Full Band Calculations Research Articles

Interlayer exchange coupling in (111)Co/Cu/Cotrilayers: Full-band versus reflection-amplitude calculations

We calculate the interlayer exchange coupling in (111) $\mathrm{Co}/\mathrm{Cu}/\mathrm{Co}$ trilayers with tight-binding band structures in two independent ways: a full-band calculation and an approximate calculation in terms of reflection amplitudes. An analytic Green's function method is used for the full-band calculation. The calculation involves solving the Green's function analytically for each fixed in-plane wave vector $({\mathbf{k}}_{\ensuremath{\Vert}})$ and an accurate integration over ${\mathbf{k}}_{\ensuremath{\Vert}}$ with nonuniform sampling. In the approximate method, we calculate the spin-dependent reflection amplitudes, which are complex numbers due to the effects of evanescent waves, and their phase factors depend on both the energy and wave vector. In the approximate method, the integration over ${\mathbf{k}}_{\ensuremath{\Vert}}$ near the extremal points is carried out analytically with the stationary phase approximation and contributions for ${\mathbf{k}}_{\ensuremath{\Vert}}$ far away from the extremal points are ignored. The method describes well the overall behavior of the interlayer exchange coupling for thick spacers. However, for thin spacers we found that the stationary phase approximation gives poor results and the full-band calculation is needed for accurate estimate of the coupling strength.

Characterization and modeling of threshold voltage shift due to quantum mechanical effects in pMOSFET

The threshold voltage shift due to quantum mechanical effects (QMEs) has substantial influences on modern pMOSFET characteristics. Investigations of QMEs in a pMOSFET are classified into two approaches: full-band calculation and effective mass approximation. In this paper, formulation of carrier distribution in the pMOSFET inversion layer in the threshold region based on the effective mass approximation is presented, and a new method to calculate the threshold voltage shift due to QMEs is developed. The results with the effective mass approximation are compared with the full-band calculation and show satisfactory coincidence. Based on the present model, the subband structure of the hole inversion layer and the carrier distribution characteristics are investigated. The quantum mechanical and semi-classical two-dimensional density of states (2D DOS) are calculated and compared. The dependence of the threshold voltage shift due to QMEs on substrate doping concentration is then analyzed from the DOS point of view.

Alloy corrections to the virtual crystal approximation and explicit band structure calculations for silicon-germanium

Cluster and band structure calculations for the electronic energy levels of silicon-germanium systems are presented. Much of the electronic structure information concerning silicon-germanium alloys to date relies upon the virtual crystal approximation (VCA). A discussion is presented for corrections beyond the VCA and in particular the use of full band calculations for device modeling of SiGe technologies.

Valence band parameters of wurtzite materials

We deduced the valence band parameters of several würtzite materials (ZnS, CdS, CdSe, and GaN) by matching the results of existing full-band calculations of the energy spectrum with analytical expressions of the envelope-function formalism. The calculated A-, B-, and C-type hole dispersion relations show strongly anisotropic characteristics and anti-crossing features in spectrum due to band mixing effects. We demonstrated that for all materials considered except CdSe, the spherical cubic approximation for six Luttinger-like parameters holds with good accuracy, so that the anisotropy arises mainly owing to the crystal field splitting term. Thus, the top valence band may be described with (in addition to crystal field and spin-orbit splitting energies) only two Luttinger-like parameters, γ 1 and γ 2.

Periodic-cluster calculations of the valence states and native defects in diamond, silicon, germanium, ZnS, ZnSe, and SiC.

We present the results of band calculations and of native defects on diamond, Si, Ge, ZnS, ZnSe, and SiC by means of periodic clusters. We study the convergence of the results with cluster size. For clusters as small as eight atoms, eigenvalues and eigenfunctions seem to be converged. Compared to nonperiodic clusters, the present technique has the advantage of establishing a one-to-one correspondence to the eigenvalues of a full band calculation (infinite-sized cluster). Compared to the full band calculation, the periodic-cluster technique is equivalent to a calculation with ``special points'' integration in the Brillouin zone. The periodic-cluster calculation is useful when a spectrum ``discretization'' is desired, for instance, in the calculation of defects. We illustrate this point by presenting the results of calculations on native defects in these semiconductors. Our results for the valence states are in excellent agreement with experiment. In other instances, our results are able to give a theoretical interpretation to experiment.

Approximate treatment for semi-core states

The “semi-core” states which are not completely localized on a single atomic site are nonetheless simple enough in structure as not to require a full energy band calculation for their inclusion into the charge density. A simple approximate technique to accomplish this is presented.

Some aspects of the electronic structure of uranium pnictides and chalcogenides

A simple model for the bonding between uranium f and pnictogen or chalcogen p electrons is described and estimates of the atom and angular momentum resolved contributions to the equation of state obtained. The results of full band calculations for the uranium chalcogenides and pnictides are summarized and the equation of state for UN analysed. The trends of the lattice parameter across the two series is reproduced theoretically by semirelativistic self-consistent calculations. Results of fully relativistic and semi-relativistic calculations compare favourably with photoemission results showing that in the pnictides the anion p-valence band lies at least 1 eV closer to the Fermi level than in the corresponding chalcogenide.