Ran Hee Shin

Control of defect-mediated tunneling barrier heights in ultrathin MgO films

The impact of oxygen vacancies on local tunneling properties across rf-sputtered MgO thin films was investigated by optical absorption spectroscopy and conducting atomic force microscopy. Adding O2 to the Ar plasma during MgO growth alters the oxygen defect populations, leading to improved local tunneling characteristics such as a lower density of current hotspots and a lower tunnel current amplitude. We discuss a defect-based potential landscape across ultrathin MgO barriers.

Room-temperature polarization switching and antiferromagnetic coupling in epitaxial (Ga,Fe)2O3/SrRuO3 heterostructures

Room-temperature reversible remnant polarization of gallium ferrite thin-films is reported as a multiferroic material with non-zero order parameters of polarization and magnetization. With the addition of Fe ions in Ga sites, Ga0.6Fe1.4O3 (GFO) thin films have been considered as potentially promising of multiferroicity. The b-axis oriented epitaxial GFO films were grown on SrRuO3(111)/SrTiO3(111). The six-fold symmetric in-plane epitaxy of the GFO films was confirmed using X-ray diffraction. The magnetic moment of the films was measured as a function of temperature and external magnetic field, which shows a room-temperature non-zero magnetization. Macroscopic and microscopic methods have been applied to demonstrate the polarization switching of the films. The remnant polarization is measured as 0.05 μC/cm2. Reduction of leaky behaviors of the GFO films owing to the conducting oxide of SrRuO3 will pave a way to take advantage of the room-temperature non-zero multi-orders for future non-volatile memory devi...

Structural properties and phase formation of epitaxial PbVO3 thin films grown on LaAlO3(0 0 1) by pulsed laser deposition

We report epitaxial synthesis of PbVO3 (PVO) thin films on LaAlO3 (LAO) (0?0?1) substrates under reduction atmosphere from a Pb2V2O7 sintered target over a wide range of temperatures using laser ablation. Polar magnetic PVO has a non-centrosymmetric structure (tetragonality c/a?=?1.23). The effects of substrate temperature on structural properties and chemical composition of PVO thin films were studied by x-ray diffraction and energy-dispersive spectrometry. The c-axis of the PVO films was abnormally elongated as much as 5?7% on the well-lattice-matched LAO substrates. Using Raman scattering spectroscopy, single phase formation of PVO with the shift of phonon modes was confirmed and epitaxial growth of the PVO films was also confirmed with polarization-dependent measurements. The strained and epitaxial growth of the PVO films will lead to a new class of multiferroic materials with future electronic device applications.

Nanoscale Observation of Poling Behaviors in Nd-Substituted Bi4Ti3O12 Ferroelectric Thin Films

In this study, the effects of oxygen vacancies on the local poling properties of Bi-layered ferroelectric thin films were examined. Bi3.15Nd0.85Ti3O12 (BNT) thin films were coated on Pt(111)/Ti/SiO2/Si substrates using a sol–gel method. An oxygen vacancy of 2.4±0.5% was observed mainly at the Bi2O22+ layers in the film that annealed in an oxygen-deficient atmosphere. The film containing oxygen vacancies showed a shift in the polarization–electric field hysteresis loop. Contact-mode electrostatic force microscopy revealed a domain pinning behavior in the BNT film with oxygen vacancies. The unpolarized domains were explained by the built-in field formed by the oxygen vacancies in the Bi2O22+ layers.

Oxygen Stability and Leakage Current Properties of La-Modified Bismuth Titanate Ferroelectric Thin Films

The effects of oxygen annealing on the structural properties and leakage current behaviors of sol–gel-derived Bi3.25La0.75Ti3O12-y ferroelectric thin films have been studied. The thin films were pyrolyzed in different atmospheric conditions including a fully oxygenized ambient of 1 bar and an evacuated ambient of less than 10 mbar. X-ray diffraction patterns of the deoxygenated films showed measurable shifts to a higher angle, indicating that each deoxygenated film had a smaller lattice constant than the oxygenated one. Scanning probe analysis showed well-developed grains and inhomogeneous and asymmetric bound charges in both samples. The leakage current behavior of the films was measured as a function of voltage and temperature and was explained on the basis of the Schottky–Simmons thermionic emission model. X-ray photoemission spectra showed that oxygen stability in Bi2O2 layers plays an important role in determining the level of leakage current in the films.

Synthesis and physical properties of Cu(In,Ga)Se 2 nanoparticles and CuGaSe 2 thin-films for tandem cell photovoltaic applications

For tandem cell photovoltaic applications, we have investigated Cu(In,Ga)Se2 (CIGS) nanoparticles and CuGaSe2 (CGS) thin-films. Alternative structures with metal Mo and transparent conductive oxide (TCO) back contacts were also studied.

Kelvin probe microscopic studies on grain boundaries of Cu(In, Ga)Se 2 and Cu 2 ZnSnSe 4 thin-films grown by co-evaporation methods

Cu2ZnSnSe4 (CZTSe) thin-films is a promising candidate for absorber layer as an alternative to Cu(In, Ga)Se2 (CIGS) solar cells. However, they have been recorded lower efficiency than that of CIGS until now. In CIGS, local electrical property is an important issue for high efficiency such as Na interstitial in the grain boundaries (GBs). Therefore, difference between CIGS with CZTSe in local electrical property can be one of the clues for explaining lower efficiency of CZTSe thin-film solar cells than CIGS. We studied local surface potential using Kelvin probe force microscopy (KPFM) around GBs. The results reveal difference of electron-hole transport behavior on both thin-films at GBs. Eventually, we can understand relation between solar cell efficiency and local electrical property.