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Applied Physics Letters
   
 
 
 

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8 February 2010

Volume 96, Issue 6,  partial issue

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Cover image from Takashi Yokoyama, Appl. Phys. Lett. 96, 063101 (2010).

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LASERS, OPTICS, AND OPTOELECTRONICS

Group velocity dispersion and self phase modulation in silicon nitride waveguides

D. T. H. Tan, K. Ikeda, P. C. Sun, and Y. Fainman

Appl. Phys. Lett. 96, 061101 (2010) (3 pages)

Online Publication Date: 8 February 2010

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The group velocity dispersion (GVD) of silicon nitride waveguides, prepared using plasma enhanced chemical vapor deposition, is studied and characterized experimentally in support of nonlinear optics applications. We show that the dispersion may be engineered by varying the geometry of the waveguide and demonstrate measured anomalous GVD values as high as −0.57  ps2/m and normal GVD values as high as 0.86  ps2/m. We also experimentally demonstrate the absence of any observed nonlinear loss at the telecommunications wavelength at peak intensities of up to 12  GW/cm2. Spectral broadening due to self phase modulation in silicon nitride waveguides with a nonlinear parameter of 1.4  W−1/m is also demonstrated.
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81.15.Gh, 42.65.Jx, 42.79.Gn

Efficiency droop in 245–247 nm AlGaN light-emitting diodes with continuous wave 2 mW output power

W. Sun, M. Shatalov, J. Deng, X. Hu, J. Yang, A. Lunev, Y. Bilenko, M. Shur, and R. Gaska

Appl. Phys. Lett. 96, 061102 (2010) (3 pages)

Online Publication Date: 9 February 2010

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We report on 245–247 nm AlGaN-based deep ultraviolet (DUV) light-emitting diodes with continuous wave output power up to 2 mW. DUV diodes with peak emission wavelength of 245 and 247 nm exhibit turn-on voltage less than 10 V. At room temperature and cw operation the maximum external quantum efficiency was close to 0.18%, which is the highest value published to date for devices with peak emission wavelength shorter than 250 nm. A large external efficiency droop observed at current densities above 100  A/cm2 is attributed to self-heating, carrier spillover from the QWs into the barrier layers or the p-type cladding layer, and/or Auger recombination. A semiempirical equation was proposed to describe the efficiency droop in DUV diodes at a high current injection.
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85.60.Jb, 72.80.Ey, 81.05.Ea, 72.20.Jv

Reactive radical facilitated reaction-diffusion modeling for holographic photopolymerization

Jianhua Liu, Haihui Pu, Bin Gao, Hongyue Gao, Dejin Yin, and Haitao Dai

Appl. Phys. Lett. 96, 061103 (2010) (3 pages)

Online Publication Date: 9 February 2010

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A phenomenological concentration of reactive radical is proposed to take the role of curing light intensity in explicit proportion to the reaction rate for the conventional reaction-diffusion model. This revision rationally eliminates the theoretical defect of null reaction rate in modeling of the postcuring process, and facilitates the applicability of the model in the whole process of holographic photopolymerizations in photocurable monomer and nematic liquid crystal blend system. Excellent consistencies are obtained in both curing and postcuring processes between simulated and experimentally measured evolutions of the first order diffraction efficiency of the formed composite Bragg gratings.
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82.50.-m, 42.79.Dj, 42.40.Eq, 61.30.-v, 82.30.Cf, 82.35.-x

Single-walled carbon-nanotube-deposited tapered fiber for four-wave mixing based wavelength conversion

K. K. Chow, M. Tsuji, and S. Yamashita

Appl. Phys. Lett. 96, 061104 (2010) (3 pages)

Online Publication Date: 9 February 2010

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We report the observation of four-wave mixing (FWM) in single-walled carbon nanotubes (CNTs) deposited around a tapered fiber. The third-order nonlinearity of CNTs is originated from the interband transitions of the pi-electrons causing nonlinear polarization similar to other highly-nonlinear organic optical materials. CNTs with suitable distributed tube diameter are optically deposited around a tapered fiber and FWM effect is generated by launching a probe and a pump light into the fiber device. FWM-based wavelength conversion is further demonstrated using the generated nonlinear effects and tunable conversion of 10 Gb/s nonreturn-to-zero signal is obtained.
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42.65.Hw, 42.65.Ky, 42.81.Gs

Soliton self-deflection via power-dependent walk-off

Armando Piccardi, Alessandro Alberucci, and Gaetano Assanto

Appl. Phys. Lett. 96, 061105 (2010) (3 pages)

Online Publication Date: 9 February 2010

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We demonstrate, both experimentally and theoretically, excitation-dependent self-bending of spatial solitons in nematic liquid crystals. The observed deflection is explained by nonlinear changes in walk-off, as induced by the rotation of the optic axis via power-driven reorientation.
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78.20.Ek, 42.70.Df, 42.65.Tg

High-Q silica microsphere by poly(methyl methacrylate) coating and modifying

C.-H. Dong, F.-W. Sun, C.-L. Zou, X.-F. Ren, G.-C. Guo, and Z.-F. Han

Appl. Phys. Lett. 96, 061106 (2010) (3 pages)

Online Publication Date: 9 February 2010

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We experimentally characterize the Q-factor in a silica microsphere with the poly(methyl methacrylate) coating. Experimental results demonstrated that the Q-factor of the coated microcavity is higher than 108. Moreover, this coating is able to eliminate scattering from the unsmoothed surface and tune the optical field in the cavity. In the cavity interaction with quantum dots, the coating can not only increase the Q-factor of the microsphere with deposited quantum dots but also draw the maximal field to the interface where the quantum dots exist. Therefore, this coating method can highly enhance the interaction strength, and it will contribute to the cavity quantum electrodynamics study.
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42.79.Wc, 78.67.Hc, 42.70.Jk, 81.65.-b
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PLASMAS AND ELECTRICAL DISCHARGES

