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28 Oct 2002

Volume 81, Issue 18, pp. 3311-3500

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Upside-down tuning of light- and heavy-hole states in GaNAs/GaAs single quantum wells by thermal expansion and quantum confinement

M. H. Ya, W. Z. Cheng, Y. F. Chen, and T. Y. Lin

Appl. Phys. Lett. 81, 3386 (2002); http://dx.doi.org/10.1063/1.1518777 (3 pages) | Cited 4 times

Online Publication Date: 22 October 2002

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Polarization resolved photoluminescence from a cleaved sample edge (edge photoluminescence) was used to investigate the valence-band splitting in GaNAs/GaAs single quantum wells. The spectra resulting from the heavy- and light-hole transitions shows an interesting phenomenon, in which the light-hole state is above the heavy-hole state at low temperatures, they degenerate at about 195 K, and finally the light-hole state is below the heavy-hole state at higher temperatures. We point out that the underlying origin of our observation can be attributed to the competitive effect between the tensile strain induced by lattice mismatch and quantum confinement. It is known that the large density of heavy-hole states is beneficial for laser diodes. Our result is therefore very useful for the application of GaNAs/GaAs quantum wells in optical devices. © 2002 American Institute of Physics.
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73.21.Fg Quantum wells
78.67.De Quantum wells
81.07.St Quantum wells
78.55.Cr III-V semiconductors
81.05.Ea III-V semiconductors
68.65.Fg Quantum wells
65.40.De Thermal expansion; thermomechanical effects

Angle-dependent x-ray absorption spectroscopy study of Zn-doped GaN

J. W. Chiou, S. Mookerjee, K. V. R. Rao, J. C. Jan, H. M. Tsai, K. Asokan, W. F. Pong, F. Z. Chien, M.-H. Tsai, Y. K. Chang, Y. Y. Chen, J. F. Lee, C. C. Lee, and G. C. Chi

Appl. Phys. Lett. 81, 3389 (2002); http://dx.doi.org/10.1063/1.1518776 (3 pages) | Cited 9 times

Online Publication Date: 22 October 2002

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As-grown and Zn-implanted wurtzite GaN samples have been studied by angle-dependent x-ray absorption near edge structure (XANES) measurements at the N and Ga K-edges and the Ga L3-edge. The angle dependence of the XANES spectra shows that the Ga–N bonds lying in the bilayer have lower energies than bonds along the c-axis, which can be attributed to the polar nature of the GaN film. The comparison of the Ga L3-edge XANES spectra of as-grown and Zn-doped GaN reveals significant dopant induced enhancement of near-edge Ga d-derived states. © 2002 American Institute of Physics.
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61.05.cj X-ray absorption spectroscopy: EXAFS, NEXAFS, XANES, etc.
61.72.uj III-V and II-VI semiconductors

Frequency characterization and modeling of interface traps in HfSixOy/HfO2 gate dielectric stack from a capacitance point-of-view

Pascal Masson, Jean-Luc Autran, Michel Houssa, Xavier Garros, and Charles Leroux

Appl. Phys. Lett. 81, 3392 (2002); http://dx.doi.org/10.1063/1.1518561 (3 pages) | Cited 31 times

Online Publication Date: 22 October 2002

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A time-resolved analysis of the capacitance–voltage (C–V) technique and an inverse modeling approach have been developed to determine the energy distribution and the capture cross section of interface traps in the silicon band gap from multifrequency C–V measurements. In this work, our method is performed on n-type metal-oxide-semiconductor capacitors with HfSixOy/HfO2 gate dielectric stack and polysilicon gate. From the frequency dispersion of C–V data, we evidence a peak of acceptor states in the upper half of the band gap at 0.81 eV above the valence band and characterized by a capture cross section of 1.5×10−17 cm2. This value is approximately ten times lower than typical capture cross sections relative to the dangling bonds (Pb centers) at the Si/SiO2 interface, which is in good agreement with a Coulombic center model predicting a capture cross section inversely proportional to the square of the dielectric permittivity. © 2002 American Institute of Physics.
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73.20.Hb Impurity and defect levels; energy states of adsorbed species
73.40.Qv Metal-insulator-semiconductor structures (including semiconductor-to-insulator)
81.05.Cy Elemental semiconductors
77.84.Bw Elements, oxides, nitrides, borides, carbides, chalcogenides, etc.
71.20.Mq Elemental semiconductors
71.55.Cn Elemental semiconductors
72.20.Jv Charge carriers: generation, recombination, lifetime, and trapping
77.22.Ch Permittivity (dielectric function)
77.55.-g Dielectric thin films

