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17 Nov 2008

Volume 93, Issue 20, Articles (20xxxx)

Issue Cover Spotlight Figure

Appl. Phys. Lett. 93, 201101 (2008); http://dx.doi.org/10.1063/1.3025818 (3 pages)

W. Dai and C. M. Soukoulis
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Alleviation of Fermi-level pinning effect on metal/germanium interface by insertion of an ultrathin aluminum oxide

Yi Zhou, Masaaki Ogawa, Xinhai Han, and Kang L. Wang

Appl. Phys. Lett. 93, 202105 (2008); http://dx.doi.org/10.1063/1.3028343 (3 pages) | Cited 36 times

Online Publication Date: 18 November 2008

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Due to the strong Fermi-level pinning close to the germanium (Ge) valence band edge, all metal/p-type Ge contacts show Ohmic characteristics, while metal/n-type Ge contacts exhibit rectifying behaviors. In this paper, we report a simple method to alleviate this Fermi-level pinning effect by inserting a thin layer of aluminum oxide (Al2O3), formed by oxidation of aluminum (Al), between the metal/Ge interface. The effective Schottky barrier heights of nickel (Ni)/n-type Ge, cobalt (Co)/n-type Ge, and iron (Fe)/n-type Ge decrease from 0.54, 0.62, and 0.61 eV to 0.39, 0.23, and 0.18 eV, respectively, with this thin layer of Al2O3. The tunneling oxide significantly suppresses the Fermi-level pinning, and yet does not restrict the current density. This method seems promising to realize low resistance metal contact to n-type Ge, which is essential to realize n-channel Ge complementary metal-oxide-semiconductor field-effect transistor with metal source and drain.
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73.40.Ns Metal-nonmetal contacts
73.20.At Surface states, band structure, electron density of states
73.30.+y Surface double layers, Schottky barriers, and work functions
73.40.Ei Rectification
85.30.Tv Field effect devices
73.40.Gk Tunneling

Synthesis and transport property of AgSbTe2 as a promising thermoelectric compound

Heng Wang, Jing-Feng Li, Minmin Zou, and Tao Sui

Appl. Phys. Lett. 93, 202106 (2008); http://dx.doi.org/10.1063/1.3029774 (3 pages) | Cited 18 times

Online Publication Date: 18 November 2008

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Polycrystalline AgSbTe2 ternary compound materials with high phase purity were fabricated using a combined process of mechanical alloying and spark plasma sintering. It was found that stoichiometric AgSbTe2 is a promising composition for low-and-mediate temperature applications, whose ZT reaches 1.59 at 673 K, benefiting from its extremely low thermal conductivity (0.30 W/mK) in addition to its low electrical resistivity (<1.1×10−4 Ω m) and large positive Seebeck coefficient (260 μV/K). On the other hand, deviating from this formula would lead to unstable phase structures and higher thermal conductivity, which make the samples less attractive as thermoelectric materials or components of thermoelectric systems.
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72.20.Pa Thermoelectric and thermomagnetic effects
81.20.Ev Powder processing: powder metallurgy, compaction, sintering, mechanical alloying, and granulation
81.05.Hd Other semiconductors
81.05.Mh Cermets, ceramic and refractory composites

Epitaxial growth of high mobility Cu2O thin films and application to p-channel thin film transistor

Kosuke Matsuzaki, Kenji Nomura, Hiroshi Yanagi, Toshio Kamiya, Masahiro Hirano, and Hideo Hosono

Appl. Phys. Lett. 93, 202107 (2008); http://dx.doi.org/10.1063/1.3026539 (3 pages) | Cited 26 times

Online Publication Date: 20 November 2008

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Cu2O epitaxial films were grown for high mobility p-channel oxide thin-film transistors (TFTs). The use of a (110) MgO surface and fine tuning of a growth condition produced single phase epitaxial films with hole Hall mobilities ∼ 90 cm2 V−1 s−1 comparable to those of single crystals ( ∼ 100 cm2 V−1 s−1). TFTs using the epitaxial film channels exhibited p-channel operation although the field-effect mobilities and the on-to-off current ratio were not yet satisfactory ( ∼ 0.26 cm2 V−1 s−1 and ∼ 6, respectively).
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81.15.Fg Pulsed laser ablation deposition
68.55.ag Semiconductors
85.30.Tv Field effect devices

