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15 Aug 2009

Volume 106, Issue 4, Articles (04xxxx)

Issue Cover Spotlight Figure

J. Appl. Phys. 106, 041101 (2009); http://dx.doi.org/10.1063/1.3207769 (21 pages)

Robert C. Runkle, L. Eric Smith, and Anthony J. Peurrung
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Effects of tube diameter and chirality on the stability of single-walled carbon nanotubes under ion irradiation

Zijian Xu, Wei Zhang, Zhiyuan Zhu, Cuilan Ren, Yong Li, and Ping Huai

J. Appl. Phys. 106, 043501 (2009); http://dx.doi.org/10.1063/1.3194784 (13 pages) | Cited 5 times

Online Publication Date: 17 August 2009

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Using molecular dynamics method, we investigated the influence of tube diameter and chirality on the stability of single-walled carbon nanotubes (CNTs) under ion irradiation. We found that in the energy range below 1 keV, the dependence of CNT stability on the tube diameter is no longer monotonic under C ion irradiation, and the thinner (5, 5) CNT may be more stable than the thicker (7, 7) CNT, while under Ar irradiation, the CNT stability increases still monotonically with the CNT diameter. This stability behavior was further verified by the calculations of the threshold ion energies to produce displacement damage in CNTs. The abnormal stability of thin CNTs is related to their resistance to the instantaneous deformation in the wall induced by ion pushing, the high self-healing capacity, as well as the different interaction properties of C and Ar ions with CNT atoms. We also found that under ion irradiation the stability of a zigzag CNT is better than that of an armchair CNT with the same diameter. This is because of the bonding structure difference between the armchair and the zigzag CNTs with respect to the orientations of graphitic networks as well as the self-healing capacity difference.
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61.82.Rx Nanocrystalline materials
61.48.De Structure of carbon nanotubes, boron nanotubes, and other related systems
61.80.Jh Ion radiation effects
62.25.-g Mechanical properties of nanoscale systems

A plasmonic “ac Wheatstone bridge” circuit for high-sensitivity phase measurement and single-molecule detection

T. J. Davis, K. C. Vernon, and D. E. Gómez

J. Appl. Phys. 106, 043502 (2009); http://dx.doi.org/10.1063/1.3195071 (6 pages) | Cited 6 times

Online Publication Date: 17 August 2009

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In this paper, a plasmonic “ac Wheatstone bridge” circuit is proposed and theoretically modeled for the first time. The bridge circuit consists of three metallic nanoparticles, shaped as rectangular prisms, with two nanoparticles acting as parallel arms of a resonant circuit and the third bridging nanoparticle acting as an optical antenna providing an output signal. Polarized light excites localized surface plasmon resonances in the two arms of the circuit, which generate an optical signal dependent on the phase-sensitive excitations of surface plasmons in the antenna. The circuit is analyzed using a plasmonic coupling theory and numerical simulations. The analyses show that the plasmonic circuit is sensitive to phase shifts between the arms of the bridge and has the potential to detect the presence of single molecules.
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84.30.-r Electronic circuits
84.40.Ba Antennas: theory, components and accessories

Ambipolar diffusion and spatial and time-resolved spectroscopies in semiconductor heterostructures

Áurea R. Vasconcellos, M. J. S. P. Brasil, Roberto Luzzi, A. A. P. Silva, and A. H. S. Leite

J. Appl. Phys. 106, 043503 (2009); http://dx.doi.org/10.1063/1.3173176 (9 pages) | Cited 2 times

Online Publication Date: 17 August 2009

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An analysis of the hydrodynamic motion of the fluid of photoinjected carriers in polar semiconductors is presented. Experiments of time-resolved photoluminescence, which provide relevant insights into the dynamical behavior of heterostructures, are analyzed. We study the propagation and recombination of carriers in semiconductor devices with a large cap layer, where carriers are photoinjected, and a quantum well where they recombine. The movement of the photoinjected, and away from equilibrium, carriers along such cap layer consists, to a good degree of approximation, in an ambipolar diffusivelike one, which decays in time as a result of recombination and the coupling, via Coulomb interaction, with the optical and acoustic plasma waves. The density of the electrons arriving at the interface with the quantum well can be determined; these electrons are transferred through the interface to recombine in the quantum well, and the resulting intensity of the time-resolved luminescence is obtained. Comparison with experimental data shows a good agreement.
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78.47.D- Time resolved spectroscopy (>1 psec)
73.63.Hs Quantum wells
73.40.Kp III-V semiconductor-to-semiconductor contacts, p-n junctions, and heterojunctions
66.30.Ny Chemical interdiffusion; diffusion barriers
78.66.Fd III-V semiconductors
78.55.Cr III-V semiconductors

