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14 Oct 2002

Volume 81, Issue 16, pp. 2917-3103

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Ge–Sn semiconductors for band-gap and lattice engineering

M. Bauer, J. Taraci, J. Tolle, A. V. G. Chizmeshya, S. Zollner, David J. Smith, J. Menendez, Changwu Hu, and J. Kouvetakis

Appl. Phys. Lett. 81, 2992 (2002); http://dx.doi.org/10.1063/1.1515133 (3 pages) | Cited 54 times

Online Publication Date: 7 October 2002

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We describe a class of Si-based semiconductors in the Ge1−xSnx system. Deuterium-stabilized Sn hydrides provide a low-temperature route to a broad range of highly metastable compositions and structures. Perfectly epitaxial diamond-cubic Ge1−xSnx alloys are grown directly on Si(100) and exhibit high thermal stability, superior crystallinity, and crystallographic and optical properties, such as adjustable band gaps and lattice constants. These properties are completely characterized by Rutherford backscattering, low-energy secondary ion mass spectrometry, high-resolution transmission electron microscopy, x-ray diffraction (rocking curves), as well as infrared and Raman spectroscopies and spectroscopic ellipsometry. Ab initio density functional theory simulations are also used to elucidate the structural and spectroscopic behavior. © 2002 American Institute of Physics.
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68.55.-a Thin film structure and morphology
68.60.Dv Thermal stability; thermal effects
71.20.Nr Semiconductor compounds
78.66.Li Other semiconductors
73.20.At Surface states, band structure, electron density of states
78.30.Hv Other nonmetallic inorganics
71.15.Mb Density functional theory, local density approximation, gradient and other corrections
79.20.Rf Atomic, molecular, and ion beam impact and interactions with surfaces
82.80.Yc Rutherford backscattering (RBS), and other methods of chemical analysis

Integrity of hafnium silicate/silicon dioxide ultrathin films on Si

J. Morais, L. Miotti, G. V. Soares, S. R. Teixeira, R. Pezzi, K. P. Bastos, I. J. R. Baumvol, A. L. P. Rotondaro, J. J. Chambers, M. R. Visokay, and L. Colombo

Appl. Phys. Lett. 81, 2995 (2002); http://dx.doi.org/10.1063/1.1515112 (3 pages) | Cited 23 times

Online Publication Date: 7 October 2002

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Rapid thermal annealing at 1000 °C of (HfO2)1−x(SiO2)x pseudobinary alloy films deposited on Si were performed in N2 or O2 atmospheres. The effects on the atomic transport, structure, and composition were investigated using isotopic substitution of oxygen, high-resolution transmission electron microscopy, nuclear reaction analyses, narrow nuclear reaction resonance profiling, and grazing angle x-ray reflection. © 2002 American Institute of Physics.
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61.72.Cc Kinetics of defect formation and annealing
73.61.Ng Insulators
73.40.Qv Metal-insulator-semiconductor structures (including semiconductor-to-insulator)
68.60.-p Physical properties of thin films, nonelectronic
68.55.-a Thin film structure and morphology
68.37.Lp Transmission electron microscopy (TEM)

Sub-picosecond electron relaxation of near-infrared intersubband transitions in n-doped (CdS/ZnSe)/BeTe quantum wells

R. Akimoto, K. Akita, F. Sasaki, and T. Hasama

Appl. Phys. Lett. 81, 2998 (2002); http://dx.doi.org/10.1063/1.1515371 (3 pages) | Cited 28 times

Online Publication Date: 7 October 2002

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We report on intersubband (ISB) absorption and ultrafast ISB energy relaxation of carriers in n-type doped (CdS/ZnSe)/BeTe quantum wells (QWs), grown by molecular-beam epitaxy. The highly n-type doped QW samples were obtained by introducing a few monolayer CdS into a ZnSe/BeTe QW, and ISB absorption with a peak wavelength as short as 1.62 μm, covering 1.55 μm within its absorption bandwidth, was achieved. The ISB carrier relaxation was investigated by means of femtosecond ( ∼ 150 fs) one-color pump and probe technique at the ISB absorption peak. The ISB carrier relaxation time of 270 fs was observed in the sample with the absorption peak at 1.82 μm. The slow decay component with a time constant of a few ps, which has been observed in ZnSe/BeTe QWs, was not observed in the (CdS/ZnSe)/BeTe QWs, indicating that the Γ(ZnSe)–X(BeTe) electron transfer is suppressed, as expected from the band alignment. © 2002 American Institute of Physics.
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73.50.Gr Charge carriers: generation, recombination, lifetime, trapping, mean free paths
78.66.Hf II-VI semiconductors
78.67.De Quantum wells
78.30.Fs III-V and II-VI semiconductors
78.47.-p Spectroscopy of solid state dynamics
73.50.Dn Low-field transport and mobility; piezoresistance

