Electron Capture Rates Research Articles

Carrier capture in quantum well embedded quantum wire structures

We propose a novel quantum wire (QWR) laser structure with improved carrier capture characteristics, where the carriers are injected into a quantum well (QWL) and subsequently recombine within an embedded QWR. The corresponding electron capture rates via polar optical phonon scattering are calculated for this system. An oscillatory behavior of the electron capture rate is observed as a function of the QWR thickness at the temperatures considered (77 K and 300 K). The amplitude of these oscillations also increases as the QWR width decreases. Our calculations show that the electron capture rate in the QWL embedded QWR structure can be improved by more than 30% when compared to single QWLs. Therefore, the proposed QWR laser design provides improved modulation bandwidth and optical gain over conventional QWL and QWR structures.

Evidence for resonant electron capture and charge buildup in GaAs/AlxGa1-xAs quantum wells.

Evidence for the resonant capture of electrons into quantum wells (QW's) is demonstrated in the photoluminescence (PL) of GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As QW's doped with Be. We find that the PL intensity ratio between the free-to-bound and excitonic transitions exhibits a strong oscillation as a function of the QW width bearing a striking resemblance to the theoretical prediction of the electron capture rate. It is shown that this PL behavior reflects the buildup of excess negative charge in the QW arising from the different capture efficiencies of electrons and holes. \textcopyright{} 1996 The American Physical Society.

Importance of (n,p) reactions for stellar beta decay rates.

Recent (n,p) experiments on $^{54}\mathrm{Fe}$, $^{56}\mathrm{Fe}$, $^{58}\mathrm{Ni}$, and $^{59}\mathrm{Co}$ have measured Gamow-Teller resonances which are crucial for determining stellar beta decay rates in $^{54}\mathrm{Mn}$, $^{56}\mathrm{Mn}$, $^{58}\mathrm{Co}$, and $^{59}\mathrm{Fe}$, respectively. These nuclei are important players in the iron-core URCA process, which is a dynamic balancing of electron capture and beta decay rates during the final stage of nuclear burning in massive stars. We find that three of the nuclei, after calibration from (n,p) measurements, have significantly stronger decay rates than currently expected. These stronger rates will cause a more vigorous URCA process, resulting in presupernova cores which are cooler and less neutron rich than is presently used in core collapse calculations. \textcopyright{} 1996 The American Physical Society.

Carrier capture in cylindrical quantum wires

We present quantum mechanical calculations of electron capture rates in cylindrical quantum wires via polar-optical phonon scattering. The capture rate dependence on quantum wire radius and lattice temperature is investigated. An oscillatory behavior of the electron capture rate is observed as a function of the quantum wire radius at the temperatures considered in this study (20–300 K). However, the amplitude of these oscillations decreases significantly at large wire radii and high lattice temperatures.

On the spectral lineshapes, lifetimes and deexcitation processes of autoionizing states produced in inelastic ion-surface collisions

The natural lineshapes of peaks in electron spectra due to decay of autoionizing states of Ne, formed in collisions of Ne with Mg and Al solid targets, are modelled. The modelling takes into account the level shifts of the excited and ionic states of the atom in front of the surface and various resonant and Auger capture and loss processes. It is shown that rather complex lineshapes may be expected. The electron spectrum will generally possess a peak at energies in the vicinity of that corresponding to autoionization of a free atom, as well as a high-energy tail and secondary maximum. It is suggested that a structure observed at 22.2 eV in the experimental Ne** electron spectrum is just such a secondary maximum associated with the decay of the Ne** 3P 3S2 near the metal surface. The effects of varying the model lifetimes, electron capture and deexcitation rates are discussed.

Rapid approximate methods for calculating electron capture and auto-ionization rates

The factorized form 2 for distorted wave excitation collision strengths is applied to electron capture and the inverse process of auto-ionization, which are needed in obtaining dielectric recombination rates and the resonance contribution to excitation rates. Two simplifying approximations are considered and the corresponding formulae are obtained. Some numerical tests of accuracy are given for the relatively simple case of electron capture by He-like ions.

Theoretical study of electron transport in gallium nitride

This paper describes characteristic electron transport properties for GaN in bulk and quantum well structures. First, ensemble Monte Carlo calculations of steady-state electron drift velocity in bulk GaN are presented as a function of applied electric field for different lattice temperatures. At 300 K, the calculated peak steady-state drift velocity is 2.8×107 cm/s and the threshold field is 160 kV/cm. It is found that the peak steady-state electron drift velocity decreases only slightly by about 20% as the temperature increases from 300 to 600 K while the threshold field increases slightly by about 20%. Therefore, in addition to its high temperature stability, GaN has a low temperature coefficient making it ideal for high temperature applications. For electron transport in heterostructures, quantum mechanical calculations of the electron capture rate in GaN-based quantum wells as a function of well thickness are also presented. An oscillatory behavior of the electron capture rate as a function of quantum well thickness is observed. It is found that the electron capture time oscillates between 2 and 30 ps, which is about an order of magnitude greater than capture times in GaAs quantum wells. The amplitude of oscillations decreased as the well thickness increased. These results suggest that electron transport and carrier collection in GaN are efficient processes for improved electronic and optoelectronic devices.

