Defects and transport in complex oxide thin films

Abstract

Epitaxial oxide thin films are at the heart of new “oxide electronic” applications, such as excitonic ultraviolet light-emitting diodes and resistive switching memories. Complex oxide films are often grown by pulsed laser deposition (PLD) because the technique is believed to be material agnostic. Here, we show that one of the fundamental premises used to justify the use of PLD, that material is transferred from an ablation target to the film without stoichiometry deviations, is incorrect even when no volatile elements are involved. Even more importantly, the commonly used solution of increasing the laser energy density above a material-specific threshold value to obtain stoichiometric films cannot be used in the case of low carrier density systems such as SrTiO3, where even minute 1018 cm−3 order cation nonstoichiometry can have a dramatic effect on transport. Lattice parameter deviations in oxide films, which are often incorrectly ascribed to oxygen loss, correlate very well with cation nonstoichiometry. We show that proper simultaneous choice of ablation laser fluence and ablation area is essential and often more important than the growth temperature and oxygen pressure for obtaining bulklike properties in oxide heterostructures.

Magnetic-field-induced spontaneous superlattice formation via spinodal decomposition in epitaxial strontium titanate thin films

“…The image shows that the film includes local and random structural changes, which might derive from randomly distributed RP type lattice displacements caused by the insertion of an extra SrO layer because the overall composition of the film is Sr-excess (Sr/(Sr+Ti) = 58.3 at.%). Such randomly distributed RP type defects are also reported by Ohnishi et al 25 for the epitaxial grown SrTiO 3 thin film with a Srexcess composition. As described above, the Sr-Ti-O thin film diffraction peak is observed at a lower angle than that of the SrTiO 3 single crystal, which suggests that the insertion of an extra SrO layer occurs at least along the out-of-plane (c-axis) direction.…”

“…However, for the film with a Sr-excess composition (Sr/(Sr+Ti) = 58.3 at.%), a considerable increase of Δc ( = 0.0073 nm) was observed, which suggests that the insertion of an additional SrO layer is enhanced by the application of the magnetic field during deposition. In this figure, the relation between Δc and composition estimated by Ohnishi et al 25 for RP (SrTiO 3 ) x -SrO phases when treated as pseudocubic cells is also shown. In their estimation, Δc was calculated under the assumption that the insertion of an extra SrO layer occurs only along the out-of-plane (c-axis) direction.…”

“…The results further show that the application of a magnetic field during deposition enhanced the degree of lattice expansion, Δc, which is calculated as the difference of the out-of-plane lattice parameter of the thin films from that of bulk SrTiO 3 . The lattice expansion (Δc) of SrTiO 3 thin film was investigated extensively by Ohnishi et al 25 They found that the lattice expansion is brought about not by oxygen deficiency but by cation nonstoichiometry. They reported that the lattice expansion occurs for both Sr-excess and Ti-excess compositions.…”

Constructing oxide interfaces and heterostructures by atomic layer-by-layer laser molecular beam epitaxy

“…When the film is not stoichiometric, regardless of Sr rich or deficient, a diffraction peak from the film at a smaller angle than the substrate peak is seen, indicating a c-axis lattice expansion well established for homoepitaxial off-stoichiometric SrTiO 3 . 8,9 The c lattice constant vs. x is plotted for the films in Fig. 2c.…”

“…On the other hand, conventional PLD using a compound target often results in cation off-stoichiometry in the films. 9,10 In this paper we present an approach that combines the strengths of reactive MBE and PLD: atomic layer-by-layer laser MBE (ALL-Laser MBE) using separate oxide targets. Ablating alternately the targets of constituent oxides, for example SrO and TiO 2 , a SrTiO 3 film can be grown one atomic layer at a time.…”