Formation and stability of enhanced superhard nanostructured AlN/VN and AlN/TiN superlattice materials
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Y6.5.1
Formation and stability of enhanced superhard nanostructured AlN/VN and AlN/TiN superlattice materials C. Stampfl and A. J. Freeman Department of Physics and Astronomy, Northwestern University, Evanston, Illinois 60208-3112 ABSTRACT Using density functional theory and the full-potential linearized augmented plane wave (FLAPW) method, we investigate the formation and stability, and atomic structure, of rocksalt AlN/TiN and AlN/VN systems, including properties of the clean surfaces of the constituent materials. Calculations of the adlayer formation energy highlights the effect and interplay of the various energetic contributions on the growth of these strained systems, where the so-called “surface-interface” interaction energy is found to be important for the initial stages of AlN epitaxy. A significant strain energy builds up for increasing number of layers, where it is greater in the AlN/TiN system, which limits the thickness of rocksalt AlN regions that can grow before a structural transition to the lower energy wurtzite phase takes place. From our calculations, together with the known experimental critical thicknesses, we can obtain an accurate estimate of the wurtzite/substrate interface energy. That these values are high explains why the metastable rocksalt phase, which has significantly lower interface energies, is stabilized.
INTRODUCTION The ability to deliberately stabilize non-equilibrium metastable structures as desired, on an atomic level, affords tremendous potential for the control of the physical and chemical properties of a system. Strained-layer superlattices are an example, in particular, where precise control of the grown materials can be obtained and where often the formed structures are not in thermodynamic equilibrium but are “epitaxially stabilized” in a metastable state. Such manmade structures have no analogue in nature and exhibit properties that are neither observed for the constituents nor their alloys. In this respect, transition metal nitride superlattices of nanoscale dimensions are presently of great technological interest [1], e.g. in the area of wear-resistant coatings for mechanical systems such as cutting tools, due to the fact that these fabricated structures can exhibit enhanced hardness that far exceeds that of the constituent materials [2]. In particular, it has been found for AlN/VN [3] and AlN/TiN [4] that only for the high-pressure metastable rocksalt phase of AlN does the hardness enhancement occur; the rocksalt structure, however, is found to be stable only up to a critical thickness, beyond which a phase transition occurs to the stable wurtzite structure, resulting in a loss of hardness. Thus, it is of considerable interest to understand the factors that govern the delicate energy balance between various contributions. The above-mentioned strained superlattice systems, AlN/VN and AlN/TiN, all form sharp epitaxial (1 × 1) interfaces and represent ideal systems for fundamental theoretical studies.
CALCULATION METHOD
Y6.5.2
In the present work we perform first-princip
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