Photoemission spectra of 3d core levels excited with synchrotron radiation reveal a multicomponent substructure which increases in complexity with oxygen exposures over the range 106–1014L (langmuir). Spectral changes are already evident for Ga at 104 L, and for As near 106 L. Two oxide components shifted by 0.45 and 1 eV relative to the bulk Ga-3d core level are evident throughout the exposure range, but shift to 0.8 and 1.4 eV for 1014 L. With increasing exposure the As-3d core level develops a sequential set of shifted components at 0.8, 2.3, 3.2, and 4.2 eV relative to the bulk position in GaAs, which are attributed to single through fourfold coordinated bond formation to oxygen. Both surface and bulk-sensitive core spectra reveal a nearly equally intense oxide substructure, which indicates that contrary to previous notions subsurface oxidation is the dominant mechanism throughout the exposure range. The core spectra furthermore indicate preferential Ga oxidation—which suggests that separate Ga and As oxide phases form. Thus the oxidation of GaAs(110) is both spatially and chemically inhomogeneous. Changes in the position of the Fermi energy at the surface correlate well with the initial oxidation of surface sites and the onset of subsurface oxidation near 106 L. A final pinning position of the Fermi energy was not observed.
Ordered Bi overlayers on GaAs(110) were investigated with scanning tunneling microscopy (STM), STM spectroscopy, low-energy electron diffraction (LEED) studies, and with angle-integrated and angle-resolved photoemission spectroscopies. Two-dimensional (2D) layer growth occurs to ∼1 monolayer (ML), thereafter three-dimensional (3D) growth dominates. A Bi phase pseudomorphic with the GaAs is observed to ∼10 ML. Interdiffusion or chemical reactions between the components were not detected. STM images for coverages of 0.5 and 1 ML reveal the Bi structure to consist of chains of atoms aligned above and in between the Ga–As zigzag surface chains. Additional evidence for dual bonding sites of the Bi suggests that the Bi chains exhibit a zigzag structure similar to the Sb/GaAs system. Near 1 ML the Bi chains are interrupted by a periodic array of dislocations, ∼25 Å apart, that consist of missing Bi atoms. STM spectroscopy reveals that the Bi ML is semiconductorlike with a 0.7-eV band gap, and that the dislocations generate a band of empty, acceptorlike states within this gap. Band bending measurements indicate that the position of the Fermi level on n-type GaAs is determined by these acceptor states. Gap states were also associated with Bi terrace edges for coverages <1 ML. The position of the Fermi level for Bi on p-type GaAs is determined by the top of the Bi valence band, which overlaps that of GaAs by 0.4 eV. The dispersion of this band and another lower Bi band were measured using angle resolved photoemission spectroscopy.
In the presence of metallic states, deposition-generated midgap levels at the semiconductor surface evolve into resonances that accommodate the fractional charge density that ultimately determines the Fermi level and hence the Schottky-barrier height. This concept is applied to calculate both the barrier heights of GaAs for nonalloyed metal-semiconductor interfaces, and the index-of-interface behavior for 15 tetrahedrally coordinated semiconductors.
Abstract Growth of epitaxial thin films of sphalerite and wurtzite CdSe on cleaved BaF 2 substrates is reported. Optical transmission measurements to 6.5 eV in the ultraviolet at 300 and 80 °K were made. The spin‐orbit splitting of the A 3 ‐;A 1 transitions in cubic CdSe is 0.26 eV, in close agreement with seemingly spin‐orbit split transitions of 0.28 eV in the hexagonal phase, attributed to transitions along the Δ‐axis (the c ‐axis of the hexagonal Brillouin zone). Our findings substantiate this identification. Other optical structure is reinterpreted based on our measurements and some recent band calculations of wurtzite compounds. A band structure for wurtzite CdSe is proposed.
The crystallographic relationships, growth morphology, chemical activity, and electronic properties of Ag deposited at room temperature on GaAs(100) c(2×8) and (4×6) surfaces were investigated. Ag(110) growth was observed independent of growth rates. The growth is three- dimensional (nucleated) and the interfaces are abrupt. Stabilization of the Fermi level occurs beyond Ag coverages of 10 Å, is uncorrelated with the appearance of the metallic Ag phase at ∠0.5 Å and appears to be dependent on the formation of atomiclike interfacial states near the bottom of the bandgap. Schottky barrier heights of 0.83 and 0.97 eV were determined for Ag on the c(2×8) and (4×6) surfaces, respectively. The results are at variance with current Schottky barrier models.
Epitaxial growth of alternating thin films of GaAs and Al was achieved on GaAs substrates of various orientations. In situ high-energy electron diffraction (HEED) analysis was used to monitor the molecular beam growth process. The following crystallographic relationship was observed: (100) GaAs on (110) Al on (100) GaAs substrates, with [110] GaAs parallel to [100] Al. An atomic interface model is proposed to explain the observed growth.
