Terahertz emission upon the interband excitation of GaN layers

The experimental observation and study of terahertz photoluminescence upon the steady-state interband excitation of epitaxial $n$-GaN(Si) layers are reported. The properties of the terahertz emission spectrum and the dependence of the spectrum on temperature and photoexcitation intensity suggest that emission occurs due to the trapping of nonequilibrium electrons at charged donors. In the $n$-type material at low temperatures, charged donors can be formed as a result of the recombination of nonequilibrium holes with electrons localized at donor centers. The main contribution to terahertz photoluminescence is made by $2P\to1S$ optical transitions between the first excited state and ground state of the donors. In addition, optical transitions of electrons from the conduction band states to the ground state as well as to the excited states of donors are evident in the terahertz emission spectrum.