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21 May 2013

Volume 113, Issue 19 (partial)

Issue Cover Spotlight Figure

J. Appl. Phys. 113, 191101 (2013); http://dx.doi.org/10.1063/1.4802504 (16 pages)

R. Farshchi and M. Ramsteiner
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Spin injection from Heusler alloys into semiconductors: A materials perspective

R. Farshchi and M. Ramsteiner

J. Appl. Phys. 113, 191101 (2013); http://dx.doi.org/10.1063/1.4802504 (16 pages)

Online Publication Date: 15 May 2013

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The notion of using electron spins as bits for highly efficient computation coupled with non-volatile data storage has driven an intense international research effort over the past decade. Such an approach, known as spin-based electronics or spintronics, is considered to be a promising alternative to charge-based electronics in future integrated circuit technologies. Many proposed spin-based devices, such as the well-known spin-transistor, require injection of spin polarized currents from ferromagnetic layers into semiconductor channels, where the degree of injected spin polarization is crucial to the overall device performance. Several ferromagnetic Heusler alloys are predicted to be half-metallic, meaning 100% spin-polarized at the Fermi level, and hence considered to be excellent candidates for electrical spin injection. Furthermore, they exhibit high Curie temperatures and close lattice matching to III-V semiconductors. Despite their promise, Heusler alloy/semiconductor heterostructures investigated in the past decade have failed to fulfill the expectation of near perfect spin injection and in certain cases have even demonstrated inferior behavior compared to their elemental ferromagnetic counterparts. To address this problem, a slew of theoretical and experimental work has emerged studying Heusler alloy/semiconductor interface properties. Here, we review the dominant prohibitive materials challenges that have been identified, namely atomic disorder in the Heusler alloy and in-diffusion of magnetic impurities into the semiconductor, and their ensuing detrimental effects on spin injection. To mitigate these effects, we propose the incorporation of half-metallic Heusler alloys grown at high temperatures (>200 °C) along with insertion of a MgO tunnel barrier at the ferromagnet/semiconductor interface to minimize magnetic impurity in-diffusion and potentially act as a spin-filter. By considering evidence from a variety of structural, optical, and electrical studies, we hope to paint a realistic picture of the materials environment encountered by spins upon injection from Heusler alloys into semiconductors. Finally, we review several emerging device paradigms that utilize Heusler alloys as sources of spin polarized electrons.
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72.25.Mk Spin transport through interfaces
73.40.Ns Metal-nonmetal contacts
75.30.Hx Magnetic impurity interactions
75.70.Cn Magnetic properties of interfaces (multilayers, superlattices, heterostructures)
66.30.J- Diffusion of impurities
75.76.+j Spin transport effects
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back to top Lasers, Optics, and Optoelectronics

Polarization properties of single and ensembles of InAs/InP quantum rod nanowires emitting in the telecom wavelengths

R. Anufriev, N. Chauvin, H. Khmissi, K. Naji, J.-B. Barakat, J. Penuelas, G. Patriarche, M. Gendry, and C. Bru-Chevallier

J. Appl. Phys. 113, 193101 (2013); http://dx.doi.org/10.1063/1.4804327 (6 pages)

Online Publication Date: 15 May 2013

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The absorption and emission polarization properties of InAs quantum rods embedded in InP nanowires (NWs) are investigated by mean of (micro-)photoluminescence spectroscopy. It is shown that the degree of linear polarization of emission (0.94) and absorption (0.5) of a single NW can be explained by the photonic nature of the NW structure. Knowing these parameters, optical properties of single NWs and ordered ensembles of these NWs can be correlated one to another via proposed model, so that polarization properties of NWs can be studied using ordered ensembles on as-grown samples. As an example, the polarization anisotropy is investigated as a function of the excitation wavelength on a NW ensemble and found to be in agreement with theoretical prediction.
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78.55.Cr III-V semiconductors
78.66.Fd III-V semiconductors
81.05.Ea III-V semiconductors
78.67.Pt Multilayers; superlattices; photonic structures; metamaterials
78.67.Uh Nanowires
78.67.Qa Nanorods

Polarization dependence of absorption by bound electrons in self-assembled quantum dots

Tarek A. Ameen and Yasser M. El-Batawy

J. Appl. Phys. 113, 193102 (2013); http://dx.doi.org/10.1063/1.4805061 (5 pages)

Online Publication Date: 16 May 2013

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In this paper, the effects of the incident light polarization on the bound to continuum linear absorption coefficient of quantum dot devices have been investigated. The study is based on the effective mass theory and the Non Equilibrium Green's Function formalism. For the bound to continuum component of the absorption coefficient, both of in-plane and perpendicular polarization effects are studied for different sizes of conical quantum dots. Generally, decreasing the dot's dimensions results in an increase of the in-plane polarized light absorption and in moving the absorption peak towards longer wavelengths. On the other hand, decreasing the dot's dimensions results in a decrease of the perpendicularly polarized light absorption coefficient and in moving the absorption peak towards longer wavelengths.
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78.67.Hc Quantum dots
42.25.Ja Polarization
71.18.+y Fermi surface: calculations and measurements; effective mass, g factor
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)

