Index of content:
Volume 115, Issue 15, 21 April 2014
Multicrystalline silicon (mc-Si) is currently dominating the silicon solar cell market due to low ingot costs, but its efficiency is limited by transition metals, extended defects, and light-induced degradation (LID). LID is traditionally associated with a boron-oxygen complex, but the origin of the degradation in the top of the commercial mc-Si brick is revealed to be interstitial copper. We demonstrate that both a large negative corona charge and an aluminum oxide thin film with a built-in negative charge decrease the interstitial copper concentration in the bulk, preventing LID in mc-Si.
- Lasers, Optics, and Optoelectronics
115(2014); http://dx.doi.org/10.1063/1.4870828View Description Hide Description
We investigate the effect of a perpendicular static magnetic field on the optical bandgap of a one-dimensional (1D) graphene-dielectric photonic crystal in order to examine the possibility of reaching a rich tunable photonic bandgap. The solution of the wave equation in the presence of the anisotropic Hall situation suggests two decoupled circularly polarized wave each exhibiting different degrees of bandgap tunability. It is also numerically demonstrated that applying different values of field intensity lead to perceptible changes in photonic bandgap of such a structure. Finally, the effect of opening a finite electronic gap in the spectrum of graphene on the optical dispersion solution of such a 1D photonic crystal is reported. It is shown that increasing the value of the electronic gap results in the shrinkage of the associated photonic bandgaps.
Temporal characteristics of polarization holographic gratings formed in a photosensitive polymeric film containing N-benzylideneaniline derivative side groups115(2014); http://dx.doi.org/10.1063/1.4871681View Description Hide Description
A polarization holographic grating was recorded in a transparent thin film formed from polymethacrylate with N-benzylideneaniline (NBA) derivative side groups. We measured the real time diffraction properties. The data were analyzed based on a theoretical model that accounted for the distribution of optical anisotropy caused by molecular reorientation as well as for surface relief (SR) deformation caused by molecular motion. Optical anisotropy rapidly increased and then slowly decreased, with increasing recording time. This phenomenon was described based on photoisomerization and photocleavage reactions of the NBA side groups. SR deformation was also induced in the film by polarization holographic recording, without any subsequent processes. The photoinduced optical anisotropy and SR deformation were retained after the recording was turned off.
115(2014); http://dx.doi.org/10.1063/1.4871977View Description Hide Description
The Tungsten (W) and Silver (Ag) codoped TiO2 samples were successfully synthesized by hydrothermal method without any post calcination. To understand the correlation between electronic structure and photocatalytic properties, the synthesized samples were characterized using X-ray diffraction, Brunauer–Emmett–Teller specific surface area, transmission electron microscopy, ultra-violet–visible absorption spectra, and X-ray photoelectron spectroscopy (XPS), and the photocatalytic properties were evaluated under visible light irradiations. Codoping could not induce any changes in the phase and all the synthesized samples displayed pure anatase phase with spherical morphology. Visible light absorptions of the codoped samples were dramatically improved compared to the corresponding mono-doped samples. XPS analysis indicated that the dopant atoms successfully entered the TiO2 network. Results from the visible light photodegradation experiments showed that tungsten-silver codoped TiO2 possessed strong ability in photo-degrading methylene blue compared to tungsten doped TiO2 and silver doped TiO2, which was attributed to the smaller particle size, higher specific surface area, enhanced visible light absorption, and improved separation of photogenerated carriers.
Study of the shape of an optical window in a super-resolution state by electromagnetic-thermal coupled simulation: Effects of melting of an active layer in an optical disc115(2014); http://dx.doi.org/10.1063/1.4871858View Description Hide Description
We performed a multi-physics simulation for the propagation of electromagnetic waves and heat conduction in a super-resolution optical disc that includes an active layer of InSb. Because the change in the optical constant of InSb due to the phase transition is taken into account, the melting of the active layer can be realistically simulated in our calculation. It was found that in the case of an incident light power (P) of 2 mW, a profile of the electric field intensity transmitted through the InSb layer has an asymmetric shape with a narrow peak. This beam-narrowing was suggested to be an essential mechanism of the super-resolution, because a narrower light beam allows the detection of a smaller pit structure than the optical diffraction limit. This beam-narrowing was found to be originating from a small molten region produced in the InSb layer, which works as a mask for light exposure.
