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Low-Power, Flexible Nonvolatile Organic Transistor Memory Based on an Ultrathin Bilayer Dielectric Stack

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Advanced Electronic Materials
Authors:
Kwanyong Pak
Kwanyong Pak
Kwanyong Pak
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Junhwan Choi at Chonbuk National University
Junhwan Choi
Changhyeon Lee
Changhyeon Lee
Changhyeon Lee
  • This person is not on ResearchGate, or hasn't claimed this research yet.
Sung Gap Im at Korea Advanced Institute of Science and Technology
Sung Gap Im
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Abstract and Figures

Organic thin film transistor nonvolatile memories (OTFT‐NVMs) with polymeric electret layers have attracted research attention for the application to emerging wearable electronics. However, it is challenging to develop low‐power flexible OTFT‐NVMs due to the lack of candidate polymers for flexible electret and blocking dielectric layer (BDL) equipped with the thickness downscalability and sufficiently strong insulating properties. Here, this study reports a low‐power, flexible OTFT‐NVM fabricated with a bilayer dielectric stack composed of a 3 nm thick polymer electret layer and a high‐performance BDL prepared via an initiated chemical vapor deposition process. Especially, a crosslinked poly(1,4‐butanediol diacrylate) film is newly synthesized as a BDL, which shows excellent insulating properties with high breakdown field (Ebreak > 8 MV cm−1 with its thickness of 21.3 nm). Coupled with a 3 nm thick polymer electret layer (poly(1,3,5‐trimethyl‐1,3,5‐trivinyl cyclotrisiloxane)), the fabricated NVMs exhibit a tunable memory window with dramatically reduced programming/erasing voltages less than 15 V and an extrapolated retention time as long as 108 s. Moreover, the device maintains their memory performance up to 1.6% of applied tensile strain. The OTFT‐NVMs with the ultrathin dielectric stack can serve as a promising dielectric for stable data storage in various future wearable electronics. An ultrathin polymer electret layer with extreme downscalability is developed for low‐power, nonvolatile memory via initiated chemical vapor deposition process. By combining a newly synthesized high‐performance, flexible blocking dielectric with the ultrathin (3 nm) polymer electret, the devices exhibit excellent memory characteristics with low‐power consumption and superb mechanical flexibility, presenting an important milestone for developing a low‐power, flexible nonvolatile memory.
A schematic diagram of the flexible OTFT‐NVM with the iCVD dielectric bilayer composed of pV3D3 electret layer (3–15 nm) and pBDDA blocking dielectric layer (20 nm).
A schematic diagram of the flexible OTFT‐NVM with the iCVD dielectric bilayer composed of pV3D3 electret layer (3–15 nm) and pBDDA blocking dielectric layer (20 nm).
… 
Characteristics of the pBDDA layer: a) An illustration of the synthetic scheme of pBDDA. b) FT‐IR spectra of the BDDA monomer and the corresponding pBDDA polymer film. c) An AFM image of the pBDDA surface. Root mean squared roughness (Rq) information of the surface is also noted below the image (scale bar = 2 µm). The inset is optical microscope image of water droplet with the WCA value of the pBDDA. d) A cross‐sectional TEM image of the MIM device with 17 nm thick pBDDA on glass substrate. e) J−E characteristics of pBDDA with four different thicknesses. J of the pBDDA (about 23 nm) at 3 and 6 MV cm⁻¹ with respect to f) the applied tensile strain and g) bending cycles at a fixed tensile strain of 1.2%, respectively.
Characteristics of the pBDDA layer: a) An illustration of the synthetic scheme of pBDDA. b) FT‐IR spectra of the BDDA monomer and the corresponding pBDDA polymer film. c) An AFM image of the pBDDA surface. Root mean squared roughness (Rq) information of the surface is also noted below the image (scale bar = 2 µm). The inset is optical microscope image of water droplet with the WCA value of the pBDDA. d) A cross‐sectional TEM image of the MIM device with 17 nm thick pBDDA on glass substrate. e) J−E characteristics of pBDDA with four different thicknesses. J of the pBDDA (about 23 nm) at 3 and 6 MV cm⁻¹ with respect to f) the applied tensile strain and g) bending cycles at a fixed tensile strain of 1.2%, respectively.
