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15 May 2003

Volume 93, Issue 10, pp. 5855-8792

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Amorphous-crystalline transition at the Ir/Si(100) interface

C.-P. Ouyang, J.-J. Chang, J.-F. Wen, L.-C. Tien, J. Hwang, and Tun-Wen Pi

J. Appl. Phys. 93, 6248 (2003); http://dx.doi.org/10.1063/1.1563296 (4 pages)

Online Publication Date: 9 May 2003

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The amorphous-crystalline transition at the Ir/Si(100) interface has been characterized by using both low-energy electron diffraction (LEED) and synchrotron-based photoemission. Solid-state amorphization occurred at the Ir/Si(100)-2×1 interface deposited at 600 °C. The double domain Si(100)-2×1 LEED pattern disappeared when 1 ML Ir was deposited onto Si(100). Three types of Ir–Si bonding formed on Si(100) at 1 ML Ir coverage and gradually evolved to be amorphous IrSi, Ir3Si5, and IrxSiy (unidentified) bonding environments at Ir coverage less than ∼3 ML. The amorphous Ir–Si reacted layer was grown layer–by–layer-like. An Ir3Si5 crystalline phase, accompanying with amorphous IrSi and IrxSiy alloys, started to form on top of the amorphous Ir–Si reacted layer at Ir coverage near ∼3 ML. The Ir3Si5 crystalline phase was evolved from its corresponding Ir3Si5 amorphous bonding environment in the Ir–Si reacted layer. © 2003 American Institute of Physics.
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68.35.Ct Interface structure and roughness
61.43.Dq Amorphous semiconductors, metals, and alloys
81.05.Bx Metals, semimetals, and alloys
68.35.Fx Diffusion; interface formation
66.30.Ny Chemical interdiffusion; diffusion barriers
61.05.jh Low-energy electron diffraction (LEED) and reflection high-energy electron diffraction (RHEED)
79.60.Jv Interfaces; heterostructures; nanostructures

Ultraviolet-emitting ZnO nanowhiskers prepared by a vapor transport process on prestructured surfaces with self-assembled polymers

M. Haupt, A. Ladenburger, R. Sauer, K. Thonke, R. Glass, W. Roos, J. P. Spatz, H. Rauscher, S. Riethmüller, and M. Möller

J. Appl. Phys. 93, 6252 (2003); http://dx.doi.org/10.1063/1.1563845 (6 pages) | Cited 34 times

Online Publication Date: 9 May 2003

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ZnO wires were grown by a vapor–liquid–solid phase transport process. Self-assembled Au nano-clusters act as a catalyst or seed for the highly oriented growth of so-called ZnO whiskers on sapphire substrates by a vapor–liquid–solid phase transport process. The ZnO nanowires were more than 500 nm high and smaller than 30 nm in diameter. Low-temperature photoluminescence measurements reveal intense and detailed ultraviolet light emission near the opitical band gap of ZnO at 3.37 eV. The ZnO nanowires show almost no broad green photoluminescence emission band related to oxygen defects and only a weak signal due to donor–acceptor pair recombination. X-ray diffraction proves that the ZnO wires were grown c-plane oriented on an a-plane sapphire substrate with high crystal quality most likely because of a kind of self-purification during the growth process. © 2003 American Institute of Physics.
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68.70.+w Whiskers and dendrites (growth, structure, and nonelectronic properties)
61.46.-w Structure of nanoscale materials
78.55.Et II-VI semiconductors
78.67.Bf Nanocrystals, nanoparticles, and nanoclusters
81.05.Dz II-VI semiconductors
68.65.La Quantum wires (patterned in quantum wells)
64.60.Q- Nucleation
81.16.Dn Self-assembly
81.10.-h Methods of crystal growth; physics and chemistry of crystal growth, crystal morphology, and orientation
81.07.Bc Nanocrystalline materials

