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Indium selenide is a polymorphous material system that has attracted considerable attention due to the outstanding electronic, optical, ferroelectric, and antiferroelectric properties of its various crystalline phases; however, this polymorphism adds a layer of complexity to the analysis of Raman and x-ray photoelectron (XPS) data of these materials. In this report, a method of quantitative analysis of combined Raman and XPS data is developed to determine the stoichiometry of indium selenide thin films, as well as the phase fraction of β-In2Se3, γ-In2Se3, and κ-In2Se3 within the films. A 13:1 ratio of the indium to selenium sensitivity factors (σIn/σSe = 13) was found to yield the most accurate stoichiometry results. Further structural characterization of the κ-In2Se3 film was performed and points to its existence in these films as a distinct phase, rather than strained γ-In2Se3. [ABSTRACT FROM AUTHOR]

Theoretical models have predicted that pH-responsive surface groups can reduce the sensitivity of field-effect transistor biosensors. However, attempts to prove this experimentally have shown conflicting results. In this work, a graphene field effect transistor (gFET) biosensor is used, which, without modification, is pH insensitive. The surface of the graphene is then functionalized using 1-hydroxypyrene to modulate the pH sensitivity of the gFET. A pH sensitivity ranging from 3.9 to 36.8 mV/pH was demonstrated. The biosensitivity of the gFETs was tested using streptavidin–biotin as a model system. The experimental results showed no correlation between biosensitivity and pH sensitivity. An electrochemical membrane model was used to determine the expected relationship between biosensitivity and pH sensitivity. The model results show that biosensitivity does not decrease until a certain threshold pH sensitivity is reached. This threshold is dependent on factors such as the acid dissociation constants of the surface hydroxyl groups and ion concentration. Furthermore, the differences between the simulation and experiment suggest that the effect of screening is greatly reduced when the analyte binds within a membrane. [ABSTRACT FROM AUTHOR]

As the demand for computing applications capable of processing large datasets increases, there is a growing need for new in-memory computing technologies. Oxide-based resistive random-access memory (RRAM) devices are promising candidates for such applications because of their industry readiness, endurance, and switching ratio. These analog devices, however, suffer from poor linearity and asymmetry in their analog resistance change. Various reports have found that the temperature in RRAM devices increases locally by more than 1000 K during operation. Therefore, temperature control is of paramount importance for controlling their resistance. In this study, scanning thermal microscopy is used to map the temperature of Au/Ti/HfOx/Au devices at a steady power state and to measure temperature dynamics of the top electrode above the filament location during both resistive switching loops and voltage pulsing. These measurements are used to verify the thermal parameters of a multiphysics finite elements model. The model is then used to understand the impact of thermal conductivities and boundary conductances of constituent materials on resistance change during the first reset pulse in RRAM devices. It is found that the resistance change can be reduced significantly when the temperature in the titanium capping layer is reduced. We find that the greatest temperature reduction and, therefore, the lowest resistance change in the device are afforded by capping layers with increased thermal conductivities. This work links thermal properties to the resistance change in RRAM devices, providing critical insights into engineering devices with improved switching dynamics. [ABSTRACT FROM AUTHOR]

A Ti2AlN MAX phase layered thin film electrode and oxygen getter layer for HfO2‐based two‐terminal memristors is presented. The Ti2AlN/HfOx/Ti memristor devices exhibit enhanced resistive switching performance, including an ultra‐low reset current density (< 10−8 MΩ cm2), substantial on‐off ratio (≈ 6000), excellent multi‐level functionality (≈ 9 distinct states), impressive retention (up to 300 °C), and robust endurance (>200 million) as compared to conventional TiN and other alternative materials based memristors. Experimental measurements and modeling suggest that the distinctive combination of low thermal conductivity, high electrical conductivity, and unique ultra‐thin layer‐by‐layer structure of the Ti2AlN MAX phase thin film contribute to this exceptional performance with good reproducibility and stability. The results demonstrate for the first‐time the potential of this innovative sputtered MAX phase material for engineering energy‐efficient, high‐density non‐volatile digital, and analog memory devices aimed toward next‐generation sustainable artificial intelligence. [ABSTRACT FROM AUTHOR]

