Joan M. Redwing

In situ epitaxial MgB2 thin films for superconducting electronics

The newly discovered 39-K superconductor MgB21 holds great promise for superconducting electronics. Like the conventional superconductor Nb, MgB2 is a phonon-mediated superconductor2, with a relatively long coherence length3. These properties make the prospect of fabricating reproducible uniform Josephson junctions, the fundamental element of superconducting circuits, much more favourable for MgB2 than for high-temperature superconductors. The higher transition temperature and larger energy gap4,5 of MgB2 promise higher operating temperatures and potentially higher speeds than Nb-based integrated circuits. However, success in MgB2 Josephson junctions has been limited because of the lack of an adequate thin-film technology6,7. Because a superconducting integrated circuit uses a multilayer of superconducting, insulating and resistive films, an in situ process in which MgB2 is formed directly on the substrate is desirable. Here we show that this can be achieved by hybrid physical–chemical vapour deposition. The epitaxially grown MgB2 films show a high transition temperature and low resistivity, comparable to the best bulk samples, and their surfaces are smooth. This advance removes a major barrier for superconducting electronics using MgB2.

Bottom-up assembly of large-area nanowire resonator arrays

Directed-assembly of nanowire-based devices will enable the development of integrated circuits with new functions that extend well beyond mainstream digital logic. For example, nanoelectromechanical resonators are very attractive for chip-based sensor arrays because of their potential for ultrasensitive mass detection. In this letter, we introduce a new bottom-up assembly method to fabricate large-area nanoelectromechanical arrays each having over 2,000 single-nanowire resonators. The nanowires are synthesized and chemically functionalized before they are integrated onto a silicon chip at predetermined locations. Peptide nucleic acid probe molecules attached to the nanowires before assembly maintain their binding selectivity and recognize complementary oligonucleotide targets once the resonator array is assembled. The two types of cantilevered resonators we integrated here using silicon and rhodium nanowires had Q-factors of approximately 4,500 and approximately 1,150, respectively, in vacuum. Taken together, these results show that bottom-up nanowire assembly can offer a practical alternative to top-down fabrication for sensitive chip-based detection.

Schottky barrier engineering in III–V nitrides via the piezoelectric effect

A method for enhancing effective Schottky barrier heights in III–V nitride heterostructures based on the piezoelectric effect is proposed, demonstrated, and analyzed. Two-layer GaN/AlxGa1−xN barriers within heterostructure field-effect transistor epitaxial layer structures are shown to possess significantly larger effective barrier heights than those for AlxGa1−xN, and the influence of composition, doping, and layer thicknesses is assessed. A GaN/Al0.25Ga0.75N barrier structure optimized for heterojunction field-effect transistors is shown to yield a barrier height enhancement of 0.37 V over that for Al0.25Ga0.75N. Corresponding reductions in forward-bias current and reverse-bias leakage are observed in current–voltage measurements performed on Schottky diodes.

The role of the tunneling component in the current–voltage characteristics of metal-GaN Schottky diodes

The temperature dependence of the current–voltage characteristics of Ni–GaN Schottky barriers have been measured and analyzed. It was found that the enhanced tunneling component in the transport current of metal-GaN Schottky barrier contacts is a likely explanation for the large scatter in the measured Richardson constant.

High-field superconductivity in alloyed MgB 2 thin films

We investigated the effect of alloying on the upper critical field

AN OPTICALLY PUMPED GAN-ALGAN VERTICAL CAVITY SURFACE EMITTING LASER

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Two‐dimensional electron gas properties of AlGaN/GaN heterostructures grown on 6H–SiC and sapphire substrates

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Highly Scalable, Atomically Thin WSe2 Grown via Metal–Organic Chemical Vapor Deposition

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Dependence of Ni/AlGaN Schottky barrier height on Al mole fraction

films, in which disorder was introduced by growth, carbon doping or He-ion irradiation, finding a significant

Capacitance–voltage characterization of AlN/GaN metal–insulator–semiconductor structures grown on sapphire substrate by metalorganic chemical vapor deposition

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