Decay Lifetime Measurements Research Articles

Defect-related versus excitonic visible light emission from ion beam synthesized Si nanocrystals in SiO2

Two sources of room temperature visible luminescence are identified from SiO2 films containing ion beam synthesized Si nanocrystals. From a comparison of luminescence spectra and photoluminescence decay lifetime measurements between Xe+-implanted SiO2 films and SiO2 films containing Si nanocrystals, a luminescence feature attributable to defects in the SiO2 matrix is unambiguously identified. Hydrogen passivation of the films selectively quenches the matrix defect luminescence, after which luminescence attributable to Si nanocrystals is evident, with a lifetime on the order of milliseconds. The peak energy of the remaining luminescence attributable to Si nanocrystals ‘‘redshifts’’ as a function of different processing parameters that might lead to increased nanocrystal size and the intensity is directly correlated to the formation of Si nanocrystals. Upon further annealing hydrogen-passivated samples at low temperatures (<500 °C), the intensity of nanocrystal luminescence increases by more than a factor of 10.

Identification Possibility of Metallic Impurities in p‐Type Silicon by Lifetime Measurement

The prospect of the identification of metallic impurities in p‐type silicon has been studied using the photoconductive decay lifetime measurement technique at various high excitation levels (1011 to 1.5 × 1013 photon/pulse). The excitation level dependence of the recombination lifetime shows unique characteristics for recombination centers attributed to Al, Cr, or Fe related levels. The unambiguous identification of iron and chromium is performed by the analysis of the excitation level dependence curves measured before and after a low temperature (180°C) thermal treatment. The calculation of the iron concentration in the presence of other recombination centers is discussed. Contrary to the low excitation case the determination of iron concentration from the lifetime measured at a high excitation level requires only one correlation constant for a wide range of resistivity.

Luminescence of strained Si 1− xGe x/Si quantum wells and microstructures

We present a study on luminescence of strained Si 1− x Ge x /Si heterostructures, highlighting features characteristic of “indirect gap” strained Si 1− x Ge x /Si quantum wells (QWs). It is shown that the QW alone cannot be viewed as an independent system isolated from Si barriers. Rather, the important role of Si barrier layers is addressed on the basis of luminescence decay lifetime measurements and steady-state luminescence observations. Conversely, the QW should not stand as a strictly two-dimensional system and a volume effect is playing a major role in the interpretation of optical properties of the QWs when the excitation light is in the energy range above the Si barrier band gap. This is in marked contrast to the direct gap materials system and reflects the intrinsic aspect of the “indirect gap” nature of the SiGe QW systems. The Si layer quality was further assessed in terms of spectral intensity of double QW systems by systematically changing the cap Si layer thickness. It will be finally shown that an appropriate sample geometry taking account of carrier generation and transport leads to the development of quantum confined photoluminescence at room temperature.

Development of a fiber-optic probe for thermographic phosphor measurements in turbine engines

A laser-based system for remotely measuring the surface temperature of parts coated with thermographic phosphor (Y 2O 3:Eu) is described. The obtained temperature-decay lifetime calibration measurements and empirical fits for Y 2O 3:Eu ( 4.52% Eu) over the temperature range 200–1000 °C are presented. In addition, the results of a proof-of-principle test conducted on a stationary coupon coated with Y 2O 3:Eu immersed in an atmospheric-combustor, chamber are described. Furthermore, a design of a fiber optic probe for fluorescence decay lifetime measurements within turbine engines is presented. A surfacedamage threshold of ~3.5 J/cm 2 ( 266 nm/4–6 ns pulse width) was measured for the fused silica step-index optical fiber used.

Production rate and decay lifetime measurements ofB s 0 mesons at LEP usingD s and? mesons

The study of the properties of inclusive production ofD s mesons and of events in which a ϕ and a muon are present in the same jet provides two independent measurements of the probability,f ws , for a heavy quark to hadronize into a strangeB orD meson. The data sample analysed corresponds to 243,000 hadronicZ 0 decays. The combined value of these measurements isf ws =0.19±0.06±0.08. From the flight distance distributions ofD s and of (ϕ-lepton) secondary vertices, with the lepton emitted at high transverse momentum relative to the jet axis, two values are obtained for theB 0s meson lifetime. Combining these measurements with a previous result based on the study ofD s-μ events, theB 0s meson lifetime is measured to be: 0.96±0.37 ps.

