Roll-off Behavior Research Articles

Efficient Stochastic Computing FIR Filtering Using Sigma-Delta Modulated Signals

This work presents a soft-filtering digital signal processing architecture based on sigma-delta modulators and stochastic computing. A sigma-delta modulator converts the input high-resolution signal to a single-bit stream enabling filtering structures to be realized using stochastic computing’s negligible-area multipliers. Simulation in the spectral domain demonstrates the filter’s proper operation and its roll-off behavior, as well as the signal-to-noise ratio improvement using the sigma-delta modulator, compared to typical stochastic computing filter realizations. The proposed architecture’s hardware advantages are showcased with synthesis results for two FIR filters using FPGA and synopsys tools, while comparisons with standard stochastic computing-based hardware realizations, as well as with conventional binary ones, demonstrate its efficacy.

Enhancement of Reverse Intersystem Crossing in Charge‐Transfer Molecule through Internal Heavy Atom Effect (Adv. Funct. Mater. 50/2021)

Internal Heavy Atom Effect In article number 2104646, Jihoon Lee, Min Chul Suh, Seunghyup Yoo, and co-workers show how incorporating the internal heavy atom (IHA) effect can impact the reverse intersystem crossing (RISC) and device performance. The spectroscopic and theoretical results indicate that a fast RISC may not be the sole factor important for reducing efficiency roll-off and that the spin-flip cycles considering both T1 → S1 and S1 → T1 should be carefully taken into account to derive a complete picture of the IHA effect on efficiency roll-off behavior.

Excess Ion-Induced Efficiency Roll-Off in High-Efficiency Perovskite Light-Emitting Diodes.

Applying extensively excess ammonium halides in forming perovskites is a widely used approach to achieve high-performance perovskite light-emitting diodes (PeLEDs). However, most of these PeLEDs suffer from severe external quantum efficiency (EQE) roll-off at high current densities, thereby restricting the realization of high-brightness PeLEDs and laser diodes. In this work, we explore the underlying mechanism of the EQE roll-off in high-efficiency formamidinium lead iodide (FAPbI3)-based PeLEDs. By combining voltage-dependent electrical stress measurements and ex situ ion distribution analysis of PeLEDs, we found that the electric field-driven migration and local segregation of excess iodide ions, originated from nonstoichiometric precursors, trigger the EQE roll-off via promoting imbalanced charge injection. Based on this discovery, we introduced a simple wash-off treatment with chloroform to remove the excess iodides from the perovskite surface and demonstrated that the treatment is highly effective in suppressing the roll-off behavior. By combining the treatment and the use of an ultrathin poly(methyl methacrylate) (PMMA) interlayer, we achieved a high-brightness PeLED with an EQEmax of 19.6%, a critical current density of 1550 mA cm-2, and a radiancemax of 875 W sr-1 m-2. The study reveals the double-edge sword effect of precursor nonstoichiometry and highlights the importance of managing excess ions in perovskite films.

Low-Noise Si-JFETs Enhanced by Split-Channel Concept

We present the results of low-noise silicon junction field-effect transistors (JFET) with a split-channel concept. The device can be manufactured as a module in a standard CMOS process. The channel is split under the top gate of the JFET into a source-side main channel and a drain-side-extended drain channel. Devices with design gate length between 1.0 and 3.0 μm and effective channel length between 0.1 and 2.2 μm were fabricated. It is shown that transconductance and channel resistance are dominated by the overlap between the main channel and the top gate. Output resistance can be enhanced by increasing the overlap of the extended drain channel with the top gate. A cutoff frequency of up to 2.5 GHz, 60-mS/mm maximum transconductance, and an intrinsic gain of 2200 were achieved. For the main channel length below 1 μm, a strong roll-off behavior of the threshold voltage is observed. The flicker-corner frequency for 1/f noise is 500 Hz. Above 1 kHz, √ a value of 2.5-nV/ Hz input-referred voltage noise density at 0.3-pF input capacitance was achieved. Also, a brief comparison to devices from other publications is presented.

