Uma banca de DEFESA de DOUTORADO foi cadastrada pelo programa.
DISCENTE: GUILHERME SEVERINO MENDES DE ARAÚJO
DATA: 18/06/2024
HORA: 14:00
LOCAL: Presencial/Remota
TÍTULO: Caracterização da eletroformação e da comutação resistiva de memristores orgânicos ITO/MEH-PPV/Al
PALAVRAS-CHAVES: resistive switching device, electroforming, quantum point contact (QPC), cycle endurance, memory failure, impedance spectroscopy
PÁGINAS: 133
GRANDE ÁREA: Ciências Exatas e da Terra
ÁREA: Física
RESUMO:

Resistive memories have been the focus of intense research in
recent years due to their applications in non-volatile memory and
neuromorphic computing. Improving the reliability of resistive memory
requires understanding certain bottlenecks, such as the comprehension
of control parameters for electroforming and electrical conduction
processes in device. Additionally, it is commonly necessary to control and
stabilize the memory margin during the cycle endurance and address the
physical degradation observed in memories containing metal oxides. In
this context, this work presents a study on the electrical properties of
resistive memory in indium tin oxide/poly[2-methoxy-5-(2'-ethyl-hexyloxy)-
1,4-phenylenevinylene]/aluminum (ITO/MEH-PPV/Al) devices. The results
are consistent with the soft dielectric breakdown of an Al 2 O 3 oxide layer at
the polymer/metal interface. Analyses were performed using current-
voltage (I-V) sweeps, cycle endurance (CE), physical and electrical
degradation mechanisms (relative to memory), and impedance
spectroscopy. The device exhibits diode-like in its pristine state,
corresponding to the initial I-V sweeps. This highly resistive diode
requires an electroforming process to transform it into a memory with
resistive switching. During electroforming, the I-V sweeps are fitted using
the Schottky Emission (SE) and Fowler-Nordheim (FN) models, resulting
in an injection barrier ~0.8 to 0.9 eV, which may be associated with
injection through the polymer/Al interface. In the high resistance state
(HRS), the linear coefficient of the logI vs. logV curve is n ≅ 1 at low
voltages, transitioning to n ≅ 2 as the device approaches the switching
voltage. In the low resistance state (LRS), n ≅ 1 is observed up to the
Negative Differential Resistance (NDR). By exploring reverse bias NDR at
different levels, multi-state conduction can be observed in forward bias
sweeps (opposite polarizations), in contrast to unipolar memories, which
show multi-states by exploring NDR at different levels in the same
polarization direction. The multi-state I-V sweeps are consistent with the
Quantum Point Contact (QPC) model. The resulting fitting parameters
indicate that the number of filaments increases from 1 (in HRS) to 70 as
the multi-state approaches LRS. Furthermore, the gap distance (t gap )
decreases from (0.59 ± 0.02) nm to (0.44 ± 0.01) nm. The conductance
found in terms of G 0 (= 2e 2 /h = 7.75×10 −5 S) ranged from 1G 0 to 11.5G 0 ,
compatible with values derived from the QPC model in multi-state I-V
sweeps. Additionally, studies were conducted using I-V sweeps and cycle
resistance with different compliance currents (CC), as well as scanning
electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS),
and Raman spectroscopy. The devices exhibited a dependence of
conductance quantization G 0 on CC, with the G 0 histogram showing a
different distribution form after CC alteration. After many cycles, the
devices exhibited partial destruction in the active area (physical
alteration) followed by degradation of memory parameters and, ultimately,
failure. SEM/EDS measurements in the deteriorated region revealed that
the aluminum electrode was removed, leaving only polymer remnants.
Raman spectroscopy analyses focused on changes in the polymer
structure caused by CE, such as changes in chain planarity, which is
related to the band around 966 cm −1 that shifted to lower wavenumbers
(965 cm −1 ) along with increased intensity, indicating less planar
conformations compared to the non-degraded area. The alteration in
conjugation length was evidenced by the contribution around 1581 cm −1 ,
with a shift from 1581 cm −1 to 1584 cm −1 for the non-degraded and
degraded areas, respectively. Both results indicate a decrease in
conjugation length. All these changes demonstrate the fragmentation of
the polymer chain as a result of CE.

MEMBROS DA BANCA:
Externo ao Programa - 1534112 - ANGEL ALBERTO HIDALGO
Presidente - 2056226 - CLEANIO DA LUZ LIMA
Externo à Instituição - DOUGLAS JOSÉ COUTINHO - UTFPR
Interno - 1720015 - GARDENIA DE SOUSA PINHEIRO
Externo à Instituição - GREGORIO COUTO FARIA - USP
Interno - 1714296 - MARIA LETICIA VEGA
Externo à Instituição - ROBERTO MENDONÇA FARIA - USP

News registered on: May 27, 2024