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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

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
back to top Electronic Structure and Transport

Improved conductivity of Sb-doped SnO2 thin films

A. A. Alsac, A. Yildiz, T. Serin, and N. Serin

J. Appl. Phys. 113, 063701 (2013); http://dx.doi.org/10.1063/1.4790879 (4 pages)

Online Publication Date: 8 February 2013

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Sb-doped SnO2 thin films at different thickness have been grown by sol-gel dip-coating method. All of the films exhibit degenerate semiconductor behavior and high free carrier concentrations. In the films, electrical transport can be explained reasonably well by assuming the electron-electron interactions (EEIs) contribution to the measured electrical conductivity. Our experimental observations are consistent with the theoretical description of the EEI. The effect of films thickness on the EEI contribution is also discussed. When the thickness of film reaches to 1550 nm, the agreement between the EEI theory and experimental data becomes unsatisfactory.
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81.10.Dn Growth from solutions
81.10.Fq Growth from melts; zone melting and refining
81.15.Lm Liquid phase epitaxy; deposition from liquid phases (melts, solutions, and surface layers on liquids)
68.55.ag Semiconductors
73.61.Le Other inorganic semiconductors

Negative magnetoresistance and anomalous Hall effect in GeMnTe-SnMnTe spin-glass-like system

L. Kilanski, R. Szymczak, W. Dobrowolski, A. Podgórni, A. Avdonin, V. E. Slynko, and E. I. Slynko

J. Appl. Phys. 113, 063702 (2013); http://dx.doi.org/10.1063/1.4790321 (7 pages)

Online Publication Date: 11 February 2013

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Magnetotransport properties of spin-glass-like Ge1−x−ySnxMnyTe mixed crystals with chemical composition changing in the range of 0.083 ≤ x ≤ 0.142 and 0.012 ≤ y ≤ 0.119 are presented. The observed negative magnetoresistance we attribute to two mechanisms, i.e., weak localization occurring at low fields and spin disorder scattering giving contribution mainly at higher magnetic fields. A pronounced hysteretic anomalous Hall effect (AHE) was observed. The estimated AHE coefficient shows a small temperature dependence and is dependent on Mn-content, with changes in the range of 10−7<RS<10−6m3/C. The scaling law analysis has proven that the AHE in this system is due to the extrinsic mechanisms, mainly due to the skew scattering accompanied with the side jump processes.
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72.20.My Galvanomagnetic and other magnetotransport effects
73.43.Qt Magnetoresistance
75.47.Pq Other materials
75.50.Pp Magnetic semiconductors

Roles of core-shell and δ-ray kinetics in layered BN α-voltaic efficiency

Corey Melnick, Massoud Kaviany, and Moo-Hwan Kim

J. Appl. Phys. 113, 063703 (2013); http://dx.doi.org/10.1063/1.4790506 (8 pages)

Online Publication Date: 11 February 2013

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α-voltaics harvest electron-hole pairs created as energetic α particles collide with and ionize electrons in a semiconductor, creating δ-rays. After ionization, charged pair production continues through δ-ray impact ionization events and the Auger relaxation of core-shell holes created through K-shell ionization events. Secondary ionization events are quantified using the TPP-2M model, the fraction of K-shell ionization events is determined using the energy-loss Coulomb-repulsion perturbed-stationary-state relativistic theory, and the relaxation of the resulting holes is treated with a fully ab initio approach using multiple Fermi golden rule calculations for ranges of carrier concentrations and temperatures. The limiting rate is 15 ns−1 for small carrier concentrations and high temperatures, as compared to the radiative core-shell relaxation rate estimated here at 20 ns−1, indicating that Auger modes contribute significantly. Moreover, the K-shell ionization events are shown to dominate for low energy α particles and vanish for high energy ones. Thus, the efficiency loss due to energy dissipation in the fuel layer is mitigated, which is demonstrated by the analysis of a layered fuel-voltaic device with an efficiency from 20% to 14% for fuel layers between 5 and 10 μm thick. The design of a α-voltaic integrated with a thermoelectric generator is suggested for improved efficiency and the system-level mitigation of radiation damage and geometric inefficiency.
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79.20.Fv Electron impact: Auger emission
61.85.+p Channeling phenomena (blocking, energy loss, etc.)
71.15.-m Methods of electronic structure calculations

Effective electron mean free path in TiN(001)

