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12 Nov 2007

Volume 91, Issue 20, Articles (20xxxx)

Issue Cover Spotlight Figure

Appl. Phys. Lett. 91, 203501 (2007); http://dx.doi.org/10.1063/1.2806922 (3 pages)

Michael N. Feiginov and Dibakar Roy Chowdhury
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Polycrystalline silicon thin-film transistors with location-controlled crystal grains fabricated by excimer laser crystallization

Chun-Chien Tsai, Yao-Jen Lee, Ko-Yu Chiang, Jyh-Liang Wang, I-Che Lee, Hsu-Hsin Chen, Kai-Fang Wei, Ting-Kuo Chang, Bo-Ting Chen, and Huang-Chung Cheng

Appl. Phys. Lett. 91, 201903 (2007); http://dx.doi.org/10.1063/1.2801525 (3 pages) | Cited 2 times

Online Publication Date: 13 November 2007

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In this paper, location-controlled silicon crystal grains are fabricated by the excimer laser crystallization method which employs amorphous silicon spacer structure and prepatterned thin films. The amorphous silicon spacer in nanometer-sized width formed using spacer technology is served as seed crystal to artificially control superlateral growth phenomenon during excimer laser irradiation. An array of 1.8-μm-sized disklike silicon grains is formed, and the n-channel thin-film transistors whose channels located inside the artificially-controlled crystal grains exhibit higher performance of field-effect-mobility reaching 308 cm2/V s as compared with the conventional ones. This position-manipulated silicon grains are essential to high-performance and good uniformity devices.
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85.30.Tv Field effect devices

The effect of substrates on the Raman spectrum of graphene: Graphene- on-sapphire and graphene-on-glass

Irene Calizo, Wenzhong Bao, Feng Miao, Chun Ning Lau, and Alexander A. Balandin

Appl. Phys. Lett. 91, 201904 (2007); http://dx.doi.org/10.1063/1.2805024 (3 pages) | Cited 74 times

Online Publication Date: 13 November 2007

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The authors investigated the influence of substrates on Raman scattering spectrum from graphene. The room-temperature Raman signatures from graphene layers on GaAs, sapphire, and glass substrates were compared with those from graphene on the standard Si/SiO2 (300 nm) substrate, which served as a reference. It was found that while G peak of graphene on Si/SiO2 and GaAs is positioned at 1580 cm−1, it is downshifted by ∼ 5 cm−1 for graphene on sapphire and, in some cases, splits into doublets for graphene on glass with the central frequency around 1580 cm−1. The obtained results are important for nanometrology of graphene and graphene-based devices.
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78.30.Na Fullerenes and related materials
78.30.Hv Other nonmetallic inorganics

Connected Au network in annealed Ni/Au thin films on p-GaN

S. P. Lee, H. W. Jang, D. Y. Noh, and H. C. Kang

Appl. Phys. Lett. 91, 201905 (2007); http://dx.doi.org/10.1063/1.2809635 (3 pages)

Online Publication Date: 13 November 2007

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We report the formation of a connected Au network in annealed Ni/Au thin films on p-GaN, which was studied by scanning electron microscopy, transmission electron microscopy, and synchrotron x-ray diffraction. As the Ni was oxidized into NiO upon annealing at 530 °C in air, the Au layer was transformed to an interconnected network with an increased thickness. During annealing, Ni atoms diffuse out onto the Au through defects to form NiO, while Au atoms replace the Ni positions. The Au network grows downward until it reaches the p-GaN substrate, and NiO columns fill the space between the Au network.
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81.05.Bx Metals, semimetals, and alloys
81.15.-z Methods of deposition of films and coatings; film growth and epitaxy
81.65.Mq Oxidation
81.40.Gh Other heat and thermomechanical treatments
66.30.Lw Diffusion of other defects

Formation and stability of rocksalt ZnO nanocrystals in MgO

S. W. H. Eijt, J. de Roode, H. Schut, B. J. Kooi, and J. Th. M. De Hosson

Appl. Phys. Lett. 91, 201906 (2007); http://dx.doi.org/10.1063/1.2809651 (3 pages) | Cited 11 times