Functionally grading the shape memory response in NiTi films: Laser irradiation

A. J. Birnbaum, G. Satoh, and Y. L. Yao

J. Appl. Phys. 106, 043504 (2009); http://dx.doi.org/10.1063/1.3183950 (8 pages) | Cited 3 times

Online Publication Date: 18 August 2009

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A new process and mechanism are presented for controlling the shape memory response spatially within monolithic NiTi thin film structures. This technique is shown to effectively control the martensitic phase transformation temperature and exhibits control over aspects of the mechanical and shape memory responses as well. Specifically, the martensitic phase transformation temperature decreases with incident laser energy density. Concomitant modifications are observed in both the mechanical and shape memory responses in laser processed films. Analysis and characterization are performed via temperature controlled optical microscopy, x-ray diffraction, atomic force microscopy, and nanoindentation.
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81.30.Kf Martensitic transformations
81.40.Lm Deformation, plasticity, and creep
62.20.fg Shape-memory effect; yield stress; superelasticity
64.70.kd Metals and alloys
68.60.Bs Mechanical and acoustical properties
61.80.Ba Ultraviolet, visible, and infrared radiation effects (including laser radiation)

Amorphization of crystalline Si due to heavy and light ion irradiation

P. D. Edmondson, D. J. Riley, R. C. Birtcher, and S. E. Donnelly

J. Appl. Phys. 106, 043505 (2009); http://dx.doi.org/10.1063/1.3195081 (8 pages) | Cited 6 times

Online Publication Date: 19 August 2009

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The formation of amorphous silicon in crystalline silicon by bombardment with light (Si) and heavy (Xe) ions has been investigated by transmission electron microscopy with in situ ion irradiation. Experiments have been carried out at room temperature and low temperature (50 K) and the results are compared to a simple numerical model for amorphization. The results indicate that the amorphization mechanisms for both irradiations are heterogeneous in nature and that numerous overlaps of the collision cascade are generally required to render the crystal amorphous. Following from this, the nature of the material within the confines of collision cascades will be discussed and it will be shown that the individual cascade volume is not necessarily amorphous as previously described in the scientific literature but contains varying degrees of damage depending on the energy deposited within the cascade.
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64.70.kg Semiconductors
61.80.Jh Ion radiation effects

Experimental and theoretical studies on the elasticity of molybdenum to 12 GPa

Wei Liu, Qiong Liu, Matthew L. Whitaker, Yusheng Zhao, and Baosheng Li

J. Appl. Phys. 106, 043506 (2009); http://dx.doi.org/10.1063/1.3197135 (4 pages) | Cited 2 times

Online Publication Date: 19 August 2009

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Experiments have been conducted to measure compressional (VP) and shear wave (VS) velocities as well as unit-cell volumes (densities) of molybdenum to 12.0 GPa at room temperature using ultrasonic interferometry in conjunction with synchrotron x-radiation. Both VP and VS as well as the adiabatic bulk (KS) and shear (G) moduli exhibit monotonic increase with increasing pressure. A finite strain equation of state analysis of the directly measured velocities and densities yields KS0 = 260.7(5) GPa, G0 = 125.1(2) GPa, KS0′ = 4.7(1), and G0′ = 1.5(1) for the elastic bulk and shear moduli and their pressure derivatives at ambient conditions. Complimentary to the experimental data, VP and VS as well as the elastic bulk and shear moduli were also computed using density functional theory (DFT) at pressures comparable to the current experiment. Comparing with experimental results, the velocities and elastic moduli from DFT calculations exhibit close agreement with the current experimental data both in their values as well as in their pressure dependence.
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81.40.Jj Elasticity and anelasticity, stress-strain relations
62.20.de Elastic moduli
81.40.Lm Deformation, plasticity, and creep
62.20.F- Deformation and plasticity
62.50.-p High-pressure effects in solids and liquids

Re-entrance phase and excited metastable electronic spin states in one-dimensional spin crossover compounds explained by atom-phonon coupling model

Aurelian Rotaru, Jorge Linares, Sebastien Mordelet, Alexandru Stancu, and Jamil Nasser

