LaAlO3 Films Research Articles

Polar asymmetry of La(1−δ)Al(1+δ)O3/SrTiO3 heterostructures probed by optical second harmonic generation

By combining transport measurements and optical second harmonic generation, we have investigated heterostructures formed between crystalline thin films of LaAlO3, with varying stoichiometry and TiO2-terminated SrTiO3(001) substrates. Optical second harmonic generation directly probes the polarity of these heterostructures, thus complementing the transport data. The stoichiometry and the growth temperature are found to be critical parameters for controlling both the interfacial conductivity and the heterostructure polarity. In agreement with the previous work, all of the samples display an insulator-to-metal transition in the Al-reach region, with the conductivity first increasing and then saturating at the highest Al/La ratios. The second harmonic signal also increases as a function of the Al/La ratio, but, at the highest growth temperature, it does not saturate. This unusual behavior is consistent with the formation of an ordered structure of defect dipoles in the LaAlO3 film caused by the off-centering of excess Al atoms in agreement with the theory.

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.

Electron Scattering: Nature of Electron Scattering in LaAlO3/SrTiO3 Interfaces Near the Critical Thickness (Adv. Mater. Interfaces 1/2015)

M. Lü, T. Venkatesan, and co-workers demonstrate in article 1400437 the percolation of two dimensional electron gases (2DEG) at the LaAlO3/SrTiO3 perovskite oxide interface. The fourth unit cell islands on top of a uniformly plane of a three unit cell of a LaAlO3 film generate 2DEG in the underneath SrTiO3 layer locally. When the island coverage ratio reaches a critical value of ca. 65%, 2DEG islands percolate to form a continuous conducting network.

First-principles characterization of the critical thickness for forming metallic states in strained LaAlO₃/SrTiO₃(001) heterostructure.

The emerging two-dimensional electron gas (2DEG) at the interface between polar LaAlO3 (LAO) and nonpolar SrTiO3 (STO) provides potential applications in low-dimensional nanoelectronic devices because of its exceptional electron transport property. To form 2DEG in the LAO/STO heterostructure (HS), a minimum thickness of approximately 4 unit cells of LAO is necessary. Herein, we modeled the n-type (TiO2)(0)/(LaO)(+1) HS by depositing (LAO)n (n = 4, 5, and 6) thin films on the STO substrate and explored strain effects on the critical thickness for forming 2DEG in the LAO/STO HS-based slab systems using first-principles electronic structure calculations. A vacuum layer was added along the [001] direction on the LAO film to resemble the actual epitaxial growth process of the materials. An insulator-to-metal transition is predicted in unstrained (LAO)n/STO systems when n ≥ 5. Our calculations indicate that O 2px/py states give rise to the surface conductivity, while Ti 3dxy states are responsible for the interfacial conductivity. For the tensilely strained HS system, an increased film thickness of LAO (n ≥ 6) is required to form the 2DEG, and a remarkable shift of O 2p orbitals toward higher energy in LAO layers is found, which is caused by the strain-induced change of the electrostatic potential. For the compressively strained HS system, the critical thickness of LAO film for forming 2DEG is between 5 and 6 unit cells of LAO. In addition, our calculations suggest that the interfacial charge carrier density and magnetic moment can be optimized when a moderate tensile strain is applied on the STO substrate in the ab-plane.

Preparation of thin films of perovskite-type YAlO3:Gd3+–Pr3 + UV phosphors

Thin films of the perovskite-type YAlO3:Gd3+–Pr3+ UV phosphor were deposited on a silica glass substrate, a perovskite-type SrTiO3 (001) single crystal substrate, and a LaAlO3 (001) single crystal substrate by a radio-frequency sputtering method. All thin films were noncrystalline in the as-deposited state, and crystallized after annealing at 1000°C in air. Then, films on the silica glass and SrTiO3 substrates frequently showed cracks, in contrast with films on the LaAlO3 substrates which led to smooth surfaces. Such YAlO3:Gd3+–Pr3+ thin films on LaAlO3 substrates exhibited a high crystallinity and strong preferential (00l)- and (hh0)-orientations of the crystallites. Consequently, although UV emission at 314nm from Gd3+ was observed under 217nm excitation in all annealed films, the YAlO3:Gd3+–Pr3+/LaAlO3 film gave the most intense UV emission. It was concluded that fine luminescent YAlO3:Gd3+–Pr3+ thin films were obtained especially on LaAlO3 substrates because of the small thermal expansion difference and lattice mismatch between the YAlO3 phosphor and the LaAlO3 substrate.