115(2014); http://dx.doi.org/10.1063/1.4872140View Description Hide Description
Recently, diffuse reflectors are being incorporated into solar cells, due to the advantage of no metallic absorption loss, higher reflectance, decent light scattering property by embedded TiO2 scatterers, and the ease of fabrication. Different methods have been employed to analyze diffuse reflectors, including Monte Carlo method, N-flux method, and a one-dimensional approximation based on semi-coherent optics, and the calculated reflectance is around 80% by these methods. In this work, rigorous wave optics solution is used, and it is shown that the reflectance for diffuse medium mirrors can actually be as high as >99% over a broad spectral range, provided the TiO2 scatterer geometry is properly optimized. The bandwidth of diffuse reflectors is un-achievable by other dielectric mirrors such as distributed Bragg reflectors or high index contrast grating mirror, using the same index contrast. Finally, it is promisingly found that even if the distribution of TiO2 is random, the wide-band reflection can still be achieved for the optimized TiO2 geometry. Initial experimental result is included in the supplementary material which shows the high feasibility of diffuse medium mirrors for solar cells.
- Plasmas and Electrical Discharges
115(2014); http://dx.doi.org/10.1063/1.4871635View Description Hide Description
The metal ionized flux fraction and production of double charged metal ions Me 2+ of different materials (Al, Cu, Fe, Ti) by High Power Impulse Magnetron Sputtering (HiPIMS) operated with and without a pre-ionization assistance is compared in the paper. The Electron Cyclotron Wave Resonance (ECWR) discharge was employed as the pre-ionization agent providing a seed of charge in the idle time of HiPIMS pulses. A modified grid-free biased quartz crystal microbalance was used to estimate the metal ionized flux fraction ξ. The energy-resolved mass spectrometry served as a complementary method to distinguish particular ion contributions to the total ionized flux onto the substrate. The ratio between densities of doubly Me 2+ and singly Me + charged metal ions was determined. It is shown that ECWR assistance enhances Me 2+ production with respect of absorbed rf-power. The ECWR discharge also increases the metal ionized flux fraction of about 30% especially in the region of lower pressures. Further, the suppression of the gas rarefaction effect due to enhanced secondary electron emission of Me 2+ was observed.
115(2014); http://dx.doi.org/10.1063/1.4871755View Description Hide Description
Reactive gases such as oxygen and water vapor modify the surface morphology of BaO dispenser cathodes and degrade the electron emission properties. For vacuum cathodes operating at fixed temperature, the emission current drops rapidly when oxygen adsorbs on top of the low work function surface. Previous experiments have shown that plasma cathodes are more resistant to oxygen poisoning and can operate with O2 partial pressures one to two orders of magnitude higher than vacuum cathodes before the onset of poisoning occurs. Plasma cathodes used for electric thrusters are typically operated with xenon; however, gas phase barium, oxygen, and tungsten species may be found in small concentrations. The densities of these minor species are small compared with the plasma density, and thus, their presence in the discharge does not significantly alter the xenon plasma parameters. It is important, however, to consider the transport of these minor species as they may deposit on the emitter surface and affect the electron emission properties. In this work, we present the results of a material transport model used to predict oxygen fluxes to the cathode surface by solving the species conservation equations in a cathode with a 2.25 mm diameter orifice operated at a discharge current of 15 A, a Xe flow rate of 3.7 sccm, and 100 ppm of O2. The dominant ionization process for O2 is resonant charge exchange with xenon ions. Ba is effectively recycled in the plasma; however, BaO and O2 are not. The model shows that the oxygen flux to the surface is not diffusion-limited; therefore, the high resistance to oxygen poisoning observed in plasma cathodes likely results from surface processes not considered here.
