Ph.D. Dissertation Defense, Feb 9th-Hemant Adhikari

Hemant Adhikari adhikari at stanford.edu
Mon Feb 5 09:04:01 PST 2007


Growth and Passivation of Germanium Nanowires
Hemant Adhikari
Thesis Advisors: Prof. Paul C. McIntyre, Prof. Christopher E.D. Chidsey
Location: CIS-X 101
Time: Feb, 9th 2007. 1.00 PM. (Refreshments served at 12.45PM)

One-dimensional structures such as nanotubes and nanowires are being
actively investigated for various applications in nanotechnology,
including nanoelectronics. Silicon- or germanium-based nanowire
devices are particularly desirable for electronic and other
applications because of their compatibility with silicon integrated
circuits. In 3-dimensional nanoelectronics, vertically aligned
nanowires have been proposed to provide a solution to attain ultra
high density nanoscale device arrays. This study demonstrates the
growth of vertically aligned single-crystal germanium nanowires
(GeNWs) at temperatures of 400°C or less by metal
nanoparticle-catalyzed chemical vapor deposition. Because wires grown
at higher temperatures are tapered, a two-temperature growth procedure
was used to obtain epitaxial GeNWs of constant diameter. Epitaxially
oriented GeNWs are also demonstrated on Ge (110), Ge (001) and on a
hetero-epitaxial Ge film on Si (001) substrates.
The mechanisms governing low temperature epitaxial growth of Ge
nanowires (NWs) during gold nanoparticle-catalyzed chemical vapor
deposition remain controversial.  For the experimental conditions
studied, temperatures close to the bulk Au-Ge eutectic are required
for efficient nanowire nucleation, but subsequent growth of GeNWs
could occur at undercoolings as large as 90°C below the eutectic. The
generally accepted vapor-liquid-solid (VLS) mechanism of NW growth
requires the presence of a eutectic liquid. We have investigated
possible sub-eutectic VLS growth of Ge NWs both experimentally and
theoretically. This study presents the equilibrium phase diagrams for
the Au-Ge binary in the nanometer-scale regime. We find that
equilibrium arguments, including capillary effects, do not explain VLS
for the growth conditions studied. Observations from ex-situ heating
and cooling behavior of GeNWs (without Ge deposition) inside a
transmission electron microscope column suggest that there is a
kinetic barrier to solid Au nucleation which can cause a substantial
undercooling of the liquid below the bulk eutectic temperature. We
have also explored the possibility of the presence of liquid catalyst
at large undercoolings because of Ge supersaturation of the Au-Ge
catalyst particle during NW growth.
Surface passivation of GeNWs before deposition of dielectric and metal
layers has been identified as a key step to fabricate high-performance
devices. A detailed investigation of the surface chemistry of as-grown
and air-exposed GeNWs and exploration of various chemical passivation
pathways was undertaken by photoemission using a low energy
synchrotron source. We also demonstrate uniform encapsulation of
vertically aligned dense array of germanium nanowires by a highly
conformal SiO2¬ layer synthesized by alternating-layer deposition.
While most of the conventional techniques either require high
deposition temperatures or else fail to fill such high aspect ratio
nanostructures without introducing voids in the oxide, this
alternating-layer-deposition process results in rapid conformal
deposition of several monolayers of silica in each cycle at
temperatures as low as 250°C.
The growth and surface passivation of GeNWs demonstrated forms a sound
basis for application of NWs in ultra-high areal density devices for
dimensional scaling of semiconductor memory and logic.



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