T.-C. Chiang

C60 encapsulation of the Si(111)‐(7×7) surface

The structure of a Si(111)-(7[times]7) surface capped by a 200 A film of C[sub 60] was studied by grazing-incidence x-ray diffraction. The Si(111)-(7[times]7) reconstruction prepared in vacuum, including the loosely bonded adatoms on the surface, is preserved under the C[sub 60] overlayer. This result illustrates that C[sub 60] can be used as an inert cap for surfaces and suggests potentially interesting applications in surface science research and electronic device engineering.

Imaging phonons in a fcc Pu-Ga alloy by thermal diffuse x-ray scattering

X-ray thermal diffuse scattering intensity patterns from phonons in a fcc δ-Pu–Ga alloy have been recorded using an 18 keV undulator x-ray beam with a beam diameter of 25 μm. The results are consistent with patterns calculated using the Born–von Karman force constant model of lattice dynamics, and support the pronounced softening of the transverse acoustic branch along the [111] direction observed from inelastic x-ray scattering measurements. This work demonstrates the feasibility of using a “large-grain, small beam” approach to study lattice properties, such as phonon dispersion curves, of materials not readily available in the form of large single crystals.

C[sub 60] encapsulation of the Si(111)-(7[times]7) surface

The structure of a Si(111)-(7[times]7) surface capped by a 200 A film of C[sub 60] was studied by grazing-incidence x-ray diffraction. The Si(111)-(7[times]7) reconstruction prepared in vacuum, including the loosely bonded adatoms on the surface, is preserved under the C[sub 60] overlayer. This result illustrates that C[sub 60] can be used as an inert cap for surfaces and suggests potentially interesting applications in surface science research and electronic device engineering.

Reflection surface x-ray diffraction patterns: k-space images

For the past two decades, x-ray diffraction has been utilized for surface structural determination. Unlike reflection high-energy electron diffraction (RHEED) which is a complicated dynamical scattering process, x-ray surface analysis is simple and straightforward due to the kinematic nature of x rays. Using high brilliance x rays from an undulator beamline and a highly sensitive charge coupled device detector, we successfully observed RHEED-like x-ray diffraction patterns. The patterns were recorded during the preparation of Si(111)-(7×7), transformation to Ge/Si(111)-(5×5) and Ge growth. Also, simultaneous measurements of x-ray reflectivity and crystal truncation rods are shown feasible with this technique.

Photoelectron holography of PbSi(111)-(√3 × √3) R30°-β

Abstract The technique of holographic image reconstruction based on photoelectron detection has been applied to the Pb Si (111) -(√3 × √3) R 30°-β phase for a determination of the surface structure. Angle-resolved photoemission from the Pb 5d core level shows intensity oscillations as a function of photon energy. These constant-initial-state spectra were taken at various emission angles and Fourier inverted to yield three-dimensional real-space images of the nearest neighbors to the emitter atom. These images clearly show Pb atoms adsorbed at the T4 site.

Photoemission study of the final band in Ag(111)

For direct transitions in angle-resolved photoemission from a single crystal, the final electronic states inside the crystal can be approximately described by a free-electron-like band for energies higher than approximately=30 eV above the Fermi level. The authors report a determination of the dispersion relation for this band in Ag along the (111) direction over a wide photon energy range (40-180 eV), from which the inner potential (average electrostatic potential in the solid within the Hartree approximation) and the energy-dependent electron effective mass are deduced. The effective mass is found to increase from approximately 1.00 m at 54 eV to 1.08 m at 149 eV above the Fermi level (m is the free electron mass). This result is compared with existing many-body model calculations. The present study is also relevant to the problem of surface state resonance in this system discussed previously.

X-ray studies of the growth of smooth Ag films on Ge(111)-c(2×8)

We have performed in situ reflectivity measurements using synchrotron radiation of Ag films deposited on Ge(111) over the thickness range of 3–12 atomic layers. The films deposited at a substrate temperature of 110 K are not well ordered, but become well ordered upon annealing, as evidenced by substantial changes in the x-ray reflectivity data. The thickness distribution for each annealed film, deduced from a fit to the reflectivity data, is remarkably narrow, with just two or three adjacent discrete thicknesses present, despite the large lattice mismatch between Ag and Ge. In some cases, the film thickness is nearly atomically uniform. The results are discussed in connection with recent models and theories of electronic effects on the growth of ultrathin metal films.

Structural determination of the C60Ge(111) interface via X-ray diffraction

Abstract An X-ray diffraction study was performed to determine the nature of the C 60 Ge (111) interface formed by depositing C60 on a Ge(111)−c(2 × 8) surface at room temperature. In-plane k-scans show a (1 × 1) periodicity at the C 60 Ge (111) interface with no trace of the c(2 × 8) reconstruction, indicating that the Ge adatoms on the clean c(2 × 8)-reconstructed surface are displaced. Scans along the (10) rod indicate that these adatoms are transferred from the T4 bonding site to the H3 site after C60 deposition. A model consisting of three relaxed bilayers of Ge and randomly distributed adatoms in the H3 site explains our results.

Photoemission Circular Dichroism and Spin Polarization of the Topological Surface States in Ultrathin Bi 2 Te 3 Films

Circular dichroism (CD) observed by photoemission, being sensitive to the orbital and spin angular momenta of the electronic states, is a powerful probe of the nontrivial surface states of topological insulators, but the experimental results thus far have eluded a comprehensive description. We report a study of Bi2Te3 films with thicknesses ranging from one quintuple layer (two-dimensional limit) to 12 layers (bulk limit) over a wide range of incident photon energy. The data show complex variations in magnitude and sign reversals, which are nevertheless well described by a theoretical calculation including all three photoemission mechanisms: dipole transition, surface photoemission, and spin-orbit coupling. The results establish the nontrivial connection between the spin-orbit texture and CD.

Atomic-level investigation of the growth of Si/Ge by ultrahigh vacuum chemical vapor deposition

Si and Ge films can be prepared under ultrahigh vacuum conditions by chemical vapor deposition using disilane and digermane as source gases. These gases offer a high sticking probability, and are suitable for atomic layer epitaxy. Using synchrotron radiation photoemission spectroscopy and scanning tunneling microscopy, we have examined the surface processes associated with the heteroepitaxial growth of Ge/Si. The measured surface-induced shifts and chemical shifts of the Si 2p and Ge 3d core levels allow us to identify the surface species and to determine the surface chemical composition, and this information is correlated with the atomic features observed by scanning tunneling microscopy. Issues related to precursor dissociation, attachment to dangling bonds, diffusion, surface segregation, growth morphology, and pyrolytic reaction pathways will be discussed.Si and Ge films can be prepared under ultrahigh vacuum conditions by chemical vapor deposition using disilane and digermane as source gases. These gases offer a high sticking probability, and are suitable for atomic layer epitaxy. Using synchrotron radiation photoemission spectroscopy and scanning tunneling microscopy, we have examined the surface processes associated with the heteroepitaxial growth of Ge/Si. The measured surface-induced shifts and chemical shifts of the Si 2p and Ge 3d core levels allow us to identify the surface species and to determine the surface chemical composition, and this information is correlated with the atomic features observed by scanning tunneling microscopy. Issues related to precursor dissociation, attachment to dangling bonds, diffusion, surface segregation, growth morphology, and pyrolytic reaction pathways will be discussed.