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15 Sep 2005

Volume 98, Issue 6, Articles (06xxxx)

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Interface alloy mixing effect in the growth of self-assembled InP quantum dots on InAlGaP matrices by metalorganic chemical-vapor deposition

X. B. Zhang, J. H. Ryou, R. D. Dupuis, G. Walter, and N. Holonyak

J. Appl. Phys. 98, 063501 (2005); http://dx.doi.org/10.1063/1.2043234 (3 pages) | Cited 2 times

Online Publication Date: 16 September 2005

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We report on the growth of InP self-assembled quantum dots (SAQDs or simply QDs) on In0.5(AlxGa1−x)0.5P matrices grown on (001) GaAs substrates by metalorganic chemical-vapor deposition. We find that the size of InP QDs grown on the In0.5Al0.5P matrix decreases and the density increases when the QD growth temperature decreases. However, cathodoluminescence (CL) studies of the capped InP QDs show that the CL peak of InP QDs redshifts when the QD growth temperature decreases. These results suggest that there is a strong interface mixing during the growth of InP/In0.5Al0.5P QDs, in which some InAlP materials in the lower and upper barriers mix with the InP QDs and form InAlP alloy QDs. Postgrowth thermal annealing of these QDs in N2 shows that our QDs are quite stable after annealing at the growth temperature for a time equal to the upper In0.5Al0.5P barrier growth time. Combined with the thermal annealing study, we think the interface mixing between the InP QDs and In0.5Al0.5P layers happens mainly during the initial growth of InP and the initial capping of InP QDs with In0.5Al0.5P.
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68.35.Fx Diffusion; interface formation
78.66.Fd III-V semiconductors
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
61.72.Cc Kinetics of defect formation and annealing
78.60.Hk Cathodoluminescence, ionoluminescence
66.30.Ny Chemical interdiffusion; diffusion barriers
81.16.Dn Self-assembly
78.67.Hc Quantum dots

Theory of strain states in InAs quantum dots and dependence on their capping layers

Y. Nabetani, T. Matsumoto, G. Sasikala, and I. Suemune

J. Appl. Phys. 98, 063502 (2005); http://dx.doi.org/10.1063/1.2041846 (7 pages) | Cited 9 times

Online Publication Date: 16 September 2005

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The dependence of strain states in InAs self-assembled quantum dots (QDs) on their capping layers was investigated by valence-force field model calculations. An InAs QD on (001) GaAs and embedded in a GaNAs capping layer and the one with its dot surface terminated with nitrogen (N) and embedded in a GaAs capping layer show reduced compressive strain within the QDs in the (001) growth plane due to the lateral expansion of the QDs, while the one embedded in an InGaAs capping layer shows enhanced tensile strain along the [001] growth direction. The strain energies around the center of the InAs QDs with the GaNAs capping layer and with the N-surface termination are lowered compared with those for conventional GaAs capping layers. The burying conditions of InAs QDs also modify the sizes of QDs. The stress distributions obtained by strain energy mapping showed that In atoms around the top of QDs undergo inward stress. This inward stress prevents In segregation and explains the experimentally observed improved optical properties of GaNAs-embedded and N-terminated QDs.
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68.65.Hb Quantum dots (patterned in quantum wells)
64.75.-g Phase equilibria

Improved emission of Eu3+ by energy transfer via Tb3+ in coprecipitates TbxEu(1−x)(aspirin)3(phen)

T. Zhang, Z. Xu, L. Qian, F. Teng, X. R. Xu, and D. L. Tao

J. Appl. Phys. 98, 063503 (2005); http://dx.doi.org/10.1063/1.2042532 (4 pages) | Cited 2 times

Online Publication Date: 19 September 2005

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In order to increase the red emission of Eu complex doped in poly(N-vinylcarbazole) (PVK), series of molar ratio coprecipitates [TbxEu(1−x)(aspirin)3(phen)∙H2O] (phen = 1,10-phenanthroline) were prepared, in which Tb3+ ion acts as an energy bridge to enhance the energy transfer between the ligands and europium ion. The intramolecular energy transfer from Tb3+ to Eu3+ was studied via a time-resolved spectrum. It gave a direct evidence of the intramolecular energy transfer between the two lanthanide ions of terbium and europium. The photoluminescence and electroluminescence spectra as well as the performance of the device indium tin oxide/poly-(3,4-ethylenedioxythiophene):poly-(styrenesulphonic acid)/PVK:TbxEu(1−x)(aspirin)3(phen)/Al were also studied. Compared with that of PVK:Eu(aspirin)3phen device, the result showed an enhanced emission of Eu3+ ion due to the energy transfer via Tb3+ ion.
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78.55.Kz Solid organic materials
78.60.Fi Electroluminescence
78.47.-p Spectroscopy of solid state dynamics

