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5 Apr 2010

Volume 96, Issue 14, Articles (14xxxx)

Issue Cover Spotlight Figure

Appl. Phys. Lett. 96, 143103 (2010); http://dx.doi.org/10.1063/1.3378684 (3 pages)

Joshua A. Kellar, Justice M. P. Alaboson, Qing Hua Wang, and Mark C. Hersam
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Laser-induced Zn doping in GaN based light-emitting diode

Zheng-Qun Xue, Sheng-Rong Huang, Bao-Ping Zhang, and Chao Chen

Appl. Phys. Lett. 96, 141101 (2010); http://dx.doi.org/10.1063/1.3367725 (3 pages) | Cited 1 time

Online Publication Date: 5 April 2010

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By laser-induced Zn doping, hole concentration in the p-type GaN contact layer of conventional GaN-based light-emitting diodes (LEDs) is increased and improvement of the LED property is confirmed. Compared with LED with no use of laser-induced doping, the forward voltage under 20 mA current is decreased from 3.33 to 3.13 V and the thermal resistance of the chip is decreased from 18.6 to 9.7 K/W. In addition, the lifetime of the device is increased about 41%. These results are attributed to the improvement of the p-type Ohmic contact due to laser-induced doping of Zn to the p-GaN contact layer.
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85.60.Jb Light-emitting devices
73.40.Ns Metal-nonmetal contacts
61.72.uj III-V and II-VI semiconductors
81.05.Ea III-V semiconductors
42.62.-b Laser applications

Utilizing the interface adsorption of nitrogen for the growth of high-quality GaInAsN/GaAs quantum wells by metal organic chemical vapor deposition for near infrared applications

Asaf Albo, Catherine Cytermann, Gad Bahir, and Dan Fekete

Appl. Phys. Lett. 96, 141102 (2010); http://dx.doi.org/10.1063/1.3360216 (3 pages) | Cited 8 times

Online Publication Date: 5 April 2010

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We have investigated the composition and optical properties of GaInAsN/GaAs single quantum wells grown using metal organic chemical vapor epitaxy at 500 °C. Using time-of-flight secondary ion mass spectrometry and photoluminescence spectroscopy, we have shown the presence of a 1–2 nm thick nitrogen-rich interfacial layer at the first interface grown. The inhomogeneous asymmetric distribution of nitrogen atoms along the growth direction is attributed to the dominance of surface kinetics, nonlinear dependence of N incorporation on In content, and the strain gradient effect on the effective diffusion of N. We have utilized this finding to grow high quality quantum wells.
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81.05.Ea III-V semiconductors
81.07.St Quantum wells
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
68.35.Fx Diffusion; interface formation
68.43.Mn Adsorption kinetics
78.67.De Quantum wells

Characterization and modeling of a terahertz photoconductive switch

J. Y. Suen, W. Li, Z. D. Taylor, and E. R. Brown

Appl. Phys. Lett. 96, 141103 (2010); http://dx.doi.org/10.1063/1.3374404 (3 pages) | Cited 2 times

Online Publication Date: 5 April 2010

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We examine the terahertz (THz) performance of an ErAs:GaAs photoconductive switch under varying bias conditions and optical drive power. Despite THz power up to 287 μW, saturation effects were not seen. In addition, the THz power spectra were measured with a Fourier transform infrared spectrometer, and the roll-off was found to be invariant to bias voltage and consistent with a THz pulsewidth of 1.59 ps and a peak power of 3.1 W. These results are confirmed by a large-signal, high-frequency circuit model that suggests that further increase in THz power and efficiency are possible through an increase in the mode-locked laser power and reduction in its pulse width. The model is useful in designing both the laser and photoconductive switches to maximize available power and efficiency.
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85.60.-q Optoelectronic devices
84.40.-x Radiowave and microwave (including millimeter wave) technology

Circularly and elliptically polarized near-field radiation from nanoscale subwavelength apertures

Erdem Öğüt and Kürşat Şendur

Appl. Phys. Lett. 96, 141104 (2010); http://dx.doi.org/10.1063/1.3371696 (3 pages) | Cited 8 times

