Volume 113, Issue 12, 28 March 2013
Index of content:
- Lasers, Optics, and Optoelectronics
113(2013); http://dx.doi.org/10.1063/1.4796101View Description Hide Description
Measurements of photoluminescence and its dependence on hydrostatic pressure are performed on a set of InN/n GaN superlattices with one InN monolayer and with different numbers of GaN monolayers. The emission energies, E PL , measured at ambient pressure, are close to the value of the band gap, Eg , in bulk GaN, in agreement with other experimental findings. The pressure dependence of the emission energies, dE PL /dp, however, resembles that of the InN energy gap. Further, the magnitudes of both E PL and dE PL /dp are significantly higher than those obtained from ab-initio calculations for 1InN/n GaN superlattices. Some causes of these discrepancies are suggested.
113(2013); http://dx.doi.org/10.1063/1.4795807View Description Hide Description
The modulated photocurrent (MPC) technique is employed to study the charge carriers' trapping states of pentacene films. The characteristics of the experimental MPC spectra were found to be compatible with trapping-detrapping process of holes in gap states in which their occupancy can be modified by the bias illumination. A demarcation energy level separating empty from partially occupied traps was deduced from the MPC spectra, which can be used to monitor bias-light induced changes in the quasi Fermi level. An exponential trap distribution from structural disorder and a deep metastable gaussian trap distribution from adsorbed environmental impurities were extracted by means of the MPC spectroscopy. An attempt to escape frequency of the order of was deduced for the gap sates. The derived trap distributions agree with those found before by means of other techniques. The present results indicate that the MPC technique can be used as a valuable tool for pentacene films characterization since it can be also applied to field effect samples.
113(2013); http://dx.doi.org/10.1063/1.4797492View Description Hide Description
The dynamic reflection spectra of amorphous blue phase III were investigated. When an electric field is applied to a blue phase III cell, the reflected wavelength does not shift obviously, but the intensity of reflection increases or decreases in a few ms. This fast intensity-tunable phenomenon in blue phase III relates to the dielectric anisotropies and chiralities of the liquid crystal and can be explained by the double twist model consisting of randomly orientated double-twisted cylinders. This study shows that blue phase III can act as a fast intensity-tunable reflector for a specific wavelength.
Modelling of infrared optical constants for polycrystalline low pressure chemical vapour deposition ZnO:B films113(2013); http://dx.doi.org/10.1063/1.4795809View Description Hide Description
Doped zinc oxide films are of high interest in thin film solar cell technology for application as transparent conducting oxide. Rapid and detailed characterisation of ZnO thin film properties is required for quality control and optimisation of the deposited films. In the present work, a new model of dielectric functions based on the effective medium approximation (EMA) is developed and is applied for characterisation of polycrystalline boron doped zinc oxide (ZnO:B) films, deposited by low pressure chemical vapour deposition (LPCVD) technique onto glass substrates. The model takes into account that polycrystalline ZnO is considered to consist of crystal grains surrounded by depletion layers. Using this model and Fourier Transform Infrared Spectroscopy (FTIR) performed in reflection configuration over a wide mid-infrared spectral region (from 2 μm up to 25 μm), the properties of depletion layer and the bulk of the grains in ZnO can be rapidly characterised in detail, and the volume fraction of the depletion layer can be extracted. The results are in good agreement with previously presented theories of electron transport in polycrystalline materials. Using electrical measurements like conductivity and Hall techniques in addition to the optically determined parameters, predominant electron scattering mechanisms in polycrystalline films for different doping levels are identified. The measurements show the impact of the doping level on depletion layer of the crystallites. It is shown, furthermore, that under a water vapour rich environment the volume fraction of the depletion layer may increase up to 5 times and more, while the mobility of the charge carriers in the depletion layer drops drastically from about 31 cm2V−1s−1 to about 8 cm2V−1s−1. This indicates that water vapour exposure causes an increase of the potential barrier in the grain boundary depletion layer, limiting the electron transport across the grain boundaries to a classical thermionic emission mechanism.
Quantitative isolation of band-gap formation mechanisms by randomizing the lattice arrangement in photonic crystals113(2013); http://dx.doi.org/10.1063/1.4797482View Description Hide Description
The band-gap formation mechanisms for photonic crystals (PCs) have been theoretically investigated on the basis of the analogy of the band gap to that for the semiconductors. On the reasonable assumption that the photonic band-gaps are created through both or either of the Bragg and the Mie processes, we attempted to quantitatively determine which process is more responsible for the band-gap formation. To achieve this purpose, we introduced the randomness of the lattice arrangement into the PCs. Through the analysis of the band-gap shrinkage as a function of the randomness, we have succeeded for the first time in quantitatively isolating the gap formation mechanisms in the dielectric PCs. From these results, we further conclude that the dielectric PCs have a greater tolerance than the metallic PCs for the fluctuation of the lattice arrangements, which would undoubtedly facilitate the fabrication of the crystals.
