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@PHDTHESIS{Wiefels:817653,
author = {Wiefels, Stefan},
othercontributors = {Waser, Rainer and Jungemann, Christoph},
title = {{R}eliability aspects in resistively switching valence
change memory cells},
school = {Rheinisch-Westfälische Technische Hochschule Aachen},
type = {Dissertation},
address = {Aachen},
publisher = {RWTH Aachen University},
reportid = {RWTH-2021-04029},
pages = {1 Online-Ressource : Illustrationen, Diagramme},
year = {2021},
note = {Veröffentlicht auf dem Publikationsserver der RWTH Aachen
University; Dissertation, Rheinisch-Westfälische Technische
Hochschule Aachen, 2021},
abstract = {For over 50 years, Moore‘s law functioned as road map for
advancements in the semiconductor industry. Soon, the
predicted exponential increase in the number of devices per
microchip will reach physical limitations. In order to
overcome these limitations, redox-based resistive switching
random access memory (ReRAM) is discussed as promising
candidate for future memory applications. Recently, also a
potential application of ReRAM in neuro-inspired
architectures is gaining a lot of attention. Among other
approaches, valence change based memory (VCM) is studied
intensively. Regardless of an application as classical
memory or as neuronal network component, the reliability of
ReRAM devices is the key attribute for industrial adaption.
This dissertation addresses the three main components of the
reliability VCM ReRAM devices, being variability, retention
and endurance. Here, VCM ReRAM cells based on ZrO2
fabricated under laboratory conditions are characterized as
well as industrial devices based on HfO2 as switching oxide.
Throughout this work, a focus on large arrays instead of
single cells is emphasized. The evaluation and
interpretation is focused on the internal statistics rather
than on the behavior of individual devices. The variability
of VCM ReRAM remains one of the largest challenges for their
large scale adaption in industrial applications. Whereas the
stochastic nature of the switching process can be
significantly reduced by appropriate programming algorithms,
random fluctuations occur also between read operations. This
read to read (R2R) variability is identified as key
challenge in the short term stability of VCM ReRAM. It
determines the intrinsic statistics of large memory arrays
and effectively limits the read window between the low
resistive (LRS) and high resistive state (HRS). The random
R2R fluctuations are attributed to random jumps of the
conduction supporting oxygen vacancies. In the HRS, these
jumps lead to a log-normal read current distribution. Via an
empirical model as well as kinetic Monte Carlo (KMC)
methods, the most likely origin of these statistics is found
to be tunneling across a normally distributed gap in HRS.
Here, the exponential dependence of the read current on the
tunneling gap results in the observed log-normal statistics.
Investigating the long term stability or retention, the R2R
variability remains a key characteristic of the investigated
devices. The most critical aspect of the long term
degradation of a programmed state is found to be a
broadening of the whole distribution, i.e. increasing
variability. The trend of the degradation is fitted by an
empirical tunneling model which allows for extrapolation of
data measured at higher temperatures towards the target
retention time at lower operating temperature. Additionally,
a statistical model based on the work of Abbaspour et al. is
developed which explains the observed degradation by
diffusion of oxygen vacancies from a confined filament
region towards the active electrode. Finally, an algorithm
is developed which increases the number of possible
switching cycles, also referred to as endurance, of a
device. It dynamically adjusts the programming parameters to
ensure reliable switching. Since the frequency of applied
adjustments determines the speed of the experiment, the
algorithm dynamically adjusts this frequency to the tested
cell. It therefore increases the measurement speed if a cell
requires less adjustments. The algorithm is used to
determine the maximum endurance for different material
combinations. Thus, it is demonstrated that ohmic electrode
metals with lower oxygen chemical potential ensure higher
endurance which verifies the theoretical findings of Guo et
al. All in all, this dissertation proposes to evaluate the
reliability of VCM ReRAM for its intrinsic statistics rather
than tracing single cells.},
cin = {611610},
ddc = {621.3},
cid = {$I:(DE-82)611610_20140620$},
pnm = {DFG project 167917811 - SFB 917: Resistiv schaltende
Chalkogenide für zukünftige Elektronikanwendungen:
Struktur, Kinetik und Bauelementskalierung "Nanoswitches"
(167917811)},
pid = {G:(GEPRIS)167917811},
typ = {PUB:(DE-HGF)11},
doi = {10.18154/RWTH-2021-04029},
url = {https://publications.rwth-aachen.de/record/817653},
}
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