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14 Feb 2013

Volume 113, Issue 6, Articles (06xxxx)

Issue Cover Spotlight Figure

J. Appl. Phys. 113, 064301 (2013); http://dx.doi.org/10.1063/1.4789897 (11 pages)

Y. G. Marinov, G. B. Hadjichristov, A. G. Petrov, S. Marino, C. Versace, and N. Scaramuzza
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back to top Structural, Mechanical, Thermodynamic, and Optical Properties of Condensed Matter

Effects of parameter variations on negative effective constitutive parameters of non-metallic metamaterials

Yang Li and Nicola Bowler

J. Appl. Phys. 113, 063501 (2013); http://dx.doi.org/10.1063/1.4790714 (12 pages)

Online Publication Date: 8 February 2013

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Analytical expressions describing the variability of effective constitutive parameters of non-metallic metamaterials, as a function of the constituent geometric and material parameters and their variations, have been developed from the total differential of Clausius-Mossotti expressions (using Mie dipole polarizabilities) for the effective (bulk) constitutive parameters of the metamaterial. In practice, these expressions are important for estimating the performance of a metamaterial with particular variations in the parameters of its constituents that arise during the fabrication process, and can be used to guard against extinction of desired double negative (DNG) behavior. With the derived expressions, the effects of parameter variations on effective constitutive parameters of non-metallic metamaterials have been analyzed for three types of metamaterials: (i) cubic arrays of identical magnetodielectric spheres; (ii) cubic arrays of dielectric spheres with equal radius but two different permittivities; and (iii) cubic arrays of dielectric spheres with equal permittivity but two different radii. These effects are evaluated in terms of the calculated variations in values of the effective constitutive parameters of the metamaterial in the vicinity of the DNG or single negative (SNG) band for particular geometric and material parameters and their variations. Results show that variation in the following parameters impacts DNG bandwidth. Listed in order from greatest to least influence: (i) sphere radius; (ii) sphere permittivity and permeability; (iii) lattice constant of the array; and (iv) the constitutive parameters of the array medium, all impact the width of the achievable DNG band. For particular cases studied here, results also show that the DNG behavior may be extinguished if there are 0.78%, 0.016%, and 0.016% variations in all parameters of metamaterial types (i), (ii), and (iii), respectively, as defined above. For the design of non-metallic metamaterials with inclusions, having arbitrary material parameters, in either periodic or random arrangement, the presented results can give a qualitative guide on the level of fabrication tolerances that should be achieved in order to observe the predicted SNG or DNG behavior experimentally.
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81.05.Xj Metamaterials for chiral, bianisotropic and other complex media
61.72.Qq Microscopic defects (voids, inclusions, etc.)
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
77.22.Ch Permittivity (dielectric function)
75.85.+t Magnetoelectric effects, multiferroics

Local stress determination in chromia-former thanks to micro-Raman spectroscopy: A way to investigate spontaneous delamination processes

M. Guerain, P. Goudeau, B. Panicaud, and J. L. Grosseau-Poussard

J. Appl. Phys. 113, 063502 (2013); http://dx.doi.org/10.1063/1.4790308 (7 pages)

Online Publication Date: 11 February 2013

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Spontaneous delamination process for α-Cr2O3 thermal oxide films growing on NiCr-30 alloys is studied thanks to micro Raman spectroscopy. In particular, stress maps are performed through and around buckles developed on chromia films. Depending on the cooling rate at the end of the oxidation process, different buckle types appear which are investigated. Associated residual stress distribution clearly evidences the stress release field. In addition, geometrical features are determined for the different buckle types, and from comparison with modelling describing buckle formation and propagation, it is possible to get the interface toughness distribution.
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68.60.Bs Mechanical and acoustical properties
78.30.Hv Other nonmetallic inorganics
78.66.Nk Insulators
81.40.Lm Deformation, plasticity, and creep
62.20.mq Buckling

The impact of substrate miscut on the morphology of InGaN epitaxial layers subjected to a growth interruption

