摘要:

The mathematical analysis and parameter‐extraction process for a new characterization method are presented. This method allows simultaneous measurement of the minority‐carrier lifetime, diffusion coefficient, and diffusion lengths as well as surface recombination velocity. The technique employs semi‐infinite two‐dimensional photodiodes and uniform, instead of focused, illumination. The paper deals with the derivation of exact closed‐form solutions associated with two‐dimensional devices and discusses the simultaneous extraction of minority‐carrier transport parameters.

ISSN:0021-8979    

DOI:10.1063/1.345256

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

A backside secondary ion mass spectrometry technique is employed to examine elemental redistribution in the Ge/Pd/GaAs ohmic contact as a function of annealing conditions. Dilute Al containing marker layers (Ga1−xAlxAs) in the GaAs permit precise calibration and alignment of the elemental depth profiles. Double etch‐stop thinning yields high depth resolution. The onset of ohmic behavior is found to occur when Ge is detected at the GaAs surface. Good ohmic behavior is observed when an interfacial layer of reacted Pd4GaAs is dispersed and complete coverage of Ge occurs. The Ge/GaAs interface is abrupt with the Ge concentration dropping by over three orders of magnitude within 100 A˚. About 40 A˚ of GaAs is found to be consumed during the ohmic contact formation. Degradation of the ohmic contacts, as a result of further heat treatment, was found to correlate with Ge in‐diffusion into the GaAs. The results place strict limitations on doping and heterointerface models of ohmic behavior for this contact.

ISSN:0021-8979    

DOI:10.1063/1.345258

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

GaAs‐AlGaAs quantum wires with various widths have been fabricated using electron beam lithography and Ar ion etching. Conductance was measured as a function of the etched depth and the wire width. The results indicate that defects diffuse at room temperature. Shubnikov–de Haas oscillations in GaAs‐AlGaAs film and wires with different widths have been measured to investigate the effect of the surface damage on the electron transport characteristics. The analysis of the results shows that both electron density and scattering time decrease with decreasing the width of the channel. These are indicative of the side wall scattering and the damages induced during the sample preparation process.

ISSN:0021-8979    

DOI:10.1063/1.345259

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

Single‐quantum‐well structures were grown by atmospheric pressure organometallic vapor‐phase epitaxy, with GaxIn1−xAs layers (0≤x≤1) coherently strained to match the lattice parameter of the InP barrier layers in the (100) growth plane. The strain effects on the band lineups were analyzed using the ‘‘model solid’’ theory of Van de Walle and Martin. The hydrostatic strain component for alloys withx≊1 is shown to be sufficient to marginally convert the type‐II lineups for the unstrained case to type I. The band lineups remain type I forx≊0. Considering the effect of strain, the ‖ (3)/(2) , (1)/(2) ⟩ valence subband becomes a slowly varying function ofx. Band offsets are predicted over the entire alloy composition and compared with the reported data. The photoluminescence (10 K) peak energies for the 100‐A˚ GaxIn1−xAs/InP single quantum wells compare quite favorably with the calculated strained band gap versusx. For nominal monolayer quantum wells, the peak energies are slightly above 1.1 eV over the entire alloy range. It is shown that forx>0.47, the LH1 and HH1 subbands cross at a smaller well width asxincreases. The ground‐state exciton is light hole‐like for larger values ofxandLz, and heavy hole‐like for smallerxandLz.

ISSN:0021-8979    

DOI:10.1063/1.345260

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

Changes in various superconducting properties as a function of silver substitution in the bulk granular high‐Tcsystem Y‐Ba‐Cu‐O have been investigated. In the low doping limit, Ag impurities behave as an excellent oxygen stabilizer and the critical current density is enhanced by an order of magnitude while theTcremains practically unchanged. When the Ag content is increased to above 20%, theTcstarts to drop and the effects of Ag clusters as well as impurity phases become more important. However, the compound is still a superconductor with aTcaround 20 K by full substitution of Ag for Cu. The problems concerning the location of Ag atoms and formation of Ag clusters are examined with measurements of x‐ray absorption fine structure using synchrotron radiation. Our results indicate that at least some Ag atoms occupy the Cu(1) and Cu(2) sites in the material. Magnetic field dependence shows that the critical current density of a bulk sintered high‐Tcsuperconductor below a characteristic valueH’cis mainly controlled by intergranular weak‐link coupling. A shift ofHcwith the addition of Ag helps explain the variation of critical current density in this compound system.

