77. Photoluminescence

Spontaneous emission (radiation) of light from an electronically excited material (except for thermal radiation) is called “luminescence.” In particular, photoluminescence (PL) is light emission occurring after absorption of shorter-wavelength light (higher-energy photons) (Fox in Optical properties of solids. Oxford University Press Inc 2001 [1]). In a typical direct-gap semiconductor, electrons (solid circles) and holes (open circles) that are optically excited into conduction and valence bands, respectively, first relax to lower available energy levels by emitting multiple low-energy phonons and then relax to the ground state by recombination and emission of a photon (Fig. 77.1, E_Laser and E_PL are the excitation and emission photon energies, respectively). Since the photoexcited electron and hole are oppositely charged, they can attract each other through mutual Coulomb interaction and form a hydrogen-like electron–hole pair called an “exciton.”