Excess Lifetime Research Articles

Optical properties of disperse-red-1-doped nematic liquid crystal

We report on the optical properties of nematic liquid crystal cells containing the commercially available liquid crystal E7 doped with the azo dye Disperse Red 1 at concentrations ranging from 0.3% to 1.0%. From polarized absorption measurements an order parameter of S=0.569 was obtained, thus showing that the dye molecules align themselves very well with the liquid crystal's director axis. Optical index gratings were written in the material by overlapping two coherent laser beams at 532 nm. Self-diffraction of the writing beams and diffraction of a probe beam at 670 nm were measured. First-order self-diffraction efficiencies and rise times were found to be linearly dependent upon the dye concentration with a maximum efficiency of 3.29% and a minimum rise time of 10.6 ms. The non-linear index coefficient, n 2, was calculated to be 7.6 × 10-3cm2W-1. The data are consistent with a grating formation based on trans–cis photoisomerism of the dye molecules that leads to a reorientation of the liquid crystal phase. These photonic devices are functional without the application of any external field, they are easily prepared, and have lifetimes in excess of two years without any indication of degradation when stored at ambient temperature.

The covariance of the number of renewals in a fixed time and the ensuing excess life

The covariance of the number of renewals in a fixed time N t and the ensuing excess life time Y t is derived using matrix-analytic methods for the stationary PH-renewal process. Specific results for the Erlang and hyperexponential processes are provided to illustrate the ease of computation. Properties concerning the sign and the behavior of the covariance as t→∞ are provided throughout. Parameter estimation for renewal processes which cannot be fully observed serves as the motivation for our derivations. These statistical applications as well as links to estimation for service time distributions in queues shed light on the type of problems for which the covariance is useful.

Experimental Detection of Tetraoxygen.

Although suggested by Lewis in 1924 and theoretically predicted, O4 has so far proved extremely elusive, defying all attempts at the positive experimental detection of a bound, intact O4 species. The search has now been brought to an end by the conclusive proof, achieved by neutralization-re-ionization mass spectrometry, of the existence of intact O4 as a gas-phase species with a lifetime in excess of 1 μs, whose dissociation requires overcoming a barrier of the order of 10 kcal mol-1 .

Experimental Detection of Tetraoxygen

Although suggested by Lewis in 1924 and theoretically predicted, O4 has so far proved extremely elusive, defying all attempts at the positive experimental detection of a bound, intact O4 species. The search has now been brought to an end by the conclusive proof, achieved by neutralization-re-ionization mass spectrometry, of the existence of intact O4 as a gas-phase species with a lifetime in excess of 1 μs, whose dissociation requires overcoming a barrier of the order of 10 kcal mol-1 .

Discovery and characterization of atmospherically relevant inorganic species by structurally diagnostic mass spectrometric techniques

The role of structurally diagnostic techniques such as Mass-analyzed Kinetic Energy (MIKE), Collisionally Activated Dissociation (CAD), Neutralization Reionzation (NR) and Fourier-Trena form Ion Cyclotram Reionization (FT-ICR) mass spectrometry, complemented by theoretical calculations, in atmospheric chemistry is illustrated by representative examples. The results outlined include the recent experimental discovery of the (H2O+O2−) charge-transfer complex, long postulated as the key agent in atmospheric photonucleation theory. Next, the successful application of NR mass spectrometry to the detection of hydrogen trioxide, HOOO, as a gaseous species with a lifetime in excess of 1 μs at 298 K is discussed. The radical, whose role in the stratospheric ozone depletion and in the formation of the excited OH radicals responsible for night sky afterglow had long been postulated, defied all previous efforts of experimental detection. A new ionic route to tropospheric N2O from electrical discharges associated with thunderstorms and power-line coronas has been reported, involving the intermediacy of isomeric [N2O3]+ complexes. Finally, the new OSOSO sulfur oxide and its cation have been detected and characterized, and their presence in the atmosphere of Io, consisting of SO2 and SO, is briefly discussed.

Risk assessment for 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) based on an epidemiologic study.

