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17 Oct 2005

Volume 87, Issue 16, Articles (16xxxx)

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Appl. Phys. Lett. 87, 161907 (2005); http://dx.doi.org/10.1063/1.2105998 (3 pages)

J. Das, K. B. Kim, F. Baier, W. Löser, and J. Eckert
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Influence of Al/N flux ratio during nucleation layer growth on the structural properties of AlN grown on sapphire by molecular beam epitaxy

J. F. Fälth, S. K. Davidsson, X. Y. Liu, and T. G. Andersson

Appl. Phys. Lett. 87, 161901 (2005); http://dx.doi.org/10.1063/1.2093923 (3 pages) | Cited 7 times

Online Publication Date: 10 October 2005

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AlN nucleation and buffer layers have been grown with different Al/N flux ratio by molecular beam epitaxy on sapphire. Thick AlN on top of a nucleation layer grown with Al/N flux ratio = 1.0 exhibited deep hexagonal holes (diameter 100–200 nm, density ∼ 109 cm−2). Investigation of the nucleation layer surface revealed that the holes were formed already during the nucleation. The formation of holes in AlN buffer layers could be avoided by using a N-rich (Al/N = 0.5) nucleation layer. It is demonstrated that the holes in AlN buffer layers can be effectively avoided choosing a N-rich nucleation layer growth and then switch to Al/N flux ratio = 1.0 for buffer layer growth. The two-step AlN growth gave high quality AlN, with excellent crystalline quality and smooth surface.
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81.05.Ea III-V semiconductors
68.55.A- Nucleation and growth
68.55.-a Thin film structure and morphology
81.15.Hi Molecular, atomic, ion, and chemical beam epitaxy
68.47.Fg Semiconductor surfaces

Gallium nitride microcavities formed by photoenhanced wet oxidation

L.-H. Peng, C.-Y. Lu, W.-H. Wu, and S.-L. Wang

Appl. Phys. Lett. 87, 161902 (2005); http://dx.doi.org/10.1063/1.2103423 (3 pages) | Cited 5 times

Online Publication Date: 10 October 2005

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We report the formation of gallium nitride (GaN) microcavities by manipulating a photoenhanced oxidation rate difference between the polar and nonpolar crystallographic planes of GaN. When immersed in a buffered acetic (CH3COOH) electrolyte of pH ∼ 6.2 at room temperature, it is shown that the photo-oxidation can proceed at a rate that is one order of magnitude slower on the nonpolar plane of {1math00}GaN than on the polar plane of {000math}GaN due to the reduced surface field action. Gallium nitride microcavities bounded by optically smooth {1math00} and {1math03} facets can thus be preferentially formed on the c-plane sapphire substrate after dissolving the oxide layer. The optical properties of these GaN hexagonal cavities reveal characteristic peaks of whispering gallery modes in resonance with the GaN band edge emission spectrum. A typical cavity Q factor of 103 is observed in these GaN microcavities due to a reduced optical scattering loss in the wet chemical reaction process.
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81.65.Mq Oxidation
78.67.-n Optical properties of low-dimensional, mesoscopic, and nanoscale materials and structures
82.50.-m Photochemistry

Deducing nanopore structure and growth mechanisms in porogen-templated silsesquioxane thin films

Hua-Gen Peng, Richard S. Vallery, Ming Liu, William E. Frieze, David W. Gidley, Jin-Heong Yim, Hyun-Dam Jeong, and Jongmin Kim

Appl. Phys. Lett. 87, 161903 (2005); http://dx.doi.org/10.1063/1.2103402 (3 pages) | Cited 7 times

Online Publication Date: 10 October 2005

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Adjusting the functional group of a porogen is found to have a tremendous effect on the pore structre of porous low dielectric constant films with silsesquioxane as the matrix precursor. The pore size and interconnection length measured by positronium annihilation lifetime spectroscopy can be used to deduce the pore shape and its evolution with porosity from templates of isolated porogen molecules through film percolation. Inert, self-linkable, and amphiphilic porogens are demonstrated to randomly aggregate three-dimensionally, linearly polymerize, and form micelles, respectively.
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81.05.Rm Porous materials; granular materials
81.07.-b Nanoscale materials and structures: fabrication and characterization
81.05.Lg Polymers and plastics; rubber; synthetic and natural fibers; organometallic and organic materials
77.84.Jd Polymers; organic compounds
77.55.-g Dielectric thin films
61.43.Gt Powders, porous materials
61.46.-w Structure of nanoscale materials
61.41.+e Polymers, elastomers, and plastics
68.55.A- Nucleation and growth
68.55.-a Thin film structure and morphology
77.22.Ch Permittivity (dielectric function)
82.35.-x Polymers: properties; reactions; polymerization
82.70.Dd Colloids
82.70.Uv Surfactants, micellar solutions, vesicles, lamellae, amphiphilic systems, (hydrophilic and hydrophobic interactions)

