Following the trend of miniaturization as per Moore's law, and facing the strong demand of next-generation electronic devices that should be highly portable, wearable, transplantable, and lightweight, growing endeavors have been made to develop novel flexible data storage devices possessing nonvolatile ability, high-density storage, high-switching speed, and reliable endurance properties. Nonvolatile organic data storage devices including memory devices on the basis of floating-gate, charge-trapping, and ferroelectric architectures, as well as organic resistive memory are believed to be favorable candidates for future data storage applications. In this Review, typical information on device structure, memory characteristics, device operation mechanisms, mechanical properties, challenges, and recent progress of the above categories of flexible data storage devices based on organic nanoscaled materials is summarized.

Recent Advances of Flexible Data Storage Devices Based on Organic Nanoscaled Materials.

Ferroelectric non-volatile memory devices are the most promising type of memory for future use in commercial applications. However, conventional planar organic ferroelectric memory transistors usually show poor performance in current density, operating speed and mechanical stability. There are three main factors which restrict the performance of organic ferroelectric memory transistors: the intrinsic low charge mobility of the organic semiconductor material; the large surface roughness of the ferroelectric layer; and the relative long channel length. In this research, a novel device architecture with an ultra-short channel length was invented. Benefiting from the short channel length, the memory device devised in this research exhibits a large current density of up to 6.32 mA cm−2 and a fast operating speed. Furthermore, the current flows across the semiconductor layer from the bottom mesh source electrode to the top drain electrode, which reduced the influence of the poor interface quality caused by the use of poly(vinylidene fluoride-co-trifluoroethylene) [P(VDF–TrFe)] and enhances the performance of mechanical stability under bending. In addition, the main memory properties are comparable or superior to previously reported organic ferroelectric memories and inorganic ferroelectric memory with a vertical structure. Therefore, benefiting from the ultra-short channel length and special device structure, the ferroelectric memory in this research exhibited a large driving force, excellent bending characteristics and outstanding storage performance, which has great potential for wearable and flexible electric devices.

Flexible ultra-short channel organic ferroelectric non-volatile memory transistors

We report a concept fabrication method that helps to improve the performance and stability of copper phthalocyanine (CuPc) based organic field-effect transistors (OFETs) in ambient. The devices were fabricated using a trilayer dielectric system that contains a bilayer polymer dielectrics consisting of a hydrophobic thin layer of poly(methyl methacrylate) (PMMA) on poly(vinyl alcohol) (PVA) or poly(4-vinylphenol) (PVP) or polystyrene (PS) with Al2O3 as a third layer. We have explored the peculiarities in the device performance (i.e., superior performance under ambient humidity), which are caused due to the polarization of dipoles residing in the polar dielectric material. The anomalous behavior of the bias-stress measured under vacuum has been explained successfully by a stretched exponential function modified by adding a time dependent dipole polarization term. The OFET with a dielectric layer of PVA or PVP containing hydroxyl groups has shown enhanced characteristics and remains highly stable without any...

Enhanced environmental stability induced by effective polarization of a polar dielectric layer in a trilayer dielectric system of organic field-effect transistors: a quantitative study.

Assembly of organic semiconductors with ordered crystal structure has been actively pursued for electronics applications such as organic field-effect transistors (OFETs). Among various film deposition methods, solution-based film growth from small molecule semiconductors is preferable because of its low material and energy consumption, low cost, and scalability. Here, we show scalable and controllable directed assembly of highly crystalline 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C8-BTBT) films via a dip-coating process. Self-aligned stripe patterns with tunable thickness and morphology over a centimeter scale are obtained by adjusting two governing parameters: the pulling speed of a substrate and the solution concentration. OFETs are fabricated using the C8-BTBT films assembled at various conditions. A field-effect hole mobility up to 3.99 cm2 V–1 s–1 is obtained. Owing to the highly scalable crystalline film formation, the dip-coating directed assembly process could be a great candidate for m...

Scalable Directed Assembly of Highly Crystalline 2,7-Dioctyl[1]benzothieno[3,2- b][1]benzothiophene (C8-BTBT) Films.

