摘要:

The conditions for three-peak current–voltage(I–V)characteristics with two discrete resonant tunneling diodes (RTDs) connected in series are discussed and analyzed. With suitable design, one can obtain three-peakI–Vcharacteristics with two series-connected RTDs, and some equations are derived to help the design. The effect of parasitic resistance on theI–Vcharacteristics is investigated. A large-signal model of the RTD is developed by using the piecewise-linear approximation technique, and this model can be easily implemented in PSpice. The simulatedI–Vcurve of the series circuit is in good agreement with the measured results. The results in this paper can be applied to the design of multipeakI–Vcurve with less RTD structure in comparison with the traditional device that stacks the same RTDs to obtain a multipeakI–Vcurve. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.365241

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

A reproducible process for fabricatingBa1−xKxBiO3/Nb-dopedSrTiO3(BKBO/STNO) all-oxide-type Schottky junctions has been established, and the electrical properties of the junctions have been investigated at room temperature. Improving the STNO surface conditions by high-temperature oxygen-annealing made it possible to fabricate junctions with good rectification properties. The current–voltage characteristics of the junctions were explained by conventional thermionic emission theory. Anomalous capacitance–voltage characteristics of the junctions were observed and were analyzed with a model taking into account the electric-field-dependent permittivity of STNO and the presence of the interfacial layer. The potential barrier profile of the BKBO/STNO interface was calculated and it can be concluded that this model quantitatively explains the capacitance–voltage characteristics. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.365242

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

The optical properties of(InP)2/(GaP)2bilayer superlattice (BSL) structures have been examined with linearly polarized cathodoluminescence (CL), time-resolved CL spectroscopy, and cathodoluminescence wavelength imaging. An In and Ga composition modulation of∼18&percent;forms during the metalorganic chemical vapor deposition growth of short period(InP)2/(GaP)2bilayer superlattices. Transmission electron microscopy showed a period of∼800 Åalong the [110] direction, resulting in coherently strained quantum wires. A strong excitation dependence of the polarization anisotropy and energy of excitonic luminescence from the quantum wires was found. The results are consistent with a phase-space and band filling model that is based on ak⋅pand two dimensional quantum confinement calculation which takes the coherency strain into account. CL images reveal that defects in the BSL originate from the GaAs substrate and/or the initial stages of InGaP growth. The effects of defects on the band filling, carrier relaxation kinetics, and nonlinear optical properties were examined. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.365243

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

We have measured the energy loss rate as a function of carrier temperature for hot holes inSi/Si0.8Ge0.2quantum wells with sheet carrier densities in the range(3–7)×1011 cm−2at lattice temperatures of 0.35 and 2.0 K. Calculations of the energy loss rate for acoustic phonon deformation potential scattering with coupling constant 4.5 eV show good agreement with measurement. The deformation potential is consistent with a linear interpolation between the bulk Si and Ge values and is in agreement with that deduced from measurements of thermopower in similar samples. In contrast to previous work, we find no evidence for hole coupling to acoustic phonons via a piezoelectric interaction. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.365244

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

Electron transport properties in cubic silicon carbide (&bgr;-SiC) quantized inversion layers have been studied and the results of electron mobility calculations at room and higher temperatures have been reported. To do so, we have developed a Monte Carlo simulator used in conjunction with the self-consistent solution of the Poisson and Schroedinger equations. We show that for a fixed inversion charge concentration, &bgr;-SiC inversion layer electrons spread less into the bulk than Si ones as a consequence of the effective mass values. Therefore, the defects of theSiO2/&bgr;-SiC(interface roughness, charged centers) will strongly affect electron transport properties. We present simulated mobility curves for quantized &bgr;-SiC inversion layers taking into account different scattering mechanisms which are then compared to Si mobility curves. Special attention has been paid to the effect of Coulomb scattering due to both interface- and oxide-trapped charges. Mobility curves obtained for different interface-trapped charge concentrations show that electrons in silicon carbide inversion layers are more affected by surface defects at room and higher temperatures than they are in silicon inversion layers. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.365245

