Real-Time Observation of Reactive Spreading of Gold on Silicon

The spreading of a bilayer gold film propagating outward from gold clusters, which are pinned to clean Si(111), is imaged in real time by low-energy electron microscopy. By monitoring the evolution of the boundary of the gold film at fixed temperature, a linear dependence of the spreading radius on time is found. The measured spreading velocities in the temperature range of $800<T<930\mathrm{K}$ varied from below $100\mathrm{pm}/\mathrm{s}$ to $50\mathrm{nm}/\mathrm{s}$. We show that the spreading rate is limited by the reaction to form Au silicide, and the spreading velocity is likely regulated by the reconstruction of the gold silicide that occurs at the interface.

Figure 1

3D gold-silicon phase diagram. At temperatures above the eutectic $T_E=636\mathrm{K}$, the composition of the eutectic melt in the cluster is readjusted to follow the eutectic liquidus (bold line). Inset: Schematic diagram of an Au-Si cluster in thermodynamic equilibrium on the Si surface. Under equilibrium condition, the surface is covered by an intermetallic crystalline monolayer of covalently bonded gold silicide with $\sqrt{3}\times \sqrt{3}$ reconstruction and by additional mobile gold atoms, fed by the Au-Si cluster, with thickness of up to a second monolayer.

Figure 2

The increment in the radius of the gold-silicide layer is measured directly from the LEEM images. In each LEEM image, the position of the Au-Si eutectic microparticle is within the dark circle [19], while the surrounding bright disc corresponds to the $\sqrt{3}$ reconstructions of the gold-silicide layer that forms as a consequence of the reactive spreading of Au over the Si(111) surface. The average radius of this circular spreading annulus is measured from the LEEM images from the average of parallel line profiles (lower plot) acquired from radial rectangular selections, as described in the text. Scale bar: $1\mu \mathrm{m}$.

Figure 3

Gold-silicide spreading velocities are measured at different temperatures. The persistence of linear time evolution of the spreading of the silicide layer, incompatible with diffusive spreading, indicates that reaction-limited kinetics endure over a wide temperature range.

Figure 4

Schematics of the evolution of the concentration $C$ of the Au atoms spreading.