Transient optical studies of charge recombination dynamics in a polymer/fullerene composite at room temperature

Ivan Montanari, Ana F. Nogueira, Jenny Nelson, James R. Durrant, Christoph Winder, Maria Antonietta Loi, Niyazi Serdar Sariciftci, and Christoph Brabec

Appl. Phys. Lett. 81, 3001 (2002); http://dx.doi.org/10.1063/1.1512943 (3 pages) | Cited 91 times

Online Publication Date: 7 October 2002

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The recombination kinetics of photogenerated charge carriers in a composite of poly[2-methoxy-5- (3′,7′-dimethyloctyloxy)-1-4-phenylene vinylene], (MDMO–PPV) and the functionalised fullerene 1-(3-methoxycarbonyl)-propyl-1-phenyl-(6,6)C61 are investigated at room temperature by transient absorption spectroscopy. The decay dynamics of positively charged MDMO–PPV polarons were found to be either monophasic or biphasic, depending upon the laser excitation density employed. The slower, power law, decay phase (100 ns–10 ms) is attributed to recombination dynamics of localized polarons, while the fast decay component (<20 ns) is attributed to recombination of relatively mobile polarons observed when the density of localized states is exceeded by the density of photogenerated polarons ( ∼ 1017 cm−3). The implications of these observations are discussed in relation to polymer/C60 photovoltaic cells. © 2002 American Institute of Physics.
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73.61.Ph Polymers; organic compounds
73.61.Wp Fullerenes and related materials
73.50.Pz Photoconduction and photovoltaic effects
73.50.Gr Charge carriers: generation, recombination, lifetime, trapping, mean free paths
78.66.Qn Polymers; organic compounds
78.66.Tr Fullerenes and related materials
73.50.Dn Low-field transport and mobility; piezoresistance
71.38.-k Polarons and electron-phonon interactions
78.47.-p Spectroscopy of solid state dynamics

Carrier profiling of Al-doped 4H-SiC by scanning spreading resistance microscopy

J. Osterman, A. Hallén, and S. Anand

Appl. Phys. Lett. 81, 3004 (2002); http://dx.doi.org/10.1063/1.1514829 (3 pages) | Cited 9 times

Online Publication Date: 7 October 2002

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Epitaxially-grown Al-doped 4H-SiC has been studied by scanning spreading resistance microscopy. The measured current shows good quantitative agreement with the chemical Al concentration in the range 2×1016 to 2×1020 atoms cm−3. Simulations of the sample temperature distribution using finite element calculations predict a maximum temperature exceeding 750 K within 100 nm of the contact region at 7.5 V dc bias for an Al doping of 1020 cm−3. The heating causes a significant increase in the ionization of the dopants relative to that at room temperature. Due to the strong voltage dependence, the effect can be avoided by operating below 5 V dc bias where the temperature rise is shown to be negligible for all dopant concentrations. © 2002 American Institute of Physics.
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72.20.Jv Charge carriers: generation, recombination, lifetime, and trapping
72.20.Fr Low-field transport and mobility; piezoresistance
72.80.Jc Other crystalline inorganic semiconductors

Self-assembled GaAs antiwires in In0.53Ga0.47As matrix on (100) InP substrates

S. D. Lin, C. P. Lee, W. H. Hsieh, and Y. W. Suen

Appl. Phys. Lett. 81, 3007 (2002); http://dx.doi.org/10.1063/1.1515878 (3 pages) | Cited 3 times

Online Publication Date: 7 October 2002

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The growth of GaAs antiwires in the In0.53Ga0.47As matrix on InP substrate has been investigated. The periodic, wire-like structure was obtained when a proper amount of GaAs was deposited. The grown antiwires have a height about 1.2–2.0 nm and a period about 23 nm. Using an In0.53Ga0.47As/In0.52Al0.48As modulation-doped structure, the effect of the GaAs antiwires on the two-dimensional electron gas mobility was investigated. For the sample with antiwires near the two-dimensional channel, a significant anisotropy in low temperature mobility was observed. © 2002 American Institute of Physics.
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81.07.Vb Quantum wires
68.65.La Quantum wires (patterned in quantum wells)
81.16.Dn Self-assembly
73.63.Nm Quantum wires
73.50.Dn Low-field transport and mobility; piezoresistance
81.15.Hi Molecular, atomic, ion, and chemical beam epitaxy
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Hall effect and hole densities in Ga1−xMnxAs