Developing antibacterial superhydrophobic coatings based on polydimethylsiloxane/silver phosphate nanocomposites: Assessment of surface morphology, roughness and chemistry

Nanocomposite coatings based on polydimethylsiloxane (PDMS) and silver phosphate (Ag3PO4) nanoparticles were developed to achieve superhydrophobicity, antibacterial behavior and low protein adsorption. Since the as-synthesized nanoparticles were hydrophilic, octadecanethiol was added into the coatings’ solutions. Wettability results demonstrated that the higher the nanoparticle content, the higher the water contact angle (WCA). The highest WCA was observed for 7 wt% inclusion of nanoparticles (152°). Morphological analysis revealed the surface localization of nanoparticles and a packed structure in case of 7 wt% nanoparticle inclusion. From the roughness results, the ratio of texture surface area to cross-sectional area was found to be notably increased at higher nanoparticle contents. The presence of Ag3PO4 nanoparticles at the coatings’ top layer was confirmed by X-ray photoelectron spectroscopy. Antibacterial activity of the coatings significantly enhanced upon increasing the nanoparticle content. As a result of a true superhydrophobic (roll-off) behavior for 7 wt% nanoparticle inclusion, the protein adsorption was highly reduced (∼83 %) due to the enclosed air layer within the surface cavities leading to a lowered contact area of proteins with the coating’s surface. The combination of superhydrophobicity, antibacterial behavior and low protein adsorption leads to a biocompatible coating which could have many biomedical applications needing further investigations.

Replacement Gate High-k/Metal Gate nMOSFETs Using a Self-Aligned Halo-Compensated Channel Implant

A device design technique for boosting output resistance (Rout) characteristics of long-channel halo-doped nMOSFETs for replacement gate (RMG) high-k/metal gate (HK/MG) devices is proposed based on numerical simulations. We show that the self-aligned halo-compensated channel implant (HCCI) that is carried out after dummy poly gate removal provides compensation for the conventional halo doping. This can be utilized to mitigate the magnitude of potential drop beyond saturation at the drain end so that the long-channel Rout can be boosted relative to the conventional halo-doped device. In addition, we achieve an improved Vt roll-off behavior, an advantage to hot carrier injection (HCI) characteristics at high voltage operation, and the lowered short-channel drain-induced barrier lowering (DIBL) and intrinsic delay via the optimization of the source/drain (S/D) doping profile. HCCI technique can be implemented using a process flow that is compatible with the RMG HK/MG system-on-chip (SoC) technology in the mass production.

Utilization of Multi-Heterodonors in Thermally Activated Delayed Fluorescence Molecules and Their High Performance Bluish-Green Organic Light-Emitting Diodes.

We report a series of pentacarbazolyl-benzonitrile derivatives such as 2,4,6-tri(9H-carbazol-9-yl)-3,5-bis(3,6-di(pyridin-3-yl)-9H-carbazol-9-yl)benzonitrile (mPyBN), 3,5-bis(3,6-bis(4-(trifluoromethyl)phenyl)-9H-carbazol-9-yl)-2,4,6-tri(9H-carbazol-9-yl)benzonitrile (pCF3BN), 2,4,6-tri(9H-carbazol-9-yl)-3-(3,6-di(pyridin-3-yl)-9H-carbazol-9-yl)-5-(3,6-diphenyl-9H-carbazol-9-yl)benzonitrile (PyPhBN), 3-(3,6-bis(4-(trifluoromethyl)phenyl)-9H-carbazol-9-yl)-2,4,6-tri(9H-carbazol-9-yl)-5-(3,6-di(pyridin-3-yl)-9H-carbazol-9-yl)benzonitrile (PyCF3BN), and 3-(3,6-bis(4-(trifluoromethyl)phenyl)-9H-carbazol-9-yl)-2,6-di(9H-carbazol-9-yl)-5-(3,6-di(pyridin-3-yl)-9H-carbazol-9-yl)-4-(9H-pyrido[3,4-b]indol-9-yl)benzonitrile (CbPyCF3BN) in which some of the carbazoles are substituted with modified 3,5-diphenyl carbazoles, exhibiting thermally activated delayed fluorescence (TADF) properties. These emitters comprised two, three, and four different types of donors, capable of bluish-green emission of around 480 nm with relatively high photoluminescence quantum yields over 90% in solution. Emitters, namely, PyPhBN, PyCF3BN, and CbPyCF3BN, composed of three and four different types of donors endowed a rather short delayed lifetime (τd) of 4.25, 5.01, and 3.65 μs in their film state, respectively. Bluish-green organic light-emitting diodes based on PyPhBN, PyCF3BN, and CbPyCF3BN exhibit a high external quantum efficiency of 20.6, 19.5, and 19.6%, respectively, with unsurpassed efficiency roll-off behavior. These results indicate that the TADF properties of multidonor type molecules can be manipulated by controlling the types and number of electron donor units.

