High-mobility Electron Gas Research Articles

High valley-degeneracy electron gas at double perovskite - strontium titanate interface

Emergent phenomena such as two-dimensional electron gas (2DEG) and interfacial superconductivity and ferromagnetism are generally built on the interface between insulating oxide thin films and substrates, e.g., LaAlO3/SrTiO3, where the 2D profiles of these electronic states are precisely confined at the interface of two insulators. Herein we report a high-mobility electron gas state with unusual symmetry at the interface of the Sr2CrMoO6/SrTiO3 (110) heterostructures, the fermiology of which follows the cubic crystallographic symmetry rather than the two-dimensional interface itself, resulting in the identical Shubnikov-de Haas oscillations with applied magnetic field along all the twelve equivalent [110] crystallographic directions of SrTiO3, distinctly different from the 2D nature of the electron gas reported previously. Neutron diffraction verifies the predicted ferrimagnetic ordering between Cr and Mo moments. This, together with the magnetic hysteresis loops and negative magnetoresistance in low-field region, suggests possible spin polarization of itinerant electrons. Therefore, a quasi-3D profile, high mobility (up to 104 cm2 V−1 s−1) and possibly spin polarized electronic state is observed in the double-perovskite-based oxide heterostructures. This finding of the electronic properties in Sr2CrMoO6/SrTiO3 (110) heterostructure expands the knowledge of interfacial physics, as well as shines light on oxide-based electronics and spintronics research.

Effects of phase transition driven by cation vacancies on magnetism and electricity in LaxMnO3 thin films

The attractive physical properties of perovskite manganese make them fundamental building blocks for oxide spintronic devices. However, defects and lattice distortions can lead to extrinsic properties, especially cation vacancies, severely compromising device performance. In particularly, many intriguing properties, such as the emergence of magnetism and two-dimensional high-mobility electron gases, are closely related to cation vacancies. In this study, we deposited a series of LaxMnO3 epitaxial thin films with varying contents of La3+ vacancies on SrTiO3 (100) substrates, with the presence of La3+ vacancies verified by using X-ray diffraction, X-ray photoemission spectroscopy, X-ray absorption spectroscopy, and energy dispersive spectroscopy spectrum. The reduction in La3+ vacancies led to stronger ferromagnetism with an easy axis oriented along the (110) direction in LaMnO3 thin films, along with the appearance of a monoclinic phase, indicating that the ferromagnetism possibly originated from the three-dimensional d3z2−r2/dx2−y2-alternated orbital order rather than the Mn3+/Mn4+ double exchange effect. This result implies that the consistent regulation of cation vacancies by pulsed laser deposition can modulate both the structure and physical properties of manganate films, thereby enabling control over device properties.

High mobility electron gas with quasi-two-dimensional characteristics at the interface of Cr2O3/SrTiO3 heterostructures

Two-dimensional electron gas is precisely confined at the interface of insulating oxide thin films and substrates, e.g., LaAlO3/SrTiO3(STO) and, thus, shows 2D electronic transport features. Here, we report a high mobility electron state at the interface of a Cr2O3 film and a STO substrate, which is realized by depositing a Cr film onto a STO (111) substrate in high vacuum (1 × 10−10 mbar) using molecular beam epitaxy. At a substrate temperature of 700 °C, the deposited Cr films capture oxygen atoms from STO substrates, resulting in the formation of an insulating Cr2O3 layer and an oxygen-deficient STO layer. Due to the presence of high mobility electrons [1.5 × 104 cm2V−1 s−1 at 1.8 K] at the Cr2O3/STO interface, both out-of-plane and in-plane Shubnikov–de Haas oscillations are observed at low temperatures (<3 K), which suggests that the highly conducting electron gas has extended into the STO bulk along the thickness direction with a certain depth to allow electrons to complete the cyclotron motion.

