Volume 97, Issue 18, 01 November 2010
- lasers, optics, and optoelectronics
- structural, mechanical, thermodynamic, and optical properties of condensed matter
- electronic transport and semiconductors
- magnetism and superconductivity
- dielectrics and ferroelectricity
- nanoscale science and design
- organic electronics and photonics
- device physics
- biophysics and bio-inspired systems
Index of content:
We report first-principles calculations of transport behaviors for a molecular device whose electrode surface is contaminated by various diatomic groups. It has been found that such a device demonstrates less transport variations for the contamination of the group PO or SO in the whole bias range but it shows more transport variations for contamination of the group CN, HS, or NO only under low bias, which suggests that contamination of all diatomic groups studied here always affects high-bias transport properties of a device in an extremely gentle manner.
- LASERS, OPTICS, AND OPTOELECTRONICS
Efficiency droop alleviation in InGaN/GaN light-emitting diodes by graded-thickness multiple quantum wells97(2010); http://dx.doi.org/10.1063/1.3507891View Description Hide Description
InGaN/GaN light-emitting diodes(LEDs) with graded-thickness multiple quantum wells (GQW) was designed and grown by metal-organic chemical vapor deposition. The GQW structure, in which the well-thickness increases along  direction, was found to have superior hole distribution as well as radiative recombination distribution by performing simulation modeling. Accordingly, the experimental investigation of electroluminescencespectrum reveals additional emission from the narrower wells within GQWs. Consequently, the efficiency droop can be alleviated to be about 16% from maximum at current density of , which is much smaller than that for conventional LED (32%). Moreover, the light output power was enhanced from 18.0 to 24.3 mW at .
97(2010); http://dx.doi.org/10.1063/1.3507289View Description Hide Description
The effect of strain on the valence-band structure of semipolar InGaN grown on GaN substrates is studied. A analysis reveals that anisotropic strain in the -plane and shear strain are crucial for deciding the ordering of the two topmost valence bands. The shear-strain deformation potential is calculated for GaN and InN using density functional theory with the Heyd–Scuseria–Ernzerhof hybrid functional [J. Heyd, G. E. Scuseria, and M. Ernzerhof, J. Chem. Phys.124, 219906 (2006)]. Using our deformation potentials and assuming a pseudomorphically strained structure, no polarization switching is observed. We investigate the role of partial strain relaxation in the observed polarization switching.
Bias-dependent absorption coefficient of the absorber section in GaN-based multisection laser diodes97(2010); http://dx.doi.org/10.1063/1.3514232View Description Hide Description
We measure the modal absorption coefficient of the InGaN quantum wells(QWs) in the absorber section of (Al,In)GaN multisection laser diodes as a function of bias voltage and photon energy using optical gain-spectroscopy. In the examined laser diodes, the modal absorption at the laser wavelength of 430 nm has a maximum of at low negative bias and decreases with increasing negative bias. We explain this behavior by comparing the measurements to absorption coefficients calculated from a band-edge profile simulation. The decrease of the absorption at large negative bias is caused by a shift in the transition energies in the quantum wells due to the quantum confined Stark effect.
97(2010); http://dx.doi.org/10.1063/1.3510469View Description Hide Description
We demonstrate semiconductorquantum dots coupled to photonic crystalcavity modes operating in the visible spectrum. We present the design, fabrication, and characterization of two dimensional photonic crystalcavities in GaInP and measure quality factors in excess of 7500 at 680 nm. We demonstrate full control over the spontaneous emission rate of InPquantum dots and by spectrally tuning the exciton emission energy into resonance with the fundamental cavity mode we observe a Purcell enhancement of .
Strain evaluation in AlInN/GaN Bragg mirrors by in situ curvature measurements and ex situ x-ray grazing incidence and transmission scattering97(2010); http://dx.doi.org/10.1063/1.3514241View Description Hide Description
Strain in lattice matched and mismatched AlInN/GaN Bragg mirrorstructures were studied by in situ curvature and various ex situ x-ray measurements. In the case of lattice mismatched structures considerable deviations of the in-plane lattice parameters were evidenced near the surface region as well as in depth using x-ray grazing incidence and x-ray transmission scattering in Laue geometry. The experimental findings are explained in terms of partial stress relaxation of the AlInN/GaN Bragg layer stack with respect to the underlying GaN buffer and a mutual tensioning of the GaN and AlInN layers with respect to each other.
97(2010); http://dx.doi.org/10.1063/1.3507288View Description Hide Description
We demonstrate an ultrahigh- slotted two-dimensional photonic crystalcavity capable of obtaining strong interaction between the internal light field and the mechanical motion of the slotted structure. The measured optical quality factor is for a cavity with an effective modal volume of . Optical transduction of the thermal motion of the fundamental in-plane mechanical resonance of the structure is performed, from which a zero-point motion optomechanical coupling rate of is inferred. Dynamical back-action of the optical field on the mechanical motion, resulting in cooling and amplication of the mechanical motion, is also demonstrated.
