The structure and growth direction of ErSi_{2-x} nanowires on Si(001)

In this work the structure and interface structure of ErSi_{2-x} nanostructures grown on Si(001) are investigated by aberration corrected high angle annular dark field scanning transmission electron microscopy. The initial nucleation and growth mechanism are investigated by direct observation of the structure of ultra-small nanowires and it was found that the preferred structure does not fit with the currently accepted growth model. The nanowires nucleate in the hexagonal phase with an orientation relationship with the silicon substrate of (001)_{Si}/(01\bar{1}0)_{ErSi_{2-x}}, [110]_{Si}/[0001]_{ErSi_{2-x}} which is the exact opposite of the currently accepted strained growth model. As the nanowires increase in size it was observed that the nanostructures transform their orientation relationships to reduce their strain resulting in the structure that is expected from the strained growth model. A new growth model is suggested in that the observed orientation relationship has been found by previous calculations to be lower in energy than the strained growth equivalent for ultra-small nanowires.

The interfaces of the ErSi_{2-x} nanostructures with Si($001$) are found to be highly varied and complex, containing many defects which contribute to a lowering of the intensity of the Er columns directly at the interface. A new strain reduction mechanism is observed where the silicide nanostructure drops down a single Si step on the substrate surface. A structure model of this mechanism is proposed. The nanostructures have shown a preference for the triangular projection of Er columns at the interface which is not thought to be a remnant of the new orientation relationship, but a strain reduction mechanism that competes with the double stepped interface. Some interfaces have been found to have Er columns shifted from their expected positions up into the silicide. This was attributed to an ordering of the vacancies within the ErSi_{2-x} silicon lattice at the interface.