ScGaN Alloy Growth by Molecular Beam Epitaxy: Evidence for a Metastable Layered Hexagonal Phase

TL;DR

This study investigates the growth of ScGaN alloys via molecular beam epitaxy, revealing distinct structural regimes and providing evidence for a metastable layered hexagonal phase of ScN, expanding understanding of alloy phase stability.

Contribution

It presents experimental evidence for a metastable layered hexagonal phase of ScN in ScGaN alloys, supported by optical and structural analysis across different growth regimes.

Findings

Identification of three growth regimes with distinct structures

Linear decrease in optical transition with increasing Sc content in regimes I and III

Evidence of anisotropic lattice expansion supporting the layered hexagonal phase

Abstract

Alloy formation in ScGaN is explored using rf molecular beam epitaxy over the Sc fraction range x = 0-100%. Optical and structural analysis show separate regimes of growth, namely I) wurtzite-like but having local lattice distortions in the vicinity of the ScGa substitutions for small x (x < 0.17), II) a transitional regime for intermediate x, and III) cubic, rocksalt-like for large x (x > 0.54). In regimes I and III, the direct optical transition decreases approximately linearly with increasing x but with an offset over region II. Importantly, it is found that for regime I, an anisotropic lattice expansion occurs with increasing x in which a increases much more than c. These observations support the prediction of Farrer and Bellaiche [Phys. Rev. B 66, 201203-1 (2002)] of a metastable layered hexagonal phase of ScN, denoted h-ScN.