Charge Trapping Properties Research Articles

Characterization of the charge trapping properties in p-channel silicon–oxide–nitride–oxide–silicon memory devices including SiO2/Si3N4 interfacial transition layer

The role of SiO2/Si3N4 interfacial transition (IFT) layer in the oxide–nitride–oxide (ONO) tri-layer is quantitatively analyzed for the first time by simulating the temperature and stress-accelerated retention characteristics of p-channel silicon–oxide–nitride–oxide–silicon (SONOS) devices. The ONO tri-layer is modeled as an alloy-dielectric by changing the atomic concentration of silicon, oxygen and nitrogen. It is revealed that simulated results including the IFT layer are more consistent with the experimental data than those neglecting the IFT layer. In addition, the results show that the trapped charge density in IFT layer is two times larger than in the bulk Si3N4 film, due to the oxygen atoms penetrated from SiO2 cause the extrinsic defects in the IFT layer. The energy levels of the trapped charge are continuously distributed, and the peak value is ∼1.6 eV below the conduction band of the ONO tri-layer with a full width at half maximum of 0.45 eV.

Comparative study on the charge-trapping properties of TaAlO and ZrAlO high-k composites with designed band alignment

The charge-trapping memory (CTM) structures Pt/Al2O3/TaAlO/Al2O3/p-Si and Pt/Al2O3/ZrAlO/Al2O3/p-Si were fabricated by using rf-sputtering and atomic layer deposition techniques, in which the potentials at the bottom of the conduction band (PBCB) of high-k composites TaAlO and ZrAlO were specially designed. With a lower PBCB difference between TaAlO and p-Si than that between ZrAlO and p-Si, TaAlO CTM device shows a better charge-trapping performance. A density of trapped charges 2.88 × 1013/cm2 at an applied voltage of ±7 V was obtained for TaAlO CTM device, and it could keep about 60% of initially trapped charges after 10 years. It was suggested that the PBCB difference between high-k composite and p-Si dominates their charge-trapping behaviors.

Surface Engineering of Reduced Graphene Oxide for Controllable Ambipolar Flash Memories

Tunable charge-trapping behaviors including unipolar charge trapping of one type of charge carrier and ambipolar trapping of both electrons and holes in a complementary manner is highly desirable for low power consumption multibit flash memory design. Here, we adopt a strategy of tuning the Fermi level of reduced graphene oxide (rGO) through self-assembled monolayer (SAM) functionalization and form p-type and n-type doped rGO with a wide range of manipulation on work function. The functionalized rGO can act as charge-trapping layer in ambipolar flash memories, and a dramatic transition of charging behavior from unipolar trapping of electrons to ambipolar trapping and eventually to unipolar trapping of holes was achieved. Adjustable hole/electron injection barriers induce controllable Vth shift in the memory transistor after programming operation. Finally, we transfer the ambipolar memory on flexible substrates and study their charge-trapping properties at various bending cycles. The SAM-functionalized rGO can be a promising candidate for next-generation nonvolatile memories.

Metal-HfO2-Ge capacitor: Its enhanced charge trapping properties with S-treated substrate and atomic-layer-deposited HfO2 layer

The charge trapping properties of metal-HfO2-Ge capacitor as a nonvolatile memory have been investigated with (NH4)2S-treated Ge substrate and atomic-layer-deposited HfO2 layer. The interfacial layer generated by (NH4)2S-treated Ge substrate reveals a trace of -S- bonding, very sharp interface edges, and smooth surface morphology. The Ru-HfO2-Ge capacitor with (NH4)2S-treated Ge substrate shows an enhanced interface state with little frequency dispersion, a lower leakage current, and very reliable properties with the enhanced endurance and retention than Ru-HfO2-Ge capacitor with cyclic-cleaned Ge substrate.

Charge transition level of GePb1 centers at interfaces of SiO2/GexSi1−x/SiO2 heterostructures investigated by positron annihilation spectroscopy

In this work, we address the charge trapping properties of Ge dangling bond (DB) defects – GePb1 centers as typified by electron spin resonance spectroscopy (ESR) – found at the interfaces between condensation‐grown Si1−xGex (0.28 < x < 0.8) alloys and insulating SiO2. The ESR observation of singly‐occupied paramagnetic GePb1 centers, carried out at 4.3 K, is complemented by temperature‐dependent positron annihilation spectroscopy (PAS) in the Doppler broadening mode, which enables observation of the neutral‐to‐negative defect transitions as the temperature increases from 50 to 300 K. Through correlation of this re‐charging behavior with the temperature‐induced shift of the Fermi energy, the energy of the Ge DB −/0 transition in the Si0.27Ge0.73 alloy is inferred to be energetically distributed in a ∼0.1 eV interval above the top of the semiconductor valence band. This result refines previous estimates from capacitance–voltage measurements, thus providing independent affirmation that the energy levels of Ge DBs lie inside the band gap of the Si1−xGex alloys.

