Two-dimensional Electron Gas Research Articles

Different spectral features near the energy bandgaps of normal and inverse heterostructures of In<sub>0.52</sub>Al<sub>0.48</sub>As/InP

Photoconductivity (PC) spectroscopy and photoluminescence (PL) spectroscopy were used to characterize two heterostructure configurations of InAlAs/InP grown by molecular beam epitaxy (MBE) on the InP (100) substrate. The sample A is the type called normal heterostructure, which has an In<sub>0.52</sub>Al<sub>0.48</sub>As layer grown on InP, while sample B is called the inverse type formed by an InP cap layer on In<sub>0.52</sub>Al<sub>0.48</sub>As. The front excitation was employed in both PC experiment and PL experiment and the measurements were conducted at 77 K. The PC spectrum of sample A shows an abnormal step-like drop when the photon energy is larger than the energy band gap of In<sub>0.52</sub>Al<sub>0.48</sub>As. The phenomenon implies that the conductance of sample is a multilayer effect including the contribution of interfacial two-dimensional electron gas (2DEG). Moreover, a conductance peak is observed at 916 nm below the bandgap of InP. Accordingly, an intense luminescent peak at the wavelength manifests in the PL spectrum. The origin of the 916 nm peak is attributed to the recombination of 2DEG electrons with the valence band holes excited near the interface. However, the spectral feature of the above energy does not exist in both PC and PL spectra of sample B. This difference may be explained by the different interface electronic structures of the inverse interface. For the latter case, considering that a graded variation in In-As-P composition is related to the inverse interface of InP/InAlAs, the band bending effect should be weak. In such a case, the bound energy of 2DEG in the interface potential well is raised closer to the conductance band of the bulk. Consequently, the recombination energy of 2DEG at the inverse interface with the holes in the valence band is close to the band-to-band transition of InP bulk and the luminescence is difficult to be distinguished from that of bulk InP. The work also demonstrates that the comparative study with both PC technique and PL technique is helpful to provide a full insight into the interface electronic property.

ADVANCEMENT IN NANOSCALE ELECTRONICS IN MOSFET

This research explores a novel approach for designing Cylindrical Surrounding Double-Gate (CSDG) MOSFETs with nanometer-scale precision through a layer- by-layer fabrication technique. Traditional top- down methods, such as photolithography and ion beam deposition, are widely used for semiconductor design but present challenges in creating complex heterostructures. In response, this study proposes an alternative method focusing on cylindrical structures to develop high- resolution morphological designs of CSDG MOSFETsThe research emphasizes the versatility of the proposed method, allowing the design of various cylindricalstructures with distinct properties, including periodic and non-periodic arrangements, symmetric andasymmetric configurations, and nanometer-scale gaps/dots. The focus is on the development of a symmetricCSDG MOSFET concerning the center core, providing valuable insights for fabricating structures over thecore of 2D Electron Gas (2DEG). Keywords— Cylindrical Surrounding Double-Gate MOSFET, Fabrication Steps Layer-by-Layer Approach Semiconductor Devices,Nanoscale Electronics ,Transistor Design ,Oxidation Process, Scalability, Simulation ,Oxidant Concentration.

Spin-filter device using the Zeeman effect with realistic channel and structure parameters

We have theoretically calculated the performance of a Zeeman-type spin polarizer, which consists of ferromagnetic (FM) and metal–insulator–semiconductor (MIS) gate nanostructures on top of an InAs two-dimensional electron gas (2DEG) channel. For the calculations, we have taken a realistic electron concentration, electron effective mass and the effective g-factor of the InAs 2DEG into account. In addition, we have assumed realistic FM and MIS structure sizes by conventional electron beam lithography. In the calculations, we have demonstrated clear oscillation of spin polarization over ±85%. Furthermore, we have shown that it works not only as a spin-polarized current generator but also a detector. In addition, we have proposed a novel spin-filter device utilizing the Zeeman-type spin polarizers. We have found that its conductance characterization allows us to evaluate the operation of Zeeman-type spin polarizers. We expect the Zeeman-type spin polarizers and spin-filter devices to open up a new field of spintronics.

