Czochralski method – Crystal Pulling Techniques
In Czochralski method, the material to be grown is melted by induction or resistance
heating under a controlled atmosphere in a suitable non-reacting container. By controlling the
furnace temperature, the material is melted. A
seed crystal is lowered to touch the molten
charge. When the temperature of the seed is
maintained very low compared to the temperature
of
the
melt,
by
suitable
water
cooling
arrangement, the molten charge in contact with
the seed will solidify on the seed. Then the seed is
pulled with simultaneous rotation of the seed rod
and the crucible in order to grow perfect single
crystals.
The Czochralski process is a method of crystal growth used to obtain single crystals of
semiconductors (e.g), metals (e.g. palladium, platinum, silver, gold), salts and many oxide
crystals ( LaAlO3, YAG etc ). Simply this technique is widely adopted to grow III-V compound
semiconductors.
Large, single crystal of Si are grown by the Czochralki method.It involves growing a
single crystal ingot from the melt,using solidification on a seed crystal. Molten Si is held in a
quartz crucible in a graphite suspetor. It is heated by radio frequency induction coil. A small
dislocation – free crystal, called a seed, is moved down to touch the melt and then slowly pulled
out of the melt. A crystal grows by solidifying on the seed crystal. Seed is rotating during the
pulling stage, to obtain a cylindrical ingot.
Growth from Vapour
Crystals can be grown from a vapour when the molecules of the gas attach themselves to
a surface and move into the crystal arrangement. Several important conditions must be met for
this to occur. At constant temperature and equilibrium conditions, the average number of
molecules in the gas and solid states is constant; molecules leave the gas and attach to the surface
at the same rate that they leave the surface to become gas molecules. For crystals to grow, the
gas-solid chemical system must be in a nonequilibrium state such that there are too many
gaseous molecules for the conditions of pressure and temperature. This state is
calledsupersaturation. Supersaturation can be induced by maintaining the crystal at a lower
temperature than the gas. The advantage of vapour growth is that very pure crystals can be
grown by this method, while the disadvantage is that it is slow.
Epitaxy
Process of growing a crystal of a particular orientation on top of another crystal. Growth
of one crystal on the surface of another crystal in which the growth of the deposited crystal is
oriented by the lattice structure of the substrate. The term epitaxy comes from the Greek roots
epi, meaning "above", and taxis, meaning "in ordered manner".
Homoepitaxy is a kind of epitaxy performed with only one material. In homoepitaxy,
a crystalline film is grown on a substrate or film of the same material. This technology is used to
grow a film which is more pure than the substrate and to fabricate layers having
differentdoping levels. In academic literature, homoepitaxy is often abbreviated to "homoepi".
Heteroepitaxy is a kind of epitaxy performed with materials that are different from each
other. In heteroepitaxy, a crystalline film grows on a crystalline substrate or film of a different
material. This technology is often used to grow crystalline films of materials for which crystals
cannot otherwise be obtained and to fabricate integrated crystalline layers of different materials.
Examples
include gallium nitride (GaN)on sapphire, aluminium galliumindium
phosphide (AlGaInP) on gallium arsenide (GaAs) or diamond or iridium. [1]
Heterotopotaxy is a process similar to heteroepitaxy except for the fact that
thin film growth is not limited to two dimensional growth. Here the substrate is similar
only in structure to the thin film material.
Epitaxy is used in silicon-based manufacturing processes for BJTs and modern
CMOS, but it is particularly important for compound semiconductors such as gallium
arsenide. Epitaxy is used in nanotechnology and in semiconductor fabrication. Indeed,
epitaxy is the only affordable method of high quality crystal growth for many
semiconductor materials.