Nitride Metal/Semiconductor Superlattices for High Temperature Direct Thermal-to-Electrical Energy Conversion

We have identified TiN/GaN and ZrN/ScN as two possible pure rocksalt structured metal/semiconductor combinations for fabrication of solid-state thermionic energy converters for high operational temperatures. The selection of the materials was constrained by issues that are critical to the integration of heterogeneous materials such as crystallographic compatibility and thermodynamic stability of the metal/semiconductor combinations at high operating temperatures. The first nitride superlattice system consists of TiN as the metal layer and GaN, in its metastable rocksalt phase, as the semiconductor layer, grown on rocksalt MgO substrates. The metastable rocksalt GaN (rs-GaN) phase is stabilized by pseudomorphic epitaxy on a metallic rocksalt TiN underlayer, and its existence has been verified using high-resolution x-ray diffraction and transmission electron microscopy. The critical thickness for the rocksalt-to-wurtzite phase transition has been empirically determined to be between 1 and 2 nm, although much thicker rocksalt GaN films, up to approximately 6 nm, can be maintained for several superlattice periods. The second pure rocksalt-structured superlattice system analyzed consists of alternating layers of metallic ZrN and semiconducting ScN. These epitaxial superlattices were grown on rocksalt MgO substrates using dc magnetron sputtering in a nitrogen-argon ambient.