Effective Recombination Coefficient Research Articles

Optical analysis of III-nitride micro-light-emitting diodes with different sidewall treatments at low current density operation

In this work, the optical efficiency of III-nitride blue micro-LEDs (μLEDs) ranged from 5 × 5 to 60 × 60 μm2 with different sidewall treatments at low current density range was investigated. The results showed dielectric sidewall passivation using atomic layer deposition (ALD) has superior optical enhancement compared to conventional RF sputtering, where most of the enhancement occurred at low current density range. Additionally, the use of ALD sidewall passivation and chemical treatment offered significant efficiency improvement for different sizes of μLEDs at operating less than 1 A cm−2 and the devices without sidewall treatments did not emit light. The effect of sidewall treatments to the effective Shockley–Read–Hall (SRH) nonradiative recombination coefficient, or the effective A coefficient from the ABC model, was estimated. The effective SRH nonradiative recombination coefficient was suppressed by two orders of magnitude for devices with sidewall treatments compared to devices without sidewall passivation.

Case A or Case B? The Effective Recombination Coefficient in Gas Clouds of Arbitrary Optical Thickness

In calculations of the ionization state, one is often forced to choose between the Case A recombination coefficient α A (sum over recombinations to all hydrogen states) or the Case B recombination coefficient α B (sum over all hydrogen states except the ground state). If the cloud is optically thick to ionizing photons, α B is usually adopted on the basis of the “on-the-spot” approximation, wherein recombinations to the ground state are ignored because they produce ionizing photons absorbed nearby. In the opposite case of an optically thin cloud, one would expect the Case A recombination coefficient to better describe the effective recombination rate in the cloud. In this paper, I derive an analytical expression for the effective recombination coefficient in a gas cloud of arbitrary optical thickness which transitions from α A to α B as the optical thickness increases. The results can be readily implemented in numerical simulations and semi-analytical calculations.

Lower-ionosphere electron density and effective recombination coefficients from multi-instrument space observations and ground VLF measurements during solar flares

A new model to predict the electron density and effective recombination coefficient of the lower ionosphere under solar flare conditions is presented. This model relies on space-borne solar irradiance measurements in coincidence with ground recorded active transmissions of Very Low Frequency (VLF), (<30 kHz) signals. Use is made of the irradiance measured by broad-band radiometers onboard the satellites: GOES, SDO, and PROBA2. Measurements are made over succeeding and partly overlapping wavelength intervals of the instrument bandpass ranges altogether covering the range 0.1–20 nm. The aim is to determine the effectiveness of the particular instrument bandpass in producing changes in the ionization of the lower ionosphere (D-region) during solar X-ray flares. Ionization efficiency is evaluated using modelled Solar Spectral Irradiance for each flare separately and for each instrument as a function of its bandpass.The new model is based on coupling of the continuity equation with the Appleton relation and uses the concept of time delay – the time lag of the extreme VLF amplitude and phase behind the flare irradiance maximum. The solution of the continuity equation predicts the electron density time - height profile for 55–100 km altitude.An analysis of M to X class flares shows the flare-enhanced electron densities due to a particular ionizing wavelength domain are in good agreement for the case where irradiance is taken over the bandpass of (1) either GOES (0.1–0.8 nm) or SDO/ESP (0.1–7 nm) for up to 90 km (2) either SDO/ESP or PROBA2/LYRA (1–2 +6–20 nm) at heights above 90 km. The results agree within 22% for heights up to 90 km, and differ by at most a factor of 2 for heights above 90 km. Remarkable agreement is shown between measured and evaluated time delay; discrepancies are generally less than 8%. The effective recombination coefficient is deduced from the model itself and is found to be consistent with other independent estimates.

