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The effect of heating rate on the structural and electrical properties of sol–gel derived Al-doped ZnO films
Applied Surface Science 257 (2011) 6919–6922
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
The effect of heating rate on the structural and electrical properties of sol–gel derived Al-doped ZnO films Meizhen Gao ∗ , Xiaonan Wu, Jing Liu, Wenbao Liu Key Lab for Magnetism and Magnetic Materials of MOE, School of Physical Science and Technology, Lanzhou University, Lanzhou 730000, China
a r t i c l e
i n f o
Article history: Received 29 November 2010 Received in revised form 4 March 2011 Accepted 4 March 2011 Available online 10 March 2011 Keywords: AZO nanorods Sol–gel method Heating rate Electrical property
a b s t r a c t Al-doped ZnO (AZO) films are prepared by sol–gel method with a proper annealing procedure. For the first time, we find that the heating rate which is normally neglected during the post annealing process plays a significant role in improving AZO properties. The AZO film with nanorod structure is obtained by using a rapid heating rate. The AZO nanorods can provide a faster conduction pathway for charge transport due to the high crystal quality and thus enhance the conductivity of the film significantly. After hydrogen treatment, the AZO nanorod film exhibits a minimum resistivity of 1.4 × 10−3 cm. This approach to the preparation of AZO nanorods by a simple rapid annealing process may be helpful for the development of sol–gel-derived TCO films. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Transparent conductive oxide (TCO) films have been widely used in solar cells, flat-panel displays and other optoelectronic devices because of their high visible transmittance and low resistivity [1–3]. Recent developments support that Al-doped ZnO (AZO) used as the TCO material, combining low fabrication cost with nontoxic property, could be a very promising candidate for replacing the traditional indium tin oxide [4–6]. Of the numerous techniques used to prepare AZO films, the sol–gel method has received considerable attention due to its simplicity and ease of dopantintroduction [5–7]. This method is usually carried out by the spin-coating technique and the post annealing treatment [8]. The post annealing process is an important step for desired functionality of the film [9]. Most of the current researches focus on optimizing the post annealing condition by adjusting the annealing temperature, changing the annealing atmosphere and delaying the holding time [5–7,10]. However, reports about controlling the heating rate are rarely seen, although it is a necessary parameter during the post annealing process. On the other hand, as compared to AZO nanoparticle films, the AZO nanorod films are particularly attractive owing to its excellent photoelectric properties [11]. Although the synthesis of ZnO or AZO nanorod films have been extensively studied, only few works have been done on their preparation via a simple sol–gel route.
∗ Corresponding author. Fax: +86 931 8913554. E-mail address: [email protected] (M. Gao). 0169-4332/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2011.03.031
In this work, we investigated the effect of heating rate during the post annealing process on the structural and electrical properties of sol–gel derived AZO films. The AZO film with nanorod structure was obtained by using a rapid heating rate. The results verify that the AZO nanorods enhance the conductivity of the film significantly in comparison with typical AZO nanoparticle film. This work might provide a new approach to improving the electrical property of sol–gel derived TCO thin films. 2. Experimental procedures 2.1. Synthesis The Al-doped ZnO thin films were prepared using the sol–gel method modified from the optimal condition reported by Lee and Park [6] and Wu et al. [7]. In brief, zinc acetate dihydrate [Zn(CH3 COO)2 ·2H2 O, 99.0%] and aluminum chloride hexahydrate (AlCl3 ·6H2 O, 97.0%) were dissolved in a mixture of 2-methoxyethanol (C3 H8 O2 , 99.5%) and monoethanolamine (MEA, C2 H7 NO, 99.0%) which served as the solvent and stabilizer, respectively. The concentration of zinc acetate was 0.5 M and the molar ratio of MEA to zinc acetate was maintained at 1.0. The amount of aluminum, defined as 100 [Al]/[Al + Zn], was kept at 1.0 at.%. The resultant solution was stirred at 60 ◦ C for 3 h and then aged for 2 days. The ready solution was then deposited on normal glass substrates using the spin-coating technique at 3000 rpm for 30 s. The as-deposited films were immediately placed into a furnace of 300 ◦ C and hold for 10 min. The deposition and preheating processes were repeated for 10 times in order to get a desired thickness (250 nm). Thereafter, the AZO films were heated in air to 550 ◦ C with different
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M. Gao et al. / Applied Surface Science 257 (2011) 6919–6922
Fig. 1. SEM images of AZO films with different heating rates during post annealing process: (a) 10 ◦ C/min, (b) 20 ◦ C/min, (c) 40 ◦ C/min, and (d) 50 ◦ C/min.