The inelastic behavior and failure of dense glass under shock loading to 15 GPa

D. D. Radford

J. Appl. Phys. 98, 063504 (2005); http://dx.doi.org/10.1063/1.2041840 (9 pages)

Online Publication Date: 19 September 2005

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The in-material longitudinal and lateral stress histories in an extra-dense, silica-based glass have been measured at impact pressures well above the Hugoniot elastic limit using embedded manganin stress gauges during plate impact experiments. The measurements of longitudinal stress extend the current data and are used to provide an extensive principal Hugoniot for the material. The lateral stress measurements obtained at high impact pressures show the pressure-dependent nature of the failure process, consistent with previous studies on dense glasses. The longitudinal and lateral data obtained are used to determine the shear strength versus time history during high-pressure loading. These data in conjunction with high-speed photographic records provide insight into the inelastic response and evolution of failure in brittle materials during shock loading.
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81.40.Jj Elasticity and anelasticity, stress-strain relations
62.20.D- Elasticity
62.50.-p High-pressure effects in solids and liquids
81.40.Lm Deformation, plasticity, and creep
62.20.F- Deformation and plasticity
81.40.Np Fatigue, corrosion fatigue, embrittlement, cracking, fracture, and failure
62.20.M- Structural failure of materials

Effective parameters of resonant negative refractive index metamaterials: Interpretation and validity

Divitha Seetharamdoo, Ronan Sauleau, Kouroch Mahdjoubi, and Anne-Claude Tarot

J. Appl. Phys. 98, 063505 (2005); http://dx.doi.org/10.1063/1.2041841 (4 pages) | Cited 26 times

Online Publication Date: 19 September 2005

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We present a numerical study of resonant negative refractive index (NRI) metamaterials in which their effective parameters are calculated. For a periodic media consisting of split-ring resonators and metallic wires the effective refractive index is compared by means of two different methods. The first one is an inversion procedure in which the effective refractive index is calculated from the reflection and transmission coefficients of a finite structure and the second one consists its calculation from the phase velocity issued from the dispersion diagram. A significant difference between the two cases is highlighted in the frequency interval of interest (NRI regime) and for the finite media, counterintuitive observations are made. These anomalous features are observed in a frequency range in which there is a non-negligible contribution of the higher-order modes to propagation inside the periodic metamaterial. Hence in this particular frequency interval, the media cannot be described by an effective refractive index in a Fresnel sense. In other frequency regions, both methods converge and it will be shown that the metamaterial exhibits a negative refractive index.
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42.70.-a Optical materials

A near-room-temperature all-optical polarization switch based on the excitation of spin-polarized “virtual” carriers in quantum wells

Murat Yildirim, J. P. Prineas, Eric J. Gansen, and Arthur L. Smirl

J. Appl. Phys. 98, 063506 (2005); http://dx.doi.org/10.1063/1.2042531 (4 pages) | Cited 3 times

Online Publication Date: 20 September 2005

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Near-room-temperature operation of an all-optical polarization switch based on the virtual excitation of spin-polarized carriers in semiconductor quantum wells is demonstrated. The device is shown to exhibit a pulse-width-limited switching time, a contrast ratio of >18 dB, an optical bandwidth of ∼ 3 THz, and an energy throughput of >0.1% using a thin (40 wells) GaAs/AlGaAs sample. The results of differential transmission measurements are used to identify the dominant switching mechanisms and to monitor the spin and temporal dynamics of the carriers excited during the operation of the switch.
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42.65.Pc Optical bistability, multistability, and switching, including local field effects
72.25.Dc Spin polarized transport in semiconductors

Wrinkling of a two-layer polymeric coating

Soumendra K. Basu, Aaron M. Bergstreser, L. F. Francis, L. E. Scriven, and A. V. McCormick