Online Publication Date: 5 April 2010

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With advances in nanotechnology, obtaining circularly and elliptically polarized optical spots beyond the diffraction limit is an emerging need for plasmonic applications. Two techniques are suggested to obtain circularly and elliptically polarized near-field radiation using subwavelength apertures. It is demonstrated that a square aperture can mediate diffraction limited circularly or elliptically polarized radiation into an optical spot with circular or elliptical polarization beyond the diffraction limit. Linearly polarized diffraction limited radiation is converted into a circularly or an elliptically polarized optical spot beyond the diffraction limit by creating asymmetry in the subwavelength aperture.
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42.79.-e Optical elements, devices, and systems
42.25.Ja Polarization
73.22.Lp Collective excitations
42.25.Fx Diffraction and scattering

Electrically tunable, high performance quantum cascade laser

A. Bismuto, R Terazzi, M. Beck, and Jerome Faist

Appl. Phys. Lett. 96, 141105 (2010); http://dx.doi.org/10.1063/1.3377008 (3 pages) | Cited 13 times

Online Publication Date: 5 April 2010

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A quantum cascade laser design for wide voltage-tuning, emitting at ∼ 8.5 μm, is presented based on a diagonal bound-to-continuum design. The relatively short period length and the diagonal nature of the laser transition guarantees a wide tuning of the emission due to the linear Stark shift effect. Tuning of both the spontaneous and stimulated emission is presented over almost 100 cm−1. In spite of the large tuning, laser performance are comparable with the best results present in literature in this spectral range. In particular, continuous wave operation up to 450 mW and pulsed wall plug efficiencies up to 11.5% were measured at 300 K. A transport model, based on the density matrix formalism, was used to simulate spontaneous and stimulated emission as function of the applied field. Same model was also used to predict light-current-voltage characteristics of the lasers.
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42.55.Px Semiconductor lasers; laser diodes
42.60.Fc Modulation, tuning, and mode locking

Lasing effect in a hybrid dye-doped biopolymer and photochromic polymer system

Jaroslaw Mysliwiec, Lech Sznitko, Anna Sobolewska, Stanislaw Bartkiewicz, and Andrzej Miniewicz

Appl. Phys. Lett. 96, 141106 (2010); http://dx.doi.org/10.1063/1.3377912 (3 pages) | Cited 3 times

Online Publication Date: 6 April 2010

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We investigate an amplified spontaneous emission phenomenon and lasing in a two-layer system: a layer of modified deoxyribonucleic acid (DNA) polymeric matrix containing dye which is superimposed on a periodic relief structure formed in photochromic polymer. The luminescent layer was prepared from DNA blended with cationic surfactant molecule cetyltrimethyl-ammonium chloride (CTMA) and a Rhodamine (Rh6G) laser dye. This layer covered a specially designed photochromic polymer layer in which a surface relief grating was inscribed by holographic method in order to form a Bragg reflector for photons. Thin film of the DNA-CTMA:Rh6G/photochromic polymer was excited with pulsed laser.
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42.55.Rz Doped-insulator lasers and other solid state lasers
42.60.By Design of specific laser systems
78.45.+h Stimulated emission
42.70.Jk Polymers and organics
42.40.Eq Holographic optical elements; holographic gratings
78.67.Pt Multilayers; superlattices; photonic structures; metamaterials

Design of plasmonic near field plate at optical frequency

Haofei Shi and L. Jay Guo

Appl. Phys. Lett. 96, 141107 (2010); http://dx.doi.org/10.1063/1.3378997 (3 pages) | Cited 1 time

Online Publication Date: 6 April 2010

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We propose a design of near field plate for deep subwavelength super-focusing at optical frequency. The method used antisymmetric surface plasmon mode to generate abrupt π phase change within small fraction of a wavelength, while varied thin metallic film thickness to control the amplitude profile of the near field distribution. The numerical simulation result shows a λ/5 resolution at focus plane for the designed near field plate at the wavelength of 1550 nm, which is due to the superposition of the near field phase and amplitude distribution.
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78.66.Bz Metals and metallic alloys
78.68.+m Optical properties of surfaces
73.20.Mf Collective excitations (including excitons, polarons, plasmons and other charge-density excitations)

