United States

ScAlN alloys are promising due to their high spontaneous and piezoelectric polarizations and ferroelectric properties for various technologies including high electron mobility transistors (HEMTs), acoustic devices, etc. Despite the common belief that Molecular Beam Epitaxy (MBE) synthesis requires N-rich conditions to avoid metallic phase inclusions, our previous studies1,2 have demonstrated that the metal modulated epitaxy (MME) allows growth of state-of-the-art structural quality ScAlN in metal-rich conditions at a low substrate temperature (Tsub) of 400 °C by using precise control of metal adlayer buildup. Additionally, these previous studies found an extremely unusual feature wherein, a significant catalytic effect of Sc leads to more available nitrogen and thus, ~3x faster growth rates when Sc is present in large coverages on the surface. Accounting for this critical new feature, MME allows improved crystal quality by x-ray diffraction (XRD) figures of merit and surface roughness as low as 250 arcsec and 0.73 nm, respectively. The HEMT structure in the previous studies have shown an improved sheet resistance of 152 W/■, a mobility of 700 cm2/Vs, and a sheet charge of 5.9 x 1013 cm-2 which is near the theoretical limit, exhibiting MME’s great potential for ScAlN HEMT applications. Although the optimization of III/V ratio and growth temperature was done in the previous studies1,2, the structural properties of ScAlN are not well investigated yet especially for the MME grown films which involves low-growth temperature. The present transmission electron microscopy (TEM) study contrasts the unique characteristics of MME grown Sc0.2Al0.8N at low Tsub of 400 °C, and 650 °C without metallic inclusions and 600 °C samples that showed the metallic inclusions. Significant twist is present in the 600 °C grown sample that is absent in the 400 °C sample. MME Sc0.2Al0.8N were grown on MME GaN on hydride vapor phase epitaxy (HVPE) GaN templates on c-plane sapphire substrates as described elsewhere1,2, using plasma-assisted molecular-beam-epitaxy (PA-MBE). TEM and four-dimensional scanning TEM (4D-STEM) are employed to evaluate the structural properties. The cross-sectional sample is prepared by focused ion beam (FIB) under cryogenic temperature. TEM images close to the zone axis, and for the sample with Tsub = 400 °C show better film quality than the two higher temperature samples, having distinctive contrasts of dislocations. The 600 °C sample showed complicated and dense contrasts. This fact correlates with the previous results in XRD 2θ-ω scans indicating that the 600 °C sample had inclusions of Al3Sc phases, which are not present in the other two samples. AFM also showed smoother surface roughness for the 400 °C sample other samples.1,2 Local diffraction patterns (DPs) obtained from 4D-STEM taken for the 600 °C sample shows ScAlN is twisted ~5° along c-axis from underlying GaN, which is confirmed by twisting the sample around the c-axis in the measurement. Some local DPs show additional crystalline tilt by ~6° off the c-axis. These twist and tilt features are supported by the existence Moire fringes in the bulk of the film from TEM images observed in 600 °C sample, but not in 400 °C sample. In addition, multiple DP spots outside the wurtzite DPs are found in some regions, implying the inclusion of multiple phases, e.g., possibly rock-salt or Al3Sc phases, which could be from phase separation as revealed in XRD 2θ-ω scan in the previous studies.1,2 These features in 4D-STEM are not observed in 400 °C sample. The present study helps to understand the mechanisms of better crystal quality in the epitaxially grown ScAlN under low substrate temperatures. 1Z. Engel, et. al., J. Appl. Phys., 132, 185302, 2022. 2 Z. Engel, Doctoral Dissertation, Georgia Institute of Technology, Approved on Dec. 5th, 2022.

EMC 2023

Structural Analysis and Observation of Tilts, Twists and Crystallographic Orientation of High-Quality, Metal Rich Grown Sc0.2Al0.8N

scaln

characterization

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). This recognizes the strong interaction between device and electronic materials research and provides fruitful exchange of information between attendees of both Conferences.

The Electronic Materials Conference (EMC) is the premier annual forum on the preparation, characterization and use of electronic materials. 

