101 research outputs found
Pseudopotential-based full zone k.p technique for indirect bandgap semiconductors: Si, Ge, diamond and SiC
The is a versatile technique that describes the semiconductor
band structure in the vicinity of the bandgap. The technique can be extended to
full Brillouin zone by including more coupled bands into consideration. For
completeness, a detailed formulation is provided where the associated parameters are extracted from the local empirical pseudopotential method in
the form of band edge energies and generalized momentum matrix elements. We
demonstrate the systematic improvement of the technique with the proper choice
of the band edge states for the group-IV indirect bandgap semiconductors: Si,
Ge, diamond and SiC of the 3C cubic phase. The full zone agreement is observed
to span an energy window of more than 20 eV for Si, and 40 eV for the diamond
with the 15-band pseudopotential-based approach.Comment: 8 pages, 6 figures, requires fizik.cls (included
High-dielectric constant and wide band gap inverse silver oxide phases of the ordered ternary alloys of SiO, GeO and SnO
High-dielectric constant and wide band gap oxides have important
technological applications. The crystalline oxide polymorphs having lattice
constant compatibility to silicon are particularly desirable. One recently
reported candidate is the inverse silver oxide phase of SiO.
First-principles study of this system together with its isovalent equivalents
GeO, SnO as well as their ternary alloys are performed. Within the
framework of density functional theory both generalized gradient approximation
and local density approximation (LDA) are employed to obtain their structural
properties, elastic constants and the electronic band structures. To check the
stability of these materials, phonon dispersion curves are computed which
indicate that GeO and SnO have negative phonon branches whereas
their ternary alloys SiGeO, SiSnO,
and GeSnO are all stable within LDA possessing dielectric
constants ranging between 10 to 20. Furthermore, the lattice constant of
SiGeO is virtually identical to the Si(100) surface. The
band gaps of the stable materials are computed which restore the wide band
gap values in addition to their high dielectric constants.Comment: Published version; two figures merged into on
Interband, intraband and excited-state direct photon absorption of silicon and germanium nanocrystals embedded in a wide band-gap lattice
Embedded Si and Ge nanocrystals (NCs) in wide band-gap matrices are studied
theoretically using an atomistic pseudopotential approach. From small clusters
to large NCs containing on the order of several thousand atoms are considered.
Effective band-gap values as a function of NC diameter reproduce very well the
available experimental and theoretical data. It is observed that the highest
occupied molecular orbital for both Si and Ge NCs and the lowest unoccupied
molecular orbital for Si NCs display oscillations with respect to size among
the different irreducible representations of the point group to which
these spherical NCs belong. Based on this electronic structure, first the
interband absorption is thoroughly studied which shows the importance of
surface polarization effects that significantly reduce the absorption when
included. This reduction is found to increase with decreasing NC size or with
increasing permittivity mismatch between the NC core and the host matrix.
Reasonable agreement is observed with the experimental absorption spectra where
available. The deformation of spherical NCs into prolate or oblate ellipsoids
are seen to introduce no pronounced effects for the absorption spectra. Next,
intraconduction and intravalence band absorption coefficients are obtained in
the wavelength range from far-infrared to visible region. These results can be
valuable for the infrared photodetection prospects of these NC arrays. Finally,
excited-state absorption at three different optical pump wavelengths, 532 nm,
355 nm and 266 nm are studied for 3- and 4 nm-diameter NCs. This reveals strong
absorption windows in the case of holes and a broad spectrum in the case of
electrons which can especially be relevant for the discussions on achieving
gain in these structures.Comment: Published version, 13 pages, 15 figures, local field effects include
Auger recombination and carrier multiplication in embedded silicon and germanium nanocrystals
For Si and Ge nanocrystals (NCs) embedded in wide band-gap matrices, Auger
recombination (AR) and carrier multiplication (CM) lifetimes are computed
exactly in a three-dimensional real space grid using empirical pseudopotential
wave functions. Our results in support of recent experimental data offer new
predictions. We extract simple Auger constants valid for NCs. We show that both
Si and Ge NCs can benefit from photovoltaic efficiency improvement via CM due
to the fact that under an optical excitation exceeding twice the band gap
energy, the electrons gain lion's share from the total excess energy and can
cause a CM. We predict that CM becomes especially efficient for hot electrons
with an excess energy of about 1 eV above the CM threshold.Comment: 4 pages, 6 figures (Published version
