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Sb2Te3 films are used for studying the epitaxial registry between two-dimensionally bonded (2D) materials and three-dimensional bonded (3D) substrates. In contrast to the growth of 3D materials, it is found that the formation of coincidence lattices between Sb2Te3 and Si(111) depends on the geometry and dangling bonds of the reconstructed substrate surface. Furthermore, we show that the epitaxial registry can be influenced by controlling the Si(111) surface reconstruction and confirm the results for ultrathin films.

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.

GaN nanowires form spontaneously on a wide variety of substrates without suffering from extended defects. However, their quasi-one-dimensional nature causes these structures to have an extended free surface, resulting in a surface-to-volume ratio orders of magnitude larger than that of a planar layer. Additionally, the high nucleation density of spontaneously formed GaN nanowire ensembles results in an unintentional, but inevitable coalescence between individual nanowires. In this work, we investigate the impact of both the surface and the coalescence of nanowires on the recombination dynamics of excitons in GaN nanowire ensembles. Using simple models to simulate the change in recombination dynamics of bound excitons in GaN NWs with varying diameter and coalescence degree, we show that the comparatively short decay times at low temperatures are not generally caused by either of these mechanisms. Furthermore, we demonstrate that the biexponential decay for the donor-bound exciton is also not related to a coexistence of nonradiative and radiative recombination channels, but originates from a coupling of the donor- and acceptor-bound exciton states in the GaN NWs.

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.

As phase-change materials such as Ge2Sb2Te5 (GST) undergo dramatic changes in material properties due to a fundamentally local change in bonding behavior, the ultimate speed limits of the atomic switching process offer hope both for ultrafast switching as well as non-adiabatic energy efficient routes for memory applications [1, 2]. To this end, we have carried out experiments using the freeelectron laser SACLA to explore sub-picosecond lattice dynamics in epitaxial GST (111) in response to 30 fs optical pump excitation. The SACLA x-ray beam was used to probe the lattice dynamics using a symmetric diffraction geometry.

A narrow growth window combined with highly temperature dependent compositional variations poses a serious problem for the growth of epitaxial GeTe–Sb2Te3 (GST) thin films. The problems are further aggravated by the weak coupling of the radiatively heated non-contact thermocouples to the substrate. An increase in surface temperature during growth as inferred from the increase in desorption of GeTe heteromolecules and the resulting change in alloy composition are studied. Using the desorption signal as a feedback to control the surface temperature, the thermocouple temperature was varied over the duration of the growth to maintain a constant desorption and hence constant surface temperature. Interestingly, the composition of the grown films varies along the GeTe–Sb2Te3 pseudobinary line just by varying the desorption without changing the supplied flux. The out-of-plane lattice constant of the epitaxially grown GST thin film increases with an increase in Ge concentration.

The desire to go beyond the current limits of phase-change memory (PCM) maximum switching speed and energy consumption has lead to increased interest to the investigation of the ultra-fast processes in PCM materials [1]. We present the results of sub-picosecond resolution time-resolved x-ray diffraction studies of the laser-induced structural dynamics in Ge2Sb2Te5 as a step towards the non-thermal switching in PCM.

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

InGaAsN layers grown by molecular beam epitaxy were investigated using Fourier transform infrared absorption spectroscopy. The nitrogen-related local mode at 471 cm −1 is used as a quantitative tool to assess the substitutional nitrogen fraction. Evidence is presented that nitrogen in the quaternary alloy is bonded only to gallium atoms as in GaAsN. No change in the local environment of nitrogen is observed after annealing. From nuclear reaction analysis of GaAsN under channeling conditions it is confirmed that the major fraction of nitrogen atoms (>95%) is localized on string positions.

We propose a method that solves the problem of the independent determination of the indium and nitrogen concentrations in a strained quaternary InGaAsN superlattice. The method is experimentally based on the simultaneous measurement: (i) of the tetragonal lattice distortion of the unit cell from high resolution micrographs and (ii) of the intensity of the chemically sensitive (002) reflection from dark field images. As an example, we evaluate InGaAsN quantum wells with a nominal N concentration of 1.7% and with In concentrations of 10%, 20%, or 35%. We reveal local fluctuations of the In and N concentrations over distances down to 4 nm with a sensitivity of 0.1% for N and 1% for In fluctuations in this distance range.

