Silicon crystal surfaces with adsorption of (sub-) monolayer Au have been one of the most popular targets of research, because of the variety of beautiful ordered structures. The atomic arrangements, electronic band structures, phase transitions, Schottky-barrier property, surface electronic transport, electromigration, surface alloy phases, physics of one-dimensional metals, have been intensively studied by using Au/Si surface systems. Here, I introduce some results mainly from my group, showing some general physics obtained by this particular system.
This article presents the interfacial reactions at Au/Si(111)-7×7 at room temperature, which were studied by high-resolution medium energy ion scattering combined with photoelectrons spectroscopy using synchrotron-radiation-light. All the experiments were performed in situ under ultra-high vacuum conditions (< 3×10-8 Pa). The results obtained are compared with those reported so far including the topics of critical thickness for silicidation reactions.
Behavior of gold (Au) on n-type Si(100) surface dipped in Au aqueous solution and thermally oxidized have been studied using by atomic force microscope (AFM), X-ray photoelectron spectroscopy (XPS), alternating current surface photovoltage (AC SPV) techniques and ellipsometer. AFM and XPS analyses identified Au to exist at both the top of SiO2 and SiO2/Si interface as a cluster. Au/n-type Si Schottky-barrier causes an occurrence of frequency-dependent AC SPV. The frequency-dependent AC SPV still appeared in Au-contaminated (∼2×1015 atoms/cm2) and thermally oxidized Si surfaces at between 550 and 700oC, demonstrating that Si surface was weakly inverted. This result indicates that Au/n-type Si Schottky-barrier is formed at SiO2/Si interface even after thermal oxidation. The Au at SiO2/Si interface diffused into bulk Si as temperature increased higher than 750oC, resulting in drastic reduction of AC SPV. On oxidation kinetics at between 750 and 900oC, Au is thought to act as catalyst to promote SiO2 growth at the Si surface, resulting in the enhanced oxidation.
We have investigated growth of nanocarbon materials from Au nanoparticle catalysts. Site- and size-controlled Au nanoparticles are formed using atomic steps of a Si surface as templates. Au nanoparticles act as catalysts for chemical-vapor-deposition growth of single-walled carbon nanotubes or carbon nanowires (CNW), depending on the particle size. CNW growth is consistent with the well-known VLS (Vapor-Liquid-Solid) mechanism for semiconductor nanowires, demonstrating that nanocarbon materials grow from liquid Au-C eutectic alloy nanoparticles.
Amorphous Si/(Ge+Au) artificial superlattice thin films were fabricated and their thermoelectric properties were investigated. With thermal annealing, the artificial superlattice structure defused easily and recrystallization started. The recrystallization process progressed gradually, even after superlattice structure collapsed. When optimum Au concentration and annealing temperature, nanocrystals of around 10 nm diameter (nano-dots) were segregated in quasi-stable condition by the metal induced recrystallization effects of Au. The extra-ordinary high thermoelectric power was observed only when nano-dots were present. The quantum size effect of the nano-dots was responsible for the high thermoelectric power of SiGe-based thin films.
This article reports the results of real-time scanning tunneling microscopy (STM) observation of Au+ ion irradiation effects on high-temperature Si surface, which was achieved by our original ion gun and STM combined system. Sequential STM images of a Si(111)-7×7 surface kept at 500oC were obtained before, during, and after Au+ ion irradiation with 3 keV. Vacancy islands, which are two-dimensional clusters of surface vacancies, and 5×2-Au structures were formed on the surface and their size were changed during the subsequent thermal treatment. This method enables us to count exact numbers of vacancies and Au atoms on the surface by measuring the sizes of vacancy islands and 5×2-Au reconstructions. The timescale of the growth of the 5×2-Au domain suggests that the implanted Au atoms diffuse to the surface almost without interacting with point defects induced by the ion irradiation.
For the further understanding of the material combination between silicon and gold, it is necessary to study silicon-gold interface at the nano scale. For this purpose, we originally built an experimental setup with a micro electric mechanical system (MEMS) probes and a transmission electron microscope (TEM). Thanks to this setup, nanoscaled interfaces were formed under in-situ TEM observation. At the nanoscaled silicon-silicon and gold-gold interfaces, nanocontacts were formed just with a mechanical contact and grown by the surface diffusion. At the interface between gold and silicon, gold and silicon diffused each other at room temperature. The activation energy to diffuse gold into silicon was 0.21 eV, which was matched well with those of bulk experiments.
We have developed an ultrahigh-resolution spin-resolved photoemission spectrometer equipped with a highly efficient mini Mott detector and an intense xenon plasma discharge lamp. Three-dimensional spin-polarization is determined by using a 90o electron deflector situated between the hemispherical electron-energy analyzer and the Mott detector. The spectrometer achieves the energy resolution of 0.9 and 8 meV for non-spin-resolved and spin-resolved modes, respectively. By using the newly developed spectrometer, we have performed spin-and angle-resolved photoemission spectroscopy of bismuth thin film on Si(111) to investigate the spin structure of surface states. Unlike the normal Rashba splitting, the magnitude of the in-plane spin polarization is asymmetric across the zone center between the two elongated surface hole pockets. More surprisingly, we observed a giant out-of-plane spin polarization as large as the in-plane counterpart which switches the sign across the Γ−-M− line. We discuss the present finding in terms of the breaking of time reversal symmetry and the many-body effects.
To understand reaction mechanism of the oxidation at an Si(111)-7×7 surface using O2 at 300 K, the determination of initial products is essentially important. Oxygen bonding configurations and silicon oxidation states were observed using real-time X-ray photoelectron spectroscopy for O 1s together with Si 2p. It is found that ins structure initially forms, where one oxygen atom inserts in the backbond of Si adatom. It is ascertained that the chemisorbed molecular oxygen, so-called paul oxygen, is the adsorbate on top of the ins structure. It is also clarified that ad-ins structure and ins-tri structure, where ad means an oxygen atom on top of Si atom and tri means the interstitial oxygen atom, appear after a short time. Our results imply the presence of a mobile O2 on the surface.