**2. Materials and Methods**

In this work, VO2 films were grown on native silicon-oxide-covered Si (100) substrates by ALD at 350 ◦C with 1000 reaction cycles. VCl4 and H2O were employed as precursors to grow the VO2 films, and Ar was used as purge gas. The reservoirs of the VCl4 and H2O precursors were kept at the temperatures of 30 and 25 ◦C, respectively. The dosing rates of VCl4 and H2O were 0.288 and 0.296 cc/pulse, respectively, as determined by the reservoir temperature and vapor injection time. The flow rate of Ar was 5 sccm, as controlled by a mass flow controller (MFC, SEC-4400M, HORIBA STEC, Kyoto, Japan). An eight-step sequence of gas injection was applied in an ALD cycle, as combined four conventional gas-injection steps and four additional pump-down steps. The pump-down steps can effectively evacuate excess precursors and byproducts to obtain high-quality films with low Cl impurity contents and ensure the achievement of "true ALD mode" growth [54,55]. The time for each step in an ALD cycle was 0.1, 1, 0.5, 1, 0.5, 1, 0.5, and 1 s for VCl4 vapor injection, pump-down, Ar purge, pump-down, H2O vapor injection, pump-down, Ar purge, and pump-down, respectively.

The crystalline structures of the VO2 films were examined by an X-ray diffractometer (XRD, D8 Advance Eco, Bruker, Karlsruhe, Germany) at 30 and 90 ◦C. The surface morphologies of the film were observed with a high-resolution scanning electron microscope (SEM, SU8000, Hitachi, Tokyo, Japan). In addition, in order to obtain real surface morphology of the film, the SEM analysis was performed without any conductive coating on the VO2 film surface. The cross-sectional microstructures of the VO2

films were observed by a high-resolution transmission electron microscope (TEM, JEM-2100F, JEOL, Tokyo, Japan). The film thickness was measured from the cross-sectional TEM micrograph and the growth rate of the VO2 film was estimated from an equation of "growth rate = (film thickness/numbers of ALD cycles)". The chemical composition of the VO2 film was analyzed by a high-resolution X-ray photoelectron spectrometer (XPS, Quantera SXM, ULVAC-PHI, Chigasaki, Japan). In addition, the XPS analysis was performed on the VO2 film surface before and after Ar ion etching with an etching depth of about 2 nm. The temperature-dependent Raman spectra of the VO2 film were examined at temperatures between 30 and 80 ◦C by a micro Raman spectrometer (Raman, UniRAM II, Uninanotech, Yongin, Korea) with a temperature-controllable sample stage. The temperature-dependent sheet resistance of the VO2 film was measured at temperatures between 30 and 90 ◦C by a Keithley 2614B SourceMeter (Keithley, Solon, OH, USA) under a four-point probing configuration with a temperature-controllable sample stage.
