摘要:

By depositing monolayer coverage of Ag on freshly cleaved, clean GaAs(110) surface, the surface Fermi level was found to pin at 0.45±0.05 eV above valence band maximum (VBM) forp‐GaAs and 0.65±0.05 eV above VBM forn‐GaAs, due to the formation of midgap electronic states. Upon annealing to 500 °C, the Ag on the surface clustered into islands and the surface Fermi level moved back to within 0.2 eV of the bulk position [conduction band minimum (CBM) and VBM fornandptypes, respectively]. Further deposition of Ag on the annealed surface moved the surface Fermi level back to the previously pinned positions. The surface Fermi level movement was monitored using Ga 3dand As 3dphotoemission signals. The Ag clustering was seen both by a dramatic decrease in the Ag 4dphotoemission signal and by scanning Auger microscopy. Scanning electron microscopy also revealed that the Ag clustering formed hemispherical‐like islands with a size distribution up to ∼500 Å in diameter. The average island separation is about a few thousand angstroms (∼2000 Å). As a comparison with a reactive metal, similar annealing experiments were performed for monolayer coverage of Al on GaAs. For the Al case, the metal remained unclustered after annealing and the surface Fermi level remained pinned.

ISSN:0734-211X    

DOI:10.1116/1.584231

出版商:American Vacuum Society    

年代:1988

数据来源: AIP

摘要:

The interaction of molecular S2with cleaved GaAs(110) surfaces was followed by using a Kelvin probe and ultraviolet photoemission spectroscopy. The adsorption of sulfur increases the ionization energy of the cleaved surfaces by up to 0.92 eV and causes a Fermi‐level pinning at 0.34 eV above the valence‐band top for both types of doping. The sign of the change of the ionization energy can be explained by a chemical charge transfer between the adsorbed sulfur and adsorbate‐induced gap states, since sulfur is more electronegative than GaAs. The sulfur‐related pinning position of the Fermi level is located below the charge‐neutrality level of the virtual gap states (ViGS) of the complex GaAs band structure, where the ViGS predominantly possess donor character. This result confirms the predictions from the ViGS model of adsorbate‐induced gap states. The energy of the sulfur as well as the oxygen‐ and chlorine‐related surface states, which are of the acceptor type, correlate with the respective atomic electron affinities.

ISSN:0734-211X    

DOI:10.1116/1.584232

出版商:American Vacuum Society    

年代:1988

数据来源: AIP

摘要:

We have studied the effects of heat treatments on the chemical and electronic properties of GaAs(110) surfaces covered with V or Pd to thicknesses of about 3 Å. The annealing improves the surface stoichiometry and reduces excess Ga generated during the deposition. Particularly forp‐type surfaces and coverages<1 Å, the anneal reduces the position of the Fermi level by as much as 0.3 eV, which indicates that deposition and post‐treatment affect the nature and energetics of defect/impurity levels. This rules out the possibility that Schottky barrier heights are universally determined by a single‐intrinsic defect. Exchanged Ga does not appear to influence the position of the Fermi level for interfaces with reactive metals. Ga itself, deposited separately on clean GaAs(110) surfaces, appears to form a fixed donor level ∼250 meV above the valence band maximum (VBM) over a broad coverage range. Beyond a coverage of ∼5 Å the Fermi levels onn‐ andp‐type GaAs essentially merge to a value of 0.63 eV above the VBM, indicating that this metal forms a conventional Schottky barrier.

ISSN:0734-211X    

DOI:10.1116/1.584234

出版商:American Vacuum Society    

年代:1988

数据来源: AIP

摘要:

Ultraviolet illumination has been used to form a few monolayers of GaAs oxide in dry oxygen on bothp‐type andn‐type GaAs(100) surfaces. Some of the oxides have been annealed to form Ga‐rich oxide surfaces. Six metals with a wide range of work functions and different reactivity with the oxide layer have been used to form Schottky contacts on clean GaAs and on the thin‐oxide layers. Large variations from the Schottky barrier height on clean GaAs have been observed for both doping types due to the photo‐oxidation of the GaAs surfaces. The trend in Schottky barrier change is toward the ideal Schottky limit where the metal work function determines the barrier height. The GaAs Fermi level is shown to move in a much wider region of the band gap for the oxidized surfaces than for clean surfaces, however, it is still confined to a midgap region.

ISSN:0734-211X    

DOI:10.1116/1.584236

出版商:American Vacuum Society    

年代:1988

数据来源: AIP

摘要:

Newsp3tight‐binding models for ZnO, ZnS, ZnSe, CdS, and CdSe are utilized to calculate the atomic geometries and surface‐state eigenvalue spectra of the (110) surfaces of zinc‐blende allotropes and the (101̄0) and (112̄0) cleavage faces of the wurtzite allotropes. The models predict that each cleavage face [i.e., zinc‐blende (110), wurtzite (101̄0), and wurtzite (112̄0)] exhibits a characteristic relaxation in which the anion relaxes outward and the cation inward. The details of these relaxations depend on the local atomic connectivity and hence vary from one cleavage face to another. The dependence of all three classes of relaxed structure on the specific material considered is shown to be described by a linear scaling of the independent surface structural parameters with the bulk lattice constant. All of these structures are shown to result from a rehybridization of surface bonds created by the movement of a dangling‐bond surface state from the energy gap into the top of the valence band as it becomes modified to contain some back bonding and/or surface bonding character. The relaxed cleavage face geometries correspond to new types of epitaxially constrained chemical bonding which exhibit no bulk or molecular analogs.

