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Herein, terahertz (THz) time domain spectroscopy is used to measure the complex conductivity of semi-insulating CoFe2O4 nanoparticles (NPs) and nano-hollow spheres (NHSs) with different diameters ranging from 100 to 350 nm having a nanocrystalline shell thickness of 19 to 90 nm, respectively. Interestingly, the magnitude of conductivity for CoFe2O4 NPs and NHSs of same average diameter (∼100 nm) for a given frequency of 0.3 THz is found to be 0.33 S/m and 9.08 S/m, respectively, indicating that the hollow structure exhibits greater THz conduction in comparison to its solid counterpart. Moreover, THz conductivity can be tailored by varying the nano-shell thickness of NHSs, and a maximum conductivity of 15.61 S/m is observed at 0.3 THz for NHSs of average diameter 250 nm. A detailed study reveals that thermally activated polaronic hopping plays the key role in determining the electrical transport property of CoFe2O4 nanostructures, which is found to solely depend on their magnitude of THz absorptivity. The non-Drude conductivity of all CoFe2O4 nanostructures is well described by the Polaron model instead of the Drude-Smith model, which is relevant for backscattering of free electrons in a nanostructured material. The Polaron model includes intra-particle and interparticle polaronic conductivities for closely spaced magnetic nanostructures and provides a mean free path of 29nm for CoFe2O4 NPs of diameter 100nm, which is comparable with its average crystallite size, indicating the applicability of the developed model for nanomaterials where charge transport is determined by polaronic hopping. Finally, we have demonstrated the morphology and size dependent magnetic measurements of ferrimagnetically aligned CoFe2O4 nanostructures through a vibrating sample magnetometer in the temperature range of 80-250 K, revealing that the disordered surface spin layer of nanostructures significantly controls their magnetism. [ABSTRACT FROM AUTHOR]

The electron g factor in an ensemble of InAs/InP quantum dots with emission wavelengths around 1.4 μm is measured using time-resolved pump-probe Faraday rotation spectroscopy in different magnetic field orientations. Thereby, we can extend recent single dot photoluminescence measurements significantly towards lower optical transition energies through 0.86 eV. This allows us to obtain detailed insight into the dispersion of the recently discovered g factor anisotropy in these infrared emitting quantum dots. We find with decreasing transition energy over a range of 50 meV a strong enhancement of the g factor difference between magnetic field normal and along the dot growth axis, namely, from 1 to 1.7. We argue that the g factor cannot be solely determined by the confinement energy, but the dot asymmetry underlying this anisotropy therefore has to increase with increasing dot size. [ABSTRACT FROM AUTHOR]

The temperature dependence of the electron Landé g-factor in bulk cubic GaN is investigated over an extremely broad temperature range from 15K up to 500K by time-resolved Kerr-rotation spectroscopy. The g-factor is found to be approximately constant over the full investigated temperature range. Calculations by k · p-theory predict a negligible temperature dependence g(T) in complete agreement with the experiment as a consequence of the large band-gap and small spin orbit splitting in cubic GaN. [ABSTRACT FROM AUTHOR]

The effect of hole transport on the performance of infrared light emitting diodes (LED) was investigated. The active area of the LEDs comprised two type-II superlattices with different periods and widths connected in series. Electroluminescence spectra of the devices with different positions of long wave and mid wave superlattice sections were mostly contributed by the superlattice closest to the p-contact. The experimental results indicate that due to suppressed vertical hole transport, the recombination of electrically injected electrons and holes in a type II superlattice LED active region takes place within a few superlattice periods near p-barrier. Possible reason for the effect is reduction of hole diffusion coefficient in an active area of a superlattice LED under bias. [ABSTRACT FROM AUTHOR]

Electrical properties and deep electron and hole traps spectra are compared for undoped n-GaN films grown by hydride vapor phase epitaxy (HVPE) in the regular process (standard HVPE samples) and in HVPE process optimized for decreasing the concentration of residual donor impurities (improved HVPE samples). It is shown that the residual donor density can be reduced by optimization from ∼1017 cm-3 to (2-5)×1014 cm-3. The density of deep hole traps and deep electron traps decreases with decreased donor density, so that the concentration of deep hole traps in the improved samples is reduced to ∼5×1013 cm-3 versus 2.9×1016 cm-3 in the standard samples, with a similar decrease in the electron traps concentration. [ABSTRACT FROM AUTHOR]

In this paper, we investigate the optical properties of confined electronic states in ultra-thin Al0.27Ga0.73As/GaAs/vacuum surface quantum wells by using photoreflectance spectroscopy at room temperature. Well-resolved doublet structures were found in the spectra. The energy of the features increases with decreasing well width in agreement with the predictions of a model of the transition energy between confined electron and hole states in a surface quantum well. Both the transition broadening and intensity behaviors are also well explained by the effective mass approximation theory. The offset between the un-perturbed theoretical transition energy and the experimental data has been explained by surface-state interaction effects. Moreover, the fact that the light hole ground state in the surface quantum well can be pushed out from the surface quantum well has been directly observed experimentally. [ABSTRACT FROM AUTHOR]

We report in-plane Cu-O atomic displacements as a function of doping in the La2-xSrxCuO4 system, determined by polarized Cu K-edge extended x-ray absorption fine structure measurements. We find a wide distribution of the Cu-O bonds showing a single peak asymmetric function for the system at low doping, which gets two peak function for the underdoped and optimally doped superconducting phases before turning to a single peak function for the overdoped phase. The results provide a direct evidence of critical local lattice fluctuations for an optimum transition temperature in the superconducting phases hosting quantum stripes. [ABSTRACT FROM AUTHOR]

