Quasi Two-dimensional Electron Gas Research Articles

Quasi two-dimensional electron gas generated by laser irradiation at rutile TiO2 surface

Two-dimensional electron gas exhibits abundant physical properties that can facilitate new physics and novel device applications. Herein, we report on a systematic investigation on rutile TiO2-based quasi two-dimensional electron gas generated by laser irradiation. A transition from nonmagnetic insulator to ferromagnetic metal has been demonstrated after laser irradiation. A joint X-ray photoelectron spectroscopy and Raman spectroscopy analysis allows us to associate the metallic ferromagnetic state with the surface formation of oxygen vacancies and Ti3+. The universality of laser irradiation implies wide potential applicability to the fabrication of multifunctional oxide devices.

Ultrafast dynamics of hot carriers in a quasi–two-dimensional electron gas on InSe

Two-dimensional electron gases (2DEGs) are at the base of current nanoelectronics because of their exceptional mobilities. Often the accumulation layer forms at polar interfaces with longitudinal optical (LO) modes. In most cases, the many-body screening of the quasi-2DEGs dramatically reduces the Fröhlich scattering strength. Despite the effectiveness of such a process, it has been recurrently proposed that a remote coupling with LO phonons persists even at high carrier concentration. We address this issue by perturbing electrons in an accumulation layer via an ultrafast laser pulse and monitoring their relaxation via time- and momentum-resolved spectroscopy. The cooling rate of excited carriers is monitored at doping level spanning from the semiconducting to the metallic limit. We observe that screening of LO phonons is not as efficient as it would be in a strictly 2D system. The large discrepancy is due to the remote coupling of confined states with the bulk. Our data indicate that the effect of such a remote coupling can be mimicked by a 3D Fröhlich interaction with Thomas-Fermi screening. These conclusions are very general and should apply to field effect transistors (FET) with high-κ dielectric gates, van der Waals heterostructures, and metallic interfaces between insulating oxides.

Solution of Self-Consistent Kohn–Sham and Poisson Equations for Quasi Two-Dimensional Electron Gas in the Accumulation Layer of Semiconductor with Nonparabolic Conduction Band

We generalize the method proposed previously for a self-consistent solution of the system of the Kohn–Sham and Poisson equations to the case of high density of electrons localized near the surface of a semiconductor due to size quantization of the electron spectrum in a potential well in the band banding region. We obtain a completely self-consistent iterative solution for a quasi two-dimensional electron gas in the accumulation layer at the surface of a semiconductor with a degenerate electron gas in the bulk. Both parabolic and non-parabolic energy spectra of conduction electrons are considered. For a parabolic conduction band, we obtain the spatial distributions of the electron density and electrostatic potential, as well as the energies of size-quantized levels and their dependence on the depth of the near-surface potential well. A significant decrease in the density of three-dimensional electrons was revealed in the region where the quasi two-dimensional electron gas occupied the accumulation layer. We calculated the dependencies of the excess electron surface density and the capacitance of the structure on the surface potential. The results are presented in dimensionless form that can be used to semi-quantitatively evaluate the parameters of accumulation layers by the known level of doping and the value of band bending. In the case of the nonparabolic conduction band, due to the finite bandgap width, the spectrum of quasi two-dimensional electron gas is obtained in the two-band Franz–Kane approximation by solving the eigenvalue problem for the new single-band equation of the effective-mass method. The density of states and the effective mass of electrons in the 2D subbands is found to increase linearly with energy. We have derived the expression that analytically approximates the calculated energy spectrum. The position of levels, the dispersion of energy spectrum, and the dependence of effective mass on energy, which were found for the size-quantized sub-bands, are in agreement with the published results for characteristics of quasi two-dimensional electrons at n-InAs surface that were directly measured by angle resolved photoelectron spectroscopy and magnetotunneling spectroscopy. Conditions are considered when the nonparabolicity of the conduction band enables quasi two-dimensional electrons to absorb the normally incident radiation due to intersubband transitions.

Investigation of an effect of electric field applied to ferroelectric film upon conductivity of quasi-two dimensional electron gas at the Ba0.8Sr0.2TiO3/LaMnO3 interface

We investigate the effect of electric field on the properties of quasi-two-dimensional electron gas at the interface between ferroelectric oxide and insulating oxide in Ba0.8Sr0.2TiO3/LaMnO3 (BSTO/LMO) heterostructure. Interface conductivity measurements were performed by a four-point probe method before and after the effect of electric field on Ba0.8Sr0.2TiO3 ferroelectric film. We have shown that when a non-uniform electric field is applied to a ferroelectric film, we can turn off the state with high conductivity. Whereas under the influence of a uniform electric field on the ferroelectric film, we can turn on the interface state of high conductivity.

