Two Quantum Photoemission in Semiconductors

Two-quantum photoemission is a process in which two times the photon energy is transferred to the photoelectron. In this process, the photoelectron observed in the vacuum keeps a memory of the intermediate empty state by which it transited after its first excitation, and before its second excitation and its final escape. We discuss here the experimental problems involved and the information obtained in the case of the semiconductors Si and InP. We show that ballistic or non-ballistic electrons can be selected by an adequate choice of experimental conditions, yielding information on both the static and dynamic properties of the empty states

Theory of Non-equilibrium Single Electron Dynamics in STM Imaging of Dangling Bonds on a Hydrogenated Silicon Surface

During fabrication and scanning-tunneling-microscope (STM) imaging of dangling bonds (DBs) on a hydrogenated silicon surface, we consistently observed halo-like features around isolated DBs for specific imaging conditions. These surround individual or small groups of DBs, have abnormally sharp edges, and cannot be explained by conventional STM theory. Here we investigate the nature of these features by a comprehensive 3-dimensional model of elastic and inelastic charge transfer in the vicinity of a DB. Our essential finding is that non-equilibrium current through the localized electronic state of a DB determines the charging state of the DB. This localized charge distorts the electronic bands of the silicon sample, which in turn affects the STM current in that vicinity causing the halo effect. The influence of various imaging conditions and characteristics of the sample on STM images of DBs is also investigated.Comment: 33 pages, 9 figure

Effect of deconfinement on resonant transport in quantum wires

The effect of deconfinement due to finite band offsets on transport through quantum wires with two constrictions is investigated. It is shown that the increase in resonance linewidth becomes increasingly important as the size is reduced and ultimately places an upper limit on the energy (temperature) scale for which resonances may be observed.Comment: 6 pages, 6 postscript files with figures; uses REVTe

A model of linear chain submonolayer structures. Application to Li/W(112) and Li/Mo(112)

We propose a lattice gas model to account for linear chain structures adsorbed on (112) faces of W and Mo. The model includes a dipole-dipole interaction as well as a long-ranged indirect interaction. We have explicitly demonstrated that the periodic ground states depend on a competition between dipole-dipole and indirect interaction. The effect of temperature is studied within the molecular-field approximation. The numerical results show that for dipole-dipole interaction only, all long periodic linear chain phases are suppressed to low temperatures. However, when the long-range indirect interaction becomes important, the long-periodic linear chain phases start to fill up the phase diagram and develop a high thermal stability. Model parameters are chosen to reconstruct a sequence of long-periodic phases as observed experimentally for Li/Mo(112) and Li/W(112).Comment: RevTeX 9 pages + 5 Postscript figures (included), uses newdoc.sty (included), to be published in Surface Scienc

Experimental evaluation of interfaces using atomic-resolution high angle annular dark field (HAADF) imaging

Aberration-corrected highangleannulardarkfield (HAADF) imaging in scanning transmission electron microscopy (STEM) can now be performed at atomic-resolution. This is an important tool for the characterisation of the latest semiconductor devices that require individual layers to be grown to an accuracy of a few atomic layers. However, the actual quantification of interfacial sharpness at the atomic-scale can be a complicated matter. For instance, it is not clear how the use of the total, atomic column or background HAADF signals can affect the measured sharpness or individual layer widths. Moreover, a reliable and consistent method of measurement is necessary. To highlight these issues, two types of AlAs/GaAs interfaces were studied in-depth by atomic-resolutionHAADFimaging. A method of analysis was developed in order to map the various HAADF signals across an image and to reliably determine interfacial sharpness. The results demonstrated that the level of perceived interfacial sharpness can vary significantly with specimen thickness and the choice of HAADF signal. Individual layer widths were also shown to have some dependence on the choice of HAADF signal. Hence, it is crucial to have an awareness of which part of the HAADF signal is chosen for analysis along with possible specimen thickness effects for future HAADF studies performed at the scale of a few atomic layers

