Volume 116, Issue 19, 21 November 2014
Index of content:
We propose a scheme of multilayer thermoelectric engine where one electric current is coupled to two temperature gradients in three-terminal geometry. This is realized by resonant tunneling through quantum dots embedded in two thermal and electrical resisting polymer matrix layers between highly conducting semiconductor layers. There are two thermoelectric effects, one of which is pertaining to inelastic transport processes (if energies of quantum dots in the two layers are different), while the other exists also for elastic transport processes. These two correspond to the transverse and longitudinal thermoelectric effects, respectively, and are associated with different temperature gradients. We show that cooperation between the two thermoelectric effects leads to markedly improved figure of merit and power factor, which is confirmed by numerical calculation using material parameters. Such enhancement is robust against phonon heat conduction and energy level broadening. Therefore, we demonstrated cooperative effect as an additional way to effectively improve performance of thermoelectrics in three-terminal geometry.
- Lasers, Optics, and Optoelectronics
The high temperature photoluminescence and optical absorption of undoped ZnO single crystals and thin films116(2014); http://dx.doi.org/10.1063/1.4901833View Description Hide Description
The photoluminescence of undoped ZnO single crystals up to 1350 °C and the optical absorption of stress-relaxed, epitaxial ZnO thin films up to 1100 °C are reported. The photoluminescence intensity and power dependence with illumination flux are related to the crystal growth methods and stabilize after high temperature annealing. The observation of excitonic recombination at very high temperatures requires high illumination flux. It is found that the zero phonon line model reproduces the shift and the band gap narrowing as well as the free excitonic transition up to the cross-over with a defect level at 2.83 eV that occurs at 800 °C. A phenomenological model of the excitonic recombination band shape, taking account exciton-phonon losses and defect levels provides an excellent fit up to 2.2–2.4 eV (1100 °C). At these cross-over temperatures, an energy transfer is observed between the free exciton transition and defect transitions. However, at temperature above 1100 °C, the decrease of the band gap and the increase of thermal radiation, as well as the restrictions of our experimental set-up and particularly the illumination flux of the exciting laser, limit the analysis of the photoluminescence spectra measurements.
116(2014); http://dx.doi.org/10.1063/1.4902083View Description Hide Description
We propose a kinetic model to account for the nonuniform adsorption and desorption processes in fringe field switching (FFS) and in-plane-switching liquid crystal displays. An equation is proposed to describe the generation mechanism of residual DC voltage and good agreements with experiment are obtained. Based on this model, the mechanisms underlying the formation and relaxation processes of residual DC voltage as well as their dependences on offset DC voltage and temperature are investigated. Moreover, the residual DC voltages of FFS cells employing positive and negative dielectric anisotropy LCs are compared and the physics responsible for the observed difference is explained.
- Plasmas and Electrical Discharges
116(2014); http://dx.doi.org/10.1063/1.4901830View Description Hide Description
Active interrogation using neutrons is an effective method for detecting shielded nuclear material. A lightweight, lunch-box-sized, battery-operated neutron source would enable new concepts of operation in the field. We have developed at-scale components for a highly portable, completely self-contained, pulsed Deuterium-Tritium (DT) neutron source producing 14 MeV neutrons with average yields of 107 n/s. A gated, field ionization ion source using etched electrodes has been developed that produces pulsed ion currents up to 500 nA. A compact Cockcroft-Walton high voltage source is used to accelerate deuterons into a metal hydride target for neutron production. The results of full scale DT tests using the field ionization source are presented.
116(2014); http://dx.doi.org/10.1063/1.4902063View Description Hide Description
In the afterglow of an inductively coupled N2 plasma, relative N atom densities are measured by ionization threshold mass spectrometry as a function of time in order to determine the wall loss time tw N from the exponential decay curves. The procedure is performed with two mass spectrometers on different positions in the plasma chamber. tw N is determined for various pressures, i.e., for 3.0, 5.0, 7.5, and 10 Pa. For this conditions also the internal plasma parameters electron density ne and electron temperature Te are determined with the Langmuir probe and the rotational temperature of N2 is determined with the optical emission spectroscopy. For , a procedure is presented to evaluate the spectrum of the transition of the second positive system ( ) of N2. With this method, a gas temperature of 610 K is determined. For both mass spectrometers, an increase of the wall loss times of atomic nitrogen with increasing pressure is observed. The wall loss time measured with the first mass spectrometer in the radial center of the cylindrical plasma vessel increases linearly from 0.31 ms for 3 Pa to 0.82 ms for 10 Pa. The wall loss time measured with the second mass spectrometer (further away from the discharge) is about 4 times higher. A model is applied to describe the measured tw N. The main loss mechanism of atomic nitrogen for the considered pressure is diffusion to the wall. The surface loss probability β N of atomic nitrogen on stainless steel was derived from twN and is found to be 1 for the present conditions. The difference in wall loss times measured with the mass spectrometers on different positions in the plasma chamber is attributed to the different diffusion lengths.
