Two-dimensional materials prospects for non-volatile spintronic memories.

Non-volatile magnetic random-access memories (MRAMs), such as spin-transfer torque MRAM and next-generation spin-orbit torque MRAM, are emerging as key to enabling low-power technologies, which are expected to spread over large markets from embedded memories to the Internet of Things. Concurrently, the development and performances of devices based on two-dimensional van der Waals heterostructures bring ultracompact multilayer compounds with unprecedented material-engineering capabilities. Here we provide an overview of the current developments and challenges in regard to MRAM, and then outline the opportunities that can arise by incorporating two-dimensional material technologies. We highlight the fundamental properties of atomically smooth interfaces, the reduced material intermixing, the crystal symmetries and the proximity effects as the key drivers for possible disruptive improvements for MRAM at advanced technology nodes.

Significant Oxygen Underestimation When Quantifying Barium-Doped SrTiO Layers by Atom Probe Tomography.

In this paper, the capability for quantifying the composition of Ba-doped SrTiO layers from an atom probe measurement was explored. Rutherford backscattering spectrometry and time-of-flight/energy elastic recoil detection were used to benchmark the composition where the amount of titanium was intentionally varied between samples. The atom probe results showed a significant divergence from the benchmarked composition. The cause was shown to be a significant oxygen underestimation (≳14 at%). The ratio between oxygen and titanium for the samples varied between 2.6 and 12.7, while those measured by atom probe tomography were lower and covered a narrower range between 1.4 and 1.7. This difference was found to be associated with the oxygen and titanium predominantly field evaporating together as a molecular ion. The evaporation fields and bonding chemistries determined showed inconsistencies for explaining the oxygen underestimation and ion species measured. The measured ion charge state was in excellent agreement with that predicted by the Kingham postionization theory. Only by considering the measured ion species, their evaporation fields, the coordination chemistry, the analysis conditions, and some recently reported density functional theory modeling for oxide field emission were we able to postulate a field emission and oxygen neutral desorption process that may explain our results.

Field-Free Spin-Orbit Torque Driven Switching of Perpendicular Magnetic Tunnel Junction through Bending Current.

Current-induced spin-orbit torques (SOTs) enable fast and efficient manipulation of the magnetic state of magnetic tunnel junctions (MTJs), making them attractive for memory, in-memory computing, and logic applications. However, the requirement of the external magnetic field to achieve deterministic switching in perpendicularly magnetized SOT-MTJs limits its implementation for practical applications. Here, we introduce a field-free switching (FFS) solution for the SOT-MTJ device by shaping the SOT channel to create a "bend" in the SOT current. The resulting bend in the charge current creates a spatially nonuniform spin current, which translates into inhomogeneous SOT on an adjacent magnetic free layer enabling deterministic switching. We demonstrate FFS experimentally on scaled SOT-MTJs at nanosecond time scales. This proposed scheme is scalable, material-agnostic, and readily compatible with wafer-scale manufacturing, thus creating a pathway for developing purely current-driven SOT systems.

Key material parameters driving CBRAM device performances.

This study is focused on Conductive Bridging Random Access Memory (CBRAM) devices based on chalcogenide electrolyte and Cu-supply materials, and aims at identifying the key material parameters controlling memory properties. The CBRAM devices investigated are integrated on CMOS select transistors, and are constituted by either Ge-Se or Ge-Te electrolyte layers of various compositions combined with a Cu2GeTe3 active chalcogenide electrode. By means of extensive physical and electrical characterization, we show for a given electrolyte system that slower write is obtained for a denser electrolyte layer, which is directly correlated with a lower atomic percentage of the chalcogen element in the layer. We also evidence that the use of Ge-Se electrolyte results in larger write energy (voltage and time), however with improved state retention properties than for Ge-Te electrolyte materials. We associate these results with the stronger chemical bonding of Cu with Se, resulting both in a stabilized Cu filament and a slower Cu cation motion. More robust processing thermal stability is also observed for Ge-Se compared to Ge-Te compounds, allowing more flexibility in the integration flow design.

Spin-Orbit Torque Vector Quantification in Nanoscale Magnetic Tunnel Junctions.

Spin-orbit torques (SOT) allow ultrafast, energy-efficient toggling of magnetization state by an in-plane charge current for applications such as magnetic random-access memory (SOT-MRAM). Tailoring the SOT vector comprising of antidamping (TAD) and fieldlike (TFL) torques could lead to faster, more reliable, and low-power SOT-MRAM. Here, we establish a method to quantify the longitudinal (TAD) and transverse (TFL) components of the SOT vector and its efficiency χAD and χFL, respectively, in nanoscale three-terminal SOT magnetic tunnel junctions (SOT-MTJ). Modulation of nucleation or switching field (BSF) for magnetization reversal by SOT effective fields (BSOT) leads to the modification of SOT-MTJ hysteresis loop behavior from which χAD and χFL are quantified. Surprisingly, in nanoscale W/CoFeB SOT-MTJ, we find χFL to be (i) twice as large as χAD and (ii) 6 times as large as χFL in micrometer-sized W/CoFeB Hall-bar devices. Our quantification is supported by micromagnetic and macrospin simulations which reproduce experimental SOT-MTJ Stoner-Wohlfarth astroid behavior only for χFL > χAD. Additionally, from the threshold current for current-induced magnetization switching with a transverse magnetic field, we show that in SOT-MTJ, TFL plays a more prominent role in magnetization dynamics than TAD. Due to SOT-MRAM geometry and nanodimensionality, the potential role of nonlocal spin Hall spin current accumulated adjacent to the SOT-MTJ in the mediation of TFL and χFL amplification merits to be explored.

