Growth of single-crystal metal superlattices in chosen orientations

What determines the interfacial configuration of Nb/Al2O3 and Nb/MgO interface

Nb films are deposited on single crystal Al₂O₃ (110) and MgO(111) substrates by e-beam evaporation technique. Structure of Nb films and orientation relationships (ORs) of Nb/Al₂O₃ and Nb/MgO interface are studied and compared by the combination of experiments and simulations. The experiments show that the Nb films obtain strong (110) texture, and the Nb film on Al₂O₃(110) substrate shows a higher crystalline quality than that on MgO(111) substrate. First principle calculations show that both the lattice mismatch and the strength of interface bonding play major roles in determining the crystalline perfection of Nb films and ORs between Nb films and single crystal ceramic substrates. The fundamental mechanisms for forming the interfacial configuration in terms of the lattice mismatch and the strength of interface bonding are discussed.

Structure and Coherence of Metallic Superlattices

Most of the papers in this symposium have dealt with superlattices formed from semiconductors with the diamond or zinc blende structure or derivative structures such as fluorite in which one sublattice has the diamond structure. Such superlattices are often called compositionally modulated alloys (CMA) because the two components have the same or related structures and because the two components have some mutual solubility. In these cases, depending on the differences in the lattice parameters, the resulting CMA will be almost a perfect single crystal, and the sharpness of the interfaces between the two alternating components will depend on the amount of interdiffusion during growth.

Structures of Nb/Al2O3 Interfaces Produced by Different Experimental Routes

Nb/Al2O3 interfaces were produced by (i) diffusion bonding of single crystalline Nb and Al2O3 at 1973 K, (ii) internal oxidation of a Nb-3at.% Al alloy at 1773 K, and (iii) molecular beam epitaxy (MBE) growth of 500 nm thick Nb overlayers on sapphire substrates at 1123 K. Cross-sectional specimens were prepared and studied by conventional (CTEM) and high resolution transmission electron microscopy (HREM). The orientation relationships between Nb and Al2O3 were identified by diffraction studies. HREM investigations revealed the structures of the different interfaces including the presence of misfit dislocations at or near the interface. The results for the different interfaces are compared.