The Importance of Thickness to the Crystallization of Amorphous thin Films
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THE IMPORTANCE OF THICKNESS TO THE CRYSTALLIZATION OF AMORPHOUS THIN FILMS
GREGORY F. CARDINALE*, S.N. FARRENS* and D.G. HOWITT* *Department of Mechanical Aeronautical and Engineering,
University of California at Davis,
Materials
California
95616
ABSTRACT The
crystallization
of an amorphous
thin film,
when
it
is
annealed, can be described in terms of the latent heat of the transformation and an activation energy which depends upon the configurational entropy of the surface of the crystallizing interface.
An
expression
is
derived
temperature form of titanium oxide, growth that is observed.
for
which is
anatase,
the
consistent
low
with the
INTRODUCTION The oriented
annealing
of amorphous
crystalline
thin ceramic
structures
is
films
to develop
one of the methods by which
intrinsic properties can potentially be developed in optical and electron-optical materials. Crystallization in some films, titanium dioxide for example, the
interface
growth
is
is
particularly fortuitous in
extremely
well
defined.
that
This growth
apparently cannot be induced in very thin films and in titanium dioxide, for example, the thicknesses must be greater than 50 nm for the
films to crystallize[l] .
It
is
also apparent
that when
the thickness of the titania film is between 50 nm and 200 nm the crystallization occurs by the propagation of an interface in the
plane
of
orientation These
the
film,
of
highly
which
oriented
Films grown terminates
in but
abruptly
to
a
to the plane
of
microstructures
respects to those observed in microstructures
gives
perpendicular
rise are
preferred the
film.
in
many
similar
silicon films after zone melting.
this way can produce a variety of different they are constrained by an interface at
the
film surface.
geometry can be treated analytically,
This
fairly
which simrle
to determine the magnitude
of the thermodynamic and kinetic growth parameters(l]. study of interface propagation in this way is not new encompasses a wide variety of
solidification
The and
and precipitation
Mat. Res. Soc. Symp. Proc. Vol. 237. @1992 Materials Research Society
644
studies(2]
the ratio of the latent
which indicate that
the melting faceted.
Thus,
in
the presence
of
of an
characteristics
that is,
energy for interface propagation, In
the
determines
temperature
interface during solidification,
heat to
whether it
is
a sufficient
rough or
activation
faceting can be expected.
the thin film crystallization studies described here,
we
have distinguished the characteristics of the amorphous to crystalline phase transformation in TiO2 and have observed that a reduction in
film thickness promotes faceted growth.
EXPERIMENTAL Thin films of titanium and niobium oxides
were prepared by
reactive ion deposition using argon as the sputtering gas. thickness was measured by ellipsometry, 200
nm.
The
as-deposited
films
Film
and ranged from 40 nm to
were
completely
amorphous
(determined by electron microscopy) and contained
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