Room Temperature Negative Differential Resistance Research Articles

Room Temperature Negative Differential Resistance with High Peak Current in MoS2/WSe2 Heterostructures.

Two-dimensional transition metal dichalcogenide (2D TMD) semiconductors allow facile integration of p- and n-type materials without a lattice mismatch. Here, we demonstrate gate-tunable n- and p-type junctions based on vertical heterostructures of MoS2 and WSe2 using van der Waals (vdW) contacts. The p-n junction shows negative differential resistance (NDR) due to Fowler-Nordheim (F-N) tunneling through the triangular barrier formed by applying a global back-gate bias (VGS). We also show that the integration of hexagonal boron nitride (h-BN) as an insulating tunnel barrier between MoS2 and WSe2 leads to the formation of sharp band edges and unintentional inelastic tunnelling current. The devices based on vdW contacts, global VGS, and h-BN tunnel barriers exhibit NDR with a peak current (Ipeak) of 315 μA, suggesting that the approach may be useful for applications.

Repeatable room temperature negative differential resistance in AlN/GaN resonant tunneling diodes grown on silicon

We report repeatable AlN/GaN resonant tunneling diodes (RTDs) grown on a silicon substrate by plasma-assisted molecular-beam epitaxy. The RTDs exhibit stable negative differential resistance without hysteresis at room temperature, where no degradation is observed even after 500 continuous bidirectional sweeps. The peak-to-valley current ratio is 1.36, and the peak current density is 24.38 kA/cm2. When the temperature is changed from 77 to 475 K, the peak current remains almost unchanged and the valley current increases gradually, resulting in a reduced peak-to-valley current ratio from 1.59 to 1.07. Our work softens the material quality constraints on realizing the room-temperature repeatable negative differential resistance and paves the way to low-cost III-nitride-based monolithic and hybrid microwave integrated circuits on large-size silicon wafers.

A Room-Temperature Ferroelectric Resonant Tunneling Diode.

Resonant tunneling is a quantum-mechanical effect in which electron transport is controlled by the discrete energy levels within a quantum-well (QW) structure. A ferroelectric resonant tunneling diode (RTD) exploits the switchable electric polarization state of the QW barrier to tune the device resistance. Here, the discovery of robust room-temperature ferroelectric-modulated resonant tunneling and negative differential resistance (NDR) behaviors in all-perovskite-oxide BaTiO3 /SrRuO3 /BaTiO3 QW structures is reported. The resonant current amplitude and voltage are tunable by the switchable polarization of the BaTiO3 ferroelectric with the NDR ratio modulated by ≈3 orders of magnitude and an OFF/ON resistance ratio exceeding a factor of 2×104 . The observed NDR effect is explained an energy bandgap between Ru-t2g and Ru-eg orbitals driven by electron-electron correlations, as follows from density functional theory calculations. This study paves the way for ferroelectric-based quantum-tunneling devices in future oxide electronics.

Defect-engineered room temperature negative differential resistance in monolayer MoS2 transistors.

The negative differential resistance (NDR) effect has been widely investigated for the development of various electronic devices. Apart from traditional semiconductor-based devices, two-dimensional (2D) transition metal dichalcogenide (TMD)-based field-effect transistors (FETs) have also recently exhibited NDR behavior in several of their heterostructures. However, to observe NDR in the form of monolayer MoS2, theoretical prediction has revealed that the material should be more profoundly affected by sulfur (S) vacancy defects. In this work, monolayer MoS2 FETs with a specific amount of S-vacancy defects are fabricated using three approaches, namely chemical treatment (KOH solution), physical treatment (electron beam bombardment), and as-grown MoS2. Based on systematic studies on the correlation of the S-vacancies with both the device's electron transport characteristics and spectroscopic analysis, the NDR has been clearly observed in the defect-engineered monolayer MoS2 FETs with an S-vacancy (VS) amount of ∼5 ± 0.5%. Consequently, stable NDR behavior can be observed at room temperature, and its peak-to-valley ratio can also be effectively modulated via the gate electric field and light intensity. Through these results, it is envisioned that more electronic applications based on defect-engineered layered TMDs will emerge in the near future.

