Bulk-Like Laminated Nitride Metal/Semiconductor Superlattices for Thermoelectric Devices

Bulk-like thermionic energy conversion devices have been fabricated from nanostructured nitride metal/semiconductor superlattices using a novel lamination process. 5-μm thick (Hf_0.5Zr_0.5)N (6-nm)/ScN (6-nm) metal/semiconductor superlattices with a 12 nm period were deposited on 100-silicon substrates by reactive magnetron sputtering followed by a selective tetra methyl ammonium hydroxide substrate etching and a gold-gold lamination process to yield 300 μm × 300 μm × 290 μm microscale thermionic energy conversion elements with 16,640 superlattice periods. The thermionic element had a Seebeck coefficient of -120 μV/K at 800 K, an electrical conductivity of ~2500 Ω^-1 m^-1 at 800 K, and a thermal conductivity of 2.9 and 4.3 W/m-K at 300 and 625 K, respectively. The temperature dependence of the Seebeck coefficient from 300 to 800 K suggests a parallel parasitic conduction path that is dominant at low temperature, and the temperature independent electrical conductivity indicates that the (Hf_0.5Zr_0.5)N/gold interface contact resistivity currently dominates the device. The thermal conductivity of the laminate was significantly lower than the thermal conductivity of the individual metal or semiconductor layers, indicating the beneficial effect of the metal/semiconductor interfaces toward lowering the thermal conductivity. The described lamination process effectively bridges the gap between the nanoscale requirements needed to enhance the thermoelectric figure of merit ZT and the microscale requirements of real-world devices.