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18 Jan 1999

Volume 74, Issue 3, pp. 329-478

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Stripe-width dependence of threshold current for gain-guided AlGaInN laser diodes

D. P. Bour, M. Kneissl, L. T. Romano, R. M. Donaldson, C. J. Dunnrowicz, N. M. Johnson, and G. A. Evans

Appl. Phys. Lett. 74, 404 (1999); http://dx.doi.org/10.1063/1.123084 (3 pages) | Cited 4 times

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The threshold current density of narrow-stripe gain-guided nitride laser diodes increases very rapidly as the stripe width is made narrow. To examine this behavior, waveguide simulations, incorporating the complex refractive indices associated with optical gain, have been used to analyze the lateral optical modes of gain-guided laser diodes. Threshold current was then determined from the gain–current relationship of our laser material, which was obtained experimentally. These evaluations reveal that gain guiding, coupled with a carrier-induced index depression, offer a reasonable explanation for the rapid increase in threshold when the stripe width becomes less than 5 μm. © 1999 American Institute of Physics.
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42.55.Px Semiconductor lasers; laser diodes
42.60.By Design of specific laser systems
78.66.Fd III-V semiconductors
42.79.Gn Optical waveguides and couplers

Proton bombardment-induced electron traps in epitaxially grown n-GaN

F. D. Auret, S. A. Goodman, F. K. Koschnick, J.-M. Spaeth, B. Beaumont, and P. Gibart

Appl. Phys. Lett. 74, 407 (1999); http://dx.doi.org/10.1063/1.123043 (3 pages) | Cited 45 times

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Using deep-level transient spectroscopy, we have studied the electrical properties of defects introduced in epitaxially grown n-GaN during 2-MeV proton bombardment. The main defects detected, ER2 and ER3, are introduced at rates of 400±150 and 600±100 cm−1, respectively, and have energy levels at 0.16±0.03 and 0.20±0.01 eV, respectively, below the conduction band. A less prominent defect, ER1, with an energy level at 0.13±0.01 eV below the conduction band, is introduced at a rate of 30±10 cm−1. The small capture cross section of ER3 [(8±4)×10−18 cm2] implies that it is in a neutral or negative state when above the Fermi level. © 1999 American Institute of Physics.
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71.55.Eq III-V semiconductors
73.61.Ey III-V semiconductors
72.20.Jv Charge carriers: generation, recombination, lifetime, and trapping
73.50.Gr Charge carriers: generation, recombination, lifetime, trapping, mean free paths
61.80.Jh Ion radiation effects
61.82.Fk Semiconductors

Band gaps and band offsets in strained GaAs1−ySby on InP grown by metalorganic chemical vapor deposition

M. Peter, N. Herres, F. Fuchs, K. Winkler, K.-H. Bachem, and J. Wagner

Appl. Phys. Lett. 74, 410 (1999); http://dx.doi.org/10.1063/1.123044 (3 pages) | Cited 32 times

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Metastable GaAs1−ySby with 0.22<y<0.70 has been grown pseudomorphically strained on (001) InP substrates using metalorganic chemical vapor deposition. The Sb concentration and layer thicknesses, ranging from 24 to 136 nm, were determined by high resolution x-ray diffraction (HRXRD) measurements. Low-temperature photoluminescence (PL) spectroscopy revealed spatially indirect band-to-band emission of electrons localized in the InP and holes in the GaAs1−ySby. At increased excitation power densities samples with layer thicknesses above 65 nm showed, also, spatially direct PL across the band gap of the strained GaAs1−ySby. From the PL data the band gap energy and the band offsets of GaAs1–ySby relative to InP were derived and compared with the predictions of the Model Solid Theory. © 1999 American Institute of Physics.
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71.20.Nr Semiconductor compounds
81.05.Ea III-V semiconductors
68.55.-a Thin film structure and morphology
78.55.Cr III-V semiconductors
78.66.Fd III-V semiconductors
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)

Strong normal-incidence infrared absorption in self-organized InAs/InAlAs quantum dots grown on InP(001)

A. Weber, O. Gauthier-Lafaye, F. H. Julien, J. Brault, M. Gendry, Y. Désieres, and T. Benyattou

Appl. Phys. Lett. 74, 413 (1999); http://dx.doi.org/10.1063/1.123045 (3 pages) | Cited 44 times

