Scale size of magnetic turbulence in tokamaks probed with 30-MeV electrons

Measurements of synchrotron radiation emitted by 30-MeV runaway electrons in the TEXTOR-94 tokamak show that the runaway population decays after switching on neutral beam injection (NBI). The decay starts only with a significant delay, which decreases with increasing NBI heating power. This delay provides direct evidence of the energy dependence of runaway confinement, which is expected if magnetic modes govern the loss of runaways. Application of the theory by Mynick and Strachan [Phys. Fluids 24, 695 (1981)] yields estimates for the "mode width" (delta) of magnetic perturbations: delta<0.5 cm in Ohmic discharges, increasing to delta = 4.4 cm for 0. 6 MW NBI.     

Phosphine dissociation and diffusion on Si(001) observed at the atomic scale

A detailed atomic-resolution scanning tunneling microscopy (STM) and density functional theory study of the adsorption, dissociation, and surface diffusion of phosphine (PH(3)) on Si(001) is presented. Adsorbate coverages from approximately 0.01 monolayer to saturation are investigated, and adsorption is performed at room temperature and 120 K. It is shown that PH(3) dissociates upon adsorption to Si(001) at room temperature to produce both PH(2) + H and PH + 2H. These appear in atomic-resolution STM images as features asymmetric-about and centered-upon the dimer rows, respectively. The ratio of PH(2) to PH is a function of both dose rate and temperature, and the dissociation of PH(2) to PH occurs on a time scale of minutes at room temperature. Time-resolved in situ STM observations of these adsorbates show the surface diffusion of PH(2) adsorbates (mediated by its lone pair electrons) and the dissociation of PH(2) to PH. The surface diffusion of PH(2) results in the formation of hemihydride dimers on low-dosed Si(001) surfaces and the ordering of PH molecules along dimer rows at saturation coverages. The observations presented here have important implications for the fabrication of atomic-scale P dopant structures in Si, and the methodology is applicable to other emerging areas of nanotechnology, such as molecular electronics, where unambiguous molecular identification using STM is necessary.     

Phosphine dissociation on the Si(001) surface

Density functional calculations are performed to identify features observed in STM experiments after phosphine (PH3) dosing of the Si(001) surface. On the basis of a comprehensive survey of possible structures, energetics, and simulated STM images, three prominent STM features are assigned to structures containing surface bound PH2, PH, and P, respectively. Collectively, the assigned features outline for the first time a detailed mechanism of PH3 dissociation and P incorporation on Si(001).     

Electronic effects induced by single hydrogen atoms on the Ge(001) surface

The properties of an isolated dangling bond formed by the chemisorption of a single hydrogen atom on a dimer of the Ge(001) surface are investigated by first-principles density functional theory (DFT) calculations, and scanning tunneling microscopy (STM) measurements. Two stable atomic configurations of the Ge-Ge-H hemihydride with respect to the neighboring bare Ge-Ge dimers are predicted by DFT. For both configurations, the unpaired electron of the HGe(001) system is found to be delocalized over the surface, rendering the isolated dangling bond of the hemihydride unoccupied. However, local surface charge accumulation, such as may occur during STM imaging, leads to the localization of two electrons onto the hemihydride dangling bond. The calculated surface densities of states for one of the charged Ge-Ge-H hemihydride configurations are found to be in good agreement with atomic-resolution STM measurements on n-type Ge(001). Comparison with a Si-Si-H hemihydride of the Si(001) surface shows similarities in structural properties, but substantial differences in electronic properties.     

Valence surface electronic states on Ge(001)

The optical, electrical, and chemical properties of semiconductor surfaces are largely determined by their electronic states close to the Fermi level (E{F}). We use scanning tunneling microscopy and density functional theory to clarify the fundamental nature of the ground state Ge(001) electronic structure near E{F}, and resolve previously contradictory photoemission and tunneling spectroscopy data. The highest energy occupied surface states were found to be exclusively back bond states, in contrast to the Si(001) surface, where dangling bond states also lie at the top of the valence band.     

Atomically precise placement of single dopants in si

We demonstrate the controlled incorporation of P dopant atoms in Si(001), presenting a new path toward the creation of atomic-scale electronic devices. We present a detailed study of the interaction of PH3 with Si(001) and show that it is possible to thermally incorporate P atoms into Si(001) below the H-desorption temperature. Control over the precise spatial location at which P atoms are incorporated was achieved using STM H lithography. We demonstrate the positioning of single P atoms in Si with approximately 1 nm accuracy and the creation of nanometer wide lines of incorporated P atoms.