Sang Yeob Sung

Low-Resistivity 10 nm Diameter Magnetic Sensors

Resistivities of 5.4 μΩ·cm were measured in 10-nm-diameter metallic wires. Low resistance is important for interconnections of the future to prevent heating, electromigration, high power consumption, and long RC time constants. To demonstrate application of these wires, Co/Cu/Co magnetic sensors were synthesized with 20-30 Ω and 19% magnetoresistance. Compared to conventional lithographically produced magnetic tunnel junction sensors, these structures offer facile fabrication and over 2 orders of magnitude lower resistances due to smooth sidewalls from in situ templated chemical growth.

Quasi-Phase-Matched Faraday Rotation in Semiconductor Waveguides With a Magnetooptic Cladding for Monolithically Integrated Optical Isolators

Strategies are developed for obtaining nonreciprocal polarization mode conversion, also known as Faraday rotation, in waveguides in a format consistent with silicon-on-insulator or III-V semiconductor photonic integrated circuits. Fabrication techniques are developed using liftoff lithography and sputtering to obtain garnet segments as upper claddings, which have an evanescent wave interaction with the guided light. A mode solver approach is used to determine the modal Stokes parameters for such structures, and design considerations indicate that quasi-phase-matched Faraday rotation for optical isolator applications could be obtained with devices on the millimeter length scale.

Magneto-optical garnet waveguides on semiconductor platforms: Magnetics, mechanics, and photonics

Garnet films with thicknesses of 100–1000 nm and waveguides with widths of 700–2000 nm were grown onto Si to characterize the mechanical stresses that occurred upon crystallization (700–800 °C) by rapid thermal annealing. These magneto-optical garnet films and also photonic crystals have proposed uses in magnetic flux indicator films, integrated photonic devices, such as isolators, circulators, and polarization transformers, because their Verdet constants per unit loss are orders of magnitude better than other magneto-optical materials. However, garnet does not match Si-based materials mechanically with thermal expansion coefficients of 10.4 ppm/°C. These waveguides were optimized to have low losses in the near infrared, including the telecommunication wavelengths (1.0–2.3 dB/mm at 1.3 μm and 0.9–1.7 at 1.55 μm). The waveguide losses increased with waveguide width. Finite difference time domain simulations were used to estimate the number, effective index, and profile of modes in each guide. The polarizat...

Metallic 10 nm Diameter Magnetic Sensors and Large-Scale Ordered Arrays

Metallic nanowires with low resistivity were grown inside insulating aluminum oxide matrices that contained very uniform columnar nanopores (10.6+/1.7 nm diameters). These nanopores can be made with large-scale order (cm2), which is desirable in applications such as hard drive read sensors and random access memories. The nanowires are grown by electrochemical deposition directly inside the alumina to avoid sidewall damage compared to nanostructures that are defined from films by lithographical patterning and etching. Specifically, trilayers of [Co(15 nm)/Cu(5 nm)/Co(10 nm)] were synthesized and measured to have 30 Ω resistance and 19% magnetoresistance. These parameters are desirable for read head sensors, especially because the nanowires described here have 1:1 aspect ratios, and 10× smaller areas and 100× lower resistances than conventional read sensors based on lithographically produced magnetic tunnel junctions. A new nanostamping technique is introduced, in which linear stamps with ordered cm2 areas are imprinted onto aluminum precursors to produce ordered nanoporous aluminum oxide upon anodization. These stamps are substantially less-time consuming and cheaper to make than dot type stamps, and the order enables closely spaced arrays of CPP-GMR sensors for one-pass 2-D recording and cross recording. Importantly, the GMR sensors are grown directly into aluminum oxide with 20 nm separation. Therefore, a relatively large pattern (30 × 100 nm) can be used to produce three 10 nm-diameter GMR sensors without roughening or redeposition on sidewalls. The sensors are also already embedded in alumina for subsequent device processing.

Integration of magneto-optic garnet waveguides and polarizers for optical isolators

YIG waveguides and polarizers were integrated monolithically onto semiconductors. The waveguide losses were 1.55 dB/mm with Faraday rotations of 0.2 dB/mm. Birefringence was minimized by optimized etching. Photonic crystal polarizers and biasing films completed the isolator.

Fabrication of Garnet Waveguides and Polarizers for Integrated Optical Isolators

YIG waveguides and photonic crystal polarizers were successfully integrated with Si using SiO2 claddings. The films were grown by room temperature reactive RF sputtering and rapid thermal annealing. Faraday rotations of 0.2deg/mum were achieved.

Galfenol Thin Films and Nanowires

Galfenol (Fe1−xGax, 10 < x < 40) may be the only smart material that can be made by electrochemical deposition which enables thick film and nanowire structures. This article reviews the deposition, characterization, and applications of Galfenol thin films and nanowires. Galfenol films have been made by sputter deposition as well as by electrochemical deposition, which can be difficult due to the insolubility of gallium. However, a stable process has been developed, using citrate complexing, a rotating disk electrode, Cu seed layers, and pulsed deposition. Galfenol thin films and nanowires have been characterized for crystal structures and magnetostriction both by our group and by collaborators. Films and nanowires have been shown to be largely polycrystalline, with magnetostrictions that are on the same order of magnitude as textured bulk Galfenol. Electrodeposited Galfenol films were made with epitaxial texture on GaAs. Galfenol nanowires have been made by electrodeposition into anodic aluminum oxide templates using similar parameters defined for films. Segmented nanowires of Galfenol/Cu have been made to provide engineered magnetic properties. Applications of Galfenol and other magnetic nanowires include microfluidic sensors, magnetic separation, cellular radio-frequency identification (RFID) tags, magnetic resonance imaging (MRI) contrast, and hyperthermia.

Opening of Hybrid Bandgaps in Two-Dimensional Photonic Crystals of Pb(Mg

The effect of anisotropy on two-dimensional photonic crystals of hexagonally arranged Pb(Mg1/3Nb1/3)O3-PbTiO3 (PMNT) rods in air background was analyzed using a finite-difference time-domain method. The refractive index contrast between PMNT (n = 2.47) and air is low compared to conventional semiconductor-air photonic crystals. Yet, hybrid (polarization-independent) photonic bandgaps were achieved for anisotropic PMNT rods by adjusting the structural parameters and the anisotropy. The maximum bandwidth found was 20 nm. It was possible to engineer the center wavelength of this bandgap to 980, 1300, and 1550 nm, for potential applications as switches, electrooptic filters, and modulators in this region.