Introduction

The sol-gel process is based on the hydrolysis and condensation of molecular precursors. These molecular precursors are usually metal alkoxides. However, hydrolyzed metal ions in aqueous solutions exhibit also sol-gel transition [1]. This technique is very convenient for the synthesis of oxides or multicomponent oxides such as ceramics or glasses. The crystalline structure and the atomic homogeneity can be tailored by controlling the process parameters. Usually, complete crystallized structures are obtained at lower temperatures than those of conventional ceramic processes. The preparation of thin films from sol-gel solutions is today one of the main applications of the sol-gel technique for the deposition of oxides with a wide variety of properties.

Thermochromic materials are characterized by a semiconductor-to-metal transition occurring from a reversible change in their crystalline structure as a function of the temperature. This change have been observed in transition-metal oxides [2,3] such as Ti₂O₃, Fe₃O₄, Mo₉O₂₆ and in several Magneli phases of vanadium oxide, V_nO_2n-1. Among them, VO₂ has been received most attention because of the large reversible change of electric, magnetic and optical properties at temperatures around 70°C (4). During the semiconductor-to metal-transition, the optical properties of vanadium dioxide are characterized by a sharp decrease in optical transmission in the infrared spectrum. This is coupled with an increasing in its reflectivity. Because of this anomalous behavior, vanadium dioxide has been presented as an attractive thin film material for electrical or optical switches, optical storage, laser protection, and solar energy control for windows space satellites.

The transition temperature of vanadium dioxide may be decreased by the addition of high-valent transition metals such as niobium, molybdenum or tungsten. Trivalent cations (Cr3+ and Al3+) increase the transition temperature. The hysteresis profile associated with the transition depends on the microstructure and crystallinity.

Thin films

Among the techniques of thin-film deposition, sol-gel technique have been used to prepare vanadium dioxide thin films [5-13]. VO₂ films can be prepared by using tetravalent alkoxide precursor such as vanadium tetrabutoxide [7,8], In this process, VO₂ films are synthesized after a heat treatment at 600°C under nitrogen atmosphere. Vanadium dioxide has been also obtained from pentavalent vanadium precursors including vanadium oxo isopropoxide [5, 6, 9, 10, 11], and vanadium pentoxide [12, 13]. These processes generally require a reducing atmosphere (vacuum, CO/CO2, H, Noxal 3) for the synthesis of crystalline VO₂ films.

Figure 1, shows a view of a sol-gel VO₂ thin films on a silica substrates.

Figure 1 : VO2 thin films of several thickness deposited onto silica substrates.

Dried films are prepared by direct deposition vanadium oxo isopropoxide /Isopropanol solution. Solvent evaporation, partial hydrolysis and condensation occur after film deposition by spin coating (3000 rpm, 15 sec) and infrared heating at 60°C in air at ambient humidity. The green color of the films suggests that some pentavalent vanadium species are reduced to tetravalent vanadium. Upon drying, vanadium oxo-polymers, [VO(OR)_3-x(OH)_x]n and [V₂O_5-x(OR)_x]n, rather than hydrated vanadium oxides, are formed. In addition, the as-deposited coatings are amorphous and do not exhibit the typical layered structure of hydrated vanadium oxides. Crystalline VO₂ is subsequently formed when the films are submitted to a heat treatment of 500°C for 2 hours under a reducing atmosphere of Noxal 3 (Ar-H2( 5%)) [11]. In figure 1, the three samples show the effect of the thickness on the visual aspect of films. Thickness is varied by depositing successive layers at room temperature, with a drying step in between depositions. From the left to the right, one, three and five layers were deposited. From SEM observations, the cross-section thickness were, 70 nm, 400 nm and 500 nm respectively.

Properties

The properties of VO₂ films are dependent of the microstructure and crystallinity. These can be controlled by the experimental process parameters, such as the molecular precursors, heat treatments and controlled atmosphere.

The change in transition temperature has also been investigated in sol-gel VO₂ doped thin films [14]. In these doped films, switching characteristics, such as transition temperature, hysteresis loop, and light blocking properties, are modified.

