Mono-dimensional non-centrosymmetric crystals are crystallographic structures of interest as their "polarities" offer additional degrees of freedom to build nanoscale systems. Such is the case for GaN wires produced at INAC, where the physicists favor one of the two crystallographic polarities through silane injection modulation (SiH4) during their production, known as Metal-Organic Vapor Phase Epitaxy. This technique allows for development of original "long" wire structures that serve as a support to other semiconductor heterostructures used in light emission/detection, or for their piezoelectric properties. By combining X-ray fluorescence and X-ray excited optical luminescence, they provided experimental evidence of the role of silane to promote the N-polarity within GaN, with a stronger incorporation of silicon (electron donor dopant) than in the domains of gallium polarity.
InGaN/GaN quantum multiwells are incorporated on the surface of the GaN wires, parallel and perpendicular to their axis. By focusing a high-energy X-ray beam across a 60 x 60 nm2, the researchers were able to highlight a correlation between crystallographic polarity maps and electronic band structure defects, in the case of individual wires. They were also able to observe the diffusion of charge carriers without electrical contact. These nanoscale synchrotron analyses lead to a better understanding of the link between optical emission and elemental composition.
X-ray excited photoluminescence and spectral analysis (in the visible and near-UV) can also be supplemented by absorption spectroscopy and μLaue diffraction to study other semiconducting, optoelectronic or photovoltaic hetero-structures.