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Records: 1 - 20 of 168 - Pages: 
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Journal of Applied Physics : The temperature dependence of the refractive indices of GaN and AlN from room temperature up to 515°C

By: Naoki Watanabe, Tsunenobu Kimoto, and Jun Suda

Description: The temperature dependence of the refractive indices of GaN and AlN was investigated in the wavelength range from the near band edge (367 nm for GaN and 217 nm for AlN) to 1000 nm and the temperature range from room temperature to 515 °C. Optical interference measurements with vertical incident configuration were employed to precisely evaluate the ordinary refractive indices....

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Journal of Applied Physics : Effect of iron in silicon feedstock on p- and n-type multicrystalline silicon solar cells

By: G. Coletti, R. Kvande, V. D. Mihailetchi, L. J. Geerligs, L. Arnberg et al

Description: The effect of iron contamination in multicrystalline silicon ingots for solar cells has been investigated. Intentionally contaminated p- and n-type multicrystalline silicon ingots were grown by adding 53 ppm by weight of iron in the silicon feedstock. They are compared to reference ingots produced from nonintentionally contaminated silicon feedstock. p-type and n-type solar cell processes were applied to wafers sliced from these ingots. The as-grown minority carrier lifetime in the iron doped ingots is about 1–2 and 6–20 μs for p and n types, respectively. After phosphorus diffusion and hydrogenation this lifetime is improved up to 50 times in the p-type ingot, and about five times in the n-type ingot. After boron/phosphorus codiffusion and hydrogenation the improvement is about ten times for the p-type ingot and about four times for the n-type ingot. The as-grown interstitial iron concentration in the p-type iron doped ingot is on the order of 1013 cm−3, representing about 10% of the total iron concentration in the ingot, and is reduced to below 1011 cm−3 after phosphorus diffusion and subsequent hydrogenation. The con...

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Journal of Applied Physics : Specific multiple-scattering process in acoustic cloak with multilayered homogeneous isotropic materials

By: Ying Cheng and XiaoJun Liu

Description: It was qualitatively demonstrated through finite-element full-wave simulations that acoustic cloak can be constructed by using concentric multilayered structure with alternating homogeneous isotropic materials [ Y. Cheng et al., Appl. Phys. Lett. 92, 151913 (2008) ]. Here we present a sequential in-depth analysis of the proposed cloak by means of the multiple-scattering algorithms. Calculated pressure fields demonstrate that the cloak possesses low-reflection and wavefront-bending properties. The scattering patterns further characterize the directional cloaking performance in the far field, which is consistent with the pressure fields. The mechanism of the cloaking is ascribed to a specific multiple-scattering process determined by the microscopic material distribution and structural details of the cloak. We also discuss the behavior of the multilayered cloak as a function of wavelength....

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Journal of Applied Physics : Effective medium theory for strongly coupled randomly oriented conducting fibers on a planar surface

By: P. Loschialpo, D. Smith, and D. Zabetakis

Description: A theoretical model has been developed for predicting the reflectivity of thin planar surfaces imbedded with long conductive fibers. Intersecting fibers in the plane form larger conducting clusters. The model includes this effect, which serves to increase the reflectivity. Magnetic coupling between the fibers is also included. This coupling is found to be particularly important at higher frequencies. Test panels were made using a direct write process to imbed the long conducting fiber regions on a thin layer of cellulose. Comparisons of the model with both method of moments calculations and experimental measurements serve to validate its accuracy....

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Journal of Applied Physics : Ultrasonic generation and rotation of a small droplet at the tip of a hypodermic needle

By: Zhi Wen Tan, Su Gui Gisela Teo, and Junhui Hu

Description: We show that a hypodermic needle can generate and rotate a small droplet of water around its needle tip when the hypodermic needle is mechanically driven by an acoustic needle. The water droplet increases in size with the driving time and begins to rotate around the tip of the hypodermic needle when the diameter of the droplet is large enough. The rotating droplet falls off when its diameter is larger than a critical value of several millimeters. For a given operating voltage, the revolution speed, maximum diameter, and lifetime of a droplet can be controlled by the operating frequency of the ultrasonic transducer. A revolution speed of up to 16.7 revolutions/s has been achieved....