Beating-free quantum oscillations in two-dimensional electron gases with strong spin-orbit and Zeeman interactions

Beating-free quantum oscillations in two-dimensional electron gases with strong spin-orbit and Zeeman interactions

Effect of SiO2 Surface Passivation on the Performance of GaN Polarization Superjunction Heterojunction Field Effect Transistors

In this article, the effects of the SiO2 surface passivation layer are reported on normally‐on 1.2 kV GaN polarization superjunction (PSJ) heterojunction field effect transistors (HFETs) by comparing the electrical performances of PSJ HFETs with and without SiO2 surface passivation. A slight recovery of the 2D electron gas sheet density is observed in the slight negative shift of Vth after SiO2 surface passivation. Passivation also increases the breakdown voltage. This improvement may result from removing positive surface charges in defects along the P‐GaN gate sidewall and top u‐GaN layer after the specifically designed SiO2 surface passivation. Furthermore, the SiO2 surface passivation can also effectively suppress the surface gate leakage currents in the PSJ HFETs by eliminating the conductive channel created by the positive surface charges in defects.

Degradation and Failure Mechanism of p-GaN Gate E-Mode GaN HEMTs

GaN-based power transistors are becoming pivotal devices for high power and high voltage operation due to their higher breakdown voltage, higher carrier mobility because of two-dimensional electron gas (2-DEG), better thermal resistance, higher frequency switching, and lower on-state resistance (RON) [1]. P-GaN gate E-mode GaN HEMTs have emerged as a center of extensive research to understand their operation and reliability [2]. However, one of the reasons the E-mode GaN HEMTs are not yet successfully utilized to their maximum potential is the lack of complete understanding of gate breakdown. Gate degradation of p-GaN gate E-mode GaN HEMT is less understood because of the complexities involving the operating conditions and variables such as gate length, the acceptor doping concentration in the p-GaN layer [3], p-GaN gate thickness, and design of the device [4-5]. To outline a consistent degradation mechanism, this work aims to present an electro-physical model of gate breakdown in p-GaN gate E-mode HEMTs.Lateral E-mode GaN HEMT devices with p-GaN gate configuration are used for this study. According to the Scanning Transmission Electron Microscopy (STEM) imaging of a new device, the AlGaN barrier is 10 nm thick, and the p-GaN cap is 65 nm. The gate-source and gate-drain lengths are 0.5 micrometer and 2.5 micrometer, respectively. The gate length defined by the p-GaN layer is 0.5 micrometer and the edges of the gate metal are pulled in from the top edge of the p-GaN layer by 40 nm. A 0.7 micrometer-long, source-connected field plate is over the p-GaN gate. The p-GaN gate is subjected to static stress to induce accelerated breakdown. Devices were stressed with HP 4155 Semiconductor Analyzer. The electrical measurements were performed with the source, drain, and substrate connected to the ground. FEI 200 Nova was used for Scanning Electron Microscopy (SEM) imaging and Focused Ion-Beam (FIB) milling. ARM JEM-200F with a Cs corrector was used for STEM.Our results show that time-to-fail is highly dependent on device operating conditions. We found no correlation between time-to-fail and initial gate current (IG0). Instead, a higher impact of temperature was found on the gate current of the device (IG) and time-to-fail at 100 °C. Gate failure at constant voltage stress was a single-stage failure. Under constant current stress, on the other hand, the gate shows a multi-stage failure. The breakdown mechanism depends on the impact of static stress on the gate degradation, starting with increased gate current leading to Schottky barrier leakage and finally to catastrophic contact failure. Our study shows that the metal/p-GaN interface at the gate finger becomes the weakest part of the gate stack, likely to be the initiation point of degradation. Metal/p-GaN interface and surface defects develop as percolation paths acting as leakage sources, and nano-cracks have been observed in the gate cap. FA also shows physical degradation at the metal/p-GaN cap due to electrical stress. Observed physical damage in the gate region is correlated to the gate leakage characteristics to develop an electro-physical failure model for gate breakdown in p-GaN gate E-mode GaN HEMTs.