J. Appl. Phys. 98, 063507 (2005); http://dx.doi.org/10.1063/1.2043255 (10 pages) | Cited 9 times

Online Publication Date: 20 September 2005

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Wrinkling of a two-layer coating, comprised of a glassy polyvinyl alcohol top layer and a rubbery polyethyl acrylate bottom layer, was studied. The coatings were prepared on polyethylene terephthalate substrates by depositing and drying the layers in sequence. Fine, short-wavelength (180–200 μm) wrinkles formed when the two-layer coating was exposed to humid atmosphere after the second drying treatment and coarse, long-wavelength (450–500 μm) wrinkles formed when the two-layer coating was heated to a temperature above the glass transition temperature of the top layer. Both types of wrinkles must have arisen from compressive stress in the top layer. The compressive stress developed by either moisture absorption (fine wrinkles) or differential thermal expansion on heating (coarse wrinkles). When compressive stress is high enough, the top layer deforms out of plane without detaching from the bottom layer to produce wrinkles. The balance of forces in systems of three adhering layers of different thicknesses and moduli in static mechanical equilibrium was analyzed, and it was found that the layer with the highest in-plane axial stiffness, the product of elastic modulus by thickness, typically dictates the dimension of the entire composite. When the mismatch of thermal expansivities of the layers generates the compressive stress that produces wrinkling, the difference between the thermal expansivity of the top layer and the axial stiffness-weighted average thermal expansivity of the three layers governs the compressive stress in the top layer. Because polymer modulus falls abruptly at the glass transition temperature Tg, the critical compressive stress required for wrinkling falls abruptly at Tg. Therefore, the two-layer coating atop a stiff substrate is more susceptible to wrinkling when the top layer is above its Tg. When absorption of moisture in the top layer generates the compressive stress that generates wrinkling, that stress goes through a minimum because modulus falls and compressive strain rises with rising moisture content. However, the critical compressive stress for wrinkling falls because modulus falls with rising moisture content. Therefore, the two-layer coating atop a stiff substrate is more susceptible to wrinkling the greater the moisture content in the top layer. The relative humidity (RH) of the sample atmosphere was raised to raise the moisture content in the top layer. The wrinkle wavelength fell with rising RH because the modulus, and therefore the bending stiffness, of the top layer fell with rising RH. The measured wavelengths were qualitatively predicted at low humidities and quantitatively predicted at high humidities.
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81.40.Jj Elasticity and anelasticity, stress-strain relations
81.40.Lm Deformation, plasticity, and creep
65.60.+a Thermal properties of amorphous solids and glasses: heat capacity, thermal expansion, etc.
68.60.Bs Mechanical and acoustical properties
62.20.D- Elasticity
62.20.F- Deformation and plasticity

Sensitivity enhancement in thermoreflectance microscopy of semiconductor devices using suitable probe wavelengths

L. R. de Freitas, E. C. da Silva, A. M. Mansanares, G. Tessier, and D. Fournier

J. Appl. Phys. 98, 063508 (2005); http://dx.doi.org/10.1063/1.2043231 (7 pages) | Cited 7 times

Online Publication Date: 20 September 2005

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In this paper we present an experimental and theoretical study of the thermoreflectance response as a function of the probe wavelength for layered microelectronics structures. The investigated sample consists of a polycrystalline silicon conducting track grown on a SiO2-coated Si substrate. Thermoreflectance measurements were carried out in the wavelength range from 450 to 750 nm with the track biased in modulated regime. An oscillating pattern is observed in the spectral region where the upper layer is transparent. Such oscillations are due to the interference resulting from the multiple reflections at the interfaces. Using a thermo-optical model, we show that the optical constants (n and k) of the materials, which are wavelength dependent, as well as their temperature derivatives (dn/dT and dk/dT), strongly influence the thermoreflectance signal. The optical thicknesses of the layers, mainly determined by the real part of the refractive indices, define the period of oscillation. On the other hand, the imaginary part of the refractive indices establishes the cutoff wavelength of the oscillations. Below this cutoff wavelength, the probe light does not penetrate the material and the upper-surface reflectance dominates the signal.
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78.20.N- Thermo-optic effects
78.20.nb Photothermal effects
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)
85.30.-z Semiconductor devices

Electromigration diffusion mechanism of electroplated copper and cold/hot two-step sputter-deposited aluminum-0.5-wt % copper damascene interconnects

T. Usui, H. Nasu, T. Watanabe, H. Shibata, T. Oki, and M. Hatano

J. Appl. Phys. 98, 063509 (2005); http://dx.doi.org/10.1063/1.2009061 (6 pages) | Cited 2 times

Online Publication Date: 20 September 2005

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The electromigration (EM) diffusion mechanisms of electroplated copper (Cu) with tantalum (Ta) barrier/dielectric diffusion barrier SiCxNy and cold/hot two-step sputter-deposited aluminum (Al)–0.5-wt %Cu damascene interconnects with niobium (Nb) liner are examined and compared using the via-EM testing pattern with different linewidths. The interface between Cu and SiCxNy is the dominant diffusion path for the Cu damascene interconnects regardless of the Cu microstructure. An activation energy (Ea) of approximately 0.9±0.03 eV is obtained for the width range of 0.1–6 μm. Therefore, the diffusion mechanism is independent of the Cu microstructure. Regarding the cold/hot two-step sputter-deposited Al–0.5-wt %Cu damascene interconnects with Nb liner, the EM median time to failure (MTF) increases with increasing the linewidth for the Al bamboolike microstructure, indicating that the interface between the Al and Nb liners is the dominant diffusion path. This is probably because a rapid diffusion path along the NbAlx reaction product is formed during the two-step cold/hot sputter deposition at 400 °C. The EM MTF does not increase for more than 4 μm. It is also found that the Ea is approximately 0.9 eV for the Al bamboolike microstructure and that it decreases with increasing linewidth for the Al polycrystalline microstructure, meaning that the grain-boundary diffusion is also included and that ratio of the interfacial diffusion and grain-boundary diffusion depends upon the linewidth. This is because the Ea of the grain-boundary diffusion is smaller than that of the interfacial diffusion for Al–0.5-wt %Cu damascene interconnects. The EM diffusion mechanism of the Cu damascene interconnects with Ta barrier/dielectric diffusion barrier SiCxNy is completely different from that of the Al damascene interconnects with Nb liner.
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85.40.Ls Metallization, contacts, interconnects; device isolation
66.30.Qa Electromigration
66.30.Ny Chemical interdiffusion; diffusion barriers
68.35.Fx Diffusion; interface formation
61.72.Mm Grain and twin boundaries
66.30.Lw Diffusion of other defects
77.84.-s Dielectric, piezoelectric, ferroelectric, and antiferroelectric materials

