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Cyclic voltammetry measurements are carried out in neutral phosphate-buffered electrolyte using n-type Ga-polar GaN thin-film photoelectrodes with and without cobalt phosphate (Co-Pi) modification. Without Co-Pi, the variation of the photocurrent with the bias potential exhibits a two-step behavior and under chopped illumination spikes occur at low bias potential. Thus in this regime surface recombination is dominant. Co-Pi modification suppresses surface recombination and significantly increases the photocurrent, especially for low bias potentials. At the same time, stability tests reveal that Co-Pi does not protect GaN against photocorrosion. Experiments using H2O2 imply that this photocorrosion is a reductive process and probably related to the presence of charged surface defects.

Graphene nanoribbons (GNRs) are promising for applications in nanoelectronics due to their unique properties. Therefore, achieving the controlled and high-quality synthesis of GNRs is anticipated to be of great importance. One of the methods which shows great potential is the growth of GNRs on surface facets of SiC(0001) by the surface graphitization method. In this report we studied the dependency of the GNR width on growth temperature and SiC substrate miscut angle (or initial step height). While a linear growth rate best describes the growth in lower step heights, a nonlinear rate is observed for substrates with higher steps, which is also associated with the formation of few-layer graphene on the step edges. The structural characterization of the samples was performed by means of atomic force microscopy, scanning electron microscopy, and Raman spectroscopy.

Hexagonal boron nitride has emerged as a fundamental building block for atomically thin electronic devices. This contribution investigates its formation on graphene via molecular beam epitaxy at temperatures well below 1300°C, where it is considered metastable as a bulk material. However, it is shown that even at lower temperatures single- to few-layer hexagonal boron nitride still nucleates on graphene, and that the formation of the sp2- and sp3-bonded phases of BN can be manipulated via an in-situ N 2 -plasma pretreatment of the substrates. Here the experimental discovery of a novel, metastable, sp3-bonded phase, which so far has been only predicted theoretically, demonstrates a rich polymorphism in BN at the nanoscale.

The photoelectrochemical properties of (InGa)N nanowire photoanodes are investigated using H2O2 as a hole scavenger in order to prevent photocorrosion. Under simulated solar illumination In0.16Ga0.84N nanowires grown by plasma assisted molecular beam epitaxy show a high photocurrent of 2.7 mA/cm2 at 1.2 V vs. reversible hydrogen electrode (RHE). This value is almost the theoretical maximum expected from the corresponding bandgap (2.8 eV) for homogeneous bulk material without taking into account surface effects. These nanowires exhibit a higher incident photon to current conversion efficiency over a broader wavelength range and a higher photocurrent than a compact layer with higher In content of 28. These results are explained by the combination of built in electric fields at the nanowire sidewall surfaces and compositional fluctuations in (InGa)N which gives rise to a radial Stark effect. This effect enables spatially indirect transitions at energies much lower than the bandgap. The resulting broad band light absorption leads to the high photocurrents. This benefit of the radial Stark effect in (InGa)N nanowires for solar harvesting applications opens up the perspective to break the theoretical limit for photocurrents.

We study the growth of InN films on In 2 O 3 (111) substrates by plasma-assisted molecular beam epitaxy under N excess. InN films deposited directly on In 2 O 3 (111) exhibit a strongly faceted morphology. A nitridation step prior to growth is found to convert the In 2 O 3 (111) surface to InN{0001}. The morphology of InN films deposited on such nitridated In 2 O 3 (111) substrates is characteristic for growth by instable step-flow and is thus drastically different from the three-dimensional growth obtained without nitridation. We show that this change originates from the different polarity of the films: while InN films deposited directly on In 2 O 3 (111) are In-polar, they are N-polar when grown on the nitridated substrate.

H and O incorporation in single-layer graphene (SLG) transferred onto SiO2/Si substrates following annealing in water vapor (H2O) was investigated. The use of isotopically enriched water in conjunction with nuclear reaction analysis enabled us to specifically investigate the effect of water annealing, among other modification agents of graphene like ambient exposure. Results revealed that incorporation of these species occurs by two distinct mechanisms, depending on the temperature range considered. From 100 to 300 °C, physisorption of H2O molecules in the SLG/SiO2/Si structure is the dominant process. For 400 °C and above, chemisorption is favored due to the creation of defects in the graphene lattice. Transport measurements demonstrated that the observed physico-chemical and structural modifications have a huge impact on the electrical characteristics of these structures.