Lifetime of nonequilibrium zone-center longitudinal optical phonons in zinc-blende materials

Saswati Barman and G. P. Srivastava

Appl. Phys. Lett. 81, 3395 (2002); http://dx.doi.org/10.1063/1.1517170 (3 pages) | Cited 10 times

Online Publication Date: 22 October 2002

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We employ a theoretical model to assess the importance of Ridley’s channel in limiting the lifeitime of nonequilibrium zone-center LO phonons in zinc-blende materials. For comparison, we also present results for the decay through Klemens’s channel as well as via Vallée–Bogani’s intraband combination channel. © 2002 American Institute of Physics.
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63.20.-e Phonons in crystal lattices

Effects of hydrostatic and uniaxial stress on the conductivity of p-type GaN epitaxial layer

Y. Liu, M. Z. Kauser, M. I. Nathan, P. P. Ruden, A. M. Dabiran, B. Hertog, and P. P. Chow

Appl. Phys. Lett. 81, 3398 (2002); http://dx.doi.org/10.1063/1.1517713 (3 pages) | Cited 8 times

Online Publication Date: 22 October 2002

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We present experimental results on the dependence of the electrical conductivity of Mg-doped, p-type GaN on hydrostatic pressure and uniaxial stress. The samples were grown by molecular beam epitaxy on sapphire substrates. Hydrostatic pressure over the range studied (0–7 kbar) leads to a relatively small decrease in the conductivity. Uniaxial stress in the basal plane results in strongly increasing conductivity parallel to the stress direction and in weakly decreasing conductivity perpendicular to the stress direction. We relate the observed symmetry of the piezoconductivity to deformation potential induced changes in the band structure near the top of the valence band. © 2002 American Institute of Physics.
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73.61.Ey III-V semiconductors
73.50.Dn Low-field transport and mobility; piezoresistance
62.50.-p High-pressure effects in solids and liquids
71.20.Nr Semiconductor compounds

Electron transport studies on In0.30Ga0.70As/GaAs-quantum-well infrared photodetectors using time-resolved photocurrent measurements

S. Steinkogler, H. Schneider, R. Rehm, M. Walther, and P. Koidl

Appl. Phys. Lett. 81, 3401 (2002); http://dx.doi.org/10.1063/1.1518153 (3 pages) | Cited 3 times

Online Publication Date: 22 October 2002

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We report on the time-resolved photocurrent response of an InGaAs/GaAs quantum-well infrared photodetector stimulated by infrared fs-laser pulses ( ∼ 9 μm wavelength). We observe two dynamic photocurrent components, which are driven by the optical excitation and by the photoinduced space charge, respectively. The ratio of the time-averaged values of these two components allows us to determine the photoconductive gain. These gain values exhibit a good quantitative agreement with the gain obtained from continuous waves measurements of the peak responsivity. Furthermore, we present a detailed study of the decay time constant of the slow component as a function of the temperature and the bias voltage and compare our results with a recent theoretical model. © 2002 American Institute of Physics.
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73.63.Hs Quantum wells
78.67.De Quantum wells
85.60.Gz Photodetectors (including infrared and CCD detectors)
85.35.Be Quantum well devices (quantum dots, quantum wires, etc.)
73.61.Ey III-V semiconductors
78.66.Fd III-V semiconductors
78.30.Fs III-V and II-VI semiconductors
73.50.Pz Photoconduction and photovoltaic effects
73.50.Gr Charge carriers: generation, recombination, lifetime, trapping, mean free paths

Explanation for the leakage current in polycrystalline-silicon thin-film transistors made by Ni-silicide mediated crystallization

P. J. van der Zaag, M. A. Verheijen, S. Y. Yoon, and N. D. Young

Appl. Phys. Lett. 81, 3404 (2002); http://dx.doi.org/10.1063/1.1517406 (3 pages) | Cited 29 times

