Tungsten Oxide Thin Films for Electrochromic Applications: Pulse Width‐Controlled Deposition by High‐Power Impulse Magnetron Sputtering

Tungsten oxide (WO3) thin films have been of prime interest among electrochromic materials because of their chemical stability, strong adherence to various substrates, and high coloration efficiency. High‐power impulse magnetron sputtering (HiPIMS) holds great potential in fabricating durable WO3‐based electrochromic layers. However, the tungsten target–plasma interactions in reactive‐HiPIMS deposition of WO3 and their role in modulating the electrochromic function of the resulting WO3 coatings are yet to be understood. Herein, by controlling the HiPIMS pulse length, the stoichiometry of tungsten oxide structures can be tuned to optimize the transparency and electrochromic function of the coatings. X‐ray photoelectron spectroscopy data shows that at pulse lengths shorter than 85 μs, the concentration of suboxide compounds is less than that of tungsten trioxide, while for pulse lengths longer than 100 μs, this balance is reversed. The average optical transparency of the coatings in the range of visible light is higher than 80%. The optical transmittance modulation (ΔT) of 38.1, 36.2, and 34.3% and coloration efficiency of 41.3, 38.4, and 35.9 cm2 C−1 are measured for the WOx samples deposited at pulse lengths of 70, 85, and 100 μs, respectively. Tuning the HiPIMS pulse characteristics is a simple strategy to deposit tungsten oxide films with tuned electrochromic properties for an array of applications, from smart windows to wearable displays.


Introduction
In recent years, transition metal oxide thin films have been prepared by various physical and chemical deposition techniques, providing a range of favorable optical and electrical properties due to their stoichiometry and structure.[6] Among these techniques, reactive magnetron sputtering offers the most precise control over the ratio between oxygen and metal atoms in the thin-film structure by varying the deposition parameters. [7,8][11] In HiPIMS, ionized deposition fluxes are created by high-power pulses and can comprise 40-80% of the sputtered target material in an ionized state. [12]19][20] Argon is often used as the unreactive gas and when mixed with varied concentrations of oxygen as the reactive gas, oxidized compounds in the form of thin films can be produced under highly ionized fluxes and enhanced energies of the bombarding ions. [21]The high electron density in this process enhances the dissociation of molecular oxygen present in the deposition chamber and improves the reactivity, leading to the deposition of films with controllable stoichiometry. [22]he oxygen index indicates the excess or deficiency of oxygen in the metal oxide structure and is a useful descriptive parameter.[31] The obtained physical properties suit the metal oxide films to a variety of applications, including photovoltaic solar cells, [32] gas sensors, [33,34] thin-film transistors, [35][36][37] light-emitting diodes, [38,39] and chromogenic devices. [40,41]hromogenic property is the reversible ability to change color in materials in response to external stimuli. [42,43]Chromogenic properties encompass thermochromic, photochromic, mechanochromic, hydrochromic, and electrochromic (EC) properties. [44,45][48] This controllable feature has important applications in smart windows, displays, antiglare mirrors, electronic skin, and adaptive camouflage, among others. [49,50]58] Recently, we have shown the excellent potential of the reactive HiPIMS technology in creating ITO-free dielectric-metaldielectric (DMD) structures using tungsten oxide and silver nanothin layers: WO 3 -Ag-WO 3 . [41,47]The critical advantage of HiPIMS deposition over other conventional physical vapor deposition methods lies in the enhanced adhesion of the deposited layers to the substrate.This quality proves highly efficient for almost any engineering application, in particular for EC devices and windows.Despite this proven success, the tungsten targetplasma interactions in reactive-HiPIMS deposition of WO 3 and their role in modulating the EC function of the WO 3 coating are yet to be understood.In reactive HiPIMS deposition using a tungsten target, an oxidized tungsten layer forms on the target during the pulse-off time, and it is sputtered during the high voltage on time.As such, the pulse characteristics, and in particular the pulse length, play a crucial role in determining the tungsten oxide film properties and functions.
In previous work, we discussed the process of depositing layers of tungsten oxide and hafnium oxide using the reactive HiPIMS process. [12]We observed that the cyclic changes in the composition of the target surface throughout this process are influenced by the target properties, particularly its affinity for the reactive gas mixture.In this study, we elucidate the influence of pulse width on the stoichiometry of WO x coatings created by reactive HiPIMS with a focus on its role in regulating the EC properties of the coatings for the fabrication of EC devices.
This study aims to shed light on the role of pulse length on the chemical and optical properties as well as the EC function of tungsten oxide coatings.The surface chemistry of the thin films deposited using varied pulse lengths is evaluated using X-ray photoelectron spectroscopy (XPS), and their optical properties are studied via spectroscopic ellipsometry and UV-vis spectroscopy.Finally, the role of pulse length on EC properties of tungsten oxide layers on ITO-coated glass is elucidated using cyclic voltammetry (CV) measurements.

