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1 Sep 1998

Volume 84, Issue 5, pp. 2371-2982

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Doping-induced band edge displacements and band gap narrowing in 3C–, 4H–, 6H–SiC, and Si

U. Lindefelt

J. Appl. Phys. 84, 2628 (1998); http://dx.doi.org/10.1063/1.368374 (10 pages) | Cited 36 times

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Models for doping-induced band edge displacements and band gap narrowing in both n-type and p-type 3C–, 4H–, and 6H–SiC are presented for the first time. For comparison, Si has also been considered. The models constitute an extension of the theory of Jain and Roulston [Solid State Electron. 34, 453 (1991)] and take into account the three different electron effective mass components associated with hexagonal lattices. Furthermore, a more careful treatment of minority carrier correlation energy has been made, applying a two-band model for the dielectric function of a hole gas in the plasmon-pole approximation. The results for the band edge displacements are expressed in simple analytical form as functions of doping concentration. © 1998 American Institute of Physics.
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71.20.Nr Semiconductor compounds
71.45.Gm Exchange, correlation, dielectric and magnetic response functions, plasmons
61.72.uf Ge and Si
61.72.up Other materials
71.20.Mq Elemental semiconductors
71.18.+y Fermi surface: calculations and measurements; effective mass, g factor

Binding energy of biexcitons in GaAs–AlxGa1−xAs quantum wells

Jian-Jun Liu, Xiao-Jun Kong, and Ying Liu

J. Appl. Phys. 84, 2638 (1998); http://dx.doi.org/10.1063/1.368375 (5 pages) | Cited 15 times

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The energy of a biexciton in a GaAs–AlxGa1−xAs quantum well is calculated variationally by use of a two-parameter trial wave function. The calculated binding energy, relative to two well-separated excitons, is greater than that given previously by Kleinman [Phys. Rev. B 28, 871 (1983)]. Our binding energy takes its greatest value when the well width is approximately 10 Å. The ratio of the binding energy of a biexciton to that of an exciton for a range of well thickness (5–300 Å) is found to be from 0.127 to 0.284, which agrees fairly well with previous experimental results. The results of our approach are compared with those of earlier theories. © 1998 American Institute of Physics.
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71.35.Cc Intrinsic properties of excitons; optical absorption spectra
73.21.-b Electron states and collective excitations in multilayers, quantum wells, mesoscopic, and nanoscale systems
73.40.Kp III-V semiconductor-to-semiconductor contacts, p-n junctions, and heterojunctions
73.61.Ey III-V semiconductors
71.15.Nc Total energy and cohesive energy calculations
85.35.Be Quantum well devices (quantum dots, quantum wires, etc.)

Switching phenomena in chromium-doped vanadium sesquioxide

F. A. Chudnovskii, A. L. Pergament, G. B. Stefanovich, P. A. Metcalf, and J. M. Honig

J. Appl. Phys. 84, 2643 (1998); http://dx.doi.org/10.1063/1.368376 (4 pages) | Cited 7 times

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(V1−xCrx)2O3 with x = 0.012 exhibits both S- and Z-type switching characteristics in studies of current–voltage curves at temperatures 170–230 K. The effects are linked to electrothermal instabilities and to filamentary conduction. The samples are stable under voltage cycling in appropriate temperature ranges. © 1998 American Institute of Physics.
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72.60.+g Mixed conductivity and conductivity transitions

Effective conductivities of aligned spheroid dispersions estimated by an equivalent inclusion model

Shih-Yuan Lu

J. Appl. Phys. 84, 2647 (1998); http://dx.doi.org/10.1063/1.368377 (9 pages) | Cited 14 times

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A simple equivalent inclusion model is proposed to estimate the effective conductivities of dispersions containing aligned spheroids. The spheroids are either in perfect contact with the matrix, possessive of certain contact resistance, or coated with a confocal layer of a third material. For the perfect contact case, our results coincide with Willis’ bounds [J. Mech. Phys. Solids 25, 185 (1977)] and, for the coating case, they compare extremely well with those of Hatta and Taya [J. Appl. Phys. 59, 1851 (1986)]. New results are readily obtained for the contact resistance case through use of the proposed model. For the present systems, there exist two independent effective conductivities, one defined in the direction parallel with the symmetric axis of the spheroid and the other defined in the perpendicular direction. Interestingly, the inclusion effect for both the contact resistance and coating cases may be enhancing in one direction, but impairing in the other. But for the perfect contact case, the inclusion effect is always consistent in both directions. It is found that, for the perfect contact case, the reduced effective conductivity (σeff) is a function of the spheroid volume fraction, spheroid aspect ratio, and reduced spheroid conductivity. One more parameter, the Biot number, is needed for the contact resistance case, while two more parameters, the reduced coating layer conductivity and the relative coating thickness, appear in the coating problem. Effects of these parameters on σeff are thoroughly investigated. © 1998 American Institute of Physics.
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72.90.+y Other topics in electronic transport in condensed matter (restricted to new topics in section 72)
82.70.-y Disperse systems; complex fluids

