Significant Charge Trapping Research Articles

Total-Ionizing-Dose Effects and Low-Frequency Noise in N-Type Carbon Nanotube Field-Effect Transistors With HfO₂ Gate Dielectrics

Total ionizing dose (TID) irradiation and low-frequency noise characterization are performed on carbon nanotube field-effect transistors (CNTFETs). Large hysteresis due to high border-trap densities is observed in as-processed and irradiated devices. TID irradiation also leads to significant radiation-induced charge trapping in the surrounding dielectrics resulting in parametric degradation. Percolation-path switching, and border traps contribute to low-frequency noise, with a relatively more prominent role for border traps after irradiation than before irradiation. These results show that process improvements are required to optimize both the initial operating characteristics and the radiation response of CNTFETs.

Evidence of cluster dipole states in germanium detectors operating at temperatures below 10 K

By studying charge trapping in germanium detectors operating at temperatures below 10 K, we demonstrate for the first time that the formation of cluster dipole states from residual impurities is responsible for charge trapping. Two planar detectors with different impurity levels and types are used in this study. When drifting the localized charge carriers created by α particles from the top surface across a detector at a lower bias voltage, significant charge trapping is observed when compared to operating at a higher bias voltage. The amount of charge trapping shows a strong dependence on the type of charge carriers. Electrons are trapped more than holes in a p-type detector, while holes are trapped more than electrons in an n-type detector. When both electrons and holes are drifted simultaneously using the widespread charge carriers created by γ rays inside the detector, the amount of charge trapping shows no dependence on the polarity of bias voltage.

Unraveling the Dynamics of Charge Trapping and De-Trapping in Ferroelectric FETs

In this work, a comprehensive study of charge trapping and de-trapping dynamics is performed on n-channel ferroelectric field-effect transistors (nFeFETs) and pFeFETs. It is discovered that: 1) the degree of charge trapping depends on the substrate that nFeFETs exhibit significant electron trapping but negligible hole trapping during memory write while pFeFETs exhibit much less electron trapping but significant hole trapping when heavily stressed; 2) due to enhanced electric field in the interlayer and semiconductor, the like initial polarization states (i.e., initialized by a pulse of the same polarity as the write pulse) could exacerbate charge trapping induced by the write pulse; 3) electron trapping is fully recoverable while hole trapping shows a semi-permanent component which involves interface trap generation; and 4) less significant charge trapping in pFeFETs allows immediate read-after-write at normal operating conditions.

Instability in In0.7Ga0.3As Quantum-Well MOSFETs with Single-Layer Al2O3 and Bi-Layer Al2O3/HfO2 Gate Stacks Caused by Charge Trapping under Positive Bias Temperature (PBT) Stress

The instability of transistor characteristics caused by charge trapping under positive bias temperature (PBT) stress in In0.7Ga0.3As metal oxide semiconductor field-effect transistors (MOSFETs) with single-layer Al2O3 and bi-layer Al2O3/HfO2 gate stacks was investigated. The equivalent field across the multi-gate stacks was compared with a single layer used to compare the instability of electrical characteristics. The observed threshold voltage degradation (ΔVT) was consistent with the phenomenon of fast transient trapping of injected electrons at pre-existing shallow defects in the high-κ dielectric of HfO2, in which this charging was recovered by applying a relaxation voltage. Excluding the fast-transient charging components, the power law exponent (n), with respect to the time-dependent threshold voltage degradation, ranged from 0.3 to 0.35 for both single-layer Al2O3 and bi-layer Al2O3/HfO2 gate stacks. This long-term (slow) VT shift, which was strongly correlated with transconductance (Gm) degradation, was attributed to significant charge trapping in the border trap or/and defect sites within the high-κ dielectric.