High-Power Impulse Magnetron Sputtering (HiPIMS)
A reactive HiPIMS system (AJA International 1800F, USA), schematically illustrated in Figure 1, was used to deposit WO x (2 < x < 3) coatings on silicon wafer, glass slide, and ITO-glass substrates.This system is described in greater detail previously. [9,12,59]A tungsten target was loaded on the magnetron inside a vacuum chamber evacuated by a turbomolecular pump (base pressure < 5 Â 10 À7 Torr).The magnetron was run in the unbalanced mode.Pulses of different duration from 50 to 200 μs at a fixed frequency of 150 Hz were applied utilizing a RUP7 pulsed power supply (GBS Electronik GmbH, Germany), delivering constant voltage pulses of 926 V and various maximum current pulses in the range from 8.5 to 31.0 A. Tungsten oxide coatings were deposited with an oxygen flow rate fraction [O 2 /O 2 þ Ar] of 80% (argon flow rate = 4 standard cubic centimeters per minute (sccm), oxygen flow rate = 16 sccm), while the total gas pressure was kept constant at 1.8 mTorr.Table 1 shows the main parameters of plasma discharge in HiPIMS for constant oxygen gas flow rate fraction of 80%.

X-ray Photoelectron Spectroscopy (XPS)
XPS measurements were performed using a SPECS (FlexMode) spectrometer to investigate the chemical composition of the thin films.An Al Kα source (1486.7 eV) was used, operating at 200 W (10 kV, 20 mA).Initially, survey spectra (0-1000 eV) of samples were taken at 30 eV pass energy, 0.5 eV resolution, and 90°t ake-off angle.Subsequently, high-resolution tungsten W4f spectra were acquired at 20 eV pass energy and 0.1 eV resolution.All XPS measurements were conducted at pressures below 5.0 Â 10 À8 mbar.CasaXPS software (version 2.3.1) was used to calculate atomic concentrations from the survey spectra as well as curve-fit the high-resolution W4f spectra.For curve fitting, we used a linear background with equal full-width at half-maximum peaks, and the line shapes were a combination of Gaussian (70%) and Lorentzian (30%).

Spectroscopic Ellipsometry
We investigated the optical properties and thicknesses of the thin films via spectroscopic ellipsometry.Coatings deposited on (2 cm Â 2 cm) silicon wafers were measured by a JA Woollam 2000D spectroscopic ellipsometer using an XLS-100 light source and an EC-400 control module coupled with WVASE32 software.The measurements spanned the wavelength range of 200-1000 nm, and data were recorded in 5 nm steps at three separate incidence angles: 65°, 70°, and 75°.

UV-Vis Spectroscopy
We used a Cary 5E UVÀvis spectrometer (Varian) to evaluate the transmission of our coatings in the 300-800 nm wavelength range.We measured the optical transmittance in both the colored and bleached states of the coatings.To assess the colored-bleached response times, we monitored the optical transmittance of the coatings in 1 M LiClO 4 -PC solution during periodic changes in applied voltage, using a range of À0.8 to þ0.8 V from a DC power supply (PowerTech, N287).

