EPITAXIAL
GROWTH
The process of growing an oriented
single crystal layer on substrate wafer is called Epitaxial growth.
The term ‘epitaxy’ is derived from greek
word ‘epi’ literally means “arranged upon”.
In this methods, a thin layer of single
crystal(typically a few nanometers to few microns) is grown on the existing
crystalline substance such that film has the same lattice structure as the
substrate.
Epitaxy is further divided into
i.
Vapor Phase Epitaxy
ii.
Liquid Phase Epitaxy
iii.
Molecular Beam Epitaxy
VAPOUR PHASE EPITAXY(VPE)
Epitaxial
growth of Silicon is exclusively carried out by VPE.
Construction:
The
process is carried out in reaction chamber consisting of along cylindrical
quartz tube encircled by radio frequency induction coil as shown in the figure.
The
silicon wafers are placed on a rectangular graphite rod called Boat. This boat
is placed in the reaction chamber and heated inductively to a temperature of
1200oC.
The
various gases required for the growth of desired epitaxial layers are
introduced into the system through a control device.
The
reaction is surface analyzed and silicon is deposited on the wafer surface.
However, the decomposition temperature is very high. Also, as the reaction is
reversible and can proceed in both directions.
Growth
process:
Mostly
epitaxial films with specific impurity construction (P-type or N-type) are
required. This is accomplished by introducing Phosphine (PH3) for
n-type doping and Biborane (B2H6) for P-Type doping.
This
is a control console which permits only the required gases at suitable pressure
so that it is possible to form an almost step p-n junction layer by the
process.
Vapour
Phase Epitaxy (VPE) and Arsenic chloride(AaCl3) as source materials.
Advantages:
Halogen
based VPE produces high pure single crystal layer with limited defects.
Hydride
VPE allows proper control over the ratio of group III to group V vapour phase
species.
Disadvantages:
Higher
growth temperature results in higher impurity diffusion. SO uniform
distribution of impurities cannot be obtained.
Applications
This
method is mainly used in the manufacture of LED’s and detedtors.
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