2DEG System Research Articles

Recent progress on quantum Hall effect in unconventional material systems

The quantum Hall effect (QHE) is an elegant macroscopic manifestation of quantum mechanical behavior on the microscopic scale, and its discovery is a major triumph in condensed matter physics. While QHE has been predominantly observed in two-dimensional electron gas (2DEG) systems, recently, many efforts have been devoted to searching for the QHE in unconventional materials platforms beyond the classical framework to extend the horizon of the QHE. In this Perspective, we highlight recent important experimental discoveries and progress on the QHE material platforms beyond 2DEG platforms, such as three-dimensional QHE, Weyl-orbit-based QHE, and QHE in two-dimensional insulators. In addition, novel phenomena arising from incorporating QHE with other exotic quantum states, such as topological band structures and superconductivity, will be discussed. We also present the emerging field-free version of QHE–quantum anomalous Hall effect on its transport characteristics, working principles as well as potential applications in quantum metrology and quantum computing. With the exploration of these unconventional QHE hosts and the development of the understanding of new physics arising from the interplay between QHE and other physical systems, QHE will continue to play a critical role in both advancing fundamental physics and developing next-generation quantum technologies.

Enabling 2D Electron Gas with High Room-Temperature Electron Mobility Exceeding 100cm2 Vs-1 at a Perovskite Oxide Interface.

In perovskite oxide heterostructures, bulk functional properties coexist with emergent physical phenomena at epitaxial interfaces. Notably, charge transfer at the interface between two insulating oxide layers can lead to the formation of a 2D electron gas (2DEG) with possible applications in, e.g., high-electron-mobility transistors and ferroelectric field-effect transistors. So far, the realization of oxide 2DEGs is, however, largely limited to the interface between the single-crystal substrate and epitaxial film, preventing their deliberate placement inside a larger device architecture. Additionally, the substrate-limited quality of perovskite oxide interfaces hampers room-temperature (RT) 2DEG performance due to notoriously low electron mobility. In this work, the controlled creation of an interfacial 2DEG at the epitaxial interface between perovskite oxides BaSnO3 and LaInO3 is demonstrated with enhanced RT electron mobility values up to 119 cm2 Vs-1-the highest RT value reported so far for a perovskite oxide 2DEG. Using a combination of state-of-the-art deposition modes during oxide molecular beam epitaxy, this approach opens up another degree of freedom in optimization and in situ control of the interface between two epitaxial oxide layers away from the substrate interface. Thus this approach is expected to apply to the general class of perovskite oxide 2DEG systems and to enable their improved compatibility with novel device concepts and integration across materialsplatforms.

Spin dynamics of 2-DEG Rashba-Dresselhaus spin-orbit systems: Quantum propagators and semi-analytical analysis in low electron density.

Abstract The spin dynamics in space and time for Dresselhaus and Rashba spin-orbit interaction 2-DEG systems can be studied for any value of their β and α coefficients by using quantum propagators. With the development of analytical expressions for the quantum propagators and numerical solutions of the wave function, it has been possible to simulate the spin propagation in space-time and analyze the spin dynamics for different α/β ratios. In the present paper, the effects of the spin diffusion for large collision time τp, was performed by using quantum propagators for different α/β ratios. It was found that the largest degree of spin polarization is achieved when |α| = |β|.

Non-classical correlations and coherence in a two-dimensional electron gas under the influence of Rashba spin–orbit coupling

This investigation delves into the dynamics of quantum correlations and coherence within a two-dimensional electron gas (2DEG) system, taking into account the influence of Rashba spin–orbit coupling (SOC). We utilize metrics such as logarithmic negativity ([Formula: see text]), local quantum uncertainty (LQU ([Formula: see text])), and relative entropy of coherence ([Formula: see text]) to specifically assess these quantum resources among electron spins within non-interacting electron gases. Our study investigates how the separation distance (R) between electrons and the intensity of Rashba SOC ([Formula: see text]), impact the behavior of quantum properties within the 2DEG system. We find that specific strengths of Rashba SOC can effectively regulate quantum correlations and coherence within this system. Particularly significant is our observation that optimal quantum indicators emerge when electron proximity is minimized, underscoring the substantial influence of electron distance on quantum characteristics. Conversely, as electron separation increases, these quantum metrics diminish. Therefore, by adjusting the Rashba parameter, we can enhance the resilience of these quantum measurements against increasing electron separations, providing valuable insights into the potential for controlling and manipulating quantum behavior in similar 2DEG systems.

