Organic Ferroelectric Field-effect Transistors Research Articles

Conductance switching in organic ferroelectric field-effect transistors

Staggered bottom-contact top-gate organic ferroelectric field-effect transistors are fabricated with poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE)) as ferroelectric gate and poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine] as semiconductor. Polarization reversal of the ferroelectric gate is monitored by displacement transients in the gate current. By varying both the source and drain biases and by using fully and partially polarized transistors, we show that conductance switching only requires polarization of P(VDF-TrFE) at the source electrode. Polarization at the drain is irrelevant and does not impede charge extraction.

Nonvolatile memory devices based on organic field-effect transistors

Among the many possible device configurations for organic memory devices, organic field-effect transistor (OFET) memory is an emerging technology with the potential to realize lightweight, low-cost, flexible charge storage media. In this feature article, the recent progress in the classes of OFET-based memory, including floating gate OFET memory, polymer electret OFET memory, ferroelectric OFET memory and several other kinds of OFET memories with unique configurations, are introduced. Finally, the prospects and problems of OFETs memory are discussed.

Surface modification of a ferroelectric polymer insulator for low-voltage readable nonvolatile memory in an organic field-effect transistor

We demonstrate that the sequential surface modification of a ferroelectric polymer insulator plays an essential role in both the enhancement of the carrier mobility and the shift in the turn-on voltage (Von) in an organic ferroelectric field-effect transistor (FeFET) for nonvolatile memory. The surface of a ferroelectric polymer insulator, poly(vinylidene fluoride-trifluoroethylene), is physicochemically modified by the successive treatments of ultraviolet-ozone (UVO) and CF4 plasma to understand how the surface morphology and the hydrophobicity affect the grain size, the mobility, and Von in the FeFET. In a pentacene-based FeFET, the CF4 plasma irradiation leads to the mobility enhancement by a factor of about 5 as well as the shift in Von toward a positive voltage direction while the UVO treatment results in only the shift in Von toward a negative voltage direction. It is found that the sequence of the two successive treatments is critical for tailoring interfacial interactions between the ferroelectric polymer insulator and the pentacene layer. The underlying mechanism for the mobility enhancement and the shift in Von is described in terms of the surface morphology and the nature of the built-in electric field.

Fabrication and Characterization of Ferroelectric Poly(vinylidene fluoride–tetrafluoroethylene) Gate Field-Effect Transistor Memories

Organic ferroelectric gate field-effect transistor (FET) memories have been fabricated using pentacene as the semiconductor and a poly(vinylidene fluoride–tetrafluoroethylene) [P(VDF–TeFE)] thin film as the ferroelectric gate. The P(VDF–TeFE) film is prepared by spin coating and annealing at 170 °C for 2.5 h, and pentacene is prepared by vacuum evaporation. The polarization–electric field (P–E) hysteresis of the P(VDF–TeFE) thin film is observed and enhanced by poling treatment. The obtained Pr of 4 µC/cm2 is sufficient for controlling pentacene surface potential. Good memory characteristics are obtained in the P(VDF–TeFE) gate FET. For such a FET, the ON/OFF ratio of drain current is 830, the carrier mobility is 0.11 cm2 V-1 s-1, and the memory retention is over 16 h.

Organic ferroelectric field-effect transistor with P(VDF-TrFE)/PMMA blend thin films for non-volatile memory applications

Among the many ferroelectrics polymers, poly(vinylidene fluoride-co-trifluoroethylene) [P(VDF-TrFE)] has drawn a great attention with its promising memory properties such as large remanent polarization, good fatigue and retention properties. Since the ferroelectric layer plays a critical role in the operation of memory device, it is important to control the structure of ferroelectric thin film. In order to improve the performance of a ferroelectric device, in this contribution we employed P(VDF-TrFE) layers blended with various compositions of an amorphous poly(methyl methacrylate) (PMMA) from 0 wt.% to 20 wt.%. In metal/ferroelectric/metal capacitor structure, we observed the decrease of remanent polarization from 9.13 μC/cm 2 to 4.7 μC/cm 2 and increase of coercive voltage from 9.5 V to 15.2 V with PMMA. Polarization switching time of the ferroelectric blend films estimated from the switched polarization vs. time curves increases with the amount of PMMA. Furthermore, ferroelectric field-effect transistors (FeFETs) based on ferroelectric P(VDF-TrFE) and PMMA blend films with single-crystalline tri-isopropylsilylethynyl pentacene (TIPS-PEN) channels show that both ON and OFF currents of the transistors are maintained with the PMMA contents despite the reduction of remanent polarization of P(VDF-TrFE)/PMMA films.