EC Evaluation
We used an eDAQ electrochemical workstation (model ER466) with a standard three-electrode system to assess the EC properties of the WO x structures.Our measurements focused on the electrochemical intercalation and deintercalation of electrons and Li þ ions.For the working electrodes, we used WO x coatings  deposited on ITO-glass substrates that had an effective surface area of 0.5 cm 2 .A 1.0 cm Â 2.0 cm section of ITO-glass was cut for all the samples before HiPIMS deposition.Following the deposition, the middle 1.0 cm of each sample was covered with varnish, leaving 0.5 cm from the top uncovered for electrode connection and 0.5 cm from the bottom that was in contact with the electrolyte solution, providing an effective surface area of 0.5 cm 2 .A platinum wire served as a counter electrode, and the reference electrode was Ag/AgCl (1 M KCl).CV was conducted at a voltage sweep rate of 100 mV s À1 (À 0.8 to þ0.8 V) in an electrolyte solution of 1 M LiClO 4 dissolved in propylene carbonate.

Results and Discussion
3.1.WO x Coatings Fabricated Using HiPIMS WO x coatings were deposited using reactive Ar/O 2 HiPIMS, while the pulse width was varied from 50 to 200 μs, providing duty cycles in the range of 0.75-3%.Figure 2 shows the target current and voltage as a function of time for the discharge of the tungsten target during the HiPIMS pulse.These data indicate that there is a strong dependence of the onset delay on the pulse width of the discharge.By increasing the pulse width, both the rate of increase in target current as a function of time and the maximum target current value decrease.As listed in Table 1, by increasing the pulse width from 50 to 200 μs, the maximum target current decreases from 31.0 to 8.2 A. We attribute this strong dependence to the difference in the secondary-electron emission coefficient between the oxidized (poisoned) and metallic target surfaces.The I-V characteristic data suggest that the tungsten target surface becomes less oxidized as the pulse width increases.
Because the discharge is pulsed, the target surface is alternately eroded by bombarding ions (during the voltage on-time) and subject to reaction with the sputtering atmosphere and afterglow plasma (during the voltage off-time).The secondary-electron emission coefficient is greater for an oxidized tungsten surface compared with a metallic tungsten surface. [12]As the tungsten target is less oxidized at the end of a pulse compared to at the pulse onset, the variation of the onset delay of the target current with the changes in the discharge pulse length can be explained by the differences in the secondary-electron emission coefficient between the oxidized and metallic target surfaces.

