Subsurface dimerization in III-V semiconductor (001) surfaces

Revealing the interfaces of the hybrid system MgO/Co/GaAs(0 0 1): a structural and chemical investigation with XPS and XPD

Bcc metals and MgO are used in technological research for building magnetic tunnel junctions (MTJs), because they yield a high tunnel magnetoresistance. Thin insulating barriers are of great importance in realizing MTJs. Combined with electrons spin-injected into GaAs, tunneled electrons can be detected and manipulated. We report on a synchrotron radiation based x-ray photoelectron spectroscopy and x-ray photoelectron diffraction study on the system MgO/Co(bcc)/GaAs(0 0 1) for ultra-low Co and MgO coverages ([Formula: see text], [Formula: see text]). As a result, we obtain a Co₃Ga alloy at the Co/GaAs interface in the rare D0₃ structure. This structure is only 6.07 Å thick, and serves as a template for the metastable Co(bcc) structure. Co(bcc) itself grows heavily distorted in the (0 0 1) direction for the first two unit cells, due to the D0₃ template. The MgO/Co interface reveals a weak bonding between MgO and Co(bcc) without Co oxidation, since no compound formation was observed. Additionally, MgO grows in an amorphous phase for a thickness of [Formula: see text]. At [Formula: see text], it crystallizes in a compressed unit cell where every second layer is shifted toward the (0 0 1) direction compared to the bulk halite structure.

Interaction of Mn with GaAs and InSb: incorporation, surface reconstruction and nano-cluster formation

The deposition of Mn on to reconstructed InSb and GaAs surfaces, without coincident As or Sb flux, has been studied by reflection high energy electron diffraction, atomic force microscopy and scanning tunnelling microscopy. On both Ga- and As-terminated GaAs(0 0 1), (2 × n) Mn-induced reconstruction domains arise with n = 2 for the most well ordered reconstructions. On the Ga-terminated (4 × 6), the Mn-induced (2 × 2) persists up to around 0.5 ML Mn followed by Mn nano-cluster formation. For deposition on initially β2(2 × 4)-reconstructed GaAs(0 0 1), the characteristic trench structure of the reconstruction is partially preserved even beyond 1 monolayer Mn coverage. On both the β2(2 × 4) and c(4 × 4) surfaces, MnAs-like nano-clusters form alongside the reconstruction changes. In contrast, there are no new Mn-induced surface reconstructions on InSb. Instead, the Sb-terminated surfaces of InSb (0 0 1), (1 1 1)A and (1 1 1)B revert to reconstructions characteristic of clean In-rich surfaces after well defined coverages of Mn proportional to the Sb content of the starting reconstruction. These surfaces are decorated with self-assembled MnSb nanoclusters. These results are discussed in terms of basic thermodynamic quantities and the generalized electron counting rule.

From the bottom up: dimensional control and characterization in molecular monolayers

Self-assembled monolayers are a unique class of nanostructured materials, with properties determined by their molecular lattice structures, as well as the interfaces with their substrates and environments. As with other nanostructured materials, defects and dimensionality play important roles in the physical, chemical, and biological properties of the monolayers. In this review, we discuss monolayer structures ranging from surfaces (two-dimensional) down to single molecules (zero-dimensional), with a focus on applications of each type of structure, and on techniques that enable characterization of monolayer physical properties down to the single-molecule scale.

Structure of InSb(001) surface

The InSb(001) surface has been studied experimentally, using room temperature scanning tunnelling microscopy (RT STM), and theoretically, using ab initio density functional theory (DFT) calculations. RT experimental STM images show bright lines running along the bulk crystal [110] direction. Resolved features between the bright lines whose appearance depends on the applied bias voltage confirm clearly the c(8×2) reconstruction of this surface. Our calculations, which are reported for this surface for the first time, include the reconstructed 4×2 and c(8×2) surfaces, the latter according to the so-called ζ-model proposed previously by Lee et al and Kumpf et al. A 'defective' structure proposed previously by Kumpf et al, which contains an extra In atom within a top bilayer is also considered. In all cases, we obtained stable structures. Calculated STM images for the c(8×2) reconstruction obtained using the Tersoff-Hamann approximation compare extremely well with the experimental ones. We also find that the defect structure may not be clearly visible in the STM images. Finally, a brief discussion is given on the other, although closely related, phase of the same surface observed previously in low temperature (LT) experimental STM images (Goryl et al 2007 Surf. Sci. 601 3605).