Intersecting basal plane and prismatic stacking fault structures and their formation mechanisms in GaN

J. Bai, X. Huang, M. Dudley, B. Wagner, R. F. Davis, L. Wu, E. Sutter, Y. Zhu, and B. J. Skromme

J. Appl. Phys. 98, 063510 (2005); http://dx.doi.org/10.1063/1.2039278 (9 pages) | Cited 5 times

Online Publication Date: 20 September 2005

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A systematic study of intersecting planar boundary structures observed in a GaN epifilm grown on a vicinal 6H-SiC substrate (offcut towards [1math10]) with an AlN buffer is presented. These structures are shown to comprise stacking faults that fold back and forth from the basal plane [I1 basal plane stacking faults (BSFs)] to the prismatic plane [prismatic stacking faults (PSFs)]. The PSFs, with fault vector ½〈10math1〉 nucleate at steps on the substrate surface as a consequence of the different stacking sequences exposed on either side of the step. Once nucleated, PSFs intersecting the vertical step risers in the AlN buffer and eventually in the GaN film are replicated during the predominantly step-flow growth and propagate into the growing crystal. As a consequence of the different growth rates experienced on either side of the intersection of a PSF with a vertical step riser, the PSF may be redirected onto an equivalent {11math0} plane, leaving an I1 BSF between the bottom of the redirected section of PSF and the top of that portion of the original PSF which was below the terrace. This leads to the formation of folded PSF/BSF fault structures which exhibit various configurations. Such folded stacking fault configurations form walls which enclose domains of different stacking sequence. Stair-rod dislocations (SRDs) are observed to form at the intersections of these various faults and the Burgers vectors of some of these are discussed. In some cases, reconstruction of these SRDs occurs through the formation of partner dislocations which serve to minimize the energy associated with the lattice disconnections located at the cores of the SRDs.
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61.72.Nn Stacking faults and other planar or extended defects
61.72.Ff Direct observation of dislocations and other defects (etch pits, decoration, electron microscopy, x-ray topography, etc.)

Impact of microstructure on grain-boundary diffusion in polycrystals

K. Bedu-Amissah, J. M. Rickman, H. M. Chan, and M. P. Harmer

J. Appl. Phys. 98, 063511 (2005); http://dx.doi.org/10.1063/1.2043257 (6 pages) | Cited 1 time

Online Publication Date: 22 September 2005

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The impact of a complex microstructure on polycrystalline diffusion is investigated using both numerical and analytical methods. In particular, the diffusion equation is numerically integrated using the finite-difference method to obtain the concentration profile for a diffusant in a simplified microstructural representation. The methodology is first validated for an idealized model of diffusion in a prototypical, single grain-boundary system and then applied to a Voronoi model of a microstructure resulting from homogeneous nucleation and growth. The diffusive behavior is quantified by obtaining uptake curves as a function of time for different ratios of grain boundary to lattice diffusivities. Such curves can be used to estimate an unknown grain-boundary diffusivity, given certain microstructural assumptions. Finally, approximate analytical equations describing a diffusant uptake in polycrystalline microstructural models are developed and found to agree well with the numerical results.
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61.72.Mm Grain and twin boundaries
66.30.Lw Diffusion of other defects

Effective permittivity of planar composites with randomly or periodically distributed conducting fibers

L. Liu, S. M. Matitsine, Y. B. Gan, and K. N. Rozanov

J. Appl. Phys. 98, 063512 (2005); http://dx.doi.org/10.1063/1.2035895 (7 pages) | Cited 14 times

Online Publication Date: 22 September 2005

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Theoretical and experimental studies have been conducted on the effective electromagnetic properties of planar composites at microwave frequencies, with embedded conductive fibers of various volume concentrations. Two types of distribution are considered: random and periodic. Experimental results for the transmission coefficient and effective permittivity are obtained via the free space method. Simulation results are obtained using the finite element method (FEM). Good agreement is found between the measured and computed results, indicating the suitability of the FEM as a theoretical modeling tool for such composites, as compared to other numerical methods, such as the method of moments. It is also found that the type of distribution affects the effective permittivity of the composites: lower microwave permittivity with broader peak response is observed for composites with randomly distributed fibers, in comparison to that with periodically distributed fibers.
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77.22.Ch Permittivity (dielectric function)
72.30.+q High-frequency effects; plasma effects
72.80.Tm Composite materials

