Reminder: University Ph.D. Oral Examination - Aditi Chandra

Aditi Chandra kitu at stanford.edu
Fri Feb 24 08:55:48 PST 2006


Stanford University Ph.D. Oral Examination


Aditi Chandra

(Advisor: Prof. Bruce M. Clemens)

Department of Materials Science and Engineering
Stanford University


Monday, February 27, 2006
9:30 AM (Refreshments served at 9:15 AM)
McCullough Building, Room 335



"Synthesis, Characterization, and Applications of Au-Si Nanostructures"

The pursuit of nanoscaled architectures has demanded new synthesis 
methodologies for creating and organizing metal particles.  The challenge 
still remains to create well-defined structures with a tight control over 
particle size and distribution, especially for structures below 10nm.  This 
talk will describe a new technique for creating Au-rich nanoparticles 
through the amorphous phase separation of Si-rich AuSi sputtered 
alloys.  Using this technique, Au-rich nanoparticles, ranging from 2-3nm in 
diameter, can be routinely grown with particle densities on the order of 
10^12 particles/cm^2.  Both high resolution TEM and electron diffraction 
studies indicate that the Au-rich nanoparticles have an amorphous-like 
character.

The phase separation process is modeled by a spinodal decomposition 
mechanism.  Fastest growing composition wavelengths are calculated and are 
found to be in close agreement with the observed average 
particle-to-particle spacing, indicating that Au-rich particles can form by 
this second-order phase transformation.  Extended anneals close to the 
eutectic temperature demonstrate the remarkable stability of these 
particles and suggest that these amorphous-like particles are 
thermodynamically stable due to their relatively low interfacial energy and 
high degree of curvature.

Fundamental studies of annealed AuSi/Si multilayers reveal that Au-rich 
particles act as catalysts for the transformation of amorphous Si via 
metal-induced-crystallization.  Crystalline silicon grains are modeled as 
tapered nanowires behind Au-rich nanoparticles and the free energy change 
is examined as a function of particle radius.  It is calculated that 
particles with a radius less than 1.2 nm are unable to induce 
crystallization, and these findings are experimentally confirmed by TEM 
characterization.

Finally, Au-rich nanoparticles are successfully incorporated into metal 
oxide semiconductor (MOS) structures for use as a charge trapping layer in 
floating gate devices.  From high frequency capacitance measurements, MOS 
structures containing these particles show a significant hysteresis as 
compared to structures without nanoparticles.  This difference in behavior 
is attributed to additional charge storage in either nanoparticle or 
nanoparticle interface states.  For devices operating in the 
Fowler-Nordheim tunneling regime, a memory window of 0.6V can be achieved 
under a 10V programming voltage.  This memory window can be enhanced with 
further increases of programming voltages and/or write times.  This work 
represents one of the first examples of metal nanoparticles, formed by 
phase separation, utilized as a floating gate layer for non-volatile memory 
applications.  
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