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Showing papers by "Hua Zhang published in 1993"


Journal ArticleDOI
TL;DR: The evolution of the low-energy excitations and dielectric function of BKBO is compared to that of cuprate superconductors and Subtle but important changes are observed in the higher-loss regions (bulk plasma) with K doping.
Abstract: Evolution of the low-energy excitations and dielectric function of ${\mathrm{Ba}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{K}}_{\mathit{x}}$${\mathrm{BiO}}_{3}$ (BKBO) single crystals in the doping range 0\ensuremath{\le}x\ensuremath{\le}0.50, i.e., spanning the semiconductor to superconductor regimes, is investigated by electron-energy-loss spectroscopy (EELS) with a cold-field-emission-gun transmission-electron microscope (cFEG TEM). A free-carrier plasmon in BKBO is observed by transmission EELS for x\ensuremath{\ge}0.40 (superconductor phase), and a local excitation for x\ensuremath{\le}0.3 (insulator-semiconductor phase). The oscillator strength of the excitation in the low-loss region increases substantially and abruptly between x=0.30 and x=0.40, signaling the semiconductor-metal transition. Subtle but important changes are observed in the higher-loss regions (bulk plasma) with K doping. The evolution of the low-energy excitations and dielectric function of BKBO is compared to that of cuprate superconductors.

12 citations


Journal ArticleDOI
TL;DR: The results are consistent with the band-structure calculations, for the infinite-layer compound, which indicate strong Cu 3[ital d]--O 2[ital p] hybridization within the [ital aa] plane, but little overlap of the wave function along the [*ital c]more » axis.
Abstract: The electronic structure and dielectric functions of single crystallites of the infinite-layer compound (Sr[sub 1[minus][ital x]]Ca[sub [ital x]])[sub [ital y]]CuO[sub 2] ([ital y]=0.90, [ital x]=0.3) are investigated by transmission-electron-energy-loss spectroscopy with a cold-field emission-gun TEM. The band gap along the [ital c] axis is estimated to be significantly greater than in the [ital aa] plane, which suggests anisotropic electronic properties. Unlike other cuprates, oxygen core-loss spectra of this compound display two distinct preedge peaks. A close correlation was observed between the valence-band excitations below 10 eV in the [ital aa] plane and the oxygen [ital K] core-loss fine structure. The [similar to]2 eV excitation in the valence-band spectrum is associated with a transition from the lower Hubbard band to the upper Hubbard band and is detected only in the [ital aa] plane. This excitation coincides with the first preedge peak in O [ital K] core-loss spectra, which is assigned to a transition from the O 1[ital s] state to the upper Hubbard band. The results are consistent with the band-structure calculations, for the infinite-layer compound, which indicate strong Cu 3[ital d]--O 2[ital p] hybridization within the [ital aa] plane, but little overlap of the wave function along the [ital c]more » axis.« less

12 citations


Journal ArticleDOI
TL;DR: In this paper, transmission high-energy electron energy loss spectrometry (EELS) with a cold field emission transmission electron microscope (TEM) is employed to analyze the evolution of electronic structure and dialectric function of oxide superconductors.

8 citations


Journal ArticleDOI
TL;DR: In this paper, the authors reported the growth of YBa2Cu3O7−δ/PrBa2cu3O 7−ε superlattices by pulsed organometallic beam epitaxy technique using X-ray diffraction and analytical high resolution transmission electron microscopy measurements.
Abstract: We report on the growth of YBa2Cu3O7−δ/PrBa2Cu3O7−δ superlattices by pulsed organometallic beam epitaxy technique. X‐ray diffraction and analytical high resolution transmission electron microscopy measurements show high‐quality superlattice crystalline structures are being grown. Electrical measurements of these layered oxides have also shown very promising results. This work demonstrates the capabilities of pulsed organometallic chemical vapor deposition as an alternative method for growing high temperature superconducting oxides in a controlled manner.

8 citations


Journal ArticleDOI
TL;DR: In this article, the authors investigated the physical properties of bulk single crystals of YBa2Cu4O8 (Y124) using conventional, high resolution and analytical electron microscopy techniques.
Abstract: Bulk single crystals of YBa2Cu4O8 (Y124) have been investigated using conventional, high resolution and analytical electron microscopy techniques. Complex and profuse dislocation structures are observed in the as-grown crystals. At least two different types of dislocations are identified, with two different Burgers vectors and slip planes. The analysis of the dislocation geometry strongly suggests that they are (or were) probably quite mobile, at least at some elevated temperature. An unusual domain structure contrast is also observed in some areas. The local chemistry and crystallography of such regions appear to be close to that of Y124. EELS O K core loss analysis indicates that such regions are devoid of holes as reflected in the lack of a pre-edge peak in O K EELS spectra. These observations are discussed in terms of the influence of microstructural details on the physical properties of Y124.

6 citations


Journal ArticleDOI
TL;DR: In this paper, the free carrier plasmons in single crystals of YBa 2 Cu 4 O 8 (Y124) were observed to have low energy excitations in EELS spectra, one at about 1.4 eV and another at about 3.2 eV.
Abstract: Orientation dependent (momentum transfer resolved) electron energy loss spectrometry (EELS) has been utilized to probe the localization of free carrier plasmons in single crystals of YBa 2 Cu 4 O 8 (Y124). Two low energy excitations are observed in EELS spectra, one at about 1.4 eV which is the free carrier plasmon,and another excitation at about 3.2 eV. The oscillator strength of the free carrier plasmon is observed to vary considerably with momentum transfer. Maximum oscillator strength of the free carrier plasmon is observed when the momentum transfer ( q ) is confined to the ab planes, while the minimum strength is observed for q parallel to the c -axis. Dielectric function calculated using the Kramers-Kronig analysis indicates that the major differences in the dielectric function ofY124 within the ab plane versus along the c -axis are confined only within the first few eVs. The O K core loss spectra exhibit subtle but important deviations from that of YBa 2 Cu 3 O 7-δ (Y123). Possible EELS spectral signatures of the double chains in Y124 are discussed.

5 citations


Journal ArticleDOI
TL;DR: In this article, physical and electrical characterization of contact structures with a TiN barrier and W plugs was presented, and the relationship between electrical properties and microstructure for the two different barrier structures was discussed.
Abstract: TiN diffusion barriers have been widely used in submicron contact structures, due to its good adhesion (to SiO 2 , W, Al and Si) properties, low diffusivity (for Si, W and Al) and compatibility with TiSi 2 processing. The purpose of this paper is to present the results of physical and electrical characterization of contact structures with a TiN barrier and W plugs. Two different barrier metal processes were compared, Viz: sputtered Ti followed by post RTN and Ti/TiN films followed by post RTA in the range of 600° to 800°C. The devices were thermal stressed at 450°C for 7 hrs after W plug formation and Al metallization. Ti/TiN films with post RTA are generally superior barrier layers than Ti films with post RTA as shown by electrical characterization of contact resistance and barrier integrity. The relationship between electrical properties and microstructure for the two different barrier structures is discussed. W/TiN and TiN/TiSi 2 interface structures were characterized using high resolution TEM. TiSi 2 was found to be epitaxially grown during RTA, under certain process conditions. The crystal structure of TiSi 2 was determined from electron diffraction patterns.

3 citations