LaAlO3 Thickness Research Articles

Optical Second Harmonic Generation on LaAlO3/SrTiO3 Interfaces: A Review.

As we approach the limits of semiconductor technology, the development of new materials and technologies for the new era in electronics is compelling. Among others, perovskite oxide hetero-structures are anticipated to be the best candidates. As in the case of semiconductors, the interface between two given materials can have, and often has, very different properties, compared to the corresponding bulk compounds. Perovskite oxides show spectacular interfacial properties due to the the rearrangement of charges, spins, orbitals and the lattice structure itself, at the interface. Lanthanum aluminate and Strontium titanate hetero-structures (LaAlO3/SrTiO3) can be regarded as a prototype of this wider class of interfaces. Both bulk compounds are plain and (relatively) simple wide-bandgap insulators. Despite this, a conductive two-dimensional electron gas (2DEG) is formed right at the interface when a LaAlO3 thickness of n≥4 unit cells is deposited on a SrTiO3 substrate. The 2DEG is quite thin, being confined in only one or at least very few mono-layers at the interface, on the SrTiO3 side. A very intense and long-lasting study was triggered by this surprising discovery. Many questions regarding the origin and characteristics of the two-dimensional electron gas have been (partially) addressed, others are still open. In particular, this includes the interfacial electronic band structure, the transverse plane spatial homogeneity of the samples and the ultrafast dynamics of the confined carriers. Among a very long list of experimental techniques which have been exploited to study these types of interfaces (ARPES, XPS, AFM, PFM, …and many others), optical Second Harmonic Generation (SHG) was found to be suitable for investigating these types of buried interfaces, thanks to its extreme and selective interface-only sensitivity. The SHG technique has made its contribution to the research in this field in a variety of different and important aspects. In this work we will give a bird's eye view of the currently available research on this topic and try to sketch out its future perspectives.

Confinement‐Enhanced Rashba Spin–Orbit Coupling at the LaAlO3/KTaO3 Interface via LaAlO3 Thickness Control

Superconductors with strong spin–orbit coupling (SOC) have attracted tremendous attention due to the great potential for realizing topological superconductivity and searching Majorana fermions. Recently, two‐dimensional (2D) superconductivity with a relatively high T c ≈ 2 K is observed in LaAlO3/KTaO3 (111) and EuO/KTaO3 (111) heterointerfaces. However, the experimental investigations of the SOC in the KTaO3 (111)‐based heterointerfaces are still lacking. Herein, the Rashba SOC of LaAlO3/KTaO3 (111) interfaces with varied LaAlO3 thickness is studied. It is revealed that the spatial confinement of the 2D electron gas tuned by the LaAlO3 thickness prominently enhances the SOC. The maximal Rashba spin‐splitting energy Δso reaches ≈48 meV in this work, which is an extremely high value ever reported in oxide‐based conducting heterointerfaces.

In situ investigation of conducting interface formation in LaAlO3/SrTiO3 heterostructure

The high-mobility conducting interface (CI) between LaAlO3 (LAO) and SrTiO3 (STO) has revealed many fascinating phenomena, including exotic magnetism and superconductivity. But, the formation mechanism of the CI has not been conclusively explained. Here, using in situ angle-resolved photoemission spectroscopy, we elucidated the mechanisms for the CI formation. In as-grown samples, we observed a built-in potential (Vbi) proportional to the polar LAO thickness starting from the first unit cell (UC) with CI formation appearing above 3 UCs. However, we found that the Vbi is removed by synchrotron ultraviolet (UV)-irradiation; The built-in potential is recovered by oxygen gas (O2(g))-exposure. Furthermore, after UV-irradiation, the CI appears even below 3UC of LAO. Our results demonstrate not only the Vbi-driven CI formation in as-grown LAO/STO, but also a new route to control of the interface state by UV lithographic patterning or other surface modification.

