Volume 114, Issue 2, 14 July 2013
Index of content:
- Lasers, Optics, and Optoelectronics
Tunable multifunctional magneto-optical devices based on magnetophotonic crystals comprising liquid crystal defect layers114(2013); http://dx.doi.org/10.1063/1.4812726View Description Hide Description
In this paper, we are going to demonstrate how realization of tunable magnetophotonic crystals (MPCs) is possible through introduction and investigation of a couple of structures containing a single liquid crystal (LC) defect layer. Our starting point is the recently discussed issue according to which in such structures any alteration in optical characteristics of the LC component will cause changes to happen regarding the overall magneto-optical (MO) response of the LC-based MPC. It will be shown that our optimized MPC structures are not only powerful in most respects but also multifunctional. For instance, while a structure is capable of being operated as a sensing tool with regard to the very factor that tends to control its MO response, other modes of operation offer a function of a switchable optical filter and also a perfect MO isolator.
An explanation for invalidity of working currents' derating on improving light-emitting diode devices' reliability114(2013); http://dx.doi.org/10.1063/1.4813092View Description Hide Description
Derating of the working current level does not work for improving GaN-based light-emitting diode (LED) devices' reliability. The present work demonstrates that it is not the levels but the specific components of the applied electrical currents weighing more on LEDs' degradation. Existing defects are sources for tunneling currents and Shockley-Read-Hall (SRH) non-radiative recombination current, and the component of tunneling currents and SRH non-radiative recombination current in the applied electrical current will in turn induce fast increase of defect density. The current component from electron tunneling to deep levels in the vicinity of mixed/screw dislocations will affect more on LEDs' degradation than other components, such as heavy-hole tunneling via intermediate state. In a whole, the overflow leakage current from the active region and Auger recombination currents in the applied electrical current will generate positive effects to alleviate LEDs' degradation.
The effect of ultrafast laser wavelength on ablation properties and implications on sample introduction in inductively coupled plasma mass spectrometry114(2013); http://dx.doi.org/10.1063/1.4812491View Description Hide Description
We investigated the role of femtosecond (fs) laser wavelength on laser ablation (LA) and its relation to laser generated aerosol counts and particle distribution, inductively coupled plasma-mass spectrometry (ICP-MS) signal intensity, detection limits, and elemental fractionation. Four different NIST standard reference materials (610, 613, 615, and 616) were ablated using 400 nm and 800 nm fs laser pulses to study the effect of wavelength on laser ablation rate, accuracy, precision, and fractionation. Our results show that the detection limits are lower for 400 nm laser excitation than 800 nm laser excitation at lower laser energies but approximately equal at higher energies. Ablation threshold was also found to be lower for 400 nm than 800 nm laser excitation. Particle size distributions are very similar for 400 nm and 800 nm wavelengths; however, they differ significantly in counts at similar laser fluence levels. This study concludes that 400 nm LA is more beneficial for sample introduction in ICP-MS, particularly when lower laser energies are to be used for ablation.
Absolute spectral characterization of silicon barrier diode: Application to soft X-ray fusion diagnostics at Tore Supra114(2013); http://dx.doi.org/10.1063/1.4813093View Description Hide Description
This paper presents an experimental protocol for absolute calibration of photo-detectors. Spectral characterization is achieved by a methodology that unlike the usual line emissions-based method, hinges on the Bremsstrahlung radiation of a Soft X-Ray (SXR) tube only. Although the proposed methodology can be applied virtually to any detector, the application presented in this paper is based on Tore Supra's SXR diagnostics, which uses Silicon Surface Barrier Diodes. The spectral response of these n-p junctions had previously been estimated on a purely empirical basis. This time, a series of second-order effects, like the spatial distribution of the source radiated power or multi-channel analyser non linearity, are taken into account to achieve accurate measurements. Consequently, a parameterised physical model is fitted to experimental results and the existence of an unexpected dead layer (at least 5 μm thick) is evidenced. This contribution also echoes a more general on-going effort in favour of long-term quality of passive radiation measurements on Tokamaks.
114(2013); http://dx.doi.org/10.1063/1.4813225View Description Hide Description
In the recent years, InGaN/GaN quantum well (QW) light emitting diodes (LEDs) have gathered much importance through the introduction of white LEDs and dual wavelength LEDs. However, the continuous tunability of InGaN/GaN QW LEDs has not been well addressed or discussed. In this paper, we introduce the tunability of an InGaN/GaN QW LED having a well width of 4 nm and In mole fraction of 0.3. The results, obtained from self-consistent solutions of the Schrödinger and Poisson equations, show that the transition energy of the LED may be continuously tuned by the device current. A prominent nonlinearity of the transition energy with the device current is generated, which should be of interest to the research workers in the field of optoelectronics.
