Hong Kyoon Choi

InGaAsSb thermophotovoltaic diode: Physics evaluation

The hotside operating temperatures for many projected thermophotovoltaic (TPV) conversion system applications are approximately 1000u200a°C, which sets an upper limit on the TPV diode band gap of 0.6 eV from efficiency and power density considerations. This band gap requirement has necessitated the development of new diode material systems never previously considered for energy generation. To date, InGaAsSb quaternary diodes grown lattice matched on GaSb substrates have achieved the highest performance. In this article we relate observed diode performance to electro-optical properties such as minority carrier lifetime, diffusion length, and mobility and provide initial links to microstructural properties. This analysis has bounded potential diode performance improvements. For the 0.53 eV InGaAsSb diodes used in this analysis (active layer doping is 2×1017u200acm−3) the dark current density measured is 2×10−5u200aA/cm2 versus a potential Auger and/or a radiative limit of 2×10−6u200aA/cm2 (no photon recycling), and an abso...

Deposition of epitaxial BiFeO3/CoFe2O4 nanocomposites on (001) SrTiO3 by combinatorial pulsed laser deposition

BiFeO3/CoFe2O4 (BFO/CFO) nanocomposites were grown on SrTiO3 by pulsed laser deposition using a combinatorial method in which Bi1.2FeO3 and CoFe2O4 targets are alternately ablated. The films had the same vertically nanostructured morphology as thin films prepared by ablation of a single target, consisting of epitaxial CoFe2O4 pillars in a BiFeO3 matrix. In a series of samples synthesized with a compositional spread, the out-of-plane magnetic anisotropy and the out-of-plane compressive strain of the CoFe2O4 pillars increased with decreasing volume fraction, and the anisotropy agreed with the value predicted from the strain state and magnetoelastic coefficients of CoFe2O4. These results show the dominant effect of magnetoelastic anisotropy in determining the magnetic hysteresis of the nanocomposite.

Design rules for self-assembled block copolymer patterns using tiled templates

Directed self-assembly of block copolymers has been used for fabricating various nanoscale patterns, ranging from periodic lines to simple bends. However, assemblies of dense bends, junctions and line segments in a single pattern have not been achieved by using sparse templates, because no systematic template design methods for achieving such complex patterns existed. To direct a complex pattern by using a sparse template, the template needs to encode the key information contained in the final pattern, without being a simple copy of the pattern. Here we develop a set of topographic template tiles consisting of square lattices of posts with a restricted range of geometric features. The block copolymer patterns resulting from all tile arrangements are determined. By combining tiles in different ways, it is possible to predict a relatively simple template that will direct the formation of non-trivial block copolymer patterns, providing a new template design method for a complex block copolymer pattern.

Ordered nanoscale Archimedean tilings of a templated 3-miktoarm star terpolymer.

The directed self-assembly of 3-miktoarm star terpolymer chains (polyisoprene-arm-polystyrene-arm-polyferrocenylethylmethylsilane (3 μ-ISF)) into 2D Archimedean tilings is described. A morphological change from (4.8(2)) to (6(3)) tiling is reported in the 3 μ-ISF thin film blended with PS homopolymer when a greater swelling of PI is achieved during the solvent annealing process. Highly oriented (4.8(2)) tilings were produced by templating the self-assembled three colored structures in blended thin films. The use of (4.8(2)) and (6(3)) tilings as nanolithographic masks to transfer square and triangular hole arrays into the substrate is also demonstrated.

Templated Self‐Assembly of Functional Oxide Nanocomposites

In perovskite/spinel self-assembled oxide nanocomposites, the substrate surface plays a dominant role in determining the final morphology. Topgraphic features, such as pits and trenches, are written in the substrate using either Focused Ion Beam or wet etching through a block co-polymer mask. These features are effective at templating the self-assembly, resulting in a wide range of attainable nano-assemblies.

Hierarchical templating of a BiFeO3-CoFe2O4 multiferroic nanocomposite by a triblock terpolymer film.