Breakdown probability of neon under the influence of field electron emission and surface charges on the cathode surface

V. Lj. Marković, S. R. Gocić, and S. N. Stamenković

Appl. Phys. Lett. 96, 061501 (2010) (3 pages)

Online Publication Date: 8 February 2010

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Improved relations are suggested to determine of the breakdown probability in neon under the influence of field electron emission and surface charges on the gold plated cathode surfaces. The influence of field electron emission from a vacuum deposited gold layer is confirmed by the Fowler–Nordheim plot based on an analysis of the breakdown time delay dependence on the voltage [overline t[sub d]](U). Surface regions with a reduced conductivity on a hard galvanic layer of gold caused by diffusion of nickel atoms from a nickel sublayer are confirmed by scanning electron microscopy and energy dispersive x-ray spectrum.
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51.50.+v, 68.35.Fx, 79.70.+q

High current diffuse dielectric barrier discharge in atmospheric pressure air for the deposition of thin silica-like films

S. A. Starostin, P. Antony Premkumar, M. Creatore, H. de Vries, R. M. J. Paffen, and M. C. M. van de Sanden

Appl. Phys. Lett. 96, 061502 (2010) (3 pages)

Online Publication Date: 9 February 2010

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The diffuse dielectric barrier discharge in atmospheric pressure air was applied for the thin film deposition on polymeric web in industrially relevant roll-to-roll configuration. The silica-like film deposition was performed using the admixture of hexamethyldisiloxane precursor to air flow. Fast discharge imaging at 2  µs exposure time confirms plasma uniformity in a single current pulse time scale. Morphology and composition analyses indicate that the process results in ultrasmooth films (roughness comparable to initial substrate roughness) and shows the possibility to synthesize carbon-free layers.
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81.15.Gh, 68.35.B-, 68.55.J-, 68.55.A-, 82.80.-d, 77.55.D-
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STRUCTURAL, MECHANICAL, THERMODYNAMIC, AND OPTICAL PROPERTIES OF CONDENSED MATTER

Temperature-dependent shear band dynamics in a Zr-based bulk metallic glass

David Klaumünzer, Robert Maaß, Florian H. Dalla Torre, and Jörg F. Löffler

Appl. Phys. Lett. 96, 061901 (2010) (3 pages)

Online Publication Date: 8 February 2010

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Flow serrations recorded during inhomogeneous deformation of Zr52.5Ti5Cu17.9Ni14.6Al10 (Vit105) were studied during compression testing at temperatures between −40 and 60 °C. The shear band velocities determined exhibit a pronounced temperature dependence covering nearly two orders of magnitude. The velocities follow an Arrhenius-type behavior with an associated activation energy of 0.3±0.05  eV. The results demonstrate a thermally activated mechanism of shear band propagation, which is similar to the behavior of other, nonmetallic amorphous materials.
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81.40.Lm, 62.20.F-, 81.05.Kf

Thermoreflectance characterization of band-edge excitonic transitions in CuAlS2 ultraviolet solar-cell material

Ching-Hwa Ho

Appl. Phys. Lett. 96, 061902 (2010) (3 pages)

Online Publication Date: 8 February 2010

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Near band edge transitions of CuAlS2 chalcopyrite crystals have been characterized using temperature-dependent thermoreflectance (TR) spectroscopy in the temperature range between 30 and 340 K. A lot of interband transition features of E<sub>A</sub><sup>n = 2</sup>, E<sub>A</sub><sup>n = 3</sup>, E<sub>B</sub><sup>n = 2</sup>, EC, ED, EE, and EF were detected in the low-temperature TR spectrum of 3–6 eV at 30 K. Transition energies of the TR features of CuAlS2 are analyzed. Transition origins of the features are evaluated. Optical-electric conversion behavior of CuAlS2 was evaluated by photoconductivity (PC) measurement at 300 K. The PC spectrum reveals that the maximum photoresponse occurs near the energy positions of E<sub>A</sub><sup>n = 2</sup>, E<sub>A</sub><sup>n = 3</sup>, and E<sub>B</sub><sup>n = 2</sup> features. The features are the fundamental band edge excitons which dominate photoelectric conversion behavior of CuAlS2 ultraviolet solar-energy material.
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78.20.nb, 78.40.Fy, 73.20.Mf, 73.50.Pz, 88.40.H-

Statistical composition-structure-property correlation and glass-forming ability based on the full icosahedra in Cu–Zr metallic glasses

Z. D. Sha, Y. P. Feng, and Y. Li

Appl. Phys. Lett. 96, 061903 (2010) (3 pages)

Online Publication Date: 8 February 2010

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Using the large-scale atomic/molecular massively parallel simulator, fraction of the Cu-centered <0,0,12,0> full icosahedra (fico) is obtained from a statistical analysis over a broad compositional range with high resolution in the Cu–Zr binary system. Weak but significant peaks are observed at certain compositions which coincide with good glass formers. This correlation implies that the change in fico is a fundamental structural factor in determining the ease of glass formation. In this regard, fico can be an indicator of glass-forming ability. Our work provides further understanding on the atomic structure of the Cu–Zr system and its effect on glass formation.
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61.43.Fs, 61.43.Bn

Effects of additional Ce3+ doping on the luminescence of Li2SrSiO4:Eu2+ yellow phosphor

Tae-Gon Kim, Hyo-Sug Lee, Chun Che Lin, Taehyung Kim, Ru-Shi Liu, Ting-Shan Chan, and Seoung-Jae Im