Modeling the effect of dislocations on the minority carrier diffusion length of a semiconductor

C. Donolato

J. Appl. Phys. 84, 2656 (1998); http://dx.doi.org/10.1063/1.368378 (9 pages) | Cited 26 times

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An analysis is given of the minority carrier diffusion and collection in a semiconductor with a surface junction, in the presence of a periodic array of straight dislocations perpendicular to the surface. A dislocation is characterized by its line recombination velocity and is attributed a finite cross section. An approximate solution is obtained for the three-dimensional charge collection probability φ(r) in the unit cell of the array. The integral of φ over the unit cell is used to introduce a new definition of effective diffusion length and to give its analytical expression. It is shown that the relative reduction of diffusion length produced by the dislocation array coincides with the fractional decrease of the total number of junction-injected carriers. This result establishes a connection between the efficiency of charge collection and luminescence of a sample containing dislocations. The analysis is extended to the case of a thin sample, and is compared to previous discussions of the same problem. The consequences of the theory are illustrated by numerical examples and applications to published experimental data. © 1998 American Institute of Physics.
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72.20.Jv Charge carriers: generation, recombination, lifetime, and trapping
61.72.Bb Theories and models of crystal defects
73.25.+i Surface conductivity and carrier phenomena
78.60.Hk Cathodoluminescence, ionoluminescence

A law of mixtures for transport properties in binary particulate composites

K. L. Duncan, J. F. Lodenquai, A. S. Wagh, and K. C. Goretta

J. Appl. Phys. 84, 2665 (1998); http://dx.doi.org/10.1063/1.368379 (8 pages) | Cited 4 times

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A connected-grain model was developed earlier to explain mechanical and thermal properties of porous ceramics and sedimentary rocks. We have now generalized this model for binary particulate composites, based on simulation of a connected-grain structure of individual components of the composites by randomly selecting individual grains and shrinking them. Repetition of this procedure results in a structure of a binary particulate composite that contains channels of individual components, through which transport occurs. We developed a generalized law of mixtures in which transport properties are expressed as scaling relationships that depend on the shrinking parameter expressed as an exponent. This parameter provides the skewness of the distribution of the grains. The model is compared with various transport properties of binary composites reported in the literature. In addition, the model is tested on YBa2Cu3Ox superconductors and Ag composites that were fabricated in our laboratory and tested for electrical conductivity and elastic modulus. This test demonstrates how the model predicts two entirely different transport properties through their common microstructure and grain-size distribution. © 1998 American Institute of Physics.
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72.80.Tm Composite materials
72.90.+y Other topics in electronic transport in condensed matter (restricted to new topics in section 72)
74.72.-h Cuprate superconductors
74.25.F- Transport properties

Electron distribution and capacitance–voltage characteristics of n-doped quantum wells