J. S. Chawla, X. Y. Zhang, and D. Gall

J. Appl. Phys. 113, 063704 (2013); http://dx.doi.org/10.1063/1.4790136 (6 pages)

Online Publication Date: 11 February 2013

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The measured resistivity ρ of smooth stoichiometric epitaxial TiN(001) is 13 and 3.0 μΩ cm at 298 and 77 K for bulk layers but is 8 and 25 times higher when the layer thickness d is reduced to 2 nm. The increase in ρ with decreasing d is attributed to diffuse electron-surface scattering and is well described by the classical Fuchs-Sondheimer (F-S) model. This is unexpected because the F-S model is based on the nearly free electron model, while TiN exhibits a highly non-spherical Fermi surface and three bands crossing the Fermi-level. The measured room temperature effective electron mean free path λ for bulk scattering is 45 ± 4 nm, which is an order of magnitude larger than the free-electron prediction. This deviation is attributed to ∼93% of charge transport in TiN being due to two slightly filled bands which represent only 4% of conduction electrons. The F-S model is applicable to TiN because these two bands are nearly parabolic and nearly degenerate, yielding a single value for λ, which is estimated based on the published band structure to be 49 nm, in excellent agreement with the experimental 45 nm. These results demonstrate that the F-S model is applicable to metals with non-spherical Fermi-surfaces as long as the charge transport is dominated by a single (or multiple degenerate) band(s).
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72.20.Jv Charge carriers: generation, recombination, lifetime, and trapping
73.50.Gr Charge carriers: generation, recombination, lifetime, trapping, mean free paths
73.61.Ng Insulators
71.18.+y Fermi surface: calculations and measurements; effective mass, g factor
71.20.Ps Other inorganic compounds
72.10.Fk Scattering by point defects, dislocations, surfaces, and other imperfections (including Kondo effect)

Effect of chiral property on hot phonon distribution and energy loss rate due to surface polar phonons in a bilayer graphene

V. S. Katti and S. S. Kubakaddi

J. Appl. Phys. 113, 063705 (2013); http://dx.doi.org/10.1063/1.4790309 (5 pages)

Online Publication Date: 11 February 2013

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Effect of chiral property on hot phonon distribution and hot electron energy loss rate due to surface polar phonons is investigated in a supported bilayer graphene. Hot phonon distribution Nq(ω0) with chiral property shows a new feature of appearance of a cusp. Interestingly, minimum of the cusp is shifting toward larger q and its width is increasing as electron concentration ns increases. The maximum value of Nq(ω0) is found to shift toward lower q with the increasing ns. This unusual behavior is not found in conventional two-dimensional electron gas (2DEG). The effect of chiral property on energy loss rate is found to be less significant. Hot phonon effect on energy loss rate is studied as a function of electron temperature and electron concentration for different substrates. It is found to reduce the energy loss rate significantly and the results are compared with those in conventional 2DEG. We believe that the magnitude of hot phonon effect depends both on dielectric constant of the substrate and surface polar phonon energy.
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63.22.Rc Phonons in graphene
72.80.Vp Electronic transport in graphene
72.20.Ht High-field and nonlinear effects
77.22.Ch Permittivity (dielectric function)
79.20.Uv Electron energy loss spectroscopy

Ultrafast switching in wetting properties of TiO2/YSZ/Si(001) epitaxial heterostructures induced by laser irradiation

M. R. Bayati, S. Joshi, R. Molaei, R. J. Narayan, and J. Narayan

J. Appl. Phys. 113, 063706 (2013); http://dx.doi.org/10.1063/1.4790327 (7 pages) | Cited 1 time

Online Publication Date: 11 February 2013

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We have demonstrated dark hydrophilicity of single crystalline rutile TiO2(100) thin films, in which rapid switching from a hydrophobic to a hydrophilic surface was achieved using nanosecond excimer laser irradiation. The TiO2/YSZ/Si(001) single crystalline heterostructures were grown by pulsed laser deposition and were subsequently irradiated by a single pulse of a KrF excimer laser at several energies. The wettability of water on the surfaces of the samples was evaluated. The samples were hydrophobic prior to laser annealing and turned hydrophilic after laser annealing. Superhydrophilic surfaces were obtained at higher laser energy densities (e.g., 0.32 J.cm−2). The stoichiometries of the surface regions of the samples before and after laser annealing were examined using XPS. The results revealed the formation of oxygen vacancies on the surface, which are surmised to be responsible for the observed superhydrophilic behavior. According to the AFM images, surface smoothening was greater in films that were annealed at higher laser energy densities. The samples exhibited hydrophobic behavior after being placed in ambient atmosphere. The origin of laser induced wetting behavior was qualitatively understood to stem from an increase of point defects near the surface, which lowered the film/water interfacial energy. This type of rapid hydrophobic/hydrophilic switching may be used to facilitate fabrication of electronic and photonic devices with novel properties.
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81.15.Fg Pulsed laser ablation deposition
65.40.gp Surface energy
68.55.at Other materials
61.80.Ba Ultraviolet, visible, and infrared radiation effects (including laser radiation)
68.35.Md Surface thermodynamics, surface energies
79.20.Ds Laser-beam impact phenomena

