Calculations on the Size Effects of Raman Intensities of Silicon Quantum Dots

Raman intensities of Si quantum dots (QDs) with up to 11,489 atoms (about 7.6 nm in diameter) for different scattering configurations are calculated. First, phonon modes in these QDs, including all vibration frequencies and vibration amplitudes, are calculated directly from the lattice dynamic matrix by using a microscopic valence force field model combined with the group theory. Then the Raman intensities of these quantum dots are calculated by using a bond-polarizability approximation. The size effects of the Raman intensity in these QDs are discussed in detail based on these calculations. The calculations are compared with the available experimental observation. We are expecting that our calculations can further stimulate more experimental measurements.Comment: 21 pages, 7 figure

Electronic Structure of a Hydrogenic Acceptor Impurity in Semiconductor Nano-structures

The electronic structure and binding energy of a hydrogenic acceptor impurity in 2, 1, and 0-dimensional semiconductor nano-structures (i.e. quantum well (QW), quantum well wire (QWW), and quantum dot (QD)) are studied in the framework of effective-mass envelope-function theory. The results show that (1) the energy levels monotonically decrease as the quantum confinement sizes increase; (2) the impurity energy levels decrease more slowly for QWWs and QDs as their sizes increase than for QWs; (3) the changes of the acceptor binding energies are very complex as the quantum confinement size increases; (4) the binding energies monotonically decrease as the acceptor moves away from the nano-structures’ center; (5) as the symmetry decreases, the degeneracy is lifted, and the first binding energy level in the QD splits into two branches. Our calculated results are useful for the application of semiconductor nano-structures in electronic and photoelectric devices

Electric field and exciton structure in CdSe nanocrystals

Quantum Stark effect in semiconductor nanocrystals is theoretically investigated, using the effective mass formalism within a $4\times 4$ Baldereschi-Lipari Hamiltonian model for the hole states. General expressions are reported for the hole eigenfunctions at zero electric field. Electron and hole single particle energies as functions of the electric field ($\mathbf{E}_{QD}$) are reported. Stark shift and binding energy of the excitonic levels are obtained by full diagonalization of the correlated electron-hole Hamiltonian in presence of the external field. Particularly, the structure of the lower excitonic states and their symmetry properties in CdSe nanocrystals are studied. It is found that the dependence of the exciton binding energy upon the applied field is strongly reduced for small quantum dot radius. Optical selection rules for absorption and luminescence are obtained. The electric-field induced quenching of the optical spectra as a function of $\mathbf{E}_{QD}$ is studied in terms of the exciton dipole matrix element. It is predicted that photoluminescence spectra present anomalous field dependence of the emission lines. These results agree in magnitude with experimental observation and with the main features of photoluminescence experiments in nanostructures.Comment: 9 pages, 7 figures, 1 tabl

Evolution of the electronic structure with size in II-VI semiconductor nanocrystals

In order to provide a quantitatively accurate description of the band gap variation with sizes in various II-VI semiconductor nanocrystals, we make use of the recently reported tight-binding parametrization of the corresponding bulk systems. Using the same tight-binding scheme and parameters, we calculate the electronic structure of II-VI nanocrystals in real space with sizes ranging between 5 and 80 {\AA} in diameter. A comparison with available experimental results from the literature shows an excellent agreement over the entire range of sizes.Comment: 17 pages, 4 figures, accepted in Phys. Rev.

STABLE AMORPHOUS GERMANIUM FILMS PREPARED IN ULTRA HIGH VACUUM AND MEASURED IN-SITU : STRUCTURE AND ELECTRONIC PROPERTIES

Stable, homogeneous germanium films have been prepared by slow evaporation onto heated sapphire substrates in ultra high vacuum. Their properties do not change on annealing until they crystallise at 250°C (1,2). The structure of these films is characterised by low density, by a shift of the first diffraction peak to higher scattering angle, with respect to the crystal and by the existance of a diffraction pre-peak. The optical absorption edge is relatively sharp, has a shoulder at 0.85 eV, and the low photon energy refractive index has a lower value than for the crystal. The temperature T dependence of the d.c. electrical conductivity σ is characterised by an "S" shaped curve, when plotted as log σ versus T-¼. The photoconductivity shows marked long time ( t > 103 sec) relaxation effects at 77K and exhibits photoconductivity fatigue at 5K. These results are compared with amorphous germanium (a-Ge) films prepared under different conditions and with other amorphous and glassy materials. Some common trends in the structure and electronic properties are pointed out

ELECTRICAL AND OPTICAL PROPERTIES OF UHV SUBLIMED a-As

Thin films of amorphous arsenic have been prepared by sublimation on to substrates held at ~ 100 K in ultra high vacuum. Annealed films show an optical bandgap of ~1.0 eV. Low temperature ion bombardment greatly enhances the de conductivity, which after bombardment follows the T-¼ law. Interpretation in terms of variable range hopping leads to a Fermi level density of states after bombardment of ~5 x 1017 eV-1 cm-3, which falls to ≤ 1017 eV-1 cm-3 after annealing at 295 K. Comparison with similar experiments on amorphous r.f. sputtered phosphorus lends support to those models of defect states which suggest that charged defects are more stable in a-P than in a-As