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24 Jun 2002

Volume 80, Issue 25, pp. 4687-4873

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Changes in Al-related photoluminescence in 4H-SiC caused by hydrogenation

Yaroslav Koshka, Michael S. Mazzola, and William A. Draper

Appl. Phys. Lett. 80, 4762 (2002); http://dx.doi.org/10.1063/1.1489483 (3 pages) | Cited 5 times

Online Publication Date: 17 June 2002

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Both reduction of the intensity of aluminum-related photoluminescence after hydrogenation and the phenomenon of the optical quenching of the Al bound exciton (Al–BE) previously reported for hydrogenated 6H-SiC was observed now in the 4H-SiC polytype. Hydrogenation caused also a reduction of the nitrogen–aluminum donor-acceptor pair emission. Prolonged excitation of hydrogenated samples with above-band gap light resulted in additional quenching of the residual Al–BE photoluminescence. The quenching kinetics and some differences between the 6H and 4H polytypes are investigated. The observed phenomena are attributed to optically enhanced passivation of Al acceptors with hydrogen. © 2002 American Institute of Physics.
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78.66.Li Other semiconductors
78.55.Hx Other solid inorganic materials
71.35.Cc Intrinsic properties of excitons; optical absorption spectra

Influences of mask width, fill factor, HCl addition and C doping on wing tilts in the epitaxial laterally overgrown GaN films by hydride vapor phase epitaxy

F. Wang, R. Zhang, W. S. Tan, X. Q. Xiu, D. Q. Lu, S. L. Gu, B. Shen, Y. Shi, X. S. Wu, Y. D. Zheng, S. S. Jiang, and T. F. Kuech

Appl. Phys. Lett. 80, 4765 (2002); http://dx.doi.org/10.1063/1.1489099 (3 pages)

Online Publication Date: 17 June 2002

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Crystal tilts in the epitaxial lateral overgrown (ELO) GaN region over SiO2 mask by hydride vapor phase epitaxial away from the opening region were investigated by scanning electron microscope and the four-circle x-ray diffraction method. The increased mask width and ratio of stripe opening width to stripe period (fill factor) lead to increased wing tilts. The introduction of additional HCl and C3H8/H2 can decrease wing tilts and improve the ELO GaN surface morphologies significantly. Moreover, it has been observed that a high growth rate will bring more defects and large wing tilts into the GaN materials. © 2002 American Institute of Physics.
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68.55.-a Thin film structure and morphology
81.05.Ea III-V semiconductors
81.15.Kk Vapor phase epitaxy; growth from vapor phase
68.35.B- Structure of clean surfaces (and surface reconstruction)

Arsenic surface segregation during the molecular-beam epitaxial growth of GaAs embedded in wurtzite GaN

Hyonju Kim and T. G. Andersson

Appl. Phys. Lett. 80, 4768 (2002); http://dx.doi.org/10.1063/1.1489476 (3 pages) | Cited 5 times

Online Publication Date: 17 June 2002

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We report on the growth of a thin GaAs layer embedded in wurtzite GaN prepared on (0001) sapphire substrate by rf-plasma molecular-beam epitaxy. It was found that the As surface segregation during the growth of the GaN cap layer can be controlled by the growth parameters, i.e., the growth interruption and subsequent in situ annealing process implemented in this study. The arsenic profile of the cap layer was accurately evaluated using an empirical segregation model. The surface morphology was also investigated using atomic force microscopy. © 2002 American Institute of Physics.
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68.35.Dv Composition, segregation; defects and impurities
81.05.Ea III-V semiconductors
81.15.Hi Molecular, atomic, ion, and chemical beam epitaxy
61.72.Cc Kinetics of defect formation and annealing
68.37.Ps Atomic force microscopy (AFM)

Above-band gap two-photon absorption and its influence on ultrafast carrier dynamics in ZnTe and CdTe

M. Schall and P. Uhd Jepsen

Appl. Phys. Lett. 80, 4771 (2002); http://dx.doi.org/10.1063/1.1489480 (3 pages) | Cited 15 times

