J. G. Partridge

From the adhesion of atomic clusters to the fabrication of nanodevices

An experimental study of Bi, Sb, and Cu clusters incident at velocities ≳50m∕s on SiO2, Si3N4, polymethylmethacrylate, and photoresist surfaces shows that the clusters adhere much more strongly to SiO2 and Si3N4 than to the polymer materials. The differences in adhesion properties allow assembly of a range of nanowire-based electronic devices from cluster building blocks using lithographically patterned polymer layers. Clusters adhere to the substrate but not to the surface of the polymer template, eliminating parasitic conduction. Molecular dynamics simulations show that differing cluster-surface interactions affect adhesion most strongly when high incident velocities cause significant plastic deformation of the clusters.

Electrical Characteristics of Top-Down ZnO Nanowire Transistors Using Remote Plasma ALD

Top-down fabrication is used to produce ZnO nanowires by remote plasma atomic layer deposition over a SiO₂ pillar and anisotropic dry etching. Nanowire field-effect transistors (FETs), with channel lengths in the range of 1.3-18.6 μm, are then fabricated using these 80 nm × 40 nm nanowires. Measured electrical results show n-type enhancement behavior and a breakdown voltage ≥75 V at all channel lengths. This is the first report of high-voltage operation for ZnO nanowire FETs. Reproducible well-behaved electrical characteristics are obtained, and the drain current scales with 1/<i>L</i>, as expected for long-channel FETs. A respectable <i>I</i>_ON/<i>I</i>_OFF ratio of 2×10⁶ is obtained.

Stable n-channel metal-semiconductor field effect transistors on ZnO films deposited using a filtered cathodic vacuum arc

We report on the properties of metal-semiconductor-field-effect-transistors (MESFETs) on ZnO films grown using the filtered cathodic vacuum arc (FCVA) technique. FCVA ZnO films deposited on a-plane sapphire at 200u2009°C showed good structural and electrical properties that improved further on annealing at 800u2009°C in oxygen, due to the formation of larger grains with lower inter-grain transport barriers. MESFETs with silver oxide and iridium oxide Schottky gates on these annealed films showed excellent long-term stability with low ideality factors (<1.3), low gate leakage, and channel mobilities up to 50 cm2/Vs that were unchanged with both age and stress testing.

Structure of oxidized bismuth nanoclusters

Synchrotron X-ray diffraction has determined that beta-Bi(2)O(3) is the dominant oxide phase covering hexagonal bismuth nanoclusters produced in an inert gas aggregation source. Simulated Debye-Scherrer patterns have indicated that the oxide is 20 +/- 5 Angstroms thick on average, at the surface of 320 +/- 40 Angstroms diameter clusters. A Williamson-Hall analysis of the peak broadening was used to measure the non-uniform strain in clusters. The oxidized clusters were in -0.11 +/- 0.06% uniform compressive strain compared with other clusters without oxides detectable by X-ray diffraction which only have a small tensile uniform strain. High-resolution transmission electron microscopy (HRTEM) and multislice image simulations indicated a beta-Bi(2)O(3) thickness of 20-50 Angstroms. The HRTEM micrographs show the relative orientation between the oxide and the cluster core.

Experimental and simulational study of the operation conditions for a high transmission mass filter

The operation conditions of a double pulsed field mass filter were studied using both experiment and simulation. The mass filter consists of two pairs of parallel plates and operates on the time-of-flight principle. The study showed that the ions beam deflection angle is a critical factor in optimizing the mass filter transmission efficiency. This angle is dependent on the accelerating voltage, ion mass, and horizontal velocity of the ions. The optimum operating conditions for the mass filter were found and used to study the mass distribution of palladium ions produced by a magnetron sputtering source. The study shows that this mass filter is suitable for technological applications because of its high transmission and wide mass range.

