Index of content:
Volume 113, Issue 9, 07 March 2013
- Lasers, Optics, and Optoelectronics
113(2013); http://dx.doi.org/10.1063/1.4793490View Description Hide Description
The saturation behavior in exciting power is the specific characteristic in low voltage cathodoluminescence due to the high resistivity of the phosphor, causing the saturation behavior in field emission display. With the increasing of current density , the saturation behavior in directly results in the saturation of electron-hole (e-h) generation rate . The e-h generation region would shrink, resulting in the drastic increase of e-h concentration generation rate , causing the decrease in probability of an e-h pair exciting an activator. In addition, the radiative transition probability of an excited activator would be decreased due to the shrinkage of and increase of temperature. To restrain the saturation behavior in low voltage cathodoluminescence, the ultimate method is to lower the resistivity of the phosphor. By introducing In2O3 conductive component into Y2O3:Eu3+ phosphor, restrained saturation behavior in cathodoluminescence of Y2O3:Eu3+ phosphor was effectively achieved.
113(2013); http://dx.doi.org/10.1063/1.4794018View Description Hide Description
Emission from InAs/GaAs quantum dots (QDs) treated with Sb sprays of 7.5, 15, 22.5, and 30 s duration immediately prior to capping with GaAs has been studied via temperature dependent photoluminescence. Room temperature spectra show a significant increase in output intensity as the quantum dots are exposed to the Sb spray, but this improvement is lost when the Sb exposure is extended beyond 15 s. For the 7.5 s and 15 s Sb spray samples, temperature-dependent photoluminescence taken between 20 and 300 K show an increase in emission for increasing temperature from 30 to ∼100 K, for samples with an Sb spray before rolling off at temperatures in excess of 100 K, an effect ascribed to a small energy barrier close to the dots. Fitting of the temperature dependent data suggests that the impact of the energy barrier is only seen for the samples with lower defect densities in the immediate vicinity of the quantum dots. Results found when varying the excitation wavelength suggest the energy barrier is most likely located away from the top of the quantum dots, with it suggested that the inferred energy barrier may be due to Sb clustering around the base of the quantum dots in the capping layer, or incorporated Sb in the wetting layer of the QDs.
113(2013); http://dx.doi.org/10.1063/1.4793633View Description Hide Description
Optical phenomena affecting the performance of (Al x Ga1− x )0.52In0.48P based optoelectronic devices, such as solar cells and light emitting diodes, depend critically on the determination of the optical constants of these alloys throughout their compositional range. In this work, a generalization of the description of the dielectric function of (Al x Ga1− x )0.52In0.48P alloys as a function of the Al composition x is obtained for 0.052 < x < 1, over the energy range of 1.25–5 eV. By comparison to previous approaches used to describe the dielectric function of AlInGaP alloys, the parametric model of Johs et al. [Thin Solid Films 313–314, 137 (1998)] represents a generalization of previous models, being fully Kramers-Kronig consistent and flexible enough to describe the non-parabolic behavior of the joint density of states away from given critical points of the Brillouin zone. It also matches our experimental data for the absorption coefficient below the fundamental band edge. The results suggest that part of the states originating from the L region of the Brillouin zone become available at Γ, leading to a decrease and an increase of the oscillator strengths of the E 1 and E 0 transitions, respectively. In addition, it is found that the contributions of indirect transitions from the X point of the Brillouin zone can be neglected as the vertical transitions largely dominate the description of the dielectric function of such alloys.
113(2013); http://dx.doi.org/10.1063/1.4794001View Description Hide Description
Formation of intrinsic acceptor defects in ZnO is rare due to the low formation energy of donors. Understanding this phenomenon is of interest for the fabrication of high quality light emitting diodes. Herein, we examine the temperature dependent formation of defects in nanocrystalline ZnO through a combination of X-ray excited optical luminescence (XEOL) together with X-ray absorption near edge structures (XANES) and electron spin resonance (ESR). Certain defects are shown to form under low temperature and are unstable above 700 °C. These defects have high g-values characteristic of acceptors and short spin-lattice relaxation times. XEOL measurements show that acceptor defects with a characteristic red luminescence are also formed under these conditions. Low g-value (donor) defects forming at temperatures >700 °C are shown to have spin-lattice relaxation characteristic of nonradiative recombination centers.
