Exciton impact-ionization dynamics modulated by surface acoustic waves in GaN

The quenching of the photoluminescence induced by the fundamental and guided high-order Rayleigh-type modes propagating in the GaN∕6H-SiC(0001) heterostructure is compared. Two material types have been considered: conventional GaN and GaN underneath the Ga droplets formed during the molecular-beam-epitaxy growth. The different piezoelectric field-depth profile of each surface acoustic wave (SAW) is used to analyze their characteristics. A coupled-rate-equations model based on the impact ionization of the free and donor-bound excitons under the SAW-generated piezoelectric fields is presented, which satisfactorily reproduces the experimental results. The exciton impact-ionization rate reveals two regimes when the SAW power is increased, each of which seems to be plausibly dominated by the hole- and electron-initiated impact ionization. The SAW-induced impact-ionization model is also indicated to be valid for other wide direct band-gap piezoelectric semiconductors, such as CdS, ZnO, and AlN, where the field ionization requires an even larger electric field than in GaN.