Backward Diodes Research Articles

Advanced Terahertz Sensing and Imaging Systems Based on Integrated III-V Interband Tunneling Devices

Owing to the unprecedented development in terahertz (THz) sources and detectors in the last decade, technologists and researchers have intensified their efforts to develop advanced THz sensing and imaging systems with superior performance and unique functionalities. One of the key elements to realize such systems is the ability to monolithically integrate high-performance semiconductor devices with THz antennas and other passive structures. In recent years, devices based on interband tunneling in III-V heterostructures have emerged as promising candidates for THz detection that offer extremely high nonlinearity, high sensitivity, low noise, fast response, and room temperature operation. In this paper, we first review the development of heterostructure backward tunnel diodes (HBDs) in the InAs/AlSb/AlGaSb material system that have been demonstrated with detection sensitivity that outperforms the current state of the art (e.g., the fundamental limit of Schottky diodes) and with noise-equivalent power (NEP) below 0.2 pW/Hz½. We then present the monolithic integration of HBDs with planar folded dipole antennas (FDAs) using submicrometer-scale airbridges to achieve optimized impedance matching for high-performance and compact THz detectors and focal-plane array (FPA) imaging systems. In addition, the potential of using HBDs for realizing THz systems with advanced functionalities such as spectroscopic FPAs (using frequency-tunable THz antennas) and polariametric detection/imaging systems will be discussed. Finally, the integration of HBDs into waveguides for more advanced THz sensing and imaging (e.g., a six-port reflectometer for near-field imaging) will be discussed.

The CO2 absorption spectrum in the 2.3 µm transparency window by high sensitivity CRDS: (I) Rovibrational lines

The absorption of carbon dioxide is very weak near 2.3µm which makes this transparency window of particular interest for the study of Venus’ lower atmosphere. As a consequence of the weakness of the transitions located in this region, previous experimental data are very scarce and spectroscopic databases provide calculated line lists which should be tested and validated by experiment. In this work, we use the Cavity Ring Down Spectroscopy (CRDS) technique for a high sensitivity characterization of the CO2 absorption spectrum in two spectral intervals of the 2.3µm window: 4248–4257 and 4295–4380cm−1 which were accessed using a Distributed Feed Back (DFB) diode laser and a Vertical External Cavity Surface Emitting Laser (VECSEL) as light sources, respectively. The achieved sensitivity (noise equivalent absorption, αmin, on the order of 5×10−10cm−1) allowed detecting numerous new transitions with intensity values down to 5×10−30cm/molecule.The rovibrational assignments were performed by comparison with available theoretical line lists in particular those obtained at IAO Tomsk using the global effective operator approach. Hot bands of the main isotopologue and 16O12C18O bands were found to be missing in the HITRAN database while they contribute importantly to the absorption in the region.Additional CRDS spectra of a CO2 sample highly enriched in 18O were recorded in order to improve the spectroscopy of this isotopologue. As a result about 700 lines of 16O12C18O, 16O12C17O, 17O12C18O, 12C18O2 and 13C18O2 were newly measured.The status of the different databases (HITRAN, CDSD, variational calculations) in the important 2.3µm transparency window is discussed. Possible improvements to correct evidenced deficiencies are suggested.

Integration and fabrication of high-performance Sb-based heterostructure backward diodes with submicron-scale airbridges for terahertz detection

In this work, the authors report integration and fabrication of high-performance Sb-based heterostructure backward diodes (HBDs) with planar folded dipole antennas (FDAs) using submicron-scale airbridges for terahertz (THz) detection. By integrating HBDs into FDAs, high detector responsivity of 20 000 V/W at 200 GHz and 9500 V/W at 585 GHz could be potentially achieved due to the optimized impedance matching between the antenna and HBD detector. In order to minimize interconnect parasitics, the HBD integration is accomplished using submicron-scale airbridges. Electromagnetic simulations coupled to device models show that by introducing submicron-scale airbridges and optimizing the device layout, parasitic capacitance and spreading resistance can be significantly reduced. This allows performance nearly equal to the intrinsic device performance to be obtained. To achieve this level of performance, a novel fabrication and integration process has been developed. The process includes mix-and-match electron beam and optical lithography to span the size scales required for both the FDA and small-area HBDs, and offers high accuracy and reproducibility while requiring fewer critical fabrication steps compared to conventional hybrid integration techniques. Devices fabricated using this process have obtained a record-high device curvature coefficient of −58 V−1, indicating the quality of the devices that can be achieved. The process is scalable—in terms of device size, frequency range, and array size—enabling the development of THz focal plane arrays in a wide frequency range (100 GHz to beyond 1 THz).

