Homoepitaxial SrTiO3 Thin Films Research Articles

Engineering antiphase boundaries in epitaxial SrTiO3 to achieve forming free memristive devices

We here present a method to engineer Ruddlesden-Popper-type antiphase boundaries in stoichiometric homoepitaxial SrTiO3 thin films. This is achieved by using a substrate with an intentionally high miscut, which stabilizes the growth of additional SrO at the bottom interface. We prove the success of this strategy utilizing transmission electron microscopy. We find that these antiphase boundaries significantly influence the resistive switching properties. In particular, devices based on SrTiO3 thin films with intentionally induced antiphase boundaries do not require a forming step, which is ascribed to the existence of preformed filaments.

Sequential pulsed laser deposition of homoepitaxial SrTiO3 thin films

The control of thin film stoichiometry is of primary relevance to achieve desired functionality. Pulsed laser deposition ablating from binary-oxide targets (sequential deposition) can be applied to precisely control the film composition, offsetting the importance of growth conditions on the film stoichiometry. In this work, we demonstrate that the cation stoichiometry of SrTiO3 thin films can be finely tuned by sequential deposition from SrO and TiO2 targets. Homoepitaxial SrTiO3 films were deposited at different substrate temperatures and Ti/Sr pulse ratios, allowing the establishment of a growth window for stoichiometric SrTiO3. The growth kinetics and nucleation processes were studied by reflection high-energy electron diffraction and atomic force microscopy, providing information about the growth mode and the degree of off-stoichiometry. At the optimal (stoichiometric) growth conditions, films exhibit atomically flat surfaces, whereas off-stoichiometry is accommodated by crystal defects, 3D islands, and/or surface precipitates depending on the substrate temperature and the excess cation. This technique opens the way to precisely control stoichiometry and doping of oxide thin films.

Impact of the interplay between nonstoichiometry and kinetic energy of the plume species on the growth mode of SrTiO3 thin films

We studied the pulsed laser deposition of homoepitaxial SrTiO3 thin films in different deposition regimes in order to elucidate the possibility to promote two-dimensional growth by increasing the kinetic energy of the oncoming particles. The kinetic energy of the oncoming species is determined by exploiting plume diagnostics techniques and the resulting nucleation and growth processes are analysed by reflection high-energy electron diffraction and atomic force microscopy. We could show that although the kinetic energy of the oncoming species varies to a great extent, the diffusion process is mostly influenced by the stoichiometry. Under stoichiometric conditions, obtained only in a limited window of process parameters, the adatoms on the surface have the highest diffusivity, thus promoting a step-flow growth mode. Under nonstoichiometric conditions, both Sr- and Ti-rich, the diffusivity is strongly reduced. This results in a transition from a two-dimensional to a three-dimensional growth under Sr-rich conditions. Conversely, in the Ti-rich case, obtained at high laser fluence, the two-dimensional growth sustains until the end of the growth process. We attribute this to the high island density available at high laser fluence which facilitates the diffusion of adatoms to step edges despite of their reduced diffusion length.

Structural and Optical Characteristics of Ag Nanoparticles Embedded in Homoepitaxial SrTiO3 Thin Films

Silver nanoparticles embedded in homoepitaxial SrTiO3 thin films are prepared by laser molecular beam epitaxy technique combined with nanocluster beam source. Microstructural analysis reveals that Ag nanoparticles are well isolated within the film with an average size of 16 nm. The existence of Ag nanoparticles alters the epitaxial growth orientation of SrTiO3 and makes the film deviate from the epitaxial growth orientation. The surface plasmon resonance of Ag nanoparticles is studied by optical absorption spectrum and photoluminescence spectrum. The position of surface plasmon resonance peak in the absorption spectrum is explained using modified Mie's theory.

Interplay of point defects, biaxial strain, and thermal conductivity in homoepitaxial SrTiO3 thin films

Separating out effects of point defects and lattice strain on thermal conductivity is essential for improvement of thermoelectric properties of SrTiO3. We study relations between defects generated during deposition, induced lattice strain, and their impact on thermal conductivity κ in homoepitaxial SrTiO3 films prepared by ion-beam sputtering. Lowering the deposition temperature gives rise to lattice expansion by enhancement of point defect density which increases the hardness of the films. Due to a fully coherent substrate-film interface, the lattice misfit induces a large biaxial strain. However, we can show that the temperature dependence of κ is mainly sensitive on the defect concentration.