113(2013); http://dx.doi.org/10.1063/1.4794201View Description Hide Description
Thin-film CdS/CdTe solar cells fabricated on glass substrates have been considered as one of the most promising candidates for large-scale applications in the field of photovoltaic energy conversion. The recorded experimental efficiency of these cells is about 16%–17% and the corresponding theoretical values are more than 28%. The main causes of efficiency loss are due to optical and recombination losses. Most of the theoretical literatures either study the effect of recombination or optical losses on the CdS/CdTe solar cell efficiency. The present work studies the effect of both the optical and recombination losses on the current density and hence the solar cell efficiency. Calculations of optical losses have been carried out based on the optical constants (refractive index and extinction coefficient) of materials used and the thickness of ITO and CdS layers. Calculation of recombination losses has been based on the values of width of space-charge region and the absorption coefficient for CdTe. It has been found that the reflection losses of the interfaces air-glass, glass-ITO, ITO-CdS, and CdS-CdTe decrease the short-circuit current (J SC) from 31.24 to 28.2 mA/cm2 (9%). The absorption losses in ITO and CdS layers decrease J SC to 22.2 (20%). The recombination losses decrease J SC to 19.7 mA/cm2 (8%). The optical and recombination losses yield efficiency of CdS/CdTe solar cells in the range of 12%–16% at thickness 100 nm of each layer of ITO and CdS. According to these results, there is a good agreement between experimental and theoretical studies and this is the real start to develop the solar cells efficiency in the future studies.
113(2013); http://dx.doi.org/10.1063/1.4794494View Description Hide Description
Armchair graphene nanoribbons (A-GNRs), an alternative material for Infrared (IR) photodetectors, attract more attention because of those tunable energy gaps by changing the width of nanoribbons and the height of interband transition. In this paper, we calculate the dark current limited detectivity, D*, of the multi layer A-GNR based IR photodetector. For this purpose, we find the band structure of A-GNRs by tight-binding model and by considering the edge deformation, the absorption coefficient using the single electron approximation, the quantum efficiency, and the optical responsivity of photodetector. Then, the dark current of photodetector has been calculated by considering two contributions: (i) The interband tunneling generation and (ii) the thermogeneration due to the optical and acoustic phonon and line edge roughness (LER) scattering in the A-GNRs. Finally, we optimize the dark current limited detectivity of the photodetector for different structural parameters. The obtained results show that for the single layer A-GNR based photodetector with W = 5 nm, L = 20 μm, Vb = 2 V, Vg = 2 V, maximum value of dark current limited detectivity, D*, at T = 300 K is ∼2.2 × 108 (cm Hz1/2/W) and at T = 77 K is ∼2.1 × 1011 (cm Hz1/2/W). Also, for narrow A-GNRs, D* increases with increasing the gate voltage, while for wider A-GNRs decreases with increasing the gate voltage. Moreover, the dark current limited detectivity increases with increasing the number of the A-GNR layers.
- Plasmas and Electrical Discharges
113(2013); http://dx.doi.org/10.1063/1.4794031View Description Hide Description
Atmospheric-pressure gas discharge driven by high voltage pulses with fast rise-time and short duration has attracted significant attention for various plasma applications. In this paper, discharges were generated in a highly non-uniform electric field by point-plane gaps in open air by four repetitive nanosecond-pulse generators with repetition rate up to 1 kHz. The rise time of generators was 25 (generator #1), 15 (generator #2), 3 (generator #3), and 0.2 ns (generator #4) and a full width at half maximum was 40, 30-40, 5, and 1 ns, respectively. The experimental results show that there were typical discharge fashions, i.e., corona, diffuse, spark, or arc modes. The variables affecting the discharge characteristics, including the gap spacing and applied pulse parameters, were investigated. Especially, the diffuse discharges were investigated and discussed. With generator #1 at voltage 70-120 kV, characteristics of measured x-rays on the discharge modes were studied, and it indicates that counts of x-rays in a diffuse discharge are up to a peak value under the experimental conditions. With amplitude of voltage pulses in incident wave up to 18 (generator #3) and 12.5 kV (generator #4), runaway electron beam in low pressure helium, nitrogen, and air in a pulse-periodic mode of discharge with repetition rate up to 1 kHz was obtained. Electron beam was registered behind a thin foil in a pressure range from several to tens of Torr. X-ray radiation was obtained in a wide range of pressures, as well as at atmospheric pressure of helium, nitrogen, and air. Voltage pulses of positive and negative polarities were used. Generation of runaway electrons with pulses of positive polarity appeared because of reflected voltage pulses of reverse polarity.
