Thin-film materials with 'smart' properties that react to temperature variations, electric or magnetic fields, and/or pressure variations have recently attracted a great deal of attention. Vanadium dioxide (VO2) belongs to this family of 'smart materials' because it exhibits a semiconductor-to-metal first-order phase transition near 340 K, accompanied by an abrupt change in its resistivity and near-infrared transmission. It is also of great interest in condensed-matter physics because it is a classic strongly correlated electron system. In order to integrate vanadium dioxide into microelectronic circuits, thin-film growth of VO2 has been studied extensively, and studies of VO2 nanoparticles have shown that the phase transition is size-dependent. This paper presents a broad overview of the growth techniques that have been used to produce thin films and nanoparticles of VO2, including chemical vapor deposition, sol–gel synthesis, sputter deposition and pulsed laser deposition. Representative deposition techniques are described, and typical thin-film characteristics are presented, with an emphasis on recent results obtained using pulsed laser deposition. The opportunities for growing epitaxial films of VO2, and for doping VO2 films to alter their transition temperature and switching characteristics, are also discussed.

https://iopscience.iop.org/article/10.1088/0953-8984/20/26/264016/pdf

Synthesis of vanadium dioxide thin films and nanoparticles

Several oxides of vanadium undergo a transition from a semiconductor or insulating state to a metal phase at a critical temperature. Vanadium dioxide undergoes this transition near 68 degrees C, while V(2)O(5) undergoes a similar phase transition near 257 degrees C. During the transition a change in oxide crystal structure is accompanied by large changes in electrical and optical behavior. Thin films of vanadium oxides are capable of reversibly switching from the semiconductor to the metallic state at high speeds and with high spatial resolution. Therefore, these oxides have potential use, particularly in thin film form, for a wide variety of applications involving thermally activated electronic or optical switching devices. Such films are of considerable technical interest because of applications in chemical sensors, energy-conserving coatings, transparent conductors, and switching materials. The numerous potential electronic, optical, and optoelectronic device applications which have been suggested have stimulated work on the preparation of thin films by a variety of techniques, including chemical vapor deposition, solgel, evaporation, and sputter deposition. This paper reviews the optical properties of vanadium oxide coatings and stresses the dependence of film properties on sample preparation and resultant film microstructure.

Optical properties of vanadium dioxide and vanadium pentoxide thin films.

Thermochromic VO2 films for energy-efficient windows

Joule-heating induced conductance-switching is studied in VO2 , a Mott insulator. Complementary in situ techniques including optical characterization, blackbody microscopy, scanning transmission X-ray microscopy (STXM) and numerical simulations are used. Abrupt redistribution in local temperature is shown to occur upon conductance-switching along with a structural phase transition, at the same current.

Local Temperature Redistribution and Structural Transition During Joule‐Heating‐Driven Conductance Switching in VO2

Thin polycrystalline films of VO2 and V2O3 were deposited on a variety of substrates using a sol‐gel process. The orientation, microstructure, optical constants, and optical and electrical switching behavior are presented. These films exhibited sharp optical switching behavior even on an amorphous substrate such as fused silica. The method yields reproducible results and is amenable to the coating of large substrates and curved surfaces such as mirrors and lenses.

Switchable vanadium oxide films by a sol‐gel process

Reactively sputtered vanadium dioxide films have been analyzed to determine the relation between their electrical and structural properties. The structure of the thin films was determined by x-ray diffraction, reflection electron diffraction, and electron microscopy of carbon replicas of the film surfaces. The various substrates which were evaluated for thin film VO2 deposition were amorphous glass, glazed ceramic, and single-crystal sapphire and rutile. Electrical conductivity measurements were made to correlate film perfection with the conductivity ratio between the semiconducting and metallic states. Films deposited on amorphous substrates such as glass and glazed ceramic are polycrystalline in structure with small grain size. Conductivity ratios of approximately 102 were observed with a very broad transition region centered at 68 °C. These films, in addition to VO2, exhibit an extra x-ray line at d = 2.25, tentatively identified as VO1.87. On sapphire substrates, highly textured films were obtained wh...

D. H. Hensler

Papers

Structural and Electrical Properties of Vanadium Dioxide Thin Films