C. R. Moon, Byung-Doo Choe, S. D. Kwon, H. K. Shin, and H. Lim

J. Appl. Phys. 84, 2673 (1998); http://dx.doi.org/10.1063/1.368380 (11 pages) | Cited 17 times

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The effect of multiple quantum well (MQW) parameters on the free carrier distribution (FCD) and the apparent carrier distribution (ACD) obtained from capacitance–voltage (C–V) profiling has been investigated using a self-consistent simulation technique and the C–V profiling technique. The FCD in MQW structures reveals large outer peaks and small inner peaks even when all the parameters of each quantum wells (QWs) are the same. Such a feature becomes more apparent as the barrier layer becomes thinner or the barrier doping level becomes lower. These characteristics are found to originate from the fact that the density of carriers confined in each well is mostly determined by the depletion region formed alongside the well via the charge neutrality condition. The ACD is found to vary drastically as the thickness or the doping level of barrier changes. When the Debye averaging process is prominent, the ACD peaks are broader and smaller than the FCD peaks and are displaced toward the bottom layer side. The ACD inner peaks even disappear completely when the Debye screening length is comparable to or larger than the QW period, while real free carriers are well confined in each well. The effect of temperature on the ACD through the Debye averaging process is also investigated. When the Debye length is much smaller than the QW period, the full width at half maximum of the ACD is determined by the change of the position expectation value of the two-dimensional differential carriers. This change of position expectation value is found to be much smaller than the well width for relatively narrow QWs. The accuracy of our simulation results is confirmed by the excellent agreements between the simulated ACDs and the measured ACDs of In0.2Ga0.8As/GaAs MQWs. As an example of extracting the qualitative informations from the measured C–V profile, the C–V profiles of partially strain relaxed InxGa1−xAs/GaAs MQWs with x = 0.15 and x = 0.25, grown by metal organic chemical vapor deposition, are discussed. These results show systematically how the QW parameters affect the FCD and the ACD. © 1998 American Institute of Physics.
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73.61.Ey III-V semiconductors
73.21.-b Electron states and collective excitations in multilayers, quantum wells, mesoscopic, and nanoscale systems

Dynamical response in an array of quantum-dot cells

C.-K. Wang, I. I. Yakimenko, I. V. Zozoulenko, and K.-F. Berggren

J. Appl. Phys. 84, 2684 (1998); http://dx.doi.org/10.1063/1.368381 (6 pages) | Cited 10 times

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The time-dependent behavior of quantum cellular automata (QCA), chains of quantum-dot cells, is examined. Each cell consists of four quantum dots on the four corners of a square and contains two electrons. Cells interact with nearby cells via Coulomb potentials. We study the time evolution of the polarization of the cells due to the switching of a driver cell for the particular cases of linear chains and L-shaped configurations. Two different approaches are discussed. In the first one dissipation is not considered. For this case numerical results demonstrate that adiabatic switching can be used to implement QCA only for chains of two cells. In the second approach we explicitly include energy dissipation processes in a phenomenological way and obtain the time evolution of the polarization of the cells due to the switching of the driver cell. Dissipation processes are found to give rise to satisfactory response characteristics. For the cases studied no metastable states are found. © 1998 American Institute of Physics.
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73.21.-b Electron states and collective excitations in multilayers, quantum wells, mesoscopic, and nanoscale systems
85.35.Be Quantum well devices (quantum dots, quantum wires, etc.)
73.23.-b Electronic transport in mesoscopic systems

Basics mechanisms for enhanced prompt charge collection in a n+p junction following single charged particle interaction

N. E. Islam, R. D. Pugh, C. P. Brothers, W. M. Shedd, B. K. Singaraju, J. W. Howard, H. Dussault, and O. Fageeha

J. Appl. Phys. 84, 2690 (1998); http://dx.doi.org/10.1063/1.368441 (7 pages) | Cited 3 times

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This study shows that in the early stages of a single particle transient, the presence of residual holes in the depletion region of a n+p junction affect depletion profile and terminal currents. Due to their greater mobility, electrons deposited in the depletion region move out faster than holes, leaving residual holes in the region. The resulting charge imbalance in the depletion region may cause the depletion region to briefly extend into the substrate before the beginning of recovery process. This extended depletion region influences prompt collection of carriers in the vicinity. In particular, carrier response to this field produces a conduction and/or displacement current. The sum of the conduction plus displacement current is immediately seen as an enhanced prompt conduction current at the device terminals. Enhanced prompt charge collection depends on substrate doping, junction bias, and the density of charge deposited. Low-doped p-type substrates, with longer hole residence time in the depletion region, are more susceptible to enhanced prompt collection and substrate field disturbances. Response of p-Si and n-Si substrates to single event effects and the effects of laser beams for enhanced prompt collection are also analyzed. © 1998 American Institute of Physics.
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73.40.Lq Other semiconductor-to-semiconductor contacts, p-n junctions, and heterojunctions
85.30.Kk Junction diodes
85.30.De Semiconductor-device characterization, design, and modeling

Role of intermediate temperature molecular beam epitaxy grown GaAs defects in tunneling and diffusion

