Lifetime Τp Research Articles

A simple formulation of the saturation current density in heavily doped emitters

In a non-uniformly and heavily doped emitter region of a bipolar transistor, the continuity equation and the minority-current equation cannot be solved exactly in closed form. This paper shows that the calculation of minority-carrier current density can be calculated by a simple approach. This approach is based on the average value of the equilibrium hole density p0, diffusion constant Dp, and lifetime τp of minority carriers and leads to two coupled differential equations of the first order. These equations can be solved easily and can give a simple expression for the current density. Three definitions of the average values of p0, Dp, and τp are used and lead to three expressions for the emitter current density. The latter is identical to the one established by Rinaldi using another mathematical analysis and gives very accurate results for a shallow emitter (W < 1 µm), irrespective of the range peak doping level N(W) and surface-recombination velocity S. On the other hand, the other two expressions lead also to accurate results for the current density depending on the value of the surface-recombination velocity, but cannot be used when N(W) is greater than 1020 cm–3 and W is superior to 0.1 µm. PACS Nos.: 72.10.–d, 72.20.–i

Carrier lifetime measurements in 10 kV 4H-SiC diodes

The minority carrier lifetime τp has been measured by open circuit voltage decay (OCVD) and current recovery time (CRT) (or Lax-Neustadter's) techniques in high-voltage (10 kV) 4H-SiC p+n0n+ diodes. The τp value measured by OCVD is 1.55 µs at room temperature, which is the highest reported value for 4H-SiC diodes. CRT measurements indicate a homogeneous distribution of carrier lifetime across the diode base without any deteriorated layer between the p+-emitter and the n0-base. Some limitations of lifetime measurements by CRT are discussed.

Volterra series analysis of the photocurrent in an Al/6T/ITO photovoltaic device

Abstract Using Volterra series we resolve analytically the non-linear rate equations describing the time evolution of the charge carrier concentration in an Al/6T/ITO cell when the device is illuminated and charge carriers are uniformally generated in the bulk at a known rate. We present the analytic expressions of the charge carrier densities in the bulk as a function of time and the expression for the photocurrent across the device. We show that the response contains a transient part and a permanent one. Using the first and the second order permanent response the photocurrent action spectrum of the device is obtained. We use these analytic results to suggest a method to evaluate experimentally the carrier lifetimes τn and τp and the Langevin bimolecular recombination rate. The system of rate equations has also been resolved numerically, using the Runge–Kutta method. Finally we compare successfully the measured photocurrent action spectrum with those obtained numerically and analytically.

General trends of positronium pick-off annihilation in molecular substances

The characteristics of positronium (Ps) annihilation in molecular substances (rangingfrom organic liquids to molecular solids), manifested through the observed lifetimes (τp) for the‘pick-off’ process and values of angular correlation(θ1/2) ofdecay gammas, can be shown to be simply related to the size (radiusR) of thecavity which the Ps creates in a liquid or finds in a molecular solid. The measured τp andθ1/2are in turn calculable from the wavefunction describing the Ps centre-of-massmotion, which is determined from the average potential experienced byit in the confining cavity. Thus the height of this repulsive barrier(U0)at the wall of this vacant region corresponding to different materials (with varyingR) canbe obtained by fitting experimental observations, namely τp and θ1/2.The model we use is an improved version of the usual spherical well description ofthe cavity (where we take the walls to be diffuse). It is found that the values ofU0and R,taking into account all available and relevant data for different molecularsubstances, fall on a universal curve. We attempt to explain the reason behindthis ‘universality’ by relating the potential to the Ps work-function in materials.Finally, the fit provides us with a very convenient linear relationship between the size(R)of the cavity and the pick-off lifetime (τp).

Fiber-Optic System for 77 K Phosphorescence Lifetime Measurements of Polycylic Aromatic Compounds in Shpol'skii Matrices

A fiber-optic system is reported for rapid, accurate, and precise 77 K measurements of relatively long phosphorescence lifetimes ( τp = 200 ms). The instrument employs stimulated Raman scattering in connection with a pulsed laser source for sample excitation. Phosphorescence decay waveforms are collected at maximum emission wavelengths with a single channel detection system and the aid of a LabVIEW based in-house program acting as a data acquisition instrument, data storage device, and data interpreter. By means of program looping, up to 20 decays are recorded from a frozen sample in less than 5 min. A cryogenic fiber-optic probe directly frozen into the sample matrix removes the random errors associated with the classic sample freezing procedure and provides excellent reproducibility of measurements. The relative standard deviations of τp measured from several polycyclic aromatic compounds varied between 0 and 4.30%. The simplicity of the experimental procedure, the rather large τ p differences observed from compounds within the same pollutant class, and the analytical figures of merit provide a solid foundation for pursuing low-temperature phosphorescence, time-resolved low-temperature phosphorescence, and low-temperature multidimensional luminescence analysis.

