58 research outputs found
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Importance of excitonic effects and the question of internal electric fields in stacking faults and crystal phase quantum discs: The model-case of GaN
We compute using envelope function calculations the energy and the oscillator strength of excitons in zinc blende/wurtzite quantum wells (QWs), such as those that appear in many examples of semiconductor nanowires, and in basal plane stacking faults (BSFs). We address specifically the model-case of GaN. In addition to the electron-hole Coulomb interaction, we account for the quantum-confined Stark effect. We demonstrate that despite the type-II band alignment at the zinc blende/wurtzite interfaces, a significant binding and a rather strong oscillator strength are preserved by excitonic effects. When adjacent crystal phase QWs are coupled together, we compute increased as well as decreased exciton oscillator strength with respect to the single QW case, depending on the QW-QW coupling scheme. Comparing the results of our calculations with available data, we finally conclude in favor of the absence of built-in electric fields perpendicular to the BSF planes.We acknowledge financial support from the European Union Seventh Framework Programme under Grant Agreement No. 265073
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Role of the dielectric mismatch on the properties of donors in semiconductor nanostructures bounded by air
We compute by envelope function calculations the binding energy EB of donor atoms in thin slabs of semiconductor bounded by air, accounting for the dielectric mismatch between air and the semiconductor. We detail how EB depends on the donor-site and on the thickness of the slab. We show that due to the competition between surface and dielectric mismatch effects, EB does not monotonically decrease from the center to the surface of the nanostructures. Finally, we discuss our results in regard to recent photoluminescence experiments performed on ensemble and single GaN nanowires.We acknowledge financing from the European Union Seventh Framework Program under Grant Agreement No. 265073
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Importance of the dielectric contrast for the polarization of excitonic transitions in single GaN nanowires
We investigate the polarization of excitonic transitions of single dispersed GaN nanowires with a diameter of about 50 nm. We observe donor-bound exciton transitions with a linewidth narrower than 250 μeV at 10 K, whereas the luminescence from free excitons exhibits a width of up to 5 meV. This broadening is larger than that observed for free excitons in the as-grown nanowire ensemble and is the result of inhomogeneous strain introduced by the nanowire dispersion. This strain lowers the symmetry of the lattice structure and allows A excitons to emit light polarized parallel to the nanowire axis. The polarization anisotropy of A excitons, however, is found to largely vary from one nanowire to another. In addition, the various bound-exciton lines in a given nanowire do not show the same polarization anisotropies. These findings can be explained by the dielectric contrast between the nanowire and its environment, but only when taking into account the strong variations of the dielectric function of GaN at the near band-edge
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