Positioning of self-assembled Ge islands on stripe-patterned Si(001) substrates

Zhenyang Zhong, A. Halilovic, M. Mühlberger, F. Schäffler, and G. Bauer

J. Appl. Phys. 93, 6258 (2003); http://dx.doi.org/10.1063/1.1566455 (7 pages) | Cited 46 times

Online Publication Date: 9 May 2003

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Self-assembled Ge islands were grown on stripe-patterned Si(001) substrates by solid source molecular beam epitaxy. The surface morphology obtained by atomic force microscopy and cross-sectional transmission electron microscopy images shows that the Ge islands are preferentially grown at the sidewalls of pure Si stripes along the [−110] direction at 650 °C or along the trenches, whereas most of the Ge islands are formed on the top terrace when the patterned stripes are covered by a strained GeSi buffer layer. Reducing the growth temperature to 600 °C results in a nucleation of Ge islands both on the top terrace and at the sidewall of pure Si stripes. A qualitative analysis, based on the growth kinetics, demonstrates that the step structure of the stripes, the external strain field, and the local critical wetting layer thickness for the islands formation contribute to the preferential positioning of Ge islands on the stripes. © 2003 American Institute of Physics.
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81.07.Ta Quantum dots
68.35.B- Structure of clean surfaces (and surface reconstruction)
68.37.Ps Atomic force microscopy (AFM)
68.37.Lp Transmission electron microscopy (TEM)

Solid immersion lens-enhanced nano-photoluminescence: Principle and applications

S. Moehl, Hui Zhao, B. Dal Don, S. Wachter, and H. Kalt

J. Appl. Phys. 93, 6265 (2003); http://dx.doi.org/10.1063/1.1567035 (8 pages) | Cited 25 times

Online Publication Date: 9 May 2003

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We demonstrate a far-field nano-photoluminescence setup based on the combination of a hemispherical solid immersion lens (SIL) with a confocal microscope. The spatial resolution is confirmed to be 0.4 times the wavelength in vacuum in terms of half width at half maximum. The collection efficiency is found to be about five times higher than the same microscope without SIL, which is consistent with our theoretical analysis. We investigate in detail the influence of an air gap between the SIL and the sample surface on the system performance, and prove both experimentally and theoretically the tolerance of this far-field system to an air gap of several micrometers. These features make the present setup an ideal system for spatially resolved spectroscopy of semiconductor nanostructures. In particular, we show two examples of such applications in which the present setup is clearly suitable: Studies of excitonic transport in quantum wells and spectroscopy of single quantum dots with emphasis on polarization dependence and weak-signal detection. © 2003 American Institute of Physics.
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42.79.Bh Lenses, prisms and mirrors
07.60.Pb Conventional optical microscopes
78.55.-m Photoluminescence, properties and materials
81.07.St Quantum wells
81.07.Ta Quantum dots
78.67.Hc Quantum dots
78.67.De Quantum wells

Perpendicular magnetic anisotropy of Co–TiN composite film with nano-fiber structure