Online Publication Date: 17 June 2002

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We present measurements of transient carrier dynamics subsequent to intense above-band gap femtosecond excitation of the II–VI compounds ZnTe and CdTe, probed in the far infrared by transient terahertz time-domain spectroscopy. The intensity and temperature dependence of the carrier dynamics illuminates the role of two-photon absorption (TPA) of pump photons. At cold temperatures and high excitation intensities TPA results in a photoexcited carrier distribution which requires several picoseconds to thermalize. As a consequence, the dielectric function of the carrier distribution deviates strongly from the Drude model for as long as 20 ps after excitation. © 2002 American Institute of Physics.
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78.47.-p Spectroscopy of solid state dynamics
42.65.-k Nonlinear optics
72.80.Ey III-V and II-VI semiconductors
71.45.Gm Exchange, correlation, dielectric and magnetic response functions, plasmons
72.20.Fr Low-field transport and mobility; piezoresistance

Effect of vacancy and interstitial excess on the deactivation kinetics of As in Si

S. Solmi, M. Attari, and D. Nobili

Appl. Phys. Lett. 80, 4774 (2002); http://dx.doi.org/10.1063/1.1489719 (3 pages) | Cited 8 times

Online Publication Date: 17 June 2002

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The effect of a point defect excess, vacancies, or, respectively, interstitials, on the deactivation kinetics of As in Si was verified on silicon on insulator (SOI) substrates uniformly doped at concentrations in the range 1.8–7×1020 cm−3. SOI samples can provide more accurate carrier density measurements, moreover the buried oxide layer can prevent recombination of excess vacancies with the interstitials near the projected range. A dose of 5×1015 cm−2 Si+ ions was implanted at 100 keV to obtain interstitials supersaturation (Iex), while the same Si+ dose, at an energy of 2 MeV, was used to generate an excess of vacancies (Vex). These specimens were isothermally heated at different temperatures and their annealing behavior was compared with the one of reference undamaged samples of the same composition, but without point defect excess. Our results indicate that neither the excess of interstitials nor the one of vacancies appreciably affect the deactivation rate. Once the implantation damage has annealed out, the samples with point defect excess are found in the same situation of the reference ones, and the subsequent deactivation kinetics results coincident. © 2002 American Institute of Physics.
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72.20.Jv Charge carriers: generation, recombination, lifetime, and trapping
61.72.J- Point defects and defect clusters
72.80.Cw Elemental semiconductors
61.72.Cc Kinetics of defect formation and annealing
61.82.Fk Semiconductors
61.80.Jh Ion radiation effects

Deep-level defects in InGaAsN grown by molecular-beam epitaxy

R. J. Kaplar, S. A. Ringel, Steven R. Kurtz, J. F. Klem, and A. A. Allerman

Appl. Phys. Lett. 80, 4777 (2002); http://dx.doi.org/10.1063/1.1483912 (3 pages) | Cited 19 times

Online Publication Date: 17 June 2002

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Deep-level transient spectroscopy (DLTS) studies on both p-type unintentionally doped and n-type (Si-doped), 1.05 eV band gap InGaAsN grown by molecular-beam epitaxy are reported. Two majority-carrier hole traps were observed in p-type material, H3′ (0.38 eV) and H4′ (0.51 eV), and no evidence was found for the presence of minority-carrier electron traps. In n-type material, we observed a shallow distribution of electron levels, E1′, as well as a deep electron trap E4′ (0.56 eV) and a deep hole trap H5′ (0.71 eV). All DLTS peaks observed were broad and are thus consistent with continuous defect distributions and/or groups of closely spaced discrete energy levels in the band gap. Comparison of the spectra to previously reported spectra of metalorganic chemical vapor deposition-grown InGaAsN of the same composition revealed some similarities and some differences, suggesting that some of the observed deep levels are due to intrinsic physical sources, whereas others are specific to the growth technique used. © 2002 American Institute of Physics.
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71.55.Eq III-V semiconductors
68.55.-a Thin film structure and morphology
81.15.Hi Molecular, atomic, ion, and chemical beam epitaxy

Reliability of laser-activated low-temperature polycrystalline silicon thin-film transistors

Du.-Zen Peng, Ting-Chang Chang, Hsiao-Wen Zan, Tiao-Yuan Huang, Chun-Yen Chang, and Po-Tsun Liu

Appl. Phys. Lett. 80, 4780 (2002); http://dx.doi.org/10.1063/1.1489096 (3 pages) | Cited 5 times

