Schottky barrier height of epitaxial lattice-matched TiN/Al0.72Sc0.28N metal/semiconductor superlattice interfaces for thermionic energy conversion

Since the initial development of semiconductor heterostructures in the 1960s, researchers exploring the potential of artificially structured materials for applications in quantum electronic, optoelectronic, and energy conversion devices have sought a combination of metals and semiconductors, which could be integrated at the nanoscale with atomically sharp interfaces. Initial demonstration of such metal/semiconductor heterostructures employed elemental polycrystalline metal and amorphous semiconductors that demonstrated electronic tunneling devices, and more recently, such heterostructures were utilized to demonstrate several exotic optical phenomena. However, these metal/semiconductor multilayers are not amenable to atomic-scale control of interfaces, and defects limit their device efficiencies and hinder the possibilities of superlattice growth. Epitaxial single-crystalline TiN/Al0.72Sc0.28N metal/semiconductor superlattices have been developed recently and are actively researched for thermionic emission-based waste heat to electrical energy conversion, optical hyperbolic metamaterial, and hot-electron solar-to-electrical energy conversion devices. Most of these applications require controlled Schottky barrier heights that determine current flow along the cross-plane directions. In this Letter, the electronic band alignments and Schottky barrier heights in TiN/Al0.72Sc0.28N superlattice interfaces are determined by a combination of spectroscopic and first-principles density functional theory analyses. The experimental E-F(TiN)-E-VBM(Al0.72Sc0.28N) at the interfaces was measured to be 1.8 +/- 0.2 eV, which is a bit smaller than that of the first-principles calculation of 2.5 eV. Based on the valence band offset and the bandgap of cubic-Al0.72Sc0.28N, an n-type Schottky barrier height of 1.7 +/- 0.2 eV is measured for the TiN/Al0.72Sc0.28N interfaces. These results are important and useful for designing TiN/Al0.72Sc0.28N metal/semiconductor superlattice based thermionic and other energy conversion devices.