Formation and stability of self-assembled coherent islands in highly mismatched heteroepitaxy

We study the energetics of island formation in Stranski-Krastanow growth within a parameter-free approach. It is shown that an optimum island size exists for a given coverage and island density if changes in the wetting layer morphology after the 3D transition are properly taken into account. Our approach reproduces well the experimental island size dependence on coverage, and indicates that the critical layer thickness depends on growth conditions. The present study provides a new explanation for the (frequently found) rather narrow size distribution of self-assembled coherent islands.Comment: 4 pages, 5 figures, In print, Phys. Rev. Lett. Other related publications can be found at http://www.fhi-berlin.mpg.de/th/paper.htm

Self-assembly of quantum dots: effect of neighbor islands on the wetting in coherent Stranski-Krastanov growth

The wetting of the homogeneously strained wetting layer by dislocation-free three-dimensional islands belonging to an array has been studied. The array has been simulated as a chain of islands in 1+1 dimensions. It is found that the wetting depends on the density of the array, the size distribution and the shape of the neighbor islands. Implications for the self-assembly of quantum dots grown in the coherent Stranski-Krastanov mode are discussed.Comment: 4 pages, 6 figures, accepted version, minor change

The influence of surface stress on the equilibrium shape of strained quantum dots

The equilibrium shapes of InAs quantum dots (i.e., dislocation-free, strained islands with sizes >= 10,000 atoms) grown on a GaAs (001) substrate are studied using a hybrid approach which combines density functional theory (DFT) calculations of microscopic parameters, surface energies, and surface stresses with elasticity theory for the long-range strain fields and strain relaxations. In particular we report DFT calculations of the surface stresses and analyze the influence of the strain on the surface energies of the various facets of the quantum dot. The surface stresses have been neglected in previous studies. Furthermore, the influence of edge energies on the island shapes is briefly discussed. From the knowledge of the equilibrium shape of these islands, we address the question whether experimentally observed quantum dots correspond to thermal equilibrium structures or if they are a result of the growth kinetics.Comment: 7 pages, 8 figures, submitted to Phys. Rev. B (February 2, 1998). Other related publications can be found at http://www.rz-berlin.mpg.de/th/paper.htm

Effect of strain on surface diffusion in semiconductor heteroepitaxy

We present a first-principles analysis of the strain renormalization of the cation diffusivity on the GaAs(001) surface. For the example of In/GaAs(001)-c(4x4) it is shown that the binding of In is increased when the substrate lattice is expanded. The diffusion barrier \Delta E(e) has a non-monotonic strain dependence with a maximum at compressive strain values (e 0) studied. We discuss the consequences of spatial variations of both the binding energy and the diffusion barrier of an adatom caused by the strain field around a heteroepitaxial island. For a simplified geometry, we evaluate the speed of growth of two coherently strained islands on the GaAs(001) surface and identify a growth regime where island sizes tend to equalize during growth due to the strain dependence of surface diffusion.Comment: 10 pages, 8 figures, LaTeX2e, to appear in Phys. Rev. B (2001). Other related publications can be found at http://www.rz-berlin.mpg.de/th/paper.htm

Equilibrium shapes and energies of coherent strained InP islands

The equilibrium shapes and energies of coherent strained InP islands grown on GaP have been investigated with a hybrid approach that has been previously applied to InAs islands on GaAs. This combines calculations of the surface energies by density functional theory and the bulk deformation energies by continuum elasticity theory. The calculated equilibrium shapes for different chemical environments exhibit the {101}, {111}, {\=1\=1\=1} facets and a (001) top surface. They compare quite well with recent atomic-force microscopy data. Thus in the InP/GaInP-system a considerable equilibration of the individual islands with respect to their shapes can be achieved. We discuss the implications of our results for the Ostwald ripening of the coherent InP islands. In addition we compare strain fields in uncapped and capped islands.Comment: 10 pages including 6 figures. Submitted to Phys. Rev. B. Related publications can be found at http://www.fhi-berlin.mpg.de/th/paper.htm