Online Publication Date: 13 November 2007

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Coimplantation of Zn and O ions into a single crystalline MgO and subsequent thermal annealing were applied in the synthesis of ZnO nanocrystals. Electron microscopy showed that rocksalt instead of wurtzite ZnO stabilizes for relatively large nanocrystals up to ∼ 15 nm, resulting from its small lattice mismatch with MgO of ∼ 1.7%. The vacancies initially created by implantation induce favorable nanocrystal growth kinetics and are effectively absorbed during the nucleation and growth processes. The optical band edge of the ZnO nanocrystals was detected at ∼ 2.8 eV.
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81.07.Bc Nanocrystalline materials
61.72.Cc Kinetics of defect formation and annealing
81.10.Aj Theory and models of crystal growth; physics and chemistry of crystal growth, crystal morphology, and orientation
71.20.Nr Semiconductor compounds

Enhancement of yield strength in zirconium metal through high-pressure induced structural phase transition

Yusheng Zhao and Jianzhong Zhang

Appl. Phys. Lett. 91, 201907 (2007); http://dx.doi.org/10.1063/1.2802726 (3 pages) | Cited 3 times

Online Publication Date: 13 November 2007

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We report here a high-pressure phase-transition induced strengthening in ultrapure zirconium metal. The determined yield strength shows more than sixfold abrupt increase at the transition pressure of Pc = 6 GPa, from σyα ≈ 180 MPa in the low-pressure phase of α-Zr to σyω ≈ 1180 MPa in the high-pressure phase of ω-Zr. The observed enhancement provides an alternate route for material strengthening and is the most significant among the known strengthening techniques for metals. Our findings support the theoretical simulations of the substantial covalent bonding and “rougher” corrugation of slip planes for dislocations in the ω-phase of zirconium.
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62.50.-p High-pressure effects in solids and liquids
64.70.K- Solid-solid transitions
62.20.F- Deformation and plasticity
81.40.Lm Deformation, plasticity, and creep
61.72.Hh Indirect evidence of dislocations and other defects (resistivity, slip, creep, strains, internal friction, EPR, NMR, etc.)
81.30.Hd Constant-composition solid-solid phase transformations: polymorphic, massive, and order-disorder

Characteristics of selective reflection of chiral nematic liquid crystalline gels with a nonuniform pitch distribution

Zhenyu Bian, Kexuan Li, Wei Huang, Hui Cao, Huai Yang, and Haiquan Zhang

Appl. Phys. Lett. 91, 201908 (2007); http://dx.doi.org/10.1063/1.2812539 (3 pages) | Cited 19 times

Online Publication Date: 13 November 2007

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The particles of photopolymerizable monomer/chiral dopant composites with a crystalline (Cr)-chiral nematic (N*) phase transition were prepared. By mixing the particles with different pitches of the N* phase together with the Cr phase and making the liquid crystal (LC) monomer molecules cross-linked by photopolymerization in the planarly oriented N* phase, a chiral nematic liquid crystal (N*-LC) gel film with a nonuniform pitch distribution was obtained. The bandwidth of the reflection spectrum and the location of reflection band of the gel film can be controlled accurately by controlling the pitch lengths of the N* phase of the particles.
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42.70.Df Liquid crystals
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)
81.20.-n Methods of materials synthesis and materials processing
82.70.Gg Gels and sols
68.15.+e Liquid thin films

Structural and electronic properties of ultrathin polycrystalline Si layers on glass prepared by aluminum-induced layer exchange

T. Antesberger, C. Jaeger, M. Scholz, and M. Stutzmann

Appl. Phys. Lett. 91, 201909 (2007); http://dx.doi.org/10.1063/1.2803072 (3 pages) | Cited 10 times

Online Publication Date: 13 November 2007

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We prepared ultrathin polycrystalline silicon layers (5–100 nm) by the aluminum-induced layer exchange process. An Al/oxide/amorphous Si layer stack was annealed at temperatures below 577 °C, leading to a layer exchange and the crystallization of the silicon. The process dynamics, structural and electronic properties have been studied. In addition to the well known dependence on the annealing temperature, we found an increase of the nucleation density with layer thickness. Raman spectroscopy shows a good crystalline quality down to a layer thickness of 10 nm. Hole concentrations of the p-type layers are between 5×1018 and 9×1019 cm−3, depending on layer thickness and annealing temperature.
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73.61.Cw Elemental semiconductors
68.55.-a Thin film structure and morphology
81.05.Cy Elemental semiconductors
78.66.Db Elemental semiconductors and insulators

