Metal Insulator Semiconductor Structures Research Articles

Electrical and dielectric behaviors of Al/SiO2-surfactant/n-Si schottky structure in wide range of voltage and frequency

SiO2 surfactant insulator into Al/n-Si metal-semiconductor (MS) structure was fabricated into Al/SiO2-surfactant/n-Si metal–insulator–semiconductor (MIS) structure and its effect on the electrical properties of the final structure was investigated. The SiO2-surfactant layer is coated on the n-Si wafer by the spin coating technique. The I-V data is used to calculate the fundamental electrical parameters of this MIS structure. The density distribution of the surface states (Nss) is computed depending on the energy at forward potential. The current conduction mechanisms (CCMs) in the MIS structure are examined at the reverse and forward biases applied. To get more accurate and reliable results, the profiles of I-V and C/(G/ω)-V are measured at a wide range of bias voltage (0.25V-4V) and frequency (1kHz-1MHz), respectively. The performance of the MIS is significant due to the basic values of electrical parameters (n, I0, Rs, Rsh, Nss, ΦB0, and Rectifying Ration (RR)) and dielectric parameters (ε′, ε″, tan δ, M′, M″, Rs, and σ) compared with the MS structure. The other electrical parameters (ND, WD, Em, ΦΒ) are extracted from the slope and intercept of the reverse bias C−2-V plot as a function of frequency. Furthermore, the profile of voltage-dependent Rs and Nss was determined using different methods from I-V and C/G-V data and examined comparatively with each other. The changes in impedance properties with frequency and voltage of the MIS are discussed in detail.

Sol-Gel Multilayered Niobium (Vanadium)-Doped TiO2 for CO Sensing and Photocatalytic Degradation of Methylene Blue.

Multilayered TiO2 films doped either with Niobium or Vanadium (1.2 at. %) were deposited by the sol-gel dip coating method on c-Si and glass substrates. The films on glass substrates were tested for CO sensing and photocatalytic degradation of methylene blue. X-ray diffraction data analysis showed that all the TiO2:Nb(V) films were nanocrystalline in the anatase phase, with a uniform and compact microstructure and a homogeneous superficial structure of small grains with diameters in the range of 13-19 nm. For the electrical characterization, the TiO2:Nb(V) films were incorporated in Metal-Insulator-Semiconductor (MIS) structures. The specific resistivity is of the order of 104 Ωcm and its value decreases with increasing the electrical field, which testifies to the injection of electrons into these layers. From the analysis of the current-voltage curves taken at different temperature- and frequency-dependent capacitance-voltage and conductance-voltage characteristics, the density and parameters of deep levels in these TiO2 films are evaluated and the electron charge transport mechanism is established. It was shown that the current in these TiO2:Nb(V)-Si MIS structures is mainly carried out by inter-trap tunneling via deep levels energetically distributed in the TiO2 bandgap. Testing these sol-gel TiO2:Nb(V) layers for gas sensing and photocatalytic capabilities proved that they could serve such purposes. In particular, the results of the V-doped sol-gel TiO2 film confirm its CO detection capability, which is rarely reported in the literature. For the photodegradation of methylene blue, the Nb-doped TiO2 samples were superior, with nearly double the photocatalytic efficiency of undoped TiO2.

Transport and dielectric properties of MIS structure with embedded Si QDs (AuPd/SiO2:Si QDs/n-Si) grown by MBE

