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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 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.

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.

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.

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.

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.

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 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.

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 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.

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...

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.

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 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)

Three-dimensionally patterned Si(111) substrates are used to grow GaN based heterostructures by metalorganic vapour phase epitaxy, with the goal of fabricating well controlled, defect reduced GaN-based nanoLEDs. In contrast to other approaches to achieve GaN nanorods, we employed silicon substrates with deep etched nanopillars to control the GaN nanorods growth by varying the size and distance of the Si pillars. The small footprint of GaN nanorods grown on Si pillars minimise the influence of the lattice mismatched substrate and improve the material quality. For the Si pillars an inductively coupled plasma dry-etching process at cryogenic temperature has been developed. An InGaN/GaN multi quantum well (MQW) structure has been incorporated into the GaN nanorods. We found GaN nanostructures grown on top of the silicon pillars with a pyramidal shape. This shape results from a competitive growth on different facets as well as from surface diffusion of the growth species. Spatially resolved optical properties of the structures are analysed by cathodoluminescence. Strongly spatial-dependent MQW emission spectra indicate the growth rate differences on top of the rods.

We e present a combined experimental and theoretical analysis of the transport properties of silicon nanowire (NW) transistors. The NWs are grown by catalytic chemical vapour deposition and are later deposited on pre-patterned oxidized silicon substrates that provide the device source and drain electrodes. A back gate configuration is used for our study. Through a controlled nickel diffusion, parts of the nominally undoped NWs are turned into nickel silicide NWs. thus providing a direct metallic nanolead to the semiconducting wire. The transistors obtained with NWs of 10-30 nm diameters display p-type behaviour, current densities up to 0.8 MA/cm2 and on/off current ratios of up to 107. The subthreshold characteristics show a strong hysteresis. The simulation based on a drift-diffusion approach indicates that traps at the interface between the NWs and SiO2 are responsible for such behaviour. (8 References).

Silicon to nickel disilicide axial nanowire (NW) heterostructures have been fabricated and investigated extensively. To this end, intrinsic Si-NWs were grown by chemical vapor deposition using Au as the catalyst. The Si-NWs were contacted with Ni reservoirs so that upon annealing Ni diffused axially into the NWs. Single-crystalline NiSi 2 NW segments were formed at the diffusion path of Ni as proven by high-resolution transmission electron microscopy images. Further, the axial NiSi 2 to Si interfaces showed a sharpness of a couple of nanometers. Fully silicided NiSi 2 -NWs had maximal resistivities of 98 μΩ cm and conducted current densities of up to 205 MA/cm 2 before breakdown. Controlled silicidation from both NW ends gave NiSi 2 /Si/NiSi 2 axial NW heterostructures, which were implemented to fabricate Schottky contact field effect transistors (FET). The n ++ -substrate was used as a common back gate and the Si to NiSi 2 interfaces formed the Schottky source- and drain-(S/D) contacts to the active region. These Si-NW SB-FETs exhibited p-type behavior, and current densities in the on state of up to 0.8 MA/cm 2 for 1 V bias, the drain current could be modulated over a range of 10 7 . Moreover, the use of thin gate dielectrics enabled inverse subthreshold slopes as low as 110 mV/dec. These data show an efficient gate control over the devices by only using a back gate, due to an enhanced gate field coupling to the tip-like S/D-Schottky contacts.

We investigate the influence of modified growth conditions during the spontaneous formation of GaN nanowires on Si(111) in plasma-assisted molecular beam epitaxy. We find that a two-step growth approach, where the substrate temperature is increased during the nucleation stage, is an efficient method to gain control over the area coverage, average diameter, and coalescence degree of GaN nanowire ensembles. Furthermore, we also demonstrate that the growth conditions employed during the incubation time that precedes nanowire nucleation do not influence the properties of the final nanowire ensemble. Therefore, when growing GaN nanowires at elevated temperatures or with low Ga/N ratios, the total growth time can be reduced significantly by using more favorable growth conditions for nanowire nucleation during the incubation time.