Online Publication Date: 22 October 2002

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The source of the leakage current in polycrystalline-silicon (poly-Si) thin-film transistors (TFTs) made by Ni-mediated crystallization has been investigated. Studies of TFTs and of the crystallization process by in situ transmission electron microscopy show that the crystallization process is a two-stage process and that the cause of the leakage problem is associated with incomplete crystallization of amorphous-Si. By removing the last pockets of amorphous-Si, for instance, by long anneals, poly-Si TFTs can be made with adequately low leakage current <1 pA/μm (at a source–drain voltage of 5 V) for display applications, despite the presence of Ni up to 2.5×1019 atoms/cm3. © 2002 American Institute of Physics.
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85.30.Tv Field effect devices
85.30.De Semiconductor-device characterization, design, and modeling
68.37.Lp Transmission electron microscopy (TEM)
81.40.Gh Other heat and thermomechanical treatments

As-mediated stacking fault in wurtzite GaN epilayers

Hyonju Kim, T. G. Andersson, J.-M. Chauveau, and A. Trampert

Appl. Phys. Lett. 81, 3407 (2002); http://dx.doi.org/10.1063/1.1519096 (3 pages) | Cited 4 times

Online Publication Date: 22 October 2002

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Growth of a thin GaAs layer embedded in wurtzite GaN was performed on sapphire (0001) by rf-plasma assisted molecular-beam epitaxy. A well-confined As-rich layer with a tail in the cap layer was measured by secondary ion mass spectrometry. Transmission electron microscopy studies demonstrated the formation of basal plane stacking faults, which were correlated with the presence of As in the layer. High-resolution microscopy revealed the stacking fault as a thin platelike cubic inclusion in the hexagonal GaN. © 2002 American Institute of Physics.
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68.65.Ac Multilayers
61.72.Nn Stacking faults and other planar or extended defects
81.15.Hi Molecular, atomic, ion, and chemical beam epitaxy
68.37.Lp Transmission electron microscopy (TEM)

Ultrathin silicon dioxide layers with a low leakage current density formed by chemical oxidation of Si

Asuha, Takuya Kobayashi, Osamu Maida, Morio Inoue, Masao Takahashi, Yoshihiro Todokoro, and Hikaru Kobayashi

Appl. Phys. Lett. 81, 3410 (2002); http://dx.doi.org/10.1063/1.1517723 (3 pages) | Cited 34 times

Online Publication Date: 22 October 2002

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Chemical oxidation of Si by use of azeotrope of nitric acid and water can form 1.4-nm-thick silicon dioxide layers with a leakage current density as low as those of thermally grown SiO2 layers. The capacitance–voltage (C–V) curves for these ultrathin chemical SiO2 layers have been measured due to the low leakage current density. The leakage current density is further decreased to ∼ 1/5 (cf. 0.4 A/cm2 at the forward gate bias of 1 V) by post-metallization annealing at 200 °C in hydrogen. Photoelectron spectroscopy and C–V measurements show that this decrease results from (i) increase in the energy discontinuity at the Si/SiO2 interface, and (ii) elimination of Si/SiO2 interface states and SiO2 gap states. © 2002 American Institute of Physics.
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73.40.Qv Metal-insulator-semiconductor structures (including semiconductor-to-insulator)
73.61.Ng Insulators
81.65.Mq Oxidation
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
79.60.Jv Interfaces; heterostructures; nanostructures
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Noninvasive magnetic imaging and magnetization measurement of isolated mesoscopic Co rings

J. Bekaert, D. Buntinx, C. Van Haesendonck, V. V. Moshchalkov, J. De Boeck, G. Borghs, and V. Metlushko

Appl. Phys. Lett. 81, 3413 (2002); http://dx.doi.org/10.1063/1.1518564 (3 pages) | Cited 26 times

Online Publication Date: 22 October 2002

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A high-resolution scanning Hall probe microscope was used as a noninvasive technique to visualize the magnetization reversal in an array of micron-size Co rings. Two stable “onion” states at remanence and “vortex” states at switching fields were found. To rule out a possible influence of dipole–dipole interaction between ring elements on remagnetization processes, an isolated Co ring was deposited on top of a Hall magnetometer and extremely sharp transitions from onion to vortex and from vortex to onion state of opposite polarity were resolved. Our results were supported by MOKE magnetization measurements and micromagnetic simulations. © 2002 American Institute of Physics.
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75.75.-c Magnetic properties of nanostructures
75.60.Jk Magnetization reversal mechanisms
75.50.Cc Other ferromagnetic metals and alloys
07.55.Jg Magnetometers for susceptibility, magnetic moment, and magnetization measurements
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
78.20.Ls Magneto-optical effects

