2DEG System Research Articles

Thermodynamic Ground States of Complex Oxide Heterointerfaces.

The formation mechanism of 2-dimensional electron gases (2DEGs) at heterointerfaces between nominally insulating oxides is addressed with a thermodynamical approach. We provide a comprehensive analysis of the thermodynamic ground states of various 2DEG systems directly probed in high temperature equilibrium conductivity measurements. We unambiguously identify two distinct classes of oxide heterostructures: For epitaxial perovskite/perovskite heterointerfaces (LaAlO3/SrTiO3, NdGaO3/SrTiO3, and (La,Sr)(Al,Ta)O3/SrTiO3), we find the 2DEG formation being based on charge transfer into the interface, stabilized by the electric field in the space charge region. In contrast, for amorphous LaAlO3/SrTiO3 and epitaxial γ-Al2O3/SrTiO3 heterostructures, the 2DEG formation mainly relies on the formation and accumulation of oxygen vacancies. This class of 2DEG structures exhibits an unstable interface reconstruction associated with a quenched nonequilibrium state.

High-Throughput Design of Two-Dimensional Electron Gas Systems Based on Polar/Nonpolar Perovskite Oxide Heterostructures.

The two-dimensional electron gas (2DEG) formed at the interface between two insulating oxides such as LaAlO3 and SrTiO3 (STO) is of fundamental and practical interest because of its novel interfacial conductivity and its promising applications in next-generation nanoelectronic devices. Here we show that a group of combinatorial descriptors that characterize the polar character, lattice mismatch, band gap, and the band alignment between the perovskite-oxide-based band insulators and the STO substrate, can be introduced to realize a high-throughput (HT) design of SrTiO3-based 2DEG systems from perovskite oxide quantum database. Equipped with these combinatorial descriptors, we have carried out a HT screening of all the polar perovskite compounds, uncovering 42 compounds of potential interests. Of these, Al-, Ga-, Sc-, and Ta-based compounds can form a 2DEG with STO, while In-based compounds exhibit a strain-induced strong polarization when deposited on STO substrate. In particular, the Ta-based compounds can form 2DEG with potentially high electron mobility at (TaO2)+/(SrO)0 interface. Our approach, by defining materials descriptors solely based on the bulk materials properties, and by relying on the perovskite-oriented quantum materials repository, opens new avenues for the discovery of perovskite-oxide-based functional interface materials in a HT fashion.

Injection of 2D electron gas into a quantum-dot organic light-emitting diode structure on silicon substrate

The authors report a quantum-dot (QD) organic light-emitting diode (OLED) structure formed on Si substrate. Here, the junction area is defined by a lithographically patterned oxide layer on Si substrate and is designed to allow a wide range of scalability of lateral dimension down to a nanometer range. The device structure comprises: (from the top) indium-tin-oxide (70-nm thickness) as a hole-injecting transparent anode, Poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (30 nm) as a hole-transport layer, Poly(N-vinylcarbazole) (20 nm) as a hole-transport/electron-blocking layer, CdSe/ZnS core–shell quantum-dots as an emissive layer, and n-Si substrate as an electron-injecting cathode. The authors observed a carrier injection mechanism originating from the two-dimensional electron gas (2DEG) system available at the SiO2/Si interface. By varying the junction dimensions and geometry, the electron injection process is found to occur predominantly at the junction's periphery, not area, resulting in a low turn-on voltage (∼1–2 V). This 2DEG injection produces one-dimensional emission of light along junction edges. This edge injection/emission QD-OLED structure, when scaled down to a sub-10 nm range, offers an interesting approach to developing single quantum-dot light sources for quantum information processing.

Electrical detection of spin transport in Si two-dimensional electron gas systems

Spin transport in a semiconductor-based two-dimensional electron gas (2DEG) system has been attractive in spintronics for more than ten years. The inherent advantages of high-mobility channel and enhanced spin–orbital interaction promise a long spin diffusion length and efficient spin manipulation, which are essential for the application of spintronics devices. However, the difficulty of making high-quality ferromagnetic (FM) contacts to the buried 2DEG channel in the heterostructure systems limits the potential developments in functional devices. In this paper, we experimentally demonstrate electrical detection of spin transport in a high-mobility 2DEG system using FM Mn-germanosilicide (Mn(Si0.7Ge0.3)x) end contacts, which is the first report of spin injection and detection in a 2DEG confined in a Si/SiGe modulation doped quantum well structure (MODQW). The extracted spin diffusion length and lifetime are lsf = 4.5 μm and at 1.9 K respectively. Our results provide a promising approach for spin injection into 2DEG system in the Si-based MODQW, which may lead to innovative spintronic applications such as spin-based transistor, logic, and memory devices.

