Nano-scale luminescence imaging and properties of radially-heterostructured III-V nanowires

III-V compounds nanowires (NWs) have gathered considerable research interests in recent years, due to their potential applications to novel nano-scale photonic devices, such as nano-lasers, photodetectors and solar cells.... [ view full abstract ]

III-V compounds nanowires (NWs) have gathered considerable research interests in recent years, due to their potential applications to novel nano-scale photonic devices, such as nano-lasers, photodetectors and solar cells. Radial modulation of NW composition to form core-(multi)shell heterostructures promises to impact such nano-devices by adding new degrees of freedom (due to quantum confinement) to their design. However, strict control over the self-assembly of radially heterostructured NWs and detailed insights of their nano-scale (structural, radiative) properties are crucial for their exploitation in future nano-photonic devices.

We report on the nano-scale spectroscopic characterization of GaAs-AlGaAs core-shell and core-(multi)shell NWs grown by metal-catalysed metalorganic vapour phase epitaxy. High spatial resolution cathodo-luminescence (CL) spectroscopy measurements performed inside a field-emission scanning electron microscope enabled us to study the radiative properties of single core-(multi)shell NW structures in great details, and correlate them to the nanostructure dimensions. CL measurements were performed at low-temperature (7K) on single GaAs-AlGaAs core-shell NWs and GaAs-AlGaAs quantum well tube (QWT) structures. This allowed to study the free-exciton emission of GaAs core in GaAs-AlGaAs core-shell NWs and its energy shift with the heterostructure relevant size (namely, the shell thickness to core radius ratio), this being afterwards compared with the strain-shifted values of heavy- and light-hole excitons calculated for GaAs assuming an elastic energy equilibrium model in the NW. By comparing CL results with photoluminescence (PL) spectra collected from dense NW arrays, we further demonstrated the combined effect of strain-induced band-shift and the occurrence of a space-charge region (associated with materials unintentional doping) at the GaAs/AlGaAs interface, as the origin of red-shift of GaAs excitons in the PL emission of these core-shell NWs.

CL imaging of single GaAs-AlGaAs core-multishell NWs allowed to spatially resolve different contributions found in CL (and PL) spectra, namely (i) AlGaAs emission channels related to unintentional formation of Al-rich alloys within the nanostructure (a result of NW specific growth mechanism), and (ii), spatial inhomogeneity of the GaAs QWT emission, further ascribed to thickness gradient/fluctuations of the QWT along the NW axis. The results will be further discussed based on our current knowledge of the nano-scale structural properties of present NWs.