United States

Abstract Body: Spin Hall materials provide an energy efficient way to flip the orientation of a neighbouring magnetic layer and this ability could form the basis of future low power, high endurance MRAM devices. A wide array of computational techniques has been developed to investigate and optimize the different intrinsic and extrinsic aspects of the spin Hall effect (SHE) in materials. In this study, we use the screened Korringa-Kohn-Rostocker (KKR) methods[1] along with the linearized Boltzmann Transport equation, Quantum Espresso[2] and PAOFLOW[3] to reveal how dilute N or O impurities affect both the intrinsic and extrinsic SHE in bcc and β phase tungsten[4]. Our results, when examined in combination, suggest that the formation of grain boundaries with the doped system plays an important role in amplifying the SHE in β tungsten. Beyond this particular system, further techniques such as the coherent potential approximation enables us to treat not only dilute but concentrated alloys as well. With this expanded tool set, an even wider array of other spintronic materials, such as Pt based alloys will be explored. [1] L. Peti et al, Philos. Mag. B 78, 449 (1998) [2] P. Gianozzi et al, J. Phys.: Condens. Matt. 21, 395502 (2009) [3] M. B. Nardelli et al, Comp. Mat. Sci. 143, 462-472 (2018) [4] O. L. W. McHugh, M. Gradhand, W. F. Goh and D. A. Stewart, Phys. Rev. Mat. 4, 094404 (2020)

EMC 2021

Tackling the Total Spin Hall Conductivity—A Multi-Code Approach for Doped and Alloyed Materials


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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.

Materials Challenges for Memory

technical paper

The Electronic Materials Conference (EMC) is the premier annual forum on the preparation, characterization and use of electronic materials. This year’s event will feature a plenary session, topical sessions, a poster session and more.

EMC is coordinated with the Device Research Conference (DRC), which will be held during the same week, from June 20-23. This recognizes the strong interaction between device and electronic materials research and provides fruitful exchange of information between attendees of both Conferences.

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Abstract Body: Light–matter interactions that induce charge and energy transfer across interfaces form the foundation for photocatalysis, energy harvesting and photodetection, among other technologies. One of the most common mechanisms associated with these processes relies on carrier injection. However, the exact role of the energy transport associated with this hot-electron injection remains unclear. Plasmon-assisted photocatalytic efficiencies can improve when intermediate insulation layers are used to inhibit the charge transfer or when off-resonance excitations are employed, which suggests that additional energy transport and thermal effects could play an explicit role even if the charge transfer is inhibited. This provides an additional interfacial mechanism for the catalytic and plasmonic enhancement at interfaces that moves beyond the traditionally assumed physical charge injection. In this work, we report on a series of ultrafast plasmonic measurements that provide a direct measure of electronic distributions, both spatially and temporally, after the optical excitation of a metal/semiconductor heterostructure. We explicitly demonstrate that in cases of strong non-equilibrium, a novel energy transduction mechanism arises at the metal/semiconductor interface. We find that hot electrons in the metal contact transfer their energy to pre-existing free electrons in the semiconductor, without an equivalent spatiotemporal transfer of charge. Further, we demonstrate that this ballistic thermal injection mechanism can be utilized as a unique means to modulate plasmonic interactions. These experimental results are well-supported by both rigorous multilayer optical modelling and first-principle ab initio calculations.