Andrew E. Youtz and Bahram Nabet

J. Appl. Phys. 84, 2697 (1998); http://dx.doi.org/10.1063/1.368442 (8 pages) | Cited 2 times

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Current transport in GaAs grown by molecular beam epitaxy in the intermediate temperature (IT-GaAs) range of about 400 °C is investigated. A model is proposed which explains the direct role of deep defects in assisting tunneling from Schottky contacts to semiconductor, where, due to rapidly changing potential, the Wentzel–Kramers–Brillouin approximation cannot be used. The indirect role of defects in diffusion process in the IT-GaAs semiconductor is also investigated. The model is used to explain the dark I–V behavior of metal–semiconductor–metal photodetectors made on unannealed, i.e., as-grown, IT-GaAs. dc responses of annealed and unannealed IT-GaAs are compared showing much smaller values of dark current in low biases in the unannealed device but sharp increase due to defect assisted tunneling at medium bias levels. Since the defect that best fits the model has an activation energy of about 0.5 eV, the annealing behavior suggests that the As interstitial has all the requisite properties of the defect which determines conduction behavior in this material. The observed reduction of dark response by light at high biases, the negative photoresponse, can then be explained based on occupancy of these defects. Device applications include ohmic contacts based on tunneling through IT-GaAs, and, conversely, reduction of tunneling through growth of a barrier layer between Schottky metal and semiconductor. © 1998 American Institute of Physics.
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73.61.Ey III-V semiconductors
72.80.Ey III-V and II-VI semiconductors
73.30.+y Surface double layers, Schottky barriers, and work functions
71.55.Eq III-V semiconductors
85.60.Gz Photodetectors (including infrared and CCD detectors)
66.30.Lw Diffusion of other defects
72.40.+w Photoconduction and photovoltaic effects
61.72.J- Point defects and defect clusters
61.72.Cc Kinetics of defect formation and annealing

Theoretical study of Auger effect in 1.5 μm quantum-well lasers

O. Gilard, F. Lozes-Dupuy, G. Vassilieff, J. Barrau, and P. Le Jeune

J. Appl. Phys. 84, 2705 (1998); http://dx.doi.org/10.1063/1.368382 (11 pages) | Cited 2 times

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A new methodology for the derivation of the Auger recombination rate in quantum wells is presented. An expression of the Auger recombination rate is given, taking into account a realistic valence band structure, the Fermi–Dirac statistics, and the analytic expressions of the transition matrix element for both bound-bound and bound-unbound Auger processes. Using this method, distributions of carriers involved in bound-bound and bound-unbound recombination processes are carried out. The bound-unbound recombination mechanism is identified as a significant contribution to the Auger total current density. Because the transition matrix element is found to be a significantly increasing function of the quantum-well width, our computations show that the Auger effect is expected to be enhanced in narrow wells. Subsequently, strain dependence of the Auger current density is analyzed. It is found that the Auger effect is reduced by strain in some cases but it is equally shown that this is not a general rule as it depends on the valence subband structure. The effect of temperature on Auger events is also investigated. In particular, it is found, that around room temperature, the temperature sensitivity of Auger current density is quite low in good agreement with many experimental data. © 1998 American Institute of Physics.
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42.55.Px Semiconductor lasers; laser diodes
85.35.Be Quantum well devices (quantum dots, quantum wires, etc.)
42.55.Ah General laser theory
72.20.Jv Charge carriers: generation, recombination, lifetime, and trapping
73.21.-b Electron states and collective excitations in multilayers, quantum wells, mesoscopic, and nanoscale systems

Band-gap changes and band offsets for ternary Si1−x−yGexCy alloys on Si(001)

H. Jörg Osten

J. Appl. Phys. 84, 2716 (1998); http://dx.doi.org/10.1063/1.368383 (6 pages) | Cited 24 times

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An estimation for the band offsets and the fundamental band gap will be presented for Si1−x−yGexCy alloys tensile or compressive strained on Si(001). This estimation considers both the band lineup at the interface of two different materials as well as the strain effects. Unknown material parameters have been adjusted to obtain the best agreement with experimental results for tensile strained Si1−yCy layers. The obtained results agree very well with the first experimental data for the effect of C on band-structure properties in Si1−x−yGexCy. For a completely strain-compensated (cubic) Si1−x−yGexCy layer, we predict significant “Ge effects” (smaller gap than Si, valence-band offset to Si) with values depending on the Ge content. © 1998 American Institute of Physics.
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71.20.Nr Semiconductor compounds

Characterization of border trap generation in rapid thermally annealed oxides deposited using silane chemistry