We present an innovative approach to integrate arrays of isolated, strain-free GaN crystals on patterned Si substrates. First, micrometer-sized pillars are patterned onto Si(0 0 1) substrates. Subsequently, 2.5 μm Si substrates are deposited by low-energy plasma-enhanced chemical vapor deposition, forming crystals mostly bounded by {1 1 1}, {1 1 3}, and {15 3 23} facets. Plasma-assisted molecular beam epitaxy is then used for GaN deposition. GaN crystals with slanted {0 0 0 1} facets having a root-mean-square surface roughness of 0.7 nm are obtained for a deposited material thickness of >3 μm. Microphotoluminescence measurements performed at room and cryogenic temperature show no yellow luminescence and a neutral donor-bound A exciton transition at 3.471 eV (10 K) with a full width at half-maximum of 10 meV. Microphotoluminescence and micro-Raman spectra reveal that GaN grown on Si pillars is strain-free. Our results indicate that the shape of GaN crystals can be tuned by the pattern periodicity and that a reduction of threading dislocations is achieved in their top part.

We examine how substrate selection impacts the resulting film properties in graphene growth by molecular beam epitaxy (MBE). Graphene growth on metallic as well as dielectric templates was investigated. We find that MBE offers control over the number of atomic graphene layers regardless of the substrate used. High structural quality could be achieved for graphene prepared on Ni (111) films which were epitaxially grown on MgO (111). For growth either on Al2O3 (0001) or on (6√3×6√3)R30°-reconstructed SiC (0001) surfaces, graphene with a higher density of defects is obtained. Interestingly, despite their defective nature, the layers possess a well defined epitaxial relation to the underlying substrate. These results demonstrate the feasibility of MBE as a technique for realizing the scalable synthesis of this two-dimensional crystal on a variety of substrates.

In molecular beam epitaxy, the spontaneous formation of GaN nanowires on Si(111) substrates at elevated temperatures is limited by the long incubation time that precedes nanowire nucleation. In this work, we present three growth approaches to minimize the incubation time and to thus facilitate significantly higher growth temperatures (up to 875 °C). We achieve this advancement by (i) using III/V flux ratios of >1 to compensate for Ga desorption, (ii) introducing a two-step growth procedure, and (iii) using an AlN buffer layer to favor GaN nucleation. The GaN nanowire ensembles grown at so far unexplored substrate temperatures exhibit excitonic transitions with sub-meV linewidths comparable to those of state-of-the-art free-standing GaN layers grown by hydride vapor phase epitaxy.

Vertical GaN nanowires are grown in a self-induced way on a sputtered Ti film by plasma-assisted molecular beam epitaxy. Both in situ electron diffraction and ex situ ellipsometry show that Ti is converted to TiN upon exposure of the surface to the N plasma. In addition, the ellipsometric data demonstrate this TiN film to be metallic. The diffraction data evidence that the GaN nanowires have a strict epitaxial relationship to this film. Photoluminescence spectroscopy of the GaN nanowires shows excitonic transitions virtually identical in spectral position, line width, and decay time to those of state-of-the-art GaN nanowires grown on Si. Therefore, the crystalline quality of the GaN nanowires grown on metallic TiN and on Si is equivalent. The freedom to employ metallic substrates for the epitaxial growth of semiconductor nanowires in high structural quality may enable novel applications that benefit from the associated high thermal and electrical conductivity as well as optical reflectivity.

Isamu Akasaki, Hiroshi Amano, and Shuji Nakamura received the 2014 Nobel Prize in Physics for their invention of the blue light-emitting diode. This discovery has enabled bright and energy-saving white light sources for efficient solid-state lighting. The long road to this scientific development and its more recent fast-paced industrial and commercial application is illustrated. One thousand three hundred fifty modules with a total of 16,000 light-emitting diodes in handrails illuminate the steps of the Kiev sports stadium bright and safely. This kind of compact and efficient lighting was made possible by the work awarded with the Nobel Prize in 2014.

