydrogen; Sputtering; Transparent conductive oxides
1. Introduction ZnO is a wide-band-gap (3.3 eV) semiconductor that has been used extensively in many fields such as solar cells, surface acoustic wave devices, piezoelectric transducers, gas sensor, phosphors, and transparent conducting electrodes of optoelectronic devices. In serving as the transparent electrode of optoelectronic devices, ZnO film has several advantages including high transmittance in the visible range, abundance in natural resource, and stability under the exposure to hydrogen plasma. Impurity doped ZnO, such as H2, Al or Ga-doped ZnO, has recently gained much attention as an alternative material for Sndoped In2O3 thin films. As for sputtered Al-doped zinc oxide AZO films, there have been numerous reports concerning the influence of Al content, substrate temperature, sputtering ⁎ Corresponding author. State Key Laboratory for Materials modification by laser, ion, electron beams, Department of Physics, Dalian University of Technology, Dalian Liaoning 116024, China. Tel.: +86 411 84707865; fax: +86 411 84709304. E-mail address: [email protected] (W. Liu). 0040-6090/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2006.08.021
power, sputtering pressure (oxygen/argon pressure ratio), target to substrate distance, and post-deposition annealing (in air or argon) on AZO film quality, and many parameters in magnetron sputtering process may influence the electronic and optical properties of the deposited AZO films obviously [1–5]. Presently, researchers have done some work on ZnO and AZO films prepared in H2 ambient, ZnO films prepared in H2 ambient have been studied by Van de Walle [6] and in other reports [7–9]. Hydrogen incorporated into ZnO during the deposition is potentially an excellent candidate for further optimizing the properties of ZnO films. And AZO films prepared in Ar + H2 gas ambient have been developed by Rajesh Das and Swati Ray; they mainly studied the optical transmission and infrared reflectance of these films [10]. The results suggest that hydrogen incorporated into AZO during deposition also optimized the performances of AZO film. In this paper, the effects of different Ar + H2 ambient on the structural, electrical and optical properties of AZO films deposited on glass substrates were investigated. This work aims at further optimizing the conductivity and transmittance of AZO film suitable for transparent electrodes and revealing the reason
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for different stage variations of conductivity of the films prepared in different Ar + H2 ambient.
Table 1 Atomic percentage of AZO films measured by XPS Samples (H2-flow)
O (at.%)
Zn (at.%)
Al (at.%)
2. Experiments
0.0 sccm 0.7 sccm 2.2 sccm
59.0 52.8 50.1
44.0 44.9 47.6
1.9 2.2 2.2
A ceramic disc of ZnO (99.99% in purity) mixed with 2 wt.% Al2O3 (99.99% in purity) was used as the target (diameter 52 mm). AZO transparent conductive thin films were deposited by reactive frequency magnetron sputtering on microscope slide glass substrate (made by a Chinese company, Feizhou-Bosu). The substrate-to-target distance, substrate temperature, working power and depositional time was 70 mm, 200 °C, 200 W and 35 min, respectively. During the AZO deposition, H2 and Ar mixed gas was fed into the reaction chamber, the working pressure was maintained at about 4.5 Pa with 60 sccm Ar-flow, the flow rate of H2 was varied from 0.0 sccm to 2.2 sccm. To investigate the effects H2 ambient on the properties of AZO films, eight samples were deposited with different H2-flow which were 0.0 sccm, 0.2 sccm, 0.3 sccm, 0.4 sccm, 0.7 sccm, 1.0 sccm, 1.6 sccm and 2.2 sccm, and other conditions were remained unchanged.
The thickness of thin films was measured using a surface profilometer. The carrier resistivities, concentration and Hall mobility were studied by means of room temperature Bio-Rad HL5500 system by the van de Pauw method. The transmittance of the films was tested by an UV-2450 spectrophotometer in the wavelength range from 300 nm to 850 nm. The crystal structure was determined by X-ray diffraction (XRD, 2θ = 25°–75°) with a Cu Kα radiation (λ = 0.154056 nm). The surface atomic composition of O, Zn and Al were measured by an ESCALAB Mark II X-ray photoemission spectrometer (XPS) with a Mg Kα radiation source (hν = 1253.6 eV). The binding energy was calibrated by the peak energy of C1s. In order to avoid the influence of surface absorption in the atmosphere, Ar ion etching (4000 V, 50 μA) was performed for about 30 min with an etching rate of 0.5 nm/min. Survey spectra, covering a wide range of energies, have been taken from 0 eV to 1200 eV in binding energy in a constant pass energy mode of 187.85 eV. 3. Results and discussion 3.1. Electrical properties The conduction characteristics of AZO are primarily dominated by electrons generated from Al3+ ions on substitutional sites of Zn2+ ions and Al interstitial atoms. The resistivity ρ is proportional to the reciprocal of the product of carrier concentration n and mobility μ as the following equation [11]: q¼
Fig. 1. The electrical properties of AZO films: (a) resistivity; (b) carrier concentration and Hall mobility.
