Semipolar (112¯2) GaN films were obtained by epitaxial lateral overgrowth from (112¯2) GaN templates patterned with SiO2 stripes 7 μm wide with 3 μm spacing, oriented along the [11¯00] GaN in-plane direction. The growth conditions were optimized in order to promote a fast growth rate along the +c [0001] direction. The crystal expands both laterally and vertically until a situation where it overgrows the adjacent crystal, thus stopping the propagation of stacking faults and threading dislocations. The growth anisotropy and filtering of defects is observed by cross-sectional scanning electron microscopy and cathodoluminescence. The lowering of defect density is confirmed by x-ray diffraction measurements. The photoluminescence spectrum of the coalesced epitaxial lateral overgrowth of the (112¯2) epilayers exhibits a strong band edge emission and a low emission band at 3.41 eV, assigned to the remaining stacking faults.

Low-pressure metalorganic vapor-phase epitaxy growth conditions of AlxGa1−xN epilayers on c-oriented sapphire have been optimized for aluminum mole fractions x lying in the 0–0.35 range both on GaN and AlN nucleation layers, with a view to application in visible blind UV photodetectors. Good structural, electrical, and optical properties were obtained for undoped and n-type doped AlGaN alloys on (0001)-oriented sapphire substrates. A typical full width at half maximum of 670–800 arc s is measured for the (0002) x-ray double-diffraction peak in the ω mode of 1-μm-thick AlGaN epilayers grown on a GaN nucleation layer. Room-temperature electron mobilities up to 90 cm2/V s are measured on n-type (1018 cm−3) AlGaN epilayers. The low-temperature photoluminescence (T=9 K) performed on nonintentionally doped AlGaN epilayers with low-Al content (10% and 14%) exhibits reproducibly a sharp exciton-related peak, associated with two phonon replica and does not exhibit any low-photon energy transitions. Optical transmission as well as absorption coefficient measurements using photothermal deflection spectroscopy have been used to study the variation of the T=300 K energy gap of AlGaN with the aluminum concentration. Visible-blind AlGaN(Si)-based photoconductors and Schottky barrier photodiodes with good operating characteristics have been fabricated with these materials.

Selective area growth of a-plane GaN nanocolumns by molecular beam epitaxy was performed for the first time on a-plane GaN templates. Ti masks with 150 nm diameter nanoholes were fabricated by colloidal lithography, an easy, fast and cheap process capable to handle large areas. Even though colloidal lithography does not provide a perfect geometrical arrangement like e-beam lithography, it produces a very homogeneous mask in terms of nanohole diameter and density, and is used here for the first time for the selective area growth of GaN. Selective area growth of a-plane GaN nanocolumns is compared, in terms of anisotropic lateral and vertical growth rates, with GaN nanocolumns grown selectively on the c-plane.The main advantages of one-dimensional structures such as nanocolumns, compared to thin films, is the dislocation-and strain-free growth on different substrates. This higher crystal quality yields a better efficiency in devices such as light emitting diodes (LEDs) based on InGaN/GaN quantum disks (QDisks) [1][2][3][4].Much effort has been dedicated to the growth of self-assembled nanocolumns (NCs) by Plasma-Assisted Molecular Beam Epitaxy (PAMBE), allowing a better knowledge of the material physical properties and growth mechanisms [5][6][7][8]. However, the strong morphology dispersion, typical of a self-assembled process, hinders both the processing of nanodevices arrays and their electrical behavior (NCs merging, defect generation, current injection inhomogeneities). Indeed, arrays of self-assembled nanoLEDs show in general much lower electroluminescence efficiency than the corresponding one measured by photoluminescence (PL). In addition to that InGaN active regions embedded in self-assembled NCs of different diameters and lengths always show fluctuations in the composition, leading to multicolor emission. Arrays of nanostructures grown by selective area growth (SAG) provide a much better homogeneity in terms of morphology, electrical and optical characteristics. SAG of Ill-nitride NCs [9][10][11][12] and other nanostructures [13,14], is generally performed on very thin metal (Ti, Mo) or dielectric (Si0 2 , SiN) masks with an array of nanoholes.A relevant issue concerning optoelectronic devices based on Ill-nitrides is the presence of strong polarization fields that may reduce efficiency. This is the case in layers grown along the c-axis and, a huge effort is nowadays dedicated to the growth of high quality non-polar and semi-polar material [15], with a particular emphasis on non-polar LEDs [16].In the case of SAG of GaN NCs on c-plane GaN templates, the growth front is generally formed by semi-polar (r-planes) that yield a "pencil-like" profile [12]. This profile is then transferred to InGaN QDisks embedded within the GaN NC. Though in this structure the effects of internal fields in the active region of the device (i.e. nanoLED) may be reduced compared with polar planes, the most effective solution is to grow along non-polar directions, either by growing core-shell heterostructures on the latera...

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