1,000 research outputs found
Polarization-based calculation of the dielectric tensor of polar crystals
We present a novel method for the calculation of the static and electronic
dielectric tensor of polar insulating crystals based on concepts from the
modern theory of dielectric polarization. As an application, we present the
first ab initio calculation of the dielectric constants in the wurtzite III-V
nitrides AlN, GaN, and InN.Comment: RevTeX 4 pages, no figures, numbering error correcte
Gap opening in ultrathin Si layers: Role of confined and interface states
We present first principle calculations of ultrathin silicon (111) layers embedded in CaF2, a lattice matched insulator. Our all electron calculation allows a check of the quantum confinement hypothesis for the Si band gap opening as a function of thickness. We find that the gap opening is mostly due to the valence band while the lowest conduction band states shift very modestly due to their pronounced interface character. The latter states are very sensitive to the sample design. We suggest that a quasidirect band gap can be achieved by stacking Si layers of different thickness
Energetic stability and magnetic properties of Mn dimers in silicon
We present an accurate first-principles study of magnetism and energetics of single Mn impurities and Mn dimers in Si. Our results, in general agreement with available experiments, show that (i) Mn atoms tend to aggregate, the formation energy of dimers being lower than the sum of the separate constituents, (ii) ferromagnetic coupling is favored between the Mn atoms constituting the dimers in p-type Si, switching to an antiferromagnetic coupling in n-type Si, (iii) Mn atoms show donors (acceptor) properties in p-type (n-type) Si, therefore they tend to compensate doping, while dimers being neutral or acceptors allow for Si to be doped p-type. (C) 2004 American Institute of Physics
Band offsets and stability of BeTe/ZnSe (100) heterojunctions
We present ab-initio studies of band offsets, formation energy, and stability
of (100) heterojunctions between (Zn,Be)(Se,Te) zincblende compounds, and in
particular of the lattice-matched BeTe/ZnSe interface. Equal band offsets are
found at Be/Se and Zn/Te abrupt interfaces, as well as at mixed interfaces, in
agreement with the established understanding of band offsets at isovalent
heterojunctions. Thermodynamical arguments suggest that islands of non-nominal
composition may form at the interface, causing offset variations over about 0.8
eV depending on growth conditions. Our findings reconcile recent experiments on
BeTe/ZnSe with the accepted theoretical description.Comment: RevTeX 5 pages, 3 embedded figure
Theoretical evidence for the semi-insulating character of AlN
We present ab initio density-functional calculations for acceptors, donors,
and native defects in aluminum nitride, showing that acceptors are deeper (Be ~
0.25 eV, Mg_ 0.45 eV) and less soluble than in GaN; at further variance with
GaN, both the extrinsic donors Si_Al and C_Al, and the native donor V_N (the
anion vacancy) are found to be deep (about 1 to 3 eV below the conduction). We
thus predict that doped AlN will generally turn out to be semi-insulating in
the normally achieved Al-rich conditions, in agreement with the known doping
difficulties of high-x AlGaN alloys.Comment: RevTeX 3 pages, 1 figur
Polarization fields in nitride nanostructures: ten points to think about
Macroscopic polarization, both of intrinsic and piezoelectric nature, is
unusually strong in III-V nitrides, and the built in electric fields in the
layers of nitride-based nanostructures, stemming from polarization changes at
heterointerfaces, have a major impact on the properties of single and multiple
quantum wells, high mobility transistors, and thin films. The concepts involved
in the theory and applications of polarization in nitrides have encountered
some resistance in the field. Here we discuss critically ten ``propositions''
aimed at clarifying the main controversial issues.Comment: RevTeX 5 pages, 2 embedded figure
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