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17 Sep 2007

Volume 91, Issue 12, Articles (12xxxx)

Issue Cover Spotlight Figure

Appl. Phys. Lett. 91, 123101 (2007); http://dx.doi.org/10.1063/1.2784389 (3 pages)

Thierry Laroche, Alexandre Vial, and Matthieu Roussey
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Facets and surface relaxation of tetrahexahedral platinum nanocrystals

Yong Ding, Yifan Gao, Zhong Lin Wang, Na Tian, Zhi-You Zhou, and Shi-Gang Sun

Appl. Phys. Lett. 91, 121901 (2007); http://dx.doi.org/10.1063/1.2785953 (3 pages) | Cited 5 times

Online Publication Date: 17 September 2007

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Platinum tetrahexahedral (THH) nanocrystals have been prepared by an electrochemical treatment of Pt nanospheres supported on glassy carbon by square-wave potential [ N. Tian et al., Science 316, 732 (2007) ]. In this paper, the shape and facets of the THH nanocrystals have been characterized by transmission electron microscopy. Most of the exposed surfaces are close to {730} facets. Detailed relaxation of the surface atoms has been observed at surface steps. It is anticipated that the high density of atomic steps and kinks is likely related to the enhanced catalytic activity of the THH nanocrystals.
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61.46.Hk Nanocrystals
68.37.Lp Transmission electron microscopy (TEM)
61.05.J- Electron diffraction and scattering

Dynamics of elastic waves in two-dimensional phononic crystals with chaotic defect

Jie Chen, Jian-Chun Cheng, and Baowen Li

Appl. Phys. Lett. 91, 121902 (2007); http://dx.doi.org/10.1063/1.2779967 (3 pages) | Cited 4 times

Online Publication Date: 17 September 2007

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The authors study dynamics of wave function in two-dimensional phononic crystals with different billiard defects. It is found that the elastic wave is localized in the defect region with soft material. The spatial statistical properties of wave function are studied. More strikingly, they find that given the same area, the chaotic billiard contains more energy than the integrable billiards such as rectangular and circular billiards. The dependence of this “chaotic effect” on wave number k is also studied.
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62.30.+d Mechanical and elastic waves; vibrations
63.20.-e Phonons in crystal lattices

Effect of overlayers on critical-point parameters in the analysis of ellipsometric spectra

Y. W. Jung, T. H. Ghong, Y. D. Kim, and D. E. Aspnes

Appl. Phys. Lett. 91, 121903 (2007); http://dx.doi.org/10.1063/1.2784187 (3 pages) | Cited 9 times

Online Publication Date: 18 September 2007

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As the complete removal of overlayers may not be possible in general, the authors investigate the effect of incomplete removal on critical-point parameters in the analysis of ellipsometric data. Using an approximate analytic expression, they show that energies and broadening parameters are much less affected by overlayers than amplitudes and phases. These conclusions are confirmed by false-data calculations for GaAs and overlayer-removal data for CdTe.
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78.66.Fd III-V semiconductors
78.66.Hf II-VI semiconductors

How do the grains slide in fine-grained zirconia polycrystals at high temperature?

Maren Daraktchiev, Robert Schaller, Laurent Gremillard, Thierry Epicier, Jerome Chevalier, and Gilbert Fantozzi

Appl. Phys. Lett. 91, 121904 (2007); http://dx.doi.org/10.1063/1.2784210 (3 pages) | Cited 1 time

Online Publication Date: 18 September 2007

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Degradation of mechanical properties of zirconia polycrystals is hardly discussed in terms of solution-precipitation grain-boundary sliding due to experimental controversies over imaging of intergranular amorphous phases at high and room temperatures. Here, the authors applied the techniques of mechanical spectroscopy and transmission electron microscopy (TEM) to shed light on the amorphization of grain interfaces at high temperature where the interface reaction determines the behavior of fine-grained zirconia polycrystals. They present mechanical spectroscopy results, which yield evidences of an intergranular amorphous phase in silica doped and in high-purity zirconia. Quenching of zirconia polycrystals reveals an intergranular amorphous phase on TEM images at room temperature.
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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.Hh Indirect evidence of dislocations and other defects (resistivity, slip, creep, strains, internal friction, EPR, NMR, etc.)
61.72.Mm Grain and twin boundaries
61.43.-j Disordered solids
81.40.Gh Other heat and thermomechanical treatments

