4,494 research outputs found
Voltage modulated electro-luminescence spectroscopy and negative capacitance - the role of sub-bandgap states in light emitting devices
Voltage modulated electroluminescence spectra and low frequency ({\leq} 100
kHz) impedance characteristics of electroluminescent diodes are studied.
Voltage modulated light emission tracks the onset of observed negative
capacitance at a forward bias level for each modulation frequency. Active
participation of sub-bandgap defect states in minority carrier recombination
dynamics is sought to explain the results. Negative capacitance is understood
as a necessary dielectric response to compensate any irreversible transient
changes in the minority carrier reservoir due to radiative recombinations
mediated by slowly responding sub-bandgap defects. Experimentally measured
variations of the in-phase component of modulated electroluminescence spectra
with forward bias levels and modulation frequencies support the dynamic
influence of these states in the radiative recombination process. Predominant
negative sign of the in-phase component of voltage modulated
electroluminescence signal further confirms the bi-molecular nature of light
emission. We also discuss how these states can actually affect the net density
of minority carriers available for radiative recombination. Results indicate
that these sub-bandgap states can suppress external quantum efficiency of such
devices under high frequency operation commonly used in optical communication.Comment: 21 pages, 4 sets of figure
Control of threshold voltage in E-mode and D-mode GaN-on-Si metal-insulator-semiconductor heterostructure field effect transistors by in-situ fluorine doping of atomic layer deposition Al2O3 gate dielectrics
We report the modification and control of threshold voltage in enhancement and depletion mode AlGaN/GaN metal-insulator-semiconductor heterostructure field effect transistors through the use of in-situ fluorine doping of atomic layer deposition Al2O3. Uniform distribution of F ions throughout the oxide thickness are achievable, with a doping level of up to 5.5 × 1019 cm−3 as quantified by secondary ion mass spectrometry. This fluorine doping level reduces capacitive hysteretic effects when exploited in GaN metal-oxide-semiconductor capacitors. The fluorine doping and forming gas anneal also induces an average positive threshold voltage shift of between 0.75 and 1.36 V in both enhancement mode and depletion mode GaN-based transistors compared with the undoped gate oxide via a reduction of positive fixed charge in the gate oxide from +4.67 × 1012 cm−2 to −6.60 × 1012 cm−2. The application of this process in GaN based power transistors advances the realisation of normally off, high power, high speed devices
Ohmic contacts to n-type germanium with low specific contact resistivity
A low temperature nickel process has been developed that produces Ohmic contacts to n-type germanium with specific contact resistivities down to (2.3 ± 1.8) x10<sup>-7</sup> Ω-cm<sup>2</sup> for anneal temperatures of 340 degC. The low contact resistivity is attributed to the low resistivity NiGe phase which was identified using electron diffraction in a transmission electron microscope. Electrical results indicate that the linear Ohmic behaviour of the contact is attributed to quantum mechanical tunnelling through the Schottky barrier formed between the NiGe alloy and the heavily doped n-Ge.<p></p>
Ultrafast Optical Excitation of a Persistent Surface-State Population in the Topological Insulator Bi2Se3
Using femtosecond time- and angle- resolved photoemission spectroscopy, we
investigated the nonequilibrium dynamics of the topological insulator Bi2Se3.
We studied p-type Bi2Se3, in which the metallic Dirac surface state and bulk
conduction bands are unoccupied. Optical excitation leads to a meta-stable
population at the bulk conduction band edge, which feeds a nonequilibrium
population of the surface state persisting for >10ps. This unusually long-lived
population of a metallic Dirac surface state with spin texture may present a
channel in which to drive transient spin-polarized currents
Instability of the Fermi-liquid fixed point in an extended Kondo model
We study an extended SU(N) single-impurity Kondo model in which the impurity
spin is described by a combination of Abrikosov fermions and Schwinger bosons.
Our aim is to describe both the quasiparticle-like excitations and the locally
critical modes observed in various physical situations, including non-Fermi
liquid (NFL) behavior in heavy fermions in the vicinity of a quantum critical
point and anomalous transport properties in quantum wires. In contrast with
models with either pure bosonic or pure fermionic impurities, the strong
coupling fixed point is unstable against the conduction electron kinetic term
under certain conditions. The stability region of the strong coupling fixed
point coincides with the region where the partially screened, effective
impurity repels the electrons on adjacent sites. In the instability region, the
impurity tends to attract electrons to the neighboring sites, giving
rise to a double-stage Kondo effect with additional screening of the impurity.Comment: 10 pages, 2 figures, Proceedings of the NATO Workshop on "Concepts in
Electron Correlations", Hvar,October 200
A measurement of the differential cross section for the two-body photodisintegration of 3He at theta_LAB = 90deg using tagged photons in the energy range 14 -- 31 MeV
The two-body photodisintegration of 3He has been investigated using tagged
photons with energies from 14 -- 31 MeV at MAX-lab in Lund, Sweden. The
two-body breakup channel was unambiguously identified by the (nonsimultaneous)
detection of both protons and deuterons. This approach was made feasible by the
over-determined kinematic situation afforded by the tagged-photon technique.
