9 - Superlattice Structures

Abstract

This chapter explains that the lattice constant of a desired alloy differs from that of an underlying layer or the substrate. This gives rise to a layer under biaxial stress. It is no longer cubic, because the lattice constant in the growth plane differs from that in the perpendicular direction. The strained layer is often, somewhat incorrectly, termed “pseudo-morphic”. This term is to denote a situation where the crystal structure of an epitaxial layer is modified to match that of the substrate material to minimize the total energy of the system. Reduced dimensionality also reduces scattering because it reduces the number of momentum states into which a free carrier can scatter. This leads to higher free-carrier mobilities in 2D structures and even higher mobilities in quantum wires. The chapter discusses that clever configurations can be used to separate the free carriers from the fixed charges on the do-pant impurities. So far no development in semiconductor device technology has been more significant than the ability to grow hetero-structures and super-lattices by epitaxial techniques. It has resulted to a revolution in device performance.