Nacir Tit

Electronic structure of InNxAs1−x alloys from tight-binding calculations

We present tight-binding band-structure calculations for InNxAs1−x alloys as a function of the nitrogen concentration. The high-concentration regime is found to be extremely sensitive to x. The low-concentration region (0<x<0.25) is of technological interest and is examined in detail. Effects of clustering, percolation, and the dependence of the energy gap on the assumed valence-band offsets on the band gap are reported.

The electronic properties of the strained CdTe/ZnTe(001) superlattices

The common-anion II–VI semiconductor superlattices (SLs) are characterized by a vanishing or a small valence-band offset (VBO). In the case of the lattice-mismatched SLs, the biaxial strain can drastically affect the splitting of the valence-band top states, and therefore be explored in designing type-I character SLs. In the present work, we used the sp3s* tight-binding method, with the inclusion of strain and spin–orbit coupling effects, to investigate the electronic band structures of the strained CdTe/ZnTe(001) SLs versus the biaxial strain, layer thicknesses and VBO. Our results show that the electron is always confined within the CdTe slabs, whereas the hole behaviour controls the whole SL character. Our theoretical results are compared to the photoluminescence experiments and shown to be consistent with the strain morphology along the SL growth direction as well as the optical and structural qualities of the experimental samples.

Existence of direct bandgap transitions in SiSiO2 superlattices

Abstract We present tight-binding bandstructure calculations for SimSiO2n crystalline superlattices (SLs) grown along the [001] direction. The results show that strong quantum confinement occurs in the short-period SLs. A particularly interesting feature of the results is the almost nested bandstructure along the ZΓ line of the SL-Brillouin zone. This feature is even more attractive than having just a direct bandgap in obtaining high radiative efficiencies. These results suggest the possibility of novel optical devices which exploit the nested bandstructure, and have implications for our understanding of the luminescence in porous-Si and other Si-based nanostructures.

Large configuration-induced band-gap fluctuations in GaNxAs1−x alloys

The electronic band structures of GaNxAs1?x alloys were investigated versus the nitrogen mole fraction x and the nitrogen atomic configuration. The computational method is based on the sp3s* tight-binding technique. Two main nitrogen atomic distributions were considered: (i) the nitrogen atoms grouped in one region to form like a GaN dot inside the GaAs so as to have a maximally N-clustered (MNC) configuration; and (ii) the nitrogen atoms homogeneously distributed over the alloy and, of course, the minimal N-clustered distribution as the maximally As-clustered (MAsC) configuration. The former is found to always have the lowest band gaps. More interestingly, the results show that in the latter distribution the nitrogen atoms introduce resonant states above the conduction-band edge by about 230?meV, which is consistent with the literature, whereas they introduce a deep gap state above the valence-band edge at about 150?meV in the former distribution. As a suitable model for experimental samples, the MAsC configuration, was used to model some available photoluminescence data in the dilute regime.

CO2 adsorption on Fe-doped graphene nanoribbons: First principles electronic transport calculations

Decoration of graphene with metals and metal-oxides is known to be one of the effective methods to enhance gas sensing and catalytic properties of graphene. We use density functional theory in combination with the nonequilibrium Green’s function formalism to study the conductance response of Fe-doped graphene nanoribbons to CO2 gas adsorption. A single Fe atom is either adsorbed on graphene’s surface (aFe-graphene) or it substitutes the carbon atom (sFe-graphene). Metal atom doping reduces the electronic transmission of pristine graphene due to the localization of electronic states near the impurities. The reduction in the transmission is more pronounced in the case of aFe-graphene. In addition, the aFe-graphene is found to be less sensitive to the CO2 molecule attachment as compared to the sFe-graphene system. Pristine graphene is also found to be less sensitive to the molecular adsorption. Since the change in the conductivity is one of the main outputs of sensors, our findings will be useful in developi...

Dielectric constant and light emission in Si/SiO2 superlattices

The real part of the frequency-dependent dielectric function e1(ω) and the light-absorption coefficient α(ω), of Si slabs confined within SiO2 barriers have been calculated as a function of Si-slab thickness dsi. The calculation uses the imaginary part e2(ω) of the dielectric function obtained from tight-binding calculations. The resulting static dielectric constants are found to increase with the decreasing Si-slab width, contrary to some reported theoretical results for H-terminated Si or pure-Si clusters. The calculated integrated light absorption in the range 1.0–2.6 eV is a maximum for for slabs with dsi∼20 A, in agreement with experimental work on light emission from Si/SiO2 superlattices and other oxidized Si nanostructures.

