Al2O3 Interfacial Layer Research Articles

Enhancing Uniformity, Read Voltage Margin, and Retention in Three-Dimensional and Self-Rectifying Vertical Pt/Ta2O5/Al2O3/TiN Memristors.

This study introduces a Ta2O5-based self-rectifying memristor (SRM) with an Al2O3 interfacial layer adopted to improve switching uniformity, read voltage margin, and long-term retention. The Pt/Ta2O5/Al2O3/TiN (PTAT) device exhibits a 105 rectification ratio, 104 on/off ratio, 2 × 106 endurance, and retention of 104 s at 150 °C. A 3-layer 4 × 4 vertical resistive random access memory structure exhibits uniform switching parameters. The coefficient of variation (CV) for device-to-device measurements is 0.23 for the low resistance state (LRS) and 0.22 for the high resistance state (HRS), while for cycle-to-cycle measurements, the CV is 0.38 for the LRS and 0.11 for the HRS. Finally, the present study demonstrates the superior performance of the PTAT devices in the context of hardware-aware training for a fully connected neural network implementation. These advancements position the PTAT device as a promising candidate for high-density three-dimensional storage class memory and low-power neural networks, offering the consistent performance and reliability necessary for future high-density storage applications.

Imaging ultra-weak UV light below 100 pW cm-2 using a 4H-SiC photodetector with an Al2O3 interfacial layer.

Weak light detection is crucial in various practical applications such as night vision systems, flame monitoring, and underwater operations. Decreasing the dark current of a photodetector can effectively mitigate noises, consequently enhancing the signal-to-noise ratio and overall weak light detection performance. Herein, we demonstrate a 4H-SiC UV photodetector capable of detecting extremely weak UV light. This device comprises a photosensitive layer of 4H-SiC, two TiN electrodes and an atomically thin Al2O3 interfacial layer between TiN and the C surface of 4H-SiC. Under 360 nm UV light illumination, the proposed Al2O3 device demonstrates an ultra-low dark current of 18 fA, possibly benefiting from the effective passivation of interfacial carriers, and a boosted photo-to-dark current ratio of 6.7 × 107. Consequently, it achieves a weak-light detection limit as low as 31.8 pW cm-2, significantly outperforming the control device lacking Al2O3. When compared to previously reported SiC photodetectors, our Al2O3 device boasts an exceptional large linear dynamic range of 172 dB. Leveraging this, we construct a photodetector array capable of clearly imaging an object under ultra-weak light illumination below the 100 pW cm-2 level. The proposed photodetector represents a significant advancement in the development of highly sensitive image sensors for weak light detection.

Ultrathin Al2O3 interfacial layer for Hf0.5Zr0.5O2-based ferroelectric field-effect transistors

In this study, the effects of an Al2O3 interfacial layer (IL) on the characteristics of ferroelectric field-effect transistors (FeFETs) with Hf0.5Zr0.5O2 (HZO) gate dielectrics on Si substrates were investigated. FeFETs with HZO gate dielectrics have gained considerable attention owing to their compatibility with modern fabrication processes and scalability. However, during HZO deposition on Si substrates, an ultrathin metal silicate IL with a low dielectric constant is formed in an uncontrolled manner, leading to a significant voltage drop and the generation of interface traps during device operation. To address this issue, an ultrathin Al2O3 IL with a thickness less than 2 nm was introduced between the HZO film and Si substrate via in situ atomic layer deposition. The impact of this IL on a memory window (MW) and endurance characteristics was evaluated by comparing the devices with and without an intentional Al2O3 IL. The obtained results revealed that the Al2O3 IL effectively suppressed the interface trap generation, expanded the MW, and enhanced the transistor endurance characteristics. This described approach can be potentially used for improving the reliability of FeFETs fabricated on Si substrates.

