Junction Devices Research Articles

First principle atomistic modelling of magnetic tunnel junction (MTJ) to enhance its tunnel magneto-resistance (TMR) ratio

Abstract This paper investigates the impact of electrode materials on the Tunnel Magneto-Resistance (TMR) ratio of Magnetic Tunnel Junction (MTJ) device. Four different structures of MTJ have been simulated by using cobalt (Co), Nickel (Ni), Iron (Fe) and two alloy materials of nickel-iron (NiFe) and cobalt-iron (CoFe). These materials have been used as ferromagnetic electrodes. Mulliken population and transmission spectrum observed in both parallel and antiparallel configurations of these devices to understand the spin transport properties and Tunnel Magneto-Resistance (TMR) ratio has been estimated. The first principal study was performed based on density function theory (DFT) and Non-equilibrium Green’s Function (NEGF) computational methods using the QuantumATK simulation tool to study properties such as band structure, the density of states (DOS), Spin Transfer Torque (STT), I-V characteristics and TMR. This study explores how different electrode materials affect the Tunnel Magneto-Resistance (TMR) ratio in Magnetic Tunnel Junction (MTJ) devices. With these results, it is observed that cobalt-based MTJ devices (that is Co-MgO-Fe and CoFe-MgO-CoFe) exhibit higher TMR ratio as compared to Nickel- and Iron-based MTJ devices (that is NiFe-MgO-NiFe and Ni-MgO-Fe). As Cobalt has a high spin polarization this property makes it suitable for use in spintronics devices like MTJs, where the manipulation of electron spins is essential for data storage and information processing. These findings can be employed to improve the performance characteristics of the MTJ-based Random Access Memory (MRAM) in the field of spintronics.

Ab initio study of highly tunable charge transfer in β−RuCl3 /graphene heterostructures

Heterostructures of graphene in proximity to magnetic insulators open the possibility to investigate exotic states emerging from the interplay of magnetism, strain and charge transfer between the layers. Recent reports on the growth of self-integrated atomic wires of β−RuCl3 on graphite suggest these materials as versatile candidates to investigate these effects. Here we present detailed first-principles calculations on the charge transfer and electronic structure of β−RuCl3/graphene heterostructures and provide a comparison with the work function analysis of the related honeycomb family members α−RuX3 (X = Cl,Br,I). We find that proximity of the two layers leads to a hole-doped graphene and electron-doped RuX3 in all cases, which is sensitively dependent on the distance between the two layers. Furthermore, strain effects due to lattice mismatch control the magnetization which itself has a strong effect on the charge transfer. Charge accumulation in β−RuCl3 strongly drops away from the chain making such heterostructures suitable candidates for sharp interfacial junctions in graphene-based devices. Published by the American Physical Society 2024

Investigation of ferro-resistive switching mechanisms in TiN/Hf0.5Zr0.5O2/WOx/W ferroelectric tunnel junctions with the interface layer effect

This study presents an in-depth analysis of ferro-resistive switching (FRS) behaviors in a TiN/Hf0.5Zr0.5O2(HZO)/WOx/W ferroelectric tunnel junction (FTJ) device, with a particular focus on the role of the tungsten oxide (WOx) interface layer (IL). Structural examinations confirm the presence of the WOx IL, which significantly influences the FRS properties of the device. Electrical measurements indicate the devices exhibit stable and reproducible FRS characteristics with an ON/OFF ratio of 9.7, predominantly attributed to the tunneling electro-resistance (TER) effect driven by the ferroelectric polarization. Comprehensive numerical simulations, incorporating the nucleation-limited switching model and Simmons tunneling mechanism, provide detailed insights into how the WOx IL and the trapped charges at the HZO/WOx interface affect polarization switching mechanisms and the electronic potential barrier profile. These findings underscore the importance of interface effects in HfO2-based FTJs and advance the understanding of the TER mechanism in multilayer ferroelectric systems.