Imaging the doping and electroluminescence in extremely large planar polymer light-emitting electrochemical cells

Jun Gao and Justin Dane

J. Appl. Phys. 98, 063513 (2005); http://dx.doi.org/10.1063/1.2035897 (8 pages) | Cited 20 times

Online Publication Date: 22 September 2005

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An extremely large planar polymer light-emitting electrochemical cell with an interelectrode spacing of 11 mm has been demonstrated. The large planar device structure allows for the imaging of doping propagation, photoluminescence, and electroluminescence (EL) with high spatial and temporal resolutions. Several unconventional EL modes have been observed based on the direct imaging of these devices with an interelectrode spacing ranging from 0.6 to 11 mm. These include transient EL in a dynamic-junction device, EL from planar polymer/polymer heterojunction devices, and anomalous reverse-bias EL in a frozen-junction device. Transient EL occurs when the polarity of the applied bias is reversed after the device is fully turned on. The reversal of the applied bias causes the initiation and uneven propagation of fresh p doping within previously n-doped regions, and vise versa. This leads to the formation of transient, discrete, light-emitting p-n junctions near the electrodes before a continuous light-emission zone is formed by the complete reversal of the doping profile. In addition, planar cells consisting of a side-by-side polymer/polymer heterojunction have been demonstrated and imaged. The heterojunction in all working devices is found to be electronically conductive, but exhibits different ion-transport properties. Three types of polymer/polymer interface have been identified based on the imaging of doping and EL profiles. Finally, an anomalous reverse-bias EL mode has been observed in a planar frozen-junction device. The device was turned on at elevated temperature and then cooled to 200 K. Stress under a constant reverse bias leads to the activation of anomalous EL that originates from the same region as forward-bias EL. Furthermore, both forward- and reverse-bias EL have been found to increase with time under reverse-bias stress.
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85.60.Jb Light-emitting devices

Stabilization of amorphous GaN by oxygen

F. Budde, B. J. Ruck, A. Koo, S. Granville, H. J. Trodahl, A. Bittar, G. V. M. Williams, M. J. Ariza, B. Bonnet, D. J. Jones, J. B. Metson, S. Rubanov, and P. Munroe

J. Appl. Phys. 98, 063514 (2005); http://dx.doi.org/10.1063/1.2014937 (4 pages) | Cited 13 times

Online Publication Date: 22 September 2005

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We have investigated experimentally the structure of disordered GaN films. The results suggest that it is not possible to stabilize an amorphous network in stoichiometric films, and the GaN instead consists of random-stacked nanocrystals of some 3-nm diameter. However, incorporation of 15% or more oxygen stabilizes an amorphous phase, which we attribute to the presence of nontetrahedral bonds centered on oxygen. The ionic favorability of heteropolar bonds and its strikingly simple constraint to even-membered rings are the likely causes of the instability of stoichiometric a-GaN.
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68.55.-a Thin film structure and morphology
61.43.Dq Amorphous semiconductors, metals, and alloys
61.46.-w Structure of nanoscale materials

The shock response of float-glass laminates

N. K. Bourne

J. Appl. Phys. 98, 063515 (2005); http://dx.doi.org/10.1063/1.2058196 (5 pages)

Online Publication Date: 23 September 2005

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Interfaces within glass targets give rise to variations in the mode of failure of material components. The wide use of such laminates merits further investigation of the failure mechanism. It is already known that when shocked above a threshold of 4 GPa, glass fails under compression behind a propagating front following the compression front. Work is presented which indicates how this failure process is altered by bonding together two plates to introduce an interface, rather than leaving a monolithic target. After crossing an internal interface, the failure wave propagates only after a delay in soda-lime glass and the failed strength of the material is increased at the inner interface compared with that at the impact face. Addition of a second interface illustrates these effects. Recent work has shown that failure of more than two plates bonded together during impact shapes the pulse transmitted through materials. Indeed it has been suggested that glass sheets bonded together show some of the features of polycrystalline brittle materials. In this work, the stress has been monitored at different stations in the laminate to ascertain the effect of varying the number of tiles within the laminate. It is found that the pulse rises to ca. 4 GPa quickly and then is ramped more gradually as the number of glass sheets is increased.
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81.05.Kf Glasses (including metallic glasses)
62.50.-p High-pressure effects in solids and liquids
81.40.Lm Deformation, plasticity, and creep
62.20.F- Deformation and plasticity

