Sung Heo

In situ observation of filamentary conducting channels in an asymmetric Ta2O5−x/TaO2−x bilayer structure

Electrically induced resistive switching in metal insulator-metal structures is a subject of increasing scientific interest because it is one of the alternatives that satisfies current requirements for universal non-volatile memories. However, the origin of the switching mechanism is still controversial. Here we report the fabrication of a resistive switching device inside a transmission electron microscope, made from a Pt/SiO₂/a-Ta₂O5-x/a-TaO2-x/Pt structure, which clearly shows reversible bipolar resistive switching behaviour. The current-voltage measurements simultaneously confirm each of the resistance states (set, reset and breakdown). In situ scanning transmission electron microscope experiments verify, at the atomic scale, that the switching effects occur by the formation and annihilation of conducting channels between a top Pt electrode and a TaO2-x base layer, which consist of nanoscale TaO1-x filaments. Information on the structure and dimensions of conductive channels observed in situ offers great potential for designing resistive switching devices with the high endurance and large scalability.

Full Surface Embedding of Gold Clusters on Silicon Nanowires for Efficient Capture and Photothermal Therapy of Circulating Tumor Cells

We report on rapid thermal chemical vapor deposition growth of silicon nanowires (Si NWs) that contain a high density of gold nanoclusters (Au NCs) with a uniform coverage over the entire length of the nanowire sidewalls. The Au NC-coated Si NWs with an antibody-coated surface obtain the unique capability to capture breast cancer cells at twice the highest efficiency currently achievable (~88% at 40 min cell incubation time) from a nanostructured substrate. We also found that irradiation of breast cancer cells captured on Au NC-coated Si NWs with a near-infrared light resulted in a high mortality rate of these cancer cells, raising a fine prospect for simultaneous capture and plasmonic photothermal therapy for circulating tumor cells.

Electronic and optical properties of Al2O3/SiO2 thin films grown on Si substrate

The electronic and optical properties of Al2O3/SiO2 dielectric thin films grown on Si(1 0 0) by the atomic layer deposition method were studied by means of x-ray photoelectron spectroscopy and reflection electron energy loss spectroscopy (REELS). The band gaps of the Al2O3/SiO2 thin films before annealing and after annealing were 6.5 eV and 7.5 eV, respectively, and those of the γ-Al2O3 and α-Al2O3 phases were 7.1 eV and 8.4 eV, respectively. All of these were estimated from the onset values of the REELS spectra. The dielectric functions were determined by comparing the effective cross-section determined from experimental REELS with a rigorous model calculation based on dielectric response theory, using available software packages. The determined energy loss function obtained from the Al2O3/SiO2 thin films before annealing showed a broad peak at 22.7 eV, which moved to the γ-Al2O3 position at 24.3 eV after annealing. The optical properties were determined from the dielectric function. The optical properties of the Al2O3/SiO2 thin films after annealing were in good agreement with those of γ-Al2O3. The changes in band gap, electronic and optical properties of the Al2O3/SiO2 thin films after annealing indicated a phase transition from an amorphous phase to the γ-Al2O3 phase after annealing.

Band alignment of atomic layer deposited (ZrO2)x(SiO2)1−x gate dielectrics on Si (100)

The band alignment of atomic layer deposited (HfZrO4)1−x(SiO2)x (x = 0, 0.10, 0.15, and 0.20) gate dielectric thin films grown on Si (100) was obtained by using X-ray photoelectron spectroscopy and reflection electron energy loss spectroscopy. The band gap, valence band offset, and conduction band offset values for HfZrO4 silicate increased from 5.4 eV to 5.8 eV, from 2.5 eV to 2.75 eV, and from 1.78 eV to 1.93 eV, respectively, as the mole fraction (x) of SiO2 increased from 0.1 to 0.2. This increase in the conduction band and valence band offsets, as a function of increasing SiO2 mole fraction, decreased the gate leakage current density. As a result, HfZrO4 silicate thin films were found to be better for advanced gate stack applications because they had adequate band gaps to ensure sufficient conduction band offsets and valence band offsets to Si.

