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28 May 2007

Volume 90, Issue 22, Articles (22xxxx)

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Appl. Phys. Lett. 90, 221101 (2007); http://dx.doi.org/10.1063/1.2743884 (3 pages)

Siyka I. Shopova, Hongying Zhou, Xudong Fan, and Po Zhang
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Segregation and precipitation of Er in Ge

S. O. Kucheyev, J. E. Bradby, S. Ruffell, C. P. Li, T. E. Felter, and A. V. Hamza

Appl. Phys. Lett. 90, 221901 (2007); http://dx.doi.org/10.1063/1.2743881 (3 pages) | Cited 2 times

Online Publication Date: 29 May 2007

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Although Er-doped Ge nanomaterials are attractive for photonic applications, very little is known about the basic properties of Er in Ge. Here, the authors study the annealing behavior of Ge implanted with keV Er ions to doses resulting in ≲1 at. % of Er. Large redistribution of Er, with segregation at the amorphous/crystalline interface, starts at ≳500 °C, while lower temperatures are required for material recrystallization. However, even at 400 °C, Er forms precipitates. The concentration of Er trapped in the bulk after recrystallization decreases with increasing temperature but is independent of the initial bulk Er concentration for the range of ion doses studied here.
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64.75.-g Phase equilibria
81.30.Mh Solid-phase precipitation
61.72.S- Impurities in crystals
61.72.Cc Kinetics of defect formation and annealing
61.72.uf Ge and Si

Structural studies of single crystalline In2O3 films epitaxially grown on InN(0001)

Ch. Y. Wang, V. Lebedev, V. Cimalla, Th. Kups, K. Tonisch, and O. Ambacher

Appl. Phys. Lett. 90, 221902 (2007); http://dx.doi.org/10.1063/1.2743907 (3 pages) | Cited 12 times

Online Publication Date: 29 May 2007

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Single crystalline In2O3 is a prospective material to be used as a gate dielectric in InN based field effect transistors (FETs). This work addresses structural investigations of In2O3/InN heterostuctures for metal-oxide-semiconductor FET devices. Single crystalline cubic In2O3 (111) films were epitaxially grown on hexagonal InN (0001) epilayers. The epitaxial relationship between the film and the template was determined as In2O3[111]‖InN[0001] and In2O3[10math]‖InN〈11math0〉 with an effective lattice mismatch of 2.14%. On the basis of the structural investigations, a phenomenological model for the growth of In2O3 on InN (0001) is proposed.
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68.55.-a Thin film structure and morphology

Quantitative Landau potentials for the martensitic transformation in Ni–Al

E. K. H. Salje, H. Zhang, D. Schryvers, and B. Bartova

Appl. Phys. Lett. 90, 221903 (2007); http://dx.doi.org/10.1063/1.2743927 (3 pages) | Cited 5 times

Online Publication Date: 29 May 2007

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The martensitic phase transformation in Ni–Al (63.5 at. % Ni) is described by a Landau-type Gibbs free energy G(Q) = 1/2AΘs(coth(Θs/T)−coth(Θs/TC))Q2+1/4BQ4+1/6CQ6, with A = 5.6 J K−1 mol−1, B = −3493 J mol−1, C = 4901 J mol−1, TC = 86 K, and Θs = 257 K. The dispersion (Ginzburg) energy was estimated to be g = 60 J mol−1 nm2. This potential was determined from high resolution diffraction data to determine the spontaneous strain as proxy for the order parameter Q and differential scanning calorimetry measurements for the calibration of the excess entropy of the martensitic phase transformation. The Gibbs free energy is compared with equivalent functions of Ni–Ti, quartz, KMnF3, BaTiO3, and SrTiO3.
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64.70.K- Solid-solid transitions
65.40.G- Other thermodynamical quantities

Dynamics of bound excitons versus thickness in freestanding GaN wafers grown by halide vapor phase epitaxy

G. Pozina, C. Hemmingsson, J. P. Bergman, D. Trinh, L. Hultman, and B. Monemar

Appl. Phys. Lett. 90, 221904 (2007); http://dx.doi.org/10.1063/1.2743950 (3 pages) | Cited 4 times

Online Publication Date: 29 May 2007

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The bound exciton recombination properties in freestanding GaN layers of various thicknesses grown by halide vapor phase epitaxy have been characterized by time-resolved spectroscopy. Improvement of the donor bound exciton (D0X) lifetime was observed with increasing GaN layer thickness up to ∼ 400 μm, while for thicker layers the recombination time of D0X shows a tendency to saturate. The thickness-dependent behavior of the D0X decay can be understood in terms of competition between two nonradiative mechanisms: one of which is connected to structural defects, and consequently more important for thinner layers, while for layers with thickness above 400 μm with low structural defect density the recombination time is limited by point defects such as impurities and vacancies.
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78.66.Fd III-V semiconductors
73.20.Mf Collective excitations (including excitons, polarons, plasmons and other charge-density excitations)
71.35.-y Excitons and related phenomena
68.55.Ln Defects and impurities: doping, implantation, distribution, concentration, etc.
78.47.-p Spectroscopy of solid state dynamics