Thermal-drag carrier cooling in undoped semiconductors

Danhong Huang, T. Apostolova, P. M. Alsing, and D. A. Cardimona

J. Appl. Phys. 98, 063516 (2005); http://dx.doi.org/10.1063/1.2041842 (5 pages) | Cited 1 time

Online Publication Date: 23 September 2005

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An approach for carrier cooling in undoped and contactless semiconductors is proposed by using thermal-drag effects in comparison with other methods, such as direct resonant tunneling, nonresonant thermionic, and junction-tunneling cooling, as well as indirect optothermionic and thermoelectric cooling, of carriers in doped and contacted semiconductors. A four-step microscopic model is proposed for this thermal-drag carrier cooling in undoped semiconductors. Wide-band-gap semiconductors with small lattice specific heat and small exchange specific heat between carriers and phonons are found to achieve the best thermal-drag carrier cooling under near-band-edge interband pumping by a weak field. This indirect carrier cooling is accompanied by the lattice cooling. The carrier temperature is pinned to the lattice temperature due to ultrafast carrier-phonon scattering, and it is dragged down by the reduction of the lattice temperature, i.e., the thermal-drag effects.
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37.10.Vz Mechanical effects of light on atoms, molecules, and ions
78.55.Hx Other solid inorganic materials
72.10.Di Scattering by phonons, magnons, and other nonlocalized excitations
65.40.Ba Heat capacity
71.38.-k Polarons and electron-phonon interactions

Influence of Si(001) substrate misorientation on morphological and optical properties of Ge quantum dots

I. Berbezier, M. Descoins, B. Ismail, H. Maaref, and A. Ronda

J. Appl. Phys. 98, 063517 (2005); http://dx.doi.org/10.1063/1.2040004 (5 pages) | Cited 5 times

Online Publication Date: 26 September 2005

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We have investigated the correlation between morphological and optical properties of Ge dots deposited by molecular-beam epitaxy on a Si(001) surface and on a high-index Si(118) vicinal surface. Ge islands were confined on the top of an undulated Si0.5Ge0.5 template layer according to the Stranski-Krastanov growth mode. Atomic force microscopy measurements reveal that the main effect of the vicinal substrate is to transform hut islands on a nominal (001) substrate into wire-shaped islands on (118) substrates. We have observed a direct correlation between the elongated shape and polarization anisotropy of optical transitions in island. The island photoluminescence (PL) emission is partially ( ∼ 25%) polarized for dots deposited on a (118) substrate. PL spectroscopy investigations as a function of temperature and excitation power are reported. The results show that the PL of islands strongly depends on the pump excitation power: it broadens and is blueshifted by 28 and 14 meV/decade for structures grown on (001) and (118), respectively, as the excitation power density increases. The significant blueshift is interpreted in terms of band bending in type II recombination. Moreover, a detailed analysis of (118) island PL band shows the presence of two main peaks that could be attributed to the different island morphologies clearly identified by transmission electron and atomic force microscopies: wire islands and dome islands.
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81.07.Ta Quantum dots
68.65.Hb Quantum dots (patterned in quantum wells)
78.67.Hc Quantum dots
78.55.Ap Elemental semiconductors
81.15.Hi Molecular, atomic, ion, and chemical beam epitaxy
68.37.Ps Atomic force microscopy (AFM)
68.37.Lp Transmission electron microscopy (TEM)

Quantitative description of disorder parameters in (GaIn)(NAs) quantum wells from the temperature-dependent photoluminescence spectroscopy

O. Rubel, M. Galluppi, S. D. Baranovskii, K. Volz, L. Geelhaar, H. Riechert, P. Thomas, and W. Stolz

J. Appl. Phys. 98, 063518 (2005); http://dx.doi.org/10.1063/1.2058192 (7 pages) | Cited 27 times

Online Publication Date: 27 September 2005

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Photoluminescence in (GaIn)(NAs) quantum wells designed for laser emission was studied experimentally and theoretically. The observed temperature dependences of the luminescence Stokes shift and of the spectral linewidth evidence the essential role of disorder in the dynamics of the recombining excitations. The spatial and energy disorders can cause a localization of photocreated excitations supposedly in the form of excitons. Theoretical study of the exciton dynamics is performed via kinetic Monte Carlo simulations of exciton hopping and recombination in the manifold of localized states. Direct comparison between experimental spectra and theoretical calculations provides quantitative information on the energy scale of the potential fluctuations in (GaIn)(NAs) quantum wells. The results enable one to quantify the impact of annealing on the concentration of localized states and/or on the localization length of excitons in (GaIn)(NAs) quantum wells.
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78.67.De Quantum wells
78.55.Cr III-V semiconductors
78.66.Fd III-V semiconductors
71.23.An Theories and models; localized states
71.35.-y Excitons and related phenomena

A quantum-mechanical treatment of phonon scattering in carbon nanotube transistors

