We can further explain in the phenomena of absorption edge shift from the (αhν)2 vs. hν plot according to the following relation [18], since ZnGa2O4 belongs to direct-gap material [19],
(Ahv)2 = A(hv - Eg)
where hν is the photon energy. The absorption coefficient α can be calculated from the transmittance T shown in Fig. 4 by use of α=ln(1/T)/d. The thickness of films (d) grown on quartz substrates at oxygen pressure of 0 and 1 Pa is estimated to be 200 nm and 150 nm by using ellipsometry, respectively. The corresponding plots of (αhν)2 vs. hν are shown in Fig. 5.