Next-generation Memory Devices Research Articles

Atomic and Molecular Layer Deposition of Chiral Thin Films Showing up to 99% Spin Selective Transport.

Spin electronics is delivering a much desired combination of properties such as high speed, low power, and high device densities for the next generation of memory devices. Utilizing chiral-induced spin selectivity (CISS) effect is a promising path toward efficient and simple spintronic devices. To be compatible with state-of-the-art integrated circuits manufacturing methodologies, vapor phase methodologies for deposition of spin filtering layers are needed. Here, we present vapor phase deposition of hybrid organic-inorganic thin films with embedded chirality. The deposition scheme relies on a combination of atomic and molecular layer deposition (A/MLD) utilizing enantiomeric pure alaninol molecular precursors combined with trimethyl aluminum (TMA) and water. The A/MLD deposition method deliver highly conformal thin films allowing the fabrication of several types of nanometric scale spintronic devices. The devices showed high spin polarization (close to 100%) for 5 nm thick spin filter layer deposited by A/MLD. The procedure is compatible with common device processing methodologies.

Enhancement of the ferroelectricity by interface engineering observed by in situ transmission electron microscope

Hafnia-based ferroelectrics with excellent scalability and complementary metal–oxide–semiconductor technology compatibility are potential materials for next-generation memory and logic devices. Stabilizing the metastable ferroelectric phase in hafnia-based ferroelectrics is critical for realizing technological applications. Interface engineering is a critical method to stabilize the ferroelectric phase. However, the role played by the interface between the metal electrode and the hafnia-based ferroelectrics oxide remains unclear. In this work, a typical Hf0.5Zr0.5O2 (HZO) ferroelectric oxide film sandwiched between the metal electrode and the silicon substrate was fabricated with and without the interfacial layer. By using the in situ transmission electron microscope, the atomistic structure evolution of the HZO film ferroelectric phase was studied under electrical stimuli. It is found that the phase transition from ferroelectric (FE) orthorhombic phase (O-phase) to dielectric monoclinic phase (M-phase) occurs from the interface between the HZO and the metal electrode. While in the one with Al2O3 as an interfacial layer between the HZO and the metal electrode, the FE O-phase could remain without phase transition. This work shows the microscopic view to enhance the ferroelectric evolution in HfO2-based devices.

Giant tunneling magnetoresistance and electroresistance in α−In2Se3 -based van der Waals multiferroic tunnel junctions

In the multiferroic tunnel junction (MFTJ) composed of ferromagnetic and ferroelectric materials, the tunneling electroresistance (TER) coexists with the tunneling magnetoresistance (TMR), making it an ideal platform for designing multifunctional electronic devices. Recently, the rapid development of van der Waals (vdW) materials opened up a new avenue of MFTJ due to their atomic thickness and significance in miniaturizing device sizes. Here by employing the nonequilibrium Green's function combined with density-functional theory, we systemically study the spin-dependent electronic transport properties of ${\mathrm{Fe}}_{3}{\mathrm{GeTe}}_{2}$ (FGT)/bilayer $\ensuremath{\alpha}\ensuremath{-}{\mathrm{In}}_{2}{\mathrm{Se}}_{3}$ (BIS)/FGT vdW MFTJs. We find that the MFTJ can form multiple nonvolatile resistance states by altering the polarization orientation of the ferroelectric barrier BIS and the magnetization alignment of the two ferromagnetic FGT electrodes, with a maximum TMR (TER) ratio up to $1.1\ifmmode\times\else\texttimes\fi{}{10}^{7}$% (744%). The TER ratio can be further increased to 1868% by using left and right symmetrical copper electrodes. More interestingly, the perfect spin filtering effect can be realized in our MFTJs and the spin current can be controlled by the sign of bias voltages, suggesting a promising route for spin valves that can flexibly manipulate spin currents. Our results demonstrate that giant TMR, large TER, as well as a tunable spin filter can coexist in one system, and that the feasible tunability of such kind of vdW MFTJs is beneficial in designing next-generation logic and memory devices.

