Jin Pyo Hong

Hysteretic bipolar resistive switching characteristics in TiO2/TiO2−x multilayer homojunctions

TiO2 (oxygen rich, region 1)/TiO2−x (oxygen poor, region 2) multilayer homojunctions were studied as alternative resistive switching structures for both high and low resistance transitions. Stable bipolar resistive switching characteristics, including stable switching speeds (microseconds) and endurance behaviors, as well as long retention times (>104 s) were demonstrated. The nature of the resistive switching phenomenon in multilayer structures seems to be a combination of the conduction path and the redox reaction, resulting from the oxygen ions drifting between the oxygen rich and poor regions of the multilayer structures. A possible conduction sketch for bipolar switching behaviors is also discussed.

Al electrode dependent transition to bipolar resistive switching characteristics in pure TiO2 films

Stable bipolar resistive switching was demonstrated in polycrystalline TiO2 films involving two different top and bottom Al electrodes of two different structures (Al/TiO2/Pt and Pt/TiO2/Al) after a forming process. With an Al electrode, the transition to bipolar resistive switching was clearly observed, together with counterclockwise and clockwise switching directions, which depended on the position of the Al electrode. The transition from unipolar to bipolar resistive switching seems to be attributable to the redox reaction and trap/detrap at the interfaces between the Al electrode and TiO2 layer due to the migration of oxygen ions and electrons. However, current level analysis of devices reveals that the forming process method basically leads to the formation of conducting paths inside the TiO2 layers. The electrical device properties of the two different structures, the effects of compliance currents, and the operation voltages are also analyzed.

Room temperature formation of half-metallic Fe3O4 thin films for the application of spintronic devices

Half-metallic Fe3O4 films were prepared at room temperature using a rf sputtering system specially integrated with an external rf source. Primary emphasis was placed on obtaining a large amount of active oxygen radicals through an external electrode for efficient deposition. The insertion of an external electrode was found to be critical for room temperature growth of Fe3O4 thin films. The structural and electrical properties gave shift and broadening effects to the Verwey temperature at various powers. The magnetization could only be saturated when a 300 Oe field was applied along an easy axis of magnetization during growth. However, there was no sign of saturation up to 5 T under zero-field growth.

Roles of interfacial TiOxN1−x layer and TiN electrode on bipolar resistive switching in TiN/TiO2/TiN frameworks

Reversible counter-clockwise and clockwise resistive switching in a TiN/TiO2/TiN structure was studied by different polarities of bias voltage. The nature of the bipolar switching phenomenon is related to the creation and annihilation of filament paths caused by redox reactions at locally confined interfaces between the TiO2 layer and TiN electrode. The analysis of electron energy loss spectroscopy (EELS) confirmed the formation of interfacial TiOxN1−x layer between the TiO2 and TiN bottom electrode. The TiOxN1−x layer reduces current levels of ON and OFF states by partially blocking oxygen ion drift to the TiN bottom electrode.

Enhanced electrical characteristics of Au nanoparticles embedded in high-k HfO2 matrix

We present experimental results for laser-induced Au nanoparticle (NP) embedded in a HfO2 high-k dielectric matrix. Cross-sectional transmission electron microscopy images showed that the Au NPs of 8nm in diameter were clearly embedded in HfO2 matrix. Capacitance-voltage measurements of Pt∕HfO2∕AuNPs∕HfO2 on p-type Si substrate reliably exhibited metal-oxide-semiconductor behavior with a large flatband shift of 4.7V. In addition, the charge retention time at room temperature was found to exceed 105h. This longer time was attributed to the higher electron barrier height via high work function of the Au NP.

Multi-level resistive switching observations in asymmetric Pt/Ta2O5−x/TiOxNy/TiN/Ta2O5−x/Pt multilayer configurations

We examine multilevel (ML) resistance switching properties in a Pt/Ta2O5−x/TiOxNy/TiN/Ta2O5−x/Pt matrix, in which two bipolar resistive switching elements Pt/Ta2O5−x/TiOxNy and TiN/Ta2O5−x/Pt are anti-serially and electrically connected. The ML features for the three assigned, distinguishable resistance states are clearly identified by using an I–V device operation scheme, indicating that the middle TiN and TiOxNy electrodes are crucial for adjusting ML resistance states. Experimental observations suggest that the ML switching events rely on electrically induced oxygen ion drifts at interfaces between the top/bottom Ta2O5−x and middle TiN/TiOxNy layers.