Scaling graphene down to nanoribbons is a promising route for the implementation of this material into devices. Quantum confinement of charge carriers in such nanostructures, combined with the electric field-induced break of symmetry in AB-stacked bilayer graphene, leads to a band gap wider than that obtained solely by this symmetry breaking. Consequently, the possibility of fabricating AB-stacked bilayer graphene nanoribbons with high precision is very attractive for the purposes of applied and basic science. Here we show a method, which includes a straightforward air annealing, for the preparation of quasi-free-standing AB-bilayer nanoribbons with different widths on SiC(0001). Furthermore, the experiments reveal that the degree of disorder at the edges increases with the width, indicating that the narrower nanoribbons are more ordered in their edge termination. In general, the reported approach is a viable route towards the large-scale fabrication of bilayer graphene nanostructures with tailored dimensions and properties for specific applications., Bilayer graphene nanoribbons are very promising for future nanoelectronics. Here Lopes et al. show a novel approach for the fabrication of quasi-free-standing bilayer graphene nanoribbons on SiC, based on the precise control of the layer-by-layer growth of graphene and a simple annealing step in air.

Here, we report on the electrical characterization of phase change memory cells containing a Ge3Sb2Te6 (GST) alloy grown in its crystalline form by Molecular Beam Epitaxy (MBE). It is found that the high temperature growth on the amorphous substrate results in a polycrystalline film exhibiting a rough surface with a grain size of approximately 80-150 nm. A detailed electrical characterization has been performed, including I-V characteristic curves, programming curves, set operation performance, crystallization activation at low temperature, and resistance drift, in order to determine the material related parameters. The results indicate very good alignment of the electrical parameters with the current state-of-the-art GST, deposited by physical vapor deposition. Such alignment enables a possible employment of the MBE deposition technique for chalcogenide materials in the phase change memory technology, thus leading to future studies of as-deposited crystalline chalcogenides as integrated in electrical vehicles.

GeTe is an end-point of the GeTe-Sb2Te3 quasibinary alloys often referred to as phase-change memory materials. The polycrystalline nature of the crystalline films used in devices and the concomitant presence of grain boundaries complicate detailed structural studies of the local structure. Recent progress in the epitaxial growth of phase-change materials offers unique possibilities for precise structural investigations. In this work, we report on results of x-ray absorption near-edge structure (XANES) studies of GeTe and Ge2Sb2Te5 epitaxial films grown on Si and InAs substrates with (100) and (111) orientations. The results show a strong dependence of the local structure on the substrate material and especially orientation and are discussed in conjunction with polycrystalline samples and ab-initio XANES simulations.

We investigated the origin of the high reverse leakage current in light emitting diodes (LEDs) based on (In,Ga)N/GaN nanowire (NW) ensembles grown by molecular beam epitaxy on Si substrates. To this end, capacitance deep level transient spectroscopy (DLTS) and temperature-dependent current-voltage (I-V) measurements were performed on a fully processed NW-LED. The DLTS measurements reveal the presence of two distinct electron traps with high concentrations in the depletion region of the p-i-n junction. These band gap states are located at energies of $570\pm20$ and $840\pm30$ meV below the conduction band minimum. The physical origin of these deep level states is discussed. The temperature-dependent I-V characteristics, acquired between 83 and 403 K, show that different conduction mechanisms cause the observed leakage current. On the basis of all these results, we developed a quantitative physical model for charge transport in the reverse bias regime. By taking into account the mutual interaction of variable range hopping and electron emission from Coulombic trap states, with the latter being described by phonon-assisted tunnelling and the Poole-Frenkel effect, we can model the experimental I-V curves in the entire range of temperatures with a consistent set of parameters. Our model should be applicable to planar GaN-based LEDs as well. Furthermore, possible approaches to decrease the leakage current in NW-LEDs are proposed., Comment: 10 pages, 9 figures

In this work we present a study of electronic and optoelectronic properties of InGaN/GaN nanowire (NW) light-emitting diodes (LEDs) with a multiscale parametric approach. InGaN alloy bandgap bowing parameters extracted from empirical tight-binding (ETB) calculations are used in continuous level 3D simulation models. Strain and transport calculations are performed and quantum k·p-based models yield transition energies. Calculated emission energies are compared with experimental data showing a qualitative agreement within the experimental uncertainties.

III-N nanowires (NWs) are an attractive alternative to conventional planar layers as the basis for light-emitting diodes (LEDs). In fact, the NW geometry enables the growth of (In,Ga)N/GaN heterostructures with high indium content and without extended defects regardless of the substrate. Despite these conceptual advantages, the NW-LEDs so far reported often exhibit higher leakage currents and higher turn-on voltages than the planar LEDs. In this work, we investigate the mechanisms responsible for the unusually high leakage currents in (In,Ga)N/GaN LEDs based on self-induced NW ensembles grown by molecular beam epitaxy on Si substrates. The temperature-dependent current-voltage (I-V) characteristics, acquired between 83 and 403 K, reveal that temperatures higher than 240 K may activate a further conduction process, which is not present in the low temperature range. Deep level transient spectroscopy (DLTS) measurements show the presence of electron traps, which are activated in the same temperature interval. A detailed analysis of the DLTS signal reveals the presence of two distinct deep levels with apparent activation energies close to Ec-570 meV and Ec-840 meV, and capture cross sections of about 1.0x10-15 cm2 and 2x10-14 cm2, respectively. These results suggest that the leakage process might be related to trap-assisted tunneling, possibly produced by point defects located in the core and/or on the sidewalls of the NWs.