Magnetocaloric effect in layered perovskite manganese oxide La1.4Ca1.6Mn2O7

Hong Zhu, Hao Song, and YuHeng Zhang

Appl. Phys. Lett. 81, 3416 (2002); http://dx.doi.org/10.1063/1.1518160 (3 pages) | Cited 12 times

Online Publication Date: 22 October 2002

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We report the discovery of a large magnetic entropy change in La1.4Ca1.6Mn2O7, a bilayered perovskite manganese oxide that has a Curie temperature (TC) of about 270 K and which allows magnetic refrigeration at room temperature. The magnetic entropy changes reach values of 11.3 J K−1 kg−1 and 16.8 J K−1 kg−1 for field changes of 2 T and 5 T, respectively. The refrigerant capacity of our material is much larger than that of Gd. The large entropy change can be attributed to the fact that the ferromagnetic transition enhances the effect of the applied magnetic field greatly. © 2002 American Institute of Physics.
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75.30.Sg Magnetocaloric effect, magnetic cooling
75.47.Gk Colossal magnetoresistance
75.30.Kz Magnetic phase boundaries (including classical and quantum magnetic transitions, metamagnetism, etc.)
75.50.Dd Nonmetallic ferromagnetic materials

Enhancement of the critical current density and flux pinning of MgB2 superconductor by nanoparticle SiC doping

S. X. Dou, S. Soltanian, J. Horvat, X. L. Wang, S. H. Zhou, M. Ionescu, H. K. Liu, P. Munroe, and M. Tomsic

Appl. Phys. Lett. 81, 3419 (2002); http://dx.doi.org/10.1063/1.1517398 (3 pages) | Cited 391 times

Online Publication Date: 22 October 2002

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Doping of MgB2 by nano-SiC and its potential for the improvement of flux pinning were studied for MgB2−x(SiC)x/2 with x = 0, 0.2, and 0.3 and for 10 wt % nano-SiC-doped MgB2 samples. Cosubstitution of B by Si and C counterbalanced the effects of single-element doping, decreasing Tc by only 1.5 K, introducing intragrain pinning centers effective at high fields and temperatures, and significantly enhancing Jc and Hirr. Compared to the undoped sample, Jc for the 10 wt % doped sample increased by a factor of 32 at 5 K and 8 T, 42 at 20 K and 5 T, and 14 at 30 K and 2 T. At 20 K and 2 T, the Jc for the doped sample was 2.4×105 A/cm2, which is comparable to Jc values for the best Ag/Bi-2223 tapes. At 20 K and 4 T, Jc was twice as high as for the best MgB2 thin films and an order of magnitude higher than for the best Fe/MgB2 tapes. The magnetic Jc is consistent with the transport Jc which remains at 20 000 A/cm2 even at 10 T and 5 K for the doped sample, an order of magnitude higher than the undoped one. Because of such high performance, it is anticipated that the future MgB2 conductors will be made using a formula of MgBxSiyCz instead of pure MgB2. © 2002 American Institute of Physics.
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74.70.Ad Metals; alloys and binary compounds (including A15, MgB2, etc.)
74.25.Sv Critical currents
74.25.Uv Vortex phases (includes vortex lattices, vortex liquids, and vortex glasses)
74.25.Ha Magnetic properties including vortex structures and related phenomena

Influence of near-surface nonstoichiometry on the surface magnetization of mixed-valent manganite: A computer simulation study

U. P. Wad, Abhijit S. Ogale, S. B. Ogale, and T. Venkatesan

Appl. Phys. Lett. 81, 3422 (2002); http://dx.doi.org/10.1063/1.1517399 (3 pages) | Cited 6 times

Online Publication Date: 22 October 2002

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A Monte Carlo simulation study is performed to examine the effects of near-surface nonstoichiometry in a mixed-valent manganite (e.g., La0.7A0.3MnO3, where A = Sr, Ca) on the magnetization of its surface, and that of a few layers underneath. The nonstoichiometry is introduced either in the form of oxygen vacancy gradient or a gradient in the La:A ratio. The corresponding gradient in the exchange constant J is incorporated phenomenologically from the known phase diagram [P. Schiffer et al., Phys. Rev. Lett. 75, 3356 (1995)]. We show that the near-surface nonstoichiometry can account for the temperature dependence of magnetization of the surface layers as revealed by photoemission and x-ray magnetic circular dichroism (X-MCD) experiments [J. Park et al., Nature (London) 392, 794 (1998); J. Park et al., Phys. Rev. Lett. 81, 1953 (1998)]. The demagnetization shows viscous fingeringlike protrusions which may have important consequences for spin-polarized transport across interfaces. © 2002 American Institute of Physics.
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75.70.Rf Surface magnetism
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
75.47.Gk Colossal magnetoresistance
75.30.Mb Valence fluctuation, Kondo lattice, and heavy-fermion phenomena
75.20.Hr Local moment in compounds and alloys; Kondo effect, valence fluctuations, heavy fermions
75.50.Dd Nonmetallic ferromagnetic materials
78.20.Ls Magneto-optical effects
79.60.-i Photoemission and photoelectron spectra
61.66.Bi Elemental solids
61.66.Dk Alloys
75.30.Kz Magnetic phase boundaries (including classical and quantum magnetic transitions, metamagnetism, etc.)
75.30.Et Exchange and superexchange interactions
68.35.B- Structure of clean surfaces (and surface reconstruction)

