High-dielectric constant and wide band gap inverse silver oxide phases of the ordered ternary alloys of SiO2_{2}, GeO2_{2} and SnO2_{2}

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$_2$. First-principles study of this system together with its isovalent equivalents GeO$_{2}$, SnO$_{2}$ 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$_{2}$ and SnO$_{2}$ have negative phonon branches whereas their ternary alloys Si$_{0.5}$Ge$_{0.5}$O$_{2}$, Si$_{0.5}$Sn$_{0.5}$O$_{2}$, and Ge$_{0.5}$Sn$_{0.5}$O$_{2}$ are all stable within LDA possessing dielectric constants ranging between 10 to 20. Furthermore, the lattice constant of Si$_{0.5}$Ge$_{0.5}$O$_{2}$ is virtually identical to the Si(100) surface. The $GW$ 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

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

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

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

Computational modeling of quantum-confined impact ionization in Si nanocrystals embedded in SiO2

Injected carriers from the contacts to delocalized bulk states of the oxide matrix via Fowler-Nordheim tunneling can give rise to quantum-confined impact ionization (QCII) of the nanocrystal (NC) valence electrons. This process is responsible for the creation of confined excitons in NCs, which is a key luminescence mechanism. For a realistic modeling of QCII in Si NCs, a number of tools are combined: ensemble Monte Carlo (EMC) charge transport, ab initio modeling for oxide matrix, pseudopotential NC electronic states together with the closed-form analytical expression for the Coulomb matrix element of the QCII. To characterize the transport properties of the embedding amorphous SiO2, ab initio band structure and density of states of the α-quartz phase of SiO2 are employed. The confined states of the Si NC are obtained by solving the atomistic pseudopotential Hamiltonian. With these ingredients, realistic modeling of the QCII process involving a SiO2 bulk state hot carrier and the NC valence electrons is provided. © 2007 Elsevier B.V. All rights reserved

Elements of nanocrystal high-field carrier transport modeling

Embedded semiconductor nanocrystals (NCs) within wide bandgap oxide materials are being considered for light emission and solar cell applications. One of the fundamental issues is the high-field transport in NCs. This requires the combination of a number of tools: ensemble Monte Carlo carrier transport simulation, ab initio band structure of the bulk oxide, Fermi's golden rule modeling of impact ionization and Auger processes and the pseudopotential-based atomistic description of the confined NC states. These elements are outlined in this brief report. © 2007 WILEY-VCH Verlag GmbH & Co. KGaA

Gunn oscillations in GaN channels

Gallium nitride with its high negative differential mobility threshold is an appealing material for high power millimetre-wave oscillators as a Gunn diode. By means of extensive ensemble Monte Carlo simulations, the dynamics of large-amplitude Gunn domain oscillations from 120 GHz to 650 GHz is studied in detail. Their operations are checked under both impressed single-tone sinusoidal bias and external tank circuit conditions. The width of the doping notch is observed to enhance higher harmonic efficiency at the expense of the fundamental frequency up to a critical value, beyond which sustained Gunn oscillations cease. The degeneracy effects due to the Pauli exclusion principle are also considered, but their effects are seen to be negligible within the realistic bounds of the Gunn diode operation

Efficiency and harmonic enhancement trends in GaN-based Gunn diodes: Ensemble Monte Carlo analysis

Gallium nitride can offer a high-power alternative for millimeter-wave Gunn oscillators. Hence, an ensemble Monte Carlo-based comprehensive theoretical assessment of efficiency and harmonic enhancement in n-type GaN Gunn diodes is undertaken. First, the effects of doping notch/mesa and its position within the active channel are investigated which favors a doping notch positioned next to cathode. It is then observed that the width of the notch can be optimized to enhance the higher-harmonic operation without degrading its performance at the fundamental mode. Next, the effects of dc bias and channel doping density are investigated. Both of these have more significant effects on the higher-harmonic efficiency than the fundamental one. The lattice temperature is observed to have almost no influence up to room temperature but severely degrades the performance above room temperature. As a general behavior, the variations of temperature, channel doping, and the notch width primarily affect the phase angle between the current and voltage wave forms rather than the amplitude of oscillations. Finally, the physical origin of these Gunn oscillations is sought which clearly indicates that the intervalley scattering mechanism is responsible rather than the Γ valley nonparabolicity or the effective mass discrepancy between the Γ and the lowest satellite valleys. © 2004 American Institute of Physics

Hot electron effects in unipolar n-type submicron structures based on GaN, AlN and their ternary alloys

The authors present an analysis of impact ionisation (II) and related hot electron effects in submicron sized GaN, AlN and their ternary alloys, all of which can support very high field regimes, reaching a few megavolts per centimetre (MV/cm). The proposed high field transport methodology is based on the ensemble Monte Carlo technique, with all major scattering mechanisms incorporated. As a test-bed for understanding II and hot electron effects, an n+-n-n+ channel device is employed having a 0.1 μm thick n-region. The time evolution of the electron density along the device is seen to display oscillations in the unintentionally doped n-region, until steady state is established. The fermionic degeneracy effects are observed to be operational especially at high fields within the anode n+-region. For AlxGa1-xN-based systems, it can be noted that due to alloy scattering, carriers cannot acquire the velocities attained by the GaN and AlN counterparts. Finally, at very high fields II is shown to introduce a substantial energy loss mechanism for the energetic carriers that have just traversed the unintentionally doped n-region