Light induced suppression of Kondo effect at amorphous LaAlO3/SrTiO3 interface

We report photoelectric properties of two-dimensional electron gas (2DEG) at an amorphous LaAlO3/SrTiO3 interface. Under visible light illumination (650 nm), an enhancement of electric conductivity is observed over the temperature range from 2 to 300 K. Particularly, a resistance upturn appearing below 25 K, which is further proved to from the Kondo effect, is suppressed by the 650 nm visible light. From the results of light-assisted Hall measurements, light irradiation increases the carrier mobility rather than carrier density in the Kondo regime. It is suggested that light induces the decoherence effect of localized spin states, hence the electron scattering is weakened and the carrier mobility is improved accordingly. Moreover, the enhancement of electrical conductivity by visible light verifies that in-gap states located in the SrTiO3 side of the interface play an important role in the electrical transport of the amorphous SrTiO3-based oxide 2DEG system. Our results provide deeper insight into the photoinduced effects in the 2DEG system, paving the way for the design of optoelectronic devices based on oxides.

Enhanced thermopower in ZnO two-dimensional electron gas

Control of dimensionality has proven to be an effective way to manipulate the electronic properties of materials, thereby enabling exotic quantum phenomena, such as superconductivity, quantum Hall effects, and valleytronic effects. Another example is thermoelectricity, which has been theoretically proposed to be favorably controllable by reducing the dimensionality. Here, we verify this proposal by performing a systematic study on a gate-tuned 2D electron gas (2DEG) system formed at the surface of ZnO. Combining state-of-the-art electric-double-layer transistor experiments and realistic tight-binding calculations, we show that, for a wide range of carrier densities, the 2DEG channel comprises a single subband, and its effective thickness can be reduced to [Formula: see text] 1 nm at sufficiently high gate biases. We also demonstrate that the thermoelectric performance of the 2DEG region is significantly higher than that of bulk ZnO. Our approach opens up a route to exploit the peculiar behavior of 2DEG electronic states and realize thermoelectric devices with advanced functionalities.

Commensurability condition and hierarchy of fillings for FQHE in higher Landau levels in conventional 2DEG systems and in graphene—monolayer and bilayer

The structure of the filling rate hierarchy referred to as the fractional quantum Hall effect is studied in higher Landau levels using the commensurability condition. The hierarchy of fillings that are derived in this manner is consistent with the experimental observations of the first three Landau levels in conventional semiconductor Hall systems. The relative poverty of the fractional structure in higher Landau levels compared with the lowest Landau level is explained using commensurability topological arguments. The commensurability criterion for correlated states for higher Landau levels (with ) including the paired states at half fillings of the spin-subbands of these levels is formulated. The commensurability condition is applied to determine the hierarchy of the fractional fillings of Landau levels in the monolayer and bilayer graphene. Good agreement with current experimental observations of fractional quantum Hall effect in the graphene monolayer and bilayer is achieved. The presence of even denominator rates in the hierarchy for fractional quantum Hall effect in the bilayer graphene is also explained.

Radiation-assisted magnetotransport in two-dimensional electron gas systems: appearance of zero resistance states

Zero-resistance states (ZRS) are normally associated with superconducting and quantum Hall phases. Experimental detection of ZRS in two-dimensional electron gas (2DEG) systems irridiated by microwave(MW) radiation in a magnetic field has been quite a surprise. We develop a semiclassical transport formalism to explain the phenomena. We find a sequence of Zero-Resistance States (ZRS) inherited from the suppression of Shubnikov–de Haas (SdH) oscillations under the influence of high-frequency and large amplitude microwave radiation. Furthermore, the ZRS are well pronounced and persist up to broad intervals of magnetic field as observed in experiments on microwave illuminated 2DEG systems.