Terahertz emission characteristics of ErAs:InGaAs-based photoconductive antennas excited at 1.55 μm

A. Schwagmann, Z.-Y. Zhao, F. Ospald, H. Lu, D. C. Driscoll, M. P. Hanson, A. C. Gossard, and J. H. Smet

Appl. Phys. Lett. 96, 141108 (2010); http://dx.doi.org/10.1063/1.3374401 (3 pages) | Cited 6 times

Online Publication Date: 7 April 2010

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We characterize ErAs:In0.53Ga0.47As superlattices as substrates for photoconductive terahertz emitters excited at 1.55 μm. The bandwidth of the emitted radiation is studied as a function of the superlattice period (or equivalently the electron lifetime) and the applied bias field. The results show that a variation in the electron lifetime from 0.2 to 6.3 ps does not considerably influence the bandwidth of the emitted radiation. However, the bandwidth increases linearly from 2.6 to 3.0 THz as the applied bias field is increased from 7 to 30 kV/cm. At higher bias fields, saturation is observed. The largest measured bandwidth is 3.1 THz.
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84.40.Ba Antennas: theory, components and accessories
85.60.-q Optoelectronic devices
72.20.Jv Charge carriers: generation, recombination, lifetime, and trapping

Evaluation of a cesium iodide photocathode assisted with MgO-coated multiwall carbon nanotubes

Jungwoo Lee, Taehee Park, Wonjoo Lee, Eunkyung Park, Donghwan Lee, Bongwoo Han, Sung-Hwan Han, and Whikun Yi

Appl. Phys. Lett. 96, 141109 (2010); http://dx.doi.org/10.1063/1.3383220 (3 pages) | Cited 1 time

Online Publication Date: 7 April 2010

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This paper reports the enhanced photocurrent and relative quantum efficiency of cesium iodide (CsI) films on magnesium oxide (MgO)-coated multiwall carbon nanotubes (MWCNTs) on a silica substrate, i.e., CsI/MgO/MWCNTs/Si, when illuminating with 147 nm photons under an external electric field. The incorporation of MWCNTs resulted in significant enhancement of the photocurrent by several orders of magnitude compared to that of a conventional CsI. An analysis of the photoelectron energy spectrum attributed the phenomena to the creation of a very high electric field through the MgO/CsI film with the subsequent generation of avalanche secondary electrons.
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85.60.Ha Photomultipliers; phototubes and photocathodes

Quantum cascade lasers emitting near 2.6 μm

O. Cathabard, R. Teissier, J. Devenson, J. C. Moreno, and A. N. Baranov

Appl. Phys. Lett. 96, 141110 (2010); http://dx.doi.org/10.1063/1.3385778 (3 pages) | Cited 20 times

Online Publication Date: 8 April 2010

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Quantum cascade lasers (QCLs) emitting at wavelengths as short as 2.63–2.65 μm are demonstrated. The InAs/AlSb QCL design was optimized to weaken carrier leakage into the L-valley by reducing coupling between the active InAs quantum wells. The lasers with HR-coated facets operated up to 175 K.
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42.55.Px Semiconductor lasers; laser diodes
42.60.By Design of specific laser systems

Radiofrequency conical emission from femtosecond filaments in air

B. Forestier, A. Houard, M. Durand, Y. B. André, B. Prade, J.-Y. Dauvignac, F. Perret, Ch. Pichot, M. Pellet, and A. Mysyrowicz

Appl. Phys. Lett. 96, 141111 (2010); http://dx.doi.org/10.1063/1.3378266 (3 pages) | Cited 3 times

Online Publication Date: 8 April 2010

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We show that the broadband conical emission associated with filaments in air extends down to the radiofrequency region. This rf emission which originates from the longitudinal oscillation of charged ions formed during filamentation is strongly enhanced by the presence of a longitudinal static electric field.
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52.50.Jm Plasma production and heating by laser beams (laser-foil, laser-cluster, etc.)
52.25.Os Emission, absorption, and scattering of electromagnetic radiation
52.35.Fp Electrostatic waves and oscillations (e.g., ion-acoustic waves)
42.65.Re Ultrafast processes; optical pulse generation and pulse compression
52.80.Pi High-frequency and RF discharges

