Surface segregation of Ge at SiGe(001) by concerted exchange pathways

A Synchrotron Radiation Photoemission Study of SiGe(001)-2×1 Grown on Ge and Si Substrates: The Surface Electronic Structure for Various Ge Concentrations

Beyond the macroscopic perspective, this study microscopically investigates Si_1−xGe_x(001)-2×1 samples that were grown on the epi Ge(001) and epi Si(001) substrates via molecular-beam epitaxy, using the high-resolution synchrotron radiation photoelectron spectroscopy (SRPES) as a probe. The low-energy electron diffraction equipped in the SRPES chamber showed 2×1 double-domain reconstruction. Analyses of the Ge 3d core-level spectra acquired using different photon energies and emission angles consistently reveal the ordered spots to be in a Ge–Ge tilted configuration, which is similar to that in epi Ge(001)-2×1. It was further found that the subsurface layer was actually dominated by Ge, which supported the buckled configuration. The Si atoms were first found in the third surface layer. These Si atoms were further divided into two parts, one underneath the Ge–Ge dimer and one between the dimer row. The distinct energy positions of the Si 2p core-level spectrum were caused by stresses, not by charge alternations.

Recrystallization of highly-mismatched Be(x)Zn(1-x)O alloys: formation of a degenerate interface

We investigate the effect of thermally induced phase transformations on a metastable oxide alloy film, a multiphase Be(x)Zn(1-x)O (BZO), grown on Al2O3(0001) substrate for annealing temperatures in the range of 600-950 °C. A pronounced structural transition is shown together with strain relaxation and atomic redistribution in the annealed films. Increasing annealing temperature initiates out-diffusion and segregation of Be and subsequent nucleation of nanoparticles at the surface, corresponding to a monotonic decrease in the lattice phonon energies and band gap energy of the films. Infrared reflectance simulations identify a highly conductive ZnO interface layer (thicknesses in the range of ≈ 10-29 nm for annealing temperatures ≥ 800 °C). The highly degenerate interface layers with temperature-independent carrier concentration and mobility significantly influence the electronic and optical properties of the BZO films. A parallel conduction model is employed to determine the carrier concentration and conductivity of the bulk and interface regions. The density-of-states-averaged effective mass of the conduction electrons for the interfaces is calculated to be in the range of 0.31 m0 and 0.67 m0. A conductivity as high as 1.4 × 10(3) S · cm(-1) is attained, corresponding to the carrier concentration n(Int) = 2.16 × 10(20) cm(-3) at the interface layers, and comparable to the highest conductivities achieved in highly doped ZnO. The origin of such a nanoscale degenerate interface layer is attributed to the counter-diffusion of Be and Zn, rendering a high accumulation of Zn interstitials and a giant reduction of charge-compensating defects. These observations provide a broad understanding of the thermodynamics and phase transformations in Be(x)Zn(1-x)O alloys for the application of highly conductive and transparent oxide-based devices and fabrication of their alloy nanostructures.

Theoretical studies of the passivants' effect on the Si(x)Ge(1-x) nanowires: composition profiles, diameter, shape, and electronic properties

Theoretically, we have performed a systematic investigation on the passivants' effect on the geometrical and electronic properties of Si(x)Ge(1-x) nanowires. First-principles calculations revealed that, in the nanowires passivated by fluorine (F)∕chlorine (Cl)∕hydrogen (H) atoms, Si atoms preferred to segregate towards the surface due to the stronger Si-X bonds than that of Ge-X bonds (X = F, Cl, H). The energy barriers of X atoms' desorption is higher than that of the Si∕Ge atoms' exchanging, inducing a feasible and strong surface segregation of Si atoms at proper temperature. Considering the Si∕Ge interactions and mixing entropy, the composition profiles of Si∕Ge distributions are obtained by minimizing the Gibbs free energy, which indicates the outmost layer of surface should be mostly occupied by Si. With total Si surface segregation, the diameter and shape of most stable Si(x)Ge(1-x) nanowires are found to be determined by the composition x and the passivants' chemical potential. In addition, charge distribution of near-gap levels can be modulated through the surface passivants. Our finding provides a practical avenue to tune the electronic properties of Si(x)Ge(1-x) nanowires, by modulating the morphologies of nanowires with the composition control of Si∕Ge and the chemical potential of passivants.

The MIDAS project at ASU: John Cowley's vision and practical results

An overview of the conception and development of the MIDAS system at Arizona State University is given: a Microscope for Imaging, Diffraction and Analysis of Surfaces. John Cowley's vision in the early 1980s was ambitious and far-reaching, and it was because of him the authors came to ASU. We were centrally involved in the design and implementation of MIDAS from the mid 1980s onwards; the novel design features are briefly reviewed. Practical results obtained using this instrument are listed, and the scope for future development and applications are indicated. While it is clear that many new results have been demonstrated, even more possibilities still remain to be explored. Some comments are made about the feasibility of such developments in the light of competing instrumentation.

Organic functionalization of sidewall of carbon nanotubes

Using density functional theory, we have theoretically studied sidewall functionalization of carbon nanotubes (CNT) with a nucleophilic organic carbene, dipyridyl imidazolidene (DPI). When compared to the dissociated system, formation of the adduct from defect-free (5,5) tube and the DPI is weakly exothermic. However, introduction of (5,7,7,5) defect or nitrogen doping at the CNT stabilizes the adduct in both physical and chemical senses, suggesting a possible way to enrich the chemistry of sidewall functionalization. The work function of the adducts is found to decrease by approximately 0.4 eV per DPI/80 atoms. Upon binding of the DPI, electronic structures are modified in such a way that small gaps are introduced, where the size of the gap depends upon the degree of functionalization.