Identifying Ionic and Electronic Charge Transfer at Oxide Heterointerfaces.

The ability to tailor oxide heterointerfaces has led to novel properties in low-dimensional oxide systems. A fundamental understanding of these properties is based on the concept of electronic charge transfer. However, the electronic properties of oxide heterointerfaces crucially depend on their ionic constitution and defect structure: ionic charges contribute to charge transfer and screening at oxide interfaces, triggering a thermodynamic balance of ionic and electronic structures. Quantitative understanding of the electronic and ionic roles regarding charge-transfer phenomena poses a central challenge. Here, the electronic and ionic structure is simultaneously investigated at the prototypical charge-transfer heterointerface, LaAlO3 /SrTiO3 . Applying in situ photoemission spectroscopy under oxygen ambient, ionic and electronic charge transfer is deconvoluted in response to the oxygen atmosphere at elevated temperatures. In this way, both the rich and variable chemistry of complex oxides and the associated electronic properties are equally embraced. The interfacial electron gas is depleted through an ionic rearrangement in the strontium cation sublattice when oxygen is applied, resulting in an inverse and reversible balance between cation vacancies and electrons, while the mobility of ionic species is found to be considerably enhanced as compared to the bulk. Triggered by these ionic phenomena, the electronic transport and magnetic signature of the heterointerface are significantly altered.

Reduction of the forming voltage through tailored oxygen non-stoichiometry in tantalum oxide ReRAM devices.

In this study, we investigated the influence of oxygen non-stoichiometry on the resistive switching performance of tantalum oxide based memristive devices. Thin-films of tantalum oxide were deposited with varying sputter power and oxygen partial pressure. The electroforming voltage was found to decrease with increasing power density or decreased oxygen partial pressure, while the endurance remained stable and the resistance window ROFF/RON was found to increase. In-depth XPS analysis connects these observations to a controllable oxygen sub-stoichiometry in the sputter-deposited films. Our analysis shows that the decrease of the forming voltage results from an increase in carrier density in the as-prepared thin-films, which is induced by the presence of oxygen vacancies.

The influence of the local oxygen vacancy concentration on the piezoresponse of strontium titanate thin films.

In this study, the influence of the local oxygen vacancy concentration on piezoresponse force microscopy (PFM) measurements was investigated. Ultra-thin single-crystalline SrTiO3 thin films were deposited on niobium doped SrTiO3 substrates and analyzed using a combined PFM and local conductive atomic force microscopy (LC-AFM) measurement setup. After applying different polarization voltages between ±2 V and ±5 V to the thin films, we simultaneously observed an anomalous contrast in the piezoresponse amplitude and phase signal as well as a changed local conductivity in the exact same region. Since classic ferroelectricity can be excluded as the reason for the observed contrast, an influence of the local oxygen vacancy concentration on the piezoresponse is considered. Additionally, the surface potential was measured using Kelvin probe force microscopy (KPFM) revealing a change in surface potential in the regions of the applied voltage. The observed relaxation of the surface potential over time was fitted to a local oxidation reaction of the previously reduced regions of the ultra-thin SrTiO3 film. We propose a model that relates the local oxygen vacancy concentration to the surface potential. The influence of the oxygen vacancy concentration on the PFM measurements is explained.