6.8 Intersubband Emission

While momentum needs to be conserved in band-to-band transitions in indirect-gap semiconductors, transitions between two subbands in a QW may be direct without involvement of a phonon or any other momentum-conserving agent. For example, in the conduction band of Si or SiGe, k-conservation is satisfied for E1 and E2 subbands belonging to valleys located along the same directions (see Figure 3.12). For transition from unprimed subbands to prime subbands, participation of a phonon is necessary. The situation is simpler for transitions between two subbands in the valence band, for example from HH1 to HH2.

Intersubband absorption in Si and SiGe QW systems has been studied in detail in order to design quantum well infrared photodetectors (QWIPs). Although intersubband emission was investigated by several workers in both quantized Si inversion layers and QWs, the purpose was mainly to understand basic optoelectronic processes. The lasing action involving intersubband transitions was conceived quite early. However, the practical realization of the concept was achieved only in 1994 with the announcement of quantum cascade lasers (QCLs) involving III–V compounds by Faist et al. 50. A lot of work has since then been reported for QWs made of III–V and II–VI compounds. In a QCL structure there exists an injector, a QW in which transition occurs, and then a collector to collect the carrier. The pattern is repeated so that the number of emitted photons is multiplied ...

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