352 research outputs found
The influence of initial impurities and irradiation conditions on defect production and annealing in silicon for particle detectors
Silicon detectors in particle physics experiments at the new accelerators or
in space missions for physics goals will be exposed to extreme radiation
conditions. The principal obstacles to long-term operation in these
environments are the changes in detector parameters, consequence of the
modifications in material properties after irradiation. The phenomenological
model developed in the present paper is able to explain quantitatively, without
free parameters, the production of primary defects in silicon after particle
irradiation and their evolution toward equilibrium, for a large range of
generation rates of primary defects. Vacancy-interstitial annihilation,
interstitial migration to sinks, divacancy and vacancy-impurity complex (VP,
VO, V2O, CiOi and CiCs) formation are taken into account. The effects of
different initial impurity concentrations of phosphorus, oxygen and carbon, as
well as of irradiation conditions are systematically studied. The correlation
between the rate of defect production, the temperature and the time evolution
of defect concentrations is also investigated.Comment: 14 pages, 8 figures, submitted to Nucl. Instrum. Meth. Phys. Res.
Stability of an Exciton bound to an Ionized Donor in Quantum Dots
Total energy, binding energy, recombination rate (of the electron hole pair)
for an exciton (X) bound in a parabolic two dimensional quantum dot by a donor
impurity located on the z axis at a distance d from the dot plane, are
calculated by using the Hartree formalism with a recently developed numerical
method (PMM) for the solution of the Schroedinger equation. As our analysis
indicates there is a critical dot radius such that for radius less than the
critical radius the complex is unstable and with an increase of the impurity
distance this critical radius increases. Furthermore, there is a critical value
of the mass ratio such that for mass ratio less than the critical value the
complex is stable. The appearance of this stability condition depends both on
the impurity distance and the dot radius, in a way that with an increase of the
impurity distance we have an increase in the maximum dot radius where this
stability condition appears. For dot radii greater than this maximum dot radius
(for fixed impurity distance) the complex is always stable.Comment: 17 pages, 7 figures Applying a new numerical method which is based on
the adiabatic stability of quantum mechanics, we study the stability of an
exciton (X) bound in a parabolic two dimensional quantum dot by a donor
impurity located on the z axis at a distance d from the dot plan
Reversal of the Charge Transfer between Host and Dopant Atoms in Semiconductor Nanocrystals
We present ab initio density functional calculations that show P (Al) dopant
atoms in small hydrogen-terminated Si crystals to be negatively (positively)
charged. These signs of the dopant charges are reversed relative to the same
dopants in bulk Si. We predict this novel reversal of the dopant charge (and
electronic character of the doping) to occur at crystal sizes of order 100 Si
atoms. We explain it as a result of competition between fundamental principles
governing charge transfer in bulk semiconductors and molecules and predict it
to occur in nanocrystals of most semiconductors.Comment: 4 pages, 4 figures (3 in color), 2 table
The Structure of the [Zn_In - V_P] Defect Complex in Zn Doped InP
We study the structure, the formation and binding energies and the transfer
levels of the zinc-phosphorus vacancy complex [Zn_In - V_P] in Zn doped p-type
InP, as a function of the charge, using plane wave ab initio DFT-LDA
calculations in a 64 atom supercell. We find a binding energy of 0.39 eV for
the complex, which is neutral in p-type material, the 0/-1 transfer level lying
0.50 eV above the valence band edge, all in agreement with recent positron
annihilation experiments. This indicates that, whilst the formation of
phosphorus vacancies (V_P) may be involved in carrier compensation in heavily
Zn doped material, the formation of Zn-vacancy complexes is not.
Regarding the structure: for charge states Q=+6 to -4 the Zn atom is in an
sp^2 bonded DX position and electrons added/removed go to/come from the
remaining dangling bonds on the triangle of In atoms. This reduces the
effective vacancy volume monatonically as electrons are added to the complex,
also in agreement with experiment. The reduction occurs through a combination
of increased In-In bonding and increased Zn-In electrostatic attraction. In
addition, for certain charge states we find complex Jahn-Teller behaviour in
which up to three different structures, (with the In triangle dimerised,
antidimerised or symmetric) are stable and are close to degenerate. We are able
to predict and successfully explain the structural behaviour of this complex
using a simple tight binding model.Comment: 10 pages text (postscript) plus 8 figures (jpeg). Submitted to Phys.