- Structural, Mechanical, Thermodynamic, and Optical Properties of Condensed Matter
Temperature-dependent Raman and ultraviolet photoelectron spectroscopy studies on phase transition behavior of VO2 films with M1 and M2 phases115(2014); http://dx.doi.org/10.1063/1.4870868View Description Hide Description
Structural and electronic phase transitions behavior of two polycrystalline VO2 films, one with pure M1 phase and the other with pure M2 phase at room temperature, were investigated by temperature-controlled Raman spectroscopy and ultraviolet photoelectron spectroscopy (UPS). We observed characteristic transient dynamics in which the Raman modes at 195 cm−1 (V-V vibration) and 616 cm−1 (V-O vibration) showed remarkable hardening along the temperature in M1 phase film, indicating the rearrangements of V-V pairs and VO6 octahedra. It was also shown that the M1 Raman mode frequency approached those of invariant M2 peaks before entering rutile phase. In UPS spectra with high energy resolution of 0.03 eV for the M2 phase film, narrower V3d band was observed together with smaller gap compared to those of M1 phase film, supporting the nature of Mott insulator of M2 phase even in the polycrystalline film. Cooperative behavior of lattice rearrangements and electronic phase transition was suggested for M1 phase film.
115(2014); http://dx.doi.org/10.1063/1.4871190View Description Hide Description
Nanoindentation-induced phase transformations have been studied in amorphous Ge thin films. These films initially tend to deform via plastic flow of the amorphous phase under load but at a critical pressure a sudden phase transformation occurs. This transformation, to a soft metallic (β-Sn-like)-Ge phase confined under the indenter, is signified by a “pop-in” event on loading. Following “pop-in,” the indentation tests fall into two distinct types of behavior. In one case, the rate of deformation with increasing load after “pop-in” increases, and the observed end-phase following complete unloading is observed to be predominately diamond-cubic Ge. In the other case, the deformation rate (slope of the loading curve) remains the same as that before “pop-in,” and the end phases following unloading are found to contain predominantly unstable r8 and more stable hexagonal Ge phases. The different transformation pathways for these two cases are shown to be related to the probability that the soft (β-Sn-like)-Ge phase volume, which suddenly forms at the transformation pressure, is either unconstrained by the indenter tip (the first case) or totally constrained under the indenter tip (in the latter case).
Materials properties and dislocation dynamics in InAsP compositionally graded buffers on InP substrates115(2014); http://dx.doi.org/10.1063/1.4871289View Description Hide Description
The properties of InAs xP1−x compositionally graded buffers grown by metal organic chemical vapor deposition are investigated. We report the effects of strain gradient (ε/thickness), growth temperature, and strain initiation sequence (gradual or abrupt strain introduction) on threading dislocation density, surface roughness, epi-layer relaxation, and tilt. We find that gradual introduction of strain causes increased dislocation densities (>106/cm2) and tilt of the epi-layer (>0.1°). A method of abrupt strain initiation is proposed which can result in dislocation densities as low as 1.01 × 105 cm−2 for films graded from the InP lattice constant to InAs 0.15P0.85. A model for a two-energy level dislocation nucleation system is proposed based on our results.
Dual-polarity GaN micropillars grown by metalorganic vapour phase epitaxy: Cross-correlation between structural and optical properties115(2014); http://dx.doi.org/10.1063/1.4870950View Description Hide Description
Self-assembled catalyst-free GaN micropillars grown on (0001) sapphire substrates by metal organic vapor phase epitaxy are investigated. Transmission electron microscopy, as well as KOH etching, shows the systematic presence of two domains of opposite polarity within each single micropillar. The analysis of the initial growth stages indicates that such double polarity originates at the micropillar/substrate interface, i.e., during the micropillar nucleation, and it propagates along the micropillar. Furthermore, dislocations are also generated at the wire/substrate interface, but bend after several hundreds of nanometers. This leads to micropillars several tens of micrometers in length that are dislocation-free. Spatially resolved cathodoluminescence and microphotoluminescence show large differences in the optical properties of each polarity domain, suggesting unequal impurity/dopant/vacancy incorporation depending on the polarity.
Relationship between boson heat capacity peaks and evolution of heterogeneous structure in metallic glasses115(2014); http://dx.doi.org/10.1063/1.4871676View Description Hide Description
The dependence of boson heat capacity peaks of a typical Zr52.5Ti5 Cu 17.9Ni14.6Al10 metallic glass on different annealing time and quenching rates is studied. It is found that the boson heat capacity peak moves to higher temperatures and reduces intensity when the metallic glass is isothermally annealed or slowly quenched. We show that the intensity and position change of the boson heat capacity peak are associated with the evolution of heterogeneous structure and inelastic regions in metallic glasses. The results might help in understanding the structural features and evolution as well as their effects on boson peak of metallic glasses.