Cu ion-induced photoluminescence quenching of methylammonium lead bromide nanocrystals

Organic-inorganic hybrid perovskites have emerged as a promising material with excellent optoelectronic properties for applications in light harvesting and emitting devices. The doping of the bulk organohalide perovskites with homovalent or heterovalent metal cations has been performed to modify their energy band structure and the relevant optoelectronic properties. Here, we report the methylammonium lead bromide (MAPbBr3) perovskite nanocrystals doped with organometallic compounds and metal halides, which were synthesized via a ligand-assisted ball-milling. In particular, the PL intensity of Cu-doped MAPbBr3 nanocrystals was strongly quenched by three order of magnitude due to the significant charge trapping near the uncoordinated metallic Cu species. Furthermore, sensitive PL response of MAPbBr3 nanocrystals to the metal cations suggests a simple and effective strategy for colorimetric and fluorescence sensing of the heavy metal detection in the organic solution phase.

Understanding charge transport in Ir(ppy)3:CBP OLED films.

Ir(ppy)3:CBP blends have been widely studied as the emissive layer in organic light emitting diodes (OLEDs), yet crucial questions about charge transport within the layer remain unaddressed. Recent molecular dynamics simulations show that the Ir(ppy)3 molecules are not isolated from each other, but at concentrations of as low as 5 wt. % can be part of connected pathways. Such connectivity raises the question of how the iridium(iii) complexes contribute to long-range charge transport in the blend. We implement a kinetic Monte Carlo transport model to probe the guest concentration dependence of charge mobility and show that distinct minima appear at approximately 10 wt. % Ir(ppy)3 due to an increased number of trap states that can include interconnected complexes within the blend film. The depth of the minima is shown to be dependent on the electric field and to vary between electrons and holes due to their different trapping depths arising from the different ionization potentials and electron affinities of the guest and host molecules. Typical guest-host OLEDs use a guest concentration below 10 wt. % to avoid triplet-triplet annihilation, so these results suggest that optimal device performance is achieved when there is significant charge trapping on the iridium(iii) complex guest molecules and minimum interactions of the emissive chromophores that can lead to triplet-triplet annihilation.

Improved charge trapping properties by embedded graphene oxide quantum-dots for flash memory application

In this work, we have investigated two kinds of charge trapping memory devices with Pd/Al2O3/ZnO/SiO2/p-Si and Pd/Al2O3/ZnO/graphene oxide quantum-dots (GOQDs)/ZnO/SiO2/p-Si structure. Compared with the single ZnO sample, the memory window of the ZnO–GOQDs–ZnO sample reaches a larger value (more than doubled) of 2.7 V under the sweeping gate voltage ± 7 V, indicating a better charge storage capability and the significant charge trapping effects by embedding the GOQDs trapping layer. The ZnO–GOQDs–ZnO devices have better date retention properties with the high and low capacitances loss of ∼ 1.1 and ∼ 6.9%, respectively, as well as planar density of the trapped charges of ~ 1.48 × 1012 cm− 2. It is proposed that the GOQDs play an important role in the outstanding memory characteristics due to the deep quantum potential wells and the discrete distribution of the GOQDs. The long date retention time might have resulted from the high potential barrier which suppressed both the back tunneling and the leakage current. Intercalating GOQDs in the memory device is a promising method to realize large memory window, low-power consumption and excellent retention properties.

Analysis of Conduction and Charging Mechanisms in Atomic Layer Deposited Multilayered HfO2/Al2O3 Stacks for Use in Charge Trapping Flash Memories

Method for characterization of electrical and trapping properties of multilayered high permittivity stacks for use in charge trapping flash memories is proposed. Application of the method to the case of multilayered HfO2/Al2O3 stacks is presented. By applying our previously developed comprehensive model for MOS structures containing high-κ dielectrics on the J-V characteristics measured in the voltage range without marked degradation and charge trapping (from −3 V to +3 V), several parameters of the structure connected to the interfacial layer and the conduction mechanisms have been extracted. We found that the above analysis gives precise information on the main characteristics and the quality of the injection layer. C-V characteristics of stressed (with write and erase pulses) structures recorded in a limited range of voltages between −1 V and +1 V (where neither significant charge trapping nor visible degradation of the structures is expected to occur) were used in order to provide measures of the effect of stresses with no influence of the measurement process. Both trapped charge and the distribution of interface states have been determined using modified Terman method for fresh structures and for structures stressed with write and erase cycles. The proposed method allows determination of charge trapping and interface state with high resolution, promising a precise characterization of multilayered high permittivity stacks for use in charge trapping flash memories.

