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Preparation and electrochromic properties of rfsputtered molybdenum oxide films N. Miyata and S. Akiyoshi Citation: Journal of Applied Physics 58, 1651 (1985); doi: 10.1063/1.336307 View online: http://dx.doi.org/10.1063/1.336307 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/58/4?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Electrochromic properties of rfsputtered tungstic oxide film prepared from a W metal target J. Appl. Phys. 66, 4364
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  Preparation and electrochromic properties of rfsputtered molybdenum oxide films N. Miyata and S. Akiyoshi  Citation: Journal of Applied Physics 58 , 1651 (1985); doi: 10.1063/1.336307   View online: http://dx.doi.org/10.1063/1.336307   View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/58/4?ver=pdfcov   Published by the  AIP Publishing   Articles you may be interested in   Electrochromic properties of rfsputtered tungstic oxide film prepared from a W metal target J. Appl. Phys. 66 , 4364 (1989); 10.1063/1.343956 Electrochromic iridium oxide films prepared by reactive sputtering  Appl. Phys. Lett. 35 , 823 (1979); 10.1063/1.90950 Properties of rfsputtered beryllium thin films J. Vac. Sci. Technol. 14 , 210 (1977); 10.1116/1.569124 Luminescence of rfsputtered oxide films during sputtering J. Appl. Phys. 44 , 3817 (1973); 10.1063/1.1662855 Control of Film Properties by rf-Sputtering Techniques J. Vac. Sci. Technol. 8 , S12 (1971); 10.1116/1.1316386 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:137.189.170.231 On: Sun, 21 Dec 2014 23:13:15  Preparation and e~e tro hromi properties of rf-spuUered molybdenum oxide films N Miyata and S Akiyoshi Faculty o Engineering Yamaguchi University Ube 755, Japan (Received 11 February 1985; accepted for publication 24 Apri11985) Properties of rf-sputtered molybdenum oxide films for electrochromic display devices have been investigated. This report mainly concerns the dependence of the properties on the oxygen concentration in the sputtering atmosphere. The oxide films were prepared by rf sputtering from a compressed powder Mo0 3 target under an operating pressure of 4 X 10- 2 Torr using a mixture of Ar-0.5-50 O2, Electrical resistivity of the films formed increases with increasing oxygen concentration in the sputtering atmosphere and ranges from 2.5 X 1010 to 1 X 10 12 {} cm. Spectral transmittance of the films is about 85 in the visible and near-infrared region. Optical band gap and refractive index of these films are 2.67-2.76 eV and 2.01-2.26, respectively. Electrochemichromic properties of the films were also studied using asymmetric cells, and it was found that a good electrochromic performance was obtained usually by the cells composed of the films with a resistivity of 2.5 X 1010 n cm. INTRODU TION Tungsten oxide and molybdenum oxide are important materials for e1ectrochromic display devices. Although many investigations of the tungsten oxide films have been carried out,I-7 there are a few reports on the molybdenum oxide films which have been published concerning e1ectrochromic display devices. 8ã9 Usually, the molybdenum oxide films are prepared by thermal evaporation under a vacuum ofMo0 3 powder. 10.11 However, there are few reports on the oxide films formed by rf sputtering from compressed-powder Mo0 3 target. This paper concerns with the electrical, optical, and electrochromic properties of rf-sputtered molybdenum oxide films prepared from a compressed powder Mo0 3 target. The purpose of this study is to obtain the molybdenum oxide films with the properties required for e1ectrochromic display applications, and especially to obtain the dependence of properties of the oxide films on the oxygen concentration in the sputtering atmosphere during deposition. The dependence of the properties of the rf-sputtered oxide films on the total operating pressure of the sputtering atmosphere will be soon reported. EXPERIMENT The molybdenum oxide films were prepared by rf sputtering from a 80-mm-diam, compressed powder Mo0 3 (purity 99.9 ) target. The experimental apparatus was a rf-diode sputtering system (Daia Vacuum Co.); an oil diffusion pump was used to achieve a pressure of 1 X 10- 6 Torr. The target electrode was set in the upper side of the chamber and was water cooled. Substrates for deposition were soda lime gJlass, A1-coated glass, and indium-tin oxide (ITOl-coated glass and were located on the grounded substrate holder which could be heated. The sputtering chamber was equipped with a stainless-steel shutter that was placed between the target and substrates during evacuation and predeposition. The preparation of films was carried out under the following conditions. The rf power used was 100 W operating at 13.56 MHz, the target-to-substrate spacing was 6.6 cm, and the substrate temperature was 200 ·C, measured by a chromel-alumel thermocouple attached to the substrate holder. n Ar-0 2 gas