Valence band offset of the ZnO/AlN heterojunction determined by x-ray photoemission spectroscopy

T. D. Veal, P. D. C. King, S. A. Hatfield, L. R. Bailey, C. F. McConville, B. Martel, J. C. Moreno, E. Frayssinet, F. Semond, and J. Zúñiga-Pérez

Appl. Phys. Lett. 93, 202108 (2008); http://dx.doi.org/10.1063/1.3032911 (3 pages) | Cited 19 times

Online Publication Date: 20 November 2008

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The valence band offset of ZnO/AlN heterojunctions is determined by high resolution x-ray photoemission spectroscopy. The valence band of ZnO is found to be 0.43±0.17 eV below that of AlN. Together with the resulting conduction band offset of 3.29±0.20 eV, this indicates that a type-II (staggered) band line up exists at the ZnO/AlN heterojunction. Using the III-nitride band offsets and the transitivity rule, the valence band offsets for ZnO/GaN and ZnO/InN heterojunctions are derived as 1.37 and 1.95 eV, respectively, significantly higher than the previously determined values.
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73.40.Kp III-V semiconductor-to-semiconductor contacts, p-n junctions, and heterojunctions
79.60.Jv Interfaces; heterostructures; nanostructures

Selective-hydrogen sensing at room temperature with Pt-coated InN nanobelts

Wantae Lim, J. S. Wright, B. P. Gila, S. J. Pearton, F. Ren, Wei-Ta Lai, Li-Chyong Chen, Ming-Shien Hu, and Kuei-Hsien Chen

Appl. Phys. Lett. 93, 202109 (2008); http://dx.doi.org/10.1063/1.3033548 (3 pages) | Cited 8 times

Online Publication Date: 20 November 2008

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The hydrogen sensing characteristics of multiple InN nanobelts grown by metalorganic chemical vapor deposition were investigated. Pt-coated InN sensors could selectively detect hydrogen at the tens of ppm level at 25 °C, while uncoated InN showed no detectable change in current when exposed to hydrogen under the same conditions. Upon exposure to various concentrations of hydrogen (20–300 ppm) in N2 ambient, the relative resistance change increased from 1.2% at 20 ppm H2 to 4% at 300 ppm H2. Approximately 90% of the initial InN resistance was recovered within 2 min by exposing the nanobelts to air. Temperature-dependent measurements showed larger resistance change and faster response at high temperature compared to those at room temperature due to increase in catalytic dissociation rate of H2 as well as diffusion rate of atomic hydrogen into the Pt/InN interface. The Pt-coated InN nanobelt sensors were operated at low power levels ( ∼ 0.5 mW).
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07.07.Df Sensors (chemical, optical, electrical, movement, gas, etc.); remote sensing
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
81.16.Hc Catalytic methods
66.30.Ny Chemical interdiffusion; diffusion barriers
82.30.Lp Decomposition reactions (pyrolysis, dissociation, and fragmentation)

First-principles studies of water adsorption on graphene: The role of the substrate

Tim O. Wehling, Alexander I. Lichtenstein, and Mikhail I. Katsnelson

Appl. Phys. Lett. 93, 202110 (2008); http://dx.doi.org/10.1063/1.3033202 (3 pages) | Cited 49 times

Online Publication Date: 20 November 2008

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We investigate the electronic properties of graphene upon water adsorption and study the influence of the SiO2 substrate in this context using density functional calculations. Perfect suspended graphene is rather insensitive to H2O adsorbates, as doping requires highly oriented H2O clusters. For graphene on a defective SiO2 substrate, we find a strongly different behavior: H2O adsorbates can shift the substrate’s impurity bands and change their hybridization with the graphene bands. In this way, H2O can lead to doping of graphene for much lower adsorbate concentrations than for free hanged graphene. The effect depends strongly on the microscopic substrate properties.
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68.43.-h Chemisorption/physisorption: adsorbates on surfaces
71.15.Mb Density functional theory, local density approximation, gradient and other corrections
61.72.up Other materials
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Structure and magnetic properties of bulk nanocrystalline Dy metal prepared by spark plasma sintering