Jing Guo

J. Appl. Phys. 98, 063519 (2005); http://dx.doi.org/10.1063/1.2060942 (6 pages) | Cited 18 times

Online Publication Date: 27 September 2005

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Phonon scattering in carbon nanotube field-effect transistors (CNTFETs) is treated using the nonequilibrium Green’s function formalism with the self-consistent Born approximation. The treatment simultaneously captures the essential physics of phonon scattering and important quantum effects. For a one-dimensional channel, it is computationally as efficient as and physically more rigorous than the so-called “Büttiker probe” approach [ Phys. Rev. Lett. 57, 1761 (1986) ], which has been widely used in mesoscopic physics. The non-self-consistent simulation results confirm that the short mean-free-path optical phonon (OP) scattering, though expected to dominate even in a short channel CNTFET, essentially has no direct effect on the dc on current under modest gate biases. The self-consistent simulation results indicate that OP scattering, however, can have an indirect effect on the on current through self-consistent electrostatics. Using a high-κ gate insulator suppresses the indirect effect and leads to a dc on current closer to the ballistic limit. The indirect effect in a CNT Schottky barrier FET can be more important than that in a metal-oxide semiconductor FET.
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85.35.Kt Nanotube devices
85.30.Tv Field effect devices
63.22.-m Phonons or vibrational states in low-dimensional structures and nanoscale materials

Structural and magnetic properties of CrPt3(111) films grown on WSe2(0001)

I. L. Guhr, B. Riedlinger, M. Maret, U. Mazur, A. Barth, F. Treubel, M. Albrecht, and G. Schatz

J. Appl. Phys. 98, 063520 (2005); http://dx.doi.org/10.1063/1.2060943 (4 pages) | Cited 4 times

Online Publication Date: 27 September 2005

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We report on the growth-induced properties of 30 Å thick CrPt3(111) deposits on surfaces of WSe2(0001) with a van der Waals interaction. Assemblies of (111)-oriented fcc grains are obtained with average lateral sizes ranging from 5 to 15 nm for growth temperatures increasing from 50 °C to 550 °C. The grains develop a partial L12-type chemical long-range order starting at a deposition temperature as low as 50 °C, much lower than reported for films grown on other suitable substrate surfaces. An ordering parameter of about 0.3 was found over a wide temperature range between 200 °C and 550 °C, for a deposition rate of 0.1 Å/s. Lowering the deposition rate by a factor of 5 results in a higher ordering parameter of about 0.4. Above 400 °C, ordering is accompanied by a pronounced segregation of Se atoms toward the free surface, forming a (4×4) superstructure. All samples exhibit superparamagnetic behavior with blocking temperatures that scale with the activation volume of the ferrimagnetic grains. This work shows the importance of substrate choice in reducing the ordering temperature of thin CrPt3 films, an effect that might also occur in other alloy systems.
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75.50.Gg Ferrimagnetics
75.50.Tt Fine-particle systems; nanocrystalline materials
68.55.-a Thin film structure and morphology
75.70.Ak Magnetic properties of monolayers and thin films
68.55.A- Nucleation and growth
75.20.En Metals and alloys
68.35.Dv Composition, segregation; defects and impurities

Pressure-induced phase change in poly(tetrafluoroethylene) at modest impact velocities

Philip J. Rae, Eric N. Brown, Bradford E. Clements, and Dana M. Dattelbaum

J. Appl. Phys. 98, 063521 (2005); http://dx.doi.org/10.1063/1.2041845 (8 pages) | Cited 13 times

Online Publication Date: 27 September 2005

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Although poly(tetrafluoroethylene) (PTFE) is an unusually ductile polymer, it undergoes an abrupt ductile-brittle transition at modest impact velocities. No previous explanation for this behavior has been found after an extensive literature search. In this paper, we examine the role of a pressure-induced phase transition in PTFE in the dynamic failure of Taylor cylinder samples. There is a known phase transition in PTFE with a marked decrease in volume and compressibility that occurs at 0.5–0.65 GPa at 21 °C, with the transition pressure inversely related to temperature. Varying the temperature of the samples in the experiment revealed that the phase transition is probably involved in sample failure because the ductile/brittle transition velocity increased for decreasing temperature, despite the material fracture toughness decreasing. Additionally, Taylor tests were carried out on samples of poly(chlorotrifluoroethylene) PCTFE to investigate the behavior of a similar material to PTFE but without a pressure-induced phase transition. PCTFE did not exhibit an abrupt ductile to brittle transition.
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62.50.-p High-pressure effects in solids and liquids
64.70.K- Solid-solid transitions
81.40.Lm Deformation, plasticity, and creep
81.40.Np Fatigue, corrosion fatigue, embrittlement, cracking, fracture, and failure
62.20.F- Deformation and plasticity
62.20.M- Structural failure of materials

