- Email: [email protected]
Electrodeposition and characterization of transparent ZnO thin films
ARTICLE IN PRESS
Solar Energy Materials & Solar Cells 88 (2005) 227–235 www.elsevier.com/locate/solmat
Electrodeposition and characterization of transparent ZnO thin films T. Mahalingama,, V.S. Johna, M. Rajaa, Y.K. Sub, P.J. Sebastianc a
Department of Physics, Alagappa University, Karaikudi 630 003, India b Institute of Microelectronics, Department of Electrical Engineering, National Cheng Kung University, Tainan, Taiwan, ROC c Solar-Hydrogen-Fuel Cell Group, CIE- UNAM, 62580 Temixco, Morelos, Mexico Received 1 December 2003; received in revised form 1 March 2004; accepted 1 June 2004 Available online 12 January 2005
Abstract Thin films of zinc oxide (ZnO) have been grown by potentiostatic cathodic deposition onto tin oxide-coated glass from a simple aqueous zinc nitrate electrolyte. Cyclic voltammetry (CV) experiments were performed to determine the reaction kinetics of the species. The various optimum deposition parameters like potential, pH and bath temperature are found to be 1.1 V (SCE), 570.1 and 80 1C, respectively. Structural characterization by X-ray diffraction indicates the formation of ZnO film with a preferred c-axis orientation and exhibits the wurtzite structure. Optical studies revealed a band gap energy 3.32 eV which is characteristic of ZnO films. SEM micrographs show a compact structure with nodular appearance, which is in agreement with the reported value of ZnO and the results are discussed. r 2004 Elsevier B.V. All rights reserved. Keywords: Zinc oxide; Wurtzite structure; X-ray diffraction; Optical properties; Surface morphology
Corresponding authors. Tel.: +91 04565 426616; fax: +91 4565 425202.
E-mail address: [email protected] (T. Mahalingam). 0927-0248/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.solmat.2004.06.021
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1. Introduction In recent years, transparent conductive zinc oxide (ZnO) films find numerous applications in electronics and optical technology [1] because of their suitable electrical, optical and acoustic characteristics [2]. In photovoltaics, ZnO is used as an n-type window layer for thin film solar cells based on CuInSe2 [3] or amorphous silicon [4]. ZnO films are usually prepared by several techniques such as, thermal oxidation [5], electron beam evaporation [6], activated reactive evaporation [7], r.f. sputtering [8], metal organic chemical vapor deposition [9], molecular beam epitaxy [10], spray pyrolysis [11], etc. Electrodeposition technique is now emerging as an important low cost and low-temperature method to prepare semiconducting thin films [12]. Preparation of oxide films by electrodeposition from aqueous solution presents several advantages over other techniques. It has been reported that a wurtzite ZnO film with high optical transparency can be prepared by cathodic deposition from a simple zinc nitrate [Zn(NO3)2] aqueous solution [13]. Since the ZnO film showed a direct band gap energy of 3.3 eV [14], it is expected that some photocurrent can be generated with an irradiation of light with wavelength below 375 nm. An interesting characteristic of the electrodeposition of zinc oxide is the possibility to obtain different morphologies of the deposits. Films with dense or open structures have been described [15]. In this present work, semiconducting ZnO films are prepared by electrodeposition at various cathodic potentials from an aqueous solution and their characteristics were studied in detail.
2. Experimental details Zinc oxide thin films were electrodeposited on fluorine-doped tin oxide (SnO2:F) substrates in an aqueous medium of zinc nitrate with a solution pH ¼ 570.1. The electrodeposition cell employed a standard three electrode geometry comprising SnO2 glass substrate with a sheet resistance of 10 O/&, a graphite rod and a saturated calomel electrode (SCE) as working, counter and reference electrodes, respectively. Before use, the tin conductive oxide substrates were treated for 5 min with ultrasonic waves in a bath of isopropanol and then rinsed with acetone. The cyclic voltammetry (CV) studies were carried out using an EG&G scanning potentiostat (PAR 273 USA) with an aqueous solution of 0.1 M Zn(NO3)2. Electrodeposition of ZnO thin films were performed at 80 1C with an applied cathodic voltage of 1.1 V versus SCE. XRD studies were carried out with a JEOL JDX 8030 diffractometer using CuKa radiation (l ¼ 1.5418 A˚). Optical transmission measurements were performed using a JASCO UV-Vis-NIR spectrophotometer in order to evaluate the energy gap of the electrodeposited semiconductors. An SnO2:F-covered glass substrate was placed in the reference optical path to compensate the light intensity. Scanning electron microscopy (SEM) images were obtained using a JEOL, JSM 840 microscope at an acceleration voltage of 20 kV and the results are discussed in detail.
