B. Daudin

Growth kinetics and optical properties of self-organized GaN quantum dots

Self-organized GaN islands of nanometric scale were fabricated by controlling the Stranski–Krastanov growth mode of GaN deposited by molecular beam epitaxy on AlN. Evidence for ripening of dots under vacuum has been observed, resulting in changes in dot size distribution. We also show that in superlattice samples, consisting of multiple layers of GaN islands separated by AlN, the GaN islands are vertically correlated provided that the AlN layer thickness remains small enough. The luminescence peak of GaN dots is blueshifted with respect to bulk emission and its intensity does not vary with temperature, both effects demonstrating the strongly zero-dimensional character of these nanostructures.

Polarity determination of GaN films by ion channeling and convergent beam electron diffraction

By using ion channeling and convergent beam electron diffraction techniques, we have determined the absolute polarity of various GaN films grown by MOCVD on (0001) sapphire. We observe two main classes of GaN films, namely flat and rough pyramidal ones. We find that flat GaN films have a Ga polarity. Rough pyramidal samples contain many tiny columnar inversion domains (with Ga polarity) imbedded in a matrix exhibiting an N polarity.

Self-assembled InGaN quantum dots grown by molecular-beam epitaxy

Self-assembled InGaN islands were grown by molecular-beam epitaxy on GaN, following a Stranski–Krastanow growth mode. Atomic force microscopy revealed that their dimensions were small enough to expect zero-dimensional quantum effects: the islands were typically 27 nm wide and 2.9 nm high. Strong blue-violet photoluminescence of the dots is observed, persisting up to room temperature. The temperature dependence of the photoluminescence is analyzed and compared to that of InGaN quantum well and bulk samples.

Improved quality GaN grown by molecular beam epitaxy using In as a surfactant

The surfactant effect of In during the growth of GaN by molecular beam epitaxy has been investigated. It has been found that the presence of In modifies the diffusion kinetics in the growing GaN surface, leading to the observation of persistent reflection high energy electron diffraction intensity oscillations, characteristic of layer-by-layer growth. Electron microscopy studies revealed drastic modifications of the GaN structural properties associated with the presence of In during the growth. When grown in the presence of In, GaN exhibits an intense band edge luminescence, free of the component at 3.41 eV which is characteristic of defects associated with growth in N-rich conditions.

Dynamically stable gallium surface coverages during plasma-assisted molecular-beam epitaxy of (0001) GaN

The Ga surface coverage during the growth of GaN by plasma-assisted molecular-beam epitaxy (PAMBE) has been systematically studied by reflection high-energy electron diffraction as a function of the Ga flux and the substrate temperature. As a consequence, a diagram is depicted, which describes the Ga surface coverage during PAMBE as function of growth conditions. In particular, we show that a region exists in this diagram, in which the Ga surface coverage is independent of fluctuations in the Ga flux or the substrate temperature and which forms a “growth window” for GaN growth. The influence of the Ga surface coverage on the GaN surface morphology and the growth kinetics is discussed.

Phonon deformation potentials of wurtzite AlN

A strained AlN buffer layer used for the growth of a nitride-based superlattice on silicon carbide was studied by combining x-ray diffraction measurements and Raman spectroscopy. The deformation potentials have been derived from strains and frequency shifts for most long-wavelength optical phonons. The obtained values are compared with recent theoretical calculations and experimental determinations, restricted for the latter to a few accessible modes on account of constraints imposed by the methods of investigation.

Low temperature sapphire nitridation: A clue to optimize GaN layers grown by molecular beam epitaxy

The sapphire nitridation temperature is investigated as a possible parameter to improve the properties of GaN epilayers grown by molecular beam epitaxy using a radio frequency plasma source. It is found out that lowering the nitridation temperature to values as low as 200u2009°C allows us to drastically improve the GaN structural and optical properties. Careful examination of the interface by transmission electron microscopy reveals that, in this case, the interface between the nitridated sapphire and the AlN buffer consists of an ordered array of pure edge dislocations. In contrast, high nitridation temperatures result in a perturbed interface with the occurrence of cubic crystallites in the AlN buffer. These results, complemented by a thorough reflection high-energy electron diffraction analysis of the nitridation procedure and a secondary ion mass spectrometry investigation, are interpreted in the framework of a model whereby a higher oxygen concentration is extracted from the substrate at high nitridation...

Quantitative characterization of GaN quantum-dot structures in AlN by high-resolution transmission electron microscopy

GaN/AlN heterostructures grown by molecular beam epitaxy are studied by high-resolution transmission electron microscopy (HRTEM). The two-dimensional/three-dimensional Stranski–Krastanow growth mode transition of GaN allows the formation of GaN quantum-dot structures embedded in AlN. The nature of the wetting layer associated with these dots is determined by quantitative HRTEM analysis, based on comparison between interplanar distortion profiles of experimental and simulated images. This study demonstrates a low intermixing between GaN and AlN materials. Such result is also evidenced for the GaN dots.

Transmission electron microscopy structural characterisation of GaN layers grown on (0001) sapphire

Abstract GaN layers grown by MOCVD or by MBE on (0001) sapphire have been characterised by transmission electron microscopy (TEM). We make a review of the different crystallographic structures found in theses GaN layers. We comment shortly on the nitridation of the sapphire and the structure of the buffer layer (BL). We point out that the roughness of the BL can be an important parameter for releasing the residual strain of the GaN layer. We compute the Keating energies of the main inversion domain boundaries (IDBs) and translation domains boundaries (TDBs) observed in some GaN layers. The observed structures correspond to the lowest energy models. Perfect dislocations have Burgers vectors equal to a, a+c and c. The dislocation lines are generally parallel to the c-axis. a-Edge dislocations are generally not dissociated and we propose an atomic model for them. Screw dislocations with a Burgers vector equal to c, can `open and close during growth leaving holes (the so-called nanopipes) in the structure.

Strain distribution in GaN∕AlN quantum-dot superlattices

The two-dimensional strain distribution in a GaN∕AlN quantum-dot (QD) superlattice is measured from high-resolution transmission electron microscopy images using the geometrical phase analysis. The results are compared to elastic theoretical calculations using a combination of Fourier transform and Green’s function techniques. The GaN∕AlN system appears to be a model system for a comparison between theory and experiments as interdiffusion between GaN and AlN is negligible. We verify that for the case of a three-dimensional system, such as a QD, the biaxial strain approximation is not valid. Furthermore, we demonstrate that the presence of QDs induces a modulation in the strain state of the AlN barriers which is the driving force for the vertical alignment of the GaN QDs in the AlN matrix.