Field emission graphene–oxide–silicon field effect based photodetector

Silicon‐based devices keep moving into smaller dimension for improving the speed, efficiency, and low‐power consumption. Novel designed semiconductor device architectures are needed to overcome the physical limitations. An integration of well‐designed nanostructure and nanomaterials can potentially establish new principles and approaches to nanoelectronic and photonic devices. We herein demonstrate a graphene/SiO2/p‐Si (GOS) heterostructure with an embedded nanoscale mesa, forming a GOS‐Mesa field‐effect photodetector. The proposed structure exhibits that multiple exciton generation (MEG) can occur in a quantum‐confined two‐dimensional electron gas (2DEG) region via impact ionization, leading to high internal quantum efficiency (ηIQE). The numerical simulation of the carrier multiplication (CM) factor in our designed structure finds a reasonable agreement with empirical data. Simulated and measured internal quantum efficiency demonstrate ∼195% and ∼135% of UV–Vis radiation, respectively. A vertically confined 2DEG plays an important role not only in enabling the electron emission process which is responsible for the flowing of electron current, but also in developing a highly localized electric field (up to ∼106 V/cm) at the SiO2/Si interface, enabling an impact ionization process under photon energy of merely ∼1.95 eV. Our findings demonstrate that carrier multiplication can be achieved in a suitably designed nanoscale structure in conjunction with nanomaterial on silicon‐based devices, providing incentive to better understand MEG within quantum wells in 2DEG systems, and being a research path to enhancing the efficiency of future solar harvesting technologies. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)

Time-Dependent Modelling of Single-Electron Interferometry with Edge-States

We simulate the time-resolved dynamics of localized electrons in a 2DEG system, where an external magnetic field creates quantum Hall edge states, and properly polarized split gates define a Mach-Zehnder electron interferometer. The carriers travelling inside the Hall channels consist of localized wave packets of edge states: they are propagated numerically bymeans of a Fourier split-step approach. We find that the energy-dependent, scattering process at the quantum point contacts, together with the finite energy distribution of the carriers, have a remarkable effect on the transmission coefficient T of the device. We provide an analytical model to justify the characteristics of T which is in good agreement with the numerical simulations.

Quantum Oscillations in a Two-Dimensional Electron Gas at the Rocksalt/Zincblende Interface of PbTe/CdTe (111) Heterostructures.

Quantum oscillations are observed in the 2DEG system at the interface of novel heterostructures, PbTe/CdTe (111), with nearly identical lattice parameters (a(PbTe) = 0.6462 nm, a(CdTe) = 0.648 nm) but very different lattice structures (PbTe: rock salt, CdTe: zinc blende). The 2DEG formation mechanism, a mismatch in the bonding configurations of the valence electrons at the interface, is uniquely different from the other known 2DEG systems. The aberration-corrected scanning transmission electron microscope (AC-STEM) characterization indicates an abrupt interface without cation interdiffusion due to a large miscibility gap between the two constituent materials. Electronic transport measurements under magnetic field up to 60 T, with the observation of Landau level filling factor ν = 1, unambiguously reveal a π Berry phase, suggesting the Dirac Fermion nature of the 2DEG at the heterostructure interface, and the PbTe/CdTe heterostructure being a new candidate for 2D topological crystalline insulators.

Quantum coherence of electrons in random networks of c-axis oriented wedge-shaped GaN nanowalls grown by molecular beam epitaxy

The depth distribution of the transport properties as well as the temperature dependence of the low field magneto-conductance for -axis oriented GaN nanowall network samples grown with different average wall-widths () are investigated. Magneto-conductance recorded at low temperatures shows clear signature of weak localization effect in all samples studied here. The scattering mean free path and the phase coherence time , are extracted from the magneto-conductance profile. Electron mobility estimated from is found to be comparable with those estimated previously from room temperature conductivity data for these samples, confirming independently the substantial mobility enhancement in these nanowalls as compared to bulk. Our study furthermore reveals that the high electron mobility region extends down to several hundreds of nanometer below the tip of the walls. Like mobility, phase coherence length () is found to increase with the reduction of the average wall width. Interestingly, for samples with lower values of the average wall width, is estimated to be as high as 60 μm, which is much larger than those reported for GaN/AlGaN heterostructure based two-dimensional electron gas (2DEG) systems.

