Key Engineering Materials Vols. 602-603 (2014) pp 871-875 © (2014) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/KEM.602-603.871
Characteristics of Zinc Oxide Film Prepared by Chemical Bath Deposition Method Yenpei Fu a and Jianjhih Chen Department of Materials Science and Engineering, National Dong-Hwa University, Hualien 97401, Taiwan a
[email protected]
Keywords: Chemical bath deposition, ZnO thin film, Solar cell
Abstract. In this study, ZnO films, prepared by Chemical Bath Deposition (CBD), are applied as the conductive layers for thin film solar cells. Zinc acetate is used as a source of zinc, and different proportions of ammonia solution are added and well mixed. The growth of zinc oxide films in reaction solutions is taken place at 80oC and then heated to 500oC for one hour. In this study, the different ammonia concentrations and deposition times is controlled. The thin film structure is Hexagonal structure, which is determined by X-ray diffraction spectrometer (XRD) analysis. Scanning electron microscopy (SEM) is used as the observation of surface morphology, the bottom of the film is the interface where the heterogeneous nucleation happens. With the increase of deposition time, there were a few attached zinc oxide particles, which is formed by homogeneous nucleation. According to UV / visible light (UV / Vis) absorption spectrometer transmittance measurements and the relationship between/among the incident wavelength, it can be converted to the energy gaps (Eg), which are about 3.0 to 3.2eV, by using fluorescence spectroscopy analysis. The emission of zinc oxide films has two wavelengths which are located on 510nm and 570nm. According to Based on the all analytic results, the ammonia concentration at 0.05M, and the deposition time is 120 minutes, would obtain the conditions of ZnO films which is more suitable for applications of conductive layer material in thin film solar cell. Introduction Undoped ZnO, which has hexagonal wurtzite structure, is a kind of II-VI group N-type semiconductor material. At room temperature, it has a direct energy gap (Eg) 3.3eV, and the electron binding energy is 60meV [1]. It’s mainly applied to solar cells, light conductor, sensors (photoconductor), photocatalysts, optoelectronic devices [2-[4] and other fields. Since the energy gap of ZnO (Eg = 3.2 ~ 3.3eV) is higher than the visible light (Eg = 3.1eV), the energy of visible light can not excite the electrons from the valence band to the conduction band through the energy gap. In the visible range of light spectrum, the zinc oxide has a high transmittance and can be applied to transparent conductive film. The light wavelength and photon energy relationship have been calculated [5]. The relationship between optical wavelength and photon energy can be expressed by following equation:
λ=
c hc 1240 = = (nm) v hv hv(eV )
where λ is wavelength (in nm), c is light speed in vacuum, v is Frequency, h is Planck constant, eV is Electric voltage Zinc oxide is a kind of semiconductor at room temperature and usually doped with different elements, such as aluminum (Al), gallium (Ga), and indium (In), in order to replace the positions of Zn2+ or fill into the interstitial sites of the zinc oxide lattice of the crystal to increase the carrier concentration and enhance the conductivity [6]. In this study, zinc oxide film, prepared by chemical bath deposition, was first immersed in the solution containing zinc ions and ammonium ions of the complex ion solution. Seed layer then was formed on the surface, then chemical bath method was All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP, www.ttp.net. (ID: 134.208.4.209-04/03/14,10:23:34)
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applied to grow the seed layer of zinc oxide films under the conditions of atmosphere and low temperature. The effects on the properties of zinc oxide film, fabricated by the different parameters, were discussed.