Thickness-dependent local surface electronic structures of homoepitaxial SrTiO3 thin films

We have investigated the atomically-resolved substrate and homoepitaxial thin film surfaces of SrTiO3(001) using low-temperature scanning tunneling microscopy/scanning tunneling spectroscopy (STS) combined with pulsed laser deposition. It was found that a typical annealing treatment for preparation of SrTiO3 substrates, unexpectedly, resulted in a disordered surface on an atomic scale. In contrast, homoepitaxial SrTiO3 thin films grown on this disordered substrate exhibited a (2×2) surface reconstruction. The STS measurements revealed a number of surface defects in a 10 unit cell thick SrTiO3 film but much fewer in a 50 unit cell thick film, indicating nonuniform stoichiometry along the growth direction. These results suggest the possibility of using homoepitaxial SrTiO3 film surfaces as idealized substrates, opening a way to extract novel functionalities in complex oxides heterostructures.

Stoichiometry optimization of homoepitaxial oxide thin films using x-ray diffraction

Homoepitaxial SrTiO3 thin films grown by molecular beam epitaxy are analyzed using high-resolution x-ray diffraction and transmission electron microscopy. Measured 00L x-ray scans from stoichiometric and nonstoichiometric films are compared with calculations that account for the effects of film thickness, lattice parameter, fractional site occupancy, and an offset between film and substrate at the interface. It is found that thickness fringes, commonly observed around Bragg reflections even in stoichiometric homoepitaxial SrTiO3 films, arise from a film/substrate interface offset. Transmission electron microscopy studies confirm the presence of strain at those homoepitaxial interfaces that show an offset in x-ray diffraction. The consequences for stoichiometry optimization of homoepitaxial films using high-resolution x-ray diffraction and the quality of regrown oxide interfaces are discussed.

Growth of homoepitaxial SrTiO3 thin films by molecular-beam epitaxy

We report the structural properties of homoepitaxial (100) SrTiO3 films grown by reactive molecular-beam epitaxy (MBE). The lattice spacing and x-ray diffraction (XRD) rocking curves of stoichiometric MBE-grown SrTiO3 films are indistinguishable from the underlying SrTiO3 substrates. Off-stoichiometry for both strontium-rich and strontium-poor compositions (i.e., Sr1+xTiO3+δ films with −0.2<x<0.2) results in lattice expansion with significant changes to the shuttered reflection high-energy electron diffraction oscillations, XRD, and film microstructure. The dependence of lattice spacing on nonstoichiometry is smaller for MBE-grown films than for homoepitaxial (100) Sr1+xTiO3+δ films prepared by pulsed-laser deposition or sputtering.

Effect of oxygen vacancies on nonlinear dielectric properties of SrTiO3 thin films

Ferroelectrics (FE) are a class of nonlinear dielectrics which exhibit an electric field dependent dielectric constant. High temperature superconductors (HTSC) offer lower microwave surface resistance than conventional materials. There have been significant interest and effort in combining the low microwave losses of HTSC with the voltage control of FE dielectric constant er to implement high quality voltage tunable microwave devices, such as phase shifters, filters, delay lines, and tunable oscillators [1–4]. In such devices, it is desirable to have a large capacitance change ratio [tunability = (Cmax–Cmin)/Cmax] under a certain electric field range accompanied by a small dielectric loss. SrTiO3 (STO) thin films which posses a transition peak located at 60–70 K is one of the most interesting FE materials due to its high nonlinearity and reasonably low dielectric loss. A main focus on research for effective utilization of STO thin films is the development of materials with simultaneously optimized permittivity (absolute magnitude as well as its electric field dependence.) and low dielectric loss (tand). However, the tunability and dielectric loss in STO thin films have not been comparable to that of bulk materials. Comparing with STO single crystal, most STO thin films showed low dielectric constant, low tunability and large dielectric loss. Many reasons have been suggested to the degradation in dielectric properties of STO thin films. The tetragonal distortion (ratio of in-plane and surface normal lattice parameters, D = a/c), the dead layer near interface, and the local polar regions near charged defects like oxygen vacancies are expected to degrade the dielectric properties of STO thin films [5–7]. Several researchers have investigated the effect of oxygen vacancies on dielectric properties of heteroepitaxially grown STO thin films [8, 9]. It is worthwhile to note that the study of oxygen vacancies on dielectric properties of hetero-epitaxial STO thin films is complicated because of the additional effect of film stress due to film-substrate crystal lattice mismatch and difference in thermal expansion coefficients. There have been no systematic studies on the influence of oxygen vacancies on dielectric properties of STO thin films. In this study, STO thin films were homoepitaxially grown on Nb doped STO (Nb:STO) substrate. No additional stress is caused by lattice constant mismatch and thermal expansion coefficient differences between films and substrate in the homo-epitaxial STO thin films. The effect of oxygen vacancies on dielectric properties of STO thin films are presented. STO thin films were deposited on Nb:STO substrates by pulsed laser deposition (PLD). KrF excimer laser radiation (Lamda Physik Compex 201) with repetition rate of 5 Hz was focused on a rotating target set in the chamber through a quartz window. The incident angle of laser beam to the target surface was fixed to be 45 . The output laser pulse energy was kept as 150 mJ which yielded a growth rate of 0.02 nm per pulse. The laser energy density was about 2 J/cm. X. Z. Liu B. W. Tao (&) Y. R. Li The National Key Laboratory of Electronic Thin Films and Integrated Devices, University of Electronic Science & Technology of China, Jianshe Road, Chengdu 610054, China e-mail: bwtao@uestc.edu.cn

