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Applied Surface Science 253 (2007) 7011–7015 www.elsevier.com/locate/apsusc
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Electrochemical synthesis and optical properties of ZnO thin film on In2O3:Sn (ITO)-coated glass a
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Changdong Gu a, Jun Li a, Jianshe Lian a,*, Guoqu Zheng b
Key Lab of Automobile Materials, Ministry of Education, College of Materials Science and Engineering, Jilin University, Nanling Campus, Changchun 130025, China b College of Chemical Engineering and Material Science, Zhejiang University of Technology, Hangzhou 310014, China
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Received 23 November 2006; received in revised form 29 December 2006; accepted 7 February 2007 Available online 15 February 2007
Abstract
PACS : 61.46.Hk; 61.82.Fk; 67.70.+n; 78.20.e
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ZnO thin films were electrochemically deposited onto the ITO-coated glass substrate from an electrolyte consisted of 0.1 M Zn(NO3)2 aqueous solution at 65 1 8C. A compact ZnO film with (0 0 2) preferred orientation was obtained at the applied potential of 1.3 V for 1200 s. It was also found that the morphology of the ZnO films grown at the potential of 1.3 V was characterized of single or coalescent hexagonal platelets. However, the ZnO crystals grown at the potential of 2.0 V was changed to be a bimodal size distribution. The band gap energy of the as deposited ZnO films, about 3.5 eV, was independent of both the applied potential and the deposition time, respectively. The minor amount of Zn(OH)2 might be co-deposited with the formation of ZnO revealed by the FT-IR spectroscopy. Three strategies to improve the ZnO crystal quality based on the photoluminescence properties were proposed in the paper, which were (a) adopting the lower deposition potential, (b) increasing the deposition time at a certain potential, and (c) annealing after as-deposition, respectively. # 2007 Published by Elsevier B.V.
Keywords: ZnO; Photoluminescence; Nanostructures; Electrodeposition
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1. Introduction
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The interest in ZnO semiconductor is fueled and fanned by its prospects in optoelectronics applications owing to its wide direct band gap (3.37 eV) at room temperature and large exciton binding energy (60 meV) [1]. ZnO films can be prepared by several techniques such as radio frequency magnetron sputtering [2], metal organic chemical vapor deposition [3], pulsed laser deposition [4–6], and molecular beam epitaxy [7]. The electrodeposition is emerging as an efficient nanotechnology for the synthesis of semiconductor thin films and nanostructures, especially chalcogenides and oxides. Electrodeposited ZnO semiconductor, which was first introduced in 1996 [8,9], have been attracted more attentions owing to the simplicity and low cost of this technique [10–17]. Methods of electrodeposition of crystalline ZnO thin films are
* Corresponding author. Tel.: +86 431 85095875; fax: +86 431 85095876. E-mail address:
[email protected] (J. Lian). 0169-4332/$ – see front matter # 2007 Published by Elsevier B.V. doi:10.1016/j.apsusc.2007.02.024
usually fulfilled from aqueous solutions using nitrate ions [8], dissolved oxygen [9] or hydrogen peroxide oxygen (H2O2) [18–20] as the precursor, respectively. Additionally, for the sake of optical transmission measurements for ZnO thin films, ZnO is usually electrodeposited onto transparent conduction optically glasses, such as the SnO2:F-coated and In2O3:Sn (ITO)-coated glasses. In this paper, semiconducting ZnO thin films were prepared by electrodeposition on a high sheet-resistance ITO-coated glass substrate at various cathodic potentials from a Zn(NO3)2 aqueous solution and their microstructures and optical properties were studied in detail. 2. Experiments The substrate was the ITO-coated glass with a sheetresistance of about 118 V/&. The area of the substrate was about 4 cm2. Before the deposition, the substrate surface was first immersed with a 0.25:0.3:1NH3H2O:H2O2:H2O solution heated up to 80 8C for about 15 min, and then thoroughly rinsed with deionized water. Afterwards, the substrate was rinsed three
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Fig. 1. XRD patterns of the as-deposited ZnO thin films on ITO-coated glass substrates at the cathodic potentials of 1.3 (a), 1.5 (b), 1.7 (c), and 2.0 V (d) for 300 s, respectively. The peak related to ITO-coated substrate is indicated by a star.