Optical properties of highly luminescent zinc oxide tetrapod powders

A. M. Fischer, S. Srinivasan, R. Garcia, F. A. Ponce, S. E. Guaño, B. C. Di Lello, F. J. Moura, and I. G. Solórzano

Appl. Phys. Lett. 91, 121905 (2007); http://dx.doi.org/10.1063/1.2787875 (3 pages) | Cited 10 times

Online Publication Date: 19 September 2007

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Zinc oxide tetrapod powders with high luminescent characteristics have been grown by simple evaporation and direct oxidation of zinc. Their structural and optical properties have been studied by transmission electron microscopy and by cathodoluminescence. The legs of the tetrapods have a pure hexagonal structure and form an arrowhead joint at the center. The tetrapod legs exhibit strong band edge and weak green luminescence. Monochromatic cathodoluminescence images show that the band-edge emission originates from the bulk of the legs, while the green emission emanates from the surface, indicative of it being related to oxygen vacancies. Low temperature cathodoluminescence shows a direct correlation of near-band-edge emission with specific sites within the structure.
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78.60.Hk Cathodoluminescence, ionoluminescence
78.66.Hf II-VI semiconductors
81.05.Dz II-VI semiconductors

Measurement of silicon dioxide surface phonon-polariton propagation length by attenuated total reflection

Dye-Zone A. Chen and Gang Chen

Appl. Phys. Lett. 91, 121906 (2007); http://dx.doi.org/10.1063/1.2789177 (3 pages) | Cited 4 times

Online Publication Date: 19 September 2007

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The authors present an experimental measurement of the propagation length of surface phonon-polaritons on amorphous silicon dioxide using the method of attenuated total reflection. The measurements yield a propagation length of 10.8±1.3 μm for the silicon dioxide surface phonon-polariton resonance at 9 μm, which is in good agreement with the calculated value.
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78.68.+m Optical properties of surfaces
68.35.Ja Surface and interface dynamics and vibrations
71.36.+c Polaritons (including photon-phonon and photon-magnon interactions)

Self-fabricated single mode waveguide in fluoride glass excited by self-channeled plasma filaments

Sung-Hak Cho, Won-Seok Chang, Jae-Goo Kim, and Kyoung-Hyun Whang

Appl. Phys. Lett. 91, 121907 (2007); http://dx.doi.org/10.1063/1.2779247 (3 pages) | Cited 5 times

Online Publication Date: 19 September 2007

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Self-fabricated permanent structure of single mode waveguide in optical fluoride glasses was demonstrated using the self-channeled plasma filament excited by a femtosecond (130 fs) Ti:sapphire laser (λp = 790 nm). The photoinduced refractive index modification in ZrF4–BaF2–LaF3–AlF3–NaF glasses reached a length of approximately 10–15 mm from the input surface of the optical glass with the diameters ranging from 5 to 8 μm at the input intensities of more than 1.0×1012 W/cm2. The graded refractive index profiles were fabricated to be a symmetric form from the center of optical fluoride glass, and a maximum value of refractive index change (Δn) was measured to be 1.3×10−2. The beam profile of the output beam transmitted through the modified region showed that the photoinduced refractive index modification produced a permanent structure of single mode waveguide.
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42.79.Gn Optical waveguides and couplers
52.50.Jm Plasma production and heating by laser beams (laser-foil, laser-cluster, etc.)
42.70.Ce Glasses, quartz

Dielectric anisotropy and phonon modes of ordered indirect-gap Al0.52In0.48P studied by far-infrared ellipsometry

T. Hofmann, V. Gottschalch, and M. Schubert

Appl. Phys. Lett. 91, 121908 (2007); http://dx.doi.org/10.1063/1.2785949 (3 pages)

