Nano-mechanical and structural study of WO3 thin films

Thin Solid Films 606 (2016) 148–154

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Nano-mechanical and structural study of WO3 thin films Jose L. Enriquez-Carrejo a,⁎, Manuel A. Ramos a, Jose Mireles-Jr-Garcia a, Abel Hurtado-Macias b a b

Departamento de Física y Matemáticas, Instituto de Ingeniería y Tecnología, Universidad Autónoma de Cd. Juárez, Avenida del Charro 450 N, Cd. Juárez, Chihuahua, C.P., 32310, México Centro de Investigación en Materiales Avanzados S.C., Laboratorio Nacional de Nanotecnología, Miguel de Cervantes 120, Complejo Industrial Chihuahua, Chihuahua, C.P., 31109, México

a r t i c l e

i n f o

Article history: Received 23 July 2015 Received in revised form 31 January 2016 Accepted 24 March 2016 Available online 2 April 2016 Keywords: Flexible gas sensor Tungsten oxide Nanoindentation Thin film Confocal Raman imaging, GIXD

a b s t r a c t We present results of nano-mechanical and spectroscopic measurements of tungsten oxide (WO3) thin films fabricated by high vacuum DC magnetron sputtering as a step in the search for flexible gas sensors. In order to understand the mechanical properties at the nano-scale, the thin film samples were subjected to nanoindentation measurements in continuous stiffness measurement (CSM) mode, obtaining higher values for hardness (H) and elastic modulus (E) in samples annealed at 500 °C as compared to as-deposited samples (room temperature), within three depth regions of interest. Scanning electron microscopy (SEM) was used for morphological characterization. To determine their crystalline structure, grazing incidence X-ray diffraction (GIXD) analysis was performed on the films. An amorphous structure was determined for the as-deposited sample. To reach a final conclusion about the structural phases present in the material, spectroscopic measurements using confocal Raman spectroscopy and mapping were carried out, yielding two regions with different spectral characteristics within the annealed sample and enabling the visualization of those spatial domains. These measurements confirmed the polycrystalline nature of the annealed film, evidencing a triclinic (space group, P1) and a non-conventional monoclinic (space group, P21/c) structures. © 2016 Elsevier B.V. All rights reserved.

1. Introduction Thin films of tungsten oxide (WO3) have endured as a topic of substantial study due to their widespread applications in electrochromic devices, photocatalysis and gas sensors [1–7]. Their evolving sensor technology has a direct use at clean energy generation facilities like coal gasification plants, where gases may accumulate during operations, leading to potential health hazards as well as risk of explosion. A particular gas of interest is hydrogen sulfide (H2S), which is colorless, odorless, highly toxic and potentially explosive. At concentrations above 100 ppm, it is considered as immediately dangerous to life and health (IDLH) by the Occupational Safety and Health Administration (OSHA) of the U.S. Department of Labor [8]. Prospective applications of WO3 in H2S detection have been extensively reported before [9–13]. Accordingly, its physical and chemical properties must be comprehensively understood. It is well known that WO3 exhibits structural phase transformations, which are mostly dependent on the annealing temperatures [14–16]. Previously, the continuous crystallization process with substrate temperature was observed and studied by Raman and infrared spectroscopies [17]. Furthermore, the dependence of grain size with substrate ⁎ Corresponding author. E-mail addresses: [email protected] (J.L. Enriquez-Carrejo), [email protected] (M.A. Ramos), [email protected] (J. Mireles-Jr-Garcia), [email protected] (A. Hurtado-Macias).

http://dx.doi.org/10.1016/j.tsf.2016.03.054 0040-6090/© 2016 Elsevier B.V. All rights reserved.

temperature at deposition was established by scanning electron microscopy (SEM) and X-ray diffraction spectroscopy (XRD) and a preferred orientation along (002) was found [18]. Moreover, it has been determined by first principles calculations that surface stabilities of (010), (001) and (110) in h-WO3 are improved by adsorption of some non-metal atoms like H, N, O, F, Cl, Br, and I [19]. One of the emerging engineering applications is the design and production of flexible sensors suitable for insertion into irregular settings like fabrics. Therefore, the need to determine the nanomechanical properties of WO 3 thin films is essential. Although, flexible devices based on WO 3 have been successfully fabricated before [20,21], there is still much research to be done in order to achieve a fully working flexible gas sensor. Here, we present a series of experiments from thin film deposition using DC magnetron sputtering, Raman spectroscopy and imaging, scanning electron microscopy (SEM), and nanoindentation employing continuous stiffness measurement (CSM) method on as-deposited WO3 thin films as well as films annealed at 500 °C to determine their structural and mechanical properties. 2. Experimental details 2.1. DC magnetron sputtering Tungsten oxide thin films were obtained by DC magnetron sputtering using Si (100) wafers as substrates. The wafers were