High hole lifetime (3.8 [micro sign]s) in 4H-SiC diodes with 5.5 kV blocking voltage

The hole lifetime τp in the n-base of 4H-SiC diodes with 5.5 kV blocking voltage has been measured in the temperature range 300–550 K. The τp increases monotonically in this temperature range from 0.6 to 3.8 µs. Forward current-voltage characteristics demonstrate a very high level of base modulation by minority carriers.

High minority-carrier lifetimes in GaAs grown on low-defect-density Ge/GeSi/Si substrates

A high bulk minority-carrier lifetime in GaAs grown on Si-based substrates is demonstrated. This was achieved by utilizing a step-graded Ge/GeSi buffer (threading dislocation density 2×106 cm−2) grown on an offcut (001) Si wafer, coupled with monolayer-scale control of the GaAs nucleation to suppress antiphase domains. Bulk minority-carrier lifetimes (τp) were measured using room-temperature time-resolved photoluminescence applied to a series of Al0.3Ga0.7As/GaAs/Al0.3Ga0.7As double-heterojunction structures doped n=1.1×1017 cm−3 with GaAs thicknesses of 0.5, 1.0, and 1.5 μm. A lifetime τp=7.7 ns was determined for GaAs grown on Si. The extracted interface recombination velocity of 3.9×103 cm/s is comparable to recombination velocities found for Al0.3Ga0.7As/GaAs interfaces grown on both GaAs and Ge wafers, indicating that the crosshatch surface morphology characteristic of strain-relaxed Ge/GeSi surfaces does not impede the formation of high-electronic-quality interfaces. These results hold great promise for future integration of III–V minority-carrier devices with Si wafer technologies.

Relaxation oscillations in single-frequency InAsSb narrow band-gap lasers

Relaxation oscillations have been investigated in A3B5 narrow band-gap semiconductor lasers. Based on wideband intensity noise measurements, the relaxation oscillation frequency has been observed up to 2 GHz for a 2 mW cw single-frequency InAsSb laser at a 3.4 μm wavelength. Laser parameters that influence the bandwidth, including the photon lifetime τp, the differential gain A and the spontaneous recombination lifetime τs were calculated from experimental data obtained at 95 K. We found τp in the range 0.7–1.9 ps, A was estimated to be (4.7–7.3)×10−6 cm−3 s−1 and τs=2.5–6.9 ns. Relaxation oscillations occurred beyond the modulation bandwidth required for typical applications of antimonide lasers in spectroscopy.

Perimeter governed minority carrier lifetimes in 4H-SiC p+n diodes measured by reverse recovery switching transient analysis

Minority carrier lifetimes in epitaxial 4H-SiC p+n junction diodes were measured via an analysis of reverse recovery switching characteristics. Behavior of reverse recovery storage time (ts) as a function of initial ON-state forward current (IF) and OFF-state reverse current (IR) followed well-documented trends which have been observed for decades in silicon p+n rectifiers. Average minority carrier (hole) lifetimes (τp) calculated from plots of ts vs IR/IF strongly decreased with decreasing device area. Bulk and perimeter components of average hole lifetimes were separated by plotting 1/τp as a function of device perimeter-to-area ratio (P/A). This plot reveals that perimeter recombination is dominant in these devices, whose areas are all less than 1 mm2. The bulk minority carrier (hole) lifetime extracted from the 1/τp vs P/A plot is approximately 0.7 µs, well above the 60 ns to 300 ns average lifetimes obtained when perimeter recombination effects are ignored in the analysis. Given the fact that there has been little previous investigation of bipolar diode and transistor performance as a function of perimeter-to-area ratio, this work raises the possibility that perimeter recombination may be partly responsible for poor effective minority carrier lifetimes and limited performance obtained in many previous SiC bipolar junction devices.

Stochastic Cellular Automata Models of Molecular Excited-State Dynamics

A probabilistic asynchronous cellular automaton described previously [Seybold, Kier, and Cheng, J. Chem. Info. Comput. Sci. 1997, 37, 386−391] has been applied to the description of molecular excited-state dynamics. The model simulates in a visual, time-dependent fashion the variations in ground- and excited-state populations that occur under stipulated probabilistic transition rules. Both pulse mode and steady-state conditions can be simulated. The deterministic values for the fluorescence lifetime (τf), the phosphorescence lifetime (τp), and the quantum yields of fluorescence (φf), triplet state formation (φT), and phosphoresence (φp) arise as limiting cases for large numbers of cells or large numbers of trials. Since each trial is an independent “experiment”, stochastic variations in the populations and properties can be estimated from repeated trials. Phenomena such as ground-state depletion and population inversion under high radiation intensity arise naturally within the model.