Improvement of Quantum Efficiency and Efficiency Roll-off in OLED Devices Using Blue Thermally Activated Delayed Fluorescent Materials

The utilization of triplet excitons in the operation of organic light emitting diode play a crucial role for the enhancement of an external quantum efficiency, which will be maximized by the use of thermally activated delayed fluorescent (TADF) materials. In this study, a deep blue TADF materials with a donor-acceptor-donor structure was designed and prepared 9,9'-((2-methylpyrimidine-4,6-diyl) bis(2,1-phenylene)) bis(9H-carbazole) (DCz-mPym), (9,9' -((2- methylpyrimidine-4,6-diyl)bis(2,1-phenylene)) bis(3,6-di-tert-butyl-9H-carbazole) (DtBCz-mPym), and 5,5' -((2-methylpyrimidine-4,6-diyl) bis(2,1-phenylene)) bis (5H-benzofuro[3,2-c]carbazole) (DBFCz-mPym) using a pyrimidine as an acceptor and a carbazole derivatives as a donor. We fabricated an organic light emitting diode (OLED) using the designed three TADF materials as a dopant for light emitting layer of OLED. Correlation between the photophysical data, electrochemical properties, and the efficiency roll-off behavior were investigated for designed materials and device with appropriate structure. As a result, DBFCz-mPym, the material with very small difference between singlet energy and triplet energy, is superior to the DCz-mPym or DtBCz-mPym in terms of efficiency roll-off and harvesting of triplet excitons. Further, as a result of simulation of an efficiency roll-off curves by the increase of the current density, triple-triplet annihilation was confirmed to be the main cause.

Roll-Off Displacement in Ex Vivo Experiments of RF Ablation With Refrigerated Saline Solution and Refrigerated Deionized Water

The recurrence rate in the treatment of liver tumors using radio frequency ablation (RFA) is often related to incomplete tissue necrosis and consequently the limitation in the ablation volume. This paper proposes an ablation protocol combined with the infusion of saline solution and deionized water aiming at achieving a time displacement in the roll-off occurrence and consequently increasing the volume of ablation. An infusion of saline solution and deionized water at 5 and 23 °C was performed to evaluate the influence of these liquids on the RFA procedure in ex vivo bovine liver pieces. The obtained results were used to propose a mathematical model of the roll-off phenomenon by means of the system identification techniques. The RFA combined with the infusion of saline solution 0.9% at 5 °C presented optimal results, with a time delay of the roll-off occurrence in 27.8% compared to pure RFA ( p = 0.002) and an increase in the necrotic volume of 51.2% ( p = 0.0002). Two Box-Jenkins models were obtained to describe the roll-off phenomenon: 1) pure RFA; and 2) RFA combined with the saline solution 0.9% at 5 °C. The RFA therapy combined with the saline solution 0.9% at 5 °C increases the time range to the roll-off occurrence, leading to higher necrosis volumes in ex vivo bovine liver samples. The development of a mathematical model to describe the roll-off behavior demonstrated that the transient response is improved by the infusion of the saline solution 0.9% at 5 °C.

Atmospheric plasma etching of polymers: A palette of applications in cleaning/ashing, pattern formation, nanotexturing and superhydrophobic surface fabrication

Atmospheric Pressure Plasmas (APPs) are promising alternatives to their low-pressure counterparts for material surface treatment due to their potential for cost-effective continuous processing. Here we present for the first time to our knowledge a palette of applications of polymer etching showing the potential of APPs for cleaning/ashing, nanopattern formation, nanotexturing, and hierarchical superhydrophobic surface fabrication using the same plasma source configuration. First, we used a dielectric barrier discharge (DBD) plasma source to study etching/cleaning/ashing of different polymers such as PMMA films and AZ photoresist with lithographic patterns using He/O2 mixtures. We achieved high-rate, uniform etching, indicating the source potential for cleaning/ashing even in ambient air conditions. We have noticed that thick photoresist films are damaged after prolonged etching in air plasmas, a phenomenon resembling photoresist damage observed during low-pressure plasma etching. Second, we applied colloidal lithography with polystyrene (PS) nanoparticles followed by isotropic APP etching in order to form nanopatterns on both Silicon and Polymers. We observed that APP induces “cauliflower-like” nanotexturing of the PS spheres. This nanotexturing in combination with plasma etching of the underlying polymer and the spheres allow the fabrication of hierarchical surfaces. Optimization of plasma etching duration (i.e. hierarchical topography) followed by low-pressure plasma deposition of a low surface energy fluorocarbon (hydrophobic) coating led to the preparation of superhydrophobic surfaces with water contact angle 158° and hysteresis 9° (roll-off behavior).