Influence of reactant concentration on optical properties of ZnO nanoparticles

Abstract Zinc oxide (ZnO) nanoparticles have been prepared by wet chemical method from zinc acetate. Particle size was controlled by adjusting the reactant concentration. The size of nanoparticles was investigated using ultraviolet–visible absorption spectra and photoluminescence spectra. The present nanoparticles exhibit non-linear optical behaviour with blue shift of the wavelengths as the particle size decreases. Furthermore, yellow emission is observed in ambient air while it disappears in the presence of nitrogen gas and gets substituted by blue violet emissions. While the blue violet emissions are familiar and likely to be attributed to electronic transitions from localised states (e.g. shallow donor states on Zn interstitials ‘Zni’) or the conduction band edge to the valence band, the yellow emission in the absence of nitrogen remains unclear. Our results of the present investigation suggest that the bubbling with nitrogen should fill the oxygen vacancies, substitute the oxygen interstitials, passivate the dangling bonds and introduce shallow acceptor states, which allow electronic transitions with shorter wavelengths (i.e. blue violet emissions). In the absence of nitrogen, surface defects such as oxygen interstitials and Zn(OH)2 and possibly other point defects become again active and induce deep acceptor states of ∼1 eV above the valence band edge, which allow electronic transitions of longer wavelength (i.e. yellow emission). Our results are compared to several available experimental data and first principle calculations in order to support our claims and conclusions.

Strain effects in the common-cation II–VI heterostructures: case of ZnS/ZnSe superlattices

The electronic band-structures of the strained-layer ZnS/ZnSe (001) superlattices (SLs) have been investigated using the sp3s* tight-binding method, which includes the strain and spin–orbit effects. The SL band-structures are studied versus the biaxial strain, layer thickness, and band offsets. The results suggest that the common-cation II–VI heterojunction exhibit a vanishingly small conduction-band offset (CBO). It is shown that the SL valence-band top state is always a heavy-hole localized within ZnSe slabs; whereas the conduction-band edge state (electron) is sensitive to the biaxial strain (or VBO). To assess the strain effects, we considered three differently strained SLs corresponding to the three substrates: (i) ZnSe; (ii) ZnS0.5Se0.5; and (iii) ZnS. The results show that all the studied SLs are of type-I except those strained to ZnS (case iii), that may exhibit type-I to type-II transition. One striking result obtained here is the existence of a critical VBO (Vc0.76 eV) that predicts such transition, and particularly the fact that this value is independent of the strain state (substrate) (i.e. all SLs whose VBO is smaller than Vc are of type-I, else are of type-II). The comparison of our theoretical results to the photoluminescence experiments yields valuable information about the strain morphology as well as the structural and optical qualities of the experimental samples.

Empirical tight-binding parameters for solid C60

We present a tight-binding model for the electronic structure of solid C60 using four (one 2s and three 2p) orbitals per carbon atom. The model has been developed by fitting the tight-binding parameters to the ab initio pseudopotential calculation of Troullier and Martins (1992) in the face-centred cubic (Fm3) phase. Following this, calculations of the energy bands and the density of electronic states have been carried out as a function of the lattice constant. Good agreement has been obtained with the observed lattice-constant dependence of Tc using McMillans formula. Furthermore, calculations of the electronic structure are presented in the simple cubic (Pa3) phase.

Investigation of the electronic properties of strained ZnSe/ZnTe(001) superlattices

The empirical sp3s* tight-binding method, which includes both spin?orbit coupling and strain effects, is employed to investigate the electronic properties of the strained ZnSe/ZnTe(001) superlattices (SLs) versus the biaxial strain, layer thicknesses and valence band offset (VBO). The results show that the conduction-band edge state is always represented by an electron localized within the ZnSe slabs; whereas the valence-band edge state (which is mostly a heavy hole (HH) related to ZnTe) can be controlled by the biaxial strain. In this respect, the results show the existence of a certain critical VBO (?eV) separating two kinds of hole-confinement character (i.e.?for V BO> Vc, the HH gets localized within the ZnTe layers and the SL is of type-II; whereas for , the HH has a tendency to localize at the hetero-interface and, as a consequence, the radiative efficiency is enhanced). Finally, our results are compared to some available photoluminescence data and conclusions about the structural and optical qualities of the experimental samples are drawn.