Hyunjun Yoo

Distance control for a near-field scanning microwave microscope in liquid using a quartz tuning fork

We demonstrate a scanning near-field microwave microscope (NSMM) in the liquid environment using a tuning fork shear-force feedback method to control the distance between tip and sample. Only the probe tip for the NSMM is immersed in water. The dry part of the probe is attached to one prong of a quartz tuning fork and directly coupled to a high-quality dielectric resonator at an operating frequency f=4.5–5.5GHz. This distance control method is independent of the local microwave characteristics. The amplitude of the tuning fork was used as a set point of the distance control parameter in the liquid. To demonstrate the distance regulation system, we present the NSMM images of a copper film in air and liquid without and with readjustment of the distance set point, as well as an image of a DNA film in buffer solution. Imaging under buffer environments is of particular interest for future studies of biomolecular association reactions on solid supports.

Confined crystallization of anatase TiO2 nanotubes and their implications on transport properties

Nanotubes of TiO2 (anatase) and their ordered arrays are emerging, promising candidates as efficient host materials in many applications such as photovoltaic cells, batteries, sensors and catalysts/catalytic supports, but the interplay between these structures and their transport properties has been reported only rarely. Monodisperse, stoichiometric TiO2 nanotubes with smooth morphology and controlled wall thickness were fabricated by template-directed low-temperature atomic layer deposition (ALD), followed by annealing at elevated temperatures. We present a study on the wall thickness-dependent crystallization behaviors due to physical and/or self-confinement, as well as on the corresponding electrical properties. Over certain wall thicknesses, unexpectedly, our TiO2 nanotubes were found to be a new type of mesoporous wide gap semiconductor in which they possess similar porosity, but in terms of conductivity differ from previously known mesoporous photoanodes (i.e., anodized surfaces of Ti films and sintered films consisting of TiO2 nanoparticles). These results were ascribed to the large, elongated anatase domains (by a factor of up to 15–40 wall thicknesses) that developed via boosted crystal growth on porous alumina templates (physical confinement) as well as to the highly curved tubular shape (self-confinement). Indeed, nanotube arrays with walls thicker than 10 nm exhibited an enhancement in conductivity, by more than three orders of magnitude, compared to sintered, mesoporous TiO2 (anatase) particles, approaching the bulk value. The nearly single-crystalline TiO2 nanotubes presented here should allow for a good model system to study TiO2-based surface chemistry and have potential for many applications in photovoltaic and/or catalytic systems.

Visualization of three dimensional domain structures in ferroelectric PbTiO3 nanotubes

We report visualization of three-dimensional domain structures in ferroelectric PbTiO3 (PTO) nanotubes (NTs) using piezoresponse force microscopy (PFM). The domain distributions of the x, y, and z-axes in PTO NTs were separately measured out-of and in-planes at angles of 0° and 90° by vertical and lateral PFM. The separately obtained PFM images were combined to reconstruct the complex domain structures based on some basic assumptions and finite element modeling. The cross-sectional domain configurations of the PTO NTs were visualized by the proposed approach. The results can provide insight into complex domain configurations of ferroelectric nanostructures.

Nondestructive high spatial resolution imaging with a 60 GHz near-field scanning millimeter-wave microscope

We demonstrate a nondestructive millimeter-wave surface imaging technique using a near-field scanning millimeter-wave microscope (NSMM) with a resonant standard waveguide probe at an operation frequency f=60 GHz. A chemically etched metallic probe tip was coupled to the resonant rectangular waveguide. By properly tuning the tunable resonator and the probe tip we could improve sensitivity and spatial resolution of the NSMM. By measuring the change of the quality factor in the near-field zone, near-field scanning millimeter-wave images of patterned metal films and YBa2Cu3Oy thin films were obtained with a spatial resolution better than 1 μm. We observed the dependence of the current density NSMM images of patterned indium tin oxide films on the bias currents.

Charge diffusion in silicon nitrides: Scalability assessment of nitride based flash memory

Electron and hole diffusion coefficients of stoichiometric silicon nitride, silicon rich nitride, and silicon oxynitride were evaluated from variable temperature electrostatic force microscopy (EFM) analysis. Among them, stoichiometric silicon nitride is shown to have smallest diffusion coefficient although silicon oxynitride has the higher temperature activation energy. Scaling charge trap flash towards sub-20nm regime should be accompanied by hole dispersion management, minimization of internal electric field, and adjustment of retention specification.