heating rates (10 ◦ C/min, 20 ◦ C/min, 40 ◦ C/min and 50 ◦ C/min) and then hold at 550 ◦ C for 1 h. After annealing, the films were cooled in furnace to the room temperature.
with the residual strain along the c axis in the ZnO films. The strain along the c-axis is given by the following equation [12]: εzz =
2.2. Characterization The crystal structure of the AZO films was characterized by using an X-ray diffractometer (Philips X’ Pert Pro with Cu K␣, 0.154056 nm). The surface morphology of the films was observed by a filed emission scanning electron microscope (FE-SEM, Hitachi S-4800). The sheet resistance was measured by four-point probe method (SX1934 digital four-point testing instrument). The optical transmittance was obtained from a UV–vis spectrophotometer (70-1901 UV–vis spectrophotometer). The chemical composition and bond state were characterized by a Perkin-Elmer PHI-5702 multi-functional X-ray photoelectron spectroscopy (XPS) with Al K␣ radiation. 3. Results and discussion Fig. 1 shows the surface morphologies of AZO films with different heating rates during the post annealing process. Typical granular growth of thin films is observed in the heating rate range from 10 to 40 ◦ C/min (Fig. 1a–c), and the average particle size becomes smaller gradually with increasing the heating rate. However, it is interesting to find that when the heating rate reaches 50 ◦ C/min, the surface morphology of the film changes obviously, some larger grains appear suddenly and the microstructure partially transform into nanorods (Fig. 1d). Fig. 2 exhibits the X-ray diffraction patterns of AZO films with different heating rates. Only one peak around 34.4◦ appears, which is verified to be the (0 0 2) peak of ZnO with a hexagonal wurtzite structure. The intensity of the (0 0 2) peak decreases with the increase of the heating rate. Notably, at the heating rate of 50 ◦ C/min, the intensity of (0 0 2) peak increases sharply. Ong and Zhu [12] and Ghosh and Basak [13] reported that the surface morphology and crystal quality were intimately associated
c − c0 × 100% c0
(1)
where the c-axis lattice length is obtained from the position of the (0 0 2) peak using the Bragg condition and co is the unstrained lattice parameter of ZnO (JCPDS, 36-1451). The strain εzz thus determined for the AZO films annealed with different heating rates are summarized in Table 1. The results indicate that the strain increases as the heating rate increases from 10 to 40 ◦ C/min. The non-uniform heating may exist in the films when the heating rate is fast, which induces the increase in strain due to the stretch of lattice. The stretched lattice can increase the lattice energy and diminish the driving force of the growth [13], leading to the decrease of the particle size and crystallization, as observed in the SEM (Fig. 1a–c) and XRD results (Fig. 2a–c). For a much higher heating rate (50 ◦ C/min), the growth of ZnO will be further retarded by the increasing strain. Under thermodynamic equilibrium conditions, the fastest growth rate is along the c-axis due to the higher surface energy of {0 0 0 1} planes [14]. Thus, for the film annealed at 50 ◦ C/min, a faster growth along the c-axis occurs to release the strain and lower the energy, which causes the formation of the nanorods as shown in Fig. 1d. Meanwhile, some of the strain could be also relaxed at the grain boundaries [15,16], which leads to the appearance of several larger grains. In conclusion, the minimum value of the strain (−0.036%) is found when the film is annealed at the heating rate of 50 ◦ C/min and the corresponding film possesses a better crystal quality.