Atmospheric pressure resistive barrier air plasma jet induced bacterial inactivation in aqueous environment113(2013); http://dx.doi.org/10.1063/1.4794333View Description Hide Description
An atmospheric pressure resistive barrier air plasma jet is designed to inactivate bacteria in aqueous media in direct and indirect exposure modes of treatment. The resistive barrier plasma jet is designed to operate at both dc and standard 50–60 Hz low frequency ac power input and the ambient air at 50% humidity level was used as the operating gas. The voltage-current characteristics of the plasma jet were analyzed and the operating frequency of the discharge was measured to be 20 kHz and the plasma power was measured to be 26 W. The plasma jet rotational temperatures are obtained from the optical emission spectra, from the N2 C-B(2+) transitions by matching the experimental spectrum results with the Spectra Air (SPECAIR) simulation spectra. The reactive oxygen and nitrogen species were measured using optical emission spectroscopy and gas analyzers, for direct and indirect treatment modes. The nitric oxides (NO) were observed to be the predominant long lived reactive nitrogen species produced by the plasma. Three different bacteria including Staphylococcus aureus (Gram-positive), Escherichia coli (Gram-negative), and Neisseria meningitidis (Gram-negative) were suspended in an aqueous media and treated by the resistive barrier air plasma jet in direct and indirect exposure modes. The results show that a near complete bacterial inactivation was achieved within 120 s for both direct and indirect plasma treatment of S. aureus and E. coli bacteria. Conversely, a partial inactivation of N. meningitidis was observed by 120 s direct plasma exposure and insignificant inactivation was observed for the indirect plasma exposure treatment. Plasma induced shifts in N. meningitidis gene expression was analyzed using pilC gene expression as a representative gene and the results showed a reduction in the expression of the pilC gene compared to untreated samples suggesting that the observed protection against NO may be regulated by other genes.
Influence of ambient air on the flowing afterglow of an atmospheric pressure Ar/O2 radiofrequency plasma113(2013); http://dx.doi.org/10.1063/1.4794324View Description Hide Description
The influence of ambient air on the flowing afterglow of an atmospheric pressure Ar/O2 radiofrequency plasma has been investigated experimentally. Spatially resolved mass spectrometry and laser induced fluorescence on OH radicals were used to estimate the intrusion of air in between the plasma torch and the substrate as a function of the torch-to-substrate separation distance. No air is detected, within the limits of measurement uncertainties, for separation distances smaller than 5 mm. For larger distances, the effect of ambient air can no longer be neglected, and radial gradients in the concentrations of species appear. The Ar 4p population, determined through absolute optical emission spectroscopy, is seen to decrease with separation distance, whereas a rise in emission from the N2(C–B) system is measured. The observed decay in Ar 4p and N2(C) populations for separation distances greater than 9 mm is partly assigned to the increasing collisional quenching rate by N2 and O2 molecules from the entrained air. Absorption measurements also point to the formation of ozone at concentrations from 1014 to , depending both on the injected O2 flow rate and the torch-to-substrate separation distance.