Navakanta Bhat and Krishna C. Saraswat

J. Appl. Phys. 84, 2722 (1998); http://dx.doi.org/10.1063/1.368384 (5 pages) | Cited 19 times

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The border trap generation under high field stressing has been characterized in rapid thermal annealed low pressure chemical vapor deposited gate oxides. The hysteresis in high frequency capacitance–voltage curve is used to characterize the border traps. It is shown that at least some of the border traps are not associated with trapped positive charge. The border traps are charged and discharged through electrons tunneling from and to the substrate. The hysteresis is independent of temperature confirming the tunneling model. The effects of different annealing ambients suggest that the border trap generation depends on the physical stress at the substrate interface, which is qualitatively measured using Fourier transform infrared spectroscopy. The border trap generation is attributed to bond breaking at the substrate interface by energetic electrons. © 1998 American Institute of Physics.
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73.40.Qv Metal-insulator-semiconductor structures (including semiconductor-to-insulator)
72.20.Jv Charge carriers: generation, recombination, lifetime, and trapping
61.72.Cc Kinetics of defect formation and annealing
61.80.Ba Ultraviolet, visible, and infrared radiation effects (including laser radiation)
72.20.Ht High-field and nonlinear effects
73.40.Gk Tunneling
78.30.Hv Other nonmetallic inorganics
73.20.At Surface states, band structure, electron density of states

Slow photoconductivity decay in 3C-SiC on Si substrates

Masaya Ichimura, Noboru Yamada, Hirotaka Tajiri, and Eisuke Arai

J. Appl. Phys. 84, 2727 (1998); http://dx.doi.org/10.1063/1.368385 (5 pages) | Cited 10 times

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N-type 3C-SiC layers grown on p-type (001) Si substrates were characterized by the conventional photoconductivity decay method. A N2 laser (337 nm wavelength) was used as the excitation source. A very slow component with a time constant larger than 1 ms was observed in the photoconductivity decay curves. A numerical simulation considering a trap with a very small capture cross section for electrons (<1×10−21 cm2) was able to reproduce main qualitative features of the experimental results. From comparison of the experimental decay curves with the theoretical ones, the following conclusions were drawn about the trap in 3C-SiC. (1) The trap level Et is close to the conduction band edge Ec(Ec−Et = 0.1–0.15 eV). (2) The concentration is considered to decrease with increasing donor concentration. © 1998 American Institute of Physics.
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73.61.Le Other inorganic semiconductors
73.50.Pz Photoconduction and photovoltaic effects
73.50.Gr Charge carriers: generation, recombination, lifetime, trapping, mean free paths
71.55.Ht Other nonmetals

Passivation properties of the local oxidation of silicon–oxide/Si interface defects

Shinji Fujieda, Hajime Nobusawa, Masayuki Hamada, and Takaho Tanigawa

J. Appl. Phys. 84, 2732 (1998); http://dx.doi.org/10.1063/1.368386 (3 pages) | Cited 2 times

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The passivation properties of the local oxidation of silicon (LOCOS) oxide/Si interface defects were investigated by reverse current measurement and capacitance transient spectroscopy of pn junction diodes that had a large LOCOS-defined perimeter. The LOCOS/Si interface defects had some properties similar to those of the SiO2/Si(100) interface states of metal–oxide–silicon (MOS) diodes. However, there was a significant difference between the two interfaces in the rate of unpassivated defects remaining after H2 annealing: this rate was higher for the LOCOS/Si interface than for the MOS interface. © 1998 American Institute of Physics.
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73.40.Qv Metal-insulator-semiconductor structures (including semiconductor-to-insulator)
81.05.Cy Elemental semiconductors
81.65.Mq Oxidation
81.65.Rv Passivation
73.40.Lq Other semiconductor-to-semiconductor contacts, p-n junctions, and heterojunctions
81.40.Gh Other heat and thermomechanical treatments

Influence of geometry and passivation on noise in GaInP/GaAs heterojunction bipolar transistors

C. Delseny, Y. Mourier, F. Pascal, S. Jarrix, and G. Lecoy

J. Appl. Phys. 84, 2735 (1998); http://dx.doi.org/10.1063/1.368387 (5 pages) | Cited 1 time

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First order and low-frequency noise measurements are performed on GaInP/GaAs heterojunction bipolar transistors. The base current is decomposed into different components. Passivated transistors exhibit higher gain values, hence the passivation layer limits recombinations on the extrinsic device. Noise is measured in the 1 Hz–100 kHz frequency range on common-emitter-mounted transistors. The 1/f component is analyzed. The emitter series resistances are extracted. The proximity of the base contact on the smaller devices lead to a higher noise level due to enhanced recombinations. Extrinsic and intrinsic phenomena are put forward with the help of noise analysis versus base current and collector current density. An attempt is made to locate the 1/f noise sources. Unlike first order measurements, noise characterization shows that the passivation layer can be at the origin of recombination-type phenomena. © 1998 American Institute of Physics.
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85.30.Pq Bipolar transistors
72.70.+m Noise processes and phenomena
81.65.Rv Passivation
72.20.Jv Charge carriers: generation, recombination, lifetime, and trapping
72.80.Ey III-V and II-VI semiconductors

