LaAlO3 Thin Films Research Articles

Effect of Co doping on the structure, optical and magnetic properties of LaAlO3 thin films

LaAl1−x Co x O3 (x = 0, 0.05 and 0.10) thin films were fabricated on quartz substrates by sol–gel method. X-ray diffraction data indicate that all thin films belong to perovskite LaAlO3, and there is no secondary phase. Two obvious Raman peaks are observed in the Raman spectra, and the 113 cm−1 peak is assigned to A1 mode of perovskite LaAlO3 while the 696 cm−1 peak is caused by the Co–O stretching vibration. The band gap of the films decreases from 5.66 to 5.40 eV with the Co composition increasing from 0 to 10 %. The magnetization of the films was investigated, and it enhances significantly with increase of the Co content.

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.

Photoluminescent and dielectric properties of Eu3+-doped LaAlO3 thin films fabricated by chemical solution deposition method

Photoluminescent (PL) and dielectric properties of La1−xEuxAlO3 thin films were investigated in terms of Eu doping content (x=0, 0.02, 0.04, 0.06, 0.08, and 0.1) and annealing temperature. The La1−xEuxAlO3 thin films were prepared by a chemical solution deposition method and characterized by X-ray diffraction, field emission scanning electron microscopy, photoluminescence and dielectric measurement. The thin films were dense with a uniform thickness, and showed bright red-orange emissions, originated from the 5D0→7F2 and 5D0→7F1 transitions of Eu3+ ions. A stronger emission of 5D0→7F2 than that of 5D0→7F1 was attributed to Eu3+-doping induced structural distortion. The strongest PL intensity was observed in the thin films with a Eu3+-doping content x of 0.06, indicating the existence of concentration quenching effect of photoluminescence. Further lifetime study of photoluminescence indicated that the concentration quenching effect was due to the lifetime decrease of 5D0→7F1 and 5D0→7F2 transitions when Eu3+-doping content x increased. In addition, highly stable dielectric-bias electric field properties of Eu3+-doped LaAlO3 thin films have been confirmed. Our study suggests that Eu3+-doped LaAlO3 thin films have potential applications in integrated thin-film optoelectronic devices.

Effect of growth induced (non)stoichiometry on the thermal conductivity, permittivity, and dielectric loss of LaAlO3 films

The effect of growth-induced non-stoichiometry on the thermal and dielectric properties of pulsed-laser deposited LaAlO3 thin films is explored. The composition of the LaAlO3 films was characterized via X-ray photoelectron spectroscopy and Rutherford backscattering spectrometry and it is revealed that small deviations in laser fluence result in deviations of cation stoichiometry as large as a few atomic percent. The thermal conductivity is also found to be especially sensitive to non-stoichiometry, with 3%-5% La-excess and La-deficiency resulting in 60%-80% reduction in thermal conductivity. The dielectric constant decreases and the loss tangent increases with increasing non-stoichiometry with differences between La-excess and La-deficiency.

Stoichiometry of LaAlO3 films grown on SrTiO3 by pulsed laser deposition

We have studied the stoichiometry of epitaxial LaAlO3 thin films on SrTiO3 substrate grown by pulsed laser deposition as a function of laser energy density and oxygen pressure during the film growth. Both x-ray diffraction (θ-2θ scan and reciprocal space mapping) and transmission electron microscopy (geometric phase analysis) revealed a change of lattice constant in the film with the distance from the substrate. Combined with composition analysis using x-ray fluorescence we found that the nominal unit-cell volume expanded when the LaAlO3 film was La-rich, but remained near the bulk value when the film was La-poor or stoichiometric. La excess was found in all the films deposited in oxygen pressures lower than 10−2 Torr. We conclude that the discussion of LaAlO3/SrTiO3 interfacial properties should include the effects of cation off-stoichiometry in the LaAlO3 films when the deposition is conducted under low oxygen pressures.