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InAs self-assembled quantum dots in InAlAs matrix grown on InP(001) substrates have been fabricated using Stranski–Krastanov growth mode. A strong in-plane polarized intraband absorption in the 10.6–20 μm wavelength region has been observed and ascribed to a transition from the ground electron state to an excited state confined in the layer plane along the [110] direction. The absorption at normal-incidence reaches 7.8% for ten layers of n-doped quantum dots. The oscillator strength of the intraband transition is comparable to that achieved in quantum wells for a conduction band intersubband transition. The dependence of the intraband absorption on carrier concentration and temperature suggests a quantum-wire type confinement potential. © 1999 American Institute of Physics.
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78.66.Fd III-V semiconductors
78.30.Fs III-V and II-VI semiconductors
73.61.Ey III-V semiconductors
73.21.-b Electron states and collective excitations in multilayers, quantum wells, mesoscopic, and nanoscale systems
68.65.-k Low-dimensional, mesoscopic, nanoscale and other related systems: structure and nonelectronic properties

Molecular doping of gallium nitride

J. I. Pankove, J. T. Torvik, C.-H. Qiu, I. Grzegory, S. Porowski, P. Quigley, and B. Martin

Appl. Phys. Lett. 74, 416 (1999); http://dx.doi.org/10.1063/1.123046 (3 pages) | Cited 16 times

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Photoconductivity experiments were made on bulk GaN doped with Mg and O and grown using high pressures and high temperature. The bulk GaN:Mg,O was insulating, indicating compensation. The photoconductive response to photons above the energy band gap was comparable to that of epitaxially grown GaN:Mg samples. However, the UV-to-visible rejection ratio (solar blindness) was three orders of magnitude larger in the bulk GaN:Mg,O than for other epitaxially grown GaN samples. The dramatically improved visible rejection ratio is tentatively attributed to molecular doping by paired donors (O) and acceptors (Mg). Vacuum UV reflectance was performed to verify if MgO critical point transitions could be found in the GaN:Mg,O. A reflectance peak at 6.7 eV was found in both MgO and GaN:Mg,O. © 1999 American Institute of Physics.
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73.61.Ey III-V semiconductors
71.55.Eq III-V semiconductors
78.66.Fd III-V semiconductors
73.50.Pz Photoconduction and photovoltaic effects
61.72.uj III-V and II-VI semiconductors
68.55.Ln Defects and impurities: doping, implantation, distribution, concentration, etc.
85.40.Ry Impurity doping, diffusion and ion implantation technology
78.40.Fy Semiconductors
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Magnetic dissipation microscopy in ambient conditions

Roger Proksch, Ken Babcock, and Jason Cleveland

Appl. Phys. Lett. 74, 419 (1999); http://dx.doi.org/10.1063/1.123047 (3 pages) | Cited 9 times

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We have quantified sub-picowatt power losses in very soft (yttrium iron garnets) and relatively hard (metal evaporated tape) ferromagnetic materials associated with dissipative micromagnetic processes during magnetic force microscope imaging. We had a thermally limited power resolution of 2×10−15 W in a 1 kHz bandwidth operating in air. In the epitaxial garnet film, peak dissipation (3×10−13 W) was spatially correlated with domain wall motion induced by the localized field from the tip. In metal-evaporated recording tape imaged with a cantilever coated with 50 nm of CoCr, the dissipation was observed in extremely localized regions of the sample (<10 nm). Absence of dissipation features when using a different tip suggests the dissipation originated in the tip rather than the sample. This technique shows promise for mapping micromagnetic structure and dissipative processes, quantitatively evaluating magnetic force microscope (MFM) tip performance, and for detecting perturbations in MFM images. © 1999 American Institute of Physics.
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68.37.Ef Scanning tunneling microscopy (including chemistry induced with STM)
68.37.Ps Atomic force microscopy (AFM)
68.37.Rt Magnetic force microscopy (MFM)
68.37.Uv Near-field scanning microscopy and spectroscopy
07.79.Pk Magnetic force microscopes
75.70.Ak Magnetic properties of monolayers and thin films

