Fig. 2

Overview µ-PL spectra. The µ-PL spectra of all investigated samples are excited by a frequency-quadrupled, pulsed fiber laser with 258 nm wavelength and an excitation power of 15 µW at 7 K. For a fixed GaN ND thickness of 4 nm, the emission energy can be shifted by just varying the AlN barrier thickness from 6 nm down to 1 nm. A further reduction of the emission energy is achieved by reducing the ND thickness from 4 to 1 nm, while keeping the barrier thickness constant at 1 nm. In all samples, the dominant GaN ND luminescence is accompanied by the GaN guard layer luminescence (marked by the dashed, blue line), which divides the samples series into the so called QCSE regime and the confinement regime