Silicon Photonics: Fundamentals and Devices

5.1 Introduction

Optical processes in quantum structures have been intensively studied mostly for direct-gap semiconductors and their alloys. It is now known that photonic devices using quantum wells (QWs), quantum wires (QWRs), and quantum dots (QDs) show improved performance over their bulk counterparts. Optical processes in quantum structures involving Si, Ge, and their alloys are, on the other hand, less studied. Interest in this area took momentum with the announcement of light emission in porous silicon and the conclusion that quantum confinement plays an important role in the operation of the device.

There have been recent results in light emission from silicon-based quantum structures, in interband and intersubband transitions leading to photodetectors, and in strong excitonic absorption and its change with electric field in Ge–SiGe multiple quantum wells (MQWs): the quantum-confined Stark effect has sparked off renewed interest in the study of optical processes in Si-based quantum structures.

The present chapter gives the theory of different optical processes in nanostructures. Though some coverage of the processes in indirect-gap structures will be made, the present trend is to achieve direct-gap transitions in silicon-based structures. Therefore, in most cases, the existing theory for optical processes in direct-gap materials will be presented for understanding recently reported results and theoretical predictions in novel silicon-based structures of laser action and ...