Controlling the evolution of two-dimensional electron gas states at ametal/Bi2Se3interface

We have demonstrated that the evolution of the two-dimensional electron gas (2DEG) system at an interface of metal and the model topological insulator (TI) Bi$_2$Se$_3$ can be controlled by choosing an appropriate kind of metal elements and by applying a low temperature evaporation procedure. In particular, we have found that only topological surface states (TSSs) can exist at a Mn/Bi$_2$Se$_3$ interface, which would be useful for implementing an electric contact with surface current channels only. The existence of the TSSs alone at the interface was confirmed by angle-resolved photoemission spectroscopy (ARPES). Based on the ARPES and core-level x-ray photoemission spectroscopy measurements, we propose a cation intercalation model to explain our findings.

Towards enhancing two-dimensional electron gas quantum confinement effects in perovskite oxide heterostructures

We explore the possibility of achieving highly confined two-dimensional electron gas (2DEG) within one single atomic layer through a comprehensive comparison study on three prototypical perovskite heterostructures, LaAlO3/ATiO3 (A = Ca, Sr, and Ba), using first-principles electronic structure calculations. We predict that the heterostructure LaAlO3/BaTiO3 has a highly confined 2DEG within a single atomic layer of the substrate BaTiO3, and exhibits relatively higher interfacial charge carrier density and larger magnetic moments than the well-known LaAlO3/SrTiO3 system. The long Ti-O bond length in the ab-plane of the LaAlO3/BaTiO3 heterostructure is responsible for the superior charge confinement. We propose BaTiO3 as an exceptional substrate material for 2DEG systems with potentially superior properties.

LaTiO3/KTaO3 interfaces: A new two-dimensional electron gas system

We report a new 2D electron gas (2DEG) system at the interface between a Mott insulator, LaTiO3, and a band insulator, KTaO3. For LaTiO3/KTaO3 interfaces, we observe metallic conduction from 2 K to 300 K. One serious technological limitation of SrTiO3-based conducting oxide interfaces for electronics applications is the relatively low carrier mobility (0.5-10 cm2/V s) of SrTiO3 at room temperature. By using KTaO3, we achieve mobilities in LaTiO3/KTaO3 interfaces as high as 21 cm2/V s at room temperature, over a factor of 3 higher than observed in doped bulk SrTiO3. By density functional theory, we attribute the higher mobility in KTaO3 2DEGs to the smaller effective mass for electrons in KTaO3.

Solar Water Splitting Photoelectrodes Utilizing Oxide 2-Dimensional Electron Gas and Plasmonic Nanoparticles

The solar water splitting is the third generation of solar energy utilizing technology for future energy problem. Through the past years, the technology has been amazingly meliorated, and theoretical basis has been enormously progressed. In recent days, much endeavor has been dedicated to enhancing solar water splitting efficiency. For further improvement, we need to deeply reflect a perspective of charge generation and transport in photoelectrodes and at interfaces between the electrodes and water. One of the most appropriate strategy for this concept would be grafting plasmonics to the photoelectrodes gernally based on oxide semiconductors. If we put plasmonic nanoparticles onto the surface of oxide semiconductors, the photoactivity of the electrodes can be improved by the three mechanisms ; direct electron transfer of hot electrons (DET), local electromagnetic field enhancement (LEMF), and resonant energy transfer (RET). In this reason, revealing the interaction of plasmonic nanoparticles with 2-dimensional electron gas (2DEG) interface which has huge amount of charge in the interface and extremely high mobility would be very interesting. In this talk, we present photoelectrochemical properties of LaAlO3/SrTiO3 2DEG system with plasmonic nanoparticles. Our results clearly reveal the substantial enhancement of photoactivity in SrTiO3 by incorporating 2DEG interface and plasmonic nanoparticles, suggesting the synergetic effect of 2DEG and plasmon for enhaced photoactivity of oxide-based solar water splitting photoelectrodes.

Highly sensitive and wide-band tunable terahertz response of plasma waves based on graphene field effect transistors.

