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15 Jun 2007

Volume 101, Issue 12, Articles (12xxxx)

Issue Cover Spotlight Figure

J. Appl. Phys. 101, 121301 (2007); http://dx.doi.org/10.1063/1.2749198 (46 pages)

Wai Lun Chan and Eric Chason
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On the induced microstructure changes of the amorphous carbon nitride films during annealing

M. Lejeune, O. Durand-Drouhin, S. Charvet, A. Zeinert, and M. Benlahsen

J. Appl. Phys. 101, 123501 (2007); http://dx.doi.org/10.1063/1.2747218 (8 pages) | Cited 6 times

Online Publication Date: 18 June 2007

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The analysis of the stress release and structural changes caused by postdeposition thermal annealing of amorphous carbon nitride thin films (a‐CNx) has been carried out. The a‐CNx films were deposited on Si (100) using reactive radio frequency (rf) magnetron sputtering of a high-purity graphite target in a pure nitrogen plasma under various different rf powers. Combined Fourier transform infrared (FTIR), Raman spectroscopy, transmission spectroscopy, photothermal deflexion spectroscopy, and residual stress measurements were used to fully characterize the films. Annealing of the samples in vacuum at temperature up to 600 °C produces changes in their structural properties and the intrinsic stress. These changes are found to be strongly dependent on both the deposition conditions and microstructure changes occurring within the films during heating. FTIR spectra showed the existence of N–C sp3, N=C sp2, and C≣N triple bonds in the deposited films. The analysis of the spectra versus annealing temperature (TA) reveals that the major microstructure changes are the reorganization of the C–N and C–C configurations, without significant densification of the deposited films. The Raman features combined to the optical measurements reveal that this progressive graphitization of the material, with increasing TA, is accompanied by a higher disorder form of C sp2 sites. These results are used to describe the stress variation that accompanies the nitrogen evolution within the deposited films.
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68.55.-a Thin film structure and morphology
68.60.Bs Mechanical and acoustical properties
78.66.Nk Insulators
78.30.Hv Other nonmetallic inorganics
78.40.Ha Other nonmetallic inorganics
62.20.-x Mechanical properties of solids

Structural, morphological, and optical properties of AlGaN/GaN heterostructures with AlN buffer and interlayer

S. Çörekçi, M. K. Öztürk, B. Akaoğlu, M. Çakmak, S. Özçelik, and E. Özbay

J. Appl. Phys. 101, 123502 (2007); http://dx.doi.org/10.1063/1.2747216 (6 pages) | Cited 7 times

Online Publication Date: 18 June 2007

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AlxGa1−xN/GaN (x ∼ 0.3) heterostructures with and without a high-temperature (HT) AlN interlayer (IL) have been grown on sapphire (Al2O3) substrates and AlN buffer/Al2O3 templates by metal organic chemical vapor deposition. The effects of an AlN buffer layer (BL) grown on an Al2O3 substrate and an AlN IL grown under the AlGaN ternary layer (TL) on structural, morphological, and optical properties of the heterostructures have been investigated by high-resolution x-ray diffraction, spectroscopic ellipsometry, atomic force microscopy, and photoluminescence measurements. The AlN BL improves the crystal quality of the AlGaN TL. Further improvement is achieved by inserting an AlN IL between GaN BL and AlGaN TL. However, experimental results also show that a HT AlN IL leads to relatively rough surfaces on AlGaN TLs, and an AlN IL changes the strain in the AlGaN TL from tensile to compressive type. In addition, an AlN BL improves the top surface quality of heterostructures.
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78.67.-n Optical properties of low-dimensional, mesoscopic, and nanoscale materials and structures
81.15.Kk Vapor phase epitaxy; growth from vapor phase
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
68.65.-k Low-dimensional, mesoscopic, nanoscale and other related systems: structure and nonelectronic properties
68.35.Ct Interface structure and roughness

Comparison of models that interconnect point defect parameters in solids with bulk properties

P. Varotsos

J. Appl. Phys. 101, 123503 (2007); http://dx.doi.org/10.1063/1.2745359 (8 pages) | Cited 36 times

Online Publication Date: 18 June 2007

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Two models have been proposed for the interconnection of the defect Gibbs energy gi with bulk properties almost 60 and 30 years ago, respectively. The one, proposed by Zener [J. Appl. Phys. 22, 372 (1951)] , assumes that gi can account for the work that goes into straining the lattice and hence that it is proportional to the shear modulus of the solid. The other considers that since gi corresponds to an isobaric and isothermal process, it should be proportional to the isothermal bulk modulus and the mean volume per atom. The results of these two models are compared for different processes (defect formation, self-diffusion activation, and heterodiffusion) in a variety of solids including metals (fcc, bcc, and tetragonal) as well as solids that exhibit a superionic behavior. We find that the latter model does better than the former.
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61.72.J- Point defects and defect clusters
62.20.D- Elasticity
81.40.Jj Elasticity and anelasticity, stress-strain relations
66.30.-h Diffusion in solids

