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Journal of Applied Physics

Issue 8 • Date Apr 1999

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Displaying Results 1 - 25 of 698
  • Issue Cover

    Page(s): c1
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    Freely Available from IEEE
  • Issue Table of Contents

    Page(s): toc1
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    Freely Available from IEEE
  • Spontaneous emission factor for semiconductor superluminescent diodes

    Page(s): 3945 - 3948
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    The spontaneous emission factor β is an important parameter for the characterization of semiconductor light emitting devices. In the analysis of superluminescent diodes, especially in the calculation of the optical intensity using rate equations, most authors have used the estimated value of β taken from laser diodes, despite the conceptual difference involved in each device. In this article, the spontaneous emission factor β for superluminescent diodes is discussed in detail, and a new method in calculating the average value of β is introduced. Based on this method, the values of β for gain-guided and index-guided structures are obtained. © 1999 American Institute of Physics. View full abstract»

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  • A theoretical model for neutral velocity distributions at a planar target in plasma source ion implantation

    Page(s): 3949 - 3951
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    A theoretical model is developed to study the velocity distributions of neutrals striking a planar target in plasma source ion implantation (PSII) for the case in which the pressure of the gas is large enough that the fast neutrals can be produced in the sheath by ion-neutral charge exchange collisions. An analytic expression for the neutral velocity distribution at the target is provided. The theoretic results agree with Monte Carlo simulations. © 1999 American Institute of Physics. View full abstract»

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  • CF2 production and loss mechanisms in fluorocarbon discharges: Fluorine-poor conditions and polymerization

    Page(s): 3952 - 3959
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    The study of CF and CF2 radical production and loss mechanisms in capacitively-coupled 13.56 MHz CF4 plasmas has been extended to CF4 plasmas with an Si substrate, and to C2F6 plasmas, conditions where the atomic fluorine concentration is lower and where more polymer deposition occurs on the reactor surfaces. Processes in the gas phase and at the reactor surfaces were investigated by time resolved axial concentration profiles obtained by laser induced fluorescence, combined with absolute calibration techniques. The results for CF were similar to those observed in the fluorine rich case, whereas the results for CF2 were strikingly different and more complex. This paper focuses on the CF2 radical, which, under these conditions is produced at all of the surfaces of the reactor, apparently via a long-lived surface precursor. The results can only be explained if large polymeric ions and/or neutrals are produced by polymerization in the gas phase. The gas-phase CF2 concentration is high, causing the otherwise slow gas-phase concatenation reactions CXFY(CF2)n+CF2→CXFY(CF2)n+1 to occur. These processes produce high-mass neutrals (and ions) which are the real polymer precursors. The CF2 radica- l therefore circulates in a closed cycle between the surface and the gas phase. The degree of polymerization is controlled by the fluorine atom concentration, which simultaneously controls the concentrations of CF2, of chain initiating species such as CF3 and of dangling bonds on the growing oligomers. This model appears to apply to fluorocarbon discharges in general, and agrees well with other results presented in the literature. © 1999 American Institute of Physics. View full abstract»

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  • High speed conversion of infrared images with a planar gas discharge system

    Page(s): 3960 - 3965
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    The speed of conversion of infrared (IR) images by a planar semiconductor gas discharge system into the visible range has been investigated. Argon or nitrogen are used in the discharge gap having an electrode distance of 100 μm. Using pulse radiation from an IR laser to excite the system, we have shown that the characteristic response time of the device with the cryogenic discharge in the gap can lie in the submicrosecond range. This characteristic of the system can be applied for a fast IR imaging at a rate higher than 106 frame/s. © 1999 American Institute of Physics. View full abstract»

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  • Ion energy distributions and sheath voltages in a radio-frequency-biased, inductively coupled, high-density plasma reactor

    Page(s): 3966 - 3975
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    Ion energy distributions were measured at a grounded surface in an inductively coupled, high-density plasma reactor for pure argon, argon–helium, and argon–xenon discharges at 1.33 Pa (10 mTorr), as a function of radio-frequency (rf) bias amplitude, rf bias frequency, radial position, inductive source power, and ion mass. The ground sheath voltage which accelerates the ions was also determined using capacitive probe measurements and Langmuir probe data. Together, the measurements provide a complete characterization of ion dynamics in the sheath, allowing ion transit time effects to be distinguished from sheath impedance effects. Models are presented which describe both effects and explain why they are observed in the same range of rf bias frequency. © 1999 American Institute of Physics. View full abstract»