J.L. Enriquez-Carrejo et al. / Thin Solid Films 606 (2016) 148–154

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previously cleaned using the “piranha etch” method. A 99.99% pure WO3 disk (Kurt Lesker) was employed as target for deposition. The process was performed at room temperature under high vacuum. The base pressure was set to 4 × 10− 5 Torr before allowing Ar into the chamber as the plasma source. A 10 min pre-sputter sequence with closed shutter was performed to remove impurities from the target. Voltage and current were controlled and monitored by the instrument in order to reach and sustain a predetermined deposition rate. The wafer was then cut into several smaller pieces (samples). Some samples were subjected to annealing treatment at 500 °C in air for 3 h with a ramp down of ~ 10 h. Two sets of samples were obtained: the as-deposited thin films, processed at room temperature and the films annealed at 500 °C which will be named W-RT and W-500, respectively.

incidence angle was fixed at 0.5° and the scanning angle was varied from 20° to 80° with a 0.02° step size.

2.2. Nano-mechanical properties

By using CSM, continuous functions of hardness and elastic modulus vs. surface penetration depth can be obtained. Therefore, the CSM option is especially useful for evaluating thin films on substrates where the mechanical properties change as a function of surface penetration. Fig. 1 shows the hardness behavior as function of penetration depth of the representative WO3 thin film samples and that of the Si (100) substrate as reference. In this figure, three regions are described: region I corresponds to the hardness of the films (0 to ~ 30 nm penetration depth) where there is no influence of the substrate; this is a wellestablished criterion in the literature [27–29]. Accordingly, the hardness of the WO3 coating ranges between H = 13.74 ± 0.2 GPa for the W-RT sample and H = 15 ± 0.5 GPa for the W-500. In region II the hardness of the films is partially influenced by the substrate, here the hardness values of both samples decrease sharply with depth. Lastly, in region III the hardness is completely influenced by the substrate, i.e., the penetration depth is very near or at the Si substrate. The hardness values in this region are close to the theoretical value of the Si substrate. The elastic modulus measurements present a similar behavior. Fig. 2 shows the elastic modulus as function of penetration depth of the WO3 samples. Again, the graph divides into three regions. The measurements yielded a value of E = 164 ± 5 GPa for the W-RT sample, increasing with penetration depth in region I and stabilizing in region II. The W-500 sample exhibits a very stable value of E = 193 ± 1 GPa in region I, which decreases in regions II and III. These results suggest that the as-deposited thin films would suit better for flexible devices, because of the improved values of their elastic modulus. However, the annealed samples should not be discarded as other aspects must be taken into account as will be explained below.

Nano-mechanical properties, in this case, hardness (H) and elastic modulus (E) of WO3 thin films were evaluated by nanoindentation using the continuous stiffness measurement (CSM) method. A Nano-Indenter G200 coupled with a DCM II head was employed. The equipment was calibrated using a standard fused silica sample. The constants of area function were C0 = 24.04, C1 = − 182.34, C2 = 6523.20, C3 = − 25,456.44, and C5 = 17,603.60. A Berkovich diamond indenter with a tip radius of 20 ± 5 nm, depth limit of 140 nm, strain rate of 0.05 s− 1, harmonic displacement and frequency of 1 nm and 75 Hz, respectively, and Poisson's coefficient of ν = 0.25 was used. Residual indentation of samples was recorded by the AFM Nano Vision system attached to the nanoindenter system. On the other hand, for the measurement of the nano-mechanical properties of the Si (100) substrates the Oliver and Pharr method with controlled cycles [22] was used. The basic analysis of nanoindentation load–displacement curve (P–h) was established based on the elastic contact theory given by Sneddon [23] and Doerner et al. [24], yielding the nano-mechanical properties of the Si (100) substrates as: hardness H = 12.66 ± 0.2 GPa and elastic modulus E = 179 ± 2 GPa. In order to calculate hardness and elastic modulus of WO3 thin films, the following method was used: the CSM option enables a continuous measurement of elastic stiffness (S) during loading and not just at the point of initial unload [25,26]. This is achieved by superimposing a small oscillation on the primary loading signal and analyzing the resulting response of the system through a frequency-specific amplifier. The equation to determine the S in the CSM test is 1 1 1 : S ¼ F 0 cosφ ðK s mω2 Þ K f Z0

