Physical vapour deposition of vanadium dioxide for thermochromic smart window applications

Tuan Duc Vu ab, Zhang Chen c, Xianting Zeng b, Meng Jiang d, Shiyu Liu *b, Yanfeng Gao *ce and Yi Long *af
aSchool of Materials Science and Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Singapore. E-mail: longyi@ntu.edu.sg
bSingapore Institute of Manufacturing Technology (SIMTech), 2 Fusionopolis Way, #08-04 Innovis, Singapore 138634, Singapore
cSchool of Materials Science and Engineering, Shanghai University, Shanghai 200444, China
dCollege of Medical Imaging, Shanghai University of Medicine and Health Sciences, Shanghai, 201318, China
eSchool of Materials Science and Energy Engineering, Foshan University, Foshan 528000, China
fSingapore-HUJ Alliance for Research and Enterprise (SHARE), Nanomaterials for Energy and Energy-Water Nexus (NEW), Campus for Research Excellence and Technological Enterprise (CREATE), Singapore 138602, Singapore

Received 4th October 2018 , Accepted 19th December 2018

First published on 20th December 2018


Smart windows are defined by their ability to regulate incoming solar radiation in order to reduce the energy consumption of buildings by modulating the heat intake. Vanadium dioxide (VO2) is one of the most promising potential candidates for smart window materials due to its ability to reversibly transit from monoclinic VO2 (M) to rutile VO2 (R) at near room temperature. As a result of this transition, the infrared radiation (IR) transparent VO2 (M) abruptly becomes IR opaque, effectively regulating the heat intake by solar radiation. Despite their promising potential, VO2-based smart windows have various significant intrinsic limitations: a high transition temperature (τC) of 68 °C; low luminous transmission (Tlum) of around 40% and low solar modulation (ΔTsol) of less than 25%. Currently, various methods have been used to fabricate VO2 thin films in an attempt to improve their intrinsic properties. One of those methods is physical vapour deposition (PVD). In this paper, various PVD techniques, such as pulsed laser deposition (PLD), evaporation decomposition (ED) and sputtering, are examined with respect to their conditions for VO2 fabrication, film quality and the strategies for film improvements. Lastly, some challenges and opportunities for further studies into VO2-based smart windows are discussed.


1. Introduction

As the world population is reaching ever closer to 8 billion people, the global energy demand is also increasing rapidly to accommodate this number. It is estimated that by 2020, the total energy demand will be up to 80% higher than that in 1990.1 It is clearly crucial to reduce the energy demand by adopting energy conservation strategies. The energy consumption of the buildings sector, both residential and commercial, now exceeds the contributions from the transport and industrial sectors and has climbed to a staggering 40% of the total energy consumption in the last decade.2 Within building services, up to 50% of the energy is consumed by heating, ventilation and air conditioning applications (HVAC).3 Following this line of thought, conserving energy usage for HVAC applications is a logical step in order to reduce the overall energy consumption. Several methods have been proposed to follow this direction including but not limited to: adding insulation materials to walls,4 using cool coatings on roofs5 and utilizing smart window coatings.6

As one of the main gateways to conducting heat between the inner and outer environment, windows are typically energy inefficient due to an undesirable heat exchange, with heat accumulation during summer and heat leakage during winter.7,8 Thus, smart windows have received significant attention from both the industrial and academic sectors in recent years. Smart windows are defined by their ability to regulate the transmittance of a certain portion of the solar radiation spectra, which consists of ultraviolet (UV), visible and infrared (IR) radiation. Various materials with different radiation-regulating mechanisms have been studied, namely photochromic, electrochromic and thermochromic. One such material is vanadium dioxide (VO2), a thermochromic material with metal–insulator transition (MIT) at near room temperature.9 Below its transition temperature (τC), VO2 has a monoclinic structure and is transparent to near-IR. Upon reaching above its τC, VO2 adopts a rutile structure, and becomes metallic and opaque to near-IR.10 Based on its properties, VO2 is attractive as a smart window material because of a couple of simple reasons: (1) its modulating ability only affects the near IR and does not affect the visible spectrum, thus effectively controlling heat intake by solar radiation while minimizing the energy needed for illumination; (2) no external energy is required to activate the modulation, as the response is spontaneous when the environment temperature reaches τC.11,12 However, VO2 has not yet been commercialized widely due to some intrinsic limitations. First, the τC of bulk VO2 (M) is at about 68 °C, which is too high for practical usage. At the same time, VO2 has low solar modulation (ΔTsol) and luminous transmittance (Tlum). The theoretical predicted ΔTsol of VO2 is 25% with a moderate Tlum of about 40%,13 while the typical experimental results indicate ΔTsol at less than 20% and Tlum at approximately 40%.14

In a quest to improve the optical properties of VO2 and to lower its τC, various fabrication methods have been used, including but not limited to chemical vapour deposition (CVD),15 hydrothermal,16–18 sol–gel synthesis19–21 and physical vapour deposition (PVD). PVD refers to a variety of vacuum deposition methods used to deposit thin films via the condensation of a vaporised form of the desired film material onto various substrates. The coating methods involve purely physical processes, such as high-temperature vacuum evaporation with subsequent condensation or plasma sputter bombardment. In the case of VO2 (M/R) thin films fabrication, pure vanadium, V2O3, VO2 or V2O5 is commonly used as the target. The partial pressure of oxygen in the deposition chamber is accurately controlled to prevent the formation of other vanadium oxides with different valences, and the substrates are heated to relatively high temperatures to promote the crystallisation of VO2 (M/R). Compared with other techniques, PVD can achieve high film uniformity, good adhesion and a high packing density. Furthermore, PVD techniques are suitable for large-scale industrial production, which is crucial for the implementation of VO2 as a smart window solution.22–24 Specifically, some widely used PVD techniques include pulsed laser deposition (PLD), evaporation decomposition (ED) and sputtering. This review aims to understand these techniques of VO2 thin films fabrication, in terms of the fabricated film quality, process control, target requirements, film structure modification (i.e. doping) and process–microstructure–property relationship.

2. Reactive pulse laser deposition (RPLD)

RPLD is a widely used method for VO2 thin film preparation. During this method, a high-power pulsed laser beam is used to strike the target in vacuum (Fig. 1). Although there are relatively few requirements for suitable targets (available targets include metals, metallic oxides powder and ceramics), the deposition processing is complex. The laser pulse is absorbed by the target and converted to different kinds of energy, including thermal, chemical and mechanical energy. During this process, the target could be evaporated, ablated, ionized and even exfoliated. Moreover, the form of ejected species that are produced by laser include electrons, ions, atoms, molecules, clusters, particulates and molten globules.25 RPLD is a powerful technique that enables the fabrication of high-quality VO2 thin films by controlling their deposition parameters. Also the co-doping of VO2 films could be easily achieved by ablating dual targets.
image file: c8tc05014g-f1.tif
Fig. 1 A typical experimental arrangement for PLD (https://en.wikipedia.org/wiki/Pulsed_laser_deposition).

Borek et al. were the first researchers to prepare VO2 (M/R) thin films using PLD.26 They adopted a pulsed excimer laser (at a wavelength of λ = 248 nm and a pulse duration of τ = 15 ns) to ablate a metallic vanadium target in an ultrahigh-vacuum deposition chamber. The total gas (Ar and O2) pressure in the system was maintained between 100 and 200 mTorr (about 13–26 Pa). An oxygen concentration of approximately 10% was found to be optimal for the fabrication of pure VO2 films. Thin films were deposited on R-cut sapphire substrates at temperature ranging from 500 °C to 525 °C. The as-deposited thin films were all stored under the same atmosphere and temperature for approximately 1 h to obtain VO2 (M/R) polymorphs.

Various PLD processes have been reported ever since then, with various targets being used, including metallic vanadium, V2O3, VO2 and V2O5. Besides the most commonly used KrF excimer laser (λ: 248 nm), ArF (λ: 193 nm), XeCl (λ: 308 nm) and Nd-YAG (λ: 532 nm) excimer laser have also been adopted as the light source. During the deposition, the oxygen partial pressure is usually controlled between 1.3–6.5 Pa by using pure O2 or an O2/Ar mixed gas.27,28 In most of the works, the substrate is heated to 400–630 °C during the deposition, while in some works, a post-annealing process has been required to obtain high-quality VO2 thin films.29,30