Layer-by-Layer Assembly-Based Heterointerfaces for Modulating the Electronic Properties of Ti3C2Tx MXene

Two-dimensional (2D) transition-metal carbides (MXenes) are emerging as promising materials for a wide range of applications owing to their intriguing electrical, optical, and optoelectronic properties. However, the modulation of metallic Ti3C2Tx MXene electronic properties is the key challenge to fabricate functional nanoelectronic devices. Here, we demonstrate a solution-processable route to fabricate Ti3C2Tx MXene/CuI nanoparticle heterointerfaces by employing a layer-by-layer assembly process. The charge transfer at the heterointerfacial assembly is monitored qualitatively from the quenched photoluminescence emission of CuI. The stable electrical conductivity and consistent Raman spectra of the 3-LBL assembly (three sequential stacks of CuI/MXene) signify the oxidation stability of Ti3C2Tx thin films even after exposure to the ambient environment for 2 months. Furthermore, the 3-LBL assembly exhibited a three-dimensional (3D) variable-range hopping-based electrical conduction in the temperature range 2 ≤ T < 100 K, contrary to the weak localized transport phenomenon in Ti3C2Tx MXene. The difference in charge transport mechanism is supported by distinct magnetoresistance (MR) of the Ti3C2Tx MXene (negative MR, -0.4%) and 3-LBL assembly (positive MR, 1.6%). Therefore, the modulated electrical transport and superior oxidation stability of the Ti3C2Tx MXene in the 3-LBL assembly have the potential to develop next-generation optoelectronic and memory devices.

Ferroelectricity in CMOS-Compatible Hafnium Oxides: Reviving the ferroelectric field-effect transistor technology

A Ferroelectric Field-Effect Transistor (FEFET) is a promising candidate for next-generation memory devices because it offers numerous advantages, such as its high speed, low energy profile, and nondestructive readout process. FEFETs merge logic and memory functionality in a single device, allowing efficient data transfer and a high packing density, which could make them vital for use in future in-memory computing architectures. Although issues related to the integration of perovskite materials in the state-of-the art semiconductor industry limited their commercial success, the discovery of ferroelectricity in CMOS-compatible doped hafnium oxides led to a re-emergence of FEFETs in advanced microelectronics. The demonstration of ferroelectricity in doped hafnia at extremely thin film thicknesses plus its low permittivity, high coercive field (Ec), environment-friendly composition, and excellent CMOS compatibility are expected to unleash the promise of FEFETs, and significant progress has been made in this regard.

Memory Packaging Challenges for a Growing Market

Abstract Increasing demands for storage and high-performance memory in applications such as artificial intelligence (AI), machine learning and processing storage are driving long-term growth in the market. This paper will highlight the key challenges encountered in packaging next-generation memory devices and discuss some of the technological developments required to address them while considering performance, cost, and yield. An evaluation is highlighted of a remote microwave plasma process to remove damage caused by laser ablation during the wafer dicing process that shows an improvement in die strength using this process.

Interlayer engineering for enhanced ferroelectric tunnel junction operations in HfO x -based metal-ferroelectric-insulator-semiconductor stack

Ferroelectric tunnel junction (FTJ) has been considered as a promising candidate for next-generation memory devices due to its non-destructive and low power operations. In this article, we demonstrate the interlayer (IL) engineering in the FTJs to boost device performances. Through the analysis on the material and electrical characteristics of the fabricated FTJs with engineered IL stacks, it is clearly found that the insertion of an Al2O3 layer between the SiO2 insulator and the pure-HfO x FE improves the read disturbance (2V c = 2.2 V increased), the endurance characteristics (tenfold improvement), and the cell-to-cell TER variation simultaneously without the degradation of the ferroelectricity (less than 5%) and the polarization switching speeds through grain size modulation. Based on these investigations, the guidelines of IL engineering for low power ferroelectric devices were provided to obtain stable and fast memory operations.

Observation of magnetoresistance in CrI3/graphene van derWaals heterostructures* *Project supported by the National Natural Science Foundation of China (Grant No. 51872039) and Science and Technology Program of Sichuan, China (Grant No. M112018JY0025).

Two-dimensional ferromagnetic van der Waals (2D vdW) heterostructures have opened new avenues for creating artificial materials with unprecedented electrical and optical functions beyond the reach of isolated 2D atomic layered materials, and for manipulating spin degree of freedom at the limit of few atomic layers, which empower next-generation spintronic and memory devices. However, to date, the electronic properties of 2D ferromagnetic heterostructures still remain elusive. Here, we report an unambiguous magnetoresistance behavior in CrI3/graphene heterostructures, with a maximum magnetoresistance ratio of 2.8%. The magnetoresistance increases with increasing magnetic field, which leads to decreasing carrier densities through Lorentz force, and decreases with the increase of the bias voltage. This work highlights the feasibilities of applying two-dimensional ferromagnetic vdW heterostructures in spintronic and memory devices.