Formation of electrically conducting mesoscale wires through self-assembly of atomic clusters

Bi and Sb clusters deposited from an inert gas aggregation source have been used to form cluster-assembled wires on unpassivated, and SiO/sub 2/ passivated, V-grooved Si substrates. V-grooves (4-7 /spl mu/m in width, 6 /spl mu/m-1 mm in length) were prepared using optical lithography and anisotropic etching in KOH solution. The effectiveness of the surface templating technique was demonstrated by scanning electron microscope analysis carried out after deposition. When Sb clusters were deposited onto SiO/sub 2/ passivated substrates, the surface coverage was seen to vary from <20% on the unpatterned (normal-to-beam) surface (which is required to be nonconducting) to >100% at the apexes of the V-grooves used to promote growth of the wire. Sb wires produced with this technique currently have minimum widths of /spl sim/400 nm and lengths of /spl sim/1 mm. Electrical contacts can be positioned within the V-grooves prior to cluster deposition, thus enabling the initial onset of conduction and subsequent I(V) characteristic of a wire to be monitored in vacuum.

Ultraviolet detection from graphitic-C/Zn1−xMgxO Schottky devices fabricated at moderate temperatures

Ultraviolet (UV) Schottky detector devices were fabricated on polycrystalline wurtzite Zn1−xMgxO films energetically deposited onto a-plane sapphire at room-temperature (RT) and 200u2009°C. The unintentionally doped, transparent, n-Zn1−xMgxO films exhibit low surface roughness (<5% of film thickness), moderate carrier concentration, and Hall mobility up to 15u2009cm2u2009V−1u2009s−1. The direct bandgaps of the RT and 200u2009°C films (xu2009=u20090.24 and xu2009=u20090.20) were 3.57u2009eV and 3.40u2009eV. Schottky diodes with graphitic anodes formed on these films exhibited barrier heights up to 0.88u2009eV and ideality factors as low as 1.97. Spectral response measurements demonstrated UV/visible photo-current ratios up to ∼104.

Measurement of the conductivity exponent in random percolating networks of nanoscale bismuth clusters

Nanoscale bismuth clusters, produced in an inert gas aggregation source, have been deposited between lithographically defined electrical contacts. The conductivity exponent, t, of the 2D percolating network of bismuth clusters has been derived from in-situ electrical measurements. It was found to be 1.32/spl plusmn/0.25. This value is comparable with theoretical predictions of t /spl middot/1.3 for 2D random continuum percolation networks.

The relationship between microstructure and electrical breakdown in cathodic arc deposited hafnium oxide films

Hafnium oxide films were deposited with a range of substrate temperatures using a filtered cathodic vacuum arc deposition system. The microstructure, electronic structure, and electrical breakdown of the films were characterized. In films deposited at temperatures above 200u2009°C, the microstructure became more ordered and x-ray diffraction indicated that the dominant phase was monoclinic hafnium oxide. Evidence for the presence of the tetragonal phase was also found in the films deposited at temperatures above 400u2009°C. The near edge structure of the oxygen K-edge measured using x-ray absorption spectroscopy, provided further evidence that films prepared at high temperatures contained a combination of the monoclinic and tetragonal phases. Films deposited at room temperature were disordered and exhibited the best electrical breakdown characteristics. The electrical breakdown of the films deteriorated as the crystallinity increased with increasing deposition temperature. These results support the proposition th...

A Hydrogen Sensor Based on Graphitic Carbon

A Pt/oriented graphitic carbon heterojunction has been fabricated and tested as a conductometric hydrogen gas sensor. The carbon layer, deposited between planar Au substrate contacts using a filtered cathodic vacuum arc (with an applied substrate bias of -500 V), has a low density of 1.7 g/cm3 and with 67% of the atoms bonded in sp2 or graphitic configurations. Electron diffraction analysis showed evidence that the film has a microstructure consisting largely of vertically oriented graphitic sheets. These sheets have good through-film electrical conductivity and contributed to a low device resistance of ~ 60 Ω. A 3-nm Pt layer was deposited subsequently on the carbon layer. A change in the device resistance of >; 2% was exhibited upon exposure to 1% hydrogen gas (in synthetic, zero humidity air) at room temperature. The time for the sensor resistance to decrease by 2% under these conditions was 60 s and the baseline (zero hydrogen exposure) resistance remained constant to within 0.03% during and after the exposure tests.