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15 Mar 2006

Volume 99, Issue 6, Articles (06xxxx)

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Local structure of LiNi0.5Mn0.5O2 cathode material probed by in situ x-ray absorption spectroscopy

Aniruddha Deb, Uwe Bergmann, Stephen P. Cramer, and Elton J. Cairns

J. Appl. Phys. 99, 063701 (2006); http://dx.doi.org/10.1063/1.2179198 (10 pages) | Cited 7 times

Online Publication Date: 16 March 2006

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The layered nickel manganese oxide of LiNi0.5Mn0.5O2 is a technologically important and promising cathode material for lithium-ion batteries. A study of the structural perturbation, charge compensation mechanism, and the valence of the constituent transition metal elements (Ni and Mn) during electrochemical cycling was performed using x-ray absorption spectroscopy (XAS) employing an in situ technique with an electrochemical cell designed for long term x-ray experiments. We present the changes in the oxidation state, bond distance, and coordination number of the Ni and Mn absorbers as a function of the state of charge of the material during electrochemical cycling at moderate rate through a typical Li-ion battery operating voltage range (2.9–4.7 V). The oxidation states of the transition metals in LiNi0.5Mn0.5O2 are Mn4+ and Ni2+. The x-ray absorption near edge spectra reveal that on delithiation of Li1−xNi0.5Mn0.5O2 the Mn is electrochemically inactive and remains at Mn4+ whereas the Ni is oxidized from Ni2+ to almost Ni4+ through an intermediate stage of Ni3+. A quantitative picture of the Ni valence during the charging process was obtained employing Faraday’s law calculations in combination with the XAS results. The Faraday’s law calculation indicated that at the end of the charge Ni is at Ni3.91+ in close agreement with the XAS results where Ni is found to be at Ni3.88+ at the end of the charge. Analysis of the extended x-ray absorption fine structure shows that during cycling the material retains its crystallographic symmetry and good structural short-range order which should lead to stable cycling.
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82.45.Fk Electrodes
82.47.Aa Lithium-ion batteries
78.70.Dm X-ray absorption spectra

Binding energy of negative trions in a CdTe quantum well at high magnetic fields

P. Redliński

J. Appl. Phys. 99, 063702 (2006); http://dx.doi.org/10.1063/1.2178695 (4 pages) | Cited 2 times

Online Publication Date: 16 March 2006

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We present the results of numerical calculations of electronic states of an exciton (X) and of a trion (X−) in a CdTe quantum well for magnetic fields B up to 150 T. Using variational method we estimate the binding energy of the free X− in the singlet state (z component of angular momentum Lz = 0), as well the binding energy of the free X− in the triplet state (Lz = −ℏ). Recent experiments have shown that even up to B = 44 T the binding energy of the negative trion in a singlet state is greater than in the triplet state. We show, however, that the binding energies of these two states should cross one another at very high magnetic fields. Specifically, using material parameters typical for these experiments, we show that the crossing of these levels will take place at field Bc ≈ 65 T. The binding energy of free X− in the triplet state exceeding that of the singlet state for fields B>Bc.
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73.21.Fg Quantum wells
73.20.Mf Collective excitations (including excitons, polarons, plasmons and other charge-density excitations)
71.35.-y Excitons and related phenomena

