UC Berkeley

Abstract

Two-Dimensional, Individually-Addressable Nanostructure Arrays

by

Chu-Yeu Peter Yang

Doctor of Philosophy in Mechanical Engineering

University of California, Berkeley

Professor Liwei Lin, Chair

Assembly and integration of vertically-oriented, one-dimensional nanostructures into a two-dimensional array platform has been demonstrated. This 2D architecture is realized using top-down semiconductor and microelectro-mechanical system (MEMS) processes as well as bottom-up catalyst-assisted 1D nanostructure synthesis methods. Two prototype demonstrations include: (1) a 15×12 carbon nanotube (CNT) array and (2) a 2×5 ZnO nanowire array systems.

Arrays based on CNT structures have been developed to verify the feasibility of the 2D architecture using vertically-oriented nanostructures. A trench process is conducted by dry etching into a silicon substrate. A CNT growth process follows to fill the trenches while top electrodes are defined by metallization with a shadow mask. Measurement results show that the average resistance value from the 15×12 CNT electrodes with a contact area of 500×400 um^2 was several tens of ohms. To get a better understanding for the electrical characteristics of the as-fabricated CNT structures, a single 1 um-long CNT sample with measured diameter of 30 nm was attached to two nanoprobes inside a scanning electron microscope. Its resistivity was characterized as 0.056 ohm-m and the overall resistance value of the CNT nodes in the 2D array was estimated at 41 ohms;. This result correlates well with measured resistance values from the CNT nodes, suggesting that there are successful electrical connections between the top and bottom electrodes through the CNT bundles.

Individual-addressability of the 2D architecture has been enabled by replacing the metallic CNTs with semiconducting ZnO nanowires and creating rectifying, Schottky diodes at the array nodes. A 2×5 array featuring asymmetric bottom platinum-nZnO Schottky junctions and top nZnO-titanium/gold ohmic contacts was fabricated. ZnO nanowires were first synthesized atop of metal electrodes on a silicon substrate. After applying spin-on-glass, the tips of the nanowires were exposed by plasma etching and top metal electrodes were deposited. The I-V characteristics of a 150×500 um^2 node under forward and reverse biases with and without a 40 um-wide UV laser beam were tested. It is found that a photogenerated current is detectable from the 2D array with an estimated responsivity of 3×10^-4 A/W. Moving the laser beam to neighboring nodes did not result in an increased current from the interrogated node, demonstrating the individual-addressability of the 2D ZnO nanowire array and its capability for discrete pattern recognition as a UV photodetector.

This marks the first successful demonstration of vertically integrating ZnO nanowires into an individually-addressable 2D array for possible ultrahigh-density applications in nanoelectronic memory, information displays, photodetectors, and nano-lithography.