113(2013); http://dx.doi.org/10.1063/1.4798382View Description Hide Description
Laser machining is frequently utilized in the manufacture of photovoltaics. A natural by-product of these fabrication processes, heat, not only serves as a means of material removal but also modifies the material in an extended region beyond that ideally intended for alteration. This modified region, termed the heat affected zone, is detrimental to performance and should therefore be minimized. While undoubtedly thermal in origin, it is unclear exactly how the thermal environment during laser machining correlates to changes in the PN-junction that reduce performance. In response, we combine in-situ Raman based thermometry measurements with post-event failure analysis to identify the physical mechanisms damaging the junction during laser machining. From this approach, damage is shown to initiate prior to melting and be driven primarily by the diffusion of dopants for fluences that do not induce ablation. Additionally, comparatively small regions of damage are shown to have a large impact on operation.
Charge transfer assisted nonlinear optical and photoconductive properties of CdS-AgInS2 nanocrystals grown in semiconducting polymers113(2013); http://dx.doi.org/10.1063/1.4798383View Description Hide Description
Anisotropic CdS-AgInS2 nanocrystals are directly grown into benzothiadiazole-based semiconducting polymer solution. Their nonlinear optical absorption and nonlinear scattering are investigated with 7-ns laser pulses of 532-nm wavelength for optical limiting applications. Optical limiting is found to be enhanced in the polymer-CdS-AgInS2 nanocomposites, as compared to pure polymer or CdS-AgInS2 nanocrystals. The observation is explained quantitatively using a numerical model which includes one-photon-induced excited state absorption, two-photon absorption, and nonlinear scattering. Presence of charge transfer in polymer-CdS-AgInS2 nanocomposites is found from zero-biased photoconduction experiments, which plays an important role in the enhancement of nonlinear optical properties. In addition, it is also demonstrated that these nanocomposite films can be utilized for photodetection with large and fast photoconductive responses.
Improvement of the performance characteristics of deep violet InGaN multi-quantum-well laser diodes using step-graded electron blocking layers and a delta barrier113(2013); http://dx.doi.org/10.1063/1.4798388View Description Hide Description
The performance characteristics of deep violet In0.082Ga0.918N/GaN multi-quantum-well (MQW) laser diodes (LDs) with step-graded electron blocking layers (EBLs), a new EBL structure proposed by Zhang et al. and Yang et al., were numerically investigated. The simulation results indicated that the output emission wavelength was blue-shifted because of the change in the built-in polarization and internal electric field, although the use of step-graded EBLs effectively improved LD performance. To prevent the polarization effects and consequently a blue shift in the output emission wavelength, a new MQW LD structure with step-graded EBLs and a delta barrier as a last barrier is proposed. Simulation results indicated that the proposed LD structure exhibits higher values of output power, slope efficiency, and differential quantum efficiency, as well as lower threshold current, compared with conventional LD structures. The results also showed that the use of the delta barrier reduced the changes in the built-in polarization and internal electric field, and negligible changes in the output emission wavelength were observed.
113(2013); http://dx.doi.org/10.1063/1.4798267View Description Hide Description
The self-absorption of radiated photons increases the minority carrier concentration in semiconductor optoelectronic devices such as solar cells. This so-called photon recycling leads to an increase in the external luminescent efficiency, the fraction of internally radiated photons that are able to escape through the front surface. An increased external luminescent efficiency in turn correlates with an increased open-circuit voltage and ultimately conversion efficiency. We develop a detailed ray-optical model that calculates Voc for real, non-idealized solar cells, accounting for isotropic luminescence, parasitic losses, multiple photon reflections within the cell and wavelength-dependent indices of refraction for the layers in the cell. We have fabricated high quality GaAs solar cells, systematically varying the optical properties including the back reflectance, and have demonstrated Voc = 1.101 ± 0.002 V and conversion efficiencies of (27.8 ± 0.8)% under the global solar spectrum. The trends shown by the model are in good agreement with the data.
- Plasmas and Electrical Discharges
Characteristics of high-purity Cu thin films deposited on polyimide by radio-frequency Ar/H2 atmospheric-pressure plasma jet113(2013); http://dx.doi.org/10.1063/1.4795808View Description Hide Description
With a view to fabricating future flexible electronic devices, an atmospheric-pressure plasma jet driven by 13.56 MHz radio-frequency power is developed for depositing Cu thin films on polyimide, where a Cu wire inserted inside the quartz tube was used as the evaporation source. A polyimide substrate is placed on a water-cooled copper heat sink to prevent it from being thermally damaged. With the aim of preventing oxidation of the deposited Cu film, we investigated the effect of adding H2 to Ar plasma on film characteristics. Theoretical fitting of the OH emission line in OES spectrum revealed that adding H2 gas significantly increased the rotational temperature roughly from 800 to 1500 K. The LMM Auger spectroscopy analysis revealed that higher-purity Cu films were synthesized on polyimide by adding hydrogen gas. A possible explanation for the enhancement in the Cu film deposition rate and improvement of purity of Cu films by H2 gas addition is that atomic hydrogen produced by the plasma plays important roles in heating the gas to promote the evaporation of Cu atoms from the Cu wire and removing oxygen from copper oxide components via reduction reaction.
