In condensed matter physics and materials science, an amorphous or non-crystalline solid
is a solid that lacks the long-range order characteristic of a crystal.
In some older books, the term has been used synonymously with glass.
Nowadays, "amorphous solid" is considered to be the overarching concept, and glass the
more special case: A glass is an amorphous solid that exhibits a glass transition.
Polymers are often amorphous.
Other types of amorphous solids include gels, thin films, and nanostructured materials.
Nanostructured materials Even amorphous materials have some short-range
order at the atomic length scale due to the nature of chemical bonding.
Furthermore, in very small crystals a large fraction of the atoms are located at or near
the surface of the crystal; relaxation of the surface and interfacial effects distort
the atomic positions, decreasing the structural order.
Even the most advanced structural characterization techniques, such as x-ray diffraction and
transmission electron microscopy, have difficulty in distinguishing between amorphous and crystalline
structures on these length scales.
Amorphous thin films Amorphous phases are important constituents
of thin films, which are solid layers of a few nm to some tens of µm thickness deposited
upon a substrate.
So-called structure zone models were developed to describe the micro structure and ceramics
of thin films as a function of the homologous temperature Th that is the ratio of deposition
temperature over melting temperature.
According to these models, a necessary condition for the occurrence of amorphous phases is
that Th has to be smaller than 0.3, that is the deposition temperature must be below 30%
of the melting temperature.
For higher values, the surface diffusion of deposited atomic species would allow for the
formation of crystallites with long range atomic order.
Regarding their applications, amorphous metallic layers played an important role in the discussion
of a suspected superconductivity in amorphous metals.
Today, optical coatings made from TiO2, SiO2, Ta2O5 etc. and combinations of them in most
cases consist of amorphous phases of these compounds.
Much research is carried out into thin amorphous films as a gas separating membrane layer.
The technologically most important thin amorphous film is probably represented by few nm thin
SiO2 layers serving as isolator above the conducting channel of a metal-oxide semiconductor
field-effect transistor.
Also, hydrogenated amorphous silicon, a-Si:H in short, is of technical significance for
thin film solar cells.
In case of a-Si:H the missing long-range order between silicon atoms is partly induced by
the presence by hydrogen in the percent range.
The occurrence of amorphous phases turned out as a phenomenon of particular interest
for studying thin film growth.
Remarkably, the growth of polycrystalline films is often used and preceded by an initial
amorphous layer, the thickness of which may amount to only a few nm.
The most investigated example is represented by thin multicrystalline silicon films, where
such as the unoriented molecule.
An initial amorphous layer was observed in many studies.
Wedge-shaped polycrystals were identified by transmission electron microscopy to grow
out of the amorphous phase only after the latter has exceeded a certain thickness, the
precise value of which depends on deposition temperature, background pressure and various
other process parameters.
The phenomenon has been interpreted in the framework of Ostwald's rule of stages that
predicts the formation of phases to proceed with increasing condensation time towards
increasing stability.
Experimental studies of the phenomenon require a clearly defined state of the substrate surface
and its contaminant density etc., upon which the thin film is deposited.
External links http:chem.chem.rochester.educryst.htm
References
Further reading R. Zallen.
The Physics of Amorphous Solids.
Wiley Interscience.
S.R.
Elliot.
The Physics of Amorphous Materials.
Longman.
N. Cusack.
The Physics of Structurally Disordered Matter: An Introduction.
IOP Publishing.
N.H.
March, R.A.
Street, M.P.
Tosi, Eds.,.
Amorphous Solids and the Liquid State.
Springer.
D.A.
Adler, B.B.
Schwartz, M.C.
Steele, Eds..
Physical Properties of Amorphous Materials.
Springer.
A. Inoue, K. Hasimoto, Eds..
Amorphous and Nanocrystalline Materials.
Springer.
External links Journal of non-crystalline solids