arXiv subjects

A. Continenza

Publications and source records attributed to A. Continenza.

29 records · Page 2Linked to original sources

arXiv2001

Impact ionization in GaAs: a screened exchange density functional approach

Results are presented of a fully ab-initio calculation of impact ionization rates in GaAs within the density functional theory framework, using a screened-exchange formalism and the highly precise all-electron full-potential linearized augmented plane wave (FLAPW) method. The calculated impact ionization rates show a marked orientation dependence in {\bf k} space, indicating the strong restrictions imposed by the conservation of energy and momentum. This anisotropy diminishes as the impacting electron energy increases. A Keldysh type fit performed on the energy-dependent rate shows a rather soft edge and a threshold energy greater than the direct band gap. The consistency with available Monte Carlo and empirical pseudopotential calculations shows the reliability of our approach and paves the way to ab-initio calculations of pair production rates in new and more complex materials.

cond-mat.mtrl-sci↗

arXiv2001

Role of structural relaxations, chemical substitutions and polarization fields on the potential line-up in [0001] wurtzite GaN/Al systems

First-principles full-potential linearized augmented plane wave (FLAPW) calculations are performed to clarify the role of the interface geometry on piezoelectric fields and on potential line-ups at the [0001]-wurtzite and [111]-zincblende GaN/Al junctions. The electric fields (polarity and magnitude) are found to be strongly affected by atomic relaxations in the interface region. A procedure is tested to evaluate the Schottky barrier in the presence of electric fields and used to show that their effect is quite small (a few tenths of an eV). These calculations assess the rectifying behaviour of the GaN/Al contact, giving very good agreement with experimental values for the barrier. Stimulated by the complexity of the problem, we disentangle chemical and structural effects on the relevant properties (such as the potential discontinuity and electric fields) by studying auxiliary unrelaxed nitride/metal systems. Focusing on simple electronegativity arguments, we outline the leading mechanisms that result in the final values of the electric fields and Schottky barriers in these ideal interfaces. Finally, the transitivity rule in the presence of two inequivalent junctions is proved to give reliable results.

cond-mat.mtrl-sci↗

arXiv2001

Electronic and structural properties of superconducting MgB$_2$, CaSi$_2$ and related compounds

We report a detailed study of the electronic and structural properties of the 39K superconductor \mgbtwo and of several related systems of the same family, namely \mgalbtwo, \bebtwo, \casitwo and \cabesi. Our calculations, which include zone-center phonon frequencies and transport properties, are performed within the local density approximation to the density functional theory, using the full-potential linearized augmented plane wave (FLAPW) and the norm-conserving pseudopotential methods. Our results indicate essentially three-dimensional properties for these compounds; however, strongly two-dimensional $σ$-bonding bands contribute significantly at the Fermi level. Similarities and differences between \mgbtwo and \bebtwo (whose superconducting properties have not been yet investigated) are analyzed in detail. Our calculations for \mgalbtwo show that metal substitution cannot be fully described in a rigid band model. \casitwo is studied as a function of pressure, and Be substitution in the Si planes leads to a stable compound similar in many aspects to diborides.

cond-mat.supr-con↗

arXiv2001

Electronic and structural properties of superconducting diborides and calcium disilicide in the $\rm AlB_2$ structure

We report a detailed study of the electronic and structural properties of the 39K superconductor \mgbtwo and of several related systems of the same family, namely \mgalbtwo, \bebtwo, \casitwo and \cabesi. Our calculations, which include zone-center phonon frequencies and transport properties, are performed within the local density approximation to the density functional theory, using the full-potential linearized augmented plane wave (FLAPW) and the norm-conserving pseudopotential methods. Our results indicate essentially three-dimensional properties for these compounds; however, strongly two-dimensional $σ$-bonding bands contribute significantly at the Fermi level. Similarities and differences between \mgbtwo and \bebtwo (whose superconducting properties have not been yet investigated) %it is not yet know if it is a superconductor) are analyzed in detail. Our calculations for \mgalbtwo show that metal substitution cannot be fully described in a rigid band model. \casitwo is studied as a function of pressure, and Be substitution in the Si planes leads to a stable compound similar in many aspects to diborides.

