Index of content:
Volume 113, Issue 22, 14 June 2013
- Lasers, Optics, and Optoelectronics
Electroinduced structural change- and random walks-based impact on the light emission in Er3+/Yb3+ doped (Pb,La)(Zr,Ti)O3 ceramics113(2013); http://dx.doi.org/10.1063/1.4810899View Description Hide Description
Remarkable enhancement/reduction of light emission and competition among different wavebands with rare earth doped lanthanum lead zirconate titanate ceramics were observed in a corona atmosphere caused by an externally applied electric field. Quantum-mechanical analyses, based on variation of structural symmetry of the unit cell and hence the crystal field due to electrostriction, were given to elucidate the effect. Apart from the symmetry of crystal field, the obvious contribution from the optoenergy storage and weak localization of light involved were discussed. These results are promising in designing and implementation of lasers and sensors.
- Plasmas and Electrical Discharges
Laser ablation of ceramic Al2O3 at 193 nm and 248 nm: The importance of single-photon ionization processes113(2013); http://dx.doi.org/10.1063/1.4809639View Description Hide Description
The aim of this work is to demonstrate that single-photon photoionization processes make a significant difference in the expansion and temperature of the plasma produced by laser ablation of ceramic Al 2O3 in vacuum as well as to show their consequences in the kinetic energy distribution of the species that eventually will impact on the film properties produced by pulsed laser deposition. This work compares results obtained by mass spectrometry and optical spectroscopy on the composition and features of the plasma produced by laser ablation at 193 nm and 248 nm, i.e., photon energies that are, respectively, above and below the ionization potential of Al, and for fluences between threshold for visible plasma and up to ≈2 times higher. The results show that the ionic composition and excitation of the plasma as well as the ion kinetic energies are much higher at 193 nm than at 248 nm and, in the latter case, the population of excited ions is even negligible. The comparison of Maxwell-Boltzmann temperature, electron temperatures, and densities of the plasmas produced with the two laser wavelengths suggests that the expansion of the plasma produced at 248 nm is dominated by a single population. Instead, the one produced at 193 nm is consistent with the existence of two populations of cold and hot species, the latter associated to Al + ions that travel at the forefront and produced by single photon ionization as well as Al neutrals and double ionized ions produced by electron-ion impact. The results also show that the most energetic Al neutrals in the plasma produced at the two studied wavelengths are in the ground state.
- Structural, Mechanical, Thermodynamic, and Optical Properties of Condensed Matter
Virtual GaN substrates via buffers on Si(111): Transmission electron microscopy characterization of growth defects113(2013); http://dx.doi.org/10.1063/1.4809561View Description Hide Description
The defects and strain of GaN(0001) films as virtual substrate on Si(111) with step-graded buffers were investigated by means of transmission electron microscopy. The misfit dislocation network identified in the interfaces nearly fully compensates the lattice mismatch. Inversion domains and pinholes occur within the closed GaN film. The atomic structure of the inversion domain boundaries is identified. Major parts of the films were found to be N-polar. Threading dislocations were formed as remains from the coalescence of initial GaN islands. Furthermore, the formation of small cubic inclusions is found to be restricted to the vicinity of the interface only.
113(2013); http://dx.doi.org/10.1063/1.4809928View Description Hide Description
By carefully controlling the surface chemistry of the chemical vapor deposition process for silicon carbide (SiC), 100 μm thick epitaxial layers with excellent morphology were grown on 4° off-axis SiC substrates at growth rates exceeding 100 μm/h. In order to reduce the formation of step bunching and structural defects, mainly triangular defects, the effect of varying parameters such as growth temperature, C/Si ratio, Cl/Si ratio, Si/H2 ratio, and in situ pre-growth surface etching time are studied. It was found that an in-situ pre growth etch at growth temperature and pressure using 0.6% HCl in hydrogen for 12 min reduced the structural defects by etching preferentially on surface damages of the substrate surface. By then applying a slightly lower growth temperature of 1575 °C, a C/Si ratio of 0.8, and a Cl/Si ratio of 5, 100 μm thick, step-bunch free epitaxial layer with a minimum triangular defect density and excellent morphology could be grown, thus enabling SiC power device structures to be grown on 4° off axis SiC substrates.
