AlN films, a-plane and m-plane oriented, were obtained by molecular beam epitaxy overgrowth on GaN isolated nanopillars following a three-step process involving: dry etching of a GaN buffer layer to obtain nanopillars; GaN overgrowth on nanopillars and finally AlN overgrowth until nanocrystals coalescence. The resulting a-plane AlN layer shows a roughness, with a RMS of 90 nm over a 5 x 5 mu m2, higher than the m-plane AlN one with a RMS of 35 nm over the same area, which shows much better morphological quality. Plan-view transmission electron microscopy images of the m-plane AlN layer reveal stacking faults, but threading dislocations are barely seen. Cross-sectional transmission electron microscopy image shows stacking faults in bunches running through the sample volume, separated by areas (50 to 100 nm wide) which seem free of them.

Non-polar m-plane GaN films were grown by Plasma Assisted Molecular Beam Epitaxy on & gamma;-LiAlO2 (100) sub-strates by a controlled coalescence of GaN nanocolumns obtained by a two-step process including a top-down nanopillars etching from a GaN buffer and a subsequent bottom-up overgrowth. Transmission electron micro-scopy data show a significant reduction of extended defects density in the coalesced film as compared to the initial GaN buffer, most likely due to a filter effect by the regrowth process on the nanopillars inclined walls. Low temperature photoluminescence spectra back this reduction by a strong intensity decrease of the stacking faults fingerprint emission peaks, while a very intense donor-bound excitonic emission at 3.472 eV, 2.8 meV wide, becomes dominant.

Two different arrays of GaN nanocolumns, with average diameters of 140 nm and 260 nm, were selectively grown by molecular beam epitaxy on GaN-buffered on-axis Si(0 0 1) substrates. Footprint of the nanocolumns with respect to the grain size of the GaN buffer layer plays and important role on the crystalline quality of the nanocolumnar material. Despite the rather low crystalline quality of the GaN buffer, very high quality GaN nanocolumns were achieved in the case of the array with 140 nm average diameter, as indicated from a low temperature photoluminescence donor-bound exciton linewidth of 1.9 meV and negligible densities of stacking faults and threading dislocations determined by transmission electron microscopy. These results may help to the integration of GaN-based nano-devices with the conventional Si(0 0 1) electronics platform.

•Selective growth of GaN nanocolumns on GaN buffered on-axis Si(001) by MBE.•GaN nanocolumns with average diameter of 140 nm and high crystalline quality.•Findings expand the possibilities of integration of III-N and Si(001).

Ordered arrays of very high quality, defect-free GaN nanocolumns were achieved by selective area growth following a two step process involving nanopillar dry etching (top down) and overgrowth by Molecular Beam Epitaxy (bottom up). A study by transmission electron microscopy, over more than 50 individual nanocolumns, confirmed the absence of extended defects, such as dislocations, polarity inversion domain boundaries and stacking faults. Low temperature (10 K) photoluminescence spectrum is dominated by a donor-bound exciton emission line at 3.472 eV with a line width of 0.5 meV. In addition, a distinct emission line from the free-exciton A is observed at 3.479 eV. No traces of emission lines, either at 2.3 eV (Yellow Band); 3.45 eV (also labeled as UX line and recently linked to polarity inversion domain boundaries); or 3.42 eV (stacking faults) were observed.

Abstract The aim of this work is to gain insight into the selective area growth (SAG) of InGaN nanostructures by plasma-assisted molecular beam epitaxy, focusing on their potential as building blocks for next-generation LEDs. Several nanocolumn (NC)-based approaches such as standard axial InGaN/GaN structures and InGaN/GaN core–shell structures are discussed. The first section reports on the growth and characterization of ordered InGaN NCs as well as light-emitting diodes grown on c -plane GaN/sapphire templates. In particular, the growth mechanism of green-emitting InGaN/GaN NCs is discussed. In order to enable white light emission the stacking of red, green, and blue emitting segments is used to achieve the monolithic integration of these structures in one single InGaN NC allowing for the fabrication of ordered broad spectrum emitters. As alternative to axial InGaN/GaN nanostructures, the next section reports on the growth and characterization of InGaN/GaN core–shell structures with emission at around 3.0 eV. Furthermore, the successful fabrication of a core–shell pin diode structure is demonstrated. Finally, the SAG of In(Ga)N/GaN NCs on Si(111) substrates is presented. Ordered In(Ga)N/GaN NCs emitting from ultraviolet (3.2 eV) to infrared (0.78 eV) were grown on top of GaN-buffered Si substrates.