- Plasmas and Electrical Discharges
114(2013); http://dx.doi.org/10.1063/1.4812577View Description Hide Description
The present study focuses on the role of mass removal mechanisms in ns-laser ablation. A copper sample is placed in argon, initially set at standard pressure and temperature. Calculations are performed for a 6 ns laser pulse with a wavelength of 532 nm and laser fluences up to 10 J/cm2. The transient behavior in and above the copper target is described by a hydrodynamic model. Transmission profiles and ablation depths are compared with experimental results and similar trends are found. Our calculations reveal an interesting self-inhibiting mechanism: volumetric mass removal in the supercritical region triggers plasma shielding and therefore stops proceeding. This self-limiting process indicates that volumetric mass removal does not necessarily result in large ablation depths.
Study of strain propagation in laser irradiated silicon crystal by time-resolved diffraction of K-α x-ray probe of different photon energies114(2013); http://dx.doi.org/10.1063/1.4813095View Description Hide Description
An experimental study on the time resolved x-ray diffraction from laser shocked silicon crystal, carried out using a 10 TW Ti:sapphire laser system, is presented. The characteristic Kα x-ray line radiation generated by 45 fs laser produced plasmas of two different target materials (iron and copper) is used as the probe, whereas the stretched pulse of sub-nanosecond duration (pump), derived from the same laser, is used to compress the sample. The use of x-ray probe of different photon energies yields information about the strain over a greater crystal depth. The dynamics of the strain propagation is inferred by monitoring the evolution of rocking curve width of the shocked sample at different time delays between the pump and the probe pulse. The shock velocity deduced from these measurements is ∼106 cm/s, consistent with the sound velocity in bulk silicon. The maximum elastic compression observed is 0.4%, indicating a pressure of 0.8 GPa.
- Structural, Mechanical, Thermodynamic, and Optical Properties of Condensed Matter
114(2013); http://dx.doi.org/10.1063/1.4812387View Description Hide Description
Al x Ni yFe(1−x−y) alloys are structural materials with potential application in high-temperature oxidizing environments. These materials are of specific interest as they have the ability to develop an oxidation resistant surface layer. To study diffusion and oxidation processes related to this surface layer formation, the mixing behavior of different sized Al grains in pure Ni and Fe matrices, with approximate grain/matrix atom ratio of 1:3, at temperatures above and below the structure melting point, was studied using ReaxFF-based molecular dynamics simulations. The simulations have been carried out at constant pressure, with temperatures being stepwise ramped over the range of 300-3000 K. For the Ni matrix, our results indicated lower chemical strain energy for Al in the mixed alloy and completion of mixing at a lower temperature for the Fe matrix. These results confirm that the Al-Ni alloy is energetically more stable than the Al-Fe alloy, which is in agreement with experiment. Further, larger Al grains appear to be favorable for mixing with Fe matrix, whereas for Ni matrix, smaller Al grains appear to be favorable. We suggest that this Al grain size effect on mixing matrices is due to the differences in formation energies between Ni/Al and Fe/Al alloys and differences in Ni-Ni and Fe-Fe bond distances. We also performed additional cooling simulations over the temperature range of 3000-300 K. The simulations revealed that for the considered cooling rate Fe alloy solidifies at a lower temperature than Ni alloy. Moreover, both alloys solidify to chemically disordered crystalline structures, of which the Ni structure is less ordered than the Fe structure. Preliminary oxidation simulations of slab structures with single grain indicate that the dynamics of matrix/grain mixing processes have a pronounced influence on the oxidation reactions. We find that Al and Ni atoms in their unmixed state are the most active reactants towards oxygen, while the Al/Ni alloy and pure Fe layers show substantially slower oxidation kinetics.