Cellular solids with tunable positive or negative thermal expansion of unbounded magnitude

Roderic Lakes

Appl. Phys. Lett. 90, 221905 (2007); http://dx.doi.org/10.1063/1.2743951 (3 pages) | Cited 6 times

Online Publication Date: 29 May 2007

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Material microstructures are presented with a coefficient of thermal expansion larger in magnitude than that of either constituent. Thermal expansion can be large positive, zero, or large negative. Three-dimensional lattices with void space exceed two-phase bounds but obey three-phase bounds; lattices and normal materials have a trend of expansion decreasing with modulus. Two-phase composites with a negative stiffness phase exceed bounds that assume positive strain energy density. The author determined Young’s modulus and its relation to thermal expansion. Behavior of these composites is compared with that of homogeneous solids in expansion-modulus maps.
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65.40.De Thermal expansion; thermomechanical effects
81.40.Jj Elasticity and anelasticity, stress-strain relations
62.20.D- Elasticity

Thermal stability of metal organic vapor phase epitaxy grown AlInN

A. Gadanecz, J. Bläsing, A. Dadgar, C. Hums, and A. Krost

Appl. Phys. Lett. 90, 221906 (2007); http://dx.doi.org/10.1063/1.2743744 (3 pages) | Cited 24 times

Online Publication Date: 29 May 2007

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AlInN layers with a thickness of 100 nm were grown by metal organic vapor phase epitaxy on GaN buffer layers on Si(111) substrates. By varying the growth temperature, In and NH3 flows, and reactor pressure, three series with different In contents were produced and thermally treated in the temperature range from 30 to 960 °C. The as grown and annealed layers were investigated by x-ray diffraction in standard and grazing incidence geometry. Nearly lattice matched samples with an indium concentration of 17%–18% show long time stability at annealing temperatures as high as 960 °C. At higher temperatures, the onset of severe Ga–Si meltback etching prevents further measurements. Nonlattice matched samples consist of pseudomorphic and relaxed parts. In the latter, a redistribution and loss of indium is observed upon annealing.
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81.05.Ea III-V semiconductors
68.55.A- Nucleation and growth
61.72.Cc Kinetics of defect formation and annealing
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)

Void nucleation in biaxially strained ultrathin films of face-centered cubic metals

Kedarnath Kolluri, M. Rauf Gungor, and Dimitrios Maroudas

Appl. Phys. Lett. 90, 221907 (2007); http://dx.doi.org/10.1063/1.2744477 (3 pages) | Cited 6 times

Online Publication Date: 30 May 2007

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We report an analysis of void nucleation as a relaxation mechanism in freestanding biaxially strained ultrathin films of face-centered cubic metals based on large-scale molecular-dynamics simulations. Above a critical strain level, multiple threading dislocations are emitted from the film surface. The surface step traces formed by gliding dislocations on intersecting and on adjacent parallel glide planes lead to formation and growth of surface pits and grooves, while vacancies form due to gliding of jogged dislocations and dislocation intersections. Coalescence of the surface pits with vacancy clusters is the precursor to the formation of a larger void extending across the film.
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68.55.Ln Defects and impurities: doping, implantation, distribution, concentration, etc.
68.55.-a Thin film structure and morphology

Domain ordering of strained 5 ML SrTiO3 films on Si(001)

P. Ryan, D. Wermeille, J. W. Kim, J. C. Woicik, C. S. Hellberg, and H. Li

Appl. Phys. Lett. 90, 221908 (2007); http://dx.doi.org/10.1063/1.2744478 (3 pages) | Cited 2 times

Online Publication Date: 30 May 2007

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High resolution x-ray diffraction data indicate ordered square shaped coherent domains, ∼ 1200 Å in length, coexisting with longer, ∼ 9500 Å correlated regions in highly strained 5 ML SrTiO3 films grown on Si(001). These long range film structures are due to the Si substrate terraces defined by the surface step morphology. The silicon surface “step pattern” is comprised of an “intrinsic” terrace length from strain relaxation and a longer “extrinsic” interstep distance due to the surface miscut.
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68.55.-a Thin film structure and morphology
68.47.Gh Oxide surfaces
81.40.Jj Elasticity and anelasticity, stress-strain relations
62.40.+i Anelasticity, internal friction, stress relaxation, and mechanical resonances