Sustainable and long-time ‘rejuvenation’ of biomimetic water-repellent silica coating on polyester fabrics induced by rough mechanical abrasion

The economical use of water-repellent coatings on polymeric materials in commercial and industrial applications is limited by their mechanical wear robustness and long-term durability. In this study, we demonstrate that polyethylene terephthalate (PET) fabric modified with inorganic, methyltrimethoxysilane (MTMS)-based coatings shows excellent resistance against various types of wear damage, thereby mimicking superhydrophobic biological materials. These features were facilitated by the rational design of coating processing that also enabled tunable hierarchical surface structure. A series of custom and standard testing protocols revealed that coating-to-substrate adhesion was remarkably high, as was the resistance to various mechanical abradents. The most intriguing characteristic observed during aging and abrasion cycles was the enhancement in non-wettability or ‘rejuvenation’ reflected by water droplet roll-off behavior, a characteristic of self-cleaning materials. Water-repellent properties of coated polyester were also enhanced by prolonged thermal annealing and were maintained after custom laundry. The developed technology offers opportunities to design low cost, durable and functional textiles for both indoor and outdoor applications.

Shorter Exciton Lifetimes via an External Heavy-Atom Effect: Alleviating the Effects of Bimolecular Processes in Organic Light-Emitting Diodes.

Multiexcited-state phenomena are believed to be the root cause of two exigent challenges in organic light-emitting diodes; namely, efficiency roll-off and degradation. The development of novel strategies to reduce exciton densities under heavy load is therefore highly desirable. Here, it is shown that triplet exciton lifetimes of thermally activated delayed-fluorescence-emitter molecules can be manipulated in the solid state by exploiting intermolecular interactions. The external heavy-atom effect of brominated host molecules leads to increased spin-orbit coupling, which in turn enhances intersystem crossing rates in the guest molecule. Wave function overlap between the host and the guest is confirmed by combined molecular dynamics and density functional theory calculations. Shorter triplet exciton lifetimes are observed, while high photoluminescence quantum yields and essentially unaltered emission spectra are maintained. A change in the intersystem crossing rate ratio due to increased dielectric constants leads to almost 50% lower triplet exciton densities in the emissive layer in the steady state and results in an improved onset of the photoluminescence quantum yield roll-off at high excitation densities. Efficient organic light-emitting diodes with better roll-off behavior based on these novel hosts are fabricated, demonstrating the suitability of this concept for real-world applications.

Thermally activated delayed fluorescence of Bis(9,9-dimethyl-9,10-dihydroacridine) dibenzo[b,d]thiophene 5,5-dioxide derivatives for organic light-emitting diodes

Two luminescent dibenzo[b,d]thiophene 5,5-dioxide derivatives combined with 9,9-dimethyl-9,10-dihydroacridine were synthesized and the appearance of thermally activated delayed fluorescence was confirmed. Both 3,7- and 2,8-bis(9,9-dimethyl-9,10-dihydroacridine)dibenzo[b,d]thiophene 5,5-dioxide (3ASOA) and (4ASOA) showed prompt and delayed fluorescence with high total photoluminescence (PL) quantum efficiencies of 72% and 88% in their doped films, respectively. The maximum external electroluminescence (EL) efficiencies of 13.8% in 4ASOA and 10.4% in 3ASOA were obtained. Further, 4ASOA showed slightly relaxed rolloff behavior, indicating that 2,8-substitution of the donors appreciably improved both PL and EL characteristics.

Effects of underlying InGaN/GaN superlattice structures on the structural and optical properties of InGaN LEDs

This study proposes the use of InGaN/GaN superlattices grown beneath InGaN multiple quantum wells (MQWs) and designed with different well widths to act as an electron emitter layer (EEL). Cross-sectional transmission electron microscopy reveals strong indium segregation in the underlying superlattices with a 5-nm-thick In0.1Ga0.9N well, thus corrupting the crystalline perfection of the resulting LEDs, and also increasing their leakage current. It was also found that the depth of the localized states increases with the well width of the underlying superlattices. In the proposed LEDs, variation in the biaxial strains of the superlattice EELs with different well widths results in an increase in indium incorporation of InGaN MQWs, thus obtaining a redshifted photoluminescence emission with respect to that of normal LED. Furthermore, the presence of relatively strong carrier localization and the alleviation of electron leakage from the InGaN MQWs results in improved light output performance from the proposed LEDs grown with a narrow In0.1Ga0.9N well in the underlying superlattices. Although growth in a wide In0.1Ga0.9N well (~3.5nm) containing underlying superlattices suffers from poor crystalline quality due to partial strain relaxation, it resulted in improved roll-off behavior in terms of light intensity. This may be due to the improved hot electron cooling capacity mitigating the extent of carrier leakage.

