Phonon effects in intersubband lasers

The effects of dimensional confinement on optical phonons, phonon-assisted electron intersubband transition rates (Teng et al., 1998), and gain (Kisin et al., 1997) have been evaluated in a series of studies on semiconductor lasers. Many of the novel semiconductor lasers – such as the tunneling injection laser and the quantum cascade laser – contain quantum wells with confinement dimensions of about 50 Å or less. An example of such a semiconductor laser structure is given in Figure 10.1. In this laser, the conduction band is engineered so that the upper and lower energy levels are E3 and E2 respectively, with a third level, E1, such that phonon-assisted tunneling from level E2 to level E1 is promoted. The IF optical phonons are of special importance in such heterostructures. Two properties of the IF optical phonons account for their special significance (Stroscio, 1996) in narrow-well semiconductor lasers: (a) in narrow wells the interface phonons have appreciable interaction potentials throughout the quantum well since, as was demonstrated in Chapter 7, the interface optical phonons have potentials near the heterointerfaces of the form ce–q|z|e–iq·ρ, where, for example, q has values of very roughly 0.02 Å–1 for a typical intrasubband transition in GaAs; and (b) the energies associated with phonon-assisted processes in these heterostructures can be substantially different from those in semiconductor lasers without significant dimensional confinement, since the interface phonons may have frequencies ωq which are significantly different from those of the other phonons in the quantum well.