Two-dimensional Electron Gas System Research Articles

Anomalous enhancement of thermoelectric power factor in multiple two-dimensional electron gas system.

Toward drastic enhancement of thermoelectric power factor, quantum confinement effect proposed by Hicks and Dresselhaus has intrigued a lot of researchers. There has been much effort to increase power factor using step-like density-of-states in two-dimensional electron gas (2DEG) system. Here, we pay attention to another effect caused by confining electrons spatially along one-dimensional direction: multiplied 2DEG effect, where multiple discrete subbands contribute to electrical conduction, resulting in high Seebeck coefficient. The power factor of multiple 2DEG in GaAs reaches the ultrahigh value of ~100 μWcm-1 K-2 at 300 K. We evaluate the enhancement rate defined as power factor of 2DEG divided by that of three-dimensional bulk. The experimental enhancement rate relative to the theoretical one of conventional 2DEG reaches anomalously high (~4) in multiple 2DEG compared with those in various conventional 2DEG systems (~1). This proposed methodology for power factor enhancement opens the next era of thermoelectric research.

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.

Tunneling conductance regulated by the electrostatic barrier in monolayer black phosphorene

We investigate the tunneling conductance of monolayer black phosphorene through double magnetic barriers modulated by the electrostatic barrier. This magnetic barrier is generated by two ferromagnetic strips deposited on monolayer phosphorene and can form parallel (P) and antiparallel (AP) structures. Applying a voltage to a ferromagnetic strip will create an electrostatic barrier. The transmission of P and AP magnetic structures is calculated using the transfer matrix. Tunneling conductance is given by Landauer–Büttiker theory. Our results show that under double electrostatic barriers modulation, the AP conductance of the system is zero except for sharp peaks, while the P conductance has a certain value, which implies a large tunneling magnetoresistance. However, in the case of single electrostatic barrier modulation, the conductance becomes flat. For the Fermi level in the valence band, the movement of the Fermi level leads to the severe mismatching of transmitted wave vectors in the barrier region, resulting a positive and negative oscillation magnetoresistance. This may be useful for designing memory using a two-dimensional electron gas system.

Control over epitaxy and the role of the InAs/Al interface in hybrid two-dimensional electron gas systems

Control over epitaxy and the role of the InAs/Al interface in hybrid two-dimensional electron gas systems

Investigating the effects of impurity on electron mobility in quasi-one-dimensional wires

In this research, the electron mobility in GaAs quasi-one-dimensional wires with the presence of ionized impurity at zero temperatures was investigated and the results were compared with the mobility of a two-dimensional electron gas system. GaAs is a non-magnetic semiconductor with a direct band gap. Here for the calculations, the Boltzmann transport equation is used in the relaxation time approximation, taking into account the ionized impurity potential. Focusing on ionized Coulomb scattering and the short-range disorder is our goal. Electron mobility was investigated based on related parameters (Fermi energy and width of nanowires), and its diagram was drawn. In the end, the results of this research were compared with electron mobility in completely two-dimensional electronic systems. As expected, the numerical results showed that the electron mobility in extensive wires converges to the electron gas mobility of a fully two-dimensional plane.

Indium-bond-and-stop-etch (IBASE) technique for dual-side processing of thin high-mobility GaAs/AlGaAs epitaxial layers

We present a reliable flip-chip technique for dual-side processing of thin (<1 μm) high-mobility GaAs/AlGaAs epitaxial layers. The technique allows the fabrication of small (micron-scale with standard UV photolithography) patterned back gates and dual-gate structures on the thin GaAs/AlGaAs films with good alignment accuracy using only frontside alignment. The technique preserves the high-mobility (>106 cm2/V-s at 2 K) and most (>95%) of the charge density of the two-dimensional electron gas systems and allows linear control of the charge density with small (<1 V) electrostatic gate bias. Our technique is motivated by a THz quantum well detector based on intersubband transitions in a single, wide GaAs/AlGaAs quantum well, in which a symmetric, well-aligned dual-gate structure (with a typical gate dimension of ∼5 ×5 μm) is required for accurate and precise tuning of the THz detection frequency. Using our Indium-Bond-And-Stop-Etch technique, we realize such dual-gate structure on 660-nm thick GaAs/AlGaAs epitaxial layers that contain a modulation-doped, 40-nm wide, single square quantum well. By independently controlling the charge density and the DC electric field set between the gates, we demonstrate robust tuning of the intersubband absorption behavior of the 40-nm quantum well near 3.44 THz at 30 K.