C. C. Chen, M. Hashimoto, J. Shi, Y. Nakamura, O. Nittono, and P. B. Barna

J. Appl. Phys. 93, 6273 (2003); http://dx.doi.org/10.1063/1.1567032 (6 pages) | Cited 3 times

Online Publication Date: 9 May 2003

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Co–Ti–N films have been prepared by sputter deposition of Co and Ti in Ar+N2 atmosphere. Upon thermal anneal at elevated temperatures, Co (face-centered cubic) and TiN were formed in the film and separated from each other. Fiber-like microstructure developed with Co nano-fibers vertical to the substrate surface, and with their lateral size being less than 10 nm. The magnetic anisotropy of such films depends strongly on the film thickness. The Co–TiN films with their thickness above 100 nm show perpendicular magnetic anisotropy, which is explained in terms of shape anisotropy. Considering their microstructure, it is concluded that the diameter to length ratio of Co nano-fibers is an important factor controlling the magnetic anisotropy. For the Co–TiN film to show perpendicular magnetic anisotropy, the diameter to length ratio has to be smaller than 0.07 according to the experimental results. TiN in the films plays an important role in separating Co nano-fibers and thus to reduce the lateral magnetic interaction among them. The nano-scale nature and perpendicular magnetic anisotropy of the Co–TiN nanocomposite film make it a very promising candidate for future ultrahigh magnetic recording media. © 2003 American Institute of Physics.
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75.30.Gw Magnetic anisotropy
75.70.Ak Magnetic properties of monolayers and thin films
75.50.Ss Magnetic recording materials
68.55.Nq Composition and phase identification
61.72.Cc Kinetics of defect formation and annealing
81.40.Gh Other heat and thermomechanical treatments
64.75.-g Phase equilibria
68.55.-a Thin film structure and morphology
75.50.Tt Fine-particle systems; nanocrystalline materials
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
68.37.Lp Transmission electron microscopy (TEM)

Influence of the temperature on the carrier capture into self-assembled InAs/GaAs quantum dots

C. A. Duarte, E. C. F. da Silva, A. A. Quivy, M. J. da Silva, S. Martini, J. R. Leite, E. A. Meneses, and E. Lauretto

J. Appl. Phys. 93, 6279 (2003); http://dx.doi.org/10.1063/1.1568538 (5 pages) | Cited 16 times

Online Publication Date: 9 May 2003

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Photoluminescence (PL) spectroscopy and atomic-force microscopy (AFM) were used to investigate the size evolution of InAs quantum dots on GaAs(001) as a function of the amount of InAs material. Different families of islands were observed in the AFM images and unambiguously identified in the PL spectra, together with the signal of the wetting layer. PL measurements carried out at low and intermediate temperatures showed a thermal carrier redistribution among dots belonging to different families. The physical origin of this behavior is explained in terms of the different temperature dependence of the carrier-capture rate into the quantum dots. At high temperatures, an enhancement of the total PL-integrated intensity of the largest-sized quantum dots was attributed to the increase of diffusivity of the photogenerated carriers inside the wetting layer. © 2003 American Institute of Physics.
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78.67.Hc Quantum dots
78.55.Cr III-V semiconductors
68.65.Hb Quantum dots (patterned in quantum wells)
68.37.Ps Atomic force microscopy (AFM)

Simulation of the dc plasma in carbon nanotube growth

David Hash, Deepak Bose, T. R. Govindan, and M. Meyyappan

J. Appl. Phys. 93, 6284 (2003); http://dx.doi.org/10.1063/1.1568155 (7 pages) | Cited 41 times

Online Publication Date: 9 May 2003

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A model for the dc plasma used in carbon nanotube growth is presented, and one-dimensional simulations of an acetylene/ammonia/argon system are performed. The effect of dc bias is illustrated by examining electron temperature, electron and ion densities, and neutral densities. Introducing a tungsten filament in the dc plasma, as in hot filament chemical vapor deposition with plasma assistance, shows negligible influence on the system characteristics. © 2003 American Institute of Physics.
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81.07.De Nanotubes
52.77.Dq Plasma-based ion implantation and deposition
52.65.-y Plasma simulation
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
52.25.Dg Plasma kinetic equations

Structure and magnetic property changes of epitaxially grown L10-FePd isolated nanoparticles on annealing