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.

The nitrogen-related infrared absorption band at 471 cm−1 is found in the ternary and quaternary alloys GaAs1−xNx and InyGa1−yAs1−xNx, respectively, grown by solid-source molecular beam epitaxy. Implantation experiments with the isotopes 14N and 15N show that the band is due to a local vibrational mode of isolated nitrogen on the arsenic site. The strength of the band in GaAs1−xNx layers correlates for x

GaN nanorods (NRs) show promising applications in high-efficiency light emitting diodes, monolithic white light emission and optical interconnection due to their superior properties. In this work, we performed GaN nanostructures growth by pre-patterning the Si and AlN/Si substrates. The pattern was transferred to Si and AlN/Si substrates by photolithography and inductively-coupled plasma etching. GaN NRs were grown on these templates by metal-organic vapour phase epitaxy (MOVPE). GaN grown on Si pillar templates show a truncated pyramidal structure. Transmission electron microscopy measurements demonstrated clearly that the threading dislocations bend to the side facets of the GaN nanostructures and terminate. GaN growth can also be observed on the sidewalls and bottom surface between the Si pillars. A simple phenomenological model is proposed to explain the GaN nanostructure growth on Si pillar templates. Based on this model, we developed another growth method, by which we grow GaN rod structures on pre-patterned AlN/Si templates. By in-situ nitridation and decreasing of the V/III ratio, we found that GaN rods only grew on the patterned AlN/Si dots with an aspect ratio of about 1.5 - 2. (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

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.

The characteristic surface morphologies of GaN grown by plasma-assisted molecular beam epitaxy under various growth conditions have been investigated. Three growth regimes (one N stable and two Ga stable) are identified on a surface structure diagram (Ga/N ratio versus substrate temperature). The boundary between the N-stable regime (low Ga/N ratios) and the two Ga-stable regimes (high Ga/N ratios) is determined by the growth rate of the films and is constant over the range of substrate temperatures investigated. The boundary between the two Ga-stable regimes (the Ga-droplet regime and the intermediate regime) is determined by the formation of Ga droplets and has an Arrhenius dependence with substrate temperature. The characteristic morphologies of films grown within each of these regimes are investigated using atomic force microscopy and transmission electron microscopy. N-stable films have rough, heavily pitted morphologies. Films grown within the intermediate phase have areas of flat surface between la...

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.

The two main topics with respect to better gallium nitride (GaN)-based optoelectronic device performance are the improvement of crystal quality and of light extraction. Concerning the first topic, the epitaxial growth of GaN-related materials has to be improved. Since native substrates with good quality are still expensive, foreign substrates like sapphire or silicon which are much cheaper are used, but they introduce a lattice as well as a thermal mismatch leading to detrimental crystal defects. Several approaches to reduce the defect density exist, but in most cases they are coupled with a much higher technological effort. We present an alternative solution to decrease the defect density by the growth of GaN nanostructures, where the small footprint and high aspect ratio leads to a much lower influence of the substrate. Metal-organic vapour-phase epitaxy of GaN nanostructures and InGaN/GaN multi-quantum wells was carried out in a vertical reactor with close-coupled showerhead. Studies on the morphologic properties have been carried out by scanning electron microscopy and energy-dispersive X-ray spectroscopy to reveal the influence of growth parameters and material composition. Cathodoluminescence measurements show the good optical properties of the GaN as well as of the InGaN/GaN structures.

We report on a study of the growth of ternary AlGaN and InGaN by molecular beam epitaxy, leading to a quantitative model describing the alloy composition and growth rate as a function of group III fluxes, N flux and growth temperature. For low growth temperatures, the composition is exclusively determined by the different bond strengths between the group III elements and N, leading to a complete displacement of the more weakly bound species. The In loss from InGaN observed for typical growth conditions is caused by thermal decomposition of the growing layer with an activation energy between 3.5 and 3.8 eV depending on the In content.