Enhancement of Near‐Infrared Photovoltaic Response of Si/Au Schottky‐Junction Structure by Band‐Bending Approaches

A combined approach of band bending is employed to enhance the near‐infrared (NIR) photovoltaic (PV) response of a Si/Au Schottky junction (SHJ) device. As a reference, the basic architecture of the SHJ device consists of textured n‐Si and gold nanoparticles (Au NPs). Its photoelectric conversion efficiency at 1319 nm wavelength is 0.0302%. A series of band‐bending approaches are then applied to the reference device in sequence, aiming to minimize the electric loss of the structure by strengthening built‐in electric field. These approaches include introductions of an Al2O3 layer for front field passivation, a p+‐Si layer to form a p+n‐like junction at the front of the device and a SiO2 layer for rear field passivation. With these modifications, under 1319 nm illumination, the open‐circuit voltage eventually increases by 14%, the short‐circuit current increases by 18% and the photoelectric conversion efficiency of the device increases by 66% to reach 0.0501%. The results of this work propose a strategy to minimize the electric loss in an NIR PV device.

Integrated mid-infrared sensing and ultrashort lasers based on wafer-level Td-WTe2 Weyl semimetal

There is an urgent need for infrared (IR) detection systems with high-level miniaturization and room-temperature operation capability. The rising star of two-dimensional (2D) semimetals with extraordinary optoelectronic properties can fulfill these criteria. However, the formidable challenges with regard to large-scale patterning and substrate-selective requirements limit material deposition options for device fabrication. Here, we report a convenient and straightforward eutectic-tellurization transformation method for the wafer-level synthesis of 2D type-II Weyl semimetal WTe2. The non-cryogenic WTe2/Si Schottky junction device displays an ultrawide detection range covering 10.6 μm with a high detectivity of ∼109 Jones in the mid-infrared (MIR) region and a short response time of 1.3 μs. The detection performance has surpassed most reported IR sensors. On top of that, on-chip device arrays based on Schottky junction display an outstanding MIR imaging capability without cryogenic cooling, and 2D WTe2 Weyl semimetal can serve as a saturable absorber for stable Q-switched and mode-locked laser operation applications. Our work offers a viable route for wafer-scale vdW preparation of 2D semimetals, showcasing their intriguing potential in on-chip integrated MIR detection systems and ultrafast laser photonics.

Common Mode and Terminal Voltages With Effect of Device Junction Capacitance in Five-Level Transformerless PV Inverter

Common Mode and Terminal Voltages With Effect of Device Junction Capacitance in Five-Level Transformerless PV Inverter

Revealing Bipolar Photoresponse in Multiheterostructured WTe2-GaTe/ReSe2-WTe2 P-N Diode by Hybrid 2D Contact Engineering.

The van der Waals (vdW) heterostructures based on two-dimensional (2D) semiconducting materials have been thoroughly investigated with regard to practical applications. Recent studies on 2D materials have reignited attraction in the p-n junction, with promising potential for applications in both electronics and optoelectronics. 2D materials provide exceptional band structural diversity in p-n junction devices, which is rare in regular bulk semiconductors. In this article, we demonstrate a p-n diode based on multiheterostructure configuration, WTe2-GaTe-ReSe2-WTe2, where WTe2 acts as heterocontact with GaTe/ReSe2 junction. Our devices with heterocontacts of WTe2 showed excellent performance in electronic and optoelectronic characteristics as compared to contacts with basic metal electrodes. However, the highest rectification ratio was achieved up to ∼2.09 × 106 with the lowest ideality factor of ∼1.23. Moreover, the maximum change in photocurrent (Iph) is measured around 312 nA at Vds = 0.5 V. The device showed a high responsivity (R) of 4.7 × 104 m·AW-1, maximum external quantum efficiency (EQE) of 2.49 × 104 (%), and detectivity (D*) of 2.1 × 1011 Jones at wavelength λ = 220 nm. Further, we revealed the bipolar photoresponse mechanisms in WTe2-GaTe-ReSe2-WTe2 devices due to band alignment at the interface, which can be modified by applying different gate voltages. Hence, our promising results render heterocontact engineering of the GaTe-ReSe2 heterostructured diode as an excellent candidate for next-generation optoelectronic logic and neuromorphic computing.

