摘要:

A hybrid model is developed to simulate electron transport through the emitter‐base heterojunction and the base region of abrupt heterojunction bipolar transistors. The energy distribution of the injected electron flux through the emitter‐base junction is calculated using a rigorous quantum‐mechanical treatment of electron tunneling and thermionic emission across the spike at the emitter‐base junction. The results are compared with those predicted by the conventional thermionic‐field‐emission model. For both models, the electron fluxes injected across the emitter‐base junction are used as initial energy distributions in a regional Monte Carlo calculation to model electron transport through the base. The average base transit times are calculated using the impulse response technique as a function of the emitter‐base voltage. The differences between the thermionic‐field‐emission model and the rigorous quantum‐mechanical approaches to model electron transport through abrupt heterojunction bipolar transistors are pointed out. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.359224

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

The static and dynamic properties of interface states between silicon nitride andn‐GaAs have been studied. Comparisons have been made between interfaces that were untreated and ones which were plasma pretreated with Ar, N2, H2, and NH3. With the exception of hydrogen the pretreatments did not significantly alter the quiescent surface Fermi level position or the shape of the interface state distribution in the metal‐insulator‐semiconductor field effect transistor test structures. In all of the samples it required about 10 &mgr;s to complete the transfer of the induced charge from the bulk edge of the depletion region to the interface. Charge which had reached the interface equilibrated there within 100 ns. This charge gave rise to a relatively slow emission transient and had a thermal emission energy consistent with emission from the surface Fermi energy to the lower edge of an interface state band, in accord with the interface state band model. The extent in energy of this band depended on the surface treatment. The insensitivity of the emission time constant to both filling time and insulator electric field suggests that irrespective of the treatment the equilibrated charge remained at the interface and did not tunnel into the insulator. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.359594

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Ag‐doped YBa2Cu3O7−x(YBCO) thin films using 2–20 wt % Ag‐doped YBCO targets have been growninsituby the laser ablation technique. The improvement in properties in normal and superconducting states of Ag‐doped YBCO films has been interpreted using a two‐dimensional growth model. The model is simple and is based on widely accepted characteristics of Ag such as its flux action at high temperatures and its nonreactivity with YBCO phase. Experimental evidence in support of the growth model is presented by carrying out microstructural studies and measurements of room‐temperature resistivity, critical current density, and microwave surface resistance in superconducting state. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.359159

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

(110)‐ and (103)‐oriented almost stoichiometric YBa2Cu3O7−&dgr;(YBCO) films have been grown epitaxially on hot SrTiO3(110) substrates using a 90° off‐axis rf magnetron sputtering technique, for fabrication of vertical sandwich‐type YBCO/insulator/YBCO or YBCO/normal metal/YBCO Josephson junctions utilizing the high‐quality YBCO films. The YBCO epitaxial films with high transition temperaturesTcof ∼90 K have been depositedinsituonly under the conditions of substrate temperaturesTsof ∼650–∼700 °C and oxygen partial pressurePO2of ∼5×10−3–∼10×10−3Torr, which are in close proximity to the critical stability/decomposition line for YBa2Cu3O6in the ordinary Y–Ba–Cu–O phase diagram. Using a quadrupole mass spectrometer, a high density of atomic oxygen has directly been observed to be efficiently produced in the sputter glow discharge under the above optimum conditions ofPO2. This atomic oxygen has played a key role in promoting the formation of the perovskite structure and the epitaxial growth of the YBCO films. Furthermore, Shapiro steps have successfully been observed for a Nb–YBCO point‐contact junction, which is made by pressing a Nb needle on a surface‐etched YBCO epitaxial film, under 525.4 GHz submillimeter‐wave irradiation. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.359160

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

A torsional pendulum has been built using a stress relieved FeSiB amorphous wire. Torsional motion is magnetoelastically induced by means of the simultaneous application of longitudinalHzand circularH&fgr;magnetic fields. The experimental setup allows one to determine both the torsional deformation and the changes in the longitudinal magnetizationMzof the wire. Free and forced oscillations are observed depending on the parameters of the applied fields. In both forced and free oscillation modes two resonant peaks are observed. Their frequency and amplitude dependences on the axialHzand circular fieldsH&fgr;have been studied. Changes in shear modulus &Dgr;Gvalued 60% in excess have been measured. To interpret the experimental results a theoretical model is developed taking into account the existence of two different magnetic regions in the wire, core, and shell having a different distribution of axial, radial, and shear stresses. The calculations explain the more remarkable aspects of the observed behavior. ©1995 American Institute of Physics. 

