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The formation of Au/Ni and Pt/Ni contacts to p-GaN has been examined with Auger depth profiling and x-ray photoelectron spectroscopy analysis and compared to circular transmission line measurements of their respective transport properties. The metal bilayers were deposited by radio frequency argon ion sputtering, and the samples were annealed at different temperatures in a 10−3 mbar oxygen ambient. Both contacts were distinctly non-ohmic as deposited. Ohmic behaviour was achieved after oxidation at temperatures above 400 °C, and the two contacts were found to have comparable performance, with a lowest specific contact resistance ρc = 1.0 × 10−2 Ω cm2 for the Pt/Ni contact after annealing at 400 °C and ρc = 1.3 × 10−2 Ω cm2 for the best Au/Ni contact annealed at 600 °C. Au and Pt both serve to ensure a highly conducting contact layer after annealing in oxygen and help lower the Schottky barrier at the metal/p-GaN interface by virtue of their large work functions. Distinct differences were observed in the alloying properties of the Pt/Ni and the Au/Ni contact layers, however. While Pt and Ni interdiffuse to form a uniform alloy upon annealing in oxygen at 600 °C, Ni diffuses outwards to form surface nickel oxide for the Au/Ni metallization. Trace amounts of Ga were observed at the surface after annealing at 600 °C for the Pt/Ni and the Au/Ni bilayers alike, albeit in considerably larger measure for Pt/Ni. We believe that this surface Ga derives from a thermally activated reaction between Ni and GaN, which adversely affects the contact performance. The outward diffusion of Ni upon thermal oxidation of the Au/Ni contact appears to inhibit the rate of this interface reaction, which speaks in favour of using Au over Pt in ohmic contacts to p-GaN. Preconditioning of the p-GaN surface by deposition and subsequent removal of a thin coating of Ni are found to improve the contact performance, provided the coating is not annealed before removal.

In this paper we derive and discuss the boundary conditions for the electron wavefunction in general Ga1−xInxNyAs1−y-based heterostructures described by the band anticrossing model. The use of these boundary conditions greatly simplifies the calculation of, for example, transition energies in quantum wells. We then apply the derived equations to model the temperature-dependent bandgap of Ga1−xInxNyAs1−y/GaAs quantum well structures with high indium concentrations. From a fit to our experimental photoreflectance data we find evidence that the effective nitrogen level EN in the band anticrossing Hamiltonian, measured with respect to the valence band edge, shifts to higher energies with decreasing temperature. This supports and extends similar results reported in the literature for low indium content epilayers.

We review the status of InGaAsN-based vertical-cavity surface-emitting lasers (VCSELs) emitting in the wavelength range 1.2–1.3 μm and compare them with similar devices that have been realized using other approaches. To prove the potential of InGaAsN-based VCSELs, we present our results for monolithically MBE- and MOVPE-grown and electrically pumped VCSELs on GaAs substrates. Our MBE-grown devices emit at a wavelength of up to 1305 nm with cw output power at room temperature exceeding 1 mW and a threshold current of 2.2 mA. With an oxide-confined current aperture of about 5 μm diameter, they emit up to 700 μW in single-mode operation at room temperature. Bit-error rates of less than 10−11 are achieved for transmission over 20.5 km of standard single-mode fibre at 2.5 Gbit s−1. Our MOVPE-grown VCSELs with a similar device structure emit single mode at a wavelength of 1293 nm with a cw output power of 1.4 mW and a threshold current of 1.25 mA at room temperature. In back-to-back transmission, we reach a data rate of 10 Gbit s−1, proving the feasibility of high-speed data transmission using InGaAsN VCSELs.

We report on the growth and properties of InGaAsN/GaAs heterostructures and on their applications for lasers emitting at λ≈1.3 µm. Material growth was performed by molecular beam epitaxy using an RF plasma source. Broad area and ridge waveguide (RWG) laser structures based on such quantum wells (QWs) exhibit performances that can compete with those of 1.3 µm InGaAsP lasers. In particular, we have achieved 300 K operation of broad area lasers at 1.3 µm with threshold current densities down to 500 and 650 A cm-2 for 800 µm long, single and triple QW structures. Similar structures with heat-sinking at 10 °C yield a maximum CW output power of up to 8 W. RWG lasers have thresholds down to 11 mA and show CW operation up to 100 °C with a T0 of up to 110 K.

X-ray standing waves (XSW) were used to investigate the structure of molecular beam epitaxy (MBE) grown (AlAs)3(GaAs)7 short-period superlattices (SPSL). The modulation of the Al K, As L, and Ga L x-ray fluorescence induced by XSW was measured at the zero-order superlattice (SL) satellite (AlAs)(GaAs)(004,0) and the GaAs(004) substrate Bragg reflection. From the shape of the fluorescence yield modulations and the diffraction pattern, a model of the interfaces is derived by comparing the experimental data with dynamical calculations of the x-ray wave field distribution and reflectivity. A straightforward analysis of the fluorescence measurements at the SL satellite shows that in AlAs layers a high crystalline order is established, whereas in GaAs layers a fraction of the Ga and As atoms is not on ideal lattice sites, but is displaced towards the substrate. The data can be explained by a model in which, at each AlAs/GaAs interface of the GaAs layers, two Ga atom planes are displaced by 0.035 nm and 0.008 nm and one As atom plane by 0.023 nm. The displacements within the GaAs layers exhibit a mirror symmetry with respect to the centre of each layer.

An evaluation algorithm for the determination of the chemical composition of strained hexagonal epitaxial films is presented. This algorithm is able to separate the influence of strain and composition on the lattice parameters measured by x-ray diffraction. The measurement of symmetric and asymmetric reflections delivers the strained lattice parameters a and c of hexagonal epitaxial films. These lattice parameters are used to calculate the relaxed lattice parameters employing the theory of elasticity. From the relaxed parameters, the chemical composition of the epitaxial film can be determined by Vegard's rule. The algorithm has been applied to InGaN/GaN/Al2O3(00.1) heterostructures.

Studies of the annealing of GaInNAs quantum wells under argon or hydrogen atmosphere revealed a significant dependency of the annealing behavior on the growth temperature. Structural investigation by means of transmission electron microscopy reveals the formation of vacancy type dislocation loops after argon annealing only for quantum wells grown at low temperature. This was not observed for hydrogen annealing. The formation of these loops leads to enhanced nonradiative recombination reducing the luminescence efficiency. In contrast, samples grown at high temperatures show improved luminescence efficiency upon both annealing atmospheres. This is attributed to the growth-induced formation of different kinds of defects.

We investigate epitaxial layers of GaN on c-plane sapphire by photoluminescence, optical density and x-ray diffraction. Besides the well known luminescence from hexagonal GaN we have identified two emission bands from cubic GaN. We observe the emission of the donor bound exciton in cubic GaN at 3.279 eV. The luminescence at 3.15 - 3.21 eV is explained as the cubic donor - acceptor recombination. The corresponding acceptor binding energy is found to be low as . For layers of 400 nm thickness, the optical density yield values for the average contents of cubic GaN between 10 and 25%. Proper growth conditions minimize the cubic contents in the upper regions of such layers.

InN nanowires were grown on Si(111) substrates by plasma-assisted molecular beam epitaxy. Raman spectroscopy showed that the nanowires were strain-free and allowed the deduction of a free carrier concentration of 1–2 × 1018 cm−3. This value was confirmed by a Mott–Schottky analysis of electrolyte-based capacitance-voltage measurements. In addition, these measurements directly revealed the existence of a surface accumulation layer in the InN nanowires. In cyclic voltammetry measurements under irradiation from a Xe lamp with about 100 mW cm−2, high photocurrents of about 4 and 11 mA cm−2 were observed at 1.23 and 1.63 V bias potential versus reversible hydrogen electrode, respectively, using H2O2 as a hole scavenger. By comparing the photocurrent with and without H2O2, the main limiting factor in the performance of InN nanowire photoanodes was identified to be the poor catalytic efficiency for water oxidation at the surface, followed by parasitic bulk recombination.

X-ray diffraction has been extended by simultaneously measuring inelastic signals excited internally by the X-ray standing wave field. As the inelastic X-ray standing wave signal we recorded the photoelectron current from an (AlAs)m(GaAs)n short-period superlattice on a GaAs(001) substrate. It shows clear modulations due to the GaAs(004) substrate reflection, the (AlAs)(GaAs)(004; 0) superlattice main reflection and the thickness fringes. The modulation at the (AlAs)(GaAs)(004; 0) reflection is related to the content of atoms in lattice plane positions. This is of technological interest, since the coherent fraction offers a new parameter for the characterization of the quality of short-period superlattices. The X-ray diffraction measurement in a wide angular range gives the complete set of satellites between two neighbouring substrate reflections. For some of the samples investigated, a non-coincidence of the (AlAs)(GaAs) satellite groups belonging to GaAs(002) and GaAs(004) is observed, which can be shown to be due to an incommensurate modulation of the superlattice. From the satellite intensities, the Fourier coefficients of the modulation can be determined by comparison with kinematical diffraction theory. This gives a description of the (AlAs)m(GaAs)n superlattice unit cell and its interface widths.

Large InN microcrystals grown by molecular beam epitaxy are investigated by micro-Raman spectroscopy and electron backscatter diffraction (EBSD). High-quality (phonon linewidths between 1.5 and 2 cm−1) cubic and hexagonal crystals are identified with Raman mapping by the observation of the respective characteristic phonon modes. The unexpected occurrence of metastable cubic InN crystals is confirmed by EBSD measurements. The cubic microcrystals are revealed by EBSD to be single-crystalline and to exhibit orientation. The transverse (TO) and longitudinal-optical (LO) zone-center phonon frequencies of cubic InN are found to be 463 and 584 cm−1, respectively. The bulk carrier density in the microcrystals lies in the range of 2–3 × 1017 cm−3 as determined by the analysis of LO phonon-plasmon-coupled modes in the Raman spectra.

InAs segments were grown on top of GaAs islands, initially created by droplet epitaxy on silicon substrate. We systematically explored the growth-parameter space for the deposition of InAs, identifying the conditions for the selective growth on GaAs and for purely axial growth. The axial InAs segments were formed with their sidewalls rotated by 30° compared to the GaAs base islands underneath. Synchrotron X-ray diffraction experiments revealed that the InAs segments are grown relaxed on top of GaAs, with a predominantly zincblende crystal structure and stacking faults.

Graphene is grown by molecular beam epitaxy using epitaxial Ni films on MgO(111) as substrates. Raman spectroscopy and scanning tunneling microscopy reveal the graphene films to have few crystalline defects. While the layers are ultra-smooth over large areas, we find that Ni surface features lead to local non-uniformly thick graphene inclusions. The influence of the Ni surface structure on the position and morphology of these inclusions strongly suggests that multilayer graphene on Ni forms at the interface of the first complete layer and metal substrate in a growth-from-below mechanism. The interplay between Ni surface features and graphene growth behavior may facilitate the production of films with spatially resolved multilayer inclusions through engineered substrate surface morphology.

A combination of molecular beam epitaxy (MBE) and atomic layer deposition (ALD) was adopted to fabricate GaN-based core/shell NW structures. ALD was used to deposit a HfO2 shell of onto the MBE grown GaN NWs. Electron transparent samples were prepared by focussed ion beam methods and characterized using state-of-the-art analytical transmission and scanning transmission electron microscopy. The polycrystalline coating was found to be uniform along the whole length of the NWs. Photoluminescence and Raman spectroscopy analysis confirms that the HfO2 ALD coating does not add any structural defect when deposited on the NWs.

We report a Raman study of the so-called buffer layer with $(6\sqrt3\times6\sqrt3)R30^{\circ}$ periodicity which forms the intrinsic interface structure between epitaxial graphene and SiC(0001). We show that this interface structure leads to a nonvanishing signal in the Raman spectrum at frequencies in the range of the D- and G-band of graphene and discuss its shape and intensity. Ab-initio phonon calculations reveal that these features can be attributed to the vibrational density of states of the buffer-layer., Comment: 12 pages, 6 figures

The growth of Sb nanowires on GaSb(111)A substrates is studied by in situ azimuthal scan reflection high-energy electron diffraction (ARHEED). Bulk and layer contributions can be distinguished in the ARHEED transmission pattern through the Sb nanowires. The three-dimensional structure of the growing Sb nanowires is identified by post-growth atomic force microscopy (AFM) and x-ray diffraction (XRD). The lattice match of the Sb crystal along the [Formula: see text] and the GaSb crystal along [Formula: see text] directions lead to a preferential orientation of the Sb nanowires. The Sb adsorption and desorption kinetics is studied by thermal desorption spectroscopy.

Gallium nitride based core/shell nanowire heterostructures are promising components for UV optoelectronic nanodevices. A better understanding of their growth mechanisms is required to tailor the growth and the properties of these structures. We report an investigation of GaN-based radial nanowire heterostructures grown by Molecular Beam Epitaxy combined with Atomic Layer Deposition of HfO2 dielectric cladding. The structural quality of GaN nanowire samples coated with different materials, the elemental distribution along the nanowires' length, as well as the elemental incorporation mechanism are discussed.

We investigated GaN-based heterostructures grown on three-dimensionally patterned Si(111) substrates by metal organic vapour phase epitaxy, with the goal of fabricating well controlled high quality, defect reduced GaN-based nanoLEDs. The high aspect ratios of such pillars minimize the influence of the lattice mismatched substrate and improve the material quality. In contrast to other approaches, we employed deep etched silicon substrates to achieve a controlled pillar growth. For that a special low temperature inductively coupled plasma etching process has been developed. InGaN/GaN multi-quantum-well structures have been incorporated into the pillars. We found a pronounced dependence of the morphology of the GaN structures on the size and pitch of the pillars. Spatially resolved optical properties of the structures are analysed by cathodoluminescence.