Investigation of the role of silver species on spectroscopic features of Sm3+-activated sodium–aluminosilicate glasses via Ag+-Na+ ion exchange

Longji Li, Yong Yang, Dacheng Zhou, Zhengwen Yang, Xuhui Xu, and Jianbei Qiu

J. Appl. Phys. 113, 193103 (2013); http://dx.doi.org/10.1063/1.4807313 (5 pages)

Online Publication Date: 17 May 2013

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The introduction of silver into the Sm3+-doped sodium–aluminosilicate glasses prepared by Ag+-Na+ ion exchange leads to the formation of different ionic silver species. Under 270 nm/250 nm excitation, effective enhancement of Sm3+ luminescence is ascribed to radiative energy transfer from isolated Ag+ to Sm3+. Under 355 nm excitation, white light emission was realized by combining red orange light emission of Sm3+ with green light emission of Ag+-Ag+ and blue light emission of (Ag2)+. Silver nanoparticles formed by further heat treatment are effective quenchers of luminescence from the corresponding excited states of Sm3+ ions.
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78.55.Hx Other solid inorganic materials
78.55.Qr Amorphous materials; glasses and other disordered solids

Characterization of a hollow core fibre-coupled near field terahertz probe

M. Misra, Y. Pan, C. R. Williams, S. A. Maier, and S. R. Andrews

J. Appl. Phys. 113, 193104 (2013); http://dx.doi.org/10.1063/1.4804952 (10 pages)

Online Publication Date: 20 May 2013

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We describe the design and performance of a freely positionable THz near field probe based on a hollow core photonic crystal fibre-coupled photoconducting dipole antenna with an integrated sub-wavelength aperture. Experimental studies of the spatial resolution are compared with detailed finite element electromagnetic simulations and imaging artefacts that are a particular feature of this type of device are discussed. We illustrate the potential applications with descriptions of time domain near field studies of surface waves on a metamaterial and multimode propagation in a parallel plate waveguide.
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85.60.Bt Optoelectronic device characterization, design, and modeling
84.40.Ba Antennas: theory, components and accessories
42.81.Wg Other fiber-optical devices

Conversion mechanism of conductivity of phosphorus-doped ZnO films induced by post-annealing

Jichao Li, Bin Yao, Yongfeng Li, Zhanhui Ding, Ying Xu, Ligong Zhang, Haifeng Zhao, and Dezhen Shen

J. Appl. Phys. 113, 193105 (2013); http://dx.doi.org/10.1063/1.4805778 (6 pages)

Online Publication Date: 20 May 2013

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The effects of post-annealing on conductivity of phosphorus-doped ZnO (PZO) films grown at 500 °C by radio frequency magnetron sputtering are investigated in a temperature ranging from 600 °C to 900 °C. The as-grown PZO exhibits n-type conductivity with an electron concentration of 1.19 × 1020 cm−3, and keeps n-type conductivity as annealed at 600 °C-700 °C but electron concentration decreases with increasing temperature. However, it converts to p-type conductivity as annealed at 800 °C. Further increasing temperature, it still shows p-type conductivity but the hole concentration decreases. It is found that the P occupies mainly Zn site (PZn) in the as-grown PZO, which accounts for good n-type conductivity of the as-grown PZO. The amount of the PZn decreases with increasing temperature, while the amount of Zn vacancy (VZn) increases from 600 °C to 800 °C but decreases greatly at 900 °C, resulting in that the amount of PZn-2VZn complex increases with increasing temperature up to 800 °C but decreases above 800 °C. It is suggested that the PZn-2VZn complex acceptor is responsible for p-type conductivity, and that the conversion of conductivity is due to the change of the amount of the PZn and PZn-2VZn with annealing temperature.
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81.05.Dz II-VI semiconductors
72.60.+g Mixed conductivity and conductivity transitions
73.61.Ga II-VI semiconductors
81.15.Cd Deposition by sputtering
61.72.Cc Kinetics of defect formation and annealing
61.72.jd Vacancies

