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1 April 2009

Volume 105, Issue 7,  Articles (07xxxx)

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LASERS, OPTICS, AND OPTOELECTRONICS

Light scattering into silicon-on-insulator waveguide modes by random and periodic gold nanodot arrays

S. H. Lim, D. Derkacs, and E. T. Yu

J. Appl. Phys. 105, 073101 (2009) (6 pages)

Online Publication Date: 1 April 2009

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Experimental characterization and finite-element numerical simulations of the electromagnetic interaction between random or periodic Au nanodot arrays patterned atop a silicon-on-insulator (SOI) photodetector and incident electromagnetic plane waves have been performed at wavelengths of 400–1100  nm. The presence of the Au nanodots is found to lead to increased electromagnetic field amplitude within the semiconductor and, consequently, increased photocurrent response for both cases. Random arrays tend to exhibit broad increases in photocurrent over wavelength, whereas periodic arrays demonstrate sharp resonance peaks in the photocurrent absorption spectrum. Such features are due to the coupling of normally incident light into waveguide modes that satisfy the Bragg diffraction condition. Analysis of the dispersion relation of the waveguide modes allows for accurate prediction of the resonance peaks in the photocurrent absorption spectrum of the SOI photodetectors patterned with periodic nanodot arrays.
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85.60.Gz, 81.16.Rf, 42.79.Gn, 42.82.Et, 02.70.Dh

Microstructural evolution of nonpolar (11-20) GaN grown on (1-102) sapphire using a 3D-2D method

C. F. Johnston, M. J. Kappers, and C. J. Humphreys

J. Appl. Phys. 105, 073102 (2009) (5 pages)

Online Publication Date: 2 April 2009

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The microstructure of nonpolar, a-plane (11-20) GaN grown on r-plane (1-102) sapphire, using a three dimensional (3D)-two dimensional (2D) growth transition, has been studied at different stages of metal organic vapor phase epitaxy. The microstructure and morphology of GaN islands formed at the initial stages of growth, as well as the fully coalesced film, were characterized using transmission electron microscopy (TEM) and atomic force microscopy (AFM). The growth of GaN islands (bounded by {10-11} and (000-1) facets) was established under reactor conditions of relatively high pressure and high V/III ratio, whereas the island coalescence was achieved at lower pressure and low V/III ratio, leading to pit-free films with shallow striations along <0001>. Cross-sectional TEM studies, in combination with the AFM studies of the uncoalesced films, showed that there was a correlation between the point at which partial dislocation line direction changed and the point at which growth conditions changed from the 3D to 2D mode. Lengthening the 3D growth stage obviously increased the size of the GaN islands and was also found to increase the basal plane stacking fault (BSF) length and decrease the density of partial dislocations in the coalesced films: It is suggested that BSFs in adjacent islands merge when islands are sufficiently large to impinge before the 2D growth step. Their merging necessitates the annihilation of some of the partial dislocations, and this causes the decrease in dislocation density.
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68.55.ag, 68.37.Ps, 81.15.Kk, 81.15.Gh, 61.72.Ff

Efficiency of light emission in high aluminum content AlGaN quantum wells

Max Shatalov, Jinwei Yang, Wenhong Sun, Robert Kennedy, Remis Gaska, Kai Liu, Michael Shur, and Gintautas Tamulaitis

J. Appl. Phys. 105, 073103 (2009) (6 pages)

Online Publication Date: 3 April 2009

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High quality multiple quantum well Al0.35Ga0.65N active layers with narrow wells designed for ultraviolet (UV) light-emitting diodes using the phonon engineering approach are characterized using quasi-steady-state and time-resolved photoluminescence spectroscopy. The photoluminescence intensity decrease with temperature increasing from 10  to  300  K was very small, and the upper limit of the internal quantum efficiency (IQE) of up to 70% was estimated based on this temperature dependence. Carrier lifetime measurements yielded the lower bound of the IQE to be ~35% under optical pumping, whereas IQE of ~25% was estimated from the measured external quantum efficiency and the light extraction efficiency calculated by ray tracing. The observed photoluminescence features and the high IQE are interpreted as a consequence of strong carrier (exciton) localization.
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78.67.De, 63.22.-m, 78.55.Cr, 71.35.-y, 78.47.Cd, 73.63.Hs, 72.20.Jv

Raman analysis of longitudinal optical phonon-plasmon coupled modes of aligned ZnO nanorods

An-Jen Cheng, Yonhua Tzeng, Hui Xu, Siddharth Alur, Yaqi Wang, Minseo Park, Tsung-hsueh Wu, Curtis Shannon, Dong-Joo Kim, and Dake Wang

J. Appl. Phys. 105, 073104 (2009) (7 pages)

Online Publication Date: 3 April 2009

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The electronic properties of vertically aligned ZnO nanorods have been investigated using micro-Raman spectroscopy. The concentration and mobility of the charge carriers were determined via Raman line shape analysis using longitudinal-optical-phonon-plasmon coupled mode. The local laser heating and the stress effects have been considered when analyzing the Raman spectra. The mobility and carrier concentration of the aligned ZnO nanorods are 84.8  cm2/V  s and 3.8×1017  cm−3, respectively. As a comparison, the mobility and carrier concentration of the undoped bulk ZnO were also obtained from the Raman line shape analysis. The mobility of the aligned ZnO nanorods is about 20% lower than that of the undoped bulk ZnO, which can be attributed to enhanced surface scattering due to the reduction in dimension.
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78.67.-n, 78.30.Fs, 72.20.Fr, 63.22.-m, 73.25.+i, 73.63.-b

Detailed analysis of bathocuproine layer for organic solar cells based on copper phthalocyanine and C60

Jiang Huang (黄江), Junsheng Yu (于军胜), Hui Lin (林慧), and Yadong Jiang (蒋亚东)

J. Appl. Phys. 105, 073105 (2009) (5 pages)

Online Publication Date: 7 April 2009

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The electrical characteristics of double heterojunction organic solar cells with various thicknesses of bathocuproine (BCP) as an exciton-blocking layer (EBL) were studied. A theoretical analysis using optical transfer matrix theory was carried out to obtain insight into how the EBL influences light-absorbing and exciton-transporting properties in the organic layers. The results showed that by employing an 8–10 nm BCP layer exciton density was increased by 46%. By using an improved equivalent circuit model, photovoltaic characteristics were parametrized and modeled, which revealed that the thin BCP layer <10  nm did not affect the charge collection process. The mechanism of metallic cathode influence on solar cells was also discussed.
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84.60.Jt, 71.35.-y, 72.40.+w

Size dependent optical band gap of ternary I-III-VI2 semiconductor nanocrystals

Takahisa Omata, Katsuhiro Nose, and Shinya Otsuka-Yao-Matsuo

J. Appl. Phys. 105, 073106 (2009) (5 pages)

Online Publication Date: 8 April 2009

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The size dependent optical band gap of the less-toxic ternary I-III-VI2 chalcopyrite-type semiconductor quantum dots (QDs), CuInS2, CuInSe2, CuGaS2, CuGaSe2, AgInSe2, AgGaS2, and AgGaSe2, were evaluated using the finite-depth-well effective mass approximation calculation. From the comparison of the calculation result with the experimental values for the CuInS2 case, it was shown that the calculation was highly valid to predict the size dependent optical gap of the ternary semiconductor QDs. The optical band gap of the above seven I-III-VI2 QDs covers a wide wavelength range from the near-infrared to ultraviolet. It has been shown that the I-III-VI2 semiconductor QDs have a significant potential as alternatives to the highly toxic cadmium-containing II-VI semiconductor QDs and they are applicable to the wide range of light emitting devices and solar cells.
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78.67.Hc, 73.21.La, 71.18.+y, 71.20.Nr, 73.22.-f, 78.20.Ci

Electro-optical characteristics of omnidirectional liquid crystal domain mode using doughnut-shaped slit electrodes

Jin Seog Gwag, Kwang-Soo Bae, You-Jin Lee, and Jae-Hoon Kim

J. Appl. Phys. 105, 073107 (2009) (4 pages)

Online Publication Date: 10 April 2009

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This paper proposes a liquid crystal (LC) display (LCD) mode, characterized by an azimuthally continuous nematic domain, driven by patterned electrodes with circular- and doughnut-shaped slits producing conelike fields, as a vertically aligned (VA) nematic LC mode. This proposed mode is focused on achieving a high transmittance display with omnidirectionally uniform optical characteristics by utilizing the proposed electrode structure. Consequently, the experimental results of the proposed LCD mode show high brightness and wide viewing angles that correlate well to numerical calculations. Other electro-optics characteristics of this mode correspond to the patterned VA LC mode.
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85.60.Pg, 42.79.Kr, 78.20.Jq, 61.30.Dk, 61.30.Eb

Retrieving the vertical column density of NO2 by using multiaxis differential optical absorption spectroscopy technique

H. Y. Zuo, S. R. Luo, and Q. J. Liu

J. Appl. Phys. 105, 073108 (2009) (5 pages)

Online Publication Date: 13 April 2009

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A rigorous technique of monitoring the vertical column density (VCD) of NO2 by using the scattered solar radiation is reported. By analyzing the optical path, the method of collecting the useful scattered solar radiation is achieved. By using this method, the collecting equipment and procedure of solar radiation could be greatly simplified. And through the actual measurement, it is proved that the VCD of NO2 measured by using the scattered solar radiation is very close to that from the direct solar radiation.
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93.85.-q, 92.60.H-, 92.60.Vb, 42.68.Mj
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PLASMAS AND ELECTRICAL DISCHARGES

Plasma “anti-assistance” and “self-assistance” to high power impulse magnetron sputtering

André Anders and Georgy Yu. Yushkov

J. Appl. Phys. 105, 073301 (2009) (6 pages)

Online Publication Date: 10 April 2009

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A plasma assistance system was investigated with the goal to operate high power impulse magnetron sputtering (HiPIMS) at lower pressure than usual, thereby to enhance the utilization of the ballistic atoms and ions with high kinetic energy in the film growth process. Gas plasma flow from a constricted plasma source was aimed at the magnetron target. Contrary to initial expectations, such plasma assistance turned out to be contraproductive because it led to the extinction of the magnetron discharge. The effect can be explained by gas rarefaction. A better method of reducing the necessary gas pressure is operation at relatively high pulse repetition rates where the afterglow plasma of one pulse assists in the development of the next pulse. Here we show that this method, known from medium-frequency (MF) pulsed sputtering, is also very important at the much lower pulse repetition rates of HiPIMS. A minimum in the possible operational pressure is found in the frequency region between HiPIMS and MF pulsed sputtering.
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81.15.Cd, 52.50.Dg, 52.77.-j, 52.80.Hc