Online Publication Date: 19 September 2007

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The infrared (100–600 cm−1) optical properties of partially CuPt-type ordered Al0.52In0.48P deposited lattice matched on GaAs are studied by ellipsometry. The authors determine the ordinary and extraordinary dielectric functions and report on the evolution of the optical phonon mode frequencies of Al0.52In0.48P as a function of the degree of ordering. In addition to the InP- and AlP-like phonon modes, they observe two alloy-induced phonon modes which are anisotropic upon CuPt ordering. The observed modes are associated to vibrations with E and A1 symmetries. The alloy-induced phonon modes are useful for classifying the degree of ordering in this indirect band gap alloy.
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78.30.Fs III-V and II-VI semiconductors
63.20.-e Phonons in crystal lattices

Fluorescence tuning of confined molecules in porous silicon mirrors

A. G. Palestino, M. B. de la Mora, J. A. del Río, C. Gergely, and E. Pérez

Appl. Phys. Lett. 91, 121909 (2007); http://dx.doi.org/10.1063/1.2786596 (3 pages) | Cited 6 times

Online Publication Date: 19 September 2007

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Emission signal from fluorescent molecules (fluorescein-5-maleimide) in a porous silicon mirror is enhanced by tuning the pore size and reflectance spectrum of the porous silicon multilayer structure. This is achieved when the reflectance spectrum of the silicon mirror overlaps the fluorescent excitation and emission wavelengths of the fluorescent molecule, and chemical linkers assure the molecular confinement.
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78.67.Pt Multilayers; superlattices; photonic structures; metamaterials
33.50.Dq Fluorescence and phosphorescence spectra
78.55.Kz Solid organic materials

Optically stimulated luminescence in NaMgF3:Eu2+

C. Dotzler, G. V. M. Williams, U. Rieser, and A. Edgar

Appl. Phys. Lett. 91, 121910 (2007); http://dx.doi.org/10.1063/1.2786599 (3 pages) | Cited 8 times

Online Publication Date: 19 September 2007

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Optically stimulated luminescence (OSL) and thermoluminescence (TL) measurements were performed on polycrystalline NaMgF3:Eu2+ as well as sintered and quenched NaMgF3:Eu2+ after exposure to ionizing radiation. The authors find a range of TL traps and the sintering and quenching process reduces the concentration of shallow traps. The resultant time integrated OSL intensity is linear from microgray dose levels to approximately 100 Gy, and hence this material is suitable for a personal and environmental dosimetry, where low dose levels are encountered and high sensitivity is required.
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78.55.Hx Other solid inorganic materials
78.60.Kn Thermoluminescence
42.70.-a Optical materials
81.20.Ev Powder processing: powder metallurgy, compaction, sintering, mechanical alloying, and granulation
81.40.Gh Other heat and thermomechanical treatments
61.80.Jh Ion radiation effects

Fracture initiation mechanisms in α-alumina under hypervelocity impact

Cheng Zhang, Rajiv K. Kalia, Aiichiro Nakano, and Priya Vashishta

Appl. Phys. Lett. 91, 121911 (2007); http://dx.doi.org/10.1063/1.2786865 (3 pages) | Cited 8 times

Online Publication Date: 20 September 2007

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Atomistic mechanisms of crack initiation and fracture in α-alumina under hypervelocity impact are investigated using large-scale molecular-dynamics simulations involving 540×106 atoms. The simulations reveal crack initiation due to amorphization, structural phase changes, and interaction between high strain rate deformation modes, which can advance at sustained speeds of 8 km/s. Fracture toughness in the impact-damaged substrate is estimated to be around 2 MPamath. Spallation at the interface of the amorphous region and twins causes eventual failure. The favorable cleavage planes normal to basal planes are found to be {2mathmath0}.
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81.40.Np Fatigue, corrosion fatigue, embrittlement, cracking, fracture, and failure
81.40.Lm Deformation, plasticity, and creep
62.20.M- Structural failure of materials
62.20.F- Deformation and plasticity

Atomic force microscope cantilever based microcoordinate measuring probe for true three-dimensional measurements of microstructures

