SEMICONDUCTOR DEVICE PHYSICS AND DESIGN

1.3. LATTICE MISMATCHED STRUCTURES 17

SiO 2

Si

Rows

of Si

atoms

Figure 1.12: The tremendous success of Si technology is due to the Si/SiO 2 interface. In spite
of the very different crystal structure of Si and SiO 2 , the interface is extremely sharp, as shown
in the TEM picture in this figure. TEM image courtesy of Bell Labs.

1.3 LATTICEMISMATCHEDSTRUCTURES

It is relatively easy to grow heterostructures where the overlayer lattice constant is the same or
similar to that of the substrate. In such lattice matched epitaxy the interface quality can be very
high with essentially negligible interface defects and atomically abrupt interface. However one
often needs structures where there is lattice mismatch between the overlayer and the substrate.
The motivation for lattice mismatched epitaxy is two fold:
i) Incorporation of built-in strain: When a lattice mismatched semiconductor is grown on a
substrate and the thickness of the overlayer is very thin, the overlayer has a built-in strain. This
built-in strain has important effects on the electronic and optoelectronic properties of the material
and can be exploited for high performance devices. It can be exploited in nitride heterostructures
to effectively dope structures. It can also be exploited in Si/SiGe systems.