ISSN:0021-8979    

DOI:10.1063/1.359161

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

High coercivity and high wear resistance &ggr;‐Fe2O3thin films are described, which were prepared by bias sputtering followed by heat treatments. The coercivity, squareness ratio, and wear resistance of the films were increased by applying a negative bias voltage of about −60 V to the substrate. The improvements in magnetic and mechanical properties are attributed to the enhanced ion irradiation which increased both the internal strain of the films and the strength with which the films adhered to the substrates. When the bias voltage was too low (−180 V), many cracks appeared in the film and this resulted in low coercivity and poor wear resistance. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.359162

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

We present a coupled wave analysis of nonlinear magnetostatic surface waves in ferromagnetic slabs which have double periodically corrugated surfaces. Most relevant are the effects close to the Bragg condition. The nonlinearity of the system shifts downwards and widens the stop bands, however the relative spatial phase difference between the two surface corrugations has significant influence on the widths of stop bands and under certain conditions weakens the nonlinear effects of the system. ©1995 American Institute of Physics. 

ISSN:0021-8979    

DOI:10.1063/1.359163

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Investigations on unidirectional anisotropy and rotational hysteresis loss of exchange coupled Ni81Fe19/NiO films have been conducted to clarify the nature of the exchange coupling mechanism. The interfacial exchange coupling regions, which had been considered to be scattered among the nonexchange coupling regions matrix, were found to be composed of many local regions of two kinds: (i) blockable regions which can give the Ni81Fe19film a unidirectional anisotropy and (ii) unblockable regions which can have exchange coupling, but cannot give the Ni81Fe19film unidirectional anisotropy. These unblockable regions begin to change gradually to blockable regions on decreasing the temperature below around 100–110 K. This change is probably caused by the antiferromagnetic NiO anisotropy of unblockable regions being strengthened below that temperature. Moreover, the decrease in size of the exchange coupling field and lowered blocking temperature fortNiO<50 nm (tNiO:NiO film thickness) seems to originate from a decrease of antiferromagnetic NiO anisotropy with decreasingtNiO. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.359164

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

We report magnetoresistance in Cr‐Fe bulk alloys and its variations associated with the microstructure change due to the phase decomposition. Large negative magnetoresistance (MR) is observed in the as‐quenched Cr‐Fe bulk alloy at liquid helium temperature, but not at room temperature. This is attributed to the spin cluster glass in the homogeneous solid solution. A unique feature of the MR in these homogenous alloys is that it does not saturate even in the presence of very high magnetic fields. However, such MR behavior changes when ferromagnetic particles precipitate by annealing. A lower degree of compositional fluctuation of Fe progresses by annealing the alloy at 773 K for 400 h and then a higher degree of decomposition occurs after a prolonged aging for 2600 h. In the annealed specimens, MR is observed even at room temperature. The MR change becomes more prominent at a lower field region, since ferromagnetic particles precipitate from the solid solution. The degree of the phase decomposition of the Cr‐Fe solid solution is quantitatively evaluated by the atom probe analysis, and the origin of the MR dependence on the annealing conditions is discussed. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.359165

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

One of the important problems in bulk ferromagnetism is still the determination of magnetization as a function of temperature. The problem becomes more complicated when one tries to explain the magnetization of thin films nearTc. The experimental situation is far from clear, mainly because the experiments are difficult to perform. We have implemented a thermogravimetric technique using a modern microbalance to determine the magnetization behavior of thin films of pure Fe nearTc. We are able to obtain very precisely the films’ magnetization, by measuring the force exerted over the films when they are imbedded in a fixed external magnetic field, with this technique. We prepared several samples of pure Fe films, evaporated in ultrahigh vacuum conditions onto Cu foils ranging in thickness between 50 and 300 nm and measured their magnetization near the critical point. The value of theTcof the films is systematically lowered from the bulkTc, being 342 K lower for the 50 nm films. The magnetization curves show a dependence on the temperature as a series containing terms in (T/Tc)3/2, (T/Tc)5/2, and (T/Tc)7/2at temperatures low compared withTc. NearTc, the magnetization decreases linearly on temperature for films of all thicknesses. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.359591

出版商:AIP    

年代:1995

数据来源: AIP