arXiv subjects

Karen L. Kavanagh

Publications and source records attributed to Karen L. Kavanagh.

5 recordsLinked to original sources

arXiv2026

Thickness-dependent secondary-electron emission from suspended MoS$_2$ membranes in the helium ion microscope

Secondary-electron (SE) emission in the helium ion microscope (HIM) becomes sensitive to membrane thickness when the sample is thin enough for He-ion transmission and when the SEs emitted from the bottom surface are collected. We correlated the total SE intensity of suspended, nanometer-thick MoS$_2$ flakes on lacey carbon with thickness measured independently by electron energy-loss spectroscopy. The response peaks at 40-55 nm, with an apparent back-to-front SE signal ratio reaching 4.7. The peaked, thickness-dependent component is attributed primarily to SE emission at the bottom surface of the flake, rather than to transmitted ions striking instrument surfaces. Applying SRIM ionization profiles, an asymmetric SE-escape model with a longer escape depth on the exit side reproduces the response. We find an effective exit-side escape depth of approximately 10 nm, five times the assumed 2 nm entrance value, suggesting that deposited energy reaches the exit surface far more efficiently than the entrance surface or that the SRIM model's energy deposition profile is shifted by an effect such as channeling. The correlation provides a rapid thickness screen for suspended membranes and a route to testing low-energy ion-solid interaction models in thin materials.

cond-mat.mtrl-sci↗

arXiv2020

Transmission Helium Ion Microscopy of Graphene

We compare transmission He ion microscopy (HIM) to transmission electron microscopy (TEM) of graphene support films. We present spot transmission patterns that compare with scattering and range of ions in materials (SRIM) predictions, and show examples of scanning He$^+$ transmission images, based on integrated camera intensity. We also consider the potential for coherent HIM scattering.

physics.app-ph↗

arXiv2020

Abrupt degenerately-doped silicon nanowire tunnel junctions

We have confirmed the presence of narrow, degenerately-doped axial silicon nanowire (SiNW) $p$-$n$ junctions via off-axis electron holography (EH). SiNWs were grown via the vapor-solid-liquid (VLS) mechanism using gold (Au) as the catalyst, silane (SiH$_{4}$), diborane (B$_{2}$H$_{6}$) and phosphine (PH$_{3}$) as the precursors, and hydrochloric acid (HCl) to stabilize the growth. Two types of growth were carried out, and in each case we explored growth with both $n$/$p$ and $p$/$n$ sequences. In the first type, we abruptly switched the dopant precursors at the desired junction location, and in the second type we slowed the growth rate at the junction to allow the dopants to readily leave the Au catalyst. We demonstrate degenerately-doped $p$/$n$ and $n$/$p$ nanowire segments with abrupt potential profiles of $1.02\pm0.02$ and $0.86\pm0.3$ V, and depletion region widths as narrow as $10\pm1$ nm via EH. Low temperature current-voltage measurements show an asymmetric curvature in the forward direction that resemble planar gold-doped tunnel junctions, where the tunneling current is hidden by a large excess current. The results presented herein show that the direct VLS growth of degenerately-doped axial SiNW $p$-$n$ junctions is feasible, an essential step in the fabrication of more complex SiNW-based devices for electronics and solar energy.

physics.app-ph↗

arXiv2019

Three-dimensional imaging of beam-induced biasing of InP/GaInP tunnel diodes

Electron Holographic Tomography was used to obtain 3-dimensional reconstructions of the morphology and electrostatic potential gradient of axial GaInP/InP nanowire tunnel diodes. Crystal growth was carried out in two opposite directions: GaInP:Zn/InP:S and InP:Sn/GaInP:Zn, using Zn as the p-type dopant in the GaInP, but with changes to the n-type dopant (S or Sn) in the InP. Secondary electron and electron beam induced current images obtained using scanning electron microscopy indicated the presence of p-n junctions in both cases and current-voltage characteristics measured via lithographic contacts showed the negative differential resistance, characteristic of band-to-band tunneling, for both diodes. EHT measurements confirmed a short depletion width in both cases ($21 \pm 3$ nm), but different built-in potentials, $V_{bi}$, of 1.0 V for the p-type (Zn) to n-type (S) transition, and 0.4 V for both were lower than the expected 1.5 V for these junctions, if degenerately-doped. Charging induced by the electron beam was evident in phase images which showed non-linearity in the surrounding vacuum, most severe in the case of the nanowire grounded at the \emph{p}-type Au contact. We attribute their lower $V_{bi}$ to asymmetric secondary electron emission, beam-induced current biasing and poor grounding contacts.

physics.app-ph↗

arXiv2012

Lateral spin injection and detection through electrodeposited Fe/GaAs contacts

Efforts to achieve efficient injection of spin-polarized electrons into a semiconductor, a key prerequisite for developing electronics that exploit the electron's spin degree of freedom, have so far met with limited success. Here we report experimental studies of lateral spin injection and detection through electrodeposited Fe/GaAs tunnel contacts. We demonstrate spin injection efficiencies two orders of magnitude higher than for state-of-the-art contacts fabricated via ultra-high-vacuum methods, including those with MgO or Al2O3 tunnel barriers. To account for this enhancement, we propose that an iron oxide layer that forms at the Fe/GaAs interface during electrodeposition, being magnetic, acts as a tunnel barrier with a spin-dependent height, presenting quantum spin transport calculations for such systems. This serendipitous discovery of greatly enhanced efficiency of spin injection into GaAs via electrodeposited contacts introduces a promising new direction for the development of practical semiconductor spintronic devices.

cond-mat.mes-hall↗