Spatial distribution of photo-induced anomalous Hall effects of the top and bottom surface states in topological insulators Sb2Te3

Three-dimensional (3D) topological insulators (TIs) have attracted much attention due to their topologically protected surface states. Here, we use circularly polarized light to induce the photo-induced anomalous Hall effect (PAHE) in 3D TIs Sb2Te3 thin films with different thicknesses of 5, 7, 12 and 20 quintuple layers (QLs) at room temperature. The dependence of PAHE current on the light spot locations of Sb2Te3 thin films is investigated, which is found to show a completely different behavior from that observed in two-dimensional electron gas (2DEG) systems. This is due to the fact that the total PAHE current is the superposition of the PAHE of the top and bottom surface states. A theoretical model has been proposed to separate the PAHE current of the top and bottom surface states. In addition, as the thickness of the Sb2Te3 film increases from 5 QL to 20 QL, the PAHE currents of the top and bottom surface states first increase and then decrease. The photo-induced anomalous Hall, conductivity of the top surface states in the 7-QL Sb2Te3 film excited by 1064 nm light is as large as 592 nA⋅ V−1⋅ cm ⋅ W−1, which is much larger than that observed in InGaAs/AlGaAs quantum wells (4.45 nA⋅ V−1⋅ cm ⋅ W−1) and GaN/AlGaN heterostructures (1.43 nA⋅ V−1⋅ cm ⋅ W−1). The giant PAHE value observed in Sb2Te3 films suggests that 3D TIs may provide a good platform for spintronic devices.

Enhanced quantum oscillations and scattering effect in quaternary InAlGaN/GaN two-dimensional electron gas

Quantum transport properties of a large bandgap In0.15Al0.79Ga0.06N/GaN quaternary GaN high electron mobility transistor (HEMT) heterostructure are studied at low temperatures up to 2 K. Herein, we report the first evidence of weak localization in a quaternary GaN two-dimensional electron gas (2DEG) system. We observe negative magnetoresistance behavior and extracted dephasing time (τΦ) using a Hikami–Larkin–Nagaoka model at 2.2 K. Linear dependency of dephasing rate with temperature (τΦ−1∝T) is established below 20 K. Furthermore, Shubnikov–de Haas quantum oscillation induced by 2DEG is observed using perpendicular magnetic (B⊥) field strengths up to 14 T. From the temperature-dependent oscillation amplitude, we extracted an effective mass m*≈0.237me. The dominance of small-angle scattering in the 2DEG channel is identified from less than unit ratio (τq/τt≪1) of quantum lifetime (τq) to the Hall transport lifetime (τt). In our study, we have demonstrated that the In0.15Al0.79Ga0.06N/GaN quaternary heterostructure possesses high dephasing time (τΦ=5.4 ps) and larger quantum lifetime (τq=0.102 ps) indicating better suitability and a way forward to high-power–high-frequency GaN HEMT development.

Band structure of a nonparabolic two-dimensional electron gas system

Using angle-resolved photoelectron spectroscopy (ARPES), we study the band structures of two-dimensional electron gases (2DEGs) formed at Te-doped surfaces of InAs(110), as a result of band bending. The selected surface system is very clean and stable; therefore, ARPES spectra may be registered in short times, with minimal adsorption of residual gases and no photon beam-induced damage. We record a set of data that allows for a detailed analysis of the 2DEG band shapes, their thermal dependencies, and for an assessment of the many-body interactions. We find the electron-phonon interaction extremally weak and the electron-electron interactions hidden in dominant electron-donor interactions. While 2DEG band shapes are observed to be variable in correlation with the 2DEG density, this is tracked down to the variability of the band-bending potential and nonparabolicity of the system, and not to the many-body interactions. Thus, many-body renormalization of the 2DEG bands is not required, and the bands are described well by using the one-electron theoretical frame.

