Two-dimensional Electron Channel Research Articles

Drifting plasmons in two-dimensional electron channels: circuit analogy.

Plasmons in two-dimensional electron channels have potential applications in the terahertz frequency range. Equivalent circuit models provide a convenient framework for analysing the plasmons. This article introduces a circuit model for plasmons in the presence of a dc current that flows in a gated channel. It is shown that drifting plasmons can be described by an LC-transmission line with distributed dependent sources. A circuit analogue of the Dyakonov-Shur instability is demonstrated. Then, a lumped-element transmission line with dependent sources is analysed, and non-reciprocity is demonstrated for examples of a right- and a left-handed transmission line. Effects of ohmic loss are discussed. The results could be used for the design of non-reciprocal transmission line devices. This article is part of the theme issue 'Celebrating the 15th anniversary of the Royal Society Newton International Fellowship'.

Reduction of misfit dislocation density in metamorphic heterostructures by design optimization of the buffer layer with non-linear graded composition profile

Equilibrium distributions of misfit dislocation density along the growth direction of metamorphic buffer layers InxAl1-xAs/GaAs with maximum In content xmax≥0.77 and different non-linear graded composition profiles x propto z1/n are calculated. The effect of the initial In composition (n=2) of InxAl1-xAs buffer layer with convex-graded (xmin) composition profile on misfit dislocation density as well as amount of residual stresses at its top part is considered. Using computational approach, it was shown that a dislocation-free region is formed under thin tensile-strained GaAs layer (1-10 nm) inserted into InAlAs metamorphic buffer layer, which agrees with experimental data obtained early by transmission electron microscopy. Novel non-linear graded composition profile of metamorphic buffer layer has been proposed, which results in twice reduction of misfit dislocation density as compared to the convex-graded one. In addition, equilibrium distributions of misfit dislocation density in the HEMT heterostructures with two-dimensional electron channel InxAl1-xAs, which are based on In0.75Ga0.25As/In0.75Al0.25As metamorphic buffer layer of various designs, are calculated. The values of inverse steps (Delta), representing the difference between the maximum In content of InxAl1-xAs (xmax) and In content of In0.75Al0.25As virtual substrate, at which relaxation of the elastic strains in 2D channel In0.75Ga0.25As/In0.75Al0.25As doesn't occur, are calculated for metamorphic buffer layers InxAl1-xAs with convex-graded and optimized non-linear graded composition profiles. Keywords: metamorphic heterostructures, metamorphic buffer layer, misfit dislocations, elastic stresses, In(Ga,Al)As/GaAs.

Снижение плотности дислокаций в метаморфных гетероструктурах путем оптимизации конструкции буферного слоя с нелинейным профилем изменением состава

Equilibrium distributions of misfit dislocation density along the growth direction of metamorphic buffer layers InxAl1-xAs/GaAs with maximum In content xmax ≥ 0.77 and different non-linear graded composition profiles (x ∝ z1/n) are calculated. The effect of the initial In composition (xmin) of InxAl1-xAs buffer layer with convex-graded (n = 2) composition profile on misfit dislocation density as well as amount of residual stresses at its top part is considered. Using computational approach, it was shown that a dislocation-free region is formed under thin tensile-strained GaAs layer (1–10 nm) inserted into InAlAs metamorphic buffer layer, which agrees with experimental data obtained early by transmission electron microscopy. Novel non-linear graded composition profile of metamorphic buffer layer has been proposed, which results in twice reduction of misfit dislocation density as compared to the convex-graded one. In addition, equilibrium distributions of misfit dislocation density in the HEMT heterostructures with two-dimensional electron channel In0.75Ga0.25As/In0.75Al0.25As, which are based on InxAl1-xAs/GaAs metamorphic buffer layer of various designs, are calculated. The values of inverse steps (∆), representing the difference between the maximum In content of InxAl1-xAs (xmax) and In content of In0.75Al0.25As virtual substrate, at which relaxation of the elastic strains in 2D channel In0.75Ga0.25As/In0.75Al0.25As doesn’t occur, are calculated for metamorphic buffer layers InxAl1-xAs with convex-graded and optimized non-linear graded composition profiles.

