Nucleation and growth during molecular beam epitaxy (MBE) of Si on Si(111)

The nucleation of Si on Si(111) has been studied during deposition in UHV by spot profile analysis of low energy electron diffraction (SPA-LEED). A new method of evaluation is developed by separating the measured spot profile into a central spike and a broad shoulder. The energy dependence of the fraction of the central spike out of the total diffracted intensity provides the vertical distribution of surface atoms over different levels. With this method it is shown that the first nucleation occurs in islands of double height. Only after deposition of several layers a layer-by-layer growth is found with a well defined nucleation of a new layer before the former one is completed.     

Interaction of atomic hydrogen with the Si(100)2×1 surface

The interaction of atomic hydrogen with the Si(100)2×1 surface has been investigated in detail by a field ion-scanning tunneling microscope (FI-STM). At low exposure, hydrogen atoms reside singly on top of the dimerised Si atoms, and are imaged brightly. The hydrogen chemisorption induces the buckling of dimers, indicating the strong bonding between Si and hydrogen atoms. The adsorption geometry changed from the (2×1) monohydride phase to the (1×1) dihydride phase with increasing exposure of hydrogen. The former is imaged dark compared with the unreacted Si dimers due to the reduction of the density of electronic states near the Fermi level. Surface etching was also observed during the formation of the dihydride phase. The behavior of hydrogen desorption from the H-saturated Si(100) surface was investigated as a function of annealing temperatures. Our STM results suggest that the desorbing H₂ molecules are formed by two hydrogen atoms on the same dihydride species.     

Coverage- and temperature-dependent vibrational spectra of hydrogen chemisorbed on Si(100)2 × 1

The adsorption of atomic hydrogen at Si(100)2 × 1 has been studied for coverages at and below one monolayer at temperatures between 300 and 1200 K using high-resolution Electron Energy Loss Spectroscopy (EELS) and Low Energy Electron Diffraction (LEED). Measurements of EELS frequencies, linewidths and intensities are discussed for different coverages and temperatures during exposure as well as subsequent annealing. Formation of a monohydride Si(100)2 × 1 : H adsorption phase is observed at room temperature in the sub-monolayer range, at 650 K for all coverages up to the saturation, and during thermal decomposition of the low temperature dihydride Si(100)1 × 1 : : 2H adsorption phase. The latter is formed by saturating Si(100) at 300 K with atomic hydrogen.     

Energetics of atomic hydrogen diffusion on Si(100)

We present first principles calculations of the potential energy surface for the diffusion of a single hydrogen atom on Si(100)2 × 1. Surface relaxation is found to be very important for the energetics of diffusion. A strong anisotropy is predicted for hydrogen motion: H should diffuse mainly along dimer rows, where activation energies are ~ 1.3 eV, while the barrier for row-to-row hopping is ~ 0.5 eV higher. Our results indicate that diffusion can be considered a fast process compared to H₂ recombinative desorption.     

Field-dependent energy scales in URu(2)Si(2)

At low temperature, macroscopic properties of URu(2)Si(2) display a characteristic energy scale delta(0)(B) which decreases when a magnetic field is applied, and eventually vanishes at an extrapolated value of the field of about 40 T. We have performed inelastic neutron scattering measurements of the magnetic dynamics of URu(2)Si(2) in applied fields along the c axis of intensities up to 12 T. We show that delta(0)(B) is not related to gaps in the magnetic fluctuations spectra. This provides direct evidence of the fact that two distinct energy scales govern the physics of this compound.     

Rectification behaviour of molecular layers on Si(111)

Reproducible and strong diode-like behaviour is observed for molecular films of 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) on n-type Si(111)- 7×7 surfaces studied by scanning tunnelling microscopy (STM) and spectroscopy (STS) at 77 K. The mechanism behind the rectification is likely to be related to the electron distribution at the molecule-silicon interface. We suggest that the adsorption of the molecular layer profoundly modifies the electronic structure of the Si(111)- 7×7 surface.