This work uses a broad range of optical spectroscopies and electron microscopy to characterize the structure and electronic states of nanowires. We place an emphasis on understanding how to alter the electronic properties using strain and quantum confinement. We seek to develop a comprehensive understanding of NW properties through comparisons with model predictions. In addition, we adapt optical techniques traditionally used with larger structures to obtain a sub-micron measurement of nanowire diffusion and mobility.
First, we extend our optical techniques by spatially resolving the diffusion of excitons along the long axis of a nanowire using a solid immersion lens (SIL). By sampling the time decays as a function of distance along the nanowire, we can measure the diffusion of excitons directly. The extracted diffusion constants for defect free single crystal GaAs measured between 45-100 cm^2/s with resultant mobilities of 52,000-116,000 cm^2/eV s. In contrast, a mixed phase InP nanowire shows a much shorter spatial diffusion limited by defect states with measured diffusion constants of 22 cm^2/s and mobilities of 29,000 cm^2/eV s.
Turning our focus novel NW morphologies in Chapter 3-5, we first study the strain effects from a series of a lattice mismatched (3.6%) GaAs/GaP core shell NWs. Strain on a semiconductor creates deformations in the lattice of the material which in turn effect the electronic states and possibly the material quality. We compare our PL energies with theoretical predictions and find that our measurements are lower than predicted. We next exploit correlations between PL emission and TO2 phonon emission to predict the hydrostatic and sheer strains in cases when the light hole emission is not visible and/or TO1 phonon cannot be resolved.
In chapter 4, we investigate the material quality issues with these strained nanowires and find that the presence of dislocations results in non-radiative recombination centers which causes the electron-hole lifetimes to fall below the system response (<40ps). We correlate dislocation density with the PL intensity, bandgap position, and TO2 phonon. High resolution imaging shows that dislocations are both full and frank partial dislocations in the (111) axial plane which relieve the axial strain. Radial strain relief can be seen in the cross slip of dislocations to the other (111) planes.
Finally, we see evidence for growth of coherently strained NW structures without dislocations at core/shell thickness regimes predicted by theoretical predictions of their formation energies. We conclude this work by minimizing the effect of dislocations on the core by first growing a thin AlGaAs shell to protect the core from the dislocation recombination centers, and find that the 150nm core produces optimal results.
Finally, we demonstrate control of radial confinement through the growth of 7nm and 4nm GaAs radial quantum wells embedded within concentric AlGaAs barriers. PL shows confined QW emission shifted by 55meV and 185meV from the free exciton peak (1.515eV). TRPL shows good material quality with lifetimes in excess of 300ps for core and QW states. Structural HRSTEM data showing tapering of the QW matches the extracted temporal and spatial scans from TRPL which show movement of excitons from smaller wells at higher energies to wider sections at lower energies of the QW. We find that the PL is consistent with modeling results for a 6nm-7nm QW with 24% AlGaAs.