Volume 113, Issue 13, 07 April 2013
Index of content:
- Lasers, Optics, and Optoelectronics
113(2013); http://dx.doi.org/10.1063/1.4798605View Description Hide Description
A reproducible p-type P-N codoped ZnO [ZnO:(P, N)] film with high quality was achieved by magnetron sputtering and post-annealing techniques. It has room-temperature resistivity of 3.98 Ωcm, Hall mobility of 1.35 cm2/Vs, and carrier concentration of 1.16 × 1018 cm−3, which is better than electrical properties of the p-type N-doped ZnO (ZnO:N) and p-type P-doped ZnO (ZnO:P) films. Additionally, the p-ZnO:(P, N)/n- ZnO homojunction showed a clear p-n diode characteristic. The p-type conductivity of ZnO:(P, N) is attributed to the formation of an impurity band above the valance band maximum, resulting in a reduction in the band gap and a decrease in the ionization energy of the acceptor, as well as an improvement in the conductivity and stability of the p-type ZnO:(P, N).
113(2013); http://dx.doi.org/10.1063/1.4798942View Description Hide Description
A multi-layered metal-insulator-metal plasmonic Mach-Zehnder interferometer (MZI) is proposed to work as an array for multiplexed sensing. The interference patterns based on wavelength modulation and intensity modulation are modeled analytically and numerically, showing a high figure of merit over 170 for intensity-interrogated sensing. The proposed structure can overcome the one-slit illumination limitation of previously reported single-layered double-slit plasmonic MZI and will enable portable, high-throughput and sensitive biosensing applications.
113(2013); http://dx.doi.org/10.1063/1.4798353View Description Hide Description
We compare the theoretical optical properties of intersubband transitions for polar III-nitrides, nonpolar III-nitrides, and conventional GaAs. We calculate and examine the peak transition wavelengths, dipole matrix elements, and absorption spectra as a function of quantum well thickness for single quantum well structures on each platform. We show that the absence of polarization-related electric fields in nonpolar III-nitrides simplifies device design and facilitates clear performance advantages over conventional polar III-nitrides, including access to a wider range of absorption wavelengths, a several-fold increase in the dipole matrix element, and higher absorption probability. Compared to conventional GaAs-based structures, nonpolar III-nitrides exhibit a somewhat lower absorption probability but allow for a significantly wider design space, permitting devices operating at wavelengths that are unattainable using GaAs.
113(2013); http://dx.doi.org/10.1063/1.4799415View Description Hide Description
Tantalum pentoxide (Ta 2O5) and silicon dioxide (SiO2) are common high-index and low-index materials used in dielectric optical coatings for high average-power lasers since high-density sputtered oxide films with absorption losses at near- and mid-infrared wavelengths of less than 1 ppm can be obtained. These oxides have been chosen to investigate the spontaneous and optically induced absorption at λ 0 = 1064 nm that occurs due to simultaneous illumination at shorter wavelengths. The effect is measured using the photothermal common-path interferometric technique. This technique is capable of detecting sub-ppm levels of optical absorption and tracking its changes at a given wavelength when a second laser beam is also incident on a thin film oxide sample. In this work, dual beam experiments are employed to assess changes in the optical absorption at λ 0 = 1064 nm in ion beam sputtered Ta 2O5 and SiO2 thin films deposited on fused silica substrates, with stimulating illumination λ 1 ranging from λ 1 = 266 nm to λ 1 = 780 nm. The power and wavelength of the stimulating radiation were found to affect the optical absorption at λ 0 = 1064 nm. Furthermore, the relaxation dynamics of the induced infrared absorption was found to be wavelength dependent and is thought to be associated with various electron traps existing in the forbidden gap that depend essentially on the film's preparation conditions. The significantly greater effect observed in Ta 2O5 at λ 1 = 266 nm is attributed to band-to-band transitions.