K. W. Edmonds, K. Y. Wang, R. P. Campion, A. C. Neumann, C. T. Foxon, B. L. Gallagher, and P. C. Main

Appl. Phys. Lett. 81, 3010 (2002); http://dx.doi.org/10.1063/1.1512822 (3 pages) | Cited 87 times

Online Publication Date: 7 October 2002

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By studying the Hall effect in a series of low resistivity Ga1−xMnxAs samples, accurate values for the hole density p, Mn concentration x, and Curie temperature TC are obtained over the range 0.015 ⩽ x ⩽ 0.08. The hole density corresponds to 90% of the Mn concentration at low x, and has a maximum value of 1.0×1027 m−3 when TC = 125 K. These data allow the first meaningful comparison of mean field predicted Curie temperatures with experiment over a wide range of x. The theory is in qualitative agreement with experiment, but overestimates TC at large x and underestimates TC at low x. © 2002 American Institute of Physics.
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75.50.Pp Magnetic semiconductors
72.20.My Galvanomagnetic and other magnetotransport effects
75.30.Kz Magnetic phase boundaries (including classical and quantum magnetic transitions, metamagnetism, etc.)
72.80.Ey III-V and II-VI semiconductors
75.50.Dd Nonmetallic ferromagnetic materials

Ferromagnetism and interlayer exchange coupling in short-period (Ga,Mn)As/GaAs superlattices

R. Mathieu, P. Svedlindh, J. Sadowski, K. Światek, M. Karlsteen, J. Kanski, and L. Ilver

Appl. Phys. Lett. 81, 3013 (2002); http://dx.doi.org/10.1063/1.1515368 (3 pages) | Cited 22 times

Online Publication Date: 7 October 2002

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Magnetic properties of (Ga,Mn)As/GaAs superlattices are investigated. The structures contain magnetic (Ga,Mn)As layers, separated by thin layers of nonmagnetic GaAs spacer. The short-period Ga0.93Mn0.07As/GaAs superlattices exhibit a paramagnetic-to-ferromagnetic phase transition close to 60 K, for thicknesses of (Ga,Mn)As down to 23 Å. For Ga0.96Mn0.04As/GaAs superlattices of similar dimensions, the Curie temperature associated with the ferromagnetic transition is found to oscillate with the thickness of nonmagnetic spacer. The observed oscillations are related to an interlayer exchange interaction mediated by the polarized holes of the (Ga,Mn)As layers. © 2002 American Institute of Physics.
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75.70.Cn Magnetic properties of interfaces (multilayers, superlattices, heterostructures)
75.50.Pp Magnetic semiconductors
75.50.Dd Nonmetallic ferromagnetic materials
75.30.Et Exchange and superexchange interactions
75.30.Kz Magnetic phase boundaries (including classical and quantum magnetic transitions, metamagnetism, etc.)
73.21.Cd Superlattices

Angular rotation of magnetic hysteresis of ion-irradiated ferromagnetic thin films

G. S. Chang, T. A. Callcott, G. P. Zhang, G. T. Woods, S. H. Kim, S. W. Shin, K. Jeong, C. N. Whang, and A. Moewes

Appl. Phys. Lett. 81, 3016 (2002); http://dx.doi.org/10.1063/1.1515370 (3 pages) | Cited 4 times

Online Publication Date: 7 October 2002

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The magnetization direction of a cobalt-platinum film has been manipulated by magnetic-field-assisted ion-beam mixing. A uniaxial easy magnetization axis is induced along the oscillation direction of the incident beam of 80 keV Ar+ ions. The easy axis was then rotated by 60°, when the same ion irradiation process was carried out in the presence of an external magnetic field. Magnetic force microscopy measurements show that line shaped domain structures are created with their long axis along the induced magnetization directions. It is proposed that the creation of adjacent regions with orthogonal magnetization can play a crucial role in stabilizing the magnetostatic dipolar interaction between adjacent bits of magnetic data storage devices. © 2002 American Institute of Physics.
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75.70.Ak Magnetic properties of monolayers and thin films
75.50.Cc Other ferromagnetic metals and alloys
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
61.80.Jh Ion radiation effects
61.82.Bg Metals and alloys
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Decreasing giant splitting of longitudinal and transverse optical phonons in PbxSr1−xTiO3 due to Pb covalency