Appl. Phys. Lett. 96, 061904 (2010) (3 pages)

Online Publication Date: 9 February 2010

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Additional Ce3+ doping improves the luminescence of Li2SrSiO4:Eu2+, a yellow phosphor for ultraviolet or blue light-emitting diodes. By examining the photoluminescence of Li2SrSiO4:Eu2+, Li2SrSiO4:Ce3+, and Li2SrSiO4:Ce3+,Eu2+, it was confirmed that the energy transfer from Ce3+ to Eu2+ ions contributes little to the enhanced luminescence of Li2SrSiO4:Ce3+,Eu2+. Alternatively, we suggested that Ce3+ ions could stabilize the Li vacancies, inhibit the oxidization of Eu2+ to Eu3+, and consequently increase emission intensity, based on the characterizations with decay time and synchrotron light source x-ray absorption measurements. The proposed argument was validated with first principle calculations of the defect formation energies.
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78.55.Hx, 61.72.up, 85.60.Jb

Optical and electro-optic anisotropy of epitaxial Ba0.7Sr0.3TiO3 thin films

D. Y. Wang, S. Li, H. L. W. Chan, and C. L. Choy

Appl. Phys. Lett. 96, 061905 (2010) (3 pages)

Online Publication Date: 9 February 2010

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The anisotropic optical and electro-optic properties of ferroelectric Ba0.7Sr0.3TiO3 thin films, deposited on highly transparent single-crystal MgO (001), (011), and (111) substrates using pulsed laser deposition, were investigated. The experimental results show a strong correlation between optical, electro-optic properties, and the orientation of Ba0.7Sr0.3TiO3 thin films. The linear electro-optic coefficient rc of the (001), (011), and (111)-oriented Ba0.7Sr0.3TiO3 thin films are 99.1, 15.7, and 87.8 pm/V, respectively. Such a correlation may be attributed to the orientation dependent distribution and magnitude of spontaneous polarization.
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77.55.Px, 78.66.Nk, 78.20.Jq, 78.20.Ci, 77.84.Cg, 77.80.-e

Identification of extremely radiative nature of AlN by time-resolved photoluminescence

T. Onuma, K. Hazu, A. Uedono, T. Sota, and S. F. Chichibu

Appl. Phys. Lett. 96, 061906 (2010) (3 pages)

Online Publication Date: 9 February 2010

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Extremely radiative nature of high-quality AlN single crystalline epilayers was identified by means of far ultraviolet time-resolved photoluminescence using a frequency-quadrupled femtosecond Al2O3:Ti laser. The gross radiative lifetimes of a free excitonic polariton emission as short as 10 ps at 7 K and 180 ps at 300 K were revealed, which are the shortest ever reported for bulk semiconductor materials.
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78.66.Fd, 78.55.Cr, 71.35.-y, 78.47.jd, 71.36.+c

Effect of Mg doping on enhancement of terahertz emission from InN with different lattice polarities

X. Q. Wang, G. Z. Zhao, Q. Zhang, Y. Ishitani, A. Yoshikawa, and B. Shen

Appl. Phys. Lett. 96, 061907 (2010) (3 pages)

Online Publication Date: 9 February 2010

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Effect of Mg doping on terahertz (THz) emission from InN with different lattice polarities was studied. Strong enhancement of THz emission was observed from InN with appropriate Mg-concentrations (1018  cm−3), which is independent of lattice polarity. The buried p-type layers show stronger THz emission than the n-type ones. The dominant mechanism for THz emission was found to be photo-Dember effect and the emission intensity was inversely proportional to the conductivity, which is beneficial to investigate THz emission from InN since the conductivity can be more accurately measured than the carrier concentration and mobility due to the electron accumulation on surface.
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61.72.uj, 68.55.Ln, 61.72.sd, 73.61.Ey, 73.50.Pz, 73.50.Dn

The influence of Ni additions on the relative stability of eta and eta[prime] Cu6Sn5

U. Schwingenschlögl, C. Di Paola, K. Nogita, and C. M. Gourlay

Appl. Phys. Lett. 96, 061908 (2010) (3 pages)

Online Publication Date: 9 February 2010

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We investigate how 5 at. % Ni influences the relative stability of eta and eta[prime] Cu6Sn5. Synchrotron x-ray diffraction shows that, while Cu6Sn5 exists as eta[prime] at 25 and 150 °C and transforms to eta on heating to 200 °C, Cu5.5Ni0.5Sn5 is best fit to eta throughout 25–200 °C. Our first principles calculations predict that eta[prime] is stable at T=0  K in both Cu6Sn5 and Cu5.5Ni0.5Sn5, but that the energy difference is substantially reduced from 1.21 to 0.90 eV per 22 atom cell by the Ni addition. This effect is attributed to Ni developing distinct bonding to both Cu and Sn in the eta phase.
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61.66.Dk

Optical band-gap determination of nanostructured WO3 film

P. P. González-Borrero, F. Sato, A. N. Medina, M. L. Baesso, A. C. Bento, G. Baldissera, C. Persson, G. A. Niklasson, C. G. Granqvist, and A. Ferreira da Silva

Appl. Phys. Lett. 96, 061909 (2010) (3 pages)

Online Publication Date: 9 February 2010

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The optical band-gap energy of a nanostructured tungsten trioxide film is determined using the photoacoustic spectroscopy method under continuous light excitation. The mechanism of the photoacoustic signal generation is discussed. The band-gap energy is also computed by other methods. The absorption coefficient as well as the band-gap energy of three different crystal structures of tungsten trioxide is calculated by a first-principles Green's function approach using the projector augmented wave method. The theoretical study indicates that the cubic crystal structure shows good agreement with the experimental data.
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78.67.-n, 78.66.-w, 73.22.-f, 71.20.-b, 07.60.Rd, 42.25.Bs
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ELECTRONIC TRANSPORT AND SEMICONDUCTORS

