High-mobility Two-dimensional Electron Gas Research Articles

High-mobility spin-polarized two-dimensional electron gas at the interface of LaTiO3/SrTiO3 (110) heterostructures

High-mobility spin-polarized two-dimensional electron gas at the interface of LaTiO3/SrTiO3 (110) heterostructures

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.

High-mobility two-dimensional electron gas at the PbZr0.5Ti0.5O3/BaSnO3 heterostructure

High-mobility two-dimensional electron gas at the PbZr0.5Ti0.5O3/BaSnO3 heterostructure

High-mobility spin-polarized two-dimensional electron gas at the interface of EuTiO3/SrTiO3 heterostructures

Spin polarization of two-dimensional electron gas (2DEG) at the interface of EuTiO3/SrTiO3(STO) heterostructures has been theoretical predicted and experimentally observed via x-ray magnetic circular dichroism and polarized x-ray absorption spectroscopy, which, however, is lack of magnetotransport evidence. Here, we report the fabrication of high-quality EuTiO3/STO heterostructures by depositing antiferromagnetic insulating EuTiO3 thin films onto STO substrates. Shubnikov-de Haas oscillation, Hall, and magnetoresistance (MR) measurements show that the interface is not only highly conducting, with electron mobility up to cm2V−1s−1 at 1.8 K, but also shows low-field hysteretic MR effects. MR of ∼9% is observed at 1.8 K and 20 Oe, which is one order of magnitude higher than those observed in other spin-polarized 2DEG oxide systems. Moreover, the heterostructures show ferromagnetic hysteresis loops. These results demonstrate that the high-mobility 2DEG is spin polarized, whose origin is attributed to the interfacial Ti3+-3d states due to oxygen deficiency and the exchange interactions between interfacial Eu spins and itinerant Ti-3d electrons.

Superradiant emission in a high-mobility two-dimensional electron gas

We use terahertz time-domain spectroscopy to study gallium arsenide two-dimensional electron gas samples in external magnetic field. We measure cyclotron decay as a function of temperature from 0.4 to and a quantum confinement dependence of the cyclotron decay time below . In the wider quantum well, we observe a dramatic enhancement in the decay time due to the reduction in dephasing and the concomitant enhancement of superradiant decay in these systems. We show that the dephasing time in 2DEG’s depends on both the scattering rate and also on the distribution of scattering angles.

Observation of charge-to-spin conversion with giant efficiency at Ni0.8Fe0.2/Bi2WO6 interface

Magnetization switching using spin–orbit torque offers a promising route to developing non-volatile memory technologies. The prerequisite, however, is the charge-to-spin current conversion, which has been achieved traditionally by harnessing the spin–orbit interaction in heavy metals, topological insulators, and heterointerfaces hosting a high-mobility two-dimensional electron gas. Here, we report the observation of charge-to-spin current conversion at the interface between ferromagnetic Ni0.8Fe0.2 and ferroelectric Bi2WO6 thin films. The resulting spin–orbit torque consists of damping-like and field-like components, and the estimated efficiency amounts to about 0.48 ± 0.02, which translates to 0.96 ± 0.04 nm−1 in terms of interfacial efficiency. These numbers are comparable to contemporary spintronic materials exhibiting giant spin–orbit torque efficiency. We suggest that the Rashba Edelstein effect underpins the charge-to-spin current conversion on the interface side of Ni0.8Fe0.2. Further, we provide an intuitive explanation for the giant efficiency in terms of the spin-orbit proximity effect, which is enabled by orbital hybridization between W and Ni (Fe) atoms across the interface. Our work highlights that Aurivillius compounds are a potential addition to the emerging transition metal oxide-based spin–orbit materials.

Effect of Disorder on Magnetotransport in Semiconductor Artificial Graphene

Magnetotransport in mesoscopic samples with semiconductor artificial graphene has been simulated within the Landauer–Büttiker formalism. Model four-terminal systems in a high-mobility two-dimensional electron gas have a square shape with a side of 3–5 μm, which is filled with a short-period (120 nm) weakly disordered triangular lattice of antidots at the modulation amplitude of the electrostatic potential comparable with the Fermi energy. It has been found that the Hall resistance {{R}_{{xy}}}(B) in the magnetic field range of B = 10–50 mT has a hole plateau {{R}_{{xy}}} = - {{R}_{0}}, where R0 = h/2e2 = 12.9 kΩ, at carrier densities in the lattice below the Dirac point n < n1D and an electron plateau {{R}_{{xy}}} = {{R}_{0}} at n > n1D. Enhanced disorder destroys the plateaus, but a carrier type (electrons or holes) holds. Long-range disorder at low magnetic fields suppresses quantized resistance plateaus much more efficiently than short-range disorder.

