34 research outputs found
Simulation of convergent-beam low-energy electron diffraction on Si(001) reconstructions
No full textResearch results based upon this code and data are published at http://doi.org/10.1016/j.apsusc.2019.05.274.
The image simulation of convergent beam low energy electron diffraction (CBLEED) patterns are used to determine the sensitivity of CBLEED to atomic-scale displacements of several reconstructed variants of the Si(001) surface. The CAVATN code is used to determine the dynamical LEED intensities as a function of the incident electron energy (Ei), angle (theta, phi) and at each of the miller indices (h,k), up to the third order. The CBLEED code then maps these intensities into reciprocal space, allowing the visualisation of CBLEED patterns to be made as a function of incident electron energy (Ei). The data files for the CBLEED simulations are stored in a .txt format, with an accompanying .png image displaying the result of the simulation. This data is then analysed to determine the sensitivity of CBLEED patterns to small atomic displacements.
CAVATN code: Relevant documentation, electron beam files and the crystal structure files are all included. The CAVATN dynamical LEED package, developed from the CAVLEED code, is also included, where the code employs the muffin-tin potential approximation and involves a set of phase shifts for each atom type (which are treated as spherically symmetric scatterers in a crystal) that can be evaluated using phase shift calculation packages or tables. In the simulations performed here, complex phase shifts were used to simulate temperature dependent scattering effects at T = 293K. The inner potential is treated as energy independent and is split into real Uor = 5 eV and imaginary Uoi = 10 eV parts to respectively treat refraction (via the vacuum and muffin-tin zero difference) and absorption (due to in- elastic processes). Multiple scattering between atoms within a layer is calculated using the chain method and the multiple scattering between layers is included by the renormalized forward scattering perturbation method to evaluate the wave amplitudes of diffracted beams at the surface, and hence the intensities of the LEED pattern.
CBLEED code: The dynamical CBLEED package is included as ‘cbleed_analysis_script.py’, where the CBLEED patterns are simulated by uniformly partitioning the convergent cone into square areas as shown in Figure 1. An incident electron beam is located at the centre of these squares and defined directionally by and . Each of the incident electron beams of the sampled convergent cone was then used as input to the dynamical LEED program CAVATN, so that the corresponding multiply scattered intensities could be determined and mapped into reciprocal space. All the output data files from the CBLEED code is available for the following structures in the ‘output’ folder; Si(001)-1x1-ideal, Si(001)-2x1-symmetric, Si(001)-2x1-buckled, Si(001)-2x1-dH (for dimer height displacements) and Si(001)-2x1-dL (for dimer length displacements). The data for the sensitivity to atomic-scale displacements is included in the ‘sensitivity_output’ folder, which determines both the partial and whole pattern sensitivities
Fabrication and characterization of metallic, two-dimensional dopant δ-layers in silicon
No full textWith the recent advances of deterministic atomic-scale patterning of phosphorous and arsenic on silicon, proposed architectures for silicon-based quantum computation are close to being realized. For future scalable devices, the role of atomically abrupt `delta' layer interfaces will be critical to device operation, so a further understanding is required in the two-dimensional (2D) physics involved. This thesis discusses a broad range of characterization methods that are employed to measure the properties of buried, 2D dopant δ-layers in silicon, whilst also developing a new method of resistless extreme ultra-violet (EUV) lithography on hydrogen passivated silicon. The first results chapter discusses the optimal method for quantifying secondary ion mass spectrometry (SIMS) depth profile measurements and the progress made towards standardizing scanning tunnelling microscopy (STM) based hydrogen desorption lithography at UCL. We then demonstrate that photoemission electron microscopy (PEEM) can be used to laterally image atomically-thin phosphorous and arsenic δ-layer patterns buried in silicon, with a minimum feature size of 25 nm. The second results chapter establishes the use of synchrotron radiation in the EUV range to desorb hydrogen on the Si(001)-(2x1):H surface. Using x-ray photoelectron spectroscopy (XPS) and STM data, we develop a method to quantify the surface dangling bond density, where the data reveals that the desorption mechanism is associated with valence band excitations mediated via secondary electrons. The third results chapter shows the first soft x-ray angle-resolved photoemission spectroscopy (SX-ARPES) measurements of phosphorous and arsenic δ-layers in silicon. We demonstrate that by measuring the kz extension of the out-of-plane valleys, this offers by far the most sensitive probe of electronic two-dimensionality of silicon δ-layers yet achieved. We found that arsenic δ-layers exhibit considerably more electronic two-dimensionality than their phosphorus counterparts and also measure the absolute charge densities, relative occupancies and donor sub-band minima of the δ-layers, which yield an excellent corroboration with theoretical predictions
