摘要:

High quality epitaxial layers of nominally undoped ZnSe have been grown by metalorganic chemical vapor deposition at low temperature (325 °C) and pressure (30 Torr), using dimethylzinc and hydrogen selenide. All layers were unintentionally dopedntype with net carrier concentrations of 6.4×1014–1.5×1016cm−3and exhibited very high mobility at room temperature (up to 500 cm2/V s) as well as at 77 K, where the measured value of 9250 cm2/V s is the highest so far reported for vapor phase growth. Additional evidence for the high quality of the material is provided by photoluminescence. Experimental results indicate a correlation between the photoluminescence characteristics and the electrical properties that may be useful in assessing the quality of ZnSe films.

ISSN:0003-6951    

DOI:10.1063/1.100967

出版商:AIP    

年代:1989

数据来源: AIP

摘要:

The structure of as‐deposited and annealed Cr/GaAs Schottky contacts was investigated by high resolution and analytical electron microscopy. The Schottky barrier height for contacts prepared by cleavage andinsitumetallization in ultrahigh vacuum was stable upon annealing up to 370 °C in N2. In contrast, the contacts prepared on air‐exposed substrates show an increase of the barrier height by 80 meV during annealing in the same range of temperatures. Comparing these two types of contacts, distinct differences in the grain size, presence of an oxide layer at the interface, and change in stoichiometry in the substrate beneath the contact were detected.

ISSN:0003-6951    

DOI:10.1063/1.100968

出版商:AIP    

年代:1989

数据来源: AIP

摘要:

We present experimental tunneling current‐voltage characteristics for a wide (1040‐A˚‐wide well) double‐barrier structure; concomitant differential conductance data show a series of oscillations in the differential conductance‐voltage characteristics. By using a self‐consistent analysis, we show that the observed conductance oscillations arise predominantly from the structure of the local density of states of the confining well rather than the density of states of the global structure; this result shows that there is a lack of long‐range phase coherence for the tunneling electrons in this structure. Also, in this analysis we determine the importance of the band nonparabolicity.

ISSN:0003-6951    

DOI:10.1063/1.100969

出版商:AIP    

年代:1989

数据来源: AIP

摘要:

X‐ray photoelectron spectroscopy reveals that the remarkable electronic quality of GaAs/sulfide interfaces can be ascribed to the formation of AsxSyphases which grow on an oxide‐free GaAs surface. While one of these phases is akin to As2S3, another shows significant in‐plane S—S bonding. Raman experiments indicate that the band bending on this disulfide‐ terminated surface has been reduced to 0.12 eV.

ISSN:0003-6951    

DOI:10.1063/1.101451

出版商:AIP    

年代:1989

数据来源: AIP

摘要:

The effect of an ammonium sulfide treatment on the GaAs (100) surface has been investigated by x‐ray photoelectron spectroscopy. The treatment produces a slight Ga enrichment on the surface and leaves roughly 0.6 of a monolayer of sulfide which inhibits initial oxidation of the surface. The sulfide is not lost as the surface becomes oxidized but appears to remain near the GaAs/oxide interface. Furthermore, in the oxidized layer, As oxide is preferentially drawn to the surface relative to Ga oxide.

ISSN:0003-6951    

DOI:10.1063/1.100970

出版商:AIP    

年代:1989

数据来源: AIP

摘要:

High‐resolution electron microscopy studies as well as computer simulation studies on oxygen vacancies in Y‐Ba‐Cu‐O superconductors show that the existence of oxygen vacancies at {0, 1/2, 0} sites leads to the distortions of structural images. For certain imaging conditions, the spots corresponding to the column of oxygen vacancies can be distorted heavily, so that the indirect image of oxygen vacancies in Y‐Ba‐Cu‐O superconductors may be acquired. The results reveal that even in local area the oxygen vacancies may be distributed regularly.

ISSN:0003-6951    

DOI:10.1063/1.101452

出版商:AIP    

年代:1989

数据来源: AIP

摘要:

Highly oriented crystalline layers of Bi4(Sr,Ca)3Cu4O16+xhighTcsuperconductors were produced by the devitrification of aerosol suspensions of glasses prepared from the mixed oxides when the suspensions were brought into rapid contact with heated silver substrates. The best magnetic susceptibility signals, produced by superconductivity at 77 K, were obtained when the glass was rapidly heated to about 850 °C from room temperature. The deposited layers are composed of a nearly pure superconductive phase in the form of flat platelets up to several hundred microns in diameter that are oriented with thecaxis normal to the substrate. The addition of a small amount of Pb to the glass lowers the optimum deposition temperature and improves the crystal morphology.

ISSN:0003-6951    

DOI:10.1063/1.101453

出版商:AIP    

年代:1989

数据来源: AIP

摘要:

Using an electron diffraction technique, the thicknesses of the (nonorthorhombic) twin boundary layers were measured for pure and alloyed YBa2(Cu1−xMx)3O7−&dgr;. Boundary thickness varied from ∼0.7 nm forM=Ni,x=0.02, and &dgr;&bartil;0 to ∼2.6 nm forM=Al,x=0.02, and &dgr;&bartil;0, while it was ∼1.0 nm for a pure YBa2Cu3O7. High‐resolution transmission electron imaging of similar specimens supports the existence of such twin boundary layers.

ISSN:0003-6951    

DOI:10.1063/1.101349

出版商:AIP    

年代:1989

数据来源: AIP

摘要:

We have used x‐ray photoemission spectroscopy to study the core level spectra of 200 A˚ Bi‐Sr‐Ca‐Cu oxide thin films, in the hope that the importance of the surface sensitivity of photoemission is minimized for these samples, whose thickness is only 6–7 unit cells. The samples were annealed in oxygen at temperatures ranging from 600 to 870 °C, over which temperature range the thin films are converted from an amorphous insulating phase to an ordered superconducting phase withTc(R=0)=80 K. Regardless of the annealing temperature, the Cu 2pspectra of all films show satellite structure indicative of Cu2+states. Even the spectrum of an as‐deposited film has a satellite similar to those observed for the annealed films, including the superconducting film. The similarity of the satellite structure in the core level spectra for all films suggests that the Cu‐O interactions, such as the Cu d‐O porbital charge transfer energies and the hopping integrals, remain similar when the oxides undergo the insulator‐superconductor transition through high‐temperature anneal. That is, the same local Cu‐O interaction exists before and after the superconducting phase sets in. This implies that the Cu valence alone does not determine the properties of highTcsuperconductors.

ISSN:0003-6951    

DOI:10.1063/1.100971

出版商:AIP    

年代:1989

数据来源: AIP

摘要:

Highly textured single phase superconducting YBa2Cu3O7−xfilms have been successfully grown on the yttria‐stabilized zirconia (100) substrates by using the metalorganic chemical vapor deposition technique. The as‐deposited films grown at 650 °C were homogeneous mixtures of the related metal oxides and carbonates. Subsequent thermal annealing under oxygen flow yielded single phase superconducting films whose thickness corresponded to the deposition rates of approximately 10 &mgr;m/h. After the post‐annealing the films deposited on the yttria‐stabilized zirconia substrates exhibited a highly textured x‐ray pattern withcaxis perpendicular to the substrate surface. These films show an onset superconducting transition temperature of 93 K with the resistance becoming zero at 84 K.

ISSN:0003-6951    

DOI:10.1063/1.101378

出版商:AIP    

年代:1989

数据来源: AIP