Manufacturing high-quality inorganic thin films on large-area flexible substrates is a key technology to utilize for flexible devices such as wearable sensors, flexible displays, and electronic devices. For the device fabrications, epitaxial thin films were grown on two-dimensional (2-D) layered materials such as graphene and h-BN. Since the 2-D materials offer weak bonding to the as-grown films, the exfoliation of epilayers from 2-D materials is easily feasible. In particular, remote epitaxy has recently been reported by molecular beam epitaxial growth of GaN thin films on the graphene buffer layer (GBL) on SiC substrates. This remote epitaxial growth method enables to grow the single-crystal thin films on graphene-coated polar substrates by maintaining the epitaxial relationship between the substrate and the epilayer because the graphene does not wholly screen the electrostatic potential of the substrate. However, metalorganic vapor-phase epitaxy (MOVPE), commercially used for manufacturing GaN devices, could not be used for the remote epitaxial growth and exfoliation of high-quality GaN thin films on graphene. During the MOVPE growth, graphene substrates are affected by H2 and NH3 ambient gases at high growth temperatures, resulting in the failure of GaN exfoliation from the graphene substrate. Here, we report a new strategy for MOVPE growth of transferable single-crystal GaN thin films on graphene/c-sapphire by using pulse operation of NH3 flow to prevent deterioration of graphene substrates during GaN growth. The basic strategy for epitaxial growth of transferable single-crystal GaN thin films on graphene/c-sapphire is to use pulse operation of NH3 flow to prevent deterioration of graphene substrates during GaN growth. In the NH3 pulsed-flow growth, the NH3 injection into the reactor was periodically interrupted while the trimethylgallium (TMGa) was continuously injected. By a controlled V/III ratio under the pulsed NH3 injection, the lateral growth of the GaN buffer islands was enhanced merging into a continuous film. By optimizing the growth parameters, including the duration and period of the pulsed ammonia flow, high-quality GaN films were grown on the graphene-coated sapphire substrates, easily lifted off, and transferred onto a foreign substrate by simple mechanical exfoliation using a tape. Furthermore, we investigated the role of the pulsed operation of ammonia flow as a critical factor for the fabrication of high-quality free-standing GaN films. The freestanding GaN films, exhibiting excellent single crystallinity and electrical and optical characteristics, were also applied for transferrable GaN LEDs by heteroepitaxially growing InxGa1-xN/GaN multiple quantum wells and p-type GaN layers on the GaN films and transferring them onto Cu foil.

Pulsed-Mode Metalorganic Vapor-Phase Epitaxial Growth of GaN on Graphene/c-Sapphire for Freestanding GaN Thin Films