High-mobility electron gas in Sr2TiO4/SrTiO3 heterostructure engineered by vertical Ruddlesden–Popper faults

High-mobility electron gases in SrTiO3-based heterostructures have been mainly synthesized on single TiO2 terminated SrTiO3 substrates. Here, we show a high-mobility electron gas observed in a Sr2TiO4/SrTiO3 heterostructure based on an untreated SrTiO3 substrate with mixed termination, showing the mobility value up to 28 000 cm2 V−1 s−1 at 2 K. SrO–SrO vertical Ruddlesden–Popper faults in the Sr2TiO4 film provide escape channels for oxygen atoms under high temperatures, enabling the as-grown tuning of oxygen vacancies in the SrTiO3 layer during film deposition and reversible after-growth modulation in a thermal annealing process.

Superballistic electron flow through a point contact in a Ga[Al]As heterostructure

We measure electronic transport through point contacts in the high-mobility electron gas in a Ga[Al]As heterostructure at different temperatures and bulk electron densities. The conductance through all point contacts increases with increasing temperature in a temperature window around $T \sim 10 K$ for all investigated electron densities and point contact widths. For high electron densities this conductance exceeds the fundamental ballistic limit (Sharvin limit). These observations are in agreement with a viscous electron transport model and previous experiments in graphene.

Enhanced photovoltaic effect in LaAlO<sub>3</sub>/SrTiO<sub>3</sub> interface

Since high-mobility electron gas, which is also called two-dimensional electron gas, was discovered at the LaAlO<sub>3</sub>/SrTiO<sub>3</sub> (LAO/STO) interface, SrTiO<sub>3</sub>-based heterostructures and nanostructures have become an attractive platform for novel nanoelectronic devices. Exploring the novel physical properties of LAO/STO interface and the mechanisms of interface effect is the key to designing and fabricating the new photoelectric devices. The LAO/STO sample is prepared on an STO (001) substrate by pulsed laser deposition. In order to study the influence of interface effect on photovoltaic effect in the LAO/STO sample, a KrF pulse laser with a wavelength of 248 nm and an energy density of 50 mJ/cm<sup>2</sup> is chosen as an ultraviolet light source, a sampling oscilloscope of 350 MHz is used to measure the photovoltages, and a precision adjustable slit is adopted to control the size of irradiation area. The photovoltaic effect is studied under the condition of applied electric field at ambient temperature. The experimental results prove that the photovolatge of irradiating on the side of sample (LAO/STO interface) is higher than on the front of sample (film surface) under the same area of irradiation. Lateral photovoltaic effect is discovered in the LAO/STO sample. Irradiating on the side of sample (LAO/STO interface) can further improve the lateral photovoltaic effect in the LAO/STO sample. The open-circuit photovoltage depends linearly on the illuminated position, and the sensitivity reaches 36.8 mV/mm. The sensitivity of the lateral photovoltaic effect can be modified by the bias voltage. The experimental results not only contributes to better understanding the interface effect in LAO/STO interface, but also provides a basis for designing and using photoelectric devices for position-sensitive detection.

Rational Assembly of Two-Dimensional Perovskite Nanosheets as Building Blocks for New Ferroelectrics.

Artificial materials in the form of superlattices have been studied actively in quest of new engineering methods or design rules for the development of desired functionalities, in particular high-k ferroelectricity, ferromagnetism, and high mobility electron gas. This work presents a controlled assembly strategy for fabricating atomically precise interfaces of two-dimensional (2D) homologous perovskite nanosheets (Ca2Nam-3NbmO3m+1-; m = 3-6) to construct artificial superlattices. The distinctive thickness of each 2D homologous perovskite nanosheets attributed to the presence of different number of NbO6 octahedra provides an exquisite control to engineer interfacial properties for tailored design of superior high-k properties and emergence of ferroelectricity. The higher dielectric constant (εr = 427) and development of ferroelectricity for (Ca2Nb3O10-/Ca2Na2Nb5O16-)6 superlattice indicate that superlattice films with both odd number of NbO6 octahedra possess extended polarization due to the potential effect of heterointerface and ferroelectric instabilities. Furthermore, the increased discontinuities/offsets in Ca2Nb3O10- and Ca2Na3Nb6O19- nanosheets band alignment results in superior insulating properties (∼1 × 10-11 A cm-2 at 1 V) for (Ca2Nb3O10-/Ca2Na3Nb6O19-)6 superlattice. These findings exhibit new research opportunities for the development of novel artificial high-k dielectric/ferroelectric via precise control of interfaces at the atomic level and can be extended to the large family of 2D perovskite compounds.