The International Agency for Research on Cancer (Lyon, France) recently concluded that 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) is a human carcinogen. There have been few human studies and risk assessments with quantitative exposure data. The authors previously conducted exposure-response analyses based on estimated external TCDD exposure for 3,538 US male chemical workers and found a positive trend for all cancer with increasing cumulative exposure. In the present study, 1988 data from 170 workers with both estimated external exposure and known serum TCDD levels were used to derive the relation between the two. This derived relation was used to estimate serum TCDD levels over time for all 3,538 workers, and new dose-response analyses were conducted by using cumulative serum level. A positive trend (p = 0.003) was found between estimated log cumulative TCDD serum level and cancer mortality. For males, the excess lifetime (75 years) risk of dying of cancer given a TCDD intake of 1.0 pg/kg of body weight per day, twice the background intake, was an estimated 0.05-0.9% above a background lifetime risk of cancer death of 12.4%. Data from this cohort are consistent with another epidemiologic risk assessment from Germany and support recent conclusions by the US Environmental Protection Agency.

Recombination lifetimes in undoped, low-band gap InAsyP1−y/InxGa1−xAs double heterostructures grown on InP substrates

High-quality, thin-film, lattice-matched (LM) InAsyP1−y/InxGa1−xAs double heterostructures (DHs) have been grown lattice mismatched on InP substrates using atmospheric-pressure metalorganic vapor-phase epitaxy. The low-band gap InxGa1−xAs layers in the DHs have room-temperature band gaps that range from 0.47 to 0.6 eV. Both the optical and electronic properties of these films have been extensively measured. The band-to-band photoluminescence is quite strong and comparable to that found for LM InP/In0.53Ga0.47As DHs grown on InP. Recombination lifetime measurements of undoped DH structures show minority-carrier lifetimes in excess of 1 μs in most cases. The earlier properties make the band gap-flexible InAsyP1−y/InxGa1−xAs DH system attractive for applications in high-performance, infrared-sensitive devices.

High quality GaAs qrowth by MBE on Si using GeSi buffers and prospects for space photovoltaics

III-V solar cells on Si substrates are of interest for space photovoltaics since this would combine high performance space cells with a strong, lightweight and inexpensive substrate. However, the primary obstacles blocking III-V/Si cells from achieving high performance to date have been fundamental materials incompatabilities, namely the 4% lattice mismatch between GaAs and Si, and the large mismatch in thermal expansion coefficient. In this paper, we report on the molecular beam epitaxial (MBE) growth and properties of GaAs layers and single junction GaAs cells on Si wafers which utilize compositionally graded GeSi Intermediate buffers grown by ultra-high vacuum chemical vapor deposition (UHVCVD) to mitigate the large lattice mismatch between GaAs and Si. Ga As cell structures were found to incorporate a threading dislocation density of 0.9-1.5 x 10 (exp 6) per square centimeter, identical to the underlying relaxed Ge cap of the graded buffer, via a combination of transmission electron microscopy, electron beam induced current, and etch pit density measurements. AlGaAs/GaAs double heterostructures wre grown on the GeSi/Si substrates for time-resolved photoluminescence measurements, which revealed a bulk GaAs minority carrier lifetime in excess of 10 ns, the highest lifetime ever reported for GaAs on Si. A series of growth were performed to ass3ss the impact of a GaAs buffer to a thickness of only 0.1 micrometer. Secondary ion mass spectroscopy studies revealed that there is negligible cross diffusion of Ga, As and Ge at he III-V/Ge interface, identical to our earlier findings for GaAs grown on Ge wafers using MBE. This indicates that there is no need for a buffer to bury regions of high autodopjing,a nd that either pn or np configuration cells are easily accomodated by these substrates. Preliminary diodes and single junction Al Ga As heteroface cells were grown and fabricated on the Ge/GeSi/Si substrates for the first time. Diodes fabricated on GaAs, Ge and Ge/GeSi/Si substrate show nearly identical I-V characteristics in both forward and reverse bias regions. External quantum efficiencies of AlGaAs/GaAs cell structures grown on Ge/GeSi/Si and Ge substrates demonstrated nearly identical photoresponse, which indicates that high lifetimes, diffusion lengths and efficient minority carrier collection is maintained after complete cell processing.