Online Publication Date: 17 June 2002

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In this letter, the characteristics and reliability of laser-activated polycrystalline silicon thin-film transistors (poly-Si TFTs) have been investigated by stressing the devices under Vds = 12 V and Vgs = 15 V. In comparison with traditional furnace-activated poly-Si TFTs, the leakage current is relatively large for laser-activated poly-Si TFTs. Further, while the degradation rates of threshold voltage and subthreshold swing are comparable to those of traditional furnace-activated TFTs, the post-stress leakage and on/off current ratio for laser-activated poly-Si TFTs degrade much faster than those of furnace-activated counterparts. The laser activation modifies the grain structure between the drain and the channel region, and causes grain discontinuity extending from the drain to the channel region. As a result, an inferior reliability with extra trap state density and larger leakage current was observed in the laser-activated poly-Si TFTs. © 2002 American Institute of Physics.
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85.30.Tv Field effect devices
85.30.De Semiconductor-device characterization, design, and modeling
81.05.Cy Elemental semiconductors
42.62.-b Laser applications
72.20.Jv Charge carriers: generation, recombination, lifetime, and trapping

Ge/Si quantum-dot metal–oxide–semiconductor field-effect transistor

A. I. Yakimov, A. V. Dvurechenskii, V. V. Kirienko, and A. I. Nikiforov

Appl. Phys. Lett. 80, 4783 (2002); http://dx.doi.org/10.1063/1.1488688 (3 pages) | Cited 8 times

Online Publication Date: 17 June 2002

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We report on the operation of Si metal–oxide–semiconductor field-effect transistor with an array of ∼ 103 10 nm diameter Ge self-assembled quantum dots embedded into the active channel. The drain current versus gate voltage characteristics show oscillations caused by Coulomb interaction of holes in the fourfold-degenerate excited state of the dots at T ⩽ 200 K. A dot charging energy of ∼ 43 meV (i.e., >kT = 26 meV at T = 300 K) and disorder energy of ∼ 20 meV are determined from the oscillation period and the temperature dependence study of current maxima, respectively. © 2002 American Institute of Physics.
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85.30.Tv Field effect devices
73.63.Kv Quantum dots
85.35.Be Quantum well devices (quantum dots, quantum wires, etc.)
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Buried superconducting layers comprised of magnesium diboride nanocrystals formed by ion implantation

H. Y. Zhai, H. M. Christen, C. W. White, J. D. Budai, D. H. Lowndes, and A. Meldrum

Appl. Phys. Lett. 80, 4786 (2002); http://dx.doi.org/10.1063/1.1488695 (3 pages) | Cited 4 times

Online Publication Date: 17 June 2002

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Superconducting layers of MgB2 were formed on Si substrates using techniques that are widely used and accepted in the semiconductor industry. Mg ions were implanted into boron films deposited on Si or Al2O3 substrates. After a thermal processing step, buried superconducting layers comprised of MgB2 nanocrystals were obtained which exhibit the highest Tc reported so far for MgB2 on silicon (Tconset ≈ 33.6 K, ΔTc = 0.5 K, as measured by current transport). These results show that our approach is clearly applicable to the fabrication of superconducting devices that can be operated at much higher temperatures ( ≈ 20 K) than the current Nb technology ( ≈ 6 K) while their integration with silicon structures remains straight-forward. © 2002 American Institute of Physics.
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74.78.-w Superconducting films and low-dimensional structures
61.72.up Other materials
81.07.Bc Nanocrystalline materials
74.62.-c Transition temperature variations, phase diagrams
74.72.-h Cuprate superconductors
61.46.-w Structure of nanoscale materials

Magnetization reversal and configurational anisotropy of dense permalloy dot arrays

Xiaobin Zhu, P. Grütter, V. Metlushko, and B. Ilic

Appl. Phys. Lett. 80, 4789 (2002); http://dx.doi.org/10.1063/1.1489720 (3 pages) | Cited 25 times

Online Publication Date: 17 June 2002

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Electron beam patterned permalloy circular dots of 700 nm diameter with small separations were studied by magnetic force microscopy (MFM) in the presence of an in situ magnetic field. Images in the demagnetized state show that the dot is in a vortex state with a vortex core (singularity) in the center. Local hysteresis loops, measured by cantilever frequency shift in an external field, indicate that the magnetization reversal of individual disks is a vortex nucleation and annihilation process. By carefully doing MFM, nucleation and annihilation fields without MFM tip stray field distortions are obtained. Configurational anisotropy originated from magnetostatic coupling is found through hysteresis loops. © 2002 American Institute of Physics.
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75.25.-j Spin arrangements in magnetically ordered materials (including neutron and spin-polarized electron studies, synchrotron-source x-ray scattering, etc.)
75.50.Ww Permanent magnets
75.60.Jk Magnetization reversal mechanisms
75.30.Gw Magnetic anisotropy
75.50.Bb Fe and its alloys
68.37.Rt Magnetic force microscopy (MFM)
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects

High-resolution magnetic Co supertips grown by a focused electron beam

I. Utke, P. Hoffmann, R. Berger, and L. Scandella

Appl. Phys. Lett. 80, 4792 (2002); http://dx.doi.org/10.1063/1.1489097 (3 pages) | Cited 50 times

Online Publication Date: 17 June 2002

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We present a technique for local growth of high-resolution, high-aspect-ratio magnetic tips and thin adherent magnetic cap coatings on top of batch fabricated scanning force microscopy silicon tips. A focused electron beam of a scanning electron microscope is used for decomposition of a directed cobalt carbonyl vapor flux. Exposure parameters determine the tip geometry and tip length. Deposits consist of cubic Co clusters of 2–5 nm in size dispersed in a stabilizing carbonaceous matrix. Magnetic force microscope sensors having magnetic tip apex diameters between 50 and 240 nm were produced. Tracks of magnetic transitions written in recording media of hard disks were used to characterize tip performance. © 2002 American Institute of Physics.
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07.79.Lh Atomic force microscopes
81.15.-z Methods of deposition of films and coatings; film growth and epitaxy
75.70.Ak Magnetic properties of monolayers and thin films

Influence of the magnetic tip in ferromagnetic resonance force microscopy

V. Charbois, V. V. Naletov, J. Ben Youssef, and O. Klein

Appl. Phys. Lett. 80, 4795 (2002); http://dx.doi.org/10.1063/1.1489475 (3 pages) | Cited 10 times

Online Publication Date: 17 June 2002

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We compare mechanically detected ferromagnetic resonance spectra for different separations h between the magnetic tip and sample surface. When the bias field generated by the tip is smaller than a few hundred gauss, the prominent changes are shifts of the entire spectrum (without line shape distortions) to higher frequency as h decreases. These results are in agreement with the Damon and Eshbach model for spin waves propagating in a potential perturbed by the additional field of the probe magnet. It is used to predict the spatial resolution limit for magnetostatic modes bounded by the stray field of the tip. The answer is ∼ 4 μm for yttrium iron garnet. © 2002 American Institute of Physics.
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76.50.+g Ferromagnetic, antiferromagnetic, and ferrimagnetic resonances; spin-wave resonance
68.37.Rt Magnetic force microscopy (MFM)
07.79.Pk Magnetic force microscopes
75.30.Ds Spin waves

Anomalous magnetotransport properties of epitaxial full Heusler alloys

M. S. Lund, J. W. Dong, J. Lu, X. Y. Dong, C. J. Palmstrøm, and C. Leighton

Appl. Phys. Lett. 80, 4798 (2002); http://dx.doi.org/10.1063/1.1489081 (3 pages) | Cited 32 times

Online Publication Date: 17 June 2002

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We report the magnetotransport properties of epitaxial films of the full Heusler alloys Ni2MnGa, Ni2MnGe, and Ni2MnAl, grown by molecular beam epitaxy on (001) GaAs. The ferromagnetic alloys (Ni2MnGa,Ni2MnGe) exhibit an anomalous temperature dependence of the resistivity and a negative magnetoresistance peaking near the Curie temperature due to spin disorder scattering. Considering the absolute values of the resistivity, the anomalous high temperature behavior and an upturn in the resistivity below 20 K, we suggest that these Heusler alloys rather than being conventional metals are in fact strongly disordered electronic systems. © 2002 American Institute of Physics.
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73.50.Jt Galvanomagnetic and other magnetotransport effects (including thermomagnetic effects)
75.50.Cc Other ferromagnetic metals and alloys
72.15.Gd Galvanomagnetic and other magnetotransport effects
75.30.Kz Magnetic phase boundaries (including classical and quantum magnetic transitions, metamagnetism, etc.)
73.61.At Metal and metallic alloys
72.15.Qm Scattering mechanisms and Kondo effect
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Epitaxial La-doped SrTiO3 on silicon: A conductive template for epitaxial ferroelectrics on silicon

