E. Martinez-Guerrero

Self-assembled zinc blende GaN quantum dots grown by molecular-beam epitaxy

Zinc blende (ZB) GaN quantum dots have been grown by plasma-assisted molecular-beam epitaxy on AlN buffer layers using 3C-SiC(001) substrates. The two- to three-dimensional growth mode transition is studied by following the evolution of the reflection high-energy electron diffraction pattern. ZB GaN island layers are further examined by atomic force microscopy and transmission electron microscopy, extracting a mean island height of 1.6 nm and a mean diameter of 13 nm at a density of 1.3×1011 cm−2. Embedded ZB GaN quantum dots show strong ultraviolet photoluminescence without any thermal quenching up to room temperature.

Control of the morphology transition for the growth of cubic GaN/AlN nanostructures

The Stransky–Krastanow growth mode of strained layers which gives rise to a morphology transition from two-dimensional layer to three-dimensional islands is studied in details for the cubic gallium nitride on cubic aluminum nitride (GaN/AlN) system grown by molecular beam epitaxy. Besides the lattice parameter mismatch which governs this transition, we evidence the importance of two other parameters, namely the substrate temperature and the III/V flux ratio. Tuning each of these two parameters enables to control the strain relaxation mechanism of a GaN deposited onto AlN, leading to the growth of either quantum wells or quantum dots.

Self-Assembled GaN Quantum Dots Grown by Plasma-Assisted Molecular Beam Epitaxy

It is shown that under specific growth conditions the strain relaxation of GaN deposited on AlN obeys the Stranski-Krastanov mechanism. As a consequence, it is demonstrated that, for both cubic and hexagonal phases, the growth of self-organized three-dimensional islands can be achieved. These islands behave as quantum dots, exhibiting optical properties dominated by localization effects.

Electroluminescence Characterization of Cubic Gallium Nitride p–n Junctions Grown on SiC/Si Substrates by MBE

The electroluminescence (EL) properties of prospective cubic gallium nitride p–n junctions grown by molecular beam epitaxy (MBE) are studied. The samples were deposited on a 3 μm thick silicon carbide layer grown by chemical vapor deposition (CVD) on an n+ silicon substrate. At room temperature, visible and UV electroluminescence are obtained for V = 2.5 V under dc conditions. At low temperatures only UV luminescence is observed. This low temperature luminescence is mostly given by shallow donor–acceptor and band edge transitions. The activation at increasing temperatures of a visible green-blue electroluminescent band could be explained by the thermal activation of Mg deep states. Our results indicate that the MBE cubic gallium nitride material is a promising alternative for the fabrication of light emitters on silicon compatible substrates.

Optical Study of Cubic Gallium Nitride Band-Edge and Relation with Residual Strain

A photoreflectance study of excitonic transitions in cubic GaN grown on cubic SiC pseudo-substrates is reported. The determination of the temperature dependence of the two exciton energies is allowed by the rather low transition widths. Comparison of the two PR transition energies and widths with theoretical calculation as a function of biaxial strain indicates that the lowest PR excitonic transition is composed of both contributions of light hole and heavy hole valence band related excitons. The width of this transition is increasing as the residual strain in the layer increases the splitting between light and heavy hole valence bands.

Optical characterization of MBE grown zinc-blende AlGaN

Zinc-blende AlGaN epilayers were grown by plasma-assisted MBE on thick (001) SiC deposited by CVD on Si substrates. Alloy compositions were controlled in situ by reflection high energy diffraction (RHEED) oscillations, and confirmed by Rutherford backscattering (RBS) post growth analysis. The zinc-blende nature of our samples was assessed by RHEED analysis. From reflectivity measurements at room temperature, we deduce the variation of the lowest direct absorption edge of zinc-blende AlGaN as a function of the Al content x, E 0 (x) = 3.25 (1 - x) + 6.05x - 1.4x (1 - x), E 0 in eV. The dispersion of the refractive index n of cubic AlN was also extracted from our reflectivity and RBS data, and fitted by a Sellmeier-type relation, n 2 = 3.05 + 1.38 λ 2 /(λ 2 -180 2 ), λ being the wavelength in nm.

Growth Control of Cubic GaN and GaAlN (GaInN) Alloys by RHEED Oscillations

We report on the optimization of growth of cubic GaN and AlN binaries together with cubic GaInN and GaAlN ternary alloys. Reflection high energy diffraction oscillations are observed for all these cubic materials which allow us to determine precisely the stoichiometric growth conditions. The alloy compositions deduced from the growth rate variations by adding indium or aluminium are compared with those obtained by Rutherford backscattering spectra.

Modified Stranski‐Krastanov Growth in Stacked Layers of Self‐Assembled Cubic GaN/AlN Quantum Dots

In a stacked structure of cubic GaN/AlN islands grown in a Stranski-Krastanov mode, the critical thickness for the GaN islanding (2D-3D transition) decreases by a factor of up to 10 between the initial dot layer and the third one. This variation of the critical thickness is strongly dependent on the AlN spacer thickness. Moreover, we observe systematically a change of the quantum dot strain state and an increase of island size, depending on the GaN island layer number.

Atomic Layer Epitaxy of Hexagonal and Cubic GaN Nanostructures

We demonstrate that, depending on substrate temperature, exposure of cubic or hexagonal GaN, AlN or AlGaN surface to Ga leads to the formation of a self-regulated film. Subsequent exposure to N flux leads to the growth of a discrete quantity of GaN. This atomic layer epitaxy process can be repeated several times, allowing for the controlled growth of QWs or QDs.

Optical properties of cubic AlGaN

In this work we report optical characterization on several cubic c-AlGaN layers grown by MBE on SiC on Si pseudo-substrates, with different aluminum concentrations ranging from 0 to 70 %. Excitation power evolution of AlGaN photoluminescence (PL) spectra as well as reflectivity spectra allow to attribute PL peak to band gap recombination. PL energy dependence versus aluminum concentration is given. Reflectivity investigations are performed in the energy range between 1.5 eV and 4 eV on the samples. Theoretical calculations of multilayered structure reflectivity are fitted to experimental results, allowing an accurate determination of refractive index evolution versus Al concentration. From this analysis, qualitative information about interface roughness at AlGaN/SiC is also be derived.