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.