Potentiality of trap charge effects and SiON induced interface defects in a-Si3N4/SiON based MIS structure for resistive NVM device

Potential application of amorphous silicon nitride (a-Si₃N₄)/silicon oxy-nitride (SiON) film has been demonstrated as resistive non-volatile memory (NVM) device by studying the Al/Si₃N₄/SiON/p-Si metal–insulator–semiconductor (MIS) structure. The existence of several deep trap states was revealed by the photoluminescence characterizations. The bipolar resistive switching operation of this device was investigated by current–voltage measurements whereas the trap charge effect was studied in detail by hysteresis behavior of frequency dependent capacitance–voltage characteristics. A memory window of 4.6 V was found with the interface trap density being 6.4 × 10¹¹ cm⁻² eV⁻¹. Excellent charge retention characteristics have been observed for the said MIS structure enabling it to be used as a reliable non-volatile resistive memory device.     

Evolution in crystal structure and electrical performance of thiophene-based polymer field effect transistors: A remarkable difference between thermal and solvent vapor annealing

We report the various conformational structures of long pendant side-chains, and the effects of thermal and solvent vapor annealing (SVA) with the corresponding charge carrier mobilities of thiophene-based conjugated polymers, poly[5,5′-bis(3-dodecyl-2-thienyl)-2,2′-bithiophene] (PQT-12) and poly(4,4′-bis-decyloxymethylquaterthiophene) (POQT), by correlated study of their extraordinary polymorphic crystal structures. In substitution for alkyl chains in polythiophenes, ether alkyl chains induce a favorable non-covalent interaction between the oxygen and sulfur atoms and help the polymer chains planar with lower torsion angles between conjugated backbone units showing a reduced π–π stacking distance. However, the flexibility and conformational freedom with such long side-chains dominantly induce polymorphic crystallites from bent and extended side-chains. Especially, POQT exhibit two polymorphic crystallite phases in a similar ratio probably due to the increased freedom of ether alkyl chains. Therefore, the field effect mobility of POQT is decreased gradually with the increase of annealing temperature from 0.024 (at 80 °C) to 3.96 × 10⁻⁴ cm²/V s (at 170 °C). Contrary to the thermal annealing method, solvent-vapor-annealed POQT films show highly ordered and single-phase crystallites with edge-on orientation to the substrate, which ultimately provides an effectively improved charge carrier mobility from 0.023 (pristine) to 0.076 cm²/V s after adequate solvent vapor exposure.     

Flexible nonvolatile organic ferroelectric memory transistors fabricated on polydimethylsiloxane elastomer

The flexible organic ferroelectric nonvolatile memory thin film transistors (OFMTs) were fabricated on polydimethylsiloxane (PDMS) elastomer substrates, in which an organic ferroelectric poly(vinylidene-trifluoroethylene) and an organic semiconducting poly(9,9-dioctylfluorene-co-bithiophene) layers were used as gate insulator and active channel, respectively. The carrier mobility, on/off ratio, and subthreshold swing of the OFMTs fabricated on PDMS showed 5 × 10⁻² cm² V⁻¹ s⁻¹, 7.5 × 10³, and 2.5 V/decade, respectively. These obtained values did not markedly change when the substrate was bent with a radius of curvature of 0.6 cm. The memory on/off ratio was initially obtained to be 1.5 × 10³ and maintained to be 20 even after a lapse of 2000 s. The fabricated OFMTs exhibited sufficiently encouraging device characteristics even on the PDMS elastomer to realize mechanically stretchable nonvolatile memory devices.     

Operation of 4H-SiC high voltage normally-OFF V-JFET in radiation hard conditions: Simulations and experiment

Operation of high-voltage 4H-SiC vertical-JFET in radiation hard environment was investigated by simulation and experiment. Commercial 1700 V normally-OFF SiC JFETs in TO-247 package were irradiated with fast neutrons to fluences of 4.0 × 10¹⁴ cm^− 2 (1 MeV Si equivalent) and the effect of radiation on their characteristics was then thoroughly analyzed. Four degradation mechanisms were identified, of which the most important is the increase of JFETs ON-state resistance due to the mobility degradation and removal of carriers from transistor's light doped channel and drift regions. As a result, the JFET ON-state losses grow and, at fluences higher than 4 × 10¹⁴ cm^− 2, the low doped n-regions are fully compensated and transistor loses its functionality. On the contrary, irradiation slightly improves JFET's switching characteristics. The effect of neutron irradiation on operation of SiC V-JFET in a real application was then investigated on the step-UP 15 V/60 V DC-DC converter where the SiC JFET was used as an active switch. Converter characteristics were analyzed by means of the mixed-mode simulation using the developed 2D model of the neutron irradiated transistor. Results showed that the duty cycle of the PWM regulator is growing due to the increase in the voltage drop on the switching JFET. This effect, which is caused by the abovementioned increase the JFET's ON-state resistance, increases power dissipation and deteriorates converter efficiency. Finally, the effect of neutron irradiation on operation SiC V-JFET in the 850 V/24 V auxiliary flyback switching mode power supply was analyzed. We showed that the growth of the ON-state resistance increases transistor's conduction losses and decreases converter efficiency. Exceeding the fluence of 3.3 × 10¹⁴ cm^− 2 neutrons then causes JFET overheating and subsequent destruction.     

