Scientists Engineer Superconducting Thin Films

Scientists Engineer Superconducting Thin Films

One step closer to fabrication of useful devices such as superconductive transistors

October 8, 2008

UPTON, NY - One major goal on the path toward making useful superconducting devices has been engineering materials that act as superconductors at the nanoscale — the realm of billionths of a meter. Such nanoscale superconductors would be useful in devices such as superconductive transistors and eventually in ultrafast, power-saving electronics.

In the October 9, 2008, issue of Nature, scientists at the U.S. Department of Energy’s (DOE) Brookhaven National Laboratory report that they have successfully produced two-layer thin films where neither layer is superconducting on its own, but which exhibit a nanometer-thick region of superconductivity at their interface. Furthermore, they demonstrate the ability to elevate the temperature of superconductivity at this interface to temperatures exceeding 50 kelvin (-370°F), a relatively high temperature deemed more practical for real-world devices. 

“This work provides definitive proof of our ability to produce robust superconductivity at the interface of two layers confined within an extremely thin, 1-2-nanometer-thick layer near the physical boundary between the two materials,” said physicist Ivan Bozovic, who leads the Brookhaven thin film research team. “It opens vistas for further progress, including using these techniques to significantly enhance superconducting properties in other known or new superconductors.”

Bozovic foresees future research investigating different combinations of non-superconducting materials. “Further study of the temperature-enhancement mechanism might even tell us something about the big puzzle — the mechanism underlying high-temperature superconductivity, which remains one of the most important open problems in condensed matter physics,” he said.

Bozovic’s team had reported in 2002 the bizarre observation that the critical temperature — the temperature below which the sample superconducts — could be enhanced by as much as 25 percent in bilayers of two dissimilar copper-based materials. However, at that time, the scientists had no understanding of what caused this enhancement and in which part of the sample the superconductivity was located.

To investigate this further, they synthesized more than 200 single-phase, bilayer and trilayer films with insulating, metallic, and superconducting blocks in all possible combinations and of varying layer thickness. The films were grown in a unique atomic-layer-by-layer molecular beam epitaxy system designed and built by Bozovic and coworkers to enable the synthesis of atomically smooth films as well as multilayers with perfect interfaces. “The greatest technical challenge was to prove convincingly that the superconducting effect does not come from simple mixing of the two materials and formation of a third, chemically and physically distinct layer between the two constituent layers,” Bozovic said. Collaborators at Cornell University ruled out this possibility using atomic-resolution transmission electron microscopy to identify the samples’ constituent chemical elements, proving that the layers indeed remained distinct.

“It is too early to tell what applications this research might yield,” Bozovic said, “but already at this stage we can speculate that this brings us one big step closer to fabrication of useful three-terminal superconducting devices, such as a superconductive field-effect transistor.” In such a device, one would be able to switch the transistor from the superconducting to the resistive state by means of an external electric field, controlled by applying a voltage and using the third (gate) electrode. Circuits built from such devices would be much faster and use less power than the current ones based on semiconductors.

“No matter what the applications, this work is a nice demonstration of our ability to engineer and control materials at sub-nanometer scale, with designed and enhanced functionality,” Bozovic said.

The Brookhaven scientists have filed a U.S. provisional patent application for this work. For information about licensing, please contact Kimberley Elcess, 631-344-4151, elcess@bnl.gov.

In addition to Bozovic, the research team includes Adrian Gozar, Gennady Logvenov, and Anthony Bollinger of Brookhaven Lab, Lenna Fitting Kourkoutis and David A. Muller of Cornell University, and Lucille A. Giannuzzi of the FEI Company, Hillsboro, Oregon. The research at Brookhaven Lab was funded by the Office of Basic Energy Sciences within the DOE’s Office of Science; the Cornell work was funded by the Office of Naval Research.

New 'nano-positioners' may have atomic-scale precision

August 20, 2008 
New 'nano-positioners' may have atomic-scale precision

WEST LAFAYETTE, Ind. -  Engineers have created a tiny motorized positioning device that has twice the dexterity of similar devices being developed for applications that include biological sensors and more compact, powerful computer hard drives.

The device, called a monolithic comb drive, might be used as a "nanoscale manipulator" that precisely moves or senses movement and forces. The devices also can be used in watery environments for probing biological molecules, said Jason Vaughn Clark, an assistant professor of electrical and computer engineering and mechanical engineering, who created the design.

The monolithic comb drives could make it possible to improve a class of probe-based sensors that detect viruses and biological molecules. The sensors detect objects using two different components: A probe is moved while at the same time the platform holding the specimen is positioned. The new technology would replace both components with a single one - the monolithic comb drive.

The innovation could allow sensors to work faster and at higher resolution and would be small enough to fit on a microchip. The higher resolution might be used to design future computer hard drives capable of high-density data storage and retrieval. Another possible use might be to fabricate or assemble miniature micro and nanoscale machines.

Research findings were detailed in a technical paper presented in July during the University Government Industry Micro/Nano Symposium in Louisville. The work is based at the Birck Nanotechnology Center at Purdue's Discovery Park.