Nicolas Jouvet, Menno J. Kappers, Colin J. Humphreys, and Rachel A. Oliver

J. Appl. Phys. 113, 063503 (2013); http://dx.doi.org/10.1063/1.4790311 (5 pages)

Online Publication Date: 11 February 2013

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To investigate the formation of gross well width fluctuations in InGaN quantum wells subjected to a growth interruption or temperature ramp, the impact of substrate miscut on the morphology of thin InGaN epitaxial layers grown on GaN pseudo-substrates has been studied. Following a growth interruption, the InGaN layer morphology consists of interlinking strips of InGaN separated by elongated troughs. The troughs tend to align along the same direction as the step edges in the underlying GaN pseudo-substrate and the spacing of the troughs varies in direct proportion to the step edge spacing. However, the troughs are on average more widely spaced than the terrace edges, and analysis of the distribution of trough spacings and step edge spacings suggests that it may be double-monolayer steps which most influence the trough formation. A mechanism for trough formation is proposed in which indium is preferentially incorporated at double-monolayer step edges and that the resulting indium-rich regions decompose during the growth interruption, forming a trough.
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68.55.ag Semiconductors
68.60.-p Physical properties of thin films, nonelectronic
68.65.Fg Quantum wells
81.15.Kk Vapor phase epitaxy; growth from vapor phase
81.40.Lm Deformation, plasticity, and creep
62.20.F- Deformation and plasticity

Morphological evolution of InGaN/GaN light-emitting diodes grown on free-standing m-plane GaN substrates

R. M. Farrell, D. A. Haeger, K. Fujito, S. P. DenBaars, S. Nakamura, and J. S. Speck

J. Appl. Phys. 113, 063504 (2013); http://dx.doi.org/10.1063/1.4790636 (7 pages)

Online Publication Date: 11 February 2013

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We report on the morphological evolution of InGaN/GaN light-emitting diodes (LEDs) grown on nominally on-axis and intentionally misoriented free-standing m-plane GaN substrates. Large variations in p-n junction depth (±175nm) were observed for LEDs grown on nominally on-axis substrates, while negligible variations in junction depth (±20 nm) were observed for LEDs grown on intentionally misoriented substrates. The surfaces of LEDs grown on the nominally on-axis substrates were characterized by the presence of a high density of pyramidal hillocks [Hirai et al., Appl. Phys. Lett. 91, 191906 (2007)], while the surfaces of the LEDs grown on the intentionally misoriented substrates were relatively smooth and free of pyramidal hillocks. Detailed measurements indicated that the variations in junction depth observed for LEDs grown on nominally on-axis substrates were related to an evolution in the shape of individual pyramidal hillocks during the growth of the LEDs. These results indicate that growing LEDs on intentionally misoriented substrates is an effective way to eliminate the pyramidal hillocks and variations in junction depth associated with growth on nominally on-axis substrates.
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85.60.Jb Light-emitting devices
85.30.Kk Junction diodes

Dielectric function of Cu(In, Ga)Se2-based polycrystalline materials

Shota Minoura, Keita Kodera, Takuji Maekawa, Kenichi Miyazaki, Shigeru Niki, and Hiroyuki Fujiwara

J. Appl. Phys. 113, 063505 (2013); http://dx.doi.org/10.1063/1.4790174 (14 pages)

Online Publication Date: 11 February 2013

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The dielectric functions of Cu(In, Ga)Se2(CIGS)-based polycrystalline layers with different Ga and Cu compositions have been determined by applying spectroscopic ellipsometry (SE) in a wide energy range of 0.7–6.5 eV. To suppress SE analysis errors induced by rough surface and compositional fluctuation, quite thin CIGS layers (<60 nm) with high uniformity toward the growth direction have been characterized using a self-consistent SE analysis method. We find that the optical model used in many previous studies is oversimplified particularly for the roughness/overlayer contribution, and all the artifacts arising from the simplified analysis have been removed almost completely in our approach. The CIGS dielectric functions with the variation of the Ga composition [x = Ga/(In + Ga)] revealed that (i) the whole CIGS dielectric function shifts toward higher energies with x, (ii) the band gap increases linearly with x without the band-gap bowing effect, and (iii) the overall absorption coefficients are significantly smaller than those reported earlier. Furthermore, the reduction of the Cu composition [y = Cu/(In + Ga)] leads to (i) the linear increase in the band-edge transition energy and (ii) the decrease in the absorption coefficient, due to the smaller interaction of the Cu 3d orbitals near the valence band maximum in the Cu-deficient layers. When y > 1, on the other hand, the free-carrier absorption increases drastically due to the formation of a semi-metallic CuxSe phase with a constant band gap in the CIGS component. In this study, by using a standard critical-point line-shape analysis, the critical point energies of the CIGS-based layers with different Ga and Cu compositions have been determined. Based on these results, we will discuss the optical transitions in CIGS-based polycrystalline materials.
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78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)
81.05.Hd Other semiconductors
68.35.bg Semiconductors
71.20.Nr Semiconductor compounds

