Phase separation during co-deposition of Al–Ge thin films

We present the results of a combined experimental and theoretical investigation of phase separation and microstructure development in co-deposited Al–Ge thin films. For small film thicknesses and deposition temperatures above 150 °C the phase-separated films consist of an array of domains of the Al- and Ge-rich terminal phases (lateral phase separation). Films deposited at 100 °C or less contained one or both of the terminal phases plus a metastable phase. We show that the domain structure evolves during deposition in a manner consistent with a surface interdiffusion controlled process. As film thickness increases we observe a transition from the laterally phase-separated microstructure to a layered microstructure exhibiting phase separation perpendicular to the film/substrate interface (transverse phase separation), with Al segregating to the film surface. We present a thermodynamic argument based on the competition between surface and interfacial free energies to explain this transition. Finally, we discuss the stability of the transverse phase-separated microstructure in the thick-film limit in terms of the transport of Ge through the Al-rich surface layer.