Knudsen diffusion through thin fibrous films

Xiangning Li and William C. Strieder

J. Appl. Phys. 101, 123504 (2007); http://dx.doi.org/10.1063/1.2740360 (11 pages) | Cited 1 time

Online Publication Date: 19 June 2007

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Variational upper and multiple scattering lower bounds on the Knudsen transport rates are applied to several slab geometries of finite thickness made up of parallel, randomly placed, overlapping long cylindrical fibers, one with fibers aligned perpendicular and a second with fibers parallel to the film edges. Estimates and rigorous error bounds are generated for various film thickness. Insight into collision paths useful for thin film vapor deposition, reactive matrix formation of composites, and gas membrane transport are obtained.
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47.56.+r Flows through porous media
47.45.-n Rarefied gas dynamics

Dual layered surface crystallization of 30BaO–15TiO2–55GeO2 glass by stepwise heat treatment

Hirokazu Masai, Takumi Fujiwara, Hiroshi Mori, Yasuhiko Benino, and Takayuki Komatsu

J. Appl. Phys. 101, 123505 (2007); http://dx.doi.org/10.1063/1.2743827 (6 pages) | Cited 1 time

Online Publication Date: 19 June 2007

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In the crystallization process of 30BaO–15TiO2–55GeO2 (BTG55), we demonstrated by an x-ray diffraction analysis that a stepwise heat treatment enables a selective crystallization of fresnoite and benitoite, two crystalline phases obtained from the BTG55 mother glass and that these two phases form a dual surface layer. The result indicates that the stepwise heat treatment generates a compositional heterogeneity inside the glass matrix, which facilitates the nucleation of the fresnoite phase. The mechanism for the generation of the dual surface layer is discussed. Due to a marked difference in optical nonlinearity of the two phases, the selective crystallization may open the possibility of an optimized heat-treatment condition to achieve a large optical nonlinearity in crystallized glass. Also, two types of fresnoite, tetragonal and orthorhombic, were observed in the crystallized phase after a stepwise heat treatment of 690/720 °C.
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68.35.Rh Phase transitions and critical phenomena
64.70.K- Solid-solid transitions
64.60.Q- Nucleation
81.40.Gh Other heat and thermomechanical treatments
42.65.-k Nonlinear optics

Molecular dynamics simulation of the recrystallization of amorphous Si layers: Comprehensive study of the dependence of the recrystallization velocity on the interatomic potential

C. Krzeminski, Q. Brulin, V. Cuny, E. Lecat, E. Lampin, and F. Cleri

J. Appl. Phys. 101, 123506 (2007); http://dx.doi.org/10.1063/1.2743089 (11 pages) | Cited 14 times

Online Publication Date: 19 June 2007

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The molecular dynamics method is applied to simulate the recrystallization of an amorphous/crystalline silicon interface. The atomic structure of the amorphous material is constructed with the method of Wooten, Winer, and Weaire. The amorphous on crystalline stack is annealed afterward on a wide range of temperature and time using five different interatomic potentials: Stillinger-Weber, Tersoff, EDIP, SW115, and Lenosky. The simulations are exploited to systematically extract the recrystallization velocity. A strong dependency of the results on the interatomic potential is evidenced and explained by the capability of some potentials (Tersoff and SW115) to correctly handle the amorphous structure, while other potentials (Stillinger-Weber, EDIP, and Lenosky) lead to the melting of the amorphous. Consequently, the interatomic potentials are classified according to their ability to simulate the solid or the liquid phase epitaxy.
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81.05.Gc Amorphous semiconductors
68.55.A- Nucleation and growth
81.05.Cy Elemental semiconductors
64.70.K- Solid-solid transitions
61.72.Cc Kinetics of defect formation and annealing
61.43.Dq Amorphous semiconductors, metals, and alloys