2.1. Oxygen partial pressure

Oxygen partial pressure is one of the key parameters during VO2 thin film preparation. An improper oxygen partial pressure could not only lead to the formation of other vanadium oxides, such as V2O3,31 V6O1332 or V2O5,33 but could also influence the morphology and phase transition behaviour of the VO2 films (Fig. 2af).34,35 Fan et al.36 found that the obtained phases of VO2 could be controlled by adjusting the oxygen partial pressure during the deposition. That is, a higher oxygen pressure tends to form a VO2(B) structure, while a lower oxygen pressure favoured VO2(M1/R) epitaxial growth (Fig. 2g and h). Lafane's work showed that the oxygen pressure influences the process of molecular oxygen dissociation, and resulted in differences in the deposited films’ stoichiometric and morphologies.35 There are two important steps: species from the target hit the substrate, leading to re-sputtering, and a shocked layer is formed in the plume–oxygen gas boundary (Fig. 2j–l), which produces atomic oxygen that could directly be incorporated into the growing film. At a lower oxygen partial pressure, a thin shocked layer forms, while with high velocities of the species hitting the substrate, the strong re-sputtering energies produce homogeneous films, but also favour light elements (O) escaping, forming V2O3, V4O7, etc. At higher oxygen partial pressure, a thick shocked layer forms, offering plenty of atomic oxygen, and the re-sputtering process is weak, so an inhomogeneous film with a stoichiometry close to the target forms. However, there are other parameters that influence the appearance and performance of the deposited films, albeit the results reported were different in different researchers’ work at the same oxygen partial pressure (Fig. 2a and g).
image file: c8tc05014g-f2.tif
Fig. 2 (a) XRD patterns of the V2O5 target and the films and (b) the calculated average grain size deposited at oxygen pressure ranging from 0 to 5 × 10−2 mbar (0 to 5 Pa); (c) surface morphologies of VO2 thin film prepared at 5.4 Pa and 1.7 Pa; (d) temperature dependence of the electrical resistivity for VO2 films grown at various oxygen pressures; (e and f) transmittance spectra for VO2 films grown at 20 and 50 mTorr (2.67 and 6.67 Pa), respectively, inset is the experimental (dotted lines) and best-fitted (solid lines) transmittance spectra of VO2 films; (g) XRD patterns of VO2 films deposited on sapphire substrates under different oxygen pressure (0.5–3.6 Pa) with VO2 target, (h and i) are the surface morphologies of films deposited at 3.6 Pa (B-VO2) and 1.8 Pa (M1-VO2), and inserts are the related cross-section; (j) temporal evolution of the visible plume under vacuum and at an oxygen pressure of 4 × 10−3, 10−2 and 5 × 10−2 mbar represents the images of the plume obtained during the deposition with the presence of the substrate heater. The position of the ablating surface is at the limit of the left side of the images; (k) plume front position as a function of the time delay at vacuum and different oxygen pressures; (l) distance over which the plume-oxygen gas interaction occurs versus oxygen pressure.

2.2. Substrate temperature

The substrate temperature is another key parameter during RPLD. It is well-known that a higher substrate temperature would favour the formation of high-quality VO2 (M/R) films. Kim37 prepared VO2 films at a substrate temperature as high as 630 °C, where the range of electrical resistance changes (ERC) reached 4 × 105, comparable to that of single-crystal VO2 of 105.38 Besides high temperature, Nag et al. showed that a post-annealing process after deposition would also help the formation of high-quality samples.29 Samples deposited at room temperature with a following post-annealing process at 450 °C showed obvious advantages in light regulation across the MIT compared to samples deposited at 500 °C without a post-annealing process (Fig. 3). It should be mentioned that the cooling speed would also influence the performance of VO2 thin films.39
image file: c8tc05014g-f3.tif
Fig. 3 TEM and light transmission hysteresis for: (a) room temperature deposition followed with 450 °C annealing (RT-grown film) and (b) high temperature deposition at 500 °C (HT-grown film); (c) white light transmission hysteresis for RT grown (red) and HT grown (black) film; (d) SEM and relative IR (λ = 980 nm) switching curves through the structural phase transition time for 100 nm VO2 thin film fabricated at 450 °C annealing for 5–80 min in 250 mTorr (∼33.3 Pa) of O2. As annealing time is increased, hysteresis is enhanced and the sharpness of the transition and grain size are improved.

It is an attractive option to prepare VO2 thin films at a very low substrate temperature. Soltani et al.40 studied the process of depositing VO2 films on Kapton substrates using a vanadium target. Although no optical switching was observed in the prepared VO2/Kapton samples, it was discovered that the VO2/Si(100) exhibited the well-known MIT with a transition temperature about 68 °C at deposition temperatures as low as 300 °C. Maaza et al.41 prepared high stoichiometric VO2 films using a VO2 target without any substrate heating or atmosphere control. These films could be deposited on silicon, quartz and sapphire substrates. They all showed obvious MIT at ∼68 °C with an electrical resistance change of ∼102. Liu et al.42 and Suh et al.43 also reported VO2 films with obvious MIT prepared with a vanadium target on MgO, quartz and n-type silicon substrates. By improving the annealing time, Suh improved the crystalline quality of VO2 (Fig. 3d), although all these films performed poorly in MIT processing. In recent years, few works on lower temperature deposition can be found, with most studies adopting higher deposition temperatures (500–550 °C).

2.3. Epitaxial growth

Epitaxial growth is one route to obtain high-quality VO2. There are several methods to prepare epitaxial VO2 thin films, such as sputtering,44–46 electron beam deposition47 and molecular beam epitaxy,48,49 while other phases of epitaxy VO2 could also be prepared, such as VO2(B)50 and VO2(A)51 species produced by pulsed laser are high-energy species, which can assist the in situ fabrication of oriented, epitaxial thin films, leading to PLD becoming a more widely used method for the epitaxial growth of VO2 films.

The first work on VO2 films prepared by PLD was epitaxial.26 Details of the epitaxial growth of VO2 thin films can be found in Nag et al.'s review25 in 2008. Here, we just focus on the phase transition properties of epitaxial VO2 films prepared by PLD across MIT, which implies phase transition properties modulation for smart window applications.

Phase-transition properties refer not only to the phase-transition temperature (τC), but also to the phase-transition width (ΔH), speed (ΔT) and amplitude (ΔA). ΔH is the difference in the phase transition temperature from the VO2 (M) to (R) phase and from the VO2 (R) to (M) phase (τC:[thin space (1/6-em)]M–RτC:[thin space (1/6-em)]R–M), while ΔT is the temperature required for a complete phase transition from the (M) to (R) phase or (R) to (M) phase, and ΔA is the difference in the resistance of transmittance of VO2 across the MIT. For application in smart windows, the closer τC is to room temperature, the narrower the ΔH, the faster the ΔT and the larger the ΔA, the better the energy saving effect the smart window can have.52

For single crystals, the transition widths (ΔH) are less than 0.15 K, and the electrical resistance changes across MIT (ΔA) reaches as large as 5 orders of magnitude, while for un-oriented films, ΔH ranges from 5 to 40 K,53 and ΔA is usually less than 4 orders of magnitude. For most of the reported epitaxial VO2 films prepared by PLD, the ΔH could be reduced to as narrow as 2 K, and ΔA is usually larger than 4 orders of magnitude. These indicate the high quality of the oriented VO2 films. Different from bulk materials, the MIT performance of VO2 epitaxial films are diverse, mainly caused by the stress in the films. These stresses are induced by a lattice and expansion efficiency mismatch between substrates and films. It has been found that tensile stress along the c-axis of a tetragonal unit cell (metal phase) would reduce the MIT temperature, while compressive stress along this direction would improve the MIT temperature,54,55 where the thinner the deposited epitaxial VO2 films, the larger difference in MIT temperature.56,57 The MIT temperature of epitaxial VO2 films could be modified over a wide temperature range (>40 °C).54

It was thought that the smaller the mismatch between the substrate and deposited films, the less influence this would have on the MIT performance of VO2 films. To test this out, oriented VO2 on different substrates were prepared by PLD, including Al2O3 with r-, m- and c-cuts,32,58,59 MgO(100)32,58 and (111),60 MgF2(001),61,62 TiO2(001),63–65 (110),63 (111),66 (101)66 and (100).66 Different MIT performances were obtained due to lattice mismatch (Fig. 4a–d). To modify the stress, a buffer of epitaxial layers of other materials were deposited prior to VO2, such as TiO2,54,67,68 AlN,69 SnO,70 Cr2O371 and YSZ/NiO.72 However, the small misfits required a large distance to relax the misfit strain (critical thickness),73 but as the resistance of dislocation glide in VO2 is very large, this means the misfits are not able to relax totally.54 Although TiO2(001) had the lowest lattice mismatch with VO2 (R) (<1%), the MIT temperature of VO2/TiO2(001)/Al2O3 was much lower than bulk VO2 (∼25 °C)54,67,68 (Fig. 4f).


image file: c8tc05014g-f4.tif
Fig. 4 (a) Temperature-dependent resistance of VO2 films grown on different orientation TiO2 substrates and (b) VO2/TiO2 bilayers grown on sapphire with different orientations of (c-plane, m-plane, r-plane and a-plane); (c) temperature-dependent resistivity of VO2 thin films deposited on different substrates (TiO2, MgF2 or Al2O3) and (d) lattice constants of VO2 films and TiO2, MgF2 substrates, with lattice constants of the R- and M-phases of VO2 also plotted, dotted green line is for the strained VO2 (M) lattice constants; (e) a summary of MIT parameters of VO2 epitaxial thin films with different thicknesses; (f) temperature-dependent resistance of VO2/TiO2/m-sapphire, VO2/TiO2/r-sapphire and VO2/TiO2/c-sapphire heterostructures; (g–k) Jian et al.'s work; (g) temperature-dependent resistance; (h) resistance changing rate and (i) XRD of VO2 with 1 and 60 cycles of thermal treatment, (j and k) are TEM cross-section images of 1 and 60 cycles of thermal treatment samples, respectively.