Enhancement of tunneling electroresistance in metal/two-dimensional ferroelectric tunnel junctions: route for polarization-modulated interface transport

In fabricating ferroelectric tunnel junction (FTJ) devices, it is essential to employ low-resistance metals as electrodes interfacing with two-dimensional (2D) ferroelectric materials. For FTJs with a top contact configuration, two interfaces for charge transport are present, namely the vertical interface between the metal electrode and the 2D ferroelectric material, and the lateral interface between the electrode and the central scattering region. These interfaces significantly influence the tunneling electroresistance (TER) of FTJs. However, there exists a notable deficiency in comprehension concerning the physics of charge transport at the interface. In this work, we explore the interface transport properties in FTJs featuring a top contact configuration between metal and the typicalα-In2Se3monolayer. By employing the non-equilibrium Green's function method, we observe a TER ratio of1.15×105% for the Pd top contact interfacing with anα-In2Se3monolayer. The significant TER effect is attributed to polarization-controlled interface transport, which is further elucidated through an analysis of the transport mechanisms influenced by the out-of-plane polarization ofα-In2Se3at the vertical interface and the in-plane polarization at the lateral interface. This investigation of the fundamental physical mechanisms of polarization-controlled interface transport demonstrates significant potential for enhancing non-volatile memory devices.

Back-Side Stress to Ease p-MOSFET Degradation on e-MRAM Chips

Abstract Magnetoresistive random access memory process makes great contribution to threshold voltage deterioration of metal-oxide-silicon field-effect transistors, especially on p-type devices. Herein, a method was proposed to reduce threshold voltage degradation by utilizing back-side stress. Through the deposition of tensile material on the back side, positive charges generated by silicon-hydrogen bond breakage were inhibited, resulting in a potential reduction of threshold voltage shift by up to 20%. In addition, it was found that the method could only relieve silicon-hydrogen bond breakage physically, failing to provide a complete cure. However, it holds significant potential for applications where additional thermal budget is undesired. Furthermore, it was also concluded that the method used in this work was irreversible, with its effect sustained to the chip package phase, and it ensures competitive reliability of the resulting magnetic tunnel junction devices.

Density Functional Theory (DFT) Based Local Density Approximation (LDA) Study on Tailoring Electronic and Optical Properties of SnO and In-Doped SnO

The ambipolar nature of SnO has significantly increased its potential for using in p-n junction devices, for which it has drawn the attention of the scientific community. In this paper, the structural, electronic, and optical properties of SnO and the impact of Indium (In) doping into SnO are computed by Local Density Approximation (LDA) under the density function theory (DFT) framework. The calculated bond length of SnO in SnO is 2.285 Å, and that deviates  3% from the experimental value. The SnO and InO bond lengths in In-doped SnO are calculated to be 2.3094 and 2.266 Å, respectively. Interestingly, The band gap of pure SnO is calculated to be 2.61 eV, whereas it is significantly dropped down to 2.00 eV in the case of In doped SnO. The Total Density of State (DOS), Partial DOS, and electron density are depicted for SnO and In-doped SnO. As a consequence of In-doping static value of the refractive index and the real part of the dielectric function for SnO decrease from 1.9 to 1.4 and 3.6 to 1.97, respectively. Therefore, In-doping enhances the properties of the SnO film, which may lead the material to be applied in the future to develop electronic and opto-electronic devices.

On-line junction temperature identification of converter power devices

Abstract Traction converters often fail or age due to long operation hours, complex conditions, and frequent braking, mainly caused by overheating or temperature fluctuations. Monitoring the temperature at the junction of power device is crucial for system reliability. To achieve this, the electric-heat coupling models of IGBT are studied. Models for IGBT’s switching and conduction processes were created using its data sheet, yielding a power loss model linked to junction temperature. A Foster thermal network model for IGBT was then established, deriving the equation for temperature change when losses act as heat sources. Combining these models, an electric-heat coupling model was built on the SIMULINK platform, applied to predict and simulate junction temperature in a two-level inverter, generating a temperature-time image. Finally, an experimental test platform for a two-level SVPWM-modulated inverter was set up, using an infrared sensor to measure IGBT’s steady-state junction temperature. This was compared with simulation results to verify the model’s accuracy.

