Volume 101, Issue 23, 03 December 2012
- photonics and optoelectronics
- surfaces and interfaces
- structural, mechanical, optical, and thermodynamic properties of advanced materials
- magnetics and spintronics
- superconductivity and superconducting electronics
- dielectrics, ferroelectrics, and multiferroics
- nanoscale science and technology
- organic electronics and photonics
- device physics
- biophysics and bio-inspired systems
- energy conversion and storage
- interdisciplinary and general physics
Index of content:
We employ an intermediate excited charge state of a lateral quantum dot device to increase the charge detection contrast during the qubit state readout procedure, allowing us to increase the visibility of coherent qubit oscillations. This approach amplifies the coherent oscillation magnitude but has no effect on the detector noise resulting in an increase in the signal to noise ratio. In this letter, we apply this scheme to demonstrate a significant enhancement of the fringe contrast of coherent Landau-Zener-Stückelberg oscillations between singlet S and triplet two-spin states.
- PHOTONICS AND OPTOELECTRONICS
101(2012); http://dx.doi.org/10.1063/1.4768942View Description Hide Description
GaAs-based solar cells containing stacked layers of nanostructured type II GaSb quantum ring solar cells are reported which show significantly enhanced infrared photo-response extending out to 1400 nm. The ring formation reduces the net strain energy associated with the large lattice mismatch making it possible to stack multi-layers without the need for strain balancing. The (1 sun) short-circuit current for a 10 layer sample is enhanced by ∼6% compared to a GaAs control cell. The corresponding open-circuit voltage of 0.6 V is close to the theoretical maximum expected from such structures.
Microwave frequency comb attributed to the formation of dipoles at the surface of a semiconductor by a mode-locked ultrafast laser101(2012); http://dx.doi.org/10.1063/1.4768952View Description Hide Description
The generation of terahertz radiation by focusing a mode-locked ultrafast laser on the surface of a semiconductor was demonstrated by Zhang in 1990, and others have made numerous measurements and analyses of this effect. We have measured the surge current which causes this radiation, showing that this current, and presumably the radiation, are frequency combs with harmonics at integer multiples of the pulse repetition rate of the laser. The harmonics in the current are enhanced by placing the semiconductor in a tunneling junction, where the fundamental is increased by 8 dB with a DC tunnelingcurrent of 100 pA.
Experimental verification of the 3-step model of photoemission for energy spread and emittance measurements of copper and CsBr-coated copper photocathodes suitable for free electron laser applications101(2012); http://dx.doi.org/10.1063/1.4769220View Description Hide Description
This paper presents measurements and analysis of the quantum efficiency (QE) and intrinsic emittance of Cu and CsBr coated Cu photocathodes. The data analysis uses expressions for the quantum efficiency and the intrinsic emittance for metal cathodes previously derived from Spicer's three-step model of photoemission. Data taken with a 257 nm CW laser on (100) Cu crystals indicate an emittance of 0.77 (μm/mm-rms) for CsBr coated and 0.42 (μm/mm-rms) for uncoated cathodes. The high quantum efficiency and low emittance observed for CsBr coated cathodes have applications in free electron laser and other devices requiring high brightness electron beams.
Controlled energy shuttling in terpolymers enabling independent optimization of absorption and transport properties in organic solar cell materials101(2012); http://dx.doi.org/10.1063/1.4769249View Description Hide Description
The optimization of current materials for organic solar cells is complicated by the fact that individual modifications concurrently affect several efficiency-limiting factors. For example, in donor-acceptor copolymers, the chemical optimization of the light absorptionproperties concurrently alters the chargetransport properties and not necessarily in the desired direction. This interdependency frustrates step-wise optimization of materials for maximum power conversion efficiency. In this letter, we introduce a terpolymer which we demonstrate effectively decouples the light absorption and chargetransport properties onto separate monomeric units, allowing each desired property to be independently tuned in the material and opening an avenue for step-wise, material-design-based optimization of organic solar cell quantum efficiency.
Dynamics and pulse-package oscillations in broad-area semiconductor lasers with short optical feedback101(2012); http://dx.doi.org/10.1063/1.4769181View Description Hide Description
Pulse-package oscillations in broad-area semiconductor lasers subjected to short optical feedback are experimentally observed. The pulse-package oscillation consists of a frequency component that corresponds to an external optical feedback loop with an envelop of periodic low-frequency fluctuations. However, the periodicity induced by optical feedback does not always improve the time-averaged beam profiles of the laser oscillations. We also investigated time-averaged near-field beam profiles in relation with the optical feedback dynamics.
101(2012); http://dx.doi.org/10.1063/1.4769371View Description Hide Description
We demonstrate a dynamic axial mode tuning method by means of near-field probe in a rolled-up optical microcavity. The spatially selective nature of the tuning has been explored through both the lateral and transversal probing processes. A series of perturbation calculations based on the axial confinement model are performed to prove and improve the understanding of experimental results.