Comparison of resonant tunneling diodes grown on freestanding GaN substrates and sapphire substrates by plasma-assisted molecular-beam epitaxy* *Project supported by the National Key R&D Program of China (Grant No. 2018YFB0406600), the National Natural Science Foundation of China (Grant Nos. 61875224, 61804163, and 61827823), Key Laboratory of Microelectronic Devices and Integration Technology, Chinese Academy of Sciences (Grant No. Y9TAQ21), Key Laboratory

AlN/GaN resonant tunneling diodes (RTDs) were grown separately on freestanding GaN (FS-GaN) substrates and sapphire substrates by plasma-assisted molecular-beam epitaxy (PA-MBE). Room temperature negative differential resistance (NDR) was obtained under forward bias for the RTDs grown on FS-GaN substrates, with the peak current densities (J p) of 175–700 kA/cm2 and peak-to-valley current ratios (PVCRs) of 1.01–1.21. Two resonant peaks were also observed for some RTDs at room temperature. The effects of two types of substrates on epitaxy quality and device performance of GaN-based RTDs were firstly investigated systematically, showing that lower dislocation densities, flatter surface morphology, and steeper heterogeneous interfaces were the key factors to achieving NDR for RTDs.

Room temperature negative differential resistance in clay-graphite paper transistors

Paper electronics have been fast developed in recent years due to their degradability and recyclability; however, most of them are barely equipped with logic functions, impeding them from becoming autonomous on-paper systems. To provide a working unit for logic implementations, a paper field-effect transistor (FET) with a clay-graphite channel that demonstrated negative differential resistance (NDR) is fabricated in this study. The device exhibited a p-type semiconductor characteristic with an optical bandgap of ∼1.97 eV according to photoluminescence analysis, resulting from considerable defective graphite mixed with clay in the channel. Considering that the NDR behavior is theoretically predicted to be tunable by the contact resistance, we fabricated FETs with different contact resistances and observed that the NDR behavior occurred in devices with relatively lower contact resistance. The NDR effect is also tunable by varying the gate voltage, which matches well with simulation results. The possible mechanism for the NDR behavior is the decrease in carrier drift velocity induced by band-to-band tunneling, associated with impurities-induced trapping states and gate-induced confined states. This work provides a promising and easy pathway to develop digital components using paper electronics in the near future.

Demonstration of highly repeatable room temperature negative differential resistance in large area AlN/GaN double-barrier resonant tunneling diodes

Here, we present a systematical investigation of AlN/GaN double-barrier resonant tunneling diodes (RTDs) grown by plasma-assisted molecular beam epitaxy on metal-organic chemical vapor deposition GaN-on-sapphire templates. The processed devices featured an active region composed of 2.5 nm GaN quantum well sandwiched by two 1.5 nm AlN barriers and RTD mesa diameter ranging from 1 to 20 μm. Room temperature current–voltage characteristics exhibited a repeatable negative differential resistance (NDR) free of degradation and hysteresis after 1000 times subsequently up-to-down voltage sweeps across different sizes. High peak-to-valley current ratios of 1.93 and 1.58 were obtained at room temperature for 1 and 12 μm diameter devices, respectively, along with peak current densities of 48 and 36 kA/cm2 corresponding to peak voltages of 4.65 and 5.9 V. The peak current density decreased quickly initially and then was less susceptible to this averaging effect with increasing the device diameter. Temperature-dependent measurements revealed that the valley current density displayed a positive relationship to the temperature, and an abruptly increasement was observed for the devices with a diameter of 20 μm when the temperature rose over 230 K. We attributed this abnormal phenomenon to the increased contribution from acoustic and longitudinal optical (LO) phonon scattering, especially for the LO phonon scattering. The area dependence of electrical performance suggested that the leakage pathway through dislocations played a vital role for charge transport and there existed a threshold of dislocation density for NDR characteristics. These results promote further study for future implementation of III-nitride-based RTD oscillators into high-frequency and high-power terahertz radiation.

