Study of Pt-Ge interaction using thin film and lateral diffusion couples

Master Thesis


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University of Cape Town

The formation of germanides of platinum has been investigated using both conventional thin films and lateral diffusion couples. The investigation was carried out using such established techniques as XRD, RBS and SEM. Using results from both thin film and lateral diffusion couples, a comparison has been made of the behaviour of Pt-Ge system in parameters such as phase formation sequence, growth kinetics and dominant diffusing species. In their sequential order of formation, three distinct phases, Pt₂Ge, PtGe and PtGe₂, have been identified in thin films in the temperature range 200 - 300°C. The first phase, Pt₂Ge, was found not to follow a layered mode of formation. Both PtGe and PtGe₂ phases were found to obey a (t)1/2 law, indicating a diffusion limited growth process. By employing Ti as an inert marker, platinum was observed as the dominant diffusing species during Pt₂Ge formation. On the dominant diffusing species during PtGe and PtGe₂ formation, the thin film results were but tentative. Upon annealing at 500°C/30,90,180min, lateral diffusion couples of Pt rich source (on Ge thin film) resulted in only a limited lateral growth, and multiple phases were not observed. However, when samples of Ge rich source (on Pt thin film) were annealed at the same temperature and times, lateral interaction was observed proceeding on a relatively large scale. Germanium atoms were found to have encroached into the surrounding Pt thin film as far as ±30μm away from the Ge source region, with multiple phases growing simultaneously, viz PtGe₂, PtGe and Pt₂Ge₃. Inside the source region, the composition of the innermost compound corresponded to PtGe₂ phase. Pt₂Ge₃ was located between PtGe₂ and the initial island/thin film interface line. The compound outside the source region was characterised as PtGe. Pt₂Ge₃ phase was observed to have resulted from PtGe₂ disintegration through the mechanism 2PtGe₂ -+ Pt₂Ge₃ + Ge. Plots obtained from μRBS and SEM lateral measurements indicate that the growth of observed phases (PtGe₂, Pt₂Ge₃ and PtGe) all follow a square-root-of-time law, a characteristic of diffusion limited growth process.

Bibliography: 104-107.