Ciechan A, Bogusławski P
Institute of Physics, Polish Academy of Sciences, al. Lotników 32/46, 02-668 Warsaw, Poland.
J Phys Condens Matter. 2019 Jun 26;31(25):255501. doi: 10.1088/1361-648X/ab119d. Epub 2019 Mar 20.
Previous luminescence and absorption experiments in Co-doped ZnO revealed two ionization and one intrashell transition of [Formula: see text] electrons. Those optical properties are analyzed within the generalized gradient approximation to the density functional theory. The two ionization channels involve electron excitations from the two [Formula: see text] gap states, the [Formula: see text] triplet and the [Formula: see text] doublet, to the conduction band. The third possible ionization channel, in which an electron is excited from the valence band to the [Formula: see text] level, requires energy in excess of 4 eV, and cannot lead to absorption below the ZnO band gap, contrary to earlier suggestions. We also consider two recombination channels, the direct recombination and a two-step process, in which a photoelectron is captured by [Formula: see text] and then recombines via the internal transition. Finally, the observed increase the band gap with the Co concentration is well reproduced by theory. The accurate description of ZnO:Co is achieved after including +U corrections to the relevant orbitals of Zn, O, and Co. The [Formula: see text] value was calculated by the linear response approach, and independently was obtained by fitting the calculated transition energies to the optical data. The respective values, 3.4 and 3.0 eV, agree well. Ionization of Co induces large energy shifts of the gap levels, driven by the varying Coulomb coupling between the [Formula: see text] electrons, and by large lattice relaxations around Co ions. In turn, over ∼1 eV changes of [Formula: see text] levels induced by the internal transition are mainly caused by the occupation-dependent [Formula: see text] corrections.
先前对钴掺杂氧化锌进行的发光和吸收实验揭示了[公式：见原文]电子的两种电离和一种壳内跃迁。这些光学性质在密度泛函理论的广义梯度近似下进行了分析。这两种电离通道涉及电子从两个[公式：见原文]能隙态，即[公式：见原文]三重态和[公式：见原文]二重态，激发到导带。第三个可能的电离通道，即电子从价带激发到[公式：见原文]能级，需要超过4 eV的能量，并且与早期的观点相反，不会导致低于氧化锌带隙的吸收。我们还考虑了两种复合通道，直接复合和两步过程，其中光电子被[公式：见原文]捕获，然后通过内部跃迁进行复合。最后，理论很好地再现了观察到的带隙随钴浓度的增加。在对锌、氧和钴的相关轨道进行+U修正后，实现了对氧化锌：钴的准确描述。[公式：见原文]值通过线性响应方法计算，并通过将计算出的跃迁能量拟合到光学数据独立获得。各自的值，3.4和3.0 eV，吻合得很好。钴的电离导致能隙能级的大能量位移，这是由[公式：见原文]电子之间变化的库仑耦合以及钴离子周围的大晶格弛豫驱动的。反过来，由内部跃迁引起的[公式：见原文]能级超过约1 eV的变化主要是由与占据相关的[公式：见原文]修正引起的。