mixture containing 0.5-50 O 2 was continuously flowing through the sputtering chamber at a constant pressure during deposition. The purity of Ar gas and O2 gas was more than 99.99 and more than 99.8 , respectively. The procedures of sputtering are as foHows: the chamber was initially evacuated to 5 X 10- 6 Torr and then the Ar-02 mixture was introduced through a variable leak valve; the total pressure was controlled by the variable leak valve to maintain a constant operating pressure during deposition. Each sample was prepared using an Ar-02 mixture with a given oxygen concentration. The total operating pressure during deposition was 4 X 10- 2 Torr for all samples. The time of presputtering of the target was 15 min, and the deposition rate was in the 16-47 A/min range, which depended on the O2 concentration. The thickness of samples used was measured with a multiple interferometer (Sloan M-lOO . The resistivity of films was derived from the measurement of /-   characteristics. The spectral transmittance from 0.3 2.3J Lm was measured using double-beam spectrophotometer (Hitachi Works Ltd. 323). X-ray diffraction analysis (Rigaku Denki GFD-2) showed all the films formed in sputtering atmosphere of 0.5-50 O2 to be amorphous. Fabrication of the electrochemichromic (ECC) cells and measurements of those characteristics were carried out as follows. The experimental ECC cells were composed of a molybdenum oxide film deposited on ITO-coated glass with sheet resistance 40 or 300 n /square (Sanyo Vacuum Ind. Ltd.), a 1.0-mm-diam indium wire used as the counterelectrode, and a 2-mm-thick electrolyte. The electrolyte used was IN LiCI0 4 (Electronic grade, Kanto Chern. Co.) in propylene carbonate. The active area of the cells was -1.5 cm2. The coloration of the ECC cells was carried out applying a dc bias voltage of 0.5 V across the cells. The current 1651 J. Appl. Phys. 58 (4), 15 August 1985 0021-8979/85/161651-05 02.40 @ 1985 American Institute of Physics 1651 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:137.189.170.231 On: Sun, 21 Dec 2014 23:13:15  through the cells was recorded on an X Y recorder using a standard resistor 1 n Yokogawa Electric Works Ltd.) connected to the cell in series. The injected charge was measured by a coulomb/ampere hour meter (Hokuto Denlm Ltd. HF- 201). The change in optical transmittance ...t = 5000 A of the experimental cells during coloration was measured by a spectrophotometer using a reference cell without an oxide film and eletrolyte. EXPERIMENT L RESULTS Figure 1 shows the dependence of deposition rate of rf sputtered molybdenum oxide films on the oxygen concentration in a sputtering atmosphere, compared with that of the films prepared in a pure Ar gas atmosphere. Deposition rate was derived from the film thickness and depositing time. Since the sputtering was carried out under constant to tal pressure and with constant input power, the sputtering rate of compressed. powder Mo0 3 target decreases with increasing oxygen concentration. Consequently, the deposition rate of films decreases with increasing oxygen concentration, as shown in the figure. The electrical resistivity at room temperature as a function of the thickness of molybdenum oxide films is shown typically in Fig. 2. The sputtering atmosphere is an Ar-O.5 O 2 mixture, and total operating pressure is 4X 10- 2 Torr. The fiIm thickness ranged from 2500-14300 A, and the resistivity was derived from the measurement of 1- V characteristics of the films deposited on Al-coated glass substrate. General features of the relation between resistivity and thickness for the oxide films prepared under various concentrations of oxygen in sputtering atmosphere are the same as that shown in the figure. The resistivity of films thicker than -4000 A s an almost constant value, about 2.5 X 1010 n cm. The relation between the resistivity of molybdenum oxide films formed and the oxygen concentration in the mix- 80.---~--~----~---.----~--- ;:: - /) ~2 TOTAL PRESSURE 4 X 10- 2 Torr °O~ ~ ~~ ~ ~~ ~ ~ 10 20 30 40 50 60 OXYGEN CONCENTRATION (0lo) FIG. I. Dependence of the deposition rate of molybdenum oxide films on the oxygen concentration in sputtering atmosphere. 1652 J. Appl. Phys. Vol. 58 No.4, 15 August 1985 Ar-0.5°lo02 -2 4xl0 Torr ~ldl E 0 ~ 0 0 .c:. 0 Q 0 >- 0 8 0 0 > ~1d 0 /) 0 w c:: 109~ ~~ ~ ~ ~ ~ o 0.3 0.6 0.9 1.2 1.5 FI LM THICKNESS lJm) FIG. 2. Relation between electrical resistivity and thickness of the rf-sputtered molybdenum oxide films (Ar.