Dislocation generation at the coalescence of aluminum nitride lateral epitaxy on shallow-grooved sapphire substrates

J. Mei, F. A. Ponce, R. S. Qhalid Fareed, J. W. Yang, and M. Asif Khan

Appl. Phys. Lett. 90, 221909 (2007); http://dx.doi.org/10.1063/1.2745207 (3 pages) | Cited 6 times

Online Publication Date: 31 May 2007

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The joining of defect-free AlN stripes is observed to trigger the generation of a large density of threading dislocations in the vicinity of the coalescence point. The AlN structure was grown by pulsed lateral epitaxy on shallow-grooved sapphire substrates. In the precoalescence stage, the dislocation density in the lateral epitaxial region (<108 cm−2) is over two orders of magnitude less than in standard c-plane epitaxy. Basal-plane dislocations (b = a = ⅓<11math0>) are generated at the coalescence point as a result of relaxation of compressive stress that develops due to temperature gradients during growth. They bend toward the surface during the postcoalescence growth stage, leading to a high density of pure-edge threading dislocations in the lateral growth regions. Some threading dislocations form loops on prismatic planes in the crystal and the basal segments are observed to glide in the [0001] direction under the electron beam in the microscope.
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68.55.Ln Defects and impurities: doping, implantation, distribution, concentration, etc.
61.72.Ff Direct observation of dislocations and other defects (etch pits, decoration, electron microscopy, x-ray topography, etc.)
61.72.Hh Indirect evidence of dislocations and other defects (resistivity, slip, creep, strains, internal friction, EPR, NMR, etc.)
68.37.Lp Transmission electron microscopy (TEM)
68.60.Bs Mechanical and acoustical properties
81.40.Jj Elasticity and anelasticity, stress-strain relations

Impact-induced glass transition in elastomeric coatings

R. B. Bogoslovov, C. M. Roland, and R. M. Gamache

Appl. Phys. Lett. 90, 221910 (2007); http://dx.doi.org/10.1063/1.2745212 (3 pages) | Cited 20 times

Online Publication Date: 31 May 2007

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Polybutadiene (PB) has a low glass temperature Tg and exhibits rubbery behavior during mechanical perturbation. The corresponding PB-based polyurea (PU) has a higher Tg and fails in a brittle mode for high strain rates. However, unlike in glasses, this brittle failure is accompanied by large energy dissipation. Dielectric relaxation measurements demonstrate that whereas the PB segmental dynamics are faster than the strain rate during impact loading, for PU these motions are on the order of the strain rate, ∼ 105 s−1. Consequently, impact induces a transition to the glassy state, with the accompanying response markedly different from that of a rubber.
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64.70.P- Glass transitions of specific systems
64.70.Q- Theory and modeling of the glass transition
68.60.Bs Mechanical and acoustical properties
81.40.Np Fatigue, corrosion fatigue, embrittlement, cracking, fracture, and failure
62.20.M- Structural failure of materials
77.22.Gm Dielectric loss and relaxation

Partial-mediated slips in nanocrystalline Ni at high strain rate

X. L. Wu, Y. Qi, and Y. T. Zhu

Appl. Phys. Lett. 90, 221911 (2007); http://dx.doi.org/10.1063/1.2745250 (3 pages) | Cited 14 times

Online Publication Date: 31 May 2007

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Previous experiments on nanocrystalline Ni were conducted under quasistatic strain rates ( ∼ 3×10−3/s), which are much lower than that used in typical molecular dynamics simulations (>3×107/s), thus making direct comparison of modeling and experiments very difficult. In this study, the split Hopkinson bar tests revealed that nanocrystalline Ni prefers twinning to extended partials, especially under higher strain rates (103/s). These observations contradict some reported molecular dynamics simulation results, where only extended partials, but no twins, were observed. The accuracy of the generalized planar fault energies is only partially responsible, but cannot fully account for such a difference.
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61.72.Hh Indirect evidence of dislocations and other defects (resistivity, slip, creep, strains, internal friction, EPR, NMR, etc.)
61.72.Mm Grain and twin boundaries
61.46.Hk Nanocrystals

Correction for longitudinal mode vibration in thin slender beams

Cai Liang, Shakib Morshed, and Barton C. Prorok

Appl. Phys. Lett. 90, 221912 (2007); http://dx.doi.org/10.1063/1.2745262 (3 pages) | Cited 18 times