Effective radiative recombination coefficient of p-AlGaAs for varying aluminium concentrations

The effective radiative recombination coefficient of p-AlGaAs was experimentally determined for different sample geometries and compositions. Double-heterostructures with varying active layer thicknesses and aluminium concentrations between 0.08 and0.23 were grown using metalorganic vapor phase epitaxy. Combining injection-dependent and time-resolved photoluminescence measurements allowed to compute the effective radiative efficiency as a function of excess carrier density and to extract the effective radiative recombination coefficient. For increasing active layer thickness and for increasing aluminium content the effective radiative recombination coefficient decreases in agreement with numerical modelling. Optical interference effects, in particular for thin samples, make the results susceptible to small variations in sample thickness and additional layers underneath the double-heterostructure. The extracted non-radiative lifetimes imply the existence of three different defects, independent of composition and thickness, that are dominant in different carrier injection regimes. A sensitivity analysis of the presented method revealed that the low-injection carrier lifetime and the doping concentration are the two most critical input parameters whose combined uncertainties amplify the error on the obtained effective radiative recombination coefficient.

Reducing Shockley–Read–Hall recombination losses in the depletion region of a solar cell by using a wide-gap emitter layer

The conversion efficiency of a solar cell depends on the degree of non-radiative recombination, and thus efficiency improvements can also be achieved by reducing Shockley–Read–Hall (SRH) recombination losses. This type of loss depends not only on the crystal quality, but also on the device structure. A clear separation of the contributions of these factors would improve our understanding of the control of non-radiative recombination. In this work, we discuss the reduction of the integrated SRH recombination rate in the depletion region (USRHdep) by changing the emitter material, instead of fabricating a base layer with a higher crystal quality. First, we theoretically show that, by employing a suitable n-InGaP/p-GaAs heterojunction structure instead of a GaAs p–n homojunction, the integrated USRHdep can be reduced, because a significant part of the depletion region in the heterojunction is located in the wide-gap emitter material, which has a lower intrinsic carrier density. Then, the effective SRH recombination coefficient in the depletion region (Adep¯) is obtained from experiments, and the effect of the structural modification on USRHdep is analyzed. We are able to clearly assess the effect of the heterojunction structure on the non-radiative recombination because the grown samples exhibit the same radiative recombination loss. The analysis reveals that, in suitable heterojunction solar cells with an emitter layer containing a low intrinsic carrier concentration, Adep¯ (and thus also USRHdep) is effectively reduced.

On the use of recombination rate coefficients in hydrogen transport calculations

The commonly accepted picture for the uptake of hydrogen isotopes (HIs) from the gas phase across the surface into a metal with an endothermic heat of solution for HIs is that of dissociation followed by thermalisation in a chemisorbed surface state and finally overcoming a surface barrier to enter the metal bulk where the HIs occupy interstitial solute sites. To leave the metal bulk the HIs first transition to the chemisorbed surface state from which they then enter gas phase by recombining into a diatomic molecule. This model is generally attributed to the work of Pick and Sonnenberg from 1985. They clearly distinguish surface states and subsurface solute sites where the recombination flux is proportional to the square of the concentration of chemisorbed atoms due the diatomic nature of this Langmuir–Hinshelwood process. In an effort to compare their extended model with an earlier surface model by Waelbroeck, which uses an expression for the recombination flux proportional to the square of the sub-surface interstitial solute concentration, they derive an effective recombination coefficient. However, also with the so-derived Pick and Sonnenberg recombination coefficient, the Waelbroeck model is only applicable under certain conditions. But, due to its simplicity, it is often used in boundary conditions of diffusion trapping type calculations, generally ignoring whether or not these conditions are met. This paper will use the full Pick and Sonnenberg model implemented in the TESSIM-X code and in simplified algebraic approximations, to show the limits of applicability of the Waelbroeck–Ansatz in modelling hydrogen transport in metals foreseen for the first wall of magnetic confinement fusion devices.