A combination of high electron saturation velocity, high electron mobility, large breakdown field, and high electron density can be simultaneously achieved in GaN high electron mobility transistors (HEMTs), making them attractive for high-power, high-frequency power amplifiers. N-polar GaN epi-structures offer scaling of the gate-to-channel distance without losing 2DEG density and a built-in back-barrier for better pinch-off performance. Additionally, N-polar GaN enables the implementation of a thick in-situ GaN-cap layer, which keeps surface states away from the channel and minimizes their effect on device performance. By using N-polar HEMT epi-structure with a GaN cap (deep recess HEMT) 8 W mm−1 output power with 27.8% power added efficiency (PAE) at 94 GHz has been demonstrated, which is more than twice the output power and efficiency of conventional Ga-polar AlGaN/GaN HEMTs [1]. Despite numerous benefits, there are challenges associated with N-polar deep recess HEMTs. Dry etching used in recess etching causes surface damage in the gate area and the trench sidewalls leading to high gate leakage and low breakdown voltage. These issues are currently addressed by inserting SiNx gate dielectric deposited via metal organic chemical vapor deposition (MOCVD), a high temperature deposition technique that could be a limiting factor in device fabrication and integration. Moreover, inserting a thick dielectric layer under the gate increases the gate-to-channel distance, which is not desirable in scaled devices. Therefore, there is a need for developing a gate insulator with a high dielectric constant with low bulk and interface trap density. Moreover, a low-temperature deposition technique such as ALD allows for more flexibility in the fabrication process and is more desirable for manufacturing. Recent studies in our group have shown the successful application of ALD- HfO2 as gate dielectric in N-polar planar GaN HEMTs. [2] In this work, for the first time, a thermal-ALD HfO2 was used as the gate dielectric in N-polar deep recess GaN HEMTs. DC, pulsed IV, and small signal measurements were conducted to evaluate the device performance. Results show low gate leakage current, high transconductance, high dielectric breakdown field, and low dispersion, which indicates a high quality dielectric and dielectric/GaN interface. This work shows the potential for HfO2 as a gate insulator in N-polar deep recess HEMTs with a scaled channel. MOCVD-grown N-polar deep recess GaN HEMT structure with a 12 nm thick GaN channel and a 47.5 nm thick GaN-Cap was used for this study. The devices presented here have a 100 μm gate width, and 1.75 μm source to drain distance. The etched region length for the gate recess is 200 nm and placed 650 nm away from the source. Ti/Au gate metal has 450 nm thickness, 900 nm length, and 300 nm away from the source. DC measurements showed 0.9 A/mm maximum drain current, 2 Ohm.mm on-resistance, and 2.2 V knee voltage at 1 V gate bias. The threshold voltage was -1.5 V. The maximum transconductance of 420 mS/mm was measured at VG of -0.75 V and VD of 3 V. Gate leakage current of 140 nA/mm, and a 6.2 MV/cm dielectric breakdown field. More importantly, near zero hysteresis was observed in DC measurements. Pulsed IV measurements revealed less than 5% dispersion. Unity current gain frequency (fT) of 30.5 GHz and 53 GHz maximum oscillation frequency (fMAX) were measured. These results show the potential of using HfO2 as the gate dielectric in the future ultra-scaled N-polar deep recess GaN HEMTs. [1] B. Romanczyk et al., "W-band N-polar GaN MISHEMTs with high power and record 27.8% efficiency at 94 GHz," 2016 IEEE International Electron Devices Meeting (IEDM), 2016, pp. 3.5.1-3.5.4, doi: 10.1109/IEDM.2016.7838339. [2] S. Mohanty, A. Jian, K. Khan, and E. Ahmadi, “Demonstration of N-polar GaN MIS-HEMT with high-k atomic layer deposited HfO 2 as gate dielectric,” Accepted to appear in Journal of Electronic Materials (JEMS).

N-Polar GaN Deep Recess HEMTs with ALD HfO2 as Gate Dielectric

Semiconductor polytype heterostructures, which consist of chemically homogeneous structures formed via an abrupt change in crystal structure, offer opportunities for performance exceeding those of composition-based semiconductor heterostructures. Of particular interest are heterostructures formed via an abrupt change in atomic plane stacking sequence, such as the transition from the wurtzite (WZ) polytype, with AB stacking, to the zincblende (ZB) polytype, with ABC stacking. At a fixed chemistry WZ/ZB heterojunction, the lattice mismatch and thermal expansion coefficient mismatch are typically < 0.1%, leading to a negligible interfacial defect concentration. Meanwhile, the WZ/ZB band offset and polarization discontinuity are expected to lead to the confinement of a two-dimensional electron gas (2DEG) at the interface, without the need for impurity doping and/or alloying. Depending on the type I (nested) vs. type II (staggered) band offset at the WZ/ZB polytype junction, such heterostructures would also be promising for high power electronics and single photon emitters. It has been hypothesized that metastable NW polytype selection is governed by surface/interface energies, surface diffusivities, and/or droplet angles that determine ABC vs. AB stacking of atomic planes, resulting in zincblende (ZB) or wurtzite (WZ) polytypes. For ZB-polytype-preferring materials, such as III-As and III-P, ZB vs. WZ polytype selection has been described by empirical “contact angle” models [Panciera, 2020], enabling the design and fabrication of NW polytype superlattices. However, for GaN, a WZ-polytype preferring material, the “contact angle” models for NW polytype selection, using literature values for WZ and ZB GaN surface energies, predict ZB polytype formation, across contact angles, even though WZ GaN NWs are most frequently reported in the literature [Lu, 2021]. We are developing kinetic Monte Carlo (kMC) simulations based on a general lattice framework that can be used for both WZ and ZB polytypes, In these kMC simulations, the adsorption and desorption of Ga and N atoms are determined by the Ga chemical potential and temperature, and the polytype selection for an NW segment is automatically determined by the nucleation events occurring at the NW edges. Together, these thermodynamic and kinetic approaches will be utilized to unravel the mechanisms for polytype selection during the growth of self-catalyzed GaN nanowires. Lu, H., S. Moniri, C. Reese, S. Jeon, A. Katcher, T. Hill, H. Deng, and R.S. Goldman. 2021. “Influence of gallium surface saturation on GaN nanowire polytype selection during molecular-beam epitaxy.” Appl. Phys. Lett. 119:031601. Panciera, F., Z. Baraissov, G. Patriarche, V.G. Dubrovskii, F. Glas, L. Travers, U. Mirsaidov, and J.-C.Harmand. 2020. “Phase selections in self-catalyzed GaAs nanowires.” Nano Lett. 20:1669.