We observe that the growth rate of Ga_{2}O_{3} in plasma-assisted molecular beam epitaxy can be drastically enhanced by an additional In supply. This enhancement is shown to result from a catalytic effect, namely, the rapid formation of In_{2}O_{3}, immediately followed by a transformation of In_{2}O_{3} to Ga_{2}O_{3} due to an In-Ga interatomic exchange. We derive a simple model that quantitatively describes this process as well as its consequences on the formation rate of Ga_{2}O_{3}. Moreover, we demonstrate that the catalytic action of In_{2}O_{3} allows the synthesis of the metastable hexagonal phase of Ga_{2}O_{3}. Since the Ga_{2}O_{3}(0001)/In_{2}O_{3}(111) interface is closely lattice matched, this novel growth mode opens a new path for the fabrication of sesquioxide heterostructures.

Tapering of vapour-liquid-solid (VLS) grown nanowires (NWs) is a widespread phenomenon resulting from dynamics of the liquid droplet during growth anddirect vapour-solid (VS) growth on the sidewall. To investigate both effects in ahighly controlled way, we developed a novel two-step growth approach for the selective area growth (SAG) of GaAs nanowires (NWs) by molecular beam epitaxy. In this growth approach optimum growth parameters are provided for thenucleation of NWs in a first step and for the shape variation during elongationin a second step, allowing NWs with a thin diameter (45 nm) and an untapered morphology to be realized with high vertical yield. We quantify the flux dependenceof radial VS growth and build a model that takes into account diffusion on theNW sidewalls to explain the observed VS growth rates. As our model is consistent with axial VLS growth we can combine it with an existing model for the diameter variation due to the droplet dynamics at the NW top. Thereby, we achieve fullunderstanding of the diameter of NWs over their entire length and the evolutionof the diameter and tapering during growth. We conclude that only the combinationof droplet dynamics and VS growth results in an untapered morphology. This result enables NW shape engineering and has important implications for doping of NWs.

NPG Asia Materials 9(6), e396 (2017). doi:10.1038/am.2017.95, Published by Tokyo

The selective area growth of Ga-assisted GaAs nanowires (NWs) with a high vertical yield on Si(111) substrates is still challenging. Here, we explore different surface preparations and their impact on NW growth by molecular beam epitaxy. We show that boiling the substrate in ultrapure water leads to a significant improvement in the vertical yield of NWs (realizing 80%) grown on substrates patterned by electron-beam lithography (EBL). Tentatively, we attribute this improvement to a reduction in atomic roughness of the substrate in the mask opening. On this basis, we transfer our growth results to substrates processed by a technique that enables the efficient patterning of large arrays, nano imprint lithography (NIL). In order to obtain hole sizes below 50 nm, we combine the conventional NIL process with an indirect pattern transfer (NIL-IPT) technique. Thereby, we achieve smaller hole sizes than previously reported for conventional NIL and growth results that are comparable to those achieved on EBL patterned substrates.

The growth of GeTe thin films on a Si(111)-(√3 × √3)R30°-Sb surface is reported. At growth onset, the rapid formation of fully relaxed crystalline GeTe(0001)-(1 × 1) is observed. During growth, a GeTe(0001)-(√3 × √3)R30° surface reconstruction is also detected. Indeed, density functional theory (DFT) simulations indicate that the reconstructed GeTe(0001)-(√3 × √3)R30° structure is energetically competing with the GeTe(0001)-(1 × 1) reconstruction. The out-of-plane α-GeTe ||Si and in-plane α-GeTe ||Si epitaxial relationships are confirmed by X-ray diffraction (XRD). Suppression of rotational twist and reduction of twinned domains are achieved. The formation of rotational domains in GeTe grown on Si(111)-(7 × 7) is explained by domain matched coincidence lattice formation with the Si(111)-(1 × 1) surface. Atomic force microscopy (AFM) images show the coalescence of well-oriented islands with subnanometer roughness on their top part. van der Pauw measurements are performed to verify the...

The conversion of monolayer graphene grown epitaxially on SiC(0 0 0 1) into quasi-free standing bilayer graphene (QFSBG) can be achieved by decoupling the buffer layer (BL) from the substrate via an annealing step. We investigated the role of O 2 and H 2 O (possible agents of this process) by varying the annealing atmosphere and time. Raman spectroscopy reveals a synergistic effect of O 2 and H 2 O, which promotes a complete BL detachment and thus large-area QFSBG formation. Similar results are obtained in a H 2 O-rich ambient for longer annealing times. X-ray photoelectron spectroscopy (XPS) revealed that in both cases the observed effect is related to the formation of an oxide layer on the SiC surface during annealing.