Intrinsic degradation mechanism of nearly lattice-matched InAlN layers grown on GaN substrates

Guillaume Perillat-Merceroz, Gatien Cosendey, Jean-François Carlin, Raphaël Butté, and Nicolas Grandjean

J. Appl. Phys. 113, 063506 (2013); http://dx.doi.org/10.1063/1.4790424 (10 pages)

Online Publication Date: 11 February 2013

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Thanks to its high refractive index contrast, band gap, and polarization mismatch compared to GaN, In0.17Al0.83N layers lattice-matched to GaN are an attractive solution for applications such as distributed Bragg reflectors, ultraviolet light-emitting diodes, or high electron mobility transistors. In order to study the structural degradation mechanism of InAlN layers with increasing thickness, we performed metalorganic vapor phase epitaxy of InAlN layers of thicknesses ranging from 2 to 500 nm, on free-standing (0001) GaN substrates with a low density of threading dislocations, for In compositions of 13.5% (layers under tensile strain), and 19.7% (layers under compressive strain). In both cases, a surface morphology with hillocks is initially observed, followed by the appearance of V-defects. We propose that those hillocks arise due to kinetic roughening, and that V-defects subsequently appear beyond a critical hillock size. It is seen that the critical thickness for the appearance of V-defects increases together with the surface diffusion length either by increasing the temperature or the In flux because of a surfactant effect. In thick InAlN layers, a better (worse) In incorporation occurring on the concave (convex) shape surfaces of the V-defects is observed leading to a top phase-separated InAlN layer lying on the initial homogeneous InAlN layer after V-defects coalescence. It is suggested that similar mechanisms could be responsible for the degradation of thick InGaN layers.
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81.05.Ea III-V semiconductors
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
81.15.Kk Vapor phase epitaxy; growth from vapor phase
68.35.bg Semiconductors
61.72.Hh Indirect evidence of dislocations and other defects (resistivity, slip, creep, strains, internal friction, EPR, NMR, etc.)

Plasma nitriding process by direct current glow discharge at low temperature increasing the thermal diffusivity of AISI 304 stainless steel

L. V. Prandel, A. Somer, A. Assmann, F. Camelotti, G. Costa, C. Bonardi, A. R. Jurelo, J. B. Rodrigues, and G. K. Cruz

J. Appl. Phys. 113, 063507 (2013); http://dx.doi.org/10.1063/1.4790631 (5 pages)

Online Publication Date: 12 February 2013

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This work reports for the first time on the use of the open photoacoustic cell technique operating at very low frequencies and at room temperature to experimentally determine the thermal diffusivity parameter of commercial AISI304 stainless steel and AISI304 stainless steel nitrided samples. Complementary measurements of X-ray diffraction and scanning electron microscopy were also performed. The results show that in standard AISI 304 stainless steel samples the thermal diffusivity is (4.0 ± 0.3) × 10−6 m2/s. After the nitriding process, the thermal diffusivity increases to the value (7.1 ± 0.5) × 10−6 m2/s. The results are being associated to the diffusion process of nitrogen into the surface of the sample. Carrying out subsequent thermal treatment at 500 °C, the thermal diffusivity increases up to (12.0 ± 2) × 10−6 m2/s. Now the observed growing in the thermal diffusivity must be related to the change in the phases contained in the nitrided layer.
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81.65.Lp Surface hardening: nitridation, carburization, carbonitridation
52.77.-j Plasma applications
66.30.Xj Thermal diffusivity
68.35.Fx Diffusion; interface formation
81.40.Gh Other heat and thermomechanical treatments

