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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.
Pohang Accelerator Laboratory, Pohang University of Science and Technology, South Korea
Chirag Saharan
Department of Physics, Deenbandhu Chhotu Ram University of Science and Technology, India
Sunita Rani
Pohang Accelerator Laboratory, Pohang University of Science and Technology, South Korea
*Address all correspondence to: manish@postech.ac.kr
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, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 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, 17, 18]. Being a strongly correlated electron system, VO2 is equally attractive to condensed-matter physicists [19, 20, 21, 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, 24, 25, 26, 27, 28, 29, 30, 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 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, 34, 35, 36, 37, 38].
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]).
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, 43, 44, 45, 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, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 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, 25, 26, 27, 28, 29, 31]. Space group and lattice parameters of different VO2 polymorphs known to us are tabulated in Table 1.
Monoclinic VO2 (M1) (a = 5.74 Å, b = 4.52 Å, c = 5.38 Å, β = 122.6°) with space group P21/c is the most widely studied inorganic thermochromic material which is an insulator at room temperature. It shows a first-order MIT at 68°C with a concomitant structural transition into rutile tetragonal VO2 (R) (a = b = 4.55 Å, c = 2.86 Å) having space group P42/mnm [61]. Because of MIT and the associated huge changes in the structural, electronic and optical properties, VO2 (M1) and VO2 (R) are attractive for applications in smart windows, switching devices, memory materials and so on [16, 17].
Figure 2 shows the structural arrangement of four different phases of VO2 [64]. In the VO2 (R) phase, the vanadium atoms are equally spaced along the rutile c axis (cR), while in the VO2 (M1) phase, simultaneous dimerization and tilting in equivalent chains occur, leading to a zigzag pattern.
Figure 2.
The schematic structures for (a) rutile (R), (b) monoclinic M1, and (c) M2 phases of VO2. Red and blue balls denote vanadium and oxygen atoms, respectively. (d) The arrangement of vanadium chains in the four phases without oxygen atoms (a-d reprinted from Ref. [64]).
Highly oriented VO2 (M1) thin films on R-cut sapphire substrate were prepared by Borek et al. using PLD [41]. They ablated metallic vanadium target by a KrF pulsed excimer laser in an ultrahigh vacuum deposition chamber with Ar and O2 (10:1) atmosphere of 100–200 mTorr, and a substrate temperature (Ts) ∼ 500°C followed by 1 hour post deposition annealing in the same environment. Since then PLD was employed by number of groups to grow good quality VO2 (M1) thin films by varying the deposition parameters and post deposition treatment [44, 45, 46, 65]. Several other techniques such as sputtering, CVD, etc. were also employed to grow polycrystalline and epitaxial VO2 (M1) thin films on various substrates of different orientation [34, 42, 43, 66, 67, 68, 69]. To date, most VO2 (M1) films have been grown on substrates such as sapphire (c-type, m-type, r-type and a-type), TiO2, perovskite oxides, Si and Quartz. Figure 3(a) shows the grazing incidence X-ray diffraction (GIXRD) data of polycrystalline VO2 (M1) thin film by Kumar et al. which was grown on quartz substrate by sputtering VO2 at room temperature and post deposition annealing at 500°C [69]. Figure 3(b)–d depict the X-ray diffraction (XRD) patterns of VO2 (M1) thin film grown on TiO2 and Al2O3 substrates of different orientation [46, 70].
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]).
VO2 (R) is the high temperature phase of VO2 (M1). So, VO2 (M1) thin films generally transforms to VO2 (R) phase when heated above the MIT temperature. Apart from this, thin films showing VO2 (R) phase at room temperature can also be stabilized by strain, hydrogenation, oxygen vacancies and doping etc. [71, 72, 73, 74, 75, 76]. Fan et al. reported the growth of ultrathin VO2 (R) phase thin film on TiO2 (002) substrate [71]. Y. Zhao et al. showed that hydrogenation can also lead to growth of VO2 (R) phase thin film [72]. Very recently, Liang et al. described that increase in concentration of W dopant in V1−xWxO2/Si thin films favors the growth of VO2 (R) phase [73]. Figure 4 shows the XRD patterns of VO2 (R) phase thin films grown by different groups.
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]).
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 plasma-assisted (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 during the monoclinic M1 to rutile R transition [15, 69, 78, 79, 80, 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 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].
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).
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]).
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.
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]).
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 (Ts = 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 Ts 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 Ts 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 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].
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]).
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.
This work was supported by National Research Foundation of Korea (NRF) grant (Grant No. NRF-2015R1A5A1009962 and NRF-2019K1A3A7A09033398) funded by the Korean government. Authors also acknowledge the support from Pohang Accelerator Lab in Korea.
Authors are thankful to the publisher for waive off the article processing charges of the chapter.
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Written By
Manish Kumar, Chirag Saharan and Sunita Rani
Submitted: 10 June 2020Reviewed: 12 October 2020Published: 30 October 2020
Open access peer-reviewed chapter