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3. Results and discussion 3.1. Reaction kinetics of ZnO Fig. 1 shows the cyclic voltammogram on a tin oxide coated glass cathode over the potential range 0.5 to 1.5 V (SCE). In the CV studies, when both zinc and oxygen are present in the solution, cathodic current begins at 0.65 V (SCE). At the cathodic potential of 1.1 V versus SCE, a steep increase of the cathodic current takes place up to 8 mA corresponding to zinc deposition. In the reverse direction, the deposited zinc is deoxidized which indicates the stripping of metallic zinc occurs at 0.8 V (SCE). Since the anodic current is essentially zero over the range the stripping process is relatively insignificant. The anodic current is negligible indicating the high stability of the deposited ZnO, since they are not stripped when the direction of the potential is reversed. It has been observed that the total cathodic current consists of three components (i) reduction of nitrate, (ii) formation of hydrogen gas and (iii) deposition of ZnO. In principle, these reactions could occur simultaneously
Current density (mA/cm2)
0
2.0
4.0
6.0
8.0
-1.5
-1.0
-0.5
0
0.5
Potential (V) vs SCE Fig. 1. Cyclic voltammogram of a SnO2:F- coated glass electrode in 0.1 M zinc nitrate solution with deposition potential : 1.1 V (SCE); bath temperature : 80 1C, solution pH : 570.1.
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and compete with one another. These results show a window between 0.8 and 1.2 V (SCE) suitable for the formation of ZnO films. The possible reactions for the ZnO film formation are: ZnðNO3 Þ2 ! Zn2þ þ 2NO 3; NO 3 þ H2 O þ 2e ! NO2 þ 20 H ;
Zn2þ þ 2OH ! Zn ðOHÞ2 ; Zn ðOHÞ2 ! ZnO þ H2 O: Two anionic groups (NO 2 and OH ) are formed by dissolving zinc nitrate in water. 2+ Later, Zn ions combined with hydroxyl ions (OH) yielding zinc hydroxide. Finally, zinc hydroxide oxidizes into ZnO and water.
3.2. Rate of deposition The film thickness was determined gravimetrically by measuring the change in weight of the substrate due to film deposition, the area of deposition and using the bulk density of ZnO (5.6 g cm3). The rate of deposition of ZnO films as a function of zinc nitrate concentration in the electrolyte is shown in Fig. 2. ZnO films of thickness 1.4 mm were obtained as the time of deposit and the concentration of zinc nitrate increases. The rate of deposition increases rapidly and attains a maximum value of 1.4 mm min1 for 0.1 M concentration of Zn(NO3)2 to deposit ZnO films. The inset of the Fig. 2 shows the deposition rate of ZnO films as a function of deposition potential. The rate of deposition increases sharply with potential and attains a maximum in the potential range 0.8 to 1.25 V versus SCE. It was observed that a potential range of 0.8 to 1.25 V (SCE) is selected to deposit ZnO films with high deposition rates. Our CV studies also revealed the same range of deposition potential to synthesize ZnO films. 3.3. Structural analysis X-ray diffractograms of ZnO films deposited at various potentials (0.8, 1.0 and 1.2 versus SCE) are shown in Fig. 3. The zinc nitrate concentration and deposition temperature were maintained at 0.1 M and 80 1C, respectively for the preparation of ZnO films. The appearance of several peaks in the XRD reveals that the films are polycrystalline in nature and the structure is identified to be hexagonal (ASTM 5 – 0664). The most intense peak with small line width corresponds to (0 0 2) plane in the XRD which indicates a good crystallization state with a larger grain size. The strong preferential orientation along the (0 0 2) direction (c-axis) perpendicular to the substrate is consistent with the growth habit generally evidenced for ZnO films deposited by other deposition methods from the gas phase. The intensity of the (0 0 2) plane is maximum for the films deposited at a cathodic potential 1.0 V (SCE), whereas for the other two potentials (0.8 and 1.1 V versus SCE) the height
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Deposition rate (µm min-1)
Deposition rate (µm min-1)
T. Mahalingam et al. / Solar Energy Materials & Solar Cells 88 (2005) 227–235
0.4
231
0.4
0.3
0.2
-0.5 -1.0 -1.5 Deposition Potential (v)
0.3
0.2
0.1
0
0.01
0.1
1.0
Concentration of Zinc (mol L-2) Fig. 2. Variation of deposition rate as a function of zinc nitrate concentration. Inset shows the dependence of deposition rate with cathodic potential.
