Ph.D. Oral Examination - Ryan Tu - Dec. 19, 2006 11:00 am

Ryan Tu ryantu at stanford.edu
Wed Dec 13 09:06:08 PST 2006


PH.D. ORAL EXAMINATION

"GERMANIUM NANOWIRE CONTROLLED SYNTHESIS, ALIGNMENT, AND FIELD EFFECT
TRANSISTOR PROPERTIES"

Ryan Tu
Department of Materials Science and Engineering, Stanford University
Advisors: Hongjie Dai, Yoshio Nishi, and James Gibbons

Date: Tuesday, Dec. 19, 2006 11:00 am
Location: CIS-X 101
(Refreshment will be served at 10:45 am)


Abstract

Much excitement has been generated in recent years about semiconductor
nanowires (NWs) for future high performance electronics. Single crystal NWs
with diameters ranging from 5nm to 50nm can be chemically synthesized with
relative ease; thus allowing the opportunity for bottom-up fabrication of
NW-based integrated circuits.  Furthermore, the cylindrical symmetry of NWs
offers an advantage over top-down lithographically patterned circuits for
the realization of surround-gated structures to minimize short channel
effects.  As we near the end of the ITRS roadmap at a point where continued
reductions in length scales are unable to surmount the fundamental
limitations of silicon, germanium (Ge) has gained renew interest as a choice
of materials for future electronics owing to its high hole and electron
mobility. Combining both the structural symmetry of NWs and the high
mobility of Ge, GeNW-based devices have the potential to address future
device scaling limitations.
 
In this thesis defense, I review the electrical properties and discuss the
first direct gate capacitance measurements of various GeNW-based field
effect transistors (FETs).  Single crystalline GeNWs were synthesized via
the vapor-liquid-solid mechanism at a low temperature of 275°C from gold
nanoparticles.  Several methods were developed to control the registry,
orientation, and pitch of GeNW arrays, including an e-beam lithography
method to form arrays of individual 20nm gold nanoparticles followed by 100%
yield synthesis and a flow alignment method to deposit thin films of aligned
nanowires with controlled density.  Depletion mode GeNW FETs with Schottky
source-drain metal contacts were fabricated on top of thermally oxidized
silicon substrates with back-gated, top-gated, and surround-gated
geometries. An atomic layer deposition process for HfO2 and Al2O3 was
developed to form the gate oxide in top-gated and surround-gated devices.
C-V curves of disk capacitors fabricated on planar Ge substrates treated
with various surface layer nitridation and silicon interlayer deposition
processes were analyzed to reduce hysteresis and the density of interface
states.  A novel method for measuring atto-farads of capacitance was
developed and used to measure gate capacitance in individual top-gated and
surround-gated GeNW FETs.  This direct measurement enabled the first
accurate evaluation of hole mobility ~400cm2/Vs in GeNWs. 2D finite element
simulations with carefully measured oxide thicknesses and dielectric
constants shed light into the validity of using such approximations in
mobility calculations.  Optimized surround-gated GeNW FETs exhibited high
saturation current and capacitance per unit length with subthreshold slope
~100mV/dec, and could potentially be one component of future high mobility
nanowire integrated circuits.

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