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15 May 2000

Volume 76, Issue 20, pp. 2815-2963

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Ion beam synthesis of graphite and diamond in silicon carbide

V. Heera, W. Skorupa, B. Pécz, and L. Dobos

Appl. Phys. Lett. 76, 2847 (2000); http://dx.doi.org/10.1063/1.126493 (3 pages) | Cited 11 times

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A high dose of 1×1018 cm−2, 60 keV carbon ions was implanted into single crystalline 6H silicon carbide (SiC) at elevated temperatures. The formation of carbon phases in the crystalline SiC lattice was investigated by cross sectional transmission electron microscopy. An amorphous, carbon rich phase was produced at 300 °C. Precipitates of graphite were obtained at 600 °C, whereas at 900 °C small diamond grains were produced. These grains are in perfect epitaxial relation with the surrounding SiC lattice. © 2000 American Institute of Physics.
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81.05.Hd Other semiconductors
61.72.up Other materials
61.80.Jh Ion radiation effects
61.82.Fk Semiconductors
64.75.-g Phase equilibria
81.30.Mh Solid-phase precipitation

Auger recombination as a probe of the Mott transition in semiconductor nanocrystals

P. Němec, P. Malý, M. Nikl, and K. Nitsch

Appl. Phys. Lett. 76, 2850 (2000); http://dx.doi.org/10.1063/1.126494 (3 pages) | Cited 5 times

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We report on picosecond dynamics of photoexcited carriers in CsPbCl3 nanocrystals in a CsCl host. For low carrier densities, photoexcited carriers form excitons, which decay with a characteristic time constant of 170 ps at 300 K. Under strong photoexcitation, we have observed the Auger recombination with the Auger coefficient C ≈ 10−29 cm6 s−1. We have identified the onset of the Auger recombination at the carrier densities of ≈ 1020 cm−3 as the Mott-like transition from excitons to electron–hole plasma in nanocrystals. © 2000 American Institute of Physics.
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71.30.+h Metal-insulator transitions and other electronic transitions
72.20.Jv Charge carriers: generation, recombination, lifetime, and trapping
72.60.+g Mixed conductivity and conductivity transitions
71.35.-y Excitons and related phenomena
78.47.-p Spectroscopy of solid state dynamics

Si(011)16×2 gas-source molecular beam epitaxy: Growth kinetics

N. Taylor, H. Kim, P. Desjardins, Y. L. Foo, and J. E. Greene

Appl. Phys. Lett. 76, 2853 (2000); http://dx.doi.org/10.1063/1.126495 (3 pages) | Cited 3 times

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The growth rates RSi of Si layers deposited on Si(011)16×2 by gas-source molecular beam epitaxy from Si2H6 were determined as a function of temperature Ts (400–975 °C) and Si2H6 flux JSi2H6(5.0×1015–9.0×1016 cm−2 s−1). RSi ranges from 0.0015 μm h−1 at Ts = 400 °C to 0.415 μm  h−1 at Ts = 975 °C with JSi2H6 = 2.2×1016 cm−2 s−1. In the surface-reaction-limited regime at Ts<725 °C, RSi initially exhibits an exponential decrease with 1/Ts, then decreases at a slower rate at Ts ⩽ 550 °C as an additional deposition pathway becomes operative. In the impingement-flux-limited regime, 725 ⩽ Ts ⩽ 900 °C, RSi is independent of Ts but increases linearly with JSi2H6. At Ts>900 °C, RSi(Ts) increases with Ts due to surface roughening. Overall, RSi(JSi2H6,Ts) is well described at Ts ⩽ 900 °C by a kinetic model incorporating two competing film growth mechanisms: (1) dissociative chemisorption of Si2H6 onto dangling bonds followed by fast surface dissociation steps and second-order H2 desorption from the surface monohydride phase; and (2) Si2H6 insertion into Si–H surface bonds followed by second-order desorption of SiH4. © 2000 American Institute of Physics.
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81.05.Cy Elemental semiconductors
81.15.Hi Molecular, atomic, ion, and chemical beam epitaxy
71.55.Cn Elemental semiconductors
68.03.Fg Evaporation and condensation of liquids
68.43.Mn Adsorption kinetics
68.43.-h Chemisorption/physisorption: adsorbates on surfaces
82.30.Lp Decomposition reactions (pyrolysis, dissociation, and fragmentation)

