Non-equilibrium epitaxy of metastable polymorphs of ultrawide-bandgap gallium oxide

Precision synthesis of ultrawide-bandgap semiconductors with a desired crystal phase is of broad interest for developing advanced electronic devices. However, it is highly challenging for gallium oxide (Ga2O3), which is known for versatile crystal phase transition. Here, we report a non-equilibrium epitaxy strategy to confine the crystallization pathways of Ga2O3 toward two distinct metastable polymorphs during the pulsed laser deposition (PLD) growth. This is achieved by synergic control of the substrate orientation and intentional tin (Sn) doping, which dramatically modifies the nucleation and growth kinetics of Ga2O3. Using a-plane sapphires and a medium Sn doping level, we overcome the commonly observed growth limitations of α-phase Ga2O3 (α-Ga2O3) films that are only stable for the initial few monolayers in previous PLD studies. Instead, we stabilize epitaxial α-Ga2O3 films with excellent phase uniformity and crystallinity for a thickness beyond 200 nm. This contrasts to the otherwise formed ε-phase Ga2O3 films by simply switching the sapphire substrate orientation to c-plane regardless of the Sn doping level. Density functional theory calculations reveal the critical role of the surface energy minimization for selective stabilization of metastable phases. This study provides a perspective to improve the non-equilibrium synthesis capability for exploring emerging ultrawide-bandgap semiconductors.