Mechanism of Thermodynamically Rationalized Selective Growth of a Two-Dimensional Ternary Ferromagnet on Insulating Substrates

Two-dimensional (2D) semiconducting ferromagnet Fe₃GeTe₂ holds great promise for advanced spintronic applications because of its gate-tunable ferromagnetic ordering at room temperature, whereas the controllable growth of large-area single crystals remains very challenging due to its ternary nature and variable stoichiometry inducing many competitive phases. Here, we theoretically probe the mechanism of selective growth of monolayer Fe₃GeTe₂ on various epitaxial substrates. Thermodynamic analysis shows that the corresponding phase-pure chemical potential windows for the selective growth of Fe₃GeTe₂ can be reasonably attained in ternary phase space on insulating and chemically inert c-plane sapphire and Ga₂O₃(0001) substrates by properly modulating the interfacial interaction and employing suitable feedstocks to avoid competitive growth of possible impurity phases with different stoichiometry ratios. It is also revealed that both the weak edge-substrate interaction and interlayer coupling of Fe₃GeTe₂ together lead to a surface-dominated nucleation behavior and, thereby, energetically favor lateral growth of the monolayer rather than vertical growth of the multilayer. Importantly, straight protocols for the experimentally selective growth of phase-pure ternary Fe₃GeTe₂ are also provided by establishing the relationship between the feedstock chemical potential and growth parameters on a thermochemical basis. Our insightful study can also be reasonably extended to guide future experimental design for the selective growth of other multicomponent 2D materials.