Thermal properties and percolation in carbon nanotube-polymer composites

P. Bonnet, D. Sireude, B. Garnier, and O. Chauvet

Appl. Phys. Lett. 91, 201910 (2007); http://dx.doi.org/10.1063/1.2813625 (3 pages) | Cited 28 times

Online Publication Date: 14 November 2007

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Thermal properties of single walled carbon nanotube (SWNT)/polymethylmetacrylate composite thick films have been investigated at room temperature. The introduction of ∼ 7% SWNT into the polymer matrix enhances the thermal conductivity of the composite by 55% while the electrical conductivity increases by several orders of magnitude. Despite this difference, we show that the (moderate) enhancement of the thermal conductivity is quantified by the percolation of the SWNT network. A thermal conductivity of the SWNT network of ∼ 55 W/m K is estimated.
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65.80.-g Thermal properties of small particles, nanocrystals, nanotubes, and other related systems
81.07.De Nanotubes
73.22.-f Electronic structure of nanoscale materials and related systems
73.61.Ph Polymers; organic compounds

Anomalous temperature dependent elastic moduli of Ce-based bulk metallic glass at low temperatures

P. Yu, R. J. Wang, D. Q. Zhao, and H. Y. Bai

Appl. Phys. Lett. 91, 201911 (2007); http://dx.doi.org/10.1063/1.2813639 (3 pages) | Cited 9 times

Online Publication Date: 14 November 2007

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We report the abnormal temperature dependent elastic moduli of Ce-based bulk metallic glass (BMG) at low temperatures. Unlike other BMGs with stiffness mode, the Ce-based metallic glass exhibits an anomalous softening longitudinal acoustic mode with the decrease of temperature. Particularly, the bulk modulus shows a continuous decrease upon cooling indicating the softening of the BMG, which is completely different from that of other metallic glasses and conventional alloys. The physical origin of this abnormal elastic behavior is attributed to the alterative valences and electronic configurations at low temperatures.
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81.40.Jj Elasticity and anelasticity, stress-strain relations
61.43.Fs Glasses
62.65.+k Acoustical properties of solids

Three-primary-color photoluminescence from CdS/Si nanoheterostructure grown on silicon nanoporous pillar array

Hai Jun Xu and Xin Jian Li

Appl. Phys. Lett. 91, 201912 (2007); http://dx.doi.org/10.1063/1.2814036 (3 pages) | Cited 6 times

Online Publication Date: 14 November 2007

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CdS/Si nanoheterostructure was prepared by growing CdS nanocrystallites (nc-CdS) onto silicon nanoporous pillar array (Si-NPA) via a heterogeneous reaction process. The pillars of CdS/Si-NPA were constructed by an outermost shell of nc-CdS membrane, an innermost core of nanoporous silicon pillar, and an interface with nc-CdS embedded into amorphous SiO2 matrix. Blue, green, and red emissions were observed in CdS/Si-NPA and the origins were attributed to the emission from Si-NPA, the band edge transition, and the surface states of sulphur vacancies of nc-CdS, respectively. The three-primary-color photoluminescence realized in CdS/Si-NPA might be applied in solid state lighting.
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78.55.-m Photoluminescence, properties and materials
73.20.At Surface states, band structure, electron density of states
78.67.Bf Nanocrystals, nanoparticles, and nanoclusters

Inelastic deformation and phase transformation of shock compressed silicon single crystals

Stefan J. Turneaure and Y. M. Gupta

Appl. Phys. Lett. 91, 201913 (2007); http://dx.doi.org/10.1063/1.2814067 (3 pages) | Cited 6 times

