LaAlO3 Layer Research Articles

Studies of the interfacial structure of LaAlO3 thin films on silicon by x-ray reflectivity and angle-resolved x-ray photoelectron spectroscopy

The microstructures of amorphous LaAlO3 thin films deposited on silicon substrates by the laser molecular-beam epitaxy were studied by the x-ray reflectivity and the angle-resolved x-ray photoelectron spectroscopy. It was shown that the film∕substrate interface contains a La-rich LaxAlyOzSi layer and a SiOx layer. It was shown that the electron density of the LaAlO3 layer and the LaxAlyOzSi layer is not homogeneous along the growth direction due to the diffusion of La, Al, and Si. The growth kinetics of the LaAlO3 film was described by three processes: (1) formation of the SiOx layer at the early stage whose thickness saturates rapidly at about 13Å; (2) formation of the LaxAlyOzSi layer by the out diffusion of Si and the inner diffusion of La, Al (mostly La). This stage continues as the film grows (3) In the deposition process of LaAlO3, the distributions of La and Al in the LaAlO3 layer change from inhomogeneous to homogeneous.

LaAlO3 films prepared by MBE on LaAlO3(001) and Si(001) substrates

The aim of this work was to investigate the potentiality of molecular beam epitaxy techniques to prepare high-κ LaAlO3 (LAO) films on silicon (001). First, the homoepitaxial growth of LAO was demonstrated for growth temperatures higher than 520 °C. Then, amorphous LAO films were prepared on p-type Si(001) substrates with no interfacial SiO2 formation. LAO layers directly grown on Si(001) appear to be single crystalline at growth temperatures higher than 600 °C but show interfacial reactions. Electrical measurements demonstrate that the growth temperature has a critical impact on 1eakage current. LAO films grown under atomic oxygen have a higher permittivity and a lower charge density than films grown under molecular oxygen.

Atomic-scale characterization of perovskite superlattice using chemical lattice imaging and spatially resolved electron energy-loss spectroscopy

The layered structure of a perovskite tricolor superlattice {SrTiO3(STO)∕LaAlO3(LAO)∕La0.6Sr0.4MnO3(LSMO)}n on an STO substrate is investigated using transmission electron microscopy (TEM) and electron energy-loss spectroscopy (EELS). Conventional TEM imaging is not effective for thickness evaluation, in which both LAO and LSMO layers show the same image contrast because of the similarity in their mean atomic numbers. We observe 001 lattice fringes, which are chemically sensitive in the case of the perovskite structure ABO3, since the 001 structure factor is proportional to the difference between A- and B-site atomic scattering factors. The thickness of each layer is evaluated with an accuracy of one unit cell. We also apply spatially resolved EELS to analyze the depth profile of Ti, Mn, and La contents.

Perovskite oxide tricolor superlattices with artificially broken inversion symmetry by interface effects

We have designed and fabricated spin superlattices composed of three kinds of perovskite oxides: ferromagnetic La0.6Sr0.4MnO3 (LSMO) sandwiched by two nonmagnetic insulators, SrTiO3 (STO), and LaAlO3 (LAO). Conventional superlattices, structured as alternating stack of two components, preserve centrosymmetry. By stacking three compounds repeatedly in an asymmetric sequence such as (LAO/LSMO/STO)n, one can break the inversion symmetry. Magnetic and transport properties of the superlattices clearly indicate that the STO and LAO layers modulate the ferromagnetic spin order in LSMO layers adjacent to the interfaces in a different manner, giving rise to broken inversion symmetry of magnetic structure in the (LAO/LSMO/STO)n superlattices.

C–V characteristics of Pt/PbZr0.53Ti0.47O3/LaAlO3/Si and Pt/PbZr0.53Ti0.47O3/La0.85Sr0.15CoO3/LaAlO3/Si structures for ferroelectric gate FET memory