Correlation between structural parameters and the magnetocaloric effect in epitaxial La0.8Ca0.2MnO3/LaAlO3 thin film

J. C. Debnath, J. H. Kim, Y. Heo, A. M. Strydom, and S. X. Dou

J. Appl. Phys. 113, 063508 (2013); http://dx.doi.org/10.1063/1.4790876 (6 pages)

Online Publication Date: 12 February 2013

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An epitaxial La0.8Ca0.2MnO3/LaAlO3 (LCMO/LAO) thin film was fabricated using the pulsed laser deposition technique to evaluate the correlation between the crystal structure and the magnetocaloric effect. In our study, the LCMO film was 200 nm in thickness and appeared to have a strong out-of plane texture. We found that each column in the LCMO thin film layer is a domain which has a different ordering direction. These microscopic feature results in anisotropic properties of magnetization, entropy, and relative cooling power. The film exhibited a paramagnetic-to-ferromagnetic second order phase transition at 249 K. The lack of any hysteresis loss also confirmed that the material is intrinsically reversible. In addition, the large magnetization of the thin film results in an entropy change larger than those of all other perovskite type materials. Consequently, the relative cooling power is significantly enhanced.
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75.30.Sg Magnetocaloric effect, magnetic cooling
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
75.70.Ak Magnetic properties of monolayers and thin films
81.15.Fg Pulsed laser ablation deposition
68.55.aj Insulators
75.30.Kz Magnetic phase boundaries (including classical and quantum magnetic transitions, metamagnetism, etc.)

A comparative study of two molecular mechanics models based on harmonic potentials

Junhua Zhao, Lifeng Wang, Jin-Wu Jiang, Zhengzhong Wang, Wanlin Guo, and Timon Rabczuk

J. Appl. Phys. 113, 063509 (2013); http://dx.doi.org/10.1063/1.4791579 (12 pages) | Cited 1 time

Online Publication Date: 12 February 2013

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We show that the two molecular mechanics models, the stick-spiral and the beam models, predict considerably different mechanical properties of materials based on energy equivalence. The difference between the two models is independent of the materials since all parameters of the beam model are obtained from the harmonic potentials. We demonstrate this difference for finite width graphene nanoribbons and a single polyethylene chain comparing results of the molecular dynamics (MD) simulations with harmonic potentials and the finite element method with the beam model. We also find that the difference strongly depends on the loading modes, chirality and width of the graphene nanoribbons, and it increases with decreasing width of the nanoribbons under pure bending condition. The maximum difference of the predicted mechanical properties using the two models can exceed 300% in different loading modes. Comparing the two models with the MD results of AIREBO potential, we find that the stick-spiral model overestimates and the beam model underestimates the mechanical properties in narrow armchair graphene nanoribbons under pure bending condition.
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81.40.Lm Deformation, plasticity, and creep
62.20.de Elastic moduli
62.20.F- Deformation and plasticity
62.25.-g Mechanical properties of nanoscale systems
61.48.Gh Structure of graphene
81.40.Jj Elasticity and anelasticity, stress-strain relations

Viscoplastic analysis of cyclic indentation behavior of thin metallic films

C. B. Blada and Y.-L. Shen

J. Appl. Phys. 113, 063510 (2013); http://dx.doi.org/10.1063/1.4792034 (5 pages)

Online Publication Date: 12 February 2013

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The indentation behavior of metallic thin film is studied numerically, with attention devoted to cyclic response under a fixed load amplitude. The finite element model consists of an aluminum (Al) film above a silicon (Si) substrate, subject to indentation by a diamond indenter. The rate-dependent viscoplastic model of Al is employed. It is shown that, toward the end of the reloading phase of each cycle, plastic deformation continues to occur and the indenter moves deeper into the material. The advancing rate decreases with time, leading to a stabilized response after about ten cycles, in agreement with experimental measurement. Tracking of local deformation history is carried out, to corroborate with the macroscopic indentation response obtained from modeling and experiment.
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81.40.Lm Deformation, plasticity, and creep
81.40.Np Fatigue, corrosion fatigue, embrittlement, cracking, fracture, and failure
62.20.fq Plasticity and superplasticity
62.20.Qp Friction, tribology, and hardness
68.60.-p Physical properties of thin films, nonelectronic