Electron paramagnetic resonance (EPR) of Mn2+ and Fe3+ ions has been used as a probe of local static and dynamic lattice distortions in the relaxor ferroelectric PbMg1/3Nb2/3O3 (PMN) and solid solutions (1-x)PbMg1/3Nb2/3O3 - xPbTiO3 (x = 0.075-0.33) at temperatures from 290 to 800 K. We have found that the spectrum of the both ions in PMN contains two components: One component corresponds to the resonances from the lattice regions where the perovskite cubic structure is distorted statically while the second component corresponds to the regions where ionic displacements undergo significant dynamic fluctuations. The EPR spectrum from these lattice regions shows the motional narrowing behavior at 350-500 K. We assign these regions with reorientable polar nanoregions responsible for the dielectric relaxation in PMN. From the motional narrowing of the spectral lines, we have determined parameters of the Vogel--Fulcher law describing the rate of ionic hopping between symmetrically equivalent off-center positions. These parameters (Ea = 840-880 K, τ0 ∼ 10-13 s, Tg = 220 K) are in a satisfactory agreement with those reported for the dielectric relaxation. The fraction of polar nanoregions in PMN determined from the EPR intensity is about 20% to 30%. An addition of PbTiO3 to PMN decreases total volume of the polar nanoregions and also suppresses their reorientational dynamics. The EPR spectra do not change in the temperature range from 500 to 800 K, including the temperatures far above the Burns temperature 620 K. [ABSTRACT FROM AUTHOR]

Effects of Fe doping on the structural, optical and magnetic properties of sol-gel derived Zn1-xFexO (0 ≤ x ≤ 0.06) nanoparticles have been investigated. The single-phased wurtzite structure has been characterized by XRD analysis. The nanocrystalline nature of the samples and their crystallinity has been confirmed by TEM measurements. Raman spectroscopy has been employed to study the crystalline quality and structural disorders. FTIR analysis confirms the wurtzite structure and formation of tetrahedral coordination of the oxygen ions surrounding the zinc ions. UV-Vis measurements show that the band gap is fast blue shifted in lower Fe-concentrations and after that slowly red shifted in higher Fe-concentrations. This may be attributed to the Burstein-Moss effect which is prevailed throughout the range of Fe-doping and sp-d exchange interaction that plays a role in higher Fe-concentration. Magnetic hysteresis measurement (M-H) shows that Zn0.96Fe0.04O exhibits ferromagnetic behavior at room temperature. [ABSTRACT FROM AUTHOR]

The magnetoplasmon spectrum of two-dimensional electron gas systems with both Rashba and Dresselhaus spin-orbit interactions is investigated theoretically. The coexistence of the two types of spin-orbit interactions leads to resonant splittings of the collective excitation modes and makes the magnetoplasmon spectrum anisotropic with respect to the direction of the wave vector. The anticrossing gaps in the magnetoplasmon spectrum vary with the angle of the wave vector and reach their extrema at several fixed directions. Such anisotropic splittings could be used to evaluate the relative strength of the Rashba and Dresselhaus spin-orbit interactions in two-dimensional electron systems. [ABSTRACT FROM AUTHOR]

In this work we present the spectroscopic properties of KY3F10 (KY3F) single crystals activated with thulium and co-doped with ytterbium and neodymium ions. The most important processes that lead to the thulium up-conversion emissions in the blue and ultraviolet regions were identified. A time-resolved luminescence spectroscopy technique was employed to measure the luminescence decays and to determine the most important mechanisms involved in the up-conversion process that populates 1G4 and 1D2 (Tm3+) excited states. Analysis of the energy-transfer processes dynamics using selective pulsed-laser excitations in Yb:Tm:Nd, Tm:Nd, and Tm:Yb KY3F crystals show that the energy transfer from Nd3+ to Yb3+ ions is the mechanism responsible for the enhancement of the blue up-conversion efficiency in the Yb:Tm:Nd:KY3F when compared with the Yb:Tm system. A study of the energy transfer processes in Yb:Tm:Nd:KY3F crystal showed that the 1G4 excited level is mainly populated by a sequence of two nonradiative energy transfers that starts well after the Nd3+ and Tm3+ excitation at 797 nm according to: Nd3+ (4F3/2) → Yb3+ (2F7/2) followed by Yb3+ (2F5/2) → Tm (3H4) → Tm3+ (1G4). Results of numerical simulation of the rate equations system showed that a population inversion for 483.1 nm laser emission line is attained for a pumping rate threshold of 98 s-1, which is equivalent to an intensity of 3.3 KW cm-2 for a continuous laser pumping at 797 nm for Yb(30 mol%):Tm(0.5 mol%):Nd(1 mol%):KY3F. Nevertheless, best Yb3+ concentration for the laser emission near 483.1 nm was estimated to be within 40 and 50 mol%. On the other hand, a population inversion was not observed for the case of 960 nm (Yb3+) pumping. [ABSTRACT FROM AUTHOR]

It has been found that 2,4,6 trinitrotoluene (TNT) displays a 420 nm peak in differential reflection spectrometry spectra. Differential reflection spectrometry uses UV and visible light to measure the photon energy absorbed by electrons when they move to a higher energy state. This particular optical feature for TNT has been previously reported by the authors. The physical origin of the described feature is investigated using computational chemistry. Through this molecular modeling, circumstances where the 420 nm feature may occur have been isolated. In addition, the participating molecular orbitals have been suggested from these calculations as in the nitro groups and the ring carbon bonded to this group. [ABSTRACT FROM AUTHOR]

