Collaborative Research: Transport Imaging of Semiconductor Nanowires

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Technical: This project combines VLS (vapor-liquid-solid) growth/processing of bridged SiC and Si nanowires with a nanoscale imaging technique to study relationships between synthesis, processing, and contact/transport behavior. Transport imaging combines the scanning resolution of near field optics with the charge generation beam control of a scanning electron microscope (SEM). A goal is to "image transport" by monitoring the motion of charge via the recombination emission of light. The technique builds upon standard cathodoluminescence (CL), but is significantly different, since it maintains spatial information of the emitted light, which allows for direct measurement of spatially resolved minority carrier diffusion, mobility-lifetime products under applied fields, and contact resistance--parameters otherwise difficult to measure on a single nanowire. The project aims to take this imaging to the ~ 50-100 nm scale with the use of an atomic force microscope (AFM)/near field scanning optical microscope (NSOM) operating inside an SEM. By decoupling the e-beam excitation from the optical collection, one can image transport of charge over distances from ~ 100 nm to microns along wire lengths. A Nanonics MultiView 2000 system will be used, which allows for independent scanning of fiber tip or sample. Measurements will be performed on individually bridged SiC and Si nanowires, as well as ZnO and GaN wires. VLS nanowire synthesis/processing research is an integral part of the collaborative research. This technique has been successfully demonstrated for the growth of solid whiskers and nanowires; bridged growth has been demonstrated within silicon microtrenches. The bridged structures have been found to make rigid connections to the receiving side of the trench, making them of interest for large scale device fabrication. They also provide ideal structures for application of external bias for transport imaging. Specific goals include 1) demonstration of direct transport imaging in semiconductor nanowires, 2) transport measurements as a function of nanowire diameter, 3) measurement of contact resistance and carrier injection, 4) study of the role of VLS process catalyst and surface passivation on optical and transport properties. Non-technical: The project addresses basic research issues in a topical area of electronic/photonic materials science with high technological relevance. Additionally, the project provides education and training for three graduate students and a minimum of 6-8 undergraduate and high school students. NPS educates active duty officers in all branches of the US forces, as well as students from 30+ countries. The students reflect the full diversity of the US active duty force and many take their technical education into positions of leadership in the regular workforce upon completion of their service. A UC Berkeley graduate student will work in collaboration, and UCB will take advantage of its SUPERB ((Summer Undergraduate Program in Engineering Research at Berkeley) program to involve undergraduate and summer students. Special emphasis will be placed on recruitment of women and underrepresented students. Both PIs have strong records of mentoring women and underrepresented groups and have been active in undergraduate and K-12 outreach. The results will be disseminated through publication and presentation, as well as public outreach venues at both NPS and UCB.