- Plasmas and Electrical Discharges
The augmented saddle field discharge characteristics and its applications for plasma enhanced chemical vapour deposition113(2013); http://dx.doi.org/10.1063/1.4798928View Description Hide Description
A high ion flux parallel electrode plasma is proposed and studied in its DC configuration. By cascading a diode source region which supplies electrons and a saddle field region where these seed electrons are energized and amplified, the energy of ion bombardment on the substrate can be decoupled from the plasma density. The sufficiently large density of electrons and holes in the vicinity of the substrate raises the possibility to perform plasma enhanced chemical vapour deposition on insulating materials, at low sheath voltages (around 40 V in the configuration studied), at low temperatures in which the surface mobility of film growth species may be provided by the bombardment of moderate energy ions. As a benchmarking exercise, experiments are carried out on silane discharge characteristics and deposition of hydrogenated amorphous silicon (a-Si:H) on both silicon wafer and glass. The films grown at low anode voltages have excellent microstructures with predominantly monohydride bonds, sharp band tails, but relatively high integrated defect density in the mid 1016/cm3 range for the particular substrate temperature of 180 °C, indicating that further optimizations are necessary if the electrode configuration is to be used to create a-Si:H devices.
Understanding the discharge current behavior in reactive high power impulse magnetron sputtering of oxides113(2013); http://dx.doi.org/10.1063/1.4799199View Description Hide Description
The discharge current behavior in reactive high power impulse magnetron sputtering (HiPIMS) of Ti-O and Al-O is investigated. It is found that for both metals, the discharge peak current significantly increases in the oxide mode in contrast to the behavior in reactive direct current magnetron sputtering where the discharge current increases for Al but decreases for Ti when oxygen is introduced. In order to investigate the increase in the discharge current in HiPIMS-mode, the ionic contribution of the discharge in the oxide and metal mode is measured using time-resolved mass spectrometry. The energy distributions and time evolution are investigated during the pulse-on time as well as in the post-discharge. In the oxide mode, the discharge is dominated by ionized oxygen, which has been preferentially sputtered from the target surface. The ionized oxygen determines the discharge behavior in reactive HiPIMS.
- Structural, Mechanical, Thermodynamic, and Optical Properties of Condensed Matter
113(2013); http://dx.doi.org/10.1063/1.4798575View Description Hide Description
In this work, we establish a theoretical model for a cylindrical rod of radius R with opening angle illuminated by a modulated incident beam. The model uses the Green function method in cylindrical coordinates. An analytical expression for the Green function and thermal-wave field in such a solid is presented. The theory is validated in the limit of reducing the arbitrary wedge geometrical structure to simpler geometries. For acute angle wedges, it is shown that the thermal-wave field near the edge exhibits confinement behavior and increased amplitude compared to a flat (reference) solid with θ = π. For obtuse angle wedges, it is shown that the opposite is true and relaxation of confinement occurs leading to lower amplitude thermal-wave fields. The theory provides a basis for quantitative thermophysical characterization of wedge-shaped objects and it is tested using an AISI 304 steel wedge and photothermal radiometry detection.
Effect of oxygen and associated residual stresses on the mechanical properties of high growth rate Czochralski silicon113(2013); http://dx.doi.org/10.1063/1.4798599View Description Hide Description
The mechanical properties of Czochralski silicon (Cz-Si) crystals grown in vacancy rich regimes with elevated axial oxygen concentrations ranging from ∼6 × 1017 to ∼12 × 1017 atoms/cm3 have been investigated using nano- and micro-indentation techniques. Both hardness and fracture toughness were found to decrease with increasing oxygen concentration, while major differences in mechanical properties were found between the central core and the edge of the high oxygen concentration wafers. Photoluminescence imaging and Nomarski optical microscopy of high-oxygen wafers revealed the presence of a ring and swirl-like distributions of micro defects, including oxidation induced stacking faults. Micro-Raman analysis was used to measure local residual stress profiles associated with these characteristic defects. These results provide a quantitative understanding of the influence of the oxygen content and the associated defects resulting from the sub-optimal growth regimes within the Cz-Si process.