The expansion dynamics of ion and neutral species in laterally colliding laser produced aluminum plasmas have been investigated using time and space resolved optical emission spectroscopies and spectrally and angularly resolved fast imaging. The emission results highlight a difference in neutral atom and ion distributions in the stagnation layer where, at a time delay of 80 ns, the neutral atoms are localized in the vicinity of the target surface (<1 mm from the target surface) while singly and doubly charged ions lie predominantly at larger distances, <1.5 and <2 mm, respectively. The imaging results show that the ions were found to form a well defined, but compressed, stagnation layer at the collision front between the two seed plasmas at early times (Δt<80 ns). On the other hand, the excited neutrals were observed to form a V-shaped emission feature at the outer regions of the collision front with enhanced neutral emission in the less dense, cooler regions of the stagnation layer. [ABSTRACT FROM AUTHOR]

We report a systematic measurement of the space charge effect observed in the few-picosecond laser pulse regime in laser-based solid-state photoemission spectroscopy experiments. The broadening and the shift of a gold Fermi edge as a function of spot size, laser power, and emission angle are characterized for pulse lengths of 6 ps and 6 eV photon energy. The results are used as a benchmark for an N-body numerical simulation and are compared to different regimes used in photoemission spectroscopy. These results provide an important reference for the design of time- and angle-resolved photoemission spectroscopy setups and next-generation light sources. [ABSTRACT FROM AUTHOR]

Distributions of emission intensity from radicals, electron temperature, and rotational temperature at a radio frequency of 27.12 MHz plasma in water are clarified by detailed spectroscopy measurement. Through this investigation, the following were observed. The points of maximum emission intensity of Hα, Hβ, O (777 nm), and O (845 nm) are almost the same, while that of OH shifts upward. The electron temperature decreases, while the rotational temperature increases with pressure. The distribution of the electron temperature changes at a threshold pressure, which is concerned with a change in the electron discharge mechanism. The self-bias of the electrode changes from a negative to positive at a threshold pressure. The point of the maximum rotational temperature of OH radicals shifts to approximately 1 mm above that for the maximum intensity of OH emission. [ABSTRACT FROM AUTHOR]

Ho3+-doped and Ho3+/Yb3+-codoped lead bismuth gallate (PBG) oxide glasses were prepared and their spectroscopic properties were investigated. The derived Judd–Ofelt intensity parameters (Ω2=6.81×10-20 cm2, Ω4=2.31×10-20 cm2, and Ω6=0.67×10-20 cm2) indicate a higher asymmetry and stronger covalent environment for Ho3+ sites in PBG glass compared with those in tellurite, fluoride (ZBLAN), and some other lead-contained glasses. Intense frequency upconversion emissions peaking at 547, 662, and 756 nm as well as infrared emissions at 1.20 and 2.05 μm in Ho3+/Yb3+-codoped PBG glass were observed, confirming that energy transfer between Yb3+ and Ho3+ takes place, and a two-phonon-assisted energy transfer from Yb3+ to Ho3+ ions was determined by the calculation using phonon sideband theory. The 1.20 μm emission observed was primarily due to the weak multiphonon deexcitation originated from the small phonon energy of PBG glass (∼535 cm-1). A large product of emission cross-section and measured lifetime (9.93×10-25 cm2 s) was obtained for the 1.20 μm emission and the gain coefficient dependence on wavelength with population inversion rate (P) was performed. The peak emission cross-section for 2.05 μm emission was calculated to be 4.75×10-21 cm2. The relative mechanism of Ho3+-doped and Ho3+/Yb3+-codoped PBG glasses on their spectroscopic properties was also discussed. Our results suggest that Ho3+/Yb3+-doped PBG glasses are a good potential candidate for the frequency upconversion devices and infrared amplifiers/lasers. [ABSTRACT FROM AUTHOR]

This work aims specifically at studying the evolution of point defects induced by electron irradiation in the cubic polytype of SiC (3C-SiC) at temperatures ranging from 10 to 1450 K by means of photoluminescence (PL) spectroscopy. We identified a first annealing stage between 200 and 245 K, which probably results from migration of interstitials in the carbon sublattice. Moreover, we confirmed the high thermal stability of defect-related PL signals up to about 1100 K and calculated the activation energies associated with their annihilation. Finally, we studied the effect of a high temperature treatment at 1400 K on the DI center PL intensity in a single-crystal sample irradiated by electrons below the threshold displacement energy of the silicon sublattice. This allows checking the relevance of recent defect models based upon the migration of atoms in the carbon sublattice during the irradiation process. We conclude that the DI center does not involve the silicon vacancy and could be assigned to an isolated silicon antisite SiC. [ABSTRACT FROM AUTHOR]

We have performed systematic studies of wurtzite GaN/AlN quantum dots grown on polar and nonpolar surfaces. For this purpose, experimentally observed quantum dot geometries have been employed within an eight-band k·p model. The spatial separation of electrons and holes due to polarization potentials is found to be much larger in nonpolar than in polar grown quantum dots. In order to improve the electron-hole overlap and thus the recombination rates, we have varied the shape, size, and the periodic arrangement of nonpolar quantum dots. We observed the strongest improvement of the charge carrier overlap in nonpolar quantum dots that have a reduced dimension. If the size is reduced below 60% of the dimensions reported recently in literature, this increase is clearly more pronounced than for the polar quantum dots, indicating much better recombination rates in smaller nonpolar quantum dots. [ABSTRACT FROM AUTHOR]

The time dependent dielectric breakdown in Co–Fe–B/MgO/Co–Fe–B magnetic tunnel junctions was investigated by voltage ramp experiments. The measurements were done for two types of junctions: one set of junctions had exchange biased (pinned) bottom electrodes and one set had exchange biased (pinned) top electrodes with an additional artificial ferrimagnet. We found a significant polarity dependence in the dielectric breakdown: top as well as bottom pinned tunnel junctions showed higher breakdown voltage when the top electrode was biased positively compared to negative bias. In contrast to this the differential resistance (dV/dI)-V spectra revealed an asymmetry for the top pinned junctions which was reversed in comparison to the bottom pinned system. This indicates that both asymmetries have different origins. Additionally the bottom pinned junctions showed in general slightly lower breakdown voltages and stronger magnon excitation in the inelastic electron tunneling (d2I/dV2)-V spectra than the top pinned junctions. Possible reasons for these correlations are discussed. [ABSTRACT FROM AUTHOR]