Interplay of polarization fields and Auger recombination in the efficiency droop of nitride light-emitting diodes101(2012); http://dx.doi.org/10.1063/1.4769374View Description Hide Description
We use theoretical modeling to investigate the effect of polarization fields and non-radiative Auger recombination on the efficiency-droop and green-gap problems of polar and nonpolar nitride light-emitting diodes. The dependence of radiative and nonradiative recombination rates on electron-hole wave-function overlap is analyzed. Device designs that minimize the polarization fields lead to higher efficiency, not because the internal quantum efficiency is improved at a given carrier density but because they can be operated at a lower carrier density for a given current density.
Room temperature direct-bandgap electroluminescence from n-type strain-compensated Ge/SiGe multiple quantum wells101(2012); http://dx.doi.org/10.1063/1.4769834View Description Hide Description
N-type strain-compensated Ge/Si0.15Ge0.85multiple quantum wells(MQWs) were grown on a Si0.1Ge0.9 virtual substrate using ultrahigh vacuum chemical vapor deposition on a n+-Si(001) substrate. Under low forward bias voltage ranging from 0.6 to 1.2 V, narrow direct-bandgap electroluminescence(EL) peak from MQWslight emitting diode was observed at room temperature. The quantum confinement effect of the direct-bandgap transitions and the temperature dependent EL peak redshift are in good agreement with the calculated results.
Competitive carrier interactions influencing the emission dynamics of GaAsSb-capped InAs quantum dots101(2012); http://dx.doi.org/10.1063/1.4769431View Description Hide Description
The optical properties of InAs/GaAs quantum dots capped with a GaAsSbquantum well are investigated by means of power-dependent and time-resolved photoluminescence. The structure exhibits the coexistence of a type-I ground state and few type-II excited states, the latter characterized by a simultaneous carrier density shift of the peak position and wavelength-dependent carrier lifetimes. Complex emission dynamics are observed under a high-power excitation regime, with the different states undergoing shifts during specific phases of the measurement. These features are satisfactorily explained in terms of band structure and energy level modifications induced by two competitive carrier interactions inside the structure.
101(2012); http://dx.doi.org/10.1063/1.4769835View Description Hide Description
The mechanisms of size-dependent efficiency and efficiency droop of blue InGaN micro-pixel light emitting diodes (μLEDs) have been investigated experimentally and by simulation. Electrical characterisation confirms the improvement of current spreading for smaller μLEDs, which enables the achievement of the higher efficiency at high injection current densities. Owing to the higher ratio of sidewall perimeter to mesa area of smaller μLEDs, a lower efficiency was observed at a low injection current density, resulting from defect-related Shockley-Read-Hall non-radiative recombination. We demonstrate that such sidewall etch defects can be partially recovered by increased thermal annealing time, consequently improving the efficiency at low current densities.
101(2012); http://dx.doi.org/10.1063/1.4768677View Description Hide Description
We propose and demonstrate a method of “z-microscopy” by utilizing an array of 45°-tilted micro mirrors arranged along the axial direction. Image signals emitted from different axial positions can be orthogonally reflected by the corresponding micro mirrors and spatially separated for parallel detection, essentially converting the more challenging axial imaging to a lateral imaging problem. Each micro mirror also provides optical sectioning capability due to its finite dimension. Numerical analysis shows that nearly diffraction limited axial resolution can be achieved. Experimental demonstration of z-imaging of fluorescent microspheres is also presented.
101(2012); http://dx.doi.org/10.1063/1.4769838View Description Hide Description
In this letter, we observe that both the fundamental radial breathing mode and the flapping mode couple to a high Q optical mode and generate frequency mixing through the nonlinear optical transfer function in a single nitride optomechanical resonator. The harmonic generation of the flapping mode produces a comb-like frequency mixing spectrum. Instead of using the external pump modulation, we show that the regenerative oscillation of an internal mechanical mode can be used as a modulation source for optomechanical RF mixing.
- SURFACES AND INTERFACES
101(2012); http://dx.doi.org/10.1063/1.4768693View Description Hide Description
In this report, we study the effectiveness of hydrogen plasmasurfacetreatments for improving the electrical properties of GaSb/Al2O3 interfaces. Prior to atomic layer deposition of an Al2O3dielectric, p-GaSb surfaces were exposed to hydrogen plasmasin situ, with varying plasma powers, exposure times, and substrate temperatures. Good electrical interfaces, as indicated by capacitance-voltage measurements, were obtained using higher plasma powers, longer exposure times, and increasing substrate temperatures up to 250 °C. X-ray photoelectron spectroscopy reveals that the most effective treatments result in decreased SbOx, decreased Sb, and increased GaOx content at the interface. This in situ hydrogen plasmasurface preparation improves the semiconductor/insulator electrical interface without the use of wet chemical pretreatments and is a promising approach for enhancing the performance of Sb-based devices.
