JinPyo Hong

Temperature dependence of high-and low-resistance bistable states in polycrystalline NiO films

The resistance switching current-voltage (I-V) characteristics in polycrystalline NiO films were investigated in the temperature range of 10K<T<300K. Very clear reversible resistive switching phenomena were observed in the entire temperature range. An analysis of the temperature dependence of the resistance switching transport revealed additional features, not reported in previous studies, that weak metallic conduction and correlated barrier polaron hopping coexist in the high-resistance off state and that relative dominance depends on the temperature and defect configuration. In addition, the authors propose that metallic Ni defects, existing near polycrystalline (or granular) boundaries, play a key role in the formation of a metallic channel.

p‐Type Conduction Characteristics of Lithium‐Doped ZnO Nanowires

Nanostructured electronic devices are expected to facilitate the continuing miniaturization of electronic devices and enable ultralow power device operation. In particular, 1D nanostructures such as nanowires (NW) and nanotubes have attracted great interest over the past decade because of their specifi c physical properties and their potential as building blocks for next-generation nanoelectronic devices. [ 1 , 2 ] Among various 1D materials, zinc oxide (ZnO), which has a direct and wide bandgap, is a promising candidate for light-emitting diodes, UV and gas sensors, transistor channels, and other devices that can utilize the unique vertical alignment characteristics and highly ordered single crystalline properties of NW structures. [ 3–6 ] However, because undoped ZnO NWs are intrinsically n-type, their use in practical devices has been hindered and much effort has been dedicated toward the development of p-type ZnO NWs. In particular, control and manipulation of the doping process is increasingly becoming a key approach for the realization of p-type ZnO NWs. To realize p-type ZnO NWs, the initial dopant candidates tested included group V elements to substitute for O and group III elements to substitute for Zn, despite the large size mismatches in both cases. Recently, group I species such as Li and Na have been used to synthesize p-type ZnO NWs based on the expectation that these elements would function as shallow acceptors in ZnO host materials. [ 7–9 ] Li has the smallest ionic radius (0.76 Å) of group I species, which is very close to that of Zn (0.74 Å). Furthermore, several reports of excited centers observed using electron paramagnetic resonance spectroscopy have indicated that Li atoms can act as shallow acceptors in substantial forms of Zn sites (Li Zn ). [ 10 , 11 ] In addition, it is well-known that Li has specifi c advantages over other dopant

Memory characteristics of cobalt-silicide nanocrystals embedded in HfO2 gate oxide for nonvolatile nanocrystal flash devices

Cobalt-silicide (CoSi2) nanocrystals (NCs) were investigated for use in charge storage for metal oxide semiconductor (MOS) devices with thin HfO2 tunneling and control oxide layers. CoSi2 NCs were synthesized by exposure of Co∕Si∕HfO2 tunneling oxide/Si stacks to an external UV laser. Observations from transmission electron microscopy and x-ray photoelectron spectroscopy clearly confirm the formation of CoSi2 NCs and the values of Co–Si bonding energies that are shifted 0.3eV from original values, respectively. The CoSi2 NCs in MOS devices exhibited a large memory window of 3.4V as well as efficient programming/erasing speeds, good retention, and endurance times.

Electrically induced conducting nanochannels in an amorphous resistive switching niobium oxide film

Metallic nanostructures that act as electrical switches between bistable resistance states are created electrically in an insulating amorphous niobium oxide thin film. The physical formation of the metallic nanostructures are probed using in situ focused ion beam scanning electron microscopy equipped with a current-voltage measurement system. While the electroforming process changes the film, dramatically inducing metallic nanochannels across it, significant changes in the film do not occur during repeated resistance switching afterward. A qualitative resistive switching model is proposed taking into account the gradual forming process.

Ultrathin W space layer-enabled thermal stability enhancement in a perpendicular MgO/CoFeB/W/CoFeB/MgO recording frame

Perpendicularly magnetized tunnel junctions (p-MTJs) show promise as reliable candidates for next-generation memory due to their outstanding features. However, several key challenges remain that affect CoFeB/MgO-based p-MTJ performance. One significant issue is the low thermal stability (Δ) due to the rapid perpendicular magnetic anisotropy (PMA) degradation during annealing at temperatures greater than 300 °C. Thus, the ability to provide thermally robust PMA characteristics is a key steps towards extending the use of these materials. Here, we examine the influence of a W spacer on double MgO/CoFeB/W/CoFeB/MgO frames as a generic alternative layer to ensure thermally-robust PMAs at temperatures up to 425 °C. The thickness-dependent magnetic features of the W layer were evaluated at various annealing temperatures to confirm the presence of strong ferromagnetic interlayer coupling at an optimized 0.55 nm W spacer thickness. Using this W layer we achieved a higher Δ of 78 for an approximately circular 20 nm diameter free layer device.