of (0 0 2) plane decreases. As seen from the X-ray diffractograms, any peak attributed to the SnO2 substrate could not be observed, which indicates that dense and defect-free ZnO films could be prepared under this condition. Lattice constants calculated from peak angles were 3.246 A˚ in the ‘a’-axis and 5.207 A˚ in the ‘c’-axis, regardless of the electrolysis conditions. The values agreed well with those tabulated in the ASTM index. At room temperature, there are no diffraction peaks (not shown), indicating that the layer of ZnO film is amorphous or microcrystalline. An improvement of the crystallization state takes place when the temperature increases, with a reduction of the peak width indicating larger grains. These studies indicate that the optimum deposition potential and bath temperature to synthesize well-crystallized ZnO films are 1.1 V versus SCE and 80 1C, respectively. 3.4. Optical characteristics Fig. 4 shows the optical transmission spectrum for 0.3 mm thick ZnO film prepared with a potential 1.1 V (SCE) and bath temperature 80 1C. The optical transmission
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(110)
(102)
(101)
(100)
(200) (112) (201)
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(102)
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Intensity (arbitrary unit)
(c)
(b)
(a) 30
50
70
2θ (degrees) Fig. 3. X-ray diffraction patterns of the ZnO films prepared at various cathodic potentials (a) 0.8 V (SCE), (b) 1.0 V (SCE), (c) 1.2 V (SCE).
decreased with decrease in wavelength and was approximately 80% at wavelength near 600 nm. It was previously reported that the optical transparency strongly depends on the surface irregularity [16]. The poor transparency of the film could be ascribed to the surface roughness and existence of pores for the films prepared from 0.06 M zinc nitrate concentration in the electrolytic bath. The high optical transmission suggested that the ZnO films had a smooth surface. Optical band gap was calculated from the absorption edge in the absorption spectra. The inset of Fig. 4 shows the determination of the optical gap from the (ahg)2 versus hg variation. A band gap at about 3.32 eV can be deduced from these measurements and is consistent with the characteristic value of 3.30 eV for undoped ZnO film prepared by sputter deposition technique [17]. 3.5. Surface morphology Fig. 5 shows the surface morphology of ZnO films prepared at various deposition potentials with a constant bath temperature (80 1C). The SEM pictures indicate the
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50
200 (αhδ)2 (a.u)
Transmission (%)
90
0 3.1
10 200
100
400
3.3 E (ev)
600
3.5
800
Wavelenth , λ (nm) Fig. 4. Transmission spectrum of a ZnO layer deposited at potential 1.1 V (SCE) and bath temperature 80 1C. Inset shows the (ahg)2 versus hg plot representing the energy gap of ZnO films.
granular and porous character of the deposited films. The contrast is due to the different orientation of the crystallites. The surfaces of the films exhibit a nodular appearance. The orientation of the ZnO film prepared at 0.8 V (SCE) was the aggregate of hexagonal columns, and some pores were seen between the columns. However, the film prepared between 0.9 and 1.1 V versus SCE shows nodular appearance. The film appears to be dense, and consisting of large and flat hexagonal grains with the c-axis perpendicular to the substrate. Such nodular appearances are characteristics of ZnO growth reported in Ref. [16].
