We have investigated atomic arrangements and their electronic properties of the well-ordered thallium overlayer structures formed on the Si͑111͒-7ϫ7 surface. As for other trivalent atoms, Tl is found to form a well-defined ͱ3ϫͱ3 surface, indicating the absence of a so-called ''inert pair effect'' considered only for Tl.Another well ordered 1ϫ1 surface at 1.0 monolayer appears to be semiconducting in our angle-resolved photoemission spectra dominated by a unique dispersive surface band near the Fermi level. Our theoretical calculations using density-functional theory show that Tl adatoms occupy the T 4 sites and saturate all the dangling bonds of surface Si atoms to make the surface semiconducting with a band gap of 0.34 eV. The filled surface band observed has been well reproduced in our band calculations.The interaction of group III elements with Si surfaces has been extensively studied mainly because of their technological relevance as dopant materials and the presence of various adsorbate-induced phenomena at semiconductor surfaces. 1 The adsorption of thallium ͑Tl͒ on the Si͑111͒-7ϫ7 surface, in particular, is interesting since Tl has been known to behave quite differently from other group III elements in forming stable ordered surface structures; 2 the only ordered structure formed by Tl has been reported to be a 1ϫ1 surface ͓hereafter denoted as a Tl/Si͑111͒-1ϫ1 surface͔ at 1.0 monolayer͑ML͒ in sharp contrast with a variety of different structures found for other group III elements such as Al and In. 1 The absence of other ordered structures by Tl has been considered to exhibit a so-called ''inert pair effect'' of a Tl atom where the 6s 2 electrons are assumed to be inactive in chemical bonding with Si atoms. Tl, therefore, has been thought to act as a monovalent rather than trivalent atom on the Si͑111͒ surface in a sense chemically close to the alkali metals or novel metals. 2 Because of the large atomic radius and the effectively monovalent character of Tl, the on-top site T 1 directly above the Si first layer of the unreconstructed 1ϫ1 surface has been preferred as the binding site for Tl on the Si͑111͒ surface. 3,4 In addition to the presence of interesting structures such as the superlattice of metallic nanodots at low coverage ϳ0.2 ML 5 and the rotational epitaxy of an incommensurate Tl metallic overlayer, 4 the extremely inert nature of the Tl/Si͑111͒-1ϫ1 surface is another reason why it is so interesting since it may serve as a stable substrate surface to form other atomic structures.In this Brief Report we report results of combined studies for the atomic structures and their electronic properties of the ordered Tl overlayer structures focussed especially on the Tl/Si͑111͒-1ϫ1 surface. We have determined surface bands of the 1ϫ1 irreducible surface Brillouin zone ͑SBZ͒ from our angle-resolved photoemission spectroscopy ͑ARPES͒ measurements. The surface bands and the atomic structure of the Tl/Si͑111͒-1ϫ1 surface have been well reproduced in our theoretical calculations using the density function...

We find that foreign adsorbates acting as local impurities can induce a metal-insulator transition by pinning a charge-density wave (CDW) on the quasi-1D metallic In/Si(111)-(4x1) chain system. Our scanning tunneling microscopy image clearly reveals the presence of a new local 4x2 structure nucleated by Na adatoms at room temperature, which turns out to be insulating with a doubled periodicity along the chains. We directly determine a CDW gap energy Delta = 105+/-8 meV by identifying a characteristic loss peak in our high-resolution electron-energy-loss spectra. We thus report the first observation of a local impurity-derived Peierls-like reconstruction of a quasi-1D system.

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We report evidence of a disorder-driven Mott-Hubbard-type localization on the alkali-earth metal (AEM) (Mg,Ba)-adsorbed Si(111)-(7x7) surface. The clean metallic Si(111) surface is found to undergo a two-dimensional (2D) metal-insulator transition as randomly distributed AEM adsorbates cause disorder on the surface. A well-defined electron-energy-loss peak unique to the insulating phase is attributed to an interband excitation between the split Hubbard bands originated from a metallic surface band at Fermi energy. A quantitative analysis of the loss peak reveals that the AEM-induced insulating surfaces are of a Mott-Hubbard type driven essentially by disorder.

The LIGA X-ray exposure step was modeled into three inequalities from exposure requirements. From these inequalities, equations for the minimum and maximum exposure times required for a good quality microstructure were obtained. An equation for the thickness of an X-ray mask absorber was also obtained from the exposure requirement of threshold dose deposition. A power function of photon energy, approximating the attenuation length of the representative LIGA resist, PMMA, and the mean photon energy of the X-rays incident upon an X-ray mask absorber were applied to the above mentioned equations. Consequently, the trends of the minimum and maximum exposure times with respect to mean photon energy of X-rays and thickness of PMMA were examined and an equation for the maximum exposable thickness of PMMA was obtained. The trends of the necessary thickness of a gold X-ray mask absorber with respect to photon energy of the X-rays and PMMA thickness ratio were also examined. The simplicity of the derived equations has clari®ed the X-ray exposure phenomenon and the interplay of exposure times, the attenuation coef®cient and the thickness of an X-ray mask absorber, the attenuation coef®cient and the thickness of a resist, and synchrotron radiation power density.