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1 Jan 2002

Volume 91, Issue 1, pp. 1-547

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Failure of exchange-biased low resistance magnetic tunneling junctions upon thermal treatment

J. H. Lee, H. D. Jeong, H. Kyung, C. S. Yoon, C. K. Kim, B. G. Park, and T. D. Lee

J. Appl. Phys. 91, 217 (2002); http://dx.doi.org/10.1063/1.1420770 (4 pages) | Cited 10 times

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Transmission electron microscopy (TEM) and Rutherford backscattering spectroscopy (RBS) were used to characterize low resistance (100–1000 Ω μm2) tunneling junctions consisting of Ta/NiFe/Cu/NiFe/IrMn/CoFe/Al (6.6 and 7.7 Å)–oxide/CoFe/NiFe/Ta multilayers after annealing at temperatures ranging from 250 to 500 °C. The Al (7.7 Å) junction showed continual improvement in the magnetoresistance (MR) ratio when annealed up to 300 °C while the MR ratio of the Al (6.6 Å) junction dropped sharply above 250 °C in spite of the only 1 Å difference in the deposited thickness of aluminum metal prior to plasma oxidation. TEM measurement provided evidence that the annealing process improves, in general, structural uniformity in the insulation layer, but thermal treatment can also degrade junction performance at a relatively low temperature due to current leakage through the electrodes. Current leakage can be problematic for a junction whose insulation barrier may be too thin (less than ∼10 Å). Both RBS and TEM analyses indicated that the maximum annealing temperature of exchange biased junctions lies between 400 and 500 °C above which the multilayer structure in the pinned electrode is destroyed by interdiffusion. © 2002 American Institute of Physics.
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75.70.Cn Magnetic properties of interfaces (multilayers, superlattices, heterostructures)
75.45.+j Macroscopic quantum phenomena in magnetic systems
68.35.Fx Diffusion; interface formation
81.40.Gh Other heat and thermomechanical treatments
75.47.De Giant magnetoresistance
71.45.Gm Exchange, correlation, dielectric and magnetic response functions, plasmons
75.30.Et Exchange and superexchange interactions
73.40.Gk Tunneling
82.80.Yc Rutherford backscattering (RBS), and other methods of chemical analysis

Annealing effect on the magnetoresistance in La0.75Ca0.25MnO3 thin films grown on Si (100) substrates

Jong Cheol Lee, Dong Gyun You, Sang Yub Ie, Myeon Chang Sung, Ho Shik Song, Hyun Soon Park, Sei Kwon Kang, Sam Hyeon Lee, Kwangho Jeong, Sam Jin Kim, and Chul Sung Kim

J. Appl. Phys. 91, 221 (2002); http://dx.doi.org/10.1063/1.1417989 (4 pages) | Cited 1 time

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Polycrystalline La0.75Ca0.25MnO3 thin films have been grown by rf magnetron sputtering on Si (100) substrates. The physical properties (lattice parameter, transition temperature, and activation energy) of the thin films were changed after annealing in oxygen. We obtained significantly improved magnetoresistance (MR) (Δρ/ρ0) values with the annealed films; 0.34, 0.29, and 0.27 at 1.5 T field for the films with deposition temperatures 700 °C, 750 °C and 800 °C, respectively. We investigated the effect of the out-of-plane lattice parameters on MR and transition temperature. Annealing effect on the change of the physical properties is discussed for the films. We suggest that oxygen refilling decreases the Mn–O–Mn bond angles as well as increases Mn4+/Mn3+ ratio. As a result, both out-of-plane lattice parameter and resistivity are reduced, while activation energy and the metal–insulator transition temperature are increased. © 2002 American Institute of Physics.
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75.47.Gk Colossal magnetoresistance
75.47.De Giant magnetoresistance
68.55.-a Thin film structure and morphology
81.15.Cd Deposition by sputtering
61.72.Cc Kinetics of defect formation and annealing
71.30.+h Metal-insulator transitions and other electronic transitions
72.60.+g Mixed conductivity and conductivity transitions
75.50.Dd Nonmetallic ferromagnetic materials
81.40.Rs Electrical and magnetic properties related to treatment conditions