Plasmonic Photovoltaic Effect of an Original 2D Electron Gas System and Application in Mid‐Infrared Imaging

AbstractOwing to their consistently emerging applications in modern photonics, highly sensitive and rapid mid‐infrared (MIR) photodetectors, operating at high temperatures, are of great significance. Herein, a novel PbTe/Ge heterostructure is introduced. Notably, the discovery of a 2D electron gas (2DEG) system on the surface of the PbTe layer is experimentally identified for the first time. A high‐performance PbTe/Ge heterostructure MIR photodetector utilizing a plasmonic photovoltaic effect is developed by employing asymmetric electrodes. The photodetecting device demonstrates an exceptionally high detectivity of 1.1 × 1011 Jones along with a short response time of 15 ns at room temperature. Additionally, the proposed plasmonic photovoltaic mechanism of the device is investigated and mainly ascribed to the propagating plasmons, generated by the coupling of the 2DEG with incident MIR photons. Moreover, a linear detector array (LDA) of PbTe/Ge is demonstrated for a thermal imaging application, which exhibits promising high‐quality real‐time imaging via the 2D photodetector arrays on Ge‐based integrated circuits. This work opens up a new way for the development of next‐generation, advanced MIR photonic devices and integrated photonic systems.

Novel 2DEG System at the HfO2/STO Interface.

Multifunctional integration in a single device has always been a hot research topic, especially for contradictory phenomena, one of which is the coexistence of ferroelectricity and metallicity. The complex oxide heterostructures, as symmetric breaking systems, provide a great possibility to incorporate different properties. Moreover, finding a series of oxide heterostructures to achieve this goal remains as a challenge. Here, taking the advantage of different physical phenomena, we use H2 plasma to pretreat the SrTiO3 (STO) substrate and then fabricate HfO2/STO heterostructures with it. The novel, well-repeatable metallic two-dimensional electron gas (2DEG) is directly obtained at the heterointerfaces without any further complex procedures, while the obvious ferroelectric-like behavior and Rashba spin-orbit coupling are also observed. The understanding of the mechanism, as well as the modified facile preparation procedure, would be meaningful for further development of ferroelectric metal in complex oxide heterostructures.

Acoustic and optical plasmons excitation in double-channel AlGaN/GaN HEMT under asymmetric boundaries

We have simulatively studied the electrical excitation of acoustic and optical plasmons in double-channel AlGaN/GaN high electron mobility transistors (HEMT) under asymmetric boundaries. By solving the self-consistent hydrodynamic model with Maxwell’s equations, it is found that the drift plasmons are excited in the bilayer 2DEGs under asymmetric boundaries. The oscillation intensity in a bilayer system is much stronger than that in a monolayer 2DEG system because of the simultaneous excitation of the fundamental and the second harmonic components. The fundamental component always corresponds to the acoustic plasmon, while the optical plasmon can be excited when it is resonant with the second harmonic of the acoustic plasmon. Because of the out-of-phase properties of the acoustic plasmons, their radiated power is limited by the current cancellation. On the other hand, as the optical plasmons are excited, the radiated powers are much higher than those of the single channel HEMT due to the in-phase currents generated in the double layers. The effects of different parameters such as gate lengths, 2DEG spacings, and electron concentrations on plasmon excitations and THz emissions of double-channel HEMTs are analyzed in detail. The numerical results provide a more powerful terahertz radiation mechanism in double-channel HEMT compared with the Dyakonov–Shur instability in traditional single-channel HEMT.

