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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
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
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
back to top Magnetism and Superconductivity

Investigating the magnetic field effect on electron-hole pair in organic semiconductor devices

W. Qin, K. Gao, S. Yin, and S. J. Xie

J. Appl. Phys. 113, 193901 (2013); http://dx.doi.org/10.1063/1.4805044 (4 pages)

Online Publication Date: 15 May 2013

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By constructing dynamic equations including electrons, holes and their pair densities, we calculate the magnetoconductance (MC) and the magnetoelectroluminescence (MEL) separately. It is indicated that MC and MEL may result from different response on the applied magnetic field. MC is from the scattering of polarons by magnetic field related triplet excitons, while MEL is mainly from magnetic field related conversion between singlet and triplet electron-hole pairs. Furthermore, we discuss the relation between MC and MEL. The theoretical calculations are well consistent with the experimental results.
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72.20.Jv Charge carriers: generation, recombination, lifetime, and trapping
72.20.My Galvanomagnetic and other magnetotransport effects
78.20.Ls Magneto-optical effects
78.60.Fi Electroluminescence
71.35.-y Excitons and related phenomena
71.38.-k Polarons and electron-phonon interactions

Coercivity enhancement of anisotropic die-upset Nd-Fe-B powders by Pr-Cu alloy diffusion

Fangming Wan, Jingzhi Han, Yinfeng Zhang, Xiaodong Zhang, Jinbo Yang, Yingchang Yang, Boping Hu, Xiaolei Rao, Daoyan Cai, Zhian Chen, Renjie Chen, and Aru Yan

J. Appl. Phys. 113, 193902 (2013); http://dx.doi.org/10.1063/1.4805048 (4 pages)

Online Publication Date: 16 May 2013

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The grain boundary diffusion treatment using the low melting point Pr-Cu eutectic alloy was applied to enhance the coercivity of anisotropic die-upset Nd-Fe-B powders. It was found that the room temperature coercivity of the powders increases from 13 kOe to 24 kOe. The thickness of the grain boundary layer was increased and the Fe and Co content in the grain boundary phases was decreased. The modified boundary layers may thus increase the domain wall nucleation field, leading to an enhancement in the coercivity. The diffusion of Pr-Cu alloy prevents the loss of continuity of the Nd-rich grain boundary phase and retains the crystal texture in the initial grains.
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75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
61.72.Mm Grain and twin boundaries
66.30.-h Diffusion in solids
75.30.Gw Magnetic anisotropy
75.50.Bb Fe and its alloys
75.50.Tt Fine-particle systems; nanocrystalline materials

Slater-Pauling behavior in LiMgPdSn-type multifunctional quaternary Heusler materials: Half-metallicity, spin-gapless and magnetic semiconductors

K. Özdoğan, E. Şaşıoğlu, and I. Galanakis

J. Appl. Phys. 113, 193903 (2013); http://dx.doi.org/10.1063/1.4805063 (5 pages)

Online Publication Date: 17 May 2013

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We employ ab-initio electronic structure calculations to study 60 LiMgPdSn-type (also known as LiMgPdSb-type) quaternary Heusler compounds. All compounds obey the Slater-Pauling rule with diverse electronic and magnetic properties. 41 compounds are found to be half-metals, 8 spin-gapless semiconductors, and 9 semiconductors. CoVTiAl and CrVTiAl compounds are identified as ferromagnetic and antiferromagnetic semiconductors, respectively, with large energy gaps in both spin directions. All magnetic compounds are expected to have high Curie temperatures making them suitable for spintronics/magnetoelectronics applications.
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71.20.Gj Other metals and alloys
75.30.Kz Magnetic phase boundaries (including classical and quantum magnetic transitions, metamagnetism, etc.)
75.50.Bb Fe and its alloys
75.50.Ee Antiferromagnetics
75.50.Pp Magnetic semiconductors
71.15.-m Methods of electronic structure calculations
back to top Dielectrics and Ferroelectricity

Size dependent magnetic and dielectric properties of nano CoFe2O4 prepared by a salt assisted gel-combustion method

K. Vasundhara, S. N. Achary, S. K Deshpande, P. D. Babu, S. S. Meena, and A. K. Tyagi

J. Appl. Phys. 113, 194101 (2013); http://dx.doi.org/10.1063/1.4804946 (9 pages)

Online Publication Date: 15 May 2013

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In this communication, we report the preparation and properties of nano-CoFe2O4 by gel combustion in presence of KCl and subsequent heat treatments. The products were characterized by X-ray diffraction, Infrared spectroscopy, and Mössbauer Spectroscopy. Spinel type structure with all Fe in 3+ oxidation states was confirmed from the XRD and Mossbauer spectroscopy. The average crystallite sizes of the studied samples were about 6 and 50 nm. Low temperature magnetic and dielectric properties of the samples were studied by superconducting quantum interference device magnetometry and ac-impedance spectroscopy. The field and temperature dependent magnetization studies indicated superparamagnetic nature for 6 nm sample and ferromagnetic nature for 50 nm sample. The temperature-dependent dielectric properties measured over a wide range of frequencies indicated an increasing trend of dielectric permittivity with the decrease in crystallite size. Variable range polaron hopping conduction was observed in both samples.
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75.75.Cd Fabrication of magnetic nanostructures
76.80.+y Mössbauer effect; other γ-ray spectroscopy
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
75.50.Dd Nonmetallic ferromagnetic materials
77.22.Ch Permittivity (dielectric function)
72.20.Ee Mobility edges; hopping transport

Flexoelectric effect on the electroelastic responses of bending piezoelectric nanobeams