Tunable Magnetic Scattering Effects at the LaAlO3/SrTiO3 Interface by Ionic Liquid Gating

The gating effect achieved by an ionic liquid and its electric double layer allows for charge transfer, which can be an order of magnitude larger than that with conventional dielectrics. However, large charged ions also cause inevitable Coulomb scattering in the conducting channel formed at the interface, which can limit the carrier mobility enhancement. In this work, we study the effect of LaAlO3 thickness on the transport properties in LaAlO3/SrTiO3 heterostructures by ionic liquid gating. We find that the transport properties of the LaAlO3/SrTiO3 interface are dominated by intrinsic interactions rather than LaAlO3 thickness and possible effects of ions in the liquid. We observe a Kondo effect, which is enhanced while increasing the gate voltage. For the smallest thickness of the LAO layer, we also observe a gate-tunable and temperature-dependent anomalous Hall effect, which always emerges near the Kondo temperature. Our experiments pave the way to manipulate the various magnetic interactions at the LaAlO3/SrTiO3 interface.

Inevitable high density of oxygen vacancies at the surface of polar–nonpolar perovskite heterostructures LaAlO3/SrTiO3

The density of polarity-induced oxygen vacancies (VOs) at the LaAlO3 (LAO) surface of LAO/SrTiO3 (STO) (001) heterostructures (HSs) and the density of induced carriers at the interface are quantitatively investigated. Using chemical equilibrium conditions combined with density-functional-theory calculations of total energy, we set up a model for the densities of the VOs and the carriers, which are a function of the thickness of the LAO film, oxygen pressure, and temperature during the LAO/STO HSs growth. For the HSs with over three LAO unit-cell layers, our results show that the presence of a high density (∼1014cm−2) of the VOs is inevitable, even for the HSs grown at high oxygen pressures, and the densities of the VOs and the carriers mainly depend on the LAO thickness and slightly on the oxygen pressure and temperature. Our results also demonstrate that the intrinsic doping cannot occur. The stability of the VOs under high oxygen pressures is attributed to the release of electrostatic energy from the polar electric field in the LAO film.

Interface Local Magnetic Moment and Its Near Periodic Modulation in Oxide SrRuO3|LaAlO3 Heterojunctions: An <italic>Ab Initio</italic> Investigation

Using density functional theory, the interface local magnetic moment and its variation are explored in SrRuO3|LaAlO3|vacuum (001) heterostructures with increasing number of LaAlO3 unit cells. The computed magnetic moment projected on interface planes is found to modulate nearly periodically as the number of LaAlO3 unit cells is increased in SrRuO3|LaAlO3|vacuum structure. The magnetic moment localized at the interface RuO2 plane shifts from a lower value of $\sim 0.7~\mu _{B}$ to a higher value of $\sim 1.35~\mu _{B}$ and changes by ~42% as LaAlO3 changes from non-stoichiometric to stoichiometric with increase in its thickness. However, the magnitude shifts from higher to lower value as LaAlO3 changes from stoichiometric to non-stoichiometric with further increase in thickness. The large variation and nearly periodic modulation of magnetic moment localized at the interface with increasing LaAlO3 thickness arise due to spin-dependent screening of effective total charge of LaAlO3 film by accumulated free carriers at the interface in heterostructure. The modulation of interface local magnetic moment using ultrathin LaAlO3 film at the oxide heterointerfaces can be exploited as an additional functionality, in conjunction with those reported recently such as control of band alignment and enhancement of the electric conductivity at the interface.

Thickness Dependence of Magnetic Behavior of LaAlO3/SrTiO3 Heterostructures

AbstractThickness dependence of magnetic properties of LaAlO3 (LAO)/SrTiO3 (STO) heterostructures is systematically studied. It is found that the magnetic moment is rather robust at elevated temperatures in the samples grown under ultralow PO2 of 1 × 10−8 Torr, indicating room‐temperature ferromagnetism in the LaAlO3/SrTiO3 heterostructures. Spin‐glass‐like behavior is observed in the LAO/STO heterostructures with the LAO thickness ranging from 5 to 90 unit cells (2–35.2 nm). As the LAO layer becomes thicker, the blocking temperature of spin‐glass‐like behavior decreases and magnetic moment weakens, which may be related to the reduced density of oxygen vacancies at the interfacial TiO2 layer. It is believed that the well‐controlled magnetism by tuning the LAO thickness may open an avenue for the potential application of spintronics with LaAlO3/SrTiO3 heterostructures.

Growth and Electrostatic/chemical Properties of Metal/LaAlO3/SrTiO3 Heterostructures.