Shou-Yi Kuo, Chung-Ting Li, and Wen-Feng Hsieh

Appl. Phys. Lett. 81, 3019 (2002); http://dx.doi.org/10.1063/1.1513660 (3 pages) | Cited 15 times

Online Publication Date: 7 October 2002

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In contrast to the increase of giant splitting of longitudinal optical (LO) and transverse optical (TO) phonons in polycrystalline BaxSr1−xTiO3 [Phys. Rev. B. 64, 224103-1 (2001)], a decreasing splitting of the softest A1(1TO) and the hardest A1(3LO) was observed in PbxSr1−xTiO3. A similar structure but anomalous discrepancies in lattice dynamics of ABO3 perovskite are related to the diverse electronic states of Pb2+ and Ba2+ ions. In this article, Pb–O interaction is proposed to be more hybridized than Ba–O, and thus the change of effective charge to the LO–TO splitting may be the dominant mechanism in PbxSr1−xTiO3, while unit-cell volume change is in the BaxSr1−xTiO3 systems. © 2002 American Institute of Physics.
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77.84.Ek Niobates and tantalates
77.84.Cg PZT ceramics and other titanates
77.80.B- Phase transitions and Curie point
63.20.D- Phonon states and bands, normal modes, and phonon dispersion

Second-harmonic study of polar symmetry and domain structure in SrTi18O3

Lei Zhang, Wolfgang Kleemann, Ruiping Wang, and Mitsuru Itoh

Appl. Phys. Lett. 81, 3022 (2002); http://dx.doi.org/10.1063/1.1515130 (3 pages) | Cited 18 times

Online Publication Date: 7 October 2002

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The ferroelectric domain structure of SrTi18O3 is studied via the azimuthal dependence of the second-harmonic (SH) light intensity below Tc ≈ 25K. Tensor analysis of the SH response reveals locally variant mixtures of eight triclinic polar domains, which transform into an orthorhombic single domain under an electric field E applied perpendicularly to the tetragonal axis, with d22 being the dominant coupling constant. © 2002 American Institute of Physics.
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77.84.Ek Niobates and tantalates
77.84.Cg PZT ceramics and other titanates
77.80.Dj Domain structure; hysteresis
42.65.Ky Frequency conversion; harmonic generation, including higher-order harmonic generation
77.22.Ej Polarization and depolarization

Piezoelectric nonlinearity of Pb(Zr,Ti)O3 thin films probed by scanning force microscopy

V. V. Shvartsman, A. L. Kholkin, and N. A. Pertsev

Appl. Phys. Lett. 81, 3025 (2002); http://dx.doi.org/10.1063/1.1514389 (3 pages) | Cited 10 times

Online Publication Date: 7 October 2002

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Local piezoelectric response of Pb(Zr,Ti)O3 thin films is investigated by scanning force microscopy as a function of the amplitude of driving ac field. Thus obtained local piezoelectric nonlinearity is compared with the macroscopic behavior deduced earlier from interferometric measurements. The nanoscale piezoelectric properties are found to be remarkably different from the average macroscopic response. In particular, the local piezoelectric nonlinearity measured inside uniformly polarized areas (domains) is unexpectedly high, being qualitatively different for the as-grown domains of opposite polarities. The minority domains are often unstable and switch into the opposite polarization state under an electric field several times lower than that required for global polarization switching. This effect is explained by the presence of charged domain boundaries below the surface and by their field-induced depinning. © 2002 American Institute of Physics.
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77.55.-g Dielectric thin films
77.65.-j Piezoelectricity and electromechanical effects
77.84.Ek Niobates and tantalates
77.84.Cg PZT ceramics and other titanates
77.22.Ej Polarization and depolarization

Dielectric properties of random and 〈100〉 oriented SrTiO3 and (Ba,Sr)TiO3 thin films fabricated on 〈100〉 nickel tapes

J. T. Dawley and P. G. Clem

Appl. Phys. Lett. 81, 3028 (2002); http://dx.doi.org/10.1063/1.1516630 (3 pages) | Cited 46 times

