Optical and fundamental band gaps disparity in transparent conducting oxides: new findings for the and systems

Abstract

The optical band gap, extracted from absorption measurements, defines the figure of merit for transparent conducting oxides (TCOs). In many oxides, such as or , inversion symmetry introduces a selection rule that blocks transitions from the valence-band maximum to the conduction-band minimum. This raises the absorption threshold and enlarges the optical gap relative to the fundamental band gap. Here, we present density-functional computations identifying two optical gaps, either of which can be detected, depending on the optical light intensity. Under strong illumination, weak transitions from -points near the valence-band maximum contribute significantly to the absorption spectrum and define an optical gap matching the fundamental gap. Low optical intensities by contrast give prominence to the large optical gap determined by the selection rule. While experimental conditions have favored observation of the former optical gap in , in contrast, absorption measurements in have focused on the latter. Our findings explain the disparity between the optical and fundamental gaps in bixbyite and predict that, measured under low illumination, the optical gap for rutile will increase, from 3.60 eV to 4.34 eV.