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

An experimental study of the Hall effect in nonmetallic Y-Ba-Cu-O thin films is reported. Both epitaxial crystalline YBa2Cu3O6+x(x⩽0.5)and multiphase/amorphous Y-Ba-Cu-O thin films were studied. The structure of the samples was measured by x-ray diffraction and Raman microprobe. The amorphous Y-Ba-Cu-O samples were found to have a grain size of about 100 Å. The conduction properties were studied and analyzed for the two types of samples over a wide temperature range including room temperature. The Hall effect measurements showed positive charge carriers with a concentration ranging from 1017to 1020cm−3at room temperature. The mobility was found to decrease with higher Hall carrier concentration. The empirical relationship for the mobility dependence on impurity concentration agreed with the relationship between mobility and the experimental Hall carrier concentration, suggesting that the same localized states were responsible for both providing the carriers and reducing the mobility through scattering. It was also observed that the mobility values for both amorphous and crystalline samples followed the same empirical curve, a result which showed that the conduction mechanisms in the epitaxial (tetragonal) and amorphous Y-Ba-Cu-O materials are very likely to be similar despite the differences in the composition and structure of the films. The similarity is consistent with other work that concludes that the conduction mechanism occurs along the copper oxide planes. Our work implies that the conduction mechanism operates over a short range, less than the 100 Å grain size of the amorphous, such that the lack of order in the amorphous samples was essentially irrelevant to the charge transport. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.365246

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

Amorphous FeCoTb films with perpendicular anisotropy have been sputtered from FeCo, Tb, and Nd targets onto glass substrates, mounted on a rotating substrate holder. The composition of the films, the magnetic properties, and the Kerr rotation depend as well on the distance between the substrates and the rotation axis as on the angular velocity of the substrate holder. A model of anisotropic short range order is given, which results from a layered structure due to the sequential sputtering from the different targets. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.365247

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

A new technique for high precision measurement of magnetic anisotropy fields in thin and ultrathin films called modulated field magneto-optical anisometry (MFMA) is described. MFMA can be performed by a simple extension to a conventional magneto-optical Kerr effect magnetometer, and is therefore experimentally simple. It can resolve an arbitrary combination of anisotropies of different symmetries with a very high precision, even when there is little magneto-optical signal (e.g., ultrathin in-plane magnetized films) and high optical noise. It is spatially resolving and readily suited to the ultrahigh vacuum environment. MFMA thus offers many advantages over existing anisotropy measurement methods. A quantitative comparison is made between anisotropy field measurements made by MFMA and by Brillouin light scattering on an ultrathin Fe(001) epitaxial film. Agreement is found to within a high precision. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.365248

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

Thick films (10–12 &mgr;m) of NiZn–ferrite(Ni0.6Zn0.4Fe2O4)were grown on single-crystal (100) magnesium oxide substrates using pulsed laser deposition (PLD). The morphology, phase, orientation, strain, and magnetic properties of the as-deposited films were investigated as a function of substrate temperature (400–700 °C) andO2background pressure (50–200 mTorr). Compositional analysis shows that the PLD NiZn–ferrite films are about 45&percent; Zn deficient when grown using a standard polycrystalline single phaseNi0.4Zn0.6Fe2O4target regardless of substrate temperature orO2pressure. However, Zn-rich targets were successfully used to compensate for the Zn deficiency in the NiZn–ferrite films. PLD NiZn–ferrite films grown at 700 °C exhibit the highest degree of crystalline quality and nearly bulk saturation magnetization values (i.e., 5000 G). At lowO2pressures (<75 mTorr) the films, which were grown at 700 °C, are under a significant compressive stress. The stress decreases when the PLD NiZn–ferrite films are grown in higherO2pressures but the crystalline quality and surface morphology deteriorate. The compressive stress produces a planar anisotropy field of about 1000–3500 Oe depending on theO2pressure, which is consistent with the stress results from x-ray diffraction measurements on the NiZn–ferrite films. It is hypothesized that the film stress is largely the result of oxygen loss from the films during deposition. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.365249

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

Novel isolated magnetic single-domain&ggr;-Fe2O3nanoclusters have been prepared by coprecipitation of ferrous and ferric salts encapsulated within sol-gel derived silica(SiO2).The nonmagneticSiO2coating formed by hydrolysis and polycondensation of tetraethoxysilane on the surface of theFe2O3nanoclusters provides a means for thermally stable dispersion ofFe2O3clusters. The precipitated particles coated withSiO2are spherical with 4–5 nm diameters. Surface and strain effects played a critical role in determining the overall magnetic behavior of the spherical single-domain particles. Superparamagnetic behavior was observed by superconducting quantum interference device magnetometry and Mo¨ssbauer spectroscopy. Superparamagnetic barrier energies and the low-temperature coercivities were modified through cluster/support interface microstructure manipulation. The optical studies showed the absorption edge of the nanocomposites to be slightly blue shifted in the UV–VIS spectrum range when compared to that of bulk&ggr;-Fe2O3.This was attributed to the combined effects of the quantum confinement of the nanocrystalline&ggr;-Fe2O3clusters and the stress present at the particle/support interface. The magnetic properties can be manipulated via the matrix microstructure, synthesis conditions and thermal treatment. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.365233

出版商:AIP    

年代:1997

数据来源: AIP