Room-Temperature Negative Differential Resistance in Amorphous Carbon: The Role of Electron Trapping Defects at Device Interfaces

Hysteresis and NDR have been observed in the room-temperature current–voltage characteristics of lateral devices featuring planar graphitic electrodes separated by a nitrogen-doped amorphous carbon (a-CN <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><i>x</i></sub> ) layer. The devices were fabricated using energetic physical vapor deposition and electron-beam lithography. Devices formed on a-CN <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><i>x</i></sub> layers deposited with N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> pressure of 0.10 mTorr exhibited NDR. This NDR occurred at a well-defined voltage/electric field but only after the voltage of the opposite polarity was applied. Based on this behavior, a model is proposed in which the NDR occurs by field-induced trapping/detrapping at one of the graphite/a-CN <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><i>x</i></sub> interfaces within the device. The results of this investigation highlight the importance of interfaces and defects in determining the characteristics of a-C memristive devices.

Controlling Phase‐Coherent Electron Transport in III‐Nitrides: Toward Room Temperature Negative Differential Resistance in AlGaN/GaN Double Barrier Structures

AbstractResonant tunneling of electrons is important for the manufacture of high‐speed electronic oscillators and the electron injection control in quantum cascade lasers. In this work, room temperature negative differential resistance (NDR) in AlGaN/GaN double barrier structure with AlN/GaN digital alloy (DA) barriers is demonstrated. The peak‐to‐valley current ratio (PVCR) ranges from 1.1 to 1.24 at room temperature and becomes 1.5 to 2.96 at low temperatures, whereas no NDR is observed in double barrier structures with conventional ternary AlGaN barriers. The room temperature NDR together with the high PVCR at low temperature is attributed to the suppression of alloy disorder scattering by introducing AlN/GaN DA barriers. This work presents the successful control of phase‐coherent electron transport in III‐nitride heterostructures and is expected to benefit the future design of nitride‐based resonant tunneling structures and high‐speed electronic devices.

Coexistence of room-temperature negative differential resistance, negative photoconductivity modulated by UV light intensity and optical switching in Cu/Ni:ZnO/InGa structure

Cu/Ni:ZnO/InGa structure has been fabricated by depositing the chemically synthesized Ni:ZnO nanoparticles on glass substrate. The device shows a negative differential resistance (NDR) effect and negative photoconductivity (NPC) at room temperature. The presence of NDR is attributed to the electrons trapped/detrapped by interface states at Cu/Ni:ZnO junction. The origin of NPC is understood in term of competition between carriers injection due to the external field and photocarriers due to the external optical signal. Thus, these two effects (NDR and NPC) contribute to the optical switching process found in our device.

Dependence of growth temperature on the electrical properties and microstructure of MBE-grown AlN/GaN resonant tunneling diodes on sapphire

AlN/GaN double-barrier resonant tunnel diodes have been grown by rf-plasma assisted molecular beam epitaxy at temperatures between 760 and 860 °C on metalorganic chemical vapor deposition-grown GaN templates with sapphire substrates. Room temperature negative differential resistance (NDR) was observed for all samples despite the presence of higher densities of threading dislocations in the device layers than in the MOCVD GaN template. The fraction of devices exhibiting NDR and the peak-to-valley current ratio was small for each sample (typically 1%–10% and 1.003–1.1, respectively). A clear trend of increasing peak current density with increasing growth temperature was observed.

Effects of growth temperature on electrical properties of GaN/AlN based resonant tunneling diodes with peak current density up to 1.01 MA/cm2

Identical GaN/AlN resonant tunneling diode structures were grown on free-standing bulk GaN at substrate temperatures of 760 °C, 810 °C, 860 °C, and 900 °C via plasma-assisted molecular beam epitaxy. Each sample displayed negative differential resistance (NDR) at room temperature. The figures-of-merit quantified were peak-to-valley current ratio (PVCR), yield of the device with room-temperature NDR, and peak current density (Jp). The figures-of-merit demonstrate an inverse relationship between PVCR/yield and Jp over this growth temperature series. X-ray diffraction and transmission electron microscopy were used to determine the growth rates, and layer thicknesses were used to explain the varying figures-of-merit. Due to the high yield of devices grown at 760 °C and 810 °C, the PVCR, peak voltage (Vp), and Jp were plotted vs device area, which demonstrated high uniformity and application tunability. Peak current densities of up to 1.01 MA/cm2 were observed for the sample grown at 900 °C.