B. T. Liu, K. Maki, Y. So, V. Nagarajan, R. Ramesh, J. Lettieri, J. H. Haeni, D. G. Schlom, W. Tian, X. Q. Pan, F. J. Walker, and R. A. McKee

Appl. Phys. Lett. 80, 4801 (2002); http://dx.doi.org/10.1063/1.1484552 (3 pages) | Cited 32 times

Online Publication Date: 17 June 2002

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Use of an epitaxial conducting template has enabled the integration of epitaxial ferroelectric perovskites on silicon. The conducting template layer, LaxSr1−xTiO3 (LSTO), deposited onto (001) silicon wafers by molecular-beam epitaxy is then used to seed {001}-oriented epitaxial perovskite layers. We illustrate the viability of this approach using PbZr0.4Ti0.6O3 (PZT) as the ferroelectric layer contacted with conducting perovskite La0.5Sr0.5CoO3 (LSCO) electrodes. An important innovation that further facilitates this approach is the use of a low-temperature (450 °C) sol–gel process to crystallize the entire ferroelectric stack. Both transmission electron microscopy and x-ray diffraction analysis indicate the LSCO/PZT/LSCO/LSTO/Si heterostructures are epitaxial. The electrical response of ferroelectric capacitors (for pulse widths down to 1 μs) measured via the underlying silicon substrate is identical to measurements made using conventional capacitive coupling method, indicating the viability of this approach. © 2002 American Institute of Physics.
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85.50.Gk Non-volatile ferroelectric memories
77.84.Ek Niobates and tantalates
77.84.Cg PZT ceramics and other titanates

Unusual size effect on the polarization patterns in micron-size Pb(Zr,Ti)O3 film capacitors

Igor Stolichnov, Enrico Colla, Alexander Tagantsev, S. S. N. Bharadwaja, Seungbum Hong, Nava Setter, Jeffrey S. Cross, and Mineharu Tsukada

Appl. Phys. Lett. 80, 4804 (2002); http://dx.doi.org/10.1063/1.1489478 (3 pages) | Cited 31 times

Online Publication Date: 17 June 2002

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Direct observation of polarization distribution at nanoscale by scanning force microscopy across partially etched Pb(Zr,Ti)O3 film ferroelectric capacitors with size 2×3 μm and preferential orientation (111) has revealed an anomalous polarization pattern characterized by polarization inversion in the center of the capacitor. This self-structured pattern is reproducible and independent on the spontaneous polarization orientation, but strongly influenced by the film texture and capacitor size. In particular, capacitors of the same texture with size 0.5×0.5 μm as well as capacitors with (100) preferential orientation of any size did not exhibit such anomalous polarization distribution. A tentative explanation consistent with the essential features of the observed polarization behavior in terms of strain-induced phase transition is discussed. © 2002 American Institute of Physics.
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77.84.Ek Niobates and tantalates
77.84.Cg PZT ceramics and other titanates
84.32.Tt Capacitors
77.55.-g Dielectric thin films
77.22.Ej Polarization and depolarization
85.50.-n Dielectric, ferroelectric, and piezoelectric devices
68.55.-a Thin film structure and morphology
77.80.B- Phase transitions and Curie point

Low-temperature dielectric relaxation in the pyrochlore (Bi3/4Zn1/4)2(Zn1/4Ta3/4)2O7 compound

Chen Ang, Zhi Yu, H. J. Youn, C. A. Randall, A. S. Bhalla, L. E. Cross, J. Nino, and M. Lanagan

Appl. Phys. Lett. 80, 4807 (2002); http://dx.doi.org/10.1063/1.1486045 (3 pages) | Cited 19 times

Online Publication Date: 17 June 2002

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The dielectric behavior of the pyrochlore (Bi3/4Zn1/4)2(Zn1/4Ta3/4)2O7 compound has been studied. A low-temperature dielectric relaxation was observed in a low-permittivity matrix with ε = ∼ 60. The dielectric relaxation process follows a modified Debye model in the vicinity of the relaxation peak, and the relaxation rate follows the Arrhenius law in the wide frequency range 102 to ∼ 1010 Hz. The temperature intensity of dielectric peaks are independent of dc bias (⩽60 kV/cm). The dielectric relaxation is tentatively attributed to the hopping of Zn/Bi ions at A sites with more than one equivalent potential minima, and the reorientation of the dipoles probably formed through interactions with the “seventh oxygen” and the Bi/Zn A-site ions in the pyrochlore (Bi3/4Zn1/4)2(Zn1/4Ta3/4)2O7 compound. © 2002 American Institute of Physics.
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77.22.Gm Dielectric loss and relaxation
77.84.Ek Niobates and tantalates
77.84.Cg PZT ceramics and other titanates
81.05.Je Ceramics and refractories (including borides, carbides, hydrides, nitrides, oxides, and silicides)
77.22.Ch Permittivity (dielectric function)
77.80.-e Ferroelectricity and antiferroelectricity
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High-contrast photoluminescent patterns in lithium fluoride crystals produced by soft x-rays from a laser-plasma source