J. Appl. Phys. 106, 043507 (2009); http://dx.doi.org/10.1063/1.3186055 (5 pages) | Cited 4 times

Online Publication Date: 19 August 2009

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An unusual behavior of the disappearance and the reappearance of the hysteresis loop of the spin crossover compounds under external pressure was observed experimentally by Garcia et al. [Hyperfine Interact. 139/140, 543 (2002)] and modeled within the framework of the “atom-phonon coupling” model. In this contribution we explain the evolution of the hysteresis width versus the applied pressure. Using the same model, we also show the existence of stable as well as metastable and unstable states at very low temperatures. These excited metastable electronic spin states at low temperature can be associated to the “out of equilibrium” states in the light-induced electronic spin state trapping.
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75.30.Wx Spin crossover
75.30.Kz Magnetic phase boundaries (including classical and quantum magnetic transitions, metamagnetism, etc.)
75.40.Cx Static properties (order parameter, static susceptibility, heat capacities, critical exponents, etc.)
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
63.20.K- Phonon interactions

Structural properties of tensily strained Si layers grown on SiGe(100), (110), and (111) virtual substrates

V. Destefanis, D. Rouchon, J. M. Hartmann, A. M. Papon, L. Baud, A. Crisci, and M. Mermoux

J. Appl. Phys. 106, 043508 (2009); http://dx.doi.org/10.1063/1.3187925 (10 pages) | Cited 5 times

Online Publication Date: 19 August 2009

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We have studied the structural properties of tensily strained Si (t-Si) layers grown by reduced pressure–chemical vapor deposition on top of SiGe(100), (110), and (111) virtual substrates (VSs). Chemical mechanical planarization has been used beforehand to eliminate the as-grown surface crosshatch on all orientations and reduce by 10 up to 100 times the surface roughness. A definite surface roughening has occurred after the epitaxy of t-Si on (110) and (111). For the lowest Ge contents investigated, top Si(100) and (110) layers are locally “defect-free” whereas numerous {111} stacking faults are present in the t-Si(111) layers. For higher Ge content SiGe VS, a degradation of the crystallographic quality of (110) and (111) t-Si layers has been evidenced, with the presence of dislocations, stacking faults, and twins. Quantification of the strain level in the t-Si layers has been carried out using visible and near-UV Raman spectroscopy. The Ge contents in the VS determined by Raman spectroscopy were very close to the ones previously obtained by secondary ion mass spectrometry or x-ray diffraction. Stress values obtained for t-Si(100) layers were whatever the Ge content similar to those expected. Stress values corresponding to pseudomorphic t-Si growths have been obtained on (110) and (111) SiGe VSs, for Ge contents up to 35% and 25%, respectively. The stress values obtained on (110) surfaces for such Ge contents were high, with a noticeable anisotropy along the [001] and [1-10] directions. Degradations of the (110) and (111) Raman profiles likely coming from twin-assisted strain relaxation have been noticed for t-Si layers on SiGe VS with Ge contents higher than 35% and 25%, respectively. UV and visible Raman mapping of the growth plane strain fluctuations has finally been carried out. Original surface arrays have been highlighted for each surface orientation. Such strain fields are related to the plastic relaxation of strain in the SiGe graded layer underneath through the emission of misfit dislocations, twins, and stacking faults. Promising results have been obtained for t-Si layers on (110) SiGe VS while the technological usefulness of the (111) ones is more questionable.
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68.55.ag Semiconductors
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
61.72.Mm Grain and twin boundaries
81.65.Ps Polishing, grinding, surface finishing
78.30.Am Elemental semiconductors and insulators
78.66.Db Elemental semiconductors and insulators

Ultraviolet Raman microscopy of single and multilayer graphene

Irene Calizo, Igor Bejenari, Muhammad Rahman, Guanxiong Liu, and Alexander A. Balandin

J. Appl. Phys. 106, 043509 (2009); http://dx.doi.org/10.1063/1.3197065 (5 pages) | Cited 45 times

Online Publication Date: 20 August 2009

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We investigated Raman spectra of single-layer and multilayer graphene under ultraviolet laser excitation at the wavelength λ = 325 nm. It was found that while graphene’s G peak remains pronounced in UV Raman spectra, the 2D-band intensity undergoes severe quenching. The evolution of the ratio of the intensities of the G and 2D peaks, I(G)/I(2D), as the number of graphene layers n changes from n = 1 to n = 5, is different in UV Raman spectra from that in conventional visible Raman spectra excited at the 488 and 633 nm wavelengths. The 2D band under UV excitation shifts to larger wave numbers and is found near 2825 cm−1. The observed UV Raman features of graphene were explained by invoking the resonant scattering model. The obtained results contribute to the Raman nanometrology of graphene by providing an additional metric for determining the number of graphene layers and assessing its quality.
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78.67.Pt Multilayers; superlattices; photonic structures; metamaterials
78.55.Hx Other solid inorganic materials
78.30.Na Fullerenes and related materials

Structural, magnetic, and energetic properties of Na2FePO4F, Li2FePO4F, NaFePO4F, and LiFePO4F from ab initio calculations

