Correlation between structural and optical properties of single semiconductor nanowires with core-shell heterostructure

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Semiconductor nanowires have emerged as an important materials basis both for studying fundamental physics and for fabricating novel devices. In the nanowire geometry, strain can elastically relax at the free sidewalls, and thus heterostructures of dissimilar materials can be grown in a crystalline quality that is impossible to obtain in planar thin films. At the same time, the quasi one-dimensional shape, nanometric size, and high surface-to-volume ratio of nanowires can lead to unexpected effects. In order to understand, predict, and control the physical properties of nanowires, it is imperative to elucidate how the properties of interest depend on the nanowire structure. The overall objective of this project is to correlate the optical and structural properties of single GaAs nanowires with radial (In,Ga)As/GaAs quantum well. For such nanowire heterostructures a larger critical thickness for the formation of dislocations is expected than in their planar counterparts, because the strain induced by lattice mismatch is shared between (In,Ga)As quantum well and GaAs core and outer shell. Also, the optical transitions are anticipated to be different for the nanowire heterostructure, because the strain state of the quantum well is modified, the band structure is affected by lateral band bending due to Fermi level pinning at the nanowire sidewalls, and the nature of the entire structure is intrinsically three-dimensional.The ambitious experimental goal is to measure subsequently for one and the same single nanowire the optical and structural properties, and carry out such measurements for series of samples with systematic variations in structure. Samples will be grown by molecular beam epitaxy, the optical properties will be investigated by photo- and cathodoluminescence spectroscopy, and the structure will be analyzed by x-ray nanodiffraction. In order to obtain a deep understanding of the structure-property correlation, experiments will be complemented by continuum elasticity and multiband-k.p calculations of the optical transitions. We will identify the observed emission lines, assign them to structural features, and elucidate how these lines vary with In content and/or quantum well thickness.