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Journal of Applied Physics : Single dominant distribution of Ge nanogranule embedded in Al oxide thin film

By: Seishi Abe, Masato Ohnuma, D. H. Ping, and Shigehiro Ohnuma

Description: This paper investigates size distribution of Ge nanogranules embedded in Al oxide thin film prepared by rf reactive sputtering method. It is found from the results of x-ray diffraction and small angle x-ray diffraction spectroscopy that their mean sizes distribute bimodally or single dominantly with respect to sputtering process parameter of additional oxygen ratio in Ar and Ge concentrations. Compositional plane of these distribution types reveals that single dominant distribution appears along the line of stoichiometric composition of Al2O3, and deviation from stoichiometry results in bimodal distribution. Thus, size uniformity of Ge nanogranules seems to be enhanced when the Al oxide matrix forms stoichiometric composition....

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Journal of Applied Physics : Phase transition hysteresis and anomalous Curie…Weiss behavior of ferroelectric tetragonal tungsten bronzes Ba2RETi2Nb3O15:RE = Nd,Sm

By: Marta Prades, Héctor Beltrán, Nahum Masó, Eloisa Cordoncillo, and Anthony R. West

Description: The ferroelectric tetragonal tungsten bronze (TTB) phases, Ba2RETi2Nb3O15:RE = Nd,Sm, were prepared by low temperature solvothermal synthesis. The permittivity versus temperature data of sintered ceramics show two unusual features: first, a hysteresis of 50–100 °C between values of the Curie temperature Tc on heat-cool cycles and second: a huge depression in the Curie–Weiss temperature T0. Both effects are attributed to the complex nature of their TTB-related crystal structures with different superstructures above and below Tc and the difficulty in nucleating ferroelectric domains on cooling through Tc. Several factors may contribute to the latter difficulty: first, the structures contain two sets of crystallographic sites for the “active” Ti, Nb ions; second, the distribution of Ti and Nb over these two sets of sites is not random but partially ordered; and third, below Tc a weak commensurate superstructure perpendicular to the polar axis is present, but above Tc a weak incommensurate superstructure in a similar orientation is present. Hence the formation of the ferroelectric structure on cooling requires both nucleat...

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Journal of Applied Physics : Influence of Mg doping on the giant dielectric tunability in LuFe2O4

By: Yi Liu, Chang-Hui Li, Xiang-Qun Zhang, Zhao-Hua Cheng, and Young Sun

Description: A giant dielectric tunability effect in LuFe2O4 was recently discovered by our group. Here we report the influence of Mg doping on the dielectric tunability and dielectric loss in LuFe2−xMgxO4 (x ≤ 0.1) samples. It was found that the temperature interval of the giant dielectric tunability is broadened by a slight (5%) Mg doping, while the dielectric loss remains the same order. The maximum value of tunability is reduced by Mg doping, which supports the viewpoint that the giant dielectric tunability is crucially related to charge ordering of Fe2+ and Fe3+ ions. These results suggest that the giant dielectric tunability in LuFe2O4 can be finely tuned by Mg doping....

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Journal of Applied Physics : Influence of interfacial dislocations on hysteresis loops of ferroelectric films

By: Y. L. Li, S. Y. Hu, S. Choudhury, M. I. Baskes, A. Saxena et al

Description: We investigated the influence of dislocations, located at the interface of a ferroelectric film and its underlying substrate, on the ferroelectric hysteresis loop including the remanent polarization and coercive field using phase-field simulations. We considered epitaxial ferroelectric BaTiO3 films and found that the hysteresis loop is strongly dependent on the type and density of interfacial dislocations. The dislocations that stabilize multiple ferroelectric variants and domains reduce the coercive field, and consequently, the corresponding remanent polarization also decreases....