Preparation and optical properties of a poly(N-vinylcarbazole) material

We report on the photorefractive properties of a stable polymeric material based on the host polymer, poly(N-vinylcarbazole). Two-beam coupling and four-wave mixing experiments were used to measure the performance of devices constructed by sandwiching the material, composed of host, plasticizer, photo-sensitizer, and non-linear optical chromophore, between transparent electrodes. A steady-state diffraction efficiency of 60% and a two-beam coupling gain of 22 cm−1 in excess of the absorption of 4 cm−1 were observed at the laser's operating wavelength of 670 nm. The devices are easily prepared and have lifetimes in excess of one year with no indication of degradation when stored at ambient temperatures.

Effect of Laser Pulse Width on Ionization Efficiency in Supersonic Beam-Multiphoton Ionization-Mass Spectrometry

The efficiency in multiphoton ionization is calculated for various molecules with different lifetimes, based on a kinetic model using rate equations. The results are strongly dependent on the types of resonance ionization, i.e., ionization through congested intermediate levels or ionization through a discrete intermediate level. In the former case, a laser pulse shorter than the lifetime is always preferred for efficient ionization. In the latter case, a 10-ps laser pulse provides the maximum ionization efficiency for a molecule with a lifetime in excess of 10 ps. For a molecule having a lifetime shorter than 10 ps, the use of a laser whose pulse width is adjusted to the same value as the lifetime is suggested. The results obtained by a computer simulation are in reasonably good agreement with the experimental data obtained for monohalogenated benzenes.

An order( N) tight-binding molecular dynamics study of intrinsic defect diffusion in silicon

We present calculations of the intrinsic vacancy and interstitial diffusivities, D V and D I , in silicon using order( N) tight-binding molecular dynamics simulations. Vacancy diffusion was found to occur rapidly, with a diffusivity of around 10 −4 cm 2/s in the temperature range 900–1200°C. Interstitial diffusion was found to be a factor of 10–100 times slower than vacancy diffusion, being on the order of 10 −5 cm 2/s for the same temperature range. These diffusivities have the same order of magnitude as previous molecular dynamics calculations performed with both classical and Car-Parrinello models. The interstitial diffusion was found to occur via two different paths, one involving motion of a single interstitial down the open (110) channels in the lattice, and another involving an intermediate 〈110〉 split interstitial which facilitates the interstitial crossing from one (110) channel to another. Within the tight-binding model we use, the split interstitial path is more important than drift of a single interstitial at higher temperatures (here, above around 1000°C). The reverse is true below this temperature, with few, if any, formations of split interstitials and a diffusion dominated by traversal down the open channels of the lattice. New LDA data shows that the energetic advantage of the split interstitial over the tetrahedral interstitial is smaller than previously calculated, lending credence to the tight-binding results. The split interstitials were found to be relatively long-lived (in some cases, lifetimes in excess of 15 ps), even in a potential that favors tetrahedral interstitial formation. In order to perform these calculations, we developed a constant N elec version of the forces in the Goedecker and Colombo O( N) tight-binding algorithm originally written for systems with a constant chemical potential. Omission of this correction can lead to errors approaching 40% in the forces.

Fotólise no estado estacionário e com pulso de laser de 1-benzociclanonas e de seus derivados <FONT FACE=Symbol>a</font>,<FONT FACE=Symbol>a</font> -dimetilados

Laser excitation of 0.01 M solutions of 1-indanone (Ia), 1-tetralone (Ib), 1-benzosuberone (Ic), and their a,a -dimethyl derivatives IIa-c, respectively, in benzene, produced transients with maximum absorption at 425 nm, and lifetimes ranging from 62 ns (IIa) to 5.5ms (Ic). Quenching studies using well known triplet quenchers such as 1,3-cyclohexadiene and oxygen demonstrated the triplet nature of these transients. In the presence of hydrogen donors, such as 2-propanol, the triplet state decay of the ketones Ia-c leads to the formation of the corresponding ketyl radicals, i.e. IIIa-c, which show absorption spectra very similar to the parent ketone, with lmax at 430 nm and lifetime in excess of 20 ms. Steady state irradiations show that the a,a -dimethyl ketones IIa and IIc form ortho-alkyl benzaldehydes probably derived from an initial a-cleavage of the corresponding triplet excited states.