The effects of substrate choice, substrate temperature, Se/In flux ratio, and cooling rate after deposition on the phase composition, surface morphology, and stoichiometry of indium selenide films synthesized via molecular beam epitaxy are presented. In2Se3 films were synthesized on sapphire, Si(111) and highly oriented, pyrolytic graphite (HOPG) substrates. The phase composition, stoichiometry, and surface morphology of the films were characterized via Raman spectroscopy, x-ray photoelectron spectroscopy, and atomic force microscopy, respectively. Higher substrate temperature combined with higher Se/In ratio promoted formation of β-In2Se3 over γ and/or κ-In2Se3 on all substrates. Higher Se/In ratio also independently promoted β-In2Se3 over γ and/or κ-In2Se3 on all substrates at 673 K. The lateral dimensions of In2Se3 flakes increased as the substrate temperature increased on all substrates, and the largest lateral dimensions were observed for β-In2Se3 flakes on HOPG at 973 K. No evidence of α-In2Se3 was observed in the Raman spectra of any of the films at any of the synthesis conditions in this study. β-In2Se3 films on HOPG were cooled at 1200, 120, and 12 K/h and no evidence of a β to α-In2Se3 phase transition was observed. Some evidence of β to α-In2Se3 phase transition was observed in temperature-dependent XRD of In2Se3 powders, suggesting that another parameter besides cooling rate is locking the In2Se3 films into the β-phase. [ABSTRACT FROM AUTHOR]

Hafnium oxide non-volatile memories have shown promise as an artificial synapse in neuromorphic computing architectures. However, there is still a need to fundamentally understand how to reliably control the analog resistance change induced by oxygen ions that partially rupture or re-form the conductive filament. In this work, the impact of measurement conditions (pulse amplitude and pulse width) and titanium dopants on the analog resistance change of atomic layer deposited hafnium oxide memristor synapses are studied. A lower pulse amplitude improves the linearity of resistance change as a function of the number of pulses but results in a smaller memory window. The addition of titanium dopants does not substantively change the analog resistance modulation of hafnium oxide. Density functional theory calculations show that titanium strongly impacts oxygen ion motion in the HfxTiyOz matrix but does not impact significantly in the HfTi metallic filament. This study demonstrates that the analog characteristic of HfxTiyOz artificial synapses is largely independent of the titanium doped bulk oxide since the resistance change is primarily controlled by the HfTi metallic conducting filament. [ABSTRACT FROM AUTHOR]

Changes in the resistance of Ti/HfOx synapses are known to be governed by a thin-oxide barrier associated with the oxidation/reduction of a Hf-rich conducting filament (CF). However, experimental characterization of the CF is challenging. Critical physical properties and processes, such as the barrier location, time-dependent thickness during analog pulsing, and the temperature-effect on current, need to be better established. In this work, a compact model based on Trap-Assisted-Tunneling and Ohmic transport is utilized to analyze the analog switching of HfOx synapses. The model agrees well with the experimentally observed current–voltage relation and its temperature dependence. The extracted barrier heights during analog pulsing are consistent with a barrier situated near the reset anode; the electrode is opposite to the Ti oxygen-reservoir layer. A Finite Element Analysis simulation, which incorporates oxygen-vacancy migration, independently supports this conclusion. The model further permits extraction of the barrier thickness in relation to the analog pulses. [ABSTRACT FROM AUTHOR]

Self-aligned metal-oxide-semiconductor (MOS) capacitors are studied with several low-temperature, wet chemical silicon dioxide (SiO2) interlayers to understand their impact on electrical performance. Self-aligned MOS capacitors are fabricated with a bottom-up patterning technique that uses a poly(methyl methacrylate) brush and dopant-selective KOH etch combined with area-selective atomic layer deposition of hafnium dioxide (HfO2) and Pt. The wet chemical pretreatments used to form the SiO2 interlayer include hydrofluoric acid (HF) etch, 80 °C H2O, and SC-2. Capacitance-voltage measurements of these area-selective capacitors exhibit a HfO2 dielectric constant of ∼19, irrespective of pretreatment. After a forming gas anneal, the average interface state density decreased between 1.8 and 7.5 times. The minimum observed Dit is 1 × 1011 eV−1 cm−2 for the HF-last treatment. X-ray photoelectron spectroscopy shows an increase in stoichiometric SiO2 in the interfacial layer after the anneal. Additional carbon is also observed; however, comparison with capacitors fabricated in a nonselective process reveals minimal impact on performance. [ABSTRACT FROM AUTHOR]

Resistive random-access memory (RRAM) devices have been widely studied for neuromorphic, in-memory computing. One of the most studied RRAM structures consists of a titanium capping layer and a HfOx adaptive oxide. Although these devices show promise in improving neuromorphic circuits, high variability, non-linearity, and asymmetric resistance changes limit their usefulness. Many studies have improved linearity by changing materials in or around the device, the circuitry, or the analog bias conditions. However, the impact of prior biasing conditions on the observed analog resistance change is not well understood. Experimental results in this study demonstrate that prior higher reset voltages used after forming cause a greater resistance change during subsequent identical analog pulsing. A multiphysics finite element model suggests that this greater analog resistance change is due to a higher concentration of oxygen ions stored in the titanium capping layer with increasing magnitude of the reset voltage. This work suggests that local ion concentration variations in the titanium capping layer of just tens of atoms cause significant resistance variation during analog operation. [ABSTRACT FROM AUTHOR]