High-temperature remote thermometry using laser-induced fluorescence decay lifetime measurements of Y/sub 2/O/sub 3/:Eu and YAG:Tb thermographic phosphors

An Nd:YAG-laser-based remote sensing system for determining the surface temperature of substrates coated with thermographic phosphors has been developed. The fluorescence decay lifetime versus temperature measurements obtained for Y/sub 2/O/sub 3/:Eu (4.52% Ec) and YAG:Tb (5% Tb) over the temperature range 27 degrees C-1095 degrees C are presented. Both phosphors can be used for continuous temperature measurements over the range of approximately 550 degrees C-1000 degrees C. Stepwise regression analysis is used to calculate empirical relationships expressing temperature as a function of the measured fluorescence lifetimes for both phosphors beyond the onset temperature quenching point. The accuracy of the temperature measurements was estimated to be +or-5%. The intensities of the 611-nm Y/sub 2/O/sub 3/:Eu and the 544-nm YAG:Tb emission lines as a function of temperature are compared.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Non-contact observations of photoconductivity decay and carrier lifetime measurements in epitaxial silicon wafers

A new non-contact measurement method using a millimetre wave is proposed to observe the photoconductivity decay of an epitaxial silicon layer grown on a substrate with very low resistivity. The photoconductivity decay of the epitaxial layer was measured for p/p+ and n/n+ samples. The apparent carrier lifetime (or the effective lifetime) was determined directly from the gradient of the decay. The surface recombination velocity at the epilayer surface was decreased by using an ion exposure technique. Pairs of epilayer lifetime and effective recombination velocity at the interface between the epilayer and the substrate were determined by curve-fitting the measured photoconductivity decay to a theoretical one calculated by taking the skin effect of the electromagnetic wave into consideration.

Construction and evaluation of a frequency-domain epifluorescence microscope for lifetime and anisotropy decay measurements in subcellular domains

The measurement of time-resolved fluorescence parameters in living cells provides a powerful approach to study cell structure and dynamics. An epifluorescence microscope was constructed to resolve multi-component fluorescence lifetimes and complex anisotropy decay rapidly in labile biological samples. The excitation source consisted of focused, polarized laser light modulated by an impulse-driven Pockels' cell; parallel acquisition of phase angles and modulation amplitudes at more than 40 frequencies (5-250 MHz) was obtained by multi-harmonic cross-correlation detection. Lifetime decay was measured against standard solutions introduced into the light path proximal to the microscope objective. Anisotropy decay was measured by rotation of a Glan-Thompson polarizer in the emission path. Phase reference light was split from the beam proximal to the microscope. Optical components were selected to avoid depolarization and to optimize fluorescence detection efficiency. The dichoric was replaced by a 1 mm square mirror. Fitting routine statistics were optimized for model discrimination in realistic biological samples. Instrument performance was evaluated using fluorescein in H2O/glycerol and H2O/ethylene glycol mixtures and in Swiss 3T3 fibroblasts in monolayer culture. Objective depolarization effects were evaluated by measurement of anisotropy decay using objectives of different numerical aperture. Lifetime and anisotropy decay measured by microscopy (0.5 micron laser spot) agreed with data obtained by cuvette fluorimetry. New biological applications for time-resolved fluorescence microscopy are discussed.

Successive single rotational level radiative decay lifetime measurements of CS 2: evidence of state mixing caused by rotational coupling

We have observed that the state of the R-branch K = 0, v = (0, 10, 0) a 3A 2 of CS 2 shows two radiative decay curves. For example, for J′ = 17 measured at a pressure of 10 mTorr, the two corresponding short and long lifetimes are 0.71 ± 0.16 and 4.81 ± 0.13 μs, respectively. The lifetime of individual rotational level in this band varies with the total angular momentum quantum number J′. A plot of lifetime versus J′ shows that the value of lifetime decreases with J′ and reaches a minimum value at J′ = 17, then it increases with J′. This is interpreted to mean that three spin components of a 3A 2 are mixed together by strong rotational coupling. At some particular values of J′ the lifetimes are anomalous. This may be caused by the state of the ν 2 mode mixing with the ν 3 mode through anharmonic coupling.