2.5. Raman spectroscopy Raman scattering and imaging measurements were performed with a WiTec Alpha 300 RA confocal Raman system, equipped with a 532 nm Nd:YAG laser. A 100 × 0.9 NA objective was employed. No special sample preparation was needed for the measurements. 3. Results and discussion 3.1. Mechanical nanoindentation

ð1Þ

In a CSM experiment, the excitation frequency, ω, is set. The displacement amplitude (Zo), phase angle (φ), and excitation amplitude (F0) are measured if the machine parameters load-frame stiffness, Kf, the stiffness of the support springs, Ks, and mass m are known. 2.3. Scanning electron microscopy A survey of the surface microstructure of the films was accomplished using a JEOL JSM-7000F field emission analytical scanning electron microscope. Energy-dispersive X-ray spectroscopy (EDS) elemental analysis was executed with a Hitachi S5500 field emission scanning electron microscope. 2.4. Grazing incidence X-ray diffraction (GIXD) The GIXD experiments were executed using a Panalytical X-Pert system, employing Cu Kα radiation at 40 kV and 35 mA. The diffracted beam path included a graphite flat crystal monochromator. The grazing

Fig. 1. Hardness vs. penetration depth of the as-deposited (W-RT) and annealed (W-500) WO3 thin films.

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Pc phases, it suggests that a coexistence of different phases is present in the sample. Nonetheless, because of that similarities, it becomes challenging to unequivocally determine the correct mixture from these data; therefore, a Raman study was performed. 3.4. Raman scattering and imaging analysis

Fig. 2. Elastic modulus vs. penetration depth of the as-deposited (W-RT) and annealed (W-500) WO3 thin films.

3.2. Microstructural analysis by scanning electron microscopy Using scanning electron microscopy it was possible to determine a uniform thickness of ~ 112 nm for WO3 thin films processed at room temperature (as-deposited) as shown in Fig. 3 (left). Fig. 3 (right) portrays a film thickness of ~136 nm for the sample annealed at 500 °C. The surface examination of the W-RT sample did not reveal any evidence of crystallite formation, as depicted in Fig. 4 (left). Whereas in the W-500 sample, it was possible to observe a granular microstructure with grain sizes in the range of ~60–1100 nm, as shown in Fig. 4 (right). Grain growth arises during the annealing treatment in air [30]. The chemical composition revealed by EDS analysis is presented in Fig. 5 confirming the presence of tungsten and oxygen atoms on the surface.

3.3. Grazing incidence X-ray diffraction analysis The crystalline structure of the samples was analyzed by GIXD. The W-RT sample was determined to be amorphous. Meanwhile, the diffractogram of the W-500 sample is presented in Fig. 6, where a remarkable preferred orientation along (002) is observed. After the corresponding analysis, four possible phases were identified: the triclinic P1 (PDF: 010830947), monoclinic P21/c (PDF: 010880550), monoclinic Pc (PDF: 010872393), and monoclinic P21 /n (PDF: 010830950). Since the Bragg peaks for the P21/n and P1 phases are very similar and the same happens to the peaks for the P2 1 /c and

It is expected that the properties of thin films of a particular material exhibit variations depending on the crystal structure of that material [31–34]. With the purpose of identifying the structural phase(s) present in the WO3 thin films, their Raman spectra and images were analyzed. Fig. 7 shows a graph of the normalized Raman spectra of the WO3 samples processed at room temperature (gray) and annealed at 500 °C (blue), along with a spectrum of the Si (100) substrate (black) for comparison. In order to minimize the locality of confocal Raman single point spectra, the measurements were performed over an area of 15 × 15 μm2 and then the data averaged. All the spectra were smoothed using a Savitzky–Golay filter [35]. The bottom spectrum (black) clearly exhibits the 84, 306, and 525 cm−1 peaks as vibrations related to the Si substrate, i.e., Si–O and Si–Si modes, along with the weak features around 432 and 623 cm−1. The W-RT spectrum (gray) reveals weak and broad features around 680 and 808 cm−1 which confirm the amorphous nature of the sample, but with a tendency towards crystallization. The W-500 spectrum (blue) shows a strong peak at 811 cm−1 and a slightly weaker and broader band peaking around 714 cm−1. Those two vibrations correspond to the stretching ν(O–W–O) modes. The peak at 272 cm− 1 is ascribed to the bending δ(O–W–O) mode. The peak at 135 cm− 1 is a lattice mode [36]. These data are in good agreement with previous work [17]. All the vibrations, especially the 135 cm− 1 peak, suggest a monoclinic or triclinic structure for the W-500 sample. However, there exists an inherent risk of error if an assignment to a specific crystal structure was to be realized because, as stated before, this spectrum was averaged from thousands of point spectra over a large microscopic area. To be able to provide a more accurate statement, confocal Raman mapping measurements were performed on the area of interest. An image produced by the Raman mapping technique allows the visualization of the spatial distribution of domains within a measured area in which a predetermined characteristic molecular vibration is present. Those domains can be processed with a user-defined false-color, with brightness representing the peak intensity, so that differentiation between two or more vibrations is perceptible. The Raman image obtained from the W-500 sample is shown in Fig. 8 (left). The domains that presented a strong 714 ± 5 cm−1 vibration were highlighted with red and the domains that contained a strong 811 ± 5 cm− 1 vibration were highlighted with green. The observed spatial distribution of the two domains is noteworthy; it is