Experiment Procedures ZnO thin film. Zinc oxide (ZnO) films were prepared by chemical bath deposition method, and the starting materials are zinc nitrate (Zinc Nitrate) Zn(NO3)2.6H2O, ammonia (ammonia, NH4OH). Sodium glass was used in experiments as substrates. After 30 minutes of boiling in deionized water, the substrate then placed in acetone and alcohol, and ultrasonic vibration was introduced for 20 minutes. The substrate was washed by using deionized water to rinse. After the procedure mentioned above, high pressure air spray gun was using to remove the surface dust and water. Bath deposition solution was using 200 ml of deionized water 0.1 M of zinc nitrate and ammonia concentrations were controlled at 0.1M, 0.05M and 0.01M. when the configuration was complete, the substrate was put in and followed the set of different deposition time (30 to 120 minutes, every 30 minutes as a group) to pick up specimens. During the process, instruments were controlled in order to maintain a constant temperature at 80oC and continue to control the mixing speed at 50 rpm. After deposition, the use of deionized water to clean and dry surface under the atmosphere condition from room temperature to 500oC for 1 hour heat treatment, the heating rate was 1oC / min. Characteristics. X-ray diffraction (Rigaku D/MAX-2500V, CuKα, scanning range 20o ~ 80o) was used as the film structure and composition analysis. Scanning electron microscope (Hitachi S-3500H, Japan) observed the film surface appearance. UV / visible spectrophotometer (LAMBDA 750 UV / Vis / NIR Spectrophotometer) determined the film in the wavelengths of sunlight (300~800nm) for the transmission. Fluorescence spectrometer (Fluorescence Spectrometer) was used as the light sources, and a wavelength of 380nm light excited the sample, and then got the emission light of zinc oxide. Results and Discussions Reaction mechanic. In this study, the mechanism of formation of ZnO thin films is a continuous ionic layer adsorption and reaction type (SILAR). First, zinc nitrate was dissociated in water to form the zinc ions and nitrate ions, the reaction have to be carried out in an base environment. Ammonia was added, hydrolysis of ammonium (NH4+) and hydroxyl (OH-), at this time, is much like reaction formula (1), the balance of product ion Zn(OH)2 will be more than soluble Zn(NO3)2 product, so the solution is white liquid [7, 8]. Zn(NO3)2 +2NH4OH → n(OH)2 + 2NH4+ + 2NO3-
(1)
In chemical bath deposition method, the addition of ammonia is to restore the Zn2+, so that the root of ammonia (NH4+) ions would combine to form Zn (NH3) n2+ (n = 1~4, for the most stable ionic compounds n = 4), The formation of ZnO thin films can be divided into homogeneous (homogeneous) nucleation and heterogeneous (heterogeneous) nucleation. This reaction (2) is presented as following formula: Zn(OH)2 + 4NH4+ → Zn(NH3)n2+ + 2H2O + 2H+
(2)
In general, the reaction rate will be faster by adding a high concentration of zinc ions and ammonium ion of the precursor. So that homogeneous nucleation occurs and precipitates, generating a pellet of ZnO powder. Therefore, a lot of powder deposited on the specimen in a short period of time, so the quality of films is poor due to homogeneous nucleation and growth of films. After Zinc ion hydrolyzed, there will be forming metal complexes with ammonia (metal complexes), while the water will dissociate hydrogen ions (H +) and hydroxide ions (OH-). Reaction can be presented as (3) following:
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H2O→H++OH-
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(3)
Zn(NH3)n2++2OH-→ZnO+4NH4+H2O (4) Reaction (4), after heat treatment, zinc ions and ammonium ions form metal complexes with hydroxide ions from the ion hydrolysis reaction, and zinc oxide film. In this study, a fixed heating temperature and controlling of the deposition time are used as the analysis of various thin film quality under different concentration of ammonia during the preparation. Structure anlaysis. Fig. 1 presents the XRD pattern of ZnO films, which were deposited for 60,120 minutes after the heat treatment at 500oC for an hour. ZnO films prepared by chemical bath deposition at room temperature are wurtzite hexagonal structure. Its (100), (002), (101) diffraction peaks are more pronounced [9]. The amount of zinc oxide formation will be more with the increasing ammonia concentration which can be observed through the three diffraction peaks. So, after the heat treatment, it will increase the crystallinity and the diffraction peak will be more obvious. By increase of the deposition time, as it can be seen in Fig. 1(b), the diffraction patterns at (002) diffraction peak is slightly increased compared to Fig. 1(a) can be found at (002) position for the preferred orientation of zinc oxide surface by the accumulation.