the results from Izaki et al. [8]. However, it was contrary to those from references [11,21], where the authors claimed that the preferential orientation of the electrodeposited ZnO films changed from (1 0 0) to (0 0 2) crystal plane with decreasing the cathodic potential. During the electrodeposition, films crystallization would be affected by the kinetics of atomic arrangements. Fujimura et al. [22] reported that the (0 0 2) orientation of ZnO had the lowest surface energy among all orientations. Therefore, at a relatively low deposition rate (corresponding to a higher cathodic potential [8], such as 1.3 V in the case), adatoms on the substrate surface would have enough time to move to look for the lowest energy sites before these adatoms were covered by the next layer of atoms. However, a relatively high deposition rate (corresponding to a lower cathodic potential, such as 2.0 V in the case) would make the adatoms have no time to arrange their sites, and hence the films exhibited the random orientations. The typical surface morphologies of the ZnO thin films at the high and low cathodic potentials were given in Fig. 2(a) and (b), respectively. At the potential of 1.3 V for 300 s, the substrate was not fully covered by the ZnO thin films, as shown in Fig. 2(a). The morphology of the ZnO films grown at the potential of 1.3 V was characterized of single or coalescent hexagonal platelets, which also agreed with the XRD results (Fig. 1(a)). However, the ZnO films grown at the potential of 2.0 V (Fig. 2(b)) had been fully covered the substrate and seemed very compact and smooth, which was different with the one grown at 1.3 V. Additionally, the size of ZnO crystals exhibited bimodal distribution. Some small grains with the size of about 10 nm were interspersed on the boundaries of large grains, as shown in the inset of Fig. 2(b), which was suggested to be attributed to the high nucleation rate under the lower
3. Results and discussion 3.1. Applied potentials effects on ZnO films
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times in acetone with ultrasonic vibration with each rinse for 15 min, and again rinsed with deionized water thoroughly. The electrolytic solutions were prepared from analytical grade reagents with the following composition: 0.1 M Zn(NO3)2 aqueous solution with the initial pH value being adjusted to 6.0 by HNO3 and KOH solutions. During the electrodeposition process, the temperature of the electrolytic solution was maintained to 65 1 8C through a thermostat-controlling water tank. Electrochemical studies and film depositions were performed on a LK98 Microcomputer-based Electrochemical System (LANLIKE, Tianjin, China), which was controlled by a computer and supported by self-designed software, using a classic three-electrode cell with a platinum pole (Pt) as counterelectrode and a saturated calomel electrode as the reference. A field emission scanning electron microscope (FESEM, JEOL JSM-6700F, Japan) was employed for the observations of the surface morphology of the thin films. Crystalline structure of the sample was studied by the X-ray diffractometer (XRD, Rigaku D/max, Japan) with a Cu target and a monochronmator at 50 kV and 300 mA with the scanning rate and step being 48/min and 0.028, respectively. The optical properties of the ZnO thin films were characterized by UV–vis spectrophotometer (725PC) and Photoluminescence with an Ar ion laser as a light source using an excitation wavelength of 325 nm. All spectra were measured at room temperature. The infrared (IR) spectra of the films were measured by a Nexus670 Fourier transform spectrometer (FT-IR, Thermo Nicolet, USA). The IR experiments were performed on the pellet specimens, which were obtained by mechanically scratching the film from the substrate and mixing the obtained powder with KBr in pressed pellets.
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The linear sweep voltammetry of ZnO thin films deposition from 0.1 M Zn(NO3)2 aqueous solution at 65 1 8C in our case showed that it was difficult to electrodeposit ZnO film on the substrate at the electrode potential higher than 1.1 V because of too low deposition rate under very low current density. In this case, we chose 1.3, 1.5, 1.7, and 2.0 V as the cathodic potentials to electrochemically synthesize ZnO thin films on ITO-coated glass substrate. Fig. 1 showed the XRD patterns of the as-deposited ZnO thin films on ITO-coated glass substrates at different cathodic potentials from 1.3 to 2.0 V, with each depositing for 300 s. The diffraction peak corresponding to the ITO-coated glass substrate was indicated with a star. Additional peaks were identified to be those from the wurtzite structured ZnO. The preferred orientation of ZnO films was varying with the cathodic potentials. At the potential of 1.3 V, only the (0 0 2) preferred orientation was observed, as shown in Fig. 1(a). However, at 2.0 V, the as-deposited ZnO film had the random orientations. As a whole, the (0 0 2) preferred orientation of ZnO films was weakened with the decrease in cathodic potentials from 1.3 to 2.0 V. The result of the (0 0 2) preferential orientation weakening with decreasing the potentials was agreement to
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Fig. 3. XRD pattern (down-left inset) and FESEM morphology of the ZnO thin films obtained at the cathodic potential of 1.3 V for 1200 s. The up-right inset shows enlarged view of the corresponding image.