Long-Lived Modulation of Plasmonic Absorption by Ballistic Thermal Injection

Abstract Body: Surface plasmon polariton (SPP), which result from the strong coupling of bound electromagnetic (EM) waves and collective charge oscillations, enable subwavelength manipulation of light and matter interaction. Conventional noble metals-based SPP excited in the visible and near-Infrared spectra ranges suffer from large energy losses and cannot be dynamically tuned. Thanks to its 2D nature, graphene has emerged as a promising alternative plasmonic material with well- confined SPPs in the mid-infrared (MIR) and terahertz (THz) ranges and remarkable tunability. On the other hand, polar dielectric materials support low optical losses and sub-wavelength mode in the MIR and THz regions via surface phonon polariton (SPhP)mode' stimulation within the Reststrahlen band, a narrow spectral range between transverse optical (TO) and longitudinal optical (LO) frequencies. Due to the excellent polaritonic responses in both graphene and silicon carbide in the MIR and THz regions, devices combining the two materials are inferred to greatly advance photonics and THz technologies. We have recently theoretically demonstrated the strong confinement and large propagation figures of merit (FOM) for hybrid surface plasmon and surface phonon polariton in the epitaxial graphene on silicon carbide. The ability to confine such MIR light into small volume through excitation of SPP and SPhP has many implications for optoelectronic technologies such as the MIR photodetection[2], sensing[3], and solar cells[4]. Epitaxial graphene(EG) grown on silicon carbide(SiC) on silicon(Si) has been investigated as a suitable platform for graphene growth on semiconductors to meet the prospective technologies [5]. Despite this platform being very advantageous, experimental and theoretical works demonstrating the potentials properties/applications of EG on SiC on Si are still missing. Here we combine electromagnetic simulations based on finite elements method (FEM) with attenuated total reflectance Fourier transformed infrared (ATR-FTIR) spectroscopy to indicate improved MIR absorption, E field enhancement, and tunability for graphene-coated SiC nanowires (NWs). As anticipated, low absorption was confirmed from both simulations and measurements on bare SiC NWs, further supported by the calculated weak electric field due to the thin size of SiC NWs being incapable of absorbing sufficient light. On the other hand, enhanced absorption for the SiC NWs was measured by ATR-FTIR after a conformal graphitization of NWs[6]. Our previous study of this material system revealed a low-density oxide layer formed between graphene and SiC NWs[6]. This oxide layer between EG and SiC NW has been implemented in the FEM model, leading to field enhancement of incident EM field and strongly confined in the thin oxide. This substantial absorption and field enhancement originates from the coupling between the incident MIR photon and the plasmon/ phonon in graphene and SiC, respectively, with the oxide layer acting as a coupling medium. Furthermore, we demonstrate the modes' tunability whereby tuning the graphene's Fermi energy, the modes' resonance frequencies can be shifted to ~ 100 cm-1. Lastly, we realize substantial field enhancement, which is achieved by carefully tuning the graphene's Fermi energy and adjusting the spectra position of mode 1(mode on the left of LO frequency) closer to TO frequency. These results hold promise for various photonic applications such as perfect absorber to improve solar cell, MIR photodetectors, and other tunable photonics. [1]P. Rufangura, et al., J. Phys. Mater., vol. 3, no. 3, p. 032005, 2020. [2]T. Low and P. Avouris, ACS nano, vol. 8, no. 2, pp. 1086-1101, 2014. [3]A. G. Brolo, Nat. Photon., vol. 6, no. 11, p. 709, 2012. [4]H. A. Atwater and A. Polman, Nat. Mater., vol. 9, no. 3, p. 205, 2010. [5] N. Mishra et al., Phys. Status Solidi (a), vol. 213, no. 9, pp. 2277-2289, 2016. [6]N. Mishra et al., J. Appl. Phys., vol. 126, no. 6, p. 065304, 2019.

Enhanced Absorption with Core/Shell Silicon Carbide/Graphene Nanowires for Tunable Mid-Infrared Nanophotonics