Temperature dependence of the thermal conductivity of different forms of diamond

Saswati Barman and G. P. Srivastava

J. Appl. Phys. 101, 123507 (2007); http://dx.doi.org/10.1063/1.2747540 (8 pages) | Cited 2 times

Online Publication Date: 19 June 2007

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We present a systematic theoretical investigation of the thermal conductivity of naturally abundant, isotopically enriched, fast neutron irradiated single crystals of diamond, and chemical vapor deposited diamond films of different types over a large temperature range. Existing experimental data have been analyzed using Callaway’s theoretical model [ Phys. Rev. 113, 1046 (1959) ] for thermal conductivity based on an isotropic continuum phonon dispersion relation and using normal and umklapp phonon-phonon relaxation times derived from the application of time-dependent perturbation theory within an anharmonic continuum model. In contrast to existing theoretical studies of the thermal conductivity of diamond, our approach considers Grüneisen’s constant as the only (semi)adjustable parameter for anharmonic phonon interactions. This work quantifies the enhancement of the thermal conductivity of diamond with isotopic purity. This work also accounts for the dip in the thermal conductivity curve for hot filament chemical vapor deposition of diamond films and neutron irradiated diamond at low temperatures and provides an estimate of the amount, type, and size of defects present in such samples. We find that the N-drift term in Callaway’s theory provides a significant contribution to the thermal conductivity of all the forms of diamond studied here.
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66.70.-f Nonelectronic thermal conduction and heat-pulse propagation in solids; thermal waves
63.70.+h Statistical mechanics of lattice vibrations and displacive phase transitions
63.20.K- Phonon interactions
63.20.Ry Anharmonic lattice modes

Bond length variation in Ga1−xInxAs crystals from the Tersoff potential

J. T. Titantah, D. Lamoen, M. Schowalter, and A. Rosenauer

J. Appl. Phys. 101, 123508 (2007); http://dx.doi.org/10.1063/1.2748338 (4 pages) | Cited 7 times

Online Publication Date: 20 June 2007

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In this work we show that a reparametrized Tersoff potential accurately reproduces the bond length variations observed in ternary Ga1−xInxAs mixed crystals. The reparametrization is based on accurate first-principles electronic structure calculations. Previous parametrizations of the Tersoff potential for GaAs and InAs structures, although they accurately reproduce the properties of the zinc-blende GaAs and InAs crystals, are shown to be unable to reproduce the bond length variations in these mixed crystals. In addition to correcting the bond length inconsistencies, the new set of parameters is also shown to yield the elastic constants of GaAs and InAs that agree fairly well with measurements and to reproduce accurately their respective melting temperature.
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61.50.Lt Crystal binding; cohesive energy
81.40.Jj Elasticity and anelasticity, stress-strain relations
62.20.D- Elasticity
64.70.D- Solid-liquid transitions

Raman scattering from epitaxial TaNx (0.94 ≤ x ≤ 1.37) layers grown on MgO(001)

M. Stoehr, C.-S. Shin, I. Petrov, and J. E. Greene

J. Appl. Phys. 101, 123509 (2007); http://dx.doi.org/10.1063/1.2748354 (5 pages) | Cited 5 times

Online Publication Date: 20 June 2007

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Single-crystal group-V δ-TaNx transition-metal (TM) nitride layers, with x ranging from 0.94 to 1.37, are grown on MgO(001) and analyzed by Raman spectroscopy. In perfect NaCl-structure crystals, the first-order Raman modes are not allowed by the Oh crystal symmetry. Since first-order acoustic and optical bands are observed for all δ-TaNx samples, this indicates the presence of point defects, even for stoichiometric TaN. The large mass difference between Ta and N leads to a correspondingly large separation between the first-order acoustic and optical bands centered at 150 and 550 cm−1, respectively. A Raman peak at 440 cm−1, overlapping the first-order optical band, is identified as the difference between the first-order optical and acoustic modes. Contrary to the case for most TM nitrides, there is no peak visible in the second-order high-frequency range (700–1500 cm−1). In addition, the first-order transverse (120 cm−1) and longitudinal acoustic peaks (175 cm−1) do not exhibit the frequency shift with N/Ta ratio x (i.e., with the valence electron density) that has been reported for group-IV TM nitrides. This is consistent with the resistivity of TaNx remaining essentially constant, as well as with the very weak dependence of the lattice parameter as a function of the N/Ta ratio, and suggests that the wide single-phase region in this system is due to the presence of isoelectronic antisite substitutions rather than, as has been proposed previously, cation and anion vacancies.
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78.66.Nk Insulators
78.30.Hv Other nonmetallic inorganics
68.55.-a Thin film structure and morphology
63.20.-e Phonons in crystal lattices
68.55.Ln Defects and impurities: doping, implantation, distribution, concentration, etc.
73.61.Ng Insulators

