Optical properties of GaAs-AlGaAs core-multishell nanowire quantum structures grown by MOVPE

III-V compounds nanowires (NWs) have gathered considerable research interests in recent years, in reason of their potential applications to novel and high-performance nano-scale light emitting diodes, lasers, photodetectors... [ view full abstract ]

III-V compounds nanowires (NWs) have gathered considerable research interests in recent years, in reason of their potential applications to novel and high-performance nano-scale light emitting diodes, lasers, photodetectors and solar cells. In particular, radial modulation of NW composition in the form of core-(multi)shell heterostructures promises to impact these nano-devices by adding new degrees of freedom to their design. Besides, strict control over the epitaxial self-assembly of such radially heterostructured NWs and detailed insights of their nano-scale (structural, radiative) properties are necessary.
We report on the luminescence properties of dense arrays of vertically-aligned GaAs-AlGaAs core-multishell NW quantum heterostructures grown on (111)B-GaAs wafers by metalorganic vapor phase epitaxy. Au-catalysed GaAs NWs were radially overgrown by two AlGaAs shells between which a few-nm thin GaAs shell was introduced to form a quantum well tube (QWT).
Besides the GaAs nanowire core emission band peaked at around 1.503 eV, 7K PL spectra showed an additional broad peak in the 1.556-1.583 eV energy interval, ascribed to the transition between electron and hole confined states within the QWT. The emission blue-shifts with the shrinkage of as-grown GaAs well tubes, as the nanowire local (on the substrate) density and height change.
High spatial resolution cathodo-luminescence (CL) spectroscopy inside a field-emission scanning electron microscope was performed at low-temperature (7K) on single GaAs-AlGaAs core-multishell NWs and GaAs-AlGaAs QWT structures. CL imaging of single QWT heterostructures allowed us to spatially resolve different contributions found in the CL (and PL) spectra. Furthermore, we show for the first time the occurrence of both large-scale spatial inhomogeneities (along the NW trunk) and nano-scale fluctuations of QWT emissions. Our findings are finally analysed as function of QWT parameters and growth conditions.