X-ray diffraction imaging of Al2O3 nanoparticles embedded in an amorphous matrix

Ruben A. Dilanian and Andrei Y. Nikulin

Appl. Phys. Lett. 87, 161904 (2005); http://dx.doi.org/10.1063/1.2106024 (3 pages) | Cited 3 times

Online Publication Date: 11 October 2005

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An experimental “momentum transfer” x-ray diffraction imaging technique is suggested and tested for nondestructive determination of the shape and size of Al2O3 nanoscale particles embedded in an amorphous polymer matrix. An advantage of the proposed technique is that it does not require coherent x rays or a beamstop to block the direct beam. A two-dimensional image of an average Al2O3 nanoparticle has been reconstructed using the Gerchberg–Saxton method, with a spatial resolution of 2.5 nm. The shape of the resulting image yielded the average size of the nanoparticle to be approximately 50 nm, which is in excellent agreement with the expected value.
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61.46.-w Structure of nanoscale materials
81.70.Fy Nondestructive testing: optical methods

Effect of native defects on optical properties of InxGa1−xN alloys

S. X. Li, E. E. Haller, K. M. Yu, W. Walukiewicz, J. W. Ager, J. Wu, W. Shan, Hai Lu, and William J. Schaff

Appl. Phys. Lett. 87, 161905 (2005); http://dx.doi.org/10.1063/1.2108118 (3 pages) | Cited 11 times

Online Publication Date: 11 October 2005

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The energy position of the optical-absorption edge and the free-carrier populations in InxGa1−xN ternary alloys can be controlled using high-energy math irradiation. The blueshift of the absorption edge after irradiation in In-rich material (x>0.34) is attributed to the band-filling effect (Burstein-Moss shift) due to the native donors introduced by the irradiation. In Ga-rich material, optical-absorption measurements show that the irradiation-introduced native defects are inside the band gap, where they are incorporated as acceptors. The observed irradiation-produced changes in the optical-absorption edge and the carrier populations in InxGa1−xN are in excellent agreement with the predictions of the amphoteric defect model.
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78.66.Fd III-V semiconductors
78.40.Fy Semiconductors
68.55.Ln Defects and impurities: doping, implantation, distribution, concentration, etc.
61.80.Jh Ion radiation effects
61.82.Fk Semiconductors

Vacancy formation in GaAs under different equilibrium conditions

V. Bondarenko, J. Gebauer, F. Redmann, and R. Krause-Rehberg

Appl. Phys. Lett. 87, 161906 (2005); http://dx.doi.org/10.1063/1.2084330 (3 pages) | Cited 6 times

Online Publication Date: 11 October 2005

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Defect properties of undoped semiinsulating and silicon doped n-type GaAs annealed at different arsenic vapor pressures have been studied by means of positron annihilation and Hall effect measurements. In both types of samples, formation of monovacancylike defects during annealing was observed. The concentration of these defects increases in GaAs:Si and decreases in undoped GaAs when the arsenic pressure increases. In GaAs:Si, the defect was earlier identified as SiGa–VGa complex, however, in undoped GaAs arsenic vacancies are formed, which are part of defect complex.
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81.05.Ea III-V semiconductors
61.72.J- Point defects and defect clusters
61.72.Cc Kinetics of defect formation and annealing
78.70.Bj Positron annihilation
61.82.Fk Semiconductors
61.80.Fe Electron and positron radiation effects
72.20.My Galvanomagnetic and other magnetotransport effects

High-strength Ti-base ultrafine eutectic with enhanced ductility

J. Das, K. B. Kim, F. Baier, W. Löser, and J. Eckert

Appl. Phys. Lett. 87, 161907 (2005); http://dx.doi.org/10.1063/1.2105998 (3 pages) | Cited 66 times

Online Publication Date: 12 October 2005

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(Ti0.705Fe0.295)100−xSnx (x = 0 and 3.85) ultrafine eutectics were prepared by slow cooling from the melt through cold crucible casting. The addition of 3.85 at. % Sn to the binary Ti–Fe eutectic decreases the strength slightly but considerably improves the plastic deformability under uniaxial compressive loading from εf = 2.1% to 9.6% strain to failure. The change in the morphology of the eutectic and the distribution of the FeTi phase are suggested as origin of the improvement of the mechanical properties.
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81.05.Bx Metals, semimetals, and alloys
81.40.Lm Deformation, plasticity, and creep
62.20.F- Deformation and plasticity
81.30.Fb Solidification