Tunneling spectroscopy of chiral states in ultra-thin topological insulators

Gen Yin, Darshana Wickramaratne, and Roger K. Lake

J. Appl. Phys. 113, 063707 (2013); http://dx.doi.org/10.1063/1.4790804 (8 pages)

Online Publication Date: 12 February 2013

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The temperature, Fermi-level, and bias dependencies of the inter-surface tunneling current in thin-film topological insulators show unique, identifying signatures of the surface states and their opposite chiralities. The opposite chiralities of the surface states limit the tunneling to the band edges of the gapped Dirac cones. As a result, the tunneling conductance is sensitive to the temperature, the Fermi level, and the surface-surface potential difference. The temperature dependence of the tunneling conductance changes sign as the Fermi level scans through the Dirac point. The tunneling transmission is a minimum when the opposing surface Dirac cones are perfectly aligned in energy. This minimum state of the tunneling channel can result in negative differential resistance (NDR) in the presence of a built-in Rashba-like splitting. The unique thermal response of the tunneling conductance and the existence of NDR suggest a tunneling spectroscopy experiment to demonstrate the opposite chiralities of the opposing surface states.
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73.61.Ng Insulators
73.20.At Surface states, band structure, electron density of states
73.25.+i Surface conductivity and carrier phenomena
73.40.Gk Tunneling
73.50.Fq High-field and nonlinear effects

Improved chemical and electrical stability of gold silicon contacts via epitaxial electrodeposition

Azadeh Akhtari-Zavareh, Wenjie Li, Fouad Maroun, Philippe Allongue, and Karen L. Kavanagh

J. Appl. Phys. 113, 063708 (2013); http://dx.doi.org/10.1063/1.4792000 (10 pages)

Online Publication Date: 12 February 2013

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The ballistic transport and junction stability of epitaxial Au, grown on Si (111) and (001) substrates via electrodeposition, have been investigated as a function of Au thickness (6.7–14.1 nm). Scanning tunneling microscopy (STM) with ballistic emission electron microscopy (BEEM) showed hot electron transmission for diodes with Au thicknesses greater than 7 nm, whereas for thinner samples surface or interfacial oxidation meant that macroscopic current-voltage characteristics gave high barrier heights (0.8 eV) but no detectable BEEM transport at room temperature. Diodes that remained electrically stable for several months after exposure to air also show an atomically abrupt and epitaxial Au/Si interface via cross-sectional transmission electron microscopy. Degradation in rectifying diode properties through surface or interfacial reactions is indicated by poorer STM images and decreasing or no BEEM transmission, correlated with the formation of a continuous interfacial layer.
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73.40.Ns Metal-nonmetal contacts
81.15.Pq Electrodeposition, electroplating
81.65.Mq Oxidation
68.37.Ef Scanning tunneling microscopy (including chemistry induced with STM)
73.23.Ad Ballistic transport

Critique of charge collection efficiencies calculated through small perturbation measurements of dye sensitized solar cells

Varun Sivaram, James Kirkpatrick, and Henry Snaith

J. Appl. Phys. 113, 063709 (2013); http://dx.doi.org/10.1063/1.4789966 (6 pages)

Online Publication Date: 13 February 2013

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Through experiment and simulation, we critically examine the conventional method of extracting the charge collection efficiency, ηc, of dye-sensitized solar cells (DSSCs). We demonstrate that the collection efficiency extracted by measuring transient decay rates at short circuit deviates considerably from the true value as ηc decreases. This discrepancy arises from two sources of error: first, transient voltage decay rates are especially sensitive to perturbation size near short circuit, and second, the decay rates are influenced by transient equilibration between trapped and free charge during the measurement. Only if the collection efficiency is high (>90%) do the true and measured values coincide well. This finding finally elucidates the disparity between steady-state and transient measurements of the collection efficiency; the former is more accurate for devices with a low ηc, and the latter for devices with a high ηc. For the first time, we present a fully nondimensional model of the DSSC, allowing us to fit device parameters without extensive material knowledge. The resulting simulations and fitting of solid-state dye sensitized solar cells additionally enable us to quantify the inaccuracy of small perturbation measurements of the collection efficiency when ηc is below 90%.
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88.40.jr Organic photovoltaics
88.40.hj Efficiency and performance of solar cells

