We report on GaN n-type doping using silane, germane, and isobutylgermane as Si and Ge dopants, respectively. A significant increase in tensile stress during growth is observed for Si doped samples while this is not the case for Ge doping. In addition, Ge can be doped up to 2.9 Â 10 20 cm À3 , while Si doping leads to 3-D growth already at concentrations around 1.9 Â 10 19 cm À3. The free carrier concentration was determined by Hall-effect measurements, crystal quality, and structural properties by x-ray diffraction measurements. Additionally, secondary ion mass spectroscopy and Raman measurements were performed demonstrating the high material quality of Ge doped samples. V
We present an experimental study on the near-field light-matter interaction by tip-enhanced Raman scattering (TERS) with polarized light in three different materials: germanium-doped gallium nitride (GaN), graphene, and carbon nanotubes. We investigate the dependence of the TERS signal on the incoming light polarization and on the sample carrier concentration, as well as the Raman selection rules in the near-field. We explain the experimental data with a tentative quantum mechanical interpretation, which takes into account the role of plasmon polaritons, and the associated evanescent field. The driving force for the breakdown of the classical Raman selection rules in TERS is caused by photon tunneling through the perturbation of the evanescent field, with the consequent polariton annihilation. Predictions based on this quantum mechanical approach are in good agreement with the experimental data, which are shown to be independent of incoming light polarization, leading to new Raman selection rules for TERS.
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