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15 Apr 2012

Volume 111, Issue 8, Articles (08xxxx)

Issue Cover Spotlight Figure

J. Appl. Phys. 111, 084701 (2012); http://dx.doi.org/10.1063/1.3698319 (11 pages)

Xerxes Lopez-Yglesias, Jason M. Gamba, and Richard C. Flagan
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back to top Nanoscale Science and Design

Competitive role of Mn diffusion with growth in Mn catalyzed nanostructures

S. Rehman, M. Hafeez, U. Manzoor, M. A. Khan, and A. S. Bhatti

J. Appl. Phys. 111, 084301 (2012); http://dx.doi.org/10.1063/1.3702881 (6 pages) | Cited 1 time

Online Publication Date: 16 April 2012

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The use of dopant as a catalyst in the vapor-liquid-solid mode of growth offers a unique way to dope simultaneously the growing nanostructure. This paper reports the use of Mn as a catalyst to grow ZnS nanostructures and simultaneously doping it. It is also shown here that the diffusion rate of Mn in ZnS can be varied to compete with the growth rate of the nanostructures. The diffusion of Mn is found to depend strongly on the Mn layer thickness. The composition of nanowires is determined from X-ray diffraction (XRD) and X-ray photoemission spectroscopy (XPS) measurements. XPS revealed that Mn diffuses only on the surface forming MnS2 compound and its concentration on the surface of nanowires shows a strong dependence on its droplet size. The thermodynamic models are used to examine the interplay of the growth of ZnS nanostructures and Mn diffusion in them. The results show that uniform doping during the growth can be achieved by carefully tuning the growth temperature and the catalyst layer thickness or catalyst droplet size.
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81.16.-c Methods of micro- and nanofabrication and processing
61.72.uj III-V and II-VI semiconductors
61.46.Km Structure of nanowires and nanorods (long, free or loosely attached, quantum wires and quantum rods, but not gate-isolated embedded quantum wires)
79.60.Bm Clean metal, semiconductor, and insulator surfaces
68.35.Fx Diffusion; interface formation
82.65.+r Surface and interface chemistry; heterogeneous catalysis at surfaces

Colored porous silicon as support for plasmonic nanoparticles

M. Lublow, S. Kubala, J.-F. Veyan, and Y. J. Chabal

J. Appl. Phys. 111, 084302 (2012); http://dx.doi.org/10.1063/1.3703469 (10 pages)

Online Publication Date: 16 April 2012

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Colored nanoporous silicon thin films were employed as dielectric spacing layers for the enhancement of localized surface plasmon (LSP) polaritons. Upon formation of Au nanoparticles (Au-NPs) on these layers, a visible color change is observed due to multiple LSP resonance excitations. Far-field effects were assessed by angle-resolved reflectometry. Resonance enhancements, particularly for s-polarized light, account for the observed color change and are discussed in terms of effective medium and Mie scattering theory. Enhancements of the electric field strengths in the near-field and of the absorption in the substrate were deduced from finite difference time domain calculations and exceed considerably those of the non-porous Au-NP/Si interface. First results of improved photoelectrocatalytic hydrogen evolution at these interfaces are discussed. Samples were prepared by varied procedures of metal assisted etching and dry etching with XeF2. Structural and chemical properties were investigated by scanning electron and atomic force microscopy as well as energy dispersive x-ray analysis.
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73.20.Mf Collective excitations (including excitons, polarons, plasmons and other charge-density excitations)
61.46.Df Structure of nanocrystals and nanoparticles ("colloidal" quantum dots but not gate-isolated embedded quantum dots)
81.16.-c Methods of micro- and nanofabrication and processing
68.37.Ps Atomic force microscopy (AFM)
82.65.+r Surface and interface chemistry; heterogeneous catalysis at surfaces
71.36.+c Polaritons (including photon-phonon and photon-magnon interactions)

Three-dimensional structure of (110) porous silicon with in-plane optical birefringence

Shinsuke Shichi, Minoru Fujii, Tomoki Nishida, Hidehiro Yasuda, Kenji Imakita, and Shinji Hayashi