Kazuhisa Sato and Yoshihiko Hirotsu

J. Appl. Phys. 93, 6291 (2003); http://dx.doi.org/10.1063/1.1568531 (8 pages) | Cited 31 times

Online Publication Date: 9 May 2003

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Isolated 10-nm-sized FePd nanoparticles with the L10-type ordered structure have been fabricated by electron-beam evaporation and postannealing above 773 K, and the structural details have been investigated by transmission electron microscopy. FePd particles were epitaxially grown on a cleaved NaCl(001) substrate and were two-dimensionally dispersed on the substrate. In FePd particles formation, Pd nanoparticles were first deposited as “seed” particles epitaxially on NaCl followed by a successive deposition of Fe particles. All the Fe particles were captured by Pd particles forming Fe/Pd nanocomplex particles with a mutual fixed orientation. Coalescence and growth of the particles were not prominent during annealing, indicating that the alloying and atomic ordering reactions proceeded mostly within each nanoparticle. The negligible coalescence can be attributed to an “anchoring effect” of the seed Pd to the coalescence growth. Moreover, both of these reactions are thought to proceed almost simultaneously during annealing at temperatures between 723 and 823 K. Most of the annealed particles were single crystal particles with c axes oriented both normal and parallel to the film plane. Large coercivities above 3 kOe were obtained after annealing at 873 K, though they were smaller than those expected from the theoretical model. The small coercivity value can be attributed to the low magnetocrystalline anisotropy. The magnetocrystalline anisotropy constant of the present FePd nanoparticles estimated was less than half of that of the bulk materials. © 2003 American Institute of Physics.
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61.46.-w Structure of nanoscale materials
81.07.Bc Nanocrystalline materials
75.50.Tt Fine-particle systems; nanocrystalline materials
75.50.Bb Fe and its alloys
81.40.Rs Electrical and magnetic properties related to treatment conditions
81.40.Gh Other heat and thermomechanical treatments
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
68.37.Lp Transmission electron microscopy (TEM)
68.55.-a Thin film structure and morphology
75.70.Ak Magnetic properties of monolayers and thin films
61.66.Dk Alloys
81.15.-z Methods of deposition of films and coatings; film growth and epitaxy
81.15.Kk Vapor phase epitaxy; growth from vapor phase
75.30.Gw Magnetic anisotropy
81.16.-c Methods of micro- and nanofabrication and processing

X-ray absorption and diffraction studies of thin polymer/FePt nanoparticle assemblies

S. Anders, M. F. Toney, T. Thomson, R. F. C. Farrow, J.-U. Thiele, B. D. Terris, Shouheng Sun, and C. B. Murray

J. Appl. Phys. 93, 6299 (2003); http://dx.doi.org/10.1063/1.1567802 (6 pages) | Cited 15 times

Online Publication Date: 9 May 2003

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We have produced assemblies of monodisperse 4 nm FePt nanoparticles using polymer-mediated layer–by–layer deposition at room temperature. The process leads to good control of particle assembly thickness and offers great potential for future fabrication of ultra-high density magnetic storage media. Vibrating sample magnetometry with fields up to 9 T was applied to study the magnetic properties of the particle assemblies as a function of annealing condition while near edge x-ray absorption fine structure (NEXAFS) spectroscopy and x-ray diffraction (XRD) were used to investigate the chemical nature and structural properties within particles. It was found that the coercivity can be as high as 22.7 kOe for samples annealed at 800 °C, the moment density (normalized to the particle volume) for the sample annealed at 650 °C is estimated close to the value for bulk FePt, at 1140 emu/cm3. NEXAFS spectroscopy shows that the Fe in the as-deposited assemblies is partly oxidized, and the oxidation is greatly reduced by annealing. XRD studies on the assemblies annealed at high temperature revealed the increased atomic order and the formation of the high-anisotropy L10 phase within the particles. However, the high-temperature annealing also resulted in nanoparticle agglomeration. © 2003 American Institute of Physics.
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61.46.-w Structure of nanoscale materials
75.50.Tt Fine-particle systems; nanocrystalline materials
81.07.Bc Nanocrystalline materials
78.70.Dm X-ray absorption spectra
82.35.Np Nanoparticles in polymers
82.60.Qr Thermodynamics of nanoparticles
61.72.Cc Kinetics of defect formation and annealing
81.40.Gh Other heat and thermomechanical treatments
81.40.Rs Electrical and magnetic properties related to treatment conditions
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
82.70.-y Disperse systems; complex fluids

Tunneling barrier in nanoparticle junctions of La2/3(Ca,Sr)1/3MnO3: Nonlinear current–voltage characteristics

