Structural and photoluminescence study of thin GaN and AlN/GaN nanowires

The photoluminescence (PL) of GaN and AlN/GaN nanowires (NWs) grown under by the plasma-assisted molecular beam epitaxy (PA-MBE) is presented. The structural morphology of the AlN/GaN and GaN NWs was examined using Reflection high-energy electron diffraction (REEHD), high-resolution scanning electron microscope (SEM), and X-ray diffraction (XRD) techniques. PL spectra of GaN NWs show only one dominant peak at 3.420 eV at 8 K, attributed to exciton emission close to the surface. For AlN/GaN NWs, the dominant peak is redshifted by 27 meV with respect to GaN NWs as a result of the growth of AlN NWs on top of GaN NWs. Due to the smaller unit cells of the AlN NWs grown on top of GaN NWs, a tension strain is expected to be created in the AlN lattice at the interface with GaN and results in strain-induced potential traps for excitons. In the AlN/GaN NWs spectra, additional low-energy peaks were observed at 3.15 and 3.08 related to below band-gap emissions in AlN NWs. The PL spectra of GaN and AlN/GaN NWs show the dominant peak at 3.393 eV at 300 K, which is attributed to free exciton emission in GaN. In addition, the room-temperature PL of AlN/GaN NWs show strong emission at 3.03 eV. This study shows that GaN and GaN/AlN NWs assemblies are good luminescent candidates at room temperature. The new positions of the dominant peaks as well as the observed redshift between these emission peaks by growing AlN NWs on top of GaN NWs can push LED bandgap engineering for nanotechnology applications.