Y. S. Kim

Fabrication of three-dimensional microstructures by soft molding

We have developed soft molding as a method for meso-scale-area fabrication of three-dimensional structures. The soft molding, which is a form of soft lithography, involves placing an elastomeric mold on the surface of a spin-coated polymer film with a slight pressure (<1 N/cm2), allowing the mold to absorb solvent, releasing the pressure, and then letting the mold and the substrate remain undisturbed for a period of time. The three-dimensional structure thus formed is robust in that the pattern fidelity is preserved without any distortion or defects. For the soft molding to be successful, the rate of solvent absorption by the mold should be larger than the rate of solvent evaporation. The method is demonstrated with several three-dimensional structures.

Critical thickness of ultrathin ferroelectric BaTiO3 films

To investigate the critical thickness of ferroelectric BaTiO3 (BTO) films, we fabricated fully strained SrRuO3∕BTO∕SrRuO3 heterostructures on SrTiO3 substrates by pulsed laser deposition with in situ reflection high-energy electron diffraction. We varied the BTO layer thickness from 3to30nm. By fabricating 10×10μm2 capacitors, we could observe polarization versus electric-field hysteresis loops, which demonstrate the existence of ferroelectricity in BTO layers thicker than 5nm. This observation provides an experimental upper bound of 5nm for the critical thickness. The BTO thickness-dependent scaling of the remanent polarization agrees with the predictions of recent first-principle simulations [J. Junquera and P. Ghosez, Nature 422, 506 (2003)].

Observation of inhomogeneous domain nucleation in epitaxial Pb(Zr,Ti)O3 capacitors

We investigated domain nucleation process in epitaxial Pb(Zr,Ti)O3 capacitors under a modified piezoresponse force microscope. We obtained domain evolution images during polarization switching process and observed that domain nucleation occurs at particular sites. This inhomogeneous nucleation process should play an important role in an early stage of switching and under a high electric field. We found that the number of nuclei is linearly proportional to log(switching time), suggesting a broad distribution of activation energies for nucleation. The nucleation sites for a positive bias differ from those for a negative bias, indicating that most nucleation sites are located at the ferroelectric/electrode interfaces.

Fundamental Thickness Limit of Itinerant Ferromagnetic SrRuO3 Thin Films

We report on a fundamental thickness limit of the itinerant ferromagnetic oxide SrRuO(3) that might arise from the orbital-selective quantum confinement effects. Experimentally, SrRuO(3) films remain metallic even for a thickness of 2 unit cells (uc), but the Curie temperature T(C) starts to decrease at 4 uc and becomes zero at 2 uc. Using the Stoner model, we attributed the T(C) decrease to a decrease in the density of states (N(o)). Namely, in the thin film geometry, the hybridized Ru d(yz,zx) orbitals are terminated by top and bottom interfaces, resulting in quantum confinement and reduction of N(o).

Surface versus bulk characterizations of electronic inhomogeneity in a VO2 thin film

thin film, theMIT should occur during the percolation process. We also used surface-sensitive scanning tunneling spectros-copy STS to investigate the microscopic evolution of the MIT near the surface. Similar to the XRD results,STS maps revealed a systematic decrease in the metallic phase as temperature decreased. However, this rate ofchange was much slower than the rate observed with XRD, indicating that the electronic inhomogeneity nearthe surface differs greatly from that inside the film. We investigated several possible origins of this discrepancyand postulated that the variety in the strain states near the surface plays an important role in the broad MITobserved using STS. We also explored the possible involvement of such strain effects in other correlatedelectron oxide systems with strong electron-lattice interactions.DOI: 10.1103/PhysRevB.76.075118 PACS number s : 71.30. h, 68.47.Gh, 71.27. a

Three-dimensional pattern transfer and nanolithography: modified soft molding

One-step transfer of molded three-dimensional polymer structures into underlying substrate is reported. The one-step transfer is made possible by a molding technique presented here in the form of modified soft molding. Formation of a desired three-dimensional structure in a polymer film by this method, followed by one-step reactive ion etching, is utilized for the transfer. The technique is also shown to be effective in transferring sub-100-nm features.

Energy-level engineering of self-assembled quantum dots by using AlGaAs alloy cladding layers

The ground-state energy level of an InAs quantum dot (QD) system can be changed from 1070 to 700 nm by changing the aluminum composition in the AlxGa1−xAs matrix. For all the QDs, the lattice-mismatched strains are the same as that of InAs/GaAs QDs, so that QDs are easily formed. Photoluminescence signals from the structures were strong at low temperature and stayed relatively high at room temperature. The results suggest that these highly strained QDs in the alloy matrices could be an excellent choice for the energy-level engineering of QDs.

Interplay between carrier and impurity concentrations in annealed Ga1- xMnxAs : Intrinsic anomalous hall effect

Investigating the scaling behavior of annealed Ga1-xMnxAs anomalous Hall coefficients, we note a universal crossover regime where the scaling behavior changes from quadratic to linear. Furthermore, measured anomalous Hall conductivities in the quadratic regime when properly scaled by carrier concentration remain constant, spanning nearly a decade in conductivity as well as over 100 K in T_[C] and comparing favorably to theoretically predicated values for the intrinsic origins of the anomalous Hall effect. Both qualitative and quantitative agreements strongly point to the validity of new equations of motion including the Berry phase contributions as well as the tunability of the anomalous Hall effect.

Carrier-mediated ferromagnetic ordering in Mn ion-implanted p+GaAs: C

Highly p-type GaAs:C was ion implanted with Mn at differing doses to produce Mn concentrations in the 1char21{}5 at. % range. In comparison to LT-GaAs and

2‐Amino‐N‐(2,6‐dichloropyridin‐3‐yl)acetamide derivatives as a novel class of HBV capsid assembly inhibitor

{n}^{+}mathrm{GaAs}:mathrm{Si}