Kenton D. Childs

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.

Mass-fabricated one-dimensional silicon nanogaps for hybrid organic/nanoparticle arrays

Optical lithography based on microfabrication techniques was employed to fabricate one-dimensional nanogaps with micrometre edge lengths in silicon. These one-dimensional nanogaps served as a platform on which organic/nanoparticle films were assembled. Characterization of the gaps was performed with high-resolution TEM, SEM, and electrical measurements. Novel self-assembling attachment chemistry, based on the interaction of silicon with a diazonium salt, was used to iteratively build a multi-layer nanoparticle film across a 7 nm gap. By using nanoparticles capped with an easily displaced ligand, a variable conductive path was created across the 1D nanogap. Electrical measurements of the gap showed a dramatic change in the I(V) characteristics after assembly of the nanoparticle film.

Novel one-dimensional nanogap created with standard optical lithography and evaporation procedures

This article details a simple four-step procedure to create a one-dimensional nanogap on a buried oxide substrate that relies on conventional photolithography performed on a stack of silicon/silicon oxide/silicon, metal evaporation, and hydrofluoric acid oxide removal. Once the nanogap was fabricated it was bridged with an assembly of 1,8-octanedithiol and 5 nm Au nanoparticles capped with a sacrificial dodecylamine coating. Before assembly, characterization of the nanogaps was performed through electrical measurements and SEM imaging. Post assembly, the resistance of the nanogaps was evaluated. The current increased from 70 fA to 200 microA at +1 V bias, clearly indicating a modification due to nanoparticle molecule assembly. Control experiments without nanoparticles or octanedithiol did not show an increase in current.

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.

Room temperature single ion detection with Geiger mode avalanche diode detectors

We report on the fabrication and performance of a novel single ion Geiger mode avalanche (SIGMA) diode detector that senses single ions with ∼100% detection efficiency at room temperature for 250 keV protons. The SIGMA diode detector utilizes Geiger mode operation of avalanche photodiodes, which can be sensitive to single electron-hole (e-h) pairs induced by the ion stopping. The SIGMA diode detector takes advantage of a complementary metal oxide semiconductor foundry allowing for future integration with silicon nanostructures to build novel single atom modified devices. SIGMA diode detector offers potential improvement in current integrated ion detector approaches that have noise floors in the order of 103 e-h pairs.

Ge–Si separate absorption and multiplication avalanche photodiode for Geiger mode single photon detection

A Ge–Si separate absorption and multiplication avalanche photodiode (SAM-APD) is reported. The structure is grown using a low temperature in situ clean and epitaxy process, Tinsitu and Tepitaxy<∼460°C, resulting in a Ge layer with a dislocation density of ∼5×1010cm−2. The SAM-APD has a responsivity of 3.2×10−4A∕W (1310nm) and a dark current density at punch-through of 0.2mA∕cm2. In Geiger mode (GM) at 206K, the dark count rates (DCRs) are ∼280kHz, which is within an order of magnitude of DCR reported for InGaAs∕InP GM APDs despite the high defect density in the Ge.

Final report on LDRD project : single-photon-sensitive imaging detector arrays at 1600 nm.

The key need that this project has addressed is a short-wave infrared light detector for ranging (LIDAR) imaging at temperatures greater than 100K, as desired by nonproliferation and work for other customers. Several novel device structures to improve avalanche photodiodes (APDs) were fabricated to achieve the desired APD performance. A primary challenge to achieving high sensitivity APDs at 1550 nm is that the small band-gap materials (e.g., InGaAs or Ge) necessary to detect low-energy photons exhibit higher dark counts and higher multiplication noise compared to materials like silicon. To overcome these historical problems APDs were designed and fabricated using separate absorption and multiplication (SAM) regions. The absorption regions used (InGaAs or Ge) to leverage these materials 1550 nm sensitivity. Geiger mode detection was chosen to circumvent gain noise issues in the III-V and Ge multiplication regions, while a novel Ge/Si device was built to examine the utility of transferring photoelectrons in a silicon multiplication region. Silicon is known to have very good analog and GM multiplication properties. The proposed devices represented a high-risk for high-reward approach. Therefore one primary goal of this work was to experimentally resolve uncertainty about the novel APD structures. This work specifically examined three different designs. An InGaAs/InAlAs Geiger mode (GM) structure was proposed for the superior multiplication properties of the InAlAs. The hypothesis to be tested in this structure was whether InAlAs really presented an advantage in GM. A Ge/Si SAM was proposed representing the best possible multiplication material (i.e., silicon), however, significant uncertainty existed about both the Ge material quality and the ability to transfer photoelectrons across the Ge/Si interface. Finally a third pure germanium GM structure was proposed because bulk germanium has been reported to have better dark count properties. However, significant uncertainty existed about the quantum efficiency at 1550 nm the necessary operating temperature. This project has resulted in several conclusions after fabrication and measurement of the proposed structures. We have successfully demonstrated the Ge/Si proof-of-concept in producing high analog gain in a silicon region while absorbing in a Ge region. This has included significant Ge processing infrastructure development at Sandia. However, sensitivity is limited at low temperatures due to high dark currents that we ascribe to tunneling. This leaves remaining uncertainty about whether this structure can achieve the desired performance with further development. GM detection in InGaAs/InAlAs, Ge/Si, Si and pure Ge devices fabricated at Sandia was shown to overcome gain noise challenges, which represents critical learning that will enable Sandia to respond to future single photon detection needs. However, challenges to the operation of these devices in GM remain. The InAlAs multiplication region was not found to be significantly superior to current InP regions for GM, however, improved multiplication region design of InGaAs/InP APDs has been highlighted. For Ge GM detectors it still remains unclear whether an optimal trade-off of parameters can achieve the necessary sensitivity at 1550 nm. To further examine these remaining questions, as well as other application spaces for these technologies, funding for an Intelligence Community post-doc was awarded this year.

Functionalized Nanoparticles for Sensor Applications

We will describe our work on functionalized arrays of nanoparticles crosslinked with short conducting molecules that contain sensing functionalities. These bridging ligands modulate their conductivity based on their interaction with analytes. This functionalized nanoparticles organic ligand composite material once it is assembled between nanogaps electrodes will provide nanosized sensors that can be easily interrogated. These nanogap sensors will be engineered so that they can be fabricated into arrays of different sensor elements. This project consists of a number of different requirements that must be met in order to enable the use of functionalized nanoparticles for sensor applications. The first requirement is the appropriately functionalized nanoparticle. The second is a method to assemble the particles. The third requirement is the generation of a nanogap to contain the nanoparticles. The successes in each of these areas will be discussed as will the sensing behavior of the final films.