Tunability of conduction at the LaAlO3/SrTiO3 heterointerface: Thickness and compositional studies

The role of chemistry, film thickness, and oxygen pressure in influencing the electrical and thermal transport properties of LaAlO3/SrTiO3 heterointerfaces is explored. Unit-cell precise growth was accomplished for films between 3 and 160 unit cells thick using reflection high-energy electron diffraction-assisted pulsed-laser deposition. Subsequent temperature-dependent studies of electrical resistivity reveal three important observations: (1) by tuning the laser fluence, we can systematically tune the interfacial conductance in a step-wise manner in this system, (2) all films exhibit a critical thickness of 3–4 unit cells for the onset of conduction, and (3) the nature of the conductance is highly influenced by the stoichiometry of the LaAlO3 film with La-deficient samples showing dramatic changes with thickness, while stoichiometric and La-excess films show little dependence. Time-domain thermoreflectance studies show a diminished interfacial thermal conductance for the La-deficient films when compared to La-excess and stoichiometric films, suggesting that the interfacial conductance is more influenced by extrinsic factors such as oxygen deficiency.

Potential-well depth at amorphous-LaAlO3/crystalline-SrTiO3 interfaces measured by optical second harmonic generation

By a combination of optical second harmonic generation and transport measurements, we have investigated interfaces formed by either crystalline or amorphous thin films of LaAlO3 grown on TiO2-terminated SrTiO3(001) substrates. Our approach aims at disentangling the relative role of intrinsic and extrinsic doping mechanisms in the formation of the two-dimensional electron gas. The different nature of the two mechanisms is revealed when comparing the sample response variation as a function of temperature during annealing in air. However, before the thermal treatment, the two types of interfaces show almost the same intensity of the second harmonic signal, provided the overlayer thickness is the same. As we will show, the second harmonic signal is proportional to the depth of the potential well confining the charges at the interface. Therefore, our result demonstrates that this depth is about the same for the two different material systems. This conclusion supports the idea that the electronic properties of the two-dimensional electron gas are almost independent of the doping mechanism of the quantum well.

Bandgap Control of the Oxygen‐Vacancy‐Induced Two‐Dimensional Electron Gas in SrTiO3

We report very large bandgap enhancement in SrTiO3 (STO) films (fabricated by pulsed laser deposition below 800 {\deg}C), which can be up to 20% greater than the bulk value, depending on the deposition temperature. The origin is comprehensively investigated and finally attributed to Sr/Ti antisite point defects, supported by density functional theory calculations. More importantly, the bandgap enhancement can be utilized to tailor the electronic and magnetic phases of the two-dimensional electron gas (2DEG) in STO-based interface systems. For example, the oxygen-vacancy-induced 2DEG (2DEG-V) at the interface between amorphous LaAlO3 and STO films is more localized and the ferromagnetic order in the STO-film-based 2DEG-V can be clearly seen from low-temperature magnetotransport measurements. This opens an attractive path to tailor electronic, magnetic and optical properties of STO-based oxide interface systems under intensive focus in the oxide electronics community. Meanwhile, our study provides key insight into the origin of the fundamental issue that STO films are difficult to be doped into the fully metallic state by oxygen vacancies.

Bipolar resistance switching in transparent ITO/LaAlO₃/SrTiO₃ memristors.

We report reversible bipolar resistance switching behaviors in transparent indium-tin oxide (ITO)/LaAlO3/SrTiO3 memristors at room temperature. The memristors exhibit high optical transparency, long retention, and excellent antifatigue characteristics. The high performances are promising for employing ITO/LaAlO3/SrTiO3 memristors in nonvolatile transparent memory and logic devices. The nonvolatile resistance switching behaviors could be attributed to the migration of positively charged oxygen vacancies from the SrTiO3 substrate to the LaAlO3 film, resulting in Poole-Frenkel emission for the low resistance state and thermionic emission for the high resistance state.

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.

Influence of oxygen pressure and aging on LaAlO3 films grown by pulsed laser deposition on SrTiO3 substrates

The crystal structures of LaAlO3 films grown by pulsed laser deposition on SrTiO3 substrates at oxygen pressure of 10-3 mbar or 10-5 mbar, where kinetics of ablated species hardly depend on oxygen background pressure, are compared. Our results show that the interface between LaAlO3 and SrTiO3 is sharper when the oxygen pressure is lower. Over time, the formation of various crystalline phases is observed while the crystalline thickness of the LaAlO3 layer remains unchanged. X-ray scattering as well as atomic force microscopy measurements indicate three-dimensional growth of such phases, which appear to be fed from an amorphous capping layer present in as-grown samples.

