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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 703 records · Page 39
All sourcesNASA NTRSDOE OSTI
DOE OSTI2014

Effect of rapid thermal annealing on recombination centres in boron-doped Czochralski-grown silicon

Rapid thermal annealing in a belt furnace results in a dramatic change of the recombination properties of boron-doped Czochralski silicon: (1) the lifetime degraded by applying a prolonged illumination at room temperature was significantly improved, (2) after subsequent dark recovery, the lifetime has a remarkably high value, and (3) the permanent recovery, by annealing at 185 °C under illumination, is enormously accelerated, and the finally achieved stable lifetime acquires a record value of 1.5 ms, as compared to 110 μs after permanent recovery of not-annealed reference samples.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Direct imaging of electron recombination and transport on a semiconductor surface by femtosecond time-resolved photoemission electron microscopy

Much effort has been devoted to the development of techniques to probe carrier dynamics, which govern many semiconductor device characteristics. We report direct imaging of electron dynamics on semiconductor surfaces by time-resolved photoemission electron microscopy using femtosecond laser pulses. The experiments utilized a variable-repetition-rate femtosecond laser system to suppress sample charging problems. The recombination of photogenerated electrons and the lateral motion of the electrons driven by an external electric field on a GaAs surface were visualized. The mobility was estimated from a linear relationship between the drift velocity and the potential gradient.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Generation-recombination and trap-assisted tunneling in long wavelength infrared minority electron unipolar photodetectors based on InAs/GaSb superlattice

A long wavelength infrared minority electron unipolar photodetector based on InAs/GaSb type-II superlattices is demonstrated. At 77 K, a dark current of 3 × 10{sup −5} A/cm{sup 2} and a differential resistance-area of 3700 Ω.cm{sup 2} are achieved at the turn-on bias, with a 50%-cutoff of 10.0 μm and a specific detectivity of 6.2 × 10{sup 11} Jones. The dark current is fitted as a function of bias and temperature using a model combining generation-recombination and trap-assisted tunneling. Good agreement was observed between the theory and the experimental dark current.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Excitonic recombination dynamics in non-polar GaN/AlGaN quantum wells

The optical properties of GaN/Al{sub 0.15}Ga{sub 0.85}N multiple quantum wells are examined in 8 K–300 K temperature range. Both polarized CW and time resolved temperature-dependent photoluminescence experiment are performed so that we can deduce the relative contributions of the non-radiative and radiative recombination processes. From the calculation of the proportion of the excitonic population having wave vector in the light cone, we can deduce the variation of the radiative decay time with temperature. We find part of the excitonic population to be localized in concert with the report of Corfdir et al. (Jpn. J. Appl. Phys., Part 2 52, 08JC01 (2013)) in case of a-plane quantum wells.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Temperature-dependent measurement of Auger recombination in In{sub 0.40}Ga{sub 0.60}N/GaN red-emitting (λ = 630 nm) quantum dots

We have derived the Auger recombination coefficients, as a function of temperature, for In{sub 0.4}Ga{sub 0.6}N/GaN self-organized quantum dots from large-signal modulation measurements made on lasers in which the quantum dots form the gain media. The value of C{sub a} = 1.3 ±0.2 × 10{sup −31} cm{sup 6} s{sup −1} at room temperature and the coefficient decreases with increase of temperature.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2014

Distinguishing triplet energy transfer and trap-assisted recombination in multi-color organic light-emitting diode with an ultrathin phosphorescent emissive layer

An ultrathin layer of deep-red phosphorescent emitter tris(1-phenylisoquinoline) iridium (III) (Ir(piq){sub 3}) is inserted within different positions of the electron blocking layer fac-tris (1-phenylpyrazolato-N,C{sup 2′})-iridium(III) (Ir(ppz){sub 3}) to distinguish the contribution of the emission from the triplet exciton energy transfer/diffusion from the adjacent blue phosphorescent emitter and the trap-assisted recombination from the narrow band-gap emitter itself. The charge trapping effect of the narrow band-gap deep-red emitter which forms a quantum-well-like structure also plays a role in shaping the electroluminescent characteristics of multi-color organic light-emitting diodes. By accurately controlling the position of the ultrathin sensing layer, it is considerably easy to balance the white emission which is quite challenging for full-color devices with multiple emission zones. There is nearly no energy transfer detectable if 7 nm thick Ir(ppz){sub 3} is inserted between the blue phosphorescent emitter and the ultrathin red emitter.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Temperature dependence of the radiative recombination coefficient in crystalline silicon from spectral photoluminescence

