Solid state reactions at the interface between thin films of Au and hydrogenated amorphous Si are probed by interference enhanced Raman scattering, TEM, scanning Auger microprobe, and Schottky electrical measurements. It is found that crystalline Si islands with a dimension of ∠1 μm grow dendritically after annealing to temperatures above 150 °C. The islands apparently grow laterally due to diffusion of Si through Au. This formation of Si crystallites occurs at a temperature much lower than the normal crystallization temperature of ≳650 °C for the hydrogenated amorphous Si. Through this process the Schottky barrier is not destroyed. A model is proposed to describe the formation process, which includes the low-temperature Si diffusion into Au to form a Au–Si intermixed phase, and the agglomeration of the Au-rich regions.