It is possible to achieve high-quality epitaxial VO2 films with suitable conditions, including substrate materials, orientation, and pre- and post-deposition processes. However, Jian et al.'s work (Fig. 4g–k) showed that, due to lattice constant changes and thermal expansions during thermal cycles (17–97 °C), a large strain would accumulate around the domain boundaries in the VO2 films, resulting in degradations of ΔT and ΔH.55 It is thus still a challenge to prepare high-quality VO2 films with long-term stability for practical usage.

PLD offers advantages in the concise control of the chemical composition, crystallinity and epitaxy growth, and the thinnest VO2 films with a MIT prepared by PLD is about 2–3 nm,56,74 which means the wafer-level fabrication of VO2 is possible for device applications. However, the PLD method cannot be adopted for VO2 film growth with a larger size up to one or two inches if considering the film uniformity and the surface defects, which greatly hinders their usage in smart windows and in other applications based on the phase-transition property of the VO2 material.75

3. Ion plating/ion implantation

Ion plating is a method for producing a new phase in a host material by injecting impurities followed by nucleation within the host. Strictly speaking, ion plating is not a method for thin-film preparation since the nanoparticles prepared by this method are separated by the host material. However, these nanoparticles usually disperse in a very limited depth due to the implanted ion energy (several hundred nanometres), so they effectively act as a thin film, especially in terms of their optical properties. These are called ‘nanocomposite’ films/coatings in smart window studies. Thin films fabricated by this method perform better than pure VO2 thin films in luminous transmittance and in their solar energy modulation ability.76–78

A typical ion-plating process for the preparation of VO2 contains two steps (Fig. 5a): at room temperature, stoichiometric vanadium and oxygen are co-implanted into a substrate, such as SiO2 or Al2O3,79 at an energy of about 150–30080 (10081) keV for vanadium and 56–12080 (3681) keV for oxygen, and then the implanted substrate is annealed at a temperature ranging from 700–1000 °C82,83 in high-purity flowing argon.


image file: c8tc05014g-f5.tif
Fig. 5 (a) Schematics of the ion-beam-induced nucleation and ion-implantation process; (b) cross-section TEM image (top) and thermal hysteresis loop of ion-plating-synthesized VO2 nanocrystals (NCs) in fused silica matrix, with the thermal hysteresis loop obtained by tracking transmission at 1500 nm, and red and blue curves represent the heating and cooling processes, respectively; (c) TEM images (left) and thermal hysteresis loop (right) of VO2 NCs in fused silica matrix formed at 1000 °C in flowing argon with different annealing times, VO2 NCs had sizes ranging from dozens to hundreds of nanometres and a size-dependent ΔH as large as 40 °C.

The energy of the ions determine the dispersion depth of the obtained nanoparticles. In Lopez's work, VO2 nanoparticles of a sample prepared at a vanadium ion power of 300 keV were found within 500 nm of the top surface of the SiO2 substrate, or within 300 nm when the vanadium ions were at 150 keV. While in Karl’ work, the use of a 100 keV vanadium ion source resulted in a particle depth of about 150 nm. Average particle sizes of 30–90 nm could be obtained by changing the annealing time (Fig. 5c). Doping could be easily realized in this method by co-implanting (W,Mo).81 Moreover, ion implanting could be a general method to perform the non-equilibrium doping of VO2 films pre-deposited by different methods.84,85

VO2 nanoparticles prepared by ion implanting are well separated by the matrix, have good crystallinity due to the high annealing temperature and a relatively stable surface (Fig. 5b). These attributes make them ideal samples to study the phase-transition process of VO2 nanoparticles,86–88 even in the case of single-nanoparticle studies.89 A particle-size-dependent loop width of VO2 nanoparticles during phase transition was found and well analyzed through thermodynamics and kinetics studies by Lopez. The phase transition was found to possibly be induced by infrared light pulses of less than 200 fs duration.90

The transmittance spectrum of these samples are rarely reported. Instead, the surface plasmon resonance phenomenon of VO2 nanoparticles at metal phase is better studied.90,91 The most obvious difference caused by surface plasmon resonance is a strong absorption in the near-infrared region (mainly at 800–1800 nm). This greatly increases the switching efficiency of VO2 and is later seen to be one of the greatest advantages of nanocomposite-VO2-based smart coatings. This helps to promote the usage of VO2 for smart window application.77,92

However, ion implanting is not a widely used method to prepare VO2. Compared to other methods, the limitation in substrate selection and the high annealing temperature increase the manufacturing cost and complexity of the process. At the same time, the composite structure of the VO2 and substrate does not show any advantages over nanocomposite coatings prepared using high-quality VO2 nanoparticles.77 On the contrary, this structure could limit VO2 application in areas where good electrical properties is required.

4. Vacuum evaporation

4.1. Thermal evaporation

Thermal evaporation (TE) is also called vacuum evaporation. During this method, the target is thermally evaporated in a high vacuum chamber, where the vapour condenses on the substrate, forming the thin film. Since there is no interaction between the thermal source (such as laser, plasma and electron beam) with the target, precursors or substrates, the deposition process is simpler compared to other methods (Fig. 6).
image file: c8tc05014g-f6.tif
Fig. 6 Diagram of the thermal evaporation process (http://www.semicore.com/news/71-thin-film-deposition-thermal-evaporation).

A typical process to deposit VO2 films by TE usually consists of two steps.93,94 The first step is to prepare a vanadium-rich metallic film93,94 or VOx film.95 The target (typically metallic vanadium,94,96 VO2,94 V2O595) is placed in a heated crucible made of Mo or W, located under the substrate in a high vacuum chamber (2.7–6.7 × 10−3 Pa). Then, the target is resistively heated to evaporate to the substrate, which is pre-heated to 250–300 °C. The as-deposited films required post-annealing at about 400–550 °C under an Ar–O2 mixed gas94 or air (10.7–33.3 Pa)93 to fully convert into VO2 films. Aside from oxidizing the metallic thin film, Ke et al.97 reported a method to first grow V2O5 nanowires film under atmospheric pressure, then reduced them to VO2 (R). Other phases, including VO2 (B) and V2O3, could be also obtained by changing the reduction atmosphere.

Jiang et al.93 prepared VO2 films of ∼120 nm using TE with a metallic vanadium target. Their films showed a good infrared control ability (Fig. 7c), which was comparable to films prepared by sputtering. The MIT temperature of these films were less than 60 °C (Fig. 7b). They also found that the MIT performance of the films could be related to the post-annealing time (Fig. 7). The films showed a resistance change of 1 order of magnitude after 2 h annealing and reached to 2 orders of magnitude after 3 h annealing, but would then reduce for longer annealing times. Although TE is simple and promising, the reported works using this method only centred around preparing V2O398,99 or VOx,97,100 and few works could be found for VO2 films preparation after Jiang's work, where it was found that the MIT performance of prepared VO2 films was weak compared to other methods.94,95


image file: c8tc05014g-f7.tif
Fig. 7 (a) Relative thickness change with annealing time, indicating the metal vanadium is gradually oxidized to vanadium oxides. Dots represent the measured results and the solid curve the fitted data; (b) temperature dependence of resistance and (c) transmittance curves (250–2500 nm) of samples oxidized with different times. Samples show oblivious resistance and infrared transmittance changes at ∼55 °C after 2 h oxidization, while ΔA reaches as large as 2 orders of magnitude after 6 h oxidization.

4.2. Electron beam deposition (EBD)

Electron beam deposition is a kind of vacuum evaporation coating method. Different from TE, EBD uses an electron beam to vaporize the evaporation material (Fig. 8). Any material can be evaporated by electron beam evaporation. EBD can be used to prepare multi-component and high-purity films, benefiting from the fact that the electron beam can be accurately controlled and can directly affect the material to be evaporated. EBD equipment is also less expensive than that needed for other physical vapour-deposition techniques and further it shows the potential to produce very smooth films with large size, making it an excellent low-thermal budget deposition method for VO2 films for optical and electronic applications.101
image file: c8tc05014g-f8.tif
Fig. 8 Diagram of the electron-beam deposition process (https://en.wikipedia.org/wiki/Electron-beam_physical_vapor_deposition).

The EBD of VO2 requires a basic high-vacuum chamber (below 10−4 Pa). A vanadium target is typically used as the evaporation source.102–105 The target is put in a crucible under the substrate (about 250 mm). An incident electron beam (accelerating voltage of 10 kV) is used to evaporate the vanadium under a pure oxygen atmosphere (total pressure 0.06–0.1 Pa). The deposition rate of the vanadium oxide films is controlled by a quartz oscillator at the nominal rate of 0.03–0.07 nm s−1. Doping could be done using doped targets.

VO2 powder could also be used as an evaporation resource, either in powder form101 or compressed into a sheet.106 Different from using the vanadium target, oxygen is unnecessary during the evaporation, but a pressure of ∼0.5 × 10−4 Pa is required for pre-treating the VO2 powder or sheet. System pressure is maintained at ∼1.33 × 10−2 Pa during evaporation. The deposition rate of this method is controlled at about 0.3 nm s−1 by quartz crystal microbalance.