Ohmic contacts to silicon carbide determined by changes in the surface

F. A. Mohammad, Y. Cao, and L. M. Porter

Appl. Phys. Lett. 87, 161908 (2005); http://dx.doi.org/10.1063/1.2106005 (3 pages) | Cited 5 times

Online Publication Date: 12 October 2005

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In this study, we investigated the effects of the contact phases and the underlying SiC surface on the ohmic behavior of annealed Pt and Pt/Si contacts to p-type 4H–SiC. The contacts were annealed at 1100 and 1000 °C, respectively, for 5 min in vacuum to form ohmic contacts. Our data show that although the formation of the PtSi phase was complete, and the only phase present in the Pt/Si/SiC samples annealed at 900 °C, the contacts did not become ohmic at this temperature. Removal of the annealed Pt and Pt/Si contacts and their replacement with unannealed Ni contacts resulted in ohmic behavior that was nearly identical to that of the previous annealed contacts. In contrast, as-deposited Ni contacts on as-received p-type SiC resulted in rectifying contacts. The results of this study show that changes in the SiC substrate underneath the contacts dominate the electrical behavior, and that effects of the particular phase(s) formed are small.
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73.40.Ns Metal-nonmetal contacts
73.40.Ei Rectification
61.72.Cc Kinetics of defect formation and annealing

Thermal conductivity and interfacial resistance in single-wall carbon nanotube epoxy composites

M. B. Bryning, D. E. Milkie, M. F. Islam, J. M. Kikkawa, and A. G. Yodh

Appl. Phys. Lett. 87, 161909 (2005); http://dx.doi.org/10.1063/1.2103398 (3 pages) | Cited 86 times

Online Publication Date: 12 October 2005

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We report thermal conductivity measurements of purified single-wall carbon nanotube (SWNT) epoxy composites prepared using suspensions of SWNTs in N-N-Dimethylformamide (DMF) and surfactant stabilized aqueous SWNT suspensions. Thermal conductivity enhancement is observed in both types of composites. DMF-processed composites show an advantage at SWNT volume fractions between ϕ ∼ 0.001 to 0.005. Surfactant processed samples, however, permit greater SWNT loading and exhibit larger overall enhancement (64±9)% at ϕ ∼ 0.1. The enhancement differences are attributed to a ten-fold larger SWNT/solid-composite interfacial thermal resistance in the surfactant-processed composites compared to DMF-processed composites. The interfacial resistance is extracted from the volume fraction dependence of the thermal conductivity data using effective medium theory. [ C. W. Nan, G. Liu, Y. Lin, and M. Li, Appl. Phys. Lett. 85, 3549 (2004) ].
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81.05.Qk Reinforced polymers and polymer-based composites
81.07.-b Nanoscale materials and structures: fabrication and characterization
66.70.-f Nonelectronic thermal conduction and heat-pulse propagation in solids; thermal waves
82.70.Kj Emulsions and suspensions
82.70.Uv Surfactants, micellar solutions, vesicles, lamellae, amphiphilic systems, (hydrophilic and hydrophobic interactions)

High ductility of a metal film adherent on a polymer substrate

Yong Xiang, Teng Li, Zhigang Suo, and Joost J. Vlassak

Appl. Phys. Lett. 87, 161910 (2005); http://dx.doi.org/10.1063/1.2108110 (3 pages) | Cited 48 times

Online Publication Date: 12 October 2005

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In recent development of deformable electronics, it has been noticed that thin metal films often rupture at small tensile strains. Here we report experiments with Cu films deposited on polymeric substrates and show that the rupture strains of the metal films are sensitive to their adhesion to the substrates. Well-bonded Cu films can sustain strains up to 10% without appreciable cracks and up to 30% with discontinuous microcracks. By contrast, poorly bonded Cu films form channel cracks at strains about 2%. The cracks form by a mixture of strain localization and intergranular fracture. The films rupture at large strains when the localization is retarded by the adherent substrates.
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68.60.Bs Mechanical and acoustical properties
81.05.Bx Metals, semimetals, and alloys
81.40.Lm Deformation, plasticity, and creep
81.40.Np Fatigue, corrosion fatigue, embrittlement, cracking, fracture, and failure
62.20.F- Deformation and plasticity
62.20.M- Structural failure of materials
68.35.Np Adhesion