M. Yue, K. J. Wang, W. Q. Liu, D. T. Zhang, and J. X. Zhang

Appl. Phys. Lett. 93, 202501 (2008); http://dx.doi.org/10.1063/1.3003863 (3 pages) | Cited 4 times

Online Publication Date: 17 November 2008

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The structure and magnetic properties were studied for bulk nanocrystalline dysprosium (Dy) metal prepared by spark plasma sintering method. All the as-prepared samples have hexagonal close packed structure. A decrease in grain size results in remarkable changes in magnetic ordering temperature of the nanocrystalline Dy metal. At 5 K, the magnetization drops by 3.35%, and the coercive force increases by three times for nanocrystalline Dy compared to those of coarse-grained bulk Dy sample. These results indicate the remarkable influence of the nanostructure on the magnetism of Dy due to finite size effect.
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81.07.Bc Nanocrystalline materials
81.05.Bx Metals, semimetals, and alloys
81.16.-c Methods of micro- and nanofabrication and processing
81.20.Ev Powder processing: powder metallurgy, compaction, sintering, mechanical alloying, and granulation
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
75.50.Tt Fine-particle systems; nanocrystalline materials

Giant reversible magnetocaloric effect in cobalt hydroxide nanoparticles

X. H. Liu, W. Liu, W. J. Hu, S. Guo, X. K. Lv, W. B. Cui, X. G. Zhao, and Z. D. Zhang

Appl. Phys. Lett. 93, 202502 (2008); http://dx.doi.org/10.1063/1.3028337 (3 pages) | Cited 6 times

Online Publication Date: 17 November 2008

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The magnetocaloric effect associated with magnetic phase transitions in β-Co(OH)2 nanoparticles has been investigated. A sign change in the magnetocaloric effect is induced by a magnetic field, which is related to a field-induced transition from the antiferromagnetic to the ferromagnetic state below the Néel temperature. The large reversible magnetic-entropy change −ΔSm (20.9 J/kg K at 15 K for a field change of 7 T) indicates that β-Co(OH)2 is a potential candidate for application in magnetic refrigeration in the low-temperature range.
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75.30.Sg Magnetocaloric effect, magnetic cooling
75.30.Kz Magnetic phase boundaries (including classical and quantum magnetic transitions, metamagnetism, etc.)
75.50.Tt Fine-particle systems; nanocrystalline materials
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
75.50.Ee Antiferromagnetics

Mössbauer studies on structural ordering and magnetic properties of melt-spun Ni–Fe–Ga ribbons

N. V. Rama Rao, R. Gopalan, M. Manivel Raja, V. Chandrasekaran, and K. G. Suresh

Appl. Phys. Lett. 93, 202503 (2008); http://dx.doi.org/10.1063/1.3028342 (3 pages) | Cited 1 time

Online Publication Date: 18 November 2008

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Ribbons of ferromagnetic shape memory Ni2+xFe1−xGa (x = 0 and 0.12) alloys were prepared at wheel speeds of 12 and 35 m/s by melt spinning technique. The structural (site) disorder and the associated changes in the magnetic properties of the ribbons were investigated by 57Fe Mössbauer spectroscopy and magnetization measurements respectively. Mössbauer spectra of Ni2FeGa ribbon revealed the presence of both L21 order and B2-like disorder with some amount of Fe antisite atoms on Ni site. The increase in Ni content was found to decrease the TC while its effect on TM was the opposite way.
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76.80.+y Mössbauer effect; other γ-ray spectroscopy
75.50.Bb Fe and its alloys
61.72.-y Defects and impurities in crystals; microstructure
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects

Enhanced Jahn–Teller distortion in the orthorhombic phase of La0.15Ca0.85MnO3 and Y0.15Ca0.85MnO3