Quantitative determination of mass-resolved ion densities in H2-Ar inductively coupled radio frequency plasmas113(2013); http://dx.doi.org/10.1063/1.4794165View Description Hide Description
Inductively coupled H2-Ar plasmas are characterized by an energy-dispersive mass spectrometer (plasma monitor), a retarding field analyzer, optical emission spectroscopy, and a Langmuir probe. A procedure is presented that allows determining quantitatively the absolute ion densities of , , , , and from the plasma monitor raw signals. The calibration procedure considers the energy and mass-dependent transmission of the plasma monitor. It is shown that an additional diagnostic like a Langmuir probe or a retarding field analyzer is necessary to derive absolute fluxes with the plasma monitor. The conversion from fluxes into densities is based on a sheath and density profile model. Measurements were conducted for a total gas pressure of 1.0 Pa. For pure plasmas, the dominant ion is . For mixed H2-Ar plasmas, the molecular ion is the most dominant ion species in a wide parameter range. The electron density, ne , is around and the electron temperature, Te , decreases from 5 to 3 eV with increasing Ar content. The dissociation degree was measured by actinometry. It is around 1.7% nearly independent on Ar content. The gas temperature, estimated by the rotational distribution of the Q-branch lines of the H2 Fulcher-α diagonal band is estimated to (540 ± 50) K.
- Structural, Mechanical, Thermodynamic, and Optical Properties of Condensed Matter
Relationship of microstructure properties to oxygen impurities in nanocrystalline silicon photovoltaic materials113(2013); http://dx.doi.org/10.1063/1.4794353View Description Hide Description
We have fully investigated the correlation of microstructure properties and oxygen impurities in hydrogenated nanocrystalline silicon photovoltaic films. The achievement has been realized through a series of different hydrogen dilution ratio treatment by plasma enhanced chemical vapor deposition system. Raman scattering, x-ray diffraction, and ultraviolet-visible transmission techniques have been employed to characterize the physical structural characterization and to elucidate the structure evolution. The bonding configuration of the oxygen impurities was investigated by x-ray photoelectron spectroscopy and the Si-O stretching mode of infrared-transmission, indicating that the films were well oxidized in SiO2 form. Based on the consistence between the proposed structure factor and the oxygen content, we have demonstrated that there are two dominant disordered structure regions closely related to the post-oxidation contamination: plate-like configuration and clustered microvoids.
113(2013); http://dx.doi.org/10.1063/1.4794016View Description Hide Description
The effect of self-irradiation damage on the local structure of δ-Pu, PuAl2, PuGa3, and other Pu intermetallics has been determined for samples stored at room temperature using the extended x-ray absorption fine-structure (EXAFS) technique. These measurements indicate that the intermetallic samples damage at a similar rate as indicated in previous studies of PuCoGa5. In contrast, δ-Pu data indicate a much slower damage accumulation rate. To explore the effect of storage temperature and possible room temperature annealing effects, we also collected EXAFS data on a δ-Pu sample that was held at less than 32 K for a two month period. This sample damaged much more quickly. In addition, the measurable damage was annealed out at above only 135 K. Data from samples of δ-Pu with different Ga concentrations and results on all samples collected from different absorption edges are also reported. These results are discussed in terms of the vibrational properties of the materials and the role of Ga in δ-Pu as a network former.
Surface-induced charge at a Ge (100) dimer surface and its interaction with vacancies and self-interstitials113(2013); http://dx.doi.org/10.1063/1.4794029View Description Hide Description
The behavior of intrinsic point defects near the Ge (100) surface was investigated by using ab initio calculations. A thin plate model with clean Ge surfaces including a dimer structure on both sides was examined. A decrease of the formation energies of both types of intrinsic point defects near the surface is obtained similar as was reported before for Si. An important difference, however, is that the impact of the electric charges at the Si surface vanishes around the fifth layer, while for Ge, the effect of negative charges near the surface remains and positive charges are observed even deeper than the fifteenth layer from the surface. In bulk Ge, negatively charged vacancies are reported to be stable. Opposite to this, the neighbouring atoms around a vacancy near the Ge surface have a strong positive charge, compared with the case of Si. Taking these facts into consideration, the difference of the charge state of a vacancy in the bulk and that near the surface can be explained by band bending due to the surface-induced charge. A self-interstitial in bulk Ge most likely has a positive charge state (+2 or +1). It is also shown that a self-interstitial except for the position in the second layer away from the surface is positively charged. This charging can lead to a reflection of self-interstitials by the Ge surface because of the repulsive force between the positive charges of surface atoms and those of self-interstitials. Such interstitial reflection was suggested to explain experimental diffusion observations when self-interstitials are generated inside a bulk crystal by knocking-on Ge atoms in an implantation or irradiation process during a thermal anneal.