Direct Measurement of Fusion Enthalpy of LaAlO3 and Comparison of Energetics of Melt, Glass, and Amorphous Thin Films

Enthalpy of fusion and melting temperature of perovskite LaAlO3 were measured using thermal analysis method as 124 ± 10 kJ/mol at 2134 ± 10°C, providing a value of 52 ± 4 J·(mol·K)−1 for entropy of fusion. Crystallization enthalpy of amorphous LaAlO3 thin films was found to change from −24 to −17 kJ/mol with decrease in film thickness from 100 to 20 nm. Differences in energetics of amorphous LaAlO3 films and glass cannot be explained exclusively by surface energy contribution but must reflect differences in structure between films and glasses in this system.

Fabricating superconducting interfaces between artificially grown LaAlO3 and SrTiO3 thin films

Realization of a fully metallic two-dimensional electron gas (2DEG) at the interface between artificially grown LaAlO3 and SrTiO3 thin films has been an exciting challenge. Here we present for the first time the successful realization of a superconducting 2DEG at interfaces between artificially grown LaAlO3 and SrTiO3 thin films. Our results highlight the importance of two factors—the growth temperature and the SrTiO3 termination. We use local friction force microscopy and transport measurements to determine that in normal growth conditions the absence of a robust metallic state at low temperature in the artificially grown LaAlO3/SrTiO3 interface is due to the nanoscale SrO segregation occurring on the SrTiO3 film surface during the growth and the associated defects in the SrTiO3 film. By adopting an extremely high SrTiO3 growth temperature, we demonstrate a way to realize metallic, down to the lowest temperature, and superconducting 2DEG at interfaces between LaAlO3 layers and artificially grown SrTiO3 thin films. This study paves the way to the realization of functional LaAlO3/SrTiO3 superlattices and/or artificial LaAlO3/SrTiO3 interfaces on other substrates.

Observation of strontium segregation in LaAlO3/SrTiO3 and NdGaO3/SrTiO3 oxide heterostructures by X-ray photoemission spectroscopy

LaAlO3 and NdGaO3 thin films of different thicknesses have been grown by pulsed laser deposition on TiO2-terminated SrTiO3 single crystals and investigated by soft X-ray photoemission spectroscopy. The surface sensitivity of the measurements has been tuned by varying photon energy hν and emission angle Θ. In contrast to the core levels of the other elements, the Sr 3d line shows an unexpected splitting for higher surface sensitivity, signaling the presence of a second strontium component. From our quantitative analysis we conclude that during the growth process Sr atoms diffuse away from the substrate and segregate at the surface of the heterostructure, possibly forming strontium oxide.

Conducting and insulating LaAlO3/SrTiO3 interfaces: A comparative surface photovoltage investigation

Surface photovoltage (SPV) spectroscopy, which is a versatile method to analyze the energetic distribution of electronic defect states at surfaces and interfaces of wide-bandgap semiconductor (hetero-)structures, is applied to comparatively investigate heterostructures made of 5-unit-cell-thick LaAlO3 films grown either on TiO2- or on SrO-terminated SrTiO3. As shown in a number of experimental and theoretical investigations in the past, these two interfaces exhibit dramatically different properties with the first being conducting and the second insulating. Our present SPV investigation reveals clearly distinguishable interface defect state distributions for both configurations when interpreted within the framework of a classical semiconductor band scheme. Furthermore, bare SrTiO3 crystals with TiO2 or mixed SrO/TiO2 terminations show similar SPV spectra and transients as do LaAlO3-covered samples with the respective termination of the SrTiO3 substrate. This is in accordance with a number of recent works that stress the decisive role of SrTiO3 and the minor role of LaAlO3 with respect to the electronic interface properties.

