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.
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.
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.
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.
Higher growth temperature results in higher impurity diffusion. SO uniform distribution of impurities cannot be obtained.
This method is mainly used in the manufacture of LED’s and detedtors.