arXiv subjects

Jason Pitters

Publications and source records attributed to Jason Pitters.

12 recordsLinked to original sources

arXiv2025

Engineering Quantum Wire States for Atom Scale Circuitry

Recent advances in hydrogen lithography on silicon surfaces now enable the fabrication of complex and error-free atom-scale circuitry. The structure of atomic wires, the most basic and common circuit elements, plays a crucial role at this scale, as the exact position of each atom matters. As such, the characterization of atomic wire geometries is critical for identifying the most effective configurations. In this study, we employed low-temperature (4.5 K) scanning tunneling microscopy (STM) and spectroscopy (STS) to systematically fabricate and characterize six planar wire configurations made up of silicon dangling bonds (DBs) on the H-Si(100) surface. Crucially, the characterization was performed at the same location and under identical tip conditions, thereby eliminating artifacts due to the local environment to reveal true electronic differences among the line configurations. By performing dI/dV line spectroscopy on each wire, we reveal their local density of states (LDOS) and demonstrate how small variations in wire geometry affect orbital hybridization and induce the emergence of new electronic states. Complementarily, we deploy density functional theory (DFT) and non-equilibrium Green's functions to compute the LDOS and evaluate transmission coefficients for the most promising wire geometries. Our results indicate that dimer and wider wires exhibit multiple discrete mid-gap electronic states which could be exploited for signal transport or as custom quantum dots. Furthermore, wider wires benefit from additional current pathways and exhibit increased transmission, while also demonstrating enhanced immunity to hydrogen defects.

cond-mat.mtrl-sci↗

arXiv2025

Structural Control of Atomic Silicon Wires

Bare Si(100)-2$\times$1 surface atoms exhibit a buckled structure where one Si atom in a dimer is lowered while the other is raised, leading to two possible buckling configurations equivalent in energy. The relatively low energy barrier between these configurations allows dimers to flip rapidly and uncontrollably unless stabilized by surface defects or observed at low temperatures due to reduced thermal energy using Scanning Tunneling Microscopy (STM). This rapid flipping results in a time-averaged symmetric appearance under STM. In this study, we investigated variable length buckled dimer wires on the hydrogenated Si(100) surface composed of silicon dangling bonds for the first time. We demonstrate that on degenerate p-type silicon at 4.5 K, the rapid switching of these dimers can be frozen at low scanning biases. Such buckled wires can however be controllably flipped using a bias pulse. A line as long as 37 dimers was repeatedly uniformly flipped by a single pulse delivered near one terminus of the wire. The tip-directed flipping of a particular wire does not switch adjacent wires, suggesting binary wires can make well isolated rewritable binary memory elements. Furthermore, at sufficiently high biases switching generates telegraph noise that could be of utility for random number generation. The integration and encapsulation of these wires with previously described silicon dangling bond-made logic gates and binary wires might allow for self contained actuation and readout without requiring any role of an STM tip.

cond-mat.mtrl-sci↗

arXiv2025

Roadmap on Atomic-scale Semiconductor Devices

Spin states in semiconductors provide exceptionally stable and noise-resistant environments for qubits, positioning them as optimal candidates for reliable quantum computing technologies. The proposal to use nuclear and electronic spins of donor atoms in silicon, introduced by Kane in 1998, sparked a new research field focused on the precise positioning of individual impurity atoms for quantum devices, utilising scanning tunnelling microscopy and ion implantation. This roadmap article reviews the advancements in the 25 years since Kane's proposal, the current challenges, and the future directions in atomic-scale semiconductor device fabrication and measurement. It covers the quest to create a silicon-based quantum computer and expands to include diverse material systems and fabrication techniques, highlighting the potential for a broad range of semiconductor quantum technological applications. Key developments include phosphorus in silicon devices such as single-atom transistors, arrayed few-donor devices, one- and two-qubit gates, three-dimensional architectures, and the development of a toolbox for future quantum integrated circuits. The roadmap also explores new impurity species like arsenic and antimony for enhanced scalability and higher-dimensional spin systems, new chemistry for dopant precursors and lithographic resists, and the potential for germanium-based devices. Emerging methods, such as photon-based lithography and electron beam manipulation, are discussed for their disruptive potential. This roadmap charts the path toward scalable quantum computing and advanced semiconductor quantum technologies, emphasising the critical intersections of experiment, technological development, and theory.