Backward diodes using heavily Mg-doped GaN growth by ammonia molecular-beam epitaxy

We grew heavily Mg-doped GaN using ammonia molecular-beam epitaxy. The use of low growth temperature (740 °C) allows decreasing the incorporation of donor-like defects (<3 × 1017 cm−3) responsible for p-type doping compensation. As a result, a net acceptor concentration of 7 × 1019 cm−3 was achieved, and the hole concentration measured by Hall effect was as high as 2 × 1019 cm−3 at room temperature. Using such a high Mg doping level, we fabricated GaN backward diodes without polarization-assisted tunneling. The backward diodes exhibited a tunneling-current density of 225 A/cm2 at a reverse bias of −1 V at room temperature.

Monolithically Integrated Metal/Semiconductor Tunnel Junction Nanowire Light-Emitting Diodes.

We have demonstrated for the first time an n(++)-GaN/Al/p(++)-GaN backward diode, wherein an epitaxial Al layer serves as the tunnel junction. The resulting p-contact free InGaN/GaN nanowire light-emitting diodes (LEDs) exhibited a low turn-on voltage (∼2.9 V), reduced resistance, and enhanced power, compared to nanowire LEDs without the use of Al tunnel junction or with the incorporation of an n(++)-GaN/p(++)-GaN tunnel junction. This unique Al tunnel junction overcomes some of the critical issues related to conventional GaN-based tunnel junction designs, including stress relaxation, wide depletion region, and light absorption, and holds tremendous promise for realizing low-resistivity, high-brightness III-nitride nanowire LEDs in the visible and deep ultraviolet spectral range. Moreover, the demonstration of monolithic integration of metal and semiconductor nanowire heterojunctions provides a seamless platform for realizing a broad range of multifunctional nanoscale electronic and photonic devices.

Transport Properties of a MoS2/WSe2 Heterojunction Transistor and Its Potential for Application

This paper studies band-to-band tunneling in the transverse and lateral directions of van der Waals MoS2/WSe2 heterojunctions. We observe room-temperature negative differential resistance (NDR) in a heterojunction diode comprised of few-layer WSe2 stacked on multilayer MoS2. The presence of NDR is attributed to the lateral band-to-band tunneling at the edge of the MoS2/WSe2 heterojunction. The backward tunneling diode shows an average conductance slope of 75 mV/dec with a high curvature coefficient of 62 V(-1). Associated with the tunnel-diode characteristics, a positive-to-negative transconductance in the MoS2/WSe2 heterojunction transistors is observed. The transition is induced by strong interlayer coupling between the films, which results in charge density and energy-band modulation. The sign change in transconductance is particularly useful for multivalued logic (MVL) circuits, and we therefore propose and demonstrate for the first time an MVL-inverter that shows three levels of logic using one pair of p-type transistors.

Breaking the Efficiency Barrier for Ambient Microwave Power Harvesting With Heterojunction Backward Tunnel Diodes

Harvesting low-density ambient microwave power as an alternative power source for small ubiquitous wireless nodes has been proposed in recent papers discussing emerging technologies like the Internet of Things and Smart Cities. However, a literature review of the state-of-the-art Schottky diode based microwave rectifiers shows that a maximum efficiency has been reached for such devices operating in the low-power regime, as is the case for ambient microwave power-harvesters. This work examines the underlying physical mechanisms responsible for this RF-to-dc power conversion efficiency limitation, and explores a high I-V curvature backward tunnel diode to overcome this efficiency limitation. Measurements of the 2.4 GHz RF-to-dc power conversion efficiency at $-$ 40 dBm input power demonstrates that the backward tunnel diode outperforms the HSMS-285B Schottky diode by a factor of 10.5 and the Skyworks SMS7630 by a factor of 5.5 in a lossless matching network scenario. A prototype built using a new GSG probe embedded with a matching circuit showed a total power conversion efficiency of 3.8% for $-$ 40 dBm input power and 18.2% for $-$ 30 dBm input power at 2.35 GHz.