This study focuses on the growth of silicon quantum dots (Si QDs) on insulator substrates via solid-state dewetting of ultrathin silicon films deposited by molecular beam epitaxy (MBE). The resulting Si QDs exhibit well-defined spherical shapes, low size dispersion with an average diameter of 6 nm, and a high density (1012/cm2), making them ideal for microelectronic applications. Current-voltage spectroscopy and impedance spectroscopy were employed to investigate the electrical transport and dielectric properties of Si QDs integrated into a metal-insulator-semiconductor (MIS) structure (AuPd/SiO2: Si QDs/n-Si). Current-voltage measurements reveal that the insertion of Si QDs into the MIS structure improves the electrical transport by introducing new conduction mechanisms. Impedance measurements demonstrate the influence of Si QDs on the frequency dependence of the real part of the dielectric constant (ε′) and the loss tangent (tan δ). Furthermore, Si QDs impact both the real (M′) and imaginary (M″) parts of the electric modulus. Despite slight deviations in dielectric properties, the MIS structure retains its fundamental nature. These investigations affirm the high electrical quality of Si QDs grown via solid-state dewetting and offer valuable insights for researchers engaged in the development of Si QD-based devices, such as non-volatile memories and MIS photodetectors.

Electrical and dielectric properties of ferromagnetic GeMn nanocrystals embedded in metal-oxide-semiconductor Schottky diodes (Al/SiO2:GeMn NCs/n-Si) grown by MBE

This study examines the electrical and dielectric characteristics of magnetic GeMn nanocrystals integrated into a metal-insulator-semiconductor (MOS) configuration for optoelectronic applications. The nanocrystals were produced by solid-state dewetting of an amorphous GeMn layer deposited by MBE on SiO2/Si. The nanocrystals exhibit a well-defined hemispheric shape, high homogeneity, and with a Curie temperature higher than that of the ambient temperature. Characterizations by transmission electron microscopy (TEM) coupled with energy-dispersive X-ray spectroscopy (EDS) revealed the crystallinity of the nanocrystals and optimal incorporation of manganese in the nanocrystals, eliminating the usual diffusion or segregation anomalies observed during growth of GeMn alloy on silicon or germanium. Electrical and dielectric tests conducted via current-voltage and impedance spectroscopies on these structures confirmed the integrity of the oxide layer surrounding the GeMn nanocrystals from defects associated with the manganese. In addition, these measurements validated the proper functioning of the MOS structure as a MOS-type Schottky diode. The in-depth analysis of dielectric parameters, such as permittivity (ε), tangent δ, and electric modulus (M) as a function of frequency and bias voltage, revealed no anomalies, thus reinforcing the high quality of the MOS structure enclosing the GeMn nanocrystals.These results mark a significant advance in the growth and integration of magnetic semiconductor nanostructures for optoelectronic applications, paving the way for promising technological developments.

Electrical and dielectric characterization of Ge quantum dots embedded in MIS structure (AuPd/SiO2:Ge QDs/n-Si) grown by MBE

Monocrystalline Germanium quantum dots (Ge QDs), grown by molecular beam epitaxy (MBE), are intricately embedded within a SiO2 oxide layer of a metal-insulator-semiconductor (MIS) structure, catering to optoelectronic applications. These monocrystalline Ge QDs exhibit high density (D ∼ 5 x 1012 cm-2), possess a spherical morphology, and an average size of 6 nm with narrow size distribution. A comprehensive investigation into the influence of these quantum dots on the electrical transport and dielectric properties of the metal-insulator-semiconductor (MIS) structure was conducted through current-voltage (I-V) spectroscopy and impedance spectroscopy. Notably, I-V measurements demonstrate that Ge QDs enhance conduction phenomena, thereby improving electrical transport within the MIS structure. Furthermore, the capacitance-voltage (C-V) and conductance-voltage (G-V) measurements, spanning the inversion and accumulation regions, demonstrate a memory effect induced by Ge QDs within the MOS structure. Extensive analysis of dielectric parameters, including permittivity (ε’), dielectric loss (ε”), dissipation factor (tanδ), as well as electrical modulus (M’ and M”), across a broad frequency range and under varying bias voltages, reveals no anomalous electrical or dielectric behavior attributable to the presence of Ge QDs in MIS structure. Consequently, these findings affirm the viability of the novel MIS structures with Ge QDs for the development of optoelectronic devices such as photodetectors or solar cells.