Cation and anion topotactic transformations in cobaltite thin films leading to Ruddlesden-Popper phases

The ability to control a material's physical properties by ion migration is particularly important for the design of devices for emerging applications such as neuromorphic computing, magnetoionic switches, and next-generation memory and logic devices. The perovskite oxide LaSrCoO${}_{3}$ is an ideal system for investigating ion migration due to its high oxygen vacancy conductivity, relatively low oxygen vacancy formation energy, and strong coupling of the magnetic and electronic properties to its oxygen stoichiometry. This paper shows that it is possible to obtain a series of electronic and magnetic phases by systematically exposing the parent perovskite oxide to highly reducing annealing conditions. Under these conditions, the movement of both oxygen and cobalt ions in and out of the lattice represents a new direction for discovery of functional materials for brain-inspired computing.

On-Chip Integrated Photonic Devices Based on Phase Change Materials

Phase change materials present a unique type of materials that drastically change their electrical and optical properties on the introduction of an external electrical or optical stimulus. Although these materials have been around for some decades, they have only recently been implemented for on-chip photonic applications. Since their reinvigoration a few years ago, on-chip devices based on phase change materials have been making a lot of progress, impacting many diverse applications at a very fast pace. At present, they are found in many interesting applications including switches and modulation; however, phase change materials are deemed most essential for next-generation low-power memory devices and neuromorphic computational platforms. This review seeks to highlight the progress thus far made in on-chip devices derived from phase change materials including memory devices, neuromorphic computing, switches, and modulators.

Ruderman\u2013Kittel\u2013Kasuya\u2013Yosida-type interfacial Dzyaloshinskii\u2013Moriya interaction in heavy metal/ferromagnet heterostructures

The manipulation of magnetization with interfacial modification using various spin-orbit coupling phenomena has been recently revisited due to its scientific and technological potential for next-generation memory devices. Herein, we experimentally and theoretically demonstrate the interfacial Dzyaloshinskii–Moriya interaction characteristics penetrating through a MgO dielectric layer inserted between the Pt and CoFeSiB. The inserted MgO layer seems to function as a chiral exchange interaction mediator of the interfacial Dzyaloshinskii–Moriya interaction from the heavy metal atoms to ferromagnet ones. The potential physical mechanism of the anti-symmetric exchange is based on the tunneling-like behavior of conduction electrons through the semi-conductor-like ultrathin MgO. Such behavior can be correlated with the oscillations of the indirect exchange coupling of the Ruderman–Kittel–Kasuya–Yosida type. From the theoretical demonstration, we could provide approximate estimation and show qualitative trends peculiar to the system under investigation.

Phase-pure two-dimensional FexGeTe2 magnets with near-room-temperature TC

Two-dimensional (2D) ferromagnets with out-of-plane (OOP) magnetic anisotropy are potential candidates for realizing the next-generation memory devices with ultra-low power consumption and high storage density. However, a scalable approach to synthesize 2D magnets with OOP anisotropy directly on the complimentary metal-oxide semiconductor (CMOS) compatible substrates has not yet been mainly explored, which hinders the practical application of 2D magnets. This work demonstrates a cascaded space confined chemical vapor deposition (CS-CVD) technique to synthesize 2D FexGeTe2 ferromagnets. The weight fraction of iron (Fe) in the precursor controls the phase purity of the as-grown FexGeTe2. As a result, high-quality Fe3GeTe2 and Fe5GeTe2 flakes have been grown selectively using the CS-CVD technique. Curie temperature (TC) of the as-grown FexGeTe2 can be up to ∼ 280 K, nearly room temperature. The thickness and temperature-dependent magnetic studies on the Fe5GeTe2 reveal a 2D Ising to 3D XY behavior. Also, Terahertz spectroscopy experiments on Fe5GeTe2 display the highest conductivity among other FexGeTe2 2D magnets. The results of this work indicate a scalable pathway for the direct growth and integration of 2D ternary magnets on CMOS-based substrates to develop spintronic memory devices.