Low-threshold 303 nm lasing in AlGaN-based multiple-quantum well structures with an asymmetric waveguide grown by plasma-assisted molecular beam epitaxy on c-sapphire

V. N. Jmerik, A. M. Mizerov, A. A. Sitnikova, P. S. Kop’ev, S. V. Ivanov, E. V. Lutsenko, N. P. Tarasuk, N. V. Rzheutskii, and G. P. Yablonskii

Appl. Phys. Lett. 96, 141112 (2010); http://dx.doi.org/10.1063/1.3373834 (3 pages) | Cited 11 times

Online Publication Date: 8 April 2010

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We report on AlGaN multiple-quantum-well separate confinement laser heterostructures grown by plasma-assisted molecular-beam epitaxy directly on c-sapphire at low temperatures (<800 °C). Threading dislocation density was reduced down to 109–1010 cm−2 owing to both intentionally introduced strained AlGaN/AlN superlattices and self-organized blocking structures in the AlGaN step-graded buffer layers. The quantum wells were fabricated by a submonolayer digital alloying technique. Calculations of the optical gain and confinement in the optically pumped laser structures yielded its optimum design comprising an asymmetric waveguide. Lasing at 303 nm with the relatively low threshold excitation density of 0.8 MW/cm2 at 295K has been achieved.
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42.55.Px Semiconductor lasers; laser diodes
42.60.By Design of specific laser systems

Electrical beam steering of Y-coupled quantum cascade lasers

S. I. Ahn, E. Mujagić, M. Nobile, H. Detz, S. Kalchmair, A. M. Andrews, P. Klang, W. Schrenk, and G. Strasser

Appl. Phys. Lett. 96, 141113 (2010); http://dx.doi.org/10.1063/1.3374335 (3 pages) | Cited 4 times

Online Publication Date: 8 April 2010

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The authors describe electrical beam steering of mid-infrared quantum cascade lasers with a monolithically integrated Y-coupled cavity and electrically separated branches. The deflection of the laser beam is achieved without any additional components such as optic or mechanic systems. This is done by injecting additional direct current into one of the two emitting branches, locally increasing the temperature. We estimate that the required temperature difference between left and right branch is approximately 12 K to achieve 2° of the beam steering. This value is in a good agreement with heat transfer simulations.
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42.55.Px Semiconductor lasers; laser diodes
42.60.By Design of specific laser systems
42.60.Da Resonators, cavities, amplifiers, arrays, and rings
42.60.Jf Beam characteristics: profile, intensity, and power; spatial pattern formation

Nanofluidic control of coupled photonic crystal resonators

Silvia Vignolini, Francesco Riboli, Diederik Sybolt Wiersma, Laurent Balet, Lianhe H. Li, Marco Francardi, Annamaria Gerardino, Andrea Fiore, Massimo Gurioli, and Francesca Intonti

Appl. Phys. Lett. 96, 141114 (2010); http://dx.doi.org/10.1063/1.3378690 (3 pages) | Cited 8 times

Online Publication Date: 9 April 2010

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A fine control of a photonic molecule is obtained by nanofluidic techniques. The coupling condition between the modes of two photonic crystal nanocavities is modified by spectrally tuning each single resonator. Clear mode anticrossing and transition from localized to delocalized states are observed. The detuning induced by disorder, always present in real device, is experimentally compensated by locally modifying the photonic environment of the cavity.
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47.85.Np Fluidics
42.70.Qs Photonic bandgap materials
42.79.-e Optical elements, devices, and systems
47.61.Fg Flows in micro-electromechanical systems (MEMS) and nano-electromechanical systems (NEMS)

Wide bandwidth lasing randomly assembled ZnS/ZnO biaxial nanobelt heterostructures

H. Y. Yang, S. F. Yu, J. Yan, and L. D. Zhang

Appl. Phys. Lett. 96, 141115 (2010); http://dx.doi.org/10.1063/1.3393997 (3 pages) | Cited 2 times

