Strain- and surface-induced modification of photoluminescence from self-assembled GaN/Al_0.5Ga_0.5N quantum dots: strong effect of capping layer and atmospheric condition

We report on the influence of a capping layer on the photoluminescence properties of self-assembled GaN quantum dots grown on an Al_0.5Ga_0.5N template. Self-assembled GaN quantum dots show a large quantum confined Stark shift and long carrier recombination time due to strong built-in spontaneous and piezoelectric polarization fields. Nevertheless, owing to strong carrier localization and suppressed nonradiative processes, these quantum dots have a high-quantum efficiency even at room temperature. Here, we show that the capping thickness has an important role on the optical properties of the GaN quantum dots. The radiative and nonradiative recombination processes of quantum dots are strongly affected by adjusting the capping thickness, and the GaN quantum dots with 12 monolayers-thick Al_0.5Ga_0.5N capping layer show a remarkably high internal quantum efficiency of more than 80% at room temperature. We also studied photoluminescence quenching and enhancement for surface (uncapped) quantum dots caused by photoadsorption and photodesorption of oxygen. (paper)