Online Publication Date: 14 November 2007

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Silicon crystals were shock compressed along the [100] and [111] orientations to stresses between 15.9 and 21.7 GPa. Transmitted wave profiles exhibited considerable orientation dependence for elastic and inelastic waves but very little orientation dependence for the phase transformation wave. Following the phase transformation wave, the silicon was compressed ∼ 23%. This compression is significantly greater than the previously reported compressions for silicon shocked to similar stresses, and the present data are consistent with a completed phase transformation. The measured mechanical impedance of the transformed silicon matches the bulk impedance of the simple hexagonal phase of silicon reported in static high pressure studies.
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81.40.Jj Elasticity and anelasticity, stress-strain relations
62.30.+d Mechanical and elastic waves; vibrations

Mechanical and electronic properties of hard rhenium diboride of low elastic compressibility studied by first-principles calculation

R. F. Zhang, S. Veprek, and A. S. Argon

Appl. Phys. Lett. 91, 201914 (2007); http://dx.doi.org/10.1063/1.2805196 (3 pages) | Cited 33 times

Online Publication Date: 14 November 2007

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The mechanical and electronic properties of hcp-ReB2 are calculated by a first-principles approach. The calculated extreme equilibrium mechanical properties are consistent with the available experimental and theoretical data. The calculated elastic moduli suggest that hcp-ReB2 has a low compressibility and is most likely an intrinsically brittle material, but its relatively low ideal shear strength, as compared with c-BN, indicates that it is not intrinsically stronger/harder than c-BN. Based on the calculated electronic density of states and valence charge density distribution, the bonding nature of hcp-ReB2 is examined to obtain a deeper insight into the physical origin of the mechanical properties.
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62.20.F- Deformation and plasticity
62.20.D- Elasticity
62.20.M- Structural failure of materials
61.50.Lt Crystal binding; cohesive energy
71.20.Ps Other inorganic compounds
71.15.Mb Density functional theory, local density approximation, gradient and other corrections

eneSurfactant effects associated with Te-doped InPAs alloys

J. G. Cederberg and S. R. Lee

Appl. Phys. Lett. 91, 201915 (2007); http://dx.doi.org/10.1063/1.2801385 (3 pages) | Cited 1 time

Online Publication Date: 15 November 2007

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We examine the effects of heavy Te doping on strain-relaxed InPAs alloys grown on InP using metal-organic chemical vapor deposition. Compared to Si doping, Te doping of InPAs substantially reduces film roughness at high dopant concentrations ( ∼ 1×1019 cm−3). Unlike similar Si concentrations, high Te concentrations also suppress the incorporation of As into InPAs. An analysis of depth profiles of Te, As, and P from secondary-ion mass spectrometry further reveals the segregation of 0.6 ML of Te on the InPAs surface during growth. We infer that surfactant effects produced by the segregated Te cause the observed changes in surface morphology and As incorporation.
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68.35.B- Structure of clean surfaces (and surface reconstruction)
68.35.Dv Composition, segregation; defects and impurities
68.55.-a Thin film structure and morphology
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
61.72.S- Impurities in crystals

Formation of ferromagnetic interface between β-FeSi2 and Si(111) substrate

Azusa N. Hattori, Ken Hattori, Kenji Kodama, Nobuyoshi Hosoito, and Hiroshi Daimon

Appl. Phys. Lett. 91, 201916 (2007); http://dx.doi.org/10.1063/1.2804006 (3 pages) | Cited 8 times

Online Publication Date: 15 November 2007

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Epitaxial β-FeSi2 thin films were grown on Si(111)7×7 clean surfaces by solid phase epitaxy in ultrahigh vacuum: iron deposition at low temperature and subsequent annealing. We found that a ferromagnetic interface layer of iron-rich silicides forms between a β-FeSi2 surface layer and a Si(111) substrate spontaneously from transmission electron microscopy observations and magnetization measurements.
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75.70.Cn Magnetic properties of interfaces (multilayers, superlattices, heterostructures)
75.50.Dd Nonmetallic ferromagnetic materials
61.05.jh Low-energy electron diffraction (LEED) and reflection high-energy electron diffraction (RHEED)
61.82.Fk Semiconductors
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
68.37.Lp Transmission electron microscopy (TEM)

Global melting of Zr57Ti5Ni8Cu20Al10 bulk metallic glass under microcompression

Sheng Cheng, Xun-Li Wang, Hahn Choo, and Peter K. Liaw

Appl. Phys. Lett. 91, 201917 (2007); http://dx.doi.org/10.1063/1.2813623 (3 pages) | Cited 19 times