Micromagnetics and magnetoresistance of a Permalloy point contact

R. P. van Gorkom, J. Caro, S. J. C. H. Theeuwen, K. P. Wellock, N. N. Gribov, and S. Radelaar

Appl. Phys. Lett. 74, 422 (1999); http://dx.doi.org/10.1063/1.123048 (3 pages) | Cited 21 times

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We performed micromagnetic calculations for a Permalloy point contact. The magnetization configurations show the formation of a Néel wall in the constriction. The occurrence of a Néel wall instead of a Bloch wall results from the dipole–dipole energy in this region. The evolution of the magnetization with field differs strongly for fields parallel and perpendicular to the electrodes. In the former case, the pattern evolves abruptly and in a narrow range, while in the latter case it evolves smoothly and in a rather wide range. From the magnetization patterns, we estimate the domain-wall magnetoresistance and the anisotropic magnetoresistance, which we compare with experimental data. © 1999 American Institute of Physics.
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75.70.Kw Domain structure (including magnetic bubbles and vortices)
73.50.Jt Galvanomagnetic and other magnetotransport effects (including thermomagnetic effects)
73.40.Jn Metal-to-metal contacts
72.15.Gd Galvanomagnetic and other magnetotransport effects
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
75.60.Ch Domain walls and domain structure

Suppression of the face-centered-cubic-hexagonal-close-packed stacking fault in Co/Cu(111) ultrathin films by pulsed laser deposition

M. Zheng, J. Shen, Ch. V. Mohan, P. Ohresser, J. Barthel, and J. Kirschner

Appl. Phys. Lett. 74, 425 (1999); http://dx.doi.org/10.1063/1.123049 (3 pages) | Cited 15 times

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The thermal deposition of Co onto Cu(111) results in three-dimensional island growth and a face-centered-cubic-hexagonal-close-packed stacking fault, which hinders a complete antiferromagnetic coupling in Co/Cu(111) superlattices. We report that Co/Cu(111) films can be grown with good layer-by-layer morphology and significantly less stacking faults by pulsed laser deposition. We show that a complete antiferromagnetic coupling can be achieved in the pulsed laser deposited Co/Cu trilayer. © 1999 American Institute of Physics.
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68.65.-k Low-dimensional, mesoscopic, nanoscale and other related systems: structure and nonelectronic properties
61.72.Nn Stacking faults and other planar or extended defects
81.15.Fg Pulsed laser ablation deposition

Longitudinal magnetoresistance of CrO2 thin films

Katsuhiko Suzuki and P. M. Tedrow

Appl. Phys. Lett. 74, 428 (1999); http://dx.doi.org/10.1063/1.123050 (2 pages) | Cited 25 times

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The longitudinal magnetoresistance (LMR) of thin films of the putative half-metallic ferromagnet CrO2 deposited by chemical vapor deposition onto TiO2/Si(111) substrates has been measured in the temperature range 0.55 K<T<4.2 K in magnetic fields up to 7 T. The dependence of the LMR on small changes in the angle between the applied field and the substrate plane was also measured at T=1.3 K. X-ray diffraction measurements showed that these films have a textured rutile structure with a axis and 〈110〉 orientations normal to the substrates. The LMR of these films at about 3 T was about twice that of similar films that showed only a 〈100〉 orientation normal to their ZrO2 substrates. A change in the substrate orientation relative to the applied field direction of 10° caused a 25% decrease in the magnitude of the LMR at 3 T. The results suggest that the existence of the 〈110〉 crystallite structure strongly affects the magnetoresistance. © 1999 American Institute of Physics.
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73.50.Jt Galvanomagnetic and other magnetotransport effects (including thermomagnetic effects)
73.61.At Metal and metallic alloys
75.50.Cc Other ferromagnetic metals and alloys
68.55.-a Thin film structure and morphology
75.70.Ak Magnetic properties of monolayers and thin films

Sm(Co, Cu, Ni) thin films with giant coercivity

C. Prados and G. C. Hadjipanayis

Appl. Phys. Lett. 74, 430 (1999); http://dx.doi.org/10.1063/1.123051 (3 pages) | Cited 7 times