Online Publication Date: 7 October 2002

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Properties are presented for random and enhanced 〈100〉 orientation SrTiO3 and BaxSr1−xTiO3 (x = 0.33, 0.50, and 0.67) films fabricated on base-metal 〈100〉 Ni tapes using a chemical solution deposition approach. Films were crystallized in a reducing atmosphere, which prevented Ni oxidation, but permitted growth of SrTiO3 and BaxSr1−xTiO3 films with dielectric loss tan δ between 0.003 and 0.015. For randomly oriented BaxSr1−xTiO3 (x = 0, 0.33, 0.5, and 0.67) films, zero-field 100 kHz dielectric constants ranged from 250 to 420. Films with enhanced 〈100〉 orientation exhibited zero-field dielectric constants of 980 to 1500, three times higher than random films. Development of such high dielectric constants on Ni suggests a future method for inexpensive, high performance base-metal electrode capacitor fabrication.
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77.84.Ek Niobates and tantalates
77.84.Cg PZT ceramics and other titanates
77.55.-g Dielectric thin films
68.55.-a Thin film structure and morphology
81.15.Lm Liquid phase epitaxy; deposition from liquid phases (melts, solutions, and surface layers on liquids)
77.80.-e Ferroelectricity and antiferroelectricity
68.55.A- Nucleation and growth
77.22.Gm Dielectric loss and relaxation
77.22.Ch Permittivity (dielectric function)
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Anomalous localization effects associated with excess volume of cobalt catalyst in multiwalled carbon nanotubes

Junji Haruyama, Izumi Takesue, and Tetsuro Hasegawa

Appl. Phys. Lett. 81, 3031 (2002); http://dx.doi.org/10.1063/1.1511535 (3 pages) | Cited 5 times

Online Publication Date: 7 October 2002

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We report on the anomalous localization effects strongly associated with excess volume of a cobalt catalyst in multiwalled carbon nanotubes (MWNTs) synthesized in nanoporous alumina membranes. These effects bring about the following anomalies in bulk MWNTs: (a) A slight increase in the volume of excess cobalt changes antilocalization (AL) to weak localization (WL), (b) a further increase in excess changes this WL back to the AL, but only in magnetoresistance (MR) oscillation, and (c) even under this AL in MR, AL can not be observed in the conductance versus logarithmic temperature relation. Mechanisms for these anomalies were discussed based on the unique MWNT structures. © 2002 American Institute of Physics.
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73.22.-f Electronic structure of nanoscale materials and related systems
81.07.De Nanotubes
61.46.-w Structure of nanoscale materials
81.16.Hc Catalytic methods
73.63.Fg Nanotubes
82.30.Vy Homogeneous catalysis in solution, polymers and zeolites
73.20.Fz Weak or Anderson localization
72.20.My Galvanomagnetic and other magnetotransport effects
71.55.Jv Disordered structures; amorphous and glassy solids

Second-harmonic spectroscopy of two-dimensional Si nanocrystal layers embedded in SiO2 films

Y. Jiang, L. Sun, and M. C. Downer 

Appl. Phys. Lett. 81, 3034 (2002); http://dx.doi.org/10.1063/1.1510963 (3 pages) | Cited 6 times

Online Publication Date: 7 October 2002

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We present observations of optical second-harmonic generation (SHG) from dense (1010 or 6×1011 cm−2) layers of 5 or 8 nm average diameter silicon nanocrystals (NCs) embedded in thin (6–15 nm) SiO2 films on silicon substrates. Time-dependent SHG monitors optically-driven electrostatic charging of the Si NC layer as well as subsequent charge leakage, and thus provides noncontact electrical characterization of Si-NC-based device structures. SHG intensity and phase spectra of Si NCs are distinguished from contributions of the Si substrate by polarization-dependent and frequency-domain interferometric SH spectroscopy, which reveal a NC-size-dependent blueshift of the E1 resonance consistent with quantum confinement. © 2002 American Institute of Physics.
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42.65.Ky Frequency conversion; harmonic generation, including higher-order harmonic generation
78.67.Bf Nanocrystals, nanoparticles, and nanoclusters
78.66.Db Elemental semiconductors and insulators

High-resolution atomic force microscope nanotip grown by self-field emission

C. H. Oon, J. T. L. Thong, Y. Lei, and W. K. Chim

Appl. Phys. Lett. 81, 3037 (2002); http://dx.doi.org/10.1063/1.1515120 (3 pages) | Cited 11 times