quant-ph↗

arXiv2024

Electronic Structures of Atomic Silicon Dimer Wires as a Function of Length

Bare silicon dimers on hydrogen-terminated Si(100) have two dangling bonds. These are atomically localized regions of high state density near to and within the bulk silicon band gap. We studied bare silicon dimers as monomeric units. Silicon dimer wires are much more stable than wires composed of individual dangling bonds. Dimer wires composed of 1 to 5 dimers were intentionally fabricated and characterised by STM techniques combined with density functional theory to provide detailed insights into geometric and electronic structure. Structural and dynamic qualities displayed by short wires were shown to be similar to the characteristics of a relatively long 37 dimer wire. Rather than adding two states into the band gap, experiment and theory reveal that each dimer adds one empty state into the gap and one filled state into the valence bands. Coupling among these states provides a conduction pathway with small bulk coupling.

cond-mat.mtrl-sci↗

arXiv2020

Ionic Charge Distributions in Silicon Atomic Wires

Using a non-contact atomic force microscope (nc-AFM), we examine continuous DB wire structures on the hydrogen-terminated silicon (100) 2x1 surface. By probing the DB structures at varying energies, we identify the formation of previously unobserved ionic charge distributions correlated to the net charge of DB wires and their predicted lattice distortion. Performing spectroscopic analysis, we identify higher energy configurations corresponding to alternative lattice distortions as well as tip-induced charging effects. By varying the length and orientation of these DB structures, we further highlight key features in the formation of these ionic surface phases.

cond-mat.mtrl-sci↗

arXiv2017

Binary Atomic Silicon Logic

It has long been anticipated that the ultimate in miniature circuitry will be crafted of single atoms. Despite many advances made in scanned probe microscopy studies of molecules and atoms on surfaces, challenges with patterning and limited thermal stability have remained. Here we make progress toward those challenges and demonstrate rudimentary circuit elements through the patterning of dangling bonds on a hydrogen terminated silicon surface. Dangling bonds sequester electrons both spatially and energetically in the bulk band gap, circumventing short circuiting by the substrate. We deploy paired dangling bonds occupied by one movable electron to form a binary electronic building block. Inspired by earlier quantum dot-based approaches, binary information is encoded in the electron position allowing demonstration of a binary wire and an OR gate.

cond-mat.mtrl-sci↗

arXiv2015

New fabrication technique for highly sensitive qPlus sensor with well-defined spring constant

A new technique for the fabrication of highly sensitive qPlus sensor for atomic force microscopy (AFM) is described. Focused ion beam was used to cut then weld onto a bare quartz tuning fork a sharp micro-tip from an electrochemically etched tungsten wire. The resulting qPlus sensor exhibits high resonance frequency and quality factor allowing increased force gradient sensitivity. Its spring constant can be determined precisely which allows accurate quantitative AFM measurements. The sensor is shown to be very stable and could undergo usual UHV tip cleaning including e-beam and field evaporation as well as in-situ STM tip treatment. Preliminary results with STM and AFM atomic resolution imaging at $4.5\,K$ of the silicon $Si(111)-7\times 7$ surface are presented.

cond-mat.mes-hall↗

arXiv2015

Scanning tunneling spectroscopy reveals a silicon dangling bond charge state transition

