Second-harmonic spectroscopy of Ge/Si(001) and Si1-xGex(001)/Si(001)

0.9 Ge_0.1(001)/Si(001) films with SH photon energies 3.1<2hν<3.5 eV near the bulk E₁ critical point of Si(001) or Si_0.9Ge_0.1(001). Ge was deposited on Si(001) by using atomic layer epitaxy cycles with GeH₄ or Ge₂H₆ deposition at 410 K followed by hydrogen desorption. As Ge coverage increased from 0 to 2 monolayers the SH signal increased uniformly by a factor of seven with no detectable shift in the silicon E₁ resonant peak position. SH signals from Si_0.9Ge_0.1(001)/Si(001) were also stronger than those from intrinsic Si(001). Hydrogen termination of the Si_0.9Ge_0.1(001) and Ge/Si(001) surfaces strongly quenched the SH signals, which is similar to the reported trend on H/Si(001). We attribute the stronger signals from Ge-containingsurfaces to the stronger SH polarizability of asymmetric Ge-Si and Ge-Ge dimers compared to Si-Si dimers. Hydrogen termination symmetrizes all dimers, thus quenching the SH polarizability of all of the surfaces investigated. Received: 13 October 1998 / Revised version: 18 January 1999     

Self-diffusion of (31)Si and (71)Ge in relaxed Si(0.20)Ge(0.80) layers

Self-diffusion of implanted (31)Si and (71)Ge in relaxed Si(0.20)Ge(0.80) layers has been studied in the temperature range 730-950 degrees C by means of a modified radiotracer technique. The temperature dependences of the diffusion coefficients were found to be Arrhenius-type with activation enthalpies of 3.6 eV and 3.5 eV and preexponential factors of 7.5 x 10(-3) m(2) s(-1) and 8.1 x 10(-3) m(2) s(-1) for (31)Si and (71)Ge , respectively. These results suggest that, as in Ge, in Si(0.20)Ge(0.80) both (31)Si and (71)Ge diffuse via a vacancy mechanism. Since in Si(0.20)Ge(0.80) (71)Ge diffuses only slightly faster than (31)Si , in self-diffusion studies on Si-Ge (71)Ge radioisotopes may be used as substitutes for the "uncomfortably" short-lived (31)Si radiotracer atoms.     

Surface electromigration of metals on Si(001): In/Si(001)

The possibility of surface electromigration (SE) of metals of In, Ga, Sb and Ag on a very flat Si(001)2×1 substrate (single domain 2×1) was examined by SEM, μ-RHEED and μ-AES under UHV conditions. It was found that Ga, Sb and Ag show no SE on Si(001) surface even at DC annealing temperatures for the desorption of these metals. For In on Si(001), a very fast SE (8000 μm/min) towards the cathode side was found that suddenly sets in at 450°C DC annealing, which was related to a surface phase transition. μ-RHEED and μ-AES observation showed that the SE is related to an ordered 4×3-In phase together with two-dimensional In gas phase over the 4×3-In phase and an In-disordered phase at the front end of SE. Single domain 4×3-In phases were found to occur under sequences of In deposition and DC annealing which involve the In SE on Si(001).     

Exchanges between Si and Pb adatoms on Si(111)

Real-time observation by high-temperature scanning tunneling microscopy of exchanges between Si and Pb atoms on a Si(111)-√3 × √3 surface is reported. The exchange rate is obtained as a function of the temperature. The activation energy of the exchange is about 1.2 eV, and the prefactor, shown to depend on the Pb coverage, is from 2 × 10¹⁰ to 8 × 10¹¹ s⁻¹. This prefactor is much larger than that for the exchange between Pb and Ge adatoms on a Ge(111)-c(2 × 8) surface, indicating that the adatom arrangement greatly influences the exchange mechanism. We also report that metastable 9 × 9 reconstruction appears during Pb desorption.     

Properties of Si nanostructural modification on Si (111) surface

Si nanostructures (Si-NSs) epitaxially grown or adsorbed on Si (111) surface, with various shapes including pit-like, bars, islands, hill-like, diamond-like and double cage, were studied theoretically using density-functional theory (DFT) calculations. The electronic and optical properties of these Si-NSs were calculated, showing that the designed Si-NSs modifications can enhance the optical absorption for Si surface.     

Depth profiling of melting and metallization in Si(111) and Si(001) surfaces

An original approach for measuring the depth profile of melting and metallization of the Si(111) and Si(001) surfaces is proposed and applied. The different probing depths of the Auger electron and electron energy loss (EELS) spectroscopies are exploited to study the number of molten and metallic layers within 5-30 ? from the surface up to about 1650 K. Melting is limited to 3 atomic layers in Si(001) in the range 1400-1650 K while the number of molten layers grows much faster (5 layers at about 1500 K) in Si(111) as also indicated by the L(3)-edge shift observed by EELS. The relationship between melting and metallization is briefly discussed.     

Nucleation and growth during molecular beam epitaxy (MBE) of Si on Si(111)

The nucleation of Si on Si(111) has been studied during deposition in UHV by spot profile analysis of low energy electron diffraction (SPA-LEED). A new method of evaluation is developed by separating the measured spot profile into a central spike and a broad shoulder. The energy dependence of the fraction of the central spike out of the total diffracted intensity provides the vertical distribution of surface atoms over different levels. With this method it is shown that the first nucleation occurs in islands of double height. Only after deposition of several layers a layer-by-layer growth is found with a well defined nucleation of a new layer before the former one is completed.     

Scanning tunneling microscopy observation of copper-phthalocyanine molecules on Si(100) and Si(111) surfaces

Molecules of Cu-phthalocyanine (CuPc) deposited on Si(100) and Si(111) surfaces have been observed by an ultra high vacuum field ion scanning tunneling microscope (FI-STM). On a Si(100) surface, STM images with four-fold symmetry are observed, which reflect the shape of the CuPc molecule. The STM pictures show that CuPc molecules are deposited with the molecular plane parallel to the substrate surface and have three kinds of adsorption configurations on the dimer-row of Si(100). The images of the CuPc are modified by the electronic state of the Si(100) surface. This behavior suggests strong interaction between the molecule and the substrate. The molecular images on the Si(111) surface have a unique bias-voltage dependence. At a sample bias of 1.6 V, the molecule looks transparent by STM, and becomes dark like a vacancy at 1.2 V. From the bias dependence, the electronic interaction between the CuPc molecule and the Si surface is discussed.     

Hydrogen vibrations on Si (111)

High resolution electron energy loss spectra are reported for atomic hydrogen and deuterium adsorption on the Si (111) 7x7 surface. The experiments were carried out for various hydrogen exposures and different substrate temperatures. Clear evidence is given for the formation of SiH₂ and/or SiH₃ complexes in the early stages of adsorption. Strong indications are also obtained for the removal of adsorbed hydrogen probably via the formation and desorption of silane during hydrogen exposure.     

Self-organized nanostructures in Si1-xGex films on Si(001)

Received: 30 July 1998/Accepted: 23 October 1998     

Order-disorder transition on Si(001)

A phase transition between c(4x2) and 2x1 structures on the Si(001) surface has been observed at 200 K by low-energy electron diffraction. This transition is a second order order-disorder transition of the asymmetric dimer configuration. The streak pattern remains up to well above the transition temperature. The temperature dependence of the width and the length of the streak can be described in terms of the effects of a strong anisotropic coupling between adjacent asymmetric dimers.