Silicon Photonics: Fundamentals and Devices

4.8 Intervalence Band Absorption

As noted already, the valence bands in typical semiconductors including Si comprise LH, HH, and SO bands. At elevated temperatures or in heavily doped p-type materials having Fermi energy below the valence band edge, transitions are possible from the LH band to the HH band or from the SO band to the HH or LH band. An example of such transitions is given in Figure 4.7 as process (C) indicating a transition from the SO to the HH band. The different transitions are further illustrated in Figure 4.7, in which processes A, B, and C denote, respectively, the LH–HH, SO–LH, and SO–HH transitions. The intervalence band transitions are forbidden at k = 0, due to quantum mechanical selection rules. Direct (vertical) transitions become possible at k ≠ 0. The momentum matrix elements for these transitions are proportional to k. The absorption bands are broad.

Figure 4.7 Schematic of intervalence band absorption.