It seems that substrate heating during deposition is not the key parameter to obtain high-quality VO2 films with thermochromic performance, but a post-deposition oxygen annealing step (450–550 °C) or an applied radio frequency (incident power 10–50 W) is. In Marvel et al.'s work,101 a post annealing of as short as 5 min at 450 °C could generate a pure VO2 (M) phase from the as-deposited film (Fig. 9a and b). Leroy et al.103 used radio frequency applied to the substrate to increase the adhesion and decrease the porosity of the forming films during deposition.


image file: c8tc05014g-f9.tif
Fig. 9 (a) Surface morphology of an as-grown and annealing (from 5–60 min) VO2 film prepared by EBD, and their performance (b), (b-A) temperature dependence transmittance, (b-B) switching contrast vs. annealing time and (b-C) XRD; (c) photograph of VO2 film with different thicknesses prepared by EBD; XRD; (d) temperature dependence of resistance (e) of discharge, annealed and as-grown film, showing the comparable performances of the discharge and annealed films; (f) AFM of VO2 films prepared by EBD (f-A and f-B), PLD (f-C and f-D) and by sputtering (f-E and f-F); spectra transmittance of VO2 films prepared by sputtering (g and h) and by EBD (i and j), (g and i) are transmittance at different temperatures, (h) is reflectance from 250–2500 nm, (j) reflectance from 250–25[thin space (1/6-em)]000 nm.

Théry et al.105 comprehensively studied films deposited without any treatment (as-grown) with a 50 W radio frequency treatment (discharge) and with post-deposition annealing at 550 °C for 15 min (annealed) (Fig. 9c and d). In this work, the as-grown films did not show thermochromic performance even with the substrate heated to 500 °C. These films exhibited a poor structural quality (large deviation from stoichiometry, large mosaicity and large strains) and a purely metallic behaviour. Meanwhile, the annealed films exhibited an excellent thermochromic performance, with resistivity changes of more than four orders of magnitude across the MIT. These films also exhibited excellent structural properties (a low level of heterogeneous strain, a mosaicity confined into narrow regions close to the interface and a level of strain solely due to the film/substrate thermal expansion mismatch). They also found that films with similar electrical properties could also be obtained by only radio frequency discharge treatment during deposition. Compared to annealed films, these discharge films only suffered slightly degraded structural properties. It is crucial to add radio frequency during the electron-beam deposition of VO2.

VO2 films prepared by EBD could reach a comparable performance to that of other physical vapour-deposition techniques. A resistance change of as high as five orders of magnitude during MIT indicated the fine crystallizability of the obtained VO2 thin films.105 Furthermore, uniform films with a size of 100 × 100 mm2 were prepared by EBD, which evinced the potential of this method for large-scale preparation.105 However, Marvel's work showed that the density of films prepared by EBD is slightly lower than that prepared by PLD and by sputtering with similar annealing processing.107 A similar phenomenon could be inferred from the reflective spectrum of a sample prepared by other researchers,102 which was seen to be clearly lower than that of the samples prepared by sputtering. The maximum reflectance values were 38% and 43%, respectively, as marked in Fig. 5h and j.108 The denser the films are, the higher the reflection (Fig. 9g–j). In Marvel's work, the EBD films also showed lower high-temperature stability, which may be a result of the lower density.107 A much lower energy of the ejected target material (0.2 eV vs. 50 eV and 10–40 eV of PLD and sputtering, respectively107) may be one of the reason for the lower density. That is why an applied radio frequency could decrease the porosity of the formed films in Bessaudou's work.103 It is clear that the morphological evolution is dominated by strain imparted during the sputtering process. For films deposited by PLD and EBD, the substrate plays a more important role, with substrate de-wetting and epitaxy determining the film structure during solid-phase crystallization.107

5. Molecular beam epitaxy (MBE)

MBE (Fig. 10) is a widely used method to prepare high quality and homogeneous epitaxial crystal layers. It is highly reproducible in terms of both film thickness and composition control. Although various methods to prepare VO2 thin films have been studied, a process to prepare VO2 thin films with a perfect V–O stoichiometry and near single-crystal structure is still very attractive. Studies on the preparation of VO2 thin films using MBE and their MIT performance could only be found in recent years.
image file: c8tc05014g-f10.tif
Fig. 10 Simple sketch of a molecular-beam epitaxy system (https://en.wikipedia.org/wiki/Molecular-beam_epitaxy).

There are two main methods for VO2 thin films preparation by MBE. The first one, developed by Sambi et al.,109,110 is called the “periodic annealing” method. Here, before deposition, a TiO2(110) substrate is cleaned with cycles of Ar–ion sputtering followed by annealing at 573 K in O2 (1 × 10−6 mbar), to produce sufficient bulk oxygen vacancies to avoid the charging problem. Then, 0.2–0.5 monolayers (ML), corresponding to 5.2 × 1014 vanadium atoms per cm2, of amorphous V metal are deposited in vacuum (5 × 10−11 mbar) at room temperature by electron-beam evaporation. A post annealing process (at 423 K) in an oxygen atmosphere (7.5 × 10−7–1.5 × 10−6 mbar) follows to obtain an epitaxial VO2 layer. The sample is then cooled to room temperature and the next 0.2–0.5 ML of amorphous V metal is deposited. This cycle is repeated until a 3–5 ML thick epitaxial VO2 layer is obtained.111 The films prepared by Sambi showed ordered VO2 phase grown epitaxially on TiO2(110) with a rutile-type structure, but no MIT of these films had been observed.

Tashman et al.111 followed Sambi's work and deposited VO2 on TiO2(001). The resulting films did not show MIT either. They developed the following process and alterations:

(1) Use distilled ozone instead of O2;

(2) Increase the temperature from room temperature to 395 K and from 423 K to 473 K for deposition and annealing, respectively;

(3) Add a rapid post-annealing processing to 673 K (3 K s−1, 1 × 10−6 mbar of distilled ozone).

Detailed parameters are shown in Fig. 11a–c. With these alterations, obvious MIT was observed in films even as thin as 2.3 nm, with a resistance change in orders of magnitude (log(ΔR/R)) of 1.4. Tashman's work also showed that the transition width decreases monotonically with film thickness, while the hysteresis increases monotonically with film thickness.111


image file: c8tc05014g-f11.tif
Fig. 11 (a–c), Tashman's work: (a) illustration of the substrate temperature and background pressure of distilled ozone (PO3) used during the VO2 growth cycle, (b) RHEED images, along the [100] direction of TiO2 and VO2 in the growth cycle. Numbers in (a and b) represent different steps in the growth cycle: (1) prior to vanadium deposition, (2) the time vanadium deposition, (3) post annealing, and (4) the beginning of the next cycle, (c) LAADF-STEM image of the epitaxial VO2 film; (d–j) work of Paik et al., (d) RHEED patterns observed during the growth of a 10 nm epitaxial VO2 thin film, (e–j) XRD pattern, FWHM, c-axis length, temperature dependence of resistance, resistance change and transition width of deposited VO2 films with different thicknesses, respectively; (k–m) work of Fan et al., (k) XRD of VO2 films prepared at different oxygen flux rates, (l and m) SEM cross-section images of the obtained V2O3 and VO2.

Paik et al.112 adopted this process for the study of VO2 thin films (Fig. 11d–j), but with an even higher deposition and annealing temperature (523 K), and a post-annealing process up to 623 K. This annealing step enhances the coalescence of (001)-oriented VO2 islands, and improves the abruptness of the MIT.112 They also developed a process for TiO2 substrate pre-treatment.112

The other method is RF-plasma assisted Oxide Molecular Beam Epitaxy (OMBE), as reported by Fan et al.75 (Fig. 11k–m). In this method, a standard RF-plasma source was used to provide reactive oxygen radicals. Pure metallic vanadium powder was used as the target for e-beam evaporation. An Al2O3 crystal slice or TiO2 could be used as the substrates, which were degassed and annealed at 823 K in vacuum (4 × 10−9 mbar). The chamber pressure was maintained at 1.3–4 × 10−5 mbar during the film preparation. The evaporation rate of vanadium was controlled at 0.1 A s−1. An optimized flow rate of 1.8 sccm for pure VO2 film depositions was used as a lower oxygen gas flow would cause oxygen deficiency and could result in V2O3 film being formed instead (Fig. 11).

An extremely thin VO2 film could be prepared by MBE; the thinnest one with MIT reported is 1.5 nm by Paik.112 It seems that the thinner films have a lower MIT temperature.111–113 Fan et al. studied this phenomenon using interfacial strain dynamics and theoretical calculations, and claimed that the electronic orbital occupancy is strongly affected by the interfacial strain, which also changes the electron–electron correlation and controls the phase-transition temperature.113,114 Up to now, the lowest MIT temperature of pure VO2 thin films reported by MBE was 280.5 K, in which a film thickness of 3.3 nm has resistance changes of 2.3 orders of magnitude. The thinnest film (1.6 nm) did not show the lowest MIT temperature, probably due to diffusion of Ti substrate into the thin film layers.112 However, compared with the epitaxial VO2 films prepared by PLD on the similar TiO2(001) substrate with a similar thickness (from 3–15 nm),65 these thin epitaxial VO2 films show lower MIT temperature and smaller resistance changes during MIT. This may indicate higher stresses in these epitaxial VO2 thin films. More work could still be done in the MIT investigation of MBE epitaxial VO2.

