Electronic properties of cleaved Si(111) upon room-temperature deposition of Au

Low energy electron diffraction (LEED), Auger electron spectroscopy (AES), and photoemission yield spectroscopy (PYS) measurements are performed on a set of 2 × 1 reconstructed silicon (111) surfaces with different bulk dopings as a function of gold coverage θ, from zero to a few hundred monolayers, obtained by UHV evaporation on a sample kept at room temperature. Our measurements show the formation of an Au-Si alloy with the first two monolayers of gold deposit which induces a decrease of the ionization energy Φ by about 0.15 eV while no variation of the work function is observed. In the effective density of states, the double structure related to the 2 × 1 reconstruction is then replaced by a single peak at ? 0.4 eV below the valence band edge. At larger coverages, the Au-Si alloy remains on top of a gold layer which forms an abrupt interface with the Si substrate.     

Ge adsorption on the Si(111) surface

Total energy calculations, performed for one monolayer of Ge adsorbed on Si(111), indicate that 1 × 1 models such as the atop site and hollow site adsorption geometries are unstable with respect to the formation of 2 × 1 Seiwatz chains of Ge adatoms. This result indicates that, for one monolayer coverage, Ge-Ge bonds are likely to form.     

Room temperature adsorption and growth of Ga and In on cleaved Si(111)

Low energy electron diffraction (LEED), Auger electron spectroscopy (AES) and photoemission yield spectroscopy (PYS) measurements have been performed on a set of ultrahigh vacuum cleaved Si(111) surfaces with different bulk dopings as a function of Ga or In coverage θ. The metal layers are obtained by evaporation on the unheated substrate and θ varies from zero to several monolayers (ML). First, the 2×1 reconstruction of the clean substrate is replaced by a $\text{3}\text{×}\text{3}$ R30° structure at $\text{1}\text{3}$ ML, meanwhile the dangling bond peak at 0.6 eV below the valence band edge E_vs is replaced by a peak at 0.1 eV for Ga or 0.3 eV for In, below E_vs. At the same time, the ionization energy decreases by 0.4 eV (Ga) or 0.6 eV (In), while the Fermi level pinning position gets closer to the valence band edge by about 0.1eV. Upon increasing θ, new LEED structures develop and the electronic properties keep on changing slightly before metallic islands start to grow beyond θ ～1 ML.     

Ge/Si(111)与Si/Ge(111)体系的生长特性与表面再构研究

本文利用RHEED和AES对Ge/Si(111)和Si/Ge(111)体系的生长特性与表面再构进行了研究。由此提出了其生长模式,并讨论了应力对生长特性、界面特性和表面再构的作用。     

Effect of surface reconstruction on the adsorption of Ge on clean Si(111)

The effect of ultrahigh vacuum deposition of Ge below and at monolayer (ML) coverage onto a 7 × 7 reconstructed clean Si(111) surface held at room temperature is studied by low energy electron diffraction (LEED), Auger electron spectroscopy (AES) and photoemission yield spectroscopy (PYS). The results are compared to those obtained on 2 × 1 reconstructed clean Si(111) : (i) the Si dangling bond states are replaced by Ge dangling bond states at submonolayer coverages in both cases; (ii) the 7 × 7 reconstruction persists below 1 ML, it is not replaced by a ? 3 × ? 3 R30° at 1/3ML as it was on the 2 × 1; and (iii) the coverage below 1 ML is not homogeneous on the 7 × 7 reconstruction. This behaviour can be explained by the influence of the inhomogeneties associated with the 7 × 7 reconstruction.     

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.     

Anisotropic photoemission and chemisorption-bond geometry on Ge(111) and Si(111) surfaces

In order to study the effects of photon-polarization selection rules on chemisorptionbond geometry, we have measured photoelectron spectra as a function of angle of incidence, θ_i, in the range 28° ? θ ? 80°. A noble-gas UV resonance lamp and cylindrical mirror analyzer were used to measure both bonding and non-bonding surface Orbitals. A large enhancement (200–400%) of the photocmission is found when photon electric field intensity is near the maximum normal to the surface. This indicates a spatial variation of microscopic fields which is approximately independent of adsorbate bonding since it is determined by the optical properties of the substrate. In addition, we observe some effects on adsorbate photoelectron peaks due to different orbital symmetry. The case of atomic hydrogen chemisorption is discussed as an example of this latter effect.     

Silicene on hydrogen‐passivated Si(111) and Ge(111) substrates

By means of first‐principles calculations we predict the stability of silicene as buckled honeycomb lattice on passivated substrates of group‐IV(111)1 × 1 surfaces. The weak van‐der‐Waals interaction between silicene and substrates does not destroy its linear bands forming Dirac cones at the Brillouin zone corners. Only very small fundamental gaps are opened around the Fermi level.

     

New surface phases for potassium adatoms on cleaved Si(111)

In a previous study (B. Reihl and K.O. Magnusson, Phys. Rev. B 42 (1990) 11 839) no change was observed in the LEED pattern from the 2 × 1 pattern of the clean Si(111) surface upon potassium adsorption. In contrast to this paper, we observe six surface phases during room temperature dosing of K on the cleaved Si(111) surface. In addition, we observe a 3 × 1 pattern on the cleaved K/Si(111) surface upon annealing. This paper will provide photographs of the new phases observed with LEED. It also will set limits on the K coverage required for these phases using results from photoemission and secondary electron cutoff measurements. Tentative models for these surfaces will be proposed. These new LEED results show a more complex interaction between K and the cleaved Si(111) surface than previously thought and hopefully will encourage further exploration of adsorption on the cleaved Si(111) surface.     

Two-dimensional excitonic insulators: Si and Ge (111) surfaces

We suggest that dangling electrons at (111) surfaces of group IV semiconductors should have a 2-dimensional excitonic insulator ground state. The higher order reconstructions observed in Si and Ge are accounted for in detail.     

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.     

Ⅶ族元素在Si(111)和Ge(111)表面上的吸附

本文用原子集团模型和电荷自洽的EHT方法研究了Ⅶ族元素在Si(111)和Ge(111)表面上的化学吸附。利用能量极小的原则确定了各元素的化学吸附构型。对于Cl,在这两个表面均是顶位吸附。对于Ⅰ,都呈三度空位吸附。对Br,顶位及三度空位吸附均能发生,但是在Si(111)表面顶位吸附要优于三度空位,而在Ge(111)表面则三度空位吸附优于顶位。最后对Ⅶ族元素原子在这两种表面的吸附行为作了讨论,并与实验作了比较。 关键词：     

Surfactants: Perfect heteroepitaxy of Ge on Si(111)

The hetero growth of Ge on Si results in formation of 3D clusters with an uncontrolled defect structure. Introduction of a monolayer of a surfactant completely changes the growth mode to a 2D-layer growth (Frankvan der Merwe) with a continuous and smooth Ge film on Si(111). The surfactant is not incorporated but segregates and floates on the growing Ge film. The saturation of the dangling bonds of the semiconductor reduces the surface free energy and drives the strong segregation. The effect on the growth process is the selective change of activation energies which are important for diffusion and the mobility of the Ge. Up to a thickness of 8 MLs (MonoLayers) the misfit-related strain of the pseudomorphic Ge film is relaxed by formation of a micro rough surface. This allows a partial relaxation of the Ge towards its bulk lattice constant which would not be possible for a flat and continuous film. For thicker Ge films the misfit of 4.2% is relieved by a periodic dislocation network, which is confined to the Si-Ge interface. Ge-films thicker than 20 MLs are free of defects and completely relaxed to the Ge bulk lattice constant: a model system for perfect heteroepitaxial growth.