Youn Jung Park

Control of thin ferroelectric polymer films for non-volatile memory applications

The article presents the recent research development in controlling molecular and microstructures of thin ferroelectric polymer films for the application of non-volatile memory. A brief overview is given of the history of ferroelectric memory and device architectures based on ferroelectric polymers particularly emphasizing on the device elements such as metal/ferroelectric/metal type capacitor, metal-ferroelectric-insulator-semiconductor (MFIS) diodes and ferroelectric field effect transistor (FeFET) with ferroelectric poly(vinylidene fluoride) (PVDF) and its copolymers with trifluoroethylene (TrFE). Key material and process issues for optimizing the memory performance in each device architecture and thus realizing non-volatile ferroelectric polymer memory are in details discussed, including the control of crystal polymorphs, film thickness, various hetero-material interfaces between ferroelectric polymer and either metal or semiconductor, crystallization and crystal orientation. The current effort of micro and nanopatterning techniques is also addressed for high density and flexible memory arrays.

Spin cast ferroelectric beta poly(vinylidene fluoride) thin films via rapid thermal annealing

We describe a method of fabricating ferroelectric beta-type poly(vinylidene fluoride) (PVDF) thin films on Au substrate by the humidity controlled spin casting combined with rapid thermal treatment. Our method produces thin uniform ferroelectric PVDF film with ordered beta crystals consisting of characteristic needlelike microdomains. A capacitor with a 160nm thick ferroelectric PVDF film exhibits the remanent polarization and coercive voltage of ∼7.0μC∕cm2 and 8V, respectively, with the temperature stability of up to 160°C. A ferroelectric field effect transistor also shows a drain current bistablility of 100 at zero gate voltage with ±20V gate voltage sweep.

Nonvolatile Polymer Memory with Nanoconfinement of Ferroelectric Crystals

We demonstrate significantly improved performance of a nonvolatile polymeric ferroelectric field effect transistor (FeFET) memory using nanoscopic confinement of poly(vinylidene fluoride-co-trifluoroethylene) (PVDF-TrFE) within self-assembled organosilicate (OS) lamellae. Periodic OS lamellae with 30 nm in width and 50 nm in periodicity were templated using block copolymer self-assembly. Confined crystallization of PVDF-TrFE not only significantly reduces gate leakage current but also facilitates ferroelectric polarization switching. These benefits are due to the elimination of structural defects and the development of an effective PVDF-TrFE crystal orientation through nanoconfinement. A bottom gate FeFET fabricated using a single-crystalline triisopropylsilylethynyl pentacene channel and PVDF-TrFE/OS hybrid gate insulator shows characteristic source-drain current hysteresis that is fully saturated at a programming voltage of ±8 V with an ON/OFF current ratio and a data retention time of approximately 10(2) and 2 h, respectively.

Irreversible extinction of ferroelectric polarization in P(VDF-TrFE) thin films upon melting and recrystallization

We observed the irreversible extinction of ferroelectric polarization in spun coated poly(vinylidene fluoride-co-trifluoroethylene) thin films upon melting and recrystallization. We investigate the alteration of the ferroelectric properties correlated with the preferred polymer crystal orientation with respect to the electrodes using grazing incident scattering, spectroscopy, and electron microscopes. Heat treatment above melting point gave rise to the significant reduction of the ferroelectric performance mainly caused by the modification of molecular orientation of polymer crystals whose c and b axes are perpendicular and parallel to the electrode surface, respectively, leading to almost zero effective electric field.

Tailored Single Crystals of Triisopropylsilylethynyl Pentacene by Selective Contact Evaporation Printing

Organic semiconductors have great potential for application in numerous emerging low-cost and disposable electronic devices such as organic thin fi lm transistors (OTFTs), solar cells, memories, sensors, and fl exible displays. [ 1–3 ] One of the most urgent demands for the realization of such devices is the need to develop new organic semiconductors with a high carrier mobility as well as good and cost-effective processibility for highperformance devices. Chemically-modifi ed pentacene derivatives such as triisopropylsilylethynyl pentacene (TIPS-PEN) have received much attention, in particular in OTFTs. [ 4 , 5–14 ]

SVM-based feature extraction for face recognition

The primary goal of linear discriminant analysis (LDA) in face feature extraction is to find an effective subspace for identity discrimination. The introduction of kernel trick has extended the LDA to nonlinear decision hypersurface. However, there remained inherent limitations for the nonlinear LDA to deal with physical applications under complex environmental factors. These limitations include the use of a common covariance function among each class, and the limited dimensionality inherent to the definition of the between-class scatter. Since these problems are inherently caused by the definition of the Fishers criterion itself, they may not be solvable under the conventional LDA framework. This paper proposes to adopt a margin-based between-class scatter and a regularization process to resolve the issue. Essentially, we redesign the between-class scatter matrix based on the SVM margins to facilitate an effective and reliable feature extraction. This is followed by a regularization of the within-class scatter matrix. Extensive empirical experiments are performed to compare the proposed method with several other variants of the LDA method using the FERET, AR, and CMU-PIE databases.