Inelastic and elastic mechanisms of electron capture to a quantum well

We present a calculation of electron capture rates to a single quantum well (QW) taking full account of the quasibound 2D states, which are localized in the vicinity of a QW at energies above the barrier. To find the net capture rates we calculate the rates of capture, escape and carrier relaxation within the bound 2D states, assisted by both emission and absorption of optical phonons by an electron with arbitrary positive initial energy, as well as (quasi)elastic impurity and acoustic phonon scattering. The dependences of the capture rates of the non-degenerate electrons on the well width and temperature are discussed.

Effects of the Hyperfine Interaction on Orbital Electron Capture.

Electron capture from an unfilled atomic $s$ orbital is investigated theoretically. Analyses are carried out for two simple situations, a mirror transition of a nucleus with a nucleon in an outer ${s}_{\frac{1}{2}}$ state and a Gamow-Teller transition decreasing the nuclear spin by one unit. It is shown that the hyperfine interaction between electron and nuclear spins has a great influence on the rate of electron capture at temperatures small compared to the hyperfine splitting. The possibility of inducing electron capture with resonant electromagnetic fields is discussed.

Departures from the principle of superposition in silicon solar cells

The principle of superposition forms the theoretical basis on which the comparison of illuminated and dark current-voltage (I-V) characteristics of solar cells depends. Two cases predicted from computer simulations where the superposition principle does not apply in silicon p-n junction solar cells are reported. These predictions are confirmed experimentally with measurements taken on existing high-efficiency devices, and cannot be accurately described by previous explanations for departures from this principle. The first case, which is the more important in terms of operation under 1 sun illumination, occurs in cells where recombination via defect levels (Shockley–Read–Hall recombination) with unequal electron and hole capture rates dominates the I-V characteristics. The second case is evident at small forward voltages in almost all silicon solar cells. It is shown that the former is due to a saturation in the recombination rate, while the latter is the result of a bias-dependent modification of the carrier concentrations across the p-n junction depletion region which is different under illumination from that in the dark.

Carrier capture via confined phonons in GaAs-AlGaAs multiple quantum wells

We calculate the electron capture rates for GaAs-AlxGa1-xAs multiple quantum wells by the emission of confined phonons via electron-phonon (Frohlich) interaction. The carrier capture process as detected by optical measurements is discussed and we show the importance of a knowledge of the details of carrier excitation in order to obtain an appropriate interpretation of the experimental results. In particular we show that if the carriers are excited into a confined state with a large kinetic energy intrasubband transitions strongly influence the capture rates. We obtain remarkable agreement with several experimental results.

Electron capture rate constants for solutes in alkane liquids measured by transient photoconductivity

Electron capture rate constants for several solutes in isooctane, cyclohexane and n-hexane are reported. The method of measurement, which allows determination of the rate constants with an accuracy estimated to be better than 5%, involves the use of low power pulses of 337 nm light from a nitrogen laser, and detection of the transient conductivity of escaped electrons.

Resonance Electron Capture Rate Constants for Substituted Nitrobenzenes

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTResonance Electron Capture Rate Constants for Substituted NitrobenzenesW. B. Knighton, R. S. Mock, D. S. McGrew, and E. P. GrimsrudCite this: J. Phys. Chem. 1994, 98, 14, 3770–3776Publication Date (Print):April 1, 1994Publication History Published online1 May 2002Published inissue 1 April 1994https://doi.org/10.1021/j100065a036RIGHTS & PERMISSIONSArticle Views53Altmetric-Citations18LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (841 KB) Get e-Alerts

A new URCA process

Author(s): Aufderheide, MB; Fushiki, I; Fuller, GM; Weaver, TA | Abstract: In the course of a general study of electron capture and β-decay rates on fp shell nuclei which are abundant after core silicon burning in massive stars, we have found that the Fuller, Fowler and Newman β-decay rate are much stronger than generally has been realized. In fact, they can balance the capture rates during the conditions which are prevalent after core silicon burning, resulting in a new Urca process. The strength of the β-decay rates is a result of thermal population of the Gamow-Teller back resonance in the parent nucleus and the behavior of β-decay and electron capture Q-values for ensembles of nuclei in nuclear statistical equilibrium. All β-decay rate tabulations prior to Fuller, Fowler, and Newman neglected the contribution of back resonances and thus drastically underestimated the overall rate. We use a simple analytic model to explain this balancing. The full rates are coupled to a nuclear statictical equilibrium code to demonstrate the strength of the decays. One zone models are used to examine, in a self-consistent fashion, how these rates could affect the presupernova evolution of the iron core and hence the core collapse problem.