Online Publication Date: 15 November 2007

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Global melting was observed in micron-sized pillar samples tested under microcompression. In addition to a higher strength that was reproduced in samples with different sizes, a large plastic strain was also observed prior to the final fracture, as compared to bulk counterparts. The global melting was a result of the final fracture as evidenced by the acoustic emission at the end of the microcompression. However, the increased strain rate and large plastic strain might have contributed to the temperature increase prior to the fracture.
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81.40.Np Fatigue, corrosion fatigue, embrittlement, cracking, fracture, and failure
81.40.Lm Deformation, plasticity, and creep

Nanocubic boron nitride/nanodiamond multilayer structures

H. Q. Li, K. M. Leung, K. L. Ma, Q. Ye, Y. M. Chong, Y. S. Zou, W. J. Zhang, S. T. Lee, and I. Bello

Appl. Phys. Lett. 91, 201918 (2007); http://dx.doi.org/10.1063/1.2809382 (3 pages) | Cited 3 times

Online Publication Date: 15 November 2007

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Nanocubic boron nitride/nanodiamond (N-cBN/ND) multilayer structures with each alternating layer being ∼ 100 nm thick have been prepared by magnetron sputter and microwave plasma enhanced chemical vapor depositions. These multilayers exhibit remarkable properties, in particular, the mechanical properties. The multilayer structure is characteristic with (i) extreme hardness (82 GPa) considerably surpassing the values of the individual materials from which the multilayer is composed, (ii) high surface smoothness, (iii) significantly reduced film stress when compared with a single cBN layer of equivalent thickness, and (iv) great chemical stability. The N-cBN/ND multilayers developed have therefore important implications in mechanical and chemically resistant applications.
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64.70.Nd Structural transitions in nanoscale materials
68.65.Ac Multilayers
68.49.Sf Ion scattering from surfaces (charge transfer, sputtering, SIMS)
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
82.33.Xj Plasma reactions (including flowing afterglow and electric discharges)

Growth mode mapping and structural properties of controlled perovskite BaTiO3/SrTiO3 heterostructure

Jin Long Li, J. H. Hao, Zhang Ying, and Yanrong Li

Appl. Phys. Lett. 91, 201919 (2007); http://dx.doi.org/10.1063/1.2815749 (3 pages) | Cited 1 time

Online Publication Date: 15 November 2007

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Perovskite heteroepitaxy was investigated under various conditions using laser molecular beam epitaxy. Well-controlled BaTiO3/SrTiO3 heterostructure was studied with in situ reflective high energy electron diffraction and ex situ atomic force microscopy, x-ray diffraction, and transmission electron microscopy. The growth mode map of BaTiO3 thin films was obtained as a function of substrate temperature under 600 °C for various laser repetition rates. The effective activation energy of surface migration was determined to be 0.33 eV. A mechanism of surface migration in BaTiO3 heteroepitaxy was described for a basic understanding of atomic-scale controlled preparation of ferroelectric heterostructures at low temperature.
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77.80.-e Ferroelectricity and antiferroelectricity
77.55.-g Dielectric thin films
77.84.Ek Niobates and tantalates
77.84.Cg PZT ceramics and other titanates
81.15.Hi Molecular, atomic, ion, and chemical beam epitaxy

Growth of single crystalline GaN on silver mirrors

S. Inoue, K. Okamoto, T. Nakano, J. Ohta, and H. Fujioka

Appl. Phys. Lett. 91, 201920 (2007); http://dx.doi.org/10.1063/1.2815924 (3 pages) | Cited 2 times