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shown by the down-left inset (XRD patterns) in Fig. 3. Furthermore, the ZnO film became very compact and smooth, which was different from the one obtained for a relative short time of 300 s shown in Fig. 2(a). Obviously, the hexagonal platelet crystal structure with the hexagon edge length of 200 nm and a height of 170 nm could be found in the film, as shown in the up-right inset of Fig. 3. These hexagonal platelets grew flatly or perpendicularly to the substrate. We also found that the initial deposits of ZnO on the ITOcoated glass substrate held the (0 0 2) preferred orientation whatever the applied cathodic potential was. For example, the XRD patterns of the as-deposited ZnO thin films on ITO-coated glass substrates at the cathodic potential of 1.7 V for different times were shown in Fig. 4. However, the (0 0 2) preferred orientation of ZnO films was weakening with the increase of deposition times from 120 to 1200 s. After an enough
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Fig. 2. The typical surface morphologies of the ZnO thin films at the cathodic potentials of 1.3 V (a) and 2.0 V (b) for 300 s by FESEM. The up-right insets show enlarged view of the corresponding images.
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applied potentials. Once the dense ZnO thin film was obtained, the surface concentration of electrons available for charge transfer to form ZnO would become lower. However, in the case of intensive electricity field caused by much lower cathodic potentials, e.g. 2.0 V, the electrons would easily penetrate the ZnO deposits where the film thickness was lower than other regions for reducing the NO3 in the solutions. Thus, some tiny ZnO grains were formed at the interspaces of the large ZnO crystals, as shown in Fig. 2(b). 3.2. Deposition time effects on ZnO films We also studied the deposition time effects on the microstructures of ZnO thin films. Fig. 3 gave the XRD pattern and FESEM morphology of the ZnO thin film obtained at cathodic potential of 1.3 V for 1200 s. The up-right inset showed the enlarged view of the corresponding image. It was found that the (0 0 2) preferred orientation of ZnO film was also maintained at the potential of 1.3 V after a long time depositing, which was
Fig. 4. XRD patterns of the as-deposited ZnO thin films on ITO-coated glass substrates at the cathodic potential of 1.7 V for different time: (a) 120 s, (b) 300 s, and (c) 1200 s. The peak position related to ITO-coated substrate is indicated by a star.
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deposition time, the as-deposited ZnO films changed to random orientations.
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Fig. 6. Optical transmittance spectra of the ZnO thin films grown at the cathodic potential of 1.7 V for different times. The inset is ðlnðTÞhvÞ2 vs. hv plot obtained from transmittance.
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Fig. 7 showed the room temperature photoluminescence (PL) spectra of ZnO thin films on ITO glass by electrochemical deposition. Wang et al. reported the annealing process on asdeposited ZnO films in air would result in the enhancement and sharpening of the excitonic emission band and decrease of the deep level emissions (DLE) [27]. In our case, the as-deposited ZnO film at 1.5 V for 300 s were annealed in air at 500 8C for about 60 min and its PL spectrum was also shown in Fig. 7 for comparison. From Fig. 7, the ZnO films exhibited the strong DLE centered at about 530 nm as well as the intrinsical UV emission at about 370 nm. The relatively weak UV emission may be due to the defects that may trap the photogenerated holes and/or electrons while the DLE centered at about 530 nm may imply that there exist defects in the singly negativecharged interstitial oxygen ion state [20]. Comparing the relative PL intensity of the exciton emission to the DLE
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Figs. 5 and 6 showed the optical transmittance spectra of the ZnO thin films grown at different cathodic potentials and at the cathodic potential of 1.7 V for different times, respectively. The films were transparent to visible light and the average optical transmittance values of films were high to about 80– 90%. ZnO film, being a direct band gap semiconductor, has an absorption coefficient, which obeys the following relation for high-photon energies: ðahvÞ ¼ Aðhv Eg Þ1=2 [23], where A is a constant, a is the absorption coefficient (cm1) and hv ðeVÞ is the energy of excitation. The absorption coefficient a can be obtained by a = ln(T)/d, where T is the transmittance and d is the thickness of film, respectively. In our case, the ZnO films used to estimate the band gap energy (Eg) were in the range of 0.5–2 mm. During the estimating on Eg, it was found the effect of the thickness on the estimate of Eg was not obvious. Therefore, we assumed the absorption coefficient a / ln(T) in the fundamental absorption region, and better linearity was observed from the ðlnðTÞhvÞ2 versus hv plots (the insets of Figs. 5 and 6), which were used to determine the Eg [24]. The Eg values of the electrochemical deposited ZnO thin films at various cathodic potentials and at the cathodic potential of 1.7 V for different times were 3.5 eV or so, which was larger than the band gap energy of ZnO single crystal (3.37 eV) [25]. The FT-IR spectrum for the as-deposited ZnO films was dominated by a broad peak at about 440 cm1, corresponding to the two transverse optical modes of ZnO [26] as well as the main bands appearing in the range of 3450–3500 cm1, which were corresponding to the stretching vibrations of –OH [13]. This results indicated that the minor amount of Zn(OH)2 might be co-deposited with the formation of ZnO, which was responsible for the Eg positive shift of the ZnO film [27].
