Silicon Photonics: Fundamentals and Devices

4.1 Introduction

This chapter outlines the theory of optical processes in bulk semiconductors. First of all, the relationships between various optical constants, like absorption coefficient, refractive index (RI), and so on, are developed. The three basic processes in light matter interaction, that is, the absorption, spontaneous emission, and stimulated emission, are then introduced. This is followed by a brief description of absorption processes in semiconductors.

The theory of optical processes in semiconductors is developed mostly for direct-gap materials. Though the theory for indirect-gap semiconductors is available, its applications are somewhat limited. Recent developments indicate the possibility of having direct-gap type I band alignment in group IV materials. This necessitates the inclusion of the theory of optical transitions in both direct- and indirect-gap semiconductors.

A semiclassical theory of fundamental band-to-band absorption in semiconductors is developed for both direct and indirect-gap semiconductors. The condition for obtaining gain in semiconductors is then discussed. Other important absorption processes, like intervalence band and free carrier absorption, are then treated. The opposite process of absorption, that is, recombination and luminescence, is then introduced. Emphasis is given to main recombination processes in indirect-gap materials, that is, the trap-assisted and Auger recombination. Finally, the theory of excitonic absorption in direct-gap ...