Schottky barrier field effect transistors based on individual catalytically-grown and undoped Si-nanowires (NW) have been fabricated and characterized with respect to their gate lengths. The gate length was shortened by the axial, self-aligned formation of nickel-silicide source and drain segments along the NW. The transistors with 10-30 nm NW diameters displayed p-type behaviour, sustained current densities of up to 0.5 MA/cm2, and exhibited on/off current ratios of up to 10(7). The on-currents were limited and kept constant by the Schottky contacts for gate lengths below 1 microm, and decreased exponentially for gate lengths exceeding 1 microm.

GaAs-based InGaAsN quantum well films grown by molecular beam epitaxy are characterized by photoluminescence spectroscopy. InGaAsN with high N concentrations of about 4% requires a low growth temperature of 350 °C and minimal As flux. No indications for 3D growth are detected and carrier localization is below 25 meV. Broad area lasers with InGaAsN single quantum wells of different compositions are compared. Threshold current density correlates to room temperature photoluminescence intensity. For an emission wavelength of 1400 nm a threshold current density of 690 A/cm 2 is achieved with 1200 μm long devices. At 1510 nm a value of 780 A/cm 2 sets a new record for GaAs-based lasers with any kind of active material.

Surface photovoltage in semiconductor single quantum well structures is studied. The surface photovoltage spectra of such structures contain the essential information to enable the determination of band offsets. To estimate the band offsets only one sample is necessary, which is an advantage over other methods. The cases of type I- and type II-band alignment are discussed separately. Two particular single quantum well samples are studied by this measurement method. GaAs/In0.65Ga0.35As0.983N0.017/GaAs and GaAs/Ga0.7As0.3Sb/GaAs which are of type I and type II, respectively. The values 79/21 (type I) and 12/88 (type II) are obtained for the conduction/valence-band ratio ΔEC/ΔEV in the InGaAsN and GaAsSb quantum well structures, respectively.

We have investigated the temperature and pressure dependence of the threshold current (I/sub th/) of 1.3 /spl mu/m emitting GaInNAs vertical-cavity surface-emitting lasers (VCSELs) and the equivalent edge-emitting laser (EEL) devices employing the same active region. Our measurements show that the VCSEL devices have the peak of the gain spectrum on the high-energy side of the cavity mode energy and hence operate over a wide temperature range. They show particularly promising I/sub th/ temperature insensitivity in the 250-350 K range. We have then used a theoretical model based on a 10-band k.P Hamiltonian and experimentally determined recombination coefficients from EELs to calculate the pressure and temperature dependency of I/sub th/. The results show good agreement between the model and the experimental data, supporting both the validity of the model and the recombination rate parameters. We also show that for both device types, the super-exponential temperature dependency of I/sub th/ at 350 K and above is due largely to Auger recombination.

We present experimental investigation of the optical gain in GaInNAs/GaAs multi-quantum well structures using the variable stripe length method. The amplified spontaneous edge emission was measured at different excitation intensities and stripe lengths. The obtained optical gain values at a temperature of 5 K are typically in the order of 100–150 cm−1, while at room temperature a maximum of 50 cm−1 could be observed. Optically pumped lasing has been detected from cleaved samples at room temperature for excitation densities above 300 kW/cm2.

By measuring the spontaneous emission (SE) from metal organic vapour phase epitaxy (MOVPE) grown ∼1.3 μm GaInNAs/GaAs-based lasers during normal operation, we have quantitatively determined the variation of each of the current paths present in the devices as a function of temperature from 130 to 370 K and compared these with results previously obtained for molecular beam epitaxy (MBE) grown GaInNAs lasers. From the SE measurements we determine how the current, I, close to threshold, varies as a function of carrier density, n, which enables us to separate out the main current paths corresponding to monomolecular (defect-related), radiative or Auger recombination respectively. We find that at room temperature (RT), defect-related recombination contributes ∼360 A/cm2 (MBE) and ∼565 A/cm2 (MOVPE) to the total current density at threshold. Radiative recombination accounts for ∼110 A/cm2 (MBE) and 195 A/cm2 (MOVPE) of Jth with the remaining ∼180 A/cm2 (MBE) and 760 A/cm2 (MOVPE) are due to non-radiative Auger recombination. Our results suggest that a larger threshold carrier density in the MOVPE grown device in comparison to the MBE lasers, can reasonably explain the larger current densities of the different recombination processes at RT. We tentatively associate this with higher optical loss processes in the MOVPE grown material.