Enhanced dielectric properties in polyvinyl alcohol – Multiwall carbon nanotube composites

Here we report the study of direct current (DC) and alternate current (AC) electrical transport properties and current–voltage (I–V) characteristic of Polyvinyl Alcohol – Multiwall Carbon nanotube (PVA–MWCNT) composites, prepared by an in-situ chemical oxidative preparation technique, within temperature range 300 K ≤ T ≤ 423 K and in the frequency range 150 Hz ≤ f ≤ 2 MHz. The DC conductivity gives the presence of two different activation energies at two different temperature regions. It is observed that the dielectric constant of composite film is higher in comparison to pure PVA film and is well-fitted with modified Cole–Cole equation. The presence of hydroxyl group affects the relaxation phenomenon of composite film. The frequency variation of AC conductivity follows the power law (fS) and is found to follow correlated barrier hopping (CBH) model. The imaginary part of electric modulus shows non-Debye type asymmetric behaviour. The current – voltage characteristic has been analysed by the concept of back to back Schottky diode.

Sensitivity Improvement in GaAsSb-Based Heterojunction Backward Diodes by Optimized Doping Concentration

An impedance-matched voltage sensitivity ( $\beta _{\it v,{\textrm {opt}}}$ ) of 20400 V/W at 94 GHz was achieved at zero bias and room temperature using heterojunction backward diodes that were based on a p+-GaAsSb/i-InAlAs/n-InGaAs that was lattice matched to an InP substrate. Doping concentrations in the diodes were adjusted to optimize the sensitivity and its nonlinear characteristic. The proper donor doping concentration in n-InGaAs to increase its sensitivity was determined to be $1\times 10^{18}$ cm $^{-3}$ . Meanwhile, the proper acceptor doping concentration in a p-GaAsSb layer was $5 \times 10^{18}$ cm $^{-3}$ to achieve a large nonlinearity as a curvature coefficient ( $\gamma $ ) of $-49.4~\text{V}^{-1}$ , which exceeded that of ideal Schottky diode. The dependence of doping concentration on noise performance was also considered. The lattice-matched GaAsSb-based backward diodes are applicable in millimeter wave detectors and mixers.

Dual-gated MoS2/WSe2 van der Waals tunnel diodes and transistors.

Two-dimensional layered semiconductors present a promising material platform for band-to-band-tunneling devices given their homogeneous band edge steepness due to their atomically flat thickness. Here, we experimentally demonstrate interlayer band-to-band tunneling in vertical MoS2/WSe2 van der Waals (vdW) heterostructures using a dual-gate device architecture. The electric potential and carrier concentration of MoS2 and WSe2 layers are independently controlled by the two symmetric gates. The same device can be gate modulated to behave as either an Esaki diode with negative differential resistance, a backward diode with large reverse bias tunneling current, or a forward rectifying diode with low reverse bias current. Notably, a high gate coupling efficiency of ∼80% is obtained for tuning the interlayer band alignments, arising from weak electrostatic screening by the atomically thin layers. This work presents an advance in the fundamental understanding of the interlayer coupling and electron tunneling in semiconductor vdW heterostructures with important implications toward the design of atomically thin tunnel transistors.

Band-Edge Steepness Obtained From Esaki/Backward Diode Current–Voltage Characteristics

While science has good knowledge of semiconductor bandgaps, there is not much information regarding the steepness of the band edges. We find that a plot of absolute conductance, I/V versus voltage V , in an Esaki diode or a backward diode will reveal a best limit for the band tails, defined by the tunneling joint density of states of the two band edges. This joint density of states will give information about the prospective subthreshold swing voltage that could be expected in a tunneling field-effect transistor. To date, published current-voltage characteristics indicate that the joint band-tail density of states is not steep enough to achieve <;60 mV/decade. Heavy doping inhomogeneity, among other inhomogeneities, results in a gradual density of states extending into the bandgap. The steepest measured tunnel diodes have a tunneling joint density of states >90 mV/decade.

