Electronic structure of the GeGaAs (111) and (111) heterojunctions

The electronic density of states for GeGaAs (111) and ($\text{1}$$\text{1}$$\text{1}$) heterojunctions has been calculated. No interface states in the fundamental gap are found. A sizeable density of interface states below the top of the valence band is found for GeGa bonds-(111) junctions-interface states in the ionic gap are reported. The effect of varying the amount of the valence band discontinuity across the interface is discussed.     

Tight-binding calculations of (111) surface densities of states of Ge and GaAs

We have used the tight-binding method to calculate the local densities of states of unreconstructed Ge (111) and GaAs (111), ($\text{111}$) surfaces. In the unrelaxed surface configuration we find two types of states for each surface. The effects of relaxation on Ge surface states are also discussed.     

Electronic structure of the (111) and (111) surfaces of GaAs

Inward relaxation effects of the outermost Ga layer on the electronic structure of GaAs (111) Ga and outward expansion effects of the outermost As layer on that of GaAs ($\text{111}$) As are studied by extended Hückel theory. Three different surface geometries are examined for the respective surfaces. It is shown that upon relaxation on GaAs (111) or upon expansion on GaAs ($\text{111}$) new surface states associated with dangling- and back-bonds are revealed. The character and dispersion behaviour of strongly localized surface states are described.     

Investigation of surface states on the polar (111) and (111) faces of GaAs by photoelectron spectroscopy

Photoelectron spectra from the clean polar (111) and ($\text{111}$) faces of GaAs show emission from surface states. After exposure to oxygen which produces a covarage of about one monolayer, this emission disappears.     

Structure and electronic properties of cleaved Si(111) upon Ge adsorption

The effect of ultrahigh vacuum deposition of Ge below and at monolayer coverage onto clean cleaved Si(111) surface held at room temperature is studied by low energy electron diffraction, Auger electron specroscopy and photoemission yield spectroscopy. A well ordered $\text{3}\text{×}\text{3}$ R 30° structure developes at $\text{1}\text{3}$ ML, where it replaces the 2 × 1 initial pattern; it persists at $\text{2}\text{3}$ ML before transforming into a 1 × 1 diagram which fades into increasing background at 1 ML and up. Si surface dangling bonds are replaced at $\text{1}\text{3}$ ML by states associated with Ge-Si bonds and Ge dangling bonds to which states due to Ge-Ge bonds added upon increasing coverage.     

On the interaction of cesium with cleaved GaAs(110) and Ge(111) surfaces: Work function measurements and adsorption site model

The work function of UHV cleaved p-Ge(111) and n-GaAs(110) surfaces has been measured in dependence of the Cs coverage. At very low coverages θ < 0.001 the decrease of the contact potential difference is extremely steep. For GaAs the initial slope of the CPD versus coverage curve amounts to ?740 eV for Ge to ?130 eV per monolayer. Up to the saturation coverage the curves exhibit straight line segments with breaks at distinct coverages. Breaks are found for GaAs at approximately $\text{1}\text{12}$, $\text{1}\text{6}$, and $\text{1}\text{3}$ of a monolayer, for Ge at about $\text{1}\text{12}$, $\text{1}\text{4}$, $\text{1}\text{2}$, and $\text{3}\text{4}$. A new model is developed to explain this behaviour. It is based on the assumption of specific adsorption sites for the Cs atoms at the surfaces. With this model the experimental results, including the breaks, may be described in the whole coverage range from θ = 0.03 up to the saturation. Furthermore the dipole moments derived from the straight line segments are in excellent agreement with those values calculated for different surface molecules between the adsorbed cesium and substrate atoms at the specific adsorption sites.     

Surface and interface states of (111) faces of semiconductors

The surface and interface states of the (111) and (1?1?1?) faces of Ge and GaAs have been studied self-consistently within the tight binding approach. The surface and interface states occupation and energy levels are determined. The results obtained for the surface are compared with available experimental evidence. Those obtained for the interfaces suggest that some relaxation must occur at the interfaces between Ge and GaAs.     