X-ray photoelectron diffraction study of dopant effects in La0.7X0.3MnO3 (X = La, Sr, Ca, Ce) thin films

C. Raisch, C. Langheinrich, R. Werner, R. Kleiner, D. Koelle, M. Glaser, T. Chassé, and A. Chassé

J. Appl. Phys. 113, 063511 (2013); http://dx.doi.org/10.1063/1.4789988 (7 pages)

Online Publication Date: 13 February 2013

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We present and discuss element-specific x-ray photoelectron diffraction (XPD) patterns of La, Mn, O and the dopant ions Ca, Sr and Ce of various La0.7X0.3MnO (LXMO) films grown epitaxially on SrTiO3(001) substrates. The recorded XPD polar scans are explained in the framework of multiple-scattering cluster calculations, where in general a very good agreement between experimental and theoretical data has been found. Results for all thin films are compatible with a tetragonally distorted cubic perovskite structure with similar MnO6 network. Strong evidence for Mn-site termination was found in all thin films. Dopant locations on A-type sites were clearly confirmed for Sr in LSMO and Ca in LCMO films by means of XPD polar scans. The absence of surface-sensitive Ce3d diffraction features for Ce in LCeMO points to non-equivalent Ce sites and related near-surface disorder.
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79.60.-i Photoemission and photoelectron spectra
81.15.Fg Pulsed laser ablation deposition
82.80.Pv Electron spectroscopy (X-ray photoelectron (XPS), Auger electron spectroscopy (AES), etc.)
61.72.sh Impurity distribution
68.55.aj Insulators

Viscoelastic characterization of low-velocity impact of a solid ball on an agar gel

K. Ara and H. Katsuragi

J. Appl. Phys. 113, 063512 (2013); http://dx.doi.org/10.1063/1.4790872 (10 pages)

Online Publication Date: 13 February 2013

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A viscoelastic characterization method using low-velocity impact is experimentally studied. A steel ball is dropped from a certain height and impacts on an agar gel target with 1–4 m/s velocity. The motion of the impactor ball is captured by a high-speed camera. Instantaneous penetration depth, velocity, and acceleration of the impactor are computed from the high-speed video data. The obtained kinematic data are analyzed in terms of the equation of motion of the impactor. Specifically, we compute the impact viscosity and impact elasticity, assuming a simple impact drag force model. The impact drag force model consists of a linear viscous term, a linear elastic term, and a constant term. From the estimated impact viscosity, we confirm that the Reynolds number is relatively low (less than 10). This low Reynolds number is consistent with the simple linear viscous assumption. From the estimated impact elasticity, we can calculate the speed of sound and the strength of target agar gel. In order to examine the velocity dependence of the elasticity, we also perform very slow (less than 0.1 mm/s) penetration tests using the same agar gel samples. The comparison between impact elasticity and slow penetration elasticity reveals the weak velocity strengthening of agar gel.
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81.40.Jj Elasticity and anelasticity, stress-strain relations
81.70.Bt Mechanical testing, impact tests, static and dynamic loads
62.20.D- Elasticity
62.40.+i Anelasticity, internal friction, stress relaxation, and mechanical resonances

Defect dynamics in polycrystalline zirconium alloy probed in situ by primary extinction of neutron diffraction

Saurabh Kabra, Kun Yan, David G. Carr, Robert P. Harrison, Rian J. Dippenaar, Mark Reid, and Klaus-Dieter Liss

