Ashraf Abdel Haleem 1, 2) *, Masaya Ichimura 1)

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2 1) Ashraf Abdel Haleem 1, 2) *, Masaya Ichimura 1) Department of Engineering Physics, Electronics and Mechanics, Nagoya Institute of Technology, Gokiso, Showa, Nagoya , Japan 2) Department of Engineering Physics, Faculty of Engineering, Fayoum University, Egypt Abstract Aluminum oxide thin films were successfully deposited onto transparent conductive fluorine-doped tin oxide (FTO) glass for the first time through a facile single-step potentiostatic electrochemical deposition. The precursor solution was deionized water containing aluminum sulfate and sodium thiosulfate. The Auger electron spectroscopy, scanning electron spectroscopy, and optical transmission and thickness measurements were performed for the deposited films. The films are highly transparent, and O/Al composition ratio is nearly unity. Keywords aluminum oxide, thin films, electrochemical deposition In recent years, aluminum oxide (Al 2 O 3 ) films have attracted more and more interest due to their excellent physical and chemical predominance [1, 2], such as high dielectric constant (~9), large band gap (8.7 ev), high field strength (6~8 MV/cm) and the stability of chemical and thermal properties [3]. Thus it is promising in variety applications; including microelectronic, optical application, protection layer and magnetic head recording. In particular, aiming at cost reduction, the industrial fabrication processes of silicon solar cells point to the application of thinner silicon wafers. With the reduction of wafer thickness, the influence of recombination at the surfaces becomes more and more relevant for the cell efficiency. In order to reduce the recombination losses at the surfaces, several passivation techniques were developed. Al 2 O 3 thin films provide excellent surface passivation quality on crystalline silicon [4, 5] due to both chemical and field effect passivation mechanisms. 1

3 So far, a lot of methods were utilized to prepare aluminum oxide thin films, such as chemical vapor deposition (CVD) [6], plasma enhanced chemical vapor deposition (PECVD) [7], pulsed laser deposition (PLD) [8], DC reactive magnetron sputtering [9], atomic layer deposition (ALD) [2], solvothermal deposition [10], as well as sol-gel deposition [11]. In the present study, aluminum oxide (AlO x ) thin films are deposited, to the best of our knowledge, for the first time by the electrochemical deposition (ECD) technique. ECD is a very low-cost deposition technique and suitable for large-scale deposition. 2. Experimental Aluminum oxide thin films were deposited onto transparent conductive glass substrates (FTO-glass) using the potentiostatic electrochemical deposition from an aqueous solution containing aluminum sulfate (Al 2 (SO 4 ) 3 ) and sodium thiosulfate (Na 2 S 2 O 3 ). It is worth mentioning that sodium thiosulfate is necessary to deposit homogeneous and uniform films. Films have been successfully deposited under two different conditions A and B. In condition A, films were deposited from an aqueous solution that contained 10 mm of Al 2 (SO 4 ) 3 and 200 mm of Na 2 S 2 O 3 at normal ph value (~4.5) and room temperature under the application of DC voltage equal to -1.2 V with respect to the saturated calomel electrode (SCE) as a reference electrode for deposition time equal to 15 minutes. In condition B, films were deposited from an aqueous solution containing 10 mm of Al 2 (SO 4 ) 3 and 100 mm of NaO 2 S 3 at normal ph value (~4.6) at an elevated temperature 60 o C under the application of DC voltage equal to -1.0 V vs. SCE for 3 minutes as a deposition time. Hereinafter, sample-a and sample-b refer to films deposited from condition A and B, respectively. It should be noted that a tough precipitation occured when aluminum sulfate was added to DI water due to the formation of insoluble aluminum salt (aluminum hydroxide). Therefore, the solutions of aluminum sulfate and sodium thiosulate were prepared separately. The aluminum sulfate was added to a hot water (80-90 o C) with a vigorous stirring until the solution become clear and completely transparent. The solution was left to cool down to room temperature before mixing it to the sodium thiosulfate solution. The compositional analysis and surface morphology of the deposited films were measured by Auger electron spectroscopy (AES) using the model JEOL JAMP 7800 Auger microprobe at probe voltage 10 kv and current 2x10-8 A. An argon-ion etching with acceleration voltage 3 kv and current 20 ma was used to sputter the film surface. The optical transmission 2