A series of bound exciton transitions of a bulk ZnO sample has been studied by high resolution magnetophotoluminescence spectra. Ten sharp intense bound exciton emissions are classified into three groups according to their dependences on magnetic field, angle, and circular polarization of the emission lines. The charge states of the defect centers and exciton types are identified. The effective g factors of the electrons and holes are determined. [ABSTRACT FROM AUTHOR]

Phonon-plasmon coupled mode Raman spectra of n-type GaSb were measured at room temperature as a function of electron concentration. These spectra were obtained using an optical system based on 752.55 nm excitation. Utilization of this wavelength permits greater sensitivity to GaSb coupled mode Raman scattering over a wider doping range than is possible with visible wavelength excitation-based systems. A relatively simple spectral model for the electronic contribution to the dielectric function was evaluated for determination of electron concentration from the bulk coupled mode spectra. The electron concentrations were determined from the Raman spectra by minimizing the sum of the squared residuals between a measured and a simulated spectrum as a function of Fermi energy and a plasmon damping parameter. The electron concentrations determined from the fits to the Raman spectra were compared to the electron concentrations determined from single magnetic field Hall effect measurements that were corrected to account for carriers in two conduction band minima. The electron concentrations determined using the two methods differed from ≈-16% at low doping levels to ≈+24% at high doping levels. In general, compared to the electron concentrations determined from the corrected Hall effect measurements, the electron concentrations determined from the Raman spectra were lower for epilayers with lower Hall effect electron concentrations and higher for epilayers with higher Hall effect electron concentrations. The deviations between the results obtained with the two methods were attributed to the impact of uncertainties in GaSb material properties on the accuracy of the correction to the single field Hall effect measurements and to the limitations of the spectral model employed. [ABSTRACT FROM AUTHOR]

Photo- and thermoluminescence (TL) spectra of NaLaF4:Ln3+ (Ln=Ce,Pr,Nd,Sm,Eu,Gd,Tb,Dy,Ho,Er,Tm) and NaLaF4:Ce3+, Ln3+ (Ln=Nd,Sm,Ho,Er,Tm) are presented and used together with the empirical Dorenbos model in order to establish the 4f energy level positions of all tri- and divalent lanthanide ions doped in NaLaF4. The information will be presented in the form of an energy level diagram. It is shown that in addition to this diagram only two assumptions, viz., the presence of two host related electron traps and the presence of Vk-centers, are necessary for explaining the lanthanide-specific TL glow curves of both Ln mono- and Ce–Ln codoped NaLaF4. [ABSTRACT FROM AUTHOR]

The origin of annealing-induced variations in the effective work function of Ta-based conductors (TaC, Ta2C, and TaNx) at interfaces with SiO2 and HfO2 dielectrics is investigated using the spectroscopy of internal photoemission (IPE) of electrons. Two physical mechanisms were considered that could affect the Ta-based metal/oxide barrier: (a) oxidation of the metal conductor after thermal treatment by oxygen supplied from the oxide layer, resulting in a variation in the density of electron states inside the metal emitter, and (b) annealing-induced variations in the barrier at the interface. Field-dependent and differential IPE experiments allowed us to identify the latter as the dominant mechanism causing the barrier instability after thermal treatment in nonoxidizing ambient. The incorporation of negatively charged centers in the near-interface oxide layers is the most likely explanation for the significant increase (∼0.6 eV) of the barrier height after annealing, leading us to the hypothesis that Ta may diffuse into the oxide layer, and thereby be at the origin of the negative charge creation. [ABSTRACT FROM AUTHOR]

The infrared (IR) transmission, Raman Scattering, magnetic measurement, and electronic resistance measurement have been carried out in La2-xSrxCu0.94Mn0.06O4 and La2-xSrxCu0.94Ru0.06O4 systems. The superconductivity that is suppressed by impurity reappears as Sr content increases and forms a new relationship of the Meissner volume and Tconset versus Sr doping content. The Mn4+ doping system exhibits twice superconductive phase transitions while Ru4+ doping system exhibits only one superconductive phase transition. Coexistence and competition between superconductivity and magnetism are observed. We suggest that these surprising phenomena should be attributed to the compensation effect that is the electrons on high valence impurities are compensated by holes introduced through Sr doping. This compensation effect is confirmed by our IR spectra results. We also find that the compensation effect is strong for the localized 3d electrons of Mn4+ while very weak for the itinerant 4d electrons of Ru4+. [ABSTRACT FROM AUTHOR]

Local low energy-loss functions (ELFs) of chemical ordered nanodomains in relaxor Pb(Mg1/3Nb2/3)O3 (PMN) were obtained by using Kramers–Krönig analysis of experimental electron low energy-loss spectra. Electronic structures and dielectric functions were calculated via ab initio calculation using the full-potential linearized augmented plane-wave method on a 30 atom PMN supercell with B-site cation ordering along the [111] direction. The theoretical ELFs are fundamentally in agreement with the experimental ELFs. Furthermore, we systematically elucidate all the peaks in the low ELF based on the ab initio calculated densities of states. It is found that the Nb 4d–O 2p hybridization is enhanced through ferroelectric transition, which makes the O 2p collective excitation energy upper shift in ELF, while Mg, also as the B-site ion, presents no covalent bonding effect with O. [ABSTRACT FROM AUTHOR]