The structural evolution of Ge–Sb–Te-based phase-change material during molecular beam epitaxy growth is investigated. The in situ and ex situ synchrotron-based X-ray scattering results reveal the formation and evolution of defects-induced inhomogeneities during growth. The scattered intensity in the vicinity of various Bragg reflections indicates a specific scattering vector-dependent anisotropy in the diffuse scattering regime. The average local static distortion fields of the defects-induced inhomogeneities are estimated to be in between 2.5 and 3.0 nm. The defects-induced diffuse scattering anisotropy, observed for the particular Bragg points, arises from the combination of the elastic and crystalline anisotropies that are caused by the large number of vacancies. The experiments show that the large-size defect clusters create inhomogeneity-driven tetragonal fields inserted into the metastable phase of the Ge–Sb–Te host crystal.

Semiconductor-ferromagnet GaAs-Fe3Si core-shell nanowires were grown by molecular beam epitaxy and analyzed by scanning and transmission electron microscopy, X-ray diffraction, Mossbauer spectroscopy, and magnetic force microscopy. We obtained closed and smooth Fe3Si shells with a crystalline structure that show ferromagnetic properties with magnetizations along the nanowire axis (perpendicular to the substrate). Such nanobar magnets are promising candidates to enable the fabrication of new forward-looking devices in the field of spintronics and magnetic recording.

An analysis of the strain in an axial nanowire superlattice shows that the dominating strain state can be defined arbitrarily between unstrained and maximum mismatch strain by choosing the segment height ratios. We give experimental evidence for a successful strain design in series of GaN nanowire ensembles with axial InxGa1-xN quantum wells. We vary the barrier thickness and determine the strain state of the quantum wells by Raman spectroscopy. A detailed calculation of the strain distribution and LO phonon frequency shift shows that a uniform in-plane lattice constant in the nanowire segments satisfactorily describes the resonant Raman spectra, although in reality the three-dimensional strain profile at the periphery of the quantum wells is complex. Our strain analysis is applicable beyond the InxGa1-xN/GaN system under study, and we derive universal rules for strain engineering in nanowire heterostructures.

We investigated the photoelectrochemical properties of both n- and p-type (In,Ga)N nanowires (NWs) for water splitting by in situ electrochemical mass spectroscopy (EMS). All NWs were prepared by plasma-assisted molecular beam epitaxy. Under illumination, the n-(In,Ga)N NWs exhibited an anodic photocurrent, however, no O2 but only N2 evolution was detected by EMS, indicating that the photocurrent was related to photocorrosion rather than water oxidation. In contrast, the p-(In,Ga)N NWs showed a cathodic photocurrent under illumination which was correlated with the evolution of H2. After photodeposition of Pt on such NWs, the photocurrent density was significantly enhanced to 5 mA/cm(2) at a potential of -0.5 V/NHE under visible light irradiation of ∼40 mW/cm(2). Also, incident photon-to-current conversion efficiencies of around 40% were obtained at -0.45 V/NHE across the entire visible spectral region. The stability of the NW photocathodes for at least 60 min was verified by EMS. These results suggest that p-(In,Ga)N NWs are a promising basis for solar hydrogen production.

Arrays of dislocation free uniform Ga-polar GaN columns have been realized on patterned SiOx/GaN/sapphire templates by metal organic vapor phase epitaxy using a continuous growth mode. The key parameters and the physical principles of growth of Ga-polar GaN three-dimensional columns are identified, and their potential for manipulating the growth process is discussed. High aspect ratio columns have been achieved using silane during the growth, leading to n-type columns. The vertical growth rate increases with increasing silane flow. In a core–shell columnar LED structure, the shells of InGaN/GaN multi quantum wells and p-GaN have been realized on a core of n-doped GaN column. Cathodoluminescence gives insight into the inner structure of these core–shell LED structures.

We report on the growth of nanocrystalline graphene on c -plane Al 2 O 3 substrates by molecular beam epitaxy. Graphene films are grown by carbon evaporation from a highly-oriented-pyrolytic-graphite filament and cover the entire surface of two-inch wafers. The structural quality of the material (degree of crystallinity) is investigated in detail by Raman spectroscopy and is revealed to be strongly dependent on the growth temperature and time. We observe that adjacent graphene layers grow parallel to each other and to the substrate surface with domains sizes larger than 30 nm. Transmission electron microscopy confirms the planarity of the nanocrystalline films and X-ray photoelectron spectroscopy proves the predominant sp 2 nature of the grown layers. Transport measurements reveal that the layers are p-type doped with mobility values up to 140 cm 2 /Vs at room temperature. The present results demonstrate the potential of molecular beam epitaxy as a technique for realizing the controlled growth of graphene (mono- and few-layer) over large areas directly on an insulating substrate.