Control of growth mode in SrTiO3 homoepitaxy under 500°C

Homoepitaxial SrTiO3 thin films were grown by laser molecular beam epitaxy. The growth mode was determined by in-situ reflective high energy electron diffraction, and the surface of the films was studied by ex-situ atomic force microscopy. At the deposition rate of 0.16Å∕sec and the laser energy density of 6J∕cm2, layer-by-layer growth was observed above 460°C substrate temperature, while the Stranski-Krastanov growth mode, that is layer-by-layer growth plus island growth mode, prevailed between 460°C and 410°C. On further decreasing the substrate temperature, the island growth was determined under 410°C. With the optimization of deposition process in terms of laser energy density and deposition rate, the lowest crystallization temperatures of SrTiO3 films grown in layer-by-layer growth mode were obtained as low as 280°C. The effects of laser energy density on growth temperature were studied.

Growth mode mapping of SrTiO3 epitaxy

We have mapped the growth mode of homoepitaxial SrTiO3 thin films as a function of deposition rate and substrate temperature during pulsed laser deposition. The transition from layer by layer growth to step flow growth was mapped by making 260 depositions, 3 monolayers each, on a single substrate. The growth mode was determined by time-resolved reflection high-energy electron diffraction. An atomically smooth surface was regenerated after each deposition by annealing the sample at temperatures above 1200 °C. The depositions were performed at an oxygen pressure of 10−6 Torr and covered a temperature range from 900 to 1380 °C. The effective activation energies of surface migration on Ti- and Sr-terminated surfaces were determined from the mapping results.

Characteristics of insulator diodes determined by space-charge and diffusion : N. Sinharay and B. Meltzer, Solid-State Electron., Vol. 7, 1964, pp. 125–136

We report the leakage-current characteristics of bulk vanadium (donor)- and scandium (acceptor)-doped Ba0.7Sr0.3TiO3 ceramic structures measured using gold electrical contacts. Vanadium doping reduces the ohmic leakage current that dominates the transport characteristics by up to 5 kV cm−1. The Arrhenius activation energy is 0.18, 0.20 and 0.23 eV for 1, 2 and 4 at.% V-doped samples, respectively. Above this field, the current–voltage characteristics exhibit discontinuous current transitions associated with trap filling by electronic carriers. At higher fields, trap-controlled, space-charge-limited conduction (SCLC) is observed with an effective mobility of 4 ± 1 × 10−7 cm2 V−1 s−1, characteristic of an electronic transport process that involves quasi-equilibrium between conduction in the band and trapping. In contrast, the leakage current of Sc-doped samples increases with impurity concentration and exhibits a 0.60 eV activation energy. In this case, the limiting current conduction mechanism is the transport of holes over the electrostatic barrier at grain boundaries. Comparison of these results to those on similarly doped homoepitaxial SrTiO3 thin films deposited on single-crystal and bicrystal substrates helped to identify the characteristics of transport in the bulk and across grain boundaries for this class of materials. The Sc- and V-doped thin films and ceramics did not exhibit electrical properties characteristic of shallow-level impurity doping, suggesting that V and Sc impurities form deep-level point defects and /or complexes, or are strongly compensated.