Abstract
In this work, we investigate the local atomic structure of defect-free homogeneous and self-organized core-shell structure nanowires by means of X-ray Absorption Fine Structure (XAFS) Spectroscopy at the In LIII and K edges and Multiwavelength Anomalous Diffraction. The results are interpreted by comparison of the experimental data with X-ray absorption calculations carried out with ab initio structural models. Extended-XAFS data analysis at In K-edge shows an anisotropic In distribution in the second nearest neighbors pointing out to a deviation from randomness in In distribution for the core-shell sample.
Highlights
In recent years, much progress has been made in understanding the relationships between the optical and structural properties of InGaN alloys but many important aspects remain unclear
Regarding sample 2, first, we find that the out-of-plane In concentration in II shell (44%) is equal, within the experimental errors, to the average value found by Multiwavelength Anomalous Diffraction (MAD) (46%), showing that the out-of plane In distribution is random
This last result could suggest a similar behaviour as in sample 1 for the in-plane In distribution, but the effect is weaker and, in addition, the result about coordination numbers must be taken with caution because the Extended XAFS (EXAFS) spectrum of sample 2 is affected by a low S/N ratio leading to large experimental errors
Summary
Much progress has been made in understanding the relationships between the optical and structural properties of InGaN alloys but many important aspects remain unclear. Localization of holes has been demonstrated experimentally by Chichibu et al., who proposed a hole localization process, associated with atomic condensates of In-N Such a mechanism could be associated to a deviation from InGaN alloy randomness and to the hypothetical formation of In-N-In chains or InN clusters of a few In atoms. Only two experimental observations at the atomic scale consistent with a kind of weak phase separation or In aggregation have been found, one by Kachkanov et al., and the second by Miyanaga et al.4 The existence of this kind of composition fluctuations has been discussed recently from a theoretical point of view by Zunger et al.. The theory predicts that In clustering occurs when dislocations and/or grain boundaries are present (incoherent alloy) If these extended defects are absent (coherent alloy), the elastic strain could cause In and Ga ordering. As a matter of fact, tiny structural variations in the atomic local environment, the exact nature of which is still to be established, can produce strong effects on optical properties, making InGaN materials virtually insensitive to the presence of extended defects as, for instance, dislocations
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