Cathodoluminescence and TEM investigations of structural and optical properties of AlGaN on epitaxial laterally overgrown AlN/sapphire templates

Surface steps as high as 15 nm on up to 10 μm thick AlN layers grown on patterned AlN/sapphire templates play a major role for the structural and optical properties of AlxGa1−xN layers with x ≥ 0.5 grown subsequently by metalorganic vapour phase epitaxy. The higher the Ga content in these layers is, the stronger is the influence of the surface morphology on their properties. For x = 0.5 not only periodic inhomogeneities in the Al content due to growth of Ga-rich facets are observed by cathodoluminescence, but these facets give rise to additional dislocation formation as discovered by annular dark-field scanning transmission electron microscopy. For AlxGa1−xN layers with x = 0.8 the difference in Al content between facets and surrounding material is much smaller. Therefore, the threading dislocation density (TDD) is only defined by the TDD in the underlying epitaxially laterally overgrown (ELO) AlN layer. This way high quality Al0.8Ga0.2N with a thickness up to 1.5 μm and a TDD ≤ 5x108 cm−2 was obtained

Origin of a-plane (Al,Ga)N formation on patterned c-plane AIN/sapphire templates

a-plane (Al,Ga)N layers can be grown on patterned c-plane AlN/sapphire templates with a ridge direction along [1bar 100]Al2O3. Scanning nanobeam diffraction reveals that the formation of a-plane layers can be explained by nucleation of c-plane (Al,Ga)N with [11bar 20](Al,Ga)N[0001]Al2O3 at the ridge sidewalls. Faster growth of the top (11bar 20)(Al,Ga)N facet in the vertical direction leads to the overgrowth of c-plane (Al,Ga)N nucleated on the horizontal ridge and trench surfaces. Phase separation into binary GaN and AlN takes place during the first growth stages. However, this fades out and does not influence the composition of the final thick a-plane (Al,Ga)N layer

2D layered transport properties from topological insulator Bi2_2Se3_3 single crystals and micro flakes

Low-field magnetotransport measurements of topological insulators such as Bi$_2$Se$_3$ are important for revealing the nature of topological surface states by quantum corrections to the conductivity, such as weak-antilocalization. Recently, a rich variety of high-field magnetotransport properties in the regime of high electron densities ($\sim10^{19}$ cm$^{-3}$) were reported, which can be related to additional two-dimensional layered conductivity, hampering the identification of the topological surface states. Here, we report that quantum corrections to the electronic conduction are dominated by the surface states for a semiconducting case, which can be analyzed by the Hikami-Larkin-Nagaoka model for two coupled surfaces in the case of strong spin-orbit interaction. However, in the metallic-like case this analysis fails and additional two-dimensional contributions need to be accounted for. Shubnikov-de Haas oscillations and quantized Hall resistance prove as strong indications for the two-dimensional layered metallic behavior. Temperature-dependent magnetotransport properties of high-quality Bi$_2$Se$_3$ single crystalline exfoliated macro and micro flakes are combined with high resolution transmission electron microscopy and energy-dispersive x-ray spectroscopy, confirming the structure and stoichiometry. Angle-resolved photoemission spectroscopy proves a single-Dirac-cone surface state and a well-defined bulk band gap in topological insulating state. Spatially resolved core-level photoelectron microscopy demonstrates the surface stability.Comment: Sci. Rep. (2016

High-power diode lasers with in-situ-structured lateral current blocking for improved threshold, efficiency and brightness

We present high-power GaAs-based broad-area diode lasers with a novel variant of the enhanced self-aligned lateral structure ‘eSAS’, having a strongly reduced lasing threshold and improved peak conversion efficiency and beam quality in comparison to their standard gain-guided counterparts. To realize this new variant (eSAS-V2), a two-step epitaxial growth process involving in situ etching is used to integrate current-blocking layers, optimized for tunnel current suppression, within the p-Al0.8GaAs cladding layer of an extreme-triple-asymmetric epitaxial structure with a thin p-side waveguide. The blocking layers are thus in close proximity to the active zone, resulting in strong suppression of current spreading and lateral carrier accumulation. eSAS-V2 devices with 4 mm resonator length and varying stripe widths are characterized and compared to previous eSAS variant (eSAS-V1) as well as gain-guided reference devices, all having the same dimensions and epitaxial structure. Measurement results show that the new eSAS-V2 variant eliminates an estimated 89% of lateral current spreading, resulting in a strong threshold current reduction of 29% at 90 μm stripe width, while slope and series resistance are broadly unchanged. The novel eSAS-V2 devices also maintain high conversion efficiency up to high continuous-wave optical power, with an exemplary 90 μm device having 51.5% at 20 W. Near-field width is significantly narrowed in both eSAS variants, but eSAS-V2 exhibits a wider far-field angle, consistent with the presence of index guiding. Nonetheless, eSAS-V2 achieves higher beam quality and lateral brightness than gain-guided reference devices, but the index guiding in this realization prevents it from surpassing eSAS-V1. Overall, the different performance benefits of the eSAS approach are clearly demonstrated

GaAs/GaP quantum dots: Ensemble of direct and indirect heterostructures with room temperature optical emission

Producción CientíficaWe describe the optical emission and the carrier dynamics of an ensemble of self-assembled GaAs quantum dots embedded in GaP(001). The QD formation is driven by the 3.6% lattice mismatch between GaAs and GaP in the Stranski-Krastanow mode after deposition of more than 1.2 monolayers of GaAs. The quantum dots have an areal density between 6 and 7.6 × 1010 per cm−2 and multimodal size distribution. The luminescence spectra show two peaks in the range of 1.7 and 2.1 eV. The samples with larger quantum dots have red emission and show less thermal quenching compared with the samples with smaller QDs. The large QDs luminescence up to room temperature. We attribute the high energy emission to indirect carrier recombination in the thin quantum wells or small strained quantum dots, whereas the low energy red emission is due to the direct electron-hole recombination in the relaxed quantum dots.Comisión Europea (project FP7-ICT-2013-613024-GRASP

AlN overgrowth of nano-pillar-patterned sapphire with different offcut angle by metalorganic vapor phase epitaxy

We present overgrowth of nano-patterned sapphire with different offcut angles by metalorganic vapor phase epitaxy. Hexagonal arrays of nano-pillars were prepared via Displacement Talbot Lithography and dry-etching. 6.6 µm crack-free and fully coalesced AlN was grown on such substrates. Extended defect analysis comparing X-ray diffraction, electron channeling contrast imaging and selective defect etching revealed a threading dislocation density of about 109 cm-2. However, for c-plane sapphire offcut of 0.2° towards m direction the AlN surface shows step bunches with a height of 10 nm. The detrimental impact of these step bunches on subsequently grown AlGaN multi-quantum-wells is investigated by cathodoluminescence and transmission electron microscopy. By reducing the sapphire offcut to 0.1° the formation of step bunches is successfully suppressed. On top of such a sample an AlGaN-based UVC LED heterostructure is realized emitting at 265 nm and showing an emission power of 0.81 mW at 20 mA (corresponds to an external quantum efficiency of 0.86 %)