摘要:

Scanning tunneling microscope images of native antisite defects at the relaxed (110) surface of ZnSe are predicted. The images of a particular sample depend on the sign of the voltage bias and the voltage sweep of the sample relative to the microscope tip, and whether that sweep causes a deep level to actively participate in the tunneling. Under certain conditions the images give the appearance of two defects at incorrect sites.

ISSN:0734-211X    

DOI:10.1116/1.584578

出版商:American Vacuum Society    

年代:1989

数据来源: AIP

摘要:

Investigations of the laser excited GaAs(110) surface with angle resolved, picosecond laser photoemission have revealed a previously unobserved valley of theC3surface band. The valley minimum is at theX̄ point in the surface Brillouin zone and resides within the band gap at this location. Electron population in this valley increases only as a result of scattering from the directly photoexcited valley at Γ̄. With high momentum resolution we have been able to isolate the dynamic electron population changes at both Γ̄ andX̄ and deduced the scattering time between the two valleys.

ISSN:0734-211X    

DOI:10.1116/1.584579

出版商:American Vacuum Society    

年代:1989

数据来源: AIP

摘要:

The Cs/GaAs(110) system has been characterized using the techniques of angle‐resolved ultraviolet photoemission spectroscopy and medium energy ion scattering spectroscopy. In agreement with previous studies, the adsorption of Cs on the GaAs(110) surface at room temperature is found to cause a decrease in the work function by ∼3.6 eV. In contrast to analogous measurements on metal surfaces, no minimum is observed in the work function change (Δφ) vs Cs coverage (Θ) up to saturation. Using medium energy ion scattering, the absolute saturation coverage of Cs/GaAs(110) at room temperature has been determined to be (4.0±0.1)×1014atoms⋅cm−2which corresponds to the density of metallic Cs. Based upon photoemission measurements, the lack of Fermi‐level emission indicates that the interface is nonmetallic at this concentration. The implications of these results are discussed in comparison with alkali metal adsorption on metal surfaces.

ISSN:0734-211X    

DOI:10.1116/1.584580

出版商:American Vacuum Society    

年代:1989

数据来源: AIP

摘要:

The interfaces of alkali metals (Cs and Rb) on GaAs and InP(110) surfaces prepared at 110 K low temperature have been studied using photoemission. At low temperature, multilayers of alkali metals with laminar growth can be obtained. The overlayer metallization is investigated by following the density of states near the Fermi cutoff, the free electron plasma loss, and the Fermi level movement at the semiconductor surfaces. Several criteria of metallicity are proposed. It is found that one monolayer of Cs or Rb is not metallic, and full metallicity is established at around two monolayers of coverage. The Fermi level movement relative to the semiconductor band edges as a function of alkali metal coverage has been closely followed. The semiconductor band bendings at low coverages are attributed to the surface‐donor states originating from alkali metal atom chemisorption. The Fermi level stabilization at these interfaces occurs when the overlayers become metallic. This pinning behavior is explained in terms of the metal‐induced gap states. The Fermi level pinning at the room temperature interfaces is also discussed and compared with the low‐temperature behavior.

ISSN:0734-211X    

DOI:10.1116/1.584581

出版商:American Vacuum Society    

年代:1989

数据来源: AIP

摘要:

The geometric and electronic structure of Au on the GaAs(110) surface is studied with the scanning tunneling microscope. At low metal coverage, the binding site for the Au adsorbates is found to be beside a surface Ga atom. The adsorbates usually occupy every second unit cell on the surface. In tunneling spectroscopy, a characteristic band gap state is observed for the adsorbed Au atoms, located ∼1.0 eV above the valence‐band maximum. A resonant state in the valence band is also observed. The valence‐band resonance and band gap state are interpreted as the first and second electron states of the Au–Ga bond, respectively. Both states form bands, and the surface Fermi level is pinned between these bands. At higher metal coverage, the Au atoms form clusters, which have a distinct epitaxial relationship with the substrate. Spectroscopy at high metal coverage reveals a continuum of states throughout the band gap, associated with the metallic nature of the clusters.

ISSN:0734-211X    

DOI:10.1116/1.584582

出版商:American Vacuum Society    

年代:1989

数据来源: AIP

摘要:

Bi layers deposited on (100) GaAs are investigated with low‐energy electron diffraction, Auger electron spectroscopy, and electron energy‐loss spectroscopy. The (100) substrates are prepared by simultaneous Ar+sputtering and annealing. The deposition of Bi on room‐temperature (100) GaAs eliminates the reconstruction spots at 0.5 monolayer. No long‐range order is obtained in the room temperature grown Bi layer. Post‐growth annealing at 250 °C restores long‐range order and produces a sixfold symmetric hexagonal pattern characteristic of the rhombohedral structure of bulk Bi. The attenuation of the Auger peak intensities as a function of Bi thickness indicates that the film grows in a quasi‐laminar fashion. The deposition of Bi on high‐temperature (100) GaAs (250 °C) produces a two‐dimensional ordered growth. The GaAs (4×6) structure is replaced by a (2×1) structure after deposition of one‐half monolayer Bi. These (2×1) structure remains visible up to coverages of at least 25 monolayers. Auger measurements indicate island formation, while the sharpness of the electron diffraction spots indicates good crystallinity at the surface of the Bi crystal.