P. Tong, Bongju Kim, Daeyoung Kwon, Bog G. Kim, G. Y. Ahn, J. M. S. Park, Sung Baek Kim, and S-W. Cheong

Appl. Phys. Lett. 93, 202504 (2008); http://dx.doi.org/10.1063/1.3035249 (3 pages) | Cited 2 times

Online Publication Date: 19 November 2008

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We report the results of neutron powder diffraction experiments on La0.15Ca0.85MnO3 and Y0.15Ca0.85MnO3. After an incomplete orthorhombic-monoclinic phase transition, the residual orthorhombic phase undergoes an enhancement of Jahn–Teller distortion of MnO6 octahedra. The degree of Jahn–Teller distortion is strongly influenced by coexisting monoclinic phase. Furthermore, ferromagnetism is closely associated with this Jahn–Teller distortion, rather than the buckling effect in the orthorhombic phase. Despite the low concentration of Jahn–Teller active Mn3+ ions, our result indicates an important role of Jahn–Teller effect on physical properties of the lightly electron-doped manganites.
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64.70.K- Solid-solid transitions
71.70.Ej Spin-orbit coupling, Zeeman and Stark splitting, Jahn-Teller effect
81.40.Lm Deformation, plasticity, and creep
62.20.mq Buckling
72.80.Sk Insulators
75.47.Lx Magnetic oxides

Direct observation of changes to domain wall structures in magnetic nanowires of varying width

K. J. O’Shea, S. McVitie, J. N. Chapman, and J. M. R. Weaver

Appl. Phys. Lett. 93, 202505 (2008); http://dx.doi.org/10.1063/1.3023048 (3 pages) | Cited 11 times

Online Publication Date: 20 November 2008

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Lorentz microscopy has been used to explore the structure variation of domain walls in thin Permalloy nanowires in the vicinity of symmetric triangular antinotches. The antinotches present a complex potential landscape to domain walls. Walls can be trapped in front of, partly enter, or be trapped inside the antinotches according to the geometry of the latter and, in the case of vortex domain walls, the chirality. In all cases, the magnetization distribution was determined. Of particular note was the structure the wall assumed during depinning from the antinotch, complex forms extending over distances several times the wire width being observed.
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75.60.Ch Domain walls and domain structure
61.46.Km Structure of nanowires and nanorods (long, free or loosely attached, quantum wires and quantum rods, but not gate-isolated embedded quantum wires)
75.50.Tt Fine-particle systems; nanocrystalline materials
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects

Coherent manipulation of magnetization precession in ferromagnetic semiconductor (Ga,Mn)As with successive optical pumping

Y. Hashimoto and H. Munekata

Appl. Phys. Lett. 93, 202506 (2008); http://dx.doi.org/10.1063/1.3030988 (3 pages) | Cited 12 times

Online Publication Date: 20 November 2008

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We report the dynamic control of magnetization precession by light alone. A ferromagnetic (Ga,Mn)As epilayer was used for experiments. Amplitude of precession was modulated to a large extent by tuning the time interval between two successive optical pump pulses, which induced torques on magnetization through a nonthermal process. Nonlinear effect in precession motion was also discussed.
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75.50.Pp Magnetic semiconductors
78.66.Fd III-V semiconductors
71.20.Nr Semiconductor compounds
75.50.Dd Nonmetallic ferromagnetic materials

Bloch-point-mediated magnetic antivortex core reversal triggered by sudden excitation of a suprathreshold spin-polarized current

X. J. Xing, Y. P. Yu, S. X. Wu, L. M. Xu, and S. W. Li

Appl. Phys. Lett. 93, 202507 (2008); http://dx.doi.org/10.1063/1.3033400 (3 pages) | Cited 3 times