Unipolar nonvolatile memory devices with composites of poly(9-vinylcarbazole) and titanium dioxide nanoparticles

Organic-based devices with an 8 × 8 array structure using titanium dioxide nanoparticles (TiO₂ NPs) embedded in poly(9-vinylcarbazole) (PVK) film exhibited bistable resistance states and a unipolar nonvolatile memory effect. TiO₂ NPs were a key factor for realizing the bistability and the concentration of TiO₂ NPs influenced ON/OFF ratio. From electrical measurements, switching mechanism of PVK:TiO₂ NPs devices was closely associated with filamentary conduction model and it was found that the OFF state was dominated by thermally activated transport while the ON state followed tunneling transport. PVK:TiO₂ NPs memory devices in 8 × 8 array structure showed a uniform cell-to-cell switching, stable switching endurance, and a high retention time longer than 10⁴ s.     

Solution processable poly(2,5-dialkyl-2,5-dihydro-3,6-di-2-thienyl-pyrrolo[3,4-c]pyrrole-1,4-dione) for ambipolar organic thin film transistors

Solution processable diketopyrrolopyrrole (DPP)-bithiophene polymers (PDBT) with long branched alkyl side chains on the DPP unit are synthesized. These polymers have favourable highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels for the injection and transport of both holes and electrons. Organic thin film transistors (OTFTs) using these polymers as semiconductors and gold as source/drain electrodes show typical ambipolar characteristics with very well balanced high hole and electron mobilities (μ_h = 0.024 cm² V^?1 s^?1 and μ_e = 0.056 cm² V^?1 s^?1). These simple and high-performing polymers are promising materials for ambipolar organic thin film transistors for low-cost CMOS-like logic circuits.     

Characterization of ZrTiO4 thin films prepared by sol–gel method

The electrical properties, memory switching behavior, and microstructures of ZrTiO₄ thin films prepared by sol–gel method at different annealing temperatures were investigated. All films exhibited ZrTiO₄ (111) and (101) orientations perpendicular to the substrate surface, and the grain size increased with increasing annealing temperature. A low leakage current density of 1.47×10^?6 A/cm² was obtained for the prepared films. The I–V characteristics of ZrTiO₄ capacitors can be explained in terms of ohmic conduction in the low electric field region and Schottky emission in the high electric field region. An on/off ratio of 10² was measured in our glass/ITO/ZrTiO₄/Pt structure with an annealing temperature of 600 °C. Considering the primary memory switching behavior of ZrTiO₄, ReRAM based on ZrTiO₄ shows promise for future nonvolatile memory applications.     

Multilevel characteristics and operating mechanisms of nonvolatile memory devices based on a floating gate of graphene oxide sheets sandwiched between two polystyrene layers

Nonvolatile organic memory devices were fabricated utilizing a graphene oxide (GO) layer embedded between two polystyrene (PS) layers. Scanning electron microscope images of GO sheets sandwiched between two PS layers showed that the GO sheets were clearly embedded in the PS layers. Capacitance–voltage (C–V) curves of the Al/PS/GO/PS/n-type Si devices clearly showed hysteresis behaviors with multilevel characteristics. The window margin of the nonvolatile memory devices increased from 1 to 7 V with increasing applied sweep voltages from 6 to 32 V. The cycling retention of the ON/OFF switching for the devices was measured by applying voltages between +15 and −15 V. While the capacitance of the memory devices at an ON state have retained as 230 pF up to 10⁴ cycles, that at an OFF state maintained as 16 pF during three times of repeated measurements. The extrapolation of the retention data for the devices maintained up to 10⁶ cycles. The operating mechanisms of the nonvolatile organic memory devices with a floating gate were described by the C–V results and the energy band diagrams.     

Resistive switching characteristics of ZnO–graphene quantum dots and their use as an active component of an organic memory cell with one diode-one resistor architecture

We investigated the resistive switching characteristics of a polystyrene:ZnO–graphene quantum dots system and its potential application in a one diode-one resistor architecture of an organic memory cell. The log–log I–V plot and the temperature-variable I–V measurements revealed that the switching mechanism in a low-current state is closely related to thermally activated transport. The turn-on process was induced by a space-charge-limited current mechanism resulted from the ZnO–graphene quantum dots acting as charge trap sites, and charge transfer through filamentary path. The memory device with a diode presented a ∼10³ I_ON/I_OFF ratio, stable endurance cycles (10² cycles) and retention times (10⁴ s), and uniform cell-to-cell switching. The one diode-one resistor architecture can effectively reduce cross-talk issue and realize a cross bar array as large as ∼3 kbit in the readout margin estimation. Furthermore, a specific word was encoded using the standard ASCII character code.     