Bidirectional optical switch based on electrowetting

Chao Liu, Lei Li, and Qiong-Hua Wang

J. Appl. Phys. 113, 193106 (2013); http://dx.doi.org/10.1063/1.4807285 (4 pages)

Online Publication Date: 20 May 2013

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multimedia

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In this paper, we demonstrate a bidirectional optical switch based on electrowetting. Four rectangular polymethyl methacrylate substrates are stacked to form the device and three ITO electrodes are fabricated on the bottom substrate. A black liquid droplet is placed on the middle of the ITO electrode and surrounded by silicone oil. When we apply a voltage to one ITO electrode, the droplet stretches and moves in one direction and a light beam is covered by the stretched droplet, while the droplet yields a space to let the original blocked light pass through. Due to the shift of the droplet, our device functions as a bidirectional optical switch. Our experiment shows that the device can obtain a wide optical attenuation from ∼1 dB to 30 dB and the transmission loss is ∼0.67 dB. The response time of the device is ∼177 ms. The proposed optical switch has potential applications in variable optical attenuators, electronic displays, and light shutters.
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42.79.Ta Optical computers, logic elements, interconnects, switches; neural networks
back to top Structural, Mechanical, Thermodynamic, and Optical Properties of Condensed Matter

Adhesion and friction control localized folding in supported graphene

K. Zhang and M. Arroyo

J. Appl. Phys. 113, 193501 (2013); http://dx.doi.org/10.1063/1.4804265 (8 pages)

Online Publication Date: 15 May 2013

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Graphene deposited on planar surfaces often exhibits sharp and localized folds delimiting seemingly planar regions, as a result of compressive stresses transmitted by the substrate. Such folds alter the electronic and chemical properties of graphene, and therefore, it is important to understand their emergence, to either suppress them or control their morphology. Here, we study the emergence of out-of-plane deformations in supported and laterally strained graphene with high-fidelity simulations and a simpler theoretical model. We characterize the onset of buckling and the nonlinear behavior after the instability in terms of the adhesion and frictional material parameters of the graphene-substrate interface. We find that localized folds evolve from a distributed wrinkling linear instability due to the nonlinearity in the van der Waals graphene-substrate interactions. We identify friction as a selection mechanism for the separation between folds, as the formation of far apart folds is penalized by the work of friction. Our systematic analysis is a first step towards strain engineering of supported graphene, and is applicable to other compressed thin elastic films weakly coupled to a substrate.
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81.40.Pq Friction, lubrication, and wear
62.20.Qp Friction, tribology, and hardness
81.40.Lm Deformation, plasticity, and creep
62.20.mq Buckling
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
68.55.aj Insulators

First-principles study of temperature-dependent diffusion coefficients: Hydrogen, deuterium, and tritium in α-Ti

Yong Lu and Ping Zhang

J. Appl. Phys. 113, 193502 (2013); http://dx.doi.org/10.1063/1.4805362 (5 pages)

Online Publication Date: 15 May 2013

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We report the prediction of temperature-dependent diffusion coefficients of interstitial hydrogen, deuterium, and tritium atoms in α-Ti using transition state theory. The microscopic parameters in the pre-factor and activation energy of the impurity diffusion coefficients are obtained from first-principles total energy and phonon calculations including the full coupling between the vibrational modes of the diffusing atom with the host lattice. The dual occupancy case of impurity atom in the hcp matrix is considered, and four diffusion paths are combined to obtain the final diffusion coefficients. The calculated diffusion parameters show good agreement with experiments. Our numerical results indicate that the diffusions of deuterium and tritium atoms are slower than that of the hydrogen atom at temperatures above 425 K and 390 K, respectively.
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66.30.J- Diffusion of impurities
61.72.jj Interstitials
66.30.Dn Theory of diffusion and ionic conduction in solids
63.20.kd Phonon-electron interactions

Thermal and optical properties of glass and crystalline phases formed in the binary R2O3-Al2O3 (R = La-Lu and Y) system

Malahalli Vijaya Kumar, Takehiko Ishikawa, B. Basavalingu, Junpei T. Okada, and Yuki Watanabe

J. Appl. Phys. 113, 193503 (2013); http://dx.doi.org/10.1063/1.4805536 (8 pages)

Online Publication Date: 15 May 2013

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Rare-earth aluminate (RAlO3, R = La–Lu and Y) glass and crystalline phases were prepared by containerless levitation in an aerodynamic levitation furnace. In the RAlO3 system, La, Nd and Sm aluminum perovskites solidified as glass and Eu–Lu and Y aluminum perovskites solidified as crystalline phases. The glass forming region decreased with decreasing ionic radius of the rare-earth element. Scanning electron microscopy images and x-ray diffraction results revealed the formation of a single RAlO3 phase from the undercooled melt. The glass transition temperature, Tg, and density increased and the molar volume decreased with decreasing rare-earth element ionic radius. The refractive index at 589 nm exceeds 1.85 in each composition and a transparency of approximately 72% was achieved for the LaAlO3 glass.
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78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)
61.43.Fs Glasses
64.70.D- Solid-liquid transitions
81.30.Fb Solidification
64.70.P- Glass transitions of specific systems