115(2014); http://dx.doi.org/10.1063/1.4871777View Description Hide Description
Impulse laser heating combined with a quick sample cooling in a diamond anvil cell provides unique conditions for the 3D polymerization reaction of C60. The reaction proceeds under the 8–10 GPa pressure and 2700 K temperature in the case of heating-cooling cycle time around 0.1 μs. Such a short heating time permits to increase the maximal temperature of the fullerite sample by 1700 K on conditions that C60 are still surviving. As a result, the pressure of the phase transition to 3D polymerized fullerite phase with 519 GPa bulk modulus was essentially decreased. Furthermore, the transition has proceeded at quasi-hydrostatic conditions without activation by applying a plastic deformation.
The alloying element dependence of the local lattice deformation and the elastic properties of Ni3Al: A molecular dynamics simulation115(2014); http://dx.doi.org/10.1063/1.4870235View Description Hide Description
Molecular dynamics (MD) together with the modified analytical embedded atom method (MAEAM) is employed to study the alloying elements (Re, Ru, Co, and Ta) dependence of the elastic properties of L12-Ni3 Al. The investigations indicate that the calculated elastic properties of Ni 3 Al are in reasonable agreement with the previous results. The substituting formation energies of the alloying elements in Ni 3 Al are calculated to determine the site preference. It is found out that Re, Ru, and Ta atoms prefer to occupy the Al sites, and the Co atom prefers to occupy the Ni site. Based on Re, Ru, and Ta substituting the 1st, 2nd, 3rd, and 4th nearest-neighbor atoms, we ascertain that the substituting manners of these alloying elements have a decisive effect on the bulk modulus and the local crystal lattice of Ni 3 Al. Moreover, for Re, the bonding interaction plays a predominant role in the improvement in the bulk modulus of Ni 3 Al, whereas the size effect of Ru and Ta on the improvement in the bulk modulus is more obvious. Finally, the potential reasons of which the alloying elements enhance the bulk modulus are discussed in details.
115(2014); http://dx.doi.org/10.1063/1.4871656View Description Hide Description
The heteroepitaxial growth of ScN films was investigated on various substrates by hydride vapor phase epitaxy (HVPE). Single crystalline mirror-like ScN(100) and ScN(110) layers were successfully deposited on r- and m-plane sapphire substrates, respectively. Homogeneous stoichiometric films (N/Sc ratio 1.01 ± 0.10) up to 40 μm in thickness were deposited. Their mosaicity drastically improved with increasing the film thickness. The band gap was determined by optical methods to be 2.06 eV. Impurity concentrations including H, C, O, Si, and Cl were investigated through energy dispersive X-ray spectrometry and secondary ion mass spectrometry. As a result, it was found that the presence of impurities was efficiently suppressed in comparison with that of HVPE-grown ScN films reported in the past, which was possible thanks to the home-designed corrosion-free HVPE reactor. Room-temperature Hall measurements indicated that the residual free electron concentrations ranged between 1018–1020 cm−3, which was markedly lower than the reported values. The carrier mobility increased monotonically with the decreasing in carrier concentration, achieving the largest value ever reported, 284 cm2 V−1 s−1 at n = 3.7 × 1018 cm−3.
- Electronic Structure and Transport
115(2014); http://dx.doi.org/10.1063/1.4871465View Description Hide Description
We investigate hydrogen adsorption effects on stabilities and electronic properties of nitrogen defects in graphene using first-principles electronic-structure calculations within the density-functional theory. We find that the adsorption of hydrogen atoms on the pyridine-type nitrogen defects in graphene becomes energetically favorable, whereas in the case of the substitutional nitrogen defect the hydrogen adsorption becomes unfavorable. We also find that a transition from p-type to n-type doping properties occurs by hydrogen adsorption on the pyridine-type defects, suggesting that even the carrier type is controllable in nitrogen-doped graphene.