Transition metal interaction and Ni-Fe-Cu-Si phases in silicon

M. Heuer, T. Buonassisi, A. A. Istratov, M. D. Pickett, M. A. Marcus, A. M. Minor, and E. R. Weber

J. Appl. Phys. 101, 123510 (2007); http://dx.doi.org/10.1063/1.2748346 (6 pages) | Cited 12 times

Online Publication Date: 20 June 2007

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In the present article we characterize several intermetallic phases of the Cu-Ni-Fe-Si system found as precipitates in the misfit dislocation layer of intentionally contaminated and slowly cooled Si1−xGex/Si-heterostructures. The clusters showed a characteristic phase speciation into a Cu-rich part similar to Cu3Si and an Fe-Ni-Cu-Si phase similar to NiSi2. It is suggested that the precipitate formation of the investigated intermetallic silicides involves a homogeneous precursor phase at higher temperatures that later decomposes into the observed phases. Our results indicate that chemical reactions between metals and silicon during precipitation may reduce the lattice mismatch compared to single-metal precipitates, rendering mixed-metal-silicide precipitates more stable and energetically favorable.
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81.30.Mh Solid-phase precipitation
64.75.-g Phase equilibria
61.72.Ff Direct observation of dislocations and other defects (etch pits, decoration, electron microscopy, x-ray topography, etc.)

Raman investigation of A2CoBO6 (A = Sr and Ca, B = Te and W) double perovskites

A. P. Ayala, I. Guedes, E. N. Silva, M. S. Augsburger, M. del C. Viola, and J. C. Pedregosa

J. Appl. Phys. 101, 123511 (2007); http://dx.doi.org/10.1063/1.2745088 (6 pages) | Cited 13 times

Online Publication Date: 20 June 2007

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Raman scattering is used to investigate the room temperature phonons of tetragonal (I4/m) Sr2CoWO6 and monoclinic (P21/n) Ca2CoWO6, Sr2CoTeO6, and Ca2CoTeO6 double perovskites. Both tetragonal and monoclinic structures result from small distortions of the Fmmathm cubic lattice. Therefore, phonons were assigned according to correlations with those observed in the prototype A2B′B″O6 Fmmathm cubic structure and the internal vibrational modes of the oxygen octahedra. The observed shifts of some vibrations in the A2CoBO6 compounds upon changing either A or B elements are tentatively explained.
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78.30.Hv Other nonmetallic inorganics
63.20.-e Phonons in crystal lattices

Chemically sensitive free-volume study of amorphization of Cu60Zr40 induced by cold rolling and folding

Werner Puff, Herbert Rabitsch, Gerhard Wilde, Guru P. Dinda, and Roland Würschum

J. Appl. Phys. 101, 123512 (2007); http://dx.doi.org/10.1063/1.2740347 (9 pages) | Cited 6 times

Online Publication Date: 21 June 2007

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With the aim to contribute to a microscopical understanding of the processes of solid-state amorphization, the chemically sensitive technique of background—reduced Doppler broadening of positron-electron annihilation radiation in combination with positron lifetime spectroscopy and microstructural characterization is applied to a free volume study of the amorphization of Cu60Zr40 induced by consecutive folding and rolling. Starting from the constituent pure metal foils, a nanosale multilayer structure of elemental layers and amorphous interlayers develops in an intermediate state of folding and rolling, where free volumes with a Zr-rich environment occur presumably located in the hetero-interfaces between the various layers or in grain boundaries of the Cu layers. After complete intermixing and amorphization, the local chemical environment of the free volumes reflects the average chemical alloy composition. In contrast to other processes of amorphization, free volumes of the size of few missing atoms occur in the rolling-induced amorphous state. Self-consistent results from three different methods for analyzing the Doppler broadening spectra, i.e., S−W-parameter correlation, multicomponent fit, and the shape of ratio curves, demonstrate the potential of the background-reduced Doppler technique for chemically sensitive characterization of structurally complex materials on an atomic scale.
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81.40.Ef Cold working, work hardening; annealing, post-deformation annealing, quenching, tempering recovery, and crystallization
61.43.Dq Amorphous semiconductors, metals, and alloys
61.72.Mm Grain and twin boundaries
78.70.Bj Positron annihilation