4. Conclusions Transparent conducting ZnO films were prepared by using a simple and inexpensive electrodeposition technique. Experimental observations from CV studies reveal that a window between 0.8 and 1.2 V (SCE) is suitable for the formation of ZnO films. It is observed from the rate of deposition that a maximum value of 1.4 mm min1 is obtained from 0.1 M zinc nitrate concentration. A structural analysis elucidates the polycrystalline nature of ZnO with c-axis orientation, with a preferential orientation along (OO2) plane. Optical characteristics indicate the absorption edge at 375 nm, which corresponds to the optical band gap energy of 3.32 eV.The SEM picture shows a compact structure, which denotes a high density of nucleation centers and the surface has a nodular appearance. The simplicity and
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Fig. 5. SEM pictures of ZnO films deposited at various cathodic potentials at bath temperature 80 1C (a) 0.8 V (SCE), (b) 0.9 V (SCE), (c) 1.1 V (SCE).
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low-cost production of ZnO films with reproducible structural, optical and morphological characteristics should be useful as a less expensive window material for solar cell and display devices. References [1] P. Nunes, E. Fortunato, R. Martins, Thin Solid Films 383 (2001) 277. [2] D. Gal, G. Hodes, D. Lincot, H.W. Schock, Thin Solid Films 361 (2000) 79. [3] L. Stott, J. Hedstro¨m, M. Ru¨ckh, J. Kessler, K.O. Velthaus, H.W. Schock, Appl. Phys. Lett. 62 (1993) 597. [4] T. Ikeda, J. Sato, Y. Hayashi, Y. Wakayamma, K. Adachi, H. Nishimura, Sol. Energy Mater. Sol. Cells 24 (1994) 379. [5] P. Bonasewicz, W. Hirschwald, G. Neumann, Thin Solid Films 142 (1986) 77. [6] A. Kuroyanagi, Jpn. J. Appl. Phys. 28 (1989) 219. [7] H. Gopalaswamy, P.J. Reddy, Semicond. Sci. Technol. 5 (1990) 980. [8] F.S. Mahnmood, R.D. Gould, A.K. Hassan, H.M. Salih, Thin Solid Films 270 (1995) 376. [9] W.W. Wenas, A. Yamada, M. Konagai, K. Takahashi, J. Appl. Phys. 33 (1994) 283. [10] B. Sang, M. Konagai, J. Appl. Phys. 35 (1996) 602. [11] A. Ghosh, S. Basu, Mater. Chem. Phys. 27 (1991) 45. [12] Z.H. Gu, T.Z. Tahidy, J. Electrochem. Soc. 146 (1999) 156. [13] M. Izaki, T. Omi, J. Electrochem. Soc. 143 (1996) L53. [14] J. Katayama, M. Izaki, J. Appl. Electrochem. 30 (2000) 855. [15] S. Peulon, D. Lincot, Adv. Mater. 8 (1996) 166. [16] M. Izaki, T. Omi, Appl. Phys. Lett. 68 (1996) 2439. [17] A.P. Roth, J.B. Webb, D.F. Williams, Solid State Commun. 39 (1981) 1269.
Solar Energy Materials & Solar Cells 88 (2005) 227–235 www.elsevier.com/locate/solmat
Electrodeposition and characterization of transparent ZnO thin films T. Mahalingama,, V.S. Johna, M. Rajaa, Y.K. Sub, P.J. Sebastianc a
Department of Physics, Alagappa University, Karaikudi 630 003, India b Institute of Microelectronics, Department of Electrical Engineering, National Cheng Kung University, Tainan, Taiwan, ROC c Solar-Hydrogen-Fuel Cell Group, CIE- UNAM, 62580 Temixco, Morelos, Mexico Received 1 December 2003; received in revised form 1 March 2004; accepted 1 June 2004 Available online 12 January 2005
Abstract Thin films of zinc oxide (ZnO) have been grown by potentiostatic cathodic deposition onto tin oxide-coated glass from a simple aqueous zinc nitrate electrolyte. Cyclic voltammetry (CV) experiments were performed to determine the reaction kinetics of the species. The various optimum deposition parameters like potential, pH and bath temperature are found to be 1.1 V (SCE), 570.1 and 80 1C, respectively. Structural characterization by X-ray diffraction indicates the formation of ZnO film with a preferred c-axis orientation and exhibits the wurtzite structure. Optical studies revealed a band gap energy 3.32 eV which is characteristic of ZnO films. SEM micrographs show a compact structure with nodular appearance, which is in agreement with the reported value of ZnO and the results are discussed. r 2004 Elsevier B.V. All rights reserved. Keywords: Zinc oxide; Wurtzite structure; X-ray diffraction; Optical properties; Surface morphology
Corresponding authors. Tel.: +91 04565 426616; fax: +91 4565 425202.
