Interference between Intermediate States in the Optical Properties of Nitrogen-Doped Gallium Phosphide
Nitrogen substituting for phosphorus in gallium phosphide forms an isoelectronic trap which strongly affects the optical properties of the crystal, particularly in the vicinity of the band-gap energy. Attention is directed to optical absorption and excitation spectra produced by the presence of nitrogen. The former shows a peak at the intrinsic exciton energy and unexpected dips at energies corresponding to phonon replicas of zero-phonon transitions. The fluorescence excitation spectra show quite different structure at the same energies, and this structure depends on the fluorescence that is being observed. Such anomalous structure in the absorption spectra can arise in several ways. It can occur when there are two paths to the same final state. In addition, it can occur when there are at the same energy two classes of states which are mixed by some perturbation. Absorption to each of these classes of states will have its own anomalous structure. Different final states often radiate with different efficiencies or in different spectral regions, and hence relative measurements can be made of the various absorption processes which occur at one energy. It is found that the excitation spectra confirm predictions made as a result of the interpretation of the total absorption spectra in terms of various interference effects.