Fig. 3. SEM micrographs of WO3 thin films cross sections exposing their thickness. Left: WO3 processed at room temperature (as-deposited). Right: WO3 annealed at 500 °C.


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Fig. 4. Left: Surface micrograph of W-RT sample; no evidence of crystallite formation is seen. Right: Microstructure of WO3 thin film processed at 500 °C.

possible that this indicates the presence of two different structural phases or distinct crystallographic orientations, as the domains rarely overlap. To verify the hypothesis we need to visualize the Raman spectrum at each domain. Single point spectra were taken on two random spots, one red and one green. Fig. 8 (right) displays the spectra resulting from those measurements. The red spectrum (right), corresponding to a red domain (left), looks pretty similar to the W-500 averaged spectrum (blue) in Fig. 7; however, there are some noticeable features (emphasized by arrows). First, the appearance of the 197 cm−1 peak, which is very close to the previously observed peak at 203 cm−1 that was attributed to a stressed-monoclinic phase [17,37]. It is important to mention that this peak does not appear at every red spot and does not appear in any green spot and that it is broad and its peak maximum moves in the 196–202 cm−1 region. Second, the broad peak around 714 cm−1 from the blue spectrum in Fig. 7, actually, seems to be a peak at 710 cm−1 with a shoulder at ~ 719 cm−1 in the red spectrum in Fig. 8. Then, the previous 135 cm−1 peak weakens and broadens to show a shoulder at ~ 140 cm−1, which suggests the presence of the monoclinic ε-phase [37]. Lastly, the weak features around 682 and 644 cm−1 become more visible; these peaks were attributed, again, to the monoclinic ε-phase by Cazzanelli et al. [38] who presented a Raman spectrum which agrees very well with our “red” spectrum. Based on these findings, we conclude that the “red” domains have the monoclinic ε-phase, which was early determined by Salje et al. to have a Pc space group [39,40] and later refined to a P21/c space group by Xu et al. [41], being the latter the thermodynamically stable phase at high pressure [42,43].

In previous experiments [17] we reported a stressed monoclinic phase in similar WO3 thin films, but lacked space group identification. The presence of the ε-WO3 phase at room temperature and atmospheric pressure is not common, since it is expected at temperatures below − 40 °C [44–46] and pressures between ~ 0.09 and 3.0 GPa [39,41,47–49]. However, there are reports that show that this phase was also found in powder form at room temperature after heavy mechanical treatment [38]. This could explain why it is found in thin films, as the nanometric scale in one of the film axes produces a unidirectional confinement effect in the material or, as Cazzanelli et al. suggested [38], the ε-phase is favored by the strain produced by a high concentration of polarons in the lattice. To the extent of our knowledge, this is the first time the monoclinic P21/c structure is experimentally identified in a stable WO3 thin film at room temperature and at atmospheric pressure. In the case of the green spectrum (right), corresponding to a green domain (left), it reveals an evident change in the relative intensities of the 811, 714, 272 and 135 cm−1 peaks with respect to the red spectrum. The intensity of the 811 and 135 cm−1 peaks increases notably, while the intensity of the 714 and 272 cm−1 peaks decreases dramatically; there are also “green” regions where one or both peaks completely disappear. This spectrum could belong to the monoclinic P21/n (γ-phase) or to the triclinic P1 (δ-phase) structures, since their Raman peaks are also very similar in the measured frequency region. Moreover, the coexistence of these two phases at room temperature has been reported before [50]. Also, the mixture of the P21/n and Pc symmetry phases was observed by Arai et al. [14]. However, Souza et al. [49] argued that the mixture was improbable because the P21/n phase transforms to a stable P1 under mild pressure which can only be reversed by a new annealing treatment [38,39,46]. Besides, the transition from P 1 to P21/c was confirmed as partially reversible [49]. Taking into account the above statements and that the crystallographic orientation in some crystals could have an effect on the intensity, and even on the frequency, of the Raman peaks [51–54], we suggest this spectrum (green) belongs to a highly oriented triclinic P1 phase, as it is the thermodynamically stable structure at room temperature and at atmospheric pressure [41]. This is in good agreement with our GIXD results. In order to verify the accuracy of the observed spatial domains a second Raman image was obtained on the same area with red representing the 272 cm−1 vibration and green representing the 135 cm−1 vibration; an extremely similar image was obtained validating the previous data. 3.5. The search for flexible gas sensors

