Suyeon Cho

Effect of oxygen partial pressure on the Fermi level of ZnO1−x films fabricated by pulsed laser deposition

We investigated the influence of oxygen deficiency on the Fermi level (EF) of ZnO thin film prepared by pulsed laser deposition (PLD). For this purpose, we adopted in situ x-ray photoelectron spectroscopy and ultraviolet photoelectron spectroscopy. The oxygen deficiency was effectively controlled by varying the oxygen partial pressure [P(O2)] during the PLD. The EF shifted by +0.6 eV as the P(O2) decreased from 10 to 3.3 Pa. This shift indicates a significant change in the energy balance in the oxygen-deficient ZnO films. This fact suggests that the very large change in the resistivity of ZnO thin films resulting from the oxygen deficiency could be attributed to the EF shift rather than grain boundary formation in the ZnO film.

Charge and magnetic states of rutile TiO2 doped with Cr ions

We observe that the electronic and magnetic properties of Cr-doped rutile TiO2 single crystals are highly dependent on growth conditions. The ferromagnetic component of magnetic susceptibility is observed to be enhanced for samples grown under oxygen-rich conditions. To understand the charge state of Cr dopants and their role in response to an external magnetic field, we carry out density functional theory calculations for Cr-doped rutile TiO2. Using the results of formation energy calculations in the presence of oxygen vacancies and Cr atom substitution at the Ti sites, we demonstrate that the Cr3+ state is a source of Curie-Weiss-type magnetic response, whereas the Cr4+ defect states contribute to the ferromagnetic component. We also provide the electronic structures of various defect configurations and attempt to explain the optical and electronic properties of the Cr-doped system.

Effect of Cr concentration on resistance switching in Cr-doped SrZrO3 films and surface accumulation of Cr ions

The role of Cr ions in the resistance switching (RS) of Cr-doped SrZrO3 films has been investigated by x-ray photoelectron spectroscopy and electric current measurements. The results show that the RS behavior depends on the Cr concentration, substrate temperature during film growth, and oxygen pressure during the postannealing process. Migration of Cr3+ ions to the surface makes the surface Cr rich, and thus, appreciable RS is observed. These observations suggest that the RS behavior can be controlled by trapping and releasing Cr carriers in the interface region between the film and the electrode.