Resonant Tunneling Diode Structure Research Articles

Electronic and transport properties of U-cut edge patterned AGNR superlattice for RTD application

In this paper, we first characterize a superlattice structure created by repeating a heterostructure formed from two armchair graphene nanoribbon (AGNR) segments with different widths. We investigate the electronic and transport properties of this structure by varying its widths and lengths to demonstrate the tunability of its overall band gap. The plot of the local density of states shows the formation of localized states at the low band gap segments of the superlattice. The superlattice is then used as a barrier to create a double barrier quantum well (DBQW) to design a proposed resonant tunneling diode (RTD) structure. We observe this device's negative differential resistance (NDR) operation for a range of bias voltages between the contacts. We study the effect of dimensional parameters on the RTD performance. The non-equilibrium Green's function method, based on a tight-binding model, is employed for numerical computation.

Terahertz Resonant-Tunneling Diode With Series-Fed Patch Array Antenna

Resonant-tunneling diodes (RTDs) are promising electronic sources for terahertz (THz) waves. A recently proposed slot-based RTD simplifies the fabrication process by removing the metal–insulator–metal capacitor, which is used for dc separation, and introducing an etch stopper layer for simple air bridge formation. The simplified RTD structure, however, exhibits an unexpected large tilt angle in the main lobe of its radiation pattern due to the coupling between the slot antenna and the bias pads. Here, we present a series-fed patch array design that exploits the biasing structure and directs leakage surface waves to the broadside direction. The simulation result shows that the main lobe is along the broadside, with a maximum directivity over 12 dBi and a 3-dB bandwidth around 20%. The radiation efficiency is improved from 22% to 28% at 275 GHz. An experimental validation with a bullet lens indicates that the RTD device with a series-fed patch array antenna has a narrower beamwidth and a sidelobe level much lower compared with the original RTD. With such improved radiation performance, this simple RTD device becomes a potential source for the development of future array-level designs and flat lens systems.

Analysis of output power characteristics for resonant-tunneling diode terahertz oscillator with cylindrical cavity resonator

An increasing number of novel applications has appeared in the previously unexplored frequency range of 0.3–3.0 THz, where sub-mm radio waves meet far-infrared optical waves. Resonant-tunneling diodes (RTDs) are considered as one of the promising compact and coherent room-temperature signal sources for terahertz (THz) applications. In this work, dependencies of output power on the resonator dimensions and output power limitation factors are analyzed for an RTD THz oscillator with a cylindrical cavity resonator, which can oscillate above 2 THz. Analysis of the output power dependencies on radius and height of the cylindrical resonant cavity shows that a decrease in the resonant cavity size could lead to an increase in the output power at a fixed frequency for this type of RTD oscillator. Moreover, in addition to the high-frequency oscillation limit, a rapid decrease in the output power in the lower frequency region was found for oscillator devices with larger RTD mesa areas. Rapid decrease in output power may occur even at frequencies around 1 THz, which could considerably limit the operational range for RTD oscillators with cavity-type resonators. To determine an approach for output power optimization and understand the nature of output power drop at lower frequencies, the output power behavior and connection with resonant cavity parameters were explained in detail. Results of the output power analysis and numerical calculation indicate that for the RTD structure and circular-resonator geometry considered in the present study, output powers up to 45 μW at 1.5 THz and up to 0.25 μW at 2.5 THz could be expected for single oscillator design.

Micro-PL analysis of high current density resonant tunneling diodes for THz applications

Low-temperature micro-photoluminescence (μPL) is used to evaluate wafer structural uniformity of current densities >5mA/μm2 InGaAs/AlAs/InP resonant tunneling diode (RTD) structures on different length scales. Thin, highly strained quantum wells (QWs) are subject to monolayer fluctuations, leading to a large statistical distribution in their electrical properties. This has an important impact on the RTD device performance and manufacturability. The PL spot size is reduced using a common photolithography mask to reach a typical high Jpeak for a given RTD mesa size (1 ∼ 100 μm2). We observe that for lower strain-budget samples, the PL line shape is essentially identical for all excitation/collection areas. For higher strain-budget samples, there is a variation in the PL line shape that is discussed in terms of a variation in long-range disorder brought about by strain relaxation processes. The RTD operating characteristics are discussed in light of these findings, and we conclude that strain model limits overestimate the strain budget that can be incorporated in these devices. We also highlight μPL as a powerful nondestructive characterization method for RTD structures.

