摘要:

A realistic model for predicting valence‐band discontinuities at semiconductor heterojunctions is presented. The proposed model is based on a simple picture of the semiconductor interfaces which takes into account charge transfer at the interface. Applications are made to a number of different heterojunctions and chemical trends are examined. Comparisons with experiment and with other model theories are discussed.

ISSN:0021-8979    

DOI:10.1063/1.348584

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

The effects of post‐oxidation processing on the initial performance (as measured here by pre‐irradiation fixed‐charge and interface‐state densities) and radiation response of rapid thermally processed, metal‐oxide‐semiconductor (MOS) capacitors are investigated. The processing dependencies for the major groups of processing‐ and radiation‐induced defects are discussed with respect to recent gains in understanding of the Si‐SiO2interfacial structure. Processing conditions for ideal initial properties are found to be quite different than those required for optimum radiation response. Guidelines for optimal post‐oxidation thermal processing are given, taking into consideration both the initial performance and radiation response of the MOS device. The results indicate that even when using rapid thermal processing, post‐gate oxide anneals above 900 °C degrade the radiation tolerance.

ISSN:0021-8979    

DOI:10.1063/1.348585

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

Irradiation effects on thin foils of bulk YBa2Cu3O7−xhave been studied in a transmission electron microscope using 100‐, 150‐, 200‐, 250‐, and 300‐keV electrons at 83 and 300 K. The disordering of the oxygen atoms and vacancies in the O(4) and O(5) sites in the Cu‐O planes during irradiation was monitored by measuring the splitting of the (11¯0) diffraction spots in the [001] diffraction pattern. The results show that YBa2Cu3O7−xis insensitive to 100‐keV electron irradiation. Irradiation by higher‐energy electrons leads to irradiation‐induced oxygen disordering of the oxygen atoms and vacancies, mainly by single displacement events. The excellent fit of the data to a disordering model suggests that the displacement threshold energy for oxygen in YBa2Cu3O7−xis around 18 eV and that irradiation‐assisted oxygen reordering occurs in YBa2Cu3O7−xat 300 K, but not at 83 K.

ISSN:0021-8979    

DOI:10.1063/1.348586

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

Four types of differently prepared NbN films were irradiated at 4.6 K with fast neutrons to a fluence of 5.3 × 1022m−2(E≳0.1 MeV). The critical current densitiesJcwere measured in magnetic fields up to 23 T prior to irradiation, following low‐temperature irradiation, and again after an annealing cycle to room temperature. In all films,Jcwas found to be completely unchanged by the radiation and annealing treatments in fields up to 15 T, but to increase at higher magnetic fields. At the same time, the upper critical fieldsBc2increased by about 0.5 T (∼2%). ReplottingJcversus reduced fieldB/Bc2leads to identical field dependencies also in the high‐field range. Hence, the observed increase ofJcis equantitatively explained as aBc2effect.

ISSN:0021-8979    

DOI:10.1063/1.348557

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

Photoemission results demonstrate that atom deposition of Ti, Cr, and Cu at 20 K on the high‐temperature superconductors (HTSs) dramatically reduces interfacial reaction relative to 300 K growth but does not completely eliminate it. Thin Ti‐O or Cr‐O layers are formed during atom deposition of ∼2 A˚ of Ti or Cr on YBa2Cu3O7or Bi2Sr2Ca1Cu2O8because oxygen is withdrawn from the Bi‐O and/or Cu‐O layers. Interfacial reactions are diffusion limited at 20 K, and metal overlayers nucleate on the reacted layers. These metal layers are more uniform than those grown at 300 K because clustering is suppressed. There is no additional disruption for Cr/HTS interfaces when warmed to 300 K, but increased disruption is evident for Ti/HTS interfaces. The differences reflect the stabilities of Cr and Ti in contact with their own interfacial oxide. Cu atom deposition on Bi2Sr2Ca1Cu208(100) at 20 K also leads to much less disruption than observed for deposition at 300 K.

ISSN:0021-8979    

DOI:10.1063/1.348558

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

Initial magnetization curveM(H) and magnetic relaxationM(t) were studied on the Ag‐sheathed and highlyc‐axis oriented tape of Bi‐Pb‐Sr‐Ca‐Cu‐O with transportJct=5400 and 2400 A/cm2(77 K,H=0). TheJcvalue, derived fromM(H) curve based on Bean’s critical‐state model, agrees fairly well with theJct. The lower critical fieldHc1forH⊥caxis fromM(H) curve increases with a slope of −0.2 Oe/K as temperature decreases fromTc, and it approaches the saturated value of 15 Oe atT∼0 K. The magnitude is much lower than that for the high‐Tcsuperconductors La‐Sr‐Cu‐O and Y‐Ba‐Cu‐O. The creep rate and activation energyU0are evaluated fromM(t) based on the flux creep model. The results show a significant dependence on the measuring process and magnetic fieldH. TheU0at 5 K forH=500 Oe (H⊥c) is 0.4 eV for the in‐field magnetization (IN) and 0.1 eV for the remanent magnetization (REM), which are larger than those for Bi‐based high‐Tcsuperconductors reported previously. TheU0for the IN shows a peak of 0.8 eV around 30 K and then decreases as temperature increases, while a peak in it is weakened for the REM. TheU0atH=30 Oe (H⊥c) for the IN is one order larger than that for 500 Oe and the peak temperature is shifted to the lower temperature (∼10 K).

