Hee Han

Au/Ag Bilayered Metal Mesh as a Si Etching Catalyst for Controlled Fabrication of Si Nanowires

Au/Ag bilayered metal mesh with arrays of nanoholes were devised as a catalyst for metal-assisted chemical etching of silicon. The present metal catalyst allows us not only to overcome drawbacks involved in conventional Ag-based etching processes, but also to fabricate extended arrays of silicon nanowires (SiNWs) with controlled dimension and density. We demonstrate that SiNWs with different morphologies and axial orientations can be prepared from silicon wafers of a given orientation by controlling the etching conditions. We explored a phenomenological model that explains the evolution of the morphology and axial crystal orientation of SiNWs within the framework of the reaction kinetics.

Ultrahigh density array of epitaxial ferroelectric nanoislands on conducting substrates.

An ultrahigh density array of epitaxial PbTiO(3) (PTO) nanoislands with uniform size was fabricated on a single-crystalline Nb-doped SrTiO(3) (100) substrate over a large area (cm(2) scale) by simple but robust method utilizing polystyrene-block-poly(4-vinylpridine) copolymer micelles. Each nanoisland has an average volume of 2.6 x 10(3) nm(3) (a height of 7 nm and a diameter of 22 nm). Because of uniform nanoislands over a large area, a synchrotron X-ray diffraction experiment was successfully employed to analyze the domain structures of PTO nanoislands. They showed well-defined epitaxy on the substrate, which was also confirmed by high-resolution transmission electron microscopy. All of the nanoislands existing in the entire area showed distinct piezoresponse that confirms the existence of ferroelectricity at this size. The results indicate that the critical size of ferroelectrics could be scaled-down further, thereby much increasing the density of ferroelectric devices.

Nanostructured Ferroelectrics: Fabrication and Structure- Property Relations

With the continued demand for ultrahigh density ferroelectric data storage applications, it is becoming increasingly important to scale the dimension of ferroelectrics down to the nanometer-scale region and to thoroughly understand the effects of miniaturization on the materials properties. Upon reduction of the physical dimension of the material, the change in physical properties associated with size reduction becomes extremely difficult to characterize and to understand because of a complicated interplay between structures, surface properties, strain effects from substrates, domain nucleation, and wall motions. In this Review, the recent progress in fabrication and structure-property relations of nanostructured ferroelectric oxides is summarized. Various fabrication approaches are reviewed, with special emphasis on a newly developed stencil-based method for fabricating ferroelectric nanocapacitors, and advantages and limitations of the processes are discussed. Stress-induced evolutions of domain structures upon reduction of the dimension of the material and their implications on the electrical properties are discussed in detail. Distinct domain nucleation, growth, and propagation behaviors in nanometer-scale ferroelectric capacitors are discussed and compared to those of micrometer-scale counterparts. The structural effect of ferroelectric nanocapacitors on the domain switching behavior and cross-talk between neighboring capacitors under external electric field is reviewed.

In Situ Determination of the Pore Opening Point during Wet-Chemical Etching of the Barrier Layer of Porous Anodic Aluminum Oxide: Nonuniform Impurity Distribution in Anodic Oxide

Wet-chemical etching of the barrier oxide layer of anodic aluminum oxide (AAO) was systematically investigated by using scanning electron microscopy (SEM), secondary ion mass spectrometry (SIMS), and a newly devised experimental setup that allows accurate in situ determination of the pore opening point during chemical etching of the barrier oxide layer. We found that opening of the barrier oxide layer by wet-chemical etching can be significantly influenced by anodization time (tanodi). According to secondary ion mass spectrometry (SIMS) analysis, porous anodic aluminum oxide (AAO) samples formed by long-term anodization contained a lower level of anionic impurity in the barrier oxide layer compared to the short-term anodized one and consequently exhibited retarded opening of the barrier oxide layer during the wet-chemical etching. The observed compositional dependence on the anodization time (tanodi) in the barrier oxide layer is attributed to the progressive decrease of the electrolyte concentration upon anodization. The etching rate of the outer pore wall at the bottom part is lower than that of the one at the top part due to the lower level of impurity content in that region. This indicates that a concentration gradient of anionic impurity in the outer pore wall oxide may be established along both the vertical and radial directions of cylindrical pores. Apart from the effect of electrolyte concentration on the chemical composition of the barrier oxide layer, significantly decreased current density arising from the lowered concentration of electrolyte during the long-term anodization (~120 h) was found to cause disordering of pores. The results of the present work are expected to provide viable information not only for practical applications of nanoporous AAO in nanotechnology but also for thorough understanding of the self-organized formation of oxide nanopores during anodization.