FOAM STABILIZING PROPERTIES OF SURFACTANTS DETERMINED AT CONSTANT AND VARIABLE PRESSURE IN THE FOAM LIQUID PHASE

The foam stabilizing ability of anionic, cationic and nonionic surfactants are studied with two methods, involving constant and variable capillary pressure in the foam liquid phase: the method of determining the foam lifetime at constant pressure and the Ross-Miles test. It is reported that the most important parameters for foam stability are the capillary pressure and the type of the foam films. It is proved that the foam lifetime τp determined at constant capillary pressure is a more precise and defined characteristics of foam stability.

Effects of NAD+ binding on the luminescence of tryptophans 84 and 310 of glyceraldehyde-3-phosphate dehydrogenase from Bacillus stearothermophilus.

The individual fluorescence and phosphorescence properties of W84 and W310 in Bacillus stearothermophilus glyceraldehyde-3-phosphate dehydrogenase were identified through the construction of a single tryptophan mutant (W84F) and by comparison of the emission between mutant and wild-type enzymes. The results show that the luminescence of W310 is red-shifted and substantially quenched relative to that of W84. It displays an average subnanosecond fluorescence lifetime (tau F) and a very short, 50 microseconds, room-temperature phosphorescence (RTP) lifetime (tau P). The perturbation of W310 luminescence is believed to arise from a stacking interaction with Y283. In contrast, W84 exhibits a fluorescence lifetime tau F of several nanoseconds and a long-lived phosphorescence lifetime tau P, typical of buried, unperturbed TrP residues. NAD+ binding to the tetrameric enzyme causes a 55% reduction of W310 fluorescence intensity together with a nearly complete quenching of its low-temperature phosphorescence. W84, which is located far from the nicotinamide moiety of NAD+, is much less affected by the binding of the coenzyme; the reduction in fluorescence intensity is 35%, and its phosphorescence intensity is unchanged. Another consequence of NAD+ binding is a significant decrease of the RTP lifetime tau P of W84, manifesting thereby a conformational change in the region of the coenzyme-binding domain. However, no change is observed in the RTP lifetime tau P of W310 located in the catalytic domain. These findings and those obtained at partial coenzyme saturation support the conclusions derived from high-resolution crystallographic structures [Skarzynski, T., & Wonacott, A. J., (1988) J. Mol. Biol. 203, 1097-1118] that the NAD(+)-induced conformational change is sequential and that subtle rearrangement in the structure of unligated subunits might be responsible for the negative cooperative behavior of NAD+ binding.

Measurement of hole diffusion length in heavily doped n-type silicon using SEM

The minority carrier diffusion length is measured in heavily doped silicon substrates of n+–n epitaxial wafers. Each sample is converted to a one-sided n+–p junction by diffusion of boron through the thin n-region. The junction is angle-lapped and the electron beam of a scanning electron microscope is used to inject excess electron–hole pairs in the n+–region. The hole diffusion length is deduced from the variation of the electron beam induced current (EBIC) as a function of distance from the junction. The minority carrier lifetime is obtained from the measured data using the values of hole diffusion constant in heavily doped n-type silicon. A plot of hole lifetime τp as a function of dopant concentration reveals that the band-to-band Auger effect becomes the dominant recombination mechanism for dopant concentrations in excess of about 2 × 1019 cm−3.

Transient network model for a multiphase polymeric fluid

Rheological behaviors of entangled polymer melts filled with rigid solid particles are phenomenologically described based on a Yamamoto type transient network model. Entanglement points are described by one kind of temporary junctions with a lifetime τp. Presence of fillers introduces another kind of network junctions with a lifetime τs, characteristic of adsorption of polymer chains at the filler surface. The degree of compatibility of filler surfaces with the polymer medium is characterized in terms of the adsorption time τs relative to the disentanglement time τp. By incorporating the effects of interfacial slippage into the deformation mechanism of the transient network, one can explain the variation of such rheological functions as shear viscosity with surface treatment of fillers and with molecular weight of the polymer matrix at a given filler volume fraction.