Creation and control of quasi-two dimensional electron gas at yttrium aluminum oxides/strontium titanate heterointerfaces by spin coating

Despite of extensive researches about oxide heterointerfaces, preparing method and intrinsic mechanism are all challenging topics. Here, we generate a series of stable quasi two dimensional electron gases (q-2DEGs) at yttrium aluminum oxides/strontium titanate (YAO/STO) heterointerfaces by the spin coating for the first time. It is found that the thickness and the stoichiometry ratio are the key factors for the creation of q-2DEG, which is formed under the premise of thickness (about 81.1 ∼ 356.4 nm) and the rich-Al (Al:Y≥1:1). The underlying mechanism can be attributed to oxygen vacancies derived from the redox reaction between YAO film and STO substrate. In addition, the conductive behavior can be modulated by the light illumination and the value of photoinduced change reaches 153.7% at low temperature. This work paves a novel way to fabricate 2DEG massively and helpful for a deep understanding about the conductive mechanism of oxide heterostructures.

Effect of spin-orbit interaction on the vortex dynamics in LaAlO3/SrTiO3 interfaces near the superconducting transition

Controlling spin-orbit interaction and its effect on superconductivity has been a long-standing problem in two-dimensional inversion symmetry broken superconductors. An open challenge is to understand the role of various energy scales in shaping the complex phase diagram in these systems. From a combined experimental and theoretical study of resistance fluctuations and its higher order statistics, we propose a phase diagram for the superconducting phase in the magnetic-field--spin orbit interaction energy plane for the quasi-two dimensional electron gas at the interface of LaAlO$_3$/SrTiO$_3$ heterostructures. The relative variance of resistance fluctuations increases by few orders of magnitude below the spin-orbit field B$_{SO}$ and a non-Gaussian component to the fluctuations arises for fields below the upper critical field B$_{C2}$. Theoretical calculations show that the non-Gaussian noise predominantly arises due to percolative nature of the superconducting transition. We quantify the strength and the relative importance of the spin-orbit interaction energy, Zeeman energy and the pairing potential. Our work highlights the important role played by the interplay between these energy scales in framing the fascinating phases seen in two-dimensional inversion-symmetry-broken superconductors.

Coexistence of quasi-two dimensional electron and hole gas in a single tier Ca0.5TaO3/SrTiO3 oxide heterostructure

Quasi-two-dimensional electron gas has been realized at the polar-nonpolar interface of several insulating oxide heterostructures. However, its hole counterpart remains elusive. In an attempt to find a novel system that exhibits quasi-two-dimensional hole gas (q-2DHG) at the heterointerface, we adopt to materials search, first based on phenomenology followed by a comprehensive set of calculations based on first-principles density functional theory. Our studies show the epitaxial growth of cubic Ca0.5TaO3 on TiO2 terminated substrate display (q-2DHG). The hole gas emanates from the O 2p orbitals of the TiO2 layers of the substrate. On the other hand, an electron gas is formed at the (001) TaO2 top surface, thereby representing the heterostructure as a coupled quantum well system. The partial filling of the Ta 5dt2g conduction band indicates electron reconstruction, in agreement with the polar catastrophe model. Besides, a critical thickness of three monolayers is deduced from the calculations for the formation of q-2DHG in the Ca0.5TaO3/SrTiO3 heterostructure, which is consistent with the model prediction based on the modern theory of polarization. With both cubic systems, Ca0.5TaO3 and SrTiO3, having a similar underlying symmetry and minimal lattice mismatch, epitaxial growth with an abrupt interface can be well anticipated. Such a single-tier oxide heterostructure composed of separated confined hole-electron subsystems is expected to provide a platform to unravel exciting physics and also for functional devices related to oxide electronics.

Exact exchange: A pathway for a density functional theory of the integer quantum Hall effect

It is shown here that the exact-exchange (EE) formalism provides a natural and rigorous approach for a density functional theory (DFT) of the integer quantum Hall effect (IQHE). Application of a novel EE method to a quasi–two-dimensional electron gas (q2DEG) subjected to a perpendicular magnetic field leads to the following main findings. i) the microscopic exchange energy functional of the IQHE has been obtained, whose main feature being that it minimizes with a discontinuous derivative at every integer filling factor ν; ii) an analytical solution is found for the magnetic-field dependent EE potential, in the one-subband regime; iii) as a consequence of i), the EE potential display sharp discontinuities at every integer ν; and iv) the widely used local spin density approximation (LSDA) is strongly violated for filling factors close to integer values.