On the application of a single-crystal κ-diffractometer and a CCD area detector for studies of thin films

A multipurpose six-axis κ-diffractometer, together with the brilliance of the ESRF light source and a CCD area detector, has been explored for studying epitaxial relations and crystallinity in thin film systems. The geometrical flexibility of the six-axis goniometer allows measurement of a large volume in reciprocal space, providing an in-depth understanding of sample crystal relationships. By a set of examples of LaAlO3 thin films deposited by the atomic layer deposition technique, the possibilities of the set-up are presented. A fast panoramic scan provides determination of the crystal orientation matrices, prior to more thorough inspection of single Bragg nodes. Such information, in addition to a broadening analysis of families of single reflections, is shown to correlate well with the crystallinity, crystallite size, strain and epitaxial relationships in the thin films. The proposed set-up offers fast and easy sample mounting and alignment, along with crucial information on key features of the thin film structures.

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.

Properties of LaAlO3 thin film on GaAs(1 0 0) treated by in situ NH3 plasma

In this work, we deposited LaAlO3 film on n-type GaAs by plasma enhanced atomic layer deposition (PEALD). In situ NH3 plasma nitridation of the GaAs surface was introduced to control the interface prior to plasma enhanced atomic layer deposition of LaAlO3. The chemical bonding state of NH3 plasma treated GaAs surface were investigated by X-ray photoelectron spectroscopy (XPS), which indicate that NH3 plasma nitridation can suppress interfacial growth on GaAs substrate. Transmission electron microscopy (TEM) images show the interface between LaAlO3 and GaAs was smooth and there was not obvious interface layer observed. The capacitance–voltage (C–V) and leakage current density–voltage (J–V) measurement indicate that the NH3 plasma nitridation improved the electrical properties of LaAlO3 films on GaAs substrate.

Structural and Electronic Reconstructions at the LaAlO3/SrTiO3 Interface

A full understanding of the mechanism of the formation of a two-dimensional electron gas (2DEG) at the interface between insulating LaAlO3 (LAO) thin films and bulk SrTiO3 (STO) crystals is a prerequisite for the full exploitation of this class of materials. Here, by using a combination of advanced X-ray synchrotron-based spectroscopic and structural measurements, it is shown that a structural and electronic reconstruction of the interface occurs before the realization of the 2DEG.

Influence of burst pulses on the film topography in picosecond pulsed laser deposition of LaAlO3

Using a commercial picosecond laser in a pulsed laser deposition setup for the deposition of LaAlO3 thin films, we showed that it is possible to obtain a wide range of different surface topography just by altering the temporal output of the laser pulses. In single-pulse mode, a highly structured surface is obtained independent of the laser fluence. As the number of burst pulses is increased from 2 to 10, the surface roughness gradually decreases to almost atomically smooth as investigated with scanning electron and atomic force microscopy. Studies by X-ray photoelectron spectroscopy revealed no significant variation in the film composition indicating that the burst mode only tunes the topography without affecting other parameters. The surface roughness modification is independent of the background pressure in a wide range, which opens up the possibility of using different reactive atmospheres, which could be used to tune the properties of the material even further.

ペロブスカイト酸化物薄膜の初期成長過程

We have developed a low-temperature, high-magnetic-field scanning tunneling microscope combined with pulsedlaser deposition, and studied atomic-scale growth of SrTiO3 (STO) and LaAlO3 thin films. A (√13×√13 )-R33.7o STO(001) substrate, prepared in oxygen atmosphere, is atomically resolved and plays a crucial role in elucidating the initialgrowth process. When homoepitaxial STO films were grown on the (√13×√13 ) substrate surfaces, additional TiOx layer of the substrate was transferred to the film surfaces. Atomic-scale microscopic/spectroscopic approach opens up awayto atom-by-atom controlled oxide epitaxial films and heterostructures.