Effects of crystal defects and their interactions with impurities on electrical properties of multicrystalline Si113(2013); http://dx.doi.org/10.1063/1.4798600View Description Hide Description
We investigated the effects of different crystal defects and their interactions with impurities on the electrical properties of multicrystalline Si (mc-Si) using samples with unique defect patterns and impurities. By using the floating cast method, a single grain boundary (GB), identified as a Σ27 boundary, was first formed with a high density of impurities from atmosphere, leading to an inefficient external gettering of impurities during phosphorus (P) diffusion. During crystal growth, the Σ27 GB splits into the Σ3 and Σ9 GBs with accompanying generation of dislocations and reduction in the density of impurities. The external gettering of impurities became efficient for removing impurities as evidenced by an increase in average minority carrier lifetime. At the final stage of crystal growth, the decrease in minority carrier lifetime was significant, which could not be improved by phosphorus diffusion because of the high densities of segregated impurities and crystal defects originating from the strong contact with the crucible. The increase in Σ number was found to result in more enhanced precipitation of impurities, which led to the poor gettering effect of P diffusion. These results further confirmed the importance of the reduction in the densities of impurities and dislocations for the quality and yield improvement of mc-Si ingots for solar cells.
Optical spectroscopy and spectroscopic ellipsometry as a monitor for thin film growth by dc magnetron sputtering113(2013); http://dx.doi.org/10.1063/1.4798601View Description Hide Description
Measurements of line intensity ratios have been used in astronomy to determine physical properties of plasmas such as density and temperature. Herein, this procedure was applied to monitor thin film growth during plasma-assisted deposition and useful information about the plasma was obtained. The aim of this study was to monitor plasma variations during deposition, using wide field optical spectroscopy, and to establish a relationship with thin film stoichiometry using spectroscopic ellipsometry. With this purpose, inhomogeneous thin films were grown by dc magnetron sputtering.
113(2013); http://dx.doi.org/10.1063/1.4799018View Description Hide Description
A comprehensive P-V-T dataset for bcc-tungsten was obtained for pressures up to 33.5 GPa and temperatures 300–1673 K using MgO and Au pressure scales. The thermodynamic analysis of these data was performed using high-temperature (HT) and Mie-Grüneisen-Debye (MGD) relations combined with the Vinet equations of state (EOS) for room-temperature isotherm and the newly proposed Kunc-Einstein (KE) EOS. The KE EOS allowed calibration of W thermodynamic parameters to the pressures of at least 300 GPa and temperatures up to 4000 K with minor uncertainties (<1% in calculated volume of W). A detailed analysis of room-temperature compression data with Vinet EOS yields V 0 = 31.71 ± 0.02 Å3, KT = 308 ± 1 GPa, and KT ′ = 4.20 ± 0.05. Estimated thermoelastic parameters for HT include (∂KT /∂T) P = −0.018 ± 0.001 GPa/K and thermal expansion α = a 0 + a 1 T with a 0 = 1.35 (±0.04) × 10−5 K−1 and a 1 = 0.21 (±0.05) × 10−8 K−2. Fitting to the MGD relation yielded γ0 = 1.81 ± 0.02 and q = 0.71 ± 0.02 with the Debye temperature (θ 0,) fixed at 370–405 K. The parameters for KE EOS include two Einstein temperatures, ΘE1o = 314 K and ΘE2o = 168 K, Grüneisen parameter at ambient condition γ 0 = 1.67 and infinite compression γ∞ = 0.66, with β = 1.16 (which is a power-mode parameter in the Grüneisen equation), anharmonicity (m = 3.57) and electronic (g = 0.11) equivalents of the Grüneisen parameter, and additional parameters for intrinsic anharmonicity, a 0 = 6.2 × 10−5 K−1, and electronic contribution, e 0 = 4.04 × 10−5 K−1 to the free energy. Fixed parameters include k = 2 in KE EOS and mE 1 = mE 2 = 1.5 in expression for Einstein temperature. Present analysis should represent the best fit of the experimental data for W and can be used for a variety of thermodynamic calculations for W and W-containing systems including phase diagrams, chemical reactions, and electronic structure.
113(2013); http://dx.doi.org/10.1063/1.4798576View Description Hide Description
Light emitted from a  lithium fluoride crystal was characterized under shock wave compression to 28 GPa followed by complete stress release at the edges. The light was examined using time-gated optical spectrometry and imaging, time-resolved optical emission measurements, and hydrodynamic modeling. The shock arrival at the circumference of the crystal was delayed relative to the center so that the two regions could be studied at different times. The majority of the light emission originated when the shock waves released at the circumference of the crystal. Unlike previously reported results for shocked lithium fluoride, we found that the light spectrum is not strictly broad band, but has spectral lines associated with atomic lithium in addition to a broad band background. Also, the emission spectrum depends strongly on the gas surrounding the sample. Based on our observations, the line emission appears to be related to fracture of the lithium fluoride crystal from the shock wave releasing at the edges. Experimenters frequently utilize lithium fluoride crystals as transparent windows for observing shock compressed samples. Because of the experimental geometries used, the shock wave in such cases often reaches the circumference of the window at nearly the same moment as when it reaches the center of the sample-window interface. Light generated at the circumference could contaminate the measurement at the interface when this light scatters into the observed region. This background light may be reduced or avoided using experimental geometries which delay the arrival of the shock wave at the edges of the crystal.
