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Photoluminescence spectroscopy (PL) is a powerful optical method used for characterizing materials. PL can be used to find impurities and defects in silicon and group III-V element semiconductors, and to determine semiconductor band-gaps. A material absorbs light, creating an electron-hole pair; an electron from the valence band jumps to the conduction band leaving a hole . The photon emitted upon recombination corresponds to the energy-difference between the valence and conduction bands, and is hence lower in energy than the excitation photon.