Christopher Kirkham

Dopant activation mechanism of Bi wire- δ -doping into Si crystal, investigated with wavelength dispersive fluorescence x-ray absorption fine structure and density functional theory

We successfully characterized the local structures of Bi atoms in a wire-δ-doped layer (1/8 ML) in a Si crystal, using wavelength dispersive fluorescence x-ray absorption fine structure at the beamline BL37XU, in SPring-8, with the help of density functional theory calculations. It was found that the burial of Bi nanolines on the Si(0 0 1) surface, via growth of Si capping layer at 400 °C by molecular beam epitaxy, reduced the Bi-Si bond length from [Formula: see text] to [Formula: see text] Å. We infer that following epitaxial growth the Bi-Bi dimers of the nanoline are broken, and the Bi atoms are located at substitutional sites within the Si crystal, leading to the shorter Bi-Si bond lengths.

First-Principles Study on Interlayer States at the 4H-SiC/SiO2 Interface and the Effect of Oxygen-Related Defects

We investigate the effect of SiC stacking and interfacial O defects on the electronic structure of the 4H-SiC/SiO2 interface via first-principles calculations. We find interlayer states along the SiC conduction band edge, whose location changes depending on which of two possible lattice sites, h or k, is at the interface. Excess O atoms at the interface lead to defect structures which alter the electronic structure. Changes to the valence band edge are the same whether h or k sites are at the interface. On the other hand, defects remove the interlayer state of the conduction band edge between the first and second SiC bilayers if an h site is at the interface, but have no effect when there is a k site. The variation of the conduction band edge at the interface is interpreted in terms of floating states, a particular property of SiC.

Subatomic electronic feature from dynamic motion of Si dimer defects in Bi nanolines on Si(001)

Scanning tunneling microscopy (STM) reveals unusual sharp features in otherwise defect-free Bi nanolines self-assembled on Si(001). They appear as subatomic thin lines perpendicular to the Bi nanoline at positive biases and as atomic size beads at negative biases. Density functional theory (DFT) simulations show that these features can be attributed to buckled Si dimers substituting for Bi dimers in the nanoline, where the sharp feature is the counterintuitive signature of these dimers flipping during scanning. The perfect correspondence between the STM data and the DFT simulation demonstrated in this paper highlights the detailed understanding we have of the complex Bi-Si(001) Haiku system. This discovery has applications in the patterning of Si dangling bonds for nanoscale electronics.

Electronic coupling between Bi nanolines and the Si(001) substrate: An experimental and theoretical study

Atomic nanolines are one-dimensional systems realized by assembling many atoms on a substrate into long arrays. The electronic properties of the nanolines depend on those of the substrate. Here, we demonstrate that to fully understand the electronic properties of Bi nanolines on clean Si(001) several different contributions must be accounted for. Scanning tunneling microscopy reveals a variety of different patterns along the nanolines as the imaging bias is varied. We observe an electronic phase shift of the Bi dimers, associated with imaging atomic p orbitals, and an electronic coupling between the Bi nanoline and neighboring Si dimers, which influences the appearance of both. Understanding the interplay between the Bi nanolines and Si substrate could open a novel route to modifying the electronic properties of the nanolines.

Importance of SiC Stacking to Interlayer States at the SiC/SiO2 Interface

We investigated the effect of SiC stacking on the 4H-SiC/SiO2 interface via first principles calculations. Interlayer states are observed along the SiC conduction band edge, and are affected by the local structure at the interface. The location of these states changes depending on which of two lattice sites, h or k is at the interface. This difference is important for SiC based metal-oxide-semiconductor field-effect transistors which rely on the electronic structure of the conduction band.