113(2013); http://dx.doi.org/10.1063/1.4798591View Description Hide Description
Titanium nitride (TiN), which is widely used for hard coatings, reportedly undergoes a pressure-induced structural phase transformation, from a NaCl to a CsCl structure, at ∼7 GPa. In this paper, we use first-principles calculations based on density functional theory with a generalized gradient approximation of the exchange correlation energy to determine the structural stability of this transformation. Our results show that the stress required for this structural transformation is substantially lower (by more than an order of magnitude) when it is deviatoric in nature vis-à-vis that under hydrostatic pressure. Local stability of the structure is assessed with phonon dispersion determined at different pressures, and we find that CsCl structure of TiN is expected to distort after the transformation. From the electronic structure calculations, we estimate the electrical conductivity of TiN in the CsCl structure to be about 5 times of that in NaCl structure, which should be observable experimentally.
113(2013); http://dx.doi.org/10.1063/1.4799013View Description Hide Description
The amorphous-to-crystalline transition in Al(1.0%wtSi)/Zr and Al(Pure)/Zr multilayers grown by direct-current magnetron sputtering system has been characterized over a range of Al layer thicknesses (1.0–5.0 nm) by using a series of complementary measurements including grazing incidence X-ray reflectometry, atomic force microscopy, X–ray diffraction, and high-resolution transmission electron microscopy. The Al layer thickness transition exhibits the Si doped in Al could not only disfavor the crystallization of Al but also influence the changing trends of surface roughness and diffraction peak position of phase Al〈111〉. An interesting feature of the presence of Si in Al layer is that Si could influence the transition process in Al(1%wtSi) layer, in which the critical thickness (1.6 nm) of Al(Pure) layer in Al(Pure)/Zr shifts to 1.8 nm of Al(1.0%wtSi) layer in Al(1.0%wtSi)/Zr multilayer. We also found that the Zr-on-Al interlayer is wider than the Al-on-Zr interlayer in both systems, and the Al layers do not have specific crystal orientation in the directions vertical to the layer from selected area electron diffraction patterns below the thickness (3.0 nm) of Al layers. Above the thickness (3.0 nm) of Al layers, the Al layers are highly oriented in Al〈111〉, so that the transformation from asymmetrical to symmetrical interlayers can be observed. Based on the analysis of all measurements, we build up a model with four steps, which could explain the Al layer thickness transition process in terms of a critical thickness for the nucleation of Al(Pure) and Al(1%wtSi) crystallites.
Mechanism of fatigue performance enhancement in a laser sintered superhard nanoparticles reinforced nanocomposite followed by laser shock peening113(2013); http://dx.doi.org/10.1063/1.4799154View Description Hide Description
This study investigates the fundamental mechanism of fatigue performance enhancement during a novel hybrid manufacturing process, which combines laser sintering of superhard nanoparticles integrated nanocomposites and laser shock peening (LSP). Through laser sintering, TiN nanoparticles are integrated uniformly into iron matrix to form a nanocomposite layer near the surface of AISI4140 steel. LSP is then performed on the nanocomposite layer to generate interaction between nanoparticles and shock waves. The fundamental mechanism of fatigue performance enhancement is discussed in this paper. During laser shock interaction with the nanocomposites, the existence of nanoparticles increases the dislocation density and also helps to pin the dislocation movement. As a result, both dislocation density and residual stress are stabilized, which is beneficial for fatigue performance.