(Invited) Characteristics of GaN/High-k Capacitors Under Positive Bias Stress

We investigated characteristics of the Hf0.55Al0.45Ox MOS capacitor (HfAlO) under a high sweep voltage and positive bias stress (PBS). The role of a 0.5nm-thick HfO2 (HfO/HfAlO) or a 0.5nm-thick Al2O3 (AlO/HfAlO) interfacial layer (IL) between n-GaN and Hf0.55Al0.45Ox film on electrical properties was also discussed. The HfAlO, HfO/HfAlO and AlO/HfAlO capacitors showed a similar leakage current property. Each flatband voltage (Vfb) exhibited the same value against electric field (E) from Vfb. No Vfb hysteresis of all capacitors was observed at high Es from Vfb up to 3.5MVcm-1. Under PBS, the positive Vfb shift appeared in all capacitors and increased with increasing the E from Vfb. Note that the positive Vfb shift suppressed 0.3V regardless of IL even in high effective electric field from Vfb regions of 10~15MVcm-1 while the positive Vfb shift of the Al2O3 capacitor significantly increased above 0.5V at a low 6.5MVcm-1.

Facile construction of MXene/Ge van der Waals Schottky junction with Al2O3 interfacial layer for high performance photodetection

MXene/bulk semiconductor vdW Schottky junctions are considered to be a hopeful construction strategy for photodetection applications. However, the existence of interfacial states is an obstacle for their actualization. Herein, we developed a kind of Ti3C2Tx/Ge vdW Schottky junction photodetector with Al2O3 interfacial layer through a facile drop of Ti3C2Tx MXene solution on the well-defined Al2O3/Ge substrate. The Al2O3 interfacial layer brings a positive impact on the suppression of dark current and the improvement of light current. Actually, the device shows remarkable photodetection characteristics at 1550 nm with self-powered mode in term of a high responsivity of 665 mA/W, a large specific detectivity of 1.24 × 1013 Jones, a fast response speed of 73.5/81.5 μs. The device also exhibits a broadband response spectrum from 405 to 1550 nm. These fascinating performances and the simple device preparation make the Ti3C2Tx/Al2O3/Ge vdW Schottky junction be promising for the low cost but high performance optoelectronic devices.

Al2O3 interfacial layer derived hybrid conductive filament for the reliability enhancement of Ta2O5-based resistive random access memory

This study used an Al2O3 interfacial layer to improve the reliability characteristics of Ag/Ta2O5/Au resistive random access memory (RRAM), such as endurance and retention. The Ag/Ta2O5/Au device without the Al2O3 interfacial layer exhibited a high on/off ratio, low endurance, and poor retention. In Ag/Ta2O5/Au devices, it is difficult to precisely control the formation and rupture of conductive filaments (CFs) owing to the high diffusion coefficient of Ag cations. However, in the RS mechanism of Ag/Ta2O5/Au with an Al2O3 interfacial layer, a hybrid CF composed of Ag cations in Ta2O5 and oxygen vacancies in the Al2O3 layer was formed during the set process. During the reset process, the hybrid CF was stably ruptured by the recombination of oxygen vacancies and oxygen ions at the Al2O3 interfacial layer. This RS mechanism induces stable switching characteristics, improving endurance and retention. Therefore, Ag/Ta2O5/Au with an Al2O3 interfacial layer exhibits relatively high endurance (up to 700 cycles) and excellent retention (up to 1.0 × 104 s) while maintaining a high on/off ratio (>106) compared to those of the Ag/Ta2O5/Au device.

