Ga2O3 polymorphs: tailoring the epitaxial growth conditions

Abstract

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.