4 measurement was performed using the JASCO U-570 UV/VIS/NIR spectrometer with the FTO substrate as the reference. Profile meter Accretech, Surfcom-1400D, was used to measure the thickness of the thin film keeping the scanning speed at 0.06 mm/s. 3. Results and discussion In order to detect the suitable potentials for deposition, cyclic voltamogramm (CV) scans were performed for conditions A and B under the application of linear sweep voltage in the range of +0.5 and -1.5 V vs. SCE. Figure 1 shows the CV of condition A, and that of condition B has almost the same behavior (not shown). The figure revealed that the suitable voltage bias lies between -1.0V and -1.25V vs. SCE. The reduction peak appeared at -1.45V vs. SCE could be due to the hydrogen evolution. Figure 2 shows Auger electron spectroscopy measurements for sample A. Almost the same spectrum was obtained for sample B. The measurements revealed that each film consisted of Al and O elements. The contents of aluminum and oxygen elements in each sample were estimated using the standard relative sensitivities of elements [12]. The obtained O/Al ratio equals 1.1 for both samples A and B. This means that the as-deposited films were not stoichiometric Al 2 O 3 but AlO x with x 1. Although the precursor baths contain sulfur source (Na 2 S 2 O 3 ), the deposited films are sulfur-free. As mentioned above, the sodium thiosulfate was necessary to get uniform and homogeneous films: films deposited from a Na 2 S 2 O 3 -free precursor bath were too thin for any characterization and did not cover the substrate uniformly. Chowdhury et al. reported that they electrochemically deposited gallium sulfide-oxide from a bath containing Na 2 S 2 O 3 but could not deposit gallium oxide from a Na 2 S 2 O 3 -free bath [13]. This could be due to the catalytic activity of thiosulfate ions as a reducing agent that facilitates the reduction of the metal ions. Another possibility would be that aluminum sulfide was formed firstly and then oxidized in water to form aluminum oxide. It is well known that aluminum sulfide material is sensitive to moisture, hydrolyzing to hydrated aluminum oxides/hydroxides [14]. The surface morphology measurements, shown in Fig. 3, revealed that the deposited films are uniform with fine grains and completely cover the substrate without voids or cracks. The two films have almost the same surface morphology except the aggregated white particles that are distributed nonuniformly at the surface of sample B. the optical transmission 3

5 measurements of samples A and B are shown in Fig. 4. It is worth mentioning that both the samples have almost the same thickness (~50 nm). The optical transmission of sample A in the visible and UV regions equals ~80% and that of sample B equals ~70%. It seems that the relatively rough regions appearing at the surface of sample B adversely affected the optical transmission. Since there is no absorption edge in the UV-visible range, the energy band gap of the deposited films is larger than 4 ev. Since the substrate (FTO-glass) has a smaller band gap, we cannot estimate the band gap of the film from transmission. Finally, the electrical resistivity was roughly estimated based on the deposition current and the applied potential. The deposition current decreased with time during the deposition, which would be mainly due to the ohmic potential drop in the deposited film. The estimated resistivity value is in the range of ~10 8 cm, which is much lower than those reported for Al 2 O 3 (>10 15 cm) [15]. This could be due to the nonstoichiometric (Al-rich) composition of the present film. Accordingly, a separate study for the postdeposition heat treatment in open air or oxygen atmosphere is needed to produce stoichiometric Al 2 O 3 and increase the electrical resistivity. 4. Conclusions Electrochemical deposition is a very cost-effective technique and it is suitable for large scale deposition. In the present study, for the first time, aluminum oxide thin films were cathodically deposited onto FTO-coated glass from an aqueous solution containing aluminum sulfate and sodium thiosulfate. The presence of sodium thiosulfate was necessary to deposit homogeneous and continuous films. The AES measurement confirmed pure aluminum oxide films, and O/Al ratio is nearly equal to unity. The scanning electron microscopy measurement showed a fine grains and continuous films without voids or cracks. The optical transmission measurements showed wide-band gap (>4 ev) and highly transparent films. References [1] S. F. Szymanski, P. Rowlette, and C. A. Wolden, J. Vac. Sci. Technol. 2008; 26:1079 [2] X. Li, Q. Chen, L. Sang, L. Yang, Z. Liu, Z. Wang, Physics Procedia 2011;8: [3] Yo-Sep Min, Y. J. Cho, and C. S. Hwang, Chem. Mater. 2005;17: [4] T. Lu der, G. Hahn, B. Terheiden, Energy Procedia 2011;8:

6 [5] F. Werner, W. Stals, R. Görtzen, B. Veith, R. Brendel, J. Schmidt, Energy Procedia 2011;8: [6] A. R. Chowdhuri, C. G. Takoudis, R. F. Klie, and N. D. Browning, Appl. Phys. Lett. 2002;80:4241. [7] P. Tristant, Z. Ding, Q. B. Trang Vinh, H. Hidalgo, J. L. Jauberteau, J. Desmaison, and C. Dong, Thin Solid Films 2001;390: 51. [8] C. Cibert, H. Hidalgo, C. Champeaux, P. Tristant, C. Tixier, J. Desmaison, A. Catherinot, Thin Solid Films 2008;516: [9] K. Koski, J. Holsa, P. Juliet, Thin Solid Films 1998;326: [10] X. F. Duan, N. H. Tran, N. K. Roberts, R. N. Lamb, Thin Solid Films 2010;518: [11] W. Zhang, W. Liu, Q. Xue, Materials Research Bulletin 2001;36: [12] S. Mroczkowski, D. Lichtman, J. Vac. Sci. Technol. A 1985;3: [13] S. Chowdhury, M. Ichimura, Japanese Journal of Applied Physics 2009; 48: [14] R. J. Wehmschulte, P. P. Power, J. Am. Chem. Soc. 1997;119: [15] M.D. Gronera, J.W. Elama, F.H. Fabreguettea, S.M. George, Thin Solid Films 2002;413:

7 Fig. 3. Scanning electron microscopy measurements for samples A (a) and B (b). Fig. 4. Optical transmission measurements for samples A and B. 6

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9 Figure 1: 8

10 Figure 2: O Al 9

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12 Figure 4: 11

13 Graphical abstract Aluminum oxide thin films were cathodically deposited onto FTO-coated glass from an aqueous solution containing aluminum sulfate and sodium thiosulfate. The deposited films possess good surface morphology and high optical transmission. 12