Relative stabilities of Ag multilayers on GaAs and GaSb determined from ab initio calculations

Experimental studies have shown that flat metallic overlayers can be formed on semiconducting substrates by use of a two-step deposition process in systems traditionally known to be nonwetting. Specifically, atomically smooth monolayers are formed on semiconducting substrates when the equivalence of a critical thickness of metal is deposited at low temperatures and subsequently annealed, which is in contrast to the three-dimensional islands that form in room temperature growth. Here, ab initio density functional theory calculations are used to study the energy associated with adding new layers of Ag to the Ag/GaAs and the Ag/GaSb systems. The results predict a shift in the critical thickness for Ag/GaAs compared to that of Ag/GaSb, which is in agreement with experimental findings. The role of charge spilling and quantization in the fluctuations of the adhesion energy is also explored for these systems. It is found that charge spilling saturates for a coverage of three monolyers and greater. Beyond this point, the main contribution to adhesion energy fluctuation is attributed to structural changes due to the strain induced by the presence of the interface.