Pt/PbZr0.53Ti0.47O3 (PZT)/LaAlO3 (LAO)/Si and Pt/PbZr0.53Ti0.47O3/La0.85Sr0.15CoO3 (LSCO)/LaAlO3/Si structures for ferroelectric field effect memory applications were fabricated on n-type Si substrate by pulsed laser deposition (PLD). The Auger electron spectrometry (AES) analysis shows that a LaAlO3 buffer layer can effectively prevent Si and Ti, Pb interdiffusion between PZT and Si substrate. For both of the structure, the current density–voltage measurement shows a typical leakage current density of about 10−7A/cm2 at 8V applied voltage. Furthermore, it has been demonstrated that the PbZr0.53Ti0.47O3/LaAlO3/Si structures and Pt/PbZr0.53Ti0.47O3/La0.85Sr0.15CoO3/LaAlO3/Si structures exhibit ferroelectric switching properties, showing a memory window as large as 2 and 2.9V, respectively, under a ramp rate of 200mV/s from −6 to +6V driving voltage at 1MHz. It is believed that the La0.85Sr0.15CoO3 buffer layer deposited on LaAlO3 layer can improve the crystalline properties of PZT films, and then result in lager polarization of PZT and lager memory windows for Pt/PbZr0.53Ti0.47O3/La0.85Sr0.15CoO3/LaAlO3/Si structures.

Dielectric tunability of coherently strained LaAlO3/SrTiO3 superlattices

Dielectric LaAlO3 and SrTiO3 thin films and LaAlO3/SrTiO3 multilayers were grown epitaxially by pulsed laser deposition on (001) oriented (LaAlO3)0.3(Sr2AlTaO6)0.7 substrates. Their structural characterization was carried out by x-ray diffraction and cross section transmission electron microscopy, which allowed us to determine the degree of strain in the dielectric material. For a film thickness of 200 nm we observed significant structural relaxation of the LaAlO3 and SrTiO3 single layers toward their single crystal lattice parameters in contrast to LaAlO3/SrTiO3 multilayer structures, where the dielectric material remained coherently strained. The influence of strain on the dielectric properties was studied by impedance spectroscopy in the frequency range of 40 Hz–10 MHz at room temperature. The measurements were performed on parallel plate capacitors, using epitaxial La0.4Sr0.6CoO3 films as bottom and top electrodes. The dielectric constant ε of partially relaxed and coherently strained material was nearly the same. However, the dielectric tunability, i.e., the influence of a direct current bias voltage on ε, was found to be significantly larger for coherently strained dielectrics. For [LaAlO3(30 Å)/SrTiO3(60 Å)]20 multilayers we observed a tunability of nearly 20% at room temperature for a bias voltage of only 1 V, corresponding to an electric field strength of 50 kV/cm. The total dielectric loss of the multilayer capacitors is below 1% for frequencies above 1 MHz and depends only slightly on the bias voltage.

Dispersion characteristics of magnetostatic waves propagating at a ferrite/perovskite interface

The propagation of magnetostatic waves at the interface between an yttrium-iron-garnet (YIG) film and a layer of perovskite compound LaAlO3, SrTiO3, or YBa2Cu3O7−x (YBCO) was experimentally studied. The microwave dispersion characteristics were measured at room temperature and at 77 K. An effective method for controlling the propagation speed and the phaseshift of magnetostatic waves in the YIG/YBCO structures is suggested. Specifically, the phaseshift at 3 GHz is changed by more than 1.5π by reducing the critical current density from 1 to 0.7× 106 A/cm2.

Effects of homo-epitaxial LaAlO3 layer on microstructural properties of SrTiO3 films grown on LaAlO3 substrates

Effects of homo-epitaxial LaAlO3 (LAO) layer on microstructural properties of SrTiO3 (STO) films grown on LAO substrates have been investigated. The STO films were deposited by pulsed laser deposition (PLD) on LAO (100) substrate with and without a homo-epitaxial LAO (HE–LAO) layer. The microstructure of the STO films was examined by X-ray diffraction (XRD), transmission electron microscopy (TEM) and high-resolution electron microscopy (HREM). TEM and HREM studies indicate the HE–LAO layer contains a much higher density of structural defects than the LAO substrate and the STO films grown on its top. In addition, the STO films deposited with the HE–LAO layer have a reduced planar defect (boundary) density relative to those films deposited directly on the LAO substrate. The reduced planar defect density in the STO film is strongly correlated with the previously observed improvement in microwave properties of devices based on these films.