J. Appl. Phys. 113, 063513 (2013); http://dx.doi.org/10.1063/1.4790177 (8 pages)

Online Publication Date: 13 February 2013

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After α + β-zirconium has fully transformed into β-phase upon heating, the intensities of all β-Zr Bragg reflections decrease simultaneously as a function of time. It is shown that this effect represents a transition from the kinematic to the dynamic theory of diffraction due to the ever increasing crystal perfection driven by thermal recovery of the system. The best fitting coherent crystallite size of 30 μm and other microstructural features are verified by in situ laser scanning confocal microscopy. This effect of primary extinction in neutron diffraction has been employed to further investigate the crystal perfection kinetics. Upon further heating, crystal recovery is identified as a process of dislocation annihilation, suffering from lattice friction. Upon cooling, precipitating α-Zr induces strain into the perfect β-crystallites, re-establishing the kinematic diffraction intensities. An Avrami analysis leads to the estimations of nucleation time, consumption of nucleation sites and lower-dimensional growth. Such technique bears great value for further investigation on all metal systems annealed close to the melting temperature.
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61.72.Ff Direct observation of dislocations and other defects (etch pits, decoration, electron microscopy, x-ray topography, etc.)
62.20.Qp Friction, tribology, and hardness
81.40.Gh Other heat and thermomechanical treatments
81.40.Pq Friction, lubrication, and wear
64.70.dj Melting of specific substances

Band structure and the optical gain of GaInNAs/GaAs quantum wells modeled within 10-band and 8-band kp model

M. Gladysiewicz, R. Kudrawiec, J. M. Miloszewski, P. Weetman, J. Misiewicz, and M. S. Wartak

J. Appl. Phys. 113, 063514 (2013); http://dx.doi.org/10.1063/1.4790568 (11 pages)

Online Publication Date: 13 February 2013

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The band structure and optical gain have been calculated for GaInNAs/GaAs quantum wells (QWs) with various nitrogen concentrations within the 10-band and 8-band kp models. Two approaches to calculate optical properties of GaInNAs/GaAs QWs have been compared and discussed in the context of available material parameters for dilute nitrides and the conduction band nonparabolicity due to the band anti-crossing (BAC) interaction between the N-related resonant level and the conduction band of a host material. It has been clearly shown that this nonparabolicity can be neglected in optical gain calculations since the dispersion of conduction band up to the Femi level is very close to parabolic for carrier concentrations typical for laser operation, i.e., 5 × 1018 cm−3. This means that the 8-band kp model when used to calculate the optical gain is very realistic and much easier to apply in QWs containing new dilute nitrides for which the BAC parameters are unknown. In such an approach, the energy gap and electron effective mass for N-containing materials are needed, instead of BAC parameters. These parameters are available experimentally much easier than BAC parameters.
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78.67.De Quantum wells
61.72.uj III-V and II-VI semiconductors
71.20.Nr Semiconductor compounds
73.20.At Surface states, band structure, electron density of states
73.21.Fg Quantum wells

Thermodynamic stabilization of nanocrystalline binary alloys

Mostafa Saber, Hasan Kotan, Carl C. Koch, and Ronald O. Scattergood

J. Appl. Phys. 113, 063515 (2013); http://dx.doi.org/10.1063/1.4791704 (10 pages)

Online Publication Date: 13 February 2013

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The work presented here was motivated by the need to develop a predictive model for thermodynamic stabilization of binary alloys that is applicable to strongly segregating size-misfit solutes, and that can use available input data for a wide range of solvent-solute combinations. This will serve as a benchmark for selecting solutes and assessing the possible contribution of thermodynamic stabilization for development of high-temperature nanocrystalline alloys. Following a regular solution model that distinguishes the grain boundary and grain interior volume fractions by a transitional interface in a closed system, we include both the chemical and elastic strain energy contributions to the mixing enthalpy ΔHmix using an appropriately scaled linear superposition. The total Gibbs mixing free energy ΔGmix is minimized with respect to simultaneous variations in the grain-boundary volume fraction and the solute contents in the grain boundary and grain interior. The Lagrange multiplier method was used to obtain numerical solutions with the constraint of fixed total solute content. The model predictions are presented using a parametric variation of the required input parameters. Applications are then given for the dependence of the nanocrystalline grain size on temperature and total solute content for selected binary systems where experimental results suggest that thermodynamic stabilization could be effective.
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65.80.-g Thermal properties of small particles, nanocrystals, nanotubes, and other related systems
61.72.Mm Grain and twin boundaries
81.40.Jj Elasticity and anelasticity, stress-strain relations
62.25.-g Mechanical properties of nanoscale systems
64.75.Ef Mixing
64.75.Jk Phase separation and segregation in nanoscale systems