M. Ramzan, S. Lebègue, P. Larsson, and R. Ahuja

J. Appl. Phys. 106, 043510 (2009); http://dx.doi.org/10.1063/1.3202384 (6 pages) | Cited 5 times

Online Publication Date: 21 August 2009

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In this paper, we report on Na2FePO4F and Li2FePO4F, which are materials that are used as cathodes in batteries, using density functional theory with the LDA, LDA+U, GGA, or GGA+U approximations. Specifically, we study their crystal structure, electronic structure, and magnetic properties and provide similar information about the intermediate compounds LiFePO4F and NaFePO4F. Finally, the intercalation voltages of the corresponding batteries are calculated using various exchange-correlation approximations and conclusions are drawn about which one is the most suitable to use for the study of this class of materials.
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71.20.Ps Other inorganic compounds
71.15.Mb Density functional theory, local density approximation, gradient and other corrections
82.47.Cb Lead-acid, nickel-metal hydride and other batteries
82.45.Fk Electrodes
71.70.Gm Exchange interactions
61.66.Fn Inorganic compounds

Measurement of the intersystem crossing rate in aluminum tris(8-hydroxyquinoline) and its modulation by an applied magnetic field

Sijie Zhang, Jingyao Song, T. Kreouzis, and W. P. Gillin

J. Appl. Phys. 106, 043511 (2009); http://dx.doi.org/10.1063/1.3204015 (6 pages) | Cited 6 times

Online Publication Date: 21 August 2009

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The rate constant for intersystem crossing in aluminum tris(8-hydroxyquinoline) was measured using the time dependence of the luminescence under high excitation intensity and modeling using a rate equation approach. Under high illumination levels intersystem crossing results in the transfer of singlets into triplets, which due to their long lifetime effectively remove molecules from participating in photoluminescence. The intersystem crossing rate was found to be ∼ 2.2×104 s−1 at 80 K. The presence of a magnetic field was found to increase the rate constant by ∼ 10% with applied fields of ∼ 100 mT.
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78.55.Kz Solid organic materials

Concentration dependence of the Er3+ visible and infrared luminescence in Y2−xErxO3 thin films on Si

R. Lo Savio, M. Miritello, P. Cardile, and F. Priolo

J. Appl. Phys. 106, 043512 (2009); http://dx.doi.org/10.1063/1.3195077 (10 pages) | Cited 9 times

Online Publication Date: 24 August 2009

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Y2−xErxO3 thin films, with x varying between 0 and 0.72, have been successfully grown on crystalline silicon (c-Si) substrates by radio-frequency magnetron cosputtering of Y2O3 and Er2O3 targets. As-deposited films are polycrystalline, showing the body-centered cubic structure of Y2O3, and show only a slight lattice parameter contraction when x is increased, owing to the insertion of Er ions. All the films exhibit intense Er-related optical emission at room temperature both in the visible and infrared regions. By studying the optical properties for different excitation conditions and for different Er contents, all the mechanisms (i.e., cross relaxations, up-conversions, and energy transfers to impurities) responsible for the photoluminescence (PL) emission have been identified, and the existence of two different well-defined Er concentration regimes has been demonstrated. In the low concentration regime (x up to 0.05, Er-doped regime), the visible PL emission reaches its highest intensity, owing to the influence of up-conversions, thus giving the possibility of using Y2−xErxO3 films as an up-converting layer in the rear of silicon solar cells. However, most of the excited Er ions populate the first two excited levels 4I11/2 and 4I13/2, and above a certain excitation flux a population inversion condition between the former and the latter is achieved, opening the route for the realization of amplifiers at 2.75 μm. Instead, in the high concentration regime (Er-compound regime), an increase in the nonradiative decay rates is observed, owing to the occurrence of cross relaxations or energy transfers to impurities. As a consequence, the PL emission at 1.54 μm becomes the most intense, thus determining possible applications for Y2−xErxO3 as an infrared emitting material.
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78.55.Hx Other solid inorganic materials
78.66.Nk Insulators
68.55.A- Nucleation and growth
61.72.sd Impurity concentration
81.15.Cd Deposition by sputtering
84.60.Jt Photoelectric conversion

Mechanical properties and electronic structure of superhard diamondlike BC5: A first-principles study

