GaN epilayers on misoriented substrates

Three silicon-doped 3 mu m thick GaN epilayers were grown simultaneously by metalorganic chemical vapour deposition on (0001) sapphire substrates miso rientated by 0, 4 and 10 degrees toward the m-plane (10 (1) over bar 0). A comparative study of these epilayers was undertaken using photoluminescence (PL) spectroscopy, atomic force microscopy (AFM) scanning electron microsc opy (SEM), cathodoluminescence (CL) imaging and CL spectroscopy. Low temper ature PL of the 0 and 4 degrees epilayers shows bound exciton (BE) emission between 3.47 and 3.48 eV and a low level of yellow band emission. The peak intensities of both emission bands are a factor of 2 higher for the 4 degr ees layer. In the 10 degrees epilayer, the BE band is 3 x stronger than in the 0 degrees epilayer but there is no discernible yellow band. However, a number of additional bands appear at 3.459, 3.417, 3.362, 3.345, 3.309 and 3.285 eV. These bands may be attributed to the presence of structural defec ts in this epilayer, pointing to an abrupt degradation of its structural qu ality compared to the others. This degradation is confirmed by AFM studies. On a 20 x 20 mu m(2) image the 0 and 4 degrees epilayers exhibit smooth su rface morphologies, while the 10 degrees epilayer shows a high density of h exagonal pits. Finally, SEM images reveal the surface of the 10 degrees epi layer to be 'streaked' and pitted. Low temperature CL images at 3.48 eV (bo und exciton region) show random spotty emission, while those at 3.28 eV and 3.41 eV exhibit a streaky appearance similar to the SEM image. This sugges ts that these luminescence bands are indeed associated with structural defe cts. (C) 1999 Elsevier Science S.A. All rights reserved.