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The observation of atomic ordering and signatures of defects in self-assembled Ge islands using x-ray diffraction techniques have been previously treated as unrelated subjects. However, mutual understanding can be achieved when both subjects are studied in a common frame. Here, we report on measurements and analysis of both defects and atomic ordering in Si1-xGex islands epitaxially grown on Si(001) substrates as a function of growth temperature. By using x-ray diffraction and mapping around a bulk forbidden reflection, defect sizes, and in-plane spacing between nearby dislocations are extracted and related to the composition of the islands. The results fit well with an independent determination using selective wet chemical etching and atomic force microscopy measurements. Moreover, the temperature dependence of the ordered domain size is discussed. Although both atomic ordering and defect formation take place independently in the system, it is found that the relaxation provided by the onset of defects does not affect the formation of ordered domains, recently pointed out to be stabilized by strain and surface equilibrium on islands facets. [ABSTRACT FROM AUTHOR]

Detailed results from field emission studies of lanthanum hexaboride (LaB6) coated multiwalled carbon nanotube (MWCNT) films, pristine LaB6 films, and pristine MWCNT films are reported. The films have been synthesized by a combination of chemical and physical deposition processes. An impressive increase in field enhancement factor and temporal stability as well as a reduction in turn-on field and threshold field are observed in LaB6-coated MWCNTs compared to pristine MWCNT and pristine LaB6 films. Surface morphology of the films has been examined by scanning electron microscopy. Introduction of LaB6 nanoparticles on the outer walls of CNTs LaB6-coated MWCNTs films is confirmed by transmission electron microscopy. The presence of LaB6 was confirmed by X-ray photoelectron spectroscopy results and further validated by the Raman spectra. Raman spectroscopy also shows 67% increase in defect concentration in MWCNTs upon coating with LaB6 and an upshift in the 2D band that could be attributed to p-type doping. Ultraviolet photoelectron spectroscopy studies reveal a reduction in the work function of LaB6-coated MWCNT with respect to its pristine counterpart. The enhanced field emission properties in LaB6-coated MWCNT films are correlated with a change in microstructure and work function. [ABSTRACT FROM AUTHOR]

Intensified field emission (FE) current from temporally stable cerium hexaboride (CeB6) coated carbon nanotubes (CNTs) on Si substrate is reported aiming to propose the new composite material as a potential candidate for future generation electron sources. The film was synthesized by a combination of chemical and physical deposition processes. A remarkable increase in maximum current density, field enhancement factor, and a reduction in turn-on field and threshold field with comparable temporal current stability are observed in CeB6-coated CNT film when compared to pristine CeB6 film. The elemental composition and surface morphology of the films, as examined by scanning electron microscopy, transmission electron microscopy, and energy dispersive X-ray measurements, show decoration of CeB6 nanoparticles on top and walls of CNTs. Chemical functionalization of CNTs by the incorporation of CeB6 nanoparticles is evident by a remarkable increase in intensity of the 2D band in Raman spectrum of coated films as compared to pristine CeB6 films. The enhanced FE properties of the CeB6 coated CNT films are correlated to the microstructure of the films. [ABSTRACT FROM AUTHOR]

The cross-plane thermal conductivity κ of multilayers of SiGe nanodots separated either by Si or SiGe can be decreased by reducing the period length or by increasing the nanodot density. It is, however, not clear how far κ can be reduced by using these strategies. In addition, the role of SiGe nanodots on the reduction of j is still not fully understood. In this work, we addressed these issues by studying experimentally the cross-plane j of Ge/Si superlattices with period lengths down to 1.5 nm. Although κ tends to preserve the decreasing trend with reducing the period length, for periods shorter than 2 nm we observed a drastic drop of the average thermal resistance per period. This finding indicates a weakening of the effect of the interfaces on phonon scattering and implies a lower limit for κ. To assess the role played by the nanodots in the reduction of j we studied Ge/Si superlattices with nanodot densities varying from 0 to ~8x1010 cm-2 and a fixed Si spacer thickness of 2.7 nm. The experimental results suggest that SiGe nanodots with "pyramid"-shape have an effect comparable to nominally planar wetting layers on the cross-plane thermal transport. Finally, the comparison of superlattices with nanodots separated by Si1-xGex (with x from 0 to 0.2) shows that spacer alloying is beneficial in reducing the κ by ~20%. The results presented in this work are expected to be relevant to micro/nanoscale energy conversion which requires minimizing the thermal conductivity of superlattice-based thin film thermoelectrics. [ABSTRACT FROM AUTHOR]