Yun-Jiang Wang and Chong-Yu Wang

J. Appl. Phys. 106, 043513 (2009); http://dx.doi.org/10.1063/1.3195082 (5 pages) | Cited 7 times

Online Publication Date: 25 August 2009

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The mechanical properties and electronic structure of diamondlike BC5 are studied by ab initio method. The results for elastic moduli, stress-strain relationships, and ideal strengths are presented. The calculated properties agree well with the existing experiments. BC5 possesses very large elastic moduli and ideal strengths, suggesting its intrinsical superhard nature. Its ideal tensile strengths are much larger than ideal shear strengths. Electronic structures show that strong covalent bond network through sp3 hybridization is the origin of excellent mechanical properties of BC5. The failure of BC5 is attributed to the breaking of the B–C bond. BC5 is predicted to be metallic and remains its electronic properties at finite strain. Our results show that BC5 has good prospect in electronic application as a superhard material.
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62.20.de Elastic moduli
81.40.Jj Elasticity and anelasticity, stress-strain relations
71.15.-m Methods of electronic structure calculations
71.20.Ps Other inorganic compounds
62.20.F- Deformation and plasticity

Analysis of nonhydrostatic high-pressure diffraction data (cubic system): Assessment of various assumptions in the theory

Anil K. Singh

J. Appl. Phys. 106, 043514 (2009); http://dx.doi.org/10.1063/1.3197213 (8 pages) | Cited 9 times

Online Publication Date: 25 August 2009

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The mathematical formulation commonly used to analyze the high-pressure diffraction data from the sample under nonhydrostatic compression is based on three assumptions: A1—a weighted harmonic mean of the diffraction shear moduli under Reuss and Voigt limits with a weight parameter α that lies between 0.5 and 1 describes adequately the diffraction shear modulus; A2—a stress tensor with only the diagonal terms describes the stress state at the center of the sample under nonhydrostatic compression; and A3—the lattice-strain equations derived using only the linear elasticity theory are adequate to derive strength and elastic moduli from the diffraction data. To examine A1 we derive compressive strength, diffraction shear moduli, and single-crystal elastic moduli from the experimental high-pressure x-ray diffraction data on bcc Fe, Au, Mo, and FeO. These data contain plastic deformation effects. The diffraction shear modulus in the limit of small deformation (elastic) is computed using rigorous formulae derived by Kröner [Z. Phys. 151, 504 (1958)] and de Wit [J. Appl. Crystallogr. 30, 510 (1997)] . The elastic moduli are derived from the computed shear moduli assuming the validity of A1. The results show that A1 with α ≅ 0.5 is valid for small deformation in all four cases. The analysis of the experimental data suggests that A1 is valid with α<1 for solids with x>1 where x = 2C44/(C11−C12); for solids with x<1, the validity of A1 requires α>1. At least for solids of the cubic system, the effect of plastic deformation appears to be fully contained in a single parameter α. In practice, deviations from A2 of varying magnitudes occur mainly because of the difficulty in avoiding diffraction from regions of stress gradient in the sample. A discussion of A3 is presented.
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62.50.-p High-pressure effects in solids and liquids
81.40.Vw Pressure treatment
61.05.cp X-ray diffraction
81.40.Jj Elasticity and anelasticity, stress-strain relations
62.20.de Elastic moduli
61.66.Fn Inorganic compounds
61.66.Bi Elemental solids
81.40.Lm Deformation, plasticity, and creep
62.20.fq Plasticity and superplasticity

Carrier lifetime and spin relaxation time study for electrical spin injection into GaAs

Eunsoon Oh, T. K. Lee, J. H. Park, J. H. Choi, Y. J. Park, K. H. Shin, and K. Y. Kim

J. Appl. Phys. 106, 043515 (2009); http://dx.doi.org/10.1063/1.3186026 (4 pages) | Cited 3 times

Online Publication Date: 25 August 2009

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We fabricated spin light emitting diodes using oxide tunneling barriers between ferromagnetic materials (Ni0.8Fe0.2/Co0.9Fe0.1) and semiconductors (GaAs) and investigated the temperature-dependent carrier lifetime and spin relaxation time of the active GaAs layer. We observed the circular polarization of the free exciton from the electroluminescence spectra due to the spin injection from the ferromagnetic material, whereas the circular polarization of the conduction band to acceptor transition was negligible. From the temperature-dependent carrier lifetime and spin relaxation time of the active GaAs layer, we found that the spin injection efficiency was larger than 25% between 20 and 180 K, where the magnetic field dependence of the spin lifetime was ignored.
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85.60.Jb Light-emitting devices
75.50.Bb Fe and its alloys
78.60.Fi Electroluminescence
72.25.Mk Spin transport through interfaces
72.25.Rb Spin relaxation and scattering
72.25.Hg Electrical injection of spin polarized carriers

Strain evolution in Si substrate due to implantation of MeV ion observed by extremely asymmetric x-ray diffraction

