Ga2O3 polymorphs: Tailoring the epitaxial growth conditions

Gallium oxide is a wide bandgap n-type semiconductor highly interesting for optoelectronic applications (e.g., power electronics and solar blind UV photodetectors). Besides its most thermodynamically stable monoclinic β phase, Ga2O3 can crystallize in different polymorphs; among them the corundum α and the orthorhombic ϵ phases are the most promising ones. In this review we focus on the main aspects that promote the nucleation and stable growth of these Ga2O3 polymorphs. Particular emphasis is given to the ϵ phase since it is recently gaining increasing attention in the scientific community because of: (i) its higher lattice symmetry with respect to β-Ga2O3, which could favour the realization of heterostructures, (ii) the possibility to be grown on cheap sapphire substrates and (iii) its peculiar piezoelectric properties. While the growth of β-Ga2O3 is widely studied and understood, a thorough and comprehensive analysis of the chemical and physical aspects that allow for the stabilization of the metastable Ga2O3 phases with different synthesis methods is still missing. Therefore, the present review aims at filling this gap, by analysing the relevant growth parameters for several growth techniques (MOVPE, HVPE, mist-CVD, MBE, and PLD), highlighting similarities and differences, looking for a unified framework to understand the growth and nucleation of different Ga2O3 polymorphs. As a conclusion, we highlight practical guidelines for the deposition of the different Ga2O3 polymorphs with all the discussed thin film growth techniques.