Jae M. Seo

Growth of a crystalline and ultrathin MgO film on Fe(001)

The narrow temperature-window for obtaining a crystalline MgO film on Fe(001) has been found using in-situ STM. When Mg was deposited on Fe(001) at RT, post-oxidized at 300 °C, and additionally annealed at 400 °C, an ultrathin and crystalline MgO film was formed. It has been concluded that, in order to grow a high-quality and crystalline MgO film on Fe(001), it requires two steps, i.e., Mg film formation on the substrate at RT and subsequent annealing at the proper substrate temperature under O2 exposure for Mg atoms to be oxidized and crystallized at their deposited sites without being agglomerated.

Atomic structure of Si(5512)‐2×1: Confirmation of the structural model having two kinds of chains through homoepitaxy at 550°C

Recent empty-state scanning tunneling microscopy (STM) images of Si(5512)‐2×1 have shown that this surface consists of four types of one-dimensional structures such as honeycomb (H) chain, π-bonded (π) chain, dimer-adatom (D‐A) row, and tetramer (T) row. To confirm this revised structural model of the Si(5512)‐2×1 surface, the authors performed homoepitaxy on the reconstructed Si(5 5 12) surface held at 550°C and studied the results by STM. Under equilibrium at 550°C there exists three kinds of phases composed of (225), D(337), T(337), and (112) subunits. These subunits consist of only two commutable rows, D‐A and T rows, as well as two commutable chains, H and π chains. Mutual transformations among three phases by means of additional Si atoms are well explained by a new structural model having only two kinds of chains.Recent empty-state scanning tunneling microscopy (STM) images of Si(5512)‐2×1 have shown that this surface consists of four types of one-dimensional structures such as honeycomb (H) chain, π-bonded (π) chain, dimer-adatom (D‐A) row, and tetramer (T) row. To confirm this revised structural model of the Si(5512)‐2×1 surface, the authors performed homoepitaxy on the reconstructed Si(5 5 12) surface held at 550°C and studied the results by STM. Under equilibrium at 550°C there exists three kinds of phases composed of (225), D(337), T(337), and (112) subunits. These subunits consist of only two commutable rows, D‐A and T rows, as well as two commutable chains, H and π chains. Mutual transformations among three phases by means of additional Si atoms are well explained by a new structural model having only two kinds of chains.

Bifunctional effects of the ordered Si atoms intercalated between quasi-free-standing epitaxial graphene and SiC(0001): graphene doping and substrate band bending

Bifunctional effects of the Si atoms intercalated between the n-type 6H–SiC(0001) substrate and the zero layer have been disclosed by scanning tunneling microscopy, low-energy electron diffraction, high-resolution synchrotron photoemission spectroscopy and angle-resolved photoemission spectroscopy. As a result of Si intercalation, an ordered Si interfacial layer composed of a Si adlayer and Si adatoms with dangling bonds has been formed under quasi-free-standing epitaxial graphene (EG). It turns out that the SiC(0001) band bending is determined by the Fermi level located close to the lowest states of the upper Hubbard band. The Hubbard bands originate from strong correlation effects of the electrons in the dangling bonds of the Si adatoms ordered on the Si adlayer. The doping level of the decoupled graphene is determined by the amount of charge transferred from the Si adatoms ordered on the Si adlayer to the quasi-free-standing EG.

Charge neutrality of quasi-free-standing monolayer graphene induced by the intercalated Sn layer

It has been confirmed by angle-resolved photoemission spectroscopy that decoupled quasi-free-standing monolayer graphene (QFMLG), obtained by Sn intercalation between the buffer layer and the 6H-SiC(0 0 0 1) substrate, is charge-neutral, i.e. the Dirac point matches with the Fermi level. By combined studies of scanning tunneling microscopy/spectroscopy and core-level/valence-band photoemission spectroscopy on this system, it has been found that the intercalated Sn atoms, bonding with the Si atoms of the top Si-C bilayer on the substrate comprise a hexagonal layer, which turns out to be metallic. Such a metallic character, which has never been found in intercalation using different elements, is a major cause of charge neutrality of QFMLG, since conduction electrons of the Sn layer compensate completely spontaneous polarization charges of 6H-SiC(0 0 0 1). This charge-neutral QFMLG is stable at a high temperature of 850 ?C.

Surface reconstruction at the initial Ge adsorption stage on Si(114)-2 × 1

By combined investigation of scanning tunneling microscopy and synchrotron core-level photoemission spectroscopy on the structural and chemical evolution at the initial stage of Ge adsorption on Si(114)-2 × 1, it has been observed that one-dimensional (1D) sawtooth-like nanostructures composed of (113) and (117) facets and 1D trenches adjacent to the (113) facets are readily formed without any wetting layer. Due to the absence of chain structures on the reconstructed Si(114)-2 × 1, enhanced Ge interdiffusion detected from Ge/Si(5 5 12)-2 × 1 has not been found. Instead, Si atoms originating from etched surfaces and arriving Ge atoms form the alloy facets with Ge-rich surfaces. These experimental results prove that, if the direction of the Ge overlayer corresponding to that of the substrate is unstable like the present case, the arriving atoms prefer to form facets covered with the species of lower surface free energies rather than a uniform wetting layer.

Synchrotron photoemission studies on reconstructed strained surfaces

Recently, based on scanning tunneling microscopy studies of the reconstructed Si(5 5 12)−2×1 surface, it has been suggested that its unit cell simply consists of four kinds of one-dimensional (1D) structures: π-bonded (π) chain, honeycomb (H) chain, tetramer (T) row, and dimer-adatom (D-A) row. In the present study, by angle-resolved ultraviolet photoelectron spectroscopy, it has been found out that the Si(5 5 12)−2×1 surface has two kinds of surface states, one with a negligible dispersion originating from row structures (T/D-A) and the other with a strong dispersion originating from chain structures (π/H). Also, the Si 2p core-level spectrum shows at least two kinds of surface components, one with 0.23 eV higher binding energy originating from upward-relaxed surface atoms and subsurface atoms, and the other with 0.52 eV lower binding energy originating from downward-relaxed surface atoms. It can be realized that these spectroscopic results quantitively match with the structural model of Si(5 5 12)−2×1 h...