Oxygen adsorption on Ge(111) surface: I. Atomic clean surface

Oxygen adsorption on a clean Ge(111) surface has been studied in the temperature range 300–560 K by means of Auger electron spectroscopy (AES), thermal desorption (TD), work function (WF) measurements, and electron energy loss spectroscopy (ELS). The adsorption and WF kinetics at 300 K exhibit a shape different from those observed at higher adsorption temperatures. At 300 K oxygen only removes the empty dangling bond surface state, whereas at higher temperature new loss transitions involving chemically shifted Ge 3d core levels appear. The findings imply that at 300 K only a chemisorption oxygen state exists on the Ge(111) surface whereas the formation of an oxide phase requires higher temperatures. The shapes of the TD curves show that the desorption of GeO follows $\text{1}\text{2}$ order desorption kinetics.     

Angle resolved photoemission measurements on AgSi(111) 7 × 7 interfaces

The AgSi(111) interface is investigated by LEED, AES and angle resolved photoemission spectroscopy using 50 eV synchrotron radiation in p-polarization. Results on room temperature (RT) silver growth on Si(111) 7 × 7 are characterized by an evolution of the LEED pattern and of the d band shape which is consistent with 2D island formation in the submonolayer range. When the Ag coverage (Θ) is increased, a progressive build-up of Ag layers occurs with a possible interdiffusion of the atomic constituents. The ordered $\text{Si(111)}\text{3}\text{×}\text{3}\text{R(30°)Ag}$ structure (R₃) obtained by annealing a 1 ML RT deposit gives rise to new interface states near E_F. In contrast to the RT deposit at the same Θ, two well defined d band peaks are present while the bulk Si emission near 3.4 eV is clearly seen. The R₃ data would favour recent crystallographic models which conclude to an embedment of the Ag atoms in a threefold hollow adsorption site.     

Study of Ag/Si(111) submonolayer interface: I. Electronic structure by angle-resolved UPS

Angle-resolved ultraviolet photoelectron spectra have been measured for well defined Ag/Si(111) submonolayer interfaces of (1) $\text{Si(111)(}\text{3}\text{×}\text{3}\text{)R30°-Ag}$, (2) “Si(111)(6 × 1)-Ag”, and (3) Ag/Si(111) as deposited at room temperature. Non-dispersive and very narrow (FWHM ～ 0.4–0.5 eV) Ag 4d derived peaks are found at 5.6 and 6.5 eV below the Fermi level for surface (1) and at 5.3 and 6.0 eV for surface (2). Dispersions of sp “binding” states in the energy range between E_F and Ag 4d states have been precisely determined for surface (1). Electronic structures similar to those of the Ag(111) surface, including the surface state near E_F, have been observed for surface (3).     

AES,photoemission and work function study of the deposition of Cs on (100) and (111)B GaAs epitaxial layers

Work function, photoemission and AES measurements have been made on (100) and (111)B epitaxial GaAs layers as a function of caesium coverage. It has been shown that the photoemission maximum and work function minimum occur at a coverage of $\text{2}\text{3}$ of a monolayer. The reduction of the work function takes place in distinct stages with changes in the rate of fall occurring at $\text{1}\text{8}$ and $\text{1}\text{2}$ of a monolayer for the (111)B face and $\text{1}\text{8}$, $\text{1}\text{4}$, $\text{1}\text{3}$ and $\text{1}\text{2}$ of a monolayer for the (100) face. This indicates the presence of specific adsorption sites which have not been observed in previous work on these faces.     

LEED and AES studies of the initial growth of Ge epilayers on GaAs(100)

The initial stages of growth of epitaxial Ge overlayers on the GaAs(100) surface have been studied by LEED and AES on overlayers from 0.1 monolayers (ML) to 10 ML in thickness. It is found that a coverage of about 0.2 ML converts the initial clean surface reconstruction into a single domain (1 × 2) reconstruction with a surface atomic geometry very similar to that of clean Ge. Further growth does not significantly change the arrangement of atoms at the surface. Growth from 1 to 4 ML proceeds by a double layer growth mechanism which maintains the single (1 × 2) domain. Auger measurements indicate that the growing surface has a $\text{1}\text{2}$ ML As enrichment, and that the interface is not abrupt, but has a mixed GeGa or GeAs transition layer.     