Irradiation-enhanced Interactions at UO2/Al2O3/Al Interfaces

Combustion synthesis is used to prepare thin UO2 films on aluminum alloy substrates. This simple preparation method involves electrospraying uranyl nitrate + acetylacetone + 2-methoxyethanol solution on the substrate, followed by a short annealing at 350 or 550 °C. The irradiation of films with a 40Ar2+ ion beam (energy of 1.7 MeV and fluences of 7.7 × 1016 and 1.3 × 1017 ions/cm2) is conducted to investigate irradiation-induced restructuring processes. High-resolution transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) investigations show that the annealing temperature significantly influences the crystallinity and stability of materials during irradiation. A small amount of Mg in the alloy substrate diffuses into the amorphous Al2O3 interfacial layer between the film and the substrate. Local thermal spikes from the incoming ions facilitate the irradiation-induced mixing of immiscible Al2O3 and UO2 for the materials prepared at 350 °C. This mass diffusion produces relatively large cavities at the interface. Selective diffusion of a more significant amount of Mg for the materials prepared at 550 °C suppresses the mixing of the Al2O3 interlayer with the film but forms MgyU1–yO2±x solid solutions during irradiation. Local thermal heating triggers the precipitation of a discontinuous crystalline MgO layer close to the film surface. The enhanced and selective diffusion of Mg into the film makes the materials prepared at 550 °C more robust and mechanically stable than those prepared at 350 °C.

Monolayer MoS2-based transistors with low contact resistance by inserting ultrathin Al2O3 interfacial layer

Monolayer MoS2-based transistors with low contact resistance by inserting ultrathin Al2O3 interfacial layer

(Invited, Digital Presentation) Thermal Conductance across Heterogeneously Integrated Interfaces for Thermal Management of Wide and Ultra-Wide Bandgap Electronics

Thermal management is important for wide and ultra-wide bandgap power electronics because overheating degrades device reliability and performance. High thermal conductivity substrates such as SiC and diamond facilitate heat dissipation of these devices while the thermal boundary resistance between devices and substrates accounts for a large portion of the total thermal resistance, which prevents devices from taking the full advantage of the high thermal conductivity of the substrates. Recently, heterogeneously integrated wide and ultra-wide bandgap semiconductor interfaces are found to have low thermal boundary resistances, which provides a new degree of freedom to design and fabricate thermally conductive interfaces for thermal management of related power devices. For example, GaN can be bonded with single crystal SiC or single crystal diamond directly at room temperature. β-Ga2O3 can be bonded with SiC with or without Al2O3 interfacial layers. Compared with growth, the bonded interfaces are not limited by lattice mismatch. Moreover, the room-temperature bonding process discussed in this talk can possibly eliminate the effects of thermal stress existed in high temperature growth or bonding techniques. Recent progresses in this sub-area will be discussed in this talk, especially thermal conductance across surface-activated bonded GaN and β-Ga2O3 interfaces measured by time-domain thermoreflectance (TDTR) and the effects of thermal boundary resistance values on device temperatures. Finally, the potential challenges will also be pointed out, for instance, high-throughput thermal measurements of buried interfaces, thermal property-structure relations of interfaces bonded under different conditions, theoretical understanding of interfacial thermal transport, and device demonstrations.

Surface transfer doping of MoO3 on hydrogen terminated diamond with an Al2O3 interfacial layer

A thin layer of Al2O3 was employed as an interfacial layer between surface conductive hydrogen-terminated (H-terminated) diamond and MoO3 to increase the distance between the hole accumulation layer in diamond and negatively charged states in the acceptor layer and, thus, reduce the Coulomb scattering and increase the hole mobility. The valence band offsets are found to be 2.7 and 3.1 eV for Al2O3/H-terminated diamond and MoO3/H-terminated diamond, respectively. Compared to the MoO3/H-terminated diamond structure, a higher hole mobility was achieved with Al2O3 inserted as an interface layer. This work provides a strategy to achieve increased hole mobility of surface conductive diamond by using optimal interlayer along with high high electron affinity surface acceptor materials.