Table 1 c-axis length and strain, εzz along the c-axis of AZO films with different heating rates. Heating rate (◦ C/min)
2 (◦ )
c-axis length (nm)
Strain, εzz (%)
10 20 40 50
34.42 34.40 34.36 34.44
0.5207 0.5210 0.5216 0.5204
0.014 0.065 0.181 −0.036
M. Gao et al. / Applied Surface Science 257 (2011) 6919–6922
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Fig. 4. UV–vis spectra of AZO films with different heating rates. The inset shows the plot of the absorption coefficient versus photon energy of AZO films with different heating rates.
Fig. 2. XRD patterns of AZO films with different heating rates: (a) 10 ◦ C/min, (b) 20 ◦ C/min, (c) 40 ◦ C/min, and (d) 50 ◦ C/min.
To clarify whether Al dope into the ZnO nanorods, X-ray photoelectron spectroscopy (XPS) measurement is performed on this nanorod film. As shown in Fig. 3, the survey scan indicates no impurity above the detection limit. The Al 2p peak located at 74.11 eV corresponds to Al–O bonding and the characteristic peak of metallic Al is not observed, as shown in the inset of Fig. 3, which confirms the doping of Al in the ZnO crystal lattice [17]. Based on the above results, it is concluded that the heating rate during the high temperature annealing process plays a significant role in affecting the AZO microstructure and crystal quality. When the film is annealed at the heating rate of 50 ◦ C/min, the AZO film
Fig. 3. X-ray photoelectron spectroscopy of the AZO film with the heating rate of 50 ◦ C/min. The inset shows the Al 2p spectra.
with nanorod structure is obtained and the crystal quality of the film is enhanced effectively. Fig. 4 represents the UV–vis transmission spectra of AZO films with different heating rates. The AZO nanorod film with the heating rate of 50 ◦ C has a slightly lower transmittance over a broad range of wavelengths as compared to the other nanoparticle films. This is due to the increase in light scattering caused by the non-uniform AZO nanorods [18]. Despite this, the average transmittance of the AZO nanorod film in the visible-light range (400–800 nm) is still larger than 85%, which meets the requirement of the TCO application. The optical bandgaps for the AZO films with different heating rates are determined from the slope of the curves in the plot of absorption coefficient versus the photon energy, as shown in the inset of Fig. 4. From the plot, it can be observed that the band gap of the AZO nanorod film is a little smaller than the other films, but the Eg value of 3.30 eV is still similar to related reports [19,20]. The resistivities of AZO films with different heating rates are shown in Fig. 5a. It is obvious that the resistivity of the AZO nanorod film with the heating rate of 50 ◦ C is lower than the other AZO nanoparticle films. In comparison with the nanoparticles, the nanorods have a better crystal quality. The diffusion of electron in the nanorods will not be slowed down by potential barrier induced by grain boundaries and particle interface, which is the typical case in the nanoparticle films [21,22]. Therefore, the electron transfer in AZO nanorod
Fig. 5. Electrical resistivities of AZO films as a function of heating rate: (a) before hydrogen treatment and (b) after hydrogen treatment.