GaN-HEMT on Si as a Robust Visible-Blind UV Detector With High Responsivity

This work presents performance evaluation of GaN High Electron Mobility Transistor (HEMT) based ultraviolet (UV) detector on Si substrate. In addition to the fabrication and characterization, a systematic study is presented here using simulations extensively to investigate the UV detection mechanism. Output current has been chosen as the sensing metric, the fabricated device exhibits a high UV responsivity of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$1.62\times 10^{{7}}$ </tex-math></inline-formula> A/W at <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$2.5\times 10^{-{10}}$ </tex-math></inline-formula> W, <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\text{V}_{{\text {GS}}}={0.5}$ </tex-math></inline-formula> V. Simulations have been done using optical modules available in Silvaco ATLAS TCAD to analyze the energy band bending, Two-Dimensional Electron Gas (2DEG), channel potentials and electric fields in the device. This model can aid in systematic study of HEMT based detectors in terms of dimensional and epi layer design optimizations for sensitivity enhancements. The UV response of the device is found to decrease as the wavelength approaches the visible light wavelength. This makes the photodetectors blind to visible light ensuring selective detection of UV wavelengths. It has been observed that as the area for UV absorption is increased by increasing the W/L ratio, the increases. For a W/L ratio of 100, the detector exhibits a responsivity of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$1.86\times 10^{{7}}$ </tex-math></inline-formula> A/W.

Giant inverse Faraday effect in a plasmonic crystal ring.

Circularly polarized electromagnetic wave impinging on a conducting ring with a two-dimensional electron channel generates a circulating DC plasmonic current resulting in an inverse Faraday effect in nanorings. We show that a large ring with periodically modulated width on a nanoscale, smaller or comparable with the plasmonic mean free path, supports plasmon energy bands. When circularly polarized radiation impinges on such a plasmonic ring, it produces resonant DC plasmonic current on a macro scale resulting in a giant inverse Faraday effect. The systems comprised of the concentric variable-width rings ("plasmonic disks") and stacked plasmonic disks ("plasmonic solenoids") amplify the generated constant magnetic field by orders of magnitude.

Plasmonic instabilities in two-dimensional electron channels of variable width

Understanding of fundamental physics of plasmonic instabilities is the key issue for the design of a new generation of compact electronic devices required for numerous THz applications. Variable width plasmonic devices have emerged as potential candidates for such an application. The analysis of the variable width plasmonic devices presented in this paper shows that these structures enable both the Dyakonov-Shur instability (when the electron drift velocity everywhere in the device remains smaller than the plasma velocity) and the "plasmonic boom" instability that requires drift velocity exceeding the plasma velocity in some of the device sections. For symmetrical structures, the drifting current could be provided by an RF signal leading to RF to THz and THz to RF frequency conversion using the source and drain antennas and reducing losses associated with ohmic contacts. We show that narrow regions protruding from the channel ("plasmonic stubs") could control and optimize boundary conditions at the contacts and/or at the interfaces between different device sections. These sections could be combined into plasmonic crystals yielding enhanced power and a better impedance matching. The mathematics of the problems is treated using the transmission line analogy. We show that the combination of the stubs and the variable width channels is required for the instability rise in an optimized plasmonic crystal. Our estimates show that THz plasmonic crystal oscillators could operate at room temperature.

Gated Bow-Tie Diode for Microwave to Sub-Terahertz Detection.

We propose a new design microwave radiation sensor based on a selectively doped semiconductor structure of asymmetrical shape (so-called bow-tie diode). The novelty of the design comes down to the gating of the active layer of the diode above different regions of the two-dimensional electron channel. The gate influences the sensing properties of the bow-tie diode depending on the nature of voltage detected across the ungated one as well as on the location of the gate in regard to the diode contacts. When the gate is located by the wide contact, the voltage sensitivity increases ten times as compared to the case of the ungated diode, and the detected voltage holds the same polarity of the thermoelectric electromotive force of hot electrons in an asymmetrically shaped n-n+ junction. Another remarkable effect of the gate placed by the wide contact is weak dependence of the detected voltage on frequency which makes such a microwave diode to be a proper candidate for the detection of electromagnetic radiation in the microwave and sub-terahertz frequency range. When the gate is situated beside the narrow contact, the two orders of sensitivity magnitude increase are valid in the microwaves but the voltage sensitivity is strongly frequency-dependent for higher frequencies.