Measurements of photoinduced current have been performed on thin films of porous low-k dielectric materials comprised of carbon-doped oxides. The dielectric films were deposited on silicon surfaces and prepared with a thin gold counterelectrode. From the spectral dependence of the photoinduced current, barrier heights for the dielectric/silicon and dielectric/gold interface were deduced. Transient currents were also found to flow after the photoexcitation was abruptly stopped. An estimate of the density of shallow electron traps within the low-k material was obtained from the measurement of the net charge transported from this detrapping current. A density of traps in the range of 6×1016 traps/cm3 was inferred for the low-k films, far exceeding that observed by the same technique for reference dielectric films of pure SiO2. This behavior was also compatible with photocurrent I-V measurements on the low-k dielectric films and SiO2 reference sample. [ABSTRACT FROM AUTHOR]

We show that mid-infrared transmission spectroscopy of a quantum cascade laser provides clear-cut information on changes in charge location at different bias. Theoretical simulations of the evolution of the gain/absorption spectrum for a λ∼7.4 μm InGaAs/AlInAs/InP quantum cascade laser have been compared with the experimental findings. Transfer of electrons between the ground states in the active region and the states in the injector goes hand in hand with a decrease of discrete intersubband absorption peaks and an increase of broad, high-energy absorption toward the continuum delocalized states above the barriers. [ABSTRACT FROM AUTHOR]

The effect of the Schottky barrier height on the detection of the concentration of midgap defects using deep level transient spectroscopy (DLTS) is experimentally and theoretically studied for EH6 and EH7 defects in 4H-SiC. In this special case, the DLTS signal height for EH6 and EH7 increases with increasing barrier height and saturates at values above 1.5 and 1.7 eV, respectively. Below 1.1 eV, the DLTS peak completely disappears for both defects. The experimental data are explained by a theoretical model. The course of the quasi-Fermi level in the space charge region is calculated as a function of the reverse current through it, which is determined by the barrier height, and the reverse bias applied. [ABSTRACT FROM AUTHOR]

In situ deep level transient spectroscopy has been applied to investigate the influence of 100 MeV Si7+ ion irradiation on the deep levels present in Au/n-Si (100) Schottky structure in a wide fluence range from 5×109 to 1×1012 ions cm-2. The swift heavy ion irradiation introduces a deep level at Ec-0.32 eV. It is found that initially, trap level concentration of the energy level at Ec-0.40 eV increases with irradiation up to a fluence value of 1×1010 cm-2 while the deep level concentration decreases as irradiation fluence increases beyond the fluence value of 5×1010 cm-2. These results are discussed, taking into account the role of energy transfer mechanism of high energy ions in material. [ABSTRACT FROM AUTHOR]

Characterization of dominant electron trap in as-grown SiC epilayers has been carried out using deep level transient spectroscopy. Two electron traps E1 and Z1 at Ec-0.21 and Ec-0.61 are observed, respectively; Z1 being the dominant level. Line shape fitting, capture cross section, and insensitivity with doping concentration have revealed interesting features of Z1 center. Spatial distribution discloses that the level is generated in the vicinity of epilayers/substrate interface and the rest of the overgrown layers is defect-free. Owing to the Si-rich growth conditions, the depth profile of Z1 relates it to carbon vacancy. The alpha particle irradiation transforms Z1 level into Z1/Z2 center involving silicon and carbon vacancies. Isochronal annealing study further strengthens the proposed origin of the debated level. [ABSTRACT FROM AUTHOR]

This work describes the composition and bonding structure of hydrogenated carbon nitride (a-CNx:H) films synthesized by electron cyclotron resonance chemical vapor deposition using as precursor gases argon, methane, and nitrogen. The composition of the films was derived from Rutherford backscattering and elastic recoil detection analysis and the bonding structure was examined by infrared (IR) spectroscopy and x-ray absorption near edge spectroscopy (XANES). By varying the nitrogen to methane ratio in the applied gas mixture, polymeric a-CNx:H films with N/C contents varying from 0.06 to 0.49 were obtained. Remarkably, the H content of the films (∼40 at. %) was rather unaffected by the nitrogenation process. The different bonding states as detected in the measured XANES C(1s) and N(1s) spectra have been correlated with those of a large number of reference samples. The XANES and IR spectroscopy results indicate that N atoms are efficiently incorporated into the amorphous carbon network and can be found in different bonding environments, such as pyridinelike, graphitelike, nitrilelike, and amino groups. The nitrogenation of the films results in the formation of N-H bonding environments at the cost of C-H structures. Also, the insertion of N induces a higher fraction of double bonds in the structure at the expense of the linear polymerlike chains, hence resulting in a more cross-linked solid. The formation of double bonds takes place through complex C=N structures and not by formation of graphitic aromatic rings. Also, the mechanical and tribological properties (hardness, friction, and wear) of the films have been studied as a function of the nitrogen content. Despite the major modifications in the bonding structure with nitrogen uptake, no significant changes in these properties are observed. [ABSTRACT FROM AUTHOR]

We have studied Al/Nb interface with help of spectroscopy of reflected electrons. Ion sputtering combined with monitoring of Auger peaks has been used to approach the interface. The developed method of quantitative interpretation of spectra allowed us to characterize the distribution of Al and Nb in the interface region with a nanometer depth resolution. The Al/Nb interface was found to have an intermediate layer of about 10 nm thickness, which is more likely due to the interface roughness, rather than diffusion smearing. We discuss the possibility to distinguish by means of spectroscopy of reflected electrons either material under analysis is a single phase or a mixture of two phases. [ABSTRACT FROM AUTHOR]