Proton- and deuteron-energy spectra were measured using four silicon
surface-barrier detector telescopes located at a laboratory angle of 90deg with
respect to the incident photon-beam direction. Average statistical and
systematic uncertainties of 5.7% and 6.6% in the differential cross section
were obtained for 11 photon-energy bins with an average width of 1.2 MeV. The
results are compared to previous experimental data measured at comparable
photon energies as well as to the results of two recent Faddeev calculations
which employ realistic potential models and take into account three-nucleon
forces and final-state interactions. Both the accuracy and precision of the
present data are improved over the previous measurements. The data are in good
agreement with most of the previous results, and favor the inclusion of
three-nucleon forces in the calculations.Comment: 12 pages, 13 figures; further Referee comments addresse
The Effects of Binary Evolution on the Dynamics of Core Collapse and Neutron-Star Kicks
We systematically examine how the presence in a binary affects the final core
structure of a massive star and its consequences for the subsequent supernova
explosion. Interactions with a companion star may change the final rate of
rotation, the size of the helium core, the strength of carbon burning and the
final iron core mass. Stars with initial masses larger than \sim 11\Ms that
experiece core collapse will generally have smaller iron cores at the time of
the explosion if they lost their envelopes due to a previous binary
interaction. Stars below \sim 11\Ms, on the other hand, can end up with larger
helium and metal cores if they have a close companion, since the second
dredge-up phase which reduces the helium core mass dramatically in single stars
does not occur once the hydrogen envelope is lost. We find that the initially
more massive stars in binary systems with masses in the range 8 - 11\Ms are
likely to undergo an electron-capture supernova, while single stars in the same
mass range would end as ONeMg white dwarfs. We suggest that the core collapse
in an electron-capture supernova (and possibly in the case of relatively small
iron cores) leads to a prompt explosion rather than a delayed neutrino-driven
explosion and that this naturally produces neutron stars with low-velocity
kicks. This leads to a dichotomous distribution of neutron star kicks, as
inferred previously, where neutron stars in relatively close binaries attain
low kick velocities. We illustrate the consequences of such a dichotomous kick
scenario using binary population synthesis simulations and discuss its
implications. This scenario has also important consequences for the minimum
initial mass of a massive star that becomes a neutron star. (Abbreviated.)Comment: 8 pages, 3 figures, submitted to ApJ, updated versio
Silver antimony Ohmic contacts to moderately doped n-type germanium
A self doping contact consisting of a silver/antimony alloy that produces an Ohmic contact to moderately doped n-type germanium (doped to a factor of four above the metal-insulator transition) has been investigated. An evaporation of a mixed alloy of Ag/Sb (99%/1%) onto n-Ge ( ND=1×1018 cm−3) annealed at 400 °C produces an Ohmic contact with a measured specific contact resistivity of (1.1±0.2)×10−5 Ω-cm2. It is proposed that the Ohmic behaviour arises from an increased doping concentration at the Ge surface due to the preferential evaporation of Sb confirmed by transmission electron microscope analysis. It is suggested that the doping concentration has increased to a level where field emission will be the dominate conduction mechanism. This was deduced from the low temperature electrical characterisation of the contact, which exhibits Ohmic behaviour down to a temperature of 6.5 K
Competition of Mesoscales and Crossover to Tricriticality in Polymer Solutions
We show that the approach to asymptotic fluctuation-induced critical behavior
in polymer solutions is governed by a competition between a correlation length
diverging at the critical point and an additional mesoscopic length-scale, the
radius of gyration. Accurate light-scattering experiments on polystyrene
solutions in cyclohexane with polymer molecular weights ranging from 200,000 up
to 11.4 million clearly demonstrate a crossover between two universal regimes:
a regime with Ising asymptotic critical behavior, where the correlation length
prevails, and a regime with tricritical theta-point behavior determined by a
mesoscopic polymer-chain length.Comment: 4 pages in RevTeX with 4 figure
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