arXiv subjects

Robert Wolkow

Publications and source records attributed to Robert Wolkow.

7 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↗

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↗

arXiv2023

Atomic Defect-Aware Physical Design of Silicon Dangling Bond Logic on the H-Si(100)2x1 Surface

Although fabrication capabilities of Silicon Dangling Bonds have rapidly advanced from manual labor-driven laboratory work to automated manufacturing in just recent years, sub-nanometer substrate defects still pose a hindrance to production due to the need for atomic precision. In essence, unpassivated or missing surface atoms, contaminants, and structural deformations disturb the fabricated logic or prevent its realization altogether. Moreover, design automation techniques in this domain have not yet adopted any defect-aware behavior to circumvent the present obstacles. In this paper, we derive a surface defect model for design automation from experimentally verified defect types that we apply to identify sensitivities in an established gate library in an effort to generate more robust designs. Furthermore, we present an automatic placement and routing algorithm that considers scanning tunneling microscope data obtained from physical experiments to lay out dot-accurate circuitry that is resilient against the presence of atomic surface defects. This culminates in a holistic evaluation on surface data of varying defect rates that enables us to quantify the severity of such defects. We project that fabrication capabilities must achieve defect rates of around 0.1 %, if charged defects can be completely eliminated, or < 0.1 %, otherwise. This realization sets the pace for future efforts to scale up this promising circuit technology.

physics.app-ph↗

arXiv2023

Simulating Charged Defects in Silicon Dangling Bond Logic Systems to Evaluate Logic Robustness

Recent research interest in emerging logic systems based on quantum dots has been sparked by the experimental demonstration of nanometer-scale logic devices composed of atomically sized quantum dots made of silicon dangling bonds (SiDBs), along with the availability of SiQAD, a computer-aided design tool designed for this technology. Latest design automation frameworks have enabled the synthesis of SiDB circuits that reach the size of $32\times10^3\,\text{nm}^{2}$ -- orders of magnitude more complex than their hand-designed counterparts. However, current SiDB simulation engines do not take defects into account, which is important to consider for these sizable systems. This work proposes a formulation for incorporating fixed-charge simulation into established ground state models to cover an important class of defects that has a non-negligible effect on nearby SiDBs at the $10\,\text{nm}$ scale and beyond. The formulation is validated by implementing it into SiQAD's simulation engine and computationally reproducing experiments on multiple defect types, revealing a high level of accuracy. The new capability is applied towards studying the tolerance of several established logic gates against the introduction of a single nearby defect to establish the corresponding minimum required clearance. These findings are compared against existing metrics to form a foundation for logic robustness studies.

cond-mat.mes-hall↗

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↗

arXiv2018

Initiating and monitoring the evolution of single electrons within atom-defined structures

Using a non-contact atomic force microscope we track and manipulate the position of single electrons confined to atomic structures engineered from silicon dangling bonds (DBs) on the hydrogen terminated silicon surface. By varying the probe-sample separation we mechanically manipulate the equilibrium position of individual surface silicon atoms and use this to directly switch the charge state of individual DBs. Because this mechanism is based on short range interactions and can be performed without applied bias voltage, we maintain both site-specific selectivity and single-electron control. We extract the short range forces involved with this mechanism by subtracting the long range forces acquired on a dimer vacancy site. As a result of relaxation of the silicon lattice to accommodate negatively charged DBs we observe charge configurations of DB structures that remain stable for many seconds at 4.5 K. Subsequently we use charge manipulation to directly prepare the ground state and metastable charge configurations of DB structures composed of up to six atoms.

cond-mat.mes-hall↗