Growth of La2-xSrxCuO4Thin Films with c-Axis Orientation on SiO2Layer by in situ Post-annealing in Pulsed Laser Deposition

Abstract

The first high-TC cuprate, La2-xSrxCuO4 (LSCO) is known to be correlated with Mott insulators that become metallic and superconducting upon doping with charge carriers. A large number of studies have been performed on bulk samples of LSCO due to its simple chemical composition and crystal structures as well as its rich physical properties. The high-speed switching devices applying the metalinsulator transition (MIT) were predicted as an important application. However, the preparation of the thin film of LSCO is critical in developing devices such as thermal switches, thermochromatic coatings and non-volatile memory. The electrical and transport properties of LSCO-based thin films are highly sensitive to the degree of c-axis orientation because the charge carriers move through c-axis channels. A large number of iso-structural substrates, such as NdGaO3, LaAlO3, and SrTiO3, which have small lattice mismatches with LSCO, have been used to obtain highly oriented LSCO thin films. However, these cannot be used as common substrates in developing MIT devices because it is too difficult and too expensive to prepare their single crystalline structures. In this letter, we report the first results of experiments investigating pulsed laser deposition (PLD) and high-pressure post-annealing in oxygen on a SiO2 layer which is largely lattice-mismatched with LSCO, which leads to c-axis oriented LSCO thin films. Thermal SiO2 is the common oxide most frequently used on Si wafers and is known as the highest quality oxide of low defect density and low impurity content, thereby acting as a gate oxide in the manufacture of MIT devices. The polycrystalline LSCO target was prepared by a conventional solid-state reaction. Stoichiometric amounts of La2O3 (99.9%, preheated at 900 C for 12 h), SrO (99.99%) and CuO (99.9%) were thoroughly mixed and heated at 1050 C for 24 hrs in air. The powder was ground and pressed into pellets under a pressure of about 50 MPa and sintered at 1000 C for 24 h. An amorphous 100-nm-thick SiO2 film was grown on a (100) Si wafer using thermal oxidation at 925 C. The oxidation ambient was composed of O2 and H2. LSCO thin films were deposited on SiO2 films at various temperatures using a frequency tripled Nd:YAG Laser (Quantel, Brilliant II) with 5-ns of pulse width. Laser pulses of 5 Hz were focused onto the rotating target using a quartz lens with a 35 cm focal length at a 45 oblique incidence, and the energy density of approximately 2 J/cm. A substrate was set at a distance of 50 mm parallel to the target surface. The grown films were subsequently postannealed in ambient oxygen at various temperatures. The structural and phase identifications were carried out using an X-ray diffractometer (MacScience Co. MXP-3V) with CuKα radiation. Cross-sectional views of the deposited films were obtained using a scanning electron microscope (SEM) (Hitachi Co. S-4200). Figure 1 shows the XRD spectra of thin films of the parent material, La2-xSrxCuO4 (x = 0). The as-deposited thin film at 800 C has the predominant preferential orientation of (113) direction. This is consistent with the result of Wang et al. that the (00l) orientation is always accompanied by (113) orientations, even on a substrate with small lattice mismatch such as LaAlO3. 6 However, the thin film post-annealed at 700 C for 1 hr under oxygen atmosphere of 1000 mTorr after deposition at 500 C has the diffraction peaks of (00l) with l = 2n, which suggests that the films consist of entirely c-axis-oriented grains. The post-annealing step is usually carried out to improve the properties of as-deposited thin films, such as crystallinity, grain size and stoichiometry of the light component in the thin film processes. The parent compound can be doped by substituting some of the La with Sr. As a result, the x holes are added to the Cu-O plane in LSCO and the electrical properties of the LSCO thin film are changed. Figure 2 shows the XRD diffraction patterns of LSCO thin films post-annealed at 800