Mechanisms for Polytype Selection During the Growth of Self-Catalyzed GaN Nanowires

Polar dielectric materials such as Silicon Carbide (SiC) have attracted much attention due to the ability to support low loss surface phonon polaritons (SPhPs) in the mid-infrared. As a consequence, SPhPs resonators have been reported with a high resonance quality factor, and have been leveraged for strong coupling to polar liquids. It would be highly useful to leverage SPhP modes to create surface enhancement infrared absorption (SERIA) sensors for phosphide containing molecules. However, so far there have only been a few demonstrations of surface enhancement from SPhP modes, and the results are relatively weak compared to more conventional metal SIRA sensors. This is because there is a lack of detailed models optimize the coupling between free space light, SPhPs modes, and the chemical under analysis. A framework for generalized design of SPhP resonators would solve this issue and provide the knowledge to design SEIRA sensors. Crucially, unlike in previous works, this model must be relatively simple, and quantitatively explain modal quality factors, which is missing from existing models. In the present work, we investigate the impact of geometric parameters in SPhPs resonances that are excited in periodic trenches etched into a SiC surface. Resonances for high aspect ratio grating of 4H-SiC form between the walls of the trenches, and a vertical standing waves that are comparable with resonances in an organ pipe. To model mid-infrared resonant behavior of this structure, we develop two different approaches. The simplest consists of a cavity model which considers the structure as a metal-insulator-metal waveguide, coupled to waves in free space. The second consists of a full transmission line model, which accounts for propagation of the waves in and out of the structure. For both approaches we calculate the impedance of the SPhP mode, using a new approximation which accounts for the electric field concentration inside the SiC. Our results are compared against the 3D electromagnetic solution to Maxwell equations using CST Studio Suite and we observe excellent correspondence between full wave simulations and both our models. In both analytical models, SPhPs resonance, quality factor and absorbance are in good agreement with numerical simulation and follow the same trends, which is not possible to observe using previous models in the literature. Whilst our model is only highly accurate for relatively narrow openings of organ pipe resonators, the general trends match with experimental data. Crucially, our results show the applicability of simplified cavity and transmission line models can be used to design SPhP resonators. The extension to including both SEIRA and strong coupling phenomena is relatively trivial in this domain, which will accelerate the design of SPhP based SEIRA sensors.

Critical Coupling in Phonon Polariton Organ Pipe Resonance for Infrared Sensing

Convolutional neural networks (CNNs) are a powerful tool in computer vision, commonly used for image segmentation and classification. The accuracy and scope of CNNs can generally be improved be increasing the complexity of the network, such as the number of layers, channels, or fully connected layers. However, increasing CNN complexity comes at the cost of increased power consumption and calculation time. The first layer of a CNN detects low level features, such as edges. If an optical system was designed to perform low level feature detection, that task could be offloaded from the computer, allowing for faster, lower-power consumption CNNs, without sacrificing accuracy. Optical edge detection has been explored previously, though current methods have limitations that prevent their use in many practical applications, such as requiring polarized or coherent illumination, requiring bulky 4f systems, or being limited to small apertures. We have designed and experimentally demonstrated systems which overcome these limitations, allowing for broadband optical edge detection of incoherent thermal emission with optimized signal to noise ratio. Further, UNet image segmentation networks trained using our simulated models as an optical front end demonstrated an improved Dice coefficient (from 0.877 to 0.899) with 4 times fewer FLOPs compared to a similar UNet architecture trained on raw images. Our optical edge detection system combines the design flexibility of metasurfaces with the broadband focusing capability of refractive optics. Using gradient descent based inverse design, the metasurface is designed to modify the point spread function (PSF) of the combined optic system to emulate an edge detection kernel. Edge detection kernels have both positive and negative values, which cannot normally be achieved in an incoherent PSF; this can be circumvented by splitting the PSF into positive and negative components, which produce two images that are digitally subtracted produce a single edge image. This is achieved with a single polarization sensitive metasurface which, in conjunction with a refractive lens, forms independent images corresponding to the positive and negative PSF components using two orthogonal polarization states, which can be simultaneously captured using a polarimetry enabled LWIR camera.

