Venkatesan Manivannan

Nature of luminescent centers in cerium-activated materials with the CaFe2O4 structure

Abstract The cerium luminescence in SrY 2 O 4 shows two bands emitting in the blue and green regions of the optical spectrum. Based on standard emission and excitation spectroscopy combined with time-resolved emission studies, we can attribute the blue emission to a Ce 3+ 5d–4f transition and tentatively assign the green emission to a “Ce 4+ –O 2− ”charge transfer transition.

Influence of metal thickness to sensitivity of Pt/GaN Schottky diodes for gas sensing applications

Hydrogen gas sensors based on Pt/GaN Schottky diode structures were fabricated and their responses to hydrogen were studied. These diodes were fabricated on Si doped GaN layer (N D = 1×10 17 ). Three sets of diodes were fabricated with 80 A, 240 A and 400 A of Pt for the Schottky contacts. The electronic performances of 0.25 × 0.25 mm devices were tested in up to 1 % H 2 gas in synthetic air (79% N 2 , 21% O 2 ) by volume. The devices were operated in constant current mode in a forward bias condition. The change in voltage was monitored with the diodes exposed to hydrogen and to dry air at varying temperatures. The responses increased as the thickness of the Schottky metal contact decreased at any given temperature up to 310 °C. The trend of increasing response with decreasing thickness was also observed in 0.5 × 0.5 mm and 1.0 × 1.0 mm size Schottky diodes. SEM studies of the microstructure showed that the thinner Pt devices had higher grain boundary densities. The increase in sensitivity with decreasing thickness points to the dissociation of molecular hydrogen on the surface, the diffusion of atomic hydrogen through the Pt grain boundaries and the adsorption of hydrogen to the surface as a possible mechanism of sensing of hydrogen by Schottky diodes.

Multi-Layer Photopolymer Micromachining

This paper presents a novel method to create and integrate micro-machined devices and high aspect-ratio (height-to-width ratio) microstructures in which the microstructures are built up using multiple layers of photopolymer film and/or viscous solution. Very high aspect-ratio 2-and 3-dimensional (2-D and 3-D) microstructures were constructed by stacking photo-imageable polymer films. Such films may be dry films applied by lamination or solution layers applied by bar coating, or doctor blade coating. Photolithography is used in both cases to define the microstructure. This additive process of thin-film micromachining facilitates high aspect-ratio microstructure fabrication. We have demonstrated structures of up to 12-layers comprising 2-D arrays of deep trenches (180 μm deep and 25 μm wide) and a 2-layer SU-8 micro-trench array with an aspect ratio up to 36 on glass substrates. Miniaturized structures of interconnected reservoirs as small as 50 μm × 50 μm × 15 μm (∼38 pico liter storage capacity) are also being fabricated, along with a novel 5-layer microfluidic channel array and a vacuum-infiltration process for fluid manipulation. This method has the potential to create functional large-area micro-devices at low-cost and with increased device flexibility, durability, prototyping speed, and reduced process complexity for applications in optoelectronics, integrated detectors, and bio-devices. The novel multi-layer photopolymer dry film and solution process also allows microstructures in micro-electro-mechanical systems (MEMS) to be built with ease and provides the functionality of MEMS integration with electronic devices and integrated circuits (ICs).