The quasi 2D electron system (q2DES) that forms at the interface between LaAlO3 (LAO) and SrTiO3 (STO) has attracted much attention from the oxide electronics community. One of its hallmark features is the existence of a critical LAO thickness of 4 unit-cells (uc) for interfacial conductivity to emerge. Although electrostatic mechanisms have been proposed in the past to describe the existence of this critical thickness, the importance of chemical defects has been recently accentuated. Here, we describe the growth of metal/LAO/STO heterostructures in an ultra-high vacuum (UHV) cluster system combining pulsed laser deposition (to grow the LAO), magnetron sputtering (to grow the metal) and X-ray photoelectron spectroscopy (XPS). We study step by step the formation and evolution of the q2DES and the chemical interactions that occur between the metal and the LAO/STO. Additionally, magnetotransport experiments elucidate on the transport and electronic properties of the q2DES. This systematic work not only demonstrates a way to study the electrostatic and chemical interplay between the q2DES and its environment, but also unlocks the possibility to couple multifunctional capping layers with the rich physics observed in two-dimensional electron systems, allowing the fabrication of new types of devices.

Insulator-to-Metal Transition at Oxide Interfaces Induced by WO3 Overlayers.

Interfaces between complex oxides constitute a unique playground for two-dimensional electron systems (2DESs), where superconductivity and magnetism can arise from combinations of bulk insulators. The 2DES at the LaAlO3/SrTiO3 interface is one of the most studied in this regard, and its origin is determined by the polar field in LaAlO3 as well as by the presence of point defects, like oxygen vacancies and intermixed cations. These defects usually reside in the conduction channel and are responsible for a decrease of the electronic mobility. In this work, we use an amorphous WO3 overlayer to obtain a high-mobility 2DES in WO3/LaAlO3/SrTiO3 heterostructures. The studied system shows a sharp insulator-to-metal transition as a function of both LaAlO3 and WO3 layer thickness. Low-temperature magnetotransport reveals a strong magnetoresistance reaching 900% at 10 T and 1.5 K, the presence of multiple conduction channels with carrier mobility up to 80 000 cm2 V-1 s-1, and quantum oscillations of conductance.

La interstitial defect-induced insulator-metal transition in the oxide heterostructures LaAlO3/SrTiO3

Perovskite oxide interfaces have attracted tremendous research interest for their fundamental physics and promising all-oxide electronics applications. Here, based on first-principles calculations, we propose a novel surface La interstitial promoted interface insulator-metal transition in LaAlO3/SrTiO3 (110). Compared with surface oxygen vacancies, which play a determining role on the insulator-metal transition of LaAlO3/SrTiO3 (001) interfaces, we find that surface La interstitials can be more experimentally realistic and accessible for manipulation and more stable in ambient atmospheric environment. Interestingly, these surface La interstitials also induce significant spin-splitting states with Ti dyz/dxz character at conducting LaAlO3/SrTiO3 (110) interface. On the other hand, for insulating LaAlO3/SrTiO3 (110) (<4 unit cells LaAlO3 thickness), a distortion between La (Al) and O atoms is found at the LaAlO3 side, partially compensating the polarization divergence. Our results reveal the origin of metal-insulator transition in LaAlO3/SrTiO3 (110) heterostructures, and also shed light on the manipulation of the superior properties of LaAlO3/SrTiO3 (110) for new possibilities of electronic and magnetic applications.

Tuning Up or Down the Critical Thickness in LaAlO3 /SrTiO3 through In Situ Deposition of Metal Overlayers.

The quasi 2D electron system (q2DES) that forms at the interface between LaAlO3 and SrTiO3 has attracted much attention from the oxide electronics community. One of its hallmark features is the existence of a critical LaAlO3 thickness of 4 unit-cells (uc) for interfacial conductivity to emerge. In this paper, the chemical, electronic, and transport properties of LaAlO3 /SrTiO3 samples capped with different metals grown in a system combining pulsed laser deposition, sputtering, and in situ X-ray photoemission spectroscopy are investigated. The results show that for metals with low work function a q2DES forms at 1-2 uc of LaAlO3 and is accompanied by a partial oxidation of the metal, a phenomenon that affects the q2DES properties and triggers the formation of defects. In contrast, for noble metals, the critical thickness is increased above 4 uc. The results are discussed in terms of a hybrid mechanism that incorporates electrostatic and chemical effects.