Atomic scale analysis of self assembled GaAs/AlGaAs quantum dots grown by droplet epitaxy

J. G. Keizer, J. Bocquel, P. M. Koenraad, T. Mano, T. Noda, and K. Sakoda

Appl. Phys. Lett. 96, 062101 (2010) (3 pages)

Online Publication Date: 8 February 2010

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In this letter we have performed a structural analysis at the atomic scale of GaAs/AlGaAs quantum dots grown by droplet epitaxy. The shape, composition, and strain of the quantum dots and the AlGaAs matrix are investigated. We show that the GaAs quantum dots have a Gaussian shape and that minor intermixing of Al with the GaAs quantum dot takes place. A wetting layer with a thickness of less than one bilayer was observed.
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68.65.-k, 64.75.Ef, 68.08.Bc, 68.37.Ef

Mobility analysis of highly conducting thin films: Application to ZnO

D. C. Look, K. D. Leedy, D. H. Tomich, and B. Bayraktaroglu

Appl. Phys. Lett. 96, 062102 (2010) (3 pages)

Online Publication Date: 8 February 2010

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Hall-effect measurements have been performed on a series of highly conductive thin films of Ga-doped ZnO grown by pulsed laser deposition and annealed in a forming-gas atmosphere (5% H2 in Ar). The mobility as a function of thickness d is analyzed by a simple formula involving only ionized-impurity and boundary scattering and having a single fitting parameter, the acceptor/donor concentration ratio K=NA/ND. For samples with d=3–100  nm, Kavg=0.41, giving ND=4.7×1020 and NA=1.9×1020  cm−3. Thicker samples require a two-layer formulation due to inhomogeneous annealing.
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73.61.Ga, 61.72.uj, 68.55.A-, 81.40.Gh, 81.15.Fg

Ultrafast terahertz photoconductivity in nanocrystalline mesoporous TiO2 films

H. Němec, P. Kužel, F. Kadlec, D. Fattakhova-Rohlfing, J. Szeifert, T. Bein, V. Kalousek, and J. Rathouský

Appl. Phys. Lett. 96, 062103 (2010) (3 pages)

Online Publication Date: 8 February 2010

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Terahertz time-resolved spectroscopy is used to investigate the transport of photoexcited electrons in nanocrystalline mesoporous TiO2 films prepared by the recently proposed “brick and mortar” technology [Szeifert et al.Chem. Mater. 21, 1260 (2009)] with a variable fraction of nanocrystalline titania “bricks” and amorphous titania “mortar.” Both long- and short-range conductivity is significantly enhanced upon calcination. After an ultrafast (subpicosecond) regime where the intrananograin conductivity dominates, the electron conductivity becomes limited by the interaction of electrons with the amorphous mortar. Comparison of the experimental results with Monte Carlo simulations of the electron motion allows us to determine the crystalline grain size after calcination and the yield of mobile photocarriers.
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72.40.+w, 81.05.Rm, 78.66.-w, 73.63.Bd, 61.43.Gt, 78.66.Hf

Photoconductivity in single AlN nanowires by subband gap excitation

H. M. Huang, R. S. Chen, H. Y. Chen, T. W. Liu, C. C. Kuo, C. P. Chen, H. C. Hsu, L. C. Chen, K. H. Chen, and Y. J. Yang

Appl. Phys. Lett. 96, 062104 (2010) (3 pages)

Online Publication Date: 9 February 2010

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Photoconductivity of individual aluminum nitride (AlN) nanowires has been characterized using different subband gap excitation sources. It is interesting that both positive (under 1.53 and 2.33 eV excitations) and negative (under 3.06 and 3.81 eV excitations) photocurrent responses are observed from the wide band gap nitride nanowires. The negative photoconductivity, which is attributed to the presence of electron trap and recombination center in the bulk of AlN, is capable to be inversed by a strong positive photoconductive mechanism of surface while changes the ambience from the atmosphere to the vacuum. An oxygen molecular sensitization effect is proposed to be the reason resulting in the enhancement of positive photocurrent and the inversion of negative photoresponse in the vacuum. Understanding of the diverse photoconductivity and its molecular effect is of great importance in the development of energy-selective and highly sensitive nanowire photodetector of AlN in the visible and ultraviolet ranges.
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73.63.Nm, 72.20.Jv, 72.40.+w

Measurement of the hot electron attenuation length of copper

J. J. Garramone, J. R. Abel, I. L. Sitnitsky, L. Zhao, I. Appelbaum, and V. P. LaBella

Appl. Phys. Lett. 96, 062105 (2010) (3 pages)

Online Publication Date: 9 February 2010

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Ballistic electron emission microscopy is utilized to investigate the hot-electron scattering properties of Cu through Cu/Si(001) Schottky diodes. A Schottky barrier height of 0.64±0.02  eV and a hot-electron attenuation length of 33.4±2.9  nm are measured at a tip bias of 1.0 eV and a temperature of 80 K. The dependence of the attenuation length with tip bias is fit to a Fermi liquid model that allows extraction of the inelastic and elastic scattering components. This modeling indicates that elastic scattering due to defects, grain boundaries, and interfaces is the dominant scattering mechanism in this energy range.
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72.10.Fk, 73.23.Ad, 68.37.Vj, 61.72.Mm, 72.20.Ht

Terahertz conductivity of doped silicon calculated using the ensemble Monte Carlo/finite-difference time-domain simulation technique

K. J. Willis, S. C. Hagness, and I. Knezevic

Appl. Phys. Lett. 96, 062106 (2010) (3 pages)

