Silicon Photonics: Fundamentals and Devices

6.5 Quantum Confinement

A vast body of literature has accumulated on the studies of light emission in quantum nanostructures. The impetus for such studies came immediately after the announcement of strong light emission from porous silicon observed by Canham 10.

Porous silicon belongs to the general class of quantum-confined nanostructures. In addition, other forms of nanostructures like quantum wires and quantum dots prepared by standard epitaxial process and self-assembly, are examined to obtain efficient light emission. Si nanocrystals embedded in glass and other matrixes also form an important class of materials showing prospects for efficient luminescence.

The basic idea behind the use of nanometer-sized luminescent material is increased overlap between electron and hole envelope functions leading to enhanced absorption and recombination. In Sections 6.5.1, 6.5.2, 6.5.3, and 6.5.4, we shall first explain the phenomena of increased overlap and then consider different structures and systems and their light emission characteristics.

6.5.1 Quasi-direct No-phonon Transition

The spreads in momentum and position of any quantum particle are related by Heissenberg's uncertainty relationship, that is,

(6.9)

This means that a particle having a precisely defined position (Δr = 0) will have an infinite spread in its momentum. In a nanostructure, the wavefunction of an electron is confined ...