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Crystalline Defects in Silicon (Page 3 of 3)
Area defects
in crystals consist of stacking faults, grain boundaries, and twin
boundaries. A
'stacking
fault'
pertains to a
disturbance in the regularity of the stacking of planes of atoms in a
crystal lattice. This usually occurs when a plane is inserted into
or removed from the lattice. The insertion of an extra plane in the
stacking is known as an 'extrinsic' stacking fault, while the removal of
a plane is referred to as an 'intrinsic' stacking fault.
Stacking
faults can become electrically active when decorated by impurity atoms.
Electrically active stacking faults can cause device degradation,
examples of which are higher reverse bias currents in p-n junctions and
storage time reduction in MOS circuits.
Figure 1.
Photo of a Stacking Fault
Image Source:
http://lmass.uah.edu
- J. A.
Gavira-Gallardo, J. D. Ng and M.A. George
A
'twin'
is an area defect wherein a mirror image of the regular lattice is
formed during the growth of the silicon ingot, usually caused by a
perturbation. The
'twin boundary'
is the mirror plane of the twin formation.
A
'grain
boundary'
refers to the
transition or interface between crystals whose atomic arrangements are
different in orientation with respect to each other.
Volume defects
in a crystal
are also known as
'bulk'
defects,
which include
voids
and
precipitates
of extrinsic and intrinsic point defects.
Every
impurity introduced
into a crystal has a certain level of
solubility, which defines the
concentration of that impurity that the solid solution of the host
crystal can accommodate. Impurity solubility usually decreases
with decreasing temperature. If an impurity is introduced into a crystal at the maximum
concentration allowed by its solubility at a high temperature, the
crystal will become
supersaturated with that impurity
once it is cooled
down. A crystal under such supersaturated conditions seeks and
achieves equilibrium by
precipitating
the excess impurity atoms into another phase of different composition or
structure.
Precipitates
are considered undesirable because they have been known to act as sites
for the generation of dislocations. Dislocations arise as a means
of relieving stress generated by the strain exerted by precipitates on
the lattice. Precipitates induced during silicon wafer processing
come from oxygen, metallic impurities, and dopants like boron.
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