A process route to fabricate templated BiFeO3/CoFe2O4 (BFO/CFO) vertical nanocomposites is presented in which the self-assembly of the BFO/CFO is guided using a self-assembled triblock terpolymer. A linear triblock terpolymer was selected instead of a diblock copolymer in order to produce a square-symmetry template, which had a period of 44 nm. The triblock terpolymer pattern was transferred to a (001) Nb:SrTiO3 substrate to produce pits that formed preferential sites for the nucleation of CFO crystals, in contrast to the BFO, which wetted the flat regions of the substrate. The crystallographic orientation and magnetic properties of the templated BFO/CFO were characterized.

Room-temperature continuous operation of GaAs/AlGaAS lasers grown on Si by organometallic vapor-phase epitaxy

Graded‐index separate‐confinement heterostructure single‐quantum‐well GaAs/AlGaAs diode lasers exhibiting continuous (cw) operation at room temperature have been grown on a Si substrate by organometallic vapor‐phase epitaxy, without the use of molecular‐beam epitaxy. To improve the quality of the laser structure, a defect‐filtering layer was incorporated between this structure and a GaAs buffer layer about 1.5 μm thick grown on the substrate. Of four types of defect‐filtering layers investigated, the most effective was one grown with thermal cycling, which made it possible to obtain pulsed threshold current densities as low as 350 A/cm2 for broad‐stripe lasers with a cavity length of 500 μm. Ridge‐waveguide lasers with this type of defect‐filtering layer have exhibited cw threshold currents as low as 25 mA and a differential quantum efficiency of 55%.

Thin film knitting pattern morphology from a miktoarm star terpolymer.

Thin film knitting pattern from a miktoarm star terpolymer is demonstrated. Such structures have been predicted but not observed in bulk or thin film form. The knitting pattern exhibits well organized periodic structures consisting of undulating lamellae and alternating cylinders, with well-defined defects that result in sharp 90° bends and T junctions.

Combinatorial pulsed laser deposition of magnetic and magneto-optical Sr(GaxTiyFe0.34-0.40)O3-δ perovskite films.

Ferromagnetic Sr(GaxTiyFe0.34-0.40)O3-δ (0.1 ≤ x, y ≤ 0.5) films with single-crystal perovskite structure were epitaxially grown on (001) (LaAlO3)0.3(Sr2AlTaO6)0.7 substrates by combinatorial pulsed laser deposition (CPLD) and compared with previous results from films grown from single targets. In CPLD films the Fe was present as both Fe(2+) and Fe(3+). The distribution of Sr, Ga, Ti, and O was homogeneous, but Fe-rich nanowires with diameter of 3 nm were present perpendicular to the film plane. The unit cell was tetragonally distorted with the ratio of out-of-plane to in-plane lattice parameter decreasing from 1.06 to 1.02 as the Ga content increased. The magnetic easy axis of the films changed from out-of-plane when Ti content y > 0.3 to isotropic as the Ga content increased, consistent with a reduction in magnetoelastic anisotropy. The Ga lowered the Faraday rotation and the magnetization but increased the optical transmittance.

Cell-Based Design Methods for Directed Self-Assembly

Topographic templates can direct the self-assembly of block copolymers to achieve nanoscale patterns with high order. Previously, we have demonstrated well-aligned periodic lines, bends, and meshes using a sparse array of lithographically defined posts [1,2]. However, it is generally challenging to fabricate complex and non-periodic patterns using a sparse post array because the key information contained in the final pattern must be encoded in the sparse template. In this work, we present two cell-based design methods for fabricating complex patterns using directed self-assembly of polystyrene-b-polydimethylsiloxane (PS-b-PDMS) block copolymer thin films. For the first approach, we developed a set of template tiles consisting of square post lattices with a restricted range of geometric features. For all possible tile arrangements, we examined the resulting block copolymer patterns. We were able to predict a relatively simple template that will result in a desired complex pattern by combining tiles in different ways. For the second approach, we designed a binary-state system with ladder-shaped block copolymer structures using a square confinement. We developed design-rules for controlling alignment direction of the ladder-shaped structures by introducing openings around the square cells. These methods could provide a new template design method for a complex non-trivial block copolymer patterns.