E-mail address: [email protected] (T. Mahalingam). 0927-0248/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.solmat.2004.06.021
ARTICLE IN PRESS 228
T. Mahalingam et al. / Solar Energy Materials & Solar Cells 88 (2005) 227–235
1. Introduction In recent years, transparent conductive zinc oxide (ZnO) films find numerous applications in electronics and optical technology [1] because of their suitable electrical, optical and acoustic characteristics [2]. In photovoltaics, ZnO is used as an n-type window layer for thin film solar cells based on CuInSe2 [3] or amorphous silicon [4]. ZnO films are usually prepared by several techniques such as, thermal oxidation [5], electron beam evaporation [6], activated reactive evaporation [7], r.f. sputtering [8], metal organic chemical vapor deposition [9], molecular beam epitaxy [10], spray pyrolysis [11], etc. Electrodeposition technique is now emerging as an important low cost and low-temperature method to prepare semiconducting thin films [12]. Preparation of oxide films by electrodeposition from aqueous solution presents several advantages over other techniques. It has been reported that a wurtzite ZnO film with high optical transparency can be prepared by cathodic deposition from a simple zinc nitrate [Zn(NO3)2] aqueous solution [13]. Since the ZnO film showed a direct band gap energy of 3.3 eV [14], it is expected that some photocurrent can be generated with an irradiation of light with wavelength below 375 nm. An interesting characteristic of the electrodeposition of zinc oxide is the possibility to obtain different morphologies of the deposits. Films with dense or open structures have been described [15]. In this present work, semiconducting ZnO films are prepared by electrodeposition at various cathodic potentials from an aqueous solution and their characteristics were studied in detail.
2. Experimental details Zinc oxide thin films were electrodeposited on fluorine-doped tin oxide (SnO2:F) substrates in an aqueous medium of zinc nitrate with a solution pH ¼ 570.1. The electrodeposition cell employed a standard three electrode geometry comprising SnO2 glass substrate with a sheet resistance of 10 O/&, a graphite rod and a saturated calomel electrode (SCE) as working, counter and reference electrodes, respectively. Before use, the tin conductive oxide substrates were treated for 5 min with ultrasonic waves in a bath of isopropanol and then rinsed with acetone. The cyclic voltammetry (CV) studies were carried out using an EG&G scanning potentiostat (PAR 273 USA) with an aqueous solution of 0.1 M Zn(NO3)2. Electrodeposition of ZnO thin films were performed at 80 1C with an applied cathodic voltage of 1.1 V versus SCE. XRD studies were carried out with a JEOL JDX 8030 diffractometer using CuKa radiation (l ¼ 1.5418 A˚). Optical transmission measurements were performed using a JASCO UV-Vis-NIR spectrophotometer in order to evaluate the energy gap of the electrodeposited semiconductors. An SnO2:F-covered glass substrate was placed in the reference optical path to compensate the light intensity. Scanning electron microscopy (SEM) images were obtained using a JEOL, JSM 840 microscope at an acceleration voltage of 20 kV and the results are discussed in detail.