Antiferromagnetic hysteresis in magnetoresistive multilayers investigated by x-ray resonant scattering

Carlo Spezzani, Piero Torelli, Maurizio Sacchi, Renaud Delaunay, Coryn F. Hague, Vincent Cros, and Frédéric Petroff

Appl. Phys. Lett. 81, 3425 (2002); http://dx.doi.org/10.1063/1.1517403 (3 pages) | Cited 5 times

Online Publication Date: 22 October 2002

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We have used resonant scattering of polarized soft x rays as a direct probe of the magnetic order in a weakly coupled Co/Cu multilayer. Our field dependent results, combined with in situ resistance measurements, show a direct correlation between magnetoresistance and antiparallel magnetic ordering in reversible and irreversible processes. © 2002 American Institute of Physics.
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75.47.De Giant magnetoresistance
75.70.Cn Magnetic properties of interfaces (multilayers, superlattices, heterostructures)
75.50.Ee Antiferromagnetics
78.70.Ck X-ray scattering

Magnetization dependence of training effect of exchange coupling in ferromagnet/FeMn bilayers

S. J. Yuan, L. Wang, S. M. Zhou, M. Lu, J. Du, and A. Hu

Appl. Phys. Lett. 81, 3428 (2002); http://dx.doi.org/10.1063/1.1517711 (3 pages) | Cited 11 times

Online Publication Date: 22 October 2002

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The exchange coupling and its training effect are studied as a function of the ferromagnetic layer magnetization by using various ferromagnet/FeMn bilayers with ferromagnetic materials Ni, Ni81Fe19, Ni50Fe50, Co, and Fe. The exchange coupling energy Jex increases with increasing MFM as Jex∝math. The training effect of the exchange field is related to both the ferromagnet magnetization and the magnetization reversal mechanism. For ferromagnet/FeMn bilayers with similar magnetization reversal mechanisms, the relative change of the exchange field decreases with increasing magnetization in an exponential manner. © 2002 American Institute of Physics.
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75.70.Ak Magnetic properties of monolayers and thin films
75.30.Et Exchange and superexchange interactions
75.60.Jk Magnetization reversal mechanisms
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
75.50.Bb Fe and its alloys
75.50.Cc Other ferromagnetic metals and alloys
75.50.Ee Antiferromagnetics
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Interfacial layer formation during high-temperature annealing of ZrO2 thin films on Si

J. M. Howard, V. Craciun, C. Essary, and R. K. Singh

Appl. Phys. Lett. 81, 3431 (2002); http://dx.doi.org/10.1063/1.1517407 (3 pages) | Cited 22 times

Online Publication Date: 22 October 2002

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High-k materials deposited directly on silicon exhibit an interfacial layer between the grown layer and the underlying substrate. This is of particular concern in metal–oxide–semiconductor technologies where these layers have a deleterious effect on the overall capacitance of the resulting devices. In this letter, the growth and properties of this silicatelike interfacial layer are examined after postdeposition anneals in a vacuum, inert, and oxidizing atmospheres. X-ray reflectivity, Fourier transform infrared spectroscopy, and x-ray photoelectron spectroscopy have been used to characterize the growth and properties of this interfacial layer. © 2002 American Institute of Physics.
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68.35.Fx Diffusion; interface formation
81.40.Gh Other heat and thermomechanical treatments
81.05.Je Ceramics and refractories (including borides, carbides, hydrides, nitrides, oxides, and silicides)
81.05.Cy Elemental semiconductors
77.55.-g Dielectric thin films
77.84.Bw Elements, oxides, nitrides, borides, carbides, chalcogenides, etc.
73.40.Qv Metal-insulator-semiconductor structures (including semiconductor-to-insulator)
68.35.Ct Interface structure and roughness
61.72.Cc Kinetics of defect formation and annealing
79.60.Jv Interfaces; heterostructures; nanostructures
66.30.Ny Chemical interdiffusion; diffusion barriers
78.30.-j Infrared and Raman spectra
78.66.Nk Insulators