Online Publication Date: 15 November 2007

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GaN films have been grown on highly thermally conducting Ag(111) substrates by using pulsed laser deposition. GaN(0001) grows epitaxially on Ag(111) with an in-plane epitaxial relationship of GaN[11math0]‖Ag[1math0] when low temperature AlN buffer layers are used. The heterointerfaces in the AlN/Ag structure are atomically abrupt, and that the abruptness remains unchanged during annealing up to 700 °C for the GaN growth. Neither 30° rotational domains nor cubic phase domains exist in the GaN films. It has been also confirmed that the GaN films grown on the Ag substrate exhibit strong near band edge photoluminescence emission at 3.51 eV. These results indicate that the epitaxial growth of GaN on mirror-polished single crystalline Ag substrates possibly improve the power limits and the light extraction efficiency of future light-emitting devices.
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81.15.Fg Pulsed laser ablation deposition
73.40.Kp III-V semiconductor-to-semiconductor contacts, p-n junctions, and heterojunctions
78.55.Cr III-V semiconductors
78.66.Fd III-V semiconductors

Thermal stability of CdZnO/ZnO multi-quantum-wells

A. V. Thompson, C. Boutwell, J. W. Mares, W. V. Schoenfeld, A. Osinsky, B. Hertog, J. Q. Xie, S. J. Pearton, and D. P. Norton

Appl. Phys. Lett. 91, 201921 (2007); http://dx.doi.org/10.1063/1.2812544 (3 pages) | Cited 15 times

Online Publication Date: 15 November 2007

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The thermal stability of CdZnO/ZnO multi-quantum-well (MQW) structures was studied using rapid thermal annealing in nitrogen from 300 to 750 °C. Photoluminescence (PL) emission from the MQWs was studied while varying the annealing temperature and time. For 15 min annealings, the PL center wavelength showed a 7 nm reduction for temperatures up to 650 °C. Above 650 °C, the wavelength changed rapidly, with a 50 nm reduction at 750 °C. Annealing at 700 °C for up to 20 min produced a systematic reduction in PL wavelength up to 39 nm. The data suggest that CdZnO/ZnO MQWs are relatively stable for nitrogen annealing below 650 °C for times up to 15 min.
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78.67.De Quantum wells
78.55.Et II-VI semiconductors
61.72.Cc Kinetics of defect formation and annealing

Origin of visible luminescence in hydrogenated amorphous silicon nitride

H. L. Hao, L. K. Wu, W. Z. Shen, and H. F. W. Dekkers

Appl. Phys. Lett. 91, 201922 (2007); http://dx.doi.org/10.1063/1.2814053 (3 pages) | Cited 19 times

Online Publication Date: 16 November 2007

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We present a detailed investigation on the origin of the room-temperature visible luminescence in hydrogenated amorphous silicon nitride films. In combination with Raman spectroscopy and high resolution transmission electron microscopy, we demonstrate clearly that the red light emission originates from amorphous silicon quantum dots. On the basis of the redshift of peak position, narrowing of bandwidth, and temperature quenching of luminescence, we attribute the green emission to the bandtail recombination of carriers. In addition, the blue luminescence is assigned to the silicon-related defects according to the analysis for the gap states in silicon nitride.
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78.55.Qr Amorphous materials; glasses and other disordered solids
78.30.Am Elemental semiconductors and insulators
78.66.Nk Insulators
78.67.Hc Quantum dots
72.20.Jv Charge carriers: generation, recombination, lifetime, and trapping
73.61.Ng Insulators

Radical-source molecular beam epitaxy of ZnMgO and ZnCdO alloys on ZnO substrates

S. Sadofev, P. Schäfer, Y.-H. Fan, S. Blumstengel, F. Henneberger, D. Schulz, and D. Klimm

Appl. Phys. Lett. 91, 201923 (2007); http://dx.doi.org/10.1063/1.2815662 (3 pages) | Cited 20 times

Online Publication Date: 16 November 2007

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We report on a dramatic improvement of the crystalline quality of ZnMgO and ZnCdO epilayers using Bridgman-grown ZnO substrates. ZnMgO alloys grow pseudomorphically over several 100 nm and the (0002) ω-rocking curve width is as low as 19 arc sec. Strain inhomogeneities in low-temperature grown ZnCdO are significantly reduced and the rocking width is lowered down to 45 arc sec. Despite the high crystalline perfection, the optical properties of the films are mainly determined by their ternary character.
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78.66.Hf II-VI semiconductors
81.15.Hi Molecular, atomic, ion, and chemical beam epitaxy
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An optically controlled phase shifter employing the organic semiconductor poly(3-hexylthiophene)