Influence of lithium fluoride thickness on electrical switching behavior in a cross-point structure using self-assembly molecules

A cross-point structure using an n-type organic semiconductor and a self-assembly molecule (SAM) layer was fabricated. The structure featured electrical switching behavior attributed to the charge trapping at the organic/SAM interface. We found that such an electrical transition was influenced by the thickness of a passivation layer of lithium fluoride (LiF) that led to different electrical behaviors, especially regarding the on/off ratio and stability/reversibility of the transition levels. Investigations revealed that the morphology of LiF layer changed based on its thickness, thereby influencing the subsequent deposition of organic materials and resulting in different charge trapping properties at the organic/SAM interfaces.

Charge Trapping Properties of Au Nanocrystals with Various Sizes for Non-Volatile Memory Applications

Charge trapping properties of Au nanocrystals (NCs) with various sizes and densities embedded in metal oxide semiconductor (MOS) were investigated. Comparing with the Nc size, experimental results show that the NC spacing is more influential in electron trapping. Au NCs with the size of 8~13nm have larger memory window than the Au NCs of 13-17nm and 6-8nm, which can be ascribe to the higher field-enhancement effect caused by the larger NC spacing. Optimized NC size can improve the specific characteristic of memory device effectively.

Low-Dielectric Constant SiCN Charge Trapping Layer for Nonvolatile Memory Applications

We investigated the charge trapping property of SiOx-SiCN-SiO2 stacked films formed on silicon substrates. The stacked films captured charges and the trapped charge density was found to be roughly equal to that in a conventional SiOx-SiNX-SiO2 stacked film. On the other hand, the flat-band voltage shift induced by the trapped charge in the metal-SiOx-SiCN-SiO2-silicon capacitor was greater than that in the metal-SiOx-SiNX-SiO2-silicon capacitor. The larger flat-band voltage shift is attributed to the lower dielectric constant of SiCN film. The programming and erasing characteristics of metal-SiOx-SiCN-SiO2-silicon structures were also investigated.

Higher stabilities of positive and negative charge on tetrafluoroethylene–hexafluoropropylene copolymer (FEP) electrets treated with titanium-tetrachloride vapor

Tetrafluoroethylene–hexafluoropropylene copolymer (FEP) films were treated with titanium-tetrachloride vapor in a molecular-layer deposition process. As a result of the surface treatment, significant improvements of the thermal and temporal charge stability were observed. Charge-decay measurements revealed enhancements of the half-value temperatures and the relaxation times of positively charged FEP electrets by at least 120 °C and two orders of magnitude, respectively. Beyond previous publications on fluoropolymer electrets with surface modification, we here report enhanced charge stabilities of the FEP films charged in negative as well as in positive corona discharges. Even though the improvement for negatively charged FEP films is moderate (half-value temperature about 20 °C higher), our experiments show that the asymmetry in positive and negative charge stability that is typical for FEP electrets can be overcome by means of chemical surface treatments. The results are discussed in the context of the formation of modified surface layers with enhanced charge-trapping properties.

Correlation of charge trapping and charge transport properties of blue triplet emitters with device performances of blue phosphorescent organic light-emitting diodes

The relationship between driving voltage and charge trapping in blue phosphorescent organic light-emitting diodes was investigated using two blue phosphorescent emitters based on fluorine substituted phenylpyridine ligands. It was demonstrated that electron trapping by dopant reduces electron current density in the emitting layer, while hole trapping had little effect on the hole current density in the emitting layer. It was found that the electron trapping by phosphorescent dopant was the dominant factor for the driving voltage of blue phosphorescent organic light emitting diodes.

Shadowed off-axis production of Ge nanoparticles in Ar gas atmosphere by pulsed laser deposition: Morphological, structural and charge trapping properties

In this work, a novel customized shadowed off-axis deposition set-up is used to perform an original study of Ge nanoparticles (NPs) formation in an inert Ar gas atmosphere by pulsed laser deposition at room temperature varying systematically the background Ar gas pressure (Pbase(Ar)), target–substrate distance (d) and laser repetition rate (f). The influence of these parameters on the final NPs size distributions is investigated and a fairly uniform droplets-free and non-agglomerated NPs distribution with average height 〈h〉=2.8±0.6nm is obtained for optimized experimental conditions (Pbase(Ar)=1mbar; d=3cm; f=10Hz) with a fine control in the NPs density (from 3.2×109cm−2 to 1.1×1011cm−2). The crystalline quality of as-deposited NPs investigations demonstrate a strong dependence with the Ar gas pressure and a crystalline to amorphous phase volume fraction χc>50% is found for Pbase(Ar)=2mbar. The NPs functionality for charge trapping applications has been successfully demonstrated by capacitance–voltage (C–V) electrical measurements.