Growth Temperature Dependence of the LaAlO3/SrTiO3 Interfacial Structure

The growth temperature dependence of the interfacial structure of LaAlO3 thin films on SrTiO3 substrates is clearly observed by means of surface x-ray diffraction, combined with a holographic analysis. Although the interfacial structure that is tuned by the fabrication condition is known as a key to the control of various film properties, such a minute change in structure has not been studied in detail. Our observations show that a microscopic evaluation of the interfacial structure, as a function of the growth conditions, is achievable.

The origin of oxygen in oxide thin films: Role of the substrate

During the growth of oxide thin films by pulsed laser deposition, a strong oxygen substrate-to-film transfer has been experimentally observed for SrTiO3 and LaAlO3 thin films epitaxially grown on O18 exchanged SrTiO3 and LaAlO3 substrates by secondary ion mass spectrometry depth profiling. This oxygen transfer effect can seriously change the respective thin film properties. Taking the oxygen substrate contribution to the overall oxygen balance into account, original ways to design material properties of oxide thin films can be envisioned like a controlled charge carrier doping of SrTiO3 thin films.

High-k materials and their response to gamma ray radiation

The radiation response of four different high-k materials has been investigated by irradiating them using a 979 MBq Cs137 γ-ray source and a dose absorption rate of 0.71rad(Si)∕s. Acceptorlike electron traps and donorlike traps were observed in HfO2 and ZrO2 metal-oxide-semiconductor capacitors originating from radiation-induced defects. A lower density of donor-like traps were created in LaAlO3 and NdAlO3 capacitors, but both electron and hole trapping play a role in shifting the flat band voltage. The radiation hardness of the LaAlO3 and NdAlO3 thin films is similar to thermal SiO2 but better than the HfO2 and ZrO2.

Temperature dependence of the current conduction mechanisms in LaAlO3 thin films

Metal-oxide-semiconductor capacitors and transistors with LaAlO3 dielectric films were fabricated and the current conduction mechanisms were studied. The LaAlO3 films remained amorphous with postdeposition annealing up to 1000 °C. The leakage current density was 8.3×10−5 A/cm2 at −1 V. The low leakage current was attributed to the high barrier height of Al/LaAlO3 interface. The Al/LaAlO3 barrier height and the effective electronic mass calculated from Schottky emission and Fowler–Nordheim tunneling were 1.12 eV and 0.27m0, respectively. The dominant conduction mechanism in the temperature range of 300 K<T<420 K was space-charge-limited current, and the trapping depth was determined to be 0.36±0.1 eV.

Carrier doping in anatase TiO2 film by perovskite overlayer deposition

Heterostructures composed of perovskite overlayers on anatase TiO2 films were fabricated on LaAlO3 substrates. By depositing LaTiO3 and LaAlO3 thin films on the insulating anatase TiO2 films, metallic conduction was induced in the anatase layer with the increasing thickness of the perovskite overlayer. This perovskite overlayer deposition on TiO2 provides a method to induce free carriers, which may be applied to the development of transparent conducting oxides.

Structure Stability of LaAlO3 Thin Films on SiSubstrates

A series of amorphous and single-crystalline LaAlO3 (LAO) thinfilms are fabricated by laser molecular-beam epitaxy technique on Sisubstrates under various conditions of deposition. The structurestability of the LAO films annealed in high temperature and variousambients is studied by x-ray diffraction as well as high-resolutiontransmission electron microscopy. The results show that the epitaxialLAO films have very good stability, and the structures of amorphous LAOthin films depend strongly on the conditions of deposition andpost-annealing. The results reveal that the formation of LAOcomposition during the deposition is very important for the structurestability of LAO thin films.

TEM Investigation of Near Interface Region in Modified SrTiO3 and LaAlO3 Thin Films on SrTiO3 Substrates

TEM Investigation of Near Interface Region in Modified SrTiO3 and LaAlO3 Thin Films on SrTiO3 Substrates