Microstructure and charge trapping in ZrO2- and Si3N4-based superlattice layer systems with Ge nanoparticles

Ge was deposited on silicon as a superlattice with 10 layers of Ge embedded in Si3N4 or ZrO2 matrices via plasma enhanced chemical vapor deposition or RF-sputtering, respectively. Raman spectroscopy, transmission electron microscopy and capacitance-voltage (CV) measurements were performed in order to investigate the structural and electrical properties of the superlattices. It will be shown that, in contrast to furnace annealing, flash lamp annealing of Ge-ZrO2-superlattices leads to crystalline Ge nanoparticles in an amorphous matrix. As revealed by CV measurements, these layers show excellent charge storage capabilities. In comparison, a higher thermal budget is needed to crystallize Ge in case of Si3N4-based superlattices, and no significant charge trapping could be detected during CV measurements.

Naphtho[2,3-c][1,2,5]thiadiazole and 2H-Naphtho[2,3-d][1,2,3]triazole-Containing D-A-π-A Conjugated Organic Dyes for Dye-Sensitized Solar Cells.

Dipolar dyes comprising an arylamine as the electron donor, a cyanoacrylic acid as electron acceptor, and an electron deficient naphtho[2,3-c][1,2,5]thiadiazole (NTD) or naphtho[2,3-d][1,2,3]triazole (NTz) entity in the conjugated spacer, were developed and used as the sensitizers in dye-sensitized solar cells (DSSCs). The introduction of the NTD unit into the molecular frame distinctly narrows the HOMO/LUMO gap with electronic absorption extending to >650 nm. However, significant charge trapping and dye aggregation were found in these dyes. Under standard global AM 1.5 G illumination, the best cell photovoltaic performance achieved 6.37 and 7.53% (∼94% relative to N719-based standard cell) without and with chenodeoxycholic acid (CDCA) coadsorbent, respectively. Without CDCA, the NTz dyes have higher power conversion efficiency (7.23%) than NTD dyes due to less charge trapping, dye aggregation, and better dark current suppression.

Low temperature deposition of silicon nitride using Si3Cl8

We demonstrate the deposition of SixNy thin films using octachlorotrisilane (Si3Cl8) and ammonia between 300 and 500°C by an atomic layer deposition-like growth mechanism. Although there are chemical vapor deposition growth mechanisms present, which lead to an incomplete saturation, a step coverage >80% in high aspect ratio (>60:1) trenches could be achieved. The deposited films oxidize after contact with ambient air leading to substoichiometric N/Si ratios. Increasing the deposition temperature diminishes this oxidation. At temperatures >390°C a bulk N/Si ratio of ~1.3 is achieved. The capacitance–voltage (C–V) measurements of these films yield a k value of ~6 and a strong C–V hysteresis indicates significant charge trapping.

Defect states and charge trapping characteristics of HfO2 films for high performance nonvolatile memory applications

In this work, we present significant charge trapping memory effects of the metal-hafnium oxide-SiO2-Si (MHOS) structure. The devices based on 800 °C annealed HfO2 film exhibit a large memory window of ∼5.1 V under ±10 V sweeping voltages and excellent charge retention properties with only small charge loss of ∼2.6% after more than 104 s retention. The outstanding memory characteristics are attributed to the high density of deep defect states in HfO2 films. We investigated the defect states in the HfO2 films by photoluminescence and photoluminescence excitation measurements and found that the defect states distributed in deep energy levels ranging from 1.1 eV to 2.9 eV below the conduction band. Our work provides further insights for the charge trapping mechanisms of the HfO2 based MHOS devices.