Bandgap measurement of thin dielectric films using monochromated STEM-EELS

High-resolution electron energy-loss spectroscopy (HR-EELS), achieved by attaching electron monochromators to transmission electron microscopes (TEM), has proved to be a powerful tool for measuring bandgaps. However, the method itself is still uncertain, due to Cerenkov loss and surface effects that can potentially influence the quality of EELS data. In the present study, we achieved an energy resolution of about 0.13 eV at 0.1s, with a spatial resolution of a few nanometers, using a monochromated STEM-EELS technique. We also assessed various methods of bandgap measurement for a-SiNx and SiO2 thin dielectric films. It was found that the linear fit method was more reliable than the onset reading method in avoiding the effects of Cerenkov loss and specimen thickness. The bandgap of the SiO2 was estimated to be 8.95 eV, and those of a-SiNx with N/Si ratios of 1.46, 1.20 and 0.92 were measured as 5.3, 4.1 and 2.9 eV, respectively. These bandgap-measurement results using monochromated STEM-EELS were compared with those using Auger electron spectroscopy (AES)-reflective EELS (REELS).

Dielectric and optical properties of Zr silicate thin films grown on Si(100) by atomic layer deposition

Dielectric and optical properties of (ZrO2)x(SiO2)1−x dielectric thin films, grown on Si(100) by the atomic layer deposition method, were studied by means of reflection electron energy loss spectroscopy (REELS). The quantitative analysis of REELS spectra was carried out by using the quantitative analysis of electron energy loss spectra-e(k,ω)-REELS software to determine the dielectric function and optical properties by using an analysis of experimental REELS cross sections from the simulated energy loss function (ELF). For ZrO2, the ELF shows peaks in the vicinity of 10, 15, 21, 27, 35, 42, and 57 eV. For SiO2, a broad peak at 23 eV with a very weak shoulder at 15 eV and a shoulder at 34 eV were observed, while for Zr silicates (x=0.75 and 0.5), the peak position is similar to that of ZrO2. For Zr silicates with high SiO2 concentration (x=0.25), the peak positions are similar to that of SiO2, but the peak at 42 eV, which is due to excitation of Zr N2,3 shell electrons, still exist. This indicates that the...

Deep level trapped defect analysis in CH3NH3PbI3 perovskite solar cells by deep level transient spectroscopy

We report the presence of defects in CH3NH3PbI3, which is one of the main factors that deteriorates the performance of perovskite solar cells. Although the efficiency of the perovskite solar cells has been improved by curing defects using various methods, deeply trapped defects in the perovskite layer have not been systematically studied, and their function is still unclear. The comparison and analysis of defects in differently prepared perovskite solar cells reveals that both solar cells have two kinds of deep level defects (E1 and E2). In the one-pot solution processed solar cell, the defect state E1 is dominant, while E2 is the major defect in the solar cell prepared using the cuboid method. Since the energy level of E1 is higher than that of E2, the cuboid solar cell shows higher open-circuit voltage and efficiency.

Changes in Chemical and Structural Properties of Phase-Change Material GeTe with Nitrogen Doping and Annealing

In this study, changes in the chemical, structural, and electrical properties of undoped and 8.4 at. % nitrogen-doped GeTe films were investigated. The transition temperature of sheet resistance increased as a result of nitrogen doping, which corresponded well with phase transformations. The shift of chemical potential toward lower binding energies strongly depended on crystallization. The Ge/Te ratio showed a tendency to increase towards the surface only for undoped GeTe. Nitrogen doping may suppress the instability of GeTe.

Band gap engineering for La aluminate dielectrics on Si (100)

La aluminate (La2O3)x(Al2O3)1−x films were grown by atomic layer deposition method, for which the conduction band offset, valence band offset, and band gap were obtained by using x-ray photoelectron spectroscopy and reflection electron energy loss spectroscopy. The valence band offsets were nearly unchanged within the range from 2.23to2.37eV with increasing Al content. The conduction band offsets were changed from 2.40to2.86eV for the above dielectrics. Remarkably, the band gap could be engineered from 5.75to6.35eV by increasing Al content. We also found that La aluminate films have symmetric band profiles.

Nitrogen-Doping Effect on Ge2Sb2Te5 Chalcogenide Alloy Films during Annealing

The microstructural and electrical-property changes of undoped and 5.4% nitrogen-doped Ge2Sb2Te5 were investigated. The transition temperature of sheet resistance increased owing to nitrogen doping, which corresponded well with the observed phase-change states. The lattice parameters of the undoped and nitrogen-doped Ge2Sb2Te5 exhibited the same tendency of decrease with increasing annealing temperature. Considering the increase in the Ge2Sb2Te5 energy state owing to the presence of interstitial nitrogen, the increase in the crystallization temperature is contrary to the thermodynamic viewpoint. Nitrogen atoms and N2 gas can be located at the interstitial site without distorting the crystal structure.