Structure and chemistry of Nb/sapphire interfaces, with and without interlayers of Sb and Cr

Abstract

The microstructure and chemistry of interfaces between sputter-deposited Nb coatings and (0001) sapphire surfaces, with and without interlayers of Cr and Sb, have been determined by using high resolution transmission electron microscopy (HRTEM). The changes in the microstructure and chemistry in the interfacial region resulting from annealing the samples are determined. All samples were annealed at 600°C for 24 h and 1200°C for 10 min, except the Nb/Sb sapphire samples which were heat-treated at 400 and 600°C with a 24 h hold time. All interface systems in the as-deposited state feature a 2.5 nm thick uniform Al 2O 3 layer, caused by sputter-cleaning of the sapphire surface prior to any interlayer or coating deposition. After annealing at 1200°C for 10 min, the amorphous layer completely recrystallizes, and hemispherical voids, 10–20 nm diameter, are formed at the Nb/Al 2O 3 interface. In the Nb/Cr/Al 2O 3 system, in addition to voids and the recrystallization of the amorphous layer, the continuous Cr layer is reduced to a Cr 2Nb intermetallic reaction compound. Finally, for the Nb/Sb/Al 2O 3 samples, annealing at 400°C for 24 h results in a hitherto unreported, Nb-Sb amorphization reaction. † Present address: Johnson Matthey Electronics, East 15128 Euclid Avenue, Spokane, WA 99216, U.S.A.