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Nanostructured Sm(Co, Ni, Cu) thin films have been obtained by heat treating as-deposited sputtered amorphous films. The room-temperature coercivity increases from less than 100 Oe in the amorphous state to 42 kOe in a SmCo2Cu3 sample annealed 30 min at 550 °C. Structural data in the optimum samples suggest a particle size around 10 mn. Magnetic viscosity measurements indicated that the switching volume is of the same order as the crystallite size. Remanence measurements showed that interparticulate interactions are magnetizing and rather independent of the crystallization stage. These data indicate that the huge coercivity enhancement is due to domain-wall pinning at high anisotropy Sm–Co precipitates. © 1999 American Institute of Physics.
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75.50.Vv High coercivity materials
75.70.Kw Domain structure (including magnetic bubbles and vortices)
75.50.Kj Amorphous and quasicrystalline magnetic materials
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
75.30.Gw Magnetic anisotropy
81.40.Rs Electrical and magnetic properties related to treatment conditions

Hotspot mixing: A framework for heterodyne mixing in superconducting hot-electron bolometers

D. Wilms Floet, E. Miedema, T. M. Klapwijk, and J. R. Gao

Appl. Phys. Lett. 74, 433 (1999); http://dx.doi.org/10.1063/1.123052 (3 pages) | Cited 34 times

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We propose a framework to interpret heterodyne mixing in superconducting hot-electron bolometers. The physical conversion process of the mixer is the result of an electronic hotspot, of which the length, and consequently the resistance, oscillates at the intermediate frequency. On the basis of this concept, we calculate the (un)pumped current–voltage relation, the dc voltage responsivity, and the mixer conversion efficiency. © 1999 American Institute of Physics.
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07.57.Kp Bolometers; infrared, submillimeter wave, microwave, and radiowave receivers and detectors
85.60.Gz Photodetectors (including infrared and CCD detectors)
84.30.Qi Modulators and demodulators; discriminators, comparators, mixers, limiters, and compressors
85.25.Cp Josephson devices
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Change in electromechanical properties of 0.9PMN:0.1PT relaxor ferroelectric induced by uniaxial compressive stress directed perpendicular to the electric field

J. Zhao, A. E. Glazounov, and Q. M. Zhang

Appl. Phys. Lett. 74, 436 (1999); http://dx.doi.org/10.1063/1.123053 (3 pages) | Cited 36 times

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It was shown that the uniaxial compressive stress applied perpendicular to the electric field strongly changes the dielectric and electromechanical properties of relaxor ferroelectric 0.9PMN:0.1PT at temperatures around its dielectric constant maximum, Tm. This includes: (i) a shift of the temperature of the electric field induced phase transition into the ferroelectric state and a depoling temperature to higher values which brings a large hysteresis in the material response to the electric field, and (ii) the symmetry breaking in the plane perpendicular to the direction of the electric field (3-axis). Using this symmetry breaking, from the ratio of the effective piezoelectric coefficients in the field biased state, d32/d31, the size of the micropolar regions at temperatures near Tm was estimated. © 1999 American Institute of Physics.
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77.84.Ek Niobates and tantalates
77.84.Cg PZT ceramics and other titanates
77.22.Ch Permittivity (dielectric function)
77.80.Dj Domain structure; hysteresis
77.65.Bn Piezoelectric and electrostrictive constants

Dielectric properties of fullerene films

J. S. Su, Y. F. Chen, and K. C. Chiu

Appl. Phys. Lett. 74, 439 (1999); http://dx.doi.org/10.1063/1.123054 (3 pages) | Cited 3 times

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We report the study on the dielectric properties of C60 films by means of capacitance and dissipation factor measurements at temperatures between 5 and 325 K. In addition to the structural phase transition at 260 K, we have observed a clear anomaly at T = 90 K, which did not show up in previous dielectric studies. This result confirms the fact that a glass transition exists due to the freezing in of orientational disorder in C60. A Debye-like relaxation in the dielectric response has also been observed, and the relaxation rate is thermally activated with an energy of about 277 meV, which is in good agreement with that obtained from other measurements. © 1999 American Institute of Physics.
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77.84.Bw Elements, oxides, nitrides, borides, carbides, chalcogenides, etc.
77.55.-g Dielectric thin films
61.48.-c Structure of fullerenes and related hollow and planar molecular structures
64.70.K- Solid-solid transitions
64.70.P- Glass transitions of specific systems
64.70.Q- Theory and modeling of the glass transition
77.22.Gm Dielectric loss and relaxation
68.55.-a Thin film structure and morphology
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Improved energy transfer in electrophosphorescent devices