Low leakage current and high breakdown voltage of AlGaN/GaN Schottky barrier diodes with wet-etching groove anode

AlGaN/GaN heterojunction epitaxies with wide bandgap, high critical electric field as well as high density and high mobility two-dimensional electron gas have shown great potential applications in the next-generation high-power and high-frequency devices. Especially, with the development of Si-based GaN epitaxial technique with big size, GaN devices with low cost also show great advantage in consumer electronics. In order to improve the rectification efficiency of AlGaN/GaN Schottky barrier diode (SBD), low leakage current and low turn-on voltage are important. The GaN Schottky barrier diode with low work-function metal as anode is found to be very effective to reduce turn-on voltage. However, the low Schottky barrier height makes the Schottky interface sensitive to damage to groove surface, which leads to a high leakage current. In this work, a novel wet-etching technique with thermal oxygen oxidation and KOH corrosion is used to prepare the anode groove, and the surface roughness of groove decreases from 0.57 to 0.23 nm, compared with that of the dry-etching surface of groove. Meanwhile, the leakage current is suppressed from 1.5 × 10&lt;sup&gt;–6&lt;/sup&gt; to 2.6 × 10&lt;sup&gt;–7&lt;/sup&gt; A/mm. Benefiting from the great corrosion selectively of hot KOH solution to AlGaN barrier layer and GaN channel layer after thermal oxygen oxidation, the spikes of the edge of groove region caused by the nonuniform distribution of plasma in the cavity is improved, and the breakdown voltage of the fabricated AlGaN/GaN SBDs is raised from –1.28 to –1.73 kV.

Molecular beam epitaxy of InAs quantum wells on InP(001) for high mobility two-dimensional electron gases

For InAs quantum-well structures grown on InP, the dislocations generated in the strain relaxation is confined in the compositionally graded buffer layer, leaving the two-dimensional electron gases nearly unscattered by the defects.

High-Mobility Magnetic Two-Dimensional Electron Gas in Engineered Oxide Interfaces

The engineered interfaces of complex oxides have abundant physical properties and provide a powerful platform for the exploration of fundamental physics and emergent phenomena. In particular, research on the two-dimensional magnetic systems with high mobility remains a long-standing challenge for the discovery of quantum phase and spintronic applications. Here, we introduce a few atomic layers of the delta doping layer at LaAlO3/SrTiO3 interfaces through elaborately controllable epitaxial growth of SrRuO3. After inserting a SrRuO3 buffer layer, the interfaces exhibit a well-defined anomalous Hall effect up to 100 K and their mobility is enhanced by 3 orders of magnitude at low temperatures. More intriguingly, a large unsaturated positive magnetoresistance is created at interfaces. Combining with the density functional theory calculation, we attribute our findings to the electron transfer at interfaces and the magnetic moment of Ru4+ 4d bands. The results pave a way for further research of two-dimensional ferromagnetism and quantum transport in all-oxide systems.

Low temperature photoluminescence study for identification of intersubband energy levels inside triangular quantum well of AlGaN/GaN heterostructure

A non-destructive method for extracting the true signatures of intersubband energy level states inside a single triangular quantum well for AlGaN/GaN heterostructure is presented. Herein, we report experiments showing light interaction with high-mobility two-dimensional electron gas (2DEG) in Al0.3Ga0.7N/AlN/GaN based HEMT structure using low-temperature photoluminescence spectroscopy. An interband transition from two-dimensional electron gas (2DEG) subbands to the valence band is identified. These observed transitions were confirmed by comparing the PL spectra of the as grown and top barrier layer etched samples. The luminance peak related to 2DEG disappeared when the top AlGaN barrier layer was removed using reactive ion etching (RIE) system. Furthermore, the emission peak data are also supported by a calculation based on a self-consistent solution of one-dimensional Poisson and Schrodinger equations. The three PL spectra peaks corresponding to the interband transitions from 2DEG subbands to the valence band were reported at 3.269, 3.356 and 3.438 eV respectively. The corresponding intersubband energy states inside the quantum well were extracted (simulated) with 87 (91) and 178 (186) meV energy spacing between E0 & E1 and E0 & E2 respectively. The temperature and excitation power-dependent PL measurement makes it easy to identify the transitions from the 2DEG subbands to valence bands.