Online Publication Date: 7 October 2002

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A technique to grow a single tungsten filament tip on a tapping mode atomic force microscope (AFM) tip by a process of self-field emission in the presence of tungsten carbonyl is demonstrated. Such filaments have a tip radius of 1–2 nm and are grown to lengths ranging from 400 nm to 3 μm and a shank diameter of about 60–90 nm. Images of germanium nanocrystals and porous alumina membranes show much higher resolution and definition than standard AFM tips. The tip shows no degradation even after 10 h of scanning, demonstrating its utility as a practical tip. The self-aligned nature of the growth makes it a very simple nanotip fabrication technique. © 2002 American Institute of Physics.
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07.79.Lh Atomic force microscopes
68.37.Ps Atomic force microscopy (AFM)
81.07.Lk Nanocontacts
79.70.+q Field emission, ionization, evaporation, and desorption
81.16.Be Chemical synthesis methods

Space-selective precipitation of metal nanoparticles inside glasses

Jianrong Qiu, Mitsuru Shirai, Takayuki Nakaya, Jinhai Si, Xiongwei Jiang, Congshan Zhu, and Kazuyuki Hirao

Appl. Phys. Lett. 81, 3040 (2002); http://dx.doi.org/10.1063/1.1509095 (3 pages) | Cited 53 times

Online Publication Date: 7 October 2002

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We report the precipitation and control of metal nanoparticles inside transparent glasses. An Ag+-doped silicate glass sample was first irradiated by using an 800 nm femtosecond laser at room temperature and then annealed at 550 °C. The area near the focal point of the laser beam became gray after laser irradiation and yellow after further annealing at 550 °C for 10 min. Absorption and electron spin resonance spectra of the glass sample showed that a portion of silver ions near the focused part of the laser beam inside the glass were reduced to silver atoms after the laser irradiation. These silver atoms aggregated to form nanoparticles after further annealing at temperatures above 500 °C. A mechanism is suggested that consists of multiphoton reduction, which is induced by the fundamental light of the laser beam and supercontinuum white light, and diffusion of silver atoms driven by heat energy to form nanoparticles. The observed phenomenon may have promising applications for the fabrication of three-dimensional multicolored images inside a transparent material and for integrative micro-optical switches. © 2002 American Institute of Physics.
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81.30.Mh Solid-phase precipitation
64.75.-g Phase equilibria
42.70.Ce Glasses, quartz
42.82.Cr Fabrication techniques; lithography, pattern transfer
61.46.-w Structure of nanoscale materials
66.30.Pa Diffusion in nanoscale solids
78.67.Bf Nanocrystals, nanoparticles, and nanoclusters
81.40.Gh Other heat and thermomechanical treatments
76.30.He Platinum and palladium group (4d and 5d) ions and impurities (Zr-Ag and Hf-Au)
81.07.Bc Nanocrystalline materials
78.30.-j Infrared and Raman spectra
78.40.-q Absorption and reflection spectra: visible and ultraviolet

Electrical measurements of a dithiolated electronic molecule via conducting atomic force microscopy

Adam M. Rawlett, Theresa J. Hopson, Larry A. Nagahara, Raymond K. Tsui, Ganesh K. Ramachandran, and Stuart M. Lindsay

Appl. Phys. Lett. 81, 3043 (2002); http://dx.doi.org/10.1063/1.1512815 (3 pages) | Cited 67 times

Online Publication Date: 7 October 2002

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We describe a method of measuring the electrical properties of a molecule via conducting atomic force microscopy (AFM). A dithiolated molecule is chemically inserted into defect sites in an insulating self-assembled monolayer formed on an epitaxial Au substrate and the top thiol terminus of the molecule is reacted with a Au nanoparticle. A Au-coated AFM probe is used to contact the molecule through the nanoparticle, thus electrical data can be obtained. We report preliminary transport measurements of two test molecules. Our data shows qualitative agreement with previously published results for similar molecules deposited in a nanopore containing approximately a thousand molecules. This work indicates that the measured negative differential resistance is not an intermolecular phenomenon. © 2002 American Institute of Physics.
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72.80.Le Polymers; organic compounds (including organic semiconductors)
68.37.Ps Atomic force microscopy (AFM)
81.07.Nb Molecular nanostructures
72.20.Ht High-field and nonlinear effects
73.40.Sx Metal-semiconductor-metal structures
81.16.Dn Self-assembly
73.50.Fq High-field and nonlinear effects
73.61.Ph Polymers; organic compounds