Patterning the two dimensional electron gas at the LaAlO3/SrTiO3 interface by structured Al capping

We demonstrate an approach for patterning a quasi-two dimensional electron gas (q-2DEG) at the interface of LaAlO3 (LAO) and SrTiO3 (STO) utilizing a structured Al capping layer. The capping of Al enables the formation of q-2DEG at the interface of 1–3 unit cells (uc) of LAO on STO, which was originally insulating before capping. The properties of the q-2DEG induced by the Al capping layer are essentially the same as those of q-2DEG without Al. Therefore, we can pattern q-2DEG by simply patterning the Al film on LAO (2 or 3 uc)/STO using a one-step liftoff process. Our approach circumvents the difficulty of direct patterning of oxide materials and provides a simple and robust patterning method for future device applications based on complex oxide interfaces.

Spin galvanic effect at the conducting SrTiO3 surfaces

The (001) surface of SrTiO3 were transformed from insulating to conducting after Ar+ irradiation, producing a quasi two-dimensional electron gas (2DEG). This conducting surface layer can introduce Rashba spin orbital coupling due to the broken inversion symmetry normal to the plane. The spin splitting of such a surface has recently been demonstrated by magneto-resistance and angular resolved photoemission spectra measurements. Here, we present experiments evidencing a large spin-charge conversion at the conducting surface. We use spin pumping to inject a spin current from NiFe film into the surface, and measure the resulting charge current. The results indicate that the Rashba effect at the surface can be used for efficient charge-spin conversion. The conversion efficiency measured by the ratio of the spin flow density to the charge flow density is about 0.23 nm. This value is on the same order as the interfaces in metallic multilayers and 2DEG of LaAlO3/SrTiO3. In this point of view, the conducting SrTiO3 surface provides another solution for oxide spintronics.

Mixed valence as a necessary criteria for quasi-two dimensional electron gas in oxide hetero-interfaces

Abstract The origin of quasi-two dimensional electron gas at the interface of polar-nonpolar oxide hetero-structure, such as LaAlO3/SrTiO3, is debated over electronic reconstruction and defects/disorder models. Common to these models is the partial valence transformation of substrate Ti ions from its equilibrium 4 + state to an itinerant 3 + state. Given that the Hf ions have a lower ionization potential than Ti due to the 4 f orbital screening, one would expect a hetero-interface conductivity in the polar-nonpolar LaAlO3/SrHfO3 system as well. However, our first principles calculations show the converse. Unlike the Ti 3 + − Ti 4 + valence transition which occur at a nominal energy cost, the barrier energy associated with its isoelectronic Hf 3 + − Hf 4 + counterpart is very high, hence suppressing the formation of quasi-two dimensional electron gas at LaAlO3/SrHfO3 hetero-interface. These calculations, therefore, emphasize on the propensity of mixed valence at the interface as a necessary condition for an oxide hetero-structure to exihibit quasi two-dimensional electron gas.

Structure and ferromagnetic instability of the oxygen-deficientSrTiO3surface

SrTiO$_3$ (STO) is the substrate of choice to grow oxide thin-films and oxide heterojunctions, which can form quasi-two-dimensional electronic phases that exhibit a wealth of phenomena, and, thus, a workhorse in the emerging field of metal-oxide electronics. Hence, it is of great importance to know the exact character of the STO surface itself under various oxygen environments. Using density functional theory within the spin generalized gradient approximation we have investigated the structural, electronic and magnetic properties of the oxygen-deficient STO surface. We find that the surface oxygen vacancies order in periodic arrays giving rise to surface magnetic moments and a quasi two-dimensional electron gas in the occupied Ti 3-d orbitals. The surface confinement, the oxygen-vacancy ordering, and the octahedra distortions give rise to spin-polarized $t_{2g}$ dispersive sub-bands; their energy split near the Brillouin zone center acts as an effective Zeeman term, which, when we turn on a Rashba interaction, produces bands with momentum-spin correlations similar to those recently discovered on oxygen deficient STO surface.

English

English

Magnetism in the Interfaces of the Sandwiched PbTiO\(_3\)/LaAlO\(_3\)/SrTiO\(_3\) Hetero-structure

We investigate the effect of PbTiO3 on the LaAlO\(_3\)/SrTiO\(_3\) hetero-structure by density functional theory, where the asymmetrysandwich not only forms a quasi-two dimensional electron gas but also reveals a ferromagnetic state in the TiO\(_2\) interfaces. Itis found out that magnetic moments of TiO\(_2\) interface layers are around 0.18 \(\mu_B\) while they are mostly negligible in the pure LaAlO\(_3\)/SrTiO\(_3\) system. Even though magnetic moments are mainly produced by Ti t2g orbitals, oxygen plays a key role in the formation of the ferromagnetic state since polar distortions primarily occur in oxygen positions of the SrTiO\(_3\) and PbTiO\(_3\) sides in opposite directions.