Structural and vibrational stability of M and Z phases of silicon and germanium from first principles

A. Bautista-Hernández, T. Rangel, A. H. Romero, G.-M. Rignanese, M. Salazar-Villanueva, and E. Chigo-Anota

J. Appl. Phys. 113, 193504 (2013); http://dx.doi.org/10.1063/1.4804668 (7 pages)

Online Publication Date: 15 May 2013

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First-principles calculations were performed to investigate the structural feasibility of M and Z phases (novel monoclinic and orthorhombic structures recently reported for carbon) for silicon and germanium. The lattice parameters, bulk modulus, vibrational properties, and elastic constants are calculated using the local density approximation to describe the exchange-correlation energy, while the optical properties are calculated by using Many-Body Perturbation Theory in the G0W0 approximation. Our results indicate that silicon and germanium with the proposed crystal symmetries are elastically and vibrationally stable and are small band-gap semiconductors. We discuss the possible synthesis of such materials.
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63.20.D- Phonon states and bands, normal modes, and phonon dispersion
81.40.Jj Elasticity and anelasticity, stress-strain relations
62.20.de Elastic moduli
62.20.dq Other elastic constants
61.66.Bi Elemental solids
78.30.Am Elemental semiconductors and insulators

Controlled synthesis of buried delta-layers of Ag nanocrystals for near-field plasmonic effects on free surfaces

Patrizio Benzo, Caroline Bonafos, Maxime Bayle, Robert Carles, Laura Cattaneo, Cosmin Farcau, Gérard Benassayag, Béatrice Pécassou, and Dominique Muller

J. Appl. Phys. 113, 193505 (2013); http://dx.doi.org/10.1063/1.4804171 (7 pages)

Online Publication Date: 16 May 2013

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We report on the shallow synthesis by low energy ion implantation of delta-layers of Ag nanocrystals in SiO2 at few nanometers under its free surface. Transmission electron microscopy observations, ballistic simulations, and reflectance measurements are coupled to define the conditions for which the synthesis is fully controlled and when, on the contrary, this control is lost. We show that low dose implantation leads to the formation of a well-defined single plane of nanocrystals, while for larger doses, sputtering and diffusion effects limit the control of the size, position, and volume amount of these nanocrystals. This paper provides the experimental evidence of the incorporated dose saturation predicted in the literature when implanting metal ions at high doses in glass matrices. Its consequences on the particle population and the plasmonic optical response of the composite layers are carefully analyzed. We show here that this saturation phenomenon is underestimated in standard simulation predictions due to diffusion of metal atoms towards the surface and nanocrystal nucleation during the implantation process.
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81.16.-c Methods of micro- and nanofabrication and processing
61.46.-w Structure of nanoscale materials
81.07.Bc Nanocrystalline materials
61.72.U- Doping and impurity implantation
81.15.Cd Deposition by sputtering
64.60.Q- Nucleation

Direct pulsed laser crystallization of nanocrystals for absorbent layers in photovoltaics: Multiphysics simulation and experiment

Martin Y. Zhang, Qiong Nian, Yung Shin, and Gary J. Cheng

J. Appl. Phys. 113, 193506 (2013); http://dx.doi.org/10.1063/1.4805039 (8 pages)

Online Publication Date: 17 May 2013

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Direct pulsed laser crystallization (DPLC) of nanoparticles of photoactive material—Copper Indium Selenide (nanoCIS) is investigated by multiphysics simulation and experiments. Laser interaction with nanoparticles is fundamentally different from their bulk counterparts. A multiphysics electromagnetic-heat transfer model is built to simulate DPLC of nanoparticles. It is found smaller photoactive nanomaterials (e.g., nanoCIS) require less laser fluence to accomplish the DPLC due to their stronger interactions with incident laser and lower melting point. The simulated optimal laser fluence is validated by experiments observation of ideal microstructure. Selectivity of DPLC process is also confirmed by multiphysics simulation and experiments. The combination effects of pulse numbers and laser intensity to trigger laser ablation are investigated in order to avoid undesired results during multiple laser processing. The number of pulse numbers is inversely proportional to the laser fluence to trigger laser ablation.
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81.05.Hd Other semiconductors
81.15.Fg Pulsed laser ablation deposition
81.16.-c Methods of micro- and nanofabrication and processing
72.40.+w Photoconduction and photovoltaic effects

Kinetics and thermodynamics associated with Bi adsorption transitions at Cu and Ni grain boundaries