Internal architecture and adsorption sites of Violet Lander molecules assembled on native and KBr-passivated InSb(001) surfaces

The adsorption of individual Violet Lander molecules self-assembled on the c(8x2) reconstructed InSb(001) surface in its native form and on the surface passivated with one to three monolayers of KBr is investigated by means of low-temperature scanning tunneling microscopy (STM). Preferred adsorption sites of the molecules are found on flat terraces as well as at atomic step edges. For molecules immobilized on flat terraces, several different conformations are identified from STM images acquired with submolecular resolution and are explained by the rotation of the 3,5-di-tert-butylphenyl groups around sigma bonds, which allows adjustment of the molecular geometry to the anisotropic substrate structure. Formation of ordered molecular chains is found at steps running along substrate reconstruction rows, whereas at the steps oriented perpendicularly no intermolecular ordering is recorded. It is also shown that the molecules deposited at two or more monolayers of the epitaxial KBr spacer do not have any stable adsorption sites recorded with STM. Prospects for the manipulation of single molecules by using the STM tip on highly anisotropic substrates are also explored, and demonstrate the feasibility of controlled lateral displacement in all directions.

High resolution LT-STM imaging of PTCDA molecules assembled on an InSb(001) c(8 × 2) surface

The self-assembling of 3,4,9,10-perylene-tetracarboxylic-dianhydride (PTCDA) molecules deposited on an InSb(001) c(8 × 2) surface at sub-monolayer quantities has been investigated at low temperature (77 K) using scanning tunnelling microscopy. Sub-molecular resolution was obtained on PTCDA molecules. The results reveal that individual PTCDA molecules are arranged on the substrate in chains parallel to the [110] crystallographic direction, correlated with characteristic features of the low temperature InSb(001) c(8 × 2) surface electronic structure. A structural model for PTCDA molecules adsorbed on InSb is proposed.

PTCDA molecules on an InSb(001) surface studied with atomic force microscopy

PTCDA (3,4,9,10-perylene-tetracarboxylic-dianhydride) molecular structures assembled on an InSb(001) c(8 × 2) reconstructed surface have been studied using frequency modulated atomic force microscopy. The high-resolution imaging of the structures is possible through repulsive interactions, using the constant height scanning mode. During initial stages of growth the [110] diffusion channel dominates as indicated by formation of long PTCDA molecular chains parallel to the [110] crystallographic direction on the InSb surface. For a single monolayer coverage a wetting layer of PTCDA is formed. Finally it is shown that the PTCDA/InSb is a promising system for building molecular nanostructures by manipulation of single molecules with the AFM tip.

Ga-rich limit of surface reconstructions on GaAs(001): atomic structure of the (4 x 6) phase

The Ga-rich reconstruction of the GaAs(001) surface has been studied. Using scanning tunneling microscopy (STM), we have found the existence of a well-ordered (4 x 6) reconstruction under extreme Ga-rich conditions. A structure model, consisting of subsurface Ga-Ga dimers and surface Ga-As dimers, is proposed for the (4 x 6) surface. This model is found to be energetically favorable at the Ga-rich limit and agrees well with our experimental data from STM and reflection high-energy electron diffraction.

Atomic structure of InSb(001) and GaAs(001) surfaces imaged with noncontact atomic force microscopy

Noncontact atomic force microscopy (NC-AFM) has been used to study the c(8x2) InSb(001) and the c(8x2) GaAs(001) surfaces prepared by sputter cleaning and annealing. Atomically resolved tip-surface interaction maps display different characteristic patterns depending on the tip front atom type. It is shown that representative AFM maps can be interpreted consistently with the most recent structural model of A(III)B(V)(001) surface, as corresponding to the A(III) sublattice, to the B(V) sublattice, or to the combination of both sublattices.

Direct imaging of the InSb001-c8 x 2 surface: evidence for large anisotropy of the reconstruction

We have observed the InSb(001)-c(8 x 2) surface by using high-resolution transmission electron microscopy in the profile-imaging geometry. All images observed at temperatures up to 420 degrees C agree well with the c(8 x 2) model reported by Kumpf et al. [Phys. Rev. Lett. 86, 3586 (2001)]]. 1/30 sec real-time observations at 420 degrees C evidence that a part of the subsurface and surface layers (called a gull-type segment) undergo switching to and from a bulk configuration. The finding is suggestive of large anisotropy in the mean square displacement of the c(8 x 2) surface.