Transport behaviors of topological band conduction in KTaO3’s two-dimensional electron gases

Two-dimensional electron gas systems (2DEGs) generated at the oxide interfaces that exhibit rich physics phenomena opened up an era for oxide-based electronics, photonics, and spintronics. The recent discovery of superconductivity plus the strong spin-orbital coupling naturally existing in the 2DEGs of KTaO3 (KTO) made KTO an exciting platform for the interplay of the electronic and spin degrees of freedom to create exotic physical properties. By directly placing KTO’s 2DEGs next to another strongly-correlated oxide with nontrivial topological nodes, we reveal the anomalous effects which were induced by the topological states in the electronic transport properties of the KTO’s 2DGEs, due to the electronic reconstruction caused by the proximity effect. This adds an additional prospect to the functions of KTO heterostructures.

Engineered Kondo screening and nonzero Berry phase in SrTiO3/LaTiO3/SrTiO3 heterostructures

Controlling the interplay between localized spins and itinerant electrons at the oxide interfaces can lead to exotic magnetic states. Here we devise SrTiO3/LaTiO3/SrTiO3 heterostructures with varied thickness of the LaTiO3 layer (n monolayers) to investigate the magnetic interactions in the two-dimensional electron gas system. The heterostructures exhibit significant Kondo effect when the LaTiO3 layer is rather thin (n = 2, 10), manifesting the strong interaction between the itinerant electrons and the localized magnetic moments at the interfaces, while the Kondo effect is greatly inhibited when n = 20. Notably, distinct Shubnikov-de Haas oscillations are observed and a nonzero Berry phase of {\pi} is extracted when the LaTiO3 layer is rather thin (n = 2, 10), which is absent in the heterostructure with thicker LaTiO3 layer (n = 20). The observed phenomena are consistently interpreted as a result of sub-band splitting and symmetry breaking due to the interplay between the interfacial Rashba spin-orbit coupling and the magnetic orderings in the heterostructures. Our findings provide a route for exploring and manipulating nontrivial electronic band structures at complex oxide interfaces.

Scattering-induced positive unsaturated linear magnetoresistance in LaAlO3/SrTiO3 two-dimensional electron gas system

In this paper, positive and unsaturated linear magnetoresistance (LMR) in an LaAlO3/SrTiO3 two-dimensional electron gas system is reported. LMR appears in this system over a wide range of temperatures and magnetic fields and does not tend to saturate at magnetic field up to 14 T. The spatial fluctuation in mobility is the most likely origin of positive unsaturated LMR because the existence of strong Shubnikov-de Haas oscillations rules out the extreme quantum origin. Moreover, LMR ∝ μ and HC ∝ 1/μ are confirmed and meet the requirements of Δμ/μ < 1 in the classical model proposed by Parish and Littlewood. This suggests that the observed LMR is the classical LMR effect induced by a narrow mobility distribution. These findings are of great interest for the practical application of magnetoresistive devices such as magnetic sensors, magnetometers, and magnetic storage devices.

