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

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

3 Outline Background Research Tasks Methods Results Summary of Work

4 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

5 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

6 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?

7 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

8 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

9 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 E g is the band gap of the material 1/T ln σ -E g /2k

10 How do we manipulate nanowires when they are so small? Dielectrophoresis  Force which acts on any polarizable object in a nonuniform electric field Electrodes Nanowire Electric field where

11 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

12 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

13 Finished Electrode Devices SEM images of fabricated devices Optical images of finished wafer and single device

14 Nanowire removal from template AAT removed with NaOH Nanowires released by sonication BIG Problem!! Anodized Alumina Template TiO 2 Nanowires

15 E-Field Alignment of Nanowires on Devices TiO 2 nanowires are aligned across 3 – 6 um spaced electrodes by an AC bias  25Hz – 30MHz  10 V pp

16 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

17 Ongoing Work Redesign of the test device for accommodation in the MMR cryocooler Possible new electrode design (not to scale)

18 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.

19 Work to be Completed by Aug 07 Task 3: New mask devices

20 Acknowledgements DMEA Grant H Ms. Rebka Endale, SJSU Dr. Ann Marshall, Stanford Nanocharacterization Lab Mr. Neil Peters, Microelectronics Process Engineering Lab Ms. Anastasia Micheals, SEM Lab