Mass transport to the sensor surface is a critical step in biosensing, often being the factor determining the limit of detection. Modeling transport of the analyte to a surface under convection and diffusion is a challenging task often requiring complex simulation. Herein, we provide a general model for mass transport to planar and nanowire biosensors under flow that is applicable over a wide range of variables. The model is then used to examine the importance of radial diffusion compared with planar diffusion under flow. Only under diffusive transport nanowires are found to have greatly reduced settling times compared with planar sensors due to radial diffusion. However, the presence of flow restricts the growth of the depletion region, resulting in comparable settling times between nanowires and planar sensors of the same size. Under flow conditions in typical experiments, radial diffusion associated with nanowire sensors is inessential for fast mass transport. Instead, the sensor length in the flow direction is the critical parameter as it limits the concentration drop that can occur as the analyte flows past the sensor. Decreasing the sensor length along the flow direction is found to greatly reduce settling times for both cases of planar and radial diffusion, even approaching the reaction limited case. Similarly, decreasing the channel height decreases the settling time due to restriction of the depletion region, but not as significantly as decreasing the sensor length. [ABSTRACT FROM AUTHOR]

Moving toward the future computing and non-volatile memory, memristors have gained wide attention for their in-memory computing and neuromorphic computing applications. The resistive switching dynamics in oxide-based memristors rely critically on oxygen ions and their recombination with oxygen vacancies to partially rupture and form the conductive filament (CF) for modulating the device between the low resistance state and high resistance state. In this study, we investigated the effect of oxygen ion concentration at the HfOx/Ti interface on the behavior of HfOx-based filamentary memristors. Devices with higher oxygen ion concentrations at the HfOx surface were fabricated by treating with oxygen plasma at various times on the as grown ~ 5 nm HfOx active layer via atomic layer deposition. Detailed X-ray photoelectron spectroscopy and depth profile analyses verify the excess oxygen ion presence at the HfOx surface, and the formation of a thin TiOx layer between the HfOx and Ti layers, resulting in a HfOx/TiOx/Ti interface structure. This work demonstrates the importance of interface chemistry at the HfOx/Ti interface region and its oxygen ion motion for controlled growth of the CF during memristor set operation, which is favorable for analog logic computation. While the components of improving the memristor performance, offering multilevel resistance states, and enhancing the resistance controllability are all have been achievable, displaying a gradual SET behavior in HfOx-based memristor still remains as a challenge. Overcoming this obstacle in the HfOx-based memristor can lead to a tremendous progress toward controlling the resistance on the SET side, which can be thus programmable and be useful for analog logic computation based on spiking networks. This work demonstrates the importance of oxygen ion concentration at the HfOx/Ti interface and its motion for controlled growth of the CF during memristor set operation for resistance controllability. [ABSTRACT FROM AUTHOR]

HfOx-Based resistive random-access memory (RRAM) devices are being widely considered as both non-volatile memories for digital computation and synaptic memory for neuromorphic computing applications. The resistive switching mechanism in these materials is known to be highly dependent on the presence of oxygen vacancies and other dopants. In this work, HfOx is doped with titanium using sequential atomic layer deposition (ALD) of HfOx and TiOx. An increase in the titanium dopant content results in a decrease of the forming voltage, a decrease of both allowable set and reset stop voltages, and a decrease of the high resistance state (HRS) of the RRAM. The physical mechanisms responsible for these observed behaviors are proposed. [ABSTRACT FROM AUTHOR]

We demonstrate a bottom-up process for programming the deposition of coaxial thin films aligned to the underlying dopant profile of semiconductor nanowires. Our process synergistically combines three distinct methodsâ€"vaporâ€"liquidâ€"solid nanowire growth, selective coaxial lithography via etching of surfaces (SCALES), and area-selective atomic layer deposition (AS-ALD)â€"into a cohesive whole. Here, we study ZrO2 on Si nanowires as a model system. Si nanowires are first grown with an axially modulated n-Si/i-Si dopant profile. SCALES then yields coaxial poly(methyl methacrylate) (PMMA) masks on the n-Si regions. Subsequent AS-ALD of ZrO2 occurs on the exposed i-Si regions and not on those masked by PMMA. We show the spatial relationship between nanowire dopant profile, PMMA masks, and ZrO2 films, confirming the programmability of the process. The nanoscale resolution of our process coupled with the plethora of available AS-ALD chemistries promises a range of future opportunities to generate structurally complex nanoscale materials and electronic devices using entirely bottom-up methods. [ABSTRACT FROM AUTHOR]