Online Publication Date: 9 February 2010

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We present terahertz-frequency characterization of doped silicon via a multiphysics numerical technique that couples ensemble Monte Carlo (EMC) simulation of carrier transport and a finite-difference time-domain (FDTD) solver of Maxwell's curl equations. We elucidate the importance of rigorous enforcement of Gauss's law, in order to avoid unphysical charge buildup and enhance solver accuracy. The calculated complex conductivity of doped bulk silicon shows excellent agreement with available experimental data. This comprehensive microscopic simulator is a valuable predictive tool in the terahertz frequency range, where experimental data are scarce and the Drude model inadequate.
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72.80.Cw, 72.20.Dp, 61.72.U-

A microprobe technique for simultaneously measuring thermal conductivity and Seebeck coefficient of thin films

Yanliang Zhang, Claudiu L. Hapenciuc, Eduardo E. Castillo, Theodorian Borca-Tasciuc, Rutvik J. Mehta, Chinnathambi Karthik, and Ganpati Ramanath

Appl. Phys. Lett. 96, 062107 (2010) (3 pages)

Online Publication Date: 9 February 2010

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We demonstrate a microprobe technique that can simultaneously measure thermal conductivity kappa and Seebeck coefficient alpha of thin films. In this technique, an alternative current joule-heated V-shaped microwire that serves as heater, thermometer and voltage electrode, locally heats the thin film when contacted with the surface. The kappa is extracted from the thermal resistance of the microprobe and alpha from the Seebeck voltage measured between the probe and unheated regions of the film by modeling heat transfer in the probe, sample and their contact area, and by calibrations with standard reference samples. Application of the technique on sulfur-doped porous Bi2Te3 and Bi2Se3 films reveals alpha=−105.4 and 1.96  µV/K, respectively, which are within 2% of the values obtained by independent measurements carried out using microfabricated test structures. The respective kappa values are 0.36 and 0.52 W/mK, which are significantly lower than the bulk values due to film porosity, and are consistent with effective media theory. The dominance of air conduction at the probe-sample contact area determines the microscale spatial resolution of the technique and allows probing samples with rough surfaces.
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73.50.Lw, 72.20.Pa, 66.70.-f, 44.30.+v

Electrically tunable electron g factors in coupled InAs/GaAs pyramid quantum dots

Jiqing Wang, Huibing Mao, Jianguo Yu, Qiang Zhao, Hongying Zhang, Pingxiong Yang, Ziqiang Zhu, and Junhao Chu

Appl. Phys. Lett. 96, 062108 (2010) (3 pages)

Online Publication Date: 9 February 2010

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The electron g factors of coupled InAs/GaAs quantum dots under external magnetic and electric fields are investigated by using the eight-band k•p model. The resonant coupling between the two dots remains under electric fields below 8.2 mV/nm, and is broken above the critical field due to the quantum Stark effect. By applying electric fields, a sign reverse of g factors is observed, and an electric field tunable zero g factor is found in the quantum dot molecules. Spin-orbit interactions nicely explain the transition mechanism of g factors under external electric fields.
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73.21.La, 71.18.+y, 71.15.-m, 71.70.Ej, 78.67.Hc, 78.20.Jq

Highly tensile-strained, type-II, Ga1−xInxAs/GaSb quantum wells

T. Taliercio, A. Gassenq, E. Luna, A. Trampert, and E. Tournié

Appl. Phys. Lett. 96, 062109 (2010) (3 pages)

Online Publication Date: 9 February 2010

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We have investigated the properties of highly tensile-strained (Ga,In)As layers in a GaSb matrix. In situ observations of the growth mode suggest the formation of (Ga,In)As quantum dots. In contrast, ex situ transmission electron microscopy evidences the formation of perfect quantum wells with the presence of a (Ga,In)Sb interfacial layer. This is analyzed taking into account the surfactant behavior of Sb and stabilization of the system by reduction of the overall strain. Photoluminescence is observed up to 3.0  µm at room temperature, in good agreement with calculations assuming a type-II band alignment and including the (Ga,In)Sb interfacial layer.
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68.65.Fg, 68.65.Hb, 78.67.De, 78.55.Cr, 73.21.Fg, 81.15.Hi

Observation of shallow-donor muonium in Ga2O3: Evidence for hydrogen-induced conductivity

P. D. C. King, I. McKenzie, and T. D. Veal

Appl. Phys. Lett. 96, 062110 (2010) (3 pages)

Online Publication Date: 9 February 2010

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The electrical nature of muonium in the transparent conducting oxide material Ga2O3 is investigated via muon-spin rotation and relaxation spectroscopy. It is found to be a shallow donor, with an effective donor depth of 15<=ED<=30  meV and a hyperfine splitting of 0.13±0.01  MHz. This is in contrast to the deep level observed in the majority of semiconductors but supports the recent suggestion that muonium should be a shallow donor across the class of transparent conducting oxides. These observations suggest that hydrogen will also be a shallow donor in Ga2O3, with important implications both for unintentional conductivity and deliberate n-type doping of this material.
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76.75.+i, 71.70.Jp, 71.55.Ht

Anisotropic transport in graphene on SiC substrate with periodic nanofacets

S. Odaka, H. Miyazaki, S.-L. Li, A. Kanda, K. Morita, S. Tanaka, Y. Miyata, H. Kataura, K. Tsukagoshi, and Y. Aoyagi

Appl. Phys. Lett. 96, 062111 (2010) (3 pages)

Online Publication Date: 9 February 2010

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Anisotropic transport in graphene field-effect transistors fabricated on a vicinal SiC substrate with a self-organized periodic nanofacet structure is investigated. Graphene thermally grown on a vicinal substrate contains two following regions: atomically flat terraces and nanofacets (atomically stepped slopes). The graphene film at a nanofacet is continuously connected between two neighboring terrace films. Anisotropic transport properties are clearly observed, indicating a difference in the graphene properties of the two regions. The observed anisotropic properties are discussed in terms of the effects of nanofacet structures on conductivity and electron mobility.
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85.30.Tv, 81.16.Dn, 81.05.ue

Determination of the microstructure of Eu-treated ZnO nanowires by x-ray absorption

W. L. Huang, J. Labis, S. C. Ray, Y. R. Liang, C. W. Pao, H. M. Tsai, C. H. Du, W. F. Pong, J. W. Chiou, M.-H. Tsai, H. J. Lin, J. F. Lee, Y. T. Chou, J. L. Shen, C. W. Chen et al.