113(2013); http://dx.doi.org/10.1063/1.4810846View Description Hide Description
The growth of Cu 2ZnSnS4 (CZTS) polycrystals from solid state reaction over a range of compositions, including the regions which produce the highest efficiency photovoltaic devices, is reported. X-ray measurements confirm the growth of crystalline CZTS. Temperature and intensity dependent photoluminescence (PL) measurements show an increase in the energy of the main CZTS luminescence peak with both increasing laser power and increasing temperature. Analysis of the PL peak positions and intensity behavior demonstrates that the results are consistent with the model of fluctuating potentials. This confirms that the polycrystals are heavily doped with the presence of a large concentration of intrinsic defects. The behavior of the main luminescence feature is shown to be qualitatively similar over a broad range of compositions although the nature and amount of secondary phases vary significantly. The implications for thin-film photovoltaic devices are discussed.
113(2013); http://dx.doi.org/10.1063/1.4808283View Description Hide Description
We calculate first-principles interaction energies between substitutional solutes and oxygen interstitials in α-titanium and predict the effect of solutes on oxygen diffusion from those interactions. Interaction between 45 solutes across the periodic table and three oxygen interstitial sites are calculated with density-functional theory. The interaction energies show distinct trends across the periodic table corresponding to both atomic radii and the period. Changes in diffusion barrier due to solutes are modeled with the kinetically resolved activation barrier approximation. Solute effects at infinite dilution are numerically calculated and show both accelerated and reduced oxygen diffusivity.
Bioactivity and structural properties of nanostructured bulk composites containing Nb2O5 and natural hydroxyapatite113(2013); http://dx.doi.org/10.1063/1.4809653View Description Hide Description
In this work, we investigate the bioactivity and structural properties of nanostructured bulk composites that are composed of Nb 2O5 and natural hydroxyapatite (HAp) and are produced by mechanical alloying and powder metallurgy. X-ray diffraction and Raman spectroscopy data showed that the milling process followed by a heat treatment at 1000 °C induced chemical reactions along with the formation of the CaNb2O6, PNb9O25 and Ca3(PO4)2 phases. Rietveld refinement indicated significant changes in each phase weight fraction as a function of HAp concentration. These changes influenced the in vitro bioactivity of the material. XRD and FTIR analyses indicated that the composites exhibited bioactivity characteristics by forming a carbonated apatite layer when the composites were immersed in a simulated body fluid. The formed layers had a maximum thickness of 13 μm, as measured by confocal Raman spectroscopy and as confirmed by scanning electron microscopy. The results of this work suggest that the tested bulk composites are promising biomaterials for use in implants.
113(2013); http://dx.doi.org/10.1063/1.4810847View Description Hide Description
Both the single and multiple pulse ablation of dielectrics with high-intensity femtosecond lasers have been well studied. Additionally, the “pulse-shaping” regime, in which temporal peaks are separated by picoseconds, has also been investigated. In this paper, we show that a “prepulse configuration”—where one pulse is followed several nanoseconds later by a much stronger pulse—is qualitatively different from any of the previously investigated regimes. In particular, it leads to the recently discovered “concentric rings” feature in glass. We show that the damage pattern produced in glass is very different for two pulses separated by nanoseconds than for two pulses separated by either milliseconds or picoseconds. For nanosecond separations, the second pulse interacts with the shock wave created by the first, significantly altering the damage pattern. In addition to producing novel structures, this pump-probe technique provides a new way of studying ablation plume and shockwave dynamics.
113(2013); http://dx.doi.org/10.1063/1.4810898View Description Hide Description
The upconversion properties of Tm3+ and Yb3+ codoped CaSc2O4 phosphor excited at 980 nm are reported. The blue emission of Tm3+:1G4 → 3H6, red emission of Tm3+:1G4 → 3F4, near-infrared emissions of Tm3+:3H4 → 3H6 (800 nm) and 3F4 → 3H6 (1600 nm) are observed when excited Yb3+:2F5/2 level. The spectral properties of Yb3+:2F5/2 and Tm3+:3F4, 3H4, 1G4 levels are described in detail as a function of Yb3+ and Tm3+ concentrations. By the trends of intensity ratios of Tm3+:3F4 to Yb3+:2F5/2 and Tm3+:1G4 to 3H4 with the doped concentrations, upconversion dynamics is analyzed in Tm3+ and Yb3+ codoped CaSc2O4 material. The concentration dependent lifetimes of Yb3+:2F5/2 and Tm3+:1G4, 3H4 levels measured prove the existence of the efficient Yb3+ → Tm3+ energy transfer and followed Tm3+ → Yb3+ back-energy transfer processes, respectively. The energy transfer efficiency is up to 70% before concentration quenching occurrence. The back-energy transfer process is evidenced by the presence of the Yb3+:2F5/2 → 2F7/2 in the emission spectrum excited Tm3+:1G4 level at 466 nm. The detailed spectroscopic study conduces to understand the upconversion dynamics process in Tm3+ and Yb3+ codoped excellent CaSc2O4 oxide material.
