Nucleation Window of Ga₂O₃ and In₂O₃ for Molecular Beam Epitaxy on (0001) Al₂O₃

The epitaxial growthof Ga2O3 andIn(2)O(3) on (0001)-oriented Al2O3 by plasma-assisted molecular beam epitaxy is investigated.The synthesisof both materials underlies similar growth kinetics, with the formationof volatile suboxides limiting their growth in the metal-rich regime.Quantitative analysis of beta-Ga2O3 and c-In2O3 growth kinetics has been performed on appropriatebuffer layers mimicking homoepitaxial growth conditions. In particular,we investigate the influence of the amount of supplied atomic oxygenon the growth kinetics of beta-Ga2O3. Consideringthe heteroepitaxial growth on bare (0001) Al2O3, we found that the observed nucleation window is much more limitedthan the expected growth window derived from homoepitaxial growthconditions. For Ga2O3 this was studied in detailand the upper limit of the nucleation window was determined in termsof the gallium to oxygen ratio on the growth surface and the growthtemperature. Outside the nucleation window, we report that three monolayers alpha-(Al0.75 +/- 0.04Ga0.25 -/+ 0.04)(2)O-3 formed in the initial stage of growth passivatethe surface with respect to further beta-Ga2O3 nucleation. We attribute the formation of this layer to the intermixingof a Ga-rich adlayer and Al atoms from the substrate until the solubilitylimit in the alpha-phase is reached. The observation is accompaniedby a specific RHEED pattern with satellite reflexes assigned to asurface reconstruction in the metal-rich growth regime. We report the existence of a nucleation window of beta-Ga2O3 and c-In2O3 epitaxiallygrown on (0001) Al2O3. Its size is decisivelyinfluenced by the growth temperature and the III-V surfaceratio. Outside of the nucleation window we observe solely the formationof three monolayer beta-(Al0.75Ga0.25)(2)O-3 by an intermixing process accompanied with aunique surface reconstruction.