ISSN:0021-8979    

DOI:10.1063/1.348559

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

Superconducting 326 samples, La2−xSrxCaCu2Ozwithx=0.2 and 0.4, which exhibited superconducting onsets at temperatures (Tonc’s) higher than 40 K were successfully synthesized using hot‐isostatic‐pressing (HIP) techniques. As starting materials, powders of conventional carbonates (SrCO3and CaCO3) and oxides (La2O3and CuO) were employed. Sintering of the samples was made at a variety of temperatures (Ts) for 72 h in flowing O2gas (of 1 atm). The sintered samples were post‐annealed at various temperatures (Ta) for 6 h in a mixture of 80% Ar gas and 20% O2gas of the total pressure of 1000 atm by means of HIP techniques. Optimum combinations ofTsandTawere determined for obtaining superconducting samples. Single‐phase samples were obtained only for the composition La1.8Sr0.2CaCu2Ozfor which the oxygen content,z, was determined to be 6.01±0.01 by coulometric titration. It was concluded thatTawas a crucially important factor for obtaining superconducting 326 phase withTonc≳ 40 K, that is,Ta≳ 970 °C was required (for 6‐h HIP annealing). It was demonstrated that a long HIP annealing was necessary to obtain a significant volume fraction for the superconducting 326 phase.

ISSN:0021-8979    

DOI:10.1063/1.348560

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

Amorphous rare‐earth transition‐metal alloys of composition REl−xTMxwith RE=Dy, Ho; TM=Fe,Co and 0<x<1 and Dyl−x(Fe,Co)xwere prepared by evaporation. The saturation magnetization, uniaxial anisotropy, coercivity, and Faraday rotation were investigated as a function of composition and temperature. Also, the spectral variation of the Kerr rotation was measured. The magnetization data indicate a strong dispersion of the RE moments due to randomly oriented local crystal field axes. The strong turndown of the Curie temperature for the Fe‐rich alloys suggests that an additional dispersion is present in the Fe subnetwork. The mean field theory was used to analyze the temperature variation of the magnetization yielding smaller TM spin values and exchange coupling constants as compared with those of the Gd and Tb analogs. The uniaxial anisotropy constantKufor Dy‐Co based alloys was found to vary with the square of the Dy subnetwork magnetization as predicted by the random single‐ion theory. The anisotropy of Dy‐Fe and Ho based alloys require additionally dipolar terms to account for the measured temperature dependence ofKu. The coercive fieldHcfollows a relationHc∼K1.5u/Ms. The magneto‐optical effects are primarily caused by the transition metals and therefore their compositional, temperature, and spectral dependence correspond to that of their Gd and Tb analogs. Optical recording experiments on Dy‐FeCo disks yield good write and erase sensitivities and carrier‐to‐noise ratios up to 60 dB which are comparable to those of GdTb‐Fe and Tb‐FeCo disks.

ISSN:0021-8979    

DOI:10.1063/1.348561

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

The refractive indices of AlSb grown by the solute diffusion method and GaSb‐lattice‐matched AlxGa1−xAsySb1−yalloy grown by liquid‐phase epitaxy have been determined at room temperature from accurate measurements of the reflectance ofp‐polarized light as a function of the angle of incidence. The refractive index variations versus the photon energy were obtained in the spectral range 0.5–1.5 eV. Experimental data in the transparent wavelength region could be matched by calculated curves on the basis of a single‐oscillator model.

ISSN:0021-8979    

DOI:10.1063/1.348538

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

Defects and impurities in diamond films grown by chemical vapor deposition (CVD) were analyzed by high‐resolution cathodoluminescence (CL) spectroscopy and imaging in transmission electron microscopy (TEM). The combination of CL and TEM makes it possible to correlate the film microstructure with the electronic structure due to defects. Broad CL bands observed at 428±1 nm (2.90±0.01 eV) and 551±1 nm (2.250±0.004 eV) are attributed to closely spaced and widely separated donor‐acceptor (D‐A) pairs, respectively. A narrow peak at 738.7±0.5 nm (1.679±0.001 eV) is attributed to interstitial silicon atom impurities. An additional wide band at 365±1 nm (3.40±0.01 eV) was not identified. The material was found to be type IIb (semiconducting) and varied in quality with the growth conditions. Impurities are evidently distributed nonuniformly on a submicrometer scale, and both highly faulted and defect‐free grains were found to emit no visible CL. For the first time in CVD‐grown diamond, band‐ACL due to closely spaced D‐A pairs was found to be directly correlated with dislocations. Widely spaced D‐A pairs were more uniformly distributed throughout the film. The distribution of interstitial silicon impurities varied greatly from grain to grain, but was not correlated with any microstructure.

ISSN:0021-8979    

DOI:10.1063/1.348539

出版商:AIP    

年代:1991

数据来源: AIP