Effects of dielectric barrier discharges on silicon surfaces: Surface roughness, cleaning, and oxidation

B. Michel, M. Giza, M. Krumrey, M. Eichler, G. Grundmeier, and C.-P. Klages

J. Appl. Phys. 105, 073302 (2009) (9 pages)

Online Publication Date: 14 April 2009

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Silicon wafers were exposed to a dielectric barrier discharge (DBD) at atmospheric pressure, which was ignited by applying a high voltage (>12  kV peak voltage) to a small gap (dg=300  µm) above the wafer surface in an oxygen process gas atmosphere. The effect of the DBD on H-terminated silicon and native silicon oxide surfaces was investigated in situ and ex situ by means of Fourier transform infrared spectroscopy and x-ray photoelectron spectroscopy (XPS). The influence of the treatment on surface roughness was studied by atomic force microscopy. In order to determine the thickness of the newly formed oxide under DBD influence, the method of calculating the oxide thickness from the Si 2p peak ratio in the XPS spectrum, which has so far been described for thermal oxides only, was adopted with x-ray reflectometry calibration samples. Additionally, infrared spectroscopy and spectroscopic ellipsometry were used to verify the XPS measurements. The calculated thickness values can be fitted with the growth law d=d0 ln[(t/tau)+k], with d being the oxide thickness, grown during DBD exposure time t. Oxide thicknesses of more than 3 nm could be achieved within 350 s DBD exposure time. Our analysis of infrared spectra, XPS, and ellipsometry leads us to conclude that the newly formed oxide is porous with a pore fraction of roughly 10%.
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68.37.Ps, 78.30.-j, 68.35.bg, 79.60.-i, 81.65.Mq, 81.65.Cf
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STRUCTURAL, MECHANICAL, THERMODYNAMIC, AND OPTICAL PROPERTIES OF CONDENSED MATTER

Void generation during the annealing process of very narrow copper wires

Yasushi Sasajima, Tomoaki Akabane, Takeshiro Nagai, Yasunori Chonan, and Jin Onuki

J. Appl. Phys. 105, 073501 (2009) (7 pages)

Online Publication Date: 1 April 2009

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We carried out experiments on stress-induced void formation in ultrathin Cu wires while varying heat-treatment temperature, wire dimensions, and overlayer thickness. We also did molecular dynamics simulations of void formation in a buried wire of nanometer scale and compared these results with experimental results to clarify details of the void formation mechanism. The experimental and simulation results showed good accordance in explaining the effects of wire width, overlayer thickness, and cooling rate on void formation. (1) The narrower the wire width, the easier the void formation. (2) The thicker the overlayer, the easier the void formation. (3) The larger the cooling rate, the greater the suppression of void formation. From the obtained results, we constructed a void formation model for a buried wire. The basic concept of the model describes how local strain at four trench corners is relaxed in the buried wire in the annealing process. There are two ways to relax the local strain: (1) structural relaxation to strengthen adhesion between the wire and substrate and (2) reduction of the surface area to minimize surface energy. The way preferred is dependent on how parameters such as system temperature and wire dimensions are combined. Based on the void formation model, we interpreted the effects of wire strain, wire dimensions, and overlayer thickness.
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61.72.Qq, 61.72.Cc, 84.32.Hh, 65.40.gp, 68.35.Np

Substrate temperature effects on laser crystallized NiTi thin films

A. J. Birnbaum, Ui-Jin Chung, X. Huang, James S. Im, A. G. Ramirez, and Y. L. Yao

J. Appl. Phys. 105, 073502 (2009) (10 pages)

Online Publication Date: 1 April 2009

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Amorphous sputter-deposited NiTi thin films were subjected to pulsed, melt-mediated laser crystallization techniques to engineer their microstructure. The effects of laser processing of preheated films are examined. Laser processing of films at an elevated substrate temperature has a significant effect on the rate with which solidification occurs. It is observed that the preheating temperature at which processing is carried out has significant implications for the resulting phase and microstructure, and therefore mechanical properties. Furthermore, the microstructural effects of varying incident laser energy density are examined via atomic force microscopy, scanning electron microscopy, and x-ray diffraction, and mechanical/shape memory properties are characterized via nanoindentation.
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64.70.kd, 42.62.-b, 68.60.Bs, 81.15.Cd, 81.40.Lm, 62.20.fg

Crystallographically driven Au catalyst movement during growth of InAs/GaAs axial nanowire heterostructures

Mohanchand Paladugu, Jin Zou, Ya-Nan Guo, Xin Zhang, Hannah J. Joyce, Qiang Gao, H. Hoe Tan, C. Jagadish, and Yong Kim

J. Appl. Phys. 105, 073503 (2009) (4 pages)

Online Publication Date: 1 April 2009

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The movement of Au catalysts during growth of InAs on GaAs nanowires has been carefully investigated by transmission electron microscopy. It has been found that Au catalysts preferentially stay on {112}B GaAs sidewalls. Since a {112} surface is composed of a {111} facet and a {002} facet and since {111} facets are polar facets for the zinc-blende structure, this crystallographic preference is attributed to the different interface energies caused by the different polar facets. We anticipate that these observations will be useful for the design of nanowire heterostructure based devices.
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68.65.La, 68.37.Og, 68.35.Md, 61.46.Km

Vacancy-assisted diffusion mechanism of group-III elements in ZnO: An ab initio study

Gui-Yang Huang, Chong-Yu Wang, and Jian-Tao Wang

J. Appl. Phys. 105, 073504 (2009) (7 pages)

Online Publication Date: 2 April 2009

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Based on ab initio total energy calculations, the diffusion mechanisms of group-III elements (B, Al, Ga, and In) in ZnO are investigated. The activation energy of vacancy-assisted mechanism consists of formation energy of Zn vacancy (VZn), binding energy between the dopants and VZn, as well as effective diffusion energy barrier of the dopants in ZnO. The effective diffusion energy barriers of B, Al, Ga, and In are estimated to be 1.12, 1.76, 1.45, and 1.06 eV for in-plane diffusion, and 1.12, 2.19, 1.80, and 1.06 eV for out-of-plane diffusion, respectively. The binding energies are estimated to be −0.66, −0.52, −0.48, and −0.43 eV for B-, Al-, Ga-, and In-VZn pairs, showing a size decreasing behavior.
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61.72.jd, 66.30.H-, 71.15.-m, 71.15.Nc

Femtosecond micromachining of internal voids in high explosive crystals for studies of hot spot initiation

S. D. McGrane, A. Grieco, K. J. Ramos, D. E. Hooks, and D. S. Moore

J. Appl. Phys. 105, 073505 (2009) (7 pages)

Online Publication Date: 3 April 2009

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Femtosecond micromachining was used to produce controlled patterns of internal voids in high explosive single crystals of 1,3-dinitrato-2,2-bis(nitratomethyl) propane (PETN), 1,3,5-trinitroperhydro-1,3,5-triazine (RDX) and 1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX). The micromachined voids were characterized with optical microscopy and confocal Raman microscopy. Optical microscopy established that the voids generated near the threshold energy were localized to submicrometer diameters. Increasing the micromachining energy above threshold led to microcracking along preferred crystalline planes. Consolidation of hundreds to thousands of individual voids allowed creation of defined two- and three-dimensional structures. Production of three-dimensional consolidated structures led to extended crystal damage or residual strain over tens to hundreds of micrometers. Confocal Raman microscopy established that the defects generated were voids, with no chemical products observable and with diminished crystal spectral intensity. The results of this work suggest that large controlled arrays of internal voids can be produced in explosive crystals, with the exception that continuous three-dimensional defect structures are possible only if the extended damage is acceptable. These methods and materials are expected to be valuable for controlled studies of hot spot initiation in shocked explosives.
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82.33.Vx, 61.72.Qq, 81.40.Np, 78.30.Jw, 42.62.Cf, 82.40.Fp

Issues on the pulse-width dependence and the shape of acoustic radiation induced static displacement pulses in solids

Karthik Thimmavajjula Narasimha, Elankumaran Kannan, and Krishnan Balasubramaniam

J. Appl. Phys. 105, 073506 (2009) (5 pages)

Online Publication Date: 3 April 2009

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Recent experimental results showing the pulse width independence and the flat topped shape of static displacement generated during finite amplitude sinusoidal ultrasonic tone burst propagation in solids and the contradicting previous results reported in the literature are considered. The pulse width independence is analytically confirmed and the flat topped shape is explained by considering the progressive spatial and time domain shapes of the static strain and displacement pulses. A numerical simulation of the finite amplitude longitudinal ultrasonic wave propagation in solids has been performed to further verify the pulse width independence of the static displacement pulse.
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62.65.+k, 62.20.-x

Quantum well intermixing of a quantum well structure grown on an InAsP metamorphic pseudosubstrate on InP

O. Hulko, D. A. Thompson, B. J. Robinson, and J. G. Simmons

J. Appl. Phys. 105, 073507 (2009) (6 pages)

Online Publication Date: 3 April 2009

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InGaAsP quantum well (QW) structures, with ~1% compressive strain, have been grown on an InAs0.21P0.79 metamorphic substrate layer (MSL) deposited on an InP substrate. For comparison, a similar QW structure is grown directly on InP. A consequence of growth on the MSL is to move the QW and barrier compositions outside the spinodal isotherm resulting in a significant reduction in phase separation. This is shown to increase the photoluminescence wavelength and improve its quality in terms of linewidth and intensity. Thermally induced QW intermixing (QWI) has also been carried out on these structures both with and without a low-temperature InP capping layer. The defects present in the underlying metamorphic buffer layers are shown to have no effect on the QWI process. However, the samples with the MSL are shown to exhibit more intermixing either due to a higher diffusion or greater P–As exchange probability in the different composition QW and barrier layers compared to the QW structure grown without the MSL, and possibly influenced by the large reduction in phase separation.
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68.65.Fg, 64.75.-g, 81.40.Lm, 62.20.F-, 78.55.Cr, 78.67.De

Clathrate guest atoms under pressure

M. Christensen, S. Johnsen, F. Juranyi, and B. B. Iversen

J. Appl. Phys. 105, 073508 (2009) (9 pages)

Online Publication Date: 6 April 2009

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Powder inelastic neutron scattering (INS) has been used to determine the guest atom “rattling” energy in thermoelectric clathrates Ba8YxGe46−x (Yx=Ni6,Cu6,Zn8,Ga16) under different applied conditions. Chemical pressure was exerted by the atomic substitution, and a physical pressure of 9 kbars was applied using a clamp cell. The volume reduction induced by the physical pressure increases the energy of the guest atom rattling mode, but the local chemical environment in the cage also appears to have a similar effect. The guest atom energies were investigated as function of temperature, and softening of the guest atom modes was observed upon cooling the sample. Ba8Ga16Ge30 with holes (p-type) and electrons (n-type) as charge carriers reveal similar temperature behavior, suggesting anharmonic potentials of similar shape for the Ba guest atom independent of the charge carrier type. For Sr8Ga16Ge30 a much stronger anharmonic potential was observed compared with Ba8Ga16Ge30. The guest atom energies for Ba8YxGe46−x (Yx=Ni6,Cu6,Zn8) extracted from powder INS were compared with Einstein energies obtained from atomic displacement parameters determined from multitemperature single crystal x-ray diffraction. Additionally, the Ba8YxGe46−x (Yx=Ni6,Cu6,Zn8) samples were characterized with respect to their thermoelectric properties.
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62.50.-p, 72.20.Pa, 63.70.+h, 63.20.-e