On the sensitivity of convergent beam low energy electron diffraction patterns to small atomic displacements
No full textMultiple scattering simulations are developed and applied to assess the potential of convergent beam low-energy electron diffraction (CBLEED) to distinguish between various reconstructions of the Si(001) surface. This is found to be readily achievable through changes in pattern symmetry. A displacement R-factor approach is used to incorporate the angular content of CBLEED discs and identify optimal energy ranges for structure refinement. Defining a disc R-factor, optimal diffraction orders are identified which demonstrate an enhanced sensitivity to small atomic displacements. Using this approach, it was found that respective dimer height and length displacements as small as ±0.06 Å and ±0.20 Å could be detected
Spatially Resolved Dielectric Loss at the Si/SiO2 Interface
No full textThe Si=SiO2 interface is populated by isolated trap states that modify its electronic properties. These
traps are of critical interest for the development of semiconductor-based quantum sensors and computers,
as well as nanoelectronic devices. Here, we study the electric susceptibility of the Si=SiO2 interface with
nm spatial resolution using frequency-modulated atomic force microscopy. The sample measured here is a
patterned dopant delta layer buried 2 nm beneath the silicon native oxide interface. We show that charge
organization timescales of the Si=SiO2 interface range from 1–150 ns, and increase significantly around
interfacial traps. We conclude that under time-varying gate biases, dielectric loss in metal-insulatorsemiconductor capacitor devices is in the frequency range of MHz to sub-MHz, and is highly spatially
heterogeneous over nm length scales
Spatially resolved dielectric loss at the Si/SiO interface
No full textThe Si/SiO interface is populated by isolated trap states which modify
its electronic properties. These traps are of critical interest for the
development of semiconductor-based quantum sensors and computers, as well as
nanoelectronic devices. Here, we study the electric susceptibility of the
Si/SiO interface with nm spatial resolution using frequency-modulated
atomic force microscopy to measure a patterned dopant delta-layer buried 2 nm
beneath the silicon native oxide interface. We show that surface charge
organization timescales, which range from 1-150 ns, increase significantly
around interfacial states. We conclude that dielectric loss under time-varying
gate biases at MHz and sub-MHz frequencies in metal-insulator-semiconductor
capacitor device architectures is highly spatially heterogeneous over nm length
scales.
Supplemental GIFs can be found at
https://doi.org/10.6084/m9.figshare.2554668
Impact of static disorder on quasiparticle spectra: Debye-Waller, mean free path, and potential fluctuation effects
No full textAngle-resolved photoemission spectroscopy (ARPES) is a widely used characterization technique in condensed matter physics, providing direct access to the single-electron spectral function of crystals, including their electronic band structure and Fermi surface. Measuring the band structure of novel quantum materials has been fundamentally important for determining, for example, nontrivial band topology, as in topological insulators, Weyl semimetals, and Dirac semimetals, or for identifying new classes of materials, such as altermagnets. A key challenge with these emerging quantum materials is that their initial crystalline quality is rarely optimized, which directly affects the spectra measured by ARPES. Here, we present a theoretical framework and experimental evidence addressing two common consequences of static disorder in photoemission experiments: the loss of coherent spectral weight and the broadening of spectral features. ARPES spectra can be understood as a sum of coherent and incoherent intensities, with their relative contributions controlled by atomic disorder and electron correlation effects. For disorder caused by phonons, the coherent intensity is exponentially suppressed as temperature increases, a phenomenon analogous to the Debye-Waller factor in diffraction, where Bragg peaks diminish in favor of diffuse scattering as disorder increases. In this work, we report a soft-x-ray study of the deliberately disordered (via Ar ion sputtering) InAs(110) surface, characterized by scanning tunneling microscopy, scanning tunneling spectroscopy, and low-energy electron diffraction. We introduce a framework that enables quantification of coherent photoemission intensity loss with increasing disorder, allowing both thermal and static disorder to be treated within a unified approach. Additionally, we identify a second major effect of disorder beyond lifetime broadening: inhomogeneous spectral broadening arising from local potential fluctuations. We show that such fluctuations increase the linewidths of the spectra of localized and delocalized states and contribute to the suppression of ARPES intensity from states near the Fermi level. The concepts and analysis methods presented should make ARPES useful for direct diagnosis of disorder effects on electronic states, for science as well as engineering.LT
