United States

Abstract Body: Organic photovoltaics (OPVs) based on nonfullerene small molecule acceptors have been the subject of renewable energy research in the past five years. Ternary photovoltaic blends comprising a polymer donor, a nonfullerene small molecule acceptor, and a fullerene acceptor have proved to be most efficient active layers for OPV devices. However, the morphological parameters and their relations with mechanical properties and photovoltaic performance of this type of highly efficient polymer:nonfullerene:fullerene OPVs are still unknown to date. To address this question, we characterized the thin-film microstructure and mechanical behavior of the best-performing ternary OPV active layer (PM6:N3:PC71BM). Specifically, we alter the relative weight ratio of acceptors and systematically examined the composition dependence of photovoltaic performance, surface/bulk morphology, and mechanical parameters of the ternary blend films. It is shown that the addition of 20 wt% PC71BM to the acceptor component results in the highest power conversion efficiency (PCE) and crack-onset strain (COS) (Figure 1a). Most notably, we discover that it is possible to predict the elastic modulus of the ternary blend films by an extended Halpin-Tsai model (Figure 1b). This work provides simple guidelines for predicting the mechanical properties of many electronic devices based on ternary blends.

EMC 2021

Manipulation of Morphological, Mechanical and Photovoltaic Properties of Ternary Organic Photovoltaic Blends for Optimum Operation

## Dr. Nishikant (Nish) Sonwalkar: Academician, Scientist, Musician

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Dr. Sonwalkar is regarded as a leading expert in the application of computers in education. He has extensive research and development experience in hypermedia authoring, mobile learning, adaptive learning, and brain-based synaptic learning, advanced scientific visualization, database management systems, and computer simulation.

After receiving doctoral degree from MIT in the area of Molecular Dynamics of Nano-interfaces, he joined MIT as faculty in Mechanical Engineering, but his passion for technology enabled education led him become the founding director of Hypermedia Teaching Facility (Hypermedia Lab) at MIT and later he was appointed as the Principal Educational Architect of MIT.

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NISH 2021

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: 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.

Ferroelectricity of As-Deposited HZO Fabricated by Plasma-Enhanced Atomic Layer Deposition at 300°C by Inserting TiO2 Interlayers

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)

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

Abstract Body: Molecular Spintronics devices (MSD), which possess tunable spin states, are crucial for the advancement of futuristic quantum computing technology [1]. To overcome the dominant issue of reliable mass producibility for existing MSD technology, we have designed a magnetic tunnel junction based molecular spintronics device (MTJMSD). MTJMSD is fabricated by bridging paramagnetic molecules across the edges of the ferromagnetic electrodes to create spin channels. MTJMSD enables the utilization of ferromagnetic electrodes with a wide range of magnetic anisotropies. There is a knowledge gap about the role of magnetic anisotropy on the magnetic swicthing mechanism and equilibrium magnetic prperties of the MTJMSDs. Understanding the role of anisotropy is crucial to apply this technology in futuristic spin valve type memory device applications. In this study we investigated the effect of anisotropy on the nature of the hysteresis curves generated computationally using Monte Carlo Simulations (MCS). We applied the in-plane magnetic anisotropy () along the ferromagnetic electrode of a magnetic tunnel junction. The magnitude of A varied numerically from 0 to1. A=1 represented strong anisotropy of the order of interatomic Heisenberg exchange on the ferromagnetic electrodes. We also varied the thermal energy of the device from = 0.05 to 1.1. Here, = 0.1 corresponds to the operating temperature ranges from 500C to 900C with the assumption that the Curie temperature of the device lies between 5000C to 9000C. The effect of the anisotropy transformed the regular magnetic hysteresis of MTJMSD into a “wasp-waisted” -like hysteresis curve[2]. The “wasp-waisted” -like hysteresis curve is hypothesized due to the presence of multiple magnetic phases present on the ferromagnetic electrodes caused by strong magnetic anisotropy. Here, the materials possess hard and soft magnetic phases due to the higher and lower values of coercive fields. The “wasp-waisted” -like hysteresis curve was observed relatively at lower magnitudes of anisotropy at low thermal energy. The effect was observed up to = 0.8 with the high magnitude magnetic anisotropy ( 0.9). As a result, the effect of anisotropy keeps high magnetization of the device close to Curie temperature even when all the coupling effects get nullified due to thermal agitation. This research is supported by National Science Foundation-CREST Award (Contract # HRD- 1914751), Department of Energy/ National Nuclear Security Agency (DE-FOA-0003945). References [1] A. R. Rocha, V. M. García-suárez, S. W. Bailey, C. J. Lambert, J. Ferrer, and S. Sanvito, "Towards molecular spintronics," Nature Materials, vol. 4, no. 4, pp. 335-339, 2005/04/01 2005, doi: 10.1038/nmat1349. [2] T. M. de Lima Alves et al., "Wasp-waisted behavior in magnetic hysteresis curves of CoFe 2 O 4 nanopowder at a low temperature: Experimental evidence and theoretical approach," RSC advances, vol. 7, no. 36, pp. 22187-22196, 2017.

