948 research outputs found
Novel Reconstruction mechanisms: A comparison between group-III-nitrides and "traditional" III-V-semiconductors
We have studied the driving forces governing reconstructions on polar GaN
surfaces employing first-principles total-energy calculations. Our results
reveal properties not observed for other semiconductors, as for example a
strong tendency to stabilize Ga-rich surfaces. This mechanism is shown to have
important consequences on various surface properties: Novel and hitherto
unexpected structures are stable, surfaces may become metallic although GaN is
a wide-bandgap semiconductor, and the surface energy is significantly higher
than for other semiconductors. We explain these features in terms of the small
lattice constant of GaN and the unique bond strength of nitrogen molecules.Comment: 13 pages, 5 figure
Clean and As-covered zinc-blende GaN (001) surfaces: Novel surface structures and surfactant behavior
We have investigated clean and As-covered zinc-blende GaN (001) surfaces,
employing first-principles total-energy calculations. For clean GaN surfaces
our results reveal a novel surface structure very different from the
well-established dimer structures commonly observed on polar III-V (001)
surfaces: The energetically most stable surface is achieved by a Peierls
distortion of the truncated (1x1) surface rather than through addition or
removal of atoms. This surface exhibits a (1x4) reconstruction consisting of
linear Ga tetramers. Furthermore, we find that a submonolayer of arsenic
significantly lowers the surface energy indicating that As may be a good
surfactant. Analyzing surface energies and band structures we identify the
mechanisms which govern these unusual structures and discuss how they might
affect growth properties.Comment: 4 pages, 3 figures, to be published in Appears in Phys. Rev. Lett.
(in print). Other related publications can be found at
http://www.rz-berlin.mpg.de/th/paper.htm
Observation of Buried Phosphorus Dopants near Clean Si(100)-(2x1) with Scanning Tunneling Microscopy
We have used scanning tunneling microscopy to identify individual phosphorus
dopant atoms near the clean silicon (100)-(2x1) reconstructed surface. The
charge-induced band bending signature associated with the dopants shows up as
an enhancement in both filled and empty states and is consistent with the
appearance of n-type dopants on compound semiconductor surfaces and passivated
Si(100)-(2x1). We observe dopants at different depths and see a strong
dependence of the signature on the magnitude of the sample voltage. Our results
suggest that, on this clean surface, the antibonding surface state band acts as
an extension of the bulk conduction band into the gap. The positively charged
dimer vacancies that have been observed previously appear as depressions in the
filled states, as opposed to enhancements, because they disrupt these surface
bands.Comment: 4 pages, 3 figures. TeX for OSX from Wierde
Spectral Function in Mott Insulating Surfaces
We show theoretically the fingerprints of short-range spiral magnetic
correlations in the photoemission spectra of the Mott insulating ground states
realized in the triangular silicon surfaces K/Si(111)-B and SiC(0001). The
calculated spectra present low energy features of magnetic origin with a
reduced dispersion ~10-40 meV compared with the center-of-mass spectra
bandwidth ~0.2-0:3 eV. Remarkably, we find that the quasiparticle signal
survives only around the magnetic Goldstone modes. Our findings would position
these silicon surfaces as new candidates to investigate non-conventional
quasiparticle excitations.Comment: 5 pages, 4 figures. To be published in Journal of Physics: Condensed
Matte
Systematic vertex corrections through iterative solution of Hedin's equations beyond the it GW approximation
We present a general procedure for obtaining progressively more accurate functional expressions for the electron self-energy by iterative solution of Hedin's coupled equations. The iterative process starting from Hartree theory, which gives rise to the GW approximation, is continued further, and an explicit formula for the vertex function from the second full cycle is given. Calculated excitation energies for a Hubbard Hamiltonian demonstrate the convergence of the iterative process and provide further strong justification for the GW approximation
ZnSe Heteroepitaxial Growth on Si (100) and GaAs (100)
The early stages of ZnSe heteroepitaxy on Si(100), Si(100):As and GaAs(100) are compared and contrasted, based on results of scanning tunneling microscopy and photoemission spectroscopy. High Se reactivity with the substrate constituents leads to bulk phase formation which is detrimental to heteroepitaxy. As-termination of Si(100) not only passivates the surface, but also provides an ideal buffer for ZnSe overgrowth. Lacking a similar buffer layer, stoichiometric control of the GaAs(100) surface is investigated to find a means for controlled heteroepitaxy
Violation of particle number conservation in the it GW approximation
We present a nontrivial model system of interacting electrons that can be solved analytically in the GW approximation. We obtain the particle number from the GW Green's function strictly analytically, and prove that there is a genuine violation of particle number conservation if the self-energy is calculated non-self-consistently from a zeroth order Green's function, as done in virtually all practical implementations. We also show that a simple shift of the self-energy that partially restores self-consistency reduces the numerical deviation significantly
Why is the bandwidth of sodium observed to be narrower in photoemission experiments?
The experimentally predicted narrowing in the bandwidth of sodium is
interpreted in terms of the non-local self-energy effect on quasi-particle
energies of the electron liquid. The calculated self-energy correction is a
monotonically increasing function of the wavenumber variable. The usual
analysis of photo-emission experiments assumes the final state energies on the
nearly-free-electron-like model and hence it incorrectly ascribes the non-local
self-energy correction to the final state energies to the occupied state
energies, thus leading to a seeming narrowing in the bandwidth.Comment: 9 page
Structure analysis of the Ga-stabilized GaAs(001)-c(8x2) surface at high temperatures
Structure of the Ga-stabilized GaAs(001)-c(8x2) surface has been studied
using rocking-curve analysis of reflection high-energy electron diffraction
(RHEED). The c(8x2) structure emerges at temperatures higher than 600C, but is
unstable with respect to the change to the (2x6)/(3x6) structure at lower
temperatures. Our RHEED rocking-curve analysis at high temperatures revealed
that the c(8x2) surface has the structure which is basically the same as that
recently proposed by Kumpf et al. [Phys. Rev. Lett. 86, 3586 (2001)]. We found
that the surface atomic configurations are locally fluctuated at high
temperatures without disturbing the c(8x2) periodicity.Comment: 14 pages, 4 figures, 1 tabl
Surface energy and stability of stress-driven discommensurate surface structures
A method is presented to obtain {\it ab initio} upper and lower bounds to
surface energies of stress-driven discommensurate surface structures, possibly
non-periodic or exhibiting very large unit cells. The instability of the
stressed, commensurate parent of the discommensurate structure sets an upper
bound to its surface energy; a lower bound is defined by the surface energy of
an ideally commensurate but laterally strained hypothetical surface system. The
surface energies of the phases of the Si(111):Ga and Ge(111):Ga systems and the
energies of the discommensurations are determined within eV.Comment: 4 pages RevTeX. 2 Figures not included. Ask for a hard copy (through
regular mail) to [email protected]
- …