Study and analysis of the effects of trap assisted tunneling on the sensing performance of InAs/Si hetero juncture TFET based biosensor

This article investigates the impact of trap related non-idealities on the sensing characteristics of Indium Arsenide/Silicon (InAs/Si) heterojuncture tunnel field-effect transistor (TFET) with dual metal stacked dielectric. Presence of straddling band alignment improves the tunneling probability across InAs/Si junction of the present device thereby boosting its sensing ability. Two cavities carved in the gate dielectric across the source side act as the sensing site for the detection of biomolecules. These biomolecules are immobilized within the implanted cavity to induce the drain current. Extensive simulation is carried out using Silvaco ATLAS device simulator activating trap assisted tunneling model to capture the realistic environment of biosensor in contrast to ideal condition for better reliability analysis. The study shows that trap assisted tunneling effectively modulates the important sensitivity metrics of the biosensor including ON current sensitivity, sub-threshold sensitivity with significant impact on Id-VGS characteristics, subthreshold slope profiles and Ion/Ioff ratio and hence cannot be neglected for accurate estimation of sensitivity for real time applications.

Studies on Dual Sputtered CuGaO2 Thin Films

There has been extensive research conducted on n-type transparent conducting oxide (TCO) films like Sn-doped In2O3 (ITO) [1], Al-doped ZnO (AZO) [2], etc. P-type transparent conducting oxides are yet to be researched extensively. Active devices such as p-n junction devices and transistors are required to make progress in the development of transparent electronics. In this effort, research in p-Type TCOs is important. Most of the p-Type TCOs are copper-based ternary oxides [3].In this work, CuGaO2 thin films were prepared using the dual-target RF magnetron sputtering technique. Cu2O and Ga2O3 targets were simultaneously sputtered on quartz substrates. The sputtering power of Cu2O was kept constant at 50 W and the sputtering power of Ga2O3 was varied. The films were deposited at room temperature and were subsequently annealed in nitrogen ambiance at 900 °C. The X-ray diffraction and optical studies were performed to analyze the films to optimize their properties.[1] Fallah, H.; Ghasemi, M.; Hassanzadeh, A.; Steki, H. The effect of annealing on structural, electrical and optical properties of nanostructured ITO films prepared by e-beam evaporation. Mater. Res. Bull. 2007, 42, 487–496[2] Shantheyanda, B.P.; Todi, V.O.; Sundaram, K.B.; Vijayakumar, A.; Oladeji, I. Compositional study of vacuum annealed Al doped ZnO thin films obtained by RF magnetron sputtering. J. Vac. Sci. Technol. A 2011, 29, 051514.[3] Sundaresh, S.; Bharath, A.H.; Sundaram, K.B. Effect of Cu2O Sputtering Power Variation on the Characteristics of Radio Frequency Sputtered p-Type Delafossite CuCrO2 Thin Films. Coatings 2023, 13, 395.

Organic solar cells: Exploring the future and sustainability of clean energy

As a critical technology in renewable energy, organic solar cells play an important role in energy transition and sustainable development. This paper examines the development and significance of organic solar cells in terms of energy demand, environmental requirements, and industrial growth. With the development of China's energy structure adjustment and carbon peak carbon neutral targets, interest in novel energy technologies is growing. The paper then describes the interaction between photons and materials and the electron-hole separation and charge transport mechanism of organic solar cells. In addition, the characteristics and potential applications of organic solar cells with various structures, such as bilayer heterojunction devices, intrinsic heterojunction devices, molecular DA junction devices, and stacked devices, are described in depth. In the outlook section, the paper emphasizes the future potential of organic solar cells, including efficiency enhancements, cost reductions, and integration with energy storage technologies. In conclusion, organic solar cells have tremendous potential in the field of renewable energy, and they are anticipated to lead the energy transition and contribute to sustainable development.