Numerical simulation of the plasma generated by the interaction high-current electron beam with Al target113(2013); http://dx.doi.org/10.1063/1.4798586View Description Hide Description
The results of one dimensional particle-in-cell simulations of the dynamics of plasma generated during the interaction of a high-energy (≤200 keV) and high-current (≤15 A/cm2) electron beam with an aluminum target are presented. The generated target plasma is low-ionized and characterized by non-Maxwellian electron energy distribution. The density and electron temperature of the plasma, which expands toward the anode at a typical velocity of ∼105 cm/s, does not exceed 4 × 1014 cm−3 and 1 eV, respectively, which is in satisfactory agreement with the experimental results presented in W. An et al., J. Appl. Phys. 110, 093304 (2011). The results of the simulations showed also acceleration of the ions from the target plasma toward the anode by the potential of the non-compensated space charge of the electron beam. The typical velocity of these energetic ions is ∼108 cm/s and depends on the electron current density and energy. These ions partially compensate the space charge of the electron beam, which leads to a decrease in the depth of the potential well.
113(2013); http://dx.doi.org/10.1063/1.4798361View Description Hide Description
The influence of the quantum shielding on the collisional atomic orientation phenomena is investigated for the electron-impact excitations of the hydrogenic ion. The excitation probabilities are derived as a function of the collision energy, impact parameter, and quantum wave number by using the semiclassical method and effective interaction potential. It is found that the influence of the oscillatory quantum shielding enhances the excitation probabilities. The detailed investigation on the variation of excitation preference due to the influence of the quantum shielding is also given.
- Structural, Mechanical, Thermodynamic, and Optical Properties of Condensed Matter
Effects of double layer AlN buffer layers on properties of Si-doped AlxGa1−xN for improved performance of deep ultraviolet light emitting diodes113(2013); http://dx.doi.org/10.1063/1.4798239View Description Hide Description
Si-doped Al 0.77Ga0.23N epilayers were grown on AlN/sapphire templates by metal organic chemical vapor deposition using double AlN buffer layers. It was found that the use of double AlN buffer layers improved the overall material quality of the Si-doped Al 0.77Ga0.23N epilayers, as evidenced in the decreased density of screw dislocations and surface pits and increased emission intensity ratio of the band-edge to the deep level impurity transition. Hall effect measurements also indicated improved n-type conductivity. The performance of the deep ultraviolet light-emitting diodes fabricated using double buffer layers was significantly improved, as manifested by enhanced output power and reduced turn-on voltage.
Point defects introduced by InN alloying into In x Ga1− x N probed using a monoenergetic positron beam113(2013); http://dx.doi.org/10.1063/1.4795815View Description Hide Description
Native defects in In x Ga1− x N (x = 0.06-0.14) grown by metal organic chemical vapor deposition were studied using a monoenergetic positron beam. Measurements of Doppler broadening spectra of the annihilation radiation as a function of incident positron energy for In x Ga1− x N showed that vacancy-type defects were introduced with increasing InN composition, and the major defect species was identified as complexes between a cation vacancy and a nitrogen vacancy. The concentration of the divacancy, however, was found to be suppressed by Mg doping. The momentum distribution of electrons at the In x Ga1− x N/GaN interface was close to that in defect-free GaN or In x Ga1− x N, which was attributed to localization of positrons at the interface due to the built-in electric field, and to suppression of positron trapping by vacancy-type defects. We have also shown that the diffusion property of positrons is sensitive to an electric field near the In x Ga1− x N/GaN interface.
113(2013); http://dx.doi.org/10.1063/1.4795813View Description Hide Description
VO2 films were deposited on Pt (111)/TiO2/SiO2/Si (001) substrates by means of a pulsed laser deposition technique. An x-ray diffraction peak at 2θ = 39.9° was deconvoluted into two pseudo-Voigt profiles of Pt (111) and VO x -originated components. The VO x diffraction peak was more obvious in a VO x /Pt (111)/Al2O3 (0001) sample, having a narrower width compared with a VO2/Al2O3 (0001) sample. Temperature-controlled Raman spectroscopy for the VO x /Pt/TiO2/SiO2/Si sample has revealed the monoclinic VO2 phase at low temperature and the structural phase transition at about 72 °C in a heating process. The electronic conductive nature at the high temperature phase was confirmed by near normal incidence infrared reflectivity measurements. Out-of-plane current-voltage characteristics showed an electric field-induced resistance switching at a voltage as low as 0.2 V for a 50 nm-thick film. A survey of present and previous results suggests an experimental law that the transition voltage of VO2 is proportional to the square root of the electrodes distance.