113(2013); http://dx.doi.org/10.1063/1.4794006View Description Hide Description
We describe a simple, aqueous and low thermal budget process for deposition of polycrystalline indium phosphide on silicon substrate. Using stoichiometric indium oxide films prepared from its spin-coated precursor on silicon as an intermediate step, we achieve stoichiometric indium phosphide films through phosphidisation. Both indium oxide and indium phosphide have been characterized for surface morphology, chemical composition, and crystallinity. The morphology and crystalline structure of the films have been explained in terms of the process steps involved in our deposition method. Incomplete phosphidisation of indium oxide to indium phosphide results in the restructuring of the partly unconverted oxide at the phosphidisation temperature. The optical properties of the indium phosphide films have been analyzed using micro photoluminescence and the results compared with those of a homoepitaxial layer and a theoretical model. The results indicate that good optical quality polycrystalline indium phosphide has been achieved. The Hall measurements indicate that the carrier mobilities of our samples are among the best available in the literature. Although this paper presents the results of indium phosphide deposition on silicon substrate, the method that we present is generic and can be used for deposition on any suitable substrate that is flexible and cheap which makes it attractive as a batch process for photovoltaic applications.
Predominant growth of non-polar a-plane (Al,Ga)N on patterned c-plane sapphire by hydride vapor phase epitaxy113(2013); http://dx.doi.org/10.1063/1.4794098View Description Hide Description
We report for the first time on predominant growth of non-polar a-plane (Al,Ga)N layers on patterned c-plane AlN/sapphire templates with ridges oriented along the Al2O3 direction. The layers were grown by hydride vapor phase epitaxy. During the first stages of the growth (Al,Ga)N nucleates simultaneously on top of the ridges, inside the trenches and on the trench sidewalls. As a result, two different (Al,Ga)N orientations are formed with respect to the horizontal growth front: c-plane (Al,Ga)N on the c-plane ridges as well as inside the trenches and a-plane (Al,Ga)N on the trench sidewalls. The growth rate of a-plane (Al,Ga)N exceeds that of c-plane regions, which leads to the complete overgrowth of c-plane (Al,Ga)N by the a-plane oriented material.
113(2013); http://dx.doi.org/10.1063/1.4794172View Description Hide Description
Chaotic synchronization of two identical microresonators has been theoretically achieved using open-plus-closed-loop (OPCL) method. The coupled resonating system is used as a mass detector, where one of the resonators is functioning as a reference, and the other is used as the sensor. Simulation results show that mass changes on the sensing resonator affect the overall synchronization, which is quantitatively characterized by a similarity measure. A practical system based on electrostatically driven resonators is discussed. The sensor design will possess high resistance to noise by taking advantage of the OPCL method that was traditionally used in secure communication.
113(2013); http://dx.doi.org/10.1063/1.4794127View Description Hide Description
A comprehensive P-V-T dataset for bcc-Mo was obtained at pressures up to 31 GPa and temperatures from 300 to 1673 K using MgO and Au pressure calibrants. The thermodynamic analysis of these data was performed using high-temperature Birch-Murnaghan (HTBM) equations of state (EOS), Mie-Grüneisen-Debye (MGD) relation combined with the room-temperature Vinet EOS, and newly proposed Kunc-Einstein (KE) approach. The analysis of room-temperature compression data with the Vinet EOS yields V 0 = 31.14 ± 0.02 Å3, KT = 260 ± 1 GPa, and KT ′ = 4.21 ± 0.05. The derived thermoelastic parameters for the HTBM include (∂KT /∂T) P = −0.019 ± 0.001 GPa/K and thermal expansion α = a 0 + a 1 T with a 0 = 1.55 ( ± 0.05) × 10−5 K−1 and a 1 = 0.68 ( ± 0.07) × 10−8 K−2. Fitting to the MGD relation yields γ0 = 2.03 ± 0.02 and q = 0.24 ± 0.02 with the Debye temperature (θ 0) fixed at 455-470 K. Two models are proposed for the KE EOS. The model 1 (Mo-1) is the best fit to our P-V-T data, whereas the second model (Mo-2) is derived by including the shock compression and other experimental measurements. Nevertheless, both models provide similar thermoelastic parameters. Parameters used on Mo-1 include two Einstein temperatures ΘE10 = 366 K and ΘE20 = 208 K; Grüneisen parameter at ambient condition γ 0 = 1.64 and infinite compression γ∞ = 0.358 with β = 0.323; and additional fitting parameters m = 0.195, e 0 = 0.9 × 10−6 K−1, and g = 5.6. Fixed parameters include k = 2 in Kunc EOS, m E1 = m E2 = 1.5 in expression for Einstein temperature, and a 0 = 0 (an intrinsic anharmonicity parameter). These parameters are the best representation of the experimental data for Mo and can be used for variety of thermodynamic calculations for Mo and Mo-containing systems including phase diagrams, chemical reactions, and electronic structure.