Dielectric relaxation studies of Bi-doping effects on the oxygen-ion diffusion in La2−xBixMo2O9 oxide-ion conductors

X. P. Wang, Q. F. Fang, Z. S. Li, G. G. Zhang, and Z. G. Yi

Appl. Phys. Lett. 81, 3434 (2002); http://dx.doi.org/10.1063/1.1518151 (3 pages) | Cited 27 times

Online Publication Date: 22 October 2002

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Two dielectric relaxation peaks associated with oxygen-ion diffusion in the oxide-ion conductors La2−xBixMo2O9 (x = 0.05, 0.1, and 0.15) have been studied. It is found that the activation energies of the two peaks increase with increasing Bi-doping concentration, which results from the blocking effects of the lone-pair electrons of Bi3+ ions. From the different effects of Bi doping on the two peaks, the diffusion paths of oxygen ions corresponding to each peak are confirmed. Significantly, it is revealed that Bi doping could enhance the ionic conductivity of La2Mo2O9 at lower temperatures. © 2002 American Institute of Physics.
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66.30.H- Self-diffusion and ionic conduction in nonmetals
77.22.Gm Dielectric loss and relaxation

Evidence for forward domain growth being rate-limiting step in polarization switching in 〈111〉-oriented-Pb(Zr0.45Ti0.55)O3 thin-film capacitors

Seungbum Hong and Nava Setter

Appl. Phys. Lett. 81, 3437 (2002); http://dx.doi.org/10.1063/1.1517396 (3 pages) | Cited 11 times

Online Publication Date: 22 October 2002

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We studied polarization switching of 295 nm thick 〈111〉-oriented-Pb(Zr0.45Ti0.55)O3 (PZT) thin-film capacitors through a polarization hysteresis loop and piezoelectric force microscopy (PFM) on top of the Pt electrode. Positive voltage pulses of 450 kV/cm and 2.5 ms were applied to the top as a poling pulse. Negative pulses ranging from −45 to −60 kV/cm at an interval of 3 kV/cm were applied to the top electrode. After each pulse, we performed PFM imaging by applying 17 kHz ac voltage of 1 V peak to peak to the bottom electrode while grounding the top one and the tip. The first-harmonic amplitude and phase of the tip vibration signal were mapped over the scanned area of 10×10 μm2. Subsequently, we measured the polarization hysteresis curve at 100 Hz after each domain image acquisition to estimate the volume fraction of switched polarization induced by the positive voltage pulses. We analyzed the areal fraction of switched domains as a function of pulse voltage from phase images and compared it with the volume fraction obtained from the hysteresis loops. Based on the evolution of domain images and the comparison plots of the areal and volume fractions of switched domains, we conclude that forward domain growth is a rate-limiting step during polarization reversal in 〈111〉-oriented-PZT thin-film capacitors. © 2002 American Institute of Physics.
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85.50.-n Dielectric, ferroelectric, and piezoelectric devices
84.32.Tt Capacitors
77.55.-g Dielectric thin films
77.84.Ek Niobates and tantalates
77.84.Cg PZT ceramics and other titanates
77.80.Dj Domain structure; hysteresis
77.22.Ej Polarization and depolarization
77.80.Fm Switching phenomena

Flexoelectric polarization of barium strontium titanate in the paraelectric state

Wenhui Ma and L. Eric Cross

Appl. Phys. Lett. 81, 3440 (2002); http://dx.doi.org/10.1063/1.1518559 (3 pages) | Cited 53 times

Online Publication Date: 22 October 2002

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The strain-gradient-induced polarization (flexoelectric effect) was investigated in Ba0.67Sr0.33TiO3 (BST) ceramic at temperatures above the 21 °C Curie point. At 23 °C the flexoelectric coefficient μ12 was more than one order of magnitude greater than the highest value measured in lead magnesium niobate ceramic. Over the temperature range of linear Curie–Weiss behavior, the coefficient μ12 was roughly proportional to the dielectric permittivity; however, the constant of proportionality was higher than predicted for simple ionic solids. The unexpected behavior in the BST ceramic suggests the need for a broader database of flexoelectric coefficients. © 2002 American Institute of Physics.
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77.22.Ej Polarization and depolarization
77.84.Ek Niobates and tantalates
77.84.Cg PZT ceramics and other titanates
77.80.-e Ferroelectricity and antiferroelectricity
77.22.Ch Permittivity (dielectric function)