Analysis of Operational Characteristics of AlGaN/GaN MIS-HEMT with Different Slant-Recessed-Gate Structures

Purpose of WorkRecessed-gate metal-insulator-semiconductor high electron mobility transistors (MIS-HEMTs) have emerged as a promising technology for normally-off operation because of their ability to withstand high gate voltage sweeps while maintaining low levels of leakage current [1, 2]. Despite their excellent performance, the etch to leave nanometer-thin layers of AlGaN has proven to be a significant challenge due to difficulties in precisely controlling the etching process. Achieving a positive threshold voltage (Vth) while maintaining the high electron mobility characteristic of the two-dimensional electron gas (2DEG) requires atom-level etch control. Additionally, conventional dry etching techniques can potentially cause damage, presenting a second challenge [3]. However, atomic layer etching (ALE) offers a promising solution with its potential for low damage, uniformity, and high-resolution depth control [4]. In this work, we not only focus on the ALE process to leave nanometer-thin layers of AlGaN/GaN as recessed-gate structures but also analyze the breakdown voltage characteristics of the slant-recessed gate structure.ApproachNormally-off AlGaN/GaN MIS-HEMT with a recessed gate structure was fabricated on a 6-inch commercial Si substrate by MOCVD. The process involved mesa isolation by ICP-RIE, followed by the formation of the ohmic contact (Ti/Al/Ni/Au) at 825 °C annealing for source and drain metals, and the AlGaN/GaN recessed gate structure using ALE technology by O2/BCl3 multiple cycles of oxidation and etching [5]. After the recessed-gate process, the native oxide and carbon residue were removed by a wet cleaning process and O2 plasma treatment, and then the atom layer deposition (ALD) of about 12 nm thickness Al2O3 high-dielectric layer was also deposited. The backend process included gate field plate, SiNx passivation, and contact opening. Finally, the Ni/Au metal stack was deposited as pad electrodes, as shown in Fig. 1. The specific characteristics of the fabricated recessed MIS-HEMT were Lg of 3 μm, Lgs of 5 μm, Lgd of 10 μm, and Wg of 100 μm.Results and SignificanceIn Figure 2(a) and (b), TEM profiles of the recessed gate structure were presented. Following the ALE process for the recessed gate structure, the remaining thickness of AlGaN was approximately 5.1 nm and 5.6 nm for Slant A and Slant B, respectively. Although the thickness of AlGaN remaining was similar in both cases, the slant angle of the recessed gate structure differed, significantly. Specifically, Slant A had a 65-degree recessed gate structure, while Slant B had a 45-degree recessed gate structure. In order to analyze of operational characteristics of AlGaN/GaN MIS-HEMT with different slant-recessed gate structures (Slant A and Slant B), the three-terminal OFF-state breakdown voltage (BV) was measured, as shown in Fig. 3. Recessed gate MIS-HEMT feature design of Lg/Lgd/Lgd/Wg of 3/5/10/100 μm, and VG of -10 V was used as this BV measured condition. The Slant B structure demonstrated higher breakdown voltage (BV) performance compared to the Slant A structure. Specifically, the Slant B structure exhibited a BV of 470 V at 1 μA/mm and 750 V at 1 mA/mm, while the Slant A structure had a BV of only 70 V at 1 μA/mm and 140 V at 1 mA/mm. The reason for the superior BV performance of the Slant B structure (45-degree recessed gate structure) was attributed to the gradient of the AlGaN profile, which facilitates the thin film deposition process of materials such as Al2O3 and gate metal (Ni/Au). Additionally, the Slant B structure helps to reduce the high electric field effect between the gate and drain, further contributing to its improved BV performance.[1] Y. S. Lin, Y. W. Lain, and S. S. H. Hsu, "AlGaN/GaN HEMTs With Low Leakage Current and High On/Off Current Ratio," IEEE Electron Device Letters, vol. 31, no. 2, pp. 102-104, 2010.[2] Z. Tang et al., "600-V Normally Off SiNx/AlGaN/GaN MIS-HEMT With Large Gate Swing and Low Current Collapse," IEEE Electron Device Letters, vol. 34, no. 11, pp. 1373-1375, 2013.[3] T. H. Hung, P. S. Park, S. Krishnamoorthy, D. N. Nath, and S. Rajan, "Interface Charge Engineering for Enhancement-Mode GaN MISHEMTs," IEEE Electron Device Letters, vol. 35, no. 3, pp. 312-314, 2014.[4] T. Y. Yang et al., "A Normally-Off GaN MIS-HEMT Fabricated Using Atomic Layer Etching to Improve Device Performance Uniformity for High Power Applications," IEEE Electron Device Letters, vol. 43, no. 10, pp. 1629-1632, 2022.[5] I.-H. Hwang, H.-Y. Cha, and K.-S. Seo, "Low-Damage and Self-Limiting (Al)GaN Etching Process through Atomic Layer Etching Using O2 and BCl3 Plasma," Coatings, vol. 11, no. 3, p. 268, 2021. Figure 1