Role of anisotropy in noncontacting thermoelectric materials characterization

Adnan H. Nayfeh, Hector Carreon, and Peter B. Nagy

J. Appl. Phys. 91, 225 (2002); http://dx.doi.org/10.1063/1.1416852 (7 pages) | Cited 2 times

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Inclusions and other types of imperfections in metals can be nondestructively detected by noncontacting magnetic measurements that sense the thermoelectric currents that appear when the specimen is subjected to directional heating and cooling. The detectability of small imperfections is ultimately limited by the intrinsic thermoelectric anisotropy and inhomogeneity of the material to be inspected. This article presents an analytical method for calculating the magnetic field produced by thermoelectric currents in anisotropic materials under two-dimensional directional heating and cooling. Experimental results from a textured Ti–6Al–4V titanium-alloy plate are shown to be in very good agreement with the predictions of this model. The described analytical method can be used to optimize thermoelectric inspection procedures and to evaluate the macroscopic texture of metals from their characteristic magnetic signatures. © 2002 American Institute of Physics.
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81.70.Ex Nondestructive testing: electromagnetic testing, eddy-current testing
61.72.Qq Microscopic defects (voids, inclusions, etc.)
72.15.Jf Thermoelectric and thermomagnetic effects

Polaronic and magnetic field effects on the binding energy of an exciton in a quantum well wire

M. Bouhassoune, R. Charrour, M. Fliyou, D. Bria, and A. Nougaoui

J. Appl. Phys. 91, 232 (2002); http://dx.doi.org/10.1063/1.1419261 (5 pages) | Cited 14 times

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We present a theoretical study of the binding energy of an exciton in a cylindrical quantum well wire subject to an external magnetic field. Calculations are performed using a variational approach within the effective mass approximation. We describe the effect of the quantum confinement by an infinitely deep potential well taking into consideration the interaction between the charge carriers (electron and hole) and the optical phonons. It is shown that the polaronic correction to the exciton binding energy is important and should not be neglected. The confinement introduced by a magnetic field, in addition to the geometrical one, is also discussed. © 2002 American Institute of Physics.
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73.21.La Quantum dots
73.21.Fg Quantum wells
71.18.+y Fermi surface: calculations and measurements; effective mass, g factor
71.38.-k Polarons and electron-phonon interactions
71.35.Ji Excitons in magnetic fields; magnetoexcitons

Lateral composition modulation in AlAs/InAs and GaAs/InAs short period superlattices structures: The role of surface segregation

C. Dorin and J. Mirecki Millunchick

J. Appl. Phys. 91, 237 (2002); http://dx.doi.org/10.1063/1.1421240 (8 pages) | Cited 8 times

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The effect of In surface segregation on the microstructure of short period superlattices (SPSs) in two different material systems with nominally equivalent lattice misfit, AlAs/InAs and GaAs/InAs, has been investigated and compared. It was found that the quality of the SPSs and the appearance of lateral composition modulation are remarkably different in these two systems. For AlAs/InAs SPSs grown at temperatures of T=500 °C, uniform structures devoid of lateral composition modulation were obtained. Samples grown at T>500 °C exhibit lateral composition modulation. Uniform and homogeneous SPS structures were not obtained in the GaAs/InAs structures over the entire temperature range examined in this study (475 °C⩽T⩽510 °C). Instead, lateral composition modulation with varying degrees of regularity was always observed. It was found that In segregation and roughening determine the microstructure. A kinetic exchange model predicts that at an optimum temperature the SPS layers are more intermixed for the AlAs/InAs SPSs. Thus, the lattice mismatch is lower and the driving force for roughening is reduced, resulting in uniform SPS structures. Growing the structure away from the optimum temperature for smooth growth may induce roughening-driven composition modulation. The GaAs/InAs structures are less intermixed over the temperatures studied, which results in higher mismatch between the individual layers and a higher driving force for roughening and lateral composition modulation. © 2002 American Institute of Physics.
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68.65.Cd Superlattices
68.35.Dv Composition, segregation; defects and impurities

Temperature dependent barrier characteristics of CrNiCo alloy Schottky contacts on n-type molecular-beam epitaxy GaAs

A. Gümüş, A. Türüt, and N. Yalçin

J. Appl. Phys. 91, 245 (2002); http://dx.doi.org/10.1063/1.1424054 (6 pages) | Cited 29 times

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The current–voltage (I–V) characteristics of CrNiCo alloy Schottky contacts on a molecular-beam epitaxy n-GaAs substrate have been measured over the temperature range of 130–330 K and have been interpreted based on the assumption of a Gaussian distribution of barrier heights due to barrier height inhomogeneities that prevail at the interface. It is shown that the occurrence of Gaussian distribution of then barrier heights is responsible for the decrease of the apparent barrier height Φb0, increase of the ideality factor n and nonlinearity in the activation energy plot at low temperatures. A Φb0 vs 1/T plot was drawn to obtain evidence of a Gaussian distribution of the barrier heights, and values of mathb0(T=0)=1.02 eV and σ0=0.105 V for the mean barrier height and zero-bias standard deviation, respectively, have been obtained from this plot. Thus, a modified ln(I0/T2)−q2σ02/2k2T2 vs 1/T plot gives mathb0(T=0) and A∗ as 1.02 eV and 5.13 A/cm2 K2, respectively, without using the temperature coefficient of the barrier height. It has been concluded that the temperature dependent I–V characteristics of the device can be successfully explained on the basis of a thermionic emission mechanism with Gaussian distribution of the barrier heights. Furthermore, a value of −0.305 meV/K for the temperature coefficient of the flat band barrier height has been obtained; this value for n-GaAs is in very close agreement with values in the literature. © 2002 American Institute of Physics.
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73.30.+y Surface double layers, Schottky barriers, and work functions
73.40.Ns Metal-nonmetal contacts
73.20.At Surface states, band structure, electron density of states