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  • Reduction of density of subgrain boundaries and misfit dislocations in epitaxial (001) SrTiO3 thin films: Effect on dielectric tunability

    Page(s): 3976 - 3983
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    The effect of a YBa2Cu3O7-x buffer layer on the quality of rf magnetron sputtered epitaxial (001) SrTiO3 thin films on a LaAlO3 substrate has been investigated using high resolution transmission electron microscopy. Magnetron sputtered SrTiO3 films generally exhibit a columnar subgrain morphology. By using a 75 nm thick YBa2Cu3O7-x buffer layer the subgrain boundary area was reduced considerably compared to single layer films. Subgrain widths around 130 nm were observed, which corresponded to an 80% reduction of the subgrain boundary area. The density of misfit dislocations was also reduced by 80%. By using a YBa2Cu3O7-x buffer layer, the dielectric tunability increased 30%, to a value of 1.43 (at U=16.5 V/μm, 2 MHz, 77 K). The influence of interfacial strain and misfit dislocations on the subgrain structure, and corresponding effect on the dielectric constant Єr, is elucidated. © 1999 American Institute of Physics.   View full abstract»

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  • Convergent beam guided mode technique for use in liquid crystal studies

    Page(s): 3984 - 3987
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    The use of a convergent beam to excite guided modes in a liquid crystal layer in order to determine the director profile is described. The technique has two main advantages over the conventional collimated beam procedure. First, the convergent beam guided mode (CBGM) technique uses a highly focused beam spot that simultaneously excites many guided modes and produces reflectivity data over a wide angle range. Such reflectivity can be captured in milliseconds with a charge coupled device array and can therefore potentially be used to study cell dynamics. Second, the CBGM technique removes the need to rotate the liquid crystal cell and consequently the focused beam remains completely stationary on the liquid crystal layer, allowing examination of single pixels. The work presented here is a demonstration of the CBGM technique via a study of the static director profile of a homogeneously aligned nematic (E7) liquid crystal layer. © 1999 American Institute of Physics. View full abstract»

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  • Investigation of optically active defect clusters in KH2PO4 under laser photoexcitation

    Page(s): 3988 - 3992
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    Photoexcited defect clusters in the bulk of KH2PO4 crystals are investigated using a microscopic fluorescence imaging system with 1 μm spatial resolution. The observed defect cluster concentration is approximately 104–106per mm3 depending on the crystal growth method and sector of the crystal. The intensity of the emission clusters varies widely within the image field while a nearly uniformly distributed background is present. Spectroscopic measurements provided information on the emission characteristics of the observed defect population. © 1999 American Institute of Physics. View full abstract»

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  • On the migration behavior of metal impurities in Si during secondary ion mass spectrometry profiling using low-energy oxygen ions

    Page(s): 3993 - 3998
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    Secondary ion mass spectrometry (SIMS) was used to investigate the segregation of several metal impurities in Si under low-energy oxygen ion bombardment. Our results suggested that both the segregation of Ca, Cr, and Ta at the SiO2/Si interface, and the antisegregation of Ti, Hf, and Zr into the oxide were thermodynamically driven. The migration behavior of Ca indicates that CaO, having a higher heat of formation than Si, was most probably formed under oxygen bombardment. Sharper in-depth profiles were obtained for Ti, Zr, and Hf (metals with lower heat of oxide formation than Si) by bombarding at angles of incidence for which a stoichiometric surface oxide is formed. The effect of impurity diffusivity is demonstrated through SIMS measurements at elevated temperatures (∼350–380 °C) for Cr, Zr, Ta, and Ti. © 1999 American Institute of Physics. View full abstract»

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  • Effect of annealing on the grain growth and luminescent properties of SrS:Cu blue phosphors

    Page(s): 3999 - 4002
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    Systematic studies of the postgrowth annealing of molecular beam epitaxial deposited SrS:Cu films are reported. In the as-grown SrS:Cu films, the grain size was small and the luminescence very weak. A step-annealing procedure in a H2S atmosphere was developed and found to be a very efficient way to improve the crystallinity and luminescent properties without damage to the low temperature glass substrate and insulator layer of the device. A model is presented in terms of the free energy of formation and the reaction kinetics of Cu with H2S. It was proposed that the weak luminescence in the as-grown films was attributed to Cu atoms segregated at the grain boundaries in the SrS film. The oxidation of atomic Cu by H2S and the diffusion of Cu+ into the SrS lattice during annealing were responsible for the grain growth and the improved luminescent properties. © 1999 American Institute of Physics. View full abstract»