The physical (elastic, cohesive, and electronic) properties of precipitates are important in determining factors such as their equilibrium shape, coarsening, and strengthening behavior in alloys. In this work, we use valence electron energy-loss spectroscopy (VEELS) and energy-filtering transmission electron microscopy (EFTEM) to determine quantitatively and image the elastic moduli, cohesive energy, and interstitial electron density of both metastable and equilibrium precipitates in two different metal alloys. We show that the elastic properties of θ′ and θ precipitates in Al–Cu alloys can be measured in situ as a function of temperature and during transformation from θ′ and θ phases. We also measure and image in situ the elastic and cohesive properties of metastable TiHx precipitates in a Ti–H alloy. These results demonstrate the capability of VEELS/EFTEM for real-time nanoscale determination and imaging of multiple physical properties of precipitates in solid-solid phase transformations. [ABSTRACT FROM AUTHOR]

The ion-sputtering induced intermixing is studied by Monte Carlo transport of ions in matter (TRIM), molecular-dynamics (MD) simulations, and Auger electron spectroscopy depth profiling (AES-DP) analysis in Pt/Ti/Si substrate (Pt/Ti) and Ta/Ti/Pt/Si substrate (Ti/Pt) multilayers. Experimental evidence is found for the asymmetry of intermixing in Pt/Ti, and in Ti/Pt. In Ti/Pt we obtain a much weaker interdiffusion (broadening at the interface) than in Pt/Ti. The unexpected enhancement of the interdiffusion of the Pt atoms into the Ti substrate has also been demonstrated by simulations. We are able to capture the essential features of intermixing using TRIM and MD simulations for ion-beam sputtering and find reasonable values for interface broadening which can be compared with the experimental measurements. We explain the asymmetry of IM by the possible occurrence of transient enhanced diffusion in Pt/Ti which manifests in the exponential high diffusity tail of the AES concentration profile. [ABSTRACT FROM AUTHOR]

We present a direct observation of time-resolved intervalley transitions of electrons between the conduction band Γ and L valleys in GaN single crystals using a two-color (ultraviolet and near-infrared), femtosecond, pump-probe spectroscopic technique. We have found that the threshold for the Γ↔L transition appears at the energy of 4.51±0.05 eV and the electron scattering time from the Γ to L valley is comparable to the 170 fs width of our pump pulses, while the return process of electrons from L to Γ is measurably slower. The L to Γ scattering time is 1.02 ps, while the total depopulation time of the L valley is estimated to be ∼20 ps. Our physical model, based on the three-state rate equations, fitted our experimental data very well and allowed us to calculate the optical phonon emission time of 290 fs. The deformation potential between the two valleys was also obtained from the intervalley scattering modeling and the value was 0.92×109 eV/cm. [ABSTRACT FROM AUTHOR]

The emission mechanism in organic light-emitting devices, where the emission layer is composed of Eu(DBM)3pyzphen (DBM=Dibenzoylmethane, pyzphen=pyrazino-[2,3-f][1,10]-phenanthroline) doped into electron transporting/hole blocking material BPhen (4,7-diphenyl-1, 10-phenanthroline), is investigated. Energy transfer and carrier trapping simultaneously exist in the luminescence process, and carrier trapping is a main process. Direct carrier trapping by Eu(DBM)3pyzphen molecules is confirmed by the difference of electroluminescence and photoluminescence spectra as well as J-V characteristics. Efficient Föster and Dexter energy transfer from BPhen to Eu(DBM)3pyzphen molecules were speculated in terms of analysis of photoluminescence spectra of fixed solutions, triplet energies, and phosphorescent lifetimes. Based on these mechanisms, the overall performances of these devices were improved. High efficiencies were obtained under carrier trapping by Eu(DBM)3pyzphen molecules, and the emission of BPhen was eliminated by efficient energy transfer from the BPhen to Eu(DBM)3pyzphen molecules. [ABSTRACT FROM AUTHOR]

This paper demonstrates a swept high frequency eddy current (SHFEC) methodology that can determine near-surface conductivity deviation profiles of shot-peened superalloy surfaces, from which residual stress state can be assessed nondestructively. Our methodology is built around a laboratory-grade SHFEC hardware and a model-based SHFEC data inversion software, both described in this article. For the demonstration, a series of shot-peened Inconel 718 block specimens is prepared and examined by the proposed SHFEC inversion technique. The conductivity depth profiles of the samples under various shot peening intensities have been obtained by the inversion. Several sensitivity and consistency test results are given to support the reliability of the inverted conductivity profiles. The extreme near-surface regions (10–20 μm) of the shot-peened surfaces are also examined by various microstructural characterization methods such as scanning electron microscopy, energy dispersive x-ray spectroscopy, and x-ray diffraction, to examine our inversion results microscopically. [ABSTRACT FROM AUTHOR]

The chemistry in CH4/O2 rf discharges has been investigated by comparing molecular concentrations measured by tunable diode laser absorption spectroscopy with a hypothetical plasma composition calculated by a simple chemical rate equation system. This comparison allowed us to investigate the reaction paths of molecules, which undergo long reaction chains, and to gather information on the high-energy tail of the electron energy distribution function (EEDF). The procedure has been applied to a capacitively and two inductively coupled plasma sources with strongly varied external parameters (p=10–100 Pa, Q=2–60 SCCM) and for a wide range of electron densities (ne=1014–1017 m-3). In mixtures of CH4 and O2 the main production and depletion channels of two stable products, CO and CO2, have been obtained, and their relative importance under modified operational conditions has been studied. Furthermore, the analysis of the molecular densities of CH4, CH3, C2H2, and C2H6 dependent on the input power in an inductively coupled discharge with pure CH4 as source gas allowed us to determine a range of EEDFs, which are compatible with the measured concentrations. [ABSTRACT FROM AUTHOR]

6.0 MeV F3+ ions are implanted into x-cut LiNbO3 crystals in doses ranging from 3.0×1013 to 1.0×1015 ions/cm2 at room temperature. Planar waveguide formations are confirmed and dark modes are measured by prism coupling technique with a wavelength of 633 nm. TE modes due to both raised extraordinary index and barrier-confined waveguides are demonstrated. The Rutherford backscattering/channeling technique is used to investigate the damage produced by implantation. The experimental results are analyzed. With a fiber probe technique, the waveguide loss from TE0 mode in sample implanted with the dose of 1.0×1014 ions/cm2 is measured which is about 0.76 dB/cm after annealing. [ABSTRACT FROM AUTHOR]

