The morphology and low-temperature photoluminescence spectra of GaN samples grown by plasma-assisted molecular beam epitaxy on Si(111) were systematically studied as a function of impinging Ga/N flux ratio and growth temperature (730-850ºC). Two different growth regimes were identified: compact and nanocolumnar. A growth diagram was established as a function of growth parameters, exhibiting the transition between growth regimes, and showing under which growth conditions GaN cannot be grown due to thermal decomposition and Ga desorption. The results indicate that adatoms diffusion length and the actual Ga/N ratio on the growing surface are key factors to achieve nanocolumnar growth. A correlation can be established between growth parameters such Ga flux and growth temperature, on one hand, and structural characteristics such as nanocolumn (NC) densities and diameters, on the other hand. Clear correlation is obtained, too, between sample morphology and the relative intensities of emission lines related to structural defects. All lines comprised between 3.10 and 3.42 eV rapidly lose intensity as growth conditions tend to reduce the NC density, thus relating the associated defects to the NC coalescence. On the other hand, the doublet line around 3.45 eV, well-known characteristic of GaN NC samples, shows an opposite behaviour and therefore its different nature.

The extraordinary high crystal quality of III-nitrides nanocolumns makes them very attractive in both fundamental and applied fields. Self-assembled GaN nanocolumns have been grown on Al2O3, Si(111) or Si(100) in the past. A systematic study of the diameter, density and growth rate of the nanocolumns depending on impinging Ga and N fluxes and growth temperature has been recently published. While the self-assembled growth approach is relatively-easily controlled (substrate temperature and III/V ratio), its random nature leads to significant dispersion (height and diameter) among nanocolumns, making strongly difficult the planar processing and contact of each nanocolumn at the time of fabricating nanodevices. On the other hand, the selective (ordered) growth, making use of masks or patterning on the substrate, can localize the growth, yielding a periodic, homogeneous distribution of nanostructures, what makes the processing much easier. Selective area growth using Ti mask with nanohole arrays has been recently demonstrated. This work focuses on the influence that different factors have on the successful selective area growth of GaN nanocolumns. These parameters include the fabrication method, design and nitridation of the Ti mask, as well as the standard plasma-assisted MBE growth parameters such as the growth temperature and the impinging Ga/N flux ratio. Nanohole arrays (diameter typically in the range 70 nm to 150 nm) were fabricated by e-beam lithography and etching of a continuous thin (5 nm) Ti layer or by lift-off techniques using colloidal masks, on commercial GaN templates and Si(111) substrates. Due to the high growth temperature needed to promote the selective growth (above 900ºC), a two-step nitridation process of the Ti mask was first optimized in order to avoid its degradation. Systematic studies of the growth temperature (from 900ºC to 1040ºC), the Ga flux (between 5 and 20 nm/min) and the nitrogen (from 8 to 20 nm/min) were performed.

The growth of spontaneous (catalyst-free) self-assembled and ordered (localized) III-Nitride nanocolumns (NCs) by Molecular Beam Epitaxy (MBE) is addressed, with emphasis on the nucleation process. In self-assembled NCs growth, nucleation proceeds via Volmer-Weber or Stranski-Krastanov mechanisms depending on the initial lattice mismatch between the NCs material and substrate. It is experimentally observed that a high initial mismatch (i.e. GaN on bare Si) leads to isolated, well defined NCs, whereas a smaller one (i.e. GaN on AlN buffered Si) produces a mixture of NCs and a "faceted matrix" of continuous, rough material. A successful localized growth on patterned or masked (nanoholes) substrates depends critically on a balance between metal atoms diffusion and desorption, that are both strongly influenced by the growth temperature and the III/V ratio. A proper selection of the growth conditions leads to a reasonable growth rate within the nanoholes (mask) whereas avoiding NCs nucleation on the mask (selectivity). Self-assembled III-Nitride polar GaN and InN NCs have been successfully grown by many researchers, though little work has been done on non-polar NCs. Here we present results on the self-assembled growth of non-polar (a-plane) InN NCs on m-plane GaN/r-plane Al2O3 templates. A well defined a-plane orientation is determined by HR-TEM and from the dependence of the E2 Raman mode with the polarization angle. HR-SEM pictures reveal top NC surfaces showing a- and m- plane facets. A low-temperature PL emission energy of 0.69 eV, a typical of very high quality material, is observed. Though self-assembled NCs are quite useful to understand the basic growth mechanisms and material properties, the growth of ordered arrays of NCs is essential to fabricate real devices, like LEDs and detectors, because of the need of homogeneous geometry and characteristics of individual nanodevices. Details will be given on the use of thin Ti masks with nanoholes of different diameter and pitch and on the specific conditions for localized growth, far from those to grow self-assembled NCs. Different approaches of ordered arrays will be revised with emphasis on the applications to efficient optoelectronic devices, like phosphor-free white LEDs, and multijunction solar cells.

