Aug 6, 2013
Do purely dilute rare-earth doped group-III nitrides really exist?
The random distribution of dopant atoms in semiconductors is crucial for the electronic, optical and magnetic properties of the doped material. For years, the in situ doping of group-III nitrides with rare earth elements by non-equilibrium growth techniques such as molecular beam epitaxy (MBE) was assumed to create purely dilute magnetic semiconductors in spite of the theoretically calculated large miscibility gaps being present in those material systems.
Presenting their results in the journal Nanotechnology, researchers at the Paul-Drude-Institut für Festkörperelektronik in Berlin have unambiguously identified (by transmission electron microscopy (TEM) techniques) the existence of GdN clusters in previously assumed dilute GaN:Gd thin films grown by MBE with Gd concentrations in the range between 1016 cm–3 and 1019 cm–3.
The observation of nanoscale three-dimensional objects in buried layers is highly challenging even with state-of-the-art microscopic and/or spectroscopic methods. In the current study, the team found that the incorporation of Gd atoms introduces strong lattice distortions in the surrounding GaN lattice that could easily be neglected or mistakenly interpreted as preparation artifacts.
Based on a detailed analysis of these lattice distortions in combination with strain contrast simulations and Z-contrast high-angle annular dark-field imaging in a scanning TEM, the researchers were able to directly observe small clusters of platelet shapes embedded in the GaN matrix and identify them as coherently strained GdN clusters containing only few tens of Gd atoms.
Statistical analysis indicates that almost all incorporated Gd atoms exist in homogeneously distributed clusters of uniform size. The results will help scientists to obtain a better understanding of the ambiguous magnetic properties of this material.
In light of the discovery, careful studies on similar rare-earth doped semiconductors and other dilute magnetic material systems with a large miscibility gap should be performed in order to reconsider the possible existence of clusters and to generally clarify the process conditions for phase separation.
More information can be found in the journal Nanotechnology 24 255701
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About the author
Mingjian Wu is a PhD student at Humboldt-Universität, Berlin, and a researcher at the Paul-Drude-Institut für Festkörperelektronik in Berlin, under the supervision of Dr Achim Trampert. His PhD thesis includes the study of phase stability in epitaxial semiconductor systems using transmission electron microscopy. Dr Achim Trampert is head of the microstructure department at the Paul-Drude-Institut. His research focuses on applying advanced microscopy techniques to explore structure-property relations.