… 
Electrical characterization of the OTFT‐NVMs with the pV3D3 electret and pBDDA blocking dielectric layers: a) The initial transfer characteristics of the OTFT‐NVMs with the pBDDA layer and Bilayers #1–4. b) A schematic of the proposed device structure. c) A transfer memory characteristics of the OTFT‐NVMs with Bilayers #1–4 after the corresponding programming and erasing operations (pulse time = 1 s), respectively. VT values of the devices plotted as function of d) Vprg and e) Vers, respectively. f) Memory windows of the devices with respect to the applied gate biases. g) The retention characteristics of ID at VD = −1 V and VG = −2 V in the on‐ and off‐states. Each state was obtained by biasing ±14, 16, 18, and 20 V with pulse time of 3 s to the corresponding devices with Bilayers #1–4, respectively. h) The memory speed and i) cycling endurance characteristics of the devices for the corresponding programming/erasing voltages.
Electrical characterization of the OTFT‐NVMs with the pV3D3 electret and pBDDA blocking dielectric layers: a) The initial transfer characteristics of the OTFT‐NVMs with the pBDDA layer and Bilayers #1–4. b) A schematic of the proposed device structure. c) A transfer memory characteristics of the OTFT‐NVMs with Bilayers #1–4 after the corresponding programming and erasing operations (pulse time = 1 s), respectively. VT values of the devices plotted as function of d) Vprg and e) Vers, respectively. f) Memory windows of the devices with respect to the applied gate biases. g) The retention characteristics of ID at VD = −1 V and VG = −2 V in the on‐ and off‐states. Each state was obtained by biasing ±14, 16, 18, and 20 V with pulse time of 3 s to the corresponding devices with Bilayers #1–4, respectively. h) The memory speed and i) cycling endurance characteristics of the devices for the corresponding programming/erasing voltages.
… 
The characterization of mechanical flexibility of the OTFT‐NVMs with the iCVD bilayer dielectrics: a) Transfer memory curves of the flexible OTFT‐NVM on a 100 µm thick PEN substrate under the applied tensile strain of 0% to 1.6%. b) The memory window and c) ID at VD = −1 and VG = −2 V in the on‐ and off‐states of the fabricated devices with respect to the applied tensile strain. d) The memory window and e) ID at VD = −1 V and VG = −2 V in the on‐ and off‐states of the device at the flat condition after various bending cycles with fixed tensile strain of 1.2%.
The characterization of mechanical flexibility of the OTFT‐NVMs with the iCVD bilayer dielectrics: a) Transfer memory curves of the flexible OTFT‐NVM on a 100 µm thick PEN substrate under the applied tensile strain of 0% to 1.6%. b) The memory window and c) ID at VD = −1 and VG = −2 V in the on‐ and off‐states of the fabricated devices with respect to the applied tensile strain. d) The memory window and e) ID at VD = −1 V and VG = −2 V in the on‐ and off‐states of the device at the flat condition after various bending cycles with fixed tensile strain of 1.2%.
… 
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1800799 (1 of 9) © 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.advelectronicmat.de
Low-Power, Flexible Nonvolatile Organic Transistor
Memory Based on an Ultrathin Bilayer Dielectric Stack
Kwanyong Pak, Junhwan Choi, Changhyeon Lee, and Sung Gap Im*
DOI: 10.1002/aelm.201800799
capable of storing charges have garnered
a huge research interest due to the advan-
tageous characteristics of nondestructive
read-out, single-transistor application, and
compatibility with the complementary
logic circuit.[19–21]
In general, OTFT-NVMs can be cat-
egorized into three major types of floating
gate,[20–24] ferroelectric memories,[18,25,26]
and polymer electret,[27–34] in terms of the
charge trapping or polarization methods.