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  • Growth temperature dependence of GaS thin films on GaAs(001) surface

    Page(s): 4003 - 4009
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    The growth of GaS films on GaAs(001) surfaces by using thermal evaporation of layered-compound GaS has been investigated by Auger electron spectroscopy, low-energy electron-loss spectroscopy (LEELS), x-ray photoemission spectroscopy (XPS), and atomic force microscopy (AFM) as a function of deposition temperature. The LEELS spectrum of the films grown at lower temperatures (≤400 °C) resembles that of a GaS single crystal, whereas that of the films grown at 450 °C reveals that GaAs surface was terminated by Ga2S3 layer. XPS spectra suggest that after annealing at 500 °C, S atoms moved from As atoms to Ga atoms to form stable Ga–S bonds at the interface and As–S bonds are observed to be less stable. The band discontinuity at the GaS/GaAs(001) interface estimated by XPS showed the straddling-type I band alignment. Surface morphology of the films studied by AFM reveals the layer-by-layer initial growth of GaS. © 1999 American Institute of Physics. View full abstract»

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  • Dynamics of lateral grain growth during the laser interference crystallization of a-Si

    Page(s): 4010 - 4023
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    Laser interference crystallization of amorphous silicon (a-Si) thin films, a technique that combines pulsed laser crystallization with holography, enables the fabrication of periodic arrays of polycrystalline silicon (poly-Si) lines with lateral dimensions between 0.5 and 20 μm. The lines consist of grains with well-defined grain boundary locations and lateral dimensions that are appreciably larger than the thickness of the initial a-Si:H film (up to 2 μm for a 300 nm thick film). We investigated the dynamics of the crystallization process by two-dimensional finite element computer simulations of the heat transport and phase transitions during laser crystallization. The theoretical results were compared to: (i) measurements of the crystallization kinetics, determined by recording the transient changes of the reflectance during laser exposure, and to (ii) the structural properties of the crystallized films, determined by scanning force and transmission electron microscopy. The simulations indicate that the crystallization front responsible for the large grains propagates laterally from the edges of the molten silicon lines to their centers with a velocity of ∼14 m/s. A substantial lateral growth only occurs for laser intensities large enough to melt the a-Si film around the center of the lines down to the substrate. Vertical crystallization, which is substantially slower (0.5 m/s), also participates in the solidification process. Using a transfer matrix approach, we converted the time-dependent phase and temperature distributions generated by the simulation program into values for the reflection and transmission of the film as a function of time during and after the laser exposure. A good agreement between the simulated and measured transient reflection was obtained both in the case of homogeneous crystallization as well as that of laser interference crystallization. © 1999 American - Institute of Physics.     View full abstract»

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  • The optically active center and its activation process in Er-doped Si thin film produced by laser ablation

    Page(s): 4024 - 4031
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    The local structure of erbium-doped silicon produced by the laser ablation technique is investigated by Er LIII-edge x-ray absorption fine structure analysis. The combined analysis of extended x-ray absorption fine structure analysis and an x-ray absorption near-edge structure simulation based on multiple-scattering theory reveals the most probable atomic coordination of the optically active center; Er bonded with six oxygen atoms has a C4v symmetry. The optical activation process of this system is also discussed. The Si target with 10 wt% Er2O3 has two kinds of local structures, C-rare-earth Er2O3 grain and another Er phase incorporated in Si. The laser ablation homogenizes these phases, and deposits a new single-phase structure of the octahedron (Oh point group) on the substrates. In this phase, the optical transition probability is low due to the forbidden 4f transition of Er in the crystal field originating from the higher-order symmetry of O. After annealing, degradation of the symmetry from Oh to C4v results in a crystal field suitable for inducing sufficient radiation transition. © 1999 American Institute of Physics.   View full abstract»

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  • Spectroscopic ellipsometry analyses of sputtered Si/SiO2 nanostructures

