R. G. Zhao

Surface superstructures of the Pb/Si(001) system

Abstract In the present work the Pb/Si(001) system has been studied with LEED (low-energy electron diffraction), AES (Auger electron spectroscopy), and EELS (electron energy loss spectroscopy). Five different surface superstructures, i.e., the 2 × 2, c(4 × 8), 4 × 1, 2 × 1 and c(4 × 4) were observed. Four of them are found for the first time, except for the 2 × 1. Their relationship has been investigated as a function of Pb coverage and annealing temperature. As a result, a complete phase diagram of the system has been determined. Upon annealing at 450°C the 2 × 2 superstructure undergoes an irreversible phase transformation to the c(4 × 8), while the c(4 × 4) reversibly transforms to 2 × 1 at 300°C. Strong influence of oxygen contamination on the surface superstructures has been observed.

Surface reconstruction and faceting of group IIIIV(113) systems — common characteristics of the stable surface structures

In the present paper we report on studies of the group IIIIV(113) systems of Al, Ga and In on Si(113) and Ge(113) with LEED and AES. LEED shows that after being annealed the surfaces of the AlSi, GaSi and InGe systems facet to (103), (013), (112) and (115) facets while those of the InSi, AlGe and GaGe systems reconstruct to (113)-(1 × 2). In the entire tested range of coverage and annealing temperature none of the six systems form (113)-(1 × 1). Combining this observation with the results of previous work on group IIIIV(001) and group IIIIV(111) we suggest that the common characteristics of all stable surface structures of the IIIIV systems are: (i) the group 13 atoms form sp2-like back bonds, and (ii) the surface contains some group IV atoms with a dangling bond. The former is an intrinsic requirement of group III atoms, while the latter facilitates relief of the strain induced by the former, and is thus probably more general.

Atomic structure of the Si(103)1 × 1-In surface

Abstract To test the model that was originally proposed for the Si(103)1 × 1-Al facets and was later on tested with STM to be correct for the Ge(103)1 × 1-In facets, in the present paper we have studied the Si(103)1 × 1-In surface by means of the QKLEED/CMTA technique. A unit cell of the model consists of an indium atom, which sits in an adatom position and forms three sp 2 -like bonds with bulk silicon atoms, and a surface silicon atom with a dangling bond. The model has passed the QKLEED/CMTA test and the best parameters of it have been obtained. It has been noticed in the experiment that the clean Si(103) surface has a surprisingly high thermal stability.

Oxidation of ethylene on the ordered alloy surface Pd{001}c(2 × 2)-Mn

Abstract Ethylene adsorption and coadsorption with oxygen on the ordered alloy surface Pd{001}c(2 × 2)-Mn were studied with high resolution electron energy loss spectroscopy (EELS). The vibrational spectrum for C 2 H 4 adsorbed on Pd{001}c(2 × 2)-Mn at 140 K is similar to that for C 2 H 4 adsorbed on Pd{001} at 80 K, which has been ascribed to di-σ-bonded C 2 H 4 . In contrary to the observation on Pd{001}, where oxygen preadsorption inhibits C 2 H 4 by limiting the formation of di-σ-bonded C 2 H 4 , on Pd{001}c(2 × 2)-Mn, the vibrational spectrum of C 2 H 4 adsorbed at 140 K is not affected by the preadsorbed oxygen. At 230 K, the coadsorbed C 2 H 4 and O 2 react with each other and form CO and H 2 O on the surface.

Coadsorption of carbon monoxide and oxygen on Pd{001}c(2×2)-Mn : an oxidation pathway for CO2 formation

