Eric Nordberg

RSNA/QIBA: Shear wave speed as a biomarker for liver fibrosis staging

An interlaboratory study of shear wave speed (SWS) estimation was performed. Commercial shear wave elastography systems from Fibroscan, Philips, Siemens and Supersonic Imagine, as well as several custom laboratory systems, were involved. Fifteen sites were included in the study. CIRS manufactured and donated 11 pairs of custom phantoms designed for the purposes of this investigation. Dynamic mechanical tests of equivalent phantom materials were also performed. The results of this study demonstrate that there is very good agreement among SWS estimation systems, but there are several sources of bias and variance that can be addressed to improve consistency of measurement results.

Enhancement-mode double-top-gated metal-oxide-semiconductor nanostructures with tunable lateral geometry

We present measurements of silicon (Si) metal-oxide-semiconductor (MOS) nanostructures that are fabricated using a process that facilitates essentially arbitrary gate geometries. Stable Coulomb-blockade behavior showing single-period conductance oscillations that are consistent with a lithographically defined quantum dot is exhibited in several MOS quantum dots with an open-lateral quantum-dot geometry. Decreases in mobility and increases in charge defect densities (i.e., interface traps and fixed-oxide charge) are measured for critical process steps, and we correlate low disorder behavior with a quantitative defect density. This work provides quantitative guidance that has not been previously established about defect densities and their role in gated Si quantum dots. These devices make use of a double-layer gate stack in which many regions, including the critical gate oxide, were fabricated in a fully qualified complementary metal-oxide semiconductor facility.

Single-crystal silicon/silicon dioxide multilayer heterostructures based on nanomembrane transfer

A method to fabricate single-crystal Si∕SiO2 multilayer heterostructures is presented. Heterostructures are fabricated by repeated transfer of single crystal silicon nanomembranes alternating with deposition of spin-on-glass. Nanomembrane transfer produces multilayers with low surface roughness and smooth interfaces. To demonstrate interface quality, the specular reflectivities of one-, two-, and three-membrane heterostructures are measured. Comparison of the measured reflectivity with theoretical calculations shows good agreement. Nanomembrane stacking allows for the preprocessing of individual membranes with a high thermal budget before the low thermal budget assembly of the stack, suggesting a new avenue for the three dimensional integration of integrated circuits.

Charge sensing in enhancement mode double-top-gated metal-oxide-semiconductor quantum dots

Laterally coupled charge sensing of quantum dots is highly desirable because it enables measurement even when conduction through the quantum dot itself is suppressed. In this work, we demonstrate such charge sensing in a double-top-gated metal-oxide-semiconductor system. The current through a point contact constriction integrated near a quantum dot shows sharp 2% changes corresponding to charge transitions between the dot and a nearby lead. We extract the coupling capacitance between the charge sensor and the quantum dot, and we show that it agrees well with a three-dimensional capacitance model of the integrated sensor and quantum dot system.

Double quantum dot with tunable coupling in an enhancement-mode silicon metal-oxide semiconductor device with lateral geometry

We present transport measurements of a tunable silicon metal-oxide semiconductor double quantum dot device with lateral geometry. The experimentally extracted gate-to-dot capacitances show that the device is largely symmetric under the gate voltages applied. Intriguingly, these gate voltages themselves are not symmetric. A comparison with numerical simulations indicates that the applied gate voltages serve to offset an intrinsic asymmetry in the physical device. We also show a transition from a large single dot to two well isolated coupled dots, where the central gate of the device is used to controllably tune the interdot coupling.

Effective Scatterer Diameter Estimates for Broad Scatterer Size Distributions

Acoustic form factors have been used to model the frequency dependence of acoustic scattering in phantoms and tissues. This work demonstrates that a broad range of scatterer sizes, individually well represented by Faran theory or a Gaussian form factor, is not accurately described by a single effective scatterer from either of these models. Contributions from a distribution of discrete scatterer sizes for two different form factor functions (Gaussian form factors and scattering functions from Faran’s theory) were calculated and linearly combined. Composite form factors created from Gaussian distributions of scatterer sizes centered at 50 µm with standard deviations of up to σ = 40 µm were fit to each scattering model between 2 and 12 MHz. Scatterer distributions were generated using one of two assumptions: the number density of the scatterer diameter distribution was Gaussian distributed, or the volume fraction of each scatterer diameter in the distribution was Gaussian distributed. Each simulated form factor was fit to a single-diameter form factor model for Gaussian and exponential form factors. The mean-squared error (MSE) between the composite simulated data and the best-fit single-diameter model was smaller with an exponential form factor model, compared with a Gaussian model, for distributions with standard deviations larger than 30% of the centroid value. In addition, exponential models were shown to have better ability to distinguish between Faran scattering model-based distributions with varying center diameters than the Gaussian form factor model. The evidence suggests that when little is known about the scattering medium, an exponential scattering model provides a better first approximation to the scattering correlation function for a broad distribution of spherically symmetric scatterers than when a Gaussian form factor model is assumed.

Capacitance Modeling of Complex Topographical Silicon Quantum Dot Structures

Quantum dot (QD) layouts are becoming more complex as the technology is being applied to more sophisticated multi-QD structures. This increase in complexity requires improved capacitance modeling both for the design and accurate interpretation of QD properties from measurement. A combination of process simulation, electrostatic simulation, and computer-assisted design (CAD) layout packages are used to develop a 3-D classical capacitance model. The agreement of the classical models capacitances is tested against two different, experimentally measured, topographically complex silicon QD geometries. Agreement with experiment, within 10%-20%, is demonstrated for the two structures when the details of the structure are transferred from the CAD to the model capturing the full 3-D topography. Small uncertainties in device dimensions due to uncontrolled variation in processing, like layer thickness and gate size, are calculated to be sufficient to explain the disagreement. The sensitivity of the capacitances to small variations in the structure also highlights the limits of accuracy of capacitance models for QD analysis. We furthermore observe that a critical density, the metal-insulator transition, can be used as a good approximation of the metallic edge of the QD when electron density in the dot is calculated directly with a semiclassical simulation.

Steps Towards Fabricating Cryogenic CMOS Compatible Single Electron Devices

We describe the development of a novel silicon quantum bit (qubit) device architecture that involves using materials that are compatible with a Sandia National Laboratories (SNL) 0.35 mum complementary metal oxide semiconductor (CMOS) process intended to operate at 100 mK. We describe how the qubit structure can be integrated with CMOS electronics, which is believed to have advantages for critical functions like fast single electron electrometry for readout compared to current approaches using radio frequency techniques. Critical materials properties are reviewed and preliminary characterization of the SNL CMOS devices at 4.2 K is presented.