Scanning tunneling microscopy (STM) and photoelectron spectroscopy have been used to study the nucleation and growth of Cr overlayers on GaAs(110). Initial Cr-atom deposition onto GaAs(110) at 300 K produces surface defects that appear to be Ga-As divacancies. These defects do not trap the weakly chemisorbed adatoms. Instead, the Cr-adatom effective surface mobility on GaAs(110) is high and planar clusters form. Preferential clustering along [2BAR11] step edges is observed when steps occur. The planar arrays do not exhibit any distinct growth direction or show any tendency to form epitaxial layers. Instead, the planar Cr clusters act as the precursors to Cr-GaAs reactive intermixing. Intermixing is evident from the photoemission results but the STM images show little surface modification except in the areas beneath the clusters themselves. The STM images of the clusters show that they are irregular and appear to be disordered. We attribute this to the incorporation of Ga and As atoms, i.e., mixed Cr-Ga, Cr-As, and Cr-Cr bonding configurations. Overlayers grown by approximately 3-angstrom deposition cover the surface with Cr-derived structures with a roughness of 2.5 angstrom. This roughness is reduced by continued deposition or by mild annealing. Photoemission studies of growth at 60 and 300 K show equivalent amounts of surface disruption, implying that Cr clustering is also possible at 60 K. This is consistent with the idea that surface diffusion immediately after the condensation of weakly chemisorbed atoms is largely athermal. Although reaction on the 2-3-monolayer scale is observed at 60 and 300 K, quite different tendencies are observed with respect to Ga and As segregation to the surface of thicker films. Thus, kinetic trapping yields an overlayer with excess Ga and As at the buried interface. Finally, Cr overlayers grown by direct deposition of preformed Cr clusters containing hundreds to thousands of atoms are shown to be free of disruption and intermixing. This can be understood on the basis of kinetic constraints imposed by cluster-solid reactions.

Interfaces formed by condensing Ni atoms onto YBa2Cu3O7−x (Y-123) and Bi2Sr2Ca0.8Y0.2Cu2Ox (Bi-2212) have been studied with x-ray photoelectron spectroscopy. For both Y-123 and Bi-2212, the Ni 2p3/2 and O 1s core level features indicate Ni–O reactions and changes in the Cu 2p3/2 emission that reflect reduction from nominal Cu2+ to Cu1+ oxidation states. Ni deposition onto Bi-2212 also reduces Bi–O bonding and releases Bi atoms. For Ni/Y-123, analysis of emission intensities as a function of coverage shows that O and Ba intermix in the growing metal overlayer but that Cu is trapped at the buried interface. For Ni/Bi-2212, similar analysis shows O and Bi intermixing but less effective Cu trapping.

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.

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 EF 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 YBa2Cu3O7−x.

Scanning-tunneling-microscopy studies of the interactions of Sm with GaAs(110) at 300 K show two growth structures. Low density zigzag chains form by 0.01 monolayer (ML) with (divalent) Sm atoms bound to Ga surface atoms along the [11\ifmmode\bar\else\textasciimacron\fi{}0] direction (Sm-Sm distances 6.9 and 7.9 \AA{}). These zigzag chains are limited in length but bridge two or three substrate rows. Higher-density linear chains of (divalent) Sm atoms form at the expense of the zigzag chains with increased deposition, and the adatoms are bonded beside two Ga atoms near an As atom (Sm-Sm distance \ensuremath{\sim}4 \AA{}). Only short single chains are observed, and they convert to reacted (trivalent) clusters that bridge substrate rows at higher coverage. By \ensuremath{\sim}0.33 ML, the surface structure is composed of linear chains and reacted clusters with a small number of residual zigzag chains. Synchrotron-radiation photoemission results provide a correlation of structure with Sm valence. Comparison to Cs/GaAs(110) offers insight into low-coverage nucleation.

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 the surface and did not react with the substrate. O out-diffusion was not observed for the Cu overlayer at elevated temperatures, but clustering was enhanced. The implications of these results are discussed in terms of Y-Ba-Cu-O superconductor synthesis on SrTiO3.

We report a high-resolution x-ray photoemission study of interface formation involving 1–30 Å of TiO2 grown on p-type Si(111)-2×1 by the deposition of Ti in an activated oxygen atmosphere (5×10−6 Torr). Line-shape analysis for the Si 2p, Ti 2p, and O 1s core levels and the valence bands indicates that the Ti adatoms are in the formal 4+ oxidation state in TiO2, that a thin Si oxide interlayer also forms at the interface but is uniformly covered by TiO2, and that Ti–Si reaction is suppressed. For low TiO2 coverages, we observe a nonmonotonic movement of the semiconductor Fermi level.

X-ray and inverse photoemission were used to investigate the occupied and unoccupied electronic states of ${\mathrm{Ba}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{K}}_{\mathrm{x}}$${\mathrm{BiO}}_{3\mathrm{\ensuremath{-}}\mathrm{y}}$. The results show metallic character with a low density of states on either side of ${E}_{F}$ and a broad manifold from -13 to 2 eV consisting of Bi-O sp hybrids. Generally good agreement of theory with experiment is found except that the centroid of the calculated Bi-O manifold is \ensuremath{\sim}1 eV too close to ${E}_{F}$ (as for the high-${T}_{c}$ superconductors). Core-level results indicate single chemical environments for O, Ba, and K. The Bi 4f results suggest inequivalent Bi environments. All core-level spectra show energy-loss features shifted \ensuremath{\sim}1 eV to a higher binding energy relative to the main line. Unoccupied electronic state structures at 4, \ensuremath{\sim}7, \ensuremath{\sim}9, and 14 eV reflect Bi 6p, Ba 5d, K 3d, and Ba 4f bands, respectively. Photon emission associated with plasmon decay is observed as a constant energy feature at h\ensuremath{\nu}=22.4 eV. Inverse-photoemission resonance studies for incident electron energies that would excite O 2s-2p transitions show no enhancement of the O 2p holes directly above ${E}_{F}$.