Demonstration of a tunable antenna-coupled intersubband terahertz (TACIT) mixer

A fast, voltage-tunable terahertz mixer based on the intersubband transition of a high-mobility 2-dimensional electron gas has been fabricated from a single 40 nm GaAs-AlGaAs square quantum well heterostructure. The device is called a Tunable Antenna-Coupled Intersubband Terahertz mixer and shows tunability of the detection frequency from 2.52 to 3.11 THz with small (<1 V) top gate and bottom gate voltage biases. Mixing at 2.52 THz has been observed at 60 K with a −3dB intermediate frequency bandwidth exceeding 6 GHz.

Exchange bias in La0.7Sr0.3CrO3/La0.7Sr0.3MnO3/La0.7Sr0.3CrO3 heterostructures

In the recent past, heterostructures of magnetic oxide thin films have attracted a great deal of research excitement due to very interesting physical properties such as antiferromagnetic interlayer coupling, tunable exchange-bias, interfacial driven magnetic properties and high mobility electron gas across the interfaces. In this work, we report on the comprehensive magnetic properties observed from the heterostructures of (2 unit cells) La0.7Sr0.3CrO3/(8 unit cells) La0.7Sr0.3MnO3/(2 unit cells) La0.7Sr0.3CrO3, which are epitaxially deposited on SrTiO3 substrate by plasma-assisted oxide molecular beam epitaxy. Using SQUID magnetometer, the magnetic properties are studied when the magnetic field was applied both in plane and out of plane. The Curie temperature of this structure is found to be at 290 K. Most significantly, at 2 K, we observed a complete up/down shift (along magnetization axis) of hysteresis loop when the sample was cooled under a magnetic field of ± 5000 Oe in the in-plane configuration. We believe that the strong antiferromagnetic (super) exchange coupling of Mn-Cr across the two interfaces is responsible for the observed exchange bias. We will present and discuss our in-detailed experimental findings collected on this heterostructure as a function of temperature and magnetic field.

Separating Electrons and Donors in BaSnO3 via Band Engineering.

Separating electrons from their source atoms in La-doped BaSnO3, the first perovskite oxide semiconductor to be discovered with high room-temperature electron mobility, remains a subject of great interest for achieving high-mobility electron gas in two dimensions. So far, the vast majority of work in perovskite oxides has focused on heterostructures involving SrTiO3 as an active layer. Here we report the demonstration of modulation doping in BaSnO3 as the high room-temperature mobility host without the use of SrTiO3. Significantly, we show the use of angle-resolved hard X-ray photoelectron spectroscopy (HAXPES) as a nondestructive approach to not only determine the location of electrons at the buried interface but also to quantify the width of electron distribution in BaSnO3. The transport results are in good agreement with the results of self-consistent solution to one-dimensional Poisson and Schrödinger equations. Finally, we discuss viable routes to engineer two-dimensional electron gas density through band-offset engineering.

High-${Q}$ GaN Varactors for mm-Wave Applications: A Physics-Based Simulation Study

Varactors, with its capacitance tunable by the applied voltage, are enabling components for reconfigurable RF systems. The performance of a varactor could be characterized by its ${Q}$ -factor and the capacitance tuning ratio. In this article, we seek to develop a new varactor with very high ${Q}$ -factor and large tuning ratio, enabling reconfigurable RF systems with large bandwidth. There were two main innovations in varactor diode design. One was the use of the fin structure, in which the electrodes were placed on two opposite sidewalls of the mesa structure instead of one on top and the other at the bottom or next to the mesa. This design eliminates the extrinsic series resistance in the conventional varactor diode designs, leading to substantially increased ${Q}$ -factor. The other was the use of a multichannel structure. The structure was composed of multiple AlGaN/GaN heterojunctions with high-mobility 2-D electron gas. The high-mobility electron gas helped lowering the intrinsic series resistance, thereby further improving the ${Q}$ -factor.