113(2013); http://dx.doi.org/10.1063/1.4794314View Description Hide Description
We investigate the vibrational properties of superlattices with layers of rubber and polyurethane foam, which can be either conventional or auxetic. Phononic dispersion calculations show a second pass band for transverse modes inside the lowest band gap of the longitudinal modes. In such a band, the superlattices behave as a double-negative elastic metamaterial since the effective dynamic mass density and shear modulus are both negative. The pass band is associated to a Fabry-Perot resonance band which turns out to be very narrow as a consequence of the high contrast between the acoustic impedances of the superlattice components.
113(2013); http://dx.doi.org/10.1063/1.4792755View Description Hide Description
Plate impact experiments in backward-impact geometry were performed on bismuth (Bi) in the pressure range of 11–70 GPa. The bismuth sample used as flyer impacted a LiF window, and the impact velocity and particle velocity at interface were simultaneously measured by a distance interferometer system for any reflector. Hugoniot and sound velocity data were extracted from the observed particle velocity profiles. The obtained plot of shock velocity (D) versus particle velocity (u) showed a discontinuity at u ≈ 0.9 km/s, corresponding to a pressure of ∼27 GPa. Furthermore, plate impact experiments in forward-impact geometry were conducted to measure sound velocities of bismuth. The extracted sound velocity data from backward and forward-impact experiments showed a transition from longitudinal to bulk sound velocity (18 GPa–27 GPa), and the pressure of transition to bulk sound velocity is consistent with the pressure of D-u knee at u ≈ 0.9 km/s. This D-u discontinuity at u ≈ 0.9 km/s is attributed to shock induced melting, and the onset and completion of melting on bismuth Hugoniot are estimated around 18 GPa and 27 GPa, respectively.
113(2013); http://dx.doi.org/10.1063/1.4794009View Description Hide Description
A comparative analysis of the residual stress distributions across the conductive channel of Ga-face AlGaN/GaN high electron mobility transistors (HEMTs) is presented. Stress was measured by means of micro-Raman spectroscopy and micro-photoluminescence (PL). Raman measurements probed the volume average of the stress through the GaN layer whereas the stress near the GaN surface (AlGaN/GaN heterointerface) was acquired via PL. By combining Raman, PL, and x-ray diffraction, a self-consistent method was developed to accurately determine the variation in magnitude of stress throughout the thickness of the GaN layer. Based on this framework, it is observed in AlGaN/GaN HEMTs that a depth variation in the GaN residual stress occurs near the gate and ohmic electrodes. At these regions, the stress near the AlGaN/GaN interface (or GaN surface) exhibits a tensile shift compared to the stress averaged through the entire thickness of GaN. Across the conductive channel (away from the metal pads), the bulk average stress and the stress near this interface remain nearly identical, showing little evidence of a vertical gradient. It is expected that the induced tensile strain at the drain side gate edge will have an impact on device reliability by contributing to the elastic energy built in the AlGaN barrier in addition to the inverse piezoelectric contribution at operating conditions, which may lead to formation of crystallographic defects.