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New conditions for one-step ELO were implemented to grow coalesced (11$ \bar 2 $0) non-polar and (11$ \bar 2 $2) semi-polar GaN layers starting, respectively, from R - and M -plane sapphire. A great part of the stacking faults (SFs) and dislocations are filtrated resulting in GaN material with better structural and optical properties. In the ELO-like (11$ \bar 2 $0) and (11$ \bar 2 $2) films, the near band edge emission dominates photoluminescence spectra and is in the range 3.45–3.48 eV depending on lattice deformation. The strongest emission is met for the semi-polar (11$ \bar 2 $2) ELO. When mask stripes are not normal to the c-axis, a singular ELO is developed with inclined coalescence facets. However, in this case, SFs overgrow above the mask and so lead to poor optical properties, dominated by SF and dislocation related peaks. In any case, the internal electric field reduction in (Al,Ga)N/GaN non- or semi-polar quantum-wells stacks is better viewed when the heterostructures are grown on ELO with stripes normal to the c-axis. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Strategies for maintaining a low threading dislocation density in highly resistive GaN layers grown by MOVPE on sapphire or SiC substrates using Fe modulation doping are presented. Sheet resistances as high as 108 Ω□ for dislocation densities lower than 8 × 108 cm–2 have been obtained. 2DEGs created at the AlGaN/GaN:Fe interface have good confinement while the low dislocation density benefits the mobility: room temperature values up to 2170 cm2 V–1 s–1 have been obtained for a carrier density of 9.3 × 1012 cm–2. A simplified transport model is proposed to fit the experimental data. Processed transistors showed good channel control and DC performance. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

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We have studied the growth mechanisms of GaN and AlGaN layers with low aluminium incorporation (<7%) deposited by metal–organic vapour phase epitaxy (MOVPE) on 6H–SiC. The direct growth of GaN leads to isolated islands. For AlGaN alloys, the growth can be either three-dimensional or two-dimensional depending on the proportion of aluminium in the vapour phase. Aluminium-rich seeds, whose density depends on the aluminium incorporation in the alloy and on the 6H–SiC surface preparation, act as nucleation centres and enable one to obtain flat layers after lateral coalescence.

AlGaN/GaN quantum well (QW) structures were grown entirely by molecular-beam epitaxy on c-plane sapphire substrates. Reflection high-energy electron diffraction intensity oscillations, which testify a smooth growth front surface at the molecular monolayer scale, were used to precisely measure in situ both the growth rate and the Al content in AlGaN alloys. High-quality AlGaN/GaN QWs with thicknesses varying from 10 to 80 Å were achieved. Their photoluminescence (PL) linewidth is 10 to 30 meV, and the Stokes shift, determined by temperature-dependent PL, also ranges between 10 to 30 meV. The sample geometry and the Al composition in the barriers were varied in order to assess the built-in electric field present in wurtzite nitride-based heterostructures. The magnitude of the deduced electric field points out the role of spontaneous polarization.

GaN epilayers about 600 nm thick were grown directly on 6H-SiC by metal-organic vapour phase epitaxy (MOVPE) with a three-step process. A thin GaN nucleation layer grown at 1080 °C, initially 3D, is smoothed in several minutes under ammonia flux when the growth is interrupted. A flat layer is obtained through a large mass transport mechanism. The subsequent growth of GaN is 2D and 600 nm thick GaN films were grown on both on-axis and 3.5° off substrates. The structural properties of the samples were characterised by High Resolution X-Ray Diffraction (HRXRD) and Transmission Electron Microscopy (TEM). The full width at half maximum (fwhm) in ω-scan rocking curve of the (0002) and (1-015) lines of GaN are 124 and 188 arcsec, respectively. The defect structure of the GaN film away from the substrate consists mostly of threading dislocations with a density of 3 × 109 cm—2 and of (112‐0) prismatic defects.

A side chain, mesomorphic polymer has been synthesized, starting from a commercial polybutadiene main chain. A preliminary X-ray diffraction study shows the influence of the degree of substitution and the spacer length on the interdigited SA layer thickness, particularly for strongly polar alkoxycyanobiphenyl mesogens. Some hypotheses are deduced about the particular odd-even effect observed with this backbone and on the conformation of the main and side chains.

Due to high lattice mismatch, heterostructures of III-nitrides are subject to plastic relaxation. In this paper, we focus on the relaxation of AlGaN films grown on GaN. This relaxation is realised by cracking followed by the introduction of misfit dislocations. We describe those mechanisms and present a method to grow thick high quality crack-free AlGaN layers. This method uses jointly plastic relaxation and lateral growth.

Due to high lattice mismatch, heterostructures of III-nitrides are subject to plastic relaxation. In this paper, we focus on the relaxation of AlGaN films grown on GaN. This relaxation is realised by cracking followed by the introduction of misfit dislocations. We describe those mechanisms and present a method to grow thick high quality crack-free AlGaN layers. This method uses jointly plastic relaxation and lateral growth.

Ammonia has been used to grow GaN layers by molecular beam epitaxy on c-plane sapphire substrates. The ratio of nitrogen to Ga active species, i.e., the actual V/III ratio, has been varied from 1 to 4. It is found that increasing the V/III ratio improves the material properties both in terms of optoelectronic and structural quality. This is demonstrated by photoluminescence (PL) experiments, Hall measurements, secondary ion mass spectroscopy (SIMS), and atomic force microscopy. The origin of the residual n-type doping of undoped GaN layers is also discussed on the basis of SIMS and PL results.