().5 O 2, 4 X 10- 2 Torr). ture of Ar and O 2 gas is shown in Fig. 3. Thickness of the samples used is in the range 4800-5700 A. The samples were deposited on AI-coated glass substrate under total operating pressure of 4 X 10- 2 Torr. The resistivity was derived from the measurement of 1- V characteristics. As shown in the fig ure, resistivity of the films formed by rf sputtering depends on the oxygen concentration in the sputtering atmosphere. The resistivity of the films prepared using a mixture of oxygen concentration ranging from 0.5-5 is a constant value, 2.5 X 10 10 n cm, and increases gradually with increasing oxygen concentration in the sputtering atmosphere. For ex-   TOTAL PRESSURE -2 4xl0 Torr FI LM THICKNESS 4800-5700 A l E y E .c:. S ~1011 > 0 0 0 0 0 /) if) 0 0 w 0 0 c:: do 0 0 0 g 10 9 0 1 1 10 100 OXYGEN CONCENTRA nON (0 0) FIG. 3. Dependence of the electrical resistivity of molybdenum oxide films on the oxygen concentration in sputtering atmosphere (4 X 10- 2 Torr). N. Miyata and S. Akiyoshi 1652 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:137.189.170.231 On: Sun, 21 Dec 2014 23:13:15  ample, resistivity of the films formed in the mixture of 50 O 2 is 1 X 10 12 n cm. Figure 4 shows typically the spectral transmittance of as-prepared molybdenum oxide films, compared with that of the films prepared in a pure Ar atmosphere. These films were deposited on soda lime glass substrates under a total pressure of 4 X 10- 2 Torr using an Ar-02 mixture containing O 2 concentration ranging from 0.5-50 . The thickness of samples used ranged from 4700-5100 A. As shown in the figure, all the oxide films prepared are transparent except that of the films formed in a pure Ar atmosphere, and have the average transmittance higher than 85 in the visible and near-infrared regions. The films formed in a pure Ar gas atmosphere are brown colored and conductive. The optical band gap of the molybdenum oxide films was obtained from the absorption data. The energy dependence of a in the region of absorption edge a;;. 10 4 cm -1) is givenasa = B hv - Eof/hv where a is the absorption coefficient, B is a constant, and o is the optical band gap. The data were plotted for the films prepared in the mixture of Ar O 2 containing O 2 concentration ranging from 0.5-50 , and the resultant values of o for each sample were listed in Table I, compared with the value of the films prepared in a pure Ar atmosphere. As seen in the table, the optical band gap of films formed in the mixture of 0.5-10 O 2 concentration is 2.76 e V, and the value of films prepared in the mixture of 30-50 O2 concentration is 2.67 eV. Jt is found that the optical band gap derived depends slightly on O 2 concentration in the sputtering atmosphere, except the remarkably different value, 1.67 eV for the films deposited in a pure Ar gas. The dispersion curve for the refractive index in molyb denum oxide films has been determined from the actual film thickness and the position of the interference fringes. The refractive index of the same oxide films mentioned above was derived from the complete dispersion curve in the wavelength range from 0.45-1.6 f Lm at room temperature, and is also tabulated in Table 1. As shown in the table, the refractive index of the oxide films ranges from 2.09-2.26, and has dependence on the O 2 concentration in the mixture. 80 tj60 z <l: l-   ~4 z <l: cr 20 Figure 5 shows typically the time dependence of the i i ; i I i 0.5 O B 1.1 1.4 WAVELENGTH (iJm) FI LM THICKNESS 4700-5100 A ----- 100 0 '0 Ar Ar-O.So,oOz ----- Ar-l 00f00: -.--- Ar- SOOfoO: 4XI0 '2 Torr 1.7 2.0 2.3 FIG. 4. Spectral transmittance of the rf-sputtered molybdenum oxide films prepared using a mixture of Ar and O 2 gas. The thickness of samples ranged from 4700-5100 A 1653 J. Appl. Phys., Vol. 58, NO.4, 15 August 1985 TABLE I. Optical band gap and refractive index ofthe molybdenum oxide films. Oxygen Films Optical Refractive concentration thickness band gap index (%) A.) (eV) 0 4800 1.67 0.5 5100 2.76 2.17 I 4700 2.76 2.20 5 5000 2.76 2.26 10 4800 2.76 2.09 30 4800 2.67 2.20 50 4800 2.67 2.14 current density and the normalized optical transmittance A. = 5000 A) of the experimental cells during coloration. The molybdenum oxide films used were prepared using an Ar-02 mixture of 0.5-50 oxygen concentration. The thickness of the oxide films are in the range 4800-5500 A, and the films were deposited onto ITO-coated glass substrates with a sheet resistance of -40 {J /square. In electrochemical coloration the asymmetric cells with an indium wire counterelectrode were employed. The external potential of 0.5 V was applied to the cells, and the change of current flowing through the cells and induced optical density was measured simultaneously. As shown in Fig. 5, the time dependence of current density and transmittance of each cell is found to be dependent F 5 > I--  j) Z W o I-- 0 1 z w 0:: 0:: > u 100 w u z « 1--50 :: z j) z « 0:: 1N LiCIO APPL. VOLT. O.SV 0) FIG. 5. Current and optical density time dependence of the cells composed of the molybdenum oxide films prepared using Ar-O, mixture of different oxygen concentration: (a) the relation between current density and coloration time; (b) the relation between normalized optical transmittance and coloration time. N. Miyata and S. Akiyoshi 1653 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:137.189.170.231 On: Sun, 21 Dec 2014 23:13:15
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