Sea urchin-like Ni encapsulated with BaTiO3 to form multiferroic core-shell structures for room temperature magnetoelectric sensors

Since core-shell structures are known to have a high surface area to volume ratio, they are preferred for next-generation memory device applications that require effective strain transfer at the ferroic interface. The present investigation reports the encapsulation of sea urchin-like Ni nanoparticles using BaTiO3 nanoparticles to realize room temperature ME coupled Ni@BaTiO3 system. The magnetic, electric and magnetoelectric properties of Ni nanoparticles partially coated with BaTiO3 provide enough evidence for the existence of ME coupling between Ni and BaTiO3 at their interface. Additionally, we have observed an exchange bias shift upon field cooling, which confirms the presence of the Ni-NiO-BaTiO3 interface in the Ni@BaTiO3 core-shell. The obtained magnetoelectric coupling coefficient of Ni@BaTiO3 composite is 225 µV cm−1 Oe−1. The present study reveals that chemically derived sea urchin-like Ni@BaTiO3 core-shell architecture can be a promising way to design strongly coupled multiferroic memory devices and sensors.

Interface control of tetragonal ferroelectric phase in ultrathin Si-doped HfO2 epitaxial films

Nanoscaled HfO2-based ferroelectric thin films are a favored candidate for the integration of next-generation memory and logic devices. The unique advantage is that the ferroelectric polarization becomes more robust than the traditional perovskite ferroelectrics when the size is reduced. Understanding and controlling the ferroelectricity requires high-quality epitaxial thin films to explore intrinsic ferroelectric mechanism and evaluate device applications. Here, we report a semicoherent growth of ITO as a bottom electrode that enables genuine ultrathin epitaxial films of Si-doped HfO2 on YSZ[001]/[110]/[111] substrates. The deposited films, which are under epitaxial compressive strain, display large ferroelectric polarization values up to 42 μC/cm2 and do not need wake-up cycling. Structural characterization reveals the presence of crystalline domains with short axes of the tetragonal structure oriented perpendicular to the substrate. Using piezoforce microscopy, polar domains can be written and read and can be reversibly switched with a phase change of 180o. Ferroelectric polarization can be controlled by ITO surface polarity which can easily exploit the interfacial valance mismatch to influence the electrostatic potential across the interface. These findings have implications for our understanding of ferroelectric switching and offer easy method to manipulate domain reversal state in HfO2-based ferroelectric materials.

In situ observations of topotactic phase transitions in a ferrite memristor

Oxide-based memristive devices have recently been proposed for various applications, such as next-generation memory and neuromorphic devices. Microstructural alterations depending on the oxygen ion concentration, such as the formation of conductive filaments and interface reactions, have been posited as the operating mechanism. Accordingly, it is important to explore the role of oxygen ion mobility in the behavior of memristive devices. In this study, memristive devices fabricated with brownmillerite SrFeO2.5 in the (111) and (001) orientations were studied via high-resolution transmission electron microscopy and in situ current–voltage measurements. The resistance of the devices was changed by a reversible topotactic phase change between the insulating brownmillerite SrFeO2.5 and the conductive perovskite SrFeO3−δ. Importantly, the oxygen vacancy channel was designed so that the phase change occurred across the electrodes in (111), where the channel was directly connected to both electrodes, whereas in (001) the channel is ordered along the in-plane direction and a phase change occurs only near the electrode. This work provides direct evidence of a brownmillerite-based mechanism of resistance change and a better understanding of routes to performance improvement.

Characterization and Modeling of Current Transport in Metal/Ferroelectric/Semiconductor Tunnel Junctions

Although metal/ferroelectric/semiconductor tunnel junctions have been attracting widespread interest as next-generation memory devices, only limited effort has been devoted so far to quantitatively address their current-voltage characteristics and their achievable memory window. In this article, we show experimental data on template Pt/BaTiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> /Nb:SrTiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> tunnel junctions confirming, first of all, that large memory windows can be achieved in these devices when the switching of the electric polarization in the ferroelectric layer moves the substrate between an accumulation and a depletion condition. Besides, by measuring the current-voltage curve of the devices quickly after the set/reset pulses used for polarization switching, the importance of considering the detrimental effects of the large depolarization fields in the ferroelectric layer when assessing the device memory window is demonstrated. The experimental data are, then, reproduced by a model for electron transport through the junction accounting for both drift/diffusion and distributed tunneling in the semiconductor. The model allows to catch a comprehensive physical picture of device operation and represents a powerful tool for the design and optimization of ferroelectric tunnel junctions with semiconductor electrodes.