Among them, polymer electret-based
OTFT-NVMs have been actively investi-
gated because of the simple dielectric con-
figuration consisting of a bilayer stack of
polymer electret layer and blocking dielec-
tric layer (BDL). Similar to silicon–oxide–
nitride–oxide–silicon (SONOS) memories
in the silicon technologies, the electret-
based OTFT-NVM does not require any
additional conducting or metallic floating
gate inside the gate dielectrics, which
enables further thickness reduction and
the development of high-density memo-
ries.[35–37] Moreover, their memory charac-
teristics can be controlled by engineering
the chemical structure or the functionalities of polymer elec-
tret materials,[38] such as its hydrophobicity,[28] polarity,[27]
morphology,[39,40] intramolecular charge transfer,[41] and
π
-conjugation length/strength.[30,31] However, there are still big
hurdles that limit the realization of low-power flexible OTFT-
NVMs with polymer electrets into real field application. The
programming/erasing operation of the polymer electret-based
OTFT-NVMs requires imposing sufficiently high electric field
(E) to the electret layer for the efficient charge transfer and trap-
ping in order to secure a reasonable memory window.[27,29,42] On
the other hand, the operating voltage must also be kept as low
as possible for low-power consumption. Therefore, it is neces-
sary to reduce the thickness of the electret layer to increase the
E loaded to the electret layer (Eelect) without increasing the input
voltage. A high-performance BDL is also required to ensure a
stable programming/erasing operation. In the same manner,
the thickness of the BDL must also be minimized with high
dielectric constant (k) value for low-power operation. To date,
however, decreasing the polymer electret thickness below a few
tens of nm is accompanied by the decreased retention charac-
teristics in most cases, due to the conducting pathway forma-
tion through the electret layer,[34,43] which strongly implies
the existence of a tradeoff relationship between the operating
voltages and the retention characteristics in the electret-based
Organic thin film transistor nonvolatile memories (OTFT-NVMs) with
polymeric electret layers have attracted research attention for the applica-
tion to emerging wearable electronics. However, it is challenging to develop
low-power flexible OTFT-NVMs due to the lack of candidate polymers for
flexible electret and blocking dielectric layer (BDL) equipped with the thick-
ness downscalability and sufficiently strong insulating properties. Here,
this study reports a low-power, flexible OTFT-NVM fabricated with a bilayer
dielectric stack composed of a 3 nm thick polymer electret layer and a
high-performance BDL prepared via an initiated chemical vapor deposition
process. Especially, a crosslinked poly(1,4-butanediol diacrylate) film is newly
synthesized as a BDL, which shows excellent insulating properties with high
breakdown field (Ebreak > 8 MV cm−1 with its thickness of 21.3 nm). Coupled
with a 3 nm thick polymer electret layer (poly(1,3,5-trimethyl-1,3,5-trivinyl
cyclotrisiloxane)), the fabricated NVMs exhibit a tunable memory window
with dramatically reduced programming/erasing voltages less than 15 V
and an extrapolated retention time as long as 108 s. Moreover, the device
maintains their memory performance up to 1.6% of applied tensile strain.
The OTFT-NVMs with the ultrathin dielectric stack can serve as a promising
dielectric for stable data storage in various future wearable electronics.
K. Pak, J. Choi, C. Lee, Prof. S. G. Im
Department of Chemical and Biomolecular Engineering
and KI for Nano Century
Korea Advanced Institute of Science and Technology
291 Daehak-ro, Yuseong-gu, Daejeon 34141, Republic of Korea
E-mail: sgim@kaist.ac.kr
Polymer Electret
1. Introduction
With the growing interest in the Internet of Things and human-
friendly interfaces, flexible and wearable electronics have
emerged as a next-generation future electronics. Currently,
a great deal of research efforts have been focused to develop
a high-performance electronic system on human skin or in
clothes,[1,2] such as sensor devices,[3–5] near-field communica-
tion,[6,7] optoelectronics,[8–10] and energy harvesting devices.[11,12]
In particular, wearable nonvolatile memory (NVM) devices with
low-power consumption are of central importance for the devel-
opment of the personalized information storage.[13,14] Organic
memory is regarded as a promising candidate device for wear-
able information storage application due to the lightweight,
flexibility, and low cost.[15–18] Especially, organic NVMs based
on organic thin-film transistor (OTFT) with a dielectric setup
Adv. Electron. Mater. 2019, 5, 1800799
... [30,31] Nevertheless, despite these promising approaches, there is typically a mutual exclusivity of mechanical deformability and efficient charge transport. [32][33][34] Extensive -conjugation and backbone planarity in semiconducting polymers are known to enhance macromolecule selfassembly, aggregation, and long-range packing order, yielding highly textured films and efficient charge transport, however at the expense of enhanced brittleness. [35] A few pioneering studies reported that interrupting the -conjugation of semiconducting polymers with insulating units greatly enhances ductility for both p-and n-type conjugated polymers, [23,24,36,37] mainly by suppressing long-range crystallinity of the corresponding films. ...