We present a phenomenological approach to access the coercivity enhancement which appears in all exchange bias systems. The coercivity enhancement is correlated to the structural properties of the thin film system. We find that the coercivity reacts sensitively to structural changes as subtle as a sub-monolayer interface oxidation of a ferromagnetic layer. We apply our approach to demonstrate the role of surface oxidation in a [Pt/Co]3 layer stack. The intriguing occurrence of antiferromagnetic interlayer coupling in these systems is discussed to be strongly related to the cover layer properties. [ABSTRACT FROM AUTHOR]

Very low density growth of GaAs quantum dots in self-assembled nanoholes created by gallium droplet etching is demonstrated. The emission energy of the quantum dots can be accurately controlled by the GaAs deposition amount, from 1.8 to 1.6 eV, independently of the dot density which can be reproducibly controlled over the range 0.2-2×10-8 cm-2 by the gallium deposition rate. The ensemble full-width-half-maximum is <10 meV and single-dot linewidths of 40 μeV (limited by our spectral resolution) have been measured. Additionally, shallow mounds on the sample surface allow the buried GaAs/AlGaAs dots to be located. A simple method to reliably predict the emission energy based on the shape of the nanohole is presented. [ABSTRACT FROM AUTHOR]

The authors theoretically investigate quantum confinement and transition energies in quantum wells (QWs) asymmetrically positioned in wrinkled nanomembranes. Calculations reveal that the wrinkle profile induces both blue- and redshifts, depending on the lateral position of the QW probed. Relevant radiative transitions include the ground state of the electron (hole) and excited states of the hole (electron). Energy shifts as well as stretchability of the structure are studied as a function of wrinkle amplitude and period. Large tunable bandwidths of up to 70 nm are predicted for highly asymmetric, wrinkled QWs. [ABSTRACT FROM AUTHOR]

Ordering of misfit dislocation segments in concentric polygons at the base of SiGe epitaxial islands on Si(001) has been recently indicated by in situ Transmission Electron Microscope observation. In this paper we confirm the very regular spacing by Atomic Force Microscope and He-ion Microscope measurements of the footprint carved in the Si substrate by the plastic events. We explain the intriguing ordering, as obtained with no gliding rearrangements, by cyclic occurrence of the thermodynamic critical conditions for plastic events. Quantitative predictions by a fully analytical model, which includes the dependence on island shape and composition in the generation of misfit dislocations, matched very well experimental measurements. [ABSTRACT FROM AUTHOR]

Rolled-up Au/Co/Au microtubes with up to three windings were fabricated by the combination of strain engineering, conventional photolithography, and electron beam deposition. First, magnetization properties of the initial 2D film arrays and the corresponding tube arrays were studied and strong influences of magnetostrictive and shape anisotropy are observed. Second, the mechanical deformation was examined by an instrumented indentation technique at the nanoscale and analyzed by contact mechanics theory. The loading curve fitting in the elastic regime by the Hertz model provides a first approximation of the nanomembrane radial elastic modulus of about 135 GPa. [ABSTRACT FROM AUTHOR]