J. Appl. Phys. 111, 084303 (2012); http://dx.doi.org/10.1063/1.3703522 (6 pages)

Online Publication Date: 16 April 2012

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Electrochemical etching of a (110) oriented Si wafer results in a porous silicon (PSi) layer which exhibits a strong in-plane optical birefringence. We study the refractive index ellipsoid of (110) PSi by angle-resolved optical transmittance measurements and reveal that it is a biaxial crystal. The angle-resolved transmission electron microscope observations demonstrate that pores grow along the directions in between the <100> crystal axes and the etching current flow and these directions depend on the etching current density. The etching current density dependence of the pore direction indicates that the shape of the index ellipsoid can be controlled by the etching condition.
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68.35.bg Semiconductors
78.20.Fm Birefringence
68.47.Fg Semiconductor surfaces
61.43.Gt Powders, porous materials
81.65.Cf Surface cleaning, etching, patterning
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)

Origin of white light luminescence from Si+/C+ sequentially implanted and annealed silica

X. D. Zhou, F. Ren, X. H. Xiao, J. X. Xu, Z. G. Dai, G. X. Cai, and C. Z. Jiang

J. Appl. Phys. 111, 084304 (2012); http://dx.doi.org/10.1063/1.3703668 (7 pages)

Online Publication Date: 16 April 2012

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The white light luminescence is observed from the silica slides implanted by sequential Si+ and C+ ions or only by C+ ions followed by thermal annealing. In the photoluminescence (PL) spectra, their white emissions cover the whole visible spectral range from 350 to 800 nm. The influence of thermal annealing on the PL of the implanted samples was studied. The microstructural and optical analysis allow us to figure out the origin of the white light emission, which is mainly attributed to the emission of graphite like C clusters although the contributions from the emissions of the Si and SiC nanocrystals are also included. Compared to the white light emission of C+ implanted sample, the white light emission of Si+/C+ implanted sample has higher thermal stability.
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78.55.Hx Other solid inorganic materials
61.46.Df Structure of nanocrystals and nanoparticles ("colloidal" quantum dots but not gate-isolated embedded quantum dots)
78.67.Bf Nanocrystals, nanoparticles, and nanoclusters
81.07.Bc Nanocrystalline materials
81.40.Ef Cold working, work hardening; annealing, post-deformation annealing, quenching, tempering recovery, and crystallization

Modified beam theories for bending properties of nanowires considering surface/intrinsic effects and axial extension effect

H. F. Zhan and Y. T. Gu

J. Appl. Phys. 111, 084305 (2012); http://dx.doi.org/10.1063/1.3703673 (9 pages)

Online Publication Date: 18 April 2012

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Several studies of the surface effect on bending properties of a nanowire (NW) have been conducted. However, these analyses are mainly based on theoretical predictions, and there is seldom integration study in combination between theoretical predictions and simulation results. Thus, based on the molecular dynamics (MD) simulation and different modified beam theories, a comprehensive theoretical and numerical study for bending properties of nanowires considering surface/intrinsic stress effects and axial extension effect is conducted in this work. The discussion begins from the Euler-Bernoulli beam theory and Timoshenko beam theory augmented with surface effect. It is found that when the NW possesses a relatively small cross-sectional size, these two theories cannot accurately interpret the true surface effect. The incorporation of axial extension effect into Euler-Bernoulli beam theory provides a nonlinear solution that agrees with the nonlinear-elastic experimental and MD results. However, it is still found inaccurate when the NW cross-sectional size is relatively small. Such inaccuracy is also observed for the Euler-Bernoulli beam theory augmented with both contributions from surface effect and axial extension effect. A comprehensive model for completely considering influences from surface stress, intrinsic stress, and axial extension is then proposed, which leads to good agreement with MD simulation results. It is thus concluded that, for NWs with a relatively small cross-sectional size, a simple consideration of surface stress effect is inappropriate, and a comprehensive consideration of the intrinsic stress effect is required.
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81.40.Lm Deformation, plasticity, and creep
62.20.F- Deformation and plasticity
61.46.Km Structure of nanowires and nanorods (long, free or loosely attached, quantum wires and quantum rods, but not gate-isolated embedded quantum wires)
02.60.-x Numerical approximation and analysis
81.40.Jj Elasticity and anelasticity, stress-strain relations
62.20.dq Other elastic constants