D. F. O’Brien, M. A. Baldo, M. E. Thompson, and S. R. Forrest

Appl. Phys. Lett. 74, 442 (1999); http://dx.doi.org/10.1063/1.123055 (3 pages) | Cited 331 times

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External quantum efficiencies of up to (5.6±0.1)% at low brightness and (2.2±0.1)% at 100 cd/m2 are obtained from a red electrophosphorescent device containing the luminescent dye 2,3,7,8, 12,13,17,18-octaethyl-21H23H-phorpine platinum(II) (PtOEP) doped in a 4,4′-N,N′-dicarbazolebiphenyl (CBP) host. Due to weak overlap between excitonic states in PtOEP and CBP, efficiency losses due to nonradiative recombination are low. However, energy transfer between the species is also poor. In compensation, a thin layer of 2,9-dimethyl-4,7 diphenyl-1,10-phenanthroline is used as a barrier to exciton diffusion in CBP, improving the energy transfer to PtOEP. This technique may be applied to improve the efficiency of other electrophosphorescent devices. © 1999 American Institute of Physics.
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85.60.Jb Light-emitting devices
78.60.Fi Electroluminescence
78.55.Kz Solid organic materials
71.35.-y Excitons and related phenomena
72.20.Jv Charge carriers: generation, recombination, lifetime, and trapping

Fabry–Perot cavity chemical sensors by silicon micromachining techniques

Jaeheon Han

Appl. Phys. Lett. 74, 445 (1999); http://dx.doi.org/10.1063/1.123056 (3 pages) | Cited 9 times

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Micromachined Fabry–Perot microcavity structures filled with polymeric layers composed of poly(3-dodecylthiophene) have been fabricated and studied for use as chemical sensors. The polymer-filled microcavity devices show reversible sensing behavior in response to the exposure of molecular iodine. Here, the chemical dosing results in a dramatic change in the fraction of transmitted light which passes through the microcavity structure (up to 50% at 633 nm). Importantly, the Fabry–Perot microcavity structure produces a significantly larger change in transmitted light intensity compared to a single membrane structure. © 1999 American Institute of Physics.
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07.07.Df Sensors (chemical, optical, electrical, movement, gas, etc.); remote sensing
82.80.-d Chemical analysis and related physical methods of analysis
07.10.Cm Micromechanical devices and systems

Low resistance spin-dependent tunnel junctions deposited with a vacuum break and radio frequency plasma oxidized

J. J. Sun, V. Soares, and P. P. Freitas

Appl. Phys. Lett. 74, 448 (1999); http://dx.doi.org/10.1063/1.123057 (3 pages) | Cited 40 times

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Spin-dependent tunnel junctions with resistance-area products (RJ×A) down to 1.8 kΩ×μm2 and tunneling magnetoresistance (TMR)⩾15% were fabricated. Junction areas vary from 6 to 45 μm2. A systematic study of junction resistance and TMR versus deposited Al thickness (tAl = 7, 9, 11, and 13 Å), and oxidation time (from 4 to 90 s) is presented. The TMR is maximum (25% to 27%) for tAl = 11 Å, with 6 s oxidation time (RJ×A = 10 to 20 kΩ×μm2). At 6–10 s oxidation time, reducing the Al thickness from 11 to 7 Å reduces the resistance-area products from 10–20 kΩ×μm2 to 1–3 kΩ×μm2, while TMR decreases from 22%–27% to 13%–17%. Excess oxidation or incomplete oxidation of the Al layer leads to current–voltage curve asymmetry. © 1999 American Institute of Physics.
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73.40.Gk Tunneling
72.15.Gd Galvanomagnetic and other magnetotransport effects
72.20.My Galvanomagnetic and other magnetotransport effects
73.50.Jt Galvanomagnetic and other magnetotransport effects (including thermomagnetic effects)
73.40.Rw Metal-insulator-metal structures
81.65.Mq Oxidation
68.55.-a Thin film structure and morphology
81.15.-z Methods of deposition of films and coatings; film growth and epitaxy
73.61.At Metal and metallic alloys