XPS Surface Chemistry and Optical Studies
The ratio of tungsten-to-oxygen atoms in the tungsten oxide (WO x ) films largely determines their transparency and EC properties. [25,60]We used XPS to study the chemical composition and tungsten oxidation states of the coatings, with the results shown in Figure 3. Figure 3a shows the chemical stoichiometry of WO x films as a function of pulse length as obtained by XPS survey spectra.By increasing the pulse length from 50 to 200 μs, the chemical stoichiometry decreases from WO 2.99 to WO 2.75 .The higher oxygen concentration in WO x films deposited at lower pulse lengths can be explained by the cyclic formation and erosion of the tungsten oxide compound layer on the target surface.
It is well documented in the literature that in the pulsed sputtering processes such as HiPIMS, compound layers, formed on the target surface during the off time, are eroded during the highvoltage pulse. [12,13,61,62]For shorter pulses, the oxide layer on the target is retained during the pulse; whereas for longer pulses, the oxide layer is eroded, leaving a metal (W)-rich target.This process explains the formation of stochiometric tungsten oxide (WO 3 ) and suboxide (WO x ) coatings at lower and longer pulse lengths, respectively.Figure 3b shows W4f high-resolution spectra obtained from WO x films deposited at various pulse lengths, providing further insight into the chemistry of the WO 3 films.The high-resolution XPS spectra have a higher signal-to-noise ratio than survey spectra and are therefore appropriate to evaluate minor changes in surface chemistry that are not detectable in survey scans.The W4f is a doublet because of the spinÀorbit splitting of W4f 7/2 and W4f 5/2 with a separation energy of 2.18 eV [ΔE = E(W4f 5/2 ) À E(W4f 7/2 )]. [63]W4f 7/2 peaks for tungsten suboxides (WO x with 2 < x < 3) and WO 3 were fitted at binding energies of 34.7 and 36.1 eV, respectively.Figure 3c shows the area percentage of components fitted in the W4f high-resolution spectra for various pulse widths.By an initial increase of pulse length from 50 to 85 μs, the concentration of WO 3 state decreases from 98.26 to 96.64%, while those of WO x (suboxides) increase from 1.74 to 3.16%.By further increasing the pulse length to values greater than 100 μs, the concentration of WO 3 decreases to less than 95.04%, and that of WO x increases to more than 4.86%.These changes in the oxidation state of tungsten agree well with the stoichiometry data obtained from XPS survey spectra (Figure 3a).Taken together, the changes we see in surface chemistry indicate that pulse lengths shorter than 85 μs are favorable for producing tungsten oxide thin films with chemistries closer to stoichiometric WO 3 .The most suitable structure for optimum EC properties is an amorphous structure, as it facilitates the injection and extraction of ions. [64,65][68] In our previous studies, [8,33] we have shown that HiPIMS tungsten oxide coatings have amorphous structures, as indicated by X-ray diffraction data.
The WO x coatings were measured to have sheet resistance values on the order of 10 10 Ω □ À1 .Varying the pulse width between 50 and 200 μs did not provide any substantial alterations in the sheet resistance of the coatings, which had a similar thickness of around 80 nm.
The thickness of WO x films, as measured by spectroscopic ellipsometry and stylus profilometry, were used to calculate deposition rates.These results are plotted as a function of the pulse length and shown in Figure 4a.The deposition rate increases with increasing pulse width as expected due to increasing duty cycle.Figure 4b shows the refractive indices of WO x films at wavelengths of 300-1000 nm for various pulse lengths.At any fixed wavelength greater than 400 nm, the refractive index of the film increases with increasing pulse length.For example, at a wavelength of 633 nm, the refractive index increases from 2.02 to 2.17 as the pulse lengthens from 50 to 200 μs.The changes we see in the refractive index values align well with changes in the stoichiometry of WO x films as a function of pulse length.These results are in good agreement with the work of Kang et al. who reported a decrease in refractive index in SiO x films as higher concentrations of oxygen were incorporated in the film structure and silicon oxide structures closer to stoichiometric composition were formed. [69]Figure 4c shows the extinction indices at wavelengths of 300-1000 nm measured for WO x coatings deposited using various pulse lengths.All the WO x coatings show a highly low extinction coefficient of less than 0.2 in the range of visible light.
The transparency of EC devices is of particular importance for optical applications, including EC devices.To study the effect of pulse length on the film transparency in HiPIMS deposition, we adjusted the deposition times to ensure a constant thickness of 80 nm for all the WO x samples.Figure 4d shows the transmittance percentage of WO x films deposited at different pulse lengths as a function of wavelength.All WO x films, regardless of the pulse length, were visually transparent.However, by decreasing the pulse length, the transmittance of WO x films increases.For example, by decreasing the pulse length from 200 to 50 μs, the transmittance increases from 79.44 to 87.18% at the wavelength of 633 nm.The increase in optical transmittance for decreasing pulse length is due to the formation of WO x structures closer to the tungsten trioxide stoichiometric composition, as explained in the discussion of XPS surface chemistry studies above.

