EE PhD Oral Examination - Nahid Harjee, Monday, May 23, 2011; 8:00 a.m.

Stanford University Oral Defense – Department of Electrical Engineering

Speaker: Nahid Harjee

Advisor: Prof. Beth Pruitt

Co-Advisor: Prof. David Goldhaber-Gordon

Date: Monday, May 23, 2011

Time: 8:00 am (refreshments at 7:45 am)

Location: McCullough 335

http://campus-map.stanford.edu/index.cfm?ID=04-490

Title

Coaxial-Tip Piezoresistive Cantilever Probes for High-Resolution Scanning Gate Microscopy

Abstract

Scanning probe techniques provide a wealth of information about the nanoscale properties of materials and devices, ranging from surface topography to the presence of magnetic domains. In scanning gate microscopy (SGM), the current through a sample is recorded as a sharp, conductive tip that modifies the local electrostatic potential is scanned above the surface. SGM has been used to map current flow, carrier density and potential barriers. However, existing SGM probes produce broad electric fields that limit lateral resolution. In order to apply SGM effectively to nanostructures of recent interest including carbon nanotubes and quantum dots, there is a need for a probe that can produce highly-localized electric fields. This probe must also self-sense topography for tip-sample alignment, as conventional laser-based detection methods can disturb photosensitive samples.

In this talk, I will present a new probe that integrates a coaxial tip, shielding electric fields up to the tip apex, and a piezoresistor to electrically measure cantilever deflection. First, I will discuss the optimization of probe geometry and operating conditions to maximize vertical displacement resolution and investigate the effect of tip shape on lateral resolution. Next, I will describe the development of a process to batch-fabricate the probes and compare two techniques to create sub-micron tip apertures with focused ion beam milling. Finally, I will provide images of the coaxial tip potential profile, obtained using a quantum point contact at cryogenic temperatures, which demonstrate that the coaxial tip can produce significantly narrower perturbations than standard, unshielded tips.