Rev.
Scenarios about the long-time damage of silicon as material and detectors operating beyond LHC collider conditions
For the new hadron collider LHC and some of its updates in luminosity and
energy, as SLHC and VLHC, the silicon detectors could represent an important
option, especially for the tracking system and calorimetry. The main goal of
this paper is to analyse the expected long-time degradation in the bulk of the
silicon as material and for silicon detectors, in continuous radiation field,
in these hostile conditions. The behaviour of silicon in relation to various
scenarios for upgrade in energy and luminosity is discussed in the frame a
phenomenological model developed previously by the authors. Different silicon
material parameters resulting from different technologies are considered to
evaluate what materials are harder to radiation and consequently could minimise
the degradation of device parameters in conditions of continuous long time
operation.Comment: submitted to Physica Scripta Work in the frame of CERN RD-50
Collaboratio
Reconstruction Mechanism of FCC Transition-Metal (001) Surfaces
The reconstruction mechanism of (001) fcc transition metal surfaces is
investigated using a full-potential all-electron electronic structure method
within density-functional theory. Total-energy supercell calculations confirm
the experimental finding that a close-packed quasi-hexagonal overlayer
reconstruction is possible for the late 5-metals Ir, Pt, and Au, while it is
disfavoured in the isovalent 4 metals (Rh, Pd, Ag). The reconstructive
behaviour is driven by the tensile surface stress of the unreconstructed
surfaces; the stress is significantly larger in the 5 metals than in 4
ones, and only in the former case it overcomes the substrate resistance to the
required geometric rearrangement. It is shown that the surface stress for these
systems is due to charge depletion from the surface layer, and that the
cause of the 4th-to-5th row stress difference is the importance of relativistic
effects in the 5 series.Comment: RevTeX 3.0, 12 pages, 1 PostScript figure available upon request] 23
May 199
Coexistence of Band Jahn Teller Distortion and superconductivity in correlated systems
The co-existence of band Jahn-Teller (BJT) effect with superconductivity (SC)
is studied for correlated systems, with orbitally degenerate bands using a
simple model. The Hubbard model for a doubly degenerate orbital with the
on-site intraorbital Coulomb repulsion treated in the slave boson formalism and
the interorbital Coulomb repulsion treated in the Hartree-Fock mean field
approximation, describes the correlated system. The model further incorporates
the BJT interaction and a pairing term to account for the lattice distortion
and superconductivity respectively. It is found that structural distortion
tends to suppress superconductivity and when SC sets in at low temperatures,
the growth of the lattice distortion is arrested. The phase diagram comprising
of the SC and structural transition temperatures and versus the
dopant concentration reveals that the highest obtainable for an
optimum doping is limited by structural transition. The dependence of the
occupation probabilities of the different bands as well as the density of
states (DOS) in the distorted-superconducting phase, on electron correlation
has been discussed.Comment: RevTex, 4 pages, 4 figuers (postscript files attached) Journal
Reference : Phys. Rev. B (accepted for publication
Theory of Superconducting of doped fullerenes
We develop the nonadiabatic polaron theory of superconductivity of
taking into account the polaron band narrowing and realistic
electron-phonon and Coulomb interactions. We argue that the crossover from the
BCS weak-coupling superconductivity to the strong-coupling polaronic and
bipolaronic superconductivity occurs at the BCS coupling constant independent of the adiabatic ratio, and there is nothing ``beyond'' Migdal's
theorem except small polarons for any realistic electron-phonon interaction. By
the use of the polaronic-type function and the ``exact'' diagonalization in the
truncated Hilbert space of vibrons (``phonons'') we calculate the ground state
energy and the electron spectral density of the molecule. This
allows us to describe the photoemission spectrum of in a wide
energy region and determine the electron-phonon interaction. The strongest
coupling is found with the high-frequency pinch mode and with the
Frenkel exciton. We clarify the crucial role of high-frequency bosonic
excitations in doped fullerenes which reduce the bare bandwidth and the Coulomb
repulsion allowing the intermediate and low-frequency phonons to couple two
small polarons in a Cooper pair. The Eliashberg-type equations are solved for
low-frequency phonons. The value of the superconducting , its pressure
dependence and the isotope effect are found to be in a remarkable agreement
with the available experimental data.Comment: 20 pages, Latex, 4 figures available upon reques
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