Modeling of apparent activation energy and lifetime estimation in NAND flash memory

Misunderstanding apparent activation energy (Eaa) can cause serious error in lifetime predictions. In this paper, the Eaa is investigated for sub 20 nm NAND flash memory. In a high-temperature (HT) regime, the interface trap (Nit) recovery mechanism has the greatest impact on the charge loss. However, the values of Eaa and Ea(Nit) have a wide difference. Also, the lifetime of the device cannot be estimated by the Arrhenius model due to the Eaa roll-off behavior. For the first time, we reveal the origin of abnormal characteristics on Eaa and derive a mathematical formula for Eaa as a function of each Ea(mechanism) in NAND flash memory. Using the proposed Eaa equation, the accurate lifetime for the device is estimated.

Depinning of drops on inclined smooth and topographic surfaces: experimental and lattice Boltzmann model study.

In this study, the dynamics of initially stationary liquid drops on smooth and topographic inclined silicon surfaces was investigated experimentally and by lattice Boltzmann simulations. The transient contact angles and the critical angle of inclination were measured systematically for different liquids, drop sizes, and surfaces having different wettability and surface roughness. In general, the critical angle of inclination is larger for hydrophilic than for hydrophobic surfaces, irrespective of the liquids, and increases with increasing contact angle hysteresis and decreasing drop sizes. A two-phase liquid-vapor lattice Boltzmann model based on the Shan and Chen approach was developed for two dimensions which incorporates the wetting and topographic characteristics of the surface. The simulation results matched the experimentally found features quantitatively and allowed one to explore the roll-off behavior even in cases that can hardly be accessed experimentally.

Engineering electromagnetic responses of bilayered metamaterials based on Fano resonances

We numerically and experimentally demonstrate engineered electromagnetic responses from a single-peak Fano resonance to a fast roll-off behavior by using planar metamaterials, which are constructed by bilayered asymmetrically split rings (ASRs) with twist angles of 0° and 180°. Since each single-layer ASR metamaterial reveals a Fano-type resonance, the dramatic transmission properties are resulted from the hybridization of electromagnetic resonances due to the near-field coupling between two Fano resonances and the far-field retardation effect of the bilayered metamaterials. The surface currents and charges distributions provide an insight into deep understanding of in-phase and out-of-phase coupling of two Fano resonances. The measured and simulated results of bilayered metamaterials agree well to each other. Especially, the proposed metamaterials can be exploited to design metamaterial-based devices in the THz and optical ranges like filters and sensors.

Origins of Effective Work Function Roll-Off Behavior for High-κ Last Replacement Metal Gate Stacks

Origins of effective work function (EWF) roll-off behavior accompanied by equivalent oxide thickness (EOT) scaling for high- κ last replacement metal gate (RMG) stacks are investigated using a low-temperature interfacial layer (IL) scavenging technique. The EWF-EOT roll-off is driven by a high work function metal and the trend is linear and reversible by means of IL scavenging and regrowth reactions. These findings are consistent with the oxygen vacancy model, indicating that the same mechanism that plagued gate-first devices emerges as the IL thickness is scaled <; 4 A (EOT 8 A) for RMG stacks.

Origin of Efficiency Roll-Off in Colloidal Quantum-Dot Light-Emitting Diodes

We study the origin of efficiency roll-off (also called "efficiency droop") in colloidal quantum-dot light-emitting diodes through the comparison of quantum-dot (QD) electroluminescence and photoluminescence. We find that an electric-field-induced decrease in QD luminescence efficiency-and not charge leakage or QD charging (Auger recombination)-is responsible for the roll-off behavior, and use the quantum confined Stark effect to accurately predict the external quantum efficiency roll-off of QD light-emitting diodes.

Impact of high temperature annealing on La diffusion and flatband voltage (Vfb) modulation in TiN/LaOx/HfSiON/SiON/Si gate stacks

We present a detailed analysis of the impact of high temperature annealing on the chemical and electronic properties of TiN/HfSixOyNz/SiOxNy/Si gate stacks, where an ultra-thin LaOx capping layer (0.4–1 nm) is inserted between the TiN metal gate and the HfSixOyNz dielectric. From our experimental results, we demonstrate that La atoms diffuse through the entire nitrided hafnium silicate and reach the SiOxNy interfacial layer to form a La-silicate. In addition, hard x-ray photoelectron spectroscopy analysis highlights the band alignments’ shift of the gate stacks, which is well related to Vfb shifts based on an interfacial dipole and/or fixed charges model. Finally, this study reveals that the Vfb roll-off phenomenon is amplified with an increasing amount of La atoms near the substrate interface. A correlation between LaOx thickness and interface trap density (Dit) is observed, and a mechanism explaining the roll-off behavior is proposed.