Thermionic Emission in Artificially Structured Single-Crystalline Elemental Metal/Compound Semiconductor Superlattices

Metal/semiconductor superlattices represent a fascinating frontier in materials science and nanotechnology, where alternating layers of metals and semiconductors are precisely engineered at the atomic and nano-scales. Traditionally, epitaxial metal/semiconductor superlattice growth requires constituent materials from the same family, exhibiting identical structural symmetry and low lattice mismatch. Here, beyond this conventional constraint, a novel class of epitaxial lattice-matched metal/semiconductor superlattices is introduced that utilizes refractory hexagonal elemental transition metals and wide-bandgap III-nitride semiconductors. Exemplified by the Hf/AlN superlattices exhibiting coherent layer-by-layer epitaxial growth, cross-plane thermionic emission is observed through current-voltage measurements accomplished for the first time in any metal/semiconductor superlattices. Further, thermoreflectance measurements reveal significant enhancement in cross-plane Seebeck coefficients attributed to carrier energy filtering by Schottky barriers. Demonstration of artificially structured elemental-metal/wide-bandgap compound-semiconductor superlattices promises to usher in new fundamental physics studies and cutting-edge applications such as tunable hyperbolic metamaterials, quantum computing, and thermionic-emission-based thermoelectric and thermophotonic energy conversion devices.