Electrically induced changes in polymer dispersed liquid crystals

D. Cupelli, M. Macchione, F. P. Nicoletta, G. De Filpo, and G. Chidichimo

Appl. Phys. Lett. 76, 2856 (2000); http://dx.doi.org/10.1063/1.126496 (3 pages) | Cited 4 times

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This letter reports the changes induced in polymer dispersed liquid crystal films by applying high intensity electric fields (charge process). It has been observed that lower reorientation fields and sharper OFF–ON transitions characterize samples after the charge process. Scanning electron microscopy and dielectric investigation show that the charge process induces both a morphology and a dielectric variation in films. Nevertheless a reduction in the surface anchoring energy of the liquid crystal at droplet interface has been postulated as the changes in the dielectric properties and droplet size cannot entirely justify the enhancement of electro-optical properties. The role played by the temperature at which the charge process is performed is also discussed. © 2000 American Institute of Physics.
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61.30.Gd Orientational order of liquid crystals; electric and magnetic field effects on order
78.20.Jq Electro-optical effects

Near-band-gap optical functions spectra and band-gap energies of GaNAs/GaAs superlattice heterostructures measured by spectroscopic ellipsometry

J. Šik, M. Schubert, G. Leibiger, V. Gottschalch, G. Kirpal, and J. Humlíček

Appl. Phys. Lett. 76, 2859 (2000); http://dx.doi.org/10.1063/1.126497 (3 pages) | Cited 17 times

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The optical properties of GaAs/GaNxAs1−x superlattice structures grown by metal–organic vapor-phase epitaxy are studied by variable angle-of-incidence spectroscopic ellipsometry for photon energies from 0.75 to 1.55 eV. We employ Adachi’s critical-point composite model, and we report the direct-band-gap energy E0 and the complex index of refraction of the GaNxAs1−x sublayers for x ⩽ 3.3% from analysis of the ellipsometry data. We observe a strong redshift of E0 with increase in x, and a strong decrease of the E0 transition amplitude. The E0 values obtained for the superlattice structures are in good agreement with photoluminescence results, and also with previous reports from single epilayers. Structure, composition, layer thickness, and parallel and perpendicular lattice mismatch of the samples are studied by transmission electron microscopy and high-resolution x-ray diffraction investigations. © 2000 American Institute of Physics.
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78.66.Fd III-V semiconductors
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)
68.65.-k Low-dimensional, mesoscopic, nanoscale and other related systems: structure and nonelectronic properties
73.21.-b Electron states and collective excitations in multilayers, quantum wells, mesoscopic, and nanoscale systems

Interband luminescence and absorption of GaNAs/GaAs single-quantum-well structures

B. Q. Sun, D. S. Jiang, X. D. Luo, Z. Y. Xu, Z. Pan, L. H. Li, and R. H. Wu

Appl. Phys. Lett. 76, 2862 (2000); http://dx.doi.org/10.1063/1.126498 (3 pages) | Cited 31 times

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We have investigated the interband electron transitions in a GaNAs/GaAs single quantum well (QW) by photoluminescence and absorption spectra. The experimental results show that the dominant photoluminescence at low temperature and high excitation intensity originates from transitions within the GaNAs layer. The interband transition energy for QWs with different well widths can be well fitted if a type-II band line up of GaNAs/GaAs QWs is assumed. © 2000 American Institute of Physics.
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78.66.Fd III-V semiconductors
73.21.-b Electron states and collective excitations in multilayers, quantum wells, mesoscopic, and nanoscale systems
81.05.Ea III-V semiconductors
78.55.Cr III-V semiconductors

Local structure and bonding of Er in GaN: A contrast with Er in Si

P. H. Citrin, P. A. Northrup, R. Birkhahn, and A. J. Steckl

Appl. Phys. Lett. 76, 2865 (2000); http://dx.doi.org/10.1063/1.126499 (3 pages) | Cited 34 times