High structural and optical quality 1.3 μm GaInNAs/GaAs quantum wells with higher indium content grown by molecular-beam expitaxy

Shiyong Zhang, Zhichuan Niu, Haiqiao Ni, Donghai Wu, Zhenhong He, Zheng Sun, Qin Han, and Ronghan Wu

Appl. Phys. Lett. 87, 161911 (2005); http://dx.doi.org/10.1063/1.2108117 (3 pages) | Cited 6 times

Online Publication Date: 12 October 2005

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High structural and optical quality 1.3 μm GaInNAs/GaAs quantum well (QW) samples with higher (42.5%) indium content were successfully grown by molecular-beam epitaxy. The cross-sectional transmission electron microscopy measurements reveal that there are no structural defects in such high indium content QWs. The room-temperature photoluminescence peak intensity of the GaIn0.425NAs/GaAs (6 nm/20 nm) 3QW is higher than, and the full width at half maximum is comparable to, that of In0.425GaAs/GaAs 3QW, indicating improved optical quality caused by strain compensation effect of introducing N to the high indium content InGaAs epilayer.
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78.66.Fd III-V semiconductors
78.55.Cr III-V semiconductors
68.55.-a Thin film structure and morphology
68.65.Fg Quantum wells
81.15.Hi Molecular, atomic, ion, and chemical beam epitaxy
68.37.Lp Transmission electron microscopy (TEM)
68.60.Bs Mechanical and acoustical properties

Photovoltaic characteristics of postdeposition iodine-doped amorphous carbon films by microwave surface wave plasma chemical vapor deposition

Ashraf M. M. Omer, Sudip Adhikari, Sunil Adhikary, Hideo Uchida, and Masayoshi Umeno

Appl. Phys. Lett. 87, 161912 (2005); http://dx.doi.org/10.1063/1.2089168 (3 pages) | Cited 14 times

Online Publication Date: 14 October 2005

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The amorphous carbon thin films have been deposited on silicon and quartz substrates by microwave surface wave plasma chemical vapor deposition at low temperature (<100°C) in Ar/CH4 phase gas. Doping of iodine has been done in the postdeposited films by exposing them in iodine vapor. The photovoltaic measurements of the films were carried out before and after iodine doping. The results show dramatic decrease of optical gap from 3.4 to 0.9 eV corresponding to nondoping to iodine doping conditions, respectively. The preliminary photovoltaic characteristics of the film deposited on n-type silicon substrate under light illumination (AM1.5, 100 mW/cm2) reveal a short-circuit current density of 1.15 μA/cm2, open-circuit voltage of 177 mV and fill factor of 21.7%.
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73.61.Cw Elemental semiconductors
73.61.Jc Amorphous semiconductors; glasses
61.72.up Other materials
52.77.Dq Plasma-based ion implantation and deposition
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
72.40.+w Photoconduction and photovoltaic effects
73.50.Pz Photoconduction and photovoltaic effects

Imaging hydrogenous materials with a neutron microscope

J. T. Cremer, M. A. Piestrup, H. Park, C. K. Gary, R. H. Pantell, C. J. Glinka, and J. G. Barker

Appl. Phys. Lett. 87, 161913 (2005); http://dx.doi.org/10.1063/1.2089172 (3 pages) | Cited 1 time

Online Publication Date: 14 October 2005

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Magnified images of materials containing hydrogen, for which the main contrast mechanism for neutrons is incoherent scattering, have been obtained using a microscope employing a neutron compound refractive lens (CRL). The CRL was composed of 100 MgF2 biconcave lenses that produced magnified (22.5×) images of polyethylene and polypropylene (hydrogen-rich) grids and biological specimens using 8.5 Å cold neutrons with a 10% bandwidth. For hydrogenous materials, 98%–99% of the attenuation is by incoherent scattering and 1%–2% from neutron absorption by the hydrogen nuclei. The small angle of acceptance of the CRL discriminates against scattered neutrons from the hydrogenous object, thereby producing the needed contrast for imaging.
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87.64.-t Spectroscopic and microscopic techniques in biophysics and medical physics
07.90.+c Other topics in instruments, apparatus, and components common to several branches of physics and astronomy (restricted to new topics in section 07)

Thermoelastic effect induced by ferroelastic domain switching

Yong Chan Cho, Sang Eon Park, Chae-Ryong Cho, and Se-Young Jeong

Appl. Phys. Lett. 87, 161914 (2005); http://dx.doi.org/10.1063/1.2106003 (3 pages) | Cited 2 times