Newly Synthesized Silicon Quantum Dot–Polystyrene Nanocomposite Having Thermally Robust Positive Charge Trapping

Striving to replace the well known silicon nanocrystals embedded in oxides with solution-processable charge-trapping materials has been debated because of large scale and cost effective demands. Herein, a silicon quantum dot-polystyrene (SiQD-PS) nanocomposite (NC) was synthesized by post-functionalization of hydrogen-terminated silicon quantum dots (H-SiQDs) with styrene using a thermally induced surface-initiated polymerization approach. The NC contains two miscible components: PS and SiQD@PS which, respectively, are polystyrene and polystyrene chains-capped SiQDs. Spin-coated films of the nanocomposite on various substrate were thermally annealed at different temperatures and subsequently used to construct metal-insulator-semiconductor (MIS) devices and thin film field-effect transistors (TFTs) having a structure of p-Si++/SiO2/NC/pentacene/Au source-drain. Capacitance-voltage (C-V) curves obtained from the MIS devices exhibit a well-defined counterclockwise hysteresis with negative fat band shifts, which was stable over a wide range of curing temperatures (50-250 °C). The positive charge trapping capability of the NC originates from the spherical potential well structure of the SiQD@PS component while the strong chemical bonding between SiQDs and polystyrene chains accounts for the thermal stability of the charge trapping property. The transfer curve of the transistor was controllably shifted to the negative direction by varying applied gate voltage. Thereby, this newly synthesized and solution processable SiQD-PS nanocomposite is applicable as charge trapping materials for TFT based memory devices.

Charge trapping properties and retention time in amorphous SiGe/SiO2 nanolayers

In this paper, we report on the electrical properties of metal–oxide–semiconductor (MOS) capacitors containing a well-confined 8 nm-thick SiGe amorphous layer (a-SiGe) embedded in a SiO2 matrix grown by RF magnetron sputtering at a low temperature (350 °C). Capacitance–voltage measurements show that the introduction of the SiGe layer leads to a significant enhancement of the charge trapping capabilities, with the memory effect and charge retention time larger for hole carriers. The presented results demonstrate that amorphous floating-gate SiGe layers embedded in SiO2 may constitute a suitable alternative for memory applications.

Influence of oxygen impurity in fluorinating mixtures on charge blocking properties of fluorinated surface layer of polyethylene

Polyethylene samples were surface fluorinated by two kinds of F <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> /N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> mixture, containing 0.04% and 0.2% oxygen by volume, respectively. Infrared analyses do not clearly indicate the presence of oxygen in the surface layer fluorinated by the former, but clearly show that oxygen has been incorporated into the surface layers fluorinated by the latter. Moreover, the amount of fluorine introduced in the fluorinated layer is found to increase with the fluorine partial pressure or fluorination time. Thicknesses of the fluorinated layers are determined by SEM cross section images. Space charge measurements show that the thin fluorinated layer (0.24 μm) without appreciable oxygen can block the charge injection from electrodes very effectively, while the thick fluorinated layer (0.39 μm) with oxygen does not show the effectiveness. In this case, a further thick fluorinated layer is needed to achieve the blocking effect. The permittivity of the fluorinated layers is evaluated by the polar component of surface energy, and the results suggest an increase of the permittivity because of the incorporation of polar groups. Thermally stimulated discharge current measurements reveal different charge trapping properties between the fluorinated layers and a negative influence of oxygen on the decrease in free volume of the surface layer. The decrease in free volume is more important in suppressing the charge injection than the increase in surface layer permittivity and the change in surface trap.

Tunable electrical memory characteristics of brush copolymers bearing electron donor and acceptor moieties

A series of brush copolymers bearing N-phenylcarbazole (PK) and 2-biphenyl-5-(4-ethoxyphenyl)-1,3,4-oxadiazole (BEOXD) moieties in various compositions were studied in detail, in particular their electrical memory characteristics, optical and electrical properties, morphological structures, and interfaces. Nanoscale thin films of the brush copolymers in devices were found to exhibit excellent unipolar electrical memory versatility, which can easily be tuned by tailoring the chemical composition and by changing the film thickness. Moreover, the molecular orbitals and band gap can be tuned by changing the chemical composition. The novel memory characteristics of these copolymers originate primarily from the cooperative roles of the ambipolar PK and BEOXD moieties, which have different charge trapping and stabilization properties. The electrical memory behaviors were found to occur via a favorable hole injection from the electrode and to be governed by trap-limited space-charge limited conduction combined with ohmic conduction and local filament formation. Overall, the brush copolymers are very suitable active materials for the low-cost mass production of high performance, polarity-free digital memory devices that can be operated with very low power consumption, high ON/OFF current ratios, and high stability.

