X-ray structural characterization of individual as-grown GaAs/(In,Ga)As/(GaAs) based core-multi-shell nanowires

In recent years, owing to the major development of X-ray focusing optics such as fresnel zone plates and Kirkpatrick-Baez mirrors, highly intense and collimated X-ray beams with spot sizes in the range of a hundred nanometers and below can be achieved. Together with proper sample preparation such as selective area growth, X-ray diffraction techniques can be deployed to study single nanometer-sized objects, in particular nanowires. Typically, transmission electron microscopy based methods are used to investigate the thickness, crystal phase and alloy distribution in individual core-shell nanowires. These properties, if controlled precisely, play the key role in optimizing the optical emission of NW based devices. Nevertheless, in this work, using X-ray nano diffraction based techniques we demonstrate a new path to achieve the complete structural characterization of individual as-grown GaAs/(In,Ga)As/GaAs based core-shell nanowires in addition to correlating their phase structure to their optical properties.
In a first step, the spatial distribution of the In alloy within the (In,Ga)As shell of individual GaAs/(In,Ga)As based core-shell nanowires is inspected by means of nano-focused scanning X-ray fluorescence microscopy. In contrast to energy dispersive X-ray spectroscopy in transmission electron microscopy, using this fast probing technique one can probe several nanowires in their as-grown geometry without the need of a complicated sample preparation procedure.
In a second step, high resolution X-ray nano diffraction is applied along and perpendicular to the NW [111] growth direction to measure the structural properties of individual GaAs/(In,Ga)As/GaAs core-shell nanowires grown by vapor liquid solid epitaxy. There, we were able to calculate the thickness of the core and shells and therefore reconstruct the full NW cross-section which is forever done by means of transmission electron microscopy of the nanowire basal plane.
In a third step, monitoring the GaAs 111 Bragg reflection, scanning X-ray diffraction microscopy was used as a fast scanning technique to reveal the polytype distribution along the growth axes of several individual core-multi-shell nanowires. The phase distribution was then correlated with cathodolu-minescence measurements revealing an enhancement in the optical emission of a mixed phase formed of the cubic zinc blende crystal phase and its twin, the hexagonal wurtzite polytype and slabs of defects and stacking faults.
Finally, we reveal induced structural and optical damage of individual semi-conductor nanowires, that are supposed to be resistant to radiation, for a threshold absorbed X-ray dose. This was demonstrated only for measurements that require fixing the X-ray nano beam to the same position along the nanowire growth axis. This study will spreads awareness regarding time consuming X-ray diffraction measurements performed on nano-structures.