Resonant Raman scattering of core-shell GaN/AlN nanowires.

We have analyzed the electron-phonon coupling in GaN/AlN core-shell nanowires by means of Raman scattering excited at various wavelengths in the ultraviolet spectral range (335, 325 and 300 nm) and as a function of the AlN shell thickness. The detailed analysis of the multi-phonon spectra evidences important differences with excitation energy. Under 325 and 300 nm excitation the Raman process is mediated by the allowedA1(LO) phonon mode, where the atoms vibrate along the NW axis. Considering its selection rules, this mode is easily accessible in backscattering along the wurtzitecaxis. Interestingly, for 335 nm excitation the scattering process is instead mediated by theE1(LO) phonon mode, where atoms vibrate in thec-plane and that is forbidden in this configuration. This change is ascribed to the band anticrossing caused by the uniaxial strain imposed by the AlN shell and the proximity, at this particular excitation energy, of real electronic transitions separated by the energy of the longitudinal optical phonon modes. The energy and character of the electronic bands can be tuned by varying the AlN shell thickness, a degree of freedom unique to core-shell nanowires. The interpretation of the experimental results is supported by calculations of the electronic transitions of GaN under uniaxial strain performed within the framework of ak · pmodel.

Nanoscale x-ray investigation of composition fluctuations in AlGaN nanowires.

In the present study we combined, in the same synchrotron x-ray experiment, reciprocal space mapping, multiwavelength anomalous diffraction and diffraction anomalous fine structure, to determine the strain, crystallographic polarity, alloy composition and ordering at the atomic scale in [0001]-oriented AlGaN nanowires grown by molecular beam epitaxy on GaN nanowire bases. The information that we obtained was averaged over a macroscopic ensemble of NWs. We found from the diffraction anomalous fine structure that there were an isotropic increased number of Ga-Ga pairs in the Ga next nearest coordination shell (cation sublattice) with respect to what is expected for the AlGaN alloy composition determined by anomalous diffraction. This significant deviation from random alloy atomic distribution is present whatever the AlN molar fraction and growth conditions. Our results are consistent with nanoscale composition fluctuations expected from both alloy disorder or kinematically driven spontaneous ordering, both effects being suspected to account for the physical properties of AlGaN ternary alloys.

Impact of kinetics on the growth of GaN on graphene by plasma-assisted molecular beam epitaxy.

The growth of GaN on graphene by molecular beam epitaxy was investigated. The most stable epitaxial relationship, i.e. [00.1]-oriented grains, is obtained at high temperature and N-rich conditions, which match those for nanowire growth. Alternatively, at moderate temperature and Ga-rich conditions, several metastable orientations are observed at the nucleation stage, which evolve preferentially towards [00.1]-oriented grains. The dependence of the nucleation regime on growth conditions was assigned to Ga adatom kinetics. This statement is consistent with the calculated graphene/GaN in-plane lattice coincidence and supported by a combination of transmission electron microscopy, x-ray diffraction, photoluminescence, and Raman spectroscopy experiments.

Quantum Dot-Like Behavior of Compositional Fluctuations in AlGaN Nanowires.

We report on the structural and optical properties of AlxGa(1-x)N nanowire sections grown by plasma-assisted molecular beam epitaxy on GaN nanowire bases used as a template. Based on a combination of scanning electron microscopy, microphotoluminescence, time-resolved microphotoluminescence, and photon correlation experiments, it is shown that compositional fluctuations in AlxGa(1-x)N sections associated with carrier localization optically behave as quantum dots. Moreover, most of the micro-optical properties of such fluctuations are demonstrated to be very little dependent on kinetic growth parameters such as AlxGa(1-x)N growth temperature and AlN molar fraction in the alloy, which govern the macrostructural properties of AlxGa(1-x)N sections.

Assessment of Polarity in GaN Self-Assembled Nanowires by Electrical Force Microscopy.

In this work, we demonstrate the capabilities of atomic force microscopies (AFMs) for the nondestructive determination of the polarity of GaN nanowires (NWs). Three complementary AFMs are analyzed here: Kelvin probe force microscopy (KPFM), light-assisted KPFM, and piezo-force microscopy (PFM). These techniques allow us to assess the polarity of individual NWs over an area of tens of µm(2) and provide statistics on the polarity of the ensemble with an accuracy hardly reachable by other methods. The precise quantitative analysis of the tip-sample interaction by multidimensional spectroscopic measurements, combined with advanced data analysis, has allowed the separate characterization of electrostatic and van der Waals forces as a function of tip-sample distance. Besides their polarity, the net surface charge density of individual NWs was estimated.

