Electrical switching in VOz sol-gel films Guillermo Guzman, Fabien Beteille, Roger Morineau and Jacques Livage" Chimie de la Mati2re Condenst!e, Universitt! Pierre et Marie Curie, 4 place Jussieu, 75252 Paris, France Vanadium dioxide thin films have been made from vanadium oxoalkoxides. The precursor solution is spin-coated onto a silica substrate and heated under a reducing atmosphere. These V02 films exhibit well known insulator-metal transition at around 67°C. Electrical switches have been made from such films that reversibly switch from an OFF state to an ON state upon applying a voltage of ca. 50 V. This thermally driven process is fast and highly reversible. More than lo8 cycles have been performed without failure. Two-terminal electrical switching devices based on V205 l.8H20 xerogels have been reported by Bullot et a2.l Such devices can be easily formed via the deposition of two gold electrodes, about 100 pm apart, at the surface of a xerogel thin film.The reversible switching between ON and OFF states occurs within a few ms and involves a change in conductivity of about two orders of magnitude. Later, Zhang and Eklund showed that the switching was not observed unless the device was first 'over-biased' to a voltage around the switching voltage.' They observed the formation of a VO, filament between the electrodes and suggested that the switch- ing phenomenon was associated with an electrothermally driven metal-insulator transition of the filament.3 The forma- tion of V02 is probably due to the electrochemical reduction of Vv in the V,O, l.8H20 film.4 Such devices are easy and cheap to make but their current-voltage characteristic (I-V) is not reproducible.This arises mainly from the fact that the diameter of the VO, filament is difficult to control. All these results have been obtained with V205 * l.8H20 gels arising from the polymerization of vanadic acids in aqueous solutions. This paper shows that switching devices can be made directly with VOz thin films synthesized from vanadium alkoxides. Vanadium dioxide thin films were synthesized via the hydrolysis and condensation of vanadium alk~xides.~.~ A solu-tion (0.5 mol dm-3) of vanadium oxoalkoxide, VO(OR)3, in its parent alcohol ROH (R =Pr', tert-amyl) is spin-coated onto a silica substrate about 2cm2 in size.Spin-coating is performed in air so that the alkoxide film is partially hydrolysed by ambient moisture. Not all of the alkoxy groups are hydro- lysed. They remain bonded to the oxide network and amorph- ous 0x0-polymers [V205 -x(OR)x]n rather than hydrated vanadium oxides are formed. The coating is then dried at around 60°C in order to remove excess alcohol. The coating becomes slightly green owing to some reduction of Vv to VrV. Freshly deposited films do not exhibit the layered structure shown by V,O, * 1.8H20 films synthesized from vanadic acid in aqueous sol~tions.~ They appear to be amorphous by X-ray diffraction [Fig. l(a)]. Crystalline VO, films are formed upon heating for 2 h at 500°C under a reducing atmosphere (Ar-H,, 5%) [Fig.l(b)]. Optically transparent dense films are then obtained. They are made of VOz particles about 100 nm in size. They exhibit a homogeneous flat surface and their thickness is about 0.05 pm. Several coatings have to be made in order to obtain thicker films. The successive spin-coatings are performed after drying the previous film at 60°C. Fig. 2 I-I/ characteristic of a switching device based on VO, thin films I II I I 10 20 30 40 50 60 20/degrees Fig. 1 XRD patterns of (a) amorphous freshly deposited [V,O,-,(OR),], film, (b) crystalline VO, thin film after thermal treatment (the broad peak corresponds to the silica glass substrate) The electrical resistance of the films was measured in the 20-100°C temperature range using a two-probe device.A typical hysteresis loop is observed. Upon heating, the resistance drops rapidly by about three orders of magnitude at 70°C. Upon cooling, the insulating state is recovered around 60°C. Electrical switching devices are simply made via the depos- ition of gold electrodes, 0.4mm apart, at the surface of the VO, film (thickness ca. 0.2 pm). These electrodes are evapor- ated through a mask so that the geometry of the interelectrode space can be accurately controlled. An ac voltage is applied to the device and the I-V curve is recorded on an oscilloscope. A typical curve is shown in Fig. 2. Along the OA line, the device follows Ohm's law, V= RI. The film is in the insulating state and its resistance is quite high: this is the OFF state.However, as the current increases, the film is heated via the Joule effect. At a given threshold voltage (& M 50 V) the temperature of the film becomes higher than its transition temperature and the vanadium dioxide becomes metallic. Its resistance drops by a factor of about lo3:this is the ON state. The voltage decreases while the current increases (line AB). The device remains in the ON state as long as its temperature 0 t H VN A 4 Volts J. Muter. Chem., 1996, 6(3), 505-506 505 8 6 a 4 *4 2 '0 10 20 30 40 50 60 VN Fig. 3 Variation of the threshold voltage, Tih, with temperature is higher than the transition temperature, ie as long as the current is larger than the holding current, IhZO6mA (point C) Beyond this point, the film switches back to the OFF state (line CO) This electrical switching is due to the well known insulator- metal transition of VO, It is thermally driven and the threshold voltage depends on the amount of heat required to reach the transition temperature This amount depends on the difference between the ambient temperature and the transition temperature, T, Therefore V,, decreases regularly when the temperature increases (Fig 3) The switching behaviour disap- pears when the room temperature reaches T, The whole film is then in the metallic state Electrical switching devices can be made easily with VOz thin films deposited from vanadium alkoxides The switching mechanism is the same as previously reported for devices based on aqueous V,O, * 1 8H20 xerogels In this case, a VO, filament was electrochemically formed through the xerogel upon applying an over-voltage to the device The diameter of this filament was difficult to control as it was not formed homogeneously between the gold electrodes It was therefore difficult to build devices showing a given threshold voltage In this paper, the whole film is made of VO, The size of the active materials and the threshold voltage, T/h, depend on the geometric characteristic of the VO, film between the electrodes, R = ps/A, where R is the resistance, p the bulk resistivity, s the interelectrode separation and A the electrode contact area They are defined by the geometnc characteristics of the mask used for the deposition of the gold electrodes Reproducible results can then be obtained when these charac- teristics are carefully controlled Moreover, the transition temperature can be easily modified when doped VO, films are used Such doped thin films can be made easily by mixing molecular precursors with the alkoxide solution We have shown, for instance, that T, decreases with high-valent cations (NbV, Wv') and increases with low-valent cations such asA11116 8 The switching behaviour of VO, thin films, recorded on an oscilloscope, can be observed continuously for several weeks or even months (Fig 2) This points out the high reversibility of such devices based on VOz thin films Bulk materials do not withstand many cycles The structural distortion associated with the insulator-metal transition rapidly leads to a degra- dation of the materials Large crystals break after few cycles only References 1 J Bullot, 0 Gallais, M Gauthier and J Livage, Phys Status Solzdz A, 1982,71, kl 2 J G Zhang and P C Eklund, J Appl Phys, 1988,64,729 3 J G Zhang and P C Eklund, J Mater Res, 1993,8,558 4 J Livage, Chem Mater, 1991,3,681 5 G Guzman, R Morineau and J Livage, Muter Res Bull, 1994, 29,509 6 G Guzman, F Beteille, R Monneau and J Livage, Eur J Solid State Inorg Chem , 1995,32,851 7 F J Mom, Phys Rev Lett, 1959,3,34 8 J Livage, G Guzman, F Beteille and P Davidson, J Sol-Gel Scz Techn , in press Communication 5/073491, Received 8th November, 1995 506 J Mater Chem, 1995, VOL 6