(Invited) Benchmarking of Beyond the State-of-the-Art Vertical GaN Devices

In this paper theoretical benchmarking of semi-vertical and vertical gallium nitride (GaN) MOSFETs with rated voltage of 1.2 kV to 3.3 kV is performed against corresponding silicon carbide (SiC) devices. Specific design features and technology requirements for realization of high voltage vertical GaN MOSFETs are discussed and implemented in simulated structures. The main findings are that a) specific on-resistance of vertical GaN devices is expected to be 75% and 40% of that for 1.2 kV and 3.3 kV SiC MOSFETs, respectively, b) semi-vertical GaN do not offer any advantage over SiC MOSFETs for medium and high voltage devices (>1.0 kV), and c) vertical GaN has largest potential advantage for high and ultra-high voltage devices (>2.0 kV).Gallium nitride (GaN) devices experience an explosive and rapid development with new device providers entering the market at an impressive speed. However, the devices are HEMT (High Electron Mobility Transistor) type lateral devices utilizing the two-dimensional electron gas (2DEG) properties of the GaN/AlGaN interface. The required GaN layers in the devices are grown epitaxially on hetero substrates like Si, SiC, sapphire and poly-AlN using buffer layers to control the strain caused by lattice mismatch which prevents the vertical current flow. For power applications the lateral current flow close to the surface leads to thermal limitations and complicates thermal management in packaging. Furthermore, it leads to large footprint of the devices for large currents. The devices also lack the robust avalanche breakdown characteristics. At present the HEMT power devices are limited to operational voltages of less than 650V with some first devices from VisIC and iGaNPower demonstrating 1200V capability. For these reasons there is an interest in the development of vertical GaN devices opening the possibilities of small footprint and thus lower cost due to smaller chip sizes, high power densities and robust avalanche breakdown characteristics with large avalanche energy capability [1]. The development of vertical devices has been hindered by the lack of large area freestanding GaN substrates facilitating homoepitaxial growth of drift layers. An alternative are semi-vertical structures grown on Si which are the subject of this study along with true vertical structures on GaN. In addition to the material issues there are technological issues that require further attention like dopant compensation and activation and Mg implantation technology for junction termination. This paper focuses on prospective benchmarking of semi-vertical and vertical GaN MOSFETs against SiC devices. A beyond the state-of-the-art benchmarking, in wide voltage range, is missing in the literature. Most of the benchmarking publications report results of experimental structures based on state-of-the-art technology and simple junction termination [2], [3], [4].The main objective of the work has been to design competitive semi-vertical and vertical GaN structures and perform benchmarking against SiC devices in the voltage range 1.2 kV to 3.3 kV. The structures have been characterized by calculating the output, transfer and voltage blocking characteristics using default Sentaurus models. In the next step, the structures have been modified based on the experience and know-how from SiC devices. The electric field crowding occurring at the trench gate corners must be mitigated both with respect to the breakdown voltage in the bulk material and the reliability of the gate dielectric [5]. Efficient junction termination (JT) is also necessary for both types of structures. This is especially important in the semi-vertical structures where junction termination is an integral part of each device segment and has a significant impact on the resulting on-state resistance. The structure design was optimized by reducing the cell pitch and introducing a multi-cell-per-device-segment design in semi-vertical structures to improve the on-resistance. Removal of the performance limitations in the structures based on the state-of-the-art GaN technology and device cell optimization was done to make benchmarking with more mature SiC devices based on the prediction of what may become feasible beyond the state-of-the-art.1. Matteo Meneghini et al., Journal of Applied Physics, vol. 130, p. 181101, 2021.2. Oka, T. Ina, Y. Ueno and J. Nishii, 2019 31st International Symposium on Power Semiconductor Devices and ICs (ISPSD), pp. 303-306, 2021.3. A. Khadar, C. Liu, R. Soleimanzadeh and E. Matioli, IEEE Electron Device Letters, vol. 40, no. 3, pp. 443-446, 2019.4. Wei He et al., Nanoscale Research Letters, 17:14, 2022.5. Nakamura et al., 2011 International Electron Devices Meeting, pp. 26.5.1-26.5.3, 2011.

