Mechanism of\nThermodynamically Rationalized Selective\nGrowth of a Two-Dimensional Ternary Ferromagnet on Insulating Substrates

Two-dimensional (2D) semiconducting ferromagnet Fe<sub>3</sub>GeTe<sub>2</sub> holds great promise for advanced spintronic\napplications\nbecause of its gate-tunable ferromagnetic ordering at room temperature,\nwhereas the controllable growth of large-area single crystals remains\nvery challenging due to its ternary nature and variable stoichiometry\ninducing many competitive phases. Here, we theoretically probe the\nmechanism of selective growth of monolayer Fe<sub>3</sub>GeTe<sub>2</sub> on various epitaxial substrates. Thermodynamic analysis shows\nthat the corresponding phase-pure chemical potential windows for the\nselective growth of Fe<sub>3</sub>GeTe<sub>2</sub> can be reasonably\nattained in ternary phase space on insulating and chemically inert\nc-plane sapphire and Ga<sub>2</sub>O<sub>3</sub>(0001) substrates\nby properly modulating the interfacial interaction and employing suitable\nfeedstocks to avoid competitive growth of possible impurity phases\nwith different stoichiometry ratios. It is also revealed that both\nthe weak edge–substrate interaction and interlayer coupling\nof Fe<sub>3</sub>GeTe<sub>2</sub> together lead to a surface-dominated\nnucleation behavior and, thereby, energetically favor lateral growth\nof the monolayer rather than vertical growth of the multilayer. Importantly,\nstraight protocols for the experimentally selective growth of phase-pure\nternary Fe<sub>3</sub>GeTe<sub>2</sub> are also provided by establishing\nthe relationship between the feedstock chemical potential and growth\nparameters on a thermochemical basis. Our insightful study can also\nbe reasonably extended to guide future experimental design for the\nselective growth of other multicomponent 2D materials.