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post annealing process on the structural and electrical properties of AZO films is investigated. It is found that the AZO film with nanorod structure can be obtained when the heating rate is 50 ◦ C/min. The resultant AZO nanorods enhance the conductivity of the film significantly. Compared with typical AZO nanoparticles, the nanorod structure can provide a faster conduction pathway for charge transport due to the high crystal quality. After the reduction treatment, the conductivity of the AZO nanorod film is further improved, and a minimum resistivity of 1.4 × 10−3 cm is obtained. This work offers a new approach to the preparation of AZO or ZnO nanorods by a simple rapid annealing process, and may be helpful for the development of sol–gel derived TCO thin films. Acknowledgements This work was financially supported by the Key grant Project of Chinese Ministry of Education (grant no.309027), and by the National Science Fund for Distinguished Young Scholars (grant no. 50925103). Fig. 6. O 1s spectra of the AZO nanorod film: (a) before hydrogen treatment and (b) after hydrogen treatment. The black dot lines indicate experiment data, green lines indicate fitting results.
is much faster than in AZO nanoparticle films, which leads to the remarkable improvement in conductivity. It is well known that oxygen vacancy usually appears in the ZnO when the sample is treated under a reducing atmosphere and it is helpful for the improvement of the conductivity [23,24]. In order to obtain a much better conductivity, the AZO films that have been annealed at 550 ◦ C in air are then reduced at 400 ◦ C for 1 h in 5% H2 + 95% Ar forming gas. After the reduction treatment, the resistivities of AZO films with different heating rates are all decreased significantly as shown in Fig. 5b, and the AZO nanorod film has a minimum resistivity of 1.4 × 10−3 cm. Fig. 6 shows O 1s photoelectron peaks in the XPS spectra of the AZO nanorod film before and after hydrogen treatment. The O 1s peaks that have been normalized by the Zn 2p peaks are fitted to two peaks. The peaks at lower and higher energies can be attributed to O2− in the ZnO lattice and surface absorbed oxygen, respectively [25]. The relative chemical compositions are calculated and the oxygen vacancy contents are determined. The content of oxygen vacancy increases from 9% to 15% after hydrogen treatment. Oxygen vacancies in the crystal act as electron donors by releasing localized electrons [26,27]. Thus, the decrease of the resistivity after the reduction treatment is attributed to the increase of oxygen vacancies which can offer more conduction electrons. 4. Conclusions The AZO films with high transmittance and low electrical resistivity are prepared by cost-effective sol–gel technique under a proper annealing procedure. The effect of heating rate during the
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Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
The effect of heating rate on the structural and electrical properties of sol–gel derived Al-doped ZnO films Meizhen Gao ∗ , Xiaonan Wu, Jing Liu, Wenbao Liu Key Lab for Magnetism and Magnetic Materials of MOE, School of Physical Science and Technology, Lanzhou University, Lanzhou 730000, China
a r t i c l e
i n f o
Article history: Received 29 November 2010 Received in revised form 4 March 2011 Accepted 4 March 2011 Available online 10 March 2011 Keywords: AZO nanorods Sol–gel method Heating rate Electrical property
a b s t r a c t Al-doped ZnO (AZO) films are prepared by sol–gel method with a proper annealing procedure. For the first time, we find that the heating rate which is normally neglected during the post annealing process plays a significant role in improving AZO properties. The AZO film with nanorod structure is obtained by using a rapid heating rate. The AZO nanorods can provide a faster conduction pathway for charge transport due to the high crystal quality and thus enhance the conductivity of the film significantly. After hydrogen treatment, the AZO nanorod film exhibits a minimum resistivity of 1.4 × 10−3 cm. This approach to the preparation of AZO nanorods by a simple rapid annealing process may be helpful for the development of sol–gel-derived TCO films. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Transparent conductive oxide (TCO) films have been widely used in solar cells, flat-panel displays and other optoelectronic devices because of their high visible transmittance and low resistivity [1–3]. Recent developments support that Al-doped ZnO (AZO) used as the TCO material, combining low fabrication cost with nontoxic property, could be a very promising candidate for replacing the traditional indium tin oxide [4–6]. Of the numerous techniques used to prepare AZO films, the sol–gel method has received considerable attention due to its simplicity and ease of dopantintroduction [5–7]. This method is usually carried out by the spin-coating technique and the post annealing treatment [8]. The post annealing process is an important step for desired functionality of the film [9]. Most of the current researches focus on optimizing the post annealing condition by adjusting the annealing temperature, changing the annealing atmosphere and delaying the holding time [5–7,10]. However, reports about controlling the heating rate are rarely seen, although it is a necessary parameter during the post annealing process. On the other hand, as compared to AZO nanoparticle films, the AZO nanorod films are particularly attractive owing to its excellent photoelectric properties [11]. Although the synthesis of ZnO or AZO nanorod films have been extensively studied, only few works have been done on their preparation via a simple sol–gel route.