Degradation mechanisms of organic ferroelectric field-effect transistors used as nonvolatile memory

Organic ferroelectric field-effect transistors were fabricated by inkjet printing for use as nonvolatile memory. Changes in device hysteresis were measured for 7 days to determine the limiting properties that restrict memory retention time. It was found that shifts in threshold voltage contributed to ∼55% of the reduction in transistor current, while decreased dielectric capacitance and reduced semiconductor mobility accounted for ∼30% and ∼15% of the current decay, respectively. The decrease in mobility and the shifts in threshold voltages are caused by remnant dipolar alignment in the ferroelectric insulator, and the reduction in gate capacitance is explained by injected charges in the ferroelectric dielectric. A method to calibrate and extract the input switching voltage is presented, and this calibration accounts for variations in device characteristics with time and allows the ferroelectric transistors to be used as analog memories.

Low-voltage organic ferroelectric field effect transistors using Langmuir–Schaefer films of poly(vinylidene fluoride-trifluororethylene)

Langmuir–Schaefer transfer was used to fabricate ultrathin films of ferroelectric copolymer, poly(vinylidene fluoride-trifluoroethylene) (70–30 mol%), for non-volatile memory application at low operating voltage. Increasing the number of transferred monolayers up to 10 led to improved film crystallinity in the “in-plane” direction, which reduced surface roughness of the semicrystalline film. Treatment of the substrate surface by plasma results in different film coverage which was subsequently found to be governed by interaction of the deposited film and surface condition. Localized ferroelectric switching was substantially attained using piezo-force tip at 10 V on 10-monolayer films. Integrating this film as a dielectric layer into organic capacitor and field effect transistor yields a reasonably good leakage current (<10 −7 A/cm 2) with hysteresis in capacitance and drain current with ON/OFF ratio of 10 3 for organic ferroelectric memory application at significantly reduced operating voltage of |15| V.

Enhanced organic ferroelectric field effect transistor characteristics with strained poly(vinylidene fluoride-trifluoroethylene) dielectric

Poly(vinylidene fluoride-trifluoroethylene) (70–30 mol%) was used as the functional dielectric layer in organic ferroelectric field effect transistors (FeFET) for non-volatile memory applications. Thin P(VDF-TrFE) film samples spin-coated on metallized plastic substrates were stretch-annealed to attain a topographically flat-grain structure and greatly reduce the surface roughness and current leakage of semi-crystalline copolymer film, while enhancing the preferred β-phase of the ferroelectric films. Resultant ferroelectric properties ( P R = |10| μC/cm 2, E C = |50| MV/m) for samples simultaneously stretched (50–70% strain) and heated below the Curie transition (70 oC) were comparable to those resulting from high temperature annealing (>140 oC). The observed enhancements by heating and stretching were studied by vibration spectroscopy and showed mutual complementary effects of both processes. Organic FeFET fabricated by thermal evaporating pentacene on the smooth P(VDF-TrFE) films showed substantial improvement of semiconductor grain growth and enhanced electrical characteristics with promising non-volatile memory functionality.

Anomalous polarization switching in organic ferroelectric field effect transistors

The authors demonstrate organic ferroelectric field effect transistors using poly(vinylidene fluoride-trifluoroethylene) as dielectric in bottom common gate and patterned gate devices. Drain current hysteresis is resulted from the dipole switching at channel region due to gate-source bias. For common gate device, an additional anomalous polarization switching is observed due to gate-drain bias. This switching has no effect on the hysteresis direction yet incurs a strong peak in the off drain current leading to unstable and uncontrollable off state in memory device. Reduction of gate-drain overlapping using patterned metal gate shows diminishing the anomalous switching hence improves performance of the ferroelectric transistors.

Current status and challenges of ferroelectric memory devices

We report on the state-of-the art memory devices on the basis of ferroelectric materials. The paper starts with a short survey on competitive non-volatile memory technologies and focuses then on ferroelectric memories. This includes the ferroelectric random access memory (FeRAM) and the ferroelectric field effect transistor (FeFET). Cell layouts, material aspects and CMOS compatibility as well as fabrication issues will be discussed.Beside the current research on ferroelectric memory devices we present results on the superparaelectric limit of ferroelectric materials with respect to lateral and thickness scaling. Scanning probe techniques showed ferroelectric properties in dots as small as 20 nm. Ultra thin ferroelectric films as thin as a few unit cells can be achieved on lattice matched substrates. These investigations can be considered as a guideline for the maximum achievable packaging density of FeRAMs including low power consumption. The most challenging task to achieve storage above 128 Mb, is the conformal coverage of 3-D electrodes, e.g. by atomic layer deposition (ALD). Three dimensional capacitors are mandatory to achieve sufficient charge for clear signal sensing.In addition, we present a few new concepts based on ferroresistive films, strain induced enhanced ferroelectricity, and lead-free ferroelectrics which may be relevant for the future FeRAM technology. Finally, a new challenging concept of an entire organic ferroelectric field effect transistor (OFeFET) is briefly discussed.