D. Niebieskikwiat, R. D. Sánchez, D. G. Lamas, A. Caneiro, L. E. Hueso, and J. Rivas

J. Appl. Phys. 93, 6305 (2003); http://dx.doi.org/10.1063/1.1568156 (6 pages) | Cited 15 times

Online Publication Date: 9 May 2003

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We study the nonlinear current–voltage (I–V) characteristics and analyze the voltage-dependent tunneling conductance in nanoparticles of La2/3A1/3MnO3 (A=Ca, Sr). The powders were prepared by different wet-chemical routes and low calcination temperatures were used to obtain an average particle size D≈30 nm. The data are comprehensively explained in terms of the tunneling picture, which allows one to estimate the height of the grain boundary insulating barrier (φ) for each sample. For constant D, our results show that the sample preparation route is mainly responsible for the value of φ in nanoparticles, while the Coulomb gap in the Coulomb blockade regime is ∼3 times higher for Sr- than for Ca-doping. We also show that a small fraction of the barriers contribute to the nonlinear transport, and the current is mainly carried through low-resistive percolated paths. In addition, despite the different barrier strengths, the low-field magnetoresistance (LFMR) is similar for all samples, implying that φ is not the fundamental parameter determining the LFMR.© 2003 American Institute of Physics.
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73.63.Bd Nanocrystalline materials
81.07.Wx Nanopowders
75.47.Lx Magnetic oxides
81.16.Be Chemical synthesis methods
61.46.-w Structure of nanoscale materials
81.07.Bc Nanocrystalline materials
72.20.My Galvanomagnetic and other magnetotransport effects
85.75.-d Magnetoelectronics; spintronics: devices exploiting spin polarized transport or integrated magnetic fields
81.20.Ev Powder processing: powder metallurgy, compaction, sintering, mechanical alloying, and granulation
81.40.Gh Other heat and thermomechanical treatments
81.40.Rs Electrical and magnetic properties related to treatment conditions
61.72.Mm Grain and twin boundaries
73.23.Hk Coulomb blockade; single-electron tunneling

Mechanisms of photoluminescence from silicon nanocrystals formed by pulsed-laser deposition in argon and oxygen ambient

X. Y. Chen, Y. F. Lu, Y. H. Wu, B. J. Cho, M. H. Liu, D. Y. Dai, and W. D. Song

J. Appl. Phys. 93, 6311 (2003); http://dx.doi.org/10.1063/1.1569033 (9 pages) | Cited 33 times

Online Publication Date: 9 May 2003

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We have investigated the different mechanisms of photoluminescence (PL) of silicon nanocrystals due to the quantum confinement effect (QCE) and interface states. Si nanocrystals were formed by pulsed-laser deposition in inert argon and reactive oxygen gas. The collisions between the ejected species greatly influence the morphology of the Si nanocrystals and cause a transition from a film structure to a porous cauliflowerlike structure, as the ambient gas pressure increases from 1 mTorr to 1 Torr. The oxygen content of the Si nanocrystals increases with increasing O2 ambient pressure, and nearly SiO2 stoichiometry is obtained when the O2 pressure is higher than 100 mTorr. Broad PL spectra are observed from Si nanocrystals. The peak position and intensity of the PL band at 1.8–2.1 eV vary with ambient gas pressure, while intensity changes and blueshifts are observed after oxidation and annealing. The PL band at 2.55 eV shows vibronic structures with periodic spacing of 97±9 meV, while no peak shift is found before and after oxidation and annealing. Raman and transmission electron microscope measurements show consistent results in crystal size while more accurate atomic force microscope measurements reveal a smaller crystal size. X-ray diffraction reveals a polycrystal structure in the Si nanocrystals and the crystallinity improves after annealing. Combined with the PL spectra of Si nanocrystals obtained by crumbling electrochemically etched porous Si layer, the results clearly demonstrate that the PL band at 1.8–2.1 eV is due to the QCE in the Si nanocrystal core, while the PL band at 2.55 eV is related to localized surface states at the SiOx/Si interface. © 2003 American Institute of Physics.
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78.55.Ap Elemental semiconductors
61.46.-w Structure of nanoscale materials
78.67.Bf Nanocrystals, nanoparticles, and nanoclusters
81.15.Fg Pulsed laser ablation deposition
81.16.Pr Micro- and nano-oxidation
81.07.Bc Nanocrystalline materials
81.65.Mq Oxidation
61.72.Cc Kinetics of defect formation and annealing
81.16.Mk Laser-assisted deposition