Polarization Engineering of AlGaN/GaN HEMT With Graded InGaN Sub-Channel for High-Linearity X-Band Applications

We report on the power and linearity performance of metal-organic chemical vapor deposition grown polarization-engineered novel structure that combines the AlGaN/GaN high-electron-mobility transistor with a graded InGaN sub-channel layer. The fabricated transistors with composite two-dimensional(2D) and three-dimensional(3D) electron channels showed nearly flat transconductance and power gain profiles. The maximum f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</sub> and f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">max</sub> values of 18 GHz and 38 GHz were measured for 0.7-μm gate-length transistors. Load-pull measurement at 10 GHz revealed a maximum output power of 2.2 W/mm. Two-tone measurement at 10 GHz showed an excellent OIP3 of 38 dBm for 150-μm device width and a corresponding linearity figure of merit OIP3/P <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">DC</sub> of 9.7 dB. These results suggest that InGaN-based composite 2D-3D channel transistors could be useful for high-frequency applications requiring high linearity.

Photocurrent and photoluminescence characteristics of AlGaAs/GaAs double-heterostructures with a pair of two-dimensional electron and hole channels

AlGaAs/GaAs/AlGaAs double-heterostructures having two-dimensional electron and hole channels at the respective interfaces are studied by measuring their photocurrent and photoluminescence characteristics. Under the weak photoexcitation, it is found that photo-generated electrons and holes are driven by a built-in electric field between the two channels and flow out mostly as a photocurrent to the respective electrodes, making the photoluminescence negligibly small. When the excitation reaches a certain level, some of photo-generated electrons and holes accumulate in each channel and weaken the built-in field, leading to an exponential increase in photoluminescence or the radiative recombination of electrons and holes. When the excitation gets strong, photo-generated carriers are lost mostly in the form of photoluminescence, resulting in the saturation of photocurrent. A theoretical model to explain these findings is presented. A possibility of using this type of study to clarify operating mechanisms of super-junction devices is suggested.

Spin-polarization-induced anisotropic magnetoresistance in a two-dimensional Rashba system

In an asymmetric two-dimensional electron gas system, the Rashba effective field arises due to the intrinsic electric field. Even without ferromagnetism, the Rashba spin splitting acts as a source of spin polarization and affects the transport property of the two-dimensional electron channel. In this Rashba channel, the magnetoresistance is determined by the vector alignment between the applied field and the bias current. In addition, the channel resistance for the parallel alignment between the applied field and the Rashba field is much smaller than that for the antiparallel alignment between them, which surprisingly agrees with the spin polarization induced by the Edelstein effect. This anisotropic magnetoresistance also allows us to estimate the spin polarization in a two-dimensional quantum well channel.

Terahertz signal detection in a short gate length field-effect transistor with a two-dimensional electron gas

We consider the problem of non-resonant detection of terahertz signals in a short gate length field-effect transistor having a two-dimensional electron channel with zero external bias between the source and the drain. The channel resistance, gate-channel capacitance, and quadratic nonlinearity parameter of the transistor during detection as a function of the gate bias voltage are studied. Characteristics of detection of the transistor connected in an antenna with real impedance are analyzed. The consideration is based on both a simple one-dimensional model of the transistor and allowance for the two-dimensional distribution of the electric field in the transistor structure. The results given by the different models are discussed.

Amplification and generation of terahertz plasmons in gated two-dimensional channels: Modal analysis

A theoretical analysis is presented of plasmons in gated two-dimensional electron channels supporting dc currents. In contrast to previous treatments, the model takes into account complete mode spectra and transverse field distributions. Conditions for plasmon amplification are determined, and asymmetric plasmonic oscillators are analysed. The results are compared with the traditional treatment of the Dyakonov-Shur instability. The limitations of the traditional treatment are thus revealed. The present electrodynamically rigorous model can be used to design and analyse terahertz plasmonic oscillators.

