Growth and Characterizations of Electrochemically Deposited ZnO Thin Films

Transcription

1 Solid State Phenomena Vols (2007) pp Online available since 2007/Jun/15 at (2007) Trans Tech Publications, Switzerland doi: / Growth and Characterizations of Electrochemically Deposited ZnO Thin Films Yumeji Takashige 1,a, Takuya Nebiki 1, Tadashi Narusawa 1 1 Kochi University of Technology, Tosayamada-cho,Kochi, , Japan a Corresponding author: b@gs.kochi-tech.ac.jp Keywords: ZnO, electrodeposition Abstract. In this study,wepresent ZnO thin films using electrochemical deposition method. ZnO thin films are deposited onto metal(cu) and semiconductor (n-type ) substrates. The electrolyte consists of a 0.1M Zn(NO 3 ) 2 solution, and we applied various potentials at different bath temperatures. XRD shows preferential orientation to (002) that increases with the applied cathodic potential and the bath temperature. milar tendency is shown on both Cu and n-type substrates. SEM micrographs show ZnO surface morphology is greatly affected by the applied cathodic potentials. The RBS analysis reflects the rough morphology of ZnO thin film. The composition ratio Zn:O on n-type substrate is determined to be 1.0:1.3 ± 0.3 at the cathodic potential of -1.0[V] and the cell temperature of 70. Introduction Recently ZnO thin films attractincreasing interests as promising materials for photonic devices such as blue and UV emitters, as well as solar cells and transparent conductive films replacing ITO. This is because ZnO has a wide bandgap energy of around 3.4 ev at RT, and very low-cost and very safety material when compared with other materials. ZnO thin films have been prepared by a wide variety of techniques, sputtering, splay-pyrolysis, molecular beam epitaxy, chemical vapor deposition, sol-gel technique, and electrochemical deposition[1,2]. Among these method, the electrochemical deposition provides several advantages, such as a low equipment cost, a large scale deposition capability, and the minimum ambient impact[1]. In this paper, we investigated the ZnO thin film growth onto copper or n-type substrate by electrochemical deposition[3,4]. The surface morphology of ZnO samples on copper and its dependence on the applied cathodic potential and the cell temperature are investigated by XRD and SEM. The surface morphology and the composition ratio of ZnO thin films on n-type substrates are studied by using RBS. Experimental Procedure ZnO thin films were deposited electrochemically onto several different substrates: poly-crystal copper substrates and n-type silicon substrate(0.02ωcm, The Nilaco Corporation, Japan). Both substrates were cut into small 2 1cm 2 rectangles then first degreased with acetone and rinsed with distilled water[3]. The silicon substrate was rinsed with a 0.5:1:4 HCl:H :H 2 O mixture heated up to 80 in order to remove any trace of heavy metals. Afterwards, the oxide films on the polished face was removed by etching with a 1:5 HF:H 2 O acid solution and thoroughly rinsed with ultra pure water[4]. The Au ohmic contact was made by deposition on polished face of the silicon substrate, and annealed at 500 for 3min in N 2. The electrolytic consisted of a 0.1M Zn(NO 3 ) 2 solution. The anode was 99.9% pure platinum wire, and a Ag/AgCl electrode in saturated KCl was used as the reference electrode. The cell temperature 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, (ID: , Pennsylvania State University, University Park, United States of America-04/06/14,05:31:21)

2 136 Advances in Nanomaterials and Processing was changed from RT to 80 by using a heater. The electrochemical deposition was carried out in a potentiostatic way, the potential control was achieved by a potentiostat/galvanostat. Many samples were prepared with different values of the electrochemical deposition potential (E). This potential was varied between E = -700 and -1500mV and its effect on the resultant samples were studied. Except the cell temperature and the electrochemical potential, all other parameters were maintained constant. Results and Discussion Structural Characterization and Morphology. Fig.1 shows typical XRD patterns of ZnO thin films deposited onto copper substrates. Fig.1(a) shows diffraction spectra of samples grown with the cathodic potentials ranging from -0.7 to -1.3 V, and Fig.1(b) the cell temperature ranging from RT to 80 Each diffraction peak in Fig.1 corresponds to the reflections from (100), (002), (101), (102), (103), and (104) plane of the hexagonal(wurtzite) ZnO crystal. Fig.1(a) shows that the ZnO thin films have a preferential orientation parallel to the (002) plane when the cathodic potential is between -0.7 and -1.1 V. However, when the cathodic potential exceeds -1.2 V, the (002) peak becomes weak and other planes are intensified. Fig.1(b) shows that ZnO peaks are identified when the cell temperature is higher than 40 At 70, the (002) preferential orientation is most prominent, which indicates that this is the best temperature for ZnO growth[5]. Fig. 2 shows that ZnO thin films are grown similarly on n- substrate. (a) [v] -1.2[v] -1.1[v] Cu(111) ZnO(102) Cu(200) ZnO(110) ZnO(103) ZnO(112) ZnO(004) Cu(220) Fig.1XRD spectra of samples onto copper substrate at (a) the cathodic potentials ranging from -0.7 to -1.3 V vs. Ag/AgCl, (b) the cell temperature ranging from RT to [v] -0.9v] and potential is fixed at -1.0 V vs. Ag/AgCl -0.8[v] -0.7[v] (b) Cu(111) ZnO(102) Cu(200) ZnO(110) ZnO(103) ZnO(112) ZnO(004) Cu(220) / Fig.2 XRD spectrum from a sample on n-type substrate. The cathodic potential was -1.0 V vs. Ag/AgCl and the cell temperature was 70