Image forces of screw dislocations in a magnetoelectroelastic layered half-plane

Chien-Ching Ma and Jui-Mu Lee

J. Appl. Phys. 101, 123513 (2007); http://dx.doi.org/10.1063/1.2743740 (13 pages) | Cited 5 times

Online Publication Date: 21 June 2007

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The two-dimensional analysis of elastic, electric, and magnetic fields induced by generalized screw dislocations embedded in a transversely isotropic magnetoelectroelastic layered half-plane is presented in this study. The results indicate that explicit solutions for this problem consist only of the simplest solutions for an infinite magnetoelectroelastic medium with a generalized screw dislocation. It is shown that the physical meaning of the solution is the image method. The mathematical method used in this study provides an automatic determination for the locations and magnitudes of image screw dislocations. From the generalized Peach-Koehler equation, the explicit forms of image forces exerted on a generalized screw dislocation in a layered half-plane are derived from the full-field solutions of the generalized stresses. Numerical results for the full-field distributions of the generalized stresses in the layered half-plane medium are presented. The image forces and equilibrium positions of one screw dislocation, two screw dislocations, and an array of screw dislocations are presented by numerical calculations and are discussed in detail.
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61.72.Bb Theories and models of crystal defects
75.80.+q Magnetomechanical effects, magnetostriction

Stokes luminescence and frequency upconversion in Pr3+ doped TeO2–PbO glass

Vineet Kumar Rai, Leonardo de S. Menezes, and Cid B. de Araújo

J. Appl. Phys. 101, 123514 (2007); http://dx.doi.org/10.1063/1.2745314 (3 pages) | Cited 9 times

Online Publication Date: 21 June 2007

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Optical properties of Pr3+ doped TeO2–PbO glass were investigated by resonant excitation of 3H4→1D2 transition at ∼ 586 nm. The decay time of 1D2 level, the energy transfer (ET) rates from the 1D2 level to other Pr3+ states, and the frequency upconversion process which leads to emission at 484 nm (3P0→3H4 transition) were studied. The ET rates between Pr3+ ions were determined and their interaction was identified as due to dipole-dipole potential. A model used to describe the emission 3P0→3H4 provided information on the dynamics of Pr3+ pair states. The upconversion ET rate was also determined.
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42.70.Ce Glasses, quartz
78.66.Jg Amorphous semiconductors; glasses
78.55.Hx Other solid inorganic materials

Deep-level emissions in GaAsN/GaAs structures grown by metal organic chemical vapor deposition

J. F. Chen, C. T. Ke, P. C. Hsieh, C. H. Chiang, W. I. Lee, and S. C. Lee

J. Appl. Phys. 101, 123515 (2007); http://dx.doi.org/10.1063/1.2748613 (4 pages) | Cited 5 times

Online Publication Date: 22 June 2007

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This work presents the deep-level photoluminescence of coherently strained GaAsN/GaAs quantum-well (QW) structures with various GaAsN thicknesses and N contents. A broad deep-level emission at ∼ 1.1 eV is observed, whose wavelength is redshifted as the GaAsN thickness increases. Based on its energy separation from the QW emission, this emission is attributed to a transition between the QW electron ground state and a deep level at ∼ 0.2 eV above the GaAsN valence-band (VB) edge. This level is shown to be tied to the GaAs band edge. A transition between this level and the GaAs conduction band allows the GaAsN–GaAs band alignment to be evaluated. A type II band lineup is obtained with VB offsets of 0.03 and 0.002 eV for N = 0.6% and 1.8%, respectively. The decreased VB offset suggests a transition from type II to type I with increasing N content. Thermal annealing effectively removes this level and improves the QW emission. The concentration of this level is not clearly correlated with N content, suggesting that this level is induced by a low-temperature growth of the GaAsN layer to suppress the composition fluctuation. Given its energy, this level is tentatively assigned to VGa.
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78.67.De Quantum wells
78.55.Cr III-V semiconductors
73.21.Fg Quantum wells
71.55.Eq III-V semiconductors
61.72.Cc Kinetics of defect formation and annealing

Complementary neutron and x-ray reflectivity studies of “near-frictionless” carbon films

Jacqueline A. Johnson, Sungkyun Park, Sujoy Roy, Sunil K. Sinha, Ali Erdemir, Osman L. Eryilmaz, and John B. Woodford

