Deposition Rate and Electrochromic Properties of Ni Oxide Thin Films Deposited By Sputtering Using Water Vapor As a Reactive Gas
Authors: Kataoka Masaki , Abe Yoshio , Kawamura Midori , Kim Kyung Ho , Kiba Takayuki 
Published: December 2020

Ni oxide is well known electrochromic (EC) materials. And nickel hydroxide (Ni (OH)2) and oxyhydroxide (NiOOH) are considered to be electrochemically more active than nickel oxide (NiO). Reactive sputtering is one of the most commonly used techniques for obtaining compound thin films, however, deposition rate of compound thin film is low. We have reported that the deposition rate of sputtered nickel oxide thin film increased remarkably by substrate cooling using a liquid nitrogen and injection of water vapor (H2O) onto the target surface in argon (Ar) atmosphere [1]. In this study, we studied the effect of the direction of water vapor injection and the substrate temperature on deposition rate, composition and EC characteristics of the Ni oxide thin films.
Ni oxide thin films were prepared by an RF magnetron sputtering system at substrate temperatures (Ts) of -80 °C and -170 °C. Ar and H2O were used as sputtering gas, and the flow rates of Ar and H2O were kept constant at 2.5 cc/min. H2O was injected onto the substrate or target surface. RF power and sputtering gas pressure were constant at 50 W and 50 mTorr, respectively. Film thickness was measured using a stylus profiler, and deposition rate was calculated using the film thickness and sputtering time. Crystal structure and chemical bonding state of the films were characterized by X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FT-IR), respectively. Optical and electrical properties of the films were studied by UV-Vis spectroscopy and four probe method. EC properties of the films were evaluated using cyclic voltammetry (CV) and chronoamperometry (CA) in a 1 M KOH aqueous electrolyte.
Figure 1 shows the deposition rate of Ni oxide thin films deposited at Ts=-80 °C and -170 °C with the H2O injection onto the substrate or target surface. At Ts=-80 °C, high deposition rates of 30-35 nm/min were obtained regardless of the direction of H2O injection. The deposition rates are much higher than that obtained by conventional reactive sputtering at room temperature (4nm/min) [1]. In contrast, the deposition rate varied from 21 nm/min for H2O injection to substrate 38 nm/min (the highest deposition rate) for H2O injection to target at Ts=-170 °C. Highly transparent films with a transmittance above 80% were obtained regardless of the direction of H2O injection at Ts=-80 °C, however, the transmittances of the films deposited at Ts=-170 °C were low. From the four probe method measurement, the thin film deposited at Ts=-170 °C with H2O injection to substrate was found to be a mixed film of metal Ni and Ni oxide with a resistivity of 3.3×10-4Ωcm. The films deposited at Ts=-80 °C with H2O injection to substrate or target and the film deposited at -170 °C with H2O injection to target were considered to be NiO or Ni(OH)2. CV and CA measurements showed that the film deposited at Ts=-80 °C with H2O injection to target had good cycle stability and large transmittance change of 58% at a wavelength of 600nm.
In summary, the deposition rate, chemical composition, and EC properties of the Ni oxide films were found to be strongly dependent on the substrate temperature and the direction of H2O injection. The films deposited with H2O injection to target at Ts=-170 °C showed the highest deposition rate and that deposited at Ts=-80°C with H2O injection to target showed the largest transmittance change.
REFERENCES
[1] Y. Yokoiwa et al, Jpn. J. Appl. Phys. 58 (2019) 055504.

Figure 1