Kaiping Tai, Lin Feng, and Shen J. Dillon

J. Appl. Phys. 113, 193507 (2013); http://dx.doi.org/10.1063/1.4805361 (7 pages)

Online Publication Date: 17 May 2013

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The grain boundary diffusivity of Au in Cu and Cu-Bi, and Cu in Ni and Ni-Bi are characterized by secondary ion mass spectroscopy depth profiling. Samples are equilibrated in a Bi containing atmosphere at temperatures above and below the onset of grain boundary adsorption transitions, sometimes called complexion transitions. A simple thermo-kinetic model is used to estimate the relative entropic contributions to the grain boundary energies. The results indicate that the entropy term plays a major role in promoting thermally and chemically induced grain boundary complexion transition.
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81.05.Bx Metals, semimetals, and alloys
68.43.Mn Adsorption kinetics
66.30.Lw Diffusion of other defects
61.72.Mm Grain and twin boundaries
65.40.gd Entropy
79.20.Rf Atomic, molecular, and ion beam impact and interactions with surfaces

Modeling of plasticity and fracture of metals at shock loading

A. E. Mayer, K. V. Khishchenko, P. R. Levashov, and P. N. Mayer

J. Appl. Phys. 113, 193508 (2013); http://dx.doi.org/10.1063/1.4805713 (7 pages)

Online Publication Date: 20 May 2013

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In this paper, we present a model of dislocation plasticity and fracture of metals, which in combination with the wide-range equation of state and the continuum mechanics equations is a necessary component for simulation of the shock-wave loading. We take into account immobilization of dislocations and nucleation of micro-voids in weakened zones of substance; this is distinguished feature of the present version of the model. Accounting of the dislocations immobilization provides a better description of the unloading wave structure, while the detailed consideration of processes in the weakened zones expands the domain of applicability of fracture model to higher strain rates. We compare our results with the experimental data for the shock loading of aluminum, copper, and nickel samples; the comparison indicates satisfactory description of the elastic precursor, unloading wave, and spall pulse. Using the model, we investigate intently the early stage of the shock formation in solids; it is found out that the elastic precursor is formed even for a strong shock wave, and initially the precursor has very large amplitude and propagation velocity.
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81.40.Lm Deformation, plasticity, and creep
81.40.Np Fatigue, corrosion fatigue, embrittlement, cracking, fracture, and failure
61.72.Bb Theories and models of crystal defects
61.72.Qq Microscopic defects (voids, inclusions, etc.)
62.20.fq Plasticity and superplasticity
62.20.mm Fracture
62.50.Ef Shock wave effects in solids and liquids

Efficiency limits for the rectification of solar radiation

Heylal Mashaal and Jeffrey M. Gordon

J. Appl. Phys. 113, 193509 (2013); http://dx.doi.org/10.1063/1.4805819 (4 pages)

Online Publication Date: 20 May 2013

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Efficiency limits for rectifying (converting AC to DC) incoherent broadband radiation are presented, prompted by establishing a fundamental bound for solar rectennas. For an individual full-wave rectifier, the bound is 2/π. The efficiency boosts attainable with cascaded rectifiers are also derived. The derivation of the broadband limit follows from the analysis of an arbitrary number of random-phase sinusoidal signals, which is also relevant for harvesting ambient radio-frequency radiation from a discrete number of uncorrelated sources.
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88.40.F- Solar concentrators
84.30.Jc Power electronics; power supply circuits
84.40.Ba Antennas: theory, components and accessories

Thermoluminescence properties of Tl2Ga2S3Se layered single crystals

S. Delice, M. Isik, E. Bulur, and N. M. Gasanly

J. Appl. Phys. 113, 193510 (2013); http://dx.doi.org/10.1063/1.4807165 (5 pages)

Online Publication Date: 20 May 2013

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The trap center(s) in Tl2Ga2S3Se single crystals has been investigated from thermoluminescence (TL) measurements in the temperature range of 10–300 K. Curve fitting, initial rise, and peak shape methods were applied to observed TL glow curve to evaluate the activation energy, capture cross section, and attempt-to-escape frequency of the trap center. One trapping center has been revealed with activation energy of 16 meV. Moreover, the characteristics of trap distribution have been studied using an experimental technique based on different illumination temperature. An increase of activation energy from 16 to 58 meV was revealed for the applied illumination temperature range of 10–25 K.
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78.60.Kn Thermoluminescence

Medium range order engineering in amorphous silicon thin films for solid phase crystallization

