Sabino Fernando P, Nunes Oliveira Luiz, Wei Su-Huai, Da Silva Juarez L F
São Carlos Institute of Physics, University of São Paulo, PO Box 369, 13560-970, São Carlos, SP, Brazil.
J Phys Condens Matter. 2017 Mar 1;29(8):085501. doi: 10.1088/1361-648X/aa4e8c. Epub 2017 Jan 6.
The optical band gap, extracted from absorption measurements, defines the figure of merit for transparent conducting oxides (TCOs). In many oxides, such as [Formula: see text] or [Formula: see text], 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 [Formula: see text]-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 [Formula: see text], in contrast, absorption measurements in [Formula: see text] have focused on the latter. Our findings explain the disparity between the optical and fundamental gaps in bixbyite [Formula: see text] and predict that, measured under low illumination, the optical gap for rutile [Formula: see text] will increase, from 3.60 eV to 4.34 eV.
从吸收测量中提取的光学带隙定义了透明导电氧化物（TCO）的品质因数。在许多氧化物中，如[化学式：见原文]或[化学式：见原文]，反演对称性引入了一个选择规则，该规则阻止了从价带最大值到导带最小值的跃迁。这提高了吸收阈值，并相对于基本带隙扩大了光学带隙。在这里，我们展示了密度泛函计算，确定了两个光学带隙，根据光强度的不同，其中任何一个都可以被检测到。在强光照射下，价带最大值附近的[化学式：见原文]点的弱跃迁对吸收光谱有显著贡献，并定义了一个与基本带隙匹配的光学带隙。相比之下，低光强度突出了由选择规则决定的大光学带隙。虽然实验条件有利于观察[化学式：见原文]中的前一个光学带隙，但相比之下，[化学式：见原文]中的吸收测量则集中在后一个光学带隙上。我们的发现解释了方铁锰矿[化学式：见原文]中光学带隙和基本带隙之间的差异，并预测在低光照下测量时，金红石[化学式：见原文]的光学带隙将从3.60电子伏特增加到4.34电子伏特。