Measurements of charge accumulation induced by monochromatic low-energy electrons at the surface of insulating samples

A. D. Bass, P. Cloutier, and L. Sanche

J. Appl. Phys. 84, 2740 (1998); http://dx.doi.org/10.1063/1.368388 (9 pages) | Cited 14 times

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We investigate charging of insulators with an apparatus that allows measurements of trapped charges resulting from the impact of monoenergetic electrons of 0.1–28 eV. Details are given on the construction and operation of this instrument. A high-resolution electron monochromator provides a pulsed electron beam of variable energy and current. Accumulated surface charge is monitored using a Kelvin probe and a high-sensitivity electrometer. An ultraviolet source of adjustable maximum frequency allows the sample to be discharged for multiple measurements on the same sample. We illustrate the use of the instrument with preliminary measurements for ∼100 μm thick samples cut from an industrial polyethylene cable. The incident electron-energy dependence of the trapping probability exhibits large variation and indicates that electrons with energies <5 eV are the most efficiently trapped; charging near 10 eV is attributed to dissociative electron attachment to polyethylene molecules. © 1998 American Institute of Physics.
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84.37.+q Measurements in electric variables (including voltage, current, resistance, capacitance, inductance, impedance, and admittance, etc.)
77.84.Jd Polymers; organic compounds
73.40.-c Electronic transport in interface structures
61.80.Fe Electron and positron radiation effects
84.70.+p High-current and high-voltage technology: power systems; power transmission lines and cables
07.68.+m Photography, photographic instruments; xerography
72.20.Jv Charge carriers: generation, recombination, lifetime, and trapping

Temperature dependence of the resistivity and tunneling magnetoresistance of sputtered FeHf(Si)O cermet films

G. J. Strijkers, H. J. M. Swagten, B. Rulkens, R. H. J. N. Bitter, W. J. M. de Jonge, P. J. H. Bloemen, and K. M. Schep

J. Appl. Phys. 84, 2749 (1998); http://dx.doi.org/10.1063/1.368443 (5 pages) | Cited 11 times

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We have studied the tunneling resistivity and magnetoresistance of reactive sputter deposited FeHfO and FeHfSiO thin granular films. Maximum magnetoresistance ratios at room temperature of 2% and 3.2% were observed for films with compositions of Fe47Hf10O43 and Fe40Hf6Si6O48, respectively. The magnetoresistance shows a decrease with temperature, which cannot be explained by spin-dependent tunneling only. We propose that spin-flip scattering in the amorphous FeHf(Si)O matrix causes this decrease as function of temperature. A two current model for the tunnel magnetoresistance, taking into account spin-flip scattering, is presented which can describe the observed temperature dependence of the magnetoresistance. © 1998 American Institute of Physics.
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73.61.-r Electrical properties of specific thin films
72.20.My Galvanomagnetic and other magnetotransport effects
75.70.Ak Magnetic properties of monolayers and thin films

Space-charge effects and current self-quenching in a metal/CdS/LaS cold cathode

P. D. Mumford and M. Cahay

J. Appl. Phys. 84, 2754 (1998); http://dx.doi.org/10.1063/1.368389 (14 pages) | Cited 10 times

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We analyze the importance of space-charge effects in the cathode to anode gap region of a recently proposed metal/CdS(cadmium sulfide)/LaS(lanthanum sulfide) cold cathode. Our approach is based on an ensemble Monte Carlo description of electron transport assuming ballistic injection across the CdS and LaS layers. Under this approximation, the energy spectrum of the injected beam entering the air gap can be determined exactly as a function of the applied bias across the CdS layer. The effects of shot noise in the injected current are taken into account. For some of the biasing conditions considered here, space-charge effects are quite drastic and lead to dynamical effects which are responsible for the onset of current self-quenching similar to the Child–Langmuir regime of operation of thermionic cathodes. The limiting anode current density is found to be much larger than the Child–Langmuir limit. In the presence of strong space-charge effects, large oscillations in the minimum of the electrostatic potential in front of the cathode lead to oscillations in the measured anode current within the tens of GHz frequency range. © 1998 American Institute of Physics.
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85.45.Db Field emitters and arrays, cold electron emitters
02.70.Rr General statistical methods
02.50.Ng Distribution theory and Monte Carlo studies
73.40.Ns Metal-nonmetal contacts
77.22.Jp Dielectric breakdown and space-charge effects