Comparative study of the interface passivation properties of LiF and Al2O3 using silicon MIS capacitor

Lithium fluoride (LiF) is currently a very popular dielectric material used as a passivation or transport layer in a variety of applications, especially in high-efficiency solar cells. Despite this, its conduction properties and interface behavior with silicon remain largely unexplored. In this work, a LiF metal–insulator–semiconductor (MIS) structure is fabricated and characterized, and its properties are compared to the well-understood aluminum oxide (Al2O3) MIS structure. First, a higher current density in LiF compared to Al2O3 is highlighted, as well as its PN junction-like behavior with n-type silicon (n-Si), being rather unconventional for a dielectric layer. C–V measurements showcase the likely presence of an interface defect, causing an increase in the apparent doping and a shift in the flatband voltage VFB by +70 meV. This defect is found to be of the acceptor type, which renders the interface fixed charge more negative and improves the field-effect passivation in the case of a negative Qf. Finally, a density of interface states Dit≈2×1011 cm−2 eV−1 was found for LiF/n-Si, which is a low value showing appropriate chemical passivation at the interface. Overall, this work enables us to shed more light on the interface properties of LiF on n-Si, which is an essential step toward its wider use in state-of-the-art solar cells and other silicon-based devices.

Efficiency improvement by using metal–insulator-semiconductor structure in InGaN/GaN micro-light-emitting diodes

InGaN/GaN micro-light-emitting diodes (micro-LEDs) with a metal–insulator-semiconductor (MIS) structure on the sidewall are proposed to improve efficiency. In this MIS structure, a sidewall electrode is deposited on the insulating layer-coated sidewall of the device mesa between a cathode on the bottom and an anode on the top. Electroluminescence (EL) measurements of fabricated devices with a mesa diameter of 10 μm show that the application of negative biases on the sidewall electrode can increase the device external quantum efficiency (EQE). In contrast, the application of positive biases can decrease the EQE. The band structure analysis reveals that the EQE is impacted because the application of sidewall electric fields manipulates the local surface electron density along the mesa sidewall and thus controls surface Shockley–Read–Hall (SRH) recombination. Two suggested strategies, reducing insulator layer thickness and exploring alternative materials, can be implemented to further improve the EQE of MIS micro-LEDs in future fabrication.Graphical

A new gate design combined MIS and p-GaN gate structures for normally-off and high on-current operation

This study proposes a new gate architecture that integrates both a p-GaN gate and a metal–insulator–semiconductor (MIS) structure for a normally-off AlGaN/GaN high electron mobility transistor. Silvaco TCAD simulation software is used to assess the performance of the proposed design. A comprehensive analysis of the device’s transfer, output, and breakdown characteristics is carried out and compared with the conventional p-GaN gate AlGaN/GaN HEMT. The findings indicate that incorporating MIS in conjunction with the p-GaN gate leads to an augmentation in the on-state current density and a reduction in on-resistance. The proposed HEMT exhibits superior attributes, with an 80% increase in drain current compared to the conventional p-GaN gate HEMT, but remains similar to threshold voltage and breakdown voltage. Consequently, the proposed HEMT demonstrates elevated current density and enhances gate control over the channel without modifying the threshold voltage compared to the conventional p-GaN gate HEMT.

Dysprosium oxide (Dy2O3) layer effect on the interface possessions of Au/n-GaN Schottky diode as an interlayer and its chemical and microstructural features

The work reveals the design of Au/dysprosium oxide (Dy2O3)/n-GaN metal/insulator/semiconductor (MIS)-type structures with a dysprosium oxide interlayer and investigates its microstructural and chemical states using XRD, TEM and XPS approaches. The outcomes confirm that the Dy2O3 film exists on the n-GaN. The electrical properties of the MIS structure are probed by I–V and C–V procedures. A superior rectifying type with a small leakage current was acquired for the MIS structure rather than the Au/n-GaN Schottky diode (SD). Results showed that the MIS structure has a better barrier height (Φb) (0.91 eV) than the SD (0.82 eV), representing that the Dy2O3 layer modifies the Φb. Using Cheung's, Norde and ΨSP-V plots, the Φb of the SD and MIS structure are derived and the values are nearly similar, representing that the approaches applied here are valid. The predicted NSS of the MIS structure is two orders smaller than the SD, implying that the Dy2O3 interlayer substantially lowers the NSS. The Poole-Frenkel emission is the foremost current transport at a lower bias, though Schottky emission (SE) is ruled as the current transport at a higher bias of the SD. Nevertheless, the SE is prevailing in the current transport in the MIS structure. Consequences endorse that the Dy2O3 layer can be suitable for building of MIS/MOS devices.