- Structural, Mechanical, Thermodynamic, and Optical Properties of Condensed Matter
116(2014); http://dx.doi.org/10.1063/1.4901209View Description Hide Description
Recent advances in growth techniques have allowed the fabrication of semiconductor nanostructures with mixed wurtzite/zinc-blende crystal phases. Although the optical characterization of these polytypic structures is well reported in the literature, a deeper theoretical understanding of how crystal phase mixing and quantum confinement change the output linear light polarization is still needed. In this paper, we theoretically investigate the mixing effects of wurtzite and zinc-blende phases on the interband absorption and in the degree of light polarization of an InP polytypic superlattice. We use a single 8 × 8 k⋅p Hamiltonian that describes both crystal phases. Quantum confinement is investigated by changing the size of the polytypic unit cell. We also include the optical confinement effect due to the dielectric mismatch between the superlattice and the vaccum and we show it to be necessary to match experimental results. Our calculations for large wurtzite concentrations and small quantum confinement explain the optical trends of recent photoluminescence excitation measurements. Furthermore, we find a high sensitivity to zinc-blende concentrations in the degree of linear polarization. This sensitivity can be reduced by increasing quantum confinement. In conclusion, our theoretical analysis provides an explanation for optical trends in InP polytypic superlattices, and shows that the interplay of crystal phase mixing and quantum confinement is an area worth exploring for light polarization engineering.
116(2014); http://dx.doi.org/10.1063/1.4901577View Description Hide Description
The pressure and temperature phase diagram of Gd2Ti2O7 under irradiation are calculated by means of molecular dynamics calculations. The critical temperature for amorphization obeys a linear law with pressure. Gd2Ti2O7 under irradiation transits towards the fluorite above this temperature and amorphizes below. The configuration of the Ti interstitial reveals to be the key of the amorphizability of Gd2Ti2O7. Its stability depends upon disorder and pressure. Low pressure promotes the stabilization of Ti linked-polyhedra that drive the system to the amorphous state under irradiation. Conversely, high pressure activates its destabilization to interstitials that recombine with vacancies, driving the system to the fluorite structure under irradiation.
116(2014); http://dx.doi.org/10.1063/1.4901987View Description Hide Description
The enhanced diffusivity of oxygen in heavily boron doped silicon was obtained by analyzing oxygen out-diffusion profile changes found at the interface between a lightly boron-doped silicon epitaxial layer and a heavily boron-doped silicon substrate by secondary ion mass spectrometry. It was found that the diffusivity is proportional to the square root of boron concentration in the range of 1018 cm−3–1019 cm−3 at temperatures from 750 °C to 950 °C. The model based on the diffusion of oxygen dimers in double positive charge state could explain the enhanced diffusion. We have concluded that oxygen diffusion enhanced in heavily boron-doped silicon is attributed to oxygen dimers ionized depending on Fermi level position.
Structural, elastic, and lattice dynamic stability of yttrium selenide (YSe) under pressure: A first principle study116(2014); http://dx.doi.org/10.1063/1.4901992View Description Hide Description
Structural, elastic, and lattice dynamical stability of YSe has been investigated as a function of pressure through first principles electronic band structure calculations. The comparison of enthalpies of rocksalt type (B1) and CsCl type cubic (B2) structures determined as a function of pressure suggests that the B1 phase will transform to B2 structure at ∼32 (30 GPa at 300 K obtained from comparison of Gibbs free energy at 300 K). The transition is identified to be of first order in nature with a volume discontinuity of ∼6.2% at the transition pressure. Furthermore, the theoretically determined equation of state has been utilized to derive various physical quantities, such as zero pressure equilibrium volume, bulk modulus, and pressure derivative of bulk modulus. The single crystal elastic constants have been predicted at various pressures for both the B1 and B2 structures using the energy strain method. The activation barrier between B1 and B2 phases calculated at transition point is ∼19.7mRy/formula unit. Our lattice dynamic calculations show that both the B1 as well as B2 structures are lattice dynamically stable not only at ambient pressure but also at transition pressure. The B1 phase becomes lattice dynamically unstable at ∼112 GPa, i.e., much beyond the transition pressure. The effect of temperature on volume and bulk modulus of the YSe in B1 phase has also been examined.