G. Baldacchini, F. Bonfigli, F. Flora, R. M. Montereali, D. Murra, E. Nichelatti, A. Faenov, and T. Pikuz

Appl. Phys. Lett. 80, 4810 (2002); http://dx.doi.org/10.1063/1.1486476 (3 pages) | Cited 18 times

Online Publication Date: 17 June 2002

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A technique using soft x-rays and extreme ultraviolet light generated by a laser-plasma source has been investigated for producing low-dimensionality photoluminescent patterns based on active color centers in lithium fluoride (LiF) crystals. Strong visible photoluminescence at room temperature has been observed in LiF crystals from fluorescent patterns obtained by masking the incoming radiation. This technique is able to produce colored patterns with high spatial resolution on large areas and in short exposure times as compared with other coloration methods. © 2002 American Institute of Physics.
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78.55.Fv Solid alkali halides
61.72.J- Point defects and defect clusters
61.80.Cb X-ray effects
42.70.Nq Other nonlinear optical materials; photorefractive and semiconductor materials
71.55.Ht Other nonmetals
61.82.Ms Insulators
61.80.Ba Ultraviolet, visible, and infrared radiation effects (including laser radiation)

Field emission from open ended aluminum nitride nanotubes

V. N. Tondare, C. Balasubramanian, S. V. Shende, D. S. Joag, V. P. Godbole, S. V. Bhoraskar, and M. Bhadbhade

Appl. Phys. Lett. 80, 4813 (2002); http://dx.doi.org/10.1063/1.1482137 (3 pages) | Cited 115 times

Online Publication Date: 17 June 2002

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This letter reports the field emission measurements from the nanotubes of aluminum nitride which were synthesized by gas phase condensation using the solid-vapor equilibria. A dc arc plasma reactor was used for producing the vapors of aluminum in a reactive nitrogen atmosphere. Nanoparticles and nanotubes of aluminum nitride were first characterized by transmission electron microscope and tube dimensions were found to be varying from 30 to 200 nm in diameter and 500 to 700 nm in length. These tubes were mixed with nanoparticles of size range between 5 and 200 nm in diameter. Tungsten tips coated with these nanoparticles and tubes were used as a field emitter. The field emission patterns display very interesting features consisting of sharp rings which were often found to change their shapes. The patterns are attributed to the open ended nanotubes of aluminum nitride. A few dot patterns corresponding to the nanoparticles were also seen to occur. The Fowler–Nordheim plots were seen to be nonlinear in nature, which reflects the semi-insulating behavior of the emitter. The field enhancement factor is estimated to be 34 500 indicating that the field enhancement due to the nanometric size of the emitter is an important cause for the observed emission. © 2002 American Institute of Physics.
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79.70.+q Field emission, ionization, evaporation, and desorption
61.46.-w Structure of nanoscale materials
81.07.De Nanotubes

Controlled alignment of carbon nanofibers in a large-scale synthesis process

Vladimir I. Merkulov, A. V. Melechko, M. A. Guillorn, M. L. Simpson, D. H. Lowndes, J. H. Whealton, and R. J. Raridon

Appl. Phys. Lett. 80, 4816 (2002); http://dx.doi.org/10.1063/1.1487920 (3 pages) | Cited 61 times

Online Publication Date: 17 June 2002

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Controlled alignment of catalytically grown carbon nanofibers (CNFs) at a variable angle to the substrate during a plasma-enhanced chemical vapor deposition process is achieved. The CNF alignment is controlled by the direction of the electric field lines during the synthesis process. Off normal CNF orientations are achieved by positioning the sample in the vicinity of geometrical features of the sample holder, where bending of the electric field lines occurs. The controlled growth of kinked CNFs that consist of two parts aligned at different angles to the substrate normal also is demonstrated. © 2002 American Institute of Physics.
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81.07.De Nanotubes
61.46.-w Structure of nanoscale materials
81.05.U- Carbon/carbon-based materials
52.77.Dq Plasma-based ion implantation and deposition
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
68.55.A- Nucleation and growth