Optical probe of microwave current distributions in high temperature superconducting transmission lines

James C. Culbertson, Harvey S. Newman, and Charles Wilker

J. Appl. Phys. 84, 2768 (1998); http://dx.doi.org/10.1063/1.368390 (20 pages) | Cited 12 times

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This work develops two techniques for optically probing the spatial profile of microwave frequency superconducting currents. As an application, we measured the effects of high microwave powers on the spatial distribution of current on coplanar superconducting transmission lines fabricated using YBa2Cu3O7−δ and Tl2Ba2CaCu2O8. For both techniques, a focused light-spot served as the spatial probe whose effect was measured through the change in transmitted microwave power. For resonant geometries, the change was due to the kinetic-inductance bolometric effect; for nonresonant geometries, the change was due to the resistive-transition bolometric effect. Kinetic-inductance photoresponse measurements were acquired by setting the microwave frequency on the shoulder of a resonance and measuring the change in the microwave power transmitted through the device that occurred when the absorption of light shifted the frequency of the resonance. These kinetic-inductance measurements were performed as a function of microwave power on a YBa2Cu3O7−δ coplanar device at 74 K and on a Tl2Ba2CaCu2O8 coplanar device at 80 K. Because the photoresponse in this technique is proportional to the square of the local current density underneath the light spot (as opposed to directly proportional), this technique is sensitive to current redistribution on length scales much smaller than the ∼ 6 μm spatial resolution of our measurements. Extrinsic (defect and grain boundary associated) and intrinsic photoresponses were measured. Both coplanar samples showed no change in the intrinsic spatial distribution of the current as the microwave power was varied. At all temperatures the Tl2Ba2CaCu2O8 sample exhibited substantial extrinsic spatial variations on a distance scale equal to the film’s ∼ 5 μm grain size; these spatial variations became more dramatic as the temperature T approached the critical temperature Tc. The spatial variations for the finer-grained YBa2Cu3O7 sample were much less pronounced. Behavior consistent with Tc being several degrees lower at the film edges was observed as T approached Tc; the edge-current photoresponse started to narrow, increase faster than linearly with light power, and move away from the film edges toward the center of the strip; lowering the light power lessened these effects. Resistive-bolometric photoresponse measurements performed at temperatures within the resistive transition exhibited the same light power effects near Tc. The kinetic-inductance photoresponse can also be used as a probe of the local quality of unpatterned superconducting films; for this usage an unpatterned film serves as one wall of a resonant cavity and a focused light beam is scanned through the transparent substrate onto the underside of the superconducting film while monitoring the shift in the cavity resonance. © 1998 American Institute of Physics.
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74.78.-w Superconducting films and low-dimensional structures
74.72.-h Cuprate superconductors
84.40.Az Waveguides, transmission lines, striplines
85.25.Qc Superconducting surface acoustic wave devices and other superconducting devices
61.72.Mm Grain and twin boundaries

Planar-type tunnel junctions with Bi2Sr2CaCu2O8+δ high-temperature superconducting electrodes and Bi2Sr2YCu2O8+δ artificial barriers

E. Baca, M. Chacón, W. Lopera, M. E. Gómez, P. Prieto, J. Heiras, R. Di Leo, P. Romano, and A. M. Cucolo

J. Appl. Phys. 84, 2788 (1998); http://dx.doi.org/10.1063/1.368391 (7 pages) | Cited 1 time

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We performed a detailed study of the structure and transport properties of Bi2212/22Y2/2212 planar-type tunnel junctions. Both high-temperature superconducting electrodes and semiconducting barriers are highly epitaxial thin films deposited onto SrTiO3 single-crystal (001) substrates. Deposition of the films was carried out by a high oxygen pressure dc-sputtering technique, which produces high-quality epitaxial thin films, as determined by x-ray diffraction, lattice resolution transmission electron microscopy, and Rutherford backscattering. Critical temperatures for the superconducting electrodes of 85 K were determined by transport measurements (ρ and χ versus T). A study of resistivity as a function of temperature of the semiconducting barriers was performed. Clear quasiparticle tunneling indicating a gap structure at about 30–35 mV, a zero-bias peak, as well as linear and flat background at high voltages have been observed. For junctions with very thin barriers weak-link-type behavior was observed. An analysis of the I–V curves for these junctions has been made based on the resistively shunted junction model. © 1998 American Institute of Physics.
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74.78.-w Superconducting films and low-dimensional structures
74.50.+r Tunneling phenomena; Josephson effects
85.25.Cp Josephson devices
74.10.+v Occurrence, potential candidates
74.72.-h Cuprate superconductors
74.25.Jb Electronic structure (photoemission, etc.)