Field-dependent thermal and electrical transports in Cu/CoFe multilayer

Y. Yang, J.-G. Zhu, R. M. White, and M. Asheghi

J. Appl. Phys. 99, 063703 (2006); http://dx.doi.org/10.1063/1.2174124 (5 pages) | Cited 4 times

Online Publication Date: 17 March 2006

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This paper reports field-dependent thermal and electrical conductivity measurements of a 144 nm thick Cu/CoFe giant magnetoresistive multilayer made of extremely thin periodic layers (12 and 21 Å for CoFe and Cu layers, respectively), using steady-state Joule heating and electrical resistance thermometry in suspended bridges between 300 and 380 K. Large decreases in the electrical and thermal resistivities from antiparallel to parallel alignment of the magnetization in the film, referred to as the giant magnetoresistance (GMR) and giant magnetothermal resistance (GMTR), are observed. GMR ratios of 17% and 12% and large GMTR ratios of 25% and 58% are measured at 300 and 380 K, respectively. It is concluded that different electron scattering rates for charge and heat transports in the ferromagnetic CoFe layer are responsible for the difference between the GMR and GMTR ratios. While the previous works only reported the relative change in thermal conductance due to applied magnetic field, the present manuscript reports the absolute value of the field-dependent thermal conductivity of Cu/CoFe multilayer.
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73.63.-b Electronic transport in nanoscale materials and structures
75.70.Cn Magnetic properties of interfaces (multilayers, superlattices, heterostructures)
75.47.De Giant magnetoresistance
75.50.Bb Fe and its alloys
72.15.Eb Electrical and thermal conduction in crystalline metals and alloys
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects

Solid-state formation of titanium carbide and molybdenum carbide as contacts for carbon-containing semiconductors

W. P. Leroy, C. Detavernier, R. L. Van Meirhaeghe, A. J. Kellock, and C. Lavoie

J. Appl. Phys. 99, 063704 (2006); http://dx.doi.org/10.1063/1.2180436 (5 pages) | Cited 8 times

Online Publication Date: 20 March 2006

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Metal carbides are good candidates to contact carbon-based semiconductors (SiC, diamond, and carbon nanotubes). Here, we report on an in situ study of carbide formation during the solid-state reaction between thin Ti or Mo films and C substrates. Titanium carbide (TiC) was previously reported as a contact material to diamond and carbon nanotubes. However, the present study shows two disadvantages for the solid-state reaction of Ti and C. First, because Ti reacts readily with oxygen, a capping layer should be included to enable carbide formation. Second, the TiC phase can exist over a wide range of composition (about 10%, i.e., from Ti0.5C0.5 to Ti0.6C0.4), leading to significant variations in the properties of the material formed. The study of the Mo–C system suggests that molybdenum carbide (Mo2C) is a promising alternative, since the phase shows a lower resistivity (about 45% lower than for TiC), the carbide forms below 900 °C, and its formation is less sensitive to oxidation as compared with the Ti–C system. The measured resistivity for Mo2C is ρ = 59 μΩ cm, and from kinetic studies an activation energy for Mo2C formation of Ea = 3.15±0.15 eV was obtained.
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73.61.-r Electrical properties of specific thin films
81.65.Mq Oxidation

Thermoelectric properties of Tl–X–Te (X = Ge, Sn, and Pb) compounds with low lattice thermal conductivity

Atsuko Kosuga, Ken Kurosaki, Hiroaki Muta, and Shinsuke Yamanaka

J. Appl. Phys. 99, 063705 (2006); http://dx.doi.org/10.1063/1.2181427 (4 pages) | Cited 11 times

Online Publication Date: 21 March 2006

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We prepared polycrystalline-sintered samples of Tl2GeTe3, Tl4SnTe3, and Tl4PbTe3 and evaluated their thermoelectric properties. Although the electrical properties of these compounds were not optimized, the dimensionless figure of merit ZT was relatively high, i.e., 0.74 at 673 K for Tl4SnTe3, 0.71 at 673 K for Tl4PbTe3, and 0.29 at 473 K for Tl2GeTe3, due to the very low lattice thermal conductivity of the compounds. Low lattice thermal conductivity appears to be closely related to the weak bonding of atoms and complex crystal structures of these compounds.
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72.20.Pa Thermoelectric and thermomagnetic effects
66.70.-f Nonelectronic thermal conduction and heat-pulse propagation in solids; thermal waves

Observation of persistent photoconductivity in 2H‐MoSe2 layered semiconductors

Y. C. Lee, J. L. Shen, K. W. Chen, W. Z. Lee, S. Y. Hu, K. K. Tiong, and Y. S. Huang

J. Appl. Phys. 99, 063706 (2006); http://dx.doi.org/10.1063/1.2180398 (4 pages) | Cited 4 times

Online Publication Date: 22 March 2006

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We report the observation of persistent photoconductivity (PPC) effect in 2H‐MoSe2 layered semiconductors. The decay behavior of PPC can be well described by a stretch-exponential function. Experimental results indicate that the lattice relaxation of DX-like impurity is responsible for PPC in MoSe2. In addition, the small capture barrier created by lattice relaxation and the temperature-dependent resistance measurement exhibit that the magnitude of PPC effect in MoSe2 is small, consistent with the PPC behavior in indirect gap materials.
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72.40.+w Photoconduction and photovoltaic effects
71.20.Nr Semiconductor compounds