Najważniejsze funkcje i gatunkowe wyznaczniki anegdoty w biografii antycznej (The most important functions and genre indicators of anectode in ancient biography)
No full textThe ancient biographical literature is full of anecdotes. Though anecdote as literary form was readily applied in antiquity, it was not distinguished and didn’t have its own name. The term ‘anecdote’ appeared in the 17th century but it hasn’t been described insightfully so far. We have a few modern definitions but they are unsatisfactory. In the beginning of this article author presents shortly the origin of the term ‘anecdote’, showing similarities and differences between Procopios’ Secret History (<Ane/kdota) and earlier biographical anecdotes. The next step is rejection of modern definition of anecdote by Arnaldo Momiglianoas too narrow and normative. Author focuses on description of anecdote and shows its fundamental functions examining ancient biographies. He claims that anecdotes can argue, characterize, explain, entertain and supplement.Can be also a digression. In that point it is possible to do a preliminary distinction of the anecdote as literary form and to show its characteristic features: irremovability, similarity (and differences) to diegema (dih/ghma) and chreia (xrei/a), and credibility. In conclusion there is a short survey of ancient biographies from a point of view of presence of anecdote, an attempt of definition of anecdote included as well
Constantinople and its Inhabitants in the "New History" by Zosimos
No full textFor Zosimos Constantinople was a vital city, a city that owed a lot to Constantine the Great; all that despite the fact that both that ruler and his successors did not find much appreciation in the historian’s eyes. The new Capital city may have its problems, such as overpopulation, lack of room and safety, but it is also the place where one can easily find a job. Its inhabitants, whenever needed, can face serious threats (Gainas’ struggle with Goths), but their reactions are unpredictable and difficult to tame (Procopios’ usurpation, city unrest accompanying the deposition of John Chrysostom from bishopric). Constantinople is the place where the events essential for country’s existence take place, where there is a furious struggle for power, where one can fali with ease from the peaks of power down to the very bottoms (like e.g. Ruffinus of Eutropios). It is the place of the Imperial court, criticized so much by Zosimos himself because, as he says, of the monarchs’ weakness, but also due to bossy eunuchs, advisors and court cliąues. Such views may have resulted from the religious beliefs of the author, who could not agree to the apostasy of the rulers from religious traditions of the past. Constantinople is also the place with Christian temples and followers, led, according to the author, by arrogant individuals, for this is the way he perceives John Chrysostom. These individuals can riot the City against its rulers, while their followers from the mob may be a threat to law and public order
Resistless EUV lithography: patterning with EUV-induced surface reactions (Conference Presentation)
No full textSingle‐Atom Control of Arsenic Incorporation in Silicon for High‐Yield Artificial Lattice Fabrication
No full textArtificial lattices constructed from individual dopant atoms within a semiconductor crystal hold promise to provide novel materials with tailored electronic, magnetic, and optical properties. These custom-engineered lattices are anticipated to enable new, fundamental discoveries in condensed matter physics and lead to the creation of new semiconductor technologies including analog quantum simulators and universal solid-state quantum computers. This work reports precise and repeatable, substitutional incorporation of single arsenic atoms into a silicon lattice. A combination of scanning tunneling microscopy hydrogen resist lithography and a detailed statistical exploration of the chemistry of arsine on the hydrogen-terminated silicon (001) surface are employed to show that single arsenic dopants can be deterministically placed within four silicon lattice sites and incorporated with 97 ± 2% yield. These findings bring closer to the ultimate frontier in semiconductor technology: the deterministic assembly of atomically precise dopant and qubit arrays at arbitrarily large scales