${\rm In}_{0.53}{\rm Ga}_{0.47}{\rm As}/{\rm GaAs}_{0.5}{\rm Sb}_{0.5}$ Quantum-Well Tunnel-FETs With Tunable Backward Diode Characteristics

Vertical quantum-well (QW) tunnel-FETs are fabricated based on an ultrathin In0.53Ga0.47As/GaAs0.5Sb0.5 staggered gap (type-II) heterostructure lattice matched to InP. Area-dependent QW-to-QW tunneling current is demonstrated. Devices with HfO2 high- k gate dielectric (EOT ~ 1.3 nm) exhibit minimum subthreshold swings of 140 mV/decade at 300 K, with an ON-current density of 0.5 μA/μm2 at VDD=0.5 V. Sharp negative differential resistance is observed in the output characteristics. For the first time, gate-tunable backward diode characteristics are demonstrated in this material system, with peak curvature coefficient of 30 V-1 near VDS=0 V. These results show the potential of vertical TFETs in hybrid IC applications.

Electronic and Magneto-Transport Across the Heusler Alloy (Co2FeAl)/p-Si Interfacial Structure

Electronic and magneto-transport across the Heusler alloy Co2FeAl (CFA)/ p-Si structure have been studied. The morphology of the Heusler alloy film surface has also been characterized by atomic force microscopy and magnetic force microscopy (MFM). X-ray diffraction data revealed formation of the CFA alloy phase with the L21 structure. MFM results revealed formation of a fine domain structure of average size ∼10 nm and magnetic signal strength 0.23°. The I–V characteristics are strongly temperature-dependent between ∼80 K and 300 K for forward bias, compared with weak temperature dependence on reversing the polarity. At low temperature the I–V characteristics have the features of a backward diode. The observed strong temperature dependence is because of thermionic emission of carriers across the interface. The weak temperature dependence is because of dominant field-emission tunnelling of carriers across the interface. Large magnetic field sensitivity of the reverse current has also been observed. The observed magnetic field sensitivity for the reverse current shows the involvement of electronic spin in transport across the interface, from the Heusler alloy to the silicon. An MR of ∼35% in the presence of a magnetic field was estimated from the I–V data. The study has shown that spin-dependent tunnel transport from the CFA alloy to silicon across the interface results in the observed value of MR, which seems to be because of spin scattering.

First quadrant phototransistor behavior in CuInSe2 photovoltaics

Temperature-dependent current voltage measurements on a CuInSe2 (CIS) solar cell are described and analyzed in detail. At relatively low temperatures, the device is shown to exhibit operation as a phototransistor, evidenced by light flux dependent current in the first quadrant of the current/voltage curve prior to the onset of a high differential resistance region of the curve. The phototransistor behavior is not restricted to light with short wavelengths (such as would stimulate the front contact materials—CdS and ZnO), and has a similar sensitivity to light over a wide range of intensities. Other aspects of the device operation are analyzed including observation of a photoconductive shunt and series differential resistance component, neither of which requires short wavelength light to be observable, a non-light-sensitive diode reverse-saturation current, and characterization of the back diode in the device. It is argued that the presence of an effective back diode also requires a bias voltage to develop across that diode under a variety of operating conditions that could reduce the open circuit voltage of the device. The potential for characterization of minority carrier properties in the device absorber layer through further controlled study of phototransistor behavior in the devices is noted.

A Broadband Focal Plane Array Camera for Real-time THz Imaging Applications

We present a large-format, sub-millimeter-resolution, focal plane array sensor for THz imaging. Each pixel in the sensor array consists of broadband THz antennas monolithically integrated with ultra-fast heterostructure backward diodes for THz sensing. With the aid of in-house hybrid electromagnetic modeling tools, the focal plane array is optimized for diffraction limited image resolution and conjugate impedance matching for highest THz sensitivity. The camera is designed to operate in the 0.6–1.2 THz band with 5 frames-per-second image acquisition speeds, making it ideal for THz imaging applications, such as security screening, non-destructive evaluation and chemical, pharmaceutical, and medical imaging. The simulation results are validated by measurements to demonstrate sub-millimeter resolution with a pixel optical responsivity of 600 V/W at 0.7 THz.

