Abstract

A semiconductor could emit photons under either optical or electrical pumping. Photoluminescence (PL) and electroluminescence (EL) refer to light emission due to the excitation by optical pumping (i.e., optical radiation) and electrical pumping (i.e., application of electric field or current injection), respectively. PL and EL are usually concerned with the following optical transitions: (1) interband transition with intrinsic emission corresponding very close in energy to the bandgap of the semiconductor, where phonons and excitons may be involved, and (2) the transition involving states in the bandgap due to impurities or defects. Not all transitions can occur in the same material or under the same conditions, and not all transitions are radiative. The light emission process consists of two optical transitions: the excitation transition, and the recombination transition. In the excitation transition, the electron is excited from the ground state (e.g., a state in the valence band of the semiconductor) to the excited state (e.g., a state in the conduction band of the semiconductor) by absorption of a photon under optical pumping, or is electrically injected into the excited state under electrical pumping. In the radiative recombination transition, the electron in the excited state recombines with the hole at the ground state to emit a photon. For a given input excitation energy, the radiative recombination process competes with the nonradiative process (such as the Auger recombination). To have efficient light emission, the radiative transitions must predominate over the nonradiative ones [1–3].