ISSN:0021-8979    

DOI:10.1063/1.345261

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

The criterion of intrinsic thermal stability (ITS) of thin‐film superconductors predicts under which circumstances a thermal disturbance causes Joule heating. In the characterization method of low‐temperature scanning electron microscopy (LTSEM), the absorption of the electron beam results in a well‐controlled thermal disturbance. This work applies the ITS criterion to LTSEM of both high‐Tcand low‐Tcthin‐film superconductors and predicts the conditions for which no voltage appears along the film. The analysis of the temperature field for high‐Tcfilms differs from that for low‐Tcfilms by the absence of the acoustic‐mismatch thermal resistance between film and substrate at liquid‐nitrogen temperature. The theoretical results for low‐Tcfilms are in very good agreement with experimental data for LTSEM of lead films on sapphire substrates at 4.2 K.

ISSN:0021-8979    

DOI:10.1063/1.345262

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

Tungsten‐doped YBa2Cu3O7−xsuperconducting samples are characterized with the help of resistivity, crystal structural, microstructural, densityd, and critical current densityJcstudies for doping concentrations varying from 0.00 to 0.14 mol % of WO3. It has been found that doping of WO3has hardly any influence ofTc, whereas it seems thatdandJcare affected.dandJcof the samples increase in the range 0<WO3≤0.04, but above this range, both start decreasing. The maximumJcobtained at 77 K and zero magnetic field in YBa2Cu3O7−xdoped with 0.04 mol % of WO3is 450 A/cm2. The lower concentrations (<0.04 mol %) seem to improve the density, whereas higher concentrations lead to porous structure and also induce the formation of BaCuO2and Y2Ba1Cu1O5nonsuperconducting phase as revealed by scanning electron microscopy, energy dispersive spectral, and x‐ray diffraction analyses. The x‐ray diffraction analysis also showed no appreciable change in the lattice parameters with WO3doping. It appears that the WO3doping changes the microstructure of YBa2Cu3O7−xto influence itsJc.

ISSN:0021-8979    

DOI:10.1063/1.345264

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

We have extended our studies of a melting technique for making low‐resistance contacts to high‐temperature superconductors. We have made contacts to both YBa2Cu3O7−xand Bi2BaSr2Cu2O8, and to related superconducting compounds by melting gold or silver pads onto the samples before the final oxygen treatment. Scanning electron microscope studies show that both gold and silver do not diffuse far from the contact area. The surface contact resistivity of the best contacts made by the melting technique has an upper limit value in the 10−8&OHgr; cm2range at 77 K. This contact resistivity shows no significant change in its value over a period of 17 months. Furthermore, an electron radiation dose of 5.7×1017electron/cm2only doubled the contact resistivity. This method of making low‐resistance contacts to high‐Tcmaterials can be integrated into the final oxygen treatment of many prospective superconducting elements or devices.

ISSN:0021-8979    

DOI:10.1063/1.345265

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

Defects and structural interfaces in superconducting Bi‐Sr‐Ca‐Cu‐O have been characterized by transmission electron microscopy. The superconducting phase exhibits frequent variations in the stacking sequence (polytypoids). Dislocations, observed inside the grains, either introduce or accommodate the shear in thea‐bplane and the local composition fluctuations. In general, the grains exhibit a platelike morphology with thea‐bplane as the grain boundary plane. Grain boundaries along the short edge are generally disordered, whereas those near the long edge generally have a thin layer of the lowerTcpolytypoid. Coherent intragranular boundaries are also observed.

ISSN:0021-8979    

DOI:10.1063/1.345266

出版商:AIP    

年代:1990

数据来源: AIP

摘要:

The preparation of a nonrandom silver YBCO composite is presented. Transport and magnetic properties of these nonrandom metal superconductor composites (NRMSC) were measured. It is shown that these composites exhibit a transition to the superconducting state in transport measurements for sintering temperatures as low as 600 °C, and exhibit critical current values higher than the pure YBCO ceramic material. The new NRMSC materials exhibit hardness properties similar to those of ductile metals.

ISSN:0021-8979    

DOI:10.1063/1.345267

出版商:AIP    

年代:1990

数据来源: AIP