High-performance near-infrared photodetectors based on gate-controlled graphene–germanium Schottky junction with split active junction

Abstract The structure of a gate-controlled graphene/germanium hybrid photodetector was optimized by splitting the active region to achieve highly sensitive infrared detection capability. The strengthened internal electric field in the split active junctions enabled efficient collection of photocarriers, resulting in a responsivity of 2.02 A W−1 and a specific detectivity of 5.28 × 1010 Jones with reduced dark current and improved external quantum efficiency; these results are more than doubled compared with the responsivity of 0.85 A W−1 and detectivity of 1.69 × 1010 Jones for a single active junction device. The responsivity of the optimized structure is 1.7, 2.7, and 39 times higher than that of previously reported graphene/Ge with Al2O3 interfacial layer, gate-controlled graphene/Ge, and simple graphene/Ge heterostructure photodetectors, respectively.

Conformal MoS2/Silicon Nanowire Array Heterojunction with Enhanced Light Trapping and Effective Interface Passivation for Ultraweak Infrared Light Detection

Abstract2D layered materials have attracted considerable attention for fabricating IR photodetectors. However, performance, especially for weak signal detection, of these IR photodetectors is often limited by the low absorbance of the very thin active materials as well as unsuppressed dark currents. Herein, a photodetector with high sensitivity for ultraweak IR signals based on a conformal MoS2/silicon nanowire array heterojunction with an ultrathin Al2O3 interfacial passivation layer is reported. The conformal light‐trapping nanoarray structure can greatly enhance broadband IR absorption, while the Al2O3 layer can suppress interface carrier recombination effectively and thus lower reverse dark current. The photodetectors exhibit broadband photoresponse ranging from 300 to 1600 nm, low noise current of 0.11 pA Hz−1/2, large responsivity up to 0.61 A W−1 at zero bias voltage, and high specific detectivity of 1011–1012 Jones. Significantly, the devices are capable of detecting ultraweak IR signals (e.g., 100 pW at 808 nm and 3 nW at 1310/1550 nm) with high on–off ratios without applying any external bias. This work proposes a new strategy to enhance light absorption of 2D materials and construct high‐quality junction based on them, which is promising for low‐cost and high‐performance optoelectronic devices.

The role of oxygen defects engineering via passivation of the Al2O3 interfacial layer for the direct growth of a graphene-silicon Schottky junction solar cell

Over the past decade, graphene-based solar cells have received increasing exploration. In particular, the metal–insulator–semiconductor (MIS)-type solar cells have an inherent cost advantage compared to the p-n junction solar cells. However, the technological progress in graphene solar cells based on the MIS junction is currently facing a number of serious challenges, including long-term stability, surface recombination at the interface level, and surface defects. To overcome these barriers, a simple, cost-effective method is developed herein for the interfacial single-component passivation of Al2O3 with NH3 or H2O2, and co-passivation with NH3 and H2O2 together. The interfacial layer between the silicon and the directly-grown graphene operates as an effective electron blocking layer, thus leading to the minimization of surface recombination. Further, the NH3-H2O2 co-passivated Al2O3 performs the dual role of providing a good platform for the growth of graphene and enhancing the performance of the fabricated solar cell. Moreover, in a proof of concept demonstration, the fabricated graphene-silicon (Gr/Si) Schottky junction with an NH3-H2O2 co-passivated Al2O3 interfacial layer exhibits an efficiency of 9.49%, which is a major enhancement over that of the Gr/Si solar cell without an interfacial layer (i.e., 3.19%). These results demonstrate that the control of oxygen-related defects containing non-lattice oxygen in the Al2O3 interfacial layer remarkably impacts the charge transfer resistance and electron recombination processes. This work can be viewed as an elegant interface engineering approach to the co-passivation of Al2O3 for optoelectronic applications.