Online Publication Date: 20 November 2008

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We report on spin dynamics of single magnetic antivortices driven by sudden excitation of a spin-polarized direct current. Using micromagnetic simulations, we find that there is a critical current density where spin dynamics transition occurs. Above the critical value, the core is switched on a time scale of ∼ 200 ps through two Bloch points injection after fully suppressed by a quadruple-vortices array, not through the well-known process involving an antivortex-vortex pair creation and annihilation. As the current density decreases, the time required to switch the core increases. The state after full relaxation depends on the current density (the excitation parameter).
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75.40.Gb Dynamic properties (dynamic susceptibility, spin waves, spin diffusion, dynamic scaling, etc.)
75.30.Ds Spin waves
75.60.Jk Magnetization reversal mechanisms
72.25.Hg Electrical injection of spin polarized carriers
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Reorientation of (MnTi″−VO••)× defect dipoles in acceptor-modified BaTiO3 single crystals: An electron paramagnetic resonance study

Lixue Zhang, Emre Erdem, Xiaobing Ren, and Rüdiger-A. Eichel

Appl. Phys. Lett. 93, 202901 (2008); http://dx.doi.org/10.1063/1.3006327 (3 pages) | Cited 24 times

Online Publication Date: 18 November 2008

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The effect of external electric fields on the orientation of (MnTi″−VO••)× defect dipoles in ferroelectric BaTiO3 single crystals and its interplay with the domain structure were investigated by means of electron paramagnetic resonance (EPR) spectroscopy and optical microscopy. The results show that in specimens aged in the ferroelectric state for a defined time, so-termed ferroelectrically aged, the (MnTi″−VO••)× defect dipoles orient along the direction of spontaneous polarization and follow the domain switching upon poling with correspondingly high electric fields. A comparison of the EPR signal in poled aged single-domain samples to that in naturally aged multidomain samples indicates a similar reorientation process of the defect dipole, which means that dipole reorientation rather requires long time, thermal energy, and high electric fields, i.e., more energy.
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77.80.Dj Domain structure; hysteresis
76.30.Mi Color centers and other defects
77.84.Ek Niobates and tantalates
77.84.Cg PZT ceramics and other titanates
61.72.jd Vacancies
77.80.Fm Switching phenomena
77.22.Ej Polarization and depolarization

Half-cycle atomic layer deposition reaction studies of Al2O3 on In0.2Ga0.8As (100) surfaces

M. Milojevic, F. S. Aguirre-Tostado, C. L. Hinkle, H. C. Kim, E. M. Vogel, J. Kim, and R. M. Wallace

Appl. Phys. Lett. 93, 202902 (2008); http://dx.doi.org/10.1063/1.3033404 (3 pages) | Cited 52 times

Online Publication Date: 19 November 2008

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The reduction in III–V interfacial oxides by atomic layer deposition of Al2O3 on InGaAs is studied by interrupting the deposition following individual trimethyl aluminum (TMA) and water steps (half cycles) and interrogation of the resultant surface reactions using in situ monochromatic x-ray photoelectron spectroscopy (XPS). TMA is found to reduce the interfacial oxides during the initial exposure. Concentrations of Ga oxide on the surface processed at 300 °C are reduced to a concentration on the order of a monolayer, while AsOx species are below the level of detection of XPS.
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81.15.-z Methods of deposition of films and coatings; film growth and epitaxy
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
82.65.+r Surface and interface chemistry; heterogeneous catalysis at surfaces
79.60.Bm Clean metal, semiconductor, and insulator surfaces
68.55.aj Insulators
82.30.-b Specific chemical reactions; reaction mechanisms

Achieving a low interfacial density of states in atomic layer deposited Al2O3 on In0.53Ga0.47As

H. C. Chiu, L. T. Tung, Y. H. Chang, Y. J. Lee, C. C. Chang, J. Kwo, and M. Hong

Appl. Phys. Lett. 93, 202903 (2008); http://dx.doi.org/10.1063/1.3027476 (3 pages) | Cited 23 times