Effect of pressure on the magnetocaloric properties of nickel-rich Ni–Mn–Ga Heusler alloys

K. Mandal, D. Pal, N. Scheerbaum, J. Lyubina, and O. Gutfleisch

J. Appl. Phys. 105, 073509 (2009) (6 pages)

Online Publication Date: 6 April 2009

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Nickel-rich Ni–Mn–Ga Heusler alloys were prepared by arc melting and subsequent homogenization by annealing. A large magnetic entropy change was observed around 291 K in the alloy where martensite-austenite structural and ferro-para magnetic transitions almost coincide with each other. The effect of hydrostatic pressure of up to 8 kbar on magneto-structural transitions, magnetocaloric effect, and magnetic hysteresis was studied. The martensitic transition temperature as well as the Curie temperature TC was found to increase, whereas the magnetic entropy change |DeltaSM| decreases slightly due to the application of hydrostatic pressure. The large hysteresis observed in M versus H curve at the ambient pressure almost vanishes due to the application of 8 kbar pressure.
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75.30.Sg, 75.50.Cc, 81.30.Kf, 81.40.Gh, 75.60.Ej, 75.30.Kz

Indentation of single-crystal silicon nanolines: Buckling and contact friction at nanoscales

Bin Li, Qiu Zhao, Huai Huang, Zhiquan Luo, Min K. Kang, Jang-Hi Im, Richard A. Allen, Michael W. Cresswell, Rui Huang, and Paul S. Ho

J. Appl. Phys. 105, 073510 (2009) (7 pages)

Online Publication Date: 6 April 2009

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High-quality single-crystal silicon nanolines (SiNLs) with a 24 nm linewidth and a height/width aspect ratio of 15 were fabricated. The mechanical properties of the SiNLs were characterized by nanoindentation tests with an atomic force microscope. The indentation load-displacement curves showed an instability with large displacement bursts at a critical load ranging from 9 to 30  µN. This phenomenon was attributed to a transition of the buckling mode of the SiNLs under indentation, which occurred preceding the final fracture of the nanolines. The mechanics of SiNLs under indentation was analyzed by finite element simulations, which revealed two different buckling modes depending on the contact friction at the nanoscale.
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81.16.-c, 62.25.-g, 62.20.mq, 81.40.Lm, 62.20.mm, 62.25.Mn, 81.07.Bc, 62.20.Qp, 81.40.Pq

Sensitization of the photostimulable x-ray storage-phosphor CsBr:Eu2+ following room-temperature hydration

G. A. Appleby, J. Zimmermann, S. Hesse, O. Karg, and H. von Seggern

J. Appl. Phys. 105, 073511 (2009) (5 pages)

Online Publication Date: 6 April 2009

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The x-ray storage-phosphor CsBr:Eu2+ has been found to exhibit a strong increase in photostimulated-luminescence (PSL) intensity following hydration at room temperature. The sensitivity increases by a factor of 25 following a 60 min exposure to an atmosphere of 99% relative humidity, which is considered to be due to an incorporation of H2O molecules within the CsBr:Eu2+ matrix, which are orientated by and enhance the electric fields surrounding (Eu2+–O2−)-dipoles, increasing their charge trapping cross sections. Following irradiation the PSL increase is accompanied by the formation of mobile VK-centers, which decrease the F-center population by a recombination process, which appear as an intrinsic 360 nm emission from the CsBr matrix in low temperature thermoluminescence measurements.
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78.55.Hx, 72.20.Jv, 61.72.jn, 78.60.Kn, 61.80.-x, 72.80.Sk

Characterization of electrochemically grafted molecular layers on silicon for electronic device applications

Adina Scott and David B. Janes

J. Appl. Phys. 105, 073512 (2009) (9 pages)

Online Publication Date: 6 April 2009

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Recently, there has been considerable interest in developing organically functionalized silicon surfaces for a variety of applications including sensing and nanoelectronics. In this study, a series of as-deposited, para-substituted aryl-diazonium molecular layers covalently grafted to <111>-orientation silicon are characterized using a variety of surface analysis techniques. Collectively, these measurements suggest that relatively ideal molecular layers can be achieved with a variety of headgroups. Submonolayer amounts of silicon oxide are detected on all modified surfaces and the extent of silicon oxidation depends on the molecular substituent. For electronic device applications, it is necessary to apply contacts to molecular layers while maintaining their structural and chemical integrity. To this end, in situ spectroscopies are used to infer the effects of metallization on such molecular layers. It is found that applying gold using a soft evaporation technique does not significantly perturb the molecular layer, whereas the application of copper using the same technique induces changes in the molecular vibrational spectra. Two complementary in situ spectroscopic methods are analyzed to more accurately determine the chemical properties of gold/molecule/silicon junctions. The physical mechanisms of the measurements and consequences for interpretation of the resulting spectra are discussed.
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85.40.Ls, 78.30.-j

Amorphization and dynamic annealing of hexagonal SiC upon heavy-ion irradiation: Effects on swelling and mechanical properties

Xavier Kerbiriou, Jean-Marc Costantini, Maxime Sauzay, Stéphanie Sorieul, Lionel Thomé, Jacek Jagielski, and Jean-Jacques Grob

J. Appl. Phys. 105, 073513 (2009) (11 pages)

Online Publication Date: 7 April 2009

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Structural, mechanical, and dimensional evolutions of silicon carbide (SiC) induced by heavy-ion irradiations are studied by means of Rutherford backscattering spectrometry and channeling (RBS/C), nanoindentation, and surface profilometry measurements. 4H- and 6H-SiC single crystals were irradiated with 4 MeV Au2+ and 4 MeV Xe+ ions at room temperature (RT) or 400 °C. Using a Monte Carlo program to simulate the RBS/C spectra (MCCHASY code), we find that Au ion irradiation at RT induces a total silicon sublattice disorder related to full amorphization at a dose of about 0.4 displacement per atom (dpa). A two-step damage process is found on the basis of the disordered fractions deduced from RBS/C data. Complete amorphization cannot be reached upon both Au and Xe ion irradiations at 400 °C up to about 26 dpa because of the dynamic annealing of defects. When complete amorphization is reached at RT, the Young's modulus and Berkovich hardness of irradiated 6H-SiC samples are lower by, respectively, 40% and 45% than those of the virgin crystals. The out-of-plane expansion measured by surface profilometry increases versus irradiation dose and the saturation value measured in the completely amorphous layer (normalized to the ion projected range) is close to 25%. We show that the modifications of the macroscopic properties are mainly due to the amorphization of the material. The macroscopic elasticity constants and dimensional properties are predicted for a composite material made of crystalline matrix containing dispersed amorphous inclusions using simple analytical homogenization models. Voigt's model seems to give the best approximation for disordered fractions larger than 20% in the second step of the damage process.
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61.80.Jh, 81.05.Hd, 81.40.Jj, 62.20.de, 62.20.Qp, 62.20.dq, 61.72.Cc, 81.40.Gh, 62.20.me, 81.40.Np

Competing failure mechanisms in thin films: Application to layer transfer

L. Ponson, K. Diest, H. A. Atwater, G. Ravichandran, and K. Bhattacharya

J. Appl. Phys. 105, 073514 (2009) (9 pages)

Online Publication Date: 7 April 2009

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We investigate the origin of transverse cracks often observed in thin films obtained by the layer transfer technique. During this process, two crystals bonded to each other containing a weak plane produced by ion implantation are heated to let a thin layer of one of the material on the other. The level of stress imposed on the film during the heating phase due to the mismatch of thermal expansion coefficients of the substrate and the film is shown to be the dominent factor in determining the quality of the transferred layer. In particular, it is shown that if the film is submitted to a tensile stress, the microcracks produced by ion implantation are not stable and deviate from the plane of implantation making the layer transfer process impossible. However, if the compressive stress exceeds a threshold value, after layer transfer, the film can buckle and delaminate, leading to transverse cracks induced by bending. As a result, we show that the imposed stress sigmam—or equivalently the heating temperature—must be within the range −sigmac<sigmam<0 to produce an intact thin film where sigmac depends on the interfacial fracture energy and the size of defects at the interface between film and substrate.
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68.60.Bs, 62.20.mm, 62.20.mq, 81.40.Lm, 62.20.mt, 81.40.Np

Effect of spatial nonlocality on terahertz optical interaction with quantum wells

Guanghui Wang and Qi Guo

J. Appl. Phys. 105, 073515 (2009) (5 pages)

Online Publication Date: 7 April 2009

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Spatial nonlocal effects on optical absorption spectra from the terahertz optical interaction with semiconductor quantum wells (QWs) are investigated for two kinds of polarized states. The numerical results show that the spatial nonlocality of optical responses can lead to not only the blueshift but also the redshift of optical spectra, which depends on the QW depth. It is also demonstrated that the maximal radiation shift and the least optical absorbance can be obtained by adopting an appropriate step potential and polarization direction of incident terahertz wave. This work may provide some methods of designing the nanomaterials with large nonlocality and observing the spatial nonlocal effects by experiment.
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78.67.De, 78.70.Gq, 78.20.Bh

Modeling semiconductor nanostructures thermal properties: The dispersion role

Damian Terris, Karl Joulain, Denis Lemonnier, and David Lacroix

J. Appl. Phys. 105, 073516 (2009) (9 pages)

Online Publication Date: 7 April 2009

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We study heat transport in semiconductor nanostructures by solving the Boltzmann transport equation by means of the discrete ordinate method. Relaxation time and phase and group velocity spectral dependencies are taken into account. The Holland model of phonon relaxation time is revisited and recalculated from dispersion relations (taken in literature) in order to match bulk silicon and germanium values. This improved model is then used to predict silicon nanowire and nanofilm thermal properties in both ballistic and mesoscopic regimes.
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66.70.Lm, 63.22.Gh

Photoluminescence properties of SiOx thin films prepared by reactive electron beam evaporation from SiO and silica nanoparticles