Piezoresistive torque magnetometry below 1 K

Christian Lupien, Brett Ellman, Peter Grütter, and Louis Taillefer

Appl. Phys. Lett. 74, 451 (1999); http://dx.doi.org/10.1063/1.123058 (3 pages) | Cited 10 times

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We have investigated the performance of piezoresistive cantilevers as magnetometers in the temperature range below 1 K. The de Haas-van Alphen effect was used to study the temperature dependence of a sample of κ-(BEDT-TTF)2Cu(NCS)2, fixed on the end of a cantilever, as a function of the excitation current through the piezoresistive device. We found that by using a small thermalizing wire connected directly to the sample, large excitations were not incompatible with sample temperatures remaining low, thereby establishing the use of these devices as sensitive magnetometers well below 1 K. A large hysteretic behavior observed at low fields (below 0.01 T) and low temperature (below ∼2 K) precludes their use in that regime. © 1999 American Institute of Physics.
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07.55.Jg Magnetometers for susceptibility, magnetic moment, and magnetization measurements
71.18.+y Fermi surface: calculations and measurements; effective mass, g factor
84.32.Ff Conductors, resistors (including thermistors, varistors, and photoresistors)

Nanometer patterning of epitaxial CoSi2/Si(100) for ultrashort channel Schottky barrier metal–oxide–semiconductor field effect transistors

Q. T. Zhao, F. Klinkhammer, M. Dolle, L. Kappius, and S. Mantl

Appl. Phys. Lett. 74, 454 (1999); http://dx.doi.org/10.1063/1.123059 (3 pages) | Cited 19 times

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A nanometer patterning method, based on local oxidation of silicide layers, was used to pattern epitaxial CoSi2 layers. A feature size as small as 50 nm was obtained for 20 nm epitaxial CoSi2 layers on Si(100) after patterning by local rapid thermal oxidation in dry oxygen. A Schottky source/drain metal–oxide–semiconductor field effect transistor with epitaxial CoSi2 on p-Si(100) was fabricated using this nanopatterning method to make the 100 nm gate. The device shows good I–V characteristics at 300 K. © 1999 American Institute of Physics.
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85.30.Tv Field effect devices
81.65.Mq Oxidation
81.05.Cy Elemental semiconductors
81.07.-b Nanoscale materials and structures: fabrication and characterization
81.16.-c Methods of micro- and nanofabrication and processing
85.35.-p Nanoelectronic devices
73.30.+y Surface double layers, Schottky barriers, and work functions
85.40.Hp Lithography, masks and pattern transfer
73.61.Cw Elemental semiconductors

Analytic model for direct tunneling current in polycrystalline silicon-gate metal–oxide–semiconductor devices

Leonard F. Register, Elyse Rosenbaum, and Kevin Yang

Appl. Phys. Lett. 74, 457 (1999); http://dx.doi.org/10.1063/1.123060 (3 pages) | Cited 88 times

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An analytic model of the direct tunneling current in metal–oxide–semiconductor devices as a function of oxide field is presented. Accurate modeling of the low-field roll-off in the current results from proper modeling of the field dependencies of the sheet charge, electron impact frequency on the interface, and tunneling probability. To obtain the latter dependence, a modified WKB approximation is used. © 1999 American Institute of Physics.
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73.40.Qv Metal-insulator-semiconductor structures (including semiconductor-to-insulator)
84.32.Tt Capacitors
73.40.Gk Tunneling
85.30.De Semiconductor-device characterization, design, and modeling
85.30.Tv Field effect devices
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Extended x-ray absorption fine structure study on the cerium(IV)-induced DNA hydrolysis: Implication to the roles of 4f orbitals in the catalysis

Hidemi Shigekawa, Masahiko Ishida, Koji Miyake, Ryu Shioda, Yoshitoki Iijima, Takamitsu Imai, Hideyuki Takahashi, Jun Sumaoka, and Makoto Komiyama

Appl. Phys. Lett. 74, 460 (1999); http://dx.doi.org/10.1063/1.123036 (3 pages) | Cited 4 times

