Photonic bandgap materials defects

For Si, in order for an electron at the bottom of the CB to recombine with a hole from the top of the VB, the momentum of the electron must shift from kcb to kyi, (Figure 4.9b). However, this is not allowed by the Law of Conservation of Momentum. Instead, an indirect recombination mechanism must take place, wherein the electron is captured by an interstitial defect with energy E, which facilitates its relaxation to the top of the VB. This process is accompanied by the emission of phonons, or lattice vibrations rather than light emission. In contrast, efficient electron-hole recombination may occur without any change in momentum for direct bandgap materials, resulting in the emission of photons. We will describe some important applications for direct bandgap semiconductors later in this chapter. 

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