A full band deterministic model for semiclassical carrier transport in semiconductors

S. Smirnov and C. Jungemann

J. Appl. Phys. 99, 063707 (2006); http://dx.doi.org/10.1063/1.2181429 (11 pages) | Cited 4 times

Online Publication Date: 23 March 2006

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We present a deterministic semiclassical transport model taking into account the full band structure of a solid. The method is based on the spherical harmonics expansion of the distribution function and band structure. As an example the approach is applied to the specific case of stationary hole transport in bulk relaxed silicon and strained silicon exposed to external uniform electric and magnetic fields with arbitrary directions. To keep the structure of the method clear we only consider simple acoustic and optical phonon scatterings. First, the Boltzmann equation is rewritten in terms of energy. The limitations implied by the band structure on this form of the kinetic equation are then discussed. After expansion and projection onto the complete set of spherical harmonics the kinetic equation is reformulated as a system of ordinary differential equations which is solved numerically. Finally, the simulation results obtained for the chosen physical model are shown and discussed. It should be stressed that in contrast to early works not only the group velocity and the density of states are taken from the full band structure but the whole Boltzmann equation is completely treated within the full band picture and, therefore, when it is applicable, the method represents a deterministic equivalent of the full band Monte Carlo method.
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72.10.Di Scattering by phonons, magnons, and other nonlocalized excitations
72.10.Fk Scattering by point defects, dislocations, surfaces, and other imperfections (including Kondo effect)
71.20.Mq Elemental semiconductors

Electronic properties of the Zr–ZrO2–SiO2–Si(100) gate stack structure

C. C. Fulton, G. Lucovsky, and R. J. Nemanich

J. Appl. Phys. 99, 063708 (2006); http://dx.doi.org/10.1063/1.2181282 (10 pages) | Cited 18 times

Online Publication Date: 23 March 2006

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The interface electronic structure of a layered Zr–ZrO2–SiO2–Si(100) system was studied with x-ray (hν = 1254 eV) and ultraviolet (hν = 21.2 eV) photoemission spectroscopies. In situ growth and characterization allow the structures to be deposited and studied in a stepwise manner without the risk of contamination. This study discusses the electronic properties including electron affinities and work functions, valence band maxima, band bending in the Si, and internal fields in a layered high-κ gate stack. With this information the band alignments can be reconstructed and compared to predictions of the vacuum alignment models (i.e., the Schottky-Mott model for metal-semiconductor interfaces or the electron affinity model for heterojunctions) and the interface induced gap states model. The vacuum alignment models are first order approaches to determine the electronic barrier height for a heterojunction, and interface bonding can contribute to charge transfer across the interface, affecting the dipole contribution and altering the barrier heights. In this study, the band offsets and vacuum levels are independently measured, thereby determining the deviation from the vacuum level alignment models. The valence band offsets at the Si–SiO2, SiO2–ZrO2, and ZrO2–Zr are found to be 4.4±0.1, 0.67±0.24, and 4.9±0.44 eV, respectively. For these same interfaces the deviations from the electron affinity or Schottky-Mott model are determined to be 0.2±0.14, −1.43±0.29, and 1.3±0.39 eV, respectively.
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73.20.At Surface states, band structure, electron density of states
79.60.Jv Interfaces; heterostructures; nanostructures
73.30.+y Surface double layers, Schottky barriers, and work functions

Intrinsic exciton transitions in high-quality ZnO thin films grown by plasma-enhanced molecular-beam epitaxy on sapphire substrates