Felix Law, Per I. Widenborg, Joachim Luther, and Bram Hoex

J. Appl. Phys. 113, 193511 (2013); http://dx.doi.org/10.1063/1.4807166 (6 pages)

Online Publication Date: 20 May 2013

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In recent years, it has been recognized that medium range ordering (MRO) in amorphous silicon (a-Si:H) plays a role in controlling its solid phase crystallization (SPC) behavior. Information on the MRO can be obtained from the width of the first X-ray diffraction (XRD) peak of a-Si:H centered around 2θ = 27.5°. The broader the full width half maximum (FWHM) of the first XRD peak, the less ordered the a-Si:H material in the medium range length scale (up to 5 nm). In this work, it was found that the FWHM of the first XRD peak changes with the pressure used during the deposition of a-Si:H. A threshold SPC behavior was observed as a function of the a-Si:H deposition pressure and a good correlation between the SPC behavior and the a-Si:H XRD peak width was found. Results in this study indicate that higher MRO in a-Si:H led to faster SPC rates and smaller grain sizes, suggesting the presence of relatively active and high density of nucleation sites. High angle annular dark field scanning transmission electron microscopy and ultraviolet reflectance indicate that films with higher MRO yielded polycrystalline silicon (poly-Si) grains which were more defective and non-columnar in morphology. Results suggest that a-Si:H material with lower MRO were preferred as a precursor for SPC, which forms a better quality poly-Si thin film material. It was proposed that ion bombardment seems to play a role in altering the a-Si:H properties.
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68.55.ag Semiconductors
61.43.Dq Amorphous semiconductors, metals, and alloys
78.66.Db Elemental semiconductors and insulators
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
64.70.dg Crystallization of specific substances
68.37.Ma Scanning transmission electron microscopy (STEM)

On the structural stability of CeN at high pressures: Ab initio calculations

B. D. Sahoo, K. D. Joshi, and Satish C. Gupta

J. Appl. Phys. 113, 193512 (2013); http://dx.doi.org/10.1063/1.4803884 (7 pages)

Online Publication Date: 20 May 2013

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The structural stability of CeN under hydrostatic compression has been analyzed theoretically. The comparison of enthalpies calculated as a function of hydrostatic compression for rocksalt type (B1), tetragonal (B10), and CsCl type (B2) structures suggests that the B1 phase will transform to B10 structure at ∼53 GPa, which upon further compression will transform to B2 phase at ∼200 GPa. However, the static high pressure energy dispersive x-ray diffraction measurements on CeN by Olsen et al. [J. Alloys Compd. 533, 29 (2012)] report that the B1 phase transforms directly to B2 phase at ∼65 GPa. To resolve the discrepancy between our calculations and experimental results, we have performed lattice dynamic calculations on these structures. The phonon spectra calculated at zero pressure correctly show B1 phase to be dynamically stable, and B10 and B2 to be unstable. At 60 GPa, the B1 phase becomes dynamically unstable and the B10 structure emerges as a dynamically stable phase whereas B2 still remains unstable. At still higher pressure of ≥200 GPa, the B2 phase becomes not only the lowest enthalpy structure but also dynamically stable. These findings support the results of our static lattice calculations. Further, our calculated angle dispersive x-ray diffraction pattern of B1, B10, and B2 phases shows that most of the diffraction peaks of B10 phase except few weak peaks coincide with the peaks of either B1 or B2 phase; which may pose a difficulty in unambiguously identifying the high pressure phase until a sufficient amount of B1 phase is transformed to the new structure so that the weak peaks, if present, are also visible.
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62.50.-p High-pressure effects in solids and liquids
65.40.G- Other thermodynamical quantities
64.70.K- Solid-solid transitions
63.20.dk First-principles theory
back to top Electronic Structure and Transport

Graphene Hall bar with an asymmetric pn-junction

S. P. Milovanović, M. Ramezani Masir, and F. M. Peeters

J. Appl. Phys. 113, 193701 (2013); http://dx.doi.org/10.1063/1.4805350 (8 pages)

Online Publication Date: 15 May 2013

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We investigated the magnetic field dependence of the Hall and the bend resistances in the ballistic regime for a single layer graphene Hall bar structure containing a pn-junction. When both regions are n-type the Hall resistance dominates and Hall type of plateaus are formed. These plateaus occur as a consequence of the restriction on the angle imposed by Snell's law allowing only electrons with a certain initial angles to transmit though the potential step. The size of the plateau and its position is determined by the position of the potential interface as well as the value of the applied potential. When the second region is p-type, the bend resistance dominates, which is asymmetric in field due to the presence of snake states. Changing the position of the pn-interface in the Hall bar strongly affects these states and therefore the bend resistance is also changed. Changing the applied potential, we observe that the bend resistance exhibits a peak around the charge-neutrality point (CNP), which is independent of the position of the pn-interface, while the Hall resistance shows a sign reversal when the CNP is crossed, which is in very good agreement with a recent experiment [J. R. Williams and C. M. Marcus, Phys. Rev. Lett. 107, 046602 (2011)].
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81.05.ue Graphene
72.20.My Galvanomagnetic and other magnetotransport effects
72.20.Ht High-field and nonlinear effects
73.23.Ad Ballistic transport
72.80.Vp Electronic transport in graphene