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3. Results and discussion 3.1. Reaction kinetics of ZnO Fig. 1 shows the cyclic voltammogram on a tin oxide coated glass cathode over the potential range 0.5 to 1.5 V (SCE). In the CV studies, when both zinc and oxygen are present in the solution, cathodic current begins at 0.65 V (SCE). At the cathodic potential of 1.1 V versus SCE, a steep increase of the cathodic current takes place up to 8 mA corresponding to zinc deposition. In the reverse direction, the deposited zinc is deoxidized which indicates the stripping of metallic zinc occurs at 0.8 V (SCE). Since the anodic current is essentially zero over the range the stripping process is relatively insignificant. The anodic current is negligible indicating the high stability of the deposited ZnO, since they are not stripped when the direction of the potential is reversed. It has been observed that the total cathodic current consists of three components (i) reduction of nitrate, (ii) formation of hydrogen gas and (iii) deposition of ZnO. In principle, these reactions could occur simultaneously
Current density (mA/cm2)
0
2.0
4.0
6.0
8.0
-1.5
-1.0
-0.5
0
0.5
Potential (V) vs SCE Fig. 1. Cyclic voltammogram of a SnO2:F- coated glass electrode in 0.1 M zinc nitrate solution with deposition potential : 1.1 V (SCE); bath temperature : 80 1C, solution pH : 570.1.
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and compete with one another. These results show a window between 0.8 and 1.2 V (SCE) suitable for the formation of ZnO films. The possible reactions for the ZnO film formation are: ZnðNO3 Þ2 ! Zn2þ þ 2NO 3; NO 3 þ H2 O þ 2e ! NO2 þ 20 H ;
Zn2þ þ 2OH ! Zn ðOHÞ2 ; Zn ðOHÞ2 ! ZnO þ H2 O: Two anionic groups (NO 2 and OH ) are formed by dissolving zinc nitrate in water. 2+ Later, Zn ions combined with hydroxyl ions (OH) yielding zinc hydroxide. Finally, zinc hydroxide oxidizes into ZnO and water.
3.2. Rate of deposition The film thickness was determined gravimetrically by measuring the change in weight of the substrate due to film deposition, the area of deposition and using the bulk density of ZnO (5.6 g cm3). The rate of deposition of ZnO films as a function of zinc nitrate concentration in the electrolyte is shown in Fig. 2. ZnO films of thickness 1.4 mm were obtained as the time of deposit and the concentration of zinc nitrate increases. The rate of deposition increases rapidly and attains a maximum value of 1.4 mm min1 for 0.1 M concentration of Zn(NO3)2 to deposit ZnO films. The inset of the Fig. 2 shows the deposition rate of ZnO films as a function of deposition potential. The rate of deposition increases sharply with potential and attains a maximum in the potential range 0.8 to 1.25 V versus SCE. It was observed that a potential range of 0.8 to 1.25 V (SCE) is selected to deposit ZnO films with high deposition rates. Our CV studies also revealed the same range of deposition potential to synthesize ZnO films. 3.3. Structural analysis X-ray diffractograms of ZnO films deposited at various potentials (0.8, 1.0 and 1.2 versus SCE) are shown in Fig. 3. The zinc nitrate concentration and deposition temperature were maintained at 0.1 M and 80 1C, respectively for the preparation of ZnO films. The appearance of several peaks in the XRD reveals that the films are polycrystalline in nature and the structure is identified to be hexagonal (ASTM 5 – 0664). The most intense peak with small line width corresponds to (0 0 2) plane in the XRD which indicates a good crystallization state with a larger grain size. The strong preferential orientation along the (0 0 2) direction (c-axis) perpendicular to the substrate is consistent with the growth habit generally evidenced for ZnO films deposited by other deposition methods from the gas phase. The intensity of the (0 0 2) plane is maximum for the films deposited at a cathodic potential 1.0 V (SCE), whereas for the other two potentials (0.8 and 1.1 V versus SCE) the height
ARTICLE IN PRESS
Deposition rate (µm min-1)
Deposition rate (µm min-1)
T. Mahalingam et al. / Solar Energy Materials & Solar Cells 88 (2005) 227–235
0.4
231
0.4
0.3
0.2
-0.5 -1.0 -1.5 Deposition Potential (v)
0.3
0.2
0.1
0
0.01
0.1
1.0
Concentration of Zinc (mol L-2) Fig. 2. Variation of deposition rate as a function of zinc nitrate concentration. Inset shows the dependence of deposition rate with cathodic potential.
