Substrate Dependent Epitaxy Of Superconducting Niobium Nitride Thin Films Grown By Pulsed Laser Deposition

Niobium nitride (NbN) has suitable mechanical properties for application as protective coatings in mechanical engineering, and also its superconductivity can be utilized in thin film devices for sensorics or in combination with ferromagnet in spintronics. Long-range superconducting proximity effect at the heterostructures with a weak ferromagnet can be used for generation of spin-polarized current. For any operational heterostructure application the high quality NbN thin films need to be prepared. In this work we have investigated impact of the substrates on the structure and preferential orientation of niobium nitride thin films fabricated by pulsed laser deposition at 600 degrees C on Si (001), MgO (001), C-plane and R-plane Al2O3 substrates. Growth parameters have been tuned in order to obtain single superconducting delta-NbN phase. Microstructure was analyzed by X-ray diffraction and transmission electron microscopy. Si substrate does not induce the film preferential orientation. Films on MgO are epitaxial (001) oriented. Films on R-Al2O3 show (135) orientation with twinned crystallites on the lower symmetry substrate surface. The (111) epitaxial growth with the largest crystallites and their smallest tilting was achieved on C-Al2O3 substrate leading to the best superconducting properties.