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Journal of Applied Physics : Microwave dielectric and Raman scattering studies on bismuth zinc niobate thin films

By: K. Sudheendran, K. C. James Raju, Manoj K. Singh, and Ram S. Katiyar

Description: Bismuth zinc niobate (Bi1.5Zn1.0Nb1.5O7) thin films were deposited on fused silica substrates by pulsed laser deposition. Its phase composition and microstructure were characterized by x-ray diffraction and atomic force microscopy. The as deposited films were amorphous and all of them were crystallized after a postdeposition annealing at 600 °C for 30 min in air. The crystallite size of the films was found to be varying between 31 and 60 nm with respect to the oxygen pressure in the deposition chamber. The films were found to be less oriented with the decreasing crystallite size. The films deposited under different oxygen pressures showed a microwave dielectric permittivity in between 98–124 and a loss tangent in between 0.005 and 0.007 over the X-band frequency range. From the Raman scattering data of the bulk and thin films, it was found that the Raman modes in thin films are relatively more intense and additional Raman modes were appearing in the films in the low frequency range, which reveal that the local symmetry in the films is different from that of the bulk. The variations in the microwave dielectric properties...

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Journal of Applied Physics : Stress dependence and scaling of subcoercive field dynamic hysteresis in 0.5Pb(Zr1/2Ti1/2)O3…0.5Pb(Zn1/3Nb2/3)O3 ceramic

By: R. Yimnirun, N. Wongdamnern, N. Triamnak, M. Unruan, A. Ngamjarurojana et al

Description: Stress dependence and scaling behavior of subcoercive field dynamic ferroelectric hysteresis were investigated in rhombohedral 0.5Pb(Zr1/2Ti1/2)O3–0.5Pb(Zn1/3Nb2/3)O3 (0.5PZT–0.5PZN) bulk ceramic with profound relaxor characteristics. The scaling relation of hysteresis area 〈A〉 against frequency f, field amplitude E0, and stress σ for the minor loops takes a form of 〈A−Aσ = 0〉∝f−0.34E03.68σ0.35, which is very similar to that of soft and hard lead zirconate titanate ceramics with the same disrupted domains. This study suggests that the domain structures play a significant role in controlling the dynamic hysteresis behavior of ferroelectric materials....

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Journal of Applied Physics : Magnetic cluster glass behavior and grain boundary effect in Nd0.7Ba0.3MnO3 nanoparticles

By: B. Roy and S. Das

Description: The magnetic and transport properties of Nd0.7Ba0.3MnO3 nanoparticles were explored by transmission electron microscopy, x-ray powder diffraction, resistivity, magnetoresistance, thermopower (S), and magnetic measurements. The metal-insulator transition behavior of the temperature dependence of resistivity for the sample with the largest particle size changes to insulating nature with the decrease in the particle size due to the enhancement of the grain boundary effect. The magnetoresistance of the nanoparticles is analyzed in the light of a phenomenological model based on the spin polarized tunneling at the grain boundaries. The thermopower of the samples shows a crossover from negative to positive values and at high temperatures S follows adiabatic small polaron hopping theory. The zero field cooled and field cooled (FC) magnetizations display broad ferromagnetic transition. The Curie temperature (TC) and the irreversibility temperature (Tirr) decrease considerably with the decrease in the particle size. During cooling the ac susceptibility of the nanoparticles exhibits two magnetic phase transitions with paramagnetic...