    Page(s): 4032 - 4039
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    A spectroscopic ellipsometry technique is used to attempt a quantitative analysis of thin Si/SiO2 nanocomposite films obtained by magnetron co-sputtering. The layers are first fabricated with varying values of deposition temperature and sputtered silicon area, before being annealed at different temperatures. Using an effective medium model, the ellipsometry tool allowed the estimate of the thickness and the volume fraction of silicon agglomerates or grains, in addition to the optical parameters of the layer through a tentative determination of the dielectric function of the Si nanostructures. The Si content obtained by the ellipsometry approach agrees with those previously determined by direct measurements. Besides the obvious increase of incorporated Si with the sputtered area of this material, we noticed a maximum excess of Si for a deposition temperature in the 400–500 °C range. In this respect, the peak position of the photoluminescence (PL) detected only in the annealed samples is found to be closely connected with the amount of excess Si, and in consequence, with the mean size of the thermally grown nanocrystallites. The systematic red shift of maximum PL with the rise of Si content, and implicitly with the increase of the particle size, regardless of the deposition conditions used for the fabrication of the original layer, is liable to allow conclusive argument as for the attribution of the PL in the 1.30–1.65 eV range to the quantum confinement effect. Besides the main contribution of the nanoparticle density, the quality of the Si/SiO2 interface, as examined by infrared absorption spectroscopy, appears determining for the PL efficiency, and is increasingly improved towards stoichiometric SiO2 when the annealing temper- ature is increased up to 1100 °C. © 1999 American Institute of Physics. View full abstract»

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  • Buffer layer strain transfer in AlN/GaN near critical thickness

    Page(s): 4040 - 4044
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    X-ray diffraction has been employed to investigate the strain relaxation of both components of a GaN/AlN bilayer on sapphire (0001) as a function of the GaN layer thickness. Below a critical thickness, GaN and AlN both relax with the same in-plane lattice constant, consistent with the energy minimum condition of elasticity theory for a bilayer. Above the critical thickness, however, the strain relaxations in the two layers were different. We can fit this strain relaxation behavior with a free standing bilayer model with an additional term describing the interaction of dislocations. © 1999 American Institute of Physics. View full abstract»

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  • Shallow impurities in semiconductor superlattices: A fractional-dimensional space approach

    Page(s): 4045 - 4049
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    A thorough detailed study of donor and acceptor properties in doped GaAs–(Ga,Al)As semiconductor superlattices is performed within the fractional-dimensional approach, in which the real anisotropic “impurity+semiconductor superlattice” system is modeled through an effective isotropic environment with a fractional dimension. In this scheme, the fractional-dimensional parameter is chosen via an analytical procedure and involves no ansatz, and no fittings either with experiment or with previous variational calculations. The present fractional-dimensional calculated results for the donor and acceptor energies in GaAs–(Ga,Al)As semiconductor superlattices are found in quite good agreement with previous variational calculations and available experimental measurements. © 1999 American Institute of Physics. View full abstract»

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  • Maximum entropy principle for hydrodynamic transport in semiconductor devices

    Page(s): 4050 - 4065
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    A hydrodynamic (HD) transport approach based on a closed system of balance equations is developed from the maximum entropy principle. By considering a nonlinear expansion with respect to a local thermodynamic equilibrium, we determine an analytic expression for the distribution function as a function of macroscopic quantities such as density, velocity, energy, deviatoric stress, heat flux associated with charge carriers. From the determined distribution function and considering the collision interactions of carriers with phonons, all the constitutive functions appearing in the fluxes and collisional productions of the balance equations are explicitly calculated. The analytical closure so obtained is applied to the case of some n+nn+ submicron Si structures. Numerical HD calculations are found to compare well with those obtained by an ensemble Monte Carlo simulator thus validating the approach developed here. © 1999 American Institute of Physics. View full abstract»

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  • A comparison of projected thermoelectric and thermionic refrigerators

    Page(s): 4066 - 4070
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    Expressions for the coefficient of performance and the cooling power of a thermionic refrigerator are derived taking into account the losses associated with thermal radiation and the need for a so-called passive conductor in the electrical circuit. Specific calculations are made for a thermionic device with a work function of 0.3 eV and it is shown that the performance is generally superior to that of a thermoelectric refrigerator made from materials with a dimensionless figure of merit ZT equal to 4. © 1999 American Institute of Physics. View full abstract»

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  • Photocurrent spectroscopy and study of subband parameters for heavy holes in nanoscale In0.53Ga0.47As/In0.52Al0.48As multiquantum well structures