Solvation effect on the picosecond fluorescence lifetimes and anisotropy decays of a hydrogen-bonded biologically active furocoumarin derivative: 4,5′,8-trimethylpsoralen

The origin of non-single exponential fluorescence decays of hydrogen-bonded 4,5′,8-trimethylpsoralen (TMP) with trichloroacetic acid (TCA) in cyclohexane was elucidated from picosecond time-resolved fluorescence lifetime and anisotropy decay measurements. The lifetime of the hydrogen-bond-free TMP ( T 1) was 14.0±1.5 ps, and thereby the hydrogen-bonded TMP was identified at TCA concentrations lower than 10 −3 M, having a still longer lifetime: T 2=360±15 ps. A further increase in T 2 with increasing concentration of TCA coincided with a swelling of molecular volume of the hydrogen-bonded TMP. Moreover, time-dependent red-shifts of fluorescence maxima were observed at TCA concentrations higher than 10 −2 M, which suggests dipolar relaxation between the hydrogen-bonded TMP and excess TCA in the swelling solvation sphere.

A Novel Approach to the Assessment of Semiconductor Hetero-Interfaces in Multilayer Structures

ABSTRACTWe demonstrate the novel use of CV simulation to determine the level of charge at each interface in multilayer GaAs/AlGaAs heterostructures. The gettering of charged centres at the earliest grown interfaces is quantitatively appraised and the parallel application of PL decay lifetime measurements determines their effectiveness as recombination centres. A direct relationship is found between the magnitude of the charged states and the recombination velocity. Estimates of a 1eV activation energy and a hole capture cross section of 10−15 cm2 were also obtained. This suggests that the dominant recombination centre could be observed as a hole trap.

Relativistic theory of spontaneous emission.

We derive a formula for the relativistic decay rates in atoms in a formulation of quantum electrodynamics based upon the electron's self-energy. Relativistic Coulomb wave functions are used, the full spin calculation is carried out, and the dipole approximation is not employed. The formula has the correct nonrelativistic limit and is used for calculating the decay rates in hydrogen and muonium for the transitions 2P\ensuremath{\rightarrow}1${S}_{1/2}$ and 2${S}_{1/2}$\ensuremath{\rightarrow}1${S}_{1/2}$. The results for hydrogen are \ensuremath{\Gamma}(2P\ensuremath{\rightarrow}${1S}_{1/2}$)=6.2649\ifmmode\times\else\texttimes\fi{}${10}^{8}$ ${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$ and \ensuremath{\Gamma}(${2S}_{1/2}$\ensuremath{\rightarrow}${1S}_{1/2}$)=2.4946\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}6}$. Our result for the 2P\ensuremath{\rightarrow}1S transition rate is in perfect agreement with the best nonrelativistic calculations as well as with the results obtained from the best known radiative decay lifetime measurements. As for the hydrogen 2${S}_{1/2}$\ensuremath{\rightarrow}1${S}_{1/2}$ decay rate, the result obtained here is also in good agreement with the best known magnetic dipole calculations. For muonium we get \ensuremath{\Gamma}(2P\ensuremath{\rightarrow}${1S}_{1/2}$)=6.2382\ifmmode\times\else\texttimes\fi{}${10}^{8}$ ${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$ and \ensuremath{\Gamma}(${2S}_{1/2}$\ensuremath{\rightarrow}${1S}_{1/2}$)=2.3997\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}6}$ ${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$.