Chemically cross-linked thin poly(vinylidene fluoride-co-trifluoroethylene)films for nonvolatile ferroelectric polymer memory.

Both chemically and electrically robust ferroelectric poly(vinylidene fluoride-co-trifluoro ethylene) (PVDF-TrFE) films were developed by spin-coating and subsequent thermal annealing with the thermal cross-linking agent 2,4,4-trimethyl-1,6-hexanediamine (THDA). Well-defined ferroelectric β crystalline domains were developed with THDA up to approximately 50 wt %, with respect to polymer concentration, resulting in characteristic ferroelectric hysteresis polarization-voltage loops in metal/cross-linked ferroelectric layer/metal capacitors with remnant polarization of approximately 4 μC/cm(2). Our chemically networked film allowed for facile stacking of a solution-processable organic semiconductor on top of the film, leading to a bottom-gate ferroelectric field effect transistor (FeFET). A low-voltage operating FeFET was realized with a networked PVDF-TrFE film, which had significantly reduced gate leakage current between the drain and gate electrodes. A solution-processed single crystalline tri-isopropylsilylethynyl pentacene FeFET with a chemically cross-linked PVDF-TrFE film showed reliable I-V hysteresis with source-drain ON/OFF current bistablility of 1 × 10(3) at a sweeping gate voltage of ±20 V. Furthermore, both thermal micro/nanoimprinting and transfer printing techniques were conveniently combined for micro/nanopatterning of chemically resistant cross-linked PVDF-TrFE films.

Neural network based emotion estimation using heart rate variability and skin resistance

In order to build a human-computer interface that is sensitive to a users expressed emotion, we propose a neural network based emotion estimation algorithm using heart rate variability (HRV) and galvanic skin response (GSR). In this study, a video clip method was used to elicit basic emotions from subjects while electrocardiogram (ECG) and GSR signals were measured. These signals reflect the influence of emotion on the autonomic nervous system (ANS). The extracted features that are emotion-specific characteristics from those signals are applied to an artificial neural network in order to recognize emotions from new signal collections. Results show that the proposed method is able to accurately distinguish a users emotion.

Fabrication of micropatterned ferroelectric gamma poly(vinylidene fluoride) film for non-volatile polymer memory

We describe a facile and robust method for fabricating ferroelectric γ-type poly(vinylidene fluoride) (PVDF) thin films useful for non-volatile polymer memory. Our method is based on heating and cooling rate-independent melt-recrystallization of a thin PVDF film confined under a surface-energy-controlled top layer that strictly forbids paraelectric α crystals. Thin and uniform PVDF films with ferroelectric γ crystals consisting of characteristic twisted lamellae are formed with versatile top layers including metals, oxides, and even polymers. Micropatterns of ferroelectric γ PVDF domains isolated by paraelectric α domains are readily developed when pre-patterned top layers are employed. Our ferroelectric films are conveniently incorporated into arrays of either capacitor or transistor-type non-volatile memory units. Arrays of ferroelectric transistors with vacuum deposited pentacene channels are fabricated with micropatterned γ PVDF films. Furthermore, arrays of bottom-gate ferroelectric transistor memories are demonstrated, in which our ferroelectric PVDF film is directly micropatterned during crystallization under the patterned poly(3-hexyl thiophene) active channels.

Recent Development in Polymer Ferroelectric Field Effect Transistor Memory

The article presents the recent research development in polymer ferroelectric non-volatile memory. A brief overview is given of the history of ferroelectric memory and device architectures based on inorganic ferroelectric materials. Particular emphasis is made on device elements such as metal/ferroelectric/metal type capacitor, metal-ferroelectric-insulator-semiconductor (MFIS) and ferroelectric field effect transistor (FeFET) with ferroelectric poly(vinylidene fluoride) (PVDF) and its copolymers with trifluoroethylene (TrFE). In addition, various material and process issues for realization of polymer ferroelectric non-volatile memory are discussed, including the control of crystal polymorphs, film thickness, crystallization and crystal orientation and the unconventional patterning techniques.