Effect Of Thickness Scaling Research Articles

Ferroelectric Aluminum Scandium Nitride Transistors with Intrinsic Switching Characteristics and Artificial Synaptic Functions for Neuromorphic Computing.

Aluminum Scandium Nitride (Al1-xScxN) has received attention for its exceptional ferroelectric properties, whereas the fundamental mechanism determining its dynamic response and reliability remains elusive. In this work, an unreported nucleation-based polarization switching mechanism in Al0.7Sc0.3N (AlScN) is unveiled, driven by its intrinsic ferroelectricity rooted in the ionic displacement. Fast polarization switching, characterized by a remarkably low characteristic time of 0.00183ps, is captured, and effectively simulated using a nucleation-limited switching (NLS) model, where the profound effect of defects on the nucleation and domain propagation is systematically studied. These findings are further integrated into Monte Carlo simulations to unravel the influence of the activation energy for ferroelectric switching on the distributions of switching thresholds. The long-term reliability of devices is also confirmed by time-dependent dielectric breakdown (TDDB) measurements, and the effect of thickness scaling is discussed. Ferroelectric field-effect transistors (FeFETs) are demonstrated through the integration of AlScN and 2D MoS2 channel, where biological synaptic functions can be emulated with optimized operation voltage. The artificial neural network built from AlScN-based FeFETs achieves 93.8% recognition accuracy of handwritten digits, demonstrating the potential of ferroelectric AlScN in future neuromorphic computing applications.

Thickness-dependent topological domain textures of layered ferroelectric Bi2WO6 thin films

Topological polar structures are attracting attention as potential applications of next-generation high-density memories. We investigate how the ferroelectric domain evolves with film thickness in c-axis oriented epitaxial Bi2WO6 thin films grown on (LaAlO3)0.3(Sr2AlTaO6)0.7 substrates. In addition to the general thickness scaling effect on domain size, we find that 3- or 4-variant domains appear in a sample thicker than 120 nm, whereas ⟨100⟩-type domains compete with the ⟨110⟩ ones in thinner samples, resulting in a variety of nontrivial topological structures. By visualizing the spatial distribution of electric polarizations through angle-resolved piezoresponse force microscopy, we provide direct evidence for the spontaneous emergence of nontrivial topological polar structures. These results provide useful insights into the domain behavior of layered ferroelectric thin films.

Effect of thickness scaling on the switching dynamics of ferroelectric HfO2–ZrO2 capacitors

HfO2-based ferroelectric (FE) films have attracted immense attention in the research of ferroelectric random access memory (FeRAM) and ferroelectric field-effect transistors (FeFETs) due to their excellent ferroelectric performance and high compatibility with CMOS integration. The study of switching dynamics is essential in understanding and optimizing HfO2-based FE films for nonvolatile memory applications. To explore the effect of thickness scaling on the polarization switching speed of the HfO2-ZrO2 (HZO) FE film, we demonstrate TaN/HZO/TaN capacitors fabricated with 6, 10, and 20 nm-thick HZO films. The inhomogeneous field mechanism (IFM) model was used to characterize the polarization switching of polycrystalline HZO films. The results showed that the characteristic time constants for the 6, 10, and 20-nm-thick HZO samples were 283, 43.7, and 20.5 ps, respectively, which is inconsistent with the variation in coercive electric field (EC) with sample thickness. These results demonstrate the distribution of the local fields in capacitors with various HZO thicknesses and contribute to the design and optimization of HZO films for advanced nanonode applications.

Influence of active layer thickness on the cut-off frequency of a-IGZO thin film transistors

Design optimizations like channel length and gate/contact overlap length scaling are generally used for cut-off frequency (fT ) enhancement of thin film transistors (TFT). But channel length scaling leads to mobility degradation due to contact resistance (RC ). RC in TFTs is directly proportional to the transfer length (LT ), and is inversely proportional to the overlap length (LOV ). LT depends upon the metal/semiconductor interfacial resistivity, the vertical bulk resistance and the semiconductor sheet resistance. For the same interfacial contact properties, active layer thickness (Ta ) scaling can be effectively utilized for lowering the vertical bulk resistance thereby lowering LT and enhancing fT . To study the effect of thickness scaling on the bandwidth of TFT based amplifiers, a-IGZO thin film transistor of effective channel thickness of 70nm are first fabricated and characterized. Density of states model of this TFT is extracted and numerical simulations are used to study the influence of active layer thickness scaling. By varying the channel lengths from 10μm to 100μm, transfer line method of contact resistance estimation, is used to extract LT and RC of devices. As the active layer thickness is varied from 70μm to 30μm it is found that LT decreases from 1μm to 60 nm, due to lower vertical bulk resistance. This results in a lower RC and higher μeff, facilitating further scaling of LOV , leading to enhanced fT of the device.