Transport properties and seebeck coefficient of the quasi-twodimensional electron gas in ALP quantum wells

We consider the mobility of a quasi-twodimensional electron gas in a GaP/AlP/GaP quantum well with a valley degeneracy g 1   for quantum well width L < Lc = 45.7 Å and a valley degeneracy of g 2   for quantum well width L > Lc = 45.7 Å. We calculate the mobility as a function of electron density for interface-roughness and impurity scattering with using different approximations for the local-field correction. In the case of zero temperature and Hubbard local-field correction our results reduce to those of [16]. We also study the dependence of resistivity on temperature and parallel magnetic field. The Seebeck coefficient as a function of electron concentration and quantum well width are also calculated.

Oxygen Vacancies at the γ-Al2O3/STO Heterointerface Grown by Atomic Layer Deposition

ABSTRACTWe report the epitaxial growth of γ-Al2O3 on SrTiO3 (STO) substrates by atomic layer deposition (ALD). The ALD growth of γ-Al2O3 on STO(001) single crystal substrates was performed at a temperature of 345 °C. Trimethylaluminum and water were used as co-reactants. In-situ reflection high-energy electron diffraction and ex-situ x-ray diffraction were used to determine the crystallinity of the Al2O3 films. In-situ x-ray photoelectron spectroscopy was used to characterize the Al2O3/STO heterointerface. The formation of a Ti3+ feature is observed in the Ti 2p spectrum of STO after the first few ALD cycles of Al2O3 and even after exposure of the STO substrate to trimethylaluminum alone at 345 °C. The presence of a Ti3+ feature is a direct indication of oxygen vacancies at the Al2O3/STO heterointerface, which provide the carriers for the quasi-two dimensional electron gas at the interface.

Mechanisms of charge transfer and redistribution in LaAlO3/SrTiO3 revealed by high-energy optical conductivity

In condensed matter physics the quasi two-dimensional electron gas at the interface of two different insulators, polar LaAlO3 on nonpolar SrTiO3 (LaAlO3/SrTiO3) is a spectacular and surprising observation. This phenomenon is LaAlO3 film thickness dependent and may be explained by the polarization catastrophe model, in which a charge transfer of 0.5e(-) from the LaAlO3 film into the LaAlO3/SrTiO3 interface is expected. Here we show that in conducting samples (≥ 4 unit cells of LaAlO3) there is indeed a ~0.5e(-) transfer from LaAlO3 into the LaAlO3/SrTiO3 interface by studying the optical conductivity in a broad energy range (0.5-35 eV). Surprisingly, in insulating samples (≤ 3 unit cells of LaAlO3) a redistribution of charges within the polar LaAlO3 sublayers (from AlO2 to LaO) as large as ~0.5e(-) is observed, with no charge transfer into the interface. Hence, our results reveal the different mechanisms for the polarization catastrophe compensation in insulating and conducting LaAlO3/SrTiO3 interfaces.

Unveiling mode-selected electron–phonon interactions in metal films by helium atom scattering

The quasi two-dimensional electron gas on a metal film can transmit to the surface even minute mechanical disturbances occurring in the depth, thus allowing the gentlest of all surface probes, helium atoms, to perceive the vibrations of the deepest atoms via the induced surface-charge density oscillations. A density functional perturbation theory (DFPT) and a helium atom scattering study of the phonon dispersion curves in lead films of up to 7 mono-layers on a copper substrate show that: (a) the electron-phonon interaction is responsible for the coupling of He atoms to in-depth phonon modes; and (b) the inelastic HAS intensity from a given phonon mode is proportional to its electron-phonon coupling. The direct determination of mode-selected electron-phonon coupling strengths has great relevance for understanding superconductivity in thin films and two-dimensional systems.

Nano-patterning of the electron gas at the LaAlO3/SrTiO3 interface using low-energy ion beam irradiation

The quasi-two dimensional electron gas formed at the interface between LaAlO3 (LAO) and SrTiO3 (STO) shows fascinating properties, such as two-dimensional superconductivity, giant electric field effect, and the possible co-existence of ferromagnetic and superconducting phases. In this work, we demonstrate that the conducting LAO/STO interface can be made insulating after short irradiation by a beam of low energy Ar+ ions. The irradiation process does neither result in physical removal of the LAO film nor produces oxygen vacancies in the STO layer. Using electron beam lithography and low ion beam energy irradiation, we fabricated conducting nano-structures in the LAO/STO interface with dimensions down to 50 nm. Such a reliable and robust method of nano-patterning may be a prerequisite for future electronic applications of the LAO/STO interface.

Oxide Nanoelectronics on Demand

Electronic confinement at nanoscale dimensions remains a central means of science and technology. We demonstrate nanoscale lateral confinement of a quasi-two-dimensional electron gas at a lanthanum aluminate-strontium titanate interface. Control of this confinement using an atomic force microscope lithography technique enabled us to create tunnel junctions and field-effect transistors with characteristic dimensions as small as 2 nanometers. These electronic devices can be modified or erased without the need for complex lithographic procedures. Our on-demand nanoelectronics fabrication platform has the potential for widespread technological application.