Appl Phys A (2009) 97: 237–241 DOI 10.1007/s00339-009-5209-z

Optical properties of molybdenum oxide thin films deposited by chemical vapor transport of MoO3 (OH)2 Young Jung Lee · Young Ik Seo · Se-Hoon Kim · Dae-Gun Kim · Young Do Kim

Received: 7 July 2008 / Accepted: 4 March 2009 / Published online: 25 March 2009 © Springer-Verlag 2009

Abstract MoO3 thin films are useful for optical devices due to their electrochromic properties. In this study, deposition of MoO2 thin films was successfully accomplished by chemical vapor transport (CVT) of volatile MoO3 (OH)2 . Subsequently, a MoO3 thin film was obtained by annealing of the deposited MoO2 at 400°C in an O2 atmosphere. As annealing commenced, the optical transmittance of the films increased and their absorbance peaks were broadened and redshifted due to reduced oxygen vacancy. Thus, the molybdenum oxide thin films can successfully be deposited using the CVT technique. PACS 78.66.-w · 42.70.-a · 68.55.-a · 78.20.-e · 42.79.-e · 78.40.-q

1 Introduction Transition metal oxides such as V2 O5 , WO3 , MoO3 , and Co3 O4 have attracted considerable attention in the electrochromic (EC) device field; electrochromism is simply defined as a color change caused by an applied bias [1, 2]. In particular, WO3 and MoO3 have been extensively investigated as an electrochromic layer (film) because of their superior optical properties [3–5]. MoO3 films, for example, have fast response times, high coloration efficiency, and long lifetimes [6]. Recently, many deposition techniques have been developed to deposit Mo oxide thin films including Y.J. Lee · Y.I. Seo · S.-H. Kim · D.-G. Kim · Y.D. Kim () Division of Materials Science and Engineering, Hanyang University, Haengdang-dong 17, Seongdong-ku, Seoul 133-791, Korea e-mail: [email protected] Fax: +82-2-22204230

chemical vapor deposition (CVD) [7, 8], evaporation [9], sol-gel coating [6], various wet chemistry methods [10], RF magnetron sputtering [11, 12], and pulsed laser deposition (PLD) [13]. The extraction of metallic Mo usually involves the hydrogen reduction of Mo oxides. The reaction takes place in two distinct stages: MoO3(s) + H2(g) → MoO2(s) + H2 O(g) (1) and MoO2(s) + 2H2(g) → Mo(s) + 2H2 O(g) (2). Reaction (1) occurs over the temperature range 450◦ C to 650◦ C [14–17] and proceeds slowly, yielding intermediate oxides (MoO3−X ) between MoO3 and MoO2 below 600◦ C. Above 600◦ C, sublimation of MoO3 increases. The first stage reaction occurs mainly via chemical vapor transport (CVT) of MoO3 (OH)2(g) where the decomposition of the starting material (MoO3(s) ) results from the formation of MoO3 (OH)2(g) , which is deposited on a nucleus of the product (MoO2(s) ). By this means, the grain morphology of MoO2 is completely reformed [18]. It has been reported that the catalytic effect of copper (Cu) on the hydrogen reduction process of Mo oxides results in accelerated nucleation and subsequent growth of Mo on the Cu surface [18]. The hydrogen reduction of W oxides, especially for the WO2 →W stage with CVT of WO2 (OH)2(g) , was also shown to be strongly affected by Cu acting as a nucleation site. This was confirmed by hydrogen reduction of ball-milled WO3 –CuO powder mixtures and WO3 with coiled Cu wire [19, 20]. Thus, gases MoO3 (OH)2(g) and WO2 (OH)2(g) can be source materials for deposition of metallic and oxide phases of Mo and W on ceramic or metallic substrates by the CVT process, resulting in a kind of CVD using a solid source. Figure 1 is a schematic diagram of CVT deposition of MoO2 thin films. The substrate is buried under the MoO3 powder in a ceramic crucible that is then loaded into a furnace. The specimen is heat-treated at an appropriate tem-

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3 Results and discussion

Fig. 1 Schematic diagram of the deposition on an oxidized Si wafer by chemical vapor transport (CVT)

perature for hydrogen-reduction of MoO3 to MoO2 . At this time, the deposition of MoO2 is accomplished by the transport of MoO3 (OH)2(g) from MoO3 powder onto the substrate surface. In this study, such a CVT process during the hydrogen reduction of MoO3 is suggested as a new technique for the deposition of MoO2 thin films with subsequent annealing in O2 atmosphere for transformation of deposited MoO2 to MoO3 . Furthermore, the optical properties of deposited and annealed Mo oxide thin films are discussed with respect to their microstructures.