Photoreflectance (PR) and contactless electroreflectance (CER) spectroscopies have been applied to study optical transitions in undoped and Si-doped AlGaN/GaN heterostructures at room temperature. Spectral features related to excitonic and band-to-band absorptions in GaN layer and band-to-band absorption in AlGaN layer have been resolved and analyzed. In addition, a broad spectral feature related to two-dimensional electron gas has been observed for the Si-doped heterostructure. It has been found that some of the mentioned optical transitions are not observed in CER spectra whereas they are very strong in PR spectra. This phenomenon is associated with different mechanisms of the modulation of built-in electric field in the investigated structure. A combination of PR and CER gives the possibility of a richer interpretation of both PR and CER spectra. [ABSTRACT FROM AUTHOR]

The Stark effect on excitons in a bilayer system is investigated theoretically within the framework of the effective-mass approximation. The calculations indicate that the energy of the excitons decreases as the value of the in-plane electric field F increases at a fixed value of the distance d between the layers. However, the energy of the excitons increases with d at a fixed value of F. In particular, it increases linearly at small values of d but increases as 1/d at large values. Therefore, it can be concluded that excitons in a bilayer system have a small binding energy equal to the absolute value of the excitonic energy at large d or small F. In addition, the radiative lifetime of heavy-hole excitons in this system is calculated and is found to be short at small values of both F and d. The radiative lifetime of heavy-hole excitons in a bilayer system can be increased by two orders by an in-plane electric field of 2 kV/cm when d is twice the excitonic Rydberg. [ABSTRACT FROM AUTHOR]

Effect of growth rate on the composition fluctuation is investigated in In0.34Ga0.66As0.98N0.02/GaAs single quantum wells (QWs) by photoluminescence (PL), transmission electron microscopy, and admittance spectroscopy. In an InGaAsN layer grown at a normal growth rate, the PL spectra show a low-energy bump at the tail of an InGaAsN emission, suggesting the presence of composition fluctuation. Lowering the growth rate degrades the composition fluctuation by segregating into bimodal phases of an InGaAsN and InGaAs-rich phase. Further lowering the growth rate leads to a three-dimensional growth and enhances the InGaAs-rich phase. The carrier distribution for the InGaAsN layer grown at the normal rate shows a carrier bump at the tail of a strong accumulation peak, suggesting the presence of an electron state below the QW ground state. The admittance spectroscopy shows that the activation energy (32 meV) of this electron state is comparable to the energy separation (30 meV) between the InGaAsN emission and the low-energy bump, and thus it is possible that the composition fluctuation actually induces an electron state closely below the QW ground state. [ABSTRACT FROM AUTHOR]

We propose a simple formula for fitting the electron ionization mean free paths in solids both at high and at low electron energies. The free-electron-gas approximation used for predicting electron mean free paths is no longer valid at low impact energies [(E-EF)<50 eV], as the band structure effects become significant at those energies. Therefore, we include the results of band structure calculations in our fit. Finally, we apply the fit to nine elements and two compounds. [ABSTRACT FROM AUTHOR]

We report the bipolar transport properties of the LUMATION™ (Sumitomo Chemical) 1300 Series green-emitting polymer investigated by means of admittance spectroscopy. Analysis of the inductive response in single-carrier polymer diodes yields electron and hole mobilities which are in excellent agreement with the results of independent measurements. Admittance measurements in dual injection diodes, in combination with the analysis of current-voltage characteristics, provide evidence that the dual injection diodes operate in space-charge-limited regime, indicative of strong recombination within the material. Our results provide strong evidence that the space-charge-related admittance response of dual-carrier diodes is dominated by combined electron-hole response, which corresponds to the sum of electron and hole mobilities. This implies that electron and hole mobilities cannot be obtained separately from admittance measurements in space-charge-limited dual-carrier devices. [ABSTRACT FROM AUTHOR]

Additional to a correct color and a high efficiency, phosphors for plasma display panels must maintain their light output for thousands of hours. Often the degradation is the restricting factor in using phosphors. In this article, the mechanism of luminance decrease in blue-emitting BaMgAl10O17:Eu2+ phosphor during the operation of the PDPs has been studied. It is shown experimentally that the aging process is mainly due to the vacuum-ultraviolet excitation (VUV). It is demonstrated that the degradation mechanism can be accelerated by using a 193 nm laser excitation. Based on excitation, reflectance, thermoluminescence spectra, and aging or annealing processes by laser excitation, the main causes of the degradation are demonstrated. The aging process can be separated in two different processes according to the temperature: a first one, at low temperature, corresponding to the autoionization of luminescent centers (Eu2+→Eu3+); and a second one, at high temperature, linked to the formation of traps in the phosphor. These traps induce a perturbation of the energy migration in the phosphor. In addition, the relevant parameters of trap formation are highlighted: density of the VUV excitation, temperature, and atmosphere/pressure surrounding the phosphor. A model of BaMgAl10O17:Eu2+ phosphor degradation mechanism is proposed. [ABSTRACT FROM AUTHOR]

Trace rare-gas–optical emission spectroscopy is carried out to characterize the nitrogen plasma as a function of discharge parameters. The functional dependence of N2(C 3Πu) and N2+(B 2Σu+) excited states is monitored by measuring the emission intensities of the bandheads of second positive and first negative systems. The excited-state population density of N atoms and N2 molecules, extracted from their optical emission, is related to the ground-state population density after normalizing the changes for excitation cross section and electron energy distribution function by optical actinometry. The electron temperature is determined from the plasma-induced optical emission of trace rare gas by the line-to-line method. The obtained data may help us to adjust the optimum discharge conditions for the production of active species, which are considered to be important for the desired treatment of the samples. [ABSTRACT FROM AUTHOR]