1 nel
ð1Þ
n of AZO film is related to the O-vacancies, H- and Al-doping concentration, μ of AZO film is related to grain boundaries and impurity scattering. The electrical properties of AZO films as a function of H2flow are shown in Fig. 1. As shown in Fig. 1(a), the resistivity of AZO film prepared without H2 is 2.3 × 10− 3 Ω cm, and the resistivities of all AZO films prepared with H2 are markedly lower than that deposited without H2. It is also found that the resistivity in the order of 10− 4 Ω cm (samples prepared with H2flow of 0.4, 0.7 and 1.0 sccm) was obtained in the H2-flow range from 0.4 sccm to 1.0 sccm. But the samples prepared with H2-flow of 1.6 and 2.2 sccm show bad resistivities. In order to study the change of electrical properties, Fig. 1(b) is divided into three zones. When comparing zones I, II and III, it was found that with the H2-flow increased from 0.0 sccm to 0.3 sccm as shown in zone I, the carrier concentration increased and the Hall mobility decreased rapidly. The increase of carrier concentration is attributed to the formation of oxygen vacancy
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average transmittance in the visible range is found to be above 90% for most films, and the best transmittance is 96.3%. However the sample prepared with 2.2 sccm H2-flow shows bad transparency, which is due to the increase of intergranular zinc atoms caused by hydrogen. It is found that the transmittance edge shifts to a lower wavelength with H2-flow increasing. The phenomenon might be due to the Burstein–Moss effect [12–14], pointing out that the Fermi level inside the conduction band moves upward with increasing donor concentration due to the filling of conduction band by the increase of electron carriers. 3.3. Structure
Fig. 2. Optical transmittance spectra of the AZO films prepared with different H2-flow.
and a small quantity of hydrogen atoms situates in the Zn–O bond center [6]. And the Hall mobility decrease implies that scattering centers are contained. With H2-flow increased from 0.4 sccm to 1.0 sccm as shown in zone II, the carrier concentration and Hall mobility did not change. The irregular changes of the carrier concentration and the Hall mobility may be because the hydrogen atoms situated in the Zn–O bond center [6] reach H-doping saturated stage. At the same time, the films exhibited smaller full-width at half-maximum (FWHM) shown in Fig. 3(b), the crystallinity of the films are improved and the grain size become larger when the hydrogen atoms situated in the Zn–O bond center is at saturation stage, scattering centers of impurity defects and grain boundaries decrease, so the Hall mobility increased. However, as shown in zone III with H2-flow increased from 1.6 sccm to 2.2 sccm, the carrier concentration and Hall mobility increased and decreased, respectively. This implies that hydrogen atoms exceed Hdoping saturated stage, and many intergranular zinc atoms acting as donors of electrons are present in the films, so the carrier concentration increases. At the same time, intergranular zinc atoms form many scattering centers, the decrease of the Hall mobility being due to these scattering centers. The XPS survey spectra were used to detect the element components in the AZO thin films and the concentration of O, Zn and Al are shown in Table 1. With H2-flow increasing, the atomic concentration of Al presents only slightly changes, and the concentration of O decreases and the concentration of Zn increases. The changes of the concentration of O and Zn are mainly due to the introduction of H. These results indicate that H plays an important role in varying the electrical properties of AZO film with different electrical mechanism.
XRD results are shown in Fig. 3. All films exhibit the (0002) peak due to a self-texturing mechanism as discussed by Jiang [15]. The (0002) plane in ZnO crystals had the lowest surface energy so that continuous films tend to change into (0002) orientation films to minimize the surface energy [16]. The (0002) peak of the AZO film prepared without H2 is situated at 2θ = 34.5°, which implies that Al3+ ions substitute Zn2+ ions, and the lattice parameter of ZnO crystal decreases due to the smaller radius of Al3+ ions compared to Zn2+ ions [17]. K. C.
3.2. Optical properties The optical transmittance of the AZO thin films was measured by spectrophotometry for the wavelength ranging from 350 to 850 nm. The thickness of the AZO thin films prepared in 0.0, 0.2, 0.4, 1.0 and 2.2 sccm H2-flow ambient was 460, 450, 400, 400 and 280 nm, respectively. As shown in Fig. 2, the
Fig. 3. (a,b) X-ray diffraction patterns of AZO films of films prepared with different H2-flow.
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Park [18] reported that the length of c-axis is expected to be shorter if the Al atoms are substituted into the Zn site in the crystal because the ionic radii of Zn2+ and Al3+ were 72 pm and 53 pm, respectively. However, our results for the samples grown under Ar + H2 ambient were inconsistent with Park's conclusion. This is because hydrogen atoms could be situated in the Zn–O bond center when H2-flow is increased from 0.4 sccm to 1.0 sccm, which meant the lattice parameter of ZnO films, would increase, so the diffraction angle became smaller [6]. As H2-flow continuously increased beyond 1.0 sccm, the intensity of (0002) peak decreased markedly. It implies that when H dopant concentration exceeded the saturation stage, the increased number of grain boundary defects caused by the excess H-atoms lead to the deterioration of the crystal quality of AZO film. It is known that the grain size can be characterized by the full-width at half-maximum (FWHM) of XRD peaks generally reflects the change in the grain size of crystallites, a decrease in FWHM corresponding to an increase in grain size [19]. The FWHM values of samples prepared with H2-flow of 0.0, 0.4 sccm and 1.0 sccm were 0.361, 0.219 and 0.209, respectively; the FWHM values of samples prepared with H2flow of 0.4 sccm and 1.0 sccm show a tendency to decrease markedly. Therefore, it can be concluded that the H2-flow should be controlled in a small range of 0.4 sccm to 1.0 sccm to grow the good crystallinity AZO thin films. 4. Conclusion AZO thin films with low resistivity and high transmittance have been obtained by the magnetron sputtering using a ZnO target mixed with 2 wt.% Al2O3 in Ar + H2 ambient. The results showed that H2 has a strong influence on the properties of AZO films prepared at relatively low growth temperature. The electrical, optical and structural properties of the AZO films prepared with different H2-flow present variations related to the different H-doping state. The optimal H2-flow range for preparing low resistivity and high transmittance AZO films was 0.4 sccm to 1.0 sccm. Such method for improving the electrical properties of AZO films prepared at low growth
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