Besides the advantages in preparing extremely thin films, MBE also shows advantages in the preparation of large size thin films, accurate stoichiometry, and especially low-temperature deposition (350 °C111). Low-temperature deposition for VO2 means a reduced lattice mismatch due to the thermal expansion and diffusion of substrate elements and higher crystal quality, which can be used in basic theory research113,114 and in the development of precision components.115,116 It could also enable the application of VO2 in electronic devices, such as in uncooled infrared focal plane arrays. However, MBE requires a crystallized substrate with very little lattice mismatch with the deposition materials and expensive equipment, which limit its wide usage.

6. Sputtering

Sputtering is a PVD process in which surface particles are physically knocked off the source material, then condensing on the substrate. The depositing source, called the ‘target,’ is bombarded by energetic gaseous ions in ‘plasma.’ Kinetic energy from the incoming ions is transferred to the particles on the surface of the target, breaking the bonds between them and the bulk of the target. These surface particles are changed from solid to gas phase through the mechanical forces of the bombarding ions. Fig. 12 is a schematic of a basic sputtering system.
image file: c8tc05014g-f12.tif
Fig. 12 Basic set-up of a sputtering system (https://samaterials.wordpress.com/2015/10/28/what-is-sputtering/).

The first crucial factor is the sputtering gas, which normally consists of ultrapure argon for non-reactive deposition or a mixture of argon with other reactive gas (O2, N2, CH4, H2S, etc.) for reactive deposition. The sputtering gas mixture is ionized and turned to plasma through electrical discharge, which happens with the target as the cathode and the substrate as the anode. In this stage, the kinetic energy of ions in the plasma is dependent on the power of the electricity applied to the system. Subsequently, this also controls the kinetic energy of the sputtered particles. The operating pressure of the sputtering gas mixture is also important as it controls the flux of gaseous ions and thus the deposition rate. After being sputtered out of the target surface, ejected particles are deposited on the substrate, forming a thin film of the sputtered target, or of the product in the case of reactive sputtering. The properties of the thin film are greatly dependent on the structure and temperature of the substrate. Post-annealing can also be done after deposition to improve or change the quality of the deposited thin film. In summary, there are 4 main parameters that define a basic sputtering setup: (1) power supplied, (2) operating pressure, (3) substrate type, and (4) substrate temperature. For the same sputtering system, changes made to these parameters are the key to change and improve the thin film quality.

Currently, depending on the type of power supplied to the system during sputtering, there are several types of system, such as direct current sputtering, radio-frequency sputtering, magnetron sputtering, high-power impulse magnetron sputtering (HiPIMS), and ion-beam sputtering. Depending on the type of sputtering gas, systems can also be classified as either reactive or non-reactive sputtering. The methods to use these systems in depositing a VO2 thin film are discussed in subsequence sections.

6.1. Reactive DC-magnetron sputtering (DCMS)

Reactive DC-magnetron sputtering is characterized by its three main features:

(1) The sputtering gas is a combination of argon and reactive gas;

(2) DC power is used as the electrical source for ionization during sputtering;

(3) Magnetron sputtering is an add-on to the traditional system. It uses a magnetic field to trap and restrain electrons in plasma longer, thereby creating more gaseous ions in plasma, subsequently enhancing the ionization rate and increasing the deposition rate of the system.

By using a DC power source, this technique is the lowest cost and simplest technique for metal deposition. However, this is also a limitation because only conductive materials can be used as the target.

In the case of VO2 deposition, vanadium is a suitable target as a transition metal, and a mixture of argon and oxygen can be used as the reactive sputtering gas. Vanadium is known for its various oxidation states (II to V) so it is important to achieve the correct oxidation state for VO2. According to Kang,117 the composition of a VOx system can be influenced by the temperature during deposition and the mole fraction of V and O. Despite knowing the required mole fraction from the literature, it is hard to achieve perfect VO2 deposition in practice due to the lack of control between the ratio of sputtered vanadium and oxygen in the gas mixture. Thus, factors such as the O2/Ar ratio, overall operation pressure, substrate temperature, and structure are tuned so that the V/O mole fraction can be controlled. Fig. 13 is the phase diagram of VOx as a function of substrate temperature and oxygen partial pressure reported by Griffiths and Eastwood in 1974.118 This gives a general idea of how each stated parameter affects the phases in a sputtered VOx film. The detailed effects of each parameter are discussed in the following sections.


image file: c8tc05014g-f13.tif
Fig. 13 Phase diagram of VOx.
Without a post-annealing process. Without any post-annealing process, the effects of factors such as the reactive gas ratio and substrate temperature can be observed more clearly. Yuce et al. fabricated a VO2 thin film on a c-cut sapphire (Al2O3(0001)) substrate by optimizing the reactive oxygen flow ratio (Fig. 14b) while keeping constant the 550 °C substrate temperature, 50 W power and 0.85 Pa operational pressure.119 Raman spectra of the thin films in Fig. 14a show a direct correlation between the increased presences of V2O5 in the deposited film and the increasing oxygen flow ratio. This is consistent with how a higher ratio of reactive oxygen flow (2.50–3.00%) leads to a higher oxygen mole fraction, which then results in a higher oxidation state of vanadium after reaction. Vanadium(V) was formed instead of vanadium(IV). The optical properties and electrical properties (Fig. 14c and d) of the deposited VO2 also confirmed the abundant of VO2 at 2.25% O2 as the MIT was clear at this level but was not significant at a higher oxygen level. It can be concluded that the flow ratio of O2 directly affects the O2 mole fraction and thus, the phase formation of VOx.
image file: c8tc05014g-f14.tif
Fig. 14 Characterization of VO2 deposited by reactive DCMS without annealing. (a and b) Raman spectra of thin films produced at varying O2/Ar flow ratios. (c) Optical and (d) electrical properties of samples in (a). (e) SEM images of VO2 deposited with different substrate temperatures. (f) XRD spectra of samples in (e). (g) Optical and (h) electrical properties of samples in (e). (i) XRD spectra of VO2 on different substrates. (j) Raman spectra of VO2 on different ZnO thicknesses. (k) SEM images of samples in (j). (l) Optical and (m) electrical properties of VO2 on a 235 nm ZnO buffer layer.

Zhao et al. conducted an experiment in which seven samples of VO2 films of 120 nm thickness were produced on a c-sapphire substrate at different substrate temperatures from 550 °C to 700 °C, while the other parameters were kept constant (0.4 Pa, 11% O2/Ar flow ratio).120 From the SEM images (Fig. 14e), the samples could be divided into 3 three representative groups: G1, G2 and G3. G1 (S550, S575 and S600) had a coarse, rough surface with a relatively larger grain size. It was reported that G1 contained a porous structure as seen from the cross-section SEM images. G2 (S625 and S650) had a smooth surface and compact morphology with fine grains. On the other hand, G3 (675 and S700) had a rougher surface than G2 but less so than G1. According to the report, there was no porous structure detected in G3. These groupings in the microstructure also translated to differences seen in their XRD spectra. From Fig. 14f, the author concluded that the double peak in the XRD spectra could be attributed to the transition of vanadium oxide from V5+ → V4+ → V3+ from low to high temperature. The presence of intermediate phases during the transition from V2O5 to VO2 and from VO2 to V2O3 was the main reason why there were double peaks in the XRD spectra. This result showed the influence of substrate temperature on the stoichiometry of the deposited vanadium oxide film.

A crucial aspect of any sputtering deposition set-up is the choice of substrate. c-cut sapphire was used by both Zhao et al. and Yuce et al. due to its ability to operate at high temperature, which was higher than 550 °C in both reports. In practice, soda-lime glass is a better substrate for testing because of its potential real-life application. However, soda-lime glass at high temperature exposes the thin film to the potential for the diffusion of sodium (Na) into the thin film, thus diminishing the transformation ability of VO2.

Zhu et al.121 deposited VO2 on soda-lime glass substrate at the low temperature of 300 °C with 3 different buffer layers of TiO2, SnO2 and SiO2. The other parameters did not deviate so much from other reported reactive DCMS set-ups: 80 W power, 0.55 Pa operation pressure, 3.25% O2/Ar flow ratio. The XRD patterns in Fig. 14m show that TiO2 is not a suitable buffer layer because it was not able to hinder the diffusion of Na into VO2, thus causing the detection of NaV3O8 and NaV2O5. The SiO2-buffered sample showed the presence of (M) phase VO2 with its characteristic peak. However, peaks from the V3O5 phase were also present. This means that the SiO2 buffer layer did not manage to help create a pure VO2 thin film. Only the SnO2 buffered sample was relatively pure.

