<p>To sustain the upcoming paradigm shift in computations
technology efficiently, innovative solutions at the lowest level of the
computing hierarchy (the material and device level) are essential to delivering
the required functionalities beyond what is available with current CMOS platforms.
Motivated by this, in this dissertation, we explore ferroelectric-based devices
for steep-slope logic and energy-efficient non-volatile-memory functionalities
signifying the novel device attributes, possibilities for continual dimensional
scaling with the much-needed enhancement in performance.</p>
<p> </p>
<p>Among various ferroelectric (FE) materials, Zr doped HfO<sub>2</sub>
(HZO) has gained immense research attention in recent times by virtue of CMOS
process compatibility and a considerable amount of ferroelectricity at room
temperature. In this work, we investigate the Zr concentration-dependent
crystal phase transition of Hf<sub>1-x</sub>Z<sub>x</sub>O<sub>2</sub> (HZO)
and the corresponding evolution of dielectric, ferroelectric, and
anti-ferroelectric characteristics. Providing the microscopic insights of
strain-induced crystal phase transformations, we propose a physics-based model
that shows good agreement with experimental results for 10 nm Hf<sub>1-x</sub>Z<sub>x</sub>O<sub>2</sub>.
Further, in a heterogeneous system, ferroelectric materials can exhibit
negative capacitance (NC) behavior. Such NC effects may lead to differential
amplification in local potential and can provide an enhanced charge and
capacitance response for the whole system compared to their constituents. Such
intriguing implications of NC phenomena have prompted the design and
exploration of many ferroelectric-based electronic devices to not only achieve
an improved performance but potentially also overcome some fundamental limits
of standard transistors. However, the microscopic physical origin as well as
the true nature of the NC effect, and direct experimental evidence remain
elusive and debatable. To that end, in this work, we systematically investigate
the underlying physical mechanism of the NC effect in the ferroelectric
material. Based upon the fundamental physics of ferroelectric material, we investigate
different assumptions, conditions, and distinct features of the quasi-static NC
effect in the single-domain and multi-domain scenarios. While the quasi-static
and hysteresis-free NC effect was initially propounded in the context of a single-domain
scenario, we highlight that the similar effects can be observed in multi-domain
FEs with soft domain-wall (DW) displacement. Furthermore, to obtain the
soft-DW, the gradient energy coefficient of the FE material is required to be
higher as well as the ferroelectric thickness is required to be lower than some
critical values. Otherwise, the DW becomes hard, and their displacement would
lead to hysteretic NC effects. In addition to the quasi-static NC, we discuss
different mechanisms that can lead to the transient NC effects. Furthermore, we
provide guidelines for new experiments that can potentially provide new
insights on unveiling the real origin of NC phenomena.</p>
<p> </p>
<p>Utilizing such ferroelectric insulators at the gate stack of
a transistor, ferroelectric-field-effect transistors (FeFETs) have been
demonstrated to exhibit both non-volatile memory and steep-slope logic
functionalities. To investigate such diverse attributes and to enable
application drive optimization of FeFETs, we develop a phase-field simulation
framework of FeFETs by self-consistently solving the time-dependent
Ginzburg-Landau (TDGL) equation, Poisson’s equation, and non-equilibrium
Green’s function (NEGF) based semiconductor charge-transport equation.
Considering HZO as the FE layer, we first analyze the dependence of the multi-domain
patterns on the HZO thickness (<i>T<sub>FE</sub></i>) and their critical role
in dictating the steep-switching (both in the negative and positive capacitance
regimes) and non-volatile characteristics of FeFETs. In particular, we analyze
the <i>T<sub>FE</sub></i>-dependent formation of hard and soft domain-walls
(DW). We show that, <i>T<sub>FE</sub></i> scaling first leads to an increase in
the domain density in the hard DW-regime, followed by soft DW formation and
finally polarization collapse. For hard-DWs, we describe the polarization
switching mechanisms and how the domain density impacts key parameters such as
coercive voltage, remanent polarization, effective permittivity and memory
window. We also discuss the enhanced but positive permittivity effects in
densely pattern multi-domain states in the absence of hard-DW displacement and
its implication in non-hysteretic attributes of FeFETs. For soft-DWs, we
present how DW-displacement can lead to effective negative capacitance in
FeFETs, resulting in a steeper switching slope and superior scalability. In
addition, we also develop a Preisach based circuit compatible model for FeFET
(and antiferroelectric-FET) that captures the multi-domain polarization
switching effects in the FE layer. </p>
<p> </p>
Unlike semiconductor
insulators (e.g., HZO), there are ferroelectric materials that exhibit a
considerably low bandgap (< 2eV) and hence, display semiconducting
properties. In this regard, non-perovskite-based 2D ferroelectric
-In<sub>2</sub>Se<sub>3</sub> shows a bandgap of ~1.4eV and that
suggests a combined ferroelectricity and semiconductivity in the same material
system. As part of this work, we explore the modeling and operational principle
of ferroelectric semiconductor metal junction (FeSMJ) based devices in the
context of non-volatile memory (NVM) application. First, we analyze the
semiconducting and ferroelectric properties of the α-In<sub>2</sub>Se<sub>3</sub> van
der Waals (vdW) stack via experimental characterization and first-principles
simulations. Then, we develop a FeSMJ device simulation framework by
self-consistently solving the Landau–Ginzburg–Devonshire equation, Poisson's
equation, and charge-transport equations. Our simulation results show good
agreement with the experimental characteristics of α-In<sub>2</sub>Se<sub>3</sub>-based
FeSMJ suggesting that the FeS polarization-dependent modulation of Schottky
barrier heights of FeSMJ plays a key role in providing the NVM functionality.
Moreover, we show that the thickness scaling of FeS leads to a reduction in
read/write voltage and an increase in distinguishability. Array-level analysis
of FeSMJ NVM suggests a lower read-time and read-write energy with respect to
the HfO<sub>2</sub>-based ferroelectric insulator tunnel junction (FTJ)
signifying its potential for energy-efficient and high-density NVM applications.
| Identifer | oai:union.ndltd.org:purdue.edu/oai:figshare.com:article/15082491 |
| Date | 30 July 2021 |
| Creators | Atanu Kumar Saha (11209926) |
| Source Sets | Purdue University |
| Detected Language | English |
| Type | Text, Thesis |
| Rights | CC BY 4.0 |
| Relation | https://figshare.com/articles/thesis/Modeling_and_Applications_of_Ferroelectric_Based_Devices/15082491 |
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