2 Experimental procedure MoO3 powder (1–10 μm, 99.9%, JunTec) was used as a raw material to deposit the MoO2 thin film on two kinds of substrates: thermally oxidized Si wafers (average thickness of SiO2 layer was about 500 nm) and fused glass. The substrates were washed by sonication in acetone for 10 min and were then dried using compressed air. To deposit the MoO2 thin film, the substrates were placed under MoO3 powder in an Al2 O3 crucible, which was heated up to 550◦ C at a heating rate of 10◦ C/min. The system was held at 550◦ C for times up to 60 min under a hydrogen atmosphere with a dew point of −76◦ C. The deposited MoO2 thin films were then annealed at 400◦ C in an O2 atmosphere for various times up to 120 min to form MoO3 . To analyze the crystalline structure of the deposited and annealed thin film, X-ray diffractometry (Rigaku RINT 2500/PC, Japan, XRD) was performed. The crystalline sizes of the deposited MoO2 and oxidized MoO3 were calculated by Scherrer’s method [21] with the full width of half maximum (FWHM) values from the XRD peaks. The surface and the cross-sectional morphologies of deposited and annealed thin films were observed by a field emission scanning electron microscope (JEOL 6701F, Japan, FE-SEM). Transmittance and absorbance of deposited and annealed thin films were measured in the range from 300 nm to 900 nm using a UV-Vis spectrophotometer (Hewlett-Packard, S4100, USA).

As presented in Fig. 2(a), the MoO2 thin film was homogeneously deposited on the oxidized Si wafer at 550◦ C with no holding time at a heating rate of 10◦ C/min in a H2 atmosphere. The deposited MoO2 thin film was composed of broadly equiaxed particles that were densely cohered together. When the deposited MoO2 thin film was oxidized at 400◦ C for 60 min with a heating rate of 10◦ C/min, the particles became somewhat large and faceted as depicted in Fig. 2(b). The cross-sectional image of the deposited MoO2 film is presented in Fig. 2(c), which reveals good adhesion of the MoO2 film onto the substrate and a uniform thickness of about 0.16 μm. As shown in Fig. 2(d), the annealing process lead to an increase in the film thickness up to 0.22 μm due to a volume change between MoO2 and MoO3 . Also, the density of the annealed MoO3 film was considerably higher than that of the deposited MoO2 film. Figure 3 shows the changes in the average thickness of MoO2 thin films deposited at 550◦ C for various deposition times. As mentioned above, the thickness of MoO2 deposited at 550◦ C with no holding time was 0.16 μm. By increasing the deposition time, the film thickness linearly increased, reaching 0.96 μm at 60 min or a deposition rate of 0.013 μm/min at 550◦ C. The deposition rate is affected by the supply of source materials when using a solid source for CVD. That is, if the supplied source is exhausted in a batch chamber, the deposition rate would decrease and ultimately deposition would cease. In this study, deposition was continuously accomplished with a linear rate, indicating that sufficient MoO3 powder was provided as a source. The crystalline structures of Mo oxide films, deposited at 550◦ C for various times and subsequently annealed at 400◦ C for 60 min, were analyzed by XRD to confirm the effect of MoO2 film thickness on the formation of MoO3 film by annealing. As depicted in Fig. 4(a), the MoO2 phase in the sample deposited at 550◦ C with no holding time largely transformed to MoO3 by annealing at 400◦ C for 60 min. However, some MoO2 remained in spite of annealing for 60 min in the samples with a thickness greater than 0.25 μm held for more than 5 min at 550◦ C. The XRD peak intensities for residual MoO2 increased with increasing film thickness. Such a dependency of the film thickness on the phase change was caused by the distance required for oxygen diffusion from the supplied atmosphere to the inner film. As indicated in Fig. 4(e), annealing at 400◦ C for 60 min after deposition at 550◦ C for 60 min resulted in the majority of the phase being MoO2 . Thus, to obtain a fully annealed MoO3 film, the annealing temperature should be higher or the annealing time should be extended. Figure 5 demonstrates the change in XRD patterns of MoO2 thin films deposited at 550◦ C with no hold time that were subsequently annealed at 400◦ C for various times. At

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Fig. 2 Surface morphologies and cross-sectional images of deposited Mo oxide films on oxidized Si wafers (a), (c) after deposition at 550◦ C with no hold time and (b), (d) after heat-treatment at 400◦ C for 60 min in an O2 atmosphere

the beginning of annealing at 400◦ C, the peak intensity of the MoO2 phase decreased and the peak for the MoO3 phase appeared. With increasing annealing time, the peak intensity of the MoO3 phase increased, and finally all Mo was composed of the MoO3 phase after 120 min, as indicated in Fig. 5(e). The change of phase ratio as a function of annealing time is provided in Fig. 5, which was induced from comparison of peak intensities for each phase. When the annealing started at 400◦ C, the oxidation to MoO3 took place rapidly at the surface of the as-deposited MoO2 . Consequently, the amount of MoO3 suddenly increased at the beginning of annealing as shown in Fig. 5. However, transformation to the MoO3 phase slowed because the MoO2 available for oxidation was below the surface, inside the film, resulting in an increased diffusion length of oxygen. The average crystalline size of MoO2 as deposited at 550◦ C without holding time was about 28.3 nm, and that of fully annealed