We report on the conversion of epitaxial monolayer graphene on SiC(0 0 0 1) into decoupled bilayer graphene by performing an annealing step in air. We prove by Raman scattering and photoemission experiments that it has structural and electronic properties that characterize its quasi-free-standing nature. The (6 p 3 · 6 p 3)R30� buffer layer underneath the monolayer graphene loses its covalent bonding to the substrate and is converted into a graphene layer due to the oxidation of the SiC surface. The oxygen reacts with the SiC surface without inducing defects in the topmost carbon layers. The high-quality bilayer graphene obtained after air annealing is p-doped and homogeneous over a large area.

Scientific Reports 6: Article number: 23843; published online: 01 April 2016; updated: 16 August 2016 In this Article, Stefania Privitera and Emanuele Rimini are incorrectly listed as being affiliated with ‘Paul-Drude-Institut für Festkörperelektronik, Hausvogteiplatz 5-7, 10117 Berlin, Germany’. The correct affiliation is listed below

We present a new concept for the optical interfacing between vertical III-As nanowires and planar Si waveguides. The nanowires are arranged in a two-dimensional array which forms a grating structure on top of the waveguide. This grating enables light coupling in both directions between the components made from the two different material classes. Numerical simulations show that this concept permits a light extraction efficiency from the waveguide larger than 45% and a light insertion efficiency larger than 35%. This new approach would allow the monolithic integration of nanowire-based active optoelectronics devices, like photodetectors and light sources, on the Si photonics platform.

Scientific reports 6, 23843 (2016). doi:10.1038/srep23843, Phase Change Materials (PCMs) are unique compounds employed in non-volatile random access memory thanks to the rapid and reversible transformation between the amorphous and crystalline state that display large differences in electrical and optical properties. In addition to the amorphous-to-crystalline transition, experimental results on polycrystalline GeSbTe alloys (GST) films evidenced a Metal-Insulator Transition (MIT) attributed to disorder in the crystalline phase. Here we report on a fundamental advance in the fabrication of GST with out-of-plane stacking of ordered vacancy layers by means of three distinct methods: Molecular Beam Epitaxy, thermal annealing and application of femtosecond laser pulses. We assess the degree of vacancy ordering and explicitly correlate it with the MIT. We further tune the ordering in a controlled fashion attaining a large range of resistivity. Employing ordered GST might allow the realization of cells with larger programming windows., Published by Nature Publishing Group, London

The first experimental demonstration of a spin amplifier at room temperature is presented. An efficient, defect-enabled spin amplifier based on a non-magnetic semiconductor, Ga(In)NAs, is proposed and demonstrated, with a large spin gain (up to 2700% at zero field) for conduction electrons and a high cut-off frequency up to 1 GHz.

We investigate the formation of the ternary alloy InxGa1–xN in the growth of self-induced nanowires by plasma-assisted molecular beam epitaxy. A series of samples grown at different temperatures were analyzed in situ by line-of-sight quadrupole mass spectrometry, and the predicted composition was verified by X-ray diffraction on as-grown nanowire ensembles. The InxGa1–xN composition is determined by the thermally activated loss of In due to InN decomposition and In desorption, similar to the composition of planar layers. The convolution of decomposition, reincorporation, and desorption is described by an apparent activation energy of about 2.5 eV. We show the feasibility of in situ control of the InxGa1–xN nanowire composition by line-of-sight quadrupole mass spectrometry.

We demonstrate the potential of using Si as n-type dopant in GaAs nanowires grown by molecular beam epitaxy. The amphoteric behavior of Si that typically accompanies the vapor-liquid-solid growth mode is adequately controlled when a shell doping scheme is utilized instead, i.e. when a Si-doped GaAs shell layer is grown conformally around the undoped GaAs nanowire core in the vapor-solid mode. The incorporation site of Si was evaluated by Raman spectroscopy, and correlated with the growth conditions of the doped shell. In that way, we identified a growth window that ensures the incorporation of Si as donor, and obtained donor concentrations up to 1 × 1019 cm−3, with the compensation level by Si acceptors remaining below 10%. Finally, resistivity measurements on planarized shell-doped nanowire ensembles were employed to probe the doping efficiency and the surface depletion of free-carriers. The achievement of n-type conductivity for nanowires is essential for the realization of functional devices, and is particularly significant when a dopant as well understood and advantageous as Si is employed.