Lattice instabilities in (Pb,Cd)TiO3 alloys

S. V. Halilov, M. Fornari, and D. J. Singh

Appl. Phys. Lett. 81, 3443 (2002); http://dx.doi.org/10.1063/1.1518562 (3 pages) | Cited 19 times

Online Publication Date: 22 October 2002

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We report density functional calculations of the lattice instabilities of CdTiO3 and Pb0.5Cd0.5TiO3 supercells. The dominant instabilities in CdTiO3 are against TiO6 octahedral rotation, as may be expected from its Pnma structure. However, Pb substitution for Cd rapidly supresses this instability in favor of A-site driven ferroelectricity. At the 50% Pb composition, the rotational, rhombohedral ferroelectric and tetragonal (TET) ferroelectric instabilities of the cubic perovskite structure are comparable in energy, and moreover a rather large c/a ratio is found for the TET state. This strongly suggests investigation of the Pb rich part of the (Cd,Pb)TiO3 pseudobinary and minor modifications to it as potential high performance piezoelectric materials. © 2002 American Institute of Physics.
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77.80.B- Phase transitions and Curie point
77.84.Ek Niobates and tantalates
77.84.Cg PZT ceramics and other titanates
61.50.Ks Crystallographic aspects of phase transformations; pressure effects
61.66.Fn Inorganic compounds
64.70.K- Solid-solid transitions
71.15.Mb Density functional theory, local density approximation, gradient and other corrections
81.30.Dz Phase diagrams of other materials
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Control over exciton confinement versus separation in composite films of polyfluorene and CdSe nanocrystals

Y. C. Tseng, M. Tzolov, E. H. Sargent, P. W. Cyr, and M. A. Hines

Appl. Phys. Lett. 81, 3446 (2002); http://dx.doi.org/10.1063/1.1513214 (3 pages) | Cited 4 times

Online Publication Date: 22 October 2002

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Composite films of polyfluorene derivative poly(9,9-di-(2-ethylhexyl)-fluorenyl-2,7-diyl) and cadmium selenide nanocrystals were investigated using photomodulation spectroscopy exciting only the nanocrystal phase. Efficient charge separation resulting in hole injection into the polymer was observed in films in which the nanocrystals had been stripped of surface trioctylphosphine oxide passivating groups. The resulting induced absorption band centered at 1.2 eV was assigned to bipolarons or π-dimers formed on the polymer in the near vicinity of the polymer/nanocrystal interface. The intensity dependence of this band suggests bimolecular recombination, supporting the interpretation of the band as due to charge separation. The measured wide distribution of lifetimes for the photogenerated states confirms the glassy nature of the polymer. © 2002 American Institute of Physics.
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78.67.Bf Nanocrystals, nanoparticles, and nanoclusters
73.22.Lp Collective excitations
71.35.-y Excitons and related phenomena
71.38.Mx Bipolarons
78.66.Qn Polymers; organic compounds
61.41.+e Polymers, elastomers, and plastics
78.55.-m Photoluminescence, properties and materials
61.46.-w Structure of nanoscale materials
71.20.Rv Polymers and organic compounds

Distributions of noble metal Pd and Pt in mesoporous silica

J. Arbiol, A. Cabot, J. R. Morante, Fanglin Chen, and Meilin Liu

Appl. Phys. Lett. 81, 3449 (2002); http://dx.doi.org/10.1063/1.1518158 (3 pages) | Cited 18 times

