Suboxides and subselenides: intermediate reaction products to form Ga₂O₃, Ga₂Se₃, In₂O₃, In₂Se₃, SnO₂, and SnSe₂ during molecular-beam epitaxy

The molecular-beam epitaxial (MBE) growth of III-O and IV-O materials (e.g., Ga2O3, In2O3, and SnO2) is known to be reaction-limited by complex 2-step kinetics and the desorption of volatile suboxides (e.g., Ga2O, In2O, SnO). We find that the different surface reactivities of suboxides and respective elements (e.g., Ga, In, Sn) with active oxygen define the film-growth-windows (FGWs) and suboxide-formation-windows (SFWs) of III-O and IV-O materials, respectively. To generalize, we provide elementary reaction pathways and respective Gibbs energies to form binary III-O, III-Se, IV-O, and IV-Se ground-states as well as their subcompounds during their MBE growth. We apply the 2-step kinetics model established for oxides to identify the subselenide-limited growth of Ga2Se3 as the specific example for III-Se materials. Our kinetic and thermodynamic conclusions suggest subcompound-limited growth may be an inherent property for the growth of III–VI and IV–VI thin films by MBE and related epitaxial growth techniques.