372 research outputs found
Theory of transient spectroscopy of multiple quantum well structures
A theory of the transient spectroscopy of quantum well (QW) structures under
a large applied bias is presented. An analytical model of the initial part of
the transient current is proposed. The time constant of the transient current
depends not only on the emission rate from the QWs, as is usually assumed, but
also on the subsequent carrier transport across QWs. Numerical simulation was
used to confirm the validity of the proposed model, and to study the transient
current on a larger time scale. It is shown that the transient current is
influenced by the nonuniform distribution of the electric field and related
effects, which results in a step-like behavior of the current. A procedure of
extraction of the QW emission time from the transient spectroscopy experiments
is suggested.Comment: 5 pages, 4 figures, to be published in J. Appl. Phy
Temperature influence on the properties of thin Si₃N₄ films
Applying Raman spectroscopy, small-angle x-ray scattering, and atomic force microscopy it were studied phase composition and surface morphology of nanoscale films Si₃N₄ (obtained by RF magnetron sputtering
Strong electron correlations in cobalt valence tautomers
We have examined cobalt based valence tautomer molecules such as
Co(SQ)(phen) using density functional theory (DFT) and variational
configuration interaction (VCI) approaches based upon a model Hamiltonian. Our
DFT results extend earlier work by finding a reduced total energy gap (order
0.6 eV) between high temperature and low temperature states when we fully relax
the coordinates (relative to experimental ones). Futhermore we demonstrate that
the charge transfer picture based upon formal valence arguments succeeds
qualitatively while failing quantitatively due to strong covalency between the
Co 3 orbitals and ligand orbitals. With the VCI approach, we argue that
the high temperature, high spin phase is strongly mixed valent, with about 30 %
admixture of Co(III) into the predominantly Co(II) ground state. We confirm
this mixed valence through a fit to the XANES spectra. Moreover, the strong
electron correlations of the mixed valent phase provide an energy lowering of
about 0.2-0.3 eV of the high temperature phase relative to the low temperature
one. Finally, we use the domain model to account for the extraordinarily large
entropy and enthalpy values associated with the transition.Comment: 10 pages, 4 figures, submitted to J. Chem. Phy
Mechanisms of Manganese-Assisted Nonradiative Recombination in Cd(Mn)Se/Zn(Mn)Se Quantum Dots
Mechanisms of nonradiative recombination of electron-hole complexes in
Cd(Mn)Se/Zn(Mn)Se quantum dots accompanied by interconfigurational excitations
of Mn ions are analyzed within the framework of single electron model of
deep {\it 3d}-levels in semiconductors. In addition to the mechanisms caused by
Coulomb and exchange interactions, which are related because of the Pauli
principle, another mechanism due to {\it sp-d} mixing is considered. It is
shown that the Coulomb mechanism reduces to long-range dipole-dipole energy
transfer from photoexcited quantum dots to Mn ions. The recombination
due to the Coulomb mechanism is allowed for any states of Mn ions and
{\it e-h} complexes. In contrast, short-range exchange and
recombinations are subject to spin selection rules, which are the result of
strong {\it lh-hh} splitting of hole states in quantum dots. Estimates show
that efficiency of the {\it sp-d} mechanism can considerably exceed that of the
Coulomb mechanism. The phonon-assisted recombination and processes involving
upper excited states of Mn ions are studied. The increase in PL
intensity of an ensemble of quantum dots in a magnetic field perpendicular to
the sample growth plane observed earlier is analyzed as a possible
manifestation of the spin-dependent recombination.Comment: 14 pages, 2 figure
Influence of negative- U centers related carrier dynamics on donor-acceptor-pair emission in fluorescent SiC
Spin-orbit terms in multi-subband electron systems: A bridge between bulk and two-dimensional Hamiltonians
We analyze the spin-orbit terms in multi-subband quasi-two-dimensional
electron systems, and how they descend from the bulk Hamiltonian of the
conduction band. Measurements of spin-orbit terms in one subband alone are
shown to give incomplete information on the spin-orbit Hamiltonian of the
system. They should be complemented by measurements of inter-subband spin-orbit
matrix elements. Tuning electron energy levels with a quantizing magnetic field
is proposed as an experimental approach to this problem.Comment: Typos noticed in the published version have been corrected and
several references added. Published in the special issue of Semiconductors in
memory of V.I. Pere
Single-particle states in spherical Si/SiO quantum dots
We calculate ground and excited electron and hole levels in spherical Si
quantum dots inside SiO in a multiband effective mass approximation.
Luttinger Hamiltonian is used for holes and the strong anisotropy of the
conduction electron effective mass in Si is taken into account. As boundary
conditions for electron and hole wave functions we use continuity of the wave
functions and the velocity density at the boundary of the quantum dots.Comment: 8 pages, 5 figure
Navedennoe pogloŝenie nakački, porog generacii i kpd lazerov na rastvorah organičeskih soedinenij
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