Shock response of a model structured nanofoam of Cu

F. P. Zhao, Q. An, B. Li, H. A. Wu, W. A. Goddard, III, and S. N. Luo

J. Appl. Phys. 113, 063516 (2013); http://dx.doi.org/10.1063/1.4791758 (8 pages)

Online Publication Date: 13 February 2013

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Using large-scale molecular dynamics simulations, we investigate shock response of a model Cu nanofoam with cylindrical voids and a high initial porosity (50% theoretical density), including elastic and plastic deformation, Hugoniot states, shock-induced melting, partial or complete void collapse, nanojetting, and hotspot formation. The elastic-plastic and overtaking shocks are observed at different shock strengths. The simulated Hugoniot states can be described with a modified, power-law P−α (pressure–porosity) model, and agree with shock experiments on Cu powders, as well as the compacted Hugoniot predicted with the Grüneisen equation of state. Shock-induced melting shows no clear signs of bulk premelting or superheating. Voids collapse via plastic flow nucleated from voids, and the exact processes are shock strength dependent. With increasing shock strengths, void collapse transits from the “geometrical” mode (collapse of a void is dominated by crystallography and void geometry and can be different from that of one another) to “hydrodynamic” mode (collapse of a void is similar to one another); the collapse may be achieved predominantly by flow along the {111} slip planes, by way of alternating compression and tension zones, by means of transverse flows, via forward and transverse flows, or through forward nanojetting. The internal jetting induces pronounced shock front roughening, leading to internal hotspot formation and sizable high speed jets on atomically flat free surfaces.
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82.70.Rr Aerosols and foams
62.20.fq Plasticity and superplasticity
62.50.Ef Shock wave effects in solids and liquids
61.20.Ja Computer simulation of liquid structure
81.40.Jj Elasticity and anelasticity, stress-strain relations
81.40.Lm Deformation, plasticity, and creep

Electronic band structures of Ge1−xSnx semiconductors: A first-principles density functional theory study

Ming-Hsien Lee, Po-Liang Liu, Yung-An Hong, Yen-Ting Chou, Jia-Yang Hong, and Yu-Jin Siao

J. Appl. Phys. 113, 063517 (2013); http://dx.doi.org/10.1063/1.4790362 (5 pages)

Online Publication Date: 13 February 2013

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We conduct first-principles total-energy density functional calculations to study the band structures in Ge1−xSnx infrared semiconductor alloys. The norm-conserving optimized pseudopotentials of Ge and Sn have been constructed for electronic structure calculations. The composition-bandgap relationships in Ge1−xSnx lattices are evaluated by a detailed comparison of structural models and their electronic band structures. The critical Sn composition related to the transition from indirect- to direct-gap in Ge1−xSnx alloys is estimated to be as low as x ∼ 0.016 determined from the parametric fit. Our results show that the crossover Sn concentration occurs at a lower critical Sn concentration than the values predicted from the absorption measurements. However, early results indicate that the reliability of the critical Sn concentration from such measurements is hard to establish, since the indirect gap absorption is much weaker than the direct gap absorption. We find that the direct band gap decreases exponentially with the Sn composition over the range 0 <x<0.375 and the alloys become metallic for x> 0.375, in very good agreement with the theoretical observed behavior [D. W. Jenkins and J. D. Dow, Phys. Rev. B 36, 7994, 1987]. For homonuclear and heteronuclear complexes of Ge1−xSnx alloys, the indirect band gap at L-pointis is found to decrease homonuclear Ge-Ge bonds or increase homonuclear Sn-Sn bonds as a result of the reduced L valley. All findings agree with previously reported experimental and theoretical results. The analysis suggests that the top of valence band exhibits the localization of bond charge and the bottom of the conduction band is composed of the Ge 4s4p and/or Sn 5s5p atomic orbits.
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71.20.Nr Semiconductor compounds
71.15.Dx Computational methodology (Brillouin zone sampling, iterative diagonalization, pseudopotential construction)
71.15.Mb Density functional theory, local density approximation, gradient and other corrections
71.15.Nc Total energy and cohesive energy calculations
71.20.Gj Other metals and alloys

