In situ TEM study of nanoscale phase-change memory cells
Advisor: Prof. Yi Cui
Time: Wednesday, May 12th at 10 AM (refreshments served at 9:45AM)
Location: CIS-X Auditorium
Abstract:
Phase-change memory (PCM) utilizes the large difference in resistivity
between the crystalline and amorphous phases of chalcogenide materials
to store information. Studies have shown excellent scalability, fast
switching speeds and high endurance making this technology a promising
candidate for future non-volatile memory. While industry is making
steady progress towards implementing PCM, there are still many open
questions concerning the operation of an actual device. For example, the
size and shape of the amorphous region in a working device is largely
unknown. Similarly, the critical threshold electric field required to
switch to the conductive state, is not well understood. Finally, PCM
devices often show large variability in their electrical behavior for
unclear reasons. To address these questions, we have developed a
technique to switch individual PCM cells inside a transmission electron
microscope allowing us to directly correlate electrical behavior with
structural changes. We demonstrate reversible switching of
Germanium-Antimony-Telluride (Ge2Sb2Te5) lateral phase-change cells and
show various degrees of amorphization which result in changes in
electrical behavior. Specifically, the electrical behavior is influenced
by the purity of the amorphous domain, which can vary from completely
amorphous to an amorphous matrix with a high density of nanocrystals
interspersed. We find that the makeup of the amorphous domain strongly
depends on the geometry of the bridge and the applied pulse. We measured
a threshold field of approximately 4 V/um, which is much smaller than
what has been reported by other experiments (30-55 V/um). These insights
can assist in the design of better lateral phase-change cells capable of
reproducible switching with minimal resistance fluctuation in each state.
Since it is not clear which material will ultimately show the best
properties for use in PCM and there is a need to make phase-change
materials in the nanoscale for fundamental investigations, we developed
the vapor-liquid-solid growth of GeTe phase-change nanowires (NWs). The
NWs show a large resistance contrast upon switching. With the in situ
TEM technique we discovered that the NWs can switch via a different
mechanism that consists of opening and closing of voids giving rise to
large differences in resistivity.
--
Stefan Meister
Ph.D. Candidate, Cui Group
Materials Science and Engineering
Stanford University
McCullough Bldg., Rm 209
476 Lomita Mall
Stanford, CA 94305
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