Lithium intercalation in sputter deposited antimony-doped tin oxide thin films: Evidence from electrochemical and optical measurements115(2014); http://dx.doi.org/10.1063/1.4870958View Description Hide Description
Transparent conducting oxides are used as transparent electrical contacts in a variety of applications, including in electrochromic smart windows. In the present work, we performed a study of transparent conducting antimony-doped tin oxide (ATO) thin films by chronopotentiometry in a Li+-containing electrolyte. The open circuit potential vs. Li was used to investigate ATO band lineups, such as those of the Fermi level and the ionization potential, as well as the dependence of these lineups on the preparation conditions for ATO. Evidence was found for Li+ intercalation when a current pulse was set in a way so as to drive ions from the electrolyte into the ATO lattice. Galvanostatic intermittent titration was then applied to determine the lithium diffusion coefficient within the ATO lattice. The electrochemical density of states of the conducting oxide was studied by means of the transient voltage recorded during the chronopotentiometry experiments. These measurements were possible because, as Li+ intercalation took place, charge compensating electrons filled the lowest part of the conduction band in ATO. Furthermore, the charge insertion modified the optical properties of ATO according to the Drude model.
115(2014); http://dx.doi.org/10.1063/1.4871536View Description Hide Description
The different position VAsVGa cluster defect doping in semi-insulating (SI) GaAs has been studied by first-principles calculation based on hybrid density functional theory. Our calculated results show that EL6 level is formed due to the VAsVGa complex defect, which is very close to the experimental result. It provides the explanation of the absorption of laser with the wavelength beyond in semi-insulating GaAs. The formation energy of VAsVGa complex defect is found to decrease from surface to interior gradually. The conduction band minima and valence band maxima of GaAs (001) surface with the VAsVGa complex defect are all located at Γ point, and some defect levels are produced in the forbidden band. In contrast, the conduction band minima and valence band maxima of GaAs with the interior VAsVGa complex defect are not located at the same k-point, so it might involve the change of momentum in the electron transition process. The research will help strengthen the understanding of photoelectronic properties and effectively guide the preparation of the SI-GaAs materials.
115(2014); http://dx.doi.org/10.1063/1.4871288View Description Hide Description
The electronic properties of zigzag graphene oxide nanoribbons (ZGOR) are presented. The results show interesting behaviors which are considerably different from the properties of the perfect graphene nanoribbons (GNRs). The theoretical methods include a Huckel-tight binding approach, a Green's function methodology, and the Landauer formalism. The presence of oxygen on the edge results in band bending, a noticeable change in density of states and thus the conductance. Consequently, the occupation in the valence bands increase for the next neighboring carbon atom in the unit cell. Conductance drops in both the conduction and valence band regions are due to the reduction of allowed k modes resulting from band bending. The asymmetry of the energy band structure of the ZGOR is due to the energy differences of the atoms. The inclusion of a foreign atom's orbital energies changes the dispersion relation of the eigenvalues in energy space. These novel characteristics are important and valuable in the study of quantum transport of GNRs.
115(2014); http://dx.doi.org/10.1063/1.4871539View Description Hide Description
GaN nanofilms (NFs) with different structures are grown on SiC substrates by pulsed laser deposition under different conditions. The synthesized GaN NFs are studied by X-ray diffraction, field-emission (FE) scanning electron microscopy, X-ray photoelectron spectroscopy, and atomic force microscopy. The GaN NFs are composed of diversified GaN nanoparticles with a diameter of 9–38 nm, thickness of 10–50 nm, and roughness of 0.22–13.03 nm. FE from the GaN NFs is structure dependent, which is explained by stress changing the band gap of the NFs. By structure modulation, the turn-on field of GaN NFs can be as low as 0.66 V/μm at a current density of 1 μA/cm2, with a current density of up to 1.1 mA/cm2 at a field of 4.18 V/μm. Fowler-Nordheim curves of some samples contain multiple straight lines, which originate from the structural change and diversification of GaN nanoparticles under an applied field. Overall, our results suggest that GaN NFs with excellent FE properties can be prepared on SiC substrates, which provides a new route to fabricate high-efficiency FE nanodevices.