cond-mat.supr-con↗

arXiv2001

Energetics and bonding properties of the Ni/$β$-SiC (001) interface: an ab-initio study

We investigate the adsorption of a Ni monolayer on the $β$-SiC(001) surface by means of highly precise first-principles all-electron FLAPW calculations. Total energy calculations for the Si- and C-terminated surfaces reveal high Ni-SiC adsorption energies, with respect to other metals, confirmining the strong reactivity and the stability of the transition metal/SiC interface. These high binding energies, about 7.3-7.4 eV, are shown to be related to strong $p$-$d$ hybridization, common to both surface terminations and different adsorption sites and despite the large mismatch, can stabilize overlayer growth. A detailed analysis of the bonding mechanism, hybridization of the surface states, charge transfer and surface core level shifts reveals the strong covalent character of the bonding. After a proper accounting of the Madelung term, the core level shift is shown to follow the charge transfer trend.

cond-mat.mtrl-sci↗

arXiv2001

Coordination and chemical effects on the structural, electronic and magnetic properties in Mn pnictides

Simple structures of MnX binary compounds, namely hexagonal NiAs and zincblende, are studied as a function of the anion (X = Sb, As, P) by means of the all-electron FLAPW method within local spin density and generalized gradient approximations. An accurate analysis of the structural, electronic and magnetic properties reveals that the cubic structure greatly favours the magnetic alignment in these compounds leading to high magnetic moments and nearly half-metallic behaviour for MnSb and MnAs. The effect of the anion chemical species is related to both its size and the possible hybridization with the Mn $d$ states; both contributions are seen to hinder the magnitude of the magnetic moment for small and light anions. Our results are in very good agreement with experiment - where available - and show that the generalized gradient approximation is essential to correctly recover both the equilibrium volume and magnetic moment.

cond-mat.mtrl-sci↗

arXiv2000

Metal induced gap states and Schottky barrier heights at non-reactive GaN/noble metal interfaces

We present ab-initio local density FLAPW calculations on non-reactive N-terminated [001] ordered GaN/Ag and GaN/Au interfaces and compare the results (such as metal induced gap states and Schottky barrier heights) with those obtained for GaN/Al, in order to understand the dependence of the relevant electronic properties on the deposited metal. Our results show that the density of gap states is appreciable only in the first semiconductor layer close to the interface. The decay length of the gap states in the semiconductor side is about 2.0 $\pm$ 0.1 Å$\:$ and is independent of the deposited metal, therefore being to a good extent a bulk property of GaN. Our calculated values of the Schottky barrier heights are $Φ_{B_p}(GaN/Ag)$ = 0.87 eV and $Φ_{B_p}(GaN/Au)$ = 1.08 eV; both values are smaller than the GaN/Al value ($Φ_{B_p}(GaN/Al)$ = 1.51 eV) and this quite large spread of values excludes the possibility of a Fermi level pinning within the GaN band gap. Because of the low screening in GaN, the potential barrier at the junction is strongly affected by the structural arrangement of the first metal layer at the interface. This leads to quite large variations of the Schottky barrier height as a function of the metal, in contrast with the behavior of GaAs/metal interfaces.

cond-mat.mtrl-sci↗

arXiv1998

Influence of the exchange reaction on the electronic structure of GaN/Al junctions

