Fumiya Shoji

Initial growth process and surface structure of Ag on Si(111) studied by low-energy Ion-Scattering Spectroscopy (ISS) and LEED-AES

Abstract The growth process of silver on a Si(111) substrate has been studied in detail by low-energy ion-scattering spectroscopy (ISS) combined with LEED-AES. Neon ions of 500 eV were used as probe ions of ISS. The ISS experiments have revealed that the growth at room temperature and at high temperature are quite different from each other even in the submonolayer coverage range. The following growth models have been proposed for the respective temperatures. At room temperature, the deposited Ag forms a two-dimensional (2D) island at around 2/3 monolayer (ML) coverage, where the Ag atoms are packed commensurately with the Si(111)1 substrate. One third of the substrate Si surface remains uncovered there. Then it starts to develop into Ag crystal, and at a few ML coverage a 3D island of bulk Ag crystal grows directly on the substrate. An intermediate layer, which covers uniformly the whole surface before the growth of Ag crystal, does not exist. At high temperatures (>~200°C), the well-known Si (111)√3- Ag layer is formed as an intermediate layer, which consists of 2/3 ML of Ag atoms and covers the whole surface uniformly. These Ag atoms are embedded in the first double layer of the Si substrate. It is concluded that the formation of the √3 structure needs relatively high activation energy which may originate from the large displacement of Si atoms owing to the embedment of the Ag atoms, and does not proceed below about 200°C. The most stable state of the Ag atoms on the outermost Si layer is in the shape of an island, both for the Si(111) surface and for the Si (111)√3- Ag surface.

A TOF-ISS/ERDA apparatus for solid surface analysis

Abstract A time-of-flight (TOF) system has been constructed to perform surface-structure analysis by impact collision ion scattering spectroscopy with primary noble-gas ions and scattered-neutral detection. The structure of thin Ag films (1–5 ML thickness) deposited onto a clean Si(111) substrate has been studied.

Low energy ion scattering study of adsorption and desorption processes of Pb on Si(111) surfaces

Abstract The initial growth process and the thermal desorption process of Pb on the Si(111) substrate have been studied in detail by LEED and ISS (low-energy ion-scattering spectroscopy), where neon ions of 500 eV were used as probe ions. The growth of Pb on a Si(111)7 substrate follows the Stranski-Krastanov growth mechanism, which is common to a room temperature substrate and a 340°C one. However, the intermediate layer at room temperature and at 340°C are different from each other in its structure and formation process. The intermediate layer at room temperature is a Pb(111) layer whose orientation is identical to the substrate, while that at 340°C is a Si(111)1-Pb layer consisting of 1 ML of Pb. The ISS data suggest that extremely small 2D (two-dimensional) islands of the Pb(111) formed in the mid-course coalesce into the Pb(111) layer, and that the 1-Pb layer starts to form when the density of the 2D gas phase initially formed comes up to a critical value (which is 0.27 ML at 340°C). When a heat treatment is made in the course of the deposition, the Si (111) 3 - Pb (α) intermediate layer which probably consists of 4 3 ML of Pb is formed before the formation of 3D islands. The thermal desorption process of the Si(111)1-Pb layer can be divided into the two steps, namely, (1) the desorption of the 1-Pb layer leaving the Si (111) 3 - Pb (β) structure consisting of 1 3 ML of Pb behind and (2) that of the 3 - Pb (β) structure. The kinetics of the first step is zero-order, while the second step has a high order kinetics. It is proposed that the Pb atoms of the √3-Pb(β) structure are bonded to three Si atoms, and such a bonding configuration is responsible for the high order desorption kinetics observed.

Hydrogen-termination effects on the growth of Ag thin films on Si(111) surfaces

Abstract Using elastic recoil detection analysis (ERDA) and low energy electron diffraction (LEED), we have studied the hydrogen termination effects on the growth of Ag thin films on Si(111) surfaces. We have found for the first time that adsorbed hydrogen of about 1.5 monolayer strongly affects the initial growth process of Ag thin films and promotes the formation of parallel-oriented Ag(111) crystallites, Ag(111) [11 2 ]∥Si(111)[11 2 ] . Absolute coverage of hydrogen measured by ERDA has shown a rather large reduction during the Ag film growth, indicative of the occurrence of the atomic replacement of Ag and hydrogen.