Large-Scale Inversely Designed Metasurfaces for Broadband LWIR Optical Edge Detection

The mid-infrared wavelength range (MIR: 3-30 µm) is of significant technological importance. Many of its applications are well-served by leveraging techniques capable of enhancing light-matter interaction. Phononic systems, which leverage excitations of the crystal lattice coupled to electromagnetic modes, offer one potential approach. The negative real permittivity of a phononic material, in the material’s Reststrahlen Band, results in an optical response similar to that of plasmonic materials, though with lower loss (small imaginary permittivity). In the MIR, strongly polar AlN, with a high-energy longitudinal optical (LO) phonon at a free-space wavelength of 11.2 µm, is an intriguing phononic system, with negative permittivity extending from LO phonon wavelength to the transverse optical (TO) phonon wavelength of 15.9 µm[1]. In this work, we investigate periodic arrays of AlN-capped nanowires supporting a variety of unique phononic modes. We demonstrate localized surface phonon resonances (LSPhR) in the AlN nanostructures, in the frequency range bounded by AlN’s LO and TO phonon energies. We also demonstrate that the nano-pillar growth regime allows for normal incidence coupling to the Berreman mode which is observed in thin films at epsilon-near-zero (ENZ) permittivity (at LO phonon energy) for light incident at large grazing angles. The nanowires are grown by plasma-assisted MBE[2] on a patterned silicon wafer, which serves as the template for GaN nanowire growth, upon which AlN caps are grown. On a silicon wafer, after a series of buffer layer growths, a 70 nm Si3N4 growth mask is deposited on the top GaN buffer layer, and the growth mask is then patterned (by e-beam lithography) and etched to provide apertures to the underlying GaN with 1 µm period and varying diameters. GaN nanowires 1.8 μm in height are grown in the apertures, followed by an AlN cap of 210 nm, above the GaN nanowires. Periodic arrays of AlN-capped GaN nanowires were grown with varying aperture dimensions, resulting in nanowires with diameters of ∼ 210, 300 and 390 nm, During the growth of the AlN cap, a thin cylindrical shell of AlN with thickness in few 10s of nms (decreasing thickness from top to bottom) is deposited on the sidewalls of the GaN nanopillar. The result of the patterned growth are subwavelength AlN phononic resonators that effectively ‘float’ above the substrate, supported by the dielectric GaN nanowire bases. We measure near normal incidence reflectance (±3.5°) of the nanowire arrays using a nitrogen-purged Bruker vertex 70 FTIR with an output beam collimated and reduced to ~ 6 mm through an external double-aperture. The reflectance data is compared to the reflection simulated with a corresponding TM-polarized 2-D model in COMSOL assuming a 20 nm thick uniform AlN shell around GaN nanopillar. In this work, we study representative data from the nanowires with diameter of 390 nm. The reflectance spectra show three prominent resonant dips. The longest wavelength features at 13.3 μm wavelength can be thought as a dipole mode at the top and bottom corner GaN/AlN interface. The next longest wavelength features at wavelengths λ = 12.3 μm corresponds to the LSPhR of the AlN caps that are confirmed via simulation of the nanowire structures. The LSPhR spectrally red-shifts with increasing nanowire diameters. Even more interesting is a sharp reflectance dip in AlN nanowires at wavelength, λ = 11.2 μm (AlN LO phonon energy). The fields at λ = 11.2 μm are calculated with 2-D COMSOL model, which indicate a strong field confinement in the AlN shell surrounding the GaN nanowire. While such a resonance would not typically be seen in a planar AlN film (or even a patterned AlN film) at normal incidence, the vertical orientation of the thin (~20 nm) AlN shell offers strong coupling to the Berreman mode. 1. L. Nordin et al. ... D. Wasserman. Applied Physics Letters, 111(9):091105, 2017. 2. M. Brubaker et al. … K. Bertness, Nanotechnology 30, 234001, 2019.