116(2014); http://dx.doi.org/10.1063/1.4901994View Description Hide Description
We report on room temperature, polarization-resolved Raman scattering measurements on layered crystals of the series MoS x Se(2– x ) (0 ≤ x ≤ 2) grown by chemical vapor transport technique. The results reveal two distinct sets of features related to the and A1 g modes of pure members of series. As composition x changes, the in-plane mode shows two-mode behavior, whereas the out-of-plane A1 g mode presents more complex evolution. The MoSe2-like branch reveals the splitting associated with the altering arrangement of S and Se atoms around Mo and the resulting changes in the dipole moment of the molecule. The X-ray diffraction measurements confirm that the samples are single-phase materials of 2H-type structure over the entire range of the sulfide composition x, while the scanning transmission electron microscopy imaging reveals a random arrangement of the S and Se atoms. Modified random-element-isodisplacement model is adopted to predict the behavior of the individual modes in the alloys. The model successfully confirms the two-mode behavior exhibited by the MoS x Se(2–x) series.
116(2014); http://dx.doi.org/10.1063/1.4902064View Description Hide Description
Shock Hugoniot data have been widely used to calibrate analytic equations of state (EOSs) of condensed matter at high pressures. However, the suitability of particular analytic EOSs under off-Hugoniot states has not been sufficiently verified using experimental data. We have conducted quasi-isentropic compression experiments (ICEs) of tantalum using the compact pulsed power generator CQ-4, and explored the relation of longitudinal stress versus volume of tantalum under quasi-isentropic compression using backward integration and characteristic inverse methods. By subtracting the deviatoric stress and additional pressure caused by irreversible plastic dissipation, the isentropic pressure can be extracted from the longitudinal stress. Several theoretical isentropes are deduced from analytic EOSs and compared with ICE results to validate the suitability of these analytic EOSs in isentropic compression states. The comparisons show that the Gruneisen EOS with Gruneisen Gamma proportional to volume is accurate, regardless whether the Hugoniot or isentrope is used as the reference line. The Vinet EOS yields better accuracy in isentropic compression states. Theoretical isentropes derived from Tillotson, PUFF, and Birch-Murnaghan EOSs well agree with the experimental isentrope in the range of 0–100 GPa, but deviate gradually with pressure increasing further.
Experimental and theoretical identification of a high-pressure polymorph of Ga2S3 with α-Bi2Te3-type structure116(2014); http://dx.doi.org/10.1063/1.4902070View Description Hide Description
Since the discovery of α-phase Bi2Te3, Sb2Te3, and Bi2Se3 as 3D topological insulators, many experimental and theoretical studies of A2B3-type chalcogenides have been performed to search for new materials with interesting elastic and electric properties at ambient and extreme conditions. In this study, high-pressure properties of Ga2S3 have been characterized by in situ synchrotron X-ray diffraction (XRD), X-ray absorption near edge structure measurements, and Density-functional theory (DFT) calculations. At ∼16.0 GPa, a phase transition of α′-Ga2S3 (Cc and Z = 4) is observed experimentally to a new polymorph, which is indentified to be the tetradymite-type or α-Bi2Te3-type crystal structure (R m and Z = 3) by laser-annealing XRD experiments and DFT calculations. The isothermal pressure-volume relationship of Ga2S3 is well described by the second-order Birch-Murnaghan equation of state with K 0 = 59(2) GPa and = 4 (fixed) for the α′-Ga2S3, and K 0 = 91(3) GPa, and = 4 (fixed) for the tetradymite-type phase. In addition, band gap of α′-Ga2S3 decreases on compression and the tetradymite-type Ga2S3 exhibits metallization based on DFT calculations. The pressure-induced phase transition accompanying by changes of elastic and electrical properties may give some implications to other chalcogenides under high pressure.