Impact of Perovskite Subcell Breakdown on the Performance of Perovskite/Perovskite/Silicon Triple‐Junction Solar Cells

Perovskite‐based triple‐junction solar cells have recently gained significant attention and are rapidly developing, thanks to the insights gained from the advancement in its dual‐junction counterparts. However, employing perovskite materials in multijunction solar cells with more than two junctions brings new challenges that have not yet been addressed. One aspect is the possibility of reverse bias breakdown of perovskite subcells during operation of the triple–junction device. This is more relevant for triple‐junction solar cells because a higher reverse voltage might drop at perovskite subcells compared to the case of dual‐junction solar cells. Herein, the breakdown voltages of the two perovskite subcells in perovskite/perovskite/silicon triple‐junction solar cells are determined by progressively increasing the reverse bias applied to the subcells in a single‐junction architecture during current–voltage measurements and monitoring the appearance of shunts using illuminated lock‐in thermography measurements. Furthermore, to analyze the effect on the final triple–junction solar cell, the triple‐junction device is brought in different current limitation conditions. It is shown that the subcell breakdown can happen during the operation of the triple‐junction solar cell, especially for the case where the perovskite top cell is limiting the overall current of the device. This effect is less severe when the middle perovskite cell limits the current due to the absence of a direct contact with the silver metallization which has shown to be the major degradation site during reverse biasing of perovskite solar cells. Finally, there is no concern regarding breakdown of the silicon bottom cell due to the higher breakdown voltage of silicon compared to perovskite.

Maximizing the Synaptic Efficiency of Ferroelectric Tunnel Junction Devices Using a Switching Mechanism Hidden in an Identical Pulse Programming Learning Scheme

Memristors play a pivotal role in advanced computing, with memristor‐based crossbar arrays showing promise for various artificial neural networks. Among these, HfO2‐based ferroelectric tunnel junctions (FTJs) stand out as ideal synaptic devices for neuromorphic computing. Their compatibility with the complementary metal oxide semiconductor process and intrinsic energy efficiency make them particularly appealing. While an increasing number of studies adopt identical pulse programming (IPP) with short width to update the conductance of HfO2‐based FTJs synaptic devices, conventional ferroelectric switching models fall short in describing updates the conductance with the IPP scheme. Consequently, studies achieving conductance updates via IPP lack an underlying mechanism explanation, potentially limiting the application of HfO2‐based FTJs as synaptic devices. This study explores the potential of ferroelectric Zr‐doped HfO2 (HZO) FTJs to undergo learning through the IPP scheme. Synaptic characteristics, including the number of conductance states, symmetry, linearity, write energy, and latency by modulating IPP scheme conditions are optimized. Finally, the applicability of HZO FTJ as a synaptic device by assessing learning accuracy in pattern recognition through artificial neural network simulation based on the optimized synaptic characteristics is evaluated.

Effect of the insertion of silver nanoparticles on the performance and electrical features of Ag/SiNWs Schottky diode

In this paper, we studied the effect of the passivation of silicon nanowires (SiNWs) by silver nanoparticles (AgNPs) on the efficiency of Ag/SiNWs/Si Schottky diodes. SiNWs are obtained by one-step Ag-assisted chemical etching method. AgNPs were deposited on SiNWs using an electroless dipping method. Schottky junction devices have been successfully made using silicon nanowires coated with silver nanoparticles. The current–voltage (I-V) characteristics of Ag/AgNPs-SiNWs Schottky diode have been investigated by varying the immersion time in silver nitrate solution from 1 to 5 min. The I-V characteristics presented in logarithmic scale confirmed the domination of SCLC conduction mechanism at high voltage. The ideality factor (n), height of potential barrier (φb) and series resistance (Rs) of diodes are calculated by adopting the Cheung method. Compared to the Ag/SiNWs Schottky junction, the diode that presents AgNPs at the interface shows a significant improvement in electrical parameters. The potential barrier reaches 1.09 eV and the ideality factor is reduced to 2.64 with the presence of AgNPs for an immersion time of 1 min. Norde’s equation has also been used to calculate the series resistance and the potential barrier. The results obtained by Norde method are slightly shifted compared to those found by Cheung functions while the findings concerning the optimization of the immersion time are practically the same.