Despite the strong interest in optoelectronic devices working in the deep ultraviolet range, no suitable low cost, large-area, high-quality AlN substrates have been available up to now. The aim of this work is the selective area growth of AlN nanocolumns by plasma assisted molecular beam epitaxy on polar (0001) and semi-polar (11-22) GaN/sapphire templates. The resulting AlN nanocolumns are vertically oriented with semi-polar {1-103} top facets when grown on (0001) GaN/sapphire, or oriented at 58° from the template normal and exposing {1-100} non-polar top facets when growing on (11-22) GaN/sapphire, in both cases reaching filling factors ≥80%. In these kinds of arrays each nanostructure could function as a building block for an individual nano-device or, due to the large filling factor values, the overall array top surfaces could be seen as a quasi (semi-polar or non-polar) AlN pseudo-template.

This work reports an experimental and theoretical insight into phenomena of two-color emission and different electron-hole recombination dynamics in InGaN nanodisks, incorporated into pencil-like GaN nanowires. The studied nanodisks consist of one polar (on c facet) and six (nominally) identical semipolar (on r facets) sections, as confirmed by transmission electron microscopy. The combination of cathodoluminescence with scanning electron microscopy spatially resolves the nanodisk two-color emission, the low-energy emission (similar to 500 nm) originating from the polar section, and the high-energy emission (similar to 400 nm) originating from the semipolar section. This result has been directly linked to a "facet-dependent" nanodisk composition, the In content being significantly higher in the polar (similar to 20%) vs semipolar (similar to 10%) section (as quantified by energy dispersive x-ray spectroscopy), further leading to a strong facet-dependent strain anisotropy. Time-resolved cathodoluminescence reveals significantly different electron-hole recombination times in the two sections, moderately fast (similar to 1.3 ns) vs fast (similar to 0.5 ns) in polar/semipolar sections, respectively, the difference being linked to a strong anisotropy in the nanodisk internal electric fields. To determine the influence of each of the three contributing "facet-related" anisotropies (composition, strain, and electric field) on the two-color emission, a proper simulation [relying on virtual crystal approximation and involving three-dimensional (3D) continuum mechanical modeling, a 3D Poisson equation, and a one-dimensional Schrodinger equation] has been performed. The theoretical simulations allow the three effects to be quantitatively disentangled, revealing a clear hierarchy among their contributing weights, the facet-dependent composition inhomogeneity being identified as the dominant one (and the strain inhomogeneity being identified as the least significant one). As for different recombination times, while it is mainly linked to the internal electric field anisotropy, we also suggest that it is, very likely, influenced by gradually increasing In content along the nanodisk growth direction (lattice-pulling effect); the latter mechanism keeps electrons and holes in (relative) proximity within the polar section, enabling their relatively fast and efficient radiative recombination.

We report on the formation of polarity inversion in ordered (In,Ga)N/GaN nanocolumns grown on a Ti-masked GaN-buffered sapphire substrate by plasma assisted molecular beam epitaxy. High-resolution transmission electron microscopy and electron energy-loss spectroscopy reveal a stacking fault-like planar defect at the homoepitaxial GaN interface due to Ti incorporation, triggering the generation of N-polar domains in Ga-polar nanocolumns. Density functional theory calculations are applied to clarify the atomic configurations of a Ti monolayer occupation on the GaN (0002) plane and to prove the inversion effect. The polarity inversion leads to an enhanced indium incorporation in the subsequent (In,Ga)N segment of the nanocolumn. This study provides a deeper understanding of the effects of Ti mask in the well-controlled selective area growth of (In,Ga)N/GaN nanocolumns.

