United States

With the rapidly growing interest in cryogenic electronics, which is driven by the recent progress in spintronics and quantum / superconducting computing, there is an urgent need to develop logic and memory devices that can operate efficiently at low temperatures. To underpin these new developments there is a need for magnetic semiconductors which are compatible with cryogenic temperatures. We will discuss our recent progress in fabricating exactly these materials.

The rare-earth nitride series offer many intrinsically ferromagnetic semiconductors. They have already been incorporated into prototype field-effect transistors, spin-filters and tunnelling structures. They can be doped by controlling the inclusion of nitrogen vacancies, which donate electrons into the conduction band. They have a finite, and tuneable, conductivity even as the temperature approaches 0 K. Furthermore, the electronic and magnetic properties are not derived from the same electrons, as they are in the dilute magnetic semiconductors, i.e. the saturation magnetization and electron concentration can be tuned independently.

We will show electrical transport and magnetic measurements on GdN and DyN displaying their facility to be doped, along with density functional theory based calculations showing that the dopant states do indeed lie in the conduction band at 0~K. These materials provide cryogenically compatible ferromagnetic semiconductors for application in magnetic memory or spintronics devices.

Materials Challenges for Memory

Ferromagnetic semiconductors for low temperature devices: The rare earth nitrides

magnetic semiconductor dft memory nitride band structure


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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.

poster

The Pt/Hf-based hydroquinone (HQ)/Al2O3/TiN/Si bilayer hybrid memory was prepared by molecular layer deposition (MLD)/atomic layer deposition (ALD). The synthesis of Polymerized hybrid Hf-HQ film is demonstrated by XPS and IR. Free of electroforming process, a typical bipolar resistive switching characteristic has been confirmed with low SET/RESET voltages of -0.85 V/+1.87 V. 8 distinguishable resistance states were obtained, allowing the multi-binary data storage. Long retention time (>104 s) and good switching endurance (2500 times) with ON/OFF ratio around 70 were achieved. The I-V curve of hybrid memory units follows space-charge-limited-current theory, indicating the RS mechanism of conducting filaments. The polymer-like structure of hybrid Hf-HQ can improve the intrinsic flexibility of devices. On the basis, we’ll explore the application of polymerized hybrid Hf-based hydroquinone/Al2O3 bilayer structure in flexible RRAM devices.


Polymerized hybrid Hf-based hydroquinone /Al2O3 bilayer structure by molecular/atomic layer deposition for non-volatile resistive random access memory

Materials that demonstrate high spin-orbit torque are central to the functionality of next-generation spintronic memory and logic technologies. A promising class of materials for this purpose are topological semimetals. While first experimental results have shown the high spin hall angle of Weyl semimetals such as WTe2, these materials are not readily CMOS-compatible and cannot be easily integrated into existing industry-standard cleanroom processes on a 300 mm wafer level. Cobalt silicides are transition metal silicides that are already well-established in the semiconductor industry. Further applications have opened up with the recent identification of cobalt monosilicide (CoSi) as a chiral topological semimetal.

Here, we present material properties of CoSi thin films fabricated on 300 mm wafers using industry-standard cleanroom tools. Metalorganic chemical vapor deposition (MOCVD) was used to deposit Co layers of varied thickness onto wafers with an amorphous Si (a-Si) layer. The stacks were then annealed in the same tool in a vacuum atmosphere. We were able to control the thickness of the fabricated silicide films by adjusting the thickness of the a-Si and deposited Co layers. Relative to the a-Si thickness, the Co thickness was adjusted to obtain silicide films with a 1:1 Co:Si stoichiometry on a nanometer scale. The sensitivity of the composition of the formed silicide to the initial Co to a-Si ratio was investigated using GIXRD. The resistivity of the silicide films was also investigated.

Therefore, we speculate that the CoSi film may possess topological properties for efficient charge-to-spin conversion for use in low-power spintronic applications. This methodology paves the way for fabricating large-scale silicide films on a CMOS platform for spintronic applications.

In vacuo fabrication of thin-film cobalt monosilicide (CoSi) on 300 mm wafer level

ULTRARAM™ is an emerging III-V universal memory that exploits quantum resonant tunnelling to achieve non-volatility and fast, ultralow-low-energy switching per unit area. In this poster, we present characteristics of memories grown on Si substrates which match or exceed recent results for devices grown on GaAs substrates, revealing a path to ULTRARAM™’s commercial viability.

III-V non-volatile ULTRARAM™ 2x2 memory arrays

Temperature-dependent current-voltage (I-V) characteristics of crossbar type Pt/NiO/Pt devices with different bottom electrode thickness were measured at
cryogenic temperatures. Unipolar resistive switching occurs at a lower temperature as the thickness of the bottom electrode is reduced. The results provide important insight into the role of the bottom electrode thickness on the creation of thermally stable conductive filaments.