ISSN:0734-211X    

DOI:10.1116/1.584237

出版商:American Vacuum Society    

年代:1988

数据来源: AIP

摘要:

The atomic geometry of the GaAs(110) surface has been determined by shadow‐cone enhanced desorption. Ga+secondary ion yields were analyzed without extensive calculations or assumptions of structure except that the second and all lower layer atoms are at bulk lattice sites. The structure determined from this simple analysis shows excellent qualitative agreement with most preceding models. The surface As atomic position is determined to be displaced laterally 1.28 Å toward the second layer Ga atom and is relaxed away from the surface by 0.69 Å, while the surface Ga atom is displaced 0.37 Å laterally in the same direction as the As atom and compressed 0.45 Å toward the surface. The resulting chain rotation is 29.3° and both surface As bond lengths are determined to be lengthened. A detailed discussion of the shadow‐cone enhanced desorption method of structure analysis is presented along with areas of possible refinement. Also presented is a description of our experimental apparatus which was developed for theinsitucharacterization of molecular‐beam epitaxially grown surfaces.

ISSN:0734-211X    

DOI:10.1116/1.584238

出版商:American Vacuum Society    

年代:1988

数据来源: AIP

摘要:

It is proposed that cleavage of Si to form Si(111) (2×1) occurs by the scission of three bonds between the closely spaced double layers, rather than by the severing of the single bond that connects the double layers. It is shown that three‐bond scission occurs in the behavior of dislocations in Si. Models of the surface structure based on this cleavage process are presented and discussed in light of the current understanding of this surface.

ISSN:0734-211X    

DOI:10.1116/1.584239

出版商:American Vacuum Society    

年代:1988

数据来源: AIP

摘要:

Reflection high‐energy electron diffraction (RHEED) pattern intensity calculations were performed within the kinematical approximation for the Si(111)‐7×7 reconstructed surface using a structure model containing dimers, adatoms, and stacking faults (DAS) in the surface layers as proposed by Takayanagietal. By fitting experimental RHEED pattern intensities taken with incident electron energies of 30 and 20 keV along the [1̄1̄2] azimuth, we found that the lateral and vertical positions of atoms in the first two surface layers (i.e., adatom layer and stacking fault layer) are in good agreement with that of an energy minimization calculation (EMC). The best fit to the low‐order diffraction spots was obtained when all adatoms were moved out toward vacuum around 0.45 Å from bulk positions, and the average spacing between adatom layer and stacking fault layer was set at (1.23±0.04)Å. If we include the dimer layer in the calculation, using the dimer spacing given by EMC, we found that the relative calculated RHEED pattern intensities slightly deviated from the experimental RHEED patterns. This slight disagreement may be due to incorrect lateral positioning of the dimer atoms by EMC or the kinematical approximation. Also, the calculated relative RHEED pattern intensity is not very sensitive to variations in the spacing between the stacking fault and dimer layers. Finally, our results imply that the kinematical approximation can be used for analyzing the intensities of superlattice RHEED spots in some suitable RHEED patterns.

ISSN:0734-211X    

DOI:10.1116/1.584240

出版商:American Vacuum Society    

年代:1988

数据来源: AIP

摘要:

Tunneling microscopy and spectroscopy are used to characterize defects in Al overlayers on Si(111). At low coverages where Al and Si adatoms are both present, voltage‐dependent scanning tunneling microscopy imaging allows them to be selectively imaged due to their different electronic structures. In (3)1/2‐Al, Si adatoms substituting for Al give rise to a defect state at −0.4 eV which is strongly localized in space. Band‐bending measurements indicate that the Si substitutional defects are electrically neutral. The results are interpreted in terms of a Mott–Hubbard model for a strongly correlated system.

ISSN:0734-211X    

DOI:10.1116/1.584241

出版商:American Vacuum Society    

年代:1988

数据来源: AIP

摘要:

In order to better understand the mechanism of molecular‐beam epitaxy (MBE) growth on GaAs(001), it is essential to know the structure of the arsenic rich (2×4)/c(2×8) surface where growth usually begins and ends. We have studied this surface with the scanning tunneling microscope (STM). The specimens were grown by MBE and arsenic capped prior to transfer to the STM. In the STM chamber the arsenic capping was removed by heating to ∼450 °C. The STM images show that the (2×4) unit cell consists of three arsenic dimers and a missing dimer to give the 4× periodicity, resulting in an arsenic coverage of 0.75 monolayers. The surface consists of small domains of both (2×4) andc(2×8) reconstructions. Thec(2×8) structure is made up from the basic (2×4) units. The STM images also show other features which may be important in the growth mechanism. The most striking of these are small islands one step up and small holes one step down, typically only a few unit cells in size. The step height corresponds to the spacing between arsenic planes. The raised islands are made up of complete unit cells, and can be as small as one (2×4) unit cell wide in the 4× direction. This shows the three dimers of a (2×4) unit cell to be a very stable structure, either in the plane when it is bordered by missing dimers, or on a raised island when bordered by a step edge.

ISSN:0734-211X    

DOI:10.1116/1.584198

出版商:American Vacuum Society    

年代:1988

数据来源: AIP