Measurement of the electrical properties of a polycrystalline cadmium telluride for direct conversion flat panel x-ray detector

Cadmium telluride (CdTe) is one of the best candidate direct conversion material for medical X-ray application because it satisfies the requirements of direct conversion x-ray material such as high atomic absorption, density, bandgap energy, work fuction, and resistivity. With such properties, single crystal CdTe exhibits high quantum efficiency and charge collection efficiency. However, for the development of low-cost large area detector, the study of the improvement of polycrystalline CdTe property is desirable. In this study, in order to improve the properties of polycrystalline CdTe, we produced polycrystalline CdTe with different kinds of raw materials, high purity Cd and Te powder compounds and bulk CdTe compound synthesized from single crystal CdTe. The electric properties including resistivity, x-ray sensitivity, and charge transport properties were investigated. As a result, polycrystalline CdTe exhibited simular level of resistivity and x-ray sensitivity to single crystal CdTe. The carrier transport properties of polycrystalline CdTe showed poorer properties than those of single crystal CdTe due to significant charge trapping. However, the polycrystalline CdTe fabricated with bulk CdTe compound synthesized from single crystal CdTe showed better charge transport properties than the polycrystalline CdTe fabricated with CdTe powder compounds. This is suitable for diagnostic x-ray detectors, especially for digital fluoroscopy.

Optical spectroscopy of photovoltaic systems based on low-bandgap polymers

We report on the optical characterization of a low-bandgap thieno[3,4-b]pyrazine phenylenevinylene copolymer. Individual layers on quartz and Si substrates as well as layered structures on indium tin oxide covered glass with and without a PEDOT:PSS (poly(3,4-ethylenedioxythiophene)–poly(styrenesulfonate)) layer were examined using spectroscopic ellipsometry, transmission, reflectance, electroreflectance and external quantum efficiency measurements. For comparison, current–voltage and capacitance–voltage characteristics were also measured. Changes in optical spectra of the pristine polymer due to an applied external voltage are analyzed quantitatively in terms of the Stark effect which allows the determination of the exciton radius, binding energy and ionization field strength. For structures with an inserted PEDOT:PSS layer, a significant charge trapping in the polymer layer is observed that considerably modifies both optical and electrical properties.

Thin film properties of triphenylamine-cored dendrimers: A molecular approach to control aggregation

The solid-state photophysical and charge transport properties of two first-generation dendrimers are presented. The dendrimers are comprised of a triphenylamine core, dendrons containing a phenyl branching unit with thiophene (Dendrimer 1) or bithiophene (Dendrimer 2) moieties, and dodecyl surface groups. For Dendrimer 1, the excited state is located within the center of the dendrimer giving rise to a moderate solid-state photoluminescence quantum yield (Φpl) (0.13) and significant charge trapping, with both observations due to the degree of overlap of the main electroactive chromophores on adjacent dendrimers. For Dendrimer 2, the excited state is located within the dendron and in the solid-state this leads to a strongly red-shifted and weakened emission (Φpl~0.02) due to strong intermolecular chromophore interactions. For films of Dendrimer 2 the charge mobility was higher than Dendrimer 1 but was still limited by a low density of strongly interacting electroactive chromophores. The pronounced difference between the solid-state properties of the two dendrimers is simply engineered by the addition of an extra thiophene in each of the dendrons.

Investigation of Hole-Blocking Contacts for High-Conversion-Gain Amorphous Selenium Detectors for X-Ray Imaging

In this paper, we investigated different organic and inorganic hole-blocking contacts for amorphous selenium (a-Se)-based photodetectors: $\hbox{CeO}_{2}$ , $\hbox{TiO}_{2}$ , perylene tetracarboxylic bisbenzimidazole (PTCBI), and polyimide (PI). $\hbox{CeO}_{2}$ has previously been used as a blocking layer for high-gain a-Se devices. $\hbox{TiO}_{2}$ has similar properties to $\hbox{CeO}_{2}$ . PTCBI has a higher ionization potential compared to a-Se and has a very low hole mobility. PI is a common insulator in the semiconductor industry. It was found that an 800-nm PI layer reduces the dark current by more than two orders of magnitude in comparison with 30 nm of $\hbox{CeO}_{2}$ , 20 nm of $\hbox{TiO}_{2}$ , and 50 nm of PTCBI. No significant charge trapping was found in the devices consisting of an 800-nm PI layer. Unlike previously reported inorganic hole-blocking contact technology, PI layers further benefit from a simple spin coating fabrication step before evaporation of a-Se. Photodetector samples incorporating the PI layer are tested at high electric fields, and gains reaching 4.4 were observed at an electric field $>\hbox{80} \hbox{V}/\mu \hbox{m}$ . We conclude that using a PI layer is a promising step in the development of high-conversion-gain detectors for emerging applications in large-area medical diagnostic imaging, crystallography, and nondestructive test.