Positron lifetime spectroscopic studies of nanocrystalline ZnFe2O4

P. M. G. Nambissan, C. Upadhyay, and H. C. Verma

J. Appl. Phys. 93, 6320 (2003); http://dx.doi.org/10.1063/1.1569973 (7 pages) | Cited 28 times

Online Publication Date: 9 May 2003

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By carrying out positron lifetime measurements in zinc ferrite (ZnFe2O4) samples of various grain sizes down to 5 nm, the defect microstructures have been identified. In the bulk samples composed of grains of large sizes, positrons were trapped by monovacancies in the crystalline structure. Upon reduction of the grain sizes to nanometer dimensions, positrons get trapped selectively at either the diffused vacancies on the grain surfaces and the intergranular regions. Below about 9 nm, the grains undergo the transformation from the normal spinel structure to the inverse phase. A concomitant lattice contraction results in substantial reduction of the octahedral site volume, and hence, a fraction of the Zn2+ ions with larger ionic radius fails to occupy these sites. This leaves vacancies at the octahedral sites which then turn out to be the major trapping sites for positrons. ZnFe2O4 samples prepared through different routes were investigated, which showed similar qualitative features, although those synthesized through the hydrothermal precipitation method showed remarkably larger lifetimes for trapped positrons upon nanocrystallization in comparison to the samples prepared through the citrate route. © 2003 American Institute of Physics.
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78.70.Bj Positron annihilation
75.50.Tt Fine-particle systems; nanocrystalline materials
75.50.Gg Ferrimagnetics
61.72.J- Point defects and defect clusters
64.75.-g Phase equilibria
81.30.Mh Solid-phase precipitation

Microstructure of precipitated Au nanoclusters in MgO

C. M. Wang, S. Thevuthasan, V. Shutthanandan, A. Cavanagh, W. Jiang, L. E. Thomas, and W. J. Weber

J. Appl. Phys. 93, 6327 (2003); http://dx.doi.org/10.1063/1.1569032 (7 pages) | Cited 11 times

Online Publication Date: 9 May 2003

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Gold nanoclusters dispersed in single crystal MgO have been prepared by ion implantation at 975 K and subsequent annealing at 1275 K for 10 h. The morphological features, size, and crystallographic orientation of the Au nanoclusters with respect to the MgO matrix, as well as the interface structure between the Au nanoclusters and MgO, have been investigated using transmission electron microscopy. During annealing, the Au clusters nucleate coherently in the MgO lattice, leading to an epitaxial orientation relationship of [010]MgO//[010]Au and (200)MgO//(200)Au that is maintained for all the Au clusters. Above a critical size of ∼5 to 8 nm, a coherent-semicoherent interface transition is observed for the Au clusters in MgO. This critical cluster size is larger than the critical size ∼3 nm based on energetic considerations. This discrepancy is discussed with respect to the point and extended defect structures at the interface between the Au clusters and the MgO matrix. The Au clusters larger than this critical size exhibit faceting on the {001} planes and internal dislocations. It is further suggested that the density of quantum antidots should depend on the size of the Au clusters. © 2003 American Institute of Physics.
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61.46.-w Structure of nanoscale materials
64.75.-g Phase equilibria
61.72.up Other materials
61.72.Cc Kinetics of defect formation and annealing
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