97(2010); http://dx.doi.org/10.1063/1.3513345View Description Hide Description
The optical properties, indium concentration and distribution, defect morphology, and strain distribution of GaN/InGaN coaxial nanowiresgrown by metal organic chemical vapor deposition were investigated using spatially resolved cathodoluminescence, scanning transmission electron microscopy, and finite element analysis. The results indicate that InGaN layers with 40% or greater indium incorporation and low defect density can be achieved. The indium distribution in the InGaN shell layer was measured and qualitatively correlated with the calculated strain distribution. The three-dimensional compliance of the GaNnanowire leads to facile strain relaxation in the InGaNheteroepitaxial layer, enabling high indium incorporation and high crystalline quality.
Effect of an asymmetry AlGaN barrier on efficiency droop in wide-well InGaN double-heterostructure light-emitting diodes97(2010); http://dx.doi.org/10.1063/1.3513394View Description Hide Description
External-quantum-efficiency (EQE) and efficiency droop in wide-well InGaN double-heterostructure light-emitting diodes have been investigated. It was found that the insertion of an AlGaN barrier between the n-type GaN layer and the InGaN well resulted in higher peak EQE and reduced efficiency droop at a higher injection level. EQE was improved by 5.7% and 25.8% over that of a sample without an AlGaN barrier at a current density of and , respectively. It is suggested that the mechanism is attributed to an electron decelerating effect that enlarges the effective active region.
Reliability in room-temperature negative differential resistance characteristics of low-aluminum content AlGaN/GaN double-barrier resonant tunneling diodes97(2010); http://dx.doi.org/10.1063/1.3515418View Description Hide Description
AlGaN/GaN resonant tunneling diodes (RTDs), consisting of 20% (10%) aluminum-content in double-barrier (DB) active layer, were grown by metal-organic chemical vapor deposition on freestanding polar (c-plane) and nonpolar (m-plane) GaN substrates. RTDs were fabricated into -diameter devices for electrical characterization. Lower aluminum content in the DB active layer and minimization of dislocations and polarization fields increased the reliability and reproducibility of room-temperature negative differential resistance(NDR). Polar RTDs showed decaying NDR behavior, whereas nonpolar ones did not significantly. Averaging over 50 measurements, nonpolar RTDs demonstrated a NDR of , a current-peak-to-valley ratio of 1.08, and an average oscillator output power of 0.52 mW.
97(2010); http://dx.doi.org/10.1063/1.3511743View Description Hide Description
In this paper, we realize a scalable micromirror suitable for atom chip based cavity quantum electrodynamics applications. A very low surface roughness of 2.2 Å rms on the silicon cavitymirrors is achieved using chemical dry etching along with plasma and oxidation smoothing. Our Fabry–Perot cavity comprised of these mirrors currently demonstrates the highest finesse, , using microfabricated mirrors. We compute a single atom cooperativity for our cavities of more than 200, making them promising candidates for detecting individual atoms and for quantum information applications on a chip.
97(2010); http://dx.doi.org/10.1063/1.3514247View Description Hide Description
The authors report the fabrication of widely tunable monolithic quantum cascade lasers(QCLs) with coupled Fabry–Pérot (FP) cavities on indium phosphide. Quasicontinuous tuning of the single mode emission over a total spectral range of 242 nm was realized at two regions between 8.394 and . An absorption experiment with ammonia shows principle feasibility of gas detection with multisegment QCL devices. Good agreement of the experimentally observed tuning behavior with the one expected from calculated FP mode-combs indicates that the change in the refractive index is mainly due to thermal heating as a result of current injection.
97(2010); http://dx.doi.org/10.1063/1.3514558View Description Hide Description
We present experimental and finite-difference time-domain simulation studies on the properties of Bragg and non-Bragg band gaps; the studies are carried out by using metal slit arrays positioned at the center of the air gaps in tapered parallel-plate waveguides. Two Bragg stop bands, with a dynamic power transmission range of about 26 dB, are observed for an air gap of . Two non-Bragg stop bands are observed for an air gap of . Using the Ey field distribution and Poynting vectors, we confirm that the Ey field leaking from the slit and the Ey field propagating from another input vanish because they are out of phase. The Bragg and non-Bragg stop bands determined in the simulations show excellent agreement with those observed experimentally.
- STRUCTURAL, MECHANICAL, THERMODYNAMIC, AND OPTICAL PROPERTIES OF CONDENSED MATTER
97(2010); http://dx.doi.org/10.1063/1.3506900View Description Hide Description
The dependence of glass forming ability on liquid fragility is studied with thermodynamic and kinetic analyses. A general expression of the Gibbs free energy difference between an undercooled liquid and its equilibrium crystal is presented to address thermodynamically the fragility effect on glass formation. It is found that increasing fragility thermodynamically favors glass formation, remarkably contrasting with the fragility effect from the kinetic point of view. The dependences of glass forming ability on fragility in metallic alloys and molecular isomers are compared, and kinetics and thermodynamics of glass formation are, respectively, emphasized.