Online Publication Date: 19 November 2008

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Atomic-layer-deposited Al2O3 on In0.53Ga0.47As with short air exposure between the oxide and semiconductor deposition has enabled the demonstration of nearly ideal frequency-dependent and quasistatic capacitance-voltage (C-V) characteristics. The excellent quasistatic C-V characteristics indicate a high efficiency of 63% for the Fermi-level movement near the midgap. A low mean interfacial density of states (mathit) ∼ 2.5×1011 cm−2 eV−1 was determined under 1 MHz using a charge pumping method, which was also employed to probe the depth profile of bulk traps (Nbt) and the energy dependence of Dit at 50 kHz: a low Nbt ∼ 7×1018 cm−3 and a Dit of (2–4)×1011 cm−2 eV−1 in the lower half of the band gap and a higher Dit of ∼ 1012 cm−2 eV−1 in the upper half of the band gap. The employment of charge pumping method has given a more accurate determination of Dit, which is usually overestimated using other commonly methods such as the Terman, conductance, and high-low frequencies due to the influence of weak inversion at room temperature.
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73.20.At Surface states, band structure, electron density of states
73.40.Qv Metal-insulator-semiconductor structures (including semiconductor-to-insulator)
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)

Interface control and leakage current conduction mechanism in HfO2 film prepared by pulsed laser deposition

Hao Wang, Y. Wang, J. Zhang, C. Ye, H. B. Wang, J. Feng, B. Y. Wang, Q. Li, and Y. Jiang

Appl. Phys. Lett. 93, 202904 (2008); http://dx.doi.org/10.1063/1.3033526 (3 pages) | Cited 18 times

Online Publication Date: 20 November 2008

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The N2 atmosphere postannealing is introduced to improve the interfacial quality and the dielectric properties of HfO2 films prepared by pulsed laser deposition. The disappearance of interface layer between HfO2 film and Si substrate and the decrease of leakage current densities after annealing are further confirmed by high-resolution cross-sectional transmission electron microscopy investigation and electrical measurement. Electric conduction analysis results show that the dominant leakage current conduction mechanisms of the annealed HfO2 film are the Schottky emission at low electric field, the trap-assisted tunneling, and space-charge-limited current at high electric field for the gate and substrate injections, respectively.
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77.55.-g Dielectric thin films
81.15.Fg Pulsed laser ablation deposition
73.40.Qv Metal-insulator-semiconductor structures (including semiconductor-to-insulator)
68.37.-d Microscopy of surfaces, interfaces, and thin films

Pseudoferroelectricity: A possible scenario for doped ZnO

Alexander K. Tagantsev

Appl. Phys. Lett. 93, 202905 (2008); http://dx.doi.org/10.1063/1.3036537 (3 pages) | Cited 5 times

Online Publication Date: 21 November 2008

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A phenomenological Landau-theory model, where a crystal on cooling from a state with polar symmetry exhibits a maximum of dielectric permittivity and polarization-field hysteresis loops, is offered. The model is equivalent to that of a true ferroelectric in a weak external dc bias. Permittivity maximum occurs as a result of crossing the Widom line. In the model, the role of the external bias plays an intrinsic internal bias originating from a weak polarity of the high temperature state of the material. The results obtained show that the ferroelectriclike phenomena reported for doped ZnO are compatible with a Landau-theory description.
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77.80.Dj Domain structure; hysteresis
77.22.Ch Permittivity (dielectric function)
77.84.Bw Elements, oxides, nitrides, borides, carbides, chalcogenides, etc.
61.72.uj III-V and II-VI semiconductors
71.70.Di Landau levels
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Raman characterization of Ge distribution in individual Si1−xGex alloy nanowires

Chiharu Nishimura, Go Imamura, Minoru Fujii, Takahiro Kawashima, Tohru Saitoh, and Shinji Hayashi

Appl. Phys. Lett. 93, 203101 (2008); http://dx.doi.org/10.1063/1.3028027 (3 pages) | Cited 2 times