... The elastic moduli of PNDI-TVT x polymers with a TVT/TET content of 100 to 80, 60, 40, 20, and 0% were measured by AFM modulus mapping. [32][33][34] As shown in Figure 3f and ± 0.08 GPa). Despite the similar trends, the measured PNDI-TVT 100 modulus is substantially larger than those of the other PNDI-TVT x polymers, which is not the case for the computed values. ...
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... With rapid growth in the field of organic thin-film transistors (OTFTs), the development of application devices using OTFTs as building blocks for analog and digital integrated circuits composed of logic gates, memories, and sensors has been actively conducted. [1][2][3][4] Each constituent element of the OTFTs ensures device characteristics such as high mobility (over 10 cm 2 Vs −1 ), low-voltage driving, and operational stability. [5] In particular, it is essential to develop dielectric materials that exhibit optimal compatibility with polymer semiconductors, as charge transport occurs at the interface between the dielectric layer and the active semiconductor channel; however, existing oxide and polymer dielectric materials are not optimal candidates for OTFTs. ...
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Article
  • Nov 2017
In this paper, the development of organic field-effect transistor (OFET) memory device based on isolated and ordered nanostructures (NSs) arrays of wide-bandgap (WBG) small-molecule organic semiconductor material [2-(9-(4-(octyloxy)phenyl)-9H-fluoren-2-yl)thiophene]3 (WG3) is reported. The WG3 NSs are prepared from phase separation by spin-coating blend solutions of WG3/trimethylolpropane (TMP), and then introduced as charge storage elements for nonvolatile OFET memory devices. Compared to the OFET memory device with smooth WG3 film, the device based on WG3 NSs arrays exhibits significant improvements in memory performance including larger memory window (≈45 V), faster switching speed (≈1 s), stable retention capability (>10⁴ s), and reliable switching properties. A quantitative study of the WG3 NSs morphology reveals that enhanced memory performance is attributed to the improved charge trapping/charge-exciton annihilation efficiency induced by increased contact area between the WG3 NSs and pentacene layer. This versatile solution-processing approach to preparing WG3 NSs arrays as charge trapping sites allows for fabrication of high-performance nonvolatile OFET memory devices, which could be applicable to a wide range of WBG organic semiconductor materials.
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High Performance SONOS Flash Memory with In-Situ Silicon Nanocrystals Embedded in Silicon Nitride Charge Trapping Layer
Article
  • Oct 2017
  • SOLID STATE ELECTRON
In this paper, SONOS-type flash memory device with highly improved charge-trapping efficiency is suggested by using silicon nanocrystals (Si-NCs) embedded in silicon nitride (SiNX) charge trapping layer. The Si-NCs were in-situ grown by PECVD without additional post annealing process. The fabricated device shows high program/erase speed and retention property which is suitable for multi-level cell (MLC) application. Excellent performance and reliability for MLC are demonstrated with large memory window of ∼8.5 V and superior retention characteristics of 7% charge loss for 10 years. High resolution transmission electron microscopy image confirms the Si-NC formation and the size is around 1-2 nm which can be verified again in X-ray photoelectron spectroscopy (XPS) where pure Si bonds increase. Besides, XPS analysis implies that more nitrogen atoms make stable bonds at the regular lattice point. Photoluminescence spectra results also illustrate that Si-NCs formation in SiNx is an effective method to form deep trap states.