Self-aligned flexible organic thin-film transistors with gates patterned by nano-imprint lithography

Many applications that rely on organic electronic circuits still suffer from the limited switching speed of their basic elements – the organic thin film transistor (OTFT). For a given set of materials the OTFT speed scales inversely with the square of the channel length, the parasitic gate overlap capacitance, and the contact resistance. For maximising speed we pattern transistor channels with lengths from 10 μm down to the sub-micrometre regime by industrially scalable UV-nanoimprint lithography. The reduction of the overlap capacitance is achieved by minimising the source–drain to gate overlap lengths to values as low as 0.2 μm by self-aligned electrode definition using substrate reverse side exposure. Pentacene based organic thin film transistors with an exceptionally low line edge roughness <20 nm of the channels, a mobility of 0.1 cm²/Vs, and an on–off ratio of 10⁴, are fabricated on 4″ × 4″ flexible substrates in a carrier-free process scheme. The stability and spatial distribution of the transistor channel lengths are assessed in detail with standard deviations of L ranging from 185 to 28 nm. Such high-performing self-aligned organic thin film transistors enabled a ring-oscillator circuit with an average stage delay below 4 μs at an operation voltage of 7.5 V.     

Fluorene trimers with various 9,9′-substituents: The synthesis,characteristics, condensed state structures,and electroluminescence properties

The four fluorene-based trimers with various aromatic and alkyl substituents (T1–T4) are synthesized and characterized. These oligomers show the similar electronic absorption and emission characteristics (e.g., absorption peak at 351 nm, and highly efficient deep blue emission at 394 nm in solution), indicating that the major electronic properties of the core chromophore are essentially independent of the substituents. However, the condensed state structures and thermal properties of four trimers are found to be different from each other, from crystalline (full alkyl (T1) or full aromatic (T2) substituted trimers) to amorphous (mixed aromatic and alkyl (T4) substituted trimers). The effect of different condensed state structures on electroluminescence device properties is presented: The blue light-emitting devices with accordant structure of ITO/PEDOT:PSS/TCTA (40 nm)/trimers (40 nm)/BCP (10 nm)/Alq₃ (20 nm)/LiF/Al exhibit different EL efficiency (2.9% of T2, 1.8% of T3 and 2.7% of T4). Using amorphous T4, the white light-emitting device of ITO/TCTA (40 nm)/rubrene (0.1 nm)/T4 (8 nm)/Alq₃(52 nm)/LiF/Al is fabricated with high efficiency (6.15 cd A⁻¹), high brightness (9500 cd m⁻²) and good white light CIE coordinates (0.32, 0.37).     

Hybrid dual gate ferroelectric memory for multilevel information storage

Here, we report hybrid organic/inorganic ferroelectric memory with multilevel information storage using transparent p-type SnO semiconductor and ferroelectric P(VDF-TrFE) polymer. The dual gate devices include a top ferroelectric field-effect transistor (FeFET) and a bottom thin-film transistor (TFT). The devices are all fabricated at low temperatures (∼200 °C), and demonstrate excellent performance with high hole mobility of 2.7 cm² V⁻¹ s⁻¹, large memory window of ∼18 V, and a low sub-threshold swing ∼−4 V dec⁻¹. The channel conductance of the bottom-TFT and the top-FeFET can be controlled independently by the bottom and top gates, respectively. The results demonstrate multilevel nonvolatile information storage using ferroelectric memory devices with good retention characteristics.     

A flexible polymer memory device

A flexible polymer memory device is demonstrated in a sandwich structure of polypyrrole/P6FBEu/Au. Conductance switching at a voltage of about 4 V, with an ON/OFF current ratio up to 200, was observed in this flexible memory device. At the low-conductivity state, current density–voltage (J–V) characteristics of the device were dominated by a charge injection current. At the high conductivity state, J–V characteristics were dominated by a space-charge-limited current. Both the ON and OFF states are stable up to 10⁶ read cycles at a read voltage of 1 V. The device can be used as a write-once read-many-times (WORM) memory with good electronic stability.     