Online Publication Date: 17 November 2008

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The distribution of Ge composition in an individual Si1−xGex alloy nanowire (SiGeNW) grown by the vapor liquid solid (VLS) process was studied by micro-Raman spectroscopy by changing the measurement position from the catalyst side to the substrate side. The Si–Si mode in Raman spectra were found to split into two peaks and the intensity ratio as well as the wavenumbers of the two peaks depended on the measurement position. The Raman data revealed that SiGeNWs have a core-shell structure with a low-Ge composition core grown by the VLS process covered with a high-Ge composition shell grown by the conformal deposition.
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78.67.Lt Quantum wires
78.30.Hv Other nonmetallic inorganics
68.65.La Quantum wires (patterned in quantum wells)
81.16.-c Methods of micro- and nanofabrication and processing
68.35.Fx Diffusion; interface formation

Mechanical and electrical coupling at metal-insulator-metal nanoscale contacts

Doo-In Kim, Namboodiri Pradeep, Frank W. DelRio, and Robert F. Cook

Appl. Phys. Lett. 93, 203102 (2008); http://dx.doi.org/10.1063/1.3009211 (3 pages) | Cited 4 times

Online Publication Date: 18 November 2008

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Mechanical and electrical coupling at nanoscale metallic contacts was investigated using a conducting-probe atomic force microscope (AFM). The current-voltage responses were non-Ohmic, symmetric about zero bias, with conductance values smaller than the quantum conductance limit, which indicate electron tunneling through an insulating layer. Using a self-consistent contact mechanics model and a parabolic tunneling model for thin insulating layers, we determined the contact area, barrier height, and barrier thickness as a function of applied contact load. The results suggest the presence of two insulating layers: an oxide layer on the AFM tip and an organic contaminant layer on the metallic surface.
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73.40.Rw Metal-insulator-metal structures
73.40.Gk Tunneling
73.40.Ns Metal-nonmetal contacts

Improving light output power of InGaN-based light emitting diodes with pattern-nanoporous p-type GaN:Mg surfaces

Chung Chieh Yang, Chia Feng Lin, Chun Min Lin, Cheng Chien Chang, Kuei Ting Chen, Jui Fen Chien, and Chung Ying Chang

Appl. Phys. Lett. 93, 203103 (2008); http://dx.doi.org/10.1063/1.3027068 (3 pages) | Cited 14 times

Online Publication Date: 18 November 2008

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InGaN-based light emitting diodes (LEDs) with a top pattern-nanoporous p-type GaN:Mg surface were fabricated by using a photoelectrochemical (PEC) process. The peak wavelengths of electroluminescence (EL) and operating voltages were measured as 461.2 nm (3.1 V), 459.6 nm (9.2 V), and 460.1 nm (3.3 V) for conventional, nanoporous, and pattern-nanoporous LEDs using 20 mA operation current. The EL spectrum of the nanoporous LED had a larger blueshift phenomenon as a result of a partial compression strain release in the InGaN active layer through the formation of a top nanoporous surface. The light output power had 12.1% and 26.4% enhancements for the nanoporous and the pattern-nanoporous LEDs compared with conventional LEDs. The larger operating voltage of the nanoporous LED was due to the non-ohmic contact on the PEC treated p-type GaN:Mg surface. By using a pattern-nanoporous p-type GaN:Mg structure, the operating voltage of the pattern-nanoporous LED was reduced to 3.3 V. A lower compression strain in the InGaN active layer and a higher light extraction efficiency at the top nanoporous surface were observed in pattern-nanoporous LEDs for higher efficiency nitride-based LED applications.
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85.60.Jb Light-emitting devices
61.43.Gt Powders, porous materials
82.45.Yz Nanostructured materials in electrochemistry
82.50.-m Photochemistry

Thickness inhomogenities in the organometallic chemical vapor deposition of GaP

X. Liu and D. E. Aspnes

Appl. Phys. Lett. 93, 203104 (2008); http://dx.doi.org/10.1063/1.3029742 (3 pages) | Cited 3 times