InGaN-based quasi-one-dimensional systems (nanowires, nanorods and nanocolumns) attract increasing interest because of their ability to produce defect-free light emitters covering the entire visible range. Integration to solid-state optoelectronic devices makes epitaxial growth of vertical structures more suitable for light emission purposes, all the more that they open the way to the fabrication of complex heterostructures, such as quantum disks. In this study, we demonstrate the controlled growth of InGaN nanocolumns (NCs) by plasma-assisted molecular beam epitaxy on Si(111) substrates. The seeding of InGaN NCs by underlying GaN NCs preserves the NC morphology, while enabling the control of In content in the range of 2 to 30 %, as directly deduced from photoluminescence (PL) experiments. Similar to InGaN compact epilayers, the PL linewidth increases with In content, as a result of increased alloy disorder and carrier localization at potential fluctuations. Temperature-dependent PL emphasize the impact of this localization on the competition between radiative and nonradiative recombination processes. We find that the larger the In content the smaller the PL intensity drop measured between 8 and 300K. The well-known s-shaped T-dependence of the PL peak energy is also observed, proving, in particular, that recombination of localized carriers is still the dominant process at room temperature for In-rich InGaN NCs. Room-temperature PL experiments provided another -potentially important- result regarding both the chemical composition of InGaN NCs and their radiative efficiency. Indeed, we show how specific surface modifications of the NCs were used to reveal differences of In content between the surface and the core of the NCs, with strong impact on the room-temperature PL intensity.

In order to achieve efficient doping of GaN layers, both incorporation and activation ratio of dopants must be carefully controlled. In thick polar GaN layers, n-doping is usually obtained by the incorporation of silicon or oxygen atoms. Silicon binding energy in bulk GaN is 30.4 meV, in good agreement with the 28.8 meV calculated in the effective mass approximation. However, impurity ground states in non-flat potential band profiles are strongly modified, as it is the case in heterostructures or in presence of electric fields. In this work, we have thus calculated in the effective potential formalism the energy of an electron confined on a shallow donor nucleus located in any given conduction band-profile. We study particularly the influence on donor binding energy of basal stacking fault (BSFs) in non-polar GaN and of surface in GaN nanocolumns, and then discuss some of their consequences on the electrical and the optical properties of these two systems. We first show that in non-polar GaN, a donor in the vicinity of a BSF can exhibit a binding energy increased up to a factor of 1.8 compared to the 30 meV expected for stacking fault free GaN. Donor activation ratio is thus decreased at room temperature, in agreement with the incomplete donors ionization reported by M. McLaurin et al., J. Appl. Phys. 100, 063707 (2006). We then discuss the consequences of D°-BSF complexes on the emission properties of non-polar GaN. In particular, we propose the statistical distribution of donors in the vicinity of the BSFs planes as origin of both the broad emission from BSF-excitons and intra-BSF exciton localization. In the case nanocolumns, we find that donor binding energy goes from 28.8 meV at the core of the structure to 7.2 meV at its surface. We show that such variation in binding energy is responsible of the broad D°X emission reported for strain-free GaN nanocolumn with high surface-to-volume ratio. We finally suggest that the alteration of D°X wave functions in the vicinity of the nanocolumn surface is responsible of the strong emission at 3.449 eV typically observed in these nanostructures.