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Journal of Applied Physics : Fabrication of YBa2Cu3O7 thin film on cube-textured Cu tape

By: Makoto Tokudome, Toshiya Doi, Ryota Tomiyasu, Shinya Sato, Yoshinori Hakuraku et al

Description: We achieved a biaxially oriented YBa2Cu3O7 film on a {100} 〈001〉 textured Cu tape with Jc of 1.9×106 A cm−2 at 77 K. A Ni overlayer was deposited by electroplating technique on the {100} 〈001〉 textured Cu tape to improve oxidation resistance of Cu substrate, and then YBa2Cu3O7 was deposited on the CeO2/YSZ/CeO2 buffered Ni-electroplated Cu tape. We confirmed that the Ni-electroplated Cu tape lost its ferromagnetism after 700 °C at 1 h heat treatment. Our rolling assisted biaxially textured substrate (RABiTS) type coated conductor can be used for ac applications and is thought to be more cost feasible than the standard RABiTS architecture using Ni-alloy tapes....

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Journal of Applied Physics : Tuning of magnetism in SrRuO3 thin films on SrTiO3 (001) substrate by control of the twin and strain amount in the buffer layer

By: B. W. Lee, C. U. Jung, M. Kawasaki, and Y. Tokura

Description: Control of the magnetic easy axis in SrRuO3 thin films is important for tunnel junction studies incorporating this layer and for wider device applications. Previously, we demonstrated control of the magnetic easy axis in SrRuO3 thin films on SrTiO3 (110) substrates by introducing tunable anisotropic strain through a CaHfO3 buffer layer. However, our method was not directly applicable due to the existence of twins in the CaHfO3 layer that grew on the SrTiO3 (001) substrate. Twins appeared partly because of the crystal structure of CaHfO3, which has large orthorhombic distortion (a−b)/a = 2.5%. Here, we used a miscut SrTiO3 (001) substrate to create a twin-free CaHfO3 layer on the substrate, even though the large mismatch between the CaHfO3 and SrTiO3 (001) substrates led to relaxed growth behavior. The crystal quality of the CaHfO3 layer, as determined by the shape of the rocking curve and existence of twins, was markedly improved by use of the miscut substrate. In addition, the SrRuO3 film grown on top of this buffer layer was of high quality and tensile strain and had an in-plane magnetic easy axis....

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Journal of Applied Physics : Effect of grain refinement on enhancing critical current density and upper critical field in undoped MgB2 ex situ tapes

By: A. Malagoli, V. Braccini, M. Tropeano, M. Vignolo, C. Bernini et al

Description: Ex situ powder-in-tube MgB2 tapes prepared with ball-milled, undoped powders showed a strong enhancement of the irreversibility field H∗, the upper critical field Hc2, and the critical current density Jc(H) together with the suppression of the anisotropy of all of these quantities. Jc reached 104 A/cm2 at 4.2 K and 10 T, with an irreversibility field of about 14 T at 4.2 K, and Hc2 of 9 T at 25 K, high values for not-doped MgB2. The enhanced Jc and H∗ values are associated with significant grain refinement produced by milling of the MgB2 powder, which enhances grain boundary pinning, even if at the same time also reducing the connectivity from about 12% to 8%. Although enhanced pinning and diminished connectivity are in opposition, the overall influence of ball milling on Jc is positive because the increased density of grains with a size comparable with the mean free path produces strong electron scattering that substantially increases Hc2, especially Hc2 perpendicular to the Mg and B planes....

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Journal of Applied Physics : CoCrPt-oxide based perpendicular recording media with hybrid soft magnetic underlayers

By: S. N. Piramanayagam, K. Srinivasan, R. Sbiaa, Yan Dong, and R. H. Victora

Description: Writability of magnetic recording media with high anisotropy constant (Ku) is one of the major challenges encountered in extremely high density recording. A possible avenue to improve writability in perpendicular magnetic recording is to reduce the spacing between the writing head and the soft magnetic underlayer (SUL) or head-to-keeper spacing (HKS). A hybrid layer structure for reducing the HKS between the SUL and the writing head is investigated. The proposed structure makes use of a crystalline SUL with in-plane anisotropy together with an amorphous SUL to form a hybrid SUL (H-SUL). The H-SUL design helps to achieve better surface smoothness, thinner HKS, and good c-axis dispersion for the recording layer while still maintaining small and exchange-decoupled grains. Micromagnetic simulation demonstrates that the H-SUL scheme helps in improving writing field effectiveness and increasing field gradient along down-track and cross-track directions....

