Jinhee Heo

Synthesis and characteristics of NH2-functionalized polymer films to align and immobilize DNA molecules

We developed a method to use NH2-functionalized polymer films to align and immobilize DNA molecules on a Si substrate. The plasma-polymerized cyclohexane film was deposited on the Si substrate according to the radio frequency plasma-enhanced chemical vapor deposition method using a single molecular precursor, and it was then treated by the dielectric barrier discharge method in a nitrogen environment under atmospheric pressure. Changes in the chemistry of the surface functional groups were studied using X-ray photoelectron spectroscopy and Fourier transformed infrared spectroscopy. The wettability of the surfaces was examined using dynamic contact angle measurements, and the surface morphology was evaluated using atomic force microscopy.We utilized a tilting method to align λ-DNA molecules that were immobilized by the electrostatic interaction between the amine groups in NH2-functionalized polymer films and the phosphate groups in the DNA. The DNA was treated with positively charged gold nanoparticles to make a conductive nanowire that uses the DNA as a template. We observed that the NH2-functionalized polymer film was useful for aligning and immobilizing the DNA, and thus the DNA-templated nanowires.

Study on Ag mesh/conductive oxide hybrid transparent electrode for film heaters

Ag mesh-indium tin oxide (ITO) hybrid transparent conductive films were fabricated and evaluated for use in film heaters. PS monolayer templates were prepared using highly mono-dispersed PS spheres (11.2 μm) obtained by a filtering process with micro-sieves. At first, three Ag meshes with different sheet resistances (20, 100, and 300 Ω sq(-1)) and transmittances (70, 73, and 76%) were evaluated for film heaters in terms of voltage and long-term stability. Subsequently, in an effort to obtain better transmittance, Ag mesh-ITO hybrid heaters were fabricated utilizing finite ITO depositions. At the optimised ITO thickness (15 nm), the sheet resistance and the transmittance were 300 Ω sq(-1) and 88%, respectively, which indicates that this material is a good potential candidate for an efficient defroster in vehicles.

Fabrication of ordered anodic aluminum oxide with matrix arrays of pores using nanoimprint

Anodic aluminum oxide (AAO) with matrix arrays of pores was obtained using nanoimprint and anodizing. Si3N4 imprint stamps were fabricated using electron-beam lithography. The Si3N4 stamps were imprinted into Al films grown on Si wafers by applying a force of 250 kg for 10 s. Two different diameters (45 and 80 nm) and two different periodic distances (100 and 200 nm) of the square patterns in Si3N4 imprint stamp were transferred into an Al film as a form of shallow pores. The Al films were then anodized in 0.3M oxalic acid under conditions of 4 °C at 40 V. The authors found a linear relationship between the interpore distance and anodizing voltage (2.5 nm V−1) to obtain a square array in AAO. In addition, the matrix pores in AAO tend to form a natural hexagonal pattern as the anodizing time increases. The surface images are obtained using field-emission secondary-electron microscope and scanning probe microscopy.

Ferroelectric Properties of Very Thin Pb(Zr0.4Ti0.6)O3 Film Determined by Kelvin Force Microscope.

We attempted to examine scaling issues in terms of PbZr0.4Ti0.6O3 (PZT) film thickness. Several very thin PZT films were obtained by the combination of the wet cleaning method and the etch-back process. The polarizability of PZT film was evaluated by measuring the surface potential utilizing the Kelvin force microscope (KFM) mode. The surface potential decreased as the thickness of the PZT film decreased. In particular, the decreasing rate of the surface potential was accelerated below 60 nm, which indicates a possible limitation of the PZT film thickness in terms of the remnant polarization value. Additionally, the retention property as a function of PZT film thickness was also examined by measuring the surface potential in terms of elapsed time. The result shows that the retentivity of the remnant polarization becomes worse as the film thickness decreases.

Application of Scanning Probe Microscope for Novel Characterization of Ferroelectric Capacitor.

Novel efforts have been made to analyze the electrical properties of ferroelectric devices by utilizing scanning probe microscopy (SPM). In this study, an attempt was made to measure the hysteresis loop of submicron ferroelectric capacitors using SPM as a probe tip. Conductivity mapping images were useful for understanding the inferior process. Additionally, the surface potential measurements using the Kelvin force microscopy (KFM) mode were made for various ferroelectric films. The polarizability of Pb(Zr,Ti)O3(PZT) film can be evaluated by measuring the surface potential, and seems to be a promising aspect particularly for analyzing various ferroelectric films including hydrogen-damaged films.

