Understanding the Rocksalt-to-Wurtzite phase transformation through microstructural analysis of (Al,Sc)N epitaxial thin films

Rocksalt-to-wurtzite structural phase transitions in semiconducting materials (such as III–V nitrides, ZnO, CdSe, and others) have been studied for several decades. Almost all experimental works related to this phase transition involve diamond anvil cells to apply hydrostatic pressure, and as a result, direct observation of the microstructural transformation during the phase transition has not been possible. In this article, we have addressed and uncovered the intimate microstructural details and epitaxial relationships between phases by capturing what is essentially a thin-film snapshot of the transformation after growth of AlxSc1-xN films with a composition chosen to be close to the equilibrium phase boundary between wurtzite and rocksalt. The results support the hypothesis that the transformation is triggered by defects at rs- { 0 1 ¯ 1 } growth fronts that offer a nearly invariant plane with respect to the parallel w- { 2 1 ¯ 1 ¯ 0 } planes. The intermediate crystal structures and their epitaxial relationships are consistent with theoretical models that predict a transformation pathway involving homogeneous orthorhombic shear strain.