We report the study of single dangling bonds (DB) on the hydrogen terminated silicon (100) surface using a low temperature scanning tunneling microscope (LT-STM). By investigating samples prepared with different annealing temperatures, we establish the critical role of subsurface arsenic dopants on the DB electronic properties. We show that when the near surface concentration of dopants is depleted as a result of $1250{\deg}C$ flash anneals, a single DB exhibits a sharp conduction step in its I(V) spectroscopy that is not due to a density of states effect but rather corresponds to a DB charge state transition. The voltage position of this transition is perfectly correlated with bias dependent changes in STM images of the DB at different charge states. Density functional theory (DFT) calculations further highlight the role of subsurface dopants on DB properties by showing the influence of the DB-dopant distance on the DB state. We discuss possible theoretical models of electronic transport through the DB that could account for our experimental observations.

cond-mat.mes-hall↗

arXiv2013

Silicon Atomic Quantum Dots Enable Beyond-CMOS Electronics

We review our recent efforts in building atom-scale quantum-dot cellular automata circuits on a silicon surface. Our building block consists of silicon dangling bond on a H-Si(001) surface, which has been shown to act as a quantum dot. First the fabrication, experimental imaging, and charging character of the dangling bond are discussed. We then show how precise assemblies of such dots can be created to form artificial molecules. Such complex structures can be used as systems with custom optical properties, circuit elements for quantum-dot cellular automata, and quantum computing. Considerations on macro-to-atom connections are discussed.

cond-mat.str-el↗

arXiv2013

Characterizing the rate and coherence of single-electron tunneling between two dangling bonds on the surface of silicon

We devise a scheme to characterize tunneling of an excess electron shared by a pair of tunnel-coupled dangling bonds on a silicon surface -- effectively a two-level system. Theoretical estimates show that the tunneling should be highly coherent but too fast to be measured by any conventional techniques. Our approach is instead to measure the time-averaged charge distribution of our dangling-bond pair by a capacitively coupled atomic-force-microscope tip in the presence of both a surface-parallel electrostatic potential bias between the two dangling bonds and a tunable midinfrared laser capable of inducing Rabi oscillations in the system. With a nonresonant laser, the time-averaged charge distribution in the dangling-bond pair is asymmetric as imposed by the bias. However, as the laser becomes resonant with the coherent electron tunneling in the biased pair the theory predicts that the time-averaged charge distribution becomes symmetric. This resonant symmetry effect should not only reveal the tunneling rate, but also the nature and rate of decoherence of single-electron dynamics in our system.

cond-mat.mes-hall↗

arXiv2012

Nanoscale structuring of tungsten tip yields most coherent electron point-source

This report demonstrates the most spatially-coherent electron source ever reported. A coherence angle of 14.3 +/- 0.5 degrees was measured, indicating a virtual source size of 1.7 +/-0.6 Angstrom using an extraction voltage of 89.5 V. The nanotips under study were crafted using a spatially-confined, field-assisted nitrogen etch which removes material from the periphery of the tip apex resulting in a sharp, tungsten-nitride stabilized, high-aspect ratio source. The coherence properties are deduced from holographic measurements in a low-energy electron point source microscope with a carbon nanotube bundle as sample. Using the virtual source size and emission current the brightness normalized to 100 kV is found to be 7.9x10^8 A/sr cm^2.

cond-mat.mes-hall↗

arXiv2011

Theory of Non-equilibrium Single Electron Dynamics in STM Imaging of Dangling Bonds on a Hydrogenated Silicon Surface

During fabrication and scanning-tunneling-microscope (STM) imaging of dangling bonds (DBs) on a hydrogenated silicon surface, we consistently observed halo-like features around isolated DBs for specific imaging conditions. These surround individual or small groups of DBs, have abnormally sharp edges, and cannot be explained by conventional STM theory. Here we investigate the nature of these features by a comprehensive 3-dimensional model of elastic and inelastic charge transfer in the vicinity of a DB. Our essential finding is that non-equilibrium current through the localized electronic state of a DB determines the charging state of the DB. This localized charge distorts the electronic bands of the silicon sample, which in turn affects the STM current in that vicinity causing the halo effect. The influence of various imaging conditions and characteristics of the sample on STM images of DBs is also investigated.

cond-mat.mes-hall↗