Photocarrier generation and bipolar transport in diphenoquinone doped polymethylphenylsilane thin films

Yoshikazu Ohsawa and Hiroyoshi Naito

J. Appl. Phys. 91, 251 (2002); http://dx.doi.org/10.1063/1.1427429 (7 pages) | Cited 4 times

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Bipolar transport and photocarrier generation processes in thin films of 3,5-dimethyl-3′,5′-di- tert-butyl-4,4′-diphenoquinone (MBDQ) doped polymethylphenylsilane (PMPS) are studied in terms of optical absorption, photoluminescence, the standard time-of-flight, and electroabsorption experiments. With increasing MBDQ concentration, the electron drift mobility is observable above 30 wt. % MBDQ addition (5.0×10−7 cm2/Vs at 40 wt. % MBDQ addition) and the hole drift mobility is unchanged (∼1.0×10−4 cm2/Vs) for 0–40 wt. % MBDQ addition. The photocarrier generation efficiency is increased by an order of magnitude over pristine PMPS for 30–40 wt. % MBDQ addition in a visible spectral range. The electroabsorption experiments of MBDQ/PMPS composites show that the existence of charge transfer states becomes evident with increasing MBDQ concentration, which facilitate the photocarrier generation in the spectral range. © 2002 American Institute of Physics.
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73.61.Ph Polymers; organic compounds
78.66.Qn Polymers; organic compounds
73.50.Pz Photoconduction and photovoltaic effects
73.50.Dn Low-field transport and mobility; piezoresistance
73.50.Gr Charge carriers: generation, recombination, lifetime, trapping, mean free paths
78.20.Jq Electro-optical effects
78.40.Me Organic compounds and polymers
78.55.Kz Solid organic materials

Direct tunneling hole currents through ultrathin gate oxides in metal-oxide-semiconductor devices

Y. T. Hou, M. F. Li, Y. Jin, and W. H. Lai

J. Appl. Phys. 91, 258 (2002); http://dx.doi.org/10.1063/1.1416861 (7 pages) | Cited 34 times

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We present a physical model to calculate the direct tunneling hole current through ultrathin gate oxides from the inversion layer of metal–oxide–semiconductor field-effect transistors. A parametric self-consistency method utilizing the triangular well approximation is used for the electrostatics of the inversion layer. For hole quantization in the inversion layer, an improved one-band effective mass approximation, which is a good approximation to the rigorous six-band effective mass theory, is used to account for the band-mixing effect. The tunneling probability is calculated by a modified Wentzel–Kramers–Brilliouin (WKB) approximation, which takes the reflections near the Si/SiO2 interfaces into account. It is found that the parabolic dispersion in the SiO2 band gap used in the WKB approximation is only applicable for hole tunneling in oxides thinner than about 2 nm and for low gate voltage. A more reasonable Freeman–Dahlke hole dispersion form with significantly improved fitting to all experimental data for different oxide thickness and gate voltage range is adopted and discussed. © 2002 American Institute of Physics.
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85.30.Tv Field effect devices
85.30.De Semiconductor-device characterization, design, and modeling
71.18.+y Fermi surface: calculations and measurements; effective mass, g factor

Signature of the Meyer–Neldel rule on the correlated barrier-hopping model

Fouad Abdel-Wahab

J. Appl. Phys. 91, 265 (2002); http://dx.doi.org/10.1063/1.1416135 (6 pages) | Cited 5 times

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In this study, a modified model for bipolaron correlated barrier hopping (CBH) is proposed, since the usual bipolaron CBH model has been found to be inconsistent with experimental data for ac conductivity for chalcogenide glasses at high temperatures. The present model is based on the existence of a focal point when the experimental data are extrapolated to high temperatures on the σ(ω)–1/T plot. The correct analysis must introduce the concept of the Meyer–Neldel rule in the expression of the relaxation time. The results of the present model are in quantitative agreement with reported ac conductivity measurements. © 2002 American Institute of Physics.
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72.20.Dp General theory, scattering mechanisms
72.80.Ng Disordered solids
72.20.Ee Mobility edges; hopping transport
71.38.Mx Bipolarons
72.30.+q High-frequency effects; plasma effects
72.20.Jv Charge carriers: generation, recombination, lifetime, and trapping
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