Site-specific growth of Zno nanorods using catalysis-driven molecular-beam epitaxy

Y. W. Heo, V. Varadarajan, M. Kaufman, K. Kim, D. P. Norton, F. Ren, and P. H. Fleming

Appl. Phys. Lett. 81, 3046 (2002); http://dx.doi.org/10.1063/1.1512829 (3 pages) | Cited 136 times

Online Publication Date: 7 October 2002

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We report on catalyst-driven molecular beam epitaxy of ZnO nanorods. The process is site specific, as single crystal ZnO nanorod growth is realized via nucleation on Ag films or islands that are deposited on a SiO2-terminated Si substrate surface. Growth occurs at substrate temperatures on the order of 300–500 °C. The nanorods are uniform cylinders, exhibiting diameters of 15–40 nm and lengths in excess of 1 μm. With this approach, nanorod placement can be predefined via location of metal catalyst islands or particles. This, coupled with the relatively low growth temperatures needed, suggests that ZnO nanorods could be integrated on device platforms for numerous applications, including chemical sensors and nanoelectronics. © 2002 American Institute of Physics.
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81.15.Hi Molecular, atomic, ion, and chemical beam epitaxy
81.05.Dz II-VI semiconductors
81.07.Bc Nanocrystalline materials
61.46.-w Structure of nanoscale materials
68.55.-a Thin film structure and morphology

Size and location control of Si nanocrystals at ion beam synthesis in thin SiO2 films

Torsten Müller, Karl-Heinz Heinig, and Wolfhard Möller

Appl. Phys. Lett. 81, 3049 (2002); http://dx.doi.org/10.1063/1.1512952 (3 pages) | Cited 33 times

Online Publication Date: 7 October 2002

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Binary collision simulations of high-fluence 1 keV Si+ ion implantation into 8-nm-thick SiO2 films on (001)Si were combined with kinetic Monte Carlo simulations of Si nanocrystal (NC) formation by phase separation during annealing. For nonvolatile memory applications, these simulations help to control the size and location of NCs. For low concentrations of implanted Si, NCs form via nucleation, growth, and Ostwald ripening, whereas for high concentrations Si separates by spinodal decomposition. In both regimes, NCs form above a thin NC free-oxide layer at the SiO2/Si interface. This, self-adjusted layer has just a thickness appropriate for NC charging by direct electron tunneling. Only in the nucleation regime the width of the tunneling oxide and the mean NC diameter remain constant during a long annealing period. This behavior originates from the competition of Ostwald ripening and Si loss to the Si/SiO2 interface. The process simulations predict that, for nonvolatile memories, the technological demands on NC synthesis are fulfilled best in the nucleation regime. © 2002 American Institute of Physics.
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61.46.-w Structure of nanoscale materials
81.07.Bc Nanocrystalline materials
61.72.up Other materials
68.55.Ln Defects and impurities: doping, implantation, distribution, concentration, etc.
73.40.Qv Metal-insulator-semiconductor structures (including semiconductor-to-insulator)
68.55.A- Nucleation and growth

Engineering the nanocrystalline structure of TiO2 films by aerodynamically filtered cluster deposition

E. Barborini, I. N. Kholmanov, P. Piseri, C. Ducati, C. E. Bottani, and P. Milani

Appl. Phys. Lett. 81, 3052 (2002); http://dx.doi.org/10.1063/1.1510579 (3 pages) | Cited 35 times

Online Publication Date: 7 October 2002

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We have produced nanocrystalline titanium dioxide films with different structures (anatase or rutile) by depositing mass selected clusters from the gas phase. Nanoparticles are produced by a pulsed microplasma cluster source and are selected by aerodynamic separation effects. We have characterized nanocrystalline films by Raman spectromicroscopy and transmission electron microscopy, showing that the films assembled with very small clusters have a predominant rutile phase, whereas larger clusters form films with anatase structure. Our observations suggest that phonon confinement effects are responsible for a significant shift and broadening observed for the Raman peaks. © 2002 American Institute of Physics.
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61.46.-w Structure of nanoscale materials
68.55.-a Thin film structure and morphology
78.67.Bf Nanocrystals, nanoparticles, and nanoclusters
78.30.Hv Other nonmetallic inorganics

