Formation of KNbO3 Thin Films for Self-Powered ReRAM Devices and Artificial Synapses

116 Pages·2018·5.21 MB·English

by  Tae-Ho Lee

Springer Theses
Recognizing Outstanding Ph.D. Research
Tae-Ho Lee

Formation of KNbO
3
Thin Films for
Self-Powered ReRAM
Devices and Artificial
Synapses
Springer Theses
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Tae-Ho Lee
Formation of KNbO Thin
3
Films for Self-Powered
ReRAM Devices
fi
and Arti cial Synapses
Doctoral Thesis accepted by
Korea University, Seoul, Korea (Republic of)
123
Author Supervisor
Dr. Tae-HoLee Prof. SahnNahm
Korea Electronics Technology Institute Department ofMaterials Science
Seongnam,Korea (Republicof) andEngineering
Korea University
Seoul, Korea (Republicof)
ISSN 2190-5053 ISSN 2190-5061 (electronic)
SpringerTheses
ISBN978-981-13-2534-2 ISBN978-981-13-2535-9 (eBook)
https://doi.org/10.1007/978-981-13-2535-9
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’
Supervisor s Foreword
KNbO (KN) thin films have shown excellent electro-optic properties, high non-
3
linear optical coefficients, and good photorefractive and piezoelectric properties.
Therefore, KN films have been extensively investigated for application to the
variousnonlinearopticalandelectro-opticaldevices.However,ithasbeendifficult
togrowstoichiometricKNthinfilmsbecauseK Oevaporatedduringthegrowth.In
2
ordertoovercomethisproblem,theK Oexcesstargetanddualtargetsconsistingof
2
KNandK CO havebeenused.However,since thecomposition ofthetargetwas
2 3
differentfromthatofthefilm,itwasdifficulttocontrolthecompositionoftheKN
thin film. Dr. Tae-Ho Kim showed in his thesis how to grow the homogeneous
crystalline KN film without secondary phase and then he also investigated the
growthoftheamorphousKNthinfilmcontainingKNnanocrystals.Moreover,the
applicationsoftheKNfilmsuchasapiezoelectricnanogenerator,resistiverandom
accessmemory(ReRAM)device,andtheartificialsynapsewereinvestigatedinhis
thesis.
First, he has investigated the growth behavior of the KN film. He found that a
K Nb O secondaryphase wasformed intheKNfilmwhenthedepositionand
2.88 5 15
the annealing temperatures were greater than room temperature (RT) and 800 °C,
respectively, owing to the evaporation of K O. On the other hand, KNb O and
2 3 8
K Nb O secondary phases were formed in the KN films when the annealing
3 5.45 15
temperature was less than 800 °C. Based on these results, he showed that the
stoichiometricKNfilmswereobtainedwhentheamorphousKNfilmgrownatRT
wasannealedunderK Oatmosphereat800°CtocompensatetheevaporatedK O.
2 2
This KN film showed the promising electrical, dielectric, and piezoelectric
properties.
He has investigated on the amorphous KN films containing KN nanocrystals in
the second part of his thesis. Piezoelectric nanogenerators (PNG) were also fabri-
cated using this KN films grown on the TiN/Polyimide/PET substrates and PNG
fabricated with the KN film showed an open-circuit output voltage of 2.5 V and a
short-circuit current of 70 nA. Moreover, he found that this KN film exhibited a
bipolarresistiveswitchingbehaviorwithgoodreliabilitycharacteristicsthatcanbe
v
vi Supervisor’sForeword
explained by the formation and rupture of the oxygen vacancy filaments. He also
showed that the KN resistive random access memory device, which was powered
by the KN PNG, showed promising resistive switching behavior.
Finally, he investigated the synaptic properties of the KN thin film for the
application to the neuromorphic computing system. He proved that the Pt/KN/TiN
memristor exhibited reliable bipolar switching behavior with multiple resistance
levels, confirming that the KN film can be used as the artificial synapse and
transmission properties of a biological synapse with a good conductance modula-
tion linearity. Moreover, he showed that the KN memristor can emulate various
biological synaptic plasticity characteristics including short-term plasticity,
long-term plasticity, spike-rate dependent plasticity, paired-pulse facilitation, and
post-tetanic potentiation by controlling the number and rate of the potentiation
spike.Inhisthesis,healsoshowedthatspike-timing-dependentplasticity,whichis
an essential property of biological synapses, is realized in KN memristor. The
synaptic plasticity of the KN memristor can be explained by oxygen vacancy
movement and oxygen vacancy filaments. Moreover, he proved that the meta-
plasticity of biological synapses was also implemented in the KN memristor,
including metaplasticities of long-term potentiation and depression, and of