Elastic recoil detection analysis of hydrogen adsorbed on solid surfaces

Abstract An experimental apparatus for quantitative analysis of hydrogen adsorbed on solid surfaces is described. Elastic recoil detection analysis (ERDA) by a 6 MeV F3+ ion beam determines the hydrogen coverage and a LEED-Auger system is used to monitor the order and cleanliness of the specimen surface. The method has been applied to a H/Si(111)-7 × 7 system.

Hydrogen-induced reordering of the Si(111)-3 × 3-Ag surface

Abstract Using elastic recoil detection analysis and low energy electron diffraction we have investigated the adsorption process of hydrogen on the Si(111)- 3 × 3 -Ag surface. We find that (1) room temperature adsorption of atomic hydrogen induces a structure transformation from the Si(111)- 3 × 3 -Ag to Si(111)-1 × 1-Ag(H) structures, (2) a saturation coverage of hydrogen is 1.5 monolayer, which almost coincides with the one on a clean 7 × 7 surface, and (3) thermal desorption of hydrogen from the ordered 1 × 1-Ag(H) surface results in the recovery of the original 3 × 3 -Ag surface.

Inelastic effect in low-energy He+ ion scattering from solid surfaces

Abstract Values of the inelastic loss energy Q are investigated for the systems (He+−Ag, Au, Ta, and W) by the scattered-particle method at a fixed scattering angle of 90° and incident ion energies ranging from 300 to 1000 eV. The experiments are performed by an ISS unit with high energy resolution. It is found that the observed loss Q gradually increases and reaches about 5–6 eV even for Ag and Au surfaces at the incident energy of 1000 eV. In addition, the data for the oxygen contaminated Ta, and W surfaces suggest that two inelastic loss processes are involved and one of these is enhanced by the existence of oxygen.

Coadsorption of hydrogen and deuterium on Si(100) surfaces studied by elastic recoil detection analysis

Abstract We have carried out coadsorption experiments of hydrogen (H) and deuterium (D) to investigate the dynamic behavior of hydrogen adsorption on Si(100) surfaces. During the experiments, changes in H and D on the surface are quantitatively measured by an elastic recoil detection analysis method using a 6 MeV F 3+ ion beam. In the process of H adsorption onto D-adsorbed surfaces at both room temperature and 350°C, we found that the H concentration shows a gradual increase accompanied by a gradual decrease in the D concentration. A similar result is obtained for D adsorption onto H-adsorbed surfaces at the same temperatures. As a dynamic behavior, substitution between the adsorbed hydrogen and the atomic hydrogen impinging upon the surface is strongly suggested.

Measurements of inelastic energy loss in Ion-surface-collisions in the incident energy range 200–1500 eV, He+-Si(111) surface

Abstract The inelastic energy-loss Q has been measured for He+ ion scattering from a clean Si(111) surface. The data are obtained by measuring the energy spectra of the scattered incident ions. In all cases studied, two inelastic loss peaks, P1 and P2, are observed corresponding to the energy loss values, Q1 and Q2, respectively. The loss Q1 shows a gradual increase as a function of the distance of closest approach R. The loss Q2 shows a steep increase from a critical value of R = 0.4 A . The energy separation between the two Q values (Q2 − Q1) is found to be almost always 16 eV. This energy loss can be interpreted as being the result of the promotion of the He 1S ground state electron to a certain unoccupied Si 3p orbital.

Surface science lettersSurface structure of the Si(111)-5 × 1-Au studied by low-energy ion scattering spectroscopy

Abstract The Au/Si(111) system has been studied by low-energy ion scattering spectroscopy combined with LEED-AES. It has been found that the Au atoms causing the 5 × 1 structure are slightly embedded below the outermost Si layer. A new model of the 5 × 1-Au structure is presented.