Ionospheric Sluggishness: A Characteristic Time‐Lag of the Ionospheric Response to Solar Flares

AbstractThe term “sluggishness” was coined by E. V. Appleton in the 1950s to describe the time delay between peak irradiance at solar noon and the resulting peak in ionospheric electron density. Sluggishness can be understood as an inertial property of the ionosphere that manifests as a lag of the ionospheric response to a solar driver. As shown by Appleton, estimates of sluggishness can be used to study the chemistry of the lower ionosphere, of the D‐region in particular. In this study, for the first time, we have examined ionospheric sluggishness in terms of the time delay between the peak irradiance during a solar flare and the resulting peak in ionospheric electron density using HF instruments. Estimates of the delay are obtained using HF observations from riometers and SuperDARN radars that are primarily sensitive to absorption in the D‐region. Two new methods for measuring delay are introduced. Sluggishness is shown to be anti‐correlated with peak solar X‐ray flux and positively correlated with zenith angle and latitude. The choices of instrument, method, and reference solar waveband affect the sluggishness estimation. A simulation study was performed to estimate the effective recombination coefficient in the D‐region. The coefficient was found to vary by orders of magnitude with peak flare intensity. We argue that the variation in effective recombination coefficient with peak flare intensity is highly sensitive to changes in the negative and positive ion chemistry of the D‐region, which is sensitive to the incoming solar X‐ray and EUV radiation.

Constraining the Positive Ion Composition in Saturn’s Lower Ionosphere with the Effective Recombination Coefficient

Abstract The present study combines Radio and Plasma Wave Science/Langmuir Probe and Ion and Neutral Mass Spectrometer data from Cassini’s last four orbits into Saturn’s lower ionosphere to constrain the effective recombination coefficient α 300 from measured number densities and electron temperatures at a reference electron temperature of 300 K. Previous studies have shown an influx of ring material causes a state of electron depletion due to grain charging, which will subsequently affect the ionospheric chemistry. The requirement to take grain charging into account limits the derivation of α 300 to upper limits. Assuming photochemical equilibrium and using an established method to calculate the electron production rate, we derive upper limits for α 300 of ≲ 3 × 10−7 cm3 s−1 for altitudes below 2000 km. This suggests that Saturn’s ionospheric positive ions are dominated by species with low recombination rate coefficients like HCO+. An ionosphere dominated by water group ions or complex hydrocarbons, as previously suggested, is incompatible with this result, as these species have recombination rate coefficients &gt; 5 × 10−7 cm3 s−1 at an electron temperature of 300 K.

Measurement of the non-radiative minority recombination lifetime and the effective radiative recombination coefficient in GaAs

The combination of time-resolved (TR) and power-dependent relative (PDR) photoluminescence (PL) measurements reveals the possibility of separating the radiative and non-radiative minority carrier lifetimes and measuring the sample-dependent effective radiative recombination coefficient in direct bandgap semiconductors. To demonstrate the method, measurements on 2 μm thick p-type GaAs double-hetero structures were conducted for various doping concentrations in the range of 5x1016 and 1x1018 cm-3. With a photon recycling factor of 0.76 ± 0.04 the radiative recombination coefficient was determined to be (3.3±0.6)×10-10 cm3s-1 for the structures with a doping concentration below 1*1018 cm-3, whereas the effective radiative recombination parameter for an absorber thickness of 2 μm was directly measured to be (0.78±0.07) ×10-10 cm3s-1. For a doping concentration of 1×1018 cm-3, the radiative recombination coefficient decreases significantly probably due to the degeneracy of the semiconductor.

Variations in ionospheric D-region recombination properties during increase of its X-ray heating induced by solar X-ray flare

In this paper we present an analysis of parameters describing the effective recombination processes in the upper ionospheric D-region in the period of its additional heating by the X-radiation emitted during a solar X-ray flare. We present a procedure for calculation of the effective recombination coefficient and electron loss rate in the period when the X-radiation flux detected by the GOES satellite in the wavelength domain between 0.1 and 0.8 nm increases. The developed procedure is based on observational data obtained in the low ionospheric monitoring by the very low/low frequency radio waves and it is related to the considered area and time period. The obtained expressions are applied to data for the very low frequency signal emitted in Germany and recorded in Serbia during the solar X-ray flare detected by the GOES-14 satellite on May 5, 2010.