In this work we study the evolution of roughness in interfaces of HfO2/Al2O3 multilayers by x-ray reflectivity. It was found that, besides the reduced adatom surface mobility during atomic layer deposition, an improvement of the interface quality can be achieved upon the stacking of several layers. Although the low roughness of the initial surface could not be recovered, there was a considerable improvement of surface/interface quality along the deposition process. In particular, variations on the growth temperature were not able to tailor the surface quality, if compared to the stacking process. Finally, transmission electron microscopy analysis has shown that local defects can take place among nearly perfect interfaces. Such effect must be taken into account for nanometer-scale device fabrication. [ABSTRACT FROM AUTHOR]

The authors experimentally investigate the effects of atomic layer deposition (ALD) and laser-assisted oxidation on the optical modes in GaAs L3 photonic crystal air-bridge cavities, using layers of InAs quantum dots as internal light source. Four distinct optical mode peaks are observed in the photonic bandgap and they show different wavelength-redshifts (0-6.5 nm) as the photonic crystal surface is coated with an Al2O3 layer (0-5.4 nm thick). Numerical finite-difference time-domain (FDTD) simulations can well-reproduce the experimental result and give insight into the origin of the shifts of modes with different spatial profiles. By combining the ALD coating with in situ laser-assisted oxidation, we are able to both redshift and blueshift the optical modes and we attribute the blueshift to the formation of a GaAs-oxide at the expense of GaAs at the interface between GaAs and the Al2O3 layer. This result can be quantitatively reproduced by including a GaAs-oxide layer into the FDTD model. Selective etching experiments, confirm that this GaAs-oxide layer is mainly at the interface between GaAs and Al2O3 layers. [ABSTRACT FROM AUTHOR]

Single lateral InGaAs quantum dot molecules have been embedded in a planar microcavity in order to increase the luminescence extraction efficiency. Using a combination of metal-organic vapor phase and molecular beam epitaxy samples could be produced that exhibit a 30 times enhanced single-photon emission rate. We also show that the single-photon emission is fully switchable between two different molecular excitonic recombination energies by applying a lateral electric field. Furthermore, the presence of a polarization fine structure splitting of the molecular neutral excitonic states is reported which leads to two polarization split classically correlated biexciton-exciton cascades. The fine structure splitting is found to be on the order of 10 μeV. [ABSTRACT FROM AUTHOR]

The paper presents resistivity and magnetization measurements on nearly stoichiometric Fe3Si films epitaxially grown on GaAs substrates by electron-beam evaporation in an ultrahigh vacuum chamber. In the low-temperature resistivity a T3 term was found in all samples. A term like that is known to describe the anomalous single-magnon scattering processes in half-metallic materials and confirms so for our samples the hypothesis of half-metallic ferromagnetism in Fe3Si. The films show an anisotropic magnetoresistance in low magnetic fields. In high magnetic fields a negative longitudinal and transverse magnetoresistance (MR) has been observed linearly depending on the field strength. In the vicinity of 200 K the MR shows maximum absolute values up to 1.5% at magnetic fields of about 8 T. From the magnetization measurements a magnetic moment of 0.86μB/atom was obtained, which is close to that of bulk Fe3Si. [ABSTRACT FROM AUTHOR]

The article gives a report on resistivity measurements on Mn-doped p-type FeSi2 single crystals and analyzes the data within the framework of different hopping conductivity models. Both the Mott [N. Mott and E. A. Davies, Electron Processes in Non-Crystalline Materials (Clarendon, Oxford, 1979)] and the Shklovskii–Efros [B. I. Shklovskii and A. L. Efros, Electronic Properties o0066 Doped Semiconductors (Springer, Berlin, 1984)] regimes of the variable-range hopping (VRH) conductivity are observed. It is shown that the temperature dependence of the resistivity of Mn-doped β-FeSi2 crystals, which follows a VRH conduction mechanism, can be expressed by a scaling expression of the form ln(ρ/ρ0)=Af(T/Tx). The characteristic and transition temperatures, as well as the complete set of parameters describing the properties of the localized holes (the localization radius, the dielectric permittivity, the width of the Coulomb gap Δ, and the values of the density of states at the Fermi level) are determined. The data above indicate existence of a rigid gap δ in the spectrum of the DOS in addition to Δ and point out to the polaronic nature of the charge carriers in the investigated compound. [ABSTRACT FROM AUTHOR]