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3.3. Optical properties of as-deposited ZnO film
Fig. 5. Optical transmittance spectra of the ZnO thin films grown at different cathodic potentials. The inset is ðlnðTÞhvÞ2 vs. hv plot obtained from transmittance.
Fig. 7. Room temperature PL spectra of ZnO thin films on ITO glass by electrochemical deposition, including the films obtained at the applied potentials of 1.3, 1.5, 1.7, and 2.0 V for deposition time of 300 s, respectively; the one obtained at the applied potential of 1.3 V for deposition time of 1300 s, and the one with annealing at 500 8C for 60 min after as-depositing at 1.5 V for 300 s.
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4. Conclusions
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The authors thank Dr. Yanjie Tian from College of Physics, Jilin University for supplying the ITO-coated glass and Prof. Zuoxing Guo for kind help in the FESEM observations. This work was supported by the Foundation of National Key Basic Research and Development Program (No. 2004CB619301) and the Project 985-automotive engineering of Jilin University.
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In conclusion, ZnO thin films were electrochemically deposited onto the ITO-coated glass substrate with a sheetresistance of 118 V/& from an electrolyte containing 0.1 M Zn(NO3)2 at the temperature of 65 1 8C. It was found that the (0 0 2) preferred orientation of ZnO films was weakened with the decrease in cathodic potentials from 1.3 to 2.0 V. The morphology of the ZnO films grown at the potential of 1.3 V was characterized of single or coalescent hexagonal platelets. However, the ZnO crystals grown at the potential of 2.0 V exhibited a bimodal grain size distribution. Some grains with the size of about 10 nm were interspersed on the boundaries of large grains. The initial deposits of ZnO on the ITO-coated substrate held the (0 0 2) preferred orientation whatever the applied cathodic potential was. The asdeposited ZnO thin films were transparent to visible light and the average optical transmittance values of films were high to about 80–90%. The band gap energy of about 3.5 eV for the as-deposited ZnO films was independent of the applied potential and the deposition time. It was indicated from the FT-IR measurements that the minor amount of Zn(OH)2 might be co-deposited with the formation of ZnO, which was contributed to the high Eg of the as-deposited films. The PL spectra of the ZnO films indicated that the decrease of the applied potential, increase of the deposition time at a certain potential, and annealing process after as-deposition would improve the crystalline quality of the as-deposited ZnO films.
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(Iexc/IDLE) is a way to evaluate the quality of the ZnO films [25]. The high intensity ratio of Iexc/IDLE is an evidence of the high quality of ZnO film. In Fig. 7, the intensity ratio of Iexc/IDLE increased as decrease of the deposition potentials. In addition, the ratio of Iexc/IDLE was also enhanced as the deposition time increased from 300 to 1200 s at the applied potential of 1.3 V. After the annealing of the as-deposited ZnO films at 1.5 V, the increase of the ultraviolet emission and the decrease of the DLE from defects indicated that the crystal quality of the ZnO films were improved. These improvement in photoluminescence can be explained by the elimination of Zn(OH)2 in the ZnO film and that oxygen vacancies were combined with oxygen diffusing into the ZnO films during annealing [27,28]. The DLE is probably relative to the variation of the intrinsic defects in ZnO films, such as zinc vacancy VZn, oxygen vacancy VO, interstitial zinc Zni, interstitial oxygen Oi, and antisite oxygen OZn [20,29– 32]. Therefore, it can be deduced from Fig. 7 that in our case the processes of (a) decrease of the applied potential, (b) increase of the deposition time at a certain potential, and (c) annealing after as-deposition would reduce the intrinsic defects and improve the crystalline quality as well as the photoluminescence properties of the ZnO films.
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