Charge injection in thin dielectric layers by atomic force microscopy: influence of geometry and material work function of the AFM tip on the injection process

Optimizing ZnO-Quantum Dot Interface with Thiol as Ligand Modification for High-Performance Quantum Dot Light-Emitting Diodes

As the electron transport layer in quantum dot light-emitting diodes (QLEDs), ZnO suffers from excessive electrons that lead to luminescence quenching of the quantum dots (QDs) and charge-imbalance in QLEDs. Therefore, the interplay between ZnO and QDs requires an in-depth understanding. In this study, DFT and COSMOSL simulations are employed to investigate the effect of sulfur atoms on ZnO. Based on the simulations, thiol ligands (specifically 2-hydroxy-1-ethanethiol) to modify the ZnO nanocrystals are adopted. This modification alleviates the excess electrons without causing any additional issues in the charge injection in QLEDs. This modification strategy proves to be effective in improving the performance of red-emitting QLEDs, achieving an external quantum efficiency of over 23% and a remarkably long lifetime T95 of >12 000 h at 1000 cd m-2 . Importantly, the relationship between ZnO layers with different electronic properties and their effect on the adjacent QDs through a single QD measurement is investigated. These findings show that the ZnO surface defects and electronic properties can significantly impact the device performance, highlighting the importance of optimizing the ZnO-QD interface, and showcasing a promising ligand strategy for the development of highly efficient QLEDs.

SiC Doping Impact during Conducting AFM under Ambient Atmosphere

The characterization of silicon carbide (SiC) by specific electrical atomic force microscopy (AFM) modes is highly appreciated for revealing its structure and properties at a nanoscale. However, during the conductive AFM (C-AFM) measurements, the strong electric field that builds up around and below the AFM conductive tip in ambient atmosphere may lead to a direct anodic oxidation of the SiC surface due to the formation of a water nanomeniscus. In this paper, the underlying effects of the anodization are experimentally investigated for SiC multilayers with different doping levels by studying gradual SiC epitaxial-doped layers with nitrogen (N) from 5 × 1017 to 1019 at/cm3. The presence of the water nanomeniscus is probed by the AFM and analyzed with the force-distance curve when a negative bias is applied to the AFM tip. From the water meniscus breakup distance measured without and with polarization, the water meniscus volume is increased by a factor of three under polarization. AFM experimental results are supported by electrostatic modeling to study oxide growth. By taking into account the presence of the water nanomeniscus, the surface oxide layer and the SiC doping level, a 2D-axisymmetric finite element model is developed to calculate the electric field distribution nearby the tip contact and the current distributions at the nanocontact. The results demonstrate that the anodization occurred for the conductive regime in which the current depends strongly to the doping; its threshold value is 7 × 1018 at/cm3 for anodization. Finally, the characterization of a classical planar SiC-MOSFET by C-AFM is examined. Results reveal the local oxidation mechanism of the SiC material at the surface of the MOSFET structure. AFM topographies after successive C-AFM measurements show that the local oxide created by anodization is located on both sides of the MOS channel; these areas are the locations of the highly n-type-doped zones. A selective wet chemical etching confirms that the oxide induced by local anodic oxidation is a SiOCH layer.

Stability of Charge Distributions in Electret Films on the nm-Scale

Electret materials find use in various applications, such as microphones or filter media. In recent years, electrets have been used also increasingly on the micrometer scale, for example, in MEMS or for nano-xerography. However, for these applications, it becomes more important to prepare defined charge structures with sub-micrometer features. On the macroscopic level, the technique of isothermal potential decay at elevated temperatures has been developed to study aging effects and charge retention capabilities in electret materials. Here, we extend this technique to the nm-level by means of AFM-based methods, such as contact charging by AFM and the Kelvin probe force microscopy. Defined charge distributions in polyetherimide (PEI) ULTEM 1000 thin-film electrets have been studied for the first time with a high lateral resolution on the nanometer scale. We found a linear correlation between externally applied contact charging potential on the AFM-tip and the resulting relative surface potential on the PEI film. Charge decay at elevated temperatures is independent from the length scale. The same time dependence as for macroscopic, homogenously charged films could be established. We observe a potential decay only at an elevated temperature of 120 °C and no significant lateral charge transport. Thus, we propose a thermally enhanced charge carrier release from surface traps and a subsequent charge migration to the back electrode as the dominant mechanism. This finding is in-line with the observation that potential decay can be reduced also on the nm-level by pre-annealing the film slightly below the glass transition temperature. In contrast to many polymeric or inorganic electrets, no lateral charge migration is observed. Therefore, the charge patterns are preserved for PEI ULTEM 1000 thin-film electrets, which makes it a good candidate as electret for applications in MEMS or similar applications.

Analysis of the charging kinetics in silver nanoparticles-silica nanocomposite dielectrics at different temperatures

Dielectric nanocomposite materials are now involved in a large panel of electrical engineering applications ranging from micro-/nano-electronics to power devices. The performances of all these systems are critically dependent on the evolution of the electrical properties of the dielectric parts, especially under temperature increase. In this study we investigate the impact of a single plane of silver nanoparticles (AgNPs), embedded near the surface of a thin silica (SiO2) layer, on the electric field distribution, the charge injection and the charge dynamic processes for different AgNPs-based nanocomposites and various temperatures in the range 25°C–110°C. The electrical charges are injected locally by using an Atomic Force Microscopy (AFM) tip and the related surface potential profile is probed by Kelvin Probe Force Microscopy (KPFM). To get deeper in the understanding of the physical phenomena, the electric field distribution in the AgNPs-based nanocomposites is computed by using a Finite Element Modeling (FEM). The results show a strong electrostatic coupling between the AFM tip and the AgNPs, as well as between the AgNPs when the AgNPs-plane is embedded in the vicinity of the SiO2-layer surface. At low temperature (25°C) the presence of an AgNPs-plane close to the surface, i.e., at a distance of 7 nm, limits the amount of injected charges. Besides, the AgNPs retain the injected charges and prevent from charge lateral spreading after injection. When the temperature is relatively high (110°C) the amount of injected charges is increased in the nanocomposites compared to low temperatures. Moreover, the speed of lateral charge spreading is increased for the AgNPs-based nanocomposites. All these findings imply that the lateral charge transport in the nanocomposite structures is favored by the closely situated AgNPs because of the strong electrostatic coupling between them, additionally activated by the temperature increase.

