Efficiency of thermoelectric materials is generally discussed in terms of the dimensionless figure-of-merit, ZT
= S2σT/κ, Many researchers have found that it is possible to reduce the lattice thermal conductivity by
incorporating nanostructures (i.e. nanoparticles or heterobarriers) into materials, thereby scattering phonons.
At the same time, it has been theoretically predicted and experimentally demonstrated that barriers can be
used to "filter" the distribution of carriers which contribute to conduction. By doing so, it is possible to
significantly increase the Seebeck coefficient while only modestly decreasing the electrical conductivity. As a
result of this energy-dependent scattering of carriers, the thermoelectric power factor is increased. We present
theoretical and experimental results for metal/semiconductor nanocomposites consisting of metallic rareearth-
group V nanoparticles within III-V semiconductors (e.g. ErAs:InGaAlAs) demonstrating both an
increase in thermoelectric power factor and a decrease in thermal conductivity, resulting in a large figure of
merit. We also discuss metal/semiconductor superlattices made of lattice-matched nitride materials for
electron filtering and the prospects of these materials for efficient thermoelectrics, especially at high
temperatures. Finally, we will discuss both various synthesis techniques for these materials, including the
prospects for bulk growth, and also devices fabricated from these materials.
The delamination of a thin film heterostructure by selective absorption of pulsed laser energy at a buried interface enables the transfer of the thin film heterostructure from its growth substrate to virtually any receptor substrate without significant heating of material outside the interaction zone. By combining this "laser lift-off" process with low-temperature bonding methods, disparate classes of materials can be intimately integrated without exceeding the thermal budget of the least robust of the materials to be integrated. Furthermore, heterostructures that can be grown by epitaxy on one substrate can be transferred intact, without significant deterioration in crystal quality, to a receptor substrate that enhances the performance or functionality of the heterostructure in a device or microsystem. In this paper, applications of laser lift-off in the packaging and integration of light-emitting GaN devices are highlighted. Transfer of these devices from their sapphire growth substrates to thermally and electrically conductive receptor substrates is shown to result in improved device performance through the reduction of thermal and electrical series resistances, and by the improvement in optical design enabled by access to both sides of the heterostructure. Continued development of laser lift-off packaging has the potential to reduce manufacturing costs and complexity as well through the elimination of the sapphire dicing step. Finally, the application of the LLO technique to the assembly of functionally-enhanced microsystems is illustrated with the example of an integrated fluorescence detection microsystem.
Lead titanate PbTiO3(PT) thin films were prepared on single-crystal MgO(100) substrates by sol-gel process (SG) and pulsed laser deposition (PLD) techniques. The SG coating solutions were synthesized from lead (II) acetate, titanium isopropoxide precursors using acetic acid and diethanolamine (DEA). The SG films were spin coated at 2000 rpm, dried in air at 350 degree(s)C and rapid thermally annealed at between 400-650 degree(s)C temperature range. PLD films were grown at 2x10-6 Torr chamber pressure in flowing oxygen and at the substrate temperature of 600 degree(s)C. The films were characterized by x-ray diffraction (XRD), scanning electron microscopy (SEM), and scanning spectrophotometry. The optical properties of the PT films were evaluated from spectral transmittance and reflectance measurements. Optical and structural properties of SG and PLD thin films grown at 600 degree(s)C substrate temperature were compared. XRD analysis indicates that PLD films grow strongly textured on MgO(100) substrate. In contrast, SG films were randomly oriented. SEM showed that the surface of the SG PT films heat treated at above 600 degree(s)C had smaller grains. In contrast PLD grown films had a well-developed dense and uniform grain structure with larger grain size. The optical constants such as refractive index (n), extinction coefficient (k) and optical band gap (Eg) values were found to depend on the preparation techniques. The optical band gap values are in the range of 2.98-3.24 eV depending on the deposition method.
A morphology study was conducted on fused silica surfaces damaged by single pulse laser irradiation at 355 nm. The physical characterization of the surface showed that cracks initiate at ellipsoidal pits on both input and output surfaces. The size of the pit increases with laser pulse-length and the orientation of the ellipse is perpendicular to the electric field. The pits are less than 300 nm deep. Cracks initiate along the main axis of the ellipse. The morphology of the cracks is different for input vs. output surfaces. The output surface crack exhibits a shell- like morphology typical of localized compressive stress. The input surface crack shows, on the other hand, a star-like pattern. These differences seem to be caused by differences in plasma propagation on the surface. The extent of damage and the number of shells or branches increases with increasing fluence. A molten morphology can be observed at the center of the cracks for fluences above the pit formation threshold. This indicates that local temperatures can be in excess of several thousand degrees C.
Conference Committee Involvement (1)
Laser Applications in Microelectronic and Optoelectronic Manufacturing XI
23 January 2006 | San Jose, California, United States
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