Field dependent electrical conduction in HfO2/SiO2 gate stack for before and after constant voltage stressing

The electrical conduction mechanisms contributing to the leakage current at different field regions and different temperatures have been studied in this work. The current-voltage (I-V) measurement of TiN/HfO2/SiO2/P-Si nMOS capacitor in the temperature range from 25 °C to 125 °C, taken before stressing and in the temperature range of 25 °C to 65 °C after constant voltage stressing (CVS) at 3 V suggests that the Poole-Frenkel mechanism is the dominant conduction mechanism in the high field region. It was also observed that in the low electric field region Ohmic conduction is the dominant mechanism. Trap energy level (φt) of 0.36 eV, obtained from the Poole-Frenkel mechanism indicates that the defect is oxygen-related and is a good match with the reported value for V−/V−− in HfO2. Significant charge trapping at low level stress was observed whereas at high level and elevated temperature stressing suggests a variation of trap energy level indicating new defect formation. It is observed that the stress induced gate leakage current for the high temperature stressed devices is about three orders of magnitude more than that of room temperature stressed devices for the whole field range.

Trapping effect of metal nanoparticle mono- and multilayer in the organic field-effect transistor

The effect of silver nanoparticles self-assembled monolayer (Ag NPs SAM) on charge transport in pentacene organic field-effect transistors (OFET) was investigated by both steady-state and transient-state methods, which are current-voltage measurements in steady-state and time-resolved microscopic (TRM) second harmonic generation (SHG) in transient-state, respectively. The analysis of electronic properties revealed that OFET with SAM exhibited significant charge trapping effect due to the space-charge field formed by immobile charges. Lower transient-state mobility was verified by the direct probing of carrier motion by TRM-SHG technique. It was shown that the trapping effect rises together with increase of SAM layers suggesting the presence of traps in the bulk of NP films. The model based on the electrostatic charge barrier is suggested to explain the phenomenon.

An organic thin film transistor based non-volatile memory with zinc oxide nanoparticles

The aim of this paper is to establish a fabrication process for non-volatile memory (NVM) transistors using ZnO nanoparticles, polymethylsilsesquioxane (PMSSQ) and soluble pentacene. ZnO nanoparticles mixed into the PMSSQ solution are used to create a nanocomposite layer for charge trapping in the NVM. It has been demonstrated that the nanocomposite layer in a metal–insulator–semiconductor structure can cause a hysteresis of ~ 6 V in a capacitance–voltage (C–V) plot, indicating a significant charge trapping capability. A threshold voltage shift of ~ 2.3 V between a programmed and erased NVM transistor and a carrier mobility of ~ 0.002 cm 2/V-s are achieved. At present the fabricated NVMs have a limited life cycle of 4 program/erase cycles.

Effect of carrier injection process in the organic field-effect transistor by introducing metal nanoparticle monolayer

Silver nanoparticles (NPs) self-assembled monolayer (SAM) was deposited on pentacene organic field-effect transistors (OFET) via modified Langmuir-Schaffer (horizontal lifting) technique and its effect was being investigated by steady-state method which is current-voltage measurements. Here we focus on its effect to the carrier injection. The contact resistance, drain-source current, trapped charges and threshold voltage analysis revealed that OFET with SAM exhibited significant charge trapping effect due to the space charge field formed by immobile/ trapped charges and further influenced the carrier injection.