Chemical Processes Involved in Atomic Layer Deposition of Gallium Sulfide: Insights from Theory

Growth characteristics of atomic layer deposition (ALD) of gallium sulfide (GaxS) from tris(dimethylamino)gallium (Ga(NMe2)(3)) and hydrogen sulfide (H2S) have been investigated by density functional theory. The steady-state film growth during one ALD cycle was modeled by studying the energetics of a large set of reactions taking place on a surface described as a SH-terminated gallium sulfide (Ga2S3) cluster. The opportunity of using gas-phase cluster calculations to compute entropic contributions is discussed. Detailed reaction mechanisms are provided, along with free energy profiles and a large reaction network emerges. It confirms a ligand loss/exchange mechanism, and various aspects of the growth are explored (fate of the precursor and co-reactant, self-saturation of the surface, crawling and cooperative effects, role of the amido ligand, of the impurities, etc.) and confronted to experimental observations. All the key reaction steps are facile and thermodynamically favorable, while undesired side reactions are more demanding energetically.