Gaoliang Dai, Helmut Wolff, and Hans-Ulrich Danzebrink

Appl. Phys. Lett. 91, 121912 (2007); http://dx.doi.org/10.1063/1.2786881 (3 pages) | Cited 7 times

Online Publication Date: 20 September 2007

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An atomic force microscope (AFM) cantilever based coordinate measuring probe applicable for true three dimensional measurements of microstructures is presented. The probe has a shaft glued to a commercial AFM cantilever and a probing sphere located at the free end of the shaft. A conventional optical lever method is applied to detect the bending, torsion, and the dynamic behavior of the AFM cantilever which will change dramatically when the probe sphere is in proximity to the object to be measured. The probe has advantages such as small probing force (<1 μN), high probing sensitivity allowing nanometer measurement resolution, easy exchange of probing elements, and low cost. Measurements of a microgear and a fuel injection nozzle using the developed probe are demonstrated, indicating its promising application potential.
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07.79.Lh Atomic force microscopes

Elastic deformation of silicon (001) wafers due to thin film stresses

G. C. A. M. Janssen and B. R. Pujada

Appl. Phys. Lett. 91, 121913 (2007); http://dx.doi.org/10.1063/1.2787963 (3 pages) | Cited 3 times

Online Publication Date: 20 September 2007

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Single crystal silicon is an elastically anisotropic material. The in-plane elastic modulus of Si (001) wafers varies 30% with the orientation with respect to the crystal axes. The line tension exerted on a Si (001) wafer by a uniform isotropic film under compressive stress leads to a nonuniform deformation, reflecting the elastic anisotropy of the silicon single crystal. For low line tensions, this deformation has a fourfold symmetry, with larger deformation in the less stiff directions. For larger line tensions, a saddle point deformation superimposed on a cylindrical symmetric deformation is observed.
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62.20.F- Deformation and plasticity
81.40.Jj Elasticity and anelasticity, stress-strain relations

Effects of three-dimensional Ehrlich-Schwoebel barrier on texture selection during Cu nanorod growth

Christopher G. Johansen, Hanchen Huang, and Toh-Ming Lu

Appl. Phys. Lett. 91, 121914 (2007); http://dx.doi.org/10.1063/1.2771525 (3 pages) | Cited 7 times

Online Publication Date: 20 September 2007

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This work explores the effectiveness of the three-dimensional Ehrlich-Schwoebel (3D ES) barrier in the selection of textures during nanorod growth. The authors use molecular dynamics simulations to show that the 3D ES barrier, together with geometrical shadowing and the thermal dynamics of surface faceting, defines the texture of nanorods. The results offer insights into when and how the 3D ES barrier will be effective in texture selection, and may be applied to texture design during growth of various nanorods.
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81.16.-c Methods of micro- and nanofabrication and processing
61.46.-w Structure of nanoscale materials
68.35.B- Structure of clean surfaces (and surface reconstruction)
61.43.Bn Structural modeling: serial-addition models, computer simulation
68.35.Fx Diffusion; interface formation

Direct observation of a hypersonic band gap in two-dimensional single crystalline phononic structures

Taras Gorishnyy, Ji-Hyun Jang, CheongYang Koh, and Edwin L. Thomas

Appl. Phys. Lett. 91, 121915 (2007); http://dx.doi.org/10.1063/1.2786605 (3 pages) | Cited 15 times

Online Publication Date: 20 September 2007

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Brillouin light scattering is employed to record the phonon dispersion relation of two-dimensional (2D) hypersonic phononic crystals with square lattice plane group p4mm symmetry. The samples are single crystalline arrays of cylindrical holes with a lattice constant of 750 nm and 35% porosity patterned in epoxy using interference lithography. The dispersion relation reveals the presence of a phononic band gap between 1.21 and 1.57 GHz at the edge of the first Brillouin zone for elastic waves propagating along the [10] direction and conclusively demonstrates a band gap in a single crystalline 2D polymer based phononic structure at hypersonic frequencies.
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63.22.-m Phonons or vibrational states in low-dimensional structures and nanoscale materials
63.50.-x Vibrational states in disordered systems
61.41.+e Polymers, elastomers, and plastics
78.35.+c Brillouin and Rayleigh scattering; other light scattering
62.30.+d Mechanical and elastic waves; vibrations