Charge transport in lightly reduced graphene oxide: A transport energy perspective

R. S. Kajen, N. Chandrasekhar, K. L. Pey, C. Vijila, M. Jaiswal, S. Saravanan, Andrew M. H. Ng, C. P. Wong, and K. P. Loh

J. Appl. Phys. 113, 063710 (2013); http://dx.doi.org/10.1063/1.4792042 (7 pages)

Online Publication Date: 14 February 2013

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Significant variation in the charge transport behaviour in graphene oxide (GO) ranging from Schottky to Poole-Frenkel and to space charge limited transport exists. These have been extensively reported in the literature. However, the validity of such conventional charge transport models meant for delocalized carriers, to study charge transport through localised states in GO, a disordered semiconductor is open to question. In this work, we use the concept of transport energy (TE) to model charge transport in lightly reduced GO (RGO) and demonstrate that the TE calculations match well with temperature dependent experimental I-V data on RGO. We report on a temperature dependent TE ranging from a few 10 meV to 0.1 eV in slightly reduced GO. Last, we point out that, despite the success of several delocalised charge transport models in estimating barrier heights that resemble the TE level, they remain largely accidental and lack the insight in which the TE concept provides in understanding charge transport in RGO.
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72.80.Vp Electronic transport in graphene
72.20.Ht High-field and nonlinear effects
73.30.+y Surface double layers, Schottky barriers, and work functions
73.61.Wp Fullerenes and related materials

Low-bias electron transport properties of germanium telluride ultrathin films

Jie Liu and M. P. Anantram

J. Appl. Phys. 113, 063711 (2013); http://dx.doi.org/10.1063/1.4790801 (6 pages)

Online Publication Date: 14 February 2013

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The nanometer-scale size-dependent electronic transport properties of crystalline (c-) and amorphous (a-) germanium telluride (GeTe) ultrathin films sandwiched by titanium nitride (TiN) electrodes are investigated using ab initio molecular dynamics, density functional theory, and Green's function calculations. We find that a-GeTe ultrathin films scaled down to about 38 Å (12 atomic layers) still show a band gap and the electrical conductance is mainly due to electron transport via intra-gap states. If the ultrathin films are further scaled, the a-GeTe band gap disappears due to overlap of the two metal induced gap states (MIGS) regions near the TiN electrodes, leading to sharp increase of a-GeTe conductance and significant decrease of c-GeTe/a-GeTe conductance ratio. The c-GeTe/a-GeTe conductance ratio drops below one order of magnitude if the ultrathin films are scaled below about 33 Å, making it difficult to reliably perform read operations in thin film based phase change memory devices. This overlap of the MIGS regions sets up the ultimate scaling limit of phase change memory technology. Our results suggest that the ultimate scaling limit can be pushed to even smaller size, by using phase change material with larger amorphous phase band gap than a-GeTe.
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68.55.-a Thin film structure and morphology
71.15.-m Methods of electronic structure calculations
71.15.Mb Density functional theory, local density approximation, gradient and other corrections
73.50.-h Electronic transport phenomena in thin films
61.43.Bn Structural modeling: serial-addition models, computer simulation
61.43.Er Other amorphous solids

Effect of hydrogen incorporation on the negative bias illumination stress instability in amorphous In-Ga-Zn-O thin-film-transistors