Off-Stoichiometry Driven Carrier Density Variation at the Interface of LaAlO3/SrTiO3

The interface between LaAlO3 (LAO) and SrTiO3 (STO) has attracted enormous interests due to its rich physical phenomena, such as metallic nature, magnetism and superconductivity. In this work, we report our experimental investigations on the influence of the LAO stoichiometry to the metallic interface. Taking advantage of the oxide molecular beam epitaxy (MBE) technique, a series of high quality LAO films with different nominal La/Al ratios and LAO thicknesses were grown on the TiO2-terminated STO substrates, where systematic variations of the LAO lattice constant and transport property were observed. In particular, the sheet density can be largely reduced by nearly an order of magnitude with merely about 20% increase in the nominal La/Al ratio. Our finding provides an effective method on tuning the electron density of the two-dimensional electron liquid (2DEL) at the LAO/STO interface.

Electronic defect states at the LaAlO3/SrTiO3 heterointerface revealed by O K-edge X-ray absorption spectroscopy.

Interfaces of two dissimilar complex oxides exhibit exotic physical properties that are absent in their parent compounds. Of particular interest is insulating LaAlO3 films on an insulating SrTiO3 substrate, where transport measurements have shown a metal-insulator transition as a function of LaAlO3 thickness. Their origin has become the subject of intense research, yet a unifying consensus remains elusive. Here, we report evidence for the electronic reconstruction in both insulating and conducting LaAlO3/SrTiO3 heterointerfaces revealed by O K-edge X-ray absorption spectroscopy. For the insulating samples, the O K-edge XAS spectrum exhibits features characteristic of electronically active point defects identified as noninteger valence states of Ti. For conducting samples, a new shape-resonance at ∼540.5 eV, characteristic of molecular-like oxygen (empty O-2p band), is observed. This implies that the concentration of electronic defects has increased in proportion with LaAlO3 thickness. For larger defect concentrations, the electronic defect states are no longer localized at the Ti orbitals and exhibit pronounced O 2p-O 2p character. Our results demonstrate that, above a critical thickness, the delocalization of O 2p electronic states can be linked to the presence of oxygen vacancies and is responsible for the enhancement of conductivity at the oxide heterointerfaces.

LaAlO3 thickness window for electronically controlled magnetism at LaAlO3/SrTiO3 heterointerfaces

Complex-oxide heterostructures exhibit rich physical behavior such as emergent conductivity, superconductivity, and magnetism that are intriguing for scientific reasons as well as for potential technological applications. It was recently discovered that in-plane magnetism at the LaAlO3/SrTiO3 (LAO/STO) interface can be electronically controlled at room temperature. Here, we employ magnetic force microscopy to investigate electronically controlled ferromagnetism at the LAO/STO interface with LAO thickness t varied from 4 unit cell (u.c.) to 40 u.c. Magnetic signatures are observed only within a thickness window 8 u.c. ≤ t ≤ 25 u.c. Within this window, the device capacitance corresponds well to the expected geometric value, while for thicknesses outside this window, the capacitance is strongly suppressed. The ability to modulate electronic and magnetic properties of LAO/STO devices depends on the ability to control carrier density, which is in turn constrained by intrinsic tunneling mechanisms.

Gate Tunable In‐ and Out‐of‐Plane Spin–Orbit Coupling and Spin‐Splitting Anisotropy at LaAlO3/SrTiO3 (110) Interface

Manipulating spin–orbit coupling (SOC) is important for devices such as spin–orbit torque‐based memory and its understanding is necessary to answer several fundamental open questions in triplet state superconductivity, topological insulators, and Majorana fermions. Here spin splitting of 25 meV is reported at the LaAlO3/SrTiO3 (110) interface for in‐plane spins at a current density of 1.4 × 104 A cm−2, which is large compared to that found in semiconductor heterostructures or the LaAlO3/SrTiO3 (100) interface, and in addition it is anisotropic. The anisotropy arises from the difference in electron effective mass along the [001] and [1–10] directions. Our study predicts a spin‐splitting energy &gt;1000 meV at a current density of 107 A cm−2, which is enormous compared to metallic systems and will be an ideal spin‐polarized source. In addition to the in‐plane effect, there is an unexpected gate‐tunable out‐of‐plane SOC at the LaAlO3/SrTiO3 (110) interface when the spins lie out‐of‐plane due to broken symmetry in the plane of the interface. It is demonstrated that this can be manipulated by varying the LaAlO3 thickness showing that this interface can be engineered for spin–orbit torque devices.