A single phase InN epitaxial film is grown on a bulk In2O3(111) wafer by plasma-assisted molecular beam epitaxy. The InN/In2O3 orientation relationship is found to be (0001) ‖ (111) and [11¯00] ‖ [112¯]. High quality of the layer is confirmed by the small widths of the x-ray rocking curves, the sharp interfaces revealed by transmission electron microscopy, the narrow spectral width of the Raman E2h vibrational mode, and the position of the photoluminescence band close to the fundamental band gap of InN.

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.

The ultrafast emission dynamics of a 1.3-/spl mu/m (GaIn)(NAs)/GaAs vertical-cavity surface-emitting laser is studied by femtosecond luminescence upconversion. We obtain a minimum peak delay of 15.5 ps and a minimum pulse width of 10.5 ps. Laser operation with picosecond emission dynamics is demonstrated over a temperature range from 30 to 388 K. The bandgap shift with temperature of (GaIn)(NAs)/GaAs is determined to be about -2.9/spl middot/10/sup -4/ eV/K, which is smaller than for GaAs. Our measurements of the optical gain provide gain spectra similar to those of commercial (GaIn)(PAs)/InP-structures at moderate densities but broaden considerably for elevated carrier densities due to the stronger carrier confinement. We compare our experimental results with gain spectra calculated from a microscopic model and confirm the predictive capability of the model. The theoretical gain spectra are used as the input for a calculation of the temperature dependence of the (GaIn)(NAs)/GaAs surface-emitter emission which results in very good agreement with experiment.

Efficient infrared light emitters integrated on the mature Si technology platform could lead to on-chip optical interconnects as deemed necessary for future generations of ultrafast processors as well as to nanoanalytical functionality. Toward this goal, we demonstrate the use of GaAs-based nanowires as building blocks for the emission of light with micrometer wavelength that are monolithically integrated on Si substrates. Free-standing (In,Ga)As/GaAs coaxial multishell nanowires were grown catalyst-free on Si(111) by molecular beam epitaxy. The emission properties of single radial quantum wells were studied by cathodoluminescence spectroscopy and correlated with the growth kinetics. Controlling the surface diffusivity of In adatoms along the NW side-walls, we improved the spatial homogeneity of the chemical composition along the nanowire axis and thus obtained a narrow emission spectrum. Finally, we fabricated a light-emitting diode consisting of approximately 10(5) nanowires contacted in parallel through the Si substrate. Room-temperature electroluminescence at 985 nm was demonstrated, proving the great potential of this technology.

We used spatially resolved photoluminescence (PL) and resonant Raman spectroscopy to study the electronic structure of single GaAs nanowires (NWs) consisting of zinc-blende (ZB) and wurtzite (WZ) segments. For narrow ZB segments and stacking faults the energy range of the observed PL peak positions is found to deviate from that of the maxima in resonance Raman profiles. These different energy ranges reflect the fact that the PL recombination is dominated by spatially indirect transitions whereas the resonance enhancement of Raman scattering is caused by direct transitions. Our results provide evidence for the type II band alignment between ZB and WZ GaAs and a coherent picture of all near-band-gap transition energies in GaAs NWs.

Grazing incidence X-ray diffraction (GID) was employed to probe the structure of atomically thin carbon layers on SiC(0001): a so-called buffer layer (BL) with a $6(\sqrt{3}\times\sqrt{3})$R30$^\circ$ periodicity, a monolayer graphene (MLG) on top of the BL, and a bilayer graphene (BLG). The GID analysis was complemented by Raman spectroscopy. The lattice parameter of each layer was measured with high precision by GID. The BL possesses a different lattice parameter and corrugation when it is uncovered or beneath MLG. Our results demonstrate that the interfacial BL is the main responsible for the strain in MLG. By promoting its decoupling from the substrate via intercalation, it turns into graphene, leading to a simultaneous relaxation of the MLG and formation of a quasi-free-standing BLG., Comment: Accepted at Phys. Rev. B Rapid Communications. Supplementary material included

