3.1 Introduction

A solid, when confined in all three spatial dimensions to length scales comparable to or smaller than the electron de Broglie wavelength, gives rise to discrete electron energy levels. As a result of the attendant discrete, ideally ?-function-like density of energy states, such structures acquire new and unique electronic and optical properties not found in solid structures confined in only one direction (i.e., quantum wells) or two directions (i.e., quantum wires). These structures are called quantum dots and are a particular class of semiconductors.

QDs are the defect-free 3D islands that can form spontaneously for a deposition amount beyond a critical value during growth of a compressively strained semiconductor overlayer on an appropriate substrate [1]. The critical value depends on the lattice mismatch and material combination. A variety of III V, IV-IV, and II VI semiconductor combinations exceeding ~2% lattice mismatch exhibit the phenomenon. For the InAs on GaAs(001) combination involving ~7% mismatch, the critical deposition amount is ~1.6 ML of In. For Ge on Si(001) involving a ~4% mismatch, it is ~3 ML. For InAs/GaAs(001), the typical size of the coherent islands is ~20 nm for the base length and 3 to 8 nm in height. An atomic force microscope image of the InAs island size distribution is shown in Figure 3.1. The exciton diameter in bulk InAs is ~70 nm. Strong electronic quantum confinement...