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