High-mobility spin-polarized quasi-two-dimensional electron gas and large low-field magnetoresistance at the interface of EuTiO3/SrTiO3 (110) heterostructures

High-mobility spin-polarized two-dimensional electron gas (2DEG) at the interfaces of complex oxide heterostructures provide great potential for spintronic device applications. Unfortunately, the interfacial ferromagnetism and its associated spin polarization of mobile electrons and negative magnetoresistance (MR) are too weak. As of now, obtaining enhanced interfacial ferromagnetism and MR and strong spin-polarized 2DEG is still a great challenge. In this paper, we report on the realization of strong spin-polarized 2DEG at the interface of EuTiO3/SrTiO3 (110) heterostructures, which were prepared by directly depositing 39-nm EuTiO3 films onto as-received SrTiO3 (110) substrates. Hall and Kondo effects, low-field MR, Shubnikov–de Haas (SdH) oscillation, and magnetic hysteresis loop measurements demonstrate that high mobility electrons (1.4 × 104 cm2 V−1 s−1) accumulate at the interface of the heterostructures, which are not only highly conducting and show SdH oscillations with a non-zero Berry phase but also show a large out-of-plane and in-plane butterfly-like negative low-field MR whose magnitude is unprecedentedly large (46%–59% at 500 Oe and 1.8 K), approximately one to two orders higher than those of previously reported spin-polarized 2DEG systems. The strong spin polarization of the interfacial 2DEG is attributed to the presence of interfacial Eu2+ 4f (3.6–4 μB/f.u.) and Ti3+ 3d moments. Our results may provide guidance for exploring strong spin-polarized 2DEG at the interface of rare-earth titanate–strontium titanate heterostructures.

Formation of oxygen vacancy at surfaces of ZnO by trimethylaluminum

The two-dimensional electron gas (2DEG) is a group of electrons that can move freely in horizontal dimensions but are confined in the third direction. It has been reported that atomic layer deposition (ALD) of Al2O3 on various reducible n-type oxides can lead to the formation of 2DEG at the heterojunction interfaces, among which ZnO is known to provide promising properties. In this study, we have performed a theoretical analysis using density functional theory calculations combined with experimental investigations to elucidate the surface reactions of Al2O3 ALD on low-index nonpolar ZnO surfaces, specifically focusing on the formation of oxygen vacancies (VO). The trimethylaluminum precursor was observed to undergo sequential dissociation of CH3 ligands, leading to the removal of surface oxygen of ZnO in the form of dimethyl ether. In addition, by examining the electronic structure after the removal of oxygen, the localization of the charge density at the surface was confirmed. Experimentally, the carrier density of the 2DEG at the Al2O3/ZnO interface showed a strong dependence on the ALD process temperature of Al2O3, confirming the endothermic nature of the formation of the 2DEG. By examining the characteristics of the 2DEG induced by VO, insights into the fundamental comprehension of oxide-based 2DEG systems are provided.

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.

Anisotropic Electronic Structure of the 2D Electron Gas at the AlOx/KTaO3(110) Interface

AbstractOxide‐based 2D electron gases (2DEGs) have generated significant interest due to their potential for discovering novel physical properties. Among these, 2DEGs formed in KTaO3 stand out due to the recently discovered crystal face‐dependent superconductivity and large Rashba splitting, both of which hold potential for future oxide electronics devices. In this work, angle‐resolved photoemission spectroscopy is used to study the electronic structure of the 2DEG formed at the (110) surface of KTaO3 after deposition of a thin Al layer. The experiments reveal a remarkable anisotropy in the orbital character of the electron‐like dispersive bands, which form a Fermi surface consisting of two elliptical contours with their major axes perpendicular to each other. The measured electronic structure is used to constrain the modeling parameters of self‐consistent tight‐binding slab calculations of the band structure. In these calculations, an anisotropic Rashba splitting is found with a value as large as 4 meV at the Fermi level along the [−110] crystallographic direction. This large unconventional and anisotropic Rashba splitting is rationalized based on the orbital angular momentum formulation. These findings provide insights into the interpretation of spin‐orbitronics experiments and help to constrain models for superconductivity in the KTO(110)‐2DEG system.

