Electrical Characterization of Nanowires

Steven Kuo

San Jose State University

Thesis Advisor

Dr. Emily Allen

San Jose State University

Research Advisor

Dr. Geetha R. Dholakia

NASA AMES Center for Nanotechnology

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Task

Synthesis of nanowires by templated sol-gel growth and structural and electronic studies for applications in spin-based devices.

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Outline

Background Research Tasks Methods Results Summary of Work

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Why Nanotechnology?

Limit in today’s electronic device process

Need alternate method to continue shrinking

Nanowires - key group of nanoscale materials in developing devices

Nanoelectronics benefit from knowledge of material characteristics

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Why do we need to characterize nanowires?

Bulk properties differ from nanoscale properties Surface and grain boundary scattering

Need a method of electrical character… nanoscale materials in order to produce useful devices

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Electrical properties need to be studied…but how?

Current method of electrical characterization

Wire diameter is microns wide

What happens when…

Wire diameter is only nanometers wide?

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Research Tasks Task 1: Separation and alignment of nanowires

Removal of nanowires from Quick and easy manipulation of nanowires onto contact pads of

devices Task 2: Setup IV Measurement System

MMR Technologies Cryocooler LabVIEW Instrument interface

Task 3: IV Measurements Determine electrical characteristics of nanowires by a 4 probe

method Resistivity measurements across temperature range of

80K – 400K Determine band gap information for semiconducting nanowires

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Research Steps

Part 1: Liberate nanowires from anodized alumina template (completed)

Part 2: Align nanowires using electric field (completed)

Part 3: Setup temperature dependent measurement system (completed)

Part 4: Band gap measurements on single nanowire

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Band gap information can be determined

From 4 probe measurements Resistivity can be

determined From the relationship

ρ=1/σ Conductivity can be

determined Plot ln σ vs. 1/T Using the equation

where x is 1/T og xk

E ln

2ln

Eg is the band gap of the material

1/T

ln σ

-Eg/2k

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How do we manipulate nanowires when they are so small?

Dielectrophoresis Force which acts on any polarizable object in a

nonuniform electric field

Electrodes

NanowireElectric field

)()( 2rmsmDEP EKF

i

m

mprodK

Re)( where

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E-Field Modeling

Simulation of the expected e-field was calculated using Maxwell software

E-Field expected to be strongest at corners between the electrodes

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E-Field Alignment Device Design and Fabrication

Interdigitated electrodes fabricated at Microelectronics Process Engineering Lab at SJSU

Interdigitated electrodes 3-6 um spacings

200nm Al on 700nm SiO2 insulating layer

4 in. wafer with approx. 33 devices

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Finished Electrode Devices

SEM images of fabricated devices

Optical images of finished wafer and single device

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Nanowire removal from template

AAT removed with NaOH

Nanowires released by sonication

BIG Problem!!

Anodized Alumina Template

TiO2 Nanowires

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E-Field Alignment of Nanowires on Devices TiO2 nanowires are

aligned across 3 – 6 um spaced electrodes by an AC bias 25Hz – 30MHz 10 Vpp

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Temperature Dependent Resistivity Measurements

MMR Technologies Cryocooler 80K – 400K temperature range Verified to 80K

Keithley Electrometer and Current Source LabVIEW interface to control electrometer and current source

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Ongoing Work

Redesign of the test device for accommodation in the MMR cryocooler

Possible new electrode design (not to scale)

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Summary of Work to Date

Nanowire alignment on electrodes has been achieved

Temperature dependent resistivity measurement system completed

Publications Steven Kuo, Geetha R Dholakia and E. L. Allen, “Self

assembly of TiO2 nanowires onto devices by dielectrophoresis,” accepted for poster presentation Spring Materials Research Society Meeting, San Francisco (March 2007).

Geetha R Dholakia , Steven Kuo and E. L. Allen “Self assembly of organic nanostructures and dielectrophoretic assembly of inorganic nanowires,” accepted for presentation at American Physical Society Conference, April 2007.

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Work to be Completed by Aug 07

Task 3: New mask devices

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Acknowledgements

DMEA Grant H94003-06-2-0605 Ms. Rebka Endale, SJSU Dr. Ann Marshall, Stanford

Nanocharacterization Lab Mr. Neil Peters, Microelectronics

Process Engineering Lab Ms. Anastasia Micheals, SEM Lab