Nanoscale temperature sensing using the Seebeck effect

F. L. Bakker, J. Flipse, and B. J. van Wees

J. Appl. Phys. 111, 084306 (2012); http://dx.doi.org/10.1063/1.3703675 (4 pages)

Online Publication Date: 18 April 2012

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We experimentally study the effect of Joule heating on the electron temperature in metallic nanoscale devices and compare the results with a diffusive 3D finite element model. The temperature is probed using four thermocouples located at different distances from the heater. A good quantitative agreement, within 30%, between the experimental data and the modeling is obtained. Since we observe a strong thickness dependence of the electrical conductivity of our metals, we find that the Joule heating in nanoscale devices is often incorrectly calculated if bulk conductivities are used. Furthermore, Peltier heating/cooling is investigated and the combination with Seebeck temperature measurements provides us with a method to determine the Seebeck coefficient of a material.
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07.10.Cm Micromechanical devices and systems
02.70.Dh Finite-element and Galerkin methods

Effect of e-beam irradiation on graphene layer grown by chemical vapor deposition

M. Z. Iqbal, Arun Kumar Singh, M. W. Iqbal, Sunae Seo, and Jonghwa Eom

J. Appl. Phys. 111, 084307 (2012); http://dx.doi.org/10.1063/1.4704197 (5 pages)

Online Publication Date: 18 April 2012

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We have grown graphene by chemical vapor deposition (CVD) and transferred it onto Si/SiO2 substrates to make tens of micron scale devices for Raman spectroscopy study. The effect of electron beam (e-beam) irradiation of various doses (600 to 12 000 μC/cm2) on CVD grown graphene has been examined by using Raman spectroscopy. It is found that the radiation exposures result in the appearance of the strong disorder D band attributed the damage to the lattice. The evolution of peak frequencies, intensities, and widths of the main Raman bands of CVD graphene is analyzed as a function of defect created by e-beam irradiation. Especially, the D and G peak evolution with increasing radiation dose follows the amorphization trajectory, which suggests transformation of graphene to the nanocrystalline and then to amorphous form. We have also estimated the strain induced by e-beam irradiation in CVD graphene. These results obtained for CVD graphene are in line with previous findings reported for the mechanically exfoliated graphene [D. Teweldebrhan and A. A. Balandin, Appl. Phys. Lett. 94, 013101 (2009)]. The results have important implications for CVD graphene characterization and device fabrication, which rely on the electron microscopy.
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81.05.ue Graphene
61.80.Fe Electron and positron radiation effects
52.77.Dq Plasma-based ion implantation and deposition
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
78.67.Wj Optical properties of graphene
61.48.Gh Structure of graphene

Chromium-oxide enhancement of photo-oxidation of CdSe/ZnS quantum dot solids

S. M. Sadeghi, A. Nejat, J. J. Weimer, and G. Alipour

J. Appl. Phys. 111, 084308 (2012); http://dx.doi.org/10.1063/1.3703470 (8 pages) | Cited 2 times

Online Publication Date: 19 April 2012

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In this paper, we report the results of depositing CdSe/ZnS quantum dots (QDs) on Cr coated substrates. Compared to such QDs on glass surfaces, the emission peak blueshifts and broadens much more rapidly. The acceleration of the process is presented by showing the relative intensity, position, and full-width as a function of time for various laser intensities, and the effects on photoinduced fluorescence enhancement and photo-oxidiation are discussed. We also consider the thickness of the Cr layer and photo-induced heating, showing this factor should instead lead to a redshift of the peak. Oxidation of colloidal QDs is known to cause blueshifts and broadening of emission peaks (photobleaching). We attribute the faster photobleaching of QDs on Cr films to enhancement in the oxidation of the ZnS shell by catalytic or direct chemical reactions with chromium oxides. As a working hypothesis, this leads to additional paths for breakdown of the otherwise protective nature of the shell.
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78.67.Hc Quantum dots
68.55.ag Semiconductors
81.65.Mq Oxidation
82.70.Dd Colloids
78.55.Et II-VI semiconductors
78.66.Hf II-VI semiconductors

Mechanism of GaN quantum dot overgrowth by Al0.5Ga0.5N: Strain evolution and phase separation

