8/10/2019 Electrical Switching in VO2 Sol-gel

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8

a

4

4

2

0 10 20

30

40

5

60

V N

Fig.

3

Variation

of

the threshold voltage,

Tih

with temperature

is higher than the transition temperature,

i e

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 know n in sulator-

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 amoun t depends on the

difference between the ambient tem perature 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

V O z

thin films deposited from vanadium alkoxides Th e switching

mechanism is the same as previously reported for devices based

on aqueous

V,O,

* 18H20 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 thresh old 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, the interelectrode separation and A the electrode

contact area They are defined by the ge ome tnc 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 Mo reover, the transition

temperature can be easily modified when doped VO, films ar e

used Such dop ed 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 ) a nd increases with low -valent cations such as

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

V O z

thin films Bulk materials do

not withstand many cycles Th e structu ral distortion associated

with the insulator-metal transition rapidly leads to a degra-

datio n of the materials Large crystals break after few cycles

only

A11116 8

References

1 J Bullot, Gallais, M Gauthier and Livage, Phys Status Solzdz

A,

1982,71,k l

2 J G Zhang and

P C

Eklund,

Appl

P h y s 1988,64,729

3 G Zhang and P C Eklund,

Mater Res

1993,8,558

4 Livage, Chem Mater 1991,3,681

5 G Guzman, R Morineau and 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 M o m , Phys R e v L e t t

1959,3,34

8

Livage, G Guzman, F Beteille and P Davidson, J Sol-Gel Scz

Techn n

press

Communication 5/073491, Received 8th November 1995

5 6

J

Mater Chem 1995, VOL 6

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