Qingyu Lei

Strain-Engineered Oxygen Vacancies in CaMnO3 Thin Films

We demonstrate a novel pathway to control and stabilize oxygen vacancies in complex transition-metal oxide thin films. Using atomic layer-by-layer pulsed laser deposition (PLD) from two separate targets, we synthesize high-quality single-crystalline CaMnO3 films with systematically varying oxygen vacancy defect formation energies as controlled by coherent tensile strain. The systematic increase of the oxygen vacancy content in CaMnO3 as a function of applied in-plane strain is observed and confirmed experimentally using high-resolution soft X-ray absorption spectroscopy (XAS) in conjunction with bulk-sensitive hard X-ray photoemission spectroscopy (HAXPES). The relevant defect states in the densities of states are identified and the vacancy content in the films quantified using the combination of first-principles theory and core-hole multiplet calculations with holistic fitting. Our findings open up a promising avenue for designing and controlling new ionically active properties and functionalities of complex transition-metal oxides via strain-induced oxygen-vacancy formation and ordering.

Epitaxial strain and its relaxation at the LaAlO3/SrTiO3 interface

A series of LaAlO3 thin films with different thicknesses were deposited by pulsed laser deposition at temperatures from 720 °C to 800 °C. The results from grazing incidence x-ray diffraction and reciprocal space mapping indicate that a thin layer of LaAlO3 adjacent to the SrTiO3 substrate remains almost coherently strained to the substrate, while the top layer starts to relax quickly above a certain critical thickness, followed by a gradual relaxation at larger film thickness when they are grown at lower temperatures. The atomic force microscopy results show that the fast relaxation is accompanied by the formation of cracks on the film surface. This can be ascribed to the larger energy release rate when compared with the resistance of LaAlO3 to cracking, according to calculations from the Griffith fracture theory. For films grown at 720 °C, a drop in sheet resistance by two orders of magnitude is observed when the top layer starts to relax, indicating a relationship between the strain and the conductivity of the two-dimensional electron gas at the LaAlO3/SrTiO3 interface. The strain engineered by growth temperature provides a useful tool for the manipulation of the electronic properties of oxide heterointerfaces.

Structural Characterization of Homoepitaxial SrTiO3 Films Grown by Pulsed Laser Deposition

We report the structural characterization of homoepitaxial (001) SrTiO3 films grown by pulsed laser deposition on single crystal (001) SrTiO3 substrates. X-ray diffraction θ−2θ scan and φ scan were used to study the lattice constants and crystal quality of the SrTiO3 films. Lattice expansion in the c-axis from the bulk SrTiO3 was found in many films, which is related to the off-stoichiometry in the films. We create a contour map for both the lattice expansion and cation composition as functions of laser energy density and oxygen pressure, which shows a broad range of growth conditions to achieve stoichiometry in SrTiO3 films.