Reversible Photomodulation of Two-Dimensional Electron Gas in LaAlO3/SrTiO3 Heterostructures.

Long-lived photoinduced conductance changes in LaAlO3/SrTiO3 (LAO/STO) heterostructures enable their use in optoelectronic memory applications. However, it remains challenging to quench the persistent photoconductivity (PPC) instantly and reproducibly, which limits the reversible optoelectronic switching. Herein, we demonstrate a reversible photomodulation of two-dimensional electron gas (2DEG) in LAO/STO heterostructures with high reproducibility. By irradiating UV pulses, the 2DEG at the LAO/STO interface is gradually transformed to the PPC state. Notably, the PPC can be completely removed by water treatment when two key requirements are met: (1) the moderate oxygen deficiency in STO and (2) the minimal band edge fluctuation at the interface. Through our X-ray photoelectron spectroscopy and electrical noise analysis, we reveal that the reproducible change in the conductivity of 2DEG is directly attributed to the surface-driven electron relaxation in the STO. Our results provide a stepping-stone toward developing optically tunable memristive devices based on oxide 2DEG systems.

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.

Reconstruction of Low Dimensional Electronic States by Altering the Chemical Arrangement at the SrTiO3 Surface

AbstractDeveloping reliable methods for modulating the electronic structure of the 2D electron gas (2DEG) in SrTiO3 is crucial for utilizing its full potential and inducing novel properties. Herein, it is shown that relatively simple surface preparation reconstructs the 2DEG at the SrTiO3 (STO) surface, leading to a Lifshitz‐like transition. Combining experimental methods, such as angle‐resolved photoemission spectroscopy (ARPES) and X‐ray photoemission spectroscopy with ab initio calculations, that the modulation of the surface band structures can be effectively achieved via transforming the chemical composition at the atomic scale is found. In addition, ARPES experiments demonstrate that vacuum ultraviolet light can be efficiently employed to alter the band renormalization of the 2DEG system and control the electron‐phonon interaction . This study provides a robust and straightforward route to stabilize and tune the low‐dimensional electronic structure via the chemical degeneracy of the STO surface.

Transport Properties of the LaInO3/BaSnO3 Interface Analyzed by Poisson-Schrödinger Equation

2DEG systems formed in quantum wells of semiconductor heterostructures have been instrumental in advancing science and technology for many decades. Here, we report two unique transport properties of 2DEG formed at the interface of two perovskite oxides ${\mathrm{La}\mathrm{In}\mathrm{O}}_{3}$ ($\mathrm{LIO}$) and ${\mathrm{Ba}\mathrm{Sn}\mathrm{O}}_{3}$ ($\mathrm{BSO}$): the peculiar $\mathrm{LIO}$ thickness dependence of the high two-dimensional (2D) carrier density ($n_{2{\rm{D}}}$) and the very narrow width of the quantum well. We analyze, via Poisson-Schr\"odinger simulation, how the various materials parameters affect the 2D carrier density and its profile when using the ``interface-polarization'' model in which the polarization exists only near the interface. Our simulations show that the known material parameters of $\mathrm{LIO}$ and $\mathrm{BSO}$ are capable of generating a deep and narrow quantum well as suggested by the experimental transport properties and reveal some distinct features of the $\mathrm{LIO}$/$\mathrm{BSO}$ interface from the conventional 2DEGs. Furthermore, they predict how the $\mathrm{LIO}$/$\mathrm{BSO}$ 2DEG will evolve as the defect density decreases.

