**2.2 VO2 (T) phase and VO2 (M2) phase thin films**

VO2 (T) phase and VO2 (M2) are known to be Mott-Hubbard type insulator which may find use in Mottronics and novel electronic transport applications [15, 18]. These phases are structurally different from VO2 (M1) and VO2 (R) phase because of dissimilar types of vanadium chains and dimerization as shown in **Figure 2**. VO2 (M2) phase contains two distinct types of vanadium chains: one half of the vanadium atoms pair but do not tilt, while the other half are equidistant which tilts but do not pair. Triclinic phase i.e. VO2 (T) phase can be thought of as an intermediate phase between VO2 (M1) and VO2 (M2) phases, having two types of inequivalent vanadium chains (or sublattices) in which the vanadium atoms are paired and tilted to different degrees. VO2 (T) phase and VO2 (M2) are not as stable phase as VO2 (M1) and VO2 (R). But, doping and/or strain can stabilize these phases [15, 35, 77]. Strelcov et al. presented a phase diagram which demonstrate the influence of chemical doping and uniaxial stress on the phase structure of VO2 [35]. This phase diagram (**Figure 5(a)**) indicates that either of M1, M2, T, or R phase of VO2 can exist depending on the type of dopant and/or stress. Majid et al. reported the Cr doping driven growth of VO2 (T) phase thin films [15]. **Figure 5(b)** shows their XRD pattern of grown VO2 (M1) and VO2 (T) phase thin films. Stress-induced VO2 films with M2 monoclinic phase stable at room temperature; were grown by Okimura et al. using inductively coupled plasmaassisted (ICP) reactive sputtering technique with various rf power fed to the coil for ICP (**Figure 5(c)**) at constant Ts of 450°C and at varying Ts, under constant rf power (**Figure 5(d)**) [77]. Apart from this work, there are not much reports on the growth of single phase VO2 (M2) thin films which are stable at room temperature. But, there are numerous reports on the evolution of intermediate M2 phase in VO2 thin films

**Figure 3.**

*(a) GIXRD data of VO2 (M1) thin film prepared on quartz substrate [69]. XRD data of epitaxial VO2 (M1) thin films grown on (b) TiO2 substrates of different orientation (reprinted from Ref. [46]), (c) c-cut sapphire and (d) r-cut sapphire (c, d adopted from Ref. [70]).*

during the monoclinic M1 to rutile R transition [15, 69, 78–81]. This intermediate M2 phase in VO2 thin film can be introduced by selecting the particular substrate temperature, doping, thickness etc. Kumar et al. witnessed the intermediate M2 phase temperature dependent XRD measurements across the MIT transition in polycrystalline VO2 thin films grown on quartz substrate using sputtering technique followed by rapid thermal annealing at 530°C (**Figure 6(b)**) [69]. However, they have not observed the intermediate M2 phase for films annealed at 500°C (**Figure 6(a)**). Majid et al. noticed the evolution of intermediate M2 phase in temperature dependent Raman measurements of Cr doped VO2 thin films during T ➔ R phase transition (**Figure 6(d)**) [15]. For undoped VO2 thin films normal M1➔R phase transition crossover was observed

**Figure 4.**

*(a) XRD profiles for thickness-dependence VO2 films on TiO2 substrate [Reprinted with permission from Fan et al [71]. Copyright (2014) American Chemical Society]. (b) XRD of pure (M1 phase) and hydrogen-doping stabilized metallic (R phase) VO2 thin films prepared on sapphire substrate (Reprinted from Ref. [72], with the permission of AIP Publishing). (c) Room temperature XRD of different V1−xWxO2/Si thin films (adopted from Ref. [73]).*

without signatures of intermediate M2 phase °C (**Figure 6(c)**). Ji et al. stressed the role of microstructure on the M1-M2 phase transition in epitaxial VO2 thin films of different thicknesses [78]. Their temperature dependent Raman measurement result on 90 nm and 150 nm thick VO2 thin film sample are depicted in **Figure 6(e)** and **(f)** respectively. Azhan et al. also found intermediate M2 phase in VO2 thin films with large crystalline domains [79].

### **2.3 VO2 (A) and VO2 (B) phase thin films**

The layered polymorphs VO2 (A) and VO2 (B) are important materials from science and technology perspective. VO2 (B) has been long considered as a promising electrode material for Li ion batteries since the after report of Li et al. in 1994 [82]. It emerged as a promising cathode material owing to its layered structure and outstanding electrochemical performance [83, 84]. Also, it is important for the study of strong electronic correlations resulting from structure. On the other hand, VO2 (A) phase is highly metastable and therefore the physical properties and the potential for technical applications have not been explored in detail. This phase is an intermediate phase between VO2 (B) and VO2 (R), and has a reversible phase transition at ~162°C [85, 86]. The crystal structure of VO2 (A) and VO2 (B) phase with possible epitaxial relation on SrTiO3 substrate, are illustrated in **Figure 7(a)** and **(b)** respectively [23]. At room temperature, the metastable monoclinic VO2 (B) adopts a structure derived from V2O5 and belongs to space group C2/m while VO2 (A) adopts a tetragonal unit cell with a space group P42/ncm [23]. Growth of single crystalline VO2 (B) is very challenging due to the complex crystal structure. Similarly to VO2 (B), the study of VO2 (A) has so far been limited.

