Reminder: Ph.D. Dissertation Defense, Today 1PM Hemant Adhikari

Hemant Adhikari adhikari at
Fri Feb 9 11:44:05 PST 2007

On 2/5/07, Hemant Adhikari <adhikari at> wrote:
> 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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