The radiative recombination coefficient B(T) in crystalline silicon is determined for the temperature range 90–363 K, and in particular from 270 to 350 K with an interval of 10 K, where only sparse data are available at present. The band-band absorption coefficient established recently by Nguyen et al. [J. Appl. Phys. 115, 043710 (2014)] via photoluminescence spectrum measurements is employed to compute the values of B(T) at various temperatures. The results agree very well with literature data from Trupke et al. [J. Appl. Phys. 94, 4930 (2003).] We present a polynomial parameterization describing the temperature dependence of the product of B(T) and the square of the intrinsic carrier density. We also find that B(T) saturates at a near constant value at room temperature and above for silicon samples with relatively low free carrier densities.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2014

Reassessment of the recombination parameters of chromium in n- and p-type crystalline silicon and chromium-boron pairs in p-type crystalline silicon

Injection-dependent lifetime spectroscopy of both n- and p-type, Cr-doped silicon wafers with different doping levels is used to determine the defect parameters of Cr{sub i} and CrB pairs, by simultaneously fitting the measured lifetimes with the Shockley-Read-Hall model. A combined analysis of the two defects with the lifetime data measured on both n- and p-type samples enables a significant tightening of the uncertainty ranges of the parameters. The capture cross section ratios k = σ{sub n}/σ{sub p} of Cr{sub i} and CrB are determined as 3.2 (−0.6, +0) and 5.8 (−3.4, +0.6), respectively. Courtesy of a direct experimental comparison of the recombination activity of chromium in n- and p-type silicon, and as also suggested by modelling results, we conclude that chromium has a greater negative impact on carrier lifetimes in p-type silicon than n-type silicon with similar doping levels.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Development of the charge exchange recombination spectroscopy and the beam emission spectroscopy on the EAST tokamak

Charge eXchange Recombination Spectroscopy (CXRS) and Beam Emission Spectroscopy (BES) diagnostics based on a heating neutral beam have recently been installed on EAST to provide local measurements of ion temperature, velocity, and density. The system design features common light collection optics for CXRS and BES, background channels for the toroidal views, multi-chord viewing sightlines, and high throughput lens-based spectrometers with good signal to noise ratio for high time resolution measurements. Additionally, two spectrometers each has a tunable grating to observe any wavelength of interest are used for the CXRS and one utilizes a fixed-wavelength grating to achieve higher diffraction efficiency for the BES system. A real-time wavelength correction is implemented to achieve a high-accuracy wavelength calibration. Alignment and calibration are performed. Initial performance test results are presented.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2014

Drastic reduction in the surface recombination velocity of crystalline silicon passivated with catalytic chemical vapor deposited SiN{sub x} films by introducing phosphorous catalytic-doped layer

We improve the passivation property of n-type crystalline silicon (c-Si) surface passivated with a catalytic chemical vapor deposited (Cat-CVD) Si nitride (SiN{sub x}) film by inserting a phosphorous (P)-doped layer formed by exposing c-Si surface to P radicals generated by the catalytic cracking of PH{sub 3} molecules (Cat-doping). An extremely low surface recombination velocity (SRV) of 2 cm/s can be achieved for 2.5 Ω cm n-type (100) floating-zone Si wafers passivated with SiN{sub x}/P Cat-doped layers, both prepared in Cat-CVD systems. Compared with the case of only SiN{sub x} passivated layers, SRV decreases from 5 cm/s to 2 cm/s. The decrease in SRV is the result of field effect created by activated P atoms (donors) in a shallow P Cat-doped layer. Annealing process plays an important role in improving the passivation quality of SiN{sub x} films. The outstanding results obtained imply that SiN{sub x}/P Cat-doped layers can be used as promising passivation layers in high-efficiency n-type c-Si solar cells.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Design of charge exchange recombination spectroscopy for the joint Texas experimental tokamak