Online Publication Date: 14 October 2005

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Temperature changes induced by externally applied mechanical stress were observed in the Pb3(PO4)2 ferroelastic crystal. These characteristic features can be explained by the thermoelastic effect as in other thermoelastic martensitic materials. In this study, the mechanical stress dependence of the thermoelastic effect was studied using a modified thermomechanical analysis system. The thermoelastic effect strongly depended on the mechanical stress and was enhanced by ferroelastic domain switching, responsible for the pseudoelastic behavior of ferroelastic materials. The ∂T/∂σsin, the change of sample temperature with varying stress enhanced by pseudoelasticity, was ∼ 9.5×10−2 °C/MPa at 159 °C, ferroelastic temperature region under 1.2 MPa sinusoidal stress. This value is eight to nine times higher than the values obtained under bias stress. This result can be explained by a thermoelastic equation which includes the effective thermal expansion coefficient in the ferroelastic phase.
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81.40.Jj Elasticity and anelasticity, stress-strain relations
62.20.F- Deformation and plasticity
62.20.D- Elasticity
64.70.K- Solid-solid transitions

Temperature dependence of the mechanical properties of tetrahedrally coordinated amorphous carbon thin films

David A. Czaplewski, J. P. Sullivan, T. A. Friedmann, and J. R. Wendt

Appl. Phys. Lett. 87, 161915 (2005); http://dx.doi.org/10.1063/1.2108132 (3 pages) | Cited 7 times

Online Publication Date: 14 October 2005

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The complete elastic properties of tetrahedrally coordinated amorphous carbon (ta-C) thin films have been measured in the temperature range of 300–873 K. Flexural and torsional mechanical oscillators were fabricated from ta-C, and using the resonant frequency of the oscillators as a function of temperature, we calculated the temperature-dependent Young’s and shear moduli (658±24 and 271±6.6 GPa, at 300 K, respectively). From these values, we calculated the bulk modulus, Poisson’s ratio, and the elastic stiffness and compliance constants as a function of temperature. In addition, the temperature dependence of the coefficient of thermal expansion of ta-C was determined using a wafer curvature technique.
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68.60.Bs Mechanical and acoustical properties
81.40.Jj Elasticity and anelasticity, stress-strain relations
62.20.D- Elasticity
65.60.+a Thermal properties of amorphous solids and glasses: heat capacity, thermal expansion, etc.

Barrierless self-assembly of Ge quantum dots on Si(001) substrates with high local vicinality

P. Sutter, E. Sutter, and L. Vescan

Appl. Phys. Lett. 87, 161916 (2005); http://dx.doi.org/10.1063/1.2108133 (3 pages) | Cited 4 times

Online Publication Date: 14 October 2005

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In Ge heteroepitaxy on vicinal Si(001), miscut by 4.8° toward [100], pyramid-shaped faceted quantum dot islands (“huts”) form continuously from individual (105) facets on a wetting layer of coexisting (105) and (001) segments. Via this barrierless kinetic route the first three-dimensional islands rapidly form wherever there are substantial local gradients along ⟨100⟩ in-plane directions.
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81.07.Ta Quantum dots
81.05.Cy Elemental semiconductors
81.16.Dn Self-assembly
81.15.Cd Deposition by sputtering
68.65.Hb Quantum dots (patterned in quantum wells)

Evolution of basal plane dislocations during 4H-silicon carbide homoepitaxy

Z. Zhang and T. S. Sudarshan

Appl. Phys. Lett. 87, 161917 (2005); http://dx.doi.org/10.1063/1.2108109 (3 pages) | Cited 6 times

Online Publication Date: 14 October 2005

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A method based on the combination of molten KOH etching and reactive ion etching was developed to track dislocations from 4H-silicon carbide homoepilayer to the substrate. The conversion of basal plane dislocations (BPDs) to threading edge dislocations (TEDs) was found to occur at the epilayer/substrate interface. The BPDs with dislocation lines parallel (or approximately parallel) to the off-cut direction may propagate as BPDs into the epilayer, while those with dislocation lines forming large angles (>10°) with the off-cut direction will get converted to TEDs. A model is proposed to explain the observations.
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61.72.Hh Indirect evidence of dislocations and other defects (resistivity, slip, creep, strains, internal friction, EPR, NMR, etc.)
68.55.Ln Defects and impurities: doping, implantation, distribution, concentration, etc.
81.65.Cf Surface cleaning, etching, patterning
66.30.Lw Diffusion of other defects
81.05.Hd Other semiconductors
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