Peculiarities in optical response of hybrid-barrier GaSb/InAs/AlSb resonant tunneling diode structure

We present comprehensive investigation of the optical properties of hybrid-barrier GaSb-based resonant tunneling structures, containing a bulk-like GaInAsSb absorption layer and two asymmetric type II GaSb/InAs/AlSb quantum wells. Methods of optical spectroscopy by means of Fourier-transformed photoluminescence and photoreflectance are employed to probe optical transitions in this complex multilayer system. Based on the comparison between the absorption-like and emission-like spectra (also in function of temperature) confronted with band structure calculations four main transitions could be resolved and identified. For one of them, there has been observed unusually strong linear polarization dependence never reported in structures of that kind. It has been interpreted as related to a transition at the GaSb/GaInAsSb interface, for which various scenarios causing the polarization selectivity are discussed.

Efficient approximation algorithm for the Schrödinger–Possion system

AbstractIn this article, we study an efficient approximation algorithm for the Schrödinger–Possion system arising in the resonant tunneling diode (RTD) structure. By following the classical Gummel iterative procedure, we first decouple this nonlinear system and prove the convergence of the iteration method. Then via introducing a novel spatial discrete method, we solve efficiently the decoupled Schrödinger and Possion equations with discontinuous coefficients on no‐uniform meshes at each iterative step, respectively. Compared with the traditional ones, the algorithm considered here not only has a less restriction on the discrete mesh, but also is more accurate. Finally, some numerical experiments are shown to confirm the efficiency of the proposed algorithm.

A simple analytical model for the resonant tunneling diode based on the transmission peak and scattering effect

A new analytical model for the current–voltage characteristic of a resonant tunneling diode (RTD) is presented herein, being derived from the basic integral of the Tsu–Esaki equation. The model is physics based and developed as a function of material parameters such as the effective mass, barrier heights, and Fermi levels as well as geometrical parameters such as the barrier height and well width. The material chosen for the double-barrier RTD structure is AlGaAs/GaAs. The dependence of the peak transmission on the applied voltage and the scattering effect in the active region, which are neglected in previous models, are also taken into account. The model is implemented and validated using numerical simulations, revealing that the resulting electrical characteristic is in good agreement with numerical simulations using the Green’s function formalism.

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.

Frequency increase in resonant-tunneling diode cavity-type terahertz oscillator by simulation-based structure optimization

Terahertz (THz) radiation, in the range between the light waves and millimeter waves, has gained much attention because of its various promising applications. Resonant tunneling diodes (RTDs) have the potential for use as compact and coherent THz sources operating at room temperature. In this paper, oscillation frequency limitations and structure dependences for a RTD THz oscillator with a cylindrical cavity resonator are analyzed. Based on the dependences of the oscillation frequency on resonant cavity dimensions, a method of the oscillator structure optimization and matching with an RTD structure was proposed. It is revealed that there is only one combination of the resonant cavity dimensions which gives the maximum possible oscillation frequency for this type of structure. Analysis of the calculation results has shown that fundamental oscillation up to 2.77 THz could be expected for the RTD device considered in the present study by using the proposed optimization technique.

Long Wavelength Type II InAs/GaSb Superlattice Photodetector Using Resonant Tunneling Diode Structure

We report on a long wavelength type-II InAs/GaSb superlattice photodetector using resonant tunneling diode (RTD) structure. The linewidth of the satellite peak of the x-ray diffraction curve of the as-grown sample is only 15.7 arcsec showing a very high structural quality. The response maximum wavelength of the RTD detector is $7.5~\mu \text{m}$ and the 50% cutoff wavelength is $9.6~\mu \text{m}$ at 77 K. The measured QE is 147% at $7.5~\mu \text{m}$ when the applied bias voltage is 1.45 V and the corresponding responsivity is 8.9 A/W. This unusual QE is attributed to a large gain achieved when the device is under a resonant tunneling condition. The corresponding shot noise limited detectivity D* is $1.2\times 10^{10}\text {cm}\cdot \sqrt {Hz}/W$ at 1.45 V at 77 K.