Cross-plane thermoelectric transport in p-type La0.67Sr0.33MnO3/LaMnO3 oxide metal/semiconductor superlattices

Pankaj Jha, Timothy D. Sands, Philip Jackson, Cory Bomberger, Tela Favaloro, Stephen Hodson, Joshua Zide, Xianfan Xu, and Ali Shakouri

J. Appl. Phys. 113, 193702 (2013); http://dx.doi.org/10.1063/1.4804937 (8 pages)

Online Publication Date: 16 May 2013

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The cross-plane thermoelectric transport properties of La0.67Sr0.33MnO3 (LSMO)/LaMnO3 (LMO) oxide metal/semiconductor superlattices were investigated. The LSMO and LMO thin-film depositions were performed using pulsed laser deposition to achieve low resistivity constituent materials for LSMO/LMO superlattice heterostructures on (100)-strontium titanate substrates. X-ray diffraction and high-resolution reciprocal space mapping indicate that the superlattices are epitaxial and pseudomorphic. Cross-plane devices were fabricated by etching cylindrical pillar structures in superlattices using inductively, this coupled-plasma reactive-ion etching. The cross-plane electrical conductivity data for LSMO/LMO superlattices reveal a lowering of the effective barrier height to 223 meV as well as an increase in cross-plane conductivity by an order of magnitude compared to high resistivity superlattices. These results suggest that controlling the oxygen deficiency in the constituent materials enables modification of the effective barrier height and increases the cross-plane conductivity in oxide superlattices. The cross-plane LSMO/LMO superlattices showed a giant Seebeck coefficient of 2560 μV/K at 300 K that increases to 16 640 μV/K at 360 K. The giant increase in the Seebeck coefficient with temperature may include a collective contribution from the interplay of charge, spin current, and phonon drag. The low resistance oxide superlattices exhibited a room temperature cross-plane thermal conductivity of 0.92 W/m K, this indicating that the suppression of thermal conductivities due to the interfaces is preserved in both low and high resistivity superlattices. The high Seebeck coefficient, the order of magnitude improvement in cross-plane conductivity, and the low thermal conductivity in LSMO/LMO superlattices resulted in a two order of magnitude increase in cross-plane power factor and thermoelectric figure of merit (ZT), compared to the properties of superlattices with higher resistivity that were reported previously. The temperature dependence of the cross-plane power factor in low resistance superlattices suggests a direction for further investigations of the potential LSMO/LMO oxide superlattices for thermoelectric devices.
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72.20.Pa Thermoelectric and thermomagnetic effects
81.15.Fg Pulsed laser ablation deposition
68.65.-k Low-dimensional, mesoscopic, nanoscale and other related systems: structure and nonelectronic properties
52.77.Bn Etching and cleaning
63.20.kd Phonon-electron interactions
68.55.ag Semiconductors

The role of surface states in modification of carrier transport in silicon nanowires

Kamran Rasool, M. A. Rafiq, Mushtaq Ahmad, Zahid Imran, and M. M. Hasan

J. Appl. Phys. 113, 193703 (2013); http://dx.doi.org/10.1063/1.4805031 (5 pages)

Online Publication Date: 16 May 2013

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We investigate transport properties of polyacrylic acid (PAA) capped n and p-type silicon nanowire (SiNW) arrays. PAA diluted with deionized water at different concentrations was spun directly on vertically grown SiNW arrays prepared by metal assisted electroless chemical etching. PAA provides mechanical support to electrical contacts and acts as a source of interface doping by creating acceptor like states (holes) on SiNWs surface. PAA capping results in increase in current in p-type SiNWs and decrease in current in n-type SiNWs. Schottky emission model fits current voltage (IV) characteristics of p-type SiNWs/PAA device. Ohmic like conduction at lower voltages followed by space charge limited current (SCLC) with and without traps is observed in p-type SiNWs, n-type SiNWs, and n-type SiNWs/PAA devices. Using SCLC model with exponential distribution of traps, the extracted trap density was 7.20 × 1011/cm3 and 6.0 × 1011/cm3 for p-type SiNWs and n-type SiNWs devices, respectively. Our findings also demonstrate that the carrier concentration in SiNWs depends not only on doping concentration but also depends significantly on density of surface states.
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81.05.Cy Elemental semiconductors
81.15.Pq Electrodeposition, electroplating
81.16.-c Methods of micro- and nanofabrication and processing
72.20.Jv Charge carriers: generation, recombination, lifetime, and trapping
73.30.+y Surface double layers, Schottky barriers, and work functions
73.63.Nm Quantum wires

Progressive structural and electronic properties of nano-structured carbon atomic chains

