摘要:

The confinement of minority carriers by polycrystalline silicon (polysilicon) contacts has been studied using the photoconductivity technique. Steady‐state and transient optical measurements show a dramatic increase of stored carriers by these contacts. From a thermionic‐diffusion model, a barrier height of 130 meV for the polysilicon/silicon contact has been extracted.

ISSN:0021-8979    

DOI:10.1063/1.345573

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

Tunneling in a double‐barrier structure with a localized recombination channel is modeled by a spatially dependent imaginary potential. The general properties of nonhermitian Hamiltonians with an imaginary potential are examined. For a localized imaginary potential, the eigenvalues are found to be real for the extended states. The eigenstates with different eigenvalues are found to be nonorthogonal in general and this nonorthogonality is related to the nonconservation of matter. A continuity equation is derived from the Schro¨dinger equation showing explicitly a sink of carrier density at the imaginary potential. The lifetime of the quasi‐bound state in the quantum well in a double‐barrier structure is calculated from the width of the resonance peak in the transmission spectrum of the structure and is found to be decreasing linearly with the magnitude of the imaginary potential. The dependence agrees with a relation derived from the rate equation describing the tunneling and recombination process. A numerical simulation of the tunneling escape of a wavepacket localized initially inside the quantum well in this potential illustrates the loss of matter through the imaginary potential. The lifetime of the electron in the quantum well, the recombination time, and the tunneling time obtained from the simulation are in agreement with those calculated from the width of the resonance peak.

ISSN:0021-8979    

DOI:10.1063/1.345574

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

Tunneling current measurements onn‐type &dgr;‐doped Si(100) structures were carried out, with sheet doping concentrations ranging from ∼4×1012to 2.0×1013cm−2at 4 K. All samples have been grown by using a low‐energy ion source for antimony doping in a silicon molecular beam epitaxy system. From analysis ofdI/dVgand (dI/dVg)/ (I/Vg) spectra, tunneling associated with quantized electron subbands is identified. The subband energy positions relative to the equilibrium Fermi levelEF0under zero bias were determined from the tunneling current measurements as a function of the sheet doping concentration. Self‐consistent theoretical calculations of the electronic structure of &dgr; layers have been performed, and good agreement between theory and experiment is obtained for most structures in the tunneling spectra.

ISSN:0021-8979    

DOI:10.1063/1.345575

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

Spatial localization of Be in &dgr;‐doped GaAs within few lattice constants (<20 A˚) is achieved at low growth temperatures for concentrationsN2DBe<1014cm−2as indicated by capacitance‐voltage profiles and secondary ion mass spectroscopy. At elevated growth temperatures and at higher Be concentrations, significant spreading of the dopants occurs and is explained by (i) Fermi‐level pinning‐induced segregation, (ii) repulsive Coulomb interaction of dopants, and (iii) diffusion. The highest Be concentration achieved at low growth temperatures exceeds 2×1020cm−3and is limited by repulsive dopant interaction. It is shown that the repulsive Coulomb interaction results in a correlated, nonrandom dopant distribution. The diffusion coefficient of Be in GaAs is determined and is found to be much lower than previously reported.

ISSN:0021-8979    

DOI:10.1063/1.345576

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

Conductance measurements have been performed onp‐Si/SiO2metal‐oxide‐semiconductor capacitors fabricated by thermal oxidation of the silicon in three different reactors under different conditions in three different commercial systems. In only one case (system 3) did we find the very broad conductance curves usually associated withp‐Si/SiO2junctions, the others from systems 1 and 2 exhibiting normal interface state response. However, in the sample from system 2, the response of bulk states inside the depletion region was found to distort theGp/&ohgr; spectra, while the sample from system 1 showed no such response until annealing in H2/Ar gas at 500 °C reduced the interface state density to such a level that the bulk state responses were visible. We argue that the presence of bulk states probably explains the very broadGp/&ohgr; curves observed by us, and further propose that some such mechanism accounts for the historical difficulties encountered with the conductance technique onp‐Si/SiO2capacitors.

ISSN:0021-8979    

DOI:10.1063/1.345577

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

Self‐resonant current steps in superconducting interferometers are investigated. The case of interferometers with non‐negligible loop capacitance is considered. A theoretical analysis based on an equivalent electrical circuit is performed to get the resonance frequencies in the current‐controlled case. Careful measurements on a slightly asymmetrical (C1≠C2) interferometer are reported. The magnetic field dependence of the maximum amplitude of the resonant steps and their voltage position have been studied. A good agreement between theory and experiments is obtained.

