Defects in Crystal
•Four types of defects on the basis of their dimensionality
•Point, Line, area and volume
•Points defect - these are isolated defects scattered inside the crystal
play crucial role in doping via diffusion or ion implantation
•Anything other than a silicon atom on a lattice site, a point defect.
Two principal types of point defects:-
• A missing silicon lattice atom or vacancy (by breaking 4 bonds) ,
designated by V . It is also called a Schottky defect
• These defects can be arise due to energy supplied by external
means or due to vapor pressure effects, thermal fluctuation
effects . More than one vacancy i e two vacancies side by side –
di-vaccancy (by breaking 6-bonds)
• The concentration of both V and I increases with the increase of
temperature.
• An extra silicon atom , simply designated as I (an interstitial)
• A diamond lattice or a zincblende structure is quite loosely
packed. The packing density is only 34% compared to an fcc
atom, where the packing density is quite high, greater than
70%.
• A vacancy interstitial pair, which is also called a Frenkel
defect.
• Deliberate introduction of impurity atoms – electronically.
• Act as an active dopant, active donor or an acceptor when it is
replacing a silicon and taking up its position .
• Substitutional impurity :- one which substitutes for a silicon
atom in the lattice
• One notable exception is lithium in silicon is an interstitial impurity.
it is a monovalent atom. So, a lithium atom sitting in an interstitial
site, it can, it does not form any bonds. Very easily donate that
electron and behave as a donor.
• Does all interstitial and all vacancies are also electronically active ?
• A vacancy (4 bonds are broken) should act as an acceptor and an
interstitial (4 electrons are available) should act as a donor in a
crystal lattice.
•But, the energy levels created by these defects, vacancy or
interstitial will be fairly deep inside the energy band gap.
• These levels are created at E= Ev + 0.7 to Ev + 0.1 eV and the
interstitials which are donor-like, deep inside the energy band gap at
E = Ec – 0.9 eV.
Such deep energy levels are used to control the life time of
the minority carriers in the material.
can take part in the generation recombination phenomenon
and therefore, kill the lifetime.
can make fast switching devices, where we need to keep
the minority carrier lifetime at a minimum.
Sometimes we want to deliberately create such deep levels
by deliberately introducing impurities like gold or platinum
in order to make fast switching devices,
• The second type of defects are called dislocations.
Dislocations are one dimensional defects. a linear defect - an
extra plane of atoms . It either terminates at the edge of the
crystal or they form a closed loop within the crystal
•A dislocation is formed when the material is subjected to a lot
of shear stress, which is greater than the elastic limit for this
particular material.
•A lot of energy is required, 10 to 20 eVs,
• The crystal is subjected to this kind of stress may be during
the growth itself.
•In the growth process, silicon is first molten and then, it is
solidified in a controlled manner, so that the growth is only in a
particular crystal orientation. So, if during solidification, any part
of the melt is constrained (by the wall of the container), there will
be a lot of stress on the material. So, dislocations can form.
• During processing when the material is subjected to a lot of
thermal stress under very high temperature then also
dislocations can form.
•Another way, an excessive amount of impurity atom then due to
the misfit, there will be again a lot of stress inside the crystal and
because of this stress, dislocations will be formed .
• the two types of dislocations that are found in a crystal. One is
called a screw dislocation, the other is called an edge dislocation.
•These dislocations can move around inside the crystal which is
another very dangerous thing., undesirable since act as sink for
metallic impurities.
• like having a sort of pit or called guttering centers.
• They attract metallic impurities
• premature breakdown take place
• lot of leakage
•the device performance will be much poorer
•The third defect which is called the area defect.
• Area dislocations in a crystal, Crystal can be rejected. material
cannot be used for integrated circuit fabrication.
•Two very common area dislocation, one is called twin and the
other is called grain boundary.
• Twining is actually a change in the crystal orientation. means
two parts of the crystal are in intimate contact, but they have
some orientation with respect to each other. This is called the
twining. So, both the parts will have some crystalline orientation,
but in between you are having a change from one crystalline
orientation to another. It is not a real single crystal; that is
twinning.
•Due to some process problem, have a poly crystalline material. i.e.
various crystalline orientation in the same substrate and in
between these crystalline structures which are called the grains,
highly defective boundary region separating one crystalline region
from another, is called the grain boundary.
•So, the grain boundaries are actually a lot of defects,
•if you have a poly crystalline material with sufficiently large grain, it
is possible to separate out, in a single crystal material.
•Volume defects or three dimensional defects.
• usually created by precipitates.
• each material has a solubility limit in another material.
• normally the solubility of a material will decrease as you reduce
the temperature, is it not? Or it will increase as you increase the
temperature.
• doing the crystal growth, the temperature of the melt is very
high. So, it can contain a large amount of impurity. But then, as
the crystal is formed and then finally it is cooled to room
temperature, the solubility of these impurities will reduce, will
decrease and therefore this extra material will be precipitated
inside the crystal.
•The precipitates are highly undesirable. cannot use that portion of
the material at all.
•if there is any precipitate, then it will act as the starting point of
more dislocations because it has different coefficient of thermal
expansion