XPS studies of adatom-adatom interactions: I/Ag(111) and I/Cu(111)

We have studied submonolayer adsorption, at room temperature, of iodine on the (111) faces of silver and copper, using LEED and XPS. In both systems the √3 × √3 LEED pattern appears at ～0.2 monolayer (ML) coverage; no other superlattice pattern was observed. The $\text{I}\text{4}\text{d}{}_{\mathrm{<mtext>5</mtext><mtext>2</mtext>}}$ core electron binding energy in both cases decreases by ～0.15 eV between very dilute coverage and 0.33 ML. The leveling-off of the binding energy for I/Ag(111) for coverages >0.2 ML is shown to be a unique experimental manifestation of an indirect, substrate-mediated adatom-adatom interaction, an attraction of several meV between next-nearest neighbor iodine atoms. The more nearly linear decrease in the I binding energy on Cu(111) is shown to imply a significantly weaker next-nearest neighbor interaction on this surface. The appearance of the √3 × √3 LEED pattern at low coverages on Cu is shown to be consistent with short-range order produced merely by a size effect, that is, by nearest neighbor exclusion. These conclusions are reached with the help of Monte Carlo calculations of a triangular lattice gas.     

Theory of electron states at random alloy surfaces - surface states on Cu-Ni (111)

We present first principles calculations (based on the KKRCPA) of the angle-resolved photocurrent emitted from the (111) surfaces of single crystals of $\text{Cu}$-Ni random alloys, and compare the results with new experimental data. Surface states close to the Fermi level are observed, even for concentrated alloys, and their behaviour as a function of composition and $\text{k}$_∥ is correlated with features in the bulk spectral density. Calculations for alloys with a non-uniform concentration profile at the surface (surface segregation) are described, and the effect on the surface states is discussed.     

A study of the (00) LEED beam intensity at normal incidence from CdS(0001), Cu(001), Cu(111), and Ni(111)

A Faraday cage apparatus is used for the measurement of the (00) LEED beam intensity, I₍₀₀₎, and the total secondary emission coefficient, δ(E_k), for angles of incidence from 0° ± 2° to 8° ± 2°, with an energy resolution of ± 0.037 of the incident beam energy, in the energy range 1 to 200 eV. The data are normalized and expressed as a fraction of the incident beam intensity. The basic principle of operation is the separation of the incident and specularly diffracted beams in a uniform magnetic field. Monolayer, or in-plane, resonances associated with the emergence of nonspecular beams, as well as beam threshold minima, are observed in I₍₀₀₎ at normal incidence from clean CdS(0001), Cu(111), and Ni(111). Some major differences are observed in the I₍₀₀₎ profiles for the clean (111) surfaces of nickel and copper. All secondary Bragg peaks, except the 2$\text{2}\text{3}$ order, have greater intensities for Ni(111) in the energy range 50–150 eV, thus indicating that the atomic scattering cross-section for electrons in this energy range is larger for nickel than for copper. For the (111) surface of nickel, the (11) resonance is missing, but the (10) resonance and all $\text{1}\text{3}$ order secondary Bragg peaks between the second and fifth orders are observed. For Cu(111) both the (10) and (11) resonances are observed, but the $\text{1}\text{3}$, $\text{2}\text{3}$, 1$\text{2}\text{3}$, and 3$\text{1}\text{3}$ order secondary Bragg peaks are missing in this energy range. These data indicate that multiple scattering with evanescent intermediate waves, or “shadowing”, is predominate on the (111) surfaces on nickel and copper for energies above 30 eV, and that below 30 eV multiple scattering with propagating intermediate waves is predominate on Cu(111). Correlation of the (00) beam intensity profiles from clean Ni(111) at 0°, 2°, and 6° with the intensity profiles of the (10). (1&#x0304;0), and (11) non-specular beams is nearly one-to-one from 30 eV to 100 eV, thus supporting the dynamical theories of LEED in which peaks in the (00) beam are expected to occur at nearly the same energies as peaks in the non-specular beams.     