Composition-dependent photocatalytic activity and high-mobility carrier gas in NaTaO3/BaBiO3 heterojunctions

Perovskite oxide heterostructures have been extensively investigated for their excellent nanoelectronics and photocatalytic properties. Herein, the general features of monoclinic (001) NaTaO3/BaBiO3 heterojunctions are theoretically investigated, exploring how the interface creation affects their physicochemical properties. Density functional theory results reveal that a superlattice with 24 wt.% of BaBiO3 displays a typically desired electronic structure for photocatalytic reactions, with an intermediate band gap and highly dispersive energy levels. Hybrid functional calculations provide a band gap of 1.85 eV, with optical absorption peaks inside the visible light spectrum and band-edge potentials better aligned with water splitting levels when compared with the pristine NaTaO3 phase. Upon increasing the number of BaBiO3 layers a semiconductor-to-metal transition is observed and described based on electrostatic and polarization arguments, leading to the formation of a high-mobility two-dimensional electron gas (2DEG) at the interface. The presented results highlight the importance of such nanojunctions not only for photocatalytic but also for nanoelectronics applications.

High Mobility Two-Dimensional Electron Gas at the BaSnO3/SrNbO3 Interface.

Oxide two-dimensional electron gases (2DEGs) promise high charge carrier concentrations and low-loss electronic transport in semiconductors such as BaSnO3 (BSO). ACBN0 computations for BSO/SrNbO3 (SNO) interfaces show Nb-4d electron injection into extended Sn-5s electronic states. The conduction band minimum consists of Sn-5s states ∼1.2 eV below the Fermi level for intermediate thickness 6-unit cell BSO/6-unit cell SNO superlattices, corresponding to an electron density in BSO of ∼1021 cm-3. Experimental studies of analogous BSO/SNO interfaces grown by molecular beam epitaxy confirm significant charge transfer from SNO to BSO. In situ angle-resolved X-ray photoelectron spectroscopy studies show an electron density of ∼4 × 1021 cm-3. The consistency of theory and experiments show that BSO/SNO interfaces provide a novel materials platform for low loss electron transport in 2DEGs.

Giant magnon spin conductivity in ultrathin yttrium iron garnet films.

Conductivities are key material parameters that govern various types of transport (electronic charge, spin, heat and so on) driven by thermodynamic forces. Magnons, the elementary excitations of the magnetic order, flow under the gradient of a magnon chemical potential1-3 in proportion to a magnon (spin) conductivity. The magnetic insulator yttrium iron garnet is the material of choice for efficient magnon spin transport. Here we report a giant magnon conductivity in thin yttrium iron garnet films with thicknesses down to 3.7 nm when the number of occupied two-dimensional subbands is reduced from a large number to a few, which corresponds to a transition from three-dimensional to two-dimensional magnon transport. We extract a two-dimensional magnon spin conductivity around 1 S at room temperature, comparable to the (electronic) conductivity of the high-mobility two-dimensional electron gas in GaAs quantum wells at millikelvin temperatures4. Such high conductivities offer opportunities to develop low-dissipation magnon-based spintronic devices.

Electrical characterization and extraction of activation energies of the defect states in the LaAlO3/SrTiO3 heterostructure

In this work, we study the electronic properties of defects in the LaAlO3/SrTiO3 heterostructure, which is known to host a high mobility two-dimensional electron gas (2DEG) at the interface. This 2DEG also shows photoconductance, which could be related to defects that act as deep center trapping and releasing carriers by interaction with light. This phenomenon has raised an interest for the identification of deep energy levels in the LaAlO3/SrTiO3 heterostructure. We have studied the defect state properties using electrical characterization such as capacitance–voltage (C–V), current–voltage (I–V) measurements, and deep-level transient Fourier spectroscopy (DLTFS). From C–V and I–V analyses, a hysteresis was observed indicating an effect of mobile charges in the LaAlO3. Using DLTFS, we identify three defect states located at around 0.17 eV below conduction band and at 0.23 and 0.26 eV above the valence band. These defect states were attributed to defects in SrTiO3 such as strontium vacancies or titanium vacancies. We identify a fourth defect state having an energy of about 0.69 eV below the conduction band that could be related to oxygen vacancies in LaAlO3 or in SrTiO3. In addition, the observation of an effect of the electric field with DLTFS indicated that oxygen vacancies might be involved in Fowler–Nordheim or trap-assisted tunneling through the LaAlO3 layer.

