Fig. 1. （a）Plane-view and（b）cross-sectional SEM images of a β-Ga₂O_3-δ film deposited under the oxygen partial pressure of 20 mTorr，and（c）XRD patterns of the as-grown β-Ga₂O_3-δ films on c-sapphire substrates deposited under various oxygen partial pressures from 5 to 40 mTorr，the peaks labelled by the symbol（*）come from the sapphire substrates，（d）FIR reflectance spectra of the Ga₂O_3-δ/c-sapphire samples. The dashed lines indicate the transverse optical（TO）infrared active phonon modes. Note that the curves are shifted vertically for clarity

Fig. 2. （a）The survey XPS spectra of β-Ga₂O_3-δ films deposited under the oxygen pressure of 5 mTorr，the experimental and best-fitted XPS fitting results of the（b）Ga 2p and（c）O 1s peaks for samples deposited under the various oxygen pressures of 5，10，20，and 40 mTorr

Fig. 3. （a）Transmittance spectra of the β-Ga₂O_3-δ/c-sapphire samples deposited under the oxygen partial pressures of 5，10，20，and 40 mTorr，（b）the plots of（αE）² as a function of photon energy for direct bandgap，the arrows indicate the optical bandgap of β-Ga₂O_3-δ films，（c）experimental（dotted lines）and best-fitted（solid lines）transmittance spectra of a β-Ga₂O_3-δ film under the oxygen partial pressure of 40 mTorr，（d）refractive index n and（e）extinction coefficient κ of the β-Ga₂O_3-δ films deposited at various oxygen partial pressures，（f）the extracted absorption edge as a function of oxygen partial pressure

Fig. 4. （a）Band structure and（b）partial and total density of states（DOS）of intrinsic β-Ga₂O₃

Table 1. Parameter values of the Tauc-Lorentz model for the Ga₂O_3-δ films determined from the simulation of transmittance spectra