Auger recombination and carrier multiplication in embedded silicon and germanium nanocrystals
For Si and Ge nanocrystals (NCs) embedded in wide band-gap matrices, Auger
recombination (AR) and carrier multiplication (CM) lifetimes are computed
exactly in a three-dimensional real space grid using empirical pseudopotential
wave functions. Our results in support of recent experimental data offer new
predictions. We extract simple Auger constants valid for NCs. We show that both
Si and Ge NCs can benefit from photovoltaic efficiency improvement via CM due
to the fact that under an optical excitation exceeding twice the band gap
energy, the electrons gain lion's share from the total excess energy and can
cause a CM. We predict that CM becomes especially efficient for hot electrons
with an excess energy of about 1 eV above the CM threshold.Comment: 4 pages, 6 figures (Published version
Gain and temporal response of A1GaN solar-blind avalanche photodiodes: An essemble Monte Carlo analysis
Cataloged from PDF version of article.Multiplication and temporal response characteristics of p(+)-n-n(+) GaN and n-type Schottky Al0.4Ga0.6N avalanche photodiodes (APD) have been analyzed using the ensemble Monte Carlo method. Reasonable agreement is obtained with the published measurements for a GaN APD without any fitting parameters. In the case of AlGaN, the choice of a Schottky contact APD is seen to improve drastically the field confinement resulting in satisfactory gain characteristics. For the GaN APD, an underdamped step response is observed in the rising edge, and a Gaussian profile damping in the falling edge under an optical pulse with the switching speed degrading towards the gain region. In the AlGaN case, alloy scattering is seen to further slow down the temporal response while displacing the gain threshold to higher fields. (C) 2003 American Institute of Physics
Novel high-K inverse silver oxide phases of SiO2, GeO2, SnO2 and their alloys
Cataloged from PDF version of article.The recently reported inverse silver oxide phase of SiO2 possesses a high dielectric constant as well as lattice constant compatibility to Si. We explore the closely related oxides, GeO2, SnO2 with the same inverse silver oxide structure using ab initio density functional theory within the local density approximation (LDA). According to the phonon dispersion curves, both these structures are computed to be unstable. On the other hand, their alloys Si0.5Ge0.5O2, Si0.5Sn0.5O2, and Ge0.5Sn0.5O2, are stable with higher dielectric constants than that of SiO2 in the same phase. Their first-principles elastic constants. electronic band structures and phonon dispersion curves have been obtained with high precision. (c) 2006 Elsevier Ltd. All rights reserved
Electron initiated impact ionization in AlGaN alloys
Detailed impact ionization (II) analysis of electrons is presented for AlGaN alloys as a vital resource for solar-blind avalanche photodiode and high power transistor applications. Necessary ingredients for the II characterization are supplied from a recent experiment on the GaN end, and a Keldysh analysis for the AlN end, of the alloy AlGaN. High-field electron dynamics are simulated using an ensemble Monte Carlo framework, accounting for all valleys in the lowest two conduction bands, obtained from accurate empirical pseudopotential band structure computations. The effect of alloy scattering on II is considered and observed to be significant. For any AlxGa1-xN alloy, the electron II coefficients are found to obey the form, A exp(-K/F), for the electric field, F
Cat-state generation and stabilization for a nuclear spin through electric quadrupole interaction
Spin cat states are superpositions of two or more coherent spin states (CSSs) that are distinctly separated over the Bloch sphere. Additionally, the nuclei with angular momenta greater than 1/2 possess a quadrupolar charge distribution. At the intersection of these two phenomena, we devise a simple scheme for generating various types of nuclear-spin cat states. The native biaxial electric quadrupole interaction that is readily available in strained solid-state systems plays a key role here. However, the fact that built-in strain cannot be switched off poses a challenge for the stabilization of target cat states once they are prepared. We remedy this by abruptly diverting via a single rotation pulse the state evolution to the neighborhood of the fixed points of the underlying classical Hamiltonian flow. Optimal process parameters are obtained as a function of electric field gradient biaxiality and nuclear-spin angular momentum. The overall procedure is seen to be robust under 5% deviations from optimal values. We show that higher-level cat states with four superposed CSS can also be formed using three rotation pulses. Finally, for open systems subject to decoherence we extract the scaling of cat-state fidelity damping with respect to the spin quantum number. This reveals rates greater than the dephasing of individual CSSs. Yet, our results affirm that these cat states can preserve their fidelities for practically useful durations under the currently attainable decoherence levels. © 2017 American Physical Society
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