Ion irradiation effects on the optical properties of tungsten oxide films

Nuclear Instruments and Methods in Physics Research B 268 (2010) 3151–3154

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Ion irradiation effects on the optical properties of tungsten oxide films S. Nagata a,*, H. Fujita a, A. Inouye b, S. Yamamoto b, B. Tsuchiya a, T. Shikama a a b

Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan Japan Atomic Energy Agency, Takasaki 370-1292, Japan

a r t i c l e

i n f o

Article history: Received 7 October 2009 Received in revised form 21 May 2010 Available online 1 June 2010 Keywords: Ion irradiation Oxygen Tungsten oxide Optical absorption

a b s t r a c t The optical changes in amorphous WO3 film prepared by reactive RF sputtering and irradiated by 200–800 keV oxygen ions were measured to study the relationship between coloration and energy deposition. The color centers were effectively created by ion irradiation with contributions from nuclear collisions and electronic energy loss. The increase in the absorption coefficient was reasonably explained by a first order reaction, whose production rate depended roughly on the total deposited energy. During heat treatment in air atmosphere, transmittance recovery started at 400 K and completed at 550 K. No significant difference was found among films irradiated by different incident energies; therefore indicating that the ion-induced damage structure is not strongly influenced by the type of energy loss. Ó 2010 Elsevier B.V. All rights reserved.

1. Introduction

2. Experimental

Tungsten trioxide (WO3) film is a well-known electrochromic material, which turns blue upon reduction and transparent upon oxidation. WO3 film covered with a thin catalyst layer can be changed reversibly from transparent to deep blue by exposure to gas containing hydrogen. This optical switching behavior serves a wide range of industrial applications, such as smart windows and hydrogen gas sensors [1,2]. Although a variety of models have been proposed to explain the electrochromic and gasochromic phenomena in WO3 film, a general model that explains the coloration mechanism has not been determined. Many experimental results have been reported supporting the various models, based on preparation methods, stoichiometry, the microstructure, film crystallinity, and coloration measurement techniques [3–5]. In contrast to the reversible coloration behavior, ion bombardment causes nonreversible effects in the film, such as the formation of tungsten bronze structures [6–8], color centers [9], and compaction [10]. In addition to electron and proton injection and polaron formation, oxygen vacancies in the WO3 film play an important role in the coloration analogous to F-center formation in ionic crystals [2]. The present study investigates the changes in the optical properties of WO3 film due to ion irradiation to clarify the relationship between ion-induced coloration and energy deposition in amorphous WO3 films prepared by RF sputtering.

Tungsten oxide films of about 400 nm thick were prepared by reactive RF magnetron sputtering using a W metal target (99.9% purity). A mixture of argon (999.999% purity) and oxygen (99.99% purity) gas was introduced through a mass flow controller into a chamber originally evacuated up to a base pressure of 5  104 Pa, adjusting the partial pressure of Ar and O2 gas, to 150 and 20 mPa, respectively. The films were deposited on SiO2 glass and glassy carbon at 473 K with an average growth rate of about 0.1 nm s1. The crystal structure of the film was examined by Xray diffractometry using Cu-Ka radiation, and the chemical composition was measured by Rutherford backscattering (RBS) and elastic recoil detection (ERD). Oxygen ion irradiation was also performed in a scattering chamber connected with a 1.7 MV tandem accelerator at the Institute for Materials Research at Tohoku University. The sample was uniformly irradiated over an area of about 1 cm2 by 200, 350, and 800 keV O+ ions with a particle flux of about 2  1012 cm2 s1 up to a maximum fluence of 1  1016 cm2. After ion irradiation, the optical absorption of the film was measured with a UV–Vis spectrometer at wavelengths between 190 and 900 nm. Isochronal annealing of the sample was performed for 600 s in air atmosphere in a temperature range between 283 and 573 K, and the UV–Vis spectrum was measured after each annealing stage. 3. Results and discussion

* Corresponding author. Address: Institute for Materials Research, Tohoku University, 2-1-1, Katahira, Aoba-ku, Sendai 980-8577, Japan. Tel.: +81 22 215 2062; fax: +81 22 215 2061. E-mail address: [email protected] (S. Nagata). 0168-583X/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2010.05.076

XRD pattern showed that the crystal structure of film prepared at 473 K had an amorphous structure showing a hollow peak at around 2h = 23°. From RBS measurements of film deposited on