101(2012); http://dx.doi.org/10.1063/1.4769112View Description Hide Description
Sensor development has been reliant on planar Au and Ag nanoparticle research. The current findings explored a unique 3-D network of crystalline TiO2nanoparticles linked as nanofibers. In addition to the favorability of using TiO2 for chemical and bio-molecular sensing, the nanofiber network provides molecular diffusion control and an increased confocal volume signal. Controlled femtosecond laser synthesis is also demonstrated that directly impacts surface-enhanced Raman spectroscopy detection of two common environmentally harmful chemicals: bisphenol A and diclofenac sodium salt. These findings assert that 3-D nanofibrous network porosity optimization is crucial for Raman monitoring of drinking water.
Chemical and structural investigations of the incorporation of metal manganese into ruthenium thin films for use as copper diffusion barrier layers101(2012); http://dx.doi.org/10.1063/1.4769229View Description Hide Description
The incorporation of manganese into a 3 nm ruthenium thin-film is presented as a potential mechanism to improve its performance as a copperdiffusion barrier. Manganese (∼1 nm) was deposited on an atomic layer deposited Ru film, and the Mn/Ru/SiO2structure was subsequently thermally annealed.X-ray photoelectron spectroscopy studies reveal the chemical interaction of Mn with the SiO2 substrate to form manganese-silicate (MnSiO3), implying the migration of the metal through the Ru film. Electron energy loss spectroscopy line profile measurements of the intensity of the Mn signal across the Ru film confirm the presence of Mn at the Ru/SiO2interface.
Evolution of variable range hopping in strongly localized two dimensional electron gas at NdAlO3/SrTiO3 (100) heterointerfaces101(2012); http://dx.doi.org/10.1063/1.4768932View Description Hide Description
We report evolution of the two-dimensional electron gas behavior at the NdAlO3/SrTiO3heterointerfaces with varying thicknesses of the NdAlO3 overlayer. The samples with a thicker NdAlO3 show strong localizations at low temperatures and the degree of localization is found to increase with the NdAlO3 thickness. The T −1/3 temperature dependence of the sheet resistance at low temperatures and the magnetoresistance study reveal that the conduction is governed by a two-dimensional variable range hopping mechanism in this strong localized regime. We attribute this thickness dependence of the transport properties of the NdAlO3/SrTiO3interfaces to the interface strain induced by the overlayers.
101(2012); http://dx.doi.org/10.1063/1.4769095View Description Hide Description
Faraday rotation is a fundamental magneto-optical phenomenon used in various optical control and magnetic field sensing techniques. Recently, it was shown that a giant Faraday rotation can be achieved in the low-THz regime by a single monoatomic graphene layer. Here, we demonstrate that this exceptional property can be manipulated through adequate nano-patterning, notably achieving giant rotation up to 6THz with features no smaller than 100 nm. The effect of the periodic patterning on the Faraday rotation is predicted by a simple physical model, which is then verified and refined through accurate full-wave simulations.
101(2012); http://dx.doi.org/10.1063/1.4769358View Description Hide Description
Selenium is currently a key element for developing nano and micro-technologies. Nevertheless, the surface energy of solid selenium () reported in the literature is still questionable. In this work, we have measured = 0.291 ± 0.025 J/m2 at 293 K using the sessile drop technique with different probe liquids, namely ethylene glycol, de-ionized water, mercury, and gallium. This value is in excellent agreement with theoretical predictions.
Significant increase in conduction band discontinuity due to solid phase epitaxy of Al2O3 gate insulator films on GaN semiconductor101(2012); http://dx.doi.org/10.1063/1.4769818View Description Hide Description
We have investigated band discontinuities and chemical structures of Al2O3 gate insulator films on n-type GaNsemiconductor by photoemission and x-ray absorption spectroscopy. It is found that the solid phase epitaxy at the GaN crystal during annealing procedures at 800 °C leads to phase transformation of Al2O3films from amorphous to crystalline. Changes in crystallographic structures closely correlate with the significant increase in conduction band discontinuity at the Al2O3/GaN interface, which suggests that epitaxial Al2O3films on GaNsemiconductor, free from grain boundaries of Al2O3 polycrystalline, hold the potential for high insulation performance.
Determination of the deep donor-like interface state density distribution in metal//n-GaN structures from the photocapacitance–light intensity measurement101(2012); http://dx.doi.org/10.1063/1.4769815View Description Hide Description
We developed a method for determining of the deep donor-like interface state density distribution at the insulator/wide bandgapsemiconductorinterface in metal/insulator/semiconductor structures from the measurements of photocapacitance vs. ultraviolet light intensity . From the comparison of theoretical and experimental curves we obtained the continuous donor in the energy range between 0.15 eV and 1 eV from the valence band top for a metal/ device. In addition, the acceptor-like interface state in the upper part of the bandgap was determined from the capacitance-voltage method.