SEM characterization of heat the treated samples shown in figure 1 revealed crystallite sizes of about 50 nm for the sample having 70 nm thickness, 100 nm to 300 nm for the sample with 400 nm of thickness, and 100 to 400 for the sample with 500 nm thickness. Relatively mono-dispersed grain sizes were observed on films having thickness up to 300 nm.

The optical switching behavior of a 300 nm thick film is shown in figure 2. 

Figure 2 : Optical hysteresis at 2.5 µm of a sol-gel prepared VO2 film

The optical transmission axis is shown on a log scale to better see the transmission minimum obtained at the metal phase. A large hysteresis loop of about 20 °C is observed. This is probably related to the microstructure and some residual carbon, observed by SIMS [15]. The anomalous hysteresis shape, characterized in the cooling step, has been interpreted [16] as the result of the existence of crystallite texture domains differing in crystalline orientation and size.

VO₂ thin films were deposited also onto ZnS and Germanium substrates. Coatings having good homogeneity and optical properties were deposited on the ZnS substrates. However, poor coating adhesion was observed. Thickness greater than 100 nm could not be deposited. In addition, good quality coatings were obtained on germanium. 

The VO₂ crystalline phase was observed by XRD is shown in figure 3. 

Figure 3 : X-ray diffraction of a vanadium dioxide film using germanium as substrate

Infrared optical transmission at the low temperature (semiconductor) state and at the high temperature (metal) state is shown in the 4. Because of the wide infrared window of germanium, we could study the blocking properties of VO2 sol-gel thin films in the range of 1 µm to 20 µm. Infrared transmission lower than 0.5% was measured for VO2 coatings when heated at 100°C.

 
 

Figure 4 : Infrared transmittance at the semiconductor state (22°C) and metal state (100°C) of a sol-gel VO2 thin film on germanium.

Applications

Among the transition metal oxide exhibiting semiconductor to metal structural phase transition, vanadium dioxide is the most spectacular example. This is due to the amplitude of physical property changes during this phase transition. This has led to, vanadium dioxide being studied, not only from the fundamental point of view, but also for potential applications in electronics, optics or opto-electronics This interest is especially generated by large changes of resistivity, optical transmission, and reflection in the infrared. These changes are associated with a hysteresis loop when the material is heated and cooled around the temperature of phase transition.

Because of the ability to change the transition temperature by doping, Lee et al [17] and more recently P. Jin et al. [18], suggested that tungsten doped vanadium dioxide can be used in energy efficient-windows. These smart windows or electrochromic displays find special applications in the architectural, automotive and aerospace sectors [19].

Roach [20] pointed that, due to the changes in reflectivity during the phase transition, VO₂ films can be used as a kind of optical disc medium and demonstrate holographic storage.

Bit recording on VO₂ films using a near-infrared laser was demonstrated [21], Stability during long-term storage and over 10⁸ time-cycles of write and erase were achieved without degradation. Switching time of about 30 nsec and writing energy of the order of a few mJ/cm² were reported [22]. Bit density has been estimated to be 350 bits/mm. Such low threshold recording energy and erase-rewritabilty encourage the use of VO₂ films as a recording media [23].

More recently, the use of VO₂ thin films was suggested in ultrafast optical switching devices. The high-temperature metallic state was attained in 5ps by using femto second laser excitation at 780 nm was reported [24].

In summary, vanadium dioxide is an interesting candidate for modern applications of active thin films in optical or electric [25] switches. The sol-gel process is an attractive approach for preparing these films. The applicability of this kind of coatings needs more work on device feasibility, reproducibility and scale-up processes.
 
 

Acknowledgement This work was made in the Laboratoire de Chimie de la Matière Condensée of the University Pierre et Marie Curie where Prof. Jacques Livage generously accepted to provide me with his assistance and collaboration. In addition, this work was partially financed by the French Office National d'Etudes et de Recherches Aérospatiales.

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