The growth by molecular beam epitaxy of GeTe thin films on highly lattice-mismatched Si(111) substrates is reported. In situ reflection high-energy electron diffraction and quadrupole mass spectrometry were employed to monitor the growth process in real time and tune the deposition conditions. Epitaxy was achieved in a window of substrate temperatures between 220 and 270 °C, using a Ge/Te flux ratio of ∼0.4. Extensive ex situ X-ray diffraction characterization showed that the epitaxial layers crystallize in the rhombohedrally distorted rocksalt structure α-GeTe, with orientation relationships to the substrate α-GeTe[0001] ‖ Si[111] and α-GeTe〈100〉 ‖ Si〈〉. ω-scans of the α-GeTe(000n) reflections (n = 3, 6, 9) exhibit a full width at half maximum between 10 and 20 arcsec, indicating small mosaicity. However, a large twist is observed (∼14°), pointing to the presence of rotational domains. In addition, the layer matrix is affected by twinning. The epitaxial films exhibit a slight tensile in-plane strain, which might be due to the difference in thermal expansion coefficients between α-GeTe and Si, and/or small deviation from stoichiometric composition, i.e., vacancies in the Ge sub-lattice.

In this work we focus on the impact of different buffer and nucleation layers on the corresponding crystalline quality of gallium nitride (GaN), grown by metal-organic chemical vapour deposition (MOCVD). In situ curvature measurements, X-ray diffraction (XRD) and transmission electron microscopy (TEM) are used for advanced characterization. The influence of various growth modes on meltback etching and cracking is analyzed. Also the effect of a silicon nitride (SiN) mask on the growth of GaN and its coalescence is investigated. Furthermore, associated potential consequences for the growth of aluminium nitride (AlN) interlayers are examined to obtain a homogeneous surface without cracks and with good crystalline quality. Our studies indicate that an incomplete coalesced GaN surface located underneath the AlN interlayer leads to an increased defect density. Additionally we studied the influence of growth temperature at nucleation on the material quality and the process stability. Finally we demonstrate interlayer induced compressive strain during GaN growth with an XRD rocking-curve full width half maximum (FWHM) as low as 450 arc seconds for the reflection.

Hexagonal GaN microcrystals of a size between 1 to 3 μm are obtained by the pendeoepitaxial overgrowth of a GaN nanowire template on Si(111). The GaN microcrystals are free of threading dislocation...

Employed for a long time in optical disks, Ge2Sb2Te5 is nowadays considered the most promising material also for phase-change nonvolatile memories. In the current paper, Ge–Sb–Te phase-change mater...

The nucleation and growth of InAs nanowires on bare Si(111) has been investigated by molecular beam epitaxy. Nontapered InAs nanowires with high aspect ratio were grown perpendicular to the substrate without the use of catalyst particles, surface oxide, or other substrate mask. The nucleation of InAs takes place in In-rich areas forming spontaneously on the substrate in the beginning of the growth process. As the nucleation proceeds, the local stoichiometry on the growth interface changes from In-rich to As-rich, and the growth continues in a vapor–solid mode. This transition to As-rich conditions is correlated with the evolution of nanowire morphology, that is, with the growth becoming strictly uniaxial and with well-defined vertical sidewalls forming. The diameter, the number density, and the axial growth rate of the nanowires were found to depend exclusively on the surface diffusivity of In adatoms on the substrate.

In this paper we present the investigation of high-k dielectrics in a metal-insulator-metal structure. We show the physical and electrical properties of ZrO"2 and Sr"xZr"("1"-"x")O"y grown through sputter deposition. Uncontrolled crystallization of ZrO"2 during the growth into a mixture of different phases was observed. As a consequence, the k-value was suppressed. Stabilization of the amorphous phase of the as grown films could be achieved by the admixture of SrO. This enabled controlled crystallization into a single phase after performing post-deposition annealing. The k-value of the annealed Sr"xZr"("1"-"x")O"y was determined to be 33.

