Photoluminescence and Resonance Raman Spectroscopy of MOCVD Grown GaAs/AlGaAs Core-Shell Nanowires

This work describes experiments conducted on GaAs/AlGaAs core-shell nanowires. The GaAs/AlGaAs materials system has been a major platform in the development of heterostructure devices. Heterostructure nanowires grown using bottom-up growth techniques are a means to creating high quality nanoscaled heterostructures with a variety of potential uses, most notably in optoelectronics. The GaAs/AlGaAs nanowires that were the subject of this study were provided to us by partners in Italy, who specialize in MOCVD growth of heterostructures. These nanowires were hypothesized to have fields at the interface between core and shell that were influencing the bandstructure. We used low temperature photoluminescence (PL), resonance Raman, and polarized Raman spectroscopies to characterize the nanowires and understand the influence of the shell on the electronic structure of the nanowires. The PL and resonance Raman experiments were carried out on a custom instrument constructed as part of carrying out this study.;PL gave us a more complete picture of the influence of defect states on the electronic properties and the significance of donor-acceptor-pair (DAP) luminescence in the previously observed spectral character. The contrast in spectral character between bare GaAs nanowires and the core-shell nanowires led to an alternative hypothesis that the role of the shell in determining the spectral characteristics was largely through the thermal history imparted on the cores when they were heated to the shell's growth temperature; at the elevated temperatures, hydrogen incorporated into the cores as they were grown diffused out. Hydrogen and carbon are impurities that are difficult to avoid in MOCVD growth, because they are byproducts of the precursor breakdown and may be incorporated into the nanowires during growth. Prior to exposure to the elevated temperatures, hydrogen compensated the carbon which is useful as a p-type dopant in GaAs.;Further investigation of the nanowires using resonance Raman spectroscopy showed strong coupling between the LO-phonon and the bound exciton state, particularly in the core-shell nanowires, supporting the new hypothesis. Polarized Raman spectroscopy was used to investigate the possibility of wurtzite GaAs being present in what were otherwise zinc-blende GaAs structures.