Element-specific surface X-ray diffraction study of GaAs(001)-c(4 x 4)

In situ structure analysis of GaAs(001)-c(4 x 4) has been carried out by synchrotron surface x-ray diffraction, which is sensitive to the three-dimensional structure and the atomic species. On the basis of 98 independent in-plane diffractions and 11 fractional-order rod profiles, the atomic coordinates and thermal vibration parameters were determined. X-ray diffraction results show the buckling of surface dimers and a strain field extending up to the sixth layer from the surface. An anomalous diffraction technique has been employed to specify the atomic species of the surface dimers. It has provided direct evidence of the formation of Ga-As heterodimers.     

New structure model for the GaAs(001)-c(4x4) surface

The surface structure of the As-stabilized GaAs(001)-c(4 x 4) surface has been studied. We show that the seemingly established three As-dimer model is incompatible with experimental data and propose here a new structure model which has three Ga-As dimers per c(4 x 4) unit cell. This mixed dimer model, confirmed by the rocking-curve analysis of reflection high-energy electron diffraction and first-principles calculations, resolves disagreements in the interpretation of several previous experiments. A good agreement between the observed scanning tunneling microscopy image and the simulated one further confirms the newly proposed model.     

Atomic structure of the GaAs(001)-c(4x4) surface: first-principles evidence for diversity of heterodimer motifs

The GaAs(001)-c(4x4) surface was studied using ab initio atomistic thermodynamics based on density-functional theory calculations. We demonstrate that in a range of stoichiometries, between those of the conventional three As-dimer and the new three Ga-As-dimer models, there exists a diversity of atomic structures featuring Ga-As heterodimers. These results fully explain the experimental scanning tunneling microscopy images and are likely to be relevant also to the c(4x4)-reconstructed (001) surfaces of other III-V semiconductors.     

Two-dimensional coarsening kinetics of reconstruction domains: GaAs(001)-beta(2 x 4)

We study the nonconserved coarsening kinetics of a reconstructed semiconductor surface. The domain size evolution is obtained in situ by time-resolved surface x-ray diffraction. The system exhibits four equivalent domain types with two nonequivalent types of domain boundaries. Small domains are prepared by molecular beam epitaxy deposition of one GaAs layer. We find the correlation lengths of the domain size distribution to depend on time as l is proportional to t(0.42+/-0.05) in the half-order reflections and l is proportional to t(0.22+/-0.05) in the quarter-order reflections. The fraction of the higher energy domain boundaries increases as lnt.     

The Si(001) c(4×4) surface reconstruction: a comprehensive experimental study

We have carried out a comprehensive experimental study of the Si(001) c(4×4) surface reconstruction by scanning tunneling microscopy (STM) (at room temperature and elevated temperatures), Auger electron spectroscopy (AES), reflection high-energy electron diffraction (RHEED) and low-energy electron diffraction (LEED). Si(001) samples were kept under ultra-high vacuum (UHV) at around 550°C until the c(4×4) reconstruction appeared. STM contrast of the c(4×4) reconstruction is strongly influenced by electronic effects and changes considerably over a range of bias voltages.

The c(4×4) surface reconstruction is a result of stress which is caused by incorporation of impurities or adsorbates in sub-surface locations. The resulting c(4×4) reconstruction in the top layer is a pure silicon structure. The main structural element is a one-dimer vacancy (1-DV). At this vacancy, second layer Si-atoms rebond and cause the adjacent top Si-dimers to brighten up in the STM image at low bias voltages. At higher bias voltage the contrast is similar to Si-dimers on the (2×1) reconstructed Si(001). Therefore, besides the 1-DV and the two adjacent Si-dimers, another Si-dimer under tensile stress may complete the 4× unit cell. This is a refinement of the missing dimer model.     

Anomalous mobility of strongly bound surface species: Cl on GaAs(001)-c(8 x 2)

Although strongly bound chemisorbates at low coverage readily diffuse on metal surfaces at 300 K, they generally do not diffuse on semiconductor surfaces because of a large corrugation in the adsorbate-surface interaction potential. Chlorine chemisorbed on the Ga-rich GaAs(001)-c(8x2) surface has anomalously fast diffusion even though the chemisorption state is tightly bound and highly specific. Simple Hartree-Fock total energy calculations suggest that this diffusion of strongly bound adsorbates can occur at 300 K because there are multiple nearly degenerate adsorbate sites.     

Electron-beam-induced disordering of the Si(001)-c(4 x 2) surface structure

An electron beam (EB) irradiation effect on the Si(001)-c(4 x 2) surface was investigated by using low-energy electron diffraction. Quarter-order spots become dim and streaky by EB irradiation below approximately 40 K, indicating a disordering in the c(4 x 2) arrangement of buckled dimers. A quantitative analysis of decreasing rates of the spot intensity at various conditions of beam current, beam energy, and substrate temperature leads to a proposal for a mechanism of the disordering in the buckled-dimer arrangement in terms of electronic excitation, electron-phonon coupling, and carrier concentration.