The physics and backward diode behavior of heavily doped single layer MoS2 based p-n junctions

The single layer MoS2 is attractive for the use in the next-generation low power nanoelectronic devices because of its intrinsic bandgap compared to graphene. In this work, we investigated the transport property of a p-n junction based on two-dimensional MoS2. The n-type and p-type doping are realized through substituting sulfur with chlorine and phosphorus. The device exhibited backward diode-like behavior with large rectifying ratios. We attribute the observed current-voltage characteristics to different heavy doping effect caused by chlorine and phosphorus. Our results may throw light on the electronic modulation of MoS2 and realizations of complemented logics devices based on MoS2.

Modeling of Sb‐heterostructure backward diode for millimeter‐ and submillimeter‐wave detection

AbstractWe present modeling of Sb‐heterostructure backward tunneling diode (Sb‐HBD) for high efficient millimeter‐ and submillimeter wave detection. The diode heterostructure is modeled and optimized using TCAD software implementing a non‐local band‐to‐band tunneling model combining with the standard drift‐diffusion model. The physical device model was found to be in good agreement with reported experimental results. InAsSb/AlSb/AlGaSb structures were proposed and simulated considering the material growth. The potential of the Sb‐HBDs for high frequency operation applications is investigated. (© 2013 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

An accurate interband tunneling model for InAs/GaSb heterostructure devices

AbstractA numerical model that accurately describes interband tunneling in backward diodes and broken‐gap tunnel diode structures based on the InAs/GaSb material system is described. The model applies the transfer matrix method to discretized bias‐dependent energy band profiles to calculate the transmission probability for tunneling. The model has been validated against experimental results, with good agreement in the current‐voltage and curvature coefficient having been obtained for a range of hetero‐structure backward diode and interband tunnel diode structures. (© 2013 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Phototransistor effects in Cu(In,Ga)Se2 solar cells

Cu(In,Ga)Se2 solar cells exhibit non-idealities with respect to a standard diode especially at low temperatures: a blocking behavior of the dark current in forward direction, a crossover of the dark and illuminated current–voltage-characteristics and a saturation of the open circuit voltage with respect to temperature and illumination intensity. It will be demonstrated that at low temperatures a Cu(In,Ga)Se2 solar cell with a back barrier essentially behaves as a phototransistor with a ZnO-emitter and Schottky collector. Hereby, the base current is provided by photoexcited holes in the field-free region of the absorber. Comparing simulations with measurements it will be illustrated that these non-idealities can be quantitatively explained. It will be shown that depending on the barrier height of the back diode and the doping profile these characteristics even occur at room temperature. This back barrier might be due to a Schottky contact or to a low doping at the back contact and a higher doping level close to the heterointerface.

Effects of Transition Metal Dichalcogenide Molybdenum Disulfide Layer Formation in Copper–Zinc–Tin–Sulfur Solar Cells from Numerical Analysis

This study demonstrates the effects of transition metal dichalcogenide, MoS2 layer formation in between the copper–zinc–tin–sulphide (CZTS) absorber layer and Mo back contact from theoretical study and numerical modeling. The objective of this study is to elucidate the effects of n or p type MoS2 on the overall CZTS solar cell performance. Energy band line-up of Mo/MoS2/CZTS interface is analyzed to elucidate the interface properties. It is found out that p-MoS2 layer in CZTS solar cell induces the same adventitious effect as p-MoSe2 in CIGS solar cell. However, n-MoS2 layer has detrimental effect on the CZTS solar cell by creating an additional back contact diode with p-CZTS layer and an ohmic contact with Mo layer. Thickness, bandgap energy and carrier concentration of n-MoS2 all have been varied in the numerical simulation to observe its effects on the cell performance parameters. The results from numerical simulation show that MoS2 layer as thin as 50 nm is sufficient enough to induce adverse effect on the solar cell performance. This could be caused by the increase in series resistance of the solar cell as n-type MoS2 would inhibit hole current into Mo back contact due to the hole barrier between n-type MoS2 and Mo back contact. The increase in MoS2 bandgap and carrier concentration also results in detrimental effect to the performance of the cell mainly due to the possibility of electrons to drift towards the back contact and recombine.