Fig. 1 XRD patterns of ZnO films after heat treatment. Deposition time: (a) 60 min; (b) 120 min Surface morphology. In Fig. 2, with the ammonia concentration increased, the deposited ZnO on glass substrate by heterogeneous nucleation of zinc oxide deposits which is beneficial to the formation of zinc oxide thin film. However, prolonging deposition times, zinc oxide particles above the films began to nucleate homogeneously and to pack or fill the holes. Homogeneous nucleation will cause a greater accumulation of white particulate precipitation. In the process of filling holes in the intrinsic of the film can become much dense. Optical properties. Fig. 3 is the light transmission of ZnO thin film, deposited on sodium glass substrate, measured by UV-vis spectroscopy. Thus, it shows that the transmittance is better while the film exposes in the visible light range (400 ~ 800nm). It indicated a significant absorption peak (absorption edge) When the UV wavelength is about 380nm while transmittance decreased. With the increase of ammonia concentrations, the absorption peak appealed a red shift (365nm → 380nm). When Ammonia concentration is 0.05M, and the deposition time is 120 mins, light penetration value is 75% upward. This ZnO film
Fig. 2 ZnO films after heat treatment with different concentrations of ammonia, respectively (a,b) 0.01M (c,d) 0.05M (e,f) 0.1M with different deposition time, respectively (a,c,e) 60 minutes (b,d,f) 120 minutes
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in the long wavelength area has better light transmission, and in the short wavelength area (350 ~ 400nm) has a lower transmission capacity. The absorption (α) of relationship of ZnO band gap (Eg) may be obtained from following equation [10]:
k (hν − Eg ) n / 2 hv where k is a constant, Eg is the energy gap (Energy gap, eV as its unit), h v is the photon energy, n is a constant (n = 1 or 4), when n=1, the material is the direct band gap semiconductor known from the literature [9], (n = 4 for the indirect band gap semiconductor). The absorption coefficient is α (cm-1), and the film transmittance (T%) uses UV-visible spectrometer, it can be calculated by this relationship:
α=
α=
ln(1 / T ) d
where d is the film thickness (cm), T is transmittance (%). Plotting (αhv)2- h v as a curvature presented in Fig. 3, the intersection of curve tangent with the X-axis, is the energy gap (Energy gap) value.
Fig. 3 The transmittance measurements of ZnO films, which are all deposited 120 minutes, with wavelength range 350-800nm
Fig. 4 Deposition time 120 minutes. The energy gap of different concentration of ammonia
In the penetration experiment, zinc oxide can be found when the wavelength is near 380nm, there will be a high absorption value. In the fluorescence spectrometer, hence, we have to apply a light source with 380nm wavelength as exciting light to incident zinc oxide sample, The obtained results show the emission light wavelength is about 510nm and 570nm and a synthesis of light [11]. Fig. 5 shows that when the higher concentration of ammonia solution is involved, there is more zinc oxide deposition and attached to the substrate. When excited with an incident light, the electron and hole pairs Fig. 5 Fluorescence spectroscopy of 120 (Electron-hole pairs) will be more. According the minutes deposition time. contrast of relative intensity, the higher concentration of ammonia can be speculated that formation of zinc oxide adsorbed on the substrate is higher.
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Conclusion In this experiment, chemical bath deposition is used to fabricate zinc oxide thin films. The glass substrate immersed in the reaction solution that ion layer adsorption on the substrate is continuously occurred. The heterogeneous nuclear film has formed on surface. As time gradually increase, the zinc oxide particles accumulated on (002) surfaces while homogeneous nucleation happened. X-ray diffraction shows that the structure of ZnO thin films is the wurtzite structure, the diffraction signal could determine that prolonged deposition of zinc oxide would chose (002) surface as the preferred orientation for stacking, causing (002) diffraction peak with relative increase. First, ammonium ion will be provided, and zinc ion binding metal complex form from the SEM diagram. When the ammonia concentration is higher, the probability of the formation of metal complex is increased, and absorption in the substrate surface is beneficial to the formation of layers, combined to form pellet-like structure. But the ammonia concentration can not be too high; otherwise it would directly form a white powder of zinc oxide precipitations, causing white powder deposition on the bottom of the reaction solution. Optical properties of ZnO thin films are using UV / visible spectroscopy for light transmittance measurements. The incident range is 350 ~ 800nm, the deposition time is fixed at 120 minutes and the ammonia concentration is to be 0.05M. The obtained average penetration is the highest, and the incident wavelength and absorption using the relation[10], calculated by the concentration of 0.05M ammonia, it’s energy gap is 3.22eV. Fluorescence spectroscopy with an excitation light source 380nm excited these three different concentrations of ammonia in which the deposition time are the same, 120-minute. The films of zinc oxide, was found in the wavelength of 510nm and 570nm. In optical generation, the synthesis of these two emissions light range of wavelength is green light.
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