Silicon Photonics: Fundamentals and Devices

5.3 Intersubband Transitions

Transition between different subbands in the same band, either conduction or valence band, is involved in light emitters, like quantum cascade lasers [12, 13] and quantum well infrared photodetectors (QWIPs) [14, 15] working at mid-infrared ranges of wavelength. Here we shall discuss three different cases: interconduction subband transition in a material having an isotropic effective mass, interconduction subband transition in the case of anisotropic mass, and transitions between different-valence subbands.

Figure 5.6 illustrates the various intersubband absorption processes. In Figure 5.6a, the transition between ground and the first excited subbands in the conduction band of the QW is shown. When the QW is made very narrow in thickness, only one subband appears in the conduction band. The transition from this subband to the continuum states above the heterobarrier, as shown in Figure 5.6b, is also utilized in some devices that offer better collection of photogenerated carriers. In addition to transition between interconduction band states, intervalence subband transitions are made use of. In Figure 5.6c transitions between HH → HH and between LH → LH, subbands are shown; transitions like HH → LH and HH → SO are possible. In Figure 5.6d a transition from a HH subband to a conduction subband is illustrated, which may occur in type II QW. This is an example of spatially indirect transition.

Example 5.2

Consider the values of subband energies given in ...