MBE-grown Zincblende MnSe1−xTex Thin Films on ZnTe

We report studies on the role of a ZnTe buffer in determining the crystalline phase of MBE-grown MnSe1−xTex alloy thin films. It was found that MBE growth of MnSe and MnTe directly onto a zincblende GaAs substrate usually result in their corresponding stable phases, which are rocksalt and hexagonal, respectively. A set of zincblende MnSe1−xTex alloy thin films with Te composition covering from 0.27 to 1 were fabricated on zincblende ZnTe buffer layers. We have addressed the lattice distortion issue due to thin film effect for these films. A combination of several structural characterizations demonstrates that a perfect lattice matched MnSe1−xTex/ZnTe heterostructure can indeed be realized. The results of this study pave the way for realizing a double-barrier MnSe1−xTex/ZnTe/MnSe1−xTex resonant tunneling diode structure with perfect lattice match as a candidate for a promising THz emitter.

Modeling of tunneling current density of GeC based double barrier multiple quantum well resonant tunneling diode

The double barrier quantum well (DBQW) resonant tunneling diode (RTD) structure made of SiGeSn/GeC/SiGeSn alloys grown on Ge substrate is analyzed. The tensile strained Ge1−zCz on Si1−x−yGexSny heterostructure provides a direct band gap type I configuration. The transmission coefficient and tunneling current density have been calculated considering single and multiple quantum wells. A comparative study of tunnelling current of the proposed structure is done with the existing RTD structure based on GeSn/SiGeSn DBH. A higher value of the current density for the proposed structure has been obtained.

Influence on Characteristics of RTD Due to Variation of Different Parameters and Material Properties

In this paper, novel features offered by Resonant Tunneling Diode (RTD) are reviewed by simulating it under different conditions. GaAs/AlGaAs based RTD is used as the reference one to obtain the characteristics due to parametric variations. To fulfil this purpose a simple model of resonant electronic transport through a double-barrier structure is developed. I-V characteristics are studied by varying barrier parameters and well width. Different peak and valley currents are measured under these conditions. For the same set of parameters both symmetric and asymmetric cases are considered. Different materials of lower effective mass are also taken into consideration to improve Peak to Valley Ratio (PVR). The Indium (In) based materials are considered to compare the characteristics obtained from the conventional GaAs based RTD structure. All these proposed structures are simulated using Silvaco Atlas software.

Computational study of a new resonant tunneling diode based on an $$\hbox {MoS}_{2}$$ MoS 2 nanoribbon with sulfur line vacancies

Recent experimental studies have shown that sulfur vacancies in monolayer $$\hbox {MoS}_{2}$$ are mobile under exposure to an electron beam and tend to accumulate as sulfur line vacancies (Komsa in Phys Rev B 88: 035301, 2013). In this work, we designed a new resonant tunneling diode (RTD) based on this natural property. Two rows of sulfur vacancies are introduced into armchair $$\hbox {MoS}_{2}$$ nanoribbons ( $$\hbox {A-MoS}_{2}$$ NRs) to tune the nanoribbons’ bandgap to obtain the double-barrier quantum well structure of the resonant tunneling diode. This arrangement has a unique benefit that will result in very little physical distortion. A tight-binding (TB) model, with five 4d-orbitals of the Mo atom and three 3p-orbitals of the S atom, is employed for calculations. In the TB model, which is described in terms of Slater–Koster parameters, we also incorporate the changes of edge bonds. Density functional theory is used to determine all the necessary parameters of the TB model. They are obtained by an optimization procedure which achieves very fine parameter values, which can regenerate the most important energy bands of $$\hbox {A-MoS}_{2}$$ NRs of different widths, with highly satisfactory precision. The introduction of these new parameters is another contribution of this work. Lastly, the nonequilibrium Green’s function formalism based on the TB approximation is used to explore the properties of the new RTD structures based on $$\hbox {A-MoS}_{2}$$ NRs. Negative differential resistance with peak to valley ratio (PVR) of about 78 at room temperature is achieved for one RTD, having peak current $$I_\mathrm{p}=90$$ nA. We show that the PVR can exceed 120 when increasing the barrier length of the RTD at the expense of lower $$I_\mathrm{p}$$ .

Structures and Physics of Resonant Tunneling Diodes for Terahertz Oscillators

Resonant tunneling diode (RTD) is one candidate for compact and coherent room-temperature terahertz sources. In this paper, we introduce our recent results on RTD terahertz oscillators, including RTD materials and structures for high frequency oscillation. Up to now, oscillation at 1.98 THz has been achieved at room temperature. We also described structures for high output power and some applications of RTD oscillators.