∗ Corresponding author. Fax: +86 931 8913554. E-mail address: [email protected] (M. Gao). 0169-4332/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2011.03.031
In this work, we investigated the effect of heating rate during the post annealing process on the structural and electrical properties of sol–gel derived AZO films. The AZO film with nanorod structure was obtained by using a rapid heating rate. The results verify that the AZO nanorods enhance the conductivity of the film significantly in comparison with typical AZO nanoparticle film. This work might provide a new approach to improving the electrical property of sol–gel derived TCO thin films. 2. Experimental procedures 2.1. Synthesis The Al-doped ZnO thin films were prepared using the sol–gel method modified from the optimal condition reported by Lee and Park [6] and Wu et al. [7]. In brief, zinc acetate dihydrate [Zn(CH3 COO)2 ·2H2 O, 99.0%] and aluminum chloride hexahydrate (AlCl3 ·6H2 O, 97.0%) were dissolved in a mixture of 2-methoxyethanol (C3 H8 O2 , 99.5%) and monoethanolamine (MEA, C2 H7 NO, 99.0%) which served as the solvent and stabilizer, respectively. The concentration of zinc acetate was 0.5 M and the molar ratio of MEA to zinc acetate was maintained at 1.0. The amount of aluminum, defined as 100 [Al]/[Al + Zn], was kept at 1.0 at.%. The resultant solution was stirred at 60 ◦ C for 3 h and then aged for 2 days. The ready solution was then deposited on normal glass substrates using the spin-coating technique at 3000 rpm for 30 s. The as-deposited films were immediately placed into a furnace of 300 ◦ C and hold for 10 min. The deposition and preheating processes were repeated for 10 times in order to get a desired thickness (250 nm). Thereafter, the AZO films were heated in air to 550 ◦ C with different
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Fig. 1. SEM images of AZO films with different heating rates during post annealing process: (a) 10 ◦ C/min, (b) 20 ◦ C/min, (c) 40 ◦ C/min, and (d) 50 ◦ C/min.
heating rates (10 ◦ C/min, 20 ◦ C/min, 40 ◦ C/min and 50 ◦ C/min) and then hold at 550 ◦ C for 1 h. After annealing, the films were cooled in furnace to the room temperature.