Fig. 5. EDS profile showing the chemical composition of the film confirming the presence of tungsten and oxygen on its surface.

The measurement of the nano-mechanical properties of the WO3 thin films and their relation to the structure is just a step towards the

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Fig. 6. Diffractogram of the W-500 sample obtained by GIXD. The inset shows an amplified view of the 22–25° (2θ) region. The PDFs of the matching structures are shown at the bottom.

achievement of a fully working flexible gas sensor; nevertheless, there are still several issues to overcome. For instance, the as-deposited sample showed better mechanical properties, but it is amorphous, so it is expected to have less sensitivity than its crystalline counterparts which need a post-deposition annealing treatment or be deposited at a substrate temperature above 200 °C. A balance between crystallinity and good mechanical properties must be found. The choice of flexible substrate poses another challenge. Firstly, the annealing/deposition process could damage a polymeric substrate due to the relatively high temperature. Also, the optimal working temperature for a WO3-based

H2S (and other dangerous gases) sensor was found to be above 200 °C [55,56] which, again, discards some types of polymers to be used as substrates. Film adhesion to the substrate should also be considered. However, there are viable candidates like some commercial polyimide films which can withstand a working temperature around 300 °C and have excellent mechanical and electrical properties. Lastly, the final film should not show any piezoelectric response due to the nature of the sensor. Even though this list is not comprehensive, the issues discussed here are essential for the correct operability of the final product. 4. Conclusions Tungsten oxide thin films deposited by DC magnetron sputtering and annealed at 500 °C showed a magnitude increase in their nanomechanical properties, particularly hardness and elastic modulus, as compared to as-deposited thin films (room temperature). The values obtained for these properties in region I are substrate-independent. SEM micrographs revealed a micro/nano-grain morphology in the annealed sample and no noticeable grain formation in the asdeposited sample. GIXD measurements yielded an amorphous structure for the as-deposited films and the coexistence of at least two structural phases in the W-500 sample, which showed a remarkable preferred crystallographic orientation along (002). Confocal Raman spectroscopy and imaging confirmed the amorphous structure in the as-deposited samples and a mixture of two structural phases (monoclinic P21/c and

Fig. 7. Raman spectra of WO3 annealed at 500 °C (blue), WO3 as-deposited (gray), and Si (100) substrate (black). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

triclinic P1) for the annealed film. The Raman images allowed us to visualize those two main spatial domains with different structures. Some issues must be addressed first in order to achieve a fully viable WO3-based flexible gas sensor as discussed above. We suggest more research to be conducted using commercial polyimide films as substrates and an annealing temperature (or substrate temperature at deposition) of ~300 °C. However, it is noteworthy to mention that there are alternative approaches to achieve crystalline thin films at/near room temperature, e.g. UV radiation treatment and high-power impulse magnetron sputtering (HiPIMS). The use of those techniques applied to flexible substrates for gas sensors would require further thorough studies.


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Fig. 8. Left: Raman image of WO3 annealed at 500 °C; 811 cm−1 frequency (green) and 714 cm−1 frequency (red). Right: Raman spectra of WO3 thin film annealed at 500 °C. The red spectrum belongs to a red domain and the green spectrum to a green domain. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Acknowledgments This work was partially supported by CONACyT-México grant Fordecyt-Doctores 174509. Authors thank Kleberg Advanced Microscopy Center and Department of Physics at The University of Texas at San Antonio for access to EDS equipment and facilities.

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