Thermal cycling driven changes of the resistive switching in Pt/NiOx/Pt crossbar arrays

Ferroelectric materials are used in building non-volatile memories due to the existence of spontaneous electric polarization of which direction could be reversed by applying an electric field. This functionality represents a classic application of these materials as a binary system, but new challenges are brought by the evolution of nowadays computation needs (as increased speed or increased amount of stored data). In this way, the new paradigms of computing appeared and they address all classes of materials used in these technologies. 
In this study, we experimentally prove that multilayered ferroelectric structures present a different switching behavior than simple ferroelectric thin films. By using alternating layers of ferroelectric and isolator materials architectures we can have multiple step-like polarization switching. This behavior could be used to separately access more than 2 polarization states, which presents good time stability. In this way, we could obtain 2n different states, where n is the number of isolator layers. Experimentally, we obtained structures with 4 and 8 polarization states. Moreover, these different states could be differentiated by different capacitive values. The process of measuring the capacitance is based on applying a low voltage signal and does not determine a reversing/changing of polarization, being in this way a non-destructive reading method. The way the system could pass by different polarization states enables the algorithm to be performed for simple binary computation functions such as OR/NOR and AND/NAND. This proves that we can have data processing and storing the result on the same device, which represents a parallel computing system based on a ferroelectric memcapacitor. The current circuit architecture based on classic logic gates can be simplified and optimized using a multilayered ferroelectric capacitor, leading also to significantly reduced power consumption and reduced cost of manufacturing.


Ferroelectric based multilayered thin films structures for multi-bit memory and memcomputing

One of the most promising contenders for ultralow-energy electronic devices is resistive switching memory (RRAM). It delivers sustainably-scalable ‘neuromorphic’ computing, potentially capable of reducing energy consumption in IT by > 50% [1]. Currently, a poor understanding of the nanoscale kinetics of the switching mechanisms is preventing its commercial utilization. RRAMs are mostly studied by electron microscopy, which is destructive, invasive, and under drastically different conditions, so is not sufficient for finding true understandings [2]. 

We develop a novel non-destructive fully-optical technique to observe in-situ and in real-time switching in memristive materials under ambient conditions, otherwise invisible to conventional electron microscopy techniques [3-4]. The formation of conductive paths in a memristive material is captured within a tightly-confined plasmonic hotspot, which greatly enhances optical signals coming from a hotspot only [4]. This allows the optical measurements to be intimately linked to the electrical and material properties of the switching channel, hence accessing memristive dynamics directly. So far, we have characterized memristive switching in RRAMS based on SiO2 (electrochemical metallization mechanism (ECM)) [3] and SrTiO3 (valence change mechanism (VCM)) [4] .

For SiO2-based ECM cells, we consistently observed a change of the optical spectra only upon switching, which we correlated with SET and RESET cycles inside the memristive material in a plasmonic hotspot. This resulted in the first in-situ optical observations of the nanoscale kinetics of RRAMs under ambient conditions. Our findings indicated that the filament nucleation position can differ from cycle to cycle in the SiO2 based RRAMs [3].

The investiagtions of the SrTiO3 based RRAMs have demonstrated the method’s ability to track oxygen vacancies motion and identify the breaking mechanisms in valence change mechanism (VCM) cells. Our numerical framework couples each spectral resonance peak to the drift of oxygen vacancies and to the formation of an O2 reservoir at the STO/TiO2 interface. The reservoir might be the cause of the device breakdown upon cycling [4].

Our future plans involve investigating switching in h-BN and HfO2 based memristors.


References
[1] Miller, D. A. B., (2009) IEEE Proceedings, 97, 7, 1166.
[2] Tappertzhofen, S.; Di Martino, G.; Hofmann, S., (2019) Physica Status Solidi (a), 217, 6, 190058.
[3] Di Martino, G. et al, (2016) Small, 12, 10, 1334.
[4] Di Martino, G. et al, (2020) Nature Electronics, 3, 687.

Real-Time In-Situ Optical Tracking of Memrisitive Switching

HfOx-based resistive random access memories (RRAMs) have attracted much attention for future non-volatile memory (NVM) to realize a neuromorphic system. They demonstrated successful CMOS compatibility, good stability, and low power consumption. At room temperature and low temperature (77 K) or cryogenic temperature (4 K), HfOx-based RRAM exhibited their potential for low power memory devices. However, there are several problems to be solved to apply HfOx-based RRAM for low temperatures. For operating the devices at low temperature, a vacuum environment is needed to protect from contamination. But due to oxygen scarcity in the vacuum environment, reduction of oxygen vacancies in HfOx film can be disturbed, then redox-based HfOx RRAM characteristics are degraded. Thus, the buffer layer which plays a role as an oxygen reservoir is important in a vacuum environment.
In this work, the device performance dependence on the buffer layer thickness in a vacuum environment was studied. The titanium thin film was used as a buffer layer because of its oxygen affinity and its thickness was varied from 30 nm to 150 nm to examine the effect of oxygen gettering ability in a vacuum environment. As a result, devices show a decrement in switching voltages in a vacuum environment and thicker buffer layer, because the high concentration of oxygen vacancies makes leakier HfOx film which needs less energy for switching.


