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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.

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.

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 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.

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.

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

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.

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)

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 study the structure of epitaxial Gd2O3 layers grown on Si(0 0 1) using reflection high-energy electron diffraction (RHEED) and grazing incidence X-ray diffraction (GIXRD). When the oxygen partial pressure is not sufficiently high, GIXRD reveals a silicide phase at the interface. The use of higher oxygen pressure results in a rapid decrease in the crystallinity of the Gd2O3 layers and they become amorphous. Increasing the substrate temperature from 650 to 700 °C improves the epitaxy with larger domains and smoother surfaces. Also no silicide phase is detected by GIXRD. Therefore, the control of oxygen pressure and substrate temperature is very important to obtain high-quality epitaxial Gd2O3 layers without silicide at the interface.

Well-aligned single crystalline GaN nanowires were grown on Al2O3 (0001) by molecular beam epitaxy. An initial 0.3 nm thick Ni layer was employed as catalyst. Atomic-scale electron microscopy observations revealed the epitaxial growth and the crystal quality of the GaN nanowires with basal stacking faults being the only structural defect. Single crystalline droplets on the tip of the nanowires were identified as a Ni-based cubic oxide with a 0.208 nm {100} lattice spacing and were oriented along [001]/[0001], (010)/(100) with respect to GaN. Crystal fitting of the two mismatched lattices is realized by a dense network of misfit dislocations at the interface, so that the oxide lattice is relaxed close to its strain-free value. Analytical methods revealed the positions of the misfit dislocation cores, showing a curved and slightly rough oxide/GaN interface. The observations are consistent with the vapour-liquid-solid growth mechanism. Controlled radial growth and thickening of the nanowires was achieved by varying the growth regime. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Epitaxially grown ternary and quaternary layers of GaAs based dilute nitrides have been exposed to H+ and D+ ion bombardment to investigate the interaction between hydrogen (deuterium) and nitrogen. Low-temperature Fourier transform infrared absorption measurements reveal the formation of N–H (N–D) complexes by the appearance of high-frequency absorption bands in the mid-infrared due to vibrational modes of H(D). In thin layers, the transformation of isolated substi- tutional nitrogen, NAs, into complexes involving H (D) is complete, in thick (500–1000 nm) layers only partial. Analyses of the band intensities dependent on variations of the sample structure and parameters during ion irradiation challenge the model of a unique defect associated with the observed vibrational modes. A new model is proposed based on two different N–H (N–D) centers, each involving one H (D) atom. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We report on transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) analysis 0 of GaN and Al 0.8 Ga 0.8 N nanowires (NWs) grown by radio frequency plasma assisted Molecular Beam Epitaxy (PAMBE) on c-sapphire substrates. Conventional TEM images of single nanowires removed from the substrate exhibit a high density of basal stacking faults. Further analyses were performed using the aberration-corrected SuperSTEM facility. Geometric phase analysis (GPA) was applied to this data revealing the characteristic lattice displacement introduced by intrinsic basal stacking faults. The nickel-based "seeds", 0 which were employed to promote the NW-type growth, are found to be situated at the tip of each nanowire. Different types of analyses were applied to lattice images of these seeds including GPA, projection method analysis and Fast Fourier transform of the HREM images. All three approaches yielded f lattice constants and displacements with good agreement. The measured lattice values in the GaN nanowire body were in good agreement with the lattice constants of relaxed bulk GaN. For the seed particles the resulting lattice spacing was attributable, within the experimental error, to either nickel oxide, Ni 3 Ga, or an intermediate alloy phase. The defect density was measured in the AlGaN nanowires and was found to be in the order of 106 cm -1 l. The results are critically discussed in S; relation to the possible growth mechanisms of the NWs.

Scanning transmission electron microscopy has been used to investigate the growth mechanism of gallium nitride nanowires grown by plasma-assisted molecular beam epitaxy. The composition of nickel seeds used to promote nanowire-type growth has been studied. These seeds remain at the end of the nanowires after growth as Ni-based metallic crystallites. The concentration of gallium, nickel, oxygen and nitrogen in the seeds were quantified by analysing electron energy-loss spectra. The seed crystallites at the ends of the GaN nanowires are found to comprise of a metallic core of mainly nickel and gallium, with a surrounding oxide shell consisting of Ni, Ga, and O, attributed to the post-growth atmospheric exposure of the nanowires. Only a background concentration of nitrogen was found in the metallic nickel seed cores.

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.

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.

We have studied the pressure and temperature dependence of the threshold current density, J th , in 1.3 μm and 1.5 μm GaInNAs edge-emitting lasers. We find that J th is more temperature sensitive for the 1.5 μin devices. From analysis of the spontaneous emission from these devices we find that J th for the 1.3 μm devices is comprised of 50% non-radiative monomolecular recombination, 25% radiative recombination and 25% non-radiative Auger recombination at room temperature. In the 1.5 μm devices however it is composed of 30% monomolecular recombination, 10% radiative recombination and 60% Auger recombination. It is clear that the major difference in J th between the devices is caused by the increased Auger recombination in the 1.5 μm devices. Previously, we found that J th for 1.3 μm GaInNAs lasers increases with pressure due to the increase in Auger recombination brought about by a large increase in the cubed threshold carrier density (n 3 th ). However, for the 1.5 μm devices, we find no significant change in J th with pressures up to 8 kbar. We suggest that this is due to the higher Auger coefficient for the 1.5 μm devices, which compensates for the increase in n 3 th with pressure.

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.

The authors have investigated the adsorption and subsequent desorption of Ga on AlN (0001) with line-of-sight quadrupole mass spectrometry (QMS). The authors present desorption data consistent with a continuous Ga-flux dependent accumulation of a laterally contracted Ga bilayer on AlN (0001) from 0 to 2.7±0.3 ML GaN equivalent coverage, and further Ga accumulation in macroscopic Ga droplets. The temperature dependence of Ga-adsorbate QMS desorption transients was investigated and the authors determined that the desorption activation energies for individual monolayers of the Ga adsorbate on AlN (0001) were similar to Ga desorption from GaN (0001). For the (first) pseudomorphic Ga-adsorbate monolayer on AlN, the authors measured a maximum Ga coverage of 1.0±0.1 ML and desorption activation energy of 6.2±0.3eV. For the (second) laterally contracted Ga monolayer (1.7±0.3 ML) the desorption activation energy was 3.8±0.1eV.

The band offsets of InGaAsN single quantum well samples with different indium and nitrogen concentrations have been determined by surface photovoltage measurements. With varying indium content in the quantum well, both the conduction and the valence band states are modified. On the other hand, nitrogen content variations affect only the conduction band states and leave the valence band states basically unchanged. In particular, the value of the conduction band offset ratio increases with increasing nitrogen content and decreases with increasing indium concentration. This effect shows the possibility to design structures with a fixed band gap but varied confinement of electrons and holes by changing the In/N ratio.

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.

The boundary conditions were investigated for the electron wavefunction in Ga1−xInxNyAs1−y-based heterostructures described by the band anticrossing model. Among other advantages, the utilisation of these boundary conditions simplifies the calculation of, for example, transition energies in quantum wells. The derived equations were applied to model the temperature-dependence of the effective bandgap in Ga1−xInxNyAs1−y/GaAs quantum well structures with high indium concentration. From a fit to the experimental photoreflectance data, evidence was found to show that the nitrogen level EN in the band anticrossing Hamiltonian, measured with respect to the valence band edge, shifts to higher energies with decreasing temperature. This extends similar results reported in the literature for low indium content epilayers.

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.