Dielectric constants and phonon modes of amorphous hafnium aluminate deposited by metal organic chemical vapor deposition

C. Bundesmann, O. Buiu, S. Hall, and M. Schubert

Appl. Phys. Lett. 91, 121916 (2007); http://dx.doi.org/10.1063/1.2787962 (3 pages) | Cited 3 times

Online Publication Date: 21 September 2007

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Dielectric constants and long-wavelength optical phonon modes of amorphous hafnium aluminate films with a maximum aluminum content of 19 at. % are studied by infrared spectroscopic ellipsometry (IRSE). The hafnium aluminate films were prepared by metal organic chemical vapor deposition on silicon substrates. IRSE revealed one polar lattice mode and one impurity-type mode, which show all a systematic shift in frequency with varying Al content. The static dielectric constant decreases from 10.1 for 4.6 at. % Al to 8.1 for 19 at. % Al. The absolute values were found to be between 50% and 70% smaller than the values obtained from electrical measurements.
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77.84.Ek Niobates and tantalates
77.84.Cg PZT ceramics and other titanates
77.55.-g Dielectric thin films
77.22.Ch Permittivity (dielectric function)
63.20.D- Phonon states and bands, normal modes, and phonon dispersion
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)

Carrier dynamics in stacked InP/GaAs quantum dots

A. B. Veloso, M. K. K. Nakaema, M. P. F. de Godoy, J. M. J. Lopes, F. Iikawa, M. J. S. P. Brasil, J. R. R. Bortoleto, M. A. Cotta, P. F. P. Fichtner, M. Morschbächer, and J. R. Madureira

Appl. Phys. Lett. 91, 121917 (2007); http://dx.doi.org/10.1063/1.2789705 (3 pages) | Cited 3 times

Online Publication Date: 21 September 2007

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We investigated two stacked layers of InP/GaAs type-II quantum dots by transmission electron microscopy and optical spectroscopy. The results reveal that InP quantum dots formed in two quantum dot layers are more uniform than those from a single layer structure. The thermal activation energies as well as the photoluminescence decays are rather independent of the separation between quantum dot layers and the presence of the second layer. The quantum dot optical emission persists for thermal activation energy larger than the calculated exciton binding energy. The photoluminescence decay is relatively fast for type-II alignment.
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73.21.La Quantum dots
78.67.Hc Quantum dots
78.55.Cr III-V semiconductors

Predicting hardness of covalent∕ionic solid solution from first-principles theory

Q. M. Hu, K. Kádas, S. Hogmark, R. Yang, B. Johansson, and L. Vitos

Appl. Phys. Lett. 91, 121918 (2007); http://dx.doi.org/10.1063/1.2789708 (3 pages) | Cited 7 times

Online Publication Date: 21 September 2007

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We introduce a hardness formula for the multicomponent covalent and ionic solid solutions. This expression is tested on nitride spinel materials A3N4 (A = C,Si,Ge) and applied to titanium nitrogen carbide (TiN1−xCx with 0 ⩽ x ⩽ 1), off-stoichiometric transition-metal nitride (TiN1−x and VN1−x with x ⩽ 0.25), and B-doped semiconductors (C1−xBx, Si1−xBx, and Ge1−xBx with x ⩽ 0.1). In all cases, the theoretical hardness is in good agreement with experiments.
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62.20.Qp Friction, tribology, and hardness

Ultrafast soft x-ray absorption spectroscopy with sub-20-fs resolution

Enikoe Seres and Christian Spielmann

Appl. Phys. Lett. 91, 121919 (2007); http://dx.doi.org/10.1063/1.2789732 (3 pages) | Cited 14 times