M. Korytov, J. A. Budagosky, J. Brault, T. Huault, M. Benaissa, T. Neisius, J.-L. Rouvière, and P. Vennéguès

J. Appl. Phys. 111, 084309 (2012); http://dx.doi.org/10.1063/1.4704682 (10 pages)

Online Publication Date: 19 April 2012

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The capping of GaN quantum dots (QDs) with an Al0.5Ga0.5N layer is studied using transmission electron microscopy and atomic force microscopy in combination with theoretical calculations. The capping process can be divided into several well-distinguishable stages including a QD shape change and a local change of the Al0.5Ga0.5N capping layer composition. The phase separation phenomenon is investigated in relation with the capping layer thickness. Amount of the chemical composition fluctuations is determined from separate analysis of scanning transmission electron microscopy and high-resolution transmission electron microscopy images. The local distortion of atomic lattice in the QD surroundings is measured by high-resolution electron microscopy and is confronted with theoretically calculated strain distributions. Based on these data, a possible mechanism of alloy demixing in the Al0.5Ga0.5N layer is discussed.
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68.65.Hb Quantum dots (patterned in quantum wells)
64.75.Qr Phase separation and segregation in semiconductors
68.37.Ps Atomic force microscopy (AFM)
82.80.Fk Electrochemical methods
05.40.-a Fluctuation phenomena, random processes, noise, and Brownian motion
68.37.Ma Scanning transmission electron microscopy (STEM)

Strategies for tuning phonon transport in multilayered structures using a mismatch-based particle model

Nam Q. Le, John C. Duda, Timothy S. English, Patrick E. Hopkins, Thomas E. Beechem, and Pamela M. Norris

J. Appl. Phys. 111, 084310 (2012); http://dx.doi.org/10.1063/1.4704681 (8 pages)

Online Publication Date: 20 April 2012

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The performance of many micro- and nanoscale devices depends on the ability to control interfacial thermal transport, which is predominantly mediated by phonons in semiconductor systems. The phonon transmissivity at an interface is therefore a quantity of interest. In this work, an empirical model, termed the thermal mismatch model, is developed to predict transmissivity at ideal interfaces between semiconductor materials, producing an excellent agreement with molecular dynamics simulations of wave packets. To investigate propagation through multilayered structures, this thermal mismatch model is then incorporated into a simulation scheme that represents wave packets as particles, showing a good agreement with a similar scheme that used molecular dynamics simulations as input [P. K. Schelling and S. R. Phillpot, J. Appl. Phys. 93, 5377 (2003)]. With these techniques validated for both single interfaces and superlattices, they are further used to identify ways to tune the transmissivity of multilayered structures. It is shown that by introducing intermediate layers of certain atomic masses, the total transmissivity can either be systematically enhanced or reduced compared to that of a single interface. Thus, this model can serve as a computationally inexpensive means of developing strategies to control phonon transmissivity in applications that may benefit from either enhancement (e.g., microelectronics) or reduction (e.g., thermoelectrics) in thermal transport.
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63.22.Np Layered systems

Effect of the doping level on the radiative life time in ZnO nanowires

Ivan-Christophe Robin

J. Appl. Phys. 111, 084311 (2012); http://dx.doi.org/10.1063/1.4705370 (7 pages)

Online Publication Date: 23 April 2012

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The emission properties of ZnO nanowires grown by metal organic vapor phase epitaxy on sapphire and p-type GaN are compared using temperature dependent time resolved photoluminescence. The temperature dependence of the radiative decay time of nanowires grown on sapphire is well understood considering an exciton thermalization in a 3D density of states. In the case of growth on GaN, a great increase of the radiative decay time is observed compared to nanowires grown on sapphire. This increase of the radiative decay time could be due to a band bending effect that separates the electron and the hole. This band bending effect depends on the residual doping level and is not seen in the case of nanowires grown on sapphire probably because of a very high residual doping level that can be estimated from the radiative decay time temperature dependence.
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78.55.Et II-VI semiconductors
81.16.Be Chemical synthesis methods
71.35.-y Excitons and related phenomena
78.67.Uh Nanowires

