Bal K. Agrawal

Ab initio calculation of electronic properties of Ga1 xAlxN alloys

The electronic properties of the wide-band-gap semiconducting ordered alloys (for x = 0.0, 0.25, 0.50, 0.75 and 1.0) and the random alloys have been investigated by using a full-potential self-consistent linear muffin tin orbital (LMTO) method. The calculated direct band gap for random distribution of cation nearest-neighbour tetrahedral clusters in the alloys for any arbitrary concentration x is seen to show a quite linear variation in agreement with the experiment. On the other hand, the indirect band gap remains invariant. We observe a direct to indirect band gap crossover at x = 0.59. The band gap bowing is seen to be very small.

Vibrational properties of crystalline and amorphous Ge1−xSixalloys

Abstract A five-atom cluster Bethe lattice method treating the short-range order with proper statistical effects has been proposed for understanding the crystalline and amorphous alloys. Numerical results for Ge1−xSix alloys are in excellent agreement with the experimental data. In the system studied the network is random but the nearest-neighbour coordination should be treated exactly. The long-range coordination is seen to be concentration-dependent in the crystalline alloys but concentration-independent in the amorphous alloys.

Ab initio study of the structural, electronic and optical properties of ultrathin bismuth nanowires

The energetics, structural, electronic and optical absorption properties of the bismuth nanowires Bin with n = 1, 6 have been investigated using density functional theory (DFT) in the local density approximation (LDA) including the spin–orbit coupling (SOI). The inclusion of the SOI appreciably affects all the physical properties of the wires. The stable structures form four groups: the planar structures, the caged configurations, the pyramidal structures and the helical configurations. This finding may be a guide for the construction of atomic configurations of the nanowires possessing a larger number of atoms per unit cell. The most stable wire configurations are the 5-Bi pentagonal, and the 6-Bi hexagonal and 6-Bi triple zigzag wires, which should be seen in the experiments. All the wires are metallic. The behaviour of the electron states of the second category structures is quite near to that of a linear chain where the parabolic bands cross the EF, and the number of the channels available for the electric conduction is large. Thus, one should grow the wire structures falling into the second category for achieving high conduction. For the 5-Bi pentagonal and 6-Bi hexagonal cross-sectional wires, the number of channels available for the electric conduction are ten and twelve, respectively. The SOI drastically affects the calculated optical absorption, especially in the low energy region. The absorption peaks are different in terms of the number and the energy locations for the different wires, and may be used for the characterization of the structure of a wire. Our analysis of the calculated electronic structure and the optical data of all the studied structures supports the occurrence of the 4-Bi double and/or 6-Bi triple zigzag chains in the samples of Romanov.

Lattice dynamics of II–VI, III–V compounds

Abstract The lattice vibrations of II–VI compound ZnSe and III–V compound InSb have been calculated in the frame work of Banerjee-Varshnis second neighbour ionic [SNI] model utilizing critical-point phonons as an input to determine the required seven parameters and the experimentally determined three elastic constants C 11 , C 12 , C 44 as restraints on the values of the parameters. A reinvestigation of the experimental elastic constants particularly C 11 and C 44 for ZnSe has been suggested. Results are presented for the dispersion curves along high symmetry directions. A reasonable agreement with the recently measured inelastic nutron scattering data is observed.

Ab initio study of the physical properties of binary SimCn (m+n≤5) nanoclusters

An ab initio study of the stability, structural, electronic, vibrational and optical properties of the most stable silicon–carbon binary nanoclusters SimCn (m+n≤5) has been made. A B3LYP-DFT/6-311G(3df) method has been employed to optimize fully the geometries of the nanoclusters. The binding energies (BEs), highest occupied molecular orbital (HOMO)–lowest unoccupied molecular orbital (LUMO) gaps, bond lengths, ionization potentials (IPs), adiabatic electron affinities (EAs), vibrational frequencies, infrared intensities, relative infrared intensities and Raman scattering activities have been computed. In the more stable structures, the carbon atoms are in the majority whereas in the less stable structure the reverse is true. For the clusters containing all the carbon atoms except one silicon atom, the BE increases monotonically with the number of carbon atoms. The ground states of the clusters containing even numbers of the carbon atoms are, in general, lower than those containing odd numbers of carbon atoms. On the other hand, the lowest unoccupied states of the clusters containing even numbers of carbon atoms lie higher than those containing odd numbers of carbon atoms. All the predicted physical quantities are in good agreement with the experimental data wherever available. The growth of these most stable structures should be possible in the experiments.

