Origin of localized excitons in In-containing three-dimensional bulk (Al,In,Ga)N alloy films probed by time-resolved photoluminescence and monoenergetic positron annihilation techniques

A principal origin of the defect-insensitive emission probability of In-containing three-dimensional (3-D) bulk (Al,In,Ga)N alloy films, such as InGaN, AlInN and AlInGaN, is proposed. In contrast to In-free GaN or AlGaN, In-containing films grown on sapphire substrates show significant emission probabilities, although the threading dislocation density generated due to lattice mismatch is six orders of magnitude higher than that in conventional (Al,In,Ga)(As,P) light-emitting-diode films. According to the extremely short diffusion lengths of positrons (<4 nm) and short radiative lifetimes of excitonic emissions, we conclude that capturing of holes by localized valence states associated with atomic condensates of In-N outrun the trapping by non-radiative recombination centres, which are defect complexes associated with group III vacancies, because holes and positrons have the same positive charge. The captured holes are considered to form localized excitons with surrounding electrons to emit the light. The enterprising use of atomically inhomogeneous crystals is proposed for future innovation in light emitters even when using defective crystals.