Photoluminescence studies of GaN nanorods on Si (111) substrates grown by molecular-beam epitaxy

We have investigated the optical properties of dislocation-free vertical GaN nanorods grown on (111) Si substrates by radio-frequency plasma-assisted molecular-beam epitaxy. The hexagonal shape nanorods with lateral diameters from 80 to 190 nm are obtained. They are fully relaxed and have a very good crystal quality characterized by extremely strong and narrow photoluminescence excitonic lines near 3.47 eV. Three distinct features are observed in photoluminescence. First, free exciton transition is observed at 3.477 eV for GaN nanorods of decreased diameter. Second, the photoluminescence spectra show an abnormal behavior with increasing temperature. The third feature is the size effect in that the PL peak energies are blueshifted with decreasing diameter of the GaN nanorod. The activation energy of the free exciton for the GaN nanorods with different diameters was evaluated. (C) 2004 American Institute of Physics.