Abstract Body: Phonon polaritons are collective oscillations resulting from the coupling of photons with optical phonons in polar materials and are supported within a material-specific spectral region called the reststrahlen band, which is bounded by the transverse and longitudinal optical phonons. In this region, the material behaves optically like a metal; it is highly reflective and has a negative real part of the permittivity. When polar materials are nanostructured, phonon polaritons can enable a variety of near-field optical effects such as sub-diffraction light confinement. Polar materials that support phonon polaritons can also have anisotropic optical properties, such that the optical constants depend on the propagation direction and polarization of the incoming light. When the principal components of the permittivity tensor have opposite signs, the material is referred to as hyperbolic. Hyperbolic materials are somewhat unusual as they behave optically like a dielectric and metal along different crystal axes, and this anisotropy of the metal-like response has important implications for the physics of its supported modes. Here, we report on the first experimental observation of hyperbolic phonon polaritons (HPs) in calcite nanopillar arrays, demonstrate the aspect ratio dependence of the HP resonance frequencies, discuss fabrication challenges, observe a new, possibly higher order mode as the pitch is reduced, and compare our results to both numerical simulations and an analytical model. We also found that the simulated electric field distributions for the HP modes are not localized to the pillars but extend appreciably into the calcite substrate. In addition, we have fabricated nanohole arrays in calcite, which also support HPs. However, the HP resonances appear to exhibit a slight aspect ratio dependence but opposite in trend to what was observed for the nanopillars. Additionally, the nanohole arrays have fewer HP modes and these modes are blueshifted in comparison to the nanopillars. We are currently using finite difference time domain simulations to characterize the HP modes supported in the nanohole arrays and to understand the differences between these modes and the HP modes confined within the nanopillars. Calcite is an ideal low-loss material for studying HPs that could find applications in mid-IR nanophotonic devices, and so these results are an important step toward creating a library of materials with the appropriate phonon properties to span the infrared.

Hyperbolic Phonon Polaritons in Calcite Nanopillar and Nanohole Arrays for Nanoscale Infrared Confinement

Abstract Body: Topological photonic or plasmonic systems have recently gained significant interest for their unique optical properties and potential applications in unidirectional waveguides, field enhancement, and lasing from topologically protected states. One of the simpler models available to investigate topological systems is a 1-D diatomic staggered chain of period Δ, the Su-Schrieffer-Heeger (SSH) model where adjacent resonant particles are spaced by βΔ/2 & (2-β)Δ/2, (β/2 ∈ (0,1)). In the commonly studied quasi-static (QS) limit, radiative damping is typically neglected and only the nearest neighbor effects are considered. Such an approach is reasonably accurate when the size of an individual particle is much smaller than either of the inter-particle spacings. However, as we approach experimental sizes, where the particle radii are on the order of the interparticle gap, radiative damping and long-range effects can become too significant to neglect[1]. In this work we investigate sub-wavelength resonators leveraging localized surface phonon resonances (LSPhR), taking into account radiative damping and long-range effects. We design, fabricate and characterize topological phononic chains consisting of aluminum nitride (AlN) pillars on a molybdenum (Mo) ground plane which support edge modes in the long-wave infrared regime. AlN has a negative permittivity in the spectral range bridging AlN longitudinal optical (LO) and transverse optical (TO) phonon energies (AlN Reststrahlen band). In this spectral range, AlN can support propagating and/or localized surface phonon modes at the interface between AlN and a dielectric medium. In the QS approach, an infinite staggered chain of particles is assumed, with periodic boundary conditions. Using the nearest neighbor model, one solves the 2×2 Bloch Hamiltonian matrix to obtain the dispersion relation for the chain. The edge mode, however, cannot be observed for such a case (infinite chain assumption). In order to do so, we must apply an open boundary condition, where each particle has a unique polarization due to its interaction with all the other particles in the chain, including those at the end of the chain. Therefore, for an N-particles chain, we obtain an N×N matrix. The solution for this system gives the dispersion relation, or the allowed frequencies of the finite chain for a given Δ, r & β. One observes that set of allowed frequencies, and the phononic band gap, are the same for β & (2 – β), however, presence of the edge modes is only observed (in the band gap) for β >1. The fabricated sample consists of 1.2μm thick AlN over 200nm of Mo on a silicon substrate. The topological chain of AlN pillars of radius, r = 1.5μm, Δ = 20μm is etched using a standard photo-lithography & ICP RIE process. Similarly, an isolated 3μm wide strip of Mo is etched under the AlN pillars on the edge of the sample, to be used for thermal excitation of the topological chain. The reflectance spectrum of the pillars is measured using a Bruker V80v Fourier transform infrared (FTIR) spectrometer. The collected spectra are compared to RCWA-simulated reflection spectra, as well as reflection from a bulk AlN sample. While the bulk sample shows the absorption feature at λ ≈ 11.2μm corresponding to the AlN Berreman mode[2], the pillar spectra show coupling to LSPhR modes at λ ≈ 12.1μm, in excellent agreement with the plasma frequency calculated for the pillars and our numerical simulations. Future work will look to collect the edge mode emission from one end of chain by thermal excitation of the chain's opposite end. Authors AK and DW gratefully acknowledge Air Force award #FA8075-18-D-0002 via Macauley-Brown Inc subcontract. Authors MSA and JWA are thankful for the funding support through AFOSR Lab Task 20RWCOR090 and the AFRL Munition Directorate’s Chief Scientist Office. 1. S. R. Pocock et al. ...V. Giannini, ACS Photonics, 5(6):2271–2279, 2018. 2. L. Nordin et al. ... D. Wasserman.Applied Physics Letters, 111(9):091105, 2017.