Bistable electrical switching and nonvolatile memory effect in mixed composite of oxadiazole acceptor and carbazole donor

Bistable electrical switching and nonvolatile memory devices with the configuration of indium tin oxide (ITO)/active layer/aluminum (Al) are reported. The active layer were prepared from the mixed compositions of 2-(4-tert-butylphenyl)-5-(4-biphenylyl)-1,3,4-oxadiazole, (PBD) and poly(N-vinylcarbazole) (PVK). The as-fabricated ITO/PBD:PVK/Al sandwiched devices exhibited rewriteable flash memory property. Due to the strong interaction between oxadiazole acceptor and carbazole donor, the devices demonstrate excellent performance. The memory devices can operate over a small voltage range, the absolute value of switching-on threshold voltage is less than 1 V and the switching-off threshold voltage is less than 3.5 V. The ON/OFF ratio of current switches in the range of 10⁴–10² during the variation of applied voltage and the two different resistance states can be maintained over 4 h.     

Ferroelectric transistor memory arrays on flexible foils

In this paper, we successfully fabricated and operated passive matrix P(VDF–TrFE) transistor arrays, i.e. memory arrays in which no pass-transistors or other additional electronic components are used. Because of the smaller cell, a higher integration density is possible. We demonstrate arrays up to a size of 16 × 16, processed on thin (25 μm) poly(ethylene naphthalate) substrates, using Indium–Gallium–Zinc–Oxide (IGZO) as the semiconductor and 200 nm-thick P(VDF–TrFE) as a ferroelectric gate dielectric. The memory transistors have remnant current modulations of ～10⁵ with a retention time of more than 12 days. They can be switched in less than 1 μs at operating voltages of 25 V. Switching speed is strongly decreased with decreasing voltage: at ～10 V the transistors do not switch within 10 s. This difference in switching speed of more than 4 orders in magnitude when changing the electric field by a factor of only 2.5 makes these memories robust towards disturb voltages, and forms the basis of integration of these transistors in passive matrix-addressable transistor arrays that contains only one (memory) transistor per cell. It is shown that with current technology and memory characteristics it is possible to scale up the array size in the future.     

In-plane electrical bistability of photochromic diarylethene/Cu composite film

Electrical bistability is an essential property for memory devices. We report here the in-plane electrical bistability of photochromic diarylethene (DAE)/Cu composite film, which is prepared by Cu vapor deposition on the DAE surface with a low glass-transition temperature. The low-current level around 10⁻⁸ A was switched to a high-current level of ca. 10⁻⁴ A at a low threshold voltage (V_th) in the first voltage sweep. Once this switching occurred, the high-current level was kept in the second voltage sweep, and electrical bistability was achieved for the in-plane current. V_th was distributed in a wide range of voltages (0.5–10 V), and the colored sample obtained by the UV irradiation showed a relatively higher V_th than the colorless sample. The highest ON–OFF ratio in current was ca. 10⁶. The origin of the bistability attributed to the electrical breakdown in the insulated lines that was consisted of DAE in Cu film. The in-plane bistability of the DAE/Cu composite film has good retention time (>60 min) and readout-cycle endurance (>10⁶ cycles), indicating that it is suitable for write-once organic semiconductor memory characteristics.     

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/cm²) with hysteresis in capacitance and drain current with ON/OFF ratio of 10³ for organic ferroelectric memory application at significantly reduced operating voltage of |15| V.     

Characterization of CoxNiyO hybrid metal oxide nanoparticles as charge trapping nodes in nonvolatile memory devices

The Co_xNi_yO hybrid metal oxide nanoparticles (HMONs) embedded in the HfO_xN_y high-k dielectric as charge trapping nodes of the nonvolatile memory devices have been formed via the chemical vapor deposition using the Co/Ni acetate calcined and reduced in the Ar/NH₃ ambient. A charge trap density of 8.96 × 10¹¹ cm^?2 and a flatband voltage shift of 500 mV were estimated by the appearance of the hysteresis in the capacitance–voltage (C–V) measurements during the ±5 V sweep. Scanning electron microscopy image displays that the Co_xNi_yO HMONs with a diameter of ～10–20 nm and a surface density of ～1 × 10¹⁰ cm^?2 were obtained. The mechanism related to the writing characteristics are mainly resulted from the holes trapping. Compared with those devices with the Co_xNi_yO HMONs formed by the dip-coated technique, memory devices with the Co_xNi_yO HMONs fabricated by the drop-coated technique show improved surface properties between the Co_xNi_yO HMONs and the HfON as well as electrical characteristics.     

Achieving high sensitivity in single organic submicrometer ribbon based photodetector through surface engineering

The performance of the single organic submicrometer ribbon based photodetector has been improved by one order of magnitude by combining a better crystalline structure with proper surface engineering, such as coating a dielectric layer PMMA or PS between the substrate and the organic semiconductor. The photoconductivity gain is as much as 1.3 × 10⁴, the responsivity is about 4372 A W^?1 at the field of about 2 × 10⁶ V m^?1, and the highest on/off ratio reaches 10⁴. The extremely high gain is attributed to the high mobility and the long photo carrier’s lifetime induced by the dielectric layer.     

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/cm², 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.