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At various Ce(IV)/DPP (DPP: diphenyl phosphate) molar ratios, the structure of Ce clusters in aqueous solutions was studied by extended x-ray absorption fine structure measurements. Although the Ce–Ce bond was strong in the absence of DPP, the corresponding signal was significantly reduced when the Ce(IV)/DPP molar ratio was 1. The result indicates the existence of a strong interaction between Ce(IV) ions and DPP, and suggests the formation of a Ce–DPP structure in solution. With an increase in the molar ratio, the signal of Ce–Ce bonds increased again, and exceeded that observed in the absence of DPP, suggesting the formation of a Ce–Ce–DPP structure. Ce(IV) ions in the complexes were found to have ∼ 0.67 4f electrons. On the other hand, no measurable charge transfer was observed in the case of Ce(III) ions. These results demonstrate the role of the 4f orbital of Ce(IV) ions for its enormous activity for DNA hydrolysis. © 1999 American Institute of Physics.
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87.14.G- Nucleic acids
82.30.Vy Homogeneous catalysis in solution, polymers and zeolites
87.64.kd X-ray and EXAFS
78.70.Dm X-ray absorption spectra
68.03.Fg Evaporation and condensation of liquids
33.15.Fm Bond strengths, dissociation energies
61.25.H- Macromolecular and polymers solutions; polymer melts
64.75.-g Phase equilibria
82.60.Lf Thermodynamics of solutions
87.15.N- Properties of solutions of macromolecules
36.40.Jn Reactivity of clusters
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Mechanism of shape formation of three-dimensional periodic nanostructures by bias sputtering

Shojiro Kawakami, Takayuki Kawashima, and Takashi Sato

Appl. Phys. Lett. 74, 463 (1999); http://dx.doi.org/10.1063/1.123037 (3 pages) | Cited 49 times

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We previously demonstrated a process for fabricating three-dimensional (3D) periodic nanostructures composed of corrugated a-Si/SiO2 multilayers, which behave as 3D photonic crystals. In this process, bias sputtering is a key technique by which the pattern is self-forming. This letter clarifies the mechanism of the self-shaping effect of bias sputtering by comparing deposition simulation and experiments. The mechanism is decomposed into three main effects: diffuse incidence of neutral particles of film material, sputter etching by normally incident rare-gas ions, and subsequent redeposition of sputtered film material. Specifically, redeposition has a self-adjusting effect on the depth of holes or valleys, and is the key of formation of stable patterns. © 1999 American Institute of Physics.
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81.05.Gc Amorphous semiconductors
81.05.Cy Elemental semiconductors
81.07.-b Nanoscale materials and structures: fabrication and characterization
81.65.Cf Surface cleaning, etching, patterning
52.77.Bn Etching and cleaning
52.77.Dq Plasma-based ion implantation and deposition
42.70.Qs Photonic bandgap materials

Scanning localized viscoelastic image using a quartz crystal resonator combined with an atomic force microscopy

Jong Min Kim, Sang Mok Chang, and Hiroshi Muramatsu

Appl. Phys. Lett. 74, 466 (1999); http://dx.doi.org/10.1063/1.123033 (3 pages) | Cited 13 times

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A quartz crystal resonator and an atomic force microscopy were applied for the measurement of local viscoelasticity and surface morphology. For the reduction of signal noise from the quartz crystal resonator, we designed an oscillation circuit based on a referential quartz crystal method. A polystyrene bead-coated quartz crystal was used as a signal quartz crystal, and a bare Au quartz crystal was used as a reference. By approaching the cantilever of atomic force microscopy to the surface of working quartz crystal, the differential resonant frequency in the two quartz crystals showed changes by the interaction between the tip and the quartz crystal. The changes of differential resonant frequency in the two quartz crystals were influenced by the local viscoelasticity. The image resolution of differential resonant frequency was observed under 80 nm for the polystyrene bead-coated quartz crystal. © 1999 American Institute of Physics.
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68.37.Ef Scanning tunneling microscopy (including chemistry induced with STM)
68.37.Ps Atomic force microscopy (AFM)
68.37.Rt Magnetic force microscopy (MFM)
68.37.Uv Near-field scanning microscopy and spectroscopy
46.35.+z Viscoelasticity, plasticity, viscoplasticity
62.40.+i Anelasticity, internal friction, stress relaxation, and mechanical resonances
81.40.Jj Elasticity and anelasticity, stress-strain relations
68.35.B- Structure of clean surfaces (and surface reconstruction)