Predicting ultrafast Dirac transport channel at the one-dimensional interface of the two-dimensional coplanar ZnO/MoS2 heterostructure

The discovery of high-mobility electron gas at the two-dimensional (2D) LaAlO3/SrTiO3 heterointerface [A. Ohtomo and H. Hwang, Nature (London) 427, 423 (2004)10.1038/nature02308] has attracted great research interest due to the ballistic transport of spatially confined electrons. When such kind of valence discontinuity extends to the interface of a well-chosen in-planar 2D heterostructure, a one-dimensional (1D) confined interface can be generated. Herein, through first-principles modeling, we demonstrate that Dirac-cone-like bands emerge at the 1D zigzag interface of a ZnO/MoS2 lateral heterostructure (LHS), creating a highly mobile 1D transport channel with a high Fermi velocity of 4.5×105m/s. The metallic state at the 1D heterointerface is attributed to the polar discontinuity that introduces excess charge carriers. Nontrivial polarization (excess carriers appear at boundaries) results in a large built-in electric field within the ZnO and MoS2 monolayers. Remarkably, the ultrafast 1D conducting channel is independent of the arrangements of width (m and n) in the (ZnO)m(MoS2)n LHS, which are further illustrated by tight-binding-based orbital and polarization analysis.

Emerging many-body effects in semiconductor artificial graphene with low disorder

The interplay between electron–electron interactions and the honeycomb topology is expected to produce exotic quantum phenomena and find applications in advanced devices. Semiconductor-based artificial graphene (AG) is an ideal system for these studies that combines high-mobility electron gases with AG topology. However, to date, low-disorder conditions that reveal the interplay of electron–electron interaction with AG symmetry have not been achieved. Here, we report the creation of low-disorder AG that preserves the near-perfection of the pristine electron layer by fabricating small period triangular antidot lattices on high-quality quantum wells. Resonant inelastic light scattering spectra show collective spin-exciton modes at the M-point's nearly flatband saddle-point singularity in the density of states. The observed Coulomb exchange interaction energies are comparable to the gap of Dirac bands at the M-point, demonstrating interplay between quasiparticle interactions and the AG potential. The saddle-point exciton energies are in the terahertz range, making low-disorder AG suitable for contemporary optoelectronic applications.

Patterning of high mobility electron gases at complex oxide interfaces

Oxide interfaces provide an opportunity for electronics. However, patterning of electron gases at complex oxide interfaces is challenging. In particular, patterning of complex oxides while preserving a high electron mobility remains underexplored and inhibits the study of quantum mechanical effects where extended electron mean free paths are paramount. This letter presents an effective patterning strategy of both the amorphous-LaAlO3/SrTiO3 (a-LAO/STO) and modulation-doped amorphous-LaAlO3/La7/8Sr1/8MnO3/SrTiO3 (a-LAO/LSM/STO) oxide interfaces. Our patterning is based on selective wet etching of amorphous-LSM (a-LSM) thin films, which acts as a hard mask during subsequent depositions. Strikingly, the patterned modulation-doped interface shows electron mobilities up to ∼8 700 cm2/V s at 2 K, which is among the highest reported values for patterned conducting complex oxide interfaces that usually are ∼1 000 cm2/V s at 2 K.

Terahertz detection of magnetic field-driven topological phase transition in HgTe-based transistors

We report on terahertz photoconductivity under magnetic field up to 16 T of field effect transistor based on HgTe quantum well (QW) with an inverted band structure. We observe pronounced cyclotron resonance and Shubnikov-de Haas-like oscillations, indicating a high mobility electron gas in the transistor channel. We discover that nonlinearity of the transistor channel allows for observation of characteristic features in photoconductivity at critical magnetic field corresponding to the phase transition between topological quantum spin Hall and trivial quantum Hall states in HgTe QW. Our results pave the way towards terahertz topological field effect transistors.

Demonstration of Confined Electron Gas and Steep-Slope Behavior in Delta-Doped GaAs-AlGaAs Core-Shell Nanowire Transistors.

