Influence of the “second gap” on the transparency of transparent conducting oxides: An ab initio study
(2016) Applied Physics Letters — Vol. 108, n° 20, p. 201902 (2016)
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- Transparent conducting oxides (TCOs) are essential to many technologies. These materials are doped (n- or p-type) oxides with a large enough band gap (ideally >3 eV) to ensure transparency. However, the high carrier concentration present in TCOs leads additionally to the possibility for optical transitions from the occupied conduction bands to higher states for n-type materials and from lower states to the unoccupied valence bands for p-type TCOs. The “second gap” formed by these transitions might limit transparency, and a large second gap has been sometimes proposed as a design criteria for high performance TCOs. Here, we study the influence of this second gap on optical absorption using ab initio computations for several well-known n- and p-type TCOs. Our work demonstrates that most known n-type TCOs do not suffer from second gap absorption in the visible even at very high carrier concentrations. On the contrary, p-type oxides show lowering of their optical transmission for high carrier concentrations due to second gap effects. We link this dissimilarity to the different chemistries involved in n- versus typical p-type TCOs. Quantitatively, we show that second gap effects lead to only moderate loss of transmission (even in p-type TCOs) and suggest that a wide second gap, while beneficial, should not be considered as a needed criteria for a working TCO.
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Ha, V. A., Waroquiers, D., Rignanese, G.-M., & Hautier, G. (2016). Influence of the “second gap” on the transparency of transparent conducting oxides: An ab initio study. Applied Physics Letters, 108(20), 201902. https://doi.org/10.1063/1.4950803 (Original work published 2016)