For a comprehensive understanding of complex semiconductor heterostructures and the physics of devices based on them, a systematic determination and correlation of the structural, chemical, electronic, and optical properties on a nanometer scale is essential. Luminescence techniques belong to the most sensitive, non-destructive methods of semiconductor research. The combination of luminescence spectroscopy – in particular at liquid He temperatures with the high spatial resolution of a scanning transmission electron microscopy (STEM) as realized by the technique of low temperature cathodoluminescence microscopy in a STEM (STEM-CL), provides a unique, extremely powerful tool for the optical nano-characterization of quantum structures. Our CL-detection unit is integrated in a FEI STEM Tecnai F20 equipped with a liquid helium stage (T = 10 K / 300 K) and a light collecting parabolic mirror. Panchromatic as well as spectrally resolved (grating monochromator) CL imaging is used. In CL-imaging mode the CL-signal is collected simultaneously to the STEM signal at each pixel. The TEM acceleration voltage is optimized to minimize sample damage and to prevent luminescence degeneration under electron beam excitation.

This work reports on the effects of air exposure on the photoluminescence intensity of GaN nanocolumns, with diameters ranging from below 40nm up to around 230nm, grown selectively on GaN/sapphire and GaN/Si(111). The high control of dimensions provided by selective area growth epitaxy allowed for a better study of the relationship between the observed phenomena, namely the photoluminescence intensity quenching due to oxygen photo-adsorption, and the nanocolumns properties (morphology and dimensions). For nanocolumns with diameters below 120nm and lengths of about 300nm, photoluminescence intensity dropped by more than 90% of the initial value, while for shorter nanocolumns a reduced drop value was found.

3D InGaN/GaN microstructures grown by metal organic vapor phase epitaxy (MOVPE) and molecular beam epitaxy (MBE) have been extensively studied using a range of electron microscopy techniques. The growth of material by MBE has led to the growth of cubic GaN material. The changes in these crystal phases has been investigated by Electron Energy Loss Spectroscopy, where the variations in the fine structure of the N K-edge shows a clear difference allowing the mapping of the phases to take place. GaN layers grown for light emitting devices sometimes have cubic inclusions in the normally hexagonal wurtzite structures, which can influence the device electronic properties. Differences in the fine structure of the N K-edge between cubic and hexagonal material in electron energy loss spectra are used to map cubic and hexagonal regions in a GaN/InGaN microcolumnar device. The method of mapping is explained, and the factors limiting spatial resolution are discussed.

This work reports on the growth of (In, Ga)N core–shell micro pillars by plasma-assisted molecular beam epitaxy using an ordered array of GaN cores grown by metal organic vapor phase epitaxy as a template. Upon (In, Ga)N growth, core–shell structures with emission at around 3.0 eV are formed. With spatially resolved cathodoluminescence, an increasing In content toward the pillar top is found to be present in the (In, Ga)N shell, as indicated by a shift of CL peak position from 3.2 eV at the shell bottom to 3.0 eV at the shell top. Further, the fabrication of a core–shell pin structure is demonstrated. Spatially resolved electroluminescence measurements performed on individual pillars confirm electroluminescence from the (In, Ga)N shell (lateral diode) at around 3.0 eV, as well as from the pillar top facet (axial diode) at around 2.3 eV.

In this work, through a comparative study of self-assembled (SA) and selective area grown (SAG) (In)GaN nanocolumn (NC) ensembles, we first give a detailed insight into improved crystallographic uniformity (homogeneity of crystallographic tilts and twists) of the latter ones. The study, performed making use of: reflective high energy electron diffraction, X-ray diffraction and scanning electron microscopy, reveals that unlike their SA counterparts, the ensembles of SAG NCs show single epitaxial relationship to both sapphire(0001) and Si(111) underlying substrates. In the second part of the article, making use of X-ray diffraction, we directly show that the selective area growth leads to improved compositional uniformity of InGaN NC ensembles. This further leads to improved spectral purity of their luminescence, as confirmed by comparative macro-photoluminescence measurements performed on SA and SAG InGaN NC ensembles. An improved crystallographic uniformity of NC ensembles facilitates their integration into optoelectronic devices, whereas their improved compositional uniformity allows for their employment in single-color optoelectronic applications.