Comprehensive Performance Quasi-Non-Volatile Memory Compatible with Large-Scale Preparation by Chemical Vapor Deposition.

Two-dimensional materials with atomic thickness have become candidates for wearable electronic devices in the future. Graphene and transition metal sulfides have received extensive attention in logic computing and sensing applications due to their lower power dissipation, so that their processes have been relatively mature for large-scale preparation. However, there are a few applications of two-dimensional materials in storage, which is not in line with the development trend of integration of storage and computing. Here, a charge storage quasi-non-volatile memory with a lanthanum incorporation high-k dielectric for next-generation memory devices is proposed. Thanks to the excellent electron capture capability of LaAlO3, the MoS2 memory exhibits a very comprehensive information storage capability, including robust endurance and ultra-fast write speed of 1 ms approximately. It is worth mentioning that it exhibits a long-term stable charge storage capacity (refresh time is about 1000 s), which is 105 times that of the dynamic random access memory (refresh time is on a milliseconds timescale) so that the unnecessary power dissipation greatly reduces caused by frequent refresh. In addition, its simple manufacturing process makes it compatible with various current two-dimensional electronic devices, which will greatly promote the integration of two-dimensional electronic computing.

Ultra-low Energy Phase Change Memory with Improved Thermal Stability by Tailoring the Local Structure through Ag Doping.

Although Sb2Te3, as a candidate material for next-generation memory devices, has attractive properties such as higher operation speed and lower power consumption than Ge2Sb2Te5, its poor stability prevents its application to commercial memory devices. Transition metal dopants provide enhancements in its phase change characteristics, improving both thermal stability and operation energy. However, the enhancement mechanism remains to be sufficiently investigated, and standard properties need to be achieved. Herein, the phase change properties of Sb2Te3 are confirmed to be enhanced by the incorporation of a heavy transition metal element such as Ag. The crystallization temperature increases by nearly 40%, and the operation energy is reduced by approximately 60%. These enhancements are associated with the changes in the local Sb2Te3 structure caused by Ag incorporation. As the incorporated Ag atoms substitute Sb in the Sb-Te octahedron, this turns into a Ag-Te defective tetrahedron with a strong Ag-Te bond that induces distortion in the crystal lattice. The formation of this bond is attributed to the electron configuration of Ag and its fully filled d orbital. Thus, Ag-doped Sb2Te3 is a promising candidate for practical phase change memory devices with high stability and high operation speed.

Robust resistive switching performance of pulsed laser deposited SiC/Ag/SiC tri-layer thin films deposited on a glass substrate

In this work, the authors developed SiC(10 nm)/Ag/SiC(10 nm) thin films showing an electroforming-free resistive switching (RS) effect with a switching ratio of 102. The observed RS effect is attributed to charging and discharging of Ag nanoparticles in the film layer. Further, SiC/Ag/ SiC film shows an excellent endurance and retention as well as a good thermal stability of RS characteristics. It is also identified that the switching ratio is invariant but the switching voltage of the device greatly depends on the Ag nanoparticles concentration and the operation temperature of the device. Therefore, SiC/Ag/SiC thin films are attractive for next-generation memory devices with enhanced durability.

In Situ Transmission Electron Microscopy with Biasing and Fabrication of Asymmetric Crossbars Based on Mixed-Phased a-VO x.

Resistive switching crossbar architecture is highly desired in the field of digital memories due to low cost and high-density benefits. Different materials show variability in resistive switching properties due to the intrinsic nature of the material used, leading to discrepancies in the field because of underlying operation mechanisms. This highlights a need for a reliable technique to understand mechanisms using nanostructural observations. This protocol explains a detailed process and methodology of insitu nanostructural analysis as a result of electrical biasing using transmission electron microscopy (TEM). It provides visual and reliable evidence of underlying nanostructural changes in real time memory operations. Also included is the methodology of fabrication and electrical characterizations for asymmetric crossbar structures incorporating amorphous vanadium oxide. The protocol explained here for vanadium oxide films can be easily extended to any other materials in a metal-dielectric-metal sandwiched structure. Resistive switching crossbars are predicted to serve the programmable logic and neuromorphic circuits for next-generation memory devices, given the understanding of the operation mechanisms. This protocol reveals the switching mechanism in a reliable, timely, and cost-effective way in any type of resistive switching materials, and thereby predicts the device's applicability.