Influences of Zn flow rate and temperature on the defect state-related photoelectronic properties of MOCVD-grown ZnGa2O4

Spinel-structural ZnGa2O4 has attracted considerable attention for novel high-power device applications due to its broad transparent range, wide bandgap of ∼4.80 eV, and high breakdown voltage characteristics. Herein, we focus on the evolution of the optical, electronic, and structural properties of ZnGa2O4 films under different Zn flow rates and temperature loading, as determined by spectroscopic ellipsometry, optical transmittance, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), photoluminescence (PL), and first-principles calculations. Remarkably, non-classical temperature-dependent evolution of the crystal-related band gap Eg and defect-related Urbach tail energy EU has been observed in an ultra-wide temperature range of 10–873 K using the two optical characterization techniques combined with a temperature-loading module. Specifically, the temperature dependency of EU only partially conforms to the Bose-Einstein distribution, due to the effects of oxygen-based defect emission. The results from XRD-based structural observation, PL-based sub-bandgap emission, and XPS-based atomic-ratio scanning also collectively demonstrate a strong correlation between EU and oxygen-based defect emission. The valence-band maxima and valence-band offset, as determined by ultraviolet photoelectron spectroscopy, further support the elucidation of the impacts of the different-level oxygen-based defect states caused by the Zn flow rate on the band structure of ZnGa2O4. These systematic consequences related to the defect stage are thus promising as a beneficial reference in the continual developments of ZnGa2O4-based optoelectronic, spintronic, electronic, sensing, optical, and related applications.