    Page(s): 4071 - 4075
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    Nanoscale In0.53Ga0.47As/In0.52Al0.48As multiquantum wells structures were studied by photocurrent spectroscopy. Photocurrent spectra showed clear steplike structures accentuated by exciton peaks. Many interband transitions were assigned from the spectral structure. As peaks of forbidden transitions, which appeared in large reverse bias voltages, were extrapolated to zero bias voltage on the photocurrent spectra, transition energies were estimated in a square potential well. New estimation methods of valence band parameters, heavy hole effective mass and valence band offset, were derived from a saturation of the heavy hole subband in the valence potential well, using the envelope function model in the effective mass approximation. The heavy hole effective mass in a direction normal to the quantum well plane and the valence band offset were 0.38m0 and 0.22 eV. © 1999 American Institute of Physics. View full abstract»

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  • Contribution of interface roughness to the infrared spectra of thermally grown silicon dioxide films

    Page(s): 4076 - 4082
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    Interfacial oxide structures were studied by Infrared reflection absorption spectroscopic analysis. The longitudinal optical mode of the interfacial oxide films arose with lower frequencies and wider widths. We assumed and later verified that these changes resulted from the contribution of interfacial roughness. We therefore examined the contribution of interfacial roughness to the infrared spectra using the effective medium approximation model and via infrared spectra simulation. The experimental results explained by a model of that interface had an atomically roughness of within 1 nm and chemical structure changes effectively little fluctuation. © 1999 American Institute of Physics. View full abstract»

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  • A probe for moisture permeation using metal/organic composite films

    Page(s): 4083 - 4086
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    Composite films made by simultaneous sublimation of metal and organic materials were used as sensors for moisture in overlying polymeric films, and polymer transport properties were obtained from a spatially resolved signal. In the probe film, the molecularly dispersed metal quenched the fluorescence of the organic dye. Upon exposure to water vapor, the metal was oxidized and the fluorescence of the dye recovered. The intensity profile development over time was modeled, and a diffusion coefficient for a ultraviolet-curable adhesive was obtained which compared well to other resins of its type. This technique offers several advantages in characterizing transport properties in confined geometries, such as those used for the passivation of light-emitting diodes or other electronic devices. © 1999 American Institute of Physics. View full abstract»

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  • A study on the capacitance–voltage characteristics of metal-Ta2O5-silicon capacitors for very large scale integration metal-oxide-semiconductor gate oxide applications

    Page(s): 4087 - 4090
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    In this work, the capacitance–voltage characteristics of Au/Ta2O5/Si capacitors are studied. The Ta2O5 films are deposited by plasma-enhanced chemical vapor deposition system. Constant voltage stress is applied to the Ta2O5 capacitors. The flatband voltage VFB is measured before and after the constant voltage stress. The flatband voltage shift due to the stress is explained by the trapping of electrons and holes. An interface trapped charge density (Dit) about 2×1011eV-1 cm-2 in the midgap of silicon is extracted by using the conductance method. The discharging transient current after a constant voltage stress is measured and correlated with the electron and hole trapping. The trapped electron and hole densities in Ta2O5 are calculated by using the tunneling front model. The VFB shift and the discharging transient currents are explained based on the energy band diagram of the metal/Ta2O5/silicon system. © 1999 American Institute of Physics. View full abstract»

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  • Low-frequency noise in polycrystalline semiconducting FeSi2 thin films

    Page(s): 4091 - 4095
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    Low-frequency noise measurements have been carried out at room temperature in polycrystalline semiconducting iron disilicide (β-FeSi2) thin film with the current I as a parameter. The power spectral density of the current fluctuations exhibits a 1/f behavior at low frequencies (f≪100 Hz) and is proportional to Iβ (β≪2). The temperature dependence of the conductivity shows that, at room temperature, the measured noise is related to a thermally activated transport mechanism, which satisfies the Meyer–Neldel rule. A noise theory has been developed on the basis of trapping-detrapping of holes of the valence band and the gap states taking into account mobility inhomogeneity across the thickness of the film. Using the experimental data of Hall, conductivity, and noise measurements, the noise model provides an assessment of the distribution of traps within the energy gap of the β-FeSi2 material. © 1999 American Institute of Physics. View full abstract»

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Aims & Scope

Journal of Applied Physics is the American Institute of Physics' (AIP) archival journal for significant new results in applied physics

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Editor
P. James Viccaro
Argonne National Laboratory