Extremum method: Inverse solution of the two-layer thermal wave problem

J. L. Nzodoum Fotsing, J. Gibkes, J. Pelzl, and B. K. Bein

J. Appl. Phys. 98, 063522 (2005); http://dx.doi.org/10.1063/1.2058180 (17 pages) | Cited 12 times

Online Publication Date: 28 September 2005

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An inverse solution of the two-layer thermal wave problem has been derived, which allows us to determine the relevant thermal transport parameters, the thermal diffusion time and the thermal reflection coefficient, respectively, the ratio of the effusivities of the two layers, deduced from the relative minimum or maximum of the calibrated phase lags measured between the periodically modulated excitation of the thermal wave and the detected thermal response. Applying a functional transformation by multiplying the calibrated phase lags with the variable (1/f1/2)q, where f is the modulation frequency of excitation and q a positive or negative real number close to zero, the inversion method is extended to other values of the calibrated phase lags measured in the neighborhood of the phase minimum or maximum. The application potential of these two solution methods is studied by analyzing the phase lags measured as a function of frequency for two-layer systems of technological importance, e.g., different plasma-deposited hard coatings on tool steel, coated cutting tools after friction wear, and a sample of a shape memory alloy (NiTi) after mechanical surface treatment.
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44.10.+i Heat conduction
44.35.+c Heat flow in multiphase systems
66.30.Xj Thermal diffusivity

Above-room-temperature ferromagnetism in half-metallic Heusler compounds NiCrP, NiCrSe, NiCrTe, and NiVAs: A first-principles study

E. Şaşıoğlu, L. M. Sandratskii, and P. Bruno

J. Appl. Phys. 98, 063523 (2005); http://dx.doi.org/10.1063/1.2060941 (6 pages) | Cited 16 times

Online Publication Date: 28 September 2005

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We study the interatomic exchange interactions and Curie temperatures in half-metallic semi-Heusler compounds NiCrZ (Z = P, Se, Te) and NiVAs. The study is performed within the framework of density functional theory. The calculation of exchange parameters is based on the frozen-magnon approach. It is shown that the exchange interactions in NiCrZ vary strongly depending on the Z constituent. The Curie temperature is calculated within mean field and random phase approximations. The difference between two estimated values is related to the properties of the exchange interactions. The predicted Curie temperatures of all four systems are considerably above room temperature. The relation between half-metallicity and the value of the Curie temperature is discussed. The combination of a high spin polarization of charge carriers and a high Curie temperature makes these Heusler alloys interesting candidates for spintronics applications.
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75.50.Cc Other ferromagnetic metals and alloys
75.30.Et Exchange and superexchange interactions
71.15.Mb Density functional theory, local density approximation, gradient and other corrections

Liquid adhesion to an ultrasonically vibrating end surface

S. Tamura, Y. Tsunekawa, M. Okumiya, and Y. Furukawa

J. Appl. Phys. 98, 063524 (2005); http://dx.doi.org/10.1063/1.2058198 (6 pages) | Cited 2 times

Online Publication Date: 29 September 2005

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There exists a unique phenomenon of a large amount of liquid adhering to an ultrasonically vibrating end surface in the horizontal plane. The droplet assumes characteristic shapes that depend on the amplitude of vibration. In order to examine the mechanism of this phenomenon, the effects of the intrinsic surface tension of the adhering liquid and distribution of the intensity of the acoustic radiation pressure on the adhering droplet were investigated. Based on the experimental results, a dynamic balance model of the adhering droplet was constructed considering the balance of weight, surface tension, and acoustic radiation pressure. The calculated shape of the adhering droplet using this model is in good agreement with that observed. The mechanism that allows a large amount of a liquid to adhere to the horizontal end surface is discussed, focusing on the distribution of the acoustic radiation pressure in the droplet.
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62.60.+v Acoustical properties of liquids
47.55.D- Drops and bubbles

Early stages of soldering reactions

R. A. Lord and A. Umantsev

J. Appl. Phys. 98, 063525 (2005); http://dx.doi.org/10.1063/1.2058186 (11 pages) | Cited 13 times

Online Publication Date: 29 September 2005

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An experiment on the early stages of intermetallic compound layer growth during soldering and its theoretical analysis were conducted with the intent to study the controlling factors of the process. An experimental technique based on fast dipping and pulling of a copper coupon in liquid solder followed by optical microscopy allowed the authors to study the temporal behavior of the sample on a single micrograph. The technique should be of value for different areas of metallurgy because many experiments on crystallization may be described as the growth of a layer of intermediate phase. Comparison of the experimental results with the theoretical calculations allowed one to identify the kinetics of dissolution as the rate-controlling mechanism on the early stages and measure the kinetic coefficient of dissolution. A popular model of intermetallic compound layer structure coarsening is discussed.
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81.20.Vj Joining; welding
81.05.Bx Metals, semimetals, and alloys
64.70.D- Solid-liquid transitions
64.75.-g Phase equilibria
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