Y. Eugene Pak

Principle of ferroelectric domain imaging using atomic force microscope

The contrast mechanisms of domain imaging experiments assisted by atomic force microscope (AFM) have been investigated by model experiments on nonpiezoelectric (silicon oxide) and piezoelectric [Pb(Zr,Ti)O3] thin films. The first step was to identify the electrostatic charge effects between the tip, the cantilever, and the sample surface. The second step was to explore the tip–sample piezoelectric force interaction. The static deflection of the cantilever was measured as a function of dc bias voltage (Vdc) applied to the bottom electrode (n-type Si wafers) for noncontact and contact modes. In addition, a small ac voltage (Vac sin ωt) was applied to the tip to measure the amplitude (Aω) and phase (Φω) of the first harmonic (ω) signal as a function of Vdc. By changing from the noncontact to the contact mode, a repulsive contribution to the static deflection was found in addition to the attractive one and a 180° phase shift in Φω was observed. These results imply that in the contact mode the cantilever buckl...

Quantitative analysis of the bit size dependence on the pulse width and pulse voltage in ferroelectric memory devices using atomic force microscopy

The Bit formation using atomic force microscopy (AFM) was studied on 270-nm-thick 〈111〉 preferentially oriented Pb(Zr0.4Ti0.6)O3 (PZT) films prepared by the sol-gel process. To minimize the cantilever-sample capacitive force interaction, the experiment was carried out at or near the sample edge. Bit formation was investigated by calculating the electric field in AFM-tip/PZT film/bottom electrode configuration. It was found both experimentally and theoretically that the bit size is linearly dependent on the pulse voltage and the logarithmic value of the pulse width. The linear dependence of the bit size on the logarithmic value of pulse width was explained from the relationship between the switching time and electric field. It was found that the minimum bit size of a fully penetrating domain equals the film thickness.

A Dielectric Biosensor Using the Capacitance Change with AC Frequency Integrated on Glass Substrates

Glass-based microchannel chips were fabricated using photolithographic technology, and Pt thin-film microelectrodes as dielectric biosensors were integrated on them. From capacitance-frequency measurements at various interelectrode distances and ionic concentrations, a significant difference between deionized (DI) water and tris-ethylenediaminetetraacetic acid (EDTA) (TE) buffer was observed in the low-frequency region. Although the capacitance (CM) of the DI water decreased as the interelectrode distance increased, that of the TE buffer was similar up to a frequency of 100 Hz, after which it was spilt in the same manner as the DI water above 100 Hz. As the ionic concentration increased, the CM of the TE buffer increased and the slope in the low frequency region changed from -0.875 to -0.425. The point where the slope changed shifted towards the frequency increase. These observations were clarified from the viewpoint of interfacial phenomena, such as the electrical double layer and Faradaic reactions, the dielectric constant related to conductivity, and the capacitance inversely proportional to the interelectrode distance. The addition of deoxyribonucleic acid (DNA) molecules (10 ng/µl) increased the capacitance and dielectric loss in the TE buffer at low frequency. It is feasible to use dielectric properties for the rapid and direct detection of biomolecules, particularly DNA molecules, without using labels or indicators.

DETECTION MECHANISM OF SPONTANEOUS POLARIZATION IN FERROELECTRIC THIN FILMS USING ELECTROSTATIC FORCE MICROSCOPY

Mechanism on the detection of spontaneous polarization in a Pb(Zr0.5Ti0.5)O3 (PZT) film using contact mode of electrostatic force microscopy (EFM) is investigated. Theoretical calculations are performed on deflections induced by electrostatic force (uωe) between the tip and the sample and electromechanical vibrations (uωp) of the ferroelectric materials, respectively. From the calculation, uωe and uωp are 3.73×10-9 and 1.77×10-13 m. Enhanced mode of EFM shows the complete cancellation of the EFM image induced by the electrostatic force between the tip and the film through controlling dc voltage. Hence, electrostatic force effect is a main contributor on the detection mechanism of spontaneous polarization using EFM in contact mode.