The Ga deposition, reduction, and re-evaporation technique commonly used to produce clean n-GaN surfaces and Ag–GaN interface formation on the resultant surface, have been investigated by Soft X-ray Photoelectron Spectroscopy (SXPS) and current–voltage measurements. SXPS studies have indicated that Ga deposition produces a band-bending of ΔEk = + 1.0 eV to higher kinetic energy. Our results show this shift to be a partially reversible process: re-evaporation of the deposited Ga resulted in a Fermi shift of ΔEk = — 0.6 eV to lower energy. Ag deposition did not cause any further Fermi shift, indicating that the Fermi level is pinned (2.2 ± 0.2) eV above the valence band edge, possibly as a consequence of the cleaning procedure itself. Current voltage (I–V) measurements have shown a barrier height of 0.77 eV and an ideality factor of 1.6. Metal induced gap states and the unified defect model are discussed as possible barrier formation mechanisms.

X-ray standing-waves (XSW) are used for an investigation of the structure of (AlAs){sub m}(GaAs){sub n} short-period superlattices (SL{close_quote}s). The XSW induced modulation of x-ray fluorescence from the Al, As, and Ga atoms and the total photoelectron yield are monitored around the 0th order SL satellite (AlAs)(GaAs)(004,0) and the GaAs(004) substrate Bragg reflection. From the specific shape of these modulations and the sample reflectivity, an atomic model about the interfaces is derived. This is accomplished by comparing the experimental data with dynamical calculations of x-ray wavefield distribution and reflectivity, which are based on the Takagi-Taupin equation. The fluorescence measurements at the 0th order SL satellite reveal a high crystalline order in the AlAs layers of the short-period SL, whereas in the GaAs layers, a fraction of the Ga and As atoms is not on the ideal lattice positions. From the analysis, a model of the atomic distribution along the [001] direction can be determined. This reveals that at each internal interface in the GaAs layers, two Ga atom planes are shifted by up to 0.035 nm and one As atom plane by 0.023 nm. At each interface, the shifts are directed towards the substrate. In addition, the XSW field at themore » GaAs(004) substrate reflection results in a moir{acute e} or beating effect in the SL structure, which can be used to determine the information depth {Lambda}{sub e} of total electron-yield measurements in a more detailed approach. {copyright} {ital 1999} {ital The American Physical Society}« less

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

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

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

We have comprehensively investigated low temperature Raman and photoluminescence (PL) spectra of gas source MBE-grown GaN layers on sapphire. Strong and sharp Raman peaks are observed in the low frequency region which are enhanced by resonant excitation of yellow luminescence transitions. They are attributed to the electronic transitions related to shallow donor levels in hexagonal GaN. It is proposed that a low frequency Raman peak at 94 cm−1 may be caused by pseudo-local vibration modes related to defects involved in yellow luminescence transitions. The PL spectra are measured in dependence on temperature and excitation intensity. The correlation with residual impurities in GaN is discussed.

We investigate the influence of carrier relaxation on the emission dynamics of a semiconductor microcavity laser. The structure is optically excited with energies of 1.477 down to 1.346 eV (resonant excitation). The stimulated emission dynamics clearly becomes faster for decreasing excitation energy and the influence of the light hole on the emission dynamics is demonstrated. Theoretical calculations reproduce the results only if the nonequilibrium carrier dynamics is treated on the basis of a microscopic model.

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

The crystalline quality of molecular beam epitaxy (MBE)-grown layers of GaN on sapphire strongly depends on the initial stage of film nucleation and growth. Thus, pre-conditioning of the substrate is of vital importance. In this study we use X-ray photoelectron spectroscopy (XPS) and low energy electron diffraction (LEED) to examine in situ the case for surface cleaning and nitridation of c -plane sapphire substrates upon annealing in UHV and exposure to radio frequency (rf) plasmas of hydrogen and nitrogen, respectively. We find that low temperature (200–300 °C) heat treatment in a hydrogen plasma offers effective removal of adventitious surface contaminants, contrary to annealing in vacuum. Moreover, our XPS data provide unambiguous evidence for formation of surface nitride upon heat treatment in nitrogen plasma at 300–700 °C, in agreement with conclusions inferred from reflection high energy electron diffraction (RHEED). The surface nitride is found to remain stable on subsequent exposure to atmosphere.