Search for important weak interaction nuclei in presupernova evolution

A search is made for the most important electron captures and beta-decays after core silicon burning in massive stars. A nuclear statistical equilibrium code is used to compute isotopic abundances. Electron capture and beta-decay rates are estimated for the 150 most abundant isotopes in a simplified fashion which generally includes the strongest transitions. These estimates are made for nuclei in the fp-shell and use techniques similar to Fuller, Fowler, & Newman (1982a), and are compared to them. The general behavior of the rate of change of Y(sub e) is examined. These methods are then used to follow a typical stellar trajectory, seeking the most important weak interactions during the formation of the iron core. Ranked lists of nuclei are given, to prioritize more detailed studies on individual nuclei. Beta-decays are found to ben an important modification to the evolution below the rate of change of Y(sub e) approx. = 0.46 as the core approaches a state of dynamic equilibrium between electron captures and beta-decays.

Recombination saturation effects in silicon solar cells

Significant interest has recently been shown in the use of dark and illuminated current-voltage (I-V) measurements for the characterization of high-efficiency silicon solar cells. Similar nonideal behavior, in the form of "humps" in dark I-V curves, has been observed by various research groups but apparently different interpretations of this effect given. In this paper we present detailed computer simulations of solar cells with defects (producing recombination centers within the bandgap) at a number of specific positions in the devices. It is found that a distinct shoulder (or "hump") occurs in the I-V characteristics when the recombination centers exhibit unequal electron and hole capture rates. Furthermore, it is shown that these shoulders are a result of the saturation of (Shockley-Read-Hall) recombination via the defect levels, which dominates behavior at low forward bias. As the bias voltage is increased, recombination in the defected region increases again, beyond the saturation level. The simulations show conclusively that the shoulders in the measured dark I-V curves of high-efficiency silicon solar cells produced at the University of New South Wales arise from the rear Si-SiO/sub 2/ interface.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Nonequilibrium occupancy of tail states and defects in a-Si:H: Implications for defect structure.

A detailed investigation of the electron and hole occupancy of tail states in undoped amorphous silicon (a-Si:H) as well as changes in the dangling-bond occupancy as a function of excitation intensity was carried out using light-induced electron-spin-resonance (LESR) measurements. For very thick films the band-tail electron and hole densities are not proportional. Over a wide range of excitation conditions the excess hole density is constant, suggesting the presence of charged defects with a density that is 5--10 times larger than the neutral defect density in annealed or as-grown a-Si:H. Light soaking increases mainly the neutral defect density. The dependence of the excess hole density on film thickness and absorption profiles indicates that this effect is a bulk property, which may be masked in thinner films by the comparatively high interface defect density. Model calculations of nonequilibrium occupation statistics confirm the experimental results. For a defect distribution that includes charged defects, the calculations suggest a very small positive LESR signature of the dangling bond, in spite of the high density of charged defects in the material, as a necessary consequence of the asymmetries observed between electron and hole capture rates and tail-state distributions. The calculations demonstrate that the lack of this signature does not imply a defect structure that contains predominantly neutral defects.

The 54,56Fe(n, p) 54,56Mn reactions at En = 97 MeV

Double-differential cross sections of the 54,56Fe(n, p) reactions have been measured at 97 MeV in the angular range 0–30° for excitation energies up to 40 MeV. The spectra have been decomposed into different multipolarities by a technique based on the use of sample angular distributions calculated within the distorted-wave Born approximation. From the identified Gamow-Teller strength, S β + values were obtained for 54Fe and 56Fe. Comparisons with available shell-model calculations of the GT strength were made. The results are important for models of supernova explosions since electron-capture rates, which are proportional to S β + , in 1f2p-shell nuclei affect the dynamics of the star. At higher excitation energies, the spectra are dominated by L = 1 strength in broad distributions with a maximum at about 12 MeV. Microscopic calculations based on the random-phase approximation were performed and compared with the experimental data.

Recombination centers in Czochralski-grown p-Si

A sensitive deep-level transient spectrometer operating in the range 300–600 K has been used to detect the defect responsible for the minority-carrier lifetime in p-type Czochralski-grown Si. The characteristics of this defect (energy level, barrier for hole capture, hole and electron capture rates, and concentration) have been determined. This level is, as expected, located near the middle of the forbidden gap. We verified that it is indeed responsible for the lifetime by a comparison between the calculated value and the result of direct measurements. If this defect is an impurity, it could be manganese.

Type II supernovae from 8-10 solar mass asymptotic giant branch stars

The final evolution of 8-10 M ○. stars in the AGB phase is recalculated to clarify whether these stars undergo collapse or thermonuclear explosion. New electron capture rates based on the shell model wave functions are employed for the evolution of the degenerate O+Ne+Mg core. It is found that the difference between the old and new rates are too small to alter the earlier conclusion that the O+Ne+Mg core collapses. The optical appearance of the resultant supernova explosion of AGB stars are discussed for observational diagnostics. In particular, a possible connection of the AGB star model with SN 1993J is examined