J. Appl. Phys. 101, 123516 (2007); http://dx.doi.org/10.1063/1.2745244 (6 pages)

Online Publication Date: 22 June 2007

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“Near-frictionless carbon,” a form of amorphous hydrogenated diamondlike carbon developed at Argonne National Laboratory, has been investigated by neutron and x-ray reflectivity under ambient conditions, in vacuum, and at elevated temperature. A series of films was made with different ratios of hydrogen to carbon in the plasma; the deuterated counterparts were also synthesized. The details of scattering length density profiles, thickness, and surface and interfacial roughness of the films were obtained from neutron and x-ray reflectivity profiles. This allowed us to calculate the mass density and hydrogen composition of the bulk carbon film, leading to insight into the frictional properties. From the analysis we found that the magnitude of the scattering length density of the top surface layer was greatly affected by the experimental atmosphere in which the films were contained although the trend with hydrogen in the plasma remained the same. All samples were fitted with a three-layer model in addition to the substrate.
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68.55.-a Thin film structure and morphology
68.60.Bs Mechanical and acoustical properties
68.35.B- Structure of clean surfaces (and surface reconstruction)
68.35.Ct Interface structure and roughness
68.35.Af Atomic scale friction
78.70.Ck X-ray scattering

Stress dependence of the Raman spectrum of polycrystalline barium titanate in presence of localized domain texture

Tatsuo Sakashita, Marco Deluca, Shinsuke Yamamoto, Hirokazu Chazono, and Giuseppe Pezzotti

J. Appl. Phys. 101, 123517 (2007); http://dx.doi.org/10.1063/1.2747217 (12 pages) | Cited 5 times

Online Publication Date: 22 June 2007

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The stress dependence of the Raman spectrum of polycrystalline barium titanate (BaTiO3, BT) ceramics has been examined with microprobe polarized Raman spectroscopy. The angular dependence of the Raman spectrum of the tetragonal BT crystal has been theoretically established, enabling us to assess the stress dependence of selected spectral modes without the influence of crystallographic domain orientation. Upon considering the frequency shift of selected Raman modes as a function of orientation between the crystallographic axis and the polarization vector of incident and scattered light, a suitable instrumental configuration has been selected, which allowed a direct residual stress measurement according to a modified piezospectroscopic procedure. The analysis is based on the selection of mixed photostimulated spectral modes in two perpendicular angular orientations.
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77.84.Ek Niobates and tantalates
77.84.Cg PZT ceramics and other titanates
78.30.Hv Other nonmetallic inorganics
77.80.Dj Domain structure; hysteresis
77.22.Ej Polarization and depolarization
62.20.-x Mechanical properties of solids

Dynamics and sound attenuation in viscoelastic polymer containing hollow glass microspheres

Honggang Zhao, Yaozong Liu, Jihong Wen, Dianlong Yu, and Xisen Wen

J. Appl. Phys. 101, 123518 (2007); http://dx.doi.org/10.1063/1.2749420 (3 pages) | Cited 5 times

Online Publication Date: 25 June 2007

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The practical design of composite material suggests that there is a need for an improved quantitative understanding of the interaction between acoustic waves and microspheres. The multiple scattering method is used to investigate the sound attenuation of the viscoelastic polymer containing hollow glass microspheres. Then the dynamics of the hollow glass microspheres is investigated by reference to the elements of the scattering matrices. It shows that the dilatational vibration of the microsphere plays a key role in the sound attenuation within the viscoelastic polymer composite at the quasistatic region. The thinner glass shell undergoes more severe dilatational vibration and induces more attenuation in the polymer composite.
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62.65.+k Acoustical properties of solids
81.40.Jj Elasticity and anelasticity, stress-strain relations
62.40.+i Anelasticity, internal friction, stress relaxation, and mechanical resonances

Phonon related properties of transition metals, their carbides, and nitrides: A first-principles study

E. I. Isaev, S. I. Simak, I. A. Abrikosov, R. Ahuja, Yu. Kh. Vekilov, M. I. Katsnelson, A. I. Lichtenstein, and B. Johansson

J. Appl. Phys. 101, 123519 (2007); http://dx.doi.org/10.1063/1.2747230 (18 pages) | Cited 72 times

Online Publication Date: 25 June 2007

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Lattice dynamics of body-centered cubic (bcc) Vb-VIb group transition metals (TM), and B1-type monocarbides and mononitrides of IIIb-VIb transition metals are studied by means of first-principles density functional perturbation theory, ultra soft pseudopotentials, and generalized gradient approximation to the exchange-correlation functional. Ground state parameters of transition metals and their compounds are correctly reproduced with the generated ultrasoft pseudopotentials. The calculated phonon spectra of the bcc metals are in excellent agreement with results of inelastic neutron scattering experiments. We show that the superconductivity of transition metal carbides (TMC) and transition metal nitrides (TMN) is related to peculiarities of the phonon spectra, and the anomalies of the spectra are connected to the number of valence electrons in crystals. The calculated electron-phonon interaction constants for TM, TMC, and TMN are in excellent agreement with experimentally determined values. Phonon spectra for a number of monocarbides and mononitrides of transition metals within the cubic NaCl- and hexagonal WC-type structures are predicted. Ideal stoichiometric B1 crystals of ScC, YC, and VC are predicted to be dynamically stable and superconducting materials. We also conclude that YN is a semiconductor.
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74.25.Kc Phonons
74.70.Ad Metals; alloys and binary compounds (including A15, MgB2, etc.)
74.25.Jb Electronic structure (photoemission, etc.)

