摘要:

Photoemission spectra of 3dcore 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 104L, and for As near 106L. Two oxide components shifted by 0.45 and 1 eV relative to the bulk Ga‐3dcore level are evident throughout the exposure range, but shift to 0.8 and 1.4 eV for 1014L. With increasing exposure the As‐3dcore 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 106L. A final pinning position of the Fermi energy was not observed.

ISSN:1071-1023    

DOI:10.1116/1.582823

出版商:American Vacuum Society    

年代:1984

数据来源: AIP

摘要:

This paper is a summary of a series of experiments studying the exposure of hydrogen, oxygen, and water, on the (2×1) surfaces of Si(100) and Si(111). While the primary focus has been on high resolution electron energy loss (EELS) results, low energy electron diffraction (LEED) and x‐ray photoelectron spectroscopy (XPS) are also used in the studies. Both the (100) and cleavage (111) surfaces form a monohydride and a dihydride exhibiting a (2×1) and a (1×1) LEED pattern, respectively. These systems exhibit saturation, which is consistent with the model of hydrogen saturation of the dangling bonds. Upon water adsorption the Si–H and Si–OH vibronic modes are observed, indicating that water is decomposed. On the cleavage surface only, there is evidence of a very weak scissor mode, allowing for the possibility of a few percent of molecular water adsorption. Oxygen adsorption is complex. For samples formed at high temperatures (∼1000 K) the observed vibronic features are similar to those known for the Si–O–Si complexes in vitreous glasses. For thin oxide layers (0.5<θ<1.3 monolayers) a linear relationship is observed between oxygen coverage and asymmetric mode frequency. These data are fit with models developed for glasses which, for the monolayer regime, yield an average bond angle of about 130° and a bond distance of 1.65 Å. The results support the model in which the oxygens are envisioned as being inserted into the Si–Si back bonds.

ISSN:1071-1023    

DOI:10.1116/1.582824

出版商:American Vacuum Society    

年代:1984

数据来源: AIP

摘要:

New low‐temperature (T∼200 K) elastic low‐energy electron diffraction (ELEED) intensity data obtained for ZnSe films grown epitaxially on GaAs(110) are analyzed using anR‐factor methodology. The differences between the measured ELEED intensities for ZnSe(110) and those for GaAs(110) reveal the possibility that these two surfaces may not exhibit comparable atomic geometries. A bond‐length‐conserving top‐layer rotation of the Se species outward and the Zn inward corresponding to ω1=4° provides a description of the measured intensities (Rx=0.22,RI=0.21) comparable to that afforded by the ZnSe(110) analog of the GaAs(110) structure (a relaxed version of an ω1=29° structure,Rx=0.24,RI=0.16). Since GaAs and ZnSe exhibit essentially identical bulk lattice constants, the possibility that their (110) surfaces exhibit different atomic geometries poses a puzzle within the framework of current understanding of this topic.

ISSN:1071-1023    

DOI:10.1116/1.582825

出版商:American Vacuum Society    

年代:1984

数据来源: AIP

摘要:

Along with the growing interest in semiconductor superlattices, various theoretical schemes have been proposed to study the nature of the electronic states within these structures. The work presented here highlights a new method to investigate the electronic and optical properties of semiconductor superlattices. The backbone of the theory rests on a realistic description of the complex‐kband structure of the constituent semiconductors coupled with a suitable set of boundary conditions for the superlattice wave function. The bulk Bloch solutions, propagating and evanescent, in each semiconductor are described within a full‐zonek ⋅ pHamiltonian that provides an accurate description of the solutions up to the first Brillouin zone edge. An attractive feature of the present treatment is that the complex‐kbulk Bloch solutions ofeachconstituentsemiconductorare expanded on thesamesetof zone‐center basis functions. A new technique for constructing thek ⋅ pHamiltonian of each constituent semiconductor is presented. The superlattice wave function is described by a linear combination of propagating and evanescent bulk Bloch solutions. The expansion amplitudes are determined by imposing a set of boundary conditions on the superlattice wave function across the superlattice interfaces. These boundary conditions are used to formulate an eigenvalue problem whose solution yields directly the corresponding superlattice states associated with real or complex superlattice wave vectorq. This method provides an accurate technique to treat superlattices where one of the constituent semiconductors has an indirect energy band gap. An exposition of the formalism is presented, and the physical origin of the superlattice states is studied. The test case of the GaAs–AlAs (100) superlattice is presented. Pertinent applications are also discussed.