Probing many-body interactions in the cyclotron resonance of h -BN/bilayer graphene/ h -BN

Unlike a conventional two-dimensional electron gas system, which has parabolic band structure, the nonparabolic band dispersion of mono- to few-layer graphene violates Kohn's theorem. Thus, Landau levels (LLs) in graphene are sensitive to many-body interactions. This modifies the LL spacing, depending on the location of the Fermi energy (${E}_{\mathrm{F}}$). Such effects have been extensively studied in $h$-BN/monolayer graphene/$h$-BN through observation of inter-LL optical transitions known as cyclotron resonances (CRs). However, thus far, the influence of many-body interactions on the CR of bilayer graphene (BLG) has been rarely studied, even though BLG also possesses nonparabolic band dispersion. Here, we investigate CR in the $h$-BN/BLG/$h$-BN structure via magneto-photothermoelectric measurements under infrared laser irradiation. This method enables sensitive detection of cyclotron resonances while tuning ${E}_{\mathrm{F}}$ of BLG. The CR magnetic field value shifted significantly when ${E}_{\mathrm{F}}$ of BLG approached the charge-neutrality point (the Dirac point, DP). We attribute this to a change in the Fermi velocity of BLG near the DP, which occurs as a result of many-body interactions.

Generalized Rashba Coupling Approximation to a Resonant Spin Hall Effect of the Spin–Orbit Coupling System in a Magnetic FieldSupported by the National Natural Science Foundation of China (Grant Nos. 12075040, 11875231, 11974064, and 12047564), and the Chongqing Research Program of Basic Research and Frontier Technology (Grant No. cstc2020jcyj-msxmX0890).

We introduce a generalized Rashba coupling approximation to analytically solve confined two-dimensional electron systems with both the Rashba and Dresselhaus spin–orbit couplings in an external magnetic field. A solvable Hamiltonian is obtained by performing a simple change of basis, which has the same form as that with only Rashba coupling. Each Landau state becomes a new displaced-Fock state instead of the original Harmonic oscillator Fock state. Analytical energies are consistent with the numerical ones in a wide range of coupling strength even for a strong Zeeman splitting, exhibiting the validity of the analytical approximation. By using the eigenstates, spin polarization correctly displays a jump at the energy-level crossing point, where the corresponding spin conductance exhibits a pronounced resonant peak. As the component of the Dresselhaus coupling increases, the resonant point shifts to a smaller value of the magnetic field. In contrast to pure Rashba couplings, we find that the Dresselhaus coupling and Zeeman splittings tend to suppress the resonant spin Hall effect. Our method provides an easy-to-implement analytical treatment to two-dimensional electron gas systems with both types of spin–orbit couplings by applying a magnetic field.

A two-dimensional electron gas suspended above a neutralizing background

We consider a two-dimensional electron gas system consisting of a layer of electrons suspended above its jellium neutralizing background counterpart. We investigate the energy of the system in the Hartree–Fock approximation as a function of separation distance between the layer of electrons and the background. We find that the energy of the system is influenced by the separation distance in such a way that the new energy contribution originating from such a separation behaves as an effective kinetic energy term. The increase of the overall energy with increase of the separation distance adversely affects the stability of electron liquid states at high density. The results are derived for a fully spin-polarized (spinless) system of electrons in the thermodynamic limit, but can easily be generalized to a spin unpolarized state.

Deformation of the Fermi surface of a spinless two-dimensional electron gas in presence of an anisotropic Coulomb interaction potential

We consider the stability of the circular Fermi surface of a two-dimensional electron gas system against an elliptical deformation induced by an anisotropic Coulomb interaction potential. We use the jellium approximation for the neutralizing background and treat the electrons as fully spin-polarized (spinless) particles with a constant isotropic (effective) mass. The anisotropic Coulomb interaction potential considered in this work is inspired from studies of two-dimensional electron gas systems in the quantum Hall regime. We use a Hartree–Fock procedure to obtain analytical results for two special Fermi liquid quantum electronic phases. The first one corresponds to a system with circular Fermi surface while the second one corresponds to a liquid anisotropic phase with a specific elliptical deformation of the Fermi surface that gives rise to the lowest possible potential energy of the system. The results obtained suggest that, for the most general situations, neither of these two Fermi liquid phases represent the lowest energy state of the system within the framework of the family of states considered in this work. The lowest energy phase is one with an optimal elliptical deformation whose specific value is determined by a complex interplay of many factors including the density of the system.