Devendraprakash Gautam, Emi Koyanagi, and Takashi Uchino

J. Appl. Phys. 105, 073517 (2009) (8 pages)

Online Publication Date: 7 April 2009

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A simple reactive evaporation technique is proposed to obtain highly efficient light emitting Si nanostructures. It is shown that structurally homogeneous SiOx thin films are synthesized by a reactive electron beam evaporation from a mixture of SiO and silica nanoparticles. These SiOx thin films result in the formation of Si nanocrystals on annealing under Ar atmosphere, exhibiting efficient photoluminescence (PL) emission. The composition of SiOx thin films is varied by changing the molar concentration of silica in the mixture. X-ray photoelectron spectroscopy and Fourier transform infrared measurements demonstrate that a chemical reaction between SiO and silica nanoparticles occurs during electron beam irradiation, creating evaporated species in the form of SiOy, (1<y<2). The pivotal role of the homogeneous microscopic structure of as-deposited thin films on the resulting PL yield after annealing is proposed.
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78.66.Nk, 78.55.Hx, 81.15.Ef, 68.55.aj, 68.55.Nq, 78.30.Hv, 79.60.Dp, 81.40.Ef

Efficient multiphase green phosphor based on strontium thiogallate

Mihail Nazarov, Do Young Noh, Clare Chisu Byeon, and Hyojung Kim

J. Appl. Phys. 105, 073518 (2009) (8 pages)

Online Publication Date: 8 April 2009

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Efficient multiphase green phosphors based on strontium thiogallate were synthesized by solid state reaction at 900–1000 °C with carbon as a reduction atmosphere. The detailed x-ray diffraction analysis, energy dispersive spectroscopy, Raman spectroscopy, and photoluminescence were carried out in order to enlarge the understanding of the radiative processes in multiphase green phosphors based on strontium thiogallate. The crystal field splitting, Stokes shift, redshift, and centroidal shift were estimated, and the results are in good agreement with mathematical calculations. Our study confirms that the proposed multiphase green phosphor based on strontium thiogallate is a good candidate for solid state lighting and display devices with higher intensity and better chromaticity coordinates in comparison with the known commercial phosphors.
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81.20.Ka, 78.55.Hx, 78.30.Hv, 71.70.Ch, 85.60.Pg

Magnetic anomaly in Ni51.5Fe21.5Ga27 single crystalline ferromagnetic shape memory alloy studied by ac impedance measurements

M. L. Corró, S. Kustov, E. Cesari, and Y. I. Chumlyakov

J. Appl. Phys. 105, 073519 (2009) (5 pages)

Online Publication Date: 8 April 2009

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A magnetic anomaly in the austenitic state of Ni51.5Fe21.5Ga27 single crystalline ferromagnetic shape memory alloy has been studied by means of ac impedance measurements. A much stronger effect of the degree of atomic order on the temperature of this anomaly (as compared to the temperature of the martensitic and para-ferromagnetic transitions) has been found. It has been shown that apart from the previously reported slight variation in the saturation magnetization, the magnetic anomaly results in a nearly one order of magnitude change in the value of initial magnetic permeability. The anomaly is not revealed in the resistive impedance at low frequencies, pointing likely to its purely magnetic origin.
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75.60.Ej, 75.50.Bb, 64.70.kd, 75.30.Kz, 81.30.Kf

Optical properties of beta-Sn films

Katsuki Takeuchi and Sadao Adachi

J. Appl. Phys. 105, 073520 (2009) (6 pages)

Online Publication Date: 10 April 2009

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Optical properties of white tin (beta-Sn) have been investigated using spectroscopic ellipsometry in the photon-energy range between 0.6 and 6.5 eV at room temperature. The beta-Sn films are deposited by vacuum evaporation on Si(001) substrates. The structural properties of the films are evaluated by x-ray diffraction and ex situ atomic force microscopy. The measured epsilon(E) spectra reveal distinct structures at several interband critical points in the Brillouin zone of beta-Sn. These spectra are analyzed on the basis of a simplified model of the interband transitions, including the free-carrier absorption between the filled and empty electronic states. Dielectric-related optical constants, such as the complex refractive index, absorption coefficient, and normal-incidence reflectivity, of bulk beta-Sn films are also presented.
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78.66.Bz, 71.20.Be, 81.15.Ef, 61.05.cp, 68.37.Ps, 78.20.Ci, 68.55.-a, 81.05.Bx

Pulsed laser deposited tetrahedral amorphous carbon with high sp3 fractions and low optical bandgaps

Y. Miyajima, S. J. Henley, G. Adamopoulos, V. Stolojan, E. Garcia-Caurel, B. Drévillon, J. M. Shannon, and S. R. P. Silva

J. Appl. Phys. 105, 073521 (2009) (8 pages)

Online Publication Date: 15 April 2009

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Amorphous carbon films with sp3 bonded carbon fractions over 70% are deposited by pulsed laser deposition. However, the optical bandgap obtained from optical transmittance and spectroscopic ellipsometry analysis shows the values to be below 1.0 eV. A wide range of measurements such as electron energy loss spectroscopy, visible Raman, spectroscopic ellipsometry, optical transmittance, and electrical characterization are performed to elucidate the bonding configurations that dictate microstructural, optical and electrical properties, and their linkage to band structure changes. It is found that stress-induced electronic localized states play an important role in the physical properties of the films deposited. The optical bandgap is shown not to be a good measure of the electrical bandgap, especially for high electric field conduction in these tetrahedral amorphous carbon films.
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78.66.Nk, 73.61.Ng, 78.20.Ci, 68.55.aj, 68.55.-a, 81.15.Fg
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ELECTRONIC STRUCTURE AND TRANSPORT

Long-range order on the atomic scale induced at CoFeB/MgO interfaces

Gerrit Eilers, Henning Ulrichs, Markus Münzenberg, Andy Thomas, Karsten Thiel, and Michael Seibt

J. Appl. Phys. 105, 073701 (2009) (4 pages)

Online Publication Date: 1 April 2009

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The amorphous (a-) CoFeB/crystalline (c-) MgO based tunneling system interface has been studied by means of quantitative high resolution transmission electron microscopy from atomic to micrometer length scales with increasing annealing temperatures. On the micron scale an irregular nucleation is found. On the atomic scale a long-range order is induced by the MgO interface, explaining the high tunnel magnetoresistance values >100% even for not fully crystallized CoFeB/MgO/CoFeB tunnel junctions.
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75.70.Cn, 75.47.-m, 68.65.Ac, 81.40.Gh

Synthesis and thermoelectric properties of type-VIII germanium clathrates Sr8AlxGayGe46−x−y

Yuta Sasaki, Kengo Kishimoto, Tsuyoshi Koyanagi, Hironori Asada, and Koji Akai

J. Appl. Phys. 105, 073702 (2009) (8 pages)

Online Publication Date: 1 April 2009

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Nominal Sr8AlxGa16−xGe30 samples with x=6, 8, and 10 crystallized in the type-VIII clathrate structure (I[overline 4]3m, No. 217), while the sample with x=4 crystallized in the type-I clathrate structure (Pm[overline 3]n, No. 223). While a large number of the type-I thermoelectric clathrates exist, only three type-VIII clathrates of Ba8Ga16Sn30, Eu8Ga16Ge30, and Sr8AlxGa16−xSi30 had been synthesized before. The type-VIII Sr8AlxGayGe46−x−y samples (6<=x<=7 and 10<=y<=11) with various carrier concentrations were prepared to investigate their thermoelectric properties. They exhibited the temperature dependences of electrical conductivities and the Seebeck coefficients typical of n type degenerate semiconductors, which almost depended on their carrier concentrations systematically. A relatively large dimensionless figure-of-merit ZT of 0.56 at 800 K was obtained for the type-VIII Sr8Al6.3Ga10.3Ge29.4 sample with a carrier concentration of 3.0×1020  cm−3. This ZT value is comparable to that of 0.62 at 800 K for the type-I Sr8Ga16.5Ge29.5 clathrate. The type-VIII clathrate had a smaller effective mass, a higher mobility, and a higher lattice thermal conductivity than those of the type-I clathrate. The difference in transport properties between the type-I and type-VIII clathrates is also discussed.
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72.20.Pa, 81.20.-n, 71.18.+y, 72.20.Ee, 72.80.Jc, 66.70.Df

Quantum confinement effect on the effective mass in two-dimensional electron gas of AlGaN/GaN heterostructures

A. M. Kurakin, S. A. Vitusevich, S. V. Danylyuk, H. Hardtdegen, N. Klein, Z. Bougrioua, A. V. Naumov, and A. E. Belyaev

J. Appl. Phys. 105, 073703 (2009) (6 pages)

Online Publication Date: 1 April 2009

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We report the results of direct measurements and a theoretical investigation of the in-plane effective mass in the two-dimensional electron gas of nominally undoped AlGaN/GaN heterostructures with a different degree of quantum confinement. It is shown that in most cases the conduction band nonparabolicity effect is overestimated and the electron wave-function penetration into the barrier layer should be taken into account. The contribution of the wave-function hybridization is determined to play the dominant role. The band edge effective mass value is deduced to be (0.2±0.01)m0.
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73.21.-b, 71.18.+y

Resistance switching properties of planner Ag/Li:NiO/Ag structures induced by swift heavy ion irradiation

U. S. Joshi, S. J. Trivedi, K. H. Bhavsar, U. N. Trivedi, S. A. Khan, and D. K. Avasthi

J. Appl. Phys. 105, 073704 (2009) (4 pages)

Online Publication Date: 1 April 2009

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We report on the resistance switching induced by swift heavy ion (SHI) irradiation in lithium doped nanostructured NiO thin films grown on MgO (100) substrates by chemical solution deposition. Hysteresis in current-voltage curves were observed for the Ag/Li:NiO/Ag planner structures irradiated with 100 MeV Ag+14 ions, whereas pristine samples showed only linear I-V characteristics. No preferential oxygen loss from the film surface has been detected in on-line elastic recoil detection analysis. This suggests that change in the defect density created by SHI irradiation that may contribute to the metallic filaments play a major role as compared to the interfacial oxygen vacancies in resistance switching of NiO.
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73.40.Rw, 61.72.jd, 81.15.Lm, 61.82.Rx, 73.63.Bd, 61.80.Jh

Electrical activity of deep traps in high resistivity CdTe: Spectroscopic characterization

B. Fraboni, D. Cavalcoli, A. Cavallini, and P. Fochuk

J. Appl. Phys. 105, 073705 (2009) (6 pages)

Online Publication Date: 2 April 2009

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The electrical compensation processes of high resistivity CdTe is controlled by deep levels. We have characterized the electrical activity of deep traps by means of three different and complementary spectroscopic methods: photoinduced current transient spectroscopy, surface photovoltage spectroscopy, and space charge limited current analyses. The aim is twofold: to achieve a thorough characterization of the deep trap properties and to assess the potentiality and limitations of the three experimental techniques by a cross correlation of the results obtained with each one of them. We have obtained a direct quantitative estimate of the major deep trap concentration, and we have assessed the sensitivity limit in deep-level detection for surface photovoltage spectroscopy.
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72.80.Ey, 78.66.Hf, 61.72.uj, 72.20.Ht, 71.55.Gs, 72.40.+w