AlN layers with thicknesses between 2 and 14 nm were grown on Si(111) substrates by molecular beam epitaxy. The effect of the AlN layer thickness on the morphology and nucleation time of spontaneously formed GaN nanowires (NWs) was investigated by scanning electron microscopy and line-of-sight quadrupole mass spectrometry, respectively. We observed that the alignment of the NWs grown on these layers improves with increasing layer thickness while their nucleation time decreases. Our results show that 4 nm is the smallest thickness of the AlN layer that allows the growth of well-aligned NWs with short nucleation time. Such an AlN buffer layer was successfully employed, together with a patterned SiOx mask, for the selective-area growth (SAG) of vertical GaN NWs. In addition, we fabricated light-emitting diodes (LEDs) from NW ensembles that were grown by means of self-organization phenomena on bare and on AlN-buffered Si substrates. A careful characterization of the optoelectronic properties of the two devices showed that the performance of NW-LEDs on bare and AlN-buffered Si is similar. Electrical conduction across the AlN buffer is facilitated by a high number of grain boundaries that were revealed by transmission electron microscopy. These results demonstrate that grainy AlN buffer layers on Si are compatible both with the SAG of GaN NWs and LED operation. Therefore, this study is a first step towards the fabrication of LEDs on Si substrates based on homogeneous NW ensembles., Comment: 12 pages, 6 figures

Electron holography in a field emission gun transmission electron microscope has been used to profile the inner potential V0 across GaN/x nm In0.1Ga0.9N/GaN/(0 0 0 1) sapphire samples (x=10, 40 nm) grown by molecular beam epitaxy and viewed in cross-section. Results are presented which suggest a decrease in V0 of 3–4 V across the InGaN layer in the [0 0 0 1] direction. It is proposed that the results can be explained by charge accumulation across the InGaN layer and that the opposing contributions due to piezoelectric and polarisation fields are effectively masked by Fermi level pinning.

Focus of this work is the optimization of growth to achieve high quality laser material for emission at 1.3 μm and beyond. GaAs/GaAsN/InGaAsN heterostructures were grown by solid source molecular beam epitaxy. To achieve optimum crystal quality of InGaAsN heterostructures, growth was followed by a high temperature treatment at about 700°C. The high optical quality of our annealed material is attested by large exciton recombination lifetimes (more than 2 ns). Consequently, a decrease of single quantum well transparency current density down to 100 A/cm 2 is found and SWQ lasers with threshold current densities as low as 350 A/cm 2 have been made. This represents clearly the lowest laser thresholds reported so far for emission around 1.3 μm from the InGaAsN material system.

Self-assembled InAs quantum dots (QDs) are fabricated in In 0.03 Ga 0.97 As 0.99 N 0.01 and In 0.06 Ga 0.94 As 0.98 N 0.02 matrices on GaAs by solid source molecular beam epitaxy. The influence of InAs average layer thickness and matrix material on photoluminescence properties are studied. We observe a photoluminescence peak wavelength up to 1.49 μm from structures with a nominal InAs thickness of four monolayers (ML). For QD structures emitting at 1.3 μm, no saturation of ground state luminescence and no excited state photoluminescence are detected. This should lead to an improved performance of 1.3 μm quantum dot lasers on GaAs.

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.

We comprehensively studied InGaN/GaN heterostructures grown by molecular beam epitaxy (MBE) and metal-organic vapor deposition epitaxy (MOCVD) using a variety of methods of optical spectroscopy, such as cathodoluminescence microscopy (CL), time-integrated and time-resolved photoluminescence (PL). Micro-photoluminescence and cathodoluminescence results show the variation in emission wavelength at different scales, and this reflects the degree of compositional fluctuations in the samples. We obtain information on the decay times of the main emission lines using time-resolved photoluminescence spectroscopy and models of stretched exponentials, indicating the importance of nanoscale fluctuations for the recombination mechanism. The temperature dependent behavoir of the InGaN emission is explained in terms of a carrier freeze out at local potential fluctuations combined with a thermionic thermalization at elevated temperatures. To correlate the fluctuations in emission wavelength with values for the optical amplification we performed gain measurements in edge-stripe geometry. From all these results we conclude, that localized carriers at a statistical distribution of potential fluctuations act as recombination centers and that the degree of fluctuations determines the efficiency of optical amplification. The threshold values for lasing and the gain values are compared and discussed with respect to the different growth procedures. From all these findings we draw conclusions concerning the influence of differences in the growth conditions and their impact on the optical properties.