930 kA/cm2 peak tunneling current density in GaN/AlN resonant tunneling diodes grown on MOCVD GaN-on-sapphire template

We report on the design and fabrication of ultrahigh current density GaN/AlN double barrier resonant tunneling diodes grown via rf-plasma assisted molecular-beam epitaxy. The device structure was grown on a metal-organic chemical vapor deposition GaN-on-sapphire template. The devices displayed repeatable room temperature negative differential resistance with peak tunneling current densities (Jp) between 637 and 930 kA/cm2. Analysis of temperature dependent measurements revealed the presence of severe self-heating effects, which allow strong phonon scattering that deteriorates the electron quantum transport. Finally, a qualitative comparison to the same structure grown on a low dislocation density freestanding GaN substrate has shown that sapphire-based templates are a feasible alternative.

Repeatable Room Temperature Negative Differential Resistance in AlN/GaN Resonant Tunneling Diodes Grown on Sapphire

AbstractResonant tunneling diodes (RTDs) are candidates for high power terahertz oscillators, and form the basis for understanding the quantum confinement and vertical transport in quantum structures such as quantum cascade lasers and quantum cascade detectors. In this work, repeatable negative differential resistance (NDR) is achieved in AlN/GaN RTDs grown on sapphire substrate by plasma‐assisted molecular‐beam epitaxy. Two reproducible NDR regions sequentially following two preresonance replicas are demonstrated at room temperature. A current region exhibiting negative correlation with temperature and oscillation‐like features is first identified under reverse bias, which is interpreted as a combined contribution of weak resonant tunneling channels through different bound states in the well. The revealed peak‐to‐valley current ratio ranges from 1.1 to 1.8, and peak current density ranges from 5 to 164 kA cm−2. Using an analytic model, resonant tunneling transports in both bias directions are quantitatively characterized and show good agreements with experiment results, demonstrating the capability of accurate quantum transport control using III‐nitride grown on sapphire substrate. The findings will promote the implementation of low cost III‐nitride monolithic microwave circuits and resonant tunneling structures based on sapphire, SiC, and even silicon substrates.

Negative differential resistance in GaN homojunction tunnel diodes and low voltage loss tunnel contacts

The current-voltage characteristics and metastability in GaN p++/n++ homojunction tunnel diodes and n++/p++/i/n tunnel-contacted diodes grown via metal modulated epitaxy have been investigated. The room temperature negative differential resistance (NDR) beginning at ∼1.35 V is reported for GaN homojunction devices grown on sapphire. The NDR vanishes, and the conductivity increases as multiple I-V sweeps are performed, thus suggesting that charge trapping states with long trap lifetimes exist at defect sites, and these traps play a crucial role in the tunneling mechanism. Additionally, the use of extremely high n-type (ND ∼ 4.6 × 1020 cm−3) and p-type (NA ∼ 7.7× 1020 cm−3) doping results in a near linear characteristic with minimal rectification at current densities less than 200 A/cm2 and soft rectification above this current density. Forward-bias tunneling and NDR are still present at 77 K. The highest silicon-doped n++/p++/i/n tunnel-contacted pin diode demonstrates a turn-on voltage of 3.12 V, only 0.14 V higher than that of the pin control diode, and an improved specific on-resistance of 3.24 × 10−4 Ω cm2, which is 13% lower than that of the control pin diode.