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Effect of thickness of polymer electret on charge trapping properties of pentacene-based nonvolatile field-effect transistor memory
Article
  • Jan 2017
  • ORG ELECTRON
In this report, a set of pentacene-based organic field-effect transistor (OFET) memory devices using different thicknesses (ranged from 17.8 to 100.4 nm) of Poly (N-vinylcarbazole) (PVK) as charge trapping layers were fabricated, and the dependences of thickness on charge trapping behaviors were systematically investigated. As the thickness increased, the charge trapping capacity shows a Gaussian distributed growth behavior while the surface tunneling distance demonstrates the property of exponential decrease, which is ascribed to the synergistic effects of potential redistribution of trapped charge carriers and the co-existence of direct tunneling and Fowler–Nordheim (FN) tunneling. The optimum thickness (dot) to possess the most efficient charge trapping properties, which means a reasonably low programming voltage and high charge trapping capacity with good bias stress stability, is approximately 40 ± 5 nm. By calculating the threshold thickness (dth) of PVK for an ultrathin memory, we proposed a model of superficial tunneling distance to deconstruct the continuous chargeable polymer electret-based OFET memory. Our work provided a quantitative evaluation method and can improve the understanding of charge trapping process from the aspect of electret thickness.
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Synthesis of Ultrathin, Homogeneous Copolymer Dielectrics to Control the Threshold Voltage of Organic Thin-Film Transistors
Article
  • Aug 2016
This work demonstrates that threshold voltage (VT) of organic thin-film transistors (OTFTs) can be controlled systematically by introducing new copolymer dielectrics with electropositive functionality. A series of homogeneous copolymer dielectrics are polymerized from two monomers, 1,3,5-trimethyl-1,3,5-trivinyl cyclotrisiloxane (V3D3) and 1-vinylimidazole (VI), via initiated chemical vapor deposition. The chemical composition of the copolymer dielectrics is exquisitely controlled to tune the VT of C60 OTFTs. In particular, all the copolymer dielectrics demonstrated in this work exhibit extremely low leakage current densities (lower than 2.5 × 10−8 A cm−2 at ±3 MV cm−1) even with a thickness less than 23 nm. Furthermore, by introducing an ultrathin pV3D3 interfacial layer (about 3 nm) between the copolymer dielectrics and C60 semiconductor, the high mobility of the C60 OTFTs (about 1 cm2 V−1 s−1) remains unperturbed, showing that VT can be controlled independently by tuning the composition of the copolymer dielectrics. Coupled with the ultralow dielectric thickness, the independent VT controllability allows the VT to be aligned near 0 V with sub-3 V operating voltage, which enables a substantial decrease of device power consumption. The suggested method can be employed widely to enhance device performance and reduce power consumption in various organic integrated circuit applications.
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Flexible and Stretchable Devices
Article
No abstract is available for this article.
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Flexible Nonvolatile Polymer Memory Array on Plastic Substrate via Initiated Chemical Vapor Deposition
Article
  • May 2016
  • ACS APPL MATER INTER
Resistive random access memory based on polymer thin films has been developed as a promising flexible nonvolatile memory for flexible electronic systems. Memory plays an important role in all modern electronic systems for data storage, processing, and communication; thus, the development of flexible memory is essential for the realization of flexible electronics. However, the existing solution-processed, polymer-based RRAMs have exhibited serious drawbacks in terms of the uniformity, electrical stability, and long-term stability of the polymer thin films. Here, we present poly(1,3,5-trimethyl-1,3,5-trivinyl cyclotrisiloxane) (pV3D3)-based RRAM arrays fabricated via the solvent-free technique called initiated chemical vapor deposition (iCVD) process for flexible memory application. Because of the outstanding chemical stability of pV3D3 films, the pV3D3-RRAM arrays can be fabricated by a conventional photolithography process. The pV3D3-RRAM on flexible substrates showed unipolar resistive switching memory with an on/off ratio of over 107, stable retention time for 105 s, excellent cycling endurance over 105 cycles, and robust immunity to mechanical stress. In addition, pV3D3-RRAMs showed good uniformity in terms of device-to-device distribution. The pV3D3-RRAM will pave the way for development of next-generation flexible nonvolatile memory devices.
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