Resistive Switching Characteristics of TiO 2 Films with Embedded Co Ultra Thin Layer

We systematically investigated the resistive switching properties of thin TiO₂ films on Pt/Ti/SiO₂/Si substrates that were embedded with a Co ultra thin layer. An in-situ sputtering technique was used to grow both films without breaking the chamber vacuum. A stable bipolar switching in the current-voltage curve was clearly observed in TiO₂ films with an embedded Co ultra thin layer, addressing the high and low resistive state under a bias voltage sweep. We propose that the underlying origin involved in the bipolar switching may be attributed to the interface redox reaction between the Co and TiO₂ layers. The improved reproducible switching properties of our novel structures under forward and reverse bias stresses demonstrated the possibility of future non-volatile memory elements in a simple capacitive-like structure.

Effects of Ru diffusion in exchange-biased MgO magnetic tunnel junctions prepared by in situ annealing

We study the effect of Ru diffusion in exchange-biased MgO magnetic tunnel junctions prepared by high-temperature in situ annealing above 400 °C, circumventing Mn diffusion conventionally caused by postannealing process. The high temperature in situ annealing leads to Ru diffusion at the CoFeB/Ru interfaces, and thereby results in a reduction in tunnel magnetoresistance (TMR). The minimization of Ru diffusion during the in situ annealing provides a large TMR of 294% at room temperature with an exchange-bias field of 280 Oe. In addition, the temperature and voltage dependence of TMR reveals that there is neither significant spin-exchange scattering nor severe impurity-assisted scattering in the MgO barrier.We study the effect of Ru diffusion in exchange-biased MgO magnetic tunnel junctions prepared by high-temperature in situ annealing above 400 °C, circumventing Mn diffusion conventionally caused by postannealing process. The high temperature in situ annealing leads to Ru diffusion at the CoFeB/Ru interfaces, and thereby results in a reduction in tunnel magnetoresistance (TMR). The minimization of Ru diffusion during the in situ annealing provides a large TMR of 294% at room temperature with an exchange-bias field of 280 Oe. In addition, the temperature and voltage dependence of TMR reveals that there is neither significant spin-exchange scattering nor severe impurity-assisted scattering in the MgO barrier.

Charge trap memory characteristics of AlOx shell-Al core nanoparticles embedded in HfO2 gate oxide matrix

The memory behavior of natively oxidized AlOx shell-Al nanoparticles (NPs) in a metal oxide semiconductor (MOS) structure was investigated. Transmission electron microscopy images clearly demonstrate the formation of an AlOx shell (thicknesses of 1–1.5 nm), surrounding Al (sizes of 5–7 nm) core NPs in the MOS structure. Electrical measurements exhibited a memory window of 3.6 V, together with a promising charge retention time of about 10 years. A possible band model needed for enhanced retention characteristics was given by considering the electron/hole barrier width and the additional interface states through the AlOx shell as a method of tunneling barrier engineering.

Multifunctional resistive switching behaviors employing various electroforming steps

We examine the electroforming-dependent multifunctional resistive switching features by operating a merged Pt/Ta2O5−x/Ta–Ta/Ta2O5−x/Pt switching device under particular bias and polarity conditions. The basic Pt/Ta2O5−x/Ta resistive switching cell comprising the completely merged device shows two different bipolar switching behaviors with an initial forming process under different bias polarities. Therefore, two switching elements can be merged in various ways to produce diverse functionalities such as asymmetric complementary resistive switching (CRS) and typical CRS, achieved through control of the forming process. A possible mechanism to explain the unique features observed is discussed in terms of bias-driven oxygen ion drift and Joule-heating-based filamentary path models. This work suggests a suitable electroforming route for advancing symmetric CRS characteristics.