EC Performance of WO x Coatings
To evaluate the EC properties of the WO x coatings, CV of EC devices (effective area = 0.5 cm 2 ) in a solution containing Li þ (1 M solution of LiClO 4 in PC) was used to investigate the oxidation and reduction processes.CV measurements were recorded for potentials ranging from À0.8 to þ0.8 V (vs Ag/AgCl) at a scan rate of 100 mV s À1 .In Figure 5, the tungsten oxide oxidation peak, observed between À0.6 and À0.2 V, is clearly distinguishable in the CV curves.The internal area of the curves, which corresponds to the total exchanged electrical charges in a cycle, increases with increasing pulse width from 50 to 70 μs and then decreases as the pulse width further increases from 70 to 200 μs.The smaller total exchanged electrical charge observed for the sample deposited using 50 μs pulse length compared to that deposited at 70 μs can be explained by its significantly lower density as indicated by refractive index data (Figure 4b).The decreases in total exchanged electrical charge as a function of increases in pulse length (beyond 70 μs) agree well with the XPS results that showed increases in pulse length results in decreases in chemical stoichiometry.
Figure 6a depicts the optical transmittance spectra of the WO x coatings deposited on ITO glass substrates.The results are shown for the bleached (black line) and colored (red line) states in the wavelength range of 300-800 nm.The change in the color of the WO x coatings from dark blue (intercalation of Li þ ) to transparent (deintercalation of Li þ ) is explained by the reduction of W 6þ ions to lower-valence states such as W 5þ .
As we observed from the XPS data (Figure 3), increases in pulse length lead to the formation of suboxide states in the structure of the tungsten oxide layer, which is associated with a higher number of oxygen vacancy defects.[72] Based on the small polaron model, the pair of W þ6 ─W þ6 in the near-stoichiometric WO 3 structure and the pair of W þ6 ─W þ5 in the suboxide structure convert to W þ6 ─W þ5 and W þ5 ─W þ5 pairs, respectively, through intercalation of the ions. [72]The optical transmittance modulation (ΔT = T b -T c , where T b and T c signify transmittance in bleached and colored states, respectively) at a 633 nm wavelength (as the highest optical modulation) was calculated to be 38.1, 36.2, and 34.3% for the WO x samples deposited at pulse lengths of 70, 85, and 100 μs, respectively.The time required for 90% change in the full optical transmittance modulation at λ = 633 nm is defined as the switching time and is a useful parameter for comparison.
Figure 6b shows the optical response and corresponding switching times of the HiPIMS WO x coatings deposited at various pulse widths.The response time for bleached and colored states decreases from 7.8 and 11.6 s for the sample deposited with 100 μs pulse length to 6.0 and 8.1 s for the sample deposited with 70 μs pulse length, respectively.
The optical density (ΔOD) change curves represent the modulation ability of films in the visible light range.Figure 7a shows the OD of WO x coatings, deposited with various pulse widths, in the visible light range.The maximum OD changes are observed at the wavelength of ≈633 nm for the WO x coating deposited using a pulse width of 70 μs.
The CE is defined as the change in the ΔOD at a given wavelength.To compare the performance of ECDs, we use their coloration efficiency and OD as calculated from the following equations.

CE ¼ ΔOD=ðQ=AÞ
(1) where T b and T c indicate transmittance in bleached and colored states, respectively, at a wavelength of 633 nm per unit area of EC device.Q is calculated by integrating the current over time to become colored.Figure 7b shows the variations of OD as a function of exchanged electrical charge for the HiPIMS WO x coatings deposited with various pulse widths.The EC properties for different pulse widths are summarized in Table 2. From these results, it can be concluded that the HiPIMS tungsten oxide coating deposited using the pulse length of 70 μs shows optimum EC performance.The favorable EC performance of the HiPIMS WO x coating deposited using a pulse width of 70 μs, compared with a number of other WO 3 EC coatings (thickness ≤ 80 nm) reported in the literature, can be observed from the data listed in Table 3.Taken together, the CV and optical transmittance results indicate that tuning the pulse length in HiPIMS deposition of tungsten oxide in a reactive process is a feasible approach to tune and optimize the EC function of the coatings for a range of applications, including smart windows and antiglare mirrors.Our results here also clearly show that defects are detrimental to the EC performance in metal oxides and, contrary to what was previously thought, will not contribute to the OD parameter and, subsequently, the coloration efficiency.