Selective ion-induced intermixing and damage in low-dimensional GaN/AlN quantum structures.

Ion-induced intermixing and damage is evaluated in GaN/AlN superlattices of quantum dots (QDs) and quantum wells (QWs) using 100 keV Ar(+) implantation at low temperature (15 K). Despite the similar damage build up at low fluences, a significant increase of the damage accumulation takes place for QDs at high fluences. Elemental depth profiles were fitted with a diffusion model, revealing the higher intermixing efficiency in QD superlattices, significantly higher than for QWs. The scaling of diffusion length with the local fluence and defect concentration is understood on the basis of cascade mixing and migration of defects in the cation sublattice. The selective intermixing/damage of QDs is explained by the promotion of lateral diffusion mechanisms that result in smooth interfaces, as well as by an enhanced diffusivity due to the characteristic strain distribution in QD superlattices.

Probing alloy composition gradient and nanometer-scale carrier localization in single AlGaN nanowires by nanocathodoluminescence.

The optical properties of single AlGaN nanowires grown by plasma-assisted molecular beam epitaxy have been studied by nanocathodoluminescence. Optical emission was found to be position-dependent and to occur in a wide wavelength range, a feature which has been assigned to a composition gradient along the nanowire growth axis, superimposed on local composition fluctuations at the nanometer scale. This behavior is associated with the growth mode of such AlGaN nanowires, which is governed by kinetics, leading to the successive formation of (i) a zone with strong local composition fluctuations followed by (ii) a zone with a marked composition gradient and, eventually, (iii) a zone corresponding to a steady state regime and the formation of a homogeneous alloy.

Growth, structural and optical properties of AlGaN nanowires in the whole composition range.

We report on the growth of AlxGa1-xN nanowires by plasma-assisted molecular beam epitaxy for x in the 0.3-0.8 range. Based on a combination of macro- and micro-photoluminescence, Raman spectroscopy, x-ray diffraction and scanning electron microscopy experiments, it is shown that the structural and optical properties of AlGaN NWs are governed by the presence of compositional fluctuations associated with strongly localized electronic states. A growth model is proposed, which suggests that, depending on growth temperature and metal adatom density, macroscopic composition fluctuations are mostly of kinetic origin and are directly related to the nucleation of the AlGaN nanowire section on top of the GaN nanowire base which is used as a substrate.

Strain state of GaN nanodisks in AlN nanowires studied by medium energy ion spectroscopy.

Medium energy ion spectroscopy experiments have been performed on an ensemble of nanowires deposited by molecular beam epitaxy on Si(111), taking advantage of their reduced in-plane mosaicity. In particular, the strain in nanometric GaN insertions embedded in AlN sections deposited on top of GaN nanowires has been determined. The measured strain is consistent with atomistic valence force field calculations. This opens the way for the structural study of a new range of discontinuous nanowire-based nanostructures by medium energy ion spectroscopy and to the determination of the strain profile of nanodisks in nanowires at the monolayer scale.

Growth mechanism and properties of InGaN insertions in GaN nanowires.

We demonstrate the strong influence of strain on the morphology and In content of InGaN insertions in GaN nanowires, in agreement with theoretical predictions which establish that InGaN island nucleation on GaN nanowires may be energetically favorable, depending on In content and nanowire diameter. EDX analyses reveal In inhomogeneities between the successive dots but also along the growth direction within each dot, which is attributed to compositional pulling. Nanometer-resolved cathodoluminescence on single nanowires allowed us to probe the luminescence of single dots, revealing enhanced luminescence from the high In content top part with respect to the lower In content dot base.

Submicrometre resolved optical characterization of green nanowire-based light emitting diodes.