Memory effect in nanostructured Si-rich hafnia films

ABSTRACTMicrostructral and charge-trap properties of single Hf-silicate dielectric films are presented versus annealing treatment. The as-grown films were found to be homogeneous and amorphous. It is shown that annealing treatment results in the formation of alternated Hf-rich and Si-rich layers. The mechanism responsible for this phenomenon is found to be surface directed spinodal decomposition. The increase of annealing temperature up to 1000-1100°C resulted in the crystallization of Hf-rich phase. The stability of its tetragonal phase caused an enhancement of film permittivity was observed. The evolution of charge trapping properties of the films results in the memory effect which nature was discussed.

Effects of Substituent on Binaphthyl Hinge-Containing Conductive Polymers

Conductive polymers containing hinged 1,1′-binaphthyl were synthesized. Their conformational flexibility around the 1,1′ C–C bonds was found to vary with the substituents at the 2,2′-positions—hydroxy, linear alkyloxy, and macrocyclic alkyloxy groups were compared. Macrocyclic alkyloxy groups appeared to immobilize the binaphthyl. The connection patterns of electroactive oligothiophenes to the binaphthyl groups were also investigated with 6,6′- and 7,7′-attachments. The substituents binaphthyl polymers were examined using cyclic voltammetry, in situ conductivity measurements, and spectroelectrochemistry. Their electronic properties were found to vary greatly with the substituents and their connectivity. Binaphthyl polymer with hydroxyls and 3,4-ethylenedioxythiophenes exhibited interesting charge-trapping properties. 7,7′-Substitution led to intrachain interactions, which were promoted by the presence of linear alkyl chains. The observed properties give binaphthyl hinge-containing conductive polymers pote...

Pt and Au/TiO2 photocatalysts for methanol reforming: Role of metal nanoparticles in tuning charge trapping properties and photoefficiency

Metal-loaded TiO2 is, by far, one of the most important class of photocatalysts in hydrogen production through photoreforming of organics and water photosplitting. In this study anatase loaded with Au and Pt nanoparticles (Au/TiO2 and Pt/TiO2) by an impregnation-reduction method was investigated as for morphological, electronic (XPS) and photocatalytic properties in hydrogen production by methanol photoreforming. The electron and hole trapping centers, Ti3+ and O−, respectively, formed under UV–vis irradiation of the photocatalysts, were studied by in situ electron spin resonance (ESR) spectroscopy. The nature of the loaded metal affected both the H2 evolution rate and the distribution of the methanol oxidation products. The better performance of Pt/TiO2 is attributable to the greater ability of Pt with respect to Au to act as electron sink, slowering the recombination of photoproduced electron–hole couples. Direct evidence of this effect was obtained by ESR analysis, showing that the amount of Ti3+ active sites follows the order TiO2>Au/TiO2≫Pt/TiO2, thus confirming easier electron transfer from Ti3+ to Pt, where the H+ reduction to H2 occurs.

BaTiO3 as charge-trapping layer for nonvolatile memory applications

The charge-trapping (CT) properties of BaTiO3 are investigated by using an Al/Al2O3/BaTiO3/SiO2/Si structure. The memory device with BaTiO3 as CT layer shows promising performance in terms of large memory window (8.6V by ±12V for 1s), high program speed with low gate voltage (a VFB shift of 2.9V at +6V, 100μs), negligible VFB shift after 105-cycle program/erase stressing, and good data retention property (charge loss of 7.9% after 104-s 125°C baking time), mainly due to the high charge-trapping efficiency of the BaTiO3 film, as well as the large barrier height between the BaTiO3 charge-trapping layer and the SiO2 tunneling layer.

Improved charge-trapping properties of TiON/HfON dual charge storage layer by tapered band structure

A TiON/HfON dual charge storage layer (CSL) with tapered bandgap structure is proposed for metal-oxide–nitride-oxide–silicon-type memory by using the inter-diffusion of Ti and Hf atoms near the TiON/HfON interface to form an intermixing layer of HfxTiyON with varying Hf/Ti ratio in the dual CSL during post-deposition annealing, as confirmed by transmission electron microscopy. The memory capacitor with TiON/HfON as dual-CSL shows a large memory window of 5.0 V at ±12 V for 100 μs, improved cycling endurance with little degradation after 105 cycles and good data retention with an extrapolated 10-yr window of 4.6 V at room temperature. These are highly associated with the tapered bandgap structure and appropriate trap distribution in the dual CSL. Therefore, the TiON/HfON dual-CSL structure provides a very promising solution for future charge-trapping memory applications.