Ab initio study of anomalous band-gap bowing in alloys

A full potential self-consistent linear muffin-tin orbital method in the local density approximation (LDA) has been employed to investigate the electronic structure of the 14 ordered alloys with x = 0.0, 0.037, 0.074, 0.111, 0.125, 0.25, 0.375, 0.407, 0.50, 0.625, 0.75, 0.875, 0.963 and 1.0. The lattice parameter is seen to have a non-linear variation with the concentration of the constituent atoms. In LDA, a near closure of the fundamental energy gap appears in the concentration range 12.5-62.5% of N atoms. A strong hybridization of N s states with the Ga (s, p) and As (s, p) states is seen at the bottom of the conduction band and these states descend into the fundamental gap, filling it either partially or completely. The band gap for the random alloys also shows an anomalous bowing.

Phonon conductivity of GaAs

Abstract A modified Hollands and three-phonon processes having different temperature dependences in the varius temperature ranges have been used to analyse the lattice thermal conductivity of GaAs. A very good agreement with the experimental results has been obtained

Infrared Absorption in CsBr containing cations as impurity

Abstract The infrared absorption of CsBr doped with Na + , K + , Rb + , In + and Tl + impurity ions has been calculated. Changes in the mass and the nearest-neighbour central and noncentral force constants due to an impurity have been taken into account. An overall good agreement with the experimentally measured absorption has been observed. Some discrepancies arise in the magnitude of absorption at high frequencies which may be ascribed to the incorrect phonon frequencies obtained in the breathing shell model by Jex.

Ab initio study of small diameter (6, 6) armchair carbon nanoropes: orientational dependent properties

A comprehensive ab initio investigation of the effects of the relative orientation (RO) between the adjacent tubes in a rope on the stability, structural, electronic, optical and Raman-active properties has been performed for the ropes of small diameter carbon (6, 6) nanotubes. A number of new features not discussed earlier are observed in the present study. The symmetric rope with an RO of 0° is metallic in all directions, whereas the asymmetric ropes with a non-zero value of RO are semiconductors along the tube axis but semi-metallic normal to rope axis. The band gap increases with RO up to an angle of 15° and thereafter reveals oscillatory behaviour. No dips appear in the symmetric rope but they do exist in the asymmetric rope. Strong optical absorption appears along the axis in the energy range 2.4–4.2 eV in the isolated tube. On the other hand, for the ropes, the strong absorption extends up to the energy region 1.8–4.5 eV. Strong peaks also occur at 0.05 and 0.15 eV for the ropes with RO = 0° and 15°, respectively. The even-parity Raman-active radial breathing mode (RBM) frequencies calculated here for the isolated (n,n), n = 3–6 tubes are seen to deviate from the usual law (where d is the tube diameter). For small diameter tubes, this shows an approximate variation, ω = 1/d1/2. The RBM frequencies for the ropes are either greater or smaller compared to the isolated tube, depending on the value of RO. A cubic anharmonicity of about 14% is seen in the potential for the radial mode vibrations. The RBM frequencies calculated here for some ropes, which are lower compared to that of the isolated tube, concur with the available Raman data.

First-principles calculation of electron surface states of the zinc-blende GaN(110) surface

Abstract The surface electronic structure for an unrelaxed as well as relaxed zinc-blende GaN(110) surface has been investigated within the local density approximation of density functional theory, employing a first-principles full-potential self-consistent linear muffin-tin orbital (LMTO) method and a supercell approach. Intrinsic surface states appear in the fundamental energy gap for an unrelaxed surface. These intrinsic gap surface states shift towards the bulk valence and the conduction band region on considering the relaxation of the surface atoms. Orbitals characters of surface states at Γ, X, M, X″ symmetry points have been identified. Several localized and resonance states are predicted for the first time in different energy regions.