Microstructural study of growth of a YBa2Cu3O7−x/LaAlO3/YBa2Cu3O7−x trilayered film by pulsed laser deposition

The growth process of a YBCO/LaAlO3/YBCO trilayered film made by pulsed laser deposition has been studied by high resolution transmission electron microscopy (HRTEM). The high resolution images of the cross-section samples have shown that a 7 nm layer of LaAlO3 has been grown epitaxially between c-axis oriented YBCO layers having the nominal thickness of 250 nm. A stacking fault in the LaAlO3 layer may introduce a stacking fault into the YBCO layer, which may form nucleation sites for α-axis oriented grains. A second phase had been formed at the interface between the LaAlO3 layer and the lower YBCO layer, which has been identified by image simulation and energy dispersive x-ray (EDX) analysis as a new tetragonal La–Al–Cu–O phase based on LaAlO3 in which some of Al atoms have been replaced by Cu. The approximate lattice parameters of the new phase are a = 0.38 nm and c = 0.76 nm. However, no second phase was found at the interface between the lower YBCO layer and the LaAlO3 substrate.

Origins of conductive losses at microwave frequencies in YBa2Cu3O7??/LaAlO3/YBa2Cu3O7?? trilayers deposited by pulsed laser deposition

Oriented YBa2Cu3O7−δ/LaAlO3/YBa2Cu3O7−δ trilayers were deposited by pulsed laser deposition (PLD) onto 〈100〉 MgO and LaAlO3. Film thicknesses varied from 2000–5000 A/ layer. A comparision of structure and transport data for the bottom and top superconducting layers indicated a slight decrease in film quality for the top superconducting layer. The critical temperature was lower for the top superconducting layer (90.5 vs. ∼90 K) and the microwave surface resistance was higher (increasing from ∼2 to 18 mω at 36 GHz, 20 K). The resistivity of the dielectric was estimated to be 106 ω cm, and the loss tangent of the dielectric film at microwave frequencies had an upper limit of 0.01. Cross-sectional TEM analysis of the trilayer structure showed a high density of threading dislocations in the dielectric layer that appeared to nucleate at steps in the underlying superconducting layer. The threading dislocations may serve as conduction paths in the LaAlO3 layer.

Temperature Dependent Growth of LaAIO3 Films on YBa2Cu3O7 C-Axis Films for Multilayer Structures

AbstractFabrication of ultra smooth films, free of micro-shorts, is essential to the development of High Temperature Superconducting (HTS) thin film devices. One such example is a SNS junction consisting of two HTS layers separated by a uniformly smooth continuous barrier material. Other schemes under consideration require multilayer structures of up to 5 - 7 epitaxially grown layers of complex oxide material. Successful fabrication of such devices necessitates understanding the epitaxial growth of polycrystalline oxide films on polycrystalline film templates. Toward this end we have developed a set of deposition parameters that produce high quality epitaxial insulating layers suitable for HTS device applications. All films in this study were grown by off-axis RF magnetron sputter deposition. LaAlO3 films were deposited over MgO grown YBa2Cu3O7 (YBCO) c-axis thin films at temperatures ranging from 200 to 700C and on virgin substrates at 600C. Atomic Force Microscopy, eddy current measurements, and x-ray diffraction techniques were used to monitor the effect of growth conditions on the resulting film crystallinity, nanostructure, and electrical properties.Ex-situ interrupted growth characterization of these materials has yielded new insight into the processes that control the growth mechanism and resulting microstructure. All films were polycrystalline. Below 600C, LaAlO3 films were not epitaxial while films grown at 650C showed some <200> orientation. The shape of the underlying YBCO film is most clearly evident for the film grown at 400C. Surface roughness depended on the appearance of crystals on the film surface. The superconducting properties of the underlying YBCO film required O2 annealing prior to deposition of the LaAlO3 layer.

Improvement of the crystallinity of RF-magnetron-sputtered LaAlO3 layers on silicon (100)

Tremendous improvement of the crystallinity of RF-magnetron-sputtered LaAlO3 thin films on silicon (100) has recently been achieved by improving the vacuum prevailing prior to sputtering, particularly during thermal desorption of the artificially formed SiO2, on the surface of silicon. Additionally, the oxygen partial pressure and the substrate temperature during sputtering of these films have been carefully optimized using the appropriate phase diagram.

RF-magnetron sputtered lanthanum aluminate buffer layers on silicon

Optimization of the preparation conditions of in situ highly textured thin films of LaAlO3 on silicon (100) substrates, by planar RF-magnetron sputtering, was investigated. The parameters which have been considered for this optimization include: substrate temperature (T), gas pressure (P), the ratio of flow of oxygen to argon O2/Ar (all during sputtering of the layer), and the deposition power. Furthermore, different precleaning and oxidation treatments were applied to the silicon surface in order to remove the amorphous layer of the silicon native oxide (SiO2). This was to enhance epitaxial growth of LaAlO3 layers on silicon.