Wafer-scale CMOS-compatible graphene Josephson field-effect transistors

Electrostatically tunable Josephson field-effect transistors (JoFETs) are one of the most desired building blocks of quantum electronics. Applications of JoFETs range from parametric amplifiers and superconducting qubits to a variety of integrated superconducting circuits. Here, we report on graphene JoFET devices fabricated with wafer-scale complementary metal-oxide-semiconductor (CMOS)-compatible processing based on chemical-vapor-deposited monolayer graphene encapsulated with atomic-layer-deposited Al2O3 gate oxide, lithographically defined top gate, and evaporated superconducting Ti/Al source, drain, and gate contacts. By optimizing the contact resistance down to ∼170 Ω μm, we observe proximity-induced superconductivity in the JoFET channels with different gate lengths of 150–350 nm. The Josephson junction devices show reproducible critical current Ic tunablity with the local top gate. Our JoFETs are in the short diffusive limit with the Ic reaching up to ∼3 µA for a 50 µm channel width. Overall, our demonstration of CMOS-compatible two-dimensional (2D) material-based JoFET fabrication process is an important step toward graphene-based integrated quantum circuits.

Preparation of a/c-oriented YBa2Cu3O7−x homogeneous superconducting junctions via pulsed laser deposition

The growth orientation of YBa2Cu3O7−x (YBCO) is extremely sensitive to the growth atmosphere and the heat-treatment temperature. However, according to the existing literature, to obtain YBCO with preferred a-axis orientation, a buffer layer needs to be prepared on the substrate. In this work, YBCO films were grown via pulsed laser deposition, and the influence of N2 and O2 deposition atmospheres on the growth orientation of the YBCO films was studied. It was found that high-quality YBCO films with preferred c-axis orientation can be grown in O2 atmosphere, but it is difficult to grow a-axis-oriented YBCO films in O2 atmosphere without using a buffer layer; however, YBCO films with a high degree of preferred a-axis orientation can be grown in N2 atmosphere without using a buffer layer. Furthermore, an a/c-oriented homogeneous YBCO bilayer with c-axis orientation in the lower layer and a-axis orientation in the upper layer was prepared. X-ray diffraction and scanning transmission electron microscopy results show that the bilayer is epitaxial with a good a/c orientation, and the resistance–temperature curve shows the occurrence of two transitions at temperatures of 91 and 71 K. We believe that these temperatures correspond to the superconducting onset transition temperatures of YBCO in the c- and a-axis growth orientations, respectively. The superconducting current usually flows within the Cu–O plane. With its sudden change in the direction of the current at the interface, the homogeneous junction prepared in this work is expected to provide new ideas for the research of high-temperature superconducting junction devices.

Manipulation of Cooper-Pair Tunneling via Domain Structure in a van der Waals Ferromagnetic Josephson Junction.

Ferromagnetic Josephson junctions play a key role in understanding the interplay between superconductivity and ferromagnetism in condensed matter physics. The magnetic domain structures of the ferromagnet in such junctions can significantly affect the tunneling of the superconducting Cooper pairs due to the strong interactions between Cooper pairs and local magnetic moments in the ferromagnetic tunnel barrier. However, the underlying microscopic mechanism of relevant quasiparticle tunneling processes with magnetic domain structures remains largely unexplored. Here, the manipulation of Cooper-pair tunneling in the NbSe2/Cr2Ge2Te6/NbSe2 ferromagnetic Josephson junction is demonstrated by using a multidomain ferromagnetic barrier with anisotropic magnetic moments. The evolution of up-, down-magnetized domain and Bloch domain structures in Cr2Ge2Te6 barrier under external magnetic fields leads to the enhancement of the critical tunneling supercurrent and an unconventional dual-peak feature with two local maxima in the field-dependent critical current curve. The phenomenon of magnetic-field-modulated critical tunneling supercurrent can be well explained by the competition between the coherence length of tunneling Cooper pairs and the size of magnetic domain walls in Cr2Ge2Te6 barrier. This kind of ferromagnetic Josephson junction provides an intriguing material system for manipulating Cooper-pair tunneling by tuning the local magnetic moments within magnetic Josephson junction devices.