Localized Phonon Polariton Modes in GaN/AlN Nanowiresx

Dilute bismuth (Bi) incorporation in InGaBiAs is a promising material for extended short-wave infrared (e-SWIR, 1.7-2.5 μm wavelength) detection with a lattice match to InP-based materials. The InGaBiAs alloy is a highly mismatched semiconductor, where small concentrations of localized Bi states have a dramatic impact on the valence band electronic structure and the corresponding decrease in the bandgap energy. The use of InGaBiAs as an active material for e-SWIR photodetectors offers a means to overcome limitations of current approaches, including high dark current associated with relaxed InGaAs and low quantum efficiency of InGaAs/GaAsSb type-II superlattices on InP substrate. The epitaxial growth of InGaBiAs alloys has been previously demonstrated, with a bandgap energy shift of 56 meV/Bi% in InxGa1-xBiyAs1-y [1,2]. However, the details of optical absorption strength and minority carrier properties of key importance for photodetectors have not been explored in detail. In this work, the optical and electronic properties of InGaBiAs alloys and their relationship to epitaxial growth conditions will be reported. InGaBiAs materials were grown on InP at variable temperatures (265 and 490 °C) and V/III flux ratios. The growth parameters have a major influence on the tradeoff between efficient bismuth incorporation in the lattice and point defect density that influence the electronic properties of the material. The low temperatures (LT) grown samples are highly mismatched alloys with higher Bi incorporation. In the high temperatures (HT) grown samples, Bi acts as a surfactant. Optical absorption coefficient data were extracted from infrared transmission measurements, where the absorption edge was extended beyond 1.9 μm for Bi incorporation of 1.5% in comparison to an In0.53Ga0.47As reference sample while maintaining a high absorption coefficient (>104 cm-1) in the 1.5-1.85 μm spectral range. The minority carrier electronic properties have been investigated by temperature-dependent microwave-detected photoconductive decay (µ-PCD) measurements. For LT samples, µ-PCD measurements exhibit time constants less than 500 ns, corresponding to fast minority carrier recombination. Long-time constants of >100 µs are observed for the HT samples, which are attributed to carrier capture and re-emission from trap states with activation energies of 0.07 eV and 0.14 eV. The long photoconductive decay time constants are indicative of material with favorable minority carrier properties for photodetectors. Diode p-i-n junction devices incorporating InGaBiAs active layers were grown and fabricated into mesa devices using standard photolithography, chemical etching, and metallization processes. Dark current-voltage characteristics correlate with photoconductive decay measurements, where materials exhibiting long-time constants exhibit low dark current density (8.46x10-6 A/cm2) similar to InGaAs reference samples, while materials exhibiting short-time constants show high dark current density and/or significantly degraded diode rectification. Further development of InGaBiAs e-SWIR detectors will require a focus on co-optimization of bismuth incorporation and minority carrier properties, and continued exploration of optoelectronic device characterization. References: J. P. Petropoulos, Y. Zhong, and J. M. Zide, “Optical and electrical characterization of InGaBiAs for use as a mid-infrared optoelectronic material,” Applied Physics Letters, vol. 99, no. 3, p. 031110, 2011. R. Kudrawiec, J. Kopaczek, J. Misiewicz, J. P. Petropoulos, Y. Zhong, and J. M. Zide, “Contactless electroreflectance study of E0 and E0+Δso transitions in In0.53Ga0.47BixAs1−x alloys,” Applied Physics Letters, vol. 99, no. 25, p. 251906, 2011.