Direct Assessment of Defective Regions in Monolayer MoS2 Field-Effect Transistors through In Situ Scanning Probe Microscopy Measurements.

Implementing two-dimensional materials in field-effect transistors (FETs) offers the opportunity to continue the scaling trend in the complementary metal-oxide-semiconductor technology roadmap. Presently, the search for electrically active defects, in terms of both their density of energy states and their spatial distribution, has turned out to be of paramount importance in synthetic transition metal dichalcogenides layers, as they are suspected of severely inhibiting these devices from achieving their highest performance. Although advanced microscopy tools have allowed the direct detection of physical defects such as grain boundaries and point defects, their implementation at the device scale to assess the active defect distribution and their impact on field-induced channel charge modulation and current transport is strictly restrained. Therefore, it becomes critical to directly probe the gate modulation effect on the carrier population at the nanoscale of an FET channel, with the objective to establish a direct correlation with the device characteristics. Here, we have investigated the active channel in a monolayer MoS2 FET through in situ scanning probe microscopy, namely, Kelvin probe force microscopy and scanning capacitance microscopy, to directly identify active defect sites and to improve our understanding of the contribution of grain boundaries, bilayer islands, and defective grain domains to channel conductance.

Device modeling of two-steps oxygen anneal-based submicron InGaZnO back-end-of-line field-effect transistor enabling short-channel effects suppression.

Amorphous oxide semiconductor (AOS) field-effect transistors (FETs) have been integrated with complementary metal-oxide-semiconductor (CMOS) circuitry in the back end of line (BEOL) CMOS process; they are promising devices creating new and various functionalities. Therefore, it is urgent to understand the physics determining their scalability and establish a physics-based model for a robust device design of AOS BEOL FETs. However, the advantage emphasized to date has been mainly an ultralow leakage current of these devices. A device modeling that comprehensively optimizes the threshold voltage (VT), the short-channel effect (SCE), the subthreshold swing (SS), and the field-effect mobility (µFE) of short-channel AOS FETs has been rarely reported. In this study, the device modeling of two-steps oxygen anneal-based submicron indium-gallium-zinc-oxide (IGZO) BEOL FET enabling short-channel effects suppression is proposed and experimentally demonstrated. Both the process parameters determining the SCE and the device physics related to the SCE are elucidated through our modeling and a technology computer-aided design (TCAD) simulation. In addition, the procedure of extracting the model parameters is concretely supplied. Noticeably, the proposed device model and simulation framework reproduce all of the measured current-voltage (I-V), VT roll-off, and drain-induced barrier lowering (DIBL) characteristics according to the changes in the oxygen (O) partial pressure during the deposition of IGZO film, device structure, and channel length. Moreover, the results of an analysis based on the proposed model and the extracted parameters indicate that the SCE of submicron AOS FETs is effectively suppressed when the locally high oxygen-concentration region is used. Applying the two-step oxygen annealing to the double-gate (DG) FET can form this region, the beneficial effect of which is also proven through experimental results; the immunity to SCE is improved as the O-content controlled according to the partial O pressure during oxygen annealing increases. Furthermore, it is found that the essential factors in the device optimization are the subgap density of states (DOS), the oxygen content-dependent diffusion length of either the oxygen vacancy (VO) or O, and the separation between the top-gate edge and the source-drain contact hole. Our modeling and simulation results make it feasible to comprehensively optimize the device characteristic parameters, such as VT, SCE, SS, and µFE, of the submicron AOS BEOL FETs by independently controlling the lateral profile of the concentrations of VO and O in two-step oxygen anneal process.

Influence of the Chalcogen Element on the Filament Stability in CuIn(Te,Se,S)2/Al2O3 Filamentary Switching Devices.

In this paper, we report on the use of CuInX2 (X = Te, Se, S) as a cation supply layer in filamentary switching applications. Being used as absorber layers in solar cells, we take advantage of the reported Cu ionic conductivity of these materials to investigate the effect of the chalcogen element on filament stability. In situ X-ray diffraction showed material stability attractive for back-end-of-line in semiconductor industry. When integrated in 580 µm diameter memory cells, more volatile switching was found at low compliance current using CuInS2 and CuInSe2 compared to CuInTe2, which is ascribed to the natural tendency for Cu to diffuse back from the switching layer to the cation supply layer because of the larger difference in electrochemical potential using Se or S. Low-current and scaled behavior was also confirmed using conductive atomic force microscopy. Hence, by varying the chalcogen element, a method is presented to modulate the filament stability.