D. Jalabert

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.

Intersubband spectroscopy of doped and undoped GaN/AlN quantum wells grown by molecular-beam epitaxy

We report experimental and theoretical results on interband and intersubband transitions in GaN quantum wells with strained AlN barriers. All of the samples are grown by molecular-beam epitaxy on sapphire (0001) substrates. The results show that even at room temperature, strong electron localization occurs in the plane of the quantum wells due to the combined effect of monolayer thickness fluctuations and the high internal field in the GaN layers. We also demonstrate that the intersubband absorption is systematically blueshifted in n-doped quantum wells with respect to nominally undoped samples as a result of strong many-body effects, namely the exchange interaction. The results for both undoped and doped quantum wells are in good agreement with simulations.

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.

GaN quantum dots doped with Eu

Molecular-beam-epitaxy growth of Eu-doped GaN quantum dots embedded in AlN has been achieved. The crucial issue of Eu location has been addressed by extended x-ray absorption fine structure measurements. By comparing the signature of the Eu short-range environment for several samples, it is concluded that Eu is mostly incorporated in GaN dots. Intense cathodoluminescence associated with Eu has been measured, with no GaN bandedge emission, evidence that carrier recombination mostly occurs through rare-earth ion excitation. Persistent photoluminescence of Eu-doped GaN quantum dots as a function of the temperature is suggested to be further confirmation of the recombination of confined carriers through Eu ion excitation.

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 and optical properties of GaN/AlN quantum wells

We demonstrate the growth of GaN/AlN quantum-well structures by plasma-assisted molecular-beam epitaxy by taking advantage of the surfactant effect of Ga. The GaN/AlN quantum wells show photoluminescence emission with photon energies in the range between 4.2 and 2.3 eV for well widths between 0.7 and 2.6 nm, respectively. An internal electric field strength of 9.2±1.0 MV/cm is deduced from the dependence of the emission energy on the well width.

Strain relaxation in short-period polar GaN/AlN superlattices

We have investigated the strain relaxation mechanisms in short-period polar GaN/AlN superlattices deposited by plasma-assisted molecular-beam epitaxy, and designed to display intersubband transitions at 1.55 μm. In a first stage, we have identified the growth conditions to minimize strain relaxation, using a Ga excess to reduce the (0001) surface free energy of both GaN and AlN. Under these growth conditions, crack propagation is not observed, even for the tensile-strained superlattices grown on GaN templates. The initial misfit relaxation in the vicinity of the buffer occurs by the formation of a-type dislocations. The final strain state of the superlattice, reached after 10–20 periods, is independent of the substrate (either GaN or AlN templates). Once the steady-state conditions are reached, we observe a periodic partial relaxation of quantum wells and barriers. High-resolution transmission electron microscopy indicates that the periodic relaxation can be related to the presence of basal and prismatic ...

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.

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.

Micro-Raman characterization of In x Ga 1 − x N ∕ Ga N ∕ Al 2 O 3 heterostructures

In{sub x}Ga{sub 1-x}N/GaN/Al{sub 2}O{sub 3} (0001) heterostructures with x=10.5%, 13.5%, 19.0%, 19.6%, and 26.5% are studied, by polarized micro-Raman spectroscopy, under plane and side backscattering geometries. The combination of both scattering geometries, together with variable excitation wavelengths, enabled the possibility to check independently strain-vs-depth distribution and selective-resonance effects from In-rich regions. Several Raman modes have been detected and were attributed to either the GaN or the InGaN films. Particular modes, which are not permitted in the bulk materials, are activated in the InGaN layers. Shifts of the frequencies relative to the ones expected in the bulk materials are explained as due to the elastic strains present in the hetero-structures. The results are evaluated in combination with compositional RBS analysis and the strain values obtained are compared with high resolution x-ray diffraction results including reciprocal space mapping, leading to very good consistency between Raman and XRD. Consequent relaxation values are obtained and the underlying mechanisms are discussed.