Lateral redistribution of trapped charges in nitride/oxide/Si (NOS) investigated by electrostatic force microscopy

Charge decay and lateral spreading properties were characterized by modified electrostatic force microscopy (EFM) under a high vacuum at elevated temperatures. Variations in the charge profiles were modeled with the maximum charge density (ρ(m)) and the lateral spreading distance (Δ(s)), as extracted from the EFM potential line profiles. The scaling limitation of nitride trap memory is discussed based on the projected lateral spreading distances for holes and electrons, which were determined to be approximately 18 nm and 12 nm, respectively, at room temperature.

Enhanced electron lifetime in dye-sensitised solar cells via suppression of electron-hole recombination

Electron-hole recombination is the major problem that reduces the light-electric energy conversion efficiency of photovoltaic devices. We fabricated oxide passivation layers on the surface of transparent conducting oxides (TCO) and photoanodes by using atomic layer deposition (ALD) with the optimised thickness. The intensity-modulated spectroscopy and voltage decay measurements show that the oxide layers, which are considered as blocking layers, effectively reduce the recombination and enhance electron lifetime. Enhanced electron lifetime affects the fill factor and open circuit voltage. With the optimised thickness of the blocking layers, the fill factor and the open circuit voltage were increased by 6% and 14%, respectively. Furthermore, we separated roles of passivation layers on the photoanode and the TCO with open circuit voltage decay (OCVD) measurement. A comparison of the electron recombination characteristics with the oxide layers on the surfaces of NP and the TCO exhibits quite different behaviour, in that the recombination of electrons with higher potential mostly occurs at the surface of NPs, while others with lower potential occur at the surface of TCO.

Study of Space Charge of Metal/copper(II)-phthalocyanine Interface

We report the space charge and the surface potential of the interface between metal and copper(Ⅱ)-phthalocyanine(CuPc) thin films by measuring the microwave reflection coefficients S/sub 11/ of thin films using a near-field scanning microwave microscope(NSMM). CuPc thin films were prepared on Au and Al thin films using a thermal evaporation method. Two kinds of CuPc thin films were prepared by different substrate heating conditions; one was deposited on preheated substrate at 150。C and the other was annealed after deposition. The microwave reflection coefficients S/sub 11/ of CuPc thin films were changed by the dependence on grain alignment due to heat treatment conditions and depended on thickness of CuPc thin films. Electrical conductivity of interface between metal and organic CuPc was changed by the space charge of the interface. By comparing reflection coefficient S/sub 11/ we observed the electrical conductivity changes of CuPc thin films by the changes of surface potential and space charge at the interface.

Nano resolution imaging technique with a near-field scanning microwave microscope

We demonstrated a near-field scanning microwave microscope (NSMM) with a nano spatial resolution using a hybrid tip. In order to understand the function of the probe, we fabricated the hybrid tip with a flat shaped shoulder and a reduced length of the tapered part using a conventional chemical etching technique. Two different NSMM images were observed for patterned Cr films on glass substrates and analyzed as a function of the apex and the cone angle of the tapered part. The hybrid tips were coupled to a high quality dielectric resonator at an operating frequency f = 4.46 GHz. By using the hybrid tip, we demonstrated an improved high contrast NSMM image of lambda phage DNA on a glass substrate.

Study of Phase Transition of Copper(II)-phthalocyanine using a Near Field Scanning Microwave Microscope

We report the changes of the microwave reflection coefficients S of copper(II)-phthalocyanine (CuPc) thin films by using a near-field microwave microscope(NSMM) in order to understand the phase transition of CuPc. For a NSMM system, a high-quality microstrip resonator coupled with a dielectric resonator was used. CuPc thin films were prepared on the pre-heated glass substrates using a thermal evaporation method. The reflection coefficients S of CuPc thin films were changed by the dependence on the substrate pre-heating temperatures. By comparing reflection coefficient S and crystal structures, we found the phase transition of CuPc thin films from -phase to -phase at the substrate heating temperature 200 ./TEX>.