STDP.Therefore,heclearlyprovedinhisthesisthattheKNmemristorcanbeused
as an artificial synapse in neuromorphic computing systems.
Histhesis showsthathow togrow theKN-basedthinfilm andtheseresultscan
be used for the fabrication of the various electrical devices. Moreover, his thesis
alsoprovesthattheKNfilmcanbeusedasthepiezoelectricnanogeneratorforthe
biological device as well as the artificial synapse for the neuromorphic computing
systems.
Seoul, Korea (Republic of) Prof. Sahn Nahm
April 2018
Abstract
Biomedical devices inserted into the human body and microelectronicapplications
require miniaturization and reliable safe battery to operate. In particular, self-
poweredbiomedicalapplicationsworkbythepiezoelectricnanogenerators(PNGs),
which present numerous advantage, including high voltage, efficiency, low cost,
high versatility, simplicity in structural design and fabrication, stability and
robustness, and environmental friendliness. To improve the performance of these
self-powered biomedical devices, the development of energy efficient memory
device is important. In this study, ReRAM based memristors as artificial synapses
are proved for high efficient memory devices and combined with a piezoelectric
nanogenerators (PNGs) for self-powered biomedical devices that can be used in
neuromorphic systems.
First, Self-powered Resistive Random Access Memory (ReRAM) devices and
ArtificialSynapsesforneuromorphicdeviceswereinvestigated.AmorphousKNbO
3
(KN)filmcontainingKNnanocrystalswithasizeofapproximately5.0nmwasgrown
onTiN/SiO /Sisubstrateat350°C.ThisKNfilmshowedadielectricconstant(e)and
2 r
piezoelectricstrainconstant(d )of43and80pm/Vat10V,respectively,owingto
33
theexistenceofKNnanocrystals.PiezoelectricNanogenerators(PNGs)werefabri-
catedusingKNfilmsgrownontheTiN/Polyimide/PETsubstrates.PNGfabricated
withtheKNfilmgrownat350°Cshowedanopen-circuitoutputvoltageof2.5Vand
a short-circuit current of 70 nA. The KN film grown at 350 °C exhibited a bipolar
resistiveswitchingbehaviorwithgoodreliabilitycharacteristicsthatcanbeexplained
bytheformationandruptureoftheoxygenvacancyfilaments.TheResistiveRandom
AccessMemory(ReRAM)devicepoweredbytheKNPNGalsoshowedpromising
resistiveswitchingbehaviorwithON/OFFratioof10.Moreover,theKNfilmshows
goodbiocompatibility.
vii
viii Abstract
Taking astepforwardaparticularlygoodefficient memory device,theKNfilm
grown at 350 °C was used to synthesize the KN ReRAM based memristor for an
artificial synapse. The sensitivity of memristor synapse devices changed when
repetitive signals are applied, and this property can perform the self-learning
abilities to neuromorphic devices similar to neural network system. This charac-
teristic is expressed as the biological synapse including short-term/long-term
plasticity (STP/LTP), spike-rate-dependent plasticity (SRDP), spike-timing-
dependent plasticity (STDP), and synaptic plasticity. Synaptic characteristics of
KN memristor are controlled by voltage intensity and time tuning of pulsed signal
basedonresistanceswitching.ThesynapticpropertiesoftheKNmemristorcanbe
explained based on the oxygen vacancy movements and the shape change of
oxygenvacancyfilaments.MetaplasticityofLTPwasprovedintheKNmemristor
to achieve bio-realistic synaptic functions. Moreover, the metaplasticity of STDP
wasalsoshownintheKNmemristorwithaprimingstimulus.Alloftheresults,in
this thesis, evidently indicated the probability of ReRAM device not only in the
memory application but also in the neuromorphic system application.
Parts of this thesis have been published in the following articles:
1. B. Y. Kim, H. G. Hwang, J. U. Woo, W. H. Lee, T. H. Lee, C. Y. Kang and
S. Nahm, “Nanogenerator-induced synaptic plasticity and metaplasticity of
bio-realistic artificial synapses” NPG Asia Mater. E381, 1–9 (2017).
2. T. H. Lee, D. H. Kim, B. Y. Kim, H. Y. Choi, J. H. Oh, C. Y. Kang and
S. Nahm, “Structural and electrical properties of KNbO thin film grown on a
3
Pt/Ti/SiO /SisubstrateusingtheRFmagnetronsputteringmethod”ActaMater.
2
112, 53–58 (2016).
3. T.H.Lee,H.G.Hwang,S.Jang,G.Wang,S.Han,D.H.Kim,C.Y.Kangand
S.Nahm,“Low-Temperature-GrownKNbO ThinFilmsandTheirApplication
3
to Piezoelectric Nanogenerators and Self-Powered ReRAM device” ACS Appl.
Mater. Interfaces 9, 43220–43229 (2017).
4. T. H. Lee, H. G. Hwang, J. U. Woo, D. H. Kim, T. W. Kim and S. Nahm,
“Synaptic Plasticity and Metaplasticity of Biological Synapse Realized in a
KNbO Memristor for Application to Artificial Synapse” ACS Appl. Mater.
3
Interfaces. https://doi.org/10.1021/acsami.8b04550 (2018).
ix