Spin-filter device using the Zeeman effect with realistic channel and structure parameters

We have theoretically calculated the performance of a Zeeman-type spin polarizer, which consists of ferromagnetic (FM) and metal–insulator–semiconductor (MIS) gate nanostructures on top of an InAs two-dimensional electron gas (2DEG) channel. For the calculations, we have taken a realistic electron concentration, electron effective mass and the effective g-factor of the InAs 2DEG into account. In addition, we have assumed realistic FM and MIS structure sizes by conventional electron beam lithography. In the calculations, we have demonstrated clear oscillation of spin polarization over ±85%. Furthermore, we have shown that it works not only as a spin-polarized current generator but also a detector. In addition, we have proposed a novel spin-filter device utilizing the Zeeman-type spin polarizers. We have found that its conductance characterization allows us to evaluate the operation of Zeeman-type spin polarizers. We expect the Zeeman-type spin polarizers and spin-filter devices to open up a new field of spintronics.

Stabilization of Silicon Photoanode By Hafnium Zirconium Oxide Interlayer for Efficient Solar Water Oxidation

Photoelectrochemical devices, which convert solar energy into sustainable hydrogen, are promising for carbon neutralization. The prerequisites for the photoelectrode materials are absorbing a wide range of the solar spectrum, cost-effectiveness, long carrier lifetime, and so on. Silicon, which has a bandgap of 1.12 eV, has been of prime interest because of its excellent charge carrier mobility, earth abundance, and high crystallinity. However, instability in the electrolytes, poor catalytic activity, and negative valence band position hampers its application. The design of metal-insulator-semiconductor (MIS) structures is a promising strategy for boosting catalytic surface reactions with high stability. Herein, we report the interface engineering of MIS photoanodes with enhanced stability for water oxidation. First, we investigate the effect of poling direction of hafnium zirconium oxide (HZO) with a few-nm thickness as a surface passivation layer. Then, additional Ni/NiFe layered double hydroxide (LDH) catalysts are introduced to accelerate surface catalytic reactions. Harmonizing the HZO interlayer and Ni/NiFe LDH on silicon performs with a high current density of 26.6 mA cm-2 at 1.23 V vs. reversible hydrogen electrode with stability. This study demonstrates the rational design of MIS structure and manipulating hierarchical photoanodes' interface to boost stability and charge carrier kinetics.

Effects of ammonia and oxygen plasma treatment on interface traps in AlGaN/GaN metal–insulator–semiconductor high-electron-mobility transistors

A HfO2/Al2O3/AlN stack deposited in a plasma-enhanced atomic layer deposition system is used as the gate dielectric for GaN metal–insulator–semiconductor structures. Prior to the deposition, in situ remote plasma pretreatments (RPPs) with O2, NH3, and NH3–O2 (in sequence) were carried out. Frequency-dependent capacitance–voltage (C–V) measurements reveal that for all the RPP-treated samples, the interface trap density Dit was significantly reduced compared to the sample without RPP. In particular, the NH3-treated sample exhibits an extremely low Dit of ∼1011 cm−2·eV−1 from EC − 0.31 eV to EC − 0.46 eV. To characterize the interface traps at deeper levels, the transfer pulsed I–V measurement was employed further. In the energy range of EC − ET > 0.54 eV, the interface trap charge density Qit of various samples demonstrates the following order: NH3 RPP < NH3–O2 RPP < O2 RPP < without RPP. By adjusting multiple pulse widths, the Dit derived from the Qit result matches the C–V measurement precisely. Moreover, X-ray photoelectron spectroscopy analysis of the Ga 3d core-level indicates that NH3 RPP facilitates the conversion of Ga2O into GaN at high RF power of 2500 W, whereas O2 RPP mainly oxidizes GaN to relatively stable Ga2O3. Combined with the C–V and pulsed I–V measurements, it is confirmed that a strong positive correlation exists between Ga2O and the interface traps, rather than Ga2O3.

