Local structure of (Ga,Fe)N and (Ga,Fe)N:Si investigated by x-ray absorption fine structure spectroscopy
Mauro Rovezzi, Francesco D'Acapito, Andrea Navarro-Quezada, Bogdan Faina, Tian Li, Alberta Bonanni, Francesco Filippone, Aldo Amore Bonapasta, Tomasz Dietl
备注：
Minor changes: corrected references [3], [29] and [52]
采用密度泛函理论（DFT）中的{ \em ab initio}计算支持的X射线吸收精细结构（XAFS）测量系统地表征了金属有机气相外延生长的Fe掺杂以及Fe和Si共掺杂薄膜。扩展的XAFS数据分析表明，根据生长条件的不同，Fe原子要么占据GAN中Ga的替代位置，要么以$ε$ -Fe $_3$ N纳米晶的形式沉淀，根据DFT结果，这些纳米晶是铁磁性和金属性的。可以通过降低Fe含量，或通过增加生长速率或通过与Si共掺杂来阻碍沉淀。XAFS光谱的近边缘区域提供了关于Fe电荷状态的信息，并显示了Si共掺杂时Fe从Fe $^{+3}$到Fe $^{+2}$的部分还原，与DFT的各种实现中预期的Fe电子构型一致。
英文摘要：
X-ray absorption fine-structure (XAFS) measurements supported by {\em ab initio} computations within the density functional theory (DFT) are employed to systematically characterize Fe-doped as well as Fe and Si-co-doped films grown by metalorganic vapour phase epitaxy. The analysis of extended-XAFS data shows that depending on the growth conditions, Fe atoms either occupy Ga substitutional sites in GaN or precipitate in the form of $ε$-Fe$_3$N nanocrystals, which are ferromagnetic and metallic according to the DFT results. Precipitation can be hampered by reducing the Fe content, or by increasing the growth rate or by co-doping with Si. The near-edge region of the XAFS spectra provides information on the Fe charge state and shows its partial reduction from Fe$^{+3}$ to Fe$^{+2}$ upon Si co-doping, in agreement with the Fe electronic configurations expected within various implementations of DFT.
We report a theoretical description and experimental implementation of a novel two-dimensional coherent excitation spectroscopy based on quasi-steady-state photoinduced absorption measurement of a long-lived nonlinear population. We have studied a semiconductor-polymer:fullerene-derivative distributed heterostructure by measuring the 2D excitation spectrum by means of photoluminescence, photocurrent and photoinduced absorption from metastable polaronic products. We conclude that the photoinduced absorption probe is a viable and valuable probe in this family of 2D coherent spectroscopies.
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