Long-Wave Infrared Topological Phonon Chain

Abstract Body: The rocksalt-structure semiconductor ScN is a relatively unexplored group IIIB transition metal nitride with outstanding physical properties that, combined with its III-nitride counterparts, has the potential to dramatically expand the range of their electrical, optical, and thermoelectric device applications. Recent works have focused on the degenerate nature of ScN by substitutional impurities ON and FN, which shift the direct (X–X) gap transition to higher energies via the Burstein–Moss effect. Here, we used cathodoluminescence spectroscopy (CLS) to observe optical signatures of both the midgap VN precursor to ON doping as well as signatures above the direct (X–X) bandgap corresponding to band-to-band transitions from four separate conduction bands near the Γ point with the valence band minimum, in agreement with calculated band structure diagrams. Thin film ScN grown by reactive magnetron sputtering displays mild degenerate doping by substitutional oxygen as indicated by elevated (X–X) transition energies and the presence of Sc–O bonding determined via x-ray photoelectron spectroscopy (XPS), while ScN grown by physical vapor transport exhibited the intrinsic, non-degenerate (X–X) bandgap predicted by theory. CLS reveals a sharp, sub-bandgap emission at 1.26 eV for sputter grown ScN on GaN, which we attribute to nitrogen vacancies (VN) based on surface sensitive CLS and XPS chemical trends. This finding is in strong agreement with theoretical calculations for VN predicting the formation of a defect energy level within the gap. Since O substitution into VN defects creates the shallow donor level responsible for degenerate doping, optical identification of this doping precursor can help understand the growth and processing that influence ScN doping. L.J.B., M.S.H., and S.S. acknowledge the support from the National Science Foundation, Grant No. DMR 18-00130. A.H. and J.H.E. acknowledge the support for ScN crystal growth from the National Science Foundation under Grant No. DMR-1508172. This research was also supported by the Air Force Office of Scientific Research through Project No. FA9550-RY17COR490 (J.S.C. and A.N.R.).

Cathodoluminescence and X-Ray Photoelectron Spectroscopy of ScN: Dopant, Defects and Band Structure