Understanding Thickness-Dependent Transport Kinetics in Nanosheet-Based Battery Electrodes

There is a growing need for thicker electrode designs to achieve high energy/power for ever-increasing power needs by electronic devices and electric automobiles. Though great efforts, such as structure optimization, have been devoted on fabricating thick electrodes, understanding of performance-limiting factors essential to electrode architecture design, has not been well established. In this study, the dependence of electrochemical behavior on electrode mass loading is comprehensively investigated in nanosheet-based electrodes. In particular, the effects of electrical conductivity and porosity are illustrated. In drop-casted electrodes, where nanosheets are highly stacked, ionic diffusion in the electrolyte has been determined to be the controlling step in electrodes with high thickness. To overcome the limitation of such sluggish ionic transport, a facile ice-templating strategy was employed to create vertically aligned channels, offering fast-diffusion pathways for the Li ion in the electrolyte. Impressively, the ice-templated electrodes exhibit a specific capacity of 144 mA h g–1 at 0.2 C and retain 83 mA h g–1 at 10 C with high mass loading ∼10 mg cm–2. The enhanced ion transport kinetics was verified by various electrochemical and structural characterizations. This work demonstrates the thickness scaling effect of nanosheet-based electrodes and highlights the importance of promoting ionic transport and electrolyte access for designing thick electrodes.

Effect of thickness scaling on the permeability and thermal stability of Ta(N) diffusion barrier

Molecular and atomic permeability, thermal stability and interfacial reactions of ultrathin Ta, TaN and Ta/TaN stack as barrier to Cu/low - k interconnects have been studied. It is shown the barrier layers containing TaN and Ta/TaN have good thermal stability and barrier properties when their thickness is >3.5 nm while Ta cannot withstand 450 °C annealing. The thinner barriers are permeable for neutral molecules like heptane and demonstrate degradation of resistivity during the thermal anneal at 450 °C. Penetration of Ta precursors into low-k and formation of TaO/TaC type of bonds increase the dielectric function of low-k dielectric. Our results show that, on the low-k material with 23% porosity and 0.9 nm pore radius, the PVD deposited Ta(N) films <3.5 nm thick have poor barrier function and are sufficient for neutral molecules penetration, especially water molecules.

Layer-Dependent Chemically Induced Phase Transition of Two-Dimensional MoS2.

Two-dimensional (2D) transition metal dichalcogenides (TMDCs) with layered structures provide a unique platform for exploring the effect of number of layers on their fundamental properties. However, the thickness scaling effect on the chemical properties of these materials remains unexplored. Here, we explored the chemically induced phase transition of 2D molybdenum disulfide (MoS2) from both experimental and theoretical aspects and observed that the critical electron injection concentration and the duration required for the phase transition of 2D MoS2 increased with decreasing number of layers. We further revealed that the observed dependence originated from the layer-dependent density of states of 2H-MoS2, which results in decreasing phase stability for 2H-MoS2 with increasing number of layers upon electron doping. Also, the much larger energy barrier for the phase transition of monolayer MoS2 induces the longer reaction time required for monolayer MoS2 as compared to multilayer MoS2. The layer-dependent phase transition of 2D MoS2 allows for the chemical construction of semiconducting-metallic heterophase junctions and, subsequently, the fabrications of rectifying diodes and all 2D field effect transistors and thus opens a new avenue for building ultrathin electronic devices. In addition, these new findings elucidate how electronic structures affect the chemical properties of 2D TMDCs and, therefore, shed new light on the controllable chemical modulations of these emerging materials.