ISSN:1071-1023    

DOI:10.1116/1.582826

出版商:American Vacuum Society    

年代:1984

数据来源: AIP

摘要:

We have used the variational method to calculate the acceptor binding energies in GaAs–AlxGa1−xAs quantum wells. The calculation includes the coupling of the top four valence bands of both materials in the multiband effective mass approximation. To ensure the convergence of the calculation, a large number of basis functions which are made up of thes‐like ord‐like spatial states multiplied byj=3/2 spinors are used for the expansion of the acceptor wave function. Because the quantum well potential reduces the symmetry fromTdtoD2d, the bulk Γ8acceptor ground state splits into Γ6and Γ7states. The Γ6state is predominantly derived from the heavy‐hole subband and the Γ7state is predominantly derived from the light‐hole subband. We have calculated the binding energies of the Γ6state (measured from the top of the heavy‐hole subband) and the Γ7state (measured from the top of the light‐hole subband) for both center doped and edged doped quantum wells for various barrier heights as functions of well width. Except for well widths smaller than ≊40 Å, the Γ7binding energy is greater than the Γ6binding energy. In recent studies, the photoluminescence resulting from the acceptor levels to conduction band transition in MBE grown GaAs–AlGaAs superlattices has been measured. Our theoretical results are in excellent agreement with these experimental data.

ISSN:1071-1023    

DOI:10.1116/1.582827

出版商:American Vacuum Society    

年代:1984

数据来源: AIP

ISSN:1071-1023    

DOI:10.1116/1.582828

出版商:American Vacuum Society    

年代:1984

数据来源: AIP

摘要:

High resolution electron energy loss spectroscopy has been performed as a function of temperature (15–300 K) to determine the electronic transitions of clean and metallized Si(111) surfaces. A quantitative analysis of these EELS results is used to delineate and identify the localized and delocalized states at the Fermi level. Si(111)‐7×7 is found to have a 100 meV surface state band gap in which lies a narrow, half occupied state that determines the Fermi level. Hydrogen titration studies suggest densities of these states at ∼1.6×1013/cm2. In contrast, metal impurity stabilized Si(111)‐1×1 surfaces are not found to have these narrow states atEF. Au overlayers on Si(111)‐1×1 produce a metallic overlayer for coverages above ∼1.6×1015Au/cm2. Pd reaction with Si(111)‐1×1 forms a semimetal or semiconducting compound at coverages ≲1×1015atoms/cm2and appears to generate narrow partially occupied Si states at the interface. At higher Pd coverages a metallic silicide forms which uniformly covers the surface.

ISSN:1071-1023    

DOI:10.1116/1.582829

出版商:American Vacuum Society    

年代:1984

数据来源: AIP

摘要:

We present calculations of the surface electronic state dispersion curvesE(k̄) of the (100) (2×1) reconstructed surface of Ge, and compare them with recent angle‐resolved photoelectron measurements by Nelsonetal. We assumed Chadi’s asymmetric dimer model of the surface reconstruction and performed our calculations using the analytic Green’s function technique, with an empirical tight‐binding Hamiltonian.

ISSN:1071-1023    

DOI:10.1116/1.582830

出版商:American Vacuum Society    

年代:1984

数据来源: AIP

摘要:

We present results of a reconstruction model proposed for the (2×2) GaAs(111) surface, together with a reexamination of the (1×1) GaAs(110) surface structure. Our model indicates that the reconstruction mechanisms on the (111) and (110) surfaces are similar to one another. In both cases, surface electronic energies are lowered via orbital rehybridization between nearest neighbor Ga and As atoms with dangling bonds. Reexamination of the GaAs(110) surface structure confirms our previous result of a tilt angle of ω=27°±2° and rejects a recently proposed value of ω=7°.

ISSN:1071-1023    

DOI:10.1116/1.582831

出版商:American Vacuum Society    

年代:1984

数据来源: AIP

ISSN:1071-1023    

DOI:10.1116/1.582832

出版商:American Vacuum Society    

年代:1984

数据来源: AIP