Appl. Phys. Lett. 96, 062112 (2010) (3 pages)

Online Publication Date: 9 February 2010

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X-ray absorption near-edge structure (XANES), extended x-ray absorption fine structures (EXAFS), and photoluminescence measurements were used to elucidate the microstructural and photoluminescence properties of ZnO nanowires (ZnO-NWs) that had been treated with Eu by thermal diffusion. The O K- and Eu L3-edge XANES and EXAFS spectra at the Zn K- and Eu L3-edge verified the formation of Eu2O3-like layer on the surface of ZnO-NWs. X-ray diffraction, XANES and EXAFS measurements consistently suggest the lack of substitutional doping of Eu ions at the Zn ion sites in the interior of ZnO-NWs. The clear sharp and intense emission bands in the range 610–630 nm of Eu-treated ZnO-NWs originated from the intra-4f transition of Eu ions in the Eu2O3-like surface layer.
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61.46.Km, 78.67.Uh, 78.55.Et, 66.30.Pa, 66.70.Df, 78.70.Dm

Modeling of leakage currents in high-kappa dielectrics: Three-dimensional approach via kinetic Monte Carlo

Gunther Jegert, Alfred Kersch, Wenke Weinreich, Uwe Schröder, and Paolo Lugli

Appl. Phys. Lett. 96, 062113 (2010) (3 pages)

Online Publication Date: 9 February 2010

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We report on a simulation algorithm, based on kinetic Monte Carlo techniques, that allows us to investigate transport through high-permittivity dielectrics. In the example of TiN/ZrO2/TiN capacitor structures, using best-estimate physical parameters, we have identified the dominant transport mechanisms. Comparison with experimental data reveals the transport to be dominated by Poole–Frenkel emission from donorlike trap states at low fields and trap-assisted tunneling at high fields.
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84.32.Tt, 77.55.dj, 72.20.Ht, 73.40.Gk
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MAGNETISM AND SUPERCONDUCTIVITY

Perpendicular FePt-based exchange-coupled composite media

D. Makarov, J. Lee, C. Brombacher, C. Schubert, M. Fuger, D. Suess, J. Fidler, and M. Albrecht

Appl. Phys. Lett. 96, 062501 (2010) (3 pages)

Online Publication Date: 8 February 2010

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Exchange-coupled composite media were realized by combining perpendicular hard magnetic FePtCu alloy films with perpendicular Co/Pt multilayers which are magnetically softer. We demonstrate that the switching field of the hard layer can be efficiently altered by modifying the material properties of the soft layer by varying the number of Co/Pt bilayers. Moreover, the possibility of effectively tuning the interlayer exchange coupling using rapid thermal annealing was shown. These studies were supported by theoretical modeling revealing the relevant factors to reduce the switching field of the hard layer which are important for future media design.
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75.70.Cn, 75.50.Ww, 75.50.Vv, 68.65.Ac

Suppression of complex domain wall behavior in Ni80Fe20 nanowires by oscillating magnetic fields

K. Weerts, W. Van Roy, G. Borghs, and L. Lagae

Appl. Phys. Lett. 96, 062502 (2010) (3 pages)

Online Publication Date: 8 February 2010

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We have studied the field-induced propagation of domain walls (DW) in 750 nm wide magnetic nanowires by time-resolved magneto-optical Kerr microscopy. Two different DW propagation modes are observed, separated by the so-called Walker breakdown (WB), with velocities ranging from 250 till 500 m/s. At WB, a velocity decrease occurs because of a complex evolution of the DW shape. Through simulations and experiments, we demonstrate a method to suppress this complex DW behavior at WB by superimposing an oscillating magnetic field Hosc, parallel to the switching field Heff, at frequencies ranging from 200 to 600 MHz.
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75.78.Fg, 75.78.Jp, 78.47.D-, 75.60.Ch, 78.67.Uh

Losses in coplanar waveguide resonators at millikelvin temperatures

P. Macha, S. H. W. van der Ploeg, G. Oelsner, E. Il'ichev, H.-G. Meyer, S. Wünsch, and M. Siegel

Appl. Phys. Lett. 96, 062503 (2010) (3 pages)

Online Publication Date: 8 February 2010

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We study the loss rate for a set of lambda/2 coplanar waveguide resonators at millikelvin temperatures (20–900 mK) and different applied powers (3·10−19–10−12  W). The loss rate becomes power independent below a critical power. For a fixed power, the loss rate increases significantly with decreasing temperature. We show that this behavior can be caused by two-level systems in the surrounding dielectric materials. Interestingly, the influence of the two-level systems is of the same order of magnitude for the different material combinations. That leads to the assumption that the nature of these two-level systems is material independent.
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42.82.Et, 42.79.Gn

Structural defects in LiCoO2 studied by 7Li nuclear magnetic relaxation

J. S. Kim, K. W. Lee, J. J. Kweon, Cheol Eui Lee, K. Kim, J. Lee, S. J. Noh, and H. S. Kim

Appl. Phys. Lett. 96, 062504 (2010) (3 pages)