ISSN:0734-211X    

DOI:10.1116/1.584583

出版商:American Vacuum Society    

年代:1989

数据来源: AIP

摘要:

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 onn‐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 onp‐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.

ISSN:0734-211X    

DOI:10.1116/1.584584

出版商:American Vacuum Society    

年代:1989

数据来源: AIP

摘要:

The Au/GaAs interface is currently the subject of much investigation. However, a complete understanding of this interface, and the ability to precisely control Au/GaAs interface properties, is still lacking. Previous work has focused primarily on interfaces prepared on the GaAs(110) cleaved surface and on the chemically or thermally prepared GaAs(100) surface. This paper presents the first Schottky barrier results for the Au/GaAs(100) interface prepared completelyinsituon GaAs grown by molecular‐beam epitaxy. The resulting interface displays unexpected properties which can be interpreted in terms of enhanced electrode interdiffusion. In addition, the capability of molecular‐beam epitaxy forinsituprocessing enables the stabilization of this interface against diffusion, and allows the formation of a Au/GaAs system with nearly ideal properties. Newly developed ballistic electron spectroscopy and imaging techniques demonstrate that the heterogeneity present at the interface of Au/GaAs(100) fabricated on chemically treated GaAs substrates is removed.

ISSN:0734-211X    

DOI:10.1116/1.584585

出版商:American Vacuum Society    

年代:1989

数据来源: AIP

摘要:

It has been shown that upon annealing a 2‐monolayer coverage of Ag on UHV‐cleaved GaAs at 500 °C, the Ag clusters into islands and the surface Fermi level moves back to within 0.2 eV of the bulk position. In this study, this Fermi level unpinning behavior has been investigated further by using various substrate doping levels and varying the anneal temperature. The Ag clustering process was observed using both ultraviolet photoelectron spectroscopy (UPS) and scanning electron microscopy. Through UPS, the surface Fermi level (Efs) movement was monitored simultaneously with the clustering process. No distinct temperature for the onset of clustering is observed. Rather, the clustering process occurs continuously over the range of temperatures studied (room temperature to 500 °C), with the Ag clusters growing and increasing in separation as anneals are performed at successively higher temperatures. A 10 min annealing time was sufficient to achieve a stable, equilibrium configuration at each annealing temperature. Movement of theEfsback to near the bulk position occurred between 375–450 °C for high dopedn‐GaAs (6×1018/cm3); whereas no movement of theEfswas observed for low dopedn‐GaAs (4×1016/cm3) up to 475 °C. Similar results were obtained when the experiment was repeated for high dopedp‐GaAs (1.4×1019/cm3) and low dopedp‐GaAs (5×1016/cm3). The absence of movement ofEfsin the low doped GaAs is attributed to the longer substrate depletion length, implying that only the areas beneath and within the depletion length of the Ag clusters are pinned.

ISSN:0734-211X    

DOI:10.1116/1.584587

出版商:American Vacuum Society    

年代:1989

数据来源: AIP

摘要:

When the metal coverage at the surface of a semiconductor is sufficiently small so that there are isolated metal adatoms and therefore localized states, there is a distinction between the energy (0,+) to remove an electron from a neutral adatom and the energy (−,0) to add an electron to a neutral adatom. The acceptor level (−,0) is higher in energy than the donor level (0,+) by an energyU*, just as in the free atom, but hereU* is modified by the presence of the surface. For these isolated metal adatoms, the donor level (0,+) frequently lies in the semiconductor gap with the acceptor level (−,0) coming in the conduction band. This circumstance leads to an asymmetry between the low‐coverage band bending forn‐type semiconductors versus that forp‐type. As the number of adatoms is increased, the (0,+) and (−,0) levels shift relative to the semiconductor bands at the surface. These shifts can be understood in terms of the polar bonds between the metal adatoms and the semiconductor surface atoms. At sufficiently large metal coverages the wavefunctions of adjacent metal adatoms overlap and the discrete levels broaden into bands. Eventually the donor and acceptor bands overlap and the asymmetry betweenn‐type andp‐type band bending disappears. This picture is modified if the metal adatoms are clustered rather than uniformly distributed.

ISSN:0734-211X    

DOI:10.1116/1.584588

出版商:American Vacuum Society    

年代:1989

数据来源: AIP