Overview

The active region of a Si heterostructure consists of strained layers of different composition, thickness and doping. The composition of layer determines the bandgap and band offset with the adjacent layers. The polarity and magnitude of the band offset define the depth of a well or the height of a barrier, and thus the channel region. One can assign an average lattice constant to a strained layer. Elastic strains develop when there is a large misfit between adjacent layers. The thickness of a strained layer has a critical upper value. Some properties of binary alloys follow Vegard's law permitting interpolation between the properties of the constituents. The interpolation is not always valid however. Current state of knowledge of critical thickness is reviewed in this chapter.

The intrinsic optical and electronic properties of a semiconducting region are determined by the band structure which depends on the composition, orientation and strain. The first parameter of concern in optical transition, and many electronic processes is the bandgap. Strain in the alloy layer lifts the degeneracy of the conduction and valence bands. An important associated property is the band alignment at a heterojunction which determines whether a layer will act as a well for a channel or provide a barrier defining the channel boundary. The bandgap of a bulk material or thin film is commonly measured by optical means. Band offset can be measured by photoelectron spectroscopy and also from capacitance-voltage ( C-V) characteristics.

A basic transport property of concern is the...