Study of Optical and Electronic Properties of Individual Semiconductor Nano-structures

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1) We have investigated the transport properties of hot electrons in AlGaAs/GaAs quantum well (QW) structures in real space using a scanning tunneling microscope (STM). The electrons were injected from the STM tip at different distances from a target well on the cleaved (110) surface. Then by measuring the light emission intensity from the target well, the thermalization and diffusion lengths of the injected electrons were determined.2) Using photoluminescence (PL) microscopy with the scanning near-field optical microscope, we have investigated the formation mechanism of misfit dislocations in GaAs/InGaAs QW structures. From the density of dark lines observed in the PL images as a function of the total layer thickness, we found that there exist two critical layer thicknesses. These two critical layer thicknesses are explained by taking account of a lattice frictional force proportional to the In content.3) Using cross-sectional STM, we have investigated the electronic structures of Ga_<0.968>Mn_<0.032>As. The STM image showed a random distribution of bright spots in light and dark areas. From conductance spectra measured with the STM, the bandgap of the GaMnAs was estimated to be 1.23±0.05 eV. Finite conductance within the band gap indicates the presence of hole states in the valence band. An additional peak at 0.7 eV above the valence band edge can be assigned to electron tunneling into the ionization levels of As antisites.4) Light intensity images of self-assembled InAs/AlGaAs quantum dots (QDs) were measured using the STM. Localized bright features were observed in the images for different photon energies. The emission spectra measured over the bright features showed single emission peaks with different peak positions. By comparing the peak energies with the transition energies calculated for pyramidal QD structures, we found that the bright features correspond to the ground-state emission from individual InAs QDs.