Zhu et al.122 proceeded further in this area by testing the deposition of VO2 on different thicknesses of ZnO-coated soda-lime glass in order to investigate the effect of the buffer layer thickness on deposited VO2. A low temperature of 320 °C was used, with the other typical parameters being constant: 120 W power, 0.5 Pa operational pressure, and 4.5% O2/Ar flow ratio. It was found that a thicker buffer layer promoted the crystallinity of VO2 at a low substrate temperature. This could be observed by comparing the SEM image of VO2 with and without different ZnO buffer thicknesses in Fig. 14l and m.

The studies discussed in this section are representative of work in this field and included here to demonstrate the effect of having a buffer layer on VO2 thin films. Other materials have also been used as buffer layers in other studies and are discussed in a later section.

With a post-annealing process. Post-annealing in reactive DCMS is typically an extra step to guarantee the quality of a deposited thin film. This is done to either crystallize an amorphous film, to continue the reaction in reactive gas without further sputtering or to purge excessive reactive gas from a deposited film. Yu et al. deposited VO2 film on a polished silicon substrate at room temperature using varying O2/Ar flow ratios, 0.06 Pa operation pressure and 90 W power.123 A low temperature of the substrate undoubtedly resulted in amorphous VO2, which required recrystallization at 400 °C for 1 h in a nitrogen atmosphere. Nitrogen was used as an inert gas so it would not affect the established stoichiometry of the amorphous VO2 film. The atmospheric or ambient pressure also contributed to keeping the stoichiometry.

Not only used as a remedy to low-temperature depositions, post-annealing is also used to further improve the quality of VO2 deposited at high temperature. Dou et al. reported comprehensive findings on this by conducting a 2-step annealing process on deposited VO2 (m/a/r-plane sapphire, 2% O2/Ar flow ratio, 1 Pa, 550 °C).124 After deposition, VO2 film was crystallized at 500 °C for 400 s in 220 Pa atmosphere, followed by air oxidizing in atmospheric pressure at 450 °C for 200 s, 400 s, 600 s and 1000 s. The XRD patterns (Fig. 15a) showed a significant decrease in VO2 presence with the increase in air oxidizing time. As expected, the V2O5 presence also increased due to the further oxidation of vanadium to the V5+ state.


image file: c8tc05014g-f15.tif
Fig. 15 Characterization of VO2 thin film deposited by reactive DCMS with annealing. (a) XRD spectra of oxidized VO2 on m-plane sapphire. (b) XRD patterns of vanadium oxide film before and after annealing. (c) SEM images of VO2 samples on different substrates before and after 1000 s of annealing. Electrical properties of VO2 samples in (c) before (d) and after (e) annealing. (f) XPS spectra of VO2 thin film with different O2/Ar ratios after annealing: 25%, 32.5%, 42.5% and 50% respectively. (g) Spinodal decomposition process to achieve a TiO2–VO2 nanocomposite matrix. (h) Optical performance of A1 (sample fabricated by process in (g)), C1 (no annealed sample), V1 (pure VO2 sample) by Sun et al. (i) Chen et al.'s work. (i1) and (i3) TEM images of the V-Ti separation after annealing, inset shows the SAED pattern. (i2 and i4) Combined STEM and energy-dispersive X-ray spectroscopy mapping of V-blue and Ti-green.

In contrast, Huang et al.125 conducted a post-annealing process in an O2-lacking environment to transform the mixed phase vanadium oxide obtained from deposition to purer stoichiometry VO2. In this report, vanadium oxide film was deposited in a 12.5% O2/Ar flow ratio, 0.11 Pa operation pressure, 200 W power and 300 °C substrate temperature. XRD patterns of the deposited film revealed the presence of a mixed VO2 and V2O5 phase. After annealing in an Ar atmosphere, the V2O5 was eliminated, as seen in the XRD patterns in Fig. 15b.

The atmosphere for the post-annealing process was the deciding factor for achieving a VO2 thin film. An inert atmosphere could be used for the crystallization of an amorphous film and for purging the excessive reactive gas, while an oxygen-rich atmosphere can support the further oxidation of the film to reach the desirable stoichiometry.

VO2 deposition on m-, a- and r-plane sapphire was also reported by Dou et al. Before annealing, different substrates yielded different thin film microstructures (Fig. 15c). After 1000 s of air oxidation, all three samples turned into a similar microstructure. However, MIT investigation of these samples (Fig. 15d and e) showed they slowly lost their transition ability as they were subjected to longer post-annealing time. The similar microstructure resulting from the longer annealing time was believed to be V2O5, which agrees with XRD spectra in Fig. 15a.

While the effect of having different substrates may not last after annealing, the effect of different O2/Ar flow ratios is reported to be conserved, as reported by Xu et al. In their report, VO2 thin film was deposited on a Si(100) substrate at room temperature, 200 W power, 0.63 Pa and O2/Ar ratios of 25%, 32.5%, 42.5% and 50%. The deposited film was annealed in vacuum for 2 h at 450 °C.126 Based on the XPS spectra (Fig. 15f), it could be observed that the vanadium oxide undergoes oxidation from V3+ → V4+ → V5+ with the increasing amount of O2. This is consistent with what was reported by Yuce et al. as mentioned in the previous section. It can be concluded that if the annealing time does not transform VO2 to V2O5, it has no influence on the effect of the different reactive gas flow ratios.

Post-annealing can be a powerful tool to remedy and correct the stoichiometry of the VOx film or to complete the fabrication process. However, excessive annealing causes the formation of V2O5 from VO2 and hinders the film MIT ability. Careful planning for the annealing process is therefore needed to achieve a balance.

Aside from the traditional processes mentioned above, post-annealing can also be utilized to fabricate composite films of VO2 through a spinodal decomposition mechanism. Sun et al.127,128 and Chen et al.129 managed to fabricate a self-assembled multiplayer structure TiO2–VO2 thin film (Fig. 15g–i) by sputtering amorphous VxTi1−xO2 at room temperature, followed by annealing this amorphous film to achieve a composite structure. The final matrix, as seen in Fig. 15i, had alternating V-rich and Ti-rich phases. As reported by Sun et al.127 in Fig. 15h, the optical properties the VO2 thin film were boosted significantly by doping with TiO2 through this spinodal decomposition process.

6.2. Reactive RF-magnetron sputtering (RFMS)

The traditional sputtering system using DC power source is only capable of using conductive materials as the target. The need to sputter a wider range of materials thus led to the development of RF sputtering in which an alternate current (AC) is used as power source instead. This alternate current with a standard frequency of 13.56 MHz is used to create an alternate bias between the substrate and the target. By constantly changing the target as the cathode and anode, it prevents insulating the target to accumulate a positive charge over the course of sputtering. Thus, RF sputtering can be used for conducting, insulating and semiconducting materials. This means that metallic VOx materials can be used as a sputtering target. This is further discussed in the non-reactive sputtering section.

Reactive RF-magnetron sputtering is very similar to its DC counterpart and so the traditional parameters, such as O2/Ar flow ratio, substrate heating and substrate microstructure, still play crucial roles in deciding the quality of deposited films. However, using an AC power source allows generally a higher sputtering power to be used, ranging from 100 W130 to as high as 450 W.131 This increased sputtering power also eliminates the annealing step in most reported papers, except when room temperature deposition is attempted.132

Despite the similarity to its DC counterpart, the effects of conventional parameters are still investigated for VO2 thin film deposited with reactive RF-magnetron. Jiang et al. reported investigations on a series of VO2 samples on quartz glass at 450 °C with 200 W RF power under 1.0 Pa of varying O2 flow.133,134 Instead of the traditional stoichiometry change with varying the O2/Ar flow ratio, the report gave a fresh viewpoint on the effect of the O2/Ar flow ratio on single-phase VO2 thin film. While the XRD patterns (Fig. 16a) showed no signs of other vanadium oxide phases in the deposited thin film, AFM images (Fig. 16d) showed an increasing crystal size as the O2/Ar flow ratio was increased from 2% to 5%. Variations in the optical properties and transition temperature (Fig. 16b and c) were also reported with an exceptional transition temperature of 46 °C for the 2% sample.


image file: c8tc05014g-f16.tif
Fig. 16 Characterization of VO2 deposited by RFMS: (a) XRD patterns, (b) transmission spectra, (c) transition temperature for each sample, (d) AFM images of each sample, (e and f) XRD of VO2 deposited on different substrates at different temperatures. (g) RFMS with RF substrate bias set-up. (h) Transition temperature and (i) electrical properties changes of VO2 thin film against the substrate bias applied.

Some effects of different substrates and substrate temperatures were also reported. Panagopoulou et al. deposited VO2 at 400 °C and 300 °C on various substrate materials: glass/ZnON, glass/SnO2, commercial glass and a silicon substrate.135 XRD analysis (Fig. 16e and f) produced non-surprising results indicating a better-formed VO2 using the higher substrate temperature of 400 °C. Broader peaks from the 300 °C samples provided evidence of secondary phases, whether different VO2 phases or different VOx stoichiometry, formed during deposition.