Effects of surface oxide layer on nanocavity formation and silver gettering in hydrogen ion implanted silicon114(2013); http://dx.doi.org/10.1063/1.4812736View Description Hide Description
We have made an investigation of the surface oxide effects on nanocavity formation in hydrogen implanted silicon and the influence of resultant nanocavities on diffusion and gettering of implanted silver atoms. A wafer with a 200-nm SiO2 surface layer was implanted with 22.5 keV H ions to a dose of 1 × 1017 cm−2, yielding the concentration peak of implanted H ions at ∼140 nm below the SiO2/Si interface. Subsequently, two sets of Si samples were prepared, depending on whether the oxide layer was etched off before (Group-A) or after (Group-B) post-H-implantation annealing. As evidenced by transmission electron microscopy, Group-A samples exhibited an array of large-sized nanocavities in hexagon-like shape, extending from the surface to the depth ∼140 nm, whereas a narrow band of sphere-shaped nanocavities of small size was present around 140 nm below the surface in Group-B samples. These Si samples with pre-existing nanocavities were further implanted with Ag ions in the surface region (∼40 nm projected range) and post-Ag-implantation annealing was conducted in the temperature range between 600 and 900 °C. Measurements based on Rutherford backscattering spectroscopy revealed much different behaviors for Ag redistribution and defect accumulation in these two sets of samples. Compared to the case for Group-B Si, Group-A Si exhibited a lower concentration of residual defects and a slower kinetics in Ag diffusion as well. We discuss the role of thick surface oxide in point defect generation and recombination, and the consequence on nanocavity formation and defect retention in Si. The properties of nanocavities, e.g., their depth distribution, size, and even shape, are believed to be responsible for the observed disparities between these samples, including an interesting contrast of surface vs. bulk diffusion phenomena for implanted Ag atoms.
The effect of a thermal gradient on the electromigration-driven surface morphological stabilization of an epitaxial thin film on a compliant substrate114(2013); http://dx.doi.org/10.1063/1.4812289View Description Hide Description
We report a theoretical analysis on the surface morphological stability of a coherently strained thin film that has been grown epitaxially on a deformable substrate and is simultaneously subjected to an external electric field and a temperature gradient. Using well justified approximations, we develop a three-dimensional model for the surface morphological evolution of the thin film and conduct a linear stability analysis of the heteroepitaxial film's planar surface state. The effect of the simultaneous action of multiple external fields on the surface diffusional anisotropy tensor is accounted for. Various substrate types are considered, but emphasis is placed on a compliant substrate that has the ability to accommodate elastically some of the misfit strain in the film due to its lattice mismatch with the substrate. We derive the condition for the synergy or competition of the two externally applied fields and determine the optimal alignment of the external fields that minimizes the critical electric field-strength requirement for the stabilization of the planar film surface. We also examine the role of the temperature dependence of the thermophysical properties and show that the criticality condition for planar surface stabilization does not change when the Arrhenius temperature dependence of the surface diffusivity is considered. Our analysis shows that surface electromigration and thermomigration due to the simultaneous action of properly applied and sufficiently strong electric fields and thermal gradients, respectively, can inhibit Stranski-Krastanow-type instabilities and control the onset of island formation on epitaxial film surfaces.
114(2013); http://dx.doi.org/10.1063/1.4812581View Description Hide Description
Mechanically induced reactivity is a promising means for designing self-reporting materials. Mechanically sensitive chemical groups called mechanophores are covalently linked into polymers in order to trigger specific chemical reactions upon mechanical loading. These mechanophores can be linked either within the backbone or as crosslinks between backbone segments. Mechanophore response is sensitive to both the matrix properties and placement within the matrix, providing two avenues for material design. A model framework is developed to describe reactivity of mechanophores located as crosslinks in a glassy polymer matrix. Simulations are conducted at the molecular and macromolecular scales in order to develop macroscale constitutive relations. The model is developed specifically for the case of spiropyran (SP) in lightly crosslinked polymethylmethacrylate (PMMA). This optically trackable mechanophore (fluorescent when activated) allows the model to be assessed in terms of observed experimental behavior. The force modified potential energy surface (FMPES) framework is used in conjunction with ab initio steered molecular dynamics (MD) simulations of SP to determine the mechanophore kinetics. MD simulations of the crosslinked PMMA structure under shear deformation are used to determine the relationship between macroscale stress and local force on the crosslinks. A continuum model implemented in a finite element framework synthesizes these mechanochemical relations with the mechanical behavior. The continuum model with parameters taken directly from the FMPES and MD analyses under predicts stress-driven activation relative to experimental data. The continuum model, with the physically motivated modification of force fluctuations, provides an accurate prediction for monotonic loading across three decades of strain rate and creep loading, suggesting that the fundamental physics are captured.