Dynamic characteristics of high-speed In0.4Ga0.6As/GaAs self-organized quantum dot lasers at room temperature

S. Ghosh, S. Pradhan, and P. Bhattacharya

Appl. Phys. Lett. 81, 3055 (2002); http://dx.doi.org/10.1063/1.1514823 (3 pages) | Cited 45 times

Online Publication Date: 7 October 2002

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We have measured the room-temperature modulation characteristics of self-organized In0.4Ga0.6As/GaAs quantum dot lasers in which electrons are injected into the dot lasing states by tunneling. A small-signal modulation bandwidth of f−3 dB = 22 GHz is measured. Values of differential gain at 288 K of dg/dn ≅ 8.85×10−14 cm2 and gain compression factor ε = 7.2×10−16 cm3 are derived from the modulation data. Extremely low values of linewidth enhancement factor α ∼ 1 and chirp <0.6 Å were also measured in the devices. © 2002 American Institute of Physics.
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42.55.Px Semiconductor lasers; laser diodes
85.35.Be Quantum well devices (quantum dots, quantum wires, etc.)
42.60.Fc Modulation, tuning, and mode locking

Gradient nanostructures for interfacing microfluidics and nanofluidics

Han Cao, Jonas O. Tegenfeldt, Robert H. Austin, and Stephen Y. Chou

Appl. Phys. Lett. 81, 3058 (2002); http://dx.doi.org/10.1063/1.1515115 (3 pages) | Cited 65 times

Online Publication Date: 7 October 2002

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It is difficult to introduce long genomic DNA molecules into nanometer scale fluidic channels directly from the macroscale world because of the steep entropic barrier caused by necessary stretching of the polymer. We present a very simple technique using optical lithography to fabricate continuous spatial gradient structures which smoothly narrow the cross section of a volume from the micron to the nanometer length scale, greatly reducing the local entropic barrier to nanochannel entry. This technique, diffraction gradient lithography, can be very valuable for the fabrication of micro/nano total analysis systems. © 2002 American Institute of Physics.
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81.16.Nd Micro- and nanolithography
85.85.+j Micro- and nano-electromechanical systems (MEMS/NEMS) and devices
47.60.-i Flow phenomena in quasi-one-dimensional systems
47.85.Np Fluidics

Phase diagram for the interaction of oxygen with SiC

Y. Song and F. W. Smith

Appl. Phys. Lett. 81, 3061 (2002); http://dx.doi.org/10.1063/1.1514397 (3 pages) | Cited 12 times

Online Publication Date: 7 October 2002

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We report on experimental studies of the interactions of oxygen with the 4H– and 6H–SiC surfaces at high temperatures. It is observed that these interactions lead to the growth of passivating SiO2 layers at high O2 pressures, etching of the surfaces at lower pressures, and enhancements of the surface segregation of carbon at still lower pressures. A pressure–temperature phase diagram for the oxidation of SiC containing these three experimentally observed regions is presented. © 2002 American Institute of Physics.
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81.65.Mq Oxidation
81.65.Rv Passivation
81.65.Cf Surface cleaning, etching, patterning
68.35.Dv Composition, segregation; defects and impurities
64.70.-p Specific phase transitions

GaN islanding by spontaneous rearrangement of a strained two-dimensional layer on (0001) AlN

C. Adelmann, N. Gogneau, E. Sarigiannidou, J.-L. Rouvière, and B. Daudin

Appl. Phys. Lett. 81, 3064 (2002); http://dx.doi.org/10.1063/1.1515114 (3 pages) | Cited 37 times

Online Publication Date: 7 October 2002

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It is shown that a two-dimensional GaN layer grown on (0001) AlN under Ga-rich conditions remains two-dimensional while annealing under a Ga flux due to a surfactant effect of Ga. In contrast, further annealing under vacuum without the Ga flux leads to evaporation of excess Ga and to spontaneous transformation of the GaN layer into islands if the initial layer is thicker than about 2.5 monolayers. The resulting morphology is studied by atomic force microscopy and transmission electron microscopy. The latter reveals that these islands sit on top of a continuous 2.5 monolayer thick wetting layer, i.e., they represent a Stranski–Krastanow structure. © 2002 American Institute of Physics.
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68.55.-a Thin film structure and morphology
61.72.Cc Kinetics of defect formation and annealing
68.35.B- Structure of clean surfaces (and surface reconstruction)
81.15.Hi Molecular, atomic, ion, and chemical beam epitaxy
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