Preparation and electrochromic properties of rfsputtered molybdenum oxide filmsN. 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

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Preparation and e~ectrochromic properties of rf-spuUered molybdenum oxide films

N. Miyata and S. Akiyoshi Faculty of 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 Mo03 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 1012 {} 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.

INTRODUCTION 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.8 9 Usually, the molybdenum oxide films are prepared by thermal evaporation under a vacuum ofMo03 powder. 10.11 However, there are few reports on the oxide films formed by rf sputtering from compressed-pow-der Mo03 target.

This paper concerns with the electrical, optical, and electrochromic properties of rf-sputtered molybdenum ox-ide films prepared from a compressed powder Mo03 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 Mo03 (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. An Ar-02 gas mixture containing 0.5-50% O2 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 /- V 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 LiCI04 (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

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aHighlight

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.

EXPERIMENTAL 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 Mo03 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% O2 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 is 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.---~--~----~---.----~---,

;:: -(/)

~ ~20

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-0.5lo02 -2 4xl0 Torr

~ldl E 0

~ 0 0 .c:.

"- 0

Q 0

>- 0 8 0 0 I->

~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% O2, 4 X 10-2 Torr).

ture of Ar and O2 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 1010 n cm, and increases gradually with increasing oxygen concentration in the sputtering atmosphere. For ex-

ld 3 TOTAL PRESSURE -2 4xl0 Torr FI LM THICKNESS 4800-5700 A

ld E y E .c:. S

~1011-> 0

0 0 I- 0 0 (/) if) 0 0 w 0 0 c::

,do 0 0 0 g

109 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).

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ample, resistivity of the films formed in the mixture of 50% O2 is -1 X 1012 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 O2 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;;. 104 cm -1) is givenasa = B (hv - Eof/hv, where a is the absorption coef-ficient, B is a constant, and Eo is the optical band gap. The data were plotted for the films prepared in the mixture of Ar-O2 containing O2 concentration ranging from 0.5-50%, and the resultant values of Eo 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% O2 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 O2 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 O2 concentration in the mixture.

80

tj60 z

on the oxygen concentration in the sputtering atmosphere in which the oxide films are prepared. The cells composed of the films prepared using a mixture of Ar-0.5-5% O2 exhibit good response characteristics of coloration. When a dc vol-tage is applied, the current flowing through these cells rises initially to a maximum and then decreases with increasing time. The transmittance of these films decreases rapidly with increasing time and reaches the electrochromic steady state. The elapsed time to the steady state is approximately 200 sec. But, the current density of these ceJJs does not reach a steady state even after 1000 sec. On the contrary, the transmittance of the films formed in the mixture of Ar-50% O2 decreases slowly with increasing time.

The rdation between the optical density (A. = 5000 A) and the injected charge of the experimental ceUs is shown in Fig. 6. The oxide films used were prepared on ITO-coated glass with - 4011 /square using a mixture of Ar-0.5-50% O2 and their thickness ranged from 4800-5500 A. In the elec-trochemical coloration, a dc potential of 0.5 V was applied between the molybdenum oxide-coated electrode and the counterdectrode of the cells.

As seen in Fig. 6, the electrochemical coloration char-acteristics of the molybdenum oxide films is also dependent on the oxygen concentration in the sputtering atmosphere, and the oxide films prepared in an Ar and 0.5-5% O2 atmo-sphere are electrochemically colored in deep blue. Each film formed in the mixture of Ar and 0.5-5% O2 shows almost the same Jjnear relation up to - 1.0 o.d., and the optical density of these films into which the charge of 50 mC/cm2 was injected, is more than 1.0. On the contrary, the films formed in Ar-30-50% O2 atmosphere were not colored in

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1.5

...J 1.0 u I-0.. o

0.5

.100%Ar o Ar-0.5%02 b. Ar- 1 "1002 D Ar- 5 %Oz

v Ar-l0%02 .. Ar-30o/.0z Ar-50 /o~

TOTAL PRES. -2 4xl0 Torr

1 N LiCl04 APPL. VOLT.

0.5 V

50 laO INJECTED CHARGE (rrC/cm2 )

150

FlG. 6. Injected charge vs optical density of the molybdenum oxide films prepared using Ar-O, mixture of different oxygen concentration.

