D. M. Hill

Passivation of high Tc superconductor surfaces with CaF2 and Bi, Al, and Si oxides

X‐ray photoelectron spectroscopy has been used to investigate passivation of the high‐temperature superconductors YBa2Cu3O7−x and Bi2Ca1+xSr2−xCu2O8+y involving overlayer materials deposited in vacuum onto freshly cleaved surfaces. CaF2 was found to be completely inert and to form uniform overlayers on the surface. Deposition of Bi, Si, and Al in activated oxygen produced oxides and exhibited minimal reaction and disruption of the superconductor surface. In contrast, results for Bi, Si, and Al deposition in nonactivated oxygen or ultrahigh vacuum showed extensive substrate disruption due to surface reactions and oxygen withdrawal. This indicates that the activation technique provides oxygen from the gas phase for oxide formation and facilitates nondisruptive overlayer growth, even for reactive metals.

Single‐crystal YBa2Cu3O7−x and Bi2Ca1+xSr2−xCu2O8+y surfaces and Ag adatom‐induced modification

Comparison of x-ray photoemission results for single crystal and polycrystalline YBa/sub 2/Cu/sub 3/O/sub 7-//sub x/ allows us to identify oxygen vacancies in the Cu-O chains and determine their effects on the Cu, Ba, and Y core level line shapes. The deposition of adatoms of Ag onto single crystals of YBa/sub 2/Cu/sub 3/O/sub 7-//sub x/ and Bi/sub 2/Ca/sub 1+//sub x/Sr/sub 2-//sub x/Cu/sub 2/O/sub 8+//sub y/ leads to changes in the Cu 2p/sub 3//sub ///sub 2/ emission indicative of slight Cu/sup 2 +/ to Cu/sup 1 +/ surface reduction, with greater modification for the 1-2-3s than the 2-1-2-2s. O 1s core level results show Ag-induced broadening because of these surface effects. The Ba 3d and Y 3d structures showed minimal changes for the 1-2-3 surface. Ag deposition onto the 2-1-2-2 samples also leads to Bi dissociation and subsequent surface segregation.Comparison of x‐ray photoemission results for single crystal and polycrystalline YBa2Cu3O7−x allows us to identify oxygen vacancies in the Cu‐O chains and determine their effects on the Cu, Ba, and Y core level line shapes. The deposition of adatoms of Ag onto single crystals of YBa2Cu3O7−x and Bi2Ca1+xSr2−xCu2O8+y leads to changes in the Cu 2p3/2 emission indicative of slight Cu2+ to Cu1+ surface reduction, with greater modification for the 1‐2‐3’s than the 2‐1‐2‐2’s. O 1s core level results show Ag‐induced broadening because of these surface effects. The Ba 3d and Y 3d structures showed minimal changes for the 1‐2‐3 surface. Ag deposition onto the 2‐1‐2‐2 samples also leads to Bi dissociation and subsequent surface segregation.

Y, Ba, Cu, and Ti interface reactions with SrTiO3 (100) surfaces

Interface reactions of adatoms of Y, Ba, Cu, and Ti with ordered and disordered SrTiO3 (100) surfaces were examined with x‐ray photoemission spectroscopy, Auger spectroscopy, and low‐energy electron diffraction. Atomic distributions for these interfaces before and after annealing at 500 °C were determined using Ar ion sputter depth profiling. We observed strong reactions of Y and Ba with O extracted from the substrate for both ordered and disordered SrTiO3, with the strongest interactions occurring for Y on the disordered surface. These reactions were diffusion limited at 300 K, and the growth of a metal overlayer was observed with increasing coverage. Comparison of these results to those for Ti/SrTiO3 made it possible to determine the extent of the reactions and the reaction products. At elevated temperatures, more extended out‐diffusion of O from the substrate into the overlayer was observed, completing the conversion to the respective metal oxides. In contrast, Cu deposited on SrTiO3 formed clusters on...

Ba oxides: Core level binding energies and defect-related Fermi level pinning

Abstract X-ray photoemission spectroscopy has been used to measure core level and valence band energies for Ba oxides prepared by oxidation of Ba metal and evaporation of Ba in an activated oxygen ambient. We demonstrate that previously-observed negative binding energy shifts for oxidized Ba relative to metallic Ba are due to changes in the position of the Fermi level in the energy gap of the oxide. Shifts of the Ba and O core levels ranging from −0.4 to + 0.5 eV were induced by changing the amount of disorder in an oxide surface by sputtering or annealing without changing the chemical composition of the surface. Movement of E F toward the valence band maximum was correlated with increasing disorder. Apparent shifts of up to −1.6 eV with respect to the metal were obtained when Ba was evaporated in an activated oxygen ambient. These results are related to the unusual core level binding energies observed in the high temperature superconductors, particularly Ba in YBa 2 Cu 3 O 7− x .

