Thermal stability of ultrathin titanium films on a Pt(111) substrate

We report annealing studies using He⁺-ion scattering spectroscopy (He⁺-ISS) and X-ray photoelectron spectroscopy (XPS) to evaluate the thermal stability of ultrathin Ti films deposited on a Pt(111) single-crystal surface at 300 K. These results establish that strong Pt-Ti intermetallic bonding provides sufficient driving force for thermal interdiffusion of Ti and Pt at a (111)-oriented interface at 700 K (427 °C), even in the absence of oxygen and a minimal influence of grain boundaries. Identification of this critical temperature provides additional fundamental information for Ti/Pt bilayer film processing considerations. This data was obtained by using He⁺-ISS to monitor the concentration of Ti in the topmost surface layer as a function of temperature. In these studies, we define 1 dose unit (DU) to be the amount of deposited Ti that is required to eliminate the Pt signal in He⁺-ISS. This amount should be close to that required to produce a one-monolayer Ti film, but it could exceed one monolayer of Ti if alloying or clustering occurs on the substrate at 300 K under these deposition conditions. For Ti films less than 1 DU, heating to 700 K slightly decreased the Ti surface concentration. Higher annealing temperatures of up to 950 K caused extensive loss of surface Ti, and this is attributed to Ti diffusion into the Pt crystal sub-surface region. For a 1-DU Ti film, temperatures of 750-800 K were required to cause sufficient interdiffusion so that Pt atoms were detected in the topmost layer, strongly suggesting that the 1-DU Ti film was more thermally stable than those of lower Ti coverage. Thicker Ti films of greater than 1 DU required temperatures of 800-1050 K before any change was detected in the Ti surface concentration as probed by the He ⁺-ISS signal intensity. After annealing to temperatures higher than 1000 K, XPS revealed a chemical shift for the Ti (2p) peaks of 1.5 eV to higher binding energy, indicating formation of a Ti/Pt alloy.