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.

The electronic structure and surface interactions of vapor-deposited Cu on single-crystal and polycrystalline ${\mathrm{Bi}}_{2}{\mathrm{Ca}}_{1+x}{\mathrm{Sr}}_{2\ensuremath{-}x}{\mathrm{Cu}}_{2}{\mathrm{O}}_{8+y}$ were studied using x-ray photoelectron spectroscopy. The results are compared to the $\frac{\mathrm{Cu}}{\mathrm{Y}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}x}}$ interface. Changes in the $\mathrm{Cu} 2p$ satellite emission indicate that the Cu adatoms do not disrupt ${\mathrm{Bi}}_{2}{\mathrm{Ca}}_{1+x}{\mathrm{Sr}}_{2\ensuremath{-}x}{\mathrm{Cu}}_{2}{\mathrm{O}}_{8+y}$ as extensively as $\mathrm{Y}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}x}$. However, deposition of Cu induces changes in the Bi environment in the superconductor, and surface segregation of Bi metal was observed at high coverages. Core-level attenuation results suggest minimal out-diffusion of oxygen, in contrast with what is observed for $\frac{\mathrm{Cu}}{\mathrm{Y}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}x}}$.

Interface reactions for vapor-deposited Cu on ${\mathrm{La}}_{1.85}$${\mathrm{Sr}}_{0.15}$${\mathrm{CuO}}_{4}$ were investigated with x-ray photoemission and inverse photoemission. Strong Cu-O reactions indicate oxygen removal and the $\mathrm{Cu} {2p}_{\frac{3}{2}}$ emission shows conversion of the superconductor ${\mathrm{Cu}}^{2+}$ configuration to ${\mathrm{Cu}}^{1+}$ within 40-50 \AA{} of the surface. Valence-band results show a loss of the $\mathrm{Cu} {3d}_{{x}^{2}\ensuremath{-}{y}^{2}}\ensuremath{-}\mathrm{O} {2p}_{x,y}$ electronic states near the Fermi level, and we conclude that the surface layer is not superconducting. Changes in the occupied and empty electronic states of La at low Cu coverages provide additional evidence of Cu-induced reaction. A metallic overlayer begins to form at \ensuremath{\sim}2 \AA{} nominal Cu coverage as these surface reactions become diffusion limited at room temperature.

The distribution of atomic species across metal--III-V-compound-semiconductor interfaces has been studied with ${\mathrm{Ar}}^{+}$-ion bombardment and x-ray photoemission complemented by synchrotron radiation photoemission. Results for (Ti, Cr, Co, Au)/GaAs, (Co, Cr)/InP, and (Cr, Au)/InSb show that room-temperature metal deposition induces substrate disruption. The details of reactions at these interfaces then play a critical role in determining the distribution of semiconductor atoms in the overlayers. Strong metal-anion reactions cause the expulsion of cations from regions where there is compound formation, and there is a characteristic coverage at which this occurs. The result is a cation-deficient region near the buried interface. Weak metal-anion reactions cause no such long-range species redistribution, except for surface segregation. For Au--III-V interfaces, there is an onset for anion surface segregation as a nearly pure Au layer decorated by semiconductor atoms in supersaturation evolves from the Au-anion-cation mixture found at low coverage. The driving force for atomic redistribution is the lowering in energy of the system, but this is restricted by kinetics and diffusion at low temperature. Studies for Cr/GaAs show the effect of altering the temperature and, hence, the amount of diffusion.

High-resolution x-ray photoemission spectroscopy and Ar-ion bombardment have been used to study temperature-dependent chemical reaction and species redistribution for Ti/GaAs(100). Our results show that Ti deposited at room temperature disrupts the GaAs substrate by reacting with As and releases Ga into the overlayer. As is found to accumulate near the buried interface in the form of a Ti-As compound. Ga is depleted from, but accumulates beyond, this reacted region. Sputter-depth profiles indicate that high-temperature annealing causes Ti diffusion into the GaAs substrate and enhanced reaction with As. Ga expulsion from the forming Ti-As compound becomes more severe when the amount of Ti-As increases. Heating promotes segregation of ejected Ga atoms to the vacuum surface, but has little influence on As segregation.

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, Al, In, and Bi), semiconductors (Si and Ge) and dielectric materials (CaF2, Al2O3, SiO2, Bi2O3) are discussed in terms of adatom reactivity with, and modification of, the superconductor surface region.

Interface reactions for vapor-deposited Cu on La/sub 1.85/Sr/sub 0.15/CuO/sub 4/ were investigated with x-ray photoemission and inverse photoemission. Strong Cu-O reactions indicate oxygen removal and the Cu 2p/sub 3/2/ emission shows conversion of the superconductor Cu/sup 2+/ configuration to Cu/sup 1+/ within 40--50 A of the surface. Valenc-band results show a loss of the Cu 3d/sub x//sub <2/-y/sup =/--O 2p/sub x//sub ,//sub y/ electronic states near the Fermi level, and we conclude that the surface layer is not superconducting. Changes in the occupied empty electronic states of La at low Cu coverages provide additional evidence of Cu-induced reaction. A metallic overlayer begins to form at approx.2 A nominal Cu coverage as these surface reactions become diffusion limited at room temperature.