Study on Polarization Properties of Randomly Oriented Bi3.35La0.85Ti3O12 Ferroelectric Thin Film Utilizing Three-Dimensional Piezoresponse Image

We investigated the domain structures of randomly oriented Bi3.35La0.85Ti3O12 (BLT) ferroelectric thin films by mapping the three-dimensional piezoresponse images of the film. Such BLT films were prepared by the sol–gel method. The domain arrangement in the patterned BLT thin films without a top electrode was recorded along all three components of the x-, y- and z-axes by monitoring lateral deflection and vertical displacement. In particular, to obtain lateral deflection along the y-axis, the patterned sample was physically rotated by 90°. In addition, polarization reversal was investigated by applying dc voltage through a conductive tip during the area scanning. We observed that various types of domain behavior such as 54.7° and 90° domain switchings, and pinned domain formation occurred.

Electrical characterization of sub-micron magnetic tunneling junction cells using scanning probe microscopy

We attempted to measure the tunneling current behavior of magnetic tunneling junction (MTJ) cells utilizing scanning probe microscopy (SPM) interfaced with an external magnetic field generator. Magnetic field was generated by allowing current to flow through coils and controlled by current ieldfeedback circuit, thereby enabling the evaluation of tunneling current under various magnetic fields. I-V measurement was carried out in the contact mode using a conducting cantilever at a specific magnetic field. The obtained magnetoresistance (MR) ratios of MTJ cells were about 21% regardless of the variation in the size of MTJ cells, and RA ranged from 8.5 K to 12.5 K[Ωµm2]. In addition, we also attempted to observe magnetic images of MTJ cells under various magnetic fields. We believe that the novel characterization method utilizing SPM is greatly beneficial for the characterization of MTJ cells because it enables the measurement of the I-V behavior of ultrasmall cells without the need for a using an extra electrode. Thus, the novel method may be used to measure the electrical properties of ultrasmall MTJ cells, namely below 0.1 µm×0.1 µm.

Characterization of Domain Switching Behavior of MTJ Cells Using Magnetic Force Microscopy (MFM) and R–H Loop Analysis

Correlation between electrical and magnetic properties of magnetic tunnel junctions (MTJ) for magnetic random access memory (MRAM) was studied. The MTJ (Ta/NiFeCr/ PtMn/CoFe/Ru/CoFe/Al2O3/CoFe/NiFe/Ta) was analyzed by utilizing R-H loops and MFM images. We verified that a kink in an R-H loop comes from a vortex domain of free layer. In addition, we also observed a close relationship between a domain switching behavior and an irregular R-H curve. These results would be useful for the characterization of the MTJ cell, thereby optimizing the process to realize an ultrahigh density MRAM.

Characterization of domain switching behavior of MTJ cells using magnetic force microscopy(MFM) and H-R loop analysis

Correlationship between electrical and magnetic properties of magnetic tunnel junction (MTJ) for Magnetic Random Access Memory(MRAM) was studied. MTJ (Ta/NiFeCr/PtMn/CoFe/Ru/CoFe/AlOx/CoFe/NiFe/Ta) was analyzed utilizing H-R loop and MFM images. We verified that the kink in H-MR loop comes from the vortex domain of free layer. In addition, we also observed a close relationship between domain switching behavior and anomalous H-R curve. These results would be useful for characterization of MTJ cell, thereby optimizing the process to realize the ultra high density MRAM.

Study of the relationship between scalability of MTJ and switching field using SPM

The switching field variation in terms of the sizes of sub-micron MTJ cells was studied by measuring the I-V characteristics of MTJ cells and hysteresis I-H loop, obtained by measuring the tunneling current with sweeping magnetic field. The observed switching field using SPM was compared to the results obtained through conventional methods. The measured currents with respect to applied magnetic fields reveal hysteretic behavior with two stable states. It was also observed that the switching field of MTJ cell increase as the size of MTJ cell decreases due to the depolarization.