X. Q. Zhang, Z. G. Yao, S. H. Huang, Ikuo Suemune, and H. Kumano

J. Appl. Phys. 99, 063709 (2006); http://dx.doi.org/10.1063/1.2183354 (4 pages) | Cited 5 times

Online Publication Date: 23 March 2006

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High-quality ZnO thin films have been grown by plasma-enhanced molecular-beam epitaxy on sapphire substrates. Free-exciton absorption and exciton-LO phonon absorption peaks are observed in the films at room temperature, indicating that the exciton states are stable even at room temperature. Three excitonic transitions associated with valence bands A, B, and C are clearly revealed in the reflectance spectrum measured at low temperatures. This result indicates that the ZnO thin films have a perfect wurtzite crystal structure. Biexciton emission is observed in the photoluminescence spectra at low temperatures, from which the biexciton binding energy is estimated to be 14.5 meV, in good agreement with previous results. Exciton-LO (Ex-LO) and exciton-2LO (Ex-2LO) photon emission peaks are observed at low temperature. The energy difference between the Ex-LO and Ex-2LO bands is about 72.5 meV, which coincides with previously reported values of the LO phonon energy for ZnO thin films.
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71.35.-y Excitons and related phenomena
78.55.Et II-VI semiconductors
78.66.Hf II-VI semiconductors
63.20.kk Phonon interactions with other quasiparticles

Theoretical investigations for shot noise in correlated resonant tunneling through a quantum coupled system

Ivana Djuric, Bing Dong, and H. L. Cui

J. Appl. Phys. 99, 063710 (2006); http://dx.doi.org/10.1063/1.2183352 (12 pages) | Cited 20 times

Online Publication Date: 24 March 2006

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We carry out a theoretical analysis of the zero-frequency and finite-frequency shot noise in electron tunneling through a two-level interacting system connected to two leads, when a weak coherent coupling between the two levels is present, by means of recently developed bias-voltage and temperature-dependent quantum rate equations. For this purpose, we generalize the traditional generation-recombination approach for shot noise of two-terminal tunneling devices to properly take into account the coherent superposition of different electronic states (quantum effects). As applications, analytical and numerical investigations have been given in detail for two particular cases: (1) electron tunneling through a quantum dot connected to ferromagnetic leads with intradot spin-flip scattering and (2) spinless fermions tunneling through two coupled quantum dots, focusing on the shot noise as functions of bias-voltage and frequency.
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72.70.+m Noise processes and phenomena
73.40.Gk Tunneling
73.63.Kv Quantum dots

Preparation and properties of the trivalent-ion doped tungsten bronze LaxWO3

C. Kasl and M. J. R. Hoch

J. Appl. Phys. 99, 063711 (2006); http://dx.doi.org/10.1063/1.2180427 (6 pages) | Cited 5 times

Online Publication Date: 28 March 2006

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The production and characterization of lanthanum-doped tungsten oxide, LaxWO3, sintered samples are presented for 0.05 ⩽ x ⩽ 0.23. The x-ray diffraction results show that the cubic perovskite phase is stable for x in the range of 0.086–0.21. Outside of this range additional phases are present. Resistivity measurements made between 64 and 300 K provide information on the electronic properties. Room temperature Hall effect results show that the trivalent La ions donate their valence electrons into the WO3 conduction band. The samples have been prepared for an investigation of the metal-insulator transition in this tungsten bronze system.
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71.30.+h Metal-insulator transitions and other electronic transitions
72.60.+g Mixed conductivity and conductivity transitions
72.20.My Galvanomagnetic and other magnetotransport effects
71.20.Ps Other inorganic compounds

Acoustoelectric coupling for direct electrical characterization of semiconductor devices

Cyril Ravat, Émilie Absil, Stéphane Holé, and Jacques Lewiner

J. Appl. Phys. 99, 063712 (2006); http://dx.doi.org/10.1063/1.2186025 (5 pages) | Cited 1 time

Online Publication Date: 31 March 2006

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In this paper, the electrical characterization of semiconductor structure by acoustoelectric couplings (pressure-wave-propagation method) is theoretically studied and experimentally tested. Estimated signals for a Schottky diode structure fit very well the experimental measurements. Information on the screening length or on the voltage barrier can be directly obtained from the measurements.
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85.30.Kk Junction diodes
85.30.Hi Surface barrier, boundary, and point contact devices
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