Abstract Body: The AlN wurtzite structure retains piezoelectricity to at least 1000oC and AlN films can be grown using a wide range of techniques, making AlN an attractive high temperature piezoelectric material for harsh environment microwave acoustic sensor applications. Several theoretical predictions and experimental thin film growth studies reported in the literature have indicated that the addition of ScN into the AlN wurtzite lattice to form a ScxAl1-xN alloy can increase the value of the piezoelectric coupling coefficient, d33. The most common method that has been used for ScxAl1-xN film synthesis is RF magnetron sputtering from a fixed target composition which allows uniform film growth over large wafer areas. In this work, a plasma-assisted epitaxial growth method has been used to synthesize highly oriented (0002) ScxAl1-xN films on pre-nitrided c-sapphire substrates over a range of stoichiometries from x = 7 at.% to 55 at.% by co-evaporating Sc and Al high purity metal sources in the presence of a nitrogen plasma generated by an RF plasma source within a highly controlled ultra-high vacuum environment. This plasma-assisted epitaxy approach allows ScxAl1-xN films to be produced with the desired highly oriented (0002) grains needed for piezoelectric activity using precisely controlled Sc, Al, and N-plasma fluxes, and the method easily allows synthesis of many ScxAl1-xN film compositions. ScxAl1-xN film stoichiometries are dictated by the Sc and Al arrival rates on the 930oC growth surface during the plasma epitaxy process and were verified by in situ X-ray photoelectron spectroscopy analysis immediately after growth. X-ray diffraction pole figures and in situ RHEED indicate that the desired highly oriented ScAlN (0002) grains epitaxially nucleate on prenitrided c-sapphire substrates for Sc concentrations below 20 at.%. At Sc concentrations above 20 at.% the (0002) film texture becomes less pronounced, and above ~45 at.% the ScAlN (0002) wurtzite structure becomes unstable and non-wurtzite phases are produced. The piezoelectric activity of the ScxAl1-xN films was evaluated by measuring the response of one-port surface acoustic wave resonators (SAWRs) that were fabricated on top of the ScxAl1-xN epitaxial films with Pt-Ni interdigitated transducer electrodes patterned by a photolithographic lift-off process. To successfully fabricate the SAWRs, a 10nm thick PE-CVD SixNy layer was needed to prevent etching of the ScAlN film by the photoresist developer. The degree of piezoelectric coupling was inferred by measuring the reflection coefficient (|S11|) response of one-port resonators and calculating the respective conductance and susceptance values for the SAWR devices. In addition to the quality of (0002) epitaxy and grain mosaic spread, other ScAlN film characteristics affect the amount of piezoelectric coupling including the Sc concentration, oxygen impurity content, and other grain distributions and defects within the films, as evidenced by high resolution x-ray diffraction, RHEED, and x-ray photoelectron spectroscopy data.

Plasma-Assisted Epitaxy of Piezoelectric ScxAl1-xN Films for Use in Harsh Environment Microwave Acoustic Sensors

Abstract Body: Alloys made of III-nitrides such as GaN, AlN, InN have been and continue to be of large technological and economic importance. They have found use in devices such as light-emitting diodes and High Electron Mobility Transistors. However, lattice mismatches limit the performance of III-nitride-based devices. One proposed solution that has received interest in recent years is the incorporation of ScN to enhance optical and electronic properties. In particular, scandium aluminum nitride (ScxAl1-xN) has been demonstrated to exhibit spontaneous polarization, high piezoelectricity, a wide bandgap, and an excellent lattice match to GaN. These properties have been demonstrated to be dependent on the Sc content and growth parameters such as temperature, pressure, and sputtering mode. ScxAl1-xN-based devices are expected to find use in applications such as filters and resonators, microelectromechanical systems and transistors, among others. This work reports the characterization of ScxAl1-xN films deposited on oxidized silicon substrates by a pulsed DC magnetron sputtering technique. The Sc and Al sources used were 99.999% pure Sc and 99.9995% pure Al targets. Sample composition and crystalline orientation were investigated using X-ray photoelectron spectroscopy and X-ray diffraction. Investigation on surface roughness was conducted using atomic force microscopy. Our results contribute to the understanding and development of ScxAl1-xN synthesis and characterization for the next generation of electronic devices and sensors.

Composition and Electrical Characterization of ScxAl1-xN Grown on Si Substrate by DC Sputtering