The electroluminescent properties of InGaN/GaN nanowire-based light emitting diodes (LEDs) are studied at different resolution scales. Axial one-dimensional heterostructures were grown by plasma-assisted molecular beam epitaxy (PAMBE) directly on a silicon (111) substrate and consist of the following sequentially deposited layers: n-type GaN, three undoped InGaN/GaN quantum wells, p-type AlGaN electron blocking layer and p-type GaN. From the macroscopic point of view, the devices emit light in the green spectral range (around 550 nm) under electrical injection. At 100 mA DC current, a 1 mm2 chip that integrates around 10(7) nanowires emits an output power on the order of 10 µW. However, the emission of the nanowire-based LED shows a spotty and polychromatic emission. By using a confocal microscope, we have been able to improve the spatial resolution of the optical characterizations down to the submicrometre scale that can be assessed to a single nanowire. Detailed µ-electroluminescent characterization (emission wavelength and output power) over a representative number of single nanowires provides new insights into the vertically integrated nanowire-based LED operation. By combining both µ-electroluminescent and µ-photoluminescent excitation, we have experimentally shown that electrical injection failure is the major source of losses in these nanowire-based LEDs.

Structural and optical properties of InGaN/GaN nanowire heterostructures grown by PA-MBE.

The structural and optical properties of InGaN/GaN nanowire heterostructures grown by plasma-assisted molecular beam epitaxy have been studied using a combination of transmission electron microscopy, electron tomography and photoluminescence spectroscopy. It is found that, depending on In content, the strain relaxation of InGaN may be elastic or plastic. Elastic relaxation results in a pronounced radial In content gradient. Plastic relaxation is associated with the formation of misfit dislocations at the InGaN/GaN interface or with cracks in the InGaN nanowire section. In all cases, a GaN shell was formed around the InGaN core, which is assigned to differences in In and Ga diffusion mean free paths.

The structural properties of GaN/AlN core-shell nanocolumn heterostructures.

The growth and structural properties of GaN/AlN core-shell nanowire heterostructures have been studied using a combination of resonant x-ray diffraction, Raman spectroscopy and high resolution transmission electron microscopy experiments. For a GaN core of 20 nm diameter on average surrounded by a homogeneous AlN shell, the built-in strain in GaN is found to agree with theoretical calculations performed using a valence force field model. It is then concluded that for an AlN thickness up to at least 12 nm both core and shell are in elastic equilibrium. However, in the case of an inhomogeneous growth of the AlN shell caused by the presence of steps on the sides of the GaN core, plastic relaxation is found to occur. Consistent with the presence of dislocations at the GaN/AlN interface, it is proposed that this plastic relaxation, especially efficient for AlN shell thickness above 3 nm, is promoted by the shear strain induced by the AlN inhomogeneity.

Identification of III-N nanowire growth kinetics via a marker technique.

By using a marker technique based on nanowire (NW) heterostructure, we have identified the Ga-limited and N-limited GaN NW growth regimes, which are shifted in comparison to those in two-dimensional GaN layers. The results show that the Ga atoms diffusing along NW sidewalls have a significant contribution to the NW vertical growth. By reducing the substrate temperature, Ga-rich conditions locally activate the lateral growth. In contrast to Ga atoms, the contribution of Al and N adatom diffusion to the NW vertical growth is negligible. Finally, the control of GaN/AlN heterostructures in NWs is demonstrated.

Nucleation mechanism of GaN nanowires grown on (111) Si by molecular beam epitaxy.

We have performed a real-time in situ x-ray scattering study of the nucleation of GaN nanowires grown by plasma-assisted molecular beam epitaxy on AlN(0001)/Si(111). The intensity variation of the GaN diffraction peak as a function of time was found to exhibit three different regimes: (i) the deposition of a wetting layer, which is followed by (ii) a supralinear regime assigned to nucleation of almost fully relaxed GaN nanowires, eventually leading to (iii) a steady-state growth regime. Based on scanning electron microscopy and electron microscopy analysis, it is proposed that the granular character of the thin AlN buffer layer may account for the easy plastic relaxation of GaN, establishing that three-dimensional islanding and plastic strain relaxation of GaN are two necessary conditions for nanowire growth.

The structural properties of GaN insertions in GaN/AlN nanocolumn heterostructures.

The strain state of 1 and 2.5 nm thick GaN insertions in GaN/AlN nanocolumn heterostructures has been studied by means of a combination of high resolution transmission electron microscopy, Raman spectroscopy and theoretical modeling. It is found that 2.5 nm thick GaN insertions are partially relaxed, which has been attributed to the presence of dislocations in the external AlN capping layer, in close relationship with the morphology of GaN insertions and with the AlN capping mechanism. The observed plastic relaxation in AlN is consistent with the small critical thickness expected for GaN/AlN radial heterostructures.