Electronic structure of amorphous silicon oxynitride with different compositions

A. N. Sorokin, A. A. Karpushin, V. A. Gritsenko, and H. Wong

J. Appl. Phys. 105, 073706 (2009) (4 pages)

Online Publication Date: 2 April 2009

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A revised tight-binding Hamiltonian parametrization scheme for calculating the electronic structure of both covalent and ion-covalent solids is proposed. Unlike the conventional approaches, the present nonempirical calculation does not rely on any empirical parameter. We use the atomic properties of isolated atoms to construct the matrix elements. Results show that the predicted charge transfer in SiO2 and Si3N4, the bandgap energies of SiO2, Si3N4, and SiOxNy films, as well as the electron and hole barriers at the Si/SiO2, Si/Si3N4, and Si/SiOxNy interfaces are in good agreement with the experimental findings reported in literatures.
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71.23.An, 77.84.Bw, 71.70.-d, 71.15.Ap

Electronic properties of titanium and chromium impurity centers in germanium

J. Lauwaert, J. Van Gheluwe, J. Vanhellemont, E. Simoen, and P. Clauws

J. Appl. Phys. 105, 073707 (2009) (6 pages)

Online Publication Date: 2 April 2009

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The electronic properties of the 3d transition metal impurities titanium and chromium in crystalline germanium have been investigated by means of deep level transient spectroscopy. The metals were implanted at 90  keV and diffused deeper into the bulk during a thermal anneal at 500  °C, which yielded spectra specific for the implanted metal. It was found that Ti introduces a deep electron trap and a semishallow hole trap. For Cr one deep electron trap and three hole traps were observed. The capture cross section of the electron traps is thermally activated while three of the hole traps display electric field enhanced emission, which is in agreement with multiple-acceptor states of metal impurities on substitutional sites. It is concluded that Ti in germanium is a double acceptor, while Cr is a triple acceptor with, in addition, a donor level close to the valence band.
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71.55.Cn, 81.40.Ef, 71.20.Mq

Low temperature electrical transport properties of carbon matrix containing iron nanoparticles

E. P. Sajitha, V. Prasad, and S. V. Subramanyam

J. Appl. Phys. 105, 073708 (2009) (6 pages)

Online Publication Date: 6 April 2009

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We present a comparative study of the low temperature electrical transport properties of the carbon matrix containing iron nanoparticles and the films. The conductivity of the nanoparticles located just below the metal-insulator transition exhibits metallic behavior with a logarithmic temperature dependence over a large temperature interval. The zero-field conductivity and the negative magnetoresistance, showing a characteristic upturn at liquid helium temperature, are consistently explained by incorporating the Kondo relation and the two dimensional electron-electron interaction. The films, in contrast, exhibit a crossover of the conductivity from power-law dependence at high temperatures to an activated hopping law dependence in the low temperature region. The transition is attributed to changes in the energy dependence of the density of states near the Fermi level. The observed magnetoresistance is discussed in terms of quantum interference effect on a three-dimensional variable range hopping mechanism.
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72.20.Ee, 71.30.+h, 61.46.Df, 75.50.Tt, 71.20.Ps, 72.20.My, 73.61.Ng

Thermal transport properties of polycrystalline tin-doped indium oxide films

Toru Ashida, Amica Miyamura, Nobuto Oka, Yasushi Sato, Takashi Yagi, Naoyuki Taketoshi, Tetsuya Baba, and Yuzo Shigesato

J. Appl. Phys. 105, 073709 (2009) (4 pages)

Online Publication Date: 13 April 2009

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Thermal diffusivity of polycrystalline tin-doped indium oxide (ITO) films with a thickness of 200 nm has been characterized quantitatively by subnanosecond laser pulse irradiation and thermoreflectance measurement. ITO films sandwiched by molybdenum (Mo) films were prepared on a fused silica substrate by dc magnetron sputtering using an oxide ceramic ITO target (90  wt % In2O3 and 10  wt % SnO2). The resistivity and carrier density of the ITO films ranged from 2.9×10−4 to 3.2×10−3  Omega cm and from 1.9×1020 to 1.2×1021  cm−3, respectively. The thermal diffusivity of the ITO films was (1.5–2.2)×10−6  m2/s, depending on the electrical conductivity. The thermal conductivity carried by free electrons was estimated using the Wiedemann–Franz law. The phonon contribution to the heat transfer in ITO films with various resistivities was found to be almost constant (lambdaph=3.95  W/m K), which was about twice that for amorphous indium zinc oxide films.
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66.30.Xj, 68.55.ag, 78.20.Nv, 81.15.Cd, 72.20.Jv, 78.66.Li

Thermoelectric power factors of nanocarbon ensembles as a function of temperature

D. M. Gruen, P. Bruno, R. Arenal, J. Routbort, D. Singh, and M. Xie

J. Appl. Phys. 105, 073710 (2009) (10 pages)

Online Publication Date: 13 April 2009

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Thermoelectric power factors of nanocarbon ensembles have been determined as a function of temperature from 400 to 1200 K. The ensembles, composed of mixtures of nanographite or disperse ultrananocrystalline diamond with B4C, are formed into mechanically rigid compacts by reaction at 1200 K with methane gas and subsequently annealed in an argon atmosphere at temperatures up to 2500 K. The ensembles were characterized using scanning electron microscopy, Raman, x-ray diffraction, and high resolution transmission electron microscopy techniques and found to undergo profound nanostructural changes as a function of temperature while largely preserving their nanometer sizes. The power factors increase strongly both as a function of annealing temperature and of the temperature at which the measurements are carried out reaching 1  µW/K2 cm at 1200 K without showing evidence of a plateau. Density functional “molecular analog” calculations on systems based on stacked graphene sheets show that boron substitutional doping results in a lowering of the Fermi level and the creation of a large number of hole states within thermal energies of the Fermi level [P. C. Redfern, D. M. Greun, and L. A. Curtiss, Chem. Phys. Lett. 471, 264 (2009)]. We propose that enhancement of electronic configurational entropy due to the large number of boron configurations in the graphite lattice contributes to the observed thermoelectric properties of the ensembles.
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72.20.Pa, 73.63.-b, 78.30.Hv, 78.67.-n, 73.22.-f, 81.05.Uw

Electrical transport measurements of highly conductive carbon nanotube/poly(bisphenol A carbonate) composite

Seamus A. Curran, Jamal Talla, Sampath Dias, Donghui Zhang, David Carroll, and Donald Birx

J. Appl. Phys. 105, 073711 (2009) (5 pages)

Online Publication Date: 13 April 2009

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Acid-treated and pristine chemical vapor deposition grown multiwalled carbon nanotube (MWNT) and poly(bisphenol A carbonate) (PC) composites were prepared through a simple solution blending with varied nanotube weight fractions. The electrical conductivities of the composites can be described by the scaling law based on percolation theory with unprecedented high saturated ac conductivity of pristine nanotubes (sigmasat=1598.4  S cm−1,  pc=0.19  wt %) and acid-treated nanotubes (sigmasat=435.4  S cm−1,  pc=0.3  wt %), which correlates well with the dc behavior. We attribute the high saturated conductivities to managing the dispersions, rather than looking to have a well dispersed three-dimensional network thin film. The comparison was made between acid-treated nanotubes and pristine nanotube, both dispersed in PC at various loadings. It was found that the pristine nanotubes in PC possessed an even higher conductivity than the more evenly dispersed composites consisting of lightly acid-treated MWNT in PC.
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73.63.Fg, 72.80.Tm, 81.05.Qk, 81.07.-b, 81.16.-c, 81.15.Gh

Investigation of iron impurity gettering at dislocations in a SiGe/Si heterostructure

Jinggang Lu, Xuegong Yu, Yongkook Park, and George Rozgonyi

J. Appl. Phys. 105, 073712 (2009) (6 pages)

Online Publication Date: 14 April 2009

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This article examined the electrical activities of dislocations in a SiGe/Si heterostructure by deep level transient spectroscopy (DLTS) after iron contamination and phosphorous diffusion gettering. DLTS of iron contaminated samples revealed a peak at 210 K, which was assigned to individual iron atoms or very small (<2  nm) precipitates decorated along dislocations, considering that the iron contamination annealing was terminated by quenching and transmission electron microscopy did not reveal any precipitate at dislocations. Arrhenius plot of the 210 K peak yielded a hole capture cross section of 2.4×10−14  cm2 and an energy level of 0.42 eV above the valance band. The relatively large hole capture cross section indicates strong interactions between iron-related deep levels and the dislocation shallow bands. DLTS of the iron contaminated sample revealed that 6×1014  cm−3 of boron can more effectively trap interstitial iron at room temperatures than the strain field/defect sites at 107–108  cm−2 dislocations. Phosphorous diffusion experiments revealed that the gettering efficiency of iron impurities depends on the dislocation density. For regions of high dislocation density, phosphorous diffusion cannot remove all iron impurities decorated at dislocations, suggesting a strong binding of iron impurities at dislocation core defects.
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73.40.Lq, 71.55.Cn, 66.30.Lw, 72.80.Cw, 72.20.Jv, 61.72.Cc, 61.72.Yx

Enhanced thermoelectric performance of AgSbTe2 synthesized by high pressure and high temperature

Taichao Su, Xiaopeng Jia, Hongan Ma, Fengrong Yu, Yongjun Tian, Guihong Zuo, Youjin Zheng, Yiping Jiang, Dan Dong, Le Deng, Bingke Qin, and Shizhao Zheng

J. Appl. Phys. 105, 073713 (2009) (4 pages)

Online Publication Date: 15 April 2009

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Near single phase ternary bulk thermoelectric material AgSbTe2 was synthesized by high pressure and high temperature (HPHT) method. The temperature-dependent thermoelectric properties including Seebeck coefficient, electrical conductivity, and thermal conductivity were studied. The HPHT synthesized AgSbTe2 sample has higher thermoelectric performance in the measured temperature range than that of the same sample prepared at normal pressure. The enhanced thermoelectric properties should be attributed to the HPHT quenching which keeps partially the high electrical conductivity of AgSbTe2 under high pressure.
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72.20.Pa, 72.80.Jc, 81.05.Hd, 62.50.-p, 81.40.Gh, 66.70.Df
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MAGNETISM AND SUPERCONDUCTIVITY