Reactivity and passivation for Bi adatoms on YBa2Cu3O6.9 and Bi2Ca1+xSr2−xCu2O8+y

X‐ray photoemission spectroscopy was used to examine the reactivity of clean YBa2Cu3O6.9 and Bi2Ca1+xSr2−xCu2O8+y surfaces with adatoms of vapor‐deposited bismuth. For YBa2Cu3O6.9, depositions ≤8 A of Bi induce O 1s, Cu 2p3/2, and Bi 4f line‐shape changes due to BiO bonding, limited oxygen loss from the substrate, partial conversion of Cu2+ to Cu1+, and growth of Bi metal. For coverages greater than 8 A, there is no reaction and only Bi metal growth is observed. For Bi deposition onto Bi1Ca1+xSr2−xCu2O8+y, there is minimal reaction with only subtle O 1s and Cu 2p3/2 line‐shape changes and the nucleation and growth of Bi metal starting at low coverages (≥2 A). Comparison shows that Bi is less reactive than other metals, except Ag and Au, and that Bi2Ca1+xSr2−xCu2O8+y is more stable toward Bi deposition than YBa2Cu3O6.9.

Use of angle‐dependent photoemission for atom profiling: Au on compound semiconductors

We present studies of Au film formation on the III‐V compound semiconductors GaAs and InSb based on polar‐angle‐dependent x‐ray photoelectron spectroscopy. The results highlight the power of this technique as a nondestructive tool for atom profiling. The Au overlayer is highly heterogeneous due to disruption of the original surface and significant anion segregation in Au. Although GaAs and InSb show common behavior, significant differences are observed because the anion–cation bond strength determines the extent of the surface disruption and semiconductor atom solid solubilities in Au dominate the segregation patterns in each system. We present a model of the overlayer structure, based on thermodynamic considerations, which shows very good quantitative agreement with the observed profiles.

Ni/YBa 2 Cu 3 O 7− x and Ni/Bi 2 Sr 2 Ca 0.8 Y 0.2 Cu 2 O x interface formation: Reactivity, segregation, and chemical trapping

Interfaces formed by condensing Ni atoms onto YBa 2 Cu 3 O 7-x (Y-123) and Bi 2 Sr 2 Ca 0.8 Y 0.2 Cu 2 O x (Bi-2212) have been studied with x-ray photoelectron spectroscopy. For both Y-123 and Bi-2212, the Ni 2p 3/2 and O 1s core level features indicate Ni-O reactions and changes in the Cu 2p 3/2 emission that reflect reduction from nominal Cu 2+ to Cu 1+ oxidation states. Ni deposition onto Bi-2212 also reduces Bi-O bonding and releases Bi atoms

Silicon interfaces with high temperature superconductors

Abstract X-ray photoelectron spectroscopy was used to study vapor-deposition and growth of Si overlayers on polycrystalline YBa2Cu3O7 − x and (PbBi)2Sr2 − xCa1 + xCu2O8 − y. Interface reactions at low coverage for both superconductors were characterized by O withdrawal from the near-surface region of the superconductors and oxidation of the Si adatoms. Oxygen loss from the substrate resulted in substantial disruption and reduction of the nominal Cu oxidation state from 2 + to 1 + . Disruption and O loss were more extensive for YBa2Cu3O7 − x, demonstrating the higher surface stability of (PbBi)2Sr2 − xCa1 + xCu2O8 − y. Surface modification for Si/YBa2Cu3O7 − x resulted in the loss of Cu-O derived states near the Fermi level and a shift of the Fermi level in the resulting gap. A Si film formed on the reacted interface region as O redistribution was kinetically limited at higher coverages.

Surface and interface properties of high temperature superconductors

The energy distribution of the occupied and unoccupied electronic states of the high temperature superconductors La1.85Sr0.15CuO4, YBa2Cu3O7−x, Bi2Ca1+xSr2−xCu2O8+y and related compounds have been investigated using x‐ray and inverse photoemission spectroscopy. Results from polycrystalline and single crystal samples show very similar valence band characteristics, and comparison with theory shows the importance of correlation effects. Core level results reveal a nominally 2+ valence state for copper and inequivalent chemical environments for oxygen. The empty state spectra show structure that can be identified with La 5d and 4f levels for La1.85Sr0.15CuO4, Ba 5d and 4f and Y 4d levels for YBa2CuO3O7−x, and Bi 6p, Sr 4d and Ca 3d levels for Bi2Ca1+xSr2−xCu2O8+y. Inverse photoemission studies also show that the excitation of O 2s core levels near threshold results in resonant behavior of the unoccupied O levels just above EF. Interface studies which include overlayers of metals (Au, Ag, Ti, Fe, Cu, Pd, La, A...

Ni/YBa sub 2 Cu sub 3 O sub 7 minus sub x and Ni/Bi sub 2 Sr sub 2 Ca sub 0. 8 Y sub 0. 2 Cu sub 2 O sub x interface formation: Reactivity, segregation, and chemical trapping

Interfaces formed by condensing Ni atoms onto YBa 2 Cu 3 O 7-x (Y-123) and Bi 2 Sr 2 Ca 0.8 Y 0.2 Cu 2 O x (Bi-2212) have been studied with x-ray photoelectron spectroscopy. For both Y-123 and Bi-2212, the Ni 2p 3/2 and O 1s core level features indicate Ni-O reactions and changes in the Cu 2p 3/2 emission that reflect reduction from nominal Cu 2+ to Cu 1+ oxidation states. Ni deposition onto Bi-2212 also reduces Bi-O bonding and releases Bi atoms