Comparison of optically modulated absorption and photoconductivity decay lifetime measurements on HgCdTe

The optically modulated absorption (OMA) effect has been examined in p-type (NA−ND≊2×1015 cm−3) Hg0.785 Cd0.215Te. The effect has been used to measure the excess hole lifetime for temperatures between 85 and 250 K. The measured lifetimes agree with the results of photoconductivity (PC) decay measurements performed on the same samples. The measured lifetimes also indicate that an Auger process is the dominant recombination mechanism at temperatures above about 150 K, while a Shockley–Read–Hall process controls the lifetime below this temperature. In addition, it was found that the OMA signal exhibits extremely large phase-interference effects when coherent radiation is used for the probe beam. These effects have been both experimentally and theoretically investigated.

Tests of fundamental physical theories from measurements on free charged leptons

After a brief introduction in which the concepts of the magnetic and electric dipole moments of particles are introduced and questions are discussed associated with the discrete P and T transformations, the main part of the paper follows which consists of two sections. In the first of these the validity of quantum electrodynamics is analyzed on the basis of measurements of the anomalous magnetic moments of electrons and muons. Special emphasis is placed on experimental methods of measurement and on their historical development. The most detailed description is provided of three recent experiments: 1) measurement of (g – 2) for muons at CERN; 2) the spin resonance experiment on single electrons at the University of Washington; 3) the precise comparison of electron and positron magnetic moments carried out using the VEPP-2M storage ring at Novosibirsk. In the next section a review is given of the tests of CPT, CP, and T invariance from muon decay correlations and lifetime measurements. Finally the present status of experimental searches for an electric dipole moment (the existence of which would violate both P and T invariance) of electrons and muons is discussed.

Application of photon counting techniques to measurement of the risetime of chromium (III) phosphorescence in the near infrared

An extremely sensitive system for luminescence decay lifetime measurements based on multichannel delayed coincidence photon counting is described. The stop channel photomultiplier is an RCA C31034 gallium arsenide photocathode type, resulting an in system capable of measurements on emissions with lifetimes in the range nanoseconds to tens of microseconds, and spanning the wavelength range 300 nm to at least 900 nm. The system has been used to seek a risetime of phosphorescence from chromium (III) complexes in room temperature and cooled solutions. No risetime could be observed. An upper limit of 10 nsec can therefore be placed on the risetime for the compounds studied except for [Cr(en)3]3+, where an upper limit of 1 nsec has been established.

Luminescence in Tellurium-Doped Cadmium Sulfide

The room-temperature emission in CdS:Te is a single broad band whose half-width and peak position provide a simple means of estimating the tellurium concentration in the range 2×1018−5×1019 atoms cm−3 with a precision of 15%. Of the two discrete bands (at 2.1 and 1.7 eV) seen at low temperatures, only the higher-energy emission can be described by a simple configuration coordinate model. The nophonon line energy, not observed experimentally, is 2.36±0.05 eV, the exciton being bound to the tellurium atom by 0.22 eV. At high tellurium concentrations the exciton can be trapped at pairs or larger groupings of tellurium atoms sited on nearest-neighbor sulfur sites, the binding energy increasing correspondingly. The emission intensity from the 2.1-eV band is reduced by a factor of about 100 as the temperature is increased from near 0° to 300°K. The quenching behavior cannot be described in detail by a single decay process but the effective activation energy is about 0.20±0.03 eV. The drastic temperature quenching above 200°K restricts the usefulness of CdS:Te as a room-temperature scintillation device but can be reduced by the addition of donors. The relative efficiency at 300°K to that at low temperature increases from less than 1% in undoped material to 15%–20% in CdS:Te containing about 1018 donors cm−3. Chlorine, bromine, or iodine appear to serve equally well as donors. Donor-doped crystals exhibit two components in decay lifetime measurements at 300°K but only one (τ∼250±50 nsec) at low temperature. The extra radiative decay time (500±100 nsec) may arise from free-electron to bound-hole transitions at high donor concentrations. Energetically one expects such a process to be near degenerate with the trapped exciton decay and no obvious change occurs in the broad emission band. Hall data show a reduction in the donor binding energy roughly proportional to ND1/3, where ND is the donor concentration. Pulse heights detected after nuclear excitation of CdS:Te are increased about fivefold when 5×1017 donors cm−3 are added, chlorine giving the cleanest pulse shapes.