Online Publication Date: 9 April 2010

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Wide-bandwidth random lasing action is observed from the randomly assembled ZnS/ZnO biaxial nanobelt heterostructures under optical excitation. This is because optical gain at ultraviolet regime can be obtained from the near-band-edge radiative recombination of ZnS and ZnO. Surface defects related radiative recombination centers of ZnS and ZnO nanostructures also contribute to the visible optical gain. Hence, a broadband optical gain is obtained from the ZnS/ZnO biaxial nanobelt heterostructures. Moreover, a wide bandwidth coherent optical feedback can be achieved from the randomly assembled biaxial nanobelts due to high refractive index contrast between the nanobelts and air.
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78.67.Bf Nanocrystals, nanoparticles, and nanoclusters
73.25.+i Surface conductivity and carrier phenomena
72.20.Jv Charge carriers: generation, recombination, lifetime, and trapping
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)
61.72.-y Defects and impurities in crystals; microstructure
68.35.Dv Composition, segregation; defects and impurities

Quasicontinuous refractive index tailoring of SiNx and SiOxNy for broadband antireflective coatings

Weibin Qiu, Young Mo Kang, and Lynford L. Goddard

Appl. Phys. Lett. 96, 141116 (2010); http://dx.doi.org/10.1063/1.3380825 (3 pages) | Cited 6 times

Online Publication Date: 9 April 2010

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A broadband antireflective coating for silicon was fabricated by tailoring the compositions of SiNx and SiOxNy during conventional plasma enhanced chemical vapor deposition. The coating’s refractive index was quasicontinuously graded, e.g., from 3.22 to 1.44 at 1550 nm. Over the 280–3300 nm wavelength range, the reflectance was below 8% peak and 4.3% average. The deposited stack was composed of dense dielectric materials. This enables patterning and processing into robust devices after coating deposition. Using single layer ellipsometry data, the transfer matrix method was applied to predict the multilayer coating’s reflectance spectra. The results showed good agreement with experimental data.
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42.79.Wc Optical coatings
77.55.-g Dielectric thin films
78.67.Pt Multilayers; superlattices; photonic structures; metamaterials
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
68.55.A- Nucleation and growth

Thermal induced facet destructive feature of quantum cascade lasers

Quande Zhang, Feng-Qi Liu, Wei Zhang, Quanyong Lu, Lijun Wang, Lu Li, and Zhanguo Wang

Appl. Phys. Lett. 96, 141117 (2010); http://dx.doi.org/10.1063/1.3385159 (3 pages)

Online Publication Date: 9 April 2010

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We present a study on the facet damage profile of quantum cascade lasers (QCLs). Conspicuous cascade half-loop damage strips on front facet are observed when QCLs catastrophically failed. Due to the large difference on thermal conductivities between active region and the substrate, dominant heat is compulsively driven to the substrate. Abundant heat accumulation and dissipation on substrate build large temperature gradient and thermal lattice mismatch. Thermal-induced stress due to sequential mismatch leads to the occurrence of the multistep damages on front facet. Good agreement is achieved between the observed locations of damaged strips and the calculated results.
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42.55.Px Semiconductor lasers; laser diodes
42.60.By Design of specific laser systems
61.80.Ba Ultraviolet, visible, and infrared radiation effects (including laser radiation)
61.82.Fk Semiconductors
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A cusp electron gun for millimeter wave gyrodevices

C. R. Donaldson, W. He, A. W. Cross, F. Li, A. D. R. Phelps, L. Zhang, K. Ronald, C. W. Robertson, C. G. Whyte, and A. R. Young

Appl. Phys. Lett. 96, 141501 (2010); http://dx.doi.org/10.1063/1.3374888 (3 pages) | Cited 7 times