113(2013); http://dx.doi.org/10.1063/1.4811185View Description Hide Description
Melt quenching of B2O3 with less than 25 mol. % rare-earth oxide (RE 2O3) at ambient pressure results in a milky white glass because of liquid-liquid phase separation into B2O3 and RE 2O3·3B2O phases. In contrast, we have found that melt quenching under GPa-order pressure realizes a transparent RE-doped B2O3 glass. This study investigates the local structure around the RE ions in the B2O3 glass prepared at 3 GPa using optical measurements and electron-spin-echo envelope modulation spectroscopy. It is shown that the RE-rich microparticles disappear and the RE ions are isolated from each other in a highly symmetric crystal field formed by triangular and tetrahedral boron units. This result is consistent with that extrapolated from the data for RE-doped sodium borate glasses.
113(2013); http://dx.doi.org/10.1063/1.4811350View Description Hide Description
The microstructure and the crystallization mechanism of amorphous silicon thin films during metal induced lateral crystallization (MILC) under an electric field were investigated. With the applied electric field, the growth rate was enhanced in the anode direction, while it was retarded in the cathode direction. The microstructure of the MILC region under an electric field showed a bi-directional needle network structure, but with an enhanced directionality in the anode direction. The enhanced MILC growth under an electric field was due to the negatively charged Ni-ions, which played a vital role in the 4-step MILC growth mechanism.
Charge transfer processes and ultraviolet induced absorption in Yb:YAG single crystal laser materials113(2013); http://dx.doi.org/10.1063/1.4810858View Description Hide Description
Charge transfer (CT) transitions and UV induced color centers in Yb:YAG single crystals have been investigated. A simultaneous pair formation of a stable ion and a hole related color center (hole polaron) are observed through a CT-process. Slightly different types of hole related color centers are formed in Yb:YAG crystals containing small levels of iron impurities. Furthermore, excitation spectroscopy on the UV irradiated Yb:YAG samples could confirm an energy transfer process between and ions. The findings are important for an increased knowledge of the physical loss mechanisms observed in Yb-doped laser materials, such as the nonlinear decay process in Yb:YAG crystals as well as the photodarkening phenomenon in Yb-doped fiber lasers.
113(2013); http://dx.doi.org/10.1063/1.4810859View Description Hide Description
We studied state-filling-dependent intraband carrier dynamics in InAs/GaAs self-assembled quantum dots using two-color photoexcitation spectroscopy. The photoluminescence (PL) intensity was observed to be dramatically reduced by selectively pumping carriers from the intermediate state to the continuum state located above the conduction band edge, and the PL-intensity reduction decreased with an increase in the continuous-wave excitation power. We analyzed the observed state-filling-dependent intraband carrier dynamics by detailed modeling of carrier excitation and relaxation processes in which the two-photon absorption for the interband transition, Pauli blocking, and saturable absorption for the intraband transition is considered.
113(2013); http://dx.doi.org/10.1063/1.4810926View Description Hide Description
Unlike conventional resonant ultrasonic spectroscopy aimed at determining elastic constants and related parameters of solids, resonant ultrasound spectroscopy of defects (RUSOD) addresses an opportunity to detect, visualize, and classify mechanical defects in materials. The approach is based on the resonant ultrasonic wave-defect interaction due to local defect resonance. RUSOD is shown to be defect- and frequency selective imaging technique capable of distinguishing between different defects by variation of ultrasonic frequency.
113(2013); http://dx.doi.org/10.1063/1.4810929View Description Hide Description
A model framework for predicting the dynamic shock-compression response of heterogeneous powder mixtures using readily obtained measurements from quasi-static tests is presented. Low-strain-rate compression data are first analyzed to determine the region of the bulk response over which particle rearrangement does not contribute to compaction. This region is then fit to determine the densification modulus of the mixture, , an newly defined parameter describing the resistance of the mixture to yielding. The measured densification modulus, reflective of the diverse yielding phenomena that occur at the meso-scale, is implemented into a rate-independent formulation of the P-α model, which is combined with an isobaric equation of state to predict the low and high stress dynamic compression response of heterogeneous powder mixtures. The framework is applied to two metal + metal-oxide (thermite) powder mixtures, and good agreement between the model and experiment is obtained for all mixtures at stresses near and above those required to reach full density. At lower stresses, rate-dependencies of the constituents, and specifically those of the matrix constituent, determine the ability of the model to predict the measured response in the incomplete compaction regime.
