Ultrafast Modification of the Polarity at LaAlO3_3/SrTiO3_3 Interfaces

Oxide growth with semiconductor-like accuracy has led to atomically precise thin films and interfaces that exhibit a plethora of phases and functionalities not found in the oxide bulk material. This yielded spectacular discoveries such as the conducting, magnetic or even superconducting LaAlO$_3$/SrTiO$_3$ interfaces separating two prototypical insulating perovskite materials. All these investigations, however, consider the static state at the interface, although studies on fast oxide interface dynamics would introduce a powerful degree of freedom to understanding the nature of the LaAlO$_3$/SrTiO$_3$ interface state. Here we show that the polarization state at the LaAlO$_3$/SrTiO$_3$ interface can be optically enhanced or attenuated within picoseconds. Our observations are explained by a model based on charge propagation effects in the interfacial vicinity and transient polarization buildup at the interface

Conducting interfaces between band insulating oxides: the LaGaO3/SrTiO3

We show that the growth of the heterostructure LaGaO3/SrTiO3 yields the formation of a highly conductive interface. Our samples were carefully analyzed by high resolution electron microscopy, in order to assess their crystal perfection and to evaluate the abruptness of the interface. Their carrier density and sheet resistance are compared to the case of LaAlO3/SrTiO3 and a superconducting transition is found. The results open the route to widening the field of polar-non polar interfaces, pose some phenomenological constrains to their underlying physics and highlight the chance of tailoring their properties for future applications by adopting suitable polar materials.Comment: in press Appl. Phys. Lett. 97, 1 (2010

Universal electronic structure of polar oxide hetero-interfaces

The electronic properties of NdGaO3/SrTiO3, LaGaO3/SrTiO3, and LaAlO3/SrTiO3 interfaces, all showing an insulator-to-metal transition as a function of the overlayer-thickness, are addressed in a comparative study based on x-ray absorption, x-ray photoemission and resonant photoemission spectroscopy. The nature of the charge carriers, their concentration and spatial distribution as well as the interface band alignments and the overall interface band diagrams are studied and quantitatively evaluated. The behavior of the three analyzed heterostructures is found to be remarkably similar. The valence band edge of all the three overlayers aligns to that of bulk SrTiO3. The near-interface SrTiO3 layer is affected, at increasing overlayer thickness, by the building-up of a confining potential. This potential bends both the valence and the conduction band downwards. The latter one crossing the Fermi energy in the proximity of the interface and determines the formation of an interfacial band offset growing as a function of thickness. Quite remarkably, but in agreement with previous reports for LaAlO3/SrTiO3, no electric field is detected inside any of the polar overlayers. The essential phenomenology emerging from our findings is discussed on the base of different alternative scenarios regarding the origin of interface carriers and their interaction with an intense photon beam

Persistent photoconductivity in 2-dimensional electron gases at different oxide interfaces

We report on the transport characterization in dark and under light irradiation of three different interfaces: LaAlO3/SrTiO3, LaGaO3/SrTiO3, and the novel NdGaO3/SrTiO3 heterostructure. All of them share a perovskite structure, an insulating nature of the single building blocks, a polar/non- polar character and a critical thickness of four unit cells for the onset of conductivity. The interface structure and charge confinement in NdGaO3/SrTiO3 are probed by atomic-scale- resolved electron energy loss spectroscopy showing that, similarly to LaAlO3/SrTiO3, extra electronic charge confined in a sheet of about 1.5 nm in thickness is present at the NdGaO3/SrTiO3 interface. Electric transport measurements performed in dark and under radiation show remarkable similarities and provide evidence that the persistent perturbation induced by light is an intrinsic peculiar property of the three investigated oxide-based polar/non-polar interfaces. Our work sets a framework for understanding the previous contrasting results found in literature about photoconductivity in LaAlO3/SrTiO3 and highlights the connection between the origin of persistent photoconductivity and the origin of conductivity itself. An improved understanding of the photo- induced metastable electron-hole pairs might allow to shed a direct light on the complex physics of this system and on the recently proposed perspectives of oxide interfaces for solar energy conversion.Comment: 11 pages, 7 figure

Self-formed LaAlO3/SrTiO3LaAlO_3/SrTiO_3 Micro-Membranes

Oxide heterostructures represent a unique playground for triggering the emergence of novel electronic states and for implementing new device concepts. The discovery of 2D conductivity at the $LaAlO_3/SrTiO_3$ interface has been linking for over a decade two of the major current research fields in Materials Science: correlated transition-metal-oxide systems and low-dimensional systems. A full merging of these two fields requires nevertheless the realization of $LaAlO_3/SrTiO_3$ heterostructures in the form of freestanding membranes. Here we show a completely new method for obtaining oxide hetero-membranes with micrometer lateral dimensions. Unlike traditional thin-film-based techniques developed for semiconductors and recently extended to oxides, the concept we demonstrate does not rely on any sacrificial layer and is based instead on pure strain engineering. We monitor through both real-time and post-deposition analyses, performed at different stages of growth, the strain relaxation mechanism leading to the spontaneous formation of curved hetero-membranes. Detailed transmission electron microscopy investigations show that the membranes are fully epitaxial and that their curvature results in a huge strain gradient, each of the layers showing a mixed compressive/tensile strain state. Electronic devices are fabricated by realizing ad hoc circuits for individual micro-membranes transferred on silicon chips. Our samples exhibit metallic conductivity and electrostatic field effect similar to 2D-electron systems in bulk heterostructures. Our results open a new path for adding oxide functionality into semiconductor electronics, potentially allowing for ultra-low voltage gating of a superconducting transistors, micromechanical control of the 2D electron gas mediated by ferroelectricity and flexoelectricity, and on-chip straintronics.Comment: 8 pages, 4 figure