Silicon Photonics: Fundamentals and Devices

5.5 Effect of Electric Fields

As mentioned in Chapter 4, the effect of an electric field applied perpendicular to the QW layer plane is to shift the subband energy levels. As a result, the effective band gap decreases more and more, as the value of the field is increased. In the present section, we shall discuss the effect of an electric field on excitonic absorption in QWs.

5.5.1 Qualitative Discussion of Electroabsorption

Excitonic absorption spectra are influenced differently when the electric field is applied perpendicular or parallel to the QW layer plane. We focus our attention on the case when the field is applied perpendicular to the QW layer planes. As discussed in Chapter 3, the effect of a perpendicular electric field is to shift the energies of the electron and hole subbands. Figure 5.10 shows the subband structure without an electric field and in the presence of a perpendicular electric field. Figure 5.10b indicates that the peaks of the electron and hole envelope functions are shifted to opposite directions in the presence of a field.

Figure 5.10 The effect of an electric field applied perpendicular to the QW layer plane on subband energies (a) and envelope functions (b). Reproduced with permission from [24]. Copyright (2005) Macmillan Publishers Ltd.

The shift in exciton peak is due to the Stark shift induced by the field. The exciton peaks should occur at an energy ...