MoO3 (120 min) was around 34.0 nm, as calculated by Scherrer’s equation. Thus, annealing at 400◦ C brings about a phase change resulting only in crystalline volume change from MoO2 to MoO3 without growth. Figure 6 indicates the optical transmittance and absorbance spectra of various Mo oxide films. Interestingly, the samples’ color changes with annealing time. The MoO2 film as deposited at 550◦ C with no hold time was scarcely transparent; the MoO2 thin film was deposited on both sides of the glass because the glass was embedded in the raw powder (MoO3 ) in this study. However, the Mo oxide films became transparent by the phase change from MoO2 to MoO3 with increasing annealing time. As the annealing time increased, the transmittance band for the thin film broadened and a red-shift was observed. In particular, for samples annealed for over 30 min, the transmittance rapidly increased in the infrared region and the absorbance over

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Fig. 3 Thickness changes in MoO2 thin films at 550◦ C for various deposition times on oxidized Si wafers

Fig. 4 XRD patterns of Mo oxide thin films deposited at 550◦ C with various deposition times after oxidation at 400◦ C for 60 min. Hold times at 550◦ C are (a) 0 min, (b) 5 min, (c) 15 min, (d) 30 min, and (e) 60 min

∼500 nm gradually decreased. The color of the Mo oxide films changed from violet to bright gray as presented in Fig. 6(a). Gesheva and co-workers [22–24] have investigated the optical and electrochromic properties of mixed MoO3 –WO3 thin films and pure MoO3 and WO3 thin films made by atmospheric pressure CVD. Their thin films were generally amorphous in the as-deposited state. After annealing, absorption in pure metal oxides decreases, while in the mixed oxide films it significantly increases. Structural transformations from the amorphous to crystalline phase cause an

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Fig. 5 XRD patterns of Mo oxide thin film deposited at 550◦ C with no hold time and heat-treated 400◦ C in an O2 atmosphere for (a) 0 min, (b) 15 min, (c) 30 min, (d) 60 min, and (e) 120 min

increase of the energy band gap values for MoO3 [22]. Reddy et al. [25] studied the optical properties of MoO3 thin films made by a thermal evaporation technique under high vacuum. According to their report, thermal annealing at a suitable temperature could change the state of crystallinity. They also noted that oxygen vacancies could be created during deposition by thermal evaporation, and electrons could be trapped in such oxygen vacancies. Other papers have also reported that surface oxygen vacancies are the most abundant surface donors and trap electrons [26]. In this study, the quantity of oxygen vacancies decreased by increasing the crystallinity of Mo oxide thin film during the phase change from MoO2 to MoO3 by annealing. Because of the decrease in oxygen vacancies, electron trapping also diminished during the photon transportation. Therefore, the increase in transmittance resulted from the phase change from MoO2 to MoO3 .

4 Conclusions Mo oxide thin films were successfully fabricated by CVT of volatile MoO3 (OH)2(g) during hydrogen reduction at 550◦ C for various holding times with subsequent annealing under O2 at 400◦ C for various maintaining times. The MoO2 thin film revealed good adhesion and was homogeneously deposited at 550◦ C at a deposition rate of about 0.013 μm/min. The average crystalline sizes of the deposited and annealed (400◦ C for 120 min) films were 28.3 nm and 34.0 nm, respectively. Phase change from MoO2 to MoO3 was confirmed by XRD and was found to be a function of annealing time and film thickness of the

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in transmittance was considered to be the effect of crystallinity resulting from the phase change and subsequent decrease in the oxygen vacancies. Acknowledgements This work was supported by the Korea Research Foundation Grant funded by the Korean Government (MOEHRD, Basic Research Promotion Fund) (KRF-2006-311-D00990).

References

Fig. 6 The optical transmittance and absorption spectra of deposited Mo oxide thin films on glass after deposition of 550◦ C with no hold time and heat-treated at 400◦ C under an O2 atmosphere. Heat-treatment times in O2 are (a) as deposited (no O2 treatment), (b) no hold time (c) 15 min, (d) 30 min, (e) 60 min, and (f) 120 min

as-deposited film because the phase change requires the diffusion of oxygen from the supplied atmosphere to the inner film. The transmittance band broadened and red-shifted with increasing annealing time, and the thin films simultaneously became more transparent. Transmittance increased rapidly in the infrared region while the broadening absorbance peak, observed over ∼500 nm, gradually decreased. This increase

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