Online Publication Date: 9 February 2010

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Microscopic environments and dynamics in LiCoO2 systems were probed by 7Li nuclear magnetic relaxation measurements with regard to the structural defects as revealed by x-ray diffraction and electron spin resonance measurements. Thus, the structural defects of differing degrees, associated with Li vacancies, were well accounted for in the temperature-dependent spin-lattice relaxation rate analysis, and in the structural and dynamical inhomogeneity as derived from the relaxation patterns.
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61.72.jd, 76.60.-k, 76.30.-v, 66.30.-h
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DIELECTRICS AND FERROELECTRICITY

Ta2O5 polymorphs: Structural motifs and dielectric constant from first principles

Wanda Andreoni and Carlo A. Pignedoli

Appl. Phys. Lett. 96, 062901 (2010) (3 pages)

Online Publication Date: 9 February 2010

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Using large-scale simulations based on density-functional theory we determine the structural properties of several polymorphs of tantalia, their vibrational spectra and dielectric properties. Our calculations indicate that structurally distinct configurations can coexist, having coordination polyhedra that are dissimilar or have different relative arrangements. Interpolyhedra vibrations are discovered to be responsible for sizable changes of the static dielectric constant from one polymorph to the other. Our results for the dielectric constant (both at high and low frequency) are in excellent agreement with experiment and provide an explanation for the different measured values and their claimed dependence on growth conditions.
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61.50.Ks, 61.66.Fn, 77.22.Ch, 78.30.Hv

First-principles investigations of the dielectric properties of crystalline and amorphous Si3N4 thin films

T. Anh Pham, Tianshu Li, Sadasivan Shankar, Francois Gygi, and Giulia Galli

Appl. Phys. Lett. 96, 062902 (2010) (3 pages)

Online Publication Date: 9 February 2010

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We have investigated the dielectric properties of silicon nitride thin films with thickness below 6 nm, by using density functional theory calculations. We find a substantial decrease in the static dielectric constant of crystalline films, as their size is reduced. The variation in the response in proximity of the surface plays a key role in the observed decrease. In addition, amorphization of the films may bring further reduction of both the static and optical dielectric constants.
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77.55.-g, 77.22.Ch, 68.55.-a, 64.70.K-, 78.66.Nk, 78.20.Ci
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NANOSCALE SCIENCE AND DESIGN

Self-assembled templating for the growth of molecular nanodots

Takashi Yokoyama

Appl. Phys. Lett. 96, 063101 (2010) (3 pages)

Online Publication Date: 8 February 2010

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A checkerboard pattern of alpha-sexithiophene (alpha-6T) with an approximate lattice size of 2.6×3.3  nm2 has been achieved on a Ag(110) surface at monolayer coverage, which is formed through a lateral ordering of two oriented molecules and vacancy defects. We find that this self-assembled pattern provides a template structure for the formation of molecular nanodots and nanocolumns. Second and third-layer islands of alpha-6T are formed within the checkerboard lattice, leading to a uniform molecular nanodot array, and a nanosized columnar structure is expected if growth is continued into the multilayer regime.
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81.16.Dn, 61.72.jd, 81.07.Ta

Low-temperature solid phase epitaxy for integrating advanced source/drain metal-oxide-semiconductor structures

A. Gouyé, I. Berbezier, L. Favre, G. Amiard, M. Aouassa, Y. Campidelli, and A. Halimaoui

Appl. Phys. Lett. 96, 063102 (2010) (3 pages)

Online Publication Date: 9 February 2010

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We show that chemical vapor deposition using trisilane decomposition opens capabilities for the deposition of amorphous silicon on Si substrate at low temperature. Based on this behavior we developed a process including amorphous silicon deposition and crystallization. Transmission electron microscopy observations prove that solid phase epitaxy (SPE) occurs and produces monocrystalline layers, free of extended defects and compatible with complementary metal-oxide-semiconductor technology. We also show that during SPE films remain amorphous on oxidized areas while they transform into single crystal on Si. This process opens promising perspectives for the fabrication of advanced MOS structures.
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68.55.ag, 81.15.Np, 81.15.Gh, 64.70.kg

Strain effects on basal-plane hydrogenation of graphene: A first-principles study

Kun Xue and Zhiping Xu

Appl. Phys. Lett. 96, 063103 (2010) (3 pages)

Online Publication Date: 9 February 2010

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In this letter we discuss basal-plane hydrogenation of graphene, in the extent of intercoupling between strain and electronic structure. Our first-principles calculations reveal that the atomic structures, binding energies, mechanical and electronic properties of graphene are significantly modified by applying strain. At a biaxial strain of 10%, binding energies of hydrogen on graphene can be improved by 53.89% and 23.56% in the symmetric and antisymmetric phase. In symmetric phase, carbon-hydrogen binding is unstable in compression. In antisymmetric phase, binding of hydrogen atoms reduces the sp2 characteristic of graphene, which is partially recovered at finite tensile strain.
Show PACS
61.48.Gh, 73.22.Pr, 71.15.Nc, 71.15.-m
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ORGANIC ELECTRONICS AND PHOTONICS

FREE

Crystallitic orientation effects on charge transport in polythiophene thin-film transistors

Liping Zhou, Xue-Feng Wang, Qin Han, Jian-Chun Wu, and Zhen-Ya Li

Appl. Phys. Lett. 96, 063301 (2010) (3 pages)