NUMERICAL ANALYSIS OF GUTs PREDICTIONS IN THE LIGHT OF RECENT RESULTS FROM LEP

We find numerical best fit for sin 2 Θw(MZ), unifying mass MX and the proton lifetime τp as the outcome of analysis where experimental values of Z boson mass MZ, strong coupling constant αs(MZ) and electromagnetic coupling α0(MZ) are taken as the only input parameters. It is found that simple nonsupersymmetric models are unlikely to be realistic ones. On the other hand, we find the best numerical fit: sin 2Θw(MZ = 0.2330 ± 0.0007 (theor.) ± 0.0027 (exp.) , [Formula: see text] yr for supersymmetric unified theories with three generations. The central values require, however, that the supersymmetric mass Λs≲300 GeV . Possibilities of increasing this limit as well as cases with four generations and threshold effects are also discussed. Compact formulas for theoretical and experimental uncertainties involved in the analysis are also produced.

PROTON LIFETIME: A DEATH BLOW TO THE MINIMAL SU(5) MODEL?

This paper points out that the minimal SU(5) model predicted value of proton lifetime (τp) falls far short of the experimental hints for the same mainly because of an unrealistic assumption made in the theoretical estimates for τp. Furthermore, it has been argued that this model is neither ruled out nor even disreputed despite the so far non-observation of proton decay for the reason discussed in this paper.

Combined ac photocurrent and photothermal reflectance measurements in semiconducting p-n junctions. II

The ac photocurrent (PC) and photothermal reflectance (PTR) spectroscopic response theory of a semiconducting p-n junction developed in part I [J. Appl. Phys. 66, ⧠⧠⧠⧠ (1989)] is applied to theoretical simulations to identify the influence of the various electronic and geometrical device parameters involved. The theory is further tested in an experimental configuration with a commercial solar cell. The experimental ac photocurrent and photothermal reflectance data exhibited satisfactory agreement with the theoretical model, and yielded, respectively, experimental values for the minority lifetimes τp and τn. A combination of PC and PTR data yielded reasonable Ln estimates.

Long minority hole diffusion length and evidence for bulk radiative recombination limited lifetime in InP/InGaAs/InP double heterostructures

Minority hole diffusion length, lifetime, and mobility for undoped InGaAs were measured at 297 and 77 K in metalorganic chemical vapor deposited InP/InGaAs/InP double heterostructures using continuous-wave and pulsed optical beam induced current techniques. A hole diffusion length Lp=140 μm, a lifetime τp=18.5 μs, and a mobility μp=425 cm2/(V s) were obtained at room temperature. To the best of our knowledge, these properties are the highest reported values for InGaAs. The measured carrier lifetime and its temperature dependence are consistent with bulk radiative recombination in undoped InGaAs.

EPR relaxations of aqueous Ni2+ ion

The experimental data for proton NMR relaxation and shifts in aqueous Ni2+ solutions at 25°C have been extended, especially to high fields (8.4 T). Infinite-order perturbation theory for the EPR relaxation in the Ni2+ and a generalized set of Solomon–Bloembergen equations for the proton–electron spin–spin interaction are applied to deduce the rms value Δ and the correlation time τv for the tensor D(t) in the zero field splitting or SDS term of the electron spin Hamiltonian. Both the Brownian rotation of the Ni(H2O)62+, with correlation time τϑ, and the mean lifetime τp of protons in this complex, before exchanging with the bulk, are very important in determining the functional relation of the proton relation times and shift to the corresponding EPR quantities. The best fit of the theory to the data implies the following values for the parameters for assumed values τϑ=25 psec and τp=32 μsec: Δ=2.6 cm−1, τv=2.2 psec, TlS=T2S=2.9 psec at zero magnetic field, proton contact shift in the complex=14 ppm, proton dynamical shift in the complex at zero field=2.1 ppm. The proton dynamical shift is a second-order effect in the dipolar electron–proton spin–spin interaction. The discussion includes the comparison of some of these parameters with earlier determinations as well as a summary of additional effects that might be included in a more realistic model.

Calculated efficiency of dye lasers as a function of pump parameters and triplet lifetime

The rate equations governing pulsed laser operation in dyes have been solved numerically and the results discussed for a variety of material parameters and pump conditions, especially for pulses in excess of 1 μsec. The resulting laser parameters have been considered as a function of the triplet lifetime and the pump peak power. Calculated values of the efficiency in the conversion of pump light into laser light have been derived in general, and applied specifically to the case of Rhodamine 6G. The calculated efficiencies have been compared with known experimental results on Rhodamine 6G under laser and flashlamp excitation. The agreement is generally good. Since the predicted efficiencies for laser operation in excess of a few hundred nanoseconds depend critically on the values of the triplet lifetime τp in solution, especially for τp in the range 100–300 nsec, the results of the calculations can be reliably compared to experiment only when precise values of the triplet lifetimes become experimentally available.