Prospects of Dilute Bismuth in InGaBiAs Alloys for e-SWIR Photodetectors

Diffusive memristors, also known as volatile electrochemical metallization memory cells (ECMs), are recognized as extremely interesting devices for various applications in emerging memories and neuromorphic computing areas. For non-volatile ECM cells, it has been shown that the counter-electrode (CE) material plays a crucial role in the switching process by affecting the reactions at the CE/electrolyte interface. This is due to the different electrocatalytic activity of the CE material towards reduction-oxidation reactions, which determines the metal ion concentration in the electrolyte and ultimately impacts the switching kinetics. In this work, we look at the switching behavior of diffusive memristors and how this is affected by different CE materials with a special focus on (i) the understanding of the influence of the CE material on the volatile switching process of diffusive memristors and (ii) device engineering and optimization towards industrially used and CMOS-compatible materials. To that end, we used inert Pt metal and two non-noble metals widely used in semiconductor technology, namely, W and TiN, as CE materials in Ag/HfO2/CE-based diffusive memristor cells. Rectangular voltage pulses of different amplitudes were applied and the SET times were analyzed from the transient curves. The results show that the CE material has a significant effect on the SET kinetics, with interesting differences being observed depending on the voltage regime. It is found that non-noble CE materials such as TiN and W form a native oxide either during ambient transfer or during the ALD process of the HfO2 electrolyte film. This has been confirmed by the XPS analysis of different stacks with and without the HfO2 layer. Furthermore, the formation of interfacial oxides at the CE/electrolyte interface in the case of non-noble metals is also discussed in relation to the observed differences in the SET kinetics. The findings of this work highlight the significant role of the CE material in the switching process of Ag/HfO2-based diffusive memristors and the importance of considering interfacial oxide formation in the design of these devices. This work was in part funded by the German Research Foundation (DFG) under Grant No. SFB 917, in part by the Federal Ministry of Education and Research (BMBF, Germany) in the project NEUROTEC (Project No. 16ME0398K) and NeuroSys (Project No. 03ZU1106AB) and are based on the Jülich Aachen Research Alliance (JARA-FIT).

Effect of the Counter Electrode Material on the SET Kinetics of Ag/HfO2-Based Diffusive Memristors

To overcome the von Neumann bottleneck and increase power efficiency in computation, neuromorphic computing has been considered which has the capability for parallel computing, long-term potentiation, and adaptive learning for developing energy efficient neural networks.1 Electric double layer (EDL) gated transistor uses ions within an electrolyte to induce high carrier density (1014 cm-2) in the channel, and its short term and long term ionic responses resemble a biological synapse, making it a promising platform for synaptic applications such as artificial synapses and neurons.2 The accuracy, computational demands, and simplicity of training neural networks depend significantly on a linear synaptic weight response (or plasticity), i.e. a proportionate synaptic weight update in response to input signals.3-6 However, achieving linear weight update in EDLT based synaptic devices have proven to be challenging with fixed magnitude input pulses.7-9 In this study, we first discussed the fundamental cause for nonlinear synaptic weight update in the aforementioned EDLT synaptic devices. With fixed magnitude input voltage pulses, while the amount of EDL build up during each pulse is constant, the amount of EDL dissipation in between pulses is dependent on the instantaneous EDL concentration and follows an exponential decay with respect to time. This leads to increasing amount of dissipation as pulses progress and EDL builds up nonlinearly. This behavior was confirmed with finite element modeling using modified Nernst-Poisson-Planck equations.10,11 A train of 10 pulses with frequencies ranging from 10 – 100 Hz (VG = 1 V) was applied to a parallel plate capacitor geometry consisting of PEO:LiClO4 as the electrolyte, and the ion concentration was calculated. As expected, the linear input pulses resulted in a non-linear response, where the maximum EDL concentration at 100 Hz eventually saturated at 9 x 1013 ions/cm2, more than 2 times smaller than the maximum EDL concentration at a steady 1 V (1.9 x 1014 ions/cm2). Then, we demonstrated that linear weight update can be achieved with varying input pulse widths and/or magnitudes (i.e., nonlinear inputs). A challenge for this approach is that the EDL concentration at the end of each pulse needs to be evaluated to predict the input voltage or width of the subsequent pulse. Based on the instantaneous change in EDL concentration with respect to time (dc/dt), we obtained an empirical relationship relating dc/dt, voltage, concentration, and time. Coupling this relation with the original COMSOL model, the input signals for a linear synaptic response can be predicted in a single computation. For a series of pulses with magnitude between 0.25 V and 3 V at a fixed frequency of 100 Hz, a linear synaptic weight update was obtained with R2 = 0.996. After 10 pulses the improved model resulted with a 13% increase in EDL concentration (to 1.02 x 1014 ions/cm2) and can continue to increase with subsequent pulses. These preliminary results provide insights into extending this model for an FET and developing an EDL gated synaptic network. As part of ongoing work, the group is currently working on experimentally demonstrating the model using parallel plate capacitors (metal-electrolyte-metal) and Graphene-based EDL transistors. 1. Schuman et al. arXiv abs/1705.06963, 2017; 2. Zhu et al. Adv. Mat., 2018, 30 (21); 3. Sung et al. Nanotech., 2018, 29 (11); 4. Lee at al. Adv. Fun. Mat., 2020, 30 (45); 5. Yang et al. Small Science, 2022, 2 (1); 6. Jacobs-Gedrim et al. IEEE (ICRC), 2017; 7. Jiang et al. Nanoscale, 2019, 11 (3); 8. Wan et al. Adv. Mat., 2016, 28; 9. Wen et al. ACS App. Mat. & Int., 2017, 9 (42); 10. Woeppel et al. ACS App. Mat. & Int., 2020, 12 (36); 11. Kilic et al. Phy. Rev., 2007, 75 (2)