Dirac Electrons in a Magnetic Quantum Ring

The electronic structure of a magnetic quantum ring in monolayer graphene is investigated analytically. The magnetic quantum ring is formed from inhomogeneous magnetic fields perpendicular to its monolayer plane, which are zero inside the ring and constant elsewhere. The low-energy spectra and the probability density of the magnetic edge states around the magnetic quantum ring are calculated analytically by solving the single-particle Dirac equation on a single K point with continuity of the wavefunctions across the boundaries of the magnetic quantum ring and without electron-electron interactions. The results are analyzed through a comparison with a magnetic quantum ring in a two-dimensional electron gas system, showing similar angular momentum transitions in the energy dispersion. These results indicate that electronic properties are often caused by the geometric shape of the system and have little to do with the base material. The qualitative energy spectra are confirmed analytically even if the solution of the Dirac Hamiltonian is limited by not including the detailed atomic arrangement of the material.

Algebraic decay of the nonadiabaticity arising through chiral spin transfer torque in magnetic domain walls with Rashba spin-orbit interaction

Spin transfer torque in a two-dimensional electron gas system without space inversion symmetry was theoretically investigated by solving the Pauli-Schr\"odinger equation for the itinerant electrons inside magnetic domain walls. Due to the presence of the Rashba spin-orbit coupling induced by the broken inversion symmetry, the spin transfer torque is chiral and the nonadiabaticity, which is defined to measure the relative importance of the nonadiabatic fieldlike torque to the adiabatic dampinglike torque, exhibits an inverse power-law decay as the domain wall width is increased. This algebraic decay is much slower than the exponential decay observed for systems without the Rashba spin-orbit coupling, and may find applications in innovative design of spintronic devices utilising magnetic topological textures such as magnetic domain walls and skyrmions.

(Invited) Electron Transport in Bulk and 2DEGs in beta-Ga2O3 and kV Class Laterals Device in beta-Ga2O3

Monoclinic beta-Ga2O3 and (AlxGa1-x)2O3 alloys are an emerging ultra-widebandgap semiconductor system for next generation power, GHz power switching and RF applications. In this talk, we will present the low field and high field transport simulations in Ga2O3 using first-principles calculations. Density functional theory calculations is used to get the band structure. The density functional perturbation theory is used to understand the lattice dynamics and identify the infrared (IR) active and Raman active phonon modes. Electron-phonon scattering rates are calculated Boltzmann transport equation is solved to calculate the low field mobility, velocity-field profiles and the impact ionization coefficients. The effect of dynamic screening on the electron mobility is evaluated in a two-dimensional electron gas (2-DEG) system in (AlxGa1-x)2O3/Ga2O3 heterostructures.We will also report the Ga2O3 based power and RF devices. MOSFETs with spin-doping source/drain and n+ ohmic capped layers will be presented. kV breakdown voltages are seen with the devices limited by extrinsic breakdown. Temperature dependent characteristics show potential for high temperature applications.

Modulated in-plane carrier distribution of oxide two-dimensional electron gas systems by light assisted electrostatic gating

The oxide two-dimensional electron gases (2DEGs) have been intensively studied for both fundamental and applied research over the past decade. It is known that the electrostatic and optical gating can both effectively modulate the transport behavior of 2DEG. However, the in-plane carrier distribution under the light assisted electrostatic gating is scarcely studied. In this study, we systematically investigate this problem in the 3d and 5d oxide-based 2DEGs samples, and observe strong lateral photovoltaic effect (LPE) in both of the 2DEGs. Remarkably, the lateral voltage is strongly dependent on the gating electrical field especially in the 5d oxide-based 2DEGs and its signs are absolutely reversed as the bias voltage becomes negative. Through systematic research, it is found that the leak current caused by light assisted electrostatic gating, strongly affects the in-plane carrier distribution. These results have significance for fundamental research and device application based on the photovoltaic effect of oxide 2DEGs.