Optical properties of vacancies in thermochemically reduced Mg-doped sapphire single crystals

R. Ramírez, M. Tardío, R. González, J. E. Muñoz Santiuste, and M. R. Kokta

J. Appl. Phys. 101, 123520 (2007); http://dx.doi.org/10.1063/1.2748729 (12 pages) | Cited 9 times

Online Publication Date: 25 June 2007

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Optical absorption and emission experiments were used to characterize defects and defect aggregates in Mg-doped Al2O3 crystals due to thermochemical reduction at high temperatures. Oxygen vacancies and higher-order defects are produced much more readily in Mg-doped than in undoped Al2O3 crystals. F+ and F centers (oxygen vacancies with one or two electrons, respectively) were monitored by their optical absorption bands at about 4.8 and 6.0 eV, respectively. In contrast with undoped crystals, where the reduction produces primarily F centers and a small amount of F+ centers, in Mg-doped crystals both F and F+ centers are created in comparable concentrations. These thermally generated F and F+ centers are much more stable than those produced in undoped crystals irradiated with neutrons. Clustering of individual oxygen vacancies forming higher-order defects, such as anion divacancy F22+ and F2+ centers, was investigated by low temperature absorption and luminescence experiments, in conjunction with UV irradiation and thermal treatments. The strong absorption bands at 2.87 and 3.69 eV were shown to be due to Mg-perturbed F22+ and F2+ centers, respectively. In addition, photoconversion of F22+ and F2+ centers was observed. In crystals containing large concentrations of F22+ centers, electrons excited by 5.0 eV light are trapped by F22+ and F2+ centers resulting in the conversion of F22+ centers into F2+. A model of F-type centers was extended to F22+ and F2+ centers. The calculated optical parameters are in very good agreement with those determined experimentally. A simple analysis of the lattice energy suggests that the environments of the F2-type centers are different in TCR Al2O3 crystals and in n-irradiated undoped crystals.
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78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)
61.72.J- Point defects and defect clusters
82.30.-b Specific chemical reactions; reaction mechanisms

Fundamental band edge absorption in nominally undoped and doped 4H‐SiC

P. Grivickas, V. Grivickas, J. Linnros, and A. Galeckas

J. Appl. Phys. 101, 123521 (2007); http://dx.doi.org/10.1063/1.2749335 (10 pages) | Cited 11 times

Online Publication Date: 26 June 2007

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Fundamental band edge absorption is investigated in nominally undoped (n<1014 cm−3) and heavily doped (n ∼ 8×1018 cm−3) 4H‐SiC by a spectroscopy technique based on spatially and time-resolved free-carrier absorption. The spectra are extracted over a wide absorption range (0.02–500 cm−1) at temperatures from 75 to 450 K. The experimental results are supported by an indirect transition theory with a unique set of dominating momentum-conserving phonons, showing good correlation with earlier findings of differential absorption measurements at 2 K. Exciton binding energy of 30±10 meV is derived from fitting the data at 75 K. The detected polarization anisotropy of absorption with respect to c axis is shown to be consistent with the selection rules for the corresponding phonon branches. An analytical model related to constant degree of involved phonons describes well the obtained energy gap variation with temperature. Finally, doping induced band gap narrowing is characterized above the impurity-Mott transition and compared with theoretical calculations in the random phase approximation. The shape of the fundamental absorption edge at high carrier concentrations is discussed in terms of excitonic enhancement above the Mott transition, as recently detected in Si.
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78.30.Hv Other nonmetallic inorganics
78.47.-p Spectroscopy of solid state dynamics
63.20.D- Phonon states and bands, normal modes, and phonon dispersion
71.35.-y Excitons and related phenomena
61.72.up Other materials
71.30.+h Metal-insulator transitions and other electronic transitions

A continuum thermodynamics model for the sensing effect in ferromagnetic shape memory Ni–Mn–Ga