of (0 0 2) plane decreases. As seen from the X-ray diffractograms, any peak attributed to the SnO2 substrate could not be observed, which indicates that dense and defect-free ZnO films could be prepared under this condition. Lattice constants calculated from peak angles were 3.246 A˚ in the ‘a’-axis and 5.207 A˚ in the ‘c’-axis, regardless of the electrolysis conditions. The values agreed well with those tabulated in the ASTM index. At room temperature, there are no diffraction peaks (not shown), indicating that the layer of ZnO film is amorphous or microcrystalline. An improvement of the crystallization state takes place when the temperature increases, with a reduction of the peak width indicating larger grains. These studies indicate that the optimum deposition potential and bath temperature to synthesize well-crystallized ZnO films are 1.1 V versus SCE and 80 1C, respectively. 3.4. Optical characteristics Fig. 4 shows the optical transmission spectrum for 0.3 mm thick ZnO film prepared with a potential 1.1 V (SCE) and bath temperature 80 1C. The optical transmission
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(110)
(102)
(101)
(100)
(200) (112) (201)
T. Mahalingam et al. / Solar Energy Materials & Solar Cells 88 (2005) 227–235
(102)
232
Intensity (arbitrary unit)
(c)
(b)
(a) 30
50
70
2θ (degrees) Fig. 3. X-ray diffraction patterns of the ZnO films prepared at various cathodic potentials (a) 0.8 V (SCE), (b) 1.0 V (SCE), (c) 1.2 V (SCE).
decreased with decrease in wavelength and was approximately 80% at wavelength near 600 nm. It was previously reported that the optical transparency strongly depends on the surface irregularity [16]. The poor transparency of the film could be ascribed to the surface roughness and existence of pores for the films prepared from 0.06 M zinc nitrate concentration in the electrolytic bath. The high optical transmission suggested that the ZnO films had a smooth surface. Optical band gap was calculated from the absorption edge in the absorption spectra. The inset of Fig. 4 shows the determination of the optical gap from the (ahg)2 versus hg variation. A band gap at about 3.32 eV can be deduced from these measurements and is consistent with the characteristic value of 3.30 eV for undoped ZnO film prepared by sputter deposition technique [17]. 3.5. Surface morphology Fig. 5 shows the surface morphology of ZnO films prepared at various deposition potentials with a constant bath temperature (80 1C). The SEM pictures indicate the
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50
200 (αhδ)2 (a.u)
Transmission (%)
90
0 3.1
10 200
100
400
3.3 E (ev)
600
3.5
800
Wavelenth , λ (nm) Fig. 4. Transmission spectrum of a ZnO layer deposited at potential 1.1 V (SCE) and bath temperature 80 1C. Inset shows the (ahg)2 versus hg plot representing the energy gap of ZnO films.
granular and porous character of the deposited films. The contrast is due to the different orientation of the crystallites. The surfaces of the films exhibit a nodular appearance. The orientation of the ZnO film prepared at 0.8 V (SCE) was the aggregate of hexagonal columns, and some pores were seen between the columns. However, the film prepared between 0.9 and 1.1 V versus SCE shows nodular appearance. The film appears to be dense, and consisting of large and flat hexagonal grains with the c-axis perpendicular to the substrate. Such nodular appearances are characteristics of ZnO growth reported in Ref. [16].
4. Conclusions Transparent conducting ZnO films were prepared by using a simple and inexpensive electrodeposition technique. Experimental observations from CV studies reveal that a window between 0.8 and 1.2 V (SCE) is suitable for the formation of ZnO films. It is observed from the rate of deposition that a maximum value of 1.4 mm min1 is obtained from 0.1 M zinc nitrate concentration. A structural analysis elucidates the polycrystalline nature of ZnO with c-axis orientation, with a preferential orientation along (OO2) plane. Optical characteristics indicate the absorption edge at 375 nm, which corresponds to the optical band gap energy of 3.32 eV.The SEM picture shows a compact structure, which denotes a high density of nucleation centers and the surface has a nodular appearance. The simplicity and
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Fig. 5. SEM pictures of ZnO films deposited at various cathodic potentials at bath temperature 80 1C (a) 0.8 V (SCE), (b) 0.9 V (SCE), (c) 1.1 V (SCE).
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