Crystallographic orientation dependent maximum layer thickness of cubic AlN in CrN/AlN multilayers

Metastable rock-salt (face centered cubic, c-) AlN can be grown in CrN/AlN multilayers when the AlN layer is thin enough. Exceeding a certain critical thickness, the thermodynamically stable wurtzite (w) structure grows. In this work, a bilayer-period-gradient (21 repeated blocks, each consisting of 10 bilayers with AlN layer-thicknesses ranging from 1.0 nm to 10.0 nm), similar to 2.0 mu m-thick, reactively magnetron sputtered multilayer was characterized in detail with a spherical aberration-corrected transmission electron microscope (TEM). The studies are complemented by DFT (density functional theory) calculations. The high resolution TEM (HRTEM) studies reveal that the <111> growth-orientation is not as effective as the <110> and <100> growth-orientations in stabilizing the metastable c-AlN. The critical thickness for the c-AlN layers (before the thermodynamically stable w-AlN forms) is around-2.0 nm for the <111> growth-orientation but reaches as high as 4.1 nm for both <110> and <100> growth-orientations. Contrary to the <111> orientation, in both <110> and <100> orientations several unusually highly mismatched c-CrN/w-AlN interface structures form as soon as w-AlN is present. DFT studies suggest that the larger critical thickness of the AlN layers in <100> and <110> orientation is allowed by the lower surface energy and higher cubic/wurtzite interfacial energy. The combination of HRTEM and DFT studies allows answering open questions on the impact of crystallographic orientations and interface structures, and also provides a better understanding on the growth mechanisms of c-AlN, necessary for the outstanding mechanical properties of AlN-containing multilayers. (C) 2019 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.