Ab-initio calculations have been used to study the influence of the interface morphology and, notably, of the exchange reaction on the electronic properties of Al/GaN (100) interfaces. In particular, the effects of interface structure (i.e. interfacial bond lengths, semiconductor surface polarity, and reacted intralayers) on the SBH at the Al/GaN (001) junction are specifically addressed. The electronic structure of the following atomic configurations have been investigated theoretically: (i) an abrupt, relaxed GaN/Al interface; (ii) an interface which has undergone one monolayer of exchange reaction; and interfaces with a monolayer-thick interlayer of (iii) AlN and (iv) Ga$_{0.5}$Al$_{0.5}$N. Intermixed interfaces are found to pin the interface Fermi level at a position not significantly different from that of an abrupt interface. Our calculations also show that the interface band line--up is not strongly dependent on the interface morphology changes studied. The p-type SBH is reduced by less than 0.1 eV if the GaN surface is Ga-terminated compared to the N-terminated one. Moreover, we show that both an ultrathin Ga$_x$Al$_{1-x}$N ($x$ = 0, 0.5) intralayer and a Ga$\leftrightarrow$Al atomic swap at the interface does not significantly affect the Schottky barrier height.

cond-mat.mtrl-sci↗

arXiv1997

Electric fields and valence band offsets at strained [111] heterojunctions

[111] ordered common atom strained layer superlattices (in particular the common anion GaSb/InSb system and the common cation InAs/InSb system) are investigated using the ab initio full potential linearized augmented plane wave (FLAPW) method. We have focused our attention on the potential line-up at the two sides of the homopolar isovalent heterojunctions considered, and in particular on its dependence on the strain conditions and on the strain induced electric fields. We propose a procedure to locate the interface plane where the band alignment could be evaluated; furthermore, we suggest that the polarization charges, due to piezoelectric effects, are approximately confined to a narrow region close to the interface and do not affect the potential discontinuity. We find that the interface contribution to the valence band offset is substantially unaffected by strain conditions, whereas the total band line-up is highly tunable, as a function of the strain conditions. Finally, we compare our results with those obtained for [001] heterojunctions.

cond-mat.mtrl-sci↗

arXiv1997

Influence of growth direction and strain conditions on the band line-up at GaSb/InSb and InAs/InSb interfaces

First-principles full potential linearized augmented plane wave (FLAPW) calculations have been performed for lattice-mismatched common-atom III-V interfaces. In particular, we have examined the effects of epitaxial strain and ordering direction on the valence band offset in [001] and [111] GaSb/InSb and InAs/InSb superlattices, and found that the valence band maximum is always higher at the InSb side of the heterojunction, except for the common-anion system grown on an InSb substrate. The comparison between equivalent structures having the same substrate lattice constant, but different growth axis, shows that for comparable strain conditions, the ordering direction slightly influences the band line-up, due to small differences of the charge readjustment at the [001] and [111] interfaces. On the other hand, strain is shown to strongly affect the VBO; in particular, as the pseudomorphic growth conditions are varied, the bulk contribution to the band line-up changes markedly, whereas the interface term is almost constant. On the whole, our calculations yield a band line-up that decreases linearly as the substrate lattice constant is increased, showing its high tunability as a function of different pseudomorphic growth conditions. Finally, the band line-up at the lattice matched InAs/GaSb interface determined using the transitivity rule gave perfect agreement between predicted and experimental results.

cond-mat.mtrl-sci↗

arXiv1997

Effects of epitaxial strain and ordering direction on the electronic properties of (GaSb)_1/(InSb)_1 and (InAs)_1/(InSb)_1 superlattices

The structural and electronic properties in common anion (GaSb)_1/(InSb)_1 and common cation (InAs)_1/(InSb)_1 [111] ordered superlattices have been determined using the local density total energy full potential linearized augmented plane wave method. The influence of the ordering direction, strain conditions and atomic substitution on the electronic properties of technological and experimental interest (such as energy band-gaps and charge carrier localization in the different sublattices) were determined. The results show an appreciable energy band-gap narrowing compared to the band-gap averaged over the constituent binaries, either in [001] ordered structures or (more markedly) in the [111] systems; moreover energy band-gaps show an increasing trend as the substrate lattice parameter is decreased. Finally, the systems examined offer interesting opportunities for band-gap tuning as a function of the growth condition (about 0.7 eV in (GaSb)_1/(InSb)_1 and 0.3 eV in (InAs)_1/(InSb)_1).

cond-mat.mtrl-sci↗