N. Gogneau

Structure of GaN quantum dots grown under modified Stranski-Krastanow conditions on AlN

We propose a procedure to grow GaN quantum dots (QDs) on AlN by using the Ga surfactant effect in plasma-assisted molecular beam epitaxy. Self-formed GaN islands were spontaneously generated under vacuum, after evaporation of the Ga bilayer stabilizing the two-dimensional GaN layer grown under Ga-rich conditions. Island characteristics (size and density) are studied as a function of the nominal amount of GaN deposited. We demonstrate that the QD density can be controlled in the 3×1010 cm−2–2×1011 cm−2 range. It is shown that beyond a given amount of GaN nominally deposited, there is a coexistence between elastic and plastic relaxation, with GaN islands being formed on a partially relaxed two-dimensional GaN layer thicker than two monolayers.

Surfactant effect of In for AlGaN growth by plasma-assisted molecular beam epitaxy

In this article, the surfactant capability of In for AlGaN growth by plasma-assisted molecular beam epitaxy has been assessed. We have determined the range of substrate temperatures and In fluxes to form a self-regulated 1×1 In adlayer on AlxGa1−xN(0001). The presence of this In film favors two-dimensional growth of AlGaN under stoichiometric conditions. The formation of metal droplets on the surface is inhibited. In incorporation, if any, is lower than 0.01%. The structural quality of the layers is verified by high-resolution x-ray diffraction, both in symmetric and asymmetric reflections.In this article, the surfactant capability of In for AlGaN growth by plasma-assisted molecular beam epitaxy has been assessed. We have determined the range of substrate temperatures and In fluxes to form a self-regulated 1×1 In adlayer on AlxGa1−xN(0001). The presence of this In film favors two-dimensional growth of AlGaN under stoichiometric conditions. The formation of metal droplets on the surface is inhibited. In incorporation, if any, is lower than 0.01%. The structural quality of the layers is verified by high-resolution x-ray diffraction, both in symmetric and asymmetric reflections.

Molecular-beam epitaxial growth and characterization of quaternary III–nitride compounds

We report on the controlled growth and characterization of quaternary AlGaInN compounds by plasma-assisted molecular beam epitaxy. Two-dimensional growth is achieved with a monolayer of In segregating at the growth front. In incorporation is hindered by increasing growth temperature and Al mole fraction, which is explained by the lower binding energy of InN compared to GaN and AlN. The mosaicity of the layers is determined by the substrate quality, whereas the alloy disorder increases with the Al content, independent of the In mole fraction. Room temperature photoluminescence is dominated by a narrow band-edge emission, whose Stokes shift and activation energy increase with the In content. This behavior is interpreted in terms of carrier localization in self-formed alloy inhomogeneities. An In-related band bowing parameter of 2.5 eV has been estimated.

Growth kinetics of N-face polarity GaN by plasma-assisted molecular-beam epitaxy

We have studied the surface kinetics of N-face GaN during molecular-beam epitaxial growth by investigating the Ga wetting and the surface morphology. In the case of N-face GaN, it is not possible to establish the self-regulated Ga bilayer that is used as a surfactant for molecular-beam-epitaxy growth of Ga-face GaN. Indeed, to prevent the accumulation of Ga droplets, growth of the N-face GaN must be performed with less than one monolayer of excess Ga on the growing surface. Optimum surface morphology is achieved when growth is performed at the Ga accumulation limit.

GaN islanding by spontaneous rearrangement of a strained two-dimensional layer on (0001) AlN

It is shown that a two-dimensional GaN layer grown on (0001) AlN under Ga-rich conditions remains two-dimensional while annealing under a Ga flux due to a surfactant effect of Ga. In contrast, further annealing under vacuum without the Ga flux leads to evaporation of excess Ga and to spontaneous transformation of the GaN layer into islands if the initial layer is thicker than about 2.5 monolayers. The resulting morphology is studied by atomic force microscopy and transmission electron microscopy. The latter reveals that these islands sit on top of a continuous 2.5 monolayer thick wetting layer, i.e., they represent a Stranski–Krastanow structure.

Influence of AlN overgrowth on structural properties of GaN quantum wells and quantum dots grown by plasma-assisted molecular beam epitaxy

The effects of AlN overgrowth on the structural properties of GaN nanostructures (quantum wells and quantum dots) grown by plasma-assisted molecular beam epitaxy have been investigated using Rutherford backscattering spectroscopy, transmission electron microscopy, and reflection high-energy electron diffraction. The capping process induces a remarkable change in the dimensions of the nanostructures. The overgrowth process implies a thinning of the GaN quantum well and an isotropic reduction of the GaN island size. We demonstrate that this thickness/size reduction affects only the top GaN/AlN interface. The phenomenon is attributed to an exchange mechanism between Al atoms from the cap layer and Ga atoms in the nanostructures. We also demonstrate that this exchange is thermally activated and depends on the strain state of the nanostructures.

Effects of stacking on the structural and optical properties of self-organized GaN/AlN quantum dots

We report on the effect of vertical correlation on GaN/AlN quantum dots grown by plasma-assisted molecular-beam epitaxy using the modified Stranski–Krastanow growth mode. When increasing the number of GaN periods, we observe a homogenization of the island distribution and a redshift of the luminescence line. This redshift is attributed to an increase of the quantum Stark effect due to the increase of the piezoelectric contribution to the internal electric field.

In incorporation during the growth of quaternary III-nitride compounds by plasma-assisted molecular beam epitaxy

Epitaxial growth of quaternary AlGaInN compounds by plasma-assisted molecular beam epitaxy has been demonstrated. Two-dimensional growth is achieved under In excess, with an In film segregating at the growth front. The maximum In incorporation is significantly affected by the substrate temperature and the Al mole fraction of the alloy. This behavior has been attributed to the enhancement of In segregation due to the high binding energy of AlN compared to InN and GaN.

Metal organic vapor phase epitaxy of InAsP/InP(001) quantum dots for 1.55μm applications: Growth, structural, and optical properties

This contribution reports the metal organic vapor phase epitaxy of InAsP/InP(001) quantum dots with a voluntary V-alloying obtained owing to an additional phosphine flux during InAs quantum dot growth. The quantum dots were studied by photoluminescence and transmission electron microscopy. We show that the additional phosphine flux allows to tune quantum dot emission around 1.55 μm while improving their optical properties. The comparison of the optical and structural properties of the InAsP quantum dots allows to deduce their phosphorus composition, ranging from 0% to 30% when the phosphine/arsine flow ratio is varying between 0 and 50. On the basis of the compositions deduced, we discuss on the effects of the phosphine flow and of the alloying on the quantum dot growth, structural, and optical properties.

Development of ion sources from ionic liquids for microfabrication

In this article the authors present the potential of ionic liquid ion sources (ILISs) for direct microfabrication of silicon structures. The authors have developed a specific source geometry using the ionic liquid EMI-BF4 to obtain stable emission currents up to the 10 μA regime. ILIS (EMI-BF4) engraving properties were then investigated. The results and the chemical analysis of the patterned substrates suggest that reactive ion species can be generated from ILIS. This possibility is of major interest to allow decisive advances in the field of focused ion beam applications.