with the residual strain along the c axis in the ZnO films. The strain along the c-axis is given by the following equation [12]: εzz =
2.2. Characterization The crystal structure of the AZO films was characterized by using an X-ray diffractometer (Philips X’ Pert Pro with Cu K␣, 0.154056 nm). The surface morphology of the films was observed by a filed emission scanning electron microscope (FE-SEM, Hitachi S-4800). The sheet resistance was measured by four-point probe method (SX1934 digital four-point testing instrument). The optical transmittance was obtained from a UV–vis spectrophotometer (70-1901 UV–vis spectrophotometer). The chemical composition and bond state were characterized by a Perkin-Elmer PHI-5702 multi-functional X-ray photoelectron spectroscopy (XPS) with Al K␣ radiation. 3. Results and discussion Fig. 1 shows the surface morphologies of AZO films with different heating rates during the post annealing process. Typical granular growth of thin films is observed in the heating rate range from 10 to 40 ◦ C/min (Fig. 1a–c), and the average particle size becomes smaller gradually with increasing the heating rate. However, it is interesting to find that when the heating rate reaches 50 ◦ C/min, the surface morphology of the film changes obviously, some larger grains appear suddenly and the microstructure partially transform into nanorods (Fig. 1d). Fig. 2 exhibits the X-ray diffraction patterns of AZO films with different heating rates. Only one peak around 34.4◦ appears, which is verified to be the (0 0 2) peak of ZnO with a hexagonal wurtzite structure. The intensity of the (0 0 2) peak decreases with the increase of the heating rate. Notably, at the heating rate of 50 ◦ C/min, the intensity of (0 0 2) peak increases sharply. Ong and Zhu [12] and Ghosh and Basak [13] reported that the surface morphology and crystal quality were intimately associated
c − c0 × 100% c0
(1)
where the c-axis lattice length is obtained from the position of the (0 0 2) peak using the Bragg condition and co is the unstrained lattice parameter of ZnO (JCPDS, 36-1451). The strain εzz thus determined for the AZO films annealed with different heating rates are summarized in Table 1. The results indicate that the strain increases as the heating rate increases from 10 to 40 ◦ C/min. The non-uniform heating may exist in the films when the heating rate is fast, which induces the increase in strain due to the stretch of lattice. The stretched lattice can increase the lattice energy and diminish the driving force of the growth [13], leading to the decrease of the particle size and crystallization, as observed in the SEM (Fig. 1a–c) and XRD results (Fig. 2a–c). For a much higher heating rate (50 ◦ C/min), the growth of ZnO will be further retarded by the increasing strain. Under thermodynamic equilibrium conditions, the fastest growth rate is along the c-axis due to the higher surface energy of {0 0 0 1} planes [14]. Thus, for the film annealed at 50 ◦ C/min, a faster growth along the c-axis occurs to release the strain and lower the energy, which causes the formation of the nanorods as shown in Fig. 1d. Meanwhile, some of the strain could be also relaxed at the grain boundaries [15,16], which leads to the appearance of several larger grains. In conclusion, the minimum value of the strain (−0.036%) is found when the film is annealed at the heating rate of 50 ◦ C/min and the corresponding film possesses a better crystal quality.
Table 1 c-axis length and strain, εzz along the c-axis of AZO films with different heating rates. Heating rate (◦ C/min)
2 (◦ )
c-axis length (nm)
Strain, εzz (%)
10 20 40 50
34.42 34.40 34.36 34.44
0.5207 0.5210 0.5216 0.5204
0.014 0.065 0.181 −0.036
M. Gao et al. / Applied Surface Science 257 (2011) 6919–6922
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Fig. 4. UV–vis spectra of AZO films with different heating rates. The inset shows the plot of the absorption coefficient versus photon energy of AZO films with different heating rates.
Fig. 2. XRD patterns of AZO films with different heating rates: (a) 10 ◦ C/min, (b) 20 ◦ C/min, (c) 40 ◦ C/min, and (d) 50 ◦ C/min.
To clarify whether Al dope into the ZnO nanorods, X-ray photoelectron spectroscopy (XPS) measurement is performed on this nanorod film. As shown in Fig. 3, the survey scan indicates no impurity above the detection limit. The Al 2p peak located at 74.11 eV corresponds to Al–O bonding and the characteristic peak of metallic Al is not observed, as shown in the inset of Fig. 3, which confirms the doping of Al in the ZnO crystal lattice [17]. Based on the above results, it is concluded that the heating rate during the high temperature annealing process plays a significant role in affecting the AZO microstructure and crystal quality. When the film is annealed at the heating rate of 50 ◦ C/min, the AZO film
Fig. 3. X-ray photoelectron spectroscopy of the AZO film with the heating rate of 50 ◦ C/min. The inset shows the Al 2p spectra.