D. Usanmaz and G. P. Srivastava

J. Appl. Phys. 113, 193704 (2013); http://dx.doi.org/10.1063/1.4805047 (6 pages)

Online Publication Date: 16 May 2013

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Ab initio calculations, based on the planewave pseudopotential method and the density functional theory, have been reported on the changes in the electronic and structural properties of short carbon atomic chains held rigidly between hydrogenated thin armchair graphene nanoribbons (Na-AGNR) of dimer line numbers Na = 4 and 5. We have considered chains of several lengths (n = 4–9 atoms) and with different forms of attachment with the AGNRs. It is found that odd-numbered chains are metallic in nature, with chemical bonding more like ⋯ C=C=C=C⋯ (as in cumulene). Even numbered chains show semiconductor structure when held between 4-AGNR and semi-metallic nature when held between 5-AGNR, in both cases characterized by chemical bonding more like ⋯ C–C≣C–C≣C⋯ (as in polyyne).
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61.48.Gh Structure of graphene
73.22.Pr Electronic structure of graphene
61.46.Df Structure of nanocrystals and nanoparticles ("colloidal" quantum dots but not gate-isolated embedded quantum dots)
71.15.Ap Basis sets (LCAO, plane-wave, APW, etc.) and related methodology (scattering methods, ASA, linearized methods, etc.)
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

Electronic structure and thermoelectric properties of half-Heusler Zr0.5Hf0.5NiSn by first-principles calculations

D. F. Zou, S. H. Xie, Y. Y. Liu, J. G. Lin, and J. Y. Li

J. Appl. Phys. 113, 193705 (2013); http://dx.doi.org/10.1063/1.4804939 (7 pages)

Online Publication Date: 17 May 2013

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The electronic structures of Zr0.5Hf0.5NiSn and the parent compounds ZrNiSn and HfNiSn are investigated by using first-principles calculations, and the thermoelectric properties are calculated on the base of the semi-classical Boltzmann transport theory and the empirical thermal conductivity model. The temperature dependence of thermoelectric transport properties of these three compounds is discussed and compared with experimental data, and good agreements are observed. To further optimize the thermoelectric performance of the Zr0.5Hf0.5NiSn compound, the chemical potential dependence of electrical transport properties at three different temperatures is investigated, and the maximum power factors and corresponding optimal p- or n-type doping levels are evaluated, suggesting that the compound has better thermoelectric performance when it is p-type doped.
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72.20.Pa Thermoelectric and thermomagnetic effects
66.70.Df Metals, alloys, and semiconductors
71.15.-m Methods of electronic structure calculations
71.20.Nr Semiconductor compounds
61.72.up Other materials

Principal physical properties of GaN/AlN multiquantum well systems determined by density functional theory calculations

Pawel Strak, Pawel Kempisty, Maria Ptasinska, and Stanislaw Krukowski

J. Appl. Phys. 113, 193706 (2013); http://dx.doi.org/10.1063/1.4805057 (15 pages)

Online Publication Date: 17 May 2013

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A critical comparison of three polarization based approaches with the fields in AlN/GaN multiple quantum wells (MQWs) systems proved that they give identical results. The direct density functional theory (DFT) results, i.e., the fields, are in qualitative agreement with data obtained within the polarization theory. The results of DFT calculations of an AlN/GaN MQW system were used in the projection method to obtain a spatial distribution of the bands in the structure with atomic resolution. In parallel, the plane averaged and c-smoothed potential profiles obtained from the solution of the Poisson equation were used to determine the electric field in the multiquantum well structures and the magnitude of dipole layers at the AlN/GaN heterostructures. The dipole layers cause potential jumps of about 2.4 V that seriously affects the band offsets. The presence of the dipole layer is in good agreement with the potential measurements by electron holography. It was shown that the wells of the width up to 4 Ga layers behave as potential minima, but the wider layers behave as standard quantum wells. The barriers up to 3 Al layers do not localize the carriers. It is shown that the Quantum Confined Stark Effect causes a huge decrease of their energies and oscillator strengths of the optical transitions, especially for wider structures. For wider wells, the strengths fall much faster for perpendicular polarization which indicates the important role of the anisotropic band offsets. A direct simulation shows that the band offset for the valence band crystal field split off hole states, i.e., pz states are different from heavy and light hole (i.e., p⊥ = px⊗py) states being equal to valence band offset (VBO)⊥ = 0.85 eV and rough estimate of VBOII ≅ 0.5 eV, respectively. These values are in good agreement with the recently reported measurement of AlN/GaN offsets.
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68.65.Fg Quantum wells
71.70.Ch Crystal and ligand fields
73.21.Fg Quantum wells
78.20.Jq Electro-optical effects
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