Nano-scale charge trapping memory based on two-dimensional conjugated microporous polymer

There is a growing interest in new semiconductor nanostructures for future high-density high-performance flexible electronic devices. Two-dimensional conjugated microporous polymers (2D-CMPs) are promising candidates because of their inherent optoelectronic properties. Here, we are reporting a novel donor–acceptor type 2D-CMP based on Pyrene and Isoindigo (PI) for a potential nano-scale charge-trapping memory application. We exfoliated the PI polymer into ~ 2.5 nm thick nanoparticles (NPs) and fabricated a Metal–Insulator–Semiconductor (MIS) device with PI–NPs embedded in the insulator. Conductive AFM (cAFM) is used to examine the confinement mechanism as well as the local charge injection process, where ultrathin high-κ alumina supplied the energy barrier for confining the charge carrier transport. We have achieved a reproducible on-and-off state and a wide memory window (ΔV) of 1.5 V at a relatively small reading current. The device displays a low operation voltage (V < 1 V), with good retention (104 s), and endurance (103 cycles). Furthermore, a theoretical analysis is developed to affirm the measured charge carriers’ transport and entrapment mechanisms through and within the fabricated MIS structures. The PI–NPs act as a nanoscale floating gate in the MIS-based memory with deep trapping sites for the charged carriers. Moreover, our results demonstrate that the synthesized 2D-CMP can be promising for future low-power high-density memory applications.

Fully Deep‐UV Transparent Thin Film Transistors Based on SrSnO3

AbstractUltra‐wide bandgap semiconductors are gaining attention for their promising properties for UV optoelectronics and UV transparent electronics as well as high‐power applications. Among them, La‐doped SrSnO3 exhibits excellent properties both for deep‐UV transparent oxide semiconductors and deep‐UV transparent conducting oxide. Here, the demonstration of thin film transistors (TFTs) with full deep‐UV transparency is reported, including electrodes, gate oxide, and substrate. The lightly La‐doped SrSnO3 for the channel layer is grown on MgO (100) substrates with buffer layers by pulsed laser deposition. TFTs with a metal—insulator–semiconductor structure are fabricated using high‐k perovskite dielectric LaScO3 as the gate oxide. A degenerately La‐doped SrSnO3 is used as the gate, the source, and the drain electrodes to obtain good ohmic contact with the channel layer as well as UV transparency. The resultant device shows a field effect mobility value of ≈24 cm2 V−1 s−1 and an on/off ratio &gt;106. The optical transmittance of the entire device (including the substrate) is found to be &gt;75% at 300 nm in wavelength. Furthermore, the electrical characteristics of the device exhibit excellent stability under visible irradiation. This research highlights the potential of SrSnO3 in advancing the field of UV optoelectronics and UV transparent electronics.

Analog Synaptic Devices Based on IGZO Thin‐Film Transistors with a Metal–Ferroelectric–Metal–Insulator–Semiconductor Structure for High‐Performance Neuromorphic Systems

A ferroelectric thin‐film transistor (FeTFT)‐based synaptic device with an indium–gallium–zinc oxide (IGZO) channel and a metal–ferroelectric–metal–insulator–semiconductor (MFMIS) structure is reported. The fabricated FeTFT exhibits a highly linear conductance response (|α| = 0.21) with a large dynamic range (Gmax/Gmin ≈ 53.2), although identical program pulses are applied to the device. In addition, because the inner metal layer of the FeTFTs has an MFMIS structure, the electric field is uniformly applied to the entire IGZO channel, which reduces the cycle‐to‐cycle variation (σ = 0.47%) in the conductance responses. In the system simulation with the measured synaptic characteristics, the high classification accuracy of ≈97.0% is achieved in the MNIST image set, verifying the feasibility of FeTFT‐based neuromorphic systems.