113(2013); http://dx.doi.org/10.1063/1.4798290View Description Hide Description
Carbon nanobuds, which are hybrid fullerene-nanotube structures, have previously shown glimpses of their potential in nanotechnology applications. By performing molecular dynamics simulations, a novel study of thermal conduction in a compact form of carbon nanobuds is presented. We demonstrate that nanobuds conduct thermal energy relatively well, within an order compared to nanotubes . Alike their close relatives—the carbon peapod, this essentially extends the dimensionality of thermal management applications. A rigorous calculation of the vibrational entropy and specific heat at the interfacial locations suggests that both functions are relatively identical in magnitude at fullerene-fullerene and fullerene-nanotube interfaces. Two common nanobud configurations are compared: while both are comparable in mechanical stability, a type 2 carbon nanobud (perfect fullerene, nanotube with vacancies) is less thermally conductive than a type 1 nanobud (perfect fullerene, perfect nanotube) due to more scattering of long-wavelength modes at the defect sites of the nanotube.
113(2013); http://dx.doi.org/10.1063/1.4798294View Description Hide Description
The phase formation and the glass-forming ability (GFA) of the Cu-Zr-Co system were studied. For the latter, predictions from a modified topological instability criterion were used to select six alloys. Five new glassy compositions were found and a good correlation was observed between the predicted GFA and characteristic thermal indicators. The addition of Co stabilizes a B2-type phase at room temperature in these alloys, which could be useful for the development of shape memory bulk metallic glass composites.
113(2013); http://dx.doi.org/10.1063/1.4796098View Description Hide Description
A phase-field model accounting for elastic inhomogeneity is established for microstructure study in martensitic materials. It is motivated by Hashin-Shtrikman variational formulation by introducing a homogeneous comparison medium and a polarized stress field. As a result, the driving force due to stress can be computed in the equivalent homogeneous medium since it is formally identical to that in the actual inhomogeneous solid. The model is applied to the simulations of three-dimensional self-accommodation patterns of microstructure for tetragonal and trigonal martensite. The results show that the former is an atypical pattern while the latter exhibits a common herringbone structure. Finally, the proposed framework also offers advantages of modeling other phase-transforming materials with ability in domain simulations together with effective properties as byproduct.
113(2013); http://dx.doi.org/10.1063/1.4796185View Description Hide Description
Multichannel transition visible and near-infrared (NIR) fluorescences have been captured in Dy3+-doped fiber-adaptive Na2O–ZnO–PbO–GeO2–TeO2 glasses. The maximum stimulated emission cross-sections σ em-max were derived to be 0.33 × 10−21, 3.66 × 10−21, and 0.67 × 10−21 cm2 for conventional visible emissions assigned to 4F9/2→6H J (J = 15/2, 13/2, and 11/2) transitions, respectively. Infrequent multi-peak NIR emissions were recorded in the spectral range of 900–1500 nm, among which the values of σ em-max were solved to be 1.05 × 10−22 and 1.56 × 10−22 cm2 for ∼1.02 and ∼1.18 μm emission bands. Internal quantum efficiency for the 4F9/2 level and external quantum yield for visible emissions of Dy3+ were determined to be 88.44% and 12.38%, severally. Effective multichannel radiative emissions reveal a potential in developing fiber-lighting sources, tunable lasers, and NIR optical amplifiers.
X-ray characterization techniques for the assessment of surface damage in crystalline wafers: A model study in AlN113(2013); http://dx.doi.org/10.1063/1.4798352View Description Hide Description
A high-resolution X-ray diffraction method with enhanced surface sensitivity has been used to investigate the effects of various polishing steps on the near-surface region of single crystal substrates. The method involves the study of a highly asymmetric reflection, observable under grazing incidence conditions. Analysis of rocking curve measurements and reciprocal space maps (RSMs) revealed subtle structural differences between the polished substrates. For aluminum nitride wafers, damage induced from diamond sawing and mechanical polishing was readily identifiable by on-axis rocking curves, but this method was unable to distinguish between sample surfaces subjected to various degrees of chemical mechanical polishing (CMP). To characterize sufficiently these surfaces, (10.3) RSMs were measured to provide both qualitative and quantitative information about the near-surface region. Two features present in the RSMs were utilized to quantitatively assess the polished wafers: the magnitude of the diffuse scatter in the omega-scans and the elongation of the crystal truncation rod. The method is able to distinguish between different degrees of CMP surface preparation and provides metrics to quantify subsurface damage after this polishing step.