Effect of the Al2O3 interfacial layer thickness on the measurement temperature-induced I–V characteristics in Au/Ti/Al2O3/n-GaAs structures

Effect of the Al2O3 interfacial layer thickness on the measurement temperature-induced I–V characteristics in Au/Ti/Al2O3/n-GaAs structures

Spin-coating for fabrication of high-entropy high-k (AlTiVZrHf)Ox films on Si for advanced gate stacks

Spin-coating was performed to fabricate amorphous high-entropy oxide (HEO) (AlTiVZrHf)Ox films on Si substrates. The films were evaluated through X-ray photoelectron spectroscopy and through transmission electron microscopy (TEM)-based energy-dispersive spectroscopy (EDS) mapping, which respectively revealed all constituent elements and the homogenous distributions and no aggregation of those elements. TEM analysis revealed four distinct layers—a crystalline Al metal gate, an amorphous Al2O3 interfacial layer, amorphous (AlTiVZrHf)Ox, and native amorphous SiO2—in a patterned metal–oxide–semiconductor (MOS) gate stack on the Si substrate. The resulting stack exhibited a low leakage current density (JL) and no frequency dispersion in its capacitance–voltage characteristics after undergoing forming gas annealing. The obtained dielectric constant (k ≈ 32) for the HEO film is promising for advanced gate stacks and transistor applications. Although the film exhibited minor nanocrystallinity and the stack exhibited no interfacial Al2O3 layer after rapid thermal annealing at 900 °C, a low JL, distinct layers, and a clearly defined interface between the (AlTiVZrHf)Ox and SiO2 were observed, indicating no substantial diffusion among these layers. EDS mapping revealed the homogenous distribution of each constituent element without aggregation. Medium-entropy-oxide films and their MOS devices were fabricated for comparison; however, they exhibited inferior performance.

Improved synaptic functionalities of Li-based nano-ionic synaptic transistor with ultralow conductance enabled by Al2O3 barrier layer

In this study, we investigated the effect of an Al2O3 barrier layer in an all-solid-state inorganic Li-based nano-ionic synaptic transistor (LST) with Li3PO4 electrolyte/WO x channel structure. Near-ideal synaptic behavior in the ultralow conductance range (∼50 nS) was obtained by controlling the abrupt ion migration through the introduction of a sputter-deposited thin (∼3 nm) Al2O3 interfacial layer. A trade-off relationship between the weight update linearity and on/off ratio with varying Al2O3 layer thickness was also observed. To determine the origin of the Al2O3 barrier layer effects, cyclic voltammetry analysis was conducted, and the optimal ionic diffusivity and mobility were found to be key parameters in achieving ideal synaptic behavior. Owing to the controlled ion migration, the retention characteristics were considerably improved by the Al2O3 barrier. Finally, a highly improved pattern recognition accuracy (83.13%) was achieved using the LST with an Al2O3 barrier of optimal thickness.

Achieving a Low-Voltage, High-Mobility IGZO Transistor through an ALD-Derived Bilayer Channel and a Hafnia-Based Gate Dielectric Stack

Ultrahigh-resolution displays for augmented reality (AR) and virtual reality (VR) applications require a novel architecture and process. Atomic-layer deposition (ALD) enables the facile fabrication of indium-gallium zinc oxide (IGZO) thin-film transistors (TFTs) on a substrate with a nonplanar surface due to its excellent step coverage and accurate thickness control. Here, we report all-ALD-derived TFTs using IGZO and HfO2 as the channel layer and gate insulator, respectively. A bilayer IGZO channel structure consisting of a 10 nm base layer (In0.52Ga0.29Zn0.19O) with good stability and a 3 nm boost layer (In0.82Ga0.08Zn0.10O) with extremely high mobility was designed based on a cation combinatorial study of the ALD-derived IGZO system. Reducing the thickness of the HfO2 dielectric film by the ALD process offers high areal capacitance in field-effect transistors, which allows low-voltage drivability and enhanced carrier transport. The intrinsic inferior stability of the HfO2 gate insulator was effectively mitigated by the insertion of an ALD-derived 4 nm Al2O3 interfacial layer between HfO2 and the IGZO film. The optimized bilayer IGZO TFTs with HfO2-based gate insulators exhibited excellent performances with a high field-effect mobility of 74.0 ± 0.91 cm2/(V s), a low subthreshold swing of 0.13 ± 0.01 V/dec, a threshold voltage of 0.20 ± 0.24 V, and an ION/OFF of ∼3.2 × 108 in a low-operation-voltage (≤2 V) range. This promising result was due to the synergic effects of a bilayer IGZO channel and HfO2-based gate insulator with a high permittivity, which were mainly attributed to the effective carrier confinement in the boost layer with high mobility, low free carrier density of the base layer with a low VO concentration, and HfO2-induced high effective capacitance.

