Ab-initio Molecular Dynamics study of electronic and optical properties of silicon quantum wires: Orientational Effects

We analyze the influence of spatial orientation on the optical response of hydrogenated silicon quantum wires. The results are relevant for the interpretation of the optical properties of light emitting porous silicon. We study (111)-oriented wires and compare the present results with those previously obtained within the same theoretical framework for (001)-oriented wires [F. Buda {\it et al.}, {\it Phys. Rev. Lett.} {\bf 69}, 1272, (1992)]. In analogy with the (001)-oriented wires and at variance with crystalline bulk silicon, we find that the (111)-oriented wires exhibit a direct gap at ${\bf k}=0$ whose value is largely enhanced with respect to that found in bulk silicon because of quantum confinement effects. The imaginary part of the dielectric function, for the external field polarized in the direction of the axis of the wires, shows features that, while being qualitatively similar to those observed for the (001) wires, are not present in the bulk. The main conclusion which emerges from the present study is that, if wires a few nanometers large are present in the porous material, they are optically active independently of their specific orientation.Comment: 14 pages (plus 6 figures), Revte

Optical properties of excitons in semiconductor quantum wires

Spectra of luminescence and linear and nonlinear absorption of semiconductor quantum wires crystallized in a transparent dielectric matrix have been investigated and interpreted in terms of excitonic transitions and filling of the exciton phase space. The calculated energies of excitonic transitions are in qualitative agreement with experimental data. The estimated values of exciton binding energies (>100 meV) in semiconductor quantum wires embedded in dielectric are a factor of several tens higher than in bulk semiconductors. The cause of this increase in the exciton's binding energy is not only dimensional quantization, but also the enhancement of Coulomb interaction, i.e. stronger attraction between electrons and holes owing to the large difference between dielectric constants of semiconductor filament and dielectric matrix

Linear and nonlinear excitonic absorption in semiconducting quantum wires crystallized in a dielectric matrix

Spectra of linear and nonlinear absorption of GaAs and CdSe semiconducting quantum wires crystallized in a transparent dielectric matrix (inside chrysotile-asbestos nanotubes) have been measured. Their features are interpreted in terms of excitonic transitions and filling of the exciton phase space in the quantum wires. The theoretical model presented here has allowed us to calculate the energies of excitonic transitions that are in qualitative agreement with experimental data. The calculated exciton binding energies in quantum wires are a factor of several tens higher than in bulk semiconductors. The cause of this increase in the exciton binding energy is not only the size quantization, but also the “dielectric enhancement,” i.e., stronger attraction between electrons and holes owing to the large difference between permittivities of the semiconductor and dielectric matrix

Excitons in InP quantum wires with dielectric barriers

Spectra of linear absorption and photoluminescence (PL) at different polarization of the laser excitation; photoluminescence excitation (PLE) spectra and time - resolved PL of the InP quantum wires (QWRs) crystallized in transparent nanotubes of chrysotile asbestos have been measured. We evaluate the exciton ground state binding energy as large as 200 meV. In the presence of the strong resonant laser pumping of InP QWRs (the energy of laser photon coincides with the absorption band of QWRs) the induced suppression and enhancement of PL in different parts of the spectra and the blue shift (about 13meV) of the PL maximum relatively to the energy of laser photon have been observed. The PL features of InP QWRs has been explained by saturation of the capture centers and Auger process

Excitons in InP quantum wires with dielectric barriers

Spectra of linear absorption and photoluminescence (PL) at different polarization of the laser excitation; photoluminescence excitation (PLE) spectra and time - resolved PL of the InP quantum wires (QWRs) crystallized in transparent nanotubes of chrysotile asbestos have been measured. We evaluate the exciton ground state binding energy as large as 200 meV. In the presence of the strong resonant laser pumping of InP QWRs (the energy of laser photon coincides with the absorption band of QWRs) the induced suppression and enhancement of PL in different parts of the spectra and the blue shift (about 13meV) of the PL maximum relatively to the energy of laser photon have been observed. The PL features of InP QWRs has been explained by saturation of the capture centers and Auger process