Peculiarities of the electrophysical properties of InSb/AlInSb/AlSb heterostructures with a high electron concentration in the two-dimensional channel

The electrophysical properties of InSb/AlInSb/AlSb heterostructures with a high electron concentration are reported. We have observed anisotropy of the electron concentration and mobility measured at a low magnetic field in the [110] and \(\left[ {1\bar 10} \right]\) crystallographic directions. It has been established by analysis of Shubnikov-de Haas oscillations that the conductivity through the two-dimensional electron channel InSb/AlInSb quantum well (QW) does not depend on the crystallographic direction. However, the magnetic-field dependences of the Hall coefficient and resistivity of the structures reveal a strong influence of the crystallographic directions. This has allowed one to conclude that the anisotropy of the electron transport parameters in the QW structures measured at low magnetic fields correspond to parasitic conductivity through the Al0.09In0.91Sb buffer layer with two pronounced anisotropic contributions: the influence of metallic In nanoclusters inhomogeneously distributed within the buffer layer and conductivity of the near-interface layer with a high anisotropic density of extended defects.

High Mobility One- and Two-Dimensional Electron Systems in Nanowire-Based Quantum Heterostructures

Free-standing semiconductor nanowires in combination with advanced gate-architectures hold an exceptional promise as miniaturized building blocks in future integrated circuits. However, semiconductor nanowires are often corrupted by an increased number of close-by surface states, which are detrimental with respect to their optical and electronic properties. This conceptual challenge hampers their potentials in high-speed electronics and therefore new concepts are needed in order to enhance carrier mobilities. We have introduced a novel type of core-shell nanowire heterostructures that incorporate modulation or remote doping and hence may lead to high-mobility electrons. We demonstrate the validity of such concepts using inelastic light scattering to study single modulation-doped GaAs/Al0.16Ga0.84As core-multishell nanowires grown on silicon. We conclude from a detailed experimental study and theoretical analysis of the observed spin and charge density fluctuations that one- and two-dimensional electron channels are formed in a GaAs coaxial quantum well spatially separated from the donor ions. A total carrier density of about 3 × 10(7) cm(-1) and an electron mobility in the order of 50,000 cm(2)/(V s) are estimated. Spatial mappings of individual GaAs/Al0.16Ga0.84As core-multishell nanowires show inhomogeneous properties along the wires probably related to structural defects. The first demonstration of such unambiguous 1D- and 2D-electron channels and the respective charge carrier properties in these advanced nanowire-based quantum heterostructures is the basis for various novel nanoelectronic and photonic devices.

Amplification of drifting semiconductor plasmons and effects of carrier collisions and diffusion

Carrier collisions and diffusion are the major mechanisms influencing loss in solid-state plasmas, and their understanding is crucial for the design of terahertz plasmonic oscillators and amplifiers. This paper presents an analysis of the role collisions and diffusion play in the amplification of plasmons reflecting from conducting boundaries in two-dimensional electron channels. The theoretical model relies on the hydrodynamic description of the electron dynamics, complete mode spectra and electrodynamic boundary conditions. Diffusion has been found to reduce the amplification coefficient due to a reduced asymmetry of the counter-propagating plasmons, and both collisions and diffusion have been found to influence the plasmon propagation loss. The distances of sustained amplification have been calculated and the implications for terahertz oscillators have been analysed.