We present a novel approach for the growth of GaAs nanowires (NWs) with controllable number density and diameter, which consists of the combination between droplet epitaxy (DE) and self-assisted NW growth. In our method, GaAs islands are initially formed on Si(111) by DE and, subsequently, GaAs NWs are selectively grown on their top facet, which acts as a nucleation site. By DE, we can successfully tailor the number density and diameter of the template of initial GaAs islands and the same degree of control is transferred to the final GaAs NWs. We show how, by a suitable choice of V/III flux ratio, a single NW can be accommodated on top of each GaAs base island. By transmission electron microscopy, as well as cathodo-and photoluminescence spectroscopy, we confirmed the high structural and optical quality of GaAs NWs grown by our method. We believe that this combined approach can be more generally applied to the fabrication of different homo-or heteroepitaxial NWs, nucleated on the top of predefined islands obtained by DE. © 2013 American Chemical Society.

Nuclear spin hyperpolarization is essential to future solid-state quantum computation using nuclear spin qubits and in highly sensitive magnetic resonance imaging. Though efficient dynamic nuclear polarization in semiconductors has been demonstrated at low temperatures for decades, its realization at room temperature is largely lacking. Here we demonstrate that a combined effect of efficient spin-dependent recombination and hyperfine coupling can facilitate strong dynamic nuclear polarization of a defect atom in a semiconductor at room temperature. We provide direct evidence that a sizeable nuclear field (~150 Gauss) and nuclear spin polarization (~15%) sensed by conduction electrons in GaNAs originates from dynamic nuclear polarization of a Ga interstitial defect. We further show that the dynamic nuclear polarization process is remarkably fast and is completed in

We investigate modulation‐doped InxGa1−xAs/AlyGa1−yAs quantum wells grown by molecular beam epitaxy with respect to carrier mobility and its dependence on In content, In distribution, populations of electron subbands, and local positions of electron wave functions. We find that the room‐temperature electron mobilities are dominated by the In contents at the maxima of the electron wave functions rather than by the average In contents. At 77 K the mobilities are most strongly influenced by the distance between doping layers and the maxima of the electron wave functions. As a practical result of this study, we present a quantum well structure for high electron mobility transistors with a carrier mobility as high as 8100 cm2/V s at 295 K for an electron density of 2.5×1012 cm−2.

Experimental results on the linewidth enhancement factor α of 1.3 μm (GaIn)(NAs) lasers are presented and analyzed on the basis of a comparison with theoretical data obtained from a microscopic model. Our experimental data are obtained from the shift of the Fabry–Perot modes with injection current using an approach to eliminate temperature-dependent artifacts. At the emission wavelength at threshold we find a value of 2.5 for α which clamps for varying injection current.

We investigate designed InN/GaN superlattices (SLs) grown by plasma-assisted molecular beam epitaxy on c-plane GaN templates in situ by line-of-sight quadrupole mass spectroscopy and laser reflectivity, and ex situ by scanning transmission electron microscopy, X-ray diffraction, and photoluminescence (PL). The structural methods reveal concordantly the different interface abruptness of SLs resulting from growth processes with different parameters. Particularly crucial for the formation of abrupt interfaces is the Ga to N ratio that has to be bigger than 1 during the growth of the GaN barriers, as Ga-excess GaN growth aims at preventing the unintentional incorporation of In accumulated on the growth surface after the supply of InN, that extends the (In,Ga)N quantum well (QW) thickness. Essentially, even with GaN barriers grown under Ga-excess yielding to 1 monolayer (ML) thick QWs, there is a real discrepancy between the designed binary InN and the actual ternary (In,Ga)N ML thick QWs revealed by the above...