(Invited) Heterogeneous Integration of Ultrawide Bandgap III-Nitrides and Diamond

Industry demands for power switching, amplification and photonic applications are continually pushing the limits of semiconductor device technologies leading to increasing applied voltages and, thus, higher electric fields while simultaneously requiring reduction in the size, weight and power consumption. In addition, recent heightened interest in higher temperature (and other extreme environment) applications, while simultaneously maintaining high-voltage and high-power performance, has inexorably led to ultrawide bandgap (UWBG) semiconductor materials. This interest in UWBG materials stems from superior properties that include high material hardness, breakdown strength to withstand large electric fields and mitigation of power dissipation during device switching, deliver high current density while operating at higher junction and ambient temperature. Among these materials are boron nitride in its hexagonal and cubic phases, gallium oxide, diamond, and aluminum gallium nitride. The versatile alloys of wurtzite AlxGa1-xN have long been demonstrated across the full composition range. The variation in Eg with AlGaN composition, along with select WBG and UWBG materials, is shown in Figure 1 versus (in-plane for non-cubic) lattice constant.Premier WBG materials to date for electronics and photonics are SiC (Eg = 3.3 eV) and GaN (Eg = 3.4 eV). The latter has been successfully applied to high-power transistors and near ultraviolet photonics. In both electronics and photonics applications, the polarization fields of the wurtzite crystal structure, generally grown along (0001) direction, produce design challenges. In photonics, for example, the fields and band alignment couple to spatially separate electrically injected electrons and holes to reduce wavefunction overlap in quantum wells. In electronics, the internal fields have contributed to difficulties in the design of logic and bipolar transistor devices based on these materials. Despite this, the inherent polarization field and band offsets were successfully employed in the design of high electron-mobility transistors (HEMTs). The inherent AlGaN/GaN HEMT band offset produces a high electron concentration at the interface that spontaneously forms a two-dimensional electron gas (2DEG). Because charge flow in these devices is restricted to the 2DEG, current crowding results in intense self-heating. The local heating decreases electrical conductivity to produce a runaway scenario for elevating the Joule heating. Dissipating this heat, therefore, is the principal limiting factor in operating at power higher than 40 W/mm.The various temperature dependences of semiconductor device parameters are the principal drivers in the operation of semiconductor devices. The physics has inextricably led to UWBGs for future high temperature applications, > 300 °C, due to their lower intrinsic carrier density and thus the effects of leakage are orders of magnitudes less than Si-based devices. Furthermore, AlGaN and diamond offer superior high temperature mechanical stability due to their structural hardness and chemical inertness. Thermal management is essential for achieving higher power densities. Diamond is highly attractive for many applications due to its high hardness, transparency, chemical inertness, promising semiconductor properties, and its high thermal conductivity (κ) to 2500 W/m·K in natural material. Factors related to the consequences of self-heating in devices have motivated extensive research to improve the properties of diamond and produce better approaches to determine the thermal properties. Laboratory-grown diamond films exhibit significantly poorer κ than natural diamond and, when integrated with other electronic materials, a low thermal boundary conductance (TBC in W/m2·K) that inhibits heat transfer between materials. The former originates from the ubiquitous polycrystalline nature of chemical vapor deposition (CVD) diamond and the presence of non-diamond carbon (NDC) in the layer. The low TBC stems from fundamental barriers to phonon propagation between two materials and from practical issues of poor diamond quality in the initial growth regime. Continued research is needed to improve κ in the polycrystalline diamond and mitigate the negative impact of the initial diamond layer on TBC.The current status of hetero-integration of UWBG AlGaN and diamond will be reviewed. Particular emphasis is given to high Al content AlGaN for near-bandgap matching with diamond. Patterned diamond grown using hot-filament CVD to realize unique three-dimensional lateral device structures in a self-aligned, as-grown fabrication process is employed. Engineering the AlGaN-diamond bandgap alignment is noteworthy at the nonpolar AlGaN – diamond interface including diamond surface terminations that produce either negative or positive electron affinity. The review will focus on improvements in material properties, especially the interfaces, that are critical to both charge and heat transport for UWBG device structures. Figure 1