Antiferromagnetic-spin-fluctuation-mediated pairing as a likely mechanism for unconventional superconductivity in LaAg1−cMnc alloys

S. Kumar, S. N. Kaul, J. Rodríguez Fernández, and L. Fernández Barquín

J. Appl. Phys. 105, 073901 (2009) (14 pages)

Online Publication Date: 1 April 2009

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Electrical resistivity, ac magnetic susceptibility, specific heat, dc magnetization, and dc magnetic susceptibility of superconducting LaAg1−cMnc alloys with c=0.0, 0.025, 0.05. 0.1, 0.2, and 0.3 have been measured in the temperature range of 0.35  K<=T<=300  K at external magnetic fields ranging from 0 to 90 kOe with a view to unravel the exact nature of the superconducting ground state. In these alloys, each Mn atom carries a magnetic moment of ~=4µB. A comparison of the results of these investigations with the predictions of the existing theoretical models permits us to make a number of interesting observations that include the following. The intermetallic compound LaAg is an archetypal Bardeen–Cooper–Schrieffer (BCS) spin-singlet isotropic even-parity s-wave superconductor with a superconducting transition temperature of Tc=0.97  K. At low solute concentrations of c[approximate]0.03, Mn substitutes for La at the La sublattice sites in the LaAg parent compound and Tc suddenly drop from 0.97 to temperatures below 0.35 K, reflecting thereby the destruction of conventional phonon-mediated s-wave superconductivity of the LaAg host by pair-breaking magnetic (Mn) impurities. At a threshold concentration of Mn, c~=0.05 (which corresponds to the antiferromagnetic instability/critical phase boundary in the magnetic phase diagram), superconducting gap opens up, Tc abruptly shoots up to 5 K, and unconventional superconductivity sets in at ambient pressure for T<=Tc. Beyond this threshold concentration, Mn has exclusive site preference for Ag at the Ag sublattice sites in LaAg and Tc increases from 5 to 6 K. The unconventional nature of superconductivity at these solute concentrations is signaled by strong departures from the BCS predictions. We present ample experimental evidence that favors antiferromagnetic-spin-fluctuation-mediated pairing as the most likely mechanism for the unconventional (d-wave) superconductivity observed in LaAg1−cMnc alloys with c>=0.05.
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74.25.Ha, 74.20.Rp, 74.20.Mn, 74.25.Fy, 75.30.Ds, 74.70.Ad

Fundamental and harmonic submillimeter-wave emission from parallel Josephson junction arrays

Faouzi Boussaha, Morvan Salez, Alexandre Féret, Benoit Lecomte, Christine Chaumont, Michel Chaubet, Frédéric Dauplay, Yan Delorme, and Jean-Michel Krieg

J. Appl. Phys. 105, 073902 (2009) (7 pages)

Online Publication Date: 1 April 2009

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We report heterodyne measurements of Josephson microwave radiation emitted by a parallel array of small superconductor-insulator-superconductor (SIS) junctions at submillimeter-wave frequencies. The array consists of five Nb/Al–AlOx/Nb junctions nonevenly distributed in a niobium superconducting stripline, and is optimized for rf coupling in the 450–640 GHz range. We observed Fiske-like resonant steps in its I-V curve in the presence of magnetic field. The device was placed in a waveguide mount, and its radiation was quasioptically coupled out of the cryostat, to a SIS-mixer spectrometer in the same frequency range, with a 4–8 GHz band for spectral analysis. We detected a coherent signal in the spectra when the array was biased on the first and third steps, respectively, at the first harmonic frequency of 242 GHz and at the fundamental frequency of 493 GHz, both being the Josephson frequencies associated with their dc voltages. This strongly suggests that this type of parallel arrays optimized for wideband rf coupling, though strongly discretized, hosts dynamic fluxon regimes similar to long Josephson junctions, which could find applications in superconducting digital electronics or integrated heterodyne submillimeter-wave receivers.
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85.25.Cp, 84.40.Az

Epitaxial film growth and optoelectrical properties of layered semiconductors, LaMnXO (X=P, As, and Sb)

Kentaro Kayanuma, Hidenori Hiramatsu, Toshio Kamiya, Masahiro Hirano, and Hideo Hosono

J. Appl. Phys. 105, 073903 (2009) (7 pages)

Online Publication Date: 2 April 2009

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Thin films of LaMnXO (X=P, As, and Sb), which are isostructural compounds of the newly discovered superconductor, LaFeAsO, were grown epitaxially on MgO (001) substrates at ~680 °C by pulsed laser deposition. Postdeposition thermal annealing at 1000 °C in evacuated silica glass ampoules improved the crystallinity and orientation for the LaMnPO and LaMnAsO films, but it led to the phase segregation of the LaMnSbO film. Thermopower and optical absorption measurements revealed that all the films are p-type semiconductors with indirect bandgaps from 1.0 to 1.4 eV, which are supported by density functional calculations with the GGA+U approximation.
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68.55.ag, 78.66.Li, 72.40.+w, 73.50.Lw, 81.15.Fg, 81.16.Mk

Patterned media with composite structure for writability at high areal recording density

Rachid Sbiaa, Kyaw Oo Aung, S. N. Piramanayagam, Ei-Leen Tan, and Randall Law

J. Appl. Phys. 105, 073904 (2009) (4 pages)

Online Publication Date: 2 April 2009

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Writability in bit-patterned media (BPM) is a critical issue for high areal densities. In this study, magnetization reversal for multilayer of (Co/Pd) nanodots was investigated using magnetic force microscopy. We observed an increase of more than 15 times in switching field (Hsw) in BPM over that of continuous films. An exchange coupled structure made of a thin Co layer with in-plane magnetization and high perpendicular anisotropy layer of (Co/Pd) multilayer to reduce the switching field is proposed. When the Co layer is thinner than 2 nm, its magnetization is aligned perpendicular to the film plane due to the exchange coupling with the 15 nm thick (Co/Pd) multilayer. The thin Co layer helps in reducing the Hsw of (Co/Pd) by almost 50% and also its distribution by 57% as measured from remanence magnetization.
Show PACS
75.70.Cn, 75.60.Jk, 75.30.Gw, 85.70.Kh, 75.60.Ej, 75.50.Ss

Coexistence of superparamagnetic and superspin glass behaviors in Co50Ni50 nanoparticles embedded in the amorphous SiO2 host

M. Thakur, M. Patra, S. Majumdar, and S. Giri

J. Appl. Phys. 105, 073905 (2009) (6 pages)

Online Publication Date: 2 April 2009

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We investigate the magnetic properties of the nanoparticles of Co50Ni50 alloy embedded in the amorphous SiO2 host with volume fractions, phi=5% and 10%. The static and dynamic aspects of the magnetic properties are investigated by temperature and time dependence of low-field dc magnetization and frequency dependence of ac susceptibilities. The experimental results and analyses suggest that the dilution of the Co50Ni50 nanoparticles belongs to the crossover regime from superparamagnetic to superspin glass states, which are distinguished from the atomic canonical spin glasses.
Show PACS
75.75.+a, 75.50.Lk, 75.50.Tt, 75.40.Cx, 75.30.Cr, 75.20.En

Thermopower of Co-doped FeSe

Evan Lyle Thomas, Winnie Wong-Ng, Daniel Phelan, and Jasmine N. Millican

J. Appl. Phys. 105, 073906 (2009) (5 pages)

Online Publication Date: 3 April 2009

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The effect of cobalt doping on the thermopower of the superconductor FeSe is investigated through electrical and thermal transport measurements. Our results point to the destruction of superconductivity at very low Co concentrations. Thermopower data suggest negative charge carriers along with a large enhancement of the Seebeck coefficient, S, on the order of [approximate]−80  µV/K near T=100  K.
Show PACS
74.25.Fy, 61.72.U-, 74.70.-b

Structural and magnetic properties of magnetron sputtered Co70Fe30 films on GaAs(110)

A. T. Hindmarch, A. K. Suszka, M. MacKenzie, J. N. Chapman, M. Henini, D. Taylor, B. J. Hickey, and C. H. Marrows

J. Appl. Phys. 105, 073907 (2009) (8 pages)

Online Publication Date: 3 April 2009

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The surface morphology, crystal structure, and in-plane magnetic anisotropy of sputter deposited Co70Fe30 films on GaAs(110) epilayer substrates have been investigated. The surface morphology of thin Co70Fe30 films appears to closely follow that of the underlying GaAs(110) substrate. The study of the crystal structure by x-ray diffraction shows that no crystallographic orientation other than Co70Fe30(110) is present in our films, although we cannot unambiguously deconvolute characteristic reflections of Co70Fe30(110) from those of GaAs(110) due to the very small lattice mismatch. High-resolution transmission electron microscopy shows that the crystal structure of the Co70Fe30 film is coherently matched to the GaAs(110) substrate. The magnetic anisotropies in both 35 and 1100 Å thick films are consistent with those of similar epitaxial films, and demonstrate that a strong bcc (110) texture is propagated throughout the entire film thickness. The cubic and uniaxial anisotropy constants extracted by fitting with the Stoner–Wohlfarth model are consistent with those of molecular beam epitaxy grown bcc CoxFe(100−x)/GaAs(110). This work shows that sputter deposition may be used to produce structurally coherent bcc-Co70Fe30/GaAs(110) contacts suitable for spin-injection applications.
Show PACS
68.55.jd, 68.55.jm, 68.35.Ct, 68.37.Og, 72.25.Mk, 75.30.Gw, 75.70.Cn, 75.70.Ak

Reversal magnetoresistance effect in ferromagnet/superconductor/ferromagnet/antiferromagnet heterostructure

Lin Peng, Chuanbing Cai, Changzhao Chen, Feng Fan, Xiaoqi Wang, and Zhiyong Liu

J. Appl. Phys. 105, 073908 (2009) (4 pages)

Online Publication Date: 3 April 2009

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The oxide heterostructure consisting of La0.7Sr0.3MnO3/YBa2Cu3O7-delta/La0.7Sr0.3MnO3/La0.33Ca0.67MnO3 is fabricated by using the technique of pulsed laser deposition. The magnetoresistance (MR) for the system is investigated in the region of mixed vortex state near the onset temperature of superconducting transition. The present experimental results indicate a reversal variation in MR that is observed with increasing temperature in the system, implying the competition between a positive MR of YBa2Cu3O7-delta and a negative MR of La0.7Sr0.3MnO3. Furthermore, our analysis further suggests that the large MR peaks are mainly caused by spin-dependent interface scattering, which depends on the relative orientation of magnetization in two ferromagnetic layers. And it is proved that the superconductivity is an essential factor for the large positive MR in the artificial heterostructure system.
Show PACS
74.50.+r, 74.25.Fy, 74.78.Fk, 74.25.Ha, 75.70.Cn, 74.72.Bk