T. Emoto, J. Ghatak, P. V. Satyam, and K. Akimoto

J. Appl. Phys. 106, 043516 (2009); http://dx.doi.org/10.1063/1.3202329 (5 pages) | Cited 4 times

Online Publication Date: 25 August 2009

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We studied the strain introduced in a Si(111) substrate due to MeV ion implantation using extremely asymmetric x-ray diffraction and measured the rocking curves of asymmetrical 113 diffraction for the Si substrates implanted with a 1.5 MeV Au2+ ion at fluence values of 1×1013, 5×1013, and 1×1014/cm2. The measured curves consisted of a bulk peak and accompanying subpeak with an interference fringe. The positional relationship of the bulk peak to the subpeak and the intensity variation of those peaks with respect to the wavelengths of the x rays indicated that crystal lattices near the surface were strained; the lattice spacing of surface normal (111) planes near the surface was larger than that of the bulk. Detailed strain profiles along the depth direction were successfully estimated using a curve-fitting method based on Darwin’s dynamical diffraction theory. Comparing the shapes of resultant strain profiles, we found that a strain evolution rapidly occurred within a depth of ∼ 300 nm at fluence values between 1×1013 and 5×1013/cm2. This indicates that formation of the complex defects progressed near the surface when the fluence value went beyond a critical value between 1×1013 and 5×1013/cm2 and the defects brought a large strain to the substrate.
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81.05.Cy Elemental semiconductors
68.35.Gy Mechanical properties; surface strains
61.72.uf Ge and Si
61.72.Dd Experimental determination of defects by diffraction and scattering
81.40.Lm Deformation, plasticity, and creep
62.20.F- Deformation and plasticity

Effect of Ta migration from sidewall barrier on leakage current in Cu/SiOCH low-k dielectrics

T. L. Tan, C. L. Gan, A. Y. Du, and C. K. Cheng

J. Appl. Phys. 106, 043517 (2009); http://dx.doi.org/10.1063/1.3202387 (4 pages) | Cited 3 times

Online Publication Date: 25 August 2009

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Tantalum migration from the anode sidewall barrier was observed in Cu interconnects/SiOCH low-k dielectrics after voltage ramp and constant voltage stress. This phenomenon was found to contribute to leakage current as observed from the current-voltage (I-V) and current-time (I-t) leakage curves. The mechanism of Ta migration is proposed to be due to the ease of Ta oxidation that converts the Ta atoms into ions, which subsequently drift along with the applied electric field and into the SiOCH low-k dielectric. The Ta ionic drift and diffusion into the dielectric led to an increase in leakage current, although the eventual formation of a stable Ta oxide resulted in a leakage saturation.
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73.40.Ns Metal-nonmetal contacts
81.65.Mq Oxidation
68.35.Fx Diffusion; interface formation
85.40.Ls Metallization, contacts, interconnects; device isolation
66.30.Ny Chemical interdiffusion; diffusion barriers
77.84.Bw Elements, oxides, nitrides, borides, carbides, chalcogenides, etc.

Temperature-dependent study of photocurrent signal in CdGa2Se4 layers

S. H. You, K. J. Hong, T. S. Jeong, and C. J. Youn

J. Appl. Phys. 106, 043518 (2009); http://dx.doi.org/10.1063/1.3204027 (5 pages) | Cited 1 time

Online Publication Date: 26 August 2009

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The photoconductive CdGa2Se4 layer has been investigated using photocurrent (PC) spectroscopy as a function of temperature. Three peaks corresponding to the band-to-band transitions were observed in the PC spectra for all temperature ranges. From the relations of peak position and temperature, the temperature dependence of the band-gap energy is precisely discussed. Also, contrary to our expectation, the PC intensities decreased with decreasing temperatures. From the relation of log Jph versus 1/T, where Jph is the PC density, two dominant levels by the exponential variation in the PC with varying temperature were observed, one at high temperatures and the other at low temperatures.
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72.40.+w Photoconduction and photovoltaic effects
68.55.ag Semiconductors
71.20.Nr Semiconductor compounds
81.05.Hd Other semiconductors

Hugoniot temperatures and melting of tantalum under shock compression determined by optical pyrometry