Cluster model approach for electronic structure of Si and Ge(111) and GaAs(110) surfaces

For the purpose of exploring how realistic a cluster model can be for semiconductor surfaces, extended Huckel theory calculations are performed on clusters modeling Si and Ge(111) and GaAs(110) surfaces as prototypes. Boundary conditions of the clusters are devised to be reduced. The ideal, relaxed, and reconstructed Si and Ge(111) surfaces are dealt with. Hydrogen chemisorbed (111) clusters of Si and Ge are also investigated as prototypes of chemisorption systems. Some comparison of the results with finite slab calculations and experiments is presented. The cluster-size dependence of the calculated energy levels, local densities of states, and charge distributions is examined for Si and Ge(111) clusters. It is found that a 45-atom cluster which has seven layers along the [111] direction is large enough to identify basic surface states and study the hydrogen chemisorption on Si and Ge(111) surfaces. Also, it is presented that surface states on the clean Si and Ge(111) clusters exist independent of relaxation. Further, the calculation for the relaxed GaAs(110) cluster gives the empty and filled dangling-orbital surface states comparable to experimental data and results of finite slab calculations. The cluster approach is concluded to be a highly useful and economical one for semiconductor surface problems.     

Electronic structure of the Si (111) reconstructed surface in the vacancy model

The self-consistent pseudopotential method is applied to the Si (111) 7 × 7 reconstructed surface in the vacancy model with a simplified $\text{3}\text{×}\text{3}$ superlattice structure. Numerical results with and without relaxation of surface atoms are presented. It is concluded that the relaxation, if any, is to be much smaller than the atomic distance to explain the photoemission spectrum of the 7 × 7 surface. The importance of the many-body effect is suggested in the photoemission process associated with the dangling bond surface states of Si.     

Photoelectron diffraction effects in XPS angular distributions from GaAs(110) and Ge(110) single crystals

Angular distribution measurements of XPS intensities have been made for various spectral lines from GaAs(110) and Ge(110) single-crystal surfaces. Observed angular distribution curves (ADC's) showed steep intensity variations and sharp peaks due to X-ray photoelectron diffraction (XPED) phenomena. The effects of the type of transition process (photoelectron or Auger), electron kinetic energy and crystal structure on the XPED patterns were examined. Considerably different ADC patterns were observed for high-energy photoelectrons and Auger electrons and for low-energy photoelectrons. ADC's for Ga 3d, As 3d and Ge 3d showed almost the same patterns for scans of the type [110] → [100] → [1$\text{1}$0], but they showed substantially different patterns for [110] → [11$\text{1}$] → [00$\text{1}$] scans. These features correspond well with the structural characteristics of GaAs and Ge crystals. A discussion of the applicability of XPS angular distribution measurements to the geometric analysis of crystal surfaces is presented.     

Structure of the Si(111)−2 × 1 surface with 13 ML Ge coverage

Two different $\text{3}\text{×}\text{3}\text{R30°}$ adatom models for $\text{1}\text{3}$ monolayer Ge coverage of the Si(111)?2 × 1 surface were studied with the Pseudopotential method. Total energy calculations indicate that the T₄ model (adatom above the second layer) is preferable to the H₃ model (adatom above the hollow site)     

Electron tunneling in the GaAs/AlAs(111) structures with a smooth potential at heteroboundaries

The effect of smooth interface potential on the electron tunneling in the GaAs/AlAs(111) structures with thin layers is studied using the pseudopotential method. The transition region between the structure components is represented by a half-period of the hexagonal (GaAs) ₃(AlAs)₃ (111) superlattice. It is shown that the allowance for the smooth potential results in a decrease in the Γ-L-mixing, Fano-resonance narrowing, and disappearance of interface states at the GaAs/AlAs(111) interface as compared to the abrupt-interface model. The shifts of the lowest Γ-and L-resonances observed for the structures with the layer thickness <2 nm amount to ∼0.1 eV, which is in good agreement with the behavior of levels in quantum wells. The transmission coefficient of electrons with the energies 0–0.5 eV above the GaAs conduction-band bottom obtained by multivalley calculation is close to that calculated with allowance for the lowest conduction band states Γ ₁ ⁽¹⁾ and Γ ₁ ⁽²⁾ of superlattice and Γ ₁ and L ₁ of binary crystals. This indicates that a two-valley superlattice model of the smooth GaAs/AlAs(111) interface can be developed. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 7, pp. 7–13, July, 2007.