Scanning Tunneling Microscopy Induced Luminescence Studies of Semiconductor Nanostructures

Abstract: This thesis treats scanning tunneling luminescence (STL) investigations of semiconductor nanostructures. The STL technique combines scanning tunneling microscopy (STM) with detection of photons, induced by the tunneling electrons. The high spatial resolution in STM and the local excitation allow for optical investigations on the nanometer-scale. The work concerns design and implementation of an optical detection system used for STL studies of single InP quantum dots (QDs) overgrown with thin layers of GaInP. Constant current imaging together with STL spectra and monochromatic photon mapping were used to correlate the surface topography with the optical properties of the QDs. It was found that the QDs act as seeds for the GaInP overgrowth, where elongated GaInP islands are formed. The emission from single QDs was observed to be shifted towards higher energies with increasing cap layer thickness, which by multi-band k.p theory was determined to be induced by strain. The geometry of the overgrowth was realistically modelled in the calculations, using data from STM and transmission electron microscopy. Theoretical emission energies were also calculated, which are in good agreement with the experimental results. Studies of the GaInP islands showed that the InP QDs locally induce domains in the islands with high degree of ordering in the GaInP. The emission from these domains was found to occur at an energy below the emission from the GaInP barrier material. High polarization anisotropy for the island luminescence was observed by photoluminescence measurements, in which maximum emission intensity was detected for light polarized parallel to the elongation of the islands.