Kinetics of Zr-Al intermetallic compound formation during ultra-high vacuum magnetron sputter-deposition of Zr/Al2O3(0001) thin films

Abstract

Zr layers are grown on Al2O3(0001) substrates by ultra-high vacuum dc magnetron sputter-deposition in pure Ar discharges at temperatures Ts between 873 K and 1173 K. We investigate the Ts-dependent evolution of microstructure and composition across the Zr/Al2O3(0001) interfaces using X-ray diffraction, transmission electron microscopy, and X-ray energy dispersive spectroscopy. We obtain fully-dense, close-packed hexagonal structured Zr(0001) thin films with smooth surfaces at 873 K ≤ Ts ≤ 973 K and increasingly porous, 0002-textured layers with highly corrugated surfaces at Ts > 1023 K. At the Zr/Al2O3 interfaces, we observe thin crystalline films of Zr-Al intermetallic compounds, whose thicknesses increase from 10 ± 4 nm at Ts = 873 K to 116 ± 4 nm at Ts = 1173 K. The interfacial layers are composed of hexagonal Ni2In-structured Zr2Al phase at Ts = 973 K and both orthorhombic-structured Zr2Al3 and hexagonal Mn5Al3-structured Zr5Al3 phases at Ts > 973 K. The thicknesses of both the intermetallic phases increase with increasing Ts, however, the Zr2Al3 layer grows faster than the Zr-rich phase at higher Ts. We attribute the formation of Zr-rich Zr2Al, rather than the thermodynamically more stable Al-rich Zr2Al3 layers, to the presence of dissolved oxygen and epitaxial registry with Al2O3(0001). Our results indicate that reaction-diffusion kinetics rather than thermodynamics govern the interfacial layer composition and we suggest that the Zr/Al2O3(0001) interfacial stability is controlled by the supply of Al from the Al2O3(0001) substrate during sputter-deposition of Zr.