Surface effects on the magnetic behavior of antiferromagnetic particles

K. N. Trohidou, X. Zianni, and J. A. Blackman

J. Appl. Phys. 84, 2795 (1998); http://dx.doi.org/10.1063/1.368392 (6 pages) | Cited 14 times

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Antiferromagnetic particles can exhibit the phenomenon of superparamagnetism if the number of spins in the two sublattices is unequal (“uncompensated spins”). Monte Carlo simulations have been performed on spherical particles in which the spins interact with classical Heisenberg interactions. Anisotropy effects are also included with distinct forms in the core and at the surface. The magnetization was calculated as a function of both temperature and applied magnetic field for a range of particle sizes. Different types of behavior are found which depend on the number of uncompensated spins. Certain field dependent behavior is shown to scale with the ratio of the number of uncompensated to the total number of spins. © 1998 American Institute of Physics.
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75.50.Tt Fine-particle systems; nanocrystalline materials
75.70.Rf Surface magnetism
75.40.Mg Numerical simulation studies
75.40.Cx Static properties (order parameter, static susceptibility, heat capacities, critical exponents, etc.)
75.10.Jm Quantized spin models, including quantum spin frustration

Magnetic properties of CoFe2O4 thin films prepared by a sol-gel method

Jae-Gwang Lee, Jae Yun Park, Young-Jei Oh, and Chul Sung Kim

J. Appl. Phys. 84, 2801 (1998); http://dx.doi.org/10.1063/1.368393 (4 pages) | Cited 63 times

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Thin films with cobalt ferrite layers on thermally oxidized silicon wafers were fabricated by a sol-gel method. Magnetic and structural properties of the films were investigated with an x-ray diffractometer, a vibrating sample magnetometer and atomic force microscopy. The crystallization temperature for Co ferrite thin films was determined by using Mössbauer spectroscopy. Co ferrite films annealed at and above 450 °C have only a single phase spinel structure without any preferred crystallite orientation. Their rms surface roughness is less than 3 nm and the size of grains is about 30 nm for annealing temperatures greater than 650 °C. Films fired at and above 550 °C have moderate saturation magnetization and there is no significant difference of their magnetic properties for external fields applied parallel and perpendicular to their planes. The coercivity shows a strong dependence on the annealing temperature. © 1998 American Institute of Physics.
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75.70.Ak Magnetic properties of monolayers and thin films
75.50.Gg Ferrimagnetics
81.10.Dn Growth from solutions
81.10.Fq Growth from melts; zone melting and refining
81.15.Lm Liquid phase epitaxy; deposition from liquid phases (melts, solutions, and surface layers on liquids)
68.35.B- Structure of clean surfaces (and surface reconstruction)
76.80.+y Mössbauer effect; other γ-ray spectroscopy
81.40.Gh Other heat and thermomechanical treatments
81.40.Rs Electrical and magnetic properties related to treatment conditions
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
68.55.-a Thin film structure and morphology

Giant magnetoimpedance in a cylindrical magnetic conductor

D. Ménard, M. Britel, P. Ciureanu, and A. Yelon

J. Appl. Phys. 84, 2805 (1998); http://dx.doi.org/10.1063/1.368421 (10 pages) | Cited 52 times

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A rigorous treatment of the giant magnetoimpedance (GMI) in soft magnetic wires is presented. A small-signal approximation is used for a cylindrical magnetic conductor which is saturated along its axis by a static magnetic field. The general analysis of GMI includes a discussion of the influence of different parameters on the GMI and of how the calculation can be extended to nonsaturating fields. The comparison with high frequency impedance spectra of CoFeSiB wires measured with a network analyzer, including the observation of the ferromagnetic resonance peaks, confirms that the proposed model gives a satisfactory explanation for the linear GMI effect over a broad frequency range and opens the way to more refined calculations. © 1998 American Institute of Physics.
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72.15.Gd Galvanomagnetic and other magnetotransport effects
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