Exploring the Physical Origin of the Negative Capacitance Effect in a Metal–Ferroelectric–Metal–Dielectric Structure

AbstractThe ferroelectric negative capacitance (NC) draws a great deal of attention for low‐power negative capacitance field‐effect transistors (NCFET) and NC capacitors. The fabrication of steep‐slope FET (subthreshold swing &lt; 60mVdec−1) is reported, followed by modeling approaches. While the device fabrication favors a ferroelectric gate structure without interlayer metal, many NCFET models adopt interlayer metal between the ferroelectric layer and metal–insulator–semiconductor structure. The metal interlayer averages out spatial variation in the ferroelectric polarization, enabling a compact charge‐based relation between the layers. In addition, the approach assumes that the NC effect emerges from the ferroelectric layer regardless of the metal interlayer, which is not necessarily probable. This work reinvestigates the possible NC effect in ferroelectric–dielectric capacitors connected by a metal interlayer. The experiment confirms that the NC effect in the metal–ferroelectric–dielectric–metal structure does not appear in the metal–ferroelectric–metal–dielectric–metal structure. These results are inconsistent with the multidomain‐1D Landau–Ginzburg‐Devonshire model. In contrast, the suppression of the NC effect in the structure is fully explained by the advanced inhomogeneous stray‐field energy model, which simulates the dynamic evolution of polarization and screening charges. Therefore, an NCFET with a metal interlayer is impractical.

2D Amorphous GaOX Gate Dielectric for β-Ga2O3 Field-Effect Transistors.

Appropriate gate dielectrics must be identified to fabricate metal-insulator-semiconductor field-effect transistors (MISFETs); however, this has been challenging for compound semiconductors owing to the absence of high-quality native oxides. This study uses the liquid-gallium squeezing technique to fabricate 2D amorphous gallium oxide (GaOX) with a high dielectric constant, where its thickness is precisely controlled at the atomic scale (monolayer, ∼4.5 nm; bilayer, ∼8.5 nm). Beta-phase gallium oxide (β-Ga2O3) with an ultrawide energy bandgap (4.5-4.9 eV) has emerged as a next-generation power semiconductor material and is presented here as the channel material. The 2D amorphous GaOX dielectric is combined with a β-Ga2O3 conducting nanolayer, and the resulting β-Ga2O3 MISFET is stable up to 250 °C. The 2D amorphous GaOX is oxygen-deficient, and a high-quality interface with excellent uniformity and scalability forms between the 2D amorphous GaOX and β-Ga2O3. The fabricated MISFET exhibits a wide gate-voltage swing of approximately +5 V, a high current on/off ratio, moderate field-effect carrier mobility, and a decent three-terminal breakdown voltage (∼138 V). The carrier transport of the Ni/GaOX/β-Ga2O3 metal-insulator-semiconductor (MIS) structure displays a combination of Schottky emission and Fowler-Nordheim (F-N) tunneling in the high-gate-bias region at 25 °C, whereas at elevated temperatures it shows Schottky emission and F-N tunneling in the low- and high-gate-bias regions, respectively. This study demonstrates that a 2D GaOX gate dielectric layer can be produced and incorporated into an active channel layer to form an MIS structure at room temperature (∼25 °C), which enables the facile fabrication of MISFET devices.

