Optical Switches Based on Vanadium Dioxide Films Grown By The Sol-Gel Process
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OPTICAL SWITCHES BASED ON VANADIUM DIOXIDE FILMS GROWN BY THE SOL-GEL PROCESS Richard S. Potember and Kenneth R. Speck The Johns Hopkins University Applied Physics Laboratory Laurel, Maryland 20723, USA ABSTRACT Vanadium tetrakis
dioxide
(t-butoxide)
thin
films
by the
have been
grown
sol-gel process.
from
vanadium
A new method for
the synthesis of the vanadium precursor was also developed. Films were deposited by dipcoating glass slides from an isopropanol solution,
followed
600*C under
by post-deposition
nitrogen.
The
annealing
properties of these
of the films,
films at to a high
degree, were a function of crystalline boundaries and crystalline grain size. These gel-derived V0 2 films undergo a reversible semiconductor-to-metal
phase
transition
near
72*C,
exhibiting
characteristic resistive and spectral switching comparable with near stoichiometric V0 2 films prepared on non-crystalline substrates
by other techniques.
Films were
transition metal oxides to demonstrate
doped with hexavalent
lowering of the transition
of the transition temperature.
INTRODUCTION Vanadium dioxide is induced
known to undergo a reversible
semiconductor-to-metal
Associated
with
this
phase
transition,
changes in its optical, electrical, with respect to temperature (3,4] . prompted
considerable
interest
in
transition
the
material
at
thermally68 0 C
exhibits
(1,2]. large
and magnetic characteristics This anomalous behavior has the
fabrication
of
vanadium
dioxide thin films for scientific investigation and for use in various technological applications [5,6]. Recently, V0 2 has been studied walls.
for use as an energy-conserving coating for windows and Other applications include electro- and photo-chromic
devices, thermal sensors and transparent electrical conductors. There have also been numerous studies involving the incorporation of various transition metal ions into the V0 2 lattice.
It
was
raised or lowered,
observed that the transition temperature may be depending on the valence of the dopant ion.
Mat. Res. Soc. Symp. Proc. Vol. 180. ©1990 Materials Research Society
754
Tungsten, with
molybdenum,
increasing
effect
per
resistance quickly
and tantalum
concentration,
atomic
percent
ratio before
for low
dopant
infrared transmission is
small [7]. Processing
Figure
and
of
crystal
having
[6-9] .
after
the transformation
levels.
However,
In
of
these
structure
tungsten
each
large
films
V0 2
Tt
largest
case,
the
decreases
changes
in
the
shown
the
or
in
glass
Dopant ions were placed into
vanadium
molybdenum
schematically
deposited by dipcoating
solution.
of
is
dioxide
isopropanolic solutions of the vanadium tetrakis those
the
added
Thin films of V0 2 were
1.
are known to decrease
tungsten
are observed even when the resistance ratio
slides from an isopropanol the
ions
with
(VI)
by
combining
(t-butoxide) with
oxytetrachloride
(Alfa) .
Tungsten and molybdenum were chosen as dopants because of the very large effec
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