114(2013); http://dx.doi.org/10.1063/1.4812245View Description Hide Description
Strong modulations of the reflected x-ray intensities near the respective absorption edges of the constituent materials promise to determine layer composition of thin film structures along with spectroscopic like information. Near the absorption edge, the orders of magnitude more contrast beyond the pure electron density distributions of materials find an approach to overcome the low density difficulty of the conventional x-ray reflectivity technique. These aspects are explained by experimental studies on partially decomposed boron nitride thin films. Chemical composition profile is determined from free surface to the embedded buried layer with depth resolution in nanometer scale. The results of resonant reflectivity for chemical analysis are correlated with depth dependent x-ray photo electron spectroscopy.
114(2013); http://dx.doi.org/10.1063/1.4813134View Description Hide Description
We report the enhanced p-type conduction with Hall mobility of 53.3 cm2 V−1 s−1 in B-doped nanocrystalline diamond (NCD) films by 1000 °C annealing. High resolution transmission electronic microscopy, uv, and visible Raman spectroscopy measurements show that a part of amorphous carbon grain boundaries (GBs) transforms to diamond phase, which increases the opportunity of boron atoms located at the GBs to enter into the nano-diamond grains. This phase transition doping is confirmed by the secondary ion mass spectrum depth profile results that the concentration of B atoms in nano-diamond grains increases after 1000 °C annealing. It is also observed that 1000 °C annealing improves the lattice perfection, reduces the internal stress, decreases the amount of trans-polyacetylene, and increases the number or size of aromatic rings in the sp2-bonded carbon cluster in B-doped NCD films. These give the contributions to improve the electrical properties of 1000 °C annealed B-doped NCD films.
Predicting alloy vibrational mode properties using lattice dynamics calculations, molecular dynamics simulations, and the virtual crystal approximation114(2013); http://dx.doi.org/10.1063/1.4812737View Description Hide Description
The virtual crystal (VC) approximation for mass disorder is evaluated by examining two model alloy systems: Lennard-Jones argon and Stillinger-Weber silicon. In both material systems, the perfect crystal is alloyed with a heavier mass species up to equal concentration. The analysis is performed using molecular dynamics simulations and lattice dynamics calculations. Mode frequencies and lifetimes are first calculated by treating the disorder explicitly and under the VC approximation, with differences found in the high-concentration alloys at high frequencies. Notably, the lifetimes of high-frequency modes are underpredicted using the VC approximation, a result we attribute to the neglect of higher-order terms in the model used to include point-defect scattering. The mode properties are then used to predict thermal conductivity under the VC approximation. For the Lennard-Jones alloys, where high-frequency modes make a significant contribution to thermal conductivity, the high-frequency lifetime underprediction leads to an underprediction of thermal conductivity compared to predictions from the Green-Kubo method, where no assumptions about the thermal transport are required. Based on observations of a minimum mode diffusivity, we propose a correction that brings the VC approximation thermal conductivities into better agreement with the Green-Kubo values. For the Stillinger-Weber alloys, where the thermal conductivity is dominated by low-frequency modes, the high-frequency lifetime underprediction does not affect the thermal conductivity prediction and reasonable agreement is found with the Green-Kubo values.
Surface morphology evolution of m-plane GaN during molecular beam epitaxy growth: Impact of Ga/N ratio, miscut direction, and growth temperature114(2013); http://dx.doi.org/10.1063/1.4813079View Description Hide Description
We present a systematic study of morphology evolution of m-plane GaN grown by plasma-assisted molecular beam epitaxy on free-standing m-plane substrates with small miscut angles towards the –c and +c  directions under various gallium to nitrogen (Ga/N) ratios at substrate temperatures T = 720 °C and T = 740 °C. The miscut direction, Ga/N ratio, and growth temperature are all shown to have a dramatic impact on morphology. The observed dependence on miscut direction supports the notion of strong anisotropy in the gallium adatom diffusion barrier and growth kinetics. We demonstrate that precise control of Ga/N ratio and substrate temperature yields atomically smooth morphology on substrates oriented towards +c  as well as the more commonly studied –c miscut substrates.
114(2013); http://dx.doi.org/10.1063/1.4813482View Description Hide Description
A model was derived in this paper to calculate the high-temperature Hugoniot of solid material along isobaric path by using an enthalpy-based equation of state (EOS) model. It provides a way and complements the Mie-Grüneisen EOS for studying high-temperature Hugoniot of materials. The Hugoniot of tungsten at 1223 K in moderate pressure range (0–10 GPa) and the Hugoniot of molybdenum at 1673 K in high pressure range (10∼300 GPa) were calculated using the presented model. The calculated results fit in with the literature data. The model can satisfactorily predict the Hugoniot of solid at high temperature over a wide pressure range. The model was also extended to predict the Hugoniot of porous materials with high initial temperature along isobaric path; and the Hugoniots of multi-component solids and porous materials at high temperature were also calculated combining with the pressure equilibrium method.