Spin-resolved photoemission of a ferromagnetic Mn5Ge3(0001) epilayer on Ge(111)

Yu. S. Dedkov, M. Holder, G. Mayer, M. Fonin, and A. B. Preobrajenski

J. Appl. Phys. 105, 073909 (2009) (4 pages)

Online Publication Date: 3 April 2009

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Here we present a study of the electronic structure of epitaxial ferromagnetic Mn5Ge3(0001) films on Ge(111) by means of x-ray absorption spectroscopy, x-ray photoelectron spectroscopy, and spin-resolved photoelectron spectroscopy. Spin-polarization (P) value of +(15±5)% at the Fermi energy (EF) is measured with a photon energy of hnu=21.2  eV. Our findings are in contrast to recent band structure calculations, predicting P=−41% at EF for the ferromagnetic bulk Mn5Ge3.
Show PACS
71.20.Nr, 79.60.Bm, 78.70.Dm, 75.30.Cr, 75.70.Ak, 75.50.Dd, 75.50.Pp, 75.60.Ej, 72.25.Dc

Magnetic anisotropy and spin wave relaxation in CoFe/PtMn/CoFe trilayer films

Y. H. Ren, C. Wu, Y. Gong, C. Pettiford, and N. X. Sun

J. Appl. Phys. 105, 073910 (2009) (6 pages)

Online Publication Date: 6 April 2009

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We investigated the magnetic anisotropic properties and the spin wave relaxation in trilayer films of CoFe/PtMn/CoFe grown on the seed layer Ru or NiFeCr with CoFe compositions being Co–16  at. %  Fe. The measurements were taken in samples with the ferromagnetic layers of CoFe varying from 10 to 500 Å by the ferromagnetic resonance (FMR) technique. The magnetic anisotropic parameters were investigated by rotating the field aligned axis with respect to the spectral field in the configurations of both in plane and out of plane. We determine the effective in-plane anisotropy field of ~0.005  T, the uniaxial out-of-plane anisotropy of ~−0.3  T, and the exchange stiffness D of ~512  meV Å2. Moreover, spin wave damping was estimated by analyzing the FMR linewidth and line shape as a function of the angle between the external field and easy axis and as a function of the thickness of the CoFe layers. We identify an extrinsic contribution of the damping parameter dominated by two-magnon scattering in addition to the intrinsic Gilbert term with a damping parameter, alpha=0.012. Further, we reveal that a significant linewidth broadening could also be caused by the overlap of the surface and the uniform spin wave excitations. The FMR lines show a strong dependence of the surface anisotropy contribution of free energy in trilayer films.
Show PACS
75.30.Gw, 75.30.Ds, 76.50.+g, 75.70.Rf, 75.50.Bb, 75.70.Cn

Room-temperature ferromagneticlike behavior in Mn-implanted and postannealed InAs layers deposited by molecular beam epitaxy

R. González-Arrabal, Y. González, L. González, M. García-Hernández, F. Munnik, and M. S. Martín-González

J. Appl. Phys. 105, 073911 (2009) (6 pages)

Online Publication Date: 6 April 2009

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We report on the magnetic and structural properties of Ar- and Mn-implanted InAs epitaxial films grown on GaAs (100) by molecular beam epitaxy and the effect of rapid thermal annealing (RTA) for 30 s at 750 °C. Channeling particle induced x-ray emission (PIXE) experiments reveal that after Mn implantation almost all Mn atoms are substitutional in the In site of the InAs lattice, like in a diluted magnetic semiconductor. All of these samples show diamagnetic behavior. However, after RTA treatment the Mn–InAs films exhibit room-temperature magnetism. According to PIXE measurements the Mn atoms are no longer substitutional. When the same set of experiments was performed with Ar as implantation ion, all of the layers present diamagnetism without exception. This indicates that the appearance of room-temperature ferromagneticlike behavior in the Mn–InAs-RTA layer is not related to lattice disorder produced during implantation but to a Mn reaction produced after a short thermal treatment. X-ray diffraction patterns and Rutherford backscattering measurements evidence the segregation of an oxygen-deficient MnO2 phase (nominally MnO1.94) in the Mn–InAs-RTA epitaxial layers which might be the origin of the room-temperature ferromagneticlike response observed.
Show PACS
75.70.Ak, 61.72.Cc, 81.15.Hi, 75.50.Pp, 68.55.Ln, 75.20.Ck

Microwave-assisted switching of NiFe magnetic microstructures

M. Laval, J. J. Bonnefois, J. F. Bobo, F. Issac, and F. Boust

J. Appl. Phys. 105, 073912 (2009) (6 pages)

Online Publication Date: 7 April 2009

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We have calculated by either macrospin analytical method or micromagnetic numerical solving the microwave-assisted switching of a small rectangular platelet as a function of the amplitude and the frequency of a microwave excitation field. The spectral analysis of this phenomenon reveals that microwave-assisted switching is optimal for a rf slightly lower than the linear magnetic resonance frequency measured at low level. Similar experimental results, obtained on 100  µm scale NiFe structures, reveal the same tendency. Switching time for such a process was obtained from calculations that show that subnanosecond switching times require combinations of large dc and rf magnetic fields.
Show PACS
75.40.Mg, 75.25.+z, 75.60.Ej

Interplay between structural and magnetic properties of L10-FePt(001) thin films directly grown on MgO(001)

B. Laenens, F. M. Almeida, N. Planckaert, K. Temst, J. Meersschaut, A. Vantomme, C. Rentenberger, M. Rennhofer, and B. Sepiol

J. Appl. Phys. 105, 073913 (2009) (6 pages)

Online Publication Date: 7 April 2009

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We present a detailed study of the magnetic and structural properties of L10-FePt thin films. The films are prepared via molecular beam epitaxy directly onto MgO(001) substrates, i.e., without buffer layer. Despite the large lattice misfit between the in-plane lattice parameters of L10  FePt and MgO, highly ordered thin films are obtained with the easy magnetization c axis perpendicular to the film plane. Via high resolution transmission electron microscopy and Rutherford backscattering measurements we focus on the FePt/MgO interface to study the misfit relaxation and the defect density. Further, the influence of elevated substrate temperatures and of postgrowth high temperature annealing on the structural and magnetic properties is discussed.
Show PACS
68.55.Ln, 75.70.Ak, 61.72.Ff, 81.15.Hi, 75.60.Ej, 68.37.Og

Changing the exchange bias of spin valves by means of current pulses: Role of the Joule heating

X. L. Tang (唐晓莉), H. W. Zhang (张怀武), H. Su (苏桦), Y. L. Jing (荆玉兰), and Z. Y. Zhong (钟智勇)

J. Appl. Phys. 105, 073914 (2009) (4 pages)

Online Publication Date: 8 April 2009

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Show Abstract
In-plane alternating pulses of current applied with an external magnetic field that changes the strength and direction of the exchange bias are observed in NiFe/Cu/NiFe/FeMn spin valves. To provide clear evidence of this spin-polarized current for antiferromagnets incorporated in spin valves, a series of measurements was carried out to rule out the influence of Joule heating caused by the pulse. The measurements revealed that Joule heating plays a minor role in the impact of exchange bias, thus providing more convincing evidence of the spin torque exerted on the interfacial antiferromagnetic moments.
Show PACS
75.70.Cn, 75.47.Np, 75.50.Ee, 75.50.Bb, 75.30.Et, 75.30.Cr

Spherical magnetic nanoparticles: Magnetic structure and interparticle interaction

V. Russier

J. Appl. Phys. 105, 073915 (2009) (9 pages)

Online Publication Date: 8 April 2009

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The interaction between spherical magnetic nanoparticles is investigated from micromagnetic simulations and analyzed in terms of the leading dipolar interaction energy between magnetic dipoles. We focus mainly on the case where the particles present a vortex structure. In the first step the local magnetic structure in the isolated particle is revisited. For particles bearing a uniaxial magnetocrystalline anisotropy, it is shown that the vortex core orientation relative to the easy axis depends on both the particle size and the anisotropy constant. When the particle magnetization presents a vortex structure, it is shown that the polarization of the particles by the dipolar field of the other one must be taken into account in the interaction. An analytic form is deduced for the interaction which involves the vortex core magnetization and the magnetic susceptibility which are obtained from the magnetic properties of the isolated particle.
Show PACS
75.50.Tt, 75.30.Cr, 75.30.Gw, 75.30.Et, 75.25.+z

Trimmed-diamond shaped toggle magnetoresistive random access memory cells

Y. Fukuma, H. Fujiwara, P. B. Visscher, and G. J. Mankey

J. Appl. Phys. 105, 073916 (2009) (5 pages)

Online Publication Date: 8 April 2009

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We have performed micromagnetic simulations for the design of toggle magnetoresistive random access memory (MRAM) cells to make the operating field as low as possible while keeping a reasonable margin and thermal stability. The memory cells are composed of weakly coupled synthetic antiferromagnets. The cells are diamond-shaped to suppress the formation of edge domains, which increase the operating field. The adverse effect of the diamond shape making the remanent state too stable is prevented by trimming the sharp points. The optimization of the trimming allows us to reduce the operating field and offer a pathway for realizing high-density toggle MRAM.
Show PACS
85.75.Bb, 85.70.Ec

The effective magnetoelectric coefficients of polycrystalline Cr2O3 annealed in perpendicular electric and magnetic fields

Y. Y. Liu, S. H. Xie, G. Jin, and J. Y. Li

J. Appl. Phys. 105, 073917 (2009) (5 pages)

Online Publication Date: 8 April 2009

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Magnetoelectric annealing is necessary to remove antiferromagnetic domains and induce macroscopic magnetoelectric effect in polycrystalline magnetoelectric materials, and in this paper, we study the effective magnetoelectric properties of perpendicularly annealed polycrystalline Cr2O3 using effective medium approximation. The effect of temperatures, grain aspect ratios, and two different types of orientation distribution function have been analyzed, and unusual material symmetry is observed when the orientation distribution function only depends on Euler angle psi. Optimal grain aspect ratio and texture coefficient are also identified. The approach can be applied to analyze the microstructural field distribution and macroscopic properties of a wide range of magnetoelectric polycrystals.
Show PACS
75.80.+q, 75.60.Ch, 77.22.Ch, 75.60.Nt, 75.30.Kz

Nonferromagnetic nanocrystalline ZnO:Co thin films doped with Zn interstitials

Christoph Knies, Matthias T. Elm, Peter J. Klar, Jan Stehr, Detlev M. Hofmann, Nikolai Romanov, Tom Kammermeier, and Andreas Ney

