We study the influence of the heteroepitaxial interface on the defect content of nonpolar a-plane GaN grown on r-plane sapphire, using transmission electron microscopy techniques and the topological theory of interfacial defects. The structural mismatch is accommodated in different ways along distinct in-plane directions. For the misfit along the [0001] direction of GaN, the I1 basal stacking faults constitute the principal relaxation mechanism, through their delimiting partial dislocations. The misfit along [11¯00] is relaxed by misfit dislocations that have out-of-plane Burgers vectors causing rotational misorientations of the epilayer about [0001]. These misorientations lead to the introduction of low-angle tilt grain boundaries which are defect sources through their associated primary lattice dislocations. Furthermore, semipolar nanocrystals can nucleate on sapphire p-plane nanofacets, causing the introduction of threading dislocations (TDs) and stacking faults, when these nanocrystals are overgrown by the nonpolar epilayer. The semipolar nanocrystals bear a high-symmetry 90° [1¯21¯0] orientation relative to the nonpolar epilayer. The interfacial dislocations at the nonpolar/semipolar boundaries have suitable Burgers vectors to become TDs. These grain boundaries also promote the introduction of stacking faults and pockets of cubic GaN. The coexistence of semipolar orientation variants leads to twin boundaries and associated dislocations.

Articles you may be interested inStudy of structural properties of cubic InN films on GaAs(001) substrates by molecular beam epitaxy and migration enhanced epitaxy Effects of gallium doping on properties of a -plane ZnO films on r -plane sapphire substrates by plasma-assisted molecular beam epitaxy J. Vac. Sci. Technol. A 29, 03A111 (2011); 10.1116/1.3562162Direct growth of hexagonal InN films on 6H-SiC by radio-frequency metal-organic molecular-beam epitaxya)The heteroepitaxy of a-plane ͑1120͒ InN films on r-plane ͑1102͒ sapphire substrates, by nitrogen radio frequency plasma-assisted molecular beam epitaxy, has been investigated and compared to that of c-plane ͑0001͒ InN. The epitaxial growth of a-plane InN proceeded through the nucleation, growth, and coalescence of three-dimensional islands, resulting in surface roughness that increased monotonically with epilayer thickness. The full width at half maximum of ͑1120͒ x-ray diffraction rocking curves decreased significantly with increasing InN thickness, characteristic of structural improvement, and it reached the value of 24 arcmin for a 1 m thick film. Hall-effect measurements exhibited a similar dependence of electron concentration and mobility on thickness for both the a-and c-plane InN films. The analysis of the Hall-effect measurements, by considering the contribution of two conducting layers, indicates a similar accumulation of low mobility electrons with N s Ͼ 10 14 cm −2 at the films' surface/interfacial region for both the a-and c-plane InN films. From optical transmittance measurements, the absorption edge of 0.768 eV was determined for the 1 m a-plane film, consistent with the expected Burstein-Moss effect. Photoluminescence spectra exhibited a lower energy peak at 0.631 eV, suggesting defect-related transitions.

Self-heating effects in a molecular beam epitaxy-grown GaN/AlGaN/GaN structure on a single crystalline diamond is investigated. A transient interferometric method, in combination with a three dimensional model, is used to describe a pulsed operation of a transistor-like heater, and a micro-Raman technique is used in a steady state. Good agreement is found between the techniques. The thermal conductivity of the diamond is found to be 2200 W/m K, and the thermal boundary resistance to the III-N epi-structure is < 1 × 10−8 m2 K/W. The excellent cooling efficiency of the diamond is manifested by the fast saturation of the temperature at 1 μs and by a record low normalized thermal resistance of 3.5 K mm/W.