Strong surface and impurity scattering in III-V semiconductor-based nanowires (NW) degrade the performance of electronic devices, requiring refined concepts for controlling charge carrier conductivity. Here, we demonstrate remote Si delta (δ)-doping of radial GaAs-AlGaAs core-shell NWs that unambiguously exhibit a strongly confined electron gas with enhanced low-temperature field-effect mobilities up to 5 × 10(3) cm(2) V(-1) s(-1). The spatial separation between the high-mobility free electron gas at the NW core-shell interface and the Si dopants in the shell is directly verified by atom probe tomographic (APT) analysis, band-profile calculations, and transport characterization in advanced field-effect transistor (FET) geometries, demonstrating powerful control over the free electron gas density and conductivity. Multigated NW-FETs allow us to spatially resolve channel width- and crystal phase-dependent variations in electron gas density and mobility along single NW-FETs. Notably, dc output and transfer characteristics of these n-type depletion mode NW-FETs reveal excellent drain current saturation and record low subthreshold slopes of 70 mV/dec at on/off ratios >10(4)-10(5) at room temperature.

Controlled formation of high-mobility shallow electron gases in SrTiO3 single crystal

We report on the controlled formation of sub-100-nm-thin electron channels in SrTiO3 by doping with oxygen vacancies induced by Ar+ ion irradiation. The conducting channels exhibit a consistently high electron mobility (∼15,000 cm2 V−1 s−1), which enables a clear observation of magnetic quantum oscillations, and a gate-tunable linear magnetoresistance. Near the onset of electrical conduction, a metal–insulator transition is induced by mobility suppression. With the high electron mobility and the ease of controlled channel formation, this ion irradiation doping method may provide an excellent basis for developing oxide electronics.

Terahertz Radiation from Magnetic Excitations in Diluted Magnetic Semiconductors

We probed, in the time domain, the THz electromagnetic radiation originating from spins in CdMnTe diluted magnetic semiconductor quantum wells containing high-mobility electron gas. Taking advantage of the efficient Raman generation process, the spin precession was induced by low power near-infrared pulses. We provide a full theoretical first-principles description of spin-wave generation, spin precession, and of emission of THz radiation. Our results open new perspectives for improved control of the direct coupling between spin and an electromagnetic field, e.g., by using semiconductor technology to insert the THz sources in cavities or pillars.

Topological crystalline insulators in the SnTe material class

Topological crystalline insulators are new states of matter in which the topological nature of electronic structures arises from crystal symmetries. Here we predict the first material realization of topological crystalline insulator in the semiconductor SnTe by identifying its non-zero topological index. We predict that as a manifestation of this non-trivial topology, SnTe has metallic surface states with an even number of Dirac cones on high-symmetry crystal surfaces such as {001}, {110} and {111}. These surface states form a new type of high-mobility chiral electron gas, which is robust against disorder and topologically protected by reflection symmetry of the crystal with respect to {110} mirror plane. Breaking this mirror symmetry via elastic strain engineering or applying an in-plane magnetic field can open up a continuously tunable band gap on the surface, which may lead to wide-ranging applications in thermoelectrics, infra-red detection and tunable electronics. Closely related semiconductors PbTe and PbSe also become topological crystalline insulators after band inversion by pressure, strain and alloying.

Noise and electric field characterization of irradiated SrTiO3

SrTiO3 (STO) is a band insulator with a perovskite crystallographic structure, widely used as substrate for the deposition of other perovskite materials, for instance manganites or cuprates. STO surface becomes conductive under Ar+ ion irradiation, exhibiting similar properties to STO/LaAlO3 interface or bulk reduced STO. We report the transport and noise properties as function of temperature and electric field of Ar+ irradiated STO exhibiting at its surface a high mobility electron gas. This system shows metallic properties with low charge densities allowing mobility measurements. The low frequency 1/f noise presents an unexpected linear dependence with the mobility which cannot be described by the usual Hooge model. The effect of the electric field at a given temperature on the mobility, magnetoresistance and low frequency noise is also presented.