N. N. Sarawate and M. J. Dapino

J. Appl. Phys. 101, 123522 (2007); http://dx.doi.org/10.1063/1.2748356 (11 pages) | Cited 13 times

Online Publication Date: 26 June 2007

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A magnetomechanical model based on continuum thermodynamics is presented which describes the sensing effect in single-crystal ferromagnetic shape memory Ni–Mn–Ga. The model quantifies the stress and magnetization dependence on strain at different values of bias fields under isothermal conditions. A magnetic Gibbs energy is considered as the thermodynamic potential with Zeeman, magnetostatic, and anisotropy energy contributions. Constitutive equations for stress and magnetization are obtained in the isothermal case after restricting the process through the Clausius-Duhem inequality for the second law of thermodynamics. Mechanical dissipation and the microstructure of Ni–Mn–Ga are incorporated in the continuum model through the internal state variables volume fraction, domain fraction, and magnetization rotation angle. Closed-form solutions describing the evolution of the internal state variables are developed. The model requires only seven parameters identified from simple experiments: stress-strain curve at zero field, and easy-axis and hard-axis magnetization curves.
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75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
75.40.Cx Static properties (order parameter, static susceptibility, heat capacities, critical exponents, etc.)
75.50.Cc Other ferromagnetic metals and alloys
62.20.F- Deformation and plasticity
81.40.Jj Elasticity and anelasticity, stress-strain relations

Effects of anchoring strength on diffraction properties of liquid crystal phase gratings formed on photoalignment polymer films

Hiroshi Ono, Shinji Oikawa, and Nobuhiro Kawatsuki

J. Appl. Phys. 101, 123523 (2007); http://dx.doi.org/10.1063/1.2749485 (7 pages) | Cited 5 times

Online Publication Date: 26 June 2007

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The liquid crystal phase gratings were prepared using a polarization holographic exposure on poly(vinyl cinnamate) or photocross-linkable polymer liquid crystal (PCLC) films. Anchoring strength between the photoalignment layer and low-molar-mass liquid crystal is a very important role to realize the liquid crystal grating as a practical optical device. Anchoring strength of our PCLC film is sufficiently strong and optical properties of liquid crystal gratings were theoretically well explained assuming the strong anchoring.
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42.70.Df Liquid crystals
42.40.Eq Holographic optical elements; holographic gratings
61.30.-v Liquid crystals
42.70.Jk Polymers and organics
42.79.Wc Optical coatings

Microstructural analysis of Ga(NAs)/GaP heterostructures

I. Németh, T. Torunski, B. Kunert, W. Stolz, and K. Volz

J. Appl. Phys. 101, 123524 (2007); http://dx.doi.org/10.1063/1.2749304 (4 pages) | Cited 2 times

Online Publication Date: 27 June 2007

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We have investigated the microstructure of compressively strained Ga(NAs)/GaP quantum wells (QWs) with different N contents. This material system is a promising candidate for future integration of photonics on silicon substrates. N-induced microscopic strain fields are detected applying strain sensitive transmission electron microscopy dark-field (DF-TEM) imaging. Exceeding 7% of N concentration, we find a deterioration of the upper QW interface despite a reduction of the macroscopic strain for compositions with increasing N content. These nitrogen-induced structural characteristics of the ternary alloy are presumably correlated with the optical properties as observed by photoluminescence spectroscopy.
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68.65.Fg Quantum wells
78.67.De Quantum wells
78.55.Cr III-V semiconductors
68.37.Lp Transmission electron microscopy (TEM)

Solid solubility of Ir and Rh at the Co sites of skutterudites

X. Shi, Z. Zhou, W. Zhang, L. D. Chen, J. Yang, and C. Uher

J. Appl. Phys. 101, 123525 (2007); http://dx.doi.org/10.1063/1.2749490 (5 pages) | Cited 2 times

Online Publication Date: 27 June 2007

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Phase diagrams for skutterudite solid solutions made by alloying at the Co site of CoSb3 skutterudite structure are studied by density functional theory. A limited solid solubility is observed for IrxCo1−xSb3 and RhxCo1−xSb3, showing good agreement with experimental reports. A correlation between solubility and material preparation procedure is discussed. A simple method to synthesize skutterudite nanocomposites is proposed. Our calculations also demonstrate that IrxRh1−xSb3 is a compound with ordered structure rather than a random solid solution.
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64.75.-g Phase equilibria
81.30.Bx Phase diagrams of metals, alloys, and oxides
81.16.-c Methods of micro- and nanofabrication and processing
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