Memory properties and short-range order in silicon oxynitride-based memristors

Silicon oxynitride films of various compositions were grown by the plasma-enhanced chemical deposition method using a setup with remote plasma and an inductive excitation from a gas mixture of 10% monosilane (diluted with argon) and nitrogen in the presence of residual oxygen in working gas mixtures. The high-frequency generator power (13.56 MHz) was varied in the range of 50–150 W. The samples composition was studied using x-ray photoelectron spectroscopy. A comparison of the experimental Si 2p photoelectron spectrum with the calculation showed that, at a low generator power (50 W), the short-range order in silicon oxynitride films is better described by the “random mixture” model. As the generator power is increased (100 W and higher), the excess silicon content in a silicon oxynitride film is decreased, and the short-range order is better described by the “random bonding” model. On metal–insulator–semiconductor-structures based on the silicon oxynitride films obtained, measurements of current–voltage characteristics were carried out, and the resistive switching of the obtained structures is studied in the present contribution. It is found that, in films in which the short-range order is described by the random bonding model, a stable switching in the bipolar regime is observed.

Theory and Simulation of Metal–Insulator–Semiconductor (MIS) Photoelectrodes

A metal-insulator-semiconductor (MIS) structure is an attractive photoelectrode-catalyst architecture for promoting photoelectrochemical reactions, such as the formation of H2 by proton reduction. The metal catalyzes the generation of H2 using electrons generated by photon absorption and charge separation in the semiconductor. The insulator layer between the metal and the semiconductor protects the latter element from photo-corrosion and, also, significantly impacts the photovoltage at the metal surface. Understanding how the insulator layer determines the photovoltage and what properties lead to high photovoltages is critical to the development of MIS structures for solar-to-chemical energy conversion. Herein, we present a continuum model for charge-carrier transport from the semiconductor to the metal with an emphasis on mechanisms of charge transport across the insulator. The polarization curves and photovoltages predicted by this model for a Pt/HfO2/p-Si MIS structure at different HfO2 thicknesses agree well with experimentally measured data. The simulations reveal how insulator properties (i.e., thickness and band structure) affect band bending near the semiconductor/insulator interface and how tuning them can lead to operation closer to the maximally attainable photovoltage, the flat-band potential. This phenomenon is understood by considering the change in tunneling resistance with insulator properties. The model shows that the best MIS performance is attained with highly symmetric semiconductor/insulator band offsets (e.g., BeO, MgO, SiO2, HfO2, or ZrO2 deposited on Si) and a low to moderate insulator thickness (e.g., between 0.8 and 1.5 nm). Beyond 1.5 nm, the density of filled interfacial trap sites is high and significantly limits the photovoltage and the solar-to-chemical conversion rate. These conclusions are true for photocathodes and photoanodes. This understanding provides critical insight into the phenomena enhancing and limiting photoelectrode performance and how this phenomenon is influenced by insulator properties. The study gives guidance toward the development of next-generation insulators for MIS structures that achieve high performance.

Trap‐Assisted Memristive Switching in HfO2‐Based Devices Studied by In Situ Soft and Hard X‐Ray Photoelectron Spectroscopy

AbstractMemristive devices are under intense development as non‐volatile memory elements for extending the computing capabilities of traditional silicon technology by enabling novel computing primitives. In this respect, interface‐based memristive devices are promising candidates to emulate synaptic functionalities in neuromorphic circuits aiming to replicate the information processing of nervous systems. A device composed of Nb/NbOx/Al2O3/HfO2/Au that shows promising features like analog switching, no electro‐forming, and high current‐voltage non‐linearity is reported. Synchrotron‐based X‐ray photoelectron spectroscopy and depth‐dependent hard X‐ray photoelectron spectroscopy are used to probe in situ different resistance states and thus the origin of memristive switching. Spectroscopic evidence for memristive switching based on the charge state of electron traps within HfO2 is found. Electron energy loss spectroscopy and transmission electron microscopy support the analysis. A device model is proposed that considers a two‐terminal metal–insulator–semiconductor structure in which traps within the insulator (HfO2/Al2O3) modulate the space charge region within the semiconductor (NbOx) and, thereby, the overall resistance. The experimental findings are in line with impedance spectroscopy data reported in the companion paper (Marquardt et al). Both works complement one another to derive a detailed device model, which helps to engineer device performance and integrate devices into silicon technology.