A Novel Processing Technology for High‐Efficiency Silicon Solar Cells

A novel simultaneous boron and phosphorus diffusion technique is presented to produce simple, high‐efficiency silicon solar cells in one thermal cycle. This technique uses boron and phosphorus spin‐on dopant films to fabricate limited solid doping sources out of dummy silicon wafers. This approach results in the delivery of a fixed dose of or to the diffused sample. The resulting diffusion glass is extremely thin (∼60 Å), which allows for the in situ growth of a passivating thermal oxide without increasing the solar cell reflectance. Reverse saturation current density measurements show that the in situ oxide passivation for a light boron and phosphorus diffusion provides excellent passivation properties, resulting in values in the range. Measurements of the bulk minority carrier lifetime show that by fabricating separate boron solid sources, trace impurities in the spin‐on dopant film are not transported to the diffused sample. This filtering action is shown to result in bulk lifetimes in excess of 1 ms for silicon doped indirectly from the source wafers but gives much lower lifetimes (∼6 μs) for the wafers on which the boron spin‐on film was directly applied. This process was validated by fabricating in situ oxide passivated, solar cells in one high‐temperature cycle incorporating several high‐efficiency features including surface texturing and a back side reflector, resulting in confirmed efficiencies in the 19–20% range. © 1999 The Electrochemical Society. All rights reserved.

Determination of the excess carrier lifetime in the collector region of silicon power bipolar transistors

An accurate nondestructive method to determine the excess carrier lifetime in the collector region of silicon n/sup +/-p-v-n/sup +/ power bipolar transistors is presented for the first time. Based on the measurement of the common-emitter collector characteristics and the collector-base junction C-V characteristics of the transistors, this method is also very simple and practical. The calculation results show that the excess carrier lifetime determined using this method is almost the same (with 1% difference) as that determined using the open circuit voltage decay (OCVD) technique with the emitter removed.

Mass spectrometry of di-tert-butylnaphthalenes and benzyl- tert-butylnaphthalenes: hydrogen migration and intermediate ion–molecule complexes in tert-butylated naphthalene ions

The mass spectrometric fragmentations of isomeric di-tert-butylnaphthalenes 1-6 and benzyl-tert-butylnaphthalenes 8 and 9 have been studied using deuterated derivatives and the methods of tandem mass spectrometry. The molecular ions of 1-6, 8 and 9 decompose almost exclusively by the loss of a methyl radical. The unimolecullar fragmentations of the resulting substituted 2-naphthyl-2-propyl cations 1a-6a, 8a and 9a show analogies to the fragmentation of unsubstituted 2-naphthyl-2-propyl cations but exhibit an interesting dependence of the reactions on the relative orientation of the tert-butyl or the benzyl substituent. Thus, the losses of ethene and of a methyl radical are observed as for the unsubstituted 2-naphthyl-2-propyl cation ions by rearrangements of the ionized propyl side chain. However, in the case of tert-butyl substituted 2-naphthyl-2-propyl cations 1a-6a, the loss of methyl increases distinctly if a naphthoquinodimethane radical cation can be formed by loss of methyl from the tert-butyl substituent, This is proven by D-labeling and indicates a particular stability of the quinoid radical cations. Further, the mass-analyzed ion kinetic energy spectra of the cations 1a-6a, 8 and 9 are characterized by intense signals owing to the tert-butyl cation or benzyl cation. The exception is metastable 4a which expels almost exclusively propene, This latter fragmentation is attributed to reactive interactions between the two side chains in a crowded 2,3-position at the naphthalene ring of 4a, The release of cations corresponding to the tert-butyl or benzyl substitueut from metastable substituted 2-naphthyl-2-propyl cations occurs by proton transfer from the ionized propyl side chain to the naphthalene ring, followed by protolytic cleavage of the substituent and involves the formation of intermediate ion-molecule complexes. This is shown in the case of tert-butylated ions 1a-6a by proton transfer from the tert-butyl cation to the naphthalene moiety and subsequent elimination of isobutene, In the case of the benzylated ions 8a and 9a, the benzyl cation within the complex abstracts a hydride and dissociates as a toluene molecule. Independent proof of intermediate ion-molecule complexes, consisting of a tert-butyl or a benzyl cation and a propenylnaphthalene molecule, comes from the observation of unexpected fragmentations which require addition of the carbenium ion within the complex to the double bond of the propenyl side chain. Both for the tert-butyl cation and for the benzyl cation, the abundance of these intermolecular reactions exhibits a hitherto unknown dependence on the orientation of the two side chains and are most intense in the case of a 1,4-orientation. A possible explanation is a dependence of the excess energy and life time of the intermediate ion-molecule complex on the structure of the precursor.

