NER: Fabrication of MgB2 Quantum Devices

University Of Illinois At Urbana-Champaign, Urbana IL

Investigators

Abstract

This proposal was received in response to the Nanoscale Science and Engineering Initiative, Program Solicitation NSF 01-157, in the NER category. The proposal focuses on a novel, scanned probe approach to the growth of superconducting MgB2 wires and quantum dots. Superconductive electronics has immense appeal since it can be extremely fast and possesses inherently low power dissipation. The 39 K transition temperature of MgB2 in concert with its negligible grain boundary scattering (the central limitation of the copper oxide superconductors) make it the most ideal material known for high temperature superconductive nanoelectronics. MgB2 electronic devices with a minimum feature size of 100 nm have already been made using conventional fabrication techniques. Our approach will use a scanning tunneling microscope (STM) to directly react Mg with B in the presence of an intense tip-induced electric field. This approach has been used to locally oxide metals with features as small as 5 nm. Our project will attempt to apply this technique to the growth of MgB2. The project will require the growth of very pure, thin boron films, the identificationof efficient chemical precursors for scanned probe growth, the use of an ultrahigh vacuum STM for scanned growth, photolithography to produce submicron scale contact pads and low noise, low current cryogenic measurements of resulting devices. In addition to its very small ultimate feature size, the STM approach permits the direct fabrication of arbitrary shaped structures without the complications of conventional lithography. Several important physical problems will be explored. In extremely narrow wires, thermal and quantum fluctuations will play a crucial role in the conduction process. In a quantum dot geometry the discreteness of electrical charge becomes paramount. In a device made from a complex superconductor like MgB2 these processes are only partially understood but will have a direct bearing on the operation of future nanoscale devices. A frontier scientific question that will be explored here is how the mechanism for superconductivity itself can be altered at in sufficiently small devices. There is growing evidence that MgB2 is an admixture of two interpenetrating superconductors. How this unique "two gap" behavior is affected by shrinking the size of the system, and what new possibilities it holds for device operation and for designing new superconductors will all be questions that we hope to initiate with this project.