Linear and nonlinear optical properties of ZnO/ZnS and ZnS/ZnO core shell quantum dots: Effects of shell thickness, impurity, and dielectric environment114(2013); http://dx.doi.org/10.1063/1.4813094View Description Hide Description
In the present work, we investigated theoretically the linear, nonlinear, and total absorption coefficients and refractive index changes associated with intersubband transitions in ZnO/ZnS core shell quantum dot (CSQD) and ZnS/ZnO inverted CSQD (ICSQD), emphasizing on the influence of the shell thickness, impurity, and dielectric environment. The effect of the polarization charges due to the possible existence of the dielectric mismatch between the system and its surrounding matrix is considered. The electronic structures are numerically calculated by employing the potential morphing method in the framework of effective mass approximation. We find that in both impurity-free CSQD and ICSQD, increasing the shell thickness red shifts significantly the threshold energy and enhances drastically the nonlinear absorption coefficients and all the refractive index changes, independently on the dielectric environments. Similar behaviour has also been observed in most of the cases studied when the impurity is displaced from the core center to the shell center. In contrast, comparing to a dielectrically homogeneous system, dispersing the systems into a matrix with a lower dielectric constant blue shifts all the peak positions of the absorption coefficients and refractive index changes. However, the corresponding magnitudes (in absolute value) are substantially reduced. Finally, we find that the nonlinear properties are more sensitive to the external perturbations, while at a weak radiation intensity, the variation of the total quantities is generally dominated by that of the corresponding linear terms.
114(2013); http://dx.doi.org/10.1063/1.4812496View Description Hide Description
This paper describes the strain-field analysis of threading edge dislocations (TEDs) and basal-plane dislocations (BPDs) in 4H-SiC using x-ray microbeam three-dimensional (3D) topography. This 3D topography enables quantitative strain-field analysis, which measures images of effective misorientations (Δω maps) around the dislocations. A deformation-matrix-based simulation algorithm is developed to theoretically evaluate the Δω mapping. Systematic linear calculations can provide simulated Δω maps (Δω sim maps) of dislocations with different Burgers vectors, directions, and reflection vectors for the desired cross-sections. For TEDs and BPDs, Δω maps are compared with Δω sim maps, and their excellent correlation is demonstrated. Two types of asymmetric reflections, high- and low-angle incidence types, are compared. Strain analyses are also conducted to investigate BPD-TED conversion near an epilayer/substrate interface in 4H-SiC.
114(2013); http://dx.doi.org/10.1063/1.4812569View Description Hide Description
A combination of electron microscopy and in-situ x-ray diffraction is employed to study the thermal oxidation of brass (Cu 0.7 Zn 0.3 alloy) in order to elucidate the mechanism of one-dimensional growth of ZnO nanostructures. Oxidation of the brass alloy results in the growth of a ZnO overlayer with ZnO nanowire formation on the ZnO layer. Increasing the oxidation temperature thickens the ZnO overlayer while suppressing ZnO nanowire formation on the top, which provides clear evidence that the formation of ZnO nanowires is related to a stress-driven mechanism that involves accumulation of compressive stress generated from the ZnO/Cu-Zn interfacial reaction and relaxation of the compressive stress by outward grain-boundary diffusion of Zn.
114(2013); http://dx.doi.org/10.1063/1.4813091View Description Hide Description
Multiple thickness Fe foils were ramp compressed over several nanoseconds to pressure conditions relevant to the Earth's core. Using wave-profile analysis, the sound speed and the stress-density response were determined to a peak longitudinal stress of 273 GPa. The measured stress-density states lie between shock compression and 300-K static data, and are consistent with relatively low temperatures being achieved in these experiments. Phase transitions generally display time-dependent material response and generate a growing shock. We demonstrate for the first time that a low-pressure phase transformation ( -Fe to -Fe) can be overdriven by an initial steady shock to avoid both the time-dependent response and the growing shock that has previously limited ramp-wave-loading experiments. In addition, the initial steady shock pre-compresses the Fe and allows different thermodynamic compression paths to be explored.