Secondary resonance magnetic force microscopy

Suguru Tanaka, Yasuo Azuma, and Yutaka Majima

J. Appl. Phys. 111, 084312 (2012); http://dx.doi.org/10.1063/1.4705400 (4 pages)

Online Publication Date: 23 April 2012

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In this study, we have developed secondary resonance magnetic force microscopy (SR-MFM) for imaging alternating magnetic fields from a sample surface at the secondary resonant frequency of the magnetic cantilever at the same time as the topographic image. SR-MFM images of alternating magnetic fields diverging from the main pole in a driving perpendicular magnetic recording head are presented, and the divergence and convergence of the fields are discussed. The spatial resolution of SR-MFM is estimated to be 18 nm; this is 2.5 times smaller than that of conventional MFM.
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07.79.Pk Magnetic force microscopes
85.70.Kh Magnetic thin film devices: magnetic heads (magnetoresistive, inductive, etc.); domain-motion devices, etc.
68.37.Rt Magnetic force microscopy (MFM)

Thermal insulating layer on a conducting substrate. Analysis of thermoreflectance experiments

C. Frétigny, J.-Y. Duquesne, D. Fournier, and F. Xu

J. Appl. Phys. 111, 084313 (2012); http://dx.doi.org/10.1063/1.3702823 (7 pages)

Online Publication Date: 23 April 2012

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Thermoreflectance experiments are sensitive to the thermal properties of thin layers deposited on substrates (conductivity and diffusivity). However, retrieving these properties from experimental data remains a difficult issue. The case of a conducting layer deposited on an insulating substrate was studied previously. We present here a mathematical and experimental analysis of the thermoreflectance response in the opposite case: an insulating layer on a conducting substrate. We show theoretically that conductivity and diffusivity can be determined independently thanks to a comparison with the substrate. The method is applied to experiments performed on a silicon substrate covered with a thin layer deposited by sputtering a titanium target.
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78.20.N- Thermo-optic effects
68.55.aj Insulators
81.15.Cd Deposition by sputtering
66.70.Lm Other systems such as ionic crystals, molecular crystals, nanotubes, etc.
73.40.Qv Metal-insulator-semiconductor structures (including semiconductor-to-insulator)

Observation of negative contact resistances in graphene field-effect transistors

Ryo Nouchi, Tatsuya Saito, and Katsumi Tanigaki

J. Appl. Phys. 111, 084314 (2012); http://dx.doi.org/10.1063/1.4705367 (7 pages) | Cited 1 time

Online Publication Date: 25 April 2012

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The gate-voltage (VG) dependence of the contact resistance (RC) in graphene field-effect transistors is characterized by the transmission line model. The RC-VG characteristics of Ag, Cu, and Au contacts display a dip around the charge neutrality point and become even negative with Ag contacts. The dip structure is well reproduced by a model calculation that considers a metal-contact-induced potential variation near the metal contact edges. The apparently negative RC originates with the carrier doping from the metal contacts to the graphene channel and appears when the doping effect is more substantial than the actual contact resistance precisely at the contacts. The negative RC can appear at the metal contacts to Dirac-cone systems such as graphene.
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85.30.Tv Field effect devices
73.40.Cg Contact resistance, contact potential

Small copper clusters studied by x-ray absorption near-edge structure

H. Oyanagi, Z. H. Sun, Y. Jiang, M. Uehara, H. Nakamura, K. Yamashita, Y. Orimoto, L. Zhang, C. Lee, A. Fukano, and H. Maeda

J. Appl. Phys. 111, 084315 (2012); http://dx.doi.org/10.1063/1.3700346 (4 pages)

Online Publication Date: 25 April 2012

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The local structure of copper nanoparticles grown in organic solution by reducing Cu(II) hexafluoroacetylacetonate [Cu(hfac)2] was studied as-grown by the Cu K-edge x-ray absorption near-edge structure (XANES). Comparison of the experimental XANES spectra with reference materials indicated small copper clusters are formed by ligand-exchange with oleylamine and subsequent reducing by diphenylsilane. The multiple-scattering (MS) calculation for various model clusters consisting of 13–135 atoms suggests that small (13–19 atom) Cu clusters are stabilized without a large deformation.
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81.16.-c Methods of micro- and nanofabrication and processing
78.70.Dm X-ray absorption spectra
61.46.Df Structure of nanocrystals and nanoparticles ("colloidal" quantum dots but not gate-isolated embedded quantum dots)