with nanorod structure is obtained and the crystal quality of the film is enhanced effectively. Fig. 4 represents the UV–vis transmission spectra of AZO films with different heating rates. The AZO nanorod film with the heating rate of 50 ◦ C has a slightly lower transmittance over a broad range of wavelengths as compared to the other nanoparticle films. This is due to the increase in light scattering caused by the non-uniform AZO nanorods [18]. Despite this, the average transmittance of the AZO nanorod film in the visible-light range (400–800 nm) is still larger than 85%, which meets the requirement of the TCO application. The optical bandgaps for the AZO films with different heating rates are determined from the slope of the curves in the plot of absorption coefficient versus the photon energy, as shown in the inset of Fig. 4. From the plot, it can be observed that the band gap of the AZO nanorod film is a little smaller than the other films, but the Eg value of 3.30 eV is still similar to related reports [19,20]. The resistivities of AZO films with different heating rates are shown in Fig. 5a. It is obvious that the resistivity of the AZO nanorod film with the heating rate of 50 ◦ C is lower than the other AZO nanoparticle films. In comparison with the nanoparticles, the nanorods have a better crystal quality. The diffusion of electron in the nanorods will not be slowed down by potential barrier induced by grain boundaries and particle interface, which is the typical case in the nanoparticle films [21,22]. Therefore, the electron transfer in AZO nanorod
Fig. 5. Electrical resistivities of AZO films as a function of heating rate: (a) before hydrogen treatment and (b) after hydrogen treatment.
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M. Gao et al. / Applied Surface Science 257 (2011) 6919–6922
post annealing process on the structural and electrical properties of AZO films is investigated. It is found that the AZO film with nanorod structure can be obtained when the heating rate is 50 ◦ C/min. The resultant AZO nanorods enhance the conductivity of the film significantly. Compared with typical AZO nanoparticles, the nanorod structure can provide a faster conduction pathway for charge transport due to the high crystal quality. After the reduction treatment, the conductivity of the AZO nanorod film is further improved, and a minimum resistivity of 1.4 × 10−3 cm is obtained. This work offers a new approach to the preparation of AZO or ZnO nanorods by a simple rapid annealing process, and may be helpful for the development of sol–gel derived TCO thin films. Acknowledgements This work was financially supported by the Key grant Project of Chinese Ministry of Education (grant no.309027), and by the National Science Fund for Distinguished Young Scholars (grant no. 50925103). Fig. 6. O 1s spectra of the AZO nanorod film: (a) before hydrogen treatment and (b) after hydrogen treatment. The black dot lines indicate experiment data, green lines indicate fitting results.
is much faster than in AZO nanoparticle films, which leads to the remarkable improvement in conductivity. It is well known that oxygen vacancy usually appears in the ZnO when the sample is treated under a reducing atmosphere and it is helpful for the improvement of the conductivity [23,24]. In order to obtain a much better conductivity, the AZO films that have been annealed at 550 ◦ C in air are then reduced at 400 ◦ C for 1 h in 5% H2 + 95% Ar forming gas. After the reduction treatment, the resistivities of AZO films with different heating rates are all decreased significantly as shown in Fig. 5b, and the AZO nanorod film has a minimum resistivity of 1.4 × 10−3 cm. Fig. 6 shows O 1s photoelectron peaks in the XPS spectra of the AZO nanorod film before and after hydrogen treatment. The O 1s peaks that have been normalized by the Zn 2p peaks are fitted to two peaks. The peaks at lower and higher energies can be attributed to O2− in the ZnO lattice and surface absorbed oxygen, respectively [25]. The relative chemical compositions are calculated and the oxygen vacancy contents are determined. The content of oxygen vacancy increases from 9% to 15% after hydrogen treatment. Oxygen vacancies in the crystal act as electron donors by releasing localized electrons [26,27]. Thus, the decrease of the resistivity after the reduction treatment is attributed to the increase of oxygen vacancies which can offer more conduction electrons. 4. Conclusions The AZO films with high transmittance and low electrical resistivity are prepared by cost-effective sol–gel technique under a proper annealing procedure. The effect of heating rate during the
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