Performance of electrophoretic deposited low voltage phosphors for full color field emission display devices

Electrophoretic deposition of low voltage phosphors was successfully carried out on patterned indium–tin–oxide glass in order to develop prototype full color field emission display devices. The optimized suspension consisted of 2–3 μm size color phosphors in isopropyl alcohol, and mixed La(NO3)2 and Al(NO3)2 salts for the best display panel images. The ZnS:Cu, Al, ZnS:Ag, Cl, and Y2O2S:Eu phosphors were used to produce primary green, blue, and red colors, respectively. The compositional and structural analysis of the phosphors was performed and basic deposition parameters, such as deposition time and applied voltages, were systematically investigated in order to obtain the desirable brightness in real panels. A uniform thickness of about 4–9 μm was achieved over a whole 4 in. glass plate, depending on the electrical properties of each phosphor. Finally, the deposited phosphors were extensively characterized by a scanning electron microscopy, three-dimensional surface analysis.

Surface energy-mediated construction of anisotropic semiconductor wires with selective crystallographic polarity

ZnO is a wide band-gap semiconductor with piezoelectric properties suitable for opto-electronics, sensors, and as an electrode material. Controlling the shape and crystallography of any semiconducting nanomaterial is a key step towards extending their use in applications. Whilst anisotropic ZnO wires have been routinely fabricated, precise control over the specific surface facets and tailoring of polar and non-polar growth directions still requires significant refinement. Manipulating the surface energy of crystal facets is a generic approach for the rational design and growth of one-dimensional (1D) building blocks1234. Although the surface energy is one basic factor for governing crystal nucleation and growth of anisotropic 1D structures, structural control based on surface energy minimization has not been yet demonstrated56789. Here, we report an electronic configuration scheme to rationally modulate surface electrostatic energies for crystallographic-selective growth of ZnO wires. The facets and orientations of ZnO wires are transformed between hexagonal and rectangular/diamond cross-sections with polar and non-polar growth directions, exhibiting different optical and piezoelectrical properties. Our novel synthetic route for ZnO wire fabrication provides new opportunities for future opto-electronics, piezoelectronics, and electronics, with new topological properties.

Bipolar resistance switching characteristics in a thin Ti?Ni?O compound film

Resistance switching phenomena in an amorphous Ni-Ti-O film were investigated. Very clear bipolar resistive switching characteristics were observed with good reproducibility. Stable retention and on/off pulse switching operation was demonstrated. An analysis of x-ray photoelectron spectroscopy of the Ni-Ti-O film provided a clue that the observed unusual bipolar resistance switching in the film is due to a microscopic change in the Ni-O and Ti-O binding states at the Ni-Ti-O film/electrode interface.

Unipolar resistive switching in insulating niobium oxide film and probing electroforming induced metallic components

We have observed unipolar-type resistance switching in an ultrathin niobium oxide film. An analysis of the temperature dependence of the resistance switching transport revealed that low-resistance state showed a type of electrical conduction typically observed in metals. The modification in chemical binding states of the film in different resistance states was studied using x-ray photoelectron spectroscopy. The analysis of XPS showed that metallic suboxides NbOδ (δ ≪ 2), decomposed from some of Nb2O5 and NbO2 components of the film, were created after electroforming process, suggesting that the metallic suboxides are constituting elements of metallic channels in the low resistance state.

All MgB2 tunnel junctions with Al2O3 or MgO tunnel barriers

All MgB2 thin film tunnel junctions with Al2O3 or MgO tunnel barriers were fabricated in situ on Si substrates in a molecular beam epitaxy system and their tunneling characteristics were investigated. In the quasiparticle tunneling spectra of the junction with Al2O3 tunnel barrier, we observed both superconducting gaps of MgB2, while only a small gap was seen with MgO tunnel barrier. Using a microscopic structural analysis, we found that the difference in the spectra is due to the crystal orientation difference of the MgB2 films: the film grown on Al2O3 was polycrystalline whereas the film grown on MgO was c-axis oriented.