In situ optical characterization of LaAlO3 epitaxy on SrTiO3(001)

We followed the growth of LaAlO3 (LAO) on TiO2-terminated SrTiO3(001) in situ using a combination of a special monochromatic ellipsometry at photon energy of 1.96 eV and reflection high-energy electron diffraction (RHEED). We find that the phase of the ellipsometric ratio, defined as , changes linearly with the LAO film thickness. From the 4th unit cell (uc) up to the 11th unit cell, the slope of change in Δ is different from that for the initial three unit cells and yet there is no abrupt change in Δ when the LAO thickness increases from 3 uc to 4 uc. We explore structural and electronic processes in the LaAlO3-SrTiO3system that may be responsible for such an in situ observed optical response.

Suppression of the critical thickness threshold for conductivity at the LaAlO3/SrTiO3 interface.

Perovskite materials engineered in epitaxial heterostructures have been intensely investigated during the last decade. The interface formed by an LaAlO3 thin film grown on top of a TiO2-terminated SrTiO3 substrate hosts a two-dimensional electronic system and has become the prototypical example of this field. Although controversy exists regarding some of its physical properties and their precise origin, it is universally found that conductivity only appears beyond an LaAlO3 thickness threshold of four unit cells. Here, we experimentally demonstrate that this critical thickness can be reduced to just one unit cell when a metallic film of cobalt is deposited on top of LaAlO3. First-principles calculations indicate that Co modifies the electrostatic boundary conditions and induces a charge transfer towards the Ti 3d bands, supporting the electrostatic origin of the electronic system at the LaAlO3/SrTiO3 interface. Our results expand the interest of this low-dimensional oxide system from in-plane to perpendicular transport and to the exploration of elastic and inelastic tunnel-type transport of (spin-polarized) carriers.

Transport properties of a quasi-two-dimensional electron system formed inLaAlO3/EuTiO3/SrTiO3heterostructures

We study the structural, magnetic, and transport properties of LaAlO3/EuTiO3/SrTiO3 heterostructures grown by pulsed laser deposition. The samples are characterized in situ by electron diffraction and scanning probe microscopy and ex situ by transport measurements and x-ray absorption spectroscopy. LaAlO3/EuTiO3/SrTiO3 films show a ferromagnetic transition at T <= 7.5 K, related to the ordering of Eu2+ spins, even in samples characterized by just two EuTiO3 unit cells. A finite metallic conductivity is observed only in the case of samples composed by one or two EuTiO3 unit cells and, simultaneously, by a LaAlO3 thickness greater than or equal to four unit cells. The effect of ferromagnetic EuTiO3 on the transport properties of delta-doped LaAlO3/EuTiO3/SrTiO3 is critically discussed.

Synthesis of LaAlO3 by metal organic decomposition on SrTiO3 substrates

LaAlO3 (LAO) films on SrTiO3 (STO) substrates have been produced by metal organic decomposition using La(C5H7O2)3 and Al(C5H7O2)3 as precursors dissolved in propionic acid. The process consists of growing thin layers through dip coating and subsequent annealing. After testing different cationic ratios of La and Al, it was determined that an optimal ratio leads to a single LAO phase film that grows epitaxially (cube on cube) on top of the STO. This was shown by X-ray diffraction (XRD) and transmission electron microscopy (TEM) analyses. These analyses, as well as additional X-ray reflectivity analysis, also revealed that the LAO's thickness obtained in one dip ranges from 8 nm to 16 nm. Taking advantage of the epitaxial conditions, several layers can be stacked by successive dip coatings and annealing to form an epitaxial structure.

Quasi-two-dimensional electron gas behavior in doped LaAlO3 thin films on SrTiO3 substrates

We have demonstrated the growth of Tm and Lu doped LaAlO3 epitaxial thin films on single crystal (001) SrTiO3 substrates. These rare-earth dopants potentially act as sources of localized moment and spin-orbit scattering centers at the interface. Through structural and chemical characterization, we confirm the incorporation of Tm and Lu dopants into highly crystalline LaAlO3 films. The rare earth doping of the La site does not significantly modify the sheet carrier concentration or mobility compared to undoped samples despite the evolution of sheet carrier concentration, mobility, and sheet resistance with LaAlO3 thickness in undoped LaAlO3 films on SrTiO3.