Online Publication Date: 21 September 2007

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The most challenging application of time-resolved spectroscopy is to watch the motion of atoms directly. In the current experiment, we were able to follow the displacement of atoms in silicon after excitation with an intense near infrared femtosecond laser pulse with time-resolved x-ray absorption spectroscopy. The sub-20-fs x-ray probe pulses in an energy range up to several 100 eV were generated by high harmonic generation of intense femtosecond laser pulses at a repetition rate of 1 kHz. In a proof of principle experiment, we found an oscillatory motion of the atoms which can be explained by the excitation of coherent phonons. Our setup can be easily extended for studying structural dynamics in many important systems not only in solids but also in gases and liquids.
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78.70.Dm X-ray absorption spectra
78.47.-p Spectroscopy of solid state dynamics
42.65.Re Ultrafast processes; optical pulse generation and pulse compression

Nanostructured optical fiber by the sol-gel process in the SiO2–ZrO2 system

G. Brasse, C. Restoin, J.-L. Auguste, S. Hautreux, J.-M. Blondy, and A. Lecomte

Appl. Phys. Lett. 91, 121920 (2007); http://dx.doi.org/10.1063/1.2786604 (3 pages) | Cited 2 times

Online Publication Date: 21 September 2007

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A SiO2–ZrO2 nanostructured optical fiber developed using the chemical sol-gel method is fabricated and its waveguiding properties are studied. The refractive index difference between the core and the cladding is 0.021 for a (30%ZrO2–70%SiO2) molar core composition. Illumination of the fiber with a supercontinuum source (350–1750 nm) shows that the light is guided over this entire range. The measured attenuation losses are 0.7 dB/m at a 1064 nm wavelength and the calculated cutoff wavelength is 714 nm.
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81.16.-c Methods of micro- and nanofabrication and processing
81.15.Lm Liquid phase epitaxy; deposition from liquid phases (melts, solutions, and surface layers on liquids)
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)
42.55.Wd Fiber lasers
42.70.Hj Laser materials
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Photoemission study of the electronic structures of tris-(8-hydroxyquinoline) aluminum/Li2O/Al interfaces

K. Cho, S. W. Cho, C.-N. Whang, K. Jeong, S. J. Kang, and Y. Yi

Appl. Phys. Lett. 91, 122101 (2007); http://dx.doi.org/10.1063/1.2786837 (3 pages) | Cited 8 times

Online Publication Date: 17 September 2007

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The electronic structures of tris-(8-hydroquinoline) aluminum (Alq3)/Li2O/Al interfaces were studied using in situ ultraviolet and x-ray photoelectron spectroscopies (UPS and XPS). The UPS and XPS spectra allowed us to evaluate the complete energy level diagrams and to analyze the chemical interactions at the interfaces. Inserting Li2O between Al and Alq3 led to the highest occupied molecular orbital (HOMO) of Alq3 shifting to a higher binding energy compared to that without Li2O, which resulted in an improved electron injection. We also observed that the magnitude of the secondary cutoff shift was almost identical to that of the HOMO shift with the insertion of Li2O. This implies that the energy level alignment depends on the interface dipole and ionization energy of the adsorbate. Additionally, a gap state was observed in the gap of Alq3, which is related to the interfacial reaction. The N 1s spectra revealed that there were destructive chemical reactions between Alq3 and Al, which could be prevented by inserting Li2O between them.
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73.20.At Surface states, band structure, electron density of states
73.21.Ac Multilayers
73.20.Hb Impurity and defect levels; energy states of adsorbed species
79.60.Jv Interfaces; heterostructures; nanostructures
71.15.Nc Total energy and cohesive energy calculations

Scanning tunneling spectroscopy of inhomogeneous electronic structure in monolayer and bilayer graphene on SiC

Victor W. Brar, Yuanbo Zhang, Yossi Yayon, Taisuke Ohta, Jessica L. McChesney, Aaron Bostwick, Eli Rotenberg, Karsten Horn, and Michael F. Crommie

Appl. Phys. Lett. 91, 122102 (2007); http://dx.doi.org/10.1063/1.2771084 (3 pages) | Cited 77 times