Realizing a Linear Synaptic Weight Update in Electric-Double-Layer Gated Transistors for Achieving Spike-Timing-Dependent Plasticity in Neuromorphic Computing

A switching of resistive memory cells leads to a local accumulation of Joules heat in the device. In resistive RAM (ReRAM) arrays of memory cells, the heat generated in one cell spreads via common electrode metal lines to the neighboring cells and may cause their performance degradation. In our study, we used a baseline Cu/TaOx/Pt cell with a variety of alternative inert electrode cells replacing the inert Pt electrode. The performance degradation results in a reduced number of switching cycles and, in extreme cases, even in a loss of a bit, caused by the rupture of the nanofilament. We propose a thermal analysis of the thermal cross-talk, describe its impact on cells' electric performance, and identify three major mechanisms for the ReRAM reliability: (i) thermal conductivity, (ii) the specific heat capacity, and (iii) geometry of the electrodes. Several ReRAM arrays are manufactured to vary the thermal conductivity, specific heat, and geometry of the electrodes by depositing eight different inert electrodes: Pt(50 nm)/Ti(30 nm), Ru(50 nm)/Ti(30 nm), Co(50 nm)/Ti(30 nm), Pt(50 nm/Cu(100 nm)/Ti(30 nm), Pt(50 nm)/ Cu(200 nm)/Ti(30 nm), Ru(50 nm/Cr(30 nm), Ru(50 nm)/Ti(50 nm), and Rh(50 nm)/Cr(30 nm). The analysis handles the heat generation in nanostructures and its transfer to the nano-sized or micrometer-sized electrodes and how the heat transfer over the electrodes is transferred to nanofilaments of other memory cells sharing one common electrode. Using the proposed methodology, the experimentally found differences in the degradation of electric performance of the array cells performed under identical circumstances can be correctly predicted by the proposed thermal analysis using the material properties and geometry parameters of the electrodes. The analysis starts from the observation that the heat, Q, generated - typically between 3 μJ to 60 μJ - in a metallic nanofilament of a resistance Ron, - typically in the 1 kΩ to 100 kΩ range - if it were absorbed in the filament itself, it would lead according to the heat absorption equation of a body of mass m and specific heat capacity cs, Q=m×cs×ΔT, to a temperature higher than the temperature inside the sun of 2.7×107 oK. Temperature ΔT, this high is the result of the tiny volume of nanofilament of typically 5×10-18 cm-3. Since such a high temperature is impossible, the heat has to be accommodated in the ReRAM array otherwise, as it spreads to the more massive electrodes, depending on their mass (i.e. on their volume) and their specific heat capacity, and by the Newtonian losses which largely depend on the surface areas of the electrodes including the contact pads. Our analysis shows that the heat spreads across the electrodes w very quickly in the μs range due to the high thermal diffusivity, k, of the metallic electrodes. It can be shown that the typical time scale is given by τ=√(k×L), where L is e.g. the distance between two neighboring cells. Thus, within less than a second a steady state of temperature distribution is established resulting in linear dependence of the temperature distribution along the electrode when evaluating the heat conduction equation. The solution of the equation allows for determining the temperature of the passive neighboring cells when heated by an active memory cell. We have confirmed experimentally that the heat from the same heated filament spreads more efficiently and quickly along the Cu electrode than along the Pt electrode, caused by the thermal conductivity of Cu being 5.8 higher than that of Pt. In fact, we have confirmed experimentally that the reliability of the passive neighboring cells disposed along the Cy electrode is much worse than the reliability along the Pt electrode. The proposed thermal analysis allows predicting ReRAM critical switching voltages, the degradation of electrical performance, and the reliability of ReRAM devices contingent on the choice of the electrode materials and the total volume of the electrodes.

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