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X-ray absorption measurements from relatively high concentrations of Er (>0.1 at. %) doped in GaN films show that Er occupies the Ga site with an unprecedentedly short Er–N bond length. Electroluminescence intensities from these GaN:Er films correlate with the concentration of Er atoms that replace Ga, not with the abundantly present O impurities in the host. Simple chemical concepts are used to explain each of these results and their striking difference from those obtained for Er-doped Si. © 2000 American Institute of Physics.
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71.55.Eq III-V semiconductors
68.55.Ln Defects and impurities: doping, implantation, distribution, concentration, etc.
81.05.Ea III-V semiconductors
78.66.Fd III-V semiconductors
78.60.Fi Electroluminescence
78.70.Dm X-ray absorption spectra
61.50.Lt Crystal binding; cohesive energy

Load transfer and deformation mechanisms in carbon nanotube-polystyrene composites

D. Qian, E. C. Dickey, R. Andrews, and T. Rantell

Appl. Phys. Lett. 76, 2868 (2000); http://dx.doi.org/10.1063/1.126500 (3 pages) | Cited 744 times

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Multiwall carbon nanotubes have been dispersed homogeneously throughout polystyrene matrices by a simple solution-evaporation method without destroying the integrity of the nanotubes. Tensile tests on composite films show that 1 wt % nanotube additions result in 36%–42% and ∼25% increases in elastic modulus and break stress, respectively, indicating significant load transfer across the nanotube-matrix interface. In situ transmission electron microscopy studies provided information regarding composite deformation mechanisms and interfacial bonding between the multiwall nanotubes and polymer matrix. © 2000 American Institute of Physics.
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81.40.Lm Deformation, plasticity, and creep
62.20.F- Deformation and plasticity
81.05.Qk Reinforced polymers and polymer-based composites
81.05.ub Fullerenes and related materials
61.48.-c Structure of fullerenes and related hollow and planar molecular structures
81.70.Bt Mechanical testing, impact tests, static and dynamic loads
62.20.D- Elasticity
81.40.Jj Elasticity and anelasticity, stress-strain relations

Enhanced damage due to light in low-damage reactive-ion etching processes

L. G. Deng, M. Rahman, and C. D. W. Wilkinson

Appl. Phys. Lett. 76, 2871 (2000); http://dx.doi.org/10.1063/1.126501 (3 pages) | Cited 5 times

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Enhanced dry-etch damage in GaAs/GaAlAs and InGaAs/InAlAs quantum well structures due to additional illumination in low-power reactive-ion etching environments has been studied. We observed that damage in shallow quantum wells is increased significantly when above band-gap laser illumination is present during etching, although there is no measurable effect for illumination subsequent to etching. Contributing factors include the influence on ion channeling and also to a form of radiation-enhanced diffusion. The study shows that the effective defect diffusion coefficient is constrained to lie within the range 10−15 to 10−14 cm2/s. © 2000 American Institute of Physics.
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81.65.Cf Surface cleaning, etching, patterning
81.05.Ea III-V semiconductors
68.65.-k Low-dimensional, mesoscopic, nanoscale and other related systems: structure and nonelectronic properties
52.77.Bn Etching and cleaning
52.77.Dq Plasma-based ion implantation and deposition
61.80.Ba Ultraviolet, visible, and infrared radiation effects (including laser radiation)
61.80.Jh Ion radiation effects
61.85.+p Channeling phenomena (blocking, energy loss, etc.)
66.30.Lw Diffusion of other defects
61.82.Fk Semiconductors

Reversible phase transition between amorphous and crystalline in Zr41.2Ti13.8Cu12.5Ni10Be22.5 under high pressure at room temperature

Liling Sun, W. K. Wang, D. W. He, W. H. Wang, Q. Wu, X. Y. Zhang, Z. X. Bao, and Q. Zhao

Appl. Phys. Lett. 76, 2874 (2000); http://dx.doi.org/10.1063/1.126502 (3 pages) | Cited 11 times

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A reversible phase transition between amorphous and crystalline in bulk metallic glass (BMG) Zr41.2Ti13.8Cu12.5Ni10Be22.5 has been investigated under high pressure at room temperature. The BMG displayed a structure memory under high pressure as detected by in situ synchrotron radiation of x-ray diffraction and resistance measurement in a diamond anvil cell. Direct experimental observations found that the crystallization of the BMG occurred at 24 GPa on uploading and the crystalline phase reverted back to the amorphous state during downloading. This unusual phenomenon was discussed thermodynamically. © 2000 American Institute of Physics.
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64.70.K- Solid-solid transitions
62.50.-p High-pressure effects in solids and liquids
81.05.Bx Metals, semimetals, and alloys
61.43.Dq Amorphous semiconductors, metals, and alloys