Features of electronic transport in relaxed Si/Si1−XGeX heterostructures with high doping level

The low-temperature electrical and magnetotransport characteristics of partially relaxed Si/Si1−xGex heterostructures with two-dimensional electron channel (ne≥1012cm−2) in an elastically strained silicon layer of nanometer thickness have been studied. The detailed calculation of the potential and of the electrons distribution in layers of the structure was carried out to understand the observed phenomena. The dependence of the tunneling transparency of the barrier separating the 2D and 3D transport channels in the structure, was studied as a function of the doping level, the degree of blurring boundaries, layer thickness, degree of relaxation of elastic stresses in the layers of the structure. Tunnel characteristics of the barrier between the layers were manifested by the appearance of a tunneling component in the current–voltage characteristics of real structures. Instabilities, manifested during the magnetotransport measurements using both weak and strong magnetic fields are explained by the transitions of charge carriers from the two-dimensional into three-dimensional state, due to interlayer tunneling transitions of electrons.

V2O5 quantum dots/graphene hybrid nanocomposite with stable cyclability for advanced lithium batteries

High-speed electron transfer channels and short Li ion transport distance are beneficial to improvement of Li ion battery properties. Here, a two-step solution phase synthesis method is developed to construct the V2O5 quantum dots/graphene hybrid nanocomposite by controlling nucleation and growth processes. It is demonstrated that the V2O5 quantum dots with an average size of 2-3 nm are uniformly anchored on the graphene sheets. The specific capacity can achieve 212mAhg−1 at 100mAg−1 after 100 cycles. Significantly, the novel V2O5 quantum dots/graphene shows a stable cycling performance with 89% capacity retention after 300 cycles. The improvement in electrochemical properties could be attributed to the short Li ion transfer distance, two-dimensional electron channels, homogeneous dispersion and immobilization of V2O5 quantum dots. Meanwhile, it indicates that V2O5 quantum dots/graphene is promising cathode material for use in long-life rechargeable lithium batteries. This design conception and synthesis strategy for V2O5 could also be extended to other electrode material systems.

Amplifying mirrors for terahertz plasmons

Semiconductor plasmons have long held out a promise for terahertz generation, but competitive plasmonic mechanisms have yet to be found. Here, we introduce amplifying terahertz mirrors: planar interfaces for two-dimensional electron channels that amplify plasmons in the presence of electron drift. In contrast to existing formulations, we develop a rigorous mode matching technique that takes the complete mode spectrum into account. Mirrors are characterized by plasmon reflection and transmission coefficients whose values can increase with drift. Amplitude and power coefficients are determined, and conditions are found for their values to exceed unity. Resonators based on different combinations of amplifying mirrors are investigated, and an asymmetric configuration (consisting of two different electron channels confined between conducting planes) whose roundtrip gain can exceed unity is identified. The unusual conditions needed for oscillation are examined in detail and the general advantages of asymmetric arrangements are highlighted. Finally, the potential of mode matching as a universal tool for plasmonics is discussed.

Inducing an Incipient Terahertz Finite Plasmonic Crystal in Coupled Two Dimensional Plasmonic Cavities

We measured a change in the current transport of an antenna-coupled, multigate, GaAs/AlGaAs field-effect transistor when terahertz electromagnetic waves irradiated the transistor and attribute the change to bolometric heating of the electrons in the two dimensional electron channel. The observed terahertz absorption spectrum indicates coherence between plasmons excited under adjacent biased device gates. The experimental results agree quantitatively with a theoretical model we developed that is based on a generalized plasmonic transmission line formalism and describes an evolution of the plasmonic spectrum with increasing electron density modulation from homogeneous to the crystal limit. These results demonstrate an electronically induced and dynamically tunable plasmonic band structure.

Distributed gain in plasmonic reflectors and its use for terahertz generation

Semiconductor plasmons have potential for terahertz generation. Because practical device formats may be quasi-optical, we studied theoretically distributed plasmonic reflectors that comprise multiple interfaces between cascaded two-dimensional electron channels. Employing a mode-matching technique, we show that transmission through and reflection from a single interface depend on the magnitude and direction of a dc current flowing in the channels. As a result, plasmons can be amplified at an interface, and the cumulative effect of multiple interfaces increases the total gain, leading to plasmonic reflection coefficients exceeding unity. Reversing the current direction in a distributed reflector, however, has the opposite effect of plasmonic deamplification. Consequently, we propose structurally asymmetric resonators comprising two different distributed reflectors and predict that they are capable of terahertz oscillations at low threshold currents.