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Using hemisphere-shaped patterned r-plane sapphire substrates, high quality nonpolar (1120) a-plane gallium nitride (GaN) films have been obtained with superior structural characteristics to films grown on conventional (unpatterned) r-plane sapphire. This is evidenced by reduced x-ray rocking curve widths, smaller in-plane crystallographic anisotropy, and smoother surface morphology. Observations by transmission electron microscopy and cathodoluminescence reveal that the defect density is remarkably reduced in regions above the patterned hemispheres. The growth of a-plane GaN on the patterned substrates proceeds first by a relatively fast growth of the film on the flat surface region, and then a gradual lateral overgrowth over the protruding hemispheres, where the direct epitaxial growth is severely retarded. [ABSTRACT FROM AUTHOR]

Photoluminescence (PL) and optical gain (OG) spectra of a-plane GaN layers have been analyzed over a wide range of excitation intensities. The samples were fully coalesced layers grown by metalorganic chemical vapor deposition over r-plane sapphire substrates using epitaxial layer overgrowth (ELOG) and selective area lateral epitaxy (SALE) procedures. ELOG and SALE a-plane samples showed a strong stimulated emission line in backscattering-geometry PL spectra along with extremely high OG coefficient values (in SALE samples more than 2000 cm-1). Structures prepared with natural cleaved facet cavities based on these films were used to demonstrate optically pumped room-temperature lasing. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We report on the strain reduction in AlGaN layers grown on porous GaN (P-GaN) by metalorganic chemical vapor deposition (MOCVD). The P-GaN was obtained by ultraviolet radiation-enhanced electroless wet chemical etching of thick hydride vapor phase epitaxial grown GaN (HVPE-GaN) templates over (001) sapphire substrates. By optimizing the growth conditions, lateral growth of AlGaN was enhanced resulting in air-bridge formation between the P-GaN and the AlGaN layers. X-ray diffraction studies showed significant strain relaxation in AlGaN layers primarily due to the strain sharing between P-GaN and the AlGaN layers. This allowed us to grow crack-free good optical quality layers with thickness exceeding the critical limits for AlGaN deposition on the conventional MOCVD GaN or HVPE-GaN. The obtained results demonstrate the potential of this approach for the development of efficient ultraviolet light emitters. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We report on an AlN/AlGaN superlattice approach to grow high-Al-content thick n[sup +]-AlGaN layers over c-plane sapphire substrates. Insertion of a set of AlN/AlGaN superlattices is shown to significantly reduce the biaxial tensile strain, thereby resulting in 3-μm-thick, crack-free Al[sub 0.2]Ga[sub 0.8]N layers. These high-quality, low-sheet-resistive layers are of key importance to avoid current crowding in quaternary AlInGaN multiple-quantum-well deep-ultraviolet light-emitting diodes over sapphire substrates. © 2002 American Institute of Physics. [ABSTRACT FROM AUTHOR]