Hydrogen storage in single-walled carbon nanotubes

Seung Mi Lee and Young Hee Lee

Appl. Phys. Lett. 76, 2877 (2000); http://dx.doi.org/10.1063/1.126503 (3 pages) | Cited 127 times

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We perform density-functional calculations to search for hydrogen adsorption sites and predict maximum storage capacity in single-walled carbon nanotubes. We find two chemisorption sites at top sites of the exterior and the interior of the tube wall. We further find that a form of H2 molecule can exist in an empty space inside nanotubes. The storage capacity of hydrogen in an empty space increases linearly with tube diameter. The maximum storage capacity is limited by the repulsive energies between H2 molecules inside nanotubes and those between H2 molecules and the tube wall. We predict that hydrogen storage capacity in (10,10) nanotube can exceed 14 wt % (160 kg H2/m3). © 2000 American Institute of Physics.
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84.60.-h Direct energy conversion and storage
68.03.Fg Evaporation and condensation of liquids
68.43.Mn Adsorption kinetics
61.48.-c Structure of fullerenes and related hollow and planar molecular structures

X-ray photoelectron spectroscopy characterization of AlGaN surfaces exposed to air and treated in NH4OH solution

Tamotsu Hashizume, Shin-ya Ootomo, Ryuusuke Nakasaki, Susumu Oyama, and Michio Kihara

Appl. Phys. Lett. 76, 2880 (2000); http://dx.doi.org/10.1063/1.126504 (3 pages) | Cited 20 times

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Chemical properties of Al0.17Ga0.83N surfaces exposed to air and treated in an NH4OH solution were systematically investigated by x-ray photoelectron spectroscopy (XPS). An air-exposed sample showed highly nonstoichiometric surface which included a large amount of Ga and Al oxides. The angle-resolved XPS analysis revealed that the natural oxide layer possessed a complicated composition distribution in depth and that the Al-oxide component was dominant on the topmost layer. A drastic reduction of such Al-oxide component as well as Ga-related oxide was achieved after the treatment in an NH4OH solution at 50 °C for 10 min, resulting in a constant in-depth composition distribution. The NH4OH-based treatment was found to enhance the intensity of the E2 (high) Raman peak and to reduce the root-mean-square value of surface roughness. © 2000 American Institute of Physics.
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81.05.Ea III-V semiconductors
79.60.Bm Clean metal, semiconductor, and insulator surfaces
68.35.B- Structure of clean surfaces (and surface reconstruction)
78.30.Fs III-V and II-VI semiconductors

Single-wall carbon nanotubes as molecular pressure sensors

Jonathan R. Wood and H. Daniel Wagner

Appl. Phys. Lett. 76, 2883 (2000); http://dx.doi.org/10.1063/1.126505 (3 pages) | Cited 64 times

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Specific peaks of the Raman spectrum of single-wall nanotubes shift significantly upon immersion of the tubes in a liquid, relative to the corresponding peaks in air. This observation means that nanotubes are sensitive to molecular forces, and is interpreted by relating the corresponding molecular strain to a thermodynamic parameter, the cohesive energy density (or more loosely, the surface tension) for a range of liquids. We find that nanotubes deform by a different amount for each liquid. Calibration of this phenomenon enables the construction of a compressive stress–strain curve for carbon nanotubes. © 2000 American Institute of Physics.
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07.07.Df Sensors (chemical, optical, electrical, movement, gas, etc.); remote sensing
62.20.F- Deformation and plasticity
81.40.Jj Elasticity and anelasticity, stress-strain relations
65.20.-w Thermal properties of liquids
65.40.gd Entropy
68.03.Cd Surface tension and related phenomena
78.66.Tr Fullerenes and related materials

Local transport and trapping issues in Al2O3 gate oxide structures

R. Ludeke, M. T. Cuberes, and E. Cartier

Appl. Phys. Lett. 76, 2886 (2000); http://dx.doi.org/10.1063/1.126506 (3 pages) | Cited 47 times