Conventional comb drives have a pair of comblike sections with "interdigitated fingers," meaning they mesh together. These meshing fingers are drawn toward each other when a voltage is applied. The applied voltage causes the fingers on one comb to become positively charged and the fingers on the other comb to become negatively charged, inducing an attraction between the oppositely charged fingers. If the voltage is removed, the spring-loaded comb sections return to their original position. 

By comparison, the new monolithic device has a single structure with two perpendicular comb drives. 

Clark calls the device monolithic because it contains comb drive components that are not mechanically and electrically separate. Conventional comb drives are structurally "decoupled" to keep opposite charges separated. 

"Comb drives represent an advantage over other technologies," Clark said. "In contrast to piezoelectric actuators that typically deflect, or move, a fraction of a micrometer, comb drives can deflect tens to hundreds of micrometers. And unlike conventional comb drives, which only move in one direction, our new device can move in two directions - left to right, forward and backward - an advance that could really open up the door for many applications."

Clark also has invented a way to determine the precise deflection and force of such microdevices while reducing heat-induced vibrations that could interfere with measurements.

Current probe-based biological sensors have a resolution of about 20 nanometers. 

"Twenty nanometers is about the size of 200 atoms, so if you are scanning for a particular molecule, it may be hard to find," Clark said. "With our design, the higher atomic-scale resolution should make it easier to find."

Properly using such devices requires engineers to know precisely how much force is being applied to comb drive sensors and how far they are moving. The new design is based on a technology created by Clark called electro micro metrology, which enables engineers to determine the precise displacement and force that's being applied to, or by, a comb drive. The Purdue researcher is able to measure this force by comparing changes in electrical properties such as capacitance or voltage.

Clark used computational methods called nodal analysis and finite element analysis to design, model and simulate the monolithic comb drives. 

The research paper describes how the monolithic comb drive works when voltage is applied. The results show independent left-right and forward-backward movement as functions of applied voltage in color-coded graphics. 

The findings are an extension of research to create an ultra-precise measuring system for devices having features on the size scale of nanometers, or billionths of a meter. Clark has led research to create devices that "self-calibrate," meaning they are able to precisely measure themselves. Such measuring methods and standards are needed to better understand and exploit nanometer-scale devices. 

The size of the entire device is less than one millimeter, or a thousandth of a meter. The smallest feature size is about three micrometers, roughly one-thirtieth as wide as a human hair. 

"You can make them smaller, though," Clark said. "This is a proof of concept. The technology I'm developing should allow researchers to practically and efficiently extract dozens of geometric and material properties of their microdevices just by electronically probing changes in capacitance or voltage."

In addition to finite element analysis, Clark used a simulation tool that he developed called Sugar.

"Sugar is fast and allows me to easily try out many design ideas," he said. "After I narrow down to a particular design, I then use finite element analysis for fine-tuning. Finite element analysis is slow, but it is able to model subtle physical phenomena that Sugar doesn't do as well."

Clark's research team is installing Sugar on the nanoHub this summer, making the tool available to other researchers. The nanoHub is operated by the Network for Computational Nanotechnology, funded by the National Science Foundation and housed at Purdue's Birck Nanotechnology Center.  

The researchers also are in the process of fabricating the devices at the Birck Nanotechnology Center.

Writer: Emil Venere, (765) 494-4709, venere@purdue.edu 
Sources: Jason Vaughn Clark, (765) 494-3437, jvclark@purdue.edu 
Purdue News Service: (765) 494-2096; purduenews@purdue.edu 

PHOTO CAPTION:

This illustration depicts a tiny device called a monolithic comb drive, which might be used as a high-precision "nanopositioner" for such uses as biological sensors, computer hard drives and other possible applications. The device was created by Jason Vaughn Clark, an assistant professor of electrical and computer engineering and mechanical engineering at Purdue University. (Birck Nanotechnology Center, Purdue University)

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ABSTRACT 

Modeling a Monolithic Comb Drive for Large-Deflection Multi-DOF Microtransduction

J. V. Clark School of Electrical and Computer Engineering School of Mechanical Engineering Purdue University 

To help extend the investigation and exploitation of nanoscale phenomena, there is a need for high precision, large deflection microtransducers with multiple degrees of freedom (DOF). In this paper, we investigate the predicted performance of such a large deflection microtransducer using finite element analysis. To sense and actuate in three dimensions, we use three types of comb drives: a vertical comb drive, a planar comb drive, and a novel planar monolithic comb drive, which operates as an in-situ RC circuit. The two planar comb drives are used to translate a proof mass with independent in-plane x- and y-directions, and the vertical comb drive translates the proof mass in the out-of-plane z-direction. The device resists rotation about the z axis. We address precise sensing and actuation by using high-precision capacitance and voltage to detect position and to apply force, respectively. We explore design issues such as geometry and material properties, and we characterize the monolithic comb drive. We limit the geometry of the transducer to one structural layer, which is amenable to a simple one-mask fabrication process such as silicon-on-insulator (SOI).