Online Publication Date: 8 February 2010

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We simulate charge transport through polymer grain boundaries under the buried critical interface of polymer and dielectric in polythiophene thin-film transistors and find the recently observed enhancement of electronic conduction can be a result of optimized crystallitic orientation. The simulation is based on a first-principle description of the system with transport properties evaluated by nonequilibrium Green's function method. Highly oriented crystal is assumed inside each grain and four most probable configurations of crystallitic orientation for grain boundary are considered. The calculation confirms that the electronic conductance can differ by five orders in magnitude for different conformations.
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85.30.Tv, 81.05.Lg, 61.72.Mm
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DEVICE PHYSICS

Hierarchical nanostructured spherical carbon with hollow core/mesoporous shell as a highly efficient counter electrode in CdSe quantum-dot-sensitized solar cells

Sheng-Qiang Fan, Baizeng Fang, Jung Ho Kim, Jeum-Jong Kim, Jong-Sung Yu, and Jaejung Ko

Appl. Phys. Lett. 96, 063501 (2010) (3 pages)

Online Publication Date: 8 February 2010

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Hierarchical nanostructured spherical carbon with hollow core/mesoporous shell (HCMS) was explored as a counter electrode in CdSe quantum-dot-sensitized solar cells. Compared with conventional Pt electrodes and commercially available activated carbon, the HCMS carbon counter electrode exhibits a much larger fill factor due to the considerably decreased charge transfer resistance at the interface of the counter electrode/polysulfide electrolyte. Furthermore, a solar cell with the HCMS carbon counter electrode presents a high power conversion efficiency of up to 3.90% as well as an incident photon-to-current conversion efficiency peak of 80%.
Show PACS
88.40.jm, 88.40.hj

High effective minority carrier lifetime on silicon substrates using quinhydrone-methanol passivation

Bhumika Chhabra, Stuart Bowden, Robert L. Opila, and Christiana B. Honsberg

Appl. Phys. Lett. 96, 063502 (2010) (3 pages)

Online Publication Date: 9 February 2010

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Iodine-methanol (I2/ME), a chemical passivation method, is extensively used in silicon (Si) solar cell fabrication for measuring minority carrier lifetime in bulk regions. We demonstrate that quinhydrone-methanol (QHY/ME) provides higher lifetimes than I2/ME. For 0.01  mol/dm−3 QHY/ME on float-zone (FZ) wafers at 1×1015  cm−3 injection level, a high lifetime of 3.3 ms and surface recombination velocity of 7 cm/sec on n-type (100  Omega cm) and 1.1 ms on p-type (2  Omega cm) is reported. The surface recombination velocity is also measured out of solution for several days. Chemical characterization results indicate increasing surface oxidation with decreasing passivation, consistent with proposed bonding mechanism.
Show PACS
73.25.+i, 72.20.Jv, 68.47.Fg, 72.80.Cw, 81.05.Cy, 81.65.Rv

On the origin of leakage current reduction in TiO2 passivated porosus silicon Schottky-barrier diode

J. D. Hwang and C. H. Chou

Appl. Phys. Lett. 96, 063503 (2010) (3 pages)

Online Publication Date: 9 February 2010

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Significant reduction by 40 times was achieved in the leakage current of TiO2 passivated porous silicon (PS) Schottky-barrier diode, compared to that of as-grown PS one. It was found that the imperfect native oxide, nonstoichiometric Si-suboxide (SiOx) and Si-O bond with nonbridging oxygen, and highly chemically reactive Si–H2 species are the issues of leakage current. These issues can be drastically suppressed and replaced by a stable stoichiometric SiO2 and Si–OH bond on the TiO2 passivated PS surface. The enriched OH formation will favor the growth of TiO2 and passivate the surface states of PS, hence reducing the leakage current.
Show PACS
85.30.Hi, 85.30.Kk, 82.30.-b, 81.65.Rv, 61.50.Nw

Interband cascade photovoltaic devices

Rui Q. Yang, Zhaobing Tian, J. F. Klem, Tetsuya D. Mishima, Michael B. Santos, and Matthew B. Johnson

Appl. Phys. Lett. 96, 063504 (2010) (3 pages)

Online Publication Date: 9 February 2010

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A photovoltaic (PV) device based on an interband cascade (IC) structure is proposed for efficiently converting solar and thermal energy to electricity. These IC PV devices employ absorption and transport regions with characteristics that are favorable for achieving high open-circuit voltage and thus possibly improving conversion efficiency over conventional PV devices. Preliminary experiments carried out using IC infrared photodetectors (seven stages) and lasers (11 stages) showed open-circuit voltages that exceed the single-band gap voltage from these devices under infrared illumination. The observed open-circuit voltage demonstrates multiple stages operating in series and provides an initial proof of concept for IC PV devices.
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85.60.Gz, 42.60.-v, 81.05.Ea
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BIOPHYSICS AND BIO-INSPIRED SYSTEMS

Cleavage pattern of DNA caused by endonuclease: Theoretical modeling and experimental verification

Shio Inagaki, Li Liu, Masahiro Takinoue, and Kenichi Yoshikawa

Appl. Phys. Lett. 96, 063701 (2010) (3 pages)

Online Publication Date: 9 February 2010

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In apoptotic cells, genomic DNA molecules are fragmented into multiple fragments with lengths that are integer multiples of approximately 180–200 base pairs (bp), i.e., the size of a single nucleosome. Here we propose a simple mathematical model for interpreting this cleavage pattern of DNA. Under the condition of a purely stochastic cleavage process, we derive a time evolution of the probability distribution of the fragment length by a Poisson distribution. We examine the applicability of our model by analyzing experimental results with apoptotic cells. Our model enables us to satisfactorily interpret the experimental trends. Interestingly, this theoretical fitting of the experimental data provides kinetic information for the cleavage reaction.
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87.15.-v, 36.20.-r, 87.15.B-, 87.18.Wd, 02.50.Ey, 87.15.R-

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