On Si(1 1 1) and Si(0 0 1), GaN nanowires (NWs) form in a self-induced way without the need for any external material. On sapphire, NW growth is induced by Ni collectors. Both types of NWs exhibit the wurtzite crystal structure and grow in the Ga-polar C-direction perpendicular to the substrate. The NW sidewalls are M-plane facets, although on the Ni-induced NWs also A-plane segments form, if the growth temperature is low. Both self-induced and collector-induced NWs are free of strain and epitaxially aligned to the substrate, but in particular the former show a significant spread in tilt and twist caused by a mostly amorphous interfacial layer of Si-N. The self-induced NWs are virtually free of extended defects, but the collector-induced NWs contain many stacking faults. The photoluminescence of the former is significantly brighter and sharper. The spectra of single, dispersed, self-induced NWs contain extremely sharp excitonic lines. Significant emission is caused by excitons bound to donors close to the surface whose binding energy is reduced compared to the bulk value. In comparison, both the microstructure and optical properties of the self-induced NWs are superior. The limited material quality of the collector-induced NWs can be explained by detrimental effects of the collector.

A study of GaN–AlGaN nanowire heterostructures grown by molecular beam epitaxy on sapphire substrates is presented. Nanowire growth was promoted using nickel seeds formed by in-situ annealing of a Ni thin film. Deposition sequences were designed to form two nanowire samples with embedded alternating multilayers of GaN and Al0.2Ga0.8N of variable thicknesses. Subsequent analyses showed the formation of two types of nanostructures in both samples, namely long and tapered nanowires and short columnar structures with uniform diameters along the whole length. Ni seeds were found at the end of the long nanowires only, indicating a catalyst-induced growth mechanism. No Ni seeds were found at the top of columnar structures indicating a catalyst-free growth mechanism. Analytical electron microscopy techniques were applied to the study of the Al distribution along both types of nanostructures. The layered heterostructure was found only for the columnar structures, whereas in nanowires the mediation of the Ni seed led to an incorporation of Al varying at monolayer level unrelated to the deposition sequence. The Al to Ga concentration ratio was found to decrease from the central segment of the nanowires towards the tip. Residual radial growth was also found to interfere with the effective deposition of the multilayer NW heterostructures.

We investigate the strain in core–shell nanowires consisting of a highly mismatched materials system of a GaAs core and a MnAs shell. The strain in the GaAs core is directly traced using Raman spectroscopy, whereas that in the MnAs shell is assessed by magnetization measurements. The Raman peak positions in the MnAs-capped nanowires are shifted in comparison to those in bare GaAs nanowires in a nonuniform manner. We theoretically explore the influence of the anisotropic strain in the core–shell nanowires on the peak shift. When the shell thickness considerably exceeds the core diameter, the shell is only weakly strained. This can be interpreted as the shell taking over the role of the rigid substrate. In addition, we evaluate the diffusion length in the MnAs growth.

Digital epitaxial rare-earth oxide layers are grown on Si(1 1 1) substrates by molecular beam epitaxy at substrate temperatures as low as 200 C. It is demonstrated by X-ray diffraction and transmission electron microscopy that coherent digital layers form with an abrupt interface to the substrate. Theoretical investigations employing density functional theory demonstrate the potential in designing physical properties by strain. The large lattice mismatch of 9% between La 2 O 3 and Lu 2 O 3 allows for an intentional variation of the internal strain in the layers over a wide range.

Molecular beam epitaxy growth of GaSb growth on Si(1 1 1) substrates can be improved by pre-depositing Sb at high temperature. The (√3 × √3) reconstruction obtained by this procedure results in closed heteroepitaxial GaSb layers in contrast to the direct growth on Si(1 1 1)(7 × 7) which produces islands. The growth is characterized by atomic force microscopy, electron and X-ray diffractions. On the basis of these investigations, the formation of an interface misfit dislocation network is discussed.

The nucleation and growth of GaAs nanowires fabricated by molecular beam epitaxy (MBE) following the Au-assisted vapor-liquid-solid mechanism were compared on GaAs(1 1 1)B and on Si(1 1 1) substrates. On both substrates, reflection high-energy electron diffraction (RHEED) patterns and scanning electron microscopy (SEM) images of several samples belonging to a growth time series were analyzed. During the nucleation stage, growth on Si(1 1 1) is dominated by horizontally growing traces and coalescing islands, while growth on GaAs(1 1 1)B proceeds instantly in the vertical direction. After this nucleation stage, the Si substrate is covered by a closed, rough GaAs layer, and nanowires of similar shape grow on both substrates with similar axial and radial growth rates. However, the diameter of the nanowires on Si(1 1 1) is different than that on GaAs(1 1 1)B, because the size of the Au droplets, which result from the annealing of a thin Au layer, is different on the two types of substrates.