1654 J. Appl. Phys., Vol. 58, NO.4, 15 August 1985

deep blue, and nonlinear relation between optical. density and injected charge was observed in the initial stage of color-ation. In addition, the films formed in a pure Ar atmosphere were hardly electrochemicalIy colored. And also, it has been suggested that an optimum sputtering condition exists for the preparation of rf-sputtered films for electrochromic ap-plications.

DISCUSSION The electrical and optical properties of the rf-sputtered

molybdenum oxide films depend on the oxygen concentra-tion in the sputtering atmosphere.

The oxide films with a resistivity of2.5 X 1010 11 em and 2X 1:011-1 X 1012 11 cm can be prepared in an atmosphere containing 0.5-10% O2 and 20-50% 2, respectively. These films also can be electrochemically colored, and their colora-tion characteristics are dependent on the oxygen concentra-tion in the sputtering atmosphere.

As shown in Figs. 5 and 6, the oxide films prepared in an Ar and 0.5-5% O2 mixture have good electrochromic prop-erties, and the films prepared in an Ar and 50% O2 mixture have poor electrochromic properties. In particular, the for-mer films are electrochemically colored to a deeper blue col-or than the latter films.

As seen in Fig. 5, the coloration current flowing through the cell composed of the films with good electroch-romic properties is larger than that of the cell composed of the fUms with poor electrochromic pro~:rtie5,aJtllOu.gh these celts lJsed hav~~ the Sltme structure and dimelll,ions, em.ployed m(llybdemmt oxide tUrns with almost the same thickrw%, a.nd were opt:!rated under the s~,me bias voltage.

Furth.ennor-e, it is observed that the optical density of thl; cdb with good c{)ion:l.tion characteristics reaches an op-tic~tI. stead.y sta:te,whewas the cells with poor coloration characteristics are gradually colored. It is believed that these differences in the electrochromic properties are mainly due t(l the d.ifference of the diffusion coefficient of Li + through good or poor el.ectrochrornic molybdenum oxid.e films .

Although it will be difficult for the molybdenum oxide lilrns to be e~valuated. as good or poor electrochromic materi-als from only these physical properties, the oxygen partial pressure or the absolute oxygen concentration of the atmo-sphere in which the oxid.e films are prepared seems to be rel.ated. to the properties, and appears to playa dominant role in their col.oration chara(:teristics .

CONCLUSIONS It is concluded that tbe physical. and e1ectrochemichro-

mic properties of the rf-sputtered molybd.enum oxide films, 4800-5700 A thick, deposited on the substrate at 200 C un der a total. pressure of 4 X 10- 2 Torr in the atmosphere of Ar-O2 mixtures containing oxygen concentration of 0.5-50%, are as fonows.

(1) The electrical and optical properties of molybdenum oxide films depend on the oxygen concentration in the sput-tering atmosphere.

(2) The electrical resistivity of oxidic films formed in-creases with increasing oxygen concentration in the atmo-

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sphere. The films formed in the atmosphere of Ar-02 con-taining oxygen concentration ranging from 0.5-10% have a resistivity of - 2.5 X 1010 [J cm. But, the films formed in the atmosphere of Ar-02 containing oxygen concentration rang-ing from 20-50% have a resistivity of 2X 1011-1 X 1012 [Jcm.

(3) Spectral transmittance of the oxide films with 4700-5100 A thick prepared using a mixture of Ar-O.S-SO% O2 is higher than 85% in the visible and near-infrared regions. The optical band gap of these films ranges from 2.67-2.76 e V, depending on the oxygen concentration in the sputtering atmosphere, and their refractive index is 2.09-2.26.

(4) Electrochemichromic properties of the oxide films are dependent on the oxygen concentration in the Ar-02 atmosphere.

(5) The films with a resistivity of -2.SX 1010 [J cm, prepared in the mixture of O2 concentration ranging from 0.5-5% exhibit good response characteristics, and can be electrochemically colored deep blue. The optical density of these films was measured as greater than 1.0 when a charge of SO mC/cm2 was injected. However, the films with a resis-tivity of -1 X 1012 [J cm, prepared in the mixture of O2 con-centration of 50% exhibit poor e1ectrochromic properties.

1655 J. App/. Phys., Vol. 58, No.4, 15 August 1985

ACKNOWLEDGMENT The authors would like to thank Mr. O. Matsumoto for

critical comments and useful discussions.

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