Hyeon-Kyun Noh, Ji-Sang Park, and K. J. Chang

J. Appl. Phys. 113, 063712 (2013); http://dx.doi.org/10.1063/1.4792229 (6 pages)

Online Publication Date: 14 February 2013

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In amorphous indium-gallium-zinc oxide (a-IGZO) thin film transistors, negative shifts of the threshold voltage commonly occur under negative bias illumination stress (NBIS), and its origin is attributed to hole traps such as O-vacancy (VO) defects. We perform density functional calculations to investigate the effect of hydrogenation on the NBIS instability. We find that hydrogen passivates the electrical activity of VO in form of HO, in which H occupies the vacancy site. The activation energy for dissociating HO into VO and an interstitial H (Hi) is about 1.27 eV, much higher than the migration barrier of about 0.51 eV for Hi diffusion. Kinetic Monte Carlo simulations show that HO defects are quite stable upon post thermal annealing up to 200 °C. Thus, we propose that H incorporation into a-IGZO not only effectively reduces the density of VO defects but also mitigates the NBIS instability in devices fabricated at low temperatures.
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85.30.Tv Field effect devices
61.72.jd Vacancies
61.72.jj Interstitials
81.40.Gh Other heat and thermomechanical treatments
61.72.Cc Kinetics of defect formation and annealing
81.40.Ef Cold working, work hardening; annealing, post-deformation annealing, quenching, tempering recovery, and crystallization
02.50.Ng Distribution theory and Monte Carlo studies
71.15.Mb Density functional theory, local density approximation, gradient and other corrections
back to top Magnetism and Superconductivity

The role of octahedral tilting in the structural phase transition and magnetic anisotropy in SrRuO3 thin film

Wenlai Lu, Wen Dong Song, Kaihua He, Jianwei Chai, Cheng-Jun Sun, Gan-Moog Chow, and Jing-Sheng Chen

J. Appl. Phys. 113, 063901 (2013); http://dx.doi.org/10.1063/1.4790699 (7 pages) | Cited 1 time

Online Publication Date: 8 February 2013

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We present a stoichiometry-dependent structural phase transition in SrRuO3 film on SrTiO3 substrate. The oxygen stoichiometry in the films was varied by changing the oxygen partial pressure P(O2) during the deposition process. For SrRuO3 films with P(O2) ≥ 60 mTorr, they exhibited a pseudo-orthorhombic structure with in-plane uniaxial magnetic anisotropy. On the other hand for films with P(O2) ≤ 45 mTorr, the tetragonal SrRuO3 phase with a perpendicular uniaxial magnetic anisotropy was stabilized at room temperature. The big difference in the magnetic anisotropy of these two SrRuO3 phases was shown to be closely linked to their respective RuO6 octahedral rotation patterns: the RuO6 octahedra rotate differently along the two orthogonal in-plane directions in the pseudo-orthorhombic phase, whereas in the tetragonal phase only octahedral rotations around z-axis are present and the octahedral tilts along the in-plane axes are diminished. First-principles calculations show that such a suppression of the RuO6 octahedra tilting in the tetragonal phase arises from the oxygen vacancies at the octahedral apex (along z-axis). This work demonstrates that the stoichiometry plays an important role in determining the octahedral rotations and tilts in the perovskite materials, which may induce new phases with distinctively different structural symmetry and physical property.
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75.70.Ak Magnetic properties of monolayers and thin films
81.15.Fg Pulsed laser ablation deposition
81.30.Hd Constant-composition solid-solid phase transformations: polymorphic, massive, and order-disorder
61.72.jd Vacancies
64.70.K- Solid-solid transitions
75.30.Gw Magnetic anisotropy

Search for long-range ferromagnetism: Charge-spin co-doped Ba1−x−yLax+yTi1−xMxO3 (M = Cr, Fe, and Co)

D. F. Shao, J. Yang, H. B. Jian, X. B. Zhu, and Y. P. Sun

J. Appl. Phys. 113, 063902 (2013); http://dx.doi.org/10.1063/1.4790869 (7 pages)

Online Publication Date: 8 February 2013

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We investigate the structural, magnetic, and electrical properties of charge-spin co-doped Ba1−x−yLax+yTi1−xMxO3 (M = Cr, Fe, and Co). For Ba1−x−yLax+yTi1−xCrxO3, carrier doping induces short-range ferromagnetism, and we find a large negative magnetoresistance in these compounds. However, for Ba1−x−yLax+yTi1−xMxO3 (M = Fe and Co), the magnetism does not seem to couple with the charge carriers. The transport properties of Ba1−x−yLax+yTi1−xCrxO3 can be described by a three-dimensional variable-range-hopping model that implies itinerant charge carriers, whereas almost all the samples of Ba1−x−yLax+yTi1−xMxO3 (M = Fe and Co) exhibit high resistivity. It is known that only the itinerant charge carriers can enhance the coupling of two isolated magnetic ions. Therefore, there is almost no long-range ferromagnetism in the studied system.
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61.66.Fn Inorganic compounds
72.20.My Galvanomagnetic and other magnetotransport effects
75.50.Dd Nonmetallic ferromagnetic materials
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
61.72.up Other materials

