This work studies the effect of the growth temperature on the morphology and emission characteristics of self-assembled InGaN nanocolumns grown by plasma assisted molecular beam epitaxy. Morphology changes are assessed by scanning electron microscopy, while emission is measured by photoluminescence. Within the growth temperature range of 750 to 650°C, an increase in In incorporation for decreasing temperature is observed. This effect allows tailoring the InGaN nanocolumns emission line shape by using temperature gradients during growth. Depending on the gradient rate, span, and sign, broad emission line shapes are obtained, covering the yellow to green range, even yielding white emission.

I will present some applications of the novel technique of time-resolved cathodoluminescence to the study of diffusion/recombination processes of excitons in low-dimensional nano-objects based on wide band gap semiconductors. More specifically, I will address the case of basal stacking faults in GaN and in GaN based quantum wells, and dynamical properties of excitons in ZnO-based nanowires.

Self-organised nanocolumnar systems based on group-III nitride semiconductors present numerous original characteristics, of which the absence of extended defects is certainly the most promising for optoelectronic applications. Nevertheless, another feature, namely the large surface/volume ratio of nanocolumns with typical diameters in the 20–60 nm range, deserves further investigation. We focus for this talk on luminescence properties of GaN and InGaN nanocolumns grown by plasma-assisted molecular beam epitaxy on silicon (111) substrates. Two cases of surface-related properties will be addressed: 1) The origin of the photoluminescence doublet at 3.450-3.456 eV will be discussed in terms of the particular physics of donor-bound excitons, when donor nuclei sit within a layer of ~10 nm from the side surface of a GaN nanocolumn. Detailed studies of power- and temperature-dependent PL spectroscopy will be analyzed. 2) Changes in photoluminescence properties of nitride nanocolumnar systems placed in various environments (vacuum, air…) will be presented and discussed in terms of photochemical effects and their consequences on the dynamics of excitonic recombination.

We present an overview of the state-of-the art MBE growth and applications of GaN-based nanocolumns.

We report on plasma-assisted molecular beam epitaxy growth and characterization of InGaN/GaN quantum dots (QDs) for violet/blue applications. The growths were performed on 4μm thick (0001) GaN-on-sapphire templates. Prior to QDs growth, a GaN buffer layer, ~100 nm thick was grown at ~700 ºC, under intermediate Ga-rich conditions to burry possible impurities and to provide flat surface. The targeted 20% In content for QDs growth was calibrated by growth of thin (~20 nm) compact InGaN layers, varying growth temperature and Ga/N flux ratio. The In content in the compact layers was estimated by X-ray diffraction technique. The InGaN QDs were grown at 580 ºC with the nitrogen flux set to 3 nm/min (in (0001) GaN equivalent growth rate units) and the Ga flux set to 50% of the nitrogen flux value. The In flux was varied until the effective stoichiometry (conditioned by InN decomposition and In desorption) was achieved. To provide stable In adlayer coverage (and thus avoid transient effects), two monolayers (MLs) of metallic indium were deposited prior to growth of 4 InGaN MLs. Then, the sample was left either in vacuum or under active nitrogen flux for typically 60s before the growth temperature was sharply decreased. When 2 MLs In excess was desorbed, the Stransky-Krastanov transition occurred, as confirmed in-situ by RHEED. Both single uncovered InGaN/GaN QD layers and three period InGaN/GaN QD super-lattices, covered by 30 nm GaN cap layer, were grown. Depending on growth conditions, dot and ring morphology can be distinguished, as confirmed by atomic force microscopy (AFM), the later being linked to lower adatom surface mobility. The dots were typically formed at the edge of atomic terraces, with height of 2-3 nm, diameter of 20-30 nm and density of ~5*1010 cm-2. Low temperature photo- and cathodoluminescence revealed InGaN peak around 440 nm, with a typical full width of half maximum of ~25 nm (~16 meV).