Approximation of a Phase Space Operator for the Numerical Solution of the Wigner Equation

The application of the Wigner-Veyl transform for the analysis of quantum transport effects leads to several difficulties and to criticism. One of the difficulties is the numerical discretization of the Wigner transport equations, after the Wigner-Veyl transform is applied onto the statistical density matrix as a part of the quantum Liouville equation. The conventional discretization methods are based on a finite-difference approximation of the resulting Wigner transport equation applying upwind difference schemes or hybrid difference schemes. The results obtained do not match with the statistical density matrix based on the solution of the Schrodinger equation. The contribution addresses the difficulties when the conventional discretization approaches are applied. The difficulties arising from this approach are discussed, and the prerequisites for an improved approach are defined. On the basis of this discussion and the conclusions obtained, a new approach is proposed. This approach is based on an evaluation of the characteristics of the Wigner transport equation in the eigenvalue domain. The new approach is discussed and is applied onto a standard resonant tunneling diode structure. The numerical results are compared with the results from the application of the conventional techniques.

Photoluminescence Characterisation of High Current Density Resonant Tunnelling Diodes for Terahertz Applications

High structural perfection, wafer uniformity, and reproducibility are key parameters for high-volume, low cost manufacture of resonant tunnelling diode (RTD) terahertz (THz) devices. Low-cost, rapid, and non-destructive techniques are required for the development of such devices. In this paper, we report photoluminescence (PL) spectroscopy as a non-destructive characterisation technique for high current density InGaAs/AlAs/InP RTD structures grown by metal-organic vapour phase epitaxy (MOVPE) for THz applications. By using a PL line scanning technique across the edge of the sample, we identify characteristic luminescence from the quantum well (QW) and the undoped/n InGaAs layers. By using the Moss-Burstein effect, we are able to measure the free-electron concentration of the emitter/collector and contact layers in the RTD structure. Whilst the n InGaAs luminescence provides information on the doping concentration, information on the alloy composition and compositional variation is extracted from the InGaAs buffer layer. The QW luminescence provides information on the average well width and provides a monitor of the structural perfection with regard to interface and alloy disorder. key words: resonant tunnelling diode, high-speed electronics, terahertz, photoluminescence

A Dual-Pass High Current Density Resonant Tunneling Diode for Terahertz Wave Applications

We report on a dual-pass high current density resonant tunneling diode (RTD) for terahertz wave applications. This technique reduces the overall fabrication complexity and improves the reproducibility for creating low resistance ohmic contacts. With our dual-pass technique, we demonstrate accurate control over the final device area by measuring the RTD current–voltage characteristic during the fabrication process and guiding the emitter current through the full RTD structure with a second contact electrode on the collector side. We go on to show how we may extract important information about the RTD performance using this method.

Design and simulation of a novel GaN based resonant tunneling high electron mobility transistor on a silicon substrate

For the first time, we have introduced a novel GaN based resonant tunneling high electron mobility transistor (RTHEMT) on a silicon substrate. A monolithically integrated GaN based inverted high electron mobility transistor (HEMT) and a resonant tunneling diode (RTD) are designed and simulated using the ATLAS simulator and MATLAB in this study. The 10% Al composition in the barrier layer of the GaN based RTD structure provides a peak-to-valley current ratio of 2.66 which controls the GaN based HEMT performance. Thus the results indicate an improvement in the current–voltage characteristics of the RTHEMT by controlling the gate voltage in this structure. The introduction of silicon as a substrate is a unique step taken by us for this type of RTHEMT structure.

Enhanced current injection from a quantum well to a quantum dash in magnetic field

Resonant tunneling injection is a key ingredient in achieving population inversion in a putative quantum dot cascade laser. In a quantum dot based structure, such resonant current requires a matching of the wavefunction shape in k-space between the injector and the quantum dot. We show experimentally that the injection into an excited state of a dash structure can be enhanced tenfold by an in-plane magnetic field that shifts the injector distribution in k-space. These experiments, performed on resonant tunneling diode structures, show unambiguously resonant tunneling into an ensemble of InAs dashes grown between two AlInAs barrier layers. They also show that interface roughness scattering can enhance the tunneling current.