Effect of Electron Precipitations on the Effective Recombination Coefficient

The results of an analysis of the possible effect of auroral electron fluxes on the effective recombination coefficient αeff in the ionosphere are presented. It is shown that the αeff value in the E-region of the ionosphere is determined mainly by the physical-chemical properties of the medium. In the F1-layer of the ionosphere, the effective recombination coefficient becomes dependent on both the value of the energy flux and the type of the energy spectrum of the auroral electron flux.

Plasma Decay in the Afterglow of High-Voltage Nanosecond Discharges in Unsaturated and Oxygenated Hydrocarbons

Results of experimental and theoretical study of plasma decay in the afterglow of high-voltage nanosecond discharges in gaseous ethylene and dimethyl ether at room temperature and pressures from 2 to 20 Torr are presented. Using a microwave interferometer, the time behavior of the electron density in the range from 2 × 1010 to 3 × 1012 cm–3 during plasma decay is investigated. By processing the experimental data, the effective coefficients of electron–ion recombination as functions of the gas pressure are obtained. It is found that these coefficients substantially exceed the recombination coefficients of simple hydrocarbon ions. This distinction, as well as the increase in the effective recombination coefficient with pressure, is explained by the formation of cluster ions in three-body collisions, which recombine with electrons more efficiently than simple molecular ions. The coefficients of three-body conversion of simple molecular ions into cluster ions in the plasmas of ethylene and dimethyl ether, as well as the coefficients of recombination of electrons with cluster ions in these gases, are determined by analyzing the experimental data.

Photoionization Modeling of Titan's Dayside Ionosphere.

Previous modeling studies of Titan's dayside ionosphere predict electron number densities that are roughly a factor of 2 higher than those observed by the RPWS/Langmuir probe. The issue can equivalently be described as the ratio between the calculated electron production rates and the square of the observed electron number densities resulting in roughly a factor of 4 higher effective recombination coefficient than expected from the ion composition and the electron temperature. Here we make an extended reassessment of Titan's dayside ionization balance, focusing on 34 flybys between TA and T120. Using a recalibrated data set and by taking the presence of negative ions into account, we arrive at lower effective recombination coefficients compared with earlier studies. The values are still higher than expected from the ion composition and the electron temperature, but by a factor of ~2-3 instead of a factor of ~4. We have also investigated whether the derived effective recombination coefficients display dependencies on the solar zenith angle (SZA), the integrated solar EUV intensity (<80 nm), and the corotational plasma ram direction (RAM), and found statistically significant trends, which may be explained by a declining photoionization against the background ionization by magnetospheric particles (trends in SZA and RAM) and altered photochemistry (trend in EUV). We find that a series of flybys that occurred during solar minimum (2008) and with similar flyby geometries are associated with enhanced values of the effective recombination coefficient compared with the remaining data set, which also suggests a chemistry dependence on the sunlight conditions.

Effect of the spatial and temporal distribution of molecular ions in the ionospheric E-field on the behavior of NmF2 during the geomagnetic storm of March 17–23, 2015

We consider different approaches to calculating the ion composition in the lower ionosphere and use two methods for modeling the concentration of molecular ions. The first method allows us to solve an equation with an effective recombination coefficient for the total concentration of molecular ions, and the second method calculates their concentrations separately. Numerical experiments were performed using a global self-consistent model of the thermosphere, ionosphere, and protonosphere (GSM TIP) for the conditions of March 16–20, 2015. We show that the second method, which provides a more accurate approximation, leads to significant consequences. Due to a strong change in the calculated concentration of ions in the E-region, the conductivity varies thus affecting the qualitative and quantitative behavior of the electric dynamo field at low and equatorial latitudes. Even without mesospheric tides and the electric dynamo field in the F-region, the calculation of partial concentrations of molecular ions makes it possible to qualitatively reproduce the outburst of the eastern electric field observed at the geomagnetic equator and in its low-latitude vicinity. During geomagnetic disturbances, a more accurate approximation leads to significant changes in the pattern of electron density perturbations at the heights of the ionospheric F-region in comparison with that obtained with the calculated total concentration of molecular ions.