Oxygen and ozone oxidation-enhanced field emission of carbon nanotubes

Sheng-Chin Kung, Kuo Chu Hwang, and I. Nan Lin

Appl. Phys. Lett. 80, 4819 (2002); http://dx.doi.org/10.1063/1.1485315 (3 pages) | Cited 43 times

Online Publication Date: 17 June 2002

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Vertically aligned carbon nanotube (CNT) arrays were grown on p-type silicon wafer using acetylene and iron phthalocyanine as the sources of hydrocarbons and catalysts, respectively. The CNT arrays were treated by chemical reagents, such as oxygen (O2), ozone (O3), bromine, and acids. When treated by O2 and O3, the emission current of the CNT array was increased ∼800% along with a decrease of the onset field emission voltage from 0.8 to 0.6 V/μm. Other chemical treatments, e.g., bromination and acid oxidation, lead to poorer field emission performance. The effects of these chemical processes on the field emission properties of CNT arrays will be discussed. © 2002 American Institute of Physics.
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79.70.+q Field emission, ionization, evaporation, and desorption
81.65.Mq Oxidation
61.46.-w Structure of nanoscale materials

Structure study of Se species in channels of AlPO4-5 crystals

Irene L. Li and Z. K. Tang

Appl. Phys. Lett. 80, 4822 (2002); http://dx.doi.org/10.1063/1.1489100 (3 pages) | Cited 5 times

Online Publication Date: 17 June 2002

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The thermal adsorption/desorption process of selenium species in the channels of AlPO4-5 (AFI) single crystals is studied by simultaneous thermogravimetry (TG) and differential scanning calorimetry (DSC). There exist two peaks in the TG and DSC curves, indicating that two different structures of Se species, helical chain and 8-member ring, coexist in the AFI channels. Polarized Raman and optical absorption spectra show that the helical chains are highly aligned along the channel direction, while the Se8 rings are randomly oriented in the channels. The helical chains have a lowest optical absorption band at 2.6 eV, which is strongly polarization dependent, while the 8-member rings have a lowest absorption band at 3.0 eV that varies slowly with the light polarization. © 2002 American Institute of Physics.
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61.46.-w Structure of nanoscale materials
68.43.Mn Adsorption kinetics
78.30.-j Infrared and Raman spectra
78.67.Bf Nanocrystals, nanoparticles, and nanoclusters
68.43.Vx Thermal desorption

Triplex molecular layers with nonlinear nanomechanical response

V. V. Tsukruk, H.-S. Ahn, D. Kim, and A. Sidorenko

Appl. Phys. Lett. 80, 4825 (2002); http://dx.doi.org/10.1063/1.1486267 (3 pages) | Cited 7 times

Online Publication Date: 17 June 2002

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The molecular design of surface structures with built-in mechanisms for mechanical energy dissipation under nanomechanical deformation and compression resistance provided superior nanoscale wear stability. We designed robust, well-defined trilayer surface nanostructures chemically grafted to a silicon oxide surface with an effective composite modulus of about 1 GPa. The total thickness was within 20–30 nm and included an 8 nm rubber layer sandwiched between two hard layers. The rubber layer provides an effective mechanism for energy dissipation, facilitated by nonlinear, giant, reversible elastic deformations of the rubber matrix, restoring the initial status due to the presence of an effective nanodomain network and chemical grafting within the rubber matrix. © 2002 American Institute of Physics.
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68.60.Bs Mechanical and acoustical properties
81.07.Bc Nanocrystalline materials
62.25.-g Mechanical properties of nanoscale systems
07.10.Cm Micromechanical devices and systems
61.46.-w Structure of nanoscale materials
85.85.+j Micro- and nano-electromechanical systems (MEMS/NEMS) and devices
62.20.F- Deformation and plasticity
81.40.Jj Elasticity and anelasticity, stress-strain relations
81.40.Lm Deformation, plasticity, and creep
62.20.Qp Friction, tribology, and hardness
81.40.Pq Friction, lubrication, and wear
68.35.Af Atomic scale friction
62.20.D- Elasticity