Chengda Dai, Jianbo Hu, and Hua Tan

J. Appl. Phys. 106, 043519 (2009); http://dx.doi.org/10.1063/1.3204941 (7 pages) | Cited 9 times

Online Publication Date: 26 August 2009

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LiF single crystal was used as transparent window (anvil) to tamp the shock-induced free surface expansion of Ta specimen, and the Ta/LiF interface temperature was measured under shock compression using optical pyrometry technique. The shock temperatures and/or melting temperatures of Ta up to ∼ 400 GPa were extracted from the observed interface temperatures based on the Tan–Ahrens’ model for one-dimensional heat conduction across metal/window ideal interface in which initial melting and subsequent solidification were considered under shock loading. The obtained data within the experimental uncertainties are consistent with the results from high-pressure sound velocity measurements. The temperature of the partial melting on Ta Hugoniot is estimated to be ∼ 9700 K at 300 GPa, supported by available results from theoretical calculations.
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64.70.dj Melting of specific substances
62.50.-p High-pressure effects in solids and liquids
62.50.Ef Shock wave effects in solids and liquids
62.65.+k Acoustical properties of solids
64.30.-t Equations of state of specific substances

Raman spectra of GexAsySe1−x−y glasses

R. P. Wang, A. Smith, A. Prasad, D. Y. Choi, and B. Luther-Davies

J. Appl. Phys. 106, 043520 (2009); http://dx.doi.org/10.1063/1.3204951 (4 pages) | Cited 2 times

Online Publication Date: 26 August 2009

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Various Ge–As–Se glasses spanning a mean coordination number (MCN) from 2.2 to 2.94 have been investigated using differential scanning calorimetry and Raman spectroscopy. The glass transition temperature Tg was found to increase with increasing MCN, except for those glasses located within the nanoscale phase-separated region of the phase diagram. The evolution of Raman features at wavenumbers from 150 to 350 cm−1 exhibits two transitionlike features. Merging of the 225 and 250 cm−1 modes at MCN = 2.5 is a symbol of the extinction of Se–Se bonds. Additionally, the appearance of two modes at 280–290 and 170 cm−1 at MCN>2.7 come from the defect modes of ethanelike Ge2Se6/2. The increase in the scattering from these defects is an important factor leading to enhanced optical loss in the glasses with high MCN.
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78.35.+c Brillouin and Rayleigh scattering; other light scattering
81.05.Kf Glasses (including metallic glasses)
61.43.Fs Glasses
64.70.ph Nonmetallic glasses (silicates, oxides, selenides, etc.)

Spatially resolved x-ray diffraction study of GaSb layers grown laterally on SiO2-masked GaAs substrates

A. Wierzbicka, J. Z. Domagala, and Z. R. Zytkiewicz

J. Appl. Phys. 106, 043521 (2009); http://dx.doi.org/10.1063/1.3194319 (4 pages) | Cited 1 time

Online Publication Date: 27 August 2009

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In this work spatially resolved x-ray diffraction (SRXRD) is used to analyze strain in GaSb layers grown by epitaxial lateral overgrowth (ELO) on SiO2-masked (001) GaAs substrates. We show that this heteroepitaxial structure contains local mosaicity in the wing area that cannot be detected by selective etching. While the standard x-ray diffraction measurements only suggest the presence of grain structure of the ELO layer, SRXRD allows examining the microscopic strain distribution in the sample. In particular, size of microblocks and their relative misorientation are determined.
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68.60.Bs Mechanical and acoustical properties
68.55.ag Semiconductors

Elongation and breaking mechanisms of gold nanowires under a wide range of tensile conditions

F. Tavazza, L. E. Levine, and A. M. Chaka

J. Appl. Phys. 106, 043522 (2009); http://dx.doi.org/10.1063/1.3200957 (10 pages) | Cited 15 times

Online Publication Date: 27 August 2009

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Semistatic density functional theory is used to explore the evolution of [1 1 0] and [1 1 1] gold nanowires during tensile deformation under a wide range of conditions, including different tensile axes (along high- and low-symmetry directions), nanowire shapes, and effective strain rates. Large structural changes are observed during the elongation. The analysis of such low-energy intermediate configurations provides quantitative information about the underlying energy landscape that cannot be obtained through experiments or more approximate modeling methods, and four stable intermediate atomic structures are identified. A rich diversity of deformation pathways is uncovered that converge to only two final local configurations with reproducible breaking strengths, in agreement with experimental results. Such a high reproducibility in the breaking force makes gold nanowires excellent candidates as intrinsic force standards at the nanolevel.
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61.46.Km Structure of nanowires and nanorods (long, free or loosely attached, quantum wires and quantum rods, but not gate-isolated embedded quantum wires)
62.25.-g Mechanical properties of nanoscale systems
81.40.Lm Deformation, plasticity, and creep
62.20.F- Deformation and plasticity

Temperature dependent surface photovoltage spectroscopy characterization of highly strained InGaAs/GaAs double quantum well structures grown by metal organic vapor phase epitaxy