A multi-scale based cohesive zone model for the analysis of thickness scaling effect in fiber bridging

A cohesive zone model (CZM) is proposed to assess the thickness scaling effect associated with fiber bridging during fracture. The CZM was developed through a multi-scale simulation approach and utilizes an embedded cell model of the Double Cantilever Beam (DCB) that explicitly accounts for the bridging bundles on the fracture plane. In particular, micromechanical simulations of failure were carried out, for varying arms thickness, in order to determine the homogenized fracture behavior. To model the observed scaling effect, the conventional cohesive law, formulated as an opening-stress relation, is enriched with information on the crack opening angle. Continuum finite element simulations indicated that the proposed CZM was able to mimic very well the essential features observed in the experiments, e.g. raising R-curve behavior and thickness scaling effect on the energy dissipated at steady state.

Thickness scaling effect on interfacial barrier and electrical contact to two-dimensional MoS2 layers.

Understanding the interfacial electrical properties between metallic electrodes and low-dimensional semiconductors is essential for both fundamental science and practical applications. Here we report the observation of thickness reduction induced crossover of electrical contact at Au/MoS2 interfaces. For MoS2 thicker than 5 layers, the contact resistivity slightly decreases with reducing MoS2 thickness. By contrast, the contact resistivity sharply increases with reducing MoS2 thickness below 5 layers, mainly governed by the quantum confinement effect. We find that the interfacial potential barrier can be finely tailored from 0.3 to 0.6 eV by merely varying MoS2 thickness. A full evolution diagram of energy level alignment is also drawn to elucidate the thickness scaling effect. The finding of tailoring interfacial properties with channel thickness represents a useful approach controlling the metal/semiconductor interfaces which may result in conceptually innovative functionalities.

One-Dimensional Thickness Scaling Study of Phase Change Material $(\hbox{Ge}_{2}\hbox{Sb}_{2}\hbox{Te}_{5})$ Using a Pseudo 3-Terminal Device

To address the scalability of phase change memory (PCM), we study a 1-D thickness scaling effect on threshold switching voltage (Vth), Vth drift, high resistance state (RESET) resistance (RRESET) drift, and crystallization temperature (Tcrys). We use a pseudo three-terminal device to accurately correlate the amorphous region thickness to the observed characteristics. The pseudo 3-terminal device is a fully functional PCM cell and enables 1-D thickness scaling study down to 6 nm without the need for ultrafine lithography. Vth scales down to 0.65-0.5 V (at 25°C-75°C) for 6-nm-thick Ge2Sb2Te5 (GST), showing that stable read operation is possible in scaled PCM devices. The Vth drift measurement suggests that Vth drift can be attributed to threshold switching field (Eth) drift, whereas Vth0, i.e., Vth at zero thickness, stays almost constant. RRESET drift shows no dependence on the amorphous GST thickness. Tcrys is ~175°C for the device with 6-nm-thick GST, compared with ~145°C of thick GST. From the 1-D scaling study, no significant hurdles against scaling are found down to 6 nm. Further study of scaling effect on endurance and development of scalable selection device is needed to assess the ultimate scalability of PCM.

Size effects in ultrathin epitaxial ferroelectric heterostructures

In this letter we report on the effect of thickness scaling in model PbZr0.2Ti0.8O3(PZT)∕SrRuO3 heterostructures. Although theoretical models for thickness scaling have been widely reported, direct quantitative experimental data for ultrathin perovskite (&amp;lt;10nm) films in the presence of real electrodes have still not been reported. In this letter we show a systematic quantitative experimental study of the thickness dependence of switched polarization in (001) epitaxial PZT films, 4to80nm thick. A preliminary model based on a modified Landau Ginzburg approach suggests that the nature of the electrostatics at the ferroelectric–electrode interface plays a significant role in the scaling of ferroelectric thin films.

ONO inter-poly dielectric scaling for nonvolatile memory applications

The ONO (oxide/nitride/oxide) inter-poly dielectric thickness scaling effect on electric-field-leakage-current characteristics and charge retention characteristics in nonvolatile memories are investigated. Surface top-oxide thickness strongly affects the charge leakage and retention characteristics. Thicker than 3 nm top oxide can block hole injunction from the anode. Thick top oxide can reduce leakage current in both high and low electric field regions. Moreover, it can improve charge retention characteristics in nonvolatile memory cells. Therefore, a certain amount of top oxide is required to preserve good charge retention characteristics. SiN thickness scaling leads to an improvement in charge retention characteristics. Bottom oxide has an important role in suppressing electron leakage in a low electric field region. A degraded quality thin bottom oxide leads to charge retention capability degradation. Therefore, bottom-oxide quality and thickness control is an important subject for ONO thickness scaling. >