Effect of InGaAs interlayer on the properties of GaAs grown on Si (111) substrate by molecular beam epitaxy116(2014); http://dx.doi.org/10.1063/1.4902160View Description Hide Description
High-quality GaAs films have been epitaxially grown on Si (111) substrates by inserting an In x Ga1− x As interlayer with proper In composition by molecular beam epitaxy (MBE). The effect of In x Ga1− x As (0 < x < 0.2) interlayers on the properties of GaAs films grown on Si (111) substrates by MBE has been studied in detailed. Due to the high compressive strain between InGaAs and Si, InGaAs undergoes partial strain relaxation. Unstrained InGaAs has a larger lattice constant than GaAs. Therefore, a thin InGaAs layer with proper In composition may adopt a close lattice constant with that of GaAs, which is beneficial to the growth of high-quality GaAs epilayer on top. It is found that the proper In composition in In x Ga1− x As interlayer of 10% is beneficial to obtaining high-quality GaAs films, which, on the one hand, greatly compensates the misfit stress between GaAs film and Si substrate, and on the other hand, suppresses the formation of multiple twin during the heteroepitaxial growth of GaAs film. However, when the In composition does not reach the proper value (∼10%), the In x Ga1− x As adopts a lower strain relaxation and undergoes a lattice constant smaller than unstrained GaAs, and therefore introduces compressive stress to GaAs grown on top. When In composition exceeds the proper value, the In x Ga1− x As will adopt a higher strain relaxation and undergoes a lattice constant larger than unstrained GaAs, and therefore introduces tensile stress to GaAs grown on top. As a result, In x Ga1− x As interlayers with improper In composition introduces enlarged misfit stress to GaAs epilayers grown on top, and deteriorates the quality of GaAs epilayers. This work demonstrates a simple but effective method to grow high-quality GaAs epilayers and brings up a broad prospect for the application of GaAs-based optoelectronic devices on Si substrates.
116(2014); http://dx.doi.org/10.1063/1.4901997View Description Hide Description
We report theoretically and numerically on the sound transmission loss performance through a thick plate-type acoustic metamaterial made of spring-mass resonators attached to the surface of a homogeneous elastic plate. Two general analytical approaches based on plane wave expansion were developed to calculate both the sound transmission loss through the metamaterial plate (thick and thin) and its band structure. The first one can be applied to thick plate systems to study the sound transmission for any normal or oblique incident sound pressure. The second approach gives the metamaterial dispersion behavior to describe the vibrational motions of the plate, which helps to understand the physics behind sound radiation through air by the structure. Computed results show that high sound transmission loss up to 72 dB at 2 kHz is reached with a thick metamaterial plate while only 23 dB can be obtained for a simple homogeneous plate with the same thickness. Such plate-type acoustic metamaterial can be a very effective solution for high performance sound insulation and structural vibration shielding in the very low-frequency range.
Quantitative transmission electron microscopy analysis of multi-variant grains in present L10-FePt based heat assisted magnetic recording media116(2014); http://dx.doi.org/10.1063/1.4902082View Description Hide Description
We present a study on atomic ordering within individual grains in granular L10-FePt thin films using transmission electron microscopy techniques. The film, used as a medium for heat assisted magnetic recording, consists of a single layer of FePt grains separated by non-magnetic grain boundaries and is grown on an MgO underlayer. Using convergent-beam techniques, diffraction patterns of individual grains are obtained for a large number of crystallites. The study found that although the majority of grains are ordered in the perpendicular direction, more than 15% of them are multi-variant, or of in-plane c-axis orientation, or disordered fcc. It was also found that these multi-variant and in-plane grains have always grown across MgO grain boundaries separating two or more MgO grains of the underlayer. The in-plane ordered portion within a multi-variant L10-FePt grain always lacks atomic coherence with the MgO directly underneath it, whereas, the perpendicularly ordered portion is always coherent with the underlying MgO grain. Since the existence of multi-variant and in-plane ordered grains are severely detrimental to high density data storage capability, the understanding of their formation mechanism obtained here should make a significant impact on the future development of hard disk drive technology.
- Electronic Structure and Transport
116(2014); http://dx.doi.org/10.1063/1.4901871View Description Hide Description
A linear theory of the new non-linear photovoltaic effect in the closed circuit consisting of a non-uniformly illuminated uniform bipolar semiconductor with neutral impurities is developed. The non-uniform photo-excitation of impurities results in the position-dependant current carrier mobility that breaks the semiconductor homogeneity and induces the photo-electromotive force (emf). As both the electron (or hole) mobility gradient and the current carrier generation rate depend on the light intensity, the photo-emf and the short-circuit current prove to be non-linear functions of the incident light intensity at an arbitrarily low illumination. The influence of the sample size on the photovoltaic effect magnitude is studied. Physical relations and distinctions between the considered effect and the Dember and bulk photovoltaic effects are also discussed.