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Journal of Applied Physics : Low field giant magnetoresistance in coupled elliptical rings

By: S. Jain and A. O. Adeyeye

Description: The giant magnetoresistance response of ellipse shaped Co(10 nm)/Cu(8 nm)/Ni80Fe20(10 nm)/Cu(2 nm) rings magnetostatically coupled to external magnetic elements has been systematically investigated. Magnetoresistance responses at low field show distinct states when the field is cycled through various intermediate magnetization states of Ni80Fe20 and Co layers. Due to strong magnetostatic coupling between the ring and the external magnetic elements, these metastable intermediate states were not observed when the field is cycled through saturation. We have also investigated the temperature dependence of magnetization behavior of these rings. Our experimental results are in good agreement with micromagnetic simulations....

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Journal of Applied Physics : Gallium oxide and gadolinium gallium oxide insulators on Si d-doped GaAs/AlGaAs heterostructures

By: G. W. Paterson, P. Longo, J. A. Wilson, A. J. Craven, A. R. Long et al

Description: Test devices have been fabricated on two specially grown GaAs/AlGaAs wafers with 10 nm thick gate dielectrics composed of either Ga2O3 or a stack of Ga2O3 and Gd0.25Ga0.15O0.6. The wafers have two GaAs transport channels either side of an AlGaAs barrier containing a Si δ-doping layer. Temperature dependent capacitance-voltage (C-V) and current-voltage (I-V) studies have been performed at temperatures between 10 and 300 K. Bias cooling experiments reveal the presence of DX centers in both wafers. Both wafers show a forward bias gate leakage that is by a single activated channel at higher temperatures and by tunneling at lower temperatures. When Gd0.25Ga0.15O0.6 is included in a stack with 1 nm of Ga2O3 at the interface, the gate leakage is greatly reduced due to the larger band gap of the Gd0.25Ga0.15O0.6 layer. The different band gaps of the two oxides result in a difference in the gate voltage at the onset of leakage of ∼ 3 V. However, the inclusion of Gd0.25Ga0.15O0.6 in the gate insulator introduces many oxide states ( ≥ 4.70×1012 cm−2). Transmission electron microscope images of the interface region show that the gr...

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Journal of Applied Physics : Characterization of dilute InPN layers grown by liquid phase epitaxy

By: T. D. Das, S. Dhar, and B. M. Arora

Description: We have grown dilute InPN layers by liquid phase epitaxy and characterized them using high resolution x-ray diffraction, optical absorption, low temperature photoluminescence, and Hall measurement techniques. Our results indicate that a maximum amount of 0.2% nitrogen has been incorporated in the material with a band gap lowering consistent with expectations. The crystalline quality of the material is found to improve upon nitrogen incorporation. Large increase in luminescence from the material is observed after a high temperature annealing....

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Journal of Applied Physics : Electronic properties of (Ga,Mn)N thin films with high Mn content

By: S. Granville, B. J. Ruck, A. R. H. Preston, T. Stewart, F. Budde et al

Description: Optical and dc resistivity measurements as well as x-ray spectroscopies have been performed on (Ga,Mn)N films containing Mn at up to 11 at. %. The results indicate that at higher Mn contents, the Fermi level is situated within extended states, while GaN host interband optical transitions are unaffected. The Mn state is confirmed to be 3d5, as in the case of lower Mn content films; however, the high Mn content merges the 3d levels into a band located just below the host conduction band. The Fermi level is located within these Mn states just below the conduction band, in sharp contrast to its midgap position in fully crystalline, low Mn concentration materials. The difference in the position of the Fermi level at high Mn dopant levels has important implications for the promotion of ferromagnetism in this material....

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