Aside from the traditional parameters, different methods to modify the sputtering process are also possible under RFMS. One of these is to create a secondary bias acting on the substrate concurrently with the RF acting on the target. Azhan et al. conducted a series of experiments using reactive RF-magnetron sputtering and RF substrate bias to deposit VO2 on a single-crystal sapphire substrate.136,137 This set-up is shown in Fig. 16g. By using RF substrate bias during deposition, the reported transition temperature of the VO2 film was 36 °C for 40 W biasing power. This is an extremely good value compared to the conventional transition temperature of 68 °C. However, films deposited with a higher biasing power showed significant decreases in MIT ability. A balance between the transition temperature and MIT properties is thus needed so that VO2 films deposited by this method can be considered for further application.

6.3. Reactive pulsed direct current magnetron sputtering (pDCMS)

Pulsed direct current magnetron sputtering is the result of combining the DC power source from conventional DCMS and the concept of alternating bias to prevent target charging from RFMS. Instead of using an AC power source with frequency in the range of MHz, pDCMS still uses a DC power source with a pulsing capability in the range of kHz. This prevents charge accumulation on the insulator target surface, allowing a wider range of materials to be used with a DC power source. Additional systems, such as substrate bias, can also be used in this case while being traditionally redundant in conventional DCMS.138

6.4. Reactive high-power impulse magnetron sputtering (HiPIMS)

High-power impulse magnetron sputtering or HiPIMS is a recently developed technology that combines magnetron sputtering and pulsed power. Similar to how DC and RF sputtering are characterized by their method to create plasma from the sputtering gas, HiPIMS is characterized by the use of high voltage and a short duration energy burst (pulse) to generate plasma. Because of this plasma generation mechanism, high-density plasma can be formed on the surface of target materials, causing a higher degree of ionization of the sputtered particles as well as a higher rate of molecular gas dissociation compared to other sputtering methods. In other words, an HiPIMS deposited thin film often has a higher density than films from other methods.

Using HiPIMS for VO2 deposition is still a relatively new research area. Due to the theoretical higher film density, it is expected that high-quality VO2 could be deposited with much lower temperature (<400 °C) compared to its DC and RF counterpart without using any post-annealing process. This allows for a wider range of substrate selection; even a polymeric substrate has been attempted.139 Aside from the conventional parameters like in other previously mentioned reactive sputtering methods, HiPIMS systems are also identified by the frequency and duty cycle of their pulses. Sadly, there has been no report yet on the effect of these parameters on similarly prepared VO2 samples. HiPIMS systems are compatible with additional substrate bias. Results both with131,138–140 and without141 substrate bias have been reported.

HiPIMS vs. RFMS. Loquai et al. conducted a comparison between VO2 deposited on B270 glass using HiPIMS and RFMS.139 The processing parameters utilized, as seen in Fig. 17a, were comparable between both deposition processes, except for a different O2/Ar flow ratio. From the transmission spectra (Fig. 17b and c), the optical properties of VO2 film by both depositions processed were also similar despite the difference in O2 flow. This means that HiPIMS could produce similar grade VO2 with half the amount of O2 as compared to RFMS.
image file: c8tc05014g-f17.tif
Fig. 17 (a) Deposition parameters and (b) optical properties of VO2 thin film by RFMS and HiPIMS. (c) Measured discharge voltage during deposition vs. O2 fraction. (d) Transmission spectra of VO2 thin film with no bias and varying bias and a constant O2 fraction of 8.7%.
HiPIMS vs. pDCMS. Aijaz et al. conducted an extensive study on the effect of substrate temperature, O2/Ar flow ratio and substrate bias on the quality of VO2 thin films deposited by both HiPIMS and pDCMS.138 An average power of 600 W was used for both HiPIMS (500 Hz, pulse on-time of 100 μs) and pDCMS (100 kHz, pulse off-time 1.6 μs). By using a measured discharge voltage against O2 fraction plot (Fig. 17c), the point of transition between the metallic film and insulating film could be singled out. Thus, the stoichiometry of the VO2 film could be identified. The thermochromic performance of VO2 by HiPIMS at different O2 fractions, temperature and substrate bias is shown in Fig. 17d. As concluded in previous sections, increasing the O2 fraction leads to a transition from V3+ → V4+ → V5+. This was reaffirmed in this report. Comparison with VO2 by pDCMS showed that HiPIMS is able to produce functional thermochromic VO2 at a much lower substrate temperature.

6.5. Inductively coupled plasma-assisted sputtering (ICPS)

ICPS is an unconventional modification to conventional sputtering methods. In addition to the magnetron plasma produced by a magnetic field near the target surface, conductive coils are installed in the space between the target and the substrate (Fig. 18a). This configuration can further enhance the plasma density during sputtering, reaching a density comparable to HiPIMS but without the use of extremely high energy pulses.
image file: c8tc05014g-f18.tif
Fig. 18 (a) Schematic of an ICPS system. (b) Schematic of an ICMS system. (c) Schematic of an ion-beam-assisted sputtering system. XRD spectra of VO2 on Ti/Si substrate (d), and on an ITO/glass substrate (e).

Reports on VO2 deposition using ICPS are limited to a couple of research group due to the niche set-up.142–146 Mian and Okimura compared conventional RFMS and ICPS using low-temperature (250 °C) deposition and traditional temperature (400 °C).142 XRD spectra comparison (Fig. 18d and e) showed an exceptional quality of the VO2 thin film deposited at 250 °C by ICPS. The XRD peaks from the 250 °C samples from both the ITO and Si substrate were comparable, even sharper than the 400 °C peaks by conventional RFMS.

6.6. Inverted cylindrical magnetron sputtering (ICMS)

ICMS is another modification of the conventional RFMS set-up. Instead of additional magnetic coils, like in ICPS, a perpendicular cathode and anode set-up is used (Fig. 18b). This set-up is useful for deposition on a substrate with a complex geometry. Aside from this niche usage, other parameters for ICPS do not deviate too much from conventional RFMS. Research on VO2 deposition by ICPS is rare but the reported results to date are comparable to those achieved with a conventional system.147,148

6.7. Ion-beam-assisted sputtering (IBS)

Ion-beam-assisted sputtering is different from all the previously discussed sputtering methods. Instead of generating bombarding ions inside a plasma of sputtering gas mixture, ions are directly propagated towards target materials at an angle so that ejected particles are deposited on the substrate surface (Fig. 18c). Because ions are injected into the chamber using ion guns, the operating pressure in an IBS system is much lower than that in conventional plasma sputtering. IBS has the advantage of controlling the stoichiometry of the deposited film by tuning the power of the ion guns. This is especially useful for hard-to-control stoichiometry compounds, such as VOx. However, IBS is still mainly used for the non-reactive sputtering of vanadium metal thin films instead.149,150 Reactive IBS requires a dual ion-gun configuration to control O2 and Ar separately.151

6.8. Non-reactive sputtering methods

A non-reactive VO2 sputtering system either uses a VOx ceramic target as the source material or consists of 2 steps: deposition of a vanadium metal thin film on a substrate, and oxidation of the said film into VO2.
Ceramic target system. As mentioned in the previous section, the molar ratio of vanadium and oxygen during sputtering is one of the most crucial factors to achieve stoichiometric VO2. A method to by-pass this issue is to use a VO2 ceramic target as the source material during sputtering. Theoretically, the vanadium to oxygen ratio in the produced film should be the same as in the source. Yu et al. reported a typical process to produce stoichiometric VO2 using this method.152 Non-reactive RFMS was used to sputter VO2 ceramic onto Eagle XG glass with a pure Ar atmosphere. Post-annealing was done under vacuum to enhance the thin film crystallinity. As expected, RFMS was used due to it being the most common system for ceramic sputtering. This is because most ceramics are insulators and are not suitable to be sputtered using a DC power. However, VO2 is unique because it exhibits metallic properties at room temperature. Thus, DCMS was also attempted and shown to be successful by Sun et al.153 VO2 is not the only ceramic target that can be used as a thin film source, and a V2O5 target has also been tested.154 Due to having a different stoichiometry, an additional annealing process in an O2-rich environment is required to reach the final VO2 thin film.
Vanadium target system with pure argon sputtering. A VO2 thin film can also be achieved through the oxidation of a sputtered vanadium film. DCMS is the most common method to produce a vanadium film as it is the cheapest and most accessible sputtering system.155–160 There are also reports, however, on IBS being used instead.149,150 The deposition of a vanadium film is applicable for a wide range of substrates because it is common practice to conduct deposition at room temperature on a substrate. The crystallinity and morphology of the as-deposited vanadium film are not the emphasis of these studies. Most of them instead have been focused on the effect of the annealing and oxidation parameters on the final VO2 film. Details are not discussed here as they are not the focus of this paper. However, it is important to note that this is one of the main synthesis routes of a VO2 film through a sputtering system.

7. Strategies to improve the performance of VO2 thin films using PVD

As discussed in the beginning of this review, VO2 is not a perfect material and the multiple intrinsic limitations of VO2 need to be addressed before it can be applied commercially. Over the years, concurrently with attempts to fabricate high-quality pure VO2, various strategies have been developed to modify the matrix to improve its feasibilities. Each of the following strategies is proposed to solve at least one of the main problems of pure VO2, namely the high transition temperature τC, low Tlum and ΔTsol, and substrate effects.

