Electric current paths in a Si:P delta-doped device imaged by nitrogen-vacancy diamond magnetic microscopy.

The recently-developed ability to control phosphorous-doping of silicon at an atomic level using scanning tunneling microscopy, a technique known as atomic precision advanced manufacturing (APAM), has allowed us to tailor electronic devices with atomic precision, and thus has emerged as a way to explore new possibilities in Si electronics. In these applications, critical questions include where current flow is actually occurring in or near APAM structures as well as whether leakage currents are present. In general, detection and mapping of current flow in APAM structures are valuable diagnostic tools to obtain reliable devices in digital-enhanced applications. In this paper, we used nitrogen-vacancy (NV) centers in diamond for wide-field magnetic imaging (with a few-mm field of view and micron-scale resolution) of magnetic fields from surface currents flowing in an APAM test device made of a P delta-doped layer on a Si substrate, a standard APAM witness material. We integrated a diamond having a surface NV ensemble with the device (patterned in two parallel mm-sized ribbons), then mapped the magnetic field from the DC current injected in the APAM device in a home-built NV wide-field microscope. The 2D magnetic field maps were used to reconstruct the surface current densities, allowing us to obtain information on current paths, device failures such as choke points where current flow is impeded, and current leakages outside the APAM-defined P-doped regions. Analysis on the current density reconstructed map showed a projected sensitivity of ∼0.03 A m-1, corresponding to a smallest-detectable current in the 200µm wide APAM ribbon of ∼6µA. These results demonstrate the failure analysis capability of NV wide-field magnetometry for APAM materials, opening the possibility to investigate other cutting-edge microelectronic devices.

Fabrication and field emission properties of vertical, tapered GaN nanowires etched via phosphoric acid.

The controlled fabrication of vertical, tapered, and high-aspect ratio GaN nanowires via a two-step top-down process consisting of an inductively coupled plasma reactive ion etch followed by a hot, 85% H3PO4crystallographic wet etch is explored. The vertical nanowires are oriented in the[0001]direction and are bound by sidewalls comprising of{336¯2}semipolar planes which are at a 12° angle from the [0001] axis. High temperature H3PO4etching between 60 °C and 95 °C result in smooth semipolar faceting with no visible micro-faceting, whereas a 50 °C etch reveals a micro-faceted etch evolution. High-angle annular dark-field scanning transmission electron microscopy imaging confirms nanowire tip dimensions down to 8-12 nanometers. The activation energy associated with the etch process is 0.90 ± 0.09 eV, which is consistent with a reaction-rate limited dissolution process. The exposure of the{336¯2}type planes is consistent with etching barrier index calculations. The field emission properties of the nanowires were investigated via a nanoprobe in a scanning electron microscope as well as by a vacuum field emission electron microscope. The measurements show a gap size dependent turn-on voltage, with a maximum current of 33 nA and turn-on field of 1.92 V nm-1for a 50 nm gap, and uniform emission across the array.

Imaging of localized electronic states at a nonconducting surface by single-electron tunneling force microscopy.

Localized electronic states near a nonconducting SiO(2) surface are imaged on a approximately 1 nm scale by single-electron tunneling between the states and a scanning probe tip. Each tunneling electron is detected by electrostatic force. The images represent the number of tunneling electrons at each spatial location. The spatial resolution of the single electron tunneling force microscope is determined by quantum mechanical tunneling, providing new atomic-scale access to electronic states in dielectric surfaces and nonconducting nanostructures.

Electrostatic force microscopy analysis of lipid miscibility in two-component monolayers.

Electrostatic force microscopy (EFM) was used to assess lipid miscibility and phase behavior in two-component Langmuir-Blodgett (LB) monolayers composed of cationic dioctadecyldimethylammonium bromide (DOMA) and nonionic methyl stearate (SME) lipids. The surface potential measurements were calibrated by applying known bias voltages to the sample during several line scans, thus creating surface potential "scale bars" on the images from which it was determined that circular domains were 50 mV more positive than the surrounding phase. As the spatially averaged surface potential of DOMA was over 400 mV more positive than that of SME, this 50-mV surface potential difference is too low to correspond to lipid phase separation (immiscibility) in the two-component film. Rather, the surface potential contrast was attributed to an increased packing density and a more orthogonal orientation of lipids in the domains resulting in a greater contribution of dipoles to the measured (normal) surface potential. Monolayers prepared by sequentially spreading the two lipids resulted in irregular domains that were 50-450 mV more positive than the surrounding phase, representing varying degrees of lipid mixing, restricted by two-dimensional diffusion at the interface. Fluorescent images of monolayers stained with negatively charged dye supported the EFM miscibility prediction and assignment of surface potential. These results demonstrate a new approach using EFM to quantitatively measure surface potential in order to assess the lateral distribution of components in thin films as well as predict adsorption patterns to heterogeneous interfaces.