Jin Won Seo

Doubling the critical temperature of La1.9Sr0.1CuO4 using epitaxial strain

The discovery of high-temperature superconductivity in copper oxides raised the possibility that superconductivity could be achieved at room temperature. But since 1993, when a critical temperature (T c) of 133 K was observed in the HgBa2Ca2Cu3O8+δ (ref. 2), no further progress has been made in raising the critical temperature through material design. It has been shown, however, that the application of hydrostatic pressure can raise T c — up to ∼164 K in the case of HgBa2Ca2Cu3O8+δ (ref. 3). Here we show, by analysing the uniaxial strain and pressure derivatives of T c, that compressive epitaxial strain in thin films of copper oxide superconductors could in principle generate much larger increases in the critical temperature than obtained by comparable hydrostatic pressures. We demonstrate the experimental feasibility of this approach for the compound La1.9Sr0.1CuO4, where we obtain a critical temperature of 49 K in strained single-crystal thin films — roughly double the bulk value of 25 K. Furthermore, the resistive behaviour at low temperatures (but above T c) of the strained samples changes markedly, going from insulating to metallic.

CVD synthesis of high-purity multiwalled carbon nanotubes using CaCO3 catalyst support for large-scale production

Abstract Multiwalled carbon nanotubes (MWCNTs) were synthesized in a catalytic reaction using CaCO3 as catalyst support. Impregnated with conventional catalysts CaCO3 enabled the production of MWCNTs in a fixed-bed flow reactor at relatively low reaction temperature. The purification was performed in one step: both metallic particles and catalyst support were dissolved in a diluted acid. Hence, disadvantages, namely multi-step processes and hazardous chemicals, were avoided. A further advantage of CaCO3 support is efficient and stable growth of MWCNTs. First results obtained in our new rotary-tube oven indicated high quality and pure MWCNTs with a yield of 100 g/day by this method.

Synthesis of MWNT-based composite materials with inorganic coating

Abstract Multiwalled carbon nanotube (MWNT) based metal oxide composites were prepared by an impregnation method using organometallic compounds as precursor. Aluminium isopropoxide (AlIP), tetraethyl orthosilicate (TEOS), and tetraethyl orthotitanate (TEOTi) were used as inorganic sources and decomposed by hydrolysis on the surface of carbon nanotubes. The composites were subsequently investigated by transmission electron microscopy and their coverage was compared. A direct, solvent-free impregnation technique turned out to be the most successful for all organometallic compounds and provided homogeneous inorganic cover layer on the surface of purified MWNTs.

High-K dielectrics for the gate stack

This article gives an overview of recent developments in the search for the next-generation dielectric for the complementary metal-oxide semiconductor gate stack. After introducing the main quantities of interest, the paper concentrates on a figure of merit that connects two main properties of the gate stack, namely, the leakage current and the capacitance. This is done for single layers as well as for bilayers consisting of interfacial SiOx and a high-K dielectric. In the case of the bilayers, the impact of the interfacial layer SiOx is enormous, reducing the leakage current by an order of magnitude per monolayer. This extreme dependance makes a good correlation between the leakage and the structural parameters nearly impossible. This is illustrated using numerical examples designed to help the reader evaluate the orders of magnitude involved. The origin of the interfacial layer is traced back by means of thermodynamic considerations. As the estimates put forward in the literature do not correspond to th...

Electrical properties of La2O3 and HfO2/La2O3 gate dielectrics for germanium metal-oxide-semiconductor devices

Germanium metal-insulator-semiconductor capacitors with La2O3 dielectrics deposited at high temperature or subjected to post deposition annealing show good electrical characteristics, especially low density of interface states D-it in the 10(11) eV(-1) cm(-2) range, which is an indication of good passivating proper-ties. However, the K value is estimated to be only about 9, while there is no,evidence for an interfacial layer. This is explained in terms of a spontaneous and strong reaction between La2O3 and Ge substrate to form a low K and leaky La-Ge-O germanate over the entire film thickness, which, however, raises concerns about gate scalability. Combining a thin (similar to 1 nm) La2O3 layer with thickef HfO2 degrades the electrical characteristics, including Dit, but improves gate leakage and equivalent oxide thickness, indicating a better potential for scaling. Identifying suitable gate dielectric stack which combines good passivating/interfacial properties with good scalability remains a challenge. (c) 2008 American Institute of Physics.

Catalytic CVD Synthesis of Carbon Nanotubes: Towards High Yield and Low Temperature Growth

The catalytic chemical vapor deposition (CCVD) is currently the most flexible and economically attractive method for the growth of carbon nanotubes. Although its principle is simple, the precisely controlled growth of carbon nanotubes remains very complex because many different parameters influence the growth process. In this article, we review our recent results obtained on the synthesis of carbon nanotubes via CCVD. We discuss the role of the catalyst and the catalyst support. Our recent results obtained from the water assisted growth and the equimolar C2H2-CO2 reaction are also discussed. Both procedures lead to significantly enhanced carbon nanotube growth. In particular, the latter allows growing carbon nanotubes on diverse substrate materials at low temperatures.

Field-effect transistors with SrHfO3 as gate oxide

The authors demonstrate that the compound SrHfO3 grown epitaxially on Si(100) by molecular-beam epitaxy is a potential gate dielectric to fabricate n- and p-metal-oxide semiconductor field-effect transistors with equivalent oxide thickness (EOT) below 1nm. The electrical properties on capacitors and transistors show low gate leakage and good capacitance and I-V output characteristics. The lower electron and hole mobilities, which are strongly limited by charge trapping, nevertheless fit well with the general trend of channel mobility reduction with decreasing EOT.

Effect of electron irradiation on the electrical properties of fibers of aligned single-walled carbon nanotubes

We carried out in situ resistivity measurements on macroscopic oriented ropes of single wall carbon nanotubes in a transmission electron microscope. We have found a minimum in the resistivity as a function of irradiation dose. This minimum is interpreted as a result of a twofold effect of the irradiation: the domination of covalent bond formation between tubes in a bundle due to broken bonds in the tube walls and the amorphization of the sample at high dose. Despite this improvement, the temperature dependence of the resistivity remains hopping-like.

Controlled positioning of carbon nanotubes by dielectrophoresis: Insights into the solvent and substrate role

We demonstrate the ability to precisely control the deposition of a defined number of carbon nanotubes (CNTs) from solution onto microfabricated electrodes using dielectrophoresis. The solvation shell around the CNTs, exhibiting a high dielectric constant which is possibly larger than the intrinsic dielectric constant of CNTs, is found to play a crucial role in electrophoretic processes. Substrate resistivity is also very important: The spatial repartition of the electric field between the substrate and the microelectrodes leads to deviations from the precise location of the CNTs. A recipe is given for the dielectrophoresis of CNTs which can be extended to other nanowires or nanotubes.

Interface formation and defect structures in epitaxial La2Zr2O7 thin films on (111) Si

We have studied the growth of epitaxial La2Zr2O7 thin films on (111) Si. Although the interface structure can be strongly affected by the Si oxidation during the deposition process, epitaxial growth of La2Zr2O7 was obtained. A detailed study by means of transmission electron microscopy reveals two types of structures (pyrochlore and fluorite) with the same average chemical composition but strong differences in reactivity and interface formation. The structural complexity of the ordered pyrochlore structure seems to prevent excess oxygen diffusion and interfacial SiO2 formation.