Robust Electronic Structure of Manganite-Buffered Oxide Interfaces with Extreme Mobility Enhancement.

The electronic structure as well as the mechanism underlying the high-mobility two-dimensional electron gases (2DEGs) at complex oxide interfaces remain elusive. Herein, using soft X-ray angle-resolved photoemission spectroscopy (ARPES), we present the band dispersion of metallic states at buffered LaAlO3/SrTiO3 (LAO/STO) heterointerfaces where a single-unit-cell LaMnO3 (LMO) spacer not only enhances the electron mobility but also renders the electronic structure robust toward X-ray radiation. By tracing the evolution of band dispersion, orbital occupation, and electron-phonon interaction of the interfacial 2DEG, we find unambiguous evidence that the insertion of the LMO buffer strongly suppresses both the formation of oxygen vacancies as well as the electron-phonon interaction on the STO side. The latter effect makes the buffered sample different from any other STO-based interfaces and may explain the maximum mobility enhancement achieved at buffered oxide interfaces.

Fermi level tuning and band alignment in Mn-doped InAs/GaSb

InAs/GaSb hosts a broken gap band alignment that has been shown to generate helical topological edge states. Upon the introduction of Mn into the structure, it has been predicted to host a quantized anomalous Hall effect. Here, we show that dilute Mn doping on InAs in InAs/GaSb, allows a tuning of the Fermi level, the introduction of paramagnetism, but also has a non-trivial impact on the band alignment of the system. The measurement of Shubnikov-de-Haas oscillations, cyclotron resonance, and a non-linear Hall effect in Mn-doped samples indicate the coexistence of a high mobility two-dimensional electron gas and a hole gas. Conversely, in undoped InAs/GaSb, pure-n-type transport is observed. We hypothesize that Mn acceptor levels can pin the Fermi energy near the valence band edge of InAs, far from the interface, which introduces a strong band bending to preserve the band offset at the InAs/GaSb interface. The realization of the QAHE in this structure will thus require a careful control of the band alignment to preserve topological insulating character.

Breakdown of topological protection by cavity vacuum fields in the integer quantum Hall effect.

The prospect of controlling the electronic properties of materials via the vacuum fields of cavity electromagnetic resonators is emerging as one of the frontiers of condensed matter physics. We found that the enhancement of vacuum field fluctuations in subwavelength split-ring resonators strongly affects one of the most paradigmatic quantum protectorates, the quantum Hall electron transport in high-mobility two-dimensional electron gases. The observed breakdown of the topological protection of the integer quantum Hall effect is interpreted in terms of a long-range cavity-mediated electron hopping where the anti-resonant terms of the light-matter coupling Hamiltonian develop into a finite resistivity induced by the vacuum fluctuations. Our experimental platform can be used for any two-dimensional material and provides a route to manipulate electron phases in matter by means of vacuum-field engineering.

Ferroelectric domain induced giant enhancement of two-dimensional electron gas density in ultrathin-barrier AlGaN/GaN heterostructures

With high carrier mobility two-dimensional electron gas (2DEG), AlGaN/GaN heterostructures are widely used in radio frequency and power devices. While due to limited spontaneous and piezoelectric polarization in AlGaN/GaN heterostructures, their practical 2DEG density is around 1013 cm−2 so far. Further enhancement of the 2DEG density is always an attractive topic with both fundamental and application importance. Theoretical studies have shown that the carrier density in the AlGaN/GaN interface can be greatly enhanced by a ferroelectric layer with large polarization. Some experimental results have already been reported, showing signatures of correlation between ferroelectric polarization and 2DEG density. However, they can only realize limited enhancement of 2DEG in AlGaN/GaN heterostructures. Recently, with a simple substrate transfer technique, the hetero-integration of high-quality PbZrxTi1-xO3 (PZT) on the AlGaN/GaN was successfully fabricated by our group. A threshold voltage shifts up to 4 V was achieved by the ferroelectric polarization. Herein, we report further optimization of this structure with ultrathin-barrier layer and in-situ poled ferroelectric domain, where the 2DEG density was non-volatilely enhanced three orders of magnitude up to 2.57 × 1014 cm−2. This result provides good experimental support to the theoretical prediction and might help to further improve the performance GaN based electronic devices.