Hongyan Tang, R. S. Donnan, and T. Kreouzis

Appl. Phys. Lett. 91, 202101 (2007); http://dx.doi.org/10.1063/1.2806936 (3 pages)

Online Publication Date: 12 November 2007

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The transmission characteristics of optically controlled phase shifters employing the organic semiconductor poly(3-hexylthiophene) (P3HT) are reported. Two microstrip structures were fabricated onto a P3HT coated indium tin oxide substrate supported by glass. Experimental results on these unoptimized prototype structures yield reversible differential phase shifts >10° at 2.0 GHz under tens of milliwatt optical power illumination. These devices demonstrate great potential as linear analog phase shifters.
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42.79.-e Optical elements, devices, and systems

A comparative study of the electronic structures of oxygen- and chlorine-treated nitrogenated carbon nanotubes by x-ray absorption and scanning photoelectron microscopy

S. C. Ray, C. W. Pao, H. M. Tsai, J. W. Chiou, W. F. Pong, C. W. Chen, M.-H. Tsai, P. Papakonstantinou, L. C. Chen, and K. H. Chen

Appl. Phys. Lett. 91, 202102 (2007); http://dx.doi.org/10.1063/1.2807275 (3 pages) | Cited 4 times

Online Publication Date: 13 November 2007

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The electronic structures and bonding properties of oxygen- and chlorine-treated nitrogenated carbon nanotubes (N-CNTs) were studied using x-ray absorption near-edge structure (XANES) and scanning photoelectron microscopy. Features in the C K-edge XANES spectra are shifted by ∼ 0.3 eV toward higher energies and by ∼ 1.1 eV toward lower energies relatively to those of the more symmetrical pyridinelike and graphitelike structured N-CNTs upon chlorination and oxidation, respectively. Increases in N K-edge XANES intensities for both chlorination and oxidation reveal substitution of C–C bonds by C–N bonds consistent with the observed valence-band photoemission spectra of the decrease of the C 2s bond and the increase of the N 2s bond.
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71.20.Tx Fullerenes and related materials; intercalation compounds
78.70.Dm X-ray absorption spectra
79.60.Jv Interfaces; heterostructures; nanostructures

Migration volume for polaron dielectric relaxation in disordered materials

A. N. Papathanassiou, I. Sakellis, and J. Grammatikakis

Appl. Phys. Lett. 91, 202103 (2007); http://dx.doi.org/10.1063/1.2812538 (3 pages) | Cited 10 times

Online Publication Date: 13 November 2007

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A theoretical study of the influence of pressure on the dielectric relaxation related with polaron tunneling and phonon-assisted hopping in disordered solids is developed. The sign and absolute value of the migration volume, which is obtained by employing the present formulation, evidence the nature of the relaxation. As a paradigm, positive and negative values of migration volume are evaluated by analyzing recently published dielectric loss measurements under pressure in semiconducting polypyrrole. A straightforward relation between the value of the migration volume and the nature of short-range polaron flow and the size of polaron distortion is revealed.
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77.84.Jd Polymers; organic compounds
77.22.Gm Dielectric loss and relaxation
71.38.-k Polarons and electron-phonon interactions

Acceptor, compensation, and mobility profiles in multiple Al implanted 4H‐SiC

F. Giannazzo, F. Roccaforte, and V. Raineri

Appl. Phys. Lett. 91, 202104 (2007); http://dx.doi.org/10.1063/1.2813022 (3 pages) | Cited 15 times

Online Publication Date: 13 November 2007

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In this letter, we studied the effect of the annealing temperature (from 1400 to 1650 °C) on the acceptor, compensation, and mobility depth profiles in 4H‐SiC implanted with multiple energy (40–550 keV) and medium dose (1×1013 cm−2) Al ions. Scanning capacitance microscopy and scanning spreading resistance microscopy were jointly used to determine those depth profiles with nanometric resolution. It was demonstrated that the electrical activation in the Al implanted layer at increasing annealing temperatures was the result of a counterbalance between the increase in the acceptor concentration and the decrease in the percentage compensation.
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72.20.Fr Low-field transport and mobility; piezoresistance
61.72.Cc Kinetics of defect formation and annealing
07.79.-v Scanning probe microscopes and components
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