Ultrathin AlN/GaN crystalline porous freestanding nanomembranes are fabricated on Si(111) by selective silicon etching, and self-assembled into various geometries such as tubes, spirals, and curved sheets. Nanopores with sizes from several to tens of nanometers are produced in nanomembranes of 20-35 nm nominal thickness, caused by the island growth of AlN on Si(111). No crystal-orientation dependence is observed while releasing the AlN/GaN nanomembranes from the Si substrate indicating that the driving stress mainly originates from the zipping effect among islands during growth. Competition between different relaxation mechanisms is experimentally revealed for different nanomembrane geometries and well-described by numerical calculations. The cathodoluminescence emission from GaN nanomembranes reveals a weak peak close to the GaN bandgap, which is dramatically enhanced by electron irradiation.

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A correlation between the structural and optical properties of GaN thin films grown in the ͓1120͔ direction has been established using transmission electron microscopy and cathodoluminescence spectroscopy. The GaN films were grown on an r-plane sapphire substrate, and epitaxial lateral overgrowth was achieved using SiO 2 masks. A comparison between the properties of GaN directly grown on sapphire and GaN laterally grown over the SiO 2 mask is presented. The densities and dimensions of the stacking faults vary significantly with a high density of short faults in the window region and a much lower density of longer faults in the wing region. The low-temperature luminescence spectra consist of peaks at 3.465 and 3.41 eV, corresponding to emission from donor-bound excitons and basal-plane stacking faults, respectively. A correlation between the structural defects and the light emission characteristics is presented.

The distinctly different growth domains of a-plane epitaxial lateral overgrown GaN on stripe masks oriented along [011̱0] direction were directly visualized by highly spatially and spectrally resolved cathodoluminescence microscopy. Clear cut microscopic regions dominated by differing individual peak wavelengths originating from either basal plane stacking faults, prismatic stacking faults, impurity related donor-acceptor pair or (D0,X) emission are explicitly correlated to the different growth domains. The luminescence in the domains grown in [0001] direction over the mask [epitaxial lateral overgrown wings] is dominated by the intense and sharp (D0,X) emission at 3.471eV. Here, no luminescence originating from morphological defects is found over several micrometers. This evidences the excellent material quality of the a-plane GaN, which is fully relaxed at the surface of the wings.

Abstract