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The bias dependence of interfacial barriers in Al2O3-based metal–oxide–semiconductor structures was studied by ballistic electron emission spectroscopy. Strong image force reductions of the barriers were observed. A conduction band offset between Al2O3 and Si of 2.78 eV was obtained. Electron trapping into levels that overlap the Si band gap and are located near the Si–Al2O3 interface led to charge densities of ∼ 2.5×1012 cm−2. © 2000 American Institute of Physics.
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73.40.Qv Metal-insulator-semiconductor structures (including semiconductor-to-insulator)
73.50.Gr Charge carriers: generation, recombination, lifetime, trapping, mean free paths
73.20.At Surface states, band structure, electron density of states
68.37.Vj Field emission and field-ion microscopy
73.61.Ng Insulators

Observation of electronic Raman scattering from Mg-doped wurtzite GaN

K. T. Tsen, C. Koch, Y. Chen, H. Morkoc, J. Li, J. Y. Lin, and H. X. Jiang

Appl. Phys. Lett. 76, 2889 (2000); http://dx.doi.org/10.1063/1.126507 (3 pages) | Cited 6 times

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Electronic Raman scattering experiments have been carried out on both molecular beam epitaxy and metal-organic chemical vapor deposition-grown Mg-doped wurtzite GaN samples. Aside from the expected Raman lines, a broad structure (full width at half maximum≅15 cm−1) observed at around 841 cm−1 is attributed to the electronic Raman scattering from neutral Mg impurities in Mg-doped GaN. Our experimental results demonstrate that the energy between the ground and first excited states of Mg impurities in wurtzite GaN is about one-half of its binding energy. © 2000 American Institute of Physics.
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78.30.Fs III-V and II-VI semiconductors
78.66.Fd III-V semiconductors
71.55.Eq III-V semiconductors

Evidence for a self-organized growth in granular Co/Al2O3 multilayers

D. Babonneau, F. Petroff, J.-L. Maurice, F. Fettar, A. Vaurès, and A. Naudon

Appl. Phys. Lett. 76, 2892 (2000); http://dx.doi.org/10.1063/1.126508 (3 pages) | Cited 31 times

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By sequential deposition of thin layers of cobalt and alumina by sputtering, we have fabricated granular multilayers consisting of successive planes of nanosized cobalt clusters separated by alumina along the growth direction. Combining grazing-incidence small-angle x-ray scattering and transmission electron microscopy experiments, we show that, in a given range of thickness, the vertical arrangement of clusters from plane to plane is not random but shows a topology-induced self-organization. © 2000 American Institute of Physics.
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68.65.-k Low-dimensional, mesoscopic, nanoscale and other related systems: structure and nonelectronic properties
78.70.Ck X-ray scattering
81.15.Cd Deposition by sputtering
68.55.-a Thin film structure and morphology
61.46.-w Structure of nanoscale materials

An optimal quasisuperlattice design to further improve thermal stability of tantalum nitride diffusion barriers

G. S. Chen, S. C. Huang, S. T. Chen, T. J. Yang, P. Y. Lee, J. H. Jou, and T. C. Lin

Appl. Phys. Lett. 76, 2895 (2000); http://dx.doi.org/10.1063/1.126509 (3 pages) | Cited 13 times

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X-ray diffraction and transmission electron microscopy, along with electrical and film stress measurements, were used to evaluate the effectiveness of 40-nm-thick amorphous Ta2N and microcrystalline TaN diffusion barriers, both single and multilayered, against Cu penetration. Failure of the single-layered Ta2N diffusion barrier upon annealing is initialized by crystallization/grain growth, mainly helped by frozen-in compressive stress (3–4 GPa) to transform itself into a columnar structure with a comparable grain size to the thickness of the barrier. However, when subjected to annealing, the Ta2N/TaN alternately layered diffusion barrier with an optimum bilayer thickness (10 nm) remains almost stress-free (0–0.7 GPa) and transforms itself into an equiaxed structure with grain sizes of only ⩽ 3 nm. Such quasisuperlattice films can present lengthening and complex grained structures to effectively block Cu diffusion, thus acting as much more effective barriers than Ta2N (and TaN) single-layered films. © 2000 American Institute of Physics.
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68.35.Fx Diffusion; interface formation
85.40.Ls Metallization, contacts, interconnects; device isolation
68.60.Dv Thermal stability; thermal effects
68.65.-k Low-dimensional, mesoscopic, nanoscale and other related systems: structure and nonelectronic properties
66.30.Ny Chemical interdiffusion; diffusion barriers
68.55.-a Thin film structure and morphology
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