Selective emitters design and optimization for thermophotovoltaic applications

E. Nefzaoui, J. Drevillon, and K. Joulain

J. Appl. Phys. 111, 084316 (2012); http://dx.doi.org/10.1063/1.4705363 (8 pages)

Online Publication Date: 27 April 2012

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Among several solutions to exploit solar energy, thermophotovoltaics have been popularized and have known great breakthroughs during the past two decades. Yet, existing systems still have low efficiencies since the wavelength range of optimal photovoltaic (PV) conversion is very small compared to the emitter spectral range. Selective emitters are a very promising solution to this problem. We developed numerical tools to design and optimize such emitters. Some of the resulting structures composed of two or four layers of metals and semiconductors are presented in this paper. We also show that the usual PV devices efficiency limits (30% for crystalline silicon under solar radiation, according to Shockley-Queisser model) can be easily overcome thanks to these structures.
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88.40.jj Silicon solar cells
84.60.Jt Photoelectric conversion

The photoionization cross section of a hydrogenic impurity in a multi-layered spherical quantum dot

Mehmet Şahin, Firdes Tek, and Ahmet Erdinç

J. Appl. Phys. 111, 084317 (2012); http://dx.doi.org/10.1063/1.4705410 (8 pages)

Online Publication Date: 27 April 2012

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In this study, we have investigated the photoionization cross section of an on-center hydrogenic impurity in a multi-layered spherical quantum dot. The electronic energy levels and their wave functions have been determined fully numerically by shooting method. Also, we have calculated the binding energy of the impurity by using these energy values. The photoionization cross section has also been computed as a function of the layer thickness and normalized photon energies. We have discussed in detail the possible physical reasons behind the changes in the binding energies and photoionization cross section. It is observed that both the binding energies and the photoionization cross sections depend strongly on the layer thickness and photon energies.
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73.21.La Quantum dots
71.15.Nc Total energy and cohesive energy calculations

Unbiased line width roughness measurements with critical dimension scanning electron microscopy and critical dimension atomic force microscopy

L. Azarnouche, E. Pargon, K. Menguelti, M. Fouchier, D. Fuard, P. Gouraud, C. Verove, and O. Joubert

J. Appl. Phys. 111, 084318 (2012); http://dx.doi.org/10.1063/1.4705509 (14 pages) | Cited 3 times

Online Publication Date: 27 April 2012

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With the constant decrease of semiconductor device dimensions, line width roughness (LWR) becomes one of the most important sources of device variability and thus needs to be controlled below 2 nm for the future technological nodes of the semiconductor roadmap. The LWR control at the nanometer scale requires accurate measurements, which are inevitably impacted by the noise level of the equipment that causes bias from true LWR values. In this article, we compare the capability of two metrology tools, the critical dimension scanning electron microscopy (CD-SEM) and critical dimension atomic force microscopy (CD-AFM) to measure the true line width roughness of silicon and photoresist lines. For this purpose, we propose several methods based on previous works to estimate the noise level of those two equipments and thus extract the true LWR. One of the developed methods for the CD-SEM technique generalizes the power spectral densities (PSD) fitting method proposed by Hiraiwa and Nishida with a more universal autocorrelation function, which includes both correlation length and roughness exponent. However, PSD fitting method could not be used with CD-AFM due to the time consuming character of this technique. Hence, other experimental protocols have been set up for CD-AFM in order to accurately characterize the LWR. Our study shows that the CD-SEM technique combined with our PSD fitting method is much more powerful than CD-AFM to get all roughness information (true LWR, correlation length, and roughness exponent) with a good accuracy and efficiency on hard materials such as silicon. Concerning materials degradable under electron beam exposure such as photoresist, the choice is more disputable, since ultimately they are impacted by the electrons. Fortunately, our PSD fitting method allows working with low number of integration frames, which limits the resist degradation. Besides, we have highlighted some limitations of the CD-AFM technique due to the tip diameter. This technique can underestimate LWR if the roughness presents significant amount of high frequency components, as it is the case for photoresist patterns. So far, there is no universal technique to accurately estimate the LWR on any materials. Nevertheless, the CD-SEM protocol we propose opens a way for a better characterization of the photoresist LWR after lithography and a better understanding of the LWR transfer during the plasma etching steps involved in gate patterning processes.
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81.16.Nd Micro- and nanolithography
85.40.Hp Lithography, masks and pattern transfer
68.37.Ps Atomic force microscopy (AFM)
81.65.Cf Surface cleaning, etching, patterning
68.35.B- Structure of clean surfaces (and surface reconstruction)
81.05.Cy Elemental semiconductors