Optimization of electrical properties of Al/LaAlO3/indium tin oxide capacitor by adjusting oxygen pressures in pulsed laser deposition and applying post-deposition annealing at low temperatures

This work presents a physicochemical analysis of the characteristics of high-k lanthanum aluminate LaAlO3 (LAO) that is deposited on indium tin oxide (ITO)/glass substrates by pulsed laser deposition at various oxygen pressures and then undergoes low-temperature post-deposition annealing (PDA). Samples that are deposited at a low oxygen pressure of 0.067Pa have a higher leakage current density than samples deposited at 0.27Pa oxygen pressure owing to the imperfect stoichiometry of LAO that rises from deficient oxygen atoms in LAO. The leakage current increased with the oxygen pressure over 0.27Pa because of the degradation of either surface roughness of LAO or the interfacial layer that is composed of a mixture of metal oxides. PDA treatment at a low temperature of 200 or 300°C in O2 was applied to samples that were deposited at an oxygen pressure of 0.27Pa. These samples had a relatively low leakage current density. PDA treatment at 300°C improved the leakage current density of samples by approximately an order of magnitude at an electrical field of 1MV/cm. The incorporation of oxygen atoms during PDA at 300°C in O2 ambient increases breakdown field of capacitors with an Al/LAO/ITO structure from 7 to 14MV/cm by reducing the surface roughness of LAO and improving the stoichiometry and bond structures of the LAO films.

Modulation of conductance and superconductivity by top-gating in LaAlO3/SrTiO3 2-dimensional electron systems

We report the electrical top-gating of a 2-dimensional electron gas (2DEG) formed at the LaAlO3/SrTiO3 interface, using electron-beam evaporated Au gate electrodes. In these structures, epitaxial LaAlO3 films grown by pulsed laser deposition induce the 2DEGs at the interface to the SrTiO3 substrate and simultaneously act as the gate dielectric. The structured top-gates enable a local tuning and complete on/off switching of the interface (super-)conductivity, while maintaining the usual, intrinsic characteristics for these LaAlO3/SrTiO3 interfaces when no gate voltage is applied.

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.

Deposition and x-ray characterization of epitaxial thin films of LaAlO3

Highly epitaxial thin films of lanthanum aluminate (LaAlO3) have been obtained on strontium titanate (SrTiO3) substrates by means of atomic layer deposition using La(thd)3 (Hthd=2,2,6,6-tetramethylhepta-3,5-dione), Al(CH3)3 and ozone as precursors. The system shows a near linear relationship between pulsed and deposited composition. Thin films with stoichiometric composition have been subject to thermal annealing at 650°C under oxygen atmosphere, thereby achieving epitaxial films on Ti-O-terminated substrates of SrTiO3. The thin film||substrate epitaxial relationship is determined to be LaAlO3(100)|LaAlO3[100]||-SrTiO3(100)|SrTiO3[100] by use of synchrotron radiation. Selected films were also deposited on LaAlO3(100) to achieve homoepitaxy. This resulted in the observation of split peaks for high q-reflections, pointing towards slight differences in stoichiometry. For ultrathin films, Bragg satellites were observed around the specular reflections, coming from either surface- or interface reconstruction.

Interfacial band configuration and electrical properties of LaAlO3/Al2O3/hydrogenated-diamond metal-oxide-semiconductor field effect transistors

In order to search a gate dielectric with high permittivity on hydrogenated-diamond (H-diamond), LaAlO3 films with thin Al2O3 buffer layers are fabricated on the H-diamond epilayers by sputtering-deposition (SD) and atomic layer deposition (ALD) techniques, respectively. Interfacial band configuration and electrical properties of the SD-LaAlO3/ALD-Al2O3/H-diamond metal-oxide-semiconductor field effect transistors (MOSFETs) with gate lengths of 10, 20, and 30 μm have been investigated. The valence and conduction band offsets of the SD-LaAlO3/ALD-Al2O3 structure are measured by X-ray photoelectron spectroscopy to be 1.1 ± 0.2 and 1.6 ± 0.2 eV, respectively. The valence band discontinuity between H-diamond and LaAlO3 is evaluated to be 4.0 ± 0.2 eV, showing that the MOS structure acts as the gate which controls a hole carrier density. The leakage current density of the SD-LaAlO3/ALD-Al2O3/H-diamond MOS diode is smaller than 10−8 A cm−2 at gate bias from −4 to 2 V. The capacitance-voltage curve in the depletion mode shows sharp dependence, small flat band voltage, and small hysteresis shift, which implies low positive and trapped charge densities. The MOSFETs show p-type channel and complete normally off characteristics with threshold voltages changing from −3.6 ± 0.1 to −5.0 ± 0.1 V dependent on the gate length. The drain current maximum and the extrinsic transconductance of the MOSFET with gate length of 10 μm are −7.5 mA mm−1 and 2.3 ± 0.1 mS mm−1, respectively. The enhancement mode SD-LaAlO3/ALD-Al2O3/H-diamond MOSFET is concluded to be suitable for the applications of high power and high frequency electrical devices.