Formation of ferroelectric nano-domains using scanning force microscopy for the future application of memory devices

Abstract Applying voltage between the conductive tip in atomic force microscopy (AFM) and bottom electrode through Pb(Zr, Ti)O3 (PZT) films can cause switching of ferroelectric domains. Formation and imaging of ferroelectric domains in nanometer scale could be applied to develop the future ultrahigh-density memory device. Relevant issues, i.e. bit (induced ferroelectric domains) size dependence on applied voltage and pulse width, are discussed. The bit size showed a log-linear dependence on the pulse width and a linear dependence on the pulse voltage. Using the analysis of electric field distribution, the size of the induced bits under certain pulse voltage and width was estimated.

Formation and observation of ferroelectric domains in PbZr1-xTixO3(PZT) thin films using atomic force microscopy

Very small-sized ferroelectric domains were induced and observed using a modified atomic force microscopy (AFM). Bias voltage between a conductive AFM tip and a sol-gel processed PZT film caused the switching of small ferroelectric domains. ELectrostatic forces between the polarized area and the tip provide the imaging of the polarized small domains. Applying voltage with the opposite sign can depolarize the polarized area and the formation of a series of data dots was demonstrated. In addition, the retention phenomena of micron size domains in PZT films were investigated. The polarized images disappeared within a few days even without an application of voltage - often called the retention loss or failure. An empirical relationship between relaxation time, bit size and poling time is established and verified. Two operative processes for the retention loss are either the stray charge accumulation on the polarized surfaces or the stress relaxation of the piezoelectric films. An effective way of improving the retention characteristics is suggested. The experimental results obtained in this study provide substantial insight into the mechanism for the retention failure of the polarized domains as well as the polarization behavior in PZT films with a nanometer scale.

Fabrication of in situ patterned iron oxide films using micro contact printing and selective deposition

Abstract In situ patterning of iron oxide thin layers were fabricated via microcontact printing (μCP) and selective area deposition. μCP is used to pattern two different surface moieties of self-assembled organic monolayers (SAMs) on Au/Cr/Si substrates. An elastomeric stamp is used to transfer hydrophobic (hexadecanethiol (HDT)) SAMs that are to sustain deposition of iron oxide precipitates. Hydrophilic (dithiothreitol, DTT) SAMs were used to induce crystalline iron oxide films. 0.05 M of iron nitrate (Fe(NO3)3·9H2O) aqueous solutions containing urea under nitric acid (pH ≤ 2) were used. Selective depositions were realized through precipitation of crystalline iron oxides at ambient temperature (80°C) onto the mixed SAM surfaces.

Effect of metal-insulator-semiconductor structure derived space charge field on the tip vibration signal in electrostatic force microscopy

The role of metal–insulator–semiconductor structure derived space charge field in the tip vibration signal of electrostatic force microscopy was studied using boron doped Si tip and Pt film sputtered on Si substrate. The ohmic contact between the tip and the film was confirmed by current–voltage characterization. Then the tip was held at a position so that an air gap of 100 nm existed between the tip and the film. The tip deflection and the tip vibration signals were examined by applying dc voltage to the film and ac voltage to the tip. The asymmetry in both signals supports the existence of the space charge field, and the direction of the field at zero dc bias field is from the tip to the sample as expected from the band bending theory.

Characterization of Retention Phenomena of Micron-Size Electrical Domains in Pzt Thin Films

The retention phenomena of purposely aligned micron-size domains (defined as “bits”) in Pb(Zr,Ti)O 3 thin films were characterized by atomic force microscopy (AFM) combined with a lock-in amplifier. It is found that the retention loss occurs by “region by region” showing local variation of the rate of the loss. Furthermore, the total retention loss can be successfully described by an extended exponential decay, which implies a narrow distribution of the relaxation times of the domains. This probably comes from the fact that the micron-size bits consist a few hundreds of domains. Along with the characterization, the effects of the bit size and the poling time per unit area on the retention characteristics were investigated. Based on our observations, it is concluded that the retention time is proportional to both the poling time per unit area and the bit size. This trend is successfully explained by a kinetic model developed by our group.