Non-Kolmogorov−Avrami−Ishibashi Switching Dynamics in Nanoscale Ferroelectric Capacitors

Switching dynamics of nanoscale ferroelectric capacitors with a radius of 35 nm were investigated using piezoresponse force microscopy. Polarization switching starts with only one nucleation event occurring only at the predetermined places. The switching dynamics of nanoscale capacitors did not follow the classical Kolmogorov-Avrami-Ishibashi model. On the basis of the consideration of two separate (nucleation and growth) steps within a nonstatistical finite system, we have proposed a model which is in good agreement with the experimental results.

Nonlinear phenomena in multiferroic nanocapacitors: joule heating and electromechanical effects.

We demonstrate an approach for probing nonlinear electromechanical responses in BiFeO(3) thin film nanocapacitors using half-harmonic band excitation piezoresponse force microscopy (PFM). Nonlinear PFM images of nanocapacitor arrays show clearly visible clusters of capacitors associated with variations of local leakage current through the BiFeO(3) film. Strain spectroscopy measurements and finite element modeling point to significance of the Joule heating and show that the thermal effects caused by the Joule heating can provide nontrivial contributions to the nonlinear electromechanical responses in ferroic nanostructures. This approach can be further extended to unambiguous mapping of electrostatic signal contributions to PFM and related techniques.

First-Order Reversal Curve Probing of Spatially Resolved Polarization Switching Dynamics in Ferroelectric Nanocapacitors

Spatially resolved polarization switching in ferroelectric nanocapacitors was studied on the sub-25 nm scale using the first-order reversal curve (FORC) method. The chosen capacitor geometry allows both high-veracity observation of the domain structure and mapping of polarization switching in a uniform field, synergistically combining microstructural observations and probing of uniform-field polarization responses as relevant to device operation. A classical Kolmogorov-Avrami-Ishibashi model has been adapted to the voltage domain, and the individual switching dynamics of the FORC response curves are well approximated by the adapted model. The comparison with microstructures suggests a strong spatial variability of the switching dynamics inside the nanocapacitors.

Air-bridged Ohmic contact on vertically aligned si nanowire arrays: application to molecule sensors.

A simple, cost-effective, and highly reliable method for constructing an air-bridged electrical contact on large arrays of vertically aligned nanowires was developed. The present method may open up new opportunities for developing advanced nanowire-based devices for energy harvest and storage, power generation, and sensing applications.

Individual switching of film-based nanoscale epitaxial ferroelectric capacitors

We have investigated the individual switching of nanoscale capacitors by piezoresponse force microscopy. Nanoscale epitaxial ferroelectric capacitors with terabyte per inch square equivalent density were fabricated by the deposition of top electrodes onto a pulsed laser deposited lead zirconate titanate thin film by electron beam evaporation through ultrathin anodic aluminum oxide membrane stencil masks. Using bias pulses, the nanoscale capacitors were uniformly switched and proved to be individually addressable. These film-based nanoscale capacitors might be a feasible alternative for high-density mass storage memory applications with near terabyte per inch square density due to the absence of any cross-talk effects.

Fabrication of 3D trench PZT capacitors for 256Mbit FRAM device application

We fabricated trench PbZrxTi1-xO3 (PZT) capacitors that can be used in 256Mbit 1T-1C FRAM devices. The capacitor has 0.25mum diameter and 0.4mum depth. Three layers, Ir(20nm)/PZT(60nm)/Ir(20nm), were deposited in SiO2 trench holes by ALD and MOCVD. Both columnar and granular grains were formed on the sidewalls of the trench capacitors, and their relative portion had strong size dependence. The trench capacitors with more columnar PZT grains showed good switching behavior under 2.1V external bias and 19 to 24 muC/cm2 remnant polarization