Luminescence

Luminescence is the phenomenon in which a crystal absorbs energy and emits the energy back in the visible range. There are two varieties here-florescence and phosphorescence. Fluorescence is an instantaneous re-emission. Phosphorescence which is also known as afterglow takes a longer time to glow. Luminescence is greatly influenced by the presence of impurities and imperfections in the material. These impurities are called activators and introduce additional energy states in the bandgap of the material. These energy states are called recombination centers and traps. If the discrete energy states are located near the center of the bandgap, they are referred as recombination centers. All the other energy states in the bandgap are called traps. Thus a trap is an imperfection which is capable of capturing an electron or a hole. The existence of recombination centers and traps introduces a number of newer possible transitions in the semiconducting material. Luminescence finds applications in electroluminescent lamps, fluorescent tubes, cathode ray oscilloscopes and television displays. The phenomenon of luminescence may be classified into photoluminescence, cathodoluminescence, electroluminescence, chemiluminescence, thermoluminescence, triboluminescence and injection luminescence. Photoluminescence is produced by photons. Cathodoluminescence by high energy electrons, electroluminescence due to ac electric fields, chemiluminescence due to some chemical reactions, thermoluminescence by simulation at low temperatures, triboluminescence is due to mechanical energy and injection luminescence is due to forward biased p-n junction.