The effects of electric field on the electronic structure of a semiconductor quantum dot

The effect of electric field on the electronic structure of a spherical quantum dot is studied in the framework of the effective-mass envelope-function theory. The dependence of the energy of electron states and hole states on the applied electric field and on the quantum dot size is investigated; the mixing of heavy holes and light holes is taken into account. The selection rule for the optical transition between the conduction band and valence band states is obtained. The exciton binding energies are calculated as functions of the quantum dot radius and the strength of the electric field. (C) 1998 American Institute of Physics.

The Kiloparsec-scale structure of 3C 286

We present radio images of the compact steep spectrum (CSS) quasar 3C 286 acquired with the Very Large Array (VLA) at 8.4 and 22.5 GHz. The source exhibits a two-sided core-jet structure with a bright central component and two extended components one to the east (P.A. 100degrees) and another to the southwest (P.A. -116degrees). From the compact core, an extension runs towards the southwest component up to similar to 0.7 arcsecond. The emission between the primary central component and the southwest component exhibits a knotty structure. A gradual change of the jet position angles from -135degrees to -120degrees in the inner southwest jet suggests a local bend. The position angle changes of the major eastern components E2 and E1 suggest that the eastern jet likely follows a curved trace. The bends in the jet trace may be associated with a relativistic precession or some interaction between the jet and the ambient matter. A mean spectral index of alpha(8.4)(22.5) similar to -0.76 (S-nu proportional to nu(alpha)) is estimated for the core component. Steep spectra are also obtained for the extended southwest component (2.6", P.A. -116degrees) and eastern component (0.8", P.A. 100degrees), with alpha(8.4)(22.5) similar to -0.88 and alpha(8.4)(22.5) similar to -1.79, respectively. The radio morphologies and spectral index distributions suggest that the core seen in our images is likely to be the beamed inner jet while the real nucleus is dimmed by it beaming away from us.

Band crossing in isovalent semiconductor alloys with large size mismatch: First-principles calculations of the electronic structure of Bi and N incorporated GaAs

For large size- and chemical-mismatched isovalent semiconductor alloys, such as N and Bi substitution on As sites in GaAs, isovalent defect levels or defect bands are introduced. The evolution of the defect states as a function of the alloy concentration is usually described by the popular phenomenological band anticrossing (BAC) model. Using first-principles band-structure calculations we show that at the impurity limit the N-(Bi)-induced impurity level is above (below) the conduction- (valence-) band edge of GaAs. These trends reverse at high concentration, i.e., the conduction-band edge of GaAs1-xNx becomes an N-derived state and the valence-band edge of GaAs1-xBix becomes a Bi-derived state, as expected from their band characters. We show that this band crossing phenomenon cannot be described by the popular BAC model but can be naturally explained by a simple band broadening picture.

BAND-STRUCTURE OF MG1-XZNXSYSE1-Y

The empirical pseudopotential method within the virtual crystal approximation is used to calculate the band structure of Mg1-xZnySySe1-y, which has recently been proved to be a potential semiconductor material for optoelectronic device applications in the blue spectral region. It is shown that MgZnSSe can be a direct or an indirect semiconductor depending on the alloy composition. Electron and hole effective masses are calculated for different compositions. Polynomial approximations are obtained for both the energy gap and the effective mass as functions of alloy composition at the GAMMA valley. This information will be useful for the future design of blue wavelength optoelectronic devices as well as for assessment of their properties.

THEORY OF THE ELECTRONIC-STRUCTURE OF POROUS SI

A theoretical model for the electronic structure of porous Si is presented. Three geometries of porous Si (wire with square cross section, pore with square cross section, and pore with circular cross section) along both the [001] and [110] directions are considered. It is found that the confinement geometry affects decisively the ordering of conduction-band states. Due to the quantum confinement effect, there is a mixing between the bulk X and GAMMA states, resulting in finite optical transition matrix elements, but smaller than the usual direct transition matrix elements by a factor of 10(-3). We found that the strengths of optical transitions are sensitive to the geometry of the structure. For (001) porous Si the structure with circular pores has much stronger optical transitions compared to the other two structures and it may play an important role in the observed luminescence. For this structure the energy difference between the direct and the indirect conduction-band minima is very small. Thus it is possible to observe photoluminescence from the indirect minimum at room temperature. For (110) porous Si of similar size of cross section the energy gap is smaller than that of (001) porous Si. The optical transitions for all three structures of (110) porous Si tend to be much stronger along the axis than perpendicular to the axis.

ELECTRONIC-STRUCTURE OF ND2FE17N

The electronic structures of ternary compound Nd2Fe17N with N atoms on 9e, 3b, and 18g sites are calculated and compared. The local moments on different Fe sites are in good agreement with experiments. The mechanism of increasing Curie temperature by N doping is checked by additional calculations with lattice expansion. The results show that the change in interatomic interaction is more important than the lattice expansion effect.

ELECTRONIC AND MAGNETIC-STRUCTURE OF THE TERNARY COMPOUND ND2FE17N

The electronic and the magnetic structure of the Nd2Fe17N1 phase in the family of Nd-Fe-N ternary compounds have been calculated using the first-principles, spin-polarized orthogonalized linear-combination-of-atomic-orbitals method. Results are presented in the form of site-decomposed and spin-projected partial density of states. The occupation sites of the three N atoms are determined by an average radial distribution of all possible N site configurations. Both cases of N occupying the 3b and the 18g sites are studied. The results indicate that the 6c Fe sites have the maximum and the 18h Fe sites have the minimum local moments. This is in good agreement with experiment. It is concluded that the influence on the local moment due to lattice expansion is less important compared to that due to interatomic interaction between the N atom and its neighbors. The results also show the important role of N atoms in raising the Curie temperature of this compound.