Online Publication Date: 17 September 2007

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The authors present a scanning tunneling spectroscopy (STS) study of the local electronic structure of single and bilayer graphene grown epitaxially on a SiC(0001) surface. Low voltage topographic images reveal fine, atomic-scale carbon networks, whereas higher bias images are dominated by emergent spatially inhomogeneous large-scale structure similar to a carbon-rich reconstruction of SiC(0001). STS spectroscopy shows an ∼ 100 meV gaplike feature around zero bias for both monolayer and bilayer graphene/SiC, as well as significant spatial inhomogeneity in electronic structure above the gap edge. Nanoscale structure at the SiC/graphene interface is seen to correlate with observed electronic spatial inhomogeneity. These results are relevant for potential devices involving electronic transport or tunneling in graphene/SiC.
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73.20.At Surface states, band structure, electron density of states
68.37.Ef Scanning tunneling microscopy (including chemistry induced with STM)

Conductive atomic force microscopy investigation of transverse current across metallic and semiconducting single-walled carbon nanotubes

Chiara Baldacchini and Salvatore Cannistraro

Appl. Phys. Lett. 91, 122103 (2007); http://dx.doi.org/10.1063/1.2785168 (3 pages) | Cited 4 times

Online Publication Date: 17 September 2007

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The comprehension of conduction mechanisms in single-walled carbon nanotubes is a crucial task for developing efficient nanodevices. Appealing hybrid architectures could exploit charge transport perpendicular to the main nanotube axis in order to minimize carrier path and phonon scattering effects. Such transverse transport is investigated in metallic and semiconducting nanotubes by means of conductive atomic force microscopy. The transverse current response is interpreted in the framework of a tunneling transport model, and reveals that conduction across metallic nanotubes is either tunneling- or bandlike, depending on the force applied by the tip, while charge carriers always tunnel through the semiconducting nanotubes.
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73.63.Fg Nanotubes
73.40.Gk Tunneling

Nonlinear Peltier effect in semiconductors

Mona Zebarjadi, Keivan Esfarjani, and Ali Shakouri

Appl. Phys. Lett. 91, 122104 (2007); http://dx.doi.org/10.1063/1.2785154 (3 pages) | Cited 6 times

Online Publication Date: 18 September 2007

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Nonlinear Peltier coefficient of a doped InGaAs semiconductor is calculated numerically using the Monte Carlo technique. The Peltier coefficient is also obtained analytically for single parabolic band semiconductors assuming a shifted Fermi-Dirac electronic distribution under an applied bias. Analytical results are in agreement with numerical simulations. Key material parameters affecting the nonlinear behavior are doping concentration, effective mass, and electron-phonon coupling. Current density thresholds at which nonlinear behavior is observable are extracted from numerical data. It is shown that the nonlinear Peltier effect can be used to enhance cooling of thin film microrefrigerator devices especially at low temperatures.
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73.61.Ey III-V semiconductors
73.50.Lw Thermoelectric effects
72.20.Pa Thermoelectric and thermomagnetic effects
61.72.uj III-V and II-VI semiconductors
07.20.Mc Cryogenics; refrigerators, low-temperature detectors, and other low-temperature equipment
85.30.De Semiconductor-device characterization, design, and modeling

Low-temperature charge transport in Ga-acceptor nanowires implanted by focused-ion beams

S. J. Robinson, C. L. Perkins, T.-C. Shen, J. R. Tucker, T. Schenkel, X. W. Wang, and T. P. Ma

Appl. Phys. Lett. 91, 122105 (2007); http://dx.doi.org/10.1063/1.2786014 (3 pages)

Online Publication Date: 19 September 2007

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Ga-acceptor nanowires were embedded in crystalline Si using focused-ion beams. The dc current-voltage characteristics of these wires after annealing are highly nonlinear at low temperatures. A conductance threshold of less than 50 mV is observed independent of Ga+ dosage and implant beam overlap. These features suggest a Coulomb blockade transport mechanism presumably caused by a network of Ga precipitates in the substrate. This granular scenario is further supported by measurements of gated nanowires. Nanowires with metallic conductance at low temperatures could be achieved by reducing the current density of the focused-ion beams.
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73.23.Hk Coulomb blockade; single-electron tunneling
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