Application of neutron interferometry to the measurement of thin film density

W. E. Wallace, D. L. Jacobson, M. Arif, and A. Ioffe

Appl. Phys. Lett. 74, 469 (1999); http://dx.doi.org/10.1063/1.123038 (3 pages) | Cited 3 times

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The application of neutron interferometry to the measurement of the atom density of polymer thin films (<1 μm thick) supported on silicon substrates is described. Polymer films were chosen primarily for their fixed, well-defined stoichiometry; however, the technique is applicable to films of any elemental composition. The wavelength-independent phase shift of a beam of thermal neutrons passing through the sample gives a measure of the product of the film atom density, the film thickness, the lattice spacing of the silicon interferometer, and the scattering lengths of the constituent elements of the film. The film thickness was found by x-ray reflectivity while the other two parameters are well-defined quantities. The technique does not rely on complex mathematical modeling of physical processes nor on thin film standards for data interpretation. With some refinements, neutron interferometry is envisioned as an important tool in the creation of thin films having well-defined densities which will be useful in the calibration of many analysis techniques. © 1999 American Institute of Physics.
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03.75.Dg Atom and neutron interferometry
06.30.Dr Mass and density
61.41.+e Polymers, elastomers, and plastics
68.55.-a Thin film structure and morphology
68.60.-p Physical properties of thin films, nonelectronic

Charge storage in Co nanoclusters embedded in SiO2 by scanning force microscopy

D. M. Schaadt, E. T. Yu, S. Sankar, and A. E. Berkowitz

Appl. Phys. Lett. 74, 472 (1999); http://dx.doi.org/10.1063/1.123039 (3 pages) | Cited 61 times

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Scanning force microscopy was used to study localized charge deposition and subsequent transport in Co nanoclusters embedded in SiO2 deposited on an n-type Si substrate. Co nanoclusters were charged by applying a bias voltage pulse between tip and sample, and electrostatic force microscopy was used to image charged areas, to determine quantitatively the amount of stored charge, and to characterize the discharging process. Charge was deposited controllably and reproducibly within areas ∼20–50 nm in radius, and an exponential decay in the peak charge density was observed. Longer decay times were measured for positive than for negative charge; this is interpreted as a consequence of the Coulomb-blockade energy associated with single-electron charging of the Co nanoclusters. © 1999 American Institute of Physics.
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73.40.Ns Metal-nonmetal contacts
07.79.Lh Atomic force microscopes
73.23.Hk Coulomb blockade; single-electron tunneling
73.40.Gk Tunneling

Determination of built-in field by applying fast Fourier transform to the photoreflectance of surface-intrinsic n+-type doped GaAs

D. P. Wang, K. R. Wang, K. F. Huang, T. C. Huang, and A. K. Chu

Appl. Phys. Lett. 74, 475 (1999); http://dx.doi.org/10.1063/1.123040 (3 pages) | Cited 8 times

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Photoreflectance spectroscopy of surface-intrinsic n+-doped (s-i-n+) GaAs has been measured at various power densities (Ppu) of a pump beam. Many Franz–Keldysh oscillations (FKOs) were observed above the band-gap energy, which will enable the electric-field strength (F) to be determined from the periods of the FKOs. Field F thus obtained is subject to photovoltaic effects. In order to reduce the photovoltaic effects from the pump beam, Ppu was kept below 10 μW/cm2 in the previous experiments. Here, we demonstrate that the built-in field can be determined at a larger Ppu by using fast Fourier transform techniques. © 1999 American Institute of Physics.
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78.20.Jq Electro-optical effects
72.40.+w Photoconduction and photovoltaic effects
73.25.+i Surface conductivity and carrier phenomena
73.20.At Surface states, band structure, electron density of states
81.05.Ea III-V semiconductors
02.30.Uu Integral transforms
02.30.Vv Operational calculus
02.60.-x Numerical approximation and analysis
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