**1. Introduction**

Vanadium dioxide (VO2) has attracted extensive research interest during the past decades owing to its unique behavior, called semiconductor-to-metal transition (SMT) or insulator-to-metal transition (IMT), which accompanies the reversible and ultrafast phase transition between monoclinic VO2 [VO2(M)] and tetragonal rutile VO2 [VO2(R)] at temperatures around 340 K (~67 ◦C) [1–5]. Thus, the optical and electrical properties of VO2 can be switched by controlling the SMT behavior of VO2 [6,7]. Numerous factors for adjusting the SMT behavior of VO2 have already been established that include impurity doping [8], stoichiometry [9], strain [10], grain boundary [11], oxygen vacancy [12], applied external electrical field [13], and light irradiation [14]. Therefore, the VO2 has been widely investigated as a key material for applications in the smart thermochromic windows [15], two-terminal electronic devices [16], electric-field-effect three-terminal devices [17,18], integrated optical circuits [19], electronic oscillators [20], metamaterials [21], memristive devices [22], programmable critical thermal sensors [23], gas sensors [24], and so forth.

Various techniques had been employed for preparing VO2 films, including the sol–gel method [22,23], electron-beam evaporation [25,26], sputtering [5,17], pulsed laser deposition (PLD) [27,28], molecular beam epitaxy (MBE) [16,29], chemical vapor deposition (CVD) [30–34], and atomic layer deposition (ALD) [35–50]. Among them, ALD is an excellent technique which has drawn much attention due to its many advantages, including preparation of the highly conformal thin films with almost 100% step coverage, accurate control of film thickness at the atomic scale, low growth temperature, and wide-area uniformity. These features make ALD a powerful technique for the fabrication of emerging nanostructures and nanodevices [51–53].

Generally, organic vanadium compounds are employed as ALD precursors and reacted with H2O or O3 to grow vanadium oxide thin films, such as tetrakis(ethylmethylamino)vanadium (TEMAV) [35–42], vanadyl isopropoxide (VTIP) [43–46], and vanadyl triisopropoxide (VTOP) [47–50]. However, the organic precursors (TEMAV, VTIP, and VTOP) are only suitable for low process temperatures in ALD because the decomposition temperatures of TEMAV, VTIP, and VTOP are about 175 [35,40], 200 [43], and 180 ◦C [49,50], respectively. When the process temperature is higher than the decomposition temperature of the precursor, the growth mechanism of film will be changed from ALD to CVD-like mode [35,40,43,49,50]. In this case, the film is grown by CVD instead of ALD. This is why the low temperature of 120–170 ◦C is usually used for the film growth of ALD using TEMAV, VTIP, or VTOP as precursor. However, the low process temperature is not enough to grow crystalline films. Therefore, the as-deposited vanadium oxide films grown by ALD from organic vanadium precursors are generally reported to have amorphous structures with no significant SMT behavior and a postannealing process is necessary for converting the amorphous to crystalline VO2. Previous studies have reported that postannealing in N2, He, O2, or N2/O2 mixed gas with a low O2 partial pressure resulted in crystalline monoclinic VO2 for the temperature range of 425–585 ◦C [35–42]. Since the extra postannealing process is necessary to obtain a crystalline VO2, it results in higher manufacturing costs and increases the failure possibilities of the process and products.

This work reports that a directly crystalline VO2 film has been successfully grown by ALD using vanadium tetrachloride (VCl4) and H2O as precursors at a reaction temperature of 350 ◦C without any postannealing process. It is noticed that the inorganic VCl4 is used as an ALD precursor for the first time, although a few papers reported that the VCl4 had been used in traditional chemical vapor deposition (CVD) techniques [30–32]. The VO2 film has a significant SMT behavior with a VO2(M)-to-VO2(R) phase-transition temperature of about 61 ± 1 ◦C, which is verified from temperature-dependent Raman spectra and sheet-resistance variations of VO2 film. Besides, the VO2 film exhibits two orders of magnitude change in sheet resistance across the semiconductor-to-metal transition although the film thickness is only 30 nm. The results demonstrated that crystalline VO2 films can be directly grown by ALD using VCl4 and H2O as precursors without any postannealing process, presenting a new selection of precursors for the ALD process to grow the crystalline VO2 films.
