Exciton and biexciton luminescence from single hexagonal GaN∕AlN self-assembled quantum dots

We report single dot spectroscopy of hexagonal $\mathrm{GaN}∕\mathrm{AlN}$ self-assembled quantum dots grown by metalorganic chemical vapor deposition. Through the reduction of the number of quantum dots using submicron mesa structures, we have obtained several isolated photoluminescence peaks emitted by individual quantum dots. With increasing excitation power, an additional peak emerges in the higher-energy side of an excitonic ground state. This additional peak shows quadratic power dependence, and is attributed to the photoluminescence from a biexciton state. The biexciton binding energy is negative and the magnitude is about $30\mathrm{meV}$ , which is explained by the effects of a strong built-in electric field.