Online Publication Date: 18 November 2008

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We analyze exponential lateral-thickness variations observed in the growth of GaP on (001) GaAs, thermally generated SiO2, (001) Si, and nanoscopically roughened Si surfaces by organometallic chemical vapor deposition, using as a reference the polycrystalline GaP deposited on the Mo susceptor surrounding the 2 in. wafers. We find these variations to be due to differences in the chemical reactivities of the various surfaces toward the generation of a precursor, probably a H–P = Ga–CH3 dimer adduct, by heterogeneous catalysis followed by desorption and diffusion through the gas phase.
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68.55.ag Semiconductors
68.55.jd Thickness
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
82.65.+r Surface and interface chemistry; heterogeneous catalysis at surfaces
68.43.Nr Desorption kinetics
66.30.-h Diffusion in solids

Dynamic-gate operation in nanoelectronic amplifiers due to reduced screening

L. Worschech, D. Hartmann, and A. Forchel

Appl. Phys. Lett. 93, 203105 (2008); http://dx.doi.org/10.1063/1.3035852 (3 pages) | Cited 1 time

Online Publication Date: 19 November 2008

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Semiconductor nanostructures in shape of a Y were patterned lithographically in a two-dimensional electron gas. One of the branches was pinched off, so it can act as a gate for the other branch. Such nanoelectronic amplifiers show switching voltages smaller than the thermal limit and tunable bistability. The authors related their observations to a reduced-screening induced dynamic capacitance, which can exceed the contribution from the geometric capacitance.
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85.35.-p Nanoelectronic devices

Codoping of boron and phosphorus in silicon nanowires synthesized by laser ablation

N. Fukata, M. Mitome, Y. Bando, M. Seoka, S. Matsushita, K. Murakami, J. Chen, and T. Sekiguchi

Appl. Phys. Lett. 93, 203106 (2008); http://dx.doi.org/10.1063/1.3033226 (3 pages) | Cited 11 times

Online Publication Date: 19 November 2008

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Codoping of boron (B) and phosphorus (P) atoms was performed during the synthesis of silicon nanowires (SiNWs) by laser ablation. The observation of a local vibrational mode of B clearly showed B doping in codoped SiNWs, while Fano broadening due to heavy B doping disappeared, indicating compensation by P donors. The electrospin resonance signal of conduction electrons also disappeared due to compensation by B acceptors. These results indicate that codoping of B and P atoms was achieved in SiNWs during laser ablation.
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81.16.-c Methods of micro- and nanofabrication and processing
81.05.Cy Elemental semiconductors
61.72.uf Ge and Si
61.46.Km Structure of nanowires and nanorods (long, free or loosely attached, quantum wires and quantum rods, but not gate-isolated embedded quantum wires)

Magnetoresistance studies on Co/AlOX/Au and Co/AlOX/Ni/Au tunnel structures

R. S. Liu, C. M. Canali, L. Samuelson, and H. Pettersson

Appl. Phys. Lett. 93, 203107 (2008); http://dx.doi.org/10.1063/1.3000614 (3 pages) | Cited 2 times

Online Publication Date: 20 November 2008

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We report on magnetoresistance (MR) studies on Co/AlOX/Au and Co/AlOX/Ni/Au magnetic tunnel junctions. In spite of the fact that the difference between the two samples is merely a 3 nm thick Ni layer, there is a sharp contrast in MR behavior indicating that the electronic structure at the interface between the ferromagnetic electrodes and the insulating barrier dominates the MR signal. The former sample exhibits a clear tunneling anisotropic MR (TAMR), with the characteristic correlation between resistance and current direction, in contrast to the latter sample which displays a conventional tunneling MR (TMR) dominated by the relative orientation between the magnetization directions of the two electrodes. In addition, the TAMR has a much stronger temperature dependence than the TMR, indicating a much faster drop-off of the tunneling density of states anisotropy than the tunneling electron spin polarization with increasing temperature. Finally, we propose a possible simple way to distinguish TAMR from normal TMR by measuring the resistance of the device at different angles of the external magnetic field.
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75.47.-m Magnetotransport phenomena; materials for magnetotransport
73.50.Jt Galvanomagnetic and other magnetotransport effects (including thermomagnetic effects)
75.50.Cc Other ferromagnetic metals and alloys
72.25.-b Spin polarized transport
75.70.Cn Magnetic properties of interfaces (multilayers, superlattices, heterostructures)
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
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