Preliminary investigation on the modification of electronic properties in surface passivated SnO2 nanowires with Schottky contacts on being exposed to 137Cs γ-radiation

Ayan Kar, Ryan Ahern, N. Gopalsami, A. C. Raptis, Michael A. Stroscio, and Mitra Dutta

J. Appl. Phys. 111, 084319 (2012); http://dx.doi.org/10.1063/1.4706557 (5 pages)

Online Publication Date: 27 April 2012

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This paper demonstrates modification of SnO2 nanowire electronic and surface properties at room temperature on being exposed to γ-radiation. Electrons generated by the high energy photons are captured by oxygen sensitized nanowires and result in a change in the nanowire resistance along with an enhancement in the Schottky barrier thereby modulating current flow across the metal-semiconductor (M-S) junction. The time dependent change in device resistance shows a notably short response time to radiation. Further luminescence and transmission data from the irradiated nanowires indicate a change in the nature and density of surface defects after exposure to highly energetic γ-radiation.
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81.05.Hd Other semiconductors
78.67.Uh Nanowires
81.07.Gf Nanowires
73.22.-f Electronic structure of nanoscale materials and related systems
73.30.+y Surface double layers, Schottky barriers, and work functions
73.40.Ns Metal-nonmetal contacts

Thermal conductivity of ZnO thin film produced by reactive sputtering

Yibin Xu, Masahiro Goto, Ryozo Kato, Yoshihisa Tanaka, and Yutaka Kagawa

J. Appl. Phys. 111, 084320 (2012); http://dx.doi.org/10.1063/1.4706569 (7 pages) | Cited 2 times

Online Publication Date: 27 April 2012

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ZnO thin films have been produced by reactive sputtering with different oxygen contents in the sputtering gas. As a result of transmission electronic microscopy observation, each film consist of two layers: an interfacial layer close to the substrate, with a thickness of about 200 nm, composed of very fine crystal grains and an upper layer above the interfacial layer, composed of column-shaped grains aligned along the out-plane direction. The grain diameter ranges from 35 to 100 nm depending on the oxygen partial pressure. The in-plane and out-plane thermal conductivity have been measured at room temperature. The out-plane thermal conductivity of the interfacial layer is 2.3 W m−1 K−1, independent on the oxygen partial pressure. The out-plane thermal conductivity of the upper layer is 5.4, 7.1, and 4.0 W m−1 K−1, and the in-plane thermal conductivity 4.86, 6.01, and 2.66 W m−1 K−1, for the O2 30%, 60%, and 90% ZnO film, respectively. Both out-plane and in-plane thermal conductivity decrease with the decrease of grain diameter. The thermal conductance of grain boundary has been estimated with the phonon diffusion mismatch model, and the intrinsic thermal conductivity within ZnO grains has been calculated with a cylinder-structured composite model. The result shows that the thermal conductivity of the ZnO thin films is dominated by the intrinsic thermal conductivity, which is a function of grain size.
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66.70.Df Metals, alloys, and semiconductors
68.55.ag Semiconductors
61.72.Mm Grain and twin boundaries
81.05.Dz II-VI semiconductors
81.15.Cd Deposition by sputtering
63.20.D- Phonon states and bands, normal modes, and phonon dispersion

Magnetic properties of ZnO nanoclusters

X. G. Zhao and Z. Tang

J. Appl. Phys. 111, 084321 (2012); http://dx.doi.org/10.1063/1.4707946 (4 pages)

Online Publication Date: 30 April 2012

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First-principles calculations were employed to study the magnetic properties of ZnO nanoclusters (NCs). It is demonstrated that the magnetism of the ZnO nanoclusters is dominated by the dangling-bond states of the surface oxygen ions surrounded by two Zn cations. Such surface-state driven magnetism can be completely destroyed by hydrogen passivation and thus is very sensitive to the chemical environments of the NCs. The theory indicates that magnetism may be steadily observed in appropriately encapsulated ZnO nanoparticles or around their grain boundaries, which is consistent with the previously reported experiments.
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75.70.Rf Surface magnetism
81.65.Rv Passivation
71.55.Gs II-VI semiconductors
73.20.At Surface states, band structure, electron density of states
75.50.Pp Magnetic semiconductors
75.50.Tt Fine-particle systems; nanocrystalline materials
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