Impact of Anisotropy on the Magnetic Hysteresis of Magnetic Tunnel Junction Based Molecular Spintronics Device (MTJMSD) Properties

Abstract Body: A two-dimensional, solid-state, non-volatile memory has been demonstrated using electric double layer (EDL) gating of two-dimensional (2D) field effect transistors (FETs) with nanosecond scale switching speeds. In EDL gating, oppositely charged ions are accumulated near the 2D semiconductor surface, which induce image charges in the channel. This approach yields large capacitance densities corresponding to sheet carrier densities on the order of 1013 -1014 cm-2. The monolayer electrolyte random access memory (i.e., MERAM) consists of cobalt crown ether phthalocyanine (CoCrPc) and lithium ions deposited as a monolayer on the surface a 2D semiconductor (WSe2) by drop casting and annealing. CoCrPc is an electrically insulating (band gap ~ 1.34 eV) material with four crown ethers and it lays flat on the surface of a 2D semiconductor. Each crown solvates a Li+ ion that can diffuse up or down through the crown ether cavity. In response to an applied electric field, Li+ ions diffuse through the crown ether cavity and are positioned either near or away from the channel resulting in on and off states, respectively. We have previously shown switching behavior on 2D FETs using this material with potential application in non-volatile memory. Achieving well-ordered monolayers of CoCrPc and complete solvation of Li+ into the crown ethers are critical for device operation because the location and orientation of the molecules governs the device mechanism. When the well-ordered electrolyte layer deposited on a WSe2 FET is capped with h-BN, the off-state is stabilized (as confirmed by density functional theory (DFT)) by providing an energetically favorable state to Li+ ions. We have demonstrated the bistability using back-gated pulsed voltage testing with pulse width of 1 sec (± 30 V). Switching speeds down to 50 nanoseconds (limit of the instrument) are demonstrated with an on-off current ratio > 103. Further, we performed the endurance test on the device and demonstrated the stability of the device for 1000 cycles (max. measured) with 50 ns pulse. The on-off current ratio was maintained (> 103) even after 1000 switching cycles. Current work includes quantifying performance metrics for several devices and top-gating these devices to decrease program/erase voltages.

High-Speed Two-Dimensional Solid-State Non-Volatile Memory Based on Electric Double Layer Gating Using a Monolayer Electrolyte

Abstract Body: Nanowire light-emitting diodes (LEDs) have long been investigated as alternatives to conventional mesa LEDs, as they allow for increased light extraction efficiency (LEE) and a reduction of the internal electric field strength in AlInGaN alloys. Nanowires also show potential for use as single pixels in micro-LED (µLED) displays and photonic crystal surface-emitting laser diodes (PCSELs). While epitaxially grown nanowires allow for higher material quality in the active region and reduction of the quantum confined Stark effect, they often suffer from uniformity and yield issues, and are difficult to fabricate into ordered, addressable arrays. Nanowires can alternatively be formed through dry-etching of epitaxially grown light emitting layers, taking advantage of processes and tools developed by the semiconductor industry. The general process for fabricating a nanowire array LED involves dry-etching of an n-i-p epitaxial stack to form high aspect ratio structures, followed by deposition of the n-contact metal, application of an interlayer dielectric, and deposition of the p-contact material. As most LEDs emitting in the visible range are based on the AlInGaN material system, an aggressive Cl2 plasma must be used to perform the dry-etch. The poor selectivity of Cl2 plasmas against conventional dry-etch hardmask materials such as photoresist and SiO2 necessitates the use of a metal hardmask such as Ni, which is capable of withstanding the extended etch times required to form nanowires with heights greater than 1 µm. This thick Ni hardmask must be removed from the nanowires in order to maximize LEE. In devices which use self-masked n-metal deposition, in which the nanowires themselves act at the mask for deposition of a universal n-contact, wet etching of the Ni hardmask following deposition of the n-metal (which itself often contains Ni) is not possible, because the chemistries commonly used to etch Ni will also attack the other metals in the n-metal stack (Ti, Al, Ni, etc.). For this reason, an alternative method of removing the Ni hardmask is desired. In this work, we present a novel method for removing a metal hardmask from the tops of dry-etched GaN nanowires which does not involve metal etch chemistry, but instead relies upon an underlying “liftoff” layer to separate a thick metal “cap” from the tops of the nanowires. A 300 nm layer of SiO2 is deposited on the wafer, followed by patterning of the Ni hardmask through a liftoff process. The Ni is then used to mask a CF4/CHF3 dry-etch of the underlying SiO2, and a subsequent Cl2/Ar dry-etch of the GaN, with ~200 nm of Ni capable of withstanding etch depths of more than 7 µm. The nanowires are then etched in a KOH solution to remove dry-etch damage and smooth the sidewalls of the nanowires. A universal n-metal (Ti/Al/Ni/Ag) contact, masked by the nanowires themselves, is then deposited and annealed. A buffered oxide etch (BOE) solution is then used to remove the SiO2 layer, releasing the thick metal caps (Ni/Ti/Al/Ni/Ag) which are washed away in the etch solution. Polydimethylsiloxane (PDMS) is then coated over the “bare” nanowires and etch-back to expose the p-GaN nanowire tips. A transparent top contact, either ITO or thin (<20 nm) Ni is then deposited, making contact to the tops of the nanowires and linking many of them in series, allowing for electrical excitation of arbitrarily large arrays simultaneously. Using this process, we have fabricated high brightness arrays of 2.5 µm diameter electrically driven nanowires emitting at 550 nm with a linewidth of ~40 nm. Removal of the metal cap from the nanowires allows for increased LEE from extremely uniform arrays, while the use of a universal n-metal contact allows for decreased series resistance and more uniform emission intensity from larger arrays. This novel fabrication approach can allow for more efficient top-down fabricated light-emitting nanowire arrays, which are of interest for use in LEDs, µLED displays, and PCSELs.

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