Planar Hall resistance ring sensor based on NiFe/Cu/IrMn trilayer structure

Brajalal Sinha, Tran Quang Hung, Torati Sri Ramulu, Sunjong Oh, Kunwoo Kim, Dong-Young Kim, Ferial Terki, and CheolGi Kim

J. Appl. Phys. 113, 063903 (2013); http://dx.doi.org/10.1063/1.4790139 (5 pages)

Online Publication Date: 8 February 2013

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We have investigated the sensitivity of a planar Hall resistance sensor as a function of the ring radius in the trilayer structure Ta(3)/IrMn(10)/Cu(0.2)/NiFe(10)/Ta(3) (nm). The diagonal components of magnetoresistivity tensor in rectangular prism corresponding to anisotropic magnetoresistance are few ten times larger than that of off-diagonal component corresponding to planar Hall resistance. However, it is noteworthy that the resultant contribution is governed by the off-diagonal components due to the cancellation of diagonal components in the self-balanced bridge configuration. Both the experimental and theoretical results show that the sensitivity varies linearly with the ring radius. In multi-ring architecture, the circumference can be increased to a limit, which consequently enhances sensitivity. We found the sensitivity of the investigated 7-rings planar Hall to be more than 600 μV/Oe.
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85.75.Ss Magnetic field sensors using spin polarized transport
07.07.Df Sensors (chemical, optical, electrical, movement, gas, etc.); remote sensing
07.55.-w Magnetic instruments and components

Increasing Curie temperature in tetragonal Mn2RhSn Heusler compound through substitution of Rh by Co and Mn by Rh

Vajiheh Alijani, Olga Meshcheriakova, Juergen Winterlik, Guido Kreiner, Gerhard H. Fecher, and Claudia Felser

J. Appl. Phys. 113, 063904 (2013); http://dx.doi.org/10.1063/1.4791564 (4 pages)

Online Publication Date: 11 February 2013

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The tetragonal Mn2RhSn Heusler compound shows better lattice match with MgO than do Mn3−xGa and tetragonal Mn3−xCoxGa Heusler compounds and hence is better suited for spin transfer torque applications. Beside the improved lattice match, this compound shows a low saturation magnetic moment reduces the current to switch which makes it more relevant for application in spin transfer torque devices. This compound shows a low Curie temperature; introducing Co into this system and increasing the Rh content leads to an increase in the Curie temperature. Doping with Co retains the tetragonal structure, with improved lattice match with MgO and low magnetic moment, intact up to x = 0.6, although doping with Rh changes the structure from tetragonal to cubic.
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75.30.Kz Magnetic phase boundaries (including classical and quantum magnetic transitions, metamagnetism, etc.)
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
81.30.Hd Constant-composition solid-solid phase transformations: polymorphic, massive, and order-disorder
61.72.up Other materials
64.70.kd Metals and alloys
75.30.Cr Saturation moments and magnetic susceptibilities

Origin of the different multiferroism in BiFeO3 and GaFeO3

S. G. Bahoosh and J. M. Wesselinowa

J. Appl. Phys. 113, 063905 (2013); http://dx.doi.org/10.1063/1.4791586 (6 pages)

Online Publication Date: 11 February 2013

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We have proposed microscopic models for describing the multiferroic properties of BiFeO3 and GaFeO3. It is shown that the mechanisms of the multiferroism are different. In BiFeO3, the magnetoelectric coupling is biquadratic, whereas in GaFeO3 it is linear. The site disorder between Ga and Fe is a primary source of the net magnetic moment in GaFeO3. The temperature and magnetic field dependence of the polarization is calculated in order to show that the proposed models for these two multiferroics are correct. Near the magnetic phase transition temperature TN we obtain a kink in the electric properties.
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75.85.+t Magnetoelectric effects, multiferroics
75.30.Cr Saturation moments and magnetic susceptibilities
75.30.Kz Magnetic phase boundaries (including classical and quantum magnetic transitions, metamagnetism, etc.)
77.22.Ej Polarization and depolarization
77.80.-e Ferroelectricity and antiferroelectricity
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