|About this Abstract
||2010 Electronic Materials Conference
||TMS 2010 Electronic Materials Conference
||GG1, Bulk-like Thermionic Energy Conversion Device Fabricated from Laminated Nanostructured Metal/Semiconductor Superlattices
||Jeremy L Schroeder, David A Ewoldt, Polina V Burmistrova, Robert Wortman, Timothy D Sands
|On-Site Speaker (Planned)
||Jeremy L Schroeder
Thermionic carrier transport in metal/semiconductor superlattices is a promising energy conversion approach based on nanocomposite materials. Current research focuses on thin-film sputter deposited transition metal nitride superlattices for improving ZT via energy barrier filtering (increased power factor) and interface scattering of phonons (suppressed thermal conductivity). However, even if these superlattices prove successful in enhancing ZT, barriers exist between research-based thin-film superlattices and practical bulk-like device structures. For example, optimal power density in thin-film thermoelectric devices requires superlattice leg lengths in the range of 50-200μm. Such leg lengths are impractical for sputtered deposited films due to lengthy deposition times, high residual growth stress, and varying crystal quality. One approach to realizing long leg lengths is through laminating multiple thin-film superlattices to create bulk-like structures. Effective laminate structures require negligible parasitics be introduced by the bonding layers. Simple analysis shows that the electrical and thermal parasitics are less than 10% when using a bonding medium of high thermal and electrical conductivity, such as copper or gold, and assuming a low contact resistance (~1×10<sup>-8</sup> Ω-cm<sup>2</sup>) between the bonding medium and superlattice. The metal/semiconductor superlattice structure offers the benefit of a metal-metal contact between the bonding medium and metal nitride, which should offer low contact resistance. Five micrometer (Hf<sub>0.5</sub>, Zr<sub>0.5</sub>)N/ScN superlattices with 12nm period were deposited on 2” 100-silicon substrates by reactive DC magnetron sputtering. One micrometer of gold was deposited on the superlattices followed by Au-Au thermocompression bonding of two 2” superlattices, thereby creating a bilayer structure. The structure was diced into 5mm x 5mm pieces followed by selective etching of the silicon substrates in tetramethyl ammonium hydroxide (TMAH) with the HfN buffer layer acting as an effective TMAH etch stop. The resulting 12μm superlattice bilayer foils were stress balanced with respect to the Au-Au bond interface and they were robust enough to be handled with vacuum tweezers. One micrometer of gold was then deposited on both sides of the superlattice bilayer foils followed by stacking and Au-Au thermocompression bonding of twenty bilayer foils (although the number of stacked bilayers is practically unlimited). The final 290μm thick laminate structure was an artificial bulk-like structure fabricated from 200μm of nanostructured superlattice films and 90μm of gold bonding medium. The 5mm x 5mm laminate was subsequently diced and polished into 300μm x 300μm x 290μm devices for electrical and thermal characterization. These laminate devices demonstrate a route forward towards realizing practical thermoelectric devices based on nitride metal/semiconductor superlattices.