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Materials Challenges for Memory

Phase change memory devices for electrical and optical in-memory computing


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The energy cost of storing and moving data is enormous, and is growing at such a rapid pace that within a few years the power demand may overwhelm some developed nations. Non-volatile memory (NVM) and neuromorphic computing are enabling critical advances in this area, but there are many scientific and technological challenges. This conference will cover the materials challenges of different memory technologies, from the fundamental processes and mechanisms, to industry perspectives on materials specifications and manufacturing.

technical paper

Operando methods for determining lithiation, switching-mechanisms and phase changes in electrochemical processes

Material challenges in ferroelectric memories

Devices and materials for in-memory computing: challenges and opportunities

Industry challenges and outlook for resistive memories

Resistive +PCM

Spin-transfer-torque MRAM

Oxide perovskites have received widespread attention ever since their discovery due to the multiple physical properties they exhibit, including ferroelectricity, multiferroicity, and superconductivity. One prominent absence in this list of properties that oxide perovskites exhibit is electronic topological order. This is a consequence of the large band gaps of oxide perovskites, which make the band inversions necessary for topology impossible. Alternatively, we find that topological phonons – nodal rings, nodal lines, and Weyl points – are ubiquitous in oxide perovskites in terms of structures (tetragonal, orthorhombic, and rhombohedral), compounds (BaTiO3, PbTiO3, and SrTiO3), and external conditions (photoexcitation, strain, and temperature). In particular, in the tetragonal phase of these compounds all types of topological phonons can simultaneously emerge when stabilized by photoexcitation, whereas the tetragonal phase stabilized by thermal fluctuations only hosts a more limited set of topological phonon states. In addition, we find that the photoexcited carrier density can be used to control the emergent topological states, for example driving the creation/annihilation of Weyl points and switching between nodal lines and nodal rings. Overall, we propose oxide perovskites as a versatile platform in which to study topological phonons and their manipulation with light [1]. Topological phonons in oxide perovskites provide a promising platform to study physical phenomena, ranging from the phonon Hall effect to superconductivity, and may also offer new technological opportunities such as the realization of controllable topological quantum states for neuromorphic computing.

Reference:
[1] Bo Peng, Yuchen Hu, Shuichi Murakami, Tiantian Zhang, Bartomeu Monserrat. Topological phonons in oxide perovskites controlled by light. Science Advances 6 (46), eabd1618 (2020).

Topological phonons in oxide perovskites controlled by light

Magneto-ionics, understood as voltage-driven ion transport in magnetic materials, could represent a significant breakthrough in the search for routes to enhance energy efficiency in magnetically-actuated devices. Particularly, magneto-ionics in Co3O4 allows for voltage-controlled ON−OFF ferromagnetism, since Co3O4 (paramagnetic) can be partially reduced to Co (ferromagnetic) by voltage actuation and reversed by applying a voltage of opposite polarity [1]. This could be used to binary encode information: for example, OFF = 0 (paramagnetism) and ON = 1 (ferromagnetism). However, ion motion at room temperature is still too slow for applications. The way to apply electric field (voltage actuation) can have a strong impact on the room temperature magneto-ionic motion. Specifically, in electrolyte-gated paramagnetic Co3O4 films, the oxygen magneto-ionic motion can be increased by more than one order of magnitude by applying the electric field through an electrochemical capacitor configuration (i.e., using, as working electrode, an underlying conducting buffer layer beneath the Co3O4 film) rather than using an electric-double-layer transistor-like configuration (i.e., placing the electric contact at one side of the Co3O4 film as working electrode). This is due to a greater uniformity and strength of the electric field in the capacitor design [1,2].

[1] A. Quintana, E. Menéndez, M. O. Liedke, M. Butterling, A. Wagner, V. Sireus, P. Torruella, S. Estradé, F. Peiró, J. Dendooven, C. Detavernier, P. D. Murray, D. A. Gilbert, K. Liu, E. Pellicer, J. Nogues and J. Sort, Voltage-controlled ON−OFF ferromagnetism at room temperature in a single metal oxide film, ACS Nano 12 (2018) 10291–10300
[2] J. de Rojas, A. Quintana, A. Lopeandía, J. Salguero, J. L. Costa-Krämer, L. Abad, M. O. Liedke, M. Butterling, A. Wagner, L. Henderick, J. Dendooven, C. Detavernier, J. Sort and E. Menéndez, Boosting room-temperature magneto-ionics in a non-magnetic oxide semiconductor, Advanced Functional Materials 30 (2020) 2003704