Effect of charge trapping on effective carrier lifetime in compound semiconductors: High resistivity CdZnTe116(2014); http://dx.doi.org/10.1063/1.4901826View Description Hide Description
The dominant problem limiting the energy resolution of compound semiconductor based radiation detectors is the trapping of charge carriers. The charge trapping affects energy resolution through the carrier lifetime more than through the mobility. Conventionally, the effective carrier lifetime is determined using a 2-step process based on measurement of the mobility-lifetime product (μτ) and determining drift mobility using time-of-flight measurements. This approach requires fabrication of contacts on the sample. A new RF-based pulse rise-time method, which replaces this 2-step process with a single non-contact direct measurement, is discussed. The application of the RF method is illustrated with high-resistivity detector-grade CdZnTe crystals. The carrier lifetime in the measured CdZnTe, depending on the quality of the crystals, was between about 5 μs and 8 μs. These values are in good agreement with the results obtained using conventional 2-step approach. While the effective carrier lifetime determined from the initial portion of the photoresponse transient combines both recombination and trapping in a manner similar to the conventional 2-step approach, both the conventional and the non-contact RF methods offer only indirect evaluation of the effect of charge trapping in the semiconductors used in radiation detectors. Since degradation of detector resolution is associated not with trapping but essentially with detrapping of carriers, and, in particular, detrapping of holes in n-type semiconductors, it is concluded that evaluation of recombination and detrapping during photoresponse decay is better suited for evaluation of compound semiconductors used in radiation detectors. Furthermore, based on previously reported data, it is concluded that photoresponse decay in high resistivity CdZnTe at room temperature is dominated by detrapping of carriers from the states associated with one type of point defect and by recombination of carriers at one type of extended defects. The recombination at the extended defects produces long, logarithmic decay limiting substantially performance of CdZnTe detectors. This decay is associated with the “electrostatic trapping” of excess holes by the Schottky-type depletion space-charge regions formed around the defects.
116(2014); http://dx.doi.org/10.1063/1.4902009View Description Hide Description
The electronic and structural properties of Mo(100)/GaAs(100) interfaces and Mo diffusion into GaAs are explored using first principle calculations. Our results show that the interface undergoes substantial atomic rearrangement with respect to the bulk structures and the bilayer of the GaAs adjacent to the interface becomes conducting. We study the n-type Schottky barrier height's dependence on Mo interdiffusion in the GaAs, with values ranging from ∼0.9 eV to ∼1.39 eV. This range is caused by the diffusants acting as additional n–type doping at the surface and their interaction with the metal-induced gap states.
116(2014); http://dx.doi.org/10.1063/1.4902142View Description Hide Description
A macroscopic graphene is thermally annealed in oxygen gas, and Raman spectroscopy shows that the number of structural defects increases after thermal annealing. The temperature dependence of resistance is studied in macroscopic graphene with different numbers of structural defects in various gas environments. The temperature dependent slope of normalized resistance is independent of the number of structural defects and is small in the gas environment of heavier molecules. Following the temperature dependent slopes of normalized resistance, one can identify the molecule mass of the environmental gas. The temperature dependence of thermal electric power is sensitive to the quantity of the structural defects.
116(2014); http://dx.doi.org/10.1063/1.4901953View Description Hide Description
In this study, we address the electronic properties of conducting films constituted of an array of randomly distributed few layer graphene patches and investigate on their most salient galvanometric features in the moderate and extreme disordered limit. We demonstrate that, in annealed devices, the ambipolar behaviour and the onset of Landau level quantization in high magnetic field constitute robust hallmarks of few-layer graphene films. In the strong disorder limit, however, the magneto-transport properties are best described by a variable-range hopping behaviour. A large negative magneto-conductance is observed at the charge neutrality point, in consistency with localized transport regime.
116(2014); http://dx.doi.org/10.1063/1.4902248View Description Hide Description
We report photo-thermoelectric transport phenomena in Pb 2CrO5 single crystals. Without illumination, this material exhibits an insulating behavior characterized by an activation-type temperature variation of the electrical conductivity. The Seebeck coefficient contrastingly shows a crossover from high-temperature insulating to low-temperature metallic behavior, which is attributed to degenerate carriers in a donor level. We have found that under illumination, both the conductivity and the Seebeck coefficient increase in magnitude with increasing photon flux density in the degenerate-conduction regime. This result is difficult to understand within a simple photo-doping effect, which usually leads to a decrease in the Seebeck coefficient under illumination. The observed phenomenon is discussed in terms of a two-carrier contribution to the transport properties.