Structure and Strain Relaxation in Self-Assisted Grown InAs/GaAs Nanowires

At present the most interesting technology for the preparation of advanced opto-electronic devices seems to be the one based on semiconductor nanowires (NWs). Because of their high aspect ratio the NWs are the best candidates for the combination of highly mismatched semiconductors while maintaining good crystal quality. In particular, thanks to strain relaxation at the NW sidewalls, a reduction of the misfit dislocation density is possible, which is highly beneficial for device applications. In this regard interface sharpness is also a critical issue, especially for InAs/GaAs. In this work we present a study of the structure and strain relaxation in InAs/GaAs axial nanostructures in self-assisted NWs grown by MBE on (111) Si substrates. The growth was performed without the use of any catalyst. The top InAs segment is grown on the bottom self-assisted GaAs part of the NW via a two-step process by supplying In and As separately. The samples have been studied by High Resolution TEM (HR-TEM) as well as Scanning TEM (STEM) with a HAADF (High Angle Annular Dark Field) detector. HR-TEM showed that the InAs/GaAs is sharp at the atomic level, a result that has never been reported in the literature for MBE grown InAs/GaAs NWs. By STEM-HAADF the chemical transition between GaAs and InAs was seen to occur abruptly over a distance of 1.5 nm across the interface. The strain relaxation and related misfit dislocation (MD) generation were investigated by mapping the strain distribution obtained by processing HR-TEM images of the interface by the Geometrical Phase Analysis (GPA) method. MDs were detected as expected due to the size of about 70 nm of the NWs. However, their mutual distance increases radially on going from the NW center towards the sidewalls confirming that the strain was partially relaxed elastically at the sidewalls thus decreasing the formation probability of the MDs, i.e. their density. The latter experimental results have been compared with FEM (Finite Element Method) calculations with excellent agreement.