Intrinsic Defect Limit to the Growth of Orthorhombic HfO2 and (Hf,Zr)O2 with Strong Ferroelectricity: First-Principles Insights

We fabricated ultra-flexible organic non-volatile ferroelectric 
field-effect transistors (FeFETs) with poly(vinylidene fluoride-trifluoroethylene) [P(VDF-TrFE)] on polyimide substrates, which adopted a solution-based top-contact/bottom-gate structure for low cost process without patterning. P(VDF-TrFE) gate dielectric layers and regioregular poly(3-hexylthiophene) active layers were formed by the spin-coating method. The field-effect mobility (μ
FE) was ∼0.28 cm2/V s, the on/off ratio was approximately 5.6 × 103, and the memory window (threshold voltage shift) was approximately 7 V. In addition, FeFETs were operated even at small bend radii without considerable changes in these values.

Solution-processed organic ferroelectric field-effect transistors on ultra-flexible substrates

In this research, we have investigated the undoped HfO2 formation with a metastable orthorhombic phase on Si substrates and its substrate orientation dependence A 24-nm-thick HfO2 was deposited by radio-frequency (RF) magnetron sputtering with an Ar/O2 plasma utilizing a Hf target It was found that metastable orthorhombic phase formation was enhanced at an Ar/O2 flow ratio of 20/02 sccm followed by post-deposition annealing at 600 °C/30 s in N2 ambient The memory window (MW) extracted from the capacitance–voltage (C–V) characteristics measured at 100 kHz of Al/HfO2/p-Si(100) was increased from 039 to 084 V as the Ar/O2 flow ratio during sputtering was increased from Ar/O2 of 20/10 to 20/02 sccm In the case of p-Si(111) substrates, the MW was increased to 099 V when the Ar/O2 flow ratio was 20/02 sccm probably owing to the enhanced crystallization of the metastable orthorhombic phase

https://iopscience.iop.org/article/10.7567/JJAP.57.11UF09/pdf

Ferroelectric properties of undoped HfO2 directly deposited on Si substrates by RF magnetron sputtering

With lots of interest in negative capacitance field-effect transistors for ultralow-power complementary metal–oxide–semiconductor technology, negative capacitance thin-film transistors (NCTFTs) have also received much attention. Although previous studies on NCTFTs were done, the NC effect in organic-based TFTs was not studied yet. In this paper, P(VDF-TrFE) ferroelectric-gated P3HT semiconductor channel TFTs are experimentally demonstrated with solution-based fabrication process. Especially, this paper shed light on the NC effect in the organic-based TFTs. The step-up current-voltage characteristics are repeatedly and reliably observed in diverse TFTs, and then, with the results, transconductance ( $g_{m}$ ) amplification implemented by the negative capacitance was delved. Moreover, with the aid of ferroelectric polarization switching, super steep-switching characteristic of the organic-based TFT was experimentally confirmed. These experimental results and discussion would be helpful in understanding NC effects in TFTs.

Transconductance Amplification by the Negative Capacitance in Ferroelectric-Gated P3HT Thin-Film Transistor

We have investigated the effects of the Kr/O2-plasma reactive sputtering on the nondoped HfO2 thin-film formation for ferroelectric gate metal-ferroelectrics-Si field-effect transistor (MFSFET). The remnant polarization was increased while the coercive voltage ( ${V}_{c}$ ) was decreased for 5-nm-thick HfO2 formed by the Kr/O2-plasma sputtering followed by the postmetallization annealing (PMA) utilizing the Pt gate electrode compared to the film formed by the Ar/O2-plasma sputtering. The excellent fatigue properties until 1010 cycles were realized under the ±4 V/ $5~\mu \text{s}$ input pulses. The memory window (MW) of 1.2 V was obtained under 2.5 V operation in the drain current ( ${I}_{D}$ )–drain voltage ( ${V}_{G}$ ) characteristics for the fabricated MFSFET.

Min Gee Kim

Papers

Solution-processed organic ferroelectric field-effect transistors on ultra-flexible substrates

Ferroelectric properties of undoped HfO2 directly deposited on Si substrates by RF magnetron sputtering

Transconductance Amplification by the Negative Capacitance in Ferroelectric-Gated P3HT Thin-Film Transistor

Effect of Kr/O 2 -Plasma Reactive Sputtering on Ferroelectric Nondoped HfO₂ Formation for MFSFET With Pt Gate Electrode

The Effect of PMA with TiN Gate Electrode on the Formation of Ferroelectric Undoped HfO 2 Directly Deposited on Si(100)