A broad-spectrum gas sensor based on correlated two-dimensional electron gas

Designing a broad-spectrum gas sensor capable of identifying gas components in complex environments, such as mixed atmospheres or extreme temperatures, is a significant concern for various technologies, including energy, geological science, and planetary exploration. The main challenge lies in finding materials that exhibit high chemical stability and wide working temperature range. Materials that amplify signals through non-chemical methods could open up new sensing avenues. Here, we present the discovery of a broad-spectrum gas sensor utilizing correlated two-dimensional electron gas at a delta-doped LaAlO3/SrTiO3 interface with LaFeO3. Our study reveals that a back-gating on this two-dimensional electron gas can induce a non-volatile metal to insulator transition, which consequently can activate the two-dimensional electron gas to sensitively and quantitatively probe very broad gas species, no matter whether they are polar, non-polar, or inert gases. Different gas species cause resistance change at their sublimation or boiling temperature and a well-defined phase transition angle can quantitatively determine their partial pressures. Such unique correlated two-dimensional electron gas sensor is not affected by gas mixtures and maintains a wide operating temperature range. Furthermore, its readout is a simple measurement of electric resistance change, thus providing a very low-cost and high-efficient broad-spectrum sensing technique.

THz properties of grating-gate plasmonic crystals

This study reviews recent advances in the modern field of terahertz plasmonics concerning the control of resonant properties of grating-gate plasmonic crystal structures. Particularly, we conducted both experimental and theoretical investigations of AlGaN/GaN grating-gate structures with a focus on investigations of the resonant structure of transmission spectra associated with plasmon excitations in two-dimensional electron gas at different modulation degree of concentration profiles. Two distinct resonant phases of the plasmonic crystal structure were analyzed. The first one, the delocalized phase, is observed in the case of a small modulation degree of electron gas. In this phase, we found that plasmonic resonant absorption of incident radiation occurs across the entire grating-gate structure, with domination in the gated regions of the electron gas. In contrast, the second phase, the localized one, is realized at a strong modulation of the electron concentration profiles when the gated regions of the electron gas are completely depleted. Here, plasmon resonances are characterized by the spatial localization of absorption of incident radiation exclusively within the ungated regions of the electron gas. Moreover, in the localized phase, we observed the unexpected blue shift of plasmon resonant frequency with an increase of gate voltage. This observation was explained by the result of ‘edge gating effect’ and additional shrinking of the concentration profile of the electron gas in the ungated region. We demonstrate that the correct description of both phases requires rigorous electrodynamic simulations and cannot be achieved solely in the frameworks of simplified single-mode or single-cavity models.

Reconstructing the potential configuration in a high-mobility semiconductor heterostructure with scanning gate microscopy

The weak disorder potential seen by the electrons of a two-dimensional electron gas in high-mobility semiconductor heterostructures leads to fluctuations in the physical properties and can be an issue for nanodevices. In this paper, we show that a scanning gate microscopy (SGM) image contains information about the disorder potential, and that a machine learning approach based on SGM data can be used to determine the disorder. We reconstruct the electric potential of a sample from its experimental SGM data and validate the result through an estimate of its accuracy.

Tuning of the Berreman mode of GaN/AlxGa1-xN heterostructures on sapphire: The role of the 2D-electron gas in the mid-infrared

We investigated the mid-infrared optical properties of GaN/Al(x)Ga(1-x)N-type heterostructures on sapphire substrate grown by the Metal-Organic Chemical Vapor Deposition. We show how the polarization-dependent reflection spectrum is affected by the presence of the strain-induced 2D electron gas at the interfaces between Al(x)Ga(1-x)N and GaN layers. In particular, we show that the 2D electron gas contribution, modeled from its density and transport properties, can play a relevant role at the Berreman mode excitation condition. In this framework, our results offer an advanced approach for the optimization and design of GaN-based broadband optoelectronic and energy management devices.

Upper bound on the window of density occupied by microemulsion phases in two-dimensional electron systems

Upper bound on the window of density occupied by microemulsion phases in two-dimensional electron systems

Terahertz bow-tie diode based on asymmetrically shaped AlGaN/GaN heterostructures