3 Solid State Phenomena Vols Fig. 3 shows that the surface morphologies of resultant samples onto copper substrate with different cathodic potentials. The morphology in Fig.3(a) looks hazy for the sample grown with E = -0.9 V. The morphology in Fig.3(b) looks like an aggregation of the columns, and the vertices shows angular shapes which is close to the hexagonal structure. For higher cathodic potentials the morphology resembles the cauliflower shape, as shown Fig.1(c) for the sample grown with -1.3 V. (a) (b) (c) 10µm Fig 3. Surface morphology of samples grown at electrodeposition potentials E = -0.9 V (a), E = -1.1 V (b), and E = -1.3 V (c). RBS Analysis. Fig. 4 shows RBS spectra obtained from ZnO thin film on n-type substrate by using 2MeV He +. The arrows in Fig.3 show Channel No. for Zn,, and O which appear at approximately 750, 500, 300. The <100> channeling spectrum indicates the O signal somewhat more clearly by decrease of background from the substrate. The low energy tail of the Zn peak were observed for all samples. This is probably because of very rough surface morphology of the film. The interdiffusion at the interface is also likely as a cause of this spectral shape, however, since the rough surface morphology has been already studied by SEM in case of copper substrate, it is natural to assume that the -case has also quite a rough surface morphology. O <Random> <Channeling> 4:39 8 Zn,3304 Fig.4 RBS spectrum of ZnO thin film onto n-type substrate at applied potential -1.0 V, cell temperature 70 by using 2MeV He +.

4 138 Advances in Nanomaterials and Processing The composition of the deposited film on n- substrate is calculated based on the RBS results for the samples grown with cathodic potentials from -0.9 to -1.5 V, and the cell temperature from 60 to 80. The calculated compositions are 1.0:1.3 ±0.3, 1.0:0.8 ±0.3 with -0.9 V and -1.5 V respectively. The estimated error of ±30% came from the fact that the O signal intensity is difficult to estimate because of the background from the substrate. These results suggest that Zn rich films are grown with higher cathodic potential. The results of composition ratio for the cell temperature difference are both 1.0:1.3 ±0.3 for 60 and 80 However, we can not conclude definitely that there is no temperature dependence because the estimated error of ±30% is too large. Finally, the RBS spectrum suggests that the average ZnO film thickness is approximately 0.5 through 1 µm. If we take into account that the deposition time is only a few minutes, the growth rate is very high compared with other deposition techniques. Conclusions ZnO thin films which has the wurtzite structure were prepared by electrochemical deposition. The preferential orientation of the films on Cu substrates is the (002) crystal plane, and such tendency becomes clear with the increase of the cathodic potential from -0.7 to -1.1 V. The morphology of ZnO films surface also changes with the cathodic potential. RBS measurements suggested that the composition ratio of ZnO thin films becomes Zn rich with the increase of the cathodic potential. However, this result can not be concluded firmly because of a large error value that comes from the rough surface morphology. It has been also shown that the electrochemical potential has a great influence on the crystalline quality and the surface morphology of the grown films. References [1] T. Yoshida, D. Komatsu, N. Shimokaswa, H. Minoura: Thin Solid Films Vol. 451(2004), p. 166 [2] M.Izaki, T. Omi: J. Appl. Phys. Lett. Vol. 68(1996), p [3] R. E. Marotti, D. N. Guerra, C. Bello, G. Machado, E. A. Dalchiele: Sol. Energy Mater. Sol. Cells Vol. 82(2004), p. 85 [4] E. A. Dalchiele, P. Giorgi, R. E. Marotti, F. Martin, J. R. Ramos-Barrado, R. Ayouci, D. Leinen: Sol. Energy Mater. Sol. Cells Vol. 70(2001), p. 245 [5] Sophie Peulon, Daniel Lincont: J. Electrochem. Soc. Vol. 145(1998), p. 864

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