Until very recently silicate glasses were
the only type of materials commonly used.
Advantages over their crystalline
Amorphous solids are relatively easy to
prepare i.e. large-area, homogeneous
amorphous thin film can be prepared. For
example a-Si:H for solar cells or thin-film
Near the glass transition temperature,
which is lower than melting point, the
materials remain workable so that they
can easily be formed into various shapes.
Amorphous materials, particularly bulk
glasses are often structurally
homogeneous and isotropic and their
physical properties are also homogeneous
and isotropic, unlike crystalline materials.
Amorphous semiconductors are
promising electronic materials for
wide range of applications such as:
Thin film transistors (TFT)
Optical memory devices
Electro photographic application
X-ray image sensors
Eu-doped optical fiber
DVD (digital video/versatile disc)
Hard cover made from ta-C
Electro photographic application: one
of the most common, everyday used
application is electro photography or
xerography (Greek word, meaning is “dry
The first xerography was made by Carlson
and Kornei in 1938(!) in Astoria NY (USA).
The really first experiment was
made using sulfur, but later on Se
was the basic material. Recently a-
Si:H films have been utilized instead.
( I. Shimizu: 1985 J. Non-Cryst. Sol.
77-78, 1363 ).
Solar cells: Potentially the most
important application of the amorphous
semiconductors a-Si:H is in the direct
conversion of sunlight to electric power.
This is a cheaper raw material than
crystalline silicon. No structural damage!
For example: space shuttle use.
The conversation of solar light to electric
power is available renewaable sources of
The basic physical principle involved is the
absorption of photon resulting in the
creation of electron-hole pairs; the excess
electrons in the conduction band, and
holes in the valence band.
Internal junction field separates them
There are several conditions that a
thin film solar cell must satisfy in
order to exhibit efficient photovoltaic
e n e r g y c o n v e r s i o n :
The optical absorption coefficient (α) must
be large enough
The photogenerated electrons and holes
must be collected efficiently by contacting
electrodes on the both sides of the active
Phase change memory
~ 1 ns laser heating above melting point,
Tm causes amorphous (polycrystalline)
~50 ns laser heating above glass
transition temperature, but below Tm
forms crytalline bit.
Phase change materials are the memory
materials of the future:
1. Fast (~ 10 ns)
2. Dense (bit diameter < 50 nm)
3. Stable (several years per lost bit)
4. Long-lived (> 1012 cycles per lost bit)
5. Low manufactoring cost
6. Low power consumption