Thickness-Dependent Domain Relaxation Dynamics Study in Epitaxial K0.5Na0.5NbO3 Ferroelectric Thin Films

We explored the time dependence of the nanoscale domain relaxation mechanism in epitaxial K_0.5Na_0.5NbO₃ (KNN) thin films grown on La_0.67Sr_0.33MnO₃/SrTiO₃ (001) substrates over the thickness range 20-80 nm using scanning probe microscopy. Kelvin probe force microscopy (KFM) and piezoresponse force microscopy were performed on pulsed-laser-deposition-deposited KNN thin films for studying the time evolution of trapped charges and polarized domains, respectively. The KFM data show that the magnitude and retention time of the surface potential are the maxima for 80 nm-thick film and reduce with the reduction in the film thickness. The charging and discharging of the samples reveal the easier and stronger electron trapping compared to hole trapping. This result further indicates the asymmetry between retention of the pulse-voltage-induced upward and downward domains. Furthermore, the time evolution of these ferroelectric nanodomains are found to obey stretched exponential behavior. The relaxation time (T) has been found to increase with increase in thickness; however, the corresponding stretched exponent (β) is reduced. Moreover, the written domain can retain for more than 2300 min in KNN thin films. An in-depth understanding of domain relaxation dynamics in Pb-free KNN thin films can bridge a path for future high-density memory applications.

Temperature Influence on PI/Si3N4 Nanocomposite Dielectric Properties: A Multiscale Approach

The interphase area appears to have a great impact on nanocomposite (NC) dielectric properties. However, the underlying mechanisms are still poorly understood, mainly because the interphase properties remain unknown. This is even more true if the temperature increases. In this study, a multiscale characterization of polyimide/silicon nitride (PI/Si₃N₄) NC dielectric properties is performed at various temperatures. Using a nanomechanical characterization approach, the interphase width was estimated to be 30 ± 2 nm and 42 ± 3 nm for untreated and silane-treated nanoparticles, respectively. At room temperature, the interphase dielectric permittivity is lower than that of the matrix. It increases with the temperature, and at 150 °C, the interphase and matrix permittivities reach the same value. At the macroscale, an improvement of the dielectric breakdown is observed at high temperature (by a factor of 2 at 300 °C) for NC compared to neat PI. The comparison between nano- and macro-scale measurements leads to the understanding of a strong correlation between interphase properties and NC ones. Indeed, the NC macroscopic dielectric permittivity is well reproduced from nanoscale permittivity results using mixing laws. Finally, a strong correlation between the interphase dielectric permittivity and NC breakdown strength is observed.

Influence of dielectric layer thickness on charge injection, accumulation and transport phenomena in thin silicon oxynitride layers: a nanoscale study

Charge injection and retention in thin dielectric layers remain critical issues due to the great number of failure mechanisms they inflict. Achieving a better understanding and control of charge injection, trapping and transport phenomena in thin dielectric films is of high priority aiming at increasing lifetime and improving reliability of dielectric parts in electronic and electrical devices. Thermal silica is an excellent dielectric but for many of the current technological developments more flexible processes are required for synthesizing high quality dielectric materials such as amorphous silicon oxynitride layers using plasma methods. In this article, the studied dielectric layers are plasma deposited SiO _x N _y . Independently on the layer thickness, they are structurally identical: optically transparent, having the same refractive index, equal to the one of thermal silica. Influence of the dielectric film thickness on charging phenomena in such layers is investigated at nanoscale using Kelvin probe force microscopy (KPFM) and conductive atomic force microscopy. The main effect of the dielectric film thickness variation concerns the charge flow in the layer during the charge injection step. According to the SiO _x N _y layer thickness two distinct trends of the measured surface potential and current are found, thus defining ultrathin (up to 15 nm thickness) and thin (15-150 nm thickness) layers. Nevertheless, analyses of KPFM surface potential measurements associated with results from finite element modeling of the structures show that the dielectric layer thickness has weak influence on the amount of injected charge and on the decay dynamics, meaning that pretty homogeneous layers can be processed. The charge penetration depth in such dielectric layers is evaluated to 10 nm regardless the dielectric thickness.

Tuning Oxygen Vacancy Diffusion through Strain in SrTiO3 Thin Films

Understanding diffusion of oxygen vacancies in oxides under different external stimuli is crucial for the design of ion-based electronic devices, improvement of catalytic performance, and so forth. In this manuscript, using an external electric field produced by an atomic force microscopy tip, we obtain the room-temperature diffusion coefficient of oxygen-vacancies in thin films of SrTiO₃ under compressive/tensile epitaxial strain. Tensile strain produces a substantial increase of the diffusion coefficient, facilitating the mobility of vacancies through the film. Additionally, the effect of tip bias, pulse time, and temperature on the local concentration of vacancies is investigated. These are important parameters of control in the production and stabilization of nonvolatile states in ion-based devices. Our findings show the key role played by strain for the control of oxygen vacancy migration in thin-film oxides.