As-supplied polycrystalline ZnO samples (purity 99.9% from Sigma-Aldrich, Germany) have been annealed at different temperatures and subsequently characterized by positron annihilation spectroscopy, x-ray-diffraction (XRD) analysis, thermogravimetric analysis (TGA), and resistivity measurements. Positron annihilation lifetime analysis and coincidence Doppler-broadened electron-positron annihilation γ-radiation (CDBEPAR) line-shape measurements have been employed at a time to identify the nature of defects in differently annealed ZnO materials. Annealing up to 300 °C, an increase of defect lifetime (τ2) as well as shape parameter (S parameter) has been observed. Further annealing causes a large decrease of τ2 and S parameter. TGA study shows considerable mass loss from ZnO as the annealing temperature is increased above 300 °C. This is possibly due to oxygen evaporation from the sample. The c-axis lattice parameter, extracted from the XRD spectra, shows an increase due to annealing above 600 °C, which is a signature of the presence of a huge number of oxygen vacancies. Resistivity variations of the annealed samples are also consistent with the TG and XRD analyses. The ratio curve analysis of the CDBEPAR spectra successfully probes the change in zinc-related vacancy defects in annealed ZnO. [ABSTRACT FROM AUTHOR]

The role of the metastable Ar atoms in a 1-cm-diameter cylindrical hollow cathode discharge (HCD) is studied self-consistently based on a hybrid model and experimental measurements in the pressure range of 0.3–1 Torr and currents of 1–10 mA. The model comprises submodels based on the principles of Monte Carlo and fluid simulations. The Monte Carlo model describes the movement of the fast electrons, fast Ar and Cu atoms, and fast Ar+ and Cu+ ions as particles, while in the fluid model, the slow electrons, Ar+, Cu+ ions, Cu, and Ar metastable atoms are treated as a continuum. The population of the two metastable states within the 3p54s configuration (3P2 and 3P0) were combined into one collective level, for which the continuity equation was written. Typical calculation results are, among others, the two-dimensional profiles of the production and the loss rates of Ar metastable atoms, as well as the metastable atom densities and fluxes throughout the complete HCD. Moreover, the calculated radial profiles (averaged over the axial direction) of the Ar metastable atom density are compared with experimental radial density profiles recorded by laser absorption spectroscopy. The relative importance of the different processes determining the Ar metastable population is analyzed, as well as the influence of pressure and voltage on them. Experimental results evidence the presence of the metastable atom production source at the cathode surface, probably originating from fast Ar+ ions and Ar atoms impinging on it. Comparison between experimental and calculated Ar metastable atom densities shows a good agreement at low pressures, but at 1 Torr the calculated values differ by a factor of 2 from the measured ones. Several possible explanations for this discrepancy are discussed. [ABSTRACT FROM AUTHOR]

Accelerator-based γ-ray-induced positron annihilation spectroscopy performs positron annihilation spectroscopy by utilizing MeV bremsstrahlung radiation generated from an accelerator (We have named the technique “accelerator-based γ-ray-induced PAS,” even though “bremsstrahlung” is more correct here than “γ rays”. The reason for that is to make the name of the technique more general, since PAS may be performed by utilizing MeV γ rays emitted from nuclei through the use of accelerators as described later in this article and as in the case of positron lifetime spectroscopy [F.A. Selim, D.P. Wells, and J.F. Harmon, Rev. Sci. Instrum. 76, 033905 (2005)].) instead of using positrons from radioactive sources or positron beams. MeV γ rays create positrons inside the materials by pair production. The induced positrons annihilate with the material electrons emitting a 511-keV annihilation radiation. Doppler broadening spectroscopy of the 511-keV radiation provides information about open-volume defects and plastic deformation in solids. The high penetration of MeV γ rays allows probing of defects at high depths in thick materials up to several centimeters, which is not possible with most of the current nondestructive techniques. In this article, a detailed description of the technique will be presented, including its benefits and limitations relative to the other nondestructive methods. Its application on the investigation of plastic deformation in thick steel alloys will be shown. [ABSTRACT FROM AUTHOR]

We have studied the role of boron ion energy in the engineering of dislocation loops for silicon light-emitting diodes (LEDs). Boron ions from 10 to 80 keV were implanted in (100) Si at ambient temperature, to a constant fluence of 1×1015 ions/cm2. After irradiation the samples were annealed for 20 min at 950 °C by rapid thermal annealing. The samples were analyzed by transmission electron microscopy and Rutherford backscattering spectroscopy. It was found that the applied ion implantation/thermal processing induces interstitial perfect and faulted dislocation loops in {111} habit planes, with Burgers vectors a/2<110> and a/3<111>, respectively. The loops are located around the projected ion range, but stretch in depth approximately to the end of range. Their size and distribution depend strongly on the applied ion energy. In the 10 keV boron-implanted samples the loops are shallow, with a mean size of ∼30 nm for faulted loops and ∼75 nm for perfect loops. Higher energies yield buried, large, and irregularly shaped perfect loops, up to ∼500 nm, coexisting with much smaller faulted loops. In the latter case much more Si interstitials are bounded by the loops, which are assigned to a higher supersaturation of interstitials in as-implanted samples, due to separated Frenkel pairs. An interesting phenomenon was found: the perfect loops achieved a steady-state maximum size when the ion energy reached 40 keV. Further increase of the ion energy only increased the number of these large loops and made them bury deeper in the substrate. The results of this work contribute to laying a solid ground in controlling the size and distribution of dislocation loops in the fabrication of silicon LEDs. [ABSTRACT FROM AUTHOR]