The effect of fuel oxidation on plasma decay in combustible mixtures excited by high-voltage nanosecond repetitively pulsed discharge

The results of the experimental study of high-voltage nanosecond repetitively pulsed discharge afterglow in propane:O2, ethane:O2 and H2:O2 mixtures were presented for room temperature and pressures from 2 to 6 Torr. Time-resolved electron density during the plasma decay was measured using a microwave interferometer for initial electron densities in the range between 3 × 1011 and 3 × 1012 cm−3 and the effective recombination coefficients were obtained. In hydrocarbon-containing mixtures, the rate of plasma decay varied nonmonotonously with the oxidation degree increase. The effective recombination coefficient (i) peaked when the amount of intermediate species was expected to be high and (ii) was independent of the number of voltage pulses (the effect of saturation) when oxidation was complete. In H2:O2 mixtures, the rate of plasma decay and the effective recombination coefficient increased monotonously with increasing oxidation degree. It was shown that plasma decay in completely oxidized fuel:O2 mixtures is close to that in water vapor. It was suggested that the nonmonotonous behavior of the effective recombination coefficient during plasma decay in hydrocarbon:O2 mixtures is explained by the production of some hydrocarbon intermediates in the oxidation processes that can favor plasma decay. The possible mechanisms of plasma decay acceleration in the oxidized mixtures were discussed.

Improving the twilight model for polar cap absorption nowcasts

During Solar Proton Events (SPE), energetic protons ionize the polar mesosphere causing HF radiowave attenuation, more strongly on the dayside where the effective recombination coefficient, αeff, is low. Polar cap absorption (PCA) models predict the 30 MHz cosmic noise absorption, A, measured by riometers, based on real-time measurements of the integrated proton flux-energy spectrum, J. However, empirical models in common use cannot account for regional and day-to-day variations in the day- and nighttime profiles of αeff(z) or the related sensitivity parameter, m=A/√J. Large prediction errors occur during twilight when m changes rapidly, and due to errors locating the rigidity cutoff latitude. Modeling the twilight change in m as a linear or Gauss error-function transition over a range of solar-zenith angles (χl < χ < χu) provides a better fit to measurements than selecting day or night αeff profiles based on the Earth-shadow height. Optimal model parameters were determined for several polar cap riometers for large SPEs in 1998-2005. The optimal χl parameter was found to be most variable, with smaller values (as low as 60°) post-sunrise compared with pre-sunset, and with positive correlation between riometers over a wide area. Day and night values of m exhibited higher correlation for closely spaced riometers. A nowcast simulation is presented in which rigidity boundary latitude and twilight model parameters are optimized by assimilating age-weighted measurements from 25 riometers. The technique reduces model bias, and root-mean-squared errors are reduced by up to 30% compared with a model employing no riometer data assimilation.

Changes in the D region associated with three recent solar eclipses in the South Pacific region

AbstractWe estimate D region changes due to 22 July 2009 total solar eclipse (SE), 13–14 November 2012 total SE, and 9–10 May 2013 annular SE, using VLF navigational transmitters signal observations at Suva, Fiji. The North West Cape (NWC) signal (19.8 kHz) showed an amplitude and phase decrease of 0.70 dB and 23° during November SE and 2.0 dB and 90° during May SE. The modeling using Long Wave Propagation Capability code for NWC‐Suva path during November and May SEs showed an increase in average D region reflection height (H′) and sharpness factor (β) by 0.6 and 0.5 km and 0.012 and 0.015 km−1, respectively. The July total SE for JJI‐Suva path showed an increase in H′ of 1.5 km and a decrease in β of 0.055 km−1. The decrease in the electron density calculated using SE time H′ and β is maximum for July total SE and minimum for May annular SE. The effective recombination coefficient estimated from the decay and recovery of signal phase associated with May annular SE was higher (27%) than normal daytime value 5.0 × 10−7 cm−3 s−1 and varied between 1.47 × 10−6 and 1.15 × 10−7 cm−3 s−1 in the altitude 70 to 80 km. Morlet wavelet analysis of signals amplitude shows strong wave‐like signatures (WLS) associated with three SEs with period ranging 24–66 min, but the intensity and duration of WLS show no clear dependence on SE magnitude and type. Apart from the cooling spot, the eclipse shadow can also generate WLS associated with atmospheric gravity waves.

