Interfaces in complex InAs-GaSb heterostructured nanowires - A transmission electron microscopy study

Sammanfattning: In this project epitaxially grown InAs-GaSb complex heterostructured nanowires have been characterized by means of aberration corrected TEM techniques and energy dispersive X-ray spectroscopy (EDX). The InAs-GaSb material is of interest due to its high charge carrier mobility, which has possible applications in electronic devices such as FETs. InAs-GaSb heterostructured nanowires in zincblende structure have previously been studied and have possible electronic applications such as tunneling field effect transistors. However, heterostructured nanowires of InAs-GaSb in wurtzite structure have until now not been observed. The focus of this project have been to perform a structural characterization of InAs-GaSb core-shell and InAs-GaSb-InAs core-shell-shell nanowires with an emphasis on the interfaces. Epitaxial axial growths of GaSb are present on top of the core-shell(-shell) structures which are induced by the seed particles during the shell growth. The lattice mismatch of wurtzite InAs-GaSb is 1.15%. However, the nanowire shells were fully epitaxial with practically no misfit dislocations. This is due to that the elastic relaxation mechanisms are the dominating mechanisms for compensating the lattice mismatch induced strain. As the shells were fully epitaxial stacking faults are transferred from the core to the shells. The tapering events of the shells were found to generally coincide with these stacking faults. Analysis with high resolution EDX revealed a change of composition at the axial interface between the InAs core and the axial growth of GaSb. A short segment, approx. 6 nm, of GaAsSb ternary at the axial interface is attributed to the different solubility of the elements in the Au seed particle and to the reservoir effect of the seed particle during epitaxial growth. A one atomic bilayer thick composition change was present at the radial interfaces, which was observed by means of aberration corrected scanning TEM (STEM). The bilayer was a ternary of InAsGa between the InAs core and the GaSb shell. Such bilayers were present also at both interfaces in the GaSb-InAs double shell nanowires. This change in composition might influence the band gap alignment and subsequently the electric properties of the material. It is of importance to further evaluate the functional impact of this ternary in order to further understand and develop core-shell materials. Evidence of metastability of the core-shell nanowires were also found during the project which is another important phenomenon to further investigate.