Asymmetrical shaping of AlGaN/GaN heterostructures containing a conductive layer of two-dimensional electron gas (2DEG) was used for the development of bow-tie (BT) diodes for room temperature terahertz (THz) detection. Considering operation of the THz BT diode in the unbiased mode as preferable for practical applications, we investigated the diodes with an obvious asymmetry of IV characteristics, which was found to be more pronounced with the decrease of an apex width, resulting in the sensitive THz detection. A nonuniform heating of carriers in a metalized leaf of the BT diode was attributed as the main mechanism that caused the rectification of THz waves. The responsivity and noise-equivalent power (NEP) at the fundamental antenna frequency of 150 GHz were up to 4 V/W and 2 nW/√Hz, respectively. Such high sensitivity of BT diodes allowed us to measure for the first time the response spectrum of the asymmetric BT antenna demonstrating fundamental and higher order resonances in good agreement with finite-difference time-domain simulation data in a broad spectrum range. The detailed investigation of the lowand high-frequency noise characteristics of AlGaN/GaN BT diodes revealed that only thermal noise needs to be considered for the unbiased operation, the value of which was relatively low due to a high density of 2DEG enabling low resistivity values. Moreover, we observed that the responsivity of BT diode scales with its resistance, revealing that tapering of the diode apex below a few microns could be ineffective in applications which require low NEP values.

Quantum Hall coherent perfect absorption in graphene

100%\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$100\\%$$\\end{document} absorption in a two-dimensional electron gas (2DEG) with Dirac spectrum is demonstrated to be obtained by controlling the interference of multiple incident radiations, referred to as coherent perfect absorption (CPA). However, when a 2DEG such as graphene is exposed to a magnetostatic bias, it resonantly could absorb electromagnetic radiation by transitions of its Dirac electrons between non-equidistant and nonlinear Landau levels. Here, the magneto-optical terahertz (THz) CPA in graphene under the quantum Hall effect (QHE) regime at both strong and subtesla magnetostatic bias fields is addressed. Our findings show that an effective magneto-optical surface conductivity corresponding to right- and left-handed circular (RHC- and LHC) polarizations could model a magneto-tunable CPA in graphene in THz range. Significantly, graphene under QHE regime reveals different tunable CPA properties for each circularly polarized beams by the intensity of the applied magnetic bias. Moreover, it is observed that different phase modulations at CPA frequencies are achieved for RHC and LHC polarizations. Considering the maximum efficiency for a 2D absorber, our results demonstrate the magnetostatic tuning of CPA in 2D Dirac materials for long-wavelength sensing applications and signal processing.

Large Nonlinear Transverse Conductivity and Berry Curvature in KTaO3 Based Two-Dimensional Electron Gas.

Two-dimensional electron gas (2DEG) at oxide interfaces exhibits various exotic properties stemming from interfacial inversion and symmetry breaking. In this work, we report large nonlinear transverse conductivities in the LaAlO3/KTaO3 interface 2DEG under zero magnetic field. Skew scattering was identified as the dominant origin based on the cubic scaling of nonlinear transverse conductivity with linear longitudinal conductivity and 3-fold symmetry. Moreover, gate-tunable nonlinear transport with pronounced peak and dip was observed and reproduced by our theoretical calculation. These results indicate the presence of Berry curvature hotspots and thus a large Berry curvature triplet at the oxide interface. Our theoretical calculations confirm the existence of large Berry curvatures from the avoided crossing of multiple 5d-orbit bands, orders of magnitude larger than that in transition-metal dichalcogenides. Nonlinear transport offers a new pathway to probe the Berry curvature at oxide interfaces and facilitates new applications in oxide nonlinear electronics.

High-mobility two-dimensional electron gas at the PbZr0.5Ti0.5O3/BaSnO3 heterostructure

High-mobility two-dimensional electron gas at the PbZr0.5Ti0.5O3/BaSnO3 heterostructure

Strongly interacting two-dimensional electron systems: Evidence for enhanced one-dimensional edge-channel coupling

Strongly interacting two-dimensional electron systems: Evidence for enhanced one-dimensional edge-channel coupling

Polarization-selective equilibration and quantum interference in graphene-based quantum Hall edge states

Quantum Hall (QH) edge states in two-dimensional electron gas systems have drawn attention as potential carriers of quantum information, owing to their chiral ballistic nature, arising from Landau level formation under a strong perpendicular magnetic field. They remain stable even in the presence of material disorder, making them ideal for phase-sensitive measurements like QH interferometry. We report the equilibration processes between electron waves in spin and valley polarized QH edge states, particularly in a four-terminal measurement configuration. We compare experimental results with theoretical estimations based on the Landauer formula, taking into account selective equilibration depending on spin polarization. Furthermore, quantum interference in co-propagating QH edge states is investigated, elucidating how factors such as magnetic fields, voltage bias, and gate configurations influence quantum phase. The research offers insights into the controllability of interactions between QH edge states, vital for harnessing the potential of graphene in QH interferometers and quantum information processing.