Investigation of AlGaN/GaN Schottky barrier diodes on free-standing GaN substrate with low leakage current

Benefiting from the excellent properties of GaN with a wide bandgap of 3.4 eV as well as high critical field of 3.3 MV/cm, GaN-based devices prove to be a promising candidate in extreme conditions. Especially, high-density high-mobility two-dimensional electron gas (2DEG) induced by spontaneous piezoelectric polarization in AlGaN/GaN heterostructure enables AlGaN/GaN device to lower on-resistance (&lt;i&gt;R&lt;/i&gt;&lt;sub&gt;ON&lt;/sub&gt;). However, owing to the lack of free-standing GaN substrate with large size and high quality, the epitaxis of GaN is always based on hetero-substrate such as Al&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;3&lt;/sub&gt;, Si and SiC, which shows large lattice mismatch and thermal mismatch. The large mismatch between GaN and substrate leads to high dislocation as well as high leakage current (&lt;i&gt;I&lt;/i&gt;&lt;sub&gt;R&lt;/sub&gt;) of GaN devices. In this work, high-performance AlGaN/GaN Schottky barrier diode with low &lt;i&gt;I&lt;/i&gt;&lt;sub&gt;R&lt;/sub&gt; and low turn-on voltage (&lt;i&gt;V&lt;/i&gt;&lt;sub&gt;ON&lt;/sub&gt;) is fabricated on a 3-inch free-standing GaN substrate with C-doping GaN buffer layer to suppress &lt;i&gt;I&lt;/i&gt;&lt;sub&gt;R&lt;/sub&gt;. Owing to the suppressed dislocation density of the AlGaN/GaN epitaxial wafer on free-standing substrate, low Ohmic contact resistance (&lt;i&gt;R&lt;/i&gt;&lt;sub&gt;C&lt;/sub&gt;) is difficult to achieve the suppressed penetration of Ohmic metal into 2DEG channel, which is adverse to the high current density. In this work, a low &lt;i&gt;R&lt;/i&gt;&lt;sub&gt;C&lt;/sub&gt; of 0.37 Ω·mm is obtained by one-step self-aligned Ohmic process, including the etching of partial AlGaN barrier layer and lift-off of Ohmic metal. The 2DEG is formed under the effect of residual AlGaN barrier layer, and the short distance between 2DEG and Ohmic metal contributes to lowering the value of &lt;i&gt;R&lt;/i&gt;&lt;sub&gt;C&lt;/sub&gt;. The groove anode region is defined by the low damaged inductively coupled plasma process with a low etching rate of 1 nm/min, and the total depth is 35 nm, confirmed by atomic force microscope. Fully removing the AlGaN barrier layer from the anode region makes the anode metal directly contact the 2DEG channel, thereby improving the performance of the fabricated AlGaN/GaN Schottky barrier diode (SBD) with a low &lt;i&gt;V&lt;/i&gt;&lt;sub&gt;ON&lt;/sub&gt; of 0.67 V, low &lt;i&gt;I&lt;/i&gt;&lt;sub&gt;R&lt;/sub&gt; of 3.6 × 10&lt;sup&gt;–8&lt;/sup&gt; A/mm, and an &lt;i&gt;I&lt;/i&gt;&lt;sub&gt;ON&lt;/sub&gt;/&lt;i&gt;I&lt;/i&gt;&lt;sub&gt;OFF&lt;/sub&gt; ratio of up to 3 × 10&lt;sup&gt;7&lt;/sup&gt;. The values of differential &lt;i&gt;R&lt;/i&gt;&lt;sub&gt;ON,sp&lt;/sub&gt; are calculated to be 0.44, 0.86, 1.59, 2.55 mΩ·cm&lt;sup&gt;2&lt;/sup&gt; for GaN SBDs with various values of &lt;i&gt;L&lt;/i&gt;&lt;sub&gt;AC&lt;/sub&gt; of 6, 10, 15, 20 μm, and the values of &lt;i&gt;R&lt;/i&gt;&lt;sub&gt;ON,sp&lt;/sub&gt; determined at an anode current density of 100 mA/mm are 1.27, 2.08, 3.29, 4.63 mΩ·cm&lt;sup&gt;2&lt;/sup&gt;, respectively. As the measured temperature increases from 300 to 425 K, the &lt;i&gt;I&lt;/i&gt;&lt;sub&gt;R&lt;/sub&gt; is increased only by 3 times to 1.6 × 10&lt;sup&gt;–7&lt;/sup&gt; A/mm, which shows the great potential for next-generation power electronics.