We present a comprehensive study relating the magnetic properties to structural properties of Mn+-implanted Si1-xGex films as a function of Ge content (x=0–0.5). Ferromagnetic ordering with three critical temperatures, TB∼10–16 K, TC1∼650–780 K, and TC2∼825–860 K, are reported in this material system. Element specific x-ray absorption fine structure results show that the majority of the Mn ions are nonsubstitutional in all samples. The transmission-electron microscopy coupled with z contrast and chemical analysis reveals the presence of Mn-rich nanosized clusters including Mn4Si7 in Si-rich samples and Mn7Ge3 phases in Ge-rich samples. A composition transition occurred at x∼0.2–0.3, where we observe a change in bond lengths and defect structures. Additionally, an enhancement in magnetizations with an increase in both TB and TC1 as well as a conversion from n-type to p-type conduction are also detected. [ABSTRACT FROM AUTHOR]

We tune the emission energy of self-assembled InAs/GaAs(001) quantum dots (QDs) by partial GaAs capping and annealing. During the annealing step, the surface above the QDs flattens substantially. The existence of In-rich cores below the thin GaAs cap is directly probed by using in situ selective etching. The blueshift of the QD emission energy, resulting from the reduction of the QD height, is tuned by increasing the annealing time or by reducing the capping layer thickness. For long annealing times, the ensemble photoluminescence (PL) displays a multipeak behavior which is attributed to monolayer fluctuations of the QD height. Single-QD micro-PL shows that the quality of the QDs can be improved by desorbing the In accumulated around the QDs by performing the annealing at high substrate temperatures. [ABSTRACT FROM AUTHOR]

We systematically investigated the DC-characteristics of Si/SiGe interband tunneling diodes as a function of layer structure and device geometry. The Si spacer thicknesses Xp and XN between the intrinsic SiGe layer and the p- and n-δ-doped layers were varied. We obtained a peak current density of 16 kA/cm² and a peak-to-valley current ratio of 2.7 (with a device area of 45 μm²) for a structure with Xp= 0 nm and XN= 1 nm. A good homogeneity of all measured diode parameters was achieved over the wafer area. [ABSTRACT FROM AUTHOR]

Co films were deposited on flexible polyester substrates by dc magnetron sputtering and plastically strained in tension up to 3.7%. The magnetic measurements show that the plastic strain changes the coercivity and effective uniaxial magnetic anisotropy, as well as the magnetic domain structure of the Co films. Furthermore, the transport results show that the plastic strain strongly alters the resistance and magnetoresistance of the Co films. [ABSTRACT FROM AUTHOR]

C/β-SiC/Si hybrid microtubes have been fabricated by releasing prestressed C/Si bilayer structures and treating with a postannealing process. Detailed characterization reveals the synthesis of β-SiC via a solid phase reaction at the C/Si interface. Remarkably, the production of β-SiC is promoted in the tube wall by rolled-up bonding of adjacent windings, which increases the area of the C/Si interface by a factor of 2. The Raman spectra acquired from the hybrid microtubes disclose peaks pertaining to the optical phonon modes of β-SiC that exhibit obvious downshifts due to surface effects on the SiC nanoparticles. Moreover, two light emission bands are detected from a hybrid microtube and their origin is discussed based on spectral analyses. [ABSTRACT FROM AUTHOR]

Ultrathin Si and SiGe films on insulator are used as templates for the fabrication of hybrid metal/semiconductor rolled-up tubes. Structured Cr films are thermally evaporated on SiGe- and Si-on-insulator substrates. The metal/semiconductor bilayers are released from the substrate and formed into rolled-up tubes. The tube diameter is controlled by tuning the metal and the semiconductor layer properties. A cross section of a Cr/SiGe tube is investigated by transmission electron microscopy and electron energy loss spectroscopy. [ABSTRACT FROM AUTHOR]