Raman and photoluminescence spectroscopy of Si nanocrystals: Evidence of a form factor

Giuseppe Faraci, Giovanni Mannino, Agata R. Pennisi, Rosa Ruggeri, Paolo Sberna, and Vittorio Privitera

J. Appl. Phys. 113, 063518 (2013); http://dx.doi.org/10.1063/1.4792063 (5 pages)

Online Publication Date: 14 February 2013

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We investigated the quantum confinement in Si nanocrystals embedded in a SiO2 matrix. The size was accurately controlled in the range 3–8 nm by annealing at high temperature Si/SiO2 multilayers fabricated by chemical vapour deposition. Raman shift and line width were compared with existing theoretical models for each cluster size. We found evidence of uni-dimensional confinement in 3 nm crystals, whereas for 4.5 nm crystals the confinement appears three-dimensional. This conclusion is supported by the luminescence spectra shifting towards higher wavelengths for the smaller size, in opposite direction for larger sizes.
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78.67.Pt Multilayers; superlattices; photonic structures; metamaterials
81.40.Gh Other heat and thermomechanical treatments
68.65.Ac Multilayers
78.30.Hv Other nonmetallic inorganics
78.55.Hx Other solid inorganic materials

Influence of the supersaturation on Si diffusion and growth of Si nanoparticles in silicon-rich silica

M. Roussel, E. Talbot, P. Pareige, and F. Gourbilleau

J. Appl. Phys. 113, 063519 (2013); http://dx.doi.org/10.1063/1.4792218 (7 pages)

Online Publication Date: 14 February 2013

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SiOX/SiO2 multilayers have been prepared using magnetron sputtering and annealed in order to induce the growth of Si nanoparticles in Si-rich sublayers. This sample has undergone several successive annealing treatments and has been analyzed using a laser-assisted tomographic atom probe. This allows the phase separation between Si and SiO2 and the growth process to be studied at the atomic scale as a function of annealing temperature. Si diffusion coefficient is estimated from the accurate measurement of matrix composition and Si particle size. We demonstrate that the diffusion coefficient in SiOX is supersaturation dependent, leading to a decrease in silicon particle growth kinetics during annealing. In addition, we use our measurements to predict the critical thickness for efficient SiO2 diffusion barriers.
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81.16.-c Methods of micro- and nanofabrication and processing
61.46.Df Structure of nanocrystals and nanoparticles ("colloidal" quantum dots but not gate-isolated embedded quantum dots)
61.72.Cc Kinetics of defect formation and annealing
68.35.Fx Diffusion; interface formation
68.65.Ac Multilayers
81.05.Cy Elemental semiconductors

Mid-infrared absorptance of silicon hyperdoped with chalcogen via fs-laser irradiation

Meng-Ju Sher, Yu-Ting Lin, Mark T. Winkler, Eric Mazur, Christian Pruner, and Augustinus Asenbaum

J. Appl. Phys. 113, 063520 (2013); http://dx.doi.org/10.1063/1.4790808 (6 pages)

Online Publication Date: 14 February 2013

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Silicon hyperdoped with heavy chalcogen atoms via femtosecond-laser irradiation exhibits strong broadband, sub-bandgap light absorption. Understanding the origin of this absorption could enable applications for hyperdoped-silicon based optoelectronic devices. In this work, we measure absorption to wavelengths up to 14 μm using Fourier transform infrared spectroscopy and study sulfur-, selenium-, and tellurium-hyperdoped Si before and after annealing. We find that absorption in the samples extends to wavelengths as far as 6 μm. After annealing, the absorption spectrum exhibits features that are consistent with free-carrier absorption. Although the surface morphology influences the shape of the absorption curves, the data permit us to place an upper bound on the position of the chalcogen dopant energy levels.
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61.72.U- Doping and impurity implantation
68.35.B- Structure of clean surfaces (and surface reconstruction)
61.72.Cc Kinetics of defect formation and annealing
78.30.Am Elemental semiconductors and insulators
78.30.Hv Other nonmetallic inorganics
81.05.Cy Elemental semiconductors
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