Phase change materials for non-volatile electronic memories [Elektronische Ressource] / vorgelegt von Martin Stefan Salinga

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• Physik

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• Publié par: rheinisch-westfalischen_technischen_hochschule_-rwth-_aachen
• Publié le: 01 janvier 2008
• Nombre de lectures: 7
• Langue: English
• Poids de l'ouvrage: 18 Mo

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Phase Change Materials
for Non-volatile Electronic Memories
Von der Fakulta¨t fu¨r
Mathematik, Informatik und Naturwissenschaften
der Rheinisch-Westf¨alischen Technischen Hochschule Aachen
zur Erlangung des akademischen Grades eines Doktors
der Naturwissenschaften genehmigte Dissertation
vorgelegt von
Diplom-Physiker Martin Stefan Salinga
aus Herten in Westfalen
Berichter: Universitatsprofessor Dr. Matthias Wuttig¨
Dr. Simone Raoux
Tag der mundlichen Prufung: 4. Juni 2008¨ ¨
Diese Dissertation ist auf den Internetseiten
der Hochschulbibliothek online verfugbar.¨iiAbstract
Starting from a brief introduction into the physics of phase change materials their
applications in the field of data storage are reviewed. Without the latter there would
certainly be less attention paid to the topic today, even in basic research. For years
now,opticaldatastoragemediabasedonphasechangematerialshavebeensuccessfully
produced for the mass market. Despite its maturity there are still new ideas on how to
further improve this technology in order to stay competible with other storage media
in the future. In contrast, for the application of phase change materials in electronic
memories the situation is completely different. Although proposed decades ago, today
it is still a rather young and promising technology.
In this work both applications are analyzed with respect to their requirements to
the incorporated phase change material, with an emphasis on the second one. For
optical storage and especially in the case of electronic memories the author arrives at
the conclusion, that the crystallization kinetics of phase change materials is the most
urgent and fundamental problem that needs to be solved. The emphasis on a deep
and quantitative understanding of this phenomenon implicitely criticizes the popular
approach of studying the switching of electronic cells for phase change based memories
without an in-depth research of the crystallization kinetics.
After this identification of the research objective, the result of a thorough review
of the literature on the theory of crystallization is presented. The understanding of
glass formation turns out to be extremely important, since it deals with the stability
of an amorphous solid or undercooled liquid against structural reconfiguration. Conse-
quently, a separate chapter is dedicated to the theory of glass formation.
Based on the knowledge of the theoretical connections between glass transition
and crystallization it is investigated how meaningful a calculation of the enthalpy of
atomization is for a prediction of stoichiometric trends not only of glass transition,
but also of crystallization. The sparce experimental evidence on the glass transition
temperature of phase change materials is compared to the calculated enthalpies. The
same is done for a series of measurements of crystallization. These comparisons show
that the proposed strategy works well for predicting the influence of a stoichiometric
iiivariation of a phase change material on its stability against crystallization.
The author’s critical review of the literature on crystallization kinetics reveals that
the widely used classical crystallization theory still lacks a rigorous experimental prove
of its validity. The latter is difficult, because the multitude of quasi-free parameters
generally ensures a good mathematical agreement between theory and the often very
limited experimental data.
Such a check of validity of the theory is especially challenging for phase change
materials: In a wide range of high temperatures crystallization proceeds so fast, that
until now an experimental quantification of nucleation rate and crystal growth velocity
was only possible in a rather limited regime of low temperatures. The extrapolation
of such data over the whole temperature range up to the melting temperature by
application of the equations provided by the classical theory is assessed to be too
uncertain to be trusted.
To close that gap with experimental evidence and to advance therewith towards
a disentanglement of electronic and thermal effects involved in the switching of an
electrical cell, a new experimental setup has been designed and realized. It combines
laser induced annealing experiments with the capability to apply and measure fast
electrical pulses. The implementation of a control of the sample’s base temperature is