Defect induced shortening of excess carrier lifetime in single period nipi structures

High sensitivity for detecting optically generated excess carriers in the mid infrared spectrum is offered by nipi structures of PbTe. The built in nipi potential separates the electron–hole pairs in real space which leads to a high excess carrier lifetime τ. The temperature dependence of τ in PbTe single period nipi structures grown on BaF2 substrates under repeated thermal cycling ranging from 300 to 12 K was studied in the present paper. The typical behaviour of the lifetime τ deviates from theory. This was attributed to the fact that the lattice mismatch and the different thermal expansion of BaF2 and PbTe generate a temperature dependent strain which causes strain induced lattice defects. Besides misfit dislocations at the interface, threading dislocations are generated, which form channels for enhanced recombination of the excess carriers. A special temperature cycling technique is presented which can change the number and the arrangement of the strain induced defects and leads to higher excess carrier lifetimes τ at low temperatures.

Field emission properties of multiwalled carbon nanotubes

Field emission properties of multiwalled carbon nanotubes, in the form of both single tips and films, have been investigated. Nanotubes compare very favourably with other field emission sources, as they show low operating voltages and produce high current densities. Lifetimes in excess of 100 h in continuous operation at constant applied voltage were observed. Our measurements suggest that the good field emission properties are largely due to small tip size and to a non-metallic density of states at the tip.

Fully integrated magnetically actuated micromachined relays

A fully integrated magnetically actuated micromachined relay has been successfully fabricated and tested. This particular device uses a single-layer coil to actuate a movable upper magnetically responsive platform. The minimum current for actuation was 180 mA, resulting in an actuation power of 33 mW. Devices have been tested which can make and break 1.2 A of current through the relay contacts when the relay is electromagnetically switched. Operational lifetimes in excess of 850000 operations have been observed. Contact resistances as low as 22.4 m/spl Omega/ have been observed under electromagnetic actuation. Magnetic and structural finite-element (FE) simulations have been performed using ANSYS to calculate both the actuation and contact forces.

Polymer Light‐Emitting Diode Displays — Device Performance and Applications

ABSTRACTThe emerging technology based around polymer light‐emitting diodes (PLEDs) promises to have many advantages over existing electroluminescence technologies. Lightweight, battery operated, and highly visible pixellated displays may be beneficial for a broad range of applications in portable electronics, ranging from simple alphanumeric displays to high information content flat panel displays.PLED displays have now reached a level of performance that is attractive for commercial applications, with operating lifetimes in excess of 10,000 hrs, luminance levels of several hundred cd/m2 and battery‐compatible operating voltages. In this talk we discuss the lifetime and readability issues of passively‐addressed matrix displays developed at UNIAX Corporation.

X-ray photoelectron spectroscopic investigation of the durability of siloxane plasma polymer coatings in a steam atmosphere

Thin-film plasma polymer coatings were deposited on to titanium, stainless steel and 70/30 copper-nickel substrates from hexamethyldisiloxane (HMDSO) as potential permanent promoters of dropwise condensation of steam. Long-term durability condensation tests showed that HMDSO films on titanium exposed to low-velocity steam performed well with some lifetimes in excess of 12 000 h. X-ray photoelectron spectroscopy (XPS) was used to monitor the dependence of chemical composition of the films on the deposition conditions and it was shown that composition was not critically dependent on deposition conditions. Additionally, XPS was used to monitor chemical composition before and after exposure of specimens to steam in an attempt to identify the failure mechanisms for films which had ceased to sustain dropwise condensation over their entire surface. Furthermore, short tube sections were coated with plasma-polymerized HMDSO and exposed to steam at elevated velocities. In this case the durability was poor and this is attributed to water droplet impingement erosion on the tube surfaces. This was confirmed by a subsequent test where no water droplets were present in the steam flow and the coating lifetime was seen to increase.