Online Publication Date: 22 October 2002

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Mesoporous silica nanostructures have been synthesized and loaded with Pd and Pt catalytic noble metals. It is found that Pd forms small nanoclusters (3–5 nm) on the surface of the mesoporous structure whereas Pt impregnation results in the inclusion of Pt nanostructures within the silica hexagonal pores (from nanoclusters to nanowires). It is observed that these materials have high catalytic properties for CO–CH4 combustion, even in a thick film form. In particular, results indicate that the Pt and Pd dispersed in mesoporous silica are catalytically active as a selective filter for gas sensors. © 2002 American Institute of Physics.
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61.46.-w Structure of nanoscale materials
61.43.Gt Powders, porous materials
81.05.Rm Porous materials; granular materials
81.07.Bc Nanocrystalline materials
82.33.Ln Reactions in sol gels, aerogels, porous media
82.75.Fq Synthesis, structure determination, structure modeling
82.60.Qr Thermodynamics of nanoparticles
82.30.Vy Homogeneous catalysis in solution, polymers and zeolites
07.07.Df Sensors (chemical, optical, electrical, movement, gas, etc.); remote sensing
82.80.-d Chemical analysis and related physical methods of analysis

Numerical simulation analysis of a near-field optical virtual probe

Tao Hong, Jia Wang, Liqun Sun, and Dacheng Li

Appl. Phys. Lett. 81, 3452 (2002); http://dx.doi.org/10.1063/1.1517401 (3 pages) | Cited 8 times

Online Publication Date: 22 October 2002

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A near-field optical virtual probe based on the principle of near-field evanescent wave interference can be used in optical data storage, nanolithography, near-field optical imaging, and optical manipulation. This letter gives the numerical simulation results of the optical distribution of a near-field virtual probe and analyzes the dependence of the virtual probe’s on the aperture, the polarization, etc. The simulation results reveal that the transmission efficiency is higher than that of a nanoaperture metal-coated fiber probe. The size of the virtual probe is constant when the distance between the lens and recording media increases within a certain range so that the critical nanoseparation control in near-field system can be relaxed. © 2002 American Institute of Physics.
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42.79.Vb Optical storage systems, optical disks
42.79.Bh Lenses, prisms and mirrors
02.70.Bf Finite-difference methods
07.79.Fc Near-field scanning optical microscopes

Oxidation and reduction behavior of Ge/Si islands

T. Sass, V. Zela, A. Gustafsson, I. Pietzonka, and W. Seifert

Appl. Phys. Lett. 81, 3455 (2002); http://dx.doi.org/10.1063/1.1517715 (3 pages) | Cited 18 times

Online Publication Date: 22 October 2002

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We have investigated the oxidation/reduction behavior of dome-shaped three-dimensional islands of Ge on Si(001) grown by ultrahigh vacuum chemical vapor deposition. The oxidation was done by exposing the surfaces to H2O steam in N2. The reduction was done by H2, which selectively reduces only the GeO2. The results of the oxidation/reduction processes under varying conditions were analyzed by high-resolution transmission electron microscopy. We found that the selective reduction of such structures does not result in a perfect recovery of the former Ge dots, but results in phase-segregated Ge enrichments. In many cases these enrichments show epitaxial relationship with the underlying Si substrate. These structures are of potential interest for Ge dots embedded in an insulating material as well as for lateral epitaxial overgrowth of SiO2/Si(001) by Ge, using the reduced Ge dots as the seeds for epitaxy. © 2002 American Institute of Physics.
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81.65.Mq Oxidation
81.05.Cy Elemental semiconductors
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
82.30.-b Specific chemical reactions; reaction mechanisms
81.15.-z Methods of deposition of films and coatings; film growth and epitaxy

Growth of ordered, single-domain, alumina nanopore arrays with holographically patterned aluminum films

Zhijun Sun and Hong Koo Kim

Appl. Phys. Lett. 81, 3458 (2002); http://dx.doi.org/10.1063/1.1517719 (3 pages) | Cited 44 times

Online Publication Date: 22 October 2002

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Highly-ordered, single-domain ( ∼ cm2), alumina nanopore arrays with controlled symmetry were obtained by anodizing prepatterned aluminum films that were deposited on silica substrates. Holographic lithography, in conjunction with a conformal deposition process, was utilized to form nanoscale corrugations on aluminum film surface prior to anodization. Both the order and symmetry of pore arrays were found to be well controlled and guided by the periodic surface corrugations from the initial stage of pore growth. Ordered nanopore arrays grown on foreign substrates are promising as a host or template for forming various nanodevices and their on-chip integration. © 2002 American Institute of Physics.
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81.07.-b Nanoscale materials and structures: fabrication and characterization
81.05.Rm Porous materials; granular materials
81.16.Pr Micro- and nano-oxidation
81.16.Nd Micro- and nanolithography
61.46.-w Structure of nanoscale materials
61.43.Gt Powders, porous materials
68.35.B- Structure of clean surfaces (and surface reconstruction)
42.40.My Applications
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