Effects of catalytic recombination on the surface of metals and quartz for the conditions of entry into the Martian atmosphere

Heat transfer to water-cooled surfaces of metals and quartz is studied experimentally in subsonic jets of dissociated carbon dioxide at a stagnation pressure of 80 hPa and enthalpy of 9 and 14 MJ/kg, corresponding to the descent conditions of the ExoMars space vehicle into the Martian atmosphere, using an RF induction plasmatron at the Institute for Problems in Mechanics, Russian Academy of Sciences. The measurements of heat fluxes to surfaces of different materials showed the significant effect of the catalytic properties of surfaces that can be arranged in the following descending order of the heat flux: silver, copper, stainless steel, quartz. The effect of strong modification of the silver surface is recorded during the tests; the maximum value of the heat flux is achieved after 15-min exposure of the surface to the jet. In the computational analysis of heat transfer, we used a two-parameter model of heterogeneous recombination of O atoms and CO molecules at the surface. With this model, the effective recombination coefficient of CO molecules is determined on the water-cooled surfaces of quartz and stainless steel, based on the experimental data on heat fluxes.

Assimilation of real-time riometer measurements into models of 30 MHz polar cap absorption

Space weather events may adversely affect high frequency (HF) radio propagation, hence the ability to provide nowcasting and forecasting of HF radio absorption is key for industries that rely on HF communications. This paper presents methods of assimilating 30 MHz radio absorption measurements into two types of ionospheric polar cap absorption (PCA) model to improve their performance as nowcasting tools. Type 1 models calculate absorption as m times the square root of the flux of solar protons above an energy threshold, E t . Measurements from 14 riometers during 94 solar proton events (1995–2010) are assimilated by optimising the day and night values of m by linear regression. Further non-linear optimisations are demonstrated in which parameters such as E t are also optimised and additional terms characterise local time and seasonal variations. These optimisations reduce RMS errors by up to 36%. Type 2 models incorporate altitude profiles of electron and neutral densities and electron temperatures. Here the scale height of the effective recombination coefficient profile in the D-region is optimised by regression. Furthermore, two published models of the rigidity cut-off latitude (CL) are assessed by comparison with riometer measurements. A small improvement in performance is observed by introducing a 3-h lag in the geomagnetic index K p in the CL models. Assimilating data from a single riometer in the polar cap reduces RMS errors below 1 dB with less than 0.2 dB bias. However, many high-latitude riometers now provide absorption measurements in near-real time and we demonstrate how these data may be assimilated by fitting a low-order spherical harmonic function to both the measurements and a PCA model with optimised parameters.

Determination of effective recombination coefficient by thermodesorption method

The degassing by thermodesorption method (TDS) of structural material sample previously hydrogen saturated is considered. The mathematical model of TDS-experiment and parametric identification method are presented. The data for nickel and tungsten in numerical simulations are used. The two-peak problem and effect (“derivatives”) of diffusion and desorption parameters on the TDS-spectrum are illustrated. Diffusion coefficient is known but the recombination parameters on surface remain to be estimate. The estimation algorithm needs no special mathematical software and allows to scan a material in wide temperature range. Integral smoothing processing of measurements is applied in the algorithm to secure its noise resistance.