Online Publication Date: 5 April 2010

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The experimental results of a thermionic cusp electron gun, to drive millimeter and submillimeter wave harmonic gyrodevices, are reported in this paper. Using a “smooth” magnetic field reversal formed by two coils this gun generated an annular-shaped, axis-encircling electron beam with 1.5 A current, and an adjustable velocity ratio α of up to 1.56 at a beam voltage of 40 kV. The beam cross-sectional shape and transported beam current were measured by a witness plate technique and Faraday cup, respectively. These measured results were found to be in excellent agreement with the simulated results using the three-dimensional code MAGIC.
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84.40.Ik Masers; gyrotrons (cyclotron-resonance masers)
84.47.+w Vacuum tubes
84.40.-x Radiowave and microwave (including millimeter wave) technology

Application of microplasma discharge in a spark gap for high repetitive switching

Hasibur Rahaman, Sang Hoon Nam, Jong Woo Nam, Byung-Joon Lee, and Klaus Frank

Appl. Phys. Lett. 96, 141502 (2010); http://dx.doi.org/10.1063/1.3383240 (3 pages)

Online Publication Date: 5 April 2010

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The electrical breakdown in a spark gap for repetitive switching has been a long research interest. For this purpose, microplasma discharge is implemented in the spark gap which is further integrated inside a coaxial transmission line. This work addresses important physical properties and insights of the microplasma discharge, to be optimized, such as plasma generation in a spark channel, dielectric recovery process, and residual plasma in the postspark discharge period. Although understanding the microplasma discharge is the primary goal, considerable attention has been focused on an external circuit scheme to drive the discharge system at a high repetition rate.
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52.80.Mg Arcs; sparks; lightning; atmospheric electricity
52.75.Kq Plasma switches (e.g., spark gaps)
52.50.Dg Plasma sources

The controlling role of envelope mismatches in intense inhomogeneous charged beams

E. G. Souza, A. Endler, R. Pakter, F. B. Rizzato, and R. P. Nunes

Appl. Phys. Lett. 96, 141503 (2010); http://dx.doi.org/10.1063/1.3385393 (3 pages) | Cited 3 times

Online Publication Date: 5 April 2010

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Inhomogeneous cold beams undergo wave breaking as they move along the axis of a magnetic focusing system. All the remaining control parameters fixed, the earliest wave breaking is a sensitive function of the inhomogeneity parameter: the larger the inhomogeneity, the sooner the breaking. The present work analyzes the role of envelope size mismatches in the wave breaking process. The analysis reveals that the wave breaking time is also very susceptible to the mismatch; judiciously chosen mismatches can largely extend beam lifetimes. The work is extended to include recently discussed issues on the presences of fast and slow regimes of wave breaking, and the theory is shown to be accurate against simulations.
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42.79.-e Optical elements, devices, and systems
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On the effect of impurities in metallic glass formation

J. S. Yu, Y. Q. Zeng, T. Fujita, T. Hashizume, A. Inoue, T. Sakurai, and M. W. Chen

Appl. Phys. Lett. 96, 141901 (2010); http://dx.doi.org/10.1063/1.3373528 (3 pages) | Cited 1 time

Online Publication Date: 5 April 2010

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We report atomic-scale characterization of impurity elements in a Pd40Ni40P20 metallic glass by state-of-the-art atom probe tomography combining with transmission electron microscopy. The significant partitioning of the impurities in heterogeneous nanocrystals of the primarily crystallized glass provides compelling evidence that minor impurities dramatically influence the stability of supercooled liquids by manipulating heterogeneous crystallization of metallic glasses.
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64.70.pe Metallic glasses
64.70.Nd Structural transitions in nanoscale materials
61.43.Fs Glasses
64.70.kj Glasses

Dielectrophoresis of Janus particles under high frequency ac-electric fields

Lu Zhang and Yingxi Zhu

Appl. Phys. Lett. 96, 141902 (2010); http://dx.doi.org/10.1063/1.3378687 (3 pages) | Cited 14 times