Z. Yan and L. Y. Jiang

J. Appl. Phys. 113, 194102 (2013); http://dx.doi.org/10.1063/1.4804949 (9 pages)

Online Publication Date: 15 May 2013

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Flexoelectricity, referring to a spontaneous electric polarization induced by strain gradient in dielectrics, presents a strong size dependency at the nanoscale. In the current work, the influence of the flexoelectric effect on the mechanical and electrical properties of bending piezoelectric nanobeams with different boundary conditions is investigated. Based on the extended linear piezoelectricity theory and the Euler beam model, analytical solutions of the electroelastic fields in the piezoelectric nanobeams subjected to both electrical and mechanical loads are obtained with the inclusion of the flexoelectric effect. Simulation results show that the flexoelectric effect on the elastic behavior of bending beams is sensitive to the beam boundary conditions and the applied electrical load. In addition, for a cantilever piezoelectric nanobeam, an axial relaxation strain is induced from the piezoelectric and flexoelectric effects, while these effects induce a resultant axial force in both the clamped-clamped and simply supported piezoelectric nanobeams. Results also indicate that the flexoelectric effect plays a significant role in the contact stiffness and electric polarization of piezoelectric beams when their thickness is at the nanoscale. It is found that the flexoelectric effect on the electroelastic responses of piezoelectric nanobeams is more pronounced for the beams with smaller thickness. These results are useful for understanding the fundamental mechanical and physical properties of bending piezoelectric nanobeams.
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61.46.-w Structure of nanoscale materials
77.65.-j Piezoelectricity and electromechanical effects
77.55.H- Piezoelectric and electrostrictive films
77.84.-s Dielectric, piezoelectric, ferroelectric, and antiferroelectric materials

Mesoscopic analysis of leakage current suppression in ZrO2/Al2O3/ZrO2 nano-laminates

Dominik Martin, Matthias Grube, Wenke Weinreich, Johannes Müller, Walter M. Weber, Uwe Schröder, Henning Riechert, and Thomas Mikolajick

J. Appl. Phys. 113, 194103 (2013); http://dx.doi.org/10.1063/1.4804670 (8 pages)

Online Publication Date: 15 May 2013

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Metal-Insulator-Metal capacitors, with ZrO2/Al2O3/ZrO2 (ZAZ)-nanolaminate thin-films as a dielectric layer, exhibit reduced leakage currents compared to corresponding capacitors based on pure ZrO2 while maintaining a sufficiently high dielectric constant for the DRAM application. This work is a comparative study demonstrating how the incorporation of a small amount of Al is responsible for the suppression of crystallization during deposition. Extensive electrical characterization leads to the identification of a defect band which conductive atomic force microscopy shows to be formed along crystallite grain boundaries, extending through the entire ZrO2-film. The incorporation of a sub-layer of Al2O3 prevents these grain boundaries resulting in an effective reduction of leakage currents, despite the film being in the nanocrystalline phase, necessary for it to exhibit the required high dielectric constant. A transport model based on phonon assisted trap to trap tunneling is proposed.
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84.30.Sk Pulse and digital circuits
84.32.Tt Capacitors
85.40.-e Microelectronics: LSI, VLSI, ULSI; integrated circuit fabrication technology
back to top Nanoscale Science and Design

Voltage-driven perpendicular magnetic domain switching in multiferroic nanoislands

Jia-Mian Hu, T. N. Yang, L. Q. Chen, and C. W. Nan

J. Appl. Phys. 113, 194301 (2013); http://dx.doi.org/10.1063/1.4804157 (9 pages)

Online Publication Date: 15 May 2013

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We show that, using phase-field simulations, large voltage-driven perpendicular magnetic domain switching can be realized in magnetic-ferroelectric nanoislands with relieved substrate constraint, which is difficult in continuous multiferroic layered thin films due to significant substrate clamping. The as-grown magnetic and ferroelectric domain structures in the heterostructured nanoislands can be tailored by engineering their respective geometric sizes and/or the underlying substrate strain. Influences of the lateral size of the island on the dynamic voltage-driven magnetic domain switching are addressed, whereby an optimum lateral size is identified for illustration. Thus, such three-dimensional multiferroic nanoislands should provide great flexibilities for designing novel high-density spintronic/microelectronic devices with purely voltage-driven means.
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75.60.Ch Domain walls and domain structure
75.75.-c Magnetic properties of nanostructures
77.80.Dj Domain structure; hysteresis

Ferromagnetism, adatom effect, and edge reconstruction induced by Klein boundary in graphene nanoribbons

Zhi-qiang Bao, Jun-jie Shi, and Min Zhang

J. Appl. Phys. 113, 194302 (2013); http://dx.doi.org/10.1063/1.4805348 (7 pages)

Online Publication Date: 15 May 2013

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The electronic structure and magnetic characteristics of Klein graphene nanoribbons (KGNRs), as observed by Suenaga and Koshino [K. Suenaga and M. Koshino, Nature 468, 1088 (2010)], are investigated using first-principles calculations. We find three new characteristics induced by the Klein boundary. First, the localized edge states in the KGNRs have a ferromagnetic coupling rather than the antiferromagnetic coupling of the zigzag graphene nanoribbons (ZGNRs). Lieb's theorem is no longer applicable in the KGNRs. Second, the marginal single carbon adatom of the ZGNRs can destroy the edge states nearby. The edge states can recover if the length of the zigzag chains is equal to or greater than five times that of the lattice constant. Finally, we show that the pentagon-heptagon edge can be induced from the Klein boundary.
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73.22.Pr Electronic structure of graphene
75.75.-c Magnetic properties of nanostructures
75.50.Dd Nonmetallic ferromagnetic materials
71.15.-m Methods of electronic structure calculations
61.48.Gh Structure of graphene
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