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Journal of Applied Physics

Issue 7 • Date Apr 2009

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Displaying Results 1 - 25 of 759
  • Light scattering into silicon-on-insulator waveguide modes by random and periodic gold nanodot arrays

    Page(s): 073101 - 073101-6
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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. View full abstract»

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

    Page(s): 073102 - 073102-5
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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. View full abstract»

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  • Efficiency of light emission in high aluminum content AlGaN quantum wells

    Page(s): 073103 - 073103-6
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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. View full abstract»

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

    Page(s): 073104 - 073104-7
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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. View full abstract»

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

    Page(s): 073105 - 073105-5
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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. View full abstract»

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

    Page(s): 073106 - 073106-5
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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. View full abstract»

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

    Page(s): 073107 - 073107-4
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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. View full abstract»

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

    Page(s): 073108 - 073108-5
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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. View full abstract»

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

    Page(s): 073301 - 073301-6
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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. View full abstract»

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

    Page(s): 073302 - 073302-9
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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=d0ln[(t/τ)+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%. View full abstract»

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  • Void generation during the annealing process of very narrow copper wires

    Page(s): 073501 - 073501-7
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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. View full abstract»

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  • Substrate temperature effects on laser crystallized NiTi thin films

    Page(s): 073502 - 073502-10
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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. View full abstract»

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

    Page(s): 073503 - 073503-4
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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. View full abstract»

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

    Page(s): 073504 - 073504-7
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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. View full abstract»

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

    Page(s): 073505 - 073505-7
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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. View full abstract»

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

    Page(s): 073506 - 073506-5
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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. View full abstract»

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

    Page(s): 073507 - 073507-6
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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. View full abstract»

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  • Clathrate guest atoms under pressure

    Page(s): 073508 - 073508-9
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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 B- a8YxGe46-x (Yx=Ni6,Cu6,Zn8) samples were characterized with respect to their thermoelectric properties. View full abstract»

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

    Page(s): 073509 - 073509-6
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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 |ΔSM| 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. View full abstract»

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

    Page(s): 073510 - 073510-7
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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. View full abstract»

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

    Page(s): 073511 - 073511-5
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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. View full abstract»

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

    Page(s): 073512 - 073512-9
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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. View full abstract»

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

    Page(s): 073513 - 073513-11
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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 secon- d step of the damage process. View full abstract»

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  • Competing failure mechanisms in thin films: Application to layer transfer

    Page(s): 073514 - 073514-9
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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 σm—or equivalently the heating temperature—must be within the range c≪σm≪0 to produce an intact thin film where σc depends on the interfacial fracture energy and the size of defects at the interface between film and substrate. View full abstract»

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  • Effect of spatial nonlocality on terahertz optical interaction with quantum wells

    Page(s): 073515 - 073515-5
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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. View full abstract»

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Aims & Scope

Journal of Applied Physics is the American Institute of Physics' (AIP) archival journal for significant new results in applied physics

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P. James Viccaro
Argonne National Laboratory