Silicon Photonics: Fundamentals and Devices

5.4 Excitonic Processes in QWs

The study of excitonic processes in indirect band gap semiconductors has been mainly of academic interest so far. The same is true for QWs formed by Si and its alloys. Recently there have been reports of strong excitonic absorption due to transition from the valence band to the zone center conduction band in Ge–SiGe MQWs. The following theory of excitonic processes in direct-gap materials is presented in that context, giving in most cases the bare outline.

5.4.1 Excitons in 2D: Preliminary Concepts

In bulk semiconductors, mutual Coulomb interaction between electrons and holes becomes prominent at low temperatures. The excitons dissociate at higher temperatures owing to their small binding energy. In a QW or other low-dimensional systems, the situation is altogether different. Take, for example, the values of binding energy and Bohr radius in bulk GaAs which are, respectively, 4.2 meV and 15 nm. As shown in Figure 5.7, the excitonic orbit is spherical in bulk GaAs. However, if the pair is created in a QW of width less than 15 nm, the Bohr radius, the orbit becomes squeezed as shown in Figure 5.7. The confinement of electrons and holes reduces the physical separation between the particles along the z-direction, as a result of which the Coulombic interaction and hence the binding energy increase. Shinada and Sugano 18 (1966) first showed that the binding energy for pure 2D systems increases fourfold. The measured optical absorption spectra shown in ...