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

#### **Figure 5.**

*(a) A temperature-composition phase diagram of VO2, demonstrating the influence of chemical doping and uniaxial stress on the phase structure of VO2 (reprinted with permission from Strelcov et al. [35]. Copyright 2012 American Chemical Society). (b), room-temperature XRD patterns of the pure (M1 phase) and Cr-doped (T phase) VO2 thin films on the [001] Si substrate (adapted from Ref. [15]). (c and d) XRD patterns of the VO2 films grown on quartz substrates with various RFpower fed to the coil for ICP, at constant Ts of 450°C and at varying Ts, under constant RF power (Reprinted from Ref. [77], with the permission of AIP Publishing).*

Recently; several reports are focused on VO2 (A) and VO2 (B) phases in the form of bulk and nano-powders where annealing treatment causes them to revert to stable VO2 (M1) phase [25]. Chen et al. appears to be the first to report the growth of textured VO2 (B) films with thickness only <25 nm on SrTiO3 (001) substrate [87].

The good mathing of the a − b plane of VO2 (B) to that of (001)-oriented perovskites enables the epitaxial growth of phase-pure VO2 (B) thin films on perovskite substrates, such as SrTiO3 and LaAlO3. Srivastava et al. successfully stablized the single phase VO2 (B) and VO2 (A) thin films by tuning the laser retation rate and oxygen partical pressure during PLD while keeping the constant substrate tempearture (*T*s = 500°C) [23]. The XRD pattern of their grown films and the phase digram of used deposition parameters are shown in **Figure 7(c)** and **(d)** respectively. Lee et al. argued that a proper choice of *T*s is crtical among the deposition parameters for the growth of VO2 (A) and VO2 (B) phase thin film on perovskite substrates [60]. They found that the thin films of these phases can reproducibly grow at *T*s lower than 430°C only (**Figure 8(a)** and **(b)**). Morover, VO2 (A) phase can also appear as an intermediate phase (**Figure 8(c)**) when annealing is carried out for VO2 (B)➔ VO2 (R) conversion [60]. Wong et al. successfully synthesize thin

#### **Figure 6.**

*Temperature dependence of XRD data (at X-ray wavelength (λ) = 0.0693 nm) during heating cycle for VO2 thin film annealed at (a) 500°C and (b) 530°C (a,b adopted from Ref. [69]). Temperature-dependent Raman spectra of (c) pure and (d) Cr-doped VO2 thin films collected in the cooling cycles (c, d adopted from Ref. [15]). Temperature dependent Raman spectra of (e) 90 nm and (f) 150 nm thick VO2 thin film grown on Al2O3 substrate (e, f adopted from Ref. [78]).*

films of the metastable VO2 (B) polymorph on (001) LaAlO3 at deposition temperature Ts = 325°C (**Figure 8(d)**) [70]. Very recently, Choi et al. grown epitaxial VO2 (A) and VO2 (B) thin films having tungsten doping were grown on (011)-oriented SrTiO3 and 001)-oriented LaAlO3 substrate respectively using PLD [88].

#### **3. Conclusions**

An overview of thin film stabilization of different VO2 polymorphs i.e. VO2 (M1), VO2 (M2), VO2 (R), VO2 (T), VO2 (A) and VO2 (B) is presented in this chapter. It is understood that one can stabilize the thin film of a particular VO2 polymorph by properly selecting the deposition technique, growth parameters, type of substrate and dopant etc.

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

#### **Figure 7.**

*The schematic crystal structure representation of (a) 220 orientated VO2 (A), (b) 002 orientated VO2 (B) grown on SrTiO3 (100) substrate. (c) XRD patterns showing different phases for VO2 thin films grown at various deposition parameters. (d) Phase diagram showing the role of laser frequency and oxygen pressure during pulsed laser deposition for different polymorphs of VO2 thin films (a-d adopted from Ref. [23]).*

#### **Figure 8.**

*XRD patterns of (a) VO2 (B) and (b) VO2 (A) thin film on SrTiO3 (001) and (011) substrates respectively. (c) XRD during annealing of VO2 (B)/STO sample (a-c adopted from Ref. [60]). (d) XRD scan of VO2 (B) film grown on LaAlO3 (001) substrate (adopted from Ref. [70]).*