7.1. Elemental doping

The deposition of VO2 was performed to investigate its MIT properties with different fabrication parameters and conditions. In order to manipulate the transition temperature of VO2, the doping of various elements into the VO2 matrix has become a common strategy. Table 1161 shows a non-exhaustive list of the dopants that have been investigated over the years and their effects on the intrinsic properties of VO2. Tungsten is the most common dopant due to its ability to greatly reduce the transition temperature of VO2 at the expense of reduced optical properties. Therefore, reports of doped VO2 have mainly focused on W,161–163 followed by Al,164 Mg161,165–167 and surprisingly Si.168,169 DCMS and RFMS are most common methods used to fabricate doped VO2. For these processes, there are multiple methods where the dopant amount can be controlled:
Table 1 Effects of selected doping on the optical properties of VO2
Dopant Limit Effect on τC Effect on Tlum Effect on ΔTsol
Eu3+ 4 at% ↓6.5 °C/at%
Mg2+ 7 at% ↓3 °C/at%
W6+ 2.5 at% ↓23 °C/at%
F 2.1 at% ↓20 °C/at% N.A. N.A.
Mo6+ 2.5 at% ↓12 °C/at%
Nb5+ 4 at% ↓8 °C/at%
P3− 1 at% ↓13 °C/at% N.A. N.A.
Fe3+ 1.4 at% ↓6 °C/at% N.A. N.A.
Sb3+ 7 at% ↓1 °C/at% N.A.
Zr4+ 11 at% N.A.


(1) A limited amount of dopant can be attached to vanadium target during sputtering. An equal sputtering power is applied on the target and dopant pellets. The dopant amount in the film is controlled through the number of pellets used.162,164,165,168,169 This is the most common set-up as a single-target configuration with one power source is applicable.

(2) The dopant and vanadium can also be co-sputtered as two targets, using different power sources.166,167 This is less common due to the limited accessibility of the co-sputtering configuration.

(3) An alloy of a dopant with vanadium can also be used as a sputtering target.163 This is the least common method due to the limited choice of dopant and the dopant ratio that can be tested.

(4) An amorphous solid solution of a dopant and vanadium can be co-deposited on the substrate, followed by annealing to spinodally decompose the matrix into a nano-composite structure. Similar systems and crystal structures are needed for this to be possible.127

Aside from the common rare earth elements and transition metals listed in Table 1, noble metals, such as Au170,171 and Ag,172–174 have been studied as potential doping elements in recent years. The surface plasmon resonance (SPR) properties exhibited by those elements at the near-IR-vis wavelength open up the possibilities of localized heating to decrease the τC of the VO2 matrix.

7.2. Multilayer structures

Buffer layer. As mentioned in previous sections of the review, the usage of buffer layers has been an integral part of the deposition process of VO2. Buffer layers serve as a shortcut to broaden the possibilities of the surfaces to grow a VO2 thin film while keeping glass as the main backing material. The usage of any kind of buffer layers is largely to offset negative effects caused by the chosen substrates on the performance of VO2 thin films. Typically, it acts as a diffusion barrier to prevent impurities from the substrates, such as sodium ions in soda lime glass, from diffusing into VO2 during deposition, and it can also be used as the basis for the epitaxial growth of a VO2 thin film as well as a seed layer that can facilitate the crystallization of VO2 under unconventional deposition conditions, such as low deposition temperature.121,122,175–177 Several studies on different compounds, including but not limited to SiNx,175 ZnO,121,122,176 V2O3,177,178 TiO2121,176 and SnO2,121,176,179 have been done focusing solely on these two benefits of having a buffer layer.

However, some compounds push the advantages of buffer layers further than just being a seed layer or a diffusion barrier. Montero et al.179 used In2O3:Sn (ITO) as a conductive layer on soda lime glass to enhance the effect of substrate bias during the RFMS deposition of a VO2 thin film. In this study, a buffer layer was used to expand the fabrication condition of VO2, activating an otherwise insulating material as a suitable substrate. Other researchers attempted to directly improve the performance of VO2 thin film with an appropriate buffer layer. For instance, Chang et al.180 successfully showed that a Cr2O3 buffer layer could enhance the optical properties of VO2 thin film, achieving respectable values of ΔTsol = 12.2% and Tlum = 46.0%, which were enhanced by 4.4% and 9.6% compared with pure VO2.

Antireflection coating and multilayers structure. Because buffer layers are largely used as a strategy to achieve high-quality VO2 thin films, their effect on the optical performance of VO2 is mostly indirect and unrecognized in most studies utilizing them. This is due to the upper most layer of the matrix still being VO2. The interaction with incident light is mostly unmodified with or without a buffer. Hence, an antireflection (AR) coating has been adopted as a strategy to improve the optical properties (ΔTsol and Tlum) as well as to reduce interaction between O2 in the atmosphere with VO2 thin film, which has been proven to be easily oxidized to V2O5 with prolonged uncontrolled exposure.181 Combining the AR coating and buffer layers into a VO2 matrix opens up more avenues for development. The properties of the VO2 matrix are more flexible as they are now dependent on changes to not only the VO2 thickness and crystallinity but also to the type of buffers, type of AR coatings, and their thickness, respectively. A non-exhaustive summary of studies using AR coatings and multilayers structure is listed in Table 2. It is observed that there is a delicate balance between the enhancement of Tlum and ΔTsol. An increment of one parameter usually comes with the sacrifice of the other one. It is crucial for studies to understand this relationship and to achieve a balance of these two parameters.
Table 2 Optical performance enhancement of selected VO2 system using AR coating
Buffer layer AR coating Effect on Tlum Effect on ΔTsol Ref.
ZrO2 ↑18.2% 182
AlxOy 183
TiO2 TiO2 ↓2.9% ↑1.6% 184
↑16.9% ↑1.5% 185
↑3.2% ↑5.4%
↑0.6% ↑3.9% 186
↑24.9% ↓1% 187
SiNx SiNx Highest ↑6.65% Highest ↑3.4% 188
WO3 WO3 ↑17.4% ↓1.8% 189


8. Recommendations and future work

Despite the extensive efforts by several research groups all over the globe in recent years, VO2 thin films for smart window application are still largely theoretical and only feasible in a laboratory environment. PVD in general is an effective approach to deposit VO2 due to its relative simplicity to control the stoichiometry, high crystallinity and high performance. However, the high temperature and high-vacuum conditions required for the deposition continue to be a hindrance to the mass production of VO2-based materials for commercial usages.

While they can easily be fabricated nowadays, the optical and thermochromic performances of VO2 thin films continue to be a significant roadblock.190 At this moment, while it is possible to achieve a near room temperature τC for VO2 thin films through doping, it often comes with it a sacrifice to either Tlum or ΔTsol due to the resulting impurities and irregularities in the films caused by the introduction of foreign atoms into the matrix. Meanwhile, Tlum and ΔTsol have been increased by a variety of methods with limited effects, such as a multi-layered architecture (i.e. buffer layers,121,122,135,143,166,168 or sandwich structure181) or by the nano-patterning of VO2.191,192 However, it is of utmost important for all these three key parameters to be improved simultaneously. Further studies into VO2 performance optimization should adhere to this line of thought. On the other hand, the addition of functional layers onto the VO2-based smart window architecture has opened up the question of whether multi-functional smart windows can be achieved. Further research into this area by adding sub-functions, such as self-cleaning or self-healing ability, would keep VO2-based smart windows competitive among the energy conservation research landscape and functional windows.193–197

It is to be noted that most studies about VO2 focus on material properties and ignore the product feasibility aspect of smart windows. Currently, most deposition of high-quality VO2 thin films is limited to experimental substrates, such as fused silica, and the transition to regular soda-lime window glass is still a challenge due to the issue of sodium's diffusion and substrate mismatch. A method to circumvent this is to insert buffer layers, but this is complicated and detrimental to the crystal formation. Furthermore, the mass production of VO2 coating is still in its infancy and more effort is required to reduce the deposition temperature and enhance the crystallinity and design of the multifunctional layers.

In conclusion, it is crucial to continue to increase the performance of VO2 thin films as a thermochromic material as well as to bridge the gap between the laboratory standards and industry large-scale production. Continuing efforts to reduce the deposition temperature, enhance the crystallinity and uniformity as well as performance improvement and upscaling are needed to promote the commercialization of VO2 in thermochromic smart window applications.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

Dr Long and Prof. Gao designed the outline. T. D. V., Dr Long, and Dr Liu wrote sputtering section and strategies to improve (Sections 6 and 7). Dr Chen and Prof. Gao wrote evaporation sections (Sections 2–5). T. D. V. and Dr Long wrote the abstract, introduction, and conclusions. Dr Zeng read through the paper and advised accordingly. This research was supported by the Ministry of Science and Technology of China (2016YFB0303901), Singapore Minster of Education (MOE) Academic Research Fund Tier one RG200/17, and the National Research Foundation, Prime Minister's Office, Singapore, under its Campus for Research Excellence and Technological Enterprise (CREATE) program, the National Natural Science Foundation of China (51702209, 51702208, 51873102), and the Shanghai Municipal Science and Technology Commission (18JC1412800).

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Footnote

These authors contributed equally.

This journal is © The Royal Society of Chemistry 2019