ISSN:0021-8979    

DOI:10.1063/1.345578

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

High‐quality Nb/Al, Alox/Nb Josephson tunnel junctions using double‐oxide layers as barriers have been fabricated. The critical current density is controlled by the thickness of the second Al layer. This layer has to be oxidized completely through in order to obtain high‐quality junctions. Typically, gap voltages of 2.8–3.0 mV andVmup to 70 mV at 4.2 K were obtained.

ISSN:0021-8979    

DOI:10.1063/1.345579

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

X‐ray photoemission results make it possible to compare Ti adatom reactivity with clean surfaces of CuO, Y2BaCuO5, and YBa2Cu3O7−x. Changes in the Ti 2p1/2,3/2and O 1score level emission during overlayer growth are related to surface reactions between the Ti adatoms and oxygen atoms leached from the substrates. The results show the growth of a TiO2‐like reaction species at initial Ti depositions, the formation of Ti suboxides at intermediate coverages, and the onset of metallic behavior at higher Ti coverages. The Ti‐O reaction products grow in a layer‐by‐layer fashion initially, but Ti metal overlayer growth occurs as clusters on this Ti‐O reacted region. Substrate modifications associated with Ti‐O reactions result in significant changes in the chemical environments of Cu, Ba, and Y for Y2BaCuO5and YBa2Cu3O7−x.

ISSN:0021-8979    

DOI:10.1063/1.345580

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

rf (radio frequency) properties of polycrystalline ceramic material are substantially inferior to those of the best microtwinned crystals. At 6 GHz and 77 K, the best surface resistance values for ceramics are ∼20 m&OHgr;, compared to <0.5 m&OHgr; for high‐quality crystals. The microwave resistance is observed to increase at higher rf fields for ceramics, e.g., two orders of magnitude between 0.1 and 10 Oe, but remains low for crystals. A possible reason for the inferior properties in randomly oriented polycrystalline ceramics is related to the anisotropy of the superconducting properties; crystals aligned unfavorably with the sampling rf field could be responsible for the high resistance. Another possibility is related to the difficulty of carrying current across the grain boundaries due to weak links arising from second phases, impurities, cracks, etc., at the boundaries. To elucidate the contribution from these two potential problem sources, we have measured the rf properties at 6 GHz of an oriented polycrystalline ceramic pellet prepared from a suspension of high purity powder in a 4‐T magnetic field. Samples were characterized by x‐ray diffraction, light microscopy, transmission electron microscopy, x‐ray rocking curves, and x‐ray pole figure studies, indicating a high degree of alignment, although not as complete as in epitaxial thin films. At liquid He temperature, the surface resistance is 28 times lower when thecaxis is perpendicular to the plane in which rf currents flow than when thecaxis is in the plane. At 77 K, the surface resistance is 3 m&OHgr;, a significant improvement over the properties of the best randomly oriented material reported. At 4 K the resistance improves to 0.3 m&OHgr;. These resistances were all measured with rf fields below 0.1 Oe, but increased by one order of magnitude when the rf field was increased to 10 Oe, as in the polycrystalline material. Our results indicate that while the low‐field rf behavior is strongly improved by orientation, the high‐field behavior is dominated by the poor current carrying capacity across weak links.

ISSN:0021-8979    

DOI:10.1063/1.345581

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

The sintering of DyBa2Cu3O7−&dgr;has been improved by controlling powder size, sintering temperature, and sintering atmosphere. Sintering below the eutectic temperature has been explored for different temperatures and oxygen partial pressures. Sintering kinetics have been enhanced, yielding high density (up to 95%) and small grain size (down to 1.5 &mgr;m) with shorter sintering times and lower temperatures. Two sets of samples, one set containing 10‐wt. % Ag2O in the precursor powder and one undoped set, were studied. The sintering rate decreased with decreasing temperature, even in samples sintered in low partial pressures of oxygen (which increase the oxygen vacancy concentration). The addition of Ag2O yielded a dispersion of Ag particles about one‐half the DyBa2Cu3O7−&dgr;grain size at grain boundary triple points. Ag tended to enhance grain growth slightly and did not improve the superconducting properties. Resistivity was as low as 220 &mgr;&OHgr;‐cm at 100 K. Transport critical current densities (Jct) were up to five times higher than in previous larger grain size samples that were sintered above the eutectic temperature. The best of the new samples achieved ∼100 A/cm2at 4.2 K in fields of 1–7 T. This improvement occurred in spite of there being more than twice as much grain boundary surface area per unit volume. We conclude that the grain boundaries in sintered 123 compounds are not always barriers to current flow, and that increasing the proportion of nonbasal plane faced grain boundaries is important in raisingJct.

ISSN:0021-8979    

DOI:10.1063/1.345582

出版商:AIP    

年代:1990

数据来源: AIP