Thin Film Stabilization of Different VO2 Polymorphs

*Manish Kumar, Chirag Saharan and Sunita Rani*

### **Abstract**

In recent years, VO2 has emerged as a popular candidate among the scientific community across the globe owing to its unique technological and fundamental aspects. VO2 can exist in several polymorphs (such as: A, B, C, D, M1, M2, M3, P, R and T) which offer a broad spectrum of functionalities suitable for numerous potential applications likewise smart windows, switching devices, memory materials, battery materials and so on. Each phase of VO2 has specific physical and chemical properties. The device realization based on specific functionality call for stabilization of good quality single phase VO2 thin films of desired polymorphs. Hence, the control on the growth of different VO2 polymorphs in thin film form is very crucial. Different polymorphs of VO2 can be stabilized by selecting the growth route, growth parameters and type of substrate etc. In this chapter, we present an overview of stabilization of the different phases of VO2 in the thin film form and the identification of these phases mainly by X-ray diffraction and Raman spectroscopy techniques.

**Keywords:** thin film, VO2, thermochromic, X-ray diffraction, Raman

### **1. Introduction**

Thin film materials with 'smart' properties have attracted increasing attention in past few decades, as we move towards the smarter world [1]. This is driven by the fact that these materials react to the variation in parameters such as temperature, pressure, electric or magnetic fields etc. [2–13]. Vanadium dioxide (VO2) is a well-known 'smart material' which is popular since the Morin' work in 1959 [14]. Its monoclinic M1 phase exhibits a metal–insulator transition (MIT) near room temperature, accompanied by larges changes in the structural, electronic and optical properties [15]. These distinctive features makes it attractive in smart windows, switching devices, memory materials and so on [16–18]. Being a strongly correlated electron system, VO2 is equally attractive to condensed-matter physicists [19–22].

VO2 can exhibit various polymorphic structures (such as: A, B, C, D, M1, M2, M3, P, R and T), each having quite different physical and chemical properties [23–31]. Among these polymorphs, many are neither stable in ambient conditions nor can be easily synthesized. This happens because vanadium oxides can adopt a wide range of V:O ratios, resulting in different structural motifs. Phase space diagram (**Figure 1**) for the vanadium oxides indicates that there are more than 15 other stable vanadium oxides phases (like VO, V2O3, V3O5 etc.) and only a narrow window in phase space exist in which the pure semiconducting phase of VO2 can be grown [32]. This narrow window strongly limits the synthesis of VO2 either in the form of bulk crystals, thin films, or micro- and nanostructures. Nonetheless, different stoichiometric

#### **Figure 1.**

*Phase space diagram for the vanadium oxides. Note the narrow window within which stoichiometric VO2 can be grown for x = 2.0 (reprinted from Ref. [32]).*


#### **Table 1.**

*The crystallography data for VO2 polymorphs.*

VO2 polymorphs have been stabilized using techniques such as sputtering, pulsed laser deposition (PLD), sol–gel deposition, reactive evaporation and metal–organic chemical vapor deposition (MOCVD) etc. [15, 23, 25, 31, 33–38].

*Thin Film Stabilization of Different VO2 Polymorphs DOI: http://dx.doi.org/10.5772/intechopen.94454*

Koide and Takei appears to be the first to grow VO2 thin films by chemical vapor deposition (CVD) technique in 1967 [39]. In their deposition method, fumes of vanadium oxychloride (VOCl3) was carried by N2 gas into the growth chamber and was hydrolyzed on the surface of rutile substrates to give epitaxial VO2 films. In 1967, VO2 thin films were also grown using reactive sputtering by Fuls et al. who made the films by reactive ion-beam sputtering of a vanadium target in an argon–oxygen atmosphere [40]. PLD emerged as a deposition technique for oxide superconductors in the late 1980s, and was first used to prepare VO2 thin films by Borek et al. in 1993 [41]. Since then, consistent efforts have been made to grow thin films of various VO2 polymorphs by using different deposition techniques/routes. Sputtering and PLD are the leading deposition techniques used to grow different VO2 thin films polymorphs [42–46]. This is because of the ease with which one can play the deposition parameters in these techniques to stabilize thin films of various compounds [47–60].

In this chapter we will focus on the stabilization of thin film of six main VO2 polymorphs: VO2 (M1), VO2 (M2), VO2 (R), VO2 (T), VO2 (A) and VO2 (B). But in passing it should be noted that VO2 polymorphs likewise VO2 (M3), VO2 (P), VO2 (C) and VO2 (D) have also been mostly studied in bulk and nanostructure form and reports are missing on thin film stabilization of these phases [24–29, 31]. Space group and lattice parameters of different VO2 polymorphs known to us are tabulated in **Table 1**.