J. Appl. Phys. 105, 073918 (2009) (7 pages)

Online Publication Date: 9 April 2009

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ZnCoO thin films were synthesized via a wet chemical route and subsequently annealed in Zn vapor to increase the conductivity by introducing Zn interstitials. All samples show small hysteresis loops close to the detection limit of the magnetometer. Thus the samples were thoroughly investigated to obtain evidence for further ferromagneticlike behavior. Optical and magneto-optical experiments show the crystal field transitions of Co2+ in the near infrared and visible spectral range. At energies above 2.8 eV a charge transfer transition of Co2+ is observed. The results of magnetotransport measurements are explained by the formation of an impurity band situated below the conduction band. No further evidence for ferromagnetism is obtained.
Show PACS
73.61.Ga, 78.66.Hf, 61.72.jj, 78.20.Ls, 71.70.Ch, 72.20.My

Mössbauer investigation on Ni–Zn nanoferrite with the highest saturation magnetization

A. Mahesh Kumar, K. H. Rao, and J. M. Greneche

J. Appl. Phys. 105, 073919 (2009) (5 pages)

Online Publication Date: 9 April 2009

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The highest value of saturation magnetization (80 emu/g) ever reported has been achieved in Ni0.65Zn0.35Fe2O4 at lower sintering temperatures through sol-gel synthesis. The observed highest value of saturation magnetization has been explained on the basis of core-surface magnetic interactions facilitated through low temperature and in-field Mössbauer investigation of the system. Cation distribution for this composition has also been proposed and verified quantitatively by calculating the intensity of each peak of x-ray diffraction and estimating the lattice constant.
Show PACS
76.80.+y, 75.50.Tt, 81.20.Fw, 75.60.Ej, 61.66.Fn, 75.30.Et
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DIELECTRICS AND FERROELECTRICITY

Magnetic, ferroelectric, and dielectric properties of Bi(Sc0.5Fe0.5)O3–PbTiO3 thin films

F. Yan, I. Sterianou, S. Miao, I. M. Reaney, M. O. Lai, and L. Lu

J. Appl. Phys. 105, 074101 (2009) (6 pages)

Online Publication Date: 1 April 2009

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Bi(Sc0.5Fe0.5)O3–PbTiO3 (BSF-PT) thin films with a composition in the vicinity of a morphotropic phase boundary (MPB) between rhombohedral and tetragonal phases have been grown on LaNiO3/SiO2/Si substrates at 550 °C and 150 mTorr by pulsed laser deposition. The dielectric properties of the film were enhanced due to high crystallinity, low porosity, and the vicinity of the MPB. A saturated ferroelectric hysteresis loop was obtained with 2Pr=76  µC/cm, and the leakage current was minimized by cooling the samples, postdeposition in O2. Magnetic measurements revealed that BSF-PT thin films were antiferromagnetic confirming their anticipated multiferroic nature.
Show PACS
77.55.+f, 75.70.Ak, 77.80.Dj, 81.15.Fg, 75.80.+q, 75.50.Ee

Phonon polaritons in a nonaxial aligned piezoelectric superlattice

Zhen Qi, Zhi-qiang Shen, Cheng-ping Huang, Shi-ning Zhu, and Yong-yuan Zhu

J. Appl. Phys. 105, 074102 (2009) (7 pages)

Online Publication Date: 2 April 2009

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Propagation of phonon polaritons in a nonaxial aligned one-dimensional piezoelectric superlattice is studied theoretically. Because of the piezoelectric effect, both transverse and longitudinal superlattice vibrations are induced by one transverse polarized electromagnetic wave. Due to the coupling effect, two orthogonal modes of elliptically polarized polaritons are present with different propagation properties and can be tuned by the rotation angle.
Show PACS
68.35.Ja, 77.22.Ch, 77.22.Ej, 77.65.-j, 71.36.+c, 63.20.-e

Current conduction mechanisms in Pr2O3/oxynitride laminated gate dielectrics

Fu-Chien Chiu, Chun-Yen Lee, and Tung-Ming Pan

J. Appl. Phys. 105, 074103 (2009) (4 pages)

Online Publication Date: 2 April 2009

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Metal-oxide-semiconductor capacitors with Pr2O3/oxynitride laminated gate dielectrics were fabricated. The current transportation of Al/Pr2O3/SiON/n-Si devices was studied at temperatures ranging from 300 to 400 K. The dominant conduction mechanism at low electric field (<0.6  MV/cm) is the hopping conduction in the laminated gate dielectrics. The determined hopping distance and activation energy is about 1.5 nm and 50±1  meV, respectively. However, the dominant conduction mechanism at high electric field (>2  MV/cm) is the Poole–Frenkel emission in which the trap energy level determined from Arrhenius plot is about 0.56±0.01  eV.
Show PACS
73.40.Qv, 84.32.Tt, 73.50.Dn, 73.50.Fq, 77.55.+f

Electrical properties of Cu/a-BaTiO3/Cu capacitors studied in dc and ac regimes

F. El Kamel, P. Gonon, and G. Radnóczi

J. Appl. Phys. 105, 074104 (2009) (5 pages)

Online Publication Date: 7 April 2009

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Electrical properties of Cu/a-BaTiO3/Cu capacitors have been investigated in both dc and ac regimes as a function of temperature. A clear correlation is found between the temperature dependence of dc leakage currents and the temperature variation of the dielectric relaxation, showing that these measurement techniques are probing the same defects. Using either of these two techniques, we were able to detect at least three types of electrical active defects. Oxygen vacancy diffusion takes place at high temperature with an activation energy of around 1 eV. The diffusion of copper creates ionic defects in the a-BaTiO3 layer, which introduces two other contributions to the conduction process. The first is related to the motion of ionic species (ionic conduction, thermally activated with an activation energy of 0.3 eV). In addition, it has been argued that the presence of copper ions introduces a discrete set of shallow traps within the bandgap, resulting in a n-type conductivity (electronic conduction). The traps depth and their effective density are 0.45 eV and 4×1016  cm−3, respectively.
Show PACS
84.32.Tt, 73.40.Rw, 66.30.Dn, 66.30.Ny, 61.72.jd, 77.22.Gm

Orientation effects in 2-2 piezocomposites based on (1−x)Pb(A1/3Nb2/3)O3−xPbTiO3 single crystals (A=Mg or Zn)

V. Yu. Topolov and A. V. Krivoruchko

J. Appl. Phys. 105, 074105 (2009) (7 pages)

Online Publication Date: 14 April 2009

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Volume fraction and orientation dependences of piezoelectric coefficients and hydrostatic piezoelectric response of 2-2 parallel-connected composites with two piezoactive components are analyzed. These composites comprise relaxor-ferroelectric (1−x)Pb(A1/3Nb2/3)O3−xPbTiO3 single crystals (A=Mg or Zn) and polyvinylidene fluoride, and different variants of poling directions of these layers are considered. It is shown how the orientation of the main crystallographic axes and the anisotropy of the electromechanical properties of the single crystal influence maxima of effective piezoelectric coefficients and other parameters of the composites. Advantages of the piezocomposites based on the [011] poled single crystals are discussed.
Show PACS
77.84.Lf, 77.84.-s, 77.65.-j, 77.65.Bn
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NANOSCALE SCIENCE AND DESIGN

Silicon and silicon oxide core-shell nanoparticles: Structural and photoluminescence characteristics

Mallar Ray, Samata Sarkar, Nil Ratan Bandyopadhyay, Syed Minhaz Hossain, and Ashit Kumar Pramanick

J. Appl. Phys. 105, 074301 (2009) (7 pages)

Online Publication Date: 1 April 2009

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We report the synthesis of spherical core-shell structures of silicon and silicon oxide by a novel route of forced external oxidation of ball milled silicon. Structural investigations reveal the formation of a crystalline silicon core surrounded by an amorphous oxide shell, with core and shell dimensions varying approximately between 4–10 and 55–170 nm, respectively. The observations suggest partial amorphization of crystalline silicon, invasive oxygen induced passivation of dangling bonds, and formation of different types of defects in the nanocrystalline silicon/silicon oxide core-shell structure, particularly at the interfaces. No detectable photoluminescence (PL) is obtained from the as-milled silicon, but the oxidized core-shell structures exhibit strong room temperature PL, detectable with unaided eye. The peak energy of the PL spectra blueshifts with an increase in excitation energy, with the peak positions varying from 2.24 to 2.48 eV under external excitation ranging from 2.41 to 3.5 eV. The observed PL characteristics are explained in terms of dominant electronic transitions between the localized defect states and quantum confinement induced widened band states.
Show PACS
61.46.Df, 81.07.Bc, 61.43.Dq, 82.45.Bb, 78.55.Qr

Fabrication and characterization of DNA-templated conductive gold nanoparticle chains

Hyung Jin Kim, Yonghan Roh, Seong Kyu Kim, and Byungyou Hong

J. Appl. Phys. 105, 074302 (2009) (4 pages)

Online Publication Date: 2 April 2009

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We present a fabrication of conductive nanowires made of gold nanoparticle (AuNP) chains based on lambda-DNA molecules immobilized on a surface of (3-aminopropyl)triethoxysilane-coated Si wafer as the template. A tilting technique was used to align and stretch the lambda-DNAs on the surface. Aniline-capped AuNPs (AN-AuNPs) were electrostatically assembled along the immobilized DNAs by careful control of the AN-AuNPs treatment time and the DNA concentration. AuNPs are attached on DNA with a reduction in Au surface potential as the treatment time increases. Also, the interparticle spacing is dependent on the treatment time and the DNA concentration. AuNP chains with a complete contact between particles were obtained when the treatment time and the DNA concentration were optimized. For electrical conductivity measurements, an isolated AuNP chain was fixed between two gold electrodes. The AuNP chain based on DNA showed an Ohmic behavior at room temperature with the conductivity of two orders of magnitude lower than the bulk value.
Show PACS
81.07.Bc, 73.63.Bd, 73.63.Nm

Carrier mobility as a function of carrier density in type-II InAs/GaSb superlattices

F. Szmulowicz, S. Elhamri, H. J. Haugan, G. J. Brown, and W. C. Mitchel

J. Appl. Phys. 105, 074303 (2009) (5 pages)

Online Publication Date: 2 April 2009

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See Also: Erratum

Show Abstract
We report on a study of the in-plane carrier mobility in InAs/GaSb superlattices as a function of carrier density. Instead of using a number of differently doped samples, we use the persistent-photoconductivity effect to vary the carrier density over a wide range from n- to p-type in single samples and perform Hall effect measurements. Hence, our data are not obscured by sample to sample nonuniformities. We demonstrate that low-temperature in-plane mobilities are limited by screened interface roughness scattering (IRS), although present models of two-dimensional carrier screening of IRS lead to a limited agreement with our data.
Show PACS
73.21.Cd, 72.20.My, 72.40.+w, 72.20.Ee, 72.20.Fr, 68.65.Cd