C. H. Chan, J. D. Wu, Y. S. Huang, Y. K. Su, and K. K. Tiong

J. Appl. Phys. 106, 043523 (2009); http://dx.doi.org/10.1063/1.3208053 (5 pages) | Cited 1 time

Online Publication Date: 28 August 2009

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Highly strained InxGa1−xAs/GaAs double quantum well (DQW) structures grown by metal organic vapor phase epitaxy with different In compositions are investigated by surface photovoltage spectroscopy (SPS) in the temperature range 20–300 K. A lineshape fit of spectral features in the differential surface photovoltage (SPV) spectra determines the transition energies accurately. A comprehensive analysis of the anomalous phenomena appearing in lower temperature SPV spectra enable us to evaluate directly the band lineup of DQW and to remove the ambiguity in the identification of spectral features. The process of separation of carriers within the QW with possible capture by the interface defect traps plays an important role for phase change in SPV signal in the vicinity of light-hole related feature at low temperature. The results demonstrate the considerable diagnostic values of the SPS technique for characterizing these highly strained DQW structures.
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73.63.Hs Quantum wells
78.67.De Quantum wells
68.65.Fg Quantum wells
81.15.Kk Vapor phase epitaxy; growth from vapor phase
73.25.+i Surface conductivity and carrier phenomena
72.40.+w Photoconduction and photovoltaic effects

Response of seven crystallographic orientations of sapphire crystals to shock stresses of 16–86 GPa

G. I. Kanel, W. J. Nellis, A. S. Savinykh, S. V. Razorenov, and A. M. Rajendran

J. Appl. Phys. 106, 043524 (2009); http://dx.doi.org/10.1063/1.3204940 (10 pages) | Cited 11 times

Online Publication Date: 28 August 2009

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Shock wave profiles of sapphire (single-crystal Al2O3) with seven crystallographic orientations (c, d, r, n, s, g, and m-cut) were measured with time-resolved VISAR (velocity interferometer for a surface of any reflector) interferometry at shock stresses in the range 16–86 GPa. Shock propagation was in the direction normal to the surface of each cut. The angle between the c-axis of the hexagonal representation of the sapphire crystal structure and the direction of shock propagation varied from 0 for c-cut up to 90° for m-cut in the basal plane. Based on published shock-induced transparencies for three directions of shock propagation, shock-induced optical transparency correlates with the smoothness of the mechanical shock wave profile. The ultimate goal was to find the direction of shock propagation for which shock-compressed sapphire is most transparent as a window material. In the experiments particle velocity histories were recorded at the interface between a sapphire crystal and a LiF window. In most cases measured wave profiles are noisy as a result of heterogeneity of deformation. Measured values of Hugoniot elastic limits (HELs) depend on direction of shock compression and peak shock stress. The largest HEL values (24 GPa) were recorded for shock loading along the c-axis and perpendicular to c along the m-direction. Shock compression along the m- and s-directions is accompanied by the smallest heterogeneity of deformation and the smallest rise time of the plastic shock wave. m- and s-cut sapphires most closely approach ideal elastic-plastic flow, which suggests that m- and s-cut sapphires are probably the sapphire orientations that remain the most transparent to the highest shock pressures. Under purely elastic deformation sapphire demonstrates very high spall strength, which depends on both load duration and peak stress. Plastic deformation of sapphire causes loss of its tensile strength.
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62.50.Ef Shock wave effects in solids and liquids
62.20.D- Elasticity
61.66.Fn Inorganic compounds
81.40.Jj Elasticity and anelasticity, stress-strain relations
81.40.Lm Deformation, plasticity, and creep
62.20.fq Plasticity and superplasticity

Strain relaxation in epitaxial Pt films on (001) SrTiO3

Junwoo Son, Joël Cagnon, and Susanne Stemmer

J. Appl. Phys. 106, 043525 (2009); http://dx.doi.org/10.1063/1.3207795 (3 pages) | Cited 2 times

Online Publication Date: 31 August 2009

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The relationship between strain relaxation and microstructure evolution of epitaxial, (001)-oriented Pt thin films on (001) SrTiO3 substrates is investigated as a function of Pt film thickness. X-ray diffraction shows that the Pt films gradually relax after film coalescence with increasing film thickness. The Pt film surfaces exhibit a cross-hatched surface pattern that increases in amplitude and density with film thickness and is due to progressive relaxation of the lattice mismatch strain by twinning. The potential of these Pt films as bottom electrodes for functional perovskite films is discussed.
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68.60.Bs Mechanical and acoustical properties
62.40.+i Anelasticity, internal friction, stress relaxation, and mechanical resonances
81.40.Jj Elasticity and anelasticity, stress-strain relations
68.55.jd Thickness
68.35.bd Metals and alloys
68.35.Gy Mechanical properties; surface strains
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