ON-OFF magneto-ionics in Co3O4

Voltage-controlled magnetism (VCM) utilizes an applied electric field to modulate magnetic properties in a material instead of current, potentially representing a sizable increase in energy savings in next-generation technology and devices. However, VCM still faces several difficult challenges: a scarcity of room-temperature multiferroics, mechanical fatigue and failure in strain-mediated piezoelectric/magnetostrictive devices, and thickness limitations in charged metallic films. Voltage-driven ionic motion (magneto-ionics) presents an alternative which does not suffer from many of these drawbacks, although translating magneto-ionics into real world devices requires significant improvements in magneto-ionic rates, cyclability, and changes in magnetization. Here, we report on room-temperature voltage-driven nitrogen magneto-ionics (via N transport) by electrolyte-gating of a CoN film using a propylene carbonate liquid electrolyte[1]. Nanocrystalline (face-centered cubic structure) CoN and Co3O4 films biased using an electrochemical capacitor configuration are compared. Both materials show reversible voltage-driven ON-OFF ferromagnetism, albeit with CoN displaying a remarkable 6-fold advantage in magneto-ionic rate. Nitrogen transport in CoN is found to occur uniformly throughout the film, creating a plane-wave-like migration front, without assistance of diffusion channels, in sharp contrast with oxygen transport in Co3O4, where ion diffusion is more inhomogeneous and preferentially occurs along grain boundaries. Nitrogen magneto-ionics in CoN is also shown to require lower onset voltages than oxygen magneto-ionics, as well as enhanced rates and cyclability due to the combination of a lower energy barrier for ion diffusion (and, hence, minimum applied voltage) and the lower electronegativity of nitrogen with respect to oxygen, consistent with ab initio calculations of N vs. O ion motion in a cobalt lattice. These promising results suggest nitrogen magneto-ionics can be tantalizing alternative for efficient voltage-driven effects and, alongside oxygen magneto-ionics, may enable the use of magneto-ionics in devices such as brain-inspired/stochastic computing or magnetic micro-electro-mechanical systems.

Nitrogen Magneto-ionics

Layered magnetic semiconductors are a key resource for next-generation spintronic logic and memory applications [1]. The recent advances in growing 2D magnetic materials provide ease of thickness control, atomistic mechanical stability, measurable spin-mobility, and a practical approach for spintronic research [2]. Here, we have synthesized 2D chromium sulfide (Cr2S3) using chemical vapor deposition (CVD) on a sapphire substrate and carried out transport measurements under an external magnetic field applied out-of-plane (Figure 1). The microscopy and Raman investigations suggest layered non-van der Waal crystal growth along the c-axis (001) direction. The set of investigations suggest that the 2D-Cr2S3 consists of a layered geometry with magnetic behavior similar to that reported in CrI3 [3] i.e., intra-layer ferromagnetic and inter-layer antiferromagnetic ordering along the stacking (c-axis) direction. The Neil temperature (TN) is noted as ~120 K and small magnetic contributions are evident till room temperature (300 K) as shown in Figures 1d and 2a. 
A four-probe device structure with non-magnetic (Al) electrodes was fabricated using standard semiconductor processing[4] to carry out the transport measurements as shown in Fig. 1c. The voltage across the two inner electrodes was measured in response to a constant current of 10 µA injected into the device. The field-induced magnetoresistance (MRH) was calculated as 〖MR〗_H=(R(H)-R(0))/(R(0)), where R(H) is the resistance measured at an applied magnetic field strength of H in Oe. A large MRH of -25% was observed at 100 K, i.e., near TN. Our study shows the change in MRH magnitude from negative to positive below 20 K as shown in Figure 2a. 
To probe the spin induced magnetoresistance (MR_s=(∆R_s)/R_0 where ∆R_s is the difference in resistance between the positive sweep and the negative sweep of the applied magnetic field, as shown in Figure 2b), the resistance versus a dual sweep (0→5T→0→-5T→0→5T) of the magnetic field (Hext), has been investigated at different temperatures. For TN<T≤300 K, a small negative MRs vs. Hext was noticed. This behavior is characteristic of a disordered magnetic state, typically attributed to quenching of spin disorder upon applying a Hext [5]. On the other hand, distinct resistance states (0 and 1) are observed with Rs ≅ 3 kΩ when cooled down to 10 K at ±5 kOe and 10 µA (power = 0.2 watts). Here, MRs of 13% is observed, which has not been shown before in thin-film Cr2S3 systems. The large resistance hysteresis also matches the magnetization hysteresis (not shown here) and can be attributed to the large (2-5 x 108) spins/m3, which get flipped above the threshold magnetic field.[6]
In summary, we have successfully demonstrated the magneto-resistive effect in CVD-grown 2D Cr2S3 thin film, which makes Cr-based 2D materials novel among 2D materials. Furthermore, these Cr2S3 with ferrimagnetic and narrow bandgap characteristics provide an excellent platform for next-generation spintronic applications.

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Phase change memory devices for electrical and optical in-memory computing

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