The efficacy of Ni as a surfactant to improve the crystalline quality of graphene grown directly on dielectric Al2O3(0001) substrates by molecular beam epitaxy is examined. Simultaneously exposing the substrate to a Ni flux throughout C deposition at 950 °C led to improved charge carrier mobility and a Raman spectrum indicating less structural disorder in the resulting nanocrystalline graphene film. X-ray photoelectron spectroscopy confirmed that no residual Ni could be detected in the film and showed a decrease in the intensity of the defect-related component of the C1s level. Similar improvements were not observed when a lower substrate temperature (850 °C) was used. A close examination of the Raman spectra suggests that Ni reduces the concentration of lattice vacancies in the film, possibly by catalytically assisting adatom incorporation.

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

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

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

Phase-change materials based on Ge-Sb-Te alloys are widely used in industrial applications such as nonvolatile memories, but reaction pathways for crystalline-to-amorphous phase-change on picosecond timescales remain unknown. Femtosecond laser excitation and an ultrashort x-ray probe is used to show the temporal separation of electronic and thermal effects in a long-lived ($>$100 ps) transient metastable state of Ge$_{2}$Sb$_{2}$Te$_{5}$ with muted interatomic interaction induced by a weakening of resonant bonding. Due to a specific electronic state, the lattice undergoes a reversible nondestructive modification over a nanoscale region, remaining cold for 4 ps. An independent time-resolved x-ray absorption fine structure experiment confirms the existence of an intermediate state with disordered bonds. This newly unveiled effect allows the utilization of non-thermal ultra-fast pathways enabling artificial manipulation of the switching process, ultimately leading to a redefined speed limit, and improved energy efficiency and reliability of phase-change memory technologies., 8 pages, 4 figures

We report an investigation of selective quantum-well intermixing (QWI) in 1.3-μm GaInNAs/GaAs multi quantum wells by silica-cap-induced disordering processes. After thermal annealing under specific conditions, controlled shifts of band gap at room temperature of over 200 nm have been observed in sputtered SiO 2 -capped samples, while uncapped and SiO 2 -capped samples by plasma-enhanced chemical vapor deposition demonstrated negligible shift. This selective modification of the band gap in GaInNAs quantum wells has been confirmed by detailed photoluminescence and photoluminescence excitation spectroscopy, and by secondary ion mass spectrometry. The controlled tuning of the band gap of GaInNAs/GaAs by QWI is important for a wide range of photonic integrated circuits and advanced device applications.

The pressure dependence of the total threshold current and its respective recombination components in 1.3 μm GaInNAs single-quantum-well lasers using spontaneous emission measurements up to 13 kbar is presented. We observed an unusual increase of the nonradiative Auger recombination current with increasing pressure in this material, which is opposite to those in 1.3 μm InP-based InGaAsP and AlGaInAs devices where the Auger current decreases with pressure. It is shown that the high-pressure-induced increase of the threshold current in GaInNAs is associated with the increase of the Auger current, while the defect-related monomolecular nonradiative current remains nearly unchanged in the pressure range studied. Theoretical calculations show that the unusual increase of the Auger current with pressure in GaInNAs is due to a large increase in the threshold carrier density.

GaN nanowire ensembles with axial In(x)Ga(1-x)N multi-quantum-wells (MQWs) were grown by molecular beam epitaxy. In a series of samples we varied the In content in the MQWs from almost zero to around 20%. Within the nanowire ensemble, the MQWs fluctuate strongly in composition and size. Statistical information about the composition was obtained from x-ray diffraction and Raman spectroscopy. Photoluminescence at room temperature was obtained in the range of 2.2 to 2.5 eV, depending on In content. Contrary to planar MQWs, the intensity increases with increasing In content. We compare the observed emission energies with transition energies obtained from a one-dimensional model, and conclude that several mechanisms for carrier localization affect the luminescence of these three-dimensional structures.

High-resolution x-ray diffraction peak profiles from self-induced GaN nanowires are studied theoretically and experimentally. We show that the peak profiles can be explained as a result of an inhomogeneous fluctuating strain in nanowires. We attribute this strain to random distortions caused by lattice defects at the interface between the nanowire and the substrate and at coalescence joints. An exponential decay of the mean-squared strain along the nanowire describes the peak profiles in successive diffraction orders.