Structures of AlN/VN Superlattices with Different AlN Layer Thicknesses
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I.W. Kim, S.A. Barnett, and L.D. Marks Department of Material Science & Engineering, Northwestern University, Evanston, Illinois 60208 (Received 16 December 2001; accepted 5 March 2002)
AlN/VN superlattices with different periods were studied using x-ray diffraction and transmission electron microscopy (TEM). A phase transformation of the AlN from an epitaxially stabilized rock-salt structure to a hexagonal wurtzite structure was observed for an AlN layer thickness greater than 4 nm. A structural model is proposed on the basis of TEM results for the orientation of the transformed phase. The VN layer grown on top of the hexagonal AlN was observed to be reoriented compared to that in the stabilized B1-AlN/VN. The VN nucleated by taking the w-AlN(002) plane as its (111) plane instead of the (002) plane.
I. INTRODUCTION
AlN, one of the III–V compounds, is characterized by high ionicity, short bond length, low compressibility, high thermal conductivity, and a wide band gap. These properties make it interesting and useful in many areas, for instance applications for lasers and high-temperature transistors. At ambient temperature and pressure, AlN has a wurtzite (w-AlN) structure with alternating layers of Al and N atoms. A high-pressure rock-salt (B1-AlN) structure and a zinc-blende (zb-AlN) structure were predicted by Christensen et al. using total energy calculations.1,2 Both of them are cubic structures, as shown in Fig. 1. A pressure-induced first-order transition from the hexagonal to the B1 structure has been observed in bulk AlN, in reasonable agreement with theory.3,4 An alternative method of obtaining the metastable cubic phase is epitaxial stabilization. In a previous study, zinc-blende AlN was observed to be stabilized in an AlN/W superlattice, in which zb-AlN transformed to w-AlN at larger thickness;5 B1-AlN was observed to be stabilized in both AlN/NbN6 and AlN/TiN7 superlattices with an AlN layer thickness of less than 2.0 nm. More recently it was found that B1-AlN could also be stabilized in AlN/VN (001) superlattices with an AlN layer thickness of less than 4.0 nm, the largest critical thickness obtained to date.8 This is due to the smaller lattice mismatch between B1-AlN and VN. Relatively little is known about the B1 to hexagonal transformation.
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In this paper, we report results showing the epitaxial stabilization of B1-AlN in AlN/VN superlattices with an AlN layer thickness less than 4.0 nm. A phase transformation from B1-AlN to w-AlN takes place when the AlN layer thickness exceeds the 4.0-nm critical thickness. Cross-sectional high-resolution electron microscopy (HREM) observations of the superlattice layers also show a reorientation of the VN layer above the w-AlN layer. Both the orientation of the transformed w-AlN as well as that of the subsequent VN can be understood as minimizing the misfit stresses. The relative orientation of the transformed w-AlN, as well as observation of a very small fraction of retained B1-Al
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