Wurtzite GaN nanocolumns are reproducibly grown by plasma-assisted molecular beam epitaxy on Si͑111͒ and c-sapphire substrates. The nanocolumns density and diameter ͑600-1500 Å͒ are effectively controlled by means of the III/V ratio. The nanocolumns are fully relaxed from lattice and thermal strain, having a very good crystal quality characterized by strong and narrow ͑2 meV͒ low-temperature photoluminescence excitonic lines at 3.472-3.478 eV. In addition, the spectra reveal a doublet at 3.452-3.458 eV and a broad line centered at 3.41 eV. This broad emission shows a sample-dependent spectral energy dispersion, from 3.40 to 3.42 eV, explained as due to the effect of strain and/or electric fields associated with extended structural defects located at the nanocolumns bottom interface. From cathodoluminescence data, it is concluded that the doublet emission lines originate at the nanocolumns volume, most probably related to Ga I defects, given the column growth mode ͑Ga balling͒.

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A wet etching method for GaN and Al x Ga 1−x N, based on aqueous solutions of KOH, is presented. A detailed analysis of the etching rate dependence with temperature and concentration is described. This etching has been used for the fabrication of high optical quality pyramidal nanostructures in wurtzite N-face GaN grown on AlN-buffered Si(111) substrates by molecular beam epitaxy. These nanostructures have been studied by high-resolution transmission and scanning electron microscopy and their optical quality has been analysed by low-temperature photoluminescence (PL) measurements. The pyramids are parallel to the basal plane and limited by {11 21} planes and its presence improves the overall PL response of the sample. The relationship between the polarity of GaN and the characteristics of the AlN buffer has also been analysed.

The effect of the III/V ratio and growth temperature on the In incorporation has been studied in thick ͑Ͼ300 nm͒ InGaN layers, with In mole fractions from 19% to 37%, grown by molecular-beam epitaxy on sapphire and on GaN templates. Significant desorption of In occurs at growth temperatures above 550°C. Symmetric and asymmetric reflections from high resolution X-ray diffraction reveals that the layers are not fully relaxed. A bowing parameter of 3.6 eV is calculated from optical absorption data, once corrected for strain-free band gap values. The increase of both, the absorption band-edge broadening and the photoluminescence full width at half maximum at room temperature with the In content, is discussed in terms of a strong In localization effect. This localization effect is further evidenced by the S-shaped temperature dependence of the emission energy.

The structural, morphological, electrical and optical properties of In-rich AlxIn1−xN (0  <  x  <  0.39) layers grown by reactive radio-frequency (RF) sputtering on sapphire are investigated as a function of the deposition parameters. The RF power applied to the aluminum target (0 W–150 W) and substrate temperature (300 °C–550 °C) are varied. X-ray diffraction measurements reveal that all samples have a wurtzite crystallographic structure oriented with the c-axis along the growth direction. The aluminum composition is tuned by changing the power applied to the aluminum target while keeping the power applied to the indium target fixed at 40 W. When increasing the Al content from 0 to 0.39, the room-temperature optical band gap is observed to blue-shift from 1.76 eV to 2.0  eV, strongly influenced by the Burstein–Moss effect. Increasing the substrate temperature, results in an evolution of the morphology from closely-packed columnar to compact. For a substrate temperature of 500 °C and RF power for Al of 150 W, compact Al0.39In0.61N films with a smooth surface (root-mean-square surface roughness below 1 nm) are produced.