Highly responsive near-infrared photodetector with low dark current using graphene/germanium Schottky junction with Al2O3 interfacial layer

Abstract The performance of a graphene/Ge Schottky junction near-infrared photodetector is significantly enhanced by inserting a thin Al2O3 interfacial layer between graphene and Ge. Dark current is reduced by two orders of magnitudes, and the specific detectivity is improved to 1.9 × 1010 cm ⋅ Hz1/2W−1. The responsivity is improved to 1.2 AW−1 with an interfacial layer from 0.5 AW−1 of the reference devices. The normalized photo-to-dark current ratio is improved to 4.3 × 107 W−1 at a wavelength of 1550 nm, which is 10–100 times higher than those of other Ge photodetectors.

Enhanced High-Frequency Performance of Top-Gated Graphene FETs Due to Substrate- Induced Improvements in Charge Carrier Saturation Velocity

The high-frequency performance of top-gated graphene field-effect transistors (GFETs) depends to a large extent on the saturation velocity of the charge carriers, a velocity limited by inelastic scattering by surface optical phonons from the dielectrics surrounding the channel. In this work, we show that, by simply changing the graphene channel surrounding dielectric with a material having higher optical phonon energy, one could improve the transit frequency and maximum frequency of oscillation of GFETs. We fabricated GFETs on conventional SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> /Si substrates by adding a thin Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> interfacial buffer layer on top of SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> /Si substrates, a material with about 30% higher optical phonon energy than that of SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> , and compared performance with that of GFETs fabricated without adding the interfacial layer. From S-parameter measurements, a transit frequency and a maximum frequency of oscillation of 43 and 46 GHz, respectively, were obtained for GFETs on Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> with 0.5- <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu \text{m}$ </tex-math></inline-formula> gate length. These values are approximately 30% higher than those for state-of-the-art GFETs of the same gate length on SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> . For relating the improvement of GFET high-frequency performance to improvements in the charge carrier saturation velocity, we used standard methods to extract the charge carrier velocity from the channel transit time. A comparison between two sets of GFETs with and without the interfacial Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> layer showed that the charge carrier saturation velocity had increased from <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$1.5\cdot 10^{{7}}$ </tex-math></inline-formula> to <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$2\cdot 10^{{7}}$ </tex-math></inline-formula> cm/s.

Memristive and artificial synapse performance by using TiOx/Al2O3 interface engineering in MoS2-based metallic filament memory

The MoS2 as a switching material has recently shown promising resistive switching characteristics. In this work, we demonstrate the impact of TiOx/Al2O3 interfacial layer on memristive/artificial synapse characteristics using MoS2-based Al/Cu/TiOx (or)Al2O3/MoS2/TiN structure. Owing to its barrier properties over Cu migration, the memristor with 2 nm-thick TiOx layer shows significant improvement in resistive switching as compared to both Al2O3 interfacial layer and 4 nm-thick MoS2. The memristor shows uniform long P/E endurance of 2 × 109 cycles under a low operation current of 200 μA and the high switching speed of 100 ns is applied. The ex-situ transmission electron microscope image reveals the formation of Cu metallic filament in MoS2 layer after long P/E endurance. At high operation current (1 mA), both the TiOx and Al2O3 interfacial layers show long P/E endurance of >109 cycles. In addition, the TiOx based memory device shows excellent RESET voltage controlled gradual RESET behavior without memory loss, enhanced uniform artificial synapse behavior with a maximum long-term potentiation/depression states of 55/500 at a small energy consumption of 5.7 pJ for future memory in computing application.