CO adsorption and coadsorption with O2 on a Pd{001}c(2 x 2)-Mn ordered alloy surface were studied with low energy electron diffraction (LEED) and high-resolution electron energy loss spectroscopy (HREELS), At room temperature, the energy loss at 253 meV in the EEL spectra of CO on Pd{001}c(2 x 2)-Mn, is attributed to the C-O stretching vibration of CO molecules at on-top sites of first layer Pd atoms. This peak diminished as the surface was exposed to oxygen. At 140 K, a new feature located at 285 meV has been observed in the EEL spectra of the coadsorption of CO and O2. The peak position is near the asymmetric stretch mode of the CO2 gas phase (293 meV). As the oxygen precoverage was increased under constant CO exposure, the intensity of this peak was increased. This peak disappeared upon annealing to 300 K for 1 min. Analyses indicate that the 285 meV peak is caused by CO2 chemisorbed on Pd{001}c(2 x 2)-Mn with the molecular axis perpendicular to the surface. The reaction that produced CO2 could be explained by the microscopic process of the interaction between the oxygen atoms, which are dissociatively adsorbed at the hollow sites between two Mn and two Pd atoms, and CO molecules, at on-top sites of neighboring Pd atoms.

A comparative study of the thermal stability of the (103) surface of group-III-metal/group-IV-semiconductor systems

Abstract To investigate and understand the thermal stability of the (103) surface of the Al Si , Ga Si , In Si , Al Ge , Ga Ge , and In Ge systems, in the present article we study well-annealed surfaces of the systems as well as those of the Ga Si , Al Ge , and Ga Ge (001) systems by means of LEED and AES. The results show that the (103) surfaces of the Al Ge and Ga Ge systems are unstable while those of the other four III IV (103) systems are very stable. On the basis of the atomic structure of the III IV (103) 1 × 1 surface and the covalent bond length of the involved elements, we suggest that this is because group-III atoms would induce significant tensile stresses to the surface of the Al Ge and Ga Ge systems, while tensile stresses around group-III atoms are not favored by the III IV systems.

{310} faceting of the Ge(001) 2 × 1 surface induced by indium

Abstract In the present paper, by means of scanning tunneling microscopy, low-energy electron diffraction, and Auger electron spectroscopy, we have found and studied the {310} faceting induced by In on the Ge(001)2 × 1 surface. On the basis of the dual bias STM images a model, which contains one In atom and only one dangling bond in a unit cell, has been proposed for the atomic structure of the {310} facets. We suppose that the faceting process proceeds through expansion of many mini-{310}-facets in the cost of the original (001) surface, instead of via any other transient surface structures.

Effect of the convergence of Cu cap on the structure of Co/Cu(111) film

Annealing Cu(111) covered with more than 10 ML Co film results in substrate Cu atoms diffusing through the Co film, and forming a Cu overlayer capping the Co him. Through AES and LEED experiments and QKLEED calculation, we found that the coverage of the Cu cap has an important effect on the top layers of the Co film. When the coverage of the Cu cap is less than 0.7 ML, the stacking sequence of the top layers is hcp, the same as that of Co film. When the coverage of the Cu cap is more than 1 ML, the top layers become fee and present twinned fee because of stacking on hcp Co him. This tends to explain the results of Co/Cu(111) superlattice studies in which Co show a fee stacking sequence

{310} facets induced by submonolayer Al on Si(001)

Abstract In the present work we show that presence of about 1 3 ML of Al on the Si(001)2 × 1 surface and heating to near 700°C are two necessary conditions for faceting of the surface. The facets are {310} in stead of {111} as identified previously. A plausible model of the atomic structure of the {310} facets is proposed based on the Al coverage and the generally accepted rules regarding reconstructions on Si surfaces, i.e., reduction of the number of dangling bonds and rehybridization.

Surface reconstructions and faceting of the GaGe(113) system

Abstract Surfaces of the Ga Ge (113) system have been studied by means of low-energy electron diffraction (LEED) and scanning tunneling microscopy (STM). It has been found that the only two reconstructions of the system, i.e., ( 8 −1 5 1 ) and “1 × 2”, consist of the same (1 × 2)-like building blocks and, hence have only slightly different coverages. Coexisting with the reconstructions are small (112) and (115) facets, which develop through bunching of steps. On the basis of LEED patterns and STM images and under the guidance of the common characteristics of surface structures of III IV systems, structural models have been proposed for these reconstructions and the (112) facets for further investigation. It has been noticed that only small percentages of a monolayer of Ga can make the Ge(113) surface entirely 3 × 1; otherwise it contains a great many small 3 × 2 patches scattered inside large 3 × 1 domains.