113(2013); http://dx.doi.org/10.1063/1.4799064View Description Hide Description
The wide scatter in experimental results has not allowed drawing solid conclusions on self-diffusion in the chalcopyrite CuInSe2 (CIS). In this work, the defect-assisted mass transport mechanisms operating in CIS are clarified using first-principles calculations. We present how the stoichiometry of the material and temperature affect the dominant diffusion mechanisms. The most mobile species in CIS is shown to be copper, whose migration proceeds either via copper vacancies or interstitials. Both of these mass-mediating agents exist in the material abundantly and face rather low migration barriers (1.09 and 0.20 eV, respectively). Depending on chemical conditions, selenium mass transport relies either solely on selenium dumbbells, which diffuse with a barrier of 0.24 eV, or also on selenium vacancies whose diffusion is hindered by a migration barrier of 2.19 eV. Surprisingly, indium plays no role in long-range mass transport in CIS; instead, indium vacancies and interstitials participate in mechanisms that promote the formation of antisites on the cation sublattice. Our results help to understand how compositional inhomogeneities arise in CIS.
113(2013); http://dx.doi.org/10.1063/1.4799377View Description Hide Description
The relaxation dynamics of photoexcited carriers in a chemical vapor deposited graphene transferred on quartz substrate has been investigated by using ultrafast optical-pump terahertz (THz)-probe spectroscopy. Terahertz transmission through graphene sample is reduced by optical pumping. The change of transmission decays exponentially after the optical pulse. We find the relaxation time is insensitive to the substrate temperature from 10 K to 300 K but increases sublinearly with pump fluence. We model the relaxation process involving electron-phonon coupling together with a set of rate equations to describe the transient responses of quasi-particles and optical phonons. The increases of the extracted carrier temperature and the measured relaxation time with pump fluence are associated with the fact that high pump fluence significantly increases the carrier temperature and broadens the carrier distribution. As a result, it leads to the reduction of optical phonon emission efficiency and the decrease of cooling rate as well.
H-H interactions from SiO2 to SiO2/Si(100) interfaces and H-induced O vacancy generation via 3-fold coordinated O in SiO2113(2013); http://dx.doi.org/10.1063/1.4796146View Description Hide Description
H-H interactions and condensation of H atoms around O atoms from bulk SiO2 to SiO2/Si(100) interfaces leading to degradation modes have been extensively studied through first-principles calculations. For all charge-state-dependent H-H interactions, H atoms are found to experience mainly mutually repulsive forces in defect-free bulk SiO2, but to be attracted together near the O atoms closer to DBs in bulk SiO2 and around SiO2/Si interfaces. Through H condensation around the DBs in bulk SiO2, H-induced O vacancy generation has been found to occur even in bulk SiO2 via three-fold coordinated O atoms when a dangling bond (DB) exists in the bulk SiO2. The single DB acts as a reservoir for an extra electron, which contrasts with SiO2/Si systems that have a large reservoir for extra electrons.
113(2013); http://dx.doi.org/10.1063/1.4798359View Description Hide Description
A series of continuous-wave spectroscopic measurements elucidates the mechanism responsible for the technologically important green emission from deep-level traps in ZnO:Zn powders. Analysis of low-temperature photoluminescence (PL) and PL excitation spectra for bound excitons compared to the temperature-dependent behavior of the green emission reveals a deep correlation between green PL and specific donor-bound excitons. Direct excitation of these bound excitons produces highly efficient green emission from near-surface defects. When normalized by the measured external quantum efficiency, the integrated PL for both excitonic and green emission features grows identically with excitation intensity, confirming the strong connection between green emission and excitons. The implications of these findings are used to circumscribe operational characteristics of doped ZnO-based white light phosphors whose quantum efficiency is almost twice as large when the bound excitons are directly excited.
- Electronic Structure and Transport
113(2013); http://dx.doi.org/10.1063/1.4798593View Description Hide Description
Hexagonal boron nitride sheet has been shown to be the best insulating substrate for graphene electronics. Using first-principles calculations, we here show that BN nanoribbons (BNNRs) can not only serve as a desirable substrate but also bring new properties into the supported graphene nanoribbons (GNRs). In particular, zigzag GNRs on zigzag BNNRs become a spin-relevant semiconductor that can be easily tuned into a half-metal, thanks to polar character of the BNNRs. In contrast, armchair GNRs can basically have all their electronic properties survived from the interaction of armchair BNNRs. Our findings provide helpful guide for developing hybrid BN-graphene nanodevices.