Chang Uk Jung

Effects of heat dissipation on unipolar resistance switching in Pt/NiO/Pt capacitors

We fabricated Pt∕NiO∕Pt capacitor structures with various bottom electrode thicknesses tBE and investigated their resistance switching behaviors. The capacitors with tBE⩾50nm exhibited typical unipolar resistance memory switching, while those with tBE⩽30nm showed threshold switching. This interesting phenomenon can be explained in terms of the temperature-dependent stability of conducting filaments. In particular, the thinner tBE makes dissipation of Joule heat less efficient, so the filaments will be at a higher temperature and become less stable. This study demonstrates the importance of heat dissipation in resistance random access memory.

Ferroelectric properties of SrRuO3∕BaTiO3∕SrRuO3 ultrathin film capacitors free from passive layers

Structural studies on ultrathin SrRuO3∕BaTiO3∕SrRuO3 capacitors, with BaTiO3 thicknesses of between 5nm and 30nm, show well-defined interfaces between ferroelectric BaTiO3 and electrode SrRuO3 layers. In these capacitors, we cannot observe any extrinsic electrical effects due to either the formation of an insulating interfacial passive layer or passive-layer-induced charge injection. Such high-quality interfaces result in very good fatigue endurance, even for the 5nm thick BaTiO3 capacitor.Structural studies on ultrathin SrRuO3∕BaTiO3∕SrRuO3 capacitors, with BaTiO3 thicknesses of between 5nm and 30nm, show well-defined interfaces between ferroelectric BaTiO3 and electrode SrRuO3 layers. In these capacitors, we cannot observe any extrinsic electrical effects due to either the formation of an insulating interfacial passive layer or passive-layer-induced charge injection. Such high-quality interfaces result in very good fatigue endurance, even for the 5nm thick BaTiO3 capacitor.

Role of oxygen vacancies in resistive switching in Pt/Nb-doped SrTiO3

Oxygen vacancies at the metal/oxide interface, driven by an electric field, have been considered responsible for the switching to the low-resistance state. We studied the electrical properties, along with microscopic observations, of the Pt/Nb-doped SrTiO3 (001) single-crystal system. Electron energy loss spectroscopy revealed highly accumulated oxygen vacancies at the interface in the high-resistance state, contrasting to common explanation. Higher resistance state by more oxygen vacancies was further confirmed in Pt/H2-annealed SrTiO3. These results suggest the presence of an interfacial state which dominantly determined the resistivity by changing the barrier height at the interface.

Epitaxial growth and the magnetic properties of orthorhombic YTiO3 thin films

High-quality YTiO3 thin films were grown on LaAlO3 (110) substrates at low oxygen pressures (⩽10−8Torr) using pulsed laser deposition. The in-plane asymmetric atomic arrangements at the substrate surface allowed the authors to grow epitaxial YTiO3 thin films, which have an orthorhombic crystal structure with quite different a- and b-axis lattice constants. The YTiO3 film exhibited a clear ferromagnetic transition at 30K with a saturation magnetization of about 0.7μB∕Ti. The magnetic easy axis was found to be along the [1−10] direction of the substrate, which differs from the single crystal easy axis direction, i.e., [001].

Modification of magnetic properties through the control of growth orientation and epitaxial strain in SrRuO3 thin films

We studied the correlation between the magnetic properties and the structural modifications in SrRuO3 thin films. Previous results on the control of growth orientation in SrRuO3 thin film have been rather limited. For example, only orthorhombic a-axis growth could be obtained for SrRuO3 film on SrTiO3 (110) substrates. Here we were able to obtain various SrRuO3 films with (100)o, (010)o, and (111)o growth orientations, as well as with different amount of strain on SrTiO3 (110) substrates by using (Ca,Sr)SnO3 and CaHfO3 buffer layers. The magnetic properties were found to change systematically with the structural distortion of the SrRuO3 thin films.

Abnormal resistance switching behaviours of NiO thin films: possible occurrence of both formation and rupturing of conducting channels

We report a detailed study on the abnormal resistance switching behaviours observed in NiO thin films which show unipolar resistance switching phenomena. During the RESET process, in which the NiO film changed from a low resistance state to a high resistance state, we sometimes observed that the resistance became smaller than the initial value. We simulated the resistance switching by using a random circuit breaker network model. We found that local conducting channels could be formed as well as ruptured during the RESET process, which result in the occurrence of such abnormal switching behaviours.

Compliance current induced non-reversible transition from unipolar to bipolar resistive switching in a Cu/TaOx/Pt structure

Unipolar resistive switching (URS) as well as bipolar resistive switching (BRS) behaviors in a Cu/TaOx/Pt structure were investigated. Upon increasing the compliance current (Ic), the current-voltage characteristics of the Cu/TaOx/Pt structure showed a URS behavior at Ic = 0.1 mA then experienced a non-reversible transition from the URS to a BRS mode at Ic = 10 mA. Through a detailed analysis of the electrical properties in each resistance state of URS and BRS, we revealed that the permanent transition from the URS to the BRS mode was induced by the formation of stronger Cu metal conductive filaments within the TaOx thin film. More interestingly, both URS and BRS modes were governed by the formation and rupture of conductive filaments, whereas the rupture of these filamentary paths in BRS was proposed due to both Joule heating and electric field effects.

Resistance switching in epitaxial SrCoOx thin films

We observed bipolar switching behavior from an epitaxial strontium cobaltite film grown on a SrTiO3 (001) substrate. The crystal structure of strontium cobaltite has been known to undergo topotactic phase transformation between two distinct phases: insulating brownmillerite (SrCoO2.5) and conducting perovskite (SrCoO3−δ) depending on the oxygen content. The current–voltage characteristics of the strontium cobaltite film showed that it could have a reversible insulator-to-metal transition triggered by electrical bias voltage. We propose that the resistance switching in the SrCoOx thin film could be related to the topotactic phase transformation and the peculiar structure of SrCoO2.5.

Brush-shaped ZnO heteronanorods synthesized using thermal-assisted pulsed laser deposition.

Brush-shaped ZnO heteronanostructures were synthesized using a newly designed thermal-assisted pulsed laser deposition (T-PLD) system that combines the advantages of pulsed laser deposition (PLD) and a hot furnace system. Branched ZnO nanostructures were successfully grown onto CVD-grown backbone nanowires by T-PLD. Although ZnO growth at 300 °C resulted in core-shell structures, brush-shaped hierarchical nanostructures were formed at 500-600 °C. Materials properties were studied via photoluminescence (PL), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) characterizations. The enhanced photocurrent of a SnO(2)-ZnO heterostructures device by irradiation with 365 nm wavelength ultraviolet (UV) light was also investigated by the current-voltage characteristics.

The Effect of Plasma Treatment on the Physical Properties of SrRuO3 Films on SrTiO3 Substrate

The transport properties of as-deposited and plasma-treated SrRuO3 thin films grown on SrTiO3(001) substrates were investigated. The SrRuO3 thin films were treated with two different kinds of plasma gases: O2 and H2. The difference in the transport properties of the two plasma-treated SrRuO3 films was explained by the different oxygen vacancy concentration, based on different values for the out-of-plane lattice constant. This oxygen vacancy was significantly reduced by recovery annealing under high oxygen pressure.