- Electronic Structure and Transport
113(2013); http://dx.doi.org/10.1063/1.4809572View Description Hide Description
We use Monte-Carlo Simulations to study the conductance switching generated by gas-induced electron trapping/-releasing in films of sintered metal oxide nanoparticles by using a site-bond percolation model. We explore the possibilities of gas sensors based on these mechanisms. In our study, we model films of different thicknesses where the conductance values of the grains (sites) and of the contacts (bonds) between these grains depend on the surface density Nr of adsorbed gas molecules from the ambient atmosphere. Below a critical density , the system is insulating due to the interruption of current flow, either through the connecting bonds or through the grain interior. This leads to two competing critical gas covering thresholds and , respectively, that separate the insulating from the conducting phase. For , the characteristic curve of monodisperse sensors shows a noticeable jump from zero to a finite conductance at , while for polydisperse sensors site percolation effects modify the jump into a steep increase of the characteristic curve and thus lead to an enhanced sensitivity. For , both mono- and polydisperse systems follow the same curves that show a smoother characteristic increase which reveals that, despite the occurrence of an inherent bond percolation effect close to , the increase of the bonds is the dominating effect.
Islands stretch test for measuring the interfacial fracture energy between a hard film and a soft substrate113(2013); http://dx.doi.org/10.1063/1.4810763View Description Hide Description
We present a technique for measuring the interfacial fracture energy, Γi , between a hard thin film and a soft substrate. A periodic array of hard thin islands is fabricated on a soft substrate, which is then subjected to uniaxial tension under an optical microscope. When the applied strain reaches a critical value, delamination between the islands and the substrate starts from the edge of the islands. As the strain is increased, the interfacial cracks grow in a stable fashion. At a given applied strain, the width of the delaminated region is a unique function of the interfacial fracture energy. We have calculated the energy release rate driving the delamination as a function of delamination width, island size, island thickness, and applied strain. For a given materials system, this relationship allows determination of the interfacial fracture energy from a measurement of the delamination width. The technique is demonstrated by measuring the interfacial fracture energy of plasma-enhanced chemical vapor deposition SiNx islands on a polyimide substrate. We anticipate that this technique will find application in the flexible electronics industry where hard islands on soft substrates are a common architecture to protect active devices from fracture.
113(2013); http://dx.doi.org/10.1063/1.4810853View Description Hide Description
This paper describes studies of the effect of electron tunneling on magnetic switching in single Co particles large enough to exhibit continuous energy spectra at mK-temperatures. The ground state spin S 0, in units of , is estimated to be in these particles. The magnetic switching field decreases versus tunneling current, with the effective magnetic temperature at the switching field smaller by factor of 2–3 compared to that found previously in smaller Co particle where . We show that this relatively weak size dependence confirms that the magnetic tunneling transitions in the particle are driven by mesoscopic fluctuations in magnetic anisotropy energy.
Femtosecond laser diagnostics of the built-in electric field across the p+-Si/SiO2 interface and its ultrafast shielding113(2013); http://dx.doi.org/10.1063/1.4810902View Description Hide Description
Ultrafast shielding of the built-in electric field E 0 across the p+-Si/SiO2 interface of boron doped Si upon near infrared femtosecond (fs) laser pulse irradiation (73 ± 5 fs, 35 GW/cm2≤ ≤ 115 GW/cm2) is shown to be dominated by electron-hole (e-h) pairs generated via two-photon absorption (TPA), whereas contributions from one-photon absorption (OPA) appear negligible. E 0 shows up in the instantaneous signal of the Electric Field Induced Second Harmonic (EFISH). Its power law is derived from the linear log vs. log plots of six fs laser wavelengths 741.2 nm ≤ λ ≤ 801.0 nm for the first time. These reveal 1.2 ≤ n(λ) ≤ 2.1 with the minimum at λ = 752.4 nm (2hν = 3.3 eV) related to resonantly enhanced TPA. Shielding of E 0 by e-h pairs from OPA cannot be detected by EFISH in the same fs laser pulse as their generation requires relatively slow electron-phonon coupling.
Electrical conduction in polyethylene: The role of positive charge and the formation of positive packets113(2013); http://dx.doi.org/10.1063/1.4810857View Description Hide Description
A model for positive hole transport in polyethylene is developed which takes particular account of the crystalline-amorphous morphology of the polymer. The significant feature is the employment of super-exchange quantum mechanical tunneling to explain hole transport through the amorphous phase. The consequence is that the hole mobility exhibits a maximum as a function of the electric field, a manifestation of the inverse Marcus effect. It is shown that this feature accounts for the majority of the reported high-field hole transport effects in polyethylene, including packet formation.