The old diagnostic neutral beam injector first operated at the University of Texas at Austin is ready for rejoining the joint Texas experimental tokamak (J-TEXT). A new set of high voltage power supplies has been equipped and there is no limitation for beam modulation or beam pulse duration henceforth. Based on the spectra of fully striped impurity ions induced by the diagnostic beam the design work for toroidal charge exchange recombination spectroscopy (CXRS) system is presented. The 529 nm carbon VI (n = 8 − 7 transition) line seems to be the best choice for ion temperature and plasma rotation measurements and the considered hardware is listed. The design work of the toroidal CXRS system is guided by essential simulation of expected spectral results under the J-TEXT tokamak operation conditions.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2014

High spatial and temporal resolution charge exchange recombination spectroscopy on the HL-2A tokamak

A 32/64-channel charge exchange recombination spectroscopy (CXRS) diagnostic system is developed on the HL-2A tokamak (R = 1.65 m, a = 0.4 m), monitoring plasma ion temperature and toroidal rotation velocity simultaneously. A high throughput spectrometer (F/2.8) and a pitch-controlled fiber bundle enable the temporal resolution of the system up to 400 Hz. The observation geometry and an optimized optic system enable the highest radial resolution up to ~1 cm at the plasma edge. The CXRS system monitors the carbon line emission (C VI, n = 8–7, 529.06 nm) whose Doppler broadening and Doppler shift provide ion temperature and plasma rotation velocity during the neutral beam injection. The composite CX spectral data are analyzed by the atomic data and analysis structure charge exchange spectroscopy fitting (ADAS CXSFIT) code. First experimental results are shown for the case of HL-2A plasmas with sawtooth oscillations, electron cyclotron resonance heating, and edge transport barrier during the high-confinement mode (H-mode)

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2014

Trap and recombination centers study in sprayed Cu₂ZnSnS₄ thin films

In this work, a study of trap and recombination center properties in polycrystalline Cu₂ZnSnS₄ thin films is carried out in order to understand the poor performance in Cu₂ZnSnS₄ thin film solar cells. Thermally stimulated current has been studied in Cu₂ZnSnS₄ deposited by pneumatic spray pyrolysis method using various heating rates, in order to gain information about trap centers and/or deep levels present within the band-gap of this material. A set of temperature-dependent current curves revealed three levels with activation energy of 126±10, 476±25, and 1100±100 meV. The possible nature of the three levels found is presented, in which the first one is likely to be related to Cu{sub Zn} antisites, while second and third to Sn vacancies and Sn{sub Cu} antisites, respectively. The values of frequency factor, capture cross section, and trap concentration have been determined for each center.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Electron-hole recombination on ZnO(0001) single-crystal surface studied by time-resolved soft X-ray photoelectron spectroscopy

Time-resolved soft X-ray photoelectron spectroscopy (PES) experiments were performed with time scales from picoseconds to nanoseconds to trace relaxation of surface photovoltage on the ZnO(0001) single crystal surface in real time. The band diagram of the surface has been obtained numerically using PES data, showing a depletion layer which extends to 1 μm. Temporal evolution of the photovoltage effect is well explained by a recombination process of a thermionic model, giving the photoexcited carrier lifetime of about 1 ps at the surface under the flat band condition. This lifetime agrees with a temporal range reported by the previous time-resolved optical experiments.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Determination of CdTe bulk carrier lifetime and interface recombination velocity of CdTe/MgCdTe double heterostructures grown by molecular beam epitaxy