an additional, valuable component. Each of the sections of the new setup on its own is
already a sophisticated tool, that enables its user to investigate phase change materials
on very short time scales. Examples for this are unprecedented laser experiments
that have been performed by the author. Some of those innovatively separate crystal
nucleationandgrowth. Othersdemonstrateapathtowardsaquantitativemeasurement
of crystallization kinetics in the melt-quenched amorphous state. The latter is highly
relevantfortechnology,sinceitisthatamorphousphase,thatisrealizedinapplications.
But the new setup is more than just the sum of its parts. The combination of
optical, electrical and thermal experiments opens up a wide field of new possibilities.
This is indicated by the demonstration of an electrical experiment for the investigation
of the threshold-switching effect, the phenomenon describing an abrupt breakdown of
the resistivity of the amorphous phase upon application of a critical electric field. The
initialization of the sample by laser annealing allows for a“clean”experiment in which
the starting conditions are not dependent on the property of interest itself, in this case
the electrical behaviour. Such a successive approach from purely thermal experiments
towards the testing of the realistic but complex cells of a phase change based electronic
memory is necessary to stepwise synchronize the numerical simulations. This strategy
is essential for achieving a deep understanding of the physical processes involved in the
switching of such cells.
ivABSTRACT
Beyond these technologically important measurements the new setup is pioneering
foramultitudeoffurtheropticalandelectricalexperimentsthatarelikelytomakevalu-
able contributions to the research of the physical properties of phase change materials.
Examples for such experiments are proposed at the end of this work.
vviKurzfassung
¨Ubersetzung des englischen Originaltitels: Phasenwechselmaterialien fu¨r nicht-
flu¨chtige elektronische Datenspeicher
AusgehendvoneinerkurzenEinfuhrungindiegrundlegendenphysikalischenEigen-¨
schaftenvonPhasenwechselmaterialienwerdenzuna¨chstdieAnwendungensolcherMa-
terialien auf dem Gebiet der Datenspeicherung betrachtet, ohne welche das Thema
heute sicherlich auch auf Seiten der Grundlagenforschung weitaus weniger Beachtung
fande. OptischeDatenspeicher,diePhasenwechselmaterialienverwenden,werdenschon¨
seit vielen Jahren erfolgreich fur den Massenmarkt hergestellt. Hier liegt also eine aus-¨
gereifte Technologie vor, bei der allerdings neue Ideen fu¨r nochmalige Verbesserungen
vorliegen, mit denen fu¨r weitere Jahre Konkurrenzf¨ahigkeit gegenu¨ber anderen Spei-
chermedien erreicht werden konnte. Ganz anders sieht es fur die Verwendung von¨ ¨
Phasenwechselmaterialien in elektronischen Datenspeichern aus. Obgleich schon vor
Jahrzehnten vorgeschlagen, ist sie in ihrer heutigen Form eine recht junge, dabei aber
u¨beraus vielversprechende Technologie.
Beide Anwendungen werden in dieser Arbeit hinsichtlich ihrer Anforderungen an
die verwendeten Phasenwechselmaterialien analysiert, wobei der Schwerpunkt auf den
elektronischen Speichern liegt. Dabei wird herausgearbeitet, dass sowohl fur optische,¨
abergeradeauchfurelektronischeDatenspeicher,dastiefeunddamitauchquantitative¨
Versta¨ndnis der Kristallisationskinetik von Phasenwechselmaterialien die dringendste
und fundamentalste Fragestellung ist, die es in diesem Zusammenhang zu bearbeiten
gilt. Damit wird implizit der verbreitete Ansatz kritisiert, gerade bei elektronischen
Phasenwechselspeichern ohne eine entsprechend sorgfaltige Studie der Kristallisations-¨
kinetik das Schaltverhalten zu erforschen.
Nach der Identifizierung dieses Forschungsziels wird das Ergebnis einer ausgiebi-
gen Literaturrecherche zur Theorie der Kristallisation pra¨sentiert. Als u¨beraus wichtig
stellt sich dabei das Versta¨ndnis der Glasbildung heraus, da hierin die Stabilit¨at eines
amorphen Festko¨rpers bzw. einer unterku¨hlten Flu¨ssigkeit gegen strukturelle Umord-
nung behandelt wird. Aus diesem Grund wird der Theorie der Glasbildung ein eigenes
Kapitel gewidmet.
viiBasierend auf dem Wissen um die theoretischen Zusammenh¨ange zwischen
Glasu¨bergang und Kristallisation wird untersucht, inwieweit von einer berechneten
Atomisierungsenthalpie von Phasenwechselmaterialien ausgehend Aussagen uber sto-¨ ¨
chiometrische Trends in deren Glasubergangstemperatur oder sogar uber Veranderung¨ ¨ ¨
in der Kristallisation getroffen werden ko¨nnen. Der Vergleich der berechneten En-
thalpien mit den raren experimentellen Ergebnissen fu¨r die Glasu¨bergangstemperatur
vonPhasenwechselmaterialiensowiemitMessungenzurKristallisationzeigt,dassdiese
Strategie in der Tat eine Vorhersage uber den Einfluss von Stochiometrievariationen¨ ¨