Terahertz (THz) technology is becoming a spotlight of scientific interest due to its promising myriad applications including imaging, spectroscopy, industry control and communication. However, one of the major bottlenecks for advancing this field is due to lack of well-developed solid-state sources and detectors operating at THz gap which serves to mark the boundary between electronics and photonics. Here, we demonstrate exceptionally wide tunable terahertz plasma-wave excitation can be realized in the channel of micrometer-level graphene field effect transistors (FET). Owing to the intrinsic high propagation velocity of plasma waves (>~108 cm/s) and Dirac band structure, the plasma-wave graphene-FETs yield promising prospects for fast sensing, THz detection, etc. The results indicate that the multiple guide-wave resonances in the graphene sheets can lead to the deep sub-wavelength confinement of terahertz wave and with Q-factor orders of magnitude higher than that of conventional 2DEG system at room temperature. Rooted in this understanding, the performance trade-off among signal attenuation, broadband operation, on-chip integrability can be avoided in future THz smart photonic network system by merging photonics and electronics. The unique properties presented can open up the exciting routes to compact solid state tunable THz detectors, filters, and wide band subwavelength imaging based on the graphene-FETs.

Epitaxial growth and transport properties of BaTiO3/LaAlO3/SrTiO3 heterostructures

Epitaxial BaTiO3(BTO)/LaAlO3(LAO) heterostructure films were fabricated on TiO2-teminated SrTiO3(STO) substrates by pulsed laser deposition. The oxygen pressure effects during the growth of ferroelectric BTO overlayers on top of the LAO/STO structures with two dimensional electron gas (2DEG) were investigated. An obvious influence on the crystalline structure and microstructural characteristics was observed via X-ray diffractometry and atomic force microscopy. In addition, the interfacial sheet resistance measurements were carried out for those heterostructures with and without BTO overlayers. The results indicate that the LAO/STO interface is insulating state above oxygen pressure of 2.5Pa, which could however be tuned to conducting state through integrating ferroelectric BTO film into the LAO/STO 2DEG system.

Non-equilibrium study of spin wave interference in systems with both Rashba and Dresselhaus (001) spin-orbit coupling

We study the electron spin transport in two dimensional electron gas (2DEG) system with both Rashba and Dresselhaus (001) spin-orbital coupling (SOC). We assume spatial behavior of spin precession in the non-equilibrium transport regime, and study also quantum interference induced by non-Abelian spin-orbit gauge field. The method we adopt in this article is the non-equilibrium Green's function within a tight binding framework. We consider one ferromagnetic lead which injects spin polarized electron to a system with equal strength of Rashba and Dresselhaus (001) SOC, and we observe the persistent spin helix property. We also consider two ferromagnetic leads injecting spin polarized electrons into a pure Dresselhaus SOC system, and we observe the resultant spin wave interference pattern.

Fourfold oscillation in anisotropic magnetoresistance and planar Hall effect at the LaAlO3/SrTiO3heterointerfaces: Effect of carrier confinement and electric field on magnetic interactions

The confinement of the two-dimensional electron gas (2DEG), preferential occupancy of the Ti 3$d$ orbital, and strong spin-orbit coupling at the LaAlO${}_{3}$/SrTiO${}_{3}$ interface play a significant role in its emerging properties. Here we report a fourfold oscillation in the anisotropic magnetoresistance (AMR) and planar Hall effect (PHE) at the LaAlO${}_{3}$/SrTiO${}_{3}$ heterointerface. We evaluate the carrier confinement effects on the AMR and find that the fourfold oscillation appears only for the case of 2DEG systems while it is twofold for the three-dimensional system. The AMR behavior is further found to be sensitive to applied electric field, emphasizing the significance of spin-orbit coupling at the interface. These confinement effects suggest that the magnetic interactions are predominant at the interface, and the gate electric field modulation of AMR shows the tunability of magnetic interactions. As the fourfold oscillation fits well to the phenomenological model for a cubic symmetry system, this suggests that the origin of the oscillation may be linked to the anisotropy in magnetic scattering arising from the interaction of electrons with the localized magnetic moments coupled to the crystal symmetry. The observed large PHE further indicates the in-plane nature of magnetic ordering that arises from the in-plane Ti 3${d}_{xy}$ orbitals.