The bulk Shockley-Read-Hall carrier lifetime of CdTe and interface recombination velocity at the CdTe/Mg{sub 0.24}Cd{sub 0.76}Te heterointerface are estimated to be around 0.5 μs and (4.7 ± 0.4) × 10{sup 2 }cm/s, respectively, using time-resolved photoluminescence (PL) measurements. Four CdTe/MgCdTe double heterostructures (DHs) with varying CdTe layer thicknesses were grown on nearly lattice-matched InSb (001) substrates using molecular beam epitaxy. The longest lifetime of 179 ns is observed in the DH with a 2 μm thick CdTe layer. It is also shown that the photon recycling effect has a strong influence on the bulk radiative lifetime, and the reabsorption process affects the measured PL spectrum shape and intensity.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Dissociative recombination and mutual neutralization of heavier molecular ions: C{sub 10}H{sub 8}{sup +}, WF{sub 5}{sup +}, and C{sub n}F{sub m}{sup +}

Dissociative recombination (DR) rate coefficients for the naphthalene cation, C{sub 10}H{sub 8}{sup +}, and WF{sub 5}{sup +}, and mutual neutralization (MN) rate coefficients for these species and five C{sub n}F{sub m}{sup +} ions, were determined at 300 K using variable electron and neutral density attachment mass spectrometry (VENDAMS). DR proceeds at 9 ± 3 × 10{sup −7} cm{sup 3} s{sup −1} for C{sub 10}H{sub 8}{sup +} and at 6.1 ± 1.4 × 10{sup −7} cm{sup 3} s{sup −1} for WF{sub 5}{sup +}. Consistent with previous results, MN for the polyatomic cations with the halide anions Cl{sup −}, Br{sup −}, and I{sup −} exhibits an approximate μ{sup −1/2} reduced mass dependence of the reactant partners, demonstrating that ion collision velocities influence the rate coefficients. This work is an extension of VENDAMS to systems, where low reactant concentrations are necessary to avoid significant reaction of product ions with the neutral precursor, i.e., conditions not suitable for traditional flowing afterglow measurements, as well as to ions of masses > ∼ 100 Da, which are not amenable to the study of DR in magnetic storage rings. Our results expand the sparse literature on DR and MN of heavier ions.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2015

Variations of operations and noise of semiconductor lasers subject to optical feedback with nonlinear gain and nonradiative recombination

Influence of the nonlinear gain (NLG) and nonradiative recombination life time ratio (NRRLTR) on the rout to chaos and noise characteristics of semiconductor lasers (SLs) subject to optical feedback (OFB) are investigated. The analyses are performed in terms of the temporal trajectory of the photon numbers, bifurcation diagrams and relative intensity noise (RIN). Variations of the fluctuations of the photon number are statistically analyzed. The corresponding frequency dependence of RIN is characterized. The simulation results show that, NLG and NRRLTR can cause significant changes in the rout to chaos of the SLs and probability distribution of the photon numbers. We found that the value of the OFB rate at which the transition from CW to periodic oscillation (PO) or chaos state occur increases, as the value of the NLG/ NRRLTR is increased/decreased. The RIN is found to be suppressed when the laser is operated in CW or PO region and at relatively higher/lower values of the NLG/NRRLTR. We believe that, the instability of the SLs can be reduced by operating it under OFB and by increasing/decreasing the values of the NLG/NRRLTR parameter.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Donor-acceptor pair recombination luminescence from monoclinic Cu{sub 2}SnS{sub 3} thin film

The defect levels in Cu{sub 2}SnS{sub 3} (CTS) were investigated using photoluminescence (PL) spectroscopy. A CTS thin film was prepared on a soda-lime glass/molybdenum substrate by thermal co-evaporation and sulfurization. The crystal structure was determined to be monoclinic, and the compositional ratios of Cu/Sn and S/Metal were determined to be 1.8 and 1.2, respectively. The photon energy of the PL spectra observed from the CTS thin film was lower than that previously reported. All fitted PL peaks were associated with defect related luminescence. The PL peaks observed at 0.843 and 0.867 eV were assigned to donor-acceptor pair recombination luminescence, the thermal activation energies of which were determined to be 22.9 and 24.8 meV, respectively.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