Nitride Multilayers as a Platform for Parallel Two-Dimensional Electron-Hole Gases: MgO/ScN(111)

TL;DR

This paper proposes using nitride-oxide heterostructures, specifically MgO/ScN(111), to realize parallel two-dimensional electron and hole gases, enabling exploration of novel quantum states and superconductivity.

Contribution

It introduces a new class of nitride-oxide heterostructures capable of hosting spatially separated 2D electron and hole gases, with detailed ab initio calculations supporting their potential.

Findings

Spatially separated 2D electron and hole gases observed after 5 ScN layers

Higher density of transition metal ions at (111) interface enhances carrier density

Guidance provided for experimental realization of nitride-based conducting gases

Abstract

At interfaces between insulating oxides LaAlO$_3$ and SrTiO$_3$, a two dimensional electron gas (2DEG) has been observed and well studied, while the predicted hole gas (2DHG) has not been realized due to the strong tendency of holes in oxygen $2p$ orbitals to localize. Here we propose, via ab initio calculations, an unexplored class of materials for the realization of parallel two dimensional (2D), two carrier (electron+hole) gases: nitride-oxide heterostructures, with (111)-oriented ScN and MgO as the specific example. Beyond a critical thickness of five ScN layers, this interface hosts spatially separated conducting Sc-$3d$ electrons and N-$2p$ holes, each confined to $\sim$two atomic layers -- the transition metal nitride provides both gases. A guiding concept is that the N$^{3-}$ anion should promote robust two carrier 2D hole conduction compared to that of O$^{2-}$; metal mononitrides are mostly metallic and even superconducting while most metal monoxides are insulating. A second positive factor is that the density of transition metal ions, hence of a resulting 2DG, is about three times larger for a rocksalt (111) interface than for a perovskite(001) interface. Our results, including calculation of Hall coefficient and thermopower for each conducting layer separately, provide guidance for new exploration, both experimental and theoretical, on nitride-based conducting gases that should promote study of long sought exotic states viz. new excitonic phases and distinct, nanoscale parallel superconducting nanolayers.