431 kA/cm2 peak tunneling current density in GaN/AlN resonant tunneling diodes

We report on the design and fabrication of high current density GaN/AlN double barrier resonant tunneling diodes grown via plasma assisted molecular-beam epitaxy on bulk GaN substrates. A quantum-transport solver was used to model and optimize designs with high levels of doping and ultra-thin AlN barriers. The devices displayed repeatable room temperature negative differential resistance with peak-to-valley current ratios ranging from 1.20 to 1.60. A maximum peak tunneling current density (Jp) of 431 kA/cm2 was observed. Cross-gap near-UV (370–385 nm) electroluminescence (EL) was observed above +6 V when holes, generated from a polarization induced Zener tunneling effect, recombine with electrons in the emitter region. Analysis of temperature dependent measurements, thermal resistance, and the measured EL spectra revealed the presence of severe self-heating effects.

Nickel induced re-structuring of 2D graphene to 1D graphene nanotubes: Role of radical hydrogen in catalyst assisted growth

Here, we demonstrate for the first time the structural evolution of 1D graphene nanotubes (GNTs) by the cutting of two dimensional (2D) graphene oxide (GO) sheet in reducing environment at ambient conditions in presence of Ni metal in acidic environment. We observed that in-situ generated radical hydrogen (H) responsible for cutting of graphene sheets and re-structuring of 2D sheet structure to one 1D nanotubes. Structural evolution of GNTs was confirmed by using of transmission electron microscopy (TEM) technique. The current vs. voltage (I–V) characteristics of GNTs displayed room temperature (RT) negative differential resistance (NDR) effect which is typical in nanowires, suggested the applicability of nanomaterial for various kind of electronics applications such as memory devices and transistors fabrication.

High current density 2D/3D MoS2/GaN Esaki tunnel diodes

The integration of two-dimensional materials such as transition metal dichalcogenides with bulk semiconductors offer interesting opportunities for 2D/3D heterojunction-based device structures without any constraints of lattice matching. By exploiting the favorable band alignment at the GaN/MoS2 heterojunction, an Esaki interband tunnel diode is demonstrated by transferring large area Nb-doped, p-type MoS2 onto heavily n-doped GaN. A peak current density of 446 A/cm2 with repeatable room temperature negative differential resistance, peak to valley current ratio of 1.2, and minimal hysteresis was measured in the MoS2/GaN non-epitaxial tunnel diode. A high current density of 1 kA/cm2 was measured in the Zener mode (reverse bias) at −1 V bias. The GaN/MoS2 tunnel junction was also modeled by treating MoS2 as a bulk semiconductor, and the electrostatics at the 2D/3D interface was found to be crucial in explaining the experimentally observed device characteristics.

Highly repeatable room temperature negative differential resistance in AlN/GaN resonant tunneling diodes grown by molecular beam epitaxy

AlN/GaN resonant tunneling diodes grown on low dislocation density semi-insulating bulk GaN substrates via plasma-assisted molecular-beam epitaxy are reported. The devices were fabricated using a six mask level, fully isolated process. Stable room temperature negative differential resistance (NDR) was observed across the entire sample. The NDR exhibited no hysteresis, background light sensitivity, or degradation of any kind after more than 1000 continuous up-and-down voltage sweeps. The sample exhibited a ∼90% yield of operational devices which routinely displayed an average peak current density of 2.7 kA/cm2 and a peak-to-valley current ratio of ≈1.15 across different sizes.

Room temperature negative differential resistance in terahertz quantum cascade laser structures

The mechanisms that limit the temperature performance of GaAs/Al0.15GaAs-based terahertz quantum cascade lasers (THz-QCLs) have been identified as thermally activated LO-phonon scattering and leakage of charge carriers into the continuum. Consequently, the combination of highly diagonal optical transition and higher barriers should significantly reduce the adverse effects of both mechanisms and lead to improved temperature performance. Here, we study the temperature performance of highly diagonal THz-QCLs with high barriers. Our analysis uncovers an additional leakage channel which is the thermal excitation of carriers into bounded higher energy levels, rather than the escape into the continuum. Based on this understanding, we have designed a structure with an increased intersubband spacing between the upper lasing level and excited states in a highly diagonal THz-QCL, which exhibits negative differential resistance even at room temperature. This result is a strong evidence for the effective suppression of the aforementioned leakage channel.