S. Nagata et al. / Nuclear Instruments and Methods in Physics Research B 268 (2010) 3151–3154

CE ¼

DOD L ¼ ðac  ab Þ ; Q Q ln 10

ð1Þ

where ab and ac are the absorption coefficients of the film in the bleached and colored states, respectively, Q is the charge injection per unit area corresponding to the optical density change DOD, and L is the film thickness. Assuming that the number of color centers per unit area (Nc) corresponds to the injected charge, we can replace Q with Nc, as given below:

CE ¼ ðac  ab Þ

L Nc  1:6  1019  ln 10

ð2Þ

:

For an estimation of the ion-induced color centers, if we assume a typical value for the coloration efficiency of 50 cm2 C1 [1], we obtain about 4  1016 cm2 color centers for the 350 keV O irradiation to a fluence of 5  1013 cm2, where the change in the absorption coefficient was about 2 lm1. SRIM [12] calculation estimated the number of displacements at about 500 per incident ion, with a displacement energy of 25 eV. This discrepancy suggests smaller displacement energy or the contribution of electronic energy loss to creating the color centers. Fig. 2 shows changes in absorption coefficient averaged over wavelengths from 600 to 900 nm for film irradiated with different energies, plotted as a function of the deposited energy of the incident ions. The 200 keV O+ ions are totally stopped in the film, and

-1

the glassy carbon substrate, the oxygen concentration was estimated at O/W = 3.0 ± 0.05 throughout the film. ERD measurements revealed that the hydrogen was also uniformly contained in the film at levels as high as H/W = 0.4–0.7. Although the chemical composition of the present films was similar to tungsten bronze HxWO3, the films had transparence in the wavelengths between 400 and 900 nm, as shown in Fig. 1. Therefore, the film consists of a large number of water molecules and oxygen deficiencies corresponding to the state of W4+ [11]. The large amount of hydrogen was probably taken from the atmosphere after preparation, because water molecules were retained on the substrate held at 470 K during deposition. Fig. 1 shows optical absorption spectra of the WO3 film between 300 and 900 nm for various incident fluences of 350 keV O+ ions. The absorption coefficient increased in the visible and near infrared wavelengths with an increase in the fluence of the incident O+ ions. The O+ ions induced spectral changes resembling gasochromic changes during exposure to hydrogen gas. In general, no significant differences in the ion-induced changes of the absorption spectra were observed due to 200, 350, and 800 keV O+ ion bombardments. The coloring efficiency in an electrochromic film is defined as

Change of absorption coefficcient ( µm )

3152

5 4 3 2

200 keV 350 keV 800 keV fitted curve

1 0

0

100

200

300

400 14

500

600

2

Total energy loss (10 keV/cm ) Fig. 2. Absorption coefficient averaged over the wavelengths from 600 to 900 nm for the film irradiated at different energies, plotted as a function of the incident ion fluence.

the projected range of 300 keV O+ ions is slightly larger than the film thickness. Although the coloration mechanism is not clarified, a simple phenomenological model can be applied to describe the relation between the energy deposition and coloration. If the color center production rate is proportional to the energy deposition Ed, the number of color centers is

NðEd Þ ¼ N0 ð1  expðk  Ed ÞÞ;

ð3Þ

where k is the production rate of luminescence centers, and N0 is the density of oxygen. Fig. 2 shows a least square fitting to the increase curve, plotted as a function of deposited total energy, where k is estimated to be 1.2  1016 keV1 cm2. In the present experimental condition, electronic and nuclear energy losses in the WO3 film for different incident O+ energies calculated by SRIM [12] is shown in Fig. 3. Only a small fraction of energy was deposited by nuclear collision for 800 keV O+ ions, in contrast to its higher electronic energy loss. To examine the effect of the type of energy deposition on changes in optical absorption, the changes in absorption coefficient were re-plotted as a function of the nuclear and electronic energy loss, as shown in Fig. 4a and b, respectively. Comparing the same nuclear energy deposition, the change in induced optical absorption from the 800 keV O+ ions was most effective, suggesting that electronic energy deposition plays a role in vacancy formation. As shown in Fig. 4b, the curves for different incident energies are close to each other when plotted as a function of the electronic energy loss. A slightly higher absorption observed for

+

350 keV O irradiation

1

electronic 800 keV

1 Energy loss (keV/nm)

Absorbance

14

-2

5 x 10 cm 14 -2 2 x 10 cm 14 -2 1 x 10 cm 13 -2 5 x 10 cm unirradiated

electronic 350 keV electronic 200 keV nuclear 200 keV

0.1 nuclear 350keV nuclear 800keV

0 300

400

500 600 700 Wavelength (nm)

800

900

Fig. 1. Optical absorption spectra of the WO3 film in the wavelengths between 300 and 900 nm for various incident fluences of 350 keV O+ ions.