High-Mobility Field-Effect Transistor Using 2-Dimensional Electron Gas at the LaScO3/BaSnO3 Interface

A 2-dimensional electron gas (2DEG) system with high mobility was discovered at the interface of two perovskite oxides: a polar orthorhombic perovskite LaScO3 and a nonpolar cubic perovskite BaSnO3. Upon depositing the LaScO3 film on the BaSnO3 film, we measured the conductance enhancement and the resulting 2DEG density (n2D). Comparing the results with the previously reported LaInO3/BaSnO3 polar interface, we applied the “interface polarization” model to the LaScO3/BaSnO3 system, in which the polarization exists only over four pseudocubic unit cells in LaScO3 from the interface and vanishes afterward like the LaInO3/BaSnO3 interface. Based on the calculations of the self-consistent Poisson–Schrödinger equations, the LaScO3 thickness dependence of n2D of the LaScO3/BaSnO3 heterointerface is consistent with this model. Furthermore, a single subband in the quantum well is predicted. By use of the conductive interface and the LaScO3 as a gate dielectric, a 2DEG transistor composed of only perovskite oxides with high field-effect mobility (μFE) close to 100 cm2 V–1 s–1 is demonstrated.

Suppression of weak localization due to AlN interlayer in AlGaN/GaN 2DEG

We have conducted a comprehensive investigation of the magneto-transport properties of Al0.30Ga0.70N/GaN and Al0.30Ga0.70N/AlN/GaN based HEMT structure in terms of inelastic scattering time via weak localization (WL) measurement. Owing to 1 nm AlN interlayer, a small negative-magnetoresistance (MR) effect which appeared over the interval −0.10 T ≤ B⊥ ≤ 0.10 T is further suppressed to −0.040 T ≤ B⊥ ≤ 0.040 T. Using HRXRD measurement and fitting our experimental magnetoresistance (MR) data with theoretical model, it has been analyzed that WL suppression in 2DEG is mainly due to the interlayer while contribution due to crystalline disorder is negligible. Different weak localization parameters like elastic scattering time τe and inelastic scattering time τi are extracted by fitting temperature dependent negative MR data using Hikami-Larkin-Nagaoka (HLN) model. A linear dependence of inelastic scattering rate (τi−1∝ T) with temperature is also observed up to temperature 15 K. Our combined experimental and modeled results demonstrate a significant role of AIN interlayer in suppressing the WL in AlGaN/GaN 2DEG system. Quantitative analysis within the framework of HLN model leads to the conclusion that AlN interlayer break the electronic phase coherence, decrease τi and suppress the localization effect.

Effects of Lead-Device Interfaces on Transport Properties in a Two-Terminal 2DEG System

Effects of Lead-Device Interfaces on Transport Properties in a Two-Terminal 2DEG System

Direct Cation–Cation Interactions Induced by Mg Dopants for Electron–Gas Behavior in α-Fe2O3

α-Fe2O3 and its derivatives have been extensively studied for many years, but fascinating structural chemistry and physical behaviors continue to be reported in these systems to date. Here, we show that Mg doping in α-Fe2O3 could induce direct cation–cation interactions, which result in electron–gas behaviors. Rather than a simple replacement of Fe in FeO6 octahedra, the Mg dopant in α-Fe2O3 is eight-coordinated by six oxygen and two Fe in neighboring FeO6 octahedra via direct Fe–Mg–Fe interactions. The Mg2+ dopant takes both Fe3+ and empty octahedral interstitial sites so that one Mg-doped octahedron is sandwiched by two neighboring FeO6 octahedra, yielding an arrangement of three neighboring Fe–Mg–Fe polyhedra over the medium-range order. The valence electrons confined in the Fe–Mg bonds exhibit two-dimensional electron–gas (2DEG) characteristics, which may provide a new approach to other 2DEG systems, and the couplings with Fe 3d spins and the distortion of local symmetry lead to the spin-glass-like (SGL) magnetism, distinct from the antiferromagnetism of α-Fe2O3 and SGL/superparamagnetic behaviors in related materials due to the ultrasmall particle size. This study illustrates the complexity of the doped structure not only in local symmetries but also in the chemical bonding and electron spins for emerging physical properties even in low-cost oxides.