Conclusions
Reactive HiPIMS was used to fabricate tungsten oxide EC coatings on ITO-coated substrates.We tuned the HiPIMS pulse length from 50 to 200 μs and studied the chemical structure, transparency, and EC function of the coatings.XPS data indicated that stoichiometric tungsten oxide (WO 3 ) and suboxide  (WO x ) coatings form at lower and longer pulse lengths, respectively.The deposition rate increased by increases in pulse length because of the increasing duty cycle.In agreement with the variations in the stoichiometry of the tungsten oxide coatings, the transmittance decreased by increasing the pulse length due to an increase in the concentration of tungsten in sub-oxide environments and a decrease in that of stochiometric tungsten trioxide.
The WO 3 coating, fabricated using the relatively short HiPIMS pulse length of 70 μs, exhibited the greatest optical transmittance modulation (ΔT% = 38.1 and coloration efficiency, CE = 41.3 cm 2 C À1 ) with coloration and bleaching times of 6.0 and 8.1 s, respectively.The results presented in this paper collectively indicate that varying the pulse length during the reactive HiPIMS deposition using a tungsten target is a promising approach to fabricating tungsten oxide coatings with controlled modulation of stoichiometry and EC properties.Such transparent and electrochromic coatings hold excellent potential for modern optoelectronic applications such as smart windows and wearable electronics.

Figure 1 .
Figure 1.Schematic illustration of the HiPIMS system used for the fabrication of WO 3 EC layers.

Figure 3 .
Figure 3. XPS surface chemistry characterization of WO x films deposited at various pulse lengths by HiPIMS.a) Chemical stoichiometry for WO x films as a function of pulse length obtained from XPS survey spectra.b) W4f high-resolution spectra of WO x films deposited at various pulse lengths.The solid black lines are the recorded spectra, and the purple dashed lines represent the fitted envelope.The composite spectra are fit using doublets with W-oxidation states of W 6þ , W 5þ , and W 0 and a W4f 7/2 -W4f 5/2 spin-orbit separation of 2.18 eV with an intensity ratio of 0.75.c) Area percentage for WO x suboxide (red) and WO 3 (black) components fitted in W4f high-resolution spectra as a function of the pulse length.

Figure 4 .
Figure 4. Changes in deposition rate and optical properties of HiPIMS WO x films regulated by pulse length.a) The deposition rate of WO x films as a function of pulse length measured by spectroscopic ellipsometry and stylus profilometry.b) Refractive indices of WO x films for wavelengths in the range of λ = 300-1000 nm.c)The extinction indices at wavelengths of 300-1000 nm measured for WO x films deposited using various pulse widths.d) Optical transmittance of WO x films in the wavelength range 300-800 nm obtained for coatings deposited using pulse lengths varied from 50 to 200 μs.The deposition time was adjusted accordingly to achieve a film thickness of 80 nm for all the samples.

Figure 5 .
Figure 5. EC behavior of HiPIMS WO x films as a function of their deposition pulse width.CV of WO x coatings at various pulse widths.The measurements were carried out in 1 M LiClO 4 -PC electrolyte between þ0.8 V and À0.8 V at a sweep rate of 100 mV s À1 (effective surface area = 1 cm 2 ).

Figure 6 .
Figure 6.a) Optical transmittance spectra of WO x coatings at various pulse widths for colored (solid blue curves) and bleached (solid red curves) states in the range λ = 300-800 nm.b) Transmittance percentage as a function of time for WO x coatings deposited using various pulse widths for colored and bleached states between þ0.8 V and À0.8 V switching voltages.The response time taken to reach 90% of the maximum ΔT is indicated.

Table 1 .
HiPIMS processing parameters used to fabricate tungsten oxide thin films.
The calculated CE values of WO x coatings deposited with pulse widths of 70, 85, and 100 μs are 41.3, 38.4, and 35.9 cm 2 C À1 , respectively.The durability