In situ spectroscopic ellipsometry and ex situ structural investigations were performed
on annealed amorphous Ga2O3 thin films prepared by radio frequency sputtering. The
change of the optical parameters was monitored during the annealing in air. In situ
ellipsometry was shown to be a valuable method to determine the phase transition temperatures
of Ga2O3 during annealing in ambient conditions. X-ray diffraction and transmission
electron microscopy revealed that the initially amorphous film first crystallizes
into the metastable γ-Ga2O3 phase, which transforms into the β-Ga2O3 phase at higher
temperatures. Optical transmittance measurements were used to determine the optical
bandgap of the layers at the different annealing steps. Photoluminescence experiments
were performed on the samples after annealing at varying temperatures and atmospheres.
After the initial formation of the γ-Ga2O3 phase a broad green luminescence band emerged,
while after the transformation to the β-Ga2O3 phase blue or red emission was observed
depending on the annealing atmosphere.
