Hexagonal AlN: Dimensional-Crossover-Driven Bandgap Transition

TL;DR

This study uses first-principles calculations to explore how the structural, electronic, and vibrational properties of hexagonal AlN change with layer number, revealing a unique layer-dependent bandgap transition.

Contribution

It provides the first detailed theoretical investigation of layer-dependent properties and the dimensional crossover-driven bandgap transition in hexagonal AlN.

Findings

Bulk h-AlN is a stable direct-bandgap semiconductor.

Single-layer h-AlN is an indirect-bandgap semiconductor.

Thicker N-layered h-AlN (N>10) exhibits a direct bandgap at the Gamma point.

Abstract

Motivated by a recent experiment that reported the successful synthesis of hexagonal (h) AlN [Tsipas et al. Appl. Phys. Lett. 103, 251605 (2013)] we investigate structural, electronic and vibrational properties of bulk, bilayer and monolayer structures of h-AlN by using first-principles calculations. We show that the hexagonal phase of the bulk h-AlN is a stable direct-bandgap semiconductor. Calculated phonon spectrum displays a rigid-layer shear mode at 274 cm-1 and an Eg mode at 703 cm-1 which are observable by Raman measurements. In addition, single layer h-AlN is an indirect-bandgap semiconductor with a nonmagnetic ground state. For the bilayer structure, AA' type stacking is found to be the most favorable one and interlayer interaction is strong. While N-layered h-AlN is an indirect bandgap semiconductor for N=1-10, we predict that thicker structures (N>10) have a direct-bandgap at the Gamma-point. The number-of-layer-dependent bandgap transitions in h-AlN is interesting in that it is significantly different from the indirect-to- direct crossover obtained in the transition metal dichalcogenides.