This work reports on the selective area growth mechanism of green-emitting InGaN/GaN nanocolumns with long InGaN sections (330–410 nm) grown by molecular beam epitaxy on GaN/sapphire. The evolution of the morphology of the InGaN segment is found to depend critically on the nominal III/V ratio as well as the diameter of the GaN section, both affecting the local III/V ratio at the top of the nanocolumn. Depending on the local III/V ratio, either metal or nitrogen rich, the diameter of the InGaN segment increases or decreases, respectively, compared to the initial diameter of the GaN segment. In addition, the In distribution inside the InGaN segment is found to depend on the local III/V and In/Ga ratios, i.e., increasing In content toward the top under local metal rich conditions and decreasing In content toward the top under locally N rich conditions.

Transmission electron microscopy and spatially resolved electron energy‐loss spectroscopy have been applied to investigate the indium distribution and the interface morphology in axial (In,Ga)N/GaN nanowire heterostructures. The ordered axial (In,Ga)N/GaN nanowire heterostructures with an indium concentration up to 80% are grown by molecular beam epitaxy on GaN‐buffered Si(111) substrates. We observed a pronounced lattice pulling effect in all the nanowire samples given in a broad transition region at the interface. The lattice pulling effect becomes smaller and the (In,Ga)N/GaN interface width is reduced as the indium concentration is increased in the (In,Ga)N section. The result can be interpreted in terms of the increased plastic strain relaxation via the generation of the misfit dislocations at the interface.

The aim of this work is to provide an overview on the recent advances in the selective area growth of (In) GaN nanostructures by plasma assisted molecular beam epitaxy, focusing on their potential as building blocks for next generation light emitting diodes. The first two sections deal with the emission control in the entire ultraviolet to infrared range, including approaches for white light emission, using thick InGaN segments on axial nanocolumns. Selective area growth of axial nanostructures is developed on both GaN/sapphire templates and GaN-buffered Si(111). Ordered arrays of InGaN/GaN light emitting diodes grown by molecular beam epitaxy, emitting in the blue (441 nm), green (502 nm), and yellow (568 nm) spectral range are reported.As an alternative to axial nanocolumns, section 3 reports on the growth and characterization of InGaN/GaN core-shell structures on an ordered array of top-down patterned GaN microrods. Finally, section 4 reports on the selective area growth of GaN, with and without InGaN insertion, on semi-polar (11-22) and non-polar (11-20) templates. Upon selective area growth the high defect density present in the semi-polar templates is strongly reduced as indicated by a dramatic improvement of the optical properties. In the case of selective area growth on non-polar (11-20) templates, the formation of nanostructures with a low aspect ratio took place allowing for the fabrication of high-quality, non-polar GaN pseudo-templates by coalescence of these nanostructures.

This work presents a comprehensive optical characterization of Zn1-xMgxO thin films grown by spray pyrolysis (SP). Absorption measurements show the high potential of this technique to tune the bandgap from 3.30 to 4.11 eV by changing the Mg acetate content in the precursor solution, leading to a change of the Mg-content ranging from 0 up to 35%, as measured by transmission electron microscopy-energy dispersive x-ray spectroscopy. The optical emission of the films obtained by cathodoluminescence and photoluminescence spectroscopy shows a blue shift of the peak position from 3.26 to 3.89 eV with increasing Mg incorporation, with a clear excitonic contribution even at high Mg contents. The linewidth broadening of the absorption and emission spectra as well as the magnitude of the observed Stokes shift are found to significantly increase with the Mg content. This is shown to be related to both potential fluctuations induced by pure statistical alloy disorder and the presence of a tail of band states, the latter dominating for medium Mg contents. Finally, metal-semiconductor-metal photodiodes were fabricated showing a high sensitivity and a blue shift in the cut-off energy from 3.32 to 4.02 eV, i.e., down to 308 nm. The photodiodes present large UV/dark contrast ratios (10(2) - 10(7)), indicating the viability of SP as a growth technique to fabricate low cost (Zn, Mg)O-based UV photodetectors reaching short wavelengths.