Patterning surfaces with colloidal particles using optical tweezers

J. P. Hoogenboom, D. L. J. Vossen, C. Faivre-Moskalenko, M. Dogterom, and A. van Blaaderen

Appl. Phys. Lett. 80, 4828 (2002); http://dx.doi.org/10.1063/1.1488690 (3 pages) | Cited 44 times

Online Publication Date: 17 June 2002

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A method for positioning colloidal particles on surfaces in any designed pattern is described. Optical tweezers are used to bring particles from a reservoir to the substrate where opposite surface charges are used to immobilize particles on the surface. Both chemical surface modification and polyelectrolyte coating of either substrate or colloids make the method generally applicable. We show that using this technique large, two-dimensional patterns can be created that can be dried without distortions by critical point drying. As an example we show the positioning of 79 nm radius metallodielectric particles and we show how two-dimensional patterns can be used to direct three-dimensional epitaxial crystal growth. The method is inexpensive, relatively fast, and can be fully automated. © 2002 American Institute of Physics.
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68.35.Dv Composition, segregation; defects and impurities
82.70.Dd Colloids

Self-organized, ordered array of coherent orthogonal column nanostructures in epitaxial La0.8Sr0.2MnO3 thin films

J. C. Jiang, E. I. Meletis, and K. I. Gnanasekar

Appl. Phys. Lett. 80, 4831 (2002); http://dx.doi.org/10.1063/1.1489078 (3 pages) | Cited 16 times

Online Publication Date: 17 June 2002

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We report direct transmission electron microscopy evidence of self-organized, ordered array of coherent orthogonal column nanostructures in epitaxial La0.8Sr0.2MnO3 (LSMO) thin films grown on (001) LaAlO3 (LAO) using pulsed-laser ablation. The orthogonal column nanostructures have an orthorhombic structure and are epitaxially grown on a continuous cubic perovskite LSMO thin-film layer that is epitaxially grown on (001) LAO substrate. The orthogonal column nanostructures exhibit a narrow size distribution, with the short edges having a length of about 25 nm and the long edge (growth direction) of ∼ 70 nm. The short edges are parallel to the [100] and [010] directions of LAO. All columns are encapsulated by uniformly thick amorphous-like grain boundaries and are “quasi” periodically arranged along the [100] and [010] directions of LAO. The continuous epitaxial layer of cubic perovskite structure grows along the [001] direction with an in-plane orientation relationship with respect to the substrate of 〈100〉LSMOC∥〈100〉LAO, while the orthorhombic column structures grow along its [1math0] direction with an in-plane orientation relationship of [110]LSMOO∥[100]LAO and [001]LSMOO∥[010]LAO. The spontaneous formation of such self-organized, coherent column nanostructures can be considered to follow a type of Stranski–Krastanov growth mode without Oswald ripening. © 2002 American Institute of Physics.
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68.55.-a Thin film structure and morphology
75.70.Ak Magnetic properties of monolayers and thin films
61.46.-w Structure of nanoscale materials
81.07.Bc Nanocrystalline materials
81.15.Fg Pulsed laser ablation deposition
61.72.Mm Grain and twin boundaries

Photoluminescence of size-separated silicon nanocrystals: Confirmation of quantum confinement

G. Ledoux, J. Gong, F. Huisken, O. Guillois, and C. Reynaud

Appl. Phys. Lett. 80, 4834 (2002); http://dx.doi.org/10.1063/1.1485302 (3 pages) | Cited 134 times

Online Publication Date: 17 June 2002

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Silicon nanocrystals with diameters between 2.5 and 8 nm were prepared by pulsed CO2 laser pyrolysis of silane in a gas flow reactor and expanded through a conical nozzle into a high vacuum. Using a fast-spinning molecular-beam chopper, they were size-selectively deposited on dedicated quartz substrates. Finally, the photoluminescence of the silicon nanocrystals and their yield were measured as a function of their size. It was found that the photoluminescence follows very closely the quantum-confinement model. The yield shows a pronounced maximum for sizes between 3 and 4 nm. © 2002 American Institute of Physics.
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78.67.Bf Nanocrystals, nanoparticles, and nanoclusters
78.55.Ap Elemental semiconductors
81.15.Fg Pulsed laser ablation deposition
61.46.-w Structure of nanoscale materials
82.30.Lp Decomposition reactions (pyrolysis, dissociation, and fragmentation)
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
81.07.Bc Nanocrystalline materials
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