Online Publication Date: 5 April 2010

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Janus polystyrene particles with gold and dielectric multilayer coatings on one hemisphere are shown to drastically alter the dielectrophoresis (DEP) behavior of homogeneous precursor particles under ac-electric fields. Alkanethiol coatings on the gold-coated hemisphere can effectively modify the DEP crossover frequency (ωc) with a negative (n) to positive (p) (p→n) DEP transition as increasing ac-frequency, in contrast to p→n DEP transition observed with precursor particles. All measured n→p DEP crossover frequencies with dependence on particle size, alkanethiol thickness, and medium conductivity are collapsed to yield a scaling with the resistance-capacitance (RC) time of the alkanethiol layer capacitance and the conductive medium resistance.
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82.45.-h Electrochemistry and electrophoresis
82.45.Mp Thin layers, films, monolayers, membranes
77.55.-g Dielectric thin films

Effect of doping on the mid-infrared intersubband absorption in GaN/AlGaN superlattices grown on Si(111) templates

P. K. Kandaswamy, H. Machhadani, Y. Kotsar, S. Sakr, A. Das, M. Tchernycheva, L. Rapenne, E. Sarigiannidou, F. H. Julien, and E. Monroy

Appl. Phys. Lett. 96, 141903 (2010); http://dx.doi.org/10.1063/1.3379300 (3 pages)

Online Publication Date: 6 April 2010

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We report on the effect of Si doping on the mid-infrared intersubband absorption in GaN/AlGaN superlattices. For increasing doping levels, interband luminescence displays a blueshift and a broadening of the band edge caused by the screening of the internal electric field and band-filling effects. The intersubband absorption energy is mainly governed by many-body effects like exchange interaction and depolarization shift, which increase the e1–e2 subband separation. The ISB blueshift induced by many-body effects can be more than 50% of the e1–e2 transition energy.
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78.66.Fd III-V semiconductors
78.55.Cr III-V semiconductors
61.72.uj III-V and II-VI semiconductors
71.55.Eq III-V semiconductors

Anelastic anomalies and negative Poisson’s ratio in tetragonal BaTiO3 ceramics

Liang Dong, Donald S. Stone, and Roderic S. Lakes

Appl. Phys. Lett. 96, 141904 (2010); http://dx.doi.org/10.1063/1.3384996 (3 pages) | Cited 3 times

Online Publication Date: 6 April 2010

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Anelastic anomalies (sharp variations in modulus and damping with temperature) were observed in tetragonal BaTiO3 via broadband viscoelastic spectroscopy after aging at 50 °C for 15 h. The effect was most pronounced under electrical short circuit condition, at low frequency and under small excitation strain (10−6). Softening in bulk modulus and negative Poisson’s ratio were observed near 60 °C. Effects are attributed to an oxygen vacancy mechanism. A relaxational model cannot account for sharp response at smaller strains. Heterogeneity of negative stiffness is considered as a cause.
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81.40.Jj Elasticity and anelasticity, stress-strain relations
62.40.+i Anelasticity, internal friction, stress relaxation, and mechanical resonances
61.72.jd Vacancies
62.20.dj Poisson's ratio
81.05.Je Ceramics and refractories (including borides, carbides, hydrides, nitrides, oxides, and silicides)

Correlation of oxygen vacancy concentration and thermoelectric properties in Na0.73CoO2−δ

P. H. Tsai, T. Norby, T. T. Tan, R. Donelson, Z. D. Chen, and S. Li

Appl. Phys. Lett. 96, 141905 (2010); http://dx.doi.org/10.1063/1.3385264 (3 pages) | Cited 4 times

Online Publication Date: 6 April 2010

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In this work, oxygen vacancies were precisely controlled by manipulating oxygen partial pressure and the oxygen partial pressure dependence of thermoelectric properties of polycrystalline Na0.73CoO2−δ was investigated. The results suggest that resistivity and thermopower augment simultaneously with increasing concentration of oxygen vacancies δ due to the recombination of electrons and holes and average Co valence state. The total thermal conductivity can be reduced by minimizing δ. The dimensionless figure of merit was strongly affected by δ at lower operating temperature. The experimental results provide guideline for the variation in performance of this material system under different processing and operating environments.
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61.72.jd Vacancies
72.20.Pa Thermoelectric and thermomagnetic effects
66.70.Lm Other systems such as ionic crystals, molecular crystals, nanotubes, etc.
72.20.Jv Charge carriers: generation, recombination, lifetime, and trapping
72.80.Sk Insulators
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