Abstract Body: The rapid increase in data quantity and processing necessitate novel approaches to computing. Digital computing based on von Neumann architectures may not be optimal for data-intensive operations like machine learning due to the high energy costs of moving data between processor and memory. Neuromorphic computing that combines logic and memory functionality into a single analogue nonvolatile memory element is a promising energy-efficient solution that decreases the energy consumption by orders of magnitude. Filament-based phase-change memory and resistive random access memory are the most well-investigated approaches to neuromorphic computing, but these memories are stochastic due to the unreliability of the nanosized filaments used to store information. In this work, we develop the “bulk” resistive random access memory that stores information using the oxygen vacancy concentration in the 3D bulk. This cell contains a solid electrolyte sandwiched between two mixed ionic and electronic conductors (MIECs). The ion-conducting solid electrolyte blocks electrons and prevents the positive feedback that results in filament formation. Instead, the oxygen vacancy ions are uniformly distributed in the absence of filaments and thermal gradients. To switch the cell, electrochemical voltage pulses (+/-1.5V, 2 μs) shuttle vacancies between the MIECs; the concentration of the vacancies in one MIEC stores the analogue resistive state. This enables exceptionally reliable, linear, and deterministic switching among hundreds of analogue information states, while utilizing CMOS-compatible materials with long endurance and information retention. Atomic force microscopy and physical modeling confirms the absence of filaments. Three types of solid electrolytes are utilized: a thick model single-crystal yttrium-stabilized zirconia (YSZ), a thin-film YSZ grown by sputtering, and ultra-thin-film electrolytes of alumina, hafnia, and zirconia grown by atomic layer deposition. The oxygen vacancy ionic conductivities of these materials play a crucial role in the switching and retention times. The broad classes of oxygen conductors provide significant opportunities for future materials research towards next-generation analogue nonvolatile memory. Reference: “Filament-free bulk resistive memory enables deterministic analogue switching,” Adv Mater, 32, 2003984 (2020)

Deterministic Analogue Resistive Memory for Neuromorphic Computing Based on Transition Metal Oxides

Abstract Body: Recently, research on wearable intelligent electronics that detects, remembers, and learns external stimuli in real-time is receiving great interest. And a tactile synaptic device studies have emerged because they have the potential for human-interactive neuromorphic applications capable of large-scale parallel processing. However, the artificial tactile synaptic devices reported so far have complex physical connections between the sensor unit and the memory unit, which are inevitably not suitable for the wearable and patchable device due to the complex and costly manufacturing steps. Here, we demonstrate an artificial organic tactile synaptic device based on the integrated single device that enables the sensing, storing, and learning of a variety of tactile information. The synaptic device is able to be programmed with various tactile input pressures, by using a ferroelectric field-effect transistor structure with a pressure-sensitive ball-shaped top gate electrode. The synaptic device reliably and stably operates with high tactile reception sensitivity of 88 KPa-1 under bending conditions. And it was confirmed that synaptic plasticity was stably implemented for 10,000 inputs by various electrical/tactile stimuli, which allows for precise and robust tactile perception learning. Furthermore, we demonstrated that an integrated 4 x 4 tactile synaptic array allows for 2-dimensional tactile learning and proof-of-concept recognition simulations for diverse handwriting patterns with an outstanding error tolerance. As a result, our study proposes the novel platform for a single, integrated tactile neuromorphic system, which can simultaneously sense and learn a variety of external information.

Development of a Single Device-Based Organic Tactile Synapse for Artificial Learning Skin Applications

Abstract Body: We report the observation of ferroelectricity in hafnium-zirconium-oxide thin films in the as-deposited state, namely after deposition at a low temperature of 300 °C without post-metallization annealing. The Hf0.5Zr0.5O2 (HZO) thin film was interposed between two TiO2 interlayers, and all films were produced by plasma enhanced atomic layer deposition and integrated into a TiN-based metal-insulator-metal capacitor. The ferroelectric nature of the as-deposited HZO film was evaluated by a polarization-voltage hysteresis loop and a 2Pr value of ~7.4 μC/cm2 was achieved. Grazing incidence x-ray diffraction measurements and atomic-resolution scanning transmission electron microscopy characterization revealed the co-existence of fully crystallized polar orthorhombic and monoclinic phases of the dielectric in the as-deposited sample. We concluded that the nucleation and growth of the crystalline polar non-centrosymmetric orthorhombic phases in the 10 nm HZO thin film was prompted by the available energy from the plasma and the tensile lattice mismatch strain provided by the TiO2 interlayer.

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Long-Lived Modulation of Plasmonic Absorption by Ballistic Thermal Injection

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A multi-code investigation of promising spin Hall materials

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