0

50

100 150 200 250 300 Depth in the WO3 film (nm)

350

Fig. 3. Nuclear and electronic energy loss in the WO3 film for different incident O+ energies calculated by SRIM.

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S. Nagata et al. / Nuclear Instruments and Methods in Physics Research B 268 (2010) 3151–3154

8

4

2 800 keV 350 keV 200 keV 0

0

20

40

60 14

2

-1

Change of absorption coefficient ( µm )

Integrated nuclear energy loss (10 keV/cm ) 6

4

800 keV 350 keV 200 keV

0

0 50 100 150 200 250 300 14 2 Integrated electronic energy loss (10 keV/cm )

Fig. 4. Changes of the absorption coefficient plotted as a function of the nuclear and electronic energy loss for (a) and (b), respectively.

200 keV O+ ions is probably due to the contribution of the larger nuclear energy transfer for vacancy formation. Fig. 5 shows the optical absorption spectra of WO3 film irradiated by 350 keV O+ ions to a fluence of 5  1014 cm2 after heat treatment at various temperatures. The bleaching behavior has no particular structure in the absorption spectra; transmission recovery occurred uniformly in the observed wavelengths between 400 and 900 nm. Note that the changes in the absorption spectra during heat treatment are quite similar to those during ion irradiation, as shown in Fig. 1. Fig. 6 shows changes in the absorption coefficient averaged over wavelengths from 500 to 900 nm for film irradiated by 350 keV O+ ions to various fluences, plotted as a func-

1.5

Absorbance

10 min annealing in air after irradiation + 14 2 350 keV O 5 x 10 O/cm 1.0

6

annealed in air 16 -2 1 x 10 cm

4

2

15

-2

14

-2

as-prepared film annealed in vacuum

2 x 10 cm

1 x 10 cm

0 250 300 350 400 450 500 550 600 650 Annealing temperature (K)

Fig. 6. Changes of the absorption coefficient averaged over the wavelengths from 600 to 900 nm, plotted as a function of the annealing temperature. The absorption in an un-irradiated film during the heat treatment is shown as a solid line.

(b) absorption v.s. electronic energy loss

2

+

800 keV O

-1

(a) absorption v.s. nuclear energy loss

Change of absorption coefiicient ( µm )

-1

Change of absorption coefficient ( µm )

6

as irradiated

tion of annealing temperature. The recovery of optical transmission began at above 420 K, depending on the irradiation fluence of the film. The need for heat treatment at higher temperature for heavily irradiated film was due to the formation of complex defects. No significant difference in thermal bleaching was observed among the films irradiated at different energies. This suggests that the structure of ion-induced damage is not strongly influenced by whether the energy deposition is nuclear or electronic. In case of 200 keV irradiation, implanted oxygen atoms did not show particular structure in the thermal bleach, probably due to their relatively small amount with respect to the concentration of the created color centers. The change in the absorption coefficient of the un-irradiated sample during heat treatment at high vacuum is shown as a solid line in Fig. 6. During heat treatment in vacuum, the increase in absorption is attributed to vacancy formation due to the release of oxygen from the film, in which retained hydrogen atoms are simultaneously released as water. 4. Conclusion The ion-induced changes in the optical absorption of amorphous WO3 films indicates that both nuclear collisions and electronic energy deposition play important roles in creating color centers. The evolution of coloration from 200–800 keV O+ ion irradiation was simply described using a first order reaction, whose color center formation rate correlated with the total energy deposition rate, rather than the nuclear energy loss. Furthermore, the bleaching behavior of the irradiated film during thermal annealing in air atmosphere was independent of the incident ion energy; suggesting that the structure of the ion-induced damage was not strongly influenced by the energy deposition type (nuclear or electronic). Although the ion-induced coloration mechanism has not yet been clarified, this simple relationship between the number of ion-induced color centers and the deposited ion energy can be utilized in optical recording devices using medium- or high-energy ion irradiation.

373 K Acknowledgments

0.5 473 K

This work is partly supported by a Grant-in-Aid for Scientific Research from the Japan Society for the Promotion of Science (Nos. 20360412 and 21656236).

573 K 0.0 300

400

500 600 700 Wavelegth (nm)

800

900

Fig. 5. Optical absorption spectra of WO3 films irradiated by 350 keV O+ ions to a fluence of 5  1014 cm2 after heat treatment at various temperatures.

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