Influence of buffer layer thickness on the performance of HfOx-based RRAM at vacuum environment

In this paper, pentacene-based organic floating-gate memory characteristics utilizing N-doped LaB6 metal and LaBxNy insulating layer were investigated. The pentacene-based floating-gate memory was fabricated using N-doped LaB6 metal and LaBxNy insulator stacked layer with annealing of 200oC. The memory window (MW) of 0.77 V and 10 years retention were successfully observed with low program/erase voltage and time of ±4 V/10 ms.

Pentacene-based organic floating-gate memory utilizing N-doped LaB6 metal and LaBxNy insulating layers for flexible device applications

Charge transport across the interfaces in complex oxides attracts a lot of attention because it allows the creating of novel functionalities. The movable domain walls in epitaxial BiFeO3 films possess enhanced conductivity that can be used for reading out in ferroelectric-based memories [1]. Chemical solution deposition of BiFeO3 thin films is one of the most commercially available techniques to produce large-scale low-cost coatings for application in memory devices. In this work, complementary piezoresponse force microscopy, conductive atomic force microscopy, and electron backscattered diffraction methods are used to study the local phase distribution, morphology, piezoelectric response, and leakage current of the polycrystalline BiFeO3 thin films. The relation between the polarization, strain, and conductivity in sol-gel BiFeO3 films with special emphasis on grain boundaries as natural interfaces in polycrystalline ferroelectrics is investigated [2]. It was demonstrated that an interaction between the polarization and grain boundaries at elevated temperature leads to the formation of the highly conductive grain boundaries with the electrical conductivity about two orders higher than in the bulk. At room temperature, the conductive traces of the charged defects at the position of the grain boundaries remain and do not change upon polarization switching. These collective states provide further insight into the physics of complex oxide ferroelectrics and strongly affect their practical applications because reveal an additional mechanism of the leakage current in such systems.

The research was made possible by Russian Science Foundation (Grant 19-72-10076). The equipment of the Ural Center for Shared Use “Modern nanotechnology” UrFU was used.

[1] J. Seidel et al., “Conduction at domain walls in oxide multiferroics,” Nat. Mater., vol. 8, no. 3, pp. 229–234, Mar. 2009.
[2] D. Alikin et al., “Strain-polarization coupling mechanism of enhanced conductivity at the grain boundaries in BiFeO3 thin films,” Appl. Mater. Today, vol. 20, p. 100740, Sep. 2020.

Local electromechanical properties of the polycrystalline BiFeO3 thin films: collective polarization and transport phenomena

Ultrafast and energy-efficient writing by spin-orbit torque (SOT) prevents device degradation within thermally stable magnetic random-access memory (MRAM) with stable perpendicular magnetic anisotropy (PMA) by separating the read and write paths. However, orthogonality between spin polarization and PMA impedes its application at industrial level because of non-deterministic switching. Previously proposed approaches to break this orthogonality includes structural deformation and external magnetic field control. However, all of those approaches increase the fabrication complexity, are highly sensitive to the SOT current duration and magnitude, or increase the switching energy. By exploiting the precessional nature of the field-like SOT component, we propose toggle PMA SOT-MRAM that achieves field-free and energy-efficient switching with a simple structure, and is robust to variations in the SOT current magnitude and duration. 
The proposed three-terminal structure consists of a magnetic tunnel junction (MTJ), heavy metal, and compensating ferromagnet. A unidirectional SOT current applied through the heavy metal stabilizes the MTJ free ferromagnet magnetization at an excited steady state after crossing the hard axis plane. Switching off the excitation relaxes the magnetization to the nearest easy axis, thus completing a toggle cycle. The next excitory pulse toggles the magnetization back to its initial state. The deformation-free structure provides efficient data read-out and can also be leveraged for directional writing through a simple XOR between the stored and incoming bits. Robust MRAM writing with greater than 50% tolerance to variations in the excitory SOT current magnitude has been demonstrated through micromagnetic simulation. Moreover, room temperature simulations indicate that this switching is robust to thermal noise. The proposed device therefore provides a promising solution for compact, highly-efficient, deformation-free and robust SOT-MRAM with PMA.

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Ferromagnetic semiconductors for low temperature devices: The rare earth nitrides

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Polymerized hybrid Hf-based hydroquinone /Al2O3 bilayer structure by molecular/atomic layer deposition for non-volatile resistive random access memory

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