Experimental verification of negative refraction for a wedge-type negative index metamaterial operating at terahertz97(2010); http://dx.doi.org/10.1063/1.3511540View Description Hide Description
We report on angle-resolved time domain spectroscopy (TDS) carried out on a prismlike negative index metamaterial operating around 0.5 THz. The wedge-type devices are constituted of hole arrays etched in goldthin films, which are stacked according to a sequential mask shift. By means of a goniometric TDS setup and subsequent analysis of the temporal waveforms, negative refraction is demonstrated with values close to around 0.5 THz. The dispersion of refractive index retrieved from the Snell–Descartes law shows comparable trends in comparison with the dispersion deduced from complex transmission and reflection measurements on slab-type samples.
Retrieval of terahertz spectroscopic signatures in the presence of rough surface scattering using wavelet methods97(2010); http://dx.doi.org/10.1063/1.3507384View Description Hide Description
Scattering of terahertz waves by surface roughness can obscure spectral signatures of chemicals at these frequencies. We demonstrate this effect using controlled levels of surface scattering on -lactose monohydrate pellets. Furthermore, we show an implementation of wavelet methods that can retrieve terahertz spectral information from rough surface targets. We use a multiresolution analysis of the rough-surface-scattered signal utilizing the maximal overlap discrete wavelet transform (MODWT) to extract the resonant signature of lactose. We present a periodic extension technique to circumvent the circular boundary conditions of MODWT, which can be robustly used in an automated terahertz stand-off detection device.
Rashba and Dresselhaus spin-orbit coupling in GaN-based heterostructures probed by the circular photogalvanic effect under uniaxial strain97(2010); http://dx.doi.org/10.1063/1.3511768View Description Hide Description
The spin splitting in GaN-based heterostructures has been investigated by means of circular photogalvanic effect experiments under uniaxial strain. The ratios of Rashba and Dresselhaus spin-orbit coupling coefficients (R/D ratios) have been measured in heterostructures with various Al compositions. It is found that the R/D ratio increases from 4.1 to 19.8 with the Al composition of the barrier varied from 15% to 36%. The Dresselhaus coefficient of bulk GaN is experimentally obtained to be . The results indicate that the spin splitting in GaN-based heterostructures can be modulated effectively by the polarization-induced electric fields.
97(2010); http://dx.doi.org/10.1063/1.3509417View Description Hide Description
(BMS) can be sensed to blue light (wavelength ) by doped europium (Eu). Eu doped BMS turns to bright pink, and is bleached by green light irradiation in several seconds. The coloration-decoloration process is repeatable and first. The colored state hardly changes for over 480 h (20 days), and is stable even at . The blue-green light photochromism can be explained by electron transfers between doped Eu and oxygen defects. The doped Eu broadens the absorption band of BMS, and supplies excited electrons. The oxygen defects form electron traps, which catch excited electrons, and become F-like color centers.
97(2010); http://dx.doi.org/10.1063/1.3511451View Description Hide Description
The performance of carbon nanotube(CNT) fibers is limited by the intertube characteristics. Here we report a direct method of curing to improve mechanical properties of poly(amic acid)-infiltrated fibers. After curing at for 60 min the fibers composed of double- and triple-walled CNTs, their strength is stably improved by 30.3%, from 1.58 to 2.06 GPa. The enhancement arises from the increase in shear stress between tube surfaces, by measuring the static frictional force of CNT fibers. Due to the existence of CNTs, the imidization temperature of polyimide drops greatly from 218 to .
In situ Raman spectroscopy for characterization of the domain contributions to electrical and piezoelectric responses in films97(2010); http://dx.doi.org/10.1063/1.3502591View Description Hide Description
We employed in situRaman spectroscopy under electric field for (100)/(001)-oriented tetragonal films with (001)-volume fraction of 35%. The increase in was revealed above 200 kV/cm, which resulted in the larger remanent polarization. In addition, the application of high enough field also brings a feature, i.e., large reversible change in with/without electric field that can quantitatively explain the enhanced piezoelectric response. These demonstrate the usefulness of in situ Raman observation to probe the domain contributions to the electrical and piezoelectric responses.
97(2010); http://dx.doi.org/10.1063/1.3511472View Description Hide Description
In situ spectroscopic ellipsometry in the spectral region of 1.3–4.3 eV has been employed to monitor, in real-time, the formation of trivalent Cr process (TCP) conversion coatings on polished Al substrates. The measured ellipsometry parameters ( and ), as a function of immersion time, reveal that the initial stages of film formation include the chemical thinning of the native oxide layer, formation of a very thin initiation layer and the subsequent rapid formation of the TCP film. The filmoptical constant is modeled using Cauchy dispersion relation and its thickness was determined as a function of immersion time during growth.