Online Publication Date: 1 June 2007

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This letter reports on a correction to the theoretical prediction of longitudinal mode vibration in thin, slender beams. Thin magnetostrictive strips were fashioned from Metglas™ and subjected to a modulated magnetic field to determine resonant frequency and acoustic wave propagation speed. The results indicated that current analytical solutions were not adequate to predict behavior. Numerical simulations were performed that adjusted Poisson’s ratio until the acoustic wave speed matched that measured in the experiments. The results indicated that the current equations, formulated using the plane-strain modulus, should be modified by using the plane-stress or biaxial modulus.
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46.40.Cd Mechanical wave propagation (including diffraction, scattering, and dispersion)
75.80.+q Magnetomechanical effects, magnetostriction
46.25.Cc Theoretical studies
62.65.+k Acoustical properties of solids

Nanostructure transition: From solid solution Ti(N,C) to nanocomposite nc-Ti(N,C)/a-(C,CNx)

Y. H. Lu and Y. G. Shen

Appl. Phys. Lett. 90, 221913 (2007); http://dx.doi.org/10.1063/1.2745261 (3 pages) | Cited 6 times

Online Publication Date: 1 June 2007

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A nanostructure transition from solid solution (SS) Ti(N,C) to two-phase nanocrystalline (nc)-Ti(N,C)/amorphous (a)-(C,CNx) thin films was investigated using a combination of high-resolution transmission electron microscopy, x-ray diffraction, and x-ray photoelectron spectroscopy. The finding of the authors is that such a nanostructure transition was strongly controlled by the relative atomic ratio x[x ≡ (C+N)/Ti]. The results indicated that SS Ti(N,C) and uncompleted and completed segregated two-phase nanocomposite nc-Ti(N,C)/a-(C,CNx) were successively formed at x ⩽ 1.0, 1.0<x<1.2, and x ≥ 1.2, respectively. Increase of the x values not only decreased the grain size and promoted the formation of more [200]-oriented nanocrystallites but also produced more disorders and defects in thin films. A maximum hardness was achieved for a SS Ti(N,C) structure at x = 1.0. The corresponding nanostructure transition mechanism is also discussed.
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64.70.K- Solid-solid transitions
68.37.Lp Transmission electron microscopy (TEM)
82.80.Pv Electron spectroscopy (X-ray photoelectron (XPS), Auger electron spectroscopy (AES), etc.)
64.75.-g Phase equilibria
68.55.-a Thin film structure and morphology
81.40.Np Fatigue, corrosion fatigue, embrittlement, cracking, fracture, and failure

Temperature dependent photoluminescence in oxygen ion implanted and rapid thermally annealed ZnO/ZnMgO multiple quantum wells

Xiaoming Wen, Jeffrey A. Davis, Lap Van Dao, Peter Hannaford, V. A. Coleman, H. H. Tan, C. Jagadish, K. Koike, S. Sasa, M. Inoue, and M. Yano

Appl. Phys. Lett. 90, 221914 (2007); http://dx.doi.org/10.1063/1.2745264 (3 pages) | Cited 10 times

Online Publication Date: 1 June 2007

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The authors investigate the effect of oxygen implantation and rapid thermal annealing in ZnO/ZnMgO multiple quantum wells using photoluminescence. A blueshift in the photoluminescence is observed in the implanted samples. For a low implantation dose, a significant increase of activation energy and a slight increase of the photoluminescence efficiency are observed. This is attributed to the suppression of the point defect complexes and transformation between defect structures by implantation and subsequent rapid thermal annealing. A high dose of implantation leads to lattice damage and agglomeration of defects leading to large defect clusters, which result to an increase in nonradiative recombination.
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78.67.De Quantum wells
78.55.Et II-VI semiconductors
61.72.uj III-V and II-VI semiconductors
61.72.Cc Kinetics of defect formation and annealing

Ab Initio atomic simulations of antisite pair recovery in cubic silicon carbide

F. Gao, J. Du, E. J. Bylaska, M. Posselt, and W. J. Weber

Appl. Phys. Lett. 90, 221915 (2007); http://dx.doi.org/10.1063/1.2743751 (3 pages) | Cited 6 times

Online Publication Date: 1 June 2007

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The thermal stability of an antisite pair in cubic silicon carbide (3C-SiC) is studied using ab initio molecular dynamics within the framework of density functional theory. The lifetime of the antisite pair configuration is calculated for temperatures between 1800 and 2250 K, and the effective activation energy for antisite pair recombination is determined to be 2.52 eV. The recombination energy path and static energy barrier are also calculated using the nudged elastic band method along with the dimer method to accurately locate the transition states. The consistency of the results suggests that the antisite pair cannot be correlated with the DI photoluminescence center, as proposed previously by theoretical interpretations. An extended exchange mechanism is found for the antisite pair recombination.
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61.72.J- Point defects and defect clusters
72.20.Jv Charge carriers: generation, recombination, lifetime, and trapping
78.55.Hx Other solid inorganic materials
66.30.J- Diffusion of impurities
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