GaN nanowires are grown as templates for pendeoepitaxial coalescence overgrowth at substrate temperatures between 720 and 820 °C on Si(1 1 1) by molecular beam epitaxy. The unintentional coalescence of nanowires is suppressed with increasing substrate temperature. Simultaneously, the width of the donor-bound-exciton transition at 3.472 eV decreases down to 1.6 meV. The length distribution of the GaN nanowires strongly influences the morphology of the pendeoepitaxial layer after coalescence.

Vertically aligned GaN nanorods have recently obtained substantial interest as a new approach to solid state lighting. In comparison to conventional planar LEDs, 3D NanoLEDs are expected to offer substantial advantageous: very low defect density, quasi free-standing, no strain due to mismatch of thermal expansion coefficients, no substrate bending even when grown on large area silicon. Core-shell strategies are another very interesting aspect. The active LED surface per wafer could be increased by more than one order of magnitude. However, most of these advantages have not yet been proven in real devices, which would include a quantitative comparison of light emission. Related to the 3D character, there are also technological risks. In the following we will discuss the main developments which have paved the way up to this point, including a detailed discussion of possible benefits and risks connected with the NanoLED approach (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

GaN and ZnO are both wide band gap semiconductors with interesting properties concerning optoelectronic and sensor device applications. Due to the lack or the high costs of native substrates, alternatives like sapphire, silicon, or silicon carbide are taken, but the resulting lattice and thermal mismatches lead to increased defect densities which reduce the material quality. In contrast, nanostructures with high aspect ratio have lower defect densities as compared to layers. In this work, we give an overview on our results achieved on both ZnO as well as GaN based nanorods. ZnO nanostructures were grown by a wet chemical approach as well as by VPT on different substrates – even on flexible polymers. To compare the growth results we analyzed the structures by XRD and PL and show possible device applications. The GaN nano- and microstructures were grown by metal organic vapor phase epitaxy either in a self-organized process or by selective area growth for a better control of shape and material composition. Finally we take a look onto possible device applications, presenting our attempts, e.g., to build LEDs based on GaN nanostructures.

We studied the influence of nitrogen incorporation on the InAsN/GaAs quantum dot formation using cross-sectional scanning tunneling microscopy. Upon nitrogen exposure during InAs growth, rather strong dissolution effects occur, leading to the formation of extended almost spherical InGaAs quantum dots with a low indium content and almost free of nitrogen. In this paper we will discuss in particular the detailed stoichiometry determination of the quantum dots. Furthermore, we will demonstrate that nitrogen atoms are located in the surrounding GaAs matrix. Indium atoms are found underneath the nominal base plane, where quantum dot growth started, demonstrating strong strain-induced intermixing already during quantum dot formation. The observed strong separation of indium and nitrogen is related to a rather low solubility of nitrogen within InAs as compared with GaAs. Finally, a schematic model for the growth of InAs and InAsN quantum dots in GaAs will be discussed in detail (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

GaN nanorods (or nanowires) have attracted great interest in a variety of applications, e.g. high-efficiency light emitting diodes, monolithic white light emission and optical interconnection due to their superior properties. In contrast to the mostly investigated self-assembled growth of GaN nanorods, we performed GaN nanorod growth by pre-patterning of the Si substrates. The pattern was transferred to Si substrates by photolithography and cryo-temperature inductively-coupled plasma etching. These Si templates then were used for further GaN nanorod growth by metal-organic vapour phase epitaxy (MOVPE). The low temperature AlN nucleation layer had to be optimized since it differs from its 2